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Fundam^
botany

DATE

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60M— 048— Form 8

FUNDAMENTALS OF BOTANY

GAGER

THE LARGEST AND OLDEST
LIVING THING

The General Sherman "big tree" {Sequoia gigan-
tea). This tree was about 1,200 yrs. old when
Christ was born. At the time of the Trojan wars
and the Exodus of the Hebrews from Egypt, under
the leadership of Moses, the tree was a sapling
20-30 ft. high. It has been alive during all of
mediaeval and modern history. Its height is
279.9 ft., circumference of the trunk at the base
102.8 ft., greatest diameter 36.5 ft., diameter
100 ft. from the ground 17.7 ft. Note that its
lowest branches are at about the level of the
tops of the conifers surrounding it. (Photo by
Pillsbury.)

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(Frontispiece)

FUNDAMENTALS

OF

BOTANY

BY

C. STUART GAGER

DIRECTOR OF THE BROOKLYN BOTANIC GARDEN

WITH 437 ILLUSTRATIONS

/S S

PHILADELPHIA

BLAKISTON^S SON & CO

1012 WALNUT STREET

Copyright, 1916, by P. Blakiston's Son & Co.

PRINTED IN U. S. A.
BY THE MAPLE PRESS COMPANY, YORK, PA.

TO
M. K. G.

AND

L. J. P.

PREFACE

An introductory course of study in botany should do at
least six things for the student:

1. Teach him the fundamental elementary facts con-
cerning plant life.

2. Acquaint him with the broad, illuminating generaliza-
tions, and with the theories and working hypotheses which
have been formulated on the solid basis of observed fact.
An intelligent comprehension of these fundamental con-
cepts is far more important for purposes of general culture
and a liberal education, and also for more advanced study,
than an intimate acquaintance with the facts alone on
which the generalizations are based.

3. Familiarize him with the methods of thought and
work by means of which the science has been and is being
advanced.

4. Give him some acquaintance with the great names in
the history of the science, so that he may view our present
body of knowledge in true perspective. The student
should be made to realize that what we now know was not
obtained ''ready made," but only by painstaking investi-
gation and search. Therefore, brief references are made
in the following pages to a number of pioneers in the un-
explored fields of botanical discovery. From an educa-
tional point of view it is quite as important to understand
that our present body of knowledge has been a gradual

X PREFACE

possession of the human race, and to know how we came
to acquire it, as to possess the information which is our
rich heritage.

5. Impress upon him the present Hmitations of our
knowledge, and acquaint him with some of the more
important problems awaiting solution.

6. Enlist his interest in the science by so presenting
it as to make him sense its importance and value to human
life in general, and to his own life in particular, and thereby
stimulate him to pursue the study further, and, may be,
to the point of making his own original contributions. A
class that is properly taught can never inquire, at the end
of the course, "What is the use of knowing all that?"

Outside of technical and professional schools, no intro-
ductory course in any subject should ever be planned or
presented on the supposition that its main purpose is to
pave the way for more advanced courses. The main func-
tion of all introductory courses is, as the name implies, to
introduce the pupil to a new realm of thought, to acquaint
him with a possible new interest in life. If he be led to
discover himself in this new realm — well and good; if not
— well and good also; he will find himself elsewhere, but
will always be enriched and liberalized from the widening
of his mental horizon by contact with another discipline
than his major interest in life.

In presenting an elementary course, the aim should not
be to make the subject simple — to remove all difficulties —
but to make it really interesting, that is, significant to the
pupil in his own life, and to make it as rich as possible as
a revelation of those broad basic principles which are fun-
damental to all true culture, but which are to be amplified
and more deeply investigated only in more advanced

PREFACE xi

courses. The presentation should be made as simple as
possible, consistent with the realization of these aims.

From the standpoint of pure science, the most funda-
mental problem of botany is that of the development of
the individual plant; the ultimate problem that of the
development of the kingdom of plants. In other words
the foundation and the ultimate goal are, respectively,
ontogeny (hfe-history of the individual) and phylogeny
(life-history of the race).

Ontogeny is fundamental because without a knowledge
of its processes the processes of phylogeny cannot be com-
prehended. Phylogeny is the ultimate problem because
its complete solution involves an orderly description of all
the phenomena of plant life, and their relation to each
other.

Thanks to the nature-study movement, most students
have nowadays acquired some knowledge of the parts and
a few of the functions of a flowering plant before they take
up the study of formal botany; for such, Chapter II will
serve only as a timely review; for others, as an essential
preparation for the subsequent chapters of Part I.

From an educational point of view, the most rapid
progress and the most substantial results are to be ob-
tained by an order of topics so arranged that each will
throw the greatest amount of light on those that follow.
It is also an immense gain for the pupil to be introduced
to the broad generahzations of the science by being led
to meet them for the first time in that type where their
concrete embodiment is most clearly defined and most
easily discerned. In the author's mind, acceptance of
these two postulates points unmistakably to the fern as,
par excellence, the best plant with which to begin the study

Xll PREFACE

of life-histories. A successful teacher of zoology once
declared that frogs were created for the express purpose
of serving as the introductory type in the study of that
science; as strong a claim may be made for the fern in
botany.

Such concepts as alternation of generations, sexual vs.
asexual reproduction, fertiliza<"ion, heredity, adjustment
to environment, life-cycle are nowhere more clearly illus-
trated than in the fern; the essence of them all may be
clearly comprehended by anyone who has carefully studied
its life-history. And with what a rich equipment may the
Hfe-histories of all other forms, both higher and lower, be
then undertaken! As Athene sprang full armed from the
imperial head of Zeus, so, from a study of the fern, do all
the essentials of alternation, sex, life-cycle, et cetera, leap
clearly defined into the mind of the beginning student,
there to remain throughout the course, illuminating all
subsequent studies of life-histories.

During the past fifteen or twenty years it has been the
general, if not universal, custom to study in the laboratory
the same forms as those treated in the text. Part II of
the present book has been planned with the idea of having,
for the most part, different sets of forms discussed in the
laboratory and the lecture room. This plan not only gives
the pupil acquaintance with a wider range of types, but
also tends to insure greater independence in the laboratory
work. It has been tacitly assumed that, in connection
with the use of this text, substantially the same classic
types will be studied in the laboratory as have formed the
basis of laboratory work for two decades. They are not
only types for which material may be obtained with com-
parative ease in quantity, but also forms which have been

PREFACE xiii

demonstrated by twenty years of trial to possess large
teaching value. Thus, for example, we have Ascophyllum
Sphagnum, Anthoceros, Cycas, and Erythronium in the
text, to be supplemented by Fucus, Polytrichum, Marchan-
tia, Zamia, and Trillium in the laboratory. By this plan
the laboratory work can never degenerate into merely
having the student pretend to ''verify" the statements in
the text.

It is anticipated that a laboratory guide, planned to
accompany this text and carry out the idea just outlined,
may soon become available.

In the matter of illustrations, the author has been most
fortunate in being able to command the services of Miss
Maud H. Purdy for the preparation of all original draw-
ings, and of Mr. Louis Buhle, photographer at the Brook-
lyn Botanic Garden, in making most of the photographic
negatives and prints not otherwise acknowledged in the
legends. For those so acknowledged the author expresses
here his best thanks to authors and publishers who have
freely granted permission to reproduce copyrighted as
well as uncopyrighted illustrations. The collections of
living plants, photographs, and drawings at the Brooklyn
Botanic Garden have been freely at the disposal of the
author, and grateful recognition is here made to that insti-
tution for the exceptional opportunities which it has
afforded.

Special appreciation is here recorded for permission from
Prof. David M. Mottier and Prof. Harlan H. York to
reproduce, in advance of their own publication, Figs. 8
and 263, respectively.

Specifically, acknowledgment is made to authors and
publishers, for permission to reproduce illustrations as

XIV PREFACE

follows: Prof. W. L. Bray and the New York State College
of Forestry (Syracuse University), Fig. 266; Dr. N.
L. Britton and Charles Scribner's Sons, Figs. 341 and
379; Prof. D. H. Campbell and The Macmillan Co., Figs.
275, 283, 140, 141, and 146; Prof. F. E. Clements and
Henry Holt and Co., Fig. 27; Mr. Frederick V. Coville
and the U. S. Department of Agriculture, Division of
Publications, Fig. 336; Prof. H. H. Dixon and The
Macmillan Co., Fig. 35; Doubleday Page & Co., Fig. 225;
Prof. H. Garman and the Kentucky Agric. Exp. Station,
Fig. 228; Prof. William F. Ganong and Henry Holt and
Co., Fig. 69; Prof. Patrick Geddes and John Murray,
Fig. 240; Geological Survey of Ohio, Fig. 581; Prof. W.
D. Hoyt and the Bureau of Fisheries, Washington, D. C,
Fig. 177; the Ohio Geological Survey, Fig. 412; Prof.
A. C. Seward and The Cambridge University Press (Lon-
don), Figs. 413 and 414; Prof. D. H. Scott and A. & C.
Black, Figs. 415 and 418; Dr. Albert Schneider and
Willard N. Clute & Co., Fig. 238; Prof. Hugo de Vries
and The Open Court Publishing Co., Figs. 401 and 402;
Prof. G. R. Wieland and the Carnegie Institution of
Washington, Figs. 421-427; Prof. S. W. Williston, Fig. 432.

The conception of diagramning life-cycles, as in Figs.
321, 329, and others, appears to have originated with
Prof. John H. Schaffner, of Ohio State University. These
diagrams possess admirable teaching value.

It is a pleasure for the author to acknowledge a large
indebtedness to his colleagues on the staff of the Brooklyn
Botanic Garden, and especially to Dr. E. W. Olive, whose
careful and critical reading of the entire manuscript and
proof has robbed the reviewer of much that would have
been rightfully his, and has added much of fundamental

PREFACE XV

value. Special acknowledgment is made of Dr. Olive's
interest and assistance in supervising the preparation of
the drawing for Fig. 198, on the wheat rust, and for sup-
plying the microscopical preparations from which a
number of illustrations have been made, as indicated in
the legends.

A similar valuable service was rendered by Dr. O. E.
White in reading and discussing the manuscript and
proof of Chapters XXXII-XXXVIII, and in reading
all of the galley proof and part of the page proof; and
by Mr. Norman Taylor, in reading manuscript and proof
of Chapters XXVII-XXX.

The author is indebted to Prof. E. C. Jeffrey, of Harvard
University, and Prof. E. W. Berry, of the Johns Hopkins
University, for suggestions and criticisms in connection
with the genealogical trees (Figs. 433 and 434), and
especially to Prof. G. R. Wieland, of Yale University, who
critically read the manuscript of Chapters XXXVII and
XXXVIII, and to whose stimulating comments is due
much of whatever value these chapters may possess.

For any imperfections and inaccuracies, of whatever
sort, in the subject matter, the author alone is responsible.

C. Stuart Gager.

Brooklyn Botanic Garden.
July 19, 1916,

CONTENTS

PART I
INTRODUCTION

Chapter Page

I. Fundamental Notions i

II. The Parts of a Flowering Plant 7

III. The Cell 14

PART II
THE VEGETATIVE FUNCTIONS OF PLANTS

IV. Loss of Water 21

V. Absorption of Water 47

VI. The Path of Liquids in the Plant 61

VII. Nutrition 69

VIII. Fermentation 94

IX. Respiration 103

X. Growth 113

XI. Adjustment to Surroundings 125

PART 111

STRUCTURE AND LIFE HISTORIES

XII. Life History of a Fern 147

XIII. Life History of a Fern (concluded) 168

XIV. Fundamental Principles 179

XV. Life History of a Moss 193

XVI. Life History of a Liverwort 209

Anthoceros , 212

Other Forms 222

XVII. Life Histories of Algae 227

Ascophyllum 227

xvii

XVlll CONTENTS

Chapter Page

XVIII. Life Histories of Algae (concluded) 242

Dictyota dichotoma 242

Ulothrix 250

Pleurococcus 252

XIX. Life Histories of Fungi 256

An Alga-like Fungus (Rhizopus) 256

A Sac-fungus (Microsphaera) 268

A " Rust " Fungus (Wheat Rust) 272

A Fleshy Fungus (Agaricus) 276

Other Non-green Plants 284

XX. Economic Importance of Fungi 288

Fungi that cause Plant Diseases 290

Molds 304

Cold Storage 305

Yeasts 306

Bacteria 306

Helpful Bacteria 316

XXI. Saprophytism and Symbiosis 321

Saprophytism 322

Symbiosis 324

XXII. The Problem of Sex in Plants 344

XXIII. From Alga to Fern 362

XXIV. Calamites and Lycopods 368

I. The Horsetails (Equisetales) .368

II. The Club-mosses (Lycopodiales) 376

III. Little Club-mosses (Selaginellales) 382

XXV. Seed-bearing Plants 390

The Cycads 390

XXVI. Seed-bearing Plants (continued) 412

Gymnosperms 412

Life History of the Pine 412

Reproduction 416

XXVII. Seed-bearing Plants (continued) 433

Life History of an Angiosperm 433

XXVIII. Seed-bearing Plants (continued) 446

Angiosperms 446

Archichlamydeas 457

Apetalae 457

Polypetalae 459

XXIX. Seed-bearing Plants (continued) 473

Metachlamydeae (Sympetalae) 473

CONTENTS XIX

Chapter Page

XXX. Seed-bearing Plants (concluded) 489

Monocotyledons 489

Types of Monocotyledons 491

XXXI. Evolution 502

XXXII. Darwinism 510

XXXIII. Experimental Evolution 520

XXXIV. Heredity 541

XXXV. Experimental Study of Heredity 549

XXXVI. The Evolution of Plants 569

XXXVII. Paleobotany 578

XXXVIII. The Evolution of Plants (concluded) 502

Appendix 621

PART I
INTRODUCTION

CHAPTER I
FUNDAMENTAL NOTIONS

1. What is Botany? — The names of most sciences merely
tell what they are about. Thus the term zoology (from
the Greek, zoon^ animal, + logos, discourse) indicates
the study of animals, geology the study of the earth,
mineralogy the study of minerals. A similar name for the
study of plants would be phytology, from the Greek phyton
{(pvTov), a plant, and this word is, indeed, sometimes used.
But the generally accepted name, botany j tells more than
what the science is about; it points back to why mankind
ever came to study plants. The reason was because
plants are so intimately and fundamentally related to our
own lives that it becomes, not only interesting, but
absolutely essential to know about them, and under-
stand them.

The word botany comes from a Greek word, hosko
{^b(TKoy), meaning, ''I eat." Botany, then, was originally
the science of things good to eat, and the term recognizes
the fact that for all of our food we are either directly or

D. a HILL LIBRARY
North Carolina State College

2 INTRODUCTION

indirectly dependent upon plants. The earliest "botan-
ical" interests were naturally in plants as food. But
it must have been discovered early in the history of man-
kind that some plants were not only not good to eat, but
positively poisonous, causing sickness or even death;
while others produced marked physiological effects, acting
some on one part of the body, some on another, like medi-
cine, and thus was early developed the study of plants
in order to ascertain their medicinal properties, and
their value in the treatment of diseases. This interest is
still reflected in the Spanish name for a drug store —
hotica.

2. Relation of Plants to Man. — Thus we see that the
primary reason for our being interested in plants at all
was because they are intimately related to our physical
existence and well-being. As civilization advanced,
other uses for plants and plant products were discovered,
and thus other reasons for being interested in them.
They furnish all the wood of the world, and one has only
to consider for a moment how absolutely dependent we
are on wood, to realize still more vividly the intimate
relation between the life of plants and that of man him-
self. Our houses and furniture are of wood, our food
(the product of plants) is shipped in wooden boxes,
crates, and barrels, over rails supported by wooden ties;
most of the paper in use is made of wood pulp, and
innumerable articles in daily use — lead pencils, tool
handles, many musical instruments, et cetera — are made
largely of wood. Surely, it would be rather strange
if we did not have some interest, at least, in objects
so closely related to our daily lives, our welfare, and our
happiness.

FUNDAMENTAL NOTIONS 3

3. Relation of Botany to Other Sciences. — It is not

possible to study any one science in disregard of all the
others. Plants are related not only to man, but to the
air and soil in which they live; their life processes
are chemical or physical in nature; they are distributed
in space over the earth's surface, and in time, in the
layers of rocks of various geological ages; and so the
study of botany touches meteorology, chemistry, physics,
geography, climatology, geology, the science of soils, and
other branches of science.

4. Biology. — The science which deals with life in
general is biology, and all the sciences which deal with
living things are biological sciences. Zoology, human
anatomy and physiology, bacteriology, and botany are
some (but not all) of the biological sciences, and they are
all more or less closely related to each other. There is
no hard and fast boundary line between any of the
sciences; they represent, rather, different points of
view of nature. But it is convenient to subdivide our
knowledge more or less arbitrarily for purposes of study.

5. Systematic Botany. — Just as the various ''sciences"
or ''knowledges" represent different points of view of
nature, so each science may have subdivisions, repre-
senting different points of view of its phenomena. The
study of plants for the primary purpose of ascertaining
their genetic relationships is systematic botany. The
ultimate aim of this study is to disclose the course of
the evolution of the plant kingdom; this aim can never
be fully realized, because most of the necessary data
have been lost forever in the course of the geological
evolution of the earth. Systematic botany includes:

(a) Classification, or the arrangement of the various

4 INTRODUCTION

kinds of plants according to some system (whence the
term, systematic).

(b) Taxonomy, or the principles of classification, based
upon the facts observed and their interpretation.

(c) Nomenclature, the principles and rules adopted for
the formation and assignment of plant names.

6. Morphology. — If, in our study, our attention is
centered chiefly on structure and form, our point of view
is that of morphology, and we recognize external and
internal anatomy, microscopic anatomy {histology), com-
parative morphology, experimental morphology (which
attempts to ascertain, by experiment, the causes of form
and structure), embryology (the study of embryos), and
other subdivisions.

7. Physiology. — If we are interested primarily in w^hat
the various parts of the plant are doing, rather than in their
form and structure, our point of view is that of physiology.
We shall find, as we study, that the facts of form cannot
be understood or explained except in the light of the
physiological work of the given part; and conversely,
that physiological work cannot be explained unless the
structure is also understood.

8. Ecology. — Every plant lives in a certain place, with
certain external surroundings; in other words it has a
home, or, as we usually say, a habitat or environment.
In order to live and keep healthy the plant must be
favorably adjusted to the various features (factors) of its
environment — the range of temperature, amount of light
and moisture, components of the soil, the earth's at-
traction (gravity), and surrounding animals and other
plants. The science of the relation of living things to their
environment is Ecology.

FUNDAMENTAL NOTIONS 5

9. Plant Geography. — The study of the present dis-
tribution of plants over the earth's surface, and of the
causes and consequences of this distribution, is plant
geography (sometimes called phy to geography).

10. Fossil Botany.— The oldest known rocks contain
the remains of plants that lived thousands — probably
millions — of years ago. These remains often, though not
always, of stone, are fossils, and their study constitutes
the study of fossil botany, or paleobotany. This study
is not only interesting in itself, throwing much light
upon our knowledge of plants, but is also of great value
to the geologist, often helping him to interpret correctly
the rock-layer, and to decide to what geological age it
belongs. By means of fossils, we may also learn much
of the climate of past ages, and the great changes that
have since taken place. Thus, when we find fossil re-
mains of tropical plants, such as palms, in the rocks of
the present arctic regions, we know that there must have
been a. tropical climate in that latitude at the time the
plants, now fossils, were living and growing there.

11. Educational Value of Botany. — From the preced-
ing paragraphs it is evident that a study of plants will not
only give us valuable information that may be used to
advantage in every day life, but that it will give us a
broader outlook than w^e might otherwise obtain over the
past and present of the world in which we live; it may
not only suggest to us the vocation we would prefer to
follow, but may give us a breadth of view and a wealth
of ideas that will help to increase both our usefulness and
happiness.

12. Plan of Study. — We shall first review the structure of
a familiar type of plant, and then make an elementary study

6 INTRODUCTION

of the fundamental life-processes involved in the nutri-
tion and growth of the individual. The second part of
the book will be devoted to studying the various kinds
of plants, and the different ways in which they solve the
same life problems of nutrition and reproduction.

CHAPTER II
THE PARTS OF A FLOWERING PLANT

13. Fundamental Terms. — The plants with which we
are most familiar (flowering plants) are composed of
various parts, such as leaves, roots, tendrils; and each

H|i^> ^^'_y^^

Fig. I. — A cactus plant {Rhipsalis pachyptera) with leafless stem.
The branches are flattened, thus exposing more green tissue to the light
in the absence of leaves.

part has its special work to perform. The various parts
of a plant having their own special work are called organs,
and the special work of an organ is its function. Since

7

8

INTRODUCTION

they are composed of organs plants (and animals also)
are said to be organized, and are called organisms. For a
similar reason, the kingdom of living things is called the
organic kingdom, or the organic world.

Fig. 2.

-Kohlrabi, showing the stem modified as an organ for the storage
of food, and a well developed tap-root.

14. Root and Shoot. — Everyone knows that the plants
with which we are best acquainted have roots in the

THE PARTS OF A FLOWERING PLANT 9

ground, and a stem with leafy branches above ground.
It is well, however, to recall this elementary knowledge
and to get clear ideas of these commonly recognized parts.
The botanist recognizes leaves as merely appendages of
the stem or branches, and branches as merely subdivisions
of the stem. Stems may or may not have branches (Figs.

Fig, 3. — Fibrous roots on cutting of sugar cane.

I and 305). The stem, with its branches and leaves,
constitutes the shoot. The shoot, therefore, is all the
plant except the roots. In broad outline, the structure
of any common plant is made up as follows:

f Root (with or without branches)
Plant body \ ( Stem (with or without branches)

Shoot 1 Leaves

lO

INTRODUCTION

16. The Root. — When a plant has more than one root,
there may be a main or tap-root with branches (Fig. 2),
or there may be no clearly recognized main root, but
numerous roots of equal value, each attached directly to
the stem (Fig. 3). The root-system frequently subdi-

Fig. 4. — Aerial fibrous roots of the royal palm.

vides into smaller and smaller branches or rootlets, and
these may ramify (branch and spread) extensively in all
directions. Roots never bear leaves, but the surface of
the finer, active roots is covered, for a short distance

THE PARTS OF A FLOWERING PLANT

II

back from the tip with innumerable fine hair-Hke out-
growths, root-hairs (Fig. 36).

16. The Functions of Roots. — The functions of roots
all have to do with maintaining the life of the individual
plant to which they belong, either by holding the plant

I'iG. 5. — Portion of root-system of a yellow birch {Betiila lutca), showing
roots serving to anchor the plant to the substratum. (Photo by Elsie M
Kittredge.)

firmly fixed in the ground (anchorage) (Figs. 4 and 5),
where food elements are abundant, by taking in these food
elements from the substratum (absorption), or by storing
up, for future use, food that has been made by the plant.

12

INTRODUCTION

Fig. 6.

Fig. 6. — Leaf of
a willow {Salix ^p.).
b, blade; p, petiole;
s, stipules.

Fig. 7. — Diagram
to show the essential
parts of a "flower-
ing" plant. /.r.,
tap-root ; b .r . ,
branch root; cot.
seed-leaf (cotyle-
don); i, internodc;
a.l, leaf-axil; n, node;
a.b, axillary bud; r,
receptacle of floral
organs; ca, calyx;
per, perianth; co,
corolla; st, stamens
(androecium); pi,
pistil (gynoecium).

Fig. 7.

THE PARTS OF A FLOWERING PLANT 13

17. The Shoot. — As stated above, the shoot is composed
of a branched or an unbranched stem, usually, but not
always, bearing leaves (Figs, i and 57). The leaves are
commonly composed of a flat, expanded, and usually
green part (the blade), which may or may not be borne
on a leaf-stalk {petiole). The portion of the leaf attached
to the stem is the leaf-base, the edge of the blade is the
margin, the tip of the blade is the apex, and the portion
of the blade attached to the petiole is the base of the blade
(Fig. 6). These parts may be tabulated as follows:

f Stem

Shoot <

Branches

' Blade

Leaves

Petiole

Leaf-base

Stipules

Apex

Margin

Base of the blade

Veins

The main functions of leaves are: (i) to elaborate plant
food in the presence of sunlight; (2) to help regulate the
water content of the plant. In these two functions hes
the significance of the fact that the leaf-blade is flat,
expanded, thin, and green. This will be explained in
Chapters IV and VII. Leaves also have other important
functions, to be mentioned later.

The branches serve to support the leaves, to hold them
up into the hght and air, and to connect them with the
root-system.

18. The Flower.— The interpretation of the flower is
not essential at this point, and is reserved for a future
chapter (Chapter XXIX), when it may be better under-
stood. It is sufficient here to state that the chief function
of the flower is the production of seed.

The essential parts of a flowering plant are shown in
the diagram (Fig. 7).

CHAPTER III
THE CELL

19. Historical. — The advancement of our knowledge
of nature has often depended upon the invention of some
new instrument that made possible observations that
could not have been made without its aid. The balance
did this for chemistry, the telescope for astronomy, the
thermometer for medicine. The possibiUties for under-
standing plant life were more than doubled by the in-
vention of the compound microscope. By its aid the
study of the finer internal structure of plants was made
possible.

20. Robert Hooke. — One of the earliest to employ the
microscope in this way was Robert Hooke (163 5-1 703)
of England. He was at first interested in demonstrating
the powers of the microscope on various objects. Among
them he tried thin sections of cork, and found the cork to
be composed of little compartments, which he called
cells, since they roughly resembled the cells of a honey-
comb. Marcello Malpighi (1674), an Italian, and Nehe-
miah Grew, an EngHshman (1682), greatly extended the
microscopic study of plants, adding so much to our knowl-
edge that they are now often referred to as the fathers of
plant anatomy.

21. Protoplasm. — At first the attention of botanists
was devoted almost exclusively to the walls of these cell-
like compartments, and to their shape and arrangement.

14

THE CELL

15

The walls were considered the important feature, and the
term cell meant the space enclosed by the wall. All this
was very natural, for botanists had quite generally, up
to this time, devoted their best energies to studying the
form and structure of plants, paying relatively little at-
tention to their hfe functions, or physiology. Gradually,
however, it came to be recognized that the really impor-
tant part was the substance that filled the little compart-
ments in all living tissues} It finally came to be under-
stood that this is the only living substance in plants
(and in animals as well), and that the cell- walls, and in
fact the entire organism, are built up by the activity of
this remarkable substance. It was first called by several
different names, but Hugo Von Mohl, a noted German
botanist, called it protoplasm,'^ considering it as the first
organic substance formed from the inorganic materials
taken in by the plant. This name was generally adopted
by both botanists and zoologists.

22. The Cell-theory.— The idea that all living things
are composed of cells, that the cell is the unit of plant and
animal structure, and that the essential thing about the
cell is the protoplasm, was elaborated by Schleiden (1838)
(for plants) and by Schwann (1839) (for animals), and
was accepted as generally correct by all students of plants
and animals. This doctrine became known as the cell-
theory of Schleiden and Schwann. The term cell is now
used in biology chiefly to designate the protoplasm comprised
within the cell-wall. A cell, then, is not a compartment
containing something, but is a structural unit of living

1 An intimately connected layer, group, or body of similar cells, all
having like functions, is a tissue.

» Greek protos (first) + plasma (thing formed).

i6

INTRODUCTION

matter. Many biologists now use the term protoplast
(instead of cell) to designate the units of protoplasm.

23. Structure of the Cell. — Painstaking microscopic
study of cells has revealed the fact that they have a wonder-
fully beautiful and complex structure (Figs. 8 and 9).
The protoplast is composed of two clearly defined parts,
a denser, more or less globular portion, the nucleus, sur-
rounded by cytoplasm {i.e., cell-plasm). Nucleus and

r^Wl^mk--^^

Fig. 8. — Cross-section of a cell from the root of a marrow-fat pea.
w", nucleolus; n.p nucleoplasm; n.m, nuclear membrane; a.m, starch-
forming plastid; st, starch grain; c.w, cell-wall; c.p^ cytoplasm; ch, chon-
driosomes; they are scattered throughout the cytoplasm. (After D. M.
Mottier.)

cytoplasm together constitute protoplasm. The nucleus
was discovered by Robert Brown, in 1831. The sub-
stance of the nucleus is designated nucleoplasm, and there
is generally a still denser body in the nucleus — the nu-
cleolus (plural, nucleoli). Sometimes there is more than
one nucleolus within the nucleus. The most important
chemical substance in the nucleus is chromatin, a very
complex protein, rich in phosphorus. The name chromatin
refers to the dense color it acquires when treated with

THE CELL

17

certain stains. The cytoplasm of many, possibly of all,
})lant cells contains, scattered through it, numerous tiny,
deeply staining granules, called chondriosomes.

{a) The cytoplasm appears net-like, or reticular, in
structure, and the spaces between the meshes are vacuoles.
Each vacuole is filled with an aqueous solution of various

Fig. 9. — Diagram of a plant cell in perspective, with portions of adja-
cent cells. Note the nucleus. The lighter areas are vacuoles in the
cytoplasm.

substances, known as the cell-sap. Insoluble solids of
various nature, such as crystals and starch grains, com-
monly occur in the vacuoles.

(6) At every free surface, such as the outer surface and
the walls of the vacuoles, the cytoplasm is specially organ-
ized into a limiting membrane (or plasma-membrane).
The structure of this membrane is not well understood,

i8

INTRODUCTION

although it is one of the most important parts, and much
study is now being given to it, in an endeavor to under-
stand it better.

¥

r
I

k

^

r

i

Mi^^^^

[

■

■

".•^'»^ v ^^^^II^^^^^^^^^^^^^Hb

Fig. 10. — Robert Brown (i 773-1858). One of the greatest of English
botanists. He discovered the nucleus in cells, and also the gymnospermy
of Conifers and Cycads.

{c) The structural elements of a cell may be concisely
tabulated as follows:

Cell

Nucleus (nucleoplasm)

Nuclear membrane

Nucleolus
Cytoplasm

Limiting membrane

Vacuoles

Vacuolar membranes

Cell-sap

Other contents (inclusions)
Cell-wall

THE CELL 19

id) Quite commonly, in plants, adjacent protoplasts
are joined together by strands of cytoplasm passing
through minute pores in the cell-wall.

24. Peculiar Properties of Protoplasm. — More is known
of the structure of protoplasm than is indicated above,
but a more detailed treatment is reserved until Chapter X.
Quite as important as the structure of protoplasm are the
physiological or functional characteristics, or properties,
that distinguish it from every other known substance.
The most significant and wonderful of these is its ability
to reproduce itself. By the vital activities of animals
and plants, their living substance undergoes a continual
destruction, which is accompanied by continual construc-
tion. Parts which are destroyed are constantly replaced,
and new protoplasm is continually being formed. No
other known substance can do this. If a crystal, for ex-
ample, of salt, is suspended in a saturated solution of
salt in water, some of the salt particles in solution will
attach themselves to the crystal in a regular manner, so
as to enlarge it, while preserving its characteristic shape.
But here, as is readily recognized, the crystal itself is
entirely inactive. It does not change another kind of
matter into salt, but merely serves as a center of deposit
for more salt. Protoplasm, on the other hand, entirely
alters the nature of the substances which enter into it,
and recombines them into a substance like itself, with
entirely new properties — in fact converts the non-living
into the living.

25. Secretions. — In the course of its continual de-
struction and reconstruction, protoplasm gives off or
secretes other substances, unlike itself and unlike the
material of which it was formed; these are called

20 INTRODUCTION

secretions. Such is the origin of the cell-wall. It is
secreted by the protoplast which it encloses. Sugar (as
in the sugar-cane), starch (as in corn and nearly all
plants), fats (as in Brazil nuts), and green coloring
matter and other pigments, are among the substances
secreted by protoplasm.

26. Complexity of the Cell. — A recent writer, after
describing the minute details of cell-structure, states
that, ''The vital processes exhibited by a cell indicate a
complexity of organization and a minuteness in the
details of its mechanism which transcend our compre-
hension and bafHe the human imagination, to the same
extent as do the immensities of the stellar universe."

27. Value of the Cell-theory. — It is hardly possible to
overestimate the value of the cell-theory to botany, and
to all biological science. By means of it we are led to
see that all the vital activities of any living thing have
their seat in the protoplasts of the individual cells. If a
plant or an animal grows, it does so because the individual
cells of its body multiply and grow; if it respires, it is
because every living cell of its body respires; if a wound
heals, it is because the adjacent cells reproduce them-
selves and form new tissue to replace that destroyed by
the wound; sickness results because certain cells behave
abnormally, or perform their normal functions out of
place; reproduction is the setting free by an organism of
one or more of its cells, which become the starting point
of a new individual. In fact, all that a plant or animal
does, physiologically speaking, is the sum total of what
the cells that compose it do. Thus the cell-theory gives us
a necessary, basic idea of all life-processes.

PART II
THE VEGETATIVE FUNCTIONS OF PLANTS

CHAPTER IV
LOSS OF WATER

28. Plants are Alive. — The most fundamental concep-
tion of a plant is that it is alive, just as truly and in the
same sense as is any animal. Therefore it takes in water
and food, respires, grows, moves, responds when stimu-
lated, reproduces, grows old, and dies. We are so ac-
customed to associate life with activity that one who,
for example, views a large tree, especially in winter,
stripped of its leaves, and apparently motionless, except
when swayed by the wind, is not always conscious of the
fact that the tree really is alive. A study of p'.ants,
however, teaches us the fallacy of the idea that life is
always associated with evident motion.

29. Kinds of Vital Activity. — Everything a plant does
affects either one of two things — either (i) the main-
tenance of the individual, or (2) the perpetuation of
the race to which that individual belongs. These two
classes of functions are known, respectively, as (i)
vegetative and (2) reproductive. This and the next five
chapters will deal with the vegetative functions of plants.

30. Loss of Water Demonstrated.— If a leafy branch,
cut from any plant, is laid aside for a time it will, as is

22 THE VEGETATIVE FUNCTIONS OF PLANTS

well known, become wilted. Potted plants, if not kept

Fig. II. — A fern {Cyrlomium falcatum), well watered and turgid.
(Cf. Fig. 12).

Fig. 12. — A fern {Cyrtomiiim Jalcalum), deprived of water for 48 hours.
The same plant as shown in Fig. 11.

well watered, and plants growing out of doors, if not sup-
plied with sufficient rain, will also wilt (Figs. 11 and 12).

LOSS OF WATER

23

M C 3 <U W

a.5

■a5 fl 2 a

.24

THE VEGETATIVE FUNCTIONS OF PLANTS

Some plants will imicli more readily than others, hut in all
cases the wilting is due to the fact that water is being
continually given off by the plant. This may be easily
demonstrated by placing a living plant, or a few fresh

Fig. 14. — Asicr sp., showing transition from i)etiolate, through petio-
late-stipulate, to sessile leaves. Note the elongation of the lower petioles
thus bringing the blade out to better light exposure.

leaves of any convenient plant, under a bell-jar or a
tumbler, whose inner surface is first known to be per-
fectly dry. As a check or control on the experiment a
second bell-jar should be placed beside the first one, but
without any plant or leaves under it. The inner surface

LOSS OF WATER 2$

of the jar will soon become clouded by a thin film of
moisture, and within a very short time, this moisture will
.begin to collect in drops (Fig. 13).

i i i 4 if

4 4S

1

Fig. 15. — Asler sp. Series of leaves, all from one plant, showing gradual
transition from petiolate (lower right hand specimen) to sessile leaf.

The surface of the control jar will remain perfectly dry.
If the experiment is set in the sun, the result will be
greatly hastened. This loss of water from within living

26

THE VEGETATIVE FUNCTIONS OF PLANTS

plants is called transpiration, in recognition of the fact
that the moisture must pass through the epidermis.

31. Function and Structure. — As stated in Chapter I,
the function of an organ cannot be intelligently discussed
unless its structure is understood, and vice versa, the
structure of any part is without meaning, except as

Fig. i6. — Leaf of a banana {Musa sapientwn), showing enormous
development of the blade, and also how easily a leaf-blade may be torn
in the absence of a marginal vein. This leaf-blade {i.e. without the
petiole) measured over 15 ft. long and 3 ft. wide, thus exposing over
45 sq. ft. of green tissue to the light.

viewed in the light of its function. Therefore, if we wish
to understand transpiration and the other functions of
leaves, we must first ascertain their structure. This has
significance for us only in the light of their physiological
work.

LOSS OF WATER

27

32. External Anatomy of a Leaf. — We have seen
(Chapter II) that the three main parts of a leaf are the
blade, petiole and leaf-base. These parts may manifest
every conceivable variation as to shape and size, and bear
every relation to each other as to proportion. The petiole

Fig. 17. — ^Leaf of Hercules club {Aralia spinosa), partly thrice compound.
The leaf-blade measured 15 in. wide at the base, and 12 in. long.

maybe more or less shortened, or it may be entirely wanting
so as to make the leaf-blade sessile (seated) on the stem (Figs.
14 and 15). The blade may be greatly enlarged (Fig. 16),
or more or less branched (Fig. 17), or it may be greatly re-
duced, or even entirely wanting. In the latter case, the

28

THE VEGETATrV^E FUNCTIONS OF PLANTS

Fig. iS. — New Zealand raspberry (Rubus auslralis). The lower portion
is the petiole; the blade is reduced to three spiny veins slightly expanded
at the tip.

Fig. 19.— Homology of bud-scales in an ash {Fraxinus sp.). Series
showing gradual transition from outer bud-scale (at left) to unexpanded
foliage leaf (at right). The outer bud-scale is morphologically a leaf-base.

LOSS OF WATER

29

petiole may take on the character of the blade, and perform
all its functions, as in the case of various acacias. In certain

Fig. 20. — Tulip bulb; longitudinal section. F, solid stem; B, flower
bud; S, leaf-bases serving as bud-scales, and also for the storage of plant
food.

leaves nothing remains but base, petiole, larger veins, and
the tips of the blade, as in the New Zealand raspberry

IP^I

^n^^^^^^^^P^ 1

■

Iv'^^H

p* ^^^^^^p^ 1

H

^^.^^P^^H

MJiflH^^I

M^M

^^L' 1

Ih

i^l

1

Fig. 21. — Buds of the tulip-tree {Llriodeiidroti tulipifera), showing stipules
as bud-scales.

(Fig. 18). In some plants, as for example, the grasses
(Fig. 22), there is no distinction between petiole and blade.

30

THE VEGETATIVE FUNCTIONS OF PLANTS

Leaves may be so altered as to cease to be foliage leaves,
and serve, for example, as bud-scales (Fig. 19), or other
organs. In some bud-scales both petiole and blade may
be wanting, the leaf-base alone serving as the scale (Fig,
20). Again, the bud-scale may consist chiefly of petiole.

Fig. 22. — Various types of leaf-blade.

or of blade, or (as in the tulip-tree) of stipules only (Fig.
21). Variations of leaves are illustrated in Fig. 22.

It is not essential here to endeavor to frame a definition
of a leaf, though this would be a profitable exercise for
the reader. The main thing is to emphasize the fact that
the leaf is an exceedingly plastic organ, that is, appearing

LOSS OF WATER

31

in many forms and disguises, and readily adjusting itself
to wide variations in its surroundings. By virtue of this
characteristic, it helps to enable the plant as a whole to
become adapted to its surroundings. For the present
purpose, we are primarily interested in leaves as foliage.

33. Internal Anatomy of the Leaf-blade. — If we take
any convenient foliage-leaf, such as, for example, a leaf

g;---w-jigl!!-',.

— '-^^■^^"■-^-i^^

:*$iSl

Fig. 23. — ^Leaf of a live-for-
ever {Sedum sp.), with a por-
tion of the epidermis peeled
back. Underneath the epi-
dermis is the mesophyll.

Fig. 24. — Mullein {Verbascum Thap-
siis). L, cross-section of leaf-blade,
showing relative thickness of layer of
epidermal hairs; H, a single hair from a
leaf (greatly magnified).

of the common lilac, we may readily demonstrate, with
the aid of a scalpel or sharp knife, that the surfaces of
the blade are covered with a thin skin or epidermis,
which may be peeled off (Fig. 23), disclosing the mid-leaf
substance {mesophyll) , lying between the upper and the
lower epidermis. In many leaves (for example, those of

32

THE VEGETATIVE FUNCTIONS OF PLANTS

the great mullein), there are numerous more or less promi-
nent ''hairs/' which are outgrowths of the epidermis, and
readily come away with it when it is peeled off (Fig. 24).
We notice that the veins of the leaf appear to be imbedded
in the mesophyll, and that they lie somewhat nearer the
lower than the upper surface. The lower surface is also
seen to be usually of a lighter green color than the upper,
and the lower epidermis may be peeled off more readily
than the upper. The explanation of these facts is found
in the microscopic structure (histology) of the mesophyll,
soon to be studied.

34. Histology of the Leaf -epidermis. — When a small
portion of the lower epidermis is examined under the

Fig. 25. — ^Lizard's tail {Saururus cerniiiis). Portions of leaf-epidermis;
U , upper epidermis; L, lower epidermis; ep, epidermal cell; st, guard-cells
of the stomata. (Camera lucida drawing.)

microscope it is seen to be composed of larger cells,
irregular in shape, and of smaller cells, usually somewhat
half-moon shaped, occurring in pairs, and the pairs
irregularly distributed at frequent intervals among the
larger cells (Fig. 25). The latter possess no green color-

LOSS OF WATER

33

ing matter, but the smaller cells contain numerous green
bodies. The green substance is chlorophyll (leaf -green) ,
and the separate green particles are chlorophyll-bodies, or
chloroplasts. Between each pair of smaller cells is a tiny
hole or stoma (from a Greek word meaning mouth or
opening), and the two cells are the guard-cells of the
stoma (plural, stomata) (Fig. 26).

The structure of the upper epidermis of the same leaf
(Fig. 23) is seen to be quite similar to that of the lower,

Fig. 26. — Photomicrograph of stomata from a leaf of Verbena ciltata,
showing their condition at 9 a.m. (After Lloyd.)

except that in most plants there are fewer stomata in the
upper than in the lower epidermis. In some leaves {e.g.y
barberry, osage orange, lilac) there are no stomata in the
upper epidermis; while in other plants, such as, for ex-
ample, the water-lily whose leaves float on water,
there are stomata in the upper epidermis but none in the
lower.

35. Microscopic Structure in Cross-section. — Since all
objects examined with the aid of a microscope are observed
with transmitted light, that is, by Hght that passes through

34

THE VEGETATIVE FUNCTIONS OF PLANTS

them to the eye,^ in order to examine opaque objects,
sections of them must be cut, thin enough to be readily
transparent. The conditions of observation are also
much simplified by this means.

Thin cross-sections of leaves, that is, sections cut at
right angles to the surface, are readily made with a sharp

Fig. 27. — Cross-sections of leaves of an oak {Quercus novimexicana),
showing the effect of different light conditions on the internal anatomy.
I, from leaf growing in sunlight; 2, from leaf growing in the shade. (After
Clements.)

razor. When examined with the microscope, such sections
disclose a structure similar to that illustrated in Fig. 27.
The epidermis, both upper and lower, is seen to consist
of a single layer of cells. The free surface of the outer
cell-wall is coated with a layer of a wax-like substance,

^ Objects examined with the unaided eye are observed with light re-
fluted from their surface to the eye.

LOSS OF WATER 35

known as cuticle. The mesophyll, or leaf -parenchyma,
has two well-defined regions. In the portion next the
upper epidermis the cells are elongated, and arranged close
together at right angles to the epidermis. This portion is
the palisade layer. In the other portion of the mesophyll
the cells are of irregular shapes, and loosely arranged, with
intercellular spaces. Commonly, also, the palisade cells
contain more, and more deeply colored, chlorophyll grains
than do the cells of the spongy parenchyma. The above
facts make it clear why the upper surface of leaves is
darker green than the lower surface. Cross-sections of
veins are also seen, imbedded in the spongy parenchyma.
Details of their structure need not be considered here.
The absence of chlorophyll from the epidermal cells
(except the guard-cells) may also be noted.

36. Stomata and Guard-cells. — In the lower epidermis,
sections of the stomata are found, and it is readily seen
that the stomata are tiny holes or pores through the epi-
dermis, connecting the intercellular spaces with the out-
side air. The guard-cells are so constructed that under
changing conditions of light and moisture they may be-
come more or less turgid. When they become more
turgid, they are more convex, and thus enlarge the diam-
eter of the stoma; when less turgid, they become less
convex and this diminishes the size of the opening, and
in certain cases may even close it completely. By these
changes the passage of water-vapor or other gases through
the stomata may be either facilitated or retarded.

37. Structure of the Petiole. — The main function of
the petiole is to hold the leaf-blade well exposed to Hght,
while, at the same time, keeping it connected with the
stem. It will have been noted already that the veins

36

THE VEGETATIVE FUNCTIONS OF PLANTS

converge at the ])ase of the blade (P'ig. 22). They may be
traced from tl\is point, through the petiole, into the
branch. The veins are composed of fibers and vessels,

Fig. 28. — Horse-chestnut {Aescidus Hippocastanmn). Is, leaf-scar,
showing scars of seven fibro-vascular bundles, corresponding, in number,
to the seven leaflets of the compound leaf, formerly attached at Is. The
leaf is drawn to a smaller scale than the branch.

closely associated, and are, therefore, called fibro-vascular
bundles. A cross-section of the petiole of a horse-chestnut
leaf, for example, shows one fibro-vascular bundle for
each leaflet of the compound blade (Fig. 28) ; each of the

LOSS OF WATER

37

seven bundles extends out into the blade as the prominent
mid-veifi of a leaflet. In a common "trick" of child-
hood, the epidermis of the petiole of the common, broad-
leaved plantain is broken by sharply bending the petiole,
or by carefully cutting with a
knife. The petiole may then be
carefully pulled apart, so as to dis-
close the fibro-vascular bundles
without breaking them (Fig. 29).
These bundles are the channels
through which liquids pass between
the leaf-blade and the branch.

38. Transpiration. — In order to
understand transpiration, we
should have in mind a clear pic-
ture of the conditions within a
leaf.^ Because of moisture in the
cells, the cell- walls are saturated.
From their moist surfaces water is
continually evaporating into the
intercellular spaces (Fig. 27), so
that the air in those spaces is al-
ways nearly saturated; that is, it
holds nearly as much water as pos-
sible in the form of vapor. From
the intercellular spaces the vapor
diffuses out through the stomata,

and passes off into the air. If the outer air is also very
humid, as frequently near the ground after sunset, the

Fig. 29. — Leaf of plantain
{Plantago), with the petiole
stretched lengthwise from a
transverse cut, showing the
fibro-vascular bundles that
continue up into the five
main veins of the leaf-blade.

^ While loss of water is not confined to leaves, they are the chief organs
of transpiration, and if we understand the process in them, we shall
understand it elsewhere.

38

THE VEGETATIVE FUNCTIONS OF PLANTS

vapor from within the leaves may not be able readily to
pass off, and will accumulate in drops on the surface
of the leaves, forming dew. The passing off of water is
not confined to the stomata {stomatal transpiration),
but may take place through portions of the epidermis
where there are no stomata, the water passing through the
cuticle {cuticular transpiration).

39. Control of Transpiration. — The rate of transpiration
is controlled by both external and internal factors. If

Fig. 30. — Gasteria nigricans. Succulent leaves, with thick cuticle
serving for the storage of water.

the outer air is very humid, water cannot evaporate into
it as rapidly as when it is less humid. On humid days,
therefore, transpiration will be diminished. It is in
recognition of this fact that gardeners, in ''wetting down"
a plant house, do not confine the water to the plants.

LOSS OF WATER

39

but thoroughly wet the walks and walls so as to maintain
a favorable humidity of the air.

Naturally, anything that tends to increase humidity
will retard transpiration. If the air in the vicinity of
foliage is quiet, its humidity will increase owing tQ the

Fig. 31. — Transpiration from four leaves of oleander. At the left,
both sides coated with cocoa butter; next, under surface, only, coated;
next, upper surface, only, coated; right, uncoated. All exposed for the
same length of time,

water-vapor from the leaves, but when the wind blows,
fresh portions of less humid air are continually brought
into contact with the plants, and transpiration becomes
more rapid. This is why a plant or a bouquet, being
carried from one place to another, will keep fresh longer if
wrapped with paper.

40 THE VEGETATIVE FUNCTIONS OF PLANTS

Warm air can contain more water- vapor in a given space
than colder air. For this reason, other things being
equal, plants on which the sun is shining will transpire
more rapidly than those in the shade, or than on a cool,
cloudy day. Florists take advantage of this fact by
keeping cut flowers in a place artificially cooled by ice.

Certain structural features of the plant operate to
reduce transpiration. The epidermal hairs, as for ex-
ample on the mullein leaf, tend to retain the more humid
air near the surface of the leaf, even when the wind
blows. In some plants {e.g., the tropical gasterias, Fig.
30) the cuticle is greatly thickened, so that water can
pass off only very slowly. The very curling of leaves,
when they begin to wilt, also tends to reduce transpira-
tion by reducing the amount of surface exposed (Fig. 31).
The arrangement of leaves in a compact rosette accom-
plishes the same result (Fig. 32).

Evidence obtained by recent studies of transpiration
in several different species of flowering plants indicates
that there is no necessary nor uniform relation between
the amount of transpiration and the number of stomata
per unit of leaf-surface, nor between the amount of
transpiration and the total area of the stomata. These
studies indicate that, contrary to our earlier conceptions,
the amount of transpiration is probably regulated by a
complex of several factors, among which the stomata are
less important than was formerly supposed.

40. Advantages of Transpiration. — It might seem, at
first thought, that the loss of water by transpiration is a
disadvantage to plants. Of course this would be the case
were it not possible for the roots to take in water as fast
as it is lost. When this is not possible, transpiration

LOSS OF WATER

41

becomes a real source of danger to the plant. Indirectly,
however, transpiration performs a great service, for it
aids in, and is probably one of the chief causes of, the
ascent of liquids taken in from the soil. Were water
never given off, (either by transpiration or by secretion or

Fig. 32. — Sempervivum tabidcBJorme. The arrangement of the leaves
in a compact rosette, the hairs on their margins, their thick cuticle, and
other characters, make the plant xerophytic or drought-resistant.

both, see paragraph 41) it would not be possible for tissues,
already turgid, to receive a fresh supply, and, since all the
elements of plant-food can be carried through the plant
only in solution, the importance of this point can hardly
be overestimated.

The manner in which transpiration may facilitate the
passage of liquids through the stem may be illustrated

42

THE VEGETATIVE FUNCTIONS OF PLANTS

by a very simple experiment. A small leafy branch of
any plant (a branch of some evergreen is excellent to

Fig. ;^:^. — Experiment to illustrate the so-called "lifting power" of
transpiration, d, dish of mercury; mt, surface of mercury column that
has risen in the glass tube, gt; rt, rubber tube; br. branch of a pine tree,
whose leaves are transpiring.

use) is inserted in the end of a piece of glass tubing about
3 feet long. The joint between the glass and the branch

LOSS OF WATER 43

should be made perfectly air-tight by means of a piece of
rubber tubing, about i inch long, extending over the end
of the glass tube. Fill the glass tube with water, then
invert it, and place the lower end in a dish of mercury,
having care that the water remains up in the tube far
enough to cover the end of the branch (Fig. 2>3)- As
transpiration proceeds, the pressure of the atmosphere
will force the mercury up the glass tube as rapidly as the
water passes into the plant. This experiment is some-
times said to illustrate the ** lifting power" of transpira-
tion, but from the explanation here given, it is seen that
the mercury is not hfted, but pushed up the tube by the
pressure of the outside air. This experiment should not
be regarded as illustrating more that it really does; it
does not, for example, explain the rise of sap in plants.

41. Ascent of Sap. — It is a well-known fact that,
although living leaves deprived of water merely become
wilted, dead leaves eventually dry up; they cannot supply
themselves with water, although evaporation is taking
place from their surfaces, and although the stem to which
they are attached is abundantly supplied. We must
conclude, therefore, that merely physical forces (imbibi-
tion and evaporation) are not sufficient to account for
the rise of liquids in stems. Recent experiments indicate
that, in this connection, much importance should be
attached to the secretion of substances by the leaf — a
physiological process.

We are familiar with such action in the secretion of
nectar by the nectar-glands of flowers (Fig. 34). Some
leaves (e.g., Colocasia aniiquorum) also secrete water so
rapidly that it falls in drops from their tips. It is
probable that, in transpiration, the protoplasm in the

44 'J^HE VEGETATIVE FUNCTIONS OF PLANTS

mesophyll cells of leaves first secretes a solution to the
outer surface of the cells, and that this solution becomes
concentrated by evaporation of the liquid solvent into
the intercellular spaces. The concentrated solution, then,
by osmotic action, causes more water to be withdrawn
from the interior of the cells, and these, in turn, are

Fig. 34. — Petal of crown imperial {Frilillaria imperialis), showing the
remarkably large drop of nectar (n), secreted by the nectar gland near the
base of the petal.

replenished from the fibro-vascular bundles. On account
of the tensile strength of the water column in these bundles
the water is raised as rapidly as it is transpired from the
leaves.

Thus, while the physical processes of osmosis and
transpiration may be factors in causing the ascent of

LOSS OF WATER

45

sap, the physiological process of secretion is also of very
great importance. Moreover, on this basis we are able
to account for the ascent of sap in submerged aquatic
plants, like eel-grass, pond-weed, and others, where tran-
spiration is not possible, or in land plants in very humid
tropical regions where the nearly or quite saturated air
greatly reduces transpiration or even wholly prevents it
for extended periods of time. In fact it may be shown,
experimentally, that a leafy branch can raise water through

Fig. 35. — Experiment to show that secretory action in the cells of a
leaf are able to cause the rise of liquid in a branch, when evaporation from
the leaf-surfaces is impossible, i, Beaker containing solution of eosin;
2, cork; 3, inverted glass bell-jar containing water; 4, iron support. In
this experiment the eosin rose rapidly in the branch. (Modified from
H. H. Dixon.)

the fibro-vascular bundles, even when submerged. The
apparatus is set up as shown in Fig. 35, where the leafy
branch, immersed in water in an inverted glass bell-jar,
has the cut end of the stem in a solution of eosin or red

46 THE VEGETATIVE FUNCTIONS OF PLANTS

ink. Under these conditions only secretion can operate
to withdraw water from the fibro-vascular bundles, and
yet the eosin will rise in the branch and into the leaves.
From this and other experiments (not described here)
it is evident that the withdrawal of water from the fibro-
vascular bundles in the stem by the combined action of
secretion, osmosis, and transpiration may account for
the ascent of sap, even to the tops of very tall trees and
vines. These processes are able to raise the water column
on account of its great tensile strength, by which it does
not separate, although gravity pulls down on its lower
end, and the physiological processes in the leaves result
in a pull at its upper end.

CHAPTER V
ABSORPTION OF WATER

42. Importance of Water. — Everyone knows that plants
must have a suitable amount of water in order to live and
keep healthy. Deprived of water they wilt, ^nd finally
die. If they are given too much water they also suffer.
The water serves many purposes. In the first place, it
is needed to keep the protoplasm sufficiently moist. Pro-
toplasm may keep alive though very dry, as in the case
of dry seeds, but in order to be most active it must have
enough water to keep it in a semi-fluid condition. In
the second place, were it not for water, no food materials
could reach the protoplasts, for there are, in general, no
openings in the cell-walls large enough for solid matter to
pass through. Therefore, all substances must reach the
protoplasts in aqueous solution. Again, water is neces-
sary in the transportation of materials from one part of
a plant to another; and finally, it is necessary in order to
keep plants from wilting, for no plant can live if it is
permanently wilted.

43. Relative Water Requirement. — The amount of
water required by various kinds of plants in order to
reach maturity and produce seeds varies greatly. It
depends in part upon the weather conditions {e.g.j sun-
shine, wind, humidity, and other factors), in part upon
food supply, and in part upon the species or variety of
plant. Some species are so constructed that they con-

47

48

THE VEGETATIVE FUNCTIONS OF PLANTS

serve more of the water taken in than do other species.
The two extremes in this respect are desert plants, such
as cacti, sage-brush, and euphorbias, and water-loving
plants, such as water-lilies, ferns, touch-me-not or jewel-
weed, and cucurbits like pumpkin and squash. A con-
venient method of measuring these differences is to com-
pare the weight of water absorbed with the weight of
dry matter produced. This ratio is known as the relative
water requirement of the plant. Thus, if a given plant,
during its growth, has taken in loo pounds of water, and
the soHd matter produced, when dried out to a constant
weight in a drying oven, weighs 2 pounds, the relative
water requirement is (100 : 2) 50.

44. Government Experiments. — In experiments con-
ducted for the United States Department of Agriculture,
for the purpose of ascertaining the relative water require-
ments of various plants, it was found that the weight of
water taken in by hubbard squash plants amounted in
some cases to over 6,000 times the weight of the fruit,
and to over 900 times the weight of the total dry substance,
not including the roots. Other ratios, in round numbers,
were ascertained, as follows:

Table I. — Water Requirements of Plants

Plant

Water requirement based on

Grain

Dry matter

Rye

1,800

496

Rice.

744

Flax

Cucumber.

2,800
1,600
6,400

900
891

Sunflower

705

Purslane

300

ABSORPTION OF WATER 49

45. Importance of Root-hairs. — In Chapter II attention
was called to the root-hairs. Their chief function is the
absorption of water and dissolved substances from the
soil. This may be demonstrated by a very simple
experiment. If a young seedling of bean, corn, or any
other plant growing in soil, is pulled up, care being taken
not to loosen the dirt from the roots, then properly
transplanted in another place, and well watered, it
will continue to grow, having suffered no apparent injury.
If another seedling, of about the same size and vigor is
pulled up, and the soil removed from the roots by means
of the fingers, some of the very delicate root-hairs will
be torn off with the soil, and many others will dry up owing
to too prolonged exposure to the air. If this seedhng
is then transplanted, it will recover with difficulty, or
it will wilt and die, even though well watered, showing in
a very clear manner the inability of a plant to take in
water when deprived of its root-hairs.

46. Location of Root-hairs. — Root-hairs for study may
be easily secured by germinating seeds in a moist chamber,
formed by inverting a flower pot over a saucer of water.
Small seeds, such as flax or white mustard, will readily
adhere to the moistened inner surface of the inverted
flower pot. Within 24 to 36 hours the roots will have
developed to a length of several millimeters, and the
root-hairs will appear as a delicate white "fuzz," near
the end of the root, but not extending clear to the tip
(Fig. 36). On older roots it may be seen that the root-
hairs are confined to a relatively short zone, only a
few milhmeters long. The hairs nearer the root-tip are
shorter than those further back, indicating that they are
younger.

50 THE VEGETATIVE FUNCTIONS OF PLANTS

The tip of the root is covered by a root-cap (Fig. 37),
composed of cells that serve to protect the delicate tip as
it grows through the soil. In some cases, as in the

Fig. 36. — Germinating seeds of white mustard, showing development of

root-hairs.

water-hyacinth (Figs. 2)^ and 39), the root-cap is so well
developed that it may be easily seen with the naked eye,
and quite readily removed and replaced. Root-hairs are
never found on the region covered by the root-cap.

ABSORPTION OF WATER

51

47. Structure of Root -hairs.— The structure of root-
hairs, and their relation to the root as a whole, are illus-
trated in Figs. 40 and 41. It is seen at once that they
are epidermal cells, elongated at right angles to the sur-
face of the root, forming a thread-likfe sac, closed at both

k. ^

^

-.—re- '

Fig. 37. — Jack-in-the-pulpit (Ariscstna
triphyllum) . Longitudinal section
through a root, rt, root- tip; re, root-
cap. (After F. L. Pickett.)

Fig. 38. — Roots of the water-
hyacinth (Eichornia crassipes
Solms), showing removable
root-caps; b, root-cap removed
from c.

ends. The typical cell-structure is readily recognized —
cytoplasm, nucleus, vacuoles, sometimes merged into one
large vacuole, cell-sap, and finally the cell-wall. It has
recently been shown that the cell-wall is composed of an
inner layer of cellulose and an outer layer of calcium
pectate.

52

THE VEGETATIVE FUNCTIONS OF PLANTS

48. Relation between Root-hairs and Soil. — If young,
active roots are removed from the soil, thoroughly rinsed
in water, and examined with a compound microscope
at the zone of root-hairs, it will be seen that tiny par-
ticles of soil have adhered so closely to the cell-wall of

1' .-^m^

J

U '

Fig. 3q. — Water-hyacinth (Eichornia crassipes). Photomicrograph of
a longitudinal section of a root, showing the mode of origin of lateral roots
{i.e. endogenous), a, b, c, lateral roots; r, t, root-cap.

the hair that they were not washed ofif; in fact, they can-
not be removed without tearing the hair. They appear
to be imbedded in the cell- wall (Fig. 42), and are firmly
held by pectin mucilage, resulting from a transformation
of the outer membrane of calcium pectate. Since pectin
is a water-loving colloid its importance here is recognized
at once, in connection with the absorption of liquids by
the root-hairs.

ABSORPTION OF WATER

53

Fig. 40. — Diagram Fig. 41. — Root-hairs from the root of a mus-

sho wing relation of root- tard seedling, a, In state of turgor; h, be-

hairs to adjacent cells of ginning of plasmolysis after immersion in weak

the root, h, the oldest salt-solution; c, later stage of plasmolysis; «,

of the four hairs shown, nucleus,
and furthest from the
root-tip.

Fig. 42. — Root-hairs, with soil-particles adhering. (After Sachs.)

54

THE VEGETATIVE FUNCTIONS OF PLANTS

49. Relation between the Water and the Soil. — Fig.

43 is a diagram, showing on a greatly enlarged scale,
how the root-hairs lie in the soil, and the condition of the
soil most desirable for the well-being of the plant. It
is seen from the figure that the soil is not compact, but
open or porous, the soil particles being separated by
spaces as large or larger than themselves. Under con-
ditions most favorable for the plant, the spaces are

I

Fig. 43. — Diagram to illustrate a root-hair (A) in the soil, and its
relation to the soil-particles, the capillary film of water (w), and the air
spaces (a); e, epidermal cell of the root, of which the root-hair is an out-
growth, or branch. (After Sachs.)

filled with air, while each particle of soil is surrounded by a
thin film of water. This is the water that supplies the
plant through the root-hairs. As fast as removed it is
replenished by the capillary action of the soil. Roots
will continue to remove the capillary water from the soil
until a point is reached where the attraction of the soil-
particles for the water exceeds the absorbing power of the
root-hairs; then the plant will wilt unless more water is
added. Plants cannot absorb all the water from the soil.

ABSORPTION OF WATER 55

50. Advantage of the Air Spaces.— Living roots, like
everything else alive, need fresh air for respiration. If
the spaces between the soil particles were filled with water,
the air would be driven out, and the root-hairs could not
respire. They would soon cease to function at all, and
ultimately the whole plant would die. Thus it is seen
that plants may have too much water, as well as too
little. Farmers' crops (notably corn) often suffer from
this cause, as well as from drought. When the soil
contains too much water the leaves will commonly turn
yellow and die. In order to understand how the soil-
water passes into root-hairs, it is necessary to understand
the physical actions of diffusion and osmosis.

51. Diffusion of Gases.— If a bottle of musk, or other
perfume, is opened in one corner of a room, free from all
air currents, a person standing some distance away could,
in time, detect the odor. Now the only way we can smell
a substance is to have one or more particles of that
substance, in gaseous form, touch the olfactory surfaces
of the nose. Therefore, in the case of the musk, tiny
invisible particles must have left the surface of the sub-
stance, passed up through the neck of the bottle, out into
the room, and travelled (though without any air currents)
to the person detecting the odor. This illustrates diffusion
of gases.

52. Diffusion of Liquids. — If a small quantity of sugar
could be deposited, through a glass tube, at the bottom
of a tall tumbler filled with water, the sugar would first
dissolve, and the water near the bottom would become
sweet. If we carefully avoided stirring the water, and if
all currents in the liquid were avoided, nevertheless,
within a short time the water on the surface would taste

56 THE VEGI'/iATIVE E UNCTIONS OF PLANTS

sweet, showing that some of the dissolved sugar had
passed, by its own motion, up through the mass of the
water to the top. This simple experiment illustrates difu-
sion of liquids. The dissolved sugar behaves in a manner
quite similar to that of the gaseous "odor" of the musk.

53. Osmosis. — If, now, the denser sugar solution in the
bottom of the tumbler were separated from the less
dense water above by a porous membrane, such as a piece
of bladder or parchment, the diffusion would take place
through the porous membrane, and the water above would
soon become sweet, as in the previous case. In other
words, it is possible to have diffusion through a membrane.
Di fusion through a membrane is osmosis.

The conditions realized in the experiment described
above are a denser liquid (in the bottom of the tumbler) ,
separated from a less dense liquid (at the top of the
tumbler) by a porous membrane. Moreover, not only
would the sugar solution pass up through the membrane,
but the water above would pass in the opposite direction,
and more rapidly than the sugar solution. This would
continue until the solution was of the same density
(equal amounts of sugar in equal amounts of water)
on both sides of the membrane. Thus the action of
osmosis may be stated as follows:

When two fluids {liquids or gases) of different densities
are separated by a porous membrane, di fusion through the
membrane will take place until equilibrium results. The
difusion will be more rapid from the less dense to the more
dense fluid.

Or, again, osmosis may be defined as the interchange oj
two fluids of diferent densities when separated by a porous
membrane.

ABSORPTION OF WATER

57

54. Demonstration of Osmosis. — The contents of a
hen's egg are enclosed by a porous membrane closely ap-
pressed to the inside of the shell, except at the large end
of the egg. At this end the shell, as may readily be seen,
is more porous than elsewhere, so that air rea,dily enters,
pressing the membrane in, and forming an air-chamber
between it and the shell. With a sharp-pointed knife
the shell may here be punctured, and with the aid of small
scissors, removed so as to make an opening from J^ to
% inch in diameter. The greatest care must be taken not

Fig. 44. — Experiment with an egg to demonstrate osmosis. A, at the
beginning; B, about one hour later; w, water surface; s, support; m, egg-
membrane.

to puncture the membrane, which at this region, lies
concave (Fig. 44).

The contents of the egg are the yolk, the ''white"
(albumen) , and an aqueous solution of various salts, which
permeates the yolk and the "white." If, now, the egg
is placed upright in a glass of water, so as to be completely
covered by the water, we shall have realized the condi-
tions for osmosis, the two liquids being the water outside
and the aqueous solution inside the egg. Within a short
time the less dense water will have passed through the
membrane so much more rapidly than the dissolved salts

58 THE VEGETATIVE FUNCTIONS OF PLANTS

pass out, that the membrane will begin to bulge, becom-
ing convex and in a state of tension. This condition is
known as turgor, and the membrane is said to be turgid.
The turgor is the result of the osmotic pressure of the solu-
tion within. If osmosis is allowed to continue after this
condition is realized, the osmotic pressure will rupture
the membrane, and allow the contents of the egg to escape.
It is partly the osmotic pressure of the substances in solu-
tion in the cell-sap that keeps the lining layer of cytoplasm
closely appressed to the cell-wall of the root-hair, and other
cells.

55. Application to Root-hairs. — The application of the
experiment with the egg to root-hairs in the soil is obvious.
The cell-sap in the vacuole is the denser liquid, the soil-
water (a very weak solution of various substances dis-
solved from the soil) is the less dense, while the two
limiting membranes of the layer of cytoplasm lining the
inner surface of the cell- wall constitute two porous osmotic
membranes. Under normal conditions an interchange
(osmosis) begins between the cell-sap and the soil-solu-
tion. The latter soaks^ through the thin cell-wall, passes
by osmosis through the outer limiting membrane, diffuses
through the lining layer of cytoplasm until it reaches the
inner limiting membrane, through which it passes by
osmosis into the vacuole, and becomes a part of the cell-
sap. This process is sometimes referred to as endosmosis
(osmosis from without in). In reverse order, minute
traces of various substances pass out, by exosmosis.
Doubtless the dissolved substances that enter the cell
from without are in part altered, chemically, as they

^ It is not necessary, here, to attempt to explain this process of soaking
{imbibition), in the terms of physics.

ABSORPTION OF WATER

59

diffuse through the cytoplasm between the two limiting
or surface membranes.

66. Plasmolysis.— If the closed sac, formed by the
porous membrane, contains the less dense, instead of the
more dense liquid, then the reverse of turgor will take
place, and the sac will collapse. This may be easily
demonstrated under the microscope by irrigating root-
hairs, or other plant cells, with solutions more dense than
the cell-sap. For example, the root-hairs shown in
Fig. 41, mounted in water on a glass slide, under a cover-
glass, were found, by microscopic examination, to be
turgid. Then they were irrigated with a 5 per cent,
solution of common table salt. This solution is denser
than the cell-sap of the root-hairs, so that exosmosis was
more rapid than endosmosis, and cell-sap was withdrawn
from the vacuoles faster than liquid entered from without.
The salt-solution having soaked through the cell-wall,
passed with difficulty through the limiting membrane of
the cytoplasm, and thus began to exert an osmotic pressure
from without, which loosened the protoplasm {plasmoly-
sis^), and caused it to collapse. .When this results, the
cell is then said to be plasmolyzed.

67. Importance of Osmosis. — No physical phenomenon
is more important than osmosis. Upon it depends the
life and death of every living thing. By it, not only do
plants take in necessary substances from the soil, but all
the food assimilated by man and the lower animals passes
into their cells. It has been demonstrated that the mainte-
nance of turgidity is necessary in order that cells may con-
tinue to perform their normal functions. In a state of
plasmolysis they cannot do so. This is illustrated in a

^ From the Greek, plasma -{- luein, to loose, or set free.

6o THE VEGETATIVE FUNCTIONS OF PLANTS

simple manner by the well-known fact that, if a quantity
of salt is placed on the soil around a plant, the plant w^ill
soon die. The reason is now obvious.

68. Rigidity. — Attention has been called to the fact
that water serves as the vehicle by which substances in
the soil are carried into the roots and transported to all
parts of the plant. But the water serves another use in
helping to keep the parts of the plant rigid, and thus main-
taining their form. This service is accomplished chiefly
by means of the osmotic pressure which obtains in every
individual cell. If every cell is turgid, tissues as a whole,
and the organs of which they form a part will be rigid.
This may be easily demonstrated by plasmolyzing the
cells in a piece of rigid plant tissue, and then restoring
their turgor.

A fresh piece of a beet or turnip, about 2 inches long,
3.^ inch wide, and y^ inch thick will be found to be quite
rigid, so that it cannot be easily bent without breaking.
If the piece is now placed in a 5 per cent, solution of table
salt for 10 or 15 minutes or longer, it will be found to have
lost its rigidity, and may be bent nearly double without
breaking. The salt-solution as we know, caused the
plasmolysis, and consequent loss of turgor of every cell,
and so the entire tissue became flabby.

CHAPTER VI
THE PATH OF LIQUIDS IN THE PLANT

59. The Problem Stated. — We have seen that plants
are continually losing water by transpiration, chiefly from
the leaves, and making good the loss by absorption through
the roots. The question now arises as to how the water
passes through the stem to the leaves. Does it pass
through the entire tissue of the stem, or is it confined to
definite regions or channels?

60. Demonstration of Channels. — It will be easy to
solve our problem experimentally by placing various
stems or branches in hquid containing some coloring
substance which will stain the tissues through which it
passes. Common red ink may be used for this. Into
water, colored with red ink, may be placed young seedling
corn plants, stalks of celery, seedlings of castor-oil plants,
leaves of plantain and lily, parsnips with a portion of
the small end cut aw^ay, or any other available material.
After the stems have been allowed to stand in the ink
solution over night, they should be thoroughly rinsed, to
remove the stain from the surface^ and then examined
by cutting off a small portion of the submerged buds. It
will be clearly seen that the red coloring matter is not
deposited throughout the tissue, but is confined to
clearly marked channels — the fibro-vascular bundles. These
bundles will be found to be distributed differently ac-
cording to the kind of plant or its age, or both. Two

6i

62

THE VEGETATIVE FUNCTIONS OF PLANTS

general types of distribution will be recognized, repre-
sented respectively by the corn or the lily, and by the
castor-oil plant or the parsnip.

61. Internal Structure of the
Corn Stem. — In the corn stalk,
the fibro-vascular bundles are dis-
tributed thickly and irregularly
through the fundamental tissue
(parenchyma) of the stem (Fig. 45).
They are somewhat more numerous
near the outer rind. A longitudi-
nal section shows the bundles in side
view, extending through the stem.
The corn stalk represents a type
of structure {monocotyledonous)
common to all grasses^ and closely
related plants, and often, though
misleadingly, called endogenous.
Ingrowth, the new tissue originates
(with few exceptions, e.g., Yucca)
only at the tip of the stem. As a
rule, growth in thickness results
only by the enlargement of cells
already formed, without involving
the formation of new ones.

62. Internal Structure of the
Castor-oil Plant Stem. — A type
of structure quite different from
that of the corn stalk is illus-
trated in the stem of the castor-oil plant (Fig. 46). Here
the fibro-vascular bundles (in the young stem) are

^ The Indian corn (maize) belongs to the family of Grasses (Gramineae).

Fig. 45. — Fibro-vascular
bundles in a corn stalk {Zea
Mays).

THE PATH OF LIQUIDS IN THE PLANT 63

Fig. 46. — Diagram showing tissue-systems in young stem of castor-oil
plant (Ricinus cctnmunis), as seen in cross-section, ep, epidermis;
cor, cortex; p, pith or medulla; b, fibro-vascular bundle; ph, phloem;
ca, cambium; x, xylem; ic, interfascicular cambium.

Fig. 47, — Castor-oil plant {Ricinus communis). Cross-section of the
stem of a plant about two years old.

I

64 THE VEGETATIVE FUNCTIONS OF PLANTS

L_Jn

Fig. 48.

Fig. 49-

Fig. 48. — White lupine {Lupiniis albiis). Made
semi-transparent by being placed for several hours
in a weak aqueous solution of potassium hydroxide.
After this treatment the fibro-vascular system may
be clearly seen (through the surrounding tissues)
by transmitted light, cot, cotyledons; /;, hypocotyl;
r, main root; br, hr^, branch roots of different ages.
Note the endogenous origin of the branch roots.

Fig. 49. — The system of leaf-veins in a leaf of the
common rubber-plant {Ficiis elaslka). The veins are
connected with the root by means of the vascular
bundles extending through the stem and roots.

THE PATH OF LIQUIDS IN THE PLANT 65

distributed quite regularly in a circle, surrounding the
i:entral pith {medulla). This central tissue extends out
between the fibro- vascular bundles, forming the pith-rays,
or medullary rays. The tissue outside the zone of bundles
is the cortex.

In older stems of this type the bundles increase in
number until a nearly continuous cylinder of vascular
tissue results (Fig. 47), broken only by numerous thin
medullary rays. In cross-sectional view this cylinder, of
course, appears as a circle. Stems having this arrange-
ment of tissues grow by the formation of cylinders of new
tissue, outside of, and surrounding the older woody tissue.
On this account they are called exogenous (outside growing)
stems, or, preferably, dicotyledonous stems.

63. Extension of Vascular Tissue into the Roots and
Leaves. — As shown in Fig. 48, the vascular bundles of
the stem continue down into the roots, branching out into
the smallest rootlets, and connecting with the tissue which
lies next to the epidermis with its root-hairs. Thus it is
seen that there is an unbroken connection of vascular tissue
from the roots to the leaves. Root-hairs and leaf-veins
(Figs. 49 and 50) are the opposite extremities of this sys-
tem, which serves for the conduction of liquids through
the plant.

64. Structure of the Fibro-vascular Bundles.— When
cross-sections of the bundles of the type shown in the
castor-oil plant are examined under the microscope,
they appear somewhat wedge-shaped, with the smaller
end pointing toward the center of the stem (Fig. 51).
Three well-defined regions may be clearly distinguished,
as follows:

I. The xylem, at the pointed end of the bundle, and
s

66

THE VEGETATIVE FUNCTIONS OF PLANTS

Fig. 50. — Photomicrograph of a portion of the system of veins in a
leaf of the rubber-plant (Ficus elastica). Enlarged about six times from
a portion of Fig. 49.

Fig. 51. — The castor-oil plant {Ricinus communis). Portion of cross-
section of young stem, co, Cortex; p, pith or medulla; e, epidermis; ph,
phloem; cfl, cambium; x, xylem. The last three elements compose the
fibro-vascular bundle, the cambium being continuous from bundle to
bundle; the portion between the bundles is called interfascicular cambium.

THE PATH OF LIQUIDS IN THE PLANT 67

occupying most of its area. The cell-walls of the xylem
are thicker than those of the other cells of the bundle,
and have begun to be transformed into wood, hence the
name xylem. 2. The phloem, at the opposite, or blunt
end of the bundle. The phloem forms part of the bark.
3. The cambium, between the xylem and phloem, com-
posed of extremely thin-walled cells, and the narrowest
of the three regions. The cambium is embryonic tissue,
with its cells in a state of active division. The new cells
formed next the xylem soon become transformed into
xylem-cells; those formed next the phloem, into new
phloem. The cambium itself persists throughout the
life of the plant. It is perpetual embryonic tissue, never
becoming entirely transformed, but giving rise to new
cells on either side so long as the plant remains alive. A
strand of cambium extending between the bundles {inter-
fascicular cambium) gives rise to new bundles, as well as
to new fundamental tissue. In time the bundles increase
in thickness, and become so numerous and close together
that there is an almost continuous cylinder of wood in-
side the cambium, and a cylinder of phloem and other
tissues outside the cambium.

65. Passage of Liquids through the Stem. — The water
and dissolved mineral substances, taken in by the root-
hairs, pass up through the xylem to the leaves, while the
plant-food, manufactured in the leaves, passes down
through the phloem, and is distributed to all living
tissues. The liquids passing through the stem are
popularly called "sap.'^

66. Economic Value of Maple Sap. — In the case of
the sugar-maple, a very sweet sap flows in unusually large
quantities during the early spring period of alternate

68 THE VEGETATIVE FUNCTIONS OF PLANTS

thawing and freezing. During this season, in many parts
of the country, it is customary to ''tap" the trees, by
boring a small hole into the trunk far enough to enter the
wood, and then insert a wooden or metal "spiggot" or
spout, through which the sap flows out and is caught in
pails. It is then ''boiled down," either in the sugar bush
or in the house, until the water has passed off in large
quantities as steam, leaving a thickened maple syrup in
the kettle or evaporating pan. If the boiling is continued
the syrup, when cooled, becomes maple sugar. The saps
of the sugar-cane and of the sugar-beet are, as is well
known, the source of the ordinary sugars of commerce.

CHAPTER VII
NUTRITION

67. Organic and Inorganic. — All substances belong to
either one or the other of two classes of matter — organic
or inorganic. Organic substances are, for the most part,
those which compose the bodies of animals or plants, past
or present, or which have been, or may be, formed by the
life-processes of living things. The possibility of synthe-
sizing certain organic compounds (hydrocarbons) artifi-
cially in the laboratory has broken down the hard and
fast distinction, formerly recognized, between organic and
inorganic substances. Bone, flesh, shells, bark, wood,
leaves, gums and resins formed by plants, coal, sugar,
flour, starch, cellulose, plant and animal juices, and all
protoplasm represent organic substances. Inorganic sub-
stances are those which have never been incorporated into
the bodies of plants or animals, or if so, have since lost all
evidence of that fact. Water, salt, iron, oxygen, carbon,
glass, sulphur, air, represent inorganic substances. Some-
times the line is hard to draw. Thus a piece of wood or
of bone converted into charcoal may, if carefully handled,
retain unmistakable traces of having formed a part of
the body of an animal or a plant, but if the piece is ground
in a mortar, a fine powder may result, that has lost all
trace of its organic origin. Ordinarily, however, the
two kinds of matter are easily distinguished, either by
their structure or their known origin. So clearly distinct
and unlike are they, that one entire branch of the science

69

70

THE VEGETATIVE FUNCTIONS OF PLANTS

of chemistry, organic chemistry, is devoted to a study of
organic substances, all of which are compounds of carbon.
Inorganic chemistry is concerned with all other substances.
68. Nutrition of Plants and Animals. — The foods of
every living thing — those substances which, by digestion
and assimilation, can be incorporated into the structure

Animals

Non-
chlorophyll
bearing
plarTte

Chlorophyll-
bearing
plants

Dieintegratlon

by saprophytes

& paraeltee

Inorganic

matter

(Chemical

elements and

compounds)

Fig. 52. — The organic cycle

of the living body as a part of it — are all organic. In this
respect animals and plants are alike; they both require
organic substances as food. Thus it is evident that there
must be a continual formation of organic compounds out
of inorganic in order to maintain the food supply of the
world. As we shall soon learn, this is the great and funda-
mental role of green plants in the world's economy — to
elaborate organic compounds for food out of inorganic

NUTRITION 71

'*raw materials/' With a very few exceptions among the
bacteria, plants that are not green (such as toad-stools,
molds, and other fungi), and most animals cannot do this.
Therefore we are wholly dependent upon green plants for
the food supply of the world. Animals and non-green
plants live, either directly or indirectly, upon green plants.
The bodies of all living things are constantly giving off
inorganic compounds (such, for example, as the carbon
dioxide given off in respiration, and the water in trans-
piration), and after death all bodies are (by the action of
bacteria) gradually broken up into inorganic substances.
Thus, we see, there is a kind of perpetual cycle, or cir-
culation, from one realm to the other, as indicated in the
diagram (Fig. 52).

69. Kinds of Foods. — When we examine the bodies of
plants we find that the foods elaborated are stored in
various organs or tissues. These foods all belong to one
of three classes of substances, viz., carbohydrates^ re-
presented by starch, sugars, and cellulose; proteins,
represented by protoplasm itself, the gluten of wheat,
and other substances; and/a/5, represented by the various
oils, such, for example, as olive and cotton-seed oil.

70. Chemical Composition of These Foods. — A chemical
examination of carbohydrates reveals the fact that they
are composed of the elements, carbon, hydrogen, and
oxygen, the two latter occurring in the same proportion
as in water, namely, two parts of hydrogen to one of water
(H2O). Fats contain the same elements, only in different
proportions, while proteins, in addition to carbon, oxygen,
and hydrogen, always contain nitrogen. Other sub-
stances almost universally found in plants are calcium,
potassium, magnesium, phosphorus, sulphur, iron, and

72 THE VEGETATIVE FUNCTIONS OF PLANTS

chlorine. Calcium is not necessary for all fungi (e.^.,
not for Aspergillus niger), nor for all algae. Three main
problems now confront us:

1. What is the source of these food elements?

2. Where, in the plant, are they elaborated into plant
food?

3. How is the process accomplished?

71. Source of the Food Elements. — Since most plants
are fixed for life to a certain spot they must obtain their
food elements from their immediate surroundings; they
cannot, like animals, go in search of them. The carbon
and oxygen are obtained chiefly from the air, where the
carbon appears, in combination with oxygen, as carbon
dioxide. Free oxygen (as well as in combination with
carbon) is also obtained from the air. About four-fifths
of the air is nitrogen, and one might naturally infer
that the plant could obtain an abundant supply from
that source. We shall learn later, however, that most
plants cannot utilize the free nitrogen of the air, but
must have it supplied in chemical combination with
other elements, in the form of nitrates. These are
obtained from the soil. The calcium, potassium, mag-
nesium, phosphorus, sulphur, and iron are also obtained
from the soil in the form of soluble salts, such as phos-
phates, nitrates, sulphates, and carbonates. The hy-
drogen is obtained chiefly from water (H2O).

72. Seat of Elaboration of Carbohydrates. — Careful
and thorough studies have established, beyond all doubt,
the fact that the inorganic food elements are combined
to form carbohydrates only in the green cells of plants,
either in leaves, stems, or other parts. This is one of the
most fundamental facts in all science. Among those

NUTRITION

73

^'""'thp'^ffr ^"Sen-Housz (1730-1799). He was the first to recognize
the difiference between photosynthesis and respiration in plants

Fig. 54.-Joseph Priestley (1733-1804). He discovered oxygen, and
made pioneer studies of the function of chlorophyll.

74 THE VEGETATIVE FUNCTIONS OF PLANTS

entitled to credit for this discovery may be mentioned
Jan Ingen-Housz (i 730-1 799), a Dutch physician, and
Jean Senebier (i 742-1 809) and Nicolas Theodore de
Saussure (1767-1845), two Frenchmen. Joseph Priestley
an Englishman, also contributed to this work, both

Fig. 55. — Leaves of the tulip tree {Liriodendron tuUpifera). At left,
from a large mature tree; at right, from a young sapling. Average sized
leaves were chosen from each tree. Greatly reduced.

directly and indirectly, by his discovery, in 1774, of the
gas oxygen, and his experiments on the purification of the
air by green plants.

73. The Significance of Leaves. — While the formation
of carbohydrate food may take place in any cell con-

NUTRITION 75

taining the green substance, chlorophyll (leaf -green), the
chief organs for this work are the leaves. This explains
many facts about leaves — e.g., why they are green, why
they are thin and usually broad, why they are often
much larger in young, rapidly growing plants that need
much nourishment, than in mature plants (Fig. 55),
why they occur at or very near the tips of the branches,
where they are well exposed to light (Figs. 56 and 57).
There is no more important fact in botany, nor indeed
in all natural science, than that all the food of the world
is primarily manufactured in the chlorophyll-containing cells
of plants.

74. Importance of Sunlight. — Plants and plant parts
grown in the dark are, with rare exceptions, never green.
This means that sunlight is necessary in order to make
chlorophyll. But green plants cannot elaborate food in
the dark. This means that sunlight is necessary, not
alone for the formation of chlorophyll, but for food mak-
ing as well. Non-green tissues, even in sunlight, cannot
manufacture food; for this process both chlorophyll and
sunlight are necessary. The green cell has often been
likened to a factory; the chlorophyll is the machinery,
the sunlight is the energy, while the product of the factory
is the manufactured food.

75. Details of the Process. — The manufacture of carbo-
hydrates involves three essential steps:

1. Taking in the raw materials (water and carbon
dioxide) .

2. Recombining these parts into carbohydrates.

3. Giving off the waste material (chiefly oxygen).
Taking in the Raw Materials. — We have seen in Chapter

IV that the air spaces between the green cells of a leaf

76 THE VEGETATIVE FUNCTIONS OF PLANTS

are in direct connection with the outside air through the
stomata. In the presence of sunlight the formation of
carbohydrates begins in the green cells. This uses up
the carbon dioxide in the cells, and the supply is renewed
from the intercellular spaces. The gas passes through
the cell-walls and the layer of protoplasm in solution in

Fig. 56. — A tree (hornbeam), seen from ''outside," showing the
dense foliage at or near the tips of the branches. The same tree as in
Fig. 57-

water. This results in reducing the amount (and thus
the pressure) of this gas in the intercellular spaces, and as
a result more carbon dioxide passes by diffusion through
the stomata to the intercellular spaces. Thus, as fast
as the gas is used the supply is renewed from without.

76. Photosynthesis. — Within the cell, the carbon di-
oxide and water (or simple combinations of these) are
finally, hy a series of steps, recombined by the chlorophyll

J

NUTRITION

77

in the presence of sunlight, into a carbohydrate — probably
some form of sugar. // is this series of steps that is called
photosynthesis.'^ Not all of the oxygen contained in the
water (H2O) and carbon dioxide (CO2) used, enters into
the composition of the carbohydrate. The unused por-
tion is set free, and is either utilized in other processes,

Fig. 57. — A tree (hornbeam), seen from among the branches (as the
squirrel sees it), showing absence of leaves except at or near the tips of the
branches. The same tree as in Fig. 56.

or diffuses out through the stomata to the surrounding
air. Thus, the taking in of carbon dioxide and the
giving off of oxygen are outward indications that photo-
synthesis is going on inside the green cells.

77. Starch -making. — The sugar made by photosynthe-
sis is soluble in the cell-sap, and if it were not removed

^ The word means combining (synthesis) in the presence of, or by means
of, light (photos).

78

THE VEGETATIVE FUNCTIONS OF PLANTS

from the solution, at least as fast as it was made, it would
accumulate and thus interfere with the manufacture of
more sugar. Some of it is removed at once, either by
nourishing the protoplasm of the cell where it was made,
or by being translocated to other parts of the plant. But
some of the sugar is removed from solution by being

Fig. 58. — Cell of Pellionia Daveauana, showing starch-grains. The
black, crescent-shaped body on the end of each grain is the amyloplast.
Greatly enlarged. (Cf. Figs. 8 and 59.)

converted into starch, a substance not soluble in water.
Thus the accumulation of starch in a leaf or other green
tissue indicates that photosynthesis is in progress, and
that the resulting carbohydrate is or has been formed
faster than translocated. The conversion of sugar to
starch is accomplished by certain plastids in the cell
(Figs. 8 and 58).

NUTRITION 79

78. Enzymes. — For a long time it has been known that
during photosynthesis plants take in carbon dioxide and
water and give off oxygen, but the intermediate steps
have never been clearly understood. The appearance of
starch in green tissues is an evidence that photosynthesis
has taken place, but it was early recognized that starch
was not the first organic substance to be formed. It is
now known that some of the various steps in the process
are accomplished by means of certain substances called
enzymes, formed in every cell. Enzymes have the re-
markable power of transforming other substances, with-
out being thereby used up or permanently changed them-
selves. They belong to the class of substances known as
ferments J but their real nature and mode of action are not
well understood. Each enzyme is commonly named from
the particular substance in the transformation of which
it takes part, and this name usually ends with the termi-
nation, -ase. Thus we have oxidase, which acts upon
substances to oxidize them, maltase, which acts upon mal-
tose (a form of sugar), protease, which acts upon protein,
and so on.

79. The Steps in Starch-formation. — Careful experi-
ments have suggested that the first step in the formation
of starch may be the interaction of water and carbon
dioxide, under the agency of an oxidase, resulting eventu-
ally in formaldehyde (CH2O). The subsequent steps
may be something as follows :

2. Condensation of the formaldehyde molecules into
a simple sugar, dextrose (C6H12O6), by aldehydase.

3. Transformation of dextrose into a more complex
sugar, maltose, by maltase.

4. The changing of maltose into dextrine by dextrinase.

So THE VEGETATIVE FUNCTIONS OF PLANTS

5. The conversion of dextrin into soluble starch
(amylum) by amylase.

6. The conversion of soluble starch into insoluble starch
by yet another ferment or enzyme, coagulase.

Other analyses of the process of photosynthesis have
been suggested, and it is possible that the one outlined
above may become more or less modified in the light of
future experiments.

80. Storage of Food. — As stated above, some of the
food elaborated in the leaves is transferred to various

Fig. 59. — Starch grains from a potato tuber. (After Duncan J. Reid.)

other parts of the plant. Some of it is used immediately
to nourish these parts, but often this food accumulates
faster than needed. This is what always occurs in certain
parts of the plant, such, for example, as the tubers (under-
ground stems) of potatoes (Figs. 59 and 60), the roots of
turnips, the seeds of beans, peanuts, and other plants,
the fruits of all plants, the leaf-base of the onion (form-
ing the scaly bulbs or "onions"); and all buds when

NUTRITION

8l

they are forming. In these storage organs the soluble
sugar is generally removed from solution by being con-
verted by starch-forming leucoplasts (amyloplasls) into
starch, 1 and thus the storage organs finally become gorged
with an excess of food. It is on this account that they are
valuable as food for man.

Fig. 6o. — Young potato tuber, developed (in light) as a branch of a
sprout of an old seed-tuber. Part of the food elaborated and digested in
the leaves of the parent plant was translocated down the leaf-stalk and
stem, and stored in the older tuber, part of which is shown in the figure.
After this piece was cut off, the stored food began to be digested and trans-
located to the developing "eye" or bud, accompanied by the development
of the latter into the young tuber. Ordinarily such changes, in the potato,
occur only underground and in the dark.

81. The Need and Source of Nitrogen.— Protein foods

differ from carbohydrates and fats by containing nitrogen

which the latter lack. Notwithstanding the fact that

four-fifths of the atmosphere is nitrogen, green plants are

unable to use this free nitrogen until it has been chemically

combined with other substances, such, for example, as

1 In the case of the onion, for example, the sugar is not converted into
starch.
6

82

THE VEGETATIVE FUNCTIONS OF PLANTS

potassium, calcium, magnesium, ammonium, and others,
forming potassium nitrate (KNO3), calcium nitrate
(Ca(N03)2), ammonium nitrate (NH4)N03, and so forth.
Nitrates and ammonia may result from the disintegration
of plant and animal bodies, but the supply must con-
tinually be renewed by drawing upon the free nitrogen

Fig. 61. — Root-tubercles on yellow sweet clover {Mdilotus officinalis),
caused by and containing Psciidomonas radicicola.

of the air. The ability to ''fix" atmospheric nitrogen,
that is, to form nitrogen compounds with the free nitrogen
of the air, is possessed by several kinds of bacteria, includ-
ing species of Azotobacter, Clostridium, and Pseiidomonas.
There is also evidence that certain lower fungi possess

NUTRITION 83

this function. Pseudomonas radicicola, lives, as its name
implies, in roots, and chiefly in those of leguminous plants,
such as clover, lupine, locust, peas, beans, alfalfa, and
their near relatives. The presence of the bacterium
causes Httle swellings or nodules on the roots, commonly
known as leguminous tubercles (Figs. 61 and 228).

Nitrification, or the oxidation of ammonium compounds
into nitrates, was at first thought to be simply a chemical
process, but early in the nineteenth century it was found
to depend upon hving organisms. Numerous bacteria,
molds and other fungi {Mucor, Penicillium, Botrytis) , and
the yeast Torula are active in the formation of ammonium
salts by the disintegration of organic compounds. Follow-
ing this process nitrification results. Two important
nitrifying bacteria live in the soil; one (Nitrosomonas)
forms nitrites from ammonia, and the other {Nitrohactcr)
forms nitrates from nitrites (Fig. 229).

Root-nodules, caused by nitrogen-fixing organisms, oc-
cur also on roots of certain non-leguminous plants, includ-
ing Elaeagnaceae (Oleaster family), Myricacese (Bayberry
family), Podocarpineae, the g&nM?>Alnus, of the Betulace^
(Birch family), and Cycadaceae (Cycas family). The
roots of Cycas contain two kinds of nitrogen-fixing organ-
isms, Pseudomonas radicicola and Azotohacter (Fig. 241).

82. Value of Leguminous Crops.— Because of the
presence in their roots of organisms that can use the
free nitrogen of the air to form compounds of nitrogen,
leguminous crops are of inestimable value to agriculture.
In fact, they are absolutely necessary in order to maintain
the fertility of the soil. When any leguminous crops
are harvested, the roots are left in the soil with their
tubercles rich in compounds of nitrogen, and the com-

84 THE VEGETATIVE FUNCTIONS OF PLANTS

pounds (nitrates) are available to the next non-leguminous
crop, such as oats or corn. This is one of the main
reasons why good farmers always practice a rotation of
crops, alternating leguminous with non-leguminous plants,
for thereby the richness of the soil in available nitrogen is
maintained. Thus, for example, a certain field in Illinois
was planted to corn for 28 years in succession. The yield
of corn for the last year was 22 bushels per acre. On a
second field, planted for the same length of time with
alternate crops of corn and oats, the final yield of corn
was 36 bushels; while a third field in which corn, oats, and
clover were planted alternately, the final yield of corn
was 59 bushels — over twice the yield w^ithout rotation with
a legume. This increase in yield was due chiefly to the
enrichment of the soil in nitrates by the organisms that
form tubercles on the roots of the clover. If the entire
clover crop (tops and all) is plowed under occasionally,
so as to serve as ''green manure," the results will be
better than when the clover tops are always removed
as hay.

83. Manufacture of Proteins. — Proteins are formed
de novo only in living plant cells, by combining the prod-
ucts of photosynthesis with the nitrogen supplied in the
formi of nitrates. Their formation may be favored by
light, but unlike the process of photosynthesis, protein-
formation does -not require either light or chlorophyll.
The formation of proteins occurs in large measure in
foliage-leaves, but may take place in any living cell. The
manner of their formation is not as well understood as is
that of carbohydrates.

84. Fats. — Fats may occur in plant cells in either liquid
or soKd form. We are most familiar with the liquid fats.

NUTRITION

85

such as the various oils derived from plants. Fat occurs
in the solid form in the cocoanut. In the tallow-tree
(Sapium sehiferum) it is so abundant, as a waxy coating
on the seeds, that it is used in eastern Asia for making
candles. Fats commonly occur in droplets in the proto-
plasm, or as an emulsion in the cell-sap, but the place

1^^^^^ '

^^^^ifH

f *

1 *" ^

^-0^ J

^j^^^^^^^^Hl

^^^^^^^^^^^^^^^^^^^^^iL *'

^^^^^^^^^H

Fig 62 —Portion of a cross-section of a grain of Indian corn {Zea
Mays) G.E, glandular epithelium of the scutellum which secretes dias-
tase; G, a simple racemose gland in the tissue of the scutellum; D, duct of
the latter, emptying into the starchy endosperm surrounding the embryo.

and method of their formation have never been clearly
determined.

85. Digestion.— We have learned above (paragraph 42)
that substances can enter a plant only in solution. It is
also true that substances, even when inside the cell, can-
not be utilized as food by the plant unless they are in
solution. In order that the protoplasm can be nourished,

86

THE VEGETATIVE FUNCTIONS OF PLANTS

therefore, all insoluble stored foods must be converted
into soluble substances and dissolved. The process of
converting an insoluble food into a soluble substance and
dissolving it is digestion. This change, like the various
processes in photosynthesis, is brought about by various
enzymes, which have the power of converting insoluble

-'^'"^^1

Q

1 : — ~ r^

* -I

. #

^^1

i .^

■

^te^

^M^

H^k-

,^!^fm

^j

^^B^

Fig. 63. — Venus's flytrap {DioncEa muscipula). Insects are captured
by the rapid closing together of the two valves of the leaf-blade. When
the valves come together the rigid marginal teeth interlock, making escape
impossible. The digestive enzyme is secreted by glands distributed over
the surface of the valves. ^ (After John Ellis.)

starch, proteins, and fats, into soluble substances. Starch
is converted by diastase (one of the earliest known enzymes) ,
proteins by proteases, and fatty substances (lipoids) by
lipase.

NUTRITION

87

Digestive enzymes are probably secreted by every
active plant-cell. In the corn grain the corn starch
(endosperm), surrounding the embryo, is digested during
germination by diastase secreted by a glandular epithe-

FiG. 64.— Bladderwort {Utricularia sp.). Aquatic insects are held captive
in the tiny floating bladders, where they are digested and absorbed.

Fig. 65.— a sundew {Drosera intermedia), showing the glandular tipped
tentacles on the leaves. When an insect alights on a leaf he is held by
the sticky secretion; the tentacles then bend over in contact with his body,
holding him more firmly, and pouring out a protein-digesting enzyme
secreted by their glands. «= & /

88 THE VEGETATIVE FUNCTIONS OF PLANTS

lium of the embryo. Often this glandular layer is in-
vaginated, so as to form a true gland (Fig. 62).

A special case of plant nutrition is presented by the
insectivorous plants, of which there are many kinds.
They are all characterized by the possession of some de-
vice for capturing insects that visit them, and by the
ability to secrete a proteolytic enzyme capable of digest-
ing the protein tissues of their prey. These plants can
also digest any meat fed to them artificially. After
being digested the protein food is absorbed by osmosis.
The Venus's flytrap {Dioncea muscipula), numerous
species of bladderwort (Utricularia), and several species
of sundew {Drosera) serve to illustrate the insectivorous
plants (Figs. 63-65). Although these plants are able
to digest protein, experiments have shown that a pro-
tein diet is not necessary either for their life or healthy
growth.

86. Assimilation. — After food has been elaborated and
digested it still is not a part of the living protoplasm, but
only lies within the vacuoles of the protoplast. It is no
more a part of the plant than is food in our hands or
stomachs a part of our bodies. One step more is nec-
essary; the digested food must be incorporated into, and
made part of, the living protoplasm itself. It must be
transformed from lifeless matter into living matter. As
in the case of enzyme action, the process by which this
remarkable change is brought about is not understood.
By multitudes of accurate, painstaking experiments,
however, one great fundamental truth has been estabhshed,
namely, that non-living matter can be converted into
living matter only by the action of other living matter
already existing. Proteins, sugars, and fats have all

NUTRITION 89

been manufactured from simpler substances, artificially
in the laboratory, without the aid of living organisms;
and they may all be digested artificially in a test-tube,
also without the aid of living organisms; but, although
the attempt has often been made, no one has ever suc-
ceeded in artificially producing even the minutest drop
of living protoplasm. Only hving protoplasm, acting
directly on non-living matter, can bring about that
marvellous change. This fact is concisely expressed by
the Latin phrase, "Omne vivum e vivo'' (All life from life).

87. Biogenesis. — That living matter is always descended
from preceding living matter, and that it never arises
spontaneously from the non-living is the principle of
biogenesis.^ Opposed to this principle is the principle
of ahio genesis,"^ which teaches that living matter may
originate from non-living without the intervention of
other living matter. This was formerly quite generally
believed. Men thought, for example, that putrid meat
might become transformed directly into the maggots
(young flies) so often found in it; but we now know that
maggots in decaying meat always arise from the eggs of
flies that have previously visited the meat and deposited
their eggs there. Thanks to the painstaking experiments
and clear thinking of Redi, Pasteur, Tyndall, and others,
belief in the principle of biogenesis is now practically
universal among scientists.

That living matter could not, under favorable condi-
tions, originate from non-living matter, or that it did not
in the beginning, or never does now, cannot, of course,

^ Biogenesis, from the Greek words hios {pios), life, and genesis {y^vtais)
generation.

^ Abiogenesis. The prefix a (Greek alpha) deprives the remainder of
the word of its meaning, or reverses the meaning.

90 THE VEGETATIVE FUNCTIONS OF PLANTS

be demonstrated; one can never, experimentally or other-
wise, prove a universal negative. The principle of
biogenesis affirms that, in experiments conducted with
the utmost skill, and with every possible precaution to
exclude all traces of living matter, no faintest manifesta-
tion of life has ever been detected ; we therefore logically
conclude that, however life may have been originally
created, it never originates now from non-living matter,
but always from living matter only.

88. Dissimilation. — Nothing is more unstable than
protoplasm. No sooner is new protoplasm formed by
the process of assimilation than it begins to disintegrate,
forming various new substances, such as cell-walls, gums,
resins, latex, coloring matters (in flowers and other plant
parts), the perfumes of flowers, the poisons of poisonous
plants, and the substances that give the various flavors
and tastes to different kinds of plants. The process by
which all such substances are formed by protoplasm is
called secretion. Thus we see that protoplasm is in a state
of continual formation and disintegration. The sum total
of all these changes, both constructive and destructive, is
called metabolism. It is metabolism, above everything else,
that distinguishes living from non-living matter. We also
see that death is essential to life; unless protoplasm per-
ishes no new protoplasm can be formed. All life, it is
true, comes from life; but only on the condition that that
which is already living shall perish.

89. Economic Value of Plant Secretions. — Many of
the substances secreted by protoplasm are commercial
products. This is notably true of wood, all of which con-
sists of lignified cell-walls; of cork, which consists of sub-
erized cell-walls; of the various gums, such as gum arabic,

NUTRITION 91

gum mastic, and gum tragacanth; of the resins; and of
turpentine, rubber, vegetable dye-stuffs, perfumes, and
various other articles of commerce. Other plant secre-
tions may play an important role in practical agricul-
ture, in connection with the rotation of crops, as
indicated in paragraph 90(6), below.

90. Rotation of Crops. — Closely connected with nutri-
tion and secretion is the question of crop rotation in
agriculture. Farmers have known for ages that if one
kind of plant is grown in the same soil year after year the
yield is greatly diminished. Under such conditions, for
example, the yield of wheat will diminish from 25 or 30
bushels to 12 or 15 bushels per acre, and also deteriorate
in quality. Various hypotheses and theories have been
proposed from time to time to account for this fact, but
only three of these theories are here noted, as follows:

(a) Nutrition Theory. — We have seen above that grow-
ing plants withdraw from the soil various so-called mineral
''nutrients," in solution in the soil-water. These are
essential to the healthy, vigorous growth of the plant.
Different kinds of plants absorb these compounds in
different proportion, and one theory of crop rotation is
based upon this fact. It is argued that, by following one
kind of crop with another, different demands are made on
the soil, and the compound of which the soil was im-
poverished or ''exhausted" by the first crop is renewed
by capillary action from lower or adjacent regions. Its
renewal is also hastened by the application of suitable
fertilizer. Especially is this true in the case of nitrogen,
which is renewed by alternating with non-leguminous
plants, leguminous crops, whose root-tubercle organisms
renew the nitrates, as explained in paragraph 82. This

g2 THE VEGETATIVE FUNCTIONS OF PLANTS

theory was advocated by the great agricultural chemist,
Liebig. Undoubtedly it is correct, as far as it goes, but
a more thorough consideration of the question indicates
that it is not adequate as a complete explanation.

(b) Theory of Toxic Excreta. — A second hypothesis is
that advanced by certain investigators in the Bureau of
Soils of the United States Department of Agriculture.^
This is based on the fact that the roots of plants are known
to excrete substances which are poisonous or toxic to the
species producing them. These toxic substances accumu-
late in the soil during a succession of the same kind of
crops, and thus gradually render it toxic to that kind of
plants. By following with a succession of different kinds
of crops the toxic excreta of the first are either removed
by seepage of soil-water, or destroyed, either by being
oxidized, or by the addition of other substances which
render them harmless. By this theory the function of
fertilizers is not so much to renew exhausted mineral
''nutrients," as to render harmless the accumulated
excreta of the previous crop.

(c) Sanitary Theory. — This theory has been carefully
worked out by Professor Bolley, of the North Dakota
Agricultural College. By thorough studies of the wheat
crop he has been enabled to make the following positive
statements:^ Constant or rather constant culture of
wheat on the same lands brings about wheat-sickness, or
wheat-sick soil. Wheat does not thrive well in the
presence of its own dead bodies, no matter how fertile
the soil. Constant wheat cropping does not especially

^ A similar hypothesis was advanced by A. P. DeCandolle in his
Physiologic Vegelale, Paris, 1832.

2 The phraseology of Professor Bolley (North Dakota Agric. Coll.
Bull. 107), is closely followed.

NUTRITION 93

exhaust or use up the mineral "nutrients" more rapidly
than a succession of the different kinds of crops, nor does
it introduce into the soil any permanent excrement toxic
to wheat. It does, however, tend to introduce with the
seed, stubble, roots, et cetera a number of kinds (at least
five) of parasitic fungi that cause diseases of the wheat
plants. These fungi destroy, blight, and rot off the roots
of the plants, and live internally in the straw and the
seeds. The accumulation of these fungi in fertile soils
brings about the condition of wheat-sickness, ''wheat-
tired" soil. The fungi attack the roots, leaves, stems,
young developing grains, and seedlings, and the value of
crop-rotation lies in growing a series of different kinds of
crops that do not transmit or bear each other's diseases.
Crop-rotation is not primarily to conserve the fertility of
the soil, but is a sanitary measure, tending to eradicate
contagious disease. The reproductive bodies {spores) of
these fungi are carried from field to field and persist in the
field for some time, but lose their vitality during the few
seasons when other crops are being cultivated. It is thus
seen that one farmer, by careless methods of agriculture,
may not only suffer a loss of yield of his own crops, but
may also infect his neighbors' fields. In addition to crop-
rotation, this trouble may be reduced or avoided by care-
fully sterilizing all seed, before sowing, by soaking them
in a weak solution of formaldehyde, and also by sterilizing
the soil in a similar manner. The validity of this theory
is based upon extended studies of wheat, oats, barley,
and flax: it doubtless holds true also for other crops.

These three theories are all based upon thorough experi-
mental investigation, and it is probable that all three
contribute to a rational basis for the rotation of crops.

CHAPTER VIII
FERMENTATION

91. Importance of Fermentation. — In Chapter VII
(paragraph 78) reference was made to enzymes, which
have the power of causing marked chemical changes in
other substances without being thereby permanently
transformed or used up themselves. The number of
different kinds of enzymes now known is very great, and
it is probable that further investigation will reveal still
others not now recognized. One of the most interesting
and illuminating results of their study is the revelation
of the fact that one or more kinds of ferments or enzymes
are produced by every living cell (plant or animal), and
that life itself involves, and is in large measure depend-
ent upon, a series of fermentations. This truth, which is
becoming more and more firmly established by scientific
research, was recognized as early as 1839 by Schwann,
one of the founders of the cell-theory.^ His famous
work, '' Microscopical Researches," contains the following
passage:

"I have been unable to avoid mentioning fermentation,
because it is the most fully and exactly known operation
of cells, and represents, in the simplest fashion, the
process which is repeated by every cell of the living
body." In fact, a knowledge of enzymes and fermentation
is necessary in order to understand some of the most

^Cf. p. 15.

94

TERMENTATION

95

/f^l

fundamental processes of plant physiology. Fermen-
tation is most commonly associated in our minds with
yeast.

92. Yeast. — Practically everyone is acquainted with
yeast, which was the earliest recognized agent of fermen-
tation. We are now most familiar with it in the form
of small cakes, purchased at
the grocer's for use in making
bread and other "raised"
dough. Our grandparents
bought it in liquid form from
the local baker; and in brew-
eries and large bakeries it is
used in the liquid form in mak-
ing beer and bread. If a small
piece of a "compressed yeast"
cake, about the size of a pea
seed, is placed with a little
sugar and water in a fermenta- ^ ,, ^

. / . \ • ^'^^- ^^- — Fermentation-tube,

tion-tube (Fig. 66), and set in /, level of fermenting liqu|d

a warm place the mixture will

soon begin to "work," and tiny

bubbles of gas will be seen rising in increasing numbers

to the top of the tube. The process which gives rise to

these bubbles is alcoholic fermentation.

93. Conditions Necessary for Alcoholic Fermentation. —
If two other tubes are prepared precisely like the first
one, except that ice-water is used in one and boiling water
in the other, and are set, the first in a cold place {e.g., the
refrigerator), and the second in a very warm place, fer-
mentation will occur either very tardily or not at all.
If a third fermentation-tube is set up with sugar but no

space filled with gas (CO2)
given off by fermentation.

96 THE VEGETATIVE FUNCTIONS OF PLANTS

yeast, and a fourth tube with yeast but no sugar, no
fermentation will take place. In other words, in order
to have fermentation three conditions must be reahzed:
(i) a ferment, (2) something for the ferment to act upon,
(3) suitable external conditions.^

94. Products of Alcoholic Fermentation. — If we place
a large quantity of the fermenting mixture in a deep
glass cylinder, and cover over the top so as to hinder the
escape of the gas given off, the gas will collect in quantity
in the space above the liquid. If, after fermentation has
been allowed to proceed vigorously for a few hours, we
insert in the cylinder a lighted splinter of wood or a lighted
candle, the flame will at once go out, showing that the
oxygen of the air, which supports combustion, has largely
disappeared and has been replaced by another gas.
The test with the flame should, of course, be made also at
the very beginning of the experiment, to show that the air
above the Hquid will support combustion before fermenta-
tion has begun.

95. Carbon Dioxide Formed. — In order to ascertain
what gas has taken the place of oxygen, we may next
insert a fine wire, bent into a small circular loop at the
end, and dipped in lime-water. A film of lime-water
will extend across the space enclosed by the loop. Lime-
water has the characteristic property of turning milky
in the presence of carbon dioxide, and in this test the
film of lime-water will at once turn white or milky, show-
ing that the gas given off by fermentation is carbon dioxide.
It is the formation of bubbles of this gas in bread dough
that causes the dough to become ''light" and to ''rise."

^ The student may devise an experiment of his own to ascertain whether
or not light is necessary to fermentation.

FERMENTATION 97

96. Alcohol Formed. — If a large quantity of fermenting
liquid — as much as a pint or a quart — is boiled, and the
vapor that first comes off is condensed to a liquid, this
liquid will have the characteristic odor of alcohol, and
will burn with a pale, almost colorless flame. It is, in
fact, alcohol, and it is on this account that this kind of
fermentation is called alcoholic fermentation, to distinguish
it from other kinds. Carbon dioxide and alcohol are,
therefore, two products of the fermentation of sugar by
yeast. If the proportions of yeast, sugar, and water,
and the temperature are properly adjusted, and if the
fermentation is allowed to continue long enough, it will
be found that nearly all the sugar has disappeared, having
been converted by the yeast into carbon dioxide and
alcohol. Not all of the sugar will be converted for, as
Pasteur, the great student of fermentation, demonstrated,
small amounts of other substances such as, for example, suc-
cinic acid and glycerine, are formed by fermentation, and
these finally begin to check the activity of the ferment.

97. Heat Produced. — If a delicate and accurate ther-
mometer is inserted into a fermenting yeast-mixture, and
the temperature recorded from time to time, it will be
found that the mixture grows gradually warmer, indicating
that fermentation produces heat. The experiment will
succeed best if the yeast mixture is placed in a Dewar flask,
or, what amounts to the same thing, in a 'thermos"
bottle, which is double-walled, and thus retains more of
the heat produced than does an ordinary, single-walled
container. It is important to keep in mind the three
major results of alcoholic fermentation: (i) the formation
of alcohol; (2) the formation of carbon dioxide; (3) the in-
crease of temperature. We shall see, in the next chapter,

7

98 THE VEGETATIVE FUNCTIONS OF PLANTS

that the last two are also the results of another important
life-process of plants.

98. What is Yeast? — For centuries men employed yeast
in baking and brewing without having the remotest idea
as to what it really is, or of how it causes fermentation.
This was because they did not inquire into the matter.
It was not until 1680 that Leeuwenhoek, a Dutch natu-
ralist, discovered that liquid yeast always contains tiny
floating globules. It was 150 years after this that a
French scientist, Cagniard de la Tour, discovered that
yeast is a living organism, and soon thereafter another
observer, Turpin, demonstrated that yeast is really a
plant, closely related to the fungi. Thanks to the
painstaking work of many other students, and especially
of Pasteur, we now have a detailed knowledge of the
structure and activity of the yeast plant. It is related
to the sac-fungi (Ascomycetes) .

99. Microscopic Appearance. — If a drop of yeast mix-
ture that has been fermenting vigorously is examined

under the microscope, the indi-
vidual yeast plants may be
readily observed (Fig. 67).
They are seen to be unicellular
plants, globular or ellipsoidal in
form, of various sizes according
to age, and devoid of chloro-

FiG. 67.-Cells' of yeast P^l- The nucleus can be seen
{Saccharoynyces sp.). Some of only after the cell is stained.

the cells are budding. The ^^ c ,1 a n mi i

clear spaces are vacuoles. Many of the larger cells Will be

seen to have small outgrowths

or buds, also of various sizes according to age. It is by the

formation of the buds, that is, by budding, that the plant

FERMENTATION 99

reproduces itself. When the buds reach a certain size,
they separate from the parent plant, continue to take in
nourishment from the surrounding liquid by osmosis,
increase in size to maturity, and then give rise to other
plants, by repeating the process of budding.^ Under
favorable conditions new plants are formed very rapidly
by budding, so that in the course of a few hours the total
number of yeast plants will have enormously increased,
notwithstanding the fact that some of them in the mean-
time may have died. This increase in number may be
noted with the naked eye, by observing the increase of
turbidity or opalescence of the yeast-mixture after it has
stood for an hour or more. The yeast cakes of commerce
consist largely of starch and millions of tiny yeast plants,
skimmed from the surface of a fermenting liquid, and then
pressed together.

100. The Active Agent in Fermentation. — After it be-
came recognized that the presence of the yeast plant is
necessary, in order to have alcoholic fermentation, it re-
quired careful study before it was discovered that if the
yeast cells, after being disintegrated by grinding, are all
filtered out of the mixture, the filtered liquid still re-
tains the power to cause fermentation. From this it was
learned that the active agent, or immediate cause of the
process, is not the yeast itself, but some substance or
substances produced by the yeast. These substances are
the real ferments or enzymes, and are secreted by the
living protoplasm of the yeast. At least three different
enzymes are known to be produced by yeast. The one

^ Another process of reproduction of yeast, by the production of
endospores, need not here be described.

IOC THE VEGETATIVE FUNCTIONS OF PLANTS

that is active in converting sugar into carbon dioxide and
alcohol is a zymase, called alcoholase.

101. How Enzymes Work. — It has previously been
stated that enzymes have the ability to cause changes in
other substances without themselves being altered or
consumed thereby. The mystery of this fact has never
been fully explained, but the simile used by Huxley helps
us to form a crude mental picture of the process. ''There
can be no doubt," says Huxley, "that the constituent
elements of fully 98 per cent, of the sugar which has
vanished during fermentation have simply undergone
rearrangement; like the soldiers of a brigade, who at the
word of command divide themselves into the independent
regiments to which they belong. The brigade is sugar,
the regiments are carbonic acid, succinic acid, alcohol,
and glycerine." We may add that the commanding
officer is the enzyme, secreted by the yeast.

102. Many Kinds of Enzymes. — Two kinds of enzymes
have just been mentioned — that which converts starch
to sugar (diastase), and that which causes alcoholic
fermentation (alcoholase). In our own bodies we are
familiar with the ptyalin, or "animal diastase," of the
saliva, which can also convert starch to sugar, the pepsin
of the gastric juice, which can change the insoluble pro-
teins of meat into soluble proteins, the pancreatic juice,
and others. Among plants is found cytase, which can
liquefy the cellulose of cell-walls. It is by this means that
the delicate threads of fungi which grow on trees can pene-
trate the hard, solid wood. The enzyme, secreted by the
fungus, softens and liquefies the wood, and the delicate
fungal thread may then penetrate with ease. When
leaves fall in the autumn, the final stage in the process is

FERMENTATION lOI

the solution of a portion of the cell-walls of a layer of cells
(the ahscission-layer) at the place where the leaf joins the
branch. This greatly weakens the attachment, and the
leaf falls, often merely from its own weight. The solu-
tion of this tissue is caused by an enzyme secreted by
the cells adjacent to the abscission layer. Many of the
peculiar effects produced by molds and by bacteria are
caused by enzymes secreted by these organisms. The
ripening of cheese, the formation of alcohol in beer and
wine, the changing of sweet to sour or "hard" cider, the
turning brown of cut fruit, such as apples, when exposed
to the air, and all processes of decay are caused by fer-
mentations produced by enzymes secreted by bacteria,
molds, yeast, or other living cells.

103. The Significance of Alcoholic Fermentation. —
From what has just preceded, it is seen that the various
processes of fermentation are useful or otherwise im-
portant to mankind, but we must seek for the real sig-
nificance of alcoholic fermentation in its use to the
organism that secretes the ferment. In this connection
we must recall the fact that all the life-processes of plants
and animals require energy. The continued release of
this energy within the cells usually makes necessary a
supply of free oxygen from the air; but some organisms
can secure the necessary oxygen by decomposing com-
pounds that contain it — such as carbohydrates, proteins,
and fats. Thus we must regard alcoholic fermentation
(in part, at least) as a process by which yeast and other
organisms or cells secure the necessary energy for their
activities when deprived of the free oxygen of the air.
Alcohol and other complex compounds result because
the processes of oxidation are not completed, owing to

I02 THE VEGETATIVE FUNCTIONS OF PLANTS

the restricted supply of oxygen that can be obtained in
the absence of air. If the oxidation processes could he
completed the end products would he carhon dioxide and
water. Organisms that may continue to live without a
supply of free oxygen are called anaerohes} Either the
entire organism may live anaerobically, or this condition
may be confined to one or more of its organs, or even to
a single cell or group of cells. It is possible for yeast to
live in contact with free oxygen, but when this is not
present it can secure sufficient oxygen for its needs by
alcoholic fermentation alone.-

104. Relation of Fermentation to Our Daily Lives. —
Reference has already been made to the fact that such
diverse operations as the manufacture of all alcoholic
drinks and the making of bread are dependent upon
fermentations, and we have seen that the process funda-
mental to all life — the formation of carbohydrates by
photosynthesis — probably involves the action of at least
six different ferments. The active principles of all the
digestive juices of our own bodies are also enzymes —
diastase in the saliva, pepsin in the stomach, trypsin in the
intestine, and so on. We shall learn, in the next chapter,
that all muscular and mental activity, even life itself, is
dependent on the fermentative activity of enzymes.

^ fl + CL^^ (air) + bios (life) = living without air.

2 From a reading of Chapters VII to IX the student will learn that
the transformations wrought in other substances by the catalytic agents,
called enzymes, are not all of the same nature; some (as alcoholic fer-
mentation, and digestive processes) are destructive, others (as the poly-
merizations and other transformations in photosynthesis, mentioned on
pages 79 and 8o) are constructive. The author believes that it con-
tributes to simplicity, without sacrificing clear and accurate thinking, to
consider "enzyme" and "ferment" as synonymous terms, and to call all
activities of enzymes fermentations, whether they are analytic or synthetic.

CHAPTER IX
RESPIRATION

106. Anaerobes and Aerobes. — In the preceding chapter
we learned that all plants require energy for their ac-
tivities, and that this energy is derived by the oxidation
of substances within the cell. In the case of yeast and
other organisms, when living in an atmosphere devoid of
free oxygen, the necessary oxygen is obtained from com-
pounds which contain it, by the process of fermentation,
brought about by enzymes. As is well known, many organ-
isms, such as man and most other animals, and most
plants, cannot live in the absence of free oxygen; such
organisms, called aerobes,'^ continually take in oxygen from
the air. The using up of oxygen by the cells creates the
need for more, and in the case of man and other mammals,
the supply is obtained through the lungs by breathing.
Plants, however, have no lungs, nor any organs that
correspond to lungs. ^

When oxygen is consumed by plant tissues, its pressure
within the plant becomes less than its pressure outside
the plant, and therefore more passes in through the
stomata and epidermis by the simple physical process of
diffusion.

^ aero (air) -1- be {bios, life) living in air.

2 Leaves have sometimes been called "the lungs of plants." From our
study of nutrition, it will be readily recognized that leaves may much more
appropriately be called the stomachs of plants.

103

I04 THE VEGETATIVE FUNCTIONS OF PLANTS

106. Consumption of Oxygen Demonstrated. — The

exchange of gases between the atmosphere and the
interior of the plant may readily be demonstrated by a
simple experiment, as follows. Into each of seven glass
cyHnders (Fig. 68) fit a partition of wire gauze, so as to
divide the space vertically into equal parts. Into the
first jar place nothing, and fill the space, on one side only
of the wire gauze, in each of the other six jars respectively,
with germinating seeds (pea or lupine), living herbaceous
stems, living roots (washed free from soil but moist and
fresh), green leaves, freshly opening flower buds, and
(in the last jar) fresh mushrooms or other fleshy fungi.
Test the air in the empty half of each jar with a lighted
taper to make sure that it contains sufficient oxygen to
support combustion, and then seal all the jars air-tight
with rubber stoppers, and set them side by side in any
convenient place except in direct sunlight.^ At the end
of 12 to 24 hours again test the air in each of the jars with
the lighted taper. The air in the first jar, containing no
plant material, will still support combustion, but the
taper will be extinguished at once when lowered into each
of the other six jars.^ This shows that the air in each of
these jars has become poorer in oxygen, and that it does
not now contain enough to support combustion.^ This

1 The heat of direct sunlight is unfavorable to the best results.

^ If the taper is not extinguished in the jar containing the green leaves,
the student should be able, from his knowledge of other plant processes,
to suggest a probable reason why, and to devise suitable modifications of
th€ experiment so as to demonstrate the respiration of green leaves. In
this and other tests of the air, the cork stopper should not he removed any
longer than is absolutely necessary, and the lighted taper shoidd he lowered
and removed quickly.

^ Does the experiment show that there is no oxygen in the jars in which
the flame is extinguished?

RESPIRATION

105

io6

THE VEGETATIVE FUNCTIONS OF PLANTS

observation justifies the inference that living plants

take in oxygen.
107. Carbon Dioxide Given Off. — The air in each of

the seven cylinders may next be tested with lime-water,

by pouring in not more than
one or two tablespoonfuls, and
mixing it well with the air by
tipping the cylinders, holding
the halves with the plant ma-
terial uppermost, and allowing
the lime-water to flow back
and forth a few times from one
end of the jar to the other.
Care should be taken not to
dirty the lime-water by allow-
ing it to rinse the plant mate-
rial. After this treatment the
lime-water will be found to
have turned milky in all of the
jars except the one containing
no plant tissue. From these
results we may correctly infer
that carbon dioxide has been
given off by the plants.

108. Heat Evolved.— That
heat is evolved during the
gaseous exchange above
demonstrated may be illus-

FiG. 69. — A simple calorim-
eter for studying the tempera-
ture transformations in respira-
tion. Respiring seeds are placed
in the Dewar bulb (double walled,
with a vacuum between the
walls). At the bottom of the
bulb is a dish containing caustic
potash; cotton wool is packed
between the thermometer and
the neck of the bulb. (After
Ganong.)

trated by placing a delicate
thermometer into, say, the germinating seeds. The best
results will be obtained by placing the plant material into
a double- walled Dewar flask (or a thermos bottle), which

RESPIRATION IO7

will retain most of any heat that may be given off (Fig.
69). If the temperature is recorded at the beginning of
the experiment, and again at the end, a rise of temper-
ature will be noted. In one experiment, set up with
germinating pea seeds (air dry weight 80 grams) in a
Dewar flask as above described, a rise of 19.3° was ob-
served within 96 hours.

109. Respiration Versus Breathing. — In the case of man
and other animals, the exchange of gases and evolution
of heat, demonstrated by the experiments described above,
are an index of respiration. The process of taking in
oxygen and giving off of carbon dioxide by animals is
called breathing. It is better to restrict the term breath-
ing to the mechanical exchange of gases between the lungs
of animals adn the external air, and to confine the term
respiration to the oxidation processes of the living pro-
toplasm. It will thus be recognized that respiration is a
function of every living cell, and that the cells of our
fingers, for example, respire just as truly as do those of our
lungs and other organs. The lungs, by their mechanical
expansion and contraction, merely serve to bring the
oxygen of the external air into intimate contact with the
blood, which carries it to all respiring tissues of the body.
There is no process in plants comparable to this breath-
ing. In the case of some animals without lungs, certain
specialized organs (in fishes, the gills) are continually
bathed with external oxygen, which passes into the
blood hy diffusion. This more closely resembles the
process by which oxygen from the air passes into the
plant body. In other animals {e.g., earthworms) there
are no special organs for breathing, and the oxygen diffuses
through the moist body-walls.

I08 THE VEGETATIVE FUNCTIONS OF PLANTS

110. Stomata and Gaseous Exchange. — In Chapter IV
we described the openings or stomata in the epidermis of
leaves, through which water-vapor passed to the out-
side. We also learned in Chapter VII that the inter-
change of gases in photosynthesis takes place through the
stomata. So, also, does the exchange of gases that

Fig. 70. — White birch {Betiila populifolia). Portion of a branch showing
the prominent lenticels.

accompanies respiration. As the oxygen within the
cells is consumed in respiration, more is absorbed from the
intercellular spaces. Thus its pressure becomes less
within the leaf than without, and consequently oxygen
enters by diffusion through the stomata. At the same
time the air in the intercellular spaces becomes richer

RESPIRATION IO9

in carbon dioxide than the air outside, and therefore this
gas passes out, also by diffusion.

111. Lenticels. — Many living cells and tissues are more
deep seated than the mesophyll of leaves, and oxygen
obtains access to these cells by different ways in different
plants. Only one of these cases may be considered here.
If any young woody twig is examined, small ''dots" or
lines will be discovered on the surface. On closer exami-
nation these will be found to be small openings through
the bark (Fig. 70). They are known as lenticels. The
outer portion of the bark, and the older, inner layers
of wood are not alive, but the cambium layer, between
wood and bark is alive, and therefore needs a continual
supply of fresh oxygen. This is supplied through the
lenticels in a manner quite analogous to that by which
the supply in the leaves is maintained.

112. Respiration and Photosynthesis. — The two proc-
esses of respiration and photosynthesis are often con-
fused, but in reality thay have very little in common,
except that both result in an exchange of gases with
the external air. But it must be kept clearly in mind
that the processes themselves are quite distinct from
the exchanges of gases that accompany them, or result
because of them. Photosynthesis furnishes carbon to the
plant in a form available for use; respiration is the
physiological process by which the carbon is utilized to
supply the energy necessary for all life-processes. The
result of the two processes is the continual income and
outgo of carbon. The carbon enters and leaves the
plant in the same form, namely carbon dioxide. The
disintegration of carbohydrates is also accomplished by
bacterial decay and other fermentative processes. We

no

THE VEGETATIVE FUNCTIONS OF PLANTS

thus see that there is a continuous circulation of carbon
in nature, known as the carbon cycle (Fig. 71). A com-
parison between the two processes is shown in Table II.

Table II. — Comparison of Respiration and Photosynthesis

t. Photosynthesis

Changes inorganic
matter into plant-
foods (carbohy-
drates), which are
Assimilated and
used by the plant

To supply energy
for work,
To repair waste
(Nutrition),
In the construction
of new parts
(Development), in-
cluding Reproduc-
tion.

Photosynthesis

I. Takes place only in cells contain-
ing chlorophyll.
Requires light.
CO2 absorbed, O set free.
Carbohydrates formed.
Plant gains in dry weight.
Kinetic energy of sunlight be-
comes potential energy.

All of these proc-
esses are depend-
ent upon Oxida-
tion within the
cells.

This process of
oxidation is

called. . . .

2. Respiration,

which involves
The taking in of

oxygen,
The oxidizing of
oxidizable mat-
ter,

The release of
all products of
these oxidations.

Respiration

1. Takes place in all active cells.

2. Can proceed in darkness.

3. O absorbed, CO2 set free.

4. Carbohydrates consumed.

5. Plant loses in dry weight.

6. Potential energy becomes kinetic
energy.

113. Plant and Animal Respiration. — There is probably
no erroneous notion about plants more tenaciously held,
nor more widespread, than the belief that plant respira-
tion is the reverse of animal respiration. This error is
due entirely to a confusion of the two processes of respira-
tion and photosynthesis. From what has preceded, how-
ever, it should now be clear that plants respire in the same
way as animals, using up oxygen in the processes of oxida-
tion within their tissues, renewing the supply from the
surrounding air (or, in anaerobic respiration, from the
breaking down of chemical compounds rich in oxygen), and

RESPIRATION

III

releasing the carbon dioxide and other waste products
resulting from the oxidations. Heat is developed in both
plants and animals. The condensation of water-vapor
from the breath shows that water is formed in animal
respiration, and careful, delicate experiments have also

Carbohydrates
Plant proteins
Pats .

Fig. 71. — The carbon cycle.

shown that water is formed in plant respiration. In
both plants and animals respiration converts potential
energy (in the form of complex chemical compounds) into
kinetic energy — manifest in motion, locomotion, and the
overcoming of resistance of various kinds (that is, work).
The two processes are compared in the following table:

112 THE VEGETATIVE FUNCTIONS OF PLANTS

Table III. — Comparison of Plant and Animal Respiration

Plant respiration Animal respiration

1. Oxidations occur within the tissue i. Ditto

2. Oxygen taken in 2. Ditto

3. Carbon dioxide given off 3. Ditto

4. Heat evolved 4. Ditto

5. Water-vapor formed 5. Ditto

6. Dry weight decreased 6, Ditto

7. Potential energy becomes kinetic 7. Ditto

8. Occurs in every living cell 8. Ditto

9. Occurs without ceasing, day and night 9. Ditto

10. Accomplished by respiratory enzymes 10. Ditto

114. Respiration and Fermentation. — Perhaps one of
the most surprising and interesting of all the results of the
study of respiration is the revelation of the fact that
the real process of respiration (the oxidation of living tis-
sues, as distinguished from the exchange of gases by
breathing, or otherwise) is accomplished by enzymes
known as oxidases, and is therefore, in reality, a form of
jer meditation. In fact, the more deeply we study all the
fundamental processes of living things, the more it
seems to become evident that every chemical process in
organisms, in fact, that life itself is absolutely dependent
upon fermentations. We are brought face to face with the
almost startling fact that such commonplace phenomena as
the ripening of fruit, the raising of dough, and the decay
of plant tissues, are conditioned by the same class of sub-
stances (enzymes), and by the same kind of processes that
underlie the digestion of food, the respiration of tissues,
and the thinking of human minds. And, moreover, we
seem to be led to the odd conclusion that living organisms
do not respire because they take in oxygen, hut that they
take in oxygen because they have respired.

CHAPTER X
GROWTH

115. Definition. — Growth is increase in size of either the
organism as a whole or of any of its parts. By growth the
individual protoplast of a cell may become more bulky,
the chloroplasts or leucoplasts may become larger, the
nucleus bigger, the cell-walls thicker, the cell as a whole
may increase in any dimension, and, as a result of this,
the tissues and organs, and finally the entire organism,
may become larger. Growth does not always involve the
whole organism. Cell-walls often grow thicker while the
size of the plant as a whole does not alter. Growth often
involves a decrease in the size of one or more of the parts;
thus, when a potato ^'sprouts," the tuber itself, giving its
substance to nourish the newly formed stems, becomes
smaller and lighter in weight, while the stems increase in
size and weight. As a whole, however, the potato plant
is growing (Fig. 60).

116. Osmotic Pressure and Growth. — Growth does
not always involve increase in weight. If, for example,
the osmotic pressure increases within a turgid cell, and if
the cell- walls are elastic, the cell will grow bigger in one or
more dimensions. Not only may this growth involve no
increase in weight (as, for example, when the increase in
osmotic pressure within a cell is due merely to the chang-
ing of starch to sugar), but may even be accompanied by
loss of weight on account of waste products being given

8 113

114 THE VEGETATIVE FUNCTIONS OF PLANTS

off by the growing cell. The immediate cause of all growth
is osmotic pressure.

The great amount of force often exerted by young
growing organs, due to the osmotic pressure within their
cells, is strikingly illustrated in Fig. 72, showing the
rupturing of a concrete pavement by young fern leaves.

Fig. 72. — The rupturing of concrete by the growth of young leaves of the
ostrich fern {Onoclea SlriUhiopteris Hoflfm.). (After Stone.)

117. Experiment in Growth. — The relation between
osmotic pressure and growth may be demonstrated by a
very simple experiment, illustrated in Fig. 73. P is a
portion of the herbaceous stem of any convenient plant,
fastened securely at one end to an iron clamp (C), lying
at the bottom of a glass jar (J). The upper end of the
stem is attached by a small thread to the short arm
of an index (I), the opposite end of which may move up
and down over a graduated scale (S). If the jar is filled
with a solution of common salt water, water will pass out
of the plant tissue by exosmosis. This will reduce the
osmotic pressure {turgor) within, and the stem will shorten,
on account of the contraction of the elastic cell- walls,

GROWTH

115

causing the long arm of the index to move up over the
graduated scale. If the salt-solution is now removed, and
the jar filled with tap water, or better, distilled water,
the water will enter the cells of the stem by osmosis, in-
creasing the internal osmotic pressure and turgor of each
cell. As a result the stem as a whole will elongate or grow
in length, thereby causing the index to move down over
the scale.

Fig. 73.- — Experiment to demonstrate the relation between osmosis
and growth in length. /, jar containing water, and subsequently salt-
solution; p, portion of leaf-stalk of Rhubarb; /, index-lever (portion
omitted at b); S, scale. Explanation in text.

118. Differential Growth. — Not all the tissues of a
stem or other part grow at the same rate.^ On this ac-
count, and since adjacent tissues are closely united, those
which elongate or grow more slowly are stretched by those
which grow more rapidly. As a result either a state of
tension exists, or the organ is distorted, or both. When
one epidermis of a leaf grows more rapidly than the other,
distortion results, and the leaf becomes ''crisped" or
crinkled. This is normally the case in some plants, but

* The student should endeavor to reason out an explanation for this.

ii6

THE VEGETATIVE FUNCTIONS OF PLANTS

in others the crisping may denote a diseased condition
of the leaf.

119. Tissue-Tension. — If a thin surface strip of tissue
is cut away for a short distance from a stalk of celery,

^^■mI' s9H

VI

^^^n' -^' M^l

^^^^^^B <"^' ^^mS^^M

I' 1

^^^m '^-^^iKj^H

■f H

Kl

H^B

U

FiG.^ 74. — Longitudinal tissue-tension in leaf-stalk of rhubarb. In C
the strip of outer tissues, entirely removed from the main piece, is seen to
have shortened, showing that, before being removed, it was in a state of
longitudinal tissue-tension, owing to the fact that, in growth, the inner
tissues elongated more rapidly than the outer, thus stretching the latter
lengthwise.

Fig. 75. — Portion of dandelion scape, showing "curls" resulting from
longitudinal tissue-tension.

or the thick petiole of a burdock or other leaf, the strip
at once curves outward, on account of its longitudinal

GROWTH

117

tissue-tension. We are all familiar with this phenomenon
(Figs. 74 and 75). When boys make whistles from young
willow twigs in spring, a cylinder of bark is removed,
and may be easily replaced; but if the cylinder of
bark becomes split lengthwise, the edges cannot be
made to come together around the wood without consider-
able stretching. This illustrates transverse tissue-tension
(Fig. 76). If the preceding statements in this chapter
have been understood, the student should now be able
to explain these phenomena without further assistance.

Fig. 76. — Portion of stem of a willow, illustrating transverse tissue-
tension. By gentle tapping and twisting the cambium layer has been
bruised, so that a small cylinder of the bark was easily twisted oflf.

120. Elongation of Roots. — In order to ascertain the
manner of growth of roots, the root of a young seedling
of lupine, or other plant, may be carefully marked with
dots or lines of India ink, at intervals of i millimeter,
beginning about i millimeter back from the root-tip, and
extending for a distance of 15 to 20 millimeters. If the
root is again observed, after having been left to grow for
about 24 hours, it will be found that the first six or eight
marks near the tip have spread apart, those from 3 to 7
millimeters from the tip having separated more than those
farther back Those marks most remote from the tip
will be found to have separated very little if at all (Fig.
77). By this simple experiment we learn that the

Ill

THE VEGETATIVE FUNCTIONS OF PLANTS

growth in length of roots is largely confined to a zone a
few miUimeters in length directly back of the root-tip.
It should also be noted in this connection that the zone
of root-hairs begins just back of the zone of growth, no
root-hairs being found in the region of elongation.

Fig.

77. — Experiment to show the method of growth in length of a root.
A, 24 hours after the condition shown in the glass.

121. Elongation of Stems. — The manner of elongation
of stems may be ascertained by marking the internodes^
of a young growing stem throughout their entire length.
Two or three adjacent internodes near the tip of the stem
should be marked. After a period of from 24 to 36 hours
the marks will be found to have separated, but, in con-
trast to the behavior of the root, the marks will be found
to have separated through the entire extent of the inter-
node, and several adjacent internodes will be found to
have elongated at the same time (Fig. 78). This con-
tributes to the more rapid elongation of the stem, and is

^ An internode is the space between two successive leaves on the stem.

GROWTH

119

an advantage to the plant, since
the leaves are thereby more
rapidly brought into positions
of best exposure to air and sun-
light. The growth of several
internodes at the same time,
and their elongation throughout
their entire length, carries the
tip of the stem forward with
much greater force than if
growth were confined to a short
zone, as in the case of the grow-
ing root. But a more rapid and
forceful advance of the root-tip
through the soil might result in
serious or fatal injury, on ac-
count of the resistance and ob-
stacles encountered in the soil.
Thus the different manner of
growth of stems and roots is seen
to be of direct advantage to the
plant as a whole.

122. Growth of Leaves. — In
tropical climates leaves that
have once begun to form con-
tinue to grow until they reach
full size; but in temperate cli-
mates, having an alteration of
summer and winter, this is not
the case. Here the leaves of any
given season are all formed dur-
ing the preceding growing sea-
son, and remain over winter

A

Fig. 78. — Diagram showing
mode of growth in length of a
portion of a stem of bindweed
{Convolvulus) . A, stem with in-
ternodes marked off into inter-
vals of I cm.; B, the same stem
24 hours later, showing the rela-
tive elongation of the various
internodes. (Cf. Fig. 77.) (Re-
drawn from Bonnier and Leclerc
du Sablon.)

I20 THE VEGETATIVE FUNCTIONS OF PLANTS

in a resting state in buds. The outer coverings or scales
of the bud are modified leaves or parts of leaves. They
have almost or entirely lost their character as foliage
organs, and while they are forming, their outer (dorsal)
surfaces elongate more rapidly than their upper (ventral)
surfaces. This causes them to curve together, so as to

H^^^^^^^B

in

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i^^^^V^^H

mm

SI

Fig, 79. — Opening buds of horse-chestnut {AJlsculus Hippocaslanum).
(Cf. Fig. 80.) (Photo by E. M. Kittredge.)

overlap, and form a protection to the embryonic stem,
leaves, and other parts within the bud. With the return
of warmth and moisture the following spring, the bud-
scales resume their growth, but now their inner surfaces
elongate more rapidly than their outer, reversing the
method of their growth when forming. As a result of
this, they begin to open outward. At the same time the

GROWTH

121

embryonic parts within the bud begin to enlarge and this
helps to force the bud-scales apart. The young stem-in-
ternodes rapidly elongate, the petioles of the leaves in-
crease in length, and gradually the leaf-blades expand

Fig. 8o. — Sapling of horse-chestnut {Msculus Hippocastanwn), with young
leaves not yet wholly expanded. (Cf. Fig. 79.)

as their cells become more and more turgid (Figs. 79
and 80).

123. Permanent and Temporary Growth. — The size
finally attained by stems, roots, leaves, and other parts
is usually permanent; but some growth is temporary,
and certain tissues may manifest various alterations in

122 THE VEGETATIVE FUNCTIONS OF PLANTS

Fig. 8i. — Sensitive plant {Mimosa pudica). B, normal position of foliage
in light; A, position of foliage after the plant was slightly shaken.

Fig. 82. — Oxalis buplenrifolia. p, Petiole; b, trifoliolate blade. At the
left, leaflets in normal attitude in light; at the right, attitude of leaflets at
night or after sudden shock.

GROWTH

123

size according to circumstances. This is illustrated by the
petals of flowers (such for example as the tulip), that open
and close several times before they drop off. This motion
is caused by temporary fluctuations of growth of the upper
and lower surfaces of the petals In a similar manner is
explained the change of position of the leaflets of certain
plants, such as clover, oxalis, bean, and others, at night
or in cloudy weather, and the more rapid motion of the
leaves of the "sensitive" and other plants (Figs. 81, 82,
and 96).

1

II

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19

i^s

• •

t

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• «

1

Fig. 83. — Structure 'of seeds. Bean '{Phaseoliis), pea (Pw^^w), castor oil
{.Ricinus), lupine {Lupinus), Indian corn {Zea Mays).

124. Growth and Nourishment. — When a plant or
plant organ is growing, the protoplasm is constantly
forming new parts, and therefore must continually be
renewed or nourished. The more rapidly new parts are
formed, the greater the need for food. This need is pro-

124 THE VEGETATIVE FUNCTIONS OF PLANTS

vided for in many v/ays. Young embryo plants in the
seed find a rich supply of nourishment, ready made by the
parent plant, stored within their tissues (as in the bean
or pea), or around them (as in the corn, or castor-oil seed,
Fig. 83). They do not have to manufacture their own
food at first. Young and rapidly growing plants (seed-
lings and young saplings) have much larger leaves than
mature plants of the same species (Fig. 55). On this
account food making, from the raw materials taken in
by the plant, may proceed much more rapidly. Plants
commonly store food in quantity where it will be needed
in the future by rapidly growing new parts, and such
storage organs usually become swollen by the abundance
of stored food. This is illustrated by ''potatoes," which,
as is well known, are underground branches (tubers) stored
with food for the use of young sprouts when they begin
to grow in the spring. All bulbs are to be interpreted in
the same way. Farmers recognize the need on the part
of growing plants for an abundance of food, when they
fertilize their fields, thereby placing in the soil a rich
supply of the raw materials out of which the growing
crops can manufacture food to meet their needs.

CHAPTER XI
ADJUSTMENT TO SURROUNDINGS

126. Environment. — Not only must plants be nour-
ished, and respire in order to live; they must also be in
general harmony with their surroundings. The sum total
of these surroundings is called the environment. Among
the factors of environment are temperature, water, light,
gravity, air, electricity, soil, animals, and other plants.
It will not be possible, here, to study the adjustments of
the plant to all of these factors, but only to the more
important ones, such as gravity, water, and light,

126. Stimulus and Response. — It is the nature of the
various parts of a plant to grow in a certain definite rela-
tion to their environment. Thus, for example, main stems
and roots normally grow parallel to the plumb-line, while
their branches grow at an angle to it ; foliage-leaves grow
naturally in the light, while roots grow naturally in the
dark. Any change in the environment requires a re-
adjustment on the part of the plant, if the latter is to
remain healthy. If the readjustment cannot be made
the given organ, or the entire plant, may become un-
healthy, or may die. The change in the environment,
considered from the standpoint of its effect on the plant,
is called a stimulus; the readjustment or attempt at re-
adjustment, a response. Thus, if a plant is growing at a
certain rate at a certain temperature, any change in the
temperature becomes a stimulus to which the plant

126

THE VEGETATIVE FUNCTIONS OF PLANTS

responds by growing either more or less rapidly, as the

case may be.

127. Relation of Roots and
Stems to Gravity. — It is common
knowledge that, in general, roots
grow downward, while stems
grow upward. If a germinating
seed of lupine is so placed that
the axis of the emerging embryo-
plant is horizontal, the position
of the elongating root and shoot
after a given interval of time
{e.g. 48 hours) will be as shown
in Fig. 84. The result is the
same whether the plant is placed
in diffused light or in the dark.
The difference in the direction of
growth may not, therefore, be
attributed to the action of light.
If the seedling is so placed that
the only external influence is
gravity — the attraction of the
earth — the result is as shown in
the figure. We must, therefore,
conclude that the result is due
to the earth's gravitational at-

Fig. 84. — Seedling of white
lupine {Lnpinus albus), after traction,
having been placed horizon-
tally in the dark for 48 hours.
The hypocotyl (h) has re-
sponded negatively, and the
root (r) positively to the stim-

The correctness of this conclu-
sion may be assured by subject-
ing the seedling to another influ-
ulus of gravity. The portion ence than gravity, but, like it,

(0) remains in the original . , , . ,

horizontal position. actmg on both root and shoot, at

ADJUSTMENT TO SURROUNDINGS

127

right angles to their long axis. Such an influence is the
centrifugal tendency produced by motion in a circle. If

-^

Fig. 85.— Knight's experiment, substituting centrifugal "force" for
gravity. (After Knight.)

seedlings are fastened to a wheel, with the root and shoot
pointing in various directions, and the wheel rapidly ro-
tated, the growing root will bend until it points in the di-

128 THE VEGETATIVE FUNCTIONS OF PLANTS

rection of the *'puH" (centrifugal tendency), while the
shoot will curve in the opposite direction (Fig. 85).

The causal relationship between gravity and the direc-
tion of growth of roots and shoots was first established
by the English botanist, Thomas Andrew Knight, who
devised the experiment illustrated in Fig. 78.

128. Geotropism. — Careful thought about these results
will make it clear that the horizontally placed root, in
the first experiment, does not merely bend down because
of its weight. If this were so, we would expect the shoot
to bend down also. The curving is the response of the
organs to the stimulus of the pull. The property of an
organ by virtue of which it may detect the direction of
the pull of gravity is geotropism. It is thus seen that
geotropism is a particular kind of irritability. Organs
which respond by a curvature in the direction of the pull
are positively geotropic; those which respond by a curvature
in the opposite direction are negatively geotropic.

129. Zone of Curvature. — The following simple experi-
ment shows that the geotropic curvature always takes
place in a definite region. A germinating bean or other
seed, with the sprout Qiypocotyl) about 15 to 20 milli-
meters long, is pinned to a strip of cork, fastened to the
bottom of a Petri dish (Fig. 86). The sprout is marked
with fine lines of India ink 2 millimeters apart, beginning
2 millimeters back from the tip, as in the study of growth
(page 118). Up to this point in the operations care must
be exercised to keep the sprout as nearly parallel with the
plumb-line as possible. By rotating the cork, or the
entire Petri dish, the sprout is now fixed at right angles
to the plumb-line, and the Petri dish covered and set in

ADJUSTMENT TO SURROUNDINGS

129

the dark.^ The air in the dish may be kept moist by a
strip of damp filter-paper placed around the circumference
of the dish, inside.

Fig. 86. — Experiment to demonstrate positive geotropism in the root
of a seedling of lupine {Lupinus albus). S, metal stand; P, Petri dish,
with edges lined with moist filter-paper. The seedling is pinned to a strip
of sheet cork. The four views are of the same seedling at the successive
hours as indicated. (Apparatus after W. T. Bovie.)

At the end of 12 hours, more or less, the sprout, on ac-
count of its positive geotropism, will have responded to
the pull of gravity by curving downward, until the por-

Why set in the dark?
0

I30

THE VEGETATIVE FUNCTIONS OF PLANTS

tion between the curvature and the tip is parallel to the
plumb-line. Examination of the ink-marks will show that
the zone of curvature is the same as the zone of maximum

Fig. 87. — XcgaLive geotropism in
a barrigona palm {C ol pothrinax
Wrighlii). By some accident the
palm, when younger, had been bent
over.

Fig. 88. — Mullein {Vcrhascum
thapsus) showing geotropic recov-
ery of the terminal inflorescence,
after having been bent over.

growth in length. The stimulus of gravity has modified
the distribution of the rate of growth in such a way that
the upper side of the sprout has grown more rapidly than
the under side. After the sprout has become oriented

ADJUSTMENT TO SURROUNDINGS

131

in a vertical line, growth takes place by equal amounts,
in equal periods of time, throughout the entire diameter,
thus resulting in growth directly downward.^

Geotropic response in nature is illustrated in Figs.

Fig.

-Negative geotropism in a fleshy fungus growing on a treetrunk.

87-89. Its value is evident in insuring good light-ex-
posure and the efficient distribution of seeds and spores.
130. Problems to Solve. — Many interesting questions
now arise. How does the root, for example, " detect '*

^ Granting, of course, that all environmental conditions remain uniform
on all sides.

132

THE VEGETATIVE FUNCTIONS OF PLANTS

the fact that its axis is not parallel to the plumb-line?
Have the root and the shoot a nervous system, or a brain?
How is the bending accomplished? These questions can-
not be discussed here, but they should be given careful
thought. They lead us into one of the most fascinating
realms of plant study.

Fig. 90. — Phototropic response of a seedling of white mustard (Brassica
alba), b, box, admitting light only through a narrow slit at the right;
V, vial; w, water-surface; cl, cheese-cloth; sh, shoot; r, root.

131. Effect of Light on Direction of Growth.— The

fact that stems ordinarily grow toward, and roots away
from the light, as mentioned above, is common knowl-
edge. That this is the normal response of roots and stems

ADJUSTMENT TO SURROUNDINGS

^33

to the stimulus of light may be shown by a very simple
experiment. A young seedling of mustard, or any other
convenient plant, is allowed to develop in complete dark-
ness in ordinary tap-water, until its root and stem are

!.;::v.

f-^ y

Fig. 91. — Seedling of white mustard (Brasslca alba) showing the com-
bined effect of light and gravity on the direction of growth of root and
shoot. The dotted Hne figure indicates the position of root and shoot at
the beginning of the experiment. The change indicated was accomplished
in about 48 hours. (Cf. Fig. 84.)

each about i inch long. Being subjected to only the
influence of gravity, the root grows vertically downward,
the stem vertically upward. If the bottle and plant are

134

THE VEGETATIVE FUNCTIONS OF PLANTS

then placed inside a covered box, to which the Kght is ad-
mitted only through a narrow sht in one side, the stimulus
of the one-sided illumination will be followed by a curva-
ture of the root away from the light, and of the stem
toward the light, as shown in Fig. 90.

Fig. gj.- Mountain palm {Gaiissia sp.), growing on a steep, west-facing
cliff. The stems show a phototropic curvature toward the source of
most abundant light.

The result of the simultaneous stimulation of root and
shoot by light and gravity is illustrated in Fig. 91.

132. Phototropism. — The property of an organ by virtue
of which it may detect the direction of the source of incident
light rays is phototropism. Organs which respond by a

ADJUSTMENT TO SURROUNDINGS 135

curvature in the direction of the source of Hght are
positively phototropic; those which respond by a curva-
ture in the opposite direction are negatively phototropic.
Like geotropism, phototropism is a special kind of irri-
tabiHty. Organs growing in the light are, of course,
subject to the influence of both light and gravity at the
same time.

Fig. 93. — Seedlings of t\iQ whittlM^me. {Liipinus alb us). At the left,
grown under normal illumination; at the right, grown in darkness. Both
cultures are of the same age,

Phototropic response on a large scale, in nature, is
shown in Fig. 92.

133. Effect of Light on Rate of Growth. — Every one is
famihar with the fact that stems grown in darkness, or
in reduced light, are commonly much elongated, and

136

THE VEGETATIVE FUNCTIONS OF PLANTS

bleached to a pale yellow color, or even to white. The
chlorophyll has failed to develop. This condition is
called etiolation. Early studies of this phenomenon
seemed to indicate that light retarded growth in length,
but more thorough and more extended observations
clearly showed that such is not always the case. The

I"iG. 94. — Calla paliistris. -A , Normal plant, grown in daylight; B etiolated
plant of the same age, grown in darkness. (After D. T. MacDougal.)

stems of many kinds of plants {e.g., potato, pea, bean)
undoubtedly grow much longer in darkness than in light
(Fig. 93), but in other species the difference, if any, is
much less (Fig. 94). A disturbance of the normal illumi-
nation causes a general disturbance of the functions of the
organ or of the entire plant, so that not only growth, but
differentiation of tissues (development), nutrition, and
metabolism in general are more or less upset, and proceed

ADJUSTMENT TO SURROUNDINGS

137

138

THE VEGETATIVE FUNCTIONS OF PLANTS

in an abnormal manner.^ Under conditions of normal illu-
mination (granted, of course, that all other conditions are
normal), the physiological processes of the plant are regu-
lated in a normal manner; but when the illumination is
abnormal, healthy regulation ceases, and the organ be-
haves abnormally. This abnormal behavior includes the
failure of chlorophyll to develop, and irregularities of
growth. With many plants the stems grow abnormally
long and slender, suggesting that absence of light favors
more rapid growth in length.

,^Mp^

m^'^~

^^^km

M^ji'k^ <:-^m

Fig. 96. — Potted plants of an oxalis showing the position of the leaflets
during the day {A) and the night {B) — the so-called "sleep" of plants.

134. Relation of Leaves to Light. — More than all other
organs of the plant, the foliage-leaf is developed and ad-
justed with reference to illumination. Its form, dimen-
sions, and internal structure, and its attitude and posi-
tion on the stem are chiefly expressions of the surround-

^ It is often stated in "popular" writings that stems grown in darkness
"reach for" or "seek" the hght. A careful consideration by the student
of all that these terms imply, when predicated of a plant, will lead at once
to a recognition of their incorrectness, and even of their absurdity.

ADJUSTMENT TO SURROUNDINGS 1 39

ing conditions of light, and clearly indicate that the chief

Fig. 97. — House geranium (Pelargonium), showing back and side views
of the same plant, grown with the same side always facing a window. (Cf
Fig. 98.)

Fig. 98. — House geranium {Pelargonium). At the left, front view of
the same plant as shown in Fig. 97. At the right, about three days after
having been reversed, in front of the window. Note that only the upper,
younger leaves are properly adjusted to receive the light rays on their
upper surfaces; the lower, older leaves were not able to change the "fixed"
light position they previously assumed.

function of leaves is to bring the plant into suitable rela-
tion to light. Its flat expanded form makes possible the

140

THE VEGETATIVE FUNCTIONS OF PLANTS

favorable exposure to light of a large amount of chloro-
phyll, upon which the plant is absolutely dependent for
the manufacture of its food. Leaves that develop in
reduced light-intensity (shade), ordinarily dispose of their

Fig. qq. — Geotropic correlation among the branches of a young spruce
tree. After the terminal bud of the sapling was destroyed one of the lat-
eral branches (normally transversely geotropic) became negatively geo-
tropic; ultimately it assumed a vertical position and became the "leader"
of the tree.

tissue in such a way as to become thinner and of larger
area than when developed in more intense light. When
the light-intensity is increased, the paHsade layer often
becomes double (Fig. 27). If a leafy stem is bent, so

ADJUSTMENT TO SURROUNDINGS

141

as to bring the leaf-blade into poor light-exposure, the
leaf-stalk, in some plants, will bend and twist so as to
restore the blade to a suitable light-relation.

The leaves of some plants (e.g., nasturtium) remain
adjustable to the direction of incident light during their
active life (Fig. 95) ; the leaflets of legumes and some other

Fig. 100. — Vertically growing branch of a maple, side view, showing
elongation of the petioles of the lower leaves, which serves to prevent their
being shaded too much by the leaf-blades above them. (Cf. Fig. loi.)

plants (Fig. 96) fold together each night and on cloudy
days, thus manifesting the so-called ''sleep" of plants.
Whether this response is of value to the plant is not
entirely certain. In other plants {e.g., the house geranium)
the leaves have a fixed light- position, and after reaching
maturity are not able to readjust themselves to changed
conditions of illumination (Figs. 97 and 98).

142

THE VEGETATIVE FUNCTIONS OF PLANTS

135. Correlation within the Plant. — It is of interest to
note that the parts of a plant are not only sensitive to the
stimulus of their surroundings, but are correlated, or ad-
justed to each other. This correlation modifies their
reaction to external stimulation. Thus, if the

''leader,"

Fig. ioi. — Adjustment of leaves, by the lengthening of the petioles, to
secure the best illumination, in maple. Top view of same branch as in
Fig. I GO.

or main stem, of a tree is destroyed, one or more of the
lateral branches will turn upward, in what appears to
be an ''endeavor" to perform the functions of a leader
(Fig. 99). In other words, the destruction of the
leader alters the mode of reaction of the lateral branches
to the pull of gravity.

ADJUSTMENT TO SURROUNDINGS

143

136. Correlation among Leaves.— On a vertical and
equally illuminated branch, as of maple, the petioles of
the lower leaves are longer than those above them, and
the stalks of opposite leaves are usually about equal in

the same seedlings. ' """P""^' ^«- 95, showing other views of

length (Figs. 100 and loi). But on horizontally growing
branches, the attitude of the leaves is profoundly altered
The length of the petioles, and their attitude is such as
to msure the placing of the blades in positions for the
most favorable illumination. This often results in a
marked twisting and bending of the leaf-stalks. Blades

144

THE VEGETATIVE FUNCTIONS OF PLANTS

of opposite leaves often do not appear to be opposite each
other, and are often of very unhke size. Such an arrange-
ment of the blades forms a leaf-mosaic (Figs. 102 and 103).
If a leaf is removed from a group, or even if a leaflet is

t^^-7m''^\^m

mx^^^

^:^i
v-^^:,

Fig. 103. — Leaf-mosaic in the Boston ivy.

removed from the blade of a compound leaf, the remaining
leaves or leaflets will alter their positions with reference
to each other so as to occupy the space most advan-
tageously and economically (Fig. 104).

137. Advantages of Power of Adjustment. — Very little
thought will enable one to understand at once the pro-

ADJUSTMENT TO SURROUNDINGS

145

Fig. 104. — Effect of removal of a leaflet from a palmately compound
leaf {e.g. Woodbine). B, normal leaf; C, after removal of upper right-hand
leaflet. A, Unbroken lines represent average normal position of leaflets;
dotted lines, average position of leaflets after operation; barred line, posi-
tion of leaflet removed. (A, after Zeleny.)

Fig. 105. — Motherwort (Lconurus Cardiaca), showing adjustment of
leaves to light. This plant grew at the edge of shrubbery, receiving rela-
tively little light from behind.

146 THE VEGETATIVE FUNCTIONS OF PLANTS

found significance to the plant of its ability so to adjust its
organs as to bring them into harmony with surrounding
influences. If a stem, bent over, could not erect itself,
if leaves could not assume positions that secure the
most favorable illumination, if stems and leaves were not
correlated to each other, most plants would soon be out
of harmony with their environment and would sicken and
die. Fully one-half the leaves of the motherwort, illus-
trated in Fig. 105, would have been deprived of suitable
illumination by adjacent plants, had they not possessed
this power of adjustment.

138. Purpose of Part m. — Chapters I to XI have dealt
with all parts of the plant except the flower, and all the
activities studied have been primarily for the sake of pre-
serving the life of the individual plant. Flowers function
primarily for the race to which the individual plant
belongs; but they can be really understood only after a
thorough study of the life histories of some of the lower,
non-flowering plants. The study of life histories will be
taken up in Part III.

PART HI

STRUCTURE AND LIFE HISTORIES

CHAPTER XII

LIFE HISTORY OF A FERN

139. Morphology. — In the preceding chapters we con-
sidered various physiological processes, the primary
result of which was to maintain the life of the individual
plant. Most of those processes are carried on by all
plants. Every one knows, however, that plants differ
widely from each other in both structure and habit of
life. In other words, we recognize the fact of variation.
This means that different plants solve the same problems
of life in different ways. For example, some plants expose
a large amount of chlorophyll to sunlight by forming thin
leaf-blades of relatively large area; while others, such as
the cactus, accomplish the same result by developing
thick, succulent green stems, and dispensing with leaves
entirely. Some leafy plants raise their foliage up to the
light on strong woody stems, able to stand alone; while
others secure this result by climbing up on other plants.
In many cases the organs of plants are disguised, appear-
ing to be what, in reality, they are not; stems may mas-
querade as leaves, and leaves as stems. That phase of
botany which concerns itself with a comparative study of
structures, and seeks to interpret the real structural

147

148

STRUCTURE AND LIFE HISTORIES

nature of an organ, no matter how it may be disguised, is
termed the science of form, or morphology.

140. Life History. — Every plant, in the course of its
existence, passes through a series of changes in orderly
sequence. Like an animal, every plant begins life as a
single cell, the egg, or the equivalent of an egg. Except
in some of the lower forms, the egg develops into an

Fig. io6. — A fei

(Afiisosorus hirsukis), showing portion of the stem
above ground.

embryo, and the embryo matures into an adult. By a
series of more or less complicated processes the adult
eventually gives rise to another egg, like the one from
which it came, thus completing one life-cycle and initiat-
ing another. These various changes constitute the life
history of the individual. The various stages of life
history common to most plants are nowhere more clearly

LIFE HISTORY OF A FERN

149

Fig. 107. — Portion of the rhizome of the common brake {Pteris aquilina)
showing a cross-section view at the right. The two dark areas are scler-
enchyma.

Fig. 108. — Cross-section of the rhizome of the bracken fern {Pteris aqiii
Una), showing the tissue systems. Greatly magnified.

ISO

STRUCTURE AND LIFE HISTORIES

illustrated than in the ferns. We shall therefore begin
our study of life histories with that group of plants.

141. Description of a Fern Plant. — The more common
ferns of temperate regions have underground stems or
rhizomes (sometimes called root-stocks) , so that only the

Fig. 109. — Tree ferns on the military road between Cayey and Caguas,
Porto Rico. (Photo by M. A. Howe.)

leaves appear above ground.-^ The stem may be branched
or unbranched. When branched, the branches are pro-
duced without reference to the insertion of the leaves,
in contrast to the habit of higher plants of forming
branches only in the upper angle {axil) between the stem

1 The leaves of ferns are often called fronds.

LIFE HISTORY OF A FERN

151

and leaf-stalk. There is always a terminal bud at the
tip of the fern-stem (and of the branches when any oc-
cur) ; and the leaves are usually attached just back of this
tip. The stems are commonly (though not always)
covered by hairs or scales (Fig. 106), and on their older
portions, at some distance back from the tip, may be seen

C^Mr

Fig. 1 10. — A, Upper epidermis; B, lower epidermis, of the fern, Drynaria
meyeniana. (Camera lucida drawing.)

the scars, or the ends of leaf-stalks, left by old leaves that
have died and fallen away. The rhizome bears true roots
(Fig. 107), and its tissues are differentiated into epider-
mal, fundamental, mechanical, and conducting systems
(Fig. 108). In tropical countries there are "tree ferns,"
with upright stems, and this type of fern is common
among the fossil plants of earlier geological ages (Fig. 109).

152

STRUCTURE AND LIFE HISTORIES

142. Two Kinds of Fern-leaves. — Careful examination
of the leaves of some mature ferns will disclose the fact
that they are not all alike. Some of them are merely
foliage-leaves, and do not differ in any essential point from

Fig. III. — Osmunda Claytoniana. Young sporophylls, showing circinate
vernation. Note the spore-bearing pinnae.

the foliage-leaves of higher plants, such as the maple or
lily; they possess stomata (Fig. no), and also resemble
the leaves of higher plants in their internal structure.
All fern-leaves, however, have a very characteristic ar-
rangement in the embryonic or bud condition, being

LIFE HISTORY OF A FERN

153

coiled up from the tip. As the leaves grow they unroll,
and in some ferns, at certain stages, they often closely re-
semble the neck of a violin (Fig. in). The leaf -blade
possesses veins of fibro-vascular bundles that pass down

Fig. 112. — Portions of the sporophylls of two ferns to show the sori.
On the left Poly podium piinctaliim (L,)Sw.; on the right a variety of Pteris
longifolia, with sporangia marginal on the pinnules.

the leaf-stalk and through the stem to the roots. Because
of the possession of these vascular bundles, ferns (and
all other plants of which this is true) are called vas-
cular plants. These leaves perform all the functions

154

STRUCTURE AND LIFE HISTORIES

performed by the foliage-leaves of other plants, the most
important of which are photosynthesis and transpiration.
143. Spore-bearing Leaves. — The second type of fern-
leaf bears, on its underside, numerous "fruit-dots" or sori

Fig. 113. — Sporophylls of two ferns. At the left, a species oi Poly podium
(Phymatodes), having no indusium; at the right, Diplazium zelanicum.

(singular sorus) (Figs. 112 and 113). These structures
have to do with reproduction. A single sorus of such a fern
as, for example, Polypodium irioides, is composed of a

LIFE HISTORY OF A FERN

155

cluster of tiny stalked cases. The cases contain minute
•unicellular reproductive bodies called spores, and the en-

FiG, 114. — Cross-section through the marginal sorus of a sporophyll of
the bracken fern (Pteris aquilina). I, palisade layer; /&, vascular bundle;
sp, sporangium; in, indusium. (Greatly magnified.)

tire structure is a sporangium. The place where the
sporangia are attached to the leaf is the sporangiophore^

Fig. 115. — Cyrtomiiim falcatum. Under (dorsal) surface of a portion of
a sporophyll, showing the numerous sori on the pinnae.

(Fig. 114), and over all is often found a thin membranous
covering, the indusium (Figs. 114 and 115). In some ferns
the indusium is lacking, and the sorus is naked. Spore-

1 Also called receptacle.

156

STRUCTURE AND LIFE HISTORIES

bearing leaves are called sporophylls, and plants that bear
sporophylls are called sporophytes.

144. Types of Foliage-leaf. — In some ferns the foliage-
leaf presents a simple, unbranched blade, and petiole;
but in other species the blade is variously branched. In
such cases the larger, primary divisions are called pinn(Bj

Fig. 1 1 6. — Fern leaves, showing various degrees of subdivision or branch-
ing of the blade. A, Phyllitis; B, Polypodium; C, Pteris; D, Adiantnm.

and the secondary subdivisions pinnules. Illustrations
of these various types are shown in Fig. ii6.

145. Sporangia. — As noted above each sporangium con-
sists of a spore-case borne on a stalk (Fig. 117). The struc-
ture of the case varies considerably in various groups of
ferns, but it usually possesses walls only one cell thick, with

LIFE HISTORY OF A FERN

157

a clearly differentiated region, the annulus, composed of
cells whose radial and inner cell-walls are greatly thick-
ened. Various types of spore-cases are illustrated in

Fig. 117. — Sporangia of an undetermined species of fern; //, lip-cells;
an, annulus; st, stalk; sp, mature spores. Each of the four nuclei in the
upper cells of the stalk is in the terminal cell of one of the four rows of cells
that compose the stalk.

Fig. 118. Among the group of ferns which are now most
common, and to which the bracken fern (or ^' brake"),
the maiden-hair fern, the common polypody, and others

Fig. 118. — Types of fern sporangia. A, Loxsoma Cunninghami; B,
Gleichenia circinata; C, Todea harbara; D, Thyrsoptcris elcgans; E, Matonia
pectinata; F, Lygodium ja ponicum . (Redrawn from various sources.)

belong, the sporangium always originates from a single
epidermal cell. Ferns whose sporangia thus originate are
called leptosporangiate ferns. The walls of their spore-

158 STRUCTURE AND LIFE HISTORIES

cases are always only one cell thick, and always possess
some form of annulus.

146. Spores. — As the sporangia mature the spore-case
itself becomes differentiated into two distinct kinds of
tissue, namely, sterile tissue on the outside, forming the
wall, and fertile tissue within. In most species of ferns
the fertile tissue of each sporangium becomes organized
into 16 relatively large cells, rich in protoplasm. As soon
as these cells are mature they divide once, and then, with-
out resting, a second time, thus giving rise to four cells
each. Each group of four cells is called a tetrad; each cell
of a tetrad becomes a spore. There is always an even
number of spores (usually 64) formed in each spore-case,
and the cells from which they are formed by the two suc-
cessive cell-divisions (tetrad-divisions) are spore-mother-
cells. The spores finally become separated from each
other by the dissolving of the middle layer (middle lamella)
of the cell-wall between adjacent spores. The solution
of this middle layer is accomplished by an enzyme
secreted by the cells, and which acts upon this par-
ticular layer. The spores finally come to lie dry and
perfectly free from each other within the spore-case.
Their purpose is to reproduce the plant, and especially
to multiply the number of individual plants.

147. Number of Spores. — The number of spores pro-
duced by a vigorous fern is a great revelation to one who
has never given such matters careful thought. Pro-
fessor Bower, of Glasgow, has called attention to this fact
in the following words:

"a rough estimate may be made of the numerical output of spores from
a large plant of the Shield fern, as follows: In each sporangium 48^

^ Bower gives this number as the characteristic output for the species
Aspidium Filix-mas. In other species the number may be 64.

LIFE HISTORY OF A FERN

159

spores may be formed; a sorus wiU consist of fully 100 sporangia, usually
more; 20 is a moderate estimate of the sori on an average pinna; there may
be fully so fertUe pinnae on one well-developed leaf, and a strong plant
would bear 10 fertile leaves. 48 X 100 X 20 X 50 X 10 = 48,000,000.
The output of spores on a strong plant in the single season will thus, on a
moderate estimate, approach the enormous number of fifty millions."

148. Types of Sporophylls.— In many ferns the leaves
serve both vegetative and reproductive functions in about

Fig. 119— The cinnamon fern (Osmunda cinnamomea), showing foliage
leaves and sporophylls. (Photo by Elsie M. Kittredge.)

equal degree, as in the case of Polypodium mentioned
above. In some species, however, there are two kinds of
leaves— one devoted entirely to vegetative functions, and
another to the reproductive, or spore-producing function
(Fig. 119) ; between these two extremes all grades of trans-
ition are found (Fig. 120) . But however widely the sporo-

i6o

STRUCTURE AND LIFE HISTORIES

Fig. 120.— Clayton's fern {Osmunda Claytoniana), showing sporophylls
in the center, surrounded by foliage leaves.

phyll departs from a foliage-leaf in appearance, it must,

nevertheless, be regarded as morphologically a leaf. As par-

LIFE HISTORY OF A FERN

i6i

tial evidence of the true foliar nature of sporophylls, there
may be cited the interesting experiment of Atkinson, who,
by removing the true fohage-leaves just beginning to unfold

r.

4

Fig. 121.-— a fern (Tectoria cicutaria) that bears bulbils on both the
upper and lower surfaces of its leaves. Plantlets develop from the bulbils
while they are still attached.

in the spring, was able to induce developing sporophylls
to alter their character, and become transformed into
foliage-leaves. Similar results were also obtained by
Goebel. These experiments indicate that foliage-leaves

l62

STRUCTURE AND LIFE HISTORIES

and sporophylls are very closely related to each other,
and demonstrate clearly that foUage-leaves may be

Fig. 12 2— Walking fern {Camptosorus rhizophyllus). The smaller,
lower plant originated at the tip of a leaf of the larger plant, and one of its
leaves has, in turn, struck root.

produced by the sterilization of spore-bearing leaves.
The interesting question here naturally arises as to

LIFE HISTORY OF A FERN 1 63

whether, in the evolutionary development of the plant
kingdom, through long geological ages, foliage-leaves have
in general originated by the sterilization of spore-bearing
organs.

149. Vegetative Multiplication. — In addition to re-
production by spores, ferns may also be propagated vege-
tatively in at least four ways. By one of these methods,
the rhizome is cut into several pieces, and from every

Fig. 123. — A Boston fern (Nephrolepis), reproducing vegetatively by
means of runners or stolons. The parent plant is in the round pot. (After
R. C. Benedict.)

piece that contains a bud a new plant will develop. By
the second method, the plant is propagated by means of
bulbils^ which occur on the foliage-leaves of several
species. In the fern Tectoria cicutaria, bulbils occur on
both the upper and under sides of the leaf (Fig. 121). These
bulbils fall to the ground, and under suitable conditions of
light, moisture, and temperature give rise to new fern-
plants. One of the ferns native to the eastern United States

164

STRUCTURE AND LIFE HISTORIES

{Cystopleris bulbijcra) received its specific name from the
fact that it bears bulbils. A third method is illustrated
in the very interesting ''walking fern" {Camptosorus
rhizophyllus) , where the tips of the long acuminate leaves
rest upon the moist ground, take root, and develop an
entire new plant at the distance of the leaf's length from
the parent fern (Fig. 122). The result of several repeti-
tions of this suggested the common name '^ walking fern."
A fourth method is by means of stolons or "runners"
(Fig. 123).

150. Dispersal of Spores. — After the spores are mature
the essential need is that they become dispersed, so that

Fig. 124. — Tips of two sporophylls of the fern, Drynarla mcyeniana,
showing the large marginal sori. The black dots adjacent to the leaf-tips
are spores projected onto white paper by the snapping of the sporangia.
The specimens were covered with a bell-jar.

they may find favorable conditions of moisture, tem-
perature, light, and soil for development; for, with rare
exceptions, such conditions do not obtain within the
spore-case. Moreover, if the spores remained within the
sporangia they would be so greatly crowded that only a

LIFE HISTORY OF A FERN

165

very small percentage of them would be able to develop
into new plants. When the spores are ripe, the spore-case
opens, and by various movements the spores are expelled.
That sporangia are able to throw the spores to a
considerable distance may be shown in a very simple
way by placing a portion of a sporophyll with mature
sporangia on a sheet of white paper, with the fruit-dots
uppermost, and covering it with a large bell- jar. Within
a few hours the scattered spores may be seen against
the white background of the paper, and the greatest dis-
tance to which they have been thrown may be easily
measured (Fig. 124).

Before germination;
b, early stage, showing protonema (pr.), and first rhizoid (rh); c, d, e, f,
successive stages in the development of the prothallus.

151. Germination of Spores. — After dispersal, and
under favoring conditions of temperature, moisture
and light, the spore begins to absorb water, and soon
commences to grow. As the internal pressure in-
creases, the walls of the spore are burst apart, and a tiny

l66 STRUCTURE AND LIFE HISTORIES

tube, the germ-tube, or protonenia (first thread), begins
to develop. This process is germination. Shortly, near
the wall of the spore, a smaller, slender tube develops as
a branch of the germ-tube (Fig. 125 ). This is the first of
innumerable root-like bodies, or rhizoids, which will
help to hold the new plant firmly to the soil, and also serve
to take in water and dissolved mineral nutrients.

152. The Prothallus. — Before the germ- tube has greatly
enlarged, it becomes divided into two cells, and then, by

Fig. 126. — Prothallus of a fern. Archegonia on the (central) cushion,
near the notch; antheridia among the rhizoids, below. (After Margaret
C. Ferguson.)

successive cell-divisions, into an increasing number.
Meanwhile chlorophyll bodies begin to appear, but never
in the rhizoids. The final product of these cell-divisions
and growth is a tiny, flat, green body, often (but not
always) heart-shaped, with a central portion, the cushion,
several cells thick, and a marginal part, the wings, of

LIFE HISTORY OF A FERN 1 67

only one cell in thickness. Because of its flatness this
little plant (for such it is) is called a thallus; and because
it precedes, in the order of reproduction, the new sporo-
phyte, it is called the prothallus (Fig. 126). It is usually
possible to divide the prothallus into right and left
halves, similar in shape and in other characters, and hence
it is said to possess bilateral symmetry.

CHAPTER XIII
LIFE HISTORY OF A FERN (Concluded)

The prothallus, as just described, bears little resem-
blance, indeed, to the fern plant with which we are com-
monly familiar. In fact the relation between the two was
not understood, nor even suspected, until about 1848,
when Count Lesczyc-Suminski, a Polish botanist, first
gave a connected description of the life history of the fern.
We shall now proceed to follow the steps which lead from
the prothallus to the new sporophyte.

153. Dorso-ventral Differentiation. — The appearance
of the first root-like body, or rhizoid, was noted above.
As the prothallus develops the rhizoids become more and
more numerous, forming a mass of fine thread-like bodies
on the under side, opposite the notch, of the heart-shaped
prothallus. The presence of rhizoids, and of other
structures soon to be described, make it easy to dis-
tinguish at once the surface that bears them from the
opposite surface. Since the surface bearing the rhizoids
lies normally next to the substratum it was called the
dentral surface, while the opposite surface was called
vorsal. As now used, the terms dorsal and ventral are
morphological terms, and have no reference to the manner
in which the prothallus lies. Normally the ventral surface
is the under one and the dorsal surface the upper, but
the application of the terms would not be changed if
the differentiated prothallus should happen, by any

168

LIFE HISTORY OF A FERN

169

chance, to lie upside down. The dorsal surface would
then be the under surface, and the ventral surface the
upper one. Organisms or organs having two such surfaces
clearly distinguishable are said to have dorso-ventral
differentiation. Among many other structures thus dif-
ferentiated are foliage-leaves, sporophylls, man, fishes, and
other animals.

154. Reproductive Organs : Archegonia.^Examination
of the ventral surface of a mature prothallus with a lens

Fig. 127. — Archegonia of a fern {Adiantum). A, young archegonium;
B, mature; C, top view, showing terminal cells of the four rows of wall
cells; V, wall of venter; e, egg; v.cx, ventral canal-cell; n.c, neck-canal;
sp, sperms entering the neck-canal. A and B in longitudinal section.

will reveal near the notch and on the cushion, several
tiny flask-shaped bodies, the archegonia. Each arche-
gonium consists of a wall, one cell thick, and contents
(Fig. 127). The neck projects away from the surface,
and is usually slightly curved, while the remainder, the
venter, is imbedded in the tissue of the cushion. As the

lyo STRUCTURE AND LIFE HISTORIES

archegonium approaches maturity it is seen to contain
three cells; a long neck-canal cell, nearly filling the neck,
an egg-cell or ovum, filling the venter, and between these
two a ventral-canal cell. The egg is the female reproduc-
tive cell. As it matures, the other two cells become disin-
tegrated into a mucilaginous mass that fills the neck-canal.
Since the archegonia contain the eggs they are the female
reproductive organs.

155. Reproductive Organs : Antheridia.^ — Search among
the rhizoids will reveal another class of organs, the

Fig. 128. — Portion of a cross-section of a prothallus of a fern (Adian-
tum), showing an antheridium (an), and sporogenous cells within. (Drawn
from preparation of E. W. Olive.)

antheridia, globular and also having walls only one cell
thick. These are the male reproductive organs. At
maturity they contain a large number of tiny motile cells,
composed chiefly of a coiled nucleus, and able to swim
about in water by the vigorous lashing of numerous little
thread-like cilia attached to one end. These are the
sperms, or male reproductive cells (Figs. 128 and 129).

156. Fertilization. — Neither the eggs nor the sperms are
able, independently, to reproduce their kind. In order

LIFE HISTORY OF A FERN

171

to accomplish this they must unite, and the fusion of the
sperm and egg is fertilization. One of the most significant
facts about fertilization in ferns is that free water is re-
quired, in order that the sperms may reach the egg by their
own locomotion. When the antheridia and archegonia
are mature, a suitable amount of water (such as would
result from a rain or a copious dew), soaking through the

Fig. 129. — Fern prothallus; cross-sections showing antheridia (aw),
sperms (sp), and rhizoids (rh). Below at the right is a sperm (sp) greatly
enlarged.

archegonial walls, will cause the mucilaginous matter in
the neck-canal to swell. This in turn will rupture the
archegonia at their distal ends, and a portion of the con-
tents of the neck-canal will become extruded, while the
egg will remain in the venter. The same conditions of
moisture will cause the rupture of the antheridia, and the
sperms will be set free (Fig. 129). The mucilaginous matter

172

STRUCTURE AND LIFE HISTORIES

extruded from the archegonia contains a substance (malic
acid, in some ferns) which stimulates the sperms to swim
toward it. This they are enabled to do by the free
external water. On reaching the archegonia, they enter
it, and swim down the neck-canal to the egg. The sperm
that first reaches the egg penetrates it, and passes through

Fig. 130. — Fertilization in the fern, Onoclca. A, longitudinal section
of archegonium, showing the egg in the venter, and numerous sperms
passing down the neck-canal. B, an egg-cell in the venter. One sperm
has entered the nucleus, three sperms have failed to enter the egg. (After
W. R. Shaw.)

its cytoplasm until it reaches the egg-nucleus, with which
it fuses, thus completing the act oi fertilization (Fig. 130).
As soon as one sperm enters the egg-cell, the latter at once
forms a. fertilization-membrane about itself, through which
the remaining sperms cannot enter.

157. Nature of the Fertilized Egg. — It will at once be
recognized that the fertiHzed egg, resulting from a union

LIFE HISTORY OF A FERN

173

with the sperm, possesses a double or diploid nature.^
In recognition of its dual nature it is called the oosperm
(egg and sperm). ^ The oosperm, however, like the un-
fertilized Qggj is still only one cell, though its nucleus com-
prises substances contributed by both egg and sperm.
In some cases the Qgg and sperm that unite in fertilization
may come from different parents; their fusion is then
called cross-fertilization.

Fig. 131. — Young embryo of a maidenhair fern {Adiantum concinnum);
still surrounded by the archegonium, which has grown in size, L, leaf,
S, stem; R, root; F, foot. (After Atkinson.)

158. Development of the Fertilized Egg. — After fertili-
zation the egg begins to develop, undergoing a series of
nuclear and cell-divisions, accompanied by increase in

^ As distinguished from the unfertilized egg, which is of a single, or
haploid nature.

2 The term oospore is often used here, but this term lacks the advan-
tage of indicating the real nature of the fertiUzed egg.

174 STRUCTURE AND LIFE HISTORIES

size. The cell-wall of the first division (in all of the family
Polypodiaceae) is parallel to the axis of the archegonial
neck. The second wall, at right angles to the first, di-
vides the oosperm into four cells. The beginning of these
divisions marks the beginning of the embryo. By further
cell-divisions each of the first four cells develops a mass of
embryonic tissue. The two cells on one side of the first
wall formed represent, the one the embryonic stem, and
the other the embryonic leaf, or cotyledon. One of the
two cells on the opposite side of the first wall, develops
into the embryonic root, while the other develops into an
organ peculiar to the embryonic stage, and known as the
foot (Fig. 131). The function of the foot is to absorb
nourishment for the young embryo from the prothallus,
by osmosis. The need of such an organ becomes ap-
parent when it is recalled that the oosperm, and conse-
quently the embryo, lie free in the venter of the arche-
gonium, without any organic or structural connection
with the prothallus. This necessary connection is early
established by the foot.

159. Growth of the Embryo.— As the embryo continues
to grow, the root develops first. The advantage of this
will become evident when we remember that the primary
and most fundamental need of the young plant is water,
which is taken in by the roots. The next most funda-
mental need is nourishment, and as plant food is manufac-
tured in chlorophyll-bearing organs, and usually in
leaves, we would expect the early development of leaves.
Such is the case, the growth of the first leaf being second-
ary only to that of the root, and in advance of the stem.
The development of the stem follows, and finally spore-
bearing leaves appear (Fig. 132). We then have an

LIFE HISTORY OF A FERN

175

organism similar to that with which we started — a full-
grown fern-plant, capable of producing spores, which can
develop into prothaUia again, with antheridia and arche-
gonia, producing sperms and eggs, and so on. Thus we
see that the steps in the life history of a fern constitute
a life-cycle. At whatever point or with whatever struc-
ture we start, if we follow the course of development we
are brought back again ' to the same point, or the same
kind of structure with which we began.

Fig. 132. — ProthaUia of a fern, i, Before the sporophyte had appeared;
2-5, with sporophytes attached; I, cotyledon or first leaf of the sporophyte;
V, circinate vernation of a leaf; s, mass of soil adhering to the rhizoids and
roots.

160. Simpler Ferns. — In addition to the leptosporan-
giate ferns, which have served as a basis for the general-
ized description given above, there is another group,
having a more primitive type of organization. Repre-
sentatives of this group include the ''moonworts'^ (species
of Botrychium, Fig. 133), and the ''adder's tongue"
{Ophioglossum vulgatunij Fig. 134). The species of

176 STRUCTURE AND LIFE HISTORIES

Botrychiiim usually (though not invariably) possess but
one foHage-leaf, and a fertile spike, both of which are

Fig. 133. — Rattlesnake fern {Botrychiiim virginlanmn {h.) S\v.),

more or less branched. Abnormal forms are not un-
common in which the fertile spike is more or less steril-

LIFE HISTORY OF A FERN

177

Fig. 134.— Adder's tongue fern {Ophioglossum vulgatum L.). R, ru

or stolon.

1 78 STRUCTURE AND LIFE HISTORIES

ized, sometimes being entirely so; while in other cases
sporangia occur on the foliage-leaf. As in the replace-
ment of sporophylls by sterile leaves in the ostrich fern,
Onoclea struthiopteris (paragraph 148), these abnormalities
indicate the close relationship between leaves and spore-
bearing organs, and clearly show that the latter may be
completely transformed, by sterilization, into foliage-
leaves.

In Ophioglossum the foliage-leaf and spore-bearing spike
are both unbranched, the latter suggesting an adder's
tongue, whence the name, Ophioglossum. In both OpJdo-
glossum and Botrychium the sporangia originate horn a group
of epidermal and sub-epidermal cells, and are consequently
imbedded in the surrounding tissue. Their walls are
more than one cell in thickness, the annulus is lacking,
and they open by a sHt. Ferns of this type are called
eusporangiate. Their prothahia are usually fleshy and
subterranean, bear the antheridia and archegonia on the
dorsal instead of on the ventral surface, and are perennial,
often living on after the sporophyte has died. In general
the sporophyte possesses less sterile tissue in proportion
to the fertile tissue than is the case with the leptosporan-
glate forms. These characters mark the group as more
primitive than the leptosporangiate ferns, and they are
much less numerous, only about 100 species being known
from the entire world, while of the leptosporangiate ferns
between 3,000 and 4,000 species have been described.

CHAPTER XIV

FUNDAMENTAL PRINCIPLES

161. Two Kinds of Reproduction. — In the two preced-
ing chapters attention has been called to three ways of ob-
taining new fern-plants, namely, by spores, by vegetative
multiplication, and by fertilized eggs. The first two
methods may be grouped together as asexual, while the
second is sexual, as shown in the following table.

Artificial (slips,
f By the giving off of i cuttings, etc.).
Asexual, in- | multi-cellular por- 1 Natural (tubers,
volving cell- I tions or outgrowths [ bulbs, gemmae) .
divisions ! of vegetative tissue,
only. I By the giving off of

special reproductive
bodies of one or few
cells, called spores.

Reproduction

Sexual, in-
volving cell-
fusions.

162. Vegetative Multiplication. — Vegetative multipli-
cation may be accomplished either without or with the
intervention of man. In the first case the plant produces
special reproductive bodies such as tubers, bulbs, offsets
and stolons, which become separated from the plant with-
out assistance, and develop into new individuals. In
the second case a similar result is accomplished through
the removal by the gardener of portions of the parent
plant, such as slips, cuttings, leaves {e.g.j in the begonia),
or by bending branches over until they touch the ground,
and there take root, after which the newly rooted portion
may be severed from the parent plant. This is called
layering. The production of new individuals by the arti-

179

l8o STRUCTURE AND LIFE HISTORIES

ticial methods of the gardener is called propagation; but
between these methods and the multiplication by special
bodies, given off spontaneously by the plant, no hard and
fast line can be drawn. Some plants, for example, be-
come layered without the gardener's assistance; other
plants (as the willow), by self-pruning, spontaneously
give off branches from which new plants may develop;
while, on the other hand, the gardener may cut a tuber,
such as the "potato" into a number of pieces, from each of
which a new plant will develop. In this practice artificial
propagation and vegetative multiplication are combined.

163. Reproduction by Spores. — The essential fact about
a spore is that it is an individual cell or small group of
cells, produced primarily for reproductive purposes,
given off by the plant, and capable hy itself of producing
a new individual. The essence of all reproduction is the
separation of the reproducing cell or body from the parent
plant. If a bud or a bulb remains attached to the plant
that formed it, it produces only a branch or other organ,
but not a new individual. So, also, a spore must be sepa-
rated from the parent plant in order to reproduce the
latter. In many cases spores may germinate before they are
set free, but the separation must come sooner or later. No
hard and fast line can be drawn between spores and gemmae.

164. Sexual Reproduction. — In marked contrast to
reproduction by spores, is the reproduction by means of
sperms and eggs, involving cell- and nuclear-fusions, known
as fertilization. Eggs and sperms are called gametes,'^
the egg being the female gamete, the sperm the male
gamete. The diploid cell, resulting from the union of two
gametes, is called a zygote, and this term is often extended

^ From the Greek word, 7d|Lios (gamos)j meaning marriage.

FUNDAMENTAL PRINCIPLES l8l

to apply to the resulting diploid organism through all
stages of its development to maturity.

165. Two Kinds of Generivtions. — A study of the life
history of the fern disclosed two distinct phases or genera-
tions, one bearing spores, and therefore called the sporo-
phyte (spore-bearing plant) , the other bearing gametes and
for that reason called the gametophyte (gamete-bearing
plant). The gametophyte of the fern was seen to be
entirely independent of the sporophyte, capable of manu-
facturing its own food by means of its own chlorophyll,
not dependent upon any other plant, and in some groups
being perennial, living on from year to year, and giving
rise to sporophytes that live for only one season. The
sporophyte, on the other hand, is at first, entirely de-
pendent upon the gametophyte for its nutrition, living as
a parasite upon the prothallus, from which it absorbs its
nourishment by means of the special organ, the foot.
Gradually, however, the sporophyte puts forth roots,
capable of taking in water and dissolved mineral sub-
stances from the soil, and chlorophyll-bearing organs (the
fronds or leaves), capable of manufacturing organic food.
As the sporophyte becomes independent, the gameto-
phyte (with few exceptions, as noted above), perishes.
A comparison of the two generations shows that the
sporophyte is the much more complex of the two, being
clearly differentiated into roots, and leafy shoot. The
difference in the origin of these two generations results in
a very fundamental difference in the nature of all the
cells in each. Since the sporophyte is derived from an
oosperm (zygote), formed by the fusion of the two
gametes, all of its cells are diploid, containing material
derived from both its male and female parentage. The

l82 STRUCTURE AND LIFE HISTORIES

gametophyte, on the other hand, being derived from a sin-
gle reproductive cell (the spore) , without nuclear or cell-fu-
sions, is composed of cells of a single or haploid nature.

166. Alternation of Generations. — Our study of the
fern also brought out another fact of very fundamental
importance. Sporophytes do not produce sporophytes,
nor gametophytes, gametophytes; but there is always
an alternation of generations, sporophytes producing
gametophytes, and gametophytes, sporophytes.

The order of sequence in the life-cycle is as follows:

sporophyte — ^•spore — ^gametophyte — ^gametes — >oosperm — >sporophyte.

The order of structures and processes involved in the
life-cycle is as follows:

OUTLINE OF LIFE HISTORY OF A FERN

Gametophyte (prothallus)

Antheridium Archegonium

i i

Sperm (male gamete) Egg (female gamete)

Fertilization

Oosperm (zygote)

4.4.

Embryo

ii

Mature sporophyte (mature zygote)

4.4.

Sporophyll

ii

Sporangium

4.4.

Spore-mother-cell

Ar Ar Ar Ar

Spore Spore Spore Spore

i

Gametophyte

Reduction

FUNDAMENTAL PRINCIPLES

183

The fact of a cycle in the life history is brought out
clearly in the following diagram :

Fig. 135. — Diagram of life-cycle of a fern.

167. Reduction. — Since the sporophyte (descended from
the diploid oosperm) has cells of a double nature, resulting
from fertilization, and since the spores which give rise to
the gametophyte are of a single (or haploid) nature,
there must occur, at some stage in the life of the sporo-
phyte, a process of reduction, restoring the cells, made
diploid by fertilization, to the haploid condition. Pains-
taking studies of cellular structure and processes has
disclosed the fact that this reduction takes place during
the two successive divisions (tetrad-divisions) of the spore-
mother-cell, resulting in the formation of four spores.
The diploid condition persists in all the cells of the
sporophyte, and through every cell-division, up to the two
divisions preceding spore-formation, just as the single or
haploid condition persists in all the cells of the gameto-
phyte, up to the very act of fertiHzation.

i84

STRUCTURE AND LIFE HISTORIES

168. Nature and Method of Reduction. — In order
thoroughly to understand fertihzation and reduction one
must have a knowledge of the structure and behavior of
the nucleus in cell-division and cell-fusion. This subject

Fig. 136.- — Diagram illustrating various stages of indirect nuclear
division (mitosis). A, resting nucleus of the mother-cell; B, formation
of nuclear skein or spirem; C, longitudinal splitting of the spirem; D, the
chromosomes (four in number) have been formed by the transverse seg-
mentation of the spirem; E, chromosomes arranged on the equator of the
nuclear spindle; F andC, early and late anaphase, the chromosomes moving
to the pales of the spindle; H, formation of daughter spirems; /, resting
stage of the two daughter-cells.

is too difficult and too extended to be thoroughly treated
in an introductory study, but the sahent facts are as
follows. The nucleus of all cells comprises at least four
substances: nuclear sap, a threadwork of linin, and a

FUNDAMENTAL PRINCIPLES

i8s

^ <^ 'cl.B -^ h

<-l-H 1^ "'"^ ■*-' «J_

o a o ^^ o

;^ O S O - S

i

f

IV

\ \

\\

\ \

NE^^CT

/

l\\

//4

1.

o

(U o

en

s

o

E
2

"So

o
6

V

>»J3

<*>■

<4-4

w

u

o

O

i86

STRUCTURE AND LIFE HISTORIES

substance called chromatin;'^ all these are enclosed by a
nuclear membrane. In the non-dividing nucleus the
chromatin is distributed on the linin threads in the
form of minute granules (Fig. 136). At one of the stages
prehminary to nuclear division the linin network, with the
chromatin, becomes transformed into a thickened skein,

D \ VI S \0N

Fig. 138. — Diagram of a cytological life-cycle, based on a hypothetical
fern with four chromosomes in the sporophyte. The nuclear phenomena
are based on those of the thread- worm {Ascaris). Each chromosome is
designated by a characteristic mark so that it may be traced throughout
the diagram. (After R. F. Griggs.)

which shortly becomes split into two, throughout its entire
length. The skein finally becomes divided transversely
into a number of double chromatin bodies or chromosomes.
The number of these chromosomes is characteristic, and
always the same for each species of plant. The nuclear

^ Because it stains readily when treated with aniline dyes.

J

FUNDAMENTAL PRINCIPLES 187

membrane then disappears, and, by a complicated mechan-
ism, not entirely understood, the two halves of the chro-
mosomes are separated and carried apart to opposite sides
of the cell. After this division of the nucleus, a new cell-
wall forms, dividing the entire cell into halves; new nuclear
membranes develop, and the chromosomes in esich daughter-
nucleus become gradually retransformed into a resting
nucleus, like the one with which we started.

In reduction (Fig. 137) a new resting nucleus is not
organized after the first nuclear division, but this divi-
sion is followed at once by a second, or reducing division,
(maiosis) by which the number of chromosomes in each
nucleus is reduced by one-half. This is the process of
tetrad-division, by which spores are formed from the
spore-mother-cells. The reduced number of chromosomes
persists throughout the gametophyte-phase, including the
formation of both egg and sperm. When the latter unite
the nucleus of the zygote will, of course, possess the doubled
number of chromosomes, which then persists throughout
the body of the sporophyte (mature zygote), until the
stage of spore-formation is again reached. These facts
are shown diagrammatically in Fig. 138.

169. Inheritance. — It is, of course, common knowledge
that men do not gather grapes of thorns, nor figs of
thistles. A given species of fern always reproduces the
same species, and this is true of all plants. It requires
only a brief reflection to realize that this must be so, for
the beginning of every living thing is always merely a
piece of an antecedent organism, the parent. The off-
spring would, therefore, naturally partake of the nature of
its parent — it is a piece of it — was originally a part of
it. Resemblance between ancestor and descendant is

l88 STRUCTURE AND LIFE HISTORIES

commonly regarded as inheritance, but only a little
careful thinking will lead us to see that, resemblance
and inheritance are by no means synonymous. The real
nature of inheritance is well illustrated by the inheritance
of property by a son from his father. The thing inherited
is not an external appearance, but a material substance
(land, buildings, a business), which is handed from one
to another. So it is in reproduction. That which one
generation of plants inherits from another is the substance
of the reproductive cells — the protoplasm of the spore,
oosperm, tuber, or bulb — plus a certain characteristic
organization of this protoplasm, and the effects of its past
history.

170. Inheritance Versus Expression.— That inheritance
and expression are not the same thing is plainly shown in
the life history of the fern, for the gametophyte clearly
derives its living matter by inheritance from the sporo-
phyte, and the sporophyte, in turn, its living matter from
the gametophyte, and yet the two generations look so
little alike that men for centuries knew them both with-
out recognizing the fact that they were merely two dif-
ferent phases in the life history of the same species of
plant. So, often, among human beings, children may
bear very little if any resemblance to their parents, but
may closely resemble their grandparents. Clearly we
do not inherit the color of our eyes or hair, the shapes of
our noses, the peculiarities of our voices, or our mental
traits from our parents, nor even from our more remote
ancestors. What we do inherit is a tiny particle of proto-
plasm having a certain characteristic composition, struc-
ture, and past history. This protoplasm is capable, under
certain combinations of circumstances, of developing

FUNDAMENTAL PRINCIPLES 1 89

into a mature organism, resembling the one from which
it came, but under other combinations of circumstances
the external appearance — the expression — may resemble
that of the parent only a very little, or not at all. In-
heritance may therefore he defined as the recurrence in suc-
cessive generations, of a similar cellular constitution} Ex-
pression of this cellular condition is greatly modified by
circumstances, which are never precisely the same for
any two individuals.

171. Variation.— The preceding sentence explains, in
part, why it is that no two individuals are ever precisely
alike — precisely similar circumstances surrounding de-
veloping organisms never occur twice ; that is, the environ-
ment varies. Besides this, internal changes may take
place in the reproductive cells. For either one or both of
these reasons, constant variation is the rule for living
things. This subject will be considered more at length
in Chapters XXXII and XXXIII.

172. Adjustment to Environment. — By the term envir-
onment is meant all the circumstances that surround a
cell, tissue, organ, or organism at any given time, or
throughout its existence. The environment of tissues
and organs includes surrounding tissues and organs,
and the environment of cells includes the neighboring
tissues and cells. The most essential thing in the
life of every plant or animal is to keep in harmony with
its environment. Every change of environment neces-
sitates an adjustment on the part of the plant in order to
maintain this harmony. Adjustments are most easily
made when the plant is young and plastic, and especially
while it is developing to maturity. If the amount of
water in the soil is diminished the young plant will send

^ Following Johannsen.

1 90 STRUCTURE AND LIFE HISTORIES

its roots deeper, if light is entirely cut off no chlorophyll
will form. A leaf, or the prothallus of ferns, is bilaterally
symmetrical because the environment is uniform on all
sides; the same organs have dorso-ventral differentiation
largely because the environment is unlike above and be-
low. The motility of sperms is an adjustment to water in
the environment. Thus variations in the environment
may result in different expressions of inheritance, just as
variations in inheritance would be followed by differences
in expression, even in an unchanging environment. In
order correctly to understand a plant nothing is more
necessary than to remember that its characteristics are
the result, not of its inheritance alone, nor of its environ-
ment only, hut of the interaction between the two.

173. Struggle for Existence. — In Chapter XII atten-
tion was called to the fact that a moderate-sized fern pro-
duces each year about 50,000,000 spores. If each one of
these spores ultimately produced a mature fern-plant, and
if we allowed only i square foot of ''elbow room" for each
plant, the progeny of one parent only, in one season would
require at least 50,000,000 square feet, or nearly 1%
square miles. If each of these plants in turn, produced
50,000,000 offspring the next season, the descendants of
only one fern plant would, in only two years, cover the
stupendous area of over 83,000,000 square miles, or an
area equal to that of the North American Continent.
It has been calculated that a single plant of hedge mustard
may produce as many as 730,000 seeds. If each seed
developed another full-grown plant, and if the plants were
distributed 73 to each square meter, there would be enough
mustard plants to cover an area equal to 2,000 times
the dry surface of the earth. One may easily imagine

FUNDAMENTAL PRINCIPLES I9I

the result if all the seeds produced by one of our large
forest trees were able to mature. And yet the total
number of any given kind of fern, of hedge mustard, or
of forest tree does not appreciably change from year to
year. The reason, of course, is that not all of the spores
and seeds produced are allowed to come to maturity.
The direct result of the enormous number of spores and
seeds produced is a struggle for existence — for sufficient soil,
water, light, and food to insure a healthy, mature plant.

174. Elimination of the Unfit. — As a result of variation
certain individuals will succeed better than others in the
struggle for existence. Those most poorly adapted to
their surroundings will perish, and only the more vigorous
ones — those best adjusted to their surroundings — will
persist. The result of this struggle for existence was
called by Herbert Spencer the ^^ survival of the fittest.''
What really takes place in nature is the elimination, by
death, of the unfit. Darwin called this natural selection,
implying that the result is similar to that when plant
breeders select out of a progeny the best individual for
further breeding. What really takes place in nature,
however, is not so much the selection of the fittest, but a
rejection of the unfit. Thus, among the 50,000,000
progeny of a single fern-plant, some are sure to have a
weaker constitution than others; to develop a weaker root-
system, less chlorophyll in their leaves, a less number
of sporophylls or spores, or to be inferior in other ways.
The result will be that, in the course of only a few years,
the descendants of the most vigorous or otherwise superior
plants will alone be left to perpetuate the race.

175. Problems to Solve. — In the preceding paragraphs
we have called attention to a number of the problems

192 STRUCTURE AND LIFE HISTORIES

which arise from the study of so lowly an organism as a
fern. Some of these have been partially solved — prob-
ably none of them has been completely solved. In fact,
we may say that our ignorance of life-processes greatly ex-
ceeds our knowledge. Very much more remains to be
ascertained than has already been found out; for example,
what is protoplasm? Nobody really knows. We have
analyzed the substance chemically, we have carefully
examined and tried (but without complete success) to
describe its structure. We know it is more than merely
a chemical compound. It is a historical substance. A
watch, as such, is not. The metal and parts of which a
watch is made, have, it is true, a past history; but the
watch comes from the hands of its maker de novo, without
any past history as a watch. But not so the plant cell.
It has an ancestry as a cell; its protoplasm has what we
may call a physiological memory of the past. It is what
it is, not merely because of its present condition, but
because its ancestral cells have had certain experiences.
We can never understand a plant protoplast merely by
studying it; we must know something of its genealogy and
its past history.

What is the origin of the sporophyte, and how did
there come to be two alternating generations? What is
the meaning of fertilization; what the mechanism and
laws of inheritance? How did there come to be on the
earth such plants as ferns? What was the origin of Hfe?
What is Hfe? No one can give complete answers to these
questions; but the purpose of the study of botany is to
help fit us to seek the answers intelHgently. To those
who are interested in problems of this sort, nothing can
be more fascinating, nor more profitable.

CHAPTER XV
LIFE HISTORY OF A MOSS

176. Variety of Mosses. — There have been described and
named over 12,000 different species of Musci, or mosses.
Obviously, in an introductory study, we can only get a
glimpse of so large a group. A comparative study of the
species has led to the recognition of three distinct orders
as follows:

[ I. Sphagnales (the peat-mosses)
Musci 2. Andreasales (the black mosses)
1 3. Bryales (the true mosses)

Of these the Sphagnales are considered the most primitive,
and the Bryales most highly developed. The Sphagnales
will be considered first.

177. Habitat of Sphagnum. — Peat-mosses, as the name
implies, grow in swamps and lake margins,' usually in
dense clumps or thick mats, in places forming the familiar
peat-bogs of northern regions. They are usually of a very
pale green color, often almost white, especially just
below the top, and frequently with a tinge of red or
yellow.

178. Description of Sphagnum. — The plant consists of
an upright central axis or stem, with a central, pith-like
portion of thin-walled parenchyma (Fig. 139.) The cell-
walls of the outer portion, or cortex, are thicker and often
tinted with a reddish pigment. The cortex varies in thick-

? 193

194

STRUCTURE AND LIFE HISTORIES

ness from four cells in the main stem, to one or two cells
in the smaller branches. The leaves are only one cell thick,
and are densely crowded on the stem, having, at maturity,
what is known as the two-fifths arrangement; that is,
if one starts with a given leaf and follows upward in a
spiral around the stem, he will pass five leaves before he
comes to one vertically above that with which he started.

Fig. 139. — Sphagnum sp. Upper portions of leafy plants, showing
sporogonia.

and in doing this he will have passed twice around the
stem. The leaves of sphagnum never have a mid-rib
or other veins, and correlated with this is the entire
absence of any fibro-vascular bundles in the stem. This
is one of the features that marks the plant as of lower
organization than the fern. The stem forms numerous
branches, usually one for every fourth leaf, and glandular
hairs are usually met with at the bases of very young
leaves (Figs. 140 and 141).

LIFE HISTORY OF A MOSS
C G

195

Fig. 140. — Sphagnum cymbifolium, Ehrb. A, B, C, cells from a
young leaf, X about 300; D, cells from a mature leaf; E, section of a
similar leaf; F, cross-section of an old stem, showing the thick, large celled
cortex, X about 25; G, Sclerenchyma cells from the central portion of the
stem, X about 300. (After Campbell.)

Fig. 141. — Sphagnum cymbifolium, Ehrb. A, median longitudinal
section of a slender branch; x, the apical cell; B, part of a section of the
same farther down, showing the enlarged cells at the bases of the leaves,
and the double cortex (cor); C, cross-section near the apex of a slender
branch; D, glandular hair at the base of a young leaf; all X 525. (After
Campbell.)

196 STRUCTURE AND LIFE HISTORIES

As the leaves mature, part of their cells increase greatly
in size, and the protoplasm becomes entirely transformed
into thickenings of the cell-walls, leaving the cells quite
empty of everything but air and water. These large,
empty cells greatly mask the smaller ones containing
chlorophyll, and this accounts for the pale color of the
plants. The walls of the empty cells are commonly per-
forated with several pores. A similar type of cell is also
developed in the outer layers of the stem (Fig. 141).
These cells are extremely hygroscopic, and absorb water
rapidly and in large quantities, so that the entire living
plant is usually thoroughly saturated with water. On
account of its sponge-like nature, sphagnum is much used
by florists in packing plants for shipment, and in other
ways.

179. Sterile and Fertile Branches. — Two kinds of
branches occur: sterile and fertile. The organs of repro-
duction (antheridia and archegonia) occur only on the
fertile branches, but antheridia and archegonia never on
the same branch. In some species they occur on separate
plants {dioecious — two households) ; in other species on the
same plant {monoecious — one household).

180. Antheridial Branches. — The male branches, or
antheridiophores, bear leaves that vary in color from
green to yellow and red, and the antheridia occur in the
axils of these leaves (Fig. 142). They consist of a rela-
tively long stalk, composed of four rows of cells, or less,
and bearing, at maturity, the globular capsule containing
the sperms (Fig. 143). The sperms are coiled, with about
two complete turns, and bear two long thread-like cilia at
their anterior end. In locomotion the end bearing the cilia
precedes. At the opposite or posterior end occurs a small

LIFE HISTORY OF A MOSS

197

appendage composed of starch, which ultimately drops
off. It will be seen at once that the sperms of sphagnum
differ from those of ferns in having only two cilia instead
of many. When the antheridium is ripe the cells swell,
and the capsule is thus forced open.

Fig. 142. — Sphagnum. Photomicrograph of a longitudinal section of an
antheridial branch, showing five antheridia. (Cf. Fig. 143.)

181. Archegonial Branches. — The female branches, or
archegoniophoreSy usually occur near the upper end of
the plant, and bear the archegonia at their tips. As in
the case of the fern, each archegonium consists of a
neck (slightly twisted in Sphagnum), with neck-canal, a
venter, containing the egg, and a basal stalk, or pedicle,
not found in the ferns. Several archegonia commonly

iqS

STRUCTURE AND LIFE HISTORIES

Fig. 143. — Sphagnum acutifoliutn, Ehrb. A, prothallus (pr), with a
young leafy branch just developing from it; B, portion of a leafy plant; a,
male cones; ch, female branches; C, male branch or cone, enlarged with
a portion of the vegetative branch adhering to its base; D, the same, with
a portion of the leaves removed so as to disclose the antheridia; E, an-
theridium discharging spores; F, a single sperm; G, longitudinal section
of a female branch, showing the archegonia (ar); H, longitudinal section
through a sporogonium; sg^, the foot; ps, pseudopodium; c, calyptra; sg,
sporogonium, with dome of sporogenous tissue; ar, old neck of the arche-
gonium; /. Sphagnum squarrosum Pers.; d, operculum; c, remains of calyp-
tra; qs, mature pseudopodium; cA, perichaetium. (Cf. Figs. 139 and 142.)
(From Schimper.)

LIFE HISTORY OF A MOSS

199

occurs together in a group on a single female branch. A
number of enlarged leaves surrounding the archegonia con-
stitute a perichcBtium. The antheridial and archegonial
branches at first occur close together near the summit of
the branch, but the branch often elon-
gates in the region between the two,
thus separating them.

182. Asexual Multiplication. — One
of the sterile branches, near the apex
of the plant, usually develops more
strongly than the others, and each year
the old stem below dies off, and the
young branch becomes established as a
new plant. Under favorable condi-
tions young plantlets, called innovation
branches, may develop on the sterile
branches, at the tip and back from the
tip, strike root, and become established
as independent plants (Fig. 144).

183. Sexual Reproduction. — Fertili
zation is accomplished in a manner
similar to that in the fern, a film of
water being required in order that the
motile sperm may swim to the neck-canal, down which it
passes, to the venter and into the egg, where the two nuclei
unite. Fertilization probably occurs, as a rule, in winter,
for young embryos are usually found in very early spring.
The first division-wall of the oosperm is horizontal, or
nearly at right angles to the axis of the neck, and thus
at right angles to the position of the wall in the first divi-
sion of the leptosporangiate ferns. As the cell-divisions
follow each other in rapid succession, the upper cells

Fig. 144. — Sphag-
num cuspidatum,
showing innovation,
or short, branches.
(After Schimper.)

200

STRUCTURE AND LIFE HISTORIES

of the developing embryo become organized into a large
globular spore-case, containing a thick central column
(columella), surrounded by a dome of spores, and, out-
side of all, the wall of the sporangium. As the spore-case

Fig. 145. — Sphagnum sp. Photomicrograph of a longitudinal section
through a sporogonium and portion of the pseudopodium. op^ operculum;
r, annulus; ca, wall of capsule; 5/>, spores; col, columella;/, foot; ps, pseu-
dopodium.

enlarges the wall of the archegonium is ruptured and the
top portion of it is carried up as a cap on the spore-case,
forming the calyptra. The lower cells produced by the
dividing oosperm become organized into a much swollen

LIFE HISTORY OF A MOSS 20I

foot, imbedded in the tissues below, and connected with
the spore-case by a very short stalk (Figs. 143 and 145).

184. The Sporoph3rte. — It will have been recognized
already that the simple structure just described, since it
bears spores, is the sporophyte stage of Sphagnum}
While the sporophyte is maturing, the apex of the female
branch elongates, forming a leafless stalk, a half inch or
more in length. This stalk is called the pseudo podium
(false foot). The development of the pseudopodium,
coincident with that of the sporophyte is very interest-
ing, and the question at once naturally arises, as to how
this correlation is brought about. No positive explana-
tion has ever been given, but it seems probable that, as the
sporophyte begins to develop, the cells of the foot excrete
some substance which stimulates the cells in which it is
imbedded to divide and enlarge, resulting finally in the
formation of the pseudopodium. The advantage of the
pseudopodium in facilitating the distribution of spores
by raising the spore-case higher into the air and well
above the perichaetial and other leaves, is obvious.

The sporophyte of Sphagnum possesses no chlorophyll,
and consequently does not elaborate any food, obtaining
its entire supply from the sphagnum-plant by absorption
through the foot. Numerous groups of stomatal guard-
cells occur on the wall of the spore-case, but they have no
slit between them — no true stomata — and are there-
fore functionless. There are also, underneath the guard-
cells, no intercellular spaces, such as are always associated
with true stomata. The presence of these functionless
stomata is thought by some botanists to indicate that the

^ Such simply organized sporophytes are commonly called sporogonia
(singular sporogonium).

202

STRUCTURE AND LIFE HISTORIES

sporophytes of the ancestors of Sphagnum possessed true
stomata and the function of photosynthesis.

185. Formation of Spores. — As the spore-case develops,
the inner cells become differentiated into two kinds, one
composing the larger part of the tissue, and the other,
larger and richer in protoplasm, forming a dome of sporo-

genous or spore-forming tissue
near the upper wall (Fig. 145).
From this tissue, spore-
mother-cells are developed,
and from each of these, by
reducing divisions, as in the
fern, four spores.

186. Asexual Reproduc-
tion.— While the spores are
maturing, a circular groove
{annulus) is formed near the
apex of the spore-case, and
the cells in this zone have
thinner walls than those ad-
jacent (Fig. 145). At the
maturity of the spore-case these cells become dry, and are
easily torn apart, thus forming a lid, or operculum^ at the
summit of the spore-case. The falling away of the
operculum affords an opportunity for the scattering of the
spores. Under favorable conditions the spores germinate,
putting forth a very short, green protonema, as in the case
of fern-spores. The tip of the protonema soon broadens
out, forming a prothallus, much like that of the fern in
shape, but being only one cell thick (Fig. 146). Rhizoids
form on the under side, and from the margin other
threads develop, having chlorophyll, and resembling the

Fig. 146. — Spliagnumsp. A,B,
young protonemata; C, older pro-
tonema with leafy bud, k; r, mar-
ginal rhizoids. (After Campbell.)

LIFE HISTORY OF A MOSS 203

protonema. At the tips of each of these threads a thallus
may also form, and in this way vegetative multiplication
is brought about. From each thallus arises an upright
leafy branch — the sphagnum-plant described above. The
reader has already recognized that this complex phase of
sphagnum is the gametophyte.

187. Life-cycle of Sphagnxim. — The life-cycle of sphag-
num may be summarized as follows:

OUTLINE OF LIFE HISTORY OF SPHAGNUM
Sphagnum-plant {gametophyte)

— i ^

T T

Anthericlial branch Archegonial branch

i 4-

Antheridia Archegonia

4. I

Sperm (male gamete) Egg (female gamete)

Fertilization

Oosperm (zygote)

ii

Embrvo

a

Mature Sporophyte (mature zygote)

ii

Sporangium

ii

S pore-mother-cell

Reduction

1 1 1 1

Spore Spore Spore Spore

i

Protonema

4-

Thallus

i

Sphagnum-plant {gametophyte)

188. Diagram of Life-cycle. — The life-cycle of Sphag-
num may be diagrammatically represented as follows:

204

STRUCTURE AND LIFE HISTORIES

Fig. 147. — Diagram of life-cycle of sphagnum.

189. Other Mosses. — The so-called ''true mosses,"
with which we are perhaps more familar than with
sphagnum, cannot be studied here in detail, but it may be
said that, in broad outline, their life histories are closely
similar to that of sphagnum. The protonema does
not produce a thallus, but the leafy branches, or moss-
plants, arise directly from the filamentous protonema
(Fig. 148). They are both monoecious and dioecious. In
the "true mosses" no pseudopodium is formed, but the
stalk of the sporophyte (so very short in Sphagnum)^
elongates to form a seta, often over i inch in length. In
the spore-case, or capsule, there is much less sporogenous
or fertile tissue, in proportion to sterile tissue, than in
Sphagnum. Moreover, at the base of the capsules, in
the true mosses, occur functional stomata, opening into
intercellular spaces, and surrounded by chlorophyll-

LIFE HISTORY OF A MOSS

205

bearing cells. Thus photosynthesis may be carried on,
though of course to a very limited degree. The sporo-
phyte of the true mosses seems to occupy an intermediate
position between those of Sphagnum and the fern, and, as
we ascend from the lower form in Sphagnum to the higher
form in the fern, the transition is largely characterised by a
decrease in the amount of fertile tissue and an increase in
the relative amount of sterile tissue of the sporophytes.

Fig. 148. — Protonemata of a moss bearing young gametophyte buds.

190. Vegetative Multiplication. — Extensive experiments
^seem to indicate that every living cell of a moss-plant can
develop protonemata — or in other words is a potential
spore. These protonemata like those produced by the
germination of spores, produce buds which may develop
into mature plants. The production of entire plants or
of parts of plants in this way, by portions of the vegetative
body; is called regeneration. In some species of mosses the

2o6

STRUCTURE AND LIFE HISTORIES

Fig. 149. — Hair-cap moss {Polytrichum commune). A, male plant; B,
same, proliferating; C, female plant, bearing sporogonium; D, same; g,
gametophyte; s, seta; c, capsule; 0, operculum; a, calyptra, E, top view of
male plant.

207

Fig. 150.-A moss (T^^/raM^'^^^^^), showing gemmae; G, a gemma enlarged.
(Cf. Fig. 151.)

^'°* ^5;.— Photomicrograph of a longitudinal section of
{.letraphts), showing gemmaj (g). (Cf. Fig. 150.)

2o8 STRUCTURE AND LIFE HISTORIES

leafy-shoot, and in others the protonemata, may give rise
to special small bodies, called genimcB, which may become
separated from the parent plant and give rise to new plants
(Figs. 150 and 151). Gemmae will be illustrated more
fully in the liverworts to be discussed in the next chapter.
191. Comparison with the Fern. — By comparing Sphag-
num with a fern several points of interest are brought
out. In the first place, we learn that, while the "fern-
plant" with which we are familiar is a sporophyte, the
sphagnum-plant is a gametophyte. In the second place,
while the sporophyte of the fern is at first dependent on
the gametophyte for its nutrition, the sporophyte soon
becomes entirely independent, and the simply organized
gametophyte perishes; while in sphagnum the sporo-
phyte is the much more simply organized, and is depend-
ent upon the gametophyte for nutrition throughout its
entire life. In their mode of reproduction, however, the
two plants are very similar, each producing haploid
gametes of two sexes, male and female, that need to fuse
in fertilization; the product of fertilization (zygote) being
diploid, and producing a spore-bearing phase; and the
spores, haploid again, through reduction, giving rise,
without nuclear and cell-fusion to the haploid gameto-
phyte. In each case, in the life-cycle, gametophyte
alternates with sporophyte, fertilization with reduction,
gametes with spores, haploid cells with diploid. What
takes place in the cells between fertilization and reduc-
tion, and between reduction and fertilization? This is one
of the many fascinating problems in botany still awaiting
solution. There is only one way by which the answers
to these problems may be ascertained; namely, by accu-
rate, persistent, painstaking observation and experiment.

CHAPTER XVI

LIFE HISTORY OF A LIVERWORT

192. Habitat. — The group of plants ranking next below
the mosses in the scale of Hfe is the liverworts (Hepaticse).
They are widely distributed over the earth's surface;
being found in a wide climatic range, but usually in moist
situations. Some forms (e.g., Riccia natans) may grow
floating on the surface of water, others {e.g., Riccia fluitans)

Fig. 152. — Anlhoceros fusiformis. Portion of lamellate, cristate thallus,
which easily retains water. (After M. A. Howe.)

submerged; but, as in mosses, no salt-water forms have
been found. A few species grow on other plants {epi-
phytic), or in other situations where the water supply
may at times fall very low. Such forms have various
contrivances which serve to retain water. Thus, some
species of Anthoceros {e.g., A. fusiformis, A. fimhriatus)
possess crisped lobes, forming a fringe around the margin
which helps the plant to retain water (Figs. 152 and
153). In another species {A. punctatus), water is retained

14

209

2IO

STRUCTURE AND LIFE HISTORIES

in little pits or depressions on the upper surface. The
habitat of the Hverworts illustrates a step forward in the
abandonment by plants of a wholly aquatic Hfe and the
establishment of a land vegetation; but the prevailingly
moist situations in which most of the species are found,

^l^M

^,

^■

^;

,fe

t --.
\

i
" '\

Fig. 153. — Anthoceros fimhriatus. Portion of a thallus viewed from
below, with the rhizoids omitted. The one-layered crisped lobes at the
margin serve to retain moisture. (After Goebel.)

and the need of water for fertilization by swimming
sperms (soon to be described), points to an ancestral
habitat truly aquatic.

193. Description of the Plant Body. — The plant body
of the liverworts shows a marked departure from that of
the mosses in the direction of simplicity. There are four
main groups or orders of Hepaticae, as follows:

I I. Ricciales.

TT 4.' J 2. Marchantiales.
Hepaticae \

3. Jungermanmales

. 4. Anthocerotales.

i

LIFE HISTORY OF A LIVERWORT

211

In the first two and last of these orders the plant body is a
thallus, either closely resembling the prothallus of the
ferns, or freely branching. In the third order the plant

Fig. IS4.-A leafy liverwort {Porella navicularis) . Male plant, about
natural size. (After M. A. Howe.) ^ ^ ^ ^

body is leafy (Joliose), and in this respect somewhat re-
sembles certain true mosses, for which it is often mis-
taken (Figs. 154 and 155). In the first two and last orders
the thallus always shows dorso-ventral differentiation.

212 STRUCTURE AND LIFE HISTORIES

ANTHOCEROS

194. The Gametophyte. — One of the most important
groups is the genus Anthoceros^ including several different
kinds or species. The plant body, or thallus (Fig. 156), is
roughly circular or semicircular,, with numerous rhizoids
growing from the ventral surface. It increases in size at
numerous growing points on the margin of the thallus, and

Fig. 155. — A leafy liverwort (Porella navicularis) . Female plant, about
natural size. (After M. A. Howe.)

is green from the presence of chlorophyll in the cells.
There is only one chloroplast in a cell, in contrast to the
numerous chloroplasts in each cell of the mosses and ferns.
195. Reproductive Organs. — The antheridia are found
just back of the growing points, near the middle of the
lobe. In some species they occur in groups (Figs. 157
and 1 59), in other species singly (Fig. 158) . They develop^

LIFE HISTORY OF A LIVERWORT

213

not from epidermal cells as in ferns, but each one from a
single cell just underneath the epidermis (subepidermal),

Fig. 156. — Anthoceros IcBvis, showing the lobed thallus of the gameto-
phyte, bearing several upright sporogonia in various stages of develop-
ment. At the right and left sporogonia dehiscing, and scattering the
spores. Note the slender, thread-like columellas, and the lack of differ-
entiation of the sporogonium into seta and capsule. The sheoth (calyp-
tra) at the base of the sporogonium is formed chiefly from the vegetative
tissues of the gametophyte, and only to a slight extent by the walls of the
archegonium.

Fig. 157. — Anthoceros Pearsont. Longitudinal section through a well-
developed, glandular thickening, in which are embedded a number of
antheridia, X 53. (After M. A. Howe.)

near the dorsal surface. They remain imbedded in the
surrounding tissue until mature, closely resembling, in

214

STRUCTURE AND LIFE HISTORIES

their location and mode of origin, the antheridia of some
of the lower, or eusporangiate, ferns, such as Ophioglosum
and BotrycJiium. The archegonia are also imbedded, with
the tip of the neck reaching to the surface (Fig. 159).
They are further concealed at maturity by the growth of

Fig. 158. — Cross-section of the thallus of a horn wort (Anthoceras sp.).
The oval area is an antheridium, containing sperms, or sperm-mother-
cells.

Fig. 159. — Anthoceros fusiformis. Vertical longitudinal section near
the apex of the thallus, showing archegonia (at the left), and antheridia
(at the right). X about 53. (After M. A. Howe.)

a small dome of tissue over the opening where the neck
comes to the surface. Both antheridia and archegonia
occur on the same plant, sometimes closely intermingled.
196. Symbiosis. — A most interesting case of symbiosis
occurs between Anthoceros and a much more lowly organ-

LIFE HISTORY OF A LIVERWORT

215

ized plant — a blue-green alga, of the genus Nostoc (Fig. 160).
On the ventral surface of the thallus slits occur in the
epidermis. These are not stomata, and the intercellular
spaces into which they open are fitted with a mucilaginous
substance produced by a transformation of the adjacent
cell-walls. This mucilage furnishes ideal conditions of
food and moisture for the alga, which flourishes there.

Fig. 160. — Photomicrograph of a cross-section of a liverwort {Antho-
ceros fusiformis). The dark, oval area is a colony of a species of Nostoc,
an alga that lives symbiotically in the tissues of the liverwort. (Micro-
scopic preparation by M. A. Howe.)

Whether the presence of the alga is of any advantage to
the liverwort is not known, but apparently it is of no
disadvantage.

197. Vegetative Multiplication. — ^Liverworts present
many interesting devices for vegetative multiplication by
the giving off or separation of a portion of the vegetative
tissue, and the establishment of this separated piece as an
independent plant. No group of plants excels the
liverworts in their power to regenerate new individuals
from pieces of the plant body. If the thallus is cut into

2l6

STRUCTURE AND LIFE HISTORIES

many small pieces with a pair of scissors, each piece can
regenerate a new plant. In many species the tips of the
lobes of the thallus become separated from the plant
naturally, by the dying off of portions back from the tip.
In such cases each tip develops an entire new individual.
A thorough study of these phenomena has led botanists

Fig. i6i. — A liverwort {Lunularia). Below, portions of the thallus,
showing the lunar-shaped cupules, with brood-buds, or gemmae. Above
a single gemma, greatly magnified.

to the conclusion that every cell of a liverwort is able to
reproduce an entire plant, just as effectually as though
it were a spore. Some species produce little multi-
cellular bodies called gemmcs (Fig. i6i). Other species
produce fleshy tubers, richly stored with reserve food-
materials,^ and specially valuable in helping the species

^ Analogous to tuber-formation in the potato.

LIFE HISTORY OF A LIVERWORT

217

to tide over periods of drought. Tubers in liverworts were
first discovered and recognized in a species of Anthoceros.
At least two species {Fossomhronia tuherifera and An-
thoceros tuberosus) received their specific names from their

Fig. 162. — Anthoceros phymatodes. Portion of thallus showing develop-
ing tubers. X about 15. (After M. A. Howe.)

characteristic of forming tubers. In some species the
tubers appear as swellings or outgrowths on the under-
side of the thallus; in others (Figs. 162 and 163) as en-
largements of the tips of thallus-lobes. The leafy liver-
wort, Bryopteris filicina (Fig. 164), illustrates vegetative
multiplication by stolons.

Fig. 163. — Anthoceros phymatodes. Mature tuber, sprouting. X about
21. (After M. A. Howe.)

198. The Sporophyte. — After fertilization the oosperm
develops a young embryo, and from the lower or basal
half the foot develops, with projections reaching down
into the tissue of the gametophyte (Fig. 165). After the

2l8

STRUCTURE AND LIFE HISTORIES

1>^^

early stages of development the sporophyte ceases to grow
at the apex, and elongates only by the formation and
enlargement of new cells just above the foot {intercalary
growth). Around the base of the sporophyte there
develops from the tissue of the gametophyte a sheath,
but, in contrast with the mosses, no seta is formed. The

appearance of the sporophytes,
as they appear in clusters, has
been aptly likened to tufts of
delicate blades of grass (Fig.
156). Spores are formed from
the cylindrical archesporium,
between outer and inner layers
of sterile tissue. The inner
thread-like layer is the colu-
mella (Fig. 156). Chlorophyll
develops in the sterile cells, and
intercellular spaces open to the
surface through true stomata
(Fig. 166). The sporophyte,
therefore, has the function of
photosynthesis, and if only a
root, or roots, would develop
from the basal portion, it could
well become established as an
independent plant. As it is, it can live only as long as
the gametophyte remains active, so as to maintain the
supply of water to the sporophyte. The columella serves
to conduct water up the sporophyte.

199. Formation of Spores. — As in the mosses and
ferns, spores arise from spore-mother-cells by reducing
divisions (tetrad-formation). In Anthoceros they do

Fig. 164. — Bryopteris fili-
cina, showing long shoots and
short shoots. The stolons at
the base, with reduced leaves,
serve not only in vegetative
propagation, but also to anchor
the plant. (After Goebel.)

LIFE HISTORY OF A LIVERWORT

219

not all mature at the same time, as in the mosses and
ferns, but new spores continue to form in the region of
intercalary growth so long as growth continues. As the
spores mature the tip of the sporophyte splits open, and
the walls spread apart progressively, as spores lower down
come to maturity.

i ' i "

i

^ 1

I'., 'i-'

ra ffl^

^IKl".

ii

Fig. 165. — Anthoceros. Photomicrograph of a longitudinal section of
the sporogonium, and portion of the gametophytic thallus. Note the
foot of the sporogonium, and the more darkly stained spores at the center,
above.

200. Distribution of Spores. — In addition to spores, a
portion of the tissue of the archesporium develops sterile
cells or cell-rows, called elaiers (Fig. 166). When these
become dry, as the sporogonium splits open from the tip,

220

STRUCTURE AND LIFE HISTORIES

Fig. i66. — Anthoceros. At the left, a stoma with guard-cells,
right, spores and pseudo-elaters.

At the

Fig. 167. — Diagram of the life-cycle of Anthoceros.

LIFE HISTORY OF A LIVERWOtlT 221

they twist with a jerking motion, which helps to project
the spores out to a distance from the parent plant. The
advantage of this is self-evident.

201. Germination.— In some, if not all, species of
Anthoceros, the spores germinate best after a resting
period of several weeks or months. Early in germination
chlorophyll develops in a plastid contained in the spore,
and a germination-tube, or protonema, forms, long in
some species, shorter in others, and at the tip of this tube
a thallus develops.

202. Summary of Life History. — The life history of
Anthoceros is, in outline, as follows (it is illustrated dia-
grammatically in Fig. 167):

OUTLINE OF LIFE HISTORY OF ANTHOCEROS

Anthoceros-plant {garnet ophyte) .

i
—I T^

Antheridia Archegonia

4. i

Sperms Egg

Fertilization

Oosperm (zygote)

I I

Embryo

4- 4-

Mature sporophyte (mature zygote)

4- Ar

Spore-mother-cell

X. 4, 4. 4, 1 Reduction

Spore Spore Spore Spore J

\r

Protonema

4.

Anthoceros-plant (gametophyte)

2 22 STRUCTURE AND LIFE HISTORIES

OTHER FORMS

203. Riccia. — About 4,000 species of liverworts have
been described, and it is, of course, possible here to refer
to only a very few of the forms, chosen because they
illustrate some special idea or step in the evolutionary
development of plants. In addition to the forms already
mentioned, attention should be called to the genus Riccia,

Fig. 168. — A IWtrwoTt {Riccia trichocar pa), X about 35. Cross-section
of the thallus, showing young sporogonium in the enlarged venter of the
archegonium. (After M. A. Howe.)

which is of interest because of its aquatic mode of life,
and also because of its extremely simple sporophyte — the
simplest sporophyte, in fact, of all plants that possess
archegonia (Archegonates) . The fertilized egg of Riccia
develops a sporophyte which has only fertile cells (spores),
except for a wall, one cell thick, enclosing the spores
(Fig. 168). In fact, the sporophyte consists of only a
very simple spore-case, of short duration; it never pro-
jects beyond the venter of the archegonium. Spores
are formed in the usual way, as described for the forms

LIFE HISTORY OF A LIVERWORT

^n

previously studied. Except for the sterile wall-cells, all
the cells formed by the successive divisions of the oopserm

^h^M^f^

Fig. i69.~Ricciocarpus. sp., sporogonium; gam, tissue of gametophyte.

Fig. 170.— Life cycle of Ricciocarpus. (After Cardiflf.)

become spore-mother-cells, each of which, by the tetrad-
division, gives rise to four spores. Meanwhile the wall-

224 STRUCTURE AND LIFE HISTORIES

cells become disorganized and their substance goes to nour-
ish the spores. By this time the cells of the inner layer
of the venter have also become disorganized and their sub-
stance, in like manner, is absorbed by the spores (Fig. 169).
Thus the tissue of the gametophyte serves as nourish-
ment for the developing, primitive sporophyte. The phe-
nomenon of an embryo developing into a mature sporophyte
does not appear in Riccia. As we descend the scale of plant
life we find the sporophyte increasingly simpler; and this
simplicity consists in the diminution of the amount of
sterile tissue.

204. Life History of Riccia. — The life history of Riccia
is shown in outline as follows, and diagrammatically in
Fig. 170. Carefully compare this history with that of
Anthoceros.

OUTLINE OF LIFE HISTORY OF RICCU
Riccia-plant {gametophyte)

Oosperm (zygote)

Cell-divisions

Sporophyte (transient spore-case)

44-

S pore-mother-cclls

\r X^ X- X ] Reduction

Spore Spore Spore Spore

i

Protonema

i

Riccia-plant {gametophyte)

LIFE HISTORY OF A LIVERWORT 225

205. Comparison with Mosses and Ferns. — {a) The

Gametophyte. — The gametophytes of the various kinds of
liverworts differ greatly among themselves, but on the
whole they are more simply organized than those of the
mosses, lacking especially the highly developed, leafy
branches or gametophores. The moss-plant represents
the highest degree of gametophytic organization known
among land-plants, and the leafy branch is practically
universal in that group. On the other hand, the vege-
tative body of the liverworts is, in some forms, simpler
than the prothallus of the fern, while in other forms it is
much more complicated, becoming 'a leafy branch in the
Jungermanniales, and bearing complex gametophores and
other organs in the Marchantiales. But while it may be-
come complex, its organization is always of a lower type
than that of the moss-plant. The antheridia are much
alike in both mosses and liverworts, and on the whole
differ but little from that of the true ferns; but the sperma-
tozoids of the former are always hiciliate, while those of
the true ferns are always multiciliate. The archegonia
of mosses and liverworts may or may not be stalked, but
they are never stalked in the true ferns. With the ex-
ception of Anthoceros they are never sunk beneath the
surface in either mosses or liverworts, bi:^t in the ferns the
venter is commonly sunk in the tissue of the prothallus.

(h) The Sporophyte. — The typical sporogonium or
sporophyte of liverworts and mosses consists of a stalk
or seta, with a foot at one end, imbedded in the tissue
of the gametophyte, and a spore-case at the opposite
end. There are, however, all degrees of variation of this
type of structure. The stalk and foot may be entirely
wanting, as in the simple sporophyte of Riccia; the stalk

IS

2 26 STRUCTURE AND LIFE HISTORIES

may be very greatly reduced, as in Sphagnum; or the
spore-case may not appear as a clearly defined organ,
but may appear to merge very gradually into the stalk, as
in Anthoceros. In liverworts the spore-case never opens
by a lid or operculum, as is universally the case in mosses,
but always by valves, formed by longitudinal splittings
of the sporangial walls. Elaters may or may not occur in
liverworts, but never occur in mosses. The sporogonium
of liverworts and mosses never possesses a leafy stem, and
never possesses true roots; only one case (that of the moss,
Eriopus remotifolius) has ever been reported where the
sporogonium produces rhizoids from its basal end. To
compare the simple sporogonium of Hverworts and mosses
with the leafy plant of the true ferns, would be quite super-
fluous. It should, however, be pointed out that the sporo-
phyte of liverworts and mosses lives always, throughout its
life, as a parasite on the gametophy te, while the sporophy te
of ferns always becomes established, sooner or later, as an
independent plant. Except for the very simple columella
of Anthoceros and the central strand in the seta of mosses,
nothing approaching a true vascular bundle occurs in the
liverworts and mosses; while, as stated before, the well-
developed fibro-vascular system of the ferns has caused
them to be known as vascular cryptogams.

CHAPTER XVII

LIFE HISTORIES OF ALG^E

206. The Main Groups of Algae. — The plants that rank
next in the scale of life below the liverworts are the Algae.
On the basis of color they fall naturally into four main
groups or phyla, as follows:

1. Blue-green algae (Cyanophyceae)

2. Green algse (Chlorophyceae)

3. Brown algae (Phasophyceae)

4. Red algae (Rhodophyceae)

Associated with these differences in color are certain dif-
ferences of structure, which also lead to a similar grouping.
In recent years the above four groups have been further
subdivided into seven phyla, on the basis of other char-
acters than color. A study of one of the commoner brown
algae, Ascophyllum, will serve to illustrate many funda-
mental facts about the algae in general.

ASCOPHYLLUM

207. Habitat of Ascophyllimi. — This plant, and the
closely related Fucus, have become familiar in inland
markets, because they are commonly used as a packing in
the shipment of crabs and other kinds of "shell fish."
Ascophyllum and Fucus, and their near relatives, consti-
tute the family FucacecB. For the most part they are
inhabitants of the ocean, or of brackish water, or, in rare
cases, of fresh water. The two genera mentioned are

227

228

STRUCTURE AND LIFE HISTORIES

LIFE HISTORIES OF ALG^ 229

commonly found attached to rocks between the lines of
high and low tide, where they are subjected to alternate
submersion and exposure (Fig. 171). The Fucace^ have
an added interest because of their economic uses. They
serve as food for the inhabitants of the west coast of
South America, and in other countries, and are also widely
used as fertilizer, and as a source of iodine. They include

Fig. 172.— Portion of plant of Ascophyllufn nodosum. X %.

some of the largest plants in the ocean, and one of the
genera, Sargassum, floating on the surface, helps to form
the well-known - Sargasso Sea," of the middle Atlantic
ocean.

208. Description of AscophyUum.— The plant body of
Ascophyllum is a branched thallus, the branches being

330

STRUCTURE AND LIFE HISTORIES

in

relatively narrow (from J^ to J-^ inch), several feet
length, and interrupted at frequent intervals by swellings
or nodes, which are air sacs, and add greatly to the buoy-
ancy of the plant in water (Fig. 172). Many of the Fucaceae
possess two kinds of branches, more or less distinct from each
other — long branches and short branches, or spurs. This
is a phenomenon which occurs in several groups of plants.

Fig. 173. — Ascophyllum nodosum (L) Lejol. Radical longitudinal sec-
tion of an old branch of the thallus. c, cortical tissue, the seat of photo-
synthesis; m, central tissue, or medulla. (Redrawn from Reinke.)

and notably in the pines, to be studied later. In Ascophyll-
um the distinction between long and short branches is not
as strongly marked as in some other forms, such, for ex-
ample, as Scaheria. The short branches have enlarged
tips, which somewhat resemble the swellings of the main
stem. The plant has a "rubbery" appearance, with a
smooth, slippery surface, and is usually attached to rocks
by a "hold-fast" organ.

209. Anatomy. — A study of the internal structure
(Fig. 173) reveals two systems of tissues, more or less
clearly distinct:

LITE HISTORIES OF ALG.E 23 1

1. The cortex, composed of several external layers of
cells, somewhat resembling, in arrangement, the palisade
parenchyma of the leaf and, like the latter, having the
function of photosynthesis. The outer portion of this
layer is further loosely differentiated into an epidermoidal
tissue, but there is no true epidermis. The outer cell-
walls of this layer, forming the external surface of the
plant, possess a thick layer of cuticle. The cells of the
cortex retain their embryonic character for a long time,
and by successive divisions favor the growth of the branch
in thickness.

2. The medulla, or central tissue, is composed of cells
arranged for the most part in rows, so as to form filaments.
This tissue serves chiefly for the conduction of liquids.
The walls of its cells are mucilaginous and much swollen,
except at certain small pits, the "sieve tubes," closed by
a perforated membrane.

210. Photosynthesis.— The cells of the cortex possess
several chromoplasts or chromatophores, each containing
chlorophyll, by which photosynthesis is possible; but, in
addition to chlorophyll, the chromatophores contain also
a brown pigment {phycophwin) , which masks the chloro-
phyll, and explains the external color that gives the name
Phaeophyceae to the family.

211. Vegetative Multiplication. — Vegetative multiplica-
tion does not occur in Ascophyllum, nor in most of the
genera of Phaeophyceae. In the few genera where it has
been observed, it is accomplished by a fragmentation of the
plant body, or by the death of the older part of the thallus.
The pieces thus set free may develop new plants, but these
usually remain sterile.

232

STRUCTURE AND LIFE HISTORIES

212. Sexual Reproduction. — The reproductive organs
of Ascophyllum (Fig. 174) are borne in chambers
(conceptacles) beneath the surface of the enlargements at
She tips of the short branches. Since the branches bear

^.f^H'lS?-^.

^^^^^l^?

Fig. 174. — Ascophyllum nodosum. A, Cross-section through a female
receptacle; 5, spermagonia; C, ripe oogonium; D, eggs, freed from the
oogonium, but still enclosed by the separated inner layer of the oogonial
wall. (Redrawn from Thuret and Bornet.)

the gametes they are sometimes referred to as gameto-
phores. These chambers open to the exterior by short,
narrow canals, the openings of which may be easily seen
on the surface of the swollen tips. The inner surface of

LIFE HISTORIES OF ALG^ 233

the conceptacles is covered with more or less branched
hairs (paraphyses) , and associated with these hairs are the
organs that bear the sperms and eggs. In some species
of Fucaceae both sperms and eggs are borne in the same
conceptacle, and the plant is, accordingly, moncscious.
This is the case with one of the species of Fucus {F,
platycarpus). In other species, such as those of Asco-
phyllum and Fucus vesiculosus, sperms and eggs are borne
in separate conceptacles, and even on separate plants, in
which latter case the species are dioecious.

213. Gametangia.— The organs that bear either kind
of gametes (sperms or eggs) are termed gametangia. The
female gametangium differs in a very fundamental manner
from the complex archegonium of the mosses and ferns, for
it consists of only one cell, called the oogonium. The
male gametangium, or spermagonium, is likewise unicel-
lular, and the wall is composed of two layers, an inner and
an outer layer. The spermagonia are in reaKty modified
branches of the hairs that line the conceptacles. The
oogonia are not attached to the hairs, but directly to the
inner surface of the conceptacle by a short unicellular
stalk.

214. Gametes.— The male gametes, or sperms, are
formed by successive divisions of the protoplast of the
unicellular spermagonium. Like those of the Hverworts
and mosses, they bear two long ciHa, attached to the side.
They also possess a pigment body, usually reddish in
color. The oogonia of Ascophyllum commonly bear only
four eggs, organized out of the protoplast of the oogonium,
but in rare cases three or five. In some genera {e.g.,
Fucus) there are eight eggs. The nucleus of the oogonium
cell usually divides into eight daughter-nuclei, but in

234 STRUCTURE AND LIFE HISTORIES

Ascophyllum all except four (or the exceptional three or
five) fail to organize daughter-cells about themselves, and
abort. Forms having the full complement of eight eggs,
are, therefore, considered more primitive than those with a
less number.

216. Fertilization. — The eggs are never fertiHzed while
in the oogonia, nor even while in the conceptacle. The
walls of the oogonium burst, and the eggs pass out into the
surrounding water. They are covered with a thick layer
of mucilaginous substance, and by means of some material,
not definitely known, they attract the sperms that happen
to have been discharged at the same time and near the
same place. No other case is known in plants where the
difference in size between egg and sperm is so great as in
the Fucaceae (Fig. 175). The sperms swarm about an egg,
and finally one of them enters it and its nucleus unites
with that of the egg, thus completing fertilization. Soon
after fertilization the oosperm or zygote becomes sur-
rounded by a dehcate cellulose wall, the fertilization-
membrane. The setting free of the egg before fertilization
marks a lower stage of development than is found in the
mosses and ferns.

The process of fertilization in Fucus may be easily
observed by placing mature eggs and sperms together in
sea-water in a watch glass, under the microscope. The
sperms, attracted by the chemical stimulus of the sub-
stance excreted by the egg, swim toward it, and within
about five minutes large numbers of them have become
attached to its surface. By the vigorous lashing of their
ciha the egg is set in vigorous motion. One of the sperms
succeeds in penetrating the cytoplasm of the egg, and
reaches its nucleus. The fusion of the two nuclei may

LIFE HISTORIES OE ALG^E

235

occur within lo minutes after the gametes have been
placed together in the water. After the formation of the

Fig. 175. — Ascophyllum nodosum (L.) Lejol. Ay egg at the time of
fertilization, surrounded by numerous sperms. X about 200; B, oosperm
germinating, 6 days old; C, D, somewhat older stages than B. (Redrawn
from Thuret and Bornet.)

fertilization-membrane the sperms seem to avoid the
oosperm, quite as though they were repelled by some
substance formed at its surface.

236 STRUCTURE AND LIFE HISTORIES

216. The Result of Fertilization.— (a) The immediate
result of fertiUzation is physical— the formation of the
fertiHzation-membrane. Just how this is accompHshed is
not clearly understood. We know, of course, that the
surface of the egg, as in every free mass of protoplasm,
acts as a semipermeable membrane or surface, allowing
some substances in solution, but not all, to pass through
by osmosis. It has been suggested that, when the sperm
enters the egg, chemical changes at once occur, which
alter the permeability of its surface-membrane, thus per-
mitting, for the first time, the exosmosis of some substance
(or substances) which become transformed into the fertiliza-
tion-membrane on contact with the sea-water. It may
be, of course, that the substance composing the mem-
brane is not formed until the sperm enters the egg. How-
ever it may be caused, the formation of the membrane is
a necessary antecedent to all subsequent changes in the
fertilized egg; without its formation the egg dies and dis-
integrates.

(b) The ultimate result of fertilization, as noted in the
preceding chapters, is biological — the intermingHng of the
germ-plasms of the egg and sperm, involving the fusion
of the two nuclei, the doubling of the chromosome number,
and the combination, in one zygote, of the inheritances
from two individuals.^

217. Artificial Fertilization. — Considering that the for-
mation of the fertiHzation-membrane is purely physical,
biologists began to reason that it ought to be possible to
induce it artificially. The experiment was first success-
fully made by a zoologist, Loeb, with the eggs of sea-urchins

^ The commingling of the two inheritances was called, by Weismann,
amphimixis.

LIFE HISTORIES OF ALGiE 237

and other marine animals. In 19 13 it was successfully
accomplished by Overton with the eggs of Fucus. The
eggs were dipped for about a minute, or a minute and a
half to two minutes, in a mixture of 50 cc. of sea-water
plus 3 cc. of a very weak solution of acetic, butyric, or
other fatty acid, and then transferred to normal sea-
water. This treatment caused the formation of the fertiH-
zation-membrane, quite as in natural fertilization by the
sperm. If, after the formation of the membrane, the eggs
are placed for 30 minutes in hypertonic sea-water (50 cc.
of normal sea-water plus 8 to 10 cc. of a weak solution of
sodium chloride (common salt), or potassium chloride),
and then back into normal sea-water, the eggs begin to
divide and continue to develop into young plants. The
question as to the chromosome number in the cells of
plants formed by artificial fertilization is of very great
interest, but has not yet been investigated.

218. Germination of the Oosperm. — After either natural
or artificial fertilization the young zygote begins at once
to divide, without any period of rest. Of the two cells
formed by the first division, one gives rise to the hold-fast
organ, by which the new plant is attached to the rocks,
while the other develops into the main body of the plant,
which resembles the parent plant in all external characters
(Fig. 175)-

219. Reduction. — As always in normal fertilization, the
nucleus of the oosperm is diploid, and the Ascophyllum
plant that develops from it is also diploid. It is therefore
the sporophytic generation. At the end of the first two
nuclear divisions of the spermagonia and oogonia, re-
duction has been accomplished, and the four nuclei that
result are haploid. They therefore belong to the haploid

238 STRUCTURE AND LIFE HISTORIES

or gametophytic generation. In other words they have
the same value as spores, and the one-celled stage of the
spermagonia and oogonia the same value as spore-mother-
cells.

220. Female Gametophyte. — Each of the four cells re-
sulting from the reduction-divisions in the oogonial
protoplast divides again, producing a total of eight cells,
which constitute a very simple gametophyte. No further
divisions occur in the oogonia. In some of the Fucaceae
{e.g.f Fucus vesiculosus) each of these eight daughter-cells
functions as a female gamete or egg; but in Ascophyllum,
and a few other species, part of the daughter-cells, as
stated above, disintegrate or abort, leaving only from one
to five. In Ascophyllum nodosum one-half of them abort,
leaving only four eggs. The female gametophyte is thus
seen to be reduced to merely its gametes.

221. Male Gametophyte. — Each of the four cells
resulting from the reduction-divisions of the spermagonial
protoplast undergoes four divisions in succession, re-
sulting in 64 cells or a total of 256, all of which develop
into a male gamete, or sperm. The four daughter-cells,
therefore represent a very elementary or simple male
gametophyte.

222. Simplification of the Gametophytes. — The im-
portant point to note in connection with the life history of
Ascophyllum is, not only the great simplicity of the game-
tophyte, but the fact that it consists of nothing but
fertile or reproductive cells. Each of the four spores
gives rise only to gametes; no sterile cells, or gametophytic
plant bodies are produced.

223. Gametophyte or Sporophyte.— In light of the facts
above related, the question as to the real nature of the

LIFE HISTORIES OF ALGJE 239

plant body of Ascophyllum becomes of very great interest.
Its cells possess the double or sporophytic number of
chromosomes, but it bears organs (spermagonia and
oogonia) that ultimately contain gametes. Is it, therefore,
a gametophyte or a sporophyte? For a long time it was
considered a gametophyte, but a clear understanding of
the divisions that take place in the gametangia, accom-
panied by reduction, and the fact that the body-cells are
all diploid, lead unmistakably to the conclusion that it is
a sporophyte. We have seen that the protoplasts of the
young spermagonium and the young oogonium are in
reality equivalent or analogous to spore-mother-cells, and
that, in each case, their four daughter-cells, with their
reduced number of chromosomes, are functionally equiva-
lent or analogous to spores. The spermagonia and
oogonia, therefore, which seem at first thought to be sexual
organs — simplified antheridia and archegonia — come to be,
finally, more truly comparable to sporangia, from which
the spores are not set free, as spores, but, while still in the
spore-case, develop into either a male or a female gameto-
phyte consisting of nothing but gametes.

The real nature of Ascophyllum (and of the other
Fucaceae, for that matter) is just opposite from what a
superficial examination would lead us to infer, and we
have, in this low form, a condition just the reverse from
what is found in the liverworts, mosses, and ferns; in other
words, a prominent sporophyte, bearing a very simple
gametophyte, that lives upon it as a parasite deriving all
of its nourishment from the sporophyte.

224. Alternation of Generations. — Although the game-
tophytic generation is reduced to its lowest terms, the
fundamental fact of alternation is not affected. As soon

240

STRUCTURE AND LIFE HISTORIES

as the gametes are set free fertilization takes place, pro-
ducing a fertilized egg, with, of course, the double or diploid
number of chromosomes. Every cell of the sporeling,
resulting from the germination of the oosperm, and every
cell of the mature plant into which it develops, possesses
the double number, up to the first two divisions of the

■•■■^^ 1 Stage

Fig. 176. — Diagram of life-cycle of Ascophyllum nodosum.

young gametangia, when reduction gives rise to gameto-
phytic cells. The fusion of these — egg with sperm — in
fertilization restores the double or sporophytic number
and so on in cycle after cycle (Fig. 176).

225. Diagram of Life History. — The successive steps in
the life cycle of Ascophyllum may be briefly summarized
as follows:

LIFE HISTORIES OF ALG^E 24I

OUTLINE OF LIFE HISTORY OF ASCOPHYLLUM

Spermagonial-plant Oogonial-plant

(Sporophyte) (Sporophyte)

Spermagonium Oogoniun

(Sporangium) (Sporangium)

Protoplast of spermagonium Protoplast of oogonium

(spore-mother-cell) (spore-mother-cell)

4- >l< 4^ 4^ Reduction '^ ^ ^ ^

Spore Spore Spore Spore Spore Spore Spore Spore

64 sperms 8 less 4 eggs

(Reduced male gametophyte) (Reduced female gametophyte)

Oosperms (rest)

Spermagonial-plants Oogonial-plants
(Sporophytes) (Sporophytes)

226. The Cause of Sex. — One of the many very inter-
esting things revealed by a study of the life history of
Ascophyllum nodosum is the fact that part of the fertilized
eggs give rise to plants that bear only male gametes, and
the remainder to plants that bear only female gametes.
Since the fundamentally different individuals develop
under identically the same environmental influences, we
are forced to the inference that the difference lies in the
fertilized eggs themselves. Some of them appear to be
male-producing, others female-producing. In what does
this fundamental difference consist? Here is a very
important problem, but further consideration of it must
be postponed until Chapter XXII.

x6

CHAPTER XVIII
LIFE HISTORIES OF ALG^ (CONCLUDED)
DICTYOTA DICHOTOMA

227. Habitat. — ^Like Ascophyllum and Fucus, Dictyota
dichotoma grows chiefly along the ocean margins in the
zone between the lines of high and low tide. It is thus
subjected to alternate wetting and drying, and to rhyth-
mically alternating changes of light and temperature. It

m^^'^^'^iK^TTlH

^Br\' 1 :-k , ' i ^ . ' ^jI^ ^\_ 1 \ k M^^^^^k

B^S^

Fig. 177. — Dictyota dichotoma. Left, sporogonial plant; right sperma-
gonial (gametophytic) plant. (After W. D. Hoyt.)

is found from as far north as the middle of the Scandi-
navian peninsula to about 40° south latitude.

228. Description.— The plant body (Fig. 177) is a
thallus, flat, and branching profusely by forking at the

242

LIFE HISTORIES OF ALGiK 243

tips, i.e.y dichotomously (whence the nsime dicholoma) , but
showing no differentiation into anything like stem and
leaf. One end of the plant is differentiated into a special
branching organ, the hold-fast, by which it becomes
attached to rocks, shells, and other convenient solids.
As in Fucus, again, the green chlorophyll is masked by a
brown pigment, which indicates the relationship of the
plant to the Brown seaweeds, or Phaeophyceae. There are
two kinds of branches — cylindrical ones which are sterile,
and others more strap-shaped, or ribbon-like, which
eventually bear the reproductive organs.

Vegetative multiplication may occur by the separation
of portions of the thallus, which may become established as
independent plants. In some species of Didyota, specially
differentiated bodies have been noted, resembling the
brood-buds or gemmae, such as occur in some of the liver-
worts and mosses.

229. Reproduction. — With reference to reproductive
organs, three kinds of plants occur :

1. Plants bearing asexual spores only (asexual).

2. Plants bearing oogonia only (female).

3. Plants bearing antheridia only (male).

But the most interesting feature in this connection is that
the plants of all three groups look very much alike, except, of
course, for their different kinds of reproductive organs,
and for unimportant individual variations.

Oogonia and antheridia are both produced from surface
cells. The surface cell first divides into a smaller stalk-
cell, and a larger external one, rich in protoplasm, which
forms the gametes. The protoplast of the oogonium does
not divide, as in Ascophyllum and Fucus, but develops
into only one very large egg; while the protoplast of the

244

STRUCTURE AND LIFE HISTORIES

antheridium continues to divide until as many as 1,500 or
more sperms are formed in each one. Since the antheridia
occur in groups oisori (Figs. 178 and 179), the total num-

FiG. 178. — Diclyola dichotoma. Cross-section of a male thallus, show-
ing the comparative development of the antheridial sori, and the tufts of
hairs which are scattered over the frond, a, young sorus; h, older sorus;
c, sorus opened. The sperms have been set free. There remain only the
cells which form the involucre, d, tuft of very young hairs; e, tuft of
older hairs; /, the same fully developed. X about 35. (After Thuret.)

SoOOiOSoo 0

SgS;

Qino-

Fig. 179. — Dictyota dichotoma. A, vertical section through portion
of sorus, showing antheridia; B, sperms. (Redrawn from J. Lloyd Wil-
liams.)

ber of sperms formed on one plant, or even in one sorus,
is enormous. The oogonia may occur singly or in groups
(Fig. 180).

LIFE HISTORIES OF ALGiE

245

230. Discharge of Gametes. — As in Ascophyllum, both
kinds of gametes are set free at the same time, before
fertilization. Recent studies have disclosed the very
interesting fact that their discharge occurs at rhythmic
intervals of about two weeks, synchronizing with the
periods of high and low tide. The advantages of this, if
any, are not apparent, and the periodicity persists in plants
placed in jars of sea-water in the laboratory, and even
with branches newly developed in the laboratory, and thus

Fig. 180. — Dictyota dichotoma (Huds.) Lamx. Longitudinal section of an
oogonial sorus. (After Bornet and Thuret.)

never (as branches) subjected to the variations of the tide.
So close, however, is che harmony between tidal periods
and discharge of gametes, that the exact day of their dis-
charge, at any given station, can be predicted (with an
error, at most, of only one day) by consulting the tide-
tables for the given station.

231. Fertilization. — From the freshly liberated eggs
there diffuses through the water some unknown substance
which attracts the sperms. The latter respond to this
stimulus by swimming toward the egg. One of them

246

STRUCTURE AND LIFE HISTORIES

enters it, and makes its way through the cytoplasm to the
egg-nucleus (Fig. 181), with which it fuses, thus completing
the act of fertilization. The other sperms come to rest
outside the egg, and finally disintegrate. If eggs liberated
at different periods occur in the water together, the sperms
will swim toward those liberated last, in preference to the
others. As soon as fertilization has been accomplished,
the oosperm begins to divide, and develops into a new
plant , which finally comes to resemble externally the ones
that bore the antheridia and oogonia.

4 r ,/

Fig. 181. — Dictyota dicholoma. At the left, section of newly liberated
egg; «, egg; cyto, cytoplasm of egg; sp, three of the numerous sperms ap-
proaching the egg to fertilize it. At the right, portion of a section of an
egg after one of the sperms (shown at the right of the egg-nucleus, en) has
entered; sp, sperms that have not entered. The fusion of the sperm with
the egg nucleus has been delayed, and the sperm has greatly increased in
size. (Redrawn from J. Lloyd Williams.)

232. Asexual Reproduction. — The plants that develop
from fertilized eggs never bear antheridia and oogonia, but
non-motile, asexual spores only. These are set free at
irregular intervals, and never at rhythmic periods like the
gametes. They are produced by two successive divisions
of a spore-mother-cell, and thus occur in groups of four
(tetrads) ; the plants that bear them are commonly referred

LIFE HISTORIES OF ALG^E

247

to as tetrasporic plants. When the tetraspores are set free
they soon become attached to some solid object, and, like
the fertilized eggs, develop into plants that externally
resemble, at maturity, those bearing tetraspores. Thus,
the plants produced by the fertilized eggs and by the tetra-
spores closely resemble each other in all vegetative characters;
they differ externally only in the kind of reproductive
organs they bear.

233. Alternation of Generations. — Although the Dic-
tyota plants developed from zygotes and spores look alike,

/.

Fig. 182. — Dictyota dichotoma. A, Vertical section, transverse to the
axis of the thallus, showing a polar view of the nuclear plate in the first
division of the antheridium. C, Similar view of the first division of the
oogonium; B, similar view of the first nuclear division of the fertilized egg.
Note that the reduced (haploid) number of chromosomes in A and C is 16,
while the fertilized egg (B) shows the diploid number (32). (Redrawn
from J. Lloyd Williams.)

it is obvious that the products of the tetraspore, since they
bear gametes, and never spores, are gametophytes; and
the products of the fertilized egg, since they bear spores
only, and never gametes, are sporophytes. These facts
have only recently been established by careful experi-
mental cultures. There is thus a true alternation of
generations, although, in marked contrast to the ferns and
mosses, the plant bodies of the two generations are

248 STRUCTURE AND LIFE HISTORIES

vegetatively alike. Obviously we would expect all the
cells of the gametophyte to bear the reduced or haploid
number of chromosomes, and all the cells of the zygote or
sporophyte, the diploid number. Such is the case. Fig.
182, C, shows a section of an oogonium with the nucleus
in its first division, and the number of chromosomes (16)
may be easily counted. In Fig. 182, A, is shown the divid-
ing nucleus of the antheridium also with 16 chromosomes.
In the fertilized egg (Fig. 182, B) the diploid number (32)
may readily be counted. Since Dictyota is dioecious, we
must infer that the tetraspores, though looking alike ex-
ternally, are fundamentally different internally, since part
of them give rise to antheridial or male plants^ and part
to oogonial or female plants.

234. Reduction. — Since all the cells of the sporophyte
(tetrasporic plant) contain the diploid number of chromo-
somes, and all the cells of the gametophytes the reduced
number, the question naturally arises, where does re-
duction take place? It occurs in the two divisions that
result in the formation of the tetraspores; each of the
latter possess the reduced number, whereas the spore-
mother-cell is diploid. From this we learn that reduction
may occur at different points in the life-cycle in different
forms. In Ascophyllum it occurs in the nuclear divisions
immediately preceding the formation of the gametes; in
Dictyota, immediately preceding the formation of the
spores, while between reduction and the formation of
gametes occur the innumerable cell-divisions that give
rise to the body of the gametophyte.

235. Life-cycle of Dictyota. — The life-cycle of Dictyota
may be diagrammed thus :

LIFE HISTORIES OF ALG^ 249

OUTLINE OF LIFE HISTORY OF DICTYOTA

Antheridial-plant Oogonial-plant

(Male gametophyte) (Female gametophyte)

4, 4.

Antheridium Oogonium

4- 4.

Sperm Fertilization

Oosperm (zygote)

4.4,

Tetraspo ric-plant
(Sporophyte)

II

Sporangium

Spore-mother-cell

\, \. \, \. \Reduction

Tetra- Tetra- Tetra- Tetra-

spore spore spore spore

4^ 4* 4" 4'

Male Male Female Female

gameto- gameto- gameto- gameto

phyte phyte phyte phyte

236. Effect of Environment. — How is the similarity in
external appearance of the alternating generations of
Dictyota to be explained? We cannot say for certain.
The two generations start from reproductive cells (zygotes
and spores) that are profoundly unlike in their internal
organization, but it must be borne in mind that both
generations develop under practically identical external
conditions — the surrounding ocean-water, with alternate
exposure and immersion at low and high tides. This
would tend to influence the plant bodies alike, and has
been suggested by some botanists as an explanation of the
resemblance of the two kinds of generations. In the
case of ferns and mosses the environment of the two

250 STRUCTURE AND LIFE HISTORIES

generations is not alike. The sporophyte, for example,
begins life as a parasite on the gametophyte, while the
gametophyte leads an independent existence from the
start. The reader may also recall other differences. The
question we have here raised, however, is a very funda-
mental one, and will be further discussed in Chapter
XXII.

ULOTHRIX

237. Habitat. — The genus Ulothrix occurs everywhere,
from pole to pole, in fresh water.

238. Description. — The plant body (Fig. 262) is usually
a simple thread of cells, though in exceptional cases the
cell-divisions result in a cell-plate instead of a thread.^ All
the cells are similar in appearance and structure, except
the basal one, by which the plant is attached to some solid
body. This cell is somewhat larger than the others,
possesses less pigment, and is suitably modified to serve
as an organ of attachment, or hold-fast. The protoplast
of each cell possesses one nucleus, surrounded by the green,
cylindrical chloroplast. No other pigment occurs, and
therefore Ulothrix belongs to the Chlorophyceae, or green
algae. Photosynthesis, of course, takes place, and a
portion of the photosynthate may become transformed
into starch, the presence of which is easily demonstrated
by the usual test with iodine.

239. Asexual Reproduction. — Every cell of the plant,
except the hold-fast, is capable of functioning as a re-
productive cell, and two methods of reproduction are
common. In one case the entire protoplast of a cell

^ These two forms of plant bodies are sometimes designated by the
terms "linear aggregate" (a filament) and "superficial aggregate" (a
plate).

LIFE HISTORIES OF ALG^ 25 1

becomes organized into from one to eight motile spores
(zoospores, or swarm-spores), each with, four cilia and a
red ''eye-spot." The zoospore escapes by swimming
through a small opening in the cell-wall, attaches itself
by its ciliate end, and by a series of cell-divisions produces a
new filament like the one from which it came. In this
case it is evident that the zoospore, reproducing without
cell-fusion, is an asexual spore, and that the mother-cell
from which it came functioned as a sporangium.

240. Sexual Reproduction. — Other cells of the same
filament that produced the asexual zoospores, may, by
successive nuclear and cell-divisions, become divided into
as many as 1 6 to 64, or even more, independent motile cells,
each with a red "eye-spot," but. with only two cilia.
Like the zoospores, they escape by swimming through an
opening in the wall of the mother-cell. Occasionally one
of them comes to rest and begins germination, but the
process never continues far enough to produce a new plant.
More commonly, two of these cells come together and fuse,
showing that they are, in reality, gametes. Since the
gametes are similar in size they are termed equal gametes,
or isogametes. The fusion of two isogametes is called
conjugation, to distinguish it from the fusion of un-
equal gametes; it is essentially the same as fertilization.
Whether or not reduction occurs, in the nuclear and cell-
divisions that result in the formation of the isogametes,
is not known.

241. Germination. — After the fusion of the gametes the
zygote at once begins to increase in size, but soon its cell-
wall becomes thickened, and then the protoplast divides
into a number of swarm-spores, each of which, when set
free, may develop into a new plant.

252

STRUCTURE AND LIFE HISTORIES

OUTLINE OF LIFE HISTORY OF ULOTHRIX

Plant Body
(Undifferentiated into gametophyte and sporophyte stages)

i i

i i

Physiological

Physiological

Sporangium

Gametangium

4- 4-

4, >t ) Reduction

Zoospore

Isogamete Isogamete

I i

^^-^„^Conj ligation ^^0^

Plant Body

^"^■^-^ l^X^^

(Undifferentiated into

Zygote

gametophyte and sporo-

I I

phyte stages)

Swarm-spores

4. 4.

Plant Body

PLEUROCOCCUS

242. Habitat. — Everyone is familiar with the green
layer found on the outer surface of the bark of trees, on

Fig. 183. — Individual plants of green slime {Pleurococcus vulgaris), show-
ing the tendency of the cells to remain attached after cell-divisions, thus
causing transitions from a one-celled to a multi-cellular plant.

wooden fences, and the moist shaded surfaces of stone
steps or rocks. Careful observation will show that this
crool is largely due to the presence of countless numbers

LIFE HISTORIES OF ALG^

253

of a tiny green plant, called Pleurococcus, usually of the
species Pleurococcus vulgaris. The individual plants are
so small that they may be seen only with the aid of the
microscope (Fig. 183). An examination of the trees in
any given locality will disclose the fact that Pleurococcus
prefers one side of the tree to the other, and that the
choice of sides has a direct relation to light, temperature,
or moisture — one or all.

Fig. 184. — Pleurococcus vulgaris. Sections of one-, two-, and four-celled
plants, showing the nuclei and the large chlorophyll bodies (chb) to which
the green color of the plants is due. In D, the larger chloroplast is shown
in perspective. (Camera lucida drawings from a microscopic preparation
by E. W. Olive.)

243. Structure. — No plant structure could be much
simpler than that of Pleurococcus, for the plant body is a
single cell, the simplest organic unit capable of independent
existence. The protoplast possesses a well-defined nucleus
and a chloroplast, and is surrounded by a cellulose cell-

2 54 STRUCTURE AND LIFE HISTORIES

wall (Fig. 184). We have here, in fact, a unicellular
organism.

244. Reproduction. — The reproduction of Pleurococcus
consists merely of the processes of cell-division, and the
final separation of the daughter-cells. The latter may
adhere until several divisions have occured, but eventually
the middle layer of the cell-wall common to the two
adjacent daughter-cells is dissolved, probably by an
enzyme. The cells then separate from one another, and
become independent plants, increasing in size, and soon
repeating the simple reproductive process just described.

245. Simplicity of Pleurococcus. — The structure, life-
processes, and life-relationships of plants could hardly
find a more simple expression than in Pleurococcus.
Morphologically the green plant is here reduced to its
lowest terms. There is no differentiation into parts —
no hold-fast, roots, or rhizoids, no shoot, no special
reproductive organs. From the standpoint of physiology,
every essential function is performed by the one cell —
absorption of water and dissolved nutrient substances
by osmosis through the cell- wall and plasma-membrane;
photosynthesis, with entrance of carbon dioxide and
exit of oxygen by diffusion through the membrane and
cell- wall; respiration, with the accompanying exchange
of gases in the same way; digestion, assimilation, and
growth, resulting finally in reproduction by the division
of the entire plant body into new individuals. So far
as known, such processes as cell-fusion (in fertilization
or conjugation), reduction, and alternation of genera-
tions are entirely absent. Pleurococcus is a generalized
plant, with almost no division of physiological labor.
At one time the entire plant body functions vegetatively,

LIFE HISTORIES OF ALG^ 255

that is, for the maintenance of the individual; at another
time the entire plant body functions reproductively,
that is, for the maintenance of the race to which the
individual belongs.

246. Life History of Pleurococcus. — The simple life-
history of Pleurococcus may be briefly indicated as follows :

OUTLINE OF LIFE HISTORY OF PLEUROCOCCUS

Unicellular plant

4-

Cell-divisions

i

Temporary cell-aggregates

I

Separation of daughter-cells

Ar Ar Ar •\'

Unicellu- Unicellu- Unicellu- Unicellu-
lar plant lar plant lar plant lar plant

CHAPTER XIX
LIFE HISTORIES OF FUNGI

247. The Groups of Ftingi. — We are all more or less
familiar with fungi, as represented by the common
molds, mildews, toadstools, and mushrooms. They
are all plants without chlorophyll, and are therefore
dependent upon green plants for their nourishment.
The Greek word for fungi is mycetes, and this word
terminates the names of the various groups, as follows:

1. Phycomycetes, alga-Hke fungi; so-called
because they closely resemble certain
algae, except for the lack of chlorophyll.

2. Ascomycetes, sac-fungi; so-called because
their asexual spores are formed in tiny
sacs (asci)

3. Basidiomycetes, with spores borne on
little club-shaped hy^Yidd, or basidia. In-
clude the smuts, rusts, and mushrooms.

AN ALGA-LIKE FUNGUS (RHIZOPUS)

248. Habitat. — Everyone is acquainted with "bread
mold," a plant without chlorophyll, and having a fila-
mentous plant body. There are many kinds of fila-
mentous fungi, more or less closely related to Rhizopus,
and growing on various substances or "substrata."
They all agree in at least three points: (i) they are always
filamentous; (2) they never possess chlorophyll; (3) they
always grow on some organic substratum. The sub-

256

LIFE HISTORIES OF FUNGI 257

stratum is also usually moist. Partly as a result of
their presence, the substratum on which they grow is
usually disintegrating with decay. From these facts
Rhizopus is called a saprophyte} The most common
species is Rhizopus nigricans.

249. How Obtained. — Rhizopus, or almost any other
filamentous fungus, may be readily obtained by sowing
its spores on a suitable substratum. But, unlike the
higher plants, Rhizopus may ordinarily be obtained merely
by exposing moist bread, or other nutritive substance, to
the air. In the course of time, without one's sowing any
spores, colonies of the plant will appear, and this clearly
demonstrates the very interesting fact that these spores
are always floating in the air in greater or less abundance.
When the moist bread is exposed, some of the floating
spores come to rest upon it, and there, under favoring
conditions of moisture, temperature, and light they
germinate, and develop new plants.

250. Description of Plant Body. — The plant body of
Rhizopus (Fig. 185) consists of a slender, thread-like
filament, called the mycelium, branching freely, devoid
of chlorophyll, and growing over the surface of the
substratum. The threads of mycehum are termed
hyphce. Careful examination with the microscope dis-
closes the fact that the hyphae are largely or wholly
aseptate, that is, not divided by cross-walls or septa.
The plant body, therefore, consists of one cell, though
there are several to many nuclei distributed through the
cytoplasm; such a structure is termed a coenocyte}

^ From the Greek, sapros (aairpos), rotten, + phyton {ipvrSv), a plant.
2 Greek, koivos {koinos), common, -1- kvtos (kutos), a hollow (cell).
17

258

STRUCTURE AND LIFE feCSTORIES

The protoplasm appears to be streaming in a constant
current in one direction, and this is thought to be due to
the evaporation of water from surfaces more exposed to
the air, and the intake of more water from the substratum,
by osmosis.

Fig. 185. — Bread mold {Rhizopus nigricans). A, older plant; myc,
mycelia; sph, sporangiophore; sp, sporangium; st, stolon produced by A,
and giving rise at its tip to a new plant, B. Greatly enlarged.

251. Secretion of a Powerful Poison. — In the course
of some experiments, made in order to determine the
cause of sex in the mucors, Blakeslee and Gortner in-
jected into the ear of a healthy rabbit some of the juice,
squeezed out of the mycelium of Rhizopus nigricans.
To their great surprise the animal died almost instantly,
before the injection was completed. Further experi-
ments clearly demonstrated that Rhizopus contains a
powerful poison (or toxin) ^ which is soluble in water,
but which produces its effect only when introduced into
the circulatory system. When this expressed juice was
fed to the rabbits, or when they ate the mycelium, no

LIFE HISTORIES OF FUNGI 259

harmful result followed. The discoverers of the toxin
suggest that the discovery may possibly throw light on
''pellagra" and some of the destructive diseases of cattle,
such as "cornstalk-disease," and the "horse-disease,"
prevalent in some of our western states, and for some
time thought to be due to some food-impurity.

262. Vegetative Mtiltiplication. — At various points the
mycelium produces one or more upright, aerial hyphae,
15 to 20 millimeters high, and of larger diameter than the
mycelial hyphae. One or more of these hyphse eventu-
ally bend over and grow for a short time along the
surface of the substratum, like the stolons or "runners"
of such plants as strawberries. Finally, at their free ends
there develop slender hyphse that penetrate the sub-
stratum, like rhizoids, and another group of the larger,
aerial hyphae, one or more of which may repeat the process
just described. The formation of these stolons suggested
a former name of the plant, viz., Mucor stolonifer.

253. Asexual Reproduction. — At the tip of each up-
right hypha there develops a globular sporangium, and
hence the hypha is called a sporangiophore (Fig. i86). The
protoplasm is constantly streaming up the sporangiophore,
getting more and more dense toward the sporangium, and
contains large numbers of nuclei. Further down, to-
ward the base of the sporangiophore, the protoplasm
becomes thinner and thinner, and the nuclei, fewer in
number, finally disintegrate. In the meantime the tip
of the sporangiophore increases in size until the globular
sporangium is formed, rich in cytoplasm and nuclei.
By degrees the protoplasm becomes more dense toward
the periphery, and gradually thinner toward the central
portion, where large numbers of vacuoles develop. Soon

26o

STRUCTURE AND LIFE HISTORIES

a well-marked cleft separates the denser peripheral pro-
toplasm from the thinner central portion, which now
develops into a little column, ox columella (Fig. i86). In the

Fig. 1 86. — Rhizopus nigricans, i, Young sporangium, showing cyto-
plasm and nuclei streaming up the sporangiophore into the sporangium
and out toward the periphery. 2, Sporangium of nearly full size. The
differentiation between the dense peripheral and the looser central plasms
is clearly shown. Nuclei are still passing from the central to the periph-
eral regions along the fine strands of cytoplasm. X about 200. (After
D. B. Swingle.) (Cf. Figs. 187 and 188.)

meantime innumerable furrows develop in the denser plasm,
first at the periphery and then working their way in from
the columella cleft. By the time the two sets of furrows

LIFE HISTORIES OF FUNGI

261

meet, the dense protoplasm has been cut up into innumer-
able, more or less angular masses, each surrounded by a
plasma-membrane, and containing from two to six nuclei

Fig. 187. — RhizopHS nigricans, i, Full-sized sporangium, showing
layer of vacuoles nearly formed along the inner surface of the denser plasm,
X about 200. 2, Section passing to one side of the sporangiophore,
showing columella-cleft being formed by fusion of the vacuoles shown in i,
and. by a furrow from the base of the sporangium, X about 200; 3, Detail
of another part of same sporangium as shown in 2, showing early cleavage
furrows, X about 850. (After D. B. Swingle.) (Cf. Figs. 186 and 188).

each (Fig. 187). These finally become entirely separated
from each other, oval in shape, and surrounded by a wall
of fungus cellulose (chitin?) having clearly marked ridges
(Fig. 188). They are asexual spores, but differ from the

262

STRUCTURE AND LIFE HISTORIES

spores of ferns, mosses, liverworts, and algae by usually, if
not always, possessing more than one nucleus. They
secrete a slimy substance in which they are imbedded.

Fig. 188. — Rhizopus nigricans, i, Section of sporangium, showing
cleavage of peripheral cytoplasm much further advanced than in Fig. 187.
Furrows are here cutting outward from the columella cleft, X about 200;
2, section of sporangium in which the spores are completely formed,
rounded up, and surrounded by thin walls. The columella wall is also
formed, X about 200; 3, ripe spores in their living condition, showing
variations in size, and ridges on their walls, X over 350. (After D. B.
Swingle.) (Cf. Figs. 186 and 187.)

When ripe the wall of the sporangium bursts open (Fig.
i8q), and the spores, thus set free, float away through the
air in countless millions. So widely and so thickly are

LIFE HISTORIES OF FUNGI

263

they distributed that a piece of moist bread, or a jar of
canned fruit, left exposed for only a brief period, in almost
any locality, will, as noted above, soon become "moldy"
from the growth of mycelium produced by their germina-
tion. On account of the practically universal distribution
of ''bread mold" its spores are, of course, commonly

Fig. iBg—Rhizoptis nigricans. A, Young sporangium, showing col-
umella within; B, older sporangium, with the wall removed, showing ripe
spores covering the columella; C, D, views of the collapsed columella after
dissemination of the spores.

present in the air of laboratories, where the mold is a great
pest, and has therefore won the appropriate title of
''laboratory weed."

254. Sexual Reproduction. — When the hyphae of
mycelia derived from spores of different sex-value
are intermingled they frequently develop short lateral
branches, which grow out toward each other until their
tips come in contact (Fig. 190). The rich protoplasmic
contents at the tips are cut off from the remainder of
the coenocytic mycelium, the walls in contact with each
other become dissolved,^ and the two protoplasmic masses
fuse. This will be recognized as conjugation; the fusing
masses of protoplasm are isogametes, and the cut-off tips
of the conjugating branches function as gametangia.

^ Probably by enzyme action, though this has not been actually
demonstrated.

264

STRUCTURE AND LITE HISTORIES

After the zygote is formed, the outer wall thickens and
assumes certain external characteristics, easily recog-
nized. It also becomes black as it ripens, and this fact
has given rise to the common name, "black mold." In
this condition the zygote rests, as a zygospore.

Fig. 190. — Sexual reaction between a hermaphroditic Mucor and (+)
and ( — ) races of a dioecious species. Diagrammatic representation of a
Petri dish culture showing a heterogamic hermaphroditic mucor (^) in
the center separated by channels on either side from the (+) and (— )
races, respectively, of a dioecious species. Sp., sporangia containing
spores by means of which the plant may be reproduced nonsexually. 1-6,
stages in development of a hermaphroditic zygospore from unequal male
and female gametes. A, sexual reaction between a ( — ) filament and fe-
male gamete. B, sexual reaction between a ( +) filament and male gamete.
C, a male zygospore formed at stimulus of contact with a (+) filament.
(After Blakeslee.)

256. Germination. — At the close of the resting period,
and under favorable conditions of temperature and mois-
ture, the zygote germinates, sending out an erect hypha,
which at once develops a globular sporangium at its apex.
The asexual spores from this sporangium become the
starting point of another series of changes like those
just described.

256. Sexuality of Rhizopus. — It is well known that,
while conjugation often occurs freely between mycelia
from different spores, in other cases it fails entirely. The
explanation of this was not known until about eight years

LIFE HISTORIES OF FUNGI 265

ago, when it was discovered that there are, in reality, two
unlike strains of Rhlzoptis-mycelmm, and therefore two
kinds of spores. The two kinds of mycelia are designated

Fig. 191. — Petri-dish culture showing dark line of zygospores between
the (+) and ( — ) strains of a dioecious species of mo\d,'C/i(>anephora (Mc)
and white lines of "imperfect hybridization" between the strains of this
species and the opposite strains of JMucor V (Mv). Note the total absence
of zygospore formation between two (+) or two ( — ) strains. (After
Blakeslee.)

as (+) and (— ). When the interminghng hyphae are of
h'ke strains, either all (+) or all ( — ), conjugation fails
completely, but when they are from unlike strains, zygo-
spores form in great abundance. This fact is strikingly

266

STRUCTURE AND LIFE HISTORIES

shown in Fig. 191, which illustrates the result of growing
mycelia from unlike strains side by side. The zone where
they come into contact is sharply defined by the line of
black zygospores, resulting from conjugation.

257. Distinction of Sexes. — When first discovered, the
two unlike strains were designated as (+) and (— ),
because it was not possible to decide, with certainty,
which was male and which female. Externally both

Fig. 192. — Diagram of life-cycle of a dioecious mold.

strains looked very much alike, except that one appeared
to be vegetatively more vigorous than the other. Recent
experiments seem to indicate that the vegetatively more
vigorous (+) strain is female, while the less vigorous
(— ) strain is male. Not all molds are dmcious, like
Rhizoi>tis, In some species the mycelium from a single
spore produces conjugating branches of both (+) and

LIFE HISTORIES OF FUNGI 267

(— ) value, which fuse and form a zygospore. Such
mycelia are, of course, monoecious.

The life-cycle of a dioecious mold is illustrated in
Fig. 192.

258. Phycomycetes. — Rhizopus nigricans represents the
group of Phycomycetes, lower fungi, often called "tube-
fungi." The group is characterized by the tubular hyphae
without septa. In this coenocytic character they re-
semble the filamentous or tubular green algae (such as
Vaucheria, or "green felt"), of which they are supposed
to be degenerate descendants.

259. Life -cycle of Rhizopus. — The life-cycle of Rhizopus
may be diagrammed as follows:

OUTLINE OF LIFE HISTORY OF RHIZOPUS

(+) Spore (-) Spore

4- I

Mycelium Mycelium

i i

Gametangium Gametangium

i 4.

(+) Gamete (— ) Gamete

Conjugati

Zygospore

4.4.

Germ-tube

4, I

Sporangium

4^ I Reduction

(-|-) Spores ( — ) Spores

i i

(-I-) Mycelium ( — ) Mycelium

i 4,

Sporangium Sporangium

4, 4-

(+) Spore ( — ) Spore

268

STRUCTURE AND LIFE HISTORIES

A SAC-FUNGUS (MICROSPHiERA)

260. Habitat and Structure. — The fungus, Micro-
sphcera, may be found growing on the leaves of the lilac,
and is commonly called ''powdery mildew." The my-

FiG. 193. — Lilac mildew {Microsphara Alni). The white areas are the
mycelia of the fungus, growing over the surface of the leaf. The tiny
black dots are the perithecia, which contain the asci.

celium is septate, and forms a web over the surface of the
leaf (Fig. 193), sending down haustoria into the epidermal
cells, to secure nourishment, and sending up short branches
(conidiophores), each bearing a chain of colorless spores

{conidia) .

LIFE HISTORIES OF FUNGI

269

261. Reproduction by Asci. — In late summer or early
fall one may notice, on an infected leaf, among the my-
celium, tiny black dots or spheres (Fig. 194), whence the
name, Microsphcera. Examined with the microscope, these
bodies are seen to bear numerous appendages, branched
at the end, and with the tips of the branches curved back to
form miniature hooks (Fig. 195). When these little spheres
are crushed, or when they burst open, they are found to con-

FiG. 194. — Powdery mildew {MicrospJmra Alni) on lilac leaf. An in-
fected area from the leaf in Fig. 193, greatly magnified.

tain a number of tiny sacs or asci (singular ascus), whence
the name "sac-fungus," or Ascomycete. In each ascus are
a number of spores or ascospores, formed from the con-
tents of the ascus. The young ascus is, therefore, a spore-
mother-cell. There are usually eight ascospores in an
ascus, but the number may vary.

The spherical case containing the asci is the '^spore-
fruit" (ascocarp or perithecium) , and results from the
fusion of the contents of an antheridium and an oogonium

270

STRUCTURE AND LIFE HISTORIES

(Fig. 185). The cells from this point on, to and including
the formation of the ascus, are diploid, and therefore
constitute the sporophytic generation.

Fig. 195. — Lilac mildew {Microsphcera Alni). A, perithecium, with
appendages; B, perithecium, showing asci (a); C, an ascus, containing
ascospores; D, conidiophore (cph), bearing a chain of conidia (conidio-
spores, c.s); E, beginning of fertilization; antli, antheridium; car, carpo-
gonium; F, later stage in fertilization; the contents of the antheridium and
carpogonium have fused; /, fusion of the two nuclei; G, germination of
ascospore (a.s); g.t, germ tube. (£ and F after R. A. Harper.)

262. Germination. — Reduction occurs during the forma-
tion of the ascospores. When an ascospore germinates it
develops directly into a mycelium.

263. Life-cycle. — The Hfe-cycle of Microsphcera may
be tabulated as follows:

UTE HISTORIES OF FUNGI 27 1

OUTLINE OF LIFE HISTORY OF MICROSPH^RA

Spore (conidium)

I

Mycelium

4.

Antheridium^

Spore (conidium)

4.

Mycelium

i

Oogonium*

Zygote in ascocarp (Sporophyte)
Ascogenous hyphae

4. 4.

Young asci

^ } Reduction
Ascospores (Gametoph^'te')

I

Mycelium

I "

Conidiophore

4.

Conidium (Conidiospore)

Fig. 196. — A "cup-fungus," Peziza sylvestris. (Photo by F. J. Seaver).

^ In MicrosphcBra there is only a slight morphological diflferentiation of
the gametes.

272

STRUCTURE AND LIFE HISTORIES

264. Other Sac -fungi. — Between 25,000 and 30,000
species of sac-fungi have been described. Some of them
are filamentous, like the lilac-mildew, while some are
fleshy, like the common ''cup-fungi'' (Fig. 196). The
edible morel, Morchella esculenta, (Fig. 197) is an Asco-
mycete, and the common yeast referred to in the chapter
on fermentation (Fig. 60) is also classed here because, in
one of its methods of reproduction the unicellular plant

Fig. 197. — The morel, Morchella esculenta. (Photo by W. A. Murrill.)

body functions as a spore-mother-cell, the protoplast
becoming organized into spores (ascospores) , and the wall
of the mother-cell serving as an ascus.

A "RUST" FUNGUS (WHEAT RUST)

265. Importance. — One of the most important, as well
as most difficult, fungi to understand is the wheat rust
(Puccinia graminis). This fungus is important because

LIFE HISTORIES OF FUNGI 273

it attacks some of the most valuable of all agricultural
crops (wheat, oats, rye, barley, etc.), causing, at times,
millions of dollars worth of damage.* It is difficult to
understand because it is heterxcious — that is, lives alter-
nately on two different plants, the barberry and the grain.
For many years the form on the grain was supposed to
be quite another plant from that on the barberry, which
was called jEcidium herheridis.

266. Life History. — a. Red Rust Stage. — The mycelium
of red rust (uredo stage) grows between the cells of the
stem and leaves of the wheat, or other grain, and finally
during the summer, numerous sporophores, bearing red
spores {uredinios pores), break through the epidermis
(Fig. 198), producing reddish or rusty-looking dots and
lines, whence the name ''rust" for the plant. The one-
celled uredinio-spores are easily blown by the wind in
great numbers to other wheat plants, where they germ-
inate, and thus spread the rust widely and rapidly.

b. Black Rust Stage. — In late summer the same myce-
lium develops an entirely different kind of spore, two-
celled, and black. These are the final spores of the season,
the teliospores (or teleutos pores), and in them the nuclear
fusions occur. They rest over winter, and germinate
the following spring, each cell usually sending forth a
hypha commonly composed of four cells, which constitute
the hasidium (promycelium) . Each of these cells produces
a tiny sporophor.e, bearing at its tip a single basidiospore
(sporidium). Reduction occurs during germination.

c. Barberry Stage. — The basidiospores are blown by the

*The financial loss from wheat-rust in the United States amounted
to $67,000,000 in 1891; in 1904 the loss in North Dakota, South Dakota,
and Minnesota alone was estimated at $25,000,000.
18

274

STRUCTURE AND LIFE HISTORIES

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LIPE HISTORIES OF FUNGI 275

wind to the leaves of nearby barberry bushes, where they
germinate. The resulting myceHum penetrates the epi-
dermis, and ramifies between the cells of the leaf paren-
chyma, from which it absorbs nourishment. Under
certain circumstances, as for example in Austraha where
barberries do not grow, the barberry stage is eliminated.
Soon this mycehum develops a reproductive structure
called the pycnium (pycnidium or spermagonium), which
breaks through the upper epidermis and produces innu-
merable pycnios pores (spermatia). The real nature of
this structure is not known.

OUTLINE OF LIFE HISTORY OF WHEAT RUST
Rust on Grain

,~ TT TT

Urediniospores (early summer) Teliospores (late summer)

4-4- . J-i

Germination Rest (Nuclear fusion)

Mycelium Germination

ii 4-4-

Urediniospores Basidium

4-4' 4' \Redtiction

Germination Sterigmata

ii 4.

Etc. Basidiospores (sporidia)

I

To barberry

4. 4.

Pycnia ^cia [Cell-fusion)

I i4.

? ^ciospores

Rust- on grain

Near the lower epidermis are formed the ''cluster-
cups," or (Ecia^ (aecidia). They break through to the
^ Sing, oscium.

276

STRUCTURE AND LIFE HISTORIES

outer surface, and the countless numbers of ceciospores
are carried by the wind to nearby wheat, where they, in
turn, germinate, and the complicated series of events
begins again.

267. Diagram of Life History. — The Kfe history of
Puccinia graminis may be outlined as shown on page 275.

A FLESHY FUNGUS (AGARICUS)

268. Habitat. — Fleshy fungi are found widely dis-
tributed, growing in the soil of fields, pastures, and

Fig. 199 — Meadow mushroom {Agaricus campestris L.). A, view
showing under side of pileus; g, gills; a, annulus, or remains of the veil
attached to the stipe; B, side view; s, stipe; a, annulus; p, margin of pileus,
showing at intervals the remains of the veil. (After W. A. Murrill.)

woods, or on tree trunks, decaying logs, and elsewhere.
The familiar edible mushroom (Agaricus campestris),
as its name implies, grows commonly in meadows, and
is hence often called the "meadow mushroom'^ (Fig. 199)-
One of the most poisonous species, Amanita phalloides,

LIFE HISTORIES OF FUNGI 277

(Fig. 200), resembles Agaricus superficially and is often
mistaken for it. Amanita always has, at the base of the
stalk, a cup, which Agaricus lacks.
269. Description. — The body of Agaricus consists of a

Fig. 200. — The deadly amanita, Amanita phalloidcs. Note the cup at
the base of the stipe. (Photo by E. M. Kittredge.)

short fleshy stalk (the stipe), having numerous root-like
hyphge {rJiizomorphs) penetrating the soil from its lower
end, and bearing at its upper end an umbrella-shaped
expansion, the pileus. On the under side of the pileus are
numerous thin lamellae or gills. The stalk and pileus are

278

STRUCTURE AND LIFE HISTORIES

composed of innumerable hyphae (Fig. 201), closely asso-
ciated together in a pseudo-tissue, but there is no real

Fig. 201. — Coriinarhis cinnamomeus. Above, longitudinal section
through stipe and pileus, showing gills (g); below, longitudinal section
through primary and secondary gills; h, hymenium or sporogenous surf ace.
(After Gertrude E. Douglas.)

tissue-differentiation, such as occurs in the mosses and
ferns and higher plants, though the external layer may
usually be peeled off with ease.
270. Reproduction.— If the pileus of Agaricus is placed

LIFE HISTORIES OF FUNGI

279

Fig. 202. — Spore-print of the fly agaric {Amanita muscaria L.). The
spore-print is made by laying the pileus, gills downward, on a sheet of
black paper. The white spores fall onto the paper, making the print.
(Photo by W. A. Murrill.)

Fig. 203. — Fruiting surface (hymenium) of a mushroom {Agaricus).
m, hyphse of the trama and sub-hymenium; 6, basidium; st, sterigma; sp,
spores. (Diagrammatic.)

28o STRUCTURE AND LIFE HISTORIES

with the gills down over a piece of white paper, and left
for a few hours, a dark purplish deposit will form in lines
under each gill. With Amanita the color is white. This
deposit (the ''spore-print ") is composed of spores, shed from
the surfaces of the gills (Fig. 202). Microscopic examina-

FiG. 204. — •The meadow mushroom {Agaricus campestris, var. Colum-
bia.) Young fruiting bodies (carpophores). The mycelial hyphae are in
the substratum. (Photo by G. F. Atkinson.)

tion discloses the fact that the gills are composed of a net-
work of hyphae. Their surface is covered with innumer-
able short, thick, club-shaped bodies, filled with proto-
plasm (Fig. 203 ) . These are the basidia (singular basidium) .
Fungi which bear basidia are grouped together as Basidio-
mycetes. At the tip of each basidium are two tiny projec-

LIFE HISTORIES OF FUNGI

281

tions, the sterigmata (singular, sterigmaY, and on the end
of each sterigma is a spore (basidiospore). In the early
stages of development the gills appear white, at maturity
they are purplish, and, at about the time the spores are
shed, they become dark brown. Although many pains-
taking experiments have been made in order to secure the
germination of the spores, no one has ever been successful.
Whether or not the mushroom is produced from spores
in nature we can only conjecture. No sexual organs have
ever been discovered on any of the fleshy Basidiomycetes.

Fig. 205. — The common mushroom [Agaricus campestris). Young "but-
tons." (Photo by G. F. Atkinson.)

271. Vegetative Propagation.— The meadow-mushroom
is propagated for food from "bricks" of mycelium, or
''spawn" that may be purchased from seedsmen. When
portions of these ''bricks" are placed in suitably prepared
soil, under favorable conditions of moisture and heat,
the mycelium resumes its growth, ramifying in all di-
rections through the soil. At numerous points on the
mycehum tiny little "buttons" form (Fig. 204). As the

^ In most of the gill-bearing fungi there are four sterigmata on each
basidium, as shown in Fig. 203.

282

STRUCTURE AND LIFE HISTORIES

Fig. 206. — A pore-bearing fungus {Poly poms hetulimis). Under (fruit-
ing) surface, showing the pores.

^ Fig. 207.— Giant puflfballs {Lycoperdon giganteum Botsch.). Note the
size of the four specimens as compared with the egg and fruit in the center.
(After W. A. Murrill.)

LIFE HISTORIES OF FUNGI 283

buttons enlarge, a little chamber forms near the tip (Fig,
205), and into this chamber some of the hyphae grow, form-
ing the ''gills," with their ''fruiting" surface. While the
gills are forming other hyphas form a veil, extending from
the stipe to the edge of the pileus, and protecting the
gills until the spores are ripe. By continued growth of
the pileus the veil becomes ruptured, thus allowing the
spores to escape. It has been found that the spores do
not merely fall from the sterigmata f rom their own weight,
but that they are forcibly expelled.

Entire new mushrooms may also be obtained by growing
pieces of a young plant on suitable nutrient media. From
such pieces mycehum is produced which ultimately bears
the "buttons."

OUTLINE OF LIFE HISTORY OF MEADOW-MUSHROOM

Spore (Basidiospore)

4.

Mycelium

J-

Binucleate cells

4.4.

Young sporophore ("Buttons")

Mature sporophore (the "Mushroom") \ ^!^^

Gills

II

Hymenium

4.4,

Basidium {Nuclear fusion)

4, \ Reduction
Sterigma

i

Spore (Basidiospore)

272. Other Classes of Fleshy Fungi.— In addition to
the gill-bearing fungi (Agaricaceae) , there are many other
families of Basidiomycetes, one bearing the fruiting sur-

284 STRUCTURE. AND LIFE HISTORIES

face in tubes (Polyporaceae, Fig. 206), another on the sur-
face of teeth or spines, on the under surface of the pileus
(Hydnaceae) . The common "pufT-balls" are Basidio-
mycetes, with the fruiting surface entirely enclosed in
the more or less globular fruiting body, almost the entire
contents of which break down into the powder or dust of
the ripe puff-ball (Fig. 207). Over 14,000 species of
Basidiomycetes have been described.

273. Life-cycle.- — The life-cycle of the meadow-mush-
room (Agaricus campestris) may be indicated as shown on

page 283.

OTHER NON-GREEN PLANTS

In addition to the true fungi, there are two other groups
of non-green thallophytes which ought to be mentioned
here, one (the Myxomycetes) because of their scientific
interest; the other (bacteria) because of their economic
importance.

274. Myxomycetes.— The myxomycetes are on the
border-line between the kingdom of plants and that of
animals. In some of their characters they so closely re-
semble lower animals like the Amceba, that they have
been claimed by the zoologists, under the name Mycetozoa
(fungus-like animals). In their method of reproduction
they are more like plants than like animals.

The body of the organism is a large naked mass of
protoplasm, called a Plasmodium, commonly found on
old decaying logs, in tan-bark, and similar places, where
moisture and organic food are abundant. The proto-
plast flows over the surface on which it grows, like the
Amodba, taking in nourishment, at times flowing around
and thus injecting particles of food, assimilating, and
growing in size. The protoplasm may be spread out in

LIFE HISTORIES OF FUNGI

285

sheets or in a delicate network of strands, or both
(Fig. 208). When viewed under the microscope the

Fig. 208. — Plasmodium of a slime-mold (myxomycete), Fuligo sept lea,
growing on the inner surface of a glass jar. The natural color was bright
orange.

protoplasm is seen to be in almost constant motion, flow-
ing for about a minute in one direction; gradually slowing

Fig. 209. — Fruiting bodies of a myxomycete {Comatricha suksdorfii
E. and E.), greatly enlarged. (Photo from tyoe specimen.)

up, it comes to rest, and then resumes its flowing, but in the
opposite direction, also for about a minute. Different

286

STRUCTURE AND LIFE HISTORIES

species possess different colors, but they never possess
chlorophyll.

275. Reproduction. — At a certain stage of development,
the Plasmodium will begin to form tiny upright stalks,
at the top of which will develop a spore-case, containing
spores and capillitium (Fig. 209) . The capillitium consists
of hygroscopic threads which aid in the dissemination of
the spores. When the spores are ripe they are scattered

Fig.

-Kohlrabi, showing club-root, caused by a myxomycete,
Plasmodia phor a hrassiccB. (Cf. Fig. 2.)

by the wind, and each, on germination, produces a swarm-
spore. A new Plasmodium results from the fusion of these
swarm-spores.

276. Economic Importance. — The Myxomycetes have
very little economic importance, but the disease of cab-
bages, kohlrabi, and turnips, known as club-root, is caused
by a siime-mold growing on the roots (Fig. 210).

277. Bacteria. — The bacteria are among the very

LIFE HISTORIES OF FUNGI 287

simplest plant structures known. They are one-celled,
but of many shapes, and with or without motile cilia.
They include the smallest living things known. There
are even reasons for believing that some forms are ultra-
microscopic, that is, too small to be seen with the most
powerful microscopes that can be made. Some forms are
less than one fifty-thousandth of an inch in diameter.
Of some kinds of bacteria, as many as 300 could be placed
side by side on the period at the end of this sentence. In
fact the word "microbe" means "tiny living thing, ''^
though not all microbes are bacteria. The germ of
malaria, for example, is a microscopic animal (protozoon),
resembling an Amesba.

Several genera of bacteria are distinguished according
to shape as, for example, Bacterium (non-motile rods),
Bacillus (motile rods), Micrococcus (spherical). Spirillum
(spiral threads) (Fig. 211).

Fig. 211. — Various forms of bacteria, a, Spirillum; b, Bacillus typhosus;
c, Staphylococcus; d, e, j, h, Micrococcus; f, k, I, Bacillus; g, Pseudomonas
Pycocyanea; i, Streptococcus.

They are found everywhere and multiply rapidly by
cell-division, whence they are called "fission-fungi," or
Schizomycetes. Not possessing chlorophyll, they are all,
of course, either parasitic or saprophytic, some being
highly beneficial — in fact indispensable — to man; others
highly harmful, causing some of the worst known diseases
of both plants and animals.

^ From the Greek ixiKpSs (mikros), small, + /3/os (bios), life.

CHAPTER XX

ECONOMIC IMPORTANCE OF FUNGI

In the preceding chapter we have frequently referred
to ways in which our own lives are related to the life of
plants. It would be difficult to say what particular group
of plants affects us most, but certainly none more than
the fungi. They are among the direct causes of human
sorrow and happiness, of health and disease, of poverty
and wealth, hfe and death. They are at once the founda-
tion and the arch enemy of agriculture, the objects and ob-
stacles of commerce, the source and hindrance of human
industry.

EDIBLE FUNGI.

278. Mushrooms and Toadstools. — Everyone is familiar
with edible ''mushrooms;" they are now on sale at every
grocery. Nearly everyone thinks there is a difference
between mushrooms and ^' toad-stools," Such, however, is
not the case. These two terms are applied indiscrim-
inately by the botanist to any ''fleshy " fungus. The word
"mushroom" is used by most people to designate the
meadow-agaric (Agaricus campestris) , which is the mush-
room of commerce, par excellence. There are over i,ooo
fleshy fungi that are good to eat, and many more that
are not poisonous, but non-edible because they are
tough, or fibrous, or ill-tasting. Many of those good
to eat resemble the meadow-mushroom in having
a stalk and gills, and are closely related to
it (Fig. 212); while others, like the "puff-balls,"

288

ECONOMIC IMPORTANCE OF FUNGI 289

belong to an entirely diiferent group. All puff-balls are
good to eat when young. Many esteem the morel
(Morchella esculenta) as a great delicacy.

Fig. 212. — Shaggy-mane mushroom (Coprinus comatus). Edible before
the spores turn black.

279. Criterion of Edibility. — Here, as elsewhere, there
is no royal road, no short cut to knowledge. There are
absolutely no external characteristics which distinguish
edible from poisonous fungi. The only way to tell whether
a given species is poisonous or not is to try it. Since this
is so, it is best for the amateur not to make the endeavor,
but to depend only upon the knowledge of an experienced
mycologist. One should first seek to attain skill in
determining the exact species of his specimen, and then
follow the assurance, and especially the warnings, of
some reliable book. In general, one should avoid all
bright-colored species (although some of them are not
poisonous), and all species that have a ''cup" at the
base of the stalk, or stipe. To insure no mistak^^ in this
latter point, one should always be sure that he has the
base of the stipe, and has not broken it off above the
base. Beginners should also avoid all specimens in the
*' button" stage of development, as it is more difficult
to determine the exact species at that stage.
19

290 STRUCTURE AND LIFE HISTORIES

280. Mushroom Culture. — The growing of mushrooms
for the market is a very important industry, especially
in some localities. Cultures are usually started from
''spawn," obtained from the seedsman in the form of
''bricks." These bricks consist largely of mycelium,
tightly pressed together. When the bricks are broken
up and distributed through a "bed" of soil and manure,
properly prepared, the mycelium resumes its growth, and
soon begins to produce the "buttons" (Fig. 204), which
finally develop into mature mushrooms.

The industry is commonly carried on in cellars and
caves. This is not necessary, for the meadow-mushroom,
as its name clearly implies, grows in nature in open
meadows and pastures. But, since the fungi have no
chlorophyll, they do not need the light, and so space
can be used for their culture that would not well serve
any other useful purpose.

FUNGI THAT CAUSE PLANT DISEASES

281. Govermnent Regulation. — Fungi that grow as
parasites on green plants cause serious disturbances of
the normal life-processes and structure of their hosts,
interfering with healthy growth, and causing plant
diseases. Since the fungi are reproduced by spores, these
diseases may rapidly spread by contagion. On this
account state legislatures and the national Congress have
been obliged to pass stringent laws governing international
and interstate traffic in plants liable to disease, providing
for their careful inspection and quarantine. The United
States Government maintains an expert pathologist
continuously at the port of New York to inspect plants

ECONOMIC IMPORTANCE OF FUNGI 29 1

imported from foreign countries. Sometimes whole
cargoes of potatoes or other vegetables are refused en-
trance at the port, and must then be taken to sea and
dumped into the ocean, or else taken to the port of some
other country where the regulations are less stringent or
less rigidly inforced.

282. Diseases Caused by Phycomycetes. — Among the
plant diseases caused by the alga-like fungi may be
mentioned:

Fig. 213. — "Little potatoes." A disease caused by the parasitic fungus,
Rhizodonia {Corticium vagum var. solani Burt).

1. The ''damping-off fungus" {Pythium de Baryanum
Hesse), which attacks young seedlings of beans and other
plants near the surface of the ground, causing the tissues
there to disintegrate, and the entire plant finally to wilt
and die.

2. Brown rot of lemons, commonly seen in fruit that
has been kept too long or in too damp a place.

3. *' Blister-blight" or white ''rust" of radishes and
their relatives, such as shepherd's purse and mustard.

292

STRUCTURE AND LIFE HISTORIES

This fungus is known as Albugo Candida, or more recentl)*
as Cystopiis candidus.

4. Downy mildew^ of grapes and cucumbers.

Fig. 214. — Witches' brooms on the hackberry {Celtis occidentalis),
caused by a gall-mite {Phyioplus sp.), or possibly by the mite in conjunc-
tion with a powdery mildew {Sphcerotheca phytoptophyla), which is usually
found on the "brooms."

5. Potato rot and "late blight," Phytophthora injestans
(Mont.) DeBary. This disease was the cause of the
failure of the potato crop and the consequent famine in

ECONOMIC IMPORTANCE OF FUNGI

293

Ireland, in 1846-47. (The disease known as "little pota-
toes" {Rhizoctonia) and by various other names (Fig. 213),
is caused by one of the Basidiomycetes.)

283. Ascomycetes. — Among diseases caused by various
species of sac-fungi are the following:

Fig. 215. — Ergot {Claviceps purpurea). Sclerotia on wild rye {Elymus
virginicus), at left; marsh grass {Spartina sp.), middle; cultivated rye
{Secale cereale), at right.

1. Peach leaf-curl, plum pockets, and "witches brooms"
(Fig. 214).

2. Brown rot of peach and plum. The damage caused
by this disease in the state of Georgia alone, in 1900,
amounted to nearly $700,000.

294

STRUCTURE AND LIFE HISTORIES

Fig 2i6 — Two chestnut trees {Castanea dentata), killed by the destructive
chestnut blight. (After W. A. Murrill.)

ECONOMIC IMPORTANCE OF FUNGI

295

3. Alfalfa leaf-spot, the sooty mold of the orange, the
powdery mildew of grapes and apples, the wilt disease of
cotton and watermelon, the ergot of rye and other cereals
(Fig. 215), the black knot of plums and cherries, and the
disastrous chestnut disease of the eastern United States.

284. Chestnut Disease. — The chestnut disease (Fig. 216)
first appeared in the vicinity of New York City about

Fig. 217. — Map of the northeastern United States, showing the approxi-
mate distribution of the chestnut bark disease in 1911. The disease has
spread further since the map was made. Horizontal lines, area where
majority of trees are dead; vertical lines, approximate area where infection
is complete; dots, location of advance infections. (After Metcalf. U. S.
Dept. Agr., Farmers Bull. 467.)

1904, and from there as a center it has rapidly spread
until it has destroyed most of the chestnut trees within
a radius of 150 to 200 miles of the city (Fig. 217). As
many as 17,000 trees have been destroyed in the city of
Brooklyn alone, entailing a total loss of several million

:^96

STRUCTURE AND LIFE HISTORIES

dollars. The mycelium of the fungus that causes it
(Endothia parasitica) grows underneath the bark, and
for this reason it is practically impossible to check it

Fig. 2 1 8. — Chestnut blight. Portion of a branch of an American chest-
nut {Castanea dentata), which had been artificially inoculated with the
spores of the chestnut-blight fungus, Endothia parasitica (Murr.) Anders.
The white areas are infected spots. (After W. A. Murrill.)

by spraying, as the bark protects the mycelium from all
known spraying solutions. The fruiting pustules of the
fungus form on the surface (Figs. 218 and 219). The

ECONOMIC IMPORTANCE OF FUNGI

297

only method of checking the spread of the disease is to
cut down and burn all affected trees. Millions of dollars
worth of damage has been caused by this disease within
the past seven or eight years. The financial loss in New
York City and vicinity, alone, has been estimated at
much more than $5,000,000, while the loss for the entire
United States, up to 191 1, was estimated by the Federal

Fig. 219. — Chestnut-blight fungus {Endothia parasitica). Fruiting
pustules and spore-masses from cultures. X about 8. A, stages in the
development of the pustules; B, C, D, various forms of spore discharge
in a moist atmosphere. (After Murrill.)

Government at not less than $25,000,000. In 19 10, the
state of Pennsylvania appointed a special commission of
experts for the investigation and control of the disease,
and appropriated over $275,000 to meet the necessary
expense of the work.

286. Grain Smut. — The smuts of wheat, oats, barley,
and corn are among the commoner pests of the farmer.

298

STRUCTURE AND LIFE HISTORIES

These diseases are caused by fungi of the genus Ustilago}
The species are often named from the plant affected,
for example Ustilago Avence on oats (Avena, Fig. 220),

Fig. 220. — Panicles of oats (Avena saliva), with the grains almost com-
pletely destroyed and replaced by the oat smut {Ustilago Avence).

Ustilago Tritici on wheat (Triticum), Ustilago Zece on
corn (Zea, Fig. 221). The mycelium extends through the

^ A Hemibasidiomycete.

ECONOMIC IMPORTANCE OF FUNGI

299

Stem, fruiting in the tissues, and commonly destroying
the kernel of grain. The innumerable black spores form
a sooty powder — whence the common name of '^smut/'

Fig. 221.— Corn-smut {Ustilago maydis) on stalk, tassel, ear, and leaf of

Zea Mays.

286. Rusts.— The life history of the wheat rust (Puc-
cinia graminis) was outlined" in Chapter XIX. This
fungus has not only caused millions of dollars worth of
damage to the wheat crop of the world but has been the
cause of legislative enactments. As early as 1760 there
was passed in Massachusetts ''An Act to prevent Damage
to English Grain arising from Barberry Bushes." This
act read, in part, as follows:

"Whereas it has been found by experience, that the Blasting of Wheat
and other English Grain is often occasioned by Barberry Bushes, to the
great loss and damage of the inhabitants of this province:

300 STRUCTURE AND LIFE HISTORIES

"Be it therefore enacted by the Governour, Council, and House of
Representatives, that whoever, whether community or private person,
hath any Barberry Bushes standing or growing in his or their Land, within
any of the Towns in this Province, he or they shall cause the same to be
extirpated or destroyed on or before the thirteenth Day of June, Anno
Domini One Thousand Seven Hundred and Sixty.

"Be it further enacted that if there shall be any Barberry Bushes
standing or growing in any land within this Province, after the said loth
day of June, it shall be lawful, by Virtue of this Act, for any Person who-
soever to enter the Lands wherein such Barberry Bushes are, first giving
one month's notice of his intention to do so to the Owner or Occupant
thereof, and to cut them down, or pull them up by the root, and then to
present a fair account of his labour and charge therein to the owner or
occupant of the said land; and if such owner or occupant shall neglect or
refuse by the space of two months next after the presenting said account,
to make to such person reasonable payment as aforesaid, then the person
who cut down or pulled up such bushes, may bring the action against such
owner or occupant, owners or occupants, before any Justice of the Peace,
if under forty shillings, or otherwise before the Inferior Court of Common
Pleas in the County where such Bushes grew, who upon proof of the cut-
ting down or pulling up of such bushes by the person who brings the action,
or such as were employed by him, shall and is hereby respectively em-
powered to enter up judgment for him to recover double the value of the
reasonable expense and labour in such service and award execution
accordingly.

"Be it further enacted, that the Surveyors of the Highways, whether
public or private, be and hereby are empowered and required ex officio to
destroy and extirpate all such Barberry Bushes as are or shall be in the
Highways in their respective Wards or Districts, and if any such shall
remain after the aforesaid tenth Day of June, Anno Domini One Thou-
sand Seven Hundred and Sixty, that then the Town or District in which
such bushes are shall pay a Fine of two shillings for every bush standing or
growing in such Highway, to be recovered by Bill Plaint, Information, or
on the Presentment of a Grand Jury, and to be paid one Half to the In-
former and the other Half to the Treasury of the County in which such
bushes grew for the use of the County."

In addition to the rust of wheat, there are also rusts of
the carnation and the clover caused by species of Uro-
myces. The carnation rust, which first appeared about

ECONOMIC IMPORTANCE OF FUNGI 30I

1892, is common in the plant houses of commercial
florists.

287. Pine Tree Blister-rust. — Among the more im-
portant plant diseases recently appearing in the United
States is the pine tree blister-rust, introduced from
Europe about 1909. One species is Cronartium pyri-
forme, which is the telial stage of Peridermium pyriforme-
The aecial stage {Peridermium) appears on the pine,
while the alternating host is the ''false toad-flax" {Com-
andra umbellata and C. pallida).^ This fungus attack.*
species of pine that have less than five leaves to the fascicle,
such 2is Pinus contorta, P. ponderosa, and P. rigida.

Another species (Cronartium rihicola) passes its aecial
stage on five-leaved pines, where it is commonly known
as Peridermium Strobi;^ the telial stage, as its name in-
dicates, is passed on species of Ribes (gooseberries and
currants) (Fig. 222).

The importance of such a disease as this may be inferred
when we consider that the value of the white pine grow-
ing in the New England states is estimated at $75,000,000.
that of the Lake states at $96,000,000, of the Western
states at $60,000,000, and of other National forests at
$30,000,000, a total of $261,000,000. The western sugar
pine {Pinus Lambertiana) has a total value estimated at
$150,000,000. Thus, timber to the value of $411,000,000
is threatened with destruction by this one parasitic dis-
ease. In order to reduce the danger of infection from the
blister-rust, and also from the pine-shoot moth {Evetria

^ The Comandra is itself a parasite on the roots of various species of
blueberry (Vacciniiint), and other woody plants.

2 From Strohus, the specific name of the common white pine {Pin*^
Strohus).

302

STRUCTURE AND LIFE HISTORIES

hiioliana), the United States Department of Agriculture
has issued quarantine regulations forbidding the impor-

FiG. 2 2 2. — White pine blister-rust. A, portion of diseased tree, show-
ing pycnidial blisters broken open; from these blisters the disease spreads
to neighboring currant or gooseberry bushes; B, early summer stage on
under surface of a currant leaf; these spores repeat during the summer,
at intervals of two weeks; C, early summer stage, much magnified; D,
late summer and fall stage, on the under surface of a currant leaf; from
this stage the disease spreads again to pine trees. (After Perley Spaulding,
by courtesy of the U. S. Dept. of Agriculture.)

tation of all five-leaved pines and all species and varieties
of Ribes, except for experimental or scientific purposes by

ECONOMIC IMPORTANCE OF FUNGI

303

the Department of Agriculture. Since July i, 191 5, the
importation of every spe-
cies of the genus Pinus has
been forbidden from all
European countries and lo-
calities. In March, 19 16,
the Federal Horticultural
Board requested all nursery-
men in the eastern United
States not to ship white
pine, gooseberry or currant
stock into the Rocky Moun-
tain and Western white pine
forest areas.

288. Timber-destroying
Fungi. — Everyone recalls
the "shelf -fungi," so often
seen growing on the trunks
of trees (Fig. 223). These
forms are the fruiting bodies
of the fungus, while the my-
celium ramifies through the
wood, often in such quan-
tities as to form the ''punk,"
formerly much used in set-
ting off fireworks. The soft
fungal threads are enabled
to make their way through
the hard woody tissue by
means of an enzyme which
they secrete. The enzyme
softens and dissolves the cell-walls of the wood, thus

Fig. 223. — A shelf-fungus {Fomes
applanatus) on sugar maple.

304

STRUCTURE AND LIFE HISTORIES

making it possible for the mycelium to penetrate with ease.
This process disintegrates the wood, weakens the tree so that
it eventually dies or is easily blown over by the wind, and
of course renders the wood of little or no value for timber.
The fungus gains admission to the tree by means of the
spores falling on some surface freshly exposed by trim-
ming the tree, by the accidental breaking of branches,
by the ''barking" caused by lawnmowers, and in other
ways; on account of the disastrous results, all such sur-
faces should be protected by be-
ing painted over as soon as a
branch is cut or broken off, or a
portion of bark removed. There
are many species of wood-de-
stroying fungi, and the financial
loss they cause to the lumber
industry, not to mention the
losses of beautiful shade trees
in lawns, parks, and streets, is
very considerable.

MOLDS

The filamentous fungi, com-
monly known as molds, be-
long to various species. The
black mold {Mucor mucedo) is
common on bread, and the blue
mold {Penicillium) (Fig. 224)
on decaying fruit and on fruit canned at home. The
appearance of the mold indicates that the fruit was not
sufficiently sterilized before the cover was screwed down
on the fruit jar. In fact, the entire process of canning is a

Fig. 224. — Penicillium glau-
cum. h, hypha; b, basal cell; st
sterigma; c, spore {conidium).

ECONOMIC IMPORTANCE OF FUNGI 305

series of operations intended primarily to kill all germs of
bacteria and fungi. The sterilized fruit is then sealed
from the air and from access of other germs while it is
still hot. The ''keeping" of canned goods depends upon
the successful exclusion of every living spore or other
germ. If goods preserved in tin cans have been im-
perfectly sterilized the gases produced by fermentation
will exert a pressure upon the can from the inside, often
strong enough to cause a bulging of the ends.

COLD STORAGE

Just as canning has for its object the preservation of
vegetable or animal tissues by killing the germs with
heat, so cold storage accomplishes the same end by means
of extreme cold. Most germs remain inactive below a
certain temperature, which may be readily ascertained
by experiment. The spoiKng of eggs is caused by the
presence within the egg of a germ-" flora," which lives
upon the yolk and white, producing by its life proc-
esses, the noxious gases of stale eggs. At certain low
temperatures most of the life-processes of the germs
are either stopped entirely, or greatly retarded. In the
colonial days of America, and later, it was common for
dwellers on farms and in villages to preserve meat by
burying a quantity of it in the snow during the winter
season — a primitive cold storage. During cold storage,
however, certain chemical changes take place in the
preserved tissue, due to enzyme action. As a result
there is a limit to the period that food can be kept in cold
storage without deteriorating. All cold-storage plants
and refrigerator cars are evidence of the fact that our
own lives are profoundly affected by the existence and
activity of microscopic forms of plant life.

30

3o6 STRUCTURE AND LIFE HISTORIES

YEASTS

We are all familiar with "yeast/' but many persons do
not realize that yeast is a plant, and that there are vari-
ous species. The baker's yeast is different from the
brewer's, and there are more than one kind of the latter —
one for example causing "top fermentation," another
"bottom fermentation." There are also various "wild"
yeasts, present in the air. The nature of fermentation
has been discussed in Chapter VIII. It is interesting to
reflect that this group of microscopic plants, in its relation
to bread making (from one kind of grain) adds to the
wealth and happiness of the world, and ministers to one
of the most fundamental needs of our physical being,
while in its relation to brewing (the formation of alcohol
from another kind of grain) it contributes to one of the
greatest sources of poverty, misery, and crime.

The use of yeast in bread making dates from prehistoric
ages. It is mentioned in old testament history as early
as the patriarchal age. It is of interest also to reflect
that in the various great migrations of the human race,
this little plant must have been preserved and transported
with as much care as domestic animals. In such a
movement as the colonization of a new continent, as,
for example. North and South America, and Australia,
provision must have been made for maintaining the
supply of yeast for the making of bread.

BACTERIA

289. Extent of Their Influence. — The study of bacteria,
the smallest of all known plants, has become so extensive
as to form a separate science, bacteriology; and it will not
be possible here to do more than suggest in outline the

ECONOMIC IMPORTANCE OF FUNGI

307

numerous important ways in which these tiny plants affect
our daily lives. In fact their discovery has practically
revolutionized our method of living in many ways.

Fig. 225. — Louis Pasteur, founder of the science of bacteriology.

Modern methods of sanitation and hygiene, public and
private, modern house furnishing, as, for example, the
substitution of bare floors with rugs in place of carpets,
modern views of disease and methods of treating it, and

3o8

STRUCTURE AND LIFE HISTORIES

the technique of innumerable industries, both agricultural
and manufacturing, have all been profoundly modified

Fig. 226. — Tumors on a black walnut {Juglans nigra); probably crown
galls caused by bacteria.

or determined altogether by the discovery of bacteria,
and the extension of our knowledge concerning them.

ECONOMIC IMPORTANCE OF FUNGI 309

290. Bacteria and Plant Diseases. — In addition to the
plant diseases caused by fungi, as mentioned above, a
number are known to be caused by bacteria. The "wilt"
of sweet corn, a disease first discovered on Long Island,
is caused by bacteria, as is also the crown gall, a tumorous
or cancerous-like disease common in the rose family
(peaches, apples, roses, raspberries), and the walnut,
grape, and willow (Fig. 226). The "bean blight " and pear
blight, the soft rot of the calla-lily, and the "wilt" of
cucumbers and melons, are also caused each by its own
peculiar kind of bacterium. On account of their nature
these diseases may all be transmitted from one plant to
another of the same kind.

291. Contagious and Infectious Diseases. — The ease
with which such tiny organisms as bacteria can be trans-
ferred from one place to another makes the diseases they
cause easily transmissible or "catching." We actually
do "catch cold"; that is, our all too common "colds" are
due to the presence of " cold "-producing germs. Arctic
explorers testify to the fact that, notwithstanding the
great exposures to which they are subjected, they never
"catch cold." This is explained by the absence of the
"cold" germs that cause colds in other climates or
regions.

When the members of the Peary arctic expedition of
1908-09 were in the field away from the heat and infective
dust of the ship, they were practically immune from colds
and respiratory troubles. "The fact that colds are due
to bacteria was clearly demonstrated in the Arctic.
We might be precipitated into icy water with the air many
degrees below zero; our clothing saturated with moisture

3IO STRUCTURE AND LIFE HISTORIES

on the march; yet we could sleep in our damp garments
without fear of taking cold/'^

292. Immimity. — This is not the place to discuss the
various theories of immunity, but the fact should be noted
in passing. It is a common belief that one who has had
the measles or the mumps cannot have the same disease
again. While this is extremely doubtful, it is known that
once having a disease does render one less liable to con-
tract it a second time. When the germs multiply in the
body with the first attack, the toxin they produce stimulates
the various cells to secrete an antitoxin, which counteracts
the toxin, or poison. Some persons appear to be naturally
immune to certain diseases {e.g., hay fever), while others
are specially susceptible.

293. Disease Carriers. — Persons who are immune may,
however, unknowingly transmit the disease-germ from
one person to another. They are called ''carriers."
Typhoid fever, caused by Bacillus typhosus, is often trans-
mitted by "typhoid carriers," a recent case being that
of "Typhoid Mary," a domestic servant near New York
City, who for several years endangered the lives of others
in homes and hospitals where she was employed. The
menace of such persons to public health justifies their
permanent isolation.

294. Combating Disease. — There are two ways of
combating disease in either plants or animals: (i) to
guard against it in advance {prophylaxis), thereby en-
deavoring to prevent its appearance; (2) to treat it after
it has appeared. Obviously the former is the more im-

^ From a letter to the author from John W. Goodsell, M. D., Surgeon
of the Peary arctic expedition of 1908-09.

ECONOMIC IMPORTANCE OF FUNGI 31I

portant and effective. The most effective prophylactic
measures are the following:

1. Personal hygiene ^ involving bodily cleanliness, tem-
perate habits, and careful diet. With plant diseases
hygienic measures include such practices as sterilizing
seeds before planting, by rinsing them in solutions of
some germ-killing substance, like formaldehyde; spray-
ing diseased trees with fungicides (fungus-kilhng solu-
tions) ; washing the branches and foliage with various solu-
tions, such as whale-oil soap (good for scale insects) ; and
painting the cut surfaces of trees, after trimming, to
prevent the mycelium of germinating fungus spores from
entering the woody tissue.

2. Public hygiene, or sanitation, which means maintain-
ing healthful surroundings. In the case of am'mal dis-
eases this includes preserving a pure pubHc water supply,
proper sewage systems, clean streets, a pure milk supply
(especially a healthful condition of the cows and their
surroundings), and a careful inspection of meat and all
other foods shipped and sold in public.

In the case of plants, sanitation includes preserving
a proper drainage and aeration of the soil; maintaining a
pure atmosphere, and especially one free from smoke and
the poisonous gases that accompany it; fumigating in
plant houses with potassium cyanide fumes, or with to-
bacco smoke to kill scale insects, and other insects, the
burning up of trees or other plants infected with a trans-
missible disease, such for example, as the chestnut bark
disease, or barberry bushes carrying wheat rust, and
eradication of injurious fungi from the soil of cultivated
fields by crop-rotation, as indicated in Chapter VII.

3. Quarantine. This is a method of sanitation by which

312

STRUCTURE AND LIFE HISTORIES

diseased individuals are prevented by isolation from com-
ing into contact with those who are well. When one
member of the family is sick with a contagious disease he

Fig. 227. — Map, illustrating the inter-continental migration of plant
diseases. No. i, potato blight: Chili-Colorado-Europe. No. 2, asparagus
rust: Europe, 1805; New Jersey, 1896; South Carolina, 1897; Michigan,
1898; Illinois, 1899; Dakota, Nebraska and Texas, 1900; California, 1901.
No. 3, potato cercosporose: Europe, 1854; United States, 1903. No. 4,
rice smut: Japan-South Carolina, 1898. No. 5, sorghum smut: Japan-
United States, 1884. No. 6, grape anthracnose: Europe- America, 1880,
or earlier, now widespread. No. 7, cucumber downy mildew: Cuba, 1868;
United States, 1889. No. 8, grape black rot: N)rth America, early;
France, 1885; Italy and the Caucasus, 1898. No. 9, potato vvart: Hun-
gary, 1896; England, 1900; Newfoundland, 1909; Boston and New York,
1910. No. 10, grape downy mildew: America early; France, 1873; the
Rhineland, Savoy and Italy, 1879; The Tyrol and Algiers, 1880; Portugal
and Greece, 1881; Alsace, 1882; the Caucasus, 1887; Brazil, 1890. Now
known in all countries except Australia. No. 11, grape powdery mildew:
United States, early; England, 1845; Belgium and France, 1848; all Europe
1849; Madeira, 1852. Known everywhere now. No. 12, chrysanthemum
rust: Japan-England, 1895; America, 1896. (After F. L. Stevens.)

should be confined to one part of the house, apart from
the others, and not allowed to mingle with them until well.
Hospitals have *' isolation wards" where persons with
communicable diseases are kept apart from other patients.

ECONOMIC IMPORTANCE OF FUNGI 313

Immigrants coming to America from foreign countries are
carefully examined, and if they have a contagious disease
they are isolated (placed in quarantine) until well.

The United States Government maintains a stringent
quarantine against the shipment of diseased plants from
one state to another, or from foreign countries into the
United States. Some, if not all, of our worst plant diseases
have been imported. The map (Fig. 227) shows how the
rice smut travelled from Japan to South CaroHna in 1898,
the chrysanthemum rust from Japan through England
(1895) t^ America (1896), and the potato blight from Chili
to Colorado and across North America to Europe (1845).
The downy mildew of the grape is an example of a disease
probably originating in North America, where it has been
known from the earliest times, and travelling thence to
France (1873) ^-nd other parts of Europe, reaching as far
as Greece by 1881, and to Brazil by 1890.

These brief references indicate the importance of main-
taining a strict quarantine on plants at all our ports of
entry.

4. Breeding of resistant varieties. This is one of the
most important of all prophylactic measures. Just as
some persons are immune to certain contagious diseases,
so certain plants in a crop are less susceptible than others,
or even entirely immune, to a given disease. By choosing
seed each year only from the immune or most resistant
individuals, a crop may sometimes be obtained which not
only withstands the disease itself, but interferes with or
finally stops entirely its spread. No phase of plant breed-
ing is more important than this.

5. Vaccination. When bacteria produce poisons (/oa:^^)
in the system, the cells affected are stimulated to secrete

314 STRUCTURE AND LIFE HISTORIES

an antitoxin which counteracts the influence of the toxin
(Cf. p. 310). The production of these antitoxins may
finally completely nullify the effect of the toxin, and then
the patient ''gets well." The presence of the antitoxin,
thus produced, explains why one who has recently
recovered from a contagious disease, like measles, or
mumps, or whooping cough, is more or less immune for
a longer or shorter period. In 1796 the English physician
Jenner observed that persons who had cowpox, a mild
form of smallpox, were commonly immune to the latter.
Reasoning from this he developed the method of vaccina-
tion. By this method the cowpox is first given to a calf
or a heifer, or sometimes to an adult cow. At the end
of five to seven days pustules occur on the infected surface
of the animal. A watery substance within these pustules
is then collected by sterile instruments and carefully
tested to make sure that it does not contain any germs
of tuberculosis or other disease. This substance is the
vaccine, and in vaccination a small portion of it is applied
to a scratched or slightly lacerated area on a person's
arm. A mild form of the disease results, causing the
formation of an antitoxin in the person's blood, and
thus rendering him actively immune. The word vaccina-
tion is derived from the Latin word vacca (a cow) , in allu-
sion to the method of obtaining the vaccine. It has been
calculated that, in large armies, fully as many lives have
been saved from disease by vaccination against typhoid,
cholera, and other diseases as are lost in battle.

6. Serum-therapy. The treatment of germ diseases
by serum-therapy consists in injecting into the blood of
the patient an antitoxin, specific for the disease to be
treated. The antitoxin is contained in the blood-serum

ECONOMIC IMPORTANCE OF FUNGI 315

of some animal that has been rendered immune to the
disease. As in the case of the preparation of vaccine, the
animal is first placed in quarantine, under the most perfect
sanitary surroundings; if found free from all contagious
diseases, and otherwise satisfactory, he is given increas-
ingly large doses of the toxin or the virus that causes the
given malady. This treatment may require as long as
six weeks, and results in the formation of quantities of the
antitoxin in the blood. A quantity of blood is then drawn
from the animal, and the blood-serum isolated, filtered,
carefully tested for purity, content of antitoxin, and free-
dom from disease-germs, and finally put up in glass
syringe containers ready for use. When a person is
exposed to the given disease {e.g., diphtheria), or has
actually contracted it, the serum is injected into his
circulatory system, where the antitoxin counteracts the
toxin of the disease. The patient is thus rendered
passively^ immune. Serum-therapy is now successfully
employed in the treatment of diphtheria, tetanus (lock-
jaw), hog cholera, and, with more or less success, of
infantile paralysis and certain other diseases.

Nothing corresponding to vaccination and serum therapy
is known for a certainty in the treatment of plant diseases.

7. Antiseptic surgery. The greatest obstacle to suc-
cessful surgery has always been the presence of the rich
and varied microscopic flora, or plant life, in the air.
When a wound was opened or a cut made the germs com-
posing this flora found on the cut surface the most favor-
able conditions for their growth and multiplication, and
the poisons they secreted interfered with the healing of

^ Passively, because the antitoxin is not produced by the activity of his
own cells, as it is in the case of vaccination.

3l6 STRUCTURE AND LIFE HISTORIES

the wound and caused gangrene, or inflammations, more
fatal than the disease for which the operation was under-
taken. When the discovery of bacteria, and of their
universal presence, enabled men to understand these
facts it was only a step to the practice of making the
operating rooms, the surgical instruments, and the surface
to be cut, absolutely aseptic or sterile, thus making possible
the almost unbelievable achievements of antiseptic surgery.
Only a step! But what a wonderful and all-important
step for the human mind to take. The honor and credit
for taking it belong chiefly to the famous English surgeon,
Lord Lister.

HELPFUL BACTERIA

295. Bacteria and the Dairy. — Not all bacteria, by
any means, are harmful to man. Many of the practices
of the dairy, for example, are dependent upon the action
of bacteria. This, in part, is thought to explain the
peculiarly delicious flavor of June butter. The souring
of milk is caused by the action of substances produced
by the bacteria present in all unsterilized milk. In
fact the familiar flavor of milk is due in large measure
to the presence in it of certain bacteria and the sub-
stances they produce. When milk is obtained under
strictly sanitary conditions ("certified" milk) it loses much,
if not all, of its characteristic and familiar flavor. The
ripening, flavoring, and other peculiarities of the various
varieties of cheese are due in part to the fact that they
ripen under the influence of different kinds of bacteria
or molds. The '^ ripening" is caused, in part, by the
action on the substance of the cheese of the enzymes
peculiar to the various fungi or bacteria growing in it.

ECONOMIC IMPORTANCE OF FUNGI

317

and in part by the action of an enzyme {galadase) inherent
in the milk from which the cheese is made.

296. Vinegar. — Cider vinegar is in every respect a
plant product. It is formed from the juice of apples, and
ripens or ''ages" under the influence of bacteria. These
bacteria produce an enzyme which causes ''acetic acid

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u'ai(

Hi

- >

71 /M

^M

IJ|M

m

sM/ J V

m

\'

mvlt

H

iH

w

f

1* \iS

1

I

Fig. 228. — Roots of soy bean plants. A, from seeds that were soaked
in soy bean bacterial culture over night; B, roots of check plant (not inocu-
lated). (After Garman and Didlake. Courtesy of Kentucky Agr. Exp.
Station.)

fermentation." This is the acid to which vinegar owes
its sourness. There is not room here to explain the
process of vinegar making in detail.

297. The Secret of Clover. — Farmers have known for
many years that a given piece of land produces a larger
yield if the ame crop is not grown continuously, season

3lS STRUCTURE AND LIFE HISTORIES

after season, but if there is an alternation or rotation oj
crops of different kinds. It was also recognized that the
best results are obtained when one of the crops in rota-
tion is a legume (a member of the family Leguminosae)
such as clover, peas, beans, vetch, lentils, and others.-^
A clear explanation of the value of rotation was not
possible until, in 1889, Hellriegel discovered that legumes
are able to utilize the free nitrogen of the air, and that
this is made possible by the bacteria that produce the
characteristic tubercles on their roots. The researches
of Hellriegel and others, at about this time, proved that
the fixation of nitrogen is due to the bacterium that
causes the tubercles, Pseudomonas radicicola (Fig. 61). If
clover or other legumes are grown in a sterile soil, free
from the presence of all bacteria, the tubercles do not
form (Fig. 228), and the fixation of free nitrogen ceases.
Thus it is seen that bacteria are essential to one of the
oldest and most fundamental practices of agriculture.

289. Nitrifying Bacteria in the Soil. — In addition to
the symhionts causing leguminous tubercles, there exist,
in all soil, at least two other forms of nitrifying bacteria,
which grow independently of other plants. The first, by
the addition of oxygen, transforms the ammonia in the
soil into nitrites, while the second, by the addition of
more oxygen, transform nitrites into nitrates.'^ It is only
in the latter form that nitrogen, so indispensible to
nutrition, can be utilized at all by plants.

299. The Nitrogen Cycle.— From the above facts it
is seen that there is, in nature, a nitrogen cycle quite as

^ The subject of rotation of crops is more fully discussed in para-
graph 90 (pp. 91-93).

2 Cf. Chapter VII, pp. 82-83.

ECONOMIC IMPORTANCE OF FUNGI

319

remarkable as the carbon cycle. By respiration and
burning, complex compounds of carbon (carbohydrates
and hydrocarbons) are broken down and the carbon re-
leased, either as pure carbon (C) or as the simple com-
pound, carbon dioxide (CO2), which is taken in by green
plants and recombined into the complex carbohydrates
by the process of photosynthesis (page 77).

n\

. Nitrate -.
■.'Bacteria ■.•■..
(Nitroliacter)^

.;- Nitrite ;••
■.".'•" Bacteria •
(NitroBOraonae);

(Nitrites]

Denitrifying
. .■ Bacteria ••.•

[Nitrogen^

Fig. 229. — The nitrogen cycle.

So also, by the action of proteolytic (protein dissolving)
enzymes produced by bacteria, certain complex com-
pounds of nitrogen, the proteins, are broken down, by
putrefaction and similar processes, into simpler com-
pounds, such as ammonia (NH3), or still further dis-
integrated until free nitrogen results. Were it not for

320 SlRUCTURE AND LIFE HISTORIES

the nitrogen-fixing bacteria this process would continue
until all complex nitrogenous compounds were reduced
to ammonia or free nitrogen. All life would then cease,
because green plants, upon which animal life is de-
pendent, are unable to live without nitrogen, and they
cannot utilize it in the form of ammonia nor of free
nitrogen. Without chlorophyll and the nitrogen- fixing
bacteria, therefore, all life would cease. Thus it is that
life and death, health and disease, wealth and poverty,
misery and happiness, are dependent upon the activity
of tiny plants, too small to be seen, except under the
highest powers of the microscope.

CHAPTER XXI
SAPROPHYTISM AND SYMBIOSIS

300. All Food Organic. — In more ways than one it is
true that life is dependent upon antecedent life. An
illustration of this principle is seen in the case of nutri-
tion, for no Kving thing, neither plant nor animal, can
utilize directly as food the unaltered inorganic elements
and compounds derived from the air and soil. This was
clearly shown in Chapter VII. While plants are com-
monly said to obtain their food from the air and soil,
strictly speaking this is not true. Carbon, oxygen,
hydrogen, phosphates, nitrates, sulphates, et cetera are
no more plant food than are flour, water, sugar, and salt
bread. Just as the elements composing bread would,
if eaten separately, make a very unsavory and poor diet,
so the inorganic elements and compounds, as such, would
not be able at all to nourish plants. They must first be
broken down (if compounds), and then recombined into
the organic compounds of carbohydrates, proteins, and
fats.

301. — ^Necessity for Chlorophyll. — As we have seen in
Chapter VII, this recombination of inorganic chemical
elements into organic compounds is the function of
chlorophyll. Thus it is that green plants are absolutely
essential to all life, and as we learned in Chapter XIX,
this explains why all non-green plants are found only in
intimate association, either with living green plants or
with the organic remains or products of other living
91 321

32 2 STRUCTURE AND LIFE HISTORIES

things — plants or animals. Not being able to manu-
facture their own food, they must find it ready made.

SAPROPHYTISM

302. Decay. — Perhaps the simplest case of the nutrition
of non-green plants is the absorption of food from the
organic remains of other plants or of animals. When
the spores or other reproductive bodies of such plants
begin to grow upon such a substratum, they secrete
various enzymes which begin to disintegrate it, reducing
it to simpler, soluble substances. This is the process
commonly known as *' decay," and the plants which
cause it are called saprophytes} The word ''decay"
is derived from a Latin word, decidere, which means
to fall apart, in allusion to the fact that the decaying
substance is being disintegrated or broken down into
simpler substances, which are recombined by assim-
ilation, in the cells of the saprophyte, into protoplasm
like its own. Such a state of existence is called sapro-
phytism.

303. Fungus -saprophytes. — Among the more familiar
saprophytic plants may be mentioned the common bread-
mold, the fungi that are instrumental in ripening cheese,
the so-called ''mildews," which often grow on old moist
pieces of leather, and numerous other filamentous fungi;
the bacteria which cause the decay of meat and other
substances, bacteria which cause the retting of flax stems,
thus freeing the bast fibers from which linen is made by
causing the decay or rotting away of the remainder of
the tissue, the bacteria which convert cabbage leaves

^ From the Greek words sapros, rotten + phyton, plant.

SAPROPHYTISM AND SYMBIOSIS 323

into sauerkraut, the numerous slime-molds (Myxo-
mycetes), and various other forms, which are more or
less intimately connected with human industries, or with
public or private hygiene.

Humus, one of the most important compounds of soil,
is composed almost entirely of the remains ot plant
and animal bodies in various stages of disintegration,

Fig. 230. — Indian pipe {Monotropa uniflora). (Photo by Elsie M.

Kittredge.)

and the disintegration is caused, almost entirely, by the
action of enzymes secreted by bacterial and fungal
saprophytes. Thus we come to reahze that these sapro-
phytic plants are absolutely essential to the most funda-
mental of all human occupations, namely agriculture.

304. Saprophytic Flowering Plants. — Not all sapro-
phytes are fungi. The Indian-pipe {Monotropa uniflora^

324 STRUCTURE AND LIFE HISTORIES

Fig. 230), and its close relative, the false beech-drops
(Monotropa Hypopitys, Fig. 231), are examples of flowering
plants, wholly devoid of chlorophyll, and therefore unable
to manufacture their food, which is absorbed entirely
from the humus in which they grow. Other examples
are Lathrcca, and the coral-root {Corallorhiza).

Fig. 231. — False beech drops {Monotropa Hypopitys). (Photo by Elsie
M. Kittredge.)

SYMBIOSIS

305. Different Kinds of Symbiosis. — The absorption of
nourishment from one plant by another involves, of course,
the intimate association of the two organisms. Such a
vital association is called symbiosis (living together), and
we find organisms living together in all degrees of intimacy

SAPROPHYTISM AND SYMBIOSIS 325

and independence, or of interdependence. One plant
may merely live upon another, without deriving any
nourishment from it {epiphytism) \ or two plants may
be mutually helpful, each contributing something of
advantage to the other {mutualism)', one plant may
live at the expense of the other, deriving nourishment
from it, but contributing little or nothing in return {para-
sitism); or the two organisms may maintain a loose or
disjunctive symbiosis, which may be either (i) nutritive,
as in those cases where ants cultivate filamentous fungi,
maintaining fungus-farms; or (2) non-nutritive, as in the
cases where certain plants like clover or orchids, are de-
pendent upon insects for the transfer of pollen from one
flower to another. These phases of symbiosis are indi-
cated in the following table:

Symbiosis

1. Disjunctive or "social."

{a) Nutritive {e.g., ants and fungus-farms).
{b) Non-nutritive (insects and pollination).

2. Epiphytism.

3. Mutualism.

4. Parasitism.

306. Social Symbiosis.— As an illustration of social
symbiosis of a nutritive character may be mentioned the
interesting relation established between certain leaf-
cutting ants and a filamentous fungus. The ants remove
the foHage-leaves from certain trees and use them as
''fungus-farms," or a suitable substratum on which to
cultivate a certain fungus, portions of which serve as
food for the ants (Fig. 232). The spores are sown by the
ants and the ''crop" harvested in a very systematic man-
ner. The loss of leaves, however, is very deleterious to
the life of the tree, and certain species {e.g., Cecropia and

326

STRUCTURE AND LIFE HISTORIES

Fig. 232. — Diagram of a nest of a fungus-growing ant {Trachymyrmex
obscurior), showing four chambers. The pendant white masses in three
of the chambers are the mycelium of the fungus — the so-called "fungus-
gardens." The species of the fungus has not been definitely determined,
but they are thought to belong to the Ascomycetes. (After W. M.
Wheeler.)

Fig. 233.— Epiphytes or "air" plants {Tillandsia sp.), growing on tele-
phone wires at Ponce, P. R,

SAPROPHYTISM AND SYMBIOSIS 327

Acacia), secrete a substance which is greatly liked by
another kind of ants, a smaller, war-like species. These
ants, attracted by the much-prized food, make their home
on the tree or in special cavities in it, and repel all at-
tempts of the leaf-cutting species to reach the foliage.

Fig. 234.— Epiphytic group of bromeliads and orchids on a tree, in Cuba
(Photo by M. T. Cook.)

Such trees are called ant-loving (myrmecophilous), or
myrmecophytes.

307. Epiphytism.— Any plant (whether parasite or not)
that lives on another, or upon any other convenient
support (Fig. 233), is an epiphyte, but the term is com-

328 STRUCTURE AND LIFE HISTORIES

monly restricted to those cases where there is no phys-
iological or nutritional relationship between the two. A
vine climbing up a tree is an epiphyte, as are also the
Pleurococcus and mosses growing on the tree trunks. Epi-
phytism is specially common in the tropics where orchids,

Fig. 235.' — An epiphytic Clusia. (From photo by G. V. Nash,
taken in Flaiti.)

ferns, hohenbergias, and great lianas (vines) are found
growing in profusion on other plants (Figs. 234, 235, and
236).

308. Mutualism. — The associating together of two
plants in intimate physiological relationship, to their

SAPROPHYTISM AND SYMBIOSIS

329

Fig. 236.-

-An orchid {Cattleya sp.) growing as an epiphyte on a portion
of a branch of white birch. Note the aerial roots.

Fig. 237. — A thallose lichen, Physcia stellaris (L.) Nyb., growing on a
rock. The cup-shaped structures are the fruiting bodies (apothecia).
At the left are seen two very young plants,.

330

STRUCTURE AND LIFE HISTORIES

Fig 238.— A lichen, Parmelia perlata (L.) Ach. i. Plant, slightly
reduced in size; a, apothecia; b, lobe of thallus; c, soredial patches. The
fI\AtZ^7^^^^fT ^^P^od^f tive bodies composed of both algal and fun-
gal elements, and there/ore able to reproduce the lichen; the ascospores.

SAPROPHYTISM AND SYMBIOSIS 33 1

mutual advantage, is admirably illustrated by lichens.
These plants grow commonly on the trunks and stems of
trees, on old boards and fences, and on rocks (Fig. 237).
The plant body is a thallus, and, when its inner structure
is examined, it is seen to be a composite plant, formed by
a species of green alga, resembling Pleurococcus, and sur-
rounded by the mycelium of a filamentous fungus (Fig.
238). If supplied with suitable moisture, the alga can
live alone, because it has chlorophyll, but the fungus,
not having chlorophyll, cannot live alone. By uniting
into a common body, each plant supplies what the other
needs.

The fungal portion of lichens reproduces by means of
spores borne in asci, and is therefore an ascomycete. The
apothecium ("fruiting" portion of the lichen) is in reality
a modified ascocarp (Fig. 238). In some species the
apothecia occur at the summit of specialized, upright
branches or podetia (Fig. 239). Only a few years ago
the interesting discovery was made that lichens may be
experimentally produced by the artificial union of certain
algae and fungi (Fig. 240). Some of the lichens thus pro-
duced resembled those found in nature, while other com-
binations were entirely new.

An interesting case of the symbiotic association of four
genera, if not to their mutual benefit, at least without
apparent detriment to either, is found in the roots of some
of the Cycadaceae. All the genera of this family produce

alone, cannot do this. 2, Longitudinal section of apothecium; a, thecium;
h and c, the two layers of the hypothecium; d, upper algal layer; e, colonies
of algae distributed through the medullary layer; /, lower algal layer; g,
lower cortical layer. 3, Cross-section of vegetative portion of thallus. 4,
Paraphyses (sterile fungal filaments), and spore-sac (ascus), containing
ascospores. 5, Ascospores. 6, Algal cells, surrounded by fungal hyphae
with haustoria (absorbing branches). (After Schneider.)

332

STRUCTURE AND LIFE HISTORIES

branched, coralloid nodules on their roots (Fig. 241).
These roots are caused primarily by infection with the

Fig. 239. — 'Thallus and erect portions (podetia) of two species of lichen
of the genus Cladonia. The larger podetium is proliferating, and bears a
number of apothecia on the margin of the podetium of the first rank.
Apothecia are also shown on the margin of the larger podetia at the right.
(Drawn from nature.)

Fig. 240. — A lichen produced synthetically by cultivating the spores
of a fungus in association with a green alga. The culture was carried on
in a test-tube on sterilized culture media, thus preventing contamination
by bacteria and foreign spores. (After Bonnier.)

nitrogen-fixing organism, Pseudomonas radicicola. By
the development of lenticels at the base of each nodule

SAPROPHYTISM AND SYMBIOSIS

333

the epidermis is ruptured, thus affording entrance to
another nitrogen-fixer, Azotobacter, and also under favor-
able conditions, to a blue-green alga (Nostoc). This is the
only known case in which four organisms are associated

Fig. 241.— Root-nodules of Cycas revoluta. n, one of several cross-
sectional views, showing the zone of the symbiont alga, Nostoc.

together symbiotically. The alga has never been found
in the nodules of Bowenia, Ceratozamia, Macrozamia, nor
Zamia (all genera of cycads).

309. Grafting.— One of the oldest practices of horti-
culture is that of grafting a twig or a bud of one species

334

STRUCTURE AND LIFE HISTORIES

(i) Root grafting in its differ-
stock prepared to receive the

Fig. 242. — Various methods of grafting.
ent stages, a, Scion cut for insertion; b,

scion; c, stock and scion united; d, the same tied up with waxed cord.
(2) Cleft grafting (Herbaceous), a, Scion ready for insertion; b, stock;
c, stock and scion united; d, the same tied up with raffia; e, cleft grafting
(woody). Stock with two scions. (3) Saddle grafting, a, Scion; b,
stock; c, scion and stock joined. (4) Budding, a, Budstick; b, T-shaped
cut in bark of stock; c, bud ready for insertion; d, stock with bud inserted;
e, the same tied up with raffia. (From Brooklyn Botanic Garden Leaflets.)

SAPEOPHYTISM AND SYMBIOSIS

335

onto the main stem or branch of another species. This is
accomplished by placing the freshly cut surface of
the scion against the freshly cut surface of the stock,

Fig. 243.— a tomato {Ly coper sicmn), grafted on a potato {Solatium)
Note the potato tuber on the surface of the soil. Cf. Fig. 404.

in such a way that the cambium layers of both come in
contact, and then binding the two together (Fig. 242).
In time the two tissues become firmly united, grow-

336

STRUCTURE AND LIFE HISTORIES

ing as one continuous stem. In all grafting the scion
maintains essentially its true nature, seldom, if ever, being
affected by the characteristics of the stock, which only
serves as a channel for the passage of water and food ele-
ments to the scion, and receiving in return from the scion
the elaborated carbohydrate and other food (Figs. 243
and 404). Scion and stock therefore represent a case of
symbiosis artificially brought about. In some cases
branches of the same tree rub against each other until
the bark is worn through, bringing the cambial layers in
contact, and resulting in a '^ natural" graft.

310. Mycorrhizas. — The roots of many plants (espe-
cially of woody plants) enter into intimate association
with the mycelia of various fungi growing in the soil.

^.^,

Fig. 244. — Ectotrophic micorhizas. At left, micorhizal mantle on root
of hickory {Carya ovata), in cross-section; at right, root- tip of an oak
{Quercus), covered by fungus mantle. (After W. B. McDougall.)

The mycelia either form a mantle or jacket at or near
the surface of the young roots {ectotrophic, Fig. 244), or
they penetrate through the cell-walls into the cell-cavities
{endotrophic, Fig. 245). Recent careful studies seem to
demonstrate that the ectotrophic mycorrhizas, common
on the roots of many kinds of trees (hickory, oaks, birch,
sugar-maple, larch, beech, hornbeam), are, in reality.

SAPROPHYTISM AND SYMBIOSIS 337

evidence of the parasitism of some fungus on the tree.
Several species of fleshy fungi are, in this way, parasitic
or partly parasitic. In ectotrophic infection the hyphae
penetrate through the epidermis and then grow and branch
underneath it, some of the branches growing between the
individual epidermal cells by dissolving the middle lamella
with enzymes which they secrete. In this manner there
is formed a pseudo-tissue, closely analogous to a lichen,
which replaces the true epidermis. The fungus doubtless
derives some nourishment from the dissolved (digested)

Fig. 245.— Tangential section of root of the red maple {Acer rubrum),
showing endotrophic micorhiza in the cells. (After W. B. McDougall.)

substance of the middle lamella, as well as from nutrient
substances that diffuse out from adjacent cells; but there
is no evidence that the tree is in any way benefited by the
presence of the fungus.

In some cases the development of the mycorrhiza-
mantle inhibits the growth of the root, and stimulates a
profuse branching, which is repeated as the branches are
infected. This gives rise to a malformation known as
''coral root," which is so well developed in one herbaceous
species as to give the plant its scientific as well as its
common name — Corallorhiza, or coral-root.

In endotrophic mycorrhizas the hyphae penetrate
through the cell-walls into the cell-cavities, and in such

338

STRUCTURE AND LIFE HISTORIES

cases all stages are found from true parasitism of the fungus
on the root, through a mutually beneficial relationship,
to a parasitism of the root-cells on the fungus. Among
common trees having endotrophic mycorrhizas, may be

Fig. 246. — Cancer-root {Conopholis americana), of the Broom-rape
family {Orohanchacea). The ovaries are developing into capsules.^ The
plant derives its name from the scaly cone. (Photo by Elsie M.
Kittredge.)

mentioned the black maple, horse-chestnut, and black
walnut, and most of the heath family (Ericaceae), such
as trailing arbutus, huckleberry, wintergreen, heather,
laurel, and rhododendron. This is probably the chief

SAPROPHYTISM AND SYMBIOSIS

339

reason why it is so difficult to transplant many of the
heaths; the delicate adjustment between the plant and
the mycorrhizal fungus is disturbed in transplanting, and
the soil conditions in the new habitat are not favorable
to its reestablishment before the plant dies.

The Indian-pipe and the false ''beech-drops'' (Figs. 230
and 231), both belonging to the heath family, also pos-
sess endotrophic mycorrhizas.

Fig. 247.— Dodder {Cuscuta sp.), parasitic on geranium {Pelargonium).
A few seedlings at the left are still rooted in the soil, and are not yet at-
tached to the host-plant. They eventually sever all relation with the soil.

311. Parasitism. — In some cases of symbiosis, as stated
above, only one plant derives any benefit from the union,
which may or may not be of positive injury to the other.
Such is the case with the endotrophic mycorrhizas, already
mentioned. There are many instances of the parasitism

340

STRUCTURE AND LIFE HISTORIES

of one flowering plant on another (Fig. 246). In some of
these cases as, for example, the dodder (Cuscuta), the para-
site may have completely lost the power of elaborating
chlorophyll, and thus lack the function of photosynthesis;

Fig. 248. — Dodder {Cuscuta sp.), in flower. Parasitic on a golden rod
{Solidago ulmifolia). (Photo by Elsie M. Kittredge.)

the parasitism is then complete (Figs. 247, 248 and 249).
In other cases the parasite may retain its chlorophyll-
apparatus, and hence be only partly dependent upon
the host, as in the case of the mistletoe (Fig. 250).
Such plants are semi- parasites. Another example of

SAPROPHYTISM AND SYMBIOSIS

341

semi-parasitism is that of the blue-green alga, Nostoc,
species of which grow in little pockets or cavities in the
tissues of the water-fern Salvinia, of Gunnera manicata,

luG. 249.— Photomicrograph of a cross-section of the stem of a dicoty-
ledonous host-plant infested with the parasite, dodder {Cusciila sp.).
Note the haustoria extending from the dodder (D, D') into the cortex of
the host (H). Greatly enlarged.

of Anthoceros, and of other plants, without apparent
injury to the host (Fig. 160).

312. Artificial Parasites. — By recent experiments cer-
tain plants have been induced, by experimental treatment,

342

STRUCTURE AND LIFE HISTORIES

to grow as parasites on other plants (Fig. 2500). The con-
dition to success in such experiments is that the osmotic
strength of the cell-sap of the host must be less than, or
at least not greater than that of the parasite.

Fig. 250, — Cross-section of a branch of live oak, showing live stems
of mistletoe, parasitic on the oak; the upper stem with foliage and
fruit. Note the prominent "sinkers" of the parasite, some of them
growing laterally for a short distance, close under the surface of the
bark, and then radially, deep into the tissue of the wood.

313. Fungal Parasites. — Mention has already been
made in Chapter XIV of the parasitism of the entire group
of fungi, including the smuts, rusts, and other disease-
producing fungi, on flowering plants. The ''shelf-fungi,"
commonly found on forest trees, are economically impor-

SAPROPHYTISM AND SYMBIOSIS 343

tant because of the enormous financial losses occasioned
by the timber-decay which they induce.

314. Parasitism Means Degeneration. — Most parasites
among the flowering plants have suffered the loss of some
organ or organs, and of one or more functions as a result
of the parasitic habit. In fact, parasitism must be re-
garded as an acquired habit, and the parasite among
plants, as in human society or elsewhere, as a degenerate
form of life. Some plants can live only as parasites
{obligate parasitism), while others may live either as
parasites or as saprophytes (facultative parasitism).

Fig. 250, a. — Cissus laciniata, parasitic on the cactus {Opuntia Blakeana).
The parasitism was artificially induced (xeno-parasitism). The host
plant has been sectioned to expose the roots of the xeno-parasite. (Re-
drawn from D. T. MacDougal.)

316. Flowers and Insects. — The dependence of certain
plants upon insects to secure the transfer of pollen from
one flower or plant to another, will be mentioned more
in detail in Chapters XXVII-XXIX.

CHAPTER XXII

THE PROBLEM OF SEX IN PLANTS

316. Cell-division and Reproduction. — As stated in
Chapter XIV, the essence of reproduction is the separation,
from the body of the parent, of a cell or larger portion,
which becomes the starting point of a new individual.
In some of the lowest plants, such as certain species of
bacteria, cell-division always results in reproduction; that
is, the two halves of the divided, one-celled body always
separate at the close of cell-division, thus giving rise to
two new individuals. A little higher in the scale of life
we find such plants as Pleurococcus, where cell-division
may result at once in reproduction, but where there is
also a marked tendency for the cells to adhere together
at the close of division, thus forming a loosely organized,
multi-cellular plant body (Fig. 183) . A further advance is
illustrated by Spirogyra, where the cells normally do not
separate at the close of division, but remain together, end
to end, producing a multi-cellular body in the form of a
filament (Fig. 251). From this simple condition we have
seen transitions to the flat thallus of the liverworts; the
simple, leafy axis of the mosses, the leafless axis of the
moss-sporophyte, and the leafy sporophyte of the ferns.
Not that these forms are derived from each other; tut they
illustrate various degrees of complexity from the simplest
unicellular plant body to a complex, multi-cellular body.
By a comparison of these forms, we see that while, in

344

THE PROBLEM OF SEX IN PLANTS

345

some of the lowest forms, cell-division always results in
reproduction, in the higher forms it only paves the way

Fig. 251.- Spirogyra sp. A, terminal portion of vegetative filament;
B, stages of scalariform conjugation; C, preparation for lateral conjuga-
tion; Z>, zygospores formed by lateral conjugation.

to increasing the size of the given individual, that is, to
growth. In other words, between cell-division and re-

346 STRUCTURE AND LIFE HISTORIES

production there is interposed the enlargement or growth
of the parent.

317. Vegetative Multiplication. — Attention has also
been called to the various methods by which the number
of individual plants is increased by the separation of multi-

FiG. 252. — Branch of a willow (Salix sp.), showing the formation of
fibrous roots. The lower portion of the stem was placed in water for a
few days.

cellular portions of the body of the parent. The most
familiar of these processes being the artificial propagation
of plants by means of cuttings. A portion of stem (Figs.
3 and 252), or sometimes of leaf (Fig. 253), stuck into
moist sand will form new roots and ultimately develop an

THE PROBLEM OF SEX IN PLANTS

347

entire new plant. A leaf of begonia laid on moist sand
will give rise to several new plants wherever it is cut (Fig.
254). Leaves of the sundew (Drosera) frequently strike

Fig. 253. — Regeneration at the leaf-base of potato leaves {Solanum
tuberosum), a, roots formed; h, tuber-like enlargement; c, same as h,
with roots; d, formation of true tuber. (After Miss Elsie Kupfer.)

Fig. 254. — Young plantlets developing from the edges of lacerations
made in a large leaf of Rex begonia. '

root at the tip and develop new plants (Fig. 255), while
the leaves of Bryophyllmn normally produce marginal
buds from which new plants develop (Fig. 256). As

34S

STRUCTURE AND LIFE HISTORIES

noted in Chapter XVI, the thallus of a liverwort may be
chopped fine and every isolated, intact cell will give rise
to a new plant.

Growing plants of the liverwort, Marchantia, isolated
by the dying of older tissue develop new individuals;
the tips of the leaves of the walking fern may strike root

Fig. 255. — Drosera rotmidifolia. Production of a young plant from the
leaf of an older plant.

and originate new plants (Fig. 122), the tips of stolons or
runners (as in ferns, eel-grass, strawberries, etc.) may
do the same (Figs. 257-259, and 123), isolated sterile
branches and "innovation-branches" of Sphagnum moss
become new individuals (Fig. 144), as may also the famihar
'.ubers and bulbs (such as those of the potato and onion),

THE PROBLEM OF SEX IN PLANTS

349

Fig. 256. — Bryophyllum crenatum. A leaf which has given rise to three
plantlets along the margin of the blade.

Fig. 257. — Eel-grass {Valtisneria spiralis), showing vegetative propaga-
tion by stolons. Young plants at P^ and P^.

350

STRUCTURE AND LIFE HISTORIES

IHbb^,

,A.:,..^

-i

.p^

F 's^^

-

,:,^ ■."'■

■^•■•/^' 5-.,

1^

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tt

4'^'

UB4l .

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ife.-.^

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w"

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fl'1 }>. '•

nf

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VI

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Fig. 258. — Scmperviviim tectoruniy with stolons.

Fig. 259. — Chlorophytum elatmn R. Br., showing vegetative propagation
by runners. P^, P^, P^, new plants; P, parent plant. From P^ one root,
and from P^ two roots hang down.

THE PROBLEM OF SEX IN PLANTS

351

and the gemmcB of various species of liverworts, mosses,
and ferns (Fig. 260).

An interesting form of vegetative multiplication is
illustrated by the fern Woodwardia orientalis, where
new plantlets arise at numerous points on the upper
surface of the leaves (Fig. 261). The number of in-
dividuals is thereby increased or multiplied, hence the
term vegetative multiplication.

Fig. 260.— Hymenophyllum sp. Prothallus. a, antheridia; b, arche-
gonia; g, gemmae. (After Winifred J. Robinson.)

318. Reproduction by Spores.— In many plants such,
for example, as the fern, the parent plant, while retaining
its own vegetative organs intact, gives off individual
cells (the spores), which become the starting points of
new individuals. The most distinctive thing about a spore
is that it escapes, or becomes separated from the parent, while
all the other cells remain organically united.

In some one-celled plants {e.g., yeast. Fig. 67), the en-
tire plant body (except the cell-wall) may become organ-

352 STRUCTURE AND LIFE HISTORIES

ized into spores, which escape from the cell-wall of the
mother-cell. In a number of many-celled plants {e.g.,
Ulothrix) practically every protoplast has the capacity of
becoming organized into one or more spores which escape
from the old cell-cavity. The next higher step is the
restriction of spore formation to certain cells in special
organs (sporangia), while the other cells function only
vegetatively.

'iG. 261. — IVoodwardia orientalis. Portion of a leaf bearing numerous
young plantlets on its upper surface.

319. Cell-fusions. — Through all the variations of re-
production by spores there is, as a rule, only the separation
of protoplasts from the parent body, never a cell-fusion or
nuclear-fusion. Some plants, however, such as Ulothrix
(Fig. 262), have been found to produce two sizes of
spores, and the small spores must always unite before
they can develop into full-sized, mature individuals.^
Attention has already been called (Chapter XVIII) to
the condition of similar sized gametes {isogamy), as in
Spirogyra, in contrast to that of unequal gametes (hetero-
gamy), as in Ascophyllum and Fuciis.

1 In certain cases {e.g., Ulothrix) the microspores may develop small,
imperfect individuals without fusion.

THE PROBLEM OF SEX IN PLANTS . 353

320. Maleness and Femaleness. — Spirogyra. In nearly
all cases of cell-fusion it is possible to recognize some
difference, either of structure or of behavior, between the
gametes. In Spirogyra, for example, it has long been
noticed that if one of the cells of a filament passes over to
the other filament through the conjugation-tube, all the
cells of that filament will ordinarily do the same. Thus
after conjugation is over all the cells in one filament will
be found empty, while all the cells of the adjacent, con-
jugating filament will contain zygotes. This behavior,
however, varies under certain conditions and with differ-
ent species.

Recent studies by York have revealed the fact that the
supplying gamete of Spirogyra always possesses less starch
and a less number of starch-formers (pyrenoids) than does
the receiving gamete (Fig. 263). It has also been noted
that the supplying filaments (male ?) are less vigorous,
vegetatively, than the receiving filaments (female ?).

321. Sexuality in Molds.— One of the most interesting
of recent discoveries in connection with sex in plants, is
that of the existence of two strains of different sexual value
in the molds. It was known for a long time that conjuga-
tion and the formation of zygotes in these plants could not
always be secured when desired; that is, conjugation would
not occur between every two individuals. At first it was
thought that the explanation lay in the fact that the ex-
ternal conditions (temperature, light, moisture), might
not be just right, or that the two adjacent plants were
not in the right condition as to age, or nutrition, or
otherwise. Finally, as stated in Chapter XIX, Blakeslee,
by careful experimental studies, found that there are two
unlike strains of many of the molds, and that whenever a

23

354

STRUCTURE AND LIFE HISTORIES

to o
o O-

u c o .s o
— ' en (/I ^> N

THE PROBLEM OF SEX IN PLANTS

355

plant of one strain grows next to a plant of another
strain, conjugation will always take place between them.
These strains he designated, provisionally as (+) and (— )
(Fig. 190). The f+) strain is vegetatively more vigorous

Fig. 263, — Spirogyra sp., illustrating sexual differentiation. ^ Receiving
(female) gamete at the left; supplying (male) gamete at the right. (Re-
drawn from camera lucida drawing by H. H. York.)

than the (— ) strain, and the conclusion seems warranted
that the (+) race is female and the (— ) race male.

322. Sexual Differentiation of Spores. — i. Physiolog-
ical. Even an elementary study of reproduction reveals
the fact that spores from the same plant, and even from
the same sporangium (as in some of the molds just men-

356

STRUCTURE AND LIFE HISTORIES

tioned), and to all external appearances entirely alike,
may produce individuals of different sex- value, some being
male and some female.-^ Such was also seen to be the
case with the marine alga Dictyota,^ the externally similar
male and female gametophytes being produced by spores
that are alike in size and other external features.

As a further advance spores that appear to be morpho-
logically alike may produce plants morphologically as well
as physiologically different {Anthoceros, some molds).

2. Structural. In the little club-mosses {Selaginella)
we found the spores unlike, not only in function but in
structure, those producing males being smaller than those
producing females — the condition of heterospory.

Fig. 264. — Vaucheria terrestris. anth, antheridium (empty); o, oogonia.

323. Differentiation of Sex-organs. — In many lower
plants there is no recognizable structural difference be-
tween the organs that produce the heterogametes, but a
step in advance in this direction is found in such a plant
as Vaucheria (Fig. 264), where the antheridia are struc-

1 See, also, pp. 246 and 248.

2 See Chapter XVIII.

THE PROBLEM OF SEX IN PLANTS 357

turally very unlike the oogonia. This differentiation is
carried a step further with the appearance of the multi-
cellular archegonium in liverworts and mosses.

324. Structural Differentiation of the Sexes. — i. Par-
tial.— In some species (though not in all) of the fresh-
water alga, CEdogonium, the spores are unlike. Large
zoospores produce normal-sized plants that bear eggs
and smaller androspores. The androspores, intermediate
in size between sperms and zoospores, produce smaller,
male indimdiials only, of simple structure, which fasten
themselves to the egg-bearing plants, and give rise to
sperms, which fertilize the tgg.^

2. Complete. — In the liverworts and mosses commonly,
and in the higher plants always, the gametophytes are
clearly differentiated into male and female, with unlike
vegetative characters which clearly distinguish them.
These unlike structural features are called the secondary
sexual characters. In these groups the sporophytes are
not differentiated in structure, but the spores they pro-
duce, though structurally alike, are physiologically unlike,
some producing male gametophytes, others female.

The most complete expression of maleness and female-
ness is found 'in certain seed-bearing plants, where the
sporophytes are differentiated - into microsporophytes
(staminate plants), bearing only microspores which pro-
duce male gametophytes (pollen-grains), and megasporo-
phytes (pistillate plants), bearing only megaspores which
produce female gametophytes (embryo-sacs) . Usually the
two kinds of sporophytes are essentially alike, except for
the sporophyll-bearing branches (the flowers), but the

^ It has been suggested that these androspores might be regarded as
sperms developing without fertilization, i.e., by parthenogenesis.

358 STRUCTURE AND LIFE HISTORIES

male and female gametophytes are as unlike as could well
be imagined. Illustrations of this condition will be
found when we study the Gymnosperms (zamia, cycas,
ginkgo), and the Angiosperms (willow, poplar, hop, etc.).

325. Determination of Sex. — i. Effect of Nutrition. —
Nobody knows the real cause of sex — of maleness and
femaleness. We may arrange plants (and animals) in
a series so as to show the gradual transition from the
simple non-sexual condition to complete differentiation of
males and females, but in doing this we should clearly
recognize the fact that we have really explained nothing.
We have only described events and structures in the order
in which it seems probable that they have occurred, in
the gradual development of the earth's vegetation.

But while we have never yet been able to determine in
advance the sex of a plant or animal, we have been able
to determine which sex shall gain expression. For ex-
ample, we have seen above that male plants are frequently
less vigorous and more poorly nourished than female
plants. We would, therefore, expect that poor nutrition
would cause a suppression of femaleness, and this is pre-
cisely what has been found in certain experiments that
have been made. When the prothallia of certain ferns
that normally bear both antheridia and archegonia are
grown under conditions that result in their being poorly
nourished, the antheridia develop, but not always the
archegonia. In such cases we know that we have not
changed the sexual nature of the prothallus, but have
only modified its expression.^ This is further illustrated

^ In exceptional cases perfect flowers may appear on staminate or pistil-
ate plants, as in papaw (Carica Papaya) and willow, suggesting that
dioecious plants may, in reality, be of double sex-value, but that only one
of the sexes comes normally to expression.

THE PROBLEM OF SEX IN PLANTS

359

in the gametophytes of the Horsetails (Chapter XXIV),
which are usually differentiated into larger ones (female)
and smaller ones (male); but under certain conditions
(apparently involving differences of nutrition), the female
gametophytes may bear antheridia, and the male gameto-
phytes archegonia.

2. Efect of Constitution of Germ-cells. — If sex is not de-
termined by external conditions — by environment — then

Zygote
Mal9

(zo ox my

Sperm
No-X,or y

Sperm
Xr class

(2Z)
Zygote
Female
Fig. 265, — Diagram to illustrate determination of sex by the
ic-chromosome.

its explanation must lie in the internal constitution of the
germ-cells — in their chemical, physical, or morphological
differences. Remarkably careful and accurate observa-
tion has, in fact, revealed a constant and fundamental
morphological difference in the germ-cells of animals.
It has been found, for example in some insects, that the
nucleus of every egg possesses a certain clearly distinguished
chromosome, called the rr-chromosome,^ while in the

' The X, as in algebra, indicating an unknown, or not understood,
factor.

360 STRUCTURE AND LITE HISTORIES

sperms one-half possess it (x) and one half do not (no — x).
In other cases (first recorded for hemipterous insects by-
Wilson) the x-chromosome in the cells of the male is
accompanied by a companion chromosome of a different
type, called by Wilson the y-chromosome. In the reduction
division one half the sperms receive the a:-chromosomes,
the other half the ^'-chromosomes. In some cases the
X- and j-elements are not single chromosomes but groups
of small chromatin bodies. The x-element is always
associated with femaleness, the no — x (or y) with maleness.
If an egg (x) is fertilized by a sperm possessing the x-
chromosome a female zygote is determined (formula xx) ;
the union of an egg with a no — x or 3; sperm results in
a male zygote (formula xo or xy) , thus :

Egg X -f- sperm no — x = zygote x (male).

Egg X + sperm y = zygote xy (male) .

Egg X + sperm x = zygote xx (female) .
This condition is illustrated diagrammatically in Fig. 265.
In sea-urchins and some other animals the condition may
be reversed, the sperms being all alike and the eggs unlike.
Careful investigations have, so far, failed to reveal any-
thing corresponding to the x-chromosome in plants,
except in two species of the dioecious liverwort, Sphcero-
carpos. In 1919 Allen^ (for S. Donnellii) and Miss Schacke^
(for S. texanus) demonstrated the presence in the cell-
nuclei of the female plants (gametophytes) of one x-
chromosome, clearly distinguished from the seven other
chromosomes by its much greater size. Likewise they

1 Allen, C. E. The basis of sex inheritance in Sphcerocarpos. Proc.
Amer. Phil. Soc, 58: 289-316. 1919.

2 Schacke, Martha A. A chromosome difference between the sexes of
Splmrocarpos texanus. Science, N. S., 49: 218-219. Feb. 28, 1919.

THE PROBLEM OF SEX IN PLANTS 36 1

have identified in the cell-nuclei of the male gametophytes
one ^^-element, distinguished by its much smaller size
(Fig. 265, a). As would be anticipated, the cell-nuclei of
the sphorophyte generation contain both the x- and the
^/-elements. On the basis of these discoveries Prof. Allen
predicts that further investigation is likely to reveal the
presence of similar bodies or elements in other plants.

326. The Meaning of Sex. — Just what is accomplished
for plants by the occurrence of two sexes is not entirely
understood. Among the lower plants the primitive expres-
sions of sex seem in some cases, to have met a need for better
nutrition, or to have resulted in rejuvenating the protoplasts
of the gametes; but these explanations are not satisfactory,
especially for the higher plants. We do know that fertili-
zation always results in increasing variation. When
plants are propagated vegetatively, as by cuttings or by
grafting, the characters remain constant in the new
plants,^ but when reproduction is by seeds (resulting from
fertilization) we always observe great variation. This
is, of course, an advantage, for it is only by variation that
new characters may appear, and without the appearance
of new characters there would be no opportunity for the
improvement of plants — either by nature or by man.

The results of all studies and discussions of the question
of sex lead us to recognize the fact that it is still largely an
unsolved mystery. We must make further and more
accurate observations, and be careful and logical in our
reasoning, before we can hope to solve this difficult
problem.

^ Except in the special case of bud-sporting (p. 532, and Fig. 400).

CHAPTER XXIII
FROM ALGA TO FERN

327. Progressive Development. — The largest fact that
stands out in a hasty review of the plants we have studied
is the increasing simplicity of form and organization from
fern to alga, or in reverse order, the increasing com-
plexity from alga to fern. The Pleurococcus is a simple
globule of living matter. Its organs are all reduced to
their lowest terms — cell-wall, cytoplasm, limiting surface
(or membrane), nucleus, nuclear membrane, chromato-
phore — the parts of a cell. The one protoplast performs
all the functions of life — takes in raw materials, elaborates
food out of these raw materials, digests the food thus
made, assimilates it, respires, and reproduces itself. It is
difficult to imagine the fundamental life-functions per-
formed under simpler circumstances of structure.

But as soon as plant cells begin to remain united after
cell-divison they begin to be differentiated. The single
Pleurococcus cell is globular, but when two or more
remain attached they are flattened at the surfaces in
contact (Fig. 183). This is a simple illustration of
morphological differentiation. When a cell-mass is
formed the outer cell-walls, in contact with the air,
become covered with a layer of cuticle, which retards
the loss of water. Cell-walls in contact with each other
do not possess the cuticle. Thus, by gradual steps the
plant body becomes increasingly complex, so that we

362

FROM ALGA TO FERN 363

may arrange a series from algae to ferns, and from ferns
to the higher seed-bearing plants, showing increasing
complexity of structure.

328. Division of Physiological Labor. — The differentia-
tion of the plant body into organs — root, stem, leaf, re-
productive organs — may be considered as an expression of
the division of physiological labor. For example, when a
sufficiently thick cell-mass is formed the inner cells may
be deprived of light; no chlorophyll can then develop,
photosynthesis becomes impossible, and the outer^cells must
elaborate all the food, not only for themselves, but for the
inner, non-green cells as well. Roots that serve to hold the
plant in the soil, and to take in water and minerals to be used
in the leaves, must be nourished by food elaborated in the
green cells. The leaves and branches must be supplied
with water taken in by the roots, and a vascular system
becomes necessary. So, in these and countless other ways,
the vegetative functions become divided among organs
specially fitted by their structure to perform them well.
Then the reproductive function becomes confined to
certain cells, which are nourished by the others. Repro-
duction itself becomes complicated by the development
of two kinds of gametes, and the introduction of cell-
fusion as well as cell-division.

Among plants, organization — the development of defi-
nite organs for definite work — has the same kind of ad-
vantage as the division of labor among men. The
"jack-at-all- trades" is not as efficient at any one of them
as the specialist. The existence of carpenters, plumbers,
masons, tailors, architects, superintendents, teachers,
lawyers, stenographers, doctors, means greater efficiency,
than could be secured if everyone tried to be all of these.

364 STRUCTURE AND LIFE HISTORIES

So the differentiation of the plant body into root, stem,
leaves, and organs of reproduction means greater efficiency
in the performance of all the functions of life.

329. From Water to Land. — A careful consideration of
all available evidence leads to the conviction that plant
life originated in the water. For example, the more
primitive types of plants have no well-defined polarity)
that is, they do not present an axis with the opposite ends
clearly differentiated for the performance of different
functions under unlike surroundings, such as roots ad-
justed to a life in the soil and darkness, and leaves
adjusted to a life in the air and light. Plants submerged
in water, such as Spirogyra, commonly possess a uni-
formity of structure, in harmony with their uniform
environment. Of course, there are exceptions to this,
Ascophyllum, Dictyota, Vaticheria, and other submerged
aquatics possess, on one end, hold-fasts which anchor them
to the substratum; but these plants probably represent
early steps toward a rooted existence on land.

One of the most marked evidences of aquatic life for
primitive organisms is their method of reproduction by
motile spores and motile gametes; while, at the same time,
one of the most distinctive characteristics of the more
highly developed land-plants is reproduction by non-
motile spores, suited to distribution by wind. The
enormous number of spores produced insures a rapid
multiplication of individuals, and their dryness insures
protection during periods of more or less prolonged
drought.

330. Development of the Sporoph3rte. — For the suc-
cessful production of large numbers of spores there is
needed some provision for richly nourishing the spore-

FROM ALGA TO FERN 365

producing tissues. There must be a large amount of
chlorophyll-bearing tissue, ample provision for taking in
abundant water and minerals, and efficient channels for
conducting the raw materials and elaborated food from
one part of the plant to another. Moreover, the spore-
bearing parts need to be lifted into the air to insure the
most efficient distribution of the spores. These needs
are admirably met by the sporophyte with roots on one
end, green leaves on the other, and sporangia borne at
or very near the tips of the branches.

A review, at this time, of the sporophytic phases of the
liverworts, mosses, and ferns will show how these sporo-
phytes gradually increase in complexity and importance,
from the simple condition in Riccia, with almost no sterile
tissue, through the sporogonium of the higher liverworts
and mosses, to the leafy sporophyte of the ferns. The
final step in the development of the sporophyte was the
differentiation of megasporophytes, bearing only mega-
spores, and microsporophytes, bearing only microspores.

331. Decline of the Gametophyte. — As the sporophyte
became more highly developed and the dry-land fio-a
more firmly established, the gametophytic phase became
less essential and less in evidence, until, in the ferns,
the sporophyte became the commonly recognized "plant,"
and the very existence of the gametophytic phase was, for
a long time, not known. Reproduction by spores and
by other non-sexual means became entirely sufficient to
perpetuate the race.

332. Classification. — By a careful comparison of all
kinds of plants, it has been recognized that certain ones
are very much alike in fundamental characteristics of
structure; they fall naturally into a group. Moreover it

366 STRUCTURE AND LIFE HISTORIES

is recognized that there are numerous groups, and that
the members of any one group differ from those in every
other group in some fundamental point. By such com-
parisons botanists have been able to classify all known
plants into more or less clearly defined groups and sub-
groups. The larger the number of characters considered,
the smaller the group, and vice versa.

The divisions of the plant kingdom already studied, and
their distinguishing characters are as follows : ^

Divisions of the Plant Kingdom

1. T hallo phytes. — Plant body a thallus; no archegonia.

2. Bryophytes. — Archegonia; no vascular system.

3. Pterido phytes. — Vascular system; no seeds.

These four divisions are, of course, distinguished by
other characters than the ones just indicated, but these
stand out prominently as positive and negative character-
istics of the respective groups.

These three divisions are further subdivided, as shown
on the following page.

' Adapted from Coulter (J. M.).

FROM ALGA TO FERN 367

Table IV.— SuBorvrsiONs of the Plant Kingdom
I. Thallophyta

(a) Algae

(i) Cyanophyceae

(2) Chlorophyceae

(3) Phaeophyceae

(4) Rhodophyceae

(b) Fungi

(i) Myxomycetes

(2) Schizomycetes (bacteria)

is) Phycomycetes

(4) Ascomycetes

(5) Basidiomycetes

(6) Fungi imperfecti (life histories imperfectly known).

(c) Lichens

2. Bryophyta

(a) Hepaticae

(b) Musci

3. Pteridophyta (true ferns) ^

(a) Eusporangiatas

(b) Leptosporangiatae

4. Calamophyta (calamites)
(a) Equisetineae

5- Lepidophyta (lycopods)

(a) Lycopodineae (homosporus ciuu-mosses)
(6) Lepidodendrineae (heterosporus club-mosses)

* An older classification combined the last three groups or phyla into
one, as follows:
3. Pteridophyta (ferns and fern allies)

(a) Filicineae

(6) Equisetineae

(c) Lycopodineae

CHAPTER XXIV
CALAMITES AND LYCOPODS

I. THE HORSETAILS (EQUISETALES)
EQUISETUM

333. Habitat and Distribution. — As the names of some
of the various species indicate, representatives of the
genus Equisetum occur in a rather large variety of habitats.
Thus we have the swamp-equisetum {E. palustre) , meadow-
equisetum {E. pratense), the field-equisetum {E. arvense),
and so on. They are frequently found along railroad
embankments in exposed situations, while other species
occur only in shaded or very moist locations (Fig. 266).
They are distributed throughout the northern hemi-
sphere, but only one species has been reported from South
America. Twenty species have been described from the
temperate and tropical North America, but none has ever
been found in Australia. Fossils of near relatives of the
genus have been found in the rocks of previous geological
ages, and some of the fossils in the coal-bearing rocks of
the Carboniferous age are thought to belong to Equisetum
itself. In temperate America the horsetails vary in height
from only a few inches to several feet. One species {E.
dehile), found near Lahore, in India, attains a height of from
10 to 15 feet, needing the support of neighboring trees in
order to stand erect, while E. giganteum, found from the
West Indies to Chili, reaches a maximum height of

368

CALAMITES AND LYCOPODS

369

about 40 feet. In the island of Jamaica it is found in
thickets 10 to 15 feet high.

334. Description of the Sporophyte. — The plant body
(Fig. 267) consists of a horizontal, much-branched, under-
ground stem, or rhizome, from which spring two kinds of
sub-aerial branches — sterile and fertile. The cells that

Fig. 266. — Equisetwn fluviatile. Pure stand in shallow water, at Tully
Lake, N. Y. (Photo by W. L. Bray.)

compose the sub-aerial branches are, in many of the
species {e.g., E. aroense), nearly or quite all formed by the
close of the growing season, in the fall, so that in the
following spring all that is necessary is the expansion of
these cells, causing an elongation of the internodes and
the appearance of the branch above ground in early
spring.
24

370

STRUCTURE AND LIFE HISTORIES

The sterile branches produce smaller side branches at
the nodes. Chlorophyll is formed in the cells of the cortex,
and stomata in the epidermis permit the exchange of

Fig. 267. — Equisetum arvense. P, sterile branch; P^, fertile branch
with strobilus, or cone; R, rhizome (underground); T., cross-section of
cone, showing insertion of sporangiophores in a whorl; A", iV, sporangio-
phores with pendant sporangia; S, S\ S'-, spores with coiled elaters (el).

gases necessary in photosynthesis and respiration. The
sterile branches elaborate practically all of the food neces-
sary to nourish the underground stem and the developing

CALAMITES AND LYCOPODS 37 1

sterile and fertile branches that will appear the following
spring. We thus have an excellent illustration of the division
of physiological labor — one branch to anchor the plant in
the soil, and serve as a storehouse of food and a center of
distribution ; roots to take in water and dissolved minerals ;
sterile aerial branches to perform the functions of food-
manufacture, and the fertile branches to perform the func-
tion of reproduction — bearing the spores, and lifting them
high in the air, thus facilitating their distribution by wind.

The fertile branch commonly appears first in the spring,
usually bearing no side branches nor foliage-leaves, but
only whorls of scale-like leaves at each node. These
scales possess little or no power of photosynthesis, and
are chiefly protective (Fig. 267). In some instances the
fertile branches bear green lateral branches. At the
apex of the fertile branch is borne the strohilus, or cone,
consisting of a central axis (the prolongation of the
axis of the branch), bearing a variable number of spor-
angiophores. In the development of the fertile branch the
cone is formed first, and is raised above ground by the
subsequent formation and elongation of the sterile tissue
below it. In some species {E. arvense) the fertile branch
dies after the shedding of the spores, while in other species
{e.g., E. pratense), after the spores are shed the entire cone
falls away and the fertile branch then takes on the char-
acters of the sterile branches which occur with it.

Each sporangiophore^ consists of a stalk with a peltate
shield at the end The axis of the cone soon ceases to

^ The sporangiophores of Eqiiisetum have been interpreted as hom-
ologous with leaves, i.e., as sporophylls, but evidence derived in part from
a study of the fossil relatives of the modern horsetails indicates that this
conclusion may not be correct (Cf. Fig. 268). The term sporangiophore
is non-commital as to homology.

372

STRUCTURE AND LIFE HISTORIES

elongate between the whorls of sporangiophores, so that
the shields occur in close contact. It is this cessation of
growth, in fact, that produces the cone; otherwise the
sporangiophores would occur in whorls distributed at
wider intervals along the axis.

The sporangia arise from a single epidermal cell {euspo-
rangiate) on the underside of the shield; there are from
five to ten on each shield.

Fig. 268. — Sphenophyllum cuneijolium, a fossil species related to the
modern horsetails. Diagram of a longitudinal sectional view of an axis
bearing sporophylls (Sph); s,s, sporangia; s^, s^, sporangia in longitudinal
section, showing spores, A vascular bundle enters the stalk of each
sporangium. Enlarged. (Redrawn from Zeiller.) Cf. Fig. 280.

The spores (which are green when ripe) are alike in size
{homos porous), but they produce two kinds of gameto-
phytes, male and female (dioecious). Therefore they
must be unlike physiologically. Under certain circum-
stances, as already mentioned, (page 359), the female

CALAMITES AND LYCOPODS

373

gametophytes may ultimately produce antheridia, and
the male ones, archegonia. It is of interest to note that
some of the fossil relatives of the modern horsetails were
heterosporus.

The structure of the spores is unusual in that they bear
four ribbon-like appendages (elaters), formed from the
outer wall, and closely coiled around the spores (Fig. 267).

Fig. 269. — Equisetum palustre. Portion of a male prothallus, bearing
antherida; a, b, c, three antheridia in successive stages of development;
a. empty; sp, escaping sperms and sperm-mother-cells; c, antheridium
not yet opened; d, initial stage in the development of an antheridium.
X about 70. (After Sadebeck.)

These appendages uncoil in dry air and recoil with moisture,
with a sharp, snapping motion, thus rolling the spores
about.

The distribution of the spores is accomplished when
they are ripe, by the opening of the dry walls of the
sporangia. The shrinking of the walls gradually forces
out the spores, and by the uncoiling and snapping of the
elaters the spores become entangled and held fast to
each other in little fiocculent masses. Thus the complete

374

STRUCTURE AND LIFE HISTORIES

isolation of single spores is prevented, and the advantage
of this is recognized at once when we recall that the
prothallia are. dioecious,

335. The Gametophytes. — Under suitable conditions of
moisture and temperature the spores begin to germinate,
and by successive cell-divisions produce the lobed pro-
thallia. The male prothallia are one cell in thickness,
and bear the antheridia at the tips of the lobes or on the
margins (Fig. 269).

Fig. 270. — Female prothallus of Equisetum arvense L. cri, young
archegonium; arz, archegoium before fertilization; st, sterile lobe of pro-
thallus; hw, rhizoids. Several lobes were removed in order to show
the cushion and the archegonia. Enlarged about 20 times. (After
Sadebeck.)

The female prothallia form a cushion of relatively
thick, spongy tissue (the meristem), and on this cushion
(as in all Pteridophytes) are borne the archegonia. In
contrast to the true ferns, the archegonia are borne on
the upper surface of the prothallus, and point upward,
in consequence of being negatively geotropic (Fig. 270).
From the edges of the cushion numerous thin flaps of
green tissue form; these fold over the cushion, enclosing
the archegonia, and thus retaining the moisture of dew

CALAMITES AND LYCOPODS

375

or rain. This forms a favorable environment for the
multiciliate sperms, which are set free from the antheridia
of neighboring male prothallia; and swim to the arche-
gonia, and down their neck-canals to the eggs which they
fertilize.

336. The New Sporophyte. — As always, the fertilized
egg develops into an embryo, and the embryo, without any

5a V' ^ "-^4^

Fig. 271. — Diagram of life-cycle of Equisetum.

period of rest, continues to grow until the new sporophyte
is formed, with underground rhizome, and finally with
the sub-aerial sterile and fertile branches, thus completing
the life-cycle^ (Fig. 271).

^ In a few species of Equisetum modified underground branches, re-
sembling a string of tubers are formed, and these give rise to new plants by
vegetative multiplication.

376 STRUCTURE AND LIFE HISTORIES

The life history of Equisetum may be tabulated as
follows :

OUTLINE OF LIFE HISTORY OF EQUISETUM

Rhizome i

X. \, I Sporophyte

Sterile branch Fertile branch I

4.4.

Strobilus

4-4-

Sporangiophores

4-4-

Sporangia

Reduction
Isospore Isospore

4- i

Male gametophyte Female gametophyte

4- 4-

Antheridium Archegonium

4- 4-

Sperm Egg

FertilizatioiJ^

Oosperm

44-

Embryo

4-i

Mature sporophyte

11. THE CLUB-MOSSES (LYCOPODIALES)

LYCOPODIUM

337. Habitat and Distribution. — Nearly all the species
of Lycopodium prefer moist situations, and one or two of
them are aquatic. They are widely distributed over the
earth, in both hemispheres, from the torrid to the frigid
zones, and commonly grow in shady places. Lycopodium
Selago, and a few other species are epiphytic. They all

CALAMITES AND LYCOPODS

377

prefer a substratum rich in humus or other organic matter.
Most of the species are restricted to one hemisphere, but
a few occur in both.

338. The Sporophjrte. — There are several hundred
species of Lycopodium. Among those most common in
temperate America are L. davatum (Fig. 272), L. ohscurum

Fig. 272. — Lycopodium davatum,

dendroideum, and L. lucidulum. These species commonly
grow trailing over the surface of the ground, and from
this, and the appearance of their foliage, they are com-
monly called "ground pine," though of course they have
nothing to do with true pines. As is well shown in the
figure, the plant-body of the sporophyte of Lycopodium
davatum consists of a sterile lower region, bearing foliage-

378

STRUCTURE AND LIFE HISTORIES

leaves, but no sporophylls, while the fertile region occurs
as a clearly recognized cone, formed by the crowding of
the sporophylls at the apex of the leafy axis (Fig. 273).
The foliage-leaves are all simple and small (microphyllous) .

Fig. 273. — Lycopodium sp. Photomicrograph of longitudinal section
of a cone, showing the sporangia on the upper surface of the sporophylls,
near their insertion on the main axis.

A more primitive type is found in Lycopodium Selago
(Fig. 274). Here the lower region is sterile, but is not
as well developed as in other types, for the sporophylls
begin to appear lower down on the stem. Moreover the
sporophylls are not aggregated into a cone, but are dis-
tributed at intervals from near the base to near the apex,

CALAMITES AND LYCOPODS

379

with sterile regions intervening. The leaves usually occur
in whorls of five, but often they are arranged in spirals.

At the zone of transition from sterile to fertile regions,
imperfectly developed [aborted) sporangia are often formed,
and this (with other evidence) has suggested that, in the

Fig. 274. — Lyco podium Selago. (After Bower.)

evolution of the sporophyte, the purely vegetative regions have
resulted from a sterilization of fertile tissue. The correctness
of this interpretation of the origin of the sterile regions is
rendered more probable by the fact that the condition
found in L. Selago is characteristic of the fossil Lycopods
of the coal measures. The possession of a well-developed

380 STRUCTURE AND LIFE HISTORIES

sterile region with foliage-leaves, and the restriction of the
sporophylls to the apices of the branches is of very con-
siderable advantage, making possible an abundant supply
of food to a vast number of spores. The branching of
L. Selago is dichotomous, and the sporangia are borne
on the upper surfaces of the sporophylls, near their bases.

339. The Gametophyte. — The spores of Lycopods are
alike in size (isospores), and on germination produce
fleshy prothallia, bearing both antheridia and archegonia
(monoecious), and partially saprophytic in habit. They
commonly have a filamentous fungus growing parasitic-
ally within their tissues. The reproductive organs occur
at the upper end, surrounded with sterile hairs {para-
physes). The whole of the antheridia, and the venters of
the archegonia are imbedded in the vegetative tissue.
The gametophyte may grow more or less completely im-
bedded in the soil, but when growing on the surface chloro-
phyll is formed, and photosynthesis may take place. Its
lower end bears numerous rhizoids.

340. The Embryo. — After an ^gg is fertilized it begins
at once to divide and soon develops an embryo. Of the
two cells resulting from the first division of the fertilized
egg, the lower one serves as a suspensor, while the other
becomes the ancestor of all the cells of the embryo. The
suspensor serves to push the embryo down into the nour-
ishing tissue of the prothallus. The young embryo
(Fig. 275) soon becomes differentiated into two distinct
regions— the foot (not shown in the figure), and the shoot.
As in the mosses and true ferns, the foot serves to absorb
nourishment from the gametophyte. But the embryo is
dependent upon the gametophyte for only a relatively
brief period — shorter than in the mosses and true ferns.

CALAMITES AND LYCOPODS

381

The elongation of the embryo-stem Qiypocotyl) carries the
first leaves {cotyledons) up above the surface of the soil,
while at the same time the first root {radicle) is develop-

FiG. 275. — Lycopodium phlegmaria. Development of embryo, st, stem;
cot, cotyledon; sus, suspensor; R, root. (After D. H. Campbell.)

Fig. 276. — Young sporophyte of Lycopodium cermmm L., with the
gametophyte, having irregular lobes of chlorophyll-bearing tissue attached
on one side. (After Treub.)

ing at its base (Fig. 276). If the prothallus is deeply
buried the hypocotyl becomes more elongated before the
cotyledons are formed.

341. Vegetative Multiplication. — Several species of Ly-
copodium bear gemmae. They are conspicuous on

382 STRUCTURE AND LIFE HISTORIES

Lycopodium lucidulum (which is common in the northern
United States), and are borne near, but not in, the axils
of the leaves. A young rootlet commonly appears on the
gemma while it is still attached to the plant. After it
falls off, the axis elongates, and a new sporophyte is
formed, like the old one.

342. Life History. — The student should be able to make
his own diagram of the life history of Lycopodium, fol-
lowing the examples given in connection with forms
previously studied.

m. LITTLE CLUB-MOSSES (SELAGINELLALES)
SELAGINELLA (LITTLE CLUB-MOSS)

343. Habitat and Distribution. — The little club-mosses
(species of Selaginella) are found in every continent and
on most of the larger islands. They usually grow only in
moist situations, and are very common in conservatories,
under the plant benches, and in pots and hanging baskets.

344. The Sporophyte. — The plant body (sporophyte)
consists of a much-branched stem (Fig. 277), bearing
scale-like, but green foliage-leaves, sessile and more or less
closely appressed to the stem. At the tips of the branches
the foliage-leaves are replaced by sporophylls, so arranged
as to form a clearly distinguished cone (Figs. 278 and 281).
Near the base of the leaves, on the surface next the stem,
is formed a thin, membraneous flap, the ligule (Figs. 279
and 280). The possession of a ligule is one of the funda-
mental distinguishing characteristics of Selaginella.

345. Two Kinds of Spores. Heterospory. — If we
examine a longitudinal section of a cone, we shall normally
find a sporangium in the axil of each sporophyll. In
exceptional cases the lower or basal leaves of the cone are

CALAMITES AND LYCOPODS

383

Fig. 277. — Selaginella Wildenovii,

Fig. 278. — Selaginella anmna. Branch bearing numerous terminal cones.

384

STRUCTURE AND LIFE HISTORIES

Fig. 279. — Selaginella Watsoniana. Base of foliage leaf, showing ligule,
L. Enlarged about 36 times.

Fig. 280. — Selaginella sp. Photomicrograph of a longitudinal section
through the tip of a cone, showing sporangia (sp), and ligules (%.).
(Cf. Fig. 268.)

CALAMITES AND LYCOPODS

38s

Sterile. The lower sporangia are larger than the upper
ones, and bear four large spores {megas pores). They are
megasporangia, and the leaves are megasporophylls. The

Fig. 281. — Sdaginella Marlcnsil. a, vegetative branch; h, portion of
the stem, bearing cones {x)\ c, longitudinal section of a cone, showing
microsporangia {mic. sp.) in the axils of microsporophylls, and megaspor-
angia in the axils of megasporophylls; d, microsporangium with micro-
sporophyll; e, microspores;/, portion of wall of sporangium, greatly magni-
fied; g, megaspore; h, microsporangium opened, and most of the micro-
spores scattered; i, megasporangium, with megasporophyll; h, same,
opened, showing the four megaspores.

smaller sporangia {microsporangia) are subtended by
viicrosporophylls, and bear numerous small spores {micro-
spores). In some species the two kinds of sporophylls

25

386

STRUCTURE AND LIFE HISTORIES

alternate along the axis of the cone (Fig. 281). The
larger number of microspores results from the fact that
every spore-mother-cell, by tetrad-division, develops
spores, while in the megasporangia only one spore-mother-
cell develops spores, the other cells serving to nourish that
one. The microspores develop only male gametophytes,
the megaspores, female. In dissemination, the spores
are ejected to some distance from the parent plant
(Fig. 282).

Fig. 282. — Selaginella Martensii. Dissemination of spores. The
branch was covered with a glass bell-jar to avoid currents of air. The
"dust" is composed of both microspores and megaspores, and indicates
the distance to which the spores are projected from the dehiscing spor-
angia. X 23.

346. The Male Gametophyte. — The male gametophyte
is developed entirely within the wall of the microspore.
The first division gives rise to a vegetative (or sterile)
and a fertile cell. The vegetative tissue never develops be-
yond the one-celled stage. By several divisions the fertile
cell develops a simple antheridium containing four sperms.
Each sperm bears two long, slender cilia (Fig. 284).

347. The Female Gametophyte. — The megaspores begin
to gerviinate ivhile still in the sporangium. This will be

CALAMITES AND LYCOPODS 387

recognized at once as a new feature in life history. By
successive divisions the protoplasm of the spore becomes a
multicellular body, the prothallus, with richer cells near the
apex. Here a number of archegonia form (Fig. 283), and
the enlargement of the prothallus, or gametophyte, causes
a splitting apart of the old, thick walls of the megaspore,
so that the female gametophyte protrudes (Fig. 284, 9).

Fig. 283. — Selaginella Krausslana. C, section of mature female gam-
etophyte, showing three archegonia, two containing eggs, and one (at the
left) an embryo with suspensor {sus.). D-G, Stages in the development
the archegonium; H, very young embryo (two-celled stage), after first
division of the fertilized egg; I, older embryo (Em), with suspensor {s).
(After Campbell.)

It bears no chlorophyll, living entirely as a parasite on
the parental sporophyte, from whence it derives all the food
with which it nourishes the embryo.

348. Fertilization, — As throughout the ferns, calami tes,
and lycopods, fertilization is accomplished by the swimming
of the sperm to the mouth of the archegonium, and down
the neck-canal to the ripe egg in the venter. Thus while
Selaginella is, in other respects, a land-plant, it retains
the aquatic method of fertilization. External water is
absolutely necessary in order that the sperm may reach
the egg.

349. The Embryo. — After fertilization the oosperm
begins to divide. The cell nearest the neck of the arche-

388

STRUCTURE AND LIFE HISTORIES

gonium, after the first division of the egg, is a suspensor,
but becomes much longer than in Lyco podium, and
thrusts the developing embryo deep down among the
nourishing cells of the gametophyte. By the possession
of a suspensor the Lycopodiales and Selaginellales are
distingiiished from the Pteridophytes and Calamites.

The embryo does not cease growth, and pass through a
resting period, but continues to develop, until its root and
shoot, with two cotyledons, emerge from the prothallus.

Fig. 284. — Diagram of life-cycle of Selaginella. (Alodified from J. H.

SchafTner.)

and the young sporophyte gradually becomes established
as an independent, green plant (Fig. 284). It is only after
the development of a vigorous leafy shoot, with chloro-
phyll apparatus, capable of elaborating an abundance of
food, that the strobilus is organized with its axis and
green sporophylls.

CALAMITES AND LYCOPODS 389

OUTLINE OF LIFE HISTORY OF SELAGINELLA

Sporophvtc

TT ' TT

Microsporophyll ^Nlegasporophyll

^ >l^ >!< >K

Microsporangium Megasporangium

^ < — Reduction — > ^

Microspore ]Megaspore

4- 4.

Male gametophyte Female gametophyte

4. i

Antheridium Archegonium

4. 4-

Sperm Egg

Fertilization

Oosperm

4-4-

Emhrvo

ii

Mature sporophyte

350. Marks of Progress. — With the introduction of

heterospory we recognize a distinctly new feature of the
sporophyte generation. Structural differentiations asso-
ciated with difference in sex have hitherto been confined
to the gametophytic generation, but now such distinctions
appear for the first time in the sporophyte. This is a
long step forward, and marks Selaginella as a more highly
organized form than the lycopods, horsetails, and ferns.
Other marks of progress are:

1. The reduction of the vegetative tissue of the
gametophytes (to only one cell in the case of the male
gametophyte).

2. The entire dependence of the gametophytes upon the
sporophytes for nutrition.

3. The retention of the female gametophyte, through-
out its entire existence, almost entirely within the wall
of the megaspore.

CHAPTER XXV

SEED-BEARING PLANTS

THE CYCADS

351. Description. — The cycads, while native only in
the tropics, are familiar to all persons who have visited
conservatories. One of the commoner species in cultiva-
tion (Cycas revoJuia) is often labelled, "Sago palm." In

Fig. 285. — Cycas rcvolula, showing terminal bud of foliage-leaves just
opening. (Compare Fig. 286.)

fact, in some respects it bears a superficial resemblance to
a palm, while in other characters it suggests the ferns.
There is a short, thick, cylindrical, unbranched stem or
trunk, bearing a crown of beautiful, leathery, green leaves,

390

SEED-BEARING PLANTS

391

having prominent midribs and pinnately divided (Fig.
285). The leaves endure for a year or more (varying
with the species), and are then replaced by a fresh crown.
The duration of each crop of leaves is said to vary accord-
ing as the plant grows wild, or in botanic gardens and

Fig. 286. — Cycas revoliita. Terminal bud of foliage leaves just opening.
Nearer view of Fig. 285. Note the circinate vernation of the leaf-pinnules,
but not of the entire leaf.

conservatories. Thus, when temperature and rainfall are
excessive, Cycas circinalis may produce two crowns of
leaves a year, instead of the one crown commonly produced
in green houses.

The young leaves are curled up at the tips, unrolling
as they grow. In the genus Cycas only the leaflets are

392

STRUCTURE AND LIFE HISTORIES

r-

J^U^

i

Mk

1

1^^^ ''J ^Ih

\!^^ '^. ^^^^k

«

^t>

Fig. 287. — Macrozamia Moorei. Two staminate cones; the older one

above.

FiG._ 288— Macrozamia Moorei. Microsporophyll (lower surface), show-
ing microsporangia (pollen-sacs), after the shedding of the pollen.

J

SEED-BEARING PLANTS

393

curled (Fig. 286), while in the related American genus,
Zamia, the entire leaf is curled as well. The plants are
dioecious, but the two sexes so closely resemble each other
that they cannot be distinguished except by their sporo-
phylls when in fruit.

Fig. 289. — Macrozamia Moorei, showing two lateral carpellate cones.

362. Microsporophylls.— The microsporophylls are
grouped into a cone (Fig. 287). This means that they
are not on the main stem, for the cone is really a branch,
bearing only sporophylls. The microsporangia, bearing
microspores, occur in groups (sori), on the under surface
of the sporophylls (Fig. 288).

394

STRUCTURE AND LIFE HISTORIES

SEED-BEARING PLANTS

395

353. Megasporophylls. — The niegasporophylls, or car-
pels, occur either in axillary cones (Fig. 289), or in groups

Fig. 291, — Cycas media. Trunk, with foliage leaves removed, showing
crown of megasporophylls (carpels). (Specimen in Brooklyn Botanic
Garden.) (Cf. Fig. 289.)

at the summit of the main stejn, surrounding the growing
point of the stem, and surrounded by the foliage-leaves
(Figs. 290 and 291) . In the latter case they possess a pin-

396

STRUCTURE AND LIFE HISTORIES

nately divided blade, with mid-rib and petiole (Fig. 292).
The genera that bear the megasporophylls on the main
stem resemble the ferns, and in this respect are the simplest,
or most primitively organized, of all living seed-plants.

364. Megasporangia. — Unlike the microsporangia, the
megasporangia of Cycas occur, not in groups, but solitary

Fig. 292. — Young megasporophyll (carpel) of Cycas revolnta, bearing
six young ovules, destined, after fertilization, to mature into seeds. Note
the relatively large amount of leaf-blade above the ovules, as compared
with Cycas media (Fig. 293). (Specimen from C. C. Chamberlain.)

on the lower part of the sporophyll, at the margin, occupy-
ing the position of the pinnate divisions (Figs. 292-294).
In genera bearing carpellate cones the megasporangia
occur in pairs on the under surface of each scale (mega-

SEED-BEARING PLANTS

397

sporophyll, Fig. 295). In the Cycads and higher plants
the megasporangia are called ovules.

355. Ovules. — The young megasporangia or ovules of
Cycads consist of two distinct regions of sterile tissue — an
inner nucellus, and an outer covering, or integument^

Fig. 293. — Megasporophyll (carpel) of Cycas media, bearing one ripe
naked (gymnospermous) seed, and three ovules which failed to become
seeds, doubtless through not being fertilized. (Compare Fig, 292.)

which is an outgrowth of the ovule just below the nucellus.
The integument serves to protect the more delicate tissues
within, and later becomes transformed into the seed-
coat (Figs. 296-298). Only one of the four megaspores
develops within the nucellus.

356. Female Gametophyte. — As in Selaginella, the mega-
spores begin to germinate while still in the sporangium,
but now a new feature is introduced into life history;

398

STRUCTURE AND LIFE mSTORTES

Fig. 294. — Cycas media. Carpel (megasporophylD with six ripened
ovules or seeds.

P'iG. 295. — Macrozamia Moorei. A scale (megasporophyll) from a
carpellate cone, bearing two ovules, maturing into seeds. A, top view;
5, bottom view. About half size.

SEED-BEARING PLANTS 399

Ihe megaspores of Cycads never leave the sporangium. The
developing female gametophyte is nourished entirely as

Fig. 296. — Ovule oi Macrozamia Moorei. A, external view; B, portion
of integument removed, showing egg-shaped gametophyte. The dark
strip on the upper left-hand portion of the latter is where a piece of the
now membranous nucellus was peeled off; C, gametophyte, entire, with
nucellar cap at the upper end; D, longitudinal section; E, end view, with
part of the integument removed, disclosing the tiny crater-like pollen-
chambers, at the bottom of which the necks of the archegonia open. (Cf .
Fig. 297.) About natural size.

a parasite, by the tissue of the nucellus. The latter
becomes almost entirely consumed, so that when a longi-

400

STRUCTURE AND LIFE HISTORIES

tudinal section is made of a mature ovule (Fig. 297), tlie
remainder of the nucellus appears only as a thin mem-
brane adhering to the outer surface of the prothallus, or
endosperm, as it is here called. When the Qgg fails to
become fertilized the gametophyte may protrude, develop
chlorophyll, and lead a brief, semi-independent existence.

■ i

^^M

mi

.--4

^^^^

W"' ^

^^^^^^S^. _»* — ■

- A-T - — 3, r.

i

t

■

1

oi..-m

^

W.\i.

1

fc.

*»-— — w'

^

_^ «■• ^^r

Sti:. L-^ ■

■ - - -^.^-'^-^

Fig. 297. — Photograph of a longitudinal section of an ovule of Macro-
zaniia Moorei. mi, micropyle; n.c, nucellar cap; a.r, archegonia (the
venters only showing) ; il, inner, hard layer, o.l, outer, fleshy layer of the
integument; g, gametophyte; p.c, pollen chamber. Enlarged from D,
Fig. 296.

Several archegonia develop in the apical end, imbedded
in the tissue of the prothallus, and with their neck-canals
opening at the surface into a pollen-chamber (Fig. 297).
They resemble somewhat the archegonia of the lower orders

SEED-BEARING PLANTS

401

studied, but possess only two neck cells, and no neck-canal
cells. The large egg-cell in the venter is the largest known
in the plant kingdom.

Fig. 298. — Cycas circinalis. Diagram of longitudinal section of a
nearly mature seed; 0, outer fleshy layer, with a bundle (0^) of the outer
vascular system; s, stony layer of integument; i, inner fleshy layer, with
a bundle {i^) of the inner vascular system; c, central vascular bundle.
(After Marie C. Stopes.)

357. Male Gametophyte. — The germination of the
microspore and the development of the male gametophyte
involve only cell-divisions, but not the growth of new
tissue. The mature gametophyte is called a pollen- grain.
It consists of three cells: a prothallial cell, a tube-cell,
and a generative cell (Fig. 299). There is no structure
that can be positively identified as an antheridium, unless
the prothalHal cell is considered (as by some), as repre-
senting the antheridium. The pollen-grain has two
coats — an outer and an inner.

358. Pollination. — When the pollen is mature it is
scattered by the wind, and some of the grains lodge, by

26

402

STRUCTURE AND LIFE HISTORIES

chance, in the pollen-chamber of the ovules on neigh-
boring female plants. The transfer of pollen to the female
plant is pollination.

359. Germination of the Pollen-grain. — In the pollen-
chamber the conditions favor the germination of the

Fig. 299. — Cycas rcvoluta. a, Pollen grains at shedding stage; X 500;
i, later stage, showing prothallial cell (/>) and generative cell (g), the tube-
nucleus not shown; X 200; c, generative cell divided, giving rise to stalk-
and body-cells; X 500; d, the stalk-cell-nucleus (5) being crowded out,
and blepharoplasts appearing in the body cell ih); X 500; ^> the body-cell
shortly before division, showing two well-developed blepharoplasts; X 750;
/, the two male cells resulting from the division of the body-cell; the beaks
of the nuclei are attached to the cilia-bearing bands; X 200. Reduced
about two-thirds in reproduction. (After Ikeno.)

pollen. This is also a new feature in life history. In ger-
mination the pollen-grain develops absorbing organs
{haustoria), which penetrate the tissue of the nucellar-cap
(Fig. 300), and also larger tubes which carry the generative
cell further down into the pollen-chamber. As the tube

SEED-BEARING PLANTS

403

elongates the generative cell passes down it, becoming
divided into two sperm-cells. These sperm-cells are
remarkably interesting little bodies, bearing a large num-

T^^ ^^^

Fig. 300. — Dioon cdiile: upper part of an ovule at the time of fertiliza-
tion, showing integument, nucellus, male gametophytes (germinating
pollen-grains), and female gametophytes (embryo-sacs). Reconstructed
from sections of several ovules. (After Chamberlain.)

ber of cilia, by the motion of which they are carried, in
the completion of their journey to the egg, through the
watery solution emptied from the pollen-tube, to and

404

STRUCTURE AND LIFE HISTORIES

through the neck-canal of the archegonium, into the
archegonial chamber. FertiHzation, here as always, is
completed by the fusion of the sperm and the egg-nuclei
(Fig. 301). The behavior of these little sperms in an allied
genus, Zamia, is thus described by their discoverer,^
Webber:

Fig. 301. — Fertilization in Zamia floridana. The male and female nuclei
are fusing; h, remains of the cilia of the sperm. (After Webber.)

"In removing the nearly mature pollen-tubes the sperm-
atozoids are found to be in various stages of development,
as would be expected. In many cases tubes have been
observed, before cutting them off, in which the two sper-
matozoids had pulled apart and were swimming free in the
protoplasm. In some instances their movement in the
pollen-tube, before it is injured, can be observed with the
aid of a hand lens.

''It is an interesting sight to see the two giant sper-
matozoids moving around vigorously in the pollen-tube,

1 ^Motile sperms were discovered in Cycas in 1897, by a Japanese botanist ,
Hirase, following their discovery in Ginkgo in 1896 by another Japanese
student, Ikeno. The latter was the first discovery of motile sperms in a
spermatophyte..

SEED-BEARING PLANTS 405

bumping against each other and the wall of the tube in
their reckless haste. They seldom escape from the upper
cut end of the pollen-tube, although they as frequently-
swim toward this end of the tube as the other end, so far
as could be observed. In many cases the pollen-tubes
were cut so that the spermatozoids escaped into the solu-
tion, and in numerous other cases mature turgid tubes
burst in the process of cutting, discharging the uninjured
spermatozoids in the sugar solution. The writer was
thus able in many cases to study the spermatozoids swim-
ming free and observe their unobstructed motion.

''The motion of the spermatozoids when swimming free
in sugar solution is in no way different from their motion
when in the pollen-tube. The general motion is a con-
tinuous rotation of the body, always in the same direction,
around an axis passing through the apex of the helicoid
spiral. Viewed from the head end or apex of the spiral
the rotation is in the direction of the hands of a clock, and
contrary to the turns of the spiral band. They roll
around, first here, then there, resembling in this respect
the motion of Pandorina. After moving about rapidly
for from five to fifteen minutes they usually cease all pro-
gressive motion, but continue to rotate for a considerably
longer period. The rotary motion also soon ceases, but the
cilia continue to vibrate for a considreably longer time.
The spermatozoids of Zamia also have an amoeboid mo-
tion, which is particularly noticeable while they are in-
closed in the pollen-tube. The apex of the spiral as a
whole frequently rotates in a most remarkable way,
turning in a circle, pushing out first this way and then that
way with the greatest freedom of motion, as if selecting a
point of exit or ingress. In other cases the base or the side

4o6 STRUCTURE AND LIFE HISTORIES

of the spermatozoid body may be considerably extended
as a blunt point in pushing between two obstacles. The
whole body seems flexible and changeable in the highest
degree and is eminently fitted for its difficult task of finding
and swimming through the narrow passage between the
neck-cells of the archegonia."

A point to be specially noted here, is that while a pollen-
tube is introduced in the process of fertilization, the final
act is accomplished as in lower aquatic forms, by the
swimming of the sperms through liquid. The pollen-
tube alone, as in higher plants, should suffice in Cycads to
bring the sperm to the egg, and the retention of locomotion
of the sperm, after the appearance of the pollen-tube, can
be interpreted only as the persistence, by inheritance, of a
character that was a fundamental necessity in lower
forms. ^

360. The Seed. — During the processes of germination of
the pollen-grains and fertilization, the ovule is increasing
in size, and developing different tissues and juices. The
outer wall of the nucellus hardens, while the integument
becomes succulent and pulp-like, so that externally the
structure resembles a plum. It cannot, however, be com-
pared to a plum in morphology {i.e.j cannot be homologized
with a plum), for a plum is a ripened ovary, while the
so-called ''fruit" of the Cycads is a ripened ovule or
seed.

361. The Embryo. — After fertilization the oosperm
develops into an embryo-sporophyte (P'ig. 302). This is
often delayed until after the seed is planted, so that after
the embryo has once begun- to form it continues to grow,

^ The accomplishment of fertilization by the mediation of a pollen-tube
(siphon) is called siphonogajny.

SEED -BEARING PLANTS

407

without undergoing any resting period, until the new sporo-
phyte has emerged from the seed and become established

;.' d

. 'o,ao®

/

J

1

c t

Fig. 302. — Zamia jloridana. a, free nuclei of proembryo; X 16; b,
tissue at base of proembryo; X 24; c, diflferentiation into suspensor and
embryo; X 29; d, young embryo showing long suspensor, natural size.
(After Coulter and Chamberlain.)

in the soil as an independent plant (Fig. 303). In its early
stages the embryo derives its nourishment as a parasite
from the female gametophyte (prothallus, or endosperm).

4o8

STRUCTURE AND LIFE HISTORIES

362. Gymnospermy.- The fact that the seed is not
enclosed within the carpels, or walls of the ovary, but
continues wholly exposed throughout its existence, until

Fig. 303. — Cycas incdia. Germinating seeds, e, endosperm (gameto-
phyte); c, cotyledons; st, leaf-stalk; h, hypocotyl, showing early enlarge-
ment to form the thick trunk shown in Fig. 291.

shed, is one of the most significant of all the features of
the cycads. On this feature, and its opposite, the great
division of seed-bearing plants (Spermatophytes) is sepa-
rated into two classes as follows:

„ .1.1 Gymnosperms

Spermatophytes < . .

[ Angiosperms

The word gymnosperm means ''naked seed,"^ while

^ From the Greek, gymnos {yviivbs), naked -f- spernta {airkp/jia), seed.

SEED-BEARING PLANTS 409

angiosperm means "enclosed seed."^ The fundamental
distinction between gymnospermy and angiospermy was
first made clear by one of the greatest of English bota-
nists, Robert Brown, in 1827 (Fig. 10).

363. Comparison with Ferns. — The cycads resemble the
true ferns in several points — in the vernation of their
leaves (coiled in the bud), in the venation of the leaves
(forked veins), in the possession of sori (for microspor-
angia), in having multiciliate sperms, in having sporo-
phylls (megasporophylls) that closely resemble foliage
leaves, and in having the embryo dependent at first upon
the prothallus for nourishment, but later becoming
established as an independent plant.

They differ from ferns in having the non-green gameto-
phyte dependent for nourishment throughout its life
upon the tissues of the sporophyte. In the heterosporous
habit they differ from the true ferns, but resemble the
higher fern relatives, like Selaginella. Their greatest
step forward is the development of a seed. They are the first
true seed-bearing plants to be met with among living
species, as we ascend from the Algae. How closely
Selaginella approaches the formation of a true seed may
be seen by referring to the condition in the megaspor-
angium following fertilization (Fig. 284). If the female
gametophyte of Selaginella should remain within the walls
of the megaspore; if the embryo should undergo a period of
rest after the formation of the young stem and first leaves,
and if this entire structure should remain within the mega-
sporangium, we should have a true seed in Selaginella.

The cycad seed is primitive (or imperfect as a seed),

^ From the Greek, angcion {a-yyeiov), a vessel -}- sperma {a-irkpua),
seed.

4IO STRUCTURE AND LIFE HISTORIES

in frequently not having the embryo develop until after
the seed is planted, and in not having the embryo undergo
a period of rest between its formation and its final develop-
ment into a mature sporophy te ; the absence of an embryo
characterizes some of the fossil relatives of Cycas,

The genus Cycas is the only living genus of plants which
produces seeds w^ithout developing either a cone or a flower,
but produces the seeds on sporophy Us borne on I he main
stern. It is the simplest of all living seed-plants, to this
extent, being in this respect as lowly organized as the
ferns.

OUTLINE OF LIFE HISTORY OF A CYCAD

The Cycad plant
(Sporophyte)

Microsporophylls Megasporophylls

>l^ >!' ■A' -^

Microsporangia Megasporangia (ovules)

4, ^ — Reduction — > \.

Microspores Megaspores

4, 4.

Male gametophyte Female gamctophyte

(Pollen-grain) (Endosperm)

i i

Vestigial antheridium Archegonium

Generative cell

4.

Sperms Egg

Fcrtilizahuii

Oosperm

a

Embryo

New Cycad plant
(Sporophyte)

;on

1

SEED-BEARING PLANTS

411

364. Distribution. — The cycads and their relatives are
all tropical or subtropical. Four genera {MicrocycaSy
Zamia, Ceratozamia, Dioon) occur in the western hemi-
sphere, with the chief center of distribution in Mexico;
and five genera {Cycas, Bowenia, Macrozamia, Encephal-
artos, and Stangeria) in the eastern hemisphere, with

Fig. 303a. — Wilhelm Hofmeister (1824-1877). His comparativ^e studies
of the history of development of mosses, vascular cryptogams, and seed-
bearing plants, disclosed the fact of an alternation of generations through-
out those forms, and afforded the basis for a correct interpretation of the
genetic relationship of the great groups of plants.

the chief center of distribution for the first three in Queens-
land, Australia. Encephalartos and Stangeria are confined
to Africa.

CHAPTER XXVI

SEED-BEARING PLANTS (Continued)

GYMNOSPERMS

LIFE HISTORY OF THE PINE

Description of the Tree

365. The Trunk. — Everyone is so familiar with the
general features of pine trees as to render a detailed de-
scription unnecessary here. The main stem or trunk is
normally a straight vertical shaft, that may be traced from
the ground to the apex of the tree (Fig. 304). Such a
trunk is called excurrent, in contrast to the opposite type
which may be traced for only a short distance from the
ground, to a point where it divides into the main
branches or limbs. The latter type of trunk is termed
deliquescent (Fig. 305).

The excurrent trunk results from the fact that the
main stem, as well as the lateral branches, always has a
terminal hud, which carries the trunk upward from year
to year. Trees may continue to increase in height each
season, until they have reached the limit for the given
species. Many factors may, of course, modify the height,
among which is the limit of height to which the species
can raise sap. Some species of pine attain a height of
160 feet.

366. Branching. — At first glance pine trees appear to
bear their main branches in circles or whorls, but closer

412

SEED-BEARING PLANTS

413

lie. 304. -Glove ui puici bliowing excurrent trunks.

414

STRUCTURE AND LIFE HISTORIES

observation discloses the fact that these are really pseudo-
whorls, since the branches do not emerge at precisely the
same level on the trunk, as in true whorls. Both because
they are older, and because they receive less light, the

Fig. 305. — Elm tree {Ulnms americana), showing deliquescent trunk.

lower branches are longer, and the gradual decrease in
length from below upward, combined with the excurrent
habit of the trunk, gives the tree a pyramidal outline when
it grows free in the open. This shape is modified by

SEED-BEARING PLANTS 415

mutual shading and crowding, resulting in natural
pruning, when trees grow close together in the forest.

367. Long and Short Branches. — The pines, like several
other groups of plants, bear two distinct kinds of branches
— long branches, and short branches. The short branches
are often called " spur-shoots. 'V Long branches, in
addition to being longer and larger, commonly bear
only scale-like leaves, while the spur-shoots bear the
foliage-leaves (Fig. 308). In certain cases juvenile forms
of long branches are recognized, which bear foliage-leaves
as an exception to the general rule.

368. Leaves. — The long, needle-like leaves of the pine
are familiar to everyone. They occur on the spur-shoots
in groups or fascicles of one, two, three, or five, according
to the species. The spur-shoots are borne in the axils of
the scale-like leaves of the long shoots (Frg. 307). The
base of the leaf cluster is sheathed by the thin membranous
scales of the terminal bud of the spur. When there are two
leaves in a fascicle they are semi-circular in cross-section,
the adjacent faces being flattened as a result of their
contact in the bud; when there are three or more they
are triangular in cross-section. The white pine {Pinus
Strobus), and its nearest relatives, have five leaves to a
fascicle, the pitch-pine (P. rigida), and its nearest relatives,
bear the leaves in threes, the scrub-pines and the Euro-
pean pine {P. sylvestris), in twos.

369. Leaf -fall. — The duration of the leaves varies with
the species and the locality from two to ten years. As a
result there are always green leaves on the tree, whence the
term '' evergreen." That the leaves are shed may be
easily determined by examining the ground under any
pine tree. The duration of the leaves is also easily

4i6

STRUCTURE AND LIFE HISTORIES

ascertained by observing the oldest annual growth still
bearing spur-shoots. When the leaves are shed the
entire spur-shoot falls away.

Reproduction

370. Staminate Cones. — Most of the cone-bearing trees
(Coniferae) are monoecious, i.e., bear both microspores
and megaspores on the same tree. The staminate cones
appear in the spring, usually in May in the northern states,

Fig. 306. — Staminate cones of the Austrian pine {Pinus auslriaca).
Below, before shedding pollen; above, after shedding.

and persist for only a few weeks. They are borne in
clusters on the long branches on the current year's growth,
and occupy the lateral position of spur-shoots; they are
never terminal (Fig. 306). They are, in reality, modified

SEED-BEARING PLANTS

417

branches, consisting of the main axis, bearing the scales
(microsporophylls, or stamens) arranged in spirals. On
the underside of the microsporophyll are the spore-cases,
containing the microspores. The staminate cones pass the
winter inside the bud, and during this period the sporangia
contain only spore-mother-cells. The tetrad-division of

Fig. 307. — Scotch pine {Piniis sylveslris). Showing carpotropism of
carpellate cones. Young cones at the left; cones one year old at the right.
(Cf. Figs. 308 and 309.)

the spore-mother-cells, occurs early in the spring, the exact
time varying with the species, the locality, and the
character of the season, but the young microspores are
usually all formed by the first of May. On either side
of the main body of the microspore is a tiny air-sac which
gives the spore great buoyancy in the air (Fig. 313).
27

4i8

STRUCTURE AND LIFE HISTORIES

371. Carpellate Cones. — One or more young carpellate
cones appear near the tip of the new growth in early
sprinc: fFigs. 307-309), and are noticeable at that time

1

^•^

v-<\

\\A \i\ ,

/

\

,^

Ail

1

^

&

W

^.^/gmfM.

t

s

^ ^

Jl

IK

fl

3

lli

W

V

/1j\ \

\

M

\ ^

/K

/

Fig. 308. — Scotch pine (Pinus sylveslris). Branch bearing carpellate
cones, one month, one year, and two years old. (Cf. Figs. 307 and 309.)

from their delicate tint of red. They terminate short,
lateral, axillary branches. Like the staminate cones, the
carpellate cones are branches, modified for the purpose of

SEED-BEARING PLANTS

419

reproduction. The central axis bears lateral scales (Figs.
309 and 310), but the homology of these scales is difficult
to determine, and is still a matter of debate. There are

Fig. 309. — Scotch pine {Finns sylvestris). A-D, stages in the develop-
ment of the carpellate cone, and its carpotropic movements. E, very
young carpellate cone much enlarged; F, ventral, G, dorsal views of a
scale from E; 1, ovuliferous scale; 2, ovule (in longitudinal section); 3,
pollen chamber and micropyle leading to the apex of the nucellus (mega-
sporangium); 4, integument of the ovule; G, i, tip of ovuliferous scale;
5, bract; 4, integument; //, longitudinal section at right angles to the
surface of the ovuliferous scale (diagrammatic); 6, megaspore; 7, pollen-
chamber; /, longitudinal section of a mature cone; 6. ovule; /, scale
from a mature cone; 6, seed; ic, wing of seed; K, dissection of mature seed;
//, hard seed coat; c, dry membraneous remains of the nucellus, here folded
back to show the endosperm and embryo; e, embryo; p, remains of
nucellus; L, embryo; c, cotyledons; e, hypocotyl; r, root-end.

good reasons for considering that they are not simple spor-
ophylls, but are of a more complex character.

Each scale comprises a bract and an ovuliferous (ovule-
bearing) scale (Fig. 309). It is probable (but not certain)

420

STRUCTURE AND LIFE HISTORIES

that the ovuliferous scale represents two sporophylls
fused together.

372. Megasporangia. — As in Cycas, the megaspor-
angium is an ovule, comprising a nucellus (sporangium
proper), surrounded by an integument. At the apex the
integument does not come quite together, so that a

Fig. 310. — Median longitudinal section of a pine cone,
scale; ov, ovule at the base of a scale.

ovuliferous

tiny opening or pore (the micropyle) is left. The micropyle
leads to the pollen-chamber below (Figs. 309 and 311).
The megaspores are more backward in development than
the microspores, the spore-mother-cell not appearing until
June. Reduction by two divisions gives rise to either
three or four megaspores in a row, but only the basal one
ever germinates; the others become disorganized and
furnish nourishment for the one that germinates.

SEED-BEARING PLANTS 42 1

373. Female Gametophyte. — During the first season the
megaspore (Fig. 311) enlarges somewhat, and its nucleus
divides several times, forming free nuclei. In this condi-
tion it remains until the next season, when the formation
of the gametophyte is carried to completion. As the
gametophyte develops it feeds on the nucellus, which is
entirely consumed except for a thin membrane, which

Fig. 311. — White pine {Pinus Slrobus). At lef t, megasporangium with
megaspore in the center; above, pollen grains in the micropyle and pollen
chamber. At right, pollen grains beginning to germinate; the cells of
the integument hav^e enlarged and closed the micropyle. (After Margaret
C. Ferguson.)

adheres to the surface of the gametophyte or endosperm,
and a cap of tissue at the tip of the gametophyte (Figs.
309, K, and 312).^ The archegonia, tw^o to five in number,
at the micropylar end of the gametophyte, are mature by
the last of May or forepart of June, in the northern states.
In Pinus the neck of the archegonium is very much re-

^ In the seeds of some of the higher plants the tissue of the nucellus
becomes filled with nourishment stored for the use of the developing
embryo, during germination. It is then called perisperm.

422

STRUCTURE AND LIFE HISTORIES

duced, consisting usually, in the white pine, of only four
cells, all in the same plane; the number of these cells is
somewhat variable. No neck-canal cells are formed; only
the egg, and the ventral canal-cell (the sister-cell of the
egg) which disorganizes early (Fig. 312).

Fig. 312. — White pine {Pinns Strohus). \'ertical section through tlie
upper part of an ovule, shortly before fertilization, s.n, sperm-nuclei;
st.c, stalk-cell; t.n, tube-nucleus; arch, archegonium; e.n, egg-nucleus.
(After Margaret C. Ferguson.)

374. Male Gametophyte. — The germination of the
microspore consists chiefly of a series of cell-divisions, all
within the wall of the microspore. The first three
divisions result in the formation of four cells, namely, two
prothallial cells, one mother-cell of the antheridium, and a

SEED-BEARING PLANTS

423

larger cell, the tube-cell, composing the larger portion of
what is now the mature male gametophyte or pollen-grain.
Commonly one of the two prothallial cells disintegrates,
so that only one is visible (Fig. 313). Thus it is seen that
the vegetative portion of the male gametophyte is reduced
to nearly its lowest terms — only one or two cells of no
known function; the antheridium is also represented only
by its mother-cell. At this stage pollination occurs.

375. Distribution of Pollen. — In all cone-bearing trees
pollination is accomplished by the wind. At about the

Fig. 313. — The white pine {Pinus Strobns). Sections through mature
pollen grains; at the left the remnants of two prothallial cells can be seen,
while at the right all signs of the first cell have disappeared. Pollen col-
lected June 9, 1898. X about 600. (After Margaret C. Ferguson.)

time the pollen-grain is mature the axes of the carpellate
cones elongate, thus separating the scales from each
other (Fig. 309, E), At the same period the axes of the
staminate cones elongate, separating the anthers from
each other (Fig. 306). The sporangial walls now become
opened by a longitudinal slit, and the least jarring of the
branch is sufficient to shake out the dry pollen-grains,
which appear in countless millions as a fine yellow dust,
the ''pollen.''

376. Abundance of Pollen. — The pollen is so abundant
that it forms a really dense cloud that is easily seen in a
photograph of a tree shedding its polien (Fig. 314). The

424

STRUCTURE AND LIFE HISTORIES

pollen-grains are, of course, blown hither and thither with
every breeze, and millions of them never reach a carpellate
cone. The writer once found an accumulation of pine
pollen in a desk drawer that had remained constantly
closed (but in the vicinity of a pine tree) during the
season of pollination. A microscopic examination of

Fig. 314. — Shedding of pollen from a xoun- pine tree. Note the cloud
of pollen at the left, caused b}^ shaking the tree.

dust from ledges, indoors and out, at the pollen season,
will usually disclose one or more pollen-grains of pines
and other species.

377. Pollen and Coal-formation. — A microscopic ex-
amination of muck from the bottom of almost any in-
land lake will disclose the fact that it contains millions of
pollen-grains of various cone-bearing trees, and spores of

SEED-BEARING PLANTS

425

Fig, 315. — The organization of shallow water accumulations in a lake
of the present geological epoch, showing remains of roots, leaves, etc.
Near the center of the figure may be seen a cross-section of the triangular
needle of a white pine. (Cf. Figs. 316 and 317.) (After E. C. Jeffrey.)

Fig. 316. — Fine muck from the bottom of a pond, as seen under the
compound microscope, showing the presence of pollen grains of pine,
spruce, and fir. (Cf. Figs. 315 and 317.) (y^.fter E. C. Jeffrey.)

426 STRUCTURE AND LIFE HISTORIES

horsetails, and other cryptogams (Figs. 315 and 316). It
is instructive in this connection to recall recent careful
studies of the structure of coal as seen in transparent
sections by the aid of the microscope. These sections,
prepared by Professor Jeffrey, reveal the remarkable fact
that the soft, or bituminous, coals contain carbonized
remains of innumerable spores of the plants which con-
stituted the dominant vegetation during the geological

Fig. 317. — Photomicrograph of a thin section of cannel coal from
Kentucky, formed under open-water conditions, i.e., of the muck at the
bottom of ancient lakes or lagoons. The light, roundish bodies are spores.
(Cf. Figs. 315 and 316.) (After E. C. Jeffrey.)

period (Carboniferous), when coal was being formed (Fig.
317). Such studies necessitate a radical change in our
earlier conception as to the conditions and method of
coal-formation.

These facts also illustrate how the study of what might,
at first thought, seem insignificant, impractical, or unim-
portant, and not closely related to our daily lives, may,
at any time, furnish the key to unlock the mystery of

SEED-BEARING PLANTS 427

5ome very important fundamental fact or scientific
principle.

The formation of pollen in such abundance is one of
the numerous instances of the ''factor of safety" in plant
organization; and the necessity for it is recognized at
once when one considers the small chance that any given
pollen-grain will reach the pollen-chamber of an ovule.

378. Pollination. — Some of the pollen-grains, of course
reach the carpellate cones, which are usually situated
higher up on the tree and higher up on the individual
shoots, than are the staminate cones. This location is
an advantage, because the light pollen-grains, specially
buoyant because of their two air-sacs, readily float up-
ward. Those that reach the carpellate cones, fall between
the ovuliferous scales, and settle down to the bases of
the scales. Some of them get caught in the sticky fluid
that fills the pollen-chamber at this time, and as the fluid
dries up the grains are drawn close down to the tip of the
nucellus (Fig. 311). Here, as always, the deposit of pollen
on the surface where it is to germinate is called pollina-
tion. PoUination in Pinus occurs in late May or early
June, depending on the species, the locality, and the nature
of the season.

379. Nodding of the Cone. — Soon after pollination the
stalk of the carpellate cone, in most species, changes its
relation to gravity, becoming negatively geotropic. One
side grows more rapidly than the others, thus causing the
cone to nod and hang pendant (Figs. 307 and 308). This
position it retains throughout the remainder of its life.

380. Germination of the Pollen-grain. — Very soon after
pollination, the tube-cell begins to develop a pollen- tube,
which secretes an enzyme that dissolves the cell-walls and

428

STRUCTURE AND LIFE HISTORIES

contents of the nucellar tissue, thus facilitating the
passage of the dehcate tube. The dissolved con-
tents nourish the growing tube, which at first serves
as a haustorium to absorb the
nourishment. Thus the male (as
well as the female) gametophyte,
lives as a parasite upon the tissue
of the sporophyte.

Soon after the pollen-tube has
begun to form, the tube-nucleus
moves down toward the tip,
where it remains, presiding over
the subsequent growth of the tube
(Fig. 318). At about this time,
also, the antheridial mother-cell
divides, forming a wall-cell^ and
a generative cell. In this condi-
tion the first winter is passed.
After pollination the carpellate
scales, by growth, are brought
„ „„ . . close together, and secrete a very

Fig. 318.— White pine . . ^

(Pinus Strobus). Germi- sticky, resmous substancc, all of
T%f"t2-l7; which very completely excludes
g.c, generative cell; /.», any water from between the scales.

tube-nucleus. X about „, . . , .

236. (After Margaret c. The cone then mcreases greatly m

Ferguson.) gj^^ (^ig. 309).

381. Fertilization. — Early in the following spring (May-
June), the generative cell, after passing into the pollen-
tube, divides to form two sperm-cells, and the pollen-tube
continues its growth toward the archegonia. By this
time (June) the egg lies mature within, and completely

^ Also commonly called ''stalk-cell."

SEED-BEARING PLANTS

429

filling, the venter of the archegonium. The pollen- tube
passes between the neck-cells of the archegonium, but
does not ordinarily enter the venter. The apex of the
tube is ruptured, probably by internal osmotic pressure,
and its entire contents are emptied into the cytoplasm
of the egg. One of the sperm-nuclei unites with the
egg-nucleus (June of the second season), and fertilization
is accomplished (Fig. 319).

Fig. 319. —White pine {Pinus Sir oh us). Longitudinal section through
an archegonium at the time of fertilization. Above the -fusing nuclei are
various other elements emptied into the egg from the pollen-tube. Col-
lected June 21, 1898. X about 62. s.g, starch grains; p.r, prothallium;
c.p.t, cytoplasm from pollen-tube; st.c, stalk-cell; t.n, tube-nucleus; s.n,
sperm-nucleus; e.n^ egg-nucleus; n.s, nutritive spheres. (After Margaret
C. Ferguson.)

382. Formation of the Seed. — After fertilization the
oosperm begins at once to develop, giving rise to three
distinct structures; the pro-embryo, the suspensor, and
the embryo-sporophyte. During the early divisions of
the fertilized egg the male and female chromatins can be
clearly distinguished (Fig. 320). At the same time the
adjacent tissues of the ovule become transformed. A
portion of the prothallus or gametophyte nourishes the
developing embryo, but the large bulk of it becomes

430 STRUCTURE AND LIFE HISTORIES

Stored as endosperm around the embryo. The remains
of the nucellus persist as a thin membrane surrounding
the endosperm (as mentioned above), the integument of
the ovule develops into the seed-coat, and from the integu-
ment there also develops a long, thin, membranous wing.
383. Seed-dispersal. — From the above description we
learn that it takes about a year and a half to make a pine
seed. When the seeds are mature, the scales of the car-

■/^^tOKiVii'^-

Fig. 320. —White pine {Pinus Strohus). Late prophase in the first
nuclear division of the fertilized egg. The nuclear membrane has disap-
peared, and the chromatin from both egg and sperm may still be dis-
tinguished. X about 236. (After JMargaret C. Ferguson.)

pellate cone, which have now become large and woody,
spread apart (Figs. 308 and 309, D), and thus permit the
loose seeds to fall out. By means of the membranous
wing, the seeds are easily dispersed by the wind.

384. Germination of the Seed. — The seeds usually do
not germinate until the spring after they are dispersed,
or two years after pollination. Under suitable condi-
tions of environment the hypocotyl elongates, forming an
arch, and drawing the cotyledons out of the ground,
while the tap-root develops from the opposite end. By
the straightening of the arch the green cotyledons are
lifted into the air and light, the hypocotyl elongates, the
root-system begins to develop, and thus the seedling
sporophyte becomes established as an independent plant.

SEED-BKARING PLANTS

OUTLINE OF LIFE HISTORY OF PINUS

Tile pine tree
(Sporophyte)

431

Carpellate cone
(Modified branch)

ii

Ovuliferous scale
(Homology obscure)

4.4.

Ovule
(Megasporangium)

ii

Megaspore-mother-cell

•4' ^ Reduction

Megaspore

I

Embryo-sac
(Female gametophyte)

i

Archegonium
Egg

TT

Staminate cone
(Modified branch)

4-4-

Stamens
(Microsporophylls)

4' 4'

Anthers
(Microsporangia)

Microspore-mother-cell

^ 4.

Microspore

4.

Pollen-grain
(Male gametophyte)

4'

(Antheridial cell)

Sperm

Oosperm

4.4.

Embryo (In a seed^
(Zygote)

4.4.

Embryo-rests

4-i

Germination of seed

4-4-

Mature sporophyte (The i)ine tree)

385. Comparison with Lower Forms.— It will be spe-
cially instructive, at this point, for the student to make a
careful comparison of the life history of Finns with the
lower forms, such as Cycas, Selaginella, Lycopodium, EquU
setum, and a true fern, noting especially the relative impor^

432

STRUCTURE AND LIFE HISTORIES

tance, in the ascending scale, of gametophyte and sporo-
phyte, and the evidences which point to a possible mode of
origin of the vegetative body of the sporophyte. A funda-
mental question, still being debated by botanists, is
whether, in racial history, the vegetative (sterile) tissue
appeared first, and the sporogenous (fertile) tissue later,

Siaqe

Fig. 321.^ — Diagram of the life-cycle of a pine. (After SchaEfner.)

or whether the fertile tissue appeared first. The question
may be put as follows: Is the cone (strobilus) the older
structure in the development of the race, and are the vege-
tative parts to be regarded as developed from it by pro-
gressive sterilization? It is better that this question be
merely stated here, as one of the larger, fundamental
problems of botany, and that the pleasure of discussing
it be reserved for the teacher and students together, in
connection with a later chapter. (Cf., also, p. 379.)

CHAPTER XXVII

SEED-BEARING PLANTS (Continued)

LIFE HISTORY OF AN ANGIOSPERM

386. Variations in Life Histories. — In the groups pre-
viously studied there is a marked uniformity or similarity
in the life histories, making it comparatively simple, once
one has the key, to interpret the structures involved.
Even as we pass from one group to the next, homologies
are detected without serious difficulty. Under various
more or less transparent disguises, we have been able to
trace such structures as the sporophyll, spore-case,
microspore and male gametophyte, megaspore and female
gametophyte, and, in the latter, the archegonium, egg, and
embryo. But studies of the highest group of plants, the
Angiosperms, soon lead us into difficulties not readily
overcome. It is not difficult to detect the sporophylls,
spore-cases, megaspores and microspores; but, just as
among the Gymnosperms the antheridium had dis-
appeared as a distinct, fully developed organ, so among
the Angiosperms the archegonium has disappeared, and
certain new structures have made their appearance. Thus
it is not possible to choose from the Angiosperms any
actual plant whose life history, in detail, is typical of the
entire group. For external features any one of several
plants might be chosen to illustrate the floral organs
commonly met with. The life history of the yellow
28 433

434

STRUCTURE AND LIFE HISTORIES

adder's-tongue, or dog's-tooth violet, illustrates the
essential points for Angiosperms.

387. Dog*s-Tooth Violet. i— The dog's-tooth violet {Ery~
thronium americamim) belongs to the order which includes

Fig. 322. — Dog's-loolh violet {Erylhronium amcricanum). Stages of
deveopment from the seed. 1-5 show the stage of development in each
of live successive years. Full explanation in the text. 6, Bulb showing
a surface bud (the sprout has been destroyed). (After F. H. Blodgett.)

the hlies (Liliales), and its structure is quite typical of
that order (Fig. 322). Its stem is a small, underground,
scaly bulb, giving rise to numerous roots. From the upper

^ The dog's-tooth violet is really not a violet at all, the common name,
as frequently, having no regard to botanical relationships. John Bur-
roughs has suggested that "fawn-lily" would be a much more appropriate
name. But common names of plants and animals are, fortunately, not
easily changed.

SEED-BEARING PLANTS 435

surface of the stem arise two smooth leaves, with a shiny,
but mottled surface, and acute apex. The petioles sheathe
the base of a flower-stalk {scape), which also arises from
the upper surface of the bulb. At the tip of the flower-
stalk is the solitary flower.

388. Blossoming. — Early in the spring the flower-stalk
begins to elongate rapidly until it has developed into a
long, slender, unbranched stem, the scape, bearing at its
tip the flower-bud, raised several inches above the ground,
and soon expanding into a flower. During the forma-
tion of the flower-bud, in the preceding autumn, the outer
surfaces of the floral envelopes grew more rapidly than the
inner surface, thus causing the formation of the bud. The
opening of the bud is caused largely by the more rapid
growth of the inner surfaces of the floral envelopes.

389. Structure of the Flower. — In Erythronium (or any
liliaceous plant; cf. Fig. 323) we may recognize all the parts
of a complete flower, as follows:

1. An outer circle of three sepals, together constituting
the calyx.

2. An inner circle of three petals, alternating with the
sepals, and together constituting the corolla. The sepals
and petals in Erythronium look very much alike, but each
petal has a nectary, or gland secreting nectar, at its base.
The calyx and corolla together constitute the perianth.

3. Two inner circles of microsporophylls, the stamens,
three in each circle, one opposite each sepal, and one
opposite each petal. All the stamens, taken together
constitute the androecium.*

* In Erythronium three of the six stamens are frequently noticeably
shorter than the other three, and mature their pollen later. This is ex-
ceptional in the Lily family, to which Erythronium belongs.

436 STRUCTURE AND LIFE HISTORIES

Each stamen consists of a slender stalk, the filament,
bearing at its tip two microsporangia {pollen-sacs) ,
united to form the anther. In the pollen-sacs are numer-
ous microspores, which finally develop into pollen-
grains.

Fig. 323. — Wood lily {Lilium philadelphicum).

A central organ, the pistil, composed of three mega-
sporophylls (carpels) united, and enclosing the mega-
sporangia (ovules). All the pistils taken together (one or
more) constitute the gynoccimn.

The pistil consists of three distinct regions as follows:

(a) The enlarged basal part, enclosing the ovules,
and hence called the ovary.

(b) A slender upward prolongation of the ovary,

SEED-BEARING PLANTS

437

called the style. Through the center of the style extends
a tiny canal, so that the style is hollow. The walls of this
canal, in Erythronium, are lined with a glandular layer of
cells forming the conducting tissue, which serves to nourish
the pollen- tubes (Fig. 324). An expansion of this tissue is
exposed at the tip of the style, forming the stigma, or surface
for the reception of the pollen-grains in polHnation. The
conducting tissue extends continuously from the stigma
down through the style to the placenta, or point of attach-
ment of the ovules.

Fig. 324. — ErytJironium americannm. i, longitudinal section of hollow
style, showing glandular cells of the conducting tissue lining the canal;
2, microspore; 3, young pollen grain (male gametophyte), showing gen-
erative nucleus and tube nucleus; 4, pollen-grain germinating, i and 4
are from E. albidum. (After J. H. Schaflfner.)

In many plants the style is not hollow, and the con-
ducting tissue completely fills the center. The stig-
matic surface secretes a sticky substance by which the
pollen-grains are held fast. In some species of plants
this surface is covered with tiny hairs, by which the
pollen-grains are held until they germinate.

390. Pollination. — The flower of Erythronium stands
out in sharp contrast to that of the gymnosperms, in two
respects, namely, the occurrence of both stamens and
carpels in the same flower, and the possession of a con-
spicuous, colored perianth. The significance of the

438 STRUCTURE AND LIFE HISTORIES

perianth can be understood only in connection with
pollination. It will be recalled that in the gymnosperms
the pollen is transferred by wind, but in Erythronium
this transfer is accomplished by means of insects. The
perianth, conspicuous to us by its petals, appears to at-
tract certain insects. Whether this is accomplished by
color, or by odor, or by some other means not clearly
demonstrated, is not absolutely known. It is generally
believed to be by color, but certain experiments seem to
disprove this theory. Be that as it may, we know that
the development of a conspicuous perianth appeared in
the same geological age (Cretaceous) as did the more
highly developed, winged insects, such as the butterflies
and moths. In fact the insects probably appeared some-
what earlier than the *^ flowers."

Attracted to the flowers by whatever means, the insect
finds, at the bases of the petals, nectar secreted by glands.
While feeding on the nectar the back of the insect becomes
dusted over with pollen from the anthers. When he
flies to another flower some of this pollen is rubbed off
on to its stigma, thus accomplishing pollination.

391. The Male Gametophyte. — The young pollen-grain
has already been recognized as a microspore. In some
species it develops into a male gametophyte before pollina-
tion, in other cases not until afterward. In either case
the gametophyte is very greatly reduced. The mature
pollen-grain of the milkweed, for example, is a mature
gametophyte, having the sperm-cells formed at about the
time the flower-buds open. In Erythronium the genera-
tive cell, formed between December i and April i, does
not divide to form the sperm-cells until after pollination,
and after the pollen-tube has begun to form (Fig. 324).

SEED-BEARING PLANTS

439

392. The Female Gametophyte. — Megaspores are
probably not formed in tetrads by two divisions of a
megaspore-mother-cell, but the ancestral-cell of the female

Fig. 325. — Dog's-tooth violet {Erylhronium). i, embryo-sac, binucle-
ate stage, the two nuclei dividing; 2, four nucleate stage; 3, mature embryo-
sac {es)) the pollen-tube {p.t) has reached the egg-apparatus and fertiliza-
tion has just taken place; the male and female nuclei are both visible in
the fertilized egg. The synergids are not shown; 4, polyembryony; four
embryos have developed from the embryogenous tissue, one embryo at
the left of a not shaded; 5, longitudinal section of an embryo {em); s,
suspensor; i, 2, and 5, E. albidum (after Schaffner); 3 and 4, E. americamim
(after Jeffrey). Only one of the multiple embryos persists.

gametophyte {archesporial cell) functions directly as a
megaspore, without division. In very early spring the
megaspore begins to germinate, increasing in size, and

440

STRUCTURE AND LIFE HISTORIES

its nucleus undergoing divisions, forming, in succession,
a two-celled, four-celled, and eight-celled embryo-sac
(Figs. 325 and 326). Three of these cells pass to one
end of the sac, opposite the micropyle, and are known

l>0 St is

Fig. 326. — At the left, diagram of the anatomy of an angiospermous
flower shortly after pollination; anlh., anther; fd., filament; st., stamen;
slig., stigma; p.g., pollen grains germinating; sly., style; pL, pollen tube;
o.'co., ovary wall; o., ovule, containing embryo-sac; pet., petal; sep., sepal.
1-8, Stages in the development of the female gametophyte (embryo-sac);
meg.sp., megaspore-mother-cell; i.i., inner integument; o.i., outer integu-
ment; fun., funiculus; dial., chalaza; mi., nucellus (megasporangium);
emb., embryo-sac. All diagrammatic.

as the antipodal cells, or antipodals, while two of them meet
in the center and fuse, forming the endosperm-nucleus.
The remaining three pass to the end near the micropyle,
where one of them becomes organized as the egg-cell; the
others are called synergids (helpers) (Cf. Fig. 326).

SEED-BEARING PLANTS

441

393. Fertilization and Formation of Embryo. — After
pollination the pollen-grain, stimulated by the sticky
substance secreted by the stigma, begins to develop a
pollen-tube, which passes down the canal that extends
through the style from stigma to placenta, nourished by
the specialized cells that line the inner surface of the

^k

i M

1

0 H^^

w>^

> 1^^^

f)~''!

\X»

W "'

^*

^

f 1

^

^ 1

3

, V,

Fig. 327. — Lilium Martagon. Longitudinal section of the stigma and
upper part of the style. The pollen-grains, caught on the papillae of the
stigma, have germinated, and the pollen-tubes are growing down along
the walls of the style-canal, nourished by the specialized cells that line it.
(After Dodel-Port.)

canal (Cf. Figs. 324 and 327). When the tip of the tube
has passed through the micropyle and into the embryo-sac,
the sperm-cells, formed during the growth of the tube, pass
out into the protoplasm of the embryo-sac, and one of
them fuses with the egg-cell, thereby accomplishing fer-
tilization (Fig. 328. Cf. Fig. 325). In some cases the
embryonic tissue that arises from the fertilized tgg gives

442

STRUCTURE AND LIFE HISTORIES

rise to as many as four embryos (Fig. 325), but usually
only one of them develops.

394. Formation of the Seed. — While the fertilized egg
is developing into the embryo, the endosperm-nucleus

Fig. 328. — Lilium canadense. Embryo-sac at the time of fertilization;
a\ a^, antipodal cells; dn., endosperm-nucleus; pi., remains of pollen-tube;
e.n., egg-nucleus; s.n.^, sperm-nucleus, fusing with the egg-nucleus; s.n.^,
second sperm-nucleus, which may later fuse with the endosperm-nucleus,
thereby accomplishing double fertilization. (Redrawn from camera
lucida drawing by O. E. White.)

is undergoing successive, rapid divisions, which finally
result in the formation of an abundance of starchy endo-
sperm, surrounding the embryo, and serving to nourish it

SEED-BEARING PLANTS 443

when the seed germinates.^ At the same tmie the integu-
ments of the ovule develop into a hard, horny seed-coat.
This kind of seed-coat is characteristic of the Liliaceae.

395. Germination of the Seed. — The seeds are mature
by about June, and lie on the ground dormant until the
following April, when they germinate. Both ends of the
embryo elongate, absorbing all the endosperm for nourish-
ment. By about the time that older plants are blossom-
ing, the young seedling has reached the stage shown in
Fig. 322,2. At one side, near the end of the hypocotyl,
there develops a root, and the tip becomes enlarged into
a bulb by the storage of starch, manufactured by the
green, cylindrical seed-leaf. Within this bulb the first
bud {plumule) develops, the seed-leaf withers, and the
young seedling remains in this condition during the
following winter.

396. Formation of Flower Bulb. — In the spring of the
second year several runners develop from the first-formed
or plumule-bulb, and at their tips bulbs also form, called
runner-bulbs. From each of the runner-bulbs, three more
runners, with bulbs, are produced, and one of these bulbs,
under favorable conditions, produces a flowering plant.
It takes at least four years to produce a bulb that will
develop a flowering plant.

''The following table illustrates the number of plants
of different ages during each of live years, supposing that
five seeds from each fruit ripen and survive the cycle, and
provided that all fourth year bulbs produce flowers."^

^ It will 6e instructive for the class to discuss the origin and mode of
formation of the starch in the endosperm.

2 Quotation and table from Frederick H. Blodgett, Bull. Torrey Club
27:307-308. 1900.

444

STRUCTURE AND LIFE HISTORIES

ist year

2d year

3d year

4th year

5th year

5 seeds

5 seeds

5 seeds

5 seeds

5 seeds

5 plumule-

5 plumule-

5 plumule-

5 plumule-

bulbs

bulbs

bulbs

bulbs

5 yearlings

5 yearlings
15 two years
old

5 yearlings
15 two years

old
45 flowers

397. Annual Bulbs. — At the base of an old flowering
bulb, and in the axil of one of the bud-scales, there de-

FiG. 329. — Diagram of life-cycle of an angiosperm (Alisma Planlago-
aquatica). 9, female gametophyte (embryo-sac); 8a and 9a, male gameto-
phyte (pollen-grain). (After J. H. Schaffner.)

velops each year an annual bulb, which attains full size
about April, and begins in May to form the bud for the
leaves and flowers of next spring.

SEED-BEARING PLANTS 445

If one digs up a bulb in the fall, he will find the flower
perfectly formed, and ready to be raised above the ground
the following spring. All the parts of the flower, at this
period, are white, on account of having been formed in
entire darkness; and they are also quite brittle.

When the buds resume their growth the second spring
they push up through the ground quickly, and with con-
siderable force. The pointed end of the sprout is covered
by a mass of hard tissue, which protects the more delicate
cells below from injury. The well-protected tip, and the
growth-force, enable the sprout to pierce even small
twigs. This has given rise to the striking name of 'Vege-
table awl."

As soon as the sprout is well above the surface of the
ground the flower bud becomes free from the parts that
enclose it, and expands into the nodding blossom; pollina-
tion is accomplished, and the life-cycle begins again.

The life cycle of another angiosperm, the water-plantain
{Alisma Plantago-aquatica) , is indicated in Fig. 329.

CHAPTER XXVIII

SEED -BEARING PLANTS (Continued)

ANGIOSPERMS

398. Essentials of a Flower. — Reduced to its lowest

terms, a flower is a branch bearing sporophylls. The

Fig. 330. — Inflorescences of Job's tears {Coix lacrima-Jobi), one of the

Gramineae.

latter may be microsporophylls only, as in the staminate
cone of Pifiiis (Fig. 306), which is thus seen to be, in

446

SEED-BEARING PLANTS 447

reality, a flower; or they may be megasporophylls. In
the latter case they may occur on the main stem, as in
Cycas (Fig. 291), or grouped on a specialized branch,
forming a cone, as in Macrozamia (Fig. 289).^

399. Perfect and Imperfect Flowers. — A flower having
stamens but no carpels, or carpels but no stamens i?

Fig. 331. — Flowers of a tuberous begonia; staminate above; pistillate
below; one of the latter with the perianth removed to show the ovary and
stigmas. (Photo by Elsie M. Kittredge.)

unable, by itself, to produce seed, and is hence called an
imperfect flower (Figs. :^2)^-2)?>s) - A species in which the
imperfect flowers occur on separate plants is dioecious.

^ Whether the carpellate cone of Finns is a flower or a cluster of flowers
(inflorescence), has long been debated. There is strong evidence for con-
sidering it a cluster of flowers, since the individual scales are probably not
simple sporophylls. (Cf., p. 419.)

448

STRUCTURE AND LIFE HISTORIES

Such is the case in the willow, hop, ailanthus and, of course,
the cycads. When the staminate and pistillate flowers
occur on the same plant, either on the same branch or
axis, as in cat-tail, "Job's tears," begonia, et cetera (Figs.

Fig. 332. — Inflorescences of the birch {Betula sp.). Below, the staminiiU'
flowers in large, pendant catkins; above, the pistillate catkins, erect.

330-332, 375) or on separate branches, as in Indian
corn, arrow-leaf, and others (Fig. 333), the species is
monoecious.

Since stamens and pistils are necessary to the formation
of seeds they are called the essential organs of the flower.
A flower like the tulip, rose, water-arum, or buttercup
(Fig. 345), having both kinds of essential organs, is a
perfect flower.

SEED-BEARING PLANTS 449

400. Complete Flowers. — In the majority of Angio-
sperms the flower, in addition to the essential organs,

Fig. 333. — Arrow-leaf (Sagiltaria). At the left, branches with staminate
flowers only; in the middle, branches with pistillate flowers only; at the
right, pistillate branch, bearing fruit.

possesses a calyx or a corolla, or both. Flowers possessing
both kinds of floral envelopes and both kinds of essential
organs are called complete.
29

450

STRUCTURE AND LIFE HISTORIES

Fig. 334. — Fruit of the tomato {Lycopersiciim esciilentum) . A berry,
showing seeds (ripened ovules) attached to the placentas, and inclosed in
the tissue of the ripened ovary {i.e., angiospermous).

Fig. 335. — Rosa rugosa, at left; CratcBgus punctata, at right. The
fruit is composed of the ripened ovary, reinforced by the enlarged
receptacle.

SEED-BEARING PLANTS 45 1

401. Essentials of a Fruit. — In the Gymnosperms we
found the seeds unprotected on the surface of the mega-
sporophyll or carpel; but in the Angiosperms the ovules
are produced in a closed ovary composed of one or more
carpels (Fig. 334). As the ovules ripen into seeds the
carpels and surrounding parts ripen into the fruit. In
some cases the fruit consists only of the ripened ovary
(Fig. 334) while in other cases it may comprise the enlarged
calyx and receptacle also (Fig. 335).

402. Immediate Effect of Pollen.— The effect of the
germinating pollen in stimulating the growth of the ovary
and adjacent tissues is a very interesting phenomenon.
A portion of the edible part of the fruit of apples is calyx,
which has developed into fleshy tissue as a result of
the stimulus of the pollen; in the case of pears the re-
ceptacle and end of the peduncle become fleshy and form
a part of the fruit; most of the strawberry fruit is the
common receptacle of the small flowers, stimulated
to a fleshy development by the growth of the pollen
on the stigmas; in the watermelon, orange, tomato, and
many other plants, it is the ovary alone that is thus
stimulated.

The immediate effect of pollen is often greatly increased
by cross-pollination. This is strikingly shown in the
blueberries (Vaccinium), as shown in Fig. 336. The
two twigs "grew in equally good situations on the same
bush, contained the same number of flowers, all pol-
linated by hand with equal care, and the fruits were pol-
linated on the same day. The only difference in treat-
ment was that the pollen used on the left-hand twig came
from other flowers on the same bush, while the pollen for
the right-hand twig was taken from another bush."

452

STRUCTURE AND LIFE HISTORIES

I

J

^

r

J^

■1

^^^i^^'^ii^^

'4

r ^\

i

r.

?

-^^ ^ J ^l^v.

1

1

\

1 '

Fig. 336. — Effect of self-pollination in the blueberry {Vacciniiim corym-
bostim), as compared with cross-pollination. These two twigs, both
natural size, were in equally good situations on the same bush, contained
the same number of flowers, all pollinated by hand at the same time with
equal care, and the fruits were photographed on the same day. The only
difference in treatment was that the pollen used on the left-hand twig
came from other flowers on the same bush, while the pollen for the right-
hand twig was taken from another bush. The cross-pollinated flowers
produced a full cluster of handsome fruit. The self-pollinated flowers
produced no ripe fruit, all the fruit that set remaining small and green and
later dropping off, until at the time the photograph was taken only two
such imperfect fruits remained. A plantation made up wholly from cut-
tings from a single bush would produce little or no fruit. At least two
original propagation stocks are necessary. (After Coville. Courtesy of
the U. S. Dept. Agric.)

SEED-BEARING PLANTS 4^3

403. Essentials of a Seed.— In the cycads and pines
we have seen accomplished the first step necessary for the
production of a true seed, namely, the retention within
the sporangium of the megaspore and the female gameto-
phyte to which it gives rise. The final step was the forma-
tion of an embryo, which usually rests before proceeding
to develop into an adult sporophyte. With few excep-
tions, the distinctive feature of a seed is a resting embryo.
The embryo may or may not be surrounded by nourish-

FiG. 337— John Ray (1628-1 705). An early and noted English botanist.
First to distinguish monocotyledons from dicotyledons.

ment stored in the form of endosperm. In the absence
of endosperm, as for example in the bean seed, the nour-
ishment is stored in the cells of the embryo itself, having
been absorbed while the embryo was forming. Enclosing
the other parts of a seed is the seed-coat which may be
derived from one integument (as in Pinus) , or from two
integuments organically united. These features are illus-
trated in Fig. 83.

404. Monocotyledons and Dicotyledons. — Except in
rare cases, all plant-embryos possess either one or more

454

STRUCTURE AND LIFE HISTORIES

''seed-leaves," or cotyledons, and on this basis Ray
(1628-1705), the noted English botanist, divided his two
major groups, flowering herbs (herbcE perfectce) and trees,

Fig. 338. — Morphology of typical monocotyledonous plant. A^ leaf,
parallel- veined; B, portion of stem, showing irregular distribution of vas-
cular bundles; C, ground plan of flower (the parts in 3's); D, top view of
flower; E, seed, showing monocotyledonous embryo.

into two sub-groups monocotyledons and dicotyledons}

These two groups are distinguished by other characters

^ Plants like Pinus having more than two cotyledons are polycotyledons.

SEED-BEARING PLANTS

455

which are quite constantly associated with the possession
of one or two cotyledons. Thus, in monocotyledons the
leaves are, with rare exceptions, parallel-veined, and the
growth of the stem is endogenous; while in dicotyledons the

Fig. 339. — Morphology of a typical dicotyledonous plant. A, leaf,
pinnately-netted veined; B, portion of stem, showing concentric layers of
wood; C, ground-plan of flower (the parts in s's); D, perspective of flower;
E, longitudinal section of seed, showing dicotyledonous embryo.

leaves are usually net- veined, and the stem exogenous. In
monocotyledons, also, the parts of the flower usually occur
in threes (as in Erythronum) , or in sixes, never in fives, while

456 STRUCTURE AND LIFE HISTORIES

in dicotyledons the parts are typically in Jours or jives.
These characters are illustrated diagrammatically in Figs.
338 and 339.

405. Groups of Dicotyledons. — There are two main
groups of dicotyledons, based on the fusion or non-fusion
of the parts of the calyx or corolla, as follows:

{A h' hi 1 [ ^P^^^^®

\ Polypetalae
Metachlamydeae Sympetalae (Gamopetalae)

The distinction between Archichlamydeae and Sym-
petalas is not absolute, since each group contains plants
having some features characteristic of the other. The
Apetalas, as the name suggests, are without corolla (in
some cases without either calyx or corolla) ; the Poly-
petalae have sepals and petals (one or both) entirely
distinct; while the Sympetalae have the sepals and the
petals wholly or partly united so as to form a tubular calyx
or corolla. In the light of our preceding study of the
fruiting branch or '^flower" of Gymnosperms, it will be
readily understood that flowers of simple structure are
presumably more primitive than those of more complex
structure. The simplest flower we can imagine is an
apetalous staminate flower of one stamen ; or an apetalous
pistillate flower of one simple pistil. Polype talous flowers
are more highly organized than apetalous, and may
therefore be less primitive; sympetalous flowers are
more complex or more highly organized and are therefore
less primitive than either Apetalae or Polypetalae.

The following examples will serve to illustrate 1 5 of the
more common or more familiar families of Dicotyledons,
Dut of a total of over 250,

SEED-BEARING PLANTS

ARCHICHLAMYDEJE

Apetal^

457

406. Willow Family (Salicaceae). — This family com-
prises both the willows (Salix) and the poplars {Populus).
There are about 20 species of poplar in North America.
The willows, in North America, comprise over 30 species,

«. !

W^

^■■W

'^^^^''■^M

'«5p/ ■

^^flf- ^^- '-^ssm- li

1^^

Fig. 340. — Inflorescence of a willow (Salix discolor). At the left pistillate,
at the right staminate catkins. (Photo by Elsie M. Kittredge.)

trees and shrubs, and are specially common in moist
situations. Several species are cultivated to furnish
twigs for basket-making. The weeping willow {Salix
bahylonica), an object of peculiar beauty along streams
and lakeshores, owes its pendulous or "weeping" character

458 STRUCTURE AND LIFE HISTORIES

to its failure to develop sufficient mechanical tissue (wood)
in its smaller branches to hold them erect.

All willows are dioecious. The imperfect, apetalous
flowers occur crowded together on scaly spikes called
catkins (Fig. 340). Each scale bears one flower in its axil.
The staminate flowers consist usually of two (sometimes

Fig. 341. — Willow (Salix exigua Nutt.) Leafy branch, bearing two
pistillate catkins. Staminate flower above, at the left; pistillate flower
below, at the right. (After Britton and Brown.)

three to ten) stamens (Fig. 341). In some species the
stamens are united. When the flower buds open, early
in spring, the numerous hairs on the scales or filaments
(one or both) give the soft, fur-like appearance, which
suggested the name ''pussy-willow." Though a perianth
is wanting pollination is accomplished by insects.

SEED-BEARING PLANTS

459

407. Lizard's-tail Family (Saururaceae). — The lizard's
tail {Saururus cernuus) , typical of this family, has apetalous
but perfect flowers, borne sessile on a moderately long,
common axis, called a spike (Figs. 342 and 343). There
are three to five more or less united ovaries. The plant
is commonly found in swamps and marshes.

Fig. 342. — Lizard's-tail {Saururus cernuus).

POLYPETAL^

408. Crowfoot Family (Ranunculaceae). — The Latin
name Ranunculus (little frog) was applied to the butter-
cups by the ancient Roman naturalist Pliny, because they
are common in wet places where frogs abound. The name
of the family is derived from this generic name, but the
family comprises about 35 genera, 28 of which occur in

460 STRUCTURE AND LIFE HISTORIES

Eastern North America. The yellow water-crowfoot
{Ranunculus delphinifolius) is one of the commoner of the
40 to 50 North American species. It is found at the bor-

FiG. 343. — Lizard's-tail {Saururus cernmis). Inflorescence, about
natural size.

ders of quiet water, and the submerged leaves are iiliformly
dissected, in marked contrast to those borne in the air.

SEED-BEARING PLANTS 46 1

The bright shining, yellow petals vary in number from
five to eight, and are much longer than the sepals; each
has a little scale at its base concealing a nectar-gland. The
simple pistils {carpels) are grouped in a round head,
surrounded by the numerous stamens (Figs. 344 and 345).

Fig. 344. — Plant of a buttercup {Ranunculus sp.). (Photo by Elsie M.

Kittredge. )

409. Spiral and Cyclic Arrangement. — It will be re-
called that in the lower type of flower, characteristic of
the Gymnosperms, the sporophylls are arranged in spirals
on the flower axis. A study of the flower in Angiosperms
discloses a tendency for the flower parts to occur in
circles; the higher the plant in the system of classification
the more completely is the cyclic arrangement realized.
In the Crowfoot family there are some species, especially

462

STRUCTURE AND LIFE HISTORIES

of Ranunculus, in which the sepals and petals are in circles,
while the stamens and pistils are in spirals — a more
primitive feature.

410. Petalody of Bracts. — It is also common in certain
species of this family (notably in the genera Trollius and

Fig. 345. — Flower of a buttercup {Ranunculus sp.); a, b, normal, show-
ing 5 petals; c, d, petalody of stamens; e, petal with nectary at its base;
f-h, ripened ovaries.

Anemone) for the green foliage-like bracts below the
perianth to assume the characters of sepals, and even of
petals, so that frequently one can hardly say whether a
given segment of the perianth is a true petal, or a trans-
formed bract. By the petalody of the bracts the flower
appears to be ''double."

411. Coalescence of Petals. — It frequently occurs in
flowers of Ranunculaceae (and in other normally poly-
petalous families also) that the initial stages or primordia

SEED-BEARING PLANTS

463

of two or more petals become wholly or partly fused or
coalesced, thus reducing the number of separate members
of the corolla (Fig. 346). Sometimes coalescence and
petalody of stamens will occur in the same specimen, so
that a flower that would normally have five petals may
have six or eight, or more, some of which have coalesced,
as is indicated by the two or more points at the tip.

Fig. 346. — Rue anemone {Ane^nonclla thalictroides) . i, normal flower
with 5 petals; 3, petalody of stamens; 4, coalescence of petals (c^); 2, coales-
cence (c), and petalody of stamens. At 2, s is shown a stamen partially
transformed into a petal, but with a portion of the anther still remaining.

412. Mustard Family (Cruciferae). — The flowers of
the mustard family are mostly characterized by having
the four narrow petals opening out at an angle of 90°
from each other, forming a Greek cross (Fig. 347), whence
the family name, Cruciferae. This character of the corolla
also appears in rare instances in other famihes {e.g.^ some
Rubiaceae), whose corolla is then said to be ''cruciferous."
The fruit, a silique, is also one of the ear-marks of the

464

STRUCTURE AND LIFE HISTORIES

family. The mustard family contains many valuable
economic plants, such as the white and black mustard,
radish, cabbage, turnip, kohlrabi, and brussels sprouts.

413. Rose Family (Rosaceae). — One of the common
wild roses (Rosa Carolina) illustrates a type of flower
structure more advanced in several ways than that of
Rammcuhts. The flowers, with rare exceptions, have

Fig. 347. — Black mustard {Brassica nigra).

sepals and petals which are borne on the margin of a
well-developed hypanthium, formed by the enlargement
of the torus, at the extremity of the peduncle. The
numerous stamens are always inserted on the sepals
[adnation of parts), and the pistils vary from one to
many. In marked contrast to the numerous horticultural
varieties of the rose, the wild roses are single; that is, they
have one circle of petals (usually five). The "doubling"
of the cultivated varieties is caused by the replacement of

SEED-BEARING PLANTS

465

stamens by petal-like organs (Fig. 346). Not that
stamens are ''transformed into petals," as is often stated,
but that petal-like organs appear at the points where
stamens normally occur in the wild form. In other words,
the supernumerary petals are homologous with stamens.

Fig. 348. — Petalody of stamens in a cultivated rose, a, indicates the
remains of anthers on petal-like organs that have replaced stamens.

This homology is made clear by transitional forms, show-
ing all gradations between true stamens and normal
petals (Figs. 348 and 349).

By comparing the methods of doubling the flower in
the buttercup and the rose, we see that double flowers may
be produced by either (or both) of two methods, often

\o

466

STRUCTURE AND LIFE HISTORIES

referred to, respectively, as ''petalody of bracts" and
''petalody of stamens." The causes of these variations
are not known.

Fig. 349. — Wild rose. A "single" flower showing incipient doubling
by the replacement of stamens by petals. Below, a series of transitional
forms from stamen to fully formed petal; an., anther, or remnant of anther.

Fig. 350.— Perennial pea {Lathyrus latijolius).

414. Leguminoseae. — The legume family is the largest,
and one of the most widely distributed of all the Archi-
chlamydeae, and includes three sub-families, as follows:

SEED-BEARING PLANTS

467

I. FapilionoidecB, containing such commonly recog-
nized plants as the peas, beans, lentils, clovers, peanuts,

Fig. 351. — Wild senna {Cassia marllandica).

lupines, lead-plant (Amorpha), locust or false-acacia
(Robinia), wistaria, and others, all of which have the

468

STRUCTURE AND LIFE HISTORIES

peculiarly modified corolla called papilionaceous, from its
fancied resemblance to a butterfly (Fig. 350).

2. CcBsalpinioidecE, containing the red-bud {Cercis), true
or honey-locust (Gleditsia), wild senna {Cassia marilan-
dica), and others, whose flowers are only imperfectly or
not at all papilionaceous (Fig. 351).

3. Mimosoidece, containing the acacias, sensitive plants
{Mimosa), and others having flowers with a regular corolla.

Fig. 352. — Legume of the edible pea {Pisum sativum), a, anther; c,
calyx; st, stigma.

Flowers that are bilaterally symmetrical, like the papilio-
naceous flowers, are called zygomorphic. Such flowers as
the buttercup, rose, and others may be divided into equal
halves by an infinite number of planes of symmetry.

The one feature that characterizes all three of the sub-
families^ is the simple pistil, composed of one carpel, and
enlarging greatly in fruit (as in peas and beans) to form a
legume (Fig. 352); whence the name of the family.

The structure of the papilionaceous corolla is illustrated
in Fig. 350. The upper and largest petal, stands erect,

* By some authors each group, designated above as a sub-family, is
considered as a separate family.

SEED-BEARING PLANTS

469

forming the standard, the two lateral petals are the
"wings;" while the two lower petals adhere along their
adjacent edges to form the keel. In these flowers there
are usually ten stamens (rarely live), which commonly
occur in two groups or "brotherhoods" (diadelphous) ,
nine in one group with their filaments united into a tube,
cleft on the upper side, the other standing alone above the
cleft.

Fig. 353. — Alsike, or Alsatian clover {Trijolium hyhridum). Inflores-
cences, showing carpotropic movements of the flowers after pollination by
an insect. At the extreme left, flowers in bud, the outermost ones just
beginning to open; next to the last, at the right, only one flower remaining
erect on account of not having yet been pollinated; at the extreme right,
every flower pollinated, turned down, and withering.

Pollination is usually accomplished in this family by
means of insects which visit the flowers for the nectar
secreted by glands. In the case of white clover and
alsike, each flower of the head, when pollinated, turns
down, and the corolla becomes brown (Fig. 353). This
change has been interpreted by some as a sign to the insects
that the nectar has been taken, and therefore that another

470 STRUCTURE AND LIFE HISTORIES

visit would not be profitable. Other students regard this
as questionable.

Since, as shown by examining a bud, the flowers of the
clover head mature from the circumference toward the
center (centripetally), the outer flowers are the first to be
visited, and hence the first to bend down (Fig. 353).^
Many of the papilionaceous legumes are self-pollinated.
This is true of the ''sweet pea" as a rule, but not without
exceptions.

The fact that the legumes furnish so many kinds of food
and fodder plants, and that the organisms causing the
tubercles on their roots (pp. 317-318) are an important
source of the nitrogen necessary for successful agriculture,
renders this family one of the most important of all the
economic plants, possibly exceeded only by the grass family.

415. Evening-primrose Family (Onagraceae). — The
evening-primroses have recently come into very great
prominence on account of the fact that they have been
extensively used for the experimental study of evo-
lution. A knowledge of their structure has, therefore be-
come increasingly important. The flowers are perfect
and symmetrical, with the parts usually in fours. The
ovary is one- to six- (usually four-) chambered, and the
calyx tube adheres to the walls. The stamens are in-
serted commonly on the summit of the calyx tube. In
the evening-primrose itself {(Enothera'^) , the pollen-
grains are held together by delicate threads that resemble
a cobweb. The seedling usually forms a rosette the first
year, and thus passes the winter (Fig. 354). The follow-

^ Often one will find a solitary, unpollinated flower left standing, and in
some localities these are sought by children as "old-maid clovers."
2 Called also Onagra.

SEED-BEARING PLANTS

471

ing season an erect stem is sent up which bears the flowers
and fruit (Fig. 400).

416. Parsley Family (Umbelliferae). — The most highly
organized family of the Archichlamydeae is that to which
the common parsley belongs. In this family the calyx-

HHJI^^^B^ i

•

1

Fig. 354. — Rosette of the evening-primrose {(Enothera hwnnis).

tube adheres throughout its length to the wall of the ovary,
and the five petals and stamens are inserted on a disk
at the base of the two styles (Fig. 355). The relatively
small flowers are, with rare exceptions, borne in clusters
known as umbels. The characteristics of an umbel are

472

STRUCTURE AND LIFE HISTORIES

that the numerous flower-stalks are of such relative
lengths as to bring all the flowers to substantially the same
plane (Fig. 356), and that the outer flower buds open first;
in other words the anthesis is centripetal. Some of the

Fig. 35S.-

-Heracleum lanatum (Parsley family),
the margin of an umbel.

Individual flower from

Fig. 356. — A compound umbel of cow parsnip (Heracleum lanatum).

genera of this family {e.g.j caraway, parsley, parsnips,
carrots) are edible, while, as often happens in other
families also, some of their near relatives {e.g., the water-
hemlock, Cicuta) are very poisonous.

CHAPTER XXIX

SEED-BEARING PLANTS (Continued)

METACHLAMYDEiE (Sympetalse)

417. Coalescence. — We have seen above that genera
normally having polypetalous flowers frequently furnish
examples of the more or less complete fusion or coalescence
of the petals. In certain entire families this fusion of
different members of the same circle of floral organs be-
comes the rule, giving rise to an entire group, the Sym-
petalcB, based upon this character. Only a few of these
families can be cited in illustration.

418. Heath Family (Ericaceae). — The heath family in
North America is composed chiefly of shrubs, though a
few, as, for example, Indian pipe {Monotropa) are herbs; ^
some of the tropical genera are trees. The beautiful
rhododendrons, azaleas, and laurels, trailing arbutus
(first harbinger of spring in the northern states) , the aro-
matic wintergreen, and the well-known huckleberries
(Gaylussacia) , and blueberries {Vaccinium) belong here.

The structure of an Ericaceous flower may be illustrated
by the common mountain laurel {Kahnia latifolia), of
the eastern states (Figs. 357 and 358). The sepals are
united below, but parted above; the sympetalous corolla

^ By some authors the large heath family is separated into a number of
smaller families, e.g., Monotropaceas, Ericaceae, Vacciniaceae. In Mono-
tropaceae, Clethraceae, and Pyrolaceae, and a few of the true heaths (Eri-
caceae), the corolla is polypetalous.

473

474

STRUCTURE AND LIFE HISTORIES

Fig. 357. — Mountain laurel {Kalmia latifolia). Note the anthers
resting in the depressions of the sympetalous corolla. (Photo by Elsie
M. Kittredge.)

Fig. 358. — Mountain laurel {Kalmia latifolia). a, stamens in their
original position, with the anthers in the pouches; b, stamens inflexed
(detail at e); c, side view; d, essential organs;/ and g, stamens.

SEED-BEARING PLANTS

475

is five-lobed having the shape of a broad, shallow bell. Its
coloration^ as viewed en masse on the bush, gave rise to
the common name "calico bush," popular in certain locali-

Fig. 359. — Common milkweed, Asclepias syriaca.

Kittredge.)

(Photo by Elsie M.

ties. The corolla has ten pouches, each of which con-
tains one anther borne on a recurved filament. When
an insect alights on the flower the anthers become loosened

^ Coloration = color pattern.

476

STRUCTURE AND LIFE HISTORIES

from the pouches, and snap over in, toward the center,
thus dusting the insect with pollen, which is then trans-
ferred to the stigma of the next flower visited by the
insect.

The corpse-plant, or Indian-pipe {Monotropa uniflora), is
of interest because, although belonging in a sympetalous

Fig. 360. — Milkweed {Asdepias sp.). a, flower-bud; b, flower; c,
very young pod; d, older pod in section, showing seeds; e, section of
flower; /, top view of flower, showing the 5 Jioods of the crown, each with
a horn incurving to the stigma; between the horns are the cleft glands
(shown enlarged at g), to which the pollinia are attached.

family, it is polypetalous, and further because, Hving
entirely as a saprophyte (or, possibly, as a root-parasite), it
has entirely lost the power to make chlorophyll, and hence
the power of photosynthesis (Fig. 230, p. 323).

419. Milkweed Family (Asclepiadaceae). — The milk-
weeds^ present a most curious and interesting modifica-

^ So called because they contain a milky juice or latex.

SEED-BEARING PLANTS 477

tion of flower-structure (Fig. 359). The deeply five-parted
and reflexed corolla bears a crown of five "hooded" bodies,
in each of which there arises a pointed, incurved ''horn"
(Fig. 360). The anthers are more or less united around the
stigma, and each cell contains a waxy, pear-shaped pollen-
mass (pollinium). The pollinia of adjacent anthers adhere
in pairs to cleft glands that grow one on each of the five
angles of the stigma. As bees climb over the flowers in
search of nectar in the bottom of the hoods, their legs
are drawn through tiny slits, and catch the cleft gland

Fig. 361. — Milkweed (Asclepias). Pollen-mass (pollinuim), showing

germination.

when pulled out. Often the gland cannot be loosened,
and the legs of the insects are pulled ofl and left attached
to the flower. When the insect visits another flower the
pollen-masses (which by this time have twisted and
folded together) become inserted into the stigmatic
chamber of the second flower, where they germinate,
sending out numerous pollen- tubes (Fig. 361). On ac-
count of the complicated nature of this process, pollina-
tion often fails, so that only a small percentage of the very
numerous flowers produces seed. After fertilization, the
carpels increase enormously in size, and ripen into a pod,
filled with a large quantity of flat, thin seeds, each of

478 STRUCTURE AND LIFE HISTORIES

which bears a tuft of long silky hairs, whence one of the
common names, ''silk weed." These hairs facilitate the
distribution of the seeds by the wind (Fig. 362).

Fig. 362. — A milkweed {Asclepias syriaca), with tufted seeds scattering
from the dehiscing pods. (Photo by Elsie M. Kittredge.)

420. Convolvulus Family (Convolvulaceae). — The fea-
tures of this family are well illustrated by the genus
from which it gets its name, Convolvulus, or bind-weed.
The five-lobed corolla is bell-shaped, all the parts of the
flower are in fives, and the pistil two-celled. Most genera
of the family are trailing or (as the name indicates)
twining vines (Fig. 363).

SEED-BEARING PLANTS

479

421. Mint Family (Labiatae).— The Mint family is
characterized by a square stem, opposite leaves, a tubular
calyx, caused by the coalescence of the five sepals, a highly
modified corolla having two lips or labia (singular labium,
whence the family name), and leaves containing many

Fig. 363. — Bindweed {Convolvulus arvensis).

small glands that secrete a volatile oil, which gives the char-
acteristic odor and taste to all the plants of the family.
The upper lip results from the fusion of two petals, the
lower lip by the fusion of three. Everyone is familiar
with one or more of these features, as embodied in the
various mints, pennyroyal, horehound, catnip, sage,
savory, thyme, hyssop, wild marjoram, and other condi-

48o

STRUCTURE AND LIFE HISTORIES

ments and drugs. The features of the flower are illus-
trated in Fig. 364.

422. Nightshade Family (Solanaceae).— The Night-
shade family is of interest chiefly because it contains

Fig. 364. — Salvia sp. (One of the Labiatae). a, flower bud; b-f,
various views of the open flower; an., anther; st., stigma; x, projections
near the base of the filaments. The lead pencil is made to imitate an
insect visiting the flower for pollen. By pressure at the base of the fila-
ments, the anthers are brought into contact with the surface of the pencil,
which thus becomes covered with pollen. When the next flower is visited
the stigma, having then bent down and spread apart, receives the pollen
from the other flower. Thus is accomplished cross-pollination. In h,
before the visit of the insect, the stigmatic surfaces are still in contact, so
that pollination is not possible.

several genera of very great economic importance, viz.:
potato {Solamim tuberosum) jtohsicco {Nicotiana Tahacum),
tomato (Lycopersicum esculentum)^ and several medi-

SEED-BEARING PLANTS

481

cinal herbs, such as belladonna, henbane, capsicum, and
others. Most of the genera are tropical. The bitter-
sweet {Solanum Dulcamara^) may serve to illustrate the
family (Fig. 365). This is a perennial, climbing vine, hav-
ing the Darts of the flower (including the stamens) in fives

Fig. 365. — Nightshade, or bittersweet {Solanum Dulcamara).

and the fruit a two-celled herry with many seeds {e.g., the
tomato. Fig. 334). The corolla is wheel-shaped and five-
parted, and the anthers converge to form a tube around the
single style. The anthers discharge the pollen through a
pore or ''chink" at the tip.

423. Figwort Family (Scrophulariaceae). — The ''butter-
and-eggs, " or "toadflax" {Linaria vulgaris), will serve to
illustrate the figworts (Fig. 366). The stamens are inserted

^ The staff- tree or waxwork {Celastrus scandens), of the Staff- tree
family, Celastraceae, is also called "bitter sweet" in certain localities.
31

482 STRUCTURE AND LIFE HISTORIES

on the tube of the irregular, two-lipped corolla, which
bears a well-developed spur at the base (Fig. 367). Not
infrequently abnormal flowers are found with five spurs,

F'iG. 366. — The toad-flax, or butter-and-eggs {Lhiaria vulgaris).

or with none, and other attendant modifications of the
corolla (Figs. 368 and 369). Such flowers are called
pelories, since they are thought to be variations indicating
the character of the ancestral form from which they are

SEED-BEARING PLANTS

483

believed to be derived. The method of pollination in
normal flowers is illustrated in Fig. 370.

424. Composite Family (Compositae). — The composites
represent the highest development of dicotyledons. The
flowers are borne on a common receptacle in a compact

Fig. 367. — Sections of flowers of the toad-flax (Linaria vulgaris).
A, front view; a, anthers; s, stigma; n, nectar-gland. B, side view; o,
ovary.

head," giving the appearance, not so much of an in-
florescence as of a compound flower, whence the family
name, assigned by early botanists, who did not understand
the morphology of the head.

The heads are surrounded by a circle of bracts, called
an involucre. The bracts which often occur on the re-

484

STRUCTURE AND LIFE HISTORIES

ceptacle among the flowers are called chaf. The family is
composed of two series: I. Tubuliflorae, with all the perfect

Fig. 368. — Toad- flax {Linaria vulgaris). Inflorescence, showing normal
(spurred) flowers, and several abnormal flowers without spurs.

flowers tubular, as in the daisy; II. Liguliflorae, with no
tubular flowers, but all the flowers having a strap-shaped

SEED-BEARING PLANTS

48s

Fig. 369. — Toad-flax (Linaria vulgaris). Abnormal flowers (pelories),
having five or more spurs. Normal, one-spurred buds are shown in A .

Fig. 370. — Toad-flax {Linaria vulgaris). Flowers being visited by an
insect for nectar. B, longitudinal section, showing the insect's proboscis
extended down the spur toward the nectar-gland; C, insect with a mass of
pollen ip), rubbed off from anthers onto the dorsal hairs of the thorax,
during successive visits.

486

STRUCTURE AND LIFE HISTORIES

{ligulate) corolla, formed by the fusion of the five petals,
as indicated by the five notches at the end (Fig. 372).
The tubuliflorae may have both tubular and ligulate
flowers, as in boneset (Eupatorium perjoliatum) or in white
daisy {Chrysanthemum leucanthemum) ^ or only tubular, as
in the burdock {Arctium, Fig. 371), or in the Canada thistle

Fig. 371. — Inflorescence and flowers of the burdock {Arctium minus),
a, Inflorescences; h, longitudinal section of the same; c, bud of individual
flower; d, mature flower; sty, stigma; stig, style; a, ring of syngenesius
anthers; c, corolla; p, pappus (calyx); ov, ovary; e, mature seed.

{Cirsium arvense). Among the liguliflorag may be men-
tioned chicory {Cichorium Intyhus, Fig. 372), dandelion
{Taraxacum), garden lettuce {Lactuca sativa), and others,
all of which possess a milky juice, or latex. The five stamens
(rarely four) are inserted on the corolla, and have their
anthers united in a tube {syngenesious) around the style.

SEED-BEARING PLANTS

487

The calyx tube is united to the one-celled ovary, and
its upper free part, or limb, is dififerentiated into hairs
{pappus), scales, teeth, or is merely cup-shaped, or in some

Fig. 372. — Chicory {Cichoriiim Intyhus). A, portion of flowering
branch; B, basal leaf (runcinate-pinnatifid) ; C, median longitudinal section
through a head, showing the insertion of the flowers; D, individual flower;
E, fruit (ripened ovary), showing the persistent pappus (calyx) of short
scales.

species entirely wanting. When the fruit is ripe, the
pappus aids in its dissemination by the wind.

While the style is growing up through the tube of anthers
the stigmatic surfaces are in contact with each other, and

488

STRUCTURE AND LIFE HISTORIES

thus protected from becoming pollinated with the flower's
own pollen. As the styles emerge above the anthers their
two tips spread apart and roll back, exposing the infacing

Fig. 373. — A composite {Coreopsis sp.). A, B, E, views of the inflor-
escence or head; C, a ray-flower; D, section through the head; F, a disc-
flower in bud; G, disc-flower just opened; H, older disc-flower, the stigmas
reflexed; /, disc-flower with corolla removed.

stigmatic surfaces so that they may receive pollen brought
by insects from other flowers (Fig. 373).^

^ The family Compositae, as recognized above, including two series,
Tubuliflorae and Liguliflorae, is restricted by some authors so as to include
only plants having florets with tubular or both tubular and ligulate cor-
ollas in the head. Such plants as the chicory, dandelion, and lettuce,
having only ligulate corollas, comprise the family Cichoriaceae. Cichor-
iaceae and Compositae (in this restricted sense) have syngenesious anthers.
Plants whose florets have tubular corollas only (sometimes none), but
anthers not truly syngenesious, comprise the Ambrosiaseae, including the
rag- weeds, cockle-bur, marsh-elder (Iva), and GcBrtneria.

CHAPTER XXX

SEED-BEARING PLANTS (Concluded)

MONOCOTYLEDONS

425. General Characters. — The monocotyledons are,
in almost every respect, of simpler structure than the
dicotyledons. As the name indicates, the embryo has
only one cotyledon; the parts of the flower are usually
in threes or sixes, but never in fives, as in dicotyledons;
the leaves are, with rare exceptions, parallel-veined,
and the early ones are always alternate on the stem. A
cross-section of the stem shows that the fibro-vascular
bundles are not arranged in a circle about a central
pith (exogenous type), but are distributed irregularly
throughout the parenchyma (endogenous). There is no
layer of perennial cambium, and consequently no cylinders
of wood and bark are formed each growing season, as in
the dicotyledons. The general characters of the group
are illustrated diagrammatically in Fig. ^;^S.

426. Relation to Dicotyledons. — A comparison of the
monocotyledons with the more highly developed dicoty-
ledons raises at once the question as to whether the former
are the more ancient forms from 'which the dicotyledons
have been evolved, or whether dicotyledons are the more
primitive, in order of development, and the monocotyle-
dons derived from them by reduction and simplification.
There is evidence on both sides of this question, which will

489

490

STRUCTURE AND LIFE HISTORIES

be considered again later on (page 608). In treating of
the dicotyledons first, however, we have tacitly assumed

YiG^ 374.— Cat-tails. Typha angustijolia at left; Typha latifolia at right.
Staminate flowers above; pistillate flowers below.

that they are the older forms, from which the mono-
cotyledons have been derived.

SEED-BEAEING PLANTS 49I

TYPES OF MONOCOTYLEDONS

427. Cat-tail Family (Typhaceae).— The cat- tails are
one of the most primitively organized family of Angio-
sperms. They grow in groups or associations in swamps
and wet places, and are conspicuous by their tall, upright
and grass-like leaves, and by the inflorescences or *' cat-
tails" in late summer and fall (Fig. 374). The monoecious

Fig. 375. — Staminate flower of the broad-leaved cat-tail {Typha lati-
folia). (Cf. Fig. 376.)

flowers are borne on a fleshy axis or spadix, the pistillate
below, and the staminate above near the tip. The flowers
have neither calyx nor corolla (Figs. 375 and 376), and the
one-celled ovary contains but one ovule. The embryo is
surrounded by abundant endosperm, often called albu-
men. The copious brown down of the fruit is composed
of club-shaped hairs on the stipe, or stalk-like support of
the pistil.

428. Water-plantain Family (Alismaceaej. — The type
genus^ of the water-plantain family is the water-plantain

^ The type genus of a family is the genus from which the family name
.-,ften is derived, and usually (though not always) embodies in a striking
way the characters which distinguish the family.

492

STRUCTURE AND LIFE HISTORIES

(Alisma Plantago-aquatica) . Like the cat-tails, the water-
plantains are marsh herbs (Fig. 377), with flowers either
perfect, monoecious or dioecious; in Alisma they are per-
fect, with usually six stamens. The three sepals are per-
sistent^ but the three white petals are deciduous (i.e., falling

Fig. 376. — Cat-tail {Typha latifolia). A, longitudinal section of
portion of inflorescence; Sp, spadix; p.f., pistillate flowers; s.f., staminate
flowers. B, pistillate flower, greatly magnified; ov, ovary; sty, style; stig.,
stigma; sc, sterile hair; P, pollen grains, in characteristic groups of four
each. (Cf. Fig. 375.)

away early). The numerous ovaries are borne in a circle
on a flattened receptacle. The possession of calyx and
corolla, together with other features, mark the family as
more highly organized than the cat-tails.

429. Grass Family (Gramineae).— The grasses consti-
tute one of the largest, one of the most important econom-
ically, and one of the most difiicult taxonomically, of all

SEED-BEARING PLANTS

493

the plant families. Here belong the *^ grains '' (wheat, rice,
oats, barley, rye, corn, and others) which furnish the most
important vegetable foods of the entire human race. The
commercial grains have been cultivated so long by man

Fig. 2>11' — Water plantain {Alisma Plantago-aqiiatica) .

that their origin is shrouded in mystery, antedating, as it
does, the dawn of written history. Except the bamboos,
which are shrubs or trees, the grasses are all herbaceous.

The chief characteristics of the family are that the fruit
is always a grain or caryopsis} The seed invariably con-

^ A one-seeded, seed-like fruit, with the wall of the ovary {pericarp)
united closely (adnate) to the seed within.

494

STRUCTURE AND LIFE HISTORIES

tains endosperm, with the embryo located at one side
(Figs. 378 and 83). The cotyledon {sciitellum) serves to
secrete enzymes that digest the endosperm, and then

Fig. 378.-Longitudinal section of a grain of wheat {Triticum vulgare),
p.s.c, pericarp and seed-coats (united); en, endosperm; al, aleurone layer;
gl, glandular layer of the scutellum; sc, scutellum; sp, sheath of plumule
{coleoptile); pi, plumule; r.cot., rudimentary cotyledon; ra, radicle; re,
root-cap; r.s., root-sheath (coleorhiza). (From microscopical preparation
of E. W. Olive.)

absorb the digested food by osmosis (Fig. 62). The cylin-
drical stems are jointed and usually hollow (except at the
joints) . The leaves are usually long, slender, and pp.rallel-

SEED-BEARING PLANTS

495

veined. The flowers are arranged in s pikelets, and the
spikelets in spikes (Fig. 379), racemes, or panicles^ (Fig.
380). The leafy parts of the flower-clusters
are modified as dry scales called glumes.
When grain is thrashed the glumes consti-
tute the so-called ''chaff." There is no
perianth.

430. Palm Family (Palmaceae). — The
palms are mostly tropical and subtropical.
With certain exceptions {e.g., Calamus — rat-
tan), the stem or caudex is normally un-
branched, varies in height with the species, and
in most species bears all the foliage near the
tip (Figs. 381 and 382). Frequently the old
leaves, or their petioles or bases only, re-
main attached to the trunk. The leaves,
though not morphologically compound, usu-
ally have their blades cleft or divided as they
mature. The flowers are complete, with three
sepals and petals, three to six stamens, and
pistil of three carpels. They are monoecious,
dioecious, or perfect, according to the species.

The flowers, intermingled with bracts, oc- yig. 379.—
cur on a more or less fleshy spadix, often Naziaracemosa

111.; , (L.) Kuntze.

much branched, and enclosed m a large spathe. Terminal spike

The inflorescence may be axial (lateral), or 'l^^^ l^i^^x^;,

terminal. When terminal the plant usually (After Britton
dies after the fruit has matured.

^ A spike is "a form of simple inflorescence with the flowers sessile or
nearly so upon a more or less elongated common axis." A panicle is "a
loose, irregularly compound inflorescence with pedicellate flowers." A
raceme is "a simple inflorescence of pediceled flowers upon a common, more
>r less elongated axis."

496

STRUCTURE AND LIFE HISTORIES

No species of palm has ever been found in both the
eastern and western hemisphere, except when introduced
artificially. Many of the palms have great commercial
value, such as the date palm, cocoanut palm (Fig. 383),
fan palm, vegetable-ivory palm (whose endosperm is
hard and white like ivory), and the oil palm. In the trop-
ics the leaves of various species are used to make thatched

Fig. 380. — Johnson-grass {Sorghum halpense). Spikelets in a panicle.
(After Britton and Brown.)

roofs, and the trunks are often used for fence posts and
porch pillars.

431. Arum Family (Araceae). — The arum family is
well illustrated by the "skunk cabbage" {Symplocarpiis
fcetidiis) (Figs. 384 and 385).^ The flower has no petals,
but four sepals, and four stamens — one opposite each
sepal. The ovary contains only one suspended ovule.
The compound globular fruit is composed of the spongy
spadix, greatly enlarged, bearing the coalesced ovaries,
with the spherical seeds just underneath the surface.

^ Called by some authors, Spathyema fcetida.

SEED-BEARING PLANTS

497

The roughness of the surface is caused by the persistence
of the styles and the fleshy sepals.

The family is divided into several sub-families on the
basis of various structural differences, such as phyllotaxy,
venation of leaves, presence or absence of milky juice,
presence or absence of perianth, and others.

"^

^

P

^

\ 4^

m,.^

, , ^

m

''%m\

H ; S

m

:=;is^^ ■•

m

. "..: ^ -^:^::3-^-1m

4

W

'•'"""

Fig. 381. — Cocoanut palms along the beach. Philippine Islands.
(Photo from Bureau of Science, Manila.)

432. Orchid Family (Orchidaceae). — The Orchidaceae
are the most highly developed of all monocotyledons.
No flower surpasses the orchids in beauty of color-
pattern, endless diversity of unusual forms, and wonder-
ful mechanisms that secure cross-pollination by insects.
As in the papilionaceous flower, the flower of orchids is
bilaterally symmetrical (zygomorphic), with (usually)
three sepals resembling in texture the three petals (Fig.

J2

498

STRUCTURE AND LIFE HISTORIES

386). One of the petals, the Up, presents a greater variety
of form in the various species than do the other petals. At
its base is the column, composed of the style, with which
are fused the one (or sometimes only two) stamens (Fig.
387). Except in the lady's slipper {Cypripedium), and its

Fig. 382.

-Sabal palmetto. (In the right distance a barragona palm).
Cuba.

nearest relatives, the pollen adheres in masses or pollinia,
as in the milk-weed. The stalked pollinia adhere to visit-
ing insects, sometimes to their eyes, and are thus trans-
ferred from one flower to another.^

^ For details of the wonderful contrivances for cross-pollination, the
student should consult some larger treatise, such as Darwin's "Cross'
fertilization of Orchids."

SEED-BEARING PLANTS

499

Fig. 383. — Germinating cocoanuts; the one seeded fruit of the cocoanut
palm. (From Bull. Agricole du Congo Beige, 1910.)

Fig. 384. — Skunk cabbage {Symplocarpiis fcctidus). Early spring growtk
and flower buds. (Photo by Elsie M. Kittredge.)

500 STRUCTURE AND LIFE HISTORIES

Fig. 385.^ — Skunk cabbage {Symplocarpus Joetidus). Inflorescence,
with portion of the fleshy spathe removed, showing the perfect flowers
covering the globose spadix. (Photo by Elsie M. Kittredge.)

Fig. 386.— Flower of an orchid {Cattleya sp.). (Cf. Fig. 387.)

SEED-BEARING PLANTS

501

Of about 24,000 known species of monocotyledons,
over one-fourth are orchids. The number of individuals,
however, of any given species is small, as compared, for

Fig. 387. — Floral organs of an orchid {Catlleya sp.). A, the entire
flower; sep, sepal; pet, petal; B, column, showing 5, stigma and r, the ros-
tellum (beak), with the small glands at the tip; to the glands are attached
the four strap-shaped caudicles of the poUinia; C, pollinia, with the four
caudicles; below, the gland; D, longitudinal section of the column; p,
poUinium; E, the same, enlarged. (Cf. Fig. 386,)

example, with the grasses. The most highly modified
forms are tropical, and are seen in temperate regions
only in plant-houses ~

CHAPTER XXXI
EVOLUTION

433. Doctrine of Special Creation. — In the time of
Linnaeus, the ''father of botany/' men believed that the
seven *'days" of creation left the world substantially as
we now find it. The stars and planets, mountains and
oceans, plants and animals were created once and for all,
and continued without important change until the present.
In the beginning, as now, there were the same oceans and
hills, the same kinds of plants, and the same kinds of
animals. Nor, it was believed, are any fundamental
changes now in progress. Creation was not continuous;
it took place within a brief period (seven ''days"), and
then ceased; after that the Creator merely watched over
the objects of his handiwork.

434. Meaning of Evolution. — Evolution means gradual
change. Applied to the natural world the theory of
evolution is the direct opposite of the doctrine of special
creation. It teaches that things were not in the beginning
as we now find them, but that there has been constant
though gradual change. Creation is regarded, not as
having taken place once and for all, but as being a con-
tinuous process operating from the beginning without
ceasing — and still in progress.

435. The Course of Evolution. — The theory teaches
that the gradual changes have been from relatively
simple conditions to those more complex. The com-

502

EVOLUTION 503

plication has been two-fold: (i) simple individuals,
whether mountains, rivers, planets, animals, or plants, have
become more complex {e.g., compare the structure of
Pleurococcus, a simple spherical cell, with that of the fern) ;
(2) the relation between living things, and between them
and their surroundings has become more complex {e.g.,
compare a unicellular bacterium, with its relatively simple
life relations, with the clover plant, highly organized,
and related to water, air, soil, light, temperature, gravity,
bacteria (in its roots), and insects (for cross-pollination).

Most of the steps of evolution have been progressive,
toward higher organization, greater perfection of parts,
increased efficiency of function, as, for example, from algae
to angiosperms; but not all the steps have been in
this direction. Some of the steps have been regressive,
toward simpler organization, less perfection of parts,
decreased efficiency of function, as, for example, from
green algae to the alga-like fungi (Phy comycetes) , from
independence to parasitism (dodder), or to saprophytism
(Indian pipe and bread-mold).

436. Inorganic Evolution. — The process of evolution is
not confined to living things, but, as indicated above,
applies to all nature. Even the chemical elements are
now believed to have been produced by evolutionary
changes, and to be even now in process of evolution. This
is one of the results of the recently discovered phenomenon
of radioactivity, which is essentially the transformation
of the atoms of one chemical element into those of another.
Fossil remains of marine animals and plants, found im-
bedded in the rocks on mountain summits, indicate,
without possibility of reasonable doubt, that what is now
mountain top was formerly ocean bottom. The mountain

504 STRUCTURE AND LIFE mSTORIES

has come to be, by a series of gradual changes. Rivers
and valleys are constantly changing so that the present
landscape is the result of evolutionary processes; climates
have changed, as we know from the fact that fossil re-
mains of tropical plants are now found in the rocks in
arctic regions; even the stars and planets, like our own
earth, are coming gradually into being, undergoing changes
of surface and interior condition, and ceasing to exist.
Nothing is constant except constant change. The main
problem of astronomy is to ascertain and record, in order,
the evolutionary changes that have resulted in the present
system of suns and planets. The main problem of
geology is to ascertain and record, in order, the evo-
lutionary steps that have resulted in the present condition
of the earth.

437. Organic Evolution. — Developmental changes in
living things constitute organic evolution. Such changes
are manifested in the development of an individual from
a spore or an Qgg. The development of a mature in-
dividual is ontogeny. The development of a group of
related forms (genera, families, orders, etc.) is phytogeny.
The chief problem of biology is to ascertain and record,
in order, the evolutionary changes that have resulted in
the appearance of life and the present condition of living
things.

The major problem of botany is to record, in order, the
evolutionary steps that have culminated in the present con-
dition of the plant world.

Organic evolution means that, after the first appearance
of life, all living things, plant or animal, have been
derived from preexisting living things, in other words, that
the present method of formation of living things, by the

EVOLUTION

505

reproduction of organisms already existing, has always
been the method — ^'Omne vivum ex ovo^^ (all life from an
egg)) ^^omne vivum e vivo'' (all life from preexisting life).
438. Method of Evolution. — To recognize that evolution
is the method of creation still leaves unanswered the im-
portant question as to the method of evolution. By what
process was the gradual development of the living world
accomplished? Various hypotheses have been elaborated
in answer to this question. We can here only briefly
outline three of the most important ones.

Fig. 388.— Louis Agassiz. (From Ballard's "Three Kingdoms.")

I. Agassiz' s Hypothesis. — The great teacher and student
of nature, Louis Agassiz, beheved that the vast array of
plant and animal species, past and present, had no material

5o6 STRUCTURE AND LIFE HISTORIES

connection, but only a mental one; that is, they merely re-
flected the succession of ideas as they developed in the
mind of the Creator, but were not genetically related to
each other. ''We must . . . look to some cause outside
of Nature, corresponding in kind to the intelligence of
man, though so different in degree, for all the phenomena
connected with the existence of animals in their wild
state .... Breeds among animals are the work of man:
Species were created by God."^

But to state that species were created by God does not
satisfy the legitimate curiosity of the scientific man.
What he wishes to know is: By what method was creation
accomplished? God might have worked in various ways.
Now, the study of Nature has never revealed to us but one
method by which living things originate, and that is hy
descent from preexisting parents. Agassiz's hypothesis
contradicts this. All oaks now-a-days are derived by
descent from preexisting oaks, but the first oak, accord-
ing to the doctrine of special creation, was created by
supernatural means; it had no ancestors. The chief objec-
tion to the acceptance of this hypothesis is that the more
profoundly and accurately we study living things, the more
obvious it becomes that truth lies in another direction.

2. Lamarck's Hypothesis. — The noted French naturaUst,
Lamarck, taught that all living things have been derived
from preexisting forms; that the effects of use and disuse
caused changes in bodily structure; that these changes
were inherited and accentuated from generation to genera-
tion; that, being of use, those individuals possessing the
changes in greatest perfection survived while others per-

* Agassiz, L. "Methods of Study in Natural History," Boston, 1893,
pp. 146, 147.

EVOLUTION 507

ished; and that the derivation of new species is thus
accounted for in a simple and logical manner. By con-
tinual reaching for tender leaves on high branches, the
long neck of the giraffe was gradually produced, the slight
gain in length in one generation being transmitted by
inheritance to the next, and so on.

The main thesis of Lamarck, as stated by himself, is
as follows:

"In animals and plants, whenever the conditions of
habitat, exposure, climate, nutrition, mode of life, et
cetera, are modified, the characters of size, shape, relations
between parts, coloration, consistency, and, in animals,
agility and industry, are modified proportionately."

As illustrating the direct effect of environment on organ-
isms, Lamarck chose a plant, the water-buttercup {Ran-
unculus aquatilis), which may grow in marshy places, or im-
mersed in water. When immersed, the leaves are all finely
divided, but when not immersed, they are merely lobed.

While plants are more passive, and are affected by their
surroundings directly, through changes in nutrition, light,
gravity, and so on, animals react to environmental changes
in a more positive and less passive manner. Thus, in
the words of Lamarck:^

''Important changes in conditions bring about impor-
tant changes in the animals' needs, and changes in their
needs bring about changes in their actions. If the new
needs become constant or durable, the animals acquire
new habits. . , . Whenever new conditions, becoming
constant, impart new habits, to a race of animals . . .
these habitual actions lead to the use of a certain part in

^Translated from his Philosophie Zoologique, vol. I, pp. 227, 223, 224,
248.

5o8 STRUCTURE AND LIFE HISTORIES

preference to another, or to the total disuse of a part which
is now useless. . . . The lack of use of an organ, made
constant by acquired habits, weakens it gradually until
it degenerates or even disappears entirely." Thus, "it
is part of the plan of organization of reptiles, as well as of
other vertebrates, that they have four legs attached to
their skeleton . . . but snakes acquired the habit of glid-
ing over the ground and concealing themselves in the grass;
owing to their repeated efforts to elongate themselves, in
order to pass through narrow spaces, their bodies have
acquired a considerable length, not commensurate with
their width. Under the circumstances, legs would serve
no purpose and, consequently, would not be used, long
legs would interfere with the snakes' desire for gliding,
and short ones could not move their bodies, for they can
only have four of them. Continued lack of use of the
legs in snakes caused them to disappear, although they
were really included in the plan of organization of those
animals."

On the other hand, ''the frequent use of an organ, made
constant by habit, increases the faculties of that organ,
develops it and causes it to acquire a size and strength it
does not possess in animals which exercise less. A bird,
driven through want to water, to find the prey on which
it feeds, will separate its toes whenever it strikes the water
or wishes to displace itself on its surface. The skin uniting
the bases of the toes acquires, through the repeated separ-
ating of the toes, the habit of stretching; and in this way
the broad membrane between the toes of ducks and geese
has acquired the appearance we observe to-day."

If such modifications are acquired by both sexes they
are transmitted by heredity from generation to generation.

EVOLUTION 509

One of the weaknesses in Lamarck's hypothesis appears
in his illustration of the snake. If we should grant that
inheritance of the effects of disuse of the legs might possi-
bly explain their absence in snakes, still it would not ex-
plain the origin of the snake's desire to glide. That is, of
course, as much a characteristic of the snake as the absence
of legs.

Other arguments against the validity of Lamarckism
are : first, that no one has ever been able to prove, by ex-
periment or otherwise, that the effects of use (the so-called

Fig. 388a. — Jean Baptiste Lamarck (i 744-1829).

"acquired characters") are inheritable, while innumerable
facts indicate that they are not; second, the hypothesis
could apply only to the animal kingdom, since plants in
general have no nervous and muscular activities like those
of animals. A hypothesis of organic evolution, to be valid,
must apply equally to both plants and animals.

3. Darwin's Hypothesis. — This will be outlined in the
next chapter.

CHAPTER XXXII
DARWINISM

439. Charles Darwin. — The question of the method of
evolution continued to be debated, with no satisfactory solu-
tion in sight, until 1859,^ when Charles Darwin pubhshed
the greatest book of the nineteenth century, and one of the
greatest in the world's history, the Origin of Species.^
This book was the result of over 20 years of careful
observation and thought. It consisted of the elaboration
of two principal theories: (i) that evolution is the method
of creation; (2) that natural selection is the method of
evolution.

440. Early Antagonism to Evolution. — The conception
that evolution (as distinguished from periodic, super-
natural interventions of the Deity) is the method of
creation was arrived at independently by Darwin, but was
not new with him. As we have just seen, it was proposed
by Lamarck. Greek philosophers 2,000 years previously
had suggested the idea; but it had never won the general
acceptance of the educated world, partly because it was
feared to be anti-religious, partly because it was never
substantiated by sufficiently convincing evidence, and
partly because of the antagonism of a few men of great

^ This date should be memorized. It is one of the most important in
the whole history of human thought.

2 The full title of the book was, "The Origin of Species by Natural Selec-
tion, or the Preservation of Favored Races in the Struggle for Life."

5 10

DARWINISM

511

influence in the world of intellect. Men preferred to fol-
low a leader, more or less blindly, rather than take the

Fig. 389. — Charles Darwin. The publication of his "Origin of Species,''
in 1859, revolutionized human thought, and gave direction to all scientific
and philosophic thinking from that time to the present.

pains to examine the voluminous evidence for themselves,
and accept the logical conclusion without prejudice or

512 STRUCTURE AND LIFE HISTORIES

fear, wherever it might lead them, or however much it
might contradict all their prejudice and preconceived
notions. But truth will always, in the end, command
recognition and acceptance, and there is almost no scien-
tific man, now-a-days, who does not regard evolution as
axiomatic. It is one of the most basic of all conceptions,
not only in the natural and the physical sciences, but also
in history, sociology, philosophy, and religion; it has, in-
deed, completely revolutionized every department of
human thought.

441. Darwinism. — It is the second of the above men-
tioned theories, i.e., natural selection, that constitutes the
essence of Darwinism. The theory is based upon five
fundamental facts, which are matters of observation, and
may be verified by anyone, as follows:

1. Inheritance. — Characteristics possessed by parents
tend to reappear in the next or in succeeding generations.
We are all familiar with the fact that children commonly
resemble one or both parents or a grandparent, or great
grandparent in some characteristic. From this we infer
that something has been inherited from the ancestor which
causes resemblance in one or more characters — physical or
mental.

2. Variation. — But the expression of the inheritance is
seldom, if ever, perfect. Eyes are a little less or a little
more brown; stature is never just the same; one-half the
face may resemble a given ancestor more than another;
petals may be more or less red or blue; no two oranges
taste exactly alike; no two maple leaves are of precisely
the same shape. There is inheritance, but inheritance is
usually expressed with modifications or variations of the
ancestral type.

DARWINISM 513

3. Fitness for Environment. — It is common knowledge
that living things must be adjusted to their environment.
Poor adjustment means sickness or weakness; complete
or nearly complete lack of adjustment means death.
Water-lilies, for example, cannot live in the desert,
cacti cannot live in salt marshes; cocoanuts cannot be
grown except in subtropical or tropical climates, edelweiss
will not grow in the tropics. This is because these various
kinds of plants are so organized that they cannot adjust
themselves to external conditions, beyond certain more or
less definite limits or extremes. A cactus is fit to live in
the desert because it is protected by its structure against
excessive loss of water, and has special provision for
storing up water that may be used in time of drought.
Deciduous trees are fitted to live in temperate regions,
partly because their deciduous habit, and their formation
of scaly buds enables them to withstand the drought of
winter. Negroes live without discomfort under the trop-
ical sun because they are protected by the black pigment
in their skin. And so, in countless ways, we might illus-
trate the fact that all living things, in order to flourish,
must be adjusted to their surroundings.

4. Struggle for Existence. — The clue to the method of
evolution first dawned upon Darwin in 1838, while reading
Malthus on "Population." Malthus emphasized the fact
that the number of human beings in the world increased
in geometrical ratio (by multiplication) , while the food sup-
ply increased much less rapidly by arithmetical ratio (by
addition). Therefore, argued Malthus, the time will soon
be reached when there will not be food enough for all;
men will then struggle for actual existence, and only the
fittest (i.e., the strongest, the fleetest, the most clever or

514 STRUCTURE AND LIFE HISTORIES

cunning) will survive. In pondering this hypothesis
Darwin at once saw its larger application.^ There are
always more progeny produced by a plant or an animal
than there is room and food for, should they all survive.
Darwin showed that the descendants of a single pair of
elephants (one of the slowest breeders of all animals)
would, if all that were born survived, reach the enormous
number of 19,000,000 in from 740 to 750 years. ^ But
the total number of elephants in the world does not appre-
ciably increase : evidently many must perish for every one
that lives. There must therefore be an intense struggle
for existence. Darwin^ gives the following illustration:

'' Seedlings, also, are destroyed in vast numbers by
various enemies; for instance, on a piece of ground 3
feet long and 2 wide, dug and cleared, and where there
could be no choking from other plants, I marked all the
seedlings of our native weeds as they came up, and out of
357 no less than 295 were destroyed, chiefly by slugs and
insects. If turf which has long been mown, and the case
would be the same with turf closely browsed by quadru-
peds, be let to grow, the more vigorous plants gradually

^ "in October 1838," says Darwin, "that is, 15 months after I had
begun my systematic inquiry, I happened to read for amusement 'Malthus
on Population,' and being well prepared to appreciate the struggle for
existence which everywhere goes on from long-continued observation of
the habits of animals and plants, it at once struck me that under these
circumstances favorable variations would tend to be preserved, and
unfavorable ones to be destroyed. The result of this would be the forma-
tion of new species. Here then I had at last got a theory by which to
work."

2 One pair of elephants produces an average of only one baby elephant
in 10 years, and the breeding period is confined to from about the 30th to
the 90th year. For illustrations of the prolific nature of plants, see
paragraph 173, pp. 190-191.

* "Origin of Species" (New York, 1902 edition), pp. 83, 84.

DARWINISM 515

kill the less vigorous, though fully grown plants; thus out
of 20 species growing on a little plot of mown turf (3 feet
by 4) nine species perished, from the other species being
allowed to grow up freely.''

^^ Struggle for Existence'^ Used in a Large Sense. — "I
should premise," said Darwin, "that I use this term in a
large and metaphorical sense including dependence of one
being on another, and including (which is more important)
not only the hfe of the individual, but success in leaving
progeny. Two canine animals, in a time of dearth, may
be truly said to struggle with each other which shall get
food and live. But a plant on the edge of a desert is said
to struggle for Hfe against the drought, though more
properly it should be said to be dependent on the moisture.
A plant which annually produces a thousand seeds, of
which only one on an average comes to maturity, may be
more truly said to struggle with the plants of the same and
other kinds which already clothe the ground. The mistle-
toe is dependent on the apple and a few other trees, but
can only in a far-fetched sense be said to struggle with
these trees, for, if too many of these parasites grow on the
same tree, it languishes and dies. But several seedling
mistletoes, growing close together on the same branch, may
more truly be said to struggle with each other. As the
mistletoe is disseminated by birds, its existence depends
on them; and it may metamorphically be said to struggle
with other fruit-bearing plants, in tempting the birds to
devour and thus disseminate its seeds. In these several
senses, which pass into each other, I use for convenience
sake the general term of Struggle for Existence."

5. Survival of the Fittest. — In this struggle for existence
only those best suited to their environment will survive.

5l6 STRUCTURE AND LIFE HISTORIES

The dandelion from the seed that germinates first secures
the best light; the one that sends down the longest and
most vigorous root-system, that produces the largest, most
rapidly growing leaves will survive, and will tend to trans-
mit its vigorous qualities to its progeny. Less vigorous
or less ''lit" individuals perish. To this phenomenon
Herbert Spencer applied the phrase, ''survival of the fit-
test." Darwin called it ''natural selection," because it
was analogous to the artificial selection of favored types
by breeders of plants and animals. It will be readily seen,
however, that the process in nature is not so much a selec-
tion of the fittest, as a rejection of the unfit; the unfit are
eliminated, while the fit survive. It has been suggested
that "natural rejection" would be a better name than
"natural selection." "Variations neither useful nor in-
jurious," said Darwin, "would not be affected by natural
selection."

442. Difficulties and Objections.^The publication of
Darwin's "Origin of Species" aroused at once a storm of
opposition. Theologians opposed the theory because they
thought it eliminated God. Especially bitter antagonism
was aroused by Darwin's suggestion that, by means of
his theory "much light will be thrown on the origin of
man and his history." The unthinking and the careless
thinkers accused Darwin of teaching that man is descended
from monkeys. Neither of these accusations, however,
was true. Darwinism neither eliminates God, nor does it
teach that monkeys are the ancestors of men.

By slow degrees, however, men began to give more care-
ful and unprejudiced attention to the new theory, and not
to pass adverse judgment upon it until they were sure they
understood it. "A celebrated author and divine has

DARWINISM 517

written to me," says Darwin, ''that he has gradually
learnt to see that it is just as noble a conception of the
Deity to believe that He created a few original forms capa-
ble of self-development into other and needful forms, as
to believe that He required a fresh act of creation to supply
the voids caused by the action of His laws."

And in closing his epoch-making book, Darwin called
attention to the fact that, in the light of evolution, all
phases of natural science possess more interest and more
grandeur.

''When we no longer look at an organic being as a savage
looks at a ship, as something wholly beyond his compre-
hension; when we regard every production of nature as
one which has had a long history; when we contemplate
every complex structure and instinct as the summing up
of many contrivances, each useful to the possessor, in the
same way as any great mechanical invention is the sum-
ming up of the labour, the experience, the reason, and even
the blunders of numerous workmen; when we thus view
each organic being, how far more interesting — I speak from
experience — does the study of natural history become!"

"It is interesting to contemplate a tangled bank, clothed
with many plants of many kinds, with birds singing on the
bushes, with various insects flitting about, and with worms
crawling through the damp earth, and to reflect that these
elaborately constructed forms, so different from each
other, and dependent upon each other in so complex a
manner, have all been produced by laws acting around us.
These laws, taken in the largest sense, being Growth with
Reproduction; Inheritance which is almost implied by
reproduction; Variability from the indirect and direct
action of the conditions of life, and from use and disuse;

5l8 STRUCTURE AND LIFE HISTORIES

a Ratio of Increase so high as to lead to a Struggle for Life,
and as a consequence to Natural Selection, entailing Diver-
gence of Character and the Extinction of less-improved
forms. Thus, from the war of nature, from famine and
death, the most exalted object which we are capable of
conceiving, namely, the production of the higher animals,
directly follows. There is grandeur in this view of life,
with its several powers having been originally breathed
by the Creator into a few forms or into one; and that,
whilst this planet has gone cycling on according to the
fixed law of gravity, from so simple a beginning endless
forms most beautiful and most wonderful have been, and
are being evolved."

443. Objections from Scientists. — Objections to Dar-
win's theory were also brought forward by scientific men —
partly from prejudice, but chiefly because they demanded
(and rightly) more evidence, especially on certain points
which seemed at variance with the theory. For example,
they said, no one has ever observed a new species develop
from another; this ought to be possible if evolution by
natural selection is now in progress. The absence of
"connecting links," or transitional forms between two
related species was noted; the presence of apparently
useless characters (of which there are plenty in both ani-
mals and plants) was not accounted for; and the geologists
and astronomers claimed that the time required for evolu-
tion to produce the organic world as we now behold it is
longer than the age of the earth as understood from geolog-
ical and astronomical evidence.

There is not space here to summarize the answers to all
these objections. Suffice it to say that scientific investi-
gation since Darwin's time has given us reasonably satis-

DARWINISM 519

factory answers to most of them, so that now practically
no scientific man doubts the essential truth of evolution;
it is the corner stone of all recent science, the foundation
of all modern thought,

444. The Modem Problem. — But Darwinism left us
with a very large and very fundamental problem unsolved.
Upon what materials does natural selection act in the
formation of species? Obviously the ''fittest" survives,
but what is the origin of the fittest? This problem Darwin-
ism did not solve. The solution of it is one of the most
fundamental and important tasks now being undertaken
by biologists. The most effective attack is by the method
of experimental evolution, which forms the subject of the
next chapter.

CHAPTER XXXIII
EXPERIMENTAL EVOLUTION

445. A New Method of Study. — Previous to Darwin's
time the study of plants and animals, was carried on
chiefly by observations in the field. The science was
largely descriptive — a record of what men had observed
under conditions over which they did not endeavor to
exercise any control; it was accurately named ''Natural
History" — a description of Nature. But Darwin and a
few of his contemporaries, especially among botanists, be-
gan to make observations under conditions which they
determined and largely regulated. In this way the
problems were simplified, observation became more ac-
curate, and the endeavor was made to assign the prob-
able causes of the observed phenomena. With the intro-
duction of this experimental method, science began to make
rapid strides, and, more than ever before, facts began to
be, not only recorded, but interpreted and explained.

446. Hugo de Vries. — The director of the Botanic Gar-
den in Amsterdam, Holland, Hugo de Vries, was among
the first to demonstrate that the method of experiment
may be applied to the study of evolution. His plan was
to secure seed of a given species from a plant which he
believed to be pure with reference to a given character,
that is, not contaminated or mixed by being cross-pollin-
ated with another variety or species. The characters of
the parent plant were carefully noted and recorded by

520

J

EXPERIMENTAL EVOLUTION

521

photographs and written descriptions, and by preserving
dried and pressed herbarium specimens. The plants of
the second generation were carefully guarded from being
cross-pollinated, and thus ''pure" seed were secured for a
third generation. This was continued often for 25 or 30
generations of the plant, requiring as many years when a

Fig. 390, — Hugo de Vries. His pioneer studies of osmosis resulted in
fundamental contributions to our knowledge of that subject; his mutation-
theory is one of the most important contributions to the study of evolu-
tion since Darwin.

species produced only one crop of seed a year. Very care-
ful records and preserved specimens were kept of the plants
of each generation, and accurate comparisons were made
to see if any individuals showed a tendency to vary widely
from their parents in any significant way, such as showing
entirely new characters, not expressed in the parents, or
failing to manifest one or more of the characters of the
parent.

52 2 STRUCTURE AND LIFE HISTORIES

447. Two Klinds of Variation. — One of the first results
of de Vries's painstaking work was the demonstration of
what he believed to be a fundamental difference between
two distinct kinds of variation — continuous (or fluctuating)
and discontinuous (or saltative, i.e., leaping).

448. Continuous Variation. — Continuous variation is
quantitative — a case merely of more or less. It deals with
averages. Some flowers on a red-flowered plant may be
lighter or darker red, but, in a series of generations, the
average of a large number in each generation does not
vary, and the departure from the average never exceeds
certain limits. The flowers of a given species may have a
certain characteristic odor, but the odor may be stronger
in some flowers than in others, or in some individual
plants than in others. The plants grown from a handful of
beans of the same variety may vary in height within limits ,
but the average height of a large number will not vary in
successive generations, and will be characteristic of the
species or variety. In other words, continuous or fluc-
tuating variation is variation about a mean. It may
be illustrated by the bob of a swinging pendulum, which
continually fluctuates within definite limits about the
mean position assumed when the pendulum is at rest
(Fig. 396).

All plants and animals manifest fluctuating variation
in all their characters (Fig. 391), and such variations are
largely, if not entirely, dependent upon the environment.
A slight change in the kind of food elements supplied, or in
the amount of water or sunlight available will make the
leaves or petals a deeper or a paler color. Rich soil,
favoring a more abundant food supply, will cause a greater
average growth than poor soil, but unless the seed for

EXPERIMENTAL EVOLUTION 523

future generations is selected from the tallest plants,
and the richness of the soil is maintained, the plants will
revert to their normal, lower average of height. In other
words, the average height of the plants of any given variety
is a constant (unvarying) character, except that it may be

Fig. 391. —Branch oi Brachychiton diver sijolmm, illustrating fluctuating
or continuous variation in the shape of the leaves on one plant.

temporarily altered by careful selection of seeds from the
tallest or shortest individuals, or by choosing the largest
or the smallest seeds from any given plant, or by making
the soil richer or poorer. When the selection ceases, and
the soil is maintained at average fertility, the characteristic
average height of the plants is restored.

524 STRUCTURE AND LIFE HISTORIES

449. Illustrations of Continuous Variation. — In a quart

of beans, for example, there are no two seeds of precisely
the same proportion or size; some are longer, some shorter.
De Vries describes^ an experiment in which about 450
beans were chosen from a quantity purchased in the mar-
ket, and the lengths of the individuals measured. The
length varied from 8 to 16 millimeters, and in the following
proportions:

Millimeters 8 9 lo 11 12 13 14 15 16

Number of beans ... . i 2 23 108 167 106 33 7 i

The beans were then placed in a glass jar divided into nine
compartments, all the beans of the same length in the
same compartment. When this was done it was found
that the beans were so grouped that the tops of the columns
in the various compartments followed a curve, known as
Quetelet's^ curve (Fig. 392).

This curve may be plotted by erecting vertical lines
(ordinates) at intervals of i millimeter on a horizontal line
or base, the height of each vertical line being proportionate
to the number of beans having the length indicated in
figures at its base. This curve shows the frequency of
occurrence of seeds of any given dimension between the
two limits or extremes, and is therefore often referred to as
a rMrve of frequency. It should be noted that, in the case
illustrated, the greatest frequency (indicated by the high-
est point of the curve) very nearly coincides with the aver-
age dimension ; in other words, the more any given character

^ De Vries. "The Mutation Theory," vol. 2, p. 47, Chicago, 1909.

^ So named from its discoverer, Qu6telet (Ket-lay). As de Vries
states: "For a more exact demonstration a correction would be necessary,
since obviously the larger beans fill up their compartment more than a
similar number of small ones."

EXPERIMENTAL EVOLUTION

525

departs from the average for
that character, the less fre-
quent is its occurrence.

In another experiment,
ears of corn, harvested from
the same crop, were meas-
ured and found to vary
in length from 4}^ inches
to 9 inches; the largest num-
ber of ears (20) were 7
inches long. The greater
the departure from this
length, in either direction,
the fewer the individuals;
for the lengths 4 inches and

Fig. 392. — Demonstration of
Quetelet's law of fluctuating varia-
bility in the length of seeds of the
common bean (Phaseolus vulgaris).
Description in the text. (Redrawn
from de Vries.)

Fig. 393. — Curve of fluctuating variation (Quetelet's curve), formed by-
arranging 82 ears of corn in ten piles, according to the length of the ears.
The extremes were 4.5 and 9 inches. The ears were taken from unselected
material from a field of corn. (After Blakeslee.)

526 STRUCTURE AND LIFE HISTORIES

pt J

9

Fig. 394. — Photograph of beans rolling down an inclined plane and
accumulating at the base in compartments, which are closed in front by
glass. The exposure was long enough to cause the moving beans to appear
as caterpillar-like objects hopping along the board. If we assume that
the irregularity of shape of the beans is such that each may make jumps
either toward the right or toward the left in rolling down the board, the
laws of chance lead us to expect that in very few cases will these jumps
be all in the same direction, as indicated by the few beans collected in the
compartments at the extreme right and left. Rather the beans will tend
to jump in both right and left directions, the most probable condition
being that in which the beans make an equal number of jumps to the right
and to the left, as shown by the large number accumulated in the central
compartment. If the board be tilted to one side, the curve of beans would
be altered by this one-sided influence. In like fashion, a series of factors —
either of environment or of heredity — if acting equally in both favorable
and unfavorable directions, will cause a collection of ears of corn to assume
a similar variability curve, when classified according to their relative size.
Such curves, called Quetelet's curves, are used by biometricians in classify-
ing and studying variations in plants and animals. (Photo by A. F.
Blakeslee. Legend slightly modified from Journal of Heredity, Tane,
1916.)

EXPERIMENTAL EVOLUTION 527

9 inches the frequency was zero. When the ears were
arranged in piles according to their length, the tops of
the piles indicated the curve of frequency (Fig. 393).

The curve of frequency indicates the quantitative dis-
tribution of any character or quality when its occurrence
is dependent largely upon chance. This is strikingly
illustrated by the grouping of bean seeds rolled down a
smooth inclined plane, and collected in receptacles at the
bottom (Fig. 394). The seeds are started rolling midway
between the edges of the plane; the chances are about
equal for some of the seeds to fall into the outside compart-
ments, but the odds are vastly in favor of their landing at
or near the center. Thus they group themselves so that
the tops of the piles form a curve of chance variation.
When the result is influenced in one direction more than
in another the crest of the curve will be nearer one extreme
than the other, and the curve is to that extent skew. The
curve of bean seeds in Fig. 394 is slightly skew toward
the right-hand extreme. Suggest one or more reasons why.

450. Fluctuating Variation and Inheritance. — When
the ancestry is not mixed or hybrid the curve of frequency
of any character in one generation ordinarily tends to
recur in successive generations of descendants,^ providing
the environment remains essentially the same.

451. Discontinuous Variation. — Long before Darwin,
students of plants and animals had observed a different
kind of variation than continuous — one which was not
quantitative but qualitative, resulting in the expression of
new characters, or of a new curve oj frequency; that is, in
fluctuation about a new mean. Plants from some of the

^ The behavior of hybrid descendants is a special case described in
::hapter XXXVII.

528 STRUCTURE AND LIFE HISTORIES

seeds of a red-flowered specimen bear flowers, not that
vary from deeper to paler red, but that suddenly, at one step,
have become pure white; one or more seeds from an odor-
less plant may give rise to individuals whose flowers are
sweet-scented; or vice versa, odorless specimens may spring
at one leap, not by gradual minute changes, from those that

Fig. 395. — Leaves of varieties of the Boston fern (Nephrolepis), showing
(from left to right) progressive branching of the pinnae and pinnules, and
illustrating so-called "orthogenetic saltation." (After R. C. Benedict.)

are fragrant; in one generation the factors controlling height
are so altered that, in successive generations, the average
of height may change by either more or less, so that the
heights of the individuals fluctuate about a new mean. In
other words, we recognize a second type of variation — not
the fluctuation of individuals about an unchanging mean,
but the appearance of a new mean, about which the given
character in individuals may fluctuate.

EXPERIMENTAL EVOLUTION

529

When discontinuous variation proceeds along a definite
line through several successive generations, each step being
an intensification of the preceding one, it is designated
^' or tho genetic saltation^' (^ig- 395)-

461. Illustration of the Pendulum.— The difference be-
tween discontinuous and fluctuating variation may be

V..,/.s,,^ Fluciuatin^
Extreme

^■2^ F/uUuatn
Extreme c

r- Mean

Extreme

Meo-n

Fig. 396. — Diagram to illustrate the difference between fluctuating
variation and mutation; 0, original point of suspension; M, new point of
suspension after the mutation has occurred.

aptly illustrated by a swinging pendulum (Fig. 396). The
vertical position, assumed when at rest, is the mean of all
positions that may be assumed as the pendulum swings;
34

530 STRUCTURE AND LIFE HISTORIES

the oscillation about this mean illustrates continuous or
fluctuating variation.

But we may conceive that the point of suspension of the
pendulum changes, as shown in the figure. The pendulum
continues to oscillate, but now about a new mean position;
a neiv character has been introduced, with its own fluctua-
tions of more or less.

453. Mutations. — Darwin, as well as others before and
after him, recognized both kinds of variation, but de Vries
was the first to work out in detail the hypothesis that
discontinuous variations furnish the material for natural
selection. Discontinuous variations he called mutations;
plants which give rise to or ''throw" them are said to
mutate. A plant that arises by mutation is an elementary
species, or mutant; and the theory that mutations (and not
fluctuations) explain the origin of the fittest, and supply
the materials upon which natural selection operates in the
formation of new species, de Vries called the ^nutation theory.

454. Examples of Mutation. — The kohlrabi, cauli-
flower, and other horticultural varieties of the wild cliff-
cabbage (Fig. 397), arc believed to be mutants, and to have
arisen, not by the prolonged selection of fluctuating varia-
tions, but at one step — in one generation — as "sports"
of the wild Brassica oleracea. Strawberry plants without
runners, green dahlias and green roses, the common seed-
less bananas of the markets, the Shirley poppies, pitcher-
leaved ash trees, Pierson's variety of the Boston fern,
5-9- "leaved" clovers (Fig. 398), white black-birds (and
other albinos, including albino men), moss-roses, thornless
cacti and thornless honey-locusts, red sunflowers, com-
posites with tubular corollas in the ray-flowers (Fig. 399),
and the innumerable white flowered varieties of colored

EXPERIMENTAL EVOLUTION

531

532

STRUCTURE AND LIFE HISTORIES

flowered species, are all illustrations of mutation. Fre-
quently the mutative change occurs in a lateral bud, pro-
ducing a ''bud-sport" (Fig. 400). Such was the origin of
the seedless navel orange from the seed-bearing orange.

Fig. 398. — Clover leaves with three to nine leaflets, illustrating a
tendency to mutate. The normal clover leaf is a pinnately compound
leaf with three leaflets. Plants with leaves having five to nine leaflets
constitute a "half-race," i.e. the normal character is active, the anomaly
semi-latent. (Photo by the author; specimens from cultures of G. H.
Shull.)

455. The Evening-primrose. — In 1886 de Vries began
to search for a species that was in a mutating condition,
believing that any given species is at some periods in its
history more labile or changeable than at other periods.
After a long search he found in an abandoned potato field
at Hilversum, near Amsterdam, a large number of plants
of Lamarck's evening-primrose {(Enothera Lamarckiana)
(Fig. 401.)

"That I really had hit upon a plant in a mutable period
became evident from the discovery, which I made a year

EXPERIMENTAL EVOLUTION

533

later, of two perfectly definite forms which were immedi-
ately recognizable as two new elementary species. One
of them was a short-styled form: O. brevistylis, which at
first seemed to be exclusively male, but later proved to
have the power, at least in the case of several individuals,

Fig, 399. — Yellow daisy, or cone-flower {Rudbeckia sp.), showing varia-
tions of the character of mutations in the ray- and disc-flowers. At d
the normally ligulate corollas are tubular; at / they have all aborted,
except two; at h many of the normally tubular disc-flowers have become
ligulate, making a nearly "double flower." (Photo by E. M. Kittredge.)

of developing small capsules with a few fertile seeds. The
other was a smooth-leaved form with much prettier foliage
than 0. Lamarckiana, and remarkable for the fact that
some of its petals are smaller than those of the parent type,
and lack the emarginate form which gives the petals of

534

STRUCTURE AND LIFE HISTORIES

Lamarckiana their cordate character. I call this form
O. IcEvifolia."

''When I first discovered them (1887) they were repre-
sented by very few individuals. Moreover each form
occupied a particular spot on the field. O. brevistylis
occurred quite close to the base from which the Oenothera

Fig. 400. — A plant of the evening-primrose {QLnolhera biennis) which,
by "bud sporting," has given rise (at the left) to a branch having the
characters of another species.

had spread; O. IcBvifolia on the other hand, in a small
group of 10 to 12 plants, some of which were flowering
whilst others consisted only of radical leaves, in a part of
the field which had not up to that time been occupied by
0. Lamarckiana. The impression produced was that all
these plants had come from the seeds of a single mutant.
Since that time, both the new forms have more or less
spread over the field" {de Vries),

EXPERIMENTAL EVOLUTION

535

Another mutant of (E. Lamarckiana was called by
de Vries (Enothera gigas (Fig. 402). The cell-nuclei of
this mutant have twice as many chromosomes as the
parent form.

Fig. 401. — Lamarck's evening-primrose {(Enolhcra Lanidrckiana). A
mutating species. (After de Vries.)

456. The Test of a Mutation.— The deciding test as to
whether a given new form, arising without crossing from
a form that has bred true for at least two generations, is
really a mutant or merely a fluctuating variant, is to see
if it breeds true to seed for the new character or characters.
If it does it is a mutant; otherwise it is not. It is clear,
therefore, that the only way the problem can be followed
out is by experiment — hence the term experimental evolu-

536

STRUCTURE AND LIFE HISTORIES

tion. The great contribution of de Vries is that he demon-
strated that evolution may be studied by the method of
experimentation. The next step for him to take after
discovering the two forms that he supposed to be mutants,
was to breed them in carefully guarded, pedigreed cultures

Fig. 402. — Giant evening-primrose QLnothera gigas, a mutant from
(Enothera Lamarckiana, originated in 1895. (Cf. Fig. 401.) (After de
Vries.)

in his garden, and also to breed the parent form, (Enothera
Lamarckiana, and see if he could observe the two forms
above mentioned, or other mutants, arise from seed pro-
duced without crossing with any other species.

The entire story of this classical series of experiments
is too long to be told here. Suffice it to say that de Vries
did observe numerous other aberrant forms arise, and also

EXPERIMENTAL EVOLUTION 537

found that they bred true (except for additional muta-
tions) when propagated by seed for over 25 years — that
is, they were true mutations.

467. Relation of Mutation Theory to Darwinism. — The
mutation theory is not intended by de Vries to supplant
the theory of natural selection, but to demonstrate that
the materials upon which selection acts in the formation
of new species are mutations, and mutations only — never
fluctuating or individual variations. In the second place
the mutation theory explains away numerous objections
to natural selection. It shows how characters that are
never of vital importance^ — i.e., matters of actual life or
death — to a species may arise and be perpetuated. With-
out mutation this is difficult to explain, ^ and yet many, if
not most, of the characteristics by which different species
are distinguished from each other are of this kind — not,
so far as we can see, absolutely essential to the life of the
species. Mutation also offers a method by which evolu-
tionary changes may take place within a much shorter
time- period than was demanded by the natural selection
of fluctuations.

Incidentally, the mutation theory clearly shows that the
absence of ''connecting links" between species is no argu-
ment against evolution, but is, on the contrary, just what
we might expect to find.

458. Value of the Mutation Theory.— As stated above,
the elaboration of the mutation theory has furnished the

^ As required by Darwin's theory. See quotation on p. 516.

2 Other explanations have been offered. For example, sometimes two
characters appear to be always associated, so that the presence of one
involves the presence of the other; as a mane and maleness in the lion,
dicotyledony and exogeny in Angiosperms.

538 STRUCTURE AND LIFE HISTORIES

biological world with a new method of study. It has
demonstrated that the method of evolution may be studied
by experimentation, and this demonstration is, probably,
de Vries's greatest service to science. The mutation
theory should also be of great service to breeders. It
has helped to establish plant and animal breeding on a
more scientific basis, has pointed the way to correct
methods where men were formerly groping in the dark,
and has showed that results of commercial value do not
require a life time, but may be obtained within two or
three seasons. By the application of modern methods it
has been possible, within a few seasons, to obtain new
strains of oats yielding as much as 14 bushels per acre
more than the variety from which they were derived, and
to produce new strains of corn not only giving a larger
yield, but maturing nearly two weeks earlier than the
parent variety.

459. Classification. — Mere information is not science.
A ''book of facts" is not a scientific treatise, for it is
composed of bits of unrelated information, presented on
some artificial basis of sequence, as for example, alpha-
betically. Scientific knowledge, in addition to being as
accurate as possible, is characterized by having an orderly
arrangement in one's mind, and this order is based on a
logical, fundamental relationship between the facts and
ideas. Only by such an arrangement of our ideas are we
able to understand their relation to each other, their rela-
tive importance, and their real significance. Classifica-
tion, therefore, is essential to all science. The very
existence and use of such words as oaks, maples, roses,
indicate that men have grouped or classified their ideas
of certain plants {e.g., red oaks, white oaks, black oaks,

EXPERIMENTAL EVOLUTION

539

bur oaks, live oaks, etc.), and have thereby recognized
that certain kinds resemble each other closely enough to be
placed in one group with a group-name. All the com-
mon names of plants indicate the recognition of classes —
a classification.

Fig. 403. — Linnaeus, the great classifier (1707-1778). lie is wearing a
sprig of the twin-flower {Linncea borealis), one of his favorite flowers, and
named after him by his friend, Gronovius. He is regarded as the father
of modern systematic botany.

460. Evolution and Classification. — Without the guiding
idea of evolution classification would be arbitrary and
artificial. Linnaeus classified plants on the basis of the
number of stamens they possessed, thus placing in one
group plants now known to be wholly unrelated, except

540 STRUCTURE AND LIFE HISTORIES

that they have a chance similarity in the number of
stamens. In like manner we may group together plants
with red flowers, blue flowers, or pink flowers, as is often
done in "popular" guides to the wild flowers. This has
its value, but it tells us really nothing about the significant
relationship between plants, does not help clear up our
own ideas, does not show the gaps in our knowledge and
tell us where to search for new facts to fill up the gaps.
Evolution, by showing that plants are all related to each
other by descent, just as are the members of a large family
of persons, discloses to us the only true basis of classifica-
tion— the plan that endeavors to arrange all plants so as to
show their descent and their relationship to each other.
Without evolution there might be any number of arbitrary
systems; on the basis of evolution there can, in the end,
be but one true system, which all students must accept,
because it will be a true record of what has actually oc-
curred in the history of development of the plant or animal
worlds. In other words, if our knowledge should ever he-
come sufficiently complete and exacts the classification oj
plants would give a summary — a bird's eye view — of the
course of evolution and the history of development. To
approximate this end is one of the largest problems of botany.

CHAPTER XXXIV
HEREDITY

461. Importance of the Study. — i. To Pure Science. —
No knowledge is more fundamental than a correct under-
standing of the laws of heredity. Its fundamental im-
portance to pure science becomes evident at once when we
consider that, since evolution has been accomplished by
the descent of one organism from another, there have been
one or more unbroken lines of inheritance from the dawn
of plant life to the present. Hence, until we know the
laws of heredity, we cannot fully understand expression,
reproduction, development, variation, sex, or evolution.

2. To Applied Science. — Correct ideas concerning he-
redity are absolutely essential to such phases of applied
science as animal and plant breeding. In the light of such
knowledge the breeder can avoid making useless experi-
ments, and can accomplish desired results more quickly,
more cheaply, and with greater certainty of success.

3. To Man. — A correct knowledge of the principles of
heredity is vital to mankind ; no knowledge is more so. To
realize this, we have only to reflect that our own characters
are very largely the result of inheritance from our ances-
tors; and not only our characters, but our physical char-
acteristics, our vigor of mind and body, our capacity for
education, our susceptibility to disease, and often the
actual existence of some disease within our bodies or minds.

462. Heredity Reduced to Its Lowest Terms. — We may
study heredity under the very simplest conditions in the

541

542 STRUCTURE AND LIFE HISTORIES

descent of one-celled organisms, such as Pleurococcus.
This plant, as we learned in Chapter XVIII, is a globule of
protoplasm, containing chlorophyll, and surrounded by a
cellulose cell-wall. But why is it globular, why does it
contain chlorophyll, why has it a cell- wall of cellulose?
Why is it not elliptical, why is it not red instead of green,
why does it have a cell-wall, instead of existing naked like
the Plasmodium of a slime-mold, why is its cell-wall of
cellulose, rather than of lignin or chitin?

The short answer is, because its ancestors, for ages and
ages, have possessed the characteristics which now char-
acterize Pleurococcus plants. But that only puts the
question back an indefinite number of generations. The
real reason is, because the Pleurococcus protoplasm pos-
sesses a physical and chemical constitution — or in other
words a mechanism — that, under normal external condi-
tions, manufactures green pigment instead of red, cellulose
instead of lignin, or any other substance, at the surface,
and makes the cell-wall of even resistance to the osmotic
pressure within, thus producing a sphere and not an ellip-
soid, or filament, or any other shape.

463. What is Inheritance? — When the Pleurococcus cell
divides, this wonderful, invisible mechanism — the certain
definite physical and chemical constitution — is transmitted
to each of the daughter-cells; each, in other words, re-
ceives Pleurococcus protoplasm. This protoplasm, with
its definite organization, constitutes the inheritance. The
daughter-cells do not inherit a spherical shape (as is evident
from Fig. 183), but a definite kind of protoplasm, cell-sap
of certain osmotic properties, and surface cellulose of even
elasticity, so that, in surroundings uniform on all sides,
a spherical shape must finally result. The shape is an

HEREDITY 543

expression of the inheritance for the given environment.
Under different external conditions the expression might
he different; hut the inheritance would he the same. The
chlorophyll in the daughter-cells, immediately after cell-
division, is a direct inheritance, but the chlorophyll subse-
quently manufactured, and the green color which it gives
to the plant, are not inherited; they are expressions of the
inheritance — which in this instance is a chloroplastid that
reproduces itself by division, and manufactures chlorophyll
in the presence of sunlight. Under abnormal external
conditions the mechanism may not act, or may act ab-
normally, so that yellow pigment appears instead of green
— or in darkness no pigment at all. In either case the in-
heritance is the same, but the expression varies. A mod-
ern writer has defined inheritance as all that an organism
has to start with. It is the protoplasmic substance, with
all its potentialities, passed on from parent to offspring.

464. Inheritance Versus Expression. — In the light of
this information, obtained by a study of the lowly Pleuro-
coccus, we are able to understand that what we inherit
from our parents or grandparents, is not a certain shape of
nose, a certain characteristic gait, a musical or mathe-
matical bent of mind, a quick temper, but a substance
(protoplasm) possessing a very delicate, intricate, and
characteristic constitution or mechanism. Under certain
conditions this inheritance may so express itself as to
cause resemblance in some physical or mental trait; or it
may find a quite different expression, as when parents of
medium height have tall children, or parents musically
inclined have children that do not care for music ; or sweet-
peas having white flowers only, produce, when crossed,
peas having colored flowers. Or again, not all that is in-

544 STRUCTURE AND LIFE HISTORIES

herited may be expressed; this is illustrated when chil-
dren resemble, not their parents, but their grandparents.
Here the parents transmitted an inheritance which, in
them, found no expression. A remarkable illustration of
inheritance without expression is seen in the case of the
alternation of generations (pages 1 81-183) The initial
protoplasm of the sporophyte is all inherited through the
fertilized egg from the gametophytes, but most of the
gametophytic characters do not appear in the sporophyte,
nor do the typically sporophytic characters find expres-
sion in the gametophyte.^

465. Inheritance Versus Heredity. — As stated above,
the inheritance is that which is actually transmitted from
parent to offspring. The fern-spore, for example, is the
inheritance of the fern gametophyte from the sporophyte.
Heredity is the genetic relationship that exists between suc-
cessive generations of organisms. The relation between two
brothers and their parents is similar — it is one of heredity ;
their inheritance may be quite different.

466. Inheritance and Reproduction. — Inheritance is, of
course, inseparably linked with reproduction and may be
studied in connection with the three following types:

I. In vegetative propagation"^ the new plant, as noted in
Chapter XVII, is obviously only a portion of the vegeta-
tive tissue of the parent plant, isolated and growing inde-
pendently by itself. The separation of the propagating
piece from the parent is often (though not always) mechan-

^ The chlorophyll, of course, is an exception. But the osmotic strength
of the cell-sap is a different expression in gametophyte and sporophyte,
otherwise the young sporophyte could not live parasitically upon the
gametophyte.

* E.g., by means of tubers, cuttings and "slips," bulbs and bulbils,
gemmae, "runners," scions, etc.

HEREDITY

545

ical and artificial. The protoplasm remains unaltered by
the act of separation, reduction divisions are not involved,
and the inheritance, except in bud-variations, is unaffected
by the change. This is evident in those cases where the

\

Wj^

^uA

^

"--,

^7^^

• vVv

A.i

-<^.

a

1

Fig. 404.^ — -Graft of tomato {Lycopersicum esculcnlum) on tobacco
{Nicoiiana tabacum). On the tomato are grafted Solanum nigrum, S.
integrifolium, and Physalis alkekengi. Cf. Fig. 243. (Graft made by
Mr. M. Free.)

isolated piece is grafted upon another plant; the specific
or varietal characteristics of the scion are seldom, if ever,
affected by the stock. Thus, in the experiment illustrated
in Fig. 404, a tomato stem was grafted upon a tobacco

35

546 STRUCTURE AND LIFE HISTORIES

plant, and upon the tomato were grafted three other
species — Solanum nigrum, Solanum integrifolium, and
Physalis alkekengi. Each species was apparently not in
the least altered by this drastic change in the conditions
of its life.

2. In asexual reproduction hy spores the situation is
quite similar to that in vegetative propagation, but in
certain cases there is abundant opportunity for the proto-
plasm to become more or less altered during the compli-
cated changes that accompany nuclear division. This is
especially the case in the reduction divisions preceding
spore-formation in the sporophytes of higher plants, espe-
cially when the plant is a hybrid; and in spore-formation
in the sporangia produced from the zygospore of some of
the filamentous fungi, like Mucor Mucedo. In the latter
case the nuclear divisions, some time preceding spore-pro-
duction, result in separating out the female (+) and male
(— ) strains, so that the spores in a given sporangium are
unlike as to sex — some being female (+), some male (— ).
This will be discussed more fully in the next chapter.
Such changes result merely in distributing the heritable
units {genes) of the mother-cell unequally to the daughter-
cells, but introducing nothing new; they may, however,
result in the complete loss of one or more heritable units,
or in the formation of a new one, not existent in the parent.
In the latter two cases we recognize a mutation. No hard
and fast line can be drawn between the various kinds of
asexual reproduction; there are various degrees of transi-
tion between reproduction by spores, gemmae, bulbs and
tubers, and the artificially severed buds and scions used
in grafting and "slipping."

3. In sexual reproduction there intervene between par-

HEREDITY 547

ents and offspring, not only the complicated reduction
divisions involved in the formation of the gametes, but
also the nuclear and cell-fusions accomplished by the union
of the egg and sperm in fertilization. Both processes —
the formation of the gametes, and their fusion — offer
almost unlimited opportunities for alterations of the pro-
toplasm— especially that of the nucleus. This method of
reproduction, therefore, has the very greatest interest and
importance for the study of heredity. In the fertilized
egg^ are united the inheritances from two parents — from
two distinct lines of ancestry — protoplasms (germ-plasms)
with two entirely different histories extending back into
the past, no one knows how far. How will these two
inheritances affect each other when they intermingle in
the fertilized egg? Will one tend to inhibit or check cer-
tain characteristics or functions of the other; will they
evenly blend, so as to produce an expression intermediate
between that of the parents; or may entirely new sub-
stances be formed or new combinations take place, result-
ing in an entirely new expression in the offspring?

467. Methods of Study. — To endeavor to answer the
questions just asked is as fascinating an occupation as it is
important, and the answers are significant for man, as well
as for plants. It is indeed, a fortunate thing that prin-
ciples ascertained by studying plants apply equally to man
and other animals, since plants are so much easier to
handle in experimental investigations.

We may go about the answering of these questions in
either of two ways. We may gather large numbers of
statistics to measure and analyze {statistical or hiometrical

* The fertilized egg (as Thomson has pointed out) is the inheritance,
and becomes, in the mature individual, the inheritor.

548 STRUCTURE AND LIFE HISTORIES

method), or we may employ the experimental method. The
method of biometry enables us to deal with a larger number
of individuals, but the material studied is usually a mixed
population, whose history is only imperfectly known, the
conditions are more complex, and little if at all under
control. By the experimental method it is not necessary
to deal with such large numbers; we may choose carefully
pedigreed material about the history of which we have
more or less accurate knowledge, and we may greatly
simplify and control the conditions under which we make
our observations. The largest advance toward the solu-
tion of the problems of inheritance has been made by the
experimental method, in the form first employed success-
fully by Gregor Mendel. This method will be briefly
explained in the next chapter.

CHAPTER XXXV

EXPERIMENTAL STUDY OF HEREDITY

468. Gregor Mendel.— Two of the most important
contributions ever made to biological science, namely,

_ Fig. 405.— Gregor Mendel, at the age of 40. His theory of alternate
inheritance (Mendelism), based largely on experiments with the garden
pea, IS the most important and most fruitful contribution ever made to
the study of inheritance.

Mendel's laws of heredity, and his method of investigating
them, were made by a teacher who studied plants as a pas-
time because he loved to do it. This man was Gregor

549

550 STRUCTURE AND LIFE HISTORIES

Mendel, a monk in the monastery at Briinn, Austria, where
he finally became abbott. In order to understand his
work clearly the student should examine carefully the
structure of the edible or garden pea, the chief plant with
which Mendel worked.

469. MendePs Problem. — It was a favorite study of
Mendel's to hybridize {i.e., cross-pollinate) plants of differ-
ent species and varieties, and observe the behavior of the
resulting hybrids in successive generations. The problem
which he endeavored to solve was the law or laws "gov-
erning the formation and development of hybrids,"^ with
special reference to the laws according to which various
characters of parents appear in their offspring.

470. MendePs Method. — He recognized that, in order
to solve the problem, attention must be given to at least
three points, as follows:

1. "To determine the number of different forms under
which the offspring of hybrids appear."

2. "To arrange these forms with certainty according to
their generations."

3. "To ascertain accurately their statistical relations,"
that is, to express the results quantitatively.

No previous student had recognized the fundamental
importance of these requirements.

471. Choice of Material. — Mendel realized that the
success of any experiment depends upon choosing the
most suitable material with which to experiment. He
laid down the requirements as follows:

^ All the quotations in this chapter are from an English translation of
Mendel's original paper. His form of expression has been preserved
as far as possible, even when the "quotes" are omitted.

EXPERIMENTAL STUDY OF HEREDITY 55 1

1. "The experimental plants must necessarily possess
constant differentiating characters.^^'^

2. "The hybrids of such plants must, during the flower-
ing period, be protected from the influence of all foreign
pollen, or be easily capable of such protection."

3. "The hybrids and their offspring should suffer no
marked disturbance in their fertility in the successive
generations."

Mendel also called attention to the advantage of choos-
ing plants which, like the peas, are easy to cultivate in
the open ground and in pots, and which have a relatively
short period of growth.

472. Characters Chosen for Observation. — "Each pair
of differentiating characters [have been thought to] unite
in the hybrid to form a new character, which in the pro-
geny of the hybrid is usually variable. The object oj the
experiment was to observe these variations in the case of each
pair of differentiating characters, and to deduce the law ac-
cording to which they appear in successive generations. The
experiment resolves itself therefore into just as many
separate experiments as there are constantly differentia-
ting characters presented in the experimental plants."
The following were the characters chosen for observation:

I. The difference in the shape of the ripe seeds (round
and smooth vs. angular and wrinkled).

^ Differentiating characters are those in respect to which the two species
or varieties to be crossed differ. The possession of chlorophyll by the leaves
of peas, for example, is a common character. " Common characters are
transmitted unchanged to the hybrids and their progeny." The color of
the corolla (for example, white in one species and purple in the other) is a
differentiating character, serving to differentiate or distinguish one species
from another.

552

STRUCTURE AND LIFE HISTORIES

2. The difference in the color of the cotyledons (pale
or bright yellow, or orange vs. light or dark green).

3. The difference in the color of the seed-coat (white
vs. gray, gray-brown, leather-
brown, with or without violet
spotting, etc.).

4. The difference in the form of
the ripe pods (deeply constricted
between the seeds and more or
less wrinkled, or the opposite).

5. The difference in the color
of the unripe pods (light or dark
green vs. vivid yellow) .

6. The difference in the posi-
tion of the flowers (i.e.j axial vs.
terminal) .

7. The difference in the length
of the stem (the extremes chosen
were ''tails" 6 to 7 feet, and
''dwarfs" % feet to i}i feet in
height).

473. Artificial Hybridizing. —
The edible pea is commonly self-
fertilized; therefore, to make
crosses it is necessary carefully to
remove the stamens of one flower
before the anthers have begun to shed their pollen, and
then place pollen from another flower on the stigma. The
flowers must then be carefully guarded, e.g., by tying
paper bags over them (Fig. 406), to prevent other pollen
being deposited by insects or otherwise. In this way the
experimenter knows just what characteristics enter into

Fig. 406. — Method of pro-
tecting flowers from foreign
pollen by paper bags, in plant-
breeding experiments. (After
O. E. White.)

EXPERIMENTAL STUDY OF HEREDITY 553

the hybrid. Careful record is kept of all data, and plants
produced in this way, with ancestral characters noted
and recorded, are called pedigreed. Plantings of such
plants are called pedigreed cultures.

In many species, in "making the cross '^ {i.e., doing the
cross-pollinating) great care must be taken to avoid con-
tamination from foreign pollen, of which the air may be
full.^ The fingers and all instruments are usually rinsed
in alcohol before each operation, to insure kilUng any
foreign pollen that might be present. Numerous other
precautions are also taken.

When the hybrid plants are mature, careful observa-
tions of whatever character is under observation are
made and recorded. Whenever possible the observation
should be quantitative.

474. MendePs Discoveries. — We may illustrate Men-
del's results in a simple manner by choosing, as the
pair of contrasted characters, smooth and wrinkled seeds
of the pea. Removing all the stamens from flowers of a
variety having smooth seeds, he pollinated those flowers
with pollen from a plant bearing wrinkled seeds.

It should now be kept clearly in mind just what the
inheritance of the fertilized egg is in such a case. From
the pistillate plant the inheritance, contributed by the
egg-cell, included the protoplasmic properties (whatever
they may be) which, when free to produce their effect,
cause smooth seeds; from the staminate parent the in-
heritance, contributed by the sperm-cell, included the
protoplasmic properties, which, when free to act, cause
wrinkled seeds.

I. Law of Dominance. — What Mendel actually found

^ See p. 423, paragraph 376.

554

STRUCTURE AND LIFE HISTORIES

by his experiments was that, in such a cross, all the seeds
of the hybrid plants are smooth. The inheritance was
"smooth" and "wrinkled," but the expression was of
only one type — smooth. A character thus expressed,

4 fc 4 4

II II nil

ckI

^

Fig. 407. — Mendelian segregation in the edible pea (Pisiim sativum).
Full explanation in the text. (Cf., Fig. 408.'^

to the exclusion of another, in the first filial (Fi) genera-
tion Mendel called dominant, and the phenomenon he
called dominance; the other character is recessive. From
such observations Mendel formulated the law of domi-
nance, as follows: When pairs of contrasting characters

EXPERIMENTAL STUDY OF HEREDITY 555

are combined in a cross, one character behaves as a dominant
over the other, which is recessive.

By similar experiments Mendel found that, in the coty-
ledons, yellow is dominant over green, tallness over dwarf-
ness, axial flowers over terminal, and so on. Such pairs
of contrasting characters are called allelomorphs.

2. Law 0] Segregation. — But what will happen if the
first filial (Fi) generation is inbred or self-poUinated. Its
inheritance included factors that make for both "smooth"
and "wrinkled," but the expression was all of one kind
only. The experiment was made, and Mendel found that
the second fihal (F2) generation included plants, part of
which possessed only smooth seeds, while the others had only
wrinkled seeds (Fig. 407). " Transitional forms were not
observed in any experiment." This illustrates in a strik-
ing way the difference between inheritance and expression.

475. Ratio of Segregation. — But now we come to that
feature of Mendel's experiments which, perhaps more than
anything else, made them superior to all others that had
preceded. He carefully counted the number of plants
bearing each kind of seed, and found that the number
of smooth-seeded plants was to those with wrinkled
seeds as 3 :i.

476. Theory of Ptirity of Gametes. — When the wrinkled
seeds (one-fourth of the total crop) were sown they all
bred true to wrinkledness — their descendants of the F3
generation bearing only wrinkled seeds. The expression
was alike in every case. The gametes that united to
produce these plants were therefore considered pure for
^^ wrinkledness;^^ that is, it was inferred that they did
not carry any inheritance tending to produce smoothness
of seed.

556

STRUCTURE AND LIFE HISTORIES

Fig. 408. — Mendelian segregation in maize, a, the starchy parent; 6,
the sweet parent; C, the first hybrid {Fi) generation, produced by crossing
a and h, showing the dominance of starchiness; d, the second hybrid (F2)
generation, showing the segregation of starchiness and sweetness with the
ratio of three starchy to one sweet (wrinkled) grain. Lower row, daughters
of d; e, /, and g resulted from planting starchy grains. One ear in three is
pure starchy, the other two mixed; h, result of planting sweet (wrinkled)
seeds. They are pure recessives, and the ear is pure sweet. (After East.)
(Cf. Fig. 407.)

EXPERIMENTAL STUDY OF HEREDITY 557

477. Not All Dominants Alike.— But when the seeds of
the F2 plants, having only smooth seeds, were sown it
was found that the dominants were not alike, except in
external appearance. The seeds, though all appeared
smooth, carried different inheritances. One-third of
them {i.e., one-fourth of all the seed produced by the F2
generation) bred true to smoothness, being therefore pure,
or homozygous, for smoothness; the other two-thirds of
the dominants {i.e., one-half of all the seed produced)
again segregated in the ratio of 3 : i — one-fourth wrinkled
and three-fourths smooth, showing that they were hetero-
zygous; that is, that they still carried inheritance from
both the wrinkled and smooth-seeded grandparents.

If we designate the first parental generation by P, the
dominant character (whatever it may be) by D, and the
recessive character by R, then the facts above described
may be diagrammed as follows:

D9 X Rd' P (ist Parental generation)

D (R) Fi (ist Hybrid generation)

4

1 T

3D iR F2 (2d Hybrid generation)

iD + 2D(R)

D 3D iR R Fs (3d Hybrid generation)

478. Significance of the Mendelian Ratio. — The ratio
3 : I or, as it appears on analysis, i : 2 : i, is the ratio that
one might expect, or that might be predicted, on the basis of
chance. Students of algebra will recognize in it the essence
of the familiar square of a + ^, namely, a^ + 2ah + 6^,

558 STRUCTURE AND LIFE HISTORIES

where a and h each equal i. In the plants the multi-
plication of inheritances (produced in fertilization) was
as follows:

eggs {s + 7C') X sperms {s + ^^0 = ^^ + 2sw -\- ww

where w = wrinkling and s = absence of wrinkling, i.e.,
smoothness.

479. Theory of Purity of Gametes. — The above ratio
is what we would expect if half of the egg-cells and half
of the sperm-cells in a heterozygous plant (one of the Fi
generation), carried only character-units or determiners^
that make for smoothness; the other half only those
factors that make for wrinkling, giving 5 and w egg-cells,
and s and w sperm-cells in equal numbers. Therefore, in
pollination the chances would be equal that an s-egg would
be fertilized with either an 5-sperm or a w-sperm, giving
{s -\- w) X {s -\- w) = ss -{- 2SW -\- WW. Since s is dominant
over w the product should be written:

ss -j- siii") + s{w) -\- ivw

giving in external appearances 35 + iw. Since the re-
sult actually observed is what it would be ij the gametes
were thus ''pure" for smoothness and wrinkling, Mendel
concluded that they really are, and moreover that each
character behaves as a unit, appearing and disappearing
in its entirety.

480. Character -units versus Unit-characters. — As just
stated, Mendel held that the various visible characters of
his plants (dwarfness, for example) behaved as units,

^ The substance or condition (protoplasmic constitution), whatever it is,
in the germ-cells that corresponds to any given character of the plant is
variously referred to by the terms factor, determiner , gene (= producer),
character-unit, and others. These terms are essentially synonyms.

EXPERIMENTAL STUDY OF HEREDITY 559

either appearing in their fullness, or not appearing at all.
From more careful observations we know that such is
not the case. A blossom may, for example, be more or
less pink, an odor more or less strong, dwarfs are not
all the same height, but fluctuate around a mean. We
conclude therefore that characters do not behave as
units, and that the conception of ''unit-characters" is
erroneous. The evidence does, however, seem to justify
the conclusion that the factor or factors, whatever they
may be,^ that are causally related to the given character
do behave as units. We may therefore designate them
as character -units. They are commonly known as genes.
Quite probably, in many if not all cases, more than one
gene or character-unit is involved in the production of
any given character.

481. Applications of MendePs Law. — Over loo pairs
of structural and color characters have been found, in
plant breeding, to behave more or less closely in accord-
ance with the Mendelian conception. In peas alone over
20 pairs of characters are expressed in successive genera-
tions, in accordance with this law. Among the more
striking results which are explainable upon Mendelian
theory are the following:

1. Mottled beans have been produced in the Fi genera-
tion by crossing two varieties, neither of which had mottled
seeds. Vaiious types appeared in the F2 generation.

2. Jet black beans have appeared in the Fi generation
from a cross between two varieties, one of which had pure
white seeds, the other light yellow. Various shades and
colors appeared in the F2 generation.

3. In one case three distinct varieties of beans, breeding

* Substance or condition, we know not what, within the germ-cells.

560 STRUCTURE AND LIFE HISTORIES

true to white seeds (when selfed^), were crossed with the
same variety of red bean. In the Fi generation each cross
gave a different color — one blue, another black, and the
third brown. A varied assortment of colors appeared in
each case in the F2 generations.

4. Two varieties of sweet peas, each breeding true to
white flowers, when crossed gave, in the Fi generation,
nothing but purple-flowered ofTspring, resembling the wild
sweet pea. A medley of white, pink, purple, and red-
flowered plants appeared in the F2 generation. Numer-
ous other cases might be cited, all of which would have
been unsolvable riddles except in the light of Mendelism.

482. Inheritance and Environment.— Emphasis should
be laid on the fact that the behavior of any plant, and
the characters it manifests, are the result of the combined
influence of inheritance and environment. A bean seed-
ling is green, not merely because it has inherited chloro-
plastids, but because it develops in sunlight; without
sunlight the green color could not come into expression.
If we vary any factor of environment (temperature, mois-
ture, illumination, food) the expression of the inheritance
may be altered, just as truly as though the inheritance
were changed. The characteristics expressed by any plant
{or animal) are the result of the combined action of inherit-
ance and environment. It is of fundamental importance
to a man, not only to be ''well-born" {eugenics), but also
to be ''well-placed" {euthenics).

483. Johannsen's Conception of Heredity. — The con-
ception that inheritance, as previously noted, is not the
transmission of external characters from parent to off-
spring, but the reappearance, in successive generations,

^ The pollination of a flower with its own pollen, or with pollen from
another flower of the same plant, is called selfing.

EXPERIMENTAL STUDY OF HEREDITY 561

of the same organization of the protoplasm with reference
to its character-units, was first developed by Johannsen,
of Copenhagen, Denmark, who proposed the term
''genes/' ''The sum total of all the 'genes' in a gamete
or zygote," is a genotype. Inheritance is the recurrence,
in successive generations, of the same genotypical constitu-
tion of the protoplasm. Johannsen does not attempt to
explain the nature of the genes, "but that the notion
'gene' covers a reality is evident from Mendelism."

This conception of heredity is diametrically opposed
to the older and popular conception, but is much more
closely in accord with the facts revealed by recent studies
of plant and animal breeding.^

484. Value of MendePs Discoveries. — The discoveries
that, in inheritance, certain characters are dominant
over certain others; that a given inheritance {e.g., condi-
tions associated with seed-color, odor, eye-color, stature,
musical ability, insanity, tendency to some disease) may
be carried and transmitted to offspring by an adult
who gives no outward signs of carrying the inheritance;
that, under certain conditions of breeding, some characters
(the recessive ones), whether good or bad, may become
permanently lost; that dominant characteristics are
certain to appear in some of the offspring — all of these
truths, learned by the study of a common garden vegetable,
will be recognized at once as of enormous importance to
the breeders of plants and animals, and above all to man-
kind, in connection with our own heredity. They point
the way to the explanation of such enigmas as the pro-
verbial bad sons of pious preachers, spendthrift children

^ A discussion of Johannsen's very fruitful method of "pure line"
breeding belongs to more advanced studies.
36

562 STRUCTURE AND LIFE HISTORIES

of thrifty parents, talented offspring of mediocre parents,
blue-eyed cliildren of brown-eyed parents,^ and so on.

485. Increased Vigor from Crossing. — Experiments
with pedigreed cultures have disclosed a principle of the
utmost practical importance for the plant breeder. A
careful analysis of a field of Indian corn {Zea Mays) has
disclosed the fact that any given variety is very complex,
being heterozygous for many characters; in other words
any horticultural variety is a composite of numerous
elementary species, and is therefore heterozygous for
most of its characters. When pollination is allowed to
take place in the corn field without interference by man,
both crossing and selfing occur. As a result the yield,
in bushels per acre, remains about stationary, or gradu-
ally becomes less and the variety changes and deterior-
ates by the segregation and recombination of the numerous
elementary species that compose it.

By artificial self-pollination for several generations {e.g.,
five or more) less complex strains result, which are homo-

^ If both parents have blue eyes the children can never have brown eyes;
if one parent has brown eyes and one blue, the children may be both blue-
and brown-eyed, or all brown-eyed, for brown eye-color in man is dominant
over blue color. "When both parents have brown eyes, part of the children
may have blue eyes and part of them brown, or they may all be brown-
eyed. As used here, the term "brown eyes" means all eyes having brown
pigment, whether in small spots (gray eyes), or traces (hazel eyes), or
generally distributed (brown, or sometimes black, eyes). The term "blue
eyes" designates only those cases in which brown pigment is entirely lacking.

Fig. 409. — Zea Mays. In the experiment, the results of which are here
illustrated, nine strains of Indian corn were selected according to the
number of rows of kernels on the cob, varying from 8 to 24 rows. These
were pollinated by hand each year, with mixed pollen, in such manner that
self-pollination was entirely prevented. An average ear of each strain is
shown in the first row above. In the second row is shown an average
ear of each strain after self-fertilization for five generations. Note the
resulting decrease in the number of rows, lack of filling out of the ears,
and other marks of inferiority. The last row shows the remarkable and
immediate increase of vigor resulting in the Fi generation of hybrids be-
tween various pairs of the selfed strains. (Photo supplied by G. H. Shull.)

EXPERIMENTAL STUDY OF HEREDITY

563

564 STRUCTURE AND LIFE HISTORIES

zygous for one or more characters, and the yield per acre
may thus become greatly reduced.^ If now, two of these
simplified strains, homozygous for many characters, and
having a low yield per acre, are crossed, there results an
Fi hybrid progeny that is heterozygous for all of these
characters. This heterozygosity is correlated with a
greatly increased vigor; the plants are much larger, and
the yield per acre is enormously increased (Fig. 409).
Thus in one experiment of this kind the average yield of
the heterozygous horticultural variety was 61.25 bushels
per acre. After self-fertilization for several generations
the yield became reduced to 29.04 bushels per acre; but
in the Fi generation of a cross between two of these self-
fertiUzed strains the yield per acre rose at once to 68.07
bushels. In the F2 generation the yield again fell to 44.62
bushels. From this, and numerous other experiments, it
is found that the biggest corn crop is to be obtained by
finding the strains that will produce the largest yield
when crossed, and thus using for seed the grains of the
first-generation hybrids each year.

486. Unsolved Problems. — ^Like all truly great con-
tributions to science, MendeFs discoveries have raised
more questions than they have answered. Therein lies,
in part, their great value. So, also, the most important
effect of Darwin's work was that it set men to asking
questions. The history of botany, as of all natural science
since 1859, is chiefly the attempts of men to answer the
questions raised by Darwin, or stimulated in their own
minds by his books. So with Mendel and de Vries;

^ If a high-yielding strain was separated out by selection, the yield
would of course be increased above the average of the mixed field.

EXPERIMENTAL STUDY OF HEREDITY 565

biological science, since 1900, has been largely occupied
in trying to answer the questions raised by these men.

What are these questions? There is not space here
even to ask them all, much less to endeavor to answer
them even briefly; but they include the following large
problems :

1. What is the mechanism of inheritance? In other
words, by what arrangement and interaction of atoms
and molecules is it made possible that the peculiar tone
of one's voice, the color of a rose, the odor of a carnation,
the evenness (or otherwise) of one's disposition, may be
transmitted from one generation to another? How may
it be transmitted through one generation, without causing
any external expression, and reappear in the second genera-
tion removed? Is the cytoplasm the carrier, or the
chromatin, or both combined, or neither? Is the transfer
accomplished by little particles (pangens), as de Vries
contends, or by chondriosomes, or otherwise? We do
not know.

2. How may dominance he explained? Why is tallness
dominant over dwarfness, brown eye-color over blue,
any one character over any other? We have not the
faintest idea.

2,. Are acquired characters inherited? In other words,
do characteristics acquired after birth by the body or
mind of the parent, either by its own activity or as a re-
sult of the immediate effects of environment, influence
the germ-cells so as to alter the inheritance which they
transmit? Some say yes, others say no; others say,
only in part. There seems to be evidence both ways.
We can arrive at the correct answer only by careful
experimentation, that is, by asking questions of nature.

566 STRUCTURE AND LIFE HISTORIES

4. Can the inheritance of a strain he artificially altered?
This is a question of the very first importance. If the
inheritance could be so altered the marvels that breeders
might perform would be greatly increased. A blue rose
(the despair of all plant breeders) might possibly be pro-
duced by sufficiently careful and extended experiment-
ing; disease and undesirable traits of character might be
eliminated from certain tainted family strains by artificial
treatment. On the other hand, by an unfortunate com-
bination of circumstances, most undesirable and re-
grettable results might be experimentally produced. The
experiment has been made of exposing the ovaries of
flowers to the rays of radium, and of injecting them with
various chemical substances, with an idea of altering the
physical or chemical nature of the egg-cells, and thus
altering the inheritance. The results of such experiments,
so far tried, need to be further confirmed before we can
say with certainty that the result sought has been
accomplished.

487. Eugenics. — Students of biology have been quick
to recognize the fact that, if we correctly understand the
laws of heredity, we are in a position to apply them, not
only to plants and the lower animals, but to mankind.
The application of the laws of heredity in a way to pro-
duce a healthier and more efficient race of men constitutes
the practice of eugenics} The underlying principles of
eugenics are of course, very largely those of heredity.
Eugenics is the applied science based upon the pure
science of heredity. The main problem of eugenics is
how to eliminate human beings with a tendency to any
physical or mental weakness, making for poverty, misery,

* Eugenics is from two Greek words meaning well born.

EXPERIMENTAL STUDY OF HEREDITY 567

ignorance, and crime; and how to increase the number
of individuals physically, mentally, and morally more
robust and sound; and withal how, if possible, to raise
the standard of all desirable human traits. A careful
study of heredity and eugenics will make possible a much
more intelligent and efficient program for charity work
and social betterment.

488. Investigations Since Mendel. — It must be re-
membered that Mendel's most valued contribution was
not the observations which he made and recorded con-
cerning the garden pea, nor the hypotheses which he ad-
vanced on the basis of those observations, but this method
of procedure, whereby he elevated the study of heredity
to the rank of an exact science. As in the case of all
hypotheses, the task for science is to subject them to the
most searching tests, to see if they invariably agree with
facts, and may be accepted as in all probability embody-
ing the actual truth — may be elevated to the rank of
theories. The testing of Mendelism has been occupying
all the best talents of many investigators since the re-
discovery of Mendel's publication, about 1900. Many
biologists are still skeptical, others reject the hypotheses,
and still others believe they contain the germ of truth,
but must be more or less modified. Whether they prove
to he erroneous or true is not so important, hut it is impor-
tant for us to know which is the case. True or not,
they, like nearly all working hypotheses (natural selec-
tion, mutation, nebular hypothesis, atomic hypothesis
in chemistry, etc.) are rendering, or have rendered, a
priceless service to science by pointing the way to further
study, which enriches our knowledge of the living world,
including ourselves, and therefore increases the intelli-

568 STRUCTURE AND LIFE HISTORIES

gence with which we may order our own conduct and lives.
If the study of plants had rendered no other service to
mankind than this contribution of an effective method of
ascertaining the laws of heredity, it would have amply
justified all the arduous labor that men have devoted to
it for 2,000 years. ^

^ Only one of the simplest cases worked out by Mendel is summarized
in this chapter. A more thorough study of his experimental results and
theories must be reserved for a more advanced course.

CHAPTER XXXVI
THE EVOLUTION OF PLANTS

489. The Problem Stated. — If we knew the entire
history of development of the plant world, we could ar-
range all plants now living, and that have lived, so as to
show their genetic relation to each other. The prob-
lem is illustrated on a small scale by various related culti-
vated plants, all known to be derived from a common
wild ancestor. Cabbage and its relatives are a case in
point. The botanical relatives of the cabbage include
such forms as kohlrabi, brussels-sprouts, collards, kale,
broccoli, and cauliflower (Fig. 397). All of these garden
vegetables are believed to have been derived from the
common wild cliff-cabbage {Brassica oleraced) of Europe
and Asia, by selecting mutations in various directions,
e.g., excessive development of the stem in kohlrabi, of
the terminal bud in cabbages, of the lateral buds in brus-
sel's sprouts, of the flower buds in cauliflower. Or, to
refer to de Vries's studies in experimental evolution, where
the course of descent was actually observed, we may
arrange the forms of Lamarck's evening-primrose so as
to show their known derivation.

The general problem, therefore, is to establish the
genetic relationship of all known plants, living and fossil.
Since the Angiosperms stand at the top of the series, the
problem resolves itself largely into ascertaining the
phytogeny, or line of ancestry, of that group.

569

570 STRUCTURE AND LIFE HISTORIES

490. Methods of Study. — In the solution of this prob-
lem two methods of attack may be employed: (i) That
of observation and comparison of structure, followed by
classification, and inference; (2) that of experiment. The
use of experiment is indicated in Chapters XXXIII and
XXXV. By this means we may learn something of the
relationship of different groups having living representa-
tives; but it chiefly serves to throw light on the method of
evolution. The course of evolution is best ascertained by
the observation and comparison of plant structures.

491. Sources of Evidence. — There are four main sources
of evidence as to the course of evolution:

1. Comparative anatomy of living forms.

2. Comparative life histories of living forms.

3. Structure of fossil forms.

4. Geological succession of fossil forms.

Studies along these four different lines have resulted
in some conflict of evidence, but on the whole the evidence
from the various sources all points to the same broad
conclusions. Conflict or contradication is in most cases
the result of insufficient evidence from one or more sources.

492. Evidence from Comparative Anatomy. — Compara-
tive study of structure has led to the conclusion that,
in its broadest aspects, the course of plant evolution has
been from the simple to the complex; that such simple
organisms as Fleurococcus , and other green algae, preceded
more complex forms like the liverworts ; that Bryophytes
appeared before ferns, and they in turn before the modern
Gymnosperms and Angiosperms.

A difficulty of accepting this conclusion as final is the
possibility that, at certain points, the course of evolution
may have been retrograde. For example it is generally

THE EVOLUTION OF PLANTS 571

accepted that the filamentous, alga-like fungi were de-
rived from green algae by retrograde evolution (degenera-
tion). Were the plants with one seed-leaf (monocoty-
ledons) derived from those with two (dicotyledons) by
retrograde evolution, or were the dicotyledons derived
from the monocotyledons by progressive evolution? Evi-
dence, recently ascertained by studies of structure and
development, points to the conclusion, that, although
monocotyledony seems the simpler, more primitive con-
dition, it is really a later phenomenon, the monocotyledons
being derived from the dictoyledons by simplification. ^
Again, a careful student of fossil plants has recently been
led to state that, ''it is beginning to appear more probable
that the Higher Cryptogams (ferns and fern allies) are a
more ancient and primitive group than the Bryophytes,
which would seem to owe 'their origin to reduction from
some higher type."^ In view of this diversity of opinion,
we learn at once that great caution must be used in in-
terpreting the evidence— that we must not ''jump at
conclusions."

493. Results of the Method of Comparative Anatomy.
— By their study of comparative anatomy and morphol-
ogy, botanists have been led to propose the following
arrangement of plant groups as representing the general
course of their evolution (Table V) :

From what has already been said, however, it should
be understood that such a table represents, not the line
of evolutionary advance, but the paths travelled by plants
in the course of their development. For example, it implies
that dicotyledons were derived from monocotyledons,

1 See paragraph 522, Chapter XXXVIII.

2 Scott, D. H. "The Evolution of Plants," p. 18.

572 STRUCTURE AND LIFE HISTORIES

Table V. — Sequence of Plant Groups, Based on the
Morphology of Living Forms

Thallophytes / AlgjE — having chlorophyll,

(no archegonia) \ Fungi — no chlorophyll.

j Bryophytes — no vascular system.
Archegoniates J Pteridophytes ]

farchegonia, but no seeds) ] Calamophytes \ vascular system.
Lepidophytes

I

Spermatophytes
(seeds)

Gymnosperms — no closed ovary.
Angiosperms — closed ovary (pistil).

Monocotyledons — one seed-leaf.

Dicotyledons — two seed-leaves.

pteridophytes from bryophytes — hypotheses which, from
other trustworthy evidence, as stated above, now seem
untenable.

Again, the table suggests that Angiosperms were de-
rived from Gymnosperms, and therefore appeared late
in the history of plant life; but the study of fossil plants
shows that they appeared in the geological past, and were
dominant in the Tertiary period, as now. We are led,
therefore, to proceed with caution in drawing inferences
based only upon a comparative study of the structure of
forms now living.

494. Evidence from Life Histories. — In the study of
the life history (ontogeny) of any higher sporophyte,
we find that vegetative (sterile) tissues develop first.
On the basis of this fact it has been inferred that all repro-
ductive organs (stamens, carpels, sporophylls) arose by
a modification of vegetative organs. Other facts, how-
ever, lead to the directly opposite conclusion.

495. Evidence from Comparative Ontogeny. — In
Chapters XVI and XXIII attention is called to the

THE EVOLUTION OF PLANTS 573

fact that the most primitive sporophytes of the lower
liverworts consist almost entirely of ''fertile" {i.e., re-
productive) cells, and that the relative amount of vege-
tative or sterile tissue gradually increases (as we pass to
more highly organized forms) by a progressive sterilization of
fertile tissue. This progressive sterihzation accompanied
a change in the habitat of plants from water to dry land.

496. Consequences of an Amphibious Habit of Life . —
The Hfe history of the fern affords a concrete illustration.
The gametophyte is semi-aquatic in habit, and the method
of fertiHzation is purely aquatic, the sperm being unable
to reach the egg except by swimming through free water.
But, when the fertihzed egg began to develop as a land
plant, the chances of fertiHzation by a sperm swimming
in free water became increasingly remote. The perpetua-
tion of the species, and the multiplication of individuals
could be insured only by the formation of a large number of
reproductive bodies (spores), capable of distribution by
wind in dry conditions, and each able to reproduce its
kind independently, without fusion with another repro-
ductive body. The larger the number of such spores, the
greater the chances of perpetuation of the given species.

497. Consequence of Enormous Spore -production. —
But the formation of a large number of spores requires a
vigorous plant body to supply them with an abundance
of water and nourishment, and to lift them up into the
air where they would stand a better chance of distribution
when dry. This is accomplished by the sporophyte,^
producing an abundance of broad, green leaves for food-

^ "The fern, as we normally see it, is an organism with, so to speak,
one foot in the water, the other on the land." Bower, F. O., "The Origin
of a Land Flora," p. 82,

574 STRUCTURE AND LIFE HISTORIES

manufacture, and of roots for absorption of water and
minerals in large quantities. From this point of view,
the plant body of the sporophyte is regarded as produced
by the progressive sterilization of tissues originally re-
productive. After the formation of a vigorous plant
body then spores, produced in special regions (sporangia) ,
could be nourished in enormous numbers.

498. Origin of Vegetative Organs. — On the basis of
the theory just outlined, we are to regard foliage-leaves
and branches, either as new formations ^ developed (by
"enation^^) on some primitive reproductive axis like a
strobilus or cone, or else as produced by the sterilization
of parts originally fertile, i.e., modifications of reproductive
tissues. The sporophyte, as we have already seen,^
has become increasingly well developed and increasingly
independent, while the gametophyte has become increas-
ingly simple and increasingly dependent. The evolution of
plants has proceeded by the progressive development
of the sporophyte, and the gradual but steady regres-
sion of the gametophyte.

499. Steps in the Evolution of the Sporophyte. — The
possible steps in the evolution of the sporophyte may be
tabulated as follows:^

1. Sterilization of fertile tissue.

2. Localization of spore-production in sporangia.

3. Origination of lateral organs (leaves), and of roots.

4. Development of heterospory.

5. Introduction of fertilization by the pollen-tube
(siphonogamy) .

6. Assumption of the seed-habit.

1 Chapter XXIII.

2 Following F. O. Bower.

THE EVOLUTION OF PLANTS 575

600. Another Hypothesis of Alternation of Genera-
tions.— Some of the facts of paleobotany support the
hypothesis that the modern sporophyte has not been
gradually developed from a simple structure like the moss
sporogonium (derived, in turn, by progressive steriliza-
tion), but that the gametophytic and sporophytic stages
were at the first vegetatively or somatically equivalent;
except for chromosome number (as is the case now, for
example, with Dictoyota^ and Polysiphonia); and that,
in the course of evolution, the sexual phase became more,
and the asexual phase less, important in certain forms
{e.g. J mosses), and the asexual phase more, and the sexual
phase less, important in other forms (e.g., ferns). This
is the hypothesis of homologous alternation, as opposed to
that of antithetic alternation.^ The structural differences
in the two generations are, on the basis of this hypothesis,
considered as due almost, if not entirely, to differences
in environment, the main factor being the gradual transi-
tion from aquatic to dry-land surroundings. Where the
environment is uniform and the same for both genera-
tions, as for Dictyota, the gametophyte and sporophyte are
identical in external organs and general appearance (Fig.
177). In any event the hypothesis postulates a homology
between the various organs of the two generations, how-
ever much these parts may differ in external appearance
as a result of individual variation and environmental
influence.

501. Lang's Ontogenetic Hypothesis. — Viewing the
matter from the standpoint of individual development
(ontogeny), Lang has developed the ontogenetic hypothesis

1 See Chapter XVII.
8 See Chapter XIV.

576 STRUCTURE AND LIFE HISTORIES

oj alternation. From this point of view two alternatives
are recognized:

1. Either the fertilized egg and the haploid spore are
potentially unlike, and will therefore produce unlike plant
bodies, even under essentially similar environment, or

2. Fertilized eggs and spores are potentially alike, but
produce unlike plant bodies as the result of the difference
in the enviromnent in which they develop.

The ontogenetic school accepts the latter alternative
as a working hypothesis, and regards the gametophytic
and sporophytic generations as essentially homologous.
The degree of homology which can actually be traced in
the vegetative structure of the two generations may vary
from substantial identity, as in Dictyota, to such wide
divergence that the tracing of homologies is quite out
of the question. In testing this hypothesis a crucial
experiment would be to obtain a gametophyte by arti-
ficially bringing a fertilized egg to mature development
outside of the archegonium and under the environment in
which the spores normally develop; or to obtain a sporo-
phyte by causing a spore to develop within the tissue of a
gametophyte, as the fertilized egg normally does.

502. Hypothetical Ancestral Tree. — From a compara-
tive study of both living and fossil forms some botanists
have been led to infer the common derivation of Filicales,
Equisetales, and Lycopodiales from the Hepaticse, and
probably through some form belonging to the Anthocero-
tales somewhat as shown in the following ancestral ''tree"
(Fig. 410). It should be clearly understood that this
tree does not illustrate known facts, but only the hypoth-
eses which have been tentatively proposed by careful
students on the basis of known facts.

THE EVOLUTION OF PLANTS

577

5

<u

d

f?.

^- o

o

0

4

^

"ni

^ci

r

"^7

>s

-^c

ci

, r?/

o

Uj

a=5

"t

FlLICAL£mi STOCK

Anccstrai. BbvqphvtaIHypothetical)

Fig. 410. — Hypothetical genealogical tree to illustrate the probable affini-
ties of the modern plant orders. This diagram is intended to indicate that
the plant orders now existing are the tips, only, of the branches of a genealog-
ical tree, whose lower limbs and roots extend into preceding geological
periods. Our knowledge is not sufficient to enable us to connect these
branches with each other, nor with the main trunk. The diagram teaches
that hypothetical (indicated by the dotted line) Anthocerotales gave rise
to a now fossil Filicalean stock, from which have been derived all the
modern orders above the mosses and liverworts.

37

CHAPTER XXXVII
PALEOBOTANY

503. The Scope of Paleobotany. — The study of fossil
plants, though of course a phase of botany, constitutes
a science by itself, not only covering a special subject
matter, but having its own methods (technique), and pos-
sessing a large literature. It is called paleobotany. One
cannot pursue this study without a knowledge of the
anatomy and morphology of living forms. This is neces-
sary in order to interpret the meaning of plant fossils,
which often occur only in small fragments of the entire
plant. Moreover, one must have a good knowledge of
at least the elements of geology, since fossils are found in
rocks. One must not only know the geological age to
which the fossil-bearing rock he studies belongs, but also
something of the geological processes by which fossils,
and even the rocks themselves, are formed.

504. What is a Fossil? — A fossil is any remains of a
plant or animal that lived in a geological age preceding
the present; these remains are preserved in rocks. ^ There
are two methods of preservation, namely, incrustation and
petrifaction. Incrustations are merely impressions or
casts resulting from the encasement of the organ or
organism in the rock-forming material. The tissue itself

^ By an extension of the term we also speak of fossil footprints of ani-
mals, fossil ripple marks, et cetera. The word fossil is derived from the
LzX\nfodere (to dig), and originally signified anything dug up.

578

PALEOBOTANY 579

either decayed or became carbonized, leaving only the
impression of its surface features. The well-known
''fossil fern-leaves," found in coal mines are of this nature.
The tissues of the plant were transformed into coal,
leaving the impression or cast on the adjacent shale. The
first stage in this process may often be observed in the
autumn, when impressions of recently fallen leaves are
made on the surface of wet mud. Obviously from such
fossils we can learn nothing of internal structure.

Petrifactions are formed by the gradual replacement
of the organic tissue by mineral matter, usually carbonate
of lime (CaCOs) or silicic acid (H4Si04). In this process
the tissues become soaked with a saturated solution of
the given mineral, which is gradually deposited from solu-
tion, and takes the place of the original organic matter.
By this means the most minute details of microscopic
structure are preserved, even in some cases the nuclei
and other cell-contents.

505. Conditions of Fossil-formation. — In order to
understand how fossils come to be formed, we must pic-
ture to ourselves certain geological processes now in
operation — the initial stages of rock-formation. Rocks
are of two kinds, igneous and sedimentary. Igneous rocks
result from the cooling of molten lava poured out on
the surface or injected into crevices by volcanic action.
Such rocks never contain fossils, as the intense heat
necessary to melt the rock destroys all trace of organic
matter.

Sedimentary rocks are formed by the deposit under
water of the sediment formed by weathering and erosion,
and transported by streams. This deposit may occur
along the flood-plains or at the mouths of streams empty-

58o

STRUCTURE AND LIFE mSTORIES

PALEOBOTANY

581

ing into inland lakes or into the ocean. In addition to
rock-sediment eroded from the surface of the land, streams

^<i;&*Kl:<f£

Bf Bs Bm 5 M O-W

MF Bf Bs.Bm.

A^l

^y:^

iir^^fi

' />o^*

Fig. 412. — Diagram illustrating the gradual filling up of lakes by the
encroachment of vegetation, and also the stages in the origin of peat and
rnarl deposits in lakes. The several plant associations of the Bog series,
displacing one another, belong to the following major groups: (i) O. W.,
open water succession; (2) M., marginal succession; (3) S., shore succes-
sion; (4) B., bog succession, comprising the bog-meadow {Bm), bog-shrub
{Bs) and bog-forest {Bf); and (5) M. F., mesophytic forest succession
(Cf. Fig. 411.)

also transport quantities of plant (and animals) frag-
ments, leaves, stems, pieces of bark, fruit, flowers, pollen
and spores, roots, and even entire plants. These natur-

582 STRUCTURE AND LIFE HISTORIES

ally become buried in the mud and sediment wherever
deposition takes place, and when the deposit becomes
converted into rock the organic remains may become con-
verted into fossils by either of the processes described
above. Swampy regions are especially favorable to the
preservation of plant and animal remains as fossils, as is
illustrated in Figs. 411 and 412.

506. Metamorphism. — After sedimentary rocks are
once formed they are subject to various changes. The
amorphous carbonate of lime, of limestone rocks, may be
transformed into crystals of calcite until marble results;
thin flakes of mica may form in clay rock in thin sheets,
transforming the rock into slate; vegetable deposits in
the form of peat may become transformed into anthracite
coal and graphite; molten lava poured out on the surface
or into crevices of sedimentary rocks may fuse the adja-
cent material, causing contact pietamorphism; while the
heat engendered over larger areas by mountain folding,
or by the weight of superincumbent strata^ may cause
regional metamorphism. Obviously such changes, espe-
cially those caused by heat, result in the complete de-
struction of all plant or animal remains or impressions,
and thus fossil records over large areas, and representing
vast periods of geologic time, have been obliterated.

507. Stratification of Rocks. — Changes in the relative
level of sea and land have occurred many times in the
geological past, so that submerged areas of sedimentation
in one period have become areas of dry land, undergoing
erosion in another; and vice versa, areas of erosion have
become areas of sedimentation. As a result of this,

^ Some rocks are buried under more than 40,000 feet of strata, and the
temperature increases approximately i°F. for every 50 to 60 feet of depth.

PALEOBOTANY 583

rocks occur in layers/ the deeper lying layers (with ex-
ceptions readily explained by geologists) being older than
those above, or nearer the surface. Moreover, as a result
of a second submersion following elevation and erosion,
subsequent layers were often deposited with an uncon-
formity on the weathered and eroded surface under-
neath.

By the presence of fossil imprints of rain drops, foot-
prints, ripple marks, and mud cracks, and by the character
of the plant and animal fossils which they contain, we
know that most sedimentary rocks were deposited in
shallow water, not far from the shore line. But since
these same rocks may have a thickness of thousands of
feet we know the area of sedimentation must have been
slowly sinking while the sediment was being deposited.
As a result of the enormous pressure of the overlying
material, of the deposit of cementing substances from
solution, and of other causes, the sedimentary deposits
became, in time, converted into solid rock.

508. Classification of Rock Strata.— By a study of the
fossils which the rocks contain, geologists have been able
to classify the various strata according to their age.
As a result of the period of erosion, indicated by un-
conformity, the transition from the stratum of one age
to that of another is often abrupt, the fossils in successive
periods being quite characteristic of the given stratum
or period. In other cases, as for example between the
Silurian and Devonian in New York State, there is no
unconformity, and this renders it more difficult to decide
just where the plane of division lies. The names and
order of occurrence of the known rock strata are given in

^ Several layers form a stratum or bed.

5^4

STRUCTURE AND LIFE HISTORIES

the following table, the older rocks being at the bottom,
the most recently formed at the top.

Table VI. — Table of Geological Time

Era

Cenozoic

Mesozoic

Quaternary

Tertiary

Secondary

Paleozoic

Primary

Archean

Period
Holocene

(recent, or the present)
Pleistocene

(ice age)

Pliocene
Miocene
Oligocene
Eocene

Upper Cretaceous
Lower Cretaceous
(Comanchean)
Jurassic
Triassic

Permian

Upper Carboniferous

(Pennsylvanian)
Lower Carboniferous

(Mississippian)
Devonian
Silurian
Ordovician
Cambrian

/ Huronian
\ Laurentian

509. Paleogeography. — By changes in the relative
level of the land and sea, above referred to, rocks contain-
ing fossils may be elevated as dry land, and frequently
as mountains, so that remains of marine organisms, as
well as of others, are often found at high elevations. In
some cases forests near the seashore have been submerged,

PALEOBOTANY

sss

Pjg 4J2. — Fossil tree stumps lu a carboniferous forest, Victoria Park,
Glasgow. (Cf. Fig. 414-) (After Seward.)

Fig. 414.— Part of a submerged forest as seen at low tide on the Cheshire
coast of England. (Cf. Fig. 413.) (After Seward.)

586 STRUCTURE AND LIFE HISTORIES

and covered over with sediment, then elevated again as
dry land, so that subsequent excavations have revealed
the fossilized trunks and stumps (Figs. 413 and 414). Thus
it is seen that, by a study of fossils, we may not only
learn of their structure and thus fill in many of the gaps
in the evolutionary sequence left by a study of forms now
living, but we may also learn of the distribution of plants
and animals in previous geological ages — in other words,
we have the basis for a science of fossil geography or
paleo geography.

610. Plant Migrations. — With the development of
Paleogeography, a clearer conception of the location and
changes of the continental areas of the past is gradually
being gained. As a consequence, plant geography is a sub-
ject of increasing interest to the paleobotanist. More-
over, geology, the fossil record, and the present zonal
grouping of plants indicate that, in the past, the polar
areas, once tropic or sub-tropic, must have been fruitful in
new species.^ High mountains or plateaus are also sug-
gested as homes of plastic races. In the tropics environ-
ments are more nearly static, and, it is reasonable to sup-
pose, less Ukely to cause variation. It is known that, once
established, many species move most readily along the
geologic formation which supplies the exact soil constitu-
ents, the rate of movement often being rapid. Flotation
of seeds is also a factor. The facts here briefly cited rest
on the observations of a large number of invesitgators,
extending over more than a century.

511. Distribution of Plants in Time.— In addition to
the distribution of plants in space (plant geography) , the

1 Owing to the precession of the equinoxes these areas undergo an extreme
variation in the length of winter and summer of 37 days every 1 2,934 years.

PALEOBOTANY

587

problem of their distribution in geologic time is one of
absorbing interest and importance. The following table
indicates the known distribution of the various plant
groups from the earliest geologic time to the present.
Table VII.— Distribution of Plants in Geologic Time^

Division

Subdivision, class,
or order

Range

Common
name or
example

I

2

5j

Dicotyledones
Monocotyledones

Comanchean to present
Comanchean to present

Oaks
Grasses

Spermato-

<

(Fossil record scant)
Permian to present
Permian to present
Devonian to Permian
Permian to present
Devonian to Jurassic

IV. i phyta

Cycadophyta

e

i
a

0

Gnetales

Coniferales

Ginkgoales

Cordaitales

Cycadales

Cycadofilicales

Ephedra

Pines

Ginkgo

Cordaites

Cycads

Neuropteris

^ Lepidophyta

III.| Calamophyta

[ Pteridophyta

Lycopodiales

Equisitales

Sphenophyllales

Filicales

Devonian to present
Devonian to present
Devonian to Permian

Devonian to present

Club mosses

Horsetails

i S p h e n 0 -

phyllum

Ferns

II. Bryophyta

Musci
Hepaticae

Tertiary to present
Tertiary to present

1 Mosses
1 Liverworts

1

I. Thallophyta

1

Fungi

Algae

Diatomeae

Schizophyta

Myxomycetae

Silurian to present
Pre-Cambrian to present
Jurassic to present
Pennsylvanian to present
(Fossil record lacking)

Fungi
Seaweeds
Diatoms
Bacteria
1 Slime-molds

1 Modified from Shimer.

612. Gaps in the Fossil Record. — In the Origin of
Species Darwin called attention to the paltry display of
fossils in our museums, as evidence of how little we really
know of the plant and animal life of past ages, "The

588 STRUCTURE AND LIFE HISTORIES

number, both of specimens and of species, preserved in
our museums,'' says Darwin, ''is absolutely as nothing
compared with the number of generations which must
have passed away during a single formation." The
meagerness of the record is, of course, due in part to the
relatively small area explored in proportion to the whole;
but there are other reasons much more serious, because
they represent opportunities lost forever. Among them
are metamorphosis, explained above, and the fact that
many of the organisms of the past were composed wholly
or partly of soft tissues, which were entirely destroyed, by
decay or otherwise, in the process of rock-formation.
Such plants, for example, as Spirogyra and many other
algae, the fleshy fungi, and, among animals, jelly-fish,
earthworms, and others, would form fossils only under
exceptionally favorable circumstances, if at all.

But there is an even more effective cause of oblitera-
tion of the fossil record in the long-continued erosion and
denudation represented by unconformity in the rock
strata. In many cases only a small proportion now re-
mains of the thickness of a rock stratum originally de-
posited, and all traces of the plant and animal life that
may have existed on the denuded area have thus been ob-
literated forever. These blank intervals between suc-
cessive periods were of vast duration.

"I look at the geological record," said Darwin, ''as a
history of the world imperfectly kept, and written in a
changing dialect; of this history we possess the last
volume alone, relating only to two or three countries.
Of this volume, only here and there a short chapter has
been preserved; and of each page, only here and there a
few lines. Each word of the slowly changing language,

PALEOBOTANY 589

more or less different in the successive chapters, may
represent the forms of life, which are entombed in our
consecutive formations, and which falsely appear to have
been abruptly introduced."^ These views have received
added emphasis from the recent development of Paleo-
geography.

513. Factors of Extinction. — The question may natu-
rally arise, ''Why did the species common in previous geo-
logical ages die out, giving place to newer forms?" The
answer is found in the facts of struggle for existence and
survival of the fittest. In the words of the great American
botanist, Asa Gray, species may continue only ''while
the external conditions of their being or well-being con-
tinue." The struggle may be with other organisms or
with the physical conditions of the environment. Among
the more important factors of extinction, may be men-
tioned the following:

1. Struggle with other plants for adequate space. This is
illustrated in a simple way by the crowding out of cultivated
plants by weeds in a garden. By more rapid germina-
tion and growth, and by other "weedy" characteristics,
the weeds get the start of the cultivated plants, occupying
all available space, and choking them out.

2. Attacks of disease-causing parasites, e.g., chestnut
trees by a parasitic fungus, elm trees by the elm tree beetle.

3. Changes of environment too great or too rapid to permit
oj readjustment. Plants are plastic organisms, and can
adapt or readjust themselves to considerable environ-
mental change, but there are limits of speed and amount
of change beyond which readjustment is not possible, and
the plant must consequently perish. If such changes

1 Darwin, C. '^Origin of Species," New York, 1902, vol. 2, p. 88.

590 STRUCTURE AND LIFE HISTORIES

involve the entire area of distribution of the species con-
cerned, the species will, obviously, become extinct. The
following seven factors are specific instances of this.

4. Diminished water supply. Aquatic plants may be
destroyed by the draining of a pond or lake; hydrophytic
forms by the drying up of a swamp. Sometimes forms
suited to conditions of moderate water supply {hydro-
phytes) are destroyed by the conversion of wide areas into
desert regions, as has doubtless occurred. If such changes
are gradual, resting spores {e.g., Spirogyra), winter buds
{e.g., Utricularia, and eel-grass), and seeds readily trans-
ported by wind {e.g., cat-tail) enable the species to become
reestablished in a new location, but not so when the
changes are too abrupt, or cover too wide an area.

5. Temperature changes, when too abrupt, too extreme,
or too long continued. When the continental ice-sheet
advanced southward during the glacial period, many
forms, adapted only to temperate conditions, became ex-
tinct. Fossils of extinct tropical plants are found in
Greenland, which is now undergoing a glacial period.

6. Volcanic eruptio7is, such, for example, as those of
Mount Pelee, which occurred in 1902, on the island of
Martinique, W. I., often destroy all signs of life over a
radius of many miles. In the states of Washington,
Oregon, and Idaho floods of molten lava, covering thous-
ands of square miles, have been poured out over the sur-
face, forming a wide plateau. It is almost certain that
many species of plants and animals have become extinct
by such agencies. Not only the lava, but poisonous
gases that fill the air during volcanic eruptions, are fatal
to plant life.

7. Encroachment of salt water in coastal regions, caused

PALEOBOTANY 59 I

by changes in the level of the land, resulting in the killing
of fresh-water vegetation.

8. Disturbance of symbiotic relationships. The inter-
relationships of organisms are very complex, affording
innumerable opportunities for extinction by a disturbance
of adjustments. Shade-loving forms in a forest may
perish by the destruction of those affording the shade;
obligate parasites may perish from the destruction of the
necessary host; plants dependent upon certain insects for
cross-pollination may perish on account of the extinction
of the necessary insects.

9. Diminution of carbon dioxide in the atmosphere. There
are reasons for thinking that in certain past ages the at-
mosphere was richer than now in carbon dioxide, and that
that condition was favorable to the development of certain
vegetatively vigorous species which cannot live in an
atmosphere like the present, having a smaller percentage
of carbon.

10. Denudation of the land surface. In the course of
ages even lofty mountains are planed down by erosion,
and the arctic and sub-arctic species of the high altitudes
thus undergo extinction. Furthermore, erosion may be
coupled with general subsidence. In fact, not only do
geologists now recognize numerous old mountain ' roots,"
such for example as the Adirondack region of New York
State, but there are also abundant evidences of periodic
emergence and subsidence of areas of continental extent,
quite throughout geologic time. The climatic and other
environmental disturbances accompanying such changes
would inevitably result in the extinction of certain species-
(See also ^[505.)

CHAPTER XXXVIII
THE EVOLUTION OF PLANTS (Concluded)

514. Evidences from Fossil Plants. — The study of fossil
plant remains has greatly enlarged our knowledge of
the course of plant evolution, filling in gaps derived from
the study of living forms, and affording new facts, not
disclosed by the study of plants now living. Like the
study of comparative anatomy and life histories, paleo-
botany teaches us that there has been a gradual evolu-
tionary progress from the simple to the more complex, but
it has also disclosed the fact that some of the complex
forms are much more ancient than had been inferred from
the study of living plants only.

515. Discovery of Seed-bearing Ferns. — For example,
remains of seed-bearing plants, quite as highly organized
as those of to-day, are found far back in the earliest fossil-
bearing strata of the Paleozoic. Great forest types ex-
isted as early as the Devonian. Later in the Carboniferous
occur many seed-bearing ferns. These have been
called Cycadofilicales (cycadaceous ferns), or, by some,
Pterido sperms. Recent studies have disclosed the fact
that most of the fossil plants from the Carboniferous coal-
bearing strata, formerly thought to be ferns, are not even
cryptogams, but are these fern-like seed-bearing plants.
The best known pteridosperm is Lyginodendron oldhamium
(Fig. 415), first described from fossil leaves, in 1829, as
a tree-fern, under the name Sphenopteris Eoeninghausi.

592

THE EVOLUTION OF PLANTS

593

After investigations extending over nearly 90 years, ''we
are now in position to draw a fairly complete picture of
the plant as it must haVe appeared when living.

"It was in effect a little tree-fern, with long, slender,

Fig. 415, — Lyginodendron oldhamium. Pinna of a microsporophyll,
found in an ironstone nodule. Before its identity was established this
specimen was named Crossotheca Hoeni7ighausi. The somewhat pellate
fertile pinules on the ultimate branches, bear each a fringe of micro-
sporangia about 3 mm. long. The appearance has been likened to that
of a fringed epaulet. (After Scott, from a photo by Kidston.)

sometimes branched, stem, 4 centimeters or less in diame-
ter, and provided with spines by means of which it
probably climbed on its neighbors. The foliage was dis-
posed spirally and consisted of relatively very large, finely

^

594

STRUCTURE AND LIFE HISTORIES

divided fronds with small, thick pinnules with revolute
margins, suggesting a xerophytic or halophytic habitat.
The stem in the lower portion gave rise to numbers
of slender roots, some of which appear to have been
aerial in their origin. These grew downward and often
branched where they entered the soil.

Fig. 416. — Young leaf of the Cycad, Bowejiia serrulata. Comparison
of this with a leaf of the fern Angiopteris (Fig. 417) shows how difficult
it might be to decide from a fossil leaf whether the plant was a cycad or a
fern. (Cf., also, Fig. 420.)

''The stems, roots, and petioles, and even the pinnules,
have been found [calcified] and so beautifully preserved that
their entire structure can be made out with certainty.
Without going into a technical description of these organs,
it may be said that the stem when young, and before
secondary growth has begun, has a very strong resemblance

THE EVOLUTION OF PLANTS

595

to the stem of [the fern] Osmunda, but when more mature
certain cycadean characters appear to predominate."^

Fig. 417. — ^Leaf of a fern (Angiopkris cvecta). (Cf. Fig. 416.)

Its foHage and other characters closely resemble some
of our modern tree-ferns (Cf. Figs. 416 and 417) but more
careful study of the calcified specimens of much beauty

^ Knowlton, F. H. American Fern Journal, 5 :85. 1915.

596

STRUCTURE AND LIFE HISTORIES

found in the English "coal balls '^^ has disclosed both the
microsporophylls, bearing pollen-sacs, and the megasporo-
phylls, bearing, not merely megasporangia, but true seeds.
The ovule has a pollen-chamber, like the cycads, except that
it projects a bit through the micropyle, and, strange as it
may seem, fossil pollen-grains have been discovered, well
preserved within this chamber. The seeds, about 3^ inch
long, have been described as resembling little acorns, en-

FiG. 418. — Restoration of a seed of Lyginodendro7i oldhaviium {Lagenos-
tema Lomaxi), from a model by H. E, Smedley. (After Scott.)

closed like hazelnuts in smaller glandular cupules (Fig.
418). They are similar to those of the cycads, except
that they are not known to have organized an embryo with
cotyledons and caulicle. Instead, the tissues of the
female gametophyte only are so far found, retained within

^ Coal balls are "concretions of the carbonates of lime and magnesia
which formed around certain masses of the peaty vegetation as centers
and, through inclosing and interpenetrating them, preserved them from
the peculiar processes of decay which converted the rest of the vegetation
into coal. In them the mineral matter slowly replaced the vegetable
matter, molecule by molecule, thus preserving the cellular structure to a
remarkable degree. Such balls are especially frequent in the coal of
certain parts of England (Lancashire and Yorkshire)."

THE EVOLUTION OF PLANTS

597

the megasporangium, which is enclosed in the integument.
In this connection it is of interest to note that the seeds of
some modern plants {e.g., orchids) do not possess differ-
entiated embryos, but whether this is a primitive or a
reduced character is not certain. The pollen was formed
in spindle-shaped pollen-sacs, having two chambers and
borne in clusters of four to six on the under side of little
oval discs from 2 to 3 millimeters long. These structures

Fig. 419. — Top, lateral pinna from a leaf of Marattia Jraxinea. (After
Bitter.) Below at left, synangium of same. (After Bitter.) At right,
cross-section of the synangium. (After Hooker-Baker.)

are found on pinnules of ordinary foliage leaves, resem.bUng
the sporophylls of certain ferns (Fig. 419) rather than the
stamens of modern flowers.

The discovery of the seed-bearing character of the
fern-like plants of the Paleozoic was predicted by
Wieland, of Yale University, nearly two years before it
was made by Oliver and Scott. It is now believed that
seed-bearing plants of the pteridosperm type were nearly
as numerous in the Paleozoic as were the cryptogams.

598

STRUCTURE AND LIFE HISTORIES

516. Significance of the "Pteridosperms." — The close
resemblance of the pteridosperms to ferns, on the one hand,
and to modern cycads on the other, justifies the conclu-
sion that they represent a "connecting link" between the
true ferns and the cycads, and that the modern cycads
have descended from the same ancestry as the modern
ferns, each developing along somewhat different lines.

Fig. 420. — Stangeria paradoxa JMoore. Specimen from the cycad
house at the New York Botanical Garden, bearing, at the apex of the stem
a carpellate cone. (Photo from New York Botanical Garden.)

It was in recognition of their vegetative resemblances that
the Pteridosperms were first called (by Potonie) Cycado-
filices, now Cycadofilicales. Van Tieghem tersely de-
scribed them as "phanerogams without flowers."

517. A Modern Fern-like Cycad. — One of the modern
cycads {Stangeria paradoxa)^ is of much interest in this

^ Stangeria paradoxa Moore = Stangeria eriopus (Kunze) Nash.

THE EVOLUTION OF PLANTS 599

connection. So closely does it resemble a certain fern
{Lomaria) that the botanist Kunze, who first described it
when it was brought from Natal to the botanic garden at
Chelsea, England, supposed it was a fern, and named it
Lomaria eriopus. The specimen possessed no fruit, which
would have helped to identify it. Its leaves, with
circinate vernation, have a pinnately compound blade,
and leaflets with pinnate dichotomous venation. Two
or three years later another botanist, examining it more
closely, pronounced it a "fern-like Zamia or a Zamia-like
fern." These facts show how puzzling the specimen was,
and how closely a plant may resemble both a cycadophyte
and a fern. In a sense this plant may be called a living
fossil. Specimens have since come into flower in botanic
gardens, and the typical cycadaceous cones (Fig. 420)
leave no doubt that the plant is a true cycadophyte.

618. Derivation of New Types. — Attention should here
again be called to the fact that the theory of evolution does
not teach that one given species becomes transformed into
another, but simply that new species are descended from
older forms which may or may not continue to exist. It
is not supposed, for example, that ferns developed into
cycads, and cycads into higher gymnosperms, but that there
has been an unbroken line of descent (possibly more than
one) in the plant kingdom ; that closely related forms (like
ferns and cycads) have descended from a common ancestral
type which may or may not now be found. We must not,
in other words, expect necessarily to find in fossil forms the
direct ancestors of those now living, although a study of
their structure is of the greatest value in enabling us to
understand the genetic relationships of the great groups
of plants.

6oo

STRUCTURE AND LIFE HISTORIES

619. Ancestors of the Angiosperms.— Just as the

Cycadofilicales indicate the ancestry of the cycads, so
fossil types of Cycadophyta have been discovered which
are interpreted by some paleobotanists as ancestors of the
modern angiosperms. Other investigators, however, dis-
sent from this view and consider that we have not yet

Fig. 421. — To the left, Cycadeoidea dacotensis Macbride. Longitudinal
section of a silicified specimen of a bisporangiate cone (unexpanded flower),
so taken that the pinnules of the microsporophylls on both sides of the
central axis, or receptacle, are successively cut throughout their entire
length. The lines indicate the planes of various sections through the cone,
published in Wieland's "American Fossil Cycads." To the right Cycado-
cephalus Sewardi Nathorst. Microsporangiate cone, natural size, preserved
as an impression on a flat slab. From a fossil-bearing bed of the Trias, at
Bjuf, Southern Sweden. (Left figure from Wieland, right figure from
Nathorst.)

sufficient knowledge of fossil forms to be justified in
designating the ancestors of the Angiosperms. This
difference of opinion is largely due to the meagerness of
the available evidence. As one writer has stated it,
''A trayful of flowers may be all the record of the Pterido-

THE EVOLUTION OF PLANTS 6oi

sperms from the Devonian on. The gaps in the evidence
are always enormous."

Although the Cycadophyta are now a very insignificant
element in the earth's flora; in the Mesozoic period they
form about one-third of the recovered vegetation of the
land. One order, the Bennetti tales, then had a cosmo-

FiG. 422. — Cycadeoidea dacotensis. Semi-diagrammatic sketch of a
flower (bisporangiate cone), cut longitudinally; one sporophyll folded, and
one (at the right) arbitrarily expanded. At the center is the apical, cone-
shaped receptacle, invested by a zone of short-stalked ovules and inter-
seminal scales. The pinnules of the sporophylls bear the compound
sporangia {Synangia). Exterior to the flower are several hairy bracts.
About three-fourths natural size. (After Wieland.)

politan distribution and seemingly was as important as
the Dicotyledons are now. Over 30 species of the petrified
stems have been found in the Mesozoic terrains of the
United States, the Black Hills of South Dakota alone
yielding a score. The Isle of Portland forms were called

6o2 STRUCTURE AND LIFE HISTORIES

Cycadeoidea by the celebrated geologist Buckland. The
name of the order is derived from the genus-name, Bennet-
tites} Other forms, usually found as casts, are called
Williams onia, still others are known mainly as genera
founded on leaf imprints.

520. Cycadeoidea. — In most of its purely vegetative
characters, such as the anatomy of the stem and the

Fig. 423. — Cycadeoidea dacolcnsis (?). Photomicrograph of a young
seed (X 15), showing a sterile scale on either side. Between them pro-
jects the entire length of the tube through which the micropyle extends.
The partially collapsed nucellus is distinctly shown in the center. (After
Wieland.)

structure of the leaves, Cycadeoidea resembled modern
cycads, but its reproductive branches were character-
istically lateral, which is one of the most fundamental
characteristics of the higher seed-bearing plants of to-
day. Only two modern cycads {Macrozamia and Bowenia)
have lateral seed-bearing cones (Fig. 289).^ Various

^ Cycadeoidea Buckland = Bennellites Carruthers.
2 The staminate cones of Zamia are lateral.

THE EVOLUTION OF PLANTS

603

Fig. 424.—Cycadoidea Wielandi. At left, a finely preserved trunk
bearing many ovulate cones with seeds approaching maturity, and a lesser
number of either young or abortive cones. /', Receptacle of a shed or
non-preserved cone with surrounding bracts 3^et present; /' , two cones
broken away during erosion, with a portion of the basal mfertile pedicel
yet remaining; m, four cones eroded down to the surface of the armor,
in this instance about or a little beneath the level of the lowermost
seeds; y, three of the dozen or more very young cones, in some cases known
to be' simply ovulate and to be regarded as having aborted or else as be-
longing to a later and sparser series of fructifications than the seed-bearing
cones present, the latter unquestionably representing the culminant fruit-
producing period in the life of this cycad; 5 (over lower arrow), the ovulate
strobilus shown at right in its natural position, this photograph having
been made before the cone was cut out by a cylindrical drill. X about
14 At right, longitudinal section of the small ovulate strobilus cut from
its natural position on the trunk as denoted by the arrow s, in photograph
I c (upper arrow), seed with dicotyledonous embryo preserved, cotyle-
dons being similarly present in the lowermost seed on the left-hand side
of the strobilus; s, traces of hypogynous staminate disk; b, bracts; /, leaf
bases. X about K- (After Wieland.)

6o4

STRUCTURE AND LIFE HISTORIES

structural characters of Cycadeoidea are shown in Figs.
421-427.

In Cycadeoidea dacotensis the ''flower," which in some
specimens was 5 inches long, was a strobilus, consisting of
a thick axis on the lower part of which were numerous
bracts arranged in spirals. The bracts surrounded a

P'v

m^^mmmm:

Fig. 425. — Cycadeoidea Wielandi. Longitudinal section through the
axis of a female inflorescence, or cone. Z, old leaf-base; d, insertion of
disc; s, erect seed, borne at summit of seed-pedicle inserted on convex
receptacle; b, hair-covered bract. (After Wieland.)

campanula of about 20 stamens. Each stamen was, in
reality, a pinnately compound sporophyll, about 4 inches
long, rolled in toward the center of the flower, and
bearing two rows of compound microsporangia (pollen-
sacs) on each leaflet. They thus closely resembled the
sporophyll of a fern.

THE EVOLUTION OF PLANTS

60s

Fig. 426. — Cycadeoidea ingens. Restoration of an expanded bispor-
angiate cone, or flower, in nearly longitudinal section. Restored from a
silicified fossil. (After Wieland.)

' ' M

■5

^^BM^

^^^mm

i^. • *

fti^^

Sl% V. ■

K.<

^^^

pl^^^

P^r'"^-'^^^'

: ^''^

^m»M

H

ij.-r'['

&.

m

,__- .-^ . . .

. jt

^ Fig. 427. — Cycadeoidea Dartoni, Tangential section through outer
tissues of the (fossilized) trunk, showing the very numerous seed-cones.
The seeds are very small (the illustration being natural size), and nearly
every one has a dicotyledonous embryo. There were over 500 such cones
on the original stem. (After a photograph loaned by Prof. Wieland.)

6o6 STRUCTURE AND LIFE HISTORIES

The axis of the flower terminated in a cone-shaped
receptacle, bearing the stalked ovules, and numerous
sterile scales (Figs. 424 and 425). The mature seeds often
contain the well-preserved fossil embryos, with two
cotyledons which quite fill out the nucellus, and show
that there was little or no endosperm. These are char-
acters never found in the lowest group of modern seed-

FiG. 428. — Flower of magnolia. (Cf. Fig. 429.)

bearing plants (the Gymnosperms), but only in the
highest group of Angiosperms, the Dicotyledons. In
fact, the French paleobotanist, Saporta, called some of the
Cycadeoids, Proangios perms.

521. Relation of Cycadeoidea to Modem Angiosperms.
— The question of the ancestry of the Angiosperms is the
most important problem of paleobotany. Although the
Bennetti tales possess many of the primitive anatomical

THE EVOLUTION OF PLANTS

607

features that characterize the Cycadofilicales, their
development of a bisporangiate strobilus with two set
of sporophylls, related to one an-
other as they are in the flower of
the Angiosperms. indicates a gen-
etic relationship to that group, as
does also the fact that the seeds,
enclosed in a fruit, possess a dicot-
yledonous embryo, without endo-
sperm. In other features the Ben-
nettitales are unlike the Angio-
sperms; the ovules, for example,
are enclosed by sterile scales, in-
stead of by the carpels on which
they are borne, and the protrusion
of the pollen-chamber through the
micropyle signifies the gymno-
spermous type of fertilization.

These and other comparisons in-
dicate that the Bennettitales were
essentially Gymnosperms having
certain Angiospermous characters,
and therefore, while they are not
to be considered as the ancestors
of the Angiosperms, it is probable
that they and the modern dicoty-
ledons are both descended from a
common branch of the ancestral
tree. Among modern plants, the
flower of the magnolias most
closely resembles that of Cycadeoidea in the spiral arrange-
ment of its stamens and pistils (Figs. 428 and 429.). How

Fig. 429. — Magnolia.
Flower with perianth re-
moved, showing the com-
pound pistil, and four of the
stamens. Most of the sta-
mens have been removed.
Note their spiral arrange-
ment as shown by the scars
at the points of attachment.
(Cf. Fig. 428.)

6o8

STRUCTURE AND LIFE HISTORIES

much significance should be attached to that fact has
been disputed by students of morphology.

The gap between the stamen of Cycadeoidea and the
type characteristic of modern Angiosperms is partially
bridged by the genus Williamsonia (which has simple
vs. pinnately compound stamens), and by another genus,
Wielandiella, both older genera than Cycadeoidea. From
this it has been inferred that the Bennet-
titales are a lateral branch, further re-
moved than their ancestors from the direct
evolutionary stock of the Angiosperms.
522. Origin of Monocotyledons. — If
the earliest Angiosperms were dicotyle-
dons, as seems to be the case, the mono-
cotyledons were probably derived from
the dicotyledons by a process of simplifi-
cation. Much light has been thrown on
this question by a study of the develop-
ment of the embryos {emhryogeny) of
certain plants. The case of Agapanthus
umbellatus L'Her. (Fig. 430), a South
African plant of the Lily family, may
be taken as illustrating the nature of
the evidence derived from embryogeny.
The sequence of events is as follows.^ As the mas-
sive proembryo enlarges, the root-end elongates, thus re-
maining narrow and pointed; while the shoot-end widens,
becoming relatively broad and flattish. At this broad
and flat end the peripheral cells remain in a state of
more active division than do the central cells, and form
what is known as the cotyledonary zone. In this zone two

1 The above description closely follows Coulter and Land (1914).

Fig. 430. — Aga-
panthus umhellatiis.
A, monocotyledon-
ous embryo; 5,
dicotyledonous em-
bryo. (Redrawn
from photo by W.
J. G. Land.)

THE EVOLUTION OF PLANTS 609

more active points (primordia) appear and begin to de-
velop. Soon the whole zone is involved in more rapid
growth, resulting in a ring or tube, but with the primordia
still evident. The cotyledonary zone continues its growth
until a tube of considerable length is developed, leaving
the apex of the proembryo depressed. At this stage either
one of two things may occur. As the cotyledonary zone
continues to grow, the two primordia on the rim of the
tube may continue to develop equally, forming two coty-
ledons; or one of the primordia may cease to grow, result-
ing in an embryo of only one cotyledon; in other words,
the entire cotyledonary zone may develop under the
guidance of only one growing point. One cotyledon is
not eliminated, but the whole growth is diverted into one
cotyledon. There thus develops what appears to be an
''open sheath" and a ''terminal" cotyledon.

In other words, monocotyledony is not the result of the
fusion of two cotyledons, nor of the suppression of one;
but is simply the continuation of one growing point on the
cotyledonary ring, rather than a division of the growth be-
tween two growing points. In a similar way, polycoty-
ledony is the appearance and continued development of
more than two growing points on the cotyledonous ring.
The rudimentary second cotyledon of a "monocotyle-
donous" grass-embryo (wheat) is shown in Fig. 378, (p. 494).

523. Ancestors of the Gymnospenns. — As far back as
Devonian time, preceding the great coal period (Carbon-
iferous), fossils have been found of a plant, Cordaites (of
the order Cordaitales) , common in that period, and
having characters which indicate that it stands in the
ancestral line of our modern conifers — that it and the
conifers had a common ancestry.
39

6io

STRUCTURE AND LIFE HISTORIES

The leaves of Cordaites resembled those of the Kauri
pines (Agathis) of the southern hemisphere (Fig. 431),
or the leaflets of Zamia. They varied from a decimeter to
over a meter in length. The male cones resembled those
of the still living Ginkgo^ each stamen having from four
to six microsporangia (pollen-sacs) on a stalk. The female

Fig. 431. — Branch, with cones, of the Kauri pine {Agalhis australis).
(From Gardener's Chronicle.)

cones resembled the male in general appearance, and
the seeds resembled those of the Cycadofilicales (Fig. 423).
The plant itself was a slender tree, some forms of which
attained a height of over 100 feet. The Cordaitales
formed the world's first great forests. They represent a
wide departure from the Cryptogams, and must be con-
sidered as true seed-bearing plants. They were closely

THE EVOLUTION OF PLANTS 6ll

related to the Ginkgo — another living fossil, ranking next
below the modern cone-bearing trees. We thus ascend
from the ferns to the conifers by a series of transitional
forms as follows (reading from the bottom, up):

6. Coniferales (modern cone-bearing trees).

•5. Ginkgoales (primitive gymnosperms).

4. Cordaitales (transitional conifers).

3. Cycadales (true cycads).

2. Cycadofilicales (cy cad-like ferns).

I. Filicales (true ferns).

524. Relation of the Above Groups.— It must not be
inferred that the above groups were derived one from the
other by descent from lower to higher. They should be
interpreted rather as samples remaining to show us, not
the steps, but the kinds of steps through which the plant
kingdom has passed in developing the more highly organ-
ized, modern cone-bearing trees from more primitive forms
like the ferns. As stated above, it is doubtful if the actual
transitional forms have been preserved, so that the entire
history of development can probably never be written.

525. A Late Paleozoic Landscape.— Fig. 432 illustrates
the kind of landscape that must have been common in the
latter part of the Paleozoic era along sluggish streams in
certain regions such as Texas and New Mexico. Of the
primitive vertebrates then abounding, only a few larger
types are shown. The dragon-flies of that time are
known to have had a spread of wing amounting, in some
cases, to as much as 2 feet. In the foreground, at the
left, are representatives of the Cycadofilicales, some of
them bushy, and others resembling our modern tree ferns.
At the right are dense thickets of Calamites, the ancient
representatives of our modern scouring rushes {Equisetum),

6l2

STRUCTURE AND LIFE HTSTORTES

THE EVOLUTION OF PLANTS 613

In the background, at the left, are the unbranched
Sigillarias, and the branched Lepidodendrons. The Cor-
daitales, which formed the Devonian forests, were not yet
extinct, but none is shown in the figure. Other forms,
ancestors of our modern conifers and angiosperms, must
be imagined as hidden in the recesses of the forest.

526. Significance of the Fossil Record. — Before the
brilliant discoveries in fossil botany, just outlined, were
made, there had been (as stated in Chapter XXXVI) a
general tendency among botanists to consider the compar-
atively simple moss-plants as an older type than the fern,
and that either they or their close relatives were the ances-
tors of Pteridophytes. As outlined in the same chapter,
the sporogonium of the moss was regarded as representing
the form from which, by elaboration of vegetative tissues
and organs, the sporophyte of the fern was derived. This
view was clearly expressed in 1884 by the noted botanist
Nageli, who considered that the sporophyte of Pterido-
phytes was derived from a moss-like sporogonium by the
development of leafy branches.

Fig. 432. — Restoration of a scene along a sluggish creek in Texas and
New Mexico during the late Carboniferous (Upper Pennsylvania) and
early Permian times. The lowlands of this period doubtless swarmed
with reptiles such as shown in the picture, and with other animals, now
extinct. Some specimens of the giant "dragon-flies" had a spread of
wings of two feet. The fern-like trees and the bushy plants in the fore-
ground are Cycadofilicales. To the right of the water are wide stretches
of the huge scouring rush (Calamites); on the left bank of the stream are
the unbranched Sigillarias (still as prominent as earlier in the coal
period), and on higher ground to the left the branched Lepidodendrons.
One must view this scene as one of many such landscapes, with ever-
varying detail, along streams and inlets. Cordaites, which in later
Devonian time made the first great forests of which there is record, is
still present, though not shown. So, too, there are hidden in the recesses
of the forest the^ forerunners of the modern coniferous types, as well as
other forms destined to give rise to the angiosperms. (Landscape from
Williston, adapted from Neumayr.)

6 14 STRUCTURE AND LIFE HISTORIES

A consideration of the fossil record, however, makes it
difficult to accept this hypothesis. Not only do we find,
in the fossil forms described above, sporophytes that do
not bear the remotest resemblance to the moss-sporo-
gonium, but fossil mosses and liverworts have never been
positively identified in either the Palaeozoic or the Meso-
zoic rocks, while the same rocks are rich in fossils of such
advanced forms as the broad-leaved sporophytes of the
Cycadofilicales and Cycadophytes. We must not, how-
ever, hastily conclude, from this lack of evidence, that
mosses and liverworts did not exist in those early ages.
Quite likely they were present when the Paleozoic rocks
were being deposited, though doubtless not represented by
the same genera, or at least not by the same species, as
are now living.

527. Summary of Results. — From what has been said,
in this and in Chapter XXXVI, we recognize that the
method of evolution is to he ascertained chiefly by experi-
ment— by studying living plants in action; but the
course of evolution chiefly by the study of comparative
morphology, with special attention to fossil forms. Other
points are necessary to complete the history of the
evolution of plants; the above paragraphs give only the
barest outline of the problem, for the entire history is
much too long and much too difficult to be treated here.
To summarize; the facts now known have led some
investigators to infer:

1. The origin of Angiosperms from Cycadophyta (pro-
angiosperms) .

2. The origin of Cycadophyta from Cycadoffiicales.

3. The origin of Cycadoffiicales from Primoffiices.

4. The origin of Filicales from Primoffiices.

THE EVOLUTION OF PLANTS

6iS

/Inceatorso/ Pr/mofihces

Fig. 433. — Genealogical tree, showing the ancestral lines of the modern
plant orders, according to a monophyletic hypothesis. Full explanation
in the text. (Cf. Fig. 434.)

6l6 STRUCTURE AND LIFE HISTORIES

5. The origin of Cordai tales from Primofilices.^

6. The origin of Coniferales from Cordaitales.

An ancestral tree embodying these views is shown in

Fig. 433-

What was the origin of the Primofilices? Here, as
often in every science, we have to acknowledge that
we do not know; the group is a hypothetical one,
and some investigators doubt its actual existence
altogether.

528. Other Views. — (a) Other and equally competent
students of the problem take exception to one or more
of the six points tabulated above. Not all of their views
can here be discussed, but mention may be made of
that first elaborated by Jeffrey, of Harvard University.
According to this view vascular plants appear at the
beginning of the fossil record as two distinct series, the
Lycopsida and Pteropsida. The Lycopsida, like the
modern Lycopodiales, are characterized by the possession
of small leaves (a primitve character), and by few spor-
angia on the upper surface of the leaves. The Pteropsida,
by contrast, are distinguished, like the modern Filicales,
by large leaves, having the numerous sporangia on the
lower surface. The two groups also have well-marked
anatomical differences. The Lycopsida reached their
greatest development in the Paleozoic period, and now
appear to be on their way to extinction. The Pteropsida,
on the other hand, although possessing many repre-
sentatives in former geological ages, still maintain
their full vigor, and are considered by this school of
paleobotanists to be in the direct ancestral line of our

^ The term Priniofdices, not hitherto used in this text, refers to a hypo-
thetical, primitive fern stock.

THE EVOLUTION OF PLANTS

617

Ancestors of LLjcop^idamd Pt^ropsida,

Fig. 434.— Genealogical tree, showing the ancestral line of the modern
plant orders according to a polyphyletic hypothesis. Full explanation
m the text. (Cf. Fig. 433.) ^^ ^^ v

6l8 STRUCTURE AND LIFE HISTORIES

modern vascular plants, substantially as indicated in

Fig. 434-^

(b) Greater precaution in drawing conclusions from the
few known facts has led still other students of fossil plants
to refrain from endeavoring to connect up the ancestral
lines, claiming that while they may converge, indicating a
common ancestry of the known forms in the geologic past,
on the other hand they may not unite, or at least may not
all converge toward the same ancestral type. In other
words, it is suggested that fossil and modern plants had a
poly genetic origin from the stage of primitive protoplasm.
Such views are illustrated in Table VII (p. 619).

It is seen from this diagram that our modern ferns have
a long ancestral history, extending from the present back
to early Palaeozoic times; the same is true of our modern
cycads, maidenhair tree (Ginkgo), club-mosses (Lyco-
podiales), and horse-tails (Equisetales). The Coniferales
may be traced back into the upper Carboniferous period,
while the most highly developed of modern plants, the
Angiosperms, appear to have come into existence as late
as about the middle of the Mesozoic era, perchance as
recently as 20 million years ago.

"The construction of a pedigree of the Vegetable King-
dom is a pious desire, which will certainly not be realized
in our time; all we can hope to do is to make some very
small contributions to the work. Yet we may at least
gather up some fragments from past chapters in the history
of plants, and extend our view beyond the narrow limits
of the present epoch, for the flora now living is after all

^ Scott restricts the name Lycopsida to the Lycopodiales, and proposes
a third group, Sphenopsida, including the Equisetales, Pseudoborniales,
Sphenophyllales, and Psilotales.

THE EVOLUTION OF PLANTS

619

nothing but one particular stage in the evolution of the
Vegetable Kingdom."^

Table VIII

Ascendancy

Periods

Persistence and relationship of
great groups

VII. Reign of Angiosperms

Tertiary

Cretaceous

Comanchian

VI. Reign of Pro-angiosperms

Jurassic
Triassic
Permian

V. Reign of Acrogens (High- Pennsylvanian
er Equisetes. Lycopods,! Mississippian
etc.) i

IV. Reign of Gymnospertns | Devonian

(A

U)

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M

^

1 ^

0

5

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0

0

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c

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III. Reign of Early Land
Plants

Silurian

Ordovician

S- Actual Fossil Land Plant rec-
ord begins
4. Primofilices — Early Equisetes
3. Basal Plant Complex with va-
riety of species

II. Reign of Algae

Cambrian
Precambrian
(Proterozoic)

Differentiation of Dry

and Aquatic Plants
(Fossil Algae abundant)

Land

Reign -of Primitive Life
(Hypothetical)

Old P r e c a m-

brian
(Archeozoic)

(Fossil Algae begin)
I. Primitive Protoplasm
Unicellular Life

and

In the above table (after Wieland), the groups are to be considered as
arranged on an unrolled cylinder, projected from a hemisphere; thus the
phyletic lines are to be pictured as diverging upward and the Cordaitales
as coming between the Ginkgoales and Filicales, to both of which the}^
are related.

629. The Element of Geological Time. — How many
years has it taken for the evolution of the higher Angio-
sperms— that is, from the dawn of the fossil record in the

* Scott, D. H. "Studies in Fossil Botany," p. 3.

620 STRUCTURE AND LIFE HISTORIES

Silurian period to the present? No one knows. From a
study of the thickness of rock strata, and a knowledge of
the probable time required for the depositing of those
strata as sediment on the floor of the ancient oceans, and
their elevation and denudation to their present condition
by weathering and erosion, geologists have been able to
suggest relative measures of geologic time. Paleozoic
time is long, twice as long as Mesozoic time, and Meso-
zoic time must be at least twice as long as Cenozoic time.
The actual age of the earth is, however, a problem which
engages the attention of physicists as well as geologists.
Sixty years ago Lord Kelvin gave a mean estimate of
100,000,000 years. With this estimate the geologists,
Walcott and Geikie, have nearly concurred; but since the
discovery of radium it has been estimated that certain
carboniferous iron ores have an age of 140,000,000 years.

Figures of such magnitude convey but little meaning to
our minds; they are too large for us to grasp their real
value. *' Therefore,'' as Darwin has said, "a man should
examine for himself the great piles of superimposed strata,
and watch the rivulets bringing down mud, and the waves
wearing away the sea-cliffs, in order to comprehend some-
thing about the duration of past time, the monuments of
which we see all around us."

530. The Essence of Science. — A careful reading of
this book will have led the student to realize that the un-
solved problems of botany are more numerous and quite
as interesting as those we have solved. The essence of
science is the endeavor to ascertain by the best method
that which is most worth knowing.

APPENDIX

THE GREAT GROUPS OF PLANTS

DiTision

Subdivision

I. Thallophyta.

A. Algse.

B. Fungi.

II. Bryophyta.

III. Pteridophyta.

IV. Calamophyta.

V, Lepidophyta.

VI. Cycadophyta.

Class

r. Cyanophyceee

2. Chlorophyceae

3. Phaeophyceae

4. Rhodophyceae

1. Myxomycetes

2. Schizomycetes

(Bacteria)

3. Phycomycetes

4. Ascomycetes

5. Basidiomycetes

6. Fungi imperfecti

(life histories
imperfectly known)

Order

Hepaticae.

Musci.

I. Eusporangiatae.

(Ricciales
Marchantiales
Jungermanniales
Anthocerotales

Andreales

Sphagnales

Bryales

Ophioglossales

Marratiales

Isoetales

2. Leptosporangiatae.

■{

1. Sphenophyllineae. . .

2. Equisetineae

3. Calamarineae

1. Lycopodineae

2. Lepidodendrineae — <

1. Cycadofilicineae. . . .

2. Cycadineas

3. Bennettitineae

4. Cordaitineae

Filicales

Marsiliales

Sphenophyllales

Equisetales

Calamarales

Lycopodiales

Selaginellales

Lepidodendrales

Cycadofilicales

Cycadales

Bennettitalei

Cordaitales

Ginkgoale&

Gnetales

621

622

APPENDIX

A. Gymno-
spermae

VII. Spermatophyta ^ ^^^.^

sperm ae

Pinoideae.

I. Monocotyledoneae

Coniferales
Tax ales
Pandanales
Naidales
Graminales
Arales
Xyridales
I Liliales
I Scitamnales
I Orchidales

Dicotyledoneae 32 Orders, including

Salicales
Polygonales
(a) Archichlamydeae | Ranunculales

ib)

Apetalae
Polypetalae

Metachlamydeae
Sympetalae ( =
Gamopetalae)

Rosales

Violales

Myrtales

Umbellales

Ericales

Polemoniales

Plantaginales

Rubiales

Campanulales

INDEX

Abiogenesis, 89
Abscission layer, loi
Absorption, 11
Acacia, 327
Acer rubrum, 337
"Adder's tongue," 175, 177
Adirondack bog, 580
Adjustment to environment, 189
Adiantum, 156, 170

concinnum, 173
Adnation, 464
Aerobes, 103
Aesculus Hippocastanum, 36, 120,

121
Agapanthus umbellatus, 608
Agaricus campestris, 288
Agassiz, Louis, 505
Agathis australis, 610
Albugo Candida, 292
Alcoholase, 100
Aldehydase, 79
Alfalfa leaf-spot, 295
Alismacese, 491
Alisma Plantago-aquatica, 444,

492, 493
Allelomorphs, 555
Alnus, 83

Alsatian clover, 469
Alsike, 469
Alternation, antithetic, 575

homologous, 575
Ambrosiaseae, 488

Amorpha, 467
Amylase, 80
Amyloplast, 78, 81
AnaerobeSj 102, 103
Anatomy, 4
Anchorage, 11
Andreaeales, 193
Androecium, 435
Anemone, 462

Anemonella thalictroides, 463
Angiopteris evecta, 595
Angiosperms, 408, 446

ancestors of, 600

life-cycle of an, 444
Annulus, 157, 202
Ant, fungus-growing, 326
Anther, 436
Antheridia, 170, 171
Antheridiophores, 196
Anthesis, 472

Anthoceros, 209, 214, 219, 220,
341, 356

fimbriatus, 210

fusiformis, 209, 214, 215

laevis, 213

Pearsoni, 213

phymatodes, 217

punctatus, 209

tuberosus, 217
Anthocerotales, 210
Antipodal cells, 440
Antiseptic surgery, 315, 316
Antitoxin, 314
Ants, leaf-cutting, 325

623

624

INDEX

Apetals, 456, 457
Apex, 13
Apical cell, 195

Apothecium, 329, 330, 331, 332
Araceae, 496
Aralia spinosa, 27
Archegonia, 169
Archegoniates, 222
Archegoniophores, 197
Archesporial cell, 439
Archichlamydeoe, 456, 457
Arctium minus, 486
Arisaema triphyllum, 51
Arrow-leaf, 449
Ascaris, 186
Asclepiadaceae, 476
Asclepias, 476, 477

syriaca, 475, 478
Ascomycetes, 98
Ascophyllum, 352, 364
Ash, 28

Aspergillus niger, 72
Aspidium Filix-mas, 158
Assimilation, 88
Aster, 24, 25
Austrian pine, 416
Avena sativa, 298
Axil, 150
Azotobacter, 83, 333

B

Bacillus typhosus, 310
Bacteria, 306

helpful, 316

nitrogen-fixing, 318
Bacteria and the dairy, 316
Banana, 26
Barley, 298

smuts of, 297
Barrigona palm, 130
Base of the blade, 13

Bean, 123, 526

blight, 309

Pond, 580
Beech, 336
Begonia, 179, 347
Belladonna, 481
Bennettites, 602
Betula, 448

lutea, II

populifolia, 108
Bindweed, 119, 478, 479
Biogenesis, 89
Biology, 3
Biometry, 548
Birch, 336, 448

yellow, II

white, 108
Bittersweet, 481
Black-mosses, 193
Black mustard, 464

walnut, 308
Bladderwort, 87, 88
Blade, 13
Blakeslee, 353
Blister-blight, 291
Blister-rust, pine tree, 301, 302
Blodgett, Frederick H., 443
Blueberries, 451, 452
Bolley, Professor, 92
Boneset, 486
Boston fern, 163

ivy, 144
Botany, educational value of, 5

fossil, 5

relation to other sciences, 3

systematic, 3

what is, I
Botrychium, 175, 178, 214

virginianum, 176
Bowenia, ss3>, 4ii

serrulata, 594
Bower, F. O., 573

INDEX

625

Brachychiton diversifolium, 523
Bracken fern, 149, 155
Bracts, petalody of, 462
Brake, 149, 155
Branches, sterile and fertile, 196

archegonial, 197

innovation, 199
Brassica alba, 132, 133

nigra, 464

oleracea, 530, 569
Breathing, respiration versus, 107
Breeding of resistant varieties, 313
Bromeliads, 327
Brood-buds, 216
Broom-rape family, 338
Brown, Robert, 16
Brussels sprouts, 464
Bryales, 193
Bryophyllum, 347

crenatum, 349
Bryophytes, 366
Bryopteris filicina, 217
Budding, 98
Bud-scales, 30
Bulbils, 163
Bundles, fibro-vascular, 36, 61

structure of, 65, 66
Burdock, 116, 486
Burroughs, John, 434
Butter, June, 316

Butter-and-eggs, 481, 482, 483-485
Buttercup, 462

Cabbage, 464
Caesalpinioideae, 468
Calamites, 368, 388, 612
Calamus, 495
Calla-lily, soft rot of, 309
Calla palustris, 136
Calorimeter, 106
40

Calyptra, 200
Cambium, 67

interfascicular, 67
Camptosorus rhizophyllus, 162, 164
Canada thistle, 486
Cancer-root, 338
Cannel coal, 426
Capsicum, 481
Carbohydrates, 71

seat of elaboration of, 72
Carbon cycle, no, iii
Carica Papaya, 358
Carpels, 436, 461
Carya ovata, 336
Caryopsis, 493
Cassia marilandica, 467, 468
Castanea dentata, 294, 296
Castor-oil seed. 123

plant, 63, 66
Catnip, 479

Cat-tails, 490, 491, 492
Cattleya, 329, 500, 501
Cecropia, 325
Cell, 15, 16, 17, 18
Cell-division and reproduction, 344
Cell-sap, 17
Cell-theory, 15, 20
Celtis occidentalis, 292
Ceratozamia, 333, 411
Cercis, 468
Chaff, 495

Character-units versus unit-char-
acters, 558
Characters, inheritance of acquired,

506-509, 565
Cheese, 316

Cherries, black knot of, 295
Chestnut, 294

American, 296

bark disease, 295

blight, 294
Chicory, 486, 487

626

INDEX

ChlorophyD, 33, 75

necessity for, 321
Chlorophytum elatum, 350
Chloroplasts, ss
Chondriosomes, 16, 17, 565
Chromatin, 16, 186
Chromosomes, 186
Cichorium Intybus, 486, 487
Cicuta, 472
Cinnamon fern, 159
Cirsium arvense, 486
Cissus laciniata, 342
Cladonia, 332
Classification, 3, 365, 538
Claviceps purpurea, 293
Clayton's fern, 160
Clethraceae, 473
Clover, leaves, 532

old-maid, 470

secret of, 317
Club-mosses, 376

little, 382
Clusia, 328
Coagulase, 80
Coal balls, 596

Coal-formation, pollen and, 424
Coalescence, 473
Cocoanuts, germinating, 499
Coix lacrima-Jobi, 446
Cold storage, 305
Colds, 309
Coleoptile, 494
Coleorhiza, 494
Colocasia antiquorum, 43
Coloration, 475
Colpothrinax Wrightii, 130
Columella, 200
Column, 498
Comandra umbelkta, 301

pallida, 301
Compositae, 483, 488
Cone-flower, 533

Conidium, 304
Coniferae, 416
Conjugation, 251

scalariform, 345
Conopholis americana, 338
Convolvulaceae, 478
Convolvulus, 119

arvensis, 479
Coprinus comatus, 289
Corallorhiza, 324, 337
Coral-root, 337
Cordaitales, 609
Cordaites, 609, 610
Coreopsis, 488
Corn, 297

Indian, 85, 123, 562

smut, 299
Corolla, 435
Correlation among leaves, 143

within the plant, 142
Cortex, 65, 195

Corticium vagum var. solani, 291
Cotyledon, 174, 381
Coville, 452
Cow parsnip, 472
Crataegus punctata, 450
Creation, special doctrine of, 502
Cronartium pyriforme, 301

ribicola, 301
Crops, rotation of, 91
Cross-fertilization, 173
Crossing, increased vigor from, 562
Crossotheca Hoenginghausi, 593
Crowfoot, yellow water, 460
Crown galls, 308

imperial, 44
Cruciferae, 463
Cryptogams, vascular, 226
Crysanthemum leucanthemum, 486

rust, 313
Cucumbers, wilt of, 309
Cultures, pedigreed, 553

INDEX

627

Cupules, 216
Currants, 301
Cuscuta, 339, 340, 341

Cushion, 166
Cuticle, 35
Cycadaceae, 83, 331
Cycadeoidea dacotensis, 600, 601,
602, 604

Dartoni, 605

Wielandi, 603, 604
Cycadocephalus Sewardi, 600
Cycadofilicales, 592, 598, 612, 613
Cycads, 390
Cycas, 83, 411, 431, 447

circinalis, 391, 401

media, 395, 397, 398, 40?

revoluta, 390, 391, 394, 396,
402
Cycle, organic, 70
Cypripedium, 498
Cyrtomium falcatum, 22, 155
Cystopteris bulbifera, 164
Cystopus candidus, 292
Cytase, 100
Cytoplasm, 16

Dandelion, 116, 486
Daisy, white. 486
Darwin, Charles, 511
Darwinism, 510
De Candolle, A. P., 92
Decay 322
Determiner, 558
Devvar flask, 106
Dextrinase, 79
Diastase, 86, 100, 102
Dicotyledons, 453, 456
Dictyota, 356, 364
Dififerentiation, dorso-ventral, 168
Diflfusion, 55
of gases, 55

Diffusion of liquids, 55
Digestion, 85
Dionaea muscipula, 86, 88
Dioon, 411

edule, 403
Diplazium zelanicum, 154
Disease carriers, 310
Diseases caused by Phycomycetes,

291
Diseases, contagious and infectious,

309
Dissimilation, 90
Dodder, 339, 340, 341
Dog's-tooth violet, 434, 439
Dominance, 554

law of, 553
Doubling, 464
Drosera, 347, 88

intermedia, 87, 88

rotundifolia, 348
Drynaria meyeniana, 151, 164

Ecology, 4

Ectotrophic micorhizas, 336

Eel-grass, 348, 349

Egg-cell, 170

Egg, fertilized, nature of, 172

development of, 1 73
Eichornia crassipes,, Solms 51, 5?
Elaters, 219, 373
Elephants, 514
Elm, 414

Elymus virginicus, 293
Embryo, 174

growth of, 174
Embryogeny, 608
Embryology, 4
Enation, 574
Encephalartos, 411
Endosmosis, 58
Endosperm, 400, 430

628

INDEX

Endosperm-nucleus, 440
Endospores, 99
Endothia parasitica, 296
Environment, 125

factors of, 125

fitness for, 513
Enzymes, 79, 86, 94, 100
Epidermis, 31
Epiphytes, 326
Epiphytism, 325, 327
Equisetales, 368, 618
Equisetum, 368, 375, 376, 431

arvense, 368, 370, 371, 374

debile, 368

fluviatile, 369

giganteum, 368

palustre, 368, 373

pratense, 368, 371
Ergot, 293, 295
Ericaceae, 473
Eriopus remotifolius, 226
Erythronium americanum, 434,

437, 439, 455

albidum, 437
Etiolation, 136
Eugenics, 560, 566
Eupatorium perfoliatum, 486
Euthenics, 560
Evening-primrose, 471, 532, 534

Lamarck's, 535

giant, 536
Evetria buoliana, 301
Evolution and classification, 539

early antagonism to, 510

experimental, 520, 535

inorganic, 503

method of, 505

of plants, 569

organic, 504
Excreta, theory of toxic, 92
Exosmosis, 58
Extinction, factors of, 589

Eye-spot, 251

Factor, 558

False beech-drops, 324, 339

Fascicles, 415

Fats, 71, 84

Fawn-lily, 434

Femaleness, maleness and, 353

Fermentation, 102

active agent in, 99

alcoholic, 95, 97

importance of, 94

products of, 96

relation of to our daily lives,
102

respiration and, 112

significance of, loi
Ferments. 79
Ferns, 348

Boston, 528

eusporangiate, 178, 372

leptosporangiate, 157

shield, 158

walking, 162, 348
Fertilization, 170, 172, 180

double, 442
Fertilization-membrane, 172
Fever, typhoid, 310
Ficus elastica, 64, 66
Filament, 436

Fittest, survival of, 191, 515
Flowers and insects, 343
Flowers, perfect and imperfect, 447
Foliage-leaf, types of, 156
Fomes applanatus, 303
Food elements, source of, 72
Foods, kinds of, 71
Foot, 174

Forests, world's first, 610
Fossil, what is a, 578

record, gaps in the, 587

INDEX

629

Fossombronia tuberifera, 217

Fraxinus, 28

Fritillaria imperialis, 44

Fronds, 150

Fruit, essentials of, 451

Fucus, 352

Function, 7

Fungi, timber-destroying, 303

Fungus-farms, 325

Gaertneria, 488
Galactase, 317
Gall-mite, 292
Gametes, 180

theory of purity of, 555
Gametophyte, 181

decline of, 365
Gamopetalae, 456
Gangrene, 316
Gasteria nigricans, 38
Gaussia, 134
Gaylussacia; 473
Gemma, 208, 216
Gene, 558, 559
Genotype, 561

Generations, alternation of , 182, 575
Geography, plant, 5
Geological time, table of, 584
Geotropism, 128
Geranium, 339

house, 139
Germination, 166
Germ-tube, 166
GinkgO; 610
Gleditsia, 468
Gleichenia circinata, 157
Glumes, 495
Golden rod, 340
Goodsell, John W., 310
Gooseberries, 301
Grafting, 333

Grafting, various methods of, 334

Grain smut, 297

Gramineae, 492

Grape, downy mildew of, 313

Gravity, relation of roots and

stems to, 126
Grew, Nehemiah, 14
Groups of plants, 366

the great, 621
Growing points, 212
Growth, 113

and nourishment, 123

differential, 115

intercalary, 218

permanent and temporary, 121
Guard-cells, 33, 35
Gunnera manicata, 341
Gymnosperms, 408, 412

ancestors of, 609
Gymnospermy, 408
Gynoecium, 436

H

Hackberry, 292

Haustoria, 402

Head, 483

Hedge mustard, 190

Hellriegel, 318

Henbane, 481

Hepaticae, 209, 210

Heracleum lanatum, 472

Hercules club, 27

Heredity, 541

experimental study of, 549
Johannsen's conception of, 560
vs, inheritance, 544

Heterogamy, 352

Heterospory, 356, 382

Hickory, 336

Hiras6, 404

Histology, 4

Hofmeister, 411

630

INDEX

Homology, 28
Hooke, Robert, 14
Horehound, 479
Hornbeam, 76, 77, 336
Horsechestnut, 36, 120, 121
Horsetails, 359, 368
Hybridizing, artificial, 552
Hygiene, personal, 311

public, 311
Hymenophyllum, 351
Hypanthium, 464
Hypocotyl, 381
Hypothecium, 331

Jungermanniales, 210
K

Kalmia latifolia, 474
Kingdom, organic, 8
Kingdom, divisions of plant, 366
Knight, Thomas Andrew, 128
Knight's experiment, 127
Knowlton, F. A., 595
Kohlrabi, 464
Kunze, 599

Ikeno, 404

Imbibition, 58

Incrustation, 578

Indian pipe, 323, 339, 473, 476

Indusium, 155

Ingen-Housz, Jan, 73, 74

Inheritance, alteration of, 566
and environment, 560
and reproduction, 544
versus expression, 188, 543

heredity, 544
mechanism of, 565
what is, 542

Innovation-branches, 348

Internode, 118

Involucre, 483

Isogametes, 251

Isogamy, 352

Isospores, 380

Iva, 488

J

Jack-in-the-pulpit, 51
Jeffrey, 426
Job's tears,. 446
Johannsen, 561
Johnson-grass, 496
Juglans nigra, 308

LabiatcC, 479

Labium, 479

Labor, division 01 physiological, 363

Lagenostema Lomaxi, 596

Lamarck, 506, 509

Lamella, middle, 158

Lang, ontogenetic hypothesis of,

575
Larch, 336
Latex, 476
Lathraea, 324
Lathyrus latifolius, 466
Layering, 179
Leaf -base, 13
Leaf-blade, internal anatomy of,

31,34
types of, 30

Leaf-fall, loi, 415

Leaf -mosaic, 144

Leaves, correlation among, 143
growth of, 119
lungs of plants, 103
relation of, to light, 138
spore-bearing, 154
stomach of plants, 103

Leeuwenhoek, 98

Legume, 318, 468

Leguminoseae, 466

INDEX

631

Leguminous crops, value of, 83
Lenticels, 109
Leonurus cardiaca, 145
Lesczyc-Suminski, Count, 168
Lichen, 330, 331, 332

produced synthetically, 332
thallose, 329
Life-cycle, 175

cytological, 186

of an angiosperm, 444

of a fern, 183

of a pine, 432

of Selaginella, 388.
Life history, 148

of angiosperm, 433

of the pine, 412
Life from life, all, 89
Light on rate of growth, effect of,

13s
Light-position, fixed, 141
Ligule, 382, 384
Ligulifloras, 484, 488
Lilium canadense, fertilisation of,
442

Martagon, 441

philadelphicum, 436
Limb, 487
Linaria vulgaris, 481, 482, 483

484, 485
Linin, 184
Linnaeus, 502, 539
Lip, 479
Lipase, 86
Lipoids, 86

Liriodendron tuhpifera, 29, 74
Lister, Lord, 316
Little potatoes, 291
Live-forever, 31 ,

Lizard's tail, 32, 459, 460
Lomaria, 599

eriopus, 599
Loxsoma Cunninghami, 157

Lunularia, 216
Lupine, 123, 126, 129

white, 64, 135
Lupinus, 123

albus, 64, 126, 129, 135
Lycopersicum, 335

esculutum, 450, 480, 545
Lycopodiales, 376, 388, 618
Lycopodium, 376, 378, 431

cernum, 381

clavatum, 377

lucidulum, 377, 382

obscurum dendroideum, 377

phlegmaria, 381

Selago, 376, 378, 379, 380
Lycopods, calamites and, 368
Lycopsida, 616, 618
Lyginodendron oldhamium, 592,

593, 596
Lygodium japonicum, 157

M

Macrozamia, 333, 411, 447

Moorei, 392, 393, 398, 399, 400

Magnolia, 606, 607

Maidenhair fern, 173

Maiosis, 185

Maize, Mendelian segregration in,
556

Maleness and femaleness, 353

Malpighi, Marcello, 14

Maltase, 79

Malthus, 513

Maple, 141

' red, 337

sap, economic value of, 67
silver, 142

Marattia fraxinae, 597

Marchantia, 348

Marchantiales, 210

Marjoram, wild, 479

632

INDEX

Marsh grass, 293
Mary, typhoid, 310
Matonia pectinata, 157
Medullary rays, 65
Megasporangia, 385
Megaspores, 385
Megasporophylls, 385
Melilotus lutea, 82
Melons, wilt of, 309
Membrane, limiting, 17

nuclear, 186
Memory, physiological, 192
Mendel, Gregor, 549
Mendelism, 549
Meristem, 374
Mesophyll, 31
Metabolism, 90
Metachlamydeae, 456, 473
Metamorphism, 582
Micorhiza, ectotrophic, 336

endotrophic, 337, 339
Microcycas, 411
Microsporangia, 385
Microspores, 385
Mid-vein, 37

Migration of plant diseases, 312
Migrations, plant, 586
Mildew, downy, 292, 313

powdery, 292, 295
Milk, souring of, 316

"certified," 316
Milkweed, 475, 476, 477, 478
Mimosa, 468

pudica, 122
Mimosoideae, 468
Mitosis, 184
Mohl, Hugo von, 15
Molds, 304

sexuality in, 355

sooty, 295
Monocotyledons, 489

and dicotyledons, 453

Monocotyledons origin of, 571, 608

types of, 491
Monotropa, 473

Hypopitys, 324

uniflora, 323, 476
Monotropaceae, 473
Moon worts, 175
Morchella esculenta, 289
Morphology, 4, 147
Mosses, true, 193, 204
Motherwort, 145
Mountain laurel, 474

palm, 134
Mucor mucedo, 304
Mullein, 31, 130
Multiplication, vegetative, 163,

179, 346, 351
Musa sapientum, 26
Musci, 193
Mushrooms, 288
Mustard, white, 50, 133
Mutation theory to Darwinism,
relation of, 537

test of, 535

value of; 537
Mutations, 530, 546
Mutualism, 325, 328
Muricaceae, 83
Myrmecophytes, 327
Myxomycetes, 284, 285, 323

N

Nasturtium, 137
garden, 143
Nazia racemosa, 495
Neck, 169
Neck-canal, 170
Neck-canal-cell, 170
Nephrolepis, 163, 528
New Zealand raspberry, 28
Nicotiana Tabacum, 480, 545
Nightshade, 481

INDEX

^33

Nitrobacter, 83
Nitrogen cycle, 318, 319
Nitrosomonas, 82
Nodules, 83
Nomenclature, 4
Nostoc, 215, 333, 341
Nourishment, growth and, 123
Nuclear division, indirect, 184
Nucleolus, 16
Nucleoplasm, 16
Nucleus, 16
Nutrition theory, 91

Osmosis, 55, 56

demonstration of, 57

importance of, 59
Osmunda cinnamomea, 159

claytonia, 152, 160
Ostrich fern, 114, 178
Ovary, 436
Ovules, 436
Ovum, 170
Oxalis, 138

bupleurifolia, 122
Oxidase, 79

Oak, 34, 336
Oats, smuts of, 297
(Enothera, 470

biennis, 471, 534

brevistylis, 533

gigas, 535, 536

Lamarckiana, 532, 533, 535
Oleander, 39
Onagra, 470
Onagraceae, 470
Onion, 348
Onoclea, 172

struthiopteris, 114, 178
Ontogeny, 504
Oosperm, 173
Operculum, 202
Ophioglossum, 178, 214

vulgatum, 175, 177
Opuntia Blakeana, 342
Orange, 295
Orchidaceae, 497
Orchids, 327, 500, 501
Organic and inorganic, 69
Organisms, 8
Organs, 7

essential, 448
Orobanchaceae, 338

Paleobotany, 5, 578
Paleogeography, 584
Palisade, 35
Palmacese, 495
Palmetto, sabal, 498
Palms, barragona, 498

cocoanut, 497
Pandorina, 405
Pangens, 565
Papaw, 359
Papillionoideae, 467
Pappus, 487
Paraphyses, 331, 380
Parasites, artificial, 341

fungal, 342
Parasitism, 325, 339

facultative, 343

obligate, 343
Parenchyma, 35
Parmelia perlata, 330
Parthenogenesis, 357
Pasteur, 89, 98, 307
Pea, 123

edible, 468, 554
Peach, brown rot of, 293

leaf-curl, 293
P^ar blight, 309

634

INDEX

Peary, 309

Peat-mosses, 193

Pedicle, 197

Pelargonium, 139, 339

Pellionia daveaueana, 78

Pelories, 482, 485

Pendulum, illustration of the, 529

Penicillium glaucum, 304

Pennyroyal, 479

Pepsin, 102

Perennial pea, 466

Pericarp, 493

Perichastium, 199

Peridermium pyriforme,. 301

Strobi, 301
Perisperm, 421
Petalody of bracts, 462, 466

of stamens, 462, 465, 466
Petals, 435

coalescence of, 462
Petiole, 13

structure of, 35
Petrifaction. 578
Phaseolus, 123

vulgaris, 525
Phloem, 67
Photosynthesis, 77

respiration and, no
Phototropism, 135
Phyllitis, 156
Phylogeny, 504, 569
Phymatodes, 154
Physalis alkekengi. 545
Physcia stellaris, 329
Physiology, 4
Phytogeography, 5
Phytology, i

Phytophthora infestans, 292
Phytoptus, 292
Pine cone, 420
Pine, Kauri, 610

Scotch, 417, 418, 419

Pine white, 421, 422, 423, 428, 429,
430

Pine shoot moth, 301
Pine tree blister-rust, 301
Pines, 413
Pinnae, 156
Pinnules, 156
Pinus, 447

austriaca, 416

contorta, 301

ponderosa, 301

rigida, 301, 415

Strobus, 415, 421, 422, 423,
428, 429, 430

sylvestris, 415, 417, 418, 419
Pistil, 436
Pisum; 123

sativum, 468, 554
Placenta, 437

Plant and animal respiration, 112
Plant-respiration, 105
Plant secretions, economic value

of, 90
Plantago, 37
Plantain, 37

Plants and animals, nutrition of, 70
Plants to man, relation of, 2
Plasma-membrane, 17
Plasmolysis, 59

Pleurococcus, 252, 253, 328, 344, 362
Plum, brown rot of, 293

pockets, 293
Plums, black knot of, 295
Plumule, 443
Podetia, 331, 332
Podocarpineae, 83
Polarity, 364
Pollen, abundance of, 423

and coal-formation, 424

immediate effect of, 451

shedding of, 424
Pollen-chamber, 400 •

INDEX

63s

Pollen-grain, 401, 436
Pollen-sacs, 436
Pollination, 401, 427
Pollinium, 477, 501
Polypetalse, 456
Polypodium, 154, 156, iS^
irioides, 154

punctatum, 155
Polytrichum commune, 206
Populus, 457

Porella navicularis, 211, 212
Potato, 335» 347, 480
blight, 313
rot, 292
tuber, 81
Potatoes, little, 291
Potonie, 598
Pressure, osmotic, 58

and growth, osmotic, 113
Priestley, Joseph, 73, 74
PrimofiUces, 616
Proangiosperms, 606
Propagation, 180
Prophylaxis, 310
Protease, 79, 86
Protein, 16
Proteins, 71

manufacture of, 84
Prothallus, 166, 167
Protonema, 165, 166
Protoplasm, 14, 15

properties of, 19
Protoplast, 16
Pseudoborniales, 618
Pseudomonas radicicola, 82, 83,

318, 332
Pseudopodium, 200, 201
Psilotales, 618
Pteridophytes, 366, 388
Pteridosperms, 592

significance of, 598
Pteris, 156

Pteris aquilina. 149, iS5

longifolia, 153
Pteropsida, 616
Ptyalin, 100
Puccinia graminis, 299
Punk, 303

Purity of gametes, theory of, 555
Pussy-willow, 458
Putrefaction, 319
Pyrenoids, 353
Pyrolaceae, 473
Pythium de Baryanum, 291

Quarantine, 311
Quercus, 336

novimexicana, 34
Qu6telet. 524
Qu6telet*s curve, 525, 526

R

Radicle, 381, 464
Ranunculacese, 459
Ranunculus, 459, 460, 461, 462

aquatilis, 507
Rattan, 495
Rattlesnake fern, 176
Ray, John, 453
Recptacle, 155
Redi, 89
Reduction, 183, 184

division, 185, 187
Regeneration, 205
Reproduction, asexual, 179

by spores, 180

cell-division and, 344

essence of all, 180

inheritance and, 544

sexual, 179, 180

636

INDEX

Respiration, 105, iii

and fermentation, 112

and photosynthesis, no

versus breathing, 107
Response, stimulus and, 125
Rex begonia, 347, 447
Rhizoctonia, 291, 293
Rhizomes, 150
Rhubarb; 116
Ribes, 301
Riccia, 222

fluitans, 209

life history of, 224

natans, 209

trichocarpa, 222
Ricciales, 210
Ricciocarpus, 223
Ricinus, 123

communis, 63, 66
Robinia, 467
Rock strata, classification of, 583

Root, ID

elongation'of, 117
Root-cap, 50
Root-hairs, 11, 53, 54> 58

and soil, relation between, 52

importance of, 49

location of, 49

structure of, 51
Root-nodules, 83

of Cycas revolutia, 333
Root-stocks, 150
Rosa Carolina, 464

rugosa, 450
Rosacese, 464
Rose, 465

wild, 466
Rosette, 40
Rostellum, 501
Rotation of crops, 318
Rubber-plant, 64, 66
Rubiaceae, 463

Rubus australis, 28
Rudbeckia, 533
Rue anemone., 463
Runners, 164, 350
Rye, cultivated, 293
wild, 293

Saccharomyces, 98
Sage, 479
Sagittaria, 449
Sago palm, 390
Salix, 12, 346, 457, 458

babylonica, 457

discolor, 457
Salvia, 480
Salvinia, 341
Salicaceae, 457
Saltation, orthogenetic, 528
Sanitary theory, 92
Sanitation, 311
Sap, nuclear, 184
Sapium sebiferum, 84
Saprophytes, fungus, 322
Saprophytism, 321, 322
Saururaceae, 459
Saururus cernuus, 32, 459, 460
Saussure, Nicolas Theodore de, 74
Savory, 479
Schleiden, 15
Schwann, 15, 94
Science, essence of, 620
Scion, 335
Scott, D. H., 619
Scrophulariaceae, 481
Scutellum, 494
Secale cereale, 293
Secretions, 19
Sedum, 31

Seed, essentials of, 453
'Seed-coat, 397, 453

INDEX

637

Segregation, law of, 555

Mendelian, 554, 556
Selaginella, 382, 384, 431

amoena, 383

Kraussiana, 387

life-cycle of, 388

Martensii, 385

Watsoniana, 384

Wildenovii, 383
Selaginellales, 382. 388
Selection, natural, 191
Selfing, 560
Self-pruning, 180
Semi-parasitism, 341
Sempervivum tabulaeforme, 41

tectorum, 350
Senebier, Jean, 74
Senna, wild, 467
Sensitive plant, 122
Sepals, 435
Serum-therapy, 314
Seta, 204
Sex, determination of, 358

meaning of, 360

problem of, in plants, 344
Sex-organs, differentiation of, 356
Sexual characters, secondary, 358
Sexuality in molds, 353

in Spirogyra, 355
Shaggy-mane mushroom, 289
Shelf-fungus, 303
Shoot, 9

Sigillarias, 612, 613
Silique, 463
Siphonogamy, 406
Skein, nuclear, 186
Skunk cabbage, 499, 500
Solanaceae, 480
Solanum, 335

Dulcamara, 481

integrifolium, 545

nigrum, 545

Solanum tuberosum, 347, 480
Solidago ulmifolia, 340
Soredia, 330
Sorghum halpense, 496
Sorus, 154
Soy bean, 317
Spartina, 293
Spathyema fcetida, 496
Species, elementary, 530

origin of, 510, 587
Spermatophytes, 408
Sphaerothica phytoptophyla, 292
Sphagnum, 193, 194, 200, 202, 204,
348

acutifolium, 198

cuspidatum, 199

cymbifolium, 195

life-history of, 203

squarrosum, 198
Sphagnales, 193

Sphenophyllum cuneifolium, 372
Sphenophyllales, 618
Sphenopsida, 618
Sphenopteris Hoeninghausi, 592
Spike, 459, 495
Spikelets, 495

Spirogyra, 344, 345, 353, 355, 364
Sporangiophores, 371
Sporangium, 155, 156, 157
Spore-mother-cells, 158
Spores, 155

dispersal of, 164

germination of, 165

swarm-, 251
Sporogonium, 201
Sporophylls, 156, 159
Sporophyte, 181, 201

development of, 364

evolution of, 574
Spruce, 140
Staff-tree, 481
Stalk-cell, 428

638

INDEX

Stamens, 417
Stangeria, 411

eriopus, 598

paradoxa, 598
Starch-making, 77

steps in, 79
Stems, elongation of. 118

exogenous, 65
Sterilization, 162

progressive, 432
Stigma, 437

Stimulus and response, 125
Stock, 335

Stolons, 164, 349, 350
Stoma, 33, 35

Stomata and gaseous exchange, 108
Strata, classification of rock, 583
Stratification, 582
Strawberries, 348
Strobilus, 371

Struggle for existence, 190, 513, 515
Style, 437
Sugar-beet, 68
Sugar-cane, 68
Sugar-maple, 336
Suminski, Count Lesczyc, 168
Sundew, 87, 88, 347
Sunlight, importance of, 75
Survival of the fittest, 191, 515
Suspensor 380
Swarm-spores, 251
Sweet clover, yellow, 82
Sweet pea, 470
Symbionts, 318
Symbiosis, 214, 321, 324, 325

social, 325
Symmetry, bilateral, 167
Sympetalae, 456, 473
Symplocarpus foetidus, 496, 499,

500
Synangia, 601
Synergids, 440

Tap root, 10

Taraxacum, 486

Taxonomy, 4

Tectoria cicutaria, 161, 163

Tetrad, 158

Tetrad-divisions, 158, 187

Tetraphis, 207

Thallophytes, 366

Thallus, 167

Thecium, 330, 331

Thyme, 479

Thyrsopteris elegans, 157

Tillandsia, 326

Tissue- tension, 117

Toadflax, 481, 482, 483, 484, 485

Toadstool, 288

Tobacco, 480

Todea barbara, 157

Tomato, 335, 450, 480

Torus, 464

Tour, Cagniard de la, 98

Toxins, 313

Trachymyrmex obscurior, 326

Transpiration, advantages of, 40

control of, 38

cuticular, 38

lifting power of, 42

stomatal, 38
Tree ferns, 150
Tree, genealogical, 577
Trifolium hybridum, 469
Triticum, 298

vulgare, 494
TroUius, 462

Tropaeolum majus, 137, 143
Trunk, 412

excurrent, 413

deUquescent, 414
Trypsin, 102
Tube-cell, 423

INDEX

639

Tubers, 217

Tubuliflorae, 488

Tulip bulb, 29

Tulip-tree, 29, 74

Turgor, 58, 114

Turpin, 98

Tyndall, 89

Type genus, 491

Typhaceae, 491

Typha latifolia, 490, 491, 492

angustifolia, 490
Typhoid fever, 310

Mary, 310

U

Ulmus americana, 414
Ulothrix, 352

zonata, 354
Ultricularia, 87, 88
Umbelliferse, 471
Umbels, 471
Unconformity, 583
Unit-c h a r a c t ers, character-units

versus, 558
Ustilago, 298

Avenae, 298

maydis, 299

Tritici, 298

Zeas, 298

V

Vaccination, 313, 314
Vacciniaceae, 473
Vaccinium, 451, 473

corymbosum, 452
Vacuoles, 17
Vallisneria spiralis, 349
Van Tieghem, 598
Variability, fluctuating. 525
Variation, 189, 512

continuous, 522

discontinuous, 527

Variation, fluctuating, and inheri-
tance, 527
Vascular plants, 153
Vaucheria, 364

terrestris, 356
Venter, 169
Ventral-canal cell, 170
Venus's flytrap, 86, 88
Verbascum Thapsus, 31, 130
Verbena ciliata, 33
Vernation, circinate, 152
Vinegar, 317
Vivum e vivo, omne, 89
Von Mohl, Hugo, 15
Vries, Hugo de, 520, 521

W

Wall-cell, 428
Water and the soil, 54
Water crowfoot, 460
Water hyacinth, 51, 52
Water importance of, 47

plantain, 493

requirement, relative, 47
Webber, 404
Wheat, smuts of, 297
Wheat-sickness, 92
White pine blister rust, 302
Whorls, 412

Wild cabbage, horticultural varie-
ties of, 531
Wieland, 597
Wielandiella, 608
Williamsonia, 602, 608
Willow, 12, 117, 346, 457, 458

weeping, 457
Wilt disease of cotton and water-
melon, 295

cucumbers and melons, 309
Wings, 166

Witches' brooms, 292,293
Wood lily, 436

640 INDEX

Woodwardia orientalis, 351, 352
X

Yeast what is, 98
Yucca. 62

A'-chromosome, 359, 360
Xeno-parasitism, 343
Xylem, 65

Yeast, 95, 98, 306

Zamia, 335, 393, 411, 610

floridana, 404, 407
Zea Mays, 62, 85, 123, 299, 562, 563
Zygospores, 345
Zygote, 180

u ^