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Education Department

Attractiveness for insects in showy involucres of Cornus canadensis
After Conway MacMillan

PRINCIPLES OF

JOSEPH Y. BERGEN, A.M.

AUTHOR OF "ELEMENTS OF BOTANY," "FOUNDATIONS OF BOTANY,'
" PRIMER OF DARWINISM," ETC.

BRADLEY M. DAVIS, PH.D.

HEAD OF DEPARTMENT OF BOTANY IN THE MARINE BIOLOGICAL,

LABORATORY, RECENTLY ASSISTANT PROFESSOR OF PLANT

MORPHOLOGY IN THE UNIVERSITY OF CHICAGO

GINN & COMPANY

BOSTON • NEAV YORK • CHICAGO • LONDON

;

jpresg

G1NN & COMPANY • PRO-
PRIETORS • BOSTON • U.S.A.

«•£.

PREFACE

The present work owes its existence to the favorable reception
accorded to Bergen's Foundations of Botany. Whatever better-
ments have been suggested by five years' use of the earlier book
in the hands of expert teachers will be found here incorporated.
The Principles of Botany also attempts to supply what many
feel to be one of the most valuable portions of botany for edu-
cational purposes, namely, a consecutive series of studies of
representative spore plants, so treated as to outline the evolu-
tionary history of the plant world. Botanical technology cannot
figure largely in any brief general botany. The authors have how-
ever touched frequently upon the economic side of the subject,
and the last two chapters are wholly devoted to practical topics.

The subject-matter has been divided into three parts, treating
respectively :

I. The structure and physiology of seed plants (Bergen).
II. The morphology, evolution, and classification of plants, being
an account of the critical morphology of plants upon which is based
their relationship by descent (Davis).

III. Ecology and economic botany (Bergen).

The whole will furnish material for a full year's work, and
it will usually be found necessary to omit portions and thus
shape a course adapted for the exact conditions under which
the work in each case is to be done. It is not the intention
of the authors to frame an inflexible course, but rather to pre-
sent in orderly fashion the material from which a thoroughly
practical one can be planned. Indeed, the authors believe that
a half-year course can be readily arranged by selections from
the more general sections of the book.

iii

iv PREFACE

The planning of a course will be materially assisted by the
use of the authors' Laboratory and Field Manual, which is so
arranged as to offer a choice between the general requirements
of a shorter, elementary course and the details which are only
possible when more time can be given to the subject, under
excellent conditions of laboratory equipment and with fairly
mature students. A glossary of botanical terms employed in
this book will be found in the Laboratory Manual.

Some instructors will prefer to devote most of the year to a
study of seed plants ; others will choose to make the story of
plant evolution the chief feature and may even prefer to begin
with Part II. This portion of the book is the outgrowth of ten
years' experience of the junior author in the University of
Chicago, where he offered a year's course in general morphology
along somewhat similar lines. The treatment given to the
thallophytes in Part II will seem to some readers long in pro-
portion to that allotted to the other groups of plants. This
cannot however be avoided in any account which attempts to
present an outline of plant evolution with the important topics
of the origin and evolution of sex and of the sporophyte. Fur-
thermore, it is very desirable to describe a range of types from
which selections may be made according to the material avail-
able in different regions of the country. The adaptation of the
book to several methods of approach has obviously necessitated
slight repetitions of fundamental matter in certain parts.

Whatever the order of treatment, the authors would urge the
importance of sending the student to the plants for as many as
may be of his facts and then linking these together by read-
ing and class discussion. Undigested laboratory work is little
better than none at all, while a reading course without type
studies and physiological experiments is a quarter of a century
behind the best practice of to-day. No matter where it is to
end, the study of botany should begin with a first-hand knowl-
edge of plants themselves, — best of all, with a knowledge of their

PREFACE V

life in their own natural environment. At the outset there
may be far more botany and more reasoning power gained in
finding out for one's self the light relations of locust or bean
leaves, or in ascertaining why one pool is teeming with Spiro-
gyra and another with Oscillatoria, than in much reading of
botanical literature.

The earlier chapters of Part I are considerably less difficult
than most of the later portions of the book. It is therefore
suggested that care should be exercised not to consume too
much time in covering this ground, together with the laboratory
work which it presupposes. Classes should rather be carried
along somewhat rapidly to such more difficult topics as are dis-
cussed in Chapters v, vui, xii, and xv, and in Part II.

Except where acknowledgment is made in the text, the figures
and plates are all new or from the Foundations or Elements of
Botany of the senior author. Most of the illustrations of
Part II are original and by Dr. Davis. Special thanks for pho-
tographs and plates either reproduced in half tone or redrawn
for zinc etchings are due to F. W. Atkinson, F. Borgesen, F. E.
Clements, E. M. Freeman, G. L. Goodale, and Conway Mac-
Millan. W. M. Davis, A. E. Frye, and F. Both have kindly
permitted the use of a number of woodcuts and maps.

Parts of the manuscript were read by A. T. Bell, F. E.
Clements, I. S. Cutter, W. F. Ganong, B. A. Harper, W. M. Hays,
J. C. Jensen, Miss Lillian J. MacBae, and Miss Caroline E.
Stringer. Proof was read by W. J. Beal, F. E. Clements,
W. N. Clute, I. S. Cutter, H. S. Pepoon, B. M. Stigall, and
Miss Eva 0. Sullivan. To all of these the authors wish to
express their great appreciation for kindly criticisms and most
helpful suggestions. J Y B

B.M.D.
CAMBRIDGE, August, 1906

CONTENTS

PAGE

INTRODUCTION . . .;.... 1

PART I

THE STRUCTURE AND PHYSIOLOGY OF SEED PLANTS

CHAPTER

I. THE SEED AND ITS GERMINATION 5

II. THE STORAGE OF FOOD IN THE SEED 8

III. MOVEMENTS, DEVELOPMENT, AND MORPHOLOGY OF THE SEED-

LING 12

IV. ROOTS 19

V. SOME PROPERTIES OF CELLS AND THEIR FUNCTIONS IN THE

ROOT 34

VI. STEMS 40

VII. STRUCTURE OF THE STEM 57

VIII. LIVING PARTS OF THE STEM ; WORK OF THE STEM 71

IX. BUDS 80

X. LEAVES 88

XL LEAF ARRANGEMENT FOR EXPOSURE TO SUN AND AIR ; HELIO-

TROPIC MOVEMENTS OF LEAVES AND SHOOTS 94

XII. MINUTE STRUCTURE OF LEAVES ; FUNCTIONS OF LEAVES . . . 102

XIII. THE FLOWER OF THE HIGHER SEED PLANTS 123

XIV. INFLORESCENCE 132

XV. ORIGIN AND STRUCTURE OF FLORAL ORGANS ; POLLINATION AND

FERTILIZATION 138

XVI. THE FRUIT . . 146

PART II

THE MORPHOLOGY, EVOLUTION, AND CLASSIFICATION
OF PLANTS

XVII. THE PRINCIPLES OF MORPHOLOGY, EVOLUTION, AND CLASSIFI-
CATION 151

XVIII. THE LOWEST ORGANISMS AND THE CELL AS THE LIFE UNIT . . 156
XIX. THE THALLOPHYTES 172

vii

viii CONTENTS

CHAPTER PACK

XX. THE ALG^E, THE LOWEST GREEN PLANTS 173

XXI. SUMMARY OF THE LIFE HISTORIES AND EVOLUTION OF THE

ALG^ 221

XXII. THE FUNGI AND THEIR RELATION TO FERMENTATION AND

DISEASE 227

XXIII. SUMMARY OF THE LIFE HISTORIES AND EVOLUTION OF THE

FUNGI 272

XXIV. THE BRYOPHYTES AND THE ESTABLISHMENT OF ALTERNATION

OF GENERATIONS 275

XXV. THE PTERIDOPHYTES AND THE APPEARANCE OF HETEROSPORY 306

XXVI. ALTERNATION OF GENERATIONS 345

XXVII. HETEROSPORY 351

XXVIII. THE SPERMATOPHYTES AND THE SEED HABIT 354

XXIX. THE EVOLUTION OF THE SPOROPHYTE AND DEGENERATION

OF THE GAMETOPIIYTE . . . 402

PART III

ECOLOGY AND ECONOMIC BOTANY

XXX. PARASITES AND CARNIVOROUS PLANTS 407

XXXI. How PLANTS PROTECT THEMSELVES FROM ANIMALS . . . 413
XXXII. POLLINATION OF FLOWERS AND PROTECTION OF POLLEN . . 420

XXXIII. How PLANTS ARE SCATTERED AND PROPAGATED 436

XXXIV. SOCIAL HABITS OF PLANTS; COMPETITION AND INVASION . . 447
XXXV. PLANT SUCCESSIONS 454

XXXVI. ECOLOGICAL GROUPS AND THEIR CHARACTERISTICS . . . 459

XXXVII. PLANT FORMATIONS ; ZONATION 474

XXXVIII. PLANT GEOGRAPHY 481

XXXIX. VARIATION, MUTATION, AND ORIGIN OF SPECIES .... 496

XL. PLANT BREEDING 500

XLI. SOME USEFUL PLANTS AND PLANT PRODUCTS ...... 514

APPENDIX 537

INDEX . . 541

LIST OF PLATES

FRONTISPIECE. Attraction for pollinating insects in Cornus canadensis,
a shade plant of cold woods with inconspicuous perianth, but a large
and showy white involucre, the whole head appearing like a flower.

Faciny page
PLATE I. Sand dunes with sea rye grass (Elymus arenarius). A sand

binder, deep-rooted, with extensively running rootstocks .... 52
PLATE II. Exposure of leaves to sunlight. All the leaves are arranged
at such an angle as to receive the maximum illumination. Balsam
spruce forest along a brookside, east slope of Pikes Peak. Species
represented : Actcea eburnea, Chamcenerion angustifolium, Hera-
cleum lanatum, Rubus strigosus 94

PLATE III. Cypress swamp, with Spanish moss(Tillandsia), an epiphytic
seed plant practically leafless, the work ordinarily done by leaves
devolving on the .slender stems. The cypress trees are furnished
with "knees," or projections from the roots, which are thought

by some to absorb air 110

PLATE IV. Ilockweeds exposed at low tide 204

PLATE V. A common tree lichen (Physcia stellaris) 254

PLATE VI. A wound parasite (Pleurotus ulmarius) on a maple tree . 270
PLATE VII. Tree ferns (Dicksonia antarclica) from Tasmania . . . 310
PLATE VIII. A probable landscape in the Carboniferous Age . . . 340
PLATE IX. Belt of trees along a Nebraska river, showing dependence
of forest on water supply.

Xerophy tic grasses on Nebraska sand hills 462

PLATE X. Zonation about a pond in southwestern Ohio. The plate
shows the north end of a pond of about eleven acres in area par-
tially surrounded by seven zones beginning with submerged aquatics
and ending with a forest zone (in the portion next the pond, mainly

of maples, elms, ashes, and willows) 478

PLATE XI. Leaning trees at timber line on Pikes Peak 486

PLATE XII. A coniferous forest interior of central Colorado. The trees
are Douglas spruce (Psendotsuga mucronata). The light intensity
is so feeble, from the dense shade, that the only seed plants on the
forest floor are saprophytic or parasitic orchids and a few little
Pyrolas 494

PLATE XIII. A tropical forest in the Philippines, showing characteris-
tic dense vegetation, the trees of many species, mostly palms . ,518

PRINCIPLES OF BOTANY

INTRODUCTION

Botany is the science which treats of plants. It considers the
structure and functions of individuals, recognizes their neighbor-
hood relations as citizens of plant communities, and studies their
positions as members of the plant kingdom more or less closely
related by common descent. The study of the individual plant
embraces a variety of topics, and the examination of its relation
to others introduces many more subjects.

Morphology is the science of form and structure. Under this
head are studied the forms of plant bodies and the portions of
which they are composed. All plants except the very simplest
are made up of parts, called organs, which are structures devel-
oped for particular kinds of work. Thus the stems, roots, and
leaves are organs, and so are also the parts of a flower. Morphol-
ogy establishes the relationships of organs which seem at first
glance very dissimilar, as when leaves take the form of bud scales
or spines or tendrils. Morphology traces the degeneration of
parts which frequently cease to perform the work for which
they were originally developed and become much simplified in
structure or almost disappear. Thus the tendrils of the wood-
bine are shown to be morphologically branches reduced to mere
organs of attachment. Although morphology deals with the
plant with less regard to its character as a living being, it
should never be entirely separated from physiology, but should
go hand in hand with that sister subject, equally necessary to
an understanding of plant life.

S '".INTRODUCTION

Plant physiology treats of the plant in action, how it lives,
respires, feeds, grows, and produces others like itself. It dis-
cusses the nature of the material in which the life activities of
the plant have their origin, and the conditions as regards light,
heat, air, and moisture under which life is possible. It considers
the raw materials out of which plant food is made, the processes
by which the manufacture is carried on, and the means by which
food once produced is transported throughout the plant body.
The mode of growth of plants is an extended and most impor-
tant topic, and the processes by which reproduction is carried
on are so numerous and complicated that they constitute one
of the most difficult and interesting departments of botany. In
order to go far into the details of the life activities of plants one
needs to know a good deal of chemistry and some physics. But
there are many of the phenomena of plant physiology which
can be taken up with profit in an elementary way and investi-
gated with rather simple apparatus.

Plant geography discusses the distribution of the various
kinds of plants over the earth's surface.

Paleobotany, usually studied along with geology, considers
the history of plant life on the earth from the appearance of
the first plants until the present time.

Taxonomy, or systematic botany, is concerned with the classi-
fication of plants. By this is meant the arrangement or grouping
of the kinds of plants to show their relationships to one another.
It attempts to express the final results of the long processes of
plant evolution, and is far more than the conventional study of
flowering plants, which occupy only the highest grades in the
elaborate system of plant evolution and classification.

Plant ecology treats of the relations of the plant to the con-
ditions under which it lives, together with the origin and devel-
opment of plant associations. Under this division of the science
are studied the effects of soil, climate, and friendly or hostile
animals and plants on the external form, the internal structure,
and the habits of plants. The main lesson to be learned from

ORDER OF STUDY 3

the study of ecology is that the plant is not an organism of fixed
form, structure, and habits, sprung from a long line of precisely
similar ancestors and destined to leave an indefinite series of
forms like itself to succeed each other in the same area. On the
other hand, each generation is a little more or less numerous
than its predecessors, covering more or less territory than they
did, and varying from them this way or that under the influ-
ence of changing conditions of life. This is an interesting depart-
ment of botany, but it has to be studied mainly out of doors.
Economic botany is the study of the uses of plants to man.

Many of the topics suggested in the above outline cannot
be studied in detail in an elementary course. It ought, however,
to be possible for the student to learn a good deal about the
simpler facts of morphology and of plant physiology. It is
necessary to study plants themselves, to take them to pieces
and to make out the connection of their parts, to examine with
the microscope small portions of the exterior surface and thin
slices of all the variously built tissues of which the plant consists.
Among the lower plants there will be found a most attractive
study of cell structure, reproductive processes, and life histories,
— all requiring the use of the compound microscope. Living
plants must be watched in order to ascertain what kinds of food
they take, what kinds of waste substances they excrete, how and
where their growth takes place and what circumstances favor it,
how they move, and indeed to get as complete an idea as pos-
sible of what has been called the behavior of plants.

Since the most familiar plants spring from seeds, the beginner
in botany may well examine at the outset the structure of a few
familiar seeds, then sprout them, and watch the growth of the
seedlings which spring from them. Afterwards he can study in a
few examples the organs, structure, and functions of seed plants,
trace their life history, and so, step by step, follow the process
by which a new crop of seeds at last results from the growth and
development of such a seed as that with which he began.

4 INTRODUCTION

After he has come to know in a general way about the struc-
ture and physiology of seed plants, the student may become
acquainted with some typical spore plants. This will open up a
new world, illustrating some of the most interesting and funda-
mental principles of biological science ; for an understanding of
the cell theory of organization and development, the nature of
sexual processes, and the evolution of the plant kingdom with
its remarkable alternation of generations, can only be gained
by tracing the chief steps in the processes through the various
groups of algae, fungi, liverworts, mosses, and ferns.

For users of the book who wish to begin in the autumn with
the study of some seed plant as a whole the following scheme
is suggested:

1. Examine a seed plant in flower, to get an idea of its gross
anatomy. Then study the development, structure, and modes
of dissemination of the fruit. Outline the structure of seeds
and follow the germination of some types. Next take up the
structure and physiology of the vegetative members of the plant
body, root, stem, and leaf.

2. Cover as much as may be of Part II, working out the
story of the evolution of plants.

3. Devote the remainder of the year to study of floral struc-
tures, field work on families of angidsperms, ecological topics,
and an outline of economic botany.

If desired, the course in botany may begin with the simplest
spore plants, tracing the evolution of the plant kingdom through
a consecutive study of types, as described in Part II, followed
by somewhat detailed work on the structure and physiology of
seed plants (Part I), and ecology (Part III).

PART I

THE STRUCTURE AND PHYSIOLOGY OF
SEED PLANTS

CHAPTEE I
THE SEED AND ITS GERMINATION

The seed. A seed is a miniature plant,
or embryo, with some accessory parts, in a
resting or dormant state, and capable under
suitable conditions of reproducing the kind
of plant which bore it.

The power of producing seeds is peculiar
to the higher plants (seed plants, or sper-
matopliytes] and sharply distinguishes them
from all lower forms of plant life.

The embryo may nearly or quite fill the
interior of the seed, as in Fig. 1, or it may
constitute only a small part of the bulk of
the latter, as in Figs. 3, 4.

2. Form and position of the embryo.
The embryo shows great diversity of form;
it may have one, two, or several seed leaves,
or cotyledons (Figs. 1, 3, 12). These may
be straight, as in the squash seed, or much
curved and folded, as in the seed of the
four-o'clock, morning-glory, or buckwheat,
but they are almost always closely packed for economy of space.

-c

FIG. 1. Lengthwise sec-
tion of a squash seed

c, hypocotyl ; co, cotyle-
don; e, endosperm ; h,
hilum ; p, plumule;
t, testa. Magnified
about two and a half
times

THE SEED AND ITS GERMINATION

The cotyledons are usually borne on a little stem, called the
hypocotyl (meaning beneath the cotyledon) (Fig. 1, c ; Fig. 2, c).
Often a little seed bud, or plumule (Fig. 3), is easily recogni-
c zable in the embryo, more or

less inclosed by the cotyledons,
if there are two of these.

3. The seed coats. The em-
bryo (and sometimes other seed
contents) is inclosed by one or
more seed coats, which in many
cases preserve the embryo from
injuries of various kinds, and
also serve other purposes. The
principal seed coat is called the
testa; it varies greatly in thick-
ness, hardness, color, and mark-
ings, and also in other respects,
as is evident when one recalls
the varied appearance of such
familiar seeds as those of the
mustard, squash, bean, pea, locust,
apple, poppy, and Brazil nut.

4. Topics for investigation.
The student should learn at first
hand (that is, from the seeds and
the young seedlings themselves),

in connection with the present chapter, something about the
following topics :

1. The anatomy of a few typical seeds.

2. Some of the conditions for germination.

3. Some of the chemical changes produced in germinating
seeds, and their effect upon the surrounding air.

4. The early steps in the development of seeds into plants.
The brief outline of the structure of the seed just given should

be much enlarged by the details learned in the laboratory.

FIG. 2. The castor bean and its
germination

A, lengthwise section of ripe seed : t,
testa; co, cotyledon ; c, hypocotyl.
B, sprouting seed covered with en-
dosperm. C, same, with half of en-
dosperm removed. D, seedling: ?•,
primary root; r', secondary roots;
c, arch of hypocotyl

OXIDATION INVOLVED IN GERMINATION

Every observing person who has grown plants from the seed
has learned that heat and moisture are necessary to insure ger-
mination, but the student will readily discover, too, that air is
necessary for anything more than the
beginning of germination.

5. Oxidation involved in germina-
tion. Germinating seeds, like all liv-
ing things, consume much oxygen,—
the gas everywhere present in the
atmosphere which supports the com-
bustion of coal and other fires and of
lamps and gas flames. In place of the
oxygen which they absorb, sprouting
seeds return to the air carbon dioxide,
— the gas which is produced by burn-
ing charcoal, and which is one of the
products of burning most kinds of fuel
and of the respiration of animals.

A thermometer with its bulb im-
mersed in a jar of sprouting peas
will mark a temperature somewhat
higher than that of the room in which they stand. The eleva-
tion of temperature is at least partly due to the union of
oxygen with combustible materials in the peas. Such a combi-
nation is known as oxidation. This kind of chemical change
is universal in plants and animals while they are in an active
condition, and the energy which they manifest in their growth
and movements is as directly the result of the oxidation going
on inside them as the energy of a steam engine is the result
of the burning of coal or other fuel under its boiler. In the
sprouting seed, much of the energy produced by the action of
oxygen upon oxidizable portions of its contents is expended in
producing growth, but some of this energy is wasted by being
transformed into heat which escapes into the surrounding soil.
It is this escaping heat which is detected by the thermometer.

FIG. 3. Lengthwise section
of grain of corn

y, yellow, proteid part of endo-
sperm ; w, white, starchy part
of endosperm; p, plumule;
s, the shield (cotyledon), in
contact with the endosperm
for absorption of food from
it; r, the primary root.
Magnified about three times.
— After Sachs

CHAPTEE II

A B

FIG. 4. Seeds with endo-
sperm, longitudinal sections

A, asparagus (magnified) ; Z?,
poppy (magnified). —After
Decaisne

THE STORAGE OF FOOD IN THE SEED

6. Importance of stored food for growth of the seedling. A
very large part of the food of man and of many of the higher

animals consists of seeds of various
kinds, particularly of the grains. Every
kind of seed contains some stored food
material, though the amount in the
poppy seed is but an insignificant frac-
tion of that in a
horse-chestnut.
Very often, as
has already been
learned, the food

is stored directly in the embryo, espe-
cially in the cotyledons. Frequently, how-
ever, most of it is deposited in the en-
dosperm, which surrounds or lies along-
side of the cotyledons (Figs. 2, 3, 4). In
either case the slow germination and sub-
sequent growth of seeds from which part
or all of the food material has been
removed shows that its presence is most
important in forcing along the growth
of the seedling (Fig. 5).

7. Usefulness of rapid growth of seed-
lings. The very existence of the young

plant may depend upon its being able to FIG. 5. Germinating
make a rapid start in life. Most areas
of fertile land contain far more seeds

peas, growing in water,
one deprived of its
cotyledons

STARCH

than can mature plants under the conditions of competition
with one another which they must encounter, and so plants
which shoot up rapidly at first possess a decided advantage.
There is also a much better chance for seedlings growing in
woodlands if they can attain considerable size before they are
too much shaded by the foliage of the trees above them. This,
of course, does not
apply to evergreen
woods.

8. Kinds of food
stored in seeds.
The three princi-
pal kinds of plant
food, or reserve
material stored in
seeds, are starch,
oil, and albumi-
nous substances,
orproteids.1 A sin-

gle seed may con-
tain all three of

FIG. 6. Section through exterior part of a
grain of wheat

these in consider- c' cuticle> or outer layer of bran; ep, epidermis; ra,
middle layer; i, i^, layers of hull next to seed coats;

able proportions, s, «i, seed coats; p, layer containing proteid grains;
Or it may Contain st> cel1* °f the endosperm filled with starch. Greatly
J magnified. — After Ischirch

proteids together

with either starch or oil. Some proteids are always present,
since the power of the seed, to live and grow depends upon
these compounds.

9. Starch. Every one is familiar with the appearance of starch
in its commercial form. As found in seeds it occurs in micro-
scopic compartments known as cells (Fig. 6). Each cell contains
many small starch grains, usually of a nearly round or an ovoid

1 As in general throughout the book, the statements of the text pre-
suppose a suitable amount of laboratory work ; for example, that of the
manual of the authors.

THE STORAGE OF FOOD IN THE SEED

FIG. 7. Canna starch
Magnified 300 diameters

shape. The shape and markings of a starch grain, whether
found in the seed or in some other part of the plant, are often
sufficiently definite to serve to identify the kind of plant from
which they came. Frequently the
markings are very regular and beauti-
ful, as in canna starch (Fig. 7). They
are due to the successive layers de-
posited as the starch grain is formed.
During the growth of the seedling,
seeds containing starch rapidly lose
it, and microscopical examination of
a sprouting grain of corn or of the
cotyledons of a bean plant several
weeks old shows the cells compar-
atively emptied of starch and those
grains which remain much eaten away, as described below.

10. Action of ferments. A substance which can produce or
excite any one of the chemical changes known as fermentation
is called a ferment. The most familiar kinds of fermentation
are the alcoholic, by which alcohol is produced, and the acid, by
which solutions of alcohol (such as fermented cider) are turned
into vinegar, and by which the sugar of milk is changed into
lactic acid when sweet milk turns sour.

All these fermentations and many others are caused by the
development within the fermenting substances of minute living
organisms, either yeasts or bacteria, described in Chapter xxn,
which are consequently known as organized ferments.

There is a class of substances which, without the presence of
yeasts or bacteria, can produce active fermentation. From the
absence of the organisms above-mentioned, these are called
unorganized ferments, and they are also known as enzymes.
One of these, diastase, plays a most important part in seeds
during germination, transforming starch into sugar. Diastase is
found in considerable quantities in malt, which is barley sprouted
and then quickly killed by moderate heating. Naturally, as a

OTHER CONSTITUENTS OF SEEDS 11

result of the action of its diastase, malt tastes much sweeter
than barley. The capacity of this enzyme to change starch to
sugar is extraordinary, any quantity of diastase sufficing to trans-
form ten thousand times its weight of starch.

11. Oil. Oil occurs in many seeds — as, for example, flax, cot-
ton seed, and corn — in sufficient quantity to make it worth while
to extract it by pressure. • It may be seen under the microscope in
extremely minute droplets, inclosed in the cells of certain regions
of the seed.

12. Proteids. Sometimes, as in Fig. 6, at p, the proteid con-
stituents of the seed occur in more or less regular grains, but
often they have no well-defined form and size. They have a
chemical composition very similar to that of white of egg or
the curd of milk, and when scorched produce the familiar smell
of burnt hair or feathers, which serves as a rough test for their
presence.

13. Other constituents of seeds. Besides starch, oil, and pro-
teids, other substances occur in different seeds. Seme of these
are of use in feeding the seedling, others are of value in protect-
ing the seed itself from being eaten by animals or in rendering
it less liable to decay. In such seeds as that of the nutmeg,
the essential oil which gives it its characteristic flavor probably
makes it unpalatable to animals and at the same time preserves
it from decay.

Date seeds are so hard and tough that they cannot be eaten
and do not readily decay. Lemon, orange, horse-chestnut, and
buckeye seeds are too bitter to be eaten, and the seeds of the
apple, cherry, peach, and plum are somewhat bitter.

The seeds of larkspur (Datura}} croton, the castor-oil plant,
mix vomica, and many other kinds of plants, contain active

poisons.

1 Commonly called Jimson weed.

CHAPTER III

MOVEMENTS, DEVELOPMENT, AND MORPHOLOGY OF THE

SEEDLING

14. How the seedling breaks ground. As the student has
already learned by his own observations, the seedling does not
always push its way straight out of the ground. Corn, like all
the other grains and grasses, sends a tightly rolled, pointed leaf
vertically upward into the air ; but seedlings
in general are not found to do
anything of the sort. The squash
seedling is a good one in which

A B • c D E

FIG. 8. Successive stages in the life history of the squash seedling

GG, the surface of the ground ; r, primary root ; r', secondary root ; c, hypocotyl ;
a, arch of hypocotyl ; co, cotyledons

to study what may be called the arched type of germination.
If the seed when planted is laid horizontally on one of its broad
surfaces, it usually goes through some such changes of position
as are shown in Fig. 8.

The seed is gradually tilted until, at the time of their emer-
gence from the ground (at C), the cotyledons are almost ver-
tical. The only part above the ground line GG, at this period,

MOVEMENTS OF THE COTYLEDONS 13

is the arched hypocotyl. Once out of ground, the cotyledons
soon rise, until (at E) they are again vertical, but with the
other end up from that which stood highest in C. Then the
two cotyledons separate until they once more lie horizontally,
pointing away from each other.

Whether the first part of the seedling to emerge from the
ground is a pointed, rolled-up leaf, as in Indian corn, or the hypo-
cotyl arch, as in Figs. 2 and 8, the result is to force the earth
aside without injury to the plumule or the cotyledons.

15. What pushes the cotyledons up? A very little study of
any set of squash seedlings, or even of Fig. 8, is sufficient to
show that the portion of the plant where roots and hypocotyl
are joined neither rises nor sinks, but that the plant grows both
ways from this part (a little above rr in Fig. 8, A and B). It is
evident that as soon as the hypocotyl begins to lengthen much
it must do one of two things: either push the cotyledons out
into the air or else force the root down into the ground as one
might push a stake down. The plantlet, in passing from the
stage shown at A to that of B and of C, develops many lateral
roots, thus making it harder and harder for the root to be thrust
bodily downward.

16. Getting rid of the seed coats. In seeds with thin coats
the latter usually burst open irregularly and allow the opening
cotyledons to escape. But in seeds with as thick a testa as that
of the squash, and still more in the case of nuts, the cotyledons
find their way out through a slit, or opening, which appears in
a definite part of the seed. If for any reason the seed coat does
not open, the embryo cannot grow. In many cases the moisture
and freezing and thawing of a winter in the earth are almost
essential to germination, and some seeds grow more promptly if
they have been scorched by fire, or if they are cracked open
before planting.

17. Discrimination between root and hypocotyl. It is not
always easy to decide by their appearance and behavior what
part of the seedling is root and what part is hypocotyl. In a

14 MORPHOLOGY OF THE SEEDLING

seedling visibly beginning to germinate, the sprout, as it is com-
monly called, which projects from the seed might be either root
or hypocotyl, or might consist of both together, so far as its
appearance is concerned. A microscopic study of the cross sec-
tion of a root, compared with one of the hypocotyl, would show
decided differences of structure between the two. Their mode
of growth is also different, as the pupil may infer from his own
observations.

18. Final position of the cotyledons. As soon as the young
plants of squash, bean, and pea have reached a height of three
or four inches above the ground, it is easy to recognize important
differences in the way in which they set out in life.

The cotyledons of the squash increase greatly in surface,
acquire a green color and a generally leaf-like appearance, and,
in fact, do the work of ordinary leaves. In such a case as this
the appropriateness of the name seed leaf is evident enough, —
one recognizes at sight the fact that the cotyledons are actually
the plant's first leaves. In the bean the leaf-like nature of the
cotyledons is not so clear. They rise out of the ground like the
squash cotyledons, but then gradually shrivel away, though they
may first turn green and somewhat leaf-like for a time.

In the pea, as in the acorn, the horse-chestnut, and many
other seeds, we have quite another plan, — the underground type
of germination. Here the thick cotyledons no longer rise above
ground at all, because they are so gorged with food that they
could never become leaves ; but the young stem pushes rapidly
up from the surface of the soil.

19. Development of the plumule. The development of the
plumule seems to depend somewhat on that of the cotyledons.
The squash seed has cotyledons which are not too thick to
become useful leaves, and so the plant is in no special haste to
get ready any other leaves. The plumule, therefore, cannot be
found with the magnifying glass in the unsprouted seed, and is
almost microscopic in size at the time when the hypocotyl
begins to show outside of the seed coats.

ROOT, STEM, AND LEAF 15

In the bean, pea, and corn, on the other hand, since the cotyle-
dons cannot serve as foliage leaves, the later leaves must be
pushed forward rapidly. In the bean the first pair are already
well formed in the seed. In the pea they cannot be clearly
made out, since the young plant forms several scales on its stem
before it produces any full-sized leaves, and the embryo contains
only hypocotyl, cotyledons, and a sort of knobbed plumule, well
developed in point of size, representing the lower, scaly part of
the stem.

20. Root, stem, and leaf. By the time the seedling is well
out of the ground the plant body, in most cases, possesses the
three kinds of vegetative organs, or parts essential to growth, of
ordinary seed plants ; that is, the root, stem, and leaf, or, as they
are sometimes classified, root and shoot. All of these organs
may multiply and increase in size as the plant grows older, and
their mature structure will be studied in later chapters ; but
some facts concerning them can best be learned by watching
their growth from the outset.

21. Elongation of the root. We know that the roots of seed-
lings grow pretty rapidly from the fact that each day finds them
reaching visibly farther down into the water or other medium
in which they are planted. A sprouted Windsor bean in a ver-
tical thistle tube will send its root downward fast enough so
that ten minutes' watching through the microscope will suffice
to show growth.

22. Root hairs. Very young seedlings of the grains, or of
mustard or red clover, afford convenient material for studying
root hairs. These are most abundantly developed when the
seed is sprouted in air that is not very moist. Only a certain
zone of the young root is covered with live hairs ; the younger
portions have not developed them and the older portions show
only dead ones. Examination with a good lens or a low power
of the microscope shows the gradual lengthening of the hairs,
from very young ones near the root tip to full-grown ones
farther up.

MORPHOLOGY OF THE SEEDLING

The root hairs in plants growing under ordinary conditions
are surrounded by the moist soil and wrap themselves around
microscopical particles of earth (Fig. 9). Thus they are able rap-
idly to absorb through their thin walls the soil water, with what-
ever mineral substances it has dissolved in it.

23. The young stem. The hypocotyl, or portion of the stem
which lies below the cotyledons, is the earliest formed portion

FIG. 9
FIG. 9. Cross section of a root

FIG. 10

A good deal magnified, showing root hairs attached to particles of soil, and some-
times enwrapping these particles. — After Frank and Tschirch

FIG. 10. A turnip seedling, with the cotyledons developed into
temporary leaves

h, root hairs from the primary root ; b, bare portion of the root, on which no hairs
have as yet been produced

of the stem. Sometimes this grows but little ; often, however,
the hypocotyl lengthens enough to raise the cotyledons well
above ground, as in Fig. 10.

The later portions of the stem are considered to be divided
into successive sections called nodes (places at which a leaf,
or a scale which represents a leaf, appears) and internodes
(portions between the leaves).

THE FIRST LEAVES

The stem increases in length by the simultaneous elongation
of several internodes, as shown by Fig. 11. It will be noticed
that in the plant figured the greatest increase in
length js neither in the oldest nor the youngest
internodes which are growing at all, but in an inter-
mediate region.

Every portion of the entire shoot, shown in the
figure, has elongated except the interval 21-22.

Counting from the top the lengthening of several
of the segments is as follows :

INTERW

1
o

3
4

5
6
7
8
9
10
11

24.

are, as
which

PER CENT INCREASE
IN LENGTH
100 <4

. . . 120

140 V

Xfc

140

. . . 160

140

120

. . . . . . 110

. . . 110

100

The first leaves. The cotyledons v^
already explained, the first leaves >J
the seedling possesses. Even if A

a plumule is found well developed in FIG. 11. Growth in a hori-
zontal shoot of hedge
the seed, it was formed after the coty- bindweedi

ledons. In those plants which have so

much food stored in the cotyledons as to

render them unfit ever to become useful

foliage leaves, as in the pea, there is

little or nothing in the color, shape, or general appearance of

the cotyledon to make one think it really a leaf, and it is only

by studying many cases that the botanist is enabled to class all

the shoot divided by
ink marks into 22 equal
segments ; I>, the same,
twenty-four hours later. —
After Bonnier and Sablon

1 Convolvulus sepium.

18 MORPHOLOGY OF THE SEEDLING

cotyledons as leaves in their nature, even if they are quite
unable to do the ordinary work of leaves. In seeds which have
endosperm, or food stored outside of the embryo, the cotyledons
usually become green and leaf-like, as they do, for example, in
the four-o'clock, the morning-glory, and the buckwheat ; but in
the seeds of the true grains, which contain endosperm, as in the
familiar instance of Indian corn, a large portion of the single
cotyledon remains throughout as a thickish mass buried in the
seed. In a few cases, as in the pea, there are scales instead of
true leaves formed on the first nodes above the cotyledons, and
co it is only at about the third node above that

leaves of the ordinary kind appear. In the bean
and some other plants which in general bear one
leaf at a node along the stem, there is a pair
produced at the first node above the cotyledons,
and the leaves of this pair differ in shape from
those which arise from the succeeding portions
of the stem.

25. Classification of plants by the number of
V their cotyledons. In the pine family the germi-

FIG. 12. Germi- natiug seed often displays more than two coty-
ledons, as shown in Fig. 12 ; in the majority of
common seed plants the seed contains two coty-
ledons, while in the lilies, the rushes, the sedges, the grasses,
and some other plants there is but one cotyledon. Upon these
facts is based the division of most seed plants into two great
groups : the dicotyledonous plants, which have two seed leaves,
and the monocotyledonous plants, which have one seed leaf.
Other important differences nearly always accompany the differ-
ence in number of cotyledons, as will be seen later.

CHAPTER IV

ROOTS

26. Origin of roots. The primary root originates from the
lower end of the hypocotyl, as the student learned from his own
observations on sprout-
ing seeds. The branches

of the primary root are
called secondary roots,
and the branches of
these are known as
tertiary roots. Those
roots which occur on the
stem or in other unusual
places are known as ad-
ventitious roots. The
roots which form so
readily on cuttings of
willow, southernwood,
Tropseolum, French marigold,
cultivated " geranium " (Pelargo-
nium), Tradescantia, and many
other plants, when placed in
damp earth or water, are adven-
titious.

27. Aerial roots. While the
roots of most familiar plants

grow in the earth, there are others which are
formed in the air, called aerial roots. They serve various pur-
poses : in some tropical air plants (Fig. 1 3) they fasten the plant
to the tree on which it establishes itself, as well as take in

FIG. 13.

Aerial roots of

an orchid

ROOTS

water which drips from branches and trunks above them, so
that these plants require no soil and grow suspended in mid air
from trees which serve them merely as supports ; many such
air plants are grown in greenhouses. In such plants as the ivy
(Fig. 14) the aerial roots, which are also adventitious, hold the
plant to the wall or other surface up which it climbs.

In the Indian com (Fig. 15) roots are sent out from nodes at
some distance above the ground and descend until they enter

FIG. 14. Aerial, adventitious roots of the ivy

the ground. They serve to anchor the cornstalk so that it may
resist the wind, and to supply additional water to the plant. They
often produce no rootlets until they reach the ground.

28. Water roots. Many plants, such as the willow, readily
adapt their roots to live either in earth or in water, and some,
like the little floating duckweed, regularly produce roots which
are adapted to live in water only. These water roots often show
large and distinct sheaths on the ends of the roots, as, for
instance, in the so-called water hyacinth (Eichhorma).

AERIAL ROOTS

FIG. 15. Lower part of stem and roots of Indian corn, showing aerial roots

("brace roots")

v- c, internodes of the stem ; 6, d, e,f, nodes of various age hearing roots. Most
of these started as aerial roots, hut all except those from 6 have now reached the
earth

22 KOOTS

29. Parasitic roots. The dodder, the mistletoe, and a good
many other seed plants are called parasites, since they live, at
least in part, upon food which they steal from other plants

ABC

FIG. 16. Dodder, a parasitic seed plant

A, magnified section of stem penetrated by roots of dodder; B, dodder upon
a golden-rod stem ; (7, seedling dodder plants growing in earth ; h, stem of
host; I, scale-like leaves; r, sucking roots, or haustoria; s, seedlings. —
A and C after Strasburger

called their hosts. Parasites develop peculiar roots, which pene-
trate the tissues of the host and form most intimate connections
with the interior portions of the stem or root of the latter.

FORMS OF ROOTS

In the dodder, as is shown in Fig. 1 6, the seedling parasite is
admirably adapted to the conditions under which it is to live.
Hooted at first in the ground, it develops a slender, leafless stem,
which, leaning this way and that, no sooner comes into perma-
nent contact with a congenial host than it produces sucking
roots at many points, gives up further growth in its soil roots,
and lengthens rapidly on the strength of the supplies of ready-
made sap which it obtains from the host.

30. Forms of roots. The primary root is that which proceeds
like a downward prolongation directly from the lower end of the

FIG. 17 FIG. 18 FIG. 19

A tap root Fibrous roots Fleshy and clustered roots

hypocotyl. In many cases the mature root system of the plant
contains one main root much larger than any of its branches.
This is called a tap root (Fig. 17).

Such a root, if much thickened, may assume some such form
as that of the carrot, parsnip, beet, turnip, or radish, and is called
a fleshy root. Some plants produce a cluster of roots from the
lower end of the hypocotyl. Such roots often become thickened,
as in the sweet potato and the dahlia (Fig. 19).

Eoots of grasses, etc., are thread-like, and known as fibrous
roots (Fig. 18).

ROOTS

31. General structure of roots. The general structure of the
very young root can be partially made out by examining the
entire root with a moderate magnifying power. Often the whole
is sufficiently translucent to allow the interior as well as the

exterior portion to be
studied while the root
is still alive and grow-
ing.

The main bulk of
the root is composed of
a central cylinder and
the cortical portion
which surrounds it.
The outermost part of
the cortex is a layer of
cells forming a thin
skin known as the epi-
dermis. The tip of the
root is covered by a
mass of loosely attached
cells forming the pro-
tective root cap.

On examining Figs.
FIG. 20. Lengthwise section (somewhat dia- ..

grammatic) through root tip of Indian corn. 20 and 21» the C7lm-
x about 130 ders of which the root

W, root cap ; i, younger part of cap ; z, dead cells is made up are easily

separating from cap; *, growing point; o, epi- j^ncrni^TipH nnr| thp

dermis ; p', intermediate layer between epidermis ° BQ> a

and central cylinder ; p, central cylinder, in which main Constituent parts
the fib ro- vascular bundles arise . — A f ter Wiesner „

of each can be made

out without much trouble. The epidermal cells are seen to be
somewhat brick- shaped, many of them provided with extensions
into root hairs. Inside the epidermis lie several layers of rather
globular, thin-walled cells, and inside these a boundary layer
between the cortical or bark portion of the root and the central
cylinder. This latter region is especially marked by the presence

STORAGE OF RESERVE MATERIAL IN ROOTS 25

of certain groups of cells, shown at w, d, and b (Fig. 21), the two
former serving as channels for air and water, the latter (and w
also) giving toughness to the root.

Eoots of shrubs and trees more than a year old will be found to
have increased in thickness by the process described in Chapter
vii, and a section may look unlike that shown in Fig. 21.

32. Storage of re-
serve material in
roots. Many roots
contain large quanti-
ties of stored plant
food, usually in the
shape of starch, sugar,
proteids, or all three
together. Parsnips,
carrots, turnips, and
sweet potatoes are
familiar examples of
storage roots.

Beet roots contain
so much sugar that
a large part of the
sugar supply of
Europe, and an in-
creasing portion of
our own supply, is
obtained from them. Oftentimes the bulk of a fleshy root is
exceedingly large as compared with that of the parts of the
plant above ground.

Not infrequently roots have a bitter or nauseous taste, as in
the case of the chicory, the dandelion, and the rhubarb ; and a
good many, like the monkshood, the yellow jasmine, and the
pinkroot, are poisonous. Evidently the plant may be benefited
by the disgusting taste or poisonous nature of its roots, which
renders them uneatable.

FIG. 21. Much magnified cross section of a young
dicotyledonous root

h, root hairs with adhering bits of sand; e, epi-
dermis; 5, thin-walled, nearly globular cells of
bark; b, hard bast; c, cambium; 10, wood cells;
d, ducts

ROOTS

33. Use of the food stored in fleshy roots. The parsnip,
beet, carrot, and turnip are biennial plants ; that is, they do
not produce seed until the second summer or fall after they are
planted.

The first season's work consists mainly in producing the food
which is stored in the roots. To such storage is due their char-
acteristic fleshy appearance. If the root is planted in the fol-
lowing spring, it feeds the rapidly growing stem which proceeds
from the bud at its summit, and an abun-
dant crop of flowers and seed soon follows;
while the root, if examined in late summer,
s=^;*f:,-'^fom wiH ^e found to be withered, with its store
v ^^S^MT\ °^ reserve matei>i-al quite exhausted.
\/\ Cff \ The roots of the rhubarb (Fig. 22), the

sweet potato, and of a multitude of other
perennials, or plants which live for many
years, contain much stored plant food.
Many such plants die to the ground at the
beginning of winter, and in spring make a
rapid growth from the materials laid up in
the roots.

34. Extent of the root system. The total
length of the roots of ordinary plants is
much greater than is usually supposed.
They are so closely packed in the earth that
only a few of the roots are seen at a time during the process of
transplanting, and when a plant is pulled or dug up in the
ordinary way a large part of the whole mass of roots is broken
off and left behind. A few plants have been carefully studied
to ascertain the total weight and length of the roots. Those of
winter wheat have been found to extend to a depth of seven
feet. By weighing the whole root system of a plant, and then
weighing a known length of a root of average diameter, the
total length of the roots may be estimated. In this way the
roots of an oat plant have been calculated to measure about

FIG. 22. Fleshy roots
of garden rhubarb.
About one fifteenth
natural size

THE ABSORBING SURFACE OF ROOTS

154 feet; that is, all the roots, if cut off and strung together
end to end, would reach that distance.

Single roots of large trees often extend horizontally to great
distances, but it is not often possible readily to trace the entire
depth to which they extend. One of the most notable examples
of an enormously developed root system is found in the mesquite
of the far Southwest and Mexico. When this
plant grows as a shrub, reaching the height,
even in old age, of only two or three feet, it is
because the water supply in the soil is very
scanty. In such cases the roots extend down
to a depth of sixty feet or more, until they
reach water, and the Mexican farmers in
digging wells follow these
roots as guides. Where water
is more abundant, the mesquite
forms a "good-sized tree, with
much shorter roots.

35. The absorbing surface
of roots. The soil roots of
inost seed plants are provided
with a highly efficient means
for absorbing water in the
shape of a coating of root hairs,
with which their younger por-
tions are thickly covered.
Some idea of their abundance
may be gathered from the
estimate - that on the hair-
bearing portions of the roots
of the common pea about 1437
hairs occur on every hundredth of a square inch of surface.

A root hair is an extremely thin-walled tube, springing from
an epidermal cell, into which it opens. The way in which the
cells give rise to hairs is well shown in Figs. 21 and 23.

— n

FIG. 23

A, a very young root hair; B, an older
one (both greatly magnified) ; e, cells
of the epidermis of the root ; n, nu-
cleus; s, watery cell sap; p, proto-
plasm lining the cell wall. — After
Frank

28 ROOTS

Most water roots are destitute of root hairs, and absorb water
through the general epidermal surface of their younger portions.

Aerial roots, like those shown in Fig. 13, are in many cases
provided with an external absorbent layer of spongy tissue, by
means of which they retain some of the water which trickles
down them during rains. This stored moisture they gradually
give up to the plant.

36. Absorption of water by roots. Just how much water
some kinds of plants give off (and therefore absorb) per day
will be discussed when the uses of the leaf are studied. For
the present it is sufficient to state that even an annual plant
during its lifetime absorbs through the roots very many times
its own weight of water. Grasses have been known to take
in their weight of water in every twenty-four hours of warm,
dry weather. This absorption in most soil roots takes place
mainly through the root hairs. Their walls are extremely thin,
and have no holes or pores visible under even the highest
power of the microscope, yet the water of the soil penetrates
very rapidly to the interior of the root hairs. The soil water
brings with it all the substances which it can dissolve from the
earth about the plant; and the closeness with which the root
hairs cling to the particles of soil, as shown in Figs. 9 and 21,
must cause the water which is absorbed to contain more foreign
matter than underground water in general does, particularly
since the roots give off enough weak acid from their surface to
corrode the surface of stones which they enfold or cover.

37. Substances required by the plant for nutrition. Ordinary
seed plants require for their nutrition ten of the chemical ele-
ments. By far the greater part of the weight of the plant body
is usually due to compounds of carbon, hydrogen, oxygen, and
nitrogen. Besides these there are present the six elements,—
sulphur, phosphorus, potassium, calcium, magnesium, and iron.
In ordinary green meadow grass there is about 80 per cent of
water and 20 per cent of dry matter. On drying the grass into
hay and then burning the latter, some 2 per cent of ash will remain,

SAP PRESSURE

and in this will be found the six elements — sulphur, phos-
phorus, potassium, calcium, magnesium, and iron — in the form
of incombustible salts (sulphates, phosphates, and so on).

The plant gets its carbon and oxygen from the air, as will
be explained in Chapter xn. Deprived of air, all green plants
soon die. The hydrogen is obtained
from water.

The importance of the six ash-
forming constituents mentioned
above is most readily studied by
means of water cultures in which
plants are grown with suitable
proportions of dissolved salts. If
any one of the six elements is
omitted from a solution, the
plants grown in it are dwarfish
and unhealthy.

Ordinary soil water contains
sufficient salts in solution for the
nutrition of plants, but not always
enough to stimulate rapid growth.

38. Sap pressure. Not only
does much water gain admission FIG. 24. Apparatus to measure
to the plant through the roots, sap Pressure

Vmf nnrlpr nrrlirmrv rirrnrrmran^ps 7'' large tube fastened to the stumP of

the dahlia stem by a rubber tube ;

it is found forcing its way 011, r, r, rubber stoppers ; t, bent tube

into, and through the stem (for — ?, 7S£i differ
explanation see Sees. 48-51). The Sachs

force called sap pressure with which the upward-flowing current
of water presses may be estimated by attaching a mercury gauge
to the root of a tree or the stem of a small sapling. This is best
done in early spring after the thawing of the ground, but before
the leaves have appeared. The experiment may also be per-
formed indoors upon almost any plant with a moderately firm
stem, through which the water from the soil rises freely.

30 ROOTS

A dahlia plant or a tomato plant answers well, though the
sap pressure from one of these will not be nearly as great as that
from a larger shrub or a tree growing out of doors. In Pig. 24
the apparatus is shown attached to the stem of a dahlia. The
difference of level of the mercury in the bent tube serves to
measure the pressure. For every foot of difference in level there
must be a pressure of nearly six pounds per square inch on the
stump at the base of the tube T.1

A black birch root tested at the end of April has given a
sap pressure of thirty-seven pounds to the square inch. This
would sustain a column of water about eighty-six feet high.

39. Root absorption and temperature of the soil. The tem-
perature of roots and the earth about them has much to do
with the rate at which they absorb water. Some plants can
absorb it at temperatures as low as 25° F. (— 4° C.), while
others cannot do so at any temperature below 39° F. (4° C.).
This fact of the power to get water from the soil ceasing at tem-
peratures in the neighborhood of the freezing point has most
important consequences, since it implies that a plant may die
for lack of water with its roots immersed in cold, wet soil.
Hence the parched appearance often noticed in leaves killed
by frost.

40. Movements of young roots. The fact that roots usually
grow downward is so familiar that we do not generally think of
it as a thing that needs discussion or explanation. Since they
are pretty flexible, it may seem as though young and slender
roots merely hung down by their own weight, like so many bits
of wet cotton twine. But the root of a young Windsor bean
seedling or of a sprouting pea will force itself down into mer-
cury. By comparing the weights of equal bulks of mercury and
Windsor bean roots, it is found that the mercury is about four-
teen times as heavy as the substance of the roots. Evidently,
then, the submerged part of the root must have been held under
by a force about fourteen times its own weight.

1 For a more accurate method see Handbook-

GEOTROPISM 31

A more accurate measurement of the force exerted by the
root may be made by confining it so it cannot bend, and letting
it push down on a spring. In this way it is found that the root
of the Windsor bean can push with a pressure of about ten
ounces.

Making fine equidistant cross marks with ink along the upper
and the lower surface of a root that is about to bend downward
at the tip readily shows that those of the upper series soon
come to be farther apart, — in other words, that the root is
forced to bend downward ty the more rapid growth of its upper
as compared with its under surface.

41. Geotropism. The property which plants or their organs
manifest, of assuming a definite direction with reference to grav-
ity,1 is called geotropism. When, as in the case of the primary
root, the effect of gravity is to make the part, if unobstructed,
turn or move downward, we say that the geotropism is positive.
If the tendency is to produce upward movement, we say that
the geotropism is negative; if horizontal movement, that it is
lateral. It was stated in the preceding section that the direct
cause of the downward extension of roots is unequal growth.
We might easily suppose that this unequal growth is not due
to gravity, but to some other cause. To test this supposition,
the simplest plan, if it could be carried out, would be to remove
the plants studied to some distant region where gravity does not
exist. This of course cannot be done, but we can easily turn a
young seedling over and over so that gravity will act on it now
in one direction, now in another, and so leave no more impres-
sion than if it did not act at all. Or we can whirl a plant so
fast that not only is gravity done away with, but another force
is introduced in its place. If a vertical wheel, like a carriage
wheel, were provided with a few loosely fitting iron rings strung
on the spokes, when the wheel was revolved rapidly the rings
would all fly out to the rim of the wheel. So in Fig. 25 it will

1 Gravity means the pull which the earth exerts upon all objects on or
near its surface,

ROOTS

be noticed that the growing tips of the roots of the sprouting
peas point almost directly outward from the center of the disk

on which the seedlings are fastened.
In this case the so-called "centrif-
ugal force" due to the rotation of
the wheel is sufficient wholly to
overcome geotropism.

42. Direction taken by secondary
roots. As the student has already
noticed in the seedlings which he
has studied, the branches of the
primary root usually make a con-
siderable angle with it (Fig. 2).
Often they run out for long dis-

FIG. 25. Sprouting peas on a
rapidly whirling disk

The youngest portions of the roots

all point directly away from the tances almost horizontally. This is
axis about which they were re- especially common in the roots of

volved. — After Detmer

forest trees, above all m cone-bearing

trees, such as pines and hemlocks (Fig. 26). This horizontal,
or nearly horizontal, position of large secondary roots is the

FIG. 26. Roots of a white pine

most advantageous arrangement to make them useful in stay-
ing or guying the stem above to prevent it from being blown
over by the wind.

ADAPTATIONS TO CONDITIONS OF LIFE 33

43. Fitness of the root for its position and work. The dis-
tribution of material in the woody roots of trees and shrubs and
their behavior in the soil show many adaptations to the condi-
tions by which the roots are surrounded. The growing tip of
the root, as it pushes its way through the soil, is exposed to
bruises ; but these are largely warded off by the root cap. The
tip also shows a remarkable sensitiveness to contact with hard
objects, so that when touched by one it swerves aside and thus
finds its way downward by the easiest path. Eoots with an
unequal water supply on either side grow toward the moister
soil ; when unequally heated they grow in the direction of the
most desirable temperature, and they usually grow away from
the light. Eoots are very tough, because they need to resist
strong pulls, but not as stiff as stems and branches of the same
size, because they do not need to withstand sidewise pressure,
acting from One side only. The corky layer which covers the
outsides of roots is remarkable for its power of preventing evapo-
ration. It must be of use in retaining in the root the moisture
which otherwise must be lost on its way from the deeper root-
lets (which are buried in damp soil), through the upper portions
of the root system, about which the soil is often very dry.

CHAPTER V

SOME PROPERTIES OF CELLS AND THEIR FUNCTIONS IN

THE ROOT

44. Definition of cell. This is not the best place to consider
the nature of cells in much detail (see Chapter xvm) ; but some
of the facts learned in Chapter iv cannot be understood with-
out a few words of explanation of cell structure and functions.

Protoplasm is the nitrogenous living substance of which
the most rapidly growing parts of plants are mainly composed.
The activities of the plant are due to the peculiar qualities and
powers of protoplasm. A cell is a unit of protoplasm, called
a protoplast. The protoplast of plants is usually inclosed hi a
case or covering whose walls (cell walls) are composed of a sub-
stance known as cellulose. Each protoplast usually contains a
single denser protoplasmic structure, called the nucleus.

In form and size cells vary greatly. Those of the root hair
(Fig. 23) are good examples of the slender, thread-like form ;
those of Fig. 27 well illustrate forms commonly assumed when
cells are pressed upon by others on all sides, as they usually
are in the interior portions of the organs of higher plants.

45. Growth and reproduction. The most remarkable property
of cells is their power of growth and reproduction. Growth
results not only from an increase in the size of cells but also
in their number as a result of cell division. This is the separa-
tion of a protoplast, generally into two independent protoplasts
or daughter cells, and is the fundamental cause of all growth
and development. The full-grown seed plant, composed of
millions of cells, arises from the embryo (with perhaps only a
few thousand), which had its beginning in a single cell. Cell
division is preceded by division of the nucleus (Fig. 170).

IRRITABILITY

Reproduction, or the formation of new organisms similar to
the parents, is possible only for protoplasm, not for any other
known substance.

46. Irritabil-
ity. Another
characteristic of
protoplasm is its
irritability. By
this is meant its
power of re-
sponding in
some way to an
application of
energy which
serves as a stim-
ulus. A famous
plant physiolo-
gist1 has illus-
trated the matter
very simply
thus : A wound-
up alarm clock,
which is not

going, is given a ^ cells from ovu]e (x 340) . ^ celis fr0m an ovule further
developed (x 340) ; C, J), cells from pulp of fruit (x 110) ;
n, nucleus; p, protoplasm; s, cell sap. — After Prantl

n-'

FIG. 27. Protoplasts in ovule and fruit of snowberry
(Symphoricarpus racemosus)

In the young and rapidly growing cells A and B the cell
sap is not present, or present only in small quantities,
while in the older cells C and D it occupies a large por-
tion of the interior of the cell

shake (stimulus),

which starts the

clock, and after

an interval of

time (latent

period) rings the alarm (result). The sensitiveness of the clock

to any jar which sets it going corresponds to the irritability of

living protoplasm. This extremely delicate responsiveness may

be manifested in a simple cell or in an organ or entire plant

composed of multitudes of cells.

1 Professor W. Pfeffer, of Leipzig, Germany.

36 SOME PROPERTIES OF CELLS

Some of the most important stimuli which call out manifes-
tations of irritability in protoplasm are heat, light, electricity,
gravity, pressure of external objects, and contact with substances
which act chemically on the protoplasm. Many instances of
irritability will come up in later chapters. A notable example
of response to a stimulus is the beginning of germination in
seeds subjected to a suitable degree of heat in presence of
moisture.

The ways in which the responses to stimulation may show
themselves are very numerous, and the same individual or organ
may be favorably affected by a certain amount of a given stimu-
lus and unfavorably by a greater amount of the same stimulus.
Every one has had the experience of drawing near to a moder-
ately heated stove in cold weather and then retreating from it
when the fire grew too hot. So, too, certain microscopic uni-
cellular plants, living in water, move toward the light until it
reaches a certain intensity, but when that intensity is passed,
they move in the opposite direction, toward the dark.

47. Selective absorption. Another extremely important power
of live protoplasm is that of selective absorption. By this is
meant the ability to take up from liquids or gases certain sub-
stances and leave unabsorbed other elements or compounds
which are also present.

Thus plants of two different species, both growing in the
same soil, usually take from it very various amounts or kinds
of mineral matter. For instance, barley plants in flower and
red clover plants in flower contain about the same proportion
of mineral matter (left as ashes after burning). But the clover
contains 5| times as much lime as the barley, and the latter
contains about 18 times as much silica as the clover. This dif-
ference must be due to the selective action of the protoplasm
in the absorbing cells of the roots.

48. Osmosis. The process by which two liquids of different
densities separated by membranes pass through the latter and
mingle, as soil water does with the liquid contents of root hairs,

OSMOSIS

is called osmosis. It is readily demonstrated by experiments
with thin animal or vegetable membranes. For instance, when
prunes, raisins, or other dried fruit, are put in water to soak,
water penetrates the outer skin and swells the seed or fruit,
while some of the material from within comes out through the
skin and flavors or discolors the water. If whole cranberries,
cherries, or plums are put in-
to boiling sirup, a similar ex-
change takes place, but in this
case the fruit is shriveled.

A still better experiment is
that with an egg from which
a bit of the shell has been
chipped away at the bottom,
arranged as shown in Fig. 28.
The entrance of water is shown
by the rise of some of the con-
tents of the egg in the tube.

49. Inequality of osmotic
exchange. The nature of the
two liquids separated by any

given membrane determines FlG 28 Egg on beaker of water,
in which direction the greater to show osmosis

flow shall take place unless The tube is cemented to the eggshell, into

which it opens. At the bottom a large
piece of the shell has been chipped
away, leaving the thin skin which lines
the egg in contact with the water in the
beaker

what would naturally be the
direction of flow is overruled
by the selective action of liv-
ing protoplasm.

If one of the liquids is pure water and the other is water
containing solid substances dissolved in it, the greater flow of
liquid will be away from the pure water into the solution, and
the stronger or denser the latter, the more unequal will be the
flow. This principle is well illustrated by the egg-osmosis
experiment. Another important principle is that substances
which readily crystallize and are easily soluble, like salt or

SOME PROPERTIES OF CELLS

sugar, pass rapidly through membranes, while jelly-like sub-
stances, like white of egg, can hardly pass through them
at all.

50. Study of osmotic action of living protoplasm ; plasmol-
ysis. The obvious parts of most living and growing plant cells
are a cell wall, which is a skin or inclosure made of cellulose, and
the living, active cell contents, or protoplasm (Sec. 44). Every
one is familiar with cellulose in various forms, one of the best
examples being that afforded by clean cotton. It is a tough,
white, or colorless substance, and chemically rather inactive.

Often, in living cells,
the spaces between
'J strands and protoplas-
—p mic lining are filled
s with a watery liquid
called the cell sap.

The action of living
protoplasm in control-
ling osmosis is well
shown by the process
known as plasmolysis.
If thin-walled cells

A B

FIG. 29. Cells from root of Indian corn

A, in natural condition ; D, plasmolyzed in 5 per ,

cent solution of potassium nitrate; w, cell wall; il(lu m>

p, denser part of protoplasm ; s, cell sap. Much such as tllOSC of O116
magnified. — After Pfeffer . ,, -,

or the pond scums, are

put into a salt solution, the cell contents will shrink away from
the cell wall (Fig. 168, B) because the direction of flow, toward
the denser liquid, draws water out of the cell. Repeating the
experiment with a cell which has been killed by a few minutes'
immersion in a poisonous solution (e.g. of chromic acid) shows
no plasmolysis.

So, too, slices of a red beet impart little color to water in which
they are placed, but after the cells are killed by boiling the color
comes out freely.

BEHAVIOR OF ROOTS DUE TO IRRITABILITY 89

51. Osmosis in root hairs. The soil water, practically identi-
cal with ordinary spring or well water, is separated from the
more or less sugary or mucilaginous sap inside of the root hairs
only by their delicate cell walls, lined with a thin layer of pro-
toplasm. This soil water will pass rapidly into the plant, while
very little of the sap will come out. The selective action,
which causes the flow of liquid through the root hairs to be
almost wholly inward, is due to the living layer of protoplasm,
which covers the inner surface of the cell wall of the root hair.
Traveling by osmotic action from cell to cell, a current of water
derived from the root hairs is forced up through the roots and
into the stem, somewhat as the contents of the egg was forced
up into the tube shown in Fig. 28.

But there is this important difference in the two cases, that
while the process in the tube was all due to the impulse received
at the start from the egg membrane, in the plant stem the origi-
nal pressure due to osmosis in the root hairs may be affected by
osmosis in countless thousands of cells higher up.

52. Behavior of roots due to irritability. In Chapter iv a
little was said about the geotropism of roots, their tendency to
put themselves into the most favorable conditions as regards
moisture, heat, and light, and their manner of avoiding obstacles.
All these actions are manifestations of irritability.

The subject of geotropism of roots is a very complicated one,
but it seems pretty certain that gravity somehow acts as a stim-
ulus on the sensitive cells of the root tip, this stimulus is trans-
mitted to the cells of the most rapidly growing portion of the
root (a little farther back), unequal groivth of the upper and
under cells of this portion follows, and so the root is bent, if its
position is not vertical in the beginning.

Moisture and heat (in the case of Indian corn up to 99.5° F.
or 37.5° C.) are favorable to the growth of roots, and so as stimuli
produce growth toward the source of moisture or heat, while
light is usually slightly unfavorable and therefore generally
results in growth of the root toward darkness.

CHAPTER VI
STEMS

53. Nature of the stem. The work of taking in the raw
materials which the plant makes into its own food is done
mainly by the roots and the leaves. These raw materials are
taken from earth, from water, and from the
air (see Chapter xn). The stem is that part or
organ of the plant which serves to bring roots
and leaves into communication with each
other. In most seed plants the stem also
serves the important purpose of lifting the
leaves up into the sunlight, where they can
best do their special work.

The student has already, in Chapter in,
learned something of the development of the
stem and the seedling ; he has now to study
the external and internal structure of the
mature stem. Much in regard to this struc-
bsc ture can be learned most easily from the
examination of twigs and branches of our
common forest trees in their winter condition.
FIG. 30. A quickly 54. Position of leaf buds. The winter buds
grown twig of of most of our trees and shrubs are formed at
cherry with p0^nts on the twig ;ust above the origins of
lateral and termi- J & J

nal buds in Oc- tne leafstalks, as shown in Fig. 79. After the

tober fall of the leaves the buds by their positions

6 sc, bud-scale scars, indicate where the leaves were formerly at-

All above these , , m, , i • • i i

scars is the growth tached. They may be arranged in pairs, a bud

of the spring and on one si(je of the stem and its mate exactly

summer of the . „ . , ,

same year opposite, or they may form a spiral around

METHODS OF BRANCHING

the stem, as shown in Fig. 30. Since every leaf bud — that is,
every bud which contains rudimentary leaves — will, if success-
ful, grow into a branch, the position of
the buds is most important in deter-
mining the shape of the tree.

55. Opposite branching. Trees with
opposite leaves and buds show a tend-
ency to form twigs in four rows about
at right angles to each other along the
sides of the branch, as shown in Fig. 31.
This arrangement will not usually
be perfectly carried out, as most of the
buds never grow,
since they are
shaded and starved,
or some may grow
much faster than
others and so make
the plan of branch-
ing less evident
than it would be if all grew alike.

56. Alternate branching. In trees like
the beech the twigs will be found to be
arranged in a more or less regular spiral
line about the branch. This, which is known
as the alternate arrangement (Fig. 32), is
more commonly met with in trees and shrubs
than the opposite arrangement. It admits of
many varieties, since the spiral may wind
more or less rapidly round the stem. In the
FIG. 32. Alternate apple, pear, cherry, poplar, oak, and walnut,
branching m a very one passes OVer five spaces before coming to
young apple tree , „ , . , . -i • -i •

a leaf which is over the first, and in doing

this it is necessary to make two complete turns around the
stem (Fig. 100).

FIG. 31. Opposite branch-
ing in a very young sap-
ling of ash

STEMS

57. Growth of the terminal bud. In some trees the termi-
nal bud from the outset keeps
the lead and produces a slen-
der, upright tree (Fig. 33), as
in the pines, spruces, and firs.

In such trees as the apple
and many oaks the terminal
bud has no preeminence over
others, and the form of the
tree is round-topped and
spreading (Fig. 34). Most
forest trees are intermediate
between these extremes.

Branches owe their char-
acteristics to several factors.
Most of our trees and larger
shrubs make a definite annual
growth, with the buds ripened
before the coming of winter
(Fig. 79). In these the ter-
minal bud is likely to grow
and continue the branch.
Such shrubs and trees as the
raspberry and blackberry, the
sumach and the ailanthus,
make an indefinite annual
growth, that is, the tips of the
branches are usually killed
by frost, and so the tree forks
often. Terminal flower buds
(Figs. 36, 37) also cause fork-
ing and allow the tree to form

i O

FIG. 33. California giant redwoods no long, straight branches.

), illustrating upright growth jf the terminal buds of

After J. H. White branches keep the lead of the

FORMS OF TREES

FIG. 34. An American elin, illustrating spreading growth

lateral ones, but the latter are numerous and most of those
which survive grow into slender twigs, the delicate spray of
the elm and many birches is produced (Fig. 38).

The general effect of the branching depends much upon the
angle which each branch or twig forms with that one from which

STEMS

it springs. The angle may be quite acute, as in the birch
(Fig. 38) ; or more nearl)7 a right angle, as in the ash (Fig. 31).
The inclination of lateral branches is due to geotropism, just
as is that of the branches of primary roots. The vertically
upward direction of the shoot which grows from the terminal
bud is also due to geotropism, which, however, in the shoot, is
exactly opposite to that in the root.

This is really only a brief way of saying that the growing tip
of the main stem of the tree, or of any branch, is made to take
and keep its proper direc-
tion, whether vertically
upward or at whatever
angle is desirable for the
tree, by the steering action
of gravity. After growth
has ceased this steering
action can no longer be
exerted, and so a tree
that has been bent over
— as, for instance, by a
heavy load of snow —
cannot right itself unless
it is elastic enough to

spring back when the load is removed. The tip of
the trunk and of each branch can grow and thus
become vertical, but the old wood cannot do so.

58. Thorns as branches. In many trees some
branches show a tendency to remain dwarfish
and incompletely developed. Such imperfect
branches may form thorns, as in the familiar wild
crab-apple trees and in the pear trees which occur in old pas-
tures in the northeastern states. In the honey locust very for-
midable brandling thorns spring from adventitious or dormant
buds on the trunk or limbs. They sometimes show their true
nature as branches by bearing leaves (Fig. 35).

FIG. 35. Leaf-bearing
thorn of honey locust

TREES, SHRUBS, AND HERBS

59. Trees, shrubs, and herbs. Plants of the largest size, with
a main trunk of a woody structure, are called trees. Shrubs
differ from trees in their smaller size, and generally in having
several stems which proceed from the ground or near it, or in
having much-forked stems. The witch-hazel, the dogwoods, and

FIG. 30. Tip of a branch of magnolia, illustrating forking due to
terminal flower buds

A, oldest flower-bud scar; B, C, D, scars of successive seasons after A; L, leaf
buds ; F, flower buds

the alders, for instance, are most of them classed as shrubs for
this reason, though in height some of them equal the smaller
trees. Some of the smallest shrubby plants, like the dwarf blue-
berry, the wintergreen, and the trailing arbutus, are only a few

STEMS

inches in height, but are ranked as shrubs because their woody
stems do not die to the ground in winter.

Herbs are plants whose stems above ground die every winter.

60. Annual, biennial,
and perennial plants.
Annual plants are those
which live but one year,
biennials those which
live two years or nearly so.

Some winter annuals
do not flower until
their second summer.
This is true of the even-
ing primrose and the
fringed gentian, and of winter wheat and rye
among cultivated plants.

Perennial plants live for a series of years. Many kinds of
trees last for centuries. The California giant redwoods, or
Sequoias (Fig. 33), which reach a height of over 300 feet under

FIG. 37. A portion of a
branch of Fig. 36

Natural size

„. _„. Twigs and
branches of the
birch

v-^J 7

favorable circumstances, live nearly 2000 years ; and some
enormous cypress trees found in Mexico were thought by Pro-
fessor Asa Gray to be from 4000 to 5000 years old.

STEMS

•4w

61. Climbing and twining stems.1 Since it is essential to the
health and rapid growth of most plants that they should have
free access to the sun and air, it is not strange that many should
resort to special devices for lifting themselves above their neigh-
bors. In tropical forests, where the darkness of the shade any-
where beneath the tree tops is so great that few flowering
plants can thrive in it, the climbing plants, or lianas (Fig. 39),
often run like great cables for hundreds of feet before they can
emerge into the sunshine above. In temperate climates no such
remarkable climbers are found, but many plants raise themselves
for considerable distances. The principal means by which they

accomplish this result are :

1. Producing roots at many points
along the stem above ground and
climbing on suitable objects by. means
of these, as in the English ivy (Fig. 14).

2. Laying hold of objects by means
of tendrils or twining branches or leaf-
stalks, as shown in Figs. 40 and 41.

3. Twining about any slender up-
right support, as shown in Fig. 42.

4. Clambering upon bushes and
other supports by means of hooked
prickles, as is done by some roses,
blackberries, and cleavers (Galium).

62. Tendril climbers. The plants
which climb by means of tendrils are
important subjects for study. Con-
tinued observation soon shows that
the tips of tendrils' sweep slowly about
in a circular or oval course until they come in contact with some
object around which they can coil. After the tendril lias taken
a few turns about its support, the free part of the tendril coils
into a spiral and thus draws the whole stem toward the point
1 See Kerner and Oliver, Natural History of Plants, Vol. I, p. 669.

FIG. 40. Coiling of a tendril
of bryony
After Sachs

CLIMBERS AND TWINERS

of attachment, as shown in Fig. 40. Some tendrils are modified
leaves or stipules, as shown in Fig. 98 ; others are modified stems.
63. Irritability of tendrils. The coiling of tendrils is due
to their irritability, aroused by the stimulus of contact with a
solid object. After a latent period, varying with different species
from a few seconds to more than an hour, the bending begins.
It is caused either by contraction of the side in contact or by
expansion of the opposite side; the exact mechanism of the
process is not yet fully under-
stood. The tendrils of the passion-
flower plant will respond to the

FIG. 41. Coiling of petiole of dwarf
nasturtium ( Tropceolum)

FIG. 42. Twining stem of hop
After Decaisne

pressure of a bit of thread, hung on the tendril and kept in
motion, whose weight is only a few millionths of a grain.

64. Twiners. Only a few of the upper internodes of the stem
of a twiner are concerned in- producing the movements of the
tip of the stem. This is kept revolving in an elliptical or cir-
cular path until it encounters some roughish and not too stout
object, about which it then proceeds to coil itself.

The movements of the younger internodes of the stems of
twiners are among the most extensive of all the movements

STEMS

made by plants. A hop vine which has climbed to the top of
its stake may sweep its tip continually around the circumfer-
ence of a circle two feet in diameter, and the common wax
plant (Hoya) of the greenhouses sometimes describes a five-foot
circle, the tip moving at the rate of thirty-two inches per hour.1

This circular motion is
produced by unequal
growth of the two sides
of the stem.2

The direction in
which twiners coil
about a supporting ob-
ject is almost always
the same for each spe-
cies of plant, but not
the same for all species.
In the hop it is as shown
in Fig. 42, but in many
plants the movement is
in the reverse direction.
65. Short-stemmed
plants. As will be

FIG. 43. The dandelion, a short-stemmed plant shown later (Chapter

xxxiv), plants live sub-
ject to a very fierce competition among themselves, and they are
exposed to almost constant attacks from animals.

While plants with long stems find it to their advantage to
reach up as far as possible into the sunlight, the dandelion, the
cinquefoil, the white clover, some spurges, the knotgrass, and
hundreds of other species, living in open places, have found
safety in hugging the ground. The dandelion, fall dandelion,

1 See article on " Climbing Plants," by Dr. W. J. Beal, in the American
Naturalist, Vol. IV, pp. 405-415.

2 See Strasburger, Noll, Schenk, and Schimper, Text-Book of Botany,
pp. 257-260, New York, 1903.

SHORT-STEMMED PLANTS

shepherd's purse, and the like, with radiating leaves, are known
as rosette plants, while those with radiating stems, like knotgrass,

FIG. 45. Roots, rootstocks, and
leaves of Iris

FIG. 44. Rootstock of cotton grass (Eriophorum)

the clovers, and black medick (Medicago) , are known as mat plants.

Any plant which can grow in safety under the very feet of

grazing animals will be especially likely to make its way in

STEMS

the world, since there are many places where it can nourish
while ordinary plants would be destroyed. The bitter dandelion,
which is almost uneatable for most animals on account of its
taste, which lies too near the earth to be fed upon by grazing
animals, and which bears being trodden on with impunity, is a
type of a large class of hardy weeds.

The plants incorrectly called "stemless," like the dandelion
(Fig. 43) and some violets, are not really stemless, but send out

b'.

FIG. 46. Rootstock of
caladium (Colocasia)

b, terminal bud; &', buds
arranged in circles where
bases of leaves were at-

FIG. 47. Part of a potato plant

tached ; s, scars left by The dark tuber in the middle is the one from which
sheathing bases of leaves the plant has grown

their leaves and flowers from a very short stem which hardly
rises from the surface of the ground.

66. Underground stems. Stems which lie mainly or wholly
underground are of frequent occurrence and of many kinds.
Some of the simplest kinds are called rootstocks. Familiar ex-
amples are those of some mints, of bloodroot, of Solomon's seal,
and of many grasses, sedges, and ferns. The real nature of the
creeping underground stem is frequently shown by the pres-
ence upon its surface of many scales, which are reduced leaves.

UNDERGROUND STEMS

Exterior view, and split lengthwise. —
After Faguet

Rootstocks of this sort often extend horizontally for long dis-
tances in the case of grasses like the sea rye grass (Plate I),

which roots itself firmly and
thrives in shifting sand dunes.
In the stouter rootstocks, like
that of the iris (Fig. 45) and
the caladium (Fig. 46), this
stem-like character is less evi-
dent. The potato is an excel-
lent example of the short and
much-thickened underground
stem known
as a tuber.
FIG. 48. Bulb of hyacinth Jt may be

seen from

Fig. 47 that

the potatoes are none of them borne on true
roots, but only on subterranean branches,
which are stouter and more cylindrical than
most of the roots. The " eyes " of the potato
are rudimentary leaves and buds.

Bulbs, whether coated like those of the
onion or the hyacinth (Fig. 48), or scaly like
those of the lily, are merely very short and
stout underground stems, covered with closely
crowded scales or layers which represent FIG. 49. Longitudi-

leaves or the bases of leaves (Fig. 49). na! sef i(f of an

onion leaf
The variously modified forms of under-

. sea, thickened base of

ground stems just discussed illustrate in a ieaf , forming a bulb
marked way the storage of nourishment during scale ; s> thin sheath

J fe of leaf; bl, blade of

the winter, or the rainless season, as the case the leaf -int, hollow
may be, to provide the material for rapid
growth during the active season. It is inter-
esting to notice that a majority of the early flowering herbs
in temperate climates, like the crocus, the snowdrop, the spring

sea

STEMS

beauty, the tulip, and the skunk cabbage, owe their early bloom-
ing habit to richly stored underground stems of some kind, or
to thick fleshy roots. Many of these very early blooming plants
are woodland species which must hurry through most of the
season's growth and begin to mature seed before the shade of
the trees above them cuts off most of the necessary supply
of light and before the drought of summer begins.

67. Condensed stems. The plants of desert regions require,
above all, protection from the extreme dryness of the surround-

FIG. 50. A globular cactus

ing air, and usually from the excessive heat of the sun. Ac-
cordingly, many desert plants are found quite destitute of ordi-
nary foliage, exposing to the air only a small surface. In the
globular cactuses (Fig. 50) the stem appears reduced to the
shape in which the least possible surface is presented by a plant
of given bulk, - - that is, in a somewhat spherical form. Other

LEAF-LIKE STEMS

cactuses are cylindrical or prismatic, while still others consist
of flattened joints ; but all agree in offering much less area to
the sun and air than is exposed by an ordinary leafy plant.

68. Leaf-like stems. The flattened stems of some kinds of
cactus, especially the common showy Phyllocactus, are suffi-
ciently like fleshy leaves, with their dark green color and imita-
tion of a midrib, to pass for leaves. There are, however, a good
many cases in which the stern takes on a more strikingly leaf-
like form. The common asparagus sends up in spring shoots
that bear large scales which are really reduced leaves. Later in

FIG. 61. A spray of a common asparagus (not the edible species)

the season, what seem like thread-like leaves cover the much-
branched mature plant, but these green threads are actually
minute branches, which perform the work of leaves (Fig. 51).
The familiar greenhouse climber, wrongly known as smilax,
properly called Myrsipliyllum, bears a profusion of what ap-
pear to be delicate green leaves (Fig. 52). Close study, how-
ever, shows that these are really short flattened branches, and
that each little branch springs from the axil of a true leaf, I, in
the form of a minute scale. Sometimes a flower and a leaf-like
branch spring from the axil of the same scale.

STEMS

Branches which, like those of Myrsiphyllum, so closely re-
semble leaves as to be almost indistinguishable from them are
called dadopliylls, meaning branch leaves.

69. The range of modification of the stem. The stem may
reach a length of many hundred feet, as in the tallest trees, in
the great lianas of South American forests, or in the rattan of
Indian jungles. On the other hand, in such plants as the prim-
rose and the dandelion the stem may be reduced to a fraction

FIG. 52. Stem of Myrsiphyllum

I, scale-like leaves; cl, cladophyll, or leaf-like branch, growing in the axil of the
leaf; ped, flower stalk, growing in the axil of a leaf

of an inch in length. It may take on apparently root-like forms,
as in many grasses and sedges, or become thickened by under-
ground storage of starch and other plant food, as in the iris, the
potato, and the crocus. Condensed forms of stem may exist
above ground, or, on the other hand, branches may be flat and
thin enough closely to imitate leaves. In short, the stem mani-
fests great readiness in adapting itself to the most varied con-
ditions of existence.

CHAPTER VII

STRUCTURE OF THE STEM
STEM OF MONOCOTYLEDONOUS PLANTS

70. External characters. The most familiar of the larger
monocotyledonous plants are the grass-like ones, such as Indian
corn, broom corn, and bamboo, the green briers (Smilax), and the
palms. The stem

of Indian corn con-
sists of a series o^
smooth, slightly
tapering internodes
connected by en-
larged nodes. Palm
stems often have a
very uneven sur-
face, due to the
projecting remains
of old leafstalks
(Fig. 53).

71. Internal
structure. A
cross section of a
corn stem shows
it to be composed
of a hard, flinty
rind, inclosing a

very soft pith, which is traversed lengthwise by many slender
fibers (Fig. 54). The fibers are arranged in a somewhat definite
way, the smaller ones thickly clustered near the rind, the larger
ones, less abundant, toward the center.

FIG. 53. Group of date palms

STRUCTURE OF THE STEM

FIG. 54. Diagrammatic cross section
of stem of Indian corn

cv, fibro-vascular bundles; gc, pithy
material between bundles. — After
Strasburger

In the bamboo, as in the cane
of our southern canebrakes, the
interior is hollow, with a hard,
transverse partition at each
node.

The fibers which traverse the
pith of the com stem are not
solid cylinders, but are built
up of cells of several kinds,
around and between tubes,
somewhat like those of Fig. 62.
The whole structure is known
as a fibro-vascular bundle; that
is, a bundle of fibers and ves-
sels, or tubes. In wroody stems,
such as those of the bamboo
or palm, the bundles are closer

together and much harder than in the corn stem. The outer

rind of the latter is composed of long, thick-walled, slender

cells, containing much silica and known as sclerencliyma fibers.
72. Mechanical function of the manner

of distribution of material in monocoty-

ledonous stems. The well-known strength

and lightness of the straw of our smaller

grains and of rods of cane or bamboo are

due to their form. It can readily be shown

by experiment that an iron or steel tube of

moderate thickness, like a piece of gas pipe

or of bicycle tubing, is much stiff er than a

solid rod of the same weight per foot. The

oat straw, the stems of bulrushes, the cane

of our southern canebrakes, and the bam-
boo are hollow cylinders ; the cornstalk is

a solid cylinder, but filled with a very light pith. The flinty outer

layer of the stalk, together with the closely packed sclerencliyma

FIG. 55. Diagrammatic
cross section of stem
of bulrush (Scirpus), a
hollow cylinder with
strengthening fibers
After Kerner

THE DICOTYLEDONOUS STEM 59

fibers of the outer rind and the frequent fibre-vascular bundles
just within this, are arranged in the best way to secure stiffness.
In a general way, then, we may say that the pith, the bundles,
and the sclereiichymatous rind are what they are and where
they are to serve important mechanical purposes. But they have
other uses fully as important (see Chapter vin).

73. Growth, of monocotyledonous stems in thickness. In
most woody monocotyledonous stems, for a reason which will
be explained later in this chapter, the increase in thickness is
strictly limited. Such stems, therefore, as in many palms and
in rattans, are less conical and more cylindrical than the trunks
of ordinary trees, and are also more slender in proportion to
their height.

STEM OF DICOTYLEDONOUS PLANTS

74. External characters. It is not easy to make any gen-
eral statements about the external characters of dicotyledonous
stems, on account of their very great variety of form. The stu-
dent in his examination of twigs in connection with Chapter VI
has learned a little about the appearance of a few woody stems.
In general, the nodes are much less marked than in stems of
corn, bamboo, and other grass-like forms. In the case of decid-
uous-leaved dicotyledonous plants, the scars left by fallen leaves
are characteristic, quite unlike those mentioned in Sec. 70.

75. Internal structure.1 If one begins his study of the struc-
ture of dicotyledonous stems with the one-year-old stem of a
woody plant or with the stem of some such robust annual as
hemp, sunflower, or the great ragweed, he will find it to be com-
posed of a somewhat cylindrical pith, surrounded by a layer
of wood usually of pretty even thickness, which is in its turn
surrounded by a layer of bark (Fig. 56).2

1 For an account of the structure of the pine stem, see Sec. 352.

2 Of course these layers are nearly cylindrical tubes, filled by pith or by
wood and pith respectively. They are not of perfectly circular cross section,
and they taper somewhat.

STRUCTURE OF THE STEM

The wood cylinder may be discontinuous, that is, broken up
into separate fibre-vascular bundles, as shown in Fig. 57 ; but
even then the position of the wood between an inclosed pith

FIG. 56. Diagrammatic cross section of an annual dicotyledonous stem

p, pith; fv, woody or fibro-vascular bundles; e, epidermis; b, bundles of hard-
bast fibers of the bark. Somewhat magnified. — After Frank

e b c p

FIG. 57. Diagrammatic cross section of one-year-old Aristolochia stem

c, region of epidermis; b, hard-bast fibers; o, outer or bark part of a bundle; w,
inner or woody part of bundle; c, cambium layer; p, region of pith; m, a
medullary ray. Considerably magnified

The space between the hard bast and the bundles is occupied by thin-walled,
somewhat cubical cells of the bark 1

and an inclosing bark is notably different from the way in
which the bundles are scattered in monocotyledonous stems.

1 In this and the following figure the relative prominence of the cambium
layer is a good deal exaggerated.

FIG. 58. One bundle from the preceding figure

to, wood cells ; d, vessels. The other letters are as in Fig. 57. Many sieve cells
occur in the region just outside of the cambium of the bundle, x 100

FIG. 59. Stem of box elder one year old

A, lengthwise (radial) section; B, cross section; e, epidermis; ck, cork; 6, hard
bast ; s, sieve cells ; c, cambium ; w, wood cells ; m, medullary rays ; d, vessels ;
p, pith. Much magnified

ftl

FIG. 60. Part of cross section of stem of flax

e, epidermis; 6, hard bast; s, sieve cells; w, wood. Much magnified. — After

Tschirch

FIG. 61

FIG. 62

FIG. 61. A group of hard-bast fibers
a, cut-off ends ; 6, lengthwise section of fibers. Greatly magnified. —After Tschirch

FIG. 62. A lengthwise section of a group of spiral vessels from the stem

of sunflower

At the top of the figure some of the spiral threads which line the vessels are seen

partly uncoiled. Greatly magnified. — After Frank

MATERIAL FOR STRENGTHENING PURPOSES

76. Disposition of material for strengthening purposes. Only
two of the many ways in which the stem is strengthened need
be mentioned here. In a majority of cases it owes its stiffness
mainly to the wood, as shown in Fig. 56. But not infrequently

FIG. 63 FIG. 64

FIG. 63. Part of a sieve tube from linden
s, sieve plates on the cell wall, x about 900. — After Thome

FIG. 64. Parts of sieve tubes as found in plants of the gourd family

ss, a sieve plate seen edgewise ; above it a similar one, surface view. Greatly
magnified. — After Thome

most of the stiffening material consists of the hard-bast fibers
found in the bark. It is this layer in flax (Fig. 60) which is
utilized in the manufacture of linen thread and linen fabrics.

64 STRUCTURE OF THE STEM

77. Structural units of the dicotyledonous stem. The stu-
dent should already, from his own examinations, have learned
a good deal about the kinds of cells and cell aggregates which
compose the stem. The preceding figures (Figs. 56-60) will serve
to illustrate the most important of these, and Figs. 61-64 show
some of them more in detail.

78. Parenchyma, prosenchyma, and collenchyma. A mass
of similar cooperating cells is called a tissue.1 Two of the prin-
cipal classes not previously mentioned which occur in the stem
are paremhymatous tissue and prosencliymatous tissue. Paren-
chyma is well illustrated by the green layer of the bark, by
wood parenchyma, and by pith. Its cells are usually somewhat
roundish or cubical, at any rate not many times longer than
wide, and at first rather full of protoplasm. Their walls are
not generally very thick. Prosenchyma, illustrated by hard

bast and masses of wood cells, consists of
thick-walled cells many times longer than
wide, containing little protoplasm and often
having little or no cell cavity.

As a rule the stems of the most highly
developed plants owe their toughness and
their stiffness mainly to prosenchymatous
tissue. In some stems, particularly the
fleshy ones, the stiffness is, however, largely
FIG. 65. Collenchyma- due to collenchyma, a kind of parenchyma
tous and other tissue in which the cellg are thickened or reen-
from stem of balsam . . _r

(Impatient) forced at their angles, as shown in Fig. 65.

e, epidermis; c, coiien- 79. The early history of the stem. In
chyma; i, intercellular ^he earliest stages of the growth of the

spaces between large • . . . . . ,, ,

parenchyma cells.— stem it consists entirely of thin-walled
After strasburger an(j rapidly dividing cells. Soon, however,

the various kinds of tissue which are found in the full-grown

stem begin to appear.

1 See Strasburger, Noll, Schenck, and Schimper, Text-Book of Botany,
pp. 92-95, 2d ed., London, 1903.

SECONDARY GROWTH

In Fig. 66 the process is shown as it occurs in the castor
bean. At ra, in B, is the central column of pith surrounded
by eight fibre-vascular bundles, fvy each of which contains a
number of vessels arranged in a somewhat regular manner and
surrounded by the forerunners of the
true wood cells.

In C the section shows a consider-
able advance in growth : the nbro-
vascular bundles are larger and are
now connected by a rapidly growing
layer of tissue c.

As growth continues, this layer
becomes the cambium layer, com-
posed of thin-walled and rapidly
dividing cells, as shown in Figs.
67 and 68.

80. Secondary growth. From the
inside of the cambium layer the wood
cells and ducts of the mature stem
are produced, while from its outer
circumference the new layers of the
bark proceed. From this mode of in-

crease the Stems of dicotyledonous

plants are called exogenous, that is,

outside growing. The presence of the

cambium layer on the outside of the

wood in early spring is a fact well

known to the schoolboy who pounds

the cylinder cut from an alder, willow,

or hickory branch until the bark will

slip off and so enable him to make a whistle. The sweet taste

of this pulpy layer, as found in the white pine, the slippery elm,

and the basswood, is a familiar evidence of the nourishment

which the cambium layer contains. It is also, as might be

supposed, very rich in proteids.

FIG. 66. Transverse section
through the hypocotyl of
the castor-oil plant at vari-
ous stages

A, after the root has just ap-
peared outside the testa of the
seed ; J3, after the hypocotyl
is nearly an inch long; C, at
the end of germination ; r,
cortex (undeveloped bark) ;
m, pith ; st, medullary rays ;
fv, h'bro-vascular bundles ;
c, layer of tissue which is to
develop into cambium. Con-
siderably magnified. — After
Sachs

STRUCTURE OF THE STEM

With the increase of the fibro-vascular bundles of the wood,
the space between them, at first large, becomes less, and the
pith, which extended freely out toward the bark, becomes com-
pressed into thin plates so as to form medullary rays.

These are, as already stated, of value in storing the food which
the plant in cold and temperate climates lays up in the sum-
mer and fall for use in the
following spring, and in
the very young stem they
serve as an important
channel for the transfer-
ence of fluids across the
stem from bark to pith, or
in the reverse direction.
On account, perhaps, of
their importance to the
plants, the cells of the
medullary rays are among
the longest lived of all
vegetable cells, retaining
their vitality in the beech
tree sometimes, it is said,
for more than a hundred
years.

After the interspaces be-
tween the first fibro-vascu-

FIG. 67. Cross section of a three-year-old
linden twig

e, epidermis and corky layer of the bark;
b, bast ; c, cambium layer ; r, annual
rings of wood. Much magnified. — After

Kny

lar bundles have become

filled up with wood, the subsequent growth must take place in
the manner shown in Fig. 68. The cambium of the original
wedges of wood fc, and the cambium ic formed between these
wedges, continues to grow from the inner and from the outer
surface, and thus causes a permanent increase in the diameter
of the stem and a thickening of the bark, which, however, usu-
ally soon begins to peel off from the outside and thus remains
pretty constant in thickness.

STEM STRUCTURE OF CLIMBING SHRUBS 67

It is the lack of any such ring of cambium as is found in
dicotyledonous plants, or even of permanent cambium in the
separate bundles, that makes it impossible for the trunks of
most palm trees to grow indefinitely in thickness, like that of
an oak or an elm.

81. Stem structure of climbing shrubs. Some of the most
remarkable kinds of dicotyledonous stems are found in climbing
shrubs. The bundles, as shown in Fig. 57, are much more dis-
tinct than in most other woody stems. It is evident that this

Jc
FIG. 68. Diagram to illustrate secondary growth in a dicotyledonous stem

R, the first-formed bark ; p, mass of sieve cells ; ifp, mass of sieve cells between
the original wedges of wood ; /c, cambium of wedges of wood ; ic, cambium
between wedges ; b, groups of bast cells ; //?, wood of the original wedges ; ifh,
wood formed between wedges; x, earliest wood formed ; M, pith. — After Sachs

is for the sake of leaving the stem flexible for twining purposes,
just as a wire cable is adapted to be wound about posts or other
supports, while a solid steel or iron rod of the same size would
be too stiff for this use.

82. Interruption of annual rings by branches ; knots. When
a leaf bud is formed on the trunk or branch of a dicotyledonous
tree it is connected with the wood by fibro-vascular bundles.

STRUCTURE OF THE STEM

As the bud develops into a branch, the few bundles which it
originally possessed increase greatly in number, and at length,
as the branch grows, form a cylinder of
wood which cuts across the annual rings,
as shown in Fig. 69. This interruption to
the rings is a knot, such as one often sees
in boards and planks. If the branch dies
long before the tree does, the knot may
be buried under many rings of wood. What
is known as "clear" lum-
ber is obtained from trees
that have grown in a
dense forest, so that the
lower branches of the
larger trees were killed
by the shade many years
before the tree was felled.
In pruning fruit trees
a knot in a tree trunk or shade trees the

R, cut-off end of stick, branches which are re-
showing annual rings ; ^ ^ •, ^ -.

K, knot formed by m0ved should be Cut

growth of a branch. — close to the trunk. If

After Roth A. . . ,

this is done, the growth
of the trunk will bury the scar before decay
sets in.

83. Grafting. When the cambium layer
of any vigorously growing stem is brought in FIG. 70. Grafting
contact with the same layer in another stem At the left scion and
of the same kind or a closely similar kind of
plant, the two may grow together to form a
single stem or branch. This process is called
grafting, and is much resorted to in order to
secure apples, pears, etc., of any desired kind
(Fig. 70). A twig known as the scion from a plant of the chosen
variety may be grafted upon another individual of similar kind

FIG.

Formation of

stock are shown
ready to be united ;
at the right they are
joined and ready to
cover with grafting
wax. — After Perci-
val

GRAFTING

known as the stock, and the resulting stems will bear the
wished-for sort of fruit. Often one species is grafted on another,
as the pear on the quince or the apple. Karely trees differing
as much as the chestnut and the oak may be grafted together.

FIG. 71. Two ash trees naturally grafted together
After Werthner

Sometimes grafting comes about naturally by the branches of a
tree chafing against one another until the bark is worn away
and the cambium layer of each is in contact with that of the
other, or two separate trees may be joined by natural grafting
into a nearly cylindrical double trunk, as is shown in Fig. 71.

STRUCTURE OF THE STEM

84. Comparison of the monocotyledonous and the dicotyledo-
nous stem.1

General
Structure :

Structure of
Bundles :

MONOCOTYLEDONOUS
STEM

A hard rind of rather
uniform structure.
Bundles intermixed
with the pith.

Bundles dosed; that
is, without permanent
cambium.

DICOTYLEDONOUS
STEM

A complex bark , usually
on young shoots, consist-
ing of an epidermis, a
corky layer, a green layer,
and a layer of bast. A
layer of cambium. Wood
in annual rings. Pith in
a cylinder at the center.

Bundles open, with per-
manent cambium.

Growth in
Thickness :

Cells of mature parts
of stem expand some-
what, but (in most
palms) new ones are
not formed.

New wood cells formed
throughout growing sea-
son from cambium ring.

1 This comparison applies only to most of the woody or tree-like stems.

CHAPTER VIII
LIVING PARTS OF THE STEM; WORK OF THE STEM

85. Active portions of the stems of trees and shrubs. In

annual plants generally, and in the very young shoots of shrubs
and trees, there are stomata (singular stoma, meaning mouth),
or breathing pores, which occur abundantly in the epidermis,
serving for the admission of air and the escape of moisture,
while the green layer of the bark answers the same purpose
that is served by the green pulp of the leaf (Chapter xn). For
years, too, the spongy lenticels, which succeed the stomata and
occur scattered over the external surface of the bark of trees
and shrubs, serve to admit air to the interior of the stem. The
lenticels at first appear as roundish spots, of very small size; but
as the twig or shoot on which they occur increases in diameter,
the lenticel becomes spread out at right angles to the length of
the stem, so that it sometimes becomes a longer transverse slit
or scar on the bark, as in the cherry and the birch and the
elder. But in the trunk of a large tree often no part of the
bark except the inner layer is alive. The older portions of
the bark, such as the highly developed cork of the cork oak,
sometimes cling for years after they are dead and useless ex-
cept as a protection for the parts beneath against mechanical
injuries or against cold. But in many cases, as in the shellbark
hickory and the grapevine, the old bark soon falls off in strips ;
or as in birches it finally peels off in bands around the stern.

The cambium layer is very much alive, and so is the young
outer portion of the wood. Testing this sapwood, particularly
in winter, shows that it is rich in starch and proteids.

The heartwood of a full-grown tree is hardly living, unless
the cells of the medullary rays retain their vitality, and so it is

72 WORK OF THE STEM

probable that wood of this kind is chiefly useful to the tree by
giving stiffness to the trunk and larger branches, thus preventing
them from being easily broken by storms.

It is therefore possible for a tree to flourish, sometimes for
centuries, after the heartwood has much of it rotted away and
left the interior of the trunk hollow, as shown in Fig. 72.

86. Uses of the components of the stem. There is a marked
division of labor among the various groups of cells that make
up the stem of ordinary dicotyledons, particularly in the stems
of trees, and it will be best to explain the uses of the kinds of
cells as found in trees rather than in herbaceous plants. A few
of the ascertained uses of the various tissues are these :

The pith forms a large part of the bulk of very young shoots,
since it is a part of the tissue of comparatively simple structure
amid which the fibre-vascular bundles arise. In mature stems
it becomes rather unimportant, though it often continues for a
long time to act as a storehouse of food.

The medullary rays in the young shoot serve as a channel
for the transference of water and plant food in a liquid form
across the stem, and they often contain much stored food.

The vessels carry water upward through the stem in certain
plants.

The wood cells of the heartwood are useful only to give stiff-
ness to the stem. Those of the sapwood, in addition to this
work, have to carry most of the water from the roots to the
leaves and other distant portions of the plant.

The cambium layer is the region in which the annual growth
of the tree takes place.

Sieve tubes form the most important portion of the inner bark,
carrying elaborated plant food from the leaves toward the roots.

The green layer of the bark in young shoots does much toward
collecting nutrient substances, or raw materials, and preparing
the food of the plant from air and water, but this work may
be best explained in connection with the study of the leaf
(Chapter xn).

MOVEMENT OF WATER IN THE STEM

FIG. 72. Pioneer's cabin, a hollow giant redwood (Sequoia)
After White

87. Movement of water in the stem. The student has already
learned that large quantities of water are taken up by the roots
of plants.

WORK OF THE STEM

Having become somewhat acquainted with the structure of
the stem, he is now in a position to investigate the question how
the various fluids, commonly known as sap, travel about in it.1

It is important to notice
that sap is by no means
the same substance every-
where and at all times.
As it first makes its way
by osmotic action inward
through the root hairs of
the growing plant it differs
but little from ordinary
spring water or well water.
The liquid which flows
from the cut stem of a
" bleeding " tree or grape-
vine which has been
pruned just before the buds
have begun to burst in the
spring is mainly water,
often with a little dissolved
organic acids, proteids, and
sugar. The sap which is

FIG. 73. Channels for the movement of °btained f rom maPle tr66S
water, upward and downward in late winter or early

The heavy black lines in roots, stems, and Spring, and is boiled down
leaves show the course of the fibro-vascular £or gjrup or Sugar is richer
bundles through which the principal move-
ments of water take place. — After Frank ill nutritious material than
and Tschirch j_v

while the elaborated sap which is sent so abundantly into the ear
of corn at its period of filling out, or into the growing pods of
beans and peas, or into the rapidly forming acorn or the chestnut,
contains great stores of food suited to sustain plant or animal life.

1 See the paper on "The So-called Sap of Trees and its Movements," by
Professor Charles R. Barnes, Science, Vol. XXI, p. 535.

MOVEMENT OF LIQUIDS IN THE STEM

From the familiar facts that ordinary forest trees apparently
flourish as well after the almost complete decay and removal of
their heartwood, and that many kinds will live and grow for a
considerable time after a ring of bark extending all round the
trunk has been removed, it may readily be inferred that the
crude sap in trees must rise through some portion of the newer
layers of the wood. A tree girdled by the removal of a ring of

FIG. 74. A cutting girdled and
sending down roots from the
upper edge of the girdled
ring

FIG. 75. Diagrammatic cross section of a
bundle from sugar cane, showing chan-
nels for water and dissolved plant food

Water travels upward through the vessels d
and through the wood cells in the region
marked w. Water with dissolved plant
food travels downward through the sieve
tubes in the region s. Magnified

sapwood promptly dies. After the removal of a ring of bark a
tree dies from starvation of the roots (Sec. 88 ; also see Fig. 394).
88. Downward movement of liquids. Most dicotyledonous
stems, when stripped of a ring of bark and then set in water,
as shown in Fig. 74, and covered with a bell jar, develop roots
only at or near the upper edge of the stripped portion. This
would seem to prove that such stems send their building mate-
rial — the elaborated sap — largely, at any rate, down through

76 WORK OF THE STEM

the bark. Its course is undoubtedly for the most part through
the sieve tubes (Figs. 63, 64), which are admirably adapted to
convey liquids. In addition to these general upward and down-
ward movements of sap, there must be local transfers laterally
through the stem, and these are at times of much importance
to the plant.

Since the liquid building material travels straight down the
stem, that side of the stem on which the manufacture of such

material is going on most rapidly
should grow fastest. Plant food is
made out of the raw materials by the
leaves, and so the more leafy side of a
tree forms thicker rings than the less
leafy side, as shown in Fig. 76.

89. Rate of movement of water in
the stem. There are many practical
FIG. 76. Unequal growth of difficulties in the way of ascertaining
rings of wood in a nearly exactly how fast the watery sap travels
horizontal stem of juniper from the root to the leaveg jfc ^ how_

Natural size ever^ eagv to ^ justrate experimentally

the fact that it does rise, and to give an approximate idea of
the time required for its ascent. The best experiment for be-
ginners is one which deals with an entire plant under natural
conditions ; that is, by allowing a plant to wilt from lack of
water, then watering it freely and noting how soon the leaves
begin to recover their natural appearance and positions.

The interval of time will give a very rough idea of the time
of transfer of water through the roots and the stem of the plant.
From this, by measuring the approximate distance traveled, a
calculation could be made of the number of inches per minute
that water travels in this particular kind of plant, through a
route which is partly roots, partly stem, and partly petiole.
Still another method is to immerse the cut ends of leafy stems
in eosin solution and note carefully the rate of ascent of the
coloring liquid. This plan is likely to give results that are too

CAUSES OF MOVEMENTS OF WATER IN THE STEM 77

low ; still it is of some use. It has given results varying from
34 inches per hour for the willow to 880 inches per hour for
the sunflower. A better method is to introduce the roots of the
plant which is being experimented upon into a weak solution
of some chemical substance which is harmless to the plant and
which can readily be detected anywhere in the tissues of the
plant by chemical tests. Proper tests are then applied to por-
tions of the stem which are cut from the plant at short intervals
of time.

Compounds of the metal lithium are well adapted for use in
this mode of experimentation, if a spectroscope is available to
test for its presence.

90. Causes of movements of water in the stem. Some of the
phenomena of osmosis were explained in Sees. 48—51, and the
work of the root hairs was described as due to osmotic action.

That portion of the sap pressure which originates in the roots
(Sec. 38), being apparently able to sustain a column of water
only 80 or 90 feet high at the most (and usually less than half
this amount), would be quite insufficient to raise the sap to the
tops of the tallest trees, since many kinds grow to a height
of more than 100 feet. Our California "big trees," or Sequoias,
reach the height of over 300 feet, and an Australian species of
Eucalyptus, it is said, sometimes towers up to 470 feet. Eoot
pressure, then, may serve to start the soil water on its upward
journey, but some other force or forces must step in to carry it
the rest of the way. What these other forces are is still a matter
of discussion among botanists.

The slower inward and downward movement of the sap may
be explained as due to osmosis. For instance, in the case of
growing wood cells, sugary sap descending from the leaves into
the stem gives up part of its sugar to form the cellulose of
which the wood cells are being made.

This loss of sugar leaves the sap rather more watery than
usual, and osmosis carries it from the growing wood to the
leaves, while at the same time a slow transfer of the dissolved

78 WORK OF THE STEM

sugar is set up from leaves to wood. The water is thrown off
in the form of vapor as fast as it reaches the leaves, so that they
do not become distended with water, while the sugar is changed
into cellulose and built into new wood cells as fast as it reaches
the region where such cells are being formed.

Plants in general 1 readily change starch to sugar, and sugar
to starch. When they are depositing starch in any part of the
root or stem for future use, the withdrawal of sugar from those
portions of the sap which contain it most abundantly gives rise
to a slow movement of dissolved particles of sugar in the direc-
tion of the region where starch is being laid up.

91. Storage of food in the stem. The reason why the plant
may profit by laying up a food supply somewhere inside its
tissues has already been suggested (Sec. 33).

The most remarkable instance of storage of food in the stem
is probably that of sago palms, which contain an enormous
amount, sometimes as much as eight hundred pounds, of starchy
material in a single trunk. But the commoner plants of tem-
perate regions furnish abundant examples of deposits of food
in the stem.

92. Storage in underground stems. The branches and trunk
of a tree furnish the most convenient place in which to deposit
food during winter to begin the growth of the following spring.
But in those plants which die down to the ground at the begin-
ning of winter the storage must be either in the roots or in
underground portions of the stem.

Kootstocks, tubers, and bulbs seem to have been developed
by plants to answer as storehouses through the winter (or in
some countries through the dry season) for the reserve materials
which the plant has accumulated during the growing season.
The commonest tuber is the potato, and this fact and the points
of interest which it represents make it especially desirable to
use for a study of the underground stein in a form most highly
specialized for the storage of starch and other valuable products.
1 Not including most of the spore plants.

OCCURRENCE OF SUGAR IN THE STEM 79

It is evident that in the potato we have to do with a very
highly modified form of stem. The corky layer of the bark is
well represented, and the loose cellular layer beneath is much
developed ; ' wood is almost lacking, but the pith is greatly
developed and constitutes the principal bulk of the tuber.
All this is readily understood if we consider that the tuber,
buried in and supported by the earth, does not need the kinds
of tissue which give strength, but only those which are well
adapted to store the requisite amount of food.

93. Occurrence of sugar in the stem. Grape sugar is an
important substance among those used for food by the plant. It
received its name from the fact that it was formerly obtained
for chemical examination from grapes. Old dry raisins usually
show little masses of whitish material scattered over the skin
which are nearly pure grape sugar. Commercially it is now
manufactured on an enormous scale from starch by boiling with
diluted sulphuric acid. In the plant it is made from starch
by processes as yet imperfectly understood, and another sugar,
called maltose, is made from starch in the seed during germina-
tion. Sugar is not as well adapted for reserve deposits as starch,
since it ferments easily and may escape by osmosis from tissues
which contain it. In the onion bulb it is stored in considerable
quantities and may be detected by a simple chemical test.

CHAPTER IX

BUDS

94. Structure of winter buds. Dissection of most winter
buds shows that they are composed of an outer covering of

tough, often hairy or resin-covered
scales and an interior mass of small
undeveloped leaves, closely packed
together. Not infrequently a rudi-
mentary flower cluster occupies the
central portion of the bud.

95. Nature of bud scales. The
fact that the bud scales are in cer-
tain cases merely imperfectly de-
veloped leaves or leafstalks is often
clearly manifest from the series of
steps connecting the bud scale on
the one hand with the young leaf
on the other, which may be found
in many opening buds, as illus-
trated by Fig. 77. In other buds
the scales are not imperfect leaves,
but the little appendages (stipules,
Figs. 89, 90) which occur at the
bases of leaves. This kind of bud

FIG. 77. Dissected bud of buck- ^ 1S 6SPecially wel1 shown in

eye (sEsculus macrostachya), the magnolia and the tulip tree

showing transitions from bud and in the familiar " rubber plant "

scales to leaves (picus dastica)

96. Naked buds. All the kinds above mentioned are resting
buds, and in temperate or cold climates winter luds, capable of

SCALY BUDS AND NAKED BUDS

FIG. 79. Alternate leaves of cultivated cherry, with
buds in their axils, in October

living through the colder months of the
year, and are also scaly buds.

In the herbs of temperate climates, and
even in shrubs and trees of tropical regions,
the buds are often naked; that is, nearly or
quite destitute of scaly coverings (Fig. 78).
These are best suited for a season or a
climate which is both warm and moist.
The scales, of whatever sort, with their coat-
ings of hair or of resinous material, are of
use mainly in protecting buds from sudden
changes of temperature or too rapid loss
of water. The latter, in climates like that
of southern California or the Mediterranean
coast, would be during the rainless summer.

FIG. 78

Tip of branch of
Ailanthus in winter
condition, showing
very large leaf scars
and nearly naked
buds

BUDS

In most cold or temperate climes it would be during the winter,
when little water can be drawn from the soil (Sec. 39).

97. Position of buds. The distinction between lateral and
terminal buds has already been alluded to (Sec. 57).

The plumule is the first terminal bud
which the plant produces. Lateral buds
are usually axillary, as shown in Fig. 79,
that is they grow in the angle formed
by the leaf with the stem (Latin, axilla,
armpit) ; but not infrequently there are
several buds grouped in some way about
a single leaf axil, either one above the
other, as in the butternut (Fig. 80), or

FIG. 80. Accessoiy
buds of butternut

I, leaf scar ; ax, axil-
lary bud ; a, a', ac-
cessory buds; t,
terminal bud. Re-
duced.

FIG. 81. Accessory buds of box elder
(Negundo)

A, front view of group; />, two groups
seen in profile. Magnified

grouped side by side, as in the red maple, the cherry, and the
box elder (Fig. 81).

In these cases, all the buds, except the axillary one, are
called accessory or supernumerary buds. Those which appear in

LEAF BUDS AND FLOWER BUDS

irregular positions, as on roots, on unusual parts of the stem, or
on leaves (Fig. 88), are called adventitious buds.

98. Leaf buds and flower buds ; the bud an undeveloped
branch. Buds are of three principal classes : leaf buds, in which
the parts inside of the scales develop into leaves, and their cen-
tral axes into stems ; mixed buds, which contain both leaves and
flowers in an undeveloped condition ; and flower buds, which
contain the rudiments of flowers only.

Sometimes, as in the black walnut and the butternut, the
leaf buds and flower buds are readily distinguishable by their

A B

FIG. 82

A, a pear leaf bud in autumn; 7>, a leafy shoot-derived from A, as seen in the
middle of the following summer, with flower bud at tip; C, the fruit spur, B,
in autumn, after the fall of the leaves. — After Percival

difference in form ; while in other cases, as in the cultivated
cherry, the difference in form is but slight. In many plants, as
the lilac, there is a notable difference in size.

The rings of scars about the twig, shown in Figs. 79 and 84,
mark the place where the bases of bud scales were attached.
A little examination of the part of the twig which lies above
this ring, as shown in Fig. 79, will lead one to the conclu-
sion that this portion has all grown in the one spring and sum-
mer since the bud scales of that particular ring dropped off.

BUDS

Following out this suggestion, it is easy to reckon the age of
any moderately old portion of a branch, since it is equal to the

— '1905

FIG. 83. Fruit bud of pear (same as C, of Fig. 82), showing tits development

A, opening in spring; B, later, developing flowers and leaves; C, later still: only
one flower has produced a fruit, the rest having fallen off. Below it is a lateral
hud which will continue the spur next year. — After Percival

number of segments between the rings. In
/ rapidly growing shoots of willow, poplar, and
-1906 similar trees, five or ten feet may be the
growth of a single year, while in the lateral
twigs of the hickory, apple, or cherry, the
yearly increase may be but a fraction of an
inch. Such " spurs " as are shown in Figs.
82-84 are of little use in the permanent
growth of the tree, and poplars, elms, soft
maples, and other trees shed the oldest of
these every year. In any case the growth is
but the development of the bud, which may be
F,G. 84. A slowly grown twig of regarded as an undeveloped stem
cherry, three inches long and or branch, with its internodes so
about ten years old shortened that successive leaves

The pointed hud / is a leaf hud ; the seem almost to spring from the

more obtuse accessory huds /, f

are flower huds Same point.

VERNATION

99. Vernation. The arrangement of leaves in the bud is called
vernation; some of the principal modes are shown in Fig. 86.
In the cherry the two halves of the leaf are folded together flat,
with the under surfaces outward ; in the walnut the
separate leaflets, or parts of the leaf, are folded flat

and then grouped into a sort of cone ; in the snow-
ball each half of the leaf is plaited in a somewhat
fan-like manner, and the edges of the two halves
are then brought round so as to meet ; in the lady's
mantle the fan-like plaiting is
very distinct ; in the wood sorrel
each leaflet is folded smoothly,
and then the three leaflets
packed closely side by side. All
these modes of vernation, and ax
many others, often characteristic
of groups of plants, have received
descriptive names by which they
are known to botanists.

100. Importance of verna-
tion. The significance of verna-
tion is best understood by
considering that there are two
important purposes to be served :
the leaves must be stowed as
closely as possible in the bud,
and upon beginning to open

they must be protected from too great heat and dryness until
they have reached a certain degree of firmness. It may be
inferred from Fig. 86 that it is common for very young leaves
to stand vertically. This protects them considerably from the
scorching effect of the sun at the hottest part of the day.
Many young leaves, as, for instance, those of the silver-leafed
poplar, the pear, the beech, and the mountain ash, are sheltered
and protected from cold, dryness, and the attacks of small

FIG. 85

B, a twig of European elm; A, a
longitudinal section of the buds of
B (considerably magnified) ; ax, the
axis of the bud, which will elongate
into a shoot ; sc, leaf scars. — After
Behrens

BUDS

insects by a coating of wool or down, which they afterwards
lose. The leaves of the tulip tree are inclosed for a little time

FIG. 86. Types of vernation

1, 2, cherry; 3, 4, European walnut; 5, 6, snowball; 7, lady's mantle; 8, oxalis.

After Kerner

A\\ B C D

FIG. 87. Development of an oxalis leaf

-4, full-grown leaf ; B, rudimentary leaf, the leaflets not yet evident ; C, more
advanced stage, the leaflets appearing ; D, a still more advanced stage. B, C,
and Z), considerably magnified. — After Frank

ADVENTITIOUS BUDS 87

in thin pouches, which serve as bud scales, and are thus entirely
shielded from direct contact with the outside air.

101. Dormant buds. Generally some of the buds on a branch
remain undeveloped in the spring, when the other buds are
beginning to grow, and this inactive condition may last for
many seasons. Finally the bud may die, or some injury to the
tree may destroy so many other buds as to leave the dormant
ones an extra supply of food, and this, with other causes, may
force them to develop and to grow into branches.

Sometimes the tree altogether fails to produce buds at places
where they would regularly occur. In the lilac the terminal
bud usually fails to appear, and the result is constant forking
of the branches.

102. Adventitious buds. Buds which occur in irregular places,
that is, not terminal nor in or near the axils of leaves, are called
adventitious buds;

they may spring from
the roots, as in the
silver-leafed poplar, or
from the sides of the

trunk, as in our Amer-

-, T FIG. 88. Budding leaf of Bryophyllum

ican elm. In many

trees, for instance willows and maples, they are sure to appear
after the trees have been cut back. Willows are thus cut back,
or pollarded, in order to cause them to produce a large crop of
slender twigs suitable for basket making.

Leaves rarely produce buds, but a few kinds do so when they
are injured. Those of the Bryopliyllum (Fig. 88), a plant allied
to the garden live-forever, when they are removed from the plant
while they are still green and fresh, almost always send out
buds from the margin. These do not appear at random, but
are borne at the notches in the leaf margin and are accompa-
nied almost from the first by minute roots. This plant seems to
rely largely upon leaf budding to reproduce itself, for in a cool
climate it rarely flowers or seeds.

CHAPTEE X

LEAVES

103. The leaf as a member of the plant body. Among seed
plants the plant body consists of root and shoot. The latter
is made up of stem and leaves. It is diffi-
cult to frame a simple and exact definition
for the leaf, but every one is sufficiently
familiar with the appearance of the ordi-
nary foliage leaves of plants, and there is
no difficulty in identifying these. The un-
usual scale-like, bristle-shaped, tendril-
shaped, or pitcher-form leaves are often
hard to recognize as such.

104. Parts of the leaf. In the typical
foliage leaf there are three parts, — the
expanded portion, or blade
(lamina), the leafstalk
(petiole), and a pair of ap-
pendages at the base of the

FIG. 89. Leaf of apple, petiole known as stipules.
with stipules Many leaves have no

After Thome petiole and are said to be

sessile (meaning sitting). Others have no blade
and perform their functions as foliage by means
of a flattened petiole or large stipules. Most pIG. 90.
leaves are bilaterally symmetrical ; that is, they
have a right and a left half, which, if folded
together along the middle line of the leaf, would
nearly coincide. Usually the upper and the under surface differ
from each other in color, smoothness, and other respects.

Leaf of
pansy, with leaf-
like stipules

After Decaisne

MODES OF VEINING

105. Veining. The blade of the leaf is traversed by a frame-
work of tibro-vascular bundles known as veins. These are
arranged in many ways, but the two prin-
cipal types are closed, or parallel-veined,

and open, or netted-veined, leaves. In the
former the veins run more or less nearly
parallel, either from base to tip of the
leaf, or from a mid-
rib outward. In the
latter the veins are
branched so as to
form a network.

106. Palmate and
pinnate veining. In
netted-veined leaves
several ribs may
radiate from the end
of the petiole, like
the sticks of a fan.
Such veining is said
to be palmate. If

there is only one midrib, from which smaller ribs extend both
ways, the veining is said to be pinnate (meaning feather-like).

Often the veining is intermediate be-
tween these two types.

107. Relation of shape to mode of
veining. Since the water supply of
the leaf is carried through the veins,
and since they support the softer
parts between them, one would ex-
pect to find that the form of the leaf
would bear a close relation to its

,-, no AT 4 mode of veining. This is the case,

FIG. 93. Netted veining (pal-
mate) in leaf of melon and m general palmately veined
After Decaisne leaves are roundish, while pinnately

FIG. 91. Parallel-veined
leaf of Solomon's seal

After Strasburger

FIG. 92. Parallel vein-
ing in canna. Veins
running from midrib
to margin

LEAVES

veined ones are longer than they are wide. These differences
are particularly noticeable in leaves in which the leaf blade is
not all of one piece, — divided leaves (Figs. 95, 96).

Usually veins, near then- origin, follow a pretty straight
course. This is desirable, in order to carry water as speedily
as possible from the base of the leaf to its tip. The arrange-
ment of the veins in the leaves of most land
plants is admirably adapted to strengthen
the leaf and protect it from being torn.
In many cases the last-named result is
secured by a sort of " binding " of looped
veins running around the margin, as is
fairly well shown in Fig. 94.

108. Description of leaf forms. The
various forms of leaves are classed and
described by botanists with great minute-
ness,1 not simply for the study of leaves
themselves, but also because in classify-
ing and describing plants the characteristic
shapes of the leaves of many kinds of plants
form a simple and ready means of distin-

FIG. 94. Netted vein- guishing them from each other and identi-
ing (pinnate) in leaf fying them.

109. Occurrence of netted or parallel
veining. With few exceptions, the leaves

of monocotyledonous plants are parallel-veined and those of
dicotyledonous plants netted-veined.

The needle-like leaves of the pines, spruces, firs, larches, and
other coniferous trees have but a single vein, or two or three
parallel ones ; but in their case the veining could hardly be other
than parallel, since the leaves are so narrow that no veins of
any considerable length could exist except in a position length-
wise of the leaf.

of foxglove
After Planchon

1 See Kerner and Oliver, Natural History of Plants, Vol. I, pp. 623-637.
See also Appendix to this book.

SIMPLE AND COMPOUND LEAVES

Monocotyledonous plants seldom have leaves with notched or

cut margins, while dicotyledonous plants frequently have them.
A certain plan of venation is found

mainly in plants with a particular mode

of germination, of stem structure, and of

arrangement of floral parts, and this is

but one of the frequent cases in botany

in which the structures of plants are

correlated in a way which is not easy

to explain.

No one knows why plants with two

cotyledons should have netted-veined

leaves, but many such facts as this are

familiar to every botanist.

110. Simple and compound leaves.

The leaves so far studied are simple

leaves, that is, leaves of which the

blades are more or less entirely united
into one piece. But
while in the elm
the margin is cut in
only a little way, in
some maples it is

deeply cut in toward the bases of the veins.
In some leaves the gaps between the adja-
cent portions extend all the way down to
the petiole (in palmately veined leaves) or
to the midrib (in pinnately veined ones).

FIG. 96. Pinnately gucn divided leaves are shown in Figs. 95
divided leaf of -, n~
celandine and 96'

Iii still other leaves, known as compound

After Decaisne . ,

leaves, or branched leaves, the petiole, as
shown in Fig. 99 (palmately compound), or the midrib, as shown
in Fig. 97 (pinnately compound), bears -what look to be separate
leaves. These differ in their nature and mode of origin from the

FIG. 95. Palmately divided
leaf of buttercup

The blade of the leaf is dis-
continuous, consisting of
several portions. — After
Decaisne

LEAVES

portions of the blade of a divided leaf. One result of this differ-
ence appears in the fact that some time before the whole leaf is
ready to fall in autumn, the leaflets of a compound leaf are seen
to be jointed at their attachments. In Fig. 99 the horse-chestnut

FIG. 97. Pinnately com-
pound leaf of locust,
with spines for stipules

FIG. 98. Pinnately com-
pound leaf of pea

A tendril takes the place
of a terminal leaflet

leaf is shown at the time of falling, with some of the leaflets
already disjointed. .

That a compound leaf, in spite of the joints of the separate
leaflets, is really only one leaf is shown: (1) by the Absence of
buds in the axils of leaflets (see Fig. 97) ; (2) by the horizon-
tal arrangement of the blades of the leaflets, without any twist
in their individual leafstalks ; (3) by the fact that their arrange-
ment on the midrib does not follow any of the systems of leaf

COMPOUND LEAVES

arrangement on the stem (Sec. 111). If each leaflet of a com-
pound leaf should itself become compound, the result would be
to produce a twice compound leaf (Fig. 108).

FIG. 99. The fall of the horse-chestnut leaf

CHAPTER XI

LEAF ARRANGEMENT FOR EXPOSURE TO SUN AND AIR;
HELIOTROPIC MOVEMENTS OF LEAVES AND SHOOTS

111. Leaf arrangement.1 Leaves are quite generally arranged
so as to secure the best possible exposure to the sun and air.
This, in the vertical shoots of the elm, the oak (Fig. 100), the
apple, beech, and other alternate-leaved trees, is quite consistent
with their spiral arrangement. In horizontal twigs and branches

FIG. 100. Leaf arrangement
of the oak

FIG. 101. Leaf arrangement
of European beech

of the elm, the beech (Fig. 101), the chestnut, the linden, and
many other trees and shrubs, the desired effect is secured by the
arrangement of all the leaves in two flat rows, one on each side
of the twig. The rows are produced, as is easily seen on exam-
ining such a leafy twig, by a twisting about of the leafstalks.
The adjustment in many opposite-leaved trees and shrubs con-
sists in having each pair of leaves cover the spaces between
the pair below it, and sometimes in the lengthening of the lower

i See Kerner and Oliver, Natural History of Plants, Vol. I, pp. 390-424.

PLATE II. Leaves arranged for maximum illumination
After F. E. Clements

LEAF MOSAICS

fe/ ,

leafstalks so as to bring the blades of the lower leaves out-
side those of the upper leaves. Examination of Figs. 102 and
103 will make the
matter clear.

The student who
observes the leafage
of trees of different
kinds on the growing
tree itself may notice
how circumstances
modify the position
of the leaves. Maple
leaves, for example,
on the ends of the
branches are ar-
ranged much like
those of the horse-
chestnut, but they

are found to be arranged more nearly flatwise along the inner
portions of the branches, that is, the portions nearer the tree.

Figs. 104 and 105
^ show the remarkable

difference in arrange-
ment in different
^^B^te^v branches of the
Dcul/iu, and equally
i? -^ali interesting modifica-
tions may be found in
alternate-leaved trees,
such as the elm and
FIG. 103. Leaf arrangement of horse-chestnut the cherry.

FIG. 102. Leaf arrangement of horse-chestnut on
vertical shoots (top view)

After Kerner

on vertical shoots (side view)
After Kerner

112. Leaf mosaics.
Jn very many cases

the leaves at the end of a shoot are so arranged as to form a
rather symmetrical pattern, as in the horse-chestnut (Fig. 102).

LEAF ARRANGEMENT AND MOVEMENTS

When this is sufficiently regular, usually with the spaces between
the leaves a good deal smaller than the areas of the leaves them-
selves, it is called a leaf mosaic (Fig. 106). Many of the most
interesting leaf groups of this sort, as in the figure above men-
tioned, are found in the rosettes of the so-called root leaves of
plants. Good examples of these are the dandelion, chicory, fall
dandelion, thistle, hawkweed, Pyrola, and plantain. The leaves
of these plants are kept from shading each other, sometimes by

FIG. 104. Opposite leaves of Deutzia 1 (from the same shrub as Fig. 105)
as arranged on a horizontal branch

the narrowness of the leaves and sometimes by the lengthening
of the leafstalks of the lower ones.

113. Much-divided leaves. Not infrequently leaves are cut
into slender, fringe-like divisions, as in the carrot, tansy, south-
ernwood, wormwood, yarrow, dog fennel, cypress vine, and many
other common plants. This kind of leaf seems to be adapted to
offer considerable surface to the sun without cutting off too
much light from other leaves underneath. Such a leaf is in
much less danger of being torn by severe winds than are broader
ones with undivided margins. The same purposes are served by

1 Deutzia crenata.

DAILY MOVEMENTS OF LEAVES

compound leaves with very
many small leaflets, such as
those of the honey locust, the
mimosa, acacia (Fig. 108), and
other trees and shrubs of the
pea family.

114. Daily movements of
leaves. Many compound
leaves have the power of
changing the position of their
leaflets to accommodate them-
selves to varying conditions of
light and temperature. Some FIG. 105. Opposite leaves of Deutzia,
plants have the power of direct- as arra^ed on a vertical branch l
ing the leaves or leaflets edgewise towards the sun during the
hottest parts of the day, allowing them to extend their sur-
faces more nearly in a horizontal
direction during the cooler hours.

The so-called "sleep" of plants
has long been known, but this sub-
ject has been most carefully studied
rather recently. The wood sorrel,
or oxalis, the common bean, clovers,
and the locust tree are some of the
most familiar of the plants whose
leaves assume decidedly different
positions at night from those which
they occupy during the day. Some-
times the leaflets rise at night, and
in many instances they drop, as in the red clover (Fig. 107)
and the acacia (Fig. 108). One useful purpose, at any rate, that
is served by the nocturnal position of the leaf is protection
from frost. It has been proved experimentally that when

FIG. 106. Leaf mosaic of a
Campanula

After Kerner

1 It will be noticed that the exposure to sunlight is here not nearly as
favorable as in Fig. 104.

LEAF ARRANGEMENT AND MOVEMENTS

part of the leaves on a plant are prevented from assuming the
folded position, while others are allowed to do so, and the plant is

then exposed during a frosty
night, the folded ones may
escape, while the others are
killed. Since many plants in
tropical climates fold their
leaves at night, it is certain
that this movement has other

FIG. 107. A leaf of red clover purposes than protection

, leaf by day ; B, the same leaf at night frorn

probably

there is much yet to be learned about the meaning and impor-
tance of leaf movements.

115. Self-induced movements ; sensitive plants. Some leaves,
notably those of the so-called telegraph plant,1 have the power
of maintaining pretty rapid movements without external stimuli.
The small lateral
leaflets of this
plant, through a
cons iderable
range of temper-
atures above
72° F. (22°C.),in
light or darkness
alike, continue to
move first up,
then down, so
that their tips
make a complete FIG. 108. A leaf of acacia

circle ill from one A, as seen by day; B, the same leaf at night. — After
to three or more Darwin

minutes. The motion is jerky, like that of the second hand of
a watch, and gives one a vivid impression of the plant as a
living thing.

1 Desmodium gyrans.

MECHANISM FOR LEAF MOTIONS

A good many plants of the pea family have leaflets which are
sensitive to the touch. The best-known species is the common
sensitive plant of the florists,1 the leaflets of which close and
drop, like those of Fig. 108, and the leafstalks droop when the
plant is touched or jarred. Some of our common wild plants of
the same family 2 have leaves which promptly show irritability

when touched, and one species is
locally known as "shame vine,"
from this peculiarity.

116. Structure of the parts
which cause leaf motions. In a
great number of cases the daily
movements of leaves are produced
by special organs at the bases of
the leafstalks. These
cushion-like organs,
called pulvini (Fig.
JjL 109), are composed
!j mainly of paren-
| chymatous tissue,
\\ which contains much
water. It is impossi-
ble fully to explain in
simple language the way in which the cells of the pulvini act, but
in a general way it may be said that changes in the light to which
the plant is exposed cause rather prompt changes in the amount
of water in the cells in one portion or other of the pulvinus. If
the cells on one side are filled fuller of water than usual, that
side of the pulvinus will be expanded and make the leafstalk
bend toward the opposite side. The promptness of these move-
ments is no doubt in considerable measure due to the fact that
in the pulvini, as in many other parts of plants, the protoplasm
of adjacent cells is connected. Delicate threads of protoplasm
extend through the cell walls, making the whole tissue a living
1 Mimosa pudica. '2 Species of Cassia and Desmanthus.

FIG. 109. Compound leaf of bean with
pulvinus

The pulvinus shows as an enlargement in
the figure about three-eighths inch long,
at the base of the petiole. —After Sachs

100

LEAF ARRANGEMENT AND MOVEMENTS

web, so that any suitable stimulus or excitant which acts on
one part of the organ will soon affect the whole organ.

117. Vertically placed leaves. Many leaves, like those of
the olive (Fig. Ill), always keep their principal surfaces nearly

FIG. 110. Leaves standing nearly vertical in compass plant (Silphium
laciniatum)

A, view from east or west ; B, from north or south. — After Kerner

vertical. Thus they receive the morning and evening sun upon
their faces, and the noonday sun (which is so intense as to
injure them when received full on the surface) upon their edges.

HELIOTROPIC MOVEMENTS:" ./>, \ \

This adjustment is most perfect in the compass plant of the
prairies of the Mississippi basin. Its leaves stand nearly upright,
many with their edges just about north and south (Fig. 110), so
that the rays of the midsummer sun will, during every bright
day, strike the leaf surfaces nearly at right angles during a
considerable portion of the forenoon and afternoon, while at
midday only the edge of each leaf is exposed to the sun.

FIG. 111. Nearly vertical leaves of the olive

118. Helio tropic movements. The whole plant above ground
usually bends toward the quarter from which most light comes.

Any set of flowering plants growing close to a wall, or of
house plants in a window, generally offers many illustrations of
this principle. Movements caused by light are called heliotropic
movements (from two words meaning turning toward light).

119. Positive and negative heliotropic movements ; how
produced. Plants may bend either toward or away from the
strongest light. In the former case they are said to show posi-
tive Jieliotropism, in the latter negative heliotropism. In both
cases the movement is produced by unequal growth, brought
about by the stimulus of unequal lighting of different sides of
the stem. A plant if placed on a revolving table before a window
and slowly turned during the hours of daylight grows upright,
like a plant out of doors. This is because it is not left with a
one-sided illumination long enough to produce any bending.

•

CHAPTER XII
MINUTE STRUCTURE OF LEAVES; FUNCTIONS OF LEAVES*

120. Outline of leaf structure. Most foliage leaves of seed
plants contain a rather complicated system of nbro-vascular
bundles forming the veins (Sec. 105), which, taken together, con-
stitute a framework by which the leaf is supported and strength-
ened. Over and around these veins lies a mass of green pulpy
material, the spongy parenchyma. . The whole leaf is covered by
an epidermis. Frequently, especially in soft and rather thick
leaves, such as those of the garden live-forever, the epidermis
can be readily peeled off as a thin, transparent skin.

The epidermis and the spongy parenchyma decay far more
readily than the woody framework, and so skeleton leaves may
often be found on the ground in the spring, showing plainly the
arrangement of the veins of the leaf.

121. Details of a leaf section. .The relative positions and
the detailed structure of the parts mentioned in Sec. 120 are
best understood by reference to the magnified cross section of a
typical foliage leaf.

In the ordinary leaf (Fig. 112) a section shows at the upper
surface a layer of transparent cells of the epidermis e, ' Beneath
this lies a layer of elongated cells p, of a green color, standing
at right angles to the epidermis. These are called palisade cells,
from a fancied resemblance of their shape and relative position
to palisades. Under this layer the leaf interior is filled with an
irregularly grouped mass of green cells known as the spongy
parenchyma sp, throughout which occur numerous air spaces a,

* To THE INSTRUCTOR : As the present chapter takes up its topics in con-
siderable detail, it is suggested that it may be found expedient, if time is
limited, to omit Sees. 129, 130, 132, 134, 139 (table), 145-147.

102

DETAILS OF LEAF STRUCTURE

103

and in which is an occasional nbro-vascular bundle I. The
palisade layer or layers and the spongy parenchyma are together
known as mesophyll (meaning middle of leaf).

The lower surface of the leaf is covered by a layer of color-
less epidermal cells e', differing somewhat in size and shape from
those of the upper epidermis.

The lower epidermis is pierced by many openings or stomata s.
Each stoma opens into an air chamber. The upper epidermis

FIG. 112. Cross section of privet leaf

e, upper epidermis; p, palisade cells; sp, spongy parenchyma; a, air spaces;
6, fibre-vascular bundle; ef, lower epidermis; 5, stoma. Much magnified. —
Modified after Giesenhagen

of this leaf contains far less stomata than the lower one, and
this is true of most leaves, — often the upper surface contains
none.

122. Uses of the parts above mentioned. It will be most
convenient to discuss the uses of the parts of the leaf in detail
a little later, but it will make matters simpler to state at once
that the epidermis serves, as a mechanical protection to the parts
beneath and prevents excessive evaporation ; that the palisade
cells hold large quantities of the green coloring matter of the
leaf in a position where it can receive enough but not too much
sunlight ; and that the cells of the spongy parenchyma share the

104

STRUCTURE AND FUNCTIONS OF LEAVES

-work of the palisade cells, besides evaporating much water. The
stomata admit air to the interior of the leaf, where the air spaces
serve to store and to distribute it ; they allow oxygen and carbon
dioxide gas to escape ; and, above all, they regulate the evapora-
tion of water from the plant.

123. The epidermis. The cells of the epidermis are very gen-
erally rilled with water. Their form and the thickness and
material of their walls depend largely on the kind of soil and

FIG. 113. Surface view of the epidermis
of a buttercup leaf l

e, cells of epidermis ; n, nuclei of epidermal
cells ; #, guard cell of stoma ; s, stoma.
Much magnified. — After Giesenhagen

FIG. 114. Section through
stoma of a buttercup
leaf, at right angles to
epidermis

e, epidermal cells; g, guard
cell of stoma ; s, stoma ;
ch, air chamber. Much
magnified. — After Bonnier
and Sablon

climate to which the plant ^is adapted. In most herbs the epi-
dermal cells form only a single layer and are not greatly
thickened.

The stomata are not mere holes in the epidermis, but have a
somewhat complicated structure. Each stoma consists of two
kidney-shaped guard cells inclosing a slit-like opening into the
leaf (Fig. 113).

When the stoma is viewed in a section at right angles to the
surface of the leaf (Fig. 114) it appears as a narrow passage
communicating with an air chamber inside the epidermis.

The number of stomata in a square inch of leaf surface is
very great. An apple leaf contains about 24,000 and a black

1 Fig. 113 is from Ranunculus Ficaria ; Figs. 114-118 from E. acris.

CHLOROPLASTS AND CHLOROPHYLL

105

FIG. 115. Upper epidermis and palisade cells of
a buttercup leaf

A, section perpendicular to upper surface; B, exte-
rior view of upper surface with palisade cells seen
through epidermis ; e, epidermis ; p, palisade cells.
Much magnified. — After Bonnier and Sablon

walnut leaf about
300,000 per square
inch of the lower
epidermis.

124. The meso-
phyll ; chloroplasts ;
chlorophyll. The
mesophyll appears to
the naked eye of a
uniform green, but
under the microscope
its cells are seen to
contain many green
structures called chlorophyll bodies or chloroplasts (" chlorophyll"
meaning leaf green and " chloroplast " meaning molded out of
green material). The color of the leaf, as well as that of green
stems and other parts of the plant body, is due to these. A

chloroplast is usually, in seed plants and

in the higher spore plants, of an ellipsoidal

form or lens-shaped

a-nd somewhat

translucent. Its

color is due to a

green substance,

FIG 116. Passage of a soM)le in alcohol
fibro-vascular bundle

from stem to leaf of b^t not in water,
a buttercup (diagram- known as cllloro-

matic)

phyll.

FIG. 117. Diagram of

125. Woody tis- distribution of fibro-

vascular bundles in the

sue in leaves. The leafstalk of a buttercup
veins of leaves con-

s, stem; 10, woody part of
bundle ; b, sieve cells of
bundle. — After Bonnier
and Sablon

e, epidermis ; iv, woody
part of bundle; b, sieve
cells of bundle ; /, fibrous
layer on outer part of

of the stem of the plant. Indeed, these bundle. Magnified.—

,,,..,,„ ,. .,,

bundles m the lear are continuous with

sist of fibro-vascular bundles containing
wood fibers and vessels much like those

After Bonnier and Sa-

106

STRUCTURE AND FUNCTIONS OF LEAVES

those of the stem and consist merely of portions of the latter
which pass outward and upward from the stem into the leaf
under the name of leaf traces.

The manner in which fibre-vascular bundles pass from the
stem through the petiole into the leaf and are there distributed
can readily be gathered from an examination of Figs. 116—118.
Their wood cells and vessels serve to
carry water into the leaf, while their
sieve cells carry plant food from its
place of manufacture in the blade of
the leaf down into the stem.

FIG. 118. Part of the fibro-
vascular skeleton of a but-
tercup leaf

Much magnified. — After Bon-
nier and Sablon

FIG. 119. Termination
of a vein in a leaf

v, spirally thickened
cells of the vein; p,
parenchyma cells of
the spongy interior
of the leaf, with chlo-
rophyll bodies ; n, nu-
cleated cells. x about 345 diameters

126. Nutrition. The series of processes by which the plant
(1) takes up the raw materials to form its food, (2) unites these
into foods, and finally (3) constructs tissue from these foods, or
(4) stores them, constitutes nutrition.

A good deal of that portion of nutrition included under
(1) is carried on by the roots. But all kinds of nutritive work
are carried on in green leaves, and the portion numbered (2) is
a specialty of green plant cells, particularly of those in leaves.

PHOTOSYNTHESIS 107

127. The work of leaves. A leaf has four principal functions:

1. Photosynthesis. 3. Assimilation.

2. Respiration. 4. Transpiration.

128. Photosynthesis. All green leaves, when in healthy con-
dition, at suitable temperatures and with sufficient illumination
can produce carbohydrates (starch or sugars} from carbon dioxide
and water.

This process is of the greatest importance, since directly or
indirectly all plants and animals depend upon it for their food
supply. The manufacture by the plant of carbohydrates from
the raw materials is known as photosynthesis (from two words
meaning light and putting together). It is often called fixation
of carbon or assimilation of carbon. Photosynthesis is per-
formed by the chloroplasts, especially in the palisade cells, and
goes on imperfectly or not at all in plants or parts of plants,
as in certain parasites and other forms, in which no chlorophyll
exists (Chapters xxn, xxx).

129. Chemical formula for photosynthesis. The details of
the photosynthetic process are not wholly known, and it is not
at all likely that in starch-producing plants starch is the first
substance formed from carbon dioxide and water, but it is one
of the early products of the action of the chloroplasts and is
the easiest to detect by chemical tests applied to the leaf. In
some plants, as the onion, the products of photosynthesis are
all stored in the form of sugar.

If the chloroplast produced starch as the direct result of
combining carbon dioxide and water, the chemical equation for
the process would in its simplest form be J :

I Six molecules

Six molecules 1 f Five mole- "1 f One mole-

of carbon L + J cules of L = J cule of > -t- < f
dioxide J [^ water J [^ starch J [

6C02 + 5H20 - C6H1005 + 602

1 Really some multiple of C6Hi005 probably more nearly expresses the
composition of starch than the simple formula given. It is certain that the
photosynthetic process is much more complicated than a mere combination
of carbon dioxide with water to form either starch or sugar.

108 STRUCTURE AND FUNCTIONS OF LEAVES

If glucose (grape sugar) were the first product, the simplest
equation would be :

6 CO2 + 6 H2O = C6H12O6 (one molecule of glucose) + 6 O,,.1

It should be noticed that each of the processes above formu-
lated results in the disappearance of six molecules of carbon
dioxide and the production of six molecules of oxygen as a
waste product.

These facts, namely, that in the green parts of plants exposed
to sunshine carbon dioxide is consumed and oxygen liberated,
form the foundation of our knowledge of photosynthesis. The
first step in the study of the subject was taken by Joseph
Priestley in 1771, by his discovering that air in which candles
had been burned until they went out could be restored to
something like its original condition by leaving in it for some
time vigorous leafy sprigs of mint.

130. External conditions for photosynthesis. Photosynthesis
can only occur :

1. When the plant is supplied with air containing carbon dioxide.

2. When the temperature is neither too high nor too low.

3. When the illumination is sufficient.

Ordinary air contains about one twenty-fifth of one per cent
of its bulk of carbon dioxide. An increase of this amount up
to four per cent, or one hundred times the normal quantity, in-
creases photosynthesis, but a larger proportion usually at length
proves injurious to the health of the plant.

Some arctic and alpine plants can perform the work of mak-
ing carbohydrates at temperatures as low as the freezing point
of water, but plants of warmer climates require a higher tem-
perature. The rate of photosynthesis usually increases with
rise of temperature up to about 77° F. (25° C.), after which
it decreases.

Photosynthesis may go on very feebly, even in compara-
tive darkness, but the light of the interior of ordinary rooms is

1 See Peirce, Plant Physiology, pp. 58-66.

FORMATION AND ACTION OF CHLOROPHYLL 109

insufficient for the vigorous growth of most seed plants except-
ing those which, in a wild condition, flourish in the shade. The
rate of photosynthesis for most of the higher plants increases
with the illumination up to a light intensity equal to that of
full sunlight.

131. Conditions for formation of chlorophyll; its mode of
action. Chlorophyll is usually produced only in plants grown
in the light. Seedlings which have been sprouted in total dark-
ness almost always have a white or very pale yellow color, and
blanched celery affords a familiar example of the appearance of
leaves grown in comparative darkness. Microscopical examina-
tion of thoroughly blanched plants shows them to be destitute
of any decidedly green chloroplasts, and alcohol fails to extract
from them the green chlorophyll solution which is readily ob-
tained from ordinary leaves.

Iron must be present in the soil in order to enable the plant
to form chlorophyll, and plants developed in water cultures abso-
lutely free from iron remain yellow and grow feebly.

Chlorophyll appears to act by intercepting a considerable
portion of the light rays which strike the leaf, thus compelling
them to expend their energy on the chloroplasts and so to pro-
duce photosynthesis. If light traverses a substance with great
ease, as it does pure dry air, for example, comparatively little
effect is produced. On the other hand, when it strikes a sub-
stance which readily absorbs it, heating or chemical effects or
both are produced, as is evident when a rough sheet of iron, a
sensitized photographic dry plate, or blue-print paper is exposed
to sunlight. Chlorophyll cannot itself do the work of photo-
synthesis, but it causes the light rays to act on the chloro-
plasts so that their protoplasm carries on the manufacture of
carbohydrates from the raw materials.

132. Rate of starch making. The amount of starch manu-
factured daily by a given area of foliage must depend on the
kind of leaves, the temperature of the air, the intensity of the
sunlight, and some other conditions. Sunflower leaves and

110 STRUCTURE AND FUNCTIONS OF LEAVES

pumpkin or squash leaves produce starch at about the same rate.
In a summer day fifteen hours long they can make nearly three
quarters of an ounce for each square yard of leaf surface. A
full-grown squash leaf has an area of about one and one-eighth
square feet, and a plant may bear as many as a hundred of them.
The entire plant would then produce nearly nine and a half
ounces of starch per day.

Another way to emphasize the amount of work done by the
leaves is to consider how much air would be needed to supply
the carbon in a given weight of wood ; for all this carbon has
probably been derived from carbohydrates made in the leaves
(or other green parts) by photosynthesis. If the wood of a tree
after drying weighs 11,000 pounds and is half carbon, the latter
would weigh 5500 pounds. Taking the carbon dioxide contents
of the air at ^m, there would be more than 20,000,000 cubic
yards of air needed to furnish the carbon of such a tree.1

The enormous amounts of carbon dioxide annually removed
from the air by the growth of plants are continually being
replaced by the respiration of animals, the decay of animal and
vegetable material, and by the burning of fuel. From the burn-
ing of coal alone it is estimated that nearly 3,000,000 million
pounds of carbon dioxide are every year returned to the
atmosphere.

133. Respiration. Plants cannot carry on their life processes
without consuming oxygen and giving off carbon dioxide and
water. This oxygen consumption is the respiration of plants.
Like animals, plants are dependent on the union of oxygen with
oxidizable substances in their tissues for the energy with which
they do the work of assimilation, growth, and reproduction,—
in other words perform their life processes.

How oxygen can be made to combine with the carbon- and
hydrogen-containing compounds in the plant at moderate tem-
peratures is a problem which plant physiologists have not yet
fully solved ; but the union does constantly go on, and as a

1 Taken with slight alterations from Peirce, Plant Physiology, p. 44.

PLATE III. A cypress swamp, the trees draped with Spanish moss

(Tillandsia)
Modified, after H. J. Webber

RESPIRATION

111

result of the combination, water and carbon dioxide are con-
tinually excreted.

The amount of oxygen absorbed and of carbon dioxide given
off is, however, so trifling compared with the amount of each
gas passing in the opposite direction, while starch making is
going on in sunlight at temperatures most favorable for photo-
synthesis, that under such circumstances it is difficult to observe
the occurrence of respiration.

When the illumination is
very feeble, from -^ to ^
that of bright, diffuse day-
light, the manufacture of
carbon dioxide by respira-
tion and its consumption by
photosynthesis are equal.

At high temperatures,
such as 104° F. (40° C.),
respiration may produce car-
bon dioxide more rapidly
than photosynthesis can con-
sume it, even with brilliant
illumination.

In ordinary leafy plants
the leaves, through their
stomata, are the principal organs for absorption of air, but much
air passes into the plant through the lenticels of the bark.

In partly submerged aquatics especial provisions are found
for carrying the air absorbed by the leaves down to the sub-
merged parts. This is accomplished in pond lilies by ventilating
tubes which traverse the leafstalks lengthwise. In many cases
such channels run up and down the stem (Fig. 120). In the
American cypress (Taxodium) the "knees," which rise from the
roots, as shown in Plate III, are thought to be for use in respira-
tion, obtaining oxygen from the air and carrying it into the
roots beneath the water.

FIG. 120. Cross section of stem of mares-
tail (Hippuris), with air passages a

After Baillon

112 STRUCTURE AND FUNCTIONS OF LEAVES

134. Resting condition and diminished respiration. The

whole plant body or parts of it may pass into a resting condi-
tion, in which growth is suspended and few manifestations of life
are discernible. Familiar examples of this inactive condition are
leafless trees in winter, and rootstocks, tubers, and bulbs during
the winter of ordinary temperate climates or the rainless sum-
mer of southern California and the Mediterranean coast region.
Seeds and many kinds of resting spores afford extreme instances
of the possibility of a suspension of activity for years, followed
by prompt growth when suitable conditions are supplied. In
general, a moderately low temperature and dryness favor the
resting state. During the resting period respiration is greatly
diminished, so much so in the case of thoroughly dry seeds as
to be almost or quite imperceptible.

When resting protoplasm is placed in circumstances which
enable it to begin active respiration, growth and development
soon appear. Thus twigs of lilac or other shrubs will flower
after a time, when placed in water and brought into a warm
room in winter.

In many cases, as with most seeds, the period of repose is
essential- for growth. Potato tubers will not sprout as soon as
they are mature: some varieties need only two months and
others four or five months of rest.

135. Assimilation. By most American plant physiologists l
the word assimilation is used as a name for the series of changes
by which the plant transforms absorbed or manufactured food
into the materials of its own tissues.

The transformation of starch or sugar into substances, like
cellulose, which consist of the same elements (carbon, hydrogen,
and oxygen) differently combined, is a relatively simple matter ;
but the manufacture from carbohydrates of such very complex
nitrogenous substances as the proteids and living protoplasm
is a most complicated process, and imperfectly understood.

1 European botanists often include in the term assimilation both photo-
synthesis and the processes discussed in this section.

PHOTOSYNTHESIS AND RESPIRATION

113

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114 STRUCTURE AND FUNCTIONS OF LEAVES

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137. Metabolism. It is convenient to have a single word to
express all the chemical changes which are controlled by the
living protoplasts. Such a word is metabolism. It embraces all
the nutritive processes mentioned in Sec. 126, as well as respi-
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SUMMARY OF METABOLIC AND OTHER PROCESSES 115

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116 STRUCTURE AND FUNCTIONS OF LEAVES

the newly formed starch from the green cells of leaves for use
in other parts of the plant body. Much of this food (carried
about in the form of a solution of sugar) is used for building
material, as suggested in Sec. 136 ; but a good deal of it is often
transported to parenchyma cells of the stem and the roots,
where it is changed back into starch for storage. This change
is accomplished by small structures known as leucoplasts in the
cells. Each leucoplast may cause a deposit, upon some part of
its outer surface, of successive layers which finally develop into
a complete starch grain. How the leucoplast is able to bring
about the change from starch to sugar is unknown.

139. Transpiration. The process of giving off water in the
form of vapor from the stomata of plants is called transpiration.
It is not a mere drying up, such as occurs when a pile of sea-
weeds or a split stick of cord wood is exposed to dry air, but is
an important function of the leaves of most seed plants and of
the higher spore plants. In such forms as the cactuses (Fig. 50),
which are practically leafless, transpiration is performed by the
epidermis of the stem.

As already mentioned (Sec. 36), ordinary terrestrial seed plants
are, during the active periods of their lives, continually absorbing
water through the roots. This water brings with it dissolved
salts from the soil, many of which are used in the tissue-
forming work of the plant body. Some of the water, but only
an insignificant portion of the wliole amount, is needed for
photosynthesis, and a good deal of it is useful in carrying the
soluble plant foods, such as sugars, to the growing parts; but
there remains a large excess of water to be excreted, and this
duty is mainly performed by the mesophyll, and its amount is
regulated by the epidermis of the leaves. The air within the
intercellular spaces of the mesophyll is surrounded by thin-
walled cells filled with watery protoplasm, and it must there-
fore be nearly or quite saturated with moisture. When allowed
to escape from the leaf this air rapidly carries off quantities of
watery vapor.

USES OF THE EPIDERMIS

117

140. Uses of the epidermis.1 The epidermis, by its tough-
ness, tends to prevent mechanical injuries to the leaf. After
the change of the outer portions of its cell walls into a corky
substance it greatly diminishes evaporation from the general
surface. This process of becoming filled with cork material,
suberin (or a substance of similar properties known as cutin), is
essential to the safety of leaves or of young stems which have
to withstand heat and dryness. The corky or cutinized cell
wall is waterproof, while ordinary cellulose allows water to
soak through it with ease.
Merely examining sec-
tions of the various kinds
of epidermis will not give
nearly as good an idea of
their properties as can be
obtained by studying
during severe droughts
the behavior of plants
which have strongly cuti-
nized surfaces and of
those which have not, or
by exposing thin-leaved
plants and thick leath-
ery-leaved ones to a very dry atmosphere without watering.
Fig. 121, however, may convey some notion of the difference
between the two kinds of structure.

In A the shaded part is all cutinized; it consists of the
thick cuticle proper and, beneath this, cutinized layers of cell
wall, under which is a heavy layer of cellulose. In B the cuticle
is thin, and the outer portion of the cell walls consists wholly
of cellulose.

In most cases, as in the india-rubber tree, the external
epidermal cells (and often two or three layers of cells beneath
these) are filled with water, and thus serve as reservoirs from
1 See Kerner and Oliver, Natural History of Plants, Vol. I, pp. 273-362.

FIG. 121. Unequal development of cuticle
by epidermis cells

A, epidermis of butcher's broom (Ruscus) ; J5,
epidermis of sunflower ; c, cuticle ; e, epider-
mis cells. — After Frank and Tschirch

118 STRUCTURE AND FUNCTIONS OF LEAVES

which the outer parts of the leaf and the stem are at times
supplied.

In many cases, noticeably in the cabbage, the epidermis is
covered with a waxy coating, which doubtless increases the
power of the leaf to retain needed moisture, and which certainly
prevents rain or dew from covering the leaf surfaces, especially
the lower surfaces, so as to hinder the operation of the stornata.
Many common plants, like the meadow rue and the nasturtium,
possess this power to shed water to such a degree that the under
surface of the leaf is hardly wet at all when immersed in water.
The air bubbles on such leaves give them a silvery appearance
when held under water.

141. Operation of the stomata. The stomata serve to admit
air to the interior of the leaf and to allow moisture in the form
of vapor to pass out of it. They do this not in a passive way,
as so many mere holes in the epidermis might, but to a con-
siderable extent they regulate the rapidity of transpiration, open-
ing more widely in damp weather and in sunlight, and closing
in very dry weather. The opening is caused by each of the
guard cells bending into a more kidney-like form than usual,
and the closing by a diminution of twrgor and straightening out
of the guard cells.

The details of the mechanical explanation of stoma move-
ments are complicated, and it is difficult fully to account for
their irritability in response to light, heat, and moisture stimuli,
and to variations in the amount of salts in the water absorbed
by the roots.

142. Location of the stomata. The under side of the leaf,
free from palisade cells, abounding in intercellular spaces, and
pretty well protected from becoming covered with rain or dew,
is especially adapted for the working of the stomata, and accord-
ingly we usually find them in much greater numbers on the
lower surface. On the other hand, stomata occur only on
the upper surface of the leaves of pond lilies, which lie flat
on the water. In those leaves which stand with their edges

HAIRS ON LEAVES 119

nearly vertical, the stomata are distributed somewhat equally
on both surfaces. Stomata occur in the epidermis of young
stems, being replaced later by the lenticels.

The health of the plant depends largely on the proper work-
ing condition of the stomata, and one reason why plants in cities
often fail to thrive is that the stomata become choked with dust
and soot. If the stomata were to become filled with water, their
activity would cease until they were freed from it ; hence many
plants have their leaves, especially the under surface, protected
by a coating of wax which sheds water.

143. Hairs on leaves. Many kinds of leaves are more or less
hairy or downy, as those of the mullein, the " mullein pink,"
many cinquefoils, and other common plants. In some instances
this hairiness may be a protection against snails or other small
leaf-eating animals, but in other cases it seems to be pretty clear
that the woolliness (so often confined to the under surface) is
to lessen the loss of water through the stomata. The Labrador
tea is an excellent example of a plant with a densely woolly
coating on the lower surface of the leaf. The leaves, too, are
partly rolled up like those of the crowberry (Fig. 361), but less
completely, with the upper surface outward, so as to give the
lower surface a sort of deeply grooved form, and on the lower
surface all of the stomata are placed. This plant, like some
others with the same characteristics, ranges far north into regions
where the temperature, even during summer, often falls so low
that absorption of water by the roots ceases, since it has been
shown that this nearly stops a little above the freezing point of
water. Exposed to cold, dry winds, the plant would then often
be killed by complete drying if it were not for the protection
afforded by the woolly, channeled under surfaces of the leaves.

144. Total amount of transpiration. In order to prevent
wilting, the rise of sap during the life of the leaf must have
kept pace with the evaporation from its surface. The total
amount of water that travels through the roots, stems, and
leaves of most seed plants during their lifetime is large, relative

120 STRUCTURE AND FUNCTIONS OF LEAVES

to the weight of the plant itself. During 173 days of growth a
corn plant has been found to give off nearly 3 1 pounds of water.
During 140 days of growth a sunflower plant gave off about
145 pounds. A grass plant has been found to give off its own
weight of water every twenty-four hours in hot, dry summer
weather. This would make about 6^ tons per acre every twenty-
four hours for an ordinary grass field, or rather over 2200 pounds
of water from a field 50 X 150 feet, — that is, from a tract not
larger than a good-sized city lot. Calculations based on obser-
vations made by the Austrian forest experiment stations showed
that a birch tree with 200,000 leaves, standing in open ground,
transpired on hot summer days from 700 to 900 pounds, while
at other times the amount of transpiration was probably not
more than 18 to 20 pounds.1

145. Accumulation of mineral matter in the leaf. Just as a
deposit of salt is found in the bottom of a seaside pool of salt
water which has been dried up by the sun, so old leaves are
found to be loaded with mineral matter left behind as the sap
drawn up from the roots is evaporated through the stomata.
A bonfire of leaves makes a surprisingly large heap of ashes. An
abundant constituent of the ashes of burnt leaves is silica, a
substance chemically the same as sand. This the plant is forced
to absorb along with the potash, compounds of phosphorus, and
other useful substances contained in the soil water ; but since
the silica is of hardly any value to most plants, it often accumu-
lates in the leaf as so much refuse. Lime is much more useful
to the plant than silica, but a far larger quantity of it is absorbed
than is needed ; hence it, too, accumulates in the leaf.

146. The fall of the leaf. In the tropics trees retain most of
their leaves the year round ; a leaf occasionally falls, but no con-
siderable portion of them drops at any one season.2 The same

1 See B. E. Fernow's discussion in Report of Division of Forestry of United
States Department of Agriculture, 1880 ; also the article, " Water as a Factor
in the Growth of Plants," by B. T. Galloway and Albert F. Woods, Year-
Book of United States Department of Agriculture, 1894.

2 Except where there is a severe dry season.

THE FALL OF THE LEAF 121

statement holds true in regard to our cone-bearing evergreen
trees, such as pines, spruces, and the like. But the impossibility
of absorbing soil water when the ground is at or near the freez-
ing temperature would cause the death, by drying up, of trees
with broad leaf surfaces in a northern winter. And in countries
where there is much snowfall, most broad-leafed trees could
only escape injury to their branches from overloading with snow,
by encountering winter storms in as close-reefed a condition as
possible. For such reasons our common shrubs and forest trees
(except the cone-bearing, narrow-leafed ones already mentioned)
are mostly deciduous, — that is, they shed their leaves at the
approach of winter. There are, however, in the eastern United
States a few species of broad-leafed evergreen trees and large
shrubs, such as the live oak, some Rhododendrons, the mountain
laurel (Kalmia), and the hollies. Along the Pacific coast there
are many more forms, including five fairly common species of
evergreen oaks, the beautiful Arbutus, and the manzanitas
(Arctostaphylos).

Looking somewhat closely into the matter of deciduousness
of the trees and shrubs of temperate climates (not including the
coniferous species), one finds that they may be classed as follows :

I. Leaves simultaneously deciduous . .\*> winter deciduous

j B, summer deciduous

C, leaves some of them
lasting two years or

II. Leaves not simultaneously deciduous

(evergreen)

D, leaves lasting more
than one year but less
than two

The only one of the four subdivisions which shows fairly con-
stant leafage at all seasons is the one designated as C. Leaves
of the subdivision D often fall when about fifteen months old,
so that the tree is unusually leafy during the three months
when the new leaves are developing to their full size, but before
the old ones begin to fall. It is a noteworthy fact that in many
species of broad-leafed evergreens, for example the ilex oak, the

122 STRUCTURE AND FUNCTIONS OF LEAVES

oleander, and Smilax aspera, the leaves do not attain their maxi-
mum power of transpiration as soon as they are fully grown.
Such a leaf transpires more when fifteen months old than when
three months old.

The fall of the leaf is preceded by important changes in the
contents of its cells. Much of the sugary and protoplasmic con-
tents of the leaf disappears before it falls. These valuable mate-
rials have been absorbed by the branches and roots, to be used
again the following spring.

The separation of the leaf from the twig is accomplished by
the formation of a layer of cork cells across the base of the
petiole in such a way that the latter finally breaks off across
the surface of the layer. A waterproof scar is thus already
formed before the removal of the leaf, and there is no waste of
sap dripping from the wound where the leafstalk has been
removed, and no chance for fungi to attack the bark or wood
and cause it to decay. In compound leaves each leaflet may
become separated from the petiole, as is notably the case with
the horse-chestnut leaf (Fig. 99). In woody monocotyledons,
such as palms, the leafstalks do not commonly break squarely
off at the base, but wither and leave projecting stumps on
the stem.

The brilliant coloration, yellow, scarlet, deep red, and purple,
of autumn leaves is popularly but wrongly supposed to be due
to the action of frost. "It depends merely on the changes in the
chlorophyll grains and the liquid cell contents that accompany
the withdrawal of the proteid material from the tissues of the
leaf. The chlorophyll turns into a yellow, insoluble substance
after the valuable materials which accompany it have been
taken away, and the cell sap at the same time may turn red.
Frost perhaps hastens the break-up of the chlorophyll, but indi-
vidual trees often show bright colors long before the first frost,
and in very warm autumns most of the changes in the foliage
may come about before there has been any frost.

CHAPTEE XIII
THE FLOWER OF THE HIGHER SEED PLANTS

147. Organs of the flower. The parts found in a complete
flower of the higher seed plants (angiosperms) are sepals, petals,
stamens, and pistils (Fig. 122). The sepals, taken together, con-
stitute the calyx ; the petals, taken together, constitute the

FIG. 122. Face view and dissection of an angiospermous flower
r, receptacle; .96, sepal; pe, petal; st, stamen; pi, pistil; o, ovule

corolla. The calyx and corolla are collectively known as the
floral envelopes, or perianth.

Many angiospermous flowers consist of five circles, or whorls,
two of which belong to the perianth, two to the stamens, and one
to the pistils. The parts of each circle alternate in position with
those of the preceding or following one, and all the members of
each circle are alike (Fig. 122).

148. Suppression and multiplication of circle. Any circle, or
part of a circle, may be suppressed. If one set of parts of the
perianth is lacking it is assumed to be the corolla (Fig. 123).1

1 For other instances of suppression of various sets, see Bergen, Flora
of the Northern States (Figs. 3, 8, 9, 11, 16).

123

124

FLOWER OF THE HIGHER SEED PLANTS

A whorl of stamens is frequently suppressed, so that only one
circle is present in the flower (Fig. 128).

Multiplication of whorls is particularly frequent among the
stamens, but other whorls may also show it (see Figs. 149, 150).

FIG. 123. Flower of
(European) wild
ginger,with calyx
but no petals

After Wossidlo

A B

FIG. 124. Flowers of willow

A, staminate flower; B, pistil-
late flower. Magnified. — After
Decaisiie

149. Unisexual flowers. The stamens and pistils may be
produced in separate flowers, which are unisexual (often called
imperfect) flowers. In the very simple unisexual flowers of the
willow (Fig. 124) each flower of the catkin (Fig. 143) consists
merely of a pistil or a group of (usually two) stamens springing
from the axil of a small bract.

Staminate and pistillate flowers may be
borne on different plants, as they are in
the willow, or they may be borne on the
same plant, as in the hickory and the hazel
among trees, or in the castor-oil plant,
Indian corn, and the begonias. When stam-
inate and pistillate flowers are borne on
separate plants, such a plant is said to be
dioecious, that is of two households ; when
both kinds of flower appear on the same
individual, the plant is said to be moncecious, that is of one
household.

FIG. 125. Bilaterally
symmetrical flower
of pansy

SYMMETRY OF THE FLOWER

125

150. Symmetry of the flower. Most angiosperms have sym-
metrical flowers. The simplest are those whose parts are ar-
ranged as iii Figs. 122, 128, and 149, having radial symmetry.1

FIG. 126. Bilaterally symmetrical flower of sweet pea
A, side view; B, front view, dissected; s, standard; w, w, wings; k, keel

A higher type of flower is that which shows bilateral symmetry?
as in Figs. 125 and 126.

If the drawing of such a flower were folded along the axis of
symmetry, one half of the drawing would cover and correspond
with the other half. Some flowers are wholly
irregular, showing no sort of symmetry.

151. The receptacle. The parts of the flower
are borne on a variously formed expansion
of the flower stalk known as the recepta-
cle. Usually, as in Fig. 122, this is only a
slight enlargement of the flower stalk, but
in the rose (Fig. 127), the pond lily (Fig. 137),
the magnolia, the Calycanthus, and a good
many other familiar flowers it is large and
conspicuous.

FIG. 127. A rose

Longitudinal section
After Decaisne

1 Such flowers are also called actinomorphic, meaning ray-formed.

2 These are called zygomorphic flowers (from Greek words signifying yoke
and/orm). In many floras these are described, as irregular flowers,

126 FLOWER OF THE HIGHER SEED PLANTS

152. The perianth. In dicotyledonous plants the sepals, or
divisions of the calyx, are commonly green and somewhat leaf-
like. The petals in showy flowers are of many colors, ranging
all the way from violet to red. Either whorl of the perianth
may be found to have assumed some very peculiar form, to
carry out the purpose of the flower, as is briefly explained in
Chapter xxxn.

Among the lower families of angiosperms the parts of the
perianth are frequently all distinct, as shown in Figs. 122 and

134. Among the higher families
the members of the perianth are

9 '» "— -^ \s£~_ / / ,p often borne upon a tubular or cup-

like outgrowth from the recep-
tacle (Fig. 136, B), so that the
sepals or petals, or both, appear
to have grown together more or
less completely.1

When the calyx or the corolla

is borne upon a tubular, bowl-
FIG. 128. Flower of Hydrophyllum 11,1

shaped, or other extension of the

cal, lobe of calyx ; cor, lobe of co-

roiia; st, stamens; p, pistil, receptacle, there are often divi-
Modified.- After Lindley sionSj teeth, or lobes at the rim

of the tube (Figs. 128, 144, Appendix) showing how many
sepals or petals the flower possesses. Special names are given
to a large number of forms of the sympetalous corolla, and
these are of much use in accurate descriptions of seed plants.
A few of these are illustrated in Chapter xxxn and in the
Appendix.

1 When the parts of the perianth are distinct the calyx is said to be chori-
sepalous and the corolla choripetalous ; other terms are polysepalous and
polypetalous. When the receptacle forms a cup-like or tubular outgrowth
so that the teeth or lobes of this alone are sepals or petals, the flower is said
to be synsepalous or sympetalous ; other terms are gamosepalous orgamopeta-
lous. Choris means apart, poly means many, syn means together, gamos
means marriage. Botanists have until recently used such expressions as
"sepals united into a tube," etc., but these are incorrect.

FORMS AND UNION OF STAMENS

127

FIG. 129. Parts of
a stamen

153. Form of the stamen ; union of stamens. Stamens are
of many specialized forms, to adapt them to their functions in
flowers of various shapes, but many are of the shape shown in
Fig. 129. Such a stamen consists of an ex-
panded part, the anther, borne on a stalk
called i\\Q filament. Anthers are often nearly
or quite sessile (seated, i.e. destitute of fila-
ments). Inside the anther is the powdery or
pasty substance called pollen (Fig. 153).

Stamens may be wholly unconnected with
each other, or distinct, as shown in Figs. 122,
124, and 128, or they may be really or ap-
parently more or less united to each other.

In Fig. 130 the stamens

have arisen separately,

but finally become joined

together by their anthers

(as is always the case in

the family Composite).

In other cases the stamens appear united

when they are not really so, because they

are borne on a ring or tube of tissue, as

already explained in connection with the

perianth (Sec. 152).

Without regard to whether the union is

real or apparent, stamens which occur in a
FIG. 130. Stamens of . -, ,,•, £1 .....

a thistle, with an- smgle 8rouP (the filaments appearing joined)
there united into a are said to be monadelphous (Fig. 131), in
rinS two groups, diadelphous (Fig. 132), in many

a, united anthers; /, groups, polyadelphous (the terms meaning

filaments, bearded * , *

on the sides. — After one brotherhood, two brotherhoods, many

Baiiion brotherhoods).

154. The carpel. The simplest form of the organ which bears
the structures called ovules, that are to mature into seeds, is
known as the carpel.

ive ; /, filament. —
After Strasburger

128

FLOWER OF THE HIGHER SEED PLANTS

In the lowest of the two great groups of seed plants, the
gymnosperms (meaning naked seeds), to which the pines, spruces,
cedars, and the like belong, the ovules are
borne exposed on the surface of the carpels,
which usually have the form of scales. But
in the higher group of seed plants, the angio-
sperms (meaning seeds in a vessel), the car-
pels constitute a part of cases or chambers
in which the ovules are formed and which
are generally quite closed.

FIG. 131. Monadel- 155- The Pistil- The term pistil (Latin
phous stamens of for pestle) is applied to the closed structure
mallow which contains the

ovules and is formed by the carpels of the
angiosperms. This is a more general term
than carpel, for it applies to organs com-
posed of one or of several FlG- 132' Diadelphous
. . stamens of sweet pea

carpels. If a pistil is of one

carpel it is said to be simple, if of two or more car-
pels it is compound.

The pistil often consists of an enlarged, hol-
low portion containing ovules and known as the
ovary,1 a stalk-like style, and a knob or ridged
expansion called the stigma (Fig. 133). Not infre-
quently the style is wanting and the stigma is
FIG 133 Parts sess^e (seated) on the ovary.

of the pistil A flower may contain several or many carpels

ow, ovary; *ty, in the form of simple pistils separate from one

style; stig, another, as in the stonecrop and the buttercup

(Figs. 134, 161). When several carpels form a

compound pistil the manner and extent of the union is various.

1 The term ovary is an unfortunate one, since it would seem to mean the
organ which bears eggs. Those who wish to avoid the use of the term may
substitute the word ovulary, proposed by Professor Charles R. Barnes, or
may simply say ovule case.

stig—GS

sty----

SIMPLE AND COMPOUND PISTILS

129

The union generally forms the ovary, although this is sometimes
developed in large part as a cup-like or tubular growth from the
stem under the carpels. Sometimes the union is so complete

FIG. 134. Flower of stonecrop
A, entire flower; Z>, vertical section. — After Decaisne

that the compound pistil has only one style and one stigma ; but
frequently the styles remain separate, or the styles may be
united and the stigmas separate, or at least lobed so as to show
of how many carpels the pistil is made up (Figs. 123, 124).
Even when there is no external sign to indicate the' compound
nature of the pistil, it can usually be recognized from a study of
a cross section of the ovary.

156. Locules of the ovary; placentas. Compound ovaries
very commonly consist of a number of separate chambers known
as locules.1 Fig. 135, B, shows
a three-loculed ovary seen in
cross section. The ovules are
not borne indiscriminately by
any part of the lining of the
ovary. In one-loculed pistils
they frequently grow in a line
running along one side of the
ovary, as in the pea pod (Fig.
343). The ovule-bearing line is

FIG. 135. Principal types of placenta

A, parietal placenta ; B, central placenta ;
C, free central placenta ; A and B, trans-
verse sections; C, longitudinal section.
— After Strasburger

called a placenta ; in compound pistils there are commonly as
many placentas as there are separate carpels joined to make the

Often (less correctly) called cells.

130

FLOWER OF THE HIGHER SEED PLANTS

pistil. Placentas on the wall of the ovary, like those in Fig. 13 5,
A, are called parietal placentas ; those which occur as at B

are said to be axial; and those
which, like the form repre-
sented in C, consist of a col-
umn rising from the bottom of
the ovary are called free cen-
tral placentas.
157. Superior, half -inferior,

anfl

FIG. 136. Insertion of the floral organs

A, hypogynous, all the other parts on the as in the diagrammatic flower

receptacle, beneath the pistil ; P>, perig- . &

ynous, petals and stamens apparently of Fig. 122, the receptacle IS

growing out of the calyx, around the rounded or club-shaped, and
pistil ; C, cpigynous, all the other parts

appearing to grow out of the pistil.— the floral organs arise from it

After Strasburger

in successive gets>

flower

is said to be liypogynous (from two Greek words here applied
to mean under the pistil), and the ovaries are said to be
superior (Fig. 136, A).

When the receptacle is concave, or when it grows up about
the pistil, carrying the other floral parts with it, so that
the pistil is inserted on the same
level with the stamens or lower,
but not at all united with the re-
ceptacle, the flower is said to be
perigynous (meaning around the
pistil) and the ovary is half infe-
rior (Fig. 136, B).

When the ovary is united with

the receptacle the flower is said

v . .

to be epigynous (meaning upon

FIG. 137. White water lily

,i .,.-,. ,, . „ The inner petals and the stamens

the pistil), Or the OVary IS infe- growing from the ovary. - After
rior (Fig. 136, C}. Decaisne

158. Floral diagrams. Sections (real or imaginary) through
the flower lengthwise, like those of Fig. 136, help greatly in
giving an accurate idea of the relative position of the floral

FLORAL DIAGRAMS

131

organs. Equally important in this way are cross sections,
which may be recorded in diagrams like those of Fig. 138.1
In constructing such diagrams it will often be necessary to
suppose some of the parts of the flower to be raised or lowered
from their true position, so as to bring them into such rela-

tions that all could be cut by a single
section. This would, for instance, be
necessary in making a diagram for the

A B C

FIG. 138. Diagram of cross sections of flowers

A, columbine; B, heath family; C, Iris family. In each diagram the dot along-
side the main portion indicates a cross section of the stem of the plant. In B
every other stamen is more lightly shaded, because some plants of the heath
family have five and some ten stamens. — After Sachs

cross section of the flower of the white water lily, of which a
partial view of one side is shown in Fig. 137.

It is found convenient, in diagrams of cross sections, to dis-
tinguish the sepals from the petals by representing the former
with midribs. The diagrammatic symbol for a stamen stands
for a cross section of the anther, and that for the pistil is a
section of the ovary. If any part is lacking in the flower (as
in the case of flowers which have some antherless filaments),
the missing or abortive organ may be indicated by a dot. In
the diagram of the Iris family (Fig. 138, C) the three dots
inside the flower indicate the position of a second circle of
stamens, found in most flowers of monocotyledons but not in
this family.

1 For floral diagrams see Le Maout and Decaisne, Traite General de
Botanique, or Eichler, Bluthendiagramme.

CHAPTER XIV

INFLORESCENCE

159. Definition of inflorescence and flower cluster. The man-
ner in which flowers are arranged on the stem is known as
inflorescence.1 Not infrequently the flowering shoot bears only
a single flower, but very generally among seed plants these
shoots are grouped into definite systems, which are called
flower clusters.

160. Advantage of grouping flowers. Flowers when clus-
tered, as in Figs. 140—143, on special nearly leafless shoots are
much more conspicuous than they would be if scattered along

ordinary leafy branches and partly hidden
by the leaves. This is a decided advantage
in securing many visits from insects which
carry pollen from plant to plant (Chapter
xxxn) and leads to a more abundant pro-
duction of seed.

161. Regular positions for flower buds.
Flower buds, like leaf buds, occur regularly
either in the axils of leaves or at the end
of the stem or branch, and are therefore
either axillary or terminal (Sec. 168).

162. Axillary and solitary flowers ; inde-
terminate inflorescence. The simplest pos-
sible arrangement for flowers which arise
from the axils of leaves is to have a single

flower spring from each leaf axil. Fig. 139 shows how this
plan appears in a plant with opposite leaves. As long as the

FIG. 139. Axillary
and solitary flowers
of pimpernel

1 Sometimes (but
flower cluster.

correctly) the word inflorescence is used to mean
132

THE RACEME AXD RELATED FORMS

133

FIG. 140. Raceme of
common red currant

p, peduncle ; p', pedicel

br, bract

stem continues to grow the production of new leaves may be
followed by that of new flowers. Since there is no definite
limit to the number of flowers which may appear in this way,
the mode of flowering just de-
scribed (with many others of
the same general character) is
known as indeterminate inflo-
rescence.

163. The raceme and re-
lated forms. If the leaves
along the stem were to become

very much dwarfed and the flowers brought closer together,
as they frequently are, a kind of flower cluster like that of
the currant (Fig. 140) or the lily of the valley would result.
Such an inflorescence is called a raceme ; the main flower
stalk is known as the peduncle ; the little individual flower

stalks are pedicels, and the small,
more or less scale-like leaves of
the peduncle are bracts.

Frequently the lower pedicels
of a cluster on the general plan of
the raceme are longer than the
upper ones and make a somewhat
flat-topped cluster, like that of the
hawthorn, the elder, the sheep lau-
rel, or the trumpet creeper. This
is called a corymb.

In many cases, for example the
parsnip, the sweet cicely, the gin-
seng, and the cherry, a group of pedicels of nearly equal
length spring from about the same point. This produces a
flower cluster called the umbel (Fig. 141).

164. Sessile flowers and flower clusters. Often the pedicels
are wanting, or the flowers are sessile, and then a modification
of the raceme is produced which is called a spike, like that of

FIG. 141. Simple umbel
of cherry

134

INFLORESCENCE

the plantain (Fig. 142). The willow, alder, birch,
poplar, and many other common trees bear a short,
flexible, rather scaly spike (Fig. 143), which is called
a catkin.

The axis of the inflorescence is often so much short-
ened as to bring the flowers into a somewhat globular
mass. This is called a head (Fig. 142). Around the
base of the head usually occurs
a circle of bracts known as the
involucre. The same name is
given to a set of bracts which
often surround the bases of the
pedicels in an umbel.

165. The composite head. The
plants of one large group — of
which the dandelion, the daisy,
the thistle, and the sunflower
are well-known members — bear
their flowers in close involucrate heads on a common recep-
tacle. The whole cluster looks so much like a single flower
that it is usually taken for
one by non-botanical people.
In many of the largest and
most showy heads, like that
of the sunflower and the
daisy, there are two kinds
of flowers, — the ray flowers,
around the margin, and the
tubular disk flowers of the in-
terior of the head (Fig. 144).

FIG. 142. Spike of plantain and
he\id of red clover

FIG. 143. Catkins of willow
A, staminate flowers ; B, pistillate flowers

The early botanists supposed

the whole flower cluster to

be a single compound flower.

This belief gave rise to the name of one family of plants,

Compositce, — that is, plants with compound flowers. In such

THE COMPOSITE HEAD

135

FIG. 144. Head of yarrow

A, top view (magnified); B, lengthwise section (magnified); re, receptacle; i,
involucre ; r, ray flowers ; d, disk flowers ; c, corolla ; s, stigma ; ch, chaff, or
bracts of receptacle

FIG. 145
Panicle of oat

FIG. 146. Compound umbel
of carrot

heads as those of the tansy, the thistle, the cudweed, and the
everlasting, there are no ray flowers, and in others, like those of
the dandelion and the chicory, all the flowers are ray flowers.

136

INFLORESCENCE

166. Compound flower clusters. If the pedicels of a raceme
branch, they may produce a compound raceme, or panicle, like
that of the oat (Fig. 145).1 Other forms of compound racemes
have received other names.

An umbel may become compound by the branching of its
flower stalks (Fig. 146), each of which then bears a little umbel,
called an iimbellet.

167. Inflorescence diagrams. The plan of inflorescence may
readily be indicated by diagrams like those of Fig. 147.

p
P
f

A BCD

FIG. 147. Diagrams of inflorescence
A, panicle; B, raceme; C, spike; D, head; E, umbel

168. Terminal flowers ; determinate inflorescence. The ter-
minal bud of a stem may be a flower bud. In this case the
direct growth of the stem is stopped or determined by the
appearance of the flower ; hence such plants are said to have a
determinate inflorescence. The simplest possible case of this
kind is that in which the stem bears but one flower at its
summit.

169. The cyme. Very often flowers appear from lateral
(axillary) buds, below the terminal flower, and thus give rise to
a flower cluster called a cyme. This may have only three flowers,
and in that case would look very much like a three-flowered

1 Panicles may also be formed by compound cymes (see Sec. 169).

THE CYME 137

umbel. But in the indeterminate inflorescence, such as the
raceme, corymb, and umbel, the order of flowering is from below
upward, or from the outside of the cluster inward, because the
lowest or the outermost flowers are the oldest, while in deter-
minate forms of inflorescence the central flower is the oldest,

FIG. 148. Compound cyme of mouse-ear chickweed
t, the terminal (oldest) flower

and therefore the order of blossoming is from the center out-
wards. Cymes are very commonly compound, like those of
the elder and of many plants of the pink family, such as the
sweet william and the mouse-ear chickweed (Fig. 148). They
may also, as already mentioned, be panicled, thus making a
cluster much like Fig. 147, A.

CHAPTER XV

ORIGIN AND STRUCTURE OF FLORAL ORGANS ; POLLINATION
AND FERTILIZATION

170. The flower a shortened and greatly modified branch.

In Chapter IX the leaf bud was explained as being an unde-
veloped branch, which in its growth would develop into a real
branch (or a prolongation of the main stem). Now, since flower
buds appear regularly either in the axils of leaves or as terminal

FIG. 149. Transition from bracts to sepals in a cactus flower

buds, there is reason to regard them as of a nature similar to
leaf buds. This would imply that the receptacle corresponds to
the axis of the buds shown in Fig. 85, and that at least some
of the parts of the flower- correspond to leaves. There is plenty
of evidence that this is really true. Sepals frequently look very
much like leaves, and in many cactuses the bracts about the
flower are so sepal-like that it is impossible to tell where the
bracts end and the sepals begin (Fig. 149). The same thing is
true of sepals and petals in such flowers as the white water lily.
In this flower there is also a remarkable series, ranging all the

138

DEVELOPMENT OF THE ANTHER 139

way from petals tipped with a bit of anther, through stamens
with a broad petal-like filament, to regular stamens, as is shown
in Fig. 150, A, B, C, D. The same thing is shown in many double
roses. In completely double flowers the stamens and pistils are
transformed into petals by cultivation. In the flowers of the
cultivated double cherry the pistils occasionally take the form
of small leaves, and some roses turn wholly into green leaves.

Summing up, then, we know that flowers are altered and
shortened branches, (1) because flower buds have, as regards

BCD

FIG. 150. Transitions from petals to stamens in white water lily
A, B, C, D, various steps between petal and stamen. — After Brown

position, the same kind of origin as leaf buds ; (2) because all
the intermediate steps are found between bracts on the one
hand and petals on the other.

171. Development of the anther. If the development of an
anther is followed throughout, it will be found at an early stage
to contain, usually, four regions, where rapid cell division is
going on, which become organized into pollen sacs. These cavi-
ties (Fig. 151) are filled with pollen grains and finally merge into
two pollen chambers which, in the commonest type of anther,
split open lengthwise to allow the escape of the pollen.

172. Relation of stamens and carpels to structures in the
lower plants. The exact significance of the stamens and car-
pels as organs of the plant body set apart for the purpose of

140 STRUCTURE OF FLORAL ORGANS; FERTILIZATION

FIG. 151. Cross section of anther
of mint

reproduction can only be understood by means of a study of
certain forms in the fern group, or pteridophytes ; for these

structures had their origin in
connection with the develop-
ment, from simpler conditions
among the fern group, of the
habit of producing seeds. The
subject is treated at some
length in Part II, Chapters
xxvi to xxx inclusive.
173. The anther and its

s, pollen sacs, with grains of pollen; contents. Some of the shapes
d, groove along which the anther will £ ,1 i i

split open. Somewhat magnified. - of anthers may be learned

After Bonnier and Sablon from FigS. 129, 130, and 152.1

The shape of the anther and the way in which it opens depend
largely upon the manner in which the pollen is to be discharged
and how it is carried from flower to flower. The commonest
method is that in which the
anther cells split lengthwise, as
in Fig. 152, A. A few anthers
open by trapdoor-like valves, as
in B, and a larger number by
little holes at the top, as in C.

The pollen in many plants
with inconspicuous flowers (as
the evergreen cone-bearing trees,
the grasses, rushes, and sedges)
is a fine, dry powder. In plants
with showy flowers it is often
somewhat sticky or pasty. The
forms of pollen grains are ex-
tremely various. Fig. 153 will serve to furnish examples of
some of the shapes which the grains assume ; c in that figure
is perhaps as common a form as any. Each pollen grain
1 See Kerner and Oliver, Natural History of Plants, Vol. II, pp. 86-95.

FIG. 152. Modes of discharging
pollen

A, by longitudinal slits in the an-
ther cells (amaryllis) ; B, by uplift-
ing valves (barberry) ; (7, by a pore
at the top of each anther lobe (night-
shade) . — After Baillon

GERMINATION OF POLLEN GRAINS

141

consists mainly of a single cell, and is covered by a moderately
thick outer wall and a thin inner one. Its contents are thickish
protoplasm, full of little opaque particles and usually containing

bed
FIG. 153. Pollen grains

a, pumpkin; b, enchanter's nightshade; c, Albuca ; d, pink; e, hibiscus. Very
greatly magnified. — After Kerner

grains of starch and small drops of oil. During the germination
of a pollen grain the outer coat bursts at some point, forced out-
ward by the pressure of a tube formed from the tough inner coat.
Sometimes, as in Fig. 153, b, there are knobs or other indications
of the places at which the outer coat
is most easily ruptured. After the tube
has pushed its way out it continues to
elongate rather rapidly.

174. Microscopical structure of the
stigma and style. Under a moderate
power of the microscope the stigma is
seen to consist of cells set irregularly
over the surface, and secreting a moist
liquid to which the pollen grains ad-
here (Fig. 154). Beneath these super- FlG 154 stigma of thorn
ficial cells is spongy parenchyma, which apple (Datura), with
runs down through the style, if there is P°llen
one, to the ovary. In some pistils the Magnified. — After Faguet
pollen tube proceeds through the cell walls, which it softens by
means of a substance which it exudes for that purpose. In other
cases (Fig. 155) there is a canal or passage along which the pollen
tube travels on its way to the ovule.

142 STRUCTURE OF FLORAL ORGANS; FERTILIZATION

175. Pollination. The transference of pollen from anthers to
stigmas is called pollination. In the case of plants with dry,
dust-like pollen this is generally due to the action of the wind.
Moist, sticky pollen is generally carried by some kind of animal,
usually by insects. The subject of pollination is so important,

especially in relation to the visits of
insects, that it needs a chapter by
itself (see Chapter xxxn).

176. Fertilization. By fertiliza-
tion in seed plants the botanist
means the union of a male sexual
nucleus from a pollen grain with the
female nucleus of the egg cell at the
apex of the embryo sac (Fig. 157).
This process gives rise to a cell
which contains protoplasm derived
from the pollen tube and from the
egg cell. In many plants the pol-
len, in order best to secure fertiliza-
tion, must come from another plant
of the same kind, and not from the
individual which bears the ovules to
be fertilized.

Pollen tubes (Fig. 156) begin to
form soon after pollen grains lodge on
the stigma. The time required for
the process to begin varies in differ-
ent kinds of plants, requiring in many cases twenty-four hours
or more. The length of time needed for the pollen tube to make
its way through the style to the ovary depends upon the length of
the style and other conditions. In the crocus, which has a style
several inches long, the descent takes from one to three days.

Finally the tube penetrates the opening at the apex of the
ovule (Fig. 157, m), called the micropyle (meaning little gate),
and transfers a male nucleus into the egg cell.

FIG. 155. Pollen grains produ-
cing tubes, on stigma of a lily

g, pollen grains ; t, pollen tubes ;
p, papillae of stigma; c, canal
or passage running toward
ovary. Much magnified. —
After Dodel-Port

NATURE OF THE FERTILIZING PROCESS

143

177. Nature of the fertilizing process. The necessary feature
of the process of fertilization is the union of the essential contents
of two cells, especially the nuclei, to form a new one from which
the future plant is to spring. This kind of union also occurs in all
the lower plants (Chapters xx-xxxi), resulting in the formation of
a spore capable of growing into a complete plant like that which

FIG. 156. Germination of the pollen grain of an angiosperm

A, inner coat of the pollen grain distended hy osmosis from contact with the moist
stigma, and protruding slightly at the print i; />, the pollen tube beginning
to form ; C, the pollen tube more elongated, with the tube nucleus t at its tip,
the generative cell g having begun to enter the tube ; D, the pollen tube still
farther elongated ; E, the division of the nucleus of the generative cell to form
the two sperm nuclei si and $2; f\ the sperm cells s\ and ,s2 fully formed,
and the tube nucleus t breaking down; G, the tube nucleus has disappeared,
and the sperm cells are about to be discharged near the tip of the pollen tube.
Somewhat diagrammatic and much magnified. — After Bonnier and Sablon

produced it. It is a sexual act and can be studied much better in
some of the algre, mosses, and ferns than in seed plants.

178. Development of the embryo. After fertilization the
egg cell finally develops the embryo of the future seed. This
formation of the embryo is always a complicated process and
varies much in different groups of seed plants. Briefly stated,

144 STRUCTURE OF FLORAL ORGANS ; FERTILIZATION

the process in angiosperms is as follows. The egg cell (Fig.

158, A) some time after fertilization forms a transverse partition

and is thus divided into two
cells, one of which (Fig. 158,
B, s) is to form the slender
suspensor of the embryo
(which serves various pur-
poses, such as forcing the
embryo into the nutritive
tissue of the seed, absorb-
ing food from the wall of the
ovary, or storing food for
the growing embryo) and the
other (e) is to form the embryo
itself. These cells in turn
subdivide, as shown in C, D,
and E. The whole pear-
shaped body in parts B-E is
called the pro-embryo, and
this continues to grow and
its cells to subdivide until
its structure becomes highly
complex. Finally it con-

i, inner coating of ovule; o, outer coating

of ovule ; p, pollen tube proceeding from tains many sharply defined

one of the pollen grains on the stigma; iong wMdl gradually de-

c, the place where the two coats of the

ovule blend. (The kind of ovule here velop into the Several organs

FIG. 157. Diagrammatic representation
of fertilization of an ovule

along one side of the ovule.) a to e, em- 179. Number of pollen

^ai/SfcrJ^SS grains to each ovule. Only

nucleus of the embryo sac; e, nucleated one pollen grain is necessary

cells, one of which, the egg cell, receives , » . M. -, i i

the male nucleus of the pollen tube ; /, f u- to fertilize each OVule, but SO

nicuius or stalk of ovule ; m, micropyle or many pollen grains are lost

opening into the ovule. — After Luerssen . , ,

that plants produce many

more of them than they do ovules. The ratio, however, varies
greatly. In the night-blooming cereus there are about 250,000

NUMBER OF POLLEN GRAINS TO EACH OVULE 145

pollen grains for 30,000 ovules, or rather more than 8 to 1 ;
in the common garden wistaria there are about 7000 pollen
grains to every ovule, and in Indian corn, the cone-bearing
evergreens, and a multitude of other plants, there are many

FIG. 158. First stages in the development of the egg cell of the
European ivy (Hedera Helix)

A, egg cell. B : s, cell which will form the suspensor; e, cell which will form the
embryo. C, showing first subdivision of the suspensor-forming cell ; D, show-
ing subdivision of the embryo-forming cell; E, showing subdivision of both
regions, slightly more advanced. — After Bonnier and Sablon

times more than 7000 to 1. These differences depend upon the
mode in which the pollen is carried from the stamens to the
pistil. Plants which are pollinated by the wind must produce
far more pollen, to allow for inevitable waste, than those which
are self-pollinated, or pollinated by insects (Chapter xxxn).

CHAPTEE XVI

THE FRUIT1

180. What constitutes a fruit. It is not easy to make a
short and simple definition of what botanists mean by the tejm
fruit. It has very little to do with the popular use of the
word. Briefly stated, the definition may be given as follows : The
fruit of a seed plant consists of the matured ovary and contents,
together with any intimately connected parts. Botanically speak-
ing, the bur of beggar's ticks (Fig. 344), the
three-cornered grain of buckwheat, and
such true grains as wheat and oats are as
much fruits as is an apple or a peach.

181. Classes of fruits. Fruits may be
divided into four classes as follows :

(a) unipistillary fruits, those which re-
sult from the ripening of a single pistil ;

(b) aggregate fruits, those which result
FIG. 159. Group of fol- from tlie ripening of a cluster of carpels

licles and a single of one flower, massed together; (c) acces-
follicle of the monks- sory fruits those in which the main bulk
hood.

of the fruit consists of something else be-
After Faguet ., , , , .

sides the carpels, — e.g. calyx or receptacle,

— added to a simple or an aggregate fruit ; (d) multiple or col-
lective fruits, those which result from the combination of the
ripened pistils of two or more flowers into one mass.

182. Forms of unipistillary fruits : the capsule. This is a
dry fruit, splitting open (dehiscing] to allow the seeds to escape.
Capsules of simple pistils may either open along one line, as

1 See Gray, Structural Botany, chap, vii, also Kerner and Oliver, Natural
History of Plant*, Vol. II, pp. 227-438.

146

FORMS OF UNIPISTILLARY FRUITS

147

in the follicles of monkshood (Fig. 159), or along two lines,
as in the legumes of the pea (Fig. 343). Many capsules result
from the ripening of compound pistils, as the poppy, Datura, or
jimson weed (Fig. 343), and crocus
(Fig. 166, I, B}.

The schizocarp. This is a dry
fruit, breaking into pieces which do
not split open, the name meaning
breaking fruit (Figs. 160, 166, II).

The akene, grain, and nut. These
are dry fruits which never split
open (indehiscent fruits).

Under the general name akene
are grouped several types of fruits.
Many, like those of Fig. 161, are small one-seeded carpels.
Another large group, the fruits of the family Composite, has
akenes which result from the ripening of an inferior ovary, fre-
quently crowned by the limb of the calyx (Fig. 166, III).

FIG. 160. Schizocarp of maple
After Faguet

FIG. 161. Akenes of a buttercup

A, head of akenes; B, section of
single akene (magnified) ; a, seed

FIG. 162. Chestnuts

Grains, such as corn, wheat, oats, barley, rice, and so on, have
the interior of the ovary completely filled by the seed, and the
seed coats and the wall of the ovary are firmly united, as shown
in Fig. 3. Naturally, therefore, they are popularly supposed to
be seeds and are always so called by non-botanical people.

148

THE FRUIT

A nut (Fig. 162) is larger than an akene, usually has a
harder shell, and commonly contains a seed which springs from

a single ovule in one locule
of a compound ovary, which
develops at the expense of all
the other ovules. The chestnut
bur is a kind of involucre, and
so is the acorn cup. The name
nut is often incorrectly ap-
plied in popular language ; for
example, the " Brazil nut" is
really a large seed with a very

FIG. 163. Cross section of an orange
a, axis of fruit, with dots showing cut-off

hard testa.

183. The berry. This is a

ends of fibro-vascular bundles ; p, par- generally fleshy fruit, which
tition between cells of ovary ; S, seed ; ,, , , .

c,loculeofovaryfilledwithapulpcom- usually does not Split Open,
posed of irregular sacs full of juice;
o, oil reservoirs near outer surface of

Such berries as the tomato,

rind ; e, corky layer of epidermis. — gra<Pe> and persimmon result
After Decaisne from t]ie ripening of a supe-

rior ovary. Those of the gooseberry, currant, and many others
result from half-inferior or inferior
ovaries, and therefore a consider-
able part of the bulk of the fruit
is receptacle. The leathery-
skinned fruit of the orange family
is a true berry.

The fruit of the apple, pear, and
quince is called a pome. It con-
sists of a several-loculed ovary,—
the seeds and the tough membrane
surrounding them in the core, —
inclosed by a fleshy edible portion
which makes up the main bulk of
the fruit. In the apple and the pear much of the fruit is
receptacle.

FIG. 164. Peach. Longitudinal
section of drupe

After Decaisne

AGGREGATE AttD ACCESSORY FRUITS 149

In the squash, pumpkin, and cucumber the ripened ovary,
together with the receptacle, makes up a peculiar fruit (with a
firm outer rind) known as the pepp. The relative bulk of the
greatly enlarged hollow receptacle and of the ovary in such
fruits is not always the same.

The drupe. This fruit is often fleshy, and usually does not
split open. The pericarp, or wall of the ripened ovary (meaning
round about and fruit), consists of an outer fleshy (or fibrous
or leathery) layer, the exocarp, and an inner, somewhat hard or
stony layer, the endocarp. In common language the endocarp
with its contained seed is called a "stone"; hence drupes are

FIG. 165
A, strawberry; B, raspberry; C, mulberry. — After Faguet

often known as stone fruits. Most drupes, as in the case of the
peach (Fig. 164), cherry, plum, cocoanut, and walnut, are one-
stoned and one-seeded.

184. Aggregate fruits. The raspberry (Fig. 165,1?), blackberry,
and similar fruits consist of many carpels, each of which ripens
into a part of a compound mass which, for a time at least, clings
to the receptacle. The whole is called an aggregate fruit.

185. Accessory fruits. Not infrequently, as in the strawberry
(Fig. 165, A), the main bulk of the so-called "fruit" consists
rather of the receptacle than of the ripened ovary or its append-
ages. Such a combination is called an accessory fruit.

186. Multiple fruits. The fruits of two or more flowers may
blend into a single mass, known as a multiple fruit. Perhaps

150

THE FRUIT

the best-known edible examples of multiple fruits are the mul-
berry (Fig. 165, C) and the pineapple. The last-named fruit is an
excellent instance of the seedless condition which often results
from long-continued cultivation.

Pt
,-s

FIG. 166. Comparative sections of fruits

I, capsule : A, unilocular liquorice pod, cross section (magnified) ; B, trilocular
crocus pod, cross section (magnified). II, schizocarp, double fruit of poison
hemlock (Conium), cross section (magnified) ; III, akene of arnica, longitudinal
section (magnified) ; IV, berry of pepper (Capsicum), cross section (reduced) ;
V, drupe of cocoanut, longitudinal section (reduced) ; VI, aggregate and acces-
sory fruit of blackberry, longitudinal section (reduced). — I-IV, after Schmidt;
V, after Decaisne ; VI, (modified) after Gray

c, limb of calyx ; en, endocarp ; ex, exocarp ; p, pericarp ; pa, partition between
locules ; r, receptacle^; s, seed

PAET II

THE MORPHOLOGY, EVOLUTION, AND
CLASSIFICATION OF PLANTS

CHAPTEE XVII

THE PRINCIPLES OF MORPHOLOGY, EVOLUTION, AND
CLASSIFICATION

187. Morphology. Morphology treats of the form and
ture of a plant or animal. The lowest organisms have a simple
morphology, but the higher plants and animals are made up
of many parts or organs, and consequently their morphology
is very complex. Organs are structures set apart or developed
for a definite kind of work. Thus the roots of a plant are organs
usually employed to attach the plant to the ground in order that
it may absorb soil moisture.

One department of morphology (comparative morphology)
deals with the various forms or disguises which the same sort
of organ may take in different kinds of plants, and compares
these structures with one another. For example, the foliage leaf
is a well-defined organ which can be recognized at a glance ; but
it requires some study to understand that the scales on the bud
and around an onion, and also some forms of spines and tendrils
are morphologically leaves, that is are leaves variously modified.
Because all of these structures are related to one another they
are called homologous, and morphology studies the homologies, or
relationships, of organs. Comparative morphology is one of the
most interesting subjects of biological study, since it furnishes

151

152 MORPHOLOGY, EVOLUTION, AND CLASSIFICATION

the basis for the established belief in the evolution or develop-
ment of the higher plants and animals from simpler forms.

188. Classification. The classifications of animals and plants
are attempts to express the actual kinships, or what among
human beings are called blood relationships, which are believed
to exist among them. To illustrate the principles of classifica-
tion let us consider the position of the pines among plants. All
of the pines have for their fruit a scaly cone whose seeds are
borne naked at the base of each scale and mature the second
year. The leaves are needle-shaped, evergreen, and clustered. Any
tree which has all the characteristics above given is a pine.

The spruces, hemlocks, firs, and larches agree with the pines
in many respects, but all of them mature their seeds the first
year, and their foliage is different. The American cypress has a
globular woody cone and deciduous leaves in two rows. The
arbor vitse and the juniper have awl-shaped or scale-like leaves,
not in clusters.

All of these cone-bearing trees are distinct kinds, but they
are grouped together because the seeds are borne naked on the
scales of the cones. This peculiarity separates the group from
a much larger assemblage of seed plants in which the seeds
are borne inclosed in seed cases, pods, or other types of fruit.
Finally, all of the s<?ec?-bearing plants are separated from the
spore-bearing groups by the possession of methods of repro-
duction which develop seeds.

Thus the pines find their place in the classification of plants
through clearly marked characters which define several different
groups. These characters are (1) the presence of the seed, (2) the
fact that the seeds are exposed or naked, (3) the development of
the seeds in a cone type of fruit, and finally (4) some peculiar-
ities of the cone, and the character of the foliage. The process
of classification leads from an assemblage of more than one
hundred thousand kinds of plants (the seed plants), through
successively smaller divisions, to the relatively small group of
the pines, with hardly more than seventy known kinds.

ORGANIC EVOLUTION 153

189. Nomenclature. It was long ago found convenient to
give Latin names to the kinds of animals and plants and to their
various natural groups. These names constitute the nomencla-
ture of botany and zoology. Each kind of plant or animal is
termed a species. A group of closely related species constitutes
a genus (plural, genera). Every species is given a name that
consists of two parts. There is the specific name, which defines
the species, and the generic name, which includes the more im-
mediate relatives. The specific name follows the generic, just as
the first name of a man follows his family name or his surname
in a directory. Furthermore, an abbreviation of the name of the
botanist who first described the species follows the combination
of generic and specific names. Thus the name of the pitch pine
is written Pinus rigida Mill., this species having been described
by a botanist named Miller. This universally adopted system of
designating species by two names, known as the binomial system
of nomenclature, was perfected by the famous Swedish natu-
ralist Linnaaus, and the edition of his Species Plantarum, which
is the basis of all botanical classification, bears the date 1753.

Closely related genera are grouped into families, whose names
generally terminate in the ending -acece, and families are brought
together into orders, whose names are written with the uniform
ending -ales. Orders are further assembled into classes, and the
classes into subdivisions, or more frequently into divisions, of
the plant kingdom. Applying this system of classification, we
ha.ve all the species of pines in the genus Pinus, in the family
Pinacece, in the order Conifer ales, in the class Coniferce, in the
subdivision Gymnospermw of that highest division of the plant
kingdom, the Spermatophyta.

190. Organic evolution. In the times of Linnaaus, who lived
in the eighteenth century, almost all naturalists believed that
the species or kinds of animals and plants had never changed
in their characters during their long history on the earth. They
believed that new kinds could only arise by special acts of cre-
ation. This doctrine of special creation gave way to the present

154 MORPHOLOGY, EVOLUTION, AND CLASSIFICATION

belief in organic evolution, or the theory of descent, chiefly through
the work of Charles Darwin, whose famous book, The Origin of
Species, appeared in 1859. The theories of organic evolution
hold that all the existing species of animals and plants have
been derived or evolved through the geological ages from the
simplest forms of life in the beginning. These theories also
hold that the kinds now on the earth are subject to change, and
that very many of them are in process of developing new species.
There are varying opinions as to the causes which bring about
changes in species, and there are several schools of evolutionists
whose theories are the subject of constant discussion arid inves-
tigation.1 But all botanists and zoologists believe in the main
principles of organic evolution ; and the theory is the framework
of biology. Indeed, the theory of organic evolution is as impor-
tant to biology as the atomic theory is to chemistry and as the
doctrine of the conservation of energy is to physics.

191. An outline of the classification of plants. We shall
present at this point a classified arrangement *of the most im-
portant of the larger groups of plants. It is quite impossible
to develop a classification very far in the compass of this book,
but this outline will serve to indicate the field covered in the
succeeding chapters.2 The thallophytes are especially difficult
to classify, for the groups are not as clearly understood as those
of the higher plants, and there are complex relationships, espe-
cially between the algse and the fungi. The classification of
the green algse offers exceptionally difficult problems, and the
arrangement presented here is largely one of convenience in the
present state of our knowledge of this puzzling assemblage of
forms. Classifications are, of course, subject to constant modi-
fication, as groups receive more and more careful study, and
authors frequently differ widely in their systems.

1 See Chapter xxxix, Variation, Mutation, and Origin of Species.

2 For the most recent and detailed classification of plants the reader is
referred to Engler, Syllabus der Pflanzenfamilien, 1903, or to Engler and
Prantl, Die Naturlichen Pflanzenfamilien.

CLASSIFICATION OF PLANTS 155

AN OUTLINE OF THE CLASSIFICATION OF PLANTS
DIVISION I. Thallophyta, the thallus plants, or thallophytes.

SERIES or THE ALG^E.

CLASS I. Cyanophycece, the blue-green algae. ,
II. Chlorophycece, the green algae.

Order 1. Protococcales, the one-celled green algae.

2. Confervales, the confervas and sea lettuce.

3. Conjugates, the pond scums

4. Diatomales, the diatoms.

5. Siphonales, the siphon algae.

6. Charales, the stoneworts.

III. Phceophycece, the brown algae.

IV. Rhodophycece, the red algae.

SERIES OF THE FUNGI.

CLASS V. Schizomycetes, the bacteria.
VI. Saccharomycetes, the yeasts.
VII. Phycomycetes, the alga-like fungi.
VIII. Ascomycetes, the sac fungi.
IX. Basidiomycetes, the basidia fungi.

D [VISION II. Bryophyta, the liverworts and mosses, or bryophytes.
CLASS I. Hepaticce, the liverworts.

Order 1. Ricciales, the Riccia forms.

2. Marchantiales, the Marchantia forms.

3. Jungermanniales, the Jungermannia forms, or

leafy liverworts.

4. Anthocerotales, the Anthoceros forms.
II. Musci, the mosses.

Order 1. Sphagnales, the peat mosses.
2. Bryales, the common mosses.

DIVISION III. Pteridophyta, the ferns and their allies, or pteridophytes.
CLASS I. Filicinece, the true ferns.
II. Equisetinece, the horsetails.
III. Lycopodinece, the club mosses.

DIVISION IV. Spermatophyta, the seed plants, or spermatophytes.
SUBDIVISION I. Gymnospermce, the gymnosperms.

II. Angiospermce, the angiosperms.1
CLASS I. Monocotyledoneoe,, the monocotyledons.
II. Dicotyledonece, the dicotyledons.

1 The reader should note that in this classification the angiosperms con-
tain only two out of sixteen classes of somewhat equivalent value.

CHAPTER XVIII

THE LOWEST ORGANISMS AND THE CELL AS THE
LIFE UNIT

192. The process of evolution. The higher complex animals
and plants are readily distinguished from one another, but the
differences become less apparent in the lower, simpler forms.
There are indeed groups of uncertain position, some authors
placing them among the plants and some among the animals.

The animal and plant kingdoms, in the process of evolution,
followed a tree-like method of development. The forms and
groups split up into divergent lines which constantly gave off,
and are still giving off, new shoots. Thus from a number of
trunks in the beginning there have been derived a multitude
of smaller branches, and from these in turn have arisen count-
less twigs. It is impossible to construct accurately these genea-
logical trees, because the species now living occupy the position
of buds on the structure, some relatively low down and some at
the highest points, but all at the ends of their respective lines
of development. The forms which represented the lowest and
intermediate stages of development are almost all extinct, — that
is, have long ago died out on the earth, — and we can only judge
of their structure by the fragmentary remains which are left as
fossils, or by comparative studies on the structure and develop-
ment of living species, which frequently give us suggestions of
what took place in the long process of organic evolution.

193. The simplest living unit a cell. The living material of
organisms, that is the part which possesses life, is called pro-
toplasm. Protoplasm is not a simple substance, but, on the
contrary, is the most involved mixture of the most complex
substances which the chemist knows. These belong to the group

166

THE SIMPLEST LIVING UNIT A CELL 157

called proteids, a familiar example of which is the white of egg
(albumen). Very little is known of the exact chemical structure
of the numerous proteids. Their molecules are very complex,
for they contain a large number of elements of remarkably
varied chemical properties, — carbon, nitrogen, oxygen, hydro-
gen, sulphur, and in some cases phosphorus. But besides the
proteids and many other organic compounds (substances usually
formed only in association with life processes, as, for example,
the sugars, starch, and fats), protoplasm also contains certain
necessary inorganic substances, such as salts of sodium, potas-
sium, calcium, magnesium, and iron, and in addition to these a
very large amount of water.

Although we know very little about the chemical nature
of protoplasm, certain remarkable structural peculiarities have
been recognized for a long time. Protoplasm always exists in
the form of units which are called cells. The simplest organisms
consist of solitary units, and are consequently termed one-celled
(unicellular). The higher organisms are made up of aggregates
of cells, and are termed many-celled (multicellular).

The cells in many-celled organisms have each a separate indi-
viduality, but they are usually set apart for particular kinds of
work and depend upon one another for mutual assistance. The
many-celled organism has been termed a cell republic, because
all the cells, as individuals, work for the common good of the
community, and by a system of helpful division of labor benefit
one another.

There is a large group of one-celled microscopic animals called
the Protozoa. This constitutes the lowest division of the animal
kingdom, and is quite distinct from the groups of many-celled
animals, although they are believed to have been derived
from it. There are likewise numerous one-celled plants, but
they are related to the higher many-celled forms by very com-
plete and interesting connecting links, so that botanists do not
make a group of one-celled plants, and can readily understand
the evolution of the many-celled forms from the single-celled.

158

THE LOWEST ORGANISMS AND THE CELL

It is much easier to understand the structure of the plant
cell by comparing it with one of the simplest of the one-celled
animals ; so at this point there will be given a brief account of
one of the best-known protozoans, the Amoeba.

194. The Amoeba.* The Amoeba under the microscope appears
as a minute, irregularly shaped body of a jelly-like consistency.
Its form when active constantly changes. A finger-like extension

or process is thrust
out from one side
(Fig. 167,^) and the
somewhat granular
protoplasm flows into
this from neighbor-
ing regions. Other
processes are succes-
sively withdrawn, so
that the protoplasm
actually moves or
flows slowly forward
into the newly formed
lobe, and thus the
Amceba glides along.
There is present in the
protoplasm a denser
protoplasmic struc-
ture termed the nucleus, which is known to be the center of
very important activities in the cell. The protoplasm also con-
tains numerous small granules, and frequently large food parti-
cles, and there are also globules, called vacuoles, of a watery
fluid, which appear and disappear in the thicker substance.
Such is the structure of a typical cell, which may be defined as
a small mass of protoplasm containing a nucleus.

* To THE INSTRUCTOR : If material of Amceba is available, its study will
furnish an excellent introduction or accompaniment to laboratory work on
the plant cell.

FIG. 167. The Amceba

A, an individual moving in the direction of the
arrows; n, nucleus ; v, pulsating vacuole; /, food
body. B, the process of cell division by constric-
tion, a nucleus in each half. — H, after Jordan,
Kellogg, and Heath

THE PLANT CELL 159

The Amoeba feeds upon smaller organisms. These may be
drawn in at any point on the surface of the cell, whose proto-
plasm simply flows around the bodies and thus takes them into
the interior. The oxygen gas held in the water which bathes
the Amoeba is also absorbed all over its surface. Food materials
which cannot be digested, together with the waste products, are
left behind by the protoplasm as it moves from place to place.

When the Amoeba reaches a certain size there takes place
the interesting event called cell division. The cell divides,
by a process of constriction (Fig. 167, B}, into similar halves,
which separate from one another as two independent daughter
Amoeboe. Previous to the division of the cell there has been a
division of the nucleus, so that each daughter Amoeba is pro-
vided with a daughter nucleus, and therefore has exactly the
same structure as the parent cell, but is, of course, only about
half as large. Cell division is the method of cell reproduction.
It is interesting to note that in this process of reproduction
there has been no loss of protoplasm, no death of any region of
the parent Amoeba, but from the division of one have come two.
There is, therefore, no death from old age in one-celled organ-
isms. They are being killed constantly, of course, by adverse
conditions, or eaten by other animals. These are the accidents
of life. However, the Amoeba and other one-celled animals and
plants need never die of old age ; that is, there is nothing in the
constitution of such an organism to prevent its living forever.

195. The plant cell. The plant cell generally differs from
the animal cell in two important respects.

First. The protoplasm is inclosed in a little box-like chamber
with transparent walls. The substance of the walls is called cellu-
lose, — a compound belonging to the great group of the starches
and sugars (carbohydrates). Such an envelope is termed a cell
wall, and is peculiar to plants. Indeed, the term cells, as used in
biology, was first applied to the chambers inclosed by cell walls,
which may be seen in thin sections of cork, pith, and other
plant structures.

160 THE LOWEST ORGANISMS AND THE CELL

Second. The protoplasm of green portions of plants will be

found to contain green bodies called chromatopliores (meaning

color bearers). Chromatopliores have a great variety of forms in

different plants and are sometimes very complex and beautiful,

— as the spiral band in the cells of the pond scum, Spirogyra

(Fig. 168, A). The green color-
ing matter in a chromatophore
is called chlorophyll (meaning
leaf green). Green chromato-
pliores are called chloroplasts
when small and numerous in
a cell. Chloroplasts are char-
acteristic of the cells in plants
above the thallophytes, and
may be readily, studied in the
leaves of mosses (Fig. 169, A),

ferns, and seed plants. Chro-
matopliores are sometimes col-
ored brown or red, as in the
cells of the brown and the red

algae. Chromatophores are

FIG. 168. Cell structure of the pond
scum (Spirogyra)

A, living cell, showing spiral band-like peculiar to plants, never being

chromatophore with pyrenoids p, and found in typical animal cells.

centrally placed nucleus n; fi, living

cell after treatment with a salt solu- The protoplasm of the plant

tion, the protoplasm contracted away U alw H directly Under

from the cell wall ; C, pyrenoid stained J J

with iodine and very greatly magni- the Cell wall, Sometimes COm-

fied (ahout 1000 diameters), a circle of ^Ipfplv fillino- thp ravirv hnf-

starch grains around the pyrenoid ?i€ cavity, but

more frequently forming a lin-
ing which surrounds one or more spaces, or vacuoles, which
contain a watery fluid called cell sap. The relation of the proto-
plasm to the cell wall is easily understood when the protoplasm
is made to contract from the wall by the withdrawal of the
watery cell sap from the vacuoles. Thus if a filament of a pond
scum or a portion of a moss leaf be placed in an aqueous solu-
tion of common salt (5 or 10 per cent), the cell sap is drawn

THE PLANT CELL

161

out of the vacuole (osmotically) and the bounding layer of proto-
plasm shrinks away from the wall (Figs. 168, 7?; 169,1?). The
force that keeps the layer of protoplasm against the cell wall is
called cell turgor.

The mass of protoplasm inclosed by the cell wall is called
the protoplast, and always contains at least one nucleus. Some
plant cells have many
nuclei. The posi-
tion of the nucleus
is somewhat variable.
In the cell of the pond
scum (Fig. 168, A, a)
it is situated in the
middle region and
held in place by deli-
cate strands of proto-
plasm which run out
to the protoplasmic
layer under the cell
wall, but the nucleus
frequently lies just

under the wall, as in FIG. 169. Cell structure of the moss leaf

(Funaria)

, two living cells from a leaf, showing the numer-
ous chloroplasts and the position of the nucleus
n in the layer of protoplasm under the cell wall ;

B, living cell after treatment with a salt solution,
the protoplast contracted away from the cell wall ;

C, stages illustrating the division of the plastids,
starch grains shown in their interiors

the moss leaf (Fig.

matophores are gen-
erally found in the
outer layer of proto-
plasm under the cell
wall. There are also many granules in the protoplasm, some of
them minute globules of oils and fats and others of a proteid
character. Many of these are food products in the cell. Finally,
the central portion of the cell generally contains a single vacuole
filled with cell sap.

It is clear that the protoplast of the plant cell corresponds to
the entire Amozbq, or any other animal cell. The cell wall is a

162 THE LOWEST ORGANISMS AND THE CELL

formation outside of the protoplast and is not a living part
of the plant cell. Many lower plants form reproductive cells
(zoospores, gametes, etc.), which for some time are without a
cellulose wall, and in this condition are motile and behave like
animal cells. However, the cell walls and the chromatophores
are responsible for the most conspicuous differences between
plants and animals, as is noted in Sec. 202.

196. Photosynthesis.1 Chromatophores and chloroplasts in
the presence of sunlight are able to manufacture from water
and the simple gas carbon dioxide certain complex organic
foods of which starch is generally the first visible product.
This process is called photosynthesis, which signifies a putting
together by light. The chemical formula for carbon dioxide is
C02, for water H2O, and for starch C6H1005. The chemistry of
the manufacture of starch cannot be truthfully shown by a
simple equation, for starch is not formed directly from carbon
dioxide and water, but by several steps through invisible sub-
stances that have not been isolated and therefore have never
been studied. The chemical processes in these steps are not
well understood. The final results may be roughly expressed
as follows :

6 C02 + 5 H20 = C6H1005 + 6 02.

This shows why free oxygen is formed during the processes of
photosynthesis. In some plants starch is never manufactured,
but instead sugars, which are substances closely related to
starch, some of them having the formula C6H1206. The sugars
are in solution and invisible. Oil is formed in some plants, as
in the 'green felt (Vaucheria), diatoms, etc., in place of starch, as
the first visible product of photosynthesis.

Many chromatophores have well-defined denser regions called
pyrenoids, which are the centers of starch formation, as is
well illustrated in the pond scum (Fig. 168, C). Chloroplasts

1 The subject of photosynthesis is treated in greater detail in connection
with the structure and functions of leaves (Chapter xn), especially in
Sees. 127-132.

THE FOOD OF PLANTS; ASSIMILATION 163

frequently contain starch grains, as may be readily shown in
the cells of the moss leaf (Fig. 169, C) when colored (stained)
with iodine. Photosynthesis is only found in plants containing
chlorophyll or other pigments of a similar physiological nature.
The sun furnishes the energy in the form of light for the build-
ing up of the simplest food products, and the plant cell is the
main factory which supplies the food of the world.

197. The food of plants ; assimilation. All plants with
chlorophyll can manufacture their own food by the processes of
photosynthesis. Moreover, it is manufactured directly within
the protoplasm of the cell and does not have to be absorbed
from without, as in the case of the animal cell (see account
of Amoeba, Sec. 194).

As we have already noted, starch is generally the first visible
product of this process of food manufacture (photosynthesis).
Starch and the related substances, sugars, are the primary foods
of green plants, and the most important, but they are merely the
starting point for a complex series of processes through which
the highly organized proteids of the protoplasm are derived.
There are some plants which lack chlorophyll, as the fungi and
certain plant parasites, and they, like the animals, depend upon
food absorbed from without the body. The food of plants is
broken down and recombined in various ways to form the pro-
toplasm, as it is in animals, and the breaking down of some of
the substances sets free energy in the form of plant heat (corre-
sponding to animal heat), as is easily proved in the germination
of seeds (see Sec. 5). So the processes of food absorption, or
assimilation, in plants are essentially the same as in animals,
but the manufacture of food (photosynthesis) is an entirely dif-
ferent process and peculiar to plants.

198. The food cycle. There is a circulation of certain ele-
ments (especially carbon, nitrogen, sulphur, and phosphorus)
through the bodies of plants and animals which may be called
the food cycle (see diagram, Fig. 207). It begins in the plant
cell with the manufacture of starch, and related substances

164 THE LOWEST ORGANISMS AND THE CELL

(carbohydrates) by photosynthesis. This makes carbon, obtained
from the carbon dioxide of the air, available in these primary
foods. Nitrogen is obtained from the nitrates dissolved in water
and drawn up through the roots, and sulphur and phosphorus in
a similar manner from sulphates and phosphates. The proteids
of protoplasm are built up from these elements, with the addi-
tion of hydrogen and oxygen. Plants are able to form some
very complex organic substances, but animals are able to carry
the building-up process still farther, for the highest forms of
proteids known are found in their substance.

There is, however, a turning point in the building-up activi-
ties when complex compounds begin to break down into simpler
substances. Some of these are the daily waste products of
an animal or plant. The most striking phenomena are those
which occur during the processes of decay, which begin at once
with the death of an organism. Decay is the process by which
highly complex organic compounds are broken down into suc-
cessively simpler substances. The final steps return the ele-
ments carbon, nitrogen, sulphur, and phosphorus to the earth
and air in very simple forms available again for the constructive
work of green plants. The processes of decay are due to the
growth and activities of bacteria and other fungi, and the sub-
ject is treated at some length in Sec. 252.

199. Cell division, growth, and reproduction. Assimilation
increases the amount of protoplasm, and this results in growth
and reproduction through cell and nuclear division. Cell divi-
sion in plants, as in animals, is preceded by nuclear division,
after which a cell wall is formed between the daughter pro-
toplasts. The nucleus in the resting condition contains granular
material called chromatin, which may be readily colored (stained)
by certain dyes. Generally there are also present one or more
globular bodies called nucleoles (Fig. 170, A). Chromatin is a
proteid and is believed to be the essential substance of the
nucleus and necessary for the life of the cell, because protoplasm
will not live if deprived of nuclei. Just previous to nuclear

THE CELL THEORY OF ORGANIZATION

165

division the chromatin becomes organized into a number of
bodies called chromosomes, each of which splits, and the halves
are distributed in two sets to the daughter nuclei. The distri-
bution of the chromosomes is effected by an interesting appa-
ratus called a spindle (Fig. 170, B), which consists of delicate
fibers (spindle fibers) formed in the early stages of nuclear
division. The two sets of daughter chromosomes (Fig. 170, C)
collect at the poles of the spindle to organize the daughter
nuclei, which then pass into the resting condition (Fig. 170, D),
and a cell wall is formed between, that divides the original

FIG. 170. Stages in nuclear and cell division from the root tip of an onion

A, resting nucleus with the chromatin in the form of a network and two nucleoles ;
B, a spindle with the divided chromosomes gathered in the middle region and
about to separate into two groups of daughter chromosomes ; (7, the two sets of
daughter chromosomes at the poles of the spindle ; D, formation of the new wall
between the daughter nuclei

cell into two daughter cells. It is a remarkable fact that the
number of chromosomes in the nucleus is fixed for different
plants, — a point which we shall have occasion to consider in
other connections.

Chromatophores reproduce themselves by simple fission, or
splitting, very plainly illustrated in the cells of the moss leaf
(Fig. 169, C), and are thus passed on with each cell division.

200. The cell theory of organization. The process of growth
and development of a many-celled organism is through con-
tinuous cell multiplication. Development generally begins with
a cell, which both in animals and in plants is called the egg.

166

THE LOWEST ORGANISMS AND THE CELL

The egg is a female sexual element which normally cannot
develop into a new organism until a male sexual cell, called
the sperm, has united with it. This union is called fertilization ^
and the fertilized egg is a sexually formed cell because it results

from the fusion of two sexual cells,
the egg and sperm. The fertilized
egg is termed an oospore (meaning
an egg spore), when there is a rest-
ing period before its further develop-
ment, or germination. A sexual cell,
such as the egg or sperm, is called
a gamete. The protoplasmic union
of egg and sperm is very complete,
for the two nuclei come together in
the center of the egg and fuse to
form a large nucleus which has, of
course, about twice as much of that
important nuclear substance, chro-
matin, as the single nucleus of either
egg or sperm.

Frequently there are present in
plants other forms of reproductive
cells called spores, which are not
formed sexually but are simply spe-
cial cells which can develop at once

zygospores ; B, another species into new plants.

(S. lonqata), in which the cell rrn • p

unions occur between adjacent The umon of gametes to give

gametes in the same filament.— sexually formed cells is especially

After Schenck ,, .,, , , . ,, ,, ... i <?

well illustrated in the fruiting1 of

the pond scum (Spirogyra). In most species the cells of fila-
ments lying side by side put forth short processes which fuse in
pairs, thus presenting a characteristic ladder-like arrangement
(Fig. 171, A). The contents of one cell then pass over and

1 The terms fruit and fructification will be used in Part II in an untech-
nical sense to designate various forms of reproductive organs and processes.

FIG. 171. The union of the
gametes in Spirogyra

A, two filaments of Spirogyra
quinina, side by side, show-

PROPERTIES PECULIAR TO LIVING MATTER 167

unite with that of the other, giving a large fusion protoplast
which develops a heavy protective wall and is a sexually formed
spore. It is called a zygospore (meaning yoked spore) because
the gametes are similar, like the halves of a yoke. This cell
union is the same in all essentials, including the final fusion of
the two nuclei, as the fertilization of the egg, except that the
two sexual cells, or gametes, are not different in form as are
eggs and sperms (examine illustrations of Volvox (Fig. 178),
(Edogonium (Fig. 182), Fucus (Fig. 199), etc.). The fruiting of
Spirogyra is a relatively simple illustration of a sexual process,
for the gametes are similar and have never become differen-
tiated into eggs and sperms.

Development proceeds through continued cell divisions, which
lead to growth and a gradual specialization or setting apart of
certain cells for particular kinds of work in the body. This
specialization of cells results in the various forms of cell struc-
tures, or tissues, of the mature organism. So the life history is a
succession of cell divisions, and the reproduction of the species
is a return to a one-celled condition through the reproductive
cells (gametes and spores). The animal and plant body dies,
but the stream of life flows on through the reproductive cells.
This is the outline of the cell theory of organization, which
perhaps ranks next to the theory of organic evolution as one of
the fundamental principles of biological science.

201. Properties peculiar to living matter. We have noted
that the chemical composition and reactions of protoplasm are
exceedingly complex, but nevertheless there are no reasons for
supposing that they are outside of chemical and physical laws.
However, protoplasm has properties which distinguish it from
lifeless matter (see also Sees. 45-47).

Protoplasm has the power of growth and repair. This means
that protoplasm can manufacture living substance out of the
lifeless and add the same to itself. It can replace with new
and fresh living matter the waste material which is used up
or discarded during the life processes.

168 THE LOWEST ORGANISMS AXD THE CELL

Protoplasm lias the power of reproduction. Reproduction ac-
companies growth and depends upon cell division. Of course
the surface area of a cell cannot increase in the same ratio as
its bulk. The surface is the region of the cell through which
some of the most important life processes of assimilation and
respiration take place, and a certain amount of cell surface is
necessary for a given bulk of protoplasm. Therefore the cell
divides when, after a period of growth, the bulk of protoplasm
becomes proportionally too great for the amount of surface area.
The sum total of the surfaces of the daughter cells is materially
increased by their division, while the combined bulk of their
protoplasm remains the same as before.

A living being is like a machine in that it requires fuel to
generate its energy or power of doing work, but the organism
has the peculiar ability of making its own repairs, of increas-
ing in size, and of detaching from itself portions which can in
their turn attain the structure and efficiency of the parent. The
process of life is continuous, although the material of protoplasm
is constantly changing, — that is, substances are constantly
going into the organism and substances are going out. It may
be compared to a whirlpool in a river: the form and action of
the whirlpool is constant, although the water which enters and
leaves remains for only a short time in circular movement.

Protoplasm always comes from preexisting protoplasm. This
means that protoplasm, so far as we know, never springs into
existence from inorganic material. It is never formed de novo.
There have been naturalists and philosophers who believed that
life might arise spontaneously under favorable conditions in
suitable nutrient solutions. They cited such illustrations as
the swarming microscopic life which appears in extracts or infu-
sions of animal and vegetable matter as examples of sponta-
neous generation. These theories were overthrown chiefly by
the work of Pasteur and Tyndall, who showed that life never
appears in these extracts and infusions provided proper care
is taken to kill all organisms that may be in them, together

DISTINCTIONS BETWEEN ANIMALS AND PLANTS 169

with all spores or other reproductive cells, and then to pre-
vent the entrance of any more germs. An experiment in this
line can be performed by heating an extract in a flask, closed
by a plug of cotton, until all germs have been killed. Such a
solution then will not even produce the bacterial growths that
cause decomposition, for the cotton plug prevents the entrance
of any dust. It is now established that all organisms at present
on the earth are generated only by their like ; that life only
comes from life, and protoplasm from preexisting protoplasm.

202. The distinctions between animals and plants. Plants
in general are distinguished from animals by two important
peculiarities.

First. The presence of chlorophyll, or equivalent pigments,
enables the plant to manufacture its own food by photosynthesis
in the interior of its own cells. Animals require foods already
manufactured by other animals or plants, and this food is ab-
sorbed from without the cell.

Second. Plants, when growing, are generally stationary, with
a firm, widely expanded, rigid structure, while animals are more
rounded, compact, and yielding. These differences are deter-
mined by the fact that the protoplasts of plants are inclosed
in cellulose compartments. The cell wall gives to plant struc-
ture a degree of stiffness which greatly limits or almost pre-
vents movement, but the individual protoplasts of plants have
all the characteristics of life in common with animal cells, —
sensation, movement, and the powers of growth, repair, and
multiplication.

203. Some organisms of doubtful position. Several groups
of lowly organisms have characters which are in part plant-like
and in part animal-like. We shall consider briefly only one of
these groups, the flagellates.1 .

1 Another large group of doubtful position is the slime molds, or
Myxomycetes, more frequently included among plants than among ani-
mals, but too special for this account. See MacBride, The North American
Slime Moulds, 1899.

170

THE LOWEST ORGANISMS AND THE CELL

204. The flagellates.1 The flagellates (Flagellata) are aquatic,
motile forms, either one-celled or consisting of colonies of cells
held together in a common gelatinous secretion. The individual

FIG. 172. Two flagellate forms

A, Euglena, the motile cell shown above, with its cilium and pigment spot at the
forward end, the process of reproduction by simple division while in a resting
condition being illustrated below; B, Uroglena Americana, a large colonial
flagellate. — B, adapted after Moore

cells are provided with one, two, or sometimes more delicate
hair-like appendages called cilia (singular, cilium, meaning an
eyelash), which move rapidly in the water and are organs of
locomotion. Some forms have chromatophores and can therefore
manufacture their own food, while others are colorless and take
their food in animal fashion through a funnel-like depression into

1 The best account of the flagellates will be found in Engler and Prantl,
Die Naturlichen Pflanzenfamilien.

THE FLAGELLATES 171

the interior of the cell. A bright red pigment spot, frequently
found in each cell, is regarded as a structure sensitive to light,
for these organisms generally move towards the source of bright
illumination. The flagellates are believed to be related to the
lowest green plants, the algae, and some groups of algae are
thought to have been derived from them.

Euglena (Fig. 172, A) is a common flagellate found in stag-
nant pools. The cells are generally green, but some of the spe-
cies and related forms are colorless, having adopted the habit
of living exclusively on organic food substances in the drain-
age water which they frequent. Euglena gracilis becomes quite
colorless when cultivated in solutions of sugar away from the
light, thus suggesting the way in which colorless plants, such
as the fungi, may have arisen from chlorophyll-bearing ances-
tors under an environment which supplied an abundance of
organic food.

Uroglena (Fig. 172, B) is a colonial flagellate which frequently
appears during the summer months in reservoirs and gives a
fishy, oily taste and odor to the water, making it unfit for use.
The taste and odor are caused by globules of oil that are set
free by the rupture of the delicate cells when the water is
carried through pipes. This is one of the organisms which can
easily be destroyed by treating reservoirs with copper sulphate.1

1 See papers by Moore and Kellerman, United States Department of
Agriculture, Bureau of Plant Industry, Bulletin 64, 1904 ;.also Bulletin 76,
1905.

CHAPTEK XIX
THE THALLOPHYTES

205. The thallophytes. The branch Thallophyta (meaning
thallus plants) contains the lowest forms in the plant kingdom.
A thallus is a simple vegetative body, without stems, leaves, or
roots, in the usual sense. The groups of the thallophytes fall
naturally into two series known as algae and fungi.

"The algce. The algae contain chlorophyll or other pigments
which can do the work of photosynthesis.

The fungi. The fungi have no chlorophyll, and must there-
fore obtain their food either as parasites from the tissues of
living plants or animals, called their hosts, or they may live as
saprophytes (meaning decay plants) upon the products of decay.

The fungi are believed to have been derived from algae which
lost their color and gave up the processes of photosynthesis
because they happened to be placed under conditions favorable
to a life of saprophytism or parasitism. A perfect classification
of the thallophytes should show the relationships of the fungi
to the algae, but these are so little understood that it seems
best for the present to treat the two groups separately.

The thallus is not really the distinguishing character of the
thallophytes, for some higher plants, as the liverworts, have
thalloid plant bodies, and some of the algae have a stem and leaf
structure as complex as that of the mosses. The thallophytes
are separated from the next higher group, the bryophytes (liver-
worts and mosses), by the absence of a peculiar type of life
history characterized by certain complicated reproductive organs.
These peculiarities cannot be understood until the liverworts
and mosses have been studied, so a full definition of the thallo-
phytes will be deferred until the end of Chapter xxiv.

172

CHAPTEK XX
THE ALG.2E, THE LOWEST GREEN PLANTS

206. The algae.* For the present we may think of the thallo-
phytes as the immense assemblage of plants below the liver-
worts, mosses, ferns, and seed plants. In number of species and
divergent evolutionary lines the group is much the largest of
the four divisions of the plant kingdom (Thallopliyta, Bryophyta,
Pteridopliyta, and Spermatophyta).

The algae are thallophytes whose plant bodies are colored
because the cells contain chromatophores. Almost all of the
fresh-water forms are green, but the majority of the marine
algae, or seaweeds, are either brown or of beautiful shades of red.
The green color is, of course, due to chlorophyll, while the brown
and red tints are caused by other pigments. The algse are
divided into four classes as follows :

Class I. The blue-green algae, or Cyanopliycece.

Class II. The green algae, or Chlorophycece.

Class III. The brown algae, or Phceophycece.

Class IV. The red algae, or Rhodophycece.

It might appear from the above that the algaB are classified by
their color, but this is not true. These four groups are defined
by peculiarities of cell structure, life history, and methods of
reproduction which can only be understood through a study of
types in the laboratory, and the summaries of these characters

*To THE INSTRUCTOR : This chapter describes many more types than it
would be desirable to present in a general course. The instructor should
make selections according to the material available (which varies greatly in
different sections of the country), and the time at the disposal of the class.
A brief discussion of the best and most available types, and the reasons why
they are desirable for laboratory work will be found in the laboratory
manual of the authors.

173

174

THE ALG^E

must follow the accounts of the groups. However, it is an inter-
esting fact that representative algre of these four classes can
generally be picked out at a glance by their color alone.

CLASS I. THE BLUE-GKEEN ALG.E, OE
CYANOPHYCE^E

207. The blue-green algae. The simplest types of plants are
found among the blue-green algae and in that related group of

the fungi called
the bacteria.
Some of these
plants are the
most primitive
forms of life
now present on
the earth.

. i / / //v.*'*ain//7.S'\\ \ \

A*3^A

208. The one-
celled blue-
green algae.

These forms
develop as

FIG. 173. One-celled blue-green algae and their
cell colonies

A, Gloeocapsa, solitary cell and small groups held together
by the thick gelatinous envelopes ; B, Clathrocystis serugi-
nosa, cell colony of many hundreds of protoplasts im- slimy growths
bedded in a jelly-like substance ; x, single cells illustrating on £jie surface
division by fission

of stones, wood-
work, and other objects, but certain types float freely in the
water in small groups, or sometimes in large cell colonies. The
following types are representative.

Gloeocapsa1 (Fig. 173, A) consists of cells with peculiar soft
walls which form concentric envelopes around the groups of
protoplasts. It is evident that the wall of each protoplast per-
sists for a long tune after every cell division, so that groups of

1 Chroococcus is an excellent substitute for Gloeocapsa, and is not uncom-
mon in stagnant pools and on wet clay banks. Its cells are solitary and lack
the gelatinous envelopes of Glxocapsa.

THE FILAMENTOUS BLUE-GREEN ALGJ5 175

daughter, granddaughter, and even great-granddaughter cells
may remain inclosed in the envelope of the original mother cell,
which becomes very much swollen and jelly-like. The outer
walls of the groups of cells finally become changed to a soft
mucilage, so that the groups of Glceocapsa cells form at times
slimy, dark green patches over damp earth, rocks, and logs.
The individual protoplasts have an exceedingly simple structure,
for the coloring matter is uniformly distributed through the
cells and no nucleus can be seen.

ClatJirocystis and Ccdosphcerium are free, floating cell colonies,
often forming greenish scums during the summer months on
the surface of park ponds, reservoirs, and other small bodies of
water. The colonies of Ccdospheerium are spherical, while those
of ClatJirocystis (Fig. 173, B) become irregular in shape through
the development of holes, so that the structure is somewhat
net-like.

209. The filamentous blue-green algae. These frequently
form felted or tufted growths or gelatinous expansions of con-
siderable size. There are a number of complex branching types,
but the following are good examples of the assemblage.

Oscillatoria is the most interesting type of the Cyanopliycece
if only one form can be studied. The filaments are generally
made up of flattened disk-shaped cells, placed face to face within
an exceedingly delicate sheath, much like a roll of coins wrapped
in paper. Cell division takes place in all portions of the fila-
ment, and several stages are illustrated in Fig. 174, A. Growth
is therefore not confined to the tip or any other special region
of the plant. New filaments arise by the breaking apart of the
older ones, generally at some point where one or more cells, have
died (Fig. 174, A, d). The end cells of filaments or fragments of
filaments are always rounded, illustrating beautifully the phe-
nomenon of cell turgor or pressure from within the protoplast
upon the cell membrane. The cell structure of Oscillatoria is
very typical of the blue-green algae. The blue-green pigment
gives color to the entire outer region of the protoplast, which

176

THE ALG^E

may be considered a diffused chromatophore. There is no nucleus
in the usual sense of the term, although the central region of the
cell has a different structure from the outer and probably contains
chromatin. The small granules arranged along the cross walls
are believed to be food products built up by the activities of
the blue-green pigment in sunlight (photosynthesis). Oscillato-

ria takes its name from
the remarkable move-
ments of the filaments,
whose free ends swing
back and forth describ-
ing a circle or an ellipse,

h ^^^~ mi while the filaments may

glide slowly forward.
The cause of this move-
ment is not understood.
Oscillator ia is found in
greatest abundance in
open drains, ditches, or
pools, where the water
is foul with decaying
There

it may form thick felts
on the bottom, or rise
to the surface in slimy
masses because of the
bubbles of gas, largely
oxygen, formed during the processes of photosynthesis and held
within the tangle of filaments.

Anabcena and Nostoc are closely related genera. The fila-
ments are chains of round or elliptical cells. Besides the blue-
green vegetative cells there are present at intervals curious cells
termed heter ocysts (meaning other cells), which are generally
larger than the vegetative cells, lighter in color, and often empty
of protoplasm. Their function is not clearly understood. The

FIG. 174. Filamentous blue-green algae

A, Oscillatoria ; d, dead cell, indicating a point organic matter,
where the filament might break apart ; f, stages
of cell fission; B, Anabsena; h, heterocyst;
s, resting cells ; C, Nostoc, habit sketch of a col-
ony and the details of a single filament ; h, heter-
ocyst; D, Gloeotrichia, portion of a colony and
the base of a single filament in detail; h, heter-
ocyst ; s, resting cell

LIFE HABITS OF THE BLUE-GREEN ALG^E 177

filaments may break apart on either side of the heterocyst, set-
ting free chains of cells which grow into new filaments. Certain
vegetative cells in Anabcena increase greatly in size and become
densely filled with protoplasm and food material and sur-
rounded by a thick protective wall (Fig. 174, B, s). Such cells
are called resting cells, or spores, and they are able to live through
seasons of drought or a winter's cold and with the return of
favorable conditions to germinate and form new filaments. The
filaments of Anabcena are held in a soft slime, but those of Nostoc
are surrounded by a stiff jelly, so that the mass of much-coiled
chains of cells has a firm boundary. Consequently, Nostoc colo-
nies (Fig. 174, C) may have a spherical form and become as
large as marbles. The slimy or jelly-like 'substance of Anabcena
and Nostoc is a modification of the delicate sheath around the
filaments and corresponds to the envelopes about the cells of
Grlceocapsa.

Glceotricliia (Fig. 174, D) sometimes develops in such quan-
tities in ponds and lakes during the summer as to form a
brilliant green scum on the surface of the water. The fila-
ments have a radiate arrangement in a soft, gelatinous sub-
stance and end in long hairs, and a very large resting cell may
be formed at the base of each filament adjacent to the terminal
heterocyst.

210. Life habits of the blue-green algae. The Cyanophycece
have some peculiar life habits of ecological interest. They are
generally found in warmish waters, both fresh and salt, and
many of the forms prefer those which are foul with decaying
organic matter. Thus open drains and reeking pools of stagnant
water present luxuriant growths of these algae. It is probable
that the plants actually use for food certain of the organic sub-
stances in such waters. Some of the most conspicuous green
scums on ponds and small lakes are composed of certain of these
algse (Ccelo splicer ium, Clathrocystis, Anabcena, and Glceotrichia f
etc.). Such scums may be called water blooms, after the German
term Wasserbliithe. The coloration of the Red Sea is due to an

178 THE ALG^E

extensive water bloom caused by a filamentous blue-green alga
(Trichodesmium) which at times fills the water, and whose color,
a reddish brown, gives then a peculiar tint to the sea.

Some forms (Anabcena, Clatlirocystis, and certain species of
Oscillatoria) are frequently responsible for the fouling of water
supplies which take on what is called the "pigpen odor," and
are otherwise unfit for public use. All of these blue-green algae,
together with the flagellate Uroglcna, can be killed by treating
the reservoir or other body of water with copper sulphate (see
Sec. 204), a perfectly safe and inexpensive remedy for contami-
nated water supplies.

Perhaps the most remarkable display of the blue-green algae
is in the waters of certain hot springs, as in Yellowstone
National Park. It is doubtful whether any algae except the
Cyanophyccce can live in water warmer than 100° F. (40° C.),
but some of the blue-green algae grow luxuriantly in hot springs
at 137°-145° F. (58°-63° C.). It is probable that their simple
cell structure makes possible a greater power of resistance to
these extraordinary life conditions.

211. Summary of the blue-green algae. The Cyanophycea
are distinguished from other groups of algae by the simpli-
city of their cell structure, the absence of sexual reproduction,
and the presence of a blue-green pigment uniformly diffused
through the outer region of the cells. The method of growth
by rapid divisions or splitting of the cells throughout the entire
plant body is a very characteristic feature of the group, and
the blue-green algae are sometimes called the "fission algae"
(Schizophycece).

The blue-green algae agree with the bacteria, or "fission fungi"
(Scliizomycetes), in their simplicity of cell structure and methods
of reproduction, but the bacteria are of course generally without
pigment. It is quite clear that the Cyanopliycece and the Scliizo-
mycetes are closely related, and some authors place them together
in a separate division of the plant kingdom below the Thallo-
phyta, named the Schizophyta, or " fission plants."

PLEUROCOCCUS 179

CLASS II. THE GREEN ALGLE, OK CHLOROPHYCE^

212. The green algae. The green algae comprise a large and
varied assemblage of groups, many of which are widely different
from one another. Some forms of the Chlorophycece are believed
to stand rather close to what was the main line of ascent from
the algie to the liverworts and mosses. Consequently the class
has an important place in an account of the evolution of the
plant kingdom. The green algse illustrate better than any other
group the origin and evolution of sexual processes in plants.
Since the more familiar algal growths of fresh water are green
algse, a more extended treatment of the Chloropliycece will be
given than of the less familiar groups of the Cyanopliycece,
Phceopliycece, and Rhodophycece, and the following six orders will
be considered together with a " Summary of the Green Algse."

Order 1. The one-celled green algse, or Protococcales.

Order 2. The confervas, or Confervales.

Order 3. The pond scums and desmids, or Conjugates.

Order 4. The diatoms, or Diatomales.

Order 5. The siphon algse, or Siphonales.

Order 6. The stone worts, or Charales.

ORDER 1. THE ONE-CELLED GREEN ALG^E, OR PROTOCOCCALES

213. The one-celled green algae. This order contains almost
all of the one-celled green algse excepting the large but very
special groups of the desmids and diatoms. We can only
describe briefly five types.

214. Pleurococcus. Pleurococcus (family Pleurococcacece) forms
the green coating or stain that is very common on the north
sides of tree trunks, fences, and stone walls. The cells (Fig. 175)
may be solitary, but they usually remain associated in small
groups for some time after the cell divisions. The protoplast
contains generally a single chromatophore of irregular shape
which, as a rule, fills the greater part of the cell. The chroma-
tophore is, however, variable in size and may or may not have

180

THE ALG^E

FIG. 175. Pleurococcus vul-
garis, a one-celled green

Several cells illustrating the
method of cell division and
their association in small
groups

a pyrenoid (Sec. 196). The nucleus can sometimes be seen in
the center of the cell. The cells are exceedingly resistant to
cold and drought, but under very severe conditions they protect
themselves by forming a heavy cell wall, thus becoming resting
cells. Sometimes the contents of a cell
break up into several daughter proto-
plasts, but as a rule the only method
of reproduction is by simple cell divi-
sion. Other forms of one-celled algae,
with more complicated methods of re-
production (by zoospores and gametes),
are frequently found associated with
Pleurococcus, but should be carefully
distinguished from this simple alga.

Pleurococcus may seem almost as
simple as the one-celled blue-green alga
Gloeocapsa, but its cell structure with
a chromatophore and well-defined nucleus is far in advance of
the Cyanopliycece.

215. Sphaerella and Volvox.* These forms are representatives
of one of the most interesting families of the green algse, the
Volvox family1 (Volvocacece). The lowest members are one-
celled and resemble the flagellates (Sec. 204), but the higher
forms are cell colonies of remarkable structure and life histories.
The vegetative cells are motile, being always provided with two

* To THE INSTRUCTOR : It is rather difficult to obtain material of the
Volvox family, and it cannot be depended upon for type study. Therefore
laboratory work on the reproductive processes in the algse can much better
be arranged with such types as Ulothrix, or Ulva, or some form of the Chce-
tophoracece, or Cladophora, CEdogonium, or Vaucheria and Fucus. But the
Volvocacece and Flagellata are so important to a conception of certain primi-
tive conditions of algal life that they should be discussed in any extended
general course. The fact that zoologists have found Volvox and its relatives
of interest should not deter botanists from making use of their own.

1 For a detailed account of the Volvocacece see Goebel, Outlines of Classi-
fication and Special Morphology of Plants, and Engler and Prantl, Die Natiir-
lichen Pflanzenfamilien.

SPHSERELLA

181

hair-like cilia, whose incessant whipping of the water carries the
organism along. There is also a red pigment spot at the ciliated
end of the cell (Fig. 176, A, p). This free-swimming, ciliated
cell is of a type strikingly different from Pleurococcus, but it
is believed to represent very nearly the ancestral condition of
many groups of algse.

Sphcerella lacustris (Hcematococcus pluvialis) is found freely
swimming in rock pools and sometimes in troughs and basins,

FIG. 176. Sphcerella

A, />, Sphserella lacustris: a single cell in detail and a group of daughter proto-
plasts within the parent cell. C, D, Sphserella Butschlii: numerous small sex-
ual elements or gametes are shown in the parent cell C, and D illustrates their
fusion in pairs to give the sexually formed cell or zygospore z. — Z>, after
Schenck ; C, I), after Blochmaun

and is frequently so abundant as to color the water a bright
green. The organism multiplies very rapidly, for the larger indi-
viduals (Fig. 176, A) form 2-16 daughter cells (Fig. 176, B),
which escape from the mother-cell membrane, swim away, and
after a period of growth form in their turn a new set of
daughter cells. The free-swimming cells come to rest at times,
becoming thick-walled resting cells, which are colored red by
a peculiar pigment. These resting cells carry the plant over

182

THE ALG^E

unfavorable seasons and are sometimes developed so numerously
as to cover the bottom of pools and rock hollows with a red
deposit. The phenomenon called " red snow " is due to deposits
of the resting cells of Sphcerella nivalis on fields of snow and ice.
Some species of Sphcerella (Fig. 176, C) develop a much greater
number of daughter elements, 32 to 64, which are much smaller

than those just described, but
have the same structure. These
smaller cells swim about freely
for a short time, and then come
together in pairs and completely
fuse with one another, beginning
at the ciliated ends (Fig. 176, D).
A cell fusion of this character is a
sexual act (Sec. 200) and the cells
which unite are gametes. The sex-
ually formed fusion cell or zyg-
ospore of Sphcerdla soon settles
down on some surface and, losing-
its four cilia, remains quiet for
several days or weeks, finally de-
veloping within itself several
motile cells of the usual type.

Volvox (Fig. 178, A) is a colo-
nial form consisting of many hun-
dreds of cells (sometimes more
than twenty thousand) imbedded
in a gelatinous substance in the
form of a sphere, with the pairs of cilia pointing outwards.
These remarkable organisms, as large as pin heads, roll slowly
through the water of quiet pools and ponds, sometimes gather-
ing in great numbers in open sunlit portions, free from water
weeds and algal growths. Daughter colonies (Fig. 178, A, d) are
formed from certain cells which after a period of growth develop
a large number of motile cells like the parent. These small cells

EIG. 177. Chlamydomonas
Braunii

Chlamydomonas is not uncommon
in the same sort of situations as
Sphserella. It may be distinguished
from the latter by the absence of
a thick gelatinous envelope around
the cells. Some of the forms show
important advances over Sphse-
rella in their sexual processes, for
the gametes may be of two sizes,
large female and small male cells,
as shown above, z, the zygospore.
— After Goroschankin

VOLVOX

183

become arranged to form a daughter colony which swims around
in the interior of the mother colony. Sometimes several of the
daughter colonies may be developed, and they finally escape
by the rupture of the parent structure. The sexual cells, or
gametes, are of two sorts : (1) large female cells, which are called
eggs because they are without cilia and consequently never
motile, and (2) small male cells, or sperms, of peculiar form,
with two cilia, and consequently very actively motile. The eggs

FIG. 178. Volvox globator, a colonial form of the Volvocacece

A, mature colony, with four daughter colonies developing in its interior; B, sec-
tion of the edge of the colony, showing three vegetative cells and a developing
egg; C, a packet of sperms within the parent cell and a single sperm very
much magnified at the side; D, an egg surrounded by a swarm of sperms;
E, an oospore with heavy protective wall. — C, after Colin.

(Fig. 178, B, Z>), formed by the enlargement of vegetative cells,
escape into the interior of the colony as naked spherical proto-
plasts. The sperms (Fig. 178, C) are developed in great num-
bers within enlarged vegetative cells. They are also set free
within the parent colony and gather about the eggs in swarms
(Fig. 178, D). Finally, a single sperm fuses with each egg, which
is then said to be fertilized. The fertilized egg immediately
forms a cell wall about itself and passes through a period of
rest as an oospore (Sec. 200).

184

THE

Volvox thus presents a great advance over Sphoerella,
Chlamydomonas, and other one-celled members of the Volvo-
cacece, in the highly developed sexual process as well as in the

complex cell colony. There is, how-
ever, a series of genera (G-onium, Pan-
dorina, Eudorina, Pleodorina, etc.) in
the family, illustrating intermediate
conditions between these extreme
forms, which makes it clear that Vol-
vox stands at the head of a remark-
able line of development in the algae.
It may be considered the climax type
of a side line of evolution, — that is,
a branch which departs widely from
the main line of ascent.

ORDER 2. THE CONFERVAS, OR

CONFERVALES

216. The confervas. The Confer-
vales comprise many very common
filamentous algae and also such mem-
branous forms as the sea lettuce. The
algae which seem to be nearest to the
main line of ascent to the liverworts
and mosses are found in this group.
Some of the types illustrate espe-
cially well the principal forms of sex-
ual reproduction in the algae and
various types of life histories.
217. Ulothrix. This confervoid alga (family Ulothricacece)
is abundant on stones and rocks along the shores of the great
lakes, in quieter waters at the seaside, and frequently grows in
stone fountains or on stonework around park ponds. The fila-
ments are unbranched, and each consists of a row of similar

FIG. 179. The water net
(Hydrodictyon]

This is a remarkable form of
the Protococcales, whose cells
form the meshes of a net-like
cell colony, A. New nets are
formed in the interior of large
cells, B, which develop an
immense number of zoospores
that never escape from the
mother cell, but join with one
another to form daughter
nets, which are set free by the
breaking down of the mother-

' cell wall

ULOTHRIX

185

cells. Each cell contains a single chromatophore with pyrenoids,
which has the form of a wide band, or girdle, just under the
cell wall, and generally surrounds the nucleus in the middle
region of the cell (Fig. 180, A, B). The filaments are attached
at one end (Fig. 180, A), and the growth by cell division takes
place throughout the entire length and is not confined to the

FIG. 180. Ulothrix zonata

A, base of filament, showing its attachment and cells containing band-shaped
chromatophores with pyrenoids; B, portion of a filament about fifty cells
above the base, showing a vegetative cell below and two cells which have
formed 4 and 8 zoospores respectively ; C, the zoospores, each with a pigment
spot and four cilia; D, stages in the germination of the zoospore; E, portion
of a filament illustrating the formation of gametes, 64 in each cell; F, the
gametes, showing pigment spot and two cilia, and stages in their fusion to form a
four-ciliate zygospore with two pigment spots ; G, germination of the zygospore,
which develops a number of zoospores. — G, after Dodel

tip as in some algae. The cells in the upper portions of older
filaments (Fig. 180, B} develop a type of reproductive cell very
common among the algae, called the zoospore (meaning animal
spore) because of its animal-like habit of swimming about.

Zoospores are naked ciliated protoplasts formed within parent
cells called sporangia. They swim rapidly through the water,
and each generally contains a red pigment spot. Zoospores are

186 THE ALG^E

attracted by light and collect at the illuminated side of a vessel,
forming a green cloud in the water. Because of these habits,
and their rapid darting to and fro in the water, they are often
called swarm spores.

The zob'spores of Ulothrix are developed most numerously at
night and are set free from the parent filaments chiefly during
the morning hours. Sometimes the entire protoplast slips out as
a single large zoospore, but more often 2, 4, or 8 zoospores are
formed in each parent cell (Fig. 180, B). They are roundish or
pear-shaped (Fig. 180, (7), with four cilia at the pointed end, and
each contains a red pigment spot, chromatophore, and nucleus.
The zoospores thus resemble the organisms called flagellates
(Sec. 204), and like them swim freely around in the water by
the lashing movements of their cilia. But the zoospores have
a relatively short free-swimming period, for after perhaps an hour
or more they attach themselves by the ciliated ends to various
objects and grow into new Ulothrix filaments (Fig. 180, D).

At times a much greater number of zoospores may be de-
veloped in the parent cells, — perhaps 32 -or 64, or even more
than a hundred (Fig. 180, 7?) . These have generally only two
cilia and are much smaller than the four-ciliate zoospores, but
otherwise have the same structure. They swim very actively
for a short time, and then come together in pairs in the water
and fuse with one another (Fig. 180, F). This cell union is a
sexual process (see Sec. 200), and the small two-ciliate zoospores
are therefore sexual elements and are called gametes. The result
of this fusion is a sexually formed cell called a zygospore
(meaning a yoked spore), because the two gametes are similar,
like the halves of a yoke, and not different in form, as the sperm
and egg (see Fig. 178, Volvox). This simple type of sexual
reproduction is termed isogamy, because the gametes have the
same form, or morphology. The zygospore, of course, corresponds
to the fertilized egg, or oospore, characteristic of higher plants.

The zygospore of Ulothrix swims about for a short time with
its four cilia, and may only be distinguished from the large

THE ORIGIN OF SEX IN PLANTS

187

four-ciliate asexual zoospores by its two pigment spots. It finally
comes to rest and remains quiet for many weeks or several
months, but slowly increases in size. Finally, the zygospore
develops several zoospores (Fig. 180, G), which escape from the
cell, swim off, and develop new Ulotlirix plants.

218. The sea lettuces. The sea lettuces include Ulva and
its relatives (family Ulvacece), and are very common along the
seacoast, forming green

fringes on the rocks and
wharves near low water-
mark. The thallus is a
thin green membrane (Fig.
181,^4) instead of a fila-
ment as in Ulotlirix. Zoo-
spores and gametes (Fig.
181, B, C) are developed in
the cells along the edge
of the membranes. Their
structure, methods of
formation, and habits are
essentially the same as in
Ulothrix, and the sea let-
tuces are equally good for
the study of these points,
and they are sometimes
more available than Ulo-
tlirix for those living at or
near the seacoast.

219. The origin of sex in plants. Ulotlirix, Ulva, and some
other types show clearly that the simplest forms of gametes
in plants are closely related to zoospores, for they are devel-
oped in the same way and have a similar structure. Indeed,
the gametes of these lower plants frequently germinate directly
like zoospores, thus showing that the sexual habit of fusing
with one another is not firmly fixed. However, the plants that

FIG. 181. The sea lettuce (Ulva)

A, habit sketch; B, cells forming four-ciliate
zoospores ; C, two-ciliate gametes. — Adapted
after Thuret

188 THE

develop from such gametes are generally smaller and weaker
than those which come from the usual zoospores. For these
reasons it seems evident that the gametes of plants arose from
zoospores, or motile cells similar to zoospores, which, adopting
the habits of fusing in pairs, became sexual cells. Such types
as Ulotlirix and Ulva have an especial interest because they
illustrate the general conditions which must have been present
with the origin of sex in any group of plants.

220. (Edogonium. (Edogonium (family (Edogoniacece) is one
of the best illustrations in the green algae of the higher sexual
condition where the gametes become differentiated and specialized
as eggs and sperms.

The species are unbranched, filamentous, fresh-water forms,
attached by a disk-like development from the lowest cell (Fig.
182, A) called a holdfast. The cells have large chromatophores
of irregular form, containing pyrenoids. There are sets of curi-
ous lines called caps across the ends of many of the cells
(Fig. 182, B, c), — structures peculiar to this family, — which
result from a method of cell division too complicated to be
described here.1 Zoospores are developed singly in the cells,
and are large protoplasts with a circle of cilia at a colorless end
(Fig. 182, Z>).- After swimming about for a short time the
zoospores settle down on the ciliated end, develop the hold-
fasts, and grow at once into new filaments.

The sexual cells, or gametes, of (Edogonium are eggs and sperms.
The eggs are developed singly in enlarged cells, which are the
female sexual organs (Fig. 182, B] and are called oogonia
(singular, oogonium). The entire protoplast of the oogonium
becomes the egg (Fig. 182, B, e), which remains within the oogo-
nium as a naked, motionless cell, without cilia, and is richly
supplied with chromatophores and food material. The sperms
are developed in pairs in short, disk-shaped cells, which are
found in small groups, forming the male sexual organ, or

1 See Goebel, Outlines of Classification and Special Morphology of Plants,
p. 44.

(EDOGONIUM

189

antheridium (Fig. 182,
B, a). The sperms, fre-
quently called anthero-
zoids by botanists, are
small, almost colorless
protoplasts, with a cir-
cle of cilia at one end
(Fig. 182, C) like the
zoospore. They are in
sharp contrast to the
eggs, being actively mo-
tile, ciliated, and with
very much reduced
chromatophore and food
contents.

A cleft, or pore (Fig.
182, B, e), is formed in
the oogonium so that
the sperms may enter,
and one of them, fusing
with the egg, fertilizes
it. The egg after fertili-
zation develops a heavy
wall (Fig. 182, jB, o)
and becomes an oospore
(meaning an egg spore).
The obspores, thus pro-
tected, can live through
drought or winter's cold
and so survive seasons
of the year impossible
for vegetative growth.
On the return of favor-
able conditions they
germinate, each oospore

FIG. 182. (Edogoniiim nodulosum

A, base of filament showing holdfast; B, filaments
with oogonia and antheridia ; e, an egg ready
for fertilization, showing the cleft for the en-
trance of the sperm ; o, the thick-walled oospore ;
a, antheridium, composed of four cells ; c, caps ;
C, sperms, showing crown of cilia ; I), zoospore ;
E, germination of the oospore, producing four
zoospores. — C, D, E, after Juranyi

190

THE ALG^E

giving rise to four large zoospores (Fig. 182, E), which develop
at once into (Edogonium plants.

Both eggs and sperms are believed to have been derived from
simpler ancestral types of ciliated gametes, similar in structure
to one another and to the zoospore. These ancestral sexual
conditions must have been those of isogamy, somewhat as is
at present illustrated in Ulothrix. The originally similar
ciliated gametes varied in size. The smaller reduced
their chromatophore and food contents because they
were formed in large numbers but retained their cilia
and thus became the small active sperms. The larger
gametes accumulated rich supplies of food, became slug-
gish, finally lost their cilia and swim-
ming habits, and at last were retained
within and protected by the oogonia,
thus becoming large nonmotile eggs.

It is clear that the plant gains very
much by differentiating and specializ-
ing its gametes as eggs and sperms, for
the eggs are protected and richly sup-
plied with protoplasm and food, while
the sperms are developed very numer-
ously and are well adapted to swim FlG- m Draparnaldia, one
actively about in the water4 where they
are attracted to the eggs by substances
secreted by its protoplasm. The higher
sexual condition, as in (Edogonium,
where the gametes are eggs and sperms, is called heterogamy
because the gametes are dissimilar, in contrast to isogamy (see
account of Ulothrix, Sec. 217).

221. The Chaetophoraceae.* The members of this family,
including such common genera as Stigeodonium, Draparnaldia

* To THE INSTRUCTOR : It does not seem to be generally known that forms
of the Chcetophoracece are almost equally good types for the study of zoo-
spores as Ulothrix and may be readily substituted for that form. Stigeodonium

of the Chattophoracece

zoospores formed singly in
the cells

COLEOCHJETE

191

(Fig. 183), and Clicetoplwra, are inore
complex than Ulotlirix, for they consist
of branching filaments of peculiar forms.
However, the cell structure, life his-
tories, methods of reproduction, and low
sexual conditions (isogamy) of these
types all show relationships to the Ulo-
thricacece. They are of especial interest
as leading up from the level of Ulo-
thrix to the highest form of the Con-
fervales, the genus Coleochcete.

222. Coleochaete. Coleochcete (family
Coleochoetacece) contains a number of
species which live in fresh water, at-
tached to the stems and leaves of water
weeds, and they frequently appear on
the sides of aquaria. Some of the forms
are circular flat plates or cushions of
cells that really consist of systems of
filaments radiating out from a center. ElG- 184- Cladophora
Large, two-ciliate zoospores are formed This large, much-branched, fii-

A amentous alga, A, has many

singly in the cells. The female organ,
oogonium, is a large, flask-shaped cell
with a long neck (Fig. 185 A, o). Its
protoplasm forms a single spherical egg
which nearly fills the lower swollen
portion of the oogonium, and the neck
finally opens above to allow the en-
trance of the sperms. The male organs,
antheridia (Fig. 185, A, a), are small
cells, generally in groups, each of which develops a two-ciliate
sperm.

and Draparnaldia are common on stones in clear brooks and in springs.
Cladophora (Fig. 184) is also a good type for laboratory study and very
common.

species common in both fresh
and salt water. Zoospores,
B, are formed generally
in terminal sporangia, and
there are gametes which fuse
in pairs, C, as in Ulothrix.
The older cells contain large
numbers of nuclei, and this
form, with certain relatives
(family Cladophoracese) , oc-
cupy a position somewhat
intermediate between the
Confervales and Siphonales

192

THE ALG^E

The fertilized egg becomes an oospore within the oogonium
and is further protected by a cellular envelope (Fig. 185, £>)
developed from short filaments which grow up around the
structure (Fig. 185, A,f), making a conspicuous fructification.

The oospore germinates the
following spring, forming
within itself a small group
of cells (Fig. 185, C), each of
which gives rise to a zoo-
spore. The decay and ruptur-
ing of the fructification allows
the zoospores to escape and
start new Coleochcete plants
at the beginning of each sea-
son. The fructification of Co-
leochcete serves to multiply
FIG. 18o. Coleochcete pulmnata

the number of zoospores pro

A, filaments with an oogonium o, anthend-

ium a, and a sperm above ; /, fertilized duced by the OOSpore, whicll

egg in its oogonium becoming surrounded ig d rl Rn advantage,
by short filaments from the cell below ; J

JB, oospore completely inclosed in a eel- Coleochcete stands at the

™;;±t t^™™sr* ^ <* °°* <* ^ ***^

oospore, each cell in the interior develops fined lines of evolution ill the
a zoospore. — After Pringsheim

ft ^ which ^^ afc

the lowly level of the Ulothricacece and runs upwards through
the Chcetophoraccce. Authors have generally regarded these
forms as leading almost directly to the bryophytes, with Coleo-
chcete just a little below the liverworts; but, as we shall see
later, there are serious difficulties in the way of this view.1
Nevertheless, these forms are perhaps nearer than any other
living algae to the theoretical main line of ascent.

1 The fructification of Coleochcete has been regarded as similar to the so-
called fruit or sporophyte of the liverworts, but, as explained in Sec. 336,
there is strong evidence against this interpretation. The one-celled sexual
organs of Coleochoete are also very different from the many-celled sexual
organs of the bryophytes, and this is also evidence against the existence of a
close relationship between the groups (see Sec. 299).

THE POND SCUMS AND DESMIDS

193

ORDER 3. THE POND SCUMS AND DESMIDS, OR CONJUGALES

223. The pond scums and desmids. The pond scums and
desmids (order Conjugates) are remarkable for the beauty and
symmetry of their cell structure, and especially for their large
and complicated chrornatophores. There are no motile stages in
their life histories, and the sexual processes consist in the union
or conjugation of similar nonmotile gametes (isogamy). These
characters distinguish the group sharply from all other algse, but

FIG. 186. Desmids

A, Closterium, a vegetative cell at the left and a zygospore at the right between
the halves of two empty cells whose contents have fused ; B, Cosmarium; the
desmid at the right has just divided and is forming two new parts between the
old halves of the parent cell. C, Micrasterias, a very elaborate form in its out-
line and markings ; D, Hyalotheca, a common filamentous desmid ; the appear-
ance of the cells in face view is shown at the right

make the relationships of the forms very uncertain. These plants
live only in fresh water and seek the sunshine, being abundant
in clear, shallow pools. The desmids live chiefly along the mar-
gin and on the bottom, while the pond scums frequently form
growths upon the surface of the water, which appear frothy
because of the bubbles of gas (largely oxygen) held among the
filaments. The filaments are very slippery to the touch on
account of a slimy excretion from the cells.

224. The desmids. There are about one thousand species in
this large group whose forms are generally one-celled, although
some desmids are filamentous (Fig. 186, D). Each cell has two

194

THE

parts, which duplicate one another even in the details of pro-
toplasmic structure, and the nucleus lies in the middle region,
with the chromatophores arranged symmetrically in the halves.

The desmids multiply rapidly by
cell division, each daughter cell
taking one half of the old cell
wall and adding to it a repro-
duction of the other half (Fig.
186, B). The gametes are naked
protoplasts, which escape by the
breaking apart of the halves
of the cells and fuse in pairs,
forming thick-walled zygospores
(Fig. 186, A). In some common
forms (Closterium and Cosma-
rium) the zygospore on germi-
nating produces two desmids.

225. The pond scums. Some
of the commonest and most
beautiful of filamentous green
algae, such as Spirogyra and Zyg-
nema, belong here. The com-
plex chromatophores with their

A, Spirogyra, illustrating stages in the Qi1firr.-iv rlifTprpntintprl iwrpnnirk

conjugation between cells of differ- sharply ditt erentiated pyrenoids
ent filaments, two zygospores shown give an especial interest to the

above; since the cells in the filament -,-, JIT ± j- 4.- • i

on the left are shorter than those on cells and helP to distinguish

the right, some of them must be left the genera. Thus the chroma-

out in the pairing of the gametes. , -, £ & • i

^, the conjugation between adjacent tophores of Spirogyra are spiral

cells of the same filament in Spirogyra bands (FigS. 168,187); Zygnema
quadrata: C.Ziiqneina(Ziiqoqoniurt%) , 111

pectinatum, showing zygospores has two star-shaped chromato-

formed in the conjugating tubes be- phores, and Mougeotid has a
tween two filaments 1,1- i i • ,1

broad, thin band in the center

of the cell. The method of sexual reproduction is very charac-
teristic, but exceptional among the algae. Generally the cells of
different filaments unite or conjugate with one another by the

FIG. 187. Pond scums

THE DIATOMS 195

fusion of processes, one put out from each gamete (Fig. 187, A).
The gamete protoplast of one of the cells in the pair then
passes over into the other, or in certain forms the two protoplasts
unite more or less midway between the two cells (Fig. 187, C).
In some species, however, there is a conjugation between adja-
cent cells of the same filament (Fig. 187, B), the contents of
one cell entering the other. The fusion of the two gamete pro-
toplasts results in a zygospore, which develops a heavy wall
about itself as in the desmids and, as a resting spore, carries
the plant over unfavorable seasons. On germination each zygo-
spore puts forth a filament, which grows by repeated cell divisions
all along its length.

ORDER 4. THE DIATOMS, OR DIATOMALES

226. The diatoms. The diatoms (order Diatomales) comprise
a remarkable group of one-celled plants, containing several thou-
sand species, everywhere present in both fresh and salt water.
They compose the greater part of the floating microscopic life,
called the plankton, of the ocean and lakes, and are the most
important source of food for some of the smaller animal forms,
and through them for the fish life. The cell walls are filled with
the mineral silica and consist of two parts, called valves, which
fit together something like the halves of a pill box (Fig. 188, A).
Diatom cells have a great variety of forms, elliptical and cir-
cular, wedge-shaped and triangular, cylindrical and rhomboidal
(see Fig. 188). The cells are solitary in many forms, but in
others are arranged in filaments, or borne at the ends of gelati-
nous stalks, or held in filamentous sheaths or jelly-like masses.
Many of the diatoms, and especially the boat-shaped forms,
glide to and fro in the water. The cells contain chromato-
phores which are generally colored brown (although certain
species are green), but in spite of this color the most natural
position of the diatoms seems to be among the green algae, with
possible distant relationships to the desmids.

196

THE ALG^E

The diatoms resemble the desmids in the similar halves of
the cell and in the development of a peculiar type of spore called
an auxospore, which probably corresponds to a zygospore. Some

auxospores are formed by
the fusion of two gamete
protoplasts which leave
the diatom shells at one
side (Fig. 188, B), and
these are true zygospores
very similar to those of
the desmids (Fig. 186, A).
Other auxospores are de-
veloped without proto-
plasmic fusions and are
probably examples of sex-
ual degeneration ; that is,
cells which were origi-
nally gametes now develop
directly into auxospores.
The auxospores are rest-
ing spores and appear to
be formed after long
periods of vegetative cell
divisions to stimulate or

FIG. 188. Diatoms

A, Navicula, the boat diatom, the side view at
the right showing the two overlapping shells

or valves; J3, Acnanthes, an auxospore with rejuvenate the protoplasm

for further activities.
The shells of diatoms,

the four empty shells of the two diatoms
whose contents united to produce this sex-
ually formed spore similar to the zygospore
of the desmids (see Closterium (Fig. 186, A) ;

C, Tabellaria, groups of cells united with being composed of silica,

one another to form a zigzag filament; D,
Licmophora, groups of cells borne on gelati-

resist solution in water

nous stalks ; ~E, Epithemia ; F, Triceratium. and are constantly being

-B, after West deposited at the bottom

of seas, lakes, ponds, and marshes, sometimes in such quantities
as to form so-called siliceous or diatomaceous earths. There are
some geological deposits (Tertiary) of diatomaceous earth many
feet in thickness, as that at Eichmond, Virginia. Some of these

THE SIPHON ALG.E 197

earths have so large a proportion of hard diatom shells as to
he valuable as polishing powders, and they are also used as the
absorbent of nitroglycerin in the manufacture of dynamite.

ORDER 5. THE SIPHON ALG^E, OR SIPHONALES

227. The siphon algae. The siphon algae (order SipJionales)
differ from all other groups of algse in the striking peculiarity
that the protoplasm, with thousands of nuclei, is not separated
into compartments or cells, but is all contained within a com-
mon filament or other cell cavity. Such a many-nucleate struc-
ture is called a ccenocyte (meaning a vessel in common). The
siphon alg« are chiefly marine, and many large and complicated
forms are found in the warmer seas (Caulerpa, Udotea, etc.).
Some of these are heavily incrusted with lime (Acetabularia,
Penicillus, Halimeda, etc.). Two-ciliate zobspores and gametes
are developed by certain types in cells cut off from the ends of
the filaments. The gametes fuse in pairs on their escape into
the water, forming zygospores. All of the siphon algse are isoga-
mous, when sexual at all, except the green felt, Vaucheria, which
stands quite alone as the only heterogamous type in the group.

228. Vaucheria. Vaucheria, the green felt, is a very common
alga, forming mats of coarse filaments on the muddy bottom -of
shallow pools and ditches. Some species are terrestrial and may
often be found as thread-like growths over the damp earth of
flowerpots in greenhouses. The filaments are long and sparingly
branched, and are, of course, continuous tubes without cross
wralls except where reproductive organs are developed. Immense
numbers of small, disk-shaped chloroplasts (Fig. 189, F) are
present in the layer of protoplasm under the cell wall, and the
very small nuclei lie among them. The protoplasm contains
numerous globules of oil, which in this plant takes the place of
starch as the first visible product of photosynthesis.

The zoospores of Vaucheria are very large, many-nucleate
and many-ciliate structures, visible to the naked eye. They are

198 THE ALG^E

developed singly in terminal cells (sporangia), which are cut off
at the ends of the filaments by the formation of a cross wall
(Fig. 189, A). The protoplasm in the sporangium contains
hundreds of nuclei from the beginning, and pairs of cilia are
formed all over the surface of the protoplast opposite them

FIG. 189. Asexual reproduction of the green felt (Vaucheria)

A, formation and discharge of the large, many-ciliate zoospore from the terminal
sporangium ; B, the zoospore showing the ciliated surface ; C\ section through
the surface of the zoospore showing the pairs of cilia above the nuclei and
the layer of plastids beneath ; D, germination of zoospore ; E, young plant of
Vaucheria, the two filaments having arisen at opposite ends of the zoospore,
one having developed an organ of attachment or holdfast h ; F, a group of
plastids, the lower in process of division. — A, B, after Gotz; 6', after Stras-
burger; D, E, after Sachs

(Fig. 189, C). The entire mass of protoplasm then slips out
from the end of the sporangium (Fig. 189, B) and swims slowly
away, but soon comes to rest and puts forth one or more fila-
ments (Fig. 189, D). The nuclei and pairs of cilia in this inter-
esting zoospore of Vaucheria unquestionably represent the two-
ciliate zoospores characteristic of most of the Siphonales. and
the green algae in general. The protoplasmic divisions neces-
sary to cut out the numerous zoospores in a sporangium are

VAUCHERIA

199

suppressed in Vaucheria, so that the entire mass of protoplasm
remains together as a many-nucleate and many-ciliate unit,
which is really a protoplast or cell in spite of its complicated
structure. Some authors have regarded this zoospore as a com-
pound structure, — that is, a mass of small, two-ciliate zoospores,

FIG. 190. Sexual reproduction of the green felt (Vaucheria)

A, Vaucheria sessilis ; o,oogonium; a, antheridium ; os, the thick-walled oospore,
and-beside it an empty antheridium ; B, Vaucheria geminata, a short lateral
branch developing a cluster of oogonia and a later stage with mature oogonia
o and empty antheridium a; C, sperms; D, germinating oospore. — C, after
Woronin ; D, after Sachs

— but it is more correct to consider it a large, undivided, many-
nucleate, protoplast.

The sexual organs of Vaucheria are oogonia and antheridia,
sometimes found side by side, as in Vaucheria sessilis (Fig.
190, A), and sometimes in groups on special side branches, as in
Vaucheria geminata (Fig. 190, .Z?). The oogonium. is a large
oval cell separated from the parent filament by a wall, and each
develops a single egg (Fig. 190, A, B, o). The young oogonium
contains numerous nuclei, but all of these degenerate except
one, which lies near the center of the cell and becomes the
single nucleus of the egg. The antheridium is a cell formed

200

THE ALG^E

FIG. 191. Botrydium and
Protosiphon

These forms of the siphon algae
are almost indistinguisha-
ble in the vegetative condi-
tion. The plants are little
green globes, somewhat
larger than pin heads, at-
tached to the surface of mud
and wet earth by a branch-
ing system of filaments
(rhizoids). These s-ingle
coenocy tic cells are therefore
differentiated into a green
part above ground, exposed
to the air and sunlight, and
a colorless underground por-
tion in contact with mois-
ture. A single cell may thus
show the same relation of
parts as a complex plant with
aerial stems and leaves and
a subterranean root system.

at the end of a branch, which is gener-
ally bent like a crook (Fig. 190, A, B, a),
and discharges a very large number of
small, two-ciliate sperms (Fig. 190, C).
The sperms enter the oogonium through
a pore in the beak-like extension at one
side, and one of them, fusing with the
egg, fertilizes it. The fertilized egg sur-
rounds itself at once with a heavy wall,
becoming an oospore (Fig. 190, A, os),
which is a resting spore in this form, as
in Volvox, CEdogonium, Coleochcete, etc.
Vauclieria has been made the sub-
ject of some interesting experimental
studies on the conditions which deter-
mine the formation of zoospores and
sexual organs respectively. Zoospores
are generally developed at once, follow-
ing some marked change in the exter-
nal conditions, as in the character of
the water, or an increase in light expo-
sure, or a rise in temperature. Sexual
organs are formed when plants are cul-
tivated in a 2-5 per cent solution of
cane sugar at a fairly high temperature
(50°-68° F. ; 10°-20° C.), and in the
presence cf light. The conditions in
Vaucheria probably illustrate very well
the kinds of influences which cause an
alga to begin its various forms of fruc-
tifications, but very few algse have been
studied in detail.

229. The coenocyte. The large, many-
nucleate structures called ccenocytes,
so well illustrated by the siphon algae

THE STONEWORTS 201

and such fungi as the molds (Sec. 261) and water molds
(Sec. 262), are peculiar to plants. The question may be asked,
Why are coenocytes considered single cells, and not a com-
pound structure made up of a united mass of protoplasts repre-
sented by the numerous nuclei ? It is known that the nuclei
do not occupy fixed positions in the coenocytes, as if they
represented the cells of a compound structure. On the con-
trary, they shift with the movements of the protoplasm which
behaves as a unit, like a gigantic protoplast growing in differ-
ent directions in obedience to various stimuli, and carrying on
the usual cell activities. For these reasons the co3nocyte must
be regarded as a many-nucleate cell and not a compound struc-
ture or mass of protoplasts.

ORDER 6. THE STONEWORTS, OR CHARALES

230. The stoneworts.* The stoneworts (order Charales) are
the most complex of the green algse. The plant body (Fig. 192, A)
consists of long, jointed stems, which bear circles of lateral
branches at the joints. The sexual organs (Fig. 192, B, C,D) are
borne on these branches, but are too complicated for considera-
tion here. Many of the Charales are heavily incrusted with
lime, which peculiarity gives them their popular name of stone-
worts. They sometimes grow in great masses attached to the
bottom of ponds and shallow lakes.

Some forms of stoneworts (Nitella), which are free from
incrusting lime, frequently illustrate very beautifully the move-
ments of protoplasm in the large cells (internodes) which lie

* To THE INSTRUCTOR : The Charales is such a highly special group that
it is hardly wise to give it much attention in a general course, especially if
time and material is available for a more thorough study of the Confervales.
Nevertheless, material of the stoneworts is frequently easily obtained, espe-
cially in the Middle West, where it is difficult to do justice to the brown and
red algse, and it might be substituted for certain work in those groups. One
of the best accounts of the Charales will be found in Goebel, Outlines of
Classification and Special Morphology of Plants.

202

THE ALGvE

between the circles of lateral
branches. The protoplasm
passes in two streams in op-
posite directions somewhat
diagonally around the cell.
The edges of the stream
form a line of stationary
protoplasm (free from chlo-
roplasts), because the move-
ments of the currents, which
may be seen on either side
of the line, neutralize one
another.

The Charales stand en-
tirely by themselves at the
end of a line of ascent whose
developmental history is a
mystery. They are very
far above most of the green
algse in the complexity of
the plant body and sex-
ual organs, which are not,
however, like those of the
liverworts and mosses. The
FIG. 192. The stonewort (Cham) antheridium of the stone-
A, portion of plant showing circles of lat- W0rts is a V61T puzzling
eral branches at the joints (nodes) ; B, a structure but the OOgonium
lateral branch bearing the sexual organs ; --i j i mi

C, the sexual organs! o, oogonium, with 1S easil7 understood. The
spirally wound filaments encircling the jointed stems with circles
egg and forming a crown above; a, the P , , , , , f

antheridium, composed of eight flattened of lateral brandies are, of
cells (shields), inclosing the antheridial course, much more COm-
filaments; I), portion of an antheridial fila- ,. -, ,, ,-. •,

ment, each cell developing a single sperm; Plated than the typical

E, two-ciiiate sperms tliallus, but the simple life

history, with no trace of an alternation of generations, makes it
necessary to include them among the thallophytes.

SUMMARY OF THE GREEN ALG^E 203

SUMMARY OF THE GREEN ALG^E

23 1\ Summary of the green algae. The green algaa comprise
a number of well-defined groups which are evidently widely
separated from one another. The most conspicuous of these are
the Volvocacece, the desmids and pond scums, the diatoms, the
siphon algse, and the stone worts. They constitute independent
evolutionary lines of varying importance, but each one clearly
developing in ways peculiar to itself and quite apart from the
theoretical main line of ascent to the liverworts and mosses
(Bryophyta). A discussion of the origin of these groups and
their possible relationships to one another would be much too
complicated for the present account. The forms of the green
algaa which seem to be nearest to the main line of ascent are in
certain related families ( Ulothricacece, Clicetoplioracece, and Coleo-
clicetacece), but it is very doubtful if any of them are directly
on the main line, and there are no living algse known which
show clearly the origin of the bryophytes.

Almost all of the green algaa at some stage in their life
history form zoospores or motile gametes (the sperm being
motile in heterogamous forms). These ciliate cells point clearly
to an ancestry of the green algaa from groups comprising one-
celled motile organisms something like the flagellates (Sec. 204)
and lower forms of the Volvocacece (Sec. 215). The formation
of the zoospore and the motile gamete is considered to be a
return in the life history, for a short time, to the primitive
one-celled conditions from which the various lines of the green
algaa are believed to have arisen. The motile conditions which
occupy practically all of the life history of the flagellates and
Volvocacece become reduced to a short reproductive period in
most of the green algae. The most important forward steps in
the evolution of the green algaa came with the introduction of
long vegetative periods in the life histories when the proto-
plasts remained quiet and formed many-celled plant bodies
(coenocytic siphon algaa excepted) of various structure. All the

204 THE ALG^E

possibilities of development into such complex attached organ-
isms as the higher spore plants and seed plants were determined
by those changes in the habits of algae by which the motile
periods in the life history became reproductive stages, and the
quiescent conditions came to be the conspicuous part of the life
history as the vegetative plant body was gradually developed.

232. Summary of the reproductive organs and processes of
the green algae. Zoo spores, also called swarm spores, are ciliate
asexual cells (generally two- or four-ciliate), and are developed
as a rule numerously (in some forms singly, or in twos, fours, etc.)
in a mother cell called a sporangium, or zoosporangium.

Gametes are sexual cells. The simplest forms are ciliate and
have the same form and structure (morphology) as the zoospores,
to which they are related. These in the process of sexual evo-
lution became differentiated into eggs and sperms. Gametes are
developed in cells called gametangia.

Eggs are never ciliated, and are consequently nonmotile.
They are generally large cells, with abundant chromatophores
and food material. Eggs are formed in cells called ob'gonia.

Sperms, frequently called antherozoids by botanists, are always
ciliated in the green algae and are very actively motile. They
are smaller than zoospores and colorless, or almost colorless.
Sperms are developed in cells called antheridia, or a group of
such cells is frequently termed an antlieridium.

Isogamy is the sexual condition in which the gametes are
similar in form and structure ; that is, they have the same mor-
phology. They may differ in size. The sexually formed cell is
called a zygospore.

Heterogamy is the sexual condition in which the gametes are
different in form and structure, as the sperm and egg, and there-
fore have a different morphology. They are always very unlike
in size, but this does not make heterogamy, because morphology
does not deal with size but with form. The egg is fertilized by
the fusion and entrance of a sperm and thus becomes a fertilized
egg, or, if it develops a protective cell wall, an oospore.

1 II

! II

'??

THE BROWN ALG^E 205

CLASS III. THE BROWN ALGLE, OR PH^OPHYCE^E

233. The brown algae. The Pliceopliycece comprise a very
large assemblage of marine algae, or seaweeds, called the brown
algae because their chromatophores are colored brown instead of
green. The brown pigment, however, performs the same sort
of work (photosynthesis) as the chlorophyll of the green algae.
The brown algae can generally be recognized at a glance by
their color, but the group is really separated from all other
classes of algae by certain peculiarities of structure, or mor-
phology. The plant bodies in most of the forms are very much
larger and more complex than those of any green algae and fre-
quently have a degree of differentiation quite above that of the
typical thallus. Indeed, some of the higher brown algae have
well-defined stems fastened to the rocks by remarkable hold-
fasts, resembling clusters of roots, and bearing expanded leaf-
like structures of complex and striking forms. Certain types
develop swollen bladders, which contain considerable oxygen,
given off from the tissues, and serve to float parts of the plant
in the water. Besides the complexity of the plant body the
brown algae are also distinguished by peculiarities of the repro-
ductive organs that can only be understood through a study
of types. Iodine is obtained from the ash of certain kelps and
rockweeds. These larger brown algae are also gathered from
the rocks and beaches by the peasantry of certain European
countries and by farmers on the New England coast and spread
over farm lands to fertilize the soil.

234. Life habits. The brown algae are most luxuriant in the
colder waters of the oceans, where they form extensive growths
along the coasts. Some of the larger forms, as Fucus and
Ascophyllum, are known as rockweeds because they cover the
rocks between low and high tide marks with heavy fringes of
brown vegetation (Plate IV). Other forms, known as the kelps,
or devil's aprons, grow below or just at low watermark and some-
times form large beds attached to the rocks. These larger types

206

THE ALG^E

can withstand the beating of the heaviest surf because of their
firm texture and strong holdfasts, and some of them grow on
the most exposed points and reefs. There are, however, many
smaller brown alga3, membranous and cord-like forms, and some

delicate filamentous
types (Ectocarpus)
which are as simple as
many green algae and
grow generally in
rather quiet waters.

We can only illus-
trate the brown algae
by representatives of
three orders, — the
Ectocarpus group, the
kelps, and the rock-
weeds.

235. Ectocarpus.
This alga (order Ecto-
carpales) is a branching
filamentous type which
forms tufts attached

to the larger alg;e, eel-

, J ,. ,

grass> and to the wood-
work ot wharves. Its

chief interest for US lies
m the reproductive or-

which are of two

FIG. 193. A filamentous brown alga (Ecto-
carpus siliculozics)

A, unilocular sporangia, one containing zoospores,
the other empty ; a zoospore, z, shown at the left ;
B, plurilocular sporangia, the larger mature, the
smaller still showing the outlines of the original
cells in the branch from which it arose; C, the
union of the gametes to form the zygospore;
note that the chromatophores with the pigment Sorts, and illustrate
spots remain separate. — C, after Oltmanns very well the reproduc-

tive processes of the lower brown algse. The asexual organs are
one-celled sporangia (Fig. 193, A), which develop large numbers
of kidney-shaped zoospores, each with a pair of cilia attached
at the side (Fig. 193, A, z). Because the zoospores are all de-
veloped in a single cell, the sporangia are called unilocular

ECTOCARPUS

207

sporangia, to distinguish them from the sexual organs, but the
structure is clearly the same as that of the one-celled sporangium
of the green algse.

The sexual organs are devel-
oped from side branches, most
of whose cells divide repeatedly
until an immense number of
small compartments are formed.
The filament thus becomes
transformed into a complicated
many-celled organ (Fig. 193, B)
made up almost wholly of small
cubical cells, each of which de-
velops a single two-ciliate ga-
mete similar to a zoospore, or,
perhaps, two or three of these
motile elements. Because the
gametes are developed in small
compartments, the organ has
been termed a plurilocular spo-
rangium. It is clear that this
many-celled organ is a very dif-
ferent sort of structure from the
one-celled reproductive organs.
It marks an important advance
in the evolution of reproductive
structures in plants and suggests
the many-celled sexual organs
characteristic of the bryophytes
and pteridophytes.

The gametes are known to
fuse in pairs (Fig. 193, C), as in

, , i

many simple green algae, and
since they have a similar struc-
ture, the sexually formed cell is

FIG. 194. Kelps from the North

Atlantic

, the simple type of Laminaria, some
of whose species grow to be thirty or
more feet long; B, the digitate type
(Laminaria digitatd), which is never
very long, but is broad at the base

208

THE ALG^E

a zygospore and the sexual condition is that of isogamy. It is
interesting to note that these motile cells sometimes germinate
without conjugation, just like the zobspores which they resem-
ble, — a fact which shows that sexuality has not become very
firmly established in the simplest of the brown algae and illus-
trates, as in Ulothrix (Sec. 217), the general conditions which are
to be expected with the origin of sex in any group of plants.

236. The kelps. The kelps (order Laminariales), also known
as the devil's aprons, are the largest types of the brown algae.
Those of the North Atlantic coast have comparatively simple

FIG. 195. The giant kelp (Macrocystis)
Adapted from Hooker and Harvey

forms (Fig. 194). There is always a stalk (stipe) attached to the
rocks by a holdfast consisting of a cluster of strong outgrowths,
and the stalk bears a long, leaf-like expansion called the Made.
The blades of some kelps are divided lengthwise into segments,
as in Laminaria digitata (Fig. 194, B).

Certain kelps of the Pacific coast are much more complex,
consisting of numerous large, leaf-like blades variously arranged
on different forms of stems. Some of the stems attain great
lengths. Thus the giant kelp (Macrocystis, Fig. 195) has been
reported six hundred to nine hundred feet long, which is two
or three times the height of the giant redwoods of California.
The sea otter's cabbage (Nereocystis, Fig. 196) frequently has a
stem more than one hundred feet long, which is enlarged above

THE KELPS 209

into a hollow float that rests on the surface of the water and
bears a number of strap-shaped leaves. The sea palm (Postelsia,
Fig. 197) is another remarkable form, with a thick, strong stalk
about a foot high, which bears a crown of small leaves and
somewhat resembles a palm tree in miniature. Macrocystis and

FIG. 196 FIG. 197

FIG. 196. The sea otter's cabbage (Nereocystis)
Adapted from Postels and Ruprecht

FIG. 197. The sea palm (Postelsia)

Nereocystis grow in deep water, but the sea palms are found on
the rocks, where the surf breaks so heavily that the tough
elastic stems are bent over at right angles by the force of
every wave.

The kelps reproduce by zob'spores developed in one-celled
sporangia that are formed in large patches upon the leaves.

210

THE ALG^E

There is no method of sexual reproduction known, and this
is remarkable, for groups as highly developed and eminently
successful as the kelps almost always have well-established

and complex methods
of sexual reproduction.
237. The rock-
weeds. The rock-
weeds (order Fucales)
are the highest forms
of the brown alga3,
both in vegetative
structure and because
of the complex sexual
conditions (heterog-
amy), with character-
istic eggs and sperms.
The commonest
genus is Fucus (Fig.
198), which is very
widely distributed in
the colder seas and
forms the bulk of the
algal vegetation be-
tween tide marks
(Plate IV). The plant
body of Fucus forks
very regularly (dichot-
omous branching),
and the growth is from
a region of cells situ-
ated in a pit at the end

EIG. 198. A rockweed (Fucus vesiculosus)

A, habit sketch, showing the forking of the branches ;
6, air bladders; r, swollen fruiting tips (recepta-
cles) , with the sunken cavities (conceptacles) which The Sexual

of each branch (Fig.

\
? p).

contain the sexual organs ; p, pit at a growing
point. JB, base of a plant ; h, holdfast

arise from the sides

FIG. 199. The sexual organs of a rockweed (Fucus vesiculosus)

A, section of a female conceptacle with oogonia, showing the hairs which pro-
ject through the opening of the conceptacle into the water, and the loose net-
work of filaments in the interior of the plant; B, mature oogon,ium containing
eight eggs; (7, the discharge of the eggs from an oogonium; D, a group of
antheridia on the hranching filaments which grow in tufts over the sides and
bottom of the male conceptacles ; E, sperms very highly magnified, showing
elongated form and the two cilia at the side ; F, an egg lying free in the water
and surrounded by sperms. — B, (7, D, E, F, after Thuret

211

212

THE ALG^E

and bottom of small cavities, called conceptacles (Fig. 199, A),
which are developed in swollen tips of older branches termed
receptacles (Fig. 198, r). Some species of Fucus (as F. edentatus)
have both male and female organs in the same conceptacle, but
in other species they are formed in different conceptacles, and
even upon different plants, as in F. vesiculosus. The female organ
is a large cell, or oogonium (Fig. 199, B, C}, which in Fucus
develops eight eggs. The male organs, antheridia, are also single
cells (Fig. 199, D), but they are generally borne in dense clusters
upon branching stalks, and each produces more than a hundred
very small sperms with two cilia at the side
(Fig. 199, E). The eggs and sperms are
forced out of the conceptacles by the swell-
ing of mucilage that is developed within
the structure, aided by the contraction of
the tissue when the plants are exposed at
low tide to the drying action of the air.

The eggs are fertilized in the sea water
outside of the conceptacles. The sperms
gather around an egg in great numbers
(Fig. 199, F), making it revolve, and finally
one enters. The male nucleus passes rap-
idly to the center of the egg and in a few

minutes begins to fuse with the female
FIG. 200. Sargassum , , ., . .

Filipendula nucleus. The striking differences m size

Tipofpiantshowingieaf- and structure between the large egg and
like lateral branches, minute sperm make the sexual process of
hollow fl^a'ts^and the Fiicus one of the best illustrations of heter-
f ruiting branches, or ogamy in the plant kingdom. Such ferti-
lized eggs as are fortunate enough to find
favorable resting places begin to germinate within twenty-four
hours, and develop directly into young Fucus plants.

238. Sargassum. Sargassum (Fig. 200) is one of the Fucales
that deserves special mention for the complexity of its plant
body, which bears three forms of lateral structures: (I) thin,

THE &ED ALG^E 213

leaf-like branches which resemble foliage, (2) berry-like floats,
and (3) small reproductive branches, or receptacles (Fig. 200, r).
Some species of Sargassum, when torn away from their at-
tachment to rocks, are able to vegetate in the open sea, where
they are called gulf weed, but they are not known to fruit in
the free floating condition. Certain ocean currents carry and
accumulate immense quantities of this floating gulf weed in
great eddies in the ocean, forming the Sargasso seas.

239. Summary of the brown algae. The Phceophycece stand
entirely apart from the green algae as a side line of plant evolu-
tion. There is much evidence that it is a group of very ancient
origin, probably arising from an ancestry of motile organisms
(somewhat like the flagellates) just as did the green algae in
early geological ages. The brown algae have developed in their
own peculiar ways the largest and most complex forms of plant
bodies in the thallophytes, and also some very high types of
sexual reproduction. It is clear, however, that these have no
relation to higher plants, bryophytes and pteridophytes, and are
also entirely independent of other groups of algae. Thus heter-
ogamy in the brown algae has been developed entirely independ-
ently of heterogamy in the green algae, illustrating very well
how similar results may be worked out through different evo-
lutionary lines independently of one another.

CLASS IV. THE RED ALGLE, OR RHODOPHYCEJE

240. The red algae.* The Rliodopliycece include the most
beautiful of the marine algae, for many of the forms are exqui-
sitely colored in clear shades of red, and are extremely delicate
in structure. Other forms are brownish red or purplish, and cer-
tain types are greenish. The pigment is held in chromatophores,

* To THE INSTRUCTOR : In a brief course where only one type can be
studied in the laboratory, Nemalion or Batrachospermum is preferable, fol-
lowed by a study of the life habits of the group and demonstrations of
herbarium material.

214

THE ALG^E

and performs the photosynthetic work of the plant. Although
the red algae can generally be distinguished by their color,
the fundamental characters of the group are based on the
structure of the sexual organs and certain complications in
the life histories which will be explained in the accounts of
Nemalion and Polysiplionia. The plant body of the red algaa

ranges from filamentous types
of great delicacy (as Callitliam-
nion) to such coarse forms as
the Irish moss (Chondrus) and
the dulse (Rhodymenia). It is a
remarkable fact that the large
types are really composed of
complicated systems of filaments
so closely associated, however,
as to give the appearance of a
cell tissue. Adjacent cells in
the same filament are usually
connected by strands of proto-
plasm, a striking feature of the

group. Some of the red alga?,
FIG. 201. The Irish moss (Chondrus as the Irigh mogs and the dulsej

are eaten, and a number of them

About one half natural size; the shaded , ,. , 1,1

spots are sexually formed fruits, or are used as relishes by the na-
cystocarps tives of the Hawaiian Islands,

China, and Japan. Certain forms have an economic value for
gelatin, which is obtained from their tissues ; thus agar-agar
comes from the stems of a red alga (Gracilaria) which grows
in the seas of the Orient.

241. Life habits. The life habits of the red algae are in strik-
ing contrast to those of the brown. They prefer warmer waters,
and the best displays are on such coasts as the Mediterranean,
the islands of the West Indies, southern California, and Aus-
tralia, They generally nourish in deeper waters than the green
and brown algae, and form the greater part of the seaweed growth

THE DISTRIBUTION OF ALGJE ON ROCKS

215

below the fringe of green and brown which is frequently quite
conspicuous on rocks near low-water mark. Some of the forms
are found at depths of two hundred feet or more, the depth limit
varying with the clearness of the water. Most of the red algae
seem to prefer shaded situations among the rocks, and it is
probable that their characteristic color is associated with these
9

FIG. 202. The distribution of conspicuous algae on two rocks of Spindle
Ledge, Woods Hole, Massachusetts, in September, 1905

The dotted line is low-water mark, and the rocks are completely covered at high
tide. Blue-green algae: 1, Calothrix scopulorum, on the highest part of the
rock. Green algae : 2, Ulva lactuca var. rigida (sea lettuce) ; 3, Enteromorpha
prolifera (sea lettuce) ; both forms grow well above low-water mark. Brown
algae: 4, Fucusvesiculosus(rockweed), in patches, but not plentiful in thesummer;
5, Chordaria flagelliformis (shoestrings), heavy growths, well below low-water
mark. Red algae: (5, Nemalion multiftdum, on the higher parts of the rock;
7, Ceramium rubrum, a well-marked fringe at low-water mark; 8, Polysi-
phonia violacea, a well-marked fringe just below low-water mark ; 9, Chondrus
crispus (Irish moss) , large patches from one to three feet below low-water mark

peculiar subdued light relations, so different from those of other
algse. Some types are incrusted with lime and form the curious
growths on rocks called corallines.

242. The distribution of algae on rocks. Many seaweeds are
only found in certain situations upon rocks, where they grow in
patches and fringes, and frequently exhibit a sort of zonation
somewhat similar to the distribution of plant life around the

216 THE ALG^E

•

margins of ponds and lakes. Fig. 202 presents a diagram of
such distribution on two isolated rocks. It will be seen that
there is a clear zone of a red alga (Polysiphonia) just below
low-water mark, another zone (Ceramium) 'at or a little above
this mark, while the Irish moss grows at some depth. The
sea lettuces and rockweed are well above low-water mark, as
is also Nemalion, which is exceptional in its habits for a red
alga. On the northerly New England coast and beyond there
are usually two distinct zones on the rocks, — one well above
low-water mark, composed chiefly of rockweeds, and the other
near this point, but below, and made up mostly of Irish moss
with other red algae, including the dulse.

243. Nemalion. Nemalion illustrates excellently the structure
of the sexual organs and the sexually formed fructification of
the red algee, called the cystocarp (meaning a fruit cavity). The
plant body is a rather soft, cord-like, branching structure, com-
posed of an immense number of filaments held together by a
stiff gelatinous substance around the cells. There is a central
axis of delicate threads,' while the outer regions consist of short
filaments pointing outward. The cells of the outer filaments
contain each a single chromatophore, and the vegetative activi-
ties (photosynthesis) as well as the reproductive take place in
this region of the plant.

The male organs, or antheridia, consist of clusters of small cells
at the surface of the plant (Fig. 203, A], each of which develops
a single sperm, spherical in form .and without cilia, and con-
sequently nonmotile. The female organ (Fig. 203, B) is devel-
oped at the end of a short branch and consists of a cell which
bears a long, hair-like extension called the tricliogyne (meaning
female hair), which is the receptive organ for the sperms. The
sperms are applied to the trichogynes largely by the contact of
male plants with the female as they are washed about by the
movements of the water. When a sperm fuses with the tri-
chogyne its nucleus (male) passes down into the swollen base of
the female cell and unites there with a female nucleus. The

NEMALION

217

trichogyne then withers above the fertilized female cell. The
female cell is called the carpogonium, but it corresponds ex-
actly to an oogonium, and indeed resembles very closely the
oogonium of some species of Cole-
ocliccte (Sec. 222). Its peculiar
form, with a receptive organ, the
trichogyne, is undoubtedly asso-
ciated with the nonmotile habits
of the sperms. The red algae are
clearly heterogamous in their
methods of sexual reproduction.

The female cell, or carpogoni-
um, after fertilization, gives rise
to a dense cluster of short fila-
ments, called fertile filaments,
which all together form a globular
fructification called the cystocarp
(Fig. 203,Z>). The terminal cells
of the fertile filaments become
spores, termed carpospores, which
develop new Nemalion plants.
The cystocarp is clearly a new
type of fructification in the algae.
It is a structure which begins
with the fertilization of the carpo-
gonium and ends with the forma-
tion of carpospores, and thus
stands as a phase in the life his-
tory inserted between two gener-
ations of the sexual plants.

244. Batrachospermum.
Batrachospcrmum is one of the
few fresh-water forms of the red
algaB, and is also an exceptional
type for its color, which is

FIG. 203. Nemalion multifidum

J[,antheridia, consisting of groups of
small cells, each of which develops
a single sperm; the vegetative
branch at the right illustrates the
method of terminal growth and the
protoplasmic connections between
the cells. B, the female cell, or car-
pogonium, c, with its trichogyne,
t, to which are attached three
sperms. C, early stage in the de-
velopment of the cystocarp; the
trichogyne above has begun to
wither. D, mature cystocarp com-
posed of fertile filaments which
develop the carpospores cs termi-
nally ; wt, withered trichogyne

218 THE ALG^E

generally some shade of green. The sexual organs and cystocarps
are enough like those of Nemalion to make it as good a form to
illustrate the sexual processes and life history of the red algae
as the latter type, and it is sometimes more available for inland
classes. Batrachospermum grows in clear brooks and is generally
found in its best condition in late winter and early spring.

245. Polysiphonia. Polysiphonia illustrates some further
complexities in the life history of the red algse which are not
present in Nemalion and Batracliospermum. The filaments of
these beautiful plants consist of rows of cells, called siphons,
connected with one another in an elaborate manner. There is
a central siphon, around which are arranged a circle of outer
siphons variable in number for different species.

The sexual organs are found 011 separate plants. The male
organs, antheridia, are modified branches (Fig. 204, A) that de-
velop an outer covering of small cells which form the sperms
(Fig. 204, B). The female organ is found on a small branch
(Fig. 204, C) and consists of a carpogonium, with its trichogyne,
accompanied by a number of vegetative cells which later take
part in the development of the cystocarp. The fusion of a sperm
with the trichogyne fertilizes the carpogonium as in Nemalion.
There are two sets of activities concerned with the development
of the cystocarp: (1) there are some remarkable cell unions
between the fertilized carpogonium and neighboring cells (aux-
iliary cells) for nutritive purposes, and then the development of
carpospores from the large fusion cell which is formed ; (2) ac-
companying this activity there is the development of an urn-
shaped envelope (Fig. 204, D), from some of the vegetative cells
around the carpogonium, and this is clearly a protective structure
to contain the carpospores. The first set of activities corresponds
to the development of the simple cystocarp of Nemalion. The
second set forms the additional urn-shaped protective case. The
cystocarp of Polysiphonia is therefore a system of two tissues,
one derived from the fertilized carpogonium, and the other from
the vegetative cells of the parent plant.

POLYSIPHONIA

219

Besides the sexual plants (male and female) there is an asexual
condition in Polysiplionia called the tetrasporic plant. Tetra-
sporic plants are individuals which develop asexual spores, called
tetraspores because they are formed in groups of four, termed
tetrads, in mother cells (Fig. 204, E, F}. The tetraspore mother
cells arise from the central siphon near the ends of the branches.

FIG. 204. Polysiplionia violacea

A, tip of filament showing two antheridia, a; B, cross section of a portion of an
antheridium illustrating the development of the sperms at the ends of the very
numerous short branches ; C, a procarp with the projecting trichogyne t, from
the female cell (carpogonium), which is hidden by the surrounding sterile cells;
D, mature cystocarp with the urn-shaped envelope inclosing the cluster of
carpospores, a single spore shown at the right; E, a short branch from a tetra-
sporic plant ; F, two groups of tetraspores from a branch similar to E ; note
the peculiar arrangement of the tetraspores in a group of four, or tetrad

Some recent investigations clearly indicate that the tetrasporic
plants come from carpospores, and that the tetraspores develop
into sexual plants. So there is an alternation of sexual and
tetrasporic plants in the life history of Polysiplionia.

246. Summary of the red algae. It is quite certain that the
red algae have had their origin from a very much higher level

220 THE

than the green or brown algae because of the complicated sexual
organs and life histories and the absence of motile stages repre-
sented by the zoospores and motile gametes of those groups.
The red algae resemble ColeocJicete (Sec. 222) in a number of
features, and it is possible that this type may be rather close to
the starting point of the group. The peculiar structure of the
female sexual organ (carpogonium), which is really anoogonium
with a receptive organ (trichogyne), is undoubtedly associated
with the loss of motility on the part of the sperms. But the
most remarkable peculiarity is the development from the ferti-
lized carpogonium of a tissue which produces carpospores. This
structure (from the fertilized carpogonium to the carpospores)
is a new phase in the life history of algae, and together with
protective envelopes, when present, constitutes the cystocarp. It
is probable that the asexual tetrasporic plants found in most
species of the red algae arise from the carpospores, and that the
sexual plants in these types are developed from the tetraspores.
The structure developing from the fertilized carpogonium and
ending with the carpospore, together with the tetrasporic plant,
when present, therefore forms an asexual phase in the life history,
alternating with the sexual plants. Such asexual phases are
called sporophytes (meaning spore-bearing plants), to distinguish
them from the sexual plants, called gametophytes (meaning
gamete-bearing plants), and their following one after another
in a life history constitutes an alternation of generations. Such
an alternation of generations is found in very few groups of
the thallophytes, but it is characteristic of the life histories
of all higher groups beginning with the liverworts and mosses
(Sec. 285). Its significance is discussed in Chapter xxvi.

CHAPTEK XXI

SUMMARY OF THE LIFE HISTORIES AND EVOLUTION OF
THE ALGJE

247. The life histories of the algae. The life history of a
plant is the succession of stages leading from one generation to
another and of course includes the reproductive periods. Kepro-
duction may be as simple a process as the breaking off of
portions from the parent plant, called vegetative reproduction.
Almost all groups of plants have developed some forms of
vegetative reproduction. The detached portions may be merely
fragments, as in Oscillatoria (Sec. 209), or much more compli-
cated, as certain bud-like structures in some of the brown and
red algae. Methods of vegetative reproduction give very simple
life histories, which are merely a succession of similar forms
such as may be represented by the formula

P-P-P-P, etc.,
P standing for the plant type.

The commonest methods of reproduction in the algae are
through the special cells called spores, which may be asexual
in character or formed sexually. The commonest form of spore
reproduction in the algae is through the zoospore. When there
is no sexual process in the life history, but some method of
asexual spore reproduction, the formula of the life history
becomes p _ asex^ s _ P _ asex^ s _ P etc ?

asex. s. standing for asexual spore.

The development of sex in a plant complicates at once the
life history. Gametes are formed, which unite to give sexually
formed cells or spores. These spores may develop directly into
plants like the parents, as in Splicerella and Volvox, Spirogyra,

221

222 LIFE HISTORIES AND EVOLUTION OF THE ALG^E

Vaucheria, Chara, and Fucus, or they may form zoospores, which
swim off and grow at once into plants like the parents, as in
CEdogonium, Ulothrix, and Coleochcete, the latter type developing
the zoospores somewhat indirectly through a group of cells. But
in all these forms the essentials of the life history are expressed
by the formula

^>sex. s. — P^^>sex. s. — P, etc.,
^ ^9

g and sex. s. standing for gamete and sexually formed spore,
respectively.

It must always be remembered that the algae with sexual
methods of reproduction frequently have also asexual zoospores
or other forms of asexual reproduction, which may produce a
number of successive generations between the development of
sexual plants. This happens very frequently among the green
algae (as in Ulothrix, CEdogonium, Coleochcete, and Vaucheria)
and in the lower brown algae (as Ectocarpus). The life history
formula of the sexual algae may then be variously broken by the
introduction of successive generations developed asexually.

In the red algae the sexually formed cell corresponding to
an oospore does not give rise at once to a generation like the
parent plant, but an asexual generation is inserted between
successive sexual plants, alternating with them. This asexual
phase may be represented by the tissue which produces the car-
pospores within the cystocarp, or it may be represented by this
structure plus the tetrasporic plant. There is, then, in the red
algae an alternation of generations, an asexual phase, or sporo-
phyte (the cystocarp and tetrasporic plant), alternating with the
sexual plant, or gametopliyte. This is the most complex type of
life history in the algae and may be expressed by the formula

/cystocarp and \ /carpospore andx

G <C Q > — Si tetrasporic plant, 1 — asex. s.( tetraspore,

\when present / \when present

> — S — asex. s. — G, etc.,
9

THE EVOLUTION OF SEX

223

G and S standing for the gametophyte and sporophyte gener-
ations, respectively. It will appear later that the higher plants
have essentially the same life history formula as this.

248. The evolution of sex. The account of the algae has
given material for a brief discussion of the origin and evolution
of sex. It has been shown that the simplest forms of sexual
cells, or gametes, have essentially the same structure and origin
as the zoospores. These conditions are illustrated by Sphcerella,
Ulotlirix, Ulva, and Cladophora. The difference between the
gamete and zoospore is chiefly one of size. The gametes are
smaller because they are generally formed more numerously in
their mother cells. Gametes sometimes are able to germinate
like zoospores, but such gametes are
apt to develop small and weak plants,
and as a rule they must fuse with
one another in pairs in order to live.
It seems clear that sex arose with
the development of a type of zoospore
smaller and apparently weaker in its
power of vegetative growth than the
normal zoospore. These smaller zoo-
spores formed the habit of fusing in
pairs, and this habit, finally becoming
fixed in the plant's life history, de-
veloped into a method of sexual repro-
duction.

After the establishment of sex in a FlG- 205- Cutleria mulffida
group of plants, further developments A> the larse female gamete;

. />, the same at rest and sur-

will tend to modify the form of the

gametes, the process finally ending in

their differentiation into eggs and

sperms. The simplest gametes are so similar in form and size

that they cannot be distinguished as male and female, but

a number of algse have gametes which are different in size,

although similar in structure, or morphology. This condition

rounded by small male ga-
metes, one of which, tn, is
shown in the act of fusion

224 LIFE HISTORIES AND EVOLUTION OF THE ALG^E

has been noted in species of Chlamydomonas (Fig. 177), and some
other green algse (as the siphon alga Bryopsis) show it, while
among the brown algse certain species of Ectocarpus and Cut-
leria (Fig. 205) furnish especially good illustrations. The larger
gamete is female and often has a relatively short motile period,
being fertilized when at rest by the smaller male gamete, thus
resembling an egg (Fig. 205, 7?). These are transitional condi-
tions leading towards the highest types of gametes, — the egg
and sperm. The term isogamy (similar gametes) is applied to
sexual conditions when the gametes are similar in form, —
that is, have the same structure, or morphology, even though
they may be very different in size. The sexually formed cell is
called a zygofepore.

Heterogamy (dissimilar gametes) is the sexual condition in
which the gametes are unlike inform, — that is, have a dif-
ferent structure, or morphology, one being the larger nonnio-
tile egg, and the other the small, specialized, motile sperm. The
sexually formed cell is called an oospore, or the egg is said to
be fertilized after the union with the sperm. Several eggs may
be formed in the mother cell, or oogonium, as in Fucus, but
they are generally developed singly. This latter condition is the
result of evolutionary processes by which all of the protoplasm
and nutritive material in the oogonium is preserved for a single
egg, thus giving it all the energy and power of growth possible.
The sperms, on the contrary, are frequently developed in very
great numbers in their parent cells, and are consequently small,
and must die quickly if they are unable to fertilize eggs.

It is very important to note that the principles affecting the
evolution of sex are always at work and have undoubtedly
operated separately in various groups of plants. Thus heter-
ogamy has developed independently in the lines of the green
algse, ending in Volvox, CEdogonium, Coleochcete, Vaucheria, and
Char a, and in the rockweeds (Fucales) as well. Heterogamy
is the highest point of sexual evolution, but plants above the
thallophytes show some important advances in their methods

THE EVOLUTION OF THE ALG^ 225

of protecting the egg. It will appear later that the eggs of liver-
worts, mosses, and ferns are retained in special, protective, cellular
structures called arcliegonia, which are the female reproduc-
tive organs. The presence of this organ is one of the important
characters of these groups (bryophytes and pteridophytes), and
its absence is one of the peculiarities of the thallophytes.

249. The evolution of the algae. The evolution of the algae
is the result of many factors which affect their life habits and
life histories. Sexual processes have been the chief factors
modifying life histories, for they are always a stimulus to devel-
opment, and have been the starting points for some of the most
important complications in the life histories and developments
of groups. The most conspicuous illustration of this principle
appears in the red algae, where an asexual generation follows the
sexual process, and similar conditions are present in a peculiar
group of the brown algae represented by Dictyota.

One of the most clearly marked evolutionary principles illus-
trated in the algae is the tendency to establish fixed or attached
forms and to limit the motile stages in the life history to the re-
productive cells (zoospores or gametes). Some of the lower algae
are motile throughout almost all of their life histories, as in the
Volvocacece (Sec. 215) and that group of uncertain relationships,
the flagellates (Sec. 204). But the motile stages become merely
reproductive phases in the higher forms. Thus the appearance
of zoospores and motile gametes in the life histories of higher
types of algae is believed to represent a return for a short time
to the motile conditions and habits of their ancestors.

The establishment of attached plant bodies opened immense
possibilities of plant development, and resulted at once in a
great variety of structures. The first of these were simple forms
of thalli, such as filaments, membranes, and plates of cells. But
later the plant structures became more complex, developing
holdfasts and stems, which bore leaf-like lateral structures,
evidently differentiated to give a large exposure to sunlight,
and for the work of photosynthesis. Thus types of plant bodies

226 LIFE HISTORIES AND EVOLUTION OF THE

arose which were more complex than the thallus, since they
showed three regions, — stems, leaf-like blades, and holdfasts.
We shall see later that the stems, blades, and holdfasts of the
highest algae do not correspond to the stems, leaves, and roots of
fern and seed plant, which developed very much later through
a complicated history. But this differentiation of the plant body
in the thallophytes is, at least, a response to the same sort
of influences as guided the development of the higher plants.
These influences were in part the evident advantages to a plant
of being fastened to a suitable attachment, from which it can
grow and present as much surface as possible to the sunlight.

In conclusion, one should think of the algae as comprising a
large number of divergent lines, whose relationships are some-
times so distant that one cannot make even a good guess as to
the evolutionary history. The stoneworts (Char ales} constitute
perhaps the best illustration of such an isolated group. Very
few of the algae now living are near the theoretical main line
of ascent to the liverworts and mosses. The algae should be
thought of as spreading out in many directions, each group
developing in its own particular line of evolution, adjusting
itself as best it may to its particular sort of life. Some pos-
sible relationships have been suggested in the accounts of the
various groups, but the subject is too complex to be given
detailed consideration here.

CHAPTER XXII

THE FUNGI AND THEIR RELATION TO FERMENTATION
AND DISEASE

250. The fungi.* The fungi are thallophytes whose plant
bodies have no chlorophyll or other coloring matter capable
of doing the work of photosynthesis. Consequently fungi are
unable to manufacture the primary foods of plants, such as
starch, and are absolutely dependent upon organic substances
obtained from animals and plants. Fungi must therefore live
either as parasites upon living plants or animals, called their
hosts, or as saprophytes (meaning decay plants) upon dead organic
matter or the products of decay. Fungi are frequently spoken of
as colorless plants, because they have no chlorophyll, but many
forms are brilliantly colored by special pigments.

The fungi have undoubtedly been derived from the algae,—
not from a single group of the algae, however, but from several
widely separated groups. Consequently the classes of the fungi
have not developed one from another, but in most cases are
believed to be either of entirely independent origin or of very
remote relationship through ancient forms of algae no longer
living. The chief peculiarities of the structures and life histo-
ries of fungi are largely the results of their adaptations to lives
of parasitism or saprophytism. One of the results of these adap-
tations has been the development of a much greater number of
species than is found in the algae.

* To THE INSTRUCTOR : As in the account of the algae, this chapter describes
more forms than should be given in a general course. Many of them must
be omitted or merely discussed in the class. They have been included in order
to provide a range of material for selection adaptable to various sections of
the country and the different conditions under which the subject must be
presented.

227

228 THE FUNGI

We shall consider five classes in the series of the fungi among
the thallophytes (see Outline of Classification, p. 155).

Class V. The bacteria, or Schizomycetes.
Class VI. The yeasts, or Saccharomycetes.
Class VII. The alga-like fungi, or Phycomycetes.
Class VIII. The sac fungi, or Ascomycetes.
Class IX. The basidia fungi, or Basidioinycetes.

CLASS V. THE BACTERIA, OR SCHIZOMYCETES

251. The bacteria. The bacteria are the smallest living
beings known. The single cells of many species are less than
one ten thousandth of an inch in diameter, and some are very
much smaller still. Most of the bacteria are one-celled. Some
types are spherical or oval, some are straight or. slightly bent
rods, and some are spirally twisted forms of various lengths
(Fig. 206). Certain species are provided with numerous cilia and
are actively motile. The cells may be loosely joined together
in chains or collected in jelly-like masses or colonies, which are
sometimes brightly colored, yellow, red, blue, or green. Some of
the bacteria are filamentous and made up of rows of cells. The
cells are very simple in structure, since they do not have a
clearly defined nucleus, and in this important respect they
resemble the blue-green algce, from which they are believed to
have been derived (Sec. 211).

The cells of the bacteria multiply by simply splitting apart,
which gives them their name of Schizomycetes, or fission fungi.
These cell divisions, under favorable conditions, take place in
some forms as frequently as once every half hour, and the de-
scendants from a single individual may number many millions
in a few days. The bacteria are only limited in their remarkable
powers of multiplication by lack of food or other unfavorable
conditions. Many bacteria have the power of developing thick-
walled resting cells, or spores, within the parent cell, which can
survive a temperature above the boiling point of water and also

FIG. 206. Groups of bacteria stained to show their cilia

Bacillus subtilis, an organism of decay characteristic of hay infusions ; B, Bacil-
lus typhi, the germ of typhoid fever; C, Bacillus vulgaris, single cells and
filaments ; D, Planococcus citreus, which forms yellow colonies on various sub-
strata ; E, Pseudomonas syncyanea, which turns milk blue ; F, other species
of Pseudomonas ; G, Microspira comma, the germ of Asiatic cholera ; H, Spi-
rittum undula, in water containing decaying fish, alga?, etc. ; /, another species
of Spirillum. — After Migula

229

230 THE FUNGI

below freezing, and are able to live for very long periods. But
other forms, as the Bacillus of typhoid fever (Fig. 206, B), may
be certainly killed within a few minutes by boiling the water
in which they live. Certain bacteria, as the species which pro-
duce lockjaw and cause butter to become rancid, will live with-
out air, and are even injured by contact with free oxygen. They
obtain the oxygen necessary for respiration from compounds,
such as the carbohydrates, which contain it.

The bacteria are present almost everywiiere, floating in the
air on particles of dust, in the water, in the soil, and always
living within and upon the bodies of animals. Thus the bacte-
ria are ready to grow and multiply wherever they find favorable
conditions, but these are exceedingly various for the different
species. Some forms are restricted to a parasitic life on particu-
lar hosts, as certain animals or plants, or man. Other types are
connected with special chemical reactions, as in the processes of
decay, fermentation, nitrification, etc. Many bacteria are indis-
pensable to life on the earth, and of the greatest service to man.
Many forms are harmless, but of no special value to man.
Some cause dangerous contagious diseases.

252. Decay. Decay is the destruction or decomposition of
highly complex organic compounds, such as the proteids, fats,
sugars, and cell walls of plants, by which they are broken down
into successively simpler substances, and finally into fluids and
gases, some of which are very ill smelling. The products of
decomposition form various chemical combinations, and are
finally used again in the constructive processes of life. The bac-
teria and other fungi are the chief agents of decay, and if it
were not for them the world would soon be filled with organic
waste products, together with the dead bodies of animals and
plants of no value as food. Thus all the chemical elements
capable of sustaining life would long ago have been used up and
life on the earth would have ceased. The bacteria are there-
fore chiefly responsible for a circulation of elements (see dia-
gram, Fig. 207), from the highly complex organic compounds of

DECAY 231

animals and plants back to the simpler substances from which
green plants manufacture their food and build up protoplasm.

Food may be kept indefinitely when under conditions that
hinder the growth of bacteria, as in cold storage. The exclusion
of all bacteria from hermetically sealed tinned foods, in which
all germs have been previously killed by heat, is the chief prin-
ciple in the success of the canning industry. The agreeable
flavors of high-grade butter and certain cheeses, as well as the
gamy taste of meat, are largely due to bacteria, and really indi-
cate the first stages in the process of decay, although usually not
at all harmful or distasteful. Not infrequently, however, incip-
ient putrefaction forms certain organic poisons, called ptomaines,
in nitrogenous foods, and these may give rise to distressing
symptoms, or even prove fatal to the consumer.

253. Fermentation. Decay rnay take place in two very dif-
ferent classes of substances : (1) the carbohydrates, such as cel-
lulose, starch, sugar, etc., and (2) the proteids or nitrogenous
substances that make up protoplasm, flesh, and many food prod-
ucts. The breaking down of the carbohydrates is called fermen-
tation, and many other fungi besides the bacteria are concerned
with the process. Yeasts, for example, are the most important
organisms in the fermentation of sugar, and the decay of cell
walls in timber is chiefly due to some of the higher fungi.

The best-known types of fermentation are the alcoholic and
the acid. Alcoholic fermentation involves the change of sugars
to alcohols, accompanied by the formation of large quantities of
carbon dioxide, and will be considered more especially in the
account of the yeasts. Acid fermentation is the transformation
of sugars and alcohols into organic acids, and bacteria play the
most important part in this process. Thus the change of cider
to vinegar is one of sugars and alcohols into acetic acid, and
the souring of milk is the formation of lactic acid from milk
sugar. Both processes are caused by bacteria. There are a
number of stages in the processes of fermentation. For exam-
ple, cellulose is first changed into some kind of sugar, and this

232

THE FUNGI

later into alcohols and organic acids. The last stages result in
the formation of the gas carbon dioxide (C02) and sometimes
marsh gas (CH4), which, when mixed with hydrogen phosphide,
becomes the " will-o'-the-wisp " of swamps.

FIG. 207. Diagram illustrating the circulation of nitrogen

Nitrogen is taken by green plants from the nitrates, and through energy derived
from the sunlight the proteids are formed. Animals carry the process of
proteid manufacture somewhat farther. The nitrogen of the proteids is then
returned, through the decay of waste products (urea, etc.) and dead tissues, to
simpler substances, and finally to ammonia, which is worked over into nitrates
by the nitrifying bacteria. Free nitrogen is brought into the circle by the
nitrogen-fixing symbiotic bacteria

Some important forms of fermentation have no connection
with living organisms, but are due to special substances called
unorganized ferments, or enzymes (Sec. 10). Such a ferment is
diastase, which converts starch to sugar.

THE CIRCULATION OF NITROGEN 233

254. Nitrification. The decay of proteid matter involves,
first, the change of the insoluble proteids into soluble substances
called peptones, — a similar process to that of digestion in the
stomach. This liquefaction is due to the secretion of special
ferments by certain bacteria. Then follow further complicated
changes until the nitrogenous substances are broken down, and
ammonia (NH3), a relatively simple compouud,is formed, together
with various organic acids and other compounds. Two forms of
bacteria which are abundant in almost all soils cooperate to trans-
form the ammonia first into nitrous acid, and then into nitric
acid, the latter forming at once nitrates, or salts of nitric acid.
The nitrates are the chief source of the nitrogen supply of green
plants. The process by which the ammonia of decay becomes
available through the nitrates for plant use instead of passing
into the air is called nitrification.

255. The circulation of nitrogen. There is a circulation of
nitrogen in nature, which is indicated in the diagram (Fig. 207).
This circulation starts with the nitrates, which are taken up
in solution by the cells of green plants, — in the higher plants,
of course, through the root system. The nitrogen in the nitrates
is combined with carbon compounds obtained from the carbo-
hydrate food manufactured by the processes of photosynthesis.
Hydrogen, oxygen, sulphur, and often phosphorus also enter
into the resultant substances, which are proteids. The energy
which makes possible this building up of the complex proteids
comes from the sunlight, as is indicated in the diagram. Animals
are able to carry the building-up processes somewhat higher,
obtaining their energy from food which comes directly or indi-
rectly from plants. Then the breaking-down process begins
through the decay of nitrogenous waste products and of dead
matter, and this is accomplished as described in the previous
sections through the activities of fungi and chiefly the bacteria.
Finally, simple ammonia is produced, and this, by the process of
nitrification, enters into the formation of nitrates, and the nitro-
gen is then available again for green plants.

234 THE FUNGI

256. The fixation of free nitrogen. One of the most impor-
tant relations of bacteria to agriculture and to plant life gener-
ally lies in the ability of some species to put the free nitrogen
of the air into chemical compounds that are available for absorp-
tion by green plants growing in barren soil. When crops are
taken off the land through a series of years the supply of nitrates

FIG. 208. Tubercles on the roots of red clover

I, section of ascending branches; b, enlarged base of stem; t, root tubercles
containing bacteria

is largely used up and the soil becomes impoverished or ex-
hausted. The nitrogen may be brought back to such soil by
fertilizers, but this is expensive. The restoration of nitrogen to
barren land has been one of the most serious problems of agri-
culture. There is one of the bacteria (Pseudomonas radicicola),
which lives on the roots of members of the legume, or pea fam-
ily, including such forms as the clover, alfalfa, and soy bean, and
develops swollen regions called root tubercles (Fig. 208). This
remarkable organism is able to take the free nitrogen from

THE GERM DISEASES 235

the air and pass it through complicated chemical changes to
the clover and alfalfa. Consequently these crops can be grown
on worn-out soil or in waste land that is deficient in nitrates.
Indeed, soils may now be inoculated with fluid cultures of these
" nitrogen-fixing bacteria," so that the organisms will immedi-
ately establish root tubercles on the seedlings of these legumes,
when sown, or the seeds themselves may be soaked in cul-
tures insuring the application of the bacteria.1 Therefore, when
a soil becomes barren of nitrogen through successive crops of
wheat, for example, the nitrogen may be largely restored by
planting clover or alfalfa and plowing the crops under. Barren
soil may also be inoculated more certainly by distributing over
it earth from an old clover field.

The " nitrogen-fixing bacteria " make available the almost in-
exhaustible supply of free nitrogen in the air which cannot be
absorbed by green plants and which consequently has been of no
service to agriculture. As indicated in the diagram (Fig. 207),
free nitrogen is constantly being brought into the nitrogen
circle through the bacteria which form root tubercles (symbiotic
bacteria), and this helps to make up the loss of nitrogen from
the nitrogen circle, which comes in various ways, as by fire or
the escape of ammonia into the air.

257. The germ diseases. There is a class of contagious, and
in some cases very dangerous, diseases caused by certain bacte-
ria which are frequently called microbes, or germs. The most
serious are diphtheria, typhoid fever, tuberculosis (consumption),
cholera, leprosy, bubonic plague, pneumonia, influenza or grippe,
and whooping cough. Some other germ diseases, such as malaria,
tropical dysentery, and possibly smallpox, are caused by lowly
organisms which are not, however, bacteria. The germ diseases
are due to the parasitic development of the organism within the

1 See Moore, " Soil Inoculation for Legumes," United States Department
of Agriculture, Bureau of Plant Industry, Bulletin 71, 1905, and Wood,
" Inoculation of Soil with Nitrogen-Fixing Bacteria,1' Bulletin 72, Part IV,
1905.

236 THE FUNGI

human or other host. They are contagious because the germs
can be easily passed directly or indirectly in various ways from
the ill person to those around him.

'The active substances which affect the patient are known in
all cases to be certain poisons called toxins, which are, for the
most part, secretions, less often decomposition products, accom-
panying the growth of the bacteria. These poisons become dis-
tributed by the blood and cause the fevers. The body resists
the effects of the toxins to the best of its ability, and in some
cases substances are formed called antitoxins, which neutralize
the poisons. The injection into the human body of an antitoxin,
which is obtained from the blood of a horse infected with diph-
theria, is the chief principle in the " antitoxin " treatment of this
very serious disease. Kecovery from a germ disease generally
renders the person safe, or immune from further attack for a
long time, because the body has developed resistant powers
to the poisons and growth of that particular germ. The viru-
lent poisons called ptomaines are usually the result of bacterial
growths in foods that have not been properly kept.

Inflammation of wounds is caused by germs, and the forma-
tion of pus is in large part the gathering of white blood corpus-
cles which feed on the germs as they multiply in the infected
tissues. The whole practice of modern surgery is based on
absolute cleanliness in the treatment of wounds to prevent the
entrance of bacteria during operations.

There are some serious bacterial diseases of plants, as the
pear and apple blight, cucumber and melon wilt, black rot of
cabbage, wet rot of potatoes, and hyacinth blight, and probably
peach yellows is also of this class.

258. Public health. The matter of public health and
hygiene calls for constant attention on the part of physicians
and health officers to the possible sources of germ diseases.
For example, contaminated water and impure milk are the
commonest means of infection for typhoid fever, and epidemics
of this disease are frequently traced to these sources. We

PUBLIC HEALTH 237

cannot emphasize these points better than by studying the his-
tory of a typical typhoid epidemic, taking as our illustration
the well-known outbreak in 1885, in Plymouth, Pennsylvania,
a town of about eighty-five hundred inhabitants. Typhoid fever
appeared in the spring with such violence that from fifty to
two hundred cases developed daily, until about eleven hundred
persons were stricken (about one eighth of the population),
more than one hundred of whom died. The disease appeared
only in persons who drank the hydrant water from certain
town reservoirs, and those who used private wells escaped. On
investigation the following facts were established. During the
winter a case of typhoid fever, contracted in Philadelphia, had
been cared for in a house which stood close to a stream that
flowed into the town reservoirs. During the illness intestinal
discharges from the patient had been thrown out upon the
snow within a few feet of this stream. During late March and
early April the snow melted and there were frequent rains that
washed the germ-laden material into -the stream, which carried
it into the reservoirs. The first cases of typhoid fever in the
epidemic appeared within two or three weeks (the period of
incubation in typhoid fever) after the infected water had been
distributed through the town. Thus the entire epidemic was
due to the carelessness or ignorance of attendants who did not
safely dispose of the germ-filled wastes from a typhoid patient.
The terrible outbreaks of cholera are usually due to infection
of water supplies. The germs of tuberculosis are very widely
distributed by means of the dried sputum of diseased persons,
hence the importance of rules against spitting in public places.
The common diseases incident to the association of children in
school, such as diphtheria, scarlet fever, measles, and mumps,
make necessary the strict isolation of all cases until there is no
possible danger of contagion. As the sources of germ infection
are reduced or stamped out, the possibilities of germ diseases
become at once lessened. The healthy human body is wonder-
fully resistant, and the problem of public health is largely the

238 THE FUNGI

practical one of combating germs. So important are the bacte-
ria in disease and hygiene that a science has developed, called
bacteriology, with elaborate methods of its own to which special-
ists give their entire attention.

CLASS VI. THE YEASTS, OK SACCHAROMYCETES

259. The yeasts. The yeasts are much larger than the bac-
teria, and have a more complex cell structure, for there is
present a clearly defined nucleus. The cells reproduce in a
peculiar manner called budding, and the yeasts are frequently
termed budding fungi. Small extensions are put forth from the
cells (Fig. 209, A), which, after increas-
ing in size, become cut off from the par-
ent structure. The parent and daughter
cells frequently remain attached in short
chains or clusters. The relationships of
the* yeasts are very obscure, but there
are reasons for believing them to be de-
generate conditions derived from some
types of higher fungi whose spores are

FIG. 209. Yeast (Saccha- kllOWn at timeS tO PaSS lnt° yeast-like

romyces cerevisice) forms when cultivated in sugary solutions.

A, vegetative cells, show- Yeasts are chiefly interesting as the

ing method of budding ; agents of alcoholic fermentation by which

B, spore formation TIT. . i

sugar dissolved in water is changed into

alcohol and the gas carbon dioxide. The alcoholic nature of
wines, beers, ales, and hard cider is due to the fermentation of
grape juice, wort, or sweet cider, all of which contain sugar, and
the froth and bubbles of gas which escape from the fermenting
fluid is carbon dioxide. The yeasts are distributed very widely,
and they are sure to be introduced by dust into any sugar solu-
tion that is not sealed up. Therefore weak sugar solutions fer-
ment spontaneously if left exposed, although it is the practice
in the manufacture of beers and some wines to use special kinds

THE ALGA-LIKE FUNGI 239

of yeasts that are cultivated for the purpose. The yeasts that are
distributed indiscriminately by the air are called wild yeasts, to
distinguish them from those which are cultivated for the pur-
poses of brewing and bread making. The wild yeasts some-
times become established in cheeses and other dairy products,
and also in breweries, where they set up fermentations that
render the food or drink unfit for use.

The raising of bread results from the fermentation by yeast
of sugar that is present in the dough.1 The cavities, or holes, in
the dough are formed by bubbles of carbon dioxide- which, with
the small percentage of alcohol developed, is driven off in the
baking. Compressed yeast is made in certain distilleries from
cultures in large vats, whose yeast scum is removed and pressed
into the yeast cakes that are sold for domestic use.

CLASS VII. THE ALGA-LIKE FUNGI, OR
PHYCOMYCETES

260. The alga-like fungi. The Pliycomycetcs (meaning alga-
fungi) comprise a large number of forms which resemble the
alga3 in their structure and methods of reproduction. Some of
them are one-celled and microscopic, but others are very con-
spicuous mold forms, and certain types are destructive parasites
that cause some very serious plant diseases. The interesting
fungus (Empusa) which kills the house flies, that are frequently
found attached by their mouth parts to window panes and
woodwork, is in a special group of this assemblage. We shall
only be able to consider representatives of the following three
orders of this interesting class of the fungi: (1) the molds,
(2) the water molds, and (3) the blights and rots.

261. The molds. The molds (order Mucorales) form very ex-
tensive and conspicuous shining cobweb-like growths (Fig. 210)
through and upon the material of manure heaps and other

1 See the paper by Helen W. Atwater, " Bread and the Principles of Bread
Making," United States Department of Agriculture, Farmer's Bulletin 112,
1900.

240

THE FUNGI

masses of decaying matter. It is desirable that the term mold
should be restricted to fungi of this group.

The bread mold (Rhizopus nigricans) illustrates well the
characters of the group. An extensive growth may always be
obtained on bread by placing it in air saturated with moisture,
as under a bell jar set in a dish of water. The vegetative body
consists of large branched filaments which generally appear

FIG. 210. The mycelium of a mold (Mucor Mucedo) developed from
a single spore

a, 6, and c, erect branches which are to bear the sporangia, showing three stages
of development. — After Brefeld

glistening white because they are covered with minute drops
of moisture. The individual filament of a fungus is called a
hypka (meaning a web), and a mass of hyphse is termed a
mycelium. The hyphse of the bread mold resemble the fila-
ments of Vaucheria (Sec. 228) in having no cross partitions,
the filaments being a single chamber from end to end, and
consequently a coenocyte (Sec. 229). The multinucleate proto-
plasm forms a layer under the wall of the hypha and contains

THE MOLDS

241

minute globules of a fatty nature. The bread mold is an excel-
lent example of a saprophytic fungus. The hyplise grow all
through the porous substance of the moist bread and absorb
fluids containing products of the bread's incipient decay. The
material over which a saprophytic fungus grows and upon
which it lives is called its substratum.

The fructifications of the bread mold are very characteristic.
Numerous erect branches arise, several in a group, from creeping
hyphae that develop clusters of short, root-like filaments at these
points (Fig. 211). The end of each erect branch then gradually

FIG. 211. Growth habit of the bread mold (Rhizopus nigricans)

Sketch showing two groups of erect hyphse bearing sporangia, with root-like
clusters of filaments at their bases

enlarges and becomes separated from the stalk below by a dome-
shaped cross wall called the columella (Fig. 212, A). The ter-
minal cell becomes a spore case, or sporangium, and develops a
multitude of smoke-colored spores, which make the spore cases
appear as black heads upon the upright stalks (Fig. 212, B).
The spores are distributed by the breaking of the sporangium
wall, exposing the dome-shaped columella which remains at the
end of the stalks after the dispersal of the spores (Fig. 212, Z>).
The molds have a remarkable method of sexual reproduction,
which is, however, rarely found in the bread mold (Rhixopus),

242

THE FUNGI

but is not uncommon in other genera, as Mucor and Sporo-
dinia. Two short branches from the mycelium become applied
to one another, end to end (Fig. 213, A). The tip of each then
becomes cut off as a sexual cell, or gamete (Fig. 213, B, C),
peculiar in having very many nuclei, and consequently called a
ccenogamete. The two gametes finally fuse, and a large zygo-
spore (Fig. 213, D) with heavy black walls is formed between

FIG. 212. The sporangium of the bread mold (Rhizopus nigricans)

A, young sporangium, showing dome-shaped cross wall (columella) shortly after its
formation; B, mature sporangium, the columella being hidden by the spores;
C, diagram of a lengthwise section of a sporangium ; s, spore cavity ; c, col-
umella. D, columella after the rupturing of the sporangium wall, which was
attached along the line a corresponding to similar line in B ; clusters of
spores still clinging to the columella

the filaments. It is probable that the sexual nuclei from the
two gametes fuse in pairs within the zygospore.

262. The water molds. The water molds (order Saprolegni-
ales) are very remarkable aquatic fungi which grow on the dead
bodies of insects when immersed in pond or ditch water (Fig.
214, A). Certain species attack the gills and mouths of young

THE WATER MOLDS

243

fishes in hatcheries and may be very destructive. The coenocytic
hyphse live in the tissues of the animal, and filaments grow out
from them freely into the water,
where they develop the repro-
ductive organs.

Zoospores are formed numer-
ously in terminal club-shaped
sporangia and are discharged
into the water (Fig. 214, (7, D).
They are two-ciliate and consti-
tute the method of rapid mul-
tiplication, swimming about in
the water, seeking a favorable
substratum on which to settle
down.

The sexual organs are male
and female. Globular oogonia
are formed at the ends of cer-
tain hyphse, and each develops
a number of eggs (Fig. 214, F).
The male organs are delicate
antheridial filaments which
arise below the oogonia or from
neighboring hyphse. These ap-
ply themselves to the oogonia
and send delicate tubes (con-
jugation tubes) into the interior,
which in some forms are said
to unite with the eggs. How-
ever, it is known that the antheridial filaments in many of
the water molds perform no function, and indeed are not even
present in some types. In such cases the eggs mature into
oospores without fertilization. The water molds furnish, then,
excellent illustrations of the degeneration of a sexual process, a
phenomenon found in other groups of fungi.

FIG. 213. Formation of zygospores
in a mold (Mucor Mucedo)

A, two hyphaj in contact, end to end;
fi, the terminal gametes; C, later
stage, the gametes fusing; D, a ripe
zygospore ; E, germination of a zygo-
spore, the filament forming a spo-
rangium at once in this case.
Brefeld

After

244

THE FUNGI

The suppression of a sexual act is termed by botanists apog-
amy (meaning without marriage), or sometimes parthenogenesis,
when the egg itself develops without fertilization. Apogamy is
found in many groups of plants, — in the algae and fungi, among
the ferns, and even in the seed plants.

FIG. 214. A water mold (Saproleynia mixta)

A, habit sketch of the mycelium around a fly ; sporangia being formed at the tips
of the longest hypha} and sexual organs nearer the body of the insect; />, tip
of hypha ; C, terminal sporangium filled with zoospores ; J), empty sporangium
with a group of zoospores near the opening; E, empty sporangium with the
hypha continuing its growth inside ; f\ an oogonium containing many eggs
and with three antheridial filaments applied to it

263. The blights. The blights (order Pcronosporales) are
parasitic fungi which cause some very destructive plant diseases.
Some of them are also called " downy mildews," but it would be
better if the term mildew were reserved for a peculiar group of
sac fungi (Sec. 266). The hyphae form extensive growths in the
tissues of the hosts. The asexual fructifications appear on the
surface, but the sexually formed oospores are developed within

THE BLIGHTS

245

the host. The type most available for study is the blister
blight (Albugo), but the potato blight, or rot,
and the grapevine blight (downy mildew) are,
for economic reasons, the most important forms
in the group.

The blister blight. The blister blight (Albugo)
grows on the shepherd's purse (Capsclla) and
not infrequently on the radish, appearing as
white blisters on the leaves and stems (Fig. 215).
The blisters are formed by the asexual fructifica-
tions, which consist of masses of spores called
conidia that are developed in chains from the
ends of hyphse just underneath the epidermis Blisters containing
(Fig. 216, A, B). Conidia are air spores of fungi, conidia on the

V f, ' ' . . . L . . . stem of the shep-

— that is, spores formed singly or in chains at herd's purse
the ends of special branches and scattered in (Capselia)
the air. Those of Albugo are distributed by the wind after the
breaking of the blisters, and germinate in moisture, developing

FIG. 215

The blister blight
(Albugo Candida)

FIG. 216. Reproductive organs of the blister blight (Albugo Candida)

A, section through the edge of a blister on a leaf; the air spores, or conidia, are
f.ormed in chains under the epidermis from the swollen tips of fungal filaments
growing between the cells of the leaf; B, tips of two filaments, showing devel-
opment of the conidia serially ; C, a filament showing sucker-like structures
(haustoria) which enter the cells of the host ; I), the sexual organs ; the male
cell, or antheridium, a, has just discharged its nucleus through a beak-like
process into the single egg within the oogonium.

246

THE FUNGI

several two-ciliate zoospores. If the conidium has germinated
on the proper host after a rain or heavy dew, the zoospores
swim over the moist surface, and finally coming to rest they

put forth delicate germ tubes that enter
the host through one of the breathing
pores or stomata. The sexual organs are
generally found in portions of the leaves
and stems which become much swollen
and colored reddish or purplish. The large
oogonium forms a single egg and is ac-
companied by a single antheridial filament
which develops from the hypha below
(Fig. 216, D). The antheridial filament
puts forth a tube-like process which en-
ters the oogonium and discharges one or
more nuclei into the egg, fertilizing it.
The fertilized egg develops heavy walls,
becoming an oospore, which rests during
the winter, and on germinating in the
spring produces a large number of zoo-
spores that infect new hosts.

•The potato blight, or rot. The potato

tification of the potato blight (fkytof^thora infcstans) has a dil-
blight (Phytophthora ferent type of conidial fructification from
infestans) Albugo. The hyphse emerge from the

A, the air spores (conidia) leaves through the stoinata (Fig. 217, A),

formed on long fila- . ,. „ i r i • .1

merits which grow out and conidia are formed freely in the air
from the interior of the in immense quantities. These air spores

potato leaf througll the ... J .. . ,

stomata; B, the devel- are distributed by the wind, and germi-
opment of zoospores in nating in moisture develop zoospores (Fig.

a conidium; a single • */•>•• •

zoosporeisshownatthe 217, B), which infect 116W hosts, as 111

right.- After Schenck Albugo. Cloudy, wet, and windy seasons
are naturally especially favorable to the spread of the potato
blight. The green parts of a blighted potato plant wither, and
the potatoes either cannot be formed, or rot in the ground. The

FIG. 217. Conidial fruc-

SUMMARY OF THE ALGA-LIKE FUNGI 247

disease is carried over from one year to the next in diseased
potatoes that are planted. The potato blight came originally
from South America (perhaps Peru) and first appeared in Europe
in 1845, probably introduced from North America. The disease
spread very rapidly, causing local famines in various countries,
notably in Ireland. It is now, however, largely held in check
by spraying the plants with Bordeaux mixture, which contains
copper and is poisonous to the fungus.

The grapevine blight, or downy mildew. This genus (Plas-
mopara) develops conidia 011 hyphse outside of the host plant,
as in the potato blight, but they germinate by tubes instead of
forming zoospores. The disease had its origin in America, but
our vines are not generally very seriously injured by it. How-
ever, when it was accidentally introduced into Europe it proved
a terrible menace to the vine-growing industries there. The
European varieties of grapes are largely grafted upon American
rootstocks because the latter resist the attacks of a very destruc-
tive insect pest called Phylloxera. But the American grapevine
blight was for a time more injurious than the insect, until means
were discovered of keeping it in check by spraying the .vines
with Bordeaux mixture.

The interesting genus Pythium, which causes the " damping
off " of seedlings, and is sometimes very destructive in green-
houses, is related to the blights.

264. Summary of the alga-like fungi. The chief points of
resemblance of the Phycomyeetes to certain algse lie in the
coenocytic structure of the fungal filaments and the develop-
ment of zoospores in terminal sporangia. The sexual organs are
likewise similar to those of algae in that they are developed
terminally, but there are important modifications because motile
sperms are not generally formed. However, motile sperms are
known for one type (Monollepharis). The conidia are plainly
modified sporangia, which become detached from the parent fila-
ments and are distributed as special reproductive spores. The
algae which most resemble the larger filamentous Phycomyeetes

248

THE FUNGI

are such forms as Vaucheria (Sec. 228), and other types of
the Siphonales, and some authors believe that the molds, water
molds, and blights have been derived from that general region
of the algas.

CLASS VIII. THE SAC FUNGI, OE ASCOMYCETES

265. The sac fungi. The sac fungi are distinguished by a
peculiar type of reproduction, through spores which are devel-
oped, generally eight in number, in a special unicellular organ
called an ascus (plural, asci), which means
a sac. The asci are produced sometimes
in very great numbers in a fructification
termed an ascocarp, or sac fruit, which is
a structure of importance. The filaments,
or hyphse, of the sac fungi are divided by
cross walls into cells, and are never long
coenocytes, as in the alga-like fungi (Phy-
covtiycetes). The Ascomycetes is one of the
two largest groups of the fungi, comprising
more than fifteen thousand species. We
can only describe a few forms from the
following groups : (1) the mildews, (2) the
cup fungi, and (3) the knot and wart fungi.
FIG. 218 Sac fruits (as- 266 The mildews. The true mildews

cocarps) of the lilac

mildew (Microsphcera (order Perisporiales) are a very clearly de-

Alni) on the lower sur- fined group of fungi, and it is desirable
face of a lilac leaf thafc the term mMew be restricted to

them. They are mostly external parasites, very common 011
the leaves of many seed plants, such as wheat, lilac, Virginia
creeper, grapes, verbena, cherry, oak, willow, etc. The hyphse
form a cobweb-like growth (mycelium) over the leaves, and put
forth sucker-like processes called haustoria, which enter the
epidermal cells of the host. There is a method of rapid multi-
plication during the summer months by air spores, or conidia,

THE MILDEWS

249

which are formed in chains from the ends of erect hyphse (Fig.
219, A) and give the leaves a powdery appearance. But the
most important fructifications are the sac fruits (ascocarps),
which appear later in the season as black dots on the leaves.
They can be most conveniently studied in the lilac mildew.

The lilac mildew. This type (Microsplicera Alni) forms white
blotches on the leaves of the lilac, especially over somewhat

FIG. 219. Reproductive organs of the mildews

A, B, the lilac mildew (Microsphxra Alni) : A, a chain of air spores (conidia)
formed from the tip of an erect filament; B, a sac fruit (ascocarp) cracked
open, with two spore sacs (asci) protruding, one of the appendages shown
in detail. C, I) (Podosphsera) : C, the sexual organs, — a the antheridium, b
the female gamete or ascogonium; 1), the development of the cellular envel-
ope of the sac fruit around the fertilized female gamete. — C, D, after Harper

shaded portions of the plant. The sac fruits are found in the
autumn as black globular bodies made up of filaments so closely
united that they form a cellular mass (Fig. 219, B), in the
interior of which are developed the spore sacs (asci). The sac
fruit of Microsphcera has several radiating appendages with
peculiar tips. It is developed as the result of a sexual process
involving the fusion of two sexual cells, or gametes (Fig. 219, C),
The asci are formed at the ends of hyphse that arise from the

250

THE FUNGI

fertilized female cell, while the wall of the ascocarp is formed
from neighboring filaments (Fig. 219, D). The ascocarp thus
resembles in its development the sexually formed fructification
(cystocarp) of certain red algse such as Polysiplwnia (Sec. 245).
The ascocarp, like the cystocarp, is a system of .two tissues, one
derived from the fertilized female gamete (called an ascogonium)
and the other from the vegetative cells of the parent plant.
The phase in the life history beginning with the fertilized asco-
gonium and ending with the production of ascospores is an
asexual or sporophyte generation alternating with the sexual

generation or gametophyte, as in the
red alga3 (Sec. 246). The wall of the
sac fruit is clearly a protective struc-
ture for the sacs, each of which gen-
erally develops six spores in the lilac
mildew, although eight nuclei are
present in the sac.

The green and yellow mildews.
These are very common saprophytes
on bread, cheese, shoes, clothing, and
other substances that mildew or
"mold" in dampness. They are
easily distinguished by their colors
and the structure of the conidial
fructifications. The green mildew is
Penicillium (Fig. 220, A), which is believed to give the peculiar
flavor to Eoquefort cheese. The yellow mildew is Aspergillus
(Fig. 220, B). Their ascocarps are rather uncommon, especially
those of Penicilliiim.

267. The cup fungi. Most of the conspicuous forms in this
very large assemblage belong to the order Pezizales. The sac
fruits are saucer--, cup-, or funnel-shaped (Fig. 221, J, J5), fre-
quently colored yellow, orange, red, brown, or bluish, and in some
forms are three or more inches in diameter. The cup fungi are
almost all saprophytes, and are found on rotten logs and earth in

FIG. 220. Green and yellow

mildews

A, the green mildew (Penicil-
lium) ; K, the yellow mildew
(Aspergillus)

TIIE CUP FUNGI

251

damp woods, forming very striking and beautiful growths. The
chief peculiarity of the ascocarps is the fact that the entire
inner surface of the cup is a fruiting surface, consisting of im-
mense numbers of asci, arranged upright and all parallel with
one another, among delicate sterile filaments (Fig. 221, C). The

FIG. 221. Cup fungi

A, Lachnea, a small hairy form frequently growing on wood ; B, Peziza, a large
form growing on earth ; C, section through the fruiting surface of a Peziza
type, showing asci in various stages of development among delicate sterile
filaments (paraphyses)

asci are thus exposed, imbedded in a fruiting surface, and are
not inclosed in a case, as in the mildews.

The sac fruits of some cup fungi (notably Pyronema) are
known to be developed as the result of a sexual process, but
there is probably a great deal of sexual degeneration in this
group of the fungi, as in the water molds (Sec. 262).

The morel. Some very striking large forms are closely related
to the cup fungi. Among them is the morel (Morchella), much
prized as one of the best of the edible fungi (Fig. 222), and some
other curiously shaped types (Helvella, Mitrula, Geoglossum, etc.).

252

THE FUNGI

The fruiting surface of the ascocarps is
sometimes very extensive, and is thrown
up into irregular lobes and ridges.

268. The knot and wart fungi. This
large group contains forms with peculiar
hard black or brown wart and scab-like
fructifications, which are found on the bark
of trees. Most of the species are sapro-
phytic, but some, as the
black knot (Fig. 223), on
the plum and cherry, are
very destructive parasites.
The outer parts of the sac
fruits contain immense
numbers of small cavities
(perithecia) that are lined

Thft convoluted upper with asci- VeiT little is

portion is an exposed known of the development

fruiting surface PI i <• •,

of such complex sac fruits,
but it is probable that many of these fungi
are sexually degenerate, as are some of the
cup fungi. Xylaria, with its large finger-like
fructifications, belongs to this group.

269. Other sac fungi. Several exceptional
sac fungi deserve special mention.

Ergot. Ergot grains (Fig. 224, A) are hard FlG 223 The black
black structures found in heads of barley,
rye, wheat, and certain grasses, notably the
wild rice. They are really the mummified

FIG. 222. The morel
(Morchella), an edi-
ble sac fungus

knot (Plowrightia)
on a branch of the
cherry

, , . . . , . The branches become

and distorted ovaries, or grains whose tis- distorted, and long
sues have become filled and destroyed by cracks are formed,

.1 ,. P ,, P /^VT . m-i greatly impairing

the mycelium of the fungus (Claviceps). The the strength of the
ergot represents a sort of resting stage in trees
the life history of the fungus, and from it are developed in the
spring purplish stalks bearing the sac fruits (Fig. 224, B).

OTHER SAC FUNGI

253

The caterpillar and grub fungi. These extraordinary parasites
(Cordyceps) grow in the bodies of certain caterpillars and other
larvae, and in their pupae. The body cavity of
the insect becomes filled with the mycelium,
and generally mummified, after which a long-
stalked sac fruit grows out from between cer-
tain segments (Fig. 225).

The truffles. The truffles are very remark-
able sac fruits, sometimes as large as pota-
toes, which are developed on my-
celium that is generally associated
with the roots of certain trees. The
commonest truffle on the market
(Tuber Irumale) comes from the re-
gion of Perigord, in central France,
and is the most prized of all the edi-
ble fungi. It grows under certain
kinds of oak trees, and is found by
dogs and swine that are trained to
discover its location, and which detect
the fungus by a characteristic odor.
The association of the mycelium of
the truffle with the roots of
the oak tree is an excellent
example of what is called
a mycorrhiza, and is dis-

A, ergot grains on a

head of barley; 13, CUSSed in Sec. 278.
small sac fruits (as- ^ t j • ^ t Flo. 225

cocarps) developing r J y

from an ergot grain. The Spot diseases of plants Caterpillar fungus

larva of the May
beetle, which lives

underround

upon the leaves and fruit. Many of them are

caused by sac fungi, as the strawberry-leaf spot

(Sphcerella), black spots on grasses and clover

(Phyllachora) resembling rust spots, tar spots on willow and

maple (Rhytisma), and the apple scab (Venturia). Some of the

254 THE FUNGI

most destructive rots are sac fungi, though frequently caused
by some kind of conidial fructification rather than by the sac
fruit. Among them are the bitter rot of apples (Glomerella],
brown rot of peaches and plums (Sclerotinia), and plum pockets
(Exoascus).

270. The imperfect fungi. Some other spot diseases and rots
are caused by fungi which are known only through conidial
or other types of asexual fructification. More species of these
forms have been described than of all the sac fungi together,
and they are assembled in a group called the Fungi imperfccti.
Some of them are very important economically, causing such
diseases as the potato scab (Oospora), tar spots (anthracnose) on
beans (Colletotrichum), black rot of tomato (Macrosporium), and
black rot of apples (Spliccropsis). Most of the imperfect funyi,
however, are saprophytes, and play an important part with
other saprophytic fungi in bringing about the decay of vege-
table debris.

271. The lichens. The lichens deserve special consideration
as a very remarkable group. They are not single plants, but
composite organisms made up of algae which are contained in
an enveloping mesh of fungal filaments. The algal cells show-
ing through the fungal layers frequently give the lichen a
greenish color, but other pigments may be present, and some
lichens have brilliant yellow, orange, brownish, and reddish
tints. Lichens have a great variety of forms. Some grow closely
pressed against rocks and tree trunks (crustaceans, Plate V, A),
some are leaf -like (foliose, Fig. 226), and some are much branched
(fruticose, Fig. 227).

The fructification of a lichen is most commonly a saucer-
or cup-shaped structure. The inner surface is a fruiting layer
(Plate V, B), and contains numerous eight-spored sacs, or asci
(Plate V, D), showing clearly that the fungi concerned in the
lichen are sac fungi, or Ascomycetcs. The fructifications are
therefore sac fruits, or ascocarps, and these are known in some
forms to develop as the result of a sexual process. Most of the

PLATE V. A Common Tree Lichen (Physcia stellaris}

A, habit sketch ; B, diagram of a section through a sac fruit (ascocarp), showing
the fruiting surface and layer of algal cells; C, section showing a group of
algal cells (Pleurococcus), held in the network of fungal filaments; I), section
of the fruiting surface, showing sacs (asci) in stages of development among
the sterile filaments (paraphyses)

THE LICHENS 255

lichens have sac fruits closely resembling those of the cup
fungi. There is one small group of tropical lichens whose
fungal portions are basidia fungi, or Basidiomycctes, and not
Ascomycetes.

The algal portions of a lichen may be scattered, but in some
types they are arranged in definite layers. The kinds of algae
differ in various lichens. Some of them are unicellular green
forms, evidently of the genus Pleurococcus (Plate V, C). Most
of the species belong to the blue-green algae, one-celled forms
being commonest, though some complicated filamentous types,
such as Nostoc, are found in certain lichens. One curious lichen,
which grows 011 the leaves of the coffee plant, contains a species
of Coleochccte (Sec. 222).

The development of the present clear understanding of the
composite, or fungal and algal, nature of lichens makes one of
the most interesting chapters in the history of botanical science.
First came the recognition of the colorless portion of the lichen
as fungal and the colored elements as algal in character. Then
these portions were separated and cultivated independently of
one another, which proved that they remained respectively algae
and fungi ; for example, the lichen spore never developed into
algal cells, but only into fungal filaments. Finally, lichens were
created by bringing germinating spores in contact with wild
algae of a suitable kind, and these lichens have in some cases
lived for many months, finally developing typical lichen sac
fruits (ascocarps), thus completing the life history.

The lichens are perhaps chiefly interesting for the relations
which the algae and fungi bear to one another. When two
organisms live in intimate physiological association, so that
both receive some benefit from the partnership, the condition
is called symbiosis (meaning a living together). The mycorrhiza
relationship (Sec. 278) is an excellent illustration of symbiosis.
It is not easy to analyze critically the relationships between
the algae and fungi in a lichen association, but some points
seem clear.

256

THE FUNGI

First. The fungi are absolutely dependent upon the algae for
their organic food (such as the carbohydrates), which, of course,
the algae are able to manufacture in the manner characteristic
of green plants (photosynthesis). The relation of the fungus to
the alga is then in all essentials that of a parasite to its host.

Second. The algae receive a certain
sort of protection in the lichen thallus.
Thus they have fixed positions on ex-
posed rocks, cliffs, trees, and other
objects where they could hardly grow
otherwise, or at least not in the same
luxuriance. The substance of the lichen
also retains moisture, so that the algal
cells are not so subject to drought.

It is well known that many of the
lowly algae would grow in situations
frequented by lichens if left alone, and
it is evident that the lichens arise be-
cause fungus spores fall among the algae,
and germinating produce hyplue which
live parasitically upon them as hosts.

The algae are then, in a sense, slaves of
FIG. 226. A leaf-like, or fo- the f m { Th are not killed for tha(.

Hose, lichen (Cetraria) , J, '.

would be oi no advantage to the fungus,

s, sac fruits ... « . i P

which requires them to manufacture

its organic foods. The term slavery perhaps best expresses the
relation of the algae to the fungi in the lichens.

Life habits of the lichens. Lichens are found on rocks, cliffs,
branches and trunks of trees, and on the ground, when the latter
cannot support green vegetation, either because it is too bar-
ren, or is exposed to unfavorable climatic conditions. They are
most luxuriant in temperate and sub-arctic regions, especially
where there is much rain. They form the bulk of the vegeta-
tion on the tops of mountains and in the arctics, where grass
and other alpine seed plants cannot grow. They are abundant

SUMMARY OF THE SAC FUNGI

257

along storm-swept seacoasts. Some forms actually cover large
areas, as the reindeer moss (Cladonia rangiferina, Fig. 227, A),
which in extreme northern countries furnishes an important
source of food for herbivorous animals, as the reindeer. Since
the lichens are the first plants to grow on exposed rocks, they
form there the first soil, mingled with decayed vegetable matter
(humus), which may furnish a foothold for higher plants, such as
the mosses and grasses,
that are constantly try-
ing to establish them-
selves in the territory of
the lichens.

Some uses of lichens.
Some lichens (Roccella)
yield beautiful purple,
blue, and crimson dyes
called orchil and cud-
bear, much used in
former centuries in
Italy, and later in other
parts of Europe. Orchil
when prepared with
soda or potash yields
the dye litmus, em-

-i i • ,1 P A, the reindeer moss (Cladonia rangiferina) ;

ployed in the manufac- B> Cladoniacornucopioides- C, Usneabarbata;

ture of litmus paper, s, sac fruits

Other lichens, as Iceland moss (Cetraria), are ground up and

mixed with wheat and made into cakes.

272. Summary of the sac fungi. The most remarkable fea-
ture of the life history of the Ascomycetes is the position of the
ascocarp as a sporophytic phase following the sexual process
and alternating with sexual plants, or gametophytes. The asco-
carp holds a place in the life history similar to that of the
cystocarp in the red algse (Sec. 246). There are numerous types
of asexual spores (such as conidia) in the Ascomycetes, which

FIG. 227. Some branching, or fruticose,
lichens

258 THE FUNGI

cannot be described here but greatly complicate the classification
of the forms. Some authors believe that the sac fungi hold rela-
tions to the red algse, and, indeed, have been derived from them.

CLASS IX. THE BASIDIA FUNGI, OR
BA SIDIOMYCETES

273. The basidia fungi. The Basidiomycetes come next to the
Ascomycetes in number of known species, which is about fourteen
thousand. The group takes its name from a peculiar type of
reproductive organ called a basidium (meaning a small pedestal).
The basidium (Fig. 238) is a somewhat swollen terminal cell
of a filament, or hypha, from which are developed a group of
four spores on delicate stalks called sterigmata. The hyphse of
the basidia fungi are divided into cells, as in the sac fungi.

The basidium is a very characteristic structure of the higher
forms of the Basidiomycetes. However^ there are some types, as
the smuts and rusts, in which the basidium is represented by a
peculiar phase in the life history (the promycelium), which does
not at first thought seem to resemble the basidium. These
points can only be made clear after a study of representative
types, and they will be referred to later in the summary of the
basidia fungi (Sec. 279). This peculiarity is the basis of a classi-
fication of the basidia fungi into two series: (1) the Protobasid-
iomycetes, which are preliminary to (2) the Eukasidiomycetes,
or typical basidia fungi. The representatives that can be con-
sidered here will accordingly be grouped as follows :

SERIES I. The simpler basidia fungi, or Protobasidlomycetes.

1. The smuts, or Ustilaginales.

2. The rusts, or Uredinales.

SERIES II. The typical basidia fungi, or Eubasidiomycetes.

3. The coral fungi, the pore fungi, the tooth fungi, the gill fungi, col-

lectively called Hymenomycetes; and divided into several orders.

4. The puffballs, the earth stars, the nest fungi, the carrion fungi,

collectively called Gastromycetes, and divided into several orders.

THE SMUTS 259

SERIES I. THE SIMPLER BASIDIA FUNGI, OR

Pli O TOBAS IDIOM YCE TES

274. The smuts. The smuts (order Ustilaginales) are para-
sites which have the peculiar habit of attacking the floral parts,
and especially the ovaries, of various members of the grass
family. The hyphae fill these parts with a dense mycelium,
destroying the tissue of the host. Finally, most of the cells in
the mycelium take on heavy walls and become
resting cells, or winter spores, which form the
black powdery mass so characteristic of the
smut fructification. These resting cells sur-
vive the winter and germinate in the spring.
Each cell then puts forth a short filament
called the promycelium (Fig. 228, A), upon
which are developed a number of small spring
spores called sporidia, and these in some cases
germinate upon the sprouting host plants, as FJG 228. Promyce-
in oats, putting forth filaments that enter the liuui of the corn

host and develop a mycelium within, which smut (
may not be noticed until the fructifications

,-, n i -f. . . A. with spriner spores

appear in the floral organs. It is important (sporidia) attached ;

to note that the sporidia multiply rapidly by ^> spring spores

, , ,. /T7. ooo -ox • n j * budding like yeast

budding (Fig. 228, B), especially under favor- ceils. — After

able conditions, as in heavily manured soils,
and these buds, or conidia, will infect like the sporidia. These
habits of budding led to the theory that the yeasts have been
derived from the smuts.

Various smuts. The corn smut is, perhaps, the most con-
spicuous form and very destructive. The infection in the corn
is local ; that is, the spore masses are formed close to the point
of entrance of the fungus. Any tender growing region is sub-
ject to infection. The corn smut can only be held in check by
burning the spore masses as soon as discovered and by avoid-
ing the use of manure, which gives favorable nutrition for the

260

THE FUNGI

germination of the spores. The smuts of oats and wheat often
cause enormous loss in these crops. The best preventive measures
seem to be, treatment of the grains with solutions of copper sul-
phate, or formalin, or steeping them in hot water for a short time
before planting, which kills the smut
spores without injuring the grain.1

275. The rusts. The rusts (order
Uredinales) cause some of the most

FIG. 229. The wheat
rust (Puccinia gram-
inis)

A, spots of the red rust
on a wheat leaf, com-
posed of the summer
spores (uredospores) ;
H, spots of the black
rust on wheat straw,
composed of the
winter spores (teleuto-
spores)

FIG. 230. The winter spores (teleuto-
spores) of the wheat rust (Puccinia
graminis)

Section through a spot of the black rust on
oats, the epidermis of the leaf being
thrown back and the two-celled teleuto-
spores raised above the surface on stalks ;
note the web of fungal filaments (hyphre)
around the very much enlarged (hyper-
trophied) cells of the host under the spot

disastrous diseases of such grains as wheat, oats, barley, and
rye. They are all parasites, forming yellow or black spots on

1 See Swingle, " The Prevention of Stinking Smut of Wheat and Loose
Smut of Oats," United States Department of Agriculture, Farmer's Bulletin
250, 1906.

THE RUSTS

261

the leaves and stems of their hosts. The most complicated life
histories in the fungi are found in this group, for many spe-
cies require two different hosts to complete their life cycle and
form a number of different reproductive spores during their
development. These peculiarities are best illustrated by the
rust of wheat.

The wheat rust. The wheat rust (Puccinia graminis) appears
on wheat, oats, and other grains and grasses, first as red or
yellow streaks or spots upon the leaves and
stems (Fig. 229, A). The host is greatly weak-
ened and consequently matures only a small
yield of grain. Towards the end of the season
black streaks (Fig. 229, B) are formed in ad-
dition to the red-rust spots, and these indicate
the development of resting cells, or winter
spores, which are peculiar two-celled struc-
tures in Puccinia (Fig. 230). The winter
spores, called teleutospores, germinate in the
spring, and each cell gives rise to a short
filament, the promycelium, usually consisting
of four cells (Fig. 231), from which are gen-
erally developed four spring spores, or sporidia.
The winter spores, promycelium, and spring
spores probably correspond to the same stages
in the life history of a smut.

Wherever the barberry is common, as in

Europe and New England, the spring spores *io. 231. Promyce-

7 1mm of the wheat

(sporidia) infect these plants and produce on rust (Puccinia

their leaves peculiar fructifications called graminis)
cluster cups, or cecidia (Fig. 232, A, B), in After Tuiasne
which are developed chains of cluster-cup spores, or cccidiospores
(Fig. 232, C). There is considerable evidence to prove that
the cluster cups represent the remains of what was once a
sexual phase in the life history of the rust, but which is now
much modified, and indeed entirely suppressed in some forms.

262

THE FUNGI

Curious structures called spermogonia (Fig. 232, C) frequently
accompany the cluster cups and are believed to be the remains
of male sexual organs now no longer functional. They develop
immense numbers of minute cells, termed spermatia, which may
at one time have been functional sperms, but apparently serve
no useful purpose now.

The secidiospores are distributed by the wind and germinate
upon young wheat, putting forth tubes which enter the host

FIG. 232. Cluster cups (secidia) on barberry leaves

A, habit sketch showing groups of cluster cups on a leaf; B, a group enlarged ;
C, section through a leaf showing cluster cups on the lower surface, with the
chains of secidiospores and the male organs (spermogonia) on the upper sur-
face. The latter develop immense numbers of minute cells which probably
represent sperms, but are now functionless

through the stomata. The infected wheat then develops a
number of crops of one-celled summer spores called uredo-
spores (Fig. 233). The first crops of summer spores are widely
scattered in high winds and infect more wheat, thus spreading
the disease very rapidly. The spots of uredospores are reddish
or yellowish, and this is the stage known as the red rust of
wheat. Finally, at the end of the season, the black spots of
teleutospores appear, and the rust's life history is completed.

THE RUSTS

263

This long life history, which is thoroughly known in Europe,
becomes much shortened in the Middle West, California, and
Australia, where there is no barberry, by the omission from
it of that host. In these regions the uredospores (summer
spores) may survive the winter or
dry season, or be carried over from
summer to summer through the
winter wheat and germinate di-
rectly upon the new developing wheat
of the following year, so that the re-
production of the rust is by a succes-
sion of the uredospores.

There is no method known of killing
the wheat rust on the living host ; but
it has been found that certain varie-
ties of wheat, as the macaroni wheats,
are far more resistant to the rust than
others. There is some hope that varie-
ties may be bred by crossing our wheats
with macaroni wheat that will be
largely immune to this disease, which
annually causes losses of many million FlG- 233- The summer spores
dollars in the United States alone. 1 £tS3 ^ ^ "*
There are a large number of varieties A single twolLteieutospore,
of Puccinia graminis, and also several t, happens to be presentamong
other species of Puccinia which attack them' ~ After De Bary
various grains, grasses, and other plants. One of these (P. as-
paragi) sometimes causes great damage to asparagus.

Other rusts. The group of the rusts is very large, the genera
being distinguished chiefly by the structure of the teleutospores

1 For a discussion of the rusts and rust problems of the United States, see
papers of Carleton from the publications of the United States Department of
Agriculture, "Cereal Rusts of the United States," Division of Vegetable
Physiology and Pathology, Bulletin 16, 1899 ; " Macaroni Wheats," Bureau
of Plant Industry, Bulletin 3, 1901; "Investigations of Rust," Bureau of
Plant Industry, Bulletin 63, 1904.

264

THE FUNGI

and the different types of life histories affecting various hosts;
but many of the forms have no economic importance, being
found on such plants as the violet, May apple, cocklebur,
asters, golden-rods, members of the pea family, etc. However,
there are destructive rusts on the roses (Phragmidium), clovers
(Uromyces], blackberries (Ceoma), etc. An interesting type is
the rust (Gymnosporangium,) which causes the distortions called
cedar apples on the junipers, and the much-branched stunted
growths called witches' brooms. This rust has a cluster-cup
stage (once named Rwstelia) on the hawthorn and apple.

SERIES II. THE TYPICAL BASIDIA FUNGI, OR EUBASIDIOMYCETES

276. The Hymenomycetes. This group, which may be con-
sidered a sub-class of the Basidiomycetes, comprises all of the
higher basidia fungi whose spores are
developed on a fruiting surface, called
an liymenium (meaning a membrane),
which is exposed. This condition is
thus contrasted with that in the Gas-
tromycetes (puffballs, etc.), where the
spores are developed within a case. The
types of fructification are exceedingly
various in this group, which includes
the pore, the tooth, and the gill fungi
in the various forms of toadstools and
brackets. But there are also some
simpler types, as the coral fungus (Cla-
varia), with irregular branches (Fig.
234), and also some expanded forms.
The pore fungi.- The pore fungi
(family Polyporacece) have commonly the shape of brackets and
grow on the trunks of trees, although some are large, fleshy toad-
stools, as Boletus (Fig. 235). The hymenium lines the cavities
of the numerous pores which are found on the under surfaces.

FIG. 234. A coral fungus
(Clavarid)

THE IIYMENOMYCETES

265

Many of the pore fungi are perennial, increasing in size from

year to year by adding new
layers of growth outside of
the old. The bracket or toad-
stool is merely the fructifica-
tion which receives its nour-
ishment from an extensive
mycelium growing in the
wood, and under the bark of
trees, or in the soil. Many
of the pore fungi are very
destructive parasites, greatly
injuring and sometimes kill-
ing forest trees. They may
cause great injury to growing
timber.1 Most of the pore
fungi are, however, sapro-
phytic in their manner of
life.

The tooth fungi. The tooth fungi (family Hydnacece) are less
common than the pore and
gill fungi. Some of them have
bracket forms, and some are
toadstools (Fig. 236). The
fruiting surface is distrib-
uted over tooth or spine-like
processes.

The gill fungi. The gill
fungi (family Agaricacecc} in-
clude most of the toadstool
and mushroom forms (Fig.
237). A toadstool consists of FlG< 236. A tooth fungus (Hydnum)

1 See von Schrenk, "The Decay of Timber and Methods of Preventing
It," United States Department of Agriculture, Bureau of Plant Industry,
Bulletin 14, 1902.

FIG. 235. A pore-bearing toadstool
(Boletus)

266

THE FUNGI

--cap

a stalk (stipe)) which in some genera arises from a cup (volva)
and is expanded above into the cap (pileus). The under surface

of the cap bears many thin plates
which hang down in a radiating ar-
rangement and are called gills. The
gills illustrate very well the struc-
ture and position of the basidia on
a fruiting surface, or hymenium,
and cross sections are shown in Fig.
238. It will be seen that the basidia
are the swollen terminal cells of a
compact mesh of hyphae, and that
each bears a group of four spores
on short stalks or sterigmata.

The toadstool is really a fructi-
fication. It is attached to an ex-
tensive mass of mycelium, which
is the vegetative portion of the
plant. This mycelium generally
lives saprophytically in the soil,
frequently around buried roots of
trees, but there are some para-
sitic gill fungi (Plate VI) which
cause the decay and final death of
valuable timber. The toadstool
develops from an. accumulation of
hyphse in small structures called

FIG. 237. A group of mushrooms
(Armillaria mellea)

my, mycelial attachment ; c, cf, c",
young stages called buttons;
mature mushroom with expanded
cap (pileus) shown ahove; st,
stem (stipe); g, gills; r, ring. —
After Hartig, through Bennet
and Murray

buttons (Fig. 237, c,

The

cap region with the gills and stalk
become differentiated within the
button, and finally break out from
the surrounding envelope and ex-
pand in a few hours to their full size ; hence the expression a
" mushroom growth." The remains of the envelope are found in
some forms as scales on the top of the cap (see mature mushroom

THE HYMENOMYCETES

26T

of Fig. 237) and in a ring attached to the stalk below the gills
(Fig. 237, r), while in certain types (Amanita, etc.) there is a
large cup (volva) at the base of the
plant out of which the stalk rises.

It is becoming rather general popu-
lar usage to apply the term mushroom
to all toadstools and other fleshy
fungi which are edible. There are no
general rules for distinguishing mush-
rooms from toadstools which do not
have exceptions; but the collector
may readily learn the characters of
the most -poisonous species, and like-
wise become acquainted with a num-
ber of choice forms which are easily
recognized.1 It is a good principle,
however, to rest satisfied with a
knowledge of a few absolutely safe
mushrooms and not to experiment
with those that are not fully known.

The most poisonous species of the gill ^ ««*•.•••'*

r FIG. 238. Gills of mushroom

fungi are in the genus Amanita and . (Coprinus comatus)

have large volvas, rings, and white Across section of gills showing

spores, and may be readily recognized fruiting surface (hymenium) ;

when carefully examined. There are

also some very poisonous species of

Boletus among the pore fungi. The

commonest mushroom of the market

(Agaricus campestris) is a form extensively cultivated, but

which also grows in the fields. These mushrooms are raised

in cellars and caves, in specially prepared, heavily manured

beds, which are planted with masses of mycelium called

1 See Farlow, " Some Edible and Poisonous Fungi," United States Depart-
ment of Agriculture, Division of Vegetable Physiology and Pathology, Bul-
letin 15, 1898.

, portion of fruiting surface
illustrating three basidia with
spores and two from which
the spores have fallen off,
showing the spore-bearing
stalks (sterigmata) s

268

THE FUNGI

spawn.1 Some species of Boletus are edible, and they, with the
morels (Sec. 267) and truffles (Sec. 269), are sold in the Euro-
pean markets with edible gill fungi.

277. The Gastromycetes. This group, in contrast with the
Hymenomycetes, includes forms in which the basidia line the
interior of chambers, or cavities, in the fructifications and are
consequently inclosed until the fructification matures. Here

are found the puffballs, earth
stars, nest fungi, and carrion
fungi.

The puffballs. These are
the fructifications (Fig. 239)
of an extensive underground
saprophytic mycelium, as in
the toadstools and mushrooms.
The young puff ball has a
white flesh made up of hyphse
and filled with small irregular
cavities lined with the fruiting
FIG. 239. Apuffball(L^erdon) gurface (hvmenium). The

spores when ripe lie freely as a brown powder in the dried-up
fibrous tissue inclosed .in an outer parchment-like envelope.
The spores may be discharged through a special opening at the
top or scattered by the irregular rupture and decay of the puff-
ball. Young puffballs are edible, and there is one extraordinary
species (Lycoperdon giganteum] which grows to be a foot or
more in diameter and is much prized as a delicacy.

The earth stars. The earth stars (Gfcaster, Fig. 240) are modi-
fied forms of puffballs. The envelope is very thick, and the
outer portion splits lengthwise into segments which, when wet,
curve back from above and raise the fructifications from the
ground. In dry weather the segments are usually rolled up

1 See Duggar, " The Principles of Mushroom Growing and Mushroom
Spawn Making," United States Department of Agriculture, Bureau of Plant
Industry, Bulletin 85, 1905,

THE GASTROMYCETES

269

tightly around the fructifications. These movements of the seg-
ments in certain species when alternately wet and dry sometimes

tear the earth stars loose from
the ground so that they may
roll about, thus assisting in
the distribution of the spores.
The puffballs and earth
stars are in the same order
(Ly coper dales).

The nest fungi. These beauti-
ful little forms (order Nidula-

riales) grow on the earth and
FIG. 240. An earth star (Geaster) decaying wood and when Qpen

resemble a nest rilled with eggs (Fig. 241). The egg-like struc-
tures are portions of the interior of the fructification, and each
contains a chamber filled with spores.

The carrion fungi. These very malodorous fungi (order
Phallalcs) grow in rich humus and mulchings. They are com-
plicated stalked types first formed within a 'large globular
structure which remains around the base of the stalk as a cup.
The top of the stalk bears a dark-colored, sticky mass of spores,
that has the odor of carrion
and attracts carrion flies,
which probably assist in the
distribution of the spores.

278. Mycorrhiza. Mycor-
rhiza (meaning fungus-in-
fected roots) is a remarkable
association of the mycelium
of certain fungi with the
roots of many seed plants,
notably trees. The fungal filaments surround the roots with a
web (Fig. 242) and enter the outer regions of the root tissue,
probably living somewhat parasitically upon the plant as a
host. They are in close contact with the soil around the roots,

FIG. 241. A nest fungus (Cyathus)

The section at the right shows the egg-like
structures containing the spores

270

THE FUNGI

and are believed to be of great assistance to them in their work
in the following way. It is necessary for the roots, of course, to
establish a close relation to the moisture of the soil in order to
obtain water for the green parts of the plant above ground. The
surface of the older portions is without root hairs and is sur-
rounded by a hard outer layer which cannot come into very
close contact with the minute moist soil particles. But it is
thought that the fungal filaments act as root hairs, and perhaps
through them the root can absorb a much greater quantity of

water and can well afford to give
them what nourishment they require
in exchange for such valuable
services. It is probable that most
trees and many others of the larger
plants have formed this partnership
with the fungi. The kinds of fungi
concerned with mycorrhizas are not
well understood, but some of them
are known to be the mycelia of
toadstools and puffballs. The sac
fungi also furnish notable examples
in the truffles (Sec. 269). The my-
corrhiza relationship is an excellent
illustration of symbiosis (which
means a living together), for two

FIG. 242. Mycorrhiza surround-
ing the tip of a beech root

After Pfeffer

organisms exist here in intimate physiological association and
both apparently receive benefit from the partnership.

279. Summary of the basidia fungi. The relationships
between the different groups of the Basidiomycetes cannot be
discussed further than to state that the promycelium of smuts
and rusts, with its sporidia, is believed to correspond to the
basidium with its four spores. There are two small groups called
the jelly fungi (orders Auricularales and Tremellales), includ-
ing the rather common Jew's-ear fungus, whose basidia become
divided into four parts. In the Jew's-ear fungus the basidium

PLATE VI. A wound parasite (Pleurotus ulmarius) on the trunk of a

maple tree
After E. M. Freeman

SUMMARY OF THE BASIDIA FUNGI 271

is indeed a four-celled filament resembling the promycelium of
a rust, each cell developing a spore at one side on a sterigmata.
The winter spores, or teleutospores, of the smuts and rusts are
considered to be special resting cells of the fungi, developed to
carry these parasitic forms over unfavorable seasons of cold or
drought when the host plants are not alive. There is thus a
break in the life history at the point where the basidium should
normally appear. The germination of these spores continues
the life history with the immediate development of a structure
(the promycelium) which corresponds to a basidium with its
spores.

The higher basidia fungi have apparently lost all trace of
sexual organs, but the cluster-cup stage in the rusts is believed
to indicate the remains of a modified sexual generation in their
life histories. The evidence for this view rests chiefly upon the
behavior of the nuclei throughout the life history of the rust
and is too complicated for treatment here. The basidia fungi
are therefore chiefly, if not wholly, apogamous. The origin and
evolution of the Basidiomycetes is a problem as yet unsolved,
which cannot be here considered. The basidia fungi are, how-
ever, by far the most wonderfully varied and specialized assem-
blage of the fungi.

CHAPTEE XXIII

SUMMARY OF THE LIFE HISTORIES AND EVOLUTION
OF THE FUNGI

280. The life histories of the fungi. To understand the
types of life histories in the different groups of the fungi one
must bear in mind the life histories of the most nearly related
groups of algse (Sec. 247), for those of the fungi are based, of
course, on the life histories of their algal ancestors. But there
have been some very important modifications as the result of
the parasitic and saprophytic modes of life of the fungi, and
especially because the highest groups of fungi present much
sexual degeneration, or apogamy, which of course in some
respects simplifies the life histories.

The life history of the bacteria is essentially as simple as
that of the blue-green algse. The alga-like fungi (Pliy corny cdes)
is a group, however, whose highest members (the molds, water
molds, and blights) have reproductive organs with many points
of similarity to the siphon algre, and more especially to Vau-
cheria (Sec. 228). The sexually formed spores generally develop
directly into plants like the parents,1 so that the formula for the
life history is

P— <>sc£. s. — P<>se£. s. — P, etc.,
^9 9

the abbreviations g and sex. s. standing for gamete and sexually
formed spore, respectively. There is often extensive reproduc-
tion through various forms of asexual spores between succes-
sive sexual generations. And indeed sexual organs may only
be formed at rare intervals, as in the bread mold, or they may

1 They form zoospores, however, in some of the blights.

272

THE LIFE HISTORIES OF THE FUNGI 273

not be functional so that a condition of apogamy is present, as
in the water molds.

The life histories of the sac fungi (Ascomycetcs) are especially
interesting in relation to those of the red algse (Sec. 246). It is
known in regard to a number of types that the sac fruits (asco-
carps) develop as the result of a sexual process, corresponding in
this respect to the cystocarps. The ascospores are formed at the
end of the ascocarp phase of the life history just as the carpo-
spores are formed at the end of the cystocarp phase in the red
algse. Both ascocarps and cystocarps are, then, new genera-
tions developed between and alternating with the sexual plants.
They are sporophytes alternating with gametophytes. The for-
mula for the life history of a sac fungus with functional sexual
organs is then

£<;^> — S (ascocarp) — asex. s. (ascospore)

— £ <^ > — S— asex.' s. — G, etc.,

G and S standing for garnetophyte and sporophyte, respectively,
and asex. s. for asexual spore.

It must always be remembered, however, that the sac fungi
have a great variety of methods of asexual reproduction through
conidia, etc. Consequently sexual organs may be formed only
occasionally, as in the green mildew (Penicillium). There is
also probably much apogamy in the group, so that the sac fruits
are apogamously developed.

The basidia fungi present the remains of an alternation of
generations in the rusts somewhat similar to that of the sac
fungi. The cluster cups are believed to be the beginning of a
phase that formerly followed a sexual process just as do the
ascocarps and cystocarps. However, the male organs (spermo-
gonia) of the rusts are no longer functional, and the cluster cups
must be considered as developing apogamousy, although there
is now a complicated history substituted for the original sexual

274 LIFE HISTORIES AND EVOLUTION OF THE FUNGI

process. The cluster-cup stage is omitted entirely in some of
the rusts and in all of the smuts, and there are likewise no
traces of it in the higher basidia fungi (Eiibasidiomycetes).
Sexual degeneration in these forms, then, has apparently been
carried so far that the sexual organs have disappeared entirely
from the life histories.

281. The origin and evolution of the fungi. The study of the
evolution of the fungi must be taken up for each of the larger
classes separately, for there is every probability that each has had
an independent origin from widely separated groups of the algre.
The bacteria have probably been derived from the blue-green algse.
The higher alga-like fungi (molds, water molds, and blights)
apparently show relationships to the siphon algae. Some authors
believe that the sac fungi have come from the red algse. The
origin of the basidia fungi is very much in doubt and that of
the yeasts also, although it is generally held that the latter are
degenerate forms from some of the higher fungi. The evolu-
tion of the fungal forms in each group becomes very complicated,
because the fungi have such wonderfully varied habits resulting
from their parasitic and saprophy tic ways of living. In fact, these
life habits have produced the greatest variety of structures and
adaptations known in any group of spore plants. Still more
remarkable, perhaps, is the widespread tendency towards sexual
degeneration, which is also believed to be associated with the
parasitic and saprophytic life habits.

CHAPTEK XXIV

THE BRYOPHYTES AND THE ESTABLISHMENT OF ALTER-
NATION OF GENERATIONS

282. The bryophytes.* The division Bryophyta (meaning
moss plants) is the next great group of plants above the division
Tliallopliyta (Chapter xix), and includes two classes: (1) the
liverworts, or Hepaticw, and (2) the mosses,
or Musci. It is not best to define these
classes until the structure and life histories
of types from each group have been studied.
Furthermore, it is impossible fully to under-
stand the characters of the bryophytes and
thallophytes except when compared with
one another. Accordingly these matters
have been reserved for the final section of
this chapter under the heading Summary
of the Bryophytes and Thallophytes (Sees.
300, 301).

However, the bryophytes differ from the
thallophytes in two very important respects Antheridium in section,

showing the outer cap-

which may be briefly stated at once, for

they must be thoroughly comprehended in

order to understand the life histories of the

liverworts and mosses. They can only be

made clear when illustrated through labora-

tory studies. These two differences are (1) in the sexual organs,

which are many-celled, and (2) in the appearance of a new stage

in the life cycle called the sporopliyte.

* To THE INSTRUCTOR : The introduction to this chapter assumes that the
life history of a liverwort or moss has been studied in the laboratory.

275

G< 243. The anther-
idium of a liverwort

Suie and the mass of
^n cells within, in

which are developed

the minute two-ciiiate

276

THE BRYOPHYTES

283. The Sexual organs. The sexual organs of the bryophytes
are many-celled. They are male and female and each consists
of a cellular case, or capsule, in which
are formed the respective gametes, which
are sperms and eggs. It will be remem-
bered that the sexual organs of the thal-
lophytes are, with very few exceptions,
one-celled. The conspicuous exceptions
are the plurilocular sexual organs of the
lower brown algae (see Ectocarpus, Sec.
235) and the antheridium of the stone-
worts (Sec. 230).

The sperm-producing organ, or anther-
idium. The antheridium (Fig. 243) is a
stalked, oval or elliptical structure, with
an outer cellular envelope inclosing a
dense mass of very small cubical cells
in which are developed the sperms. The
sperms are minute elongated or slightly
coiled protoplasts, with a pair of cilia at
one end. The mature antheridia only dis-
charge their sperms when wet, as after
heavy rains or dews, and the sperms then
FIG. 244. The a r die go- swim about in the moisture. At these
nium of a liverwort times the plants are practically leading
an aquatic life like their algal ancestors,
Thearchegoniainthisgenus d ^ development of motile sperms

hang down from a special r

receptacle (Fig. 251). The in these land plants shows clearly that
£rj§Ltai±07.£ they must have come from forms with

archegonium, while the aquatic life habits.

^"cT^l1^ Th* egg-prodncint, organ, or archego-
down into mucilage as the nium. The female organ is called an

archegonium matures. archegonium. It IS flask-shaped (Fig. 244),

and the outer cellular envelope incloses at maturity a row of cells.
The cell situated in the enlarged portion of the archegonium

THE SPOROP.HYTE 277

(venter) becomes the single egg, while the others in the neck
region (Fig. 244, ri), called canal cells, break down and their
substance becomes changed into mucilage. The -archegonia, like
the antheridia, open only when wet, the cells at the tip separat-
ing so as to give a clear passage for the entrance of the sperms
into the neck. The sperms are attracted to the opening by cer-
tain substances such as sugar contained in the liquefying muci-
lage. The sperms swim down the neck, and one of them, fusing
with the egg, fertilizes it. There is much evidence that the
canal cells are degenerate gametes, and that the archegonium
came from a type of sexual organ that originally produced a
number of gametes, as does the antheridium.

284. The sporophyte. The term sporophyte has appeared
before in the accounts of the red algae (Sec. 246) and sac fungi
(Sees. 266, 272) where certain peculiar fructifications (cystocarps
and ascocarps), following the sexual process, alternated with the
sexual plants. The sporophytes of the liverworts and mosses
have a similar position in the life history, and are likewise
borne on the parent plants and frequently called their " fruits."

The sporophyte of the liverworts and mosses develops at
once from the fertilized egg, which never becomes a resting
spore (oospore), as in the algae. The form is various in differ-
ent groups. Most of the mosses have long, stalked sporophytes
(Figs. 261, 265), which end in swollen spore cases. The liver-
worts generally have much smaller sporophytes, some of which
have no stalk at all and consist of the spore case alone. If the
sporophyte is small it may remain inclosed in the base of the
archegonium, which becomes much enlarged. But the stalked
sporophytes either burst out of the archegonium, or frequently,
as in the common mosses, tear it off and carry it upwards as a
cap-like structure (Fig. 265, B,cal} called the calyptra (meaning
a veil). The sporophytes always remain attached to the parent
plant, and finally develop spores asexually in the spore cases.
The spores are formed in groups of four, called tetrads, within
spore mother cells (Figs. 245, B, s; 258, J5). These asexual

278 THE BRYOPHYTES

spores have heavy walls and can survive the winter, frequently
protected by the spore case. It will be remembered that in the
algse the sexually formed spore is generally the protected resting
spore.

285. Alternation of generations. There are thus two phases
in the life history of a liverwort or moss. First, there is the
plant which bears the sexual organs, and this is called the
gametopliyte (meaning a gamete-bearing plant) ; second, there is
the structure which arises from the fertilized egg and ends its
history by developing asexual spores, and for this reason it is
called a sporophyte (meaning a spore-bearing plant). The game-
tophyte is developed from the spore, and the sporophyte from
the fertilized egg. So there is a regular alternation of these two
phases in the life history, the gametopliyte producing sexual
cells, or gametes, and the sporophyte producing asexual spores.
The two phases are regarded as separate generations because
each has its origin from a distinct kind of reproductive cell
(egg or spore). The gametophyte is of course a sexual genera-
tion and the sporophyte an asexual one. Their following one
after the other makes an alternation of generations,— a phrase
which from now on will be frequently used, because it signifies
the most remarkable feature in the evolution of all plants above
the thallophytes. The simple sporophytes of ancient bryophytes
gave rise to the fern plants and through them to the large and
complicated seed plants.

A life history which consists of an alternation of sporophyte
and gametophyte, as in the liverworts and mosses, may be
expressed by the formula

Gametopliyte <^ ^ ' ^> — Sporophyte — asexual spore
This in an abbreviated form becomes

— Gametophyte, etc-

&<l>-S-<#-G<*>-S-*p-ef, etc.

t/ t/

THE RICCIA GROUP 279

One must bear in mind these general characters of the bryo-
phytes, as the liverworts and mosses are separately taken up
and their characters finally summarized by treating the sub-
jects under the four heads :

Class I. The liverworts, or Hepaticce
Class II. The mosses, or Musci.
The origin and evolution of the bryophytes.
Summary of the bryophytes and thallophytes.

CLASS I. THE LIVEKWOKTS, OE HEPATIC^

286. The liverworts. The liverworts grow most luxuriantly
, in moist and shaded situations, some forms on the ground, some

on rocks, and some on trees. There are also certain aquatic
liverworts which float on the surface of the water, and a few
very simple ones which are entirely submerged like the algae.
Thus, although most of the types have the land habit, some show
very clearly adaptations for the aquatic life of their ancestors
among the algae. The creeping habits of the liverworts probably
indicate the way in which land plants arose and became estab-
lished first along the margins of streams, ponds, and marshes
where algal growths emerged from the water or were left
stranded on the wet earth. These first land liverworts naturally
clung close to the wet earth in the beginning, until the devel-
opment of root-like systems of filaments (rhizoids), which could
gather moisture, permitted them to develop upright stems as
in the mosses. The forms of the liverworts are various, as will
appear in the following brief account of the four orders.

287. The Riccia group. The simplest liverworts (order Eic-
ciales) have a flat plant body (gametophyte), some forms float-
ing on the surface of the water and others submerged, while
certain types grow close to moist earth. The plant body is a
true thallus (Fig. 245, A), and indeed is much simpler than the
plant bodies of many thallophytes among the brown and red
algae. The lower surface of the thallus bears numerous filaments,

280

THE BRYOPHYTES

caUed rhizoids (from their resemblance to roots), and delicate
membrane-like fringes, which draw up water from the soil like

root hairs if the plant has
the land habit. The thal-
lus grows from a number
of points (Fig. 245, A, gp)
situated in notches at the
ends of lobes which fork in
pairs and finally split apart,
so that the plants multiply
very rapidly during the veg-
etative season. The sexual
organs are borne on the up-
per surface of this game-
tophyte, and the sporophyte
is a simple globular case
(Fig. 245, B), filled with
spores, which remains in-
closed in the base of the
archegonium, so that the
spores are not set free until
the decay of the plant.*

288. The Marchantia
group. This large group

FIG. 245. A floating liverwort (Riccio-
carpus) and its sporophyte

A, habit sketch of the sexual plant (game- (order Marchantiales) is

tophyte) viewed from above, showing the well represented bvthe com-
position of the sporophytes in lines back /

of the growing points gp. B, section of mon liverwort (Marchantia

a sporophyte contained within the parent p0lym0rplia\ which grows

archegonium, whose neck n is still pres- ± , . .

ent: a, spores in groups of four (tetrads) Oil the ground in moist

within the spore mother cells; w, remains
of the wall of the sporophyte

situations. The ribbon-like
thallus of species of Mar-
chantia (Fig. 246) forks regularly, but one of the branches is

*To THE INSTRUCTOR : These points are admirably illustrated in the large
floating form (Ricciocarpusnatans), which is not uncommon and is an excellent
type for laboratory study, although Marchantia is the form most generally used.

THE MARCHANTIA GROUP

281

almost always larger and stronger than
the other. The lower surface bears nu-
merous filaments and fringes which are
formed in front of the growing points,
protecting them, and later
become distributed along
the lower surface. The
upper surface is marked
by diamond-shaped areas
(Fig. 247, A) which show
the position of curious
large air chambers (Fig.

Fi<;. 24(>. A Marchantia form (Marchantia
disjuncta)

247 13} that contain very ./>, female receptacle ; c, cups producing buds.

— After Sullivant

numerous filaments whose

cells have well-developed chloroplasts. These filaments perform

FIG. 247. Structure of the thallus of Marchantia

A, surface of thallus, the diamond-shaped areas marking air chambers ; 7>, a section
through the middle region of the thallus showing air chambers above, filled with
branching green filaments, and the fringes and root-like hairs (rhizoids) on the
lower surface ; C, surface view of the pore which opens into an air chamber

282

THE RRYOPHYTES

the greater part of the chlo-
rophyll work (photosynthesis)
of the plant, and the chambers
are developed as protective
structures around them. Each
chamber is open above to the
air by a circular pore (Fig.
247, (7), which can be easily
seen in the center of each
diamond-shaped area. This
specialization of the upper sur-

F.o. 248. The cups and buds of faCe °f tlle *"««*«»«« P^nt

Marchantia to a light relation gives it a

A, cup-bearing plant; B, section of a cup general resemblance to the
showing the buds arising from its bot- cell structure of leaves in seed

torn ; C, a bud showing the two growing -. ,

points; D, young plant developing from plants and terns.

thebud Some individuals of Mar-

chantia (Fig. 248, A) will usually be found bearing cups (cupules)
which contain numerous green bodies. These are many-celled
reproductive organs, called
buds (gemmse), which de-
velop from the bottom of the
cup (Fig. 248, B). Each bud
has two notches at opposite
sides (Fig. 248, C), which
become two growing points
when the structure falls on
its side upon damp earth and
begins to grow (Fig. 248, D).
This is a characteristic and
very successful method of
rapid asexual multiplica-

tion in Marchantia.

The sexual organs of
Marchantia are developed

FIG. 249. The male plant of Marchantia

A, male plant bearing antheridial receptacles;
B, lengthwise section of a receptacle (semi-
diagrammatic), showing a row of sunken
antheridia upon the upper surface; the
youngest lie just back of the notches in the
receptacle, which are the growing points;
air chambers are also shown on the upper
surface

THE MARCHANTIA GROUP

283

FIG. 250. Female plant of
Marchantia

Showing the umbrella-like arche-
gonial receptacles in various
stages of development

on stalked, umbrella-like
receptacles, which are
really much-modified
branches of the thallus.
They bear either anther-
idia or archegonia, and
the two sexual organs are
not found together on the
same plant. The anther-
idial receptacle (Fig. 249,
A) is shorter than the
archegoiiial, and the top
is a flattened disk with a
lobed or scalloped margin.
The antheridia (Figs. 243,
249, B} lie in cavities or
pits along radiating lines

FIG. 251. The female receptacle of
Marchantia

A, portion of a lengthwise section of a young
receptacle (semi-diagrammatic), showing a
row of archegonia hanging down from the
lower surface, the youngest being nearest the
stalk : air chambers are present on the upper
surface ; I, one of the finger-like lobes back of
the section, the diamond-shaped areas indi-
cating air chambers. B, a young sporophyte
within the parent archegonium: the region
which is to become the spore case is indicated
by the cross lines, and the small foot is at-
tached to the base of the archegonium; e, a
special envelope developed around the arche-
gonia of Marchantia

284

THE BRYOPHYTES

on the upper surface of the receptacle, and the youngest anther-
idia are found nearest the notches which mark the position of
the growing points along the edge of the disk. The archegonial
receptacle (Fig. 250) is larger than the antheridial, and the top
is bent back into several long, finger-like projections like the
ribs of an umbrella. Numerous archegonia are formed in lines
(Fig. 251, A) on the under side between the lobes, and are pro-
tected by singular fringes. The youngest archegonia are formed

FIG. 252. Sporophytes and receptacles of Marchantia

A, lower view of an old female receptacle, showing the sporophytes in rows between
the fringes /, like peas in a pod. B, section of a receptacle (diagrammatic),
showing a mature sporophyte anchored by its foot and projecting beyond the
fringe /: the spore case is open, exposing the mass of elaters el; a young
sporophyte is shown at the right still inclosed within the archegonium (ca-
lyptra) ; e, special envelopes around the archegonia and sporophytes

nearest the stalk, so that the older ones lie farther out, — an
arrangement exactly opposite from that of the antheridia. This
is explained by the fact that the growing points which lie be-
tween the lobes grow downward and underneath towards the
stalk. The edge of the disk is thus bent back on itself, and the
lower surface is really an extension of the upper surface.

A number of archegonia may be fertilized in Marchantia, and
their eggs then develop sporophytes in radiating rows on the

THE MARCHANTIA GROUP

285

lower surface of the receptacle (Fig. 252, A) between the fringes.
The sporophyte is more complex than in the Riccia types. The
lower part (Figs. 252, B ; 253, A) becomes a small organ of at-
tachment to the gametophyte, called the foot, through which
it obtains water with food in solution. The upper part becomes a
spore case, developing numerous spores, and among them spirally
marked filaments, termed elaters (Fig.
253, B), which are stiff and elastic and
help to distribute the spores. The elaters
are developed from cells in the young
spore case. The spore case is carried
beyond the fringe of the receptacle (Fig.
252, B) by the elongation of the region
above the foot, which forms a stalk. The
presence of a foot and stalk in addition
to the spore case marks a decided ad-
vance over the simple sporophytes of the
Riccia types, which consist of the spore
case alone.

It is very important to note that the
sporophyte has this close attachment to
the gametophyte and is dependent upon
it for water and food in solution, because
it shows that the sporophyte of the liver-
worts really lives in large part like a
parasite upon the gametophyte as a host.

289. The Jungermannia group. This
assemblage (order Jungermanniales) is
very much the largest group of the liverworts and contains
more than three thousand species. They are known as the leafy
liverworts because most of them have long stems, with delicate,
moss-like leaves. The leafy liverworts are frequently mistaken
for mosses, since they are common on tree trunks and in shaded
situations. But they have a creeping habit, and there are two
crowded rows of large leaves (Fig. 254, A), one on each side of

FIG. 253. The sporophyte
of Marchantia

A, longitudinal section of
sporophyte showing spore
ease and foot attached to
the base of the archego-
nium: e, a special envel-
ope. It, an elater

FIG. 254. The female plant of a leafy liverwort (Porella)

A, habit sketch of the upper surface, with the two rows of leaves at the sides ; B,
a portion of lower surface, showing the third row of small leaves (amphigas-
tria) ; O, the stalked sporophytes with open spore cases sc ; D, a sporophyte
with the spore case split lengthwise into four parts. — After Campbell

FIG. 255. The antheridia of a leafy liverwort (Porella)

A, portion of male plant, illustrating the small antheridial branches at the side;
-B, section of an antheridial branch, showing two antheridia situated ju£t above
the attachment of the leaves I ; C, the much-elongated sperm, with the two cilia
at one end and the remains of the parent cell at the other. — C, after Campbell

286

THE JUNGERMANNIA GROUP

287

the stem, and a third row of small modified leaves ou the lower
surface (Fig. 254, B}. The stems of mosses, on the contrary, are
almost always upright, and the leaves are arranged radially, so
that the stem has no upper
or lower surface.

The antheridia of the
leafy liverworts are borne
singly along the stem at
the bases of the lateral
leaves (Fig. 255, B) on cer-
tain branches which are
frequently much smaller
than the vegetative shoots
(Fig. 255, A). The arche-
gonia are developed in clus-
ters at the ends of branches.

The sporophyte (Fig.
256, A) has a stalk which
elongates rapidly just be-
fore the spores are ready to
be shed, so that the spore

case is raised above the

FIG. 256. The sporophyte of a leafy
liverwort (Porella)

in the parent archegonium, whose neck n is
shown above, the foot deeply sunken in the
tissue of the gametophyte : a, archegonia of
the terminal group, which were not ferti-
lized ; I, leaf-like envelopes. B, the four-lobed
spore mother cell, which develops four spores
(tetrad). C, an elater

gametophyte (Fig. 254, C), A section of a sporophyte still contained with-

as in the mosses. How-
ever, the spore case is much
less complex than that of
the mosses, being a simple
capsule that splits length-
wise into four parts at maturity (Fig. 254, D). There are spirally
thickened filaments, or elaters (Fig. 256, C), among the spores,
as in Marchantia, and these structures are not found in the
mosses. The foot of the sporophyte (Fig. 256, A) is always well
developed in the leafy liverworts.*

* To THE INSTRUCTOR : Good material of the leafy liverworts frequently
furnishes better subjects for type study of the liverworts than Marchantia.

288

THE BRYOPHYTES

290. The Anthoceros group. These types (order Anthocero-
tales) are considered the highest of the liverworts because of
their more complicated sporophytes. The gametophytes are thal-
loid (Fig. 257), somewhat irregular in outline, and more simple
in structure than those of Marchantia. The sporophytes are an
inch or more in height, and grow up from the gametophyte like
blades of grass. The upper portion splits lengthwise into halves
at maturity.

The spores of Anthoceros do not all mature at once, as in other
liverworts, but new spores are formed at the base of the sporo-
phyte as the older mature (Fig. 258, A)y.
and there is a continuous elongation of
the structure during the summer from a
basal region of growth. The cells com-
posing the wall of this long sporophyte
contain large single chromatophores (Fig.
258, E), and there are breathing pores,
or stomata (singular, stoma, meaning a
mouth), on the surface (Fig. 258, D), which
lead into intercellular spaces in the green
tissue beneath. Consequently the sporo-
phyte is able to manufacture its own food
The thaiioid sexual plants ty photosynthesis, as any green plant may

(gametophytes), with the / r J

sporophytes s in various do. But it depends upon the gametophyte
stages of development f or its SUppiy Of Water, which is absorbed

through a large bulbous foot (Fig. 258,^1) that is deeply im-
bedded in the thallus of the gametophyte.

If the base of this sporophyte could establish a root-like
structure growing in the soil, it might live independently of the
parent gametophyte, for it has chlorophyll-bearing tissues in
communication with the air through stomata, just as in the
ferns and seed plants. And it has also the power of indefi-
nite growth from its basal region (Fig. 258, A), limited only by
the length of the summer season. These peculiarities of the
sporophyte of Anthoceros are very suggestive of the way in

FIG. 257. Habit sketch
of Anthoceros

THE ANTHOCEROS GROUP

which higher plants must have arisen
from forms somewhat like the liver-
worts, a subject which we shall con-
sider later in our account of the ferns
(Sec. 331). Of all the bryophytes,
this seems to be the genus which
most closely approaches the higher
plants. This account of plant evo-
lution is now well started towards
the higher conditions of plant devel-
opment, namely, those of the ferns
and seed plants whose sporophyte
generations are independent plants
with roots, stems, and leaves, and
which comprise the most independ-
ent and successful vegetation on the
earth.

CLASS II. THE MOSSES, OR
MUSCI

291. The mosses. The mosses are
very much more numerous than
the liverworts. Some of the com-
mon kinds grow in extensive carpets
on hillsides and in forests, becom-
ing important factors in the plant

A, longitudinal section (semi-diagrammatic)
through the base of the sporophyte, showing
the large foot imbedded in the tissue of the
gametophyte, the region of growth, and the
spore-producing tissue which forms a cylin-
der in the center of the stalk ; B, a group
(tetrad) of four spores (three shown) in a
spore mofher cell; C, spores; D, a stoma
viewed from the surface ; E, section through
a stoma, showing cells with large single
chromatophores under the surface layer
(epidermis)

289

FIG. 258. The sporophyte of
Anthoceros

290

THE BRYOPHYTES

formations of many regions. The peat mosses are the chief in-
habitants of certain kinds of bogs and pond margins. The mosses

therefore constitute a group of
considerable importance in the
plant population of the earth, while
the liverworts are for the most
part confined to rather special
life habits and, with the exception of the
leafy liverworts, are not rich in species or
numerous in individuals. Almost all of the
mosses fall into two groups, which may be
called the peat mosses and the common mosses.

292. The peat mosses. The peat mosses (order
Spliagnales) are very remarkable for their struc-
ture and life habits. There is only a single genus,
Sphagnum, with about two hundred and fifty
species. The plants (gametophytes) have long
stems, with delicate, leafy branches, some of
which grow downward and soak up water, while
the rest form a dense cluster at the top (Fig. 259).
The peculiar structure of these mosses allows
them to absorb and hold water like a sponge, for
which reason they are used by gardeners for
packing around plants and flowers. The dried
moss is sometimes used for bedding in stables.
The sexual organs (antheridia and archegonia)
are formed very early in the spring or in the
late winter, and the fertilization of the egg
FIG. 259 leads at once to the development of a sporophyte.
The peat moss The sporophytes are large, smooth capsules (Fig.
(Sphagnum) 260, J), which appear to have stalks, but these
are really special developments of the gametophytes. The spore
case is attached to the top of the stalk by a large foot and opens
by a cover (Fig. 260, B, c), which falls off. The spores on ger-
mination produce small flat cell plates, out of which the leafy

THE PEAT MOSSES

291

peat mosses arise from special buds. These cell plates suggest
the simple thalloid gametophytes of the liverworts, and the leafy
structure is perhaps a special development from them.

The peat mosses live in bogs and swamps and are especially
common in northerly regions and in the mountains, where they

FIG. 260. The sporophyte of the peat moss (Sphagnum)

A, group of the sporophytes on stalks, which are really growths from the game-
tophyte. n, longitudinal section through a sporophyte, showing the large foot
imbedded in the top of the stalk: o, the remains of the parent archegonium,
with the neck still present ; s, spore chamber ; c, cover

grow over wet rocks, sometimes covering large areas. They
develop so rapidly that they frequently fill ponds and bogs.
The first growth is around the edges of the pond, but this grad-
ually works inwa,rcf, until finally the whole surface is covered
with peat moss. Such conditions produce quaking bogs, for the
surface is not firm enough to hold any large animal which
might walk upon it. Quaking bogs become firmer as the lower

292 THE BRYOPHYTES

parts of the peat mosses die and form a fibrous deposit below.
These deposits may grow to be many feet in thickness, and
finally become so firm that they can be cut out in blocks. Such
blocks when dried are used for fuel, especially in Ireland and
in the Highlands of Scotland. There are regions of the north-
ern United States, Canada, Europe, and Asia where the peat
mosses cover immense territories, and there are innumerable
bogs filled with deposits of peat which may sometime become
important sources of fuel supply.

Peat bogs are generally poorly drained or not drained at all,
and the water becomes very rich in certain organic acids that
result from the partial decomposition of the vegetation. The
accumulation of these acids renders the water unfit for the
growth of bacteria and is largely responsible for the preservation
from decay not only of the remains of the peat mosses but of
other plants with them. It is said that whalers and other ships
from the New England coast when starting, on long voyages
preferred to take their supplies of drinking water from peat
bogs because of its keeping qualities. Occasionally the bones
of extinct animals, such as the mammoth and mastodon, are
found in peat, since these gigantic creatures became mired in
the soft bogs of former periods.

As a quaking bog becomes firmer, other plants begin to grow
among the peat mosses. Certain grasses appear, some charac-
teristic orchids (Calopogon, Pogonia, Arethusa, Cypripedium,
etc.), the insectivorous plants Sarracenia (Fig. 311) and Drosera
(Fig. 312), such heaths as the swamp cranberry, swamp blue-
berry, swamp azalea, and Labrador tea, and certain trees, as the
larch or tamarack (Larix), black spruce (Picea), the arbor vita?
(Thuya), and the white cedar (Chamcecyparis). These plants, in
various combinations with the peat mosses, form very character-
istic associations, and they furnish some of the best illustrations
of what the ecologist calls plant formations. The northeastern
United States and Canada are full of examples of this interest-
ing feature in the natural history of the Sphagnum swamp.

THE COMMON MOSSES

293

293. Common mosses.*

The common mosses (or-
der Bryales) are familiar
because of the occurrence
of numerous species with
conspicuous upright stems,
which develop the long-
stalked sporophytes with
characteristic terminal
spore cases (Figs. 261,265).
It is a very large group,
containing over eight thou-
sand species, and is by far
the most numerous assem-
blage in the bryophytes.
These mosses grow
in the greatest va-
riety of situations,
— in swamps and
bogs, in the water
of streams, in moist
and shaded woods,
in open fields, and
on relatively dry
hillsides and rocks.
They perform an
important service
to plant life in
holding back much
of the rainfall,
allowing it to
sink into the earth

FIG. 261. A common moss (Catharinea undulata)

Showing the branching leafy moss plants (gameto-
phytes) attached to the root-like mass of protonemal
filaments and bearing sporophytes. — After Sachs

* To THE INSTRUCTOR : In a short course it is best to present the life his-
tory of bryophytes through a somewhat detailed study of one of the common
mosses, followed by general studies of a variety of forms of mosses and liver-
worts.

294 THE BRYOPHYTES

instead of running rapidly off in floods. The lichens and mosses
are among the first plants to appear on barren soil or exposed
rocks and cliffs, and are also the plant pioneers that push
their way up mountains and into the arctic regions where no
other vegetation can live.

294. The life history of a moss. The life history of the
common mosses is more complex than that of a liverwort. The
moss spore does not develop directly into the leafy moss plant. It

prim

FIG. 262. The protonema of a common moss (Funaria)

prim, primary shoot; br, branches from primary shoot; pi, young moss plant
or bud. — After Sachs

produces a preliminary filamentous growth, called the protonema
(meaning preliminary thread), which sometimes forms an exten-
sive network over the ground, resembling at first sight such
terrestrial algse as certain species of Vaucheria. The proton emal
filaments (Fig. 262), however, consist of cells placed end to end
(they are never coenocytic) ; they have generally oblique cross
walls and contain numerous disk-shaped chloroplasts. There
are no algae known which the protonema resembles in detail,
and yet this phase in the life history suggests what may have
been the life habits of ancestors of the mosses. Certain cells of

THE LIFE HISTORY OF A MOSS

295

the protonema change their methods of cell division and develop
small buds (Fig. 262, pi) which grow into the leafy moss plants
(F"ig. 263). One moss spore may give rise to a great quantity of
protonema, which by means of the numerous buds will form a
large group or even a turf of moss plants. Therefore the protonema
is a very effective means of establishing the large carpets of moss
vegetation. The leafy moss plant develops the sexual organs in
clusters at the top of the stem
and has further peculiarities of
structure which will be described
later. The protonema together
with the leafy moss plant consti-
tute the sexual or gametophyte
phase of the life history.

The fertilization of an egg in
an arehegonium starts at once the
development of the sporophyte,
often called the moss fruit. The
fertilized egg gives rise to a many-
celled structure (Fig. 264, A),
which establishes a growing point
above and a foot attachment to
the gametophyte below. This
young sporophyte is contained at FIG 20g A young plant of a com.
first entirely within the parent mon moss (Webera)

arehegonium, which enlarges with Showing its attachment to the proto-

its development (Fig. 264, B, a). nemal filaments which bear repro-
T^ , n v, ,, ductive buds 6

But finally the growth of the

sporophyte is so rapid that the arehegonium is torn away at its
base and borne upwards on the elongating stalk of the sporo-
phyte. The remnant of the arehegonium then covers the tip of
the stalk like a cap (Fig. 265, B, cal) and is called the calyptra
(meaning a veil), which must serve a useful purpose, protecting
the delicate growing tip of the sporophyte. Finally, the tip of
the sporophyte enlarges and becomes the complex spore case

FIG. 264. Developing sporophytes of a com-
mon moss (Funaria)

A, very young stage, showing the early cell
divisions of the egg ; B, older sporophyte just
before the archegonium a is torn away from
the gametophyte and carried upward as the
calyptra. The base of the sporophyte has
now grown down into the tip of the leafy moss
plant (gametophyte) and is firmly anchored
to it. — After Sachs

296

FIG. 265. A common moss
(Polytrichum commune)

A, male plant, showing cup-like
tip containing the antheridia.
JB, female plant with the spo-
rophyte : col, cap, or calyptra,
over the developing spore case.
C, a mature spore case with
the calyptra removed

THE LEAFY MOSS PLANT 297

(Fig. 265, C). The development of the asexual spores in the
spore case ends the life history of the moss plant, which may
be formulated as follows :

t / protonema and \ ^sperm^ „
Gametophyte (£* mnsa ,. < Mlfl > - Sporophyte

Jeafy moss planty - egg

— asexual spore — Gametophyte, etc.

This in abbreviated form becomes

G<1> -S~sp- £<*> - S-sp-G, etc.

This formula is identical with the general life-history formula
presented for the bryophytes in Sec. 285, and it is clear that
gametophyte and sporophyte alternate with one another.

295. The leafy moss plant. The leafy moss plant is, of
course, the conspicuous part of the gametophyte phase of the
life history. It consists of an upright stem, branching in some
forms, with the leaves almost always distributed spirally. The
symmetry of the plant is therefore radial instead of having an
upper and a lower side (dorsiventral) as in the leafy liverworts.
The leaves consist for the most part of simple plates of cells,
which in some forms can become dry and still retain their
vitality, freshening up with the next rain.1 The moss plant is
fastened to the earth by filaments of protonema (Fig. 263),
which grow out from the base of the stem and form a dense
network underneath the moss plants (Fig. 261). This proto-
nema becomes brown with age and serves as a system of root-like
filaments, or rhizoids, by which the moss plant obtains water
from the soil. The growth of the stem normally ends with the
production of a terminal group of sexual organs, both of which
(antheridia and archegonia) are found on the same plant in
some species and on different plants (male and female) in
others. Male plants are generally smaller than the female ones

1 The cells of the moss leaf are excellent subjects for study and have
been described in Sec. 195 and illustrated in Fig. 169.

298

THE BRYOPHYTES

and more easily distinguished (Fig. 265, A) because the orange
or reddish-brown clusters of antheridia lie exposed at the tip
of the stem and are sometimes surrounded by a circle or
rosette of modified or colored leaves. Female plants are larger,
and the archegonia are hidden by
enveloping leaves, which must be
picked off to expose these sexual
organs. The antheridia (Fig. 2 6 6, a)
and archegonia (Fig. 268, A) are

^^;&.tovr>X~y*

FIG. 266. Section through the tip of the
male plant of a moss (Funaria)

a, antheridium ; /, sterile filament, or para-
physis ; I, leaf

FIG. 267. The antheridium of
a common moss (Funaria)

a, antheridium; h, escaping
sperms ; c, a single sperm in
its parent cell. — After Sachs

sometimes numerous in the clusters and lie among hair-like
structures (paraphyses).

The mature antheridia and archegonia open only when wet
by the swelling and separation of a group of cells at their tips.
The sperms (Fig. 267, b) are discharged, then, after rains or heavy
dews, so that the moss at that time is practically living an

THE SPOROPHYTE OF THE MOSS

299

I 7

aquatic life. The archegonia (Fig. 268, B) have very long necks,
and the relatively small egg lies at the bottom as in a flask.
The sperms are attracted to the
mouth of the open archegonium by
substances in the mucilage within
the neck, one of which at least is
sugar. They swim down the neck
to the egg, and one of them fertil-
izes it.

296. The sporophyte of the
moss. The sporophytes of some of
the common mosses are the most
complex found among the bryo-
phytes, with the possible exception
of those of Anthoceros. There is
generally a long stalk which bears
a large spore case (Fig. 269, A). The
structure of the spore case is very
elaborate. A cover (operculum) is
formed at the end, which falls off
so that the spores may escape from
within. In many mosses the cover
is loosened and thrown off by an
interesting mechanism, which is
sometimes very highly developed.

There may be a circle of cells with FlG- 268' Section through the
,, . , . .._ , tip of a female plant of a moss

thickened and otherwise modified (Funaria)

cell walls, forming a well-defined
ring (Fig. 269, A, r) around the spore
case underneath the cover. These
cells change their form when wret,
sometimes swelling greatly (Fig.
269, (7), and thus loosen or tear the cover away from the spore case.
The rim of the opening formed when the cover falls off is
surrounded by a circle of pointed triangular structures called

A, group of archegonia a: £,leaf. B,
an archegonium in detail, show-
ing enlarged basal portion e with
the egg, and the neck n above with
its row of canal cells: m, mouth.
— After Sachs

300

THE BRYOPHYTES

teeth (Fig. 269, B, t)t which meet at the center of the opening
when folded inwards. The number of teeth is fixed for differ-
ent mosses. Under the circle of teeth various mosses have
another circle of much more delicate segments (Fig. 269, J5, s)

FIG. 269. The spore case of a common moss (Bryum)

A, the closed spore case : c, the cover (operculum) ; r, the ring. B, the rim of
an open spore case, showing the outer circle of teeth t, inside of Which is indi-
cated the inner circle of delicate segments s: sp, spores. C, the cover after
remaining for a minute in water: the cells of the ring r have absorbed the
water and have swollen so that the ring has broken aud curled backwards on
two sides

of the same number and general form. The teeth are sensitive
to moisture, curling inwards and outwards with changes in the
amount of vapor in the air, and by these movements they prob-
ably help in some types to empty the case of its spores, retain-
ing them in wet weather and letting them fall out in dry.

THE SPOROPIIYTE OF THE MOSS

301

The lower portion of the spore case has stomata (Fig. 270, D),
and there is much chlorophyll-bearing tissue in the moss fruit
that is capable of doing the work of photosynthesis just as in
Antlioceros. But this sporophyte is, of course, dependent upon
the gametophyte for its supply of water, which is taken up
through the pointed foot of the stalk that is deeply sunken
in the top of the leafy moss plant (Fig. 264, B}. The spores

FIG. 270. The sporophyte of a common moss (Funaria)

A, young sporophyte s attached to the leafy moss plant and covered by the calyptra
ml. />, sporophyte with mature spore case sc and calyptra cat at the tip. C,
spore case with calyptra removed : o, the cover (operculum). D, a stoma from
the surface of the spore case. E, section of young spore case, showing the
cylindrical central region of spore-producing tissue sp. f\ the spore-producing
tissue in detail. — Adapted after Campbell

are developed in groups of four (tetrads) within spore mother
cells (Fig. 270, F, sp), which form a barrel-shaped tissue (Fig.
270, E, sp) within the spore case.

In spite of the immense numbers of species in the Bryales,
the order is clearly separated from other groups of bryophytes
as a side line of plant evolution, and its families and genera are
distinguished by relatively minor differences.

302 THE BRYOPHYTES

THE ORIGIN AND EVOLUTION OF THE BRYOPHYTES

297. The origin of the bryophytes. The origin of the bryo-
phytes is a mystery. They have of course arisen from the algae,
but there are no living algae that resemble the bryophytes at all
closely. Coleochcete 1 and the stoneworts (Char ales) are the types
most frequently considered in relation to the mosses. But the
sexual organs of Coleochcete are one-celled, and the female organ
of Chara bears only a superficial resemblance to an archegonium,
while its antheridium is totally unlike any other male organ.
There must have been formerly some group of the algae, prob-
ably in the Chloropliycece, distinguished by having many-celled
sexual organs from which the antheridium and archegonium of
the bryophytes arose ; for these complex sexual organs, together
with the characteristic sporophyte generation, constitute the chief
advance of the bryophytes over the algae.

298. The evolution of the bryophytes. The evolution of
the bryophytes is clearly related to the change from the aquatic
habits of the algae to the land habit. Living upon the land exposes
the plant to the drying effects of the air and demands at once im-
portant structural adaptations, that is to say the plant must either
develop a firm cell structure so that drying up will not injure
the tissues seriously, or else it must maintain a constant con-
nection with water through the surfaces of filaments (rhizoids)
which are directly in contact with moisture. Many of the
mosses and leafy liverworts have solved the problem in the first
way and may become quite dry without serious injury. The Ric-
ciz and Marchantia forms and Anthoceros, on the other hand,
are clearly adapted to the second alternative and die at once
if removed from water or moist earth. The creeping habit and
thallus structure of the simpler liverworts, while of advantage

1 The fructification of Coleich&te has frequently been compared to a sim-
ple type of sporophyte, somewhat like those found in the Riccia group of the
liverworts. Recent investigations, however, indicate that this comparison
is not justified, and that the fructification is not sporophytic at all. See
Sec. 336 on the origin of the sporophyte.

SUMMARY OF THE BRYOPHYTES 303

in some situations, d,o not constitute so effective a plant body as
a leafy stem with an erect habit, which secures a much greater
exposure to air and light. Accordingly the appearance of leafy
stems marked a great advance over the thallus structure. This
new form of bryophyte plant body reached its highest develop-
ment when the stem became erect with the leaves arranged
spirally, as in the mosses, so as to give a radial symmetry.

It is quite safe to say that the adoption of the land habit
was the chief cause of the rapid advance of the bryophytes over
the algse. The advance in vegetative structure is generally most
marked in the gametophyte phase of the life history, although
the sporophytes of such types as Anthoceros and certain mosses
are clearly higher than the gametophytes. It may be noted in
this connection that the next great forward step in the evolu-
tion of plants came in the fern group, or pteridophytes, when
the sporophyte generation adopted the land habit and became
independent of the gametophyte. However, this subject will be
taken up in the next chapter.

SUMMARY OF THE BRYOPHYTES AND
THALLOPHYTES

299. Bryophytes and thallophytes compared. It is possible
at this point to make clear the fundamental reasons for the
separation of the spore plants, so far studied, into the two great
divisions of the plant kingdom called the Thallophyta and Bryo-
phyta. It will be seen that the bryophytes have a set of very
clearly denned characters, while the thallophytes are distin-
guished largely by the absence of these.

300. Summary of the bryophytes. The sexual organs are
many-celled structures differentiated into f emaje organs (arche-
gonia) and male organs (antheridia). The fertilized egg develops
at once into an asexual generation, or sporophyte, which pro-
duces asexual spores in groups of four, or tetrads, within cer-
tain cells called spore mother cells. The sporophyte, often called

304 THE BRYOPHYTES

the fruit, alternates with the sexual generation, or gametophyte,
and is always attached to it and dependent upon it for water
and, at least in large part, for certain foods. The asexual spores
produced by the sporophyte are of a new type not found in the
thallophytes.

Class I. The liverworts, or Hepaticce. This class is character-
ized by relatively simple sporophytes (Antlioccros excepted).
The gametophytes are thalloid except in the leafy liverworts,
and have distinct upper and lower surfaces (dorsiventral
symmetry V

Class II. The mosses, or Musci. These have relatively com-
plex sporophytes, whose spore cases open by covers, and the
rim of the spore case is frequently fringed by a circle of teeth.
The gametophytes have erect leafy stems, and the leaves are
generally arranged spirally (radial symmetry).

301. Summary of the thallophytes. The sexual organs are
almost always one-celled structures. The chief exceptions are
the so-called plurilocular sporangia of the brown algse (Sec. 235)
and the peculiar antheridium of the stoneworts (Sec. 230).
There is no organ in the thallophytes resembling the archegoriium
in structure or development. There is no alternation of sexual
generations with asexual in most of the thallophytes. However,
in the red algse (Rhodophycece) and the sac fungi (Ascomycetes)
the fertilized female cell produces peculiar fructifications called
cystocarps and ascocarps, which develop asexual spores and
constitute phases in the life history, alternating with the sexual
plants. These phases are sporophytes, and there is a true alter-
nation of generations in the red algse and sac fungi, but these
structures are peculiar and are believed to be independent de-
velopments in these two remarkable groups and not related to
the sporophytes of the bryophytes. None of the thallophytes
have sexual plants resembling in detail those of the liverworts or
mosses. The plant body is generally a thallus, though the variety
of form is very great, but the highest types in the brown and
red algse are differentiated into stems, leaf-like structures, and

SUMMARY OF THE THALLOPHYTES 305

holdfasts. The cell structure of the thallophytes is generally
much simpler than that in the plant bodies of the bryophytes,
which owe their complexities of cell structure chiefly to the
varied conditions introduced by the land habit; for the land
habit requires the plant to protect itself from drying up, in
the air. This, in general, means that a land plant must obtain
water from the soil through some kind of organs adapted for that
purpose (rhizoids or roots). And, as a rule, a land plant soon
differentiates a protective layer of cells (epidermis), which helps
to hold the water within its tissues. These structures are either
entirely absent or present in greatly reduced form in aquatic
plants, and for these reasons the cell structure of the aquatic
thallophytes is generally very much simpler than that of the
bryophytes.

Nevertheless, the thallophytes have developed some compli-
cated organs with highly differentiated tissues, as in the kelps,
rockweeds, red algae, sac fungi, and the higher basidia fungi,
such as the toadstools and mushrooms, puffballs, nest fungi,
and carrion fungi. These complexities are, however, very spe-
cial in character and not related to the structure of higher
groups of plants.

CHAPTEK XXV

THE PTERIDOPHYTES AND THE APPEARANCE OF
HETEROSPORY

302. The pteridophytes.* The division Pteridopliyta (mean-
ing fern plants) comprises three classes : (1) the ferns, or Fili-
cinece, (2) the horsetails, or Equisetinece, and (3) the club mosses,
or Lycopodinece. Representatives of these groups are generally
somewhat familiar to all, and no one would think of grouping
them with the liverworts and mosses. The differences become
more conspicuous after a study of the life histories of pteri-
dophytes, which shows that the large fern plant with its roots,
stem, and leaves is really an asexual generation, or sporophyte,
and that the gametophyte is represented by a small, compara-
tively insignificant sexual generation. This condition, so dif-
ferent from anything in the bryophytes and thallophytes,
marks one of the great forward steps in the progress of plant
evolution. It leads towards the seed plants, for these highest
forms with their varied and complex structures are sporophytes,
whose gametophyte generations are so much reduced that they
can only be recognized by careful study of the processes of seed
formation.

303. The advances in plant evolution up to the pteri-
dophytes. It is well to summarize at this point the contribu-
tions of the thallophytes and bryophytes to the progress of
plant evolution.

1. The algce. The chief contributions of the algre to plant
evolution were four in number: (1) the attached many-celled
plant body arose from the single-celled condition of the lowest

* To THE INSTRUCTOR : The introduction to this chapter assumes that the
life history of some fern has been studied in the laboratory.

306

THE ADVANCE IN PLANT EVOLUTION 307

alg?e and soon became established as the vegetative period in
the plant's life history ; (2) as a result of this, the motile
stages (zobspores) became set apart as reproductive phases in
the life histories, such reproductive motile stages, with other
reproductive cells, being called spores ; (3) certain of the repro-
ductive cells became sexual in character, and these gametes at
first similar (isogamy) were later differentiated into eggs and
sperms (heterogamy) ; (4) alternation of generations developed
'in the red algae and sac fungi, but probably independently of
the same phenomenon in the bryophytes.

2. The fungi. The fungi as special and peculiar offshoots
from the algae have of course contributed nothing to the main
evolutionary line running up to the higher plants.

3. The liverworts and mosses. The chief advances of the
liverworts and mosses over the algae were three in number:

(1) many-celled sexual organs (antheridia and archegonia) took
the place of the one-celled reproductive organs of the algse;

(2) an alternation of generations (gametophyte with sporo-
phyte) became well established, together with the origin of a
new type of asexual spore developed in groups of four (tetrads)
by the sporophyte ; (3) there was a general advance in the cell
structure of the plant bodies because of adaptations to the more
complex conditions of the land habit.

The sporophyte of the bryophytes is always attached to the
gametophyte, and except in Anthoceros and some mosses it is
not as complex as the gametophyte. In the pteridophytes, how-
ever, the conditions are reversed. The sporophyte is the large,
conspicuous phase in the life history, and as it develops it becomes
entirely independent of the gametophyte, while the latter appears
relatively insignificant, although it holds, of course, a necessary
place in the life history. The appearance in the pteridophytes
of the sporophyte as an independent plant was the most impor-
tant advance in plant evolution at this time, for the vegetative
activities gradually became shifted, at first chiefly and finally
wholly, from the sexual to the asexual generation.

FIG. 271. A fern (Aspidium Filix-mas)

A, part of the creeping stem, or rootstock, and fronds: fr, young fronds unrolling.
B, under side of a frond, showing sori s. C, section through a sorus at right
angles to surface of the leaf, showing indusiura i and sporangia s. D, a spo-
rangium discharging spores. — After Wossidlo

THE FERNS

309

The material of this chapter will be treated under the
following headings:

Class I. The ferns, or Filicinece.

Class II. The horsetails, or Equisetinece.

Class III. The club mosses, or Lycopodineat.

Fossil plants and coal.

The origin and evolution of the pteridophytes.

Summary of the pteridophytes and their advances over the bryophytes.

CLASS I. THE FEKNS, OR FILICINEM

304. The ferns. The ferns are a very large assemblage of
more than four thousand species, and most of them can be
recognized at a glance by
the characteristic forms of
their leaves, called fronds,
and by their habits of
growth. They are widely
distributed, but reach their
greatest luxuriance in the
tropics, where they present
some very striking dis-
plays. Thus the tree ferns
have stems thirty or forty
feet high, with a crown of
fronds often fifteen or more
feet in length. The stems
of some of the tree ferns
are covered by a sheath of
fibrous roots (as in Dick-
sonia, Plate VII), and in
other types the bases of FlG< 272< The stag.horn fern (piatyCcrium
the old and withered fronds Willinki)

form a similar investment. A tropical epiphytic fern with two forms of

There are also in the tropics lea ves' °ne ?f which grows close]5r a§ainst

the bark of trees and gathers and holds
Certain Small delicate ferns moisture and humus. — After Goebel

310

THE PTERIDOPHYTES

called filmy ferns, whose stems and fronds are as delicate as
mosses. Some peculiar types, as the stag-horn fern (Platy cerium,
Figs. 272, 364), grow over the surface of tropical trees and are
consequently called epiphytes (meaning upon a plant). These have
certain flattened leaves (Fig. 2 72) closely pressed against the sur-
face to which the plants are attached, where they gather and hold
moisture and humus. The roots of the filmy
ferns and the epiphytes are very poorly devel-
oped, or entirely wanting, but the air of a tropi-
cal forest is so saturated with water that they
obtain all that they need from the dripping mois-
ture and wet surfaces upon which they grow.

All the plants which one would at a glance
call ferns are sporophytes, and they consequently
produce asexual spores, which are borne upon
their fronds. These spores correspond exactly
to those developed by the sporophytes of the

FIG. 273. The walking fern
(Camplostrus rhizophyllus)

Showing the manner in which
fronds hearing reproductive
buds at their tips bend over
and establish new plants

liverworts and mosses, and they give rise to a small sexual
generation, the gametophyte. The life history of a pteridophyte
can be most easily studied from one of the common ferns famil-
iar to us in the woods, greenhouses, and gardens.1

i The moonwort and adderVtongue (Sec. 315) illustrate more primitive
conditions in the pteridophy tes than the common ferns, but are not generally
available for type studies.

PLATE VII. Tree ferns (Dicksonia antarctica) from Tasmania

These tree ferns grow to be 30 to 40 feet high, with fronds 8 to 12 feet long, and the
trunks, densely covered with small roots, may become 3 feet thick. — After a
photograph in the Harvard Museum

THE COMMON FERNS 311

305. The common ferns. The common ferns (order Filicales)
completely outclass all other orders of pteridophytes in num-
ber of species and mass of vegetation. The forms are exceed-
ingly various. The stems may be short and close to the ground,
or upright trunks, as in the tree ferns. But many types have
creeping stems, frequently wholly buried in the earth as under-
ground stems, or rootstocks, well illustrated by the common brake,
or bracken fern (Pteris aquilina). Some ferns have peculiar
methods of reproduction by buds that are formed on the leaves,
as in the bladder fern (Cystoptcris bidbifera), or the walking
fern (Camptosorus) shown in Fig. 273.

The fronds or leaves arise from the tip of the stem and form
clusters or crowns around the top of upright structures, but are
generally somewhat scattered along the creeping sterns. Most
fronds are much cut or divided (compound) after regular and
various patterns (Fig. 271). They are developed very slowly
in some genera, remaining rolled up in the bud for several
months. However, when fully formed and in the proper season
they unroll comparatively quickly from the base in a very
characteristic manner until the apex finally appears above.

The cell structure of the leaves, steins, and roots is very
much more complex than the cell structure in the bryophy tes
and recalls at once the tissues of the seed plants (see Part I,
Chapters vii and xn). The plant body has a system of tissues,
called fibro-vascular bundles (Fig. 274), whose parts are much
modified for two important functions. One tissue is composed
of large cells (Fig. 274, t) empty of protoplasm and with heavy
thickened walls marked with curious pits. These elements,
called traclieids, compose the woody part of the fibro-vascular
bundle termed the xylem, and their purpose is to conduct water
from the roots to parts of the plant above ground. But they
are also very important for the strength that they give to stems
and leaves. Another tissue is composed chiefly of cells, called
sieve tubes (Fig. 274, st), which contain much protoplasm and
food material and make up a softer region of the bundle termed

312 THE PTERIDOPHYTES

bast or phloem. The bast regions are known to be paths for the
distribution of food material in the plant. The structure of the
fern frond is essentially similar to that of the leaf of a seed
plant. There are stomata on the lower surface and chlorophyll-
bearing tissues underneath the outer cell membrane, or epidermis.
The nbro-vascular bundles run out into the green expanded por-
tions of the leaves as forking veins, which do not, however,

FIG. 274. Fibro-vascular bundle from the underground stem, or rootstock, of
the common brake (Pteris aquilina)

g, ground tissue, or parenchyma; 6s, bundle sheath; ps, bast, or phloem, sheath
surrounding the sieve tubes (st) and bast fibers which compose the bast, or
phloem ; t, large, thick-walled cells called tracheids, which with smaller cells
in the center make up the wood, or xylem

generally unite to form the close network so characteristic of
dicotyledonous seed plants.

As stated above, the fibro- vascular bundles greatly strengthen
the tissues of the leaves and stems, for they form a sort of
skeleton in the plant. They are frequently assisted in their
strengthening functions, especially in the stem, by regions of
rigid tissue, which may be variously situated, sometimes under

SPORE FORMATION 313

the epidermis and sometimes forming broad strands in the
interior. This rigid tissue (selerenchyma) is composed of elon-
gated cells with very heavy, much-thickened walls, which are
often yellowish in color. This tissue is developed from the
thin-walled cells (parenchyma), called the ground tissue, that
compose the greater part of the interior of the stems.

306. Spore formation. The sporophyte nature of the fern
plant Becomes clear at the time of fructification. Certain ones
or sometimes all of the fern fronds as they grow older develop
spore cases, or sporangia. These are variously situated on the
fronds, sometimes appearing as clusters or spots, called sori
(singular, sorus, meaning a heap), on the under surface and some-
times in lines along the under edge. A sorus may be naked,
but it is frequently protected by a membranous outgrowth, or
indusium, from the surface of the frond (Fig. 271, C, i).

The sporangia are stalked and somewhat flattened many-
celled cases, each of which develops from a single surface cell
of the frond. There are sixteen spore mother cells in the inte-
rior of the spore case, each of which gives rise to a group of
four spores (tetrad). The method of spore formation, four spores
in each mother cell, is thus identical with that of the bryophytes.

The sporangium of many common ferns is composed of thin-
walled cells except along the edge, where there is a line with
much-thickened wralls, which extends from the stalk about two
thirds around on the outside (Fig. 275, A). This line of cells is
called the ring, and as the sporangium ripens and becomes dry,
the ring is forcibly held like a bent spring. Finally, the deli-
cate cells at the side of the spore case opposite the ring are
unable to stand the strain and are torn apart so that the ring
straightens somewhat and a wide rent is made in the side of
the sporangium (Fig. 275, B, C}. The spores are thrown out vio-
lently through the rent for a considerable distance. This is the
structure of the spore case in the family Polypodiacem, for the
several families of the Filicales have sporangia which differ from
one another in form and in the structure and position of the ring.

314

THE PTERIDOPHYTES

307. Fronds, vegetative leaves, and spore leaves (sporo-
phylls). Most fronds are vegetative, that is, perform chloro-
phyll work (photosynthesis) during the early part of the season
and develop sporangia later. However, some types, as the royal
and cinnamon ferns (Osmunda) and the sensitive and ostrich
ferns (Onoclea), devote the whole of certain leaves or portions of
them entirely to the work of spore production. The blades of
these fronds or portions of fronds never become expanded, but
remain somewhat rolled up, forming pod-like structures in which
the sporangia are developed (Fig. 276). Other fronds on these

FIG. 275. The sporangium of a common fern (Aspidium Filix-mas)

A, closed sporangium; B, sporangium opening; C, fully opened and discharging
the spores. — After Kerner

ferns are devoted entirely to vegetative activities and never de-
velop sporangia. There is thus in some ferns a division of labor
among the fronds, certain of them becoming strictly vegetative
leaves, while others are spore leaves, called sporophylls.

There is a constant tendency in the pteridqphytes to give all
the work of spore production to the specialized spore leaves
(sporophylls), which means that all the other fronds on the
plant become entirely devoted to vegetative activities and may
then be called vegetative leaves or simply leaves in the sense in
which this term is generally used in the seed plants. This
differentiation of the frond into leaves arid sporophylls reaches

THE GAMETOPHYTE OF THE FERN

315

a high point of development in
the horsetails and club mosses,
and becomes even more con-
spicuous in the seed plants, as
will appear later.

308. The gametophyte of the
fern. The fern spore germinates
readily on moist surfaces and
puts forth a delicate filament,
consisting of a row of cells (Fig.
277, A). Several oblique cell
walls at the end of this filament
cut out a triangular apical cell
(Fig. 277, B, x), which becomes
the growing point. The final de-
velopment is usually a small,
delicate, heart-shaped, thallus-
like body resembling a small
liverwort, but only one cell in
thickness, except in the middle
region. The apical cell (Fig. 277,
C, x) generally becomes situated
in a deep notch at the forward
end (Fig. 277,D) because of the
greater cell growth on either side.
The back part of the structure
becomes fastened to the earth by
numerous delicate filaments or
rhizoids which act like root hairs.
This structure develops the sex-
ual organs of the fern and is con-
sequently the gametophyte gener-
ation in the life history. It is called

FIG. 276. The sensitive fern (Onoclea
sensibilis)

the prothallium because it pre- Showing vegetative leaf and spore leaf

(sporophyll) rising from the creeping

cedes the fern plant (sporophyte). rootstock

316

THE PTERIDOPHYTES

Both sexual organs (antheridia and archegonia) are found on
the same prothallium if it is well developed. But when pro-
thallia are crowded or grown under other unfavorable conditions
they remain small and stunted and become irregular in form
(Fig. 278, A). Such dwarf gametophytes only develop antheridia.

FIG. 277. The fern prothallium and archegonium

A, stages in the germination of the spore. B, young prothallium, showing first
/ appearance of wedged-shaped, apical cell x, C, tip of prothallium beginning
to take on the heart-shaped form: x, apical cell. Z), mature prothallium,
showing group of archegonia on the cushion just hack of the notch, and anther-
idia further back : rh, rhizoids. E, an open archegonium with egg ready for
fertilization, and two sperms near the entrance of the neck. — A, B, C, E, after
Campbell; D, after Schenck

On well-nourished prothallia the antheridia are formed first on
the edge and lower surface of the back portions. The arche-
gonia are developed last when the prothallium is quite large, and
are only found on the thickened region, called the cushion,
directly back of the notch, or growing point.

The antheridia. The antheridia (Fig. 278, B, C} are very much
smaller than those of the bryophytes. They develop from a

THE GAMETOPHYTE OF THE FERN

317

single cell which projects above the surface of the prothallus.
There are only three cells forming the capsule of this structure,
— a cover cell above, a ring-shaped cell in the middle, and a
funnel-shaped, basal cell. These three cells inclose at first a large
central protoplast, from which is developed a group of one hun-
dred or more small cubical cells
that produce the sperms, as in the
bryophytes. These sperms are, how-
ever, very different in form from
the two-ciliate sperms of the liver-
worts and mosses and many algse.
Each consists of a spirally coiled
band (Fig. 278, Z>), whose narrower-
pointed end is covered with numer-
ous cilia, making it a many-ciliate
sperm.

The archegonia. The archegonia
of the ferns are also much smaller
than those of the bryophytes and
simpler in structure. The short
neck alone projects above the sur-
face of the prothallus (Fig. 277, E)
and generally bends backward,
probably because the forward part
of the prothallium is not directly
on the earth, but rises at an angle.
The egg lies beneath the surface of
the prothallium, so that the base
of the archegonium may be described as sunken. There are
only two or three canal cells (Sec. 283) in this archegonium.

The eggs are fertilized under exactly the same conditions as
in the bryophytes (Sec. 283). When the prothallia are wet the
sexual organs open, and the sperms swim over the moist surfaces
and are attracted to the necks of the archegonia by substances
secreted within, one of which at least is malic acid. The

FIG. 278. The antheridium and
sperms of a fern (Onoclea)

A, small prothallium with many
antheridia an : s, old spore wall.
B, antheridium, showing cover
cell c, ring cell r, and basal cell
b, inclosing the sperm mother
cells. C, antheridium opening.
D, sperms. — After Campbell

318

THE PTERIDOPHYTES

sperms swim down the neck to the egg, and one of them fer-
tilizes it. The fern plant then, like the liverwort and moss,
practically returns to the aquatic life of the algae at the time
when the sexual cells are functional.

309. The development of the sporophyte. The early stages in
the development of the fern sporophyte as in the bryophytes

FIG. 279. Development of the sporo-
phyte of a fern

A, section of prothallium with a young sporo-
phyte : c, thickened region, or cushion, in which
is imbedded the foot; /, first leaf; r, root; ar,
unfertilized archegonia; an, old antheridia;
rh, rhizoids. B, an old prothallium with young
fern sporophyte attached, whose first leaf I has
grown up through the notch at the forward end
of the prothallium, while the root r has entered
the earth : rh, rhizoids. — After Sachs

are passed entirely within the tissue
of the prothallium, surrounded by the
remains of the archegonium. The ferti-
lized egg cell divides, and there are
formed four regions in the embryo
fern : (1) a stem region, (2) the first leaf, (3) the first root, and
(4) an organ of attachment to the gametophyte called the foot.
The leaf and root soon break out of the archegonium, the first
growing upward and the second into the earth (Fig. 279, A, B).
The stem grows more slowly. The young fern all this time
obtains nourishment from the prothallium through the foot after
exactly the same method as in the bryophytes. However, when

SUMMARY OF THE LIFE HISTORY OF A FERN 319

the root and leaf are well established the sporophyte becomes
independent of the gametophyte, which gradually dies within
a few weeks or months. It is the development of root, stem,
and leaf on the part of the sporophyte, giving it complete inde-
pendence, which marks the greatest advance of the pteridophytes
over the bryophytes.

310. Summary of the life history of a fern. The alterna-
tion of generations in the fern is much more apparent than in
the liverworts and mosses because both gametophyte and sporo-
phyte are independent plants. The two groups (bryophytes and
pteridophytes) are in striking contrast in the relative importance
of the two generations. The gametophytes of the bryophytes
are relatively large, long-lived, and complex organisms (with
stems and leaves in the mosses and leafy liverworts), while the
sporophytes are simple and so dependent upon the gametophyte
that they were for many years called its fruit. The gameto-
phytes of the pteridophytes, on the contrary, are small, short-
lived, and simple, while the sporophytes are very large and
complex (possessing stem, roots, .leaves, and a vascular system)
and, except in their earliest stages of development, completely
independent of the gametophytes.

The life history of a fern may be formulated as follows :

Gametophyte (prothallium) <^ ^ > — Sporophyte (fern plant)

*J u

— asexual spores — Gametophyte, etc.
This in abbreviated form becomes

- S — sp — G, etc.,

and is the same life-history formula as that of the bryophytes
(Sec. 285).

311. Apogamy and apospory in the ferns. There are some
irregularities in the life histories of certain ferns which are
not uncommon in greenhouses and under cultivation (species
of Pteris, Aspidium, Athyrium, Nephrodium, etc.). Prothallia

320

THE PTERIDOPHYTES

sometime fail to develop archegonia or the archegonia do
not function, but the sporophyte generation arises as a bud-like
outgrowth from the prothallium. In other cases the egg may
develop without fertilization (parthenogenesis). Such suppres-
sions of sexuality with the development of a succeeding genera-
tion asexually are called apogamy. The phenomenon has been
noted before in the water molds (Sec. 262) and other fungi, and
it is found in various groups throughout the plant kingdom.

Apospory is the suppression of the process of spore formation
and the development of a gametophyte generation directly from
the sporophyte. It is found in many of the ferns, which are also
apogamous, and is shown by the presence of prothallia, which
are direct outgrowths from the fern frond
in the place of the sporangia, or sometimes
at the tips. Apospory is also found in
certain mosses where protonema may
develop directly from portions of the stalk
and spore cases of the sporophytes.

Apogamy and apospory are both short
cuts in the life histories, which are believed
to be due to some unusual life conditions
that interfere with the regular develop-
ment of gametes and spores in the normal
life histories, established during the evolu-
tion of plant groups.

312. The water ferns. The water ferns
(order Hydropterales] include four inter-
esting genera (Marsilia, Pilularia, Sal-
and Azolla}, each of which is
for some peculiarity of struc-
ture. Salvinia (Fig. 280) and Azolla are
floating aquatics, and Marsilia and
Pilularia are either aquatic or grow in very wet places. These
habits give the common name of water ferns to the group. They
are important illustrations of the condition called lieterospory,

FIG. 280. A water fern
(Salvinia)

/, floating leaves; r, highly
modified leaf acting as a
root; s, spore fruits. —
After Pringsheim

THE WATER FERNS

321

which is briefly described in
Sec. 214 and discussed in
some detail in Chapter XXVIL
The Hydropterales are be-
lieved to have been derived
from the Filicales, and the
development of heterospory is so
the most important advance
over that group. We can only
consider the rather widely
distributed type Marsilia.

313. Marsilia.* Marsilia,
the clover leaf fern, or pepper-
wort, is easily recognized from
the form of the leaf (Fig.
281, A). The leaves arise
from a creeping stem which
in certain species, as M.
quadrifolia, grow over the
mud in shallow water along
the margins of ponds and
streams, but often come out
of the water upon muddy
banks. Other species, as M.
vestita, grow almost entirely
on muddy banks or in wet
meadows.

The spores of Marsilia are
developed in bean-shaped

FIG. 281. Marsilia

A, creeping stem of Marsilia quadrifolia,
showing a series of leaves in various
stages of development: s, spore fruits
(sporocarps). B, a spore fruit of
M. vestita, which has opened in water
and extruded a gelatinous, worm-like
structure bearing sori so

* To THE INSTRUCTOR : If only
one heterosporous pteridophyte can
be studied in the laboratory, it is
much better that the type be Sela-
ginella. For this reason the ac-
count of Marsilia has been made short. The life-history formula is, of
course, the same as that of Selaginella, which is fully treated in Sec. 326.

322

THE PTERIDOPHYTES

spore fruits, or sporocarps, borne in groups on short stalks.

These spore fruits (Fig. 281, A, s) are really modified portions of

leaves, which are excellent illus-
trations of very highly developed
sporophylls, much more special-
ized than the spore leaves of such
ferns as Onoclea and Osmunda.

The spore fruits burst open
when soaked in water through
the swelling of mucilage within,
and the contents come out as a
gelatinous, worm-like structure
bearing large groups (sori) of spores
along the sides (Fig. 281, B, so).
The spores are developed in es-
sentially the same manner as in
the common ferns (Filicales), but
the tissues of the sporangia are
so much modified that the re-
semblances can only be followed
through a detailed developmental
study. The spores are set free
by the softening of the gelatinous
material/and they begin to germi-

A, male gametophyte within micro- • i ?

spore: p, prothaiiial cells; two nate at once in the water. -They

groups of sperm mother cells shown f fc &[ k &nd gmall

within. B, sperms. C, female

gametophyte consisting of a single and are consequently called mega-

archegonium at one end of the 8poreS Bud microtpores. This con-
megaspore, which is filled with "

starch grains. D, a week-old em- dition is termed lieterospory.

bryo (slightly magnified) still at- ^. mpo-oonnrpq are full of
tached to the megaspore: I, first

leaf; r, root. —A, B, after Camp- starch grains, which furnish the

food for the development in a

few hours of a small female gametophyte. This gametophyte
(Fig. 282, C) consists of a single archegonium at one end of the
spore. Although the cells are somewhat greenish, it is perfectly

FIG. 282. Gametophytes of
Marsilia vestita

MARSILIA 323

clear that the food of this much-reduced prothallium is fur-
nished chiefly by the sporophyte by means of the megaspore.
The gametophyte of Marsilia has therefore degenerated from
the independent condition in the common ferns and is now
no longer self-supporting, but is dependent upon food stored by
the sporophyte, a relation which is exactly the reverse of that
in the bryophytes.

The microspore develops a very small male gametophyte even
more quickly than the megaspore develops the female one. This
structure consists of a lens-shaped sterile cell called the prothal-
lial cell (Fig. 282, A, p), together with a group of cells which
probably represent a single much-reduced antheridium. The
sperms are formed within this group. They are remarkably long,
coiled bands covered with cilia (Fig. 282, B), and are among the
largest sperms in the pteridophytes.

The young sporophyte develops within the archegonium, fol-
lowing essentially the same history as that of the common
ferns, and is consequently attached to the megaspore (Fig. 282, Z>).
But there is an important peculiarity in its relation to food
supply. This sporophyte makes use of considerable food that
remains in the megaspore after the development of the female
gametophyte. The Marsilia plant, therefore, actually provides
for the next sporophyte generation by storing food in the mega-
spore. This provision is strikingly similar, as will appear later,
to the conditions in the seed where the embryo (young sporo-
phyte) is nourished by food stored in the seed by the sporophyte
of the preceding generation.

Marsilia illustrates exceptionally well three important prin-
ciples in the evolution of pteridophytes and seed plants, namely :
(1) the establishment of heterospory, resulting in the separation
of male and female gametophytes, (2) the reduction or degenera-
tion of the gametophytes, which become dependent upon food
stored in the microspores and megaspores, and (3) provision in
the megaspore for the nourishment of the embryo of the suc-
ceeding sporophyte generation.

324

THE PTERIDOPHYTES

314. Heterospory. Heterospory (meaning dissimilar spores)
arose in the pteridophytes with the establishment of two sizes
of spores, — megaspores and microspores. Pteridophytes having
these are called heterosporous, and those with spores of the same
size, as in the common ferns (Mlicales), are called homosporous

(meaning similar spores). With
heterospory came also a differen-
tiation of the gametophytes into
male and female structures, the
first developing from the micro-
spores and the second developing
from the megaspores.

315. The moonwort and
adders-tongue. The moonwort
(Botrychium, Fig. 283, A) and the
adder's-tougue (Opliioglossum,
Fig. 283, B) are in the same
group (order Opliioglossahs) and
illustrate certain primitive con-
ditions in the pteridophytes.
These forms do not have external
sporangia, as in the Filicalcs, but
the spores are developed in sunken
regions along peculiar stalks.
Such sunken sporangia are much
more primitive in structure than
those which develop upon the sur-

A, the moonwort (Botrychium terna-

tum) ; B, the adder's-tongue (Ophi- face of the plant,for they resemble
ogiossum vulgatum) more ciosely the conditions in the

bryophytes where the spore mother cells are found in the interior
of the plant. The spore-bearing stalks are accompanied by sterile
blades devoted to the vegetative activities, so that these leaves
illustrate the same sort of division of labor as is found in the
royal fern (Osmunda regalis). The gametophytes are under-
ground, tuberous bodies generally, destitute of chlorophyll and

FIG. 283. The moonwort and
adder's-tongue

THE HORSETAILS 325

saprophy tic in their life habits, being associated with certain fungi
which form a mycorrhizal partnership (Sec. 278) with them.

CLASS II. THE HORSETAILS, OR, SQUISETINE&

316. The horsetails. The horsetails, or scouring rushes, are
all comprised in the genus Equisetum, which contains about 40
living species, the sole modern representatives of the order
Eguisetales and the class Equisetinece. These plants are the
remnants of what was a very extensive flora in an early geo-
logical period, called the Carboniferous Age, when the largest
deposits of coal were formed. The ancient relatives of Equi-
setum (Plate VIII, 2), together with the club mosses, were then
trees and formed the forests in those times. The horsetails live
now under what seem to be rather severe conditions in bare or
sandy soils that are unfavorable for the growth of trees, herbs,
and grasses. They illustrate very well the way in which an
ancient group is sometimes able to avoid total extinction by
withdrawing as far as possible from competition with the
recent floras, and thus hold its own by means of peculiar life
habits and adjustments to special conditions.

The most striking feature of the morphology of Eqidsetum is
the total absence of foliage suitable for vegetative activities
(photosynthesis). The foliage is represented by sheaths (Fig. 284,
Ay IB), which are found at the joints of the hollow stem.
The points on these sheaths are the tips of small leaves
more or less united below. The vegetative functions are per-
formed by the green stems. These are fluted, that is, they have
numerous ridges which run lengthwise, and the depressions
between the ridges contain stomata, which lie above the chloro-
phyll-bearing tissues (Fig. 284, F, c). The epidermis contains
deposits of silica, which is the chief substance in glass, and con-
sequently the stems feel rough. They are sometimes used for
scouring or polishing metal; hence the origin of one of the
common names, " scouring rush."

FIG. 284. A horsetail (Equisetum arvense)

A, fertile stems, bearing cones rising from the creeping rootstock : t, tuberous
bodies; v, young vegetative stern below ground and ready to grow into the
mature structure shown in B. 13, vegetative stem as it appears perhaps three
weeks after the fertile stems have shed their spores and died. C, a group of
spores with their elaters expanded. Z), a spore with the elaters coiled around
it. E, two views of the spore leaves (sporophylls), showing the group of spo-
rangia. F, portion of a section of the stem: a, air spaces; c, chlorophyll-bear-
ing tissues ; r, rigid outer tissues ; /, fibro-vascular bundle around small air
space. — A, B, C, D, after Schenck

326

THE CONE OF EQUISETUM 327

The stems are generally of two forms. There are green aerial
stems above ground, unbranched in some species, but quite
bushy in others by the development of circles of side branches
at the joints (Fig. 284, B). The aerial stems arise from creeping
underground sterns, or rootstocks, which have the same jointed
structure and sheaths of degenerate leaves, but are not green
and often not hollow. The underground stems live from year
to year and grow rapidly through the soil, frequently estab-
lishing large beds of horsetails, as, for example, along railroad
tracks and the margins of sandy pools and ponds.

The stem has large, central air cavities, running from joint to
joint, and also a number of smaller air canals, alternating with
the fibro-vascular bundles (Fig. 284, F, a). It is strengthened
by thick- walled cells, forming a rigid tissue (Fig. 284, F, r) under
the epidermis, and is consequently well protected from the dan-
ger of drying up. These peculiarities, together with the reduced
leaf surface, are characters which the horsetails have in common
with many desert plants (xerophytes), and they permit them to
live when necessary under very severe drought conditions.

The fructification of Equisetum is a cone (Fig. 284, A) devel-
oped at the tip of the stem, and it is composed of scale-like
spore leaves (sporophylls), which fit closely together and develop
spores in sporangia upon their under surfaces (Fig. 284, E).
These cones are generally found on ordinary green stems. How-
ever, in some species, as E. arvense, the stems which first appear
above ground are pale in color and are devoted entirely to the
development of the cones and die after the spores are shed, while
the green vegetative stems appear later.

317. The cone of Equisetum. A cone, or strobttus, is a com-
pact group of spore leaves (sporophylls) distributed around the
tip of a stem and distinct from the rest of the plant. It takes
its compact form because the sporophylls are closely set together
and frequently so much modified that their structure is not
apparent at a glance. Each sporophyll in Equisetum consists
of a short stalk attached to the side of the stem and bearing an

328

THE PTERIDOPHYTES

angled shield-shaped top (Fig. 284, E~). A group of sporangia
hang down all around the stalk from the lower surface of the
shield, and each develops from a group of cells instead of from
a single cell, as in the common ferns (Filicales). The shields
separate from one another when the cone matures, and the ripe
spores escape through rents in the sporangia and sift out between

the shields. The spores are
formed in groups of four
(tetrads) in the spore mother
cells.

Each spore (Fig. 284, C, D)
bears four filaments de-
veloped from an outer layer
of the spore wall, which
splits into bands that sepa-
rate from one another, but
remain attached to the spore
at one point. The filaments
coil around it when moist,
but loosen and spread out
when dry. These movements
must assist the escape of the
spores from the sporangia.
The filaments also serve as
wings in the distribution of
the spores by the wind, and

FIG. 285. Gametophytes of Equisetum

-4, male prothallium: an, antheridium.
B, sperms. C, female prothallium : ar,

archegonium.-^, C, after Hofmeister; they become entangled with

B, after Schacht one another so that groups

cling together and are carried away and germinate in clusters.

The spores are of the same size, and therefore Equisetum is

homosporous.

318. The gametophytes of Equisetum. The spores only
retain their vitality for a few days. They produce green gameto-
phytes somewhat like fern prothallia, but very irregular in form
(Fig. 285, A, C), the larger with long lobes, at the bases of which

THE CLUB MOSSES 329

are situated the sexual organs. The prothallia are normally dioe-
cious, that is, male and female in sex, but since the spores are
distributed in groups, antheridial plants are likely to develop in
the same cluster with the archegonial. The sexual organs are
sunken in the tissues of the gametophytes. The sperms are
coiled, many-ciliate protoplasts (Fig. 285, B) resembling those
of the common ferns.

The early stages in the development of the young Equisetum
sporophyte from the fertilized egg are the same as those of the
common ferns. This together with the similar gametophytes
and sperms is believed to indicate a distant relationship between
the Equisetinece and Filicinece, even though the mature sporo-
phytes of the two groups appear so different in structure.

CLASS III. THE CLUB MOSSES, OB LYCOPODINE^E

319. The club mosses. The Lycopodinece take their common
name of club mosses from the moss-like appearance of the stems,
which in most forms are covered with small leaves (Figs. 286,
289, A), and the fructification, which is generally a club-shaped
cone developed at the end of the stem (Figs. 287, A\ 289, A).
Isoetes (Fig. 291) is, however, in these particulars a conspicuous
exception. But the club mosses are very much larger than any
of the true mosses (Musci), and are of course sporophytes, like
the horsetails and ferns, while the true mosses are gameto-
phytes. Like the horsetails, they are the remnants of a very
ancient group which formed forests in the Carboniferous Age
(Plate VIII, 3, 4) ; also, they have been able to persist only by
adapting themselves to life conditions where they do not en-
counter keen competition with grasses and herbs. Almost all
of the Lycopodinece are contained in three genera : Lycopodium
(about 100 species), Selaginella (about 500 species), and Isoetes
(some 60 species). But in addition there are several remark-
able types (Pliylloglossum, Psilotum, Tmesipteris) which are
tropical or sub-tropical and cannot be described here.

330

THE PTERIDOPHYTES

320. Lycopodium. Lycopodium includes the larger club
mosses, frequently called lycopods, and are distinguished by hav-
ing needle-like leaves arranged spirally on the stem (Fig. 286)
and similar spores (homosporous). The stems are of two forms :
(1) creeping stems, close to the ground and frequently buried

FIG. 286. A club moss (Lycopodium annotinum)
Modified after Kerner

under leaves and other forest debris, and (2) upright sterns, very
much branched in some species and bearing the cones like
clubs at their ends. Some of the larger species are very common
in the northern woods, the long, creeping stems often growing
thickly over the ground. The stem is generally quite woody in
structure, and the leaves are evergreen. They are much used
in holiday decoration, and certain species are in danger of extinc-
tion, since the club mosses reproduce very slowly.

THE CONE OF LYCOPODIUM

331

321. The cone of Lycopodium. In some species of

as L. Selago, the spore leaves (sporophylls) have the
same form and grouping as the vegetative leaves so that there
is no cone distinct from the rest of the stem. But most of the
forms have very clearly defined cones, which are sometimes raised
on long stalks, as in L. compla-
natum. The sporophylls are
generally scale-like and closely
set (Fig. 287, A, B). Each spore
leaf bears a single, large, sac-like
sporangium (Fig. 287, C) at its
base, which develops from a
group of cells. The spores (Fig.
287, D) are formed in groups
of four (tetrads) in the spore
mother cells. They are very
minute and are produced in such
immense numbers that they are
collected in quantity as the
lycopodium powder of apothe-
cary shops, used in dusting
pills to keep them from sticking
together as well as for other
purposes. This powder is also
employed in the manufacture
of fireworks under the name of
vegetable sulphur.

322. The gametophytes of
Lycopodium. The gametophytes

of the club mosses in our northern woods must be uncommon, if
they are developed at all, for they have never been found. It is
probable that the sporophy tes reproduce chiefly or perhaps entirely
by vegetative brandling of their stems and in some forms by
curious buds. The gametophytes of some tropical lycopods are
however known and have been studied. They are small, tuberous

FIG. 287. The cone of a club moss
(Lycopodium annotinum)

4, the cone, showing overlapping sporo-
phylls; i>, diagram of a longitudinal
section, illustrating the form and
position of the sporophylls and spo-
rangia ; C, the inner face of a sporo-
phyll, showing the large sporangium ;
.D, two views of spores from a group
of four (tetrad)

332

THE PTERIDOPHYTES

bodies (Fig. 288) generally

FIG. 288. Gametophytes and
young sporophy tes of a club moss
(Lycopodium complanatum)

A, gametophyte with young sporo-
phyte : /, tissue filled with the fila-
ments of a fungus situated just
outside a layer of palisade cells.

B, the fungus-infected tissue. C,
a young club moss still attached
to the subterranean gametophyte.
— A, B, from material of Bruch-
mann prepared by Miss Lyon ;

C, after Bruchmann

subterranean and practically desti-
tute of chlorophyll, like those of
the moonwort and adder's-tongue.
They are therefore saprophytic,and
associated with them are fungal
filaments to form a mycorrhizal
relation (Sec. 278). The sexual
organs are sunken structures.
The sperms are two-ciliate.

The embryo sporophyte remains
attached to the gametophyte by a
large foot (Fig. 288) for a long
time after the stem and root are
developed, and must obtain much
nourishment from the gameto-
phyte, as in the case of the ferns.

323. Selaginella. Sclaginclla
is readily distinguished from Lyco-
podium. The leaves in most
species are arranged in four rows,
two rows of smaller leaves on the
upper surface and two rows of
larger leaves somewhat at the sides
(Fig. 289, A). The cones also have
four rows of spore leaves (sporo-
phylls) and are consequently four-
angled. The spores are of two
sizes, and the type is perhaps the
best illustration of heterospory in
the pteridophytes. Forms of Sel-
aginella are frequently called
" little club mosses," for many of
them are much more delicate than
the lycopods. But some tropical
species, frequently cultivated in

SELAGINELLA

333

greenhouses, are large, much-branched, and bushy plants, very
graceful and decorative. Some forms grow in dry situations
on sand and rocks, in Mexico and the Southwest. One species
(S. lepidopJiylla) is frequently sold in the North under the name

FIG. 289. Selaginella Martensii

A, branch bearing cones and showing the leaf arrangements; B, inner face of
a megasporophyll, showing the large megasporangium containing a group
of four megaspores (tetrad); C, two views of megaspores; D, inner face of
microsporophyll, showing microsporangium ; E, microspores ; F, diagram of
a longitudinal section of cone illustrating position of microsporophylls and
megasporophylls and their microsporangia and megasporangia

of " resurrection moss." This plant protects itself during drought
by rolling up the branches to form a compact ball. When
moistened the branches spread out and become fresh and green.1

1 A botanist states that the plants sold in the North will absorb moisture
and unroll, but are generally " dead " beyond recovery.

334 THE PTERIDOPHYTES

324. The cones of Selaginella. The coues of Selaginella are
not as large as those of Lycopodium, but they are much more
complex in structure. The sporangia are of two sorts, both
developing singly from a group of cells on the stem just above
the origin of the spore leaves and later becoming attached to
their bases. The sporangia near the lower part of the cone
(Fig. 289, B) produce from one to eight very large megaspores,
and frequently a group of four (tetrad). The sporangia higher
up on the cones (Fig. 289, />) are smaller and develop a great
number of minute microspores, also in tetrads. Selaginella has,
then, different sporangia for the two forms of spores, microspores
and megaspores, which are accordingly called microsporangia
and megasporangia. Furthermore, these sporangia are borne
upon different spore leaves, which are consequently termed
microsporopliylls and megasporopliylls. It is important to note
that the few megaspores which mature, are nourished and grow
at the expense of neighboring spore mother cells which become
disorganized.

325. The gametophytes of Selaginella. The microspore
develops a reduced and degenerate male gametophyte, as in
Marsilia (Sec. 313). There is a small sterile cell (prothallial
cell) and two groups of sperm cells in a very simple cellular
structure probably representing an antheridium (Fig. 290, A).
The sperms are two-ciliate (Fig. 290, B).

The megaspore develops a female gametophyte which is larger
than that of Marsilia, but it is the same sort of a reduced struc-
ture, dependent upon food stored by the sporophyte within the
megaspore. This gametophyte at maturity fills the megaspore,
and bursting through the spore wall it presents an exposed sur-
face upon which several sunken archegonia are developed (Fig.
290, C). The female gametophyte actually begins its develop-
ment before the megaspore has attained its full size in the mega-
sporangium. It is thus parasitic upon the sporophyte during its
early history, a habit which is universal in the seed plants, but
among the pteridophytes it is only found in Selaginella.

THE GAMETOPHYTES OF SELAGINELLA

335

There are some other life habits of Sclaginella wonderfully
suggestive of the way in which seed and the seed habit arose.
It is known in some species, as S. rupestris, that the micro-
spores are thrown out from the sporangia on the upper part of
the cone and sift down like pollen grains among the megaspores

FIG. 290. The gametophytes and embryo of Selaginella

A, male gametophyte contained within the microspore : p, persistent nucleus of
prothallial cell ; s, two groups of sperm mother cells. B, two-eiliate sperms.

C, female gametophyte containing an emhryo sporophyte : a, archegonium ;
r, rhizoids. I), young sporophytes held by the spore leaves of the cone. E, a
young sporophyte still attached to the megaspore. — J5, after Belajeff ; A, C,

D, adapted after notes and sketches of Miss Lyon

336 THE PTERIDOPHYTES

into the split megasporangia below. The sperms are formed
and set free in the moisture of such situations, and the eggs of
the gametophytes may be fertilized while the megaspores are
still retained within the megasporangium. The young sporo-
phytes as they develop are thus actually held by the sporophylls
of the parent sporophyte (Fig. 290, Z>) until they reach a con-
siderable size and fall off. These habits should be noted and
this paragraph read again after the life history of the seed plant
is thoroughly understood.

The development of the young sporophytes of Selaginella
and also of Lycopodium has features resembling those of the
seed plants. The early divisions of the egg establish a structure
called the suspensor (Fig. 290, (7, Sus), which carries the devel-
. oping embryo down into the midst of the tissue of the gameto-
phyte, where it can draw nourishment from all of the cells
around it. A large foot is developed (Fig. 290) which absorbs
food from that portion of the gametophyte which lies in the
megaspore, so that the embryo sporophyte is actually nourished
with food stored in the megaspore by the sporophyte of the
previous generation.

326. Life history of Selaginella. Selaginella is an excellent
type with which to illustrate the life history of a heterosporous
pteridophyte. Since two forms of spores, microspores and mega-
spores, are present, there are two forms of gametophytes, male
and female, and this feature complicates the relatively simple
life-history formulae of bryophytes and homosporous pterido-
phytes (Sees. 285, 310).

The life history of Selaginella is as follows :

~ , ^microspore — Male Gametophyte — sperm .
P P y ^megaspore — Female Gametophyte — egg -

— Sporophyte, etc.
This in abbreviated form becomes

mi sp — M G — s . 0

SUMMARY OF SELAGIKELLA 337

which when carefully studied is essentially the same as the
general life-history formula of bryophytes and pteridophytes
(Sees. 285, 310), namely,

-S-sp-G, etc.

The differences lie in the fact, above mentioned, that there are
two forms of spores, microspores and megaspores, which develop,
respectively, male and female gametophy tes, a complication which
was introduced with heterospory and which is present, as will
appear later, in the life histories of seed plants (Sec. 356).

327. Summary of Selaginella. Selaginella is the highest of the
pteridophytes and the most important because of the evolution-
ary principles which it illustrates, leading up to the seed habit.
The first three of these principles are also illustrated by Mar-
silia and Isoetes, but the fourth and fifth are new. They are
(1) the establishment of heterospory resulting in the separation
of male and female gametophytes ; (2) the reduction or degen-
eration of the gametophyte, which becomes dependent upon
food stored in the microspores and megaspores ; (3) provision
in the megaspore for the nourishment of the embryo of the suc-
ceeding sporophyte generation; (4) the early parasitic relation
of the female gametophyte within the megasporangium upon the
sporophyte; (5) the occasional habit of developing the young
sporophyte while the megaspore is still retained within its
parent megasporangium.

328. .Isoetes. The species of Isoetes are known as quillworts.
Their position among the pteridophytes is a matter of dispute,
and some botanists place them with the ferns (Filicmece), but
the anatomy of the sporophytes is more like that of the club
mosses than any other group. They have a peculiar rush-like
habit of growth, the long leaves arising in clusters around a
short stem (Fig. 291,^4). Some forms are aquatic, growing on
mud at the bottom of ponds, while others are usually found out
of water.

338 THE PTERIDOPHYTES

Isoetes is heterosporous, and the spores are developed in sunken
sporangia at the bases of spore leaves (Fig. 291, B, s). The spore
leaves are differentiated so that only the outermost develop
megaspores and are consequently megasporophylls, while the

FIG. 291. The quill wort (Isoetes echinospora)

A, habit sketch. B, base of megasporophyll, showing inner surface : s, sporangium,
containing the large megaspores ; Z, ligule. C, a group of microspores below,
and a large megaspore above, showing comparative size

innermost are microsporophylls, producing only microspores.
Male and female gametophytes are developed slowly in the
microspores and megaspores, respectively, and are reduced or
degenerate sexual plants (Fig. 292, A, C), almost as simple as

FOSSIL PLANTS

339

those of Marsilia. Having no chlorophyll, they depend upon food
stored in the megaspore, as in Marsilia and Selayinella. The
young sporophyte also makes use of food in the megaspore as
in these other two heterosporous pteridophytes above mentioned.
The sperms (Fig. 292, B) are some-
what coiled and many-ciliate, re-
sembling in this respect those of
the Filicineoe. The life- history
formula is the same as that of Sel-
aginella (Sec. 326).

FOSSIL PLANTS AND COAL

329. Fossil plants. Plant re-
mains are not generally preserved
as fossils, partly because they do
not often have hard parts, such as
the shells and bones of animals,
and partly because the larger forms
grow on land where they are sub-
ject to rapid decay. So the record
of plant life in former geological

FIG. 292. Gametophytes of the
quillwort (Isoetes)

sperm mother cells shown within
the reduced antheridium. Z>,
sperm. (7, section of female game-
tophyte removed from megaspore,
showing sunken archegonium. A,
C, Isoetes echinospora. — A, C\
after Campbell; /?, after Belajeff

, . , , A, male gametophyte within the mi-
ages IS poor as Compared With crospore: p.prothallialcell; four

that of animal life. However, there
are some very wonderful deposits
of plant remains forming the hard
and soft coal beds, which de-
serve brief mention here, since
most of the plants composing them are fossil pteridophytes.
During the Devonian and Carboniferous Ages the most con-
spicuous vegetation was represented by tree ferns and relatives
of the horsetails and club mosses, together with certain very
primitive gymnosperms. These plants reached the height of
trees and formed forests on the land and in the marshes (see
Plate VIII). The Catamites (Plate VIII, 2) were gigantic

340 THE PTERIDOPHYTES

horsetails, so nearly like the living forms of Equisctum that we
can readily picture their appearance along the margins of swamps
and streams. Curiously some of the Catamites were heterospor-
ous, although all of the living types of the Equisctineoe are
homosporous. The ancient representatives of the club mosses
(Plate VIII, 3, 4) were among the largest plants of those times,
reaching the height of one hundred feet or more. Some of them
were true lycopods, and others, as Lepidodendron and Sigillaria,
were evidently close relatives of the club mosses. Their large
trunks were covered with leaves, which fell off, leaving curious,
diamond-shaped scars that are very conspicuous on the fossil
stems. The earliest seed plants arose in these ages, but they
were far outnumbered by the pteridophytes. They were gymno-
sperms of the group Cordaitece (Plate VIII, 5), but with very
little resemblance to any living forms. The fructifications of
some of these primitive forms, somewhat intermediate between
spermatophytes and pteridophytes, are occasionally so well pre-
served that we can learn something of the structure of the
gametophytes developed by the spores. It is possible that we
shall later know much more about the origin of the seed plants
and the seed habit from the study of these fossils.

After the Carboniferous Age the tree ferns, horsetails, and
club mosses became less abundant, and gymnosperms, like the
cycads and conifers, increased in numbers and became the
dominant forest types. There was an age of cycads in a later
period (Jurassic), when the earth was covered with these plants
as far north as Greenland and the climate must have been
tropical from pole to pole. We know very little about the
earliest forms of angiosperms. They do not appear abundantly
as fossils until a later period (Cretaceous), after the age of
cycads (Jurassic), although they doubtless had their origin
much earlier, for many insects were present which must have
had the habit of feeding on pollen or nectar.

It is clear that the horsetails and club mosses of the present
time are merely the remnants of this ancient flora once dominant

COAL 341

and perhaps as luxuriant as the tropical forests of to-day. They
have survived by adjusting themselves to very different life
conditions from those of Carboniferous times, and by adopting
life habits which remove them as far as possible from competi-
tion with the prevailing vegetation forms of to-day (trees, grasses,
herbs, etc.). The degenerate, saprophytic gametophytes of Ly co-
podium illustrate well how far such changes of life habits may
extend.

330. Coal. Coal was formed during a number of periods in
the earth's history, but the most extensive deposits were laid
down during the Carboniferous Age (frequently called the coal
age), forming the so-called coal measures. The luxuriant pterido-
phyte vegetation of tree ferns, horsetails, and club mosses formed
deposits in swamps over immense areas, probably in much the
same way as peat is being formed to-day. Such plant deposits
from time to time became covered with sediment by the sink-
ing of the land. And since the land alternately rose and sank,
successive layers or beds of plant remains were laid down.
These remains became finally buried under heavy deposits of
sediment, which pressed them into compact beds of the car-
bonaceous matter called coal.

Coal is of two sorts: (1) soft or bituminous coal, which is
hardly more than half pure carbon, the rest being composed of
a variety of carbon compounds, and (2) hard or anthracite coal,
which may be 90 per cent pure carbon. Hard coal represents a
greater degree of change than soft coal, the oils and other products
having been driven off under pressure by the heat of the earth.
The coal beds vary in thickness from small layers of only a few
inches to deposits a hundred feet deep. Those of the United
States cover several hundred thousand square miles, of which
perhaps fifty thousand square miles are being worked. Vast as
are these coal beds in the United States, there are deposits in
other lands, as in China, of even greater extent. The coal supply
of China is estimated as enough to last the world seven hundred
years. The total deposits of pteridophyte vegetation were very

342 THE PTERIDOPHYTES

much thicker than the coal beds which they formed, for it has
been estimated that it took about five feet of plant remains to
make one foot of coal.

It is interesting to think of the part which the pteridophyte
flora of the Carboniferous Age plays in the present life and
economic activities of the world, giving us a fuel whose carbon
was taken ages ago from the air, which was then much more
heavily charged with carbon dioxide than is the atmosphere of
to-day.

THE ORIGIN AND EVOLUTION OF THE
PTERIDOPHYTES

331. The origin of the pteridophytes. The pteridophytes
undoubtedly arose from a bryophyte ancestry, when the sporo-
phyte generation, in some forms having a structure capable of
doing chlorophyll work, developed a root system and vascular
tissues, and taking the land habit became independent of the
gametopliyte. This was one of the most important forward steps
in the evolution of the higher plants, for it gave the sporophyte
complete freedom to live and grow to its maximum size. It
marked a turning point in plant evolution, for after that the
sporophyte became the most complex and conspicuous phase of
the life history, and the gametopliyte grew less prominent, until
finally in the seed plants the sexual generation becomes actually
dependent or parasitic upon the asexual generation, a relation-
ship which is exactly the reverse of that between the gameto-
phyte and sporophyte in the liverworts and mosses. These very
important results in the evolution of plants are summarized in
Chapter xx IX, The Evolution of the Sporophyte and Degenera-
tion of the Gametophyte.

There are no bryophytes that show clearly how the root
system arose, but we can easily understand that so complex a
sporophyte as that of Anthoceros (which has chlorophyll-bearing
tissues with stomata, and a long, indefinite period of growth)
would at once become an independent plant, if it could develop

THE EVOLUTION OF THE PTERIDOPHYTES 343

a root system. For this reason, Anthoceros (Sec. 290) is gen-
erally considered the form among the bryophytes most closely
approaching the pteridophytes in its structure and possibilities
«pf development.

332. The evolution of the pteridophytes. After the sporophyte
became independent of the gametophyte, the next important
advance was the development of the lateral spore-bearing and
vegetative organs called fronds. Then came the differentiation
of the fronds into vegetative leaves, given up entirely to chloro-
phyll work (photosynthesis), and spore leaves, or sporophylls,
devoted chiefly or wholly to spore production. With this also
came the massing of the sporophylls in cones, which was really
the beginning of the structures called flowers in the seed plants.

Finally, the condition of heterospory was attained independ-
ently in several groups of the pteridophytes, as the water ferns,
Selaginella, and Isoetes. Heterospory soon led to very signifi-
cant changes in the structure and behavior of the gametophyte
generations. They became differentiated in sex, the microspores
producing male prothalli, and the megaspores female ones. Fur-
thermore, the gametophytes became greatly reduced, finally de-
pending wholly, or almost wholly, on food stored in the spores.
The food in the female gametophyte also came to contribute to
the development of the embryo sporophyte, which was thus fur-
nished with food by the sporophyte of the previous generation.
At last, in the highest form, Selaginella, the female gametophyte,
begins its development while still retained within the megaspore,
a condition approximating very closely to the seed habit.

SUMMARY OF THE PTERIDOPHYTES AND THEIR
ADVANCES OVER THE BRYOPHYTES

333. Summary of the pteridophytes. The chief characters
of the pteridophytes and their advances over the bryophytes are :

1. The development of a leafy shoot and root system with
vascular tissues in the sporophyte generation, rendering it

344 THE PTERIDOPHYTES

independent of the gametophyte, giving it the land habit, allow-
ing it to attain a large size, and making it by far the most
conspicuous phase in the life history.

2. The development and differentiation of fronds into vege-
tative leaves and sporophylls, and the grouping of the latter
into cones.

3. The development of heterospory, which differentiated the
gametophytes as male and female in sex.

4. The degeneration of the gametophytes (in heterosporous
forms) so that they finally became dependent upon food supplied
by the sporophyte in the spore. In Selaginella the female
gametophyte even begins its development at the expense of
neighboring cells in the megasporangium. These conditions are
an exact reversal of the relations between the generations which
exist in the bryophytes.

The three classes of the Pteridophyta are readily distin-
guished by the f ollowing, characters :

Class I. Filicinece. Fronds large and few in number ; those
bearing spores generally similar to the strictly vegetative leaves
and not grouped in cones.

Class II. Equisetinece. Leaves reduced to mere scales, form-
ing sheaths around jointed stems, which have many peculiarities
of structure ; sporophylls of peculiar form, each bearing several
sporangia, and grouped in a characteristic cone.

Class III. Lycopodinece. Vegetative leaves, small and very
numerous (except in Isoetes}, covering the stem ; sporophylls
generally grouped in cones, each bearing a single sporangium ;
gametophytes much degenerate, especially in the heterosporous
Selaginella and Isoetes; sperms two-ciliate, except in Isoetes,
and not spiral, and many-ciliate as in the Filicinece and Equise-
tinece.

CHAPTER XXVI
ALTERNATION OF GENERATIONS

334. The protoplasmic basis of an alternation of genera-
tions.* The history of the alternation of generations in plants
has now been traced from the relatively simple beginnings in the
thallophytes, as illustrated by the life histories of the red algre
(Sec. 246) and sac fungi (Sec. 26,6) through the more clearly
denned conditions in the liverworts and mosses, and also through
the ferns, horsetails, and club mosses. It is clear that in the
latter groups and the pteridophytes the asexual, or sporophyte,
generation had become much the more complex of the two, and
that the sexual generation, or gametophyte, had begun to degen-
erate. This degeneration is carried much further in the seed
plants, as will be described in Chapter xxvm, and summarized
in Chapter XX ix.

It is now time to try to determine some of the reasons for
the establishment of a sporophyte generation following the game-
tophyte one, or the basis in the protoplasm itself of the alternation
of sexual and asexual generations. The basis undoubtedly rests
on the effects of the sexual process upon the nature of the pro-
toplasm in the succeeding generation. The union of gametes is
so great a physiological stimulus that the sexually formed cell
(generally a fertilized egg) is given the possibilities of a develop-
ment different from that of either parent plant or gametophyte ;
for the protoplasm of a fertilized egg is not the same as that of
either gamete which entered into its formation. It is a mixture
of protoplasms and therefore must be different from the proto-
plasm of the parent plants, and this difference is the basis for

* To THE INSTRUCTOR : In a brief course or with immature students this
chapter should be omitted.

345

346 • ALTERNATION OF GENERATIONS

the peculiarities of the generation which arises from a sexually
formed cell

Protoplasm has so far proved much too complex for an analy-
sis into the structures which determine its qualities and possi-
bilities of development; that is, we do not know why the egg
of a fern develops into a fern and that of a club moss into a
club moss; both are cells with a general similarity of cell struc-
ture. But the possibilities of fern and club moss are nevertheless
present in the respective eggs, and the one could not possibly be
made to produce the other plant. It is generally believed that
the characteristics of eggs are determined by the structure of
their protoplasm, represented perhaps by means of the invisible
molecules and groups of molecules in its chemical and physical
composition. The structures that are assumed to give distinct
character or possibilities of development to protoplasm are called
rudiments.

It is doubtful whether we shall ever be able to distinguish
the rudiments, but there are some larger structures in the cell
which with care can be followed through the cell divisions from
generation to generation. The most interesting of these are
the chromosomes, which are very characteristic structures most
clearly seen during the processes of nuclear division (Sec.
199). The substance of the chromosomes, called chromatin,
is the most important material in the nucleus. Chromatin can
be deeply colored or stained in thin sections of tissue after
special methods of treatment. It is present in the resting
nucleus, generally in the form of an irregular network. The
chromosomes are formed from the chromatin and appear during
the early stages of nuclear division. Each chromosome then
divides into halves, and the two sets of daughter chromosomes
are distributed with each nuclear division.

It is an important fact that the number of chromosomes for
the nuclei of each plant is fixed, and the number is usually
not very large. Thus the gametophytes of a red alga (Poly-
siphonia, Sec. 245) have about 20 chromosomes, but those of

BASIS OF ALTERNATION OF GENERATIONS 347

the liverwort (Anthoceros, Sec. 290) have only 4 and the fern
(Osmunda) 12. The most important feature of the process of
fertilization is the union of the two gamete nuclei, that of the
sperm with that of the egg. These nuclei have an equal number
of chromosomes in the same species (the number characteristic
of the gametophyte), and the egg and sperm are therefore equiv-
alent in their nuclear structure, whatever may be the differences
in their size. This nuclear fusion doubles the number of chromo-
somes, and the fertilized egg begins the development of the
sporophyte (when present) with twice as many chromosomes as
the gametophytes which produced the eggs and sperms.

The double number of chromosomes appears in all of the
nuclear divisions throughout the development of the sporo-
phyte up to the time of spore formation. Thus the sporophyte
phases of Polysiphonia have about 40 chromosomes, the sporo-
phyte of Anthoceros 8, and Osmunda 24. The lilies have
24 chromosomes, and the gametophyte phase only 12. The
chromosomes have been counted in more than fifty different
kinds of plants, mostly seed plants, and it is established that
sporophytes have normally double the number of chromosomes
of their respective gametophytes.

Spore formation at the end of the sporophyte generation is a
very significant period in the life history, for at this time the
double number of chromosomes is reduced by half. The spores
have then the original number of the gametophyte. The reduc-
tion of the chromosomes is effected by processes too complicated
to be described here, but the formation of the asexual spores in
groups of four, called tetrads (see Figs. 204, 245, 258, 289, 298,
302, 304), is rather characteristic of the phenomenon. There are
thus fundamental reasons for the identical methods of spore for-
mation in the bryophytes and pteridophytes, and, as will appear
later, for the methods of pollen formation and the embryo sac
in the seed plants. For the same reasons, groups of four spores,
(tetraspores), are developed at the end of the sporophyte genera-
tion in the red algae.

348 ALTERNATION OF GENERATIONS

The chromosomes are generally believed to be the actual
bearers of the qualities (represented perhaps by rudiments)
which are inherited, that is, passed on from one generation to the
next. The chief reasons for this view are their importance as
the essential structures of the nucleus, their regular behavior
throughout the cell divisions, and the evidence that they never
lose their identity completely, even in the resting nucleus, but
remain perhaps as the only permanent organs in the cell.

335. The life-history formula, showing the chromosome
count. The life-history formula which has been employed for
the bryophytes and pteridophytes becomes much more interest-
ing when considered in reference to the chromosome count.
The formula has been :

Gametophyte <^ ^> — Sporophyte — asexual spore

UtJ

— GametopJiyte, etc.

Representing the gametophyte number of chromosomes by x
and the sporophyte number by 2 a?, these may accompany the
formula as follows :

. sperm

Gametophyte <^ x chro" ^> — Sporophyte — asexual spore
x chromosomes ^^ egg ^ 2 x chromosomes x.chromosomes
x chro.

— Gametophyte, etc.
x chromosomes

Examining this formula, it is clear that there are two periods
when the number of chromosomes changes abruptly: (1) at fer-
tilization, when the number is doubled, and (2) at spore forma-
tion, when the number is reduced. The fertilized egg develops
into the sporophyte because its protoplasm has different qualities
from that of the gametophyte. The asexual spore develops into
the gametophyte because its qualities have become again the same
as those of the former gametophyte generation. Spore formation,
then, in bryophytes and pteridophytes is a return of the plant
in its life history to the conditions of ancestral gametophytes.

THE ORIGIN OF THE SPOROPHYTE 349

336. The origin of the sporophyte. It seems clear that the
sporophyte had its origin through the stimulus of the union
of gametes, and especially the union of gamete nuclei, to give
a fusion nucleus with double the number of chromosomes char-
acteristic of the gametophytes. It is probable that there is a
reduction of this number in many thallophytes before or during
the germination of the zygospore or oospore, so that there is
no opportunity for a sporophyte generation. This condition has
been reported for Coleochcete (Sec. 222), and it is probably also
true of (Edogonium, Spiroyyra, the desmids, Vaucheria, Ulothrix,
and other types.

The sporophyte arose when nuclear divisions appeared with
the double number of chromosomes, thus postponing the time
of chromosome reduction to a later period in the life history,
which became generally characterized by the formation of
asexual spores in tetrads. Sporophytes undoubtedly appeared
thus in several groups of plants entirely independently of one
another, as illustrated in the divergent lines of development of
the red algae, the sac fungi, the Dictyotacece (a small group of
the brown algae), and the bryophytes leading up to the
pteridophytes and spermatophytes.

337. Summary. The alternation of generations in plants
takes on added interest when considered in relation to the
behavior of the chromosomes, for the importance of the two
critical stages in the life history — (1) fertilization, and (2) spore
formation — becomes at once apparent. Fertilization doubles the
number of chromosomes in the egg and gives it the possibilities
of the sporophyte's development. Spore formation reduces the
double number of chromosomes by half and brings the plant's
protoplasm back to the condition where it may develop the
gametophyte. The two processes follow one another as the life
history is repeated again and again with machine-like regularity,
and there is undoubtedly a chemical and physical basis for the
life history. And, as before stated, it is generally believed that
the chromosomes hold the rudiments that determine in a broad

350 ALTERNATION OF GENERATIONS

way the programme of development, the double number defining
the sporophyte generation.

It must not be supposed, however, that the life history unfolds
entirely through the operation of forces within the organism, as
a watch runs on the strength of the wound-up mainspring.
While the organism is truly a machine, it is a machine which
is constantly influenced by forces from without which modify
its complex adjustments, and, above all, it is a self-perpetuating
machine which makes its own repairs.

There are two prominent theories respecting the manner in
which an organism develops from an egg or other reproductive
cell. The first, called preformation, assumes that the characters
of the adult are preformed or represented in miniature by rudi-
ments or other structures in the protoplasm. Development is,
therefore, something like the unfolding of a bud, and the results
are determined by conditions within the organism. The second
theory, termed epigenesis, is not willing to grant such a compli-
cated architecture to protoplasm, but holds that development
is guided chiefly by conditions without the organism. It is
probable that the correct interpretation lies between the two
extreme views, that the cell does have a complicated structure
far beyond our present possibilities of knowledge, but that the
processes of development are largely guided and controlled by
outer influences.

CHAPTEE XXVII
HETEROSPORY

338. Heterospory.* Heterospory arose in the pteridophytes
with the establishment of two sizes of spores, called megaspores
(large spores) and microspores (small spores). Heterospory and
the independence of the sporophyte are the chief contributions
of the pteridophytes to the progress of plant evolution. The
establishment of megaspores and microspores was merely the
beginning of a number of far-reaching developments in plants,
all of which are really parts of the general principle of hetero-
spory. They all reach their highest degrees of specialization in
the seed plants, as will be described in the next chapter and
summarized in Chapter xxix, but most of them are clearly
illustrated in the pteridophytes.

These developments resulting from heterospory are :

1. The gametophytes became differentiated in sex so that
the megaspore always develops a female gametophyte and the
microspore a male gametophyte.

2. The sporangia assumed two forms : megasporangia de-
voted to the production of megaspores and microsporangia
devoted to the production of microspores, as illustrated by Mar-
silia, Isoetes, Selaginella, and, as will appear in the next chapter,
the seed plants.

3. The spore leaves, or sporophylls, were differentiated into
megasporopliylls and microsporopliylls which develop, respec-
tively, megasporangia and microsporangia, as illustrated by
Isoetes, Selaginella, and the seed plants ; the sporophylls of
Marsilia bear both forms of sporangia.

* To THE INSTRUCTOR : In a brief course or with immature students this
chapter should be omitted.

351

352 HETEROSPORY

4. A tendency was developed to reduce the number of mega-
spores by sacrificing many of the cells which might be fertile so
that relatively few megaspores are formed, but these are very
large and richly supplied with food material, as illustrated by
Selaginella and the seed plants. This principle is clearly similar
to that by which plants have found it advantageous to produce
a limited number of large eggs well stocked with food, even at
the sacrifice of cells which may have been originally gametes,
such as the canal cells in the archegonium.

5. The gametophytes degenerated, as self-supporting green
plants, to a condition in which they lost their chlorophyll and
became dependent upon food stored in the megaspores and mi-
crospores and even live somewhat parasitically upon the sporo-
phytes, as is illustrated in the early stages in the development of
the female gametophyte of Selaginella and in the gametophytes
(pollen tube and embryo sac) of the seed plants.

There is another important advance in plant evolution which
is closely related to heterospory, but may be treated to better
advantage in the account of the origin of the seed habit
(Sec. 367). This advance arose in the seed plants when the
megaspore became retained within the megasporangium (a por-
tion of the ovule), so that the female gametophyte (embryo sac)
developed like a parasite upon the parent sporophyte, and the
male gametophyte (pollen tube) was required to grow down to the
female gametophyte somewhat parasitioally through the tissues
of the ovule to bring about the fertilization of the egg cell.

339. Sexual characteristics given to the megaspore and mi-
crospore by means of heterospory. The megaspore and micro-
spore are of course asexual spores because they are formed by
an asexual plant, the sporophyte. They are simply specialized
forms of the similar spores present in the liverworts, mosses,
the common ferns, horsetails, and lycopods, as shown by their
similar origin in tetrads at the end of the sporophyte generation.

But when the microspore and megaspore became clearly dif-
ferentiated through heterospory from the earlier conditions of

SEXUAL CHARACTERISTICS GIVEN BY HETEROSPORY 353

homospory they took on certain characteristics of sex. This does
not mean that the microspores and megaspores became gametes,
for their spore nuclei have never become sexual nuclei in any
group of plants. But microspore and megaspore did assume
sexual characters to this extent that they always give rise,
respectively, to male and female gametophytes.

Furthermore, the degeneration of the gametophytes steadily
reduced the number of the nuclear divisions between the germ-
ination of these spores and the formation of the gametes until
the gamete nuclei have been brought very close indeed to the
spore nuclei. An examination of the figures of the male game-
tophytes of Marsilia (Fig. 282, A}, Selaginella (Fig. 290, A), and
Isoetes (Fig. 292, A) will show that there can hardly be more
than from six to ten nuclear divisions in these types before the
sperms are developed. There are even fewer nuclear divisions
in some groups of seed plants where the degeneration of the
gametophyte is carried much further than in the pteridophytes.
Some forms of angiosperms present but a single division of the
spore nucleus before the female gamete nuclei are formed, as
in the embryo sac of the lily (Sec. 360, note), and there are only
two nuclear divisions in the male gametophytes (pollen grain
and tube) of the angiosperms.

This gradual transference of sexual characteristics to portions
of the asexual generation, accompanying the reduction of the
sexual generation, is one of the most interesting results of the
evolution of the sporophyte and degeneration of the gameto-
phyte (summarized in Chapter xxix), for it makes clear many
puzzling conditions in the seed plants. Thus it shows why the
pollen grain (which is a microspore) is really functionally a male
reproductive structure and the stamen a male organ ; and why
the carpels and pistil are functionally female organs, even though
they have had their origin on asexual plants (sporophytes).

CHAPTER XXVIII
THE SPERMATOPHYTES AND THE SEED HABIT

340. The spermatophytes.* The division Spermatophyta
(meaning seed plants) contains not only the groups frequently
called " flowering plants " but also other groups which do not
have flowers in the popular sense of the word, for the repro-
ductive organs are .borne in cones or clusters which are not
at all showy, but rather inconspicuous. These are, however,
flowers in the scientific sense, as are also the cones of the
horsetails and club mosses. The spermatophytes have also been
called phanerogams, or plicenogams (meaning evident marriage),
to distinguish them from all the lower groups of plants which
were called cryptogams (meaning hidden marriage). However,
this separation was made before the sexual processes of the
lower plants were understood, for as a matter of fact they are
much more evident than the complicated ones in the seed plants.
The seed is a more significant structure in the group than the
flower, so the name spermatophytes has in recent years come
into general favor.

The seed plant, like the fern, is a sporophyte. There is a
gametophyte generation in the life history which is, however, so
much reduced in structure that it can only be understood
by a careful study of the reproductive processes in seed for-
mation. It is the main purpose of this chapter to make clear
the 'position of the gametophyte generation in the life history,
together with the origin and evolution of the flower. The struc-
ture and physiology of the sporophyte are considered in Part I,

* To THE INSTRUCTOR : The introduction to this chapter assumes that the
life history of some seed plant, as the pine or lily, has been studied in the
laboratory.

354

THE SEED 355

and only brief reference will be made to these features, which
are treated there in detail and should follow this account if they
have not already been studied.

341. The seed. The importance of the seed in the develop-
ment of plant and also of animal life can hardly be exaggerated.
For the plant it furnishes one of the surest means of reproduc-
tion not only because of protective structures, means of dispersal,
long vitality, etc. (see Chapter xxxin), but also because the
embryo plant is carried so far forward in its development that
it is able to take root and establish itself at once. And fur-
ther to aid the embryo, the seed is a storage organ of the most
condensed forms of food material found in plants. In this
respect, also, the seed has proved a most important influence in
shaping the habits and in a large measure the evolution of some
forms of animal life ; for the highest groups of animals live to
a very great extent directly or indirectly upon food stored in
seeds and certain fruits, finding there some of the richest and
most nutritious proteid and carbohydrate foods. The animal
life of the Carboniferous Age (coal age) and the periods imme-
diately following comprised animals of great bulk of body, but of
low nervous organization. They browsed on the vegetation like
the hay and grass-eating animals (herbivora) of to-day, and like
them their bodily structure and nervous system were adapted
to such life habits. But, later, groups arose with digestive organs
adapted to richer foods, and this diet became associated with
varied life habits, which led ,to much higher types of nervous
organization and bodily structure.

342. The morphology of the seed. The morphology of the
seed can only be understood when the spermatophytes are
studied in relation to the pteridophytes. The seed plant is a
heterosporous sporophyte. The pollen grain is a microspore.
The megaspores of the seed plant are never shed. They are
retained in the megasporangium and never even lie freely as
independent cells, but are always in close physiological relation
to the tissue of the megasporangium. The cell which is the

356 THE SPERMATOPHYTES

equivalent of the megaspore, or rnegaspore mother cell, is called
the embryo sac. The megasporangium, termed the nucellus,
with the embryo sac is contained within one or two protective
envelopes, called integuments, and this group of structures con-
stitutes the ovule. There is developed within the embryo sac a
much-reduced female gametophyte which lives entirely on foods
supplied by the sporophyte. The ovule at maturity then con-
sists of the embryo sac (megaspore or rnegaspore mother cell)
with the female gametophyte, the nucellus (megasporangium),
and the integuments.

The female gametophytes are quite different in the two great
subdivisions of seed plants (gymnosperms and angiosperms).
In the first group (gymnosperms) several archegonia are gener-
ally, formed, each containing a single large egg. In the second
group (angiosperms) the female gametophyte is very much re-
duced and only one egg is formed. The fertilization of an egg
leads at once to the development of an embryo sporophyte
within the embryo sac. The embryo sporophyte of the second
generation is thus nourished through the ovule by the parent
sporophyte of the first.

The seed is a ripened ovule, that is, an ovule containing an
embryo sporophyte so far along in its development that the seed
may safely be separated from the parent plant. Morphologically,
the seed is composed of tissues representing three generations:

(1) the integuments and nucellus are of the parent sporophyte;

(2) the embryo sac contains more or less tissue of gametophyte
origin called endosperm l; (3) an embryo sporophyte of the next
generation lies within the embryo sac.

343. Pollination and fertilization. The retention of the mega-
spore (embryo sac) within the megasporangium (nucellus) so
that the female gametophyte is contained in the tissues of the
sporophyte has resulted in modifications of the structure and

1 The endosperm of the angiosperm seed has special peculiarities involved
•with the fertilization of the egg and development of the embryo, as explained
in Sees. 362 and 363.

POLLINATION AND FERTILIZATION 357

life habits of the male gametophyte quite as remarkable as those
of the female. These peculiarities are concerned with two dis-
tinct processes necessary to insure the development of seeds,
namely, pollination and fertilization.

The pollen grain is a microspore developed in groups of four
(tetrads) in pollen mother cells in essentially the same manner
as the spores are developed in all bryophytes and pteridophytes
(Fig. 302, B). The pollen grain forms a very much reduced
male gametophyte, which is represented by the protoplasmic
contents of the pollen grain and pollen tube. It would be use-
less Tor the male gametophyte to form and discharge sperms
which could not possibly reach the embryo sac imbedded in the
nucellus of the ovule. So the sperm-forming habits of the
pteridophytes, bryophytes, and the algse are generally given up,
although curiously they still persist, as will be described later,
in the cycads and Ginkgo (Sec. 348). The sperms are repre-
sented by two sperm nuclei developed by each male gameto-
phyte and discharged from the tip of the pollen tube.

The pollen tube is an outgrowth from the pollen grain. Its
purpose is to carry the sperm nuclei to the embryo sac, where
one of the two may unite with the egg nucleus and fertilize the
egg. In one of the two subdivisions of seed plants called the
gymnosperms (meaning naked seeds) the pollen grains are
applied directly to the ovules, and the pollen tube need only
grow through the tissue of the nucellus (megasporangium) to
reach the embryo sac. In the other large group called the angio-
sperms (meaning seeds inclosed in a vessel) the pollen tubes
must penetrate a case (the pistil) which contains the ovules
before they can reach the ovules themselves. There is a special
receptive surface, called the stigma, upon this structure, where
the pollen grains find moisture and other conditions favorable
for their germination.

Pollination is the application of the pollen to the ovule or to the
stigma. This application is effected in various ways, sometimes
by the wind, sometimes by other chance processes, but many

358 THE SPERMATOPHYTES

plants have developed elaborate devices to insure pollination, as
through the visits of insects to flowers. (see Chapter xxxn).

Fertilization is effected when the pollen tube pierces the embryo
sac and one of its two sperm nuclei fuses with the egg nucleus.
When one considers the extraordinary modifications of the
male gametophytes of the seed plants, the process of pollination
and the development of the pollen tube seem quite as remark-
able as the retention of the female gametophyte in the mega-
sporangium. They are both essential features of the seed habit.
344. The flower. The term flower in the popular sense gen-
erally means some showy structure such as is only found in
certain groups of the angiosperms. The flower in the scientific
sense consists of a group of spore leaves, or sporophylls, with
or without surrounding envelopes, which may or may not be
showy. It has been defined as " a shoot beset with sporophylls."
Since the seed plants are heterosporous, the spore leaves are
either microsporophylls, called stamens (producing pollen] , or
they are megasporophylls, called carpels (producing embryo sacs
in the ovules). The stamens and carpels of the gymnosperms
are generally grouped in cones which resemble the cones of the
horsetails and club mosses. But the carpels of the angiosperms
form, often with adjacent tissue, closed cases called pistils.

It should be noted that the cones of the horsetails and club
mosses are as truly flowers in the scientific sense as the cones
of the gymnosperms, and also that certain groups of angio-
sperms (grasses, sedges, and most trees) have flowers which are
not showy.

The material of this chapter will be treated under the follow-
ing headings :

Subdivision I. The gymnosperms, or Gymnospermce.

Subdivision II. The angiosperms, or Anyiospermce.

The origin of seed plants and the seed habit.

The evolution of the flower.

The classification of the angiosperms.

Summary of the spermatophytes and their relationships to the
pteridophytes.

THE GYMNOSPERMS 359

SUBDIVISION I. THE GYMNOSPEKMS, OR
G YMNOSPERM^E

345. The gymnosperms. The gymnosperms (meaning naked
seeds) are distinguished from the angiosperrns because their
seeds -are borne exposed on the carpels. They comprise not
only the familiar cone-bearing trees, or conifers, generally with
needle-shaped leaves, such as the pines, spruces, firs, hemlocks,
larches, cedars, etc., but also the large-leaved cycads, the
straggling, shrubby Ephedras, the climbing Gfnetums, and that
interesting Japanese tree Ginkgo. The gymnosperms contain
the most ancient groups of living seed plants, and the fossil
remains of primitive types are found in the Carboniferous Age
and even earlier periods, with those of the giant horsetails and
club mosses (see Cordaites, Plate VIII). Tlie study of ancient
gymnosperms, together with a fossil group, Ptcridospermoe,
intermediate between the pteridophytes and spermatophytes,
may throw much light on the origin of the seed and seed habit.
. The living groups of the gymnosperms comprise in all less
than 450 species, of which more than 300 are conifers and
about 80 are cycads. With the exception of the conifers, these
groups are hardly more than remnants of the ancient gymno-
sperm floras. But the conifers are a very successful group,
which still forms extensive forests in some temperate regions
and covers mountain sides and certain large rather barren areas,
although such forests are being rapidly cut off for timber. Of
the smaller groups the cycads are mostly tropical, the Ephedras
are chiefly desert plants, and the Gnetums tropical vines with
large-veined leaves. Like the horsetails and club mosses, the
Epliedras have for the most part developed peculiar life habits
under unfavorable conditions, and so have been able to avoid
total extinction . by withdrawing as far as possible from compe-
tition with the more recent floras.

This account can only consider the two largest groups, the
cycads and the conifers.

360

THE SPERMATOPHYTES

THE CYCADS

346. The cycads. The cycads (order Cycadales) have thick
stems which rarely branch and are generally rather short, resem-
bling immense tubers partly buried in the ground (Fig. 293, A).

A, plant bearing a
carpellate cone. B,
a carpel in side
view, showing the
two very large
ovules: ra, mycro-
pyle. — A, adapted
from a photograph
by Land

FIG. 293. A cycad (Zamia)

Some of the cycads have, however, stems which rise like col-
umns ten to forty feet high. The compound leaves, like immense
stiff feathers, form a crown at the top of the stem so that the
general habit of the cycads is somewhat like that of the tree

THE CONES OF THE CYCADS

361

ferns and palms. One form (Cycas revoluta), incorrectly called
the sago palm (since it is not a palm), is valuable for the sago
of commerce which is obtained from the stem.

347. The cones of the cycads. Some of the cycads bear
cones composed either of carpels (megasporophylls), or stamens
(microsporophylls) which resemble large scales. Carpellate and
staminate cones are always borne on
separate plants. In other types, how-
ever, as Cycas rcvoluta, the carpels,
especially, have more nearly the ap-
pearance of vegetative leaves (Fig.
294), and form rosettes at the top of
the stems. Cycas revoluta is fre-
quently grown in park conservatories,
and occasionally produces these ro-
settes of hairy, orange-colored carpels,
which bear a series of ovules as large
as plums on either side. Well-differ-
entiated cones are present in Zamia
(Fig. 293, A), which is quite' common
in southern Florida.* The carpel
(Fig. 293, J5), in this genus, bears two
ovules and the stamen, a group of
pollen sacs (Fig. 295, A). The ovule
(Fig. 295, D) has a thick integument FlG- 294. Carpel of Cycas revo-
surrounding the large nucellus, in

which lies the embryo sac containing the female gametophyte.
The pollen grains of Zamia enter the opening called the micro-
pyle (meaning little gate), where the integument fails entirely
to inclose the nucellus, and so come to lie in a small cavity

* To THE INSTRUCTOR : It ought to be possible to obtain Zamia in quantities
for advanced classes. The type is most admirable for the study of the
gametophytes of gymhosperms. The best account of these is given by Web-
ber, United States Department of Agriculture, Bureau of Plant Industry,
Bulletin 2, 1901.

362

THE SPERMATOPHYTES

termed the pollen chamber (Fig. 295, D,p). The pollen grains
germinate in the pollen chamber, forming male gametophytes.
whose development disorganizes much of the tissue at the tip

of the nucellus, so that the
pollen grain end of the male
gametophytes finally hang
down just above the em-
bryo sac.

348. The gametophytes
of the cycads. The embryo
sac of the cycads is said to
develop from one of a group
of four cells in the interior
of the nucellus. Such a
group is undoubtedly a
tetrad, and each of the four
cells corresponds to a mega-
spore, but only one produces
a female gametophyte, and
thus becomes an embryo sac.
The nucleus of the em-
bryo sac (megaspore nucleus)
gives rise to a great many

FIG. 295. The sperms and ovule of a
cycad (Zamia)

A, lower surface of a stamen, with numer-
ous pollen sacs in two groups. JJ, the two

hundred nuclei, and the

large top-shaped motile sperms at the end amount of protoplasm ill-

creases very greatly until
the embryo sac occupies the

of the pollen tube ready to be discharged

above the archegonia. C, a sperm viewed

from the end, showing the spiral band

which bears the cilia. D, diagram of a - P ,

section of an ovule after pollination : m, larger Parl

micropyle; i, integument; p, pollen cham- of the nucellus in this large

her; n, nucellus containing developing , ml n . , n , -,.

pollen tubes; a, archegonia, with large ovule. The nuclei at first lie

eggs imbedded in the endosperm (female freely in the protoplasm, but,

gametophyte). — B, C, after Webber ,, , , ,

later, walls are formed and

the embryo sac becomes filled with a delicate tissue, called the
endosperm (Fig. 295, D), which corresponds to the vegetative part
of a prothallium in a fern. Several archegonia are developed at

THE GAMETOPHYTES OF THE CYCADS 363

the micropylar end of the endosperm (Fig. 295, D, a). These are
very much reduced in structure, the neck being represented prob-
ably by two small cells and the very large eggs lying imbedded
in the cells of the endosperm.

The male gametophyte consists of the protoplasm with
several nuclei contained in the pollen grain and tube. Some of
the nuclei near the pollen grain end of the tube lie within deli-
cate cell walls. One of these cells termed the generative cell
develops two sperm mother cells which become organized into
two very large motile sperms (Fig. 295, B, C), each with a spiral
band or line bearing hundreds of cilia. The two sperms finally
begin to move around in the fluid of the pollen tube and are
discharged from the end nearest the pollen grain (which now
hangs down over the embryo sac) into the fluid within the
cavity formed from the disorganized tissue at the tip of the
nucellus. The pollen tube in the cycads grows off to one side
in the nucellus and seems to be a sort of absorbing organ, so
that it does not carry the sperms to the embryo sac as the
sperm nuclei are carried in most seed plants.

The motile sperms are set free in the fluid above the embryo
sac, whose female gametophyte at that time bears mature arche-
gonia. They have been observed swimming about for many min-
utes in sections of the living ovules, and probably have a long
motile period in the ovule. One of them is finally able to enter
the neck of an archegonium, and fusing with an egg fertilizes it.

The finding of motile sperms in the cycads and in Ginkgo1
fyy two Japanese botanists in 1896-1897 proved two of the most
interesting botanical discoveries of the past decade. It is very
remarkable that the sperm-forming habits of the bryophytes and
pteridophytes should have persisted so long after the seed habit
became established in a group. The free swimming of these
motile sperms is actually a return, such as occurs in the bryo-
phytes and pteridophytes, for a short time in the life history of the
cycad to the aquatic habits of an algal ancestry of ages ago.

1 A beautiful Japanese tree, not uncommon under cultivation.

364 THE SPERMATOPHYTES

THE CONIFERS

349. The conifers. This group (order Coniferales) has repre-
sentatives distributed all over the earth, some of them forming
the most extensive forests and having the greatest value as
timber trees. There are not many more than 300 species of
conifers, of which the pines (Pinus) have 70; Podocarpus
(growing in South America and eastern Asia), 40 ; the junipers
(Jnniperus), 30; certain cedars (Cnpressw), 20; the firs (Abies),
20; and the spruces (Picea), 12. Others have few species and
a very limited distribution. Such a form is the giant redwood
of California (Sequoia gig anted), which is found only in a few
scattered groves in the Sierra Nevada Mountains (Fig. 33).

350. The form and foliage of the conifers. The form and
foliage of the conifers is • generally very characteristic. The
trees have, as a rule, a single central stem which rises vertically
from the ground, and the side branches spread out horizontally
from this shaft so that the trees are very symmetrical and taper
to a point like a cone. The foliage, as a rule, consists of scale-
or needle-shaped leaves, which usually remain on the trees for
a number of years so that most of the trees are evergreen. But
there are some exceptions to the rule, as the larch or tamarack
(Larix), which sheds its needles every year.

The needle leaves can endure severe cold, fierce heat, and
drought. This is made possible by their very compact structure
(Fig. 296), which presents a minimum of surface exposure and
the protective layer of thick-walled cells under the heavy epideii-
mis. The chlorophyll-bearing tissue is closely packed in the pine
leaf and consists of cells with peculiar infolding walls. Some
species of pine have needles with one fibro-vascular bundle, e.g.
the white pine; others have two bundles, e.g. the Scotch and
the Austrian pine. The buds, leaves, and stems contain much
resin and turpentine, which render them unpalatable to grazing
animals and cover them with a film which sheds water and
protects the plant both from the winter's cold and the summer's

THE FORM AND FOLIAGE OF THE CONIFERS 365

drought. Resins and turpentines are also very effective in pro-
tecting young conifers from the attacks of parasitic fungi, espe-
cially when the trees are wounded.

Certain pines furnish the resin and turpentine of commerce.
Incisions are made through the bark, penetrating the wood.
A thick liquid oozes out which is a mixture of resins and oil of
turpentine. This liquid is then distilled, driving off the fluid
oil of turpentine which is collected. The resin remains behind

FIG. 296. Structure of a pine needle (Pinus Laricio)

The compact green tissue, or mesophyll, with resin ducts d, surrounds an area con-
taining two fibro-vascular bundles, which lie in a peculiar region of transfu-
sion tissue t, bounded by the bundle sheath 66'. Outside of the green tissue are
thick-walled cells forming a rigid tissue r, and around the whole is the heavy
epidermis e with lengthwise grooves containing the stomata st

in the still, and when cool is no longer semi-fluid, but becomes
quite hard and brittle. The timber value of certain conifers is
much greater than that of most other kinds of trees because the
wrood is soft, splits regularly, is easily worked, and also because
the tree trunks are so straight. The problems of forestry (see
Chapter XLI) are largely concerned with the preservation of the
pine forests, which are being cut off with little regard to the future.

366

THE SPERMATOPHYTES

351. The tissues of the pine stem. The pine is an excellent
subject for the study of stem structure and growth in a timber
tree. There are five principal regions in the stem: (1) the pith,

FIG. 297. Structure of the stein of the Scotch pine (Pinus sylvestris)

A, diagram of the arrangement of the fibre-vascular bundles at a growing point :
the shaded parts are wood. .Bt diagram of the position of the principal tissues
shown in a cross section of a four-year-old stem. C, cross section of a region
of cambium Cam, with adjacent wood and bast. D, cross section of wood at
an annual ring: d, resin duct. E, radial section of wood. F, longitudinal sec-
tion of wood. G, section of bordered pit. Medullary rays m appear in most
of tbe figures

(2) the wood, (3) the cambium, (4) the bast, and (5) the outer
bark (see Fig. 297, #).

The pith occupies the very center of the stem, and is the
remains of the undifferentiated primitive tissue present at its

THE TISSUES OF THE PINE STEM 367

growing point before the fibro-vascular bundles and bark are
formed. It practically disappears as the stem grows older and
the wood increases by a number of years of annual growth.

The wood, or xylem, comprises by far the greater part of older
stems, becoming proportionally greater as each annual ring is
added. It is composed of very much elongated' cells, called
tracheids, with firm, somewhat yellowish, thick walls. Cell
walls- of this character are said to be lignified. These cells con-
tain pits (Fig. 297, E, F, G) surrounded by a circle and termed
bordered pits, the circle being a feature characteristic of this
group of plants. There are resin ducts among the wood cells,
and also peculiar plates of cells called medullary rays which
extend through the cambium and bast into the outer wood. The
medullary rays have the form of thin knife blades penetrating
the wood for various distances.

The cainbium is a cylinder of thin- walled dells just outside
of the wood, and is the most active region of growth in the
stem. This cylinder (Fig. 297, C) is only two or three cells
wide, and the cells are continually dividing by walls parallel to
the surface (tangentially) during the season of growth. The
-daughter cells on the inside of the cambium become firm wood
cells by the thickening of their walls together with certain
changes (lignification) that give them firmness ; they also become
empty of protoplasm. The daughter cells on the outside of the
cambium form the bast, remaining soft and containing proto-
plasm and much food material. The cambium thus adds cells to
the wood on the inside and the bast on the outside. The wood is
deposited in annual rings during the season of growth, and these
are sharply distinguished from one another because the wood
cells formed at the beginning of one season are larger than those
formed in the latter part of the previous season (Fig. 297, D).

The last is difficult to study chiefly because the cells are
under severe pressure from the growing cambium on the inside
and the restraining bark on the outside, and the cell arrange-
ments are frequently distorted.

368

THE SPERMATOPHYTES

The outer lark is developed from the primitive or ground
tissue which lay outside of the circles of wood and bast when
these circles were first formed by the union of the primary
fibro-vascular bundles (Fig. 297, A), as described in Sec. 79.
There is much actively growing tissue in the bark, but the outer

FIG. 298. The staminate cone, stamen, and pollen of the Scotch pine
(Pinw.s sylvestris)

A, young growth, with staminate cones about two weeks after the opening of the
terminal hud. B, details of cone. C, end view of stamen. D, side view of
stamen. E, pollen mother cell developing four pollen grains in a tetrad. F,
pollen grain showing the two wings : p, prothailial 'tell ; g, generative cell ;
t, tube nucleus. — E, after Miss Ferguson

regions become quite dead, and crack under the pressure of the
growing cambium, thus forming scales. The cracks are healed
by the living tissue of the bark. The bast is generally so closely
attached to the outer bark that it peels off with it, and therefore

THE STAMINATE CONE

is a sort- of inner bark and must be included in any account
of this region of the stem.

The functions of these tissues are discussed in Part I,
Chapter vm.

352. The cones of the pine. The cones of the pine, as in all
conifers, are of two sorts: (1) staminate, when made up of stamens
(microsporophylls), and (2) carpellate, when composed of carpels
(rnegasporophylls).

The staminate cone. The staminate cones are developed in
clusters on the young growth that appears late in the spring
with the opening of the terminal buds (Fig. 298, A). Each
cone consists of a large number of stamens closely packed
together and arranged somewhat spirally around the central axis
(Fig. 298, B). The stamen bears two pollen sacs (Fig. 298, C, D),
within which the pollen grains are developed. The pollen grains
are formed in groups of four, or tetrads (Fig. 298, E), just like
the spores of the bryophytes and pteridophytes, and their further
history shows them to correspond exactly to the microspores.1
The pollen sac is then a microsporangium, and the stamen a
microsporophyll. The pollen sacs develop from a group or region
of cells as in the horsetails, lycopods, and Selaginella, and not
from a single surface cell as in the common ferns.

The pollen grains are produced in enormous quantities, and
being set free by the splitting of the pollen sacs, they are scat-
tered as fine yellow dust by the wind. Sometimes pollen is carried
from pine forests by the wind for many mile^, falling as so-called
showers of sulphur. The pollen grains are especially adapted
for distribution by the wind, for the outer layer of the cell wall
is swollen on two sides.to form outstanding wings (Fig. 298, F).

1 This relationship is further established by the count of the chromosomes
in the Scotch pine (Pinua^ylvestris), which shows that the pollen grain has
12, while certain tissues of the pine sporophyte have 24. Pollen formation
is then the period of chromosome reduction when the sporophyte generation
passes over to the gamptophyte, as explained in Sees. 334 and 335. Similar
chromosome reduction undoubtedly takes place with the formation of the
embryo sac in the nucellus'.

FIG. 299. Carpellate cone, carpels, and seed of the Scotch pine
(Pinus sylvestris)

A, young growth with carpellate cones, about three weeks after the opening of
the terminal bud : n, young pine needles. 7>, inner and side view of a cone
scale at the time of pollination as shown in A : b, bract ; o, ovules. C, inner
and side view of scales from a two-year-old cone as shown in D: b, bract;
o, fertilized ovules now rapidly maturing into winged seeds ; w, the developing
wings. D, a two-year-old cone. E, a mature winged seed. F, section of
mature seed : t, hard seed coat, or testa, developed from the integument of the
ovule (see Fig. 300, A, i) ; n, a membranous seed coat which is the remains of
the nucellus (see Fig.. 300, A'ri) ; en, endosperm or tissue of the female gameto-
phyte (see Fig. 300, A) ; era, e.nbryo with group of cotyledons c and the
suspensor s ; m, micropylar end of seed

370

THE CARPELLATE CONE 371

The carpellate cone. The carpellate cones have a complex
structure that cannot here be described in detail. They are
borne singly or in groups of two or three at the 'ends of the new
growth in the spring (Fig. 299, A) simultaneously with the
staminate cones. Each cone is composed of scales arranged
somewhat spirally. Each scale (Fig. 299, B) is believed to be
a group of three fused carpels (the point representing a sterile
carpel between two fertile ones). The scale bears a pair of
ovules below on the inner face, near the place where it is
attached to the axis of the cone.

The ovule has a large nucellus, surrounded by an integu-
ment, which bears two appendages looking like a pair of horns
in miniature (Fig. 299, B, o). The embryo sac which develops
in the center of the nucellus is one of a group of four cells, or
tetrad, which shows its relationship to a spore (megaspore) and
to the pollen grain. The other three cells of the tetrad fail to
develop, so that all the strength of the ovule is given to this
single functional megaspore which produces the female gameto-
phyte. The ovule is an outgrowth from the surface of the car-
.pel, its nucellus (Fig. 300 A, n) corresponds to a megasporangium,
and the integuments (Fig. 300, A,i) are probably protective
investments. The integuments do not completely inclose
the nucellus, but there is left a small opening at the tip
(Fig. 300, A, m) called the micropyle.

353. Pollination in the pine. The young carpellate cones are
upright when they first appear, and the scales are slightly sepa-
rated from one another. When the pollen is shed in clouds
from the stamens some of the grains are carried by the wind
to the carpellate cones and sift in between the scales, collecting
in little drifts near the ovules. This is the process of pollination.
At this time there are globules of moisture between the two horn-
like appendages of the ovules, and the pollen grains are caught
by these. The fluid gradually dries up, drawing the pollen grains
toward the micropyle, and finally into a cavity'ealled the pollen
chamber (Fig. 300, A, pc), which lies just above the nucellus.

372

THE SPERMATOPHYTES

Meanwhile the scales of the cone close together and the cone
bends over until it hangs downward. This is a curious behavior,
although there is evident advantage to the plant, for the cone is now
in a better position to protect the ovules from rain or dust which
might enter between the scales if the cones remained upright.

FIG. 300. "The gametophytes of the pine

, diagram of a section of a year-old ovule : embryo sac with mature archegonia
ar imbedded in the tissue of the endosperm (female gametophyte) ; pollen
tubes (male gametophytes) growing down through the tissue of the nucellus
nj pc, pollen chamber; m, micropyle; i, integument. Z>, germinating pollen
grain, showing young male gametophyte : t, tube nucleus ; ff, generative nucleus ;
p, prothallial cell. C, tip of pollen tube applied to the egg: t, tube nucleus;
s, the two sperm nuclei. 7), a mature archegonium sunken in the tissue of the
endosperm, showing the large egg surrounded by a jacket of cells rich in proto-
plasm : two neck cells of the archegonium shown just above the egg. — /?, C',
after Miss Ferguson

THE GAMETOPHYTES OF THE PINE 373

354. The gametophytes of the pine. The pine, like all seed
plants, is of course lieterosporous because it has microspores
(pollen grains) and megaspores (embryo sacs) ; so there are two
gametophytes, male and female.

The male gametophyte. The male gametophyte, as in most, if
not all, seed plants, begins to develop before the pollen is shed.
There are three nuclear divisions .which cut off two small cells,
called prothallial cells, of which traces may sometimes be found
against the wall of the pollen grain (Figs. 298, F, p ; 300, B,p}.
The third division leaves the pollen grain with a nucleus (the
tube nucleus) in the central region and a small lens-shaped cell
(the generative cell) at one side (Fig. 298. F, g). This is the
condition when the pollen is shed.

Shortly after pollination the pollen tubes begin to develop
in the pollen chamber (Fig. 300, A,p c)pbut their development
is very slow until the following spring. Then the large tube
nucleus passes to the tip of the tube, which grows rapidly
towards the center of the nucellus (disorganizing the surround-
ing tissue as it does so), where the female gametophyte lies
within the embryo sac. The generative cell now divides into a
stalk and body cell which pass into the tube. The body cell
forms two sperm nuclei a few weeks later. Four nuclei are
then finally present at the end of the pollen tube (two sperm
nuclei, the tube nucleus, and that of the stalk cell). The pollen
tube has now reached the embryo sac and is ready to discharge
its contents into one of the eggs developed by the gametophyte
(Fig. 300, D).

The female gametophyte. The embryo sac (megaspore) is a
one-nucleate cell at about the time of pollination. This nucleus
gives rise by repeated divisions to a large number of nuclei that
lie at first freely in the protoplasm as the embryo sac gradually
increases in size. Later, cell walls are formed around the free
nuclei, and the entire embryo sac becomes filled with a delicate
tissue called the endosperm (Fig. 300, A), which corresponds to
the vegetative portion of a prothallium. It takes almost a full

374 THE SPERMATOPHYTES

year for the female gametophyte to reach this stage of devel-
opment, when it occupies the greater part of the nucellus. In
the spring following the pollination of the cone, the endosperm
forms a group of several archegonia at its micropylar end. Each
archegonium (Fig. 300, D) consists of a much-reduced neck
region, generally composed of four cells, and the very large
egg which lies imbedded in the endosperm, whose cells form an
investment around it called the jacket. The egg is filled with
dense protoplasm and contains much food material supplied
through the cells of the jacket.

This is the condition of the female gametophyte thirteen
months after pollination. At about this time the pollen tube
reaches the embryo sac and entering it passes between the
neck cells of an archegonium, where its tip fuses with the egg
membrane. The contents at the end of the pollen tube are dis-
charged into the egg, including not only the two sperm nuclei,
but' also the tube nucleus and that of the stalk cell. One of
the sperm nuclei moves towards the egg nucleus, which lies
near the center of the egg, and fusing with it completes the
act of fertilization. The other three nuclei break down and
soon disappear.

355. The development of the embryo in the pine. Fertiliza-
tion takes place, as described above, a little more than a year
after pollination. The cone during this time has increased
greatly in size, but is generally hardly a third as large as the
mature seed-bearing cone.

The fertilized egg soon begins to develop the pine embryo.
This is a complicated history, which cannot be described here in
detail. The embryo is however formed at the end of a structure
called the suspensor (Fig. 299, F, s), whose development carries
the embryo into the center of the endosperm, where it lies
in a favorable situation for its nourishment. The embryo
(Fig. 299, F, em) is straight, and the stem part is surrounded
by a circle of seed leaves called cotyledons. The pine seedling
is shown in Fig. 12.

THE LIFE HISTORY OF A GYMNOSPERM 375

Meanwhile the integument becomes firmer and finally forms
the hard, protective seed coat, or testa (Fig. 299, F, t). Adjacent
tissue of the cone scale above the ovule develops a membranous
wing (Fig. 299, C, w), which separates from the scale of the cone
with the ovule as a part of the seed. It takes another full year
for these changes to take place, and the cone is not fully mature
(Fig. 299, D) and the seeds ripe until somewhat more than two
years after pollination. Then the scales of the cone, now quite
woody in texture, separate, and the seeds are shaken out, and
since they are winged (Fig. 299, .Z?) they may be carried for a
considerable distance by the wind.

356. The life history of a gymnosperm. The life history of
a gymnosperm, beginning with the sporophyte (for the gameto-
phyte phases are now so inconspicuous that they only appear
during the process of seed formation), may be formulated as
follows :

pollen grain — Male Gametophyte— sperm nucleus ,

SpOTO- (<-,,,„„,,)

P y embryo sac — Female Gametophyte — egg

(megasporc) (protoplasmic contents

of embryo sac)

This in abbreviated form becomes
.p a — M G — s

v n — $ etc-

es — te G — e -^

This formula should be compared with that of some hetero-
sporous pteridophyte, as Selaginella (Sec. 326), to make clear the
relationships. When carefully studied it will be found to be
merely an elaborated form of the simple formula of alternation
of generations.

G<Se>-S-sp-G<Se>-S~sp-G, etc.

The peculiarities of the life history of a gymnosperm are due
to heterospory (and this is true of all seed plants), by means of
which two sexual plants, male and female, have been differen-
tiated, and the fact that both gametophytes live wholly or
almost wholly as parasites upon the sporophyte.

376 THE SPERMATOPHYTES

SUBDIVISION II. THE ANGIOSPEEMS, OK
ANGIOSPERMS

357. The angiosperms. The angiosperms (meaning seeds in
a vessel) are distinguished by the fact that the ovules are devel-
oped in a closed case (ovule case or ovary) formed by .the car-
pels, sometimes alone but often together with adjacent tissue of
the stem. This immense assemblage of plants, with more than
120,000 species, forms the greater part of the earth's vegetation
and includes the most successful groups, dominating most of
the land floras. It is a much more varied assemblage than the
gymnosperms, and successful in every vegetation form (herb,
shrub, or tree). The angiosperms adapt themselves to all sorts
of life conditions, some of them being aquatics, others covering
the meadows, prairies, and heaths, certain groups entering the
deserts, and the trees forming forests generally accompanied
by undergrowths of shrubs. They occupy the highest points of
plant evolution, but along a great many very divergent lines, for
some of the culminating groups are the grasses, the hardwood
trees, the composite groups, the orchids, etc.

The general structure of the angiosperms, including the roots,
stems, leaves, flowers, and fruits, together with many principles
of plant physiology best illustrated in this group, have been
described in Part I. This account will consider chiefly the life
history, with especial reference to the gametophyte generations
and significance of the flower.

358. The angiosperm flower. The essential structures of
the angiosperm flower (Fig. 301), as of the gymnosperms, are
the stamens (microsporophylls) and the carpels (megasporo-
phylls) ; but in addition to these some accessory parts are gen-
erally present, which are either modified leaves of the plant,
or sometimes stamens and carpels that have become sterile.
These accessory parts constitute the perianth (Fig. 301, p),
situated on the stem just below the stamens and carpels, and
are generally showy structures, but also protective, at leact in

THE ANGIOSPERM FLOWER

377

the bud. The perianth, as a rule, gives the characters of color
and form which in popular usage define a flower. It is a very
important accession, for it has
resulted in some remarkable
adaptations and devices on the
part of the plant to insure pol-
lination by the visits of insects
(see Chapter xxxn). The struc-
ture of the perianth, with its
parts, — sepals and petals, —
is described in Chapter xm.
Besides having the perianth,
the angiosperm flower is pecul-
iar in that the ovules are not
normally exposed on the sur-
face of the carpels. This means
that the carpels, either singly
or in groups, form closed struc-
tures, which may be termed
ovule cases. The ovule case,
generally called the ovary (an
unfortunate term, for it does
not produce eggs but ovules),
bears a receptive surface,
termed the stigma, upon which
the pollen grains may ger-
minate. The stigma may be
raised upon a stalk, or style.

FIG. 301. The lily (Lilium
Philadelphia um}

Ovule -case, Style, and Stigma ^dissected flower, showing the pistil and

constitute the pistil (meaning
a pestle), which is said to be
simple when only a single car-
pel is involved, and compound
if there is a group of carpels.
The various arrangements of

stamens : p, parts of the perianth which
have been cut away ; s, bases of stamens
cut off. B, floral diagram : p, perianth,
composed of two circles of similar and
petal-like parts; s, stamens, likewise in
two circles ; section of ovule case (ovary)
shown in the center, composed of three
carpels (c) so united as to form three
locules containing the ovules

378

THE SPERMATOPHYTES

the carpels to form different types of pistils are described in
Sees. 156 and 157.

Another characteristic of the angiosperm is the production
of fruit. A fruit is a ripened ovule case, or ovary, frequently

FIG. 302. Anther and pollen of the lily

A, mature anther, showing the four locules, or chambers, containing pollen grains :
the anther opens lengthwise on both sides along the lines of cells shown at x.
B, stages in the formation of pollen grains in a group of four (tetrad) within
the pollen mother cell. (7, mature pollen grain with early stages in the devel-
opment of the male gametophyte : t, tube nucleus ; g, generative nucleus

THE STAMEN AND THE FORMATION OF POLLEN 379

with accessory parts. The gymnosperms do not have the exact
equivalents of fruits, although the berry-like structures of the
yew appear at first glance to be similar and the cone is, of
course, a protective structure for the seeds. True fruits, as the
term is used when applied to the angiosperms, are seed cases of
various forms, — structures which are sometimes merely protec-
tive, and sometimes fleshy and attractive to animals for food.
They are described in Chapter xvi.

The pistil distinguishes the angiosperms from the gymno-
sperms, and is a more important feature of the angiosperm
flower than the perianth, which is frequently inconspicuous,
and sometimes wholly or almost wholly absent. But the pistil
in combination with a showy perianth of some peculiar and
specialized form gives the highest types of flower structure.
The most important of these are discussed in Chapter xm.
This account will only describe the stamens and carpels in their
functions as spore-producing organs developing microspores
(pollen) and megaspores (embryo sacs).

359. The stamen and the formation of pollen. The parts
of a stamen are described in Sees. 171 to 173. Pollen formation
takes place generally in four regions of the anther, which become
pollen sacs, or loculcs (Fig. 302, A). The cells of these regions
develop the pollen grains in groups of four, or tetrads (Fig. 302,7?),
and are consequently pollen mother cells. This process is iden-
tical with that of spore formation in the pteridophytes and
bryophytes.1 The pollen mother cell is a spore mother cell, and
the pollen grain a spore, or more exactly a microspore.

The pollen sacs are . sporangia, and like the sporangia of
the horsetails, lycopods, Selaginella, and the pollen sacs of the

1 As in the case of the gymnosperms, the count of the chromosomes during
pollen formation shows it to be a period of chromosome reduction, when
the sporophyte generation passes over to the gametophyte, as explained in
Sees. 334 and 335. Thus 24 chromosomes have been counted in various tissues
of the lily plant, but only 12 appear in the nuclear divisions in the pollen
mother cell (Fig. 302, B). These cells, it may be remarked, are exceedingly
good subjects for the study -of nuclear division

380

THE SPERMATOPHYTES

gymnosperms (Sec. 352), they develop from a group or large region
of cells, and not from a single surface cell as in the sporangium
of the common ferns. The pollen sacs open along certain lines
(Fig. 302, x) or by pores, and the pollen is thus set free. The
pollen is carried in various ways to the stigma of the pistil, as
described in Chapter xxxn, and its application to this structure

FIG. 303. Section of the ovule case (ovary) of the lily

Diagram of a cross section of a young ovule case, showing the three carpels c ; each
young ovule o has a large embryo sac mother cell e within the small nucellus
n, and shows the developing inner and outer integuments ii and oi

constitutes pollination in the angiosperms. Wind, direct con-
tact of the anthers with the stigma, or the visits of insects are
means by which pollination is effected in this group of plants.

360. The carpel and the formation of the ovule. The ovules
are developed as outgrowths from the surface of the carpels
(Fig. 303), or in some cases from regions of the stem, when this

THE GAMETOPHYTES OF AN ANGIOSPERM

381

structure enters into the formation of the ovule case. Each
ovule consists of a central region called the nucellus (Figs. 303;
306, A ; 309, A, n), which becomes en-
veloped by two protective integuments
(Figs. 303 ; 306, A ; 309, A, B} C, ii, oi)
that arise from its base and grow up
around it, forming a small opening
above termed the micropyle (meaning
little gate). A cell in the interior of
the nucellus becomes the embryo sac
(Figs. 303, e; 306, A, B), which in
most cases is the exact equivalent of
a megaspore. This is proved by the
fact that the embryo sac in such forms
is one of a group of four .cells, or tetrad
(Fig. 304), and that the development
of this group follows the same history
as in pollen and spore formation. The
nucellus is therefore a megasporangium.
Certain forms of angiosperms, as the FIG. 304. A group of four
lily, have given up the formation of inegaspores (tetrad) in the
tetrads, and the spore mother cell de-
velops directly into the embryo sac.1

361. The gametophytes of an an-
giosperm. The male gametophyte
(contents of the pollen grain and tube)
is clearly similar to that of the gymno-
sperm ; but the female gametophyte of the angiosperm is a
very much more reduced structure than anything in the gym-
nosperms.

1 In these cases the first two nuclear divisions within the embryo sac
have the peculiarities of those in all spore mother cells. In the lily the
nuclei of the nucellus have 24 chromosomes, but the nuclei of the embryo
sac have 12. This shows that the two nuclear divisions characteristic of
spore formation have become a part of the gametophyte phase of the plant's
life histoiy.

nucellus of an ovule
(Canna)

The upper three megaspores
of the group are breaking
down, while the lower is
rapidly enlarging to become
the embryo sac. — After
Wiegand

382

THE SPERMATOPHYTES

The male gametophyte. As in the gymnosperms, the male
gametophyte begins its history in the pollen grain before the lat-
ter has been shed. The first division forms a tube nucleus and a
generative cell (Fig. 302, C). The nucleus of the generative cell
divides sooner or later to form two sperm nuclei. These three
nuclei, with the rest of the protoplasm, constitute all there is of

the male gametophyte (Fig. 305).
The pollen grains germinate on
the stigma of the pistil, finding
there suitable fluids to start their
growth. Each puts forth a tube
(Fig. 156, A, B, C) which pene-
trates the stigma and grows down-
ward toward the ovule case
(ovary). The tube nucleus and the
generative cell (or the two sperm

nuclei if already formed) enter
FIG. 305. Pollen grain of the elder the tube and? passing to the tip>

accompany its growth (Fig. 156,

(Sambucus)

The two sperm cells s and the tube ^ ™ ^
nucleus t, with the remaining pro- ' ' /'
toplasm, constitute the entire male The pollen tubes grow through

the tissues of the stigma and

style (if present) frequently over definite paths and enter the
micropyles of the ovules. This behavior resembles the way in
which parasitic fungi grow through the tissues of their hosts,
and it is clear that the pollen tubes live largely or wholly para-
sitically on the sporophyte. On entering an ovule the pollen
tube penetrates the nucellus and grows toward the embryo
sac, which by this time has developed the female gameto-
phyte.

The female gametophyte. The mature female gametophyte
of an angiosperm (Fig. 306, B) contains only eight nuclei, the
products of three nuclear divisions in the embryo sac. These
are distributed as follows : There is a group of three nuclei at
the micropylar end of the embryo sac (Fig. 306, B, m), forming the

FERTILIZATION AND DOUBLE FERTILIZATION 383

egg apparatus, of which one, with surrounding protoplasm, con-
stitutes the egg, and the other two are called synergids (meaning
co-workers). There is a group of three nuclei at the opposite
end of the sac, called antipodal nuclei (Fig. 306, B,ant], which
frequently become inclosed by delicate walls and possibly repre-
sent a prothallial region. The remaining two nuclei, called polar

FIG. 306. The ovule and embryo sac of the lily

A, ovule with mature embryo sac: the inner integument ii has grown beyond the
nucellus n; oi, outer integument; m, micropyle. JB, mature embryo sac: egg
apparatus at the micropylar end m ; e, egg; s, synergids ; the two polar nuclei
p are about ready to fuse near the center of the sac ; ant, antipodal nuclei

nuclei (Fig. 306, B, p), pass from the opposite ends to the center
of the embryo sac, where they later unite.

362. Fertilization and double fertilization. The tip of the
pollen tube fuses with the end of the embryo sac, near the
synergids, and the two sperm nuclei are discharged into the sac.
The tube nucleus has generally broken down and disappeared
entirely by this time (Fig. 156, F, G). One of the sperm nuclei
unites with the egg nucleus (Fig. 307, e, fs), and this is the
process of fertilization.

384

THE SPERM ATOPHYTES

The other sperm nu-
cleus is known in a
number of forms to pass
to the center of the sac
and unite with the two
polar nuclei, constitut-
ing a triple fusion (Fig.
307, p, p, ss), and form-
ing a large nucleus,
called the endosperm
nucleus. Since the en-
dosperm nucleus has
an important history in
the development of the
seed, this peculiar behav-
ior of the second sperm
nucleus is important, and
it is called the double
fertilization of the em-
bryo sac.1

1 Double fertilization is
probably the explanation of
the phenomenon called xenia,
which is the appearance at
once in the seed of some
character of the male parent.
Thus a yellow or white kind
of corn, when pollinated from
a blue or red variety, will
produce blue or red kernels.

This color in the corn is pres-
The nrst sperm nucleus fa fusing with the egg . . ,, endosi)erm and
nucleus e ; the second sperm nucleus ss is fusing ^ospei m, am

with the two polar nuclei p near the center of the character comes into the
the sac, constituting the so-called double ferti- seed through the second
lization ; p, pollen tube ; s, synergid breaking sperm nucleus. For an ac-
down; ant, antipodals; ii, inner integument; count of xenia see Webber,
m, micropyle "Xenia, or the Immediate

Effect of Pollen in Maize," United States Department of Agriculture,
Division of Vegetable Physiology and Pathology, Bulletin 22, 1900.

FIG. 307. Fertilization in the embryo sac
of the lily

DEVELOPMENT OF THE EMBRYO AND ENDOSPERM 385

363. The development of the embryo and endosperm.

fertilized egg develops the em-
bryo, but as in gymnosperms,
there is generally a preliminary
growth called the suspensor
(Fig. 309, D, E, H, s), which car-
ries the young embryo into the
center of the sac. The endo-
sperm nucleus begins to divide
at once after its formation, by
the triple fusion of the second
sperm nucleus with the two po-
lar nuclei (Fig. 307). It gives
rise to a large number of nuclei,
which become distributed in the
protoplasm of the rapidly enlar-
ging embryo sac (Figs. 308, e ;
309, H, e). Later, walls begin to
form around these endosperm
nuclei, first in the outer regions
of the embryo sac, and finally
the whole sac becomes filled
with a delicate tissue.

This tissue is called the endo-
sperm, and the embryo becomes
imbedded within it as in the pine.
But this endosperm has, of course,
a very different origin from that
Of the gymnosperms, and is a
special development peculiar to

The

• 308. Development of the embryo

and the endosperm of the lily

, f . m. The embryo em has developed from

the anglOSpermS. The group of the fertilized egg; e, endosperm

antipodal cells possibly corre- nuclei which have been derived fr°m

f • . the triple-fusion nucleus, — that is,

Sponds to the endosperm in the the two polar nuclei united with the

gymnosperms, and the egg ap-

paratus has been regarded as a

!f.conQ(JJperm nucleu.s

Fig. 307) ; ii, inner integument ; m,

micropyie

FIG. 309. Development of the ovule and embryo of the shepherd's
purse (Capsella)

A, young ovule, showing origin of two integuments at base of nucellus n. B, outer
integument growing beyond the inner, and the ovule beginning to bend over:
es, embryo sac. C, diagram of a later stage with mature embryo sac. D, devel-
opment of the suspensor s. E, early divisions of the terminal cell (embryo
cell). F, later stage, showing the differentiation of an outer cell layer in the
embryo, which is to become the epidermis. G, the two cotyledons c and the
root region r now clearly defined. H, lengthwise section of an ovule, show-
ing the position of an embryo in an embryo sac : em, embryo ; s, suspensor ; e,
endosperm; U, inner integuments; oi, outer integument; m, micropyle. — A,
B, C, adapted after Campbell

386

THE DEVELOPMENT OF THE FLOWER

387

reduced archegonium. However, it is possible that all three of
the nuclei in the egg apparatus represent eggs, only one of
which is functional.

While the embryo and endosperm are developing, the ovule

increases greatly in size, and its integuments change into the

E D

FIG. 310. Development of the flower of the shepherd's purse (Capsella)

A, tip of stem, showing the origin of the flowers: s, first appearance of the sepals
in the flower /. B, sepals well along in their development and stamens st
appearing. 6', later stage, showing the two young carpels c and the beginnings
of the petals p. D, later stage lettered as in the preceding. E, the petals now
well developed, and the ovules beginning to arise on the inner face of the
carpels, not yet united above to form the closed pistil

seed coats. In some plants, as in the squash (Fig. 1), peas, and
beans, the embryo finally fills the entire seed, and the endo-
sperm is almost completely crowded out, being represented by a
thin membrane around the embryo. In other forms, as the corn
(Fig. 3), asparagus, and poppy (Fig. 4), the embryos remain small,
and the endosperm is conspicuous as a tissue richly stored with
food material.

388 THE SPERMATOPHYTES

364. The development of the flower. The development of the
parts of a flower would be expected to progress in the order of
sepals, petals, stamens, and carpels, for this is, of course, the
order of their position on the flower stalk, beginning from below.
The parts of many flowers do arise in this order, but there are
often irregularities due to the delayed appearance of some organs.
For example, in the shepherd's purse the petals are formed last,
arising between the sepals and carpels when the latter are far
along in their development (Fig. 310, C,p).

The carpels are clearly separate in the beginning (Fig. 310,
C, D, c), and the ovules at first may be exposed on their surface
(Fig. 310, E), but sooner or later the carpels unite above, so that
the ovules are finally contained in the ovule case (ovary).

A study of flower development makes clear the significance of
perigyny and epigyny (Sec. 157), for it shows that the apparent
fusion of parts, frequently called coalescence, when sepals, petals,
or stamens seem to be united to one another or to the carpels
(see diagrams, Fig. 136), is due to the formation of tubular out-
growths from zones of tissue below the floral parts. The parts
which are most frequently affected by these zonal outgrowths
are the carpels, and it seems probable that the compound pistil
may have arisen from their activities. In many cases the ovules
are developed from tissue that is probably really a part of the
tip of the flower stalk.

365. The life history of an angiosperm. The formula for
the life history of an angiosperm is the same as that of a gym-
nosperm (Sec. 356). The gametophyte phases, however, occupy
generally a much shorter period, so that the seeds are matured
in the same season and sometimes within a few weeks after
pollination.

The formula is then as follows :

pollen grain — Male Gametophyte — sperm nucleus -.

SpOTO- / (.microspore) (proto,>la*nn^contents \

Phyte \ embryosac _ tfemaie Gametophyte - egg etc>

(megaxpore) {protoplasmic contents

of embryosac)

THE ORIGIN OF SEED PLANTS 889

THE OKIGIN OF SEED PLANTS AND THE
SEED HABIT*

366. The origin of seed plants. We shall never know exactly
when and how seed plants arose, for that important event in
plant evolution probably took place earlier than the Carbon-
iferous Age. We can, however, form some idea of the chief
factors that brought about the seed habit from a study of
the life histories of living pteridophytes and spermatophytes.
As with a number of other forward steps in the evolution of
plants, such as the origin of sex, alternation of generations,
and heterospory, the seed habit probably was developed by a
number of different groups of pteridophytes independently one
of another. Thus the cycads and the conifers among the gym-
nosperms are so widely separated that it seems possible that
they may have come from different pteridophyte parentage.
Therefore the gymnosperms are generally regarded as a group of
divergent evolutionary lines. The angiosperms are even more
puzzling. Some botanists believe that they arose quite inde-
pendently of the gymnosperms, but others hold that they may
be distantly related to Gnetum. Some think that the mono-
cotyledons and dicotyledons even have had independent origins.
However, the view which seems to be finding greatest favor at
present regards the monocotyledons not as the ancestors of the
dicotyledons, but as derived from primitive dicotyledons.1

367. The origin of the seed habit. The most important fac-
tors leading to the seed habit appear to have been (1) heter-
ospory, (2) the retention of the megaspore in the megasporangium
to become the embryo sac in which the female gametophyte
develops parasitically, and (3) the development of the pollen tube
and its parasitic habit of growth through the tissues of the

* To THE INSTRUCTOR : This subject is very difficult and may be omitted.

1 These topics are far too technical for consideration here. For reviews of
the various theories with their evidence, the reader is referred to Coulter
and Chamberlain, Morphology of Spermatophytes (Gymnosperms), 1901 and
Morphology of Angiosperms^ 1903.

390 THE SPERMATOPHYTES

sporophyte to reach the embryo sac. These principles are asso-
ciated with the last stages in the long processes of the evolution
of the sporophyte and the degeneration of the gametophyte,
which is briefly outlined in the next chapter.

Heterospory (Sec. 314, Chapter xxvn) has differentiated the
spores of the pteridophytes and established male and female
gametophytes, the first always developing from the microspores,
and the second from the megaspores. At the same time the
gametophytes became largely or wholly dependent upon food
material stored in the spores, and smaller and simpler in their
organization, until they degenerated into structures somewhat
similar to those now illustrated in the heterosporous pterido-
phytes (Marsilia, Selaginella, Isoetes, etc.). Some of them
finally lost all their chlorophyll, and adopted parasitic habits.

Heterospory also resulted in the differentiation of the spore
leaves into microsporophylls and megasporophylls, and at last,
as in many seed plants, the sporophytes themselves became
differentiated, some producing only pollen (microspores) and
some only embryo sacs (megaspores) in the ovules. In this way
sexual characters of the gametophytes were gradually taken
up first by the sporophylls and later by the sporophytes them-
selves, and thus the asexual generation began to assume the
peculiarities of sex. The microsporophyll of the seed plant
(stamen) took on characteristics of a male organ, and the mega-
sporophyll (carpel) characteristics of a female one. The early
botanists regarded the pollen grain as a male element and the
stamen as a male organ, and it is true that these structures have
male characters ; but of course the actual male gametes are the
sperm nuclei with closely associated protoplasm in the pollen
tube whose contents represent a male gametophyte. And simi-
larly, although the carpel has female characters, the female
gamete is the egg within the embryo sac whose contents rep-
resent a female gametophyte.1

1 This subject is considered more at length in Chapter xxvn, Heter-
ospory, Sec. 339.

THE ORIGIN OF THE SEED HABIT 391

The retention of the megaspore in the megasporangium was,
perhaps* the most important step in the development of the seed
habit. This retention was possibly at first somewhat accidental;
that is, the megaspore simply failed to fall out of the megaspo-
rangium (as actually happens in. some species of Selaginella,
Sec. 325), and consequently developed its gametophytes while
mechanically held on the sporophyte. Later the retention be-
came more intimate and less mechanical, so that the female
gametophyte established a close physiological association with
the sporophyte, obtaining protection and certain foods, and per-
haps most important of all it was kept moist. At last the mega-
spore, instead of being developed as a free cell, remained a part
of the tissue of the megasporangium (nucellus of the ovule) and
at that stage became the embryo sac with its clearly estab-
lished parasitic relations to the sporophyte.

Some forms of Selaginella actually illustrate a beginning of
such parasitic relations in the early stages of the development
of its megaspore (Sec. 325), for the female gametophyte begins
to develop before the spores are full grown and ready to be
discharged. But the seed habit could not have been entirely
formed until the megaspore became physiologically a part of
the megasporangium, and the latter (as a nucellus), together
with the protective integuments, became the ovule.

The development of the pollen tube is perhaps even more
remarkable than the retention of the megaspore in the mega-
sporangium. It seems clear that the pollen tube is a develop-
ment in response to the stimulus of the moisture (containing
food substances) which is excreted by the ovule in the gymno-
sperms and the stigma of the angiosperms. The habit may
readily have had a very simple beginning if microspores fell
into partially opened megasporangia, as indeed occurs in a spe-
cies of Selaginella (Sec. 325), or among a group of megasporo-
phylls. They would have found in such situations moisture
and other conditions favorable for the development of out-
growths which later became tubes. These outgrowths and

392 THE SPERMATOPHYTES

tubes would be expected to become more and more specialized
as conditions arose which led to the final retention of the mega-
spore within the megasporangium, and at last they assumed pro-
nounced parasitic habits.

With the parasitic habits of the pollen tube established it is
not difficult to imagine the gradual adjustment of the peculiar-
ities of pollination to those of ovule formation. It seems prob-
able that the earliest forms of pollen tubes carried motile sperms
to the embryo sac 1 (for motile sperms are even now present in
the cycads and Ginkgo, Sec. 348), but later the complex struc-
ture of the sperm degenerated, with that of the whole male
gametophyte, until the sperm nuclei became practically all that
was left to represent the male gametes of the pteridophytes,
bryophytes, and algse. The simplification of the sperm and
egg in the spermatophytes does not, however, affect the signifi-
cance of these sexual elements, because it is known that the
nuclei are the most essential structures of gametes.

Thus the peculiarities of the ovule and the pollen tube prob-
ably developed side by side, adjusting themselves to one another
until the complex phenomena of pollination became established.
These processes are relatively simple in the gymnosperms, where
the pollen is applied directly to the ovule ; but in the angiosperms
a new feature was introduced when carpels, or groups of carpels,
frequently with adjacent tissue of the stem, developed the ovule
cases (ovaries). Yet it is not very difficult to understand how
they may have arisen, for the same principles of protecting
the megaspore (embryo sac) and providing for the germination

1 In certain fossil groups (Pteridospermce), intermediate between the
pteridophytes and spermatophytes, the evidence indicates that motile sperms
were discharged into large pollen chambers filled with water, into which
the necks of the archegonia opened so that the sperms were able to swim
directly to the eggs. The pollen tube was probably at first an absorbing
organ, or haustorium (as in the cycads and Gink^o to-day, Sec. 348), pene-
trating the tissue of the ovule to obtain nourishment for the parasitic male
gametophyte. Later, with the disappearance of the pollen chamber and
motile sperms, the pollen tube took on the added function of carrying the
sperm nuclei directly to the embryo sac.

THE EVOLUTION OF THE FLOWER 393

of the pollen grains are simply carried one step farther, and
the megasporophylls (carpels) become factors in the processes.
Thus a receptive surface, the stigma, was developed as a special
organ to receive and start the pollen tube in its parasitic devel-
opment, which is to end with the fertilization of the egg.

The seed is the ripened ovule, for the principle of protection
is continued after fertilization, and the integuments form hard
seed coats, inclosing the developing embryo, supplied with food
material by the parent sporophyte until it has reached an ad-
vanced stage of development.

THE EVOLUTION OF THE FLOWER

368. The evolution of the flower.* The higher types of
flowers have been developed by long processes of evolution from
the simpler structure of the primitive flowers. We do not
know exactly what the primitive flowers were like, but some
of their characters may be inferred from the structure of the
simplest flowers of the angiosperms and the cones of gymno-
sperms and certain pteridophytes, as the horsetails and club
mosses, which are truly flowers, if one accepts the definition of
a flower as a " shoot beset with sporophylls." The most elabo-
rately developed theory of floral evolution is that of Engler, and
this brief outline will be a general statement of his views.

Primitive flowers were characterized by indefinite numbers
of sporophylls, usually distributed in spirals, and the absence of
the floral envelopes constituting a perianth. These conditions
are illustrated in the cones of the pteridophytes and in many

* To THE INSTRUCTOR : This subject should only be presented to classes
with a fairly wide range of experience with flower structure in various
groups of angiospern.s. An excellent study would be a series of types from
such an assemblage as the buttercup order, Ranunculales, as the mouse-
tail (Myosurus), buttercups, magnolia, white water lilies, columbine, lark-
spur, aconite, etc., where many of the principles of flower evolution are
illustrated in a single group. Similar studies might be planned for the rose
order, Rosales, or the lily order, Liliales, followed by the orchids.

394 THE SPERMATOPHYTES

gymnosperms. Spiral arrangements of sporophylls (stamens and
carpels) and floral envelopes are also not uncommon in many
flowers with well-developed perianths, as in representatives of the
buttercup order, fianunculales, namely, mousetail (Myosurus), but-
tercups, magnolia, white water lilies, and the rose order, Mosaics.
It is not at all probable that the various advances over the
primitive conditions followed any regular order. Some of them
were concerned with the differentiation of a perianth ; some had
to do with the arrangements of the sporophylls and parts of the
perianth; some dealt with the apparent fusion of parts, and
some concerned the symmetry of the flower.

The differentiation of a perianth has clearly taken place in
some flowers through the transformation of sporophylls, which
became sterile and assumed perianth characters (generally those
of petals). Such transformations are admirably shown in the
passage of stamens into the parts of the perianth in the white
water lily, and in the doubling of flowers, where stamens and
frequently carpels become petals. It is possible, however, that
parts of a perianth may be derived in a reverse direction, — that
is, from leaves or bracts on the stem just below the sporophylls.
That ordinary, leaves can become highly modified and colored
to serve the purpose of a perianth is illustrated by the showy
bracts of the painted cup, or the flowering dogwood and other
species of Cornus (Frontispiece). The parts of the simplest types
of perianth were probably all similar and largely protective,
especially to the flower bud. These later became differentiated
into the two sets, sepals and petals, — the latter, and frequently
also the former, showy and clearly related to pollination by
insects or birds (Chapter xxxn).

The arrangements of the sporophylls and parts of the peri-
anth are, as a rule, spiral in simpler types of flowers, but gener-
ally in circles or whorls in higher types. In passing from the
spiral to the cyclic arrangements the variable and indefinite
numbers of parts tend to become constant. Thus three and
multiples of three are the prevailing numbers in the flowers of

THE EVOLUTION OF THE FLOWER 395

monocotyledons,, while four and five are common numbers in
the dicotyledons. A settling of the parts into fixed numbers
would be always an important forward step in floral evolution,
according to Engler, whether it concerns the perianth, the
sporophylls, or both together; for it tends to give definite form
to the flower, and thus leads toward the higher conditions.
Sometimes a flower will be mixed in the arrangement of its
parts, the perianth being cyclic and the stamens and carpels
spiral, as in certain buttercups. The establishment of fixed
numbers is frequently accompanied by the suppression of some
parts (sepals, petals, stamens, or carpels), so that the numbers
are variable in different circles.

The apparent fusion of parts, frequently called coalescence,
results from the formation of tubular or cup-like outgrowths
from zones of tissue below the floral parts, so that they seem to
be united. The most complex conditions of flower structure,
called epigyny (Fig. 136, C) and perigyny (Fig. 136, B), are due
to these zonal growths (see Sees. 152, 157, 364). The contrast
to epigyny and perigyny is hypogyny (Fig. 136, A). When
petals or sepals are borne on zonal outgrowths the conditions
are called, respectively, sympetaly and synsepaly (Sec. 152). The
compound pistil, — that is, a pistil involving two or more car-
pels, — is one of the highest expressions of zonal growth and is
called syncarpy (meaning united fruits).

The symmetry of the flower may be either radial or bilateral,
that is with a right and a left half (Sec. 150). Primitive flowers
were radially symmetrical, as would be expected from an in-
definite number of parts spirally arranged. Bilateral symmetry
appears, however, in very many groups and always represents a
high degree of floral evolution. It is found more commonly in
epigynous and perigynous flowers than in hypogynous, but there
is no rule about its relations to these conditions. Bilateral sym-
metry is usually directly related to methods of flower pollination
by insects, for the forms of such flowers are especially adapted to
the habits of bees, which light on some expanded lip-like region

396 THE SPERMATOPHYTES

of the perianth and rummage around, gathering pollen and
nectar, and incidentally effecting the pollination of the stigma
(Sec. 401).

Bilateral symmetry is generally accompanied by dorsiven-
trality, which means that the flower hangs in such a position
that there is an upper and a lower portion as well as a right
and a left half. Excellent illustrations are such lipped flowers as
the snapdragons, the mints, and many orchids. An epigynous
flower whose symmetry is bilateral and dorsiventral and whose
parts, through suppression or other developments, show irregu-
larities which have a clear relation to insect visitations, — these
characters give the highest types of flower evolution.

According to Engler, the chief steps in the evolution of the
flower may be :

1. The differentiation of a perianth.

2. The change from spiral arrangement of parts, with indefi-
nite numbers, to cyclic arrangements, with fixed numbers.

3. The grouping of parts through zonal growths (coalescence),
resulting in syncarpy, perigyny, and epigyny.

4. The change from radial to bilateral symmetry, accompa-
nied by dorsiventrality.

5. To these stages in floral evolution should be added the
complexity attained by the massing of numerous flowers in
groups or heads (Sec. 165), as in the composite family (daisies,
sunflowers, etc.). In the highest expressions of this development
the flowers are differentiated so that the outermost of the groups
become sterile, but by a remarkable lengthening of their corollas
into rays the flower cluster becomes very conspicuous.

All flowers do not, by any means, follow the order of evolution
as outlined above, and there are very many special irregularities
in different groups. Thus certain flowers of the legume family
are bilaterally symmetrical and dorsiventral, but there is no
perigyny or epigyny. It is important to note that the higher
levels of flower evolution have been developed again and again in
unrelated groups of angiosperms independently one of another

THE MONOCOTYLEDONS 397

(as, for example, among the orchids, the legumes, the snap-
dragons, the mints, etc.). While there is generally an upward
evolution of flowers, especially when insect-pollinated, there are
in some groups numerous illustrations of floral degeneration.

THE CLASSIFICATION OF THE ANGIOSPEEMS

369. The classification of the angiosperms.1 The subdivision
Angiospermce contains two classes :

CLASS I. The monocotyledons, or Monocotyledonece , with an embryo

having a single lateral cotyledon.
CLASS II. The dicotyledons, or Dicotyledonece, with an embryo having

two terminal cotyledons (including a few exceptions).
SUB-CLASS 1. The Arcliicldamydece (meaning primitive floral envel-
opes), in which the perianth is wanting, or, if pres-
ent, has its parts entirely separate from one another.
SUB-CLASS 2. The Metacldamydece (meaning later floral envelopes), or
Sympetalce, in which the petals are united or borne
on tubular, ^cup-like, or other forms of zonal out-
growths from the receptacle (Sec. 152).

370. The monocotyledons. Besides having the single coty-
ledon in the embryo, this group is distinguished from the dicoty-
ledons by having a stem structure, with scattered fibro-vascular
bundles, instead of a cyclic arrangement. Consequently there
can be no development of a central shaft of wood surrounded
by a cylinder of bast, with a cambium tissue lying between the
two, as is commonly found in the larger dicotyledons. The leaves
are generally closed (parallel) veined instead of open (netted)
veined, and rarely notched, which means that their fibro-vascular
bundles come together at the tip or along the edge of the leaf,
instead of ending freely as they do in the dicotyledons. The
parts of the flower are generally in three or multiples of three.

1 The most generally accepted classification of the angiosperms is that of
Engler, presented in the Syllabus der Pflanzenfamilien, 1903. A brief state-
ment of the chief features of this system will be found in Coulter and Cham-
berlain, Morphology of Angiosperms, 1903.

398 THE SPERMATOPHYTES

There are more than 20,000 species of monocotyledons, which
are arranged by Engler into 11 orders, the chief of which are :

1. The grass and sedge order, Graminales, including more
than 6000 species, one of the most successful assemblages of
angiosperms and by far the largest in the number of individuals.

2. The palm order, Palmales, a very characteristic tropical
and sub-tropical group.

3. The lily order, Liliales, a large group of almost 5000
species, remarkable for the showiness and symmetry of its
flowers.

4. The orchid order, Orchidales, containing the large orchid
family with more than 5000 species, the largest family in the
Monocotyledonece, and one of the most remarkable groups of seed
plants for the beauty and complexity of its flowers and for its
peculiar life habits.

371. The dicotyledons. Besides having two cotyledons in the
embryo, this group is distinguished from the monocotyledons
by having its fibro-vascular bundles formed in a circle. This
arrangement makes possible the development of a central shaft
of wood (xylem), since the cambium regions of the bundles
unite into a cylinder which adds successive layers of wood if
the plant is perennial. The bundles in the leaves are strongly
developed, much branched, and end freely, so that the leaves
are conspicuously open (netted) veined, generally notched, and
frequently deeply divided, or compound. The parts of the flowers
are mostly in fours and fives in the higher types, except that the
number of carpels is commonly less.

There are more than 100,000 species of dicotyledons, and
these are arranged by Engler into 34 orders (26 in the Archi-
chlamydece, and 8 in the Metaclilamydece).

372. The Archichlamydeae. This sub-class is an immense
assemblage, very diverse in character, whose flowers range from
primitive types, with indefinite numbers of parts in spiral arrange-
ments, to cyclic flowers with definite numbers, perigyny, epig-
yny, and syncarpy. Some of the chief orders are :

THE ARCHICHLAMYDE/E 399

1. The tree orders, including the willows and poplars (Sali-
cales); the walnuts and hickories (Juglandales) ; the birches,
alders, beech, chestnut, and oaks (Fag ales) ; the elms, figs, mul-
berries, etc. (Urticales).

2. The buttercup order, Ranunculales, a large assemblage of
about 4000 species, full of interesting gradations in floral evolu-
tion, the buttercup family (Ranunculacece) being an especially
good group for such studies.

3. The poppy order, Papaverales, comprising the poppies and
the large mustard family.

4. The rose order, Rosales, an immense group of over 14,000
species, with several large families, such as the legume or pea
family, the rose family, etc. The flowers present a greater range
of structure than in the buttercup order. Some large groups in
the legume family have flowers with well-developed bilateral
symmetry and dorsiventrality.

5. The geranium order, Geraniales, containing the geraniums,
flax, Euphorbias, etc.

6. The violet order, Violales, comprising a large number of
families and more than 4000 species.

7. The cactus order, Cactales, a very remarkable American
group of more than 900 species, mostly adapted to desert
conditions.

8. The umbel order, Umbellales, containing more than 2500
species, mostly in the umbel (parsley) and dogwood families, —
the highest order in the series of the ArchiMamydece on account
of its epigynous flowers, the reduced number of carpels, and the
massing of the flowers in the characteristic umbel, or in close
heads surrounded by a corolla-like involucre of bracts, as in the
dogwoods (Cornacece, see Frontispiece).

373. The Metachlamydeae. The general flower characters of
this sub-class are cyclic arrangements of parts with definite
numbers, perigyny or epigyny, and a reduced number of carpels
in the compound pistil (syncarpy). The corollas are usually
showy, the petals being borne on tubular or cup-like outgrowths

400 THE SPERMATOPHYTES

(sympetaly). The stamens are usually also borne on the same
outgrowth with the petals, so that they appear to arise from
them (epipetaly). The chief orders are:

1. The ericad order, Ericales, containing the heath family, a
very characteristic group in the northerly parts of America,
Europe, and Asia, especially in the mountains.

2. The gentian order, Gentianales, with more than 4000
species, including the gentians, olive family, milkweeds, etc.

3. The phlox order, Polemoniales, with more than 14,500 spe-
cies, containing a number of prominent families, as the phloxes,
borrages, nightshades, figworts, mints, verbenas, and others. The
two-lipped flowers of the mints, figworts, etc., distinguish these
families among the MetacJilamydece as the legumes are distin-
guished among the ArchiMamydece, and the orchids among the
monocotyledons.

4. The madder order, Rubiales, including the large madder
family, the honeysuckles, the valerian family, and the teasels.

5. The bellwort order, Campanulales, containing the highest
of all angiosperm families, the Composite, the largest in the
number of species (more than 12,000), and one of the most
successful groups of plants.

SUMMARY OF THE SPEKMATOPHYTES AND THEIE
ADVANCES OVER THE PTEKIDOPHYTES

374. Summary of the spermatophytes. The chief charac-
ters of the spermatophytes and their advances over the pteri-
dophytes are :

1. The retention of the inegaspore as an intimate part of the
megasporangium (nucellus) to become the embryo sac, and the
development of the female gametophyte parasitically within this
structure. The degeneration of the female gametophyte in the
angiosperms to a group of nuclei within the embryo sac.

2. The origin of the ovule as a new structure from the mega-
sporangium (nucellus), together with enveloping integuments.

SUMMARY OF THE SPERMATOPHYTES 401

3. The development of the pollen tube from the microspore
(pollen grain) as a result of the habit of pollination, by which
microspores enter the micropyles of the ovules in gymnosperms,
and fall upon a receptive structure, the stigma, in angiosperms.

4. The degeneration .of the male gametophyte until it is
hardly more than a group of nuclei, with accompanying pro-
toplasm, in the pollen grain and its tube. The degeneration of
the motile sperms until they are represented by two sperm
nuclei alone (cycads and Ginkgo excepted, Sec. 348), which are
carried by the pollen tube into the embryo sac.

5. The development and retention of the embryo sporophyte
within the embryo sac, and the ripening of the ovule into
the seed.

6. The massing of the sporophylls on the shoot, accompanied
by envelopes which constitute the perianth of the flower. The
development in the angiosperms of the megasporophyll, or carpel,
into the simple pistil, and the grouping of carpels through zonal
growth (syncarpy) to form the compound pistil, so that the
ovules become inclosed in an ovule case (ovary). The differ-
entiation of a receptive surface, the stigma, on the pistil upon
which the pollen grain may germinate.

7. The differentiation of the parts of the perianth into sepals
and petals, and their grouping through zonal growth, together
with the stamens, to give perigyny, epigyny, sympetaly, syn-
sepaly, and epipetaly. The development of bilateral symmetry
and dorsiventrality.

8. A general development of the sporophytes in many par-
ticulars, giving them much greater complexities of tissue struc-
ture, growth, and form than those of the pteridophytes.

CHAPTEE XXIX

THE EVOLUTION OF THE SPOROPHYTE AND DEGENERATION
OF THE GAMETOPHYTE

375. The evolution of the sporophyte. Alternation of genera-
tions had its beginnings among the thallophytes, and is clearly
shown in the life histories of the red algae and the sac fungi,
but is not so conspicuous there as in the higher divisions of
the plant kingdom. Furthermore, the sporophyte generations of
these thallophytes do not seem to be related to the sporophytes
of the liverworts and the groups above them, but are probably
of independent origin.

Consequently the line of evolution, with the remarkable
development of the sporophyte and degeneration of the game-
tophyte, as illustrated by the pteridophytes and spermatophytes,
really had its beginning in the lower bryophytes and in the algal
ancestry, probably Chloropliycece, from which they were derived.
This algal ancestry, however, is not known, for there are no
living algse that have the combination of characters which would
be expected of the ancestors of the bryophytes, — namely, the
multicellular sexual organs, together with clearly established
sporophyte and gametophyte phases in the life histories.

The bryophytes were responsible for the first great steps in
the evolution of the sporophyte toward the conditions presented
in the ferns and seed plants. All of the sporophytes of the
liverworts and mosses are to a great extent parasitic upon the
gametophytes; that is, they take water from them, and probably
certain foods in solution. Two important advances appeared
in the bryophytes.

First. The spore-forming tissue gradually came to occupy a
relatively smaller part of the sporophyte (compare the sporophytes

402

THE EVOLUTION OF THE SPOROPHYTE 403

of the Riccia group with those of Marchantia, Porella, and
Anthoceros). Thus tissue which originally developed spores,
or had spore-forming possibilities, became set apart for vegeta-
tive functions alone. In this manner the foot and stalk became
established in Marcliantia and Porella, and the heavy walls of
the spore case in Anthoceros. This principle has been called the
" sterilization of potential sporogenous tissue," but a simpler ex-
pression would be " the assumption of vegetative functions by
tissues with spore-forming possibilities."

Second. Portions of the chlorophyll-bearing regions of the
sporophytes developed stomata in Anthoceros and in some of
the mosses, and this was the beginning of the elaborate mechan-
ism for chlorophyll work (photosynthesis), which is developed to
such a high degree in the leaves of ferns and seed plants.

The pteridophytes carried the advance much farther, through
the third, fourth, fifth, and sixth great steps in the development
of the sporophyte.

Third. The sporophyte became independent of the gameto-
phyte by developing roots, and to these added stems and fronds.

Fourth. This condition was associated with the differentia-
tion of a vascular tissue that made it possible for the sporophyte
to grow to a considerable height above the ground, (1) by ena-
bling it to maintain a connection with a water supply through the
roots, and (2) by providing it with a strong framework through-
out the stem and leaves. In their strengthening functions the
nbro-vascular bundles were greatly assisted by the development
of rigid tissues (schlerenchyma). In other respects, also, the
entire tissue structure, or histology, of the sporophyte became
much more complicated.

Fifth. Fronds were differentiated into spore leaves, or spo-
rophylls, and vegetative, or foliage leaves. The spore leaves
became grouped into cones, and by heterospory were differen-
tiated into microsporophylls and megasporophylls.

Sixth. The embryo sporophyte of heterosporous pteridophytes,
through the shortening of the gametophytic phases, came to use

404 THE EVOLUTION OF THE SPOROPHYTE

and depend upon food stored in the megaspores by the previous
sporophyte generation.

The spermatophytes added the final stages in the evolution
of the sporophyte, as follows :

Seventh. The ovule arose through the retention of the mega-
spore (embryo sac) in the megasporangium (nucellus) inclosed
by integuments. The development of the embryo sporophyte
within the embryo sac, and the ripening of the ovule, produced
the seed.

Eighth. The ovule case, or ovary, appeared with the devel-
opment from one or more megasporophylls, or carpels (frequently
with adjacent tissue), of an inclosing structure, the pistil, upon
which was differentiated a special region, the stigma, for the
reception of the pollen.

Ninth. The stamen was developed from the microsporo-
phyll.

Tenth. Complicated flowers arose by various groupings of the
carpels and stamens, together with showy or protective envelopes
constituting the perianth.

Eleventh. The flower cluster, or inflorescence, appeared, culmi-
nating in the composite head.

Twelfth. The tissues of the spermatophytes became more com-
plicated in many respects than those of the pteridophytes.

376. The degeneration of the gametophyte. Many steps in
the degeneration of the gametophyte were closely related to the
advances of the sporophyte.

In most of the bryophytes the gametophytes appear as
organisms equally complex with the sporophytes, and in many
forms they are more complex. Thus the gametophytes of the-
mosses and leafy liverworts show a considerable advance over
the thalloid gametophytes of the simple bryophytes. The thal-
loid gametophyte, however, seems to have been the type that
was passed over to the pteridophytes, and Anthoceros probably
gives a fair idea of the relative complexity of the two genera-
tions at the time when the first pteridophytes arose.

DEGENERATION OF THE GAMETOPHYTE 405

The beginnings of the degeneration of the gametophyte
became clearly evident in the pteridophytes when the rela-
tively small and simple prothallium took the place of the large
gametophytes, as illustrated in the Riccia and Marchantia
groups. Its further simplification was greatly accelerated by
heterospory, passing through four prominent stages :

First. Dependence upon food stored in the microspore and
rnegaspore, together with gradual loss of chlorophyll, reduced
the gametophytes to small structures producing relatively few
sexual organs and gametes. Thus the gametophytes in the
pteridophytes became dependent upon food supplied by the spo-
rophytes by way of the spores, — a relation exactly the reverse
of that in the bryophytes.

Second. The gametophytes became differentiated as male and
female in sex, associated with the microspores and megaspores,
respectively.

The spermatophytes, by means of the seed habit, brought
about the greatest changes in the gametophytes, as follows :

Third. The female gametophyte degenerated to such an
extent by the retention of the megaspore (embryo sac) in the
megasporangium (nucellus) that the archegonium lost its form
and finally became represented in its essentials by the egg alone.
The vegetative tissue became reduced until only a few nuclei
of uncertain relationship (antipodal and polar nuclei) remain.

Fourth. The male gametophyte degenerated in structure in
a similar manner until the antheridium disappeared, and the
numerous ciliated sperms of the pteridophytes were represented
by only two sperm nuclei, with associated protoplasm. Vegeta-
tive tissue was reduced until only a single nucleus remained in
the angiosperms to represent sterile cells of a male gametophyte.

There arose, however, by means of the seed habit an activity
on the part of the male gametophyte which is one of the most
remarkable developments in plant evolution. The appearance of
the pollen tube, with its parasitic relations to the sporophy te, is
a very complex life relation. This was the chief cause of floral

406 THE EVOLUTION OF THE SPOROPHYTE

evolution, with all its wonderful diversities of form and struc-
ture in relation to insect life, — diversities assumed to carry out
the relation of flowers to insect carriers of pollen.

Part II of this work has given an outline of the evolution
and classification of plants based on comparative studies of
their morphology. The conclusions are necessarily speculative
and philosophical, for we have no means of knowing exactly
what has happened throughout the geological ages. The fossil
remains of plants are very helpful in certain groups, as the pteri-
dophytes and spermatophytes, but they are fragmentary and
relatively few, except for such periods as those when coal or
coal-like deposits were formed. Consequently our conclusions
as to the evolutionary history of plants must be founded chiefly
on studies of life histories and the comparative morphology of
living groups. In spite of difficulties, the plant morphologist has
been able to establish a classification of plants, based on kinship,
so as to determine the framework of evolutionary lines with
remarkable clearness, and these conclusions give to botany its
chief interest on the side of morphology. It is not strange that
the development of exact ideas in regard to plant evolution
should have lagged behind the progress made in that line in
animal evolution, since the paleontological evidence available for
the botanist, as above stated, is so scanty. It is only within a
very few years that any attempt has been made to introduce
beginners in botany to the evolutionary history of plants, and
popular knowledge of the subject is now no farther advanced
than was knowledge of animal evolution more than thirty
years ago.

PART III

ECOLOGY AND ECONOMIC BOTANY

CHAPTER XXX
PARASITES AND CARNIVOROUS PLANTS

377. Ecology.* Plant ecology discusses the way in which
plants get on with their animal and plant neighbors and, above
all, the way in which they adjust themselves to the nature of
the soil and climate in which they live. Ecology, in short,
treats of the relations of plants to the world about them. A
good deal of what has been said in previous chapters on such
topics as parasitic plants, climbing plants, the movements of
leaves, the coating of hairs on stems and leaves, the storage
of water in epidermis cells, is really ecological botany, although
it is not so designated in the sections where it occurs. It is
evident enough that much of the subject-matter of ecology is
merely a special department of physiology, but another portion
of it forms an important part of plant geography.

378. Parasites. By the term parasite in botany, a plant
is meant which draws its food supply wholly or partially
from another living plant or animal called the host. In Sec. 29
the life history of a familiar parasite, the dodder, was briefly
sketched, and the parasitic fungi among spore plants have been
discussed in Chapter xxn.

* To THE INSTRUCTOR : The treatment of the subject of ecology will per-
tain almost entirely to seed plants. Many ecological topics relating to spore
plants have been discussed, under the various groups described in Part II.

407

408 PARASITES AND CARNIVOROUS PLANTS

379. Half -parasitic seed plants. Half parasites, or partial
parasites, are those which take a portion of their food, or of
raw materials to make food, from their host and manufacture
the rest for themselves. Usually they take mainly the newly
absorbed soil water from the host and do their own starch
making by combining the carbon dioxide, which they absorb
through their leaves, with the water stolen by the parasitic
roots, or Jiaustoria, imbedded in the wood of the host. Evidently
the needed water may just as well be taken from the under-
ground parts of the host as from the upper portions, and accord-
ingly many half parasites are parasitic on roots. This is the
case with many of the beautiful false foxgloves (Gerardia),
with the painted cup (Castillea), and some species of false
toadflax (Comandra) and some orchids.1 Usually these root
parasites are not recognized by non-botanical people as para-
sites at all, but in Germany a species common in grain fields2
and the eyebright, which abounds in grass fields, are respectively
known as " hunger " and " milk thief," from the injury they do
to the plants on which they fasten themselves. The mistletoe
is a familiar example of a half parasite which roots on branches.
Among the scanty belts of cotton wood trees along streams in
New Mexico it is necessary to lop off the mistletoe every year
to give the tree any chance to grow. Half parasites may be
known from plants that are fully parasitic by having green
or greenish foliage, while complete parasites have no chlorophyll
and so are not at all green.

380. Wholly parasitic seed plants. These are so nearly
destitute of the power of photosynthesis that they must rob
other plants of all needed food or die of starvation. Some, like
the cancer root (Aphyllon) are root parasites ; others, like the
dodder (Fig. 16), are parasitic on stems above ground. The
most dependent species of all, such as the flax dodder, can live
on only one kind of host, while the coarse orange-stemmed

1 See Bergen, Flora of the Northeastern States.

2 Alectorolophus hirsutus.

SAPROPHYTES AND CARNIVOROUS PLANTS 409

dodder,1 which is common all over the central and the north-
eastern states, grows freely on many kinds of plants, from
golden-rods to willows.

381. Saprophytes. A saprophyte (meaning decay plant) is
a plant of which the nutrition is largely or wholly dependent
on the absorption of organic

material, usually when in a
state of fermentation or de-
cay. Most plants of this kind
are fungi (Chapter xxn), but
there are a few saprophytic
seed plants, the Indian pipe,
so common in coniferous
woods, being one of the
most familiar. In appear-
ance the saprophytes re-
semble parasites so far as
the absence of green color
is concerned and of course
they do little or no photo-
synthetic work.

382. Carnivorous plants.
In the ordinary pitcher
plants (Fig. 311) the leaf
appears in the shape of a
more or less hooded pitcher.

These pitchers are usually FIG. 311. Common pitcher plant
partly filled with water, and (Sarracenia purpurea)

in this water very many At the ^-^^,Hke leaves is
drowned and decaying

insects are commonly to be found. The insects have flown or
crawled into the pitcher, and, once inside, have been unable
to escape on account of the dense growth of bristly hairs about
the mouth, all pointing inward and downward. How much the
,l Cusruta Gronovii.

410

PARASITES AND CARNIVOROUS PLANTS

FIG. 312. Sundew (Drosera rotundifolia)

common American pitcher plants depend for nourishment on
the drowned insects in the pitchers is not definitely known, but
it is certain that some of the tropical species require such food.1

1 Where the Sarracenia is abundant it will be found interesting and profit-
able to make a careful class study of its leaves. See Geddes, Chapters in
Modern Botany, Chapters i and n.

SUNDEWS AND VENUS FLYTRAP 411

In other rather common plants, the sundews, insects are
caught by a sticky secretion which proceeds from hairs on the
leaves. In one of the commonest sundews (Fig. 312) the leaves
consist of a roundish blade, borne on a moderately long petiole.
On the inner surface and round the margin of the blade are
borne a considerable number of short bristles, each terminating
in a knob, which is covered with a clear, sticky liquid. When a
small insect touches one of the sticky knobs it is held fast, and
the hairs at once begin to
close over it, as shown in
Fig. 313. Here it soon
dies and then usually re-
mains for many days,
while the leaf pours out
a juice by which the
soluble parts of the insect .
are digested. The liquid
containing the digested ]; ':'| f-

portions is then absorbed

: , . , „ ., FIG. 313. Leaves of sundew

by the leaf and contrib-

„ The one at the left has all its tentacles closed

utes an important part of over captured prey . the one at the right has

the nourishment of the only half of them thus closed. Somewhat
T i -i j_i T i magnified. — After Darwin

plant, while the undigested

fragments, such as legs, wing cases, and so on, remain on the
surface of the leaf or may drop off after the hairs let go their
hold on the captive insect.

In the Venus flytrap, which grows in the sandy regions of
eastern North Carolina, the mechanism for catching insects is
still more remarkable. The leaves, as shown in Fig. 314, termi-
nate in a hinged portion, which is surrounded by a fringe of
stiff bristles. On the inside of each half of the trap grow three
short hairs. The trap is so sensitive that when these hairs are
touched it closes with a jerk and very generally succeeds in
capturing the fly or other insect which has sprung it. The
imprisoned insect tlien-'dies and is digested, somewhat as in the

412

PARASITES AND CARNIVOROUS PLANTS

case of those caught by the sundew, after which the trap reopens
and is ready for fresh captures.

383. Object of catching animal food. It is easy to under-
stand why a good many kinds of plants have taken to catch-
ing insects and absorbing the digested products. Carnivorous,
or flesh-eating, plants belong usually to one of two classes as

FIG. 314. Venus flytrap (Dioncea muscipula)

regards their place of growth ; they are bog plants or air plants.
In either case their roots find it difficult to secure much nitrogen-
containing food, — that is, much food out of which proteid mate-
rial can be built up. Animal food, being itself largely proteid, is
admirably adapted to nourish the growing parts of plants, and
those which could develop insect-catching powers would stand
a far better chance to exist as air plants or in the thin, watery
soil of bogs than plants which had acquired no such resources.

CHAPTEK XXXI
HOW PLANTS PROTECT THEMSELVES FROM ANIMALS

384. Destruction by animals All animals are supported
directly or indirectly by plants. In some cases the animal
secures its food without much damage to the plant on which it
feeds. Browsing on the lower branches of a tree may do it little
injury, and grazing animals, if not numerous, may not seriously
harm the pasture in which they feed. Fruit-eating animals
may even be of much service by dispersing seeds (Sec. 420).
But seed-eating birds and quadrupeds, animals which (like the
hog) dig up fleshy roots, rootstocks, tubers, or bulbs, and eat
them, or animals which (like the sheep) graze so closely as to
expose the roots of grasses or even of forest trees to be parched
by the sun, destroy immense numbers of plants. Many trees,
as the apple, peach, and black locust, have the trunk fatally
weakened by the boring larvae of insects. Leaf-eating insects,
such as grasshoppers and caterpillars, cause immense damage
to foliage, and others, like the chinch bug so destructive to
grain crops, suck the juices from roots, stems, or leaves.

385. Some modes of protection from animals. Many of the
characteristics of plants may be wholly or partly due to adapta-
tions for protective purposes, while in particular cases we cannot
be sure of the fact. Perching on lofty rocks or on branches of
trees, burying the perennial part (bulb, rootstock, etc.) under-
ground, growing in dense masses, like a canebrake or a thicket
of blackberry bushes, — all such habits of plants may be partly
or altogether valuable to the plant as means of avoiding the
attacks of animals, but this cannot be proved. On the other
hand, there are plenty of instances of structures, habits, or accu-
mulations of stored material in their tissue which plants seem

413

414

HOW PLANTS PROTECT THEMSELVES

to have acquired mainly or entirely as means of defense. Some
of the most important are :

1. The habit of keeping a bodyguard of ants.

2. Forming tough, corky, woody, limy, or flinty, and therefore

nearly uneatable, tissue.

3. Arming exposed parts with cutting edges, sharp or stinging hairs,

prickles, or thorns.

4. Accumulating unpleasant or poisonous substances in exposed

parts.

386. Ant plants. Some ants live on vegetable food, but most
of them eat only animal food, and these latter are extremely
voracious. It has been estimated by a careful scientist, an

FIG. 315. An ant plant (Acacia sphcerocephala)

t, thorns; h, hole in thorn; n, nectary; 6, Belt's body on tip of leaflet. — After

Schimper

authority on this subject, that the ants of a single nest some-
times destroy as many as one hundred thousand insects in a
day. The Chinese orange growers in the province of Canton
have found how useful ants may be as destroyers of other
insects, and so they place ant nests in the orange trees and
extend bamboos across from one tree to another, to serve as
bridges for the ants to travel on.

ANT PLANTS; UNEATABLE PLANTS 415

Certain tropical trees offer ants special inducements to estab-
lish colonies on their trunks and 'branches. The attractions
which are offered to ants by various kinds of trees differ greatly.
One of the most interesting adaptations is that of an acacia
(Fig. 315), which furnishes little growths at the ends of the
leaflets which serve as ant food. These little growths are known
from their discoverer as Belt's bodies. The ants bore holes into
the large, hollow, stipular thorns shown in the figure, live in
these thorns, feed on the Belt's bodies, and protect the acacia
from insect and other enemies. A nectary on the leaf furnishes
additional food to the ant inhabitants of the tree. A great multi-
tude of plants, some of them herbs, offer more or less impor-
tant inducements to attract ant visitors ; the species which are
known to do this number over three thousand.1

387. Plants of uneatable texture. Whenever tender and
juicy herbage is to be had, plants of hard and stringy texture
are left untouched by grazing animals. The flinty-stemmed
horsetails (Equisetum, Sec. 316) and the dry, tough rushes are
familiar examples of uneatable plants of damp soil. In pastures
there grow such perennials as the bracken fern and the hard-
hack of New England and the ironweed and vervains of the
central states, which are so harsh and woody that the hungriest
browsing animal is rarely, if ever, seen to molest them. Still
other plants, like the knotgrass and cinquefoil of our dooryards,
are doubly safe, from their growing so close to the ground as to
be hard to graze, and from their woody and unpalatable nature.
The date palm, which can easily be raised from the seed in the
botanical laboratory, is an excellent instance of the same un-
eatable quality, found in a tropical or sub-tropical plant. Other
good examples are the shrubs of heath lands and of such coria-
ceous, or leathery-leafed, thickets as the Australian scrub and
the California chaparral.

1 Possibly in many cases the attractiveness of plants for ants is only
incidental and has not been evolved with direct reference to the protection
to be rendered by these insects.

416

HOW PLANTS PROTECT THEMSELVES

388. Plants with weapons for defense.1 Multitudes of plants,
which might otherwise have been subject to the attacks of graz-
ing or browsing animals, have acquired what
have with reason been called weapons.
Shrubs and trees not infrequently produce
sharp-pointed branches, familiar in our own
crab apple, wild plum, thorn trees, and,
above all, in the honey locust (Fig. 35),

FIG. 316. Spiny leaves of barberry

whose formidable thorns often branch in a very complicated
manner. It is noteworthy that the protection given by thorns

is not from those of the
season, but from the dry
and hard ones of preceding
years.

Leaves modified into
thorns are very perfectly
exemplified in the barberry
(Fig. 316). It is much com-
moner, however, to find the
leaf extending its midrib or

its veins out into sPin7 Points>
as the thistle does, or bearing

spines or prickles on its midrib, as is the case with some night-
shades (Fig. 317) and with so many roses.

1 See Kerner and Oliver, Natural History of Plants, Vol. I, p. 430.

FIG. 317. Spiny leaf of a
nightshade (Solarium
atropurpureum)

PRICKLES, THORNS, AND STINGING HAIRS 417

Prickles, which are merely hard, sharp-pointed projections
from the epidermis, are of too common occurrence to need
illustration.

Thorns are often found to be modified stipules, and in our
common locust (Fig. 319) the bud, or the very young shoot

stipules of
locust

FIG. 318. Euphorbia splendens

The spines are dead and
dry stipules

which proceeds from it, is admi-
rably protected by the jutting thorn
of the previous year on either side.

389. Pointed, barbed, and sting- FIG. 319. Thorn
ing hairs. On many plants needle-
pointed hairs form efficient defensive weapons.
Sometimes these hairs are roughened, like those of the bugloss
(Fig. 320, 6) ; sometimes they are decidedly barbed. If the barbs
are well developed, as they are in the small but formidable
bristles of prickly pear cactuses, they may cause the hairs to
travel far into the flesh of animals and cause intense pain. In
the nettle (Fig. 320, a) the hairs are efficient stings, with a
brittle tip, which on breaking off exposes a sharp, jagged tube
full of irritating fluid. These tubular hairs, with their poisonous
contents, will be found sticking in the skin of the hand or the
face after incautious contact with nettles, and the violent itch-
ing which follows is only too familiar to most people.

418

HOW PLANTS PROTECT THEMSELVES

390. Cutting leaves. Some grasses and sedges are generally
avoided by cattle because of the sharp, cutting edges of their
leaves, which will readily slit the skin of one's hand if they are
drawn rapidly through the ringers. Under the microscope the
margins of such leaves are seen to be regularly and thickly set
with sharp teeth like those of a saw (Fig. 320, c, d).

EIG. 320. Stinging hairs and cutting leaves

«, stinging hairs on leaf of nettle; b, bristle of the bugloss; c, barbed margin of
a leaf of sedge; d, barbed margin of a leaf of grass. All much magnified. —
After Kerner

391. Weapons of desert plants. In temperate regions, where
vegetation is usually abundant, such moderate means of protec-
tion as have just been described are generally sufficient to insure
the safety of the plants which have developed them. But in
desert or semi-desert regions the extreme scarcity of plant life
especially exposes the few plants that occur there to the attacks

OFFENSIVE OR POISONOUS PLANTS 419

of herbivorous animals. Accordingly, great numbers of desert
plants are characterized by nauseating or poisonous qualities or
by the presence of astonishingly developed thorns (Figs. 50,
357), while some combine both of these means of defense.

392. Offensive or poisonous plants. A disgusting smell is
one of the common safeguards which keep .plants from being
eaten. The dog fennel (Fig. 364), the hound's tongue (Cyno-
glossum), the Martynia, and the tomato plant are common exam-
ples of rank-smelling plants which are offensive to most grazing
animals and so are let alone by them. Oftentimes, as in the
case of the jiinson weed (Datura), the tobacco plant, and the
poisonous hemlock (Conium), the smell serves as a warning of
the poisonous nature of the plant.

A bitter, nauseating, or biting taste protects many plants from
destruction by animals. Buckeye, horse-chestnut, and buck-
thorn twigs and leaves are so bitter that browsing animals and
most insects let them alone. Tansy, ragweed, boneset, southern-
wood, and wormwood are safe for the same reason. The nau-
seous taste of many kinds of leaves and stems, such as those of
the potato, and the fiery taste of peppercorns, red peppers, mus-
tard, and horse-radish, make these substances uneatable for most
animals. Probably both the smell and the taste of onions serve
to insure the safety of the bulbs from the attacks of most grubs,
and the hard corm of the jack-in-the-pulpit (Arisamia) is care-
fully let alone on account of the blistering nature of its contents.

Poisonous plants are usually shunned by grown-up animals
unless they are famished, though the young ones will some-
times eat such plants and may be killed by them. Almost any
part of a poisonous species may contain the poison character-
istic of the plant, but, for obvious reasons, such substances are
especially apt to be stored in the parts of the plant where its
supply of reserve food is kept.

CHAPTER XXXII
POLLINATION OF FLOWERS AND PROTECTION OF POLLEN

393. Topics of the Chapter. The ecology of flowers is con-
cerned mainly with the means by which the transference of
pollen, or pollination, is effected, and with the ways in which
pollen is kept away from undesirable insect visitors and from
rain.

394. Cross pollination and self pollination. It was long sup-
posed by botanists that the pollen of any bisexual flower needed
only to be placed on the stigma of the same flower to insure sat-
isfactory fertilization. But in 1857 and 1858 the great English
naturalist, Charles Darwin, stated that certain kinds of flowers
were entirely dependent for fertilization on the transference
of pollen from one plant to another. It was also shown that
probably nearly all attractive flowers, even if they can produce
some seed when self-pollinated, do far better when pollinated
from the flowers of another plant of the same kind.1 This im-
portant fact was established by a long series of experiments
on the number and vitality of seeds produced by a flower when
treated with its own pollen, or self -pollinated, and when treated
with pollen from another flower of the same kind, or cross-
pollinated?

Another important advantage of cross pollination is that it
tends to give the offspring additional variability (Chapter XL),
and thus enables them better to adapt themselves to changing
environment or to any difficult conditions.

1 See Darwin, Cross and Self Fertilization in the Vegetable Kingdom
(especially Chapters i and n).

2 On dispersion of pollen, see Kernel* and Oliver, Natural History of
Plants, Vol. II, pp. 129-287.

420

WIND-POLLINATED FLOWERS 421

It should always be kept in mind that many of the most
successful plants, including a large number of troublesome
weeds, are capable of self pollination.

395. Wind-pollinated flowers.1 It has already been mentioned
that some pollen is dry and powdery, and other kinds are more
or less sticky. Pollen of the dusty sort is light, and therefore
adapted to be blown about by the wind. Any one who has been
much in cornfields after the corn has " tasseled " has noticed the
pale yellow, dusty pollen which flies about when a cornstalk is
jostled, and which collects in considerable quantities on the
blades of the leaves. Corn is monoecious, but fertilization is
best accomplished by pollen blown from the " tassel " (stamens)
of one plant being carried to the " silk " (stigma and style of
the pistils) of another plant. This is well shown by the fact,
familiar to every observing farmer's boy, that solitary cornstalks,
such as often grow very luxuriantly in an unused barnyard or
similar locality, bear very imperfect ears or none at all. The
common ragweed is remarkable for the great quantities of
pollen which shake off on to the shoes or clothes of the
passer-by, and it is wind-pollinated. So,
too, are the pines, and these produce so
much pollen that it has been mistaken
for showers of sulphur, falling often at
long distances from the forests where it FIG. 321. Pistil of a grass,
was produced. The pistil of wind-polli- provided with a feath-

, -, n f^ P ,1 i ^i ery stigma, adapted for

nated flowers is often feathery and thus wind_pollh;ation

adapted to catch flyinsr pollen grains

r After Thome

(Fig. 321). Other characteristics of

such flowers are the inconspicuous character of their perianth,
which is usually green or greenish, the absence of odor and
of nectar, the regularity of the corolla, and the development
of the flowers before the leaves or their occurrence on stalks
raised above the leaves.

1 See Miss Newell, Botany Reader, Part II, Chapter vn.

422

POLLINATION OF FLOWERS

Pollen is, in the case of a few aquatic plants, carried from
flower to flower 'by the water in which the plant grows.

396. Insect-pollinated flowers. Most plants which require
cross pollination depend upon insects as pollen carriers,1 and it
may be stated as a general fact that the showy colors and mark-
ings of flowers and their odors all serve as so many advertise-
ments of the nectar (commonly but wrongly called honey) or of
the nourishing pollen which the flower has to offer to insect
visitors.

Many insects depend mainly or wholly upon the nectar and
the pollen of flowers for their food. Such insects usually visit
during any given trip only one kind of flower, and therefore
carry but one kind of pollen. Going straight from one flower to
another with this, they evidently waste far less pollen than the

wind or water must waste. It is
therefore clearly advantageous to
flowers to develop such adapta-
tions as fit them to attract insect
visitors, and to give pollen to the
latter and receive it from them.
397. Pollen-carrying appara-
tus of insects.2 Ants and some
beetles which visit flowers have
smooth bodies, to which little
FIG. 322 ^ pollen adheres, so that their visits

A, right hind leg of a honeybee (seen are often of slight value to the

from behind and within); B the fl b t beetles; all but-

tibia; ti, seen from the outside, *

showing the collecting basket formed terflies and moths, and most

of stiff hairs. - After Muller ^ ^ye bodieg roughened with

scales or hairs, which hold a good deal of pollen entangled. In
the common honeybee (and in many other kinds) the greater
part of the insect is hairy, and there are special collecting
baskets, formed by bristle-like hairs, on the hind legs (Fig. 322).

1 A few are pollinated by snails ; many more by humming birds and other
birds. 2 See P. Kimth, Handbuch der Bluthenbiologie, Vol. I.

-ti

ATTRACTIONS FOR INSECTS 423

It is easy to see the load of pollen accumulated in these baskets
after such a bee has visited several flowers. Of course the pollen
which the bee packs in the baskets and carries off to the hive,
to be stored for food, is of no use in pollination. In fact, such
pollen is in one sense entirely wasted. But since such bees as
have these collecting baskets are the most industrious visitors
to flowers, they accomplish an immense share of the work of
pollination by means of the pollen grains, which stick to their
hairy coats and are then transferred to
other flowers of the same kind next visited
by the bee.

398. Nectar and nectaries. Nectar is a
sweet liquid which flowers secrete and

which attracts insects. After partial diges- VYV — Y-£?— -9

tion in the crop of the bee, nectar be-
comes honey. Those flowers which secrete
nectar usually do so by means of nectar

glands, small organs situated often near FlG: 3f ' Stamens and

pistil of the grape
the base of the flower, as shown in Fig. 323. (magnified), with a

Sometimes the nectar clings in droplets to nectar gland g be-
the surface of the nectar glands ; some- tween the stamens
times it is stored in little cavities or After Decaisne
pouches called nectaries. The pouches at the bases of columbine
petals are among the most familiar of nectaries.

399. Odors of flowers. The acuteness of the sense of smell
among insects is a familiar fact. Flies buzz about the wire
netting which covers a piece of fresh meat or a dish of sirup,
and bees, wasps, and hornets will fairly besiege the window
screens of a kitchen where preserving is going on. Many
plants find it possible to attract as many insect visitors as they
need without giving off any scent perceptible to us, but small
flowers, like the mignonette, and night-blooming ones, like the
white tobacco and the evening primrose, are sweet-scented to
attract night-flying moths. It is interesting to observe that
the majority of the flowers which bloom at night are white or

424 POLLINATION OF FLOWERS

yellow, and that they are much more generally sweet-scented
than flowers which bloom during the day. Many are odorous
during only a few hours of the twenty-four, just at the time
when the special insects which pollinate them are on the wing.
A few flowers (purplish, brownish, or greenish colored) are car-
rion-scented and attract flies.

400. Colors of flowers. Flowers which are of any other color
than green probably in most cases display their colors to attract
insects, or occasionally birds. The principal color of the flower
is most frequently due to showy petals; sometimes, as in the
anemone and marsh marigold, it belongs to the sepals ; and not
infrequently, as in some cornels (Frontispiece) and Euphorbias
(Fig. 318), the involucre is more brilliant and conspicuous than
any part of the flower strictly so called. In the willows and
chestnuts the stamens are the conspicuous parts.

Different kinds of insects appear to be especially attracted
by different colors. In general, dull yellow, brownish, or dark
purple flowers, especially if small, seem to depend largely on
the visits of flies. Eed, violet, and blue are the colors by v/hich
bees and butterflies are most readily enticed. The power of
bees to distinguish colors has been shown by a most interesting
set of experiments in which daubs of honey were put on slips
of glass laid on separate pieces of paper, each of a different
color, and exposed where bees would find them.1

It is certain, however, that colors are less important means
of attraction than odors from the fact that insects are extremely
near-sighted. Butterflies and moths cannot see distinctly at a
distance of more than about five feet, bees and wasps at more
than two feet, and flies at more than two and a fourth feet.
Probably no insects can make out objects clearly more than
six feet away.2 Yet it is quite possible that their attention is
attracted by colors at distances greater than those mentioned.

1 See Lubbock, Flowers, Fruits, and Leaves, Chapter i. On the general
subject of colors and odors in relation to insects, see P. Knuth, Handbuch
der Bliitheribiologie. 2 See Packard, Text-Book of Entomology, p. 260.

FACILITIES FOR INSECT VISITS 425

401. Facilities for insect visits. Regular flowers with radial
symmetry usually have no special adaptations to make them
singly accessible to insects, but lie open to all comers. They
do, however, make themselves much more attractive and afford
especial inducements in the matter of saving time to flower-
frequenting insects by being grouped. This purpose is undoubt-
edly served by dense flower clusters, such as those of the lilac,
the phlox, and the elder, and especially by heads like those of
the button bush (Ceplialanthus) and by the peculiar form of
head found in so-called composite flowers, like the sunflower,
the bachelor's button, and the yarrow
(Fig. 144). In many such clusters the
flowers are specialized, some carrying
a showy strap-shaped corolla, to serve
as an advertisement of the nectar and
pollen contained in the inconspicuous
tubular flowers. Flowers with bilateral
symmetry probably always are more
or less adapted to particular insect (or
other) visitors. The adaptations are FJG 324 A beetle on the
extremely numerous ; here only a very flower of the twayblade
few of the simpler ones will be pointed slightly enlarged.— After
out. Where there is a drooping lower Behrens

petal or, in the case of a sympetalous corolla, a lower lip,
this serves as a perch upon which flying insects may alight and
stand while they explore the flower, as the beetle is doing in
Fig. 324. In Fig. 325 one bumblebee stands with her legs
partially encircling the lower lip of the dead-nettle flower, while
another perches on the sort of grating made by the stamens
of the horse-chestnut flower. The honeybee entering the violet
clings to the beautifully bearded portion of the two lateral
petals, while she sucks the nectar from the spur beneath. All
bilaterally symmetrical flowers seem to be specially adapted to
compel visiting insects to enter them in the best way to
secure transference of -pollen.

426

POLLINATION OF FLOWERS

402. Protection of pollen from unwelcome visitors. It is

usually desirable for the flower to prevent the entrance of
small creeping insects, such as ants, which carry little pollen
and eat a relatively large amount of it. The means adopted to
secure this result are many and curious. In some plants, as
the common catchfly, there is a sticky ring about the peduncle,
some distance below the flowers, and this forms an effectual
barrier against ants and like insects. In a few plants, as the

FIG. 325. Bees visiting flowers

At the left, a bumblebee on the flower of the dead nettle; below, a similar bee in
the flower of the horse-chestnut ; above, a honeybee in the flower of a violet.
Modified. — After Behrens

teasel and the cup plant (Silphium perfoliatum), rain water col-
lects at the junctions of the leaves with the stem and forms
an effectual barrier against creeping insects. Very frequently
the calyx tube is covered with hairs, which are sometimes
sticky. How these thickets of hairs may appear to a small
insect can perhaps be realized from Fig. 32 6.1

1 On protection of pollen, see Kerner and Oliver, Natural History of
Plants, Vol. II, pp. 95-109.

PROTECTION FROM UNWELCOME VISITORS 427

Sometimes the recurved petals or divisions of the corolla
stand in the way of creeping insects. In other cases the throat

FIG. 326. Branching hairs from the outside of the corolla of the
common mullein

Magnified. — After Tschirch

of the corolla is much narrowed or closed by hairs, or by ap-
pendages. Those flowers which have one or more sepals or
petals prolonged into spurs, like the nasturtium and the colum-
bine, are inaccessible to most insects except those which have

FIG. 327. A sphinx moth, with a long sucking tube

a tongue or a sucking tube long enough to reach to the nectary
at the bottom of the spur. The large sphinx moth, shown in
Fig. 327, which is a common visitor to the flowers of the

428 POLLINATION OF FLOWERS

evening primrose, is an example of an insect especially adapted
to reach deep into long tubular flowers.

A little search among flowers, such as those of the columbine
and the foxglove, will usually disclose many which have had the
corolla bitten through by bees which are unable (or unwilling
to take the trouble) to get at the nectar by fair means, and
which therefore steal it.

403. Bird-pollinated flowers. Some flowers with very long
tubular corollas depend entirely upon birds to carry their pollen
for them. Among garden flowers the gladiolus, the scarlet salvia,
the canna, and the trumpet honeysuckle are largely dependent
upon humming birds for their pollination. The wild balsam,
or jewelweed, the swamp thistle, and the trumpet creeper are
also favorite flowers of the humming bird.

404. Prevention of self pollination. Dioecious flowers are,
of course, quite incapable of self pollination. Pistillate monoe-
cious flowers may be pollinated by staminate ones on the same
plant, but this does not secure as good seed as is secured by
having pollen brought to the pistil from a different plant of the
same kind.

In perfect flowers self pollination would commonly occur un-
less it were prevented by the action of the essential organs
or by something in the structure of the flower. In reality,
many flowers which at first sight would appear to be designed
to secure self pollination are almost or quite incapable of it.
Frequently the pollen from another plant of the same species
prevails over that which the flower may shed on its own pistil,
so that when both kinds are placed on the stigma at the same
time it is the foreign pollen which causes fertilization. But
apart from this fact there are several means of insuring the
presence of foreign pollen, and only that, upon the stigrna, just
when it is mature enough to receive pollen tubes.

405. Stamens and pistils maturing at different times. If
the stamens mature at a different time from the pistils, self
pollination is as effectually prevented as though the plant were

PREVENTION OF SELF POLLINATION

429

dioecious. This unequal ma-
turing, or dichogamy, occurs
in many kinds of flowers. In
some, the figwort and the com-
mon plantain for example, the
pistil develops before the sta-
mens, but usually the reverse
is the case. The Clerodendron}
a tropical African flower (Fig.
328), illustrates in a most
striking way the development
of stamens before the pistil. B
The insect visitor, on its way FlG> 328> Flower of Clerodendron
to the nectary, can hardly fail in two stages

to brush against the protrud- jn A (earlier stage) the stamens are ma-
ing Stamens of the flower in ture, while the pistil is still undeveloped

and bent to one side. In B (later stage)
its earlier Stage, A, but it can- the stamens have withered, and the

not deposit any pollen on the stigmas have seParated> 1>eady for the
J r reception of pollen. — After Gray

stigmas, which are imma-
ture, shut together, and tucked aside out of reach. On flying
to a flower in the later stage the pollen just acquired will be

—stiff

A B C D

FIG. 329. Provisions for cross pollination in the high mallow

A, essential organs as found in the bud; B, same in the staminate stage, the
anthers discharging pollen, pistils immature; C, intermediate stage (stig, the
united stigmas) ; Z), pistillate stage, the stigmas separated, stamens withered.
— After Muller

1%C. Thompsonioe.

430

POLLINATION OF FLOWERS

lodged on the prominent stigmas and thus produce the desired

cross pollination.

Closely related flowers often differ in their plan of pollination.

The high mallow (a plant cultivated for its purplish flowers),
which has run wild to some extent, is ad-
mirably adapted to secure cross pollination,
since when its stamens are shedding pollen,
as in Fig. 329, B, the pistils are incapable of
receiving it, while when the pistils are ma-
ture, as in D, the stamens are quite withered.
In the common low mallow of our door-
yards and waysides insect pollination may

FIG. 330. Stamens
and pistils of round-

occur, but if it does not, the curling stigmas

leafed mallow

The stigmas curled fina% come in contact with the projecting
round among the sta- stamens and receive pollen from them, as is

mens to admit of self . -,. , i • TTI- OOA

pollination. -After indicated in Fig. 330.

Muiier 406. Movements of floral organs to aid in

pollination. Besides the slow movements which the stamens
and pistil make in such cases as those of the Clerodendron and

FIG. 331. Two flowers of common sage, one of them visited by a bee
After Lubbock

the mallow, already described, the parts of the flower often
admit of considerable and rather quick movements that assist
the insect visitor to become dusted or smeared with pollen.

MOVEMENTS OF FLORAL ORGANS

431

In some flowers whose stamens perform rapid movements
when an insect enters, it is easy to see how directly useful the
motion of the stamens is in securing cross pollination. The
stamens of the laurel (Kalmia) are held in a bent position by
the expanded corolla, and when liberated by a touch throw little
masses of pollen, with a quick jerk, against the body of the
visiting insect. Barberry flowers have filaments which are sen-
sitive on the inner side near the base, and when touched make
the anther spring up against the visitor and dust him with
pollen. The common garden sage matures its anthers earlier
than its stigmas. In Fig. 331, A, the young flower is seen, vis-
ited by a bee, and one anther,

an, is shown pressed closely
against the side of the bee's
abdomen. The stigma, st, is

FIG. 332. Flower and stamens of common sage

A, p, stigma; a, anthers. JB, the two stamens in ordinary position; /, filaments;
m, connective (joining anther cells) ; a, a', anther cells. C, the anthers and
connectives bent into a horizontal position by an insect pushing against a. —
After Lubbock.

hidden within the upper lip of the corolla. In B, an older
flower, the anthers have withered and the stigma is now low-
ered so as to brush against the body of any bee which may
enter. A little study of Fig. 332 will make clear the way
in which the anthers are hinged, so that a bee striking the
empty or barren anther lobes, a, knocks the pollen-bearing
lobes, a', into a horizontal position, so that they will lie closely
pressed against both sides of its abdomen. Many stigmas, as
those of catalpa and trumpet creeper, close as soon as they
are pollinated.

432

POLLINATION OF FLOWERS

407. Flowers with stamens and pistils each of two lengths.

The flowers of bluets, partridge berry, the primroses, and a few
other common plants secure cross pollination by having stamens
and pistils of two forms (Fig. 333). Such flowers are said to be
dimorphous (of two forms). In the short-styled flowers, B, the
anthers are borne at the top of the corolla tube and the stigma
stands about halfway up the tube. In the long-styled flowers,
A, the stigma is at the top of the tube and the anthers are borne

about halfway up. An insect
pressing its head into the throat
of the corolla of B would be-
come dusted with pollen, which
would be brushed off' on the
stigma of a flower like A. On
leaving a long-styled flower the
bee's tongue would be dusted
over with pollen, some of which
might readily be rubbed off on
the stigma of the next short-
styled flower that was visited.
Cross pollination is insured,
since all the flowers on a plant
are of one kind, either long-

FIG. 333. Dimorphous flowers of
the primrose

short" styled or

the pollen is of two sorts, each
kind sterile on the stigma of any flower of similar form to
that from which it came.

TrimorpJwus flowers, with long, medium, and short styles,
are found in a species of loosestrife and in the pickerel weed
(Pontederia) .

408. Cleistogamous flowers. In marked contrast with such
flowers as those discussed in the preceding sections, which bid
for insect visitors or expose their pollen to be blown about
by the wind, are certain flowers which remain closed even
during the pollination of the stigma. These flowers are called

CLEISTOGAMOUS FLOWERS

433

cleistogamous (meaning ' with shut-in fertilization) and are of
course not cross-pollinated. Usually they occur on plants which
also bear flowers adapted for cross pollination, and in this case
the closed flowers are much less conspicuous than the others, yet

FIG. 334. A violet, with cleistogamous flowers

The structures which look like flower buds are cleistogamous flowers-in various
stages of development. The pods are the fruit of similar flowers. The plant
is represented as it appears in late July or August, after the ordinary flowers
have disappeared

434

POLLINATION OF FLOWERS

they produce much seed. Every one knows the ordinary flowers
of the violet, but most people do not know that violets very gen-
erally, after the blossoming season (of their showy flowers) is
over, produce many cleistogamous flowers, as shown in Fig. 334.

EN:

FIG. 335. Protection of pollen from moisture

At the left herb Robert and sweet scabious in sunny weather; at the right the
same flowers during rain. — After Kerner

409. Protection of pollen from rain. Pollen is very generally
protected from being soaked and spoiled by rain or dew by the
natural position of the flower, which prevents rain from entering,
as in the case of most sympetalous, nodding flowers, such as the

PROTECTION OF POLLEN FROM RAIN 435

lily of the valley and the flowers of the blueberry, huckleberry,
wintergreen, and a multitude of others. Often, in two-lipped
flowers, the anthers are more or less completely covered by
the upper lip (Fig. 331). In the salver-shaped flowers, such as
those of phlox, the mouth of the corolla tube is often so narrow
that no rain or dew can enter it. Many corollas of the same
general type as that of the sweet pea (Fig. 126) have the stamens
covered by certain petals. A large number of flowers, such as
the crocus, rose, pond lily, magnolia, and many heads, such as
those of the dandelion, the chicory, and the hawkvveed, close in
wet weather and open in the sunshine. Sometimes the flower
both changes its position and closes, as is the case with the
common cranesbill, the herb Robert, and the sweet scabious
(Fig. 335). In the linden and the jewelweed the flowers are
covered by the foliage leaves of the plant so that rain can hardly
ever enter them.

CHAPTER XXXIII
HOW PLANTS ARE SCATTERED AND PROPAGATED

410. Dispersal of plants by roots and rootstocks. Some
of the highest spore plants, as the ferns, spread freely by means
of their creeping rootstocks, and the gardener who wishes
to get large strong ferns quickly often finds it the easiest plan

to cut to pieces and
reset the rootstocks
of a well-estab-
lished plant. In
the walking fern

(Fig. 273) the tip
FIG. 336. Plant of a black v &

raspberry, showing one of the frond roots

branch (stolon) with several and begins a new

tips rooting jAant. The student has learned (in

After Beal Chapters IV and vi) that roots and

underground stems of many kinds may" serve to reproduce
the plant. Either roots or rootstocks may travel considerable
distances horizontally in the course of their growth and then
shoot up and produce a new plant, which later becomes in-
dependent of the parent. The sedges (Fig. 44) are excellent
illustrations of this process, and trees like the common locust
and the silver-leaf poplar become great nuisances in the
neighborhood of lawns and gardens by sending up sprouts
in many places. When growing wild, such trees as these

436

DISPERSAL OF SEED PLANTS BY BRANCHES 437

depend largely upon propagating by the roots to keep up
their numbers.1

411. Dispersal of seed plants by branches. There is a shrub
of the honeysuckle family,2 common in the northern woods,
which is quite generally known as hobblebush, or witch-hobble,
and sometimes as trip-toe. This is because the branches take
root at the end and so form loops which catch the foot of the
passer-by. The same habit of growth is ^^
found in the raspberry bush (Fig. 336), in
one species of strawberry bush (Euonymus),
and in some other shrubs.
Many herbs, like the
strawberry plant and the
cinquefoil, send out long
leafless runners which
root at intervals and so
propagate the plant, carry-
ing the younger individ-
uals off to a considerable
distance from the parent.

Living branches may
drop freely from the tree and then take root and grow, after
having been blown, or carried by a brook or river, to a favor-
able spot, perhaps hundreds of yards away. The so-called snap
willows lose many live twigs under conditions suitable for start-
ing new trees.

A slightly different mode of dispersal from that of the rasp-
berry is one in which buds separate from the plant and serve
to propagate it. In the bladderwort (Fig. 337), at the close of
the growing season, the terminal buds are released by the decay
of the stem and sink to the bottom of the water in which the
plants live, there to remain dormant until spring. Then each
bud starts into life and gives rise to a new individual.

FIG. 337. A free branch and two buds
of bladderwort

After Beal

1 See Beal, Seed Dispersal, Chapters n and in.

2 Viburnum Iqntanoides.

438

HOW PLANTS ARE SCATTERED

412. Dispersal of seed plants by bulblets. Almost every
farmer's boy knows what " onion sets " are. This name is often
given to little bulbs produced at the top of a naked flower stalk,
or scape, by some kinds of onions which do not usually flower
or bear seed. Tiger lilies produce somewhat similar bulblets in
the axils of the leaves, and there is a large number of species,

FIG. 338. Fruit of smoke tree (Rhus Cotinus)

Only one pedicel bears a fruit, all the others are sterile, branched, and covered
with plumy hairs

scattered among numerous families of plants, all characterized
by the habit of producing bulblets or fleshy buds, borne on the
stems or leaves above ground and of use in propagation. When
mature the bulblets fall off readily, and if they find lodgment
on unoccupied soil they grow readily into new plants. Some-
times they are carried moderate distances by wind or water,

EXPLOSIVE FRUITS; WINGED FRUITS

439

and if the ground slopes they may easily roll far enough to get
started in new places.

413. Dispersal of seeds. Seeds are not infrequently scattered
by apparatus with the aid of which the plant throws them about.
More commonly, however, they depend upon other agencies,
such as wind, water, or animals, to carry them. Sometimes the
transportation of seeds is due to the structure of the seeds them-
selves, sometimes to that of the fruit in which they are inclosed ;
the essential point is to have transportation to a long distance
made as certain as possible, to avoid overcrowding.

414. Explosive fruits. Some dry fruits burst open when
ripe in such a way as to throw their seeds violently about.
Interesting studies may

be made, in the proper
season, of the fruits of
the common blue violet,
the pansy, the wild
balsam, the garden bal-
sam, the cranesbill, the
herb Robert, the witch-
hazel, the Jersey tea, and
some other common
plants. The capsule of
the tropical American
sand-box tree bursts open
when throughly dry with FlG> 339> Fruits of linden, with a bract joined

a noise like that of a pis-

to the peduncle and forming a wing

tol shot. The explosive force of fruits is derived from the fact
that some of their parts on drying are left in a state of ten-
sion, some layers of cells being compressed or stretched and
tending to readjust their position.

415. Winged or tufted fruits and seeds. The fruits of the
ash, box elder, elm, maple (Fig. 160), and many other trees are
provided with an expanded membranous wing. Some seeds, as
those of the catalpa and the trumpet creeper, are similarly

440

HOW PLANTS ARE SCATTERED

appendaged. Winged fruits and seeds are borne on trees or
shrubs, and the wing is usually so adjusted as to make its
descent slow, with a spinning motion. As a rule, winged fruits
and seeds are much heavier than those with a tuft of hairs.
The fruits of the dandelion, the thistle, the fleabane, the arnica
(Fig. 166, III), and many other plants of the group Compositce,
to which these belong, and the seeds of the willow, the milk-
weed, the willow-herb, and other
plants, bear a tuft of hairs. All
these seeds and fruits may in
windy weather be seen traveling
often to great distances.

416. Tumble weeds. Late in
the autumn, fences, particularly
on prairie farms that are not
carefully tilled, or in pastures,
often serve as lodging places
for immense numbers of certain
dried-up plants known as tuni-
bleweeds. These blow about
over the level surface until the
first snow falls and even after
that (Fig. 341), often traveling
for many miles before they come
to a stop, and rattling out seeds
as they go. Some of the com-
monest tumbleweeds are the
Eussian thistle (Salsola Kali
var. Tragus, Fig. 340), the pigweed (Amarantus albus, Fig.
341), the tickle grass (Fig. 342), and a familiar peppergrass
(Lepidium). In order to make a successful tumbleweed, a
plant must be pretty nearly globular in form when fully grown
and dried, must be tough and light, must break off near the
ground, and drop its seeds only a few at a time as it travels.
A single plant of Russian thistle is sometimes as much as

FIG. 340. Russian thistle
After Dewey

DISPERSAL BY SHAKING AND BY WATER

441

three feet high and six feet in diameter, and carries not less
than 200,000 seeds.

417. Many-seeded pods with small openings. There are
many fruits which act somewhat like pepper boxes. The cap-
sule of the poppy is a good instance of this kind, and the fruit of
lily, monkshood (Fig. 159), columbine, larkspur, velvet leaf (Abu-
tilonAvicennce), and jimson weed (Fig. 343, C) acts in much the
same way. Clamping the dry peduncle of any one of these ripe

FIG. 341. Tumbleweeds 1 lodged against a wire fence in winter
After Millspaugh

fruits, so as to hold it upright above the table top, and swinging
it back and forth, will readily show its efficiency in seed dispersal.
418. Study of transportation by water. Nothing less than a
long series of observations by the pond margin and the brook-
side will suffice to show how general and important is the work
done by water in carrying the seeds of aquatics. Many plants
usually have their seeds transported by water, and some appear
to have no provision for dissemination in any other way.

^Amarantus albus.

442

HOW PLANTS ARE SCATTERED

Ocean currents furnish transportation for the longest journeys
that are made by floating seeds. It is a well-known fact that
cocoa palms are among the first plants to spring up on newly
formed coral islands. The nuts from which these palms grew
may readily have floated a thousand miles or more without

injury. On examining a cocoa-
nut with the fibrous husk at-
tached, just as it falls from the
tree, it is easy to see how well
this fruit is adapted for trans-
portation by water. There are
altogether about a hundred
drifting fruits known, one (the
Maldive nut) reaching a weight
of twenty to twenty-five
pounds.

419. Burs. A large class of
fruits is characterized by the
presence of hooks on the outer
surface. These are sometimes
outgrowths from the ovary, or
the style (as in avens), some-
times from the calyx, some-
times from an involucre. Their
office is to attach the fruit to
the hair or fur of passing ani-
mals. Often, as in sticktights
(Fig. 344 A, B), the hooks are
comparatively weak, but in
other cases, as in the cocklebur (Fig. 344 D), and still more in
the Martynia (the fruit of which in the green condition is much
used for pickles), the hooks are exceedingly strong. Cockleburs
can hardly be removed from the tails of horses and cattle, into
which they have become matted, without cutting out all the
hairs to which they are fastened.

FIG. 342. Panicle of tickle grass, a
common tumbleweed

After Host

BURS

443

A curious case of distribution of this kind occurred in the
island of Ternate, in the Malay Archipelago. A buffalo with his

FIG. 343. Three fruits adapted for dispersal by the shaking action of

the wind

A, celandine; 1>, pea; C, jimson weed (Datura). — After Decaisne

FIG. 344. Burs

A, sticktights; B, sticktights, two segments (magnified); (7, burdock;
D, co'ckleburs. — After Kerner

444

HOW PLANTS ARE SCATTERED

hair stuck full of the needle-like fruits of a grass l was sent as
a present to the so-called king of Ternate. Scattered from the
hair of this single animal, the grass soon spread over the whole
island.

420. Uses of stone fruits and fleshy fruits to the plant.
Besides the dry fruits, of which some of the principal kinds
have been mentioned, there are many kinds of stone fruits and

FIG. 345. Barbs and hooks of burs

A, barbed points from fruit of beggar's ticks, x 11 ; B, hook of cocklebur, x 11;
C, beggar's ticks fruit (natural size) ; I), cocklebur hook (natural size)

other fleshy fruits. Of these the great majority are eatable by
man or some of the lower animals, and oftentimes the amount
of sugar and other food material which they contain is very
considerable. It is a well-recognized principle of botany that
plants do not make unrewarded outlays for the benefit of other
species. Evidently the pulp of fruits is not to be consumed or

lAndropogon acicularis.

SEED CARRYING DONE BY ANIMALS

445

used as food by the plant itself or, in general, by its seeds.
There are, therefore, several points to be explained on the basis
of possible advantages to the plant. These are:

1. The eatable nature of the pulp of many fruits.

2. The bitter or other unpleasant taste of many seeds, as
those of the orange and lemon.

3. The hardness or toughness of many seeds of pulpy fruits,
as the date and the peach.

4. The small size and indigestibility of seeds of pulpy fruits,
as the fig and the raspberry.

A little observation in the field suffices to show that most
pulpy fruits are habitually eaten by birds or other animals large
enough to carry them away from the parent plant. Seeds of
disagreeable flavor, and very large
hard seeds are often avoided by the
animal in eating the fruit which
contains them. Small hard seeds
are commonly swallowed whole and
frequently remain nearly unacted
upon by the digestive fluids, so that
they traverse the digestive tract of
the fruit-eating animal which swal-
lowed them and remain perfectly
capable of germination. In this way
such instances of dissemination as
those of the raspberry (Fig. 346) and
the red cedars (Fig. 347) are readily
explained.

421. Seed carrying purposely
done by animals. In the cases re-
ferred to in the preceding sections, animals have been seen to
act as unconscious or even unwilling seed carriers. Sometimes,
however, they carry off seeds with the plan of storing them for
food. Ants drag away with them to their nests certain seeds
which have fleshy growths on their outer surfaces. Afterwards

C FIG. 346. Red rasp-
berry bush, in fork
of a maple

446

HOW PLANTS ARE SCATTERED

they eat these fleshy parts at their leisure, leaving the seed per-
fectly fit to grow, as it often does.

FIG. 347. Red cedar trees planted by birds roosting on fences
After Pinchot

Squirrels and blue jays are known to carry nuts and acorns
about and bury them for future use. These deposits are often
forgotten and so get a chance to grow, and in this way a good
deal of tree planting is done.

FIG. 348. Seed of bloodroot with caruncle, or crest, which serves as a handle
for ants to hold on to. Ant ready to take the seed

After Beal
1 See Beal, Seed Dispersal, pp. 69, 70.

CHAPTEE XXXIV
SOCIAL HABITS OF PLANTS; COMPETITION AND INVASION

422. Social habits. Those plants which live associated with
many individuals of the same species are called social plants.
Those kinds which are not social usually occur as members of
plant communities, or assemblages of two or more species. The
vegetation of the earth mainly consists of such assemblages, and
the total number of solitary plants is comparatively small.

Adult seed plants are usually incapable of locomotion, and
only a small proportion of all the kinds of seeds (though a some-
what larger proportion of fruits) is equipped with means for
carrying them on long journeys. It is therefore natural that
the offspring of any plant or plant community should generally
be found near the parent plants. It is not easy to trace the
working of this gradual spread of the successive broods in
the neighborhood of the parents where there is already dense
vegetation. But in any region where there are considerable areas
destitute of any given vegetation form, as in cleared land, the
young seedlings of an oak, a hickory, or a black walnut may
often be detected in many places near the parent tree.

423. Competition. Every one knows, in a general way, that
in a state of nature plants often greatly crowd each other. This
is evident enough from mere inspection of most meadows,
thickets, or tracts of woodland or waste land ; but in order to
realize how few of all the bidders for each square foot of ground
actually find a chance to occupy it, a little calculation is needed.
A single annual seed plant usually matures hundreds and often
thousands of seeds. One common weed of the Middle West, the
Russian thistle1 (Fig. 340), often produces as many as 25,000

1 Sdlsola Kali var. Tragus.
447

448 COMPETITION AND INVASION

seeds and occupies as much as four square feet of earth. The
offspring of an individual of this species, therefore, if all the seeds
grew to mature plants, would cover nearly 2.3 acres. It may
interest the student to calculate in what generation the descend-
ants of one plant would cover the entire area of his state.

424. Statistics of overcrowding. Charles Darwin seems to
have been one of the earliest observers, if not the very first, to
collect exact statistics in regard to the severity of competition
among plants. He found that out of 20 species which occurred
on a plot of turf three by four feet in area nine species died
from overcrowding by the others. On a piece of dug and cleared
ground he found that 60 weed seedlings to the square foot
sprang up and 49 of them were destroyed, chiefly by slugs
and insects.1

In a rich and weedy bit of land Professor L. H. Bailey
found in an area of twenty by twenty square inches ten spe-
cies of weeds. Reduced to the number per square foot, there
were: July 10, 30 plants; August 13, 31 plants ; September 25,
25 plants. Several of these were large weeds, such as the
redroot (Amarantus retroftexus) and the ragweed (Ambrosia
artemisicefolia) .

On June 23 of the next year there were on the same plot
(which had remained undisturbed) eleven species, numbering
108 plants to the square foot, and now the dominant plants
were red clovers. Most of the other plants were puny and
suffering from lack of light under the shade of the clovers.2

If one selects a plot hi which seedlings are just starting, the
number of individuals to the square foot will often be found to
be much greater than those above given. Under a full-grown
tree of the wild black cherry the writer has found on June 9
portions of the ground containing hardly any other seed plants
except cherry seedlings at the rate of 104 to the square foot.
Not one of all the thousands which had begun to grow could

1 Origin of Species, Chapter in.

2 The Survival of the Unlike, pp. 258-261.

STATISTICS OF OVERCROWDING

449

ever have developed into a full-sized individual on account of
the overshadowing from the parent tree,

In a weedy bit of lawn, where the grass had largely been
killed by trampling and other disturbing causes, the writer
found on June 9 plants at the rate of 1032 to the square
foot as follows :

Plantain (Plantago Riiyelii) 811

Grass (various species) 200

Knotgrass (JPolygonum aviculare) 18

Sorrel (Oxalis corniculata var. slricta) 3

1032

A B

FIG. 349. Effect of competition on radishes

Both plants were grown from the same seed and in the same soil, planted at the
same time. A was one of a lot standing so close together that their tap roots
nearly touched one another; B had several square feet of ground to itself.
About one-quarter natural size

450 COMPETITION AND INVASION

The majority of the grass plants were apparently seedlings of
the preceding autumn, and the plantains were young seedlings,
most of them an inch or less in height. A full-grown plantain
of this species occupies not less than 100 to 150 square inches,
so that of these alone more than 800 individuals were likely to
die of overcrowding.

425. How overcrowding kills. Of plants grown too close
together many die and others are dwarfed (Fig. 349) and par-
tially or wholly fail to flower or seed. This is one of the first
lessons which the beginner in gardening learns, if he neglects
properly to thin out his beds. Corn grown in closely planted
drills for fresh fodder or ensilage makes few ears, and none of
these are perfect. The weakening or destruction due to over-
crowding results mainly from these three causes 1 :

1. Insufficient light and heat for plants shaded by their more
vigorous neighbors, resulting in imperfect photosynthesis.

2. Scanty water supply, because most of the water is absorbed
by the more vigorous root systems of the stronger individuals.

3. Deficient supply of dissolved salts (nitrates, phosphates,
and so on), on account of their being largely consumed by
the stronger plants.

426. Competition most fatal between similar plants. For
obvious reasons, plants of the same general form and mode of
growth usually interfere most with each other, and plants which
are decidedly unlike interfere less, or even in some cases benefit
each other. This principle is unconsciously followed, in many
instances, by farmers and gardeners, as in the case of lawns
sown with mixed grass seed, which produce a more perfect turf
than those sown with a single species of grass. So, too, pumpkins
are often planted in cornfields, and in southern Europe beans
are raised in vineyards, in the partial shade of the vines.

If the interests of two or more kinds of plants occupying the
same area conflict little or not at all, this may be due not only
to their unlikeness of form or of requirements as regards light,
1 As far as terrestrial seed plants are concerned.

CASES OF INVASION 451

heat, or water, but also to their flowering and seeding at differ-
ent seasons. Many kinds of weeds nourish in grainfields, mak-
ing little growth until the grain is reaped, after which they
develop rapidly and flower and seed among the stubble.

427. Invasion. Some of the ways in which plants are dis-
persed have already been described (Chapter xxxin). The result
of carrying seeds or other reproductive parts into new territory
is to cause an invasion of that area. If the invaded ground con-
tains no vegetation, the newcomers take full possession. Such a
case occurs when the bed of a newly drained lake or bayou, or
soil uncovered by landslides, or newly cooled material from vol-
canic eruptions is populated by vegetation brought in by natural
agencies. If the invading species encounter other occupants of
the region invaded, the new arrivals may simply share the ter-
ritory with its previous occupants. But if the immigrants are
much better adapted to the conditions of existence in the dis-
puted area than are its actual occupants, the intruders may drive
out all before them.

428. Native species ousted by invaders. New Zealand and
the pampas of La Plata and Paraguay, in South America, have,
during the nineteenth century, furnished wonderful examples of
the spread of European species of plants over hundreds of thou-
sands of square miles of territory. The newcomers were more
vigorous, or in some way better adapted to get on in the world,
than the native plants which they encountered, and so managed
to crowed multitudes of the latter out of existence.

In our own country a noteworthy case of the kind has
occurred so recently that it is of especial interest to Ameri-
can botanists. The so-called Russian thistle (Fig. 340), which
is merely a variety of the saltwort common along the Atlan-
tic coast, was first introduced into South Dakota in flaxseed
brought from Russia and planted in 1873 or 1874. In twenty
years from that time the plant had become generally distributed
as one of the commonest weeds over an area of about 25,000
square miles.

452 COMPETITION AND INVASION

American plants, on the other hand, have in many cases
overrun other countries. -Elodea, a common water weed with
us, introduced into Great Britain about 1847, now chokes up
many pools and water courses in England and Scotland. The
prickly pear cactus (Opuntia Ficus-indica) and the century
plant, both emigrants from North America, are now the most
conspicuous plants along many cliff sides all over southern Italy.
A prickly pear has become such a nuisance in New South Wales
that large rewards are offered for its extermination.

429. Weeds. Any flowering plant which is troublesome to
the farmer or gardener is commonly known as a weed. Though
such plants are annoying from their tendency to crowd out
others useful to man, they are of extreme interest to the bot-
anist on account of this very hardiness. The principal charac-
teristics of the most successful weeds are their ability to live
in a variety of soils and exposures, their rapid growth, resist-
ance to frost, drought, and dust, their unfitness for the food
of most of the larger animals, in many cases their capacity to
accomplish self pollination, in default of cross pollination, and
their ability to produce many seeds and to secure their wide
dispersal.

Sometimes the seeds have great vitality ; those of shepherd's
purse and purslane are capable of germinating after fifteen
years or more. Many of the worst weeds, such as sow thistle,1
sorrel,2 witch grass,3 nut grass,4 and field garlic,5 have creeping
rootstocks or bulbs or tubers. Not every weed combines all of
these characteristics. For instance, the velvet leaf, or butter
print, common in cornfields, is very easily destroyed by frost;
the pigweed and purslane are greedily eaten by pigs, and the
ragweed by some horses. The horse-radish does not usually pro-
duce any seeds, but is propagated by vegetative methods.

It is a curious fact that many plants which have finally
proved to be noxious weeds have been purposely introduced
into the country. The fuller's teasel, melilot, horse-radish, wild

1 Sonchus. 2 Rumex. 3 Agropyrum. 4 Cyperus. 5 Allium.

ORIGIN OF WEEDS 453

carrot, wild parsnip, tansy, oxeye daisy, and field garlic are
instances of this.

430. Origin of weeds.1 By far the larger proportion of our
weeds are not native to this country. Some have been brought
from South America and from Asia, but most of the introduced
kinds come from Europe. The importation of various kinds of
grain and of garden seeds, mixed with seeds of European weeds,
will account for the presence of many of the latter among us.
Others have been brought over in the ballast of vessels. Once
landed, European weeds have succeeded in establishing them-
selves in so many cases, because they were superior in vitality
and in their power of reproduction to our native plants. This
may not improbably be due to the fact that the European and
western Asiatic vegetation, much of it consisting from very early
times of plants growing in comparatively treeless plains, has for
ages been habituated to flourish in cultivated ground and to
contend with the crops which are tilled there.

1 See the article, "Pertinacity and Predominance of Weeds," in Scientific
Papers of Asa Gray, selected by C. S. Sargent, Vol. II, pp. 234-242.

CHAPTEE XXXV
PLANT SUCCESSIONS*

431. Nature of plant successions. Whenever a portion of
the earth's surface is stripped of its vegetation, or undergoes
any decided change in its physical condition, the way is usually
opened for invasion of plants from the surrounding territory
(Sec. 427). In most cases the immigrants are not all of them
thoroughly adapted to their new home, and cannot become so ;
or the condition of the territory may continue to change, so
that a series of new populations appears, each in turn wholly
or partly giving way to that which follows it. Such a set of
colonizings is called a plant succession.

432. Causes of successions. It would require too much
space to state more than a very few of the causes which
originate plant successions.

First. They may be brought about by the introduction into
a region of new species which are able, without change of soil
or climate, to drive out some or all of the original occupants
(Sec. 428).

Second. They may be brought about by changing the supply
of light, heat, water, or other important factors in the surround-
ings of the plant. Such changes are sometimes natural, some-
times produced by man.

Such a river as the Mississippi, with over 12,000 square
miles in its delta, affords a good instance of the power of natural
agencies to alter the conditions of plant life. Perhaps one third
of the delta is a sea marsh, with the vegetation characteristic of

* To THE INSTRUCTOR : As this chapter is somewhat more technical than
many of the others of Part III, it may be omitted if limitations of time
demand a briefer course.

454

CAUSES OF SUCCESSIONS 455

shallow, salt, or brackish water in a warm-temperate climate,
while the remaining portion supports in places a most luxuriant
growth of land plants. Year by year, along the margin of the

FIG. 350. Aspen succession after forest fires in coniferous woods, Colorado

After Clements

submerged part of the delta, as this emerges from the water,
the change from aquatic to land vegetation goes on, and year by
year the flora which first,establishes itself in the newly emerged
mud is succeeded by others more adapted to ordinary soil.

456 PLANT SUCCESSIONS

Man produces most extensive changes in vegetation by such
operations as draining lakes and swamps, building levees, irrigat-
ing deserts and semi-deserts, clearing woodlands, and planting
treeless lands with the seeds of forest trees.

433. Order of succession in special cases. Much study has
recently been given to the exact order in which assemblages of
plants follow each other in various kinds of succession. Only
a very few cases can here be mentioned.

On the island of Krakatoa, which was completely laid waste
by a volcanic eruption in 1883, the first forms of plant life to
appear were microscopic blue-green algae (Sees. 207—211). Three
years after the eruption the flora had come to contain many
ferns, with here and there a few seed plants, on the mountains
or the coast.

In the mountains of Colorado the granite bowlders dislodged
from the faces of cliffs are covered first with incrusting lichens ;
then the gravel produced by the weathering of the granite gives
a footing to leaf -like lichens ; later the more weathered gravel
supports a growth of herbaceous seed plants ; afterward follow
thickets, then pine forests, and finally spruce forests (Plate XII).

In the pine woods 1 of central Maine when the trees have
been cut away and the clearing (as is too often the case) burned
over, the most conspicuous plants which immediately succeed
the forest are fireweed,2 raspberries, blackberries, wild cherries,3
and aspens.4 A deciduous forest of poplars and canoe birches 5
succeeds the thickets above-mentioned. This in turn would
doubtless, under natural conditions, after a long period, be dis-
placed by a pine forest.

In eastern Maine the succession is very similar, except that
blackberries are not common in the burned clearings and the
tree growth which follows the thicket stage is usually of gray
birch.6

1 Pinus Strjbus. 4 Populus tremuloides.

2 EpiloUum angustifolium. 5 Betula papyrifera.

3 Prunus pennsylvanica. 6 B. populifolia.

REASONS FOR ORDER OF SUCCESSION

457

434. Reasons for order of succession. It is not always pos-
sible to explain in detail why each set of plants in a succession
takes possession of the ground and later on is itself driven out.

FIG. 351. Young black oaks succeeding loblolly pine and shortleaf pine,

southeastern Texas

After von Schrenk

In a general way it is clear that very low spore plants can
make a living on bare rock surfaces or on partly decomposed
rock where seed plants would find too little available salts,
especially nitrates, to support their nutrition.

458 PLANT SUCCESSIONS

On lands where cultivation is abandoned, or which are in
other ways suddenly exposed to invasion, weeds of many spe-
cies often obtain a footing and nourish for some years before the
truly wild native plants of the region take final possession. This
is, in part at least, on account of the remarkable capacity of most
weeds to seed themselves (Sec. 429).

Forest or grass land is the final stage in many successions.
The former gains supremacy over the weedy thickets out of
which it rises by shading the shrubs and herbs beneath the
tree tops until all those not adapted to life in deep shade are
destroyed. Grasses have to an unsurpassed extent the power of
living with their roots (and sometimes also rootstocks) inter-
woven in a way which would prove fatal to most herbs. In
this way a lawn or meadow, on good ground, may be seen to
improve itself by choking out other plants which occur here
and there among the grass. Salt marshes, with a comparatively
scanty vegetation, are often purposely shut away from the sea,
so that the rains can wash the excess of salt out of the soil. In
four or five years they become thoroughly self-sown with the
seeds of cultivated grasses and are changed into highly produc-
tive meadows.

CHAPTEE XXXVI
ECOLOGICAL GROUPS AND THEIR CHARACTERISTICS *

435. Ecological grouping of plants. The ordinary classifi-
cation of plants, as set forth in Part II, is based, as far as
possible, on their actual relationships to each other. But when
plants are considered ecologically they are grouped according to
their relations to the world about them. They may, therefore,
be gathered into as many (or more than as many) different
groups as there are important factors influencing their modes of
life. We may, for instance, classify plants as light-loving and
shade-loving, and so on.

The most important consideration in classifying seed plants
on ecological grounds is based on their requirements in regard
to water. Grouped with reference to this factor in their life all
plants may be designated as :

1. Hydrophytes, or water-inhabiting or water -tolerating plants.

2. Xerophytes, or drought-tolerating plants.

3. Mesophytes, or plants which thrive best with a moderate supply

of water.

These three groups do not fully express all the relations of plants
to the water supply, so two others are found convenient :

4. Tropophytes, or seasonal plants which are hydrophytes during part

of the year and xerophytes during another part.1

5. Halophytes, or salt-marsh plants and " alkali " plants, species

which can flourish in a very saline soil.

* To THE INSTRUCTOR : If it is necessary to shorten the treatment of this
subject, the latter part of the chapter, beginning with Sec. 442, may be
omitted,

1 The plants which E. Warming, one of the foremost authorities, classes
as mesophytes are many of them grouped by another great authority,
A. F. W. Schirnper, as tropophytes.

459

460

ECOLOGICAL GROUPS

436. Difficulties in ecological grouping. It seems at first sight
a simple matter to group plants in regard to their need of
water. There can be no difficulty in recognizing as hydrophytes
all plants like the bladderworts, water cresses, certain mosses,
and most algse which live only in water. Cactuses, aloes, and
similar plants are recognized at sight as xerophytes. But the
chief difficulty is in dividing mesophytes from the other two
assemblages, into which they shade by indefinite gradations. In
a single mesophytic thicket, for example,
one may find such hydrophytes as the
pepper bush (Cletlirn) and such moderate

FIG. 352. Aquatic plants : pond lilies with floating leaves,
and sedges with aerial leaves

xerophytes as the catbriers (Smilax). In order to know whether
the plants of a region have plenty of water or not, we must know
not only how many inches, of yearly rainfall there are, but also
what the soil is like, what is the temperature of the soil and air?
whether or not there are dry winds, and whether there are fogs or
heavy dews. A lichen on a bare rock may be living almost under
desert conditions, while a pitcher plant in a bog near by has its
roots in standing water (or in ice) nearly all the year round.

HYDROPHYTES

461

437. Hydrophytes. Some of these are herbaceous aquatic
plants, like the duckweed, the pickerel weed, the pond lily,
and the water crowfoot ; others, such as the cultivated " calla "
(JKichardia) , the buck bean, the cat-tail, and the sweet flag, many
ferns, mosses, and liverworts, prefer damp air and soil. All of
them transpire freely, and many of them cannot live at all
under the moisture conditions which suit ordinary plants.

FIG. 353. Submerged and aerial
leaves of a crowfoot (Ranunculus
aquatilis)

The leaf with thread-like divisions is
the submerged one. — After Giesen-
hagen

FIG. 354. Cross sections of
leaves of arrowhead (Sag-
ittaria)

A, aerial leaf; B, submerged
leaf. The submerged leaf
has no ordinary epidermis
and no palisade layer, but
large air spaces. Much
magnified. — After Bonnier
and Sablon

Some aquatics have their leaves wholly submerged, others,
such as the duckweed (Fig. 355) and the pond lilies (Fig. 352),
have them floating, and still others, like the sedges in the same
illustration, have their leaves freely exposed to the air. A few
plants have both water leaves and air leaves (Fig. 353). Some
aquatic plants are rooted in the mud, while others have no roots
at all, or, like the duckweed, have only water roots.1
1 See grouping in Sec. 454.

462

ECOLOGICAL GROUPS

The leaves of land plants in very rainy sub-tropical climates
are exposed to the attacks of parasitic fungi. To ward off the

-. . ..,-._„_ attacks of these and to

allow free transpiration,
it is necessary to keep
water from accumulating
on the surfaces of the
leaves. This result is
secured by a waxy de-
posit on the epidermis,
and also by the slender

FIG. 355. The duckweed, a floating
aquatic plant

prolongation to drain off surplus water,
shown in Fig. 356. That this slender leaf
tip is useful in the way suggested is proved
by the fact that when it is cut squarely off
the leaf no longer sheds water freely.

438. Xerophytes. A xeropliyte is a plant
which is capable of sustaining life with a
very scanty supply of water. Since the first
plants which existed were aquatics, we
must consider that xerophytes are highly
specialized and modified forms adapted to East Indian fig tree1
extremely trying conditions of life. A typi-
cal xerophyte is one which can live in a
very dry soil in a nearly rainless region.
The yucca and the cactuses (Figs. 50, 357)
are good examples of such plants. Less extremely xerophytic
are plants like the date palm (Fig. 53), which flourishes in the

1 Ficus religiosa.

with a slender, taper-
ing point to drain off
water

After Schimper

PLATE IX.

The upper picture shows a belt of trees along a Nebraska river
After U. G. Cornell

The lower picture shows xerophytic grasses on Nebraska sand hills
After R. A. Emerson

FIG. 357. A field of prickly pear cactus in California

463

464

ECOLOGICAL GROUPS

oases of the Sahara, where the soil is moist from the presence
of springs, though rains are almost unknown, or the houseleeks
and stonecrops found in many gardens, the so-called Spanish
moss (Plate III), and lichens (Figs. 226, 227), all of which grow
most rapidly in moist air, but cling to bare rocks and trunks
of trees, from which they get no water.

It is important to notice that many xero-
phytes only economize water ivhen forced to
do so. With an abundant supply of water
they may transpire almost or quite as much
as mesophytes. But a drought which
would kill the latter would only cause the
xerophytes to close their stomata and greatly
lessen transpiration. A xerophyte must be
capable of storing water and transpiring
very slowly, like cactuses, aloes, stonecrops,
and such fleshy plants with a thick epider-
mis, or else it must be able to revive after
being thoroughly dried.

439. Roots and stems of xerophytic seed
plants. Some xerophytes have roots which
show no peculiarities of form or structure,
but many make special provision for storing-
food and water in their roots. Such roots

FIG. 358. Ilarpago- are fleshy and often' as in" Harpagopliytnm
phytum, a South (Fig. 358), are of great size compared with
African xerophyte the pOrtion of the plant above the ground.
After Schimper Xerophytic stems are frequently very thick
in proportion to their length, sometimes even globular (Fig. 50),
and they commonly contain large amounts of water. In leaf-
less plants, like the cactuses, the surface for transpiration is
much less than that offered by leafy plants. Many species
which bear leaves shed most of them at the beginning of the
dry season, and some remain thus in a half dormant condition
for long periods, as is the case with many Euphorbias (Fig. 318).

LEAVES OF XEROPHYTES

465

The epidermis, even on the younger portions of the stem, is
highly cutinized, and this structure makes any evaporation
very slow.

440. Leaves of xerophytes. Since the leaf is hi general the
main organ of transpiration, we might expect to find the leaves
of xerophytes highly adapted to their environment. This is the

FIG. 359 FIG. 360

FIG. 359. Cross section of leaf of Ficus elastica

•

c, cuticle ; o, opening to pit ; p, pit leading to stoma ; s, stoma ; e, epidermal cells ;
w, special cells for storage of water; ch, air chamber from stoma; sp, cells
of spongy parenchyma ; a, intercellular air spaces. Much magnified

FIG. 360. Fleshy leaves of Mesembryanthenmm, with stored water
After Giesenhagen

case, and some of their principal means of protection from
excessive transpiration are as follows l :

1. A thick epidermis, often of several layers of cells (Fig. 359).

2. Storage of water in epidermal cells.

3. Small stomata, often deeply sunken (Fig. 359).

4. Epidermal hairs or scales. These are often extraordinarily
abundant, and in some cases give one or both surfaces of the
leaf a silky or silvery luster.

1 See Warming, Lehrbuch. der (Ekologischen Pflanzengeographie, vierter
Abschnitt, Berlin, 1902.

466

ECOLOGICAL GROUPS

5. Coatings of wax or varnish or incrustations of salts.

6. Extreme development of the palisade layer.

7. Eeduction of the intercellular spaces.

8. Mucilaginous, water-retaining cell contents in the spongy

parenchyma of the
leaf (usually in fleshy
leaves, Fig. 360).

9. Permanent verti-
cal position of leaves
(Figs. 45, 110, 111).

10. Leaf move-
ments, presenting only

FIG. 361. Cross section of rolled-up leaf of
crowberry (Empetrum nigrum]

Magnified. — After Kerner

11. Rolling up of leaves, either permanent, as in Fig. 361,
or temporary, as in Indian corn and in Fig. 362.

12. Eeduction of leaf area, — the leaves either few or small,
or both. Sometimes the leaf consists of little else besides a
petiole ; sometimes, as in Figs. 50 and 357,

foliage leaves are wholly absent.

the edges to the sun
during the heat of the
day (Sec. 114).

FIG. 362. Cross section of leaves of a grass,1 unrolled for exposure to sun-
light and rolled up to prevent evaporation

r, ridges of the upper epidermis, with many stomata on their surfaces ; e, thick
lower epidermis, without stomata. — After Kerner

In regions with a long rainless summer, like that of southern
California or the coast of the Mediterranean, many shrubs are
summer deciduous, and in their leafless condition the twigs

Stipa capillata.

MESOPHYTES

467

have been found to transpire less than 3 per cent of their
maximum rate when leafy.

Some of the principal differences between hydrophytes and
xerophytes may be summed up as follows :

HYDROPHYTES

XKROPHYTKS

Roots

Few

Many

Water-conducting tissue .
Air-conducting tissue ....
Water-storage tissue ....
Epidermis

Scanty
Abundant
Wanting
Thin or wanting
Often large or dis-

Abundant
Scanty
Often abundant
Thick
Usually of reduced

sected

surface

441. Mesophytes. A mesophyte is a plant which thrives
best with a moderate supply of water. The great majority of
the wild and the cultivated plants of the United States are
mesophytes. What has been learned from Part I of this book
about the forms, structure, and habits of ordinary plants, to-
gether with what the student's own observation, aside from the
study of botany, has taught him, should suffice to give him a
fair idea of mesophytic plant life.

It is important to notice that most of our mesophytic trees
and shrubs pass the winter (or in the extreme Southwest the
dry season) in a leafless condition, and so transpire very little.
So, too, our mesophytic, herbaceous perennials, such plants as
the jack-in-the-pulpit, lilies, irises (Fig. 45), violets, and others,
lose a large portion of their evaporating surface during part of
the year by dying to the ground and leaving only the buried
bulbs, roots with buds at the crown, or rootstocks alive.

All of the plants which make decided preparations for the
season when water is hard to get may be classed as tropophytes
or periodic xerophytes.

442. Deciduousness an acquired habit. The practice of shed-
ding the leaves before the arrival of severe freezing weather,
when it becomes almost impossible to draw moisture from the

468

ECOLOGICAL GROUPS

earth, or before the culmination of the severest drought of sum-
mer, may be regarded as a habit gradually acquired by decidu-
ous trees and shrubs for their own protection. The duration of
the period of leaflessness depends on the length of the danger-
ous season. Grapevines, for instance, in central Europe are leafy
during about six months and leafless during the following six.
But near Cairo, Egypt, the leafless period is only two months
long, and in very warm and moist climates the vines are ever-
green. So, too, cherry trees are evergreen in Ceylon, and
beeches in Madeira.

A large shrubby Euphorbia* common in southern Italy, is
found absolutely leafless during July and August, when grow-
ing on the faces of limestone cliffs. But in moist soil, within

a stone's throw of the
leafless plants, there
may be found others
profusely leafy.

443. Halophytes. A
halophyte is a plant
which can thrive in a
soil containing much
common salt or other
saline substances. The
seaside is the principal
region of halophytic
vegetation, but many
halophytic plants are

FIG. 363. The mangrove, a halophyte
After W. M. Davis

also to be found in the neighborhoods about salt springs and the
" alkali " lands of the Southwest and the Pacific slope.

The mangrove tree (Fig. 363) is one of the most remarkable
of halophytes. It grows in shallow water along the seashore,
and sends out many aerial roots which at length find their way
down into the salt mud. In this way it collects drift material
and gradually extends the shore line farther out to sea.

1 E. dendroides.

HALOPHYTES 469

444. Form and structure of halophytes. Most halophytes
present certain peculiarities of form and structure, such as suc-
culence of stem or leaves, or both, a highly developed palisade
layer, small intercellular spaces, diminution of evaporating sur-
face, and often specially developed tissue for storage of water.
These are evidently xerophytic characteristics, and their pres-
ence may be due to two causes :

First, the occurrence of salt in the soil renders absorption of
the soil water comparatively difficult, since osmosis takes place
more readily between nearly pure water and the liquid con-
tents of the young roots and root hairs than between salt water
and the liquids within the root. Halophytes may therefore be
on a short allowance of water even when their roots are con-
stantly wet.

Second] the absorption of much salt would poison the plant,
and therefore it is an advantage to keep down transpiration
and with it the rate at which salt water is allowed to enter
the roots.

445. Halophytes not dependent on salts. It is worth while
to note the fact that halophytes are not usually dependent on
a highly saline soil. They are salt tolerators rather than salt
lovers.1 But they nourish in saline localities because they are
capable of enduring much more salt than ordinary plants, and
so can grow in salt marshes and such localities comparatively
unhindered by the competition of non-halophytic species.

446. Other kinds of ecological classes. One may class plants
with reference to their habits in many other regards than
according to their relative economy of water or their tolerance
of salts. Only one other kind of classification need, however, be
mentioned in this chapter, — that is, the division into sun-loving
and shade-loving plants. Even in very dense forests some plants
are found growing on the soil in the twilight formed by the
shade of the trees. Some of this undergrowth is of seed plants,

1 Or, in technical terms, the plants which grow in saline soils are facul-
tative halophytes but i\ot%obligate halophytes.

470

ECOLOGICAL GROUPS

and there are many ferns and mosses which nourish in such
situations. Shade plants commonly have large pale leaves, and
generally (except in ferns) the leaves are not much cut or lobed
(Fig. 364, A). Sun-loving plants, on the other hand, usually

have comparatively
little leaf surface, and
the leaves are often cut
into narrow divisions
(Fig. 364, B). Appar-
ently the broad leaf
surfaces in the one
class are to expose
many green cells to the
light for starch making,
while in the other class
the slender leaf divi-
sions expose enough as-
similating cells, and at
the same time the nar-
rowness of the division
permits plenty of light
to penetrate to the
plant's lower leaves. It
is also, doubtless, much
easier for leaves like
those of the yarrow,
the dog fennel, the
tansy, the carrot, and
their like to withstand the action of severe winds, to which they
are often exposed, than it would be for leaves like those of
the jack-in-the-pulpit, the trilliums, the lily of the valley, and
similar leaves.

447. Sun leaves and shade leaves on the same plant. On
plants of the same species, or even on the same individual, sun
leaves and shade leaves often differ widely-. On comparing the

FIG. 364

A, a shade plant (Clintonia) ; B, a sun plant,
dog fennel (Maruta)

SUN LEAVES AND SHADE LEAVES 471

leaves from the exterior and the interior of the crown of a de-
ciduous tree, or such an evergreen species as the live oak or the
olive, the sun leaves are usually found to be lighter-colored, of
smaller area, thicker, and of more xerophytic structure than the
shade leaves. The difference in size may be very great, the
smallest sun leaves sometimes not covering more than a tenth
of the area of the largest shade leaves on the same plant. There
is usually, also, a notable difference in the form of the two
kinds, the sun leaves being narrower in proportion to their
length. Sun leaves are often several times as thick as shade
leaves, and have far more completely developed palisade layers.
The latter may even (in leaves grown in dense shade) be quite
lacking, and the regular palisade cells be replaced by loosely
arranged, funnel-shaped cells, with their broader ends toward
the epidermis. Sun leaves have a much stronger fibre-vascular
framework than those developed in a comparatively feeble light.
In the ease of plants which have the leaves more or less hairy
or scaly, the covering of these epidermal outgrowths is, as might
be expected, much more dense on the sun leaves.

Probably the work of all kinds done by the sun leaves is far
greater than that done by shade leaves of the same species.
This is partly due to the much greater supply of energy daily
received by the former from the sun, and it is also due to their
more capacious conducting system and greater supply of chloro-
plasts. The transpiration of a given area of sun leaves is at
times tenfold that of the same area of shade leaves (both being
placed for the time in full sunlight).

448. Transition of a plant from shade conditions to sun con-
ditions. It is characteristic of many kinds of forest trees that
the young seedlings are much more tolerant of dense shade than
the adult trees. Sometimes their seeds will hardly germinate
at all unless thoroughly shaded, and the young trees for the
first few years flourish best in the shade. Afterwards most trees
need a good deal of sunlight, but they may live long with a
scanty supply of light. ''Ihe red spruce sometimes lingers on for

FIG. 365. An epiphytic fern (Platycerium) on a tree trunk

The more upright leaves next to the trunk of the tree serve to collect moisture and
to accumulate a deposit of decaying vegetable matter, while the outer leaves
serve as foliage and bear spores. — After Schimper

472

WATER SUPPLY OF EPIPHYTES 473

fifty or a hundred years, reaching meantime a diameter of not
more than two inches, and then, on getting more light, shoots
up into a large and valuable timber tree.1

449. Epiphytes. It is even easier for a plant to secure enough
sunlight in a forest region by perching itself upon the trunk,
branches, or leaves of a tree than by climbing, as our wild grape-
vines and the great tropical lianas do. There is a large number
of such perched plants, or epiphytes (meaning upon a plant), par-
tictdarly in such tropical forests as those of Fig. 39 and Plate
XIII. Epiphytic forms occur among many different groups of
seed plants and of spore plants, especially lichens. The stag-
horn fern, shown in Fig. 365, is a good example of an epiphyte.
Instances among seed plants are the so-called Florida or Spanish
moss (Plate III) and orchids like those in Fig. 13.

450. Water supply of epiphytes. Epiphytes secure their
supply of water and dissolved salts in several different ways,
some through roots by absorption from the moist bark on which
they grow, others by sending roots down until they reach the
earth, others by means of a network of aerial roots fully exposed
to the air, — as in the orchid just mentioned, — and still others
by means of leaves which function as roots. Some species, like
the Florida moss, absorb water very rapidly from dew or rains,
while others, as the stag-horn fern .(Figs. 272, 365), and Til-
landsia bulbosa, a relative of the Florida moss, hold water in
reservoirs at the bases of the. leaves, with or without the aid of
spongy decaying vegetable matter. From the great vicissitudes
in their water supply most epiphytes among seed plants possess
xerophytic characteristics.

1 See the Primer of Forestry, Part I, United States Department of Agri-
culture, 1899, pp. 33-35.

CHAPTER XXXVII
PLANT FORMATIONS; ZONATION*

451. Plant formations. One of the first things which the
young field botanist learns is the fact that the distribution of
plants depends largely on the character of the ground they occupy.
There is in any small territory, such as a Count}', for example,
one assemblage of plants for the waters of ponds and another
for their shores, one for swamps, one for moderately dry uplands,
one for very dry hilltops, and so on. The aquatic plants of the
sea are very different from those of fresh water. Sandstone and
limestone soils have vegetations peculiar to themselves;1 the
long-leaved pine, the scrub pine, and the chestnut are character-
istic trees of sandy soils, while most of the oaks, the hackberries,
and the black walnut are generally found in limestone regions.

The collection of plants as found in any given kind of station
or habitat, especially when prominent and well defined, is called
& formation. Thus we have marine aquatic formations, sea-beach
formations, pond formations, bog formations, sand-hill forma-
tions, meadow formations, heath formations, forest formations,
and many others such as the student may designate for himself.

452. Plant associations. Usually the plant formation is divis-
ible into assemblages or unit groups, which are much more alike
in their vegetation than is the formation as a whole. Thus a
woodland formation may consist of pine patches, oak patches, and

* To THE INSTRUCTOR : If it is necessary to cut down the discussion of
these topics to little more than definitions, only the first three sections of the
chapter need be read.

1 Perhaps this is sometimes due to physical rather than to chemical causes.
In other words, the chemical differences in soils are usually accompanied by
differences in their porosity, their capacity for retaining water, for absorbing
heat from the sun's rays, and so on, which greatly modify their effect on plants.

474

ZOtfATION 475

birch patches. A grass-land formation may consist of patches of
timothy and others of redtop, and so on. Such minor groups
are often called associations.

In some cases it may be possible to show that the association
is based on the mutual relations to each other of the plants
which compose it, while the formation as a whole depends on
the characteristics of the station in which it exists, i.e. on soil,
climate, and so on..1

453. Zonation. The most striking occurrence of plant forma-
tions is in localities where sharply contrasted conditions of life
exist side by side. It is often possible, within the radius of a
few hundred feet, to travel from the floating aquatic vegetation
of the deeper waters of a pond, through the rooted aquatic forms
of the shallower water, to the sub-aquatic species of the wet
shore, then past the sand-loving plants of the sand dunes farther
back from the water, and finally into the wood or meadow vege-
tation of ordinary soil. Such a series of zones is shown in
Fig. 366 and in Plate X.

Similar diagrams may be made to illustrate the distribution
of plants about a salt spring or pool, along the seashore or the
margin of a salt marsh, on the top and sides of an isolated hill
with a dry, ledgy, or sandy summit, or even about an old unused
gravel pit or a railroad embankment. Less clearly defined but
very interesting and extensive zones may be studied with rela-
tion to submerged aquatics, particularly among marine plants, as
shown in Fig. 202.

Among the most striking and symmetrical instances of zona-
tion are those to be found about the salt marshes of some of
the deserts of the far West. The waters of these marshes are
too salt to support vegetation, but encircling their borders may
sometimes be found as many as six broad concentric bands of
abundant vegetation.

1 Some authors use the term association as an equivalent for the term
formation as here employed. Consocies is sometimes used with the same
meaning as is here given to" association.

•5cm.

-500ft,

FIG. 366. Zonation about a pond

I, pond ; II, bog zone ; III, swampy thicket zone ; IV, incomplete zone in arid soil
of a sand pit; V, dry meadow zone; VI, dry woodland zone ; a, floating mass
of Eriocaulon ; b, deepest area in pond, six to ten feet; c, association of tall
rushes (Juncus militaris) ; d, birch woodland (Betula populifolid) ; WE, line
intersecting formations of west side of pond ; SN, line intersecting forma-
tions of north end of pond

476

ZONATION ABOUT A POND 477

454. Zonation in Fig. 366. The diagram represents some of
the principal formations observed in and about a rather shallow
pond in eastern Massachusetts in October, 1905.

The pond was almost encircled by six zones of vegetation,
only one of which (the bog zone) entirely surrounded the water.
All the zones except that of the meadow consisted of wild spe-
cies, growing under nearly natural conditions.

The pond itself had a rather scanty algal vegetation, among
which the most noticeable forms were a green alga (Bulbochcete),
related to (Edogonium, and a blue-green alga (Ccelosplicerium).

Seed plants were well represented in the waters of the pond.
In the deepest portion, at b, no seed plants were found extend-
ing to the surface, but there were many young specimens of a
rush (Juncus. militaris) with filiform leaves deeply submerged. In
general the pond was populated by pond lilies (NympJicea), cow
lilies (Nuphar), three or more species of potidweed (Potamoge-
tori), with much pipewort (Eriocaulori) and duckweed (Lemna).
In the shallowest portions, usually in six inches or less of water,
were found some six other herbaceous species.

The spermatophytic vegetation of the pond may be divided
according to its mode of growth into classes as follows :

Floating plants • . Lemna

Plants which grow rooted and submerged . . Potamogeton

( Juncus

Plants which grow rooted, but with more or less j Pontederia
of the stem or leaf surface in air I Nymphcea

[ Nuphar

Plants of shallow water (six inches or less), or C

, . , n A . f. TT ,, I hleochanx

which grow on floating rafts like that at a, 4 „ .

•j.1 f ,! • T Xyris. etc.

with most of the plant body aerial .....{_

455. Contents of the zones. It would involve too much
detail to enumerate the species of the several land zones
(II-VI), but they may be briefly summarized as follows :

The bog zone contained some twenty-one conspicuous species,
especially peat moss and herbaceous seed plants.

478 PLANT FORMATIONS

The swampy thicket zone contained mostly shrubs and small
trees, including an alder, a blueberry, a pepper bush (Clethra),
gray birch, and red maple.

The arid soil zone contained more than twenty species, mostly
sand-frequenting annual seed plants.

The meadow was growing under artificial conditions, and it
was merely noted that its flora consisted mainly of cultivated
grasses.

The dry woodland zone contained some twenty-three con-
spicuous forms. The three principal trees, in the order of their
numbers (omitting the region d), were white pine, northern pitch
pine, and red oak. The forest floor contained an abundant growth
of shrubs and herbs. At least five species of the latter were
common to the woodland zone and the arid sand zone.

The marked differences in the character of the vegetation
of the several zones were almost wholly due to differences in
the amount of water supply. Not only would the trees have
died if transplanted into the pond, or the pond aquatics have
died if removed to the dry sandy soil of the woodland, but in
general each set of plants was better off in its own zone than it
would have been in any other. The sand-pit flora was, however,
only a short-lived succession, soon to be followed by the wood-
land flora.

456. Similar vegetation due to similar conditions. As soon
as one begins to collect plants in a set of localities new to him,
he often discovers that his old acquaintances are still to be found
grouped as he has been accustomed to see them. The muddy
borders of ponds from Maine to Minnesota and beyond are
fringed with the same kinds of bur reeds and sedges, set with
water plantain and decorated with the soft white blossoms of
the arrowhead. The sand dunes along the northern Atlantic
coast and those that border Lake Michigan are clothed with a
sparse vegetation, which in both cases includes the little beach
plum, such coarse grasses as that shown in Plate I, and the
straggling sea rocket. Barnyards and waste grounds about farm

SIMILAR SPECIES REPLACE EACH OTHER 479

buildings from Massachusetts to Missouri contain such weeds
as the dog fennel, the low mallow (" cheeses "), motherwort,
catnip, and some smartweeds.

A little study of such cases soon leads one to the conclusion
that these plant associations and multitudes of others exist
because all the plants in each association are adapted to their
special environment. Wherever such an environment occurs
such an association will be found in it,1 or, if not already there,
will nourish when introduced.

457. Similar species replace each other. Two sets of locali-
ties alike in many respects but unlike in some points are often
inhabited by different species of the same genus. For instance,
the pine barrens of New England and the adjacent states are
commonly covered with the northern pitch pine,2 while far
southward, in sandy soil, its place is taken by the long-leaved
pine.3 Along streams and swamps northward the speckled alder 4
is generally found, while southward the smooth alder 5 is most
common. In rich w^oods of the northeastern United States the
painted trillium 6 and the erect trillium (" Benjamin," or " squaw
root ") 7 are the commonest species, while farther south, in sim-
ilar localities, the sessile trillium,8 Underwood's trillium,9 and
the large-flowered trillium10 are abundant.

In all such cases — and they are very numerous — we are
to infer that the genus is peculiarly well adapted to some spe-
cial set of conditions, as sandy soil, brooksides, or the rich,
shaded soil of woodlands. The more northerly species are capa-
ble of enduring the severe winters and brief summers of their
region, while the more southerly ones perhaps cannot do so.
The relative warmth of the climates in which they live may
not be the only reason, or even the principal reason, for the

1 That is, in localities not separated by such natural barriers as seas,
high mountains, or deserts.

2 Pinus rigida. 5 A. serrulata. 8 T. sessile.

3 P. palustris. 6 Trillium erythrocarpum. 9 T. Underwoodii.

4 Alnus incana. 7 T. erectum. 10 T. grandiflorum.

480 PLANT FORMATIONS

distribution of such plants as those just mentioned, but it is
one factor, at any rate, and it is certain that, on the whole, most
of our native and thoroughly naturalized plants are growing
under what is, for them, the best environment, since they can-
not usually be made, to exchange places with one another. If a
square mile of land in Louisiana were to be planted with Min-
nesota species, and a square mile in Minnesota with Louisiana
species, it is very improbable that either tract, if left to itself,
would long retain its artificial flora. To this rule there are,
however, important exceptions.

458. Formations of few species. It is not uncommon to find
tracts of land or water inhabited by great numbers of seed
plants of the same species, so as almost to exclude all other vege-
tation except microscopic spore plants. Ponds and slowly flow-
ing streams are often filled in this wray with the water hyacinth,1
the water cress, or the American lotus.2 The canebrakes of the
South and the wild rice swamps along northern lakes arid rivers
are other examples of an extremely simple flora spread over
large areas. Prairies not infrequently for many square miles
are covered mainly (not entirely) with a very few kinds of
grasses. The arid plains of the Eocky Mountain region, over
thousands of square miles, contain little vegetation except sage-
brush (Artemisia tridentata), and immense tracts of snow in
the arctic regions are destitute of plant life except for the
red-snow alga (Splicerella nivalis, Sec. 215), by which they are
colored pink.

In all such cases it is evident that the single species or the
few species which populate the area can endure the conditions
of existence there so well that other plants which migrate into
their territory cannot compete with them.

1 Eichhornia. 2 Nelwnbo.

CHAPTER XXXVIII

PLANT GEOGRAPHY *

459. Regions of vegetation. The earth's surface (that of the
land) has been described by one of the greatest of geographical
botanists 1 as divided into twenty -four regions of vegetation.
His grouping takes account of all the principal continental areas
which have a characteristic set of plants of their own, as well
as of the most important islands. But a simpler arrangement
is to consider the plant life of the earth as distributed among
the following regions :

1. The tropical region. 3. The arctic regions.

2. The temperate regions. 4. Mountain heights.

5. Bodies of water.

Any good geography gives some account of at least the
land vegetation of the earth. It is only necessary in the pres-
ent chapter to point out a few of the most important charac-
teristics of the plants of the areas mentioned above and to give
some reasons why the plant population of each has its special
characteristics.

460. Tropical vegetation. Within the tropics two of the
great factors of plant life and growth, namely, light and heat,
are found in a higher degree than elsewhere- on the earth.
Moisture, the third requisite, is in some regions very abundant
(over forty feet of rainfall in a year), or sometimes, in desert
areas, almost lacking. We find here, accordingly, the greatest
extremes in amount of vegetation, from the bare sands or rocks
of the Sahara desert (Fig. 367) to the densely wooded basin of

* To THE INSTRUCTOR : Unless the present chapter can he discussed in
considerable detail, it might better be omitted than hastily dealt with.
1 A. Grisebach, in Die Vegetation der Erde.

481

482 PLANT GEOGRAPHY

the Kongo and of the Amazon. The rainy forests of the tropics
contain extraordinary numbers of species. For example, near
Lagoa Santa in Brazil, in an area of three square miles, there are
found about four hundred species of trees. Xerophytic plants,
many of them with extremely complete adaptations for support-
ing life for long periods without water, are characteristic of
tropical deserts, while many of the most decided hydrophytes
among land plants are found in the dripping sub-tropical forest

in.

FIG. 367. Hills of drifted sand in the Sahara
After W. M. Davis

interiors. Throughout a large part of the belt, reaching five
degrees each way from the equator, there are daily rains the
year round.

461. Vegetation of temperate regions. We are all familiar in
a general way with the nature of the plant life of the north tem-
perate zone ; that of the south temperate is in most ways similar
to our own. Most of the annuals and biennials are of a medium
type, not decided xerophytes nor hydrophytes, and the peren-
nials are mainly tropophytes. There are no desert areas so large
or so nearly destitute of plants as those found in sub-tropical
regions, neither are there any such luxuriant growths as occur in
the rainy forest regions of the tropics. On the other hand, the
largest trees on earth, the giant redwoods, or Sequoias (Fig. 33),
occur in the temperate portion of North America, along the
Sierra Nevada, and the taller, though less bulky, gum trees
(Eucalyptus) of Australia grow in a warm temperate region.

ASSOCIATIONS DUE TO CONDITIONS OF SOIL 483

462. Temperate plant associations due to special conditions
of soil. Even where the climate is a moderate one as regards
temperature and rainfall, peculiar soils may cause the assemblage
of exceptional plant associations. Some of the most notable of
such associations in temperate North America are those of the
salt marshes, the sand dunes, and the peat bogs.

In salt, marshes the water supply is abundant, but plants do
not readily absorb salt water by their roots, so that the plants
which grow in salt marshes usually
have something of the structure and
appearance of xerophytes. Some of
them are fleshy (Fig. 368), and some
species are practically leafless.

Sand dunes, whether along the
seacoast or near the Great Lakes,
offer a scanty water supply to the
roots during much of the year, and
the soil water contains less of the
raw materials for plant food than is
offered by that of ordinary soils.
Many grasses thrive, however, in
these shifting sands (Plate I), and
some, like the beach grass (Am-
mophila) of the Atlantic coast and
the Great Lakes, will continue to grow upward as the sand is
piled about them by the winds, until they have risen to a level
of a hundred feet above the starting point.

The water of peat bogs contains little mineral matter, and
only a very scanty supply of nitrogen, in the form of nitrates
dissolved in it. The bog plants, therefore, must either get on
with an exceptionally small supply of nitrogen, or they must
get it from an unusual source. The peat mosses adopt the
former alternative, while the sundews, the pitcher plants, and
some other species adopt the latter and derive their nitrogen
supply largely from insects which they catch, kill, and digest.

FIG. 3(38. A halophyte
(Salicornia)

484 PLANT GEOGRAPHY

463. Arctic vegetation. The seed plants of the arctic flora
are mostly perennials, never trees, though many of the species,
as the willow, alder, and birch, belong to groups that are trees
in other regions. By the large bulk of the underground por-
tion as compared with that of the part above ground, they are
adapted to a climate in which they must lie dormant for not
less than nine months of the year. The flowers are often showy
and appear very quickly after the brief summer begins. Mosses

FIG. 369. A plant of arctic willow
About natural size

and lichens are abundant, — the latter of economical importance
because they furnish a considerable part of the food of reindeer.
464. Mountain or alpine vegetation. In a general way the
effect of ascending a mountain, so far as vegetation is concerned,
is like that of traveling into colder regions. It was long ago sug-
gested in regard to Mount Ararat, that on ascending it one trav-
ersed first an Armenian, then a south European, then a French,
then a Scandinavian, and finally an arctic flora. Up to a certain
height, which varies in different latitudes, the slopes of moun-
tains are very commonly forest-covered. The altitude up to

MOUNTAIN OR ALPINE VEGETATION 485

which trees can grow, or as it is commonly called in this coun-
try the " timber line," is somewhat over twelve thousand feet
in the equatorial Andes, and lessens in higher latitudes as one
goes either way from the equator, until in the arctic regions it
reaches sea level. In the White Mountains, for instance, the
timber line only rises to about forty-five hundred feet. The seed
plants of alpine regions in all parts of the earth have a peculiar
and characteristic appearance. It is easiest to show how such

FIG. 370. Trees near the timber line on the slope of Pikes Peak
After W. M. Davis

plants differ from those of the same species as they look when
growing in ordinary situations by reference to the plants them-
selves or to good pictures of them (see Fig. 372). The differences
between alpine and non-alpine plants of the same or closely
related species have been summed up as follows 1 : " The alpine
individuals have shorter stems, smaller leaves, more strongly
developed roots, equally large or somewhat larger and usually
somewhat more deeply colored flowers, and their whole structure
is drought-loving (xerophilous)."j

1 By. A. F. W. Schimper.

486

PLANT GEOGRAPHY

Trees at great elevations are stunted and gnarled by scanty
nutrition and pressure of wind and snow (Fig. 370).

FIG. 371. Decrease in size of trees at high elevations (Canadian Rockies)

Where the prevailing winds come mainly from one quarter,
all the trees of considerable areas may be inclined strongly in
one direction, as in Plate XL1

1 This phenomenon is also very noticeable along many coasts.

a «

MOUNTAIN OR ALPINE VEGETATION

487

The gradual diminution of the height of the trees on ascend-
ing a mountain is well shown in Fig. 3 7 1.1 The treeless charac-
ter of the mountain summit is also plain.

Recent experiments have shown that many ordinary plants
promptly take on alpine characteristics when they are transferred
to moderate heights on mountains. For instance, a rather com-
monly cultivated sunflower,2 when planted at a height of about
sixty-five hundred
feet, instead of
having a tall, leafy
stem, produces a
rosette of very hairy
leaves lying close to
the ground, thus be-
coming almost un-
recognizable as a
sunflower. The
change is even
greater than that
shown in the rock
rose (Fig. 372) culti-
vated by the same
experimenter. The
peculiarities of alpine
plants appear to be
due mainly to the in-
tense light which
they receive during the daytime,3 to the strongly drying char-
acter of the air in which they grow (due partly to its rarefaction),
and to the low temperature which they must endure at night.

1 Part of the diminution is only apparent, — the effect of distance, — but
the growth at the highest levels is often less than waist high.

2 Helianthus tuberosus, the so-called Jerusalem artichoke.

3 The experiments of Professor Frederic E. Clements on Pikes Peak,
however, seem to show that light is not a principal factor in the production
of alpine characteristics in plants.

FIG. 372. Two plants of rock rose (Helianthemum)

A, low ground form; B, alpine form. Both drawn
to the same scale

488 PLANT GEOGRAPHY

465. Aquatic vegetation. Plants which live wholly in water
often need a less complicated system of organs than land plants.
True roots may be dispensed with altogether, as in many
seaweeds, in most fresh-water algae, and in some seed plants.
Many such plants have mere holdfasts that keep them from
being washed out of place. In the duckweeds (Fig. 355) the
roots answer the purpose of a keel and keep the flat expanded
part of the plant from turning bottom up. The tissues that
serve to strengthen the plant body are not much developed in
submerged aquatics, since the water supports most, if not all,
of the weight of the plant. Stomata are absent, and the absorp-
tion of carbon dioxide and giving off of oxygen go on directly
through the delicate cell walls, unprotected by an epidermis
(Fig. 354,^). Submerged aquatic seed plants occur in consider-
able abundance in sea water as well as in fresh waters, but the
marine forms do not include many species.

466. Influence of rainfall in determining regions of vegeta-
tion. While the mean annual temperature and the extremes of
heat and cold, humidity of the air, force and direction of winds,
elevation above sea level, and nature of the soil are all factors
in determining the boundaries of regions of vegetation, there is
no factor more important than the annual rainfall. Of course
the rainfall itself is largely determined by several of the other
circumstances above mentioned.

In the United States this varies greatly, the yearly averages
for some of the most important areas being about as follows :

AVERAGE RAINFALL PER YEAR
REGION INCHES

New England and Middle States 43

Eastern Gulf States 55

Ohio basin 44

Missouri basin .31

Rocky Mountains, middle of eastern slope .... 20

Rocky Mountains plateau, middle 9

Pacific slope, northern portion 37

Pacific slope, southern portion 10

PLANT GEOGRAPHY OF THE UNITED STATES 489

It is evident that the rainfall incr eases southward along the
Atlantic coast, but that on the Pacific coast it diminishes south-
ward. Passing from either coast inland, one finds the rainfall
diminishing until it reaches a minimum in the Rocky Mountain
region.

467. Plant geography of the United States. All of the con-
tinuous territory of the United States1 lies in the north tem-
perate zone. There is material for volumes in the discussion of

FIG. 373. Annual rainfall of the United States

Darkest shade, over 80 inches ; lighter vertical lines, from 40 inches to 80 inches ;
horizontal lines, from 20 inches to 40 inches ; blank, from 10 inches to 20 inches ;
dotted, less than 10 inches. — After W. M. Davis

the distribution of plants over our territory in this continent
alone, but it is possible to sum up a brief outline of the matter
fcin a few pages. Excluding the floras of many single mountains
and mountain ranges, the land surface of the country may for
botanical purposes be divided into four great areas, as follows :
The forest region. This occupies the eastern and central
portion of the United States. It is bounded on the west by an

1 That is, not counting in Alaska, our West Indian possessions, the
Hawaiian Islands, or the Philippines.

490 PLANT GEOGRAPHY

irregular Hue, most of which is east of the hundredth meridian.
In some places this forest boundary lies considerably east of
the Mississippi River, while in others it extends from the river
five hundred miles or more to the westward.

The plains region. This stretches westward from the region
above-named to the Rocky Mountain plateau.

The Rocky Mountain region. This includes the Rocky Moun-
tains, the Sierra Nevada, and the various plateaus between
them.

The Pacific slope. This extends from the Cascade Range and
the Sierra Nevada to the sea.

468. The forest region. The forest region is mainly remark-
able for its great variety of hardwood trees, of which it contains
a larger number of useful species than any equal area of the
earth with a temperate climate. Perhaps the most important of
these are the oaks ; but other genera, such as the hickory, the
tulip tree, and the sassafras, are more characteristically Ameri-
can. In the northeasterly portion there are extensive forests of
the cone-bearing evergreens, such as pines, spruces, hemlocks,
and cedars ; the other trees which accompany these are mostly
deciduous hardwood species. In the southerly portion the for-
ests are partly of coniferous evergreens (Fig. 392) and partly of
deciduous mesophytes, such as hickories, beeches, oaks, elms,
hackberries, magnolias, and sycamores. There is also a consider-
able admixture of such hydrophytes as the water hickory, the
sweet bay (Magnolia), the anise tree (Illicium), the custard apple
(Anona), the red bay (Persea), the loblolly bay (Gordonia), and
the sour gum (Nyssa), due to the mild, moist climate.

This region was never completely forest-covered. Areas of
prairie, so-called "openings" in the hardwood forests (Fig. 393),
extensive marshes, and some heaths have for ages been treeless,
or nearly so. Generally, in the older states, the land most desir-
able for cultivation has been tilled so long that it is difficult to
find portions in anything like their primitive condition. It is
only in broken country like that of the mountainous regions

492 PLANT GEOGRAPHY

of eastern Tennessee and North Carolina, the Adirondack s and
the White Mountains, in swampy river valleys, in a few great
marshes, or in sterile, sandy pine barrens, that one can find the
original flora in its natural condition.

Comparing our forest region with the parts of Europe which
resemble it most in soil and climate, our flora differs notably in
possessing such leguminous trees as the locust and the honey
locusts, in the abundance of members of the heath family, and
in wealth of Composites, especially asters and golden-rods.

In very many instances our eastern flora when it differs most
notably from that of Europe greatly resembles that of China and
Japan. This is undoubtedly due to the fact that these American
species and kindred Chinese and Japanese ones had in an earlier
geological age a common ancestry.

On account of the great length of the territory along a north
and south axis and the diversified nature of its surface, the flora
of the forest region varies from a sub-tropical one in southern
Florida to one with a plentiful sprinkling of sub-arctic species
along portions of the northern border, particularly on the higher
mountains.

469. The plains region. This region rises with a gradual
ascent from the prairies (some of which occur from Ohio
westward and over great areas border the west bank of the
Mississippi), until an elevation of five thousand feet or more is
attained, when the plains reach the Rocky Mountain system.
There is no sharply defined line of demarcation between the
prairies of western Kansas, western Iowa, Minnesota, Nebraska,
and South Dakota, with less than 20 per cent of the surface
wooded, and the high plains, wholly treeless except along the
streams (Plate IX, upper figure), that flank the eastern border
of the Eocky Mountains. The lack of trees in the prairie and
plains region has been attributed to various causes, but the prin-
cipal ones are doubtless forest fires, the scanty rainfall, and the
occurrence in winter of severe drying winds, at a time when
the roots can draw no moisture from the frozen soil.

THE ROCKY MOUNTAIN REGION 493

The vegetation of the prairies consists primarily of a con-
siderable number of vigorous sod-forming grasses intermixed
with many other seed plants. Notable among these are several
species of the pea family, many golden-rods and asters, and
some larger Composites, such as sunflowers and rosinweeds
(Silphium). Especially striking is the display in late summer
and autumn of many showy Composites, such as the blazing
star (Liatris), the cone flower (Rudbeckia), and the tickseed
(Coreopsis).

The vegetation of the high treeless plains is, in the eastern
portion (Plate IX, lower figure), characterized mainly by the close
mats of the short, xerophytic buffalo grasses and grama grasses
of a grayish-green color. Among these grasses are scattered
prickly pear cactuses (Opuntia), milkweeds (Asclepias), and
thistles. After the drying up of the grasses in early July, there
is sometimes hardly any living vegetation left above ground
except that of the cactuses.

Toward the Rocky Mountains, as the soil becomes more alka-
line, various species of wormwood or sagebrush, and members
of the pigweed family (Chenopodiacece) become predominant.
The universal sagebrush (Artemisia tridentata) plainly shows
its xerophytic character by its deep-reaching roots, its reduced
leaf area, and its strongly hairy surface.

470. The Rocky Mountain region. The Rocky Mountain
region includes a very great- variety of plant formations, from
the heavily wooded mountain slopes and valleys to high sterile
plains which are almost deserts. Cone-bearing evergreen trees,
especially the true spruces, the "Douglas spruce" (Pseudotsuga),
and several pines, are very characteristic of the forests (Plate
XII). Great numbers of alpine species of herbs and shrubs are
found on the mountains at and above the timber line. In the
" alkali " regions, where the soil is too full of mineral salts to
permit ordinary plants to grow, many kinds of halophytes, such
as the salty sage (A triplex), the greasewood (Sarcobatus), Sali-
cornia, and Suceda, occur.

494 PLANT GEOGRAPHY

Most notable among the saline areas is the Great Basin, west
of the Great Salt Lake, a dreary region in general, destitute of
natural grass lands or trees, but with a scattered vegetation of
low gray or dull green shrubs and herbs. In the lower highly
alkaline valleys are found such halophytic species as those
above-named, while the drier valleys and foothills are somewhat
evenly covered with sagebrush.

In the South, cactuses, palms, and tree yuccas abound. Wher-
ever the soil is gravelly throughout the southern arid region, up
to an elevation of five thousand feet or somewhat more, the
creosote bush (Larrea tridentata) is often as exclusive in its
occupancy of the ground as the sagebrush is in the central
and northern parts of the Great Basin.

Here are some of the most notable arid regions of the United
States, such as the Mohave Desert, the Ealston Desert, and the
Colorado Desert of southern California. The intense dryness of
such areas may be understood from the fact that the average
rainfall of ten of these deserts is only five inches a year, and the
temperature in one of them (at Fort Yuma, Arizona) remains
for weeks as high as 118° during the day, with sometimes only
a little over one half inch of rain a year.

471. The Pacific slope. The Pacific coast region offers far
less marked contrasts between the summer and winter temper-
ature than are found along the Atlantic coast.

On the other hand, there is, in the southern portion of the
region, a sharply defined division of the year into a dry and a
rainy season. At San Diego the dry season begins with April
and lasts for seven months. The development of vegetation,
therefore, as in the northerly part of the plains region east of the
Rocky Mountains, is most rapid in spring and largely ceases
when the soil has become parched by the summer's heat.

The flora of the Pacific slope is best known by its extraordi-
nary coniferous evergreen trees. In the moss-carpeted woods of
the northern portion (bounded on the south by the forty-first par-
allel) are found the Port Orford cedar (Cupressus Lawsoniana),

PLATE XII. A coniferous forest in central Colorado, Douglas spruce

(Pseudotsuga mucronata)

After F. E. Clements

THE PACIFIC SLOPE 495

the red cedar, the tide-land spruce (Picea sitchensis), and the
hemlock spruce (Tsuga lieterophylla). In places there occur
dense thickets of hazel and inaple, or of shrubs of the heath
family.

In the southern portion of the Pacific slope (from the forty-
first to the thirty-fifth parallel) are found the well-known Cali-
fornia evergreen conifers, such as the sugar pine (P. Lambertiana)
of the coast, the yellow pine (P. ponder osa), and in the moun-
tains the smaller redwood (Sequoia sempervirens) and the giant
redwood (S. gigantea, Fig. 33), the largest and by far the most
monumental of trees.

Among the characteristic features of the California flora is
the abundance of xerophytic shrubs and small trees, many of
them broad-leaved (not coniferous) evergreens, forming the
chaparral thickets. Among these are members of the oak, the
rose, the sumach, the heath, the buckthorn, the composite family,
and many others.

In southern California, on account of the long dry season,
plants with large roots or rootstocks, and bulb-bearing plants,
many of them belonging to the lily family, are abundant.

In the deserts and on their borders are numerous cactuses
and other succulent forms. Among the most characteristic
desert plants are the Spanish bayonets, or Yuccas, some of
them tree-like in form and size.

CHAPTEE XXXIX
VARIATION, MUTATION, AND ORIGIN OF SPECIES

472. Variations of plants. One of the foundation principles
of scientific farming and gardening is that seeds will grow into
plants like those which produced them. Not only is it assumed
that grains of corn will grow into corn plants and beans into
bean plants, but also that any special variety of sweet corn will
produce its like, yellow-eyed beans their like, and so with mul-
titudes of familiar cases. Closer observation, however, shows
that no two of the hundreds or thousands of plants raised from
the seeds of a single parent plant will be exactly like each other
or the parent. Generally the variations are very slight, and most
of them fail to continue themselves in succeeding generations
so as to establish new varieties of plants.

473. Variations in one direction. While variation generally
goes on in all directions, so that one of a brood sprung from a
given parent will be smaller and another larger, one more and
another less hairy than the parent plant, and so on, it is not
uncommon to find what may be called definite variation, in
which the changes all lead toward a definite new type. The
behavior of lowland forms planted in alpine regions (Sec. 464)
is a good instance of the kind. It is well known, too, that seed
from northern localities when planted farther south will produce
earlier crops than can be obtained from southern seed. American
varieties of onion, after being grown for a series of years in Eng-
land, become habituated to the longer mild season there, and
when the seed is brought back to America the plants grown from
it fail to mature their bulbs before the coming of the frost.

Such facts as these seem to indicate that characteristics which
have been impressed upon the plant by external influences, such

496

IMPORTANCE OF ADAPTIVENESS IN PLANTS 497

as those of soil and climate, may be transmitted to its descend-
ants. If it be so, then the origination of new forms of plants
by the inheritance of such characteristics must be extremely
common.

474. Mutations of plants. Much attention has lately been
given to the occurrence among plants of seedlings which differ
in a marked way from the parents. It would involve too much
detail to describe the exact nature of the differences between
the seedlings of the evening primrose,1 which has been most
studied in this connection, and its offspring, but they are as
great as those between an apple tree and a pear tree. Such
abrupt and extensive changes are called mutations. A few of
the most important facts so far known in regard to mutation are :

1. New species 2 appear suddenly among the offspring of the
parent form.

2. The individuals of the new species constitute only a small
per cent of any given brood.

3. The new species reproduce themselves accurately, showing
no decided tendency to return to the parent form.

475. Importance of adaptiveness in plants. It may be in-
ferred from Chapters xxxi and xxxiv that a premium is set on all
changes in structure or habits which may enable plants to resist
their living enemies or to live amid partially adverse surround-
ings of soil or climate. It would take a volume to state, even in
a very simple way, the conclusions which naturalists have drawn
from this fact of a savage competition going on among living
things, and it will be enough to say here that the existing kinds
of plants to a great degree owe their structure and habits to the
operation of the struggle for existence, together with their response
~by means of variation to changes in the conditions ~by which they
are surrounded. How the struggle for existence has brought
about such far-reaching results will be briefly indicated in the
next section.

1 (Enothera lamarckiana.

2 For a definition of the term species, see Sec. 189.

498 VARIATION, MUTATION, AND ORIGIN OF SPECIES

476. Survival of the fittest. A change in the characteristics
of a species may have no effect on its ability to contend with
a hostile soil or climate, with parasitic plants or destructive
insects or other animal foes ; but often alterations in the struc-
ture or the habits of a plant may give it a considerable advan-
tage over its unchanged neighbors. For instance, a decided in-
crease in hairiness would tend to protect the plant from damage
by long droughts, and also (in countries where snails destroy
much vegetation) from having its leaves eaten. Nuts with
harder shells would escape being destroyed when the ordinary
ones would be cracked and eaten by wild animals. Eed berries
of the European holly are carried off by birds more extensively
than yellow ones, and thus the undigested seeds of the former
variety are more widely sown.

In meadows which are mown once a year, only those plants
can surely reproduce themselves by seed which ripen their seeds
either before or after the time when the grass is cut. Individ-
uals which can do this stand a vastly greater chance of perpetu-
ating themselves than do those which are cut down just before
their seeds have matured. For this reason certain kinds of
meadow-frequenting plants l have developed early-blooming and
late-blooming forms, which would probably never have become
abundant in regions where the grass was not mown.

Whatever the nature of the advantages given to one form or
set of forms over another in the competition which always goes
on under natural conditions, it results in what is sometimes
called survival of the fittest, and sometimes natural selection.

477. Have species arisen by variation or by mutation ? The
theory that species (and later genera and higher groups) arise
by slow degrees from the operation of natural selection acting
on the slight variations which constantly occur among animals
and plants was first fully set forth by Charles Darwin in 1858.2

1 Species of Gentiana, Euphrasia, and Rhinanthus.

2 Darwin's paper on this subject was the result of over twenty years of
study, and was read by him to accompany a paper containing similar views
which had been sent from the East Indies by Alfred Russel Wallace.

HOW SPECIES HAVE ARISEN 499

The theory that species spring suddenly from mutations was
advanced by Professor Hugo de Vries, of Amsterdam, Holland,
in a work on the mutation theory, published in 1901 and 1903.

Botanists at present are considerably divided on the question
of the origin of species, some believing that they are mainly
derived from the perpetuation and intensification of slight varia-
tions, while mutations are so infrequent as not to signify much
in this connection ; others, again, believe that mutations are
the source of species, and that variations can only give rise to
varieties. There seems to be no good reason for doubting that
both variation and mutation have been and are efficient in the
production of new species.

CHAPTER XL
PLANT BREEDING

478. Definition of plant breeding. The selection and mainte-
nance of the most desirable varieties of cultivated plants must,
to some extent, have occupied the attention of agriculturists
during all the thousands of years since farming began.1 From
the writings of Virgil and other Latin authors it is clear that
Roman farmers practiced careful selection of cereals for seed,
knowing that without this their crops would diminish. But it
is only within a short period that scientific principles have been
brought to bear on the process. In fact, it is stated that the
systematic improvement of races of cultivated plants began in
the middle of the nineteenth century. The intentional produc-
tion and perpetuation of new varieties is known as plant
breeding. It is based upon the methods outlined in Sees. 479
and 480.

479. Single selection and continued selection. New varie-
ties of plants, whether wild or cultivated, are constantly being
produced by ordinary variation and by mutation (Sees. 472-474).
In a single field, supposed to contain only one kind of wheat, a
trained botanist once found twenty-three well-marked varieties,
one of which became the parent of a sort that has remained famous
for over three quarters of a century. The plant breeder is con-
stantly on the watch for promising varieties, preserving all which
seem likely to be of use. While it is a slow, uncertain method
to await the appearance of variations in any desired direction,
and then to rely on the perpetuation of these, the large num-
ber of valuable new varieties thus secured warrants all growers

1 In China the cultivation of rice, wheat, two kinds of millet, and soy
beans dates back at least 4600 years.

500

PRODUCTION OF HYBRIDS 501

in being on the lookout for variations which promise new
values. The surer plan is to take seed from a considerable
number of parent individuals which possess the desired quality
in a high degree, raise plants from each of these, discard plants
of this second generation from all parents whose progeny does
not excel, and continue selecting from these superior stocks.
In this way many characteristics, such as abundant yield, hardi-
ness, early ripening, whiteness in the case of flour, increased
percentage of sugar in sugar beets, or improved size or flavor in
tomatoes may be secured in a few years of careful breeding.
This may be called the selection of good parent plants.

480. Production of hybrids. An important method of mak-
ing new varieties is by crossing, or hybridizing, — that is, by
pollinating the pistil of one species or variety with pollen from
another species or variety. The offspring of cross pollination is
known as a hybrid.

The process of crossing two species is comparatively easy. If
plum blossoms, for example, are to be hybridized, the operator
must gather enough of those from which pollen is to be col-
lected, brush or shake off the pollen, and, if necessary, keep it
in a cool place until needed.1 Most of the flower buds are
removed from the tree the flowers of which are to be pollinated,
and just before the opening of those buds which are left the
corolla, with its attached anthers, is cut away, as shown in
Fig. 375, and pollen applied to the stigmas with a camel's-hair
pencil or, better, with the finger tip. If fertilization results,
and plums with good seeds are produced, they must be planted,
and seedling trees grown from them. These might be allowed
to grow until they blossomed, but years of valuable time can
be saved by grafting the young seedlings upon other plum trees.
When blossoms of the hybrid form are secured, some of them
may be fertilized with pollen of either of the parent species,

1 Some kinds of pollen, as that of the pansy and the peony, are said to
remain good for weeks, and that of the date palm for more than a year ; but
in general, pollen should be used as soon as possible.

502

PLANT BREEDING

and others with pollen of different species of plum. All of the
seeds obtained from the various crosses should be planted, and the
seedlings which are produced by them should be examined, and
retained or destroyed according to their apparent value. To the
experienced plant breeder the appearance of the seedling trees,
long before they are old enough to blossom, indicates so much as

to the nature of their fruit that many
varieties can be discarded as soon
as the young plants have developed
well-grown shoots. The distinctive
work of hybridizing is to secure
parent plants better than any which
exist in the foundation species or
varieties. The work of choosing a
large number of the most promising
hybrid plants and of testing their

FIG. 375. A plum blossom pre- breeding power, so that only the
pared for hybridizing |)loodj go to speak? of tlie yery best

A, unopened blossom cut round may be retained, is the same as

just below the insertion of the , , . , , . . n

stamens, to remove the latter; breeding by selection mentioned
B, lengthwise section of a fully above. In the occasional hybrid

opened blossom, showing the , ., ,

level s at which the cutting plant, possibly one out of ten thou-
should be done sand, are combined the best in the

two parents, or possibly, as some believe, newly created char-
acters may arise.

481. Some results in breeding by selection. To give an
account of the results of selection as applied to cultivated plants
would be to write a history of the variations and improvements
in all our ornamental and useful plants under cultivation. In
this place it must suffice to give a very few illustrations of the
kind and amount of improvement brought about by such selec-
tion as is outlined in Sec. 479.

482. Selection among apples. Much of the improvement
in apples was brought about before the literature of plant breed-
ing began. It is not certainly known where the cultivated apple

SELECTION AMONG APPLES

503

originated, but an eatable variety probably occurred in prehis-
toric times throughout the territory extending from the Caspian
Sea nearly to Europe. Small forests of wild apples have been
described in modern times, growing near the southeast end of
the Black Sea.

The dwellers in pile-built houses in the lakes of northern
Italy, Savoy, and Switzerland, several thousands of years ago,
laid in stocks of apples cut and dried for whiter use. Some
of these apples appear to
have been cultivated, but
they were very small,—
inferior in size to any mod-
ern variety except some
crab apples. How great a
gain in the size of apples
has been brought about by
cultivation and selection
may be inferred from Fig.
376. This increase in bulk
is accompanied by a de-
crease in the number of
matured fruits in a cluster. Fie- 376- Mect "f cultivation upon the

size of apples

The Bismarck apple, with a, the wild Asiatic
crab apple (Pyrus baccata), and b, the Eu-
ropean wild apple (P. mains). All half nat-
ural size. — After Hodge

Originally several of the
flowers developed into ap-
ples, but in modern im-
proved varieties usually
all but one of the flowers fail, as is shown in the case of the
pear (Fig. 83).

Most of the varieties of apples in our present orchards are
descendants of seedlings sprung from trees introduced from
western Europe. In the Northwest, where only the hardiest
kinds can endure the severe climate, some of the most success-
ful sorts are importations from central Eussia, and others are
from seedlings of Eussian and the hardier American varieties, or
from hybrids produced. by accident or design.

504 PLANT BREEDING

483. Selection among beans. The common bean (Phaseolus
vulgaris) is of uncertain origin, but there is a good deal of evi-
dence to show that it came from western South America. Its
cultivation in Europe appears to have begun soon after the dis-
covery of America. As is well known, the number of varieties
in cultivation is very large, and in few plants is it easier than in
beans to produce new varieties by selection.

Bean breeding for the large seedsmen is a skilled industry.
It is said that a seedsman may even advertise a new kind of
bean under an attractive name before the variety has been pro-
duced, then order it of his bean grower, and in the course of
two or three years have seed ready for his customers. On the
farm of one large bean grower nearly 70 standard varieties are
raised for seed on a large scale, and some 200 sorts are being
tested to establish their value or to produce new kinds. All pos-
sible pains are taken by means of high cultivation to increase the
bearing qualities of the plants and also to encourage variation.
Every variety, whether a standard one or a novelty, is kept to
the desired type by the careful inspection of every plant, those
which fall short in any respect being carefully destroyed.

While new kinds are nowadays generally secured by scien-
tific plant breeding, sometimes valuable sorts are obtained from
chance seedlings, as in the case of a well-known dwarf Lima
bean which sprang from seeds gathered on a Virginia roadside
some time before 1885.

484. Selection among corn. Indian corn was cultivated by
the ancient Peruvians and the Mexicans. Its original home as
a wild plant was probably on or near the west coast of South
or Central America. Numerous rather permanent kinds which
" come true from the seed " (races), such as field corn, sweet
corn, and pop corn, have long been known, and some of these
races present many varieties.

Scientific corn breeding has been practiced for much less than
a generation, but the results already attained are of great
practical importance.

SELECTION AMONG CORN

505

Leaving out of account the very extensive use of the stems and
leaves of the corn plant for forage, and considering only the value
of the grain produced, corn breeding may be carried on to secure,
among other less important qualities, the following results :

1. A larger yield per acre.

2. A higher percentage of any one of the three principal constitu-
ents of the grain, — starch, proteids, and oil.

3. Early maturing, for growth in the more northerly states.

A B

FIG. 377. Kernels of corn with high and with low proteid contents

A, high proteids; B, low proteids; />, horny layer, consisting largely of proteids;
s, white starchy portion; e, embryo. — After University of Illinois Agricul-
tural Experiment Station, Bulletin No. 87

1. Yield. The corn crop of the United States is worth about
a billion dollars a year for the grain alone. On farms of the

FIG. 378. Kernels of corn with high and with low oil contents

A, Ai, cross and lengthwise section of high oil kernels; B, B\, sections of low
oil kernels; e, embryo. Most of the oil is contained in the embryo, so that
a large embryo means a large percentage of oil. — After University of Illi-
nois Agricultural Experiment Station, Bulletin No. 87

506 PLANT BREEDING

greatest producing state, Illinois, the average crop is hardly
thirty bushels per acre. The use of choice seed has been found
to increase the production from 10 to 20 per cent, and it is a
moderate estimate which assumes that the universal use of
improved seed would add 10 per cent to the total corn crop of
the country. This would add over $100,000,000 to the annual
receipts of our corn growers.

2. Improved quality. In every 100 pounds of ordinary shelled
corn there are, in round numbers, about

8 lb. embryo (of which 3 Ib. are oil) ;

13 lb. gluten, or proteids of the endosperm ;

64 lb. starch.

There is a demand for a limited amount of corn with a high
per cent of oil as a source of com oil. At the Illinois Agricul-
tural Experiment Station the attempt has been made to breed
varieties of corn with high and with low percentages of oil.
One variety was secured with nearly 7 per cent and another
with less than 2 per cent.

In the same way, that is by means of continued selection,
carried through many generations, varieties with much or little
starch can be obtained.

3. Early maturing. Corn was originally a tropical or sub-
tropical plant, requiring a long growing season. Quickly matur-
ing varieties had, however, been secured by the native races
even at the time of the discovery of America by Columbus. At
present there are varieties ranging all the way from the eighteen-
foot kinds that require a growing period of six months, to the
two- or three-foot kinds that mature in ninety days or less.

The most important problem that presents itself to the plant
breeder in this connection is that of increasing the yield per
acre for each of the agricultural regions where corn is produced,
whether in the North, where short-stalked, early-maturing kinds
are needed ; in the great corn belt ; or in the South, East, or
West, where varieties are needed which are bred to make the

SELECTION AMONG WHEAT 507

best yield of grain or of fodder, or of grain and fodder combined.
This plant is being especially modified for many agricultural
regions possessing distinctive soil and climatic conditions, and
is more easily adapted to locality than are most plants.

485. Selection among wheat. Wheat of many varieties has
been cultivated for thousands of years throughout a territory
ranging all the way from China to western Europe. The origi-
nal home of the plant is not known, but perhaps it was in
Mesopotamia, between the Tigris and Euphrates rivers. In
Europe systematic attempts to procure improved varieties of
wheat by selection date back well toward the beginning of the
nineteenth century. Some good varieties were originated in
our own country in the early sixties, but more wheat breeding
is now done in a single year in the Agricultural Experiment
Station of a great wheat-raising state, like Minnesota, than was
done in the whole United States prior to 1890.

It will give some idea of the extreme care with which wheat
breeding is now conducted to give the barest outline of the
mode of procedure in the Minnesota Station.

As a beginning, 10,000 good kernels of some desirable
variety of wheat, old or new, are carefully chosen. These
grains are planted 4 inches apart (or 5 inches for winter
wheat), one seed in a hill, and every plant receives a number.
About 95 per cent of the poorer plants are weeded out by
hand before harvesting the seed wheat, the heads of the re-
maining plants are cut off, and those of each plant are preserved
in an envelope. After drying, the heads are weighed, and those
of all but a few of the best-yielding plants are thrown away.

The second season there are sown in a separate plot in the
wheat-breeding nursery about a hundred seeds from each of
the plants chosen. Each of these hundred-groups (centgeners),
sprung from a single mother plant, is given a distinguishing
number. When the wheat is mature* the relative size and
strength of the plants in each plot are noted and recorded,
and by separately harvesting and weighing each little plot the

508 PLANT BREEDING

breeding power of each parent plant is measured in terms of
the average of its progeny. A select head is chosen from each
of several of the best plants in every plot, and the seed from
these is saved.

A third year and a fourth year hundred-group plots are sown
and managed as just described, and at the end of the period the
most promising varieties are taken to field trials. Here they
are tested, under ordinary farm conditions, in comparison with
the wheats commonly grown, and the best, if it stands severe
milling tests, is then propagated for distribution, under suitable
designating numbers, to wheat growers throughout the state.

The rate at which new varieties can be propagated may be
gathered from the history of one of the most famous new
wheats, "Minnesota No. 163," a variety bred by selection. This
sprang from a single grain planted in 1892. In 1893 the
product consisted of 75 plants; in 1894 a small field plot was
grown; in 1898 the crop amounted to some 300 bushels of
seed wiieat, which was distributed among about 50 farmers
throughout the state. It is estimated that in 15 years from
the time of planting the single original seed the entire wheat
crop of Minnesota, covering some 5,300,000 acres, might have
been made to consist of this variety, and that it does actually
cover millions of acres, adding about two dollars per acre to
the value of the crop.

It is not yet possible to state how much can be gained in
quality and quantity of wheat production by careful culture
and breeding. But it is interesting to note that in a good
wheat year (1895), when the average crop per acre on the Uni-
versity of Minnesota farm was 23 bushels, there were 4 im-
proved varieties which yielded over 40 bushels per acre. In
1896, when the average crop for the state was 14.2 bushels
per acre, out of 32 improved varieties on the University farm
there were 24 varieties which yielded 21 bushels per acre or
more, 2 of them yielding 33 bushels. That is, three quarters of
the varieties yielded at least 1^ times as much as ordinary

GENERAL RESULTS OF HYBRIDIZATION 509

wheat on other farms, and 2 varieties yielded about 21 times as
much. The yield was increased on the farm mentioned both
by good farm management and by breeding into the varieties
stronger power of yielding.

In 1902 one of the improved wheats, "Minnesota No. 169,"
was given an extended trial in various parts of Minnesota. It
yielded on the average 33 bushels per acre, or 18 per cent more
than the ordinary varieties. This variety probably now covers
half a million acres, in several states, and yields at least two
dollars per acre more value than the varieties (mainly the " blue
stem," its parent) which it is rapidly displacing over an area of
several million acres devoted to hard spring wheat. The impor-
tance of every increase in production is evident when one con-
siders the annual value of our wheat crop, from $250,000,000
to $500,000,000.

486. General results of hybridization. The relative impor-
tance of hybridization, and of continued selection alone as
means of securing valuable new varieties of cultivated plants,
is largely to be worked out in each class of plants. Plant
breeding as a science is too new to give material for answering
nearly all the questions that naturally arise in regard to how
varieties may be most rapidly improved. Hybridizing often
brings about great changes in the offspring, and there are
increased chances that some of the new forms will be more
valuable than any which could be discovered among the
foundation varieties. In the case of species perpetuated by
grafting, as of certain trees, and plants propagated by roots,
rootstocks, or tubers, as potatoes,1 it is very easy to secure pure-
bred stocks. In plants grown from seed, especially if the
species is more or less open-pollinated,2 there is always a most
important question as to how many generations must elapse
before the hybrid varieties can be selected " true to seed."

1 Varieties among these are called clonal varieties (from don, meaning a
cutting or scion).

2 That is, if the flowers are open to cross pollination.

510

PLANT BREEDING

Some of the most important results in variety making by
hybridization have recently been obtained in experiments on
the fruits of the rose family, particularly cherries, plums, and

FIG. 379. Five forms of leaf from hybrid blackberries, all grown from the
seed of one plant and showing extraordinary variations in the amount of
incision in the margins of the leaflets, forming a regular series from a to e

Modified after Burbank

apples, and the citrous fruits. In the case of cotton and wheat
much effective work is also being done.

The extraordinary successes of Luther Burbank in pro-
ducing new hybrid varieties of fruits and ornamental flowers

FIG. 380

a, a stoueless wild plum; 6, c, d, fruit of hybrids of a with the French prune.
All drawn to the same scale. — Modified after Burbank

have been widely discussed in the popular magazines. He has
bred some remarkable hybrids, such as those between the
strawberry and raspberry, the apple and blackberry, the petunia
and the tobacco plant. These are of little use, though of much
scientific interest. Others of his hybrids, especially the plums,
are of great commercial value. Many other investigators, whose

RESULTS OF HYBRIDIZING CITROUS FRUITS 511

results have not received popular notice, are working more
directly for useful hybrids, and a few * of these may be very
briefly summed up.

487. Results of hybridizing citrous fruits. In the plant-
breeding laboratory of the United States Department of Agri-
culture in 1896 and 1897 hybrids were made of the ordinary
sweet orange and the uneatable three-leaved orange (Citrus trifo-
liata). Three promising varieties of a new kind of fruit known
as citranges have thus been obtained. Two of these are likely
to serve as substitutes for
lemons, and the third may,
to some extent, take the
place of grape fruit. Their
main value lies in the fact
that they can be cultivated
from two hundred to four
hundred miles farther north
than ordinary citrous fruits.

Another interesting hy-
brid is that between the tan-
gerine and the grape fruit,
called the tangelo, which
shows a blending of the
characteristics of the par-

A * B

FIG. 381. The flower of the wheat plant

A, entire flower as seen at five in the morn-
ing, with the stamens protruding, the pistil
remaining inside ; 13, the anther enlarged,
showing escaping pollen ; C, the pistil en-
larged, showing the feathery stigmas. —

After University of Minnesota Agricultural
Experiment Station

ent species.

488. Results of hybridiz-
ing cotton. The cotton produced in the United States is roughly
classed as long staple and short staple. The fibers of the former
kind are about one and one-half times as long as those of the
latter. For many kinds of goods long staple cotton is indispen-
sable, and its price is from one and one-half times to nearly
twice as great as the price of short staple cotton. The short
staple sorts can be grown over a much larger territory than the
others, so that our annual production of long staple cotton is
only about one and one-half per cent of our total cotton crop.

512

PLANT BREEDING

Hybrids have been made between the very long-fibered fine sea-
island species and the ordinary upland species, and after six
generations of selection and careful cultivation some valuable
hybrid varieties seem to have been developed.

489. Results of hybridizing wheat. The flowers of wheat
are naturally self-pollinated, — that is, the stamens of each flower
commonly discharge their pollen upon the feathery stigma of
their own flower as soon as the pollen sacs open. This fact
makes hybridization much more effective in producing variation

FIG. 382. Variation in wheat, the hybrid offspring of hybrid parents

After figure redrawn from Transactions of the Highland and Agricultural
Society of Scotland

in wheat than in plants which are generally cross-pollinated ;
for in the case of wheat any kind of cross pollination, and
especially that between markedly different varieties, may be
said to give a sort of shock to the operation of reproduction,
and thus produce abundant variation. The details of the
process of artificial pollination need not be given. It is suc-
cessful in a large proportion of cases, and the offspring may be
of many types, as shown by Fig. 382. It is found that after
the fourth generation an occasional plant may be found which

RESULTS OF HYBRIDIZING WHEAT 513

yields well and will " come true to seed." More important
results may be expected in the future from hybridizing wheats
than any yet attained.1

1 The literature of plant breeding is extensive and rapidly increasing. An
excellent general account of the subject and full bibliography is contained
in Plant Breeding by L. H. Bailey, The Macmillan Company, New York and
London, 1906.

A valuable summary of the main topics of plant breeding is contained in
Bulletin No. 29, 1901, of the Division of Vegetable Physiology and Pathology
of the United States Department of Agriculture.

Much information is also given in Hugo de Vries, Species and Varieties :
their Origin by Mutation, Open Court Publishing Company, Chicago, 1905.

Other publications of the United States Department of Agriculture on
plant breeding are :

For corn, Farmer's Bulletin No. 229, 1905.

For wheat, Bureau of Plant Industry, Bulletin No. 78, 1905.

Division of Vegetable Physiology and Pathology, Bulletin No. 24, 1900.
The publications of most of the Agricultural Experiment Stations contain
much important material for the discussion of plant breeding. A few of these
are as follows :

For corn, University of Illinois Agricultural Experiment Station, Circular

No. 74, 1904 ; Bulletins Nos. 55, 82, 1902 ; 87, 1903 ; 100, 1905.
Ohio Agricultural Experiment Station, Bulletin No. 140, 1903.
Kansas Agricultural College, Bulletin No. 107, 1902.
Nebraska Agricultural Experiment Station, Bulletin No. 91, 1905.
For wheat, University of Minnesota Agricultural Experiment Station,

Bulletin No. 62, 1899.

Ohio Agricultural Experiment Station, Bulletin No. 165, 1905.
The authors wish to express their obligations to all the authorities above-
mentioned. They have also to thank Assistant Secretary Willet M. Hays,
of the Department of Agriculture, for his kindness in reading and copiously
annotating the present chapter.

CHAPTEK XLI
SOME USEFUL PLANTS AND PLANT PRODUCTS

490. Economic botany. The branch of the science which
treats of the uses of plants to man is called economic botany.
Since whole industries like agriculture, lumbering, paper making,
and a multitude of others are concerned with the utilization of
plants or parts of plants, the subject is a most extensive one and
can only be outlined in a general text-book of botany.

A partial classification of useful plant products may be sug-
gested, dividing them into

1. Food products for human use.

2. Medicinal plants and plant products.

3. Food products for domestic animals.

4. Plants used as fertilizers.

5. Plant products used in chemical and other manufactures, as tan-
ning, dyeing, etc.

6. Plant fibers and related products.

7. Timber.

8. Fuel.

9. Ornamental plants.

In general only those members of the classes above given
which are of considerable importance in our own country will
be mentioned in this chapter.

1. FOOD PRODUCTS FOR HUMAN USE

491. The grains form the most important part of our vege-
table food ; they are the fruits of the cereals, or food-producing
grasses, and for this and other reasons the grasses, which in
all number about 3500 species, are more useful to man than
any other family of plants. The principal genera of cereals are

514

THE GRAINS

515

wheat, oats, rye, barley, rice, and Indian corn. Most of the
cereals are grass-like herbs of moderate height, but corn varies
much in size, from some of the dwarf varieties of pop corn
not more than two feet high to the twenty-foot field corn of
the rich river bottoms of the Middle West. All the grains have
many varieties, but these are most familiar in the various sorts
of wheat — hard, soft, red, white, bearded, beardless, and so on
— and in the many qualities of grain of Indian corn (see
Chapter XL).

Wheat is the most important of the cereals, on account of
its palatableness, high food value, and ready digestibility. None

FIG. 383. A cornfield in Missouri
After Frye

of the other grains yield a flour which is as well adapted for
bread making as wheat flour. Eice is readily digestible, but is
inferior to most grains in the relative proportion of proteids to
other ingredients ; and oats, rye, barley, and Indian corn, as usu-
ally prepared, are somewhat difficult of digestion. Corn meal
when imperfectly cooked, and eaten to the exclusion of other
food, has often given rise in northern Italy to a much-dreaded
disease known as pellagra.

The United States is the leading wheat- and corn-raising
country, producing more than one fourth of the total world's
crop of the former grain and four fifths of the latter.

516 USEFUL PLANTS AND PLANT PRODUCTS

Any of the grains may be made to yield starch for food or
for laundry or manufacturing purposes, but the greater part of
that produced in this country is obtained from corn, which
contains about 60 per cent of it. Moldy or otherwise damaged
grain can be utilized to some extent in starch making.

Corn also contains in the embryo of the grain from 3 to 6
or more per cent of oil, which is now largely extracted for use
as food and for various manufacturing purposes.

492. Leguminous seeds. Several kinds of seeds of the pea
family (Leguminosce) — an immense family, comprising some 7000
species — are important articles of food. Beans, as every one
knows, are used as food in all stages, from the time when the
pods are half grown until the seeds are entirely ripe and dry.
In the latter condition, when properly cooked, they constitute
one of the cheapest and most concentrated forms of proteid food.

Peas, whether " green " or dry, have much the same nutritive
value as beans in the same stage of maturity. Various canned
products of beans and of peas are now prepared on a large scale.

Peanuts are the seeds of a leguminous plant which forces its
growing pods underground, where they remain during and after
the process of ripening. The domestic consumption of these seeds
is large, and they constitute a considerable article of export.

Several other kinds of leguminous seeds, such as broad bean?,
lentils, and chick peas, are extensively used as foods in parts of
Europe, but are not as yet important articles of diet in our own
country.

493. Other seeds. The remaining kinds of seeds which are
important as food are mostly known as nuts. Some of these
are really drupes, like the cocoanut and the walnut (Sec. 183,
Fig. 166, V), while others, like the Brazil nut and chestnuts, are
seeds.

From the palm family, which is of supreme importance
within the tropics, only one kind of so-called nut, the cocoa-
nut, is commonly in use among us. The abundant endosperm
of its seed is largely eaten raw and much used in cookery.

NUTS AND OTHER SEEDS

517

Three rather closely related families of trees — the walnuts,
the birches, and the beeches — furnish most of our edible nuts.
From the first come walnuts, butternuts, pecans, and hickory
nuts ; from the second, hazelnuts and filberts ; and from the
third, beechnuts and chestnuts.

The rose family furnishes almonds, which are technically
drupes, closely related to peaches (Fig. 164).

Brazil nuts are the seeds of lofty South American trees of a
tropical family allied to the mangroves and the myrtles.

FIG. 384. A grove of cocoa palms in the Philippines
After Frye

494. Chocolate, tea, and coffee. These familiar substances
are derived from plants of three different families, the first
two being somewhat nearly related tropical or sub-tropical
ones.

Chocolate consists of the ground or crushed seeds of the
cacao tree, a native of Mexico, now widely cultivated through-
out the tropics. Eemoval of a large part of the aromatic fat
known as cacao butter, which is considerably used in medicine,
leaves cocoa, which forms for some people a more digestible
beverage than chocolate.

518

USEFUL PLANTS AND PLANT PRODUCTS

Tea is made from the leaves of a shrub long cultivated in
China and Japan, and now also in India, Ceylon, and elsewhere.
Unlike chocolate, tea has no food value, but is a mild stimulant.
Coffee is made from the seeds of a small tree widely culti-
vated in hot countries and belonging to the madder family.

The seeds are pro-
duced in red ber-
ries, which are
thickly clustered
about the twigs of
the tree. Coffee
has only a trifling
food value, but is
a vigorous stimu-
lant, reenforcing

the action of the

heart.

495. Fruits
with fleshy pulp.

The kinds of fruit
with fleshy pulp,
some eaten raw
and others requir-
ing cooking, are
so numerous that
they can only be
FIG. 385. A flowering twig of the coffee tree mentioned under

Two thirds natural size, with fruit / and /* and seeds s, the families to
natural size. - After Wossidlo which theybelong.

From the palms are obtained dates, which are technically
berries with a very hard seed. In the arid portions of Africa
and northwestern Asia, where they grow, they are of the first
importance as food. Successful attempts are now in progress
to introduce the culture of the date palm into the desert regions
of the extreme southwestern United States.

PLATE XIII. A tropical forest in the Philippines, mainly palms
After F. W. Atkinson

FRUITS WITH FLESHY PULP

519

From the pineapple family our only edible fruit is the pine-
apple, largely cultivated in Florida and the West Indies.

The banana family is a very small one, but exceedingly
important, since it furnishes, in the shape of bananas, the prin-
cipal subsistence of multitudes of the inhabitants of the tropics.
The plant is herbaceous, but sometimes grows to the height of
forty feet, with enormous leaves. It is extraordinarily produc-
tive, so that a few square rods of good soil set with banana
plants will supply the fruit for an entire family.
Our importation of bananas is very large and
rapidly increasing, and what was once an arti-
cle of luxury or a curiosity is now the staple
fruit for the entire year in most of our mar-
kets. The principal supply comes from the
West Indies and Central America, but bananas
are somewhat cultivated in the extreme south-
ern portions of the United States.

The mulberry family supplies the breadfruit,

which constitutes the most important food of

i £ ^u • i i v £ ^ ^

great numbers of the inhabitants of the south

Pacific Islands. Our only fruits of this family

are the mulberry and the fig. Most of our figs one fourth nat-

are still imported, but their culture has recently ural ^ize- ~~ After

become a considerable industry in California,

since the variety which can be dried for shipment is now suc-

cessfully cultivated there.

Two closely related groups, the saxifrage family and the
rose family, furnish a large proportion of all our true berries,
and some edible fruits which are not berries. From the former
family are obtained currants and gooseberries. The rose family
consists of five sub-families. Of these the apple subdivision
furnishes quinces, pears, and apples ; the rose subdivision fur-
nishes strawberries, blackberries, and raspberries ; and the plum
subdivision furnishes plums, cherries, peaches, apricots, and
nectarines.

A cacao
d cut n to

show the seeds

520 USEFUL PLANTS AND PLANT PRODUCTS

The rue family contains a rather small number of trees and
shrubs, with only two common genera, the prickly ash and the
hop tree in temperate North America, and comprises, among
others, the orange sub-family. Under this is found the genus
Citrus, which embraces all the citrous fruits. The species and
varieties which are found in our markets may be classed as
oranges, grape fruit, and lemons.

Most of our oranges are now of American growth, coining
from California or Florida, and many of the very large fruited
species of Citrus from Polynesia, variously known as pomelo
and grape fruit, are raised in both these states, while some are
also imported from the West Indies. The best lemons are im-
ported from the Mediterranean coast, largely from Sicily.

The grape family numbers about 300 species of climbing
shrubs. Only the grape genus Vitis is a source of edible fruits,
— the berries so familiar as fresh grapes or raisins.

Of these there are two principal types, one comprising the
European (Malaga and other) varieties with solid pulp, found
also in such California varieties as the Tokay grape, all of which
are descended from one European species. The other type is
the one with soft pulp, readily separated from the skin, such
as the Catawba, Delaware, Isabella, and Concord varieties. These
have to some extent been introduced into Europe, but are
descendants of native American species. Grapes are consider-
ably cultivated in most of the states, but nowhere else so exten-
sively as in California, where they are raised for wine making, for
the manufacture of raisins, and for shipment in a fresh condition.

The heath family supplies berries of several species, such as
the familiar cranberries and the blueberries and huckleberries,
which are largely gathered for the market in several of the
northeastern states, particularly in Maine, and are somewhat
extensively canned.

From the olive family (mostly sub-tropical trees and shrubs)
are obtained olives, which constitute a table delicacy, while
the oil is a highly valuable food.

EDIBLE LEAVES AND SHOOTS 521

From the nightshade family, many of which are poisonous
plants, we get several large edible fruits (true berries, though
they are not popularly so called), — the ground cherry, or straw-
berry tomato (Physalis), the pepper (Capsicum),1 the egg plant,
and the tomato.

The gourd family furnishes all the melons, cucumbers,
squashes, and pumpkins.

496. Edible leaves and shoots. Only a few of the articles of
diet under this head have much commercial importance or form
a notable part of the subsistence of people in any portion of the
country.

From the lily family we get asparagus ; from the pigweed
family, spinach ; from the mustard family, water cress, cabbage,
cauliflower, and Brussels sprouts ; from the parsley family, celery ;
and from the Composite, lettuce and globe artichokes (Cynara).

497. Edible bulbs, rootstocks, tubers, and roots. As is else-
where explained (Sec. 66), reserve material is often stored in
underground portions of the plant body. The number of vege-
tables derived from these is not very large, but they constitute
a considerable part of the food of people, especially in temperate
and cold climates.

From the lily family onions are obtained, from the yam
family yams, from the pigweed family beets, from the mustard
family turnips and radishes, from the parsley family carrots
and parsnips, from the morning-glory family sweet potatoes,
from the nightshade family potatoes, and from the Composite
salsify and Jerusalem artichokes (Helianihus).

498. Starch and sugar from stems and roots. Sago is the
purified starchy pith of small palms, natives of Siam and of
some of the Malayan Islands. A portion of the supply also
comes from West Indian cycads (Sec. 346).

Tapioca is a starchy substance obtained from the grated roots
of plants of the spurge family (Euplwrbiacece), cultivated in
tropical America and the West Indies.

1 This is not a pulpy fruit.

522

USEFUL PLANTS AND PLANT PRODUCTS

Arrowroot is a very pure starchy food obtained from the
rootstocks of plants of two or three tropical families, espe-
cially the arrowroot family
(Marantacecc).

Sugar is largely manu-
factured in Europe and to
some extent in the United
States from the juice of the
sugar beet. The remainder
of the world's supply of
sugar comes from the stem
of the sugar cane, a grass
which grows to a height
of ten feet or more. It is
somewhat cultivated in
Louisiana, but much more
extensively in the West
Indies, Java, and the
Hawaiian Islands.

2. MEDICINAL PLANTS
AND PLANT PROD-
UCTS

499. The study of me-
dicinal plants is a special
subject, forming an im-
portant part of the course
in every college of phar-
macy. Only a few words can be given to the topic in this
chapter.

Very many of the families of angiosperms contain species

used in medicine.1 In some cases, as in the lily family, the

pea family (which furnishes sixteen remedies), the mint family,

1 In the United States Pharmacopoeia sixty-seven families are represented.

FIG. 387. Sugar cane (Saccharum)
Much reduced. — After Wossidlo

FOOD PRODUCTS FOR DOMESTIC ANIMALS 523

and the nightshade family, medical properties are quite gener-
ally distributed throughout the whole family or through certain
sections of it. In other cases, as in the poppy family (which
yields opium and morphia), the family Erytkroxylacece (which
yields cocaine), and the figwort family (which yields digitalis)
only one important remedy or group of remedies occurs. The
properties of many medicinal plants were discovered by acci-
dent in primitive times, while others have had their value
established only as a result of careful experiments on man and
the lower animals.

3. FOOD PRODUCTS FOR DOMESTIC ANIMALS

500. The most important herbivorous domestic animals —
cattle, horses, and sheep — consume'large quantities of the less
expensive grains, and in general the roots and tubers which are
useful for human food are readily eaten by these animals.

A large proportion of the grasses are utilized by grazing ani-
mals or fed as hay. Many plants of the pea family, particu-
larly alfalfa, the clovers, soy beans, and cow peas, are eaten by
domestic animals.

Both grasses and other plants are cut and fed to cattle and
horses, while fresh, as for age. Large quantities of " corn fodder "
are used in this way in many parts of the country, and the
stems and leaves of corn are also cut up, placed in large tanks
called silos, allowed to ferment, and then fed to cattle through-
out the winter.

Certain by-proc\ucts of manufacturing processes are of much
value for cattle food. Among the most important of these are
linseed meal and cotton-seed meal, which are. rich in proteids and
still retain some oil after the greater part of it has been ex-
tracted by the most powerful pressure available. The refuse
grains from breweries and the sloppy boiled corn meal from
distilleries are in a wet state extensively fed to cattle and
hogs, but are injurious if used alone. They are also dried for

524 USEFUL PLANTS AND PLANT PRODUCTS

shipment. The refuse from beet sugar manufacturing establish-
ments is used in a wet condition for cattle feeding, and is also
dried and shipped.

Some seeds not eaten by man are highly valuable when fed
to the lower animals. Acorns and beechnuts, for example, in
some of the wooded portions of the southern Middle States, fur-
nish a considerable part of the subsistence of droves of hogs.

4. PLANTS USED AS FERTILIZERS

501. For centuries the advantage of plowing under growing
crops as a means of enriching worn-out land has been well
recognized. It is only very recently that the exact significance
of this process has been understood. Even now the details are
not so fully worked out that we know just what crop will yield
the best results for every variety of soil and climate ; but in a
general way it is established that leguminous plants are the
best for this purpose on account of the power which their root
tubercles have of utilizing the nitrogen of the atmosphere
(Sec. 256). Various clovers and alfalfa are the crops most
commonly employed.

5. PLANT PRODUCTS USED IN MANUFACTURES

502. Under this head there is only space to mention a very
few of the vegetable substances used in manufacturing processes,
most of them on account of their chemical properties.

Dyeing by means of vegetable coloring matters is far less
important than it was before the introduction of the artificially
prepared aniline colors. These are so powerful that it is more
economical to use them, but they do not give soft shades. Val-
uable dyes, however, are still obtained from a considerable num-
ber of plants. Many of these belong to members of the pea
family, which furnishes Brazil wood (red), logwood (red, purple,
and black), camwood (red), indigo (dark blue).

PLANT PRODUCTS USED IN MANUFACTURES 525

From the buckthorn family are obtained yellow and green
dyestuffs, known, respectively, as Persian berries and Chinese
green indigo.

Varnishes of great value are yielded by trees of the pea family
(copal varnish) and of the sumach family (Japanese lacquer).

Tanning is largely
carried on by aid of
the bark of several
species of oak, of
which the black oak
and the Spanish oak
are two of the most
used American spe-
cies. Hemlock bark
and the leaves and
young twigs of Sicil-
ian and American
species of sumach
are also used for
tanning. Other sub-
stances employed for
the same purpose are
catechu, derived from
a species of acacia,
and gambier, derived
from the evaporated
sap of a tree of the ^IG- ^^* ^ twig °f ^ne South American rubber

tree (Hevea)

madder family, a na-
tive of the East Indies. After Schmidt

India rubber is manfactured from the sap of several tropical
trees and lianas. The principal, one of these is the Para rubber
tree (Hevea) of the spurge family.

Gutta-percha is produced by trees of the star apple family
(Sapotacece) of the Malay Archipelago, a family of much eco-
nomic importance.

526 USEFUL PLANTS AND PLANT PRODUCTS

6. PLANT FIBERS AND RELATED PRODUCTS

503. Fibrous materials for use in spinning into thread, cord-
age, and rope, also for braiding and weaving, are obtained from
many parts of the plant body. Some of the most useful of
these, such as flax and hemp, are derived from the hard bast,
others, as cotton, consist of plant hairs, and others still rep-
resent various structural elements of the plant.

A large proportion of the fibrous materials in general use
comes from monocotyledonous plants of several families.

FIG. 389. A Georgia cotton field
After Frye

Several sedges of the genus Cyperus furnish materials for
weaving, and East Indian and Chinese mattings are made from
species cultivated for the purpose.

The straw of various grains is employed for braiding into
baskets, mats, hats, and other articles. A coarse grass known
as esparto is largely exported from Spain and the North African
coast for use in paper making and for other purposes.

Many palms produce valuable fiber ; that of the husk of the
cocoanut is largely used for cordage, mats, brushes, and similar
articles.

PLANT FIBERS AND RELATED PRODUCTS 527

From material obtained from the very young leaves of
a somewhat palm-like plant (Carludovica) the well-known
Panama hats are woven.

Several plants of the lily family, especially the so-called New
Zealand flax and the century plant (Fig. 391), furnish fibers.

FIG. 300

A portion of a cotton plant in bloom with a ripe capsule or boll b and seed .s. All
slightly reduced. — After Wossidlo

From a member of the banana family, a native of the
Philippines, but cultivated also in India, is obtained the ex-
tremely valuable manila fiber, one grade of which is so fine as
to be woven into delicate shawls and similar fabrics, while
the coarser kinds are used in the manufacture of manila
rope.

Among dicotyledonous plants there are a considerable number
which serve as sources of commercial fibers.

528

USEFUL PLANTS AND PLANT PRODUCTS

To the mulberry family belong the paper mulberries, which
furnish bark from which the beautiful Japanese paper is made,
and the hemp plant, which is one of the chief rope- and cordage-
making materials.

To the nettle family belongs ramie, an eastern Asiatic
plant cultivated in Jamaica and the southern United States,
from which Chinese grass cloth and other fabrics are made.

Three closely related groups of plants — the linden family,
the mallow family, and the silk cotton family — yield many
fibrous or hair-like products of use for spinning and weaving,

or for mattress mak-
ing and similar pur-
poses. From the
bark of trees of the
first-named family is
obtained the Russian
bass, or bast, used
for making rough
mats, and the tropi-
cal product jute, used
to weave with silk,
and also for carpets,
mats, and coarse
bags. From the hairs
which clothe the seed of the cotton plant (the most important
member of the mallow family) all cotton goods are manufac-
tured. Cotton is largely cultivated in India, Egypt, and our own
country. It is an important crop in all of our Gulf states, and
in Georgia and South Carolina. The seed hairs of the tropical
silk cotton trees (Ceiba) are coming to be much used in pillows
and cushions as a substitute for feathers.

Most vegetable fibers, such as have been described in this
chapter, are useful for paper making, even after the rope or
woven fabrics made from them have been worn until they are
dropping to pieces. Large areas of forest, particularly of spruce

FIG. 391. Century plants (Agave)
After Frye

CONIFEROUS WOODS

529

and poplar growth, are now annually cut down to furnish paper
pulp. It has been recently proposed to utilize cotton stems for
paper pulp. Ten million or more tons of the raw material, worth
nearly a dollar a ton for this purpose, are now annually avail-
able in the cotton-growing states.

7. TIMBER

504. Coniferous woods. The wood of our cone-bearing trees
(mainly of the pine family) is generally known as soft wood,

y ¥.&>!>, &W

FIG. 392. Forest of hard or yellow pine (Pinus palustris] on southern
coastal plain of the United States

After Frye

and that of our broad-leaved, mostly deciduous trees is known
as hard wood. These terms are not quite correct, for the conif-
erous larches and yews furnish a harder wood than that of such
broad-leaved trees as willows, poplars, tulip trees, and buckeyes.
Out of the entire timber supply of the country more than
three quarters is at present furnished by the thirty-eight or more

530 USEFUL PLANTS AND PLANT PRODUCTS

species of cone-bearing trees, especially the pines, which grow
within our limits.

The wood of the white pine (Pinus strobus), remarkable for
its workableness and freedom from warping or cracking when
exposed to the weather, was for years the most important of all
our soft woods. Latterly, as the supply is becoming greatly
lessened, other kinds of pine, especially the long-leaf pine,
the loblolly pine of the southeastern states, and the bull pine
(P. ponderosa) of the Pacific and Rocky Mountain regions, are
to a considerable extent taking its place.

Among the other most widely used coniferous woods are two
species of true spruce (Picea), the "Douglas spruce" (Pseudotsuga),
two western species of white fir (Abies), the smaller California
redwood (Sequoia}, the American or bald cypress (Taxqdium),
and several distinct kinds of white cedar (Thuya, Chamcer
cyparis, and Libocedrus). The cypress, larch, and most of the
cedars furnish timber of great durability when exposed to the
weather or buried in the earth, and therefore are highly valued
for posts, telegraph poles, railroad ties, and similar uses.

505. Broad-leaved woods. Our native broad-leaved trees which
furnish wood for manufacturing or constructive purposes com-
prise about eighty species, a larger number than is found in
any other equal area of the temperate zones.

The principal hard-wood forests are of oak, though other valu-
able timber trees, such as maples, hickories, beeches, and elms,
are usually scattered among them. Our oak lumber is of three
kinds, — white, red, and live oak. White oak is much superior
to red for constructive purposes where strength is important, but
does not show so conspicuous a grain when polished for cabinet
work. More than half of our supply of hard woods comes from
various species of oak.

Next in importance is the wood of the tulip tree (Lirioden-
dron), generally known as yellow poplar, or whitewood. This
has largely taken the place of white pine in inside woodwork
for dwelling houses and other buildings.

BROAD-LEAVED WOODS 531

Among the most generally useful of the other broad-leaved
woods may be mentioned maple, elm, ash, and chestnut.

Several kinds are particularly valued for their durability in the
ground; among these are chestnut, black locust, and catalpa.

For cabinet work the most prized of our native woods are
black walnut, cherry, birch, and some species of oak. None of

FIG. 393. Hickory (hard wood) forest near southern end of
Appalachian highlands

After Frye

these is as beautiful as some of the finer imported kinds, such
as mahogany, rosewood, and satinwood.

506. Forestry. During the -time when the country was in
process of being settled most portions of the Atlantic coast
region, and inland as far as the prairies of what are now the
states of Illinois and Minnesota, were covered with primitive
forest. The most difficult task of the settler was to clear
enough land for tillage. The finest timber trees were destroyed
by hundreds of thousands by the process of girdling, that

532

USEFUL PLANTS AND PLANT PRODUCTS

is, by cutting away a ring of sapwood and allowing the trees
to die of starvation and lack of water. Cultivation was car-
ried on among them, until after some years the trees would

FIG. 394. A corn field in a " deadening1' or girdled forest of
deciduous trees

Modified after Ay res

fall and then were burned to rid the land entirely of their
presence. More than a century and a half of wholesale destruc-
tion of trees has finally resulted in stripping large areas of the
original forest and preventing the reforestation of the land, until

FORESTRY 533

a point has been reached when it is difficult to get lumber
of good quality for many of the most important purposes for
which it is used. In some parts of the country timber is a
profitable crop to raise, even if it has to be planted and cared
for while growing. The science and art of growing timber and
caring for tracts of wooded land is called forestry. Much atten-
tion has long been paid to it in the most enlightened countries
of Europe, but the subject is a comparatively new one in the
United States. The importance of maintaining a suitable pro-
portion of wooded land in any region does not depend merely
on the desirability of a supply of timber. The water supply of
lakes and streams, the retention of the cultivable layer of loam
on the earth's surface, the climate of any region, at least so far
as the prevention of severe winds is concerned, — all are depend-
ent on the presence of considerable forest areas.

The principles of forestry cannot be laid down in a few words,
and forest management requires years of study in the woods
themselves. The literature on the subject is extensive, and
courses in forestry are now given in a good many universities.
Evidently it is a topic of growing importance in this country.
A few useful rules can be given here.

1. Tree cutting should generally be managed on the prin-
ciple of selecting only mature trees and leaving the others to
grow up to replace those cut.

2. Forest fires should be prevented.

3. Destructive fungi should be exterminated wherever found.

4. Insect enemies of trees, such as the seventeen-year locust,
the various caterpillars, and boring insects, should be destroyed.

5. Sheep and cattle should never be pastured in woods where
they can do harm by killing young seedling trees or other
useful undergrowth.

6. Tree planting should be carried on whenever it can be
made to utilize lands not needed for other purposes, and the
species planted should be chosen with extreme care to meet the
requirements of the soil and climate.

534 USEFUL PLANTS AND PLANT PRODUCTS

8. FUEL

507. Nearly all fuel is of vegetable origin. In most civilized
countries to-day the principal fuel supply consists of various
kinds of coal, that is, of vegetable matter which has been buried
in the earth for ages and undergone many changes (Sec. 330).

Peat, the consolidated material left after the partial decay of
certain bog mosses (Sec. 292), in some countries forms a consid-
erable part of the available fuel, and the deposits in the north-
ern United States are of some importance.

Wood, in portions of the country, is still the principal fuel.
Certain varieties are preferred for household use on account of
their furnishing good beds of glowing coals, or for burning in
open fires on account of their freedom from any tendency to
snap. But in general the fuel value of thoroughly seasoned
wood is nearly proportional to its weight per cubic foot, that is
to say, the very heaviest woods, such as hickory, the white oaks,
black locust, and some kinds of ash, are worth most for heating.

Other parts of plants besides wood are used to some extent
for fuel. In large tanneries the spent bark is often compressed
to extract most of the water and then burned. Corncobs are
often burned in stoves and under steam boilers. In treeless
regions twisted ropes of straw are used as fuel.

9. ORNAMENTAL PLANTS

508. Our ornamental plants may be roughly classed into
shade trees, shrubs, herbaceous perennials, and annuals. The
total number of species and varieties cultivated in the United
States runs far into the thousands, but in many cases florists'
varieties are distinguished from one another only by color or
some other comparatively unimportant characteristic.

Most of our cultivated ornamental plants are of foreign origin,
and representatives of almost all parts of the earth except the
arctic regions are found among them. In a few instances native

ORNAMENTAL PLANTS 535

species are familiar occupants of our flower gardens, as, for
example, the native azaleas and Rhododendrons, the bee balm,
California poppy, evening primrose, Mariposa lily, Missouri
currant, purple flowering raspberry (Rubus odoratiis), cone
flower (Rudbeckia), snow on the mountains (Euphorbia), and
wild cucumber.

Some of the families which contribute most largely to our
lists of cultivated flowers are the lily, the amaryllis, the pink,
the crowfoot, the rose, the pea, the geranium, the heath, the
mint, and also the composite family.

APPENDIX

[Additional illustrations, chiefly for use with a flora in determination
of species']

I. LEAF FORMS

FIG. 1. General outline of leaves

a, linear ; 6, lanceolate ; c, wedge-shaped ; d, spatulate ; e, ovate ; /, obovate ;
g, kidney-shaped ; h, orbicular ; i, elliptical

537

538

APPENDIX

/ g hi

FIG. 2. Tips of leaves

a, acuminate or taper-pointed; b, acute; c, obtuse; d, truncate; e, retuse; /,
emarginate or notched; y (end leaflet), obcordate; h, cuspidate, — the point
sharp and rigid; i, mucronate, — the point merely a prolongation of the
midrib

FIG. 3

J, shapes of bases of leaves: 1, heart-shaped; 2, arrow-shaped; 3, halberd-shaped.
B, peltate leaf of troprcolum

APPENDIX

539

II. FORMS OF SYMPETALOUS COROLLA

FIG. 4

Bell-shaped corolla of
bell-flower (Cam-
panula)

FIG. f,

Salver-shaped corolla of
jasmine (magnified)

FIG. 0

Wheel-shaped
corolla of
potato

FIG. 7

Tubular corolla, from
head of bachelor's
button

FIG. 8

Labiate or ringent
corolla of dead
nettle

INDEX

References to illustrations are indicated by stars. App. indicates Appendix

Absorption of carbon dioxide, 107,
108

Absorption of water by roots, 28

Absorption, root, 30

Absorption, selective, 36

Acacia, leaf of, 98*

Accessory buds, 82*, 84*

Accessory fruits, 146, 149*, 150*

Actinomorphic, 125

Acuminate, App. I

Acute, App. I

Adaptations to conditions of exist-
ence, 496, 497

Adder's-tongue, 324*

Adventitious buds, 83, 87*

Adventitious roots, 19

^Ecidium, 201, 262*

Aerial roots, 19-21*

Agaricus, 267

Age of trees, 46

Aggregate fruits, 146, 149*, 150*

Ailanthus twig, 81*

Air chamber, 103*, 104*

Air passages, in Hippuris stem, 111*

Air plants, 47*

Air, relation to germination, 7

Akene, 147*

Albugo, 245*

Albuminous substances, 9

Alga-like fungi, 239-247*

Alga-like fungi, summary of, 247

Algae, 172, 173-226*

Algae, distribution of, on rocks, 215*

Algae, evolution of, 225, 226

Algae, life histories of, 221, 222

Alpine vegetation, 484-487*, 493

Alternate branching, 41*

Alternate leaves, 81*

Alternation of generations, 220, 278,
345-350

Alternation of generations, proto-
plastic basis of, 345-348

Amanita, 267

Amoeba, 158*, 159
Anabaena, 176, 177
Angiospermae, 376-388*
Angiosperm flower, 376-379*
Angiosperm, life history of, 388
Angiosperms, 128, 376-388*
Angiosperms, classification of, 397
Animal food, need of, 412
Animals and plants, distinctions

between, 168, 169

Animals, defenses against, 413-419*
Annual growth, definite, 42
Annual growth, indefinite, 42
Annual ring, 66*, 68*
Annuals, 46

Anther, 127*, 139, 140*, 378-380*
Anther, modes of opening, 140*
Antheridium, 189*, 204, 275*, 276
Antherozoid, 189, 204
Anthoceros, 288*, 289*
Anthocerotales, 288*, 289*
Antipodal cells, 144*, 383*
Antitoxins, 236
Ant plants, 414*, 415
Ants plant seeds, 446*
Apetalous, 123, 124*
Apogamy, 244, 319, 320
Apospory, 320

Apples, selection among, 503*
Aquatic plants, 459-462*, 488
Archegonium, 276*
Archichlamydeae, 897, 398
Arch of hypocotyl, 6*, 12*, 13
Arctic vegetation, 484
Arctic willow, 484*
Aristolochia stem, bundle of, 61*
Aristolochia stem, cross section of,

60*, 61*

Arrangement of leaves, 94-101*
Arrow-shaped, App. I
Artemisia, 480, 493, 494
Ascocarp, 248, 250
Ascomycetes, 248-257*

541

542

INDEX

Ascus, 248, 249*, 251*

Asexual generation, 278, 304, 342,

345-350

Ash tree, naturally grafted, 69*
Asparagus, 55*
Aspergillus, 250*
Aspidium, 308*, 314*
Assimilation, 107, 112, 115, 163
Associations, plant, 474, 475
Autumn leaves, coloration of, 121,

122

Auxospore, 196*
Axillary bud, 81*, 82*
Axillary inflorescence, 132*

Bacteria, 228-237*, 238

Bailey, L. H., 448

Barberry, spiny leaves of, 416*

Bark, 59, 60*, 65-67*, 71, 72, 366*,

368

Bark cells, 25*
Basidia fungi, 258-271*
Basidiomycetes, 258-271*
Basidium, 258-267*
Bast, 312*, 367
Bast bundle, 25*, 62*
Bast, soft (sieve tubes), 61-63*
Batrachospermum, 217
Beans, selection among, 504
Bees, 422*, 423, 426*
Beggar's ticks, 444*
Bell-shaped, App. II
Belt's bodies, 414*, 415
Berry, 148*, 150*
Biennial, 26, 46
Bilaterally symmetrical flowers,

124*, 125*

Bilaterally symmetrical leaves, 88
Birch, branching of, 46*
Bird pollination, 428
Bisexual, 123*
Bitter roots, 25
Bitter seeds, 11 *
Black knot, 252*
Bladderwort, 437*
Blights, 244-247*
Blister blight, 245*
Blue-green algse, 174-178*
Blue-green algse, life habits of, 177,

178

Blue-green algse, summary of, 178
Bog zonation, 476-478*
Boletus, 265*, 267
Botany, definition of, 1
Botany, economic, 500-535*

Botany, economic, definition of, 3

Botany, systematic, definition of, 2

Hotrychium, 324*

Botrydium, 200*

Box elder, buds of, 82*

Box elder, radial and cross sections

of stem of, 61*
Brace roots, 21*
Bract, 133*, 138*
Branched leaves, 91-93*
Branches formed from adventitious

buds, 87

Branching, alternate, 41*, 94*
Branching and leaf arrangement,

41-45*

Branching, opposite, 41*
Branch thorn, 44*
Breeding, plant, 500-513*
Brown algae, 205-213*
Brown algse, life habits of, 205, 206
Brown algse, summary of, 213
Bryales, 293-301*
Bryophyta, 275-305*
Bryophytes, 275-305*
Bryophytes, evolution of, 302
Bryophytes, origin of, 302
Bryophytes, summary of, 303, 304
Buckeye, bud of, 80*
Budding, 238*
Buds, 80-87*

Buds, adventitious, 83. 87*
Buds, dormant, 87
Buds, naked, 80, 81*
Buds, position of, 82*. 83, 87*
Buds, structure of, 80-8(5*
Bud-scale scar, 40*
Bud scales, 80*, 83
Bulb, 53*

Bulb, hyacinth, 53*
Bulblets, 438

Bulrush, cross section of stem of, 58*
Burbank, Luther, 510
Burs, 442-444*

Buttercup, leaf of, 91*. 104-106*
Butternut, buds of, 82*
Buttons, 266*

Cactus, 54*, 463*

Caladium, 52*

Calamites, 339, 340, Plate VIII

Calcium, 28, 29

Calyptra, 277, 295, 296*, 301*

Calyx, 123*, 124*, 126*

Cambium, 25*, 60-67*, 366*, 367

Cambium ring, 65, 67*, 71, 72, 367

INDEX

543

Camptosorus, 310*

Canal cells, 276*, 277

Canna, 381*

Canna, parallel veining in, 89*

Cap (pileus) of gill fungi, 266*

Capsule, 146, 150*

Carbon, 28, 29

Carbon dioxide, 7, 107, 108, 113, 115

Carbon dioxide, absorption of, 107,
108, 342

Carboniferous Age, 339-341

Carnivorous plants, 409-412*

Carpel, 127, 358, 369-371*, 380*, 381

Carpogonium, 217*

Carpospore, 217*

Carrion fungi, 269

Castor bean, germination of, 6*

Castor-oil plant, early history of
stem, 65*

Castor-oil plant, fibro-vascular bun-
dle of, 65*

Caterpillar and grub fungi, 253*

Catharinea, 293*

Catkin, 134*

Cedar apples, 264

Cedar, red, 446*

Cell, 34-39*, 129, 156-167*

Cell contents, 34, 35*, 160*, 161*

Cell division, 34, 159, 164, 165*

Cell growth, 164

Cell reproduction, 164

Cell sap, 38*, 160

Cell structure of moss leaf, 161*

Cell structure of pond scum, 160*

Cell theory of organization, 165-167

Cell turgor, 161 .

Cell wall, 34, 159, 160*, 161*

Cells, starch in, 9*

Cellulose, 34, 38, 159

Central cylinder, 24*

Central placenta, 129*, 130

Cetraria, 256*, 257

Chsetophoraceae, 190*, 191

Chara, 202*

Charales, 201, 202*

Cherry, buds in axils of leaves, 81*

Cherry twig, 84*

Chlamydomonas, 182*

Chlorophycese, 179-204*

Chlorophyll, 105, 109, 160

Chlorophyll bodies, 105, 106*

Chloroplast, 105, 106*, 107, 160,
161*

Choripetalous, 126

Chorisepalous, 126

Chromatin, 164, 165*

Chromatophore, 160*

Chromatophore, fission of, 161*, 165

Chromosomes, 165*

Cilium, 170*

Circle (whorl), 124*, 126*

Circulation of nitrogen, 231*

Circulation of protoplasm, 201

Citrous fruits, hybridizing of, 511

Cladonia, 257*

Cladophora, 191*

Cladophyll, 56*

Class, 153

Classification, 152

Clathrocystis, 174*, 175

Clavaria, 264*

Claviceps, 252, 253*

Cleistogamous flowers, 432, 433*

Clerodendron, 429*

Climbing plants, 20*, 47-50*, 92*

Climbing shrubs, stem structure, 60*,

Climbing stems, 20*, 47-50*
Closed veining, 89*
Clover leaf, 98*
Club moss, 329-339*
Cluster cup, 261, 262*
Clustered roots, 23*
Coal, 341
Coalescence, 388
Cocklebur, 442, 443*, 444*
Ccelosphgerium, 175
Ccenocyte, 197, 200, 201, 240
Coenogamete, 242
Coiling, 48*, 49*
Coleochsete, 191, 192*
Collective fruits, 146, 150
Collenchyma, 64*
Colocasia, 52*
Colorado coniferous forest, Plate

XII
Coloration of autumn leaves, 121,

122

Colors of flowers, 424
Columella, 241, 242*
Common ferns, 311-320*
Common mosses, 293-301*
Common receptacle, 134, 135*
Comparative sections of fruit, 150*
Compass plant, nearly vertical leaves

of, 100*

Competition, 447-451*
Composite, 135*, 400
Composite head, 134, 135*
Compound cyme, 137*

544

INDEX

Compound leaves, 91, 92*

Compound pistil, 128

Compound umbel, 135*

Couceptacle, 212

Condensed stems, 54*, 56

Cone, 326*, 327

Confer vales, 184-192*

Confervas, 184-192*

Conidia, 245

Coniferales, 364-375*

Coniferous forest, Plate XII

Conifers, 364-375*

Conjugates, 193*, 194*

Continuity of protoplasm, 99, 100

Coral fungus, 264*

Cordaitege, 340, Plate VIII

Cordyceps, 253*

Core, 148

Cork, 71

Corn, aerial roots of, 21*

Corn, cross section of stem of, 58*

Corn, grain of, 7*, 505*

Corn, section of root tip of, 24*

Corn, selection among, 504-507*

Corn stem, structure of, 57, 58*

Cornus canadensis, Frontispiece

Corolla, 123*, 126*

Cortex, 65*

Cortex of root, 24*

Corymb, 133

Cotton, 526*, 527*

Cotton, hybridizing of, 511, 512

Cotyledon, 5-14*, 18*, 370*, 374

Cotyledon, disposition made of, 14

Cover (operculum), 299, 300*

Crossing, 501

Cross pollination, 420, 501, 512

Crowberry, rolled-up leaf of, 466*

Cryptogams, 354

Cup fungi, 250, 251*

Cup (volva) of gill fungi, 266, 267

Cup (cupule) of Marchantia, 282*

Cuspidate, App. I

Cuticle, 9*

Cuticle, unequal development of, by

epidermis cells, 117*
Cutin, 117
Cutting leaves, 418*
Cyanophycese, 174-178*
Cycadales, 360-363*
Cycads, 360-363*
Cycas, 361*
Cyme, 136, 137*
Cypress, 111, Plate III
Cystocarp, 216, 217*, 219*

Dahlia, thickened roots of, 23*

Daily movements of leaves, 97, 98*

Damping off, 247

Dandelion, 50*

Darwin, Charles, 154, 420, 448

Date palms, 57*, 518

Datura, 11, 419, 443*

Decay, 230, 231

Deciduous, 121

Defenses against animals, 413-419*

Definite annual growth, 42

Dehiscing, 146*, 147

Descent of water, 75*, 76

Desert, Sahara, 482*

Deserts of United States, 493-495

Desmids, 193*, 194

Determinate inflorescence, 136*

Deutzia leaves, 96*, 97*

De Vries, 499, 513

Diadelphous, 127, 128*

Diagrams, floral, 131*

Diastase, 10

Diatomales, 195, 196*

Diatoms, 195, 196*

Dichogamy, 429*

Dicksonia, 309, Plate VII

Dicotyledonese, 398-400

Dicotyledonous plants, 18, 397-400

Dicotyledonous stem, annual, gross
structure of, 59, 60*

Dicotyledonous stem, cross section
of, 60-67*

Dicotyledonous stem, minute struc-
ture of, 60-66*

Dicotyledonous stem, rise of water
in, 73-75*

Dicotyledons, 398-400

Dimorphous flowers, 432*

Dioecious, 124*

Dionsea, 412*

Disk flowers, 134, 135*

Dispersal of seeds, 439-446*

Dispersal of seed plants, 436-446*

Distinct, 123*, 126, 127, 129*, 130*

Diurnal position, 97, 98*

Divided leaves, 90, 91*

Division, 153

Dodder, 22*, 23

Dormant buds, 87

Double fertilization, 383, 384*

Double flowers, 139

Downy mildew, 244, 247

Draparnaldia, 190*

Drip leaves, 462*

Drosera, 410*, 411*

INDEX

545

Drought, endurance of, 464
Drought plants, 459
Drupe, 148*, 149, 150*
Dry fruits, 140-148*
Duckweed, 462*
Duct, 25*
Dulse, 214

Earth star, 268, 269*
Ecological groups, 459-473*
Ecology, plant, definition of, 2, 3,

407

Economic botany, 500-535*
Economic botany, definition of, 3
Ectocarpus, 206*, 207
Egg apparatus, 383*
Egg cell, 165, 166, 183*, 189*, 204,

212

Egg, osmosis in, 37*
Elater, 285, 287*
Elder, pollen grain of, 382*
Elliptical, App. I
Elm, 43*
Elm bud, 85*
Emarginate, App. I
Embryo, 5-12*
Embryo sac, 144*, 356, 373, 382,

383*, 384*
Endocarp, 150*
Endosperm, 5-9*, 362*, 372*, 373,

384, 385* .
Energy, source of, in plants, 7, 110,

111

Enzymes, 10, 232
Epidermis, 24*, 25*, 60*, 61*, 64*,

103-105*

Epidermis, uses of, 117*, 118
Epigynous, 130*
Epigyny, 395
Epiphytes, 19*, 47*, 309, 310, 471-

473*

Equisetales, 325-329*
Equisetinese, 325-329*
Equisetum, 325-329*
Ergot, 253*

Eubasidiomycetes, 258, 264-269*
Euglena, 170*, 171
Euphorbia splendens, 417*
Evergreen, 121
Evolution, 153, 156
Evolution of algse, 225, 226
Evolution of sex, 223, 224
Excretion of water, 116-120
Existence, struggle for, 447-451, 497
Exocarp, 149, 150*

Exogenous, 65

Explosive fruits, 439

External character of dicotyledonous

stem, 59
External character of monocotyled-

onous stem, 57*
Eyes of potato, 52*, 53

Fall of the leaf, 93*, 120-122
Family, 153

Fermentation, 10, 11, 231, 232
Fern, life history of, 319
Ferns, 309-324*
Fertilization, 138-145*, 166
Fibrous roots, 23*
Fibrous-vascular bundles, 24, 58*,

59-61*, 65-67*, 103*, 105*, 106*
Fibro-vascular bundles of ferns, 311,

312*

Ficus elastica, 80, 465*
Ficus religiosa, drip leaf of, 462*
Filament, 127*
Filicales, 311-320*
Filicinese, 309-324*
Fission plants, 178
Fittest, survival of the, 498
Fixation of carbon, 107
Flagellates, 170*

Flax, cross section of stem of, 62*
Fleshy fruits, uses of, 444, 445, 518-

520

Fleshy roots, 23*
Floating seeds, 441, 442
Floral diagrams, 131*
Floral envelopes, 123*
Floral organs, 138-141*
Flower, 123-145*, 358
Flower, bud scar, 45*, 46*
Flower buds, position of, 45*, 46*,

83*, 84*

Flower, development of, 387*, 388
Flower, evolution of, 393-396
Flower, nature of, 138-140*
Flower, organs of, 123*
Flower, plan of, 123*, 130, 131*
Flower, symmetry of, 125*, 395
Flowers, ecology of, 420-435*
Flytrap, Venus, 412*
Follicle, 146*, 147
Food cycle, 163, 164
Food in embryo, 9-11*
Food products. See Plant products
Food, storage of, in root, 23*, 25, 26*
Food, storage of, in stem, 52-54*,

78, 79

546

INDEX

Forest map, 491
Forest region, 490-492
Formations, plant, 474-480*
Formative tissue, 64*, 65*, 67*, 366*.

See growing point
Fossil plants, 339-341, Plate VIII
Foxglove, leaf of, 90*
Free central placentation, 129*, 130
Frond, 308*, 309
Frost, action of, 121
Fruit, 146-150*
Fruit bud, 83*, 84*
Fucus, 210-212*, Plate IV
Funaria, 294*, 296*, 298*, 299*, 301*
Fungi, 172, 227-273*
Fungi, life histories of, 272, 273
Fungi, origin and evolution of, 274
Fusion of parts, 395

Gametangium, 204

Gamete, 166, 182, 185*, 186, 204

Gametophyte, 220

Gametophyte, degeneration of, 404-
406

Gamopetalous, 126

Gamosepalous, 126

Gastromycetes, 268*, 269*

Geaster, 268, 269*

Generations, alternation of, 220, 278,
345-350

Generative cells in pollen tube, 142*,
363, 372*, 373, 382*

Genus, 153

Geography, plant, definition of, 2

Geography, plant, of the United
States, 489-495

Geotropism, 31, 32*, 39, 44

Germ diseases, 235, 236

Germination, 6-14*

Germination, chemical changes dur-
ing, 6, 7

Gill fungi, 265-267*

Gills, 266, 267*

Ginkgo, motile sperms of, 363

Gloeocapsa, 174*

Glceotrichia, 176*, 177

Glucose, 108

Grafting, 68*, 69*

Grain, 7*, 147, 505*, 514-516

Grape sugar, 79, 108

Grapevine blight, 247 •

Grasses, 398, 514, 515

Grass pistil, 421*

Gravity, 31, 39, 44

Gray, Asa, 46

Green algae, 179-204*

Green algae, reproductive organs of,

204

Green algae, summary of, 203
Green felt, 197, 200*
Green layer of bark, 71, 72
Ground tissue, 313
Growing point, 24*, 280*, 316*
Growth, measurement of, in stem, 17*
Growth, secondary, 65-67*
Guard cells, 104*
Gulf weed, 212*, 213
Gymnosperm, life history of, 375
Gymnospermae, 359-375*
Gymnosperms, 128, 359-375*

Hsematococcus, 181

Hairs, 417, 418

Hairs on leaves, 119

Hairs, root, 15, 16*, 24, 25*, 27*, 28

Hairs, stinging, 418*

Halberd-shaped, App. I

Half-inferior ovary, 130*

Half parasites, 408

Halophytes, 459, 468*, 469, 483*

Hard bast, 25*, 60-66*

Haustoria, 22*, 248

Head, 134*

Heart-shaped, App. I

Heart wood, 71, 72

Helianthemum, 487*

Heliotropic movements', 101*

Heliotropism, 101*

Hepaticse, 279-289*

Herbs, 46

Heterocysts, 176*

Heterogamy, 190, 204, 212, 224

Heterospory, 320, 322, 324, 351-353

High mallow, provisions for cross

pollination of, 429*
Hilum, 5*
Hoinology, 151
Honeybee, leg of, 422*, 423
Honey gland, 423
Honey locust thorn, 44*
Hop, twining of, 49*
Horse-chestnut, leaf arrangement of,

95*

Horsetails, 325-329*
Host, 22*, 23, 407
Hot springs, plants in, 178
Hyacinth, bulb of, 53*
Hybrid, 501

Hybrid blackberries, leaves of, 510*
Hybrid plums, 510*

INDEX

547

Hybridizing, 501, 502*, 509-513*
Hybrids, production of, 501, 502*,

509-513*
Hydnum, 265*
Hydrogen, 28, 29
Hydrophytes, 459-462*, 482, 488
Hydropterales, 320
Hymenium, 264, 267*
Hymenomycetes, 264-267*
Hypha, 240

Hypocotyl, 5*, 6*, 12*, 16
Hypocotyl, cross section of, 65*
Hypogynous, 130*
Hypogyny, 395

Iceland moss, 256*, 257
Imperfect fungi, 254
Indefinite annual growth, 42
Indehiscent fruits, 147-149*
Indeterminate inflorescence, 132*,

133

Indian corn, kernel of, 7*
Indian corn, root lip of, 24*
Indian corn, structure of stem of,

58*

India-rubber plant, leaf of, 465*
Indusium, 308*, 313
Inferior ovary, 130*, 135*, 150*
Inflorescence, 132-137*
Inflorescence, determinate, 136, 137*
Inflorescence, diagrams of, 136*
Inflorescence, indeterminate, 132*,

133

Insectivorous plants, 409-412*
Insect pollination, 422-432*
Insects, pollen-carrying apparatus of,

422*, 423
Insects, sense of smell of, 423,

424

Insects, vision of, 424
Insect traps, leaves as, 409-412*
Insertion of floral organs, 130*
Integuments, 356, 362*, 370*, 371,

"381, 383*, 386*
Intercellular spaces, 103*, 111*
Internal structure of dicotyledonous

stem, 59-62*
Internal structure of monocotyledo-

nous stem, 57, 58*
Internode, 16, 17*
Invasion, 451, 452
Involucre, 134, 135*
Iris, rootstock of, 51*
Irish moss, 214*, 216
Iron, 28, 29

Irritability in plants, nature and oc-
currence of, 35, 36, 49, 97-99*
Irritability in roots, 39
Irritability of protoplasm, 35, 36
Irritability of tendrils, 48*, 49
Isoetes, 338*, 339*
Isogamy, 186, 204, 224
Ivy, aerial roots of, 20*

Jungermanniales, 285-287*

Keel, 125*

Kelps, 207*, 208*, 209*
Kidney-shaped, App. I
Knees, 111, Plate III
Knot and wart fungi, 252*
Knots, 67, 68*
Krakatoa, 456

Labiate, App. II
Lachnea, 251*
Lamina, 88
Laminaria, 207*
Lanceolate, App. I
Lateral buds, 40*, 46*, 82*
Leaf, 15-18*, 88-122*
Leaf, accumulation of mineral mat-
ter in, 120

Leaf arrangement, 94-101*
Leaf bases, 52*, App. I
Leaf blade, 88

Leaf buds, 40, 41, 45*, 46*, 83*, 84*
Leaf, fall of, 120-122
Leaf forms, 90
Leaflet, 85, 86*

Leaf, member of plant body, 88
Leaf mosaics, 95*, 96, 97*
Leaf outlines, App. I
Leaf scars, 52*, 81*
Leaf sections, 102, 103*, 465*, 466*
Leaf spine, 416*, 417*
Leafstalk, 88
Leaf tendril, 92*
Leaf tips, App. I
Leaf traces, 105*, 106
Leafy liverworts, 285-287*
Leaves as insect traps, 409-412*
Leaves, compound, 91-93*
Leaves, cutting, 418*
Leaves, divided, 90, 91*
Leaves, functions of, 102-122*
Leaves, movements of, 97-101*
Leaves of xerophytes, 465*, 466*
Leaves, simple, 88-91*
Leaves, structure of, 102-122*

548

INDEX

Legume, 147, 443*

Lenticels, 71

Lepidodendron, 340, Plate VIII

Leucoplasts, 116

Lianas, 47*, 48

Lichen, 254-257, Plate V

Lichens, nature of, 255

Life history, chromosome count in,

348

Light, exposure to, 39
Light, movements away from, 39, 101
Light, movements towards, 101
Lignification, 367
Lignin, 114
Ligule, 338*
Lilac mildew, 249*, 250
Lily, 377*, 378*, 380*, 383*, 384*,

385*
Lily, pollen grains producing tubes

on stigma, 142*
Lime, 120

Linden, fruit cluster of, 439*
Linden wood, structure of, 66*
Linear, App. I
Linnaeus, 153
Liverworts, 275, 279-289*
Living matter, properties peculiar

to, 167

Living parts of the stem, 71, 72
Lobe, 126*
Locules, 129*, 148*
Locust, pinnately compound leaf of,

92*

Locust, thorn stipules of, 417*
Lycoperdon, 268*
Lycopodinese, 329-339*
Lycopodium, 330-332*

Macrocystis, 208*

Magnesium, 28, 29

Magnolia, forking of, 42, 45*, 46*

Mallows, pollination in, 429*, 430*

Malt, 10, 11

Maltose, 79

Mangrove, 468*

Maple fruit, 147*

Marchantia, 280-285*

Marchantiales, 280-285*

Marestail, air passages of, 111*

Marsilia, 321-323*

Mat plants, 51

Mechanics of stem, 58*, 59, 60*, 62*,

312*

Medullary ray, 60*, 66, 71, 72, 367
Megasporangium, 334

Megaspore, 322
Megasporophyll, 334, 369
Melon, leaf of, 89*
Mesembryanthemum, 465*
Mesophyll, 103*, 105
Mesophytes, 459, 467, 468
Mesquite, root system of, 27
Metabolism, i 14, 115
Metabolism, digestive, 114
Metachlamydese, 397, 399, 400
Micropyle, 6*, 142, 144*, 361, 362*,

381, 383*

Microsphsera, 249*, 250
Microsporangium, 334
Microspore, 322
Microsporophyll, 334, 369
Midrib, 89*
Mildews, 248-250*
Mildews, green and yellow, 250*
Mineral matter accumulated in the

leaf, 120
Mistletoe, 408
Mixed buds, 83
Modified leaves, 80, 81*
Moisture favors root growth, 39
Molds, 239-242*
Monadelphous, 127, 128*
Monoblepharis, 247
Monocotyledoneee, 397, 398
Monocotyledon ous plants, 18
Monocotyledonous stems, 57-59*
Monocotyledorious stems, growth of,

in thickness, 59
Monocotyledonous stems, rise of

water in, 75*
Monocotyledons, 397, 398
Monoscious, 124*
Moon wort, 324*
Morchella, 251, 252*
Morel, 251, 252*
Morphology, 1, 151
Mosaics, leaf, 95*, 96, 97*
Moss, life history of, 294-297*
Mosses, 275, 289-301*
Moths, 427*
Mougeotia, 194

Movement of water in plants, 73-78*
Movements of leaves, 97-101*
Mucor, 243*
Mucorales, 239-242*
Mucronate, App. I
Mulberry, 149*
Multiple fruits, 146, 149*, 150
Musci, 275, 289-301*
Mushroom, 265, 267*

INDEX

549

Mutations, 497-499

Mutilated seedlings, growth of, 8*

Mycelium, 240*

Mycorrhiza, 269, 270*

Myrsiphyllum, 56*

Myxomycetes, 169 n.

Naked buds, 80*, 81*

Natural selection, 498

Nebraska vegetation, Plate IX

Nectar, 423

Nectar glands, 423*

Nectaries, 423

Negundo, radial and cross sections

of stem of, 61*
Nemalion, 216, 217*
Nereocystis, 209*
Nest fungi, 269*
Netted veined, 89*, 90*
Nettle, stinging hair of, 418*
Nightshade, leaf of, 416*
Nitella, 201
Nitrification, 233
Nitrogen, 28

Nitrogen, circulation of, 231*, 233
Nitrogen, fixation of, 234, 235
Nocturnal position, 97, 98*
Node, 16, 17*, 57, 59
Nomenclature, 153
Nostoc, 176, 177
Notched, App. I

Nucellus, 356, 371, 372*, 381*, 386*
Nuclear division, 165*
Nucleole, 164
Nucleus, 34, 35*, 104*
Nut, 147*, 148
Nutrient substances, 28, 29
Nutrition of plants, 106-111

Oak leaves, arrangement of, 94*

Oat, root system of, 26, 27

Obcordate, App. I

Obovate, App. I

Obtuse, App. I

Odors of flowers, 423, 424

(Edogonium, 188-190*

Offensive-smelling plants, 419

Oil, 9, 11

One-celled green algaB, 179-184*

Onion leaf, section of, 53*

Onoclea, 315*

Oogonium, 188, 189*, 204, 212

Oospore, 166, 183*, 189*, 204

Opening, 490, 531*

Open veining, 89*, 90*

Ophioglossum, 324*

Opposite branching, 41*

Orange, 148*

Orbicular, App. I

Orchid, aerial roots of an, 19*

Order, 153

Origin of sex, 187

Oscillatoria, 175, 176*

Osmosis, 36-39*

Osmosis in an egg, 37*

Osmosis in root hairs, 39

Outline of classification, 155

Ovary, 128*, 377*

Ovate, App. I

Overcrowding, 448-450*

Ovule, 123*, 128, 356

Ovule case, 377*, 380*

Ovule, structure of, 144*

Oxalis leaf, development, 86*

Oxidation, 7

Oxygen making, 7, 28, 29, 113, 115

Pacific slope, 490, 494, 495

Paleobotany, definition of, 2

Palisade cells, 102, 103*, 105*, 107

Palmate, 89, 91*, 92*

Palms, 57*, 111*, Plate XIII

Pampas region, 451

Panicle, 135*, 136

Pansy, leaf-like stipules of, 88*

Papilionaceous corolla, 125*

Papillae on stigma of a lily, 142*

Parallel veining, 89*, 90

Paraphysis, 251*, 298*

Parasites, 22*, 23, 172, 227, 407-409*

Parasitic roots, 22*, 23

Parenchyma, 64*, 106*

Parietal placenta, 129*, 130

Parthenogenesis, 244

Pea pod, 443*

Pea seedling, mutilated, 8*

Pea seedling on whirling disk, 32*

Peat, 291, 292

Peat bogs, 292

Peat moss, 290-292*

Pedicel, 133*

Peduncle, 133*

Peltate, App. I

Penicillium, 250*

Pepo, 149

Perennial, 26*, 46

Perianth, 123*, 126*, 129*, 130* 135*,

App. II

Perianth, differentiation of, 394
Pericarp, 149, 150*

550

INDEX

Perigynous, 130*

Perigyny, 395

Peronosporales, 244-247*

Petal, 123, 377*

Petiole, 88

Peziza, 251*

Phseophycese, 205-213*

Phanerogams, 354

Phloem, 312*

Phosphates, 29

Phosphorus, 28, 29

Photosynthesis, 107-115, 162, 172

Phycomycetes, 239-247*

Phylloxera, 247

Physcia, Plate V

Physiology, plant, definition of, 2

Phytophthora, 246*

Pine forests, 457*, 490, 529*

Pine needle, 364, 365*

Pine seedling, 18*

Pine stem, 366-369*

Pinnse, leaflets of a pinnately com-
pound leaf, 92*

Pinnate, 89, 90*, 91*, 92*

Pinnules, leaflets of a pinnately twice
compound leaf, 98*

Pistil, 123-126*, 128*, 358, 377*

Pistillate flower, 124*

Pitcher plant, 409*

Pith, 58-61*, 65*, 67*, 72, 366*

Placenta, 129*, 130

Plains region, 490, 492, 493

Plankton, 195

Plant breeding, 500-513*

Plant cell, 159-161*

Plant communities, 447

Plant ecology, definition of, 2, 3

Plant evolution up to pteridophytes,
306

Plant formations, 474-480*

Plant geography, 481-495*

Plant geography, definition of, 2, 3

Plant geography of United States,
489-495*

Plant fertilizers, 524

Plant fibers, 526-529*

Plant food for domestic animals, 523,
524

Plant food for human use, 514-522*

Plant fuel, 534

Plant grains, 514-516*

Plant manufactures, 524, 525*

Plant medicines, 522, 523

Plant physiology, definition of, 2

Plant products, 514-535*

Plant societies, 447

Plant successions, 454-458*

Plant timber, 529-533*

Plants, destruction of, by animals,

413
Plants, groups of, in relation to water

economy, 459

Plants of uneatable texture, 415
Plants, ornamental, 534, 535
Plasmolysis, 38*
Plasmopara, 247
Platycerium, 309*, 310, 472*
Pleurococcus, 179-180*
Pleurotus, Plate VI
Plowrightia, 252*
Plumule, 5*, 6, 7*, 14, 15
Plurilocular sporangia, 206*, 207
Pod, 441-443*
Poisonous plants, 419
Poisonous roots, 25
Poisonous seeds, 11, 419
Polar nuclei, 383*
Pollarded trees, 87
Pollen, 127, 141*, 358
Pollen-carrying apparatus, 422*, 423
Pollen chamber, 362*, 371
Pollen, discharge of, 140
Pollen grain, germination of, 142,

143*
Pollen grains, number of, per ovule,

144, 145
Pollen, protection of, 426, 427, 434*,

435

Pollen sac, 378*, 379, 380
Pollen tubes, 142*, 143*
Pollination, 142*, 144*, 145, 357,

371, 420-434*

Pollution of water supply, 171, 178
Polyadelphous, 127
Polypetalous, 126
Polysepalous, 126
IViysiphonia, 218, 219*
Poly trich urn, 296*
Pomer 148

Pond scum, 193-195*
Pond zonation, 476-478°
Pore fungi, 264, 265*
Porella, 286*, 287*
Position of buds, 82*, 83, 87*
Postelsia, 209*
Potassium, 28, 29
Potato blight, or rot, 246*
Potato tuber, 52*, 53, 78, 79
Prairies, 492, 493
Prickle, 416*, 417*

INDEX

551

Prickly leaves, 416*, 417*

Primary root, 6*, 12*, 16*, 19

Primitive flowers, 393

Procambium, 65*

Products, plant. See Plant products

Pro-embryo, 144, 145*

Promycelium, 259*, 261*, 271

Propagation by roots, 436

Propagation of plants, 436-446*

Prosenchyma, 64

Protection of plants from animals,

413-419*
Protection of pollen, 426, 427, 434*,

435

Proteids, 9*, 11*, 25
Prothallial cell, 322*, 323, 335*, 368*,

372*, 373

Prothallium, 315-317*
Protobasidiomycetes, 258-264*
Protococcales, 179, 184*
Protonema, 294*

Protoplasm, 34, 35*, 112, 156-168*
Protoplasm, characteristics of, 34-

Protoplasm, circulation of, 202
Protoplasm, continuity of, 99, 214
Protoplasm, structure of, 157
Protoplast, 34, 35*, 161
Protosiphon, 200*
Protozoa, 157
Pteridophyta, 306-344*
Pteridophytes, 306-344*
Pteridophytes, evolution of, 343
Pteridophytes, origin of, 342
Pteridophytes, summary of, 343, 344
PteridospermsB, 392 n.
Ptomaines, 232, 236
Public health, 236, 237
Puccinia, 260-263*
Puffball, 268*
Pulsating vacuole, 158*
Pulvinus, 99*
Pyrenoid, 160*, 162
Pythium, 247

Race, 504

Raceme, 133*

Radial symmetry, 123*, 125, 126*,

138*

Radiating stems, 51
Radishes, competition among, 449*
Rainfall, 481, 488, 489*
Raspberry, 436*, 445*
Ray flowers, 134*, 135*
Ray, medullary, 60*, 66, 71, 72

Receptacle, 123*, 125*, 130*
Receptacle of brown algae, 210*, 212
Receptacles of Marchantia, 282*,

284*

Red algae, 213-220*
Red algae, life habits of, 214, 215
Red algae, summary of, 219, 220
Red clover, leaf of, 98*
Red snow, 182
Regions of vegetation, 481
Reindeer moss, 257*
Reproduction, 35
Reproduction in flowering plants,

138-145*, 436-446*
Resin duct, 366*
Respiration, 107, 110-115
Resting buds, 80
Resting condition, 112
Resurrection moss, 333
Retuse, App. I
Rhizoids, 280*, 281*
Rhizopus, 240, 241*
Rhodophyceae, 213-220*
Rhubarb roots, 26*
Ricciales, 279, 280*
Ricciocarpus, 280*
Rigid tissue, 312, 313. See Scleren-

chyma

Ring, annual, 66*, 68*
Ringent, App. II
Rise of water in stems, 73-75*
Rockweeds, 210-212*, Plate IV
Rocky Mountain region, 490, 493,

494

Root, 6*, 12*, 13, 15-33*
Root absorption, 30
Root absorption and temperature, 30
Root, adaptation to work, 33
Root cap, 24*
Root climbers, 20*, 48
Root, dicotyledonous, section of, 25*
Root, fleshy, 23*

Root hair, 15, 16*, 24, 25*, 27*, 28
Root pressure, 29*, 30
Root sections, 24*, 25*
Root system, 26, 27
Root tubercles, 234*
Roots, absorbing surface of, 27
Roots, adventitious, 19
Roots, aerial, 19-21*
Roots, brace, 21*
Roots, fibrous, 23*
Roots, movements of young, 30, 31
Roots, parasitic, 22*, 23
Roots, pine, lateral extension of, 32*

552

INDEX

Roots, primary, 6*, 12*, 16*, 19
Roots, propagation by, 436
Roots, secondary, 6*, 12*, 19, 32*
Roots, selective action in, 36
Roots, storage of nourishment in,

23*, 25, 26*

Roots, structure of, 24, 25
Roots, tertiary, 19
Roots, water, 20
Rootstock, 51*, 52*, 53
Rosette plants, 50*, 51
Rotation of protoplasm, 202
Rots, 246*, 253, 254
Russian thistle, 440*, 447, 448
Russian thistle, spread of, 451, 452
Rusts, 260-264*
Rye grass, Plate I

Saccharomycetes, 238*, 239

Sac fungi, 248-257*

Sac fungi, summary of, 257

Sagebrush, 480, 493, 494

Sage, pollination in flowers of, 430*,

431*

Sagittaria, leaf of, 461*
Sago palm, 78
Sahara, 482*
Salicornia, 483*, 493
Salt marsh plants. See Halophytes
Salt marshes, 458
Salts, 29

Salver-shaped, App. II
Salvinia, 320*

Sap, descent of, 74*, 75*, 77
Sap, rise of, 29*, 30, 74*, 75*, 77
Saprolegnia, 244*
Saprolegniales, 242-244*
Saprophytes, 172, 227, 409
Sapwood, 72
Sargassum, 212*, 213
Scale, 16, 18
Scalloped, App. I
Scaly buds, 81
Schizocarp, 147*
Schizomycetes, 228-237*
Scion, 68

Scirpus, cross section of stem of, 58*
Sclerenchyma, 58*, 59
Scotch pine (Pinus sylvestris), 366*,

368*, 370*
Scouring rush, 325
Sea lettuces, 187*
Seasonal plants, 459, 467
Secondary growth, 65-67*
Secondary root, 6*, 12*, 19

Secondary roots, direction of, 32*

Sections, leaf, 102, 103*, 466*

Sections, root, 24*, 25*

Sections, wood, 59-68*

Sedge, rootstock of, 51*

Seed, 5-12*, 355, 356

Seed coats, 6, 13

Seed dispersal, 438-446*

Seed habit, origin of, 389-393

Seed leaf, 5-14*, 18*

Seedlings, 6*, 8*, 9-18*, 22

Seedlings, mutilated growth of, 8*

Seed plants, 5-14*, 18*

Seed plants, origin of, 389

Seeds, bitter, 11, 419

Seeds, poisons in, 11, 419

Selaginella, 332-337*

Selaginella, life history of, 336

Selaginella, summary of, 337

Selection by plant breeder, 500-509

Selection, natural, 498

Selective absorption, 36

Self pollination, 420

Sensitive plants, 98, 99

Sepal, 123*, 377

Sequoia, 42, 46, 73, 482, 495

Sessile anthers, 127

Sessile leaves, 88

Sessile stigma, 128

Sex, evolution of, 223, 224

Sex, origin of, 187

Sexual characteristics given by heter-

ospory, 352

Shade plants, 470*, 471
Shame vine, 98*, 99
Shepherd's purse, development of

embryo and ovule, 386*
Shepherd's purse, development of

flower of, 387*
Shoot, 15

Short-stemmed plants, 50*, 51
Shrubs, 45
Sieve plate, 63*
Sieve tubes, 61-63*, 67, 72, 76,

105*

Sigillaria, 340, Plate VIII
Silica, 120
Simple leaves, 91
Simple pistil, 128
Simple umbel of cherry, 133*
Siphonales, 197-201*
Siphon algae, 197-201*
" Sleep " of plants, 97, 98*
Slime molds, 169
"Smilax," 56*

INDEX

553

Smoke tree, 438*

Smuts, 259*

Social plants, 447

Soft bast (sieve tubes), 61-63*

Soil, arid, zonation, 476-478*

Solomon's seal, parallel-veined leaf
of, 89*

Sorus, 313

Spatulate, App. I

Species, 153

Sperm, 166, 183*, 204, 212

Spermagonia, 262*

Spermatia, 262*

Spennatophyta, 354-401*

Spermatophyte, 354-401*

Spermatophytes, 400

Sphserella, 180-182*

Sphagnales, 290*, 291*

Sphagnum, 290*, 291*

Spike, 133, 134*

Spine, 416*, 417*

Spiral vessel, 62*

Spirogyra, 166*

Sporangium, 185*, 204, 313, 314*

Spore, 166

Spore case, 313

Spore formation of ferns, 313

Spore fruit, 321*, 322

Sporidia, 259*, 261*, 271

Sporophyll, 314, 315*

Sporophyll, arrangement of, in
flowers, 394

Sporophyte, 220, 277, 299, 318

Sporophyte, evolution of, 402-404

Sporophyte, origin of, 349

Spot fungi, 253, 254

Spreading growth, 43*

Spring spores of wheat, 261*

Spruce, Douglas, 493, Plate XII

Spur, fruit, 83*, 84*

Squash seed, section of, 5*

Stamen, 123-131*, 358, 368*, 369,
377*, 378*, 379, 380

Stamen, parts of, 127*

Staminate flower, 124*

Standard, 125*

Starch, 9-11*, 25, 107-110

Starch disappears during germina-
tion, 10

Starch in leaves, 107-110

Starch making, rate of, 109, 110

Stem, 15-33*, 40-79*

Stem, active portions of, 71

Stem, comparison of monocotyledo-
nous and dicotyledonous, 70

Stem, definition of, 40
Stem, dicotyledonous, minute struc-
ture of, 59-67*

Stem, early history of, 64, 65*
Stem, functions of cells of, 71, 72
Stem, living parts of, 71-79*
Stem, modifiability of, 69
Stem, monocotyledonous, 57-59*
Stem, nature of, 40
Stem, structure of, 57-70*
Stem structure, early history of, 64,

65*

Stems, climbing, 47-50*
Stems, condensed, 54*
Stems, storage of food in, 78, 79
Stems, twining, 49*, 50*
Stemless plants, 50*, 52
Sterigmata, 266
Stiffening, mechanics for, 58*, 59,

63*, 64*

Stigma, 128*, 377
Stimulus to protoplasm, 35, 36, 39
Stinging hair, 418*
Stipa, cross section of rolled and

unrolled leaves of, 466*
Stipe, 208
Stipules, 80, 82*
Stock, 69

Stolon, with tips rooting, 436*
Stoma, 288, 289*, 301*
Stomata, 71, 103*, 118, 119
Stomata, operation of, 118
Stone fruits, uses of, 444, 445
Stone worts, 201, 202*
Storage of food in the root, 23*, 25,

26*

Storage of food in the seed, 8-11
Storage of food in the stem, 52-54*,

78, 79

Strawberry, 148*
Strobilus, 327

Struggle for existence, 447-451, 497
Style, 128*
Suberin, 117
Submerged leaves, 461*
Successions, plant, 454-458*
Sucking roots, 22*
Sugar, 10, 11, 25, 79, 107, 112-116,

522
Sugar, formed during germination,

10, 11
Sugar cane, cross section of a bundle

from, 75*
Sulphates, 29
Sulphur, 28, 29

554:

INDEX

Summer spores of wheat, 262, 263*
Sundew, 410*, 411*
Sun plants, 470*, 471
Superior ovary, 130*
Supernumerary buds, 82*
Survival of the fittest, 498
Suspensor, 144, 145*, 335*, 336, 385,

386*

Swamp zonation, 476-478*
Swarm spores, 185*, 186, 204
Sweet pea, flowers of, 125*
Symbiosis, 255

Symmetry, 123*, 125, 126*, 138*
Sympetake, 397
Sympetalous, 126
Sympetaly, 395
Syncarpy, 395
Synergids, 383*
Synsepalous, 126
Synsepaly, 395
Systematic botany, definition of, 2

Taper-pointed, App. I
Tap root, 23*
Taxonomy, definition of, 2
Teeth of moss spore case, 300*
Teleutospores, 260*, 261
Temperate plant associations, 482,

483

Temperature and germination, 7
Temperature and leaf movement, 97,

98
Temperature and photosynthesis,

108, 110
Temperature and respiration, 110-

112

Temperature and root absorption, 30
Temperature and root growth, 39
Temperature and transpiration, 120
Temperature and vegetation, 481-

488

Tendril, 48*, 49
Tendril climbers, 48*, 92*
Terminal bud, 40*, 42*, 43*, 45*, 46*,

52*, 82*, 83*, 84*
Terminal flowers, 132*
Tertiary root, 19
Testa, 5*, 6*, 370*, 375
Tetrad, 219, 277, 280*, 289*, 369
Tetraspores, 219*
Thallophyta, 172-274*
Thallophytes, 172-274*
Thallophytes, summary of, 304, 305
Thallus, 172
Thistle, Russian, 440*, 447, 448

Thorns, 416*, 417*

Thorns as branches, 44*

Tickle grass, 442*

Timber line. 485*

Tissue, 64, 167

Toadstool, 265-267*

Tooth fungi, 265*

Toxins, 236

Tracheids, 311, 312*, 366*, 367

Transition from stamens to petals,

139*

Transpiration, 107, 116
Transpiration, amount of, 119, 120
Transportation by water, 441, 442
Tree ferns, 309, Plate VII
Trees, 45
Trees, age of, 46
Trichogyne, 216, 217*
Trimorphous flowers, 432
Tropseolum, petiole, coiling of, 49*
Tropical plantassociations, 481, 482*,

Plate XIII

Tropophytes, 459, 482
Truffles, 253
Truncate, App. I
Trunk, 42*, 43*
Tube nucleus, 372*, 373, 382* '
Tuber, 52*, 53
Tuber brumale, 253
Tubercles on roots, 234*
Tubular corolla, App. II
Tumbleweeds, 440*, 441*, 442*
Turgor, 118
Turnip seedling, 16*
Twayblade, beetle on flower of, 425*
Twigs, 40*, 46*
Twiners, 48*, 49*, 50
Twining, rate of, 50

Ulothrix, 184-186*

Ulva, 187

Umbel, 133*, 135*

Umbellet, 135*, 136

Underground stems, 51*, 52*, 53*,

54, 78, 79

Uneatable plants, 50*, 52, 415-419*
Unilocular sporangia, 206*
Union of carpels, 128, 129*
Union of stamens, 127*, 128*
Unipistillary fruits, 146
Unisexual flowers, 124*
United States, plant geography of,

489-495*

Upright growth, 42*
Uredospores, 262*, 263*

INDEX

555

Uridinales, 260-264*

Uroglena, 170*, 171, 178

Uses of the components of the stem,

Usnia, 257*
Ustilaginales, 259*
Ustilago, 259*

Vacuole, pulsating, 158*
Variation, 496, 497, 499
Variety, 500, 503, 504, 506-508,

512*

Vaucheria, 197-200*
Vegetable physiology, 2
Vegetation, alpine, 484-487*, 493
Vegetation, arctic, 484*
Vegetation, regions of, 481
Vegetation, temperate, 482, 483
Vegetation, tropical, 481, 482*
Vegetative organs, 15
Vein, 89*, 10(5*
Venation, 89*, 106*
Venus flytrap, 411, 412*
Vernation, 85, 86*
Vertically placed leaves, 100*, 101*
Vessel, 61*, 72
Volvox, 182-184*

Walking fern, 310*

Water, absorption by roots, 28

Water, amount transpired, 119,

120

Water bloom, 177
Water, excretion of, 115-120
Water fern, 320*
Water lily, flower of, 130*, 139*
Water molds, 242-244*
Water, movement of, 28-30*, 73-78*,

105, 106*, 116-120
Water roots, 20

Water supply, pollution of, 171, 178
Weapons of plants, 416-419*
Wedge-shaped, App. I

Weeds, 452, 453

Wheat grain, section of, 9*

Wheat, hybridizing of, 512*, 513

Wheat rust, 260-263*

Wheat, selection among, 507-509

Wheel-shaped, App. II

Whorl, 123*

Willow, adventitious buds of, 87

Willow, arctic, 484*

Willow, flowers of, 124*

Wind pollination, 421*

Winged fruits and seeds, 439-441*,

442*

Wings, 125*
Winter buds, 80

Winter spores of wheat, 260*, 261
Witches' broom, 264
Wood cell, 25*, 61*, 62*, 72, 75*
Wood, coniferous, 366*, 367
Wood of linden, 66*
Wood parenchyma, 64
Wood sections, 59-68*
Wood, structure of, 61*, 63*, 66*

Xenia, 384 n.

Xerophytes, 459, 462-466*, 482, 485

Xerophytic leaves, 465*, 466*

Xylaria, 252

Xylem, 311, 312*, 366*, 367

Yarrow, head of, 135*
Yeast, 238*, 239
Yucca, 495

Zamia, 360*, 362*
Zonation, 475-478*, Plate X
Zones, 475-478*, Plate X
Zoosporangium, 204
Zoospores, 185*, 204
Zygnema, 194*
Zygomorphic, 125

Zygospore, 167, 182, 185*, 186, 193*,
194*, 204, 206*

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