Heredity and evolution in plants

ALBERT R. MANN
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HEREDITY AND EVOLUTION
IN PLANTS

GAGER

FRONTISPIECE

Restoration of a scene along a sluggish creek in Texas and New
Mexico during the late Carboniferous (Upper Pennsylvanian) 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.)

Heke Diy

AND

EVOLUTION IN PLANTS

BY

C. STUART GAGER

DIRECTOR OF THE BROOKLYN BOTANIC GARDEN

WITH 118 ILLUSTRATIONS

PHILADELPHIA
P. BLAKISTON’S SON & CO.
1012 WALNUT STREET
1920

CopyricHT, 1920, BY P. BLAKIsTON’s Son & Co.

We aR as

TUM MAPLE PRESS YORK PA

To the Memory of
BENJAMIN STUART GAGER

“What a science Natural History will be

when. . all the laws of change are thought
one of the most important parts of Natural
History.’"—

Charles Darwin. (Letter to J. D. Hooker.)

PREFACE

The present little book was originally intended to be
merely a reprint of Chapters XXXI to XXXVIII of the
author’s Fundamentals of Botany. The reprinting of those
chapters was suggested by comments received from various
correspondents, who pointed out that the subject matter
which they cover had not been elsewhere presented in so
concise a treatment in one volume, and in a manner
suited, not only to beginning students, but also to more
general readers. The chapter on Experimental Evolu-
tion has received the approval of the author of the mu-
tation theory, as being an accurate presentation of the
essentials of that theory. ‘‘I have especially appreci-
ated,’ writes Professor de Vries, ‘the statement of the
difference between fluctuating and saltative variation,
which is, to my mind, the real empirical basis for the
theory, far more than the experiments on mutation with
single plants. The relation of my view to Darwinism
is misunderstood by many authors, and it is a great satis-
faction to me that you have outlined it in such a plain
way.”

In the preparation of the copy for reprinting, consider-
able new matter has been added, certain sentences and
paragraphs, pertinent only to an elementary text-book,
have been omitted, and others recast, and several fresh
illustrations have been introduced, either as new or as

substitutes.
xi

xil PREFACE

Chapters X, Geographical Distribution, and XIII,
The Great Groups of Plants, and the Bibliography are
new. No attempt has been made to cite the voluminous
periodical literature in the Bibliography, but needless to
state, this has been freely consulted and drawn upon.
Numerous citations are given as foot-notes, especially in
Chapter X.

In going over the chapters it also became evident that
since, in order to read them understandingly, one must
have a clear conception of the facts of the lift history of a
vascular plant, it would be best to introduce from the
Fundamentals of Botany the three chapters (viz. XII-
XIV) on the life history of the fern. As stated in the
Preface to that book, while the ultimate problem of botany
is the development of the kingdom of plants, the more
immediate and fundamental problem is the development
of the individual plant. “‘Ontogeny is fundamental
because without a knowledge of its processes the processes
of phylogeny cannot be comprehended. Phylogeny is
the ultimate problem because its complete solution in-
volves an orderly description of all the phenomena of
plant life, and their relation to each other.”

The author is specially indebted to Dr. O. E. White,
curator of plant breeding in the Brooklyn Botanic Garden,
for a careful reading of the entire manuscript and for
many valuable suggestions; also to Mr. Norman Taylor,
curator of plants, in connection with Chapter X, and
to Dr. Alfred Gundersen, associate curator of plants in
the same institution, for numerous constructive criticisms
in connection with Chapter XIII. The diagram show-
ing the apparent affinities and approximate geological
distribution of the main groups of vascular plants (p.

PREFACE xili

248) originated with Dr. Gundersen, but has been modi-
fied, as here printed, in certain details for which the
author alone is responsible. Grateful acknowledgment is
made to Dr. Ralph E. Cleland for photo prints of Figs.
74 and 75 from negatives made by him on the summit
of Mt. Madison (Adirondacks); and to Prof. Harvey
W. Shimer, author, and The Macmillan Co., publisher,
for permission to reproduce Fig. 66.

If the following pages shall prove to be a source of re-
liable and readable elementary information to those in-
terested in the subjects treated, the object of the book
will be accomplished.

BRooKtyn BoTranic GARDEN,
March 25, 1920.

C. STUART GAGER.

CONTENTS

CHAPTER
I. Lire History or A FERN

IL. Lire History or A FERN (Concluded)
ITI. FUNDAMENTAL PRINCIPLES
IV. Hereprry
V.. EXPERIMENTAL STUDY or HEerepity
VI. Evolution
Vil. Darwinism
VIII. ExperIMENTAL EvoLution
1X. THe EvoLution or PLANts
X. GEOGRAPHICAL DISTRIBUTION
XI. PALEOBOTANY . .
XIL. Tae Evotution or Plants (Concludzd)
XIII. THe Great Groups or PLANTS
BIBLIOGRAPHY.

INDEX. .

« IOL

139

« 183
. 201
+ 243
2 252

» 257

HEREDITY AND EVOLUTION
IN PLANTS
CHAPTER I
LIFE HISTORY OF A FERN

1. Life History—Every plant, in the course of its ex-
istence, passes through a series of changes in orderly

Fic. 1.—A fern (Anisosorus hirsutus), showing portion of the stem above
ground.

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
I

2 HEREDITY AND EVOLUTION IN PLANTS

Fic. 2.—Portion of the rhizome of the common brake (Pieris aquilina)
showing a cross-section view at the right.

———

Fic. 3.—Cross-section of the rhizome of the bracken fern (Pleris aqui-
lina), showing the tissue systems. Greatly magnified.

LIFE WISTORY OF A FERN 3

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

Fic. 4.—Tree ferns on the military road between Cayey and Caguas,
Porto Rico. (Photo by M. A. Howe.)

history of the individual. The various stages of life
history common to most plants are nowhere more clearly
illustrated than in the ferns.

2. 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

4 TEEREDITY AND EVOLUTION IN PLANTS

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
and leaf-stalk. There is always a terminal bud at the

Fic. 5.—A, Upper epidermis; B, lower epidermis of the leaf of the fern,
Drynaria meyeniana. (Camera lucida drawing.)

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. 1), and on their older
portions, at some distance back from the tip, may be seen
the scars, or the ends of leaf-stalks, left by old leaves that

! The leaves of ferns are often called fronds.

LIFE HISTORY OF A TERN 5

have died and fallen away. The rhizome bears true roots
(Fig. 2), and its tissues are differentiated into epider-
mal, fundamental, mechanical, and conducting systems
(Fig. 3). In tropical countries there are ‘‘tree ferns,”

Fic. 6.—Osmunda Claytoniana. Young sporophylls, showing circinate
vernation. Note the spore-bearing pinne.

with upright stems, and this type of fern is common
among the fossil plants of earlier geological ages (Fig. 4).
There are also climbing ferns.

3. Two Kinds of Fern-leaves.—Careful examination
of the leaves of certain mature ferns will disclose the fact

6 HEREDITY AND EVOLUTION IN PLANTS

that they are not all alike. Some of them are merely
foliage-leaves, and do not differ in any essential point from
the foliage-leaves of higher plants, such as the maple or
lily; they possess stomata for the exchange of gases and

Fic. 7.—Portions of the sporophylls of two ferns to show the sori.
On the left Polypodium punctatum (L.) Sw.; on the right a variety of Pteris
longifolia, with sporangia marginal on the pinnules.

water-vapor with the outer air (Fig. 5), and they also
resemble the leaves of higher plants in their internal struc-
ture. All fern-leaves, however, have a very characteristic
arrangement in the embryonic or bud condition, being

LIFE HISTORY OF A FERN 7

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. 6). The leaf-blade

Fic. 8.—Sporophylls of two ferns. At the left, a species of Polypodium
(Phymatodes), having no indusium; at the right, Diplazium zelanicum.

possesses veins of fibro-vascular bundles that pass down
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-

8 HEREDITY AND EVOLUTION IN PLANTS

cular plants. These leaves perform all the functions
performed by the foliage-leaves of other plants, the most
important of which are the manufacture of organic, car-
bohydrate food from inorganic raw materials (photosyn-
thesis), and the giving off of water vapor from within
(transpiration).

4. Spore-bearing Leaves.—The second type of fern-
leaf bears, on its underside, numerous ‘‘fruit-dots”’ or sori
(singular sorus) (Figs. 7 and 8). These structures have
to do with reproduction. A single sorus of such a fern

Fic. 9.—Cross-section through the marginal sorus of a sporophyll of
the bracken fern (Pteris aquilina). 1, palisade layer; fb, vascular bundle;
sp, sporangium; im, indusium. (Greatly magnified.)

as, for example, Polypodium, is composed of a cluster of
tiny stalked cases. The cases contain minute unicellular
reproductive bodies called spores, and the entire structure
is a sporangium. The place where the sporangia are
attached to the leaf is the sporangiophore! (Fig. 9), and
over all is often found a thin membranous covering, the
indusium (Figs. 9 and 10). In some ferns the indusium
is lacking, and the sorus is naked. Spore-bearing leaves
are called sporophylls, and plants that bear sporophylls
are called sporophytes.

1 Also called receptacle,

LIFE HISTORY OF A FERN 9

Ns. wis aes

Vic. 10,—Cyrtomium falcatum. Under (dorsal) surface of a portion of

a ~ ONG
LE IGE: 3 ?

é
ANS 9 yt,
ypyss af i. ah
«if SS Ee

SANS ag

Fic. 11.—Fern leaves, showing various degrees of subdivision or branch-
ing of the blade. A, Phyllilis; B, Polypodiwm; C, Pieris; D, Adiantum.

To HEREDITY AND EVOLUTION IN PLANTS

5. Types of Foliage-leaf—tIn 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 pinnae,
and the secondary subdivisions pinnules. Illustrations
of these various types are shown in Fig. 11.

6. Sporangia.—As noted above, each sporangium con-
sists of a spore-case borne on a stalk (Fig. 12). The stfuc-
ture of the case varies considerably in various groups of
ferns, but it usually possesses walls only one cell thick, with
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

Fic. 12.—Sporangia of an undetermined species of fern; i, 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 vertical
rows of cells that compose the stalk.

Fig. 13. 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
belong, the sporangium always originates from a single
epidermal cell. Ferns whose sporangia thus originate are
called leptosporangiate ferns (Cf. p. 29). The walls of
their spore-cases are always only one cell thick, and

LIFE HISTORY OF A FERN II

always possess some form of annulus. As the sporangia
mature the spore-case itself becomes differentiated into
two distinct kinds of tissue, namely, vegetative tissue on
the outside, forming the wall and reproductive tissue within,
from which the spores are developed.

7. 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-

_ is

vad a
lic. 13.—Types af fern sporangia, A, Loxsoma Ciinninsianll: "B
Gleichenia circinata; C, Todea barbara; D, Bhyesoblents elegans; E, Matania
pectinata; F, Lygodium japonicum. (Redrawn from various sours. )
: ope v
fessor Bower, of Glasgow, has calléd attention to this, ‘fact
in the following words: ‘

fe fouahh estimate may be made of the numerical output of spores ‘ftom
a large plant of the Shield fern, as follows: In each sporangium 48!
spores may be formed; a sorus will consist of fully 100 sporangia, usually
more; 20 is a moderate estimate of the sori on an average pinna; there may
be fully 50 fertile pinne# on one well-developed leaf, and a strong plant
would bear ro fertile leaves. 48 X Ico 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.”

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

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

12 HEREDITY AND EVOLUTION IN PLANTS

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. 14); between these two extremes all grades of transi-
tion are found (Fig. 15). But however widely the sporo-

Fic. 14.—The cinnamon fern (Osmunda cinnamomea), showing foliage’
leaves and sporophylls.

phyll departs from a foliage-leaf in appearance, it must,
nevertheless, be regarded as morphologically a leaf. As
partial evidence of the true foliar nature of sporophylls,
there may be cited the interesting experiment of Atkinson,
who, by removing the true foliage-leaves just beginning to
unfold in the spring, was able to induce developing sporo-
“phylls to alter their character, and become transformed

LIFE HISTORY OF A FERN 13

Fic. 15.—Clayton’s fern (Osmunda Claytoniana), showing sporophylls
in the center, surrounded by foliage leaves.

14 UEREDITY AND EVOLUTION IN PLANTS

into foliage-leayes. Similar results were also obtained by
Goebel. These experiments indicate that foliage-leaves
and sporophylls are very closely related to each other,

Fic. 16.—Portion of a leaf of a fern (Tectoria cicutarig) that bears
bulbils on both the upper and lower surfaces of its leaves. Plantlets
develop from the bulbils while they are still attached.

and demonstrate clearly that foliage-leaves may be pro-
duced by the sterilization of spore-bearing leaves. The
interesting question here naturallyjarises as to whether, in
the evolutionary development of the plant kingdom

LIFE HISTORY OF A FERN 15

SAN

ad

f

rT
|

Fic. 17.—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.

16 HEREDITY AND EVOLUTION IN PLANTS

through long geological ages, foliage-leaves have in gen-
eral originated by the sterilization of spore-bearing organs.

9. Vegetative Multiplication—In addition to repro-
duction 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
piece that contains a bud a new plant will develop. By

Fic. 18.—A Boston fern (Nephrolepis), reproducing vegetatively by
means of runners or stolons. The parent plant is in the round pot.
(After R. C. Benedict.)

the second method, the plant is progagated 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. 16). 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
(Cystopteris bulbifera) received its specific name from the

LIFE HISTORY OF A FERN LT

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. 17). 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. 18).

10. Dispersal of Spores.—After the spores are mature
the essential need is that they become dispersed, so that
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
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,
often to a considerable distance; by wind and other
agencies they may be carried still further from the parent
plant.

11. 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 increases, the walls of
the spore are burst apart, and a tiny tube, the germ-tube
or protonema (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. 19). This is the first of innumerable root-like
bodies, or rhizoids, which will help to hold the new plant

HEREDITY AND EVOLUTION IN PLANTS

18

a, Before germination;

and first rhizoid (rk); c, d, e, f,

successive stages in the development of the prothallus.

showing protonema (pr.),

?

Fic. 19.—Germination of the spores of a fern.

b, early stage

i

Py
ROS

i

\)

A

A

?

(After Margaret

Archegonia on the (central) cushion

the rhizoids,

Fic. 20.—Prothallus of a fern.
near the notch; antheridia among

C. Ferguson.)

below.

LIFE HISTORY OF A FERN Ig

firmly to the soil, and also serve to take in water and dis-
solved mineral nutrients.

12. The Prothallus.—Before the germ-tube has greatly
enlarged, it becomes divided into two cells, and then,
by 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 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 sporophyte,
it is called the prothallus (Fig. 20). 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 II
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.

13. 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 struc-
tures soon to be described, makes it easy to distinguish
at once the surface that bears them from the opposite
surface. Since the surface bearing the rhizoids lies nor-
mally next to the substratum it was called the ventral
surface, while the opposite surface was called dorsal. 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 chance, to lie upside

20

LIFE HISTORY OF A FERN 21

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 differentiated are foliage-leaves,
sporophylls, man, fishes, and other animals. In buds the
dorsal surface of leaves is the upper or outer surface;
when foliage leaves are fully expanded the dorsal surface
is commonly underneath, and the ventral surface above.

Fic. 21.—Archegonia of a fern (Adiantum). A, young archegonium;
B, mature; C, top view, showing terminal cells of the four rows of wall
cells; 2, wall of venter; e, egg; v.c.c, ventral canal-cell; 2.c, neck-canal;
sp, sperms entering the neck-canal. A and B in longitudinal section.

14. Reproductive Organs: Archegonia.— Examination
of the ventral surface of a mature prothallus with a lens
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. 21). The neck projects away from the surface

22) HEREDITY AND EVOLUTION IN PLANTS

and is usually slightly curved, while the remainder, the
venter, is imbedded in the tissue of the cushion. As the
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.

Fic. 22.—Portion of a cross-section of a prothallus of a fern (Adian-
_ tum), showing an antheridium (am), and sporogenous cells within.
(Drawn from preparation of E. W. Olive.)

15. Reproductive Organs: Antheridia—Search among
the rhizoids will reveal another class of organs, the an-
theridia, 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. 22 and 23.)

; LIFE HISTORY OF A FERN 23

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

Fic. 23.—Fern prothallus; cross-sections showing antheridia (an),
sperms (sp), and rhizoids (rz). Below at the right is a sperm (sp)
greatly enlarged.

are mature, a suitable amount of water (such as would
result from a rain or a copious dew), soaking through the
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

24 HEREDITY AND EVOLUTION IN PLANTS

moisture will cause the rupture of the antheridia, and the
sperms will be set free (Fig. 23). The mucilaginous matter
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

Fic. 24.—Fertilization in the fern, Onoclea. 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 of fertilization (Fig. 24).
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.

¥

LIFE HISTORY OF A FERN 25

17. Nature of the Fertilized Egg.—It will at once be
recognized that the fertilized egg, resulting from a union
with the sperm, possesses a double or diploid nature.!
In recognition of its dual nature it is called the dosperm
(egg and sperm).? The dosperm, however, like the un-

Fic. 25.—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.)

fertilized egg, is still only one cell, though its nucleus com-
prises substances contributed by both egg and sperm.
In some cases the egg and sperm that unite in fertilization
may come from different parents; their fusion is then
called cross-fertilization.

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

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

26 HEREDITY AND EVOLUTION IN PLANTS

18. 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
size. The cell-wall of the first division (in all of the family
Polypodiacez) is parallel to the axis of the archegonial
neck. The second wall, at right angles to the first, di-
vides the odsperm 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. 25). The function of the foot is to absorb
nourishment for the young embryo from the prothallus.
The need of such an organ becomes apparent when it is
recalled that the odsperm, and consequently the embryo,
lie free in the venter of the archegonium, without any
organic or structural connection with the prothallus.
This necessary connection is early established by the foot.

19. 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.

LIFE HISTORY OF A FERN ©.

The development of the stem follows, and finally spore-
bearing leaves appear (Fig. 26). We then have. an
organism similar to that with which we started—a full-
grown fern-plant, capable of producing spores, which can
develop into prothallia 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-

Tic. 26.—Prothallia of a fern. 1, Before the sporophyte had appeared;
2-5, with sporophytes attached; /, cotyledon or first leaf of the sporophyte;
2, circinate vernation of a leaf; s, mass of soil,

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.

20. 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. 27), and the ‘‘adder’s tongue” (Oph-

2 q HEREDITY AND EVOLUTION IN PLANTS

ioglossum vulgatum, Fig. 28). The species of Botry-
chium usually (though not invariably) possess but one

Fic, 27.—Rattlesnake fern (Boirychium virginianum (L.) Sw.).

foliage-leaf, and a fertile spike, both of which are more or
less branched. . Abnormal forms are not uncommon in
which the fertile spike is more or less sterilized, sometimes

LIFE HISTORY OF A FERN 20

being entirely so; whilein other cases sporangia occur on

the foliage-leaf. As in the re-
placement of sporophylls by
sterile leaves in the ostrich fern,
Onoclea struthiopteris (para-
graph 8), these abnormalities
indicate the close relationship
between leaves and spore-bear-
ing organs, and clearly show
that the latter may be com-
pletely transformed, by sterili-
zation, into foliage-leaves.

In Ophioglossum the foliage-
leaf and spore-bearing spike
are both unbranched, the latter
suggesting an adder’s tongue,
whence thename, Ophioglossum.
In both Ophioglossum and Botry-
chium the sporangia originate
from a group of epidermal and
sub-epidermal cells, and are
consequently imbedded in the
surrounding tissue. Their walls
are more than one cell in thick-
ness, the annulus is lacking, and
they open by a slit. Ferns of
this type are called eusporangiate
(Cf.p.10). Their prothallia are
usually fleshy and subterranean,
bear the antheridia and arche-
gonia on the dorsal instead of on
the ventral surface, and are per-

Fria. 28.—Adder’s tongue
fern (Ophioglossum vulgatum
L.). #&, runner or stolon.

30 IEREDITY AND EVOLUTION IN PLANTS

ennial, often living on after the sporophyte has died. In
general the sporophyte possesses less sterile tissue in pro-
portion to fertile tissue than is the case with the lepto-
sporangiate 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.

Recent studies of the vascular anatomy of the Ophio-
glossacee have disclosed features in common with the
Osmundacee# and Polypodiacee. The fact that the vas-
cular bundles of the fertile spike originate in the same
manner as those extending into the pairs of pinne of
the sterile segment points to the conclusion that the fertile
spike represents, or is homologous with, two fused pinne
at the base of a fern leaf. From this and other evidence
the Ophioglossacee, while “simpler’’ in structural fea-
tures, have been regarded as not having had a strobilar
origin (by progressive sterilization!) from the liverworts, -
and as not standing in the ancestral line of the modern lep-
tosporangiate ferns, but as having themselves been derived
at a very early period from a primitive fern stock closely
related to the Osmundacee. On the other hand, Camp-
bell? has adduced evidence for the derivation of the fertile
spike of Ophioglossum from a sporogonium like that of the
liverwort, Anthoceros. This and other evidence indicates
that the Ophioglossacee, and the eusporangiate ferns
as a group, are the oldest fern stock, and this conclusion
is supported by the geological record, for the oldest known
fossil ferns are eusporangiate. Further investigation is
necessary before the question can be definitely settled.

‘Cf. pp. 379, 432, and 574 infra.
? Campbell, D. H., Amer. Nat. 41: 139-159. 1907.

CHAPTER III

FUNDAMENTAL PRINCIPLES

21. 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.

By the giving off of { Artificial (slips,

multi-cellular por-J| cuttings, etc.).
Asexual, in-| tions or outgrowths | Natural (tubers,

volving cell- | of vegetative tissue. | bulbs, gemmz).
divisions By the giving off of
only. special reproductive

bodies of one or few

Reproduction cells, called spores.

Sexual, in-
volving cell-
fusions.

22. 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., in the begonia),
or by bending branches over until they touch the ground,
and there take root, after which the newly rooted portion
31

32 HEREDITY AND EVOLUTION IN PLANTS

may be severed from the parent plant. This is called
layering. The production of new individuals by the arti-
ficial methods of the gardener is called propagation; but
between these methods and 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.

23. 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 by itself of producing
a new individual. The essence of all reproduction is the
separation of the reproducing cell or body from the parent
plant. Ifa 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.

24. 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

1 From the Greek word, yéuos (gamos), meaning marriage.

FUNDAMENTAL PRINCIPLES 33

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

25. Two Kinds of Generations.—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
odsperm (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
3

34 HEREDITY AND EVOLUTION IN PLANTS

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.

26. 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—odsperm—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
Sperm (male gamete) Egg (female gamete)

=

Odsperm (zygote)
Embryo
Mature soe (mature zygote)
Sporophyll

Sporangium

Spore-mother-cell

TTT [yun

Spore Spore Spore Spore

Gametophyte

FUNDAMENTAL PRINCIPLES 35

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

Gametophyte Sporophyte
Stage Stage

Fic. 29.—Diagram of life-cycle of a fern.

27. Reduction.—Since the sporophyte (descended from
the diploid odsperm) 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 fertilization.

36 HEREDITY AND EVOLUTION IN PLANTS

28. Nature and Method of Reduction.—In order
thoroughly to understand fertilization and reduction one
must have a knowledge of the structure and behavior of
the nucleus in cell-division and cell-fusion. This subject
is too difficult and too extended to be thoroughly treated

Vic. 30.—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 and G, early and late anaphase, the chromosomes moving
to the pales of the spindle; H, formation of daughter spirems; J, resting
stage of the two daughter-cells.

in an introductory study, but the salient facts are as
follows.. The nucleus of all cells comprises at least four
substances: nuclear sap, a threadwork of linin, and a
substance called chromatin,’ all these are enclosed by a
nuclear membrane. In the non-dividing nucleus the
‘Because it stains readily when treated with certain aniline dyes,

FUNDAMENTAL PRINCIPLES 37

chromatin is distributed on the linin threads in the
form of minute granules (Fig. 30.) At one of the stages
preliminary to nuclear division the linin network, with the
chromatin, becomes transformed into a thickened skein,

B

ING

S
ad
fin

Fic. 31.— Diagram illustrating various stages in the reduction division
(maiosis) of a spore-mother-cell of a plant; A, resting stage of the mother-
cell-nucleus; B, the nuclear skein or spirem, in synizesis (during synapsis);
C, the spirem after synapsis, showing its double (diploid) nature; the dot-
ted line indicates the segmentation of the spirem into two diploid chromo-
somes, each of which has split longitudinally in D; E, the diploid chromo-
somes on the equator of the spindle of the first (heterotypic) division;
F, late anaphase; G, metaphase of the second or homotypic division; H,
late anaphase of same, two haploid chromosomes approaching the poles
of each spindle; 7, the four daughter-cells (spores) of the tetrad.

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

HEREDITY AND EVOLUTION IN PLANTS

38

membrane then disappears, and, by a complicated mechan-

, not entirely understood, the two halves of the chro-
mosomes are separated and carried apart to opposite sides

ism

(S835 “AY Jey) “wWes8erp 24} NoySnosy} paoesz? oq ACU HI }eY} OS
yeu oysuayoereyD @ Aq p2}eUsIsep st suosoWIOIYD PRA *(St4D98 Y) WIOM-pBeIy} 94}
jo dsoy} wo paseq oie eusmOUaYd Ivapnu aq, -ayAqdorods 94} Ut sautosOMOIyD Inof
THA uray peoremodséy v uo paseq ‘a]94-9jT] yeows0joysd Be Jo ueBeIq—'zf *o1]

NOL\SIAIO NOILINAAH

NOILY ZN

After this division of the nucleus, a new cell-

of the cell.

’

wall forms, dividing the entire cell into halves; new nu-
clear membranes develop, and the chromosomes in-each

FUNDAMENTAL PRINCIPLES | 39

daughter-nucleus becomes gradually retransformed into a
resting nucleus, like the one with which we started.

In reduction (Fig. 31) a new resting nucleus is not
organized after the first nuclear division by which the
number of chromosomes in each nucleus is reduced by
one-half, but this division is followed at once by a second.
This is the process of tetrad-division, by which four
spores are formed from each spore-mother-cell. 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 dia-
grammatically in Fig. 32.

29. 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
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 ason from his father. The thing inherited
is not an external appearance, but a material substance

40 HEREDITY AND EVOLUTION IN PLANTS

(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,
odsperm, tuber, or bulb—plus a certain characteristic
organization of this protoplasm, and the effects of its past
history.

30. Inheritance Versus Expression.—That inherit-
ance 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
sporophyte, 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
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 be defined as the recurrence in
successive generations, of a similar cellular constitution.

1 Following Johannsen, Cf. p. 67.

FUNDAMENTAL PRINCIPLES 4I

Expression of this cellular condition is greatly modified
by circumstances, which are never precisely the same
for any two individuals (Cf. p. 48).

31. 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 V and VI.

32. Adjustment to Environment.—By the term
environment is meant all the circumstances that surround
a cell, tissue, or organism at any given time, or throughout
its existence. The environment of tissues and organs
includes surrounding tissues and organs, and the environ-
ment of cells includes the neighboring tissues and cells.
The most essential thing in the life of every plant or animal
1s to keep in harmony with its environment. Every change
of environment necessitates an adjustment on the part
of the plant in order to maintain this harmony. Adjust-
ments 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 its roots deeper, if light is entirely cut off no
chlorophyll will form. A leaf, or the prothallus of ferns, is
bilaterally symmetrical partly because the environment is
uniform on all sides; the same organs have dorso-ventral
differentiation largely because the environment is unlike
above and below. The motility of sperms is an adjustment
to water in the environment. Thus, variations in the

a2 HEREDITY AND EVOLUTION IN PLANTS

environment may result in different expressions of in-
heritance, just as variations in inheritance would be
followed by differences in expression, even in an unchang-
ing 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 environment only, but of the interaction
between the two.

33. Struggle for Existence.—In paragraph 7 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 1 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 124
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
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.

FUNDAMENTAL PRINCIPLES 43

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.

34. 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 sporophylJs 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.

35. Problems to Solve.—In the preceding paragraphs
we have called attention to a number of the problems
which arise from the study of a fern. Some of these have
been partially solved—probably none of them has been
completely solved. In fact, we may say that our igno-
rance of life-processes greatly exceeds our knowledge.
Very much more remains to be ascertained than has al-
ready been found out; for example, what is protoplasm?

44 HEREDITY AND EVOLUTION IN PLANTS

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 mem-
ory 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 by studying merely 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 life? What
is life? No one can give complete answers to these ques-
tions; but the purpose of the study of botany is to help
fit us to seek the answers intelligently. To those who are
interested in problems of this sort, nothing can be more
fascinating, nor more profitable. It is the aim of the fol-
lowing chapters to give a brief, elementary résumé of the
method employed and the results obtained during the
past fifty years by investigators in their attempts to solve
two of the more important of these problems, namely,
the nature and mechanism of inheritance and the causes
and course of plant evolution.

CHAPTER IV

HEREDITY

36. Importance of the Study.—1. To Pure Science.—
No knowledge is more fundamental than a correct under-
standing of the aws 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 moreso. 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 m’nd and body, our capacity for
education, our susceptibility to disease, and often the
actual existence of some disease within our bodies or minds.

45

46 HEREDITY AND EVOLUTION IN PLANTS

37. Heredity Reduced to Its Lowest Terms.—We may
study heredity under the very simplest conditions in the
descent of one-celled organisms, such as Pleurococcus.
This plant, a unicellular green alga, is a globule of proto-
plasm, containing chlorophyll, and surrounded by a
cellulose cell-wall (Fig. 33). 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 in-
stead 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

Fic. 33.—Individual
plants of green slime :
(Pleurococcus vulgaris) NOW characterize Pleurococcus

showing the tendency of plants. But that only puts the

the cells to remain question back an indefinite number
attached after cell-divi-

iow, thus causing dinate of generations. The real reason is,
tions from a one-celled to because the Pleurococcus protoplasm
a _multi-cellular plant. possesses a physical and chemical
(Gf. Fig. 34) constitution—or in other words a
mechanism—that, under normal external conditions,
manufactures green pigment instead of red, cellulose in-
stead 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.

38. What is Inheritance.—When the Pleurococcus cell
divides, this wonderful, invisible mechanism—the certain

HEREDITY 47

definite physical and chemical constitution—is transmitted
to each of the daughter-cells; each, in other words, re-
ceives Pleurococcus protoplasm. This protoplasm, with
us definite organization, constitutes the inheritance. ‘The
daughter-cells do not inherit a spherical shape (as is evident
from Fig. 33), but a definite kind of protoplasm, cell-sap

Fic. 34.—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.). (Cf. Fig. 33.)

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
expression of the inheritance for the given environment.
Under different external conditions the expression might
be different; but the inheritance would be the same. The
chlorophyll in the daughter-cells, immediately after cell-

48 HEREDITY AND EVOLUTION IN PLANTS

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
(Fig. 34) that reproduces itself by division, and manufac-
tures chlorophyll in the presence of sunlight. Under abnor-
mal external conditions the mechanism may not act, or
may act abnormally, so that yellow pigment appears
instead of green—or in darkness no pigment at all. In
either case the inheritance is the same, but the expression
varies. A modern writer (J. Arthur Thomson) 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.

39. Inheritance Versus Expression.—In the light of
this information, obtained by a study of the simple 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-
herited may be expressed; this is illustrated when children
resemble, not their parents, but their grandparents.
Here the parents transmitted an inheritance which, in
them, found no expression. .

HEREDITY 49

A remarkable illustration of inheritance without expres-
sion is seen in the case of the alternation of generations
(pages 33-35). The inital protoplasm of the sporophyte
is all inherited through the fertilized egg from the game-

Fic. 35.—Vegetative propagation of Haworthia sp. The new plantlet
forms on the flower stalk, below the flower-cluster. Ultimately it falls
to the ground and takes root, becoming established as an independent
plant.

tophytes, but most of the gametophytic characters do not
appear in the sporophyte, nor do the typically sporophytic
characters find expressionin the gametophyte.’ (Cf. p.4o.)

1 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.

50 HEREDITY AND EVOLUTION IN PLANTS

40. Inheritance Versus Heredity.—As stated above, the
inheritance ts that which is actually transmiited from parent
to offspring. The fern-spore, for example, is the inheri-
tance 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.

41. Inheritance and Reproduction.—Inheritance is, of
course, inseparably linked with reproduction and may be
studied in connection with the three following types:

1. In vegetative propagation, e.g., by means of tubers,
cuttings and “slips,” bulbs and bulbils, gemmez, ‘“‘run-
ners,’ scions, vegetative rejuvenation or “budding” (Fig.
35), etc., the new plant is obviously only a portion of the
vegetative tissue of the parent plant, isolated and growing
independently byitself. The separationof the propagating
piece from the parent is often (though not always) mechan-
ical and artificial. The protoplasm remains unaltered by
the act of separation, reduction divisions of cell-nuclei are
not involved, and the inheritance, except in bud-varia-
tions, is unaffected by the change. This is evident in those
cases where the 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. 36, a tomato stem was
grafted upon a tobacco 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 by spores the situation is
quite similar to that in vegetative propagation, but in

HEREDITY 51

certain cases there is abundant opportunity tor the proto-
plasm to become more or less altered during the compli-
cated changes that accompany the division of the cell-
nucleus. This is notably the case in the chromosome re-

Fic. 36.—Graft of tomato (Lycopersicum esculentum) on tobacco
(Nicotiana tabacum). On the tomato are grafted Solanum nigrum, S
integrifolium, and Physalis Alkekengi. (Graft made by Mr. M. Free.)

duction divisions preceding spore-formation in the sporo-
phytes of higher plants (p. 37), especially when the plant is
a hybrid; and in spore-formation in the sporangia produced
from the zygospore of some of the filamentous fungi, like
Rhizopus or Mucor, the common black mold of bread. In

52 HEREDITY AND EVOLUTION IN PLANTS

the latter case the nuclear divisions, some time preceding
spore-production, result in separating out the female (+)
and male (—) strains, so that the spores in a given sporan-
gium are unlike as to sex—some being female (+), some
male (—), (Fig. 37). This will be discussed more fully
in the next chapter. Such changes result merely in dis-
tributing the heritable units (genes) of the mother-cell

Fic. 37.—Sexual reaction between a hermaphroditic Mucor and (+)
and (—) races of a dicecious species. Diagrammatic representation of a
Petri dish culture showing a heterogamic hermaphroditic Mucor (2) in
the center separated by channels on either side from the (+) and (—)
races, respectively, of a dicecious 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
a female gamete. B, sexual reaction between a (+) filament and a male
gamete. C,a male zygospore formed by stimulus of contact with a (+)
filament. (After Blakeslee.)

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 transition between reproduction
5

HEREDITY 53

by spores, gemme, bulbs and tubers, and the artifically
severed buds and scions used in grafting and “slipping.”

3. In sexual reproduction there intervene between par-
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 (Fig. 38). Both proc-
esses—the formation of the gametes, and their fusion—

Fic. 38.—Fertilization in the white pine (Pinus Strobus). The smaller
sperm-nucleus (above) is imbedded in the (larger) egg-nucleus. The fu-
sion of the nucleoplasms will finally become more intimate. (After
Professor Margaret C. Ferguson.)

offer almost unlimited opportunities for alterations of the
protoplasm—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 inheritances from two parents—from two
distinct lines of ancestry—protoplasms (germ-plasms) with
two entirely different histories extending back into the

1 The fertilized egg (as Thomson has pointed out) is the inheritance,
and becomes, in the mature individual, the inheritor.

54 HEREDITY AND EVOLUTION IN PLANTS

past, no one knows how far. How will these two inheri-
tances affect each other when they intermingle in the
fertilized egg? Will one tend to inhibit or check certain
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 substances be
formed or new combinations take place, resulting in an en-
tirely new expression in the offspring?

42. 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 biometrical
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 V
EXPERIMENTAL STUDY OF HEREDITY

43. Gregor Mendel.—Two of the most important
contributions ever made to biological science, namely,

Fic. 39.—Gregor Mendel, at the age of 4o. 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
55

56 HEREDITY AND EVOLUTION IN PLANTS

Mendel, a monk in the monastery at Briinn, Austria, where
he finally became abbott. In order to understand his work
clearly the student should familiarize himself the various
characters of the edible or garden pea, the chief plant with
which Mendel worked.

44, Mendel’s Problem.—Mendel was much interested
in problems concerning the origin and evolution of species.
It was largely this interest that led him to hybridize (i.e.,
cross-pollinate) plants of different species and varieties,
and observe the behavior of the resulting hybrids in succes-
sive generations. The problem which he endeavored to
solve was the law or laws ‘‘governing the formation and
development of hybrids,’’! with special reference to the
laws according to which various characters of parents
appear in their offspring.

45. Mendel’s Method.—He recognized that, in order
to solve the problem, attention must be given to at least
four points, as follows:

1. To start with pure-breeding strains.

2. To consider each character separately.

3. To keep the different generations distinct.

4. To record, for the progeny of each generation sepa-
rately, the proportions in which the various characters
appear.

No previous student had recognized the fundamental
importance of these requirements.

46. 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 57

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.

47. 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 of 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:

1. The difference in the shape of the ripe seeds (round
and smooth vs. angular and wrinkled).

2. The difference in the color of the cotyledons (pale
or bright yellow, or orange vs. light or dark green).

1 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,

58 HEREDITY AND EVOLUTION IN PLANTS

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., axial vs.
terminal, on normal vs. fasciated
stems).

7. The difference in the length
of the stem (the extremes chosen
were ‘‘talls” 6 to 7 feet, and
“‘dwarfs” 34 feet to 115 feet in
height).

48. Artificial Hybridizing —
The edible pea is commonly self-
fertilized; therefore, to make
crosses it is necessary carefully to
Fre, 4oi-Methad af pros OMe the stamens of one flower
tecting flowers from foreign before the anthers have begun to
pollen by paper bags, in shed their pollen, and then place
oot ae pollen from another flower on the

; stigma. The flowers must then
be carefully guarded, e.g., by tying paper bags over them
(Fig. 40), to prevent other pollen being deposited by
insects or otherwise. In this way the experimenter
knows just what characteristics enter into the hybrid.

EXPERIMENTAL STUDY OF HEREDITY 59

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” (7.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 killing any
foreign pollen that might be present. Numerous other
precautions are also taken.

When the hybrid plants are mature, careful observations
of whatever character is under observation are made and
recorded. Whenever possible the observation should be
quantitative.

49. Mendel’s 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.

1. Law of Dominance.—What Mendel actually found
by his experiments was that, in such a cross, all the seeds

60 HEREDITY AND EVOLUTION IN PLANTS

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, 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
are combined in a cross, one character behaves as a dominant
over the other, which ts 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 of Segregation—But what will happen if the
first filial (F,) generation is inbred or self-pollinated. Its
inheritance included factors that make for both “‘smooth”’
and ‘“‘wrinkled,” but the expression was of one kind only.
The experiment was made, and Mendel found that the
second filial (F2) generation included plants, part of which
possessed only smooth seeds, while the others had only
wrinkled seeds (Fig. 41). “Transitional forms were not
observedin any experiment.” This illustratesin a striking
way the difference between inheritance and expression,
for a character cannot appear in a plant (or animal) unless
the plant possesses the factor or factors for that character.
Now, except for the comparatively rare cases where
mutation occurs, the factors in the F, generation must have
been derived by inheritance from the germ-cells of the F,
generation; but the experiment shows that they did not
come to expression there. The same law is illustrated in
the crossing of a sweet variety of maize (having wrinkled

EXPERIMENTAL STUDY OF HEREDITY 61

grains) with a starchy variety (having smooth grains). In
this cross starchiness is dominant over sweetness (Fig. 42).

ek % 3
ae

|
@@ EG €E@S

€ ‘

Fic. 4x—Mendelian segregation in the edible pea (Pisum sativum)
Full explanation in the text. (Cf., Fig. 42.)

50. Ratio of Segregation..-But now we come to that
feature of Mendel’s experiments which, perhaps more than

62 HEREDITY AND EVOLUTION IN PLANTS

nddRRst as Lene sigs

tanne

eee

ad

ATS

Tritt

ayrenie Horde ed

hay uct eH yh:

Pree tte

eeppad oe Seevee ee says

ei grbere

Fic. 42.—Mendelian segregation in maize. uw, the starchy parent; d,
the sweet parent; C, the first hybrid (F1) generation, produced by crossing
a and b, 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, f, and g resulted from planting starchy grains. One ear in three is
pure starchy, the other two mixed; h, result of planting sweet (wrinkled)

seed. They are pure recessives, and the ear is pure sweet. (After
East.) (Cf. Fig. 41.)

EXPERIMENTAL STUDY OF HEREDITY 63

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 : 1.

51. Theory of Purity of Gametes.—When the wrinkled
seeds (one-fourth of the total crop) were sown they all
bred true to wrinkledness—their descendants of the F;
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.

52. Not All Dominants Alike.—-But when the seeds of
’ the F, 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 (z.e., one-fourth of all the seed produced by the F;
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 3:1—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:

64 HEREDITY AND EVOLUTION IN PLANTS
Dex Re P (1st Parental generation)

D (R) F; (xst Hybrid generation)

3D 1R F, (2d Hybrid generation)
I

1D + 2D(R)
af EE

D 3D iR x F; (3d Hybrid generation)

53. Significance of the Mendelian Ratio.—The ratio
3: 1 or, asit appears on analysis, 1 : 2 : 1, 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 + b, namely, a? +
2ab + b?, where a and 8 each equal 1. In the plants the
multiplication of inheritances (produced in fertilization)
was as follows:

eggs (s + w) X sperms (s + w) = ss + 2sw + ww

where w = wrinkling and s = absence of wrinkling, 2.e.,
smoothness.

54. 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 Fy
generation), carried only character-units or determiners!
‘that make for smoothness; the other half only those
factors that make for wrinkling, giving s 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

1 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 65.

be fertilized with either an s-sperm or a w-sperm, giving
(stw) X(s+w) =ss+2sw+ww. Sincesis dominant
over w the product should be written:

ss + s(w) + s(w) + ww

giving im external appearances 3s + 1w. Since the re-
sult actually observed is what it would be zf 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.

56. Character-units versus Unit-characters.—As just
stated, Mendel held that the various visible characters of
his plants (dwarfness, for example) behaved as units,
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. Since they are causally or genetically
related to the character they have been called genes (from
the root of the Greek word, genesis). They are more
commonly known as factors. Quite probably, in many
if not all cases, more than one factor is involved in the
production of any given character.

1Substance or condition, we know not what, within the germ-cells,

66 HEREDITY AND EVOLUTION IN PLANTS

56. Applications of Mendel’s Law.— Over 100 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. Various types appeared in the F, 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. Vafious shades and
colors appeared in the F, generation.

3. In one case three distinct varieties of beans, breed-
ing true to white seeds (when selfed*), were crossed with the
same variety of red bean. In the F, 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 F. generations.

4. Two varieties of sweet peas, each breeding true to
white flowers, when crossed gave, in the F, generation,
nothing but purple-flowered offspring, resembling the
wild sweet pea. A medley of white, pink, purple, and
red-flowered plants appeared in the F, generation. Num-
erous other cases might be cited, all of which would have
been unsolvable riddles except in the light of Mendelism.

57. Inheritance and Environment.—Emphasis should
be laid on the fact that the behavior of any plant, and the

1 The pollination of a flower with its own pollen, or with pollen from an-
other flower of the same plant, is called selfing.

EXPERIMENTAL STUDY OF HEREDITY 67

characters it manifests, are the result of the combined
influence of inheritance and environment. A bean seed-
lingyis 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 inheri-
tance and environment. It is of fundamental concern to
a man, not only to be ‘“‘well-born” (eugenics), but also
to be “well-placed”? (euthenics), although the former,
according to present day conceptions appears to be more
important.

68. 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,
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 succéssive
generations, of the same genotypical constitution of the pro-
toplasm. 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
withthe facts revealed by recent studies of plant and
animal breeding (Cf. p. 40).

68 HEREDITY AND EVOLUTION IN PLANTS

59. Pure Line Breeding.—Johannsen also originated
the “pure line” theory—a theory which has done
much toward elucidating the problems of selection. He
and his followers regard genetic factors as fixed and un-
varying. Hence the results obtained in selective breeding
of a given variety of maize for high or low oil content, or
of a given variety of beans for larger or smaller size of seed,
would be interpreted on this theory, as the isolation or
separation of pure strains from a “‘mixed population”’
or “impure” variety. In practical language, several true
breeding varieties of beans, differing in seed size, might be
obtained by selection from what appeared to be a “pure”’
variety with considerable variation in size of seeds.1

60. Value of Mendel’s Discoveries.—The discoveries
that, in inheritance, certain characters are dominant over
certain others; that a given inheritance (e.g., conditions
associated with seed-color, odor, eye-color, stature, musi-
cal 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 re-
cessive ones), whether good or bad, may become perma-
nently lost; that dominant characteristics are certain to
reappear 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

1A detailed discussion of Johannsen’s method of ‘pure line” breeding

belongs to more advanced studies.
8

EXPERIMENTAL STUDY OF HEREDITY 690

of thrifty parents, talented offspring of mediocre parents,
blue-eyed children of brown-eyed parents,! and so on.

61. 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 elemen-
tary 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 gradually becomes less
and the variety changes and deteriorates 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-
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

1If both parents have blue eyes the children with rare exceptions have
blue 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 tends
to be 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 usec! 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 pig-
ment is entirely lacking.

21f a high-yielding strain was separated out by selection, the yield _
would of course be increased above the average of the mixed field.

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having a low yield per acre, are crossed, there results an
F, 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. 43).
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 F, generation of a cross between two of these self-
fertilized 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 then using for seed the grains of the
first-generation hybrids each year.

62. Breeding for Disease-resistance.—Biffen, in
England, crossed a wheat of poor quality, but resistant
to rust disease (Puccinia glumarum), with a superior
variety but very susceptible to the disease. Suscepti-
bility proved dominant in the F generation, but in the
F, generation disease-resistant forms appeared, of superior

Fic. 43.—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 Ff: generation of hybrids be-
tween various pairs of the selfed strains. (Photo supplied by G. H. Shull.)

72 HEREDITY AND EVOLUTION IN PLANTS

quality, which bred true for resistance. The water-
melon, in the southern states, is subject to a very de-
structive disease which causes a wilting of the vines and
consequent loss of fruit. By crossing the ordinary non-
resistant watermelon with the closely related common
citron, which is wilt resistant, W. A. Orton, of the United
States Department of Agriculture, produced a water-
melon resistant to this disease. Numerous other illus-
trations might be given. This is becoming one of the
common and successful methods of combating plant
disease.

63. Unsolved Problems.—Like all truly great con,
tributions to science, Mendel’s 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;
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. 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

EXPERIMENTAL STUDY OF HEREDITY 73

in part. There seems to be evidence both ways, but the
bulk of the evidence and the weight of scientific opinion
is against the inheritance of acquired characters as here
defined. We can arrive at the correct answer only by care-
ful experimentation, that is, by asking questions of nature.!

2. Can the inheritance of a strain be 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 (e.g., a weed poisonous to cattle, or a new
and virulent disease-causing bacterium) might be experi-
mentally 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 con-
firmed before we can say with certainty that the result
sought has been accomplished.

3. How may dominance be explained? Why is tallness
dominant over dwarfness, brown eye-color over blue,
any one character over any other? At present we can
only speculate on these questions.

4. What is the mechanism of inheritance? In other
words, by what arrangement and interaction of atoms

10Qn the inheritance of acquired characters, see Thomson, J. A.,
Heredity. London, 1908. Chapter VII.

74 HEREDITY AND EVOLUTION IN PLANTS

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 gen-
eration 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 definitely know, but many careful investigations point
to the chromatin as the bearer of the hereditary factors.

64. Weismannism.—It was a botanist, Nageli, who first
recognized and clearly stated that inheritance must depend
upon a least quantity of matter, and numerous experi-
ments by both botanists and zoologists soon made it
evident that the hereditary substance is in the cell-
nucleus, rather than in the cytoplasm surrounding the
nucleus. Niéageli called the hereditary substance idio-
plasm. Observations of the germ-cells of plants by Stras-
burger, and of the germ-cells of animals by O. Hertwig,
led them to conclude that the chromosomes of the dividing
nucleus (Fig. 30 )are the locus of the hereditary substance.
The subsequent evidence. upon which this conclusion rests
is too voluminous, and some of it too technical, to be pre-
sented here in any detail.’ As an illustration there may
be cited the experiment of Boveri who removed the
nucleus from the egg-cell of one species of sea-urchin, and
then caused the remaining cytoplasm to be fertilized with
a sperm-cell of another species of sea-urchin; the result-
ing larva possessed only paternal characters.

1See Morgan, T. H. The physical basis of heredity. Philadelphia,
I9IQ.

EXPERIMENTAL STUDY OF HEREDITY 75

Weismann expanded the above conception of hereditary
substance by calling attention to the fact that it must
contain elements, not only from one individual or pair,
but from a long line of ancestors. He called the idioplasm
(of Nageli) in the germ-cells germ-plasm, and the heredi-
tary units, ‘‘necessary to the production of a complete
individual,” he called ids. Each id contains a full com-
plement of hereditary factors necessary to produce a
perfect plant or animal. The germ-plasm corresponds
to the deeply staining chromatin of the cell-nucleus, and
the ids are grouped together in idants, which correspond,
in general, to the chromosomes. Weismann further
postulated that the ids were composed of ‘primary con-
stitutents,’’ which he called determinants, and that every
character independently inherited has its own determinant
in the germ-plasm. Finally Weismann postulated that
each determinant is a complex of biophors (the ultimate
units of matter in the living state), each biophore being
composed of (non-living) chemical molecules. Thus we
rise through his categories as follows, from atom to mole-
cule, from molecule to biophore, from biophore to deter-
minant, from determinant to ids, from ids to idants
(chromosomes), which are composed of the hereditary
substance or germ-plasm; schematically as follows:

germ-plasm (chromatin)
idant (chromosome)
id.
determinant (factor, of Mendel)
biophore (biogen, of Verworn)
molecule
atom

The germ-plasm is continuous from generation to genera-
tion, and therefore possesses a kind of physicalimmortality.

76 HEREDITY AND EVOLUTION IN PLANTS

65. Relation of Weismannism to Mendelism.—It will
readily be recognized that the ‘“‘determinants” of Weis-
mann are the “factors” of Mendelian nomenclature.
Morever, it follows logically from Weismann’s theory
that acquired characters are not inherited, an inference
that agrees with observation and experiment. Nageli,
director of the botanic garden in Munich, transplanted
specimens of Hawkweed (Hieraceum) from the high Alps
to the lower altitude and changed climate of his garden,
and these plants began to manifest new characters which
reappeared in successive generations for more than a
decade. This looked like the inheritance of acquired
characters, but when the plants were subsequently taken
back to the high Alps, they failed to manifest the charac-
ters expressed in the botanic garden, reverting to their
former alpine characteristics. Thus it is seen that the
reappearance of the new characters in successive genera-
tions in the botanic garden was not due to inheritance of
these acquired characters, but to the continuity of the
new environment. The inheritance had not been altered
though the expression of ithad. This isin agreement with
what we should expect from the definition of inheritance
given on page 50.

66. 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 produce
a healthier and more efficient race of men constitutes the
practice of eugenics.” The underlying principles of eugenics

1See also pages 48 and 66-67.
2 The word eugenics is from two Greek words meaning well born.

EXPERIMENTAL STUDY OF HEREDITY 77

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, 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.

67. 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, a few reject the hypotheses,
and still others believe they contain the germ of truth,
but must be more or less modified. Whether they prove
to be erroneous or true is not so important, but it 1s 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

78 HEREDITY AND EVOLUTION IN PLANTS

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-
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.?

t 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 more advanced study.

CHAPTER VI

EVOLUTION

68. Doctrine of Special Creation.—In the time of
Linnzus, 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. Opposed to this doctrine
is the theory of evolution.

69. 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.

70. The Course of Evolution—The theory teaches
that the gradual changes have been from relatively
simple conditions to those more complex. The compli-

79

80 HEREDITY AND EVOLUTION IN PLANTS

cation has been two-fold: (1) simple individuals, whether
mountains, rivers, planets, animals, or plants, have become
more complex (e.g., compare the structure of the plant,
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
alge having one or a few cells only, to flowering plants,
like roses and orchids; 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 alge to the non-green, alga-like fungi (Phycomy-
cetes, such as bread mold), from independence to parasitism
(mistletoe and dodder), or to saprophytism (Indian pipe
and toad-stools).

The thirty odd species of the Duckweed family, related
to the Arum family (Jack-in-the-pulpit, calla, skunk cab-
bage, sweet flag, etc.), illustrate regression; they comprise
the simplest, and some of them the smallest of all flowering
plants. The plant body of Lemna is a tiny disc-shaped,
thallus, having a central vein (vascular strand) with or
without branches. Each plant has one root with no
vascular tissue. The flowers, borne on the margin or
upper surface of the thallus, have one simple pistil and
only one stamen (Fig. 44). The dozen or more species

EVOLUTION 81

of Wolffia possess still simpler bodies, somewhat globose,
with neither roots, veins, nor other organs, except flowers;
even flowers are unknown in some species (e.g., Wolffia
populifera, Fig. 44). Wolffia punctata measures only
0.5-0.8 mm. long. The plants are fittingly described in
the manuals as ‘‘minute, alga-like grains,” floating on or

Fic. 44.—Lemnacee. a, b, c, Lemna trisulca; d, Wolffia punctata;
e, f, Wolfia papulifera. (Redrawn from Britton and Brown, slightly
modified.)

just beneath the surface of still water. Some botanists
consider the plant body as morphologically a frond, others
asaleaflessstem. Since the first plant-body from the seed
is only a matured cotyledon, or seed-leaf, Goebel considers
that it cannot be interpreted as other than a free-living
leaf. These tiny, simple plants are considered to have

82 HEREDITY AND EVOLUTION IN PLANTS

originated by regressive evolution, their simplification
being closely correlated with a reversion from dry land to
an aquatic habit of life. A similar reduction of structure
is found in the tiny floating ferns, Salvinia and AZolla.

71. 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
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, with-
out possibility of reasonable doubt, that what is now
mountain top was formerly ocean bottom. The mountain
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; the atmosphere and the water of the ocean
have reached their present condition as the result of gradual
transformations extending over aeons of time; 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 evolutionary steps that have resulted
in the present condition of the earth.

EVOLUTION 83

72. Fitness of the Environment.—Biological literature
has always taken account of what has been called ‘‘adapta-
tion,” or the fitness of living things for life in the surround-
ings or environment where they are placed. But a recent
writer,’ has elaborated the complimentary notion of
the fitness of the environment. Recognizing living things as
“mechanisms which must be complex, highly regulated,
and provided with suitable matter and energy as food,”
he shows that the present inorganic environment is the
best conceivable. Inorganic evolution has resulted, among
other things, in the occurrence of large quantities of water
and carbon dioxide; their physical and chemical properties,
and those of the ocean, together with the chemical properties
of the elements, carbon, oxygen, and hydrogen, and their
numerous compounds, “‘are in character or in magnitude
either unique or nearly so, and are in their effect favorable’’
to the organisms with which we are familiar, and which
possess the three fundamental characteristics of complexity,
regulation, and metabolism. The elements carbon, hydro-
gen, and oxygen, says Henderson, are uniquely and most
highly fitted to be the stuff of which life is formed, and of
the environment in which it exists.

73. 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 egg. The development of a mature in-
dividual is ontogeny. The development of a group of
related forms (genera, families, orders, etc.) is phylogeny.
The chief problem of biology is to ascertain and record,
in order, the evolutionary changes that have resulted in

1Henderson, Lawrence J. The fitness of the environment. New
York, 1913.
4

84 HEREDITY AND EVOLUTION IN PLANTS

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 preéxisting living things, in other words, that
the present method of formation of living things, by the
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 preéxisting life).

74. 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.

1. Agassiz’s Hypothesis —The great teacher and student
of nature, Louis Agassiz, believed that the vast array of
plant and animal species, past and present, had no material
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. “Wemust . 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.’”!

1 Agassiz, L. ‘‘Methods of Study in Natural History,’’ Boston, 1893,
pp. 146, 147.

EVOLUTION 85

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 by
descent from preéxisting parents. Agassiz’s hypothesis

Fic. 45.—Louis Agassiz. (From Ballard’s ‘Three Kingdom.’’)

contradicts this. All oaks now-a-days are derived by
descent from preéxisting 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.

86 HEREDITY AND EVOLUTION IN PLANTS

2. Lamarck’s Hypothesis —The noted French naturalist,
Lamarck, taught that all living things have been derived
from preéxisting 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-

Fic. 46.—Water buttercup (Ranunculus agquatilis), showing aerial
leaves (a), and aquatic leaves (w). f, fruit. Drawn from herbarium
specimen.

ished; and that the derivation of new species is thus ac-
counted for in a simple and logical manner. By continual
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:

EVOLUTION 87

“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 be-
tween 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 (Fig. 46). 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
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-

1 Translated from his Philosophie Zoologique, vol. I, pp. 227, 223, 224,
248,

88 HEREDITY AND EVOLUTION IN PLANTS

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 sepa-
rating 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.
This hypothesis is known as “the inheritance of acquired
characters.”

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

EVOLUTION 89

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

Fic. 47.—Jean Baptiste Lamarck (1744-1829). He elaborated the
hypothesis of organic evolution by inheritance of the effects of use
and disuse.

“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 VII
DARWINISM

75. Darwin and Wallace.—The question of the
method of evolution continued to be debated, with no
satisfactory solution in sight, until 1859,1 when Charles
Darwin published the greatest book of the nineteenth
century, and one of the greatest in the world’s history,
the Origin of Species.2 This book was the result of over 20
years of careful observation and thought. It consisted
of the elaboration of two principal theories: (1) that
evolution is the method of creation; (2) that natural
selection is the method of evolution.

By a strange coincidence Alfred Russell Wallace, also
by many years of thoughtful observation, reading, and
reflection, had independently formulated the conception
of natural selection in far-off Ternate, and embodied
his ideas in a paper which he sent to Darwin for the purpose
of having it read before the Royal Society. The paper,
with its accompanying letter, reached Darwin on June
18, 1858, while the latter was engaged in writing out
his own views on a scale three or four times as extensive as
that afterward followed in the Origin of Species. Asa
result of the unsurpassed magnanimity of the two men,
and their generous attitude toward each other, it was

1 This date should be memorized. It is one of the most important
in the whole history of human thought.

* The full title of the book was “The Origin of Species by Natural
Selection, or the Preservation of Favored Races in the Struggle for
Life.”

go

DARWINISM Or

arranged to have a joint paper by Darwin and Wallace
presented to the Society. This paper, entitled ‘On
the tendency of species to form varieties; and on the per-
petuation of varieties and species by natural means of
selection,’ was presented at a special meeting of the Society

Fic. 48.—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.

on July 1, 1858, being read by the secretary in the absence
of both Darwin and (of course) Wallace.

76. Early Antagonism to Evolution.—The concep-
tion that evolution (as distinguished from periodic, super-
natural interventions of the Deity) is the method of

O2 HEREDITY AND FVOGLUTION IN PLANTS

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
influence in the world of intellect. Men preferred to fol-
low a leader, more or less blindly, rather than take the
pains to examine the voluminous evidence for themselves,
and accept the logical conclusion without prejudice or
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 now almost no
scientific man 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, indeed com-
pletely revolutionized every department of human
thought.,

77. 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
ie be verified by anyone, as follows:

. Inhevitance-—Characteristics possessed by - parents
ee 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

DARWINISM 93

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.

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 tress 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-

94 NEREDITY AND EVOLUTION IN PLANTS

trate the fact that all living things, in order to flourish,
must be adjusted to their surroundings. 2

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
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.

1“Tn 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.”

°* One pair of elephants produces an average of only one baby elephant
in ro years, and the breeding period is confined to from about the 3oth to
the goth year.

DARWINISM 95

Linneus, a century before Darwin, had called attention
to the fact that if an annual plant produced only two seeds
a year, and each of the plants from these seeds, in turn,
produced two seeds the second year, and so on, there would,
if all the individuals lived, be a million plants at the end of
twenty years. But, few species breed as slowly as that.
According to Kerner, the common broad-leaved plantain
(Plantago major) produces 14,000 seeds annually; shep-
herd’s purse (Capsella Bursa-pastoris),64,000;and tobacco,
360,000. The number of seeds produced each year by the
orchid, Acropera, was carefully estimated by Darwin at
74,000,000. But these figures are wholly surpassed by
the ferns. Professor Bower estimates the number of
spores produced each year by a well grown specimen of the
shield fern (Nephrodium filix-mas) at from 50,000,000 to
100,000,000, while the estimate for the fern Angiopteris has
been placed at 4,000,000,000 spores for asingle leaf. One
plant may have as many as 50 or more spore-bearing
leaves. It has been pointed out that, at these rates of
increase, unrestricted, a given species of plant would, in
two or three years, cover an area several thousand times
that of the dry land. But nothing of the sort occurs.
There must, therefore, be an intense struggle for existence,
in which the vast majority of individuals perish. 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

1 ‘Origin of Species” (New York, 1902 edition), pp. 83, 84.

96 HEREDITY AND EVOLUTION IN PLANTS

insects. Jf 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
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 life 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 life against the drought, though more
properly it should be said to be dependent on the moisture.
A plent 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

1In the above quotation, Darwin is undoubtedly referring to the
European mistletoe (Viscum album). The American mistletoe (Phora-
dendron flavescens) is found in the eastern and central United States on
various deriduous-leaved trees, including the sour gum and red maple.

DARWINISM 97

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.
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 “fit” 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.” .

78. 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,

98 HEREDITY AND EVOLUTION IN PLANTS

is true. Darwinism neither eliminates God, nor does it
teach that monkeys were 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
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!”’

“Tt 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

DARWINISM 99

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;
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 orginally 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.”

79. 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
animals and plants) was not accounted for; and the

geologists and astronomers claimed that the time required
5

100 HMEREDITY AND EVOLUTION IN PLANTS

for evolution to produce the organic world as we now behold
it is longer than the age of the earth as understood from
geological 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-
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.

80. The Modern 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 VIII
EXPERIMENTAL EVOLUTION

81. 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, began to
make observations under conditions which they determined
and largely regulated. In this way the problems were
simplified, observation became more accurate, and the
endeavor was made to assign the probable causes of the
observed ‘phenomena. With the introduction 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.

82. 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 the origin of species. 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-pollinated with another variety or species. The
101

162 MWEREDITY AND FVOLUTION IN PLANTS

characters of the parent plant were carefully noted and
recorded by 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

Fic. 49.—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.

years when a species produced only one crop of seed a
year. Very careful records and preserved specimens were
kept of the plants of each generation, and accurate com-
parisons were made to see if any individuals showed a
tendency to vary widely from their parents in any sig-
nificant 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.

EXPERIMENTAL EVOLUTION 103

83. Two Kinds 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, z.e., leaping).

84. 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

HODODVOOO NONDOO

Semceete sas een ee ee

mPOD Ne

Fic. 50.—Fluctuating variation in the leaves of an oak (Quercus chry-
solepis), a, all the leaves of a twig; 6, younger leaves of « twig; c, con-
secutive leaves; d, some leaves on one season’s growth of a twig. (After
Copeland.)

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 thanin 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

104 HEREDITY AND EVOLUTION IN PLANTS

of the species or variety. In other words, continuous or
fluctuating 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. 56).

All plants and animals manifest fluctuating variation
in all their characters (Fig. 50), 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, fa-
voring a more abundant food supply, will cause a greater
average growth than poor soil, but unless the seed for
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
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, or otherwise. When the selection
ceases, and the soil is maintained at average fertility, the
characteristic average height of the plants is restored.

85. 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 market, and the lengths of the indi-

1De Vries. “The Mutation Theory,” vol. 2, p: 47, Chicago, 1909

EXPERIMENTAL EVOLUTION TOs

viduals measured. The length varied from 8 to 16
millimeters, and in the following proportions:

Millimeters....... 8 9 10 «Ir «12 13 14 «4S 16
Number of beans. 1 2 23 108 167 106 33 7 #1

The beans were then placed Lf U—-G

in a glass jar divided into
nine compartments, all the
beans of the same length
in the same compartments.
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 Quételet’s!
curve (Fig. 51).

This curve may be plotted
by erecting vertical lines
(ordinates) at intervals of one
millimeter on a horizontal
line or base, the height of
each vertical line being pro-
portionate to the number of
beans having the length in- Fic. 51—Demonstration of
dicated in figures at its base. Quételet’s law of fluctuating varia-
This curve shows the freq- bility in the length of seeds of the

common bean (Phaseolus vulgaris).
uency of occurrence of seeds

: Description in the text. (Redrawn
of any given dimension from de Vries.) *

Zz
Hi
H
H
H
3
3
H

ASAAASAVAVRUAASRASEADLUR LUTE ERURRE ACE!

1 So named from its discoverer, Quételet (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.”

106 HEREDITY AND EVOLUTION IN PLANTS

between the two limits or extremes, and is therefore often
referred to as a curve of frequency. It should be noted
that, in the case illustrated, the greatest frequency (in-
dicated by the highest point of the curve) very nearly
coincides with the average dimension; in other words, the
more any given character departs from the average for that
character, the less frequent is its occurrence.

In another experiment, ears of corn, harvested from.
the same crop, were measured and found to vary in length

se

Fic. 52.—Curve of fluctuating variation (Quételet’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 ginches. The ears were taken from unselected
material from a field of corn. (After Blakeslee.)

from 414 inches to 9 inches; the largest number 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 g 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. 52).

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

EXPERIMENTAL EVOLUTION 107

Fic. 53—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. Ifthe 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—it 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 Quételet’s curves, are used by biometricians in classi-
fying and studying variations in plants and animals. (Photo by A. F.
Blakeslee. Legend slightly modified from Journal of Heredity, June,

1916.)

108 HEREDITY AND EVOLUTION TN’ PLANTS

illustrated by the grouping of bean seeds rolled down a
smooth inclined plane, and collected in receptacles at the
bottom (Fig. 53). 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. 53 is slightly skew toward
the right-hand extreme. Suggest one or more reasons
why.

86. 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.!

87. 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 of frequency; that is, in
fluctuation about a new mean. Plants from some of the
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 odorless plant may give rise to individuals whose
flowers are sweet-scented; or vice versa, odorless specimens

1The behavior of hybrid descendants is a special case described in
Chapter XXXVII.

EXPERIMENTAL EVOLUTION 109

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Fic. 54.—Curves of variation, illustrating the difference between
fluctuation about a given mean, and the appearance of a new mean, 2.e.,
mutation. At the right, variations in the stature of Oenothera Lamarckiana;
at the left, variation in the stature of Oenothera nanella, a form derived
from O. Lamarckiana by mutation. (After de Vries.)

Fic. 55.—Leaves of varieties of the Boston fern (Nephrolepis), showing
(from left to right) progressive branching of the pinne and pinnules, and
illustrating ‘so-called ‘‘orthogenetic saltation.” (After R. C. Benedict.)

IIo HEREDITY AND EVOLUTION IN PLANTS

may spring at one leap, not by gradual changes, from those
that 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 (Fig. 54.)

When discontinuous variation proceeds along a definite
line through several successive generations, each step being
an intensification of the preceding one, it is designated
“‘orthogenetic saltation”’ (Fig. 55).

88. Illustration of the Pendulum.—The difference be-
tween discontinuous and fluctuating variation may be
aptly illustrated by a swinging pendulum (Fig. 56). The
vertical position, assumed when at rest, is .he mean of all
positions that may be assumed as the pendulum swings;
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 new character has been introduced, with its own fluctua-
tions of more or less.

89. 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

EXPERIMENTAL EVOLUTION IIt

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

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suspension after the mutation has occurred.

formation of new species, de Vries called the mutation
theory.

90. Examples of Mutation.—The kohlrabi, cauliflower,
and other horticultural varieties of the wild cliff-cabbage
(Fig. 57), are believed to be mutants, and to have arisen,

HEREDITY AND EVOLUTION IN PLANTS

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EXPERIMENTAL EVOLUTION 113

not by the prolonged selection of fluctuating variations,
but at one step—in one generation—as “‘sports” of the
wild Brassica oleracea. Strawberry plants without run-
ners, green dahlias and green roses, the common seedless
bananas of the markets, the Shirley poppies, pitcher-
leaved ash trees, Pierson’s variety of the Boston fern,

Fic. 58.—Clover leaves with three to nine leaflets, illustrating a tend-
ency to mutate. The normal clover leaf is a pinnately compound leaf
with three leaflets. Plants with leaves having five to nine leaflets con-
stitute a ‘“‘half-race,’’ z.c. the normal character is active, the anomaly
semi-latent. (Photo by the author; specimens from cultures of G. H.
Shull.)

5-9- “leaved” clovers (Fig. 58), 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. 48),
and the innumerable white flowered varieties of colored
flowered species, are all illustrations of mutation. Fre-
quently the mutative change occurs in a lateral bud, pro-

II4 HEREDITY AND EVOLUTION IN PLANTS

ducing a “bud-sport” (Fig. 60). Such was the origin of

the seedless naval orange from the seed-bearing orange.
91. The Evening-primrose.—In 1886 de Vries began to

search for a species that was in a mutating condition, be-

Fic. 59.—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 f they have all aborted,
except two; at # many of the normally tubular disc-flowers have become
ligulate, making a nearly “double flower.” (Photo by E. M. Kittredge.)

lieving 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 (CEnothera Lamarckiana)
(Fig. 61).

EXPERIMENTAL EVOLUTION TI5

“That I really had hit upon a plant ina mutable period
became evident from the discovery, which I made a year
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

Fic. 60,—.\ plant of the evening-primrose (Cnothera biennis) which,
by “bud sporting,” has given rise (at the left) to a branch having the
characters of another species.

the power, at least in the case of several individuals, of
developing small capsules with a few fertile seeds. The
other was a smooth-leaved form with much prettier foliage
than O. 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
6

116 HEREDITY AND EVOLUTION IN PLANTS

Lamarckiana their cordate character. I call this form
O. levifolia.”

‘‘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 Gnothera

Fic. 61.—Lamarck’s evening-primrose (CEnothera Lamarckiana), A muta-
ting species. Cf. Fig. 62. (After de Vries.)

had spread; O. levifolia 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
O. Lamarckiana. The impression produced was that all
these plants had come from the seeds of a single mutant.

EXPERIMENTAL EVOLUTION II7

Since that time, both the new forms have more or less
spread over the field” (de Vries).

Another mutant of O. Lamarckiana was called by
de Vries CEnothera gigas (Fig. 62). The cells of this
mutant have twice as many chromosomes as the parent
form.

L

Fic. 62.—Giant evening-primrose Cnothera gigas, a mutant from
Gnotheta Lamarckiana, originated in 1895. Cf. Fig. 61. (After de
Vries.)

92. 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.

118 HEREDITY AND EVOLUTION IN PLANTS

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-
tion. The next step for de Vries to take, after discovering
the two forms that he supposed to be mutants, was to breed
them in carefully guarded, pedigreed cultures in his
garden, and also to breed the parent form, Ginothera La-
marckiana, and see if he could observe the two forms above
mentioned, or other mutants, arise from seed produced
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
found that they bred true (except for additional muta-
tions) when propagated by seed for over 25 years—that
is, they were true mutations.

93. 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. Here lies the essen-
tial difference between Darwin and de Vries, for Darwin,
though recognizing, and with increasing clearness, that
mutation furnishes part of the material to be “‘selected”’
by nature, assigned a larger and more important réle to
fluctuating or individual variations. ‘Species have been
modified,” he said, ‘‘chiefly through the natural selection
of numerous successive, slight, favorable variations; aided
[however] in an important manner by . . . . variations
which seem to us in our ignorance to arise spontaneously.
It appears that I formerly underrated the frequency and

EXPERIMENTAL EVOLUTION 119g

value of these latter forms of variation, as leading ‘to
permanent modifications of structure independently of
natural selection.!. And he goes on to say that, ‘‘as my
conclusions have lately been much misrepresented, and it
has been stated that I attribute the modification of species
exclusively to natural selection, I may be permitted to
remark that in the first edition of this work (the Origin),
and subsequently, I placed in a most conspicuous position
—namely, at the close of the Introduction—the following
words: ‘I am convinced that natural selection has been
the main but not the exclusive means of modification.’’’?

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 per-
petuated. Without mutation this is difficult to explain,*

1Darwin, C. The Origin of Species. 6 Ed. New York. D. Apple-
ton & Co., 1902, p. 293.

2Darwin almost dispaired of making his position on this point under-
stood. The clear statement above quoted, he said, “has been of no
avail. Great is the power of steady misrepresentation; but the history
of science shows that fortunately this power does not long endure.’
Darwin. 1. c., p. 293-

3 As required by Darwin’s theory. See quotation above (p. 118), and
on p. 97.

“ 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. The constant association of
two characters is often (though not always) due to the fact that the factors
for those characters tend to keep together in the same chromosome, in-
stead of segregating during the formation of egg-cells and sperms. This
tendency is called linkage. The association of smooth (vs. wrinkled) seed
with tendrilled (vs. non-tendrilled) leaves in the garden pea, and of red
flower-color with round pollen in the sweet pea may be cited as examples of
linkage. In such cases one of the characters might be of vital importance
to a plant in the struggle for existence and the other not.

120 HEREDITY AND EVOLUTION IN PLANTS

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 evolutionary changes may take place within a
much shorter time-period than was demanded by the
natural selection of fluctuations. Incidentally, the muta-
tion theory clearly shows that the absence of “connecting
links” between species is-not, as was formerly urged, an
argument against evolution, but is, on the contrary, just
what we might expect to find.

94. Value of the Mutation Theory.—The elaboration of
the mutation theory (together with the rediscovery of
Mendel’s law, to be discussed in Chapter V) furnished the
biological world with a new method of study; it demon-
strated that the method of evolution, so far as it concerns
the origin of new characters, may be studied by experi-
mentation.1 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 where formerly groping
in the dark, and has showed, that results of commercial
value do not require a life time, but may be obtained with-
in two or three seasons. By the application of modern
methods it has been possible, within a few seasons, to

1 Like most great contributions to science, the elaboration of the experi-
mental method of approach to the problem of heredity and evolution cannot
be attributed solely to any one man. Students of science in any period
come into a rich inheritance in the labors of many predecessors. To
fully assign the credit for the experimental method in the study of heredity
it would be necessary to write the history of investigations extending from
Kélreuter (1760) one of the first, if not the first hybridizer, of plants, Knight
(1799), through Gaertner (1849), Jordan (1853), Naudin (1862), and others
to Mendel, de Vries, and‘those of more recent date, down to our own time.

.

EXPERIMENTAL EVOLUTION 121

obtain new strains of oats yielding as much as 14 bushels
per acre more than the variety from which they were de-
rived, and to produce new strains of corn not only giving
a larger yield, but maturing nearly two weeks earlier than
the parent variety.

Fic. 63.—Linnzus, the great classifier (1707-1778). He is wearing a
sprig of the twin-flower (Linnea borealis), one of his favorite flowers, and
named after him by his friend, Gronovius. He is regarded as the father
of modern systematic botany.

95. Classification.—Mere information is not science.
A “book of facts” is not a scientific treatise for it is com-
posed of bits of unrelated information, presented on some
artificial basis of sequence, as for example, alphabetically.

122 HEREDITY AND EVOLUTION 1N PLANTS

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, funda-
mental relationship between the facts and ideas. Only
by such an arrangement of our ideas are we able to under-
stand their relation to each other, their relative impor-
tance, and their real significance. Classification, there-
fore, is essential to allscience. 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, 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 common names of
plants indicate the recognition of classes—a classification.

96. Evolution and Classification.— Without the guiding
idea of evolution classification would be arbitrary and
artificial. Linneus 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
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

EXPERIMENTAL EVOLUTION 123

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
world. In other words, if our knowledge should ever be-
come sufficiently complete and exact, the classification of
planis 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 IX
THE EVOLUTION OF PLANTS

97. 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. 44). All of these garden
vegetables are believed to have been derived from the
common wild cliff-cabbage (Brassica oleracea) 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
phylogeny, or line of ancestry, of that group.

124

THE EVOLUTION OF PLANTS 125

98. Methods of Study.—In the solution of this prob-
lem two methods of attack may be employed: (1) That
of observation and comparison of structure, followed by
classification, and inference; (2) that of experiment. The
use of experiment is indicated in Chapters V and VIII.
By this means we may learn something of the relationship
of different groups having living representatives, but it
chiefly serves to throw light on the method of evolution.
The course of evolution is best ascertained by the observa-
tion and comparison of plant structures.

99. Sources of Evidence.—There are five main sources
of evidence as to the course of evolution:

1. Comparative life histories of living forms.

2. Comparative anatomy of living forms.

3. Geographical distribution.

4. Structure of fossil forms.

5. Geological succession of fossil forms.

Studies along these five 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.

100. 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 by some in-
vestigators? that all reproductive organs (stamens, carpels,
sporophylls) arose by a modification of vegetative organs.
Other facts, however, as set forth on pages 126-129, have
lead to the directly opposite conclusion.

1See Bower, F. O. “The origin of a land flora.” Macmillan and Co.
Ltd., London, 1908.

126 HEREDITY AND EVOLUTION IN PLANTS

101. Evidence from Comparative Ontogeny.—In zool-
ogy, evidence of the course of evolution is also seen in the
recapitulation of the characters of lower forms in the em-
bryogeny of higher forms. This is often referred to as von
Baer’s law. Evidence of that nature is less striking and
less common in plants. It is found, however, in a com-
parison of the young or embryonic stage of the sporophyte
of the higher liverwort, Marchantia, with the mature

Fic. 64.—The apical cell in the stem apex in various phyla, from
Bryophytes to Gymnosperms. A, acrogynous liverwort (Notothylus
orbicularis); B & C, eusporangiate ferns (B, Marattia Douglasii, C,
Ophioglossum pendulum); D & E, homosporous leptosporangiate ferns (D,
Osmunda Claytoniana, E, Adiantum emarginatum, representing Polypodi-
ales); F, heterosporous leptosporangiate fern (Marsilia vestita); G, a
horsetail (Calamophyte) (Equisetum telamateia); H, a late gymnosperm
(Pinus Laricio), (A-G redrawn from Campbell, H from Buchholz).

sporophyte of the lower liverwort Riccza (Fig. 65). The
latter consists almost entirely of “fertile” (7.e., reproduc-
tive) cells. As we pass to more highly organized forms,
such as Marchantia, the relative amount of vegetative
tissue gradually increases by a progressive sterilization’ of
fertile tissue. This progressive sterilization is repeated
in the ontogeny of the sporophytes of the higher forms.
The thread-like, green protonema of mosses is often in-
1$ee foot-note, p. 125.

THE EVOLUTION OF PLANTS 127

terpreted as reminiscent of an ancestral filamentous green
alga, and the appearance in the embryo of pines and
other conifers of a larger number of primordia than of
mature cotyledons, has also been regarded as a re-
capitulation of an ancestral feature (Fig. 104). Bucholz!
has demonstrated that young pine embryos possess
an apical cell similar to that characteristic of ferns
and fern-allies, this apical cell persisting until the pine
embryo comprises several hundred cells, and then loosing
its identity (Fig. 64).

102. 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 Pleurococcus, and other green alge, preceded
more complex forms like the liverworts; that Bryophytes
probably 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—.e., from the more complex
to the less complex. For example, it is generally accepted
that the filamentous, alga-like fungi were derived from
green alge by retrograde evolution (degeneration). Were
the plants with one seed-leaf (monocotyledons) derived
from those with two (dicotyledons) by retrograde evolu-
tion, or were the dicotyledons derived from the monocoty-
ledons by progressive evolution? Evidence ascertained
by comparative studies of vascular anatomy and other de-
tails of structure points to the conclusion, that, although

1Bucholz, J. T. Suspensor and Early Embryo of Pinus. Bot. Gaz.
66: 185-228, Sept., 1918.

128 HEREDITY AND EVOLUTION IN PLANTS

monocotyledony seems the simpler, more primitive condi-
tion, it is really a later phenomenon, the monocotyledons
being derived from the dicotyledons by simplification.*
As a further example there may be cited the application
of the method of comparative anatomy to solve the problem

Y

Fic. 65.—Progressive sterilization of tissue in sporophytes. a, Riccia
trichocarpa (mature); b, Marchantia polymorpha (embryo); c, Marchantia
(mature); d, Porrella, a leafy liverwort (mature); ¢, anthoceros; f, Lyco-
podium Selago; g, Lycopodium complanatum; h, Botrychium Lunaria
(Eusporangiate); i, Polypodium venosum (Leptosporangiate). (Re-
drawn from various sources.)

of the origin of the leafy sporophyte. As noted above

(101), the most primitive spore-producing phases (sporo-

phytes) of the lower liverworts (Hepatice) consist

almost entirely of “fertile” (¢.e., reproductive) cells,

and the relative amount of vegetative or sterile tissue
1See page 223.

THE EVOLUTION OF PLANTS 129

gradually increases, as we pass to more highly organized
forms, indicating a progressive sterilization of the fertile
tissue during evolutionary development. Asurvey of
the sporophytic phases of the liverworts, mosses, and
ferns will show how these sporophytes gradually in-
crease in complexity and importance, from the simple
condition in the liverwort Riccia, with almost no sterile
tissue, through the sporogonium of the higher liverworts
and mosses, to the leafy sporophyte of the ferns (Fig. 65).
The final step in the development of the sporophyte was
the differentiation of plants bearing only large spores
(megasporophytes), and those bearing only small spores
(microsporophytes), represented in the Angiosperms re-
spectively by the pistillate and staminate plants. The
progressive sterilization accompanied a change in the
habitat of the plants from water to dry land.!

On the other hand, 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 theiy
origin to reduction from some higher type.”? In view of
this diversity of opinion, we learn at once that great cau-
tion must be used in interpreting the evidence—that we
must not “jump at conclusions.”

103. Results of the Method of Comparative Anatomy.
By their study of comparative anatomy and morphol-
ogy, botanists have been led to propose the following

“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.
2Scott, D. H. The Evolution of Plants. p. 18.

9

130 HEREDITY AND EVOLUTION IN PLANTS

arrangement of plant groups as representing the general
course of their evolution (Table I):

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 im-
plies that dicotyledons were derived from monocotyledons,
pteridophytes from bryophytes—hypotheses which, from
other trustworthy evidence, as stated above, now seem
untenable. .

Taste I .—SEQUENCE OF PLANT GROUPS, BASED ON THE
MorpPHoLocy oF Livinc Forms

Thallophytes Alga—having chlorophyll.
(no archegonia) Fungi—no chlorophyll.

| Bryophytes—no vascular system.

Archegoniates Pteridophytes
(archegonia, but no seeds) | Calamophytes ; vascular system.
| Lepidophytes
Gymnosperms—no closed ovary.
Spermatophytes Angiosperms—closed ovary (pistil).
(seeds) Monocotyledons—one-seed leaf.
= Dicotyledons—two-seed leaves.

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.

104. Consequences of an Amphibious Habit of Life.—
The life history of the fern affords a concrete illustration

THE EVOLUTION OF PLANTS I31

of the consequences of a change from an aquatic to an
amphibious habit of life. The gametophyte is semi-
aquatic in habit, and the method of fertilization is purely
aquatic, the sperm being unable to reach the egg except
by swimming through free water.t But, when the ferti-
lized egg began to develop as a land plant, the chances of
fertilization by a sperm swimming in free water bécame
increasingly remote. The perpetuation 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 reproductive body. The larger the
number of such spores, the greater the chances of perpetua-
tion of the given species.

105. 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 distribu-
tion when dry. This is accomplished by the sporophyte,
producing an abundance of broad, green leaves for food-
manufacture, and of roots for absorption of water and
minerals in large quantities. From such considerations as
these the plant body of the sporophyte is regarded by Bower
and his followers as produced by the progressive sterili-
zation of tissues originally reproductive. After the for-
mation of a vigorous plant body, spores, produced in special
regions (sporangia) could be nourished in enormous
numbers.

106. Origin of Vegetative Organs.—On the basis of
Bower’s theory we are to regard foliage leaves and branches,

1See p. 23.

132 HEREDITY AND EVOLUTION IN PLANTS

. . te .
either as new formations, developed (by * enation’’) on some
primitive reproductive axis like a strobilus or cone, or else

ae 2

Fic. 66.—Diagram to show the increase in prominence of the sporo-
phyte stage of plant life from the alge to the higher seed-plants. Among
the thallophytes both the sexual and asexual methods of reproduction
are represented. A illustrates the asexual, wherein certain cells of the
plant divide into smaller cells, the zodspores, which, without union with
other cells, develop directly into new plants. BE illustrate the sexual
method, effected through an alternation of generations, wherein a
vegetative stage, the sporophyte, alternates with a reproductive stage,
the gametopyte. (After Shimer.)

as produced by the sterilization of parts originally fertile,
i.e., modifications of reproductive tissues. Thesporophyte
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 regression of
the gametophyte. This changing relationship is roughly
indicated in the following diagram (Fig. 67, and also in
Fig. 66).

107. Steps in the Evolution of the Sporophyte.—The

THE EVOLUTION OF PLANTS 133

possible steps in the evolution of the sporophyte may,
on this theory, 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.

HT

QU

SPOROPHYTE

Fic. 67.—Diagram illustrating the gradual change in the relative promi-
hence of the gametophytic and sporophytic phases in the life-cycle of
plants during their evolution from the primitive alge (at the left) to the
modern seed-bearing plants (at the right).

108. Second Hypothesis.—In a discussion of Bower’s
theory, Tansley,? considers it “a priori in the highest
degree unlikely that so fundamentally important an organ
as the foliage leaf of the vascular plant appeared in descent
as an ‘enation’ from the surface of a cylindrical body of
different morphological nature,” and states that ‘‘there
is no well established case of any such origin of an organ
of the importance and with the potentialities of the leaf in
the evolutionary history of the plant kindgom.” He also
calls attention to the fact that the sporophyte (sporo-
gonium) of mosses and liverworts has never been known to
produce by enation or otherwise, any structure resembling

1 Following F. O. Bower.
2 New Phytologist 7:177-129. April and May, 1908.

134 HEREDITY AND EVOLUTION IN PLANTS

a foliage leaf or a sporophyll, and considers that it probably
as we now know it, “represents its highest capacity for
evolution.”’

On the hypothesis of progressive sterilization and ena-
tion (strobiloid theory), one would expect more primitive
sporophytes to possess relatively small leaves, that is, to
be microphyllous, and those with relatively large leaves
(megaphyllous) to be of later evolutionary development.
But there is no fossil evidence that the microphyllous fern
allies (club-mosses, horsetails, sphenophylls) are older
groups than the megaphyllous true ferns. The suggestion
is at hand, according to Tansley, that smaller leaved forms
have been derived from the larger leaved group by reduc-
tion. The facts of embryology and gametophyte anatomy
of the Lycopods are also interpreted by Sykes! as, on the
whole, supporting the hypothesis that the simpler Lyco-
pods are reduced forms and not primitive, the entire genus
Lycopodium being regarded as formed by reduction from
some of the larger fossil cone-bearing fern-allies, such, for
example, as Lepidodendron or one of its near relatives.
Miss Sykes has further suggested that the fossil genus

, Spencerites may represent the connecting link, between the
two groups.

It is not possible nor essential, in a book of this nature
and scope, to give a detailed discussion of the evidence and
the literature bearing upon this and similar questions.
It is only intended here to call attention to the fact that
different inferences as to the origin of the leafy sporo-
phyte and the broad course of plant evolution may be

1Sykes, M. G. Notes on the morphology of the sporangium-bearing

organs of the Lycopodiacee. New Phytologist. 7:41-60. Feb. and
Mch., 1908.

THE EVOLUTION OF PLANTS 135

logically deduced from the same facts, depending on which
facts or classes of facts the emphasis is placed.!

109. Homologous Alternation.—By the theory of anti-
thetic alternation the leafy sporophyte was derived from
some such structure as the sporogonium of the Bryo-
phytes, the axis existing first, the leaves originating as out-
growths at its surface. There could thus be no true
homology between any of the organs of the sporophyte
and those of the gametophyte, however close the super-
ficial resemblance might be. The (gametophytic) leaves
of the true mosses, while of like function (analogous) to the
(sporophytic) leaves of the club-mosses, are not the same
structural elements, 7.e., are not homologous with them.
By a contrasting theory the gametophytic and sporo-
phytic stages were at the first vegetatively or somatically
equivalent (except for chromosome number), as is the case
now, for example, with the red alge, Dictyota and Polysi-
phonia, but, in the course of evolution, the sexual phase
became more, and the asexual phase less, important in other
forms (e.g., ferns). This is called the hypothesis of
homologous alternation, since the vegetative organs repre-
sent the same structural or morphological elements.
According to the antithetic theory the sporophytic phase
was originally entirely dependent on the gametophyte
(as now, e.g., in the Liverworts), while according to the
homologous theory, the sporophyte has been free-living
from the start. By the latter theory, also, leaves did
not originate as new formations at regions of the axis
previously unoccupied by lateral organs (enation), but

1 Those wishing to go more fully into this question will (in addition to
the article above cited) find much of the evidence presented and analyzed
by Lady Isabel Brown in a series of five articles on “The phylogeny and
inter-relationships of the Pteridophyta,” in The New Phytologist for 1908.
An extended bibliography accompanies each article of the series,

136 HEREDITY AND EVOLUTION IN PLANTS

axis and foliar organs were both derived from an ancestral
thallus, branching dichotomously.!

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 transition from aquatic to dry-
land surroundings. Where the environment is uniform
and the same for both generations, as for Dictyota, the
gametophyte and sporophyte are identical in external

Fic. 68.—Dictyota dichotoma, Left, sporogonial plant; right, sperma-
gonial (gametophytic) plant. (After W. D. Hoyt.)

organs and general appearance (Fig. 68). In any event
the hypothesis postulates a homology between the various
organs of the two generations, however much these parts
may differ in external appearance as a result of individual
variation and environmental influence.

110. A Third Hypothesis.—Viewing the matter from
the standpoint of individual development (ontogeny) Lang
has developed the ontogenetic hypothesis of alternation.

1In a forked manner, resulting from the occurrence of two growing
points at the tips of the axes.

THE EVOLUTION OF PLANTS 137

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 a result of the difference
in the environment 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 ex-
periment would be to obtain a gametophyte by artificially
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 sporophyte
by causing a spore to develop within the tissue of a game-
tophyte, as the fertilized egg normally does.

111. 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 Hepatice, and
probably through some form belonging to the Anthocero-
tales, somewhat as shown in the following ancestral “‘tree”’
(Fig. 69). It should be clearly understood that this
tree does not illustrate known facts, but only the hy-
potheses which have been tentatively proposed by care-
ful students on the basis of known facts.

The evidence from fossil forms will be considered more
at length in chapters XI and XII.

138 HEREDITY AND EVOLUTION IN PLANTS

Cycadales

erales
Dicat.An tosperms

Cont

FI/LICALE, STOCK
(Fossid FOAMS)

i =D sate aN Fquisetales

tral Anthocerotales._.

/Ances

Ancestrat Bavoruy ta (Hy PotHeTicat)

Fic. 69.—Hypothetical genealogical tree to illustrate the probable
affinities 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
genealogical tree, whose lower limbs and roots extend into preceding geo-
logical 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.

CHAPTER NX

GEOGRAPHICAL DISTRIBUTION

112. Significance of Geographical Distribution From
the evidence of comparative anatomy and comparative
life histories, and also from the geological record (to be
noted later), it has been possible to determine the course
of evolution, in broad outlines, with reference to certain
of the larger groups of plants. As noted above, we may
learn that, in all probability, ferns preceded gymnosperms,
and gymnosperms preceded angiosperms; but within these
various groups, and for living forms, the problem becomes
increasingly difficult. For example, how shall we deter-
mine whether the family represented by the bracken fern
(Polypodiacee) is more ancient or more modern than the
royal-fern family (Osmundacee)? Is the maiden-hair
tree (Ginkgo) a younger or an older species than the pine
and the hemlock? Did herbs precede trees in the evolu-
tion of Angiosperms, or vice versa? This question of the
relative ages of living groups is greatly illuminated by the
evidence afforded by the facts of geographical distribution
of fossil and living forms.

Darwin spoke of geographical distribution as the
‘almost keystone of the laws of creation,’ and one does
not need to pursue the study of that subject far to under-
stand the truth of his statement. Before the diffusion of

1 The interested reader will wish to consult those two remarkable chapters
(XII and XIII) of volume two of the Origin of Species.

139

I40 HEREDITY AND EVOLUTION IN PLANTS

Darwin’s teaching, which freed men’s minds from the
shackles of preconceived notions, founded, not on observa-
tion of facts, but on a more or less blind acceptance of
current theological suppositions, or on the teachings of
ancient writers, the facts of distribution had a far different
significance than they were seen to possess when men began
to interpret the present state of nature as being the result

Fic. 70.—Alexander von Humboldt (1769-1859). Founder of the
science of plant geography.

of the operation of natural causes; by most students they
had been regarded as so much information, like the matter
in a guide book, but pointed the way to no larger conception
or generalization, so far as historical evolution was con-
cerned. ‘To find giant redwood trees exclusively in Cali-
fornia meant nothing, except that they were created there
and nowhere else, and had never spread; to find the bracken

GEOGRAPHICAL DISTRIBUTION Eqi

fern of almost universal occurrence, in both temperate
and torrid latitudes, eastern and western hemispheres,
could be easily explained on the supposition that it had
gradually spread from the center of distribution where
it was created, or on the theory that it had been indepen-
dently “created” in many different localities.

The idea that the same species was “‘created”’ indepen-
dently in different localities, from which it might spread,
was taught by Gmelin as early as 1747. It is often
referred to as Schouw’s hypothesis, from the Danish
botanist who elaborated and urged it in the first part of
the nineteenth century.1. Reasoning from the facts of
discontinuous distribution (to be noted in following para-
graphs) Schouw argued for the hypothesis of the multiple
origin of species, that is, that there were originally many
primary individuals. The existing vegetation of the
globe was not created at once, argued Schouw, but by
degrees, since the surface of the earth has only gradually
become fitted for the growth of plants, and moreover
certain plants (e.g., parasites) depend upon the existence
of others, and therefore the latter must have previously
existed.2. The hypothesis of multiple origin was also, at

1Schouw, J. F. Desedibus plantarum originariis, Hauniae, 1816. His
memoir On the origin of plants was published in Danish in 1847, and the
English translation by N. Wallich was published in Hooker’s Journal of
Botany, 2: 321-326, 373-377. London, 1850; and Ibid, 3: 11-14, 1851.

2 This is an interesting illustration of how the same kind of evidence may
lead one student toward the truth and another toward error. Schouw was
proposing the ideas here set forth at the same time that Darwin was
elaborating his theory of natural selection, and only twelve years before
the appearance of the Origin. Raising the question as to whether new
species continue to be created, or whether the existing vegetable kingdom
has been finally completed, he argues that, ‘‘The most rational mode for
accounting for new species being possibly created, seems to be by suppos-

142 HEREDITY AND EVOLUTION IN PLANTS

first, adopted by Alphonse DeCandolle, but finally aban-
doned by him in his Géographie Botanique Raisonée (1855).
About the middle of the last century Agassiz was urging
his autochthonous: hypothesis, namely, that each species
originated where it is now found (indigenous), covering
from the first as large an area as at present. This hy-
pothesis, if true, would, as Gray pointed out, “remove the
whole question out of the field of inductive science.”
There would be no incentive to study the question of
geographical distribution, and little of value could result
from such an investigation. Both Schouw’s and Agassiz’s
ideas have long since been abandoned. It is no longer
considered a matter of hypothesis or theory, but. of well
established fact, that most of the existing species are im-
measurably older than the present configuration of the
continents; in fact many genera and families of Angio-
sperms of the present land flora were clearly defined as
early as the Tertiary period, and have undergone little
change since that remote time.

113. Means of Dispersal.—The question of the means
of dispersal of the seeds and spores of plants is a largé one,
and a voluminous literature exists on the subject. This
is not the place to go extensively into the matter, but a dis-
ing that a change of climate or soil produces a corresponding change in
the character of its plants; or that some casual difference in the character
of the type of any given plant, may have become permanent by its being
isolated. It is in this way that constant varieties have arisen, which may
sometimes even have become real species, ‘but on all these occasions it is
culture that has been the cause; as far as I know, we possess no facts to
prove that natural causes have produced this effect.’’ Schouw also
reached the erroneous conclusion that the present flora was probably not
derived from the plants of preceding geological periods.

1 Autochthon, from the Greek airés, self + 0av land, meaning from the
land itself.

GEOGRAPHICAL DISTRIBUTION 143

cussion of geographical distribution requires a clear under-
standing of certain points which may be briefly alluded
to here. The situation with plants is, of course, quite
different than that with animals. With the advance of a
continental ice sheet, for example, animals may actively
retreat, by their own locomotion. There are exceptions
to this method of animal dispersal. Insects and small
birds may be blown by the wind over considerable dis-
tances, and insect eggs, larve, and cocoons may be trans-
ported in the soil about the roots of floating uprooted trees
and otherwise; and instances are on record of animals
being carried as passengers on floating objects, notably,
according to Semon, in the Malay Archipelago; snakes
and crocodiles are known to have drifted in this way to
the shores of the Cocos or Keeling Islands, a group of
coral atolls in the Indian Ocean, about 700 miles south-
west of Java, the nearest land.

But plants, at all times and under all circumstances,
are wholly dependent on being carried passively by exter-
nal agencies. The chief means of seed dispersal are the
wind, streams and ocean currents, and animals—particularly
birds. For distribution over great distances it is of im-
portance to consider chiefly wind, ocean currents, and
birds.+

114. Dispersal by Wind.—DeCandolle, the great
Swiss student of plant geography, regarded the wind as
“the most general and ordinary cause of the distribution
of species over the entire surface of a country,” but re-
jected it as a means of dispersal over even narrow arms of

1 The distribution of seeds in connection with commercial shipments is
interesting, but not essential to our present purpose. The word “‘seeds’”’
is used above to designate all reproductive bodies, including fruits, spores,
and vegetative reproductive bodies, such as gemmz, bulbils, etc.

144 HEREDITY AND EVOLUTION IN PLANTS

the sea. “I have never heard,” he said,’ ‘‘of a single seed
carried from England to France, nor from Ireland to
England by the agency of the west winds, although they
are so intense and so frequent in those countries. I do
not believe it has ever been demonstrated that seeds have

Fic. 71.—Alphonse De Candolle (1806-1893). Noted Swiss botanist
and student of plant geography. Author of Géographie Botanique
Raisonée. (From Acta Horti Bergiani.)

fallen in Sardinia from Africa, in Corsica from Sardinia,
nor from Corsica to the coast of Genoa and Nice, although
the south winds are there very violent.” Other students
have reached the same conclusion on similar negative
evidence. It has also been argued, on general grounds,

1 Géographie Botanique Raisonée 2:614. Geneva, 1855.

GEOGRAPHICAL DISTRIBUTION 145

that the rate of fall of various seeds in air is such that they
would have to be carried to improbable heights by the
wind in order to travel for very great distances before
falling to the ground.

But no amount of negative evidence is conclusive in the
face of even one firmly established bit of positive evidence,
and the positive evidence is not only more conclusive but
more voluminous than the negative. Seeds of the pitcher
plant, Nepenthes ampullaria; are known to have been
transported from Ceylon to the Seychelles, a distance of
1500 miles, and Engler calculated that, out of a total of
about 675 species in Hawaii, 140 ferns and other spore-
bearing plants, and 14 angiosperms were quite certainly
transported thither by wind. In fact, a large percentage
of the vegetation of isolated oceanic islands is of plants
whose seeds could hardly have been transported there in
a viable condition except by winds.!

As Warming has noted, there is not an oceanic island
destitute of plant life, though many of them are separated
from the present mainland by hundreds and even thousands
of miles of salt water, and have never, in all probability,
been connected with any continent; all their vegetation,
therefore, must have been transported thither by some
agency. In igor there fell in Switzerland large quantities
of dust which is said to have undoubtedly come from
Africa. If this were possible it is certainly not improb-
able that light seeds of various species might be trans-
ported very long distances in a similar manner. The

1Even small animals, and especially insects, are known to be trans-
ported to considerable distances by the wind. During his voyage on
the exploring ship Beagle (1832-1836), Darwin observed spiders, buoyed
up by their webs, being wafted over the vessel by the wind as far as
60 miles from land.
10

146 HEREDITY AND EVOLUTION IN PLANTS

wind is known to be a factor in plant-distribution in the
West Indies. Thus, for example, previous to 1899, the
sedge, Fimbristylis spathacea Roth, was not known on
Great Bahama island, of the West Indies. After the hur-
ricane of August 13, 1899 this sedge appeared in clearings,
and ‘“‘soon spread as a troublesome weed through culti-
vated lands, killing out pasture grass in places; it had
therefore come to be called ‘Hurricane Grass.’’!

In August, 1883, the island of Krakatoa, west of Java
in the Sunda Strait, experienced a terrific volcanic erup-
tion, which completely destroyed every vestage of its
vegetation, converting the green island into a desolate
desert. Within three years thereafter Treub found grow-
ing there six alge and 26 vascular plants, including 11
ferns and 15 spermatophytes. A little over ten years
after Treub’s visit Penzig found 62 vascular plants, of
which 60 per cent. had been brought by ocean currents,
32 per cent. by wind, and 7 per cent. by fruit-eating birds.
Within twenty-five years from this eruption the island was
again green with forest growth and other vegetation, and
in 1906 a party of botanists confined their collecting to a
narrow zone of forest near the shore because of the diffi-
culty of “cutting a way through the dense growth of tall
grasses” between the shore and the volcanic cone in the
center of the island. Among the means of transportation
of plant life to Krakatoa, the wind is regarded by Ernst as
a factor of exceptional importance. Up to 1906, as cal-
culated by him, 39-72 per cent. of the total number of
phanerogams on the Krakatoa group were brought by ocean
currents. Ten torgper cent. of the entire flora by birds, and
16-30 per cent. by air-currents. Beccari found the same

Britton & Millspaugh. Bahama Flora., p. 51. Unpublished.

GEOGRAPHICAL DISTRIBUTION 147

species on widely separated mountain tops in the Malay
Archipelago where wind (particularly the west monsoon)
is the only agent of dispersal that may reasonably be as-
signed. The seeds of many plants are as light as dust
particles, and it has been calculated that nearly 850,000,-
ooo tons of dust are transported as far as 1,440 miles a
year in the western United States.! In the light of this
information it is not difficult to understand how seeds of
Nepenthes phyllamorpha, that weight only 0.000035 gram,
seeds of Rhododendron verticillatum and of Dendrobium
attenuatum, that weigh 0.000028 gram and 0.00000565
gram respectively, can be transported many miles, re-
sulting in a geographical distribution of those (and various
other) species, on the mountain tops of oceanic islands that
are miles apart.

James Small has carried outa series of painstaking
experiments on the transportation of the seeds of various
plants by artificially produced air currents. Among many
valuable results of these experiments, he determined that
for the seeds of the dandelion, ‘‘so long as the relative hu-
midity of the air remains above 0.77 per cent. and so long
as the fruit does not encounter an obstacle, a horizontal
wind of 1.97 miles per hour is sufficient for its dispersal to
anydistance. If theair becomes moist the pappus closes up
and the fruit falls rapidly.”” Small further concludes that
the ordinary pappose fruit of the Composite, under the
proper meteorological conditions, can be blown many hun-
dreds of miles over land and sea, and “that hypothetical
land bridges are not necessary to explain the present dis-
tribution of the Composite, so that we can take the world

1 Cited by James Small (New Phytologist 17: 226. 1918) from Evans, J.W.
The wearing down of rocks, Pt. II, Proc.Geol. Assoc. 25, Pt.4:229. 1914.

148 HEREDITY AND EVOLUTION IN PLANTS

as it is without raising and sinking continents, as Darwin
says, ‘in a quite reckless manner.’ This latter is an impor-
tant point, as the Composite are almost certainly of such
recent origin that the possibility of land bridges is in many
cases quite out of the question.”” In fact, Small contends
that a “rational study of the history of the Composite,
their migrations and colonizations, their paths of travel
and regions of concentration,’ is not possible without a
correct understanding of the conditions of wind dispersal.

The occurrence of a species of Senecio (a pappose-
fruited Composite) on the Falkland Islands, 300 miles
from the nearest land, and of another species on St.
Helena and on Prince’s Island, nearly 1500 miles from the
nearest land, are attributed by Small respectively to the
westerly and the south-east trade winds. The distri-
bution of the family, as worked out in detail by Small?
affords an instructive illustration of how geographical
distribution affords new evidence and confirms other
evidence as to the relative ages of various related groups
of plants, and as to the fact and course of evolution within
a given plant family. The immense genus, Senecio,
for example, according to Small, comprising over 2300
species, is of very wide distribution, being marked by a
concentration at high altitudes, which is not surprising in a
wind-distributed group.?. Some of the species are wide-
spread, and some are local, and the group is characterized
by its ready response to the influence of environment;
to this is attributed, in large part, its great morphological
variation. No species covers the range of the genus.

1§mall, James. The origin and development of the Composite.
Chapter X. New Phytologist 18: 1-35. Jan. and Feb., 1919.

? It has been calculated by Ball that 25-30 per cent. of the flora of the
higher Andes are Composites.

GEOGRAPHICAL DISTRIBUTION 149

Small has shown that the local species have regions of
concentration along the paths of migration of the wide-
spread species, and that they are most abundant “along
the ridge which extends around the Pacific and Indian
Oceans from Tierra del Fuego to South Africa.” The
paths of migration are chiefly coextensive with the alti-
tude of 3,000 ft., or higher, and all the facts point to the
Andes of Bolivia as the probable (hypothetical) center of
distribution for the genus, whence it has rapidly spread
“‘along the unwooded regions of the mountain ranges of the
world.” This world-wide distribution, and the posses-
sion of pappose fruits which would make possible a wide
distribution in a relatively short period of time (geologi-
cally speaking), all point (as do the facts of its morphology)
to the comparative youth of the group; while its marked
tendency to variation, its success in the struggle for exis-
tence (as may be noted everywhere), and finally the
existence of innumerable local species, with centers of
distribution along the paths of migration of the genus as a
whole, are just the facts which one would expect to find
on the basis of the theory of evolution.

115. Dispersal by Water and Birds.—Space at our
disposal will permit of only a passing reference to seed-
dispersal by water and birds. In order to be carried for
long distances by water, seeds and spores must"be able to
undergo prolonged soaking in water, and in the case of
ocean currents, in salt water. Many species of the new
strand flora of Krakatoa were certainly brought many
miles by ocean currents, and Guppy, who made a study
of ‘Plants, seeds, and currents in West Indies and Azores,”
cites the case of a ragweed (Ambrosia crithmifolia) whose

1 Guppy, H. B. London, 1917.

150 HEREDITY AND EVOLUTION IN PLANTS

seeds were dispersed on floating logs; and the small seeds
of several species were safely transported in the crevices
and holes made in small stems and branches by worms and
molluscs. Other seeds were floated on blocks of pumice.

In Hawaii, while nearly 85 per cent. of the spermato-
phyte flora is endemic (see p. 165), about 70 per cent.
of the species of the coastal zone are introduced. This
is in marked contrast to the general rule for oceanic
islands, whose littoral floras, as might be expected, are
predominantly cosmopolitan. In this particular case
MacCaughey attributes very great importance to ocean
currents as agents of dispersal. The natives of these
islands, at the time of their discovery, are reported to have
had large canoes hewn from tree trunks of the Douglas
spruce, which could have come only from the north-
west shores of North America; and considerable numbers
of tree trunks and large branches are brought from the
same coasts to Hawaii each year. Ocean currents also
bring annually large quantities of plant material to the
coast of other oceanic islands. Tansly and Fritsch have
noted large numbers of young seedlings and germinating
seeds in drift material on the coast of Ceylon, and Moseley
observed many living plants in the coastal drift of the
Moluccas, including trees, palms, epiphytic orchids, and
large quantities of fruits containing viable seeds.

Seeds are carried by birds in mud adhering to their
feet, lodged in their feathers, and in the alimentary canal.
In mud adhering to the feet of a partridge Darwin found
82 seeds that germinated. Wallace is authority for the
statement that ‘all the trees and shrubs in the Azores
bear berries or small fruits which are eaten by birds;
while all those which bear larger fruits, or are eaten

GEOGRAPHICAL DISTRIBUTION ISI

chiefly by mammals—such as oaks, beeches, hazels,
crabs, etc.—are entirely wanting.! It has been suggested
by both Guppy and Schimper that the wide distribution
of fig trees in oceanic islands (e.g. Malay and Solomon
Archipelagos) is due to their fruit being eaten by doves
and other birds capable of sustained flight. The pro-
digious powers of flight of some of the migratory birds
would make them, theoretically at least, most efficient
agents of seed dessemination over wide areas. The scarlet
tanager, for example, breeds in the eastern United States
from Oklahoma to the mountains of North Carolina, and
north to New Brunswick and Saskatchewan. At the
close of summer the birds migrate south, passing from
the Gulf coast of the United States to and along Central
America to the west tropical coast of South America.
The arctic tern nests during the northern summer along the
northeast arctic coast of North America and the southwest
coast of Greenland, but passes the northern winter within
the Antarctic Circle, 11,000 miles away. Passing as it
does through regions of similar climate in the northern
and southern hemispheres, it would theoretically be
possible for spores and light seeds to be carried to con-
genial habitats on both sides of the equator. The
American golden plover breeds in summer along the
northern coast of Canada, the parallel of 70° north latitude
passing approximately through the center of its breeding
range. In early fall the birds migrate to Labrador,
thence to Nova Scotia, and thence, after a few weeks,
in a straight flight of 2,400 miles to the north coast of
South America. From their landing point the birds
pass to Argentina where they pass the northern winter,
1Wallace, A. R., Darwinism, 3d edition, p. 361.

152 HEREDITY AND EVOLUTION IN PLANTS

returning the following spring to the Arctic coast, but
by an entirely different rout, passing over Central America,
the Gulf of Mexico, and across central North America.

The Pacific golden plover nests along the arctic shore
of Eastern Siberia and the western coast of Alaska,
but winters in southeastern Asia, eastern Australia
and generally in the islands of Oceanica, the winter home
having an east-west range of about 10,000 miles. The
journey from Alaska to Hawaii, a distance of some 3,000
miles, is made in a single flight.1| Whether seed dispersal
is actually accomplished by any of the above long distance
travelers is not definitely known to the writer; but the
flights are accomplished in a comparatively brief period,
and it seems not unreasonable, from what we actually
know of seed-transportation by birds, that lighter and
more resistant seeds and spores of plants may be thus,
transported, concealed in the plumage of the birds, or
otherwise, and between stations where no other known
agent of dispersal would appear to be adequate.

In 1911 a violent eruption of the Taal Volcano, on
Volcano Island, Luzon, Philippine Islands, ‘‘ annihilated”
(as Maso described it) every vestage of vegetation on the
island. The destruction was caused by superheated
steam, and by the deposit of a layer of fine “mud,”
which fell like rain, and carried with it large quantities
of sulphur dioxide and possibly other substances fatal
to plant life. In a study of the revegetation of the island,
made six years after the eruption, Brown, Merrill, and
Yates found evidence that birds were the most important

1 For the above data on bird migrations the author is indebted to the
article on “Our greatest travelers,” by Wells W. Cooke. Nat. Geographic
Mag. 22 : 346-365. April, rorr.

GEOGRAPHICAL DISTRIBUTION 153

agents of transportation of seeds for the new growth,
about 54 per cent. of a total of 157 species having seeds
adapted to dispersal by birds, only 21 per cent. adapted
to wind dispersal, and only 9 per cent. apparently brought
by currents of water. Of course, the distance from
Volcano Island to the nearest uninjured vegetation was
short, and the various agents of dispersal would, no doubt,
assume a different relative importance for greater dis-
tances, as they did, for example, in the case of Krakatoa,
noted above.?

116. Struggle for Existence a Factor.—DeCandolle
early contended that it was not sufficient that one or
even a few seeds be carried to a country already well
covered with vegetation in order for the new arrivals to
become established, but that a very large number of
vigorous seeds must be introduced to insure success in
the struggle for existence with the native plants. Atten-
tion has been called in a preceding chapter (pp. 94-96),
however, to the enormous rate of propagation of plants
and animals, which proceeds in geometrical progression;
so that if we allow a sufficient time period, and postulate
a species suited to the climate and soil of its newly found
home, we may expect a large degree of success in its be-

1In contradiction to the above statements of fact and logical inference,
there should be noted here Warming’s quotation (Botany of the Faeroes),
p. 676. London, 1901-1908) from the Danish Zoologist, H. Winge, of the
Zoological Museum, Copenhagen, who stated in a letter to Warming that
he had carefully examined thousands of migratory birds picked up dead at
Danish lighthouses, and had never found any seeds adhering to the feathers
beaks, or feet. Dried mud was found “‘fairly often,” but there were ad-
hering to it no seeds large enough to be seen with the naked eye or the
hand-lens. Moreover the stomachs of migrating birds were always found

to be practically empty, indicating that migrating birds travel on empty
stomachs. See, however, p. 164, infra.

154 HEREDITY AND EVOLUTION IN PLANTS

coming established, at least in limited area and numbers,
even though the number of seeds introduced was compara-
tively small. Farmers in America can bear emphatic
but sad testimony to their practical helplessness to com-
bat successfully the spread through the hay fields of the
hated daisy or white-weed (Chrysanthemum leucanthemum),
or the still more dreaded Devil’s paint brush (Hieracium
aurantiacum). Both of these species are now common
weeds in America, though introduced from Europe, the
former almost, and the latter quite within the memory of
men now living. Wallace has stated that, if a million
seeds were brought to the British Isles by wind in one year,
there would be only ten seeds to a square mile. ‘‘The
observation of a life time might never detect one, yet a
hundredth part of this number would serve in a few cen-
turies to stock an island like Britain with a great variety
of continental plants.” When we recall the enormous
mortality of seeds and seedlings, such facts as these enable
us to appreciate the importance to a species of an abundance
of spore and seed production, as, for example, in dandelions
and other composites, in ferns, and indeed in most plants.

117. Types of Distribution—There are two broad
types of geographical distribution; continuous, as in the
case of the bracken fern (Pieris acquilina); and discon-
tinuous, as in the case of the Osmunda family, where a
given species is found in widely separated localities, but
not in the intervening regions. Osmunda regalis (the
Royal Fern), for example, is known from eastern North
America, central and northern Asia, and Europe; Os-
munda Japonica from central and northern Asia and Japan
and the cinnamon fern (Osmunda cinnamomea) only from
eastern North America and Japan, The genus Diervilla,

GEOGRAPHICAL DISTRIBUTION 155

of the Honeysuckle family, is represented in the eastern
United States and Canada by the bush-honeysuckle
(Diervilla Lonicera), and in the mountains of the southern
States by D. sessilifolia and D. rivularis; it is not found
elsewhere except in eastern Asia, where it is represented
by the shrubs commonly cultivated in temperate America
under the name Weigela.

In the herbarium of the Brooklyn Botanic Garden are
two specimens of the cloud-berry, or mountain bramble
(Rubus chamemorus), collected in a bog near Montauk

Pa
Fic. 72-—Map showing the geographical distribution of the skunk-
cabbage, Symplocarpus fetidus. (After M. L. Fernald.)

Point, Long Island, by Dr. William C. Braislin, in 1908.
This is an arctic and sub-arctic bog plant, ranging from
Labrador and Newfoundland to New Hampshire, British
Columbia, and Alaska; also in Europe and Asia. It was
found on the Peary arctic expedition as far north as Lat.
64° 15’ north. Its discovery as noted above was unex-
pected, and affords an interesting example of discontinuity
of distribution. Another striking illustration is the “curly
grass” fern (Schizea pusilla), of the Polypodiacee, found in
Nova Scotia and Newfoundland, and in the pine barrens of
southern New Jersey, but not known to occur between

156 HEREDITY AND EVOLUTION IN PLANTS

these two regions. The skunk-cabbage (Symplocarpus
fetidus, Fig. 72), species of Magnolia, Hydrangea,
Hamamelis (witch-hazel), Liquidambar (sweet-gum), Ara-
lia (ginseng), Eupatorium, Onoclea (sensitive fern), Lyco-
podium (L. lucidulum), and scores if not hundreds of other
species, have a similar type of distribution.

Similarity of Floras of Eastern Asia and Eastern
North America.—The similarity in the floras of eastern
North America and eastern Asia and Japan was first
pointed out by Asa Gray,! in 1859, on the basis of his
study of the plants collected in Japan in 1855, by Charles
Wright, botanist of the U.S. North Pacific exploring ex-
pedition. Of 580 Japanese plants of this collection
Gray found only 0.37 per cent. having representatives in
western North America, while 0.61 per cent. has repre-
sentatives in eastern North America; for identical species
the corresponding percentages were 0.27 per cent. and 0.23
per cent. Of 56 Japanese species not known in Europe,
22 were known from eastern but not from western North
America. Exploration subsequent to the date of Gray’s
paper has altered our knowledge of the distribution of
many species in the region referred to, but the broad
fact pointed out by Gray has only been confirmed by more
careful investigation.

Several writers? have called attention to the fact that
various species of plants and of invertebrate animals are
confined to the west of Ireland and North America.

+ Gray, Asa. On the botany of Japan, and its relations to that of North
America, etc. Botanical Memoirs, extracted from Vol. VI (New Series) of
the Mem. Amer. Acad. Arts and Sciences. Boston and Cambridge, 25th
April, 1859.

2 E.g., Colgan, N., and R. W. Scully. Cybele Hibernica. 2d Ed. p.
71. Scharff, R. F. Proc. Royal Irish Acad. 28:13. Nov., 1909.

GEOGRAPHICAL DISTRIBUTION 157

Among the plants may be mentioned ladies’ tresses (5 pi-
ranthes Romanzofiana) and the seven-angled pipewort
(Eriocaulon septangulare); and among animals, the land
snail, Helix hortensis.

Fic. 73.—Asa Gray (1810-1888). Noted American botanist and student
of plant geography.

Of the various theories which have been advanced to
explain the occurrence of identical species on opposite
shores of the North Atlantic, Scharff enumerates the
following three.!

1. Migration from Europe across Asia and a Bering
Strait land-bridge to America, or vice versa.

1 Schartf, Robert Francis. On the evidences of a former land bridge be-
tween northern Europe and North America. Proc. Royal Irish Acad.
28B:1-28. Nov., 1909.

158 HEREDITY AND EVOLUTION IN PLANTS

2. Occasional transport by birds across the Atlantic
Ocean.

3. Migration across a direct Atlantic land-connection.
Human agency is generally rejected, except in cases where
it can positively be demonstrated.

In interpreting the above facts Scharff argues that ‘“‘the
interchange between the fauna and flora of north-western
Europe and north-eastern America was effected across
the northern land bridges,’’ which the facts of distribu-
tion and other evidence indicate existed in pre-Glacial
times, and probably in late Pliocene and early Pleistocene.

Numerous alpine species have a present discontinuous
distribution in the lowlands of arctic and sub-arctic lati-
tudes, and on lofty mountain peaks, widely separated,
in more southern latitudes. Darwin called attention to
the fact that ‘‘a list of the genera of plants collected on the
loftier peaks of Java raised a picture of a collection made
on a hillock of Europe;” and again that “‘certain plants
growing on the more lofty mountains of the tropics in
all parts of the world, and on the temperate plains of the
north and south, are the same species or varieties of the
same species.” A striking illustration of this latter fact
is the small white water-lily (Castalia tetragona), which is
found along the Misinaibi and Severn rivers in Ontario
(Canada), and at Granite Station, in northern Idaho
(U. S. A.), but is not known elsewhere except in Siberia
China, Japan, and the Himalaya mountains (Kashmir).

The flora near the summit of Mt. Washington and other
peaks of the White Mountains, in New Hampshire, has
elements in common with that of Labrador. ‘In ap-
proaching these mountain summits,” says Flint,! “one

1Vlint, William F. The distribution of plants in New Hampshire.
In Hitchcock, C. H. The Geology of New Hampshire, 1: 393. 1874.

GEOGRAPHICAL DISTRIBUTION 159

is struck by the appearance of the firs and spruces, which
gradually become more and more dwarfish, at length ris-
ing but a few feet from the ground, the branches spread-
ing out horizontally many feet and becoming thickly inter-
woven. These present a comparatively dense upper

i mae a ai ay

Fic. 74.—Lapland rhododendron (Rhododendron lapponicum). Photo-
graphed on the summit of Mt. Madison, New Hampshire, June 25, 1917,
by Ralph E. Cleland.

surface, which is often firm enough to walk upon. At
length these disappear wholly, and give place to the Lap-
land rhododendron (Fig. 74), Labrador tea, dwarf birch,
and alpine willows, all of which, after rising a few inches
above the ground, spread out over the surface of the

160 HEREDITY AND EVOLUTION IN PLANTS

nearest rock thereby gaining warmth, which enables them
to exist in spite of tempest and cold. These in their
turn give place to the Greenland sandwort, the diapensia
(Fig. 75), the cassiope, and others, with arctic rushes,
sedges, and lichens, which flourish on the very summit.””!

According to Flint, there are about fifty strictly alpine
species on these summits, found nowhere else in New
England and New York, except on similar summits, such
as Mt. Katahdin in Maine, and Mt. Marcy and Mt.
McIntyre in New York State.

Incidentally, it may be remarked that a similar state-
ment may be made for the animal life. Writing of the
insects, Scudder says? that, ‘‘in ascending Mt. Washing-
ton, we pass, as it were, from New Hampshire to northern
Labrador; on leaving the forests we first come upon animals
recalling those of the northern shores of the Gulf of St.
Lawrence and the coast of Labrador opposite Newfound-
land; and when we have attained the summit, we find
insects which represent the fauna of Atlantic Labrador
and the southern extremity of Greenland.”

118. Effects of Continental Glaciation—The above
mentioned and other similar cases of discontinuity are
satisfactorily explained by the advance and retreat of the

1 Among numerous species that have been recorded from both Labra-
dor and the peaks of the White Mountains, there may be mentioned the
following: Salix argyrocarpa, S, phylicifolia, S. herbacea, S. uva-ursi,
Comandra livida, Arenaria groenlandica, Silene acaulis, Oxyria digyna,
Cardamine bellidifolia laxa, Saxifraga rivularis, Sibbaldia procumbens,
Empetrum nigrum, Epilobium Hornemannii, Loiseleuria procumbens,
Rhododendron Lapponicum, Phyllodoce coerulea, Cassiope hypnoides,
Arctostapphylos alpina, Vaccinium cespitosum, Diapensia Lapponica,
Veronica alpina var. unalaschensis, Guaphalium supinum.

? Scudder, Samuel H. Distribution of insects in New Hampshire. In
Hitchcock, C. H. The geology of New Hampshire, 1:341. 1874.

GEOCRAPHICAL DISTRIBUTION 161

continental glacier during the Ice Age. With the advance
of the ice all vegetation was either exterminated or com-
pelled to migrate southward. With the subsequent
retreat of the ice northward the glaciated region was
gradually re-occupied by the encroachment of vegetation
from the south, and of this flora the arctic species could
become permanently re-established only in what are now
the arctic regions, and in the arctic or sub-arctic climate
of the higher mountain tops, forming there what is known
as a relict flora.' It has been suggested that, in theory,
alpine plants on high mountain peaks south of the region
covered by the continental ice sheet, should not be related
to arctic and sub-arctic forms. In harmony with this
idea Wallace has cited the volcanic Peak of Teneriffe
(Pico de Teyde), in the Canary Islands, 12,000 feet high,
where, above the timber line, von Buch found only eleven
species of plants, eight of which appeared to be endemics;
but all of them were related to the plants of the same
general region, growing at lower levels.

However, seed-distribution by birds and winds and
other agencies has been going on continually since the
continental ice sheet began to recede, with the result
that arctic-alpine and subarctic-alpine plants are numer-
ous in the alpine zone of higher peaks below the southern
limits of continental glaciation. Thus the snowy cinque-
foil (Potentilla nivea) is found, not only throughout the
arctic regions, but also in the Alps, in alpine Asia, and
in the Rocky Mountains as far south as Utah and Colo-

1 The effect of continental glaciation on the distribution of plants was
first noted by Edward Forbes, but was also worked out independently by
Darwin several years previous to the publication of Forbes’s paper.
(Darwin, C. Life and Letters, 1 :71-72, 372. New York, 1901. See also
The Origin of Species, 2: 152. New York, 1902.)

Il

162 HEREDITY AND EVOLUTION IN PLANTS

rado. According to Rydberg! there is evidence that it
has spread not only in the earlier postglacial period, but
also in recent years. The common arctic and sub-arctic
grass, Phleum alpinum, occurs as far south as Arizona and

Fic. 75.—Diapensia lapponica. Photographed on the summit of Mt.
Madison, New Hampshire, June 25, 1917, by Ralph E, Cleland.

California and the Sierra Madre of Mexico, and also in
Patagonia. It is also found throughout the Montane
zone, from which it might have spread to the subalpine,
following the woods throughout the whole mountain
system. Other similar cases might be cited.

1Letter from Dr. P. A. Rydberg to the author.

GEOGRAPHICAL DISTRIBUTION i 63

119. Escapes from Cultivation—Every case of discon-
tinuous distribution must be carefully analyzed by itself,
and care must be taken not to adopt unwarranted con-
clusions. Thus, certain cases of discontinuity are ex-
plained by the escape from cultivation of forms introduced
by human agency for economic uses, and thus have no
scientific significance. The presence in the Hawaiian
Islands of such economic plants as sugar-cane, cocoanut,
and others is an apparent case of discontinuity, but these
plants are known to have been introduced there by man,
and to have escaped from cultivation. Campbell thinks
that the candle-nut tree (Aleurites moluccana, the source
of a commercial oil) and the mountain apple (Eugenia
malaccensis), which now constitute the chief elements in
the lowland forests of Hawaii, were also introduced by
man, and are therefore only apparent cases of disconti-
nuity. Among numerous illustrations of this in North
America may be mentioned the paper mulberry (Brousone-
tia papyrifera), white mulberry (Morus alba), hemp,
(Cannabis sativa), stinging nettle (Urtica dioica), the day
lily (Hemerocallis fulva), all natives of Europe and Asia,
and the tree, Paulownia, a Japanese species now becoming
established as an escape from cultivation in New York,
New Jersey, the District of Columbia, and Georgia. The
last two species were introduced into North America as
ornamental plants, the hemp and white mulberry, of
course, as economic plants, the latter in connection with
the raising of silk worms.

Attention has recently been called to the wide and rapid
spread of the Japanese honeysuckle (Lonicera japonica)
introduced in the Eastern United States from Asia.
Twenty-five or thirty years ago this was a comparatively

164 HEREDITY AND EVOLUTION IN PLANTS

rare cultivated vine, but since that time, according to
Miss Andrews,’ ‘‘it has spread over practically the whole of
the Eastern States, from the Gulf of Mexico to the estuary
of the Hudson, making itself equally at home in the low
hammocks of the Coastal Plain, on the old red hills of the
Piedmont region, on the stony ramparts of the Lookout
Plateau, and onward for a thousand miles up the great
Appalachian Valley.” The adaptability of the plant, as
indicated by this description of its habitats, in no doubt a
large factor in its rapid spread, for while it is a profuse
bloomer under cultivation, it tends to become weedy, as
it grows wild, blossoming rarely and therefore setting few
seeds. But its wide distribution must have been accom-
plished by the dissemination of its seeds, and in this Miss
Andrews believes that the most probable agents are birds,
to whose feet the small, inconspicuous nutlets, ‘‘embedded
in a mucilaginous pulp,” readily adhere.

Several species (e.g., the fleabane, Pluchea fetida)
are found in shallow fresh water or fresh or salt water
marshes from southern New Jersey to Florida, and then
across 120 miles of salt water in Cuba. In this case it
seems clearly evident that the seeds have been able to
undergo transportation across the Florida strait within
comparatively recent times. Examples might be multi-
plied, and in such cases discontinuous distribution has
little evolutionary significance for the particular species
concerned, though the facts may serve to throw light upon
other cases that are significant.

120. Endemism.—On the basis of the evolution theory
every species originated in some one area (its center of dis-

1 Andrews, E. F. The Japanese honeysuckle in the Eastern United
States. Torreya, 19 : 37-43. Mch. toro.

GEOGRAPHICAL DISTRIBUTION 165

tribution), where it was at first endemic,! and whence it
gradually spread as far asit could. This is well illustrated
in the distribution of the Verbenacee, one of the higher and
therefore more recent families of flowering plants, com-
prising about 75 genera and 1300 species, occurring
widely throughout tropical and temperate regions. Of
104 species belonging to various genera in the Philippines,
60 per cent., according to Lam,? are apparently endemic.
These endemic forms have undoubtedly been derived from
the 39 non-endemic species, and will, in the course of time,
spread from the Philippines to neighboring islands and
thence to the mainland. About 85 per cent. of the
flora of Hawaii is endemic,*? and even the strand flora,
while cosmopolitan on the whole (the general rule for
coastal vegetation), is nearly 40 per cent. endemic, a
surprisingly high percentage.

From the facts of geographical and geological distri-
bution, Wallace deduced the following law:* Every
species has come into existence coincident both in time and
space with a pre-existing closely allied species.” ‘The law
here enunciated,” said Wallace, “‘not merely explains
but necessitates the facts we see to exist, while the vast and
long-continued geological changes of the earth readily
account for the exceptions and apparent discrepancies
that here and there occur.” And again, ‘“‘this law agrees

1 Endemic: found in a given region, but not elsewhere.

2Lam, H. J. The verbenacez of the Malayan Archipelago. Gronin-
gen, 1919.

3 Including, for example, all the native Hawaiian palms, belonging to the
genus Pritchardia. See MacCaughey, Vaughan. Bull. Torrey Bot.
Club, 45: 259-277. July, 1918, and Plant World 21: 317-328. Dec., 1918.

4 Wallace, Alfred Russel. On the law which has regulated the introduc-
tion of new species. Annals and Mag. of Nat. Hist. 16, Ser.2: 184-196.

Sept. 1855.

166 HEREDITY AND EVOLUTION IN PLANTS

with, explains and illustrates all the facts connected with
the following branches of the subject: rst, the system of
natural affinities; 2d, the distribution of animals and
plants in space; 3d, the same in time . . . 4th, the
phenomena of rudimentary organs.” And Wallace goes
on to show, in detail the bearing of the law upon each of
the four points enumerated.

Fic. 76.—Alfred Russel Wallace (1823-1913). Co-discoverer, with
Darwin, of the principle of natural selection. Noted student of geo-
phical distribution.

A quotation from Darwin is also pertinent here: “It
is . . . obvious,” said Darwin, ‘that the individuals of
the same species, though now inhabiting distant and
isolated regions, must have proceeded from one spot, where
their parents were first produced for, as has been explained,
it is incredible that individuals identically the same should
have been produced from parents specifically distinct.”

121. Mutation and Discontinuous Distribution —Read-
ing Darwin’s statement in the light of the mutation theory

GEOGRAPHICAL DISTRIBUTION 16 7

of de Vries, we must of course recognize that, if a mutating
species were widely distributed, different individuals of the
species in widely separated localities and even with a dis-
continuous distribution, might throw the same mutants.
Cnothera Lamarckiana, for example, threw the same ele-
mentary species (mutants) in experimental pedigree
cultures in Holland and in various localities in the United
States.!. Had O. Lamarckiana (contrary to fact) been
widely distributed in nature, such mutants as O. gigas,
O. scintillans, O. levifolia, and others would possibly (or
even probably) have appeared in different and widely sepa-
rated stations, and these elementary species might con-
ceivably (and not improbably) have become established as
true species of the systematist. When, therefore, we
find a given species (or a larger group) in widely separated
localities, but not in the intervening regions, we must
(barring the phenomenon of mutation referred to above)
conclude, either that it has been able to migrate across
barriers where it could not become established (as when
seeds of land plants are carried by ocean currents across
barriers of salt water), or else it has formerly had a con-
tinuous distribution, but has subsequently died out in
regions between its present localities; in the latter case it is
referred to as a relict endemic. When these localities are
distant hundreds or, as is often the case, thousands of miles
from each other, one can readily understand that species
having such discontinuity of distribution must, other
things being equal, be older than species having continuity
of distribution; they must have existed long enough for the
changes above mentioned to have taken place.

This principle is confirmed by the evidence of fossils.
A striking case is that (cited by Chodat) of Zelkowa,

1See pp. 114-117,

168 HEREDITY AND EVOLUTION IN PLANTS

related to our modern elms. This genus comprises
only four living species, which occur in only three widely
separated areas, namely, the far East (Eastern China
and Japan), the area between the Black and the Caspian
Seas (Caucasia), and islands in the eastern Mediterranean
Sea. But a study of the fossil evidence shows that during
a preceding geological age this genus had a very extended
distribution, including central Europe, the Iberian penin-
sula, Iceland, southeast Greenland, Labrador, western
North America, and Alaska. Owing to profound changes
of climate, in the transition from one geological age to
another, Zelkowa was apparently unable to survive,
except in the two restricted and widely separated areas
where it is now found.

122. Continuous Distribution.—Continuous distribution
is of two types: ubiquitous, like the bracken fern, and
isolated, like the redwoods, Sequoia. In the latter case
two suppositions are possible: either the species or genus
is very new and has not had time to spread (indigenous
endemic) ; or it is very old and a relict endemic, as defined
above. Which of these two alternatives is correct for
any given case may be ascertained only on the basis
of comparative anatomic evidence, or on fossil evidence,
or on both.

The motile sperms and the structure of the wood of the
maiden-hair tree (Ginkgo biloba), for example, point
without question to affinities with an older type of seed-
bearing plants, the Cycads. In the case of the genus
Sequoia, with only two living species, the coast redwood
(S. sempervirens) and the giant redwood (S. gigantea),
restricted in range to one state, California, the fossil
evidence shows that these two species are the meager

GEOGRAPHICAL DISTRIBUTION 169

remains (relict endemics) of a genus of several species,
which, in Tertiary times, was widespread over most of
the northern hemisphere (Fig. 77).

By a like balancing of evidence we are able to ascertain
that the ubiquitous fern family, Polypodiacee, with
some 200 genera and about 3,000 species, is a com-
paratively modern group, while the Osmunda family,

spear ee
Fic. 77.—Map showing the known geographical distribution of Se-
quoia during the Cenozoic era. The cross indicates the only known loca-

tion of living specimens. (After E. W. Berry.)

with only two (or possibly three) living genera and some
ten species, and with wide but discontinuous distribution,
is much older. The greater antiquity indicated for the
Osmundacez by the facts of their geographical distribu-
tion is also attested by fossil evidence, and further by the
nature of their spores. The spores when mature contain
chlorophyll, and this fact, of itself, indicates antiquity;
for this and other structural and physiological reasons,

170 HEREDITY AND EVOLUTION IN PLANTS

they quickly perish unless they find at once suitable
conditions for germination and development. Thus they
could not spread rapidly over large areas. In the light
of these facts the only logical inference is that their
wide and discontinuous distribution must have required
a vast period of time. The tulip tree, represented now
by only one genus (Liriodendron) and one or possibly

Fic. 78.—Map showing the known geographical distribution of the
bald cypress (Taxodium) in the Tertiary and Pleistocene. Tertiary dis-
tribution, shaded; Pleistocene occurrences north of its present limits, in
dots; present distribution, black. (From Shimer, after E. W. Berry.)

two species, and with discontinuous distribution (Eastern
North America and China), represents an old type now,
perhaps, on the way to extinction. A similar statement
may be made for Sassafras, for the bald cypress (Taxodium,
Fig. 78), and numerous other groups.

In general it may besaid that groups considered relatively
more primitive or ancient on morphological or paleonto-

GEOGRAPHICAL DISTRIBUTION I71I

logical grounds, are characterized by few genera and a
restricted or (if wide) discontinuous distribution. Thus
the Barberry family, one of the relatively primitive
groups of dicotyledons, contains only about 10 genera
and over 130 species, found in temperate North America
and Asia, temperate South America, and sparingly in the
tropics; the Nympheacee (Water lily family), more
primitive than the Berberidacee, contains only eight
genera and about 50 species, of wide but discontinuous
distribution. In contrast there may be mentioned the
gamopetalous Potato family (Solanacez), with about
70 genera and 1,600 species, found generally on every
continent, and in New Zealand, Hawaii, Australasia,
and other oceanic and continental islands, and specially
abundant in the tropics; and also the still more highly
developed Madder family (Rubiacee), with as many as 355
genera and 5,500 species, also of almost cosmopolitan
distribution. As a final example among families of
flowering plants, there may be mentioned the Orchidacez,
the most highly developed of the Monocotyledons, and, on
morphological grounds, possibly the most recent family
of seed-bearing plants. This family contains about 430
genera and over 5,000 species, of almost cosmopolitan
distribution, most abundant in the tropics, and gradually
diminishing toward the poles. The seeds of orchids are
very tiny, and the embryo consists of a few undifferenti-
ated cells. They are capable of rapid and wide
distribution (Fig. 78a).

In the Nympheacez is the relatively primitive genus,
Nelumbo, containing only two species, one the lotus
(N. lutea), in North America, the other the Oriental
lotus (N. nucifera), in Asia and Australasia. In the

172 HEREDITY AND EVOLUTION IN PLANTS

Rubiacez is the genus Mitchella, also relatively primitive,
and containing only two species, one in Japan, the other
the Partridge berry (M. repens) in North America.

123. Evidence from the Distribution of Liverworts.—
The geographical distribution of the lower cryptogams
(below the ferns and their allies) has not been the subject
of as extensive study as that of the ferns and flowering
plants, but the evidence marshalled by Campbell! in 1907,
concerning the distribution of the liverworts (Hepatice),
illustrates in a striking manner the importance of the

Fic. 78a¢.—Seed capsule and seeds of an orchid.

facts of geographical distribution in endeavoring to
determine the question of the relative age of a group of
plants. It had been argued by Scott, in 1906, that the
liverworts were probably of comparatively recent origin
because of the almost entire absence of fossil remains in the
Paleozoic rocks. But, as Scott himself records, impres-
sions have been described from Paleozoic strata of plant
forms that can be assigned only to the Hepatic, and
indeed to one of the most highly organized groups—the

‘Campbell, Douglas Houghton. On the distribution of the Heptatice,
and its significance. New Phytologist 6: 203-212. Oct. 1907.

GEOGRAPHICAL DISTRIBUTION 173

Marchantiacee. This, of course, means a long period
of evolutionary development from similar forms to the
more complex, preceding the geological age of the rocks
containing the fossil record, and one such bit of positive
evidence fully substantiated, is of itself sufficient to
establish the antiquity of the liverworts. Moreover,
when such testimony is in agreement with the evidence
derived from other sources, such as comparative mor-
phology and geographical distribution, the fact of anti-
quity would seem to be reasonably well established. Now,
in addition to the evidence of comparative morphology,
there are, as Campbell points out, certain facts of dis-
tribution that can only be satisfactorily interpreted on
the basis of the comparative antiquity of the liverworts.’
The liverworts are a widely distributed group; some of the
genera are cosmopolitan, i.e., they are found practically
everywhere, in all continents, climates, and habitats,
and widely on oceanic islands. Riccia and Marchaniia
are cosmopolitan genera of continuous distribution.
Other genera are of wide, but discontinuous distribution,
such, for example, as Targionia, a genus containing only
two species, which are found in Southern and Western
Europe, Africa, Java, Australia, and Western America,
but are absent from Eastern America and from most of
Asia. The familiar Lunularia cruciata of our greenhouses
has a distribution similar to Targionia in the eastern
hemisphere, but is unknown in the western hemisphere
except where introduced.

1 Throughout the discussion of liverworts I have drawn freely on Camp-
bell’s article, cited above, and have, to a certain extent, adopted his
wording, asking the reader and the author quoted to accept this statement
in lieu of frequent quotes.

174 HEREDITY AND EVOLUTION IN PLANTS

A third type of distribution is that of limited range,
such as has been mentioned above for the venus’s fly-
trap and the giant redwood trees. Among genera thus
distributed are Wiesnerella Javanica Schiff., known at
present only from Mt. Gedeh, in Java, and Geothallus
tuberosus Campbell, known only from near San Diego,
California. These ranges may ultimately be extended, as
was that of Treubia insignis, known for a time only from
Mt. Gedeh, but later found by its original discoverer
in New Zealand.

As already noted, in order to become widely distributed,
either continuously or discontinuously, a plant must either

1. Have reproductive bodies capable of rapid distribu-
tion over wide areas, or

2. Possess sufficient antiquity to have been in process
of dissemination for a comparatively long period of time.
Tn the former case, its reproductive bodies must be of such
nature as to resist unfavorable environment and vicissi-
tudes, during transit over long distances, and be able to
establish themselves readily in the new habitat, especially
in competition with the plants already established, and pos-
sibly also in an unfavorableenvironment. Now the spores

_of many of the most widely distributed Hepatice are not
of thisnature.. Wecan hardly explain the present distribu-
tion of such widespread tropical genera as Dendroceros,
Monoclea, and Dumortiera, says Campbell, by the theory
that their spores could be carried across the wide ocean
barriers that separate the regions where they now occur,
as the spores are not of the type that could be carried long
distances without perishing. Since there are no connecting
forms in the higher latitudes that could explain the passage
of these forms from one tropical zone to the other, we can

GEOGRAPHICAL DISTRIBUTION 175

only assume that these genera are the little changed
descendants of ancient, widely distributed types.

Although making a special search for Liverworts on
Krakatoa in 1906, Campbell found no specimens, nor up
to that time had any other collector. Professor Treub,
of the Botanic Garden at Buitenzorg, Java, had reported
two species of mosses. ‘‘Inasmuch as Krakatoa is
within sight of Java and Sumatra, both of which have
an extremely rich hepatic flora, the absence of these
plants from the new flora of Krakatoa is, to say the least,
worthy of note.” In a similar way Campbell argues
that the wide distribution of mosses (cosmopolitan in
the case of the genus Sphagnum), combined with the
inability of their reproductive bodies to withstand trans-
portation over great distances, indicates a great antiquity
for the group; and this inference is substantiated by the
meager but positive evidence of fossil remains.

In a later discussion of the origin of the Hawaiian
flora, Campbell! notes that the filmy ferns, since they are
hydrophytic with a rain-forest habit, and are, therefore,
not suited to transportation over wide stretches of ocean,
must have existed in Hawaii since those islands were
connected with some mainland, now submerged. The
relatively shallow water between Hawaii and the
Australasian-Malaysian regions, as compared to the
great depths between Hawaii and North America, in-
dicate a former mainland connection to the west, and
this inference is further substantiated by the great pre-
ponderance of Australasian-Malaysian plants in Hawaii
over those represented in America. In this connection

1Campbell, D. H. The origin of the Hawaiian flora. Mem. Torrey
Bot. Club, 17: 90-96. June, 1918.

176 TEREDITY AND EVOLUTION IN PLANTS

is should be noted that a considerable proportion of the
species of the strand vegetation of Hawaii are endemic,
but many of the introduced littorals are known to be
transported by ocean currents from the north Pacific.’

124. Distribution of Algze—And finally, to bring all the
great phyla under brief review, it may be mentioned that
facts of distribution of the Alge point to a great antiquity
for the group. This is not only in harmony with the
generally accepted evidence from comparative morphology,
but is substantiated by fossil remains, in early Paleozoic
rocks, of calcareous Siphonogamous forms related to liv-
ing calcareous forms. The absence of fossil remains of
non-calcareous green forms is readily explained by the
delicate nature of their tissues.

125. Hypothesis of ‘‘Age and Area.””—As noted above
(p. 165), an endemic species is one found in a given local-
ity but not elsewhere. According to some botanists?
endemism is a criterion of youth. The area occupied
by a species within a given country, argues Willis, varies
directly with its age within that country; that is, the longer
it has been a part of the flora, the wider the area it occupies,
so long as conditions remain constant. But Willis enumer-
ates various conditions that would interfere with the
operation of this law, including ‘“‘chance”’ (i.e., causes not
understood), the action of man (clearing of forests,* etc.),

1 Twenty-one littorals and eleven pseudo-littorals, out of a total of over
75, are listed as endemic by Vaughan MacCaughey. Bull. Torrey Bot.
Club, 45: 259-277. July, 1918.

2 Willis, J. C. The relative age of endemic species and other controver-
sial points. Ann. Bot. 31:189-208. April, 1917. James Small (see p.
148) has characterized Willis’s Age and Area hypothesis, as the most im-
portant contribution to geographical botany since the Origin of Species.

3 Macrozamia Moorei is being systematically exterminated in Australia
because it is poisonous to cattle.

GEOGRAPHICAL DISTRIBUTION 177

interposition of barriers (mountains, broad deserts, salt
water areas, sudden changes of climate from one district to
the next, geological changes, natural selection, local adapt-
ation (the possession of acharacter usefulin one country but
not in another), the dying out of occasional old species, the
arrival of a migrating species at its climate limit, et cetera.
But on the whole the endemic species, says Willis, are the
youngest. As an illustration of the operation of the hy-
pothesis of age and area, Small (/.c., p. 25-30) mentions
numerous Composite which have limited distribution,
although there would seem to be practically no limit to the
distance their pappose seeds can be transported by wind.
They are limited (endemic) because they are young.
According to another view,! endemic species are the
oldest species of a region; they are either relicts, and thus
very ancient, or they represent types which have been in
the region so long that their original characters have been
lost. The latter are indigenes, and are spoken of as
indigenous to the country. Endemics, according to
Sinnott, contain a greater percentage of trees than do
wides (or polydemics)? but, according to the same author,
trees and shrubs are older than herbs, and therefore the
endemic woody species must be older than the herbaceous
element of agivenflora. The hypothesis of Willis demands
that herbs be considered as an older form of vegetation
than trees and shrubs, which, others argue, is contrary to
amass of evidence. Trees are more common as endemics
(in Ceylon, e.g., twice as common), notwithstanding the
fact that they spread less rapidly than herbs. After its

1Sinnott, Edmund W. The “age and area” hypothesis and the
problem of endemism. Amn. Bot. 31:209-216. April, 1917.
2“ Wides” and “polydemics” are used as antonyms of endemics.
12

178 HEREDITY AND EVOLUTION IN PLANTS

first rapid spread, says Sinnott, a species becomes less
common the older its age of occupation.

Reviewing these two theories, Taylor! holds that, in the
flora of the vicinity of New York at least, endemism is not
a criterion of antiquity nor of youth, for while many
endemics of the flora of New York and vicinity are very
recent (as the hypothesis of Willis would require), and
while some of them are even found in the geologically
recent portion of the area (one, Hibiscus occuliroseus,
being a salt marsh plant and therefore very ‘new’),
other forms are relict endemics (p. 167), and could not,
therefore, be of very recent origin.?

As an example of relict (and therefore old) endemics
(outside the local flora region of New York, there may be
cited the well known case of the giant and coast redwoods
(Sequoia gigantea and S. sempervirens), and the begonia,
Hilldebrandia sandwicensis, endemic in Hawaii.®

An example of an indigenous (and therefore relatively
recent) endemic, is the well-known insectivorous plant,
Venus fly-trap (Dionea muscipula), a genus having only
one species, 7.e., monotypic (Fig. 79). This unique plant is
found in sandy swamps, only in a narrow strip of country

1 Taylor, N. Endemism in the flora of the vicinity of New York.
Torreya 16:18-27. Jan. 1916.

* Five cases of apparently relict endemism are cited by Taylor from the
vicinity of New York. Torreya 16: 18-27. Jan. 1916.

* The Begoniacee have scarcely any representatives in the islands of
the southern, equatorial, and Northern Pacific, but are abundant in the
Andes region of South America and Mexico. The endemic begonia of
Hawaii is regarded by MacCaughey (Bot. Gas. 66:273-275. Sept. 1918)
as one of several bits of evidence that “at one time in the history of the
Pacific basin the Hawaiian islands were much more closely associated with
the Andean and South Pacific regions than they are at present. See also

p. 175.

GEOGRAPHICAL DISTRIBUTION 179

about ten miles wide and extending about 4o miles south of
Wilmington, North Carolina. The yellow waterlily (Nym-
phea mexicana Zuccarini)! may also be cited as an aquatic

Fic. 79.—Venus fly trap (Dionea musci pula).

example of an indigenous endemic, being known only from
Florida, Texas, and Mexico.”

1 Castalia flava Greene (1888).
2 Conard, Henry S. The waterlilies, p. 167 and 213. Carnegie Insti-
tution of Washington, Publication No. 4. 1905.

180 HEREDITY AND EVOLUTION IN PLANTS

Again, as Taylor points out, most of the recent endem-
ics in the New York flora are not woody, the proportion
of woody species among the endemics (17 per cent.)
being essentially the same as for the entire flora (18.2
per cent.) Most of the endemics are probably accounted
for by generic and specific instability, that is, by the ten-
dency of existing forms to vary, at or near the edge of their
range, and for the variations to become established. At
least one is a case of ‘‘habitat’’ endemism; that is, the
endemic species is confined to a given locality because
suited to the environment afforded by that locality. This
is illustrated by Prunus Gravesii, a saxitile form of the
beach-plum (P. maritima).

Many factors are involved in the phenomena of en-
demism, and here, as in the case of discontinuous geo-
graphical distribution, each case must be carefully analyzed
by itself. In view of our present restricted knowl-
edge, we can generalize only with extreme caution.

126. An Illustrative Study.—As an illustration of the
application of evidence from various sources in an en-
deavor to decide the relative age of two large groups of
plants, herbs and woody plants (trees and shrubs), there
may be mentioned the recent work of Sinnott and Bailey,'
who marshalled evidence from paleobotany, anatomy,
phylogeny, and phytogeography, as bearing on the rela-
tive antiquity of herbaceous and woody plants. Very
briefly summarized, their argument runs as follows:

1. A study of fossil plants shows that the remains of

1Sinnott, Edmund W. and Irving W. Bailey. Investigations on the
phylogeny of the Angiosperms: No. 4. The origin and dispersal of
herbaceous Angiosperms. Ann. Bot. 112: 547-600. Oct. 1914. The
phraseology of the authors is freely incorporated in the above very brief
summary.

GEOGRAPHICAL DISTRIBUTION 181

Angiosperms in earlier geological periods were almost all
woody. The number of herbaceous forms increases as
we pass from older to more recent strata. Fossils of
herbaceous plants are rarely found in Cretaceous rocks
but become increasingly abundant throughout the Ter-
tiary. Caution is necessary here, however, for the foliage
and other parts of herbs are more tender and delicate
than those of woody plants, and therefore less liable to
be preserved as fossils. This evidence is significant only
in connection with evidence derived from other sources.

2. A study of the comparative anatomy of stems indi-
cates that the continuous ring of wood, which character-
izes the stems of all trees and shrubs, is a more primitive
character than the separate fibro-vascular bundles of
herbaceous stems. It is suggested that a change from a
woody to an herbaceous type may have resulted from
regional decrease in the activity of the cambium layer,
from which the wood is formed by cell-division followed
by lignification.

3. Evidence from phylogeny shows that the more
primitive groups of Angiosperms and their probable an-
cestors are composed overwhelmingly of woody plants.
In more than half of the families of Dicotyledons there are
no herbaceous species, and the few families which are
entirely herbaceous are almost all insectivorous plants,
water plants, parasites, or monotypic families, and hence
can lay no claim to great antiquity. Also, there is a
much larger proportion of woody plants in the lower
groups of Angiosperms (Apetale and Polypetale) than
in the higher groups (Sympetalz.)

4. From a study of plant geography we learn that

14 monotypic family is a family having only one genus.

182 HEREDITY AND EVOLUTION IN PLANTS

dicotyledonous herbs preponderate in north temperate
regions, and woody plants in the tropics. The latter
climate probably approaches more nearly to that under
which Angiosperms first appeared. Herbs, having a
short life cycle (one to two or three seasons) are able to
survive periods of intense cold in the form of seeds, and
would, therefore, survive in larger numbers than woody
plants on the advance and retreat of the continental ice
sheet of the Glacial period. This would account for the
fact of a much smaller proportion of woody plants in the
flora of Europe, for these could not migrate southward, as
the ice encroached, since the mountain ranges there have
a general east-west trend (in contrast to the general north-
south trend of American ranges), and southern migration
would necessitate an ascent to high altitudes that would
be fatal to temperate or subtropical species.

The above facts are not cited as established, but only
to illustrate a method. There is also evidence and argu-
ment suggesting the opposite conclusion, namely, that
herbaceous plants are older than woody.

CHAPTER XI

PALEOBOTANY

127. 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.

128. 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
peirifaction. Incrustations are merely impressions or

‘ By an extension of the term we also speak of fossil footprints of ani-
mals, fossil ripple marks, e¢ cetera. The word fossil is derivcd from the

Latin jodere (to dig), and originally signified anything dug up.
183

184 HEREDITY AND EVOLUTION IN PLANTS

casts resulting from the encasement of the organ or
organism in the rock-forming material. The tissue itself
either decayed or became carbonized, leaving only the

Nee
Fic. 80.—Fossil incrustations of the foliage of two species of Spheno-
phyllum from the coal measures of Missouri. (From U. S. Geological

Survey.)

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,

PALEOBOTANY 185

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
(Fig. 80).

Petrifactions are formed by the gradual replacement
of the organic tissue by mineral matter, usually carbonate
of lime (CaCOs) or silicic acid (H,SiO,). 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 (Figs. 97 and 100).

129. 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-
ing into inland lakes or into the ocean. In addition to
rock-sediment eroded from the surface of the land, streams
also transport quantities of plant (and animal) frag-

HEREDITY AND EVOLUTION IN PLANTS

186

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PALEOBOTANY 187

ments, leaves, stems, pieces of bark, fruit, flowers, pollen
and spores, roots, and even entire plants. These natur-

We ate pn ooh Ban Mees pee ae ee ee

Fic. 82.—Diagram illustrating the gradual filling up of lakes by the
encroachment of vegetation, and also the stages in the origin of peat and
marl deposits in lakes. The several plant associations of the Bog series,
displacing one another, belong to the following major groups: (1) 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. 81.) (After Bray.)

ally become buried in the mud and sediment wherever
deposition takes place, and when the deposit becomes

188 HEREDITY AND EVOLUTION IN PLANTS

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. 81 and 82.

130. 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 metamorphism, 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.

131. 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,
rocks occur in layers,? the deeper lying layers (with ex-

1 Some rocks are buried under more than 40,000 feet of strata, and the
temperature increases approximately 21°F. for every 50 to 60 feet of depth.
? Several layers form a siratum, or bed.

PALEOBOTANY 189

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 wncon-
ormity 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.

132. 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
the following table, the older rocks being at the bottom,
the most recently formed at the top.

Igo HEREDITY AND EVOLUTION IN PLANTS

TasLe II.—Tasie or GrorocicaL TIME
Era Period
Holocene
(recent, or the present)
Pleistocene
(ice age)

Quaternary

Cenozoic
Pliocene
Miocene
Oligocene
Eocene

Tertiary

Upper Cretaceous
Lower Cretaceous
Mesozoic Secondary (Comanchean)
Jurassic
Triassic

( Permian

Upper Carboniferous
(Pennsylvanian)

| Lower Carboniferous

Paleozoic Primary (Mississippian)

Devonian

Silurian

Ordovician

Cambrian

{ Huronian

micheant Laurentian

133. 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.
and covered over with sediment, then elevated again as
dry land, so that subsequent excavations have revealed
the fossilized trunks and stumps (Figs. 83 and 84). Thus

PALEOBOTANY Igt

Fic. 83.—Fossil tree stumps in a carboniferous forest, Victoria Park,
Glasgow. (Cf. Fig. 84.) (After Seward.)

Fic. 84.—Part of a submerged forest as seen at low tide on the Cheshire
coast of England. (Cf. Fig. 83.) (After Seward.)

192 HEREDITY AND EVOLUTION IN PLANTS

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
paleogeography.

134. 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. Asa 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, then much warmer than now, must have been fruit-
ful in new species.!_ High mountains or plateaus are also
suggested as homes of plastic races.?. In the tropics en-
vironments are more nearly static, and, it is reasonable
to suppose, less likely to favor variation. It is knownthat
once established, many species move most readily along
the geologic formation which supplies the exact soil con-
stituents most favorable to their growth, the rate of
movement often being rapid. Flotation of seeds is also
a factor. The facts here briefly cited rest on the obser-
vations of a large number of investigators, extending over
more than a century.

135. Distribution of Plants in Time.—In addition to
the distribution of plants in space (plant geography), the
problem of their distribution in geologic time is one of

1 Owing to the precession of the equinoxes these areas undergo an ex-

treme variation in the length of winter and summer of 37 days every, 12,934
years.

2 Cf. pp. 148-149.

absorbing interest and importance.

PALEOBOTANY

193

The following table

indicates the known distribution of the various plant
groups from the earliest geologic time to the present.

TasBLe III.—DistTrIBuTION OF PLANTs IN GEOLOGIC TIME}

Division eu ee ene Range ig Be
example
8g
5
2 | Monocotyled »nes | Comanchean to present | Oaks
3 Dicotyledones Comanchean to present | Grasses
Ei
Spermato- <
IV.§ phyta i
Cycadophyta & Gnetales (Fossil record scant) Ephedra
% | Coniferales Permian to present Pines
& | Ginkgoales Permian to present Ginkgo
& | Cordaitales Devonian to Jurassic Cordaites
g Cycadales Permian to present Cycads
© | Cycadofilicales Devonian toComanchian! Neuropteris
Lepidophyta Lycopodiales Devonian to present Club mosses
Calamophyta Equisitales Devonian to present Horsetails
III Sphenophyllales Devonian to Permian Spheno-
phyllum
Pteridophyta Filicales Devonian to present Ferns
IL. Bryophyta Musci Tertiary to present , Mosses
: Hepatice Tertiary? to present Liverworts
Fungi Silurian to present Fungi
Alge Pre-Cambrian to present | Seaweeds
I. Thallophyta Diatomee Jurassic to present Diatoms
Schizophyta Pennsylvanian to present! Bacteria
Myxomycete (Fossil record lacking) Slime-molds

1 Modified from Shimer.

2See, however, p. 172.

136. 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

number, both of specimens and of species, preserved in

13

194 HEREDITY AND EVOLUTION IN PLANTS

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 largely 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
alge, 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.

“T 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,
more or less different in the successive chapters, may

PALEOBOTANY 195

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.

137. 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 culti-
vated plants by weeds in a neglected garden, or of grass by
dandelions or chickweed in a lawn. By more rapid ger-
mination and growth, and by other “weedy”’ character-
istics, 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 tress by the elm tree beetle.

3. Changes of Environment too Great or too Rapid to Per-
mit of 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,” vol. 2, p. 88. New York, 19¢2.

196 HEREDITY AND EVOLUTION IN PLANTS

involve the entire area of distribution of the species con-
cerned, the species will, obviously, become extinct. The
following nine factors (paragraphs 4-12) 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 destroyd 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, Elodea, Vallisneria), and seeds readily
transported by wind (e.g., cat-tail) enable the species to
become reéstablished 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 eruptions, such, for example, as those of
Mount Pelée, 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 thou-
sands of square miles, have, during a previous geological
age, been poured out over the surface, forming a wide
plateau.

A great volcanic eruption in Alaska, in prehistoric
times, covered an area of over 140 square miles with a
deposit of ash and pumice varying in thickness from a

PALEOBOTANY 197

few inches near the margin to some 300 feet near the crater.
In 1883 the eruption of Krakatoa, in the Straits of Sunda,
killed practically all the plants and animals on an island
of five square miles in area, and on neighboring islands;
a part of the island was completely blown away, leaving
only deep water. So recently as 1912 the eruption of
Katmai, in Alaska, spread a layer of ash nearly a foot deep
over the entire surface of Kodiak Island, one hundred
miles from the volcano, and killed all the herbaceous vege-
tation, leaving only trees and bushes. It is almost certain
that many species of plants and animals have become ex-
tinct by such agencies. Not only the lava, but poisonous
gases that fiill the air during volcanic eruptions, may
prove fatal to plant and animal life.

7. Encroachment of salt water in coastal regions,
caused by changes in the level of the land, resulting in the
killing of fresh-water vegetation. According to Fernald,
one of the sundews, Drosera filiformis, is known to occur
in only two regions, namely along the Gulf coast from Flor-
ida to Mississippi, and along the Atlantic coast from Mary-
land to Massachusetts (Fig. 85). Its extinction in the
intervening region is explained by the subsidence and
drowning of a former high continental shelf, along which
this and other species migrated northward during the
late Tertiary. If a similar subsidence should occur in the
two limited regions where the species is now found it
would become extinct unless, by some combination of
circumstances, it could migrate and become established
in new localities. It is not unlikely that species have
often been exterminated in this way.

8. Encroachment of Fresh Water over Land Areas. —
Previous to about the year 1900, the Salton basin, in

198 HEREDITY AND EVOLUTION IN PLANTS

lower California, was a saline area of a so pronounced
desert type that its flora contained less than 140 species
of ferns and flowerng plants, five of which were endemic.
During the winter of 1904-1905 the fresh waters of the
Colorado River began to debouche into this basin, and
by early 1907 had formed a brackish lake, over 80 feet deep
and of about 450 square miles in area, known as the Salton
Sea. At the end of ten years it still had an area of some-
what less than 300 square miles. Some three or four
hundred years previously the entire Salton Basin was

Fic. 85.—Sketch map showing the geographical distribution of the sun-
dew, Drosera filiformis. (After M. L. Fernald.)

occupied with a lake of over 2,000 square miles in area,
which, in turn, had dried up and given place to the desert
conditions above mentioned. It is not improbable that
such drastic changes as this may have resulted in the
obliteration of one or more species, though the flora was
not well enough known previous to the last inundation
to make a definite statement on this point possible. For
example, the presence there of endemic species was not
known until the recent botanical survey of the region
lying between the late water level and that of the ancient

PALEOBOTANY 199

sea. According to MacDougal,' if the water had risen in
1907 to its ancient level of three or four hundred years
ago, it would have destroyed all these endemic species.

7. Transformation of fresh water lakes into salt lakes,
as in the case of the Caspian Sea, and the Great Salt Lake
of Utah (18percent.salt). This change gradually extermi-
nates plant and animal life until the given body of water
becomes a true “dead” sea, where practically nothing
remains alive, as in the Dead Sea (24 per cent. salt).
A more extreme case yet is Lake Van, in Turkey, where
saline matter constitutes over one-third of the contents.
In the last stages of such transformations the lake may give
place to a salt marsh or plain (salina). South of Lake
Titicaca, in the Andes Mts. of Bolivia, ar several salinas,
one of some 4000 square miles in area, with a layer of
salt three or four feet thick.

10. 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.

11. Diminution of Carbon Dioxide in the Atmosphere.—
There are reasons for thinking that in certain past ages
the atmosphere 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 per-
centage of carbon.

1TIn a letter to the author.

200 HEREDITY AND EVOLUTION IN PLANTS

12. 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
emergencies 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 J 129.)

CHAPTER XII

THE EVOLUTION OF PLANTS (Concluded)

138. 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.

139. 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. 86), first described from fossil leaves, in 1829, as
a tree-fern, under the name Sphenopteris Hoeninghaust.

After investigations extending over nearly go years, “‘ we are
201

202 HEREDITY AND EVOLUTION IN PLANTS

now in position to draw a fairly complete picture of the
plant as it must have appeared when living.

“Tt was in effect a little tree-fern, with long, slender,
sometimes branched, stem, 4 centimeters or less in diame-

Fic. 86.—Lyginodendron oldhamium.. Pinna of a microsporophyll,
found in an ironstone nodule. Before its identity was established this
specimen was named Crossotheca Hocninghausi. The somewhat peltate
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.)

ter, and provided with spines by means of which it prob-
ably climbed on its neighbors. The foliage was disposed
spirally and consisted of relatively very large, finely
divided fronds with small, thick pinnules with revolute

THE EVOLUTION OF PLANTS 203

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.

Fic. 87.—Young leaf of the Cycad, Bowenia serrulata. Comparison
of this with a leaf of the fern Angiopteris (Fig. 88) shows how difficult
it might be to decide from a fossil leaf whether the plant was a cycad ora
fern. (Cf., also, Fig. 91.) (Photo from specimen in Brooklyn Botanic
Garden.)

“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

204 HEREDITY AND EVOLUTION IN PLANTS

to the stem of [the fern] Osmunda, but when more mature
certain cycadean characters appear to predominate.”?

Its foliage and other characters closely resemble some
of our modern tree-ferns (Cf. Figs. 87 and 88), but more

Fic. 88.—Leaf of a fern (Angiopteris evecta). (Cf. Fig. 87.)

careful study of the calcified specimens of much beauty,
found in calcareous nodules (the so-called English
“‘coal balls’’?), has disclosed both the microsporophylls,

1 Knowlton, F.H. American Fern Journal, 5:85. 1915.

2 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).”” Shimer, H. W.
“ An introduction to the study of fossils,” p. 53. London, 1914.

e

THE EVOLUTION OF PLANTS 205

bearing pollen-sacs, and the megasporophylls, 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 14 inch long,
have been described as resembling little acorns, enclosed
like hazelnuts in smaller glandular cupules (Fig. 89).
They are similar to those of the cycads, except that they
are not known to have organized an embryo with cotyle-

ae

a

@
‘\,

e. @
@

Fic. 89.—Restoration of a seed of Lyginodendron oldhamium (Lagenos-
tema Lomaxi), from a model by H. E. Smedley. (After Scott.)

dons and caulicle. Instead, the tissues of the female
gametophyte only are so far found, retained within
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

206 HEREDITY AND EVOLUTION IN PLANTS

are found on pinhules of ordinary foliage leaves, resem-
bling the sporophylls of certain ferns (Fig. 90) rather than
the stamens of modern flowers.

The discovery of the seed-bearing character of the fern-
like plants of the Paleozoic has been called the most im-
portant contribution of paleobotany to botany ever made.
It was predicted by Wieland, of Yale University, nearly
two years before it was announced by Oliver and Scott.
It is now believed that seed-bearing plants of the pterido-

Fic. 90.—Top, lateral pinna from a leaf of Maraitia fraxinea. (After
Bitter.) Below at left, synangium of same. (After Bitter.) At right,
cross-section of the synangium. (After Hooker-Baker.)

sperm type were nearly as numerous in the Paleozoic
as were the cryptogams.

140. 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.

THE EVOLUTION OF PLANTS 207

It was in recognition of their vegetative resemblances that
the Pteridosperms were first called (by Potonié) Cycado-
filices, now Cycadofilicales. Van Tieghem tersely de-
scribed them as “‘phanerogams without flowers.”

141. A Modern Fern-like Cycad.—One of the modern
cycads (Stangeria paradoxa)! is of much interest in this

Fic. 91.—Slangeria paradoxa Moore. 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.)

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

1 Stangeria paradoxa Moore = Slangeria eriopus (Kunze) Nash,

208 HEREDITY AND EVOLUTION IN PLANTS

vernation, have a pinnately compound blade, and leaflets
with pinnate dichotomous yenation. Two or three years
later another botanist, examining it more closely, pro-
nounced it a “‘fern-like Zamia or a Zamia-like fern.”
These facts show how puzzling the specimen was, and how

B
8
8
\
ue

Fic. 92.—To the left, Cacadeoidea 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.)

closely a plant may resemble both acycadophyte and afern.
In asense this plant may be called a living fossil. Speci-
mens have since come into flower in botanic gardens, and
the typical cycadaceous cones (Fig. 91) leave no doubt
that the plant is a true cycadophyte.

THE EVOLUTION OF PLANTS 209

142. 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

Fic. 93.—Cycadeoidea dacolensis. 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.)

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
14

210 HEREDITY AND EVOLUTION IN PLANTS

fossil forms the direct ancestors of those now living, although
a study of their structure is of the greatest value in ena-
bling us to understand the genetic relationships of the great
groups of plants.

143. Ancestors of the Angiosperms.—Just as the Cyca-
dofilicales indicate the ancestry of the cycads, so fossil
types of Cycadophyta have been discovered which are

Fic. 94.—Cycadeoidea dacotensis (?). 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.)

interpreted by some paleobotanists as ancestors of the
modern angiosperms. Other investigators, however,
dissent from this view and consider that we have not yet
sufficient knowledge of fossil forms to be justified in desig-
nating the ancestors of the Angiosperms. This differ-
ence of opinion is largely due to the meagerness of the
available evidence. As one writer has stated it, “A

THE EVOLUTION OF PLANTS 211

trayful of flowers may be all the record of the Pterido-
sperms from the Devonian on. The gaps in the evidence
are always enormous.”’

Although the Cycadophyta are now a very insignifi-
cant element in the earth’s flora, in the Mesozoic period

Fic. 95.—Macrozamia spiralis. Tip of the trunk, showing three
lateral cones, inserted in the axils of leaves. Photo from specimen in
Brooklyn Botanic Garden. (Cf. Fig. 96.)

they form about one-third of the recovered vegetation of
the land. One order, the Hemicycadales (Bennettitales?),
then had a cosmopolitan distribution and seemingly was
as important as the Dicotyledonsarenow. Over 30species
of the petrified stems have been found in the Mesozoic

1In his paper on the Classification of the Cycadophyta (Am. Jour. Sci.
47: 391-406. June, 1919), Wieland states “Simple and good reasons”

for letting the name Bennettitales fall into disuse, and substituting there-
fore the term Hemicycadales (half-cycads).

212 HEREDITY AND EVOLUTION IN PLANTS

terrains of the United States, the Black Hills of South
Dakota alone yielding ascore. The Isle of Portland forms

Fic. 96.—Cycas circinalis. Tip of trunk, showing numerous leaf-
stalks, and the large terminal cone. Photo from specimen in Brooklyn
Botanic Garden. (Cf. Fig. 95.)

were called Cycadeoidea by the celebrated geologist Buck-
land. The original name of the order was derived from

THE EVOLUTION OF PLANTS 213

bearing many ovulate cones with seeds approaching maturity, and a lesser
number of either young or abortive cones. j’, Receptacle of a shed or
non-preserved cone with surrounding bracts yet present; /’, two cones
broken away during erosion, with a portion of the basal infertile 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; s (over lower arrow), the ovulate
strobilus, shown at the right, in its natural position, this photograph having
been made before the cone was cut out by a cylindrical drill. x 0.5.

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
1. ¢ (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; J, leaf
bases. 5. (After Wieland.)

214 HEREDITY AND EVOLUTION IN PLANTS

the genus-name, Bennettites.1 Other forms, usually found
as casts, are called Williamsonza, still others are known
mainly as genera founded on leaf imprints.

144. Cycadeoidea.—In most of its purely vegetative
characters, such as the anatomy of the stem and the

Fic. 98.—Cycadeoidea Wielandi. Longitudinal section through the
axis of a female inflorescence, or cone. J, old leaf-base; d, insertion of
disc; », erect seed, borne at summit of seed-pedicle inserted on convex
receptacle; 5, hair-covered bract. (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 Bow-

1 Cycadeoidea Buckland = Bennettites Carruthers.

THE EVOLUTION OF PLANTS 7 215

enia) have lateral seed-bearing cones (Fig. 95);! in the
other genera the carpellate cones are terminal (Fig. 96).
Various structural characters of Cycadeoidea are shown
in Figs. 92-100.

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

=

SS

Fic. 99.—Cycadeoidea ingens. Restoration of an expanded bispor-
angiate cone, or flower, in nearly longitudinal section. Restored from a
silicified fossil. (After Wieland.)

bracts arranged in spirals. The bracts surrounded a
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 bear-
ing two rows of compound microsporangia (pollen-sacs)
on each leaflet. They thus closely resembled the sporo-
phyll of a fern.

1 The staminate cones of Zamia are lateral.

216 HEREDITY AND EVOLUTION IN PLANTS

The axis of the flower terminated in a cone-shaped
receptacle, bearing the stalked ovules, and numerous
sterile scales (Figs. 97 and 98). 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 20 endosperm. ‘These are char-
acters never found in the lowest group of modern seed-

a s j

Frc. 100.—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.)

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, Proangiosperms.

145. Relation of Cycadeoidea to Modern Angiosperms.
—The question of the ancestry of the Angiosperms is the
most important problem of paleobotany. Although the

THE EVOLUTION OF PLANTS 217

Hemicycadales possess many of the primitive anatomical
features that characterize the Cycadofilicales, their
development of a bisporangiate strobilus with two sets
of sporophylls, related to one another as they are in the
flower of the Angiosperms, indicates a genetic relationship
to that group, as does also the fact that the seeds, enclosed
in a fruit, possess a dicotyledonous embryo, without endo-

Fic. 101.—Flower of magnolia. (Cf. Fig. 102.)

sperm. In other features the Hemicycadales are unlike
the Angiosperms; the ovules, for example, are enclosed
by sterile scales, instead of by the carpels on which they
are borne, and the protrusion of the pollen-chamber
through the micropyle signifies the gymnospermous type of
fertilization.

These and other comparisons indicate that the Hemi-
cycadales were essentially Gymnosperms having certain

218 HEREDITY AND EVOLUTION IN PLANTS

Angiospermous characters, and therefore, while they arenot

Fic. 102.—Magnolia.
Flower with perianth
removed, showing the
compound pistil, and four
of the stamens. Most of
the stamens have been
removed so as to bring
out their spiral arrange-
ment as shown by the
scars at the points of
attachment. (Cf. Fig.
101.)

to be considered as the ancestors of
the Angiosperms, it is probable that
they and the modern dicotyledons
are both descended from a common
branch of theancestraltree. Among
modern plants, the flower of the
magnolias most closely resembles
that of Cycadeoidea in the spiral
arrangement of its stamens and
pistils (Figs. 101 and 102). Just
what significance should be attached
to that fact has been disputed by
students of morphology. The older
view of the systematists regarded the
primitive flower as more complex in
structure, with pistils, stamens, and
floral envelopes arranged spirally in
centripetal or acropetal succession on
a fleshy axis, as in Magnolia and other
flowers of the order Ranales; other
types of floral structure were con-
sidered as derived from this one by

> reduction. This is often referred to

as the ‘“Strobiloid theory of the
flower’’ (Cf. pp. 132 and 134).

A more recent view recognizes
that simple staminate or pistillate
flowers may, im some cases, be in-
terpreted as derived by reduction
from more complex forms, but re-
gards the primitive flower as uni-

THE EVOLUTION OF PLANTS 219

sexual—in effect a microsporophyll or a megasporophyll,
from which complex forms were derived by elaboration.
This latter view, however, is not in harmony with avail-
able evidence from fossil plants, such as that afforded by
Cycadeoidea.

“The strobiloid theory of the flower seems in the present
state of our knowledge to stand alone as a working hy-

4

Fic. 103.—Theoretical stages in the reduction (from Cycadeoidea to
modern Angiosperms) of staminate discs represented as segments. 4A,
any] common campanulate form with simple stamens (e.g., morning
glory); B, hypothetical Cycadeoid reduced to a single synangium to each
frond component; C, inner view of a sector of a Williamsonia mexicana
disc; D, sector of a Cycadeoidea dacotensis disc with the pair of shoulder
spurs borne by each frond. (After Wieland.)

pothesis. If we reject it, we are left without any historical
clue to the origin of the floral structure of Angiosperms.
If we accept it, the Primitive Angiosperm must be cred-
ited with a flower resembling that of Magnolia or Lirio-
dendron in general plan.”! From this it follows that the
Magnoliacee must be among the most primitive, if not the
most primitive, of all Angiosperms, as Wieland first and
Hallier later and independently pointed out.

1Sargant, Ethel. The reconstruction of a race of primitive Angiosperms.
Ann, Bot. 22:121-186. April, 1908.

220 HEREDITY AND EVOLUTION IN PLANTS

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 (Fig.
103). From this it has been inferred that the Hemicycad-
ales are a lateral branch, further removed than their
ancestors from the direct evolutionary stock of the
Angiosperms.

146. Origin of Dicotyledony.—Two problems of major
importance are involved in the question of the evolution
of Angiosperms, namely, the origin of dicotyledony and
the origin of monocotyledony. Are dicotyledons more
ancient than monocotyledons, or vice versa? Again, in
the evolution of seed-bearing plants was the condition of
polycotyledony antecedent to that of dicotyledony, or the
reverse? This would be a comparatively easy question
to answer if we had an unbroken series of fossil remains
of the primitive and intermediate spermatophytes; but
unfortunately such evidence has not yet been discovered.
We know nothing of the embryos of the geological ances-
tors of modern conifers. The Mesozoic gymnosperms
(Cycadeoidea and other related genera) are known to have
had dicotyledonous embryos, but these forms do not stand
in the ancestral line of the (polycotyledonous) conifers
of to-day. To answer our question, therefore, we must,
for the present, depend largely on the study of living forms.
The evidence has seemed conflicting, and for nearly three-
quarters of a century opinion has varied. Adanson and
Jussieu, in the early nineteenth century, contended that
polycotyledony was derived from dicotyledony by a split-
ting of the primordia cf two original cotyledons; Sachs

THE EVOLUTION OF PLANTS 221

(1875) held the opposite opinion. Hill and de Fraine
(1908-1910) are among the recent protagonists for the
hypothesis that dicotyledons are the more primitive. One
of the most recent studies is that by Bucholz! who ex-

L ‘i K Zt

Fic. 104.—Development of stem tip and cotyledons in Pinus Bank-
siana. Dotted line represents plerome of root-tip; shaded area, meristem
of stem tip; H, J, J, K, fusing cotyledons. (After Bucholz.)

amined the embryos of pine, spruce, larch, juniper, balsam
fir, cedar of Lebanon, and others. Many instances of
the fusion of the primordia of cotyledons were found, but
no evidence of cotyledonary splitting. This fusion has
resulted in reducing the number of cotyledons, and, in

’ Bucholz, John T. Studies concerning the evolutionary status of poly-
cotyledony. Am. Journ. Bot. 6:106-119. March, 1919.

228 HEREDITY AND EVOLUTION IN PLANTS

certain species, in the formation of a cotyledonary ring,
or tube. Bucholz interprets the facts set forth by him-
self and other investigators as leading to the conclusion
that the more primitive gymnosperms had numerous coty-
ledons, that their number was reduced by the fusions of
their primordia and, in some species, a cotyledonary tube
or ring was formed. ‘‘Dicotyledony was attained either
by a general fusion of many cotyledons in two groups, or

Fig. 105.—Polycotyledonous seedlings of dicotyledonous species. A-C,
Silene odontipetala, with hemi-tricotylous, tricotylous, and tetracotylous
seedlings; D-H, Papaver Rhoeas (semi-double cultivated form), dicoty-
lous, hemi-tricotylous, tricotylous, tetracotylous, and pentacotylous
seedlings; I, Acer Pseudo-Platanus, tetracotylous seedling. (All figures
re-drawn from de Vries.)

by an extremely bilabiate development of a cotyledonary
tube” (Fig. 104).

The final conclusion of Bucholz, based on the evidence
of comparative anatomy, supplemented by studies of
development, is that the polycotyledonous condition is
the more primitive, and the dicotyledonous one derived.
On the basis of this theory, the rather common abnormal
a ppearance of supernumerary cotyledons in dicotyledonous

THE EVOLUTION OF PLANTS 223

seeds is to be interpreted as a reversion to a more primitive
condition (Fig. 105).!

147. Origin of Monocotyledony.—Ii the earliest Angio-
sperms were dicotyledons, as now seems probable, the
monocotyledons were probably derived from them by
a process of simplification. Several hypotheses have been
framed as to how the final result was accomplished, but
the voluminous evidence and the conclusions can only be
briefly summarized here.

For nearly a century it has been generally accepted
by botanists that the two seed-leaves or cotyledons of
dicotyledonous plants were lateral organs, originating
below the tip of the embryonic stem or hypocotyl, while
the single cotyledon of monocotyledonous plants was
considered as a terminal organ. The grass family offers
a casein point. The embryo of Indian corn (Zea Mays),
for example possesses a well developed cotyledon, called
the scutellum; there is little or no trace of a second
cotyledon. The embryos of many other grasses, however,
possess an organ, the epiblast, homologous in position with
the scutellum, and regarded by earlier botanists as a rudi-
mentary cotyledon (Fig. 106). Recent studies of Coulter
and Land leave little doubt of this as the correct interpre-
tation of that organ.

A study by Bruns (1882) of 82 genera of grasses, repre-

1 According to de Vries (The Mutation theory. 2: 393-456. Chicago,
1gt0) tricotylous intermediate races do not arise by selection but by
mutation, tricotyly being the expression of an ancestral character which
is alent in the normal species. If the normal character is active and
the anomaly semi-latent we have what de Vries calls a “half-race;”’ if
the normal character becomes latent and the anomaly active, we have a
“constant variety.’’? Sometimes an equilibrium is maintained in the ex-
pression of the normal character and the anomaly, giving rise to a
“middle race,’ or “eversporting variety.”

224 HEREDITY AND EVOLUTION IN PLANTS

senting 12 tribes, demonstrated the presence of the rudi-
mentary cotyledon (epiblast) in 29 of the genera, repre-

Fic. 106.—Diagram of longitudinal sections of grass-embryos (Gram-
ine) to illustrate the rudimentary cotyledon (epiblast). A-C, E+G,
redrawn from J. M. Coulter, after Bruns; D, from nature. A, Zizania
aquatica; B, Leersia clandestina; C, Leptochloa arabica; D, Triticum wul-
gare; E, Spartina cynosuroides; F, Triticum vulgare; G, Zea Mays;
s, scutellum; ¢, coleoptile; ¢, epiblast.

senting nine tribes; later studies by Van Tieghem (1897)
disclosed the presence of an epiblast in 61 out of 91 genera
examined. From these figures we may reasonably infer |

THE EVOLUTION OF PLANTS 225

that the majority of the so-called “monocotyledonous”
grasses possess two cotyledons, one of which is more or less
rudimentary, and that the grasses are primitive monocoty-
ledons, representing a transitional stage from dicotyledons
tothe higher monocotyledons. Monocotyledony, then, as
stated by Coulter, is simply one expression of a process
common to all cotyledony, gradually derived from dicotyle-
dony by reduction, and involving no abrupt transfer of a
lateral organ to aterminal origin. Variations in the rela-
tive size of the second cotyledon in grass embyros are
illustrated in Fig 106.

Henslow’ was among the first to suggest the origin of
monocotyledons from dicotyledons.? Previous to the pub-
lication of his paper, it was generally assumed that mono-
cotyledons were the older group, and Henslow stated that
no systematist of his day recognized any real points of con-
nection between the two groups. He proposed the hy-
pothesis that the monocotyledons were derived by the
arrest of the development of one seed-leaf in a primitive
dicotyledonous Angiosperm;? hence said Henslow, “only
one elongates, its superior vigour carrying it on ina straight

1 Henslow, Rev. George. A theoretical origin of endogens from exogens,
through self-adaptation to an aquatic habit. Journ. Linnean Soc. Bot.
19 3485-528. May 15, 1893.

2 The first to make the suggestion appears to have been Agardh, in his
Lérobok i Botanik, Part I. Maliné, 1829-32.

3In discussing the origin of Angiosperms, Arber (Journ. Linnean Soc.
Bot. 38: 29-80. July, 1907) calls attention to the “Law of corresponding
stages in evolution,” namely, that in the evolution of seed-plants, the stages
reached by different organs at any one period are dissimilar. From this
law it follows that such a plant as a ‘‘primitive Angiosperm,”’ in the strict
sense of the term, that is, with all its organs primitive, never existed in
reality. We must picture the ancestors of modern Angiosperms as having
certain organs in a primitive stage of evolutionary development, others
as more advanced toward the stage in which they are now found.

15.

226 HEREDITY AND EVOLUTION IN PLANTS

line with the suspensor, finally making the cotyledon ter-
minal.” This he calls ‘‘the real interpretation of a mono-
cotyledonous embryo.” Henslow further inferred a very
early origin of monocotyledons from dicotyledons, from
the fact that so many of their orders contain very few gen-
nera and monotypic groups, for groups of plants or animals
with few members, are regarded, in general, as survivals,
representing a lost ancestry. He recorded voluminous
observations in support of his theory, and, among other
evidence, called attention to “ Dicotyledonous monocoty-
ledons”’ suchas Tamus communis (black bryony), a tuberous
rhizomed species of the Yam family, where the first foliage
leaf, situated exactly opposite the cotyledon, is interpreted
(with Dutrochet) as a second cotyledon; and to ““Mono-
cotyledonous dicotyledons,” especially among aquatic
species such as the water-chestnut (Trapa natans), where
one cotyledon is arrested in its development. Other illus-
trations, not mentioned by Henslow, include such forms
as Dioscorea bonariensis and Pinguicula vulgaris (Fig. 107).
Ranunculus Ficaria is not an aquatic, but it flourishes
by the waterside, and is regarded by Henslow as descended
from an aquatic form. About one-third of the orders of
of monocotyledons are aquatic, as compared to only 4
per cent. in dicotyledons, and the monocotyledonous dicoty-
ledons are all aquatic. The final conclusion of Henslow
is, ‘‘that endogens [monocotyledons] have in the first place
descended from very early types of exogens [dicotyledons]
. ; and that, secondly, the more immediate cause of
their origin was an aquatic habit of life assumed by certain
primitive exogenous plants.”
Miss Ethel Sargant has more recently elaborated the
hypothesis of the derivation of monocotyledons from

THE EVOLUTION OF PLANTS 227

dicotyledons, by a fusion of the two cotyledons into one. *
On this basis the single seed-leaf of monocotyledons is
interpreted as homologous to the two seed-leaves of di-
cotyledons. The evidence supporting this suggestion is
derived largely from a study of the anatomy of monocoty-
ledonous seedlings. “The young epicotyl of monocotyle-
donous seedlings contains a single ring of collateral bundles
which may even show traces of cambium, much resembling

Fic. 107.——A-B, embryos of a “dicotyledonous monocotyledon,” A,
longitudinal section through an embryo of Tamus communis; B, Tamus
communis, entire (A and B enlarged after Solms-Laubach.). C-G,
embryos of “‘monocotyledonus dicotyledons;” C, D, Dioscorea bonariensis,
enlarged (after Beccari); E, Trapa natans, the water chestnut x 34
(after Barnéoud); F, Pinguicula vulgaris; G, Pinguicula caudata. (F and
G after Dickson, both greatly enlarged.)

dicotyledons.” Professor Jeffrey has also called attention
to evidence that the anatomy of the stem of the hypotheti-
cal ancestor of the Angiosperms was exogenous (dicotyle-
donous).

Miss Sargant has further pointed out that the few
dicotyledons which possess but one seed-leaf (pseudo-
monocots) are widely distributed through the dicotyle-
donous families, from Ranunculacee to Umbillifere,

1 Annals of Botany 17: 1-88. Jan., 1903; Botanical Gazette 37: 325-
345. May, 1904, ana other papers.

s

228 HEREDITY AND EVOLUTION IN PLANTS

Primulacee, and Nyctaginacee, which indicates that the
abnormality was not derived by inheritance from a com-
mon ancestor; its explanation, therefore, must be sought
in the influence of environment. Professor Henslow, as
noted above, associated the monocotyledonous tendency
with an aquatic habit of life, but Miss Sargant points out
that all the pseudo-monocots possess some underground
organ whichis thickened as a tuber, suggesting that the signi-
ficant ecological factor is a geophilous, rather than aquatic,
habit. In further confirmation Miss Sargant notes that
of twenty genera having their seed-leaves fused for some
distance upward from the base, the majority have a
tuberous hyocotyl. The dicotyledonous may-apple (Po-
dophyllum), for example, with a geophilous habit has
partially united cotyledons and a stem anatomy resembling
that of the monocots. The only exception to correlation
of this nature is the mangrove (Rhizophora Mangle),
a tropical tree whose seeds germinate in the air while still
in the fruit.

The monocotyledons are separated from the dicotyle-
dons by seven characters as follows:!

1. A single cotyledon.
Stem-anatomy.
Development of the embryo.
Parallel venation of leaves.
. Short duration of primary root.
. Seeds with endosperm.

7. Parts of the flower in threes.

Of these characters, ‘four have been shown to appear
frequently among geophytes, and to be useful to the plant
growing under conditions which determine the geophilous

AAR YES

1 As enumerated by Miss Sargant.

THE EVOLUTION OF PLANTS 220

habit. They are therefore in all probability adaptations
to that habit. Two more—stem anatomy and the ap-
parently terminal cotyledon in the embryo—may be
considered as direct consequences of such adaptations;
the stem anatomy acquiring its peculiar features from the
insertions of numerous broad-based leaves on a squat
subterranean axis, and the embryonic
cotyledonary number arising from the
congenital fusion of two ancestral cotyle-
dons. Theseventhcharacter—trimerous
floral symmetry—bears no obvious re-
lation to the geophilous habit, but is not
inconsistent with it.”

Recent evidence as to how monocoty-
ledony may have been derived from
dicotyledony has been furnished by a
study of the embryogeny of Agapanthus 5., 498.—
umbellatus L’Her (Fig. 108), a South agapanthus
African plant of the Lily family. umbellatus. A,

The sequence of events is as follows.’ a

mbryo. B, dicoty-
As the massive pro-embryo enlarges the ee emhaye:
root-end elongates, thus remaining (Redrawn from
narrow and pointed; while the shoot-end ae by W. J. G.
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 more active points (primordia)
appear and begin to develop. Soon the whole zone is
involved in more rapid growth, resulting in a ring or

1The above description closely follows Coulter and Land. The origin
of monocotyledony. Bot. Gaz. 57: 509-518, June, roT4.

230 HEREDITY AND EVOLUTION IN PLANTS

tube, but with the primordia still evident. The cotyle-
donary zone continues its growth until a tube of con-
siderable length is developed, leaving the apex of the pro-
embryo 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 cotyledons; or one of the
primordia may cease to grow, resulting in an embryo of
only one cotyledon; in other words, the entire cotyledo-
nary zone may develop under the guidance of only one
growing point. It is not that one cotyledon is eliminated,
but the whole growth is diverted into one. There thus
develops what appears to be an “‘open sheath” and a
‘‘terminal”’ cotyledon.

In other words, according to Coulter and Land, mono-
cotyledony is not the result of the fusion of two cotyledons,
nor of the suppression of one; but is simply the con-
tinuation of one growing point on the cotyledonary ring,
rather than a division of the growth between two growing
points. In a similar way, polycotyledony is the appear-
ance and continued development of more than two growing
points on the cotyledonous ring (Cf. p. 222, and Fig. 104).

We are not in possession of enough facts to construct
a genealogical tree showing the derivation of Mono-
cotyledons from Dicotyledons, nor the derivation of the
original Angiosperm stock, but the table of Arber and
Parkin (Table IV, p. 231) shows in a very general pro-
visional way a possible course of events, and the ap-
proximate geological period when the various advances
were made, beginning with the Paleozoic Cycadofilices
(Pteridosperms).

The first step in the immediate evolution of the Angio-

THE EVOLUTION OF PLANTS 231

TABLE IV
(After Arber and Parkin)
' Recent | a
qa
} q S
8 3
: 3 3
Tertiary > 6
I Ss °
! o &
a
es
—-—.
r 9 Ranalian plex} us Bs
i o
d : Ta 5 | Eu-anthostrobilateae| 5
Mesozoic ; 3 2s e
2 F 3
6) ey , 2
& | Pro-anthostrobilateae| &
§
oO
ees cs q
1
Monosporangiateae) @ Amphisporangiateae
< F |
: 8
Palaeozoic 2
=
1 be |
1 o
i aah 1
f l Ay i
;
| |
) : |

sperms, according to Arber,’ was the transfer of the pollen-
collecting surface from the ovule to the carpel or carpels,
resulting in the stigma as now known. “It was this act
which called the Angiosperms into being.”

Arber does not regard the Apetalous orders (Piperales,
Amentiferous families, and Pandanales) as primitive
Angiosperms, for that theory necessitates the view that
the perianth arose de novo, by enation.” He considers

1 Arber, E. A. Newell. On the origin of Angiosperms. Jour. Linnean.
Soc. Bot. 38: 29-80. July, 1907.

2Cf, ps 132:

232 HEREDITY AND EVOLUTION IN PLANTS

the perianth an ancient structure, present in the ancestors
of the Angiosperms, and inclosing an axis (‘“‘amphispor-
angiate cone”’) bearing both megasporophylls and micro-
sporophylls. Such a structure is called by Arber an
“anthostrobilus.’ The term “flower,” should be re-
stricted to Angiosperms, and may be termed an “ew-
anthostrobilus.” The earlier form of anthostrobilus (such
as occurs in modern Gymnosperms, and in the Mesozoic
Benettitee) is called a pro-anthostrobilus. The hypo-
thetical, direct ancestors of the Angiosperms are called
“Hemiangiosperme,’ and the possible order of evolu-
tionary development is conceived by Arber as follows:

Raeiepeias Eames and Tertiary (Recent)

? Eu-anthostrobilate.

4. Hemiangiosperme
(Fossils unknown)

3. Cycadofilices

Mesozoic—Pro-anthostrobilatz.

2. Heterosporous fern-like

ancestor Paleozoic—Non-strobilate
1. Homosporous fern-like ancestors.

ancestor

148. Ancestors of the Gymnosperms.—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.

The leaves of Cordaites resembled those of the Kauri
pines (Agathis) of the southern hemisphere (Fig. 109),
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

THE EVOLUTION OF PLANTS 233

to six microsporangia (pollen-sacs) on astalk. The female
cones resembled the male in general appearance, and
the seeds resembled those of the Cycadofilicales (F ig. 94).
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

Fic. 109.—Branch, with cones, of the Kauri pine (Agathis australis).
(From the Gardener’s Chronicle.)

wide departure from the Cryptogams, and must be con-
sidered as true seed-bearing plants. They were closely
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):

234 HEREDITY AND EVOLUTION IN PLANTS

Coniferales (modern cone-bearing trees).
Ginkgoales (primitive gymnosperms).
Cordaitales (transitional conifers).
Cycadales (true cycads).

Cycadofilicales (cycad-like ferns).

. Filicales (true ferns).

149. 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.

150. A Late Paleozoic Landscape.—The frontispiece
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 areshown. The dragon-flies of that
time are known to have had a spread of wing amounting,
in some cases, to as much as two 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).
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,

HP eens

THE EVOLUTION OF PLANTS 235

ancestors of our modern conifers and angiosperms, must
be imagined as hidden in the recesses of the forest.

151. Significance of the Fossil Record.—Before the
brilliant discoveries in fossil botany, just outlined, were
made, there had been (as stated in Chapter VI) a general
tendency among botanists to consider the comparatively
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
Na&geli, who considered that the sporophyte of Pterido-
phytes was derived from a moss-like sporogonium by the
development of leafy branches.

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 possibly 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.

1Cf., however, p. 172-

236 HEREDITY AND EVOLUTION IN PLANTS

152. Summary of Results.—From what has been said,
in this and in Chapter VI, we recognize that the method
of evolution is to be ascertained chiefly by experiment—by
studying living plants in action; but the course of evolution
chiefly by the study of comparative morphology, with special
attention to fossil forms, and supplemented by the facts of
geographical distribution. 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 Cycadofilicales.

. The origin of Cycadofilicales from Primofilices.*

. The origin of Filicales from Primofilices.

. The origin of Cordaitales from Primofilices.

. The origin of Coniferales from Cordaitales.

An ancestral tree embodying these views is shown in
Tig. 110.

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.

153. 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.

Dun pw

1 The term Primofilices, not hitherto used in this text, refers to a hypo-
thetical, primitive fern stock.

THE EVOLUTION OF .PLANTS

237
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Ancestors of Primofilices

Fic. 110.—Genealogical tree, showing the ancestral lines of the mod-
ern plant orders, according to a monophyletic hypothesis. Full ex-
planation in the text. (Cf. Fig. 111.)

238 HEREDITY AND EVOLUTION IN PLANTS

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 primitive character), and by few spor-
angia on the upper surface of the leaves. The Pteropsida,
by contrast, like the modern Filicales, are in general, dis-
tinguished 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 Pterop-
sida, 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 paleo-
botanists to be in the direct ancestral line of our modern
vascular plants, substantially as indicated in Fig. 111.1

(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
polygenetic origin from the stage of primitive protoplasm.
Such views are illustrated in Table V (p. 240).

It is seen from this diagram that our modern ferns have
a long ancestral history, extending from the present back

1 Scott restricts the name Lycopsida to the Lycopodiales, and proposes
a third group, Sphenopsida, incuding the Equisetales, Pseudoborniales,
Sphenophyllales, and Psilotales. Wieland has recently adduced reasons
for using the term Hemicycadales, vs. Bennettitales. (Cf. foot-note,
p. 211.)

THE EVOLUTION OF PLANTS 239

Legon

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Ancestors of Lycopsida.and Pteropsida

Fic. r11.—-Genealogical tree, showing the ancestral line of the modern

plant orders according to a polyphyletic hypothesis. Full explanation
in the text. (Cf. Fig. 110.)

240 TEREDITY AND EVOLUTION IN PLANTS

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

TaBLe V
Ascéndaiic Periods Persistence and relationship of
y great groups
wm) {2
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Comanchian |S) Biola] |2 =
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Jurassic a 8 _ rs
VI, Reign of Pro-angiosperms| Triassic io} g giz <
Permian S| |s Sly 3
O}..2| egal.
V. Reign of Acrogens (High-| Pennsylvanian 4 iS as i} 8
er Equisetes. Lycopods,! Mississippian | Sl eiajS) Pe
? By o a 3 ° oO
etc.) 4 SISlaS) 3s
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IV. Reign of Gymnosperms | Devonian 3
Silurian 5. Actual Fossil Land Plant rec-
III. Reign of Early Land ere Begs j
Plants 4. Primofilices—Early Equisetes
Ordovician 3. Basal Plant Complex with va-
riety of species
Cambrian 2. Differentiation of Dry Land
Il. Reign of Algze Precambrian and Aquatic Plants
(Proterozoic) (Fossil Algz abundant)
I. Reign -of Primitive Life| Old Precam- APossil Alge begin)
(Hypothetical) Baek 1, Primitive Protoplasm and
(Archeozoic) Unicellular Life

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 converging below toward the pole,
and the Cordaitales as coming between the Ginkgoales and Filicales, to
both of which they are related.

THE EVOLUTION OF PLANTS 241
!

recently as 20 million years ago. The lateness of their
appearance and the rapidity with which they have spread,
until they are now the dominant element in the flora
of the land, indicate how well they are adapted to their
environment. “Nothing is more extraordinary in the
history of the vegetable kingdom,” wrote Darwin to
Hooker, “than the apparently very sudden or abrupt
development of the higher plants.”

“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
nothing but one particular stage in the evolution of the
Vegetable Kingdom.””!

154. 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
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 Jong, 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.

1Scott, D. H. “Studies in Fossil Botany,” p. 3.
16

242 HEREDITY AND EVOLUTION IN PLANTS

Sixty years ago Lord Kelvin gave a mean estimate of
100,000,000 years. With this estimate two 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.”

CHAPTER XIII
THE GREAT GROUPS OF PLANTS

155. The entire question of taxonomic groups is very
difficult and intricate, and there is at present a consider-
able difference in opinion and usage, even among those
equally competent to judge. As set forth in Chapter IX,
the segregation and sequence of larger groups may be
based chiefly upon the morphology of living plants, or
upon that basis supplemented by the findings of anatomy
(including embryology and histology), comparative life
histories, and paleobotany.1. Manuals and “‘Floras” of
systematic botany are, for the most part, arranged upon
the former basis, which operates at present in the direction
of conservatism and few changes in connection with the
largest groups, or phyla. Regard for the evidence from
other sources is more apt to result in conflict of opinion and
more frequent revisions in the light of new studies, but it
is also more apt to result in a closer approximation to the
truth. In the former case the sequence of groups is
chiefly based upon complexity of organization, proceeding
from the simpler to the more complex. On this basis the
monocotyledons, for example, would precede the dicotyle-
dons, the order observed in the Manuals.

In the latter case the sequence of groups attempts to
indicate or reflect their order of development in time, as
indicated by the data of paleobotany, comparative life-
histories, comparative anatomy, and plant geography.
On this basis the monocotyledons would follow the

1See, also, p. 236.
243

244 HEREDITY AND EVOLUTION IN PLANTS

dicotyledons, as being derived from the latter by a process
of simplification (Cf. p. 223). The structural and ana-
tomical evidence that eusporangiate ferns are more
ancient than leptosporangiate ferns is rendered more cer-
tain by the fact that the earliest fern fossils (in Paleozoic
rocks) are eusporangiate; the leptosporangiate forms do
not appear until later, and the fossils belong to families
closely related to the more ancient eusporangiate group,
while the fossils of more recent rocks show closer affinities
with the modern living forms. (See, however, p. 30.)

In any tabular arrangement including all the great
groups or phyla every group must, of course, come defi-
nitely after some one group and precede another. Thus,
mosses logically fall between the Thallophytes and the
fern allies; but there is scarcely any evidence that they are
phylogenetically related to the groups that follow them
in the table. Strangely enough, there are few well-
authenticated fossil remains of mosses (and those not
below the Mesozoic), and it has even been seriously
suggested that they may have developed from more
complex groups by processes of reduction and simplifica-
tions; but there is little, if any, evidence to indicate from
what higher group they might have been thus derived,
and the positive, though meager, fossil evidence is suffi-
cient to render highly improbable, if not to nullify, the
suggestion of derivation by reduction.

The old group ‘“Pteridophytes,” of the manuals,
including the true ferns and their “allies” (horsetails,
lycopods, and little club-mosses), served a useful purpose
before the recent researches in fossil botany; but the
results of those studies have made it impossible consist-
ently to maintain the group longer in its former con-

THE GREAT GROUPS OF PLANTS 245

notation. The term “Pteridophyta” may still be used
to advantage in a more restricted sense, as applying
to the “true ferns,’”’ while the ‘‘fern allies” naturally
fall into two other Divisions or phyla, namely the Club-
mosses (Lepidophyta) and the Horsetails (Calamophyta
of Bessey, or Arthrophyta of Berry).

The discovery of the fossil seed-bearing ferns (Cycado-
filicales) and their fossil and living relatives (Hemicycad-
ales, Cycadales, Cordaitales, and Ginkgoales), all having
cryptogamic (i.e., centripetal) wood,! and all the living
forms distinguished from the other gymnosperns by the
possession of ciliated motile sperms, suggested the group
to which Jeffrey has given the convenient and descriptive
term Archigymnosperme (Early Gymnosperms) :in contrast
to the Yews (Taxales), Conifers (Pines, Spruces, Hem-
locks, Firs, Cypress, etc.), and Gnetales, which lack both
those characters. To this latter group Jeffrey has given
the name, Metagymnosperme (Late Gymnosperms).

Other authors have suggested grouping the woody-
stemmed and comparatively small leaved Cordaitales,
Ginkgoales, and Coniferales together, and apart from

1The first formed woody tissue is primary wood or proloxylem. It is
present when the organ (stem, root, etc.) is young, and its cell walls
are thickened in rings or spirals and thus it can readily stretch as the
organ elongates in growth, After growth in length has ceased, or has
been greatly retarded, secondary wood or metaxylem,forms. The cell
walls of this tissue have scalariform, reticulate, or pitted thickening,
and thus they cannot readily stretch. In the vascular cryptogams (e.g.,
Club-mosses and related forms) the secondary wood forms inside the
zone of primary wood; in the later or “higher” gymnosperms (Metagym-
nosperms) this order of development is reversed; while in the ferns and
lower gymnosperms (Archigymncsperms) the earlier development is cen-
tripetal and the later centrifugal. Thus the mode of development of
the woody tissue is an index of the evolutionary position of a, given
form.

246 HEREDITY AND EVOLUTION IN PLANTS

the Cycadean series (Cycadales, Hemicycadales (Bennet-
titales) , Cycadofilicales) which have pithy stems; but some
of the Cordaitean forms also have pithy stems and com-
paratively large leaves. Here again, as so often, an
attempt at a formal classification necessitates drawing
an apparently sharp line where in fact one does not exist.
As Professor Jeffrey! has said, the term Archigymno-
sperme is one of convenience, and like most scientific
terms falls short of covering the situation. On the basis
of certain criteria (e.g., the structure of the wood), the
Ginkgoales appear to be intermediate between the Coni-
ferales andthe Cordaitales. In fact, as Jeffrey” has pointed
out, the “living fossil,’ Ginkgo, may be regarded as a
connecting link or transitional form between the Archi-
gymnosperme and the Metagymnosperme.

The relationship of Jsoetes is one of the most difficult
to determine among all the vascular cryptogams. Argu-
ments for and against interpreting it as derived by re-
duction from the Lepidodendron group are given by Lady
Isabel Browne.? The secondary growth in thickness of
its stem (in such a dwarfed form) must be regarded as a
character of long standing, not recently acquired; plants
in both groups have mucilage cavities. Isoetes resembles
some of the Lepidodendrales (e.g., the so-called Stigmaria‘)
in the dichotomous branching of its roots. Other facts
of structure (e.g., the occurrence of the sporangia on the
upper side of the leaves) have also been interpreted

1Jeffrey, E.C Science, N. S. 47: 316. 1918.

*Jeffrey, E. C. The anatomy of woody ptanls, p. 315. Chicago, 1917.

3 Browne, Lady Isabel. The phylogeny and inter-relationships of the
Pteridophyta. New Phytologist 7: 93, 103, 150, 181, 230. 1908.

4The fossil remains to which the generic name Stigmaria was assigned
have long been known to be the root-system of Sigillaria.

THE GREAT GROUPS OF PLANTS 247

as pointing to the origin of Isoetes (by reduction) from
the Lepidodendrales. One of the most cogent objections
to this theory is the great amount of reduction which
must be postulated; moreover, Isoetes has no cone,
while most of the Lepidodendrales have. The absence
of secondary growth in thickness of the stem and of a ligule
on the leaves, combined with the possession of a biciliate
sperm, in Lycopodium, would tend to preclude its close
affinity with [soetes. While certain features of sporophyte
anatomy (e.g., the possession of a ligule) suggest Selag-
inella, it seems difficult to accept a close relationship
between Jsoetes and the Selaginellales, since the sperms
of the latter like those of Lycopodium are biciliate, while
those of Isoetes are multiciliate. The possession of multi-
ciliate, sperms and the structure of the archegonia suggest
affinity with the eusporangiate pteridophytes, and notably
with the Marattiales.

Without going further into details which belong to a
more advanced and technical treatise than this, and
disregarding certain mooted points, or almost equally
balanced choices like the one just mentioned, it may be said
that the following tabular statement (pp. 249-251) reflects
the present state of our knowledge concerning the rela-
tionship and developmental sequence (phylogeny) of the
great Divisions and Orders! of the Kingdom of Plants.
The same thing is shown diagrammatically in Fig. 112
(p. 248). The tabular statement aims, not only to indi-
cate the relationship and sequence of groups, but also to
help the student define the terms commonly met with in
the established literature of botany.

1 Attention is called, in passing, to the uniform termination (-ales) of
the plant Orders.

248

HEREDITY AND EVOLUTION IN PLANTS

Leaves small,
sporangia on
upper aide,
woody oylinder
continuous

LYCOPS DA

Leaves large,
sporangia on
under Bide,
woody oylinder
with foliar
gaps

PTEROPS IDA

Legend:

} living planta

OLUBMOSSES | HORSETAILS| FERNS SEED PLANTS
(Lepido~ (Calamo- (Pteridophyte (Spermatophyta }
phyta) phyta) In restricted
sense Lower
Gymmo-
sperms
g
a a a 13
r) o ” oe
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oe
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! ! Seed! s a q
| ! nake! 4 oo
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o T hot ray
ola HH i oiliatea a3
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os i=] | °
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rt A A
<j
Stem Stem with|/ Euspor- | Lepto- Eusporan- g tal
smooth, ridges & || angiate | sporan- giate ;
leaves joints, giate g
spirally | leaves With 3
arranged,| whorled, Without seeds seeds A
sporangial sporangia °
single several EY

| well known fossil plants

| foasile of doubtful affinity

MAIN GROUPS OF VASCULAR PLANTS

Their apparent affinities and approximate geological distribution

Fic.

II2.

+ exclusively foasil groups

THE GREAT GROUPS OF PLANTS 249

TaBLE VI

THE GREAT GROUPS OF THE KINGDOM OF PLANTS
MAIN GROUPS OF NON-VASCULAR PLANTS
Plants without “flowers” —Cryprocams (Nos. 1-5)
Plant body usually a thallus; sexual organs usually one

suicy cme niu eA Ree anes te co aaa ss Hd Thallophytes x
No archegonia
Chlorophyll-bearinpiny.cgee cs ceeyatoricatarneuk eaaee 6 Alge 1a
Non-chlorophyll bearing...........0.-... 000. c cece cee Fungi 1b
Plant body thalloid or leafy; sexual organs usually several
CONEM ciao a4 gulpe.nle satelite wan WRU Aodh ek ate cn etna Bryophytes 2
Archegonia

Protonema rudimentary or wanting,
sporophytes with elaters

Protonema well defined,
sporophytes without elaters

Pha ehigan ity apse fet a . .....Liverworts 2a

$Se yee ke Biaey esse wiawaas Mosses 2b
1. Thallophytes
1a. Alge
Cyanophycee (Blue-green) Pheophycee (Brown)
Chlorophycez (Green) Rhodophycee (Red) .
1b. Fungi
Myxomycetes (Slime-molds) Basidiomycetes (Spores on stalks)
Schizomycetes (Bacteria) Including the Basidiolichenes

Phycomycetes (Molds) Fungi imperfecti (Life histories
Ascomycetes (Spores in sacs) imperfectly known).

Including most Lichens

(Ascolichenes)

2-6b, Archegoniates; 2-6c, Embryophyta
2. Bryophytes
2a. Liverworts

Ricciales Jungermanniales
Marchantiales Anthocerotales

2b. Mosses
Spagnales (Peat mosses) Bryales (True mosses)

Andregales (Black mosses)

MAIN GROUPS OF VASCULAR PLANTS

Woody cylinder continuous, (¢.e., without foliar gaps),
leaves small, sporangia above——LycopsIDA

250 HEREDITY AND EVOLUTION IN PLANTS

3-5, Vascular Cryplogams
Stem smooth, leaves spirally arranged, sporangia single. Clubmosses 3
Stem with ridges and joints, leaves whorled, sporangia
SOV EL Al a ioeninacaenAe wesc ecenneetaln pie eee ees Horsetails 4
Woody cylinder discontinuous (z.e., with foliar gaps),
leaves large, sporangia below—PTEROPSIDA

Without seedse vs ok ces stis sich nodduaiduidie eee ae Seaton Ferns 5
With seeds—SprRMATOPHYTA (PHANEROGAMS)...... Seed Plants 6

Ovules naked, endosperm formed before fertilization.Gymnos perms 6
Sperms ciliated yoso¢ encae paces ce eta den Early Gymnosperms 6a
Sperms not ciliated........ 0.0... . cc eee Late Gymnos perms 6b

Ovules enclosed, endosperm formed after fertilization . Angiosperms 6c

3. Clubmosses (Lepidophyta (Bessey))

Lycopodiales Selaginellales
Isoetales (?) Lepidodendrales (Fossil)
Psilotales
4. Horsetails (Calamophyta (Bessey), Sphenopsida (Scott), Arthrophyta
(Berry))
Spenophyllales (Fossil) Calamariales (Fossil)
Pseudoborniales (Fossil) Equisetales
5. Ferns (Pteridophyia, in restricted sense; Filicinee). .
Eus porangiate Leptos porangiate
Primofilices (Ccenopteridez) Osmundales
Marattiales Polypodiales
Ophioglossales Marsiliales
(Isoetales?)

. Seed-Plants
(6a & 6b Gymnos perme of Brongniart)

6a. Early Gymnos perms (Cycadophyta (Nathorst) except Ginkgoales;
Archigymnos perme (Jeffrey))

Cycadofilicales (Fossil) Cordaitales (Fossil)

Hemicycadales (Wieland) = Ginkgoales
Bennettitales of Potonié (Fossi/)

Cycadales

6b. Late Gymnosperms (Conifere (Hallier); Metagymnosperme (Jef-
frey); Strobilophyta (Bessey))
Taxales Pinales
Araucariales Gnetales

THE GREAT GROUPS OF PLANTS 251

6c. Angiosperms (Angiosperme; Anthophyta (Braun))
Two cotyledons, leaves net-veined, parts of the flower in 5’s or 4’s

—DicotyLEepons
A petale (petals wanting)
1. Casuarinales 8. Fagales
2. Piperales 9. Urticales
3. Salicales to. Proteales
4. Myricales 11. Santalales
5. Leitneriales 12. Aristolochiales
6. Balanopsidales 13. Polygonales
7. Juglandales 14. Chenopodiales

Polypetale (petals distinct—wanting in a few exceptional cases)

1. Ranales 8. Malvales

2. Papaverales 9. Parietales
3. Sarraceniales Io. Opuntiales
4. Rosales 11. Thymeliales
5. Geraniales 12. Myrtales

6. Sapindales 13. Umbellales

7. Rhamnales

Sympetale; Gamopetale (petals more or less united)

1. Ericales 6. Plantaginales
2. Primulales 7. Rubiales

3. Ebenales 8. Valerianales
4. Gentianales 9. Campanulales

5. Polemoniales

One cotyledon, leaves usually parallel-veined, parts of the flower in 3’s
or 6’s—MOoNOCOTYLEDONS

1. Naiadales 6. Arales

2. Pandanales 7. Xyridales
3. Graminales 8. Liliales

4. Palmales (Principes) 9. Scitaminales

5. Cyclanthales (Synanthz) 10. Orchidales

BIBLIOGRAPHY!

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Coulter, John M. Evolution, heredity and eugenics. (School Science

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Delage and Goldsmith. The theories of evolution. New York, 1913.

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Gray, Asa. Darwiniana. New York, 1876.

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Lock, R. H. Recent progress in the study of variation, heredity, and
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Morgan, T. H. A critique of the theory of evolution. Princeton Univ.
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Osborn, H. F. The Origin and Evolution of Life. New York, 1917.

Osborn, H. F. From the Greeks to Darwin.

Pemberton, Henry. The path of evolution. Philadelphia, 1902.

Seward, A. C. (Editor.) Darwin and Modern Science. Cambridge
(Eng.), and New York, 1909.

Thomson, J. Arthur. Darwinism and human life. New York.

1 Omitting periodical literature
252

BIBLIOGRAPHY 253

Thomson and Geddes. Evolution. New York, ro11.

Wallace, Alfred Russel. Darwinism. 3d. Ed., London, rgor.

Fifty years of Darwinism: Modern aspects of Evolution.
New York, 1909.

MUTATION AND EXPERIMENTAL EVOLUTION

Doncaster, L. The Determination of Sex. Cambridge Univ. Press.
1914.

Gates, R.R. The Mutation Factor in Evolution. London, 1915.

Lotsy, J. P. Evolution by means of hybridization, The Hague, 1916.

Vries, Hugo de. The mutation theory. Eng. trans. by Farmer and
Darbishire. Chicago, 1909.

Vries, Hugo de. Species and Varieties, their origin by Mutation.
Chicago, 1905.

Vries, Hugo de. Intracellular pangenesis. Eng. trans, by C. Stuart
Gager, Chicago, 1910.

Vries, Hugo de. Gruppenweise Artbildung. Berlin, 1913.

Weismann, A. Theevolution theory. Trans. by J. Arthur Thomson and
M.R. Thomson. 2 vols. London, 1904.

HEREDITY

Babcock and Clausen. Genetics in relation agriculture. New York, 1919.

Babcock and Collins. Genetics laboratory manual. New York, 1918.

Bailey and Gilbert. Plant breeding. New York, rors.

Bateson, W. Materials for the study of variation. London, 1894.

Bateson, W. The Methods and Scope of Genetics. Cambridge, 1808.

Bateson, W. Problems of Genetics. Yale Univ. Press. 1913.

Bateson, W. Mendel’s Principles of Heredity. Third Impression.
Cambridge and New York. 1913.

Castle, W. E. Genetics and eugenics. Cambridge (Mass.), 1916.

Castle, Coulter, et al. Heredity and Eugenics. Chicago, 1912.

Conklin,E.G. Heredityand Environment. Princeton Univ. Press, 1915.

Coulter, John M. Fundamentals of plant breeding. New York &
Chicago, 1914.

Coulter (John M.) and Coulter (Merle C.). Plant genetics. Chicago,
1918.

Ditbkhire, A.D. Breeding and the Mendelian Discovery. London and
New York, 1913.

Darwin. C. The variation of animals and plants under domestication.
2d Ed. New York, rgoo.

254 HEREDITY AND EVOLUTION IN PLANTS

Davenport, C. B. Heredity in Relation to Eugenics. New York, 1911.

Davenport, E. Principles of breeding. Boston, 1907.

Doncaster, L. Heredity in the light of recent research. Cambridge
(Eng.), 1912.

Mendel, Gregor. Experiments in plant hybridization. Eng. trans. by
Royal Horticultural Sec. (In Bateson, W., Mendel’s principles of
heredity. Cambridge, 1909.) The original paper, Versuche sber
Pflanzen-H ybriden, was published in Verhandlungen des naturforschen-
den Vereines in Brinn. Abhandlungen IV, Band 1865. Briinn,
1866.

Morgan, T. H. ef al. Mechanism of Mendelian heredity. New York,
1915.

Morgan, T. H. The physical basis of heredity. Philadelphia, rorg.

Popenoe and Johnson. Applied eugenics. New York, ro19.

Punnett, R.C. Mendelism. 3d Ed. New York, 1911.

Thomson, J. A. Heredity. London, 1908.

Vernon, H. M. Variation in Animals and Plants. New York, 1903.

Vries, Hugo de. Plant breeding: Comments on the experiments of
Nilsson and Burbank. Chicago, 1907.

Walter, Herbert E. Genetics: An introduction to the study of heredity.
New York, rors.

Weismann, A. Essays upon heredity and kindred subjects. 2 vols.
Trans. Oxford, 1891, 1892.

Weismann, A. The germ-plasm: a theory of heredity. Trans. by W.
N. Parker and H. Rénnfeldt. London, 1893.

EVOLUTION OF PLANTS

Bailey, L.H. The Survival of the Unlike. New York, 1897.

Bower, F.O. Plant life on land. Cambridge (Eng.), 1911.

Bower, F.O. The origin of a land flora. Cambridge, 1908.

Campbell, D. H. Plant life and evolution. New York. t1og11.

Campbell, D. H. Lectures on the evolution of plants. New York, 1899.

Coulter, J. M. The evolution of sex in plants.

Jeffrey, E.C. The anatomy of woody plants. Chicago, 1917.

Scott,D.H. Studies in fossil botany. 2d Ed. London, 1909.

Scott, D. H. The evolution of plants. New York & London, 1911.

Seward, A. C. Links with the past in the plant world. Cambridge,
IQII.

Seward, A.C. Fossil plants. 3 vols. Cambridge (Eng.), 1898-1917.

Shimer, H.W. An introduction to the study of fossils. New York, 1914.

Stopes, Marie C. Ancient plants. i910.

Wieland, G.R. American fossil cycads. Carnegie Institution of Wash-
ington, Publication No. 34, Vols. 1-2, 1906.

BIBLIOGRAPHY 255

PLANT GEOGRAPHY

De Candolle, Alphonse. Géographie Botanique raisonnée. Geneva,
1855.

Humboldt and Bonpland. Essai sur la géographie des plantes. Paris,
1805.

Huxley, Leonard. Life and letters of Sir Joseph Dalton. Hooker.
New York, 1918.

Marchant, James. Alfred Russel Wallace: Letters and reminiscences.
New York, 1916.

Scharff, Robert Francis. Distribution and origin of life in America.
New York. The Macmillan Co. 1912. (Treats chiefly of Animals.)

Schimper, A. F. W. Plant geography upon a physiological basis. Eng.
translation by Fisher. Oxford, 1903.

Thistleton-Dyer, Sir William. The distribution of plants. In Darwin
and Modern Science, Chapter XVI. Ed. by A. C. Seward. Cam-
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Wallace, Alfred Russel. Geographical distribution of animals. New
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Island Life, 3d Ed. Revised. London, rg11.

Darwinism. Chapter XII. The geographical distribution of

organisms. 3d Ed. London, 1901.

My life. New York, 1906.

Warming, Eugene. Botany of the Faerées. London, 1901-1908.
(Especially the chapter on “The history of the flora of the Faerées,”’
pp. 660-681).

PERIODICALS

American Journal of Botany, Brooklyn Botanic Garden, Brooklyn,
N. Y.

American Naturalist. The Science Press, Garrison, N. Y.

Annals of Botany. Cambridge University Press, Cambridge,
England.

Botanical Abstracts, Baltimore, Md.

Botanical Gazette, The University of Chicago Press, Chicago, Ill.

Ecology, Brooklyn Botanic Garden, Brooklyn, N. Y.

Genetics. Princeton Univ. Press, Princeton, N. J.

Journal of Heredity. Amer. Genetic Association, Washington, D. C.

New Phytologist, London, Eng.

Revue général de Botanique, Paris, France.

INDEX

Acer Pseudo-Platanus, 222
Acquired characters, 72
Acropera, 95
Adiantum, 9, 21, 22
concinnum, 25
emarginatum, 126
Africa, 145
Agapanthus umbellatus, 229
Agassiz, Louis, 85
Agassiz’s Hypothesis, 84
Agathis, 232
australis, 233
Age and Area, Hypothesis, of, 176
Aleurites moluccana, 163
Algz, distribution of, 176
Alternation, homologous, 135
of generations, 132
ontogenetic hypothesis of, 136
Ambrosia crithmifolia, 149
Anaphase, 36
Anatomy, evidence from compara-
tive, 127
results of the method of com-
parative, 129
Andes, 149
Angiopteris, 95, 203
evecta, 204
Angiosperm, primitive, 225
Angiosperms, ancestors of the, 210
Anisosorus hirsulus, 1
Annulus, 10
Antheridia, 18, 22, 23
Anthostrobilus, 232
Apiéal cell, 126
Aralia, 156
Archegoniates, 130

Archegonium, 18, 21, 24
Archigymospserme, 245
Arthrophyta, 245

Ascaris, 38

Ash, 113

Aspidium Filix-mas, 11
Autochthonous hypothesis, 142
Azolla, 82

Azores, 149, 150

Bananas, 113
Barberry family, 171
Beagle, 145
Bean pond, 186
Beans, 66, 104, 105, 107
Bennettitales, 211
Bennettites, 214
Berberidacee, 171
Bibliography, 252
Biometry, 54
Biophors, 75
Birds, dispersal by, 149
Blakeslee, 52
Blue eyes, 60
Blue rose, 73
Bolivia, 149
Boston fern, 16, 109, 113
Botany, the major problem of, 84
Botrychium Lunaria, 128
Boveri, 74
Bowenia, 214

serrulata, 203
Bower, Professor, 11
Bracken fern, 2
Brake, 2
Brassica oleracea, 113, 124

17 257

258 INDEX

Bread mould, 80

Broccoli, 112

Brousonetia papyrifera, 163
Brown eyes, 69

Brown, Lady Isabel, 135
Brussels sprouts, 112
Bryophytes, 127

Bucholz, 127

Bud-sport, 114

Bulbils, 16

Cabbage, wild, 112
Cacti, thornless, 113
Calamophyta, 245
Calla, 80
Camptosorus rhizophyllus, 15, 17
Canary Islands, 161
Cannabis sativa, 163
Capsella Bursa-pastoris, 95
Castalia flava, 179
tetragona, 158
Cauliflower, 112
Ceylon, 145, 150
Character-units, 64
versus unit-characters, 65
Chromatin, 36, 75
Chromosomes, 37
Chrysanthemum leucanthemum, 154
Cinnamon fern, 12
Classification, 121
evolution and, 122
Clayton’s fern, 13
Cliff-cabbage, 124
Clover, 113
Coal balls, 204
Cocos Island, 143
Coleoptile, 224
Cone-flower, 114
Cordaitales, 233
Cordaites, 232
Corsica, 144
Cotyledonary ring, 222

Cross-fertilization, 25
Crossing, increased vigor from, 69
Crossotheca Hoeninghausi, 202,
Cultivation, escapes from, 163
Cultures, pedigreed, 59
Curve of frequency, 106
Cycadeoidea, 212, 214, 220
dacotensis, 208, 209, 210, 215,
219
Dartoni, 216
ingens, 215
Wielandi, 213, 214
Cycadocephalus Sewardi, 208
Cycadofilicales, 201, 207
Cycas circinalis, 212
Cyrtomium falcatum, 9
Cystopteris bulbifera, 16

Darwin, Charles, go, 91
Darwinism, 90, 92

mutation theory to, 118
De Candolle, 144
Dendrobium attenuatum, 147
Dendroceros. 174
Determinants, 75
Determiner, 64
de Vries, Hugo, 74, 101, 102
Diapensia lapponica, 162
Dicotyledons, monocotyledonous,

226

Dicotyledony, origin of, 220
Dictyota, 135, 137

dichotoma, 136
Diervilla, 154

Lonicera, 155

rivularis, 155

sessilifolia, 155
Differentiation, Dorso-ventral, 20
Dionaea muscipula, 178, 179
Dioscorea bonariensis, 226, 227
Diplazium selanicum, 7
Diploid, 25, 35

INDEX

Disease-resistance, breeding for, 71
Dispersal, means of, 142
Distribution, continuous, 168
types of, 154
significance of geographical, 139
Division, heterotypic, 37
homotypic, 37
Dodder, 80
Dominance, law of, 59
Dominants, 63
Drosera filiformis, 197, 198
Drynaria meyeniana, 4
Dumortiera, 174

Earthworms, 194
Egg, 21, 24
Elementary species, 111
Elephants, 94
Elodea, 196
Enation, 132, 135, 231
Endemism, 164
Environment, adjustment to, 41
fitness for, 93
fitness of the, 83
inheritance and, 66
Epiblast, 223, 224
Epidermis, 4
Equinoxes, precession of the, 192
Eequisetum, 234
telamateia, 126
Eriocaulon septangulare, 157
Eugenia malaccensis, 163
Eugenics, 76
Eupatorium, 156
Evening-primrose, 114
Evolution and classification, 122
Evolution, early antagonism to, 91
experimental, 118
inorganic, 82
meaning of, 79
method of, 84
organic, 83

259

Existence, struggle for, 42

Experimental evolution, ro1

Expression, inheritance versus, 40,
48

Extinction, factors of, 195

Factor, 64

Factors, 65

Falkland islands, 148

Fern, life-cycle of a, 35

Fern, life history of a, 1, 20, 34
Fertilization, 23, 24, 53
Fertilization-membrane, 24
Fimbristylis spathacea Roth, 146
Fittest, survival of the, 43, 97
Fleabane, 164

Foliage-leaf, 10

Foot, 25

Forbes, Edward, 161

Fossil, what is a, 183
Fossil-formation, conditions of, 185
Fossil record, significance of the, 235
Fronds, 4

Gaertner, 120

Gametes, 32

Gametophyte, 33

Gaps in the fossil record, 193

Genealogical tree, 23'7, 239

Gene, 64, 65

Generations, alternation of, 34
two kinds of, 33

Genoa, 144

Geothallus tuberosus, 174

Germination, 17

Germ-plasm, 75

Ginkgo, 139, 232, 233, 246
biloba, 168

Ginkgoales, 246

Glaciation, effects of continental,

160
Gleichenia circinata, 11

260 INDEX

Gmelin, 141 Ids, 75
Grass-embryos, 224 Incrustation, 183
Gray, Asa, 157 Indian corn, 69, 223
Green dahlias, 113 Indian pipe, 80
Green roses, 113 Indigenes, 177
Groups of plants, the great, 243 Indusium, 8
Gymnosperms, ancestors of the, 232 Inheritance, 39, 92
early, 245 and environment, 66
late, 245 and reproduction, 50

mechanism of, 73
versus expression, 40
versus heredity, 50
what is, 46

Tsoetes, 246, 247

Habit of life, consequences of an
amphibious, 130
Half-race, 223
Hamamelis, 156
Haploid, 25, 35
Hawaii, 145, 150
Hawaiian flora, origin of the, 175
Hawkweed, 76
Haworthia sp., 49
Hedge mustard, 42
Helix hortensis, 157
Hemerocallis fulva, 163
Hemiangios perma, 232
Hemicycadales, 211, 238
Hepatice, 128, 172, 174
Heredity, 45
experimental study of, 55
inheritance versus, 50
Johannsen’s conception of, 67
Heterozygous, 63
Hibiscus occuliroseus, 178
Hieraceum, 76
aurantiacum, 154
Hilldebrandia sandwicensis, 178
Honey-locusts, thornless, 113
Humbolt, 140
Hurricane grass, 146
Hybridizing, artificial, 58
Hydrangea, 156

Jack-in-the-pulpit, 80
Jelly-fish, 194

Johannsen, 40, 67

Jordan, 120

Kale, 112

Kauri pine, 233

Keeling Islands, 143

Kerner, 95

Kingdom of plants, the great
groups of the, 249

Knight, 120

Kohlrabi, 112

Kdlreuter, 120

Krakatoa, 146, 149, 175

Labrador, 160

Lagenostema Lomaxi, 205

Lakes, filling up of, 187

Lamarck, Jean Baptiste, 89
Lamarckism, arguments against, 89
Lamarck’s hypothesis, 86
Landscape, a late paleozoic, 234

Lang, 136
Hydrophytes, 196 Laverinn, 2
Idants, 75 Leaf, free-living, 82

Idioplasm, 74 Leersia clandestina, 224

INDEX

Lemna, 80
trisulca, 81
Lepidodendron, 134, 246
Lepidophyta, 245
Leptochloa arabica, 224
Leptosporangiate ferns, 10
Lesezyc-Suminski, Count, 20
Life-cycle, cytological, 38
Life-histories, evidence from, 125
Life history, 1, 3
Linin, 36
Linkage, 119
Linnaeus, 121
Linnaea borealis, 121
Liquidambar, 156
Liriodendron, 170, 219
Lomaria eriopus, 207
Lonicera japonica, 163
Loxsoma Cunninghami, 11
Lunularia cruciata, 173
Lycopersicum esculentum, 51
Lycopodium, 134, 247
complanatum, 128
lucidulum, 156
Selago, 128
Lycopods, 134
Lycopsida, 238
Lyginodendron oldhamium, 201, 202,
205
Lygodium japoricum, 11

Macrozamia, 214
Moorei, 176
spiralis, 211

Madder family, 171

Magnolia, 156, 218, 219
flower of, 217

Magnoliacee, 219

Maidenhair fern, 25

Maiosis, 37

Maize, 60, 62

Malay archipelago, 143, 147, 151

261

Malthus, 94
Marattia Douglasii, 126
fraxinea, 206
Marattiales, 247
Marble, 188
Marchantia, 173
polymorpha, 128
Marchantiacez, 173
Marsilia vestita, 126
Matonia pectinata, 11
Megasporophytes, 129
Membrane, nuclear, 36
Mendel, Gregor, 55
Mendelian ratio, significance of the,
64
Mendel, investigations since, 77
Mendelism, relation of Weismann-
ism to, 76
Mendel’s discoveries, 59
discoveries, value of, 68
law, applications of, 66
method, 56
problem, 56
Metagymnosperme, 245
Metamorphism, 188
contact, 188
regional, 188
Metaxylem, 245
Microsporophytes, 129
Migrations, plant, 192
Mistletoe, 80
American, 96
European, 96
Mitchella, 172
repens, 172
Mitosis, 36
Moluccas, 150
Monoclea, 174
Monocotyledon,
226, 227
Monocotyledony, 225
origin of, 223

dicotyledonous,

262

Monophyletic hypothesis, 237
Morus alba, 163
Moss-roses, 113
Mt. Gedeh, 174
Mt. Washington, 158
Mucor, 51
hermaphroditic, 52
Multiplication, vegetative, 16
Mutant, rrr
Mutation and discontinuous dis-
tribution, 166
examples of, 111
Mutation theory to Darwinism,
Relation of, 118
value of the, 120
Mutations, 110

Nageli, 74, 76
Naudin, 120
Nelumbo, 171
lutea, 171
nucifera, 171
Nepenthes ampullaria, 145
phyllamor pha, 147
Nephrodium filix-mas, 95
Nephrolepis, 16, 109
Nice, 144
Nicotiana tabacum, 51
Notothylus orbicularis, 126
Nymphaeacee, 171
Nymphaea mexicana, 179

Oak, 103
Objections, difficulties and, 97
Oenothera biennis, 115
brevistylis, 115, 116
gigas, 117, 167
laevifolia, 116, 167
Lamarckiana, 109, 114
115, 116, 167
nanella, 109
scintillans, 167

INDEX

Onoclea, 24, 156
Ontogeny, 83, 136
evidence from comparative,
126
Odsperm, 25, 33
Odspore, 25
Ophioglossum pendulum, 126
Orchid, seed capsule and seeds of
an, 172
Orchidaceex, 171
Organs, origin of vegetative, 131
Orton, W. A., 72
Osmunda, 169, 204
Claytoniana, 5, 13, 126
cinnamomed, 12, 154
Japonica, 154
regalis, 154
Osmundacee, 139

Paleobotany, the scope of, 183

Paleogeography, 190

Palisade layer, 8

Pangens, 74

Papaver Rhoeas, 222

Paulownia, 163

Pea, edible, 61

Peak of Teneriffe, 161

Pendulun, illustration of
110

Petrifaction, 183

Petrifactions, 185

Phaseolus vulgaris, 105

Phleum alpinum, 162

Phoradendron flavescens, 96

Photosynthesis, 8

Physalis Alkekengi, 50, 51

Phycomycetes, 80

Phyllitis, 9

Phylogeny, 83

Phymatodes, 7

Pinguicula caudata, 227
vulgaris, 226, 227

the,

Pinus Banksiana, 221
Laricio, 126
Strobus, 53
Pisum sativum, 61
Plantago major, 95
Plantain, 95
Plant groups, sequence of, 130
Plants, evolution of, 124, 201
Pleurococcus, 80, 127
vulgaris, 46, 47
Plover, American golden, 151
Pacific golden, 151
Pluchea faetida, 164
Polydemics, 177
Polyphyletic hypothesis, 239
Polypodiacee, 139, 169
Polypodium, 7, 8, 9, 12
punctatum, 6
venosum, 128
Polysiphonia, 135
Porrella, 128
Potentilla nivea, 161
Primofilices, 236
Prince’s Island, 148
Pritchardia, 165
Proangiosperms, 216
Pro-anthostrobilus, 232
Problem, the modern, 100
Propagation, 32
vegetative, 49, 5°
Prothallus, 18, 19
Protonema, 17, 18
Protoxylem, 245
Prunus Gravesii, 180
maritima, 180
Pteridophyta, 245
Pteridophytes, 244
Pteridosperms, 201
significance of the, 206
Pleris, 9
aquilina, 2, 8, 154
longifolia, 6

INDEX 263

Pteropsida, 238

Puccinia glumarum, 71

Pure line breeding, 68

Purity of gametes, theory of, 63, 64

Quercus chrysolepis, 103
Quételet, 105

Quételet’s curve, 106, 107
Quételet’s law, 105

Race, middle, 223

Radium, 73, 242

Ranales, 218

Ranunculus aquatilis, 86, 87
Ficaria, 226

Receptacle, 8

Reduction, 35
nature and method of, 36

Relict endemic, 167

Relicts, 177

Reproduction, inheritance and, 50
sexual, 32, 53

Rhizoid, 18, 17, 23

Rhizomes, 3

Rhizophora Mangle, 228

Rhizopus, 51

Rhododendron lapponicum, 159
terticillatum, 147

Riccia, 126, 129, 173
trichocarpa, 128

Rock strata, classification of, 189

Rocks, stratification of, 188

Root-stocks, 3

Rubiacee, 171, 172

Rubus chamaemorus, 155

Rudbeckia sp., 114

Rust disease, 71

Salina, 199

Saltation, orthogenetic, 109
Salvinia, 82

Sap, nuclear, 36

264

Saprophytism, 80

Sardinia, 144

Sassafras, 170

Scarlet tanager, 151

Schizaea pusilla, 155

Schouw, 142

Schouw’s hypothesis, 141

Scutellum, 224

Secd-bearing ferns, discovery of,

201

Segregation, Jaw of, 60
Mendelian, 61, 62
ratio of, 61

Selfing, 66

Self-pruning, 32

Semon, 143

Senecio, 148

Sequoia, 168, 169
gigantea, 168, 178
sempervirens, 168, 178

Seychelles, 145

Shepherd’s purse, 95

Shield fern, 11, 95

Shirley poppies, 113

Sigillaria, 246

Silene odontipetala, 222

Siphonogamy, 133

Skunk cabbage, 80

Slate, 188

Small, James, 147

Snakes, 87

Solanacee, 171

Solanum integrifolium, 50, 51
nigrum, 50, 51

Solomon archipelago, 151

Sorus, 8

Spartina cynosuroides, 224.

Special creation, doctrine of, 79

Spencer, Herbert, 43, 97

Spencerites, 134

Spermatophytes, 130

Sperms, 21, 22, 23

INDEX

Sphenophyllum, 184
Sphenopsida, 238
Sphenopteris Hoeninghausi, 201
Spiranthes Romanzofiana, 157
Spirem, 36
Spirogyra, 194, 196
Sporangia, 8, 10, 11
Sporangiophore, 8
Spore-production, consequence of
enormous, 131
Spores, dispersal of, 17
germination of, 17, 18
reproduction by, 32, 50
Sporophyll, 9
Sporophylls, 6, 7, 8, 12
Sporophyte, 33
steps in the evolution of the, 132
Sporophytes, 8
St. Helena, 148
Stangeria eriopus, 207
paradoxa, 207
Sterilization, 14
progressive, 126, 128
Stigmaria, 246
Stolons, 16, 17
Strawberry, 113
Strobiloid theory, 218, 219
Struggle for existence, 94, 96, 153
Stumps, Fossil tree, 191
Sunflowers, red, 113
Sweet flag, 80
Sweet peas, 66
Switzerland, 145
Sykes, Miss, 134
Symmetry, bilateral, 19
Symplocarpus fetidus, 155
Synangia, 209
Synapsis, 37
Synizesis, 37

Taal volcano, 152
Tamus communis, 226, 227

INDEX

Targionia, 173
Taxodium, 170
Tectoria cicutaria, 14, 16
Teneriffe, Peak of, 161
Tern, arctic, 151
Thallophytes, 130
Thomson, J. Arthur, 48
Thyrsopleris elegans, 11
Tierra del Fuego, 149
Time, distribution of plants in, r92
distribution of plants in geo-
logic, 193
table of geological, 190
the element of geological, 241
Toad-stools, 80
Todea barbara, 11
Tragspiration, 8
Trapa natans, 226
Tree ferns, 3
Tree, hypothetical ancestral, 137
Treubia insignis, 174
Triticum vulgare, 224
Twin-flower, 121

Unconformity, 189
Unit-characters,
versus, 65
Uriica dioica, 163
Utricularia, 196

character-units

Vallisneria, 196
Variation, 41, 93
and inheritance, fluctuating,
108
continuous, 103
curves of, 109
discontinuous, 108

265

Variation, fluctuating, 103, 106
illustrations of continuous, 104
two kinds of, 103

Variety, constant, 223
eversporting, 223

Vascular plants, 8

Venus fly trap, 179

Viscum album, 96

Vries, Hugo de, 74, 101, 102

Walking fern, 15, 17
Wallace, Alfred Russell, 90, 166
Wanakena, 186
Warming, 145
Water, dispersal by, 149
Water buttercup, 86, 87
Water lily family, 171
Weismannism, 74
to Mendelism, Relation of, 76
West Indies, 146
White Mountains, 160
White pine, 53
Wides, 177
Wielandiella, 220
Wiesnerella Javanica, 174
Williamsonia, 214, 220
mexicana, 219
Willow, 32
Wind, dispersal by, 143
Wolfia papulifera, 81
punctata, 81

Zamia, 215, 232

Zea Mays, 69, 223, 224
Zelkowa, 167
Zygospore, 51

Zygote, 32, 33