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THE ANATOMY OF WOODY PLANTS

THE UNIVERSITY OF CHICAGO PRESS
CHICAGO, ILLINOIS

THE BAKER & TAYLOR COMPANY 2
NEW YORK

THE J. K. GILL COMPANY
PORTLAND

THE CUNNINGHAM, CURTISS & WELCH COMPANY
LOS ANGELES

THE CAMBRIDGE UNIVERSITY PRESS
LONDON AND EDINBURGH

THE MARUZEN-KABUSHIKI-KAISHA
TOKYO, OSAKA, KYOTO, FUKUOKA, SENDAI

THE MISSION BOOK COMPANY
SHANGHAI

Diagrammatic figure of the French artichoke, Cynara Scolymus, showing distri-
bution of oil canals in the various organs and regions. For explanation see
chapter xxxi.

THE ANATOMY OF
WOODY PLANTS

By

EDWARD CHARLES JEFFREY

THE UNIVERSITY OF CHICAGO PRESS
CHICAGO, ILLINOIS

CopyRIGHT 1917 By
EpDWARD CHARLES JEFFREY

All Rights Reserved

Published October 1917

12 546

Composed and Printed By
The University of Chicago Press
Chicago, Illinois, U.S.A.

PREFACE

It is now forty years since De Bary’s classic Comparative
Anatomy of the Vegetative Organs of the Phanerogams and Ferns
made its appearance. In the interval much has been added to
our knowledge, particularly in the paleobotanical and experimental .
fields. The doctrine of descent, too, has now reached a degree of
prominence and importance which it did not possess in De Bary’s
time. As a consequence, it is desirable that the general subject
of the anatomy of the woody or so-called vascular plants should
be reviewed, with special reference to its historical and experi-
mental aspects. This is perhaps all the more desirable as an
effective counterpoise to the extreme mechanistic tendencies of
the time. It will accordingly serve a useful purpose to indicate
how large a part of the organization of existing plants is an inherit-
ance from their ancestors of earlier geological times.

In De Bary’s textbook both paleobotany and development
are deliberately eschewed. The first of these is now essential for
any adequate comprehension of comparative anatomy in its
all-important evolutionary aspects. It is abundantly clear that
the most fruitful results from the standpoint of the doctrine of
descent are to be derived from the comparative study of extinct
and existing plants belonging to the same orders, families, or
genera. It is, moreover, obvious that the living forms cannot
be interpreted without a knowledge of their past, and that to
an even greater degree the organization of fossil plants is a closed
book to those who are unfamiliar with the anatomy of allied and
still living types. The wide range of facts which must of necessity
be covered calls for a somewhat brief and even elementary treat-
ment. Fortunately, since De Bary’s time, it has become more
and more evident that the study of the development of organs
and tissues throws little trustworthy light on the processes of
evolution, and consequently that aspect of our subject need
receive no more attention than was vouchsafed to it by the great
German anatomist nearly half a century ago.

Vv

vi PREFACE

In the seventeenth chapter are summarized the important
general principles derived from the investigation of related living
and extinct organisms. The beginning of the studies leading
to the formulation of these anatomical canons stands largely to
the credit of French and English paleobotanists. Since they
have worked mainly with Paleozoic types, their activities have
been preponderantly in the direction of comparisons between the
organization of the earlier cryptogams and gymnosperms and
their still living survivors. It has been in some measure the
good fortune of American anatomists to continue the lines of
investigation thus begun and to extend them to the study of
Mesozoic and still living gymnosperms. The extremely harmoni-
ous conclusions resulting from the anatomical comparison of both
Paleozoic and Mesozoic forms with their surviving descendants
have justified the extension of the same principles to the evolu-
tionary investigation of other woody plants (particularly to the
angiosperms), concerning the geological past of which we are still
ignorant. The canons formulated in chapter xvii have as a conse-
quence been regarded of such importance that any conclusions not
in harmony with them have ordinarily not been considered in the
following pages unless they have held the ground for many years
or are at the present time advocated by anatomists of great emi-
nence. On account of the large field covered in the present neces-
sarily elementary work, this procedure has been regarded as
essential, and it is hoped that, with the explanation offered, it
will not appear to the reader too dogmatic.

The author has been fortunate in utilizing the services of his
students in the preparation and illustration of this volume. He
is particularly indebted to Mr. R. E. Torrey for his skilful and
artistic execution of a large number of the figures. Help in
this respect has also been supplied by Mr. R. C. Staebner and
Mr. Charles Drechsler. Miss Ruth Cole has rendered invaluable
aid in the onerous task of preparing the text and reading the
proof. Miss Edith S. Whitaker has also assisted in the prepa-
ration of the index.

To Professor M. A. Chrysler, of the University of Maine, the
author owes illustrations of secondary growth in monocotyledons,

PREFACE vii

and to Miss Eloise Gerry, of the United States Forest Service,
an admirable photograph elucidating the ‘‘bars of Sanio” in
coniferous wood. Last but not least, the author records the
valuable services of Mr. James Austin, assistant in the labora-
tories, in connection with the preparation of the numerous photo-
graphic illustrations. Dr. D. H. Scott and his publishers, Messrs.
A. and C. Black, have very kindly permitted the reproduction of
a number of figures from the admirable Studies in Fossil Botany,
which are among the comparatively few illustrations in the present
work which are not original. To Dr. R. T. Jackson the author
is indebted for the opportunity of photographing for illustration
a number of the sections of carboniferous plants under his care
in the Botanical Museum.

Professor John M. Coulter has been good enough to read the
proofs as the present volume passed through the press. To my
friend Mr. R. W. Sayles, Director of the Geological Museum, I
am indebted for valuable criticisms in the field of paleoclimatology
and also for other essential aid. The author is alone responsible
for the views expressed and for any errors.

BoTANICAL LABORATORIES, HARVARD UNIVERSITY
June, 1917

CONTENTS

CHAPTER

Shar (CErp
. THE TISSUE SYSTEMS .
. THE FIBROVASCULAR TISSUES: WooD—GENERAL

. THE FIBROVASCULAR TISSUES: SECONDARY Woop—TRA-

CHEIDS AND FIBERS .

. THE FIBROVASCULAR TISSUES: SECONDARY Woop—PAREN-

CHYMA

. THE FIBROVASCULAR TISSUES: SECONDARY Woop—Rays .
. THE FIBROVASCULAR TISSUES: SECONDARY WOOD—VESSELS
. THE FIBROVASCULAR TISSUES: PHLOEM

. THE EPIDERMIS

. THE FUNDAMENTAL TISSUES .

. DEFINITIONS OF THE ORGANS

. THE Root

. THE STEM

. THE LEAF

. THE MICROSPORANGIUM

. THE MEGASPORANGIUM AND SEED

. THE CANONS OF COMPARATIVE ANATOMY

. THe LycopsipA AND PTEROPSIDA

. THE LYCOPODIALES

. THE EQumsETALES (INCLUDING SPHENOPHYLLALES)

. THE FILICALES .

. THE ARCHIGYMNOSPERMAE: CYCADOFILICALES AND CyCa-

DALES

. Toe ARCHIGYMNOSPERMAE: CORDAITALES AND GINKGOALES
. THe METAGYMNOSPERMAE: CONIFERALES
. Tue METAGYMNOSPERMAE: GNETALES .

. THE ANGIOSPERMS .

PAGE

2.4 CONTENTS

CHAPTER

XXVII. THE Woopy DIcoTyLEDONS
XXVIII. THE HerBAceous DICOTYLEDONS

XXIX. THE MoNOCOTYLEDONS

XXX. ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION

XXXTI. EvoLurIONARY PRINCIPLES EXHIBITED BY THE COMPOSITAE

XXXII. ANATOMICAL TECHNIQUE

INDEX

PAGE
379
387
409
417
433
444
473

CHAPTER I
THE CELL

It has been recognized since the seventeenth century that living
beings, particularly plants, are composed of cells. The English
investigators Hooke and Grew in the latter half of that century
noted the fact that the body of vegetable organisms was often con-
stituted of minute chambers, and to these Hooke first gave the
name of cells. Grew was apparently the originator of the term
‘tissue,’ and he compared the organization of plants with the
woven texture of lace. A clear conception of the cellular structure
of living beings was not, however, reached until nearly two centu-
ries later. Plants differ from animals in the fact that their gross
internal organization is of relatively slight scientific importance
compared with the more obvious bony and muscular structures of
the animal body. The anatomy of plants is thus essentially a
matter for microscopic investigation. Some knowledge of the
general features of the structure of the cell in plants is accordingly
necessary as a preliminary to the more detailed pursuit of anatomy.

The cell in vascular plants has certain features which bring it
into sharp contrast with the corresponding unit of structure in the
higher animals. The essential substance of all living cells is proto-
plasm, simply distinguished from inanimate matter by the posses-
sion of a capacity for change and reproduction, which does not
characterize matter devoid of life. In the higher plants the proto-
plasm does not show the large degree of solidity which is a feature
of the animal cell, but is ordinarily reduced to a thin vesicle sur-
rounding a larger or smaller central cavity known as the vacuole.
The bladder-like protoplasmic vesicle is rendered possible in plants
by a containing, supporting, and likewise more resistant envelope
called the cell wall. This wall is not nitrogenous in its chemical
composition, as is true of the living protoplasmic body, but is, primi-
tively at least, a ternary compound, containing carbon, oxygen,
and hydrogen, the first-named element being the most abundant.

I

2 THE ANATOMY OF WOODY PLANTS

Fig. 1 will make clear the organization of cells in an immature
seed pod or pericarp of an iris. Each element obviously consists
of a large central cavity as noted above, variously known as hydro-
plastid, hydroleucite, tonoplast, and vacuole. The last designa-
tion seems best for descriptive purposes. Surrounding the median
space, which in life contains water with various substances in
solution, is the proto-
plasmic utricle. The
nature of the latter
varies in different
cells. In the ele-
ments to the left side
of the illustration the
protoplasm appears
as a somewhat
minutely granular
substance, while in
the cells to the right
the structural organi-
zation of the proto-
plasm is marked by
the presence of oval
bodies, the chloro-
plastids, which in life
color are green. A

Fic. 1.—Transverse section of young pericarp of
Tris species, showing organization of cells of epidermal
and fundamental tissues. dark body of larger

size is to be seen

somewhere against the cell wall and imbedded in the protoplasm.
This is the nucleus, one of the most important organs of the cell,
which appears to preside over all its changes and activities.
Another body, generally larger and lighter than the nucleus, is
frequently present in the cells under discussion—the oilplastid
or eleoplast. This is conspicuously spongy in its structure. We
may now turn with advantage to the organization of the cell
wall. On the left side of the illustration the common outer wall
is limited by a distinct membrane, the cuticle, which is chemically
different from the rest of the wall substance and is viable to

THE CELL 3

gases but not to fluids. Below the cuticle lies the cell wall
proper, in this case composed of cellulose. The cuticle and
its underlying wall are continuous except in the region of certain
apertures, the stomata, one of which is represented in the figure.
From the stoma there passes inward an air space, which quickly
divides into fine canals lying in the angles between the internal
cells. These are intercellular spaces and are a practically unfailing
accompaniment of the living elements in vascular plants.

The next illustration (Fig. 2)
visualizes the conditions in a harder
tissue, namely, one of the wood rays
of a dicotyledon. Here the mass of
cells present individually somewhat
rounded contours, and in the result-
ing angular interstices appear the
intercellular spaces. The protoplasm
in the cells under consideration is
much denser than in the elements of
the wall of the ovary figured above, Bree aential seeriantat
and accordingly the nucleus lies near cells of a ray in a dicotyledon
the geometrical center of the cell, as (Primys species), showing pitting
é : : in relation to cells and intercellu-
is commonly the case in highly proto- ,,. . ee
plasmic elements, in contrast to its

‘peripheral position in those in which the protoplasm is only
a bounding utricle. The cell wall in the present instance is, rela-
tively to the size of the cells, much thicker than in the first figure.
As a consequence of this increased thickness of the wall, special
devices are necessary for the purpose of permitting interchanges
between the cellular elements and their environment. The wall
is thin in certain definite regions for the attainment of this end, and
these locally thin spots are known as pits. It is clear that the pit
of one cell always coincides with a corresponding pit in a neighbor-
ing element. Further, it is obvious that, although the cells are
in close relation to one another by means of their pits, these are
prevented from becoming actual holes by the persistence of the
middle lamella or cement substance as the membrane of the pits.
Not all the pits, however, meet a corresponding depression. It

4 THE ANATOMY OF WOODY PLANTS

will be observed that there are well-developed pits toward the inter-
cellular aérating spaces. These are known as air pits and have
no counterpart on the side of the air space. They are commonly
found wherever the thickening of the cell wall makes difficult the
interchanges between the living protoplasm and the outside air.
In the figure under consideration the cells are united by the cement
substance of the middle lamella, which appears dark and contains
the air spaces within its sub-
stance. Most of the cells
have had both the top and
the bottom walls removed
Sent ! by the plane of section, but
= oie cs De in one or two the cell wall

Z meets the eye. Here the
pits are seen in face view,
and it becomes clear that
they vary somewhat in size
and are characteristically
outlined by a single contour.
On account of this condi-

Fic. 3.—Section of inner pericarp tion, which results from the
(“stone”) of a peach, showing cells with ex- fact that the bounding walls
tremely thick and layered walls as well as do not overhang, pits of this
numerous elongated pits. 5 5

type are called simple pits.
Communicating pores of this kind are found in cells which in
their functional condition contain living protoplasm.

Let us now turn to the examination of cells in which the wall
is so greatly thickened that the space containing the protoplasmic
body becomes much reduced in size. Fig. 3 reproduces some of
the elements of a peach stone which has been softened by a pro-
longed sojourn in hydrofluoric acid. The protoplasmic structures
have been largely consumed in the thickening of the cell wall.
The latter shows well-marked indications of layering, a feature
often present when much thickening has taken place. The layer-
ing doubtless has some numerical relation to the age of the wall in
days. The pits are of special interest in this instance. Their
correspondence in adjacent cells is as marked as in the former

THE CELL E

illustration. In many cases two or more near-lying pits of the same
cellular element become confluent as they pass inward, and open
into the cavity of the cell by a common aperture. The cement

—

Fic. 4.—Wood of the spruce (Picea canadensis), showing ray cells with simple
pits and longitudinal water-conducting elements with bordered pits.

substance is strongly developed here and the thickened walls of
the cells are lignified. The pits, in spite of the complications noted
above, are to be regarded as simple pits.

6 THE ANATOMY OF WOODY PLANTS

The examples of cells hitherto considered have been those which
in their functional condition contain living protoplasm. It is now
necessary to refer to those very important histological elements
of vascular plants, which in the mature condition normally con-
tain no protoplasm and serve to conduct water. Elements of
this nature are typically much elongated and are provided with a
different sort of intercommunicating pitting than that found in
the case of those which in the active state shelter a protoplasmic
body. In the accompanying figure (Fig. 4) are seen a number of
cells of this sort cemented together by the middle lamella (repre-

sented as a black framework around
the cell walls). The pits which
bring about intercommunication
are figured as occurring on two of
G) ©) the four nearly parallel sides of the
tracheids or water-conducting cells.
Their openings have overhanging
margins, and the membrane, the

Fic. 5.—Diagram of simple and Presence of which precludes the
bordered pits in face and profile views. possibility of actual openings, is
In the center a bordered pit from much thickened in the middle to
heartwood is shown. :

constitute the so-called torus. The
presence of overhanging margins clearly distinguishes this type
of pore from that found in the walls of living cells. The cement
substance uniting the cells with one another is in general lignified
(that is, has undergone that somewhat complex and obscure
modification chemically known as lignification) like the cell walls
which it holds together, but is likewise partially in the pectic or
mucilaginous condition, a state which causes it to absorb hema-
toxylin strongly. In the region of the pit membranes the middle
lamella becomes pectic cellulose, and here water passes through the
walls much more readily than it does elsewhere. On either side
of the figure is seen a ray of the wood, the cells of which are in
relation with the tracheids by means of pits. It is clear that the
pits which bring about intercommunication are bordered on the
side of the elongated element of the wood (the tracheid) and are

THE CELL 7

simple on the side of the medullary-ray cells. Pits of this nature
are called half-bordered.

In Fig. 5 simple and bordered pits are represented diagram-
matically side by side and from both profile and face view. Ob-
viously the simple pits in face view are single in contour, while
those which are bordered have a triple concentric outline, the outer-
most circle corresponding to the boundary of the broad membrane
of the pit, the innermost to the narrow mouth, and the intermediate
representing the outline of the torus. The distinction between
simple and bordered pits is an extremely important one, particularly
in the lower groups of vascular plants. In higher forms the dis-
tinction is of less value, but the presence of bordered pores in vas-
cular elements in general still indicates that an important function
of the element so provided is the transport of water. It should be
further noted that elements with bordered pits are usually without
intercellular spaces, while those in which the pits are of the simple
type normally possess such aérating cavities. Exceptions to this
statement are found ordinarily only in plants which have become
highly specialized in connection with resistance to drought.

CHAPTER II

THE TISSUE SYSTEMS

The cells in higher plants are generally grouped into well-marked
systems of elements which are known as the tissues. ‘Tissues are
sometimes defined as aggregations of cells performing a similar
function. This definition is, however, open to some objections, as
is also that which describes a tissue as a mass of cells of similar
origin. Characterization of tissues, either by their functions or
by their mode of origin, seems less desirable than a definition which
makes clear that the aggregations of cells have a common organiza-
tion. From the standpoint of evolution it is the structural features
of the tissues which are of the greatest significance. In works
dealing with so-called physiological plant anatomy the functions
of the tissues rather than their peculiarities of structure are
naturally most emphasized. From the point of view of the doc-
trine of descent functional features are of less significance, since
it is precisely these which are the most readily modified and as a
consequence furnish the least valuable indications of the course
of evolutionary development in any given large group. Because
the present work deals with anatomy from the outlook of evolution,
structural organization in the case of the tissues stands in the fore-
ground, although, of course, the question of the functioning of the
various tissue systems cannot be left out of view.

It is an interesting general fact that the boundaries between the
tissue systems are much more marked in the plants which are
geologically older and lower in the scale of evolution than they are
in the higher seed plants, the conifers, and the angiosperms.
Further, the more conservative organs of the higher forms—namely,
the root and leaf—exemplify a sharper delimitation of the tissue
systems than does that most progressive of all plant parts, the stem.

For the purpose of the present work the tissues of plants may
be divided into three distinct systems, which can be most easily
identified by reference to the accompanying figure (Fig. 6) of a

8

THE TISSUE SYSTEMS 9

transverse section of the creeping stem of Pteris aquilina. 'Through-
out the area of the figure are scattered oval masses, the fibrovas-
cular bundles. These are very sharply marked off from the rest
of the tissues by a boundary appearing as a dark circumscribing
line. On the outside of the stem is the integumentary system, or

Fic. 6.—Transverse section of the rootstock of Pteris aquilina, showing three
categories of tissue—namely, epidermal (external), vascular, and fundamental.

epidermal tissue, consisting of a single layer of cells. The epidermis
is characteristically uniseriate in the lower forms, and only in some
of the higher vascular plants does it become a multiple layer. The
remaining structures of the stem of the bracken fern belong to the
fundamental system. The next illustration (Fig. 7) shows a part
of the stem of Pteris more highly magnified, so that the details of

fe) THE ANATOMY OF WOODY PLANTS

structure may be more easily discerned. External is the single layer
of the epidermis, made up of cells with thick, heavily pitted walls.
With the greater magnification employed, the cellular organization
becomes very obvious. Underneath the epidermal layer is situated
the fundamental system, in turn clearly outlined against the
fibrovascular tissues by a striking boundary composed of a single
series of cells with dark tanniniferous contents. This is the most
internal layer of the fundamental system and is known either as
the endodermis or as the phloeoterma (the latter term in its ety-
mology indicating the inner layer of the cortex or fundamental
tissue). The fundamental tissues are characterized locally by

Fic. 7.—A portion of a transverse section of the rootstock of Pieris aquilina
more highly magnified, showing the three tissue systems.

certain bands of dark-brown skeletal tissue, which are very char-
acteristic of the lower vascular plants and subserve to a large extent
the mechanical function which in the higher plants is attended to
by the fibrovascular system. Those regions of the fundamental
system which are not mechanical in their nature take over the
function of storage and are crowded with granules of starch forming
a cordon around the periphery of the cells. Centrally the elements
of storage show the presence of a dark-brown substance in the
region of the vacuole. This is tannin-like in its nature and is
commonly present in the fundamental system of ferns. The
fibrovascular structures stand out sharply from the rest of the
tissues and are obviously much more complicated in their organiza-
tion than are those previously considered. The fibrovascular aggre-
gations of cells are of the greatest anatomical importance, both
because their very complexity of structure supplies many valuable

THE TISSUE SYSTEMS II

features for comparison which may be utilized in the study of
evolution, and because their conservatism makes them on the whole
the least variable of the elements entering into the composition of
the higher plants. Likewise, by reason of their resistance to decay,
they are more likely to be preserved as fossils. Beyond emphasiz-
ing the complexity of the fibrovascular system, it need not be
further considered at the present time.

a
Fic. 8.—Transverse section of the leaf of the white pine (Pinus Strobus), showing
the three tissue systems in a leaf.

We may next consider the tissue arrangements in another of
the plant organs, namely, the leaf. In the figure (Fig. 8) is repre-
sented the transverse section of the needle of Pinus strobus. Ex-
ternally the epidermis forms a boundary of a single row of cells,
continuous except where interrupted by the occurrence of stomatic

openings. Beneath the epidermis lies a layer which is ordinarily
known as the hypoderma and is more strongly developed in fossil

12 THE ANATOMY OF WOODY PLANTS

than in living pines. The fibrovascular strand is sharply bounded
in the median region by the endodermis, a layer circular in con-
figuration and well developed in Pinus, though often absent in the
higher conifers. The organization of the fibrovascular strand need
not particularly occupy our attention at this stage, as it will be
considered in detail more appropriately in the sequel. It is enough
to note that its upper or woody part is composed of empty cells
often showing bordered pits—in other words, of tracheids. These
are continuous on the
flanks of the strand with
short-pitted elements, the
transfusion cells, which are
of great interest from the
evolutionary standpoint.
The cellular complex lying
outside the endodermis
and within the epidermis
is the mesophyll, the repre-
sentative of the funda-
mental tissues in the leaf.
The cells of the mesophyll
are infolded in a manner

Fic. 9.—Transverse section of the root of characteristic of most
the sarsaparilla (Smilax), showing the three livin g pines In the meso-

systems of tissues in root organs. phyll if also, athe lower
side of the leaf, conspicuous secretory spaces, the resin canals.
In the root of the pine, as in the leaf, the same sharp distinction
between the fibrovascular structures and the fundamental tissues
is present. In the stem of the conifers generally, however, the
limit between the tissues belonging to the central conducting
cylinder (the fibrovascular system) has become obsolete and can be
judged to have been formerly present only on theoretical grounds.

In the case of the root of vascular plants in general, from the
lowest to the highest, the limit between conducting or fibrovascular
tissues and the fundamental system is usually very distinct and is
one of the features which so clearly and universally mark the root
as the most conservative of the organs of plants. Fig. g illustrates

THE TISSUE SYSTEMS 13

the situation in this respect for the monocotyledons, which may
on strong grounds be considered as the highest of the seed plants.
Externally is the piliferous layer, from which the root hairs are
derived, and which may be considered in a general way as the equiva-
lent of the epidermis of the stem and the leaf. The central region
is occupied by the fibrovascular system, sharply limited by the
endodermis, composed of cells ordinarily thick-walled. Between
the endodermal limiting membrane and the piliferous layer lies
the cortex of the root, and this corresponds to the fundamental
category of tissues in the case of the stem and leaf.

It will be clear from the account given above that there are three
tissue systems in plants which are very distinct in lower forms and
in the less changeable and more conservative parts. Of these the
epidermal tissues are always clearly limited both toward the out-
side and also in relation to the tissue system which lies inside.
The boundaries dividing the fibrovascular from the fundamental
tissues are often less plainly indicated, and in the case of the higher
groups of plants, particularly those in which the secondary growth
is strongly developed, may disappear altogether; in such cases the
limits of the tissues can only be inferred from comparative and
developmental anatomy.

CHAPTER III

THE FIBROVASCULAR TISSUES: WOOD—GENERAL

Since the fibrovascular tissues are on the whole the most impor-
tant in the organization of the higher plants both from the evolu-
tionary and from the physiological standpoint, it will be well to
begin with their anatomy. The most conspicuous and _best-
developed portion of the fibrovascular system in land plants is
the wood. The aggregation of elements assembled under this
heading affords also the best exemplification of the process of
evolution in the higher plants as the result of the progressive
development of the principle of division of labor and the gradual
adaptations of plants to the more complicated conditions of life
obtaining in later geological times. The resistance of the wood to
the organisms of decay is greater than that of any other common
plant tissue except those possessing cutinized or suberized cell walls.
We have, consequently, in the woody structures past and present
an almost perfect biological document, carrying back the history
of plants in relation to their changing conditions of environment
into remote epochs of our earth’s history.

In beginning the discussion of the organization of wood it will be
well to direct our attention in the first instance to the contrasts in
structure presented by woods of ancient and modern types. The
first illustration (Fig. 10) shows us the situation in the oak. The
wood is conspicuously marked into areas by boundaries running
at right angles. Crossing the figure from top to bottom are rows
of large openings which represent the vessels or water-conducting
tubes of our modern forest trees. It is clear that these are large
only along the lines which mark the beginning of each year’s growth.
Farther out the vessels become suddenly of much smaller caliber.
Not only is the ligneous structure of the oak transversely banded
by reason of the strikingly larger size of the vessels which signalize
the spring development, but also by an equally significant change
in the diameter and the distribution of other more or less highly

14

FIBROVASCULAR TISSUES: WOOD 15

differentiated elements, to be described in a later chapter. At
right angles to the annual zones of growth and crossing these are
the large wood rays. These are extremely conspicuous structures,
and as a result of a variation in the rate of growth due to their
presence they bring about very evident depressions on the faces
of the annual rings. The intervals between the large wood rays
are occupied by more
numerous linear stor-
age bands, which are
but a single row of
cells in thickness and
are known as uniseri-
ate rays.

A marked con-
trast to the wood of
the oak is presented !
by the ligneous
organization of the
Paleozoic gymno-
sperm Cordaites, illus-
iiabed Ane hie. Ta.
Here the annual

rings, so clearly pres- Tic. 1o.—Transverse section of the wood of the red
ent in the oak as qa ak (Quercus rubra), showing annual rings and highly
differentiated structure which characterizes the organiza-
tion of the woody cylinder in modern trees.

result of a differentia-
tion in the size and
character of the elements corresponding to regularly recurring
annual changes, are conspicuous by their absence. ‘This situation
is directly correlated with the more equable annual cycle of remote
geological times. We find illustrated in the case of Cordaites abso-
lutely no indication of seasonal changes in temperature or varia-
tions in other important conditions of a periodic or seasonal nature.
Not only is the wood monotonously the same as one passes from
the inner regions to the exterior layers, but it likewise shows slight
differentiation in the direction from left to right. Large rays of
the oak type are quite absent, and the radial storage strands are
entirely linear in their nature.

16 THE ANATOMY OF WOODY PLANTS

Having noted the varieties involved in wood structure in correla-
tion to the more variable conditions of environment present in
modern times, we may now with advantage direct our attention
to other features of organization which characterize the evolution
of the woody cylinder in the higher plants. Fig. 12 illustrates
the transverse section of the wood of a lepidodendrid, an ancient
tree of the Paleozoic
age. It as clearcin
this case that the
wood has a circular
outline correspond-
ing to that of the
stem as a _ whole.
Although there is no
indication of the ex-
istence of annual
inienements, OF
growth, the wood is
obviously divided
into a central mass,
in which the cells
are irregularly dis-
posed, surrounded
by a zone regularly

Fic. 11.—Wood of a Paleozoic gymnosperm from
Prince Edward Island, Canada, showing absence of ?
annual rings and extremely simple organization. seriate and marked

by the presence of
wood rays. The portion of the wood which shows no linear disposi-
tion of its elements is known as the primary wood. This region of
the wood is sometimes designated the ‘‘cryptogamic”’ or “‘old”’
wood, because it is particularly characteristic of the organization of
vascular cryptogams and of the older groups of plants generally.
As will be made clear later, the structure and mode of development
of wood of this category is of considerable importance from the
evolutionary point of view. The zone of secondary wood, outside
the primary or cryptogamic ligneous core, is conspicuous by reason
of its regular radial seriation and the presence of storage bands

FIBROVASCULAR TISSUES: WOOD 17

called wood rays. The secondary wood need not further occupy
us in the present chapter.

Turning our attention now to the longitudinal organization of
the primary wood, we find it characterized by the presence of certain
elements appearing in a somewhat regular sequence. The general
situation is represented in Fig. 13. On the left of the diagrammatic
illustration lies an elon-
gated thin-walled cell
marked by the presence
of spiral strengthening
bands. In its present
state this element is de-
void of protoplasmic sub-
stance, but in an earlier
phase, as indicated in the
next figure (Fig. 14), liv-
ing matter was present
and specially aggregated
in the regions of the
thickened spiral bands.
To the right lies a second
element in which the
strengthening horizontal
ridges, which reinforce

{b-=

Fic. 12.—Transverse séction of a stem of a
from the inside the gen- _lepidodendrid trunk from the Paleozoic (after
erally thin walls of the Scott), showing the strong distinction between
secondary (radially seriate) and primary (unsert
ate) xylem characteristic of ancient forms.

water-conducting cell, are
nearer to one another and
in some instances are more or less united. By accentuation of the
condition of approximation, fusion between the bands results and
we have as a consequence the presence of the scalariform or reticu-
late tracheid. In general, among the Pteridophyta this is the
extreme stage of evolution of the tracheary cells of the primary
wood, but occasionally among the more complicated and extinct
vascular cryptogams, and characteristically in all seed plants, the
final state of the primary wood is characterized by the presence of

18 THE ANATOMY OF WOODY PLANTS

the pitted element, marking a distinct advance on the scalariform
and reticulate tracheids of the primary ligneous organization of
the ferns and their allies. In the case of the primary wood those
elements which have in their walls thickenings of the nature of
rings and spirals are ordinarily designated the protoxylem. This

Fic. 13.—Diagrammatic longitudinal section
of the fibrovascular tissues of a dicotyledon (after
Sachs), showing organization of primary wood.

characterization of the first-formed portion of
the primary wood is not always justified by the
structures present, because in slowly growing
organs and in subterranean parts even of rapid
development typical ringed and spiral elements Fic. 14.—Dia-
may be nearly or quite absent. Usually, how- 8’ 23s#Be views
p i I oa heal of a young ele-
ever, the protoxylem as defined is the ligneous em a) foe See
structure present when the organ is undergoing ~=mary wood. In A
rapid elongation and by its constitution permits the normal condi-
- : ; : tion is shown,
of accommodation by stretching to correspond a
é : ph ae while in B the pro-
with the increase in length. Its elements as a toplasm has been
consequence are frequently drawn out and almost _ caused to contract
obliterated. The cells of the primary wood laa ofrples:
. : : : a molysis.
which are thickened in the reticulate, scalari-
form, or pitted manner are formed after elongation has ceased,

since by their organization they are incapable of increasing their

FIBROVASCULAR TISSUES: WOOD 19

length. Elements in the aggregation belonging to this category
are known as the metaxylem or, more specifically, as the primary
metaxylem.

In the preceding paragraph, for the sake of convenience it has
been assumed that the order of development of the primary wood
is always in the same direction. As a matter of fact the time and
the order of appear-
ance of the elements
in this, from the
evolutionary stand-
point, highly signifi-
cant tissue vary
within certain im-
portant limits. We
may first consider
the most ancient
order of seriation of
the constituents of
the primary wood—
that found in the
stems of the most
antique plants and

in the roots of all Fic. 15.—Transverse section of the upright stem of
vascular organisms Lycopodium clavatum, showing centripetal or centrad
development of the primary wood; the smaller elements
represent the protoxylem.

from the lowest to
the highest. Fig. 15
illustrates the organization of the wood in a stem of the common
club moss, Lycopodium. The tissues of the xylem constitute a sort
of star, the points of which are occupied by the small-sized elements
of the protoxylem. As the rays of the star broaden inwardly, there is
a transition from protoxylem tometaxylem. The situation becomes
more clear by reference to transverse sections of the root in a fern
shown in Fig. 16a. Spiral sculpture marks the small elements on
the outside, while toward the center of the organ the typical sculp-
ture of the metaxylem becomes more and more conspicuous. In
the club mosses and their allies, as well as in all roots, the seriation
in the development of the elements of the primary wood is very

20 THE ANATOMY OF WOODY PLANTS

generally from the exterior toward the center, and the metaxylem,
as a consequence, is more axial or central in position than the
protoxylem. This mode of development of the primary wood is
characteristic of the most ancient plants, the lycopods and their
allies, and is likewise universally present in the most conservative
organ of all plants, the root. When the primary woody tissues
develop from the outer region inward, as indicated above, they are
said to be exarch. The situation just described will become more
apparent by reference to Fig. 16, which shows 16a in an immature

Fic. 16.—a, transverse section of the bundle of an old root of Osmunda cinna-
momea, showing primary wood complete; 6, younger condition of the same with
central region (metaxylem) of bundle still immature and containing protoplasm.

condition. The outer regions of the oval mass of xylem are alone
developed, the center being still occupied with thin cells filled with
protoplasm.

In the stem organs of the ferns and lower gymnosperms a some-
what different mode of development of the primary wood is char-
acteristically present. This may be illustrated by reference to one
of the fibrovascular strands of the bracken fern, Pteris aquilina.
The smallest elements of the wood are situated in the woody tissue
constituting the center of the bundle. As in the case of Lycopodium
and its allies, the smaller first-formed cells belong to the protoxylem.
The situation which presents itself in the later development of the
woody strand of ferns, however, is usually quite different from that
found in the lycopod series. In the ferns the tissues of the primary
metaxylem, instead of lying entirely toward the center of the organ

FIBROVASCULAR TISSUES: WOOD 21

in the exarch condition featured in the lowest vascular plants,
characteristically surround the first-formed ringed and spiral ele-
ments (the protoxylem). This situation so frequently presented
by the ferns and lower gymnosperms is designated as mesarch.
The longitudinal topography of the bundle in this type is shown in
Fig. 17.

Fig. 13, described at the outset, pictures the relative position of
the constituents of the primary wood in the stem of the higher seed
plants, the Gnetales, the Con-
iferales, and the angiosperms.
In this case, since the seria-
tion of the successive elements
is always outward from an
internal starting-point, the
primary wood is known as
endarch. This condition is
the typical one for all the
higher plants, and no form Fic. 17.—Longitudinal section of a
characterized by it can, with- bundle from the rootstock of Pteris aquilina,

: showing the elements of the protoxylem in
Out che-clearest, evidence, (be 4... cantar-of tha mood:
regarded as low in the scale
of plants. Although the endarch condition is a feature of the
development of the stem and generally of the leaf in the highest
plants, it is important to emphasize at this stage that the root,
even in the most advanced organisms, betrays its extreme con-
servatism by, adhering in its primary structures to the mode of
development and seriation of the elements characteristic of the
most ancient known plants, the lycopods and their allies. Fig. 18
reveals the organization of a young root of the American larch.
We may disregard in the present connection all but the central
tissues of the root. Right and left can be distinguished two
spaces, the resin canals. Inside of each of these two secretory
cavities lies a cluster of protoxylem, distinguishable by the small
size of its elements. Toward the center from each aggrega-
tion of protoxylem extends the metaxylem, which in the young
condition represented in the figure has not yet become joined
in the center.

22 THE ANATOMY OF WOODY PLANTS

It may be summarily stated in regard to the present chapter
that wood in the older types is much simpler in structure and less
differentiated than in modern forms. Further, in most cases it is
necessary to distinguish between primary and secondary wood.

WG Y
ps a AL
OK a

)
AM

(8%

a

Fic. 18.—Transverse section of the young root of the American larch. The
primary wood has not yet been developed in the central region of the root. The
endodermis separates the central or fibrovascular region of the root from the external
fundamental tissue or cortex.

The former is first to appear and is characterized by the lack of
distinct seriation in its elements. The secondary wood, by con-
trast, is formed later and the cells which constitute its structure
are arranged in rows corresponding to radii; files of storage cells,

FIBROVASCULAR TISSUES: WOOD 23

known as wood rays, are a marked feature of its organization. The
primary wood consists typically of two regions: one composed of
elements with ringed and spiral thickenings and capable of elonga-
tion in accordance with the growth of the organs; the other con-
stituted of tracheary cells thickened in a reticulate, scalariform, or
pitted fashion and, as a consequence, incapable of extension to
meet the needs of lengthening parts. The former region of the
primary wood is designated the protoxylem and the latter the
metaxylem. The order of development of the protoxylem and
metaxylem differs significantly in different groups of plants. In
the very oldest forms the progress of differentiation is entirely
toward the center (lycopods and their allies). In the ferns and the
lower gymnosperms the sequence is first central or centripetal
and later peripheral or centrifugal, with the result that the protoxy-
lem occupies a median position. In the higher gymnosperms and
in the angiosperms the inward order of development has nearly or
quite disappeared, and, as a consequence, the protoxylem lies on
the inside of the metaxylem, which is formed characteristically
outward or centrifugally. Technically these three types of organi-
zation of the primary wood are designated exarch, mesarch, and
endarch. Finally, it may be recalled that the primary wood of
the root of all vascular plants has the exarch organization of the
older types of stems and exemplifies the fact that the root in this
respect as in so many others (to be shown in the sequel) is the most
conservative of plant organs.

CHAPTER IV

THE FIBROVASCULAR TISSUES: SECONDARY WOOD—
TRACHEIDS AND FIBERS

Very important constituents of all woody organizations are the
tracheids and fibers. These present a wide range of structure
from the lower forms to the higher and illustrate some interesting
general evolutionary principles.. Fig. 19 is a highly magnified
transverse view of the wood of the white pine (Pinus strobus) in the
region of transition from one annual ring to the next. Woods of

this type are very simple and consist
h st lAy
ry

ae mostly of elongated tapering elements
a
Lf |

2

Mi JU IL
on
2 Ora
=.

with bordered pits in their walls and
known as tracheids. The only fea-

pl
: Y ey YU} tures of organization not tracheary in
=Ssaaq =e their nature are the rays and the
=I Soeaae eX resin cavity. The tracheids are dis-
A SATs 1 my tinctly of two kinds. Some are large
eof I/ \ 4 and thinner-walled and begin the
A FoI N /] (= annual ring as the so-called spring
A SN) elements. Others, thicker as to their

walls and with a smaller lumen or cen-
Fic. 19.—Part of a transverse 4 :

section of the wood of the white) | Lalucawity. | CONSUILULe fe msunimenr
pine (Pinus Strobus), showing tracheids. The two kinds of tra-
radial and tangential pitting of Cheids are in further contrast with
the tracheids. Lite

one another because of the position
of the bordered pits on their walls. In the spring elements the
pits are confined to the radial walls—that is, those sides of the
fibers which are either in actual contact with, or are parallel to,
rays. In the case of the summer tracheary cells pits are pre-
dominant on the tangential walls which are at right angles to
the rays. In addition to being in communication with one
another by means of bordered pits, the tracheids both of the
spring and of the summer wood are likewise related to the rays by

24

FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS) 25

pits which are bordered on the side of the tracheids and are simple
on the side of the elements of the rays.

In order to form an adequate conception of the nature of the
fibrous or tracheary elements of coniferous woods it is necessary
to view them in isolation from the tissues of which they form so
essential a part. It will be advanta-
geous to consider a simpler condition
first and then to proceed to the more
complex situation presented by the
elongated elements of the wood of
the pine. In Fig. 20 are shown tra-
cheids of the Big Tree (Sequoia
gigantea) belonging to the spring and
summer growth respectively. On the
left, one of the spring tracheids is
seen from its radial face. The cell is
obviously bluntly tapering at the ends
and has a length many times that of
its diameter. The pits which orna-
ment the radial aspect are of two
sizes. The larger bordered pits are
those which connect tracheid with
tracheid. The smaller pores bring
about relations between the rays and
the tracheids. The latter are bor-
dered only on the tracheary side,
although this feature is naturally not
obvious in the illustration. The tra- Z
cheid is represented as still surrounded Fic. 20.—Tracheids of
by its cement substance, which is indi- Big Tree Sequota gigantea):

5 : planation in the text.

cated by ja) heavier) line, On the

lateral or tangential walls of the cell may be seen the profile aspect
of other bordered pits. These are tangential pits and are very
rarely present in the spring wood of conifers. Fig. 20b reproduces
the appearance of the same tracheid from the tangential side.
The general configuration of the fibrous element is now much more
pointed and only a few pits can be seen in face. On the side

26 THE ANATOMY OF WOODY PLANTS

walls in this position numerous pores may be distinguished, in
profile, forming the principal means of communication between
tracheid and tracheid and the sole one between tracheid and ray.
To the right of the figure are to be seen the radial c and tangential
d views of a summer tracheid. The narrower lumen and thicker
walls are conspicuous. In the case
of the pitting the most marked fea-
ture of contrast with the spring fibers
is the greater number of bordered
pits occurring on the tangential walls.

The situation in Pinus may
now be advantageously considered.
Figs. 21a and 6 reproduce the spring
tracheary elements of this genus from
the same aspects as represented in
the case of Sequoia. Beginning with
the spring element on the left, it is
clear that the pits in relation to the
rays show a considerable degree of
differentiation, since they consist of
two categories—namely, small, dis-
tinctly bordered pores which form
the intermediary between marginal
ray cells and tracheids, and large,
angular, scarcely bordered apertures
uniting the central ray cells with the
tracheids. The more pointed tan-
gential contour of the tracheid in }
surrounds an area entirely free from
pits, a situation nearly universal for
the spring elements of coniferous
woods. In the lateral walls of b
numerous radial pits are seen in profile. On the right of the figure
are shown the corresponding views of the summer tracheids in Pinus.
The narrower diameter and thicker walls, as well as the numerous
tangential pores, clearly differentiate elements terminating the
annual growth from those formed at the beginning of the year.

Fic. 21.—Tracheids of the pine.
Explanation in the text.

FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS = 27

Tangential pits are so obviously and constantly a criterion of
structure at the end of the annual rings in conifers that they may
be used for the purpose of distinguishing the yearly increments in
tropical species, in which, by reason of the slightly marked seasonal
conditions, the zonal woody bands are indistinctly indicated or even
apparently absent. This is notably the case, for example, in the
tropical or subtropical species of the genus Araucaria.

The phenomenon of annual rings is very closely correlated with
the appearance of tangential pits, and the general phenomena in
this respect are worthy of much greater attention than has hitherto
been devoted to them. The incipient yearly zones in trunks of
trees of earlier ages are the clearest indication of the progressive
climatic refrigeration of the earth, and these become ever more
marked in later geological times. Secondly, it will be made clear in
the sequel that the modification of the annual ring in response to
inclement seasonal conditions has been on the whole the most
important factor in the evolutionary development of plants from
the earlier epochs to the present. A consideration of the organiza-
tion of the wood in a gymnosperm without annual rings—that is, a
seed plant of Paleozoic time—is of extreme interest in the present
connection. The tracheids of the secondary wood of the gymno-
sperms of this age, without exception so far as is at present known,
had the pitting confined to the radial walls; as a consequence water
could move, easily at any rate, only in a spiral and tangentially
through the tracheary elements. The truth of this situation may
readily be grasped by reference to Fig. 11, in chapter iii, representing
wood of the Paleozoic gymnosperm Cordaites. Details of the
structure of the wood in this genus can be gathered from the
inspection of Fig. 22 in the present chapter. It is clear that
the tracheids communicate with one another and with the cells of
the uniseriate rays by radial pits. Pits are conspicuously absent
on the tangential walls of all the fibrous elements of the wood. It
will be obvious from the facts put forward in this paragraph that
the distinction between spring and summer tracheids did not
exist in the case of Paleozoic woods; in other words, the modifica-
tions in structure and pitting which have become a fixed feature of
the organization of the summer tracheids of trees of the Mesozoic

28 THE ANATOMY OF WOODY PLANTS

and Tertiary had not yet made their appearance in the Paleozoic
age. It has been suggested that the function of the tangential
pits in the summer wood is the rapid supply of water to the

Zeer
FS)

ee
SIAN aT Ace
Sa \@,
“BG FSA ESI NGI = soe
Sag Saaz Bey SIZE 22742 >
ig PE D -CA> oo <> eS
A_ & ll YN WAH NSM WG NEEL

as

i
Se =fzG: ce PNAS HES C=
VED Eo \Y) (SEF (B27 \2AV C2
ZH _ Kee Bs EC Mm AC/ MS; MEL, _ 5
Z, a = x Je = x

RUS, 5 a HAY)
S

Se —— — UfZaw. 7
AAS
“2

<a,

=
CH
ee

‘

“Pe —
TA SAN

ROK

eseSe
<a =, —F
SA

WON

=<

S eee aS Loses
= = a
5. ms Ome se Se

Seat
mals See ae
L/ CP Ps Be EZS A
Cx eS a Stee
(ag E ofA) Sf
REREAD — EEO) IS A

2

Fic. 22.—Wood of Paleozoic gymnosperm Cordaites. Explanation in the text

cambium, or zone of growth, in the reawakening of life in the trunk
after the winter period of rest. It is admissible also to interpret
the greater mechanical efficiency which marks the summer wood of
trees possessing annual rings as of advantage in resisting the

FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS 29

winter storms of later geologic times. It is unlikely that the trees
of the Paleozoic age were exposed to the often furious air currents
which characterize the present age of extreme physiographic and
climatic differentiation of the surface of our earth.

It is convenient at this point to introduce a general statement
in regard to the organization of the wood of Paleozoic and Meso-
zoic forms so far as the tracheary elements of the secondary wood
are concerned. In the case of the giant club mosses or lepi-
dodendrids of the earlier forests, the secondary growth consisted
entirely of tracheids with scalariform markings similar to those
characteristic of the primary wood of many forms. These elements
were provided with pits on both their radial and their tangential
walls, in this respect offering a distinct contrast, as likewise in their
sculpture, to the gymnosperms of the same period, which, as has
been indicated above, possessed only pitted tracheids in their
secondary growth and had the pitting strictly confined to the
radial -surfaces of the elements. In the arboreal forms included
under the general heading of sigillarians the scalariform sculpture
often gave place to the pitted condition, but the pores, as in the
lepidodendrids, were both radial and tangential in distribution.
In the sphenophyllums and their allies, the calamites, the sculpture
was transitional from scalariform to pitted and was confined to
the radial aspect of the tracheids. In the true gymnosperms of
earlier ages, including the Cycadofilicales (Pteridospermae) and
their allies, as well as that group of forms included under the con-
venient cognomen of Cordaitales, the sculpture of the secondary
elements of the wood was pitted (not scalariform) and was con-
fined to the radial aspects of the cells. It is thus clear that in the
great Paleozoic age the water-conducting elements of the secondary
woody growth, so generally prominent in both vascular cryptogams
and gymnosperms, were much more uniformly organized than is the
case with plants of later geologic times.

The further course of evolution and differentiation in tracheary |
or fibrous elements of the secondary wood can be studied to greatest
advantage in the angiosperms, which, although they first appeared
in the later Mesozoic, did not become the predominant feature
of the flora until Tertiary times. We may conveniently begin with

30 THE ANATOMY OF WOODY PLANTS

the examination of the fibrous structures of the wood of the oak.
Fig. 23 gives a general view of the organization of the wood in a
red oak (Quercus rubra). Clearly it shows a much higher degree of
complication than is found in the ligneous tissues of a conifer or

See 7

< FIAT TOLLE
SSS aq SSS

aise

oe a

GED
Y, <P <i)
ZO yy

ILA AIS IIDSO!

las
oS x

sien
bes
ANS

Fic. 23.—Wood of the red oak. Explanation in the text

forms still lower in the scale. The wood is crossed by conspicuous
bands of storage tissue, the large rays so characteristic of the
organization of oak wood. In the squares bounded by the annual
rings and conspicuous rays lie the remaining structures. Of these
the vessels are most outstanding and are very large at the beginning
of the annual-zones, becoming much smaller in caliber in the
later-formed wood. The vessels are imbedded in radially directed

FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS 31

bands of tracheids in intimate relation to the vascular ele-
ments which they surround. Laterally to the stripes of
tracheids in a general radial direction lie other denser areas
made up of mechanical elements known as fiber-tracheids.
There is scarcely any marked boundary between the fiber-
tracheid and the tracheid proper, although
the extreme conditions of both are distinct.
Fig. 24 represents these two kinds of elements
as seen in longitudinal aspect in a maceration
of the wood. The tracheid is much shorter
and broader, has thinner walls, and is abun-
dantly pitted, both radially and tangentially,
with pores provided with oblique mouths.
In the case of the fiber-tracheids we have to
do with elements of considerable length and
narrow lumen, inclosed by rather thick walls.
The pits are much more scanty, but are
equally distributed, as in the case of tracheids
proper, on both radial and tangential walls.
A marked feature of the pits in these ele-
ments is the extreme length of their oblique
mouths, which much exceeds the diameter
of the pitmembrane. This situation becomes
the rule in tracheary elements devoted more to
the mechanical than to the water-conducting
function.

In the mass of woods of the dicotyledons
the occurrence of tracheids proper and ex-
tremely differentiated mechanical elements
side by side with gradual transitions is not
seen. Usually the elongated elements of the Fic. 24.—Tra-
wood other than vessels (to be discussed in a cheids and fiber-
; tracheids of the
later chapter) are of a more or less definite ,,,.
mechanical nature and have, as a conse-
quence, lost more or less completely the characteristics of
tracheids. Fig. 25 illustrates several types of fiber-
tracheids and similar structures. In a is seen a tracheid

32 THE ANATOMY OF WOODY PLANTS

of Ephedra, a low representative of
the Gnetales. The pitting is on all
walls, and spiral bands are present
on the inner surface. In 6 is shown
a fiber-tracheid from a species of
Magnolia. Here again the pitting is
not confined to the radial aspects of
the element, but is also tangential.
The apertures of the pits are much
elongated and extend beyond the
nearly circular outline of the pit
membrane. In c appears a type of
mechanical element characteristic of
the higher dicotyledons. In this form
of fiber the pit membrane is exceed-
ingly narrow and the mouth extremely
elongated. As a result of this situa-
tion the pores of the fiber appear
to be practically without a border.
Mechanical elements of this type are
usually known as libriform fibers.
It has been considered by Strasburger
and others that the libriform mechani-
cal element is of a different morpho-
logical nature from the fiber-tracheid
and the tracheid. The distinguished
German morphologist was of the
opinion that elements of this type
were derived from the fusion of stor-
age parenchyma cells. This view,
however, as will be pointed out more
appropriately later, does not har-
monize with the general evolutionary
sequence in the development of
structures in the wood and, more-
over, meets with serious difficulties
even from the comparative stand-

SRB

Se

em
BBE BW

Fic. 25.—Types
of fibers. Explana-
tion in the text.

FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS = 33

point. For example, in the northern species which are placed
under the genus Fagus the mechanical elements are of the nature
of fiber-tracheids and possess clearly bordered pits. In the
antarctic species, on the other hand, the mechanical structures of
the wood are libriform fibers. We have thus the remarkable situa-
tion that the fibrous elements in certain species of the same genus
are derived from tracheids,
while in others they are
the result of the fusion and
modification of storage
parenchyma elements in
the wood. The situation
has only to be stated to
make the absurdity of the
interpretation obvious.
Moreover, while in the
woods of the Leguminosae
the mechanical elements

are} for the most part, of Fic. 26.—Mucilaginous fibers of the black
the nature of libriform locust (Robinia Pseudacacia).

fibers, in Bocoa provacensis

the mechanical elements are represented exceptionally by fiber-
tracheids with clearly bordered pits. In certain Rosaceae the
transition from fiber-tracheid to libriform fiber may take place
within the limits of species (Rhodotypus kerrioides). The inter-
pretation of the libriform fiber most in accord at once with the
facts of comparative anatomy and with the general trend of
evolution, as inferred from the comparison of existing with extinct
types, is that it is the product of the extreme modification of the
fiber-tracheid, just as the latter in turn takes its origin from the
less specialized tracheid.

An interesting category of mechanical fibrous elements is sup-
plied by the mucilaginous fibers found in many woods of widely
separated systematic affinities. In these elements the inner portion
of the wall has become more or less completely modified into a
mucilaginous state which causes it to stain strongly with hema-
toxylin. Fibers of this type are found commonly in certain oaks

34

wey vv VVMNTVI SD v

FIG. 27

CC ——SS=_=_

CNN

BNA

SS

WAS SAARI AAA

ee

THE ANATOMY OF WOODY PLANTS

Fic. 28

and are widespread in leguminous woods.
The presence of fibers of this description is
often of value from the hygroscopic quality
imparted to the wood, which prevents undue
shrinking or swelling. Those species of oak
with mucilaginous fibers are stated to be of
special utility for cabinet work on account of
their relative immunity from swelling and
shrinking. In the case of the black locust

(Robinia Pseudacacia) the numer-
ous mucilaginous fibers made the
wood particularly valuable for tree-
nails in the days of the construc-
tion of wooden ships. Obviously a
very important property of such a
wooden spike is that it shall neither
swell unduly in water nor shrink
extremely in the sun.

Another important group of
fibers is that included under the
head of septate fibers. These occur
very commonly in dicotyledonous
woods, particularly in shrubby and
herbaceous forms and vines belong-
ing to widely separated natural
orders. In this type the mechani-
cal element is divided transversely
by more or less delicate partitions
of a cellulosic or pectic nature,
which on staining with hematoxylin
and safranin stand out sharply
from the lignified longitudinal walls

Fic. 27.—Septate fibers from the vine
and teak (Tectona grandis).

Fic. 28.—Substitute fiber of the bar-
berry.

FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS 35

of the elements in question. The condition here present is repre-
sented in Fig. 27. To the left (a) is shown a septate fiber
from the grapevine, while on the right appears a similar element
from teak (Tectona grandis). Elements of this nature are the
probable basis of Strasburger’s apparently badly founded view that
parenchyma cells by their fusion have given rise to libriform fibers.
The pits in septate fibers are usually much larger than in the libri-
form elements and are conspicuously simple.

A last interesting category of fibrous elements, also characteristic
of extreme types, such as herbs, shrubs, and vines, is the so-called
substitute fiber (Ersaizfaser of German authors). In elements of
this nature septation is absent, but protoplasmic contents are
present in the cell when it is fully developed and in a functional
condition. Fig. 28 shows such an element from the wood of the
barberry. The pits are both radial and tangential, although the
former are obscured in illustration by the presence of protoplasm.
The mechanical elements of nearly all dicotyledonous herbs belong
in this category. The substitute fiber represents a convenient
union of the functions of strength and storage of food in the same
element, a condition particularly advantageous in the slender stems
of vines and herbs.

The final paragraph of this chapter may appropriately be
devoted to the subject of the distribution of the pitting in the fibers
of the dicotyledons. It has been noted that in the conifers in
general the pitting of the tracheary or fibrous elements is typically
confined to the radial walls, except in the case of the cells termi-
nating the year’s growth (summer tracheids). The mechanical
elements of the end of the annual ring have tangential as well as
radial pits. In the case of the Paleozoic gymnosperms, since
there are (with certain unimportant exceptions) no annual rings
marking seasonal diversity, tangential pitting is entirely absent, and
the pores of the tracheids are, as a consequence, exclusively radial.
The dicotyledons and the Gnetales present a very different situation
from that found in the seed plants characteristic of Mesozoic and
Paleozoic times; for in the former the mechanical elements,
whether of the nature of tracheids, fiber-tracheids, libriform fibers,

36 THE ANATOMY OF WOODY PLANTS

substitute fibers, or septate fibers, are particularized by the fact
that the pits or pores are not of exclusively radial distribution in
any part of the wood, but likewise occur in numbers on the tan-
gential aspects of the elements. This situation in the woods of
later times is of special significance when considered in connection
with other features of structure to be noted in a later chapter.

CHAPTER V

THE FIBROVASCULAR TISSUES: SECONDARY WOOD—
PARENCHYMA

Although the present chapter is to be devoted to the presence
of parenchyma in the secondary wood, it will be advantageous
to consider first the presence of cells belonging to this category in
the primary xylem. Parenchymatous elements, as has been made
clear in an earlier chapter, are those which are characterized in
their functional con-
dition by the pres-
ence of protoplasm,
by simple pitting of
their walls, and by
their generally more
or less isodiametric
dimensions. Cells of
this nature form a
well-marked feature
of primary wood—
that is, ligneous tis-
sues which are with-
out rays and without
regular disposition of
theirelements. The
parenchyma of the Fic. 29.—Origin of wood parenchyma in the pri-
ancient arboreal club mary wood of a lepidodendrid.
mosses of the Paleo-
zoic, the lepidodendrids, is of special interest from the standpoint
of the doctrine of descent, for here the mode of origin of the
parenchymatous elements of the primary wood is clearly indicated.
Fig. 29 shows a transverse view of the primary woody tissues of a
species of Lepidodendron from a “‘coal-ball’’ of the Carboniferous
of Lancashire in England. A number of elements of large size
and thick walls can be distinguished. These are the tracheids. In

37

38 THE ANATOMY OF WOODY PLANTS
addition there can be made out other small cells which are not so
Finally, the tissues include cells with walls

markedly thickened.
In the case of

which are distinctly thin—the wood parenchyma.
the cells with moderately thickened walls, where the bottom (or
top) is included in the plane of section, scalariform thickenings can

readily be distinguished. The thin-walled elements, when the lower

IN

YP—
TIN
iat, ll

HUAANNOHIAIECOOAOHIOTIUE
ANAT

ANN

ANN)

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WTR

Ui
RECA AHA

\
AN
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ANNI

Mill

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MM)

OY
niin

MINNA
WW
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hi

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i

ell{
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Fic. 30.—Longitudinal section of the primary wood of a lepidodendrid. Ex-

planation in the text.
or upper wall is exposed in the preparation, show it to be without
scalariform sculpture.

A highly important light is thrown on the facts recorded at
the end of the preceding paragraph by the longitudinal section of
the same primary wood exhibited in Fig. 30. Here may be seen
a number of long structures with thick walls and scalariform
sculpture. These are tracheids of the primary wood. Here and
there may be distinguished other structures of similar length,

FIBROVASCULAR TISSUES: PARENCHYMA 39

which instead of being simple are septate. In some instances the
short segments of the longer cells are obviously tracheary, since
they have scalariform sculpture on their lateral and terminal walls.
In other instances the septations of the long elements are quite
thin-walled and without any scalariform or reticulate thickenings.
Cells of the latter type are parenchymatous, and the origin of such
elements in the case of the primary wood of Lepidodendron is
clearly indicated. It is obvious that, as a consequence of the
subdivision of originally long cells, tracheary in their character,
two sorts of products result—namely, short thin-walled cells (the
wood parenchyma), and equally abbreviated elements, sisters to
these, which constitute merely segments of the subdivided tra-
cheids. It thus becomes clear that the parenchymatous elements
of the primary wood in the very ancient genus Lepidodendron are
derived from the subdivision of elements which were primitively
tracheids. It will be shown in subsequent paragraphs that the
origin of wood parenchyma in the secondary xylem is likewise due
to the septation of elements originally of the nature of tracheids.
In the present connection the question naturally arises as to the
derivation in turn of the tracheids. The following up of this
problem would lead us too far into the field of purely speculative
evolution. This much, however, may be stated, that, accepting
the thallose liverworts as the probable starting-point of the evolu-
tion of vascular forms, it becomes clear that the mass of elaters
(with spirally thickened walls) which constitute the columella in
Pellia, Aneura, and some species of Anthoceros, etc., obviously
constitute the prototype of the fibrovascular bundle of the ferns
and higher forms. If this hypothesis is to be admitted, and it is
the one assumed by the greater number of those who have worked
on the somewhat meager facts which throw light on the probable
origin of the Pteridophyta, it appears clear that in the spirally
thickened elaters we have the originals of the spiral elements of the
protoxylem. It follows that elaters are the forerunners of the
tracheids, which in turn, by subdivision, as evidenced by the lepido-
dendrids, have given rise to the parenchymatous elements of the
primary wood. There does not seem, however, to be any clear
indication in the case of other and existing lycopodineous forms

40 THE ANATOMY OF WOODY PLANTS

which throws light on this important subject. In the stem of the
remaining vascular plants the problem has likewise become obscured
by lapse of time and the blotting out of the original and primitive
conditions. There seems to be no evidence in regard to the origin
of parenchymatous elements in primary wood other than that
in the stem of the lepidodendrids, described above. The modern
descendants of the group do not furnish elucidation of the origin
of parenchymatous elements, and the same statement appears to
hold in the case of other living and extinct representatives of the
Pteridophyta.

The secondary wood of the older and Paleozoic groups of plants
was, so far as our present knowledge goes, entirely without paren-
chymatous or storage elements (with the exception of the so-called
medullary rays to be dealt with in the next chapter). This is
true not only of the arboreal cryptogams of the ancient period, the
lepidodendrids, the sigillarias, the sphenophyllums, and calamites,
but also of those lower and more primitive gymnosperms included
under the general headings of Cycadofilicales (Pteridospermae)
and Cordaitales. Not only are the Paleozoic vascular plants not
known to manifest the presence of true xyliary parenchyma, but
the same statement appears to hold in regard to the vascular plants
of the early part of the Mesozoic—that is, the Trias. Parenchyma-
tous elements have not yet been seen in any American wood of the
earlier Mesozoic. It is only in the Jurassic period that paren-
chymatous cells clearly manifest themselves as a feature of the
organization of the secondary wood, and at this time also the
annual ring as a feature of organization of the woody cylinder
first becomes well marked. There is, in fact, a distinct correlation
between the appearance of true parenchymatous storage elements
and the phenomenon of annual rings.

Before referring to true parenchymatous structures in secondary
wood it will be well to make mention of structures which may
readily be mistaken for these. In certain plants even of the
Paleozoic age the rays have attached to their margins more or
less elongated cells which are often of such length that they form
longitudinal commissures between adjacent rays. A situation of
this kind is found, for example, in the calamites and sphenophyllums

FIBROVASCULAR TISSUES: PARENCHYMA 4I

and is quite commonly present in the genus Ginkgo, a living sur-
vivor of a group anciently numerous in genera and species. A
parallel state appears in Pinus, a living genus which is represented
by nearly a hundred species in the Northern Hemisphere, but
which had probably three or four times as many species in the later
Mesozoic. These vertical commissures between the rays must
be considered in the
same category with
rays and do not
represent veritable
wood parenchyma.
Having made it
clear that all paren-
chymatous elements
in the wood cannot
be regarded as true
xylem parenchyma,
we find it possible to
discuss to greater
advantage structures
properly included
under this heading.
The first clear evi- Fic. 31.—Transverse section of the wood of the root
dence in regard to in Picea.
the presence of cells
belonging to this category dates from the Jurassic of Northern
Europe as observed by Gothan and Holden. There is good reason
to believe that the original region of appearance of the storage
elements of the secondary wood is at the end of the annual
rings. ‘There is, in fact, no good evidence that true parenchymatous
elements of the wood occur anywhere in forms not characterized
by the presence of annual rings. The conditions accompanying the
evolution of the storage elements of the xylem can best be studied
in the lower representatives of the pine family or Abietineae. The
genus Pinus itself, now known to be extremely ancient in its
occurrence, does not exhibit the elements under consideration in a
typical form. It is in the case of Picea, Larix, and Pseudotsuga

42 THE ANATOMY OF WOODY PLANTS

that they manifest themselves in the most significant manner.
The genus Picea will perhaps serve best to illustrate the conditions
involved in the appearance of parenchymatous elements in the
secondary wood. Fig. 32 illustrates the structure of a transverse
view of the wood of the root of Picea canadensis. Parts of two
annual rings are represented. The summer wood is easily recog-

nized by the thicker
walls of the tracheids
and the tangential pits.
It passes abruptly into
the spring wood of the
next annual increment,
characterized by large-
sized thin-walled ele-
ments with entirely
radial pitting. The cru-
cial feature of the figure
is presented by certain
cells portrayed in black
at the end of the annual
ring, or, as it is com-
| Fic. 32.—Detail of transverse section of the monly phrased, on the
wood of the root in Picea.
face of thesummer wood.
These elements are parenchymatous and constitute the only
representatives of this category of storage tissue in the wood of
Picea. Fig. 33 presents a longitudinal aspect of the face of the
summer wood in the same species. In order that a considerable
length may be shown, three successive segments of the plane of
section are depicted. In the median segment a row of short cells is
seen. These have obviously simple pits in their walls in relation
both to one another and to adjacent elements in the wood belonging
to a different category. In other words, the cells under considera-
tion are typical parenchymatous elements in longitudinal view.
To the left lies another and lower segment. At the top of this is
the continuation of one of the parenchymatous elements of the
first-described segment. As we follow the series of short cells
downward in the segment to the left the character of the elements

ii

SEZ |,

J
a

é\

FIBROVASCULAR TISSUES: PARENCHYMA 43

changes. ‘They now show bordered pitting instead of pores of the
simple type. The right-hand longitudinal segment in the figure
shows the region above that represented in the central segment.
Here the continuation of the parenchymatous elements of the
median segment is below, and this type of cell is again merged into a
series of short tracheids derived from the segmentation of one of
the normal longitudinal elements of the wood.

|

Fic. 33.—Longitudinal section of the wood of the root in Picea canadensis.
Explanation in the text.

52.96
SSLIoo

:

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; r OL 509}
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o
0
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0
0
Q

The general situation in regard to the mode of origin of longi-
tudinal storage elements or wood parenchyma in the gymnosperms
can perhaps be more readily illustrated by means of a diagram.
Fig. 34 represents a ray, ordinary tracheids, and a septate or
divided tracheid in relation to one another in the longitudinal
aspect. The ray need not be specially considered, as the subject
of the origin and organization of rays is to be discussed in the
following chapter. Turning our attention to the normal tracheids
represented in the diagram, we find it clear that these are char-
acterized by the possession of pits both on the radial and on the

44 THE ANATOMY OF WOODY PLANTS

tangential walls, a feature commonly found in the case of fibrous
elements in the terminal region of the annual ring in the conifers and

9 ©

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Fic. 34.—Diagrammatic
representation of the origin
of wood parenchyma in the
Picea.

allied groups. The septate tracheid is
the most striking feature of the diagram.
In this element numerous transverse walls
interrupt the continuity of the central
lumen. Certain of these transverse
partitions are characterized by the pres-
ence of bordered pits, while others offer to
the eye pits of the simple type. In a _
third condition bordered and simple pits
confront one another in the same wall.
Those cells which communicate with the
surrounding elements by means of simple
pits are typically occupied by protoplas-
mic contents, while the elements possess-
ing pores of the bordered category are
invariably without living substance. It
will be clear to the reader that, if the
diagrammatic representation presented in
Fig. 34 is correct, the parenchymatous
elements of the wood come from the sub-
division of the primordial elements which
would in other cases and under different
circumstances give rise to ordinary tra-
cheids. At an early stage these elements
became transversely septate, and in the
segments so set off the protoplasm some-
times persists (when a typical parenchy-
matous element of the wood is the result) ;
at other times it disappears with the
complete differentiation of the walls
surrounding it (in the case of so-called
short tracheids). An interesting fact in
this connection is the occurrence of paren-
chymatous storage elements in the same
region of the wood where the tangential
pits take their origin. It has been made

FIBROVASCULAR TISSUES: PARENCHYMA 45

clear in a former chapter that in the case of the Paleozoic
gymnosperms there are at the same time no annual rings (typically
“at least) and the pitting of the tracheary elements of the wood
is entirely radial in its disposition. In the gymnosperms of the
mesozoic age both annual rings and tracheids tangentially pitted
in the terminal region of the yearly zones of growth become the
rule. It is highly significant in this connection that the first
appearance of true longitudinal storage parenchyma is from the ©
Jurassic (Middle Mesozoic) onward, and that the storage cells take
their origin in the terminal region of the annual rings and clearly
show from their mode of development that they are the derivatives
of tracheary elements, since all stages of transition between septate
tracheids proper and files of parenchyma are found. It has been
suggested by Strasburger that the tangential pitting of the sum-
mer tracheids is for the purpose of easily and rapidly supplying
water to the cambium in the next opening period of growth.
Such a hypothesis would accord well with the very definite correla-
tion between tracheids of this type and the phenomenon of annual
rings. If the condition of annual growth be accepted as the prob-
able elucidation of the tangential pitting of tracheids in more
modern gymnosperms, it is not difficult to put forward a similar
claim for the terminal and tangential parenchyma which probably
represents the primitive disposition of parenchymatous elements in
gymnospermous woods. If the tangential pitting is for the purpose
of supplying the awakening cambium with water, there can be little
doubt that the later-appearing and correlated feature of terminal
longitudinal storage cells is significant in connection with the need
of the reviving initial cells of the zone of growth for readily avail-
able food.

In Pinus, as indicated above, there is no true wood parenchyma.
In the allied genera, Picea, Pseudotsuga, and Larix, it has become
clearly and unmistakably established and occurs everywhere in the
wood of the root, even when it is absent or degenerate in the stem.
As will be shown later, the root is the most conservative organ
of plants. In the case of higher members of the pine family (Abie-
tineae), Cedrus, Pseudolarix, Abies, and Tsuga, the parenchymatous
elements are still found typically at the end of the annual ring, but
they no longer normally show evidence of derivation from tracheids,

he

46 THE ANATOMY OF WOODY PLANTS

other than in the fact that they are found in spindle-shaped or fusi-
form groups resembling in contour tracheary elements. Injured |
specimens of the woods of the four genera under discussion show
(Fig. 35), however, the transition from short tracheids to true
parenchyma cells as the result of the septation of elements laid
down by the cambium as tracheids. This is an example of the

eSeud

CIN

<a\(we

Fic. 35.—Injured wood of Tsuga canadensis, showing transition of tracheids
to parenchyma.

recall of ancestral conditions as a consequence of injury, a phe-
nomenon extremely common in the case of plants with secondary
growth. ‘This situation is classified, as will be shown later, under
the caption of the evolutionary doctrine of reversion.

Already in the higher representatives of the pinelike conifers—
namely, Abies and Tsuga—the parenchymatous elements have
not only lost all normal indications of derivation from tracheary
elements at the end of the zone of annual growth, but have even
begun to abandon the strictly terminal position in the yearly rings

FIBROVASCULAR TISSUES: PARENCHYMA 47

of wood. In certain species of Abzes and Tsuga the location of
the parenchyma is on the face of the summer wood only, while
in others it is no longer strictly confined to this position. In the
genus Abies, A. webbiana and A. cephalonica are characterized by
longitudinal storage cells no longer wholly terminal in position
but scattered throughout the annual growth. The same condition
is present in Tsuga mertensiana and in smaller branches of T.
canadensis. ‘This type of disposition of parenchyma may con- |
veniently be designated diffuse to distinguish it from the terminal
condition of storage elements in their first appearance among the
conifers. It is of interest to note that, so far as the matter has been
investigated, both Abies and Tsuga in the root have terminal
parenchyma exclusively, no matter what the situation may be in the
case of the stem. ‘The diffusion of parenchymatous cells through-
out the annual ring in higher forms of seed plants has its significant
analogy in the spreading of tangential pitting, at first confined to the
later-formed tracheids of the summer wood, to the elements of the
rest of the annual ring in higher types. Diffuse wood parenchyma
never betrays its origin by normal transitions to septate tracheids.
It is only in the case of wounding that its tracheary origin can be
distinctly observed. The longitudinal storage cells, to which the
name of wood parenchyma is given, always reveal their derivation
from the fibrous elements, however, by the fact that they are
grouped in series corresponding in length and outline to the pointed
tracheids constituting primitively the sole longitudinal elements
of the wood.

In the case of the abietineous genera Abies and Tsuga, cited
above, it is quite clear that the storage elements of the woods have
in some instances departed from the exclusively terminal position
and have become distributed throughout the annual ring. In those
conifers belonging to the subtribes known as Taxodineae, Cupres-
sineae, and Podocarpineae the parenchyma of the wood is char-
acteristically diffuse and no longer, except by the grouping and
contour of its elements, gives evidence of tracheary origin. Injuries
in most cases reveal conditions of transition from septate tracheids
to pointed rows of parenchymatous elements. In the subtribes
Araucariineae and Taxineae parenchyma has disappeared from the

48 THE ANATOMY OF WOODY PLANTS

normal structure of the wood in the vegetative stem, but is ordina-
rily clearly and sometimes even abundantly present in the diffuse
condition in regions which are conservative of ancestral characteris-
tics, such as the root, cone axis, etc. It may, moreover, be easily
recalled as a consequence of experimental procedure. It is accord-
ingly clear that, from the evolutionary standpoint at any rate, con-
siderable care is necessary in deciding as to the primitive presence
or absence of parenchymatous elements in the woody tissues of the
conifers.

Ve

=
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LS

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LATVIA

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Fic. 36.—Diffuse parenchyma in a coniferous wood. Explanation in the text

_ Fig. 36 shows the topography of the diffuse condition of paren-
chyma in coniferous woods in three dimensions. In a is seen the
transverse view of a wood of this type. The longitudinal storage
elements, or wood parenchyma, are represented with heavy black
walls to bring out by contrast their distribution in the general |
structure and at the same time the frequent chemical difference
of their walls from the tracheary tissues proper. In b appears
the tangential aspect of the storage elements, in which they present
themselves in the greatest breadth and on the whole show most
clearly by pits their relation to other elements of the wood. In the
case of c these important elements of wood structure are revealed in
the radial aspect, and their diffuse disposition is as apparent as in
the transverse plane of section.

FIBROVASCULAR TISSUES: PARENCHYMA 49

It will be clear from the account of the appearance of paren-
chymatous cells given in the preceding paragraphs that their origin
in the primary tissues of the wood is revealed only in the case of that
very ancient group of cryptogamous trees, the lepidodendrids of
the Paleozoic age, where they are very clearly derived from the
septation of tracheids. Further, in the case of the secondary wood,
parenchymatous elements properly so called did not make their
appearance before the Mesozoic period and are as distinctly corre- '
lated in their origin with the phenomenon of annual rings pre-
sented by plants of the Mesozoic and later geologic time as are
tangential pits in the tracheids. As has been made clear in an
earlier chapter, the secondary wood of Paleozoic gymnosperms was
characterized, not only by the absence of annual rings, but equally
clearly by the default of pits on the tangential walls of the tracheids
and the entire absence of parenchymatous elements in the wood.
The arrival of the phenomenon of annual rings can be most satis-
factorily explained in the case of the known facts by the hypothesis
of the gradual refrigeration of the surface of our earth, with the
consequent appearance of a winter period of rest in vegetative
activity. The phenomenon of annual growth soon brought into
prominence the two striking features of terminal tangential pitting
and terminal storage parenchyma. Both features of organization
were later extended with greater or less completeness to the whole
of the annual ring. It is further quite clear that parenchymatous
storage elements came into being in the secondary wood by the
septation of tracheids, precisely as has been shown to be the case
in the similar elements of the primary wood (apparently only clearly
indicated in the case of the lepidodendroid cryptogams of the
Paleozoic age).

In the higher gymnosperms, including the greater number of
conifers as well as the Gnetales, the distribution of the longi-
tudinal storage elements of the secondary wood is typically and
primitively diffuse; that is, such cells are not confined to the end of
the recurring zones of growth, but are scattered throughout the
annual ring. It is apparently not necessary to figure the situation
for the Gnetales, since it corresponds so closely with that found in

50 THE ANATOMY OF WOODY PLANTS

the higher Coniferales. Fig. 37 illustrates the distribution of the
wood parenchyma in those dicotyledons in which the condition is
diffuse. In a is shown a transverse view of the secondary wood in
the black walnut (Juglans nigra). The distribution of the storage
cells is made clear by the outlining of their walls in solid black.
In } the same wood is depicted in longitudinal radial aspect, and the
nature and distribution of the parenchymatous elements become
doubly clear. A fact not without interest from the evolutionary

pay SE Wy lel |

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es

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is

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A NN or

Fic. 37.—Diffuse parenchyma in Juglans. Explanation in the text

standpoint is the uniform distribution of parenchyma after this
fashion in whole natural orders of dicotyledons. For example, in
the Juglandaceae, Betulaceae, Fagaceae, Ebenaceae, Ericaceae,
etc., the disposition of the longitudinal food-storing cells of the
wood is entirely of the manner designated above as diffuse.

In other orders of dicotyledons, particularly those which are
with some degree of unanimity assigned by systematists to a high
position, a strikingly different mode of parenchymatous arrange-
ment is found. This is notably the case in the Compositae,
Verbenaceae, Oleaceae, etc. The situation in this type of distribu-
tion of parenchyma can be clearly indicated by reference to the
ash. Fig. 38a shows the grouping of the parenchyma at the end

FIBROVASCULAR TISSUES: PARENCHYMA 51

of the annual ring and around the vessels in this genus. These
are the only situations in which parenchymatous cells are normally
present in orders characterized by what may be conveniently termed
vasicentric parenchyma. In 6 the longitudinal view of a vessel with
its parenchymatous jacket of vasicentric parenchyma (that is,
parenchyma confined to the vicinity of the vessels) is indicated.
The vasicentric distribution of parenchyma is, other things being
equal, an indication of an advanced systematic position among the’
dicotyledons and is, as will be shown in the sequel, accompanied

== GD — aus
=

09008

ages Sh

{a

Op
ON
A

Fic. 38.—Vasicentric parenchyma in Fraxinus. Explanation in the text

wi e/

4

by other features of organization in the tissues of the secondary
wood which are somewhat generally accepted as indicating a high
degree of specialization.

In dicotyledonous woods, with either of the two characteristic
modes of distribution of the parenchymatous elements indicated
in the preceding paragraphs, degeneracy may occur. As a con-
sequence the longitudinal elements may somewhat rarely be ab-
sent altogether or may be confined to the end of the annual ring,
the latter situation being much more commonly found. Fig. 39
illustrates the organization of the wood in Salix or Populus (or
equally well that of Liriodendron or Magnolia) as regards the

2 THE ANATOMY OF WOODY. PLANTS

erat

distribution of parenchymatous elements. In a is shown the trans-
verse view, and it is here clear that the cells in question, accentuated
by the representation of their walls in black, are confined to a posi-
tion at the end of the annual ring. Clearly, the vessels appearing
in the figure are unaccompanied by any parenchymatous elements.
In } is represented the longitudinal radial aspects of the same wood,
and it becomes clear that in this plane, too, the storage cells are
confined to the terminal region of the annual ring, for none can be
seen in relation to the vessels. The situation here indicated is not

My a

Be

ie

a

1h Ss a=
SSS SS

_]
=

vs
Ss

>———}

1 21

223 BSE Gee ee ee, eS

> —w
EQS! :

=

zs

=

NYG

Fic. 39.—Terminal parenchyma in Populus. Explanation in the text

uncommon in the case of woods with reduced vasicentric paren-
chyma. As in so many other instances, the situation is made clear
by reference to more conservative parts, such as the root and the
first annual ring of the stem, or to injured material showing a
reversion to the primitive condition. By such control of evidence
it becomes clear that in the Salicaceae, as well as in Magnolia and
Liriodendron among the Magnoliaceae, the characteristic mode of
distribution of the storage cells is vasicentric, for they occur in this
manner both in conservative organs and parts and likewise as the
result of experimental injury. It cannot, of course, be too strongly
emphasized in the case of comparative anatomical investigations
that a wide view of any particular situation is essential to an ade-

FIBROVASCULAR TISSUES: PARENCHYMA 53

quate comprehension of a given problem. The truth of this
statement will become more and more obvious as a result of repeated
illustrations in the sequel. Much more rarely does the diffuse
condition of parenchymatous disposition in the dicotyledons give
rise by reduction to a state in which storage elements are to be
found only at the end of the annual zones of growth. This situa-
tion is exemplified by certain species of the antarctic beech (Notho-
fagus), in which, in contrast to all the boreal species of the Fagaceae, :
the parenchyma is confined to the face of the summer wood and
is not distributed throughout the annual ring, as is the rule for
the family as a whole. Similar reasoning to that employed in the
case of the Salicaceae and certain Magnoliaceae results in the
correct conclusion as to the typical and primitive mode of occur-
rence of parenchymatous elements.

It is necessary to emphasize the different interpretations of
terminal parenchyma which must be adopted in the case of the
conifers and the dicotyledons. In the coniferous series the presence
of wood parenchyma on the face of the summer wood is clearly
a primitive phenomenon, both because of the comparative and
historical data and because of the equally cogent evidence derived
from the consideration of the origin of parenchyma cells in this
position in the conifers. Clearly, terminal parenchyma in the
gymnospermous series is in the act of origination in view of its
almost imperceptible transition to septate tracheids. In the
dicot¥ledons, on the other hand, comparative and experimental
data alone, in the absence at the present time of any adequate
information in regard to the historical evolution of woods of this
type, lead to the conclusion that the occurrence of storage elements
in the terminal region of the annual rings is rather the result
of reduction from a more elaborate and advanced condition (diffuse
or vasicentric) than one of primitive simplicity.

It is obvious that in general among the dicotyledons we need
not expect to have as clear evidence in regard to the problem of the
derivation of the longitudinal storage elements of the wood as in the
case of the gymnosperms, and in particular the conifers, which
present themselves to our gaze in so long and continuous a series
in geologic time. Evidence for the origin of parenchymatous

54 THE ANATOMY OF WOODY PLANTS

elements from the tracheary and other fibrous constituents of the
secondary wood in the dicotyledonous angiosperms is sufficiently

Fic. 40.—Longi-
tudinal view of
tracheids and paren-
chyma from the root
of the alder.

authentically supplied by the grouping of such
elements in longitudinal terminally pointed
groups possessing the exact configuration of
tracheids or fibers. Fig. 40 illustrates this
situation for the wood of the alder. To the
right and left are seen fiber-tracheids which
constitute the mechanical elements of the wood
in this genus. Between these two cells lies a
file of parenchymatous elements of the wood.
It is clear that the latter, represented with
heavy black boundaries, form a series which
both in length and in contour corresponds with
the adjacent fibrous elements. Normally there
are no transitions from tracheids to parenchy-
matous cells in the case of the dicotyledons, and
even experimental means do not in general
suffice to bring about the clear production of
parenchyma as the result of the progressive
septation of the original tracheary elements of
the wood. Some information on this subject,
however, is furnished by the phenomena of
injury in Liqguidambar and Prunus. Here, as a
response to experimental or accidental injuries,
tangential rows of gum or mucilage canals are
formed in the wood. About these canals may
sometimes be detected short elements which
grade from abbreviated tracheids, resulting from
the septation of tracheids of normal length, to
typical parenchymatous elements, characterized
by persistent protoplasmic contents and simple
pits in their walls.

Certain general characteristics of wood which
depend as much as anything on the condition
of the wood parenchyma may be conveniently
introduced here. Fig. 41 is a photograph of the

FIBROVASCULAR TISSUES: PARENCHYMA 5

On

polished end of the trunk of an oak. The wood is clearly separated
into two regions, a darker central and a pale peripheral. The
deeply colored central region of the trunk constitutes the heartwood
or duramen. ‘The uncolored zone which surrounds this is the
sapwood or alburnum. ‘The dark-hued heartwood is extremely
resistant to decay and constitutes the only material properly
utilizable for exposed
Seruetures. | | int the
case of dicotyledonous
trees in general, one
frequently notes even
with the naked eye a
difference in color
between the heartwood
and the sapwood.
Sometimes this impor-
tant distinction is not
revealed to the eye,
but becomes obvious
only under microscopic
investigation. Fre-
quently the sap indi-
cates its boundaries in the felled trunk by the discoloration brought
about in its tissues either by oxidases or by fungi, or by both
agencies united. In conifers, likewise, a distinction between a
darker central heartwood and a surrounding pale-hued sapwood
can be often recognized by the naked eye. The redwood or
red cedar presents the contrast in color in a very marked manner.
The larch and the spruce, which in the microscopic organization
of their woods are practically identical, can be readily distinguished
from one another by the gross aspect of their trunks. The larch
has a dark-brown heartwood, while in the species of spruce the
central region of the woody cylinder is in no way contrasted in
color with the peripheral sapwood.

It will be convenient in connection with the discussion of the
parenchymatous or storage elements of the wood to elucidate
certain features of the microscopic organization of the heartwood

Fic. 41.—Transverse view of an oak log showing
heart- and sapwood.

56 THE ANATOMY OF WOODY PLANTS

as contrasted with the sapwood. In this connection it will be well
to begin with a conifer. Fig. 42 represents side by side the micro-
scopic aspects of heart and sap tissues in the case of the white pine
(Pinus strobus). Beginning with the sapwood, which appears on
the right of the illustration, it is clear that both its rays and the
cells surrounding a resin space or canal are possessed of nuclei and
likewise contain a somewhat granular living substance or proto-
plasm. Imbedded in the protoplasm are usually found oval bodies,

aS
=

\

Se t———
fae $

Sa =
Rial s | a

Fic. 42.—Sap- and heartwood of the pine. Explanation in the text

the grains of starch, which are lacking, indeed, only in the cells
immediately surrounding the resin canal or space. By examination
of the bordered pits of the tracheids it becomes evident that in the
wood of the sap the membranes of the pits are central in position.
Turning to the left side of the figure, we find represented the corre-
sponding organization of the wood in the case of the heart. The
elements distinguished by simple pits in the delineation on the
right—in other words, the living cells—here have lost their living
contents and are quite empty. Moreover, in the water-conducting
cells, or tracheids, distinguished by the presence of bordered pits,
we discover that the membranes of the pores with their thickened
central region, or torus, are no longer median in position, but in
general have become adherent to one side or the other. Further,
the resin space has been stopped by an ingrowth known as tylosis.

FIBROVASCULAR TISSUES: PARENCHYMA 57

In other coniferous woods, particularly those higher in the scale
than Pinus, the parenchymatous elements, whether radial or
longitudinal in position, secrete antiseptic substances such as
essential oils, tannin, etc., which preserve the heart. structures
from the decay resulting from the attacks of wood-destroying
fungi. Fig. 43 throws light on the similar conditions as regards
the organization of heart and sap in the oak, as an example of the
dicotyledons. To the right, radial and longitudinal parenchymatous

i Geen) TaN a epee
ry, Se wae Lk TN = aN
ES > Bas) Bis)

Sy. of \Y) else S25 @)

an
4
mAoUn

fa\

SX \
EIN N
A \EGrs

Frc. 43.—Heart- and sapwood in the oak. Explanation in the text

la deo
Jerr S
Sy

= exam)

elements appear loaded with starch and provided with proto-
plasm and nucleus. The radial storage cells are represented with
light walls, while the true wood parenchyma is delineated with
thick, black, bounding membranes. Larger and smaller vessels
are to be seen corresponding to the spring and summer region of
the wood. The rest of the area is occupied by tracheids (larger and
thinner-walled) and fiber-tracheids (narrower and with thicker
walls). Turning now to the left, where the organization of the
heartwood is indicated, we discover the same absence of contents
in the radially and longitudinally directed elements with simple pits
(in other words, the parenchymatous elements) as in the case of
the similar structures of the pine. In this instance the larger
vessel presents a certain analogy to the resin canal of the conifer by
reason of the fact that it is occluded by an ingrowth in the form of

58 THE ANATOMY OF WOODY PLANTS

a tylosis. An adhesion of the pit membranes to one overhanging
margin or the other of the pit cannot be made out. Occasionally
a torus is present in the relatively narrow membranes of the small
bordered pits of the water-conducting elements of the dicotyledons,
but it does not present the phenomenon of fusion with the margins
of the pit characteristic of heartwood in conifers. The paren-
chymatous constituents of dicotyledonous woods in many cases give
rise to highly efficient antiseptics in the case of the heartwood.
In many instances, such as the oak, the blue gum, the quebracho,
etc., large amounts of tannin are formed which serve as an effectual
preservative. In other cases ulmic and even humic acids make
their appearance and exercise greater or less inhibitive action on
the organisms which ordinarily bring about the decay of woody
structures. In teak we have the rare example of a structurally
valuable dicotyledonous wood which in the transformation from
heartwood to sapwood elaborates, not acid substances which exer-
. cise a corrosive action on metals, particularly on iron and steel,
but an essential oil. The enduring heartwood of the teak (Tectona
grandis) is consequently valuable above all others for naval con-
struction by reason of its compatibility with iron and steel, since it
neither corrodes this fundamental structural material of present
naval architecture nor, in turn, is rotted by iron rust.

It will be apparent from the foregoing paragraphs that those
longitudinal elements which subserve the function of storage in
the woods of Mesozoic, Tertiary, and actual plants are of great
evolutionary significance. Their importance in this respect
can be gauged only after the rays or radial storage devices have
been considered in the next chapter, and they will receive their
final and fullest appreciation in connection with the highest groups
of plants. It is obvious that the incentive to the development of
longitudinal parenchymatous elements was the appearance of an
annual winter period of rest which in later geological times, begin-
ning with the earlier Mesozoic, with progressively greater emphasis
marked the originally unvarying cycle of the year. The first
parenchymatous elements came into being, so far as our knowledge
at present goes, in the earlier Jurassic. Their primitive occurrence
was at the end of the annual ring. In the conifers in this position

FIBROVASCULAR TISSUES: PARENCHYMA 59

they often clearly reveal their derivation from tracheids by almost
imperceptible transitions into elements belonging to this category.
The terminal situation of the primitive parenchymatous cells is
probably of advantage to the cambium awakening from its winter
sleep and standing much in need of instantly available food. In
general, the tangential terminal parenchyma of the more primitive
conifers has the same nutritive relation to the cambial elements as
the similarly located tangential pitting of the tracheids has to the
water supply of the cambial zone.

Later the longitudinal elements devoted to the function of
storage, like the tangential pitting of the tracheids, became dis-
tributed throughout the annual ring. So long as there was no
further differentiation of the elongated elements of the wood there
was no incentive to further evolution on the part of the elements
of the wood parenchyma. With the appearance of the vessel as
the final expression of efficiency in the transport of water on the
part. of the wood and the correlated gradual loss of the aquiferous
function on the part of the tracheids (which progressively gave rise
to fiber-tracheids and libriform fibers, respectively more and more
specialized in the mechanical direction), a new tendency found
expression in the organization of the longitudinally oriented
storage devices of the wood. In the case of the higher gymnosperms
(Gnetales) and lower dicotyledons the fibrous elements of the wood
are still largely capable, by reason of the presence of numerous
bordered pits in their walls, of the transport of water. The higher
dicotyledons, however, are in general characterized by the strict
allocation of the function of movement of water to the vessels, and
the tracheary structures of lower types become transformed into
purely mechanical or partially mechanical and partially food-storing
elements designated progressively as fiber-tracheids, libriform
fibers, septate fibers, and substitute fibers. With the appearance
of this situation the parenchymatous elements of the wood
become more and more relegated to the vicinity of the vessels for
their necessary supplies of all-important water. This situation
receives its final morphological and evolutionary expression in the
appearance of strictly localized vasicentric parenchyma in the case
of high dicotyledonous groups, such as the Compositae, Sapindales,

60 THE ANATOMY OF WOODY PLANTS

Verbenaceae, Oleaceae, etc. It must not be assumed, however,
that the localization of the parenchyma about the vessels is strictly
referable to the transformation of the tracheids into purely mechani-
calelements. It has been shown by Miss Holden in her interesting
investigations on the Sapindales that it makes no difference whether
the fibrous elements of the wood here are of the nature of tracheids
(and hence are capable of conducting water) or are libriform and
mechanical as regards the distribution of the elements of the wood
parenchyma, which in the group throughout are vasicentric. It is
thus clear that the distribution of the parenchymatous cells in dicoty-
ledonous woods is of morphological importance and is not physiolo-
gically conditioned by the nature of the fibrous portions of the wood,
whether tracheary and with bordered pits or libriform and thus
definitely relegated to purely mechanical functions.

It is finally important to note that not even the obvious facts
of distribution can without proper discrimination be subjected to
evolutionary inference. For example, in many dicotyledonous
woods terminal parenchyma occurs which might be regarded as
prima facie evidence of a primitive systematic position in view of
the situation presented by the conifers, exhibiting parenchyma on
the face of the summer wood. A comparative and experimental
investigation of this situation makes it clear in the case of the
dicotyledons that the terminal position of parenchyma is the result
of degeneracy either from the vasicentric or from diffuse modes of
parenchymatous distribution.

CHAPTER VI

THE FIBROVASCULAR TISSUES: SECONDARY WOOD—RAYS

In the preceding chapter the subject of the origin of longitudinal
storage cells has been discussed and to elements of this type the
general appellation of wood parenchyma has been given. In the
case of the ligneous tissues of vascular plants a much older type
of storage device
exists in the form of
radially directed
bands of masses of
cells which often, with
a high degree of im-
propriety, are desig-
nated medullary rays.
This denomination is
erroneous from the
historical and evolu-
tionary standpoint,
since it is clear that
in the first instance
and in the earlier and
primitive forms rays
had no relation what-
éver, to the pith or =! one SuSE

medulla and _ conse- Fic. 44.—Transverse section of the stem of a
quently cannot with lepidodendrid, showing well-developed primary and
secondary wood, the latter being radially seriate and
provided with storage rays (after Scott).

any degree of pro-
priety be designated
as medullary. Fig. 44 illustrates the conditions obtaining in the
case of the so-called medullary rays of the ancient genus Lepido-
dendron. The region of the pith () is represented largely by an
empty space, which is in turn surrounded by the primary wood.
This is distinguished by the non-seriate and irregular arrangement
61

62 THE ANATOMY OF WOODY PLANTS

of its cells in contrast to the regularly and radially disposed
structures of the secondary xylem which lies just outside the
primary region. It is clear from the figure here introduced that
the so-called medullary rays which extend from the primary wood
outward cannot be properly so designated, since they never come
in contact with the pith. The same situation occurs in the stem

,(
( sNatit—
ol
=e

ce
i;
Ml

Lk
UU,

{

i

im

\\\
\\\y)
I”

Ss

Sy A

Gee ees

Mh

Minin
= ANliny

HI
eet al

ee:

NS

ea
lf
aa

Mn

!

——

Fic. 45.—Longitudinal view of the primary wood of a lepidodendrid. Explana-
tion in the text.

of many other extinct fernlike or gymnospermous plants with
secondary growth. It is worth while to note, too, in this connec-
tion, that in the root of living plants in which the wood undergoes
secondary increase its radially disposed bands of storage cells have
nothing to do with a pith or medulla. It is accordingly evident
from an examination of older and consequently more primitive
forms, and likewise from the study of the organization of the con-
servative root structure of living groups, that the term medullary

FIBROVASCULAR TISSUES: RAYS 63

ray is a misnomer. The most appropriate name for the radial
stripes of storage elements in the secondary wood is the merely
descriptive one of wood ray.

The status of the ray having been preliminarily defined from
the evolutionary standpoint there now remains the question of the
origin of the hori-
zontally directed
bands of storage tis- 1
sue which it is cus-
tomary to consider (
under this head. I

[Fa

has been made clear | ‘a
that the lepidoden- | ig ean
drids are the only & Ge

oN =

28

plants which supply

decisive evidence as '

to the origin of the ‘

parenchymatous ele- » & & @
ments found uni- 6 Q ® /

versally in the
primary wood of

vascular plants. It be
P Fic. 46.—Transition from primary to secondary wood

will be well to recall jn a lepidodendrid. Description in the text.
the situation here by

reference to Fig. 45, which shows a longitudinal view of the
tracheary and allied structures of the first-formed wood of a speci-
men of the genus Lepidodendron from the Carboniferous of Lanca-
shire, England. It is obvious that, in addition to the longer cells
with reticulately thickened walls—the tracheids proper—there are
numerous short elements with a similar kind of sculpture. In
series with these are other cells again which are quite without the
usual tracheary thickenings and which belong, in fact, not to the
water-conducting system, but to the storage category. These are
wood parenchyma. It is evident in the present instance that the
storage cells of the primary wood have been derived from what
were originally tracheids by septation or division and subsequent
differentiation. It has been demonstrated in the previous chapter

64 THE ANATOMY OF WOODY PLANTS

that the parenchymatous structures of the secondary wood are in
the first place derived from modified tracheids.

In Fig. 46 is shown the region of transition from primary to
secondary wood in a lepidodendrid, somewhat highly magnified.
The secondary tissue is most characteristically distinguished by
its rays running in
alternation with
bands of large tra-
cherds:. dhiesie
radially directed
stripes of storage
elements at once
attract attention by
reason of the unu-
sual organization of
their component
cells. The constit-
uent units of the
rays in this instance
are reticulately
thickened after the
manner of tracheids
and, in fact, differ
from these only by
their abbreviated length and the somewhat more delicate nature
of their sculpture. There is, indeed, not the slightest doubt that
in the case of the lepidodendrids the rays are largely, and in some
instances wholly, composed of cells belonging to the category of
tracheids. This situation is clear in radial sections taken length-
wise through the wood. Fig. 47 represents such a section from
the root of the lepidodendroid type know as stigmaria. The
heavy sculpture of the tracheids composing the mass of the
wood can be easily made out. Running at right angles to the
direction of the tracheary elements of the wood are the cells of a
ray. These again show a considerable degree of scalariform
thickening, only a few being completely devoid of this form of
sculpture. It is obvious from the conditions described in the case
of the lepidodendrids, a group of arboreal club mosses flourishing

Fic. 47.—Radial view of the secondary wood of a
lepidodendrid, showing tracheary origin of the rays.

FIBROVASCULAR TISSUES: RAYS 65

in the Paleozoic, that rays in this type were more or less largely
composed of tracheids. It seems clear for this reason that the
radial bands of storage parenchyma which constitute the rays
in secondary wood, like the longitudinal parenchyma included
under the caption of wood parenchyma, are derived from the
modification of tracheary tissue. It thus becomes apparent that
originally all the parenchymatous constituents of wood, whether
primary or secondary or radial or longitudinal, in the first instance
made their appearance by the modification of tracheary tissues.
Tracheids, in fact, constitute the sole original feature of organiza-
tion in woods, and the course of evolution expressing itself in con-
tinued differentiation has led to the derivation of all the other
features of ligneous structure from this primary constituent. In
other words, wood primitively was a purely water-conducting
tissue, and the superadded mechanical and storage functions
subserved by its organization in later geologic times resulted in
appropriate modifications of the original tracheary elements. The
derivation of the rays from tracheary tissues can be distinguished
clearly only in the lepidodendrids and their allies. In others of the
arboreal cryptogams which were so characteristic of the forests of
the Paleozoic age no evidence of the origin of ray cells from tracheids
has been observed. The same statement holds for the lower and
ancient gymnosperms, the Cycadofilicales (Pteridospermae of Oliver
and Scott) and the Cordaitales and their allies. These antique
gymnospermous groups, as well as the arboreal cryptogams sig-
nalized above, had no storage tissues in their wood other than
radial parenchyma; for the longitudinal parenchymatous elements
known as wood parenchyma proper made their appearance only in
connection with the seasonal refrigeration which became ever more
pronounced during Mesozoic and later geologic time. The rays
of the older plants with secondary growth were of two main types.
In some instances (e.g., Cycadofilicales) they were composed of
bands of cells several elements in width and greatly varying in
height. In the Cordaitales and allied forms the rays were ordi-
narily uniseriate—that is, a single cell in width, in contrast to their
often multiseriate and considerable height. In a general way it
seems clear that woods of the first type are perpetuated in the still
living, although much reduced, Cycadales, while the Cordaitales,

66 THE ANATOMY OF WOODY PLANTS

according to common consent, find their successors in the conifers
of the present age.

Since it is highly probable that the Cordaitales gave origin to
the Coniferales, and since, as a consequence, they present us with

iN

VY

VO
NVQ SN

)

SZ

ra,
WY)
N
ay

WN

0 on 1\\)
WON

SG me,

AVY

8

LOK

VO.
a

Ni

Gee

WW
mM

igs

SON

wok

aN

~2-8

)

S| TS eee
NS

\ ORS

oe

AS
SAME Sz
EEENSCE ECA

So
CE

2

— = — _
—= a = Gx
= S AE BE @, H
AIS Ge— AIS].
eee —§

Zz. = = =a BG
Ae e2sa6

oN

a

Fic. 48.—View of cordaitean wood in three dimensions. Explanation in the text.

the starting-point of the radial parenchyma of the latter, it will be
well to consider the organization of this group in respect to the
structure of the rays. Fig. 48 shows the structure of a cordaitean
wood. The tracheids in this genus are usually extremely long;

FIBROVASCULAR TISSUES: RAYS 67

consequently only a portion of their length is represented in the
figure. The pitting is somewhat characteristic and is ordinarily
marked by the large number of pores and their consequent crowded
and alternating arrangement. The rays cross the direction of the
longitudinal elements and are characterized by their thin walls,
which are, however, clearly pitted laterally in relation to the radial
walls of the tracheids. It is to be noted in passing that there are
no tangential pits on the walls of the longitudinal elements of the
wood—a feature, as has been indicated in a previous chapter, very
generally characteristic of the woods of the gymnosperms of the
Paleozoic regardless of their affinities. In the transverse section
of the wood the rays stand out distinctly as uniseriate files of cells,
having their axes radially elongated. The rays are in lateral
communication with the tracheids by half-bordered pits. Other-
wise their thin walls are not characterized by the presence of pores.
The tracheary elements of the wood are tangentially in com-
munication by numerous radial pits, but their tangential walls are
quite free from pores, so that in these ancient gymnosperms water
could make progress in the trunk only in a tangential direction.
The tangential view of the wood shows the radial pits of the tra-
cheids as well as the lateral ones of the cells of the medullary rays in
profile view.

After the discussion of the organization of rays in the Cordaitales
we are in a favorable position to understand the condition in the
conifers. In this connection it will be well to start with the most
complicated condition in living representatives of the group, since
the coniferous gymnosperms, as will be shown clearly in a subse-
quent chapter, constitute a reduction series with the more complex
forms at the bottom and those with simpler organization at the
top. Fig. 49 represents radial and tangential views of the wood of
the white pine (Pinus strobus). Taking first the radial view shown
in a, a number of important contrasts in organization to cordaitean
woods are to be seen. First of all as regards the tracheids, or rather
such part of them as is included in the field of view, it is clear that
they are distinguished from similar structures in the Cordaitales
by the smaller number of pits and the considerably larger size of
these. Further, the pits in face view, instead of presenting the

68 THE ANATOMY OF WOODY PLANTS

merely double contour of the more ancient group, are marked by
triple concentric outlines. The outer circle corresponds to the
boundary of the pit membrane, while the inner one outlines the
mouth or aperture of the pit. The intermediate circular outline
delimits the thickened central region of the torus, a structure,
which so far as is known, was not present in Paleozoic gymnosperms.
Its presence in the conifers is in all probability correlated in some

SS

Ce)

P

i
[
c
LS
@

—<—

Fic. 49.—Radial and tangential sections of the wood of the white pine (Pinus
Strobus). Explanation in the text.

way with the large size of the bordered pits; it has, in fact, as
pointed out in an earlier chapter, been interpreted as a safety
device useful in preventing the rupture of the broad pit membranes
under extreme pressure. Not only are the pits of large size and
more complex organization in the pine than in cordaitean forms,
but the walls of the tracheary elements in their vicinity, partic-
ularly toward the ends of the tracheids, are distinguished by trans-
verse bands of cellulosic or pectocellulosic material, and these are
conveniently designated “bars of Sanio.”’ They should not be
confused with the trabeculae, radially directed lignified bars running
transversely through the lumina of the cells of many gymnosperms
living and extinct. (They are even found in some cases among the

FIBROVASCULAR TISSUES: RAYS 69

angiosperms.) There is good reason to believe that the “‘bars of
Sanio”’ are an original and characteristic feature of the organization
of the wood of the Coniferales and allied groups. The tracheids
shown in the figure differ from those of the cordaitean forms by
their periodic variation in size, those laid down in the beginning of
the annual increment being of larger caliber than those coming
into existence toward its close. The late tracheids are distinguished,
as indicated in earlier pages, by their tangential pits seen in profile
in the radial section. Turning our attention now to the ray
itself, we see at once from the figure that a considerably greater
degree of complication is present than that exemplified by the
similar structure in the case of the Cordaitales. Manifestly the
radial elements are of two kinds. First there are the cells which
constitute the central region of the ray and which in life are char-
acterized by protoplasmic and other contents. This situation is
indicated by the copious simple pits which ornament the vertical
and horizontal walls of the cells. The strong pitting is also a clear
feature of difference from the Cordaitales where the walls of the
ray cells are in general thin and unpitted. Laterally the central
elements of the rays are related to the tracheids by means of very
large, somewhat angular pits. The second type of element char-
acteristic of the ray in Pinus is likewise distinguished by the nature
of its pitting. All the pits seen on the walls, whether horizontal,
vertical, or lateral (related to the tracheids), belong to the bordered
type. The cells of the ray possessing this peculiar organization
are typically marginal in position and, naturally, are quite without
protoplasmic contents, except in the early stages of development.
Such tracheary elements in coniferous rays are commonly desig-
nated marginal tracheids or simply marginal cells. They obviously
permit the easy movement of water in the radial direction in those
woods characterized by their presence.

In the tangential view presented in 0, Fig. 40, it is clear that
there are two types of rays—namely, narrow uniseriate ones and
broader ones tapering at either end to the uniseriate condition.
The former are known as linear rays and the latter as fusiform rays.
It is evident from the figure that the ray of greater width is char-
acterized by the presence of a partly occluded cavity, a resin canal.

70 THE ANATOMY OF WOODY PLANTS

The stopping up of the resin canal is explained by the fact that the
section is taken from the heartwood, which, as has been pointed

Fic. 50.—Tangential view of the wood of the
Pseudotsuga. Explanation in the text.

out in a former chapter,
is distinguished by the
phenomenon of tylosis
or occlusion of the secre-
tory spaces by means of
ingrowths of the resi-
niferous parenchyma.
In the case of the nar-
row or linear ray it is
possible to distinguish
in the tangential view,
by means of their char-
acteristic pitting, both
the parenchymatous
central cells and the tra-
cheary marginal ele-
ments. "The cemtrat
elements with their
simple pits are in rela-
tion to air spaces in the
angles. The presence of
both linear and fusiform
rays is a constant fea-
ture of organization in
the pine and its nearer
allies. The linear rays
are doubtless an older
feature than the larger
ones which contain the
horizontal resin canals,
since they resemble most
nearly the radial paren-
chymatous structures of

the Cordaitales. The fusiform rays serve to bring about a connec-
tion between the vertical resin canals in different annual rings, not

FIBROVASCULAR TISSUES: RAYS qe

only with one another, but often with the similar but larger secre-
tory cavities which are present in the bark.

Fig. 50 illustrates the relation between the vertical and hori-
zontal resin canals as seen in a vertical section of the wood of the
Douglas fir (Pseudotsuga). To the left of the center lies a fusiform
ray with its included resin space, which opens broadly on the right
into a vertical secretory
canal. The plane of sec-
tion happens to lie near
the end of the annual ring
so that the cellular struc-
tures lying in view are
almost entirely parenchy-
matous. The lining of
the resiniferous spaces is
evidently largely com-
posed of elements with
thick walls and bordered
pits, a condition very com-
monly present in the
representatives of the
Pineae other than Pinus
itself.

After the discussion of
the pine and its nearer allies we may proceed with advantage to
the description of the ray structures in coniferous woods of simpler
organization. Fig. 51 illustrates the structure of the ray in the
wood of the balsam fir (Abies balsamea). It is clear that in this
case the cells of the rays are all of one kind and that the marginal
tracheids are conspicuous by their absence. The parenchymatous
elements which compose the rays in the genus under considera-
tion are in relation to one another on both vertical and hori-
zontal walls by numerous simple pits. The lateral walls show
bordered pores such as ordinarily, except in certain species of
Pinus, characterize the relation between rays and _tracheids.
The organization of the linear rays in conifers other than the
Abietineae is in general of a simple nature; and in the higher

Fic. 51.—Ray of the balsam fir. Explana-
tion in the text.

V2 THE ANATOMY OF WOODY PLANTS

subtribes, such as the Cupressineae, Taxodineae, and Taxineae,
even the intercommunicating simple pits of the horizontal and
vertical walls of the ray cells are clearly and often conspicuously

absent.

In such cases, as is to be expected, the wall of the ray

elements is in general thinner and often curved.
An interesting condition of organization of the radial paren-

Faq als “ole “oll Meals
([to Ik “all oll ell Ie
Foil cello Slee Wellollll&
Al “alle aCe ®ollNllells
Alle ilo ll(eo lst ell SIs
chase che Iolo Kell

Q|"Ol_ © Ric
2 s Hole

Fic. 52.—Ray of the Nootka cypress.

{s9|

De-

chyma is presented by
Chamaecyparis nootkaten-
sis. Here, as is shown in
Fig. 52, the rays are fre-
quently marked by the
presence of tracheary ele-
ments on one or both
margins. The illustration
represents the radial
aspect of the transition
from summer to spring
wood, and the features of
structure are in general
such as one would expect
to find under the circum-
stances, except as regards
The

ray organization.
normal presence of margi-
nal tracheids in the species figured gains a special evolutionary sig-
nificance from the fact that similar conditions are found in other
representatives of the Taxodineae, Cupressineae, and the genus
Abies among the Abietineae as a consequence of injury. For
reasons which will be fully discussed in a subsequent chapter it
seems quite clear that the structures which are found to appear as
a result of injury in vascular plants with secondary growth are often
of the nature of reversions to an ancestral condition. Fig. 53 illus-
trates in radial view the wood formed after injury in a root of the
Big Tree (Sequoia gigantea). Parts of two annual rings are included,
and the ray clearly shows features of structure which are abnormal
for the Taxodineae. To the left below and in the spring wood can
be seen several cells included in the substance of the ray which are

scription in the text.

FIBROVASCULAR TISSUES: RAYS 70

clearly tracheary in their character, since they contrast with the
adjoining elements both in the absence of protoplasmic contents
and in the occurrence of bordered pits in all their walls. It is
evident that in the case under discussion short tracheids may make
their appearance among the elements of the radial parenchyma as
a sequel to injury. The interesting investigations of Miss Holden

Fic. 53.—Ray from the injured root of Sequoia gigantea. Explanation in the
text (after Holden).

on the Cupressineae and Taxodineae as a whole make it clear that
abnormalities of this kind in these subtribes of coniferous gym-
nosperms are a common feature in the wood formed after injury.
It follows from the statements and illustrations in connection
with the last paragraph that normally in Chamaecyparis noot-
katensis and traumatically in practically all representatives of the
Cupressineae and Taxodineae, ray-tracheids are found such as
are a feature of the normal structure of the wood in the lower
members of the Abietineae. The most natural interpretation of
this phenomenon is in connection with the biological doctrine of

74 THE ANATOMY OF WOODY PLANTS

reversion. This general doctrine will receive particular considera-
tion at a later stage and consequently need not be elucidated here.
If the radial tracheids occurring under the conditions described in
the foregoing paragraph are interpreted as a reversion to an ances-
tral condition, it follows that the simple type of ray found in Abies
and in the Cupressineae or Taxodineae is by no means primitive,
but is the result of
simplification from
the more complex
state of organiza-
tion of the ray,
characteristic of
the lower living
Abietineae. Obvi-
ously an experi-
mental as well as a
purely anatomical
investigation of
ray structures is
necessary for their
complete morpho-
logical and evolu-
tionary under-
standing.

Not only does

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I'tc. 54.—Tangential section of the normal wood of

Cedrus Libant.

one find in the case of certain coniferous woods of simpler
organization evidence of derivation from ancestral types present-
ing the complication of marginal ray-tracheids, but likewise in the
genus Cedrus among the Abieteae, which has normally only
linear or uniseriate rays, fusiform radial structures containing
horizontal resin canals are found. This situation is made clear
by Figs. 54 and 55, which present tangential views of the normal
and injured wood of Cedrus Libani (the cedar of Lebanon). In the
traumatic or injured wood of cedar, which, so far as the geological
record supplies us with definite evidence, is the oldest representative
of the Abieteae or firlike conifers, we have clearly indicated a
condition ensuing from injury which definitely unites the cedar

FIBROVASCULAR TISSUES: RAYS 75

with the genus Pinus, much older geologically and more complicated
in the normal organization of the wood. It will be clear from the
statement in this connection that fusiform rays have also an impor-
tance from the experimental standpoint, quite comparable with
marginal tracheids, but less copiously expressed in traumatic
phenomena.

The rays of the
Cycadales and
their allies present
no features of spe-
cial evolutionary
interest, so far at _
any rate as is
known at the pres-
ent time; they may
therefore be dis-
missed with the
simple statement
that they are typi-
cally multiseriate
bands and not the
linear structures
constituting the
primitive condition
of organization of the radial parenchyma for the Cordaitales,
Coniferales, and Ginkgoales. The ray structures in the highest
gymnosperms, the Gnetales, are best discussed in connection with
the similar features of dicotyledonous woods, which they resemble
in so many respects. This procedure is the more desirable because
the living Gnetales are represented by a very small number of
genera of widely separated geographical ranges.

At this point the dicotyledonous angiosperms may appropriately
be considered in regard to the organization of their rays. Fig. 56
reproduces a transverse section of the wood of the oak. The struc-
ture in this case is highly complicated and corresponds to a marked
development of the principle of division of labor. The movement
of water, the functions of strength and of storage, are all distinctly

a

2
ts,
a
os
rs
°
J
°
.
8
.
e
=
e
J

Bay sed eee
LEE aan

een.
eee eet

Sevenuess’**
es
o

‘eoecseeeege--*
-— .@ Sea‘
PRR N O00 O Margy

Fic. 55.—Tangential section of the wood of Cedrits
Libani formed after injury.

76 THE ANATOMY OF WOODY PLANTS

allocated to particular and highly differentiated categories of cells.
In the present connection we are concerned only with the struc-
tures included under the heading of radial parenchyma. Clearly
the rays in the oak are of two types, even as seen in transverse
section. A small number are very broad and constitute a large
bulk of storage tissue. In contrast to these in respect both to size
and to number are linear rays, abundantly present in the figure.
It is best in this
instance to focus
our attention on
the composition of
the uniseriate or
linear rays and
their relation to
the various ele-
ments represented
in the complex
organization of the
wood in the same

genus, Quercus.
Fig. 57 repre-
sents the uniseriate
rays in radial and
; : tangential aspect.

Fic. 56.—Transverse section of the wood of the :

Meet ae In a is seen the
tangential view.
The ray is obviously composed of cells which are all alike and
related to one another and to the air spaces by simple pits. Half-
bordered pits connect ray cells with tracheids. In b and ¢ are
shown radial aspects of the ray in relation to the various structural
elements of the wood. On the left and right in 6 files of vertical
parenchyma cross its course and are related to the radial elements
by groups of clustered pits. To the left of the middle of b is shown
a vessel (this type of element will be considered in the following
chapter) which communicates with the radial storage elements by
large, generally oval, pits. The rest of the width of 6 is occupied
by tracheids, which in turn communicate with the cells of the

FIBROVASCULAR TISSUES: RAYS 77

rays by small bordered pits. Inc is seen a region where the linear
ray passes through a purely fibrous portion of the wood; and
here there are no pits at all, since the mechanical elements which,
as has been indicated in a:former chapter, have been differen-’
tiated from the tracheids no longer supply water to the other
structures of the wood. It will be clear from the foregoing
account that the uniseriate or linear rays of the oak are of uni-
form and simple organization as regards their constituent ele-
ments, but that these are characterized by a variety of pitting
corresponding to the high degree of differentiation of the wood

ACTUAL

UB

Receareber ||| k DAIL

Fic. 57.—Longitudinal and transverse views of linear rays in the oak. Explana-
tion in the text.

through which they pass. The large rays of the oak can better
be considered at a later stage after a type showing a more general-
ized condition of radial organization has been examined.

It will be convenient and profitable to consider in the present
connection the genus Casuarina, which occurs in the East Indian
and Australasian regions, since here we find in various species all
the main types of organization of the wood rays exemplified by the
dicotyledons. First is presented the tangential, longitudinal view
of the wood in Casuarina Fraseri. Here the structural conditions
are manifestly very similar to those found in the oaks of northern
latitudes, for there are two distinctly contrasted categories of rays—
namely, the numerous linear or uniseriate and the sparse broad
rays. In comparison with the wood of C. Fraseri, presenting a
strong resemblance to that of a white or black oak (Fig. 58), is
that of C. torulosa, shown in Fig. 59. Here the linear rays are as

78 THE ANATOMY OF WOODY PLANTS

in C. Fraseri, but the large band of radial parenchyma is obviously
not homogeneously parenchymatous, but is separated into pointed
groups by the presence of interspersed fibers. In other words,
instead of a continuous mass of storage tissue there is present
an aggregation of rays of a certain size, separated from one another
by strands of fibers. This condition of the large ray may be
conveniently designated as aggregate. In Fig. 60 is represented
the tangential
aspect of the wood
in C. equisetifolia.
In this type we no
longer see a sharp
contrast between
large multiseriate
or aggregate rays
and small entirely
uniseriate ones, but,
as it were, a more
democratic organi-
zation of the radial
parenchyma, in
which no extremely
large radial paren-
chymatous masses

< ————
oo ’
ase:

=a “

:@

a

Fic. 58.—Tangential section of the wood of
Casuarina Fraseri. Explanation in the text. are found, as all
grade almost imper-

ceptibly into one another in size. The last-described condition
of the rays, for reasons to be indicated later, is here designated
as diffuse. We have thus in the single genus Casuarina three
distinct types of radial parenchyma: first, the northern oak type
in which huge rays stand in the sharpest contrast to narrow uni-
seriate bands; secondly, a condition in which the contrast still
obtains as regards the dimensions of the rays, with the dis-
tinction that the large masses are not homogeneous but pene-
trated by bands of fibers; and, finally, there is to be noted a state
or organization in which all rays are of moderate width and are
scattered somewhat evenly throughout the tangential or transverse

FIBROVASCULAR TISSUES: RAYS 79

section of the wood. In the first condition the large rays are
known as compound; in the second they are designated as aggre-
gate, and in the third state, where there is generally no marked
superiority in size and the rays intergrade almost imperceptibly,
they are known as diffuse.

Of the three conditions of organization of radial parenchyma
in the dicotyledons described above, the compound is extremely
rare in trees but is
commonly found in
climbing and _her-
baceous types. The
diffuse condition of
the radial paren-
chyma is very com-
mon in forest trees,
but is much less
characteristic of
plants of herba-
ceous texture. The
genus Casuarina
has purposely been
chosen for the
exemplification of
the problems con-

A Fic. 59.—Tangential section of the wood of Casu-
nected with the arina torulosa. Explanationin the text.

evolution of radial

parenchyma in the dicotyledons, not because it is necessarily a
primitive form, but because it shows the situation synoptically
and, moreover, furnishes very clear evidence as to the relation
of the various types to one another from the evolutionary
standpoint.

An exposition of the interesting and important situation of the
ray structures in the dicotyledons can best be approached by a
diagrammatic comparison with the conditions presented by the
conifers. Fig. 61 reproduces the essential features of organization
of a coniferous stem with whorled leaves—for example, a Juniperus
or a Cupressus. The leaves, three in number, are indicated in

80

black on the periphery of the stem.

THE ANATOMY OF WOODY PLANTS

Centrally placed is the

pith, surrounded by the woody cylinder, which in turn is circled

by the phloem and the cortex.

The woody tissues are encroached

upon by three deep
bays extending
from the medulla
and subtended by
the three leaves.
These extensions
from the pith mark
the presence of the
leaf gaps, interrup-
tions in the con-
tinuity of the
woody cylinder re-
lated to the passing
out of the fobhar
traces. Later the
intervals in the
wood are covered

Fic. 60.—Tangential section of the wood of Casua-

rina equisetifolia. Explanation in the text.

that the cylinder becomes continuous
in the second or third year of growth.
Clearly there is no structural feature
of importance in the woody cylinder
related to the leaf trace other than the
foliar gap. This is the general situa-
tion in the case of coniferous stems as
well as in that of their Paleozoic ances-
tors, the Cordaitales.

Having diagrammatically compassed
the organization of the stem in the coni-
fers, we are in a position to consider the
case of such a dicotyledon as Casuarina.

by the activity of
the cambium, so

gully
PY ly

\ é [Re
wy

Dy,

WD
o»
LZR)

lr

ANN! UAL ey

< oo] >

| | \ es
mh (awe

Fic. 61—Diagrammatic
transverse section of a coniferous
twig. Explanation in the text.

In Fig. 62 are reproduced the essential features of topography of

a small branch in this genus.

Leaves, as in the case of the conifer-

FIBROVASCULAR TISSUES: RAYS 8L

ous diagram given previously, are represented in black on the
periphery of the stem. The central pith is likewise encircled in
turn by wood, phloem, and cortex. Here, too, there are excur-
sions of the pith at six points, extending into the second annual
increment of the wood; these subtend radially six corresponding
leaves. In the diagram under discussion there are, however, two
marked features of contrast to those presented in the foregoing
scheme of a coniferous axis. First of all, the wood is character- |
ized by the presence of vessels, and
secondly by a broad radial stripe

"y,
which extends from each leaf gap « Bey Has
; ' a7 \
outward. This radial stripe con- TE eS Z

trasts with the rest of the woody
cylinder by the absence of vessels
and the clustering of rays. The
radial parenchymatous stripes which
lie in the region of the six radial
bands corresponding to six leaves are
not only more prominent than the

A
Bia

Fic. 62.—Diagrammatic trans-

: : verse section of the stem in Casua-
linear and somewhat Sparse rayS IN yjna, Explanation in the text.

the rest of the wood but are of

greater width. The aggregations of rays related to the leaves
shown in the diagram are, in fact, clustered rays of the type exem-
plified by C. torulosa (Fig. 59) and are, as a consequence, aggregate
rays. Since they are in this instance clearly related to leaves,
they may at the same time be appropriately designated foliar
aggregate rays.

With the exposition of the essential features of organization
present in coniferous and dicotyledonous stems, respectively, we
are in a position to proceed further with the highly important
discussion of the evolution of the radial parenchymatous structures _
of dicotyledonous woods. Simplicity will be served and ambiguity
avoided if in further elaboration we hold to the conception of the
rays as they present themselves in a small twig of a few years’
growth. Fig. 63 illustrates synoptically the main types of rays in
the dicotyledons as seen in small branches. The only essential
departure from the conditions in nature is the delineation of the

82 THE ANATOMY OF WOODY PLANTS

three important categories of rays as occurring side by side in the
same stem. In the center of the circle representing diagram-
matically a dicotyledonous stem is figured a leaf gap and

iN

Ha

\ as

ax

l'IG. 63.—Synoptical diagram representing the transverse and longitudinal
topography of the rays related to the leaves in species of Casuarina. Explanation

in the text.

peripherally its subtending leaf. Between the two lies a corre-
sponding ray of the aggregate type. It is to be noted here that
there is a modification of the rate of growth of the annual rings

FIBROVASCULAR TISSUES: RAYS 83

in the region of the aggregate ray which results in their being
locally depressed.

To the right is to be seen another leaf with its corresponding
gap and ray. Im this case the ray structure where it is still near
the leaf trace (solid black) is the same as in that just described,
namely, aggregate. Farther out, however, the components of the
aggregate ray, instead of maintaining their original relations to one
another, begin to diverge in the tangential plane. At the same time |
vessels which are conspicuous by their absence while the ray is in
the aggregate condition begin to appear in the widening strands of
wood which separate the diverging rays. This process continues
and becomes more and more marked in successive outer annual
rings. ‘The final result is that what was once a congery or aggre-
gation of rays separated from one another by purely fibrous strands
becomes a more and more diffuse cluster of rays separated by ever-
widening vascularized intervals of wood. In this condition the
original aggregation of rays not only becomes diffuse in a fanlike
fashion in the outer region of the woody cylinder, but the individual
rays subdivide, thus accentuating the condition of diffusion. The
phenomenon of the subdivision of the rays for the sake of simplicity
is omitted in the diagrammatic representation. It is clear that the
appearance of the conditions depicted in the ray to the right of the
diagram (at 6) in the case of all the foliar rays of a stem would
result in a diffusion of rays of a medium breadth throughout the
older wood—in other words, to the condition shown in Fig. 60 for
the adult wood of Casuarina equisetifolia or an allied species.

Turning now to the left of the diagram (at c), we observe a
foliar or leaf ray of still another type. Here, as in the diffuse con-
dition of the foliar ray represented in 0, the original state is that of
aggregation with the exclusion of vessels, a situation which is per-
manent in the type diagrammed at a. In the later annual rings in
this type the aggregation becomes a homogeneous mass of paren-
chyma by the disappearance of the fibrous strands which separate
the components of the aggregation or congery from one another.
Where the clusters become fused into large homogeneous bands of
storage tissue by the parenchymatous transformation of the original
separating fibers of the aggregation, the result is the compound ray

84 THE ANATOMY OF WOODY PLANTS

represented at c in our diagram. This condition is present in the
adult structure of the wood of Casuarina Fraseri, as is shown in
Fig. 58. The primitive aggregate condition persists, on the other
hand, in C. torulosa, as is shown in Fig. 59.

1 Naame i

Fic. 64.—Diagrammatic representation of a twig of Picea canadensis. Explana-
tion in the text.

In the longitudinal aspect of the diagram presented to the reader
are shown the vertical views of the three types of rays illustrated
horizontally in a, 6, c. Fig. 63a’ shows the tangential topog-
raphy of an aggregate ray. The clustering of the masses of radial

FIBROVASCULAR TISSUES: RAYS 85

parenchyma and the exclusion of vessels can be readily seen. At
b’ is indicated the tangential projection of the diffuse condition
of rays. Here the original cluster has become scattered and
vessels are now present among the rays. In c’ is shown the longi-
tudinal tangential aspect of the compound ray—that is, the con-
dition in which the original aggregation has become fused into a
large solid mass of radial parenchyma.

The conditions
in a diagram of a
coniferous stem in
three dimensions
may now be dis-
cussed. Fig. 64
illustrates the
topography of .a
two-year-old twig
of Picea canaden-
sis. In the trans-:
verse aspect the
pith surrounded
by xylem, phloem,
and cortex can be
distinguished.
Projecting from
the surface are the
leaves or their bases. The outline of the pith is indented by a num-
ber of bays, which are the deeper the nearer they are in the vertical
plane to the departing trace of a leaf. In relation to one of these on
the side of the pith nearer the observer is an actual trace running
horizontally in the wood. The transverse aspect of the wood
shows the presence of numerous narrow rays. The face of the
stem facing the observer is cut away to show the topographical
relations of the leaf traces in the wood. It is clear that, contrary
to the conditions observed in the stem of Casuarina diagrammati-
cally represented in Fig. 62, there are no modifications in the
grouping of the rays or in other features with reference to the
foliar traces (appearing as oval dots).

Fic. 65.—Transverse section of a twig of Casuarina Fraseri

86 THE ANATOMY OF WOODY PLANTS

It will be advantageous as a sequel to the representation of the
prominent types of rays in the dicotyledons in diagram to view
them in actual photographs in the case of the genus Casuarina. We
may profitably begin here, as in the case of the diagrams, with
transverse sections. Fig. 65 reproduces the cross-section of a
small twig of Casuarina Frasert. The leaves are in the main still
present in depressions on the surface of the stem and clearly sub-
tend the foliar
rays. Rather nar-
row leaf gaps pene-
travertine ins §
annual ring and
are twice as numer-
ous as the append-
ages at a given
node (that is, there
are twelve gaps,
although only six
appendages).
This duplication
of the gaps is due
to the fact that the
whorls of leaves
alternate at differ-

Fic. 66.—Part of transverse section of a twig of
Casuarina Fraseri more highly magnified. ent nodes, and the

gaps correspond-
ing to these are persistent, with the natural result of gaps twice
as numerous as the appendages. Fig. 66 shows part of the
foregoing much more highly magnified. At the top is a persistent
leaf and in the middle line below lies the pith, which is sending off
an extension, the leaf gap; and this, on reaching the second annual
ring, undergoes considerable enlargement. Between the wide
termination of the leaf gap in the beginning of the second annual
ring and in line with and subtending the leaf on the outside of the
stem lies the foliar ray. The magnification is not sufficient in the
figure to show the organization of the leaf ray; hence a more
enlarged representation is introduced in Fig. 67. Here it is clear

FIBROVASCULAR TISSUES: RAYS 87

that the structure of the ray in the twig in C. Fraseri is different
from the adult condition, shown in Figs. 58 and 63, for in the
younger axis the ray is obviously penetrated by fibers, and these
are absent in the adult. The truth of this statement will become
still more apparent by reference to Fig. 68, which reproduces the
tangential aspect of the wood in a somewhat older branch of the
same species. The prominent mass of radial storage tissue in
the center is the
foliar ray. It is dis-
tinctly fibrous and
is consequently still
in the condition of
aggregation. As
the stem thickens
the fibers are grad-
ually transformed
into parenchyma-
tous elements more
and more like the
cells: of “the ray.
Thus it is that the
compound ray of
C. Fraserit comes
into being. It is

3 A Fic. 67.—Portion of Fig. 66 still more highly magni-
evident that its fied to show organization of the foliar ray. Explanation
early condition is __ in the text.

one of aggregation,
and that this is followed by a gradual transformation into the
compound state by the fusion of the originally separate members
of the aggregation.

The photographic representations lead likewise to conclusions
in harmony with the diagrammatic figures in the case of the diffuse
condition of the foliar ray. It will not be necessary to introduce
a total general and a partial more detailed view of the twig in this
instance, since the topographical relations are practically the same
as those shown in the case of C. Fraseri. Fig. 69 illustrates the
situation in the diffusion of the foliar ray as exemplified by

88 THE ANATOMY OF WOODY PLANTS

C. stricta. It is evident from the photograph that a mass of ray
tissues on the left (representing the end nearer the pith), charac-
terized by moderate breadth and the absence of vessels, passes
toward the right (outer side topographically) into a continually
widening fanlike cluster of rays among which vessels become more
and more prominent. Fig. 70 shows a tangential, longitudinal
section of the wood in a small branch of C. equisetifolia. Here is
present an aggrega-
tion of rays, foliar
in its character,
which as yet has
scarcely begun to
diverge into the
diffuse condition
and consequently
includes no vessels

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in its substance.
C.SinicteranduG:
equisetifolia are
both species with
the diffuse type of
ray in the adult
wood. C. stricta
has been chosen to

younger stem of Casuarina Fraseri, showing aggregate illustrate the trans-
condition. verse aspect of the

Fic. 68.—Tangential section of a large ray in the

diverging rays in
the diffuse type only because the larger size of the radial bands
in this species make the topographical relations more obvious.
The difference in gross appearance between stems with aggre-
gate or compound rays on the one hand and diffuse rays on
the other hand, is very striking. This is well illustrated by
Fig. 7oa. On the left is seen a polished segment of the trunk
of C. Fraseri (compound and aggregate rays). On the right
appears a polished trunk of C. séricta (diffuse rays). Here
the rays are at first distinct and later die out. The conspicu-
ous stage is aggregate, while that representing the disappearance

FIBROVASCULAR TISSUES: RAYS 89

of noteworthy bands of radial parenchyma marks the diffuse
condition.

It has been demonstrated in the case of Casuarina that there
are three main types of radial parenchyma in the secondary wood
—namely, the aggregate, the diffuse, and the compound. Of
these three types the aggregate is manifestly the oldest, and the
other two have originated from it by divergence in the diffuse

Fic. 69.—Transverse section of aggregate rays diverging into the diffuse condi-
tion in Casuarina stricta.

condition and by fusion in that state designated compound.
What is diagrammatically clear in Casuarina for the radial paren-
chymatous structures is much less obvious in most other dicotyle-
dons, and in no case are the relations so well shown as in the genus
named. Among arboreal forms the oak is of interest in exhibiting
both the aggregate and the compound condition of the rays. In
this genus the species occurring in warm regions are ordinarily
characterized by the possession of clustered or aggregate large
rays in contrast to the uniseriate or linear rays present in the mass
of the wood. In species of northern climates the rays are solid or
compound in their nature, but even here the condition of aggregation

go THE ANATOMY OF WOODY PLANTS

is found in the young plant as a passing phase and as frequently
persisting for many years in that most conservative of plant organs,

Fic. 70.—Tangential section of young twig
of Casuarina equisetifolia, showing an aggregate
foliar ray in center flanked on either side by
uniseriate rays.

the root. In the Betu-
laceae, by contrast, the
diffuse type of wood ray
prevails, and it is likewise
clearly derived from a
primitive state of aggre-
gation. In the birch, for
example, diffuse rays
more usually distinguish
the stem, while the aggre-
gate type is often found
present in the case of the
root. The general ana-
tomical conditions as well
as experimental data in
the Fagaceae and Betu-
laceae justify the conclu-

Fic. 7oa.—Photographs of transverse views of polished stems of Casuarina stricta
(right) and Casuarina Fraseri, showing the general topography of diffuse and compound
rays. The diffusing rays of C. stricta appear to die out, while the rays of the other
species become not only accentuated but also clearly more numerous.

FIBROVASCULAR TISSUES: RAYS gI

sion that wherever the compound or diffuse types of rays are
present in members of these groups they have been derived from
an earlier condition of aggregation. The type of ray structure
is not strictly related to any particular organization of the flower,
since amentaceous, archichlamydeous, and metachlamydeous fami-
lies present to an almost equal extent the various categories of
rays. Ina general way it may, however, be stated that the diffuse
condition of the rays is characteristic of arboreal forms, while the
compound condition occurs mainly in vines and herbs.

CHAPTER VII
THE FIBROVASCULAR TISSUES: SECONDARY WOOD—VESSELS

The vessel is an element of structure which in the higher forms
and in the secondary wood is of extremely great evolutionary
importance. Before we take up this type of ligneous element in
the seed plants it will be well to consider its occurrence in lower
forms and in the primary wood. It was pointed out by De Bary
many years ago that many of the scalariform tracheids of the
bundles in the bracken fern are of the nature of vessels, using that
term to apply to elements which are not merely pitted but actually
perforated at the ends, thus permitting a much more ready passage
of water. The statements of the distinguished anatomist of
Strassburg have been confirmed and denied by more recent inves-
tigators, but there seems to be no doubt whatever on the basis
of our improved technique that he was in every respect correct.
Fig. 71a illustrates the organization of a vessel in Pteris aquilina
isolated from the surrounding tracheids by maceration. It is
clear that the scalariform bars are separated from one another by
wider intervals at the ends of the element and that the bars them-
selves are considerably more slender in this region and are not
distinguished by as pronounced overhanging margins as is the case
in the similar structures on the lateral walls. In 6 is represented
a profile view of the terminal inclined wall of the tracheid, indicat-
ing plainly the absence of the membrane which characterized the
lateral scalariform pits. Further, the margin or border in the
case of the terminal open pits is slightly developed in comparison
with that of the lateral pores with membranes intact. Conditions
similar to those found in the bracken appear in many other repre-
sentatives of the Filicales and are by no means confined to the
Polypodiaceae, which at the present time not only are the largest,
but also are considered in many respects the most specialized family
of the group. Elements resembling vessels in the presence of
terminal perforations have likewise been described for the genus

Q2

FIBROVASCULAR TISSUES: VESSELS 93

Selaginella among the lycopodineous forms.

It is thus clear that,

so far as the primary structures of the wood are concerned, vessels

are present even in the case of
representatives of the lower vas-
cular plants.

Although in certain ferns
and lycopods structures occur
which, physiologically at any
rate, represent vessels, these
cannot be regarded as quite on
the same morphological and
evolutionary footing as the ves-
sels of the highest gymnosperms
and the angiosperms. ‘The step
from a tracheid to a vessel, where
all the tracheids are scalariform,
as is the case with the tracheids
of ferns and lycopods, is a much
shorter one than when the
fibrous elements are pitted and
not scalariform. We find as a
consequence that, although ves-
sels or elements which have been
regarded as such occur low in
the scale of the Vasculares in
the primary structures, they
make their appearance in the
secondary wood only in the
higher representatives of the
seed plants.

The secondary wood of the
extinct Paleozoic arboreal cryp-
togams, although often perfectly
preserved, in no authentic
instance has yet revealed ele-

Fic. 71.—Face and profile views of
the type of vessel found in the primary
wood of the bracken.

\

ments which may with any degree of propriety be designated as

vessels.

The same situation obtains in the case of the lower

94 THE ANATOMY OF WOODY PLANTS

gymnosperms and even in the higher representatives of the group

as far up as the conifers.

Fic. 72.—Smaller and
larger vessels of Ephedra.

the angiosperms, as will

It is in the Gnetales, the most advanced
forms among the naked-seeded Spermo-
phyta, that vessels first make their appear-
ance in the cylinder of the secondary
xylem. Fig. 72a represents a smaller ele-
ment of this type from the wood of the
genus Ephedra. The walls terminating
the structure under discussion are dis-
tinctly at angles to the lateral ones and
are remarkable for the extremely large
pits which cover their surfaces. The pits
of the terminal walls are not only very
much larger than the lateral pits, but,
with two or three exceptions, they have
lost their membranes. In the pores where
the membranes still persist a well-marked ©
torus reveals its presence in face view.
The pits of the lateral walls of the tra-
cheids in the case of Ephedra are not very
much narrower than those of the terminal
surfaces, but invariably are closed by
membranes thickened centrally by a well-
marked torus. Fig. 725 shows a some-
what larger vascular element from the
wood of Ephedra, which has its enlarged
terminal pit entirely perforate as a result
of the complete disappearance of the mem-
branes. In Fig. 73 is shown a vessel in
which, as an exceptional condition for the
genus under discussion, there is a tendency
to fusion on the part of the enlarged termi-
nal pit. Although in the Gnetales the
phenomena of fusion of the end pits of
the vessels is rare, it becomes the rule in
be shown at a later stage. At a appears

a vessel which is transitional from an element of this type to a

FIBROVASCULAR TISSUES: VESSELS 95

tracheid. It is distinguished by the fact that its end walls do
not make a definite angle with the lateral ones but taper grad-
ually. It is further peculiar in the circumstance that its terminal
pits are imperforate with one exception and have their mem-
branes marked by the presence of a very distinct torus. In ¢ is
figured a tangential view of a vessel in Ephedra, showing the
enlarged terminal pores in profile and making it clear that in this
case, as in Pteris aquilina, the vessels become patent by the dis-
appearance of the membranes of the pits. The organization of the

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Fic. 73.—Vessels of Ephedra (after Thompson). Explanation in the text

tracheae or vessels in the two remaining genera of the Gnetales—
namely, Welwitchia and Gneéum—is not essentially different from
that described for Ephedra, and consequently need not be considered
here. The Gnetales as a whole are distinguished, not only by the
presence of vessels in their wood, but by the possession of rays of
the angiospermous type, as has been indicated in the preceding
chapter.

The angiosperms are characterized throughout by the presence
of vessels, generally of a high type, but in some cases showing clear
indications of derivation from tracheids. It is only in certain
xerophytic genera among the Magnoliaceae and in certain Cactaceae
and Crassulaceae that these characteristically angiospermous

96 THE ANATOMY OF WOODY PLANTS

elements are absent in the dicotyledons. No case is at present
known of their default in monocotyledons. There are two main
modifications of vascular elements or ¢racheae in the angiosperms—
namely, vessels with scalariform perforations and those with

Fic. 74.—Vessel
from Alnus, showing
the scalariform type
of perforation.

porous perforations. The term perforation is
applied to the actual apertures which make
their appearance in the terminal or other walls
of typical vessels. The first-named condition
of perforation is characteristic of lower groups
and lower genera among the dicotyledons, while
the second is usually found in the case of the
higher orders (the Compositae, for example) of
the dicotyledons and seems to be universal for
the monocotyledons.

In Fig. 74 is represented a vessel of the
lower type—that is, one in which the perfora-
tions in the terminal wall are scalariform. This
is an element from the root of the alder. The
lateral confines of the vessel are covered with
small bordered pits which in this particular case
are arranged in a somewhat regularly banded
fashion as a result of the contact of the con-
ducting element under discussion with the cells
of a ray of the wood. If the relation had been
with tracheids or other vessels, the arrangement
of the pores would not be so regular. Pits are
clearly seen in profile on the sides of the vessels,
indicating the fact that water-conducting ele-
ments belonging to this category do not neces-
sarily have their lateral pitting confined to
the radial walls. The ends of the vessel are
strongly inclined and are manifestly at definite
angles to the lateral walls. The illustration

presents the vascular element from the radial view; hence it is
obvious in this case that the perforated end walls are radial. This
is a situation seldom departed from in the woods of dicotyledonous
trees. In other words, a radial section of a dicotyledonous wood

FIBROVASCULAR TISSUES: VESSELS 97

ordinarily reveals the face view of the characteristically perforated
terminal walls of the vessel. We find that it is distinguished by
the presence of open slits which are in general without borders and
by smaller or larger clearly bordered pits. The latter structures
retain their membranes and, where they become extremely elon-
gated, are clearly the result of the fusion of two or more
horizontally approximated
bordered pits. Fig. 75, which
represents part of the termi-
nal wall somewhat diagram-
matically and on a larger
scale, shows the mode of
fusion of pits and also shows
that the final result of this
process, when a number of
horizontally seriate pits are
concerned, is the formation
of an elongated slit which
reveals its primitive nature
only by the retention of bor-
ders at the ends. The slit as
a whole not only has lost its

Fic. 75.—Diagram showing the origin
; z of scalariform perforations by the fusion of
borders, but likewise the mem- pits in the end wall of the vessel.

brane of the row of fused pits
has disappeared. It is thus evident that the slits which occur
between the horizontal bars of the lattice work in the terminal

walls of a vessel of this type are the result of the fusion of rows of
pits, accompanied by a simultaneous loss of pit membranes.
Before we leave the subject of the mode of origin of the per-
forations which characterize the terminal walls of vessels of the
lower type in the angiosperms it will be well to discuss the situa-
tion in another group which shares with the true Amentiferae in
the minds of students of evolution the claim to be primitive repre-
sentatives of the dicotyledons. In Fig. 76a is shown a vessel from
the root of Liriodendron Tulipifera. In this instance there is no
sharp differentiation between terminal and lateral walls, although
in general in the genus such a distinction is clearly present. In

98 THE ANATOMY OF WOODY PLANTS

passing from the upper part of the figure downward there is a
transition from bordered pits arranged in horizontal rows to per-
forations of somewhat larger dimensions which are obviously
derived from bordered pits by the disappearance of borders and
membrane. The truth of this statement can be inferred from
the fact that some of the perforations still retain more or less
of the original bordered condition, particularly along their lateral
margins. Fig. 76) shows another vessel from the root of the same

Fic. 76.—Vessels from root of Liriodendron Tulipifera (a and b); c, vessel-like
elements formed after injury in the non-vascular magnoliaceous genus Drimys.

genus in which the situation is somewhat different. Here the pits
are scalariform, as in the tracheary elements of the Pteridophyta,
but with an important distinction which is not always kept in
mind in evolutionary speculations. In the ferns and allied forms
scalariform elements are a primitive feature of organization of the
wood, while in the secondary wood of the angiosperms the scalari-
form sculpture of the lateral walls is an exclusive feature of the
vessels and is the result of the lateral fusion of horizontal rows of
pits and consequently cannot in any way be regarded as a primitive
condition of structure as in the lower forms. Proceeding from top
to bottom, as in a, we find in this case the same loss of borders and
membranes leading to the appearance of perforations of the
scalariform type. The structure in question, in fact, exactly

FIBROVASCULAR TISSUES: VESSELS 99

simulates the vessel of Pteris aquilina figured in connection with
an earlier paragraph. Although the result is the same, the manner °
of reaching it is different in the two cases, and a distinction should
of course be made in drawing any evolutionary conclusions. It
will be clear to the reader from the evidence presented in the
present and in a former paragraph that the vascular structures of
the lower dicotyledons do not originate as in the Gnetales simply
by enlargements of pits and the disappearance of the pit membranes
in the terminal regions of the vessels. On the contrary, they
typically take their origin as a consequence of the lateral fusion
of horizontal rows of pits with a correlated disappearance of the
membranes. ‘The vessel in the lower angiosperms is, however, as
clearly a derivative of the tracheid as it is in the case of the highest
of the gymnosperms. Further, in the angiosperms the vessel or
trachea, as a result of its much more complex mode of evolution, is
more distinct from the fibrous or tracheary element than it is in
any of the lower groups.

The type of vessel characteristic of the higher dicotyledons and
the mass of monocotyledons may now profitably occupy our
attention. Fig. 77 illustrates three vessels belonging to the
category of elements with porous end walls (that is, vessels with
so-called porous perforations). In @ is shown such an element
from the oak in approximately radial aspect. The terminations
of the vessel taper and are distinguished by the large aperture or
pore. The lateral surfaces are covered with pits, which are of two
kinds. The smaller ones, provided with a distinct border, put the
vessel in relation with other similar structures and with tracheids.
The simple and slightly irregular pits represented in about the
middle horizontal region indicate the presence of a small ray in
contact with the vascular element. In 0 is represented a vessel
from the wood of the poplar in radial view. The same slanting
ends and large terminal pores as are present in the oak similarly
characterize the vessel of Populus. The uniform crowded lateral
pitting indicates that the face of the vessel presented to the observer
was in contact with another vessel. Inc is presented the somewhat
tangential view of a vessel in the maple. The ends of the vascular
element do not differ essentially from those of the oak and poplar.

100 THE ANATOMY OF WOODY PLANTS

In this case the wall to the left has been in contact with a ray, as
evidenced by the grouping of the pits. The region of the lateral
wall to the right has been in contact with another vessel and is

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Fic. 77.—Various types of vessels in the dicotyledons. Explanation in the text

conspicuous, not only by reason of the characteristic arrangement —
of the pits, but also by the presence of a somewhat spiral internal
sculpture, the so-called tertiary thickening, which always marks
(when present) the contact or “party wall’ between vessel and
vessel. Those surfaces of the vascular element which are in contact

FIBROVASCULAR TISSUES: VESSELS LOL

with the libriform fibers are devoid of both pits and spiral internal
sculpture.

It will be profitable to consider next the type of vessel charac-
teristic of the older wood of the stem of Lirioden-
dron. ‘This category of vessel is covered with pits
in horizontal rows, indicating contact with another
vascular element. Below and above the sharply
sloped end walls are seen; these are neither
porous, as in the case of the vessels of the last
figure, nor marked by the simultaneous presence
of narrow scalariform perforations and bordered
pits, as in the rootwood of Magnolia and in the
general secondary wood of many of the Betu-
laceae, etc. The terminal apertures of the vessel,
in fact, are crossed by a few remote bars. The
type of vascular element characteristically present
in the old wood of Liriodendron serves, indeed, as
an intermediate stage between the vessel with true
scalariform perforations and that with porous
terminal apertures.

The truth of this assertion becomes manifest
from a consideration of Fig. 79, which represents
vascular structures from the wood of the high blue-
berry, Vaccinium corymbosum. In a, 6, and ¢ are
shown successive transitions from end walls with
many pits and few narrow scalariform perforations
to fewer pits and more scalariform apertures, and
to a third condition where the pits have dis-
appeared and only somewhat wide scalariform
Openings remain. In d and e. appear further
stages in which the bars separating the terminal
large slits become fewer and more degenerate until
finally a simple porous condition is reached. It is Fic. 78.—Ves-
thus clear that the vessel with porous perforations sel from the stem
- , A A of Lirtodendron.
is a further elaboration of that with scalariform
perforations, just as this in turn has taken its origin from a vascular
element with pitted perforations of the nature found in the Gnetales.

102 THE ANATOMY OF WOODY PLANTS

The hypothesis that the scalariform vessel is the forerunner of the
porous one is also confirmed by a consideration of the first-formed
region of the woody cylinder. In the oak, for example, as well as
in many other instances, although vessels with porous terminal
apertures are characteristic of the
mature wood, elements of vascular
nature with scalariform perforations
are commonly present in the first
annual ring, particularly in the vicin-
ity of the protoxylem. It is thus evi-
dent that the vascular structures in
dicotyledonous woods supply a valu-
able argument for the general validity
of the hypothesis of organic evolution.

The lateral walls of the vessels
may next receive further considera-
tion. It has been indicated in a
former paragraph that the side walls
of a vessel have their sculptural
features largely determined by the
nature of the adjacent cells of the
wood. Where vessels are in contact
with rays, the communicating pits
are characterized by a grouping and
seriation corresponding to the outlines
and direction, whether transverse or

SE longitudinal, of the ray cells. Where
Fic. 79.—Vessel of Vaccinium the relation is with other vessels or
corymbosum, illustrating the with true tracheids, the pits are

origin of the porous type of a y di
perforation from the scalariform. numerous and likewise arranged In a

Explanation in the text. somewhat definite manner. If merely

mechanical elements abut on the
vascular walls, pitting is quite absent. Further, if so-called
tertiary spirals are present on the inner side of the walls of the
vascular elements, these are confined to those vertical regions in
contact with other vessels or with tracheids.

FIBROVASCULAR TISSUES: VESSELS 103

Where vessel comes in contact with vessel, we find frequently a
mode of pitting characteristic of a genus or even a family. Details
in this connection are beyond the range of a work so elementary
as the present one, but a few prominent features may be indicated.
As has been shown in the figure of the vessel of Liriodendron above
(Fig. 76), lateral pits may be in horizontal rows. In other instances,
as in the oak shown in Fig. 80a, the lateral pits are arranged in an
alternating fashion. Again, in 8, representing the lateral pitting
of the vessel in the poplar, alternation is accompanied by crowding
to such an extent that the pits become angular by mutual contact.

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Fic. 80.—Lateral pitting of vessels of dicotyledons. Explanation in the text

In ¢ is shown the characteristic pitting of the vessels of the vine
family (Vitaceae). Here the sculpture of the lateral walls consists
of elongated slits closed by equally elongated membranes. A
reference to the structure of the secondary wood in the region of
the pith in this instance makes it clear that the slitlike pits of the
Vitaceae are not persistent scalariform pores of the first-formed
region of the xylem, but are the result of the lateral fusion of
horizontal rows of pits in the side walls of the vessels. In d is
represented a condition of structure of the vascular wall which is
common in the higher types of dicotyledonous woods. Here the
alternating pitting is overlaid internally by spiral structures which
it is customary to call tertiary thickenings.

In concluding the statements in regard to vessels it is necessary
to refer to the phenomenon of tylosis or occlusion of the lumina
or cavities of the vessels by parenchymatous ingrowths. These

104 THE ANATOMY OF WOODY PLANTS

structures originate under various conditions, but are more usually
found in connection with those changes which precede the trans-
formation of sapwood, or alburnum, into heartwood, or duramen.
As the living cells in proximity to the vessels (whether they belong
to the radial or vertical systems of storage tissues appears not to

a

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Fic. 81.—Tyloses in oak and locust. Explanation in the text

be of importance) are about to die, they push processes into the
adjacent vessels by the bulging and growth of the cellulose mem-
branes of the pits connecting them with the vascular structures.
These ingrowths are occupied by protoplasm and, sometimes at
any rate, by nuclei as well. When abundantly developed, they
completely close the lumen of the vessel and render impossible the
passage of water. Fig. 43, on page 157, has made clear the contrast
resulting from this phenomenon in the sapwood and the heartwood

FIBROVASCULAR TISSUES: VESSELS 105

of the oak. In Figs. 81a and 6 is shown the phenomenon of tylosis
in the vessels of the white oak. In this species all the larger vessels
are more or less completely occluded |
by tyloses extending from and only
excluding the latest complete annual
ring. Inc and d a similar condition is
represented in the case of Quercus
Engelmannt. In this instance the
tyloses have developed a very thick and
strongly pitted wall, a condition more
rarely found. In e and f tylosis as
found in the case of the black locust,
Robinia Pseudacacia, is represented.
Here the ingrowths derived from adjoin-
ing parenchyma cells are very numer-
ous and form a mass completely
occluding the cavity of the vessel. In
Fig. 82 are seen, both in transverse
view, earlier stages of the invasion of a
vessel in the oak through the agency of
tyloses. The representation in this

a

Fic. 82.—Diagrammatic
representation of the process
of tylosis.

illustration is somewhat diagram-
matic to show more clearly the
entry of the ingrowths through
the medium of the pits in the
walls of the vessels.

The phenomenon of tylosis is in all probability an extremely
ancient one for the higher plants, as it is found far back into the

Fic. 83.—Tylosis in tracheids of
Liquidambar.

106 THE ANATOMY OF WOODY PLANTS

Mesozoic. It is not confined by any means to vessels and even
occurs rarely in the tracheids of dicotyledons. Fig. 83 shows the
presence of tylosis in the fiber-tracheids of the sweet gum (Liquid-
ambar styraciflua). The heavily outlined bodies in the cavities of
certain of the fibrous elements are of the nature of tyloses. Similar
conditions have been found in the case of other dicotyledonous
woods. It has been made clear in an earlier chapter that ingrowths
resembling tyloses occur in the resin canals of the pine in the region
of the heartwood. Occlusion by means of parenchymatous inva-
sions is, however, not confined to the resiniferous spaces in the
genus mentioned. In the roots and likewise in the cone the tra-
cheids of the wood often show themselves occluded by ingrowing
parenchyma cells. Such conditions are not normally found in the
vegetative stem of living pines, although they are known to occur
in the branches of cretaceous Pityoxyla from the Eastern United
States.

CHAPTER. VIII

THE FIBROVASCULAR TISSUES—PHLOEM

The ligneous or woody tissues which have been discussed under
various headings in previous chapters are of great importance on
account of their conservatism. They manifest a high degree of
differentiation and are also important by reason of the fact that
their relative imperishability has resulted in their being abundantly
preserved as fossils from the most ancient times to the present.
Woods consequently furnish on the whole the most important
historical document in favor of the hypothesis of evolution, and
those who are interested in the doctrine of descent as applied to
the case of plants cannot afford to neglect the investigation of
the ligneous tissues of the vascular plants. The situation for the
plants provided with tracheary tissues is, in fact, very different
indeed from that presented by the lower vegetable organisms, where
evolutionary data are necessarily largely speculative and experi-
mental. Invariably associated with the woody tissues are those of
the phloem. The primal function of the woody tissues was appar-
ently to conduct water into the superior or outlying parts of the
plant. To this function have been superadded in the long course
of geologic time, as pointed out in preceding chapters, the functions
of strength and storage, with corresponding and appropriate
modifications of structure. Just as the wood in the first instance
served the purpose of the movement of water from the soil, so the
primal condition of the tissues of the phloem was that of conducting
the elaborated foodstuffs manufactured in the leaves to other
parts of the plant where they are either utilized in the processes’
of growth and respiration or stored up as reserves.

The primitive type of phloem, as that of xylem, appears to have
existed in the case of the lepidodendrids. Its structure has not
been satisfactorily preserved in these primitive vascular plants, and
there is some difference of opinion as to interpretation. Some
investigators have expressed the opinion that no true phloem is

107

108 THE ANATOMY OF WOODY PLANTS

present in these ancient organisms, while others have contended
for a very simple type of organization in the arboreal club mosses
of the Paleozoic age. In view of the differences of opinion and the
uncertainty of the data in the case of the phloem of Paleozoic
Pteridophyta, it will be well to confine our attention in this
instance to the living representatives of the fern alliance.

Fig. 84 shows the detailed organization of one of the fibro-
vascular strands of the bracken Pteris aquilina. The bundle as a

Fic. 84.—Bundle of Pteris aquilina. Explanation in the text

whole is clearly limited by a dark uniseriate layer, the endodermis,
which is ordinarily interpreted as the innermost layer of the funda-
mental tissues in juxtaposition to the fibrovascular system. Within
the endodermis is situated a layer one or sometimes two cells in
breadth, the pericycle, which constitutes the external boundary
of the fibrovascular tissues, just as the endodermis marks the
internal limit of the fundamental system. Next to the pericycle
are certain extremely minute, apparently empty, cells, the first-
formed elements of the phloem, or, as they are technically desig-
nated, the protophloem. Next inward lie storage cells, nucleate

FIBROVASCULAR TISSUES: PHLOEM 109

and more or less loaded with starch. These are the parenchyma
of the phloem. We are not acquainted with the origin of paren-
chymatous cells in the phloem for reasons indicated at the outset
of the present chapter. Farther inward lie the larger conductive
elements of the phloem, known as the metaphloem. These are
cells which have no contents but a delicate lining of protoplasm,
and this does not appear in the illustration. The elements under
discussion as well as those of the protophloem are known, on °
account of certain structural features to be described later, as
sieve tubes. It is important to note in the present connection that
sieve tissues do not present the variety of modes of development
which characterizes the xylem. Almost invariably in this cate-
gory of tissues the first-formed elements are external and those of
later appearance are laid down in centripetal order. As a con-
sequence the terms exarch, mesarch, and endarch, which are so sig-
nificant in connection with the development of the primary wood,
have little bearing in the case of the sieve tissues. Internal to the
larger sieve elements of the metaphloem lies a more or less continu-
ous band of parenchyma, separating the phloem from the xylem.
The tracheary elements of the wood are individually more or less
completely surrounded with parenchyma. ‘The phloem does not
form a continuous band about the xylem, but is interrupted at the
two ends of the elongated mass of wood. In certain ferns the tissues
of the phloem occur only on one side of the bundle, in which case the
strand of fibrovascular tissue is known as collateral. Where the
phloem forms a complete jacket about the xylem the condition
is known as concentric; and where, as in the bundle appearing in
the figure under discussion, the sieve tissues are confined to two
opposite sides the condition may be designated as bicollateral.
In Fig. 85 is represented a longitudinal view of part of the
same bundle. The sieve tubes now appear as elongated cells
with tapering ends and sculptured walls. The relief of the walls
consists of somewhat angular crowded areas, although on the
whole they are tolerably evenly distributed. These areas are per-
forated with very small simple pits and are the sieve plates. It is
these structures which give to the sieve tube its name. After the
characteristic elements of the phloem have reached a certain age

110 THE ANATOMY OF WOODY PLANTS

their pores become more or less completely occluded by slimy plugs
known as callus. The details of origin and the significance of this
substance are not fully understood and from the standpoint of
the present statement are of relatively little importance.

In Fig. 86 is re-
produced a photo-
graph of the wood
and inner bark of the
pine. The xylem is
clearly very regular
in its structure and
consists of a series of
annual rings, the
organization of which
has been dealt with
in a preceding chap-
ter. In contrast to
the persisting regu-
larity of the wood the
phloem presents itself
as a mass of tissue
radially seriate only
for a short distance
outward from the
cambium, or zone of
growth. Externally

Fic. 85.—Longitudinal view of part of a bundle its elements are
of Pteris aquilina. On the left is shown a vessel, thrown into more or
while toward the right and represented in black are 2 i

less meandering lines

sieve tubes.

as a result of the col-
lapse of certain of its constituent cells, which will be considered more
in detail in the sequel. The section reproduced in the photograph
under discussion was made from material secured in the winter
period of rest. The uncollapsed part of the phloem corresponds
to a single year’s growth, and all that region of the inner bark or
phloem characterized by the meandering course of the rays is non-
functional so far as the most characteristic elements, the sieve

LAN

FIBROVASCULAR TISSUES: PHLOEM III

tubes, are concerned. Fig. 87 shows a highly magnified view of the
region immediately outside and inside the zone of growth, or cam-
bium. Internal to the active layer the wood is recognizable by its
bordered pits. The initial region, known as the cambium, is dis-
tinguishable by the richly protoplasmic character of its cells and
their thin tangential walls, which are in contrast with the thick-
ness of the radial
boundaries. Ex-
ternal to the cam-
biwm ives the
phloem, composed
most characteristi-
cally of somewhat
rectangular cells,
the sieve tubes. In
these, unlike the
cambial elements,
there is no nucleus,
and the scanty
parietal proto-
plasm presents
likewise a striking

difference from the Fic. 86.—Inner bark with cambium and adjacent
richly protoplas- wood of the pine.

mic cambial region.

Scattered thinly through the phloem are a few rounded cells
which contain protoplasm and grains of starch (represented
black as if stained with iodine). The rays cross the longitudinal
elements of the phloem, the sieve tubes, and the parenchyma cells
at right angles and appear in the figure as single files running nearly
straight for a short distance and then pursuing a meandering course
outward. The cells of the two rays shown in the figure present
a different appearance. In the file of radial parenchyma on the
right the elements are filled with protoplasm and show the presence
of a nucleus. In addition, a considerable amount of starch is
present in the form of grains tinctured black by iodine. It is clear
that the cellular elements of the ray to the right continue elements

r]
ip
Hi
ne
ir]

¢

SS

Lo

THE ANATOMY OF WOODY PLANTS

112

—S

(ki. 3) — j ra

Fic. 87.—Transverse section of xylem, phloem, and cambium of the pine, more

highly magnified.

Explanation in the text.

FIBROVASCULAR TISSUES: PHLOEM 113

of a similar nature in the portion of the ray contained in the wood.
If we now turn our attention to the ray on the left, a somewhat
different situation is presented to the eye. Here the cells are
entirely without starch, although they still show protoplasm and a
nucleus. Internally they are in line with the marginal tracheids
of the ray in the wood, which have been described in an earlier
chapter. Obviously there are two categories of cells in the rays
of the phloem in Pinus, just as has been shown to be the case
for the radial parenchyma of the xylem. It will now be con-
venient to consider the relations of the various elements of the
phloem to one another. In this connection we may first discuss
the sieve tubes. It will be noticed that there are certain dark lines
stained with iodine occurring in groups on the radial walls of the
sieve elements. These are the transverse sections of sieve pores.
Parenchymatous cells of the phloem have no intimate relation to
the tubes, as no pits facilitate interchanges between the two types
of elements. The same statement holds in regard to the cells of the
ray shown on the right side of the figure. On the left, however, the
elements of the ray are clearly related to the sieve tubes by means
of sieve plates, distinctly differentiated on the side of the sieve
tubes, but not well developed on the side of the ray. It is evident
that there is a specially intimate connection between certain ele-
ments of the ray, which are contrasted with the remaining constitu-
ents by the absence of starch, and the conductive elements of the
phloem or sieve tubes.

If the organization or, more correctly, the disorganization of the
phloem is followed more externally, it will be observed in the illustra-
tion that the ray on the right continues to retain its protoplasmic
contents and its grains of starch, while that on the left has entirely
lost its living contents. A further feature of the phloem as repre-
sented in the external region of the figure is the distortion and final
collapse of the sieve tubes, preceded by the loss of the delicate pro-
toplasmic lining surrounding the inner walls of these elements. The
parenchymatous cells retain their integrity in the collapsed region of
the phloem, precisely as is the case with the ray indicated on the
right of the figure. Another feature of the disorganization of the
phloem is the appearance of large masses or plugs of material,

114 THE ANATOMY OF WOODY PLANTS

appearing black in the figure, which are present exclusively on the
radial walls of the elements, although in some instances the col-
lapsed state of the tubes gives rise to the appearance of a tangential
position for the bodies under discussion. The plugs mentioned
above constitute the callus, a substance which in the conifers
and the great majority of seed plants higher in the scale makes
its appearance on the sieve plate at the time the tube is in the
initial stages of collapse. The callus completely blocks the pores
of the sieve plate and, after persisting for a short time, disappears,
leaving the pores of the sieve area quite open. It is also apparent
that in the case of the ray cells on the left of the figure the callus
present is unilateral and occurs only on the side of the sieve tube.
Obviously the type of ray element appearing on the left in the
illustration is definitely related in its duration to the life of
the sieve tube, and both structures cease to be functional at the
same time.

In the next figure (Fig. 88) the radial aspect of the phloem is
shown. Here the ray naturally appears in longitudinal section.
Above and below in the region of the phloem it shows vertically
placed cells which contrast in the position of their axis of greatest
length to the central elements of the radial parenchyma. These so-
called “erect cells”? correspond to the elements shown in the ray
to the left of Fig. 87 and are similarly characterized by the absence
of the starch, which is a feature of the contents of the central or
prone cells of the ray. In passing toward the right from the cam-
bium the protoplasmic filling of these elements becomes less
abundant, until finally, where the callus presents itself in face
view, it disappears altogether, in this respect presenting a con-
trast to the central cells which retain their protoplasm and starch
indefinitely. The sieve tubes.now appear in longitudinal view,
and nearer the cambium they have a delicate protoplasmic lining
often containing some small grains of so-called transitory starch.
The sieve areas, or sieve plates, are presented to the observer in
face view, and toward the extreme right callus plugs occlude their
pores and at the same time the tube has lost its living contents.
It is clear in the radial view, as in the transverse, that sieve tube
and marginal erect cell cease to exist and develop callus plugs

FIBROVASCULAR TISSUES: PHLOEM ELS

simultaneously. A single row of parenchymatous elements is
represented as crossing the axis of the ray. To the left of the
cambium lies the wood with its tracheids and the continuation of
the ray. The radial structures in the xylem show also a differentia-
tion into central and marginal cells, but here ray-tracheids take
the place of the erect cells of the ray in the phloem.

Fic. 88.—Radial section through xylem, cambium, and phloem of the pine.
Explanation in the text.

In Fig. 89 appears the tangential section of the tissues of the
phloem in Pinus. The plane of incidence of the section is tan-
gentially slightly oblique; hence elements of different ages appear
on the opposite sides. Toward the right are represented sieve
tubes in which the protoplasmic lining has disappeared and the
sieve areas (seen here in profile) are blocked by masses of callus.
In the case of the rays which are naturally shown in transverse
section the marginal or erect elements are calloused on the side of
the tubes. The central cells of the rays still show the presence of

116 THE ANATOMY OF WOODY PLANTS

protoplasm and starch. - In the middle region of the figure a file
of parenchyma cells is present. To the left of the middle line the
sieve elements show the delicate protoplasmic lining which char-
acterizes them in the functional condition. The radial parenchyma
is also entirely in a living condition, although the marginal cells
are differentiated by the absence of the starch contents found in the

Fic. 89.—Tangential section through the phloem. Explanation in the text

central region of the ray structure. The callus in both sieve tubes
and marginal cells is here conspicuous by its absence.

A somewhat complicated condition of the phloem has purposely
been chosen in the case of the coniferous gymnosperms, because
there is good reason to believe that in this group, which is a decadent
one, the more elaborately organized condition is antecedent to that
marked by a greater degree of simplicity. On the whole, the pine
and its allies represent the most highly differentiated structure of the
phloem in the group. In the Cupressineae, Taxodineae, Araucari-
neae, Podocarpineae, and Taxineae the radial structures of the
phloem show a marked degree of simplification as compared with

FIBROVASCULAR TISSUES: PHLOEM vs

the pine family proper. It is not necessary in an elementary work
like the present one to enlarge further on the organization of the
phloem of the gymnosperms in general or the conifers in particular.

The structure of the phloem in the angiosperms, and especially of
arboreal dicotyledons, will next occupy our attention. Here the

= \( ‘ % z i) Z ary Gr EC p°
A a SSE S ees s NK | BEAT) ~
Mis! Gian
= = (2g ey LS ZE \

TBA

Fic. 90.—Transverse section of the xylem, cambium, and phloem in the American
linden. Explanation in the text.

general organization is quite different from that characteristic of
gymnospermous groups and accordingly merits detailed considera-
tion. Fig. 90 shows a transverse section of the region immediately
internal and external to the cambium in the trunk of Tilia americana,
the American linden or basswood. ‘Toward the lower side of the

118 THE ANATOMY OF WOODY PLANTS

illustration appears the xylem, consisting of vessels, libriform
fibers, parenchyma, and wood rays. Above the tissue so organized
lies the cambium, or zone of growth, which alternately adds ele-
ments to the structure of the xylem and that of the phloem. The
latter tissue appears above the cambium. in the figure and outside
it in the trunk of the tree. The preparation which served as the
basis of the illustration was made from material secured during the
period of winter rest. Next to the cambium lie fibrous and similar
elements, to the abundant development of which in its inner bark
the basswood (literally bastwood or tree useful for binding) owes
its name. The fibers of the phloem are often called hard bast.
Examination of the figure will show that the fibrous elements are
separated from the cambium by a row of cells either quite empty
or containing large crystals of calcium oxalate. These are the so-
called crystallogenous cells and are very commonly present inter-
nal to the zones of fibers or hard bast in the phloem of the form
under discussion. External to the hard bast lies a zone distin-
guished by the presence of cells either richly protoplasmic as to their
contents or, in case the protoplasm is more scanty, characterized by
relatively thin and unlignified walls. This region is the so-called
soft bast and shows considerable complexity of organization.
First taking the cells with abundant protoplasm, we see clearly that
these can be divided into two categories—namely, those which
contain grains of starch (represented in black as if treated with
strong iodine solution) and those in which amylaceous substances
are absent. The latter are further distinguished by their generally
small size and somewhat triangular shape, due to the fact that they
are usually accommodated in the angles of large thin-walled ele-
ments with a delicate protoplasmic lining. The triangular cells
are known as ‘“‘companion cells’? and are a constant feature of
structure of phloem of the angiosperms as contrasted with the
gymnosperms. The larger elements of the phloem with thin walls
and scanty parietal protoplasm are the sieve tubes. In some in-
stances the sieve plate can be seen in the transverse section repro-
duced in the figure, and it is clear that its position is characteristically
radial. Farther outward the densely filled parenchymatous cells
of the soft bast give place again to the dead crystallogenous elements,

FIBROVASCULAR TISSUES: PHLOEM 119

which are in turn followed by bast fibers. The rays of the phloem
in the basswood and other dicotyledons show no special feature of
interest, since they do not in any case manifest the intimate rela-
tion to the sieve tubes described above in the pine.

Soe We.

eo oe % oh

4 a6 oe ¢ ete @-

es ose
Seeeery®

Fic. 91.—Radial section of the xylem, cambium, and phloem of the linden, or
basswood. Explanation in the text.

In the next illustration (Fig. 91) the radial aspect of the phloem
in Tilia is shown. Here the rays appear naturally in longitudinal
section and are composed of heavily pitted cells containing dense
protoplasm and much starch. Above the ray are represented the
various elements of the hard and soft bast. On the extreme left

120 THE ANATOMY OF WOODY PLANTS

are to be seen the summer elements of the wood in contact with
the cambium. They are followed by parenchyma and by crystal-
logenous cells, empty and heavily pitted on their horizontal walls.
Next, the fibers of hard bast meet the view, and these pass in suc-
cession into bast parenchyma, characterized by heavy pitting,
abundant starch, and protoplasmic contents, and by sieve tubes of
large lumen and scanty parietal protoplasm. Within, in the walls
of the sieve elements, lie the companion cells, distinguished by their
slender form and the absence of starch in their protoplasm. The
sieve tubes in the radial aspect present to the eye strongly inclined
end walls which may be compared with the similarly slanting
partitions in the vessels of the wood. The sieve element is, in
fact, the exact analogue of the vessel, and, just as the vascular
elements in angiospermous woods play the main part in the trans-
port of water, so the sieve tubes perform the principal réle in the
movement of the elaborated organic stuffs from the leaves. Another
feature of analogy between the sieve tube and the vessel is not
only the more or less inclined terminal walls at angles to the lateral
ones, but also the function of transport specially provided for in
connection with the ends of the element.

In the vessel this condition finds its expression in the develop-
ment of scalariform or porous perforations and in the case of the
sieve tubes by the appearance of particularly extensive and large-
pored sieve plates on the terminal inclined walls; and these in
highly specialized forms, such as herbaceous dicotyledons and
monocotyledons, may be the only functional regions. Following
the sieve tubes is a zone of bast parenchyma, succeeded in turn by
another zone of crystallogenous cells and hard-bast fibers.

In Fig. 92 is shown the tangential longitudinal aspect of the
phloem in the basswood. Here the rays naturally appear in trans-
verse section and are clearly of diverse sizes, but are all composed
of strongly pitted cells containing abundance of protoplasm and
starch. In this aspect the plane of section is purposely slightly
oblique so as to exhibit all categories of constituents of the phloem
at the same time. To the left lie crystallogenous cells near a large
ray. ‘The ray is in contact on the opposite side with parenchyma of
the soft bast. Then follow sieve tubes and their related com-

FIBROVASCULAR TISSUES: PHLOEM 121

panion cells. In the tangential aspect the terminal sieve plate,
since it is radial in position, is shown in profile. The protoplasm,
present more densely in the region of the terminal plates, is pur-
posely omitted in order that the sculpture of the lateral walls may

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text.

be apparent. There are manifestly strongly punctuated areas in
the side walls of the sieve element. These represent sieve plates
which have more or less completely lost their function as a result
of degeneracy. In herbaceous types lateral sieve areas frequently
have no longer even a morphological expression, the structural

122 THE ANATOMY OF WOODY PLANTS

as well as the functional plates being almost exclusively limited
to the terminal walls. Abortive and lateral sieve plates in the
angiosperms are known as lattices. In woody dicotyledons the
lattices are, in the case of lower types, often well developed and
are likewise frequently functional as actual sieve plates. Farther

Vie AC

ell
= 4 | € =

soles! es O
Me ate ans aaa
(8 (ISS I)

Fic. 93.—Transverse section of the phloem in the grapevine. Explanation in
the text.

to the right the figure shows more parenchyma and additional
fibers of the hard bast.

For comparison with the basswood is presented in Fig. 93 the
transverse view of the phloem in the grapevine (Vitis species).
To the right lies a large ray of the compound type. To the left
of this is situated the phloem proper, consisting, as is often the
case in the dicotyledons, of hard and soft bast. The former tissue is
composed of fibers which differ chiefly from the corresponding
structures in Tzlia by the fact that they are septate when viewed in

FIBROVASCULAR TISSUES: PHLOEM £22

longitudinal section. The soft bast consists of parenchyma, sieve
tubes, and their companion cells. Crystallogenous elements are
inconspicuous in the bast of the grapevine. The sieve tubes in
some cases show their terminal and mainly functional sieve
areas in transverse section. The plate in this instance is blocked
by a formation of callus which in the vine covers the plate each
autumn and disappears again in the following spring when
vegetative activity is renewed. This condition is unusual, since
quite generally in the angiosperms, as in the gymnosperms, the
appearance of a callus marks the end of the life of the sieve
tube. A seasonal callus is likewise found in the barberry and in
certain other instances. The final callus, marking the dissolution
of the characteristic functional elements of the phloem, is known as
the definitive callus. The tubes of the phloem in the grapevine
are of interest in showing a considerable amount of so-called transi-
tory starch, indicated in the figure by the larger black granules.

Both the xylem and the phloem of herbaceous dicotyledons and
of the monocotyledons show interesting modifications of the con-
ditions characteristic of arboreal and perennial forms. Here the
vessel and the sieve tube constitute to an increasing extent, and
sometimes exclusively, the functional elements of the two important
regions of the fibrovascular bundle and are present in what may be
regarded with a strong degree of probability as their highest and
most specialized form. The vessel is practically always porous in
herbs and presents as a usual peculiarity of organization terminal
walls which are nearly or quite horizontal. The sieve tube also
tends toward horizontal end walls, and the sieve plates have prac-
tically disappeared from the lateral walls, the terminal areas alone
being functionally important.

In concluding the statement regarding the organization of the
structures in the phloem it will be well to represent side by side
the stereoscopic aspects of the several types of sieve tubes in com-
parison with corresponding varieties presented by tracheids and
vessels. In Fig. 94a is shown diagrammatically the organization of
a sieve tube in the secondary phloem of a gymnosperm. It is clear
that the plates are confined to two opposite walls, which are the
radial ones. The cell, further, tapers imperceptibly to a point at

124 THE ANATOMY OF WOODY PLANTS

Fic. 94.—Diagram showing types of sieve tubes and vessels in the higher vascular
plants. Explanation in the text.

FIBROVASCULAR TISSUES: PHLOEM 125

either end. For comparison with this condition is shown in 0 the
sieve element of an arboreal dicotyledon. Here the terminal walls
are distinctly at angles with the lateral ones and bear the best-
developed sieve plates. Laterally, on two opposite and usually
tangential sides, are situated the degenerate sieve plates, or lattices.
The third item, c, visualizes the type of sieve tube characteristic
of an advanced herbaceous dicotyledon or of a monocotyledon. In
this instance the terminal wall of the sieve tube is horizontal, and
parietal lattices are conspicuous by their absence. This state
represents the highest degree of differentiation of the sieve element.
In the following items are delineated corresponding conditions for
the water-conducting elements of the xylem. In d appears a tra-
cheid from the spring wood of Pinus, showing tapering or fusiform
configuration and exclusively radial pitting. Next, in e, appear
the main vessel types of Gnetales and perennial dicotyledons,
showing inclined terminal walls which in one instance have scalari-
form perforation of the lower and in the other the porous perforation
of the higher perennial types. Finally, in f is depicted the vessel of
the monocotyledons and extreme herbaceous dicotyledons. Here
the terminal walls are practically horizontal, and the perforation is
porous (in the case of the monocotyledons invariably so).

CHAPTER IX
THE EPIDERMIS

In vascular plants the epidermis is ordinarily a single layer of
cells, and this fact, together with the external position of the
epidermis, makes the latter extremely easy to distinguish. The
organization of the epidermis is often strongly influenced by condi-
tions of environment, and as a consequence the integumentary
structures have a value from the evolutionary standpoint which can
be very easily overestimated. On account of its relatively slight
phylogenetic interest the epidermal tissue will receive small atten-
tion in the present connection.

In the case of the organs of plants normally exposed to air and
not to water or earth we find the integumentary structures con-
sisting of a well-marked layer of cells, which ordinarily remains
single and is characterized by the presence of stomata or pores.
The cells of the epidermis in lower forms often contain chloro-
plastids, while in higher forms chlorophyll-containing plastids are
usually found only in water plants or in those inhabiting shade.
Plastids or masses of organized protoplasm in the epidermal cells of
the higher plants are not indeed rare, but usually when found in the
superficial layer they lack color and consequently are included
in the category of leucoplastids. The chloroplastids of the epi-
dermal structures of Pteridophyta may possibly indicate the primi-
tive presence of these bodies in the integumentary tissues of the
higher plants, but they are perhaps equally susceptible of inter-
pretation as a response to a shaded or damp habitat. The presence
or absence of chloroplastids in the epidermal elements cannot con-
sequently receive a very high valuation from the standpoint of
determination of the relative phylogenetic position of groups of
vascular plants. The situation is particularly obscure on account
of the lack of bearing of fossil evidence on the question, both
because plastids are rarely preserved in the cells of extinct plants
and because, even if they were, it would be impossible to distin-

126

THE EPIDERMIS L277

guish between chloroplastids and leucoplastids in the altered state
which necessarily results from the condition of fossilization.

In some instances the epidermis commonly becomes a multiple
layer often known as water tissue. This is particularly the case
with plants of different systematic affinities which are exposed to
extremely arid conditions. Ficus elastica affords an excellent
illustration of the multiplication of the epidermal layer in leaves

Fic. 95.—Epidermis of the Urticaceae. Explanation in the text

of xerophytic habit. Here the epidermis, which in the young con-
dition consists of a single stratum, as the leaf matures becomes
divided into several stories of cells, all characterized, like the epider-
mis from which they are derived, by the absence of chloroplastids.
Another feature which especially distinguishes the epidermal
structures of the Urticaceae and Acanthaceae is the presence
of deposits of carbonate of lime, generally in the form of acinose
masses supported on a peduncle from one side of the cell. In
urticaceous leaves which are mesophytic in their environment,
such as the nettle or hemp, the structures under discussion, known
as cystoliths, are found in the outermost cells of the plant—that is,

128 THE ANATOMY OF WOODY PLANTS

in the elements of the simple epidermis. In Ficus elastica, on the
other hand, the multiplication of the epidermal layer has resulted
in the imbedding of the cystoliths deeply in the substance of the
water tissue. Cystoliths are a valuable diagnostic feature in the
case of the Urticaceae and Acanthaceae.

In many gymnosperms the epidermal layer is reinforced,
frequently in the leaf and more rarely and only in the most
ancient representatives of the group in the stem, by a layer of
colorless and generally strongly thickened cells known as the hy-
poderma. It is not at the present time clear that the hypoderma
so called in gymnospermous leaves is derived from the epidermis
by the multiplication of its cells, but the structure in question is
most conveniently discussed under this head. This layer occurs
very distinctly in the leaves of the cycads, Ginkgo, and the conifers,
and deepens the strata of colorless cells bounding the surfaces of the
leaf. In older types the hypodermal layers tend to become ribbed,
and in the leaves of Mesozoic pines and other conifers, as well as of
the Cordaitales of the Paleozoic, this condition is often very marked.
In the Cordaitales, Cycadofilicales, and associated groups the ribbed
condition of the tissues supporting the epidermis is often strongly
developed in the stem organs. It is not, however, by any means
settled whether the hypodermal layers are of common origin with
the epidermis and hence belong to the same histological category.

In plants exposed to extreme drought, physical or physiological,
the epidermis responds not only by the frequent multiplication of
its layers but also by the development of a well-marked cuticle and
by the cuticularization of the outer region of the external walls of its
cells. This condition is well developed in plants of desert habit
or in those rooted in poisonous soil, even when moist conditions are
present. In aquatics, on the other hand, the cuticular structures
are poorly differentiated and may be lacking even in the case of
the spores, which more constantly than any other structures of the
higher plants maintain a cuticularized exine or outer coat. The
question of the development of cuticle in plants is of some interest
from the standpoint of past climatic conditions on the surface of
our earth, although even here it is not conclusive on account of the
nearly identical influences of dry and merely poisonous substrata.

THE EPIDERMIS 1209

Structurally the value of the cuticle for purposes of identification or
as a basis of evolutionary speculations can easily be exaggerated.
There is little doubt that it shares with the general external form of
plants the shortcoming of unreliability, which results, as in the
case of formal characters, from close connection with physiological
necessities.

The epidermis is not only a limiting membrane of plants, but is
likewise charged with the important office of bringing about
regulated interchanges between internal spaces and the outside
air. The mechanism which performs the function of facilitating
gaseous and other interchanges is the stoma. This structure con-
sists of a pair of guard cells variously organized to meet conditions of
environment and bounding a pore, the stoma proper. The guard
cells in cases where they are actually functional have their inner
and outer walls so thickened that the accumulation of osmotic
pressure within the cells leads to their divergence in the middle
line with the resultant opening of the pore. No matter what may
be the conditions in regard to the presence or absence of chloro-
plastids in the general epidermal cells, the guard cells are always
provided with green corpuscles and even under conditions of
starvation retain a modicum of starch after this substance has
disappeared in the rest of the cells of the leaf. In plants exposed to
a high degree of drought and insolation the guard cells of the stomata
become very much thickened and respond to the stimulation of
light and moisture only under extreme conditions. A further
safeguarding of the epidermal pores or stomata often results from
their being depressed below the surface of the leaf or sheltered
under a hairy protection. Plainly, under the last-described con-
ditions the loss of water from the stomata will be much less
than in the unsheltered condition. The guard mechanism in
plants exposed to extreme drought is sometimes throttled, as it
were, by a surrounding zone of cells which differ from the
ordinary epidermal elements in possessing chloroplastids. Cells
of this type are known as accessory cells. The accessory device
is variously related to the guard cells, the free movement of
which it serves to check. Very frequently the braking mechan-
ism consists of a collar surrounding the guard cells, while in a large

130 THE ANATOMY OF WOODY PLANTS

number of instances the accessory cells lie over the guard cells,
so that the latter are nearly or quite shut off from the surface of the
leaf.

In many cases the guard cells have walls which differ chemically
in different regions. In the pine, for example, the walls of the
elements inclosing the pore of the stoma along their inner margins,
where they actually abut on the stomatic aperture, and on the
opposite sides, where they are in direct contact with the adjacent
cells of the epidermis, are in a condition of pectic cellulose and
absorb strongly certain of the hematoxylin stains. It has been
quite generally observed in physiological investigations that the
inner and outer walls of the stomatic guard cells are extremely
viable to water—so much so, in fact, that a deficient water supply
in plants flourishing under ordinary garden conditions results in
the rapid loss of turgescence in these elements.

The epidermal structures are frequently characterized by out-
growths known as hairs. These are most generally composed of
comparatively few cells, but in some instances may become very
complicated in their organization. When it is clear, in spite of any
degree of elaboration and magnitude, that the organs are purely
epidermal in their origin, they are properly designated as hairs
or trichomes. If tissues of the fundamental or fibrovascular
systems, or both together, enter into the structure of the processes,
they are known as emergences, provided they do not come more
accurately under the caption of modified leaves or lobes of leaves,
or, finally, of branches. The trichomes are not usually of a high
value in connection with evolutionary anatomy, since they are
subject to a very considerable degree of variability in accordance
with conditions of environment.

The sporangia of vascular plants are sometimes considered
to belong in the category of trichomes, but it is very difficult to
bring them consistently under this heading. While it may be
maintained with a certain degree of force that the spore sacs of the
polypodiaceous ferns are of the nature of hairs, since they are
clearly entirely derived from the superficial cells of the leaf, it is
difficult to homologize them on this interpretation with the spore-
producing members of the lower and more primitive groups of

THE EPIDERMIS TZ

Pteridophyta. It must be confessed that the actual organization
of the sporangium is a better indication of its morphological sig-
nificance than is any relation in origin to any particular tissue
system or any combination of tissue systems. Taking the vascular
plants as a whole, it seems evident that the sporangium probably ,
antedated the clear differentiation of the tissue systems and is
accordingly best regarded as an organ sui generis and as belonging
consequently to a distinct anatomical category. It will be shown
in what follows that there is good evidence in certain instances
that tissue systems other than the epidermis enter into the organiza-
tion of the sporangial structures of the seed plants.

In conclusion it may be stated that the epidermal structures
show an exceptional degree of plasticity to the molding influences of
environment, a condition which makes them of less value from the
phylogenetic standpoint. This highly variable system constitutes
the boundary between the organism and its surroundings, whether
fluid, gaseous, or solid. It consists normally of a single layer of
cells which in certain drought-resisting plants is multiplied to con-
stitute what is ordinarily called water tissue. The epidermis is
perforated by pores known as stomata and produces outgrowths
known as trichomes or hairs. The former structures consist of a
pair of cells usually highly responsive to the combined presence of
light and carbon dioxide and surrounding an occlusible pore. True
stomata are confined to the spore-producing generation of the
higher plants (from the Bryophyta upward). In spite of erroneous
statements to the contrary, in the case of certain badly preserved
fossil plants there are never more than two guard cells related
to a stomatic opening. Assertions contradicting this are based
on the interpretation of accessory cells as guard cells, where the
former overlie the elements guarding the stomatic pore. Neither
the shape of the epidermal cells nor the organization of the stomata
nor the structure of the hairs or trichomes can be regarded as having
a very high importance from the standpoint of the anatomical
identification of extinct plants in the absence of the confirmatory
evidence presented by external form and internal anatomy.

CHAPTER X
THE FUNDAMENTAL TISSUES

It has been indicated above that the epidermal tissues constitut-
ing the external boundary of the plant toward the outer medium
have relatively slight value from the standpoint of evolutionary
anatomy. The same statement may be made (with less emphasis)
concerning the category of cellular organization, known as the
fundamental tissues. This group is on the whole much more
extensively developed and more clearly defined in the lower forms
of vascular plants than in the case of those higher in the scale.
The more massive presence and the more distinct individualization
in the fernlike representatives of vascular plants naturally carry
with them a higher degree of histological differentiation. The
fundamental system is definitely bounded internally by the endo-
dermis (phloeoterma of Strasburger) in most of the lower groups of
plants, while in more advanced vegetable organisms the limits be-
tween the fundamental and fibrovascular systems are often obscurely
indicated except in that most conservative of all organs, the root.
Externally the limits of the fundamental system are usually clear
unless the multiplication of the epidermal layer conceals the situa-
tion. Occasionally in roots the fundamental system is sharply
marked off toward the epidermal system by the presence of a well-
developed limiting layer, the exodermis, which externally is the
counterpart of the endodermis, indicating the internal confines of
the cortex.

The accompanying figure (Fig. 96) of the subterranean stem
of the bracken fern will illustrate both the definite limits and the
considerable degree of specialization and consequent physiological
importance of the fundamental tissues. It is clear that within the
epidermis and external to the fibrovascular bundles there is present
a large amount of cellular substance. This is distinctly differ-
entiated into two categories—namely, sclerotic or supporting
tissues and storage parenchyma. The former is present in two

132

THE FUNDAMENTAL TISSUES 133

distinct, more or less continuous, zones: an outer one lying imme-
diately under the epidermis and somewhat interrupted opposite
the angular projections on the sides of the rhizome, and an inner
one coming between
the large medullary
and the smaller ex-
ternal -series of
bundles. The stor-
age cells, which are
characterized in the
figure by their walls
and in nature by
the large amount of
starch present in
their cavities, oc-
cupy all the trans-
verse section not

taken up by the epi- Fic. 96.—Rootstock of the bracken fern
dermis, the fibro-

vascular system, and
the sclerotic bands
just described. The
mechanical require-
ments of the stems
of many lower vas-
cular plants are pro-
vided for by skeletal
structures present in
the fundamental
system. This situa-
tion is common to
both the Pteropsida
and the Lycopsida.
The truth of this
statement is revealed by the figure presented of the stem of a
lepidodendrid, a very ancient representative of the group of club
mosses or Lycopsida. Here zones of thick-walled skeletal tissues

Fic. 97.—Stem of Lepidodendron S penceri

134 THE ANATOMY OF WOODY PLANTS

belonging to the fundamental system can readily be distinguished.
It may be stated in a general way that the higher vascular
plants have their skeletal tissues developed from the progressive

Fic. 98.—Transverse section of young root
of the balsam fir. Explanation in the text.

but the fundamen-
tal category of
tissues becomes
relatively insignifi-
cant in amount,
particularly in
stems with peren-
nial growth. Fur-
ther, the bound-
aries between the
fibrovascular and
fundamental tis-
sues have gen-
erally disappeared
through the de-
generacy of the
endodermis.

mechanical differentia-
tion of the primitively
solely water-conducting
fibrovascular system, in
contrast to the Pteri-
dophyta, in which the
mechanical function
mainly resides in the
fundamental system.
In the higher vascular
groups not only does
the mechanical principle
find its best expression
in connection with the
fibrovascular system,

FIG. 99.—Older root of the balsam fir

In the case of the root of all vascular plants the limit between the

conductive strands and the fundamental system is clearly marked

from the lowest forms to those at the very summit of the vascular

THE FUNDAMENTAL TISSUES 135

series. This situation is in accordance with the marked conserva-
tism of roots. The clear boundaries between fibrovascular and
fundamental structures are, for example, as well marked in the
cycads, conifers, and angiosperms as they are in the case of the
lycopods, ferns, and Equisetales. In the conifers and many dicot-
yledons the fundamental group of tissues, although well developed
in the young condition of the organ, later entirely disappears.
The accompanying illustrations (Figs. 98 and gq) of the root in the
balsam fir make the truth of this statement sufficiently clear. In
the first of the two figures the organ is shown in a young condition,
as is evidenced by the slight degree of development of the woody
cylinder. A clear boundary separates the outer region of the root
from the tissues belonging to the fibrovascular system. This is the
endodermis or innermost layer of the fundamental system. The
organization of the second figure shows a further degree of develop-
ment of the fibrovascular system, while the fundamental tissues,
inwardly limited by the endodermis, have begun to shrivel and break
away from the surface of the fibrovascular cylinder. The destruc-
tion of the jacket of fundamental tissues is the result of the appear-
ance of a corky layer immediately within the endodermis; and this,
impervious in its nature, cuts off the supply of nutrition to the exter-
nal zone. This region of the root consequently dies and flakes off.

In the case of the leaf the fundamental tissues are of consider-
able importance, since they constitute the green substance or
mesophyll of foliar organs. The limits between the stelar or
fibrovascular strands in the leaf and the surrounding mesophyll
are in general much less well marked than in the root, but on the
whole much more clearly indicated than in the stem. In the
Pteridophyta in general the endodermis is distinctly devel-
oped in the leaf. In the gymnosperms the limits between the
fundamental system and the fibrovascular strands are already less
obvious, and in the case of the angiosperms a somewhat similar
condition is to be observed. The usual situation justifies the
summary statement that in foliar organs the fundamental category
of tissues is always well developed and is physiologically of great
importance, since it subserves the cardinal functions of transpiration
and photosynthesis. Further, the morphological limitation of the
fundamental system is only less distinct in leaves than it is in roots.

CHAPTER XI

DEFINITIONS OF THE ORGANS

Not uncommonly an organ is defined as the tool of a function.
Of course from the evolutionary point of view this conception cannot
hold, because the same organ in a plant often at different times
subserves very diverse functions. For example, the stem may
function as a leaf and the leaf as a root. From the standpoint of —
the doctrine of descent the value of a particular organ in the course
of evolution is assigned, not so much on the basis of what it does,
as on that of what itis. In other words, an organ is known mainly
by its organization.

In the case of the vascular plants there are a number of distinct
structural units to which the name of organ is applied. It is not
by any means clear that the different organs of plants were always
as distinct from one another as they are at the present time. For
example, there is some reason to believe that the root may manifest
the primitive structural features of vascular plants, and in the
course of time the stem may have become differentiated from the
root, arriving finally at that degree of distinctness which at present
definitely separates it from the root. It has, moreover, often been
suggested in recent years that the leaf of the higher vascular plants
was originally of the nature of a branch and that its distinguishing
features are the product of later evolution. It would be going
beyond the range of an elementary work like the present one to
discuss the question of the origin of organs, particularly as the data
are extremely meager and not always easy to interpret. Disre-
garding, consequently, any speculations as to the appearance of the
organs of the higher or vascular plants, we may proceed at once
to the enumeration of those which are generally accepted by
anatomists.

The parts or organs of the higher plants are usually distinguished
as three—namely, root, stem, and leaf. To these may be added a
fourth, the sporangium or spore sac. Each of the organs named

136

DEFINITIONS OF THE ORGANS L327

has its particular features of organization and can be traced as a
definite and distinct structure far into the geological past of our
existing plants. Of the organs the root is the one usually distin-
guished by subserving the function of attachment to the substratum.
and also that of absorption of nutritive substances in solution from
the soil. In the case of the root, also, the direction of growth is.
normally and primitively downward. The stem has an upwardly
growing axis which serves as a support for the other parts of the
plant. The leaf is distinguished in turn by its usually flattened
form, which particularly qualifies it for its important function of
bringing the organism into relation with light and the gases of the
atmosphere. The sporangium has the particular office of producing
spores and may consequently with a certain degree of appropriate-
ness be designated as the organ of reproduction. The indications
supplied above as to the réles ordinarily played by the respective
parts of the higher plants by no means afford reliable definitions of
those organs. Very frequently stems, more rarely leaves, and
_ sometimes even sporangia serve in connection with the function of
attachment, so that the relation to the substratum cannot be
assigned as a fundamental and exclusive feature of the root.
Similar objections may be raised in regard to the functional defini-
tion of all the parts or organs of plants. In the present connection
it will be well to consider these important categories of structure
from the standpoint of organization rather than function, as that
procedure is most advantageous from the evolutionary point of
view.
THE ROOT
There is good reason to believe that the root is on the whole
the most primitive of plant organs, and it will be shown in subse-
quent chapters that, even if there be room for doubt as to its primi-
tiveness, there can be none as to its conservative character. There
is, in fact, no organ of plants so antique in its organization or so:
retentive of ancestral traits as is the root. Functionally, as has
been indicated above, it serves usually to connect the plant with
the substratum. Its unique and distinguishing features, however,
are supplied by its internal organization. Roots are characterized
by two main structural features—namely, the possession of a root

138 THE ANATOMY OF WOODY PLANTS

cap and a type of fibrovascular organization known as radial.
The root cap or pileorhiza clearly differentiates roots from other
structures in the plant which may happen under the stress of
environment to assume a subterranean mode of existence. More-
over, the root cap persists even in those cases in which the root
becomes aérial or aquatic in its habit. We need not devote atten-
tion to the mode of origin of the pileorhiza, or protective tip of
the root, as studies of this nature are at the present time of doubt-
ful morphological value, and the consideration of them in an ele-
mentary work is quite out of the question. All that need be stated
is that the cap occupies the tip of the root and is continually renewed
from behind, sometimes, but not invariably, by a well-defined active
tissue known as the calyptrogen. The protective cap is character-
istic of all roots from the lowest to the highest plants. There is
only one general condition under which this structure is ordinarily
lacking. In the fungus-infected roots or mycorrhizae of humus
plants, in which the fungal infection is superficial and does not
notably invade the internal tissues of the root, the root cap is
degenerate or absent. In the case of the haustoria of certain root
parasites, such as the dodder, not only is the root cap absent, but
also the general condition of degeneracy is so marked that frequently
the only criterion of the morphological value of the structure is its
internal or endogenous origin in the axis of the parasite.

A salient feature of the organization of the root is furnished:
by the fibrovascular tissues. Here the type of fibrovascular sys-
tem is that known as radial. In this condition the masses of
phloem, instead of surrounding the wood or xylem (a state
commonly found in the ferns and their allies) or lying just outside
of it in the same radius (seed plants) as in the stem and leaf, lie
in different and distinct radii. This mode of relative disposition
of phloem and xylem is responsible for the term radial as applied to
roots, and it is a very important characteristic of their organization.
Where the phenomenon of secondary growth in thickness is absent,
there is no subsequent modification of the general relations of
tissues in roots. In the case of roots with secondary growth, how-
ever, the situation changes with the appearance of the secondary
xylem and phloem. The secondary tissues begin to form in definite

DEFINITIONS OF THE ORGANS 139

relation to the clusters of primary phloem in all probability because
the foodstuffs are provided by that tissue. The active layer
which makes its appearance here continually adds new elements
on the outside to the phloem, and on the inside gives rise to the
secondary xylem. Asa consequence of this situation the secondary
wood, being formed opposite the clusters of primary phloem,
- naturally alternates with the primary wood. With the beginning
of the secondary growth the root therefore abandons the radial
type of organization of the primary structures. The lateral roots
originating from a given root have their position determined,
however, by the topography of the primary structures in the main
root and grow out in vertical rectilinear rows corresponding to the
primary xylem. But in the monocotyledons an exception to this
condition is found, since each lateral root originates in the interval
between two angles of the primary xylem.

Although the secondary structures of the root, as has been
indicated above, are collaterally organized, the primary plan of the
root is radial. The root, in fact, is the only organ of the plant
for which a single ground plan will illustrate the conditions found
in all groups of plants, living or extinct. The extreme conservatism
of the anatomical organization of the root is an outstanding feature
and, as will be indicated in succeeding chapters, renders it of the
greatest value in working out the evolutionary sequence of the
various groups. Extreme conservatism, radial organization of
the primary fibrovascular structures, the possession of a root cap
or pileorhiza, and an internal origin from the surface of the fibro-
vascular cylinder are the salient and important criteria of the root.

THE STEM

In the case of the axis or shoot of plants no such general formula
can be arrived at asin the root. Just as the root is the least change-
able of all the organs of the plant, so the stem is of all the most
variable. In internal organization it varies greatly from the lower
and more ancient groups to the higher and more modern. So
numerous are the types of stem as regards anatomical structure
that they can most profitably be discussed under the particular
groups of the vascular plants in later chapters of the volume. In

140 THE ANATOMY OF WOODY PLANTS

the absence of any general and common anatomical characteristics
of the stem for vascular plants as a whole, it is necessary to define
it by the important criteria supplied by the mode of attachment of
the appendages. The axis is characterized by the fact that it is
divisible into nodes from which the appendages normally take their
origin and into segments more or less elongated separating these
from one another and known as internodes. By the possession of
nodes and internodes the stem is at once distinguished from the
root and the leaf. There are usually at least two kinds of append-
ages attached to the stem in the region of the nodes—namely, the
leaves and secondary axes or branches. If the axis be subterranean,
roots may also make their appearance in the region of the nodes
and are clearly separated from the other appendages by their
internal origin. Both leaf and branch are formed superficially or
exogenously on the axis. The leaf and secondary axes or branches
are frequently related to one another in a quite definite manner, the
young branch appearing in the upper angle or axil of the leaf.
Further, the arrangement of the leaves and consequently of their
axial related structures, the branches, follows a somewhat definite
plan. Where more than one leaf occurs at a node, the foliar
structures are said to be opposite or whorled; and where they are
attached singly to the nodes, the arrangement of the leaves or
phyllotaxy is said to be spiral. The spirals are of different types,
according as the number of foliar organs intervening between two
vertically opposite leaves is greater or smaller; and the spiral
turns about the stem, drawn through the leaf bases, are more or less
numerous. In the grasses, for example, the spiral phyllotaxy
is expressed by a fraction of which one is the numerator and two the
denominator. In the alder the fraction is one over three, and in the
oak or willow two over five. The numerator indicates the number
of turns about the stem in passing from one leaf to the next one
vertically above it. The denominator is supplied by the number of
leaves attached to the stem in the interval between two which
are vertically opposite.

The stem is not only extremely variable as regards its anatomical
structure in the various groups of higher plants, but its external
form is greatly modified in correlation to different conditions of

DEFINITIONS OF THE ORGANS 141

existence or various needs on the part of the plant. In general,
however, the possession of nodes and internodes serves as a suffi-
cient diagnostic character, since this feature is rarely obliterated
even by the most extreme conditions to which the organ is exposed.

THE LEAF

This organ has a double importance, because it not only sub-
serves the vegetative functions present in the case of root and stem,
but is also charged with the extremely important office of repro-
duction. The leaf from the purely vegetative aspect is of con-
siderable evolutionary significance, since, although much less
conservative than the root, it is much more retentive of ancestral
characteristics than is the stem. Its chief evolutionary value, as
will appear in subsequent chapters, is in connection with the
unraveling of the relationships of the older and lower gymno-
sperms. In more modern types the anatomy of the root
generally throws more light on the relationships than that
furnished by the leaf. Reproductively the foliar organs are of
great importance in connection with taxonomic arrangement. In
fact, at the present time the angiosperms are almost universally
classified on the features presented by their reproductive leaves,
and in the case of the higher gymnosperms anatomical features of
the vegetative structures are only beginning to receive adequate
consideration in connection with investigations as to evolutionary
sequence.

As a reproductive structure the leaf is best considered under the
particular groups to be discussed in subsequent chapters, as no
general statements can be advantageously made at the present
stage. Asa purely vegetative organ the leaf is readily distinguished
from the stem, of which it forms an appendage, by the absence of
nodes and internodes. It is not easy to summarize the anatomical
structure of the leaf, since it is to a large degree variable in passing
from lower to higher groups. A useful criterion is its dorsiventral
organization, which is usually present, at least in the region of the
expanded part of the leaf or blade. Another outstanding feature of
the leaf is the fact that it is an axillating organ, and as a conse-
quence other appendages are commonly axillary to it. The

142 THE ANATOMY OF WOODY PLANTS

general external form of the foliar structure may reveal the presence
of a narrow base or stalk known as the petiole, and this is termi-
nated by the broad, generally horizontally placed, region called the
blade or lamina. The petiole often has related to it paired and
lateral appendages known as stipules. Occasionally an unpaired
appendage of the nature of a stipule, often called the ligule,

is present.
THE SPORANGIUM

This is the organ of reproduction. Although ordinary vegeta-
tive parts under certain conditions, and particularly in herbaceous
forms, may serve to perpetuate the plant, still the sporangium is the
special organ of reproduction and has the most important evolu-
tionary significance in this respect. The organ under discussion
is very accurately defined by the fact that it produces spores and
is, indeed, the only structure in the higher plants set apart for this
purpose. In some cases the essential fact of sporogeny is more
or less concealed as the result of the deep-seated modifications con-
nected with heterospory and the seed habit; but in such instances
a study of internal organization suffices to make the situation clear.
Normally the sporangium is a structure which is an appendage
of the leaf. Sometimes it appears to be axillary, although there
is no reason to believe that this was its primitive relation to
the foliar organs. Very frequently the reproductive leaf undergoes
profound modification in connection with the functions it has to
perform, and these features furnish a generally approved basis of
systematic grouping for the vascular plants. They need not be
considered in any detail in an elementary work on anatomy and are
best discussed in connection with particular groups in appropriate
later chapters. Unlike the other organs, the sporangium shows
no indications of derivation from another structure. It has, in fact,
been suggested that the sporogonium of the lower bryophytes is
the ancestral primordial from which all the other organs have been
derived. Professor Bower has developed this idea in his Origin of a
Land Flora.

CHAPTER Xil
THE ROOT

As has been indicated in the preceding chapter, the root is
characterized by the possession of a protective terminal structure
known as the root cap, by its radial organization, and by its endoge-
nous or internal mode of origin. The first and the last charac-
teristics are exclusive features of the root, but that of radial
organization is often
present in stems of
the more primitive
types. A comparison
of the upright stem of
the common club
moss, Lycopodium
clavatum, with the
root of the same
species makes the
truth of this state-
ment particularly
apparent. It is clear
from Figs. too and
tor that the radial
organization is com-
mon to the two Fic. too.—Transverse section of the root of
organs and that they Lycopodium clavatum.
differ only in the fact
that the stem bears leaves. This resemblance in anatomical
structure between stem and root is often present in the lower
Lycopodiales and furnishes a strong argument, taken together
with their ancient occurrence and early decline as a prominent
element of the earth’s vegetation, for the view that the lycopods
are the most primitive plants.

In the great majority of vascular plants the contrast between
the organization of the stem and that of the root is very striking.

143

144 THE ANATOMY OF WOODY PLANTS

In the case of a conifer, for example, the root is clearly distinct from
the stem by the possession of a woody cylinder devoid of pith. The
primary wood, moreover, in the root is very well developed and
forms a lens-shaped mass in the center of the cylinder which is
clearly centripetal in its development, since the smaller elements
are situated in two or more groups in an external position. The
primitive organization of
the root in contrast to the
more progressive organi-
zation of the stem is par-
ticularly well indicated in
connection with the rays.
In the cauline woody cyl-
inder these appear to take
their origin from the pith;
hence arises the com-
monly employed appella-
tion, medullary rays. The
intimate relation between
rays and pith which appa-
rently obtains in this case
is in reality merely a sem-
Fic. ror.—Transverse section of the upright blance, owing to the fact
stem of Lycopodium clavatum. that the primary wood
has become nearly obso-
lete in stem organs in the higher gymnosperms and the angio-
sperms. In the root the primitive situation in which the radial
parenchyma has no relation whatever to medullary tissues is very
clear. It is obvious that the common term medullary rays is
extremely inappropriate as applied to the radial parenchyma of
the secondary wood. The true situation is not only revealed by
the comparison of stem and root, but it also becomes even more
apparent when the stem in older types is compared with that
found in existing forms. The more detailed consideration of the
contrasts in organization between stem and root is best deferred
to later pages, after the anatomy of the various groups has been
discussed.

THE ROOT 145

We may now turn our attention to the particular features of
organization presented by the younger and older root in a conifer.
In the early stage of development presented in Fig. 102 the fibro-
vascular region is sharply separated from the rest of the transverse

DOs

‘Ga Ox wo (

ZO

jo

Fic. 102.—Transverse section of a young root of the American larch

section by a well-marked endodermis. Within this layer lies a
broad zone, the pericycle, which abuts upon the primary phloem.
Inside of the primary phloem lie two bands represented as double
rows of cells with protoplasmic contents. This is the cambium,
which provides the elements of the secondary xylem and phloem,
neither of which can be said as yet to have come into existence. In

146 ' THE ANATOMY OF WOODY PLANTS

the oval area between the cambial bands lies the primary xylem.
This consists of smaller elements at the two external angles, the
protoxylem strands. The development of the primary wood is not
complete; hence there is a considerable interval in the center
which will be later transformed into tracheids of the primary
metaxylem. In the first-formed elements of the primary wood the
bordered pits of the tracheids can be plainly seen. Subtending each
cluster of protoxylem is a resin canal, a common feature of the
organization of the root in Picea and allied genera of the Abietineae.
It is manifest from the general situation represented in the median ©
region or central cylinder of the root that the primary structures
are the only ones conspicuously in evidence, and of these the wood
is still incomplete. Further, there is a clear indication of the
presence of cambial layers, which will a little later give rise to sec-
ondary elements of both wood and bast. Surrounding the central
cylinder or stele of the root is the cortex, which is limited internally
by the endodermis and externally abuts on the root-hair-producing
stratum of the root: this is ordinarily called the piliferous layer.
As the section is taken very near the apex of the root, a few rather
poorly developed root hairs are seen. It has often been wrongly
asserted that there are no root hairs present in coniferous roots.
While this condition may be found in the case of those fungus-
inhabited radical organs known as mycorrhizae, carefully excavated
normal coniferous roots nearly always show a band of root hairs
in proximity to the root cap or pileorhiza. It will be made clear in
what follows that both root hairs and cortex are of short duration
in coniferous roots.

In the older root marked differences present themselves both
in the region of the central cylinder and in the cortex. Taking first
the fibrovascular tissues, we find that the pericycle is as well marked
as in the younger condition of the organ, but is now characterized
by the formation in its outer region of a zone of regularly arranged
cells known as the periderm. This layer has an important influ-
ence, as will be indicated later, on the tissues constituting the outer
region of the root. It is clear that the primary phloem has more
centrally been superseded by a mass of tissue of very regular radial
arrangement, the secondary phloem, derived as a result of the

THE ROOT 147

external activity of the cambium. The primary elements of the
- phloem constitute a distinct band of collapsed cells on the upper
and lower sides of the cylinder, and this is usually very conspicuous
in sections of the mature root, provided the organ has not Become
too old. Within the zones of radially disposed secondary phloem
lies the secondary xylem, which has its cells similarly arranged in
series. [The two masses of secondary wood, facing the correspond-
ing aggregations of secondary phloem, are joined internally with
the primary xylem, and they are readily distinguishable from this
by the presence of rays and the regular disposition of the tracheary
elements. The primary xylem is now complete and no longer shows
the hiatus or interruption in the center which is found in the
younger root. The two resin canals which lie opposite the groups
of small tracheids constituting the protoxylem have not collapsed.
The primary wood in the root, in contrast to the primary phloem,
does not break down, but persists in maturity. Meanwhile in the
exterior or cortical region of the root an important change has
taken place in consequence of the formation of a layer of impervious
secondary tissue known as periderm. ‘This zone, by reason of its
imperviousness, prevents the conveyance of water and nourishment
to the outer portion of the root, and the latter consequently dies
and is sooner or later exfoliated as a shriveled mass of collapsed
cells, as indicated in the diagram (Fig. 103). The result of the loss
of the cortex and piliferous layer in the older roots of the conifers
is to bring the fibrovascular tissues in immediate relation to the
soil. In this stage, however, they no longer serve the function
of absorption, but only that of conduction, since the taking up of
water and nutritive substances in solution is an activity of the
younger root when it is still provided with root hairs.

The subject of the structure and development of the root is so
important from the evolutionary standpoint that it is advisable
to supplement the description furnished in the two preceding some-
what diagrammatic drawings (Figs. 102 and 103) of the organization
of the root with actual photographic reproductions of the struc-
tures present. In order that the general validity of the principles
involved may be made apparent, a different genus of the conifers
has been purposely chosen. The diagrams (Figs. 102 and 103) refer

148 THE ANATOMY OF WOODY PLANTS

to the genus Larix, while the actual photomicrograms illustrate the
fir (Abies). In the first of these (Fig. 104) is depicted the root ina
younger stage of development. The general situation is clearly
identical with that in the larch, since the central region of the root
is sharply set off from the cortex by the presence of a well-marked
endodermis, a structure, by the way, not found in any organ of the
genus but the root. In the central cylinder may be seen the broad

F1G. 103.—Transverse section of an older root of the American larch. Explana-
tion of this and preceding figure in the text.

encircling pericycle, bounding the primary phloem above and below.
Within the first-formed phloem lies the mass of primary wood, not
as yet completed in the central region, and thus clearly indicating
the centripetal order of development of the elements of that portion
ot the woody structures. In the midst of the primary xylem is
situated a resin canal which is not yet fully developed. The singu-
larity of number and the axial position of the secretory canal
furnish practically the only features of distinction from the cor-
responding structures in the case of the larch, where the plural

THE ROOT 149

resin canals are found exclusively facing the clusters of protoxylem
and are never present in an axial position.

In Fig. 105 an older
stage of the root of
Abies is presented.
Here the development
of the primary wood is
completed, and as a con-
sequence the axial resin
canal stands out more
distinctly. The second-
ary activity has pro-
ceeded so far that there
is now a considerable
zone of radially seriate
xylem and phloem. The Fic. 104.— Young root of Abies balsamea
primary phloem has col-

lapsed. The ap-
pearance of a layer
of cork or periderm
with radially dis-
posed elements is
obvious in the re-
gion of the endo-
dermis, and the
activity in this tis-
sue is the cause of
the death and the
ultimate shedding
of the external or
cortical region of
Enes moot, Lt is
quite clear, as has,

Fic. 105.—Photograph of an older root of Abies

balsamea. Description of this and foregoing figure in ,
the text. indeed, been

pointed out above,
that the radial parenchyma of the root of the conifers can in no wise
be appropriately designated medullary rays, since the structures

150 THE ANATOMY OF WOODY PLANTS

in question have no relation whatsoever to the pith, which in this
case is non-existent. On general evolutionary grounds there are
the best of reasons for regarding the type of organization present
in the root of the conifers and, in fact, of the gymnosperms in
general as more primitive than that which characterizes the stem,
and consequently of great value from the standpoint of the doctrine
of descent.

Fic. 106.—Root of Osmunda cinnamomea. Explanation in the text

The root of the ferns and their allies differs from that found in
the gymnosperms only by the absence of secondary tissues. In
Fig. 106 is shown the root in Osmunda. Here central cylinder and
cortex are clearly delimited by the endodermis, composed of cells
with dark contents. Within this boundary lie the two masses of
phloem, and in the intervening region is seen the primary xylem.
This consists externally of narrow primitive elements which are
continued toward the center by the progressively broader tracheids
of the metaxylem. No secondary activities subsequently modify

THE ROOT 151

the topography of the root in any living representative of the fern
alliance or of the Pteridophyta in general.

The structures of the root in the dicotyledonous angiosperms
are appropriately considered at this stage. Since the early condi-
tion of development of the root has been already discussed and
figured in detail for the conifers, it will not be necessary to enlarge
upon this phase in the present connection, for the underlying
principles involved are the same in both conifers and dicotyledons.
It will be convenient to begin with a woody or arboreal root.

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Fic. 107.—Transverse section of root in Alnus incana. Explanation in the
text.

Fig. 107 illustrates the organization of an older root in Alnus incana.
Toward the right appears a layer of periderm bounding the outside
of the organ. Within lies the cortex, which terminates at a layer
of thick-walled cells, the pericycle. Within the pericyclic layer,
marking the outside of the fibrovascular cylinder, lies the phloem,
not depicted in detail. Then comes the cambium, followed in-
ternally by the secondary wood, in which conspicuous rays are
present. ‘Toward the extreme left of the illustration lies the five-
angled star indicating the topography of the primary xylem. The
stellate aggregation of elements shows at each angle of its five
points groups of cells of smaller size, the clusters of protoxylem
consisting of spiral tracheids. The mass of metaxylem into which

152 THE ANATOMY OF WOODY PLANTS

the angles of protoxylem become confluent by continued centripetal
development has interspersed with its elements a varying amount
of parenchyma. Vessels are present in the primary five-angled
region of the wood, but they do not so conspicuously differ from
the tracheids in transverse section as is the case with the similar
elements of the secondary wood.

Two features are particularly worthy of note in the present
connection. First of
all, the layer of peri-
derm does not occupy
so deep a position in
the root as in the larch
and fir shown above.
As a consequence the
central cylinder does
not generally become
stripped of its cortical
envelope as has been
found in the conifers.
Another interesting
feature is presented by
certain modifications

Fic. 108.—Transverse section of root of Alnus in the ies in relation
japonica. to the star-shaped

mass of primary wood.
This situation is more easily understood by reference to the
accompanying photographs. The first illustration (Fig. 108)
represents a transverse section of a mature root of Alnus
japonica. The secondary wood is characterized by certain
modifications in the structure of the radii corresponding to
the angles of the primary wood. These consist of clusters, more
or less pronounced, of rays of greater width than the uniseriate
condition found in the wood at large, among which the vessels,
characteristic of the organization of the remainder of the xylem,
are absent. In other words, aggregate rays are plainly present in
the secondary wood and are quite clearly related to the groups
of protoxylem marking the angles of the primary xylem. Since

THE ROOT 153

the secondary roots definitely take their origin from the same
clusters of protoxylem which are subtended by aggregate rays in
the secondary wood, it follows that the secondary roots are corre-
lated with aggregate rays precisely as the traces of the leaf are
imbedded in similar modifications of the radial parenchyma in
the case of the stem. Aggregate rays are much more commonly
found in the root than they are in the shoot organs, a consequence
of the conserv-
atism of the root,
which is to be
emphasized ina
later chapter. The
intimacy of the re-
lation between the
aggregate ray and
the trace of the
secondary root can
be readily inferred
from the inspec-
tion of Fig. 109, a
photograph of a
transverse section
of the root of Alnus

japonica. eis Fic. tog.—Transverse section of part of same, more
highly magnified. Description of this and last figure in
the text.

2 ree oe ol
Pe 2Atan. ol

?

clear that the trace
is related to a mass
of enlarged rays among which vessels are conspicuous by their
absence, precisely as in the leaf trace illustrated in Fig. 130,
page 177. It thus becomes clear that aggregate rays may be
a feature of organization of the root as they are of the stem, and
that in both organs the clusters or congeries of rays are most
conspicuously developed in relation to the appendages.

The secondary structures of the root are characterized by the
same three types of multiseriate rays found in the wood of
the stem as figured and diagrammed in chapter vi. In the case
of the root, however, the primitive aggregate condition which pre-
cedes both the compound and diffuse types tends strongly to persist.

154 THE ANATOMY OF WOODY PLANTS

The truth of this statement can be made apparent by reference to
the oak and the birch which represent, respectively, in the species

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Fic. 110 }

Fic. 110 a and b.—Tangential sections of large rays in
stem and root of Quercus rubra. Explanation in the text.

found in the
Northern United
States and Canada
the complete ex-
emplification of
the compound and
diffuse types of
rays. In Fig. r1oa
and Fig. 110) are
shown tangential
views of the wood
of the stem and the
root of Quercus
rubra, the red oak.
In each appear
large rays. Above
is the illustra-
tion of the con-
dition, in, thie
stem, and here
the ray is obvi-
ously of the type
described in an
earlier) chapter
as compound.
Below, the con-
dition typical for
the root is indi-
cated, and it is
here equally clear
that the large ray
is made up of a con-
geries of smaller
rays separated
from one another

THE ROOT hres

by fibers. The state of the ray figured for the root is the primi-
tive one for the stem and the ancestral one for the genus Quercus,

as evidenced by its
tropical and sub-
tropical represent-
atives (live or
evergreen oaks)
which typically
manifest the pres-
ence of aggregate
rays. Inthe genus
Betula the same
situation presents
itself with refer-
ence to the aggre-
gate and diffuse
rays. Inthestem,
as is shown in
Pig. ri 1a), the
multiseriate rays
ane. ditiusied
throughout the
structure of the
wood, while in the
root aggregations
of such rays are re-
lated to the out-
going traces of the
secondary roots,
Fig.111b. Similar
conditions are of
widespread occur-
rence, and the root
clearly furnishes
evidence as to the
nature of primi-
tive organization

Fic. 111}

Fic. 111 a and 6.—Transverse section of rays in stem
and root in Betula papyrifera. Explanation in text.

156 THE ANATOMY OF WOODY PLANTS

in general in vascular plants. The illustration supplied by the
rays serves as an exemplification of the original condition of the
root as regards its secondary structure. Additional examples will
present themselves repeatedly in the subsequent chapters. In the
case of the primary organization the uniformity of the radial struc-
ture presented by the root throughout the vascular series, in contrast
to the great, variety of
types exemplified by the
stem both in regard to
the arrangement and
mode of development of
the fibrovascular ele-
ments, vouches even
more emphatically for
the value of the anat-
omy of this organ in
evolutionary investiga-
tions which are not
entirely speculative in
their nature.
It will be convenient
and appropriate at this
Fic. 112.—Transverse section of a root of stage to consider the
Actaea alba. root in the herbaceous
dicotyledons. The
Ranunculaceae will serve here as an excellent exemplification of the
conditions in plants in which the annual herbaceous habit has
brought about at once reduction in the development of the second-
ary tissues and marked modifications in their topography. Fig. 112
illustrates photographically the organization of a root in the case of
Actaea alba. ‘The cortical tissues are somewhat clearly separated
from those of the fibrovascular system by an endodermal zone. The
central region of the root is distinguished by the presence of four
aggregates of phloem. A marked feature of the secondary wood is
its division into widely separated segments by broad rays of the com-
pound type. This condition is extremely common in roots of forms
in which the herbaceous habit has become strongly developed.

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THE ROOT 157

Fig. 113 shows the central region of the root under a higher degree of
magnification, and the arrangements described above now become
much more clearly visible. Another structural feature now for the
first time presents itseli—namely, the presence of groups of small-
sized elements of the primary xylem in alternation with the more

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a

ae
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Fic. 113.—Part of same, more highly magnified

pronounced radial bands of secondary wood and phloem. The
protoxylem groups are incompletely united in the center, and
a pithlike cluster of parenchymatous cells is consequently found
in the midst of the primary wood, a condition quite frequently
present both in herbaceous dicotyledons and generally in the
monocotyledons. In the woody dicotyledons and in the conifers
a structure comparable to the pith of the stem is ordinarily absent
in the root.

158 THE ANATOMY OF WOODY PLANTS

In the monocotyledons the root is characterized by the usual
absence of any indications of secondary growth. In Fig. 114 is
represented the transverse view of the root of the carrion plant,
Smilax herbacea. The root hairs are very conspicuous (Fig. 115),
and underneath the layer which forms them, the piliferous layer, is
seen a zone of cells which, with certain exceptions, possess thickened
walls. Next to this structure, known as the exodermis, lies the
cortex, which terminates
with another cellular
limiting membrane, the
endodermis. The center
of the root is occupied
by the central cylinder
and the pith. The for-
mer consists of numer-
ous and distinctly
alternating clusters of
primary xylem and
phloem which, as is
usual in roots, occupy
different radii. There
is no indication of
secondary thickening in
connection with either the xylem or the phloem. The wood
elements are smaller on the outside and become larger as they
pass inward. Although the usual gradation in size is found to
exist in the elements of the wood in monocotyledonous roots, the
order of development of the tracheary cells is not infrequently
the reverse of their gradation in diameter. In other words,
the internal elements of a tracheary nature are completed before
those lying farther outward in the region of the protoxylem. This
is one of the many abnormalities which characterize the anatomical
structure of this important group of the angiosperms. The central
region of the root is occupied by a well-marked pith, a condition
of very general occurrence in the group under discussion. A
similar situation in regard to the frequent presence of medullary
structures is found in the case of herbaceous dicotyledons. It seems

Fic. 114.—Root of Smilax herbacea

THE ROOT 159

certain that the presence of a pith in roots is not a primitive feature,
but one which has been secondarily acquired.

In the mass of mono-
cotyledonous roots there
is very little departure
from the state of affairs
found in the illustrations
shown above. At most,
the modification in the
central cylinder consists
of variations in the num-
ber of groups of xylem
and phloem. An interest-
ing complication of the
piliferous or root-hair-
forming layer is fre-
quently present in certain
orchids and aroids. Here
the normally uniseriate
piliferous layer becomes
multiplied, giving rise to
a white spongy substance
covering the surface of
the root and known as
the velamen or veil. This
structure is peculiarly
characteristic of the aérial
roots of tropical epiphytes
belonging to the two
orders mentioned above,
although sometimes found
in a condition of imper-
fect development in
terrestrial orchids of tem-

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Fic. 115.—Part of transverse section of root
of Smilax herbacea.

perate climates. The general relations presented by the velamen
are shown in Fig. 116. Here it is clear that the structure under
consideration lies external to the exodermis and is consequently

160 THE ANATOMY OF WOODY PLANTS

morphologically equivalent to the normally single piliferous layer
of terrestrial monocotyledonous roots. The cells of the velum
have banded thickenings in their walls which in a general way
simulate those found in certain types of tracheids. The elements
in question are also sometimes porous, so that water enters readily
into their cavities during the rainy
season, to be conserved for use in the
following period of drought.

In vascular plants in general the
‘a secondary roots are related in a definite
fashion to the primary structures of
YSA ARR RAARE the main root. In the greater number

JODY LIS ce of cases the young root appears as a
Sees local development of cells on the cen-
a, tral cylinder of the main root. This
-€ cellular proliferation is opposite one of
the clusters of protoxylem, so that the
arrangement of the rootlets is predeter-
mined by the organization of the pri-
mary structures of the main root. In
many monocotyledons the indication of
a developing root appears, not oppo-
nt a ae ee site one of the protoxylem groups, but
multiple piliferous layer devel. i the interval between two of these,
oped as a velamen. thus constituting a departure from the
usual topography. It is evident that
the secondary roots are formed endogenously and subsequently
bore their way outward. This internal mode of origin is
characteristic of roots and rootlets, the only exception being
found in the case of the primary root of the seedling. Fre-
quently roots are definitely related to other appendages, such
as branches and leaves, and in these instances originate at or
near the node.

SW
Nae

SIT eT

All roots, with the exception of the degenerate ones found in
certain types of parasitism, are provided with a protective cover
over their tender apex, and this is known as the root cap or pile-
orhiza. This structure is for the purpose of protecting the root

THE ROOT 161

as it forces its way through the more or less resistant soil. The
tissues of the root usually become stiffened at an early stage by
the rapid thickening of the walls of the tracheary elements of the
primary wood. Rigidity is so much a necessity in the case of the
wood of the root that ringed and spiral tracheids, such as ordinarily
characterize the first-formed region of the xylem in the aérial stem,
are conspicuous by their absence or at any rate by a scanty degree
of development. In Fig. 117 are represented the protoxylem of
the stem and root of the balsam fir. The great contrast in the
development of the extensible ringed and spiral tracheids is shown

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Fic. 117.—Longitudinal view of the primary tracheids of the root and stem of
the balsam fir, showing the different degrees of development of the protoxylem ele-
ments in the two organs.

in the diagram. It is obvious that ringed and spiral sculpture alone
is not an entirely constant character of the first-formed wood in roots.

A great deal of morphological importance has in the past been
attached to the arrangements of cells at the apices of the roots.
It is not clear, however, that inferences drawn from such data
have a very great evolutionary significance; and certainly in other
organs of the plant, such as, for example, the stem, they are of
very slight value in view of the extremely contradictory results
reached as a consequence of adherence to this criterion. At the
present time the organization of the growing point in plant organs
is regarded as of less importance than the histological structure
of the mature parts in reaching any conclusions as to equivalence
and course of evolution.

CHAPTER XIII

THE STEM

In the older types, represented by the vascular cryptogams, the
general organization of the stem presents more variety in ground
plan than that found in the case of the seed plants. In these earlier
forms the fibrovascular tissues are present in two main conditions.
In the primitive type of structure the xylem is a solid mass which
does not include any medullary substance or pith. This variety
is represented in Fig. 118 and is known as protostelic. The
protostelic condition is found in many older groups and is of almost
universal occurrence in the most conservative of all the organs of
the plant, the root. Where the mass of primary xylem is smooth
in contour and does not show on its surface any projecting salients
of protoxylem, the phloem forms a continuous layer surrounding
it, and the whole fibrovascular complex so organized is known as
concentric. Usually in concentric protosteles the protoxylem
elements are situated in relation to the general organization of the
xylem in the manner described in an earlier chapter as mesarch.
Here it will be recalled that the first-formed elements of the
wood are ultimately completely surrounded by those of later
origin. When, by reason of the projecting masses of protoxylem
not imbedded in the main mass of the primary wood as in
the mesarch type, there are more or less deep furrows on the face
of the xylem, the phloem ceases to constitute a continuous layer,
but is present in isolated strands alternating with the projecting
angles of the protoxylem, and the stelar structure is described as
radial in its organization. In this condition the development of
the wood is usually entirely centripetal, or toward the center of
the organ. This mode of organization is anatomically described as
exarch, because the protoxylem elements are external in position
to all the later-formed portion of the wood.

Another primitive type of structure presented by the fibro-
vascular organization of the stem is that known as siphonostelic.

162

THE STEM 163

Here the phloem and xylem, instead of constituting a solid mass
without any parenchymatous center or pith, are laid down in the
form of a hollow cylinder inclosing a central medulla or pith. This
type of organization of the central cylinder or stele is technically
called siphonostelic, on account of the tubular condition present.
Primitively the siphonostele or tubular central cylinder seems to

Fic. 118.—Diagram of a protostelic stem

have been organized in such a manner that phloem clothed its
inner surface as well as its outer one. Further, both inwardly
toward the pith and outwardly toward the cortex the tissues of
the central cylinder or stele are clearly delimited from those of the
fundamental system by a well-marked endodermal layer. The
siphonostele may be either concentric or radial in its organiza-
tion, precisely as in the protostele, the general situation being
identical in the two cases except for the presence of the pith. The
essential features of the siphonostele are presented in Fig. 119.
Here may be seen cortex and pith marked by a similarity of

164 THE ANATOMY OF WOODY PLANTS

organization and separated from the fibrovascular category of tis-
sues by the endodermis. To the left in the figure the siphonostele is
depicted in a continuous or closed condition, while on the right it
is open on one side. The opening corresponds to the giving off
of the trace of an appendage, which in this instance is a leaf. A
general feature of the tubular or siphonostelic central cylinder in

Fic. 119.—Diagram of a siphonostelic stem

the ferns and their immediate allies is the appearance of openings
in the tubular central cylinder which correspond to either leaves or
branches. These openings persist for a longer or shorter distance
above the point of departure of the fibrovascular supply to the
appendage and are known as leaf gaps or branch gaps according
to the organ involved. Around the edges of these gaps the endo-
dermis of the inside becomes continuous with that of the outside.
Usually, too, the phloem of the inner surface of the tubular stele
joins with that situated on the exterior. Naturally, also, the
fundamental tissue of the outside, commonly known as the cortex,

THE STEM 165

becomes reunited with that situated in the center of the stele,
designated in turn as the medulla or pith.

Fig. 119 depicts a siphonostelic central cylinder. The traces of
leaves determine the presence of /acunae, or interruptions in the
continuity of the cylinder, for some distance above their points
of departure. These are the foliar gaps or, in case they are related
to lateral branches, the branch gaps.

The protostelic and siphonostelic conditions of stem are both
subject to further modification which may now be discussed. In
the protostelic stem, for example, the phloem may more or less
completely disappear on one side of the stele, a situation found in
the stem and sometimes even in the root of certain ferns. This
condition is known as collateral and is extremely rare in proto-
stelic organs. This state of reduction is, however, very common
in the siphonostele, particularly in the higher forms where the
tubular type of central cylinder has become universal. A proper
understanding of the phenomena of reduction presented by the
siphonostele is advantageously gained by the consideration of allied
forms presenting the stelar conditions both in the normal and in the
reduced state. Illustrations of this nature can best be drawn from
the lower vascular plants, the ferns and their allies. Fig. 120
represents on the left a siphonostelic stem in which internal and
external phloem are both well developed, and likewise the endo-
dermal layers, which respectively limit these from pith and cortex.
In the illustration at 6 appears a stem in which the internal phloem
has disappeared on most of the interior of the stele and is found
merely on the margins of the foliar gap. In the case of the chan-
neled leaf trace, subtending the foliar gap, in contrast to the
situation presented by the fibrovascular system of the stem, the
internal phloem is as well marked as that occurring on the outer or
convex surface. This is in accordance with a general principle
to be more definitely elaborated at a later stage. The basal region
of the foliar fibrovascular strand in vascular plants is found in
general to retain the more primitive type of organization. In the
particular instance under consideration the retention of internal
phloem in the leaf trace is the significant feature in a consider-
able number of ferns and fern allies. It may be stated that a

166 THE ANATOMY OF WOODY PLANTS

universally accepted procedure in comparative anatomy is to judge
the former organization of the walls of the stelar tube from
pertinent conditions presented by the leaf traces which take their
origin from it. In other words, ancestral conditions which have
disappeared in the stele of the highly progressive and easily modi-
fied stem may continue almost indefinitely in the fibrovascular

Fic. 120.—Diagram showing degeneracy of internal phloem and endodermis
in the siphonostelic central cylinder. Explanation in the text.

structures of the leaf. The great value of this principle is
quite generally admitted in the case of the gymnosperms, but
its validity for the fernlike forms is unfortunately not so uni-
versally conceded in spite of cogent logical considerations in its
favor. At c is represented a further condition of reduction. Here
not only the internal phloem has disappeared in the stele, but
likewise there is no internal endodermis, although both are present
in the leaf trace. At d in the same figure is seen the anatomical

THE STEM 167

condition presented by the young plant or sporeling. Here the
tubular central cylinder is characterized by an internal as well as
an external endodermal layer. The general principle applicable to
this situation is that young stages frequently perpetuate conditions
which have disappeared in the adult. It is apparently clear from
commonly accepted canons of comparative anatomy which will be
elaborated in a subsequent chapter that leaf and seedling stem fre-
quently enable us to reconstruct the course of evolution in the
siphonostelic cylinder.

The presence of internal phloem and endodermis has been
described in the former paragraph as a primitive feature of the
earlier types with the siphonostelic central cylinder. Other
important anatomical structures of lower or cryptogamic vascular
plants are exarch and mesarch primary wood. It will be recalled
that the term exarch has reference to the fact that the protoxylem
elements—that is, the tracheary tissues which are first laid down—
are external in position, and that the subsequent development of
elements of the wood is centrad, or toward the center of the organ.
Likewise in the case of mesarch structure the situation is first
characterized by the exarch condition, but after a certain progress
has been made the formation of new elements of the xylem shifts
to the external aspect of the protoxylem, and as a result we find
the first-formed elements more or less completely surrounded by
those of later origin. Later in geological time and in still higher
types the mesarch condition in turn gives place to the endarch
by the disappearance of the centrad or centripetal wood. This
situation can best be illustrated by a diagram. In Fig. 121 at a
is shown the exarch condition of development of the structures of
the primary wood. The small-sized elements, normally ringed or
spiral, are outside the later-formed scalariform or pitted tracheids.
In dis represented the mesarch condition, in which the development
is at first centrad, but later becomes peripherad, so that the pro-
toxylem comes to occupy a central position. In c is shown the
endarch condition where the seriation from the protoxylem is
no longer centrad but has become entirely peripherad or centrifu-
gal. The condition in a characterizes the organization of the
primary xylem in the stem of the lycopods and their allies, and is

168 THE ANATOMY OF WOODY PLANTS

found without exception in the roots of all vascular plants. The
situation diagrammatically delineated in } prevails in the stem
organs of the ferns and their allies, the lower gymnosperms. In
the higher gymnosperms and the angiosperms the axial organs
exemplify almost exclusively, in living types at any rate, the
endarch condition diagrammatically represented inc. As might be
expected, the primitive occurrence of exarch and mesarch conditions

Fic. 121.—Diagram to indicate the relations of protoxylem to metaxylem in
vascular plants. Explanation in the text.

in the case of the stem can frequently be inferred from the anatom-
ical situation presented by the leaves. The universal presence of
the exarch type of primary wood in the root furnishes good evidence
that this was the original type of organization and development
of the primary wood in the vascular series.

It has been made evident in an earlier chapter that, in addition
to the primary organization of the wood consisting of tracheary and
parenchymatous elements arranged in irregular order, there is
often present, particularly in arboreal types and perennials, ancient

THE STEM 169

and modern, a subsequent ligneous development made up of cells
regularly seriate radially and including masses of radial parenchyma.
This regularized and later-appearing xylem is known as the second-
ary wood. In older types the primary wood is distinct from the
secondary ligneous structure, and its conspicuousness under these
conditions is due largely to the more abundant development of the
primary elements and their frequent exarch or mesarch configura-
tion. In the higher gymnosperms and in the angiosperms they
can be recognized only with difficulty except in the root. This
situation is the result of the slight development of the primary
elements and of the fact that they are in series with, and formed in,
the same direction as the secondary wood.

It will be apparent from the statements made in the preceding
paragraphs that the organization of the stem is characterized by
a considerable degree of variety as regards the general topography
and microscopic organization of the primary structures. When
the special consideration of the ferns and their allies is reached in
later chapters, it will be clear that the possible complexities of the
gross fibrovascular structures have been by no means exhausted.

It is now convenient to turn our attention to the secondary
fibrovascular structures of the stem. In the older vascular
plants the secondary wood was extremely simple in its organi-
zation and consisted merely of radial parenchyma and longitudinal
tracheary elements. The radial parenchyma in the lowest forms
was definitely separated from the pith, as is well illustrated in
Fig. 122, by the presence of a clearly developed zone of primary
wood. With the progress of geological time came a progressive
reduction of the primary wood and important modifications in
the structure of the secondary woody cylinder. The gradual
reduction of the primary structures of the woody cylinder has.
involved interesting conditions which must now occupy our
attention. The progressive reduction of the region of the wood
known as primary brought with it certain important topographical
changes. The primary wood was at first continuous and in most
instances constituted in the stem a siphonostele interrupted only
by the passing off of the traces of branches or leaves. The breaks
in the continuity of the primary cylinder thus determined are

170 THE ANATOMY OF WOODY PLANTS

known as leaf or branch gaps, as the case may be. In the lower
forms, with super-addition of secondary to primary woody struc-
tures, the primary cylinder of the siphonostele still maintained to
a large degree its individuality. This is, for example, true in
Fig. 122a representing the organiza-
tion of a stem in a lepidodendrid,
an arboreal club moss of the Pale-
ozoic age. The lepidodendroid
arboreal club mosses include forms
referred to the general cognomen
of Sigillaria. In lycopods of this
type, particularly in the Permian
age and toward the end of their
term of existence as an element of
our earth’s flora, the primary wood
became much reduced in amount.

5
ROSS &S
—Se5 a,
SocSy
Sosa Sa
See SSSR
SSOSSSOS SSS OO
BOSS IOS OCS OSI
/, ATW

Fic. 1226

Fic. 122 a, b, and c.—Diagrams to illustrate the effect of the degeneracy of the
primary wood on the development of the secondary xylem.

Not only did this situation express itself in the thinning down of
the primary woody cylinder, but there was also present a lack of
continuity due to the complete elimination of tracheids in certain
segments. This phase is revealed in 6. Here the primary wood
constituting a continuous cylinder in a is interrupted by intervals
resulting from the scanty development of the primary xylem.
The gaps between the resulting slender isolated strands are per-

THE STEM yi

petuated in the early development of the secondary organization,
since the radially disposed elements make their ‘first appearance
opposite the clusters of primary wood, and only as the second-
ary segments widen out later do they finally bridge over the
intervals in the continuity of the cylinder. On the upper side
a branch causes a still more prominent hiatus in the inner region
of the cylinder. The condition in a calamitean stem may next
engage our attention. Here the primary wood is much less well
developed even than in the later Sigillariae, and only in the
earliest representatives of the genus does it show an indication
of the ancestral centripetal type of development. The task of
bridging over the primary intervals by secondary growth is cor-
respondingly greater than in the sigillarian stem, and the bays
extending into the secondary wood proportionately deeper and
wider. In the upper region of the cylinder is figured a siphonostelic
branch which is responsible for a wide hiatus in the secondary
cylinder. An inspection of a, b, and c makes it clear that the
progressive degeneracy of the primary wood as well as the depar-
ture of traces belonging to the branches causes interruptions
in the continuity of the secondary wood. ‘These are filled by soft
tissues continuous with the parenchyma of the pith. The second-
ary cylinder is characterized, as has been demonstrated in the
diagrams, by bands of radial parenchyma which are often wrongly
called medullary rays. That they cannot receive this appellation
with any accuracy will be apparent from a comparison of the
conditions presented by a, 6, andc. Ina the rays in no case reach
the pith. In 6 the radial parenchyma is clearly continuous with
the soft tissues of the pith in the intervals between the isolated
segments of primary wood, while in those regions of the secondary
cylinder subtended by primary xylem, which of course represent
the primitive topographical relation, no such continuity is possible.
In c, by the still further reduction of the primary structures of the
wood, a much larger number of bands of radial parenchyma appear
to abut on the pith. It is clear from the conditions outlined in the
diagram under discussion that it is entirely inappropriate to de-
scribe the radial storage tissues of the secondary wood as medullary
rays. A satisfactory appellation for them is simply wood rays, and

172 THE ANATOMY OF WOODY PLANTS

this involves no erroneous hypothesis of their relation to the pith.
It is therefore necessary to distinguish carefully between wood rays
and gaps or discontinuities in the woody cylinder originating as a
result of conditions described above. It seems quite evident that
the failure to realize this distinction invalidates investigations on
the woody cylinder involving the confusion of thought elucidated
by the items in
t e accompanying
figure.

The last para-
graph will have con-
vinced the reader
that radial bands of
parenchyma, formed
as a result of cam-
bial activity, can-
not accurately be
described as medul-
lary rays. We are
now in a position to
consider the situa-
tion presented by

Fic. 123.—Stem of Picea, showing apparent relation the stem of peren-
of wood rays to pith. nial seed plants of

gymnospermous af-
finities. Fig. 123 reproduces part of a transverse section of a stem
of the white spruce, Picea canadensis. The woody cylinder is
characterized by numerous radial bands of parenchyma, appar-
ently in every instance taking their origin from the medulla or
pith. Further, there are present broad outwardly directed bays
from the medullary region which represent the gaps correspond-
ing to outgoing leaves (for in the gymnosperms, in contrast to
the lycopods and their allies, the leaves are related to foliar gaps).
A superficial examination of the figure would justify the applica-
tion of the term medullary ray both to the narrow radial bands of
parenchyma and to the broader bays extending from the pith.
The considerations advanced in the last paragraph make it clear,

!

THE STEM 173

however, that this is an inaccurate interpretation. The justice of
the criticism. here advanced is, moreover, evidenced by reference
to the very conserva-
tive structures of the
root. Fig. 124 illus-
trates the central
region of this organ
in the same species
Ol spmuce:. Che
primary xylem is
pronounced and dis-
tinctly centripetal in
the order of develop-
ment of its elements.
There is no medulla
present, dtherays
of the secondary
wood consequently

‘op
ry

not only have no re- — Fy¢. 124.—Root of Picea, showing primitive condition
lation to a medullary
region, but also end in the vicinity of the primary wood. It is
clear that the radial
parenchyma of the
root cannot come
Wanides «the ten mi
medullary ray, and it
will now be obvious to
the reader that in no
case can the radial
bands of storage tis-
sue in the secondary
wood be accurately
called medullary rays.
They are properly de-
scribed as wood rays.
Le will tnot= be
Fic. 125.—Stem of Ephedra gerardiana necessary to consider

174 THE ANATOMY OF WOODY PLANTS

any other lower seed plants in the present connection. The Gne-
tales and angiosperms present the next significant modification of

Fic. 126.—Tangential section of wood of Ephedra
species, showing presence of large rays.

angiosperms are
definitely corre-
lated with the ap-
pearance of vessels.
A tangential sec-
tion (Bite. an 2h6)
through the wood
of Ephedra reveals
the true nature of
the rays in the
Gnetales. Fig. 127
illustrates the
organization of an
individual large ray
in the outer region
of the woody cylin-
der. + Dheray is

structure in the
stem. auBie aos
illwstrates the
organization of the
axis of Ephedra
gerardiana, Obvi-
ously there are
numerous broad
bands of radial pa-
renchyma present.
Another feature of
interest is the pres-
ence of true vessels
in the cylinder of
secondary wood.
Broad rays of the
type found in
Ephedra and in the

127.—Portion of large ray highly magnified,
showing presence of tracheary elements in ray substance,

THE STEM 175

plainly not of homogeneous organization, but contains in its sub-
stance, in addition to true radial parenchyma, large quantities of
fibrous elements and even some vessels. The fibrous elements when
studied under higher magnification than that of the figure often show
a gradual transition to parenchyma. By referring to chapter vi
it will be seen that the large ray of Ephedra comes under the heading
of aggregate ray, described in that chapter, since it is a composite
structure made up |
partially of true
storage cells and
partially of longi-
tudinal elements of
the wood in process
of parenchymatous
transformation.
Fig. 128, which
shows the ray of
Ephedra in its nar-
rower condition
nearer the pith,
supports the view
of the nature of the
large ray arrived
at aay UBS CMence Fic. 128.—Aggregate ray of Ephedra species in region
of an examination pear the pith.

of its organization

in the outer region of the wood. In its earlier stage of develop-
ment it contains equally clearly a mixture of storage parenchyma
and of fibers more or less completely transformed into cells
resembling the ordinary elements of the rays. It is obvious
in Ephedra (and this statement holds of the Gnetales generally)
that the higher organization of the water-conducting elements of
the wood carries with it a more abundant provision for the storage
of food products elaborated by the leaves. Of course the stage
of evolution here attained is quite impossible in the case of a more
primitive type of wood in which longitudinal parenchyma either
has not yet made its appearance at all or has progressed to such

176 THE ANATOMY OF WOODY PLANTS

a slight degree that it does not constitute an important feature
of organization. With the strong development of longitudinal
storage cells, as has been explained in an earlier chapter, owing to
the septation of elements originally destined to become tracheids,
a co-ordination between these and the earlier evolved radial
parenchyma provides possibilities in connection with storage quite
adequate to accommodate the output of the more efficient and
hygrophilous leaf of
modern floras. The
aggregate rayis evi-
dently the result of the
correlation of radial and
longitudinal storage de-
vices. Its appearance
is a phenomenon of
prime biological impor-
tance and is intimately
related to the origin of
the herbaceous type in
the angiosperms on the
one hand and the evolu-
tion of the highest

Fic. 129.—Transverse section of the stem of vertebrates, the warm-
Alnus japonica, showing presence of aggregate rays. blooded mammals, on

the other. In Gnetum,
the highest genus of the Gnetales, the condition of aggregation
is no longer prominent, but has been superseded by the compound
type of ray.

The next significant forms to occupy our attention are the woody
dicotyledons. These are best introduced in connection with the
alder and the oak. In Fig. 129 appears a photograph of a branch
of Alnus japonica several years old. The central pith is triangular,
corresponding to the one-over-three phyllotaxy of the genus. The
annual rings which surround the pith are characterized by the
presence of conspicuous broad rays, of which the largest and most
striking extend from the sides of the longest angle.of the triangular
pith. These most conspicuous radial structures are related to the

THE STEM 177

two lateral traces of a leaf and may consequently be appropriately
called leaf or foliar rays. The less conspicuous rays in other radii of
the stem are in relation to other traces of other leaves lower or higher
in the stem. In the following illustration (Fig. 130) a portion of the
stem represented in Fig. 129 is shown. The greater magnification
makes it apparent that the broad radial band related to the out-
going leaf trace is in reality an aggregation of rays and not a simple
structure. Itis,in
fact, largely com-
posed of fibers as
well as of radial
parenchyma, to
the exclusion of the
vessels which form
a prominent fea-
ture of the remain-
ing organization of
the wood. In the
case of the large
ray figured it is
clear that conspic-
uous depressions
mark the surface of

the annual ring Fic. 130.—Part of transverse section of stem of Alnus
where the broad japonica, showing aggregate ray in relation to a leaf, more

Z highly magnified.
radial band crosses

it. In Fig. 131 the same ray appears in the vertical section of the
wood. It is now evident that the structure in question consists
of an aggregation of rather small rays separated from one another
by fibers and including in their midst the transverse section
of a foliar strand or leaf trace. It will be obvious from the
various figures of the stem of the alder that in this genus there are
groupings of rays in relation to the foliar traces, and that these
storage bands on account of their topographical and physiological
relations may appropriately be designated foliar rays.

It will now be convenient to refer to the conditions found in
the case of the oak. Fig. 132 illustrates a transverse section of

178 THE ANATOMY OF WOODY PLANTS

the wood of the red oak, Quercus rubra.

Large rays are plainly

present which differ from those found in the alder by the fact

Fic. 131.—Tangential section of stem
japonica, showing leaf trace imbedded in the

ray to which it is related.

foregoing paragraph.
EnPie. 1235 p. E60!
is shown a trans-
verse section of a
branch of the velvet
oak. Here there are
five pairs of rays
corresponding to the
five faces of the five-
angled pith, which
by its configuration
indicates the phyllo-
taxy precisely as
does the triangular
medulla in the alder.
In the oak there are

Fic.

that, instead of
being composed of
a IM ix tir eh io
smaller rays and
separating fibers,
they are consti-
tuted entirely of
parenchymatous
elements. In spite
of this difference in
the organization of
the broad bands of
radial parenchyma
they are correlated
with the same dips
in the annual rings
found in the alder
as described in the

132.—Transverse section of the wood of the red oak

THE STEM 179

two important lateral traces in relation to each leaf, and less
well-developed central ones. The most distinct foliar rays are
those formed in relation to the lateral traces; since there are
two of these for each leaf and five leaves in a cycle, there are ten
conspicuous rays in the woody cylinder of the stem. An inspection
of the figure makes it clear that the rays are related in approximated
pairs, and that in the narrower intervals intervening between the
foliar rays the woody cylinder is depressed. These depressed
segments are five in number; corresponding to the five pairs of
propinquitous large rays. The natural explanation of the depres-
sion is that it is the result of the local effect of the large rays on the
rate of growth of the cylinder. It has been made clear above that
wherever a large ray crosses an annual ring a depressing influence
manifests itself, resulting in a corresponding dip in the surface of
the yearly zone of growth. Obviously if two large rays occur
close to one another they will be likely to exercise a depressing
effect on the region of the annual rings lying between them. That
this is the real explanation of the depressed segments of the stem
of the oak and the stems of a similar type of organization is shown
by the fact that, where the broad rays are equidistant, as, for
example, in the grapevine, the segments fail to become depressed.
On the other hand, where broad rays happen to be absent for any
cause in the branches of the oak the depression is likewise absent.
A further elucidation of the situation is furnished by the ranun-
culaceous genus Clematis. Here in species with approximated
broad rays there are depressed intervening segments, while in the
few species where the broad rays are equidistant no such depressions
occur.

The existence of depressed segments (Fig. 133) in the stems of
woody dicotyledons with large rays has been made the basis of
an erroneous and historically incongruous hypothesis of the evo-
lution of the woody type. This misconception originated with
the Prussian botanist Sanio in the nineteenth century, and, elabo-
rated in clear diagrams in Sachs’s classic textbook, has become uni-
versal in the pedagogical literature of botany. It is considered that
the primordial condition of the woody cylinder is that of numeri-
cally varying separate bundles. These strands of fibrovascular

180 THE ANATOMY OF WOODY PLANTS

tissues are imagined to have been originally quite distinct from one
another and to have been linked up by the formation of woody
commissures organized entirely of secondary xylem and laid down
by the activity of the so-called interfascicular cambium. The
narrower and more depressed segments are supposed to owe their
peculiarities to their late and entirely secondary origin. ‘The five
outstanding segments of wood in the oak and similar forms have
been accordingly dubbed
the fascicular wood, and
the five intervening de-
pressed segments the
interfascicular wood.
An unfortunate situa-
tion often encountered
by this hypothesis is the
fact that primary wood
is frequently as well
developed on the inner
surface of the depressed
segments as on that of
the outstanding ones.
The depression, as has

; Cee been pointed out above,
Fic. 133.—Transverse section of a twig of the

velvet oak (Quercus velutina) showing five pairs of 1S susceptible of an
foliar rays. entirely different

explanation—namely, as
the result of the local inhibiting influence of approximated broad
rays on the rate of growth of the annual ring. In effect, moreover,
the hypothesis of Sanio and Sachs derives woody from herbaceous
forms, a conclusion entirely at variance with the paleontological his-
tory of plants. It is clear that the woody forms have preceded
herbaceous ones in all the main series of vascular plants. A single
illustration will serve in the present connection. Our somewhat
herbaceous existing lycopods and Equiseta are certainly known to
have come from ancestral forms which possessed so conspicuously
the arboreal and perennial habit that for many years a controversy
raged as to their affinities. The majority of paleobotanists for a

THE STEM 181

long period considered them as belonging to the seed plants, since
all the types of the present age which possess marked secondary
growth are seed plants of gymnospermous or angiospermous affini-
ties. The situation in regard to the relation of sequence between
herbaceous and woody types might be almost endlessly illustrated,
but the examples cited will serve for the present.

It has been assumed in the references to the large rays in the
alder and the oak
that.0al though
differing from one
another in organiza-
tion (in one case
aggregate and in the
other compound),
they have the same
morphological ex-
planation. It is
clear from the de-
scriptions supplied
in the present and a
foregoing chapter
that they are simi-
larly related to the
appendages. It is

Fic. 134.—Transverse section of large ray in root of
now apposite to Quercus rubra.

establish the fact

that the two types are related to one another in the particular
forms under discussion in the present connection. Fig. 134 illus-
trates the organization of the large ray in the wood of the root
of the red oak (Quercus rubra) as seen in transverse section. It
is quite obvious that the structure differs from that found in
the stem as exemplified in Fig. 135 by the fact that the ray is
not homogeneous in its organization, but has numbers of fibers
intermingled with its parenchymatous elements. In other words,
the large mass of radial parenchyma under discussion must be
considered as being of aggregate organization and not as belong-
ing to the category of rays defined in an earlier chapter as

182 THE ANATOMY OF WOODY PLANTS

compound. In Fig. 136 the longitudinal view of the root in the red
oak isshown. It is now more than ever evident that the compound
condition of the rays characteristic of the stem of the oak, at least
so far as the red oak is typical of the genus, is not present in the roots,
but that in the latter organ the broad bands of radial storage
elements belong to the type defined as aggregate. In the case
of the root, in fact, one finds for many years of development that
the more prominent
radial structures are
aggregate, a condi-
tion which gives
place in: old and
thick axes to the
compound type of
organization. In
the seedling stem
or in branches de-
rived as the result
of injuries from the
lowest region of the
main trunk, the
aggregate ray occurs
and persists often
Fic. 135.—Transverse section of large ray in stem of for a long period. In
Quercus rubra. anticipation Ofrcer
tain general prin-
ciples of comparative anatomy to be set forth in detail in a later
chapter of this work it may be stated that it is apparent in the
case of the oak that the aggregate condition of the rays has
preceded that defined as compound, and, in fact, that the aggre-
gate condition is the forerunner of the compound ray. It is
accordingly clear that as regards both the nature of the large
rays and the relationship which they bear to the appendages the
alder and the oak are substantially in a similar condition. In
other words, the question of large storage rays is one susceptible
of elucidation in accordance with well-defined evolutionary and
physiological principles.

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THE STEM 183

In the earlier chapter dealing with the various types oi rays,
particularly in those paragraphs concerning the radial parenchyma
of the higher seed plants, it has been shown that the great mass of
arboreal dicotyledons is characterized, not by extremely large
rays in marked contrast to radial parenchymatous strands of the
uniseriate type, but by rays of more moderate dimensions scattered
throughout the wood. Obviously, just as the compound type of
radial parenchyma
has taken its origin
from the fusion of
the units of the con-
geries of rays known
as aggregate, so the
diffuse condition of
rays characteristic
of the dicotyledo- \~
nous forest trees in
general has takenits |
origin by the pro-
gressive divergence
of the original com-
ponents of aggregate
rays ins the outer
annual rings of older Fic. 136.—Tangential section of large ray in root of
Stems. The phe- Quercus rubra.
nomenon of diver-
gence can actually be seen with clearness in the genus Casuarina,
while in other cases rays of the aggregate type persist in the root
or may be recalled as the result of injury even when they are not
found in the adult stem. It may accordingly be stated for the
woody dicotyledons in general that in all there was primitively
present the category of ray known as aggregate. As a result of
the fusion of the elements of such rays, compound rays have
made their appearance. Further, as a consequence of the diver-
gence of the members of the aggregations, the diffuse con-
dition is reached. The former state of rays is extremely rare
in forest trees of angiospermous affinities, while the latter

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184 THE ANATOMY OF WOODY PLANTS

prevails almost universally in the organization of their woody
cylinder.

It will now be advantageous to consider those angiospermous
stems which are united in habits under the forms known as vines
and herbs. In this connection it will be well to restrict our atten-
tion to the typical representatives of the two groups, since plants
which are not clearly marked either as herbaceous forms or as

Fic. 137.—Diagram of the organization of the stem in Leea, a shrubby tropical
representation of the Vitaceae.

vines are of very little importance in the present connection. A
general feature of the forms included under the two headings here
discussed is the presence of large rays of the oak type. This
situation has long been recognized in the case of the particular
modifications of the stem to be elucidated in the present and in a
following paragraph. The vine type may well stand first, as it is
nearer the ordinary woody perennial condition of organization
than that found in herbs. Fig. 137 illustrates the organiza-
tion of the woody cylinder of Leea, a shrubby tropical genus
belonging to the Vitaceae. Here we find very obviously both

THE STEM ries

compound and primitive rays. In shrubby representatives of
the genus Vitis occurring in the southwest region of the United
States there is sometimes present the same mode of organization
of the rays as characterizes the stem of Leea figured above. In
more northern species of Vitis, however, a very different situa-
tion presents itselfi—namely, that found in Fig. 138. Here the
only rays present are of the compound type. An interesting

Fic. 138.—Diagram of the organization of the stem in a northern species of Vitis

light is thrown on the situation by the consideration of the seed-
ling in the genus Vitis. Fig. 139 presents the stem of the young
individual. There are seen masses of wood representing leaf
traces still in position in the woody cylinder of the stem. The
segments under consideration are clearly bounded on either side
by compound rays, while in the wood are present distinct vestiges
of primitive rays. It is evident that, if the anatomy of the seedling
has any clear bearing on the problem of the origin of the type of
stem presented in our northern species of grapevine, primitive rays
were once present in the intervals of wood bounded laterally by
the large or compound rays, and that these have subsequently been

186 THE ANATOMY OF WOODY PLANTS

lost. The disappearance of the primitive rays from the bundles
of the stem is, in fact, a general feature of organization of the true
vine type and is as plainly indicated, for example, by the Ranun-
culaceae as by the Vitaceae. It is obvious that, as a result of the
considerations elucidated above, the typical vine possesses com-
pound rays, but that the primitive rays have usually quite dis-
appeared.

Fic. 139.—Diagram of the organization of the seedling stem of a northern species
of Vitis.

Very frequently the vine type passes almost imperceptibly into
that of herbs, as, for example, in the genus Clematis among the
Ranunculaceae and the genus Aristolochia belonging to an order
of the same name. It is convenient, however, to take up the ap-
pearance of the herbaceous type in an exemplification where it is
not linked in any way with the habit or conditions of organization
found in the case of vines. The genus Potentilla will serve admir-
ably in the present connection. To reduce the illustrations to the
smallest possible compass, and to make the situation at the same
time clearer, a diagrammatic mode of representation will be

' THE STEM 187

advantageous. In Fig. 140@ is shown the stem of a woody Poten-
tilla (Potentilla palustris). There are five broad segments of the
stem, and alternating with these are five narrower ones. The more
exiguous segments are related to the leaf traces and are charac-
terized, as are the similar regions of the woody axes of Casuarina
and Alnus repre-
sented in earlier GEN pa Lox ANU
illustrations in this Diliag Ze
work, by the con-
spicuous absence
of the vessels pres-
ent in the remain-
ing and broader
sectors. Although
in the outer region
the sectors under
discussion show a
complete absence
of vascular struc-
tures, in the region
near the pith a few
elements of this
nature are present.
These belong to
the leaf trace
proper, which lies
on the inner sur-
face of the narrower segments of the stem. Not only is the leaf
trace segment narrower and distinguished by the absence of vascular
structures, except in the actual trace itself, but it is depressed below
the surface of the adjoining broader sectors. In 6 these features
are indicated on a larger scale, so that the .conditions described
become the more obvious. It will be noted under the more favor-
able conditions of greater enlargement that the cluster of vessels
lying on the inside of the narrow segment and near the pith is
flanked on either hand by a region free from vessels. Inc is shown
the organization of the perennial region of the stem in another

Fic. 140.—Diagrammatic representation of stems of
species of Potentilla. Explanation in the text.

188 THE ANATOMY OF WOODY PLANTS

species of Potentilla, namely, P. intermedia. In this instance the
same outstanding and depressed segments are found as in the
previous species, but the conditions otherwise differ in important
respects. The depressed segments corresponding to leaf traces
in P. intermedia are represented in solid black, which is the
diagrammatic expression of the fact that they are no longer com-
posed of fibrous elements, as is the case with the species discussed
above. The substance of the segments is now, in fact, entirely
parenchymatous, and they are comparable to the compound type
of ray found in the oak and similar forms. On the other hand,
the fibrous regions of the cylinder in P. palustris supply the equiv-
alent of the aggregate rays of the alder and of types which
resemble it.- In d is shown a more magnified view of a part of c.
The cylinder presents three annual rings, which are seen. in both
outstanding and depressed segments of the stem. In the inner-
most annual increment of the depressed foliar region is imbedded
the leaf trace, marked by the presence of vessels. Obviously it
is not only faced by parenchymatous tissue (indicated in black),
but is likewise flanked by the same substance precisely as in 6 the
trace is faced and flanked by the fibrous modification of the wood.
In d we have the thick cylinder resulting from perennial growth
during several years. If the external region of this cylinder were
cut away to such a depth as to reach the surface of the leaf trace,
we should have the condition which is found in an annual stem of the
same plant, such indeed as is shown in e. The topographical con-
sequence of the thinning out of the woody cylinder, as a result both
of less massive development and of the annual habit, is the increas-
‘ing of the relative importance of the leaf traces as components of
the woody cylinder. This means that the storage cells which
originally not only flanked but also subtended the traces are now
confined to the sides or flanks of the foliar traces and, in fact,
separate these from the adjacent segments of the cylinder. This
situation is distinctly shown in e for the cylinder as a whole, and on
an enlarged scale for a single foliar trace and the two adjacent
segments of the woody cylinder in f. It is clear from the diagram
supplied in Fig. 140, which may be profitably compared with that
illustrating the topographical and evolutionary features of the

THE STEM 189

rays on page 82, that the herbaceous stem may result from the
breaking up of the originally continuous woody cylinder through
the formation of special storage devices in connection with traces
of the leaves. This process is far more striking in the more delicate
annual stems by reason of the slender character of the cylinder.
It is apparent that the conditions found in the stem of the oak, etc.,

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Fic. 141.—Diagram of stem of Labiatae. Explanation in the text

as regards the presence of large foliar rays, elucidate the origin of
the herbaceous type.

The situation illustrated in Fig. 140 is not, however, by any
means a universal expression of the topography in herbaceous stems.
It will be well to show further possibilities in this direction by
reference to a family of dicotyledons much higher in the scale than
are the Rosaceae. In Fig. 141a is shown the perennial stem of
the common garden sage, Salvia officinalis. The outside is covered
with an armor of periderm within which lies the narrow cortex,

Igo THE ANATOMY OF WOODY PLANTS

in turn abutting directly on the fibrovascular cylinder. The
latter is circular in outline and shows no variation of structure in
different segments. In 6 an annual branch of the sage is diagram-
matically represented so as to indicate the essential features of
organization. The stem is now square in outline, as is the usual
condition in herbaceous representatives of the Labiatae. It is to
the flat surfaces of the quadrilateral stem that the paired leaves
are attached. An examination of the fibrovascular cylinder shows
that the uniformity of structure characteristic of the round peren-
nial region is no longer present. Opposite the flat surfaces the
cylinder is much thinned out, and vessels are conspicuous by their
absence, except in small clusters marking the position of the leaf
trace, which will pass outward at a higher node. Although the
fibrous modification of the structure of the wood in relation to the
foliar fibrovascular strands is found here as in the case of Potentilla
and Alnus, it seems clear that we have to do with a less primitive
phase of the origin of the herbaceous type. In ¢ is presented a
diagram of the extremely herbaceous stem of Lamium album.
The situation as regards flattening of the stem in connection with
the quadrangular structure characteristic of the axes in the labiates
is virtually the same as in the annual shoots of the sage, with the
difference that rays are no longer seen in the purely fibrous and very
narrow portions of the cylinder facing the flat surfaces and flanking
the leaf traces on either side. In still softer and more herbaceous
stems the fibers in turn may be largely or entirely replaced by
parenchyma, so that the cylinder becomes definitely broken up
into separate strands. Similar conditions in a general way present
themselves in all cases in which the axial organs of dicotyledons
become annual and assume the herbaceous texture—for example,
the Solanaceae, as illustrated by the woody stem of the bittersweet
(Solanum Dulcamara) and the angular soft one of the potato
(S. tuberosum) and of the tomato (S. esculentum). In the two last-
named species the furrows of the stem as well as the fibrous or
parenchymatous regions of the fibrovascular cylinder correspond
in position to the leaves which are a factor in the evolutionary
processes under consideration. The Labiatae and Solanaceae
have been chosen to illustrate the development of the herbaceous

THE STEM IQI

type in higher dicotyledonous orders because the simple foliar
trace, which is a feature of the two groups under discussion, makes
the topographical conditions more easily understood. The Legu-
minosae in the case of the garden bean or, better, of the garden pea,
illustrate the situation appropriately for forms with plural traces.
At the same time the two types just mentioned, as well as the
tomato and the sage, exemplify the principle of recapitulation,
since, in all, the young stem is round and woody and only later
assumes the angular or furrowed configuration of maturity. The
hypocotyledonary region of the stem in the bean, for example, and
the lower region of the epicotyl in the pea both present a circular
and complete woody cylinder. The developmental evidence, as
also that derived from comparative anatomy, thus clearly points
to the derivation of herbaceous forms from woody ones and not
of arboreal perennials from annuals, logically following from the
account of the origin of the structures of the stem in dicotyledons
originated by Sanio and disseminated by Sachs and De Bary.

Another feature of organization of herbaceous stems which often,
although not invariably, appears is the degeneracy of the rays other
than those broad masses of storage parenchyma which, as has been
set forth in the foregoing paragraphs, flank the foliar traces in
their vertical course in the stem. This situation is well illustrated
among the ranunculaceous representatives of the Ranales, for
example, the buttercup, the meadow rue, and the clematis. It
has already been discussed in sufficient detail in connection with
the vine type of stem in a former paragraph and therefore need not
be referred to here.

The discussion of the herbaceous dicotyledons brings us to the
consideration of the stem in the monocotyledons. ‘This important
group of plants, which in physiological efficiency excels all the other
large divisions of vascular plants, is practically entirely herbaceous
in its structure; and even in those of arboreal habit the internal
organization is that of herbs. The monocotyledons on account of
the relative simplicity of their fibrovascular strands have been
referred by many to affinities with the ferns or, on the basis of their
habit, to relationship with the older gymnosperms. These attri-
butions of relationship are, however, little supported by either

192 THE ANATOMY OF WOODY PLANTS

anatomical evidence or reliable data derived from the study of the
remains of plants in the geological strata.

The stem in this large and important group is distinguished both
by the structure of the fibrovascular strands and by their mode of
arrangement in a transverse section of the stem (Fig.142). The con-
tinuous woody cylinder of the older dicotyledons in the herbaceous
types more adapted to modern climatic conditions in temperate
regions has already
given place to a con-
dition of disconti-
mu tty ene tae
monocotyledons the
process of disinte-
gration of continuity
has gone much far-
ther than even in the
herblike dicotyle-
dons. For here the
separate strands
abandon, under the
further influence of
the necessities con-
nected with the leaves, the original circular arrangement for a
scattered disposition through the transverse section of the stem.
The high efficiency of the monocotyledons in the elaboration of
foodstuffs naturally has correlated with it an ample provision
of numerous fibrovascular strands for conducting the assimilates
into the stem or axis. This situation brings it about that many
traces enter the stem at a node instead of the three or, at most,
several traces which pass into the nodal region of the axis in
the mass of dicotyledons. This multiplication of the conducting
strands brings with it complications of arrangement in the stem,
for the number is too large to be accommodated on the periphery
as is the rule even among herbaceous dicotyledons. As a con-
sequence of the necessities which have thus arisen, the leaf traces
are displaced from the margin of the central cylinder into the pith
or medulla. It is thus clear that, just as the woody stem has given

Fic. 142.—Stem of Smilax

THE STEM 193

place to the herbaceous one under the influence of necessities con-
nected with the greater elaborative efficiency of the leaves, so a
still further accentuation of foliar efficiency, correlated with a
corresponding multiplication of foliar traces, has led to the further
modification of the cylinder of the stem, resulting from the necessity
of accommodating a very much larger number of foliar traces in the
transverse area of the cylinder. This is usually effected by moving
the foliar traces from a peripheral position to the medullary region
and more rarely by the running of the foliar traces for one or more
internodes in the cortex. The multiplication of the conductive
strands from the leaves has a further effect in the organization of
the bundles which will be discussed in a later paragraph.

Not only in the arrangement of the fibrovascular bundles do
the monocotyledons differ from the dicotyledons. The bundles
themselves differ in the mass of monocotyledonous forms from the
great majority of the dicotyledonous angiosperms in the fact that
cambial activity is absent. Fig. 143 shows the general topography
of the bundle in Smilax herbacea, the carrion flower. The fibro-
vascular strand is surrounded by a well-marked sclerenchymatous
sheath in all probability corresponding to the pericycle in lower
forms. It is only rarely that in the group under discussion endo-
dermal structures are well developed about the strands in the
stem. Generally a continuous external endodermal sheath sur-
rounds the bundles in common and no internal structures of this
nature can be made out. That the parenchymatous tissue lying
between the bundles in the stem belongs to the fundamental system
can often clearly be inferred from its histological character where its
elements strongly resemble those of the cortex. The internal
organization of the bundle is distinguished by the presence of
phloem and xylem collaterally disposed in relation to one another.
The xylem consists for the most part of vessels and parenchyma,
and the vessels are of the porous type. The elements of the wood
farthest away from the phloem are typical spiral and ringed proto-
xylem. No cambium or zone of growth ordinarily is interposed
between the wood and the phloem, and this condition has gained
for the fibrovascular strands of the monocotyledons the appellation
of closed bundles. It has often been supposed that the closed

194 THE ANATOMY OF WOODY PLANTS

character of the bundles in the group is an indication of affinity
with the pteridophytes, but this view seems to have little value
when it is realized that the earlier vascular cryptogams usually

an anew
Tair rh

SY \ ae
eae

Fic. 143.—Bundle of Smilax herbacea

possessed well-marked secondary growth. ‘Toward the periphery
of the stem in each fibrovascular strand lies the phloem, composed
mostly of sieve tubes and their companion cells. The small
elements of the phloem lying on the very outside and close to the
bundle sheath represent the protophloem and are often in a some-

THE STEM Gk Co):

what swollen condition. The sieve tubes in the monocotyledons
are of a high type and are characterized by terminal walls which are
generally horizontal. The lateral abortive sieve plates, known as
lattices, present on the side walls of the sieve tubes in woody
dicotyledons and many herbaceous ones as well, are usually absent
in the group. Both phloem and xylem consequently represent
a high condition of organization and indicate an advanced phylo-
genetic position for the monocotyle- ;
dons.

In certain monocotyledonous
families cambial activity has been
detected in some instances. This
phenomenon is quite generally pres-
ent in the grasses, as has been pointed
out by Chrysler. In the Liliaceae and
aroids as well as in the Cyperaceae the
presence of a cambium has also been
recorded. Fig. 144 shows the pres- Fic. 144.—Bundle of Erianthus
ence of secondary growth in the region oe ee
of the node in Erianthus Ravennae.

Frequently the bundles of the stem in monocotyledons are of
the closed collateral character, but in some cases, particularly
in rootstocks with closely approximated nodes, they present a
different type, a concentric condition which differs from that
found in the Pteridophyta by the fact that the xylem surrounds
the phloem instead of the phloem forming a continuous girdle
about the xylem. This modification of the bundle is known as
amphivasal to distinguish it from the amphicribral condition charac-
teristic of fern allies. A strand of this nature is shown in Fig. 145.
The phloem occupies the center of the figure and is surrounded
completely by the tissues of the xylem. Bundles of this type are
apparently the result of the crowding and fusion of the foliar
strands at the nodes, for it is in the nodal region that they are best
developed. Where the nodes are remote from one another, the
amphivasal condition may entirely disappear in the internode to
reappear at the next node. With the approximation of nodes,
characteristic of many creeping monocotyledonous rootstocks with

196 THE ANATOMY OF WOODY PLANTS

tufted leaves, the amphivasal condition becomes continuous.
Fibrovascular strands of this type are, in fact, a common feature
of organization of the subterranean regions of many monocoty-
ledons, even when they are absent in the stem. This situation is
well exemplified by the orchids and Iridaceae. In Gramineae and
Cyperaceae, on the other hand, amphivasal strands are present in
the somewhat remote nodes of the annual flowering stems, but
disappear in the internodes,
ee a= and a similar situation is pres-
yer ) 4 AS, ent in the perennial subterra-
nean stem in case the nodes
are not crowded. In the latter
condition the amphivasal state
becomes continuous. In the
true palms (Principes) and in
many Scitamineae the amphi-
vasal bundle seems to have
entirely disappeared, although
this is a matter for further
investigation. There seems to
be little doubt that the forma-
tion of amphivasal bundles in the region of the nodes is an
original feature of organization of the monocotyledons, and that
it persists very strongly in the perennial subterranean stem or
rootstock, but may entirely disappear in the annual stem. The
amphivasal condition of the bundles seems to be definitely cor-
related with the numerous leaf traces passing into the stem at the
node in the monocotyledonous angiosperms. This hypothesis of
its origin is supported by the fact that in herbaceous dicotyledons
with numerous leaf traces the occurrence of amphivasal bundles
at the node can frequently be observed. The Umbelliferae may be
cited as an excellent example of this condition.

The consideration of the stem has now been completed, and
it will be advantageous in conclusion to indicate the main tendencies
manifested in its course of evolution from lower to higher forms.
It has been noted that the first important phase in the evolution
is connected with the primary structures of the fibrovascular region.

Be ai
ag> SB

Fic. 145.—Amphivasal concentric
bundle of rootstock of Smilax.

THE STEM 107

In the stems of lower types these present themselves under two
general conditions: namely, the protostelic and the siphonostelic.
In the former the xylem constitutes a solid mass without any central
pith, while in the latter the fibrovascular complex is thrown into a
hollow cylinder in which gaps are frequently present. These are
~ generally related to certain of the appendages, but may not have
any topographical connection with the lateral parts. In the
siphonostelic condition internal phloem is often seen, particularly
in the lower forms, and the stele or central cylinder is clearly
separated from the central pith or medulla by the presence of an
internal endodermis. In higher types the internal surface of the
tubular stele tends to degenerate, with the resulting disappearance
of internal phloem and endodermis. The original state of the wall
of the hollow cylindrical stele can often be inferred from the
character of the foliar strands which, in accordance with a general
principle to be enlarged upon in a later chapter, often perpetuate
a situation which has disappeared in the stem. Where the foliar
traces are concentric and separated from the fundamental tissues
by a well-marked endodermis, it may be assumed that a similar
condition was formerly present in the organization of the central
cylinder of the axis.

A further interesting phase in the evolutionary development
of the vascular cylinder is to be noted in connection with the
progressive degeneracy of the primary wood. In the lower forms
this region of the xylem is. well developed, and both by its massive
character and by the lack of seriation of its elements it is clearly
distinguishable from the secondary cylinder which surrounds it.
The stem of higher groups presents the primary structures in an
obsolete condition, which results in the bringing of the rays of the
secondary wood into apparent contact with the pith or medulla.
This advanced state of modification of the central cylinder is
responsible for the extremely inappropriate name medullary rays,
which is applied to the radial parenchyma of the secondary wood.
These structures are best called wood rays or simply rays, since
primitively in the stem and always in the root they are quite
divorced from any connection with the pith.

198 THE ANATOMY OF WOODY PLANTS

The next important modification in the case of the fibrovascular
structures of the stem is the appearance of aggregations of rays by
reason of the increased efficiency of the secondary fibrovascular
cylinder in regard both to its conducting and to its storage elements.
The clusters or congeries of rays originally diffused throughout
the woody structure tend in higher groups to become somewhat
definitely related to the appendages. By further progress in the
evolution of radial parenchyma we have the compound and diffuse
types of rays making their appearance, the former particularly
characteristic of herbaceous types, and the latter of the mass of
dicotyledonous forest trees. The appearance of large rays, inter-
rupting the continuity of the central cylinder and bringing about
the differentiation of separate fibrovascular strands, is a particular
feature of the herbaceous type representative of later geologic time
and of cooler climatic conditions.

The woody cylinder broken up into fibrovascular bundles as
the result of the evolution of storage devices in connection with the
appendages in the herbaceous dicotyledons, in the monocotyledons
undergoes further modification in relation to decrease in size and
increase in number of the bundles in correlation with the greater
efficiency and consequently more numerous conducting strands
of the leaves. The decrease in size of the bundles is definitely
related to the loss of the cambial activity which characterized the
fibrovascular structures of the monocotyledonous angiosperms as
a whole. The increase in number of the cauline bundles, resulting
from the entry of very numerous leaf traces into the stem at the
nodes, brings with it the distribution of the fibrovascular strands
throughout the transverse section of the stem. The monocotyle-
dons may, in a sense not altogether figurative, be said to represent
the second childhood of the vascular plants, just as the Pteridophyta
constitute its true infantile phase of development. It is necessary,
however, to distinguish very clearly between the primary structures
of the monocotyledonous angiosperms which are the result of the
loss of secondary growth, and the primary structures of the Paleo-
zoic forms which, so far as we know, are a primitive feature of
organization.

CHAP THe hy
THE LEAF

A general definition of the leaf or foliar organ has been supplied
on an earlier page. The leaf is important, not only on account
of its features of structure as an organ of vegetation, but also
because of the primitive and intimate relation between it and the
organs of reproduction. The latter parts will be discussed in two
chapters following
the present one,
while the main
features of organi-
zation of the leaf
itself will occupy
attention in the
DEECSEME “CONnMEC-
tion. The anat-
omy of the fohar
organs of the cryp-
togamic forms,
living or extinct,
need not be con-
sidered at this
stage, as any refer-
emcees to, them
which are neces-

Fic. 146.—Leaf bundle of Cycas revoluta. Explana-
tion in the text.

sary will appear in
later chapters dealing with particular groups.

We may conveniently begin the discussion of the leaf with the
most primitive type of living gymnosperms, the Cycadales. This
group, much reduced in numbers under modern conditions, has a
type of organization in the leaf which is of great interest from the
evolutionary standpoint. Fig. 146 presents the transverse view of
one of the bundles of the main axis or rachis of the leaf in Cycas

199

200 THE ANATOMY OF WOODY PLANTS

revoluta. ‘Toward the lower side of the figure is seen the phloem,
composed of elements which in the main are arranged in regular
radial rows indicating an origin from cambial activity. The upper
region of the bundle is composed of xylem, consisting of very large
elements which become progressively smaller and fewer in number
in the downward direction. The lower point of this aggregation of
cells is the protoxylem. This region is surrounded inferiorly by
a few rows of parenchyma which give place in turn to more

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Fic. 147.—Transverse section of leaf of Cycas revoluta. Explanation in the text

thick-walled elements of the xylem, in contact with the regular
radial files of the cambium. The lower region of the xylem is the
so-called centrifugal xylem; the upper portion broadening from the
narrow protoxylem is the centripetal or cryptogamic xylem. The
first-mentioned group of elements of the xylem are pitted in their
character, as will be made clear in a subsequent longitudinal illus-
tration. The latter, the so-called cryptogamic wood, is of impor-
tance from the evolutionary standpoint because it indicates at once
a clear relationship on the part of the Cycadales to the vascular
cryptogams and to the lower and extinct gymnosperms. In the
next figure (Fig. 147) is represented a transverse section of one of the

THE LEAF 201

divisions of the leaf. The cryptogamic wood is here relatively bet-
ter differentiated than in the axis of the leaf, and it is obvious that
the seriation of its elements is toward the upper surface of the foliar
organ. The centrifugal wood is very slightly developed. In the
next figure (Fig. 148) appears a foliar bundle of Cycas revoluta in its
course in the cortex of the stem. There is a mass of irregularly
arranged primary wood in the center, surrounded by the radially
disposed second-
ary wood, which in
turn is followed by
the tissues of the
phloem. The phloem
entirely surrounds
the bundle, and this
is consequently of
the concentric type
often found in the
living ferns and their
allies. The contrast
in organization of
the bundles of the
leaf in different
parts is very strik-
aes The lower Ror Fic. 148.—Concentric foliar bundle from the cortex
centric region, IN of Cycas revoluta.

accordance with

generally accepted principles of comparative anatomy to be eluci-
dated later in a special chapter, may be regarded as supplying
evidence of the former concentric character of the woody cylin-
der of the stem in the Cycadales, a hypothesis entirely justified
by the existence of a remarkable group of fernlike seed plants
in the Paleozoic, known as the Medulloseae, from which on both
anatomical and reproductive evidence the living group seems to
have been derived. In these forms the bundles of the stem were,
as will be subsequently shown, often concentric in their organiza-
tion. In the higher region of the leaf trace in the cycads the
concentric character is lost, but the centripetal development of

202

THE ANATOMY OF WOODY PLANTS

part of the wood still sufficiently vouches for the cryptogamic
affinities of the lowest living gymnosperms. In Fig. 149 is
shown the longitudinal view of the leaf trace in the rachis of the

leaf. To the right appears the phloem made up of elongated ele-
ments, the sieve tubes. To the left of this region lie certain pitted

tracheids belonging to the centrifugal or modern wood. Then inter-
vene a few parenchymatous cells, followed by the spiral elements of
the centripetal or cryptogamic wood. ‘These pass farther to the left

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Fic. 149.—Longitudinal section of a foliar bundle in Cycas revoluta
into ringed, scalariform, and, finally, pitted tracheids. The struc-
ture of the leaf in the Cycadales is clear evidence of their rela-
tionship with the ancient fernlike gymnosperms, which in turn were
doubtless in filiation with actual ferns in the remote geologic past.
The Cycadales have their nearest affinities in the past with a
group of gymnosperms, the Cycadofilicales or the Pteridospermeae,
which were so strikingly fernlike in their habit that until the very
end of the last century they were regarded as ferns. In contrast
to the Cycadales the mass of surviving gymnosperms have taken
their origin from forms which are known as Cordaitales. These
had nothing in their habit which recalls the ferns, but often

THE LEAF 203
present a strong superficial resemblance, particularly in the con-
formation and venation of their leaves, to the monocotyledons.
The Cordaitales had leaves varying in width, but always char-
acterized by few or numerous parallel veins. The structure of
the fibrovascular bundles constituting these veins, however, was
entirely different from the features of organization found in the
nerves of the leaves of living monocotyledons. Fig. 150 illus-
trates the general organization of the leaf in Cordaites princi-
palis. The veins are numerous and separated from one another
by intervals of the soft mesophyll of the leaf. The epidermis is

EET BS =
Wag: AGT Oo
Ms res

Fic. 150.—Transverse section of the leaf of Cordailes principalis

strengthened, particularly in the region of the fibrovascular
bundles, by hypodermal bands of. thick-walled cells. In Fig. 151
is shown an enlarged transverse section of one of the foliar bundles
of the species under consideration. The wood was entirely cen-
tripetal in its development and ended on either flank in a band of
thick-walled elements with narrow central cavities. The zone of
thick-walled elements was in contact externally with a second zone
made up of cells with thinner walls and much larger lumina. Both
the zones just described inclose the phloem, which lies below the mass
of centripetal wood. The longitudinal view (Fig. 152) throws addi-
tional light on the organization of the bundle. The double sheath
connected with the flanks of the xylem and forming a complete circle
about the phloem is composed of elongated elements with bordered
pits on their walls. These are known as transfusion cells and are

204 THE ANATOMY OF WOODY PLANTS
an important and phylogenetically interesting component of the
foliar fibrovascular strands in many gymnosperms.

They are not
a salient, and certainly not a primitive, feature of organization of

iS
Ze
%

~
SS

Fic. 151.—Transverse section of the leaf bundle in Cordaites principalis
Medulloseae.

the leaf bundles of the living Cycadales and their allies, the Paleozoic

The transfusion sheath of the foliar bundles in the

Cordaitales did not always manifest the complexity that it shows
in the case of C. principalis figured above.

In most instances only
the broader and shorter thin-walled elements were present, the

THE LEAF 205

elongated and thicker-walled inner sheath being absent. The
intimate connection between the centripetal or cryptogamic wood
and the transfusion tissue is observed in all cases. It is clear
accordingly in the Cordaitales that the transfusion tissue which, as
will be subsequently shown, plays an interesting réle in the evolu-
tionary history of higher gymnospermous groups is primarily related
to the centripetal or cryptogamic wood.

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Fic. 152.—Longitudinal section of a leaf bundle in Cordaites principalis

The organization of the leaf in the conifers will next occupy our
attention, since the present chapter deals with the leaf only in the
features which are of general evolutionary importance. Fig. 153
shows an enlarged view of the whole leaf of Pinus strobus, the white
pine. The epidermis, reinforced by a hypoderma, surrounds the
median green part of the leaf, the mesophyll, in which lie resin canals
and a single fibrovascular bundle. The latter, exceptionally for the
conifers, is marked off from the surrounding fundamental tissues
by a well-defined circular endodermis. The fibrovascular strand
consists of xylem entirely centrifugal and mostly secondary in its
origin, and this meets with the phloem on its lower border. Sur-
rounding the conducting strand of the leaf is a mixture of ordinary

206 THE ANATOMY OF WOODY PLANTS

parenchymatous elements containing protoplasmic structures and
short empty cells with bordered pits. These are the transfusion
elements, and it is easy to observe that they are most abundant on
the flanks of the xylem and, in fact, take their origin from this
general region. As in the case of the Cordaitales, the cells of the
transfusion tissue surround the phloem as well as the xylem. An
important difference, however, is the absence of intermingled

{AS
SAS ;
LEV Poy

Fic. 153.—Transverse section of leaf of Pinus Strobus

parenchymatous elements in the girdle of transfusion tissue of the
older group. Transfusion tissue is better developed in the leaf
of the pine than in any other of the subtribes of conifers. In most
of the other subgroups of the Coniferales the transfusion tissue is
mainly confined to the flanks ef the fibrovascular bundle, and the
conducting strand as a whole is not sharply separated from the
remaining tissues of the leaf on account of the absence of an endo-
dermis.

THE LEAF 207

A link between the conditions found in the case of Pinus and
the anatomical organization of the Cordaitales is presented by a
mesozoic fossil form allied to Pinus to which the name Prepinus has
been given. Fig. 154 illustrates the organization of the leaf in this
species. It is bounded by flat surfaces as a result of contact with
surrounding leaves of the many-leaved fascicle. The hypodermal
structures are strongly developed as in the Cordaitales and con-
stitute the same strengthening ribs as are found in the leaves of

Fic. 154.—Transverse section of leaf of Prepinus statenensis

that genus. On account of the changes resulting from fossiliza-
tion the endodermis is not so distinct as in living pines, and the
mesophyll or soft substance of the leaf is very poorly developed.
The bundle is surrounded by a very dense transfusion sheath in
which all the cells are empty and provided with bordered pits.
Not only is the transfusion sheath massive in Prepinus and entirely
composed of transfusion tracheids to the exclusion of paren-
chymatous elements, but it is further complicated by the presence
of an internal thick-walled zone comparable with the similar
structure in C. principalis figured above. In the comparatively

208 THE ANATOMY OF WOODY PLANTS

low magnification shown in the photomicrograph the inner zone
of the transfusion sheath appears merely as a dark boundary sur-
rounding the fibrovascular bundle proper. The conducting strand
of the leaf is represented by the xylem alone, the phloem having
disappeared during fossilization. In Fig. 155 the central region of
the foregoing is represented under a higher degree of magnification.

Fic. 155.—Portion of leaf of Prepinus statenensis, more highly magnified

The tracheary character of the transfusion sheath can now clearly
be discerned, as well as the fact that it contains no cells of the
nature of parenchyma. The narrowness of the cells constituting the
inner transfusion sheath is also now quite apparent. In Fig. 156
are shown the various structures of the fibrovascular bundles lon-
gitudinally and on a still higher scale of magnification. The outer
transfusion sheath is composed of elements with distinct and
rather large bordered pits which abut inwardly on the narrow
thick-walled cells of the inner transfusion sheath, in turn connected

THE LEAF 209

with the xylem of the fibrovascular bundle. Before we consider
the nature of this relation it will be well to examine more carefully
the bundle itself. The conductive strand is represented by its
xylem only; this consists, interestingly enough, of two regions—
a lower one in contact with the empty space once occupied by the
phloem, and an upper, consisting of distinct rows separated by
thinner-walled elements. The latter is the true centripetal or

Fic. 156.—Longitudinal section of the leaf of Prepinus statenensis. Explanation
in the text.

cryptogamic wood which in Prepinus alone among the conifers is
present in a typical form. The lower region of the xylem is cen-
trifugal and corresponds to the whole of the xylem in the leaf of
the modern Pinus. In the modern or centrifugal region of the
wood, rays are distinctly present, and these pass from its substance
into the cavity once filled by the phloem. The centrifugal region
of the wood provided with rays is related to the inner transfusion
sheath on its flanks alone. This relationship corresponds in fact
to that observed in the case of living pines, as is shown above in

210 THE ANATOMY OF WOODY PLANTS

Fig. 153. The most significant relation of the xylem in the leaf trace
is by means of the series of centripetal or cryptogamic elements.
These are numerous and serve to bring about a very intimate and
copious connection between the bundle proper and the transfusion
tissues. There is good reason on the basis of the anatomy of
Prepinus and the Cordaitales to regard the transfusion tissue which
characterizes the foliar organization of all but the very lowest of
the gymnosperms as
a product. of the
differentiation of the
centripetal or cryp-
togamic wood. In
the true pines of the
Cretaceous,the
centripetal wood
was absent, as it is
in modern species of
the genus, but the
internal transfusion
sheath was often
well developed, thus
showing a clear filia-
tion with Prepinus.
Where the centrip-
etal xylem has dis-
appeared, the relation between the bundle and the transfusion tissues
occurs on the flanks of the centrifugal wood. It will be obvious
to the reader that the tissues surrounding the fibrovascular bundle
in the conifers and their allies are of considerable interest from
the evolutionary standpoint. The transfusion tissue at the present
time has a significance in the vegetative leaves of all but the lowest
gymnosperms in connection with the storage of water and the con-
ducting of it to the green cells of the mesophyll. In the repro-
ductive leaves it has taken on another but equally interesting
function, as will be indicated in the next chapter.

Transfusion elements are found in the leaves of all seed plants
from (and excluding) the cycads upward. In the conifers they are

Fic. 157.—Leafy twig of Casuarina equisetifolia (after
Solerderer).

THE LEAF 21%

well developed, but are distinctly degenerate in the higher subtribes
of the group and are less well developed in any living conifers than
they are in Prepinus and Cretaceous species of Pinus. The
Gnetales often show the transfusion tissues in a high degree of
development. The small leaves of Ephedra naturally show them
less distinctly than the large persistent foliar organs of Welwitschia,
where they constitute a very conspicuous feature of the organiza-
tion of the leaf. In Gnetum, again, in accordance with its higher

AN : C \
di Wot
Wehie.

Fic. 158.—Base of leaf of Casuarina equisetifolia, showing transfusion tissue

systematic position, the transfusion sheath is Jess conspicuous.
Among the angiosperms transfusion tissues are present in the
dicotyledons, but present themselves in the condition typical for
the higher gymnosperms only in the genus Casuarina. Here, as is
shown in Fig. 157 illustrating the organization of a leafy twig of
Casuarina equisetifolia, there are clusters of thick-walled empty
cells flanking the leaf traces. This relation to the foliar strands
strikingly resembles that found in the higher gymnosperms and
appears to be good evidence of the primitive position of this interest-
ing genus. In Fig. 158 is shown one of the foliar bundles of the

THE ANATOMY OF WOODY PLANTS

22

genus under discussion much more highly magnified. The flanking
relation of the transfusion elements to the strand of xylem is now

very distinct and recalls that found in the leaves of the Cupres-

Less typical manifestations of the develop-

ment of transfusion tissues in the dicotyledons are provided by
those forms in which bundles related to stomata exuding fluid
water, and consequently known as water-stomata, terminate below

sineae, Taxineae, etc.

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Fic. 159.—Transverse section of leaf of Alnus incana

the stomatic pores in a mass of wide, short, tracheary elements.

This condition, although doubtless derived from ancestral gymno-

spermous structure, has departed so far from the original transfusion

tissue that it can scarcely be included in the same morphological

category.

Transfusion tissue, as will be apparent from the last paragraph,
has become a feature of very subordinate importance in the organi-

zation of the leaf in the mass of the dicotyledons.

The general

structure of the foliar organs in the group may profitably occupy

attention at this stage.

Fig. 159 reproduces somewhat diagram-

THE LEAF 213

matically the anatomical features of the leaf of Alnus incana.
Above is a layer of cells containing only protoplasm and a nucleus.
A similar situation is presented by the lower surface of the leaf,
with the exception that the continuity of the epidermis is locally
perforated by stomatic openings. The elements related to these,
the guard cells, are distinguished from the rest of the epidermal
layer by the presence of chloroplastids. The upper epidermis is
contrasted with the lower, not only by the absence of stomata, but
also by the presence of a rough impervious covering, the cuticle.
Another feature of interest in the superior epidermal cells is the
mucilaginous modification of their inner walls, and this is expressed
in the illustration by a thick layering. The central portion of the
leaf is occupied by the mesophyll, composed of the palisade (upper)
region and the spongy (lower) region. The development of palisade
parenchyma in foliar organs is definitely related to the amount of
insolation or exposure to light, while the spongy layer is less well
developed when the leaf is strongly illuminated, and becomes much
more accentuated in foliar organs exposed to shade and a damp
atmosphere. The leaf of the monocotyledons supplies little in a
general way which is of interest from the standpoint of evolutionary
anatomy, except the occasional persistence of cambial activity.

CHAPTER XV
THE MICROSPORANGIUM

The microsporangium of the vascular plants is considered first;
because there can be no question that it is the primitive type of
sporangial structure for the long series of forms which are character-
ized by the possession of water-conducting tracheary tissues. In
the lower representatives of the Vasculares the microsporangium
is the only type present, and in the heterosporous cryptogams and
the seed plants it keeps its place, with little change of its original
condition of organization, side by side with the highly modified
megasporangium and seed. The relative constancy of microspo-
rangial structures makes them in many respects of the greatest
value from the evolutionary standpoint.

If the liverworts are correctly regarded as the forms nearest
to the Pteridophyta in the series of the bryophytes, there can be
little doubt that the sporangium in its primitive form of sporogo-
nium is the forerunner of the sporophyte of the vascular series.
Professor Bower has brought forward an impressive aggregation
of evidence in favor of the hypothesis that the sporophyte is the
result of progressive sterilization of sporogenous tissue. Although
the definite mode by which the simple sporogonium of the thallose
liverworts gave rise to the sporophyte of the Pteridophyta, so
complicated in its internal structure and external organization, is
highly speculative, it will serve a useful purpose to indicate the
main probabilities in this connection based for the most part on
the investigations of Leitgeb. In certain liverworts, such as, for
example, Corsinia and Boschia, the spore sac gives rise to sterile
cells as well as spores. There is clear evidence in these and in
similar cases that the sterile cells are modified or, as Professor Bower
expresses it, sterilized potential sporogenous cells. In many
liverworts the sterile cells are useful in distributing the spores.
In this instance they are much elongated and have their walls
spirally thickened. The spirals recoil when the spores are ripe,

214

THE MICROSPORANGIUM 215

giving them an impetus which scatters them over the surface of
the ground. These spore-distributing mechanical cells are known
as elaters. In certain liverworts, such as Pellia and Aneura,
the elaters, in addition to occurring loosely among the spores
which they serve to scatter, are aggregated in a compact elon-
gated mass at one end or the other of the spore sac or theca.
This longitudinal cluster of elaters may be regarded with some
degree of probability as the prototype of the fibrovascular bundle
of the Pteridophyta and higher groups of vascular plants. In
Anthoceros and allied forms the cluster of elaters, known as the
columella, becomes a much more important structure and traverses
the sporangium from end to end. Laterally at intervals it gives
off transverse ramifications which divide the mass of spores into
separate clusters, and these may perhaps be regarded as the proto-
types of the sporangia found in the vascular series. The situation
in the horned liverwort Anthoceros, in which there is an extensive
columella with lateral ramifications, gives some support for the
hypothesis of the derivation of the sporophyte from the sporogonium
by the sterilization of potential sporogenous tissues. The mode
in which the organs, leaf, stem, and root arose from such a primitive
condition of organization is much disputed, since none of the hypo-
thetical transitional forms between the moss capsule and the sporo-
phyte of the higher plants have yet been discovered. There can be
little doubt, however, in a general way that the sporogonium is the
forerunner of the sporophyte and that the elater is the prototype
of the tracheid in vascular plants.

It will be clear from the foregoing statement that the sporangium
is so intimately involved in the primitive organization of the most
ancient spore-producing members that it is entirely proper to
consider it a definite organ of higher plants on a footing of equality
with the root, stem, and leaf. If this view of the matter is sound,
obviously no very useful purpose can be served by the examination
of the development of particular cells of the foliar organs which are
in some way or other related to the origin of sporangia. It is
likely, moreover, that the sporangia of the Pteridophyta give us on
the whole a less accurate picture of the original type of sporangium
than those of the lowest seed plants. It is therefore appropriate

216 THE ANATOMY OF WOODY PLANTS

in the present connection to begin the discussion of the sporangium
with the consideration of the situation presented by the lowest
living gymnosperms, the Cycadales. Fig. 160 shows the organiza-
tion of a microsporangium in Zamia muricata. The structure in
question is covered on the outside by an envelope of thick-walled
cells which in the condition of maturity determine its dehiscence.
The mechanical structure is known as the annulus and is of great

importance in bringing about the

Ase WO) re Ny Uy) Up distribution of the spores, particu-

Syesia ~AN fy // Yj} larly in the lower Vasculares, in

8 which it takes the place of the
elaters found in many of the liver-
worts. The annulus is plainly an
epidermal structure, both because
it is actually the external layer of
the sporangium and because its con-
tinuity is interrupted by the pres-
ence of stomata. ‘These can be seen
in the figure in profile view.

The situation in regard to the
annulus in the Pteridophyta may be
briefly summarized. In lower forms
the thick-walled epidermal cells
which serve as the mechanism for
the opening of the sporangium are
massive in their development, while in the higher forms of the
vascular cryptogams the amount of mechanical tissue tends to
become more and more restricted. Fig. 161a shows the structure
of the sporangium and its annulus in Selaginella. The mechanical
layer in this case is extensive and is almost coextensive with the
surface of the spore sac. In contrast to the conditions shown in
Selaginella are those presented by many of the ferns. In Fig. 161)
is reproduced the organization of the sporangium of Polypodium
vulgare as an illustration of the higher type of annulus in the
Pteridophyta. The opening mechanism here constitutes an incom-
plete vertical ring and in consequence literally merits the name of
annulus. The further consideration of the types of annulus

Fic. 160.—Sporangium of
Zamia muricata.

THE MICROSPORANGIUM 207

presented by those forms included under the general heading of
Pteridophyta need not occupy our attention in the present con-
nection, important as these structures are from the standpoint of
taxonomy and the evolution of particular groups.

Returning to the seed plants, we find that the Cycadales are
the only living forms in which the organization of the mechanical
tissues of the sporangium corresponds with that generally existing

Fic. 161.—Sporangium of Selaginella spc. and of Polypodium vulgare

in the Pteridophyta; and in accordance with this general situation
it will be made clear that the epidermis has not an important rela-
tion to the distribution of the microspores of plants producing
seeds. The interesting genus Ginkgo will serve to illustrate advan-
tageously the situation for the lower living seed plants. Fig. 162
shows the organization of one of the two sporangia of the micro-
sporophyll of this genus as viewed in longitudinal vertical section.
Clearly the cells of the epidermis are thin-walled and can perform
no important office in the openings of the spore cavities. Beneath
the epidermis is found a broad zone of cells provided with barred

218 THE ANATOMY OF WOODY PLANTS

thickenings in their walls, resembling, in fact, short tracheids with
reticulate thickenings. By following the mass of reticulate mechan-
ical cells to the proximal end of the sporangium we find that they
are continuous with, and pass by imperceptible transitions into,
transfusion elements related to the fibrovascular bundles of the
sporophyll. In Ginkgo it is evident that the opening mechanism
of the sporangium is a derivative of the fibrovascular system, and
does not take its origin from
the cells of the epidermis, as
is the case with the annulus of
the Cycadales, and forms lower
in the scale of vascular plants.
The situation is so important
in this respect that it is worthy
of being given a special nomen-
clature. In those types in
which the dehiscence depends
on epidermal mechanisms, in-
cluding the Pteridophyta and
the very lowest seed plants,
the term ectokinetic may be ap-
plied to the sporangium. On
the other hand, in the long
series of forms beginning with
Ginkgo and ending with the
monocotyledons, in which the opening mechanism is of internal
origin and related to the fibrovascular system, the designa-
tion endokinetic definitely indicates the origin of the apparatus
involved.
It is necessary to examine more in detail the conditions found
in the walls of the sporangia of the forms above Ginkgo. In Fig. 163
is represented a longitudinal vertical section of the microsporan-
gium in Pseudolarix Kaempferi, a representative of the Abietineae
or pinelike conifers. Here the situation resembles in a general way
that found in the case of Ginkgo, for the opening of the sporan-
gial sac is due to the presence of reticulately thickened cells which
are likewise related to the fibrovascular system of the trace of the

aE ArH
reeset
Y same

Fic. 162.—Sporangium of Ginkgo biloba

THE MICROSPORANGIUM 219

microsporophyll. As in Ginkgo, the transition from the mechan-
ical tissue to the tracheids of the fibrovascular bundle is effected
by transfusion elements. An interesting difference between the
situation found in Ginkgo and that illustrated in the Abietineae and
other conifers is the fact that the mechanical elements invade the
epidermis in Pseudolarix, but fail to do so in the more primitive
genus. In other members of the Abietineae, particularly where
the wall of the sporangium is very thin, the mechanical tissue
becomes correspond-
ingly reduced in [\
amount and no IN
\

longer shows any Kh
clear relation to the 8
fibrovascular sys-
tem. In the sub-
tribes of Coniferales
above the Abie-
tineae this condition,
in fact, becomes the
rule, and so abortive
does the mechanical
layer become that it is represented by the reticulately thickened
cells of the epidermis alone. It thus results that the opening of the
sporangial cavity is once more effected by superficial cells, but the
situation here represented should be carefully distinguished from
that in the Pteridophyta and Cycadales, since it is the result of the
invasion of the epidermis by mechanical tissues of fibrovascular
origin. Subsequently, when the dehiscing mechanism was reduced,
the epidermis once more became secondarily the seat of the opening
device. It is not without significance in this connection that the
araucarian conifers, which are often regarded as the lowest, present
the sporangial arrangements of the mass of conifers in which the
dehiscing mechanism is reduced to its lowest terms, and not that
luxuriant and apparently primitive condition exemplified by Ginkgo
and the Abietineae.

In the Gnetales the small sporangia are not characterized by the
presence of a very well-developed opening device. The situation

Fic. 163.—Sporangium of Pseudolarix Kaempferi

220 THE ANATOMY OF WOODY PLANTS

in the case of the angiosperms, however, is different. In Fig. 164,
showing transverse sections of the anthers of a tulip and a honey-
suckle, the mechanical tissues are clearly differentiated and occupy
an entirely internal position precisely as in Ginkgo. There is a
very important difference between the situation presented by
the angiosperms, whether dicotyledons or monocotyledons, and
Ginkgo and the Abietineae. In the higher group the mechan-
ical tissues, constituting the so-called fiber layer of the anther

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wall, have no longer any relation to the fibrovascular bundles of
the filament. In the case of the angiosperms the relation once
existing between the fibrovascular system and the opening mechan-
ism has apparently been lost. The dehiscing apparatus is, however,
still in a good state of development and in this respect contrasts to
the situation presented by the Gnetales and the higher Coniferales.

The structure of the walls of the microsporangia of the vascular
plants from Ginkgo upward is highly interesting from the stand-
point of the doctrine of descent. In the lower members of this
series the opening device of the sporangium is clearly in relation to
the transfusion tissue connected with the fibrovascular bundles of
the reproductive leaves. In the Cycadales, the lowest living seed

THE MICROSPORANGIUM 221

plants, we find a complete absence of typical transfusion tissue in
the leaves, although its presence has been erroneously described
for the group. The cycadean gymnosperms in the absence of
foliar transfusion tissue resemble the true ferns, which are also
characterized by the exclusion of tracheary tissues belonging to
this category from their foliar organs, whether vegetative or
reproductive. It is highly significant that an epidermal sporangial
mechanism and the absence of transfusion tissue are features which
alike mark the Pteridophyta and the seed plants most nearly allied
to these. Beginning with the Ginkgoales and proceeding upward,
we find transfusion tissue progressively taking the place of the
centripetal or cryptogamic wood in the vegetative leaves; and in
the reproductive leaves the transfusion tissues or structures
definitely associated with them assume the function of opening the
sporangium at the time of the ripening of the spores. The cor-
rectness of this interpretation of the situation is best seen in
Ginkgo, in which in the lower region of the sporophyll the trans-
fusion tissues are developed very much after the manner in which
they present themselves in the case of the vegetative leaves.
In the upper region of the sporophylls, bearing the microsporangia,
the transfusion elements grade imperceptibly into the reticulately
thickened mechanical tissues of the sporangial walls. In the sub-
tribe of the conifers which is beginning to assume importance as a
candidate for the most primitive phylogenetic position in the group
(namely, the Abietineae), we find the transfusion zone not only well
developed in the vegetative leaves of both living and fossil repre-
sentatives, but likewise occurring under highly significant condi-
tions in relation to the sporangial mechanisms. In the remaining
gymnosperms the mechanical tissue shows a strong tendency to
become reduced in amount and loses all direct relationship to the
fibrovascular tissues proper. In the angiosperms, as has been
pointed out above, the fiber layer characteristically present in the
anther wall is well developed, but no longer has any relation what-
soever to the fibrovascular system.

In conclusion, it may be stated that the opening mechanism of
the sporangia of the Pteridophyta and of the lowest gymnosperms
is epidermal in its origin, while that of the seed plants from Ginkgo

222 THE ANATOMY OF WOODY PLANTS

upward is clearly derived from transfusion tissue. This category
of tissue is the final stage of persistence of the protean centripetal
or cryptogamic wood of the lowest vascular plants. In the angio-
sperms the mechanical tissues in the walls of the anthers exemplify
the highest level of survival of the old centripetal wood of the
Pteridophyta and the lowest gymnosperms. In fact, the so-
called fiber layer of the anther in the case of the angiosperms
supplies the clearest instance of the persistence of this ancestral
structure outside of that most conservative of all organs, the root,
in which, as has been made clear in an earlier chapter, it still
maintains its pristine development in the primary organization.
The two mechanisms correlated with sporangial dehiscence pre-
viously described may appropriately be designated as ectokinetic
and endokinetic.

CHAPTER XVI
THE MEGASPORANGIUM AND SEED

In the Pteridophyta the phenomenon of heterospory has de-
veloped in many different groups. The result of the realization
of this condition has been the appearance of smaller sporangia
producing numerous small spores known as microspores and of
larger ones giving rise to a few large spores designated as mega-
spores or macrospores. In the case of the sporangia which give
rise to megaspores, or the megasporangia, the conditions connected '
with opening are the same as those exhibited by the mass of Pteri-
dophyta; in other words, the spore sacs are ectokinetic and owe
their dehiscence to the activity of a mechanical layer derived from
the epidermis. As this situation has been sufficiently discussed
in the previous chapter, it will not be profitable to return to the
matter here. Megasporangia in the proper sense of the word,
wherever they occur, are ectokinetic.

It has been recognized since the times of the great German
morphologist Hofmeister that seeds represent modified mega-
sporangia. This view of the origin of seeds is justified, not only
by their anatomical structure and by the cytological conditions
observed in the development of the endosperm, but also by the
actual persistence of the megaspore membrane in the seeds of
many of the lower gymnosperms. If any remaining doubt existed
as to the origin of seeds from megasporangia, it would be removed
by the discovery of certain interesting structures in the case of
Paleozoic lycopods which present at the same time many of the
distinctive characteristics of megasporangia and seeds. So strik-
ingly does one of these (described by Scott) resemble a seed in its
external appearance that it was for a time actually regarded as one,
until its internal organization revealed its anomalous character.
Fig. 165 illustrates the vertical section of Lepidocarpon Lomaxt,
the seedlike fructification of a lepidodendrid. Internally is shown
a mass of cells, the gametophyte, surrounded by a heavy dark line,

223

224 THE ANATOMY OF WOODY PLANTS

the section of the megaspore membrane. In addition to the
germinated megaspore shown in the figure, three other abortive
megaspores make their appearance at an earlier stage, and these
have thicker walls than the functional spore. These are not shown
in the late stage of development appearing in the figure. Not only
does the megaspore retain its thick membrane in the fructification
of Lepidocarpon, but, contrary to the conditions found in typical
seeds, the mechanical layer of
the megasporangium likewise is
well developed and was prob-
ably capable of dehiscence.
The integument with which
true seeds are provided is rep-
resented in this foreshadowing
of seminal structure by the
upfolded edges of the sporo-
phyll. The absence of an
apparatus for receiving the
microspores or pollen likewise
differentiates the fructification
under consideration from the
seeds of even the lowest of the
seed plants.
Fis. 165.—Seedlike sporangium of In Fig. 166 is shown the
Lepidocar pon (after Scott). A ‘
longitudinal view of another
seedlike structure from the Carboniferous of England known as
Miadesmia. Here the resemblance to the real seed is much more
marked than in Lepidocarpon. The sporophyll so completely
involves the megasporangium that only a small aperture is left
which corresponds physiologically, although not morphologically,
to the micropyle of the seed in the true seed plants. Within the
“integument” is inclosed, not only the megasporangium, but also
the ligule. The megasporangium is much less typical than that of
Lepidocarpon, for its mechanical layer fails to develop and it pro-
duces only one spore in contrast to the four that come into
existence in the case of the Lepidocarpon. ‘There is no good reason,
however, to regard the structure here described as representing a

THE MEGASPORANGIUM AND SEED 225

true seed any more than that delineated in connection with the
last paragraph.

An interesting condition has been described by Miss Lyon
in the American species of Selaginella, S. apus, and S. rupestris.
Here the spores germinate and are fertilized within the mega-
sporangia, a condition favored by the fact that the micro-
sporangia are situated in the upper region of the cones. The
microspores undergo development unshed, and on a wet day the
antherozoids to which they S
give rise are able to make
their way to the lower
region of the strobilus,
where the germinated
megaspores present their
archegonia for fertilization.
The development of the
embryo takes place after Fic. 166.—Seedlike sporangium of Miadesmia
the union of the sexual ele- {0 Seward, after Scott).
ments, and the sporelings later grow out among the leaves of the
cone. The situation represented by the two species mentioned
appears to be somewhat general for the genus and throws an
interesting light on the conditions which were probably present in
Lepidocarpon and Miadesmia.

The most ancient types of seeds known to us have an organiza-
tion differing in important particulars from the seedlike structures
described in the two preceding paragraphs. In the first place, the
most antique seeds are provided with a true integument and are
not merely wrapped in the sporophyll as a whole. Secondly, they
present a very important feature in the presence of a so-called
‘pollen chamber”’ which receives the microspores and provides a
fluid in which they may undergo germination and later effect
fertilization.

A primitive type of seed is diagrammatically represented in
Fig. 167. The megasporangium appears within the integument
which covers it almost completely, so that communication with the
outer world is only by a narrow canal at the apex known as the
micropyle. The megasporangium is without any mechanical layer

226 THE ANATOMY OF WOODY PLANTS

such as appears in the seedlike structures described above for the
lycopods. Such a layer was doubtless originally present, but has
ceased to be necessary as a protection on account of the shelter
afforded by the integument; moreover, it could no longer be func-
tionally useful in
opening the spo-
rangium since the
megaspores in the
case of true seeds
are permanently
inclosed. The
elimination of the
ectokinetic me-
chanical layer of
the older seeds
must lie far in the
geological past, be-
cause no evidence
of its presence has
been observed in
the oldest seeds
with the structure
of which we are
acquainted. The
pointed apex of the
megasporangium is
occupied by a
cavity, the pollen
chamber, in which
the pollen grains or microspores come to rest before germination.
This cavity has its capacity much reduced by the presence of a
central column known as the columella. Below the pollen cham-
ber lies the germinated megaspore with its somewhat thickened
megaspore membrane. ‘The membrane incloses the gametophyte,
bearing the archegonia in its upper region. Surrounding the part

Fic. 167.—Diagram of seed of a cycad

of the seed corresponding to the megasporangium and fused with
it, except in the upper region, is the integument, and this consists

THE MEGASPORANGIUM AND SEED 227

of a hard inner layer known as the sclerotesta and a softer outer
one which contains mucilage canals and to which the name of
sarcotesta is applied. Both sclerotesta and sarcotesta are pro-
vided with a system of fibrovascular strands, but tracheary ele-
ments of any kind
are absent in the
region of the nucel-
lus or megaspo-
rangium.

Dre: type ol
seed delineated in
connection with
the foregoing para-
graph is not the
only one charac-
teristic of more
ancient plants. In
Fig. 168 is shown
another category
of seed which,
although present-
ing the general
features of the
Paleozoic type, is
characterized by
certain interesting

and important

peculiarities. The Fic. 168.—Diagram of an ancient type of seed with
integument in the _ ttacheary mantle surrounding the gametophyte (modified

; : after Oliver).
diagram is repre-

sented as consisting of an outer softer sarcotesta and an inner
resistant sclerotesta. It incloses, as in the other type, the mega-
sporangium or nucellus, and this is likewise provided with a
pollen chamber. The only important difference between the seed
under discussion and that described in the preceding paragraph
is the distribution of the fibrovascular bundles. In the seeds
of the first type the tracheary strands are present in both

228 THE ANATOMY OF WOODY PLANTS

sarcotesta and sclerotesta, but are lacking in the nucellus or
megasporangium. In the seed now under consideration a fibro-
vascular envelope surrounds the megaspore and ends upwardly
in the pollen chamber. It is clear in the present instance that
the tracheary tissues invade the megasporangium precisely as
they do the microsporangium of Ginkgo and seed plants higher
in the evolutionary scale. The function of the tracheary tissue
which ends under the pollen chamber doubtless was that of
supplying water to facilitate the germination of the microspores,
and the fertilizing movements of the antherozoids originated
from these. It seems clear that we have in the type of seed
figured in connection with the present paragraph a counterpart
to microsporangia with the endokinetic mode of dehiscence. A
number of ancient seeds of the anatomical organization indicated
here have been investigated, but unfortunately they have not been
connected with absolute certainty with any definite vegetative
types.

The seed of the living Ginkgo throws no light upon the question
of the affinities of the second type of Paleozoic seeds, for, although
tracheary tissues are abundant in the base of the seed, they do not
penetrate into the megasporangium proper. It is likely that
degeneracy of the fibrovascular structures has here obscured the
real situation, since from the organization of the microsporangium
in the genus we should expect to discover tracheary elements in the
walls of the nucellus or megasporangium.

In the seeds of the Cycadales we have realized the general
features of organization depicted in Fig. 167, which is, in fact,
modeled from the young seed of Cycas revoluta. The organization
of the microsporangium in the cycads does not support the hypoth-
esis that nucellar tracheids were once present in the seed of the
group and have disappeared in its modern representatives. The
conservative tendencies of microsporangial structures as contrasted
to those of the megasporangium or seed which is considerably
more rapidly influenced by the course of evolution constitute a
valuable situation from the standpoint of comparative morphology.

The general anatomical features of the seed in the higher gymno-
sperms may next be considered. In Fig. 169 is illustrated the more

THE MEGASPORANGIUM AND SEED 229

important structures of the seed in one of the Abietineae or pine
family. It is provided with an integument in which a small pore
known as the micropyle is present at the apex of the seed. Within
the integument is accommodated the nucellus or megasporangial
portion of the seed. This is not characterized, as is that of the
older and fernlike gymnosperms,
by the presence of a pollen cham-
ber. The pollen grains or micro-
spores are received, in fact, on the
smooth apex of the nucellus and
very soon send out pollen tubes
which bore their way through the
tissues of the nucellus in order to
reach the archegonia, situated on
the apex of the gametophyte.
The absence of the pollen cham-
ber and the presence of functional
pollen tubes are features which
most clearly distinguish the seeds
of the modern gymnosperms from
those of the lower largely extinct
types included under the heading
of the Archigymnospermae. The
nucellus is not provided with a
tracheary mantle, such as is found
in the ovular structures of certain
extinct gymnosperms. The soft
sarcotesta often presented by yy, 169.—Diagram of seed of a conifer
more ancient types of seeds is

likewise conspicuous by its absence in the case of the more typical
representatives of the higher gymnosperms.

In the angiosperms the situation in regard to the seed is still
further modified by the fact that the pollen is no longer received on
the apex of the ovule or young seed, but comes to rest on a special
region of the closed sporophyll known as the stigma (Fig. 170).
The microspores or pollen in germinating send out pollen tubes,
of greater or less length, which penetrate first the tissues of the

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230 THE ANATOMY OF WOODY PLANTS

closed reproductive leaf or megasporophyll, and later enter the ovule,
either through the micropyle (in porogamous forms) or through
the chalaza or breech of the seed (a condition found in the so-called
chalazogamous dicotyledons). Not only do inclosure of the ovules

Fic. 170.—Diagram of a poroga- Fic. 171.—Diagram of a chalazog-
mous dicotyledon. amous dicotyledon.

in an ovary and the consequent exclusion of the pollen from direct
access distinguish the angiospermous seed from that found in
lower forms, but likewise the very considerable reduction in the
amount of gametophytic tissue. In this large group of seed plants
the prothallial portion of the young seed contains typically eight

THE MEGASPORANGIUM AND SEED 231

nuclei, which by the fusion of two become seven. Of these, one
nucleus surrounds itself with a protoplasmic body to become the
egg, while two others are related to the so-called synergidae. Of
the remaining four nuclear structures, three belong to the antipo-
dals, a group of cells present in the base of the embryo sac or
prothallus, while the fourth, the product of the fusion of two nuclei
as mentioned above, becomes the :
so-called endosperm nucleus, which
later develops the endosperm or
food substance of the ripened seed.
In certain of the chalazogamous
angiosperms (Fig. 171), notably
Casuarina and the hazel (Corylus),
tracheids are found present in the
nucellus. The most natural inter-
pretation of this condition is in
connection with the tracheary
apparatus in certain extinct seeds
described in a foregoing paragraph.
If the presence of tracheary tissues
in the substance of the megaspo-
rangium or nucellus in certain
angiosperms is to be interpreted as
the persistence of an ancestral char-
‘acter, it would indicate a relatively
primitive position for the chalazog-
amous forms in which it occurs.
The structure of the mature seed in the angiosperms naturally
claims a greater interest in a work devoted to anatomy. We may
first take the cases of dicotyledons and monocotyledons. Fig. 172
illustrates the organization of the seminal organ of the Indian
corn (Zea). To one side lies the embryo, which is provided with a
single seminal leaf or cotyledon. This is very large in size and has
its inner surface applied broadly to the food substance of the seed
or endosperm. The embryo proper lies to the outside of the
cotyledon and is characterized by the presence of the primary shoot,
or plumule, and the primary root, or radicle. These are inclosed

Fic. 172.—Seed of Zea mais

232 THE ANATOMY OF WOODY PLANTS

in protective sheaths respectively known as coleoptyle and cole-
orhiza. The endosperm and embryo are inclosed by a covering
composed of the fused wall of the ovary and the integuments of
the ovule. On the flatter side of the seed toward which the embryo

Fic. 173.—Seed of Celastrus scandens Fic. 174.—Seed of
Pinus palustris.

is placed a more or less prominent elevation is seen which is the
base of the style.

Fig. 173 is a photomicrograph of a dicotyledonous seed. Here
the embryo occupies a median position and is provided with
two cotyledons instead of a single one. The endosperm or food
substance, as a result of the position of the embryo, entirely sur-
rounds the rudiment of the young plant. In the seed illustrated
the coat consists of the hardened integument, and the wall of the
ovary is not involved in the formation of the seminal covering.

'

THE MEGASPORANGIUM AND SEED 233

The typical condition, of course, for both monocotyledons and
dicotyledons so far as the coat or coats of the seed are concerned is
that shown in Fig. 173, since only rarely does the wall of the ovary
participate in the formation of the protective envelope.

Certain other varieties are presented by the organization of the
seeds of angiosperms. For.example, the endosperm or food sub-
stance may be absent altogether, a condition illustrated by ‘the
legumes and the Compositae among the dicotyledons and by the
Orchidaceae among the monocotyledons. Again, the nucellar or
megasporangial substance, usually absorbed as the development
of the endosperm and embryo proceeds, sometimes persists and is
then known as perisperm. Another variation which may present
itself is the development of a supernumerary integument, often
brightly colored, after the seed has been fertilized. This subsidiary
coat is. known as the arillus and is frequently found in families not
nearly related systematically.

The organization of the seed in the pine is presented in Fig. 174
for comparison with the angiospermous conditions illustrated in
Fig. 173. The integument is clearly distinguished as a hard in-
vestment surrounding the abundant endosperm. The food sub-
stance in the seeds of the gymnosperms is derived directly from the
transformation of the gametophyte and is not a new structure, as
is the case with the endosperm of angiospermous seeds. Within
the substance of the endosperm lies the embryo, with its narrower
end toward the micropylar region of the integument. The more
slender portion of the embryo or young sporophyte is the primary
root, which is capped with more or less lax tissues. These are the
remains of the suspensors which in the developing seminal organ
forced the young embryo down into the midst of the endosperm,
The larger end of the embryonic sporophyte owes its breadth to the
presence of numerous cotyledons or seed leaves, and these dis-
tinguish the embryo of the pine from those of the angiosperms.

CHAPTER XVII
THE CANONS OF COMPARATIVE ANATOMY

With the completion of the consideration of the various tissues
and organs we are in a position to take up the relation of anatom-
ical structure to evolutionary sequence in the various groups of
vascular plants. Before we proceed to this phase of the subject,
however, it will be necessary to consider the general principles or
canons of comparative anatomy. It may be pointed out in this
connection that anatomy, in common with other branches of the
sciences, 1s based on inductive reasoning. The general principles
are consequently arrived at as the result of the consideration of
large numbers of facts with particular regard to the conclusions
which may be drawn from them! The anatomy of plants has
made great progress in recent years and in direct proportion to
our increasing knowledge of fossil forms. The most interesting
and valuable results from the evolutionary standpoint have been
reached in connection with the anatomical investigation of extinct
organisms of earlier geological ages. Naturally those of the great
coal-producing period, the Paleozoic, first received attention on
account of the importance of the study of plants of that age in
connection with the search for productive coal seams. In more
recent years the Mesozoic, which is of the greatest interest in
relation to the appearance of our modern types, has begun to be
studied. The results bearing on the advancement of our knowledge
of the general principles of the evolution of plants exemplified in
their anatomical structure cannot be too highly estimated. It will
be the aim of the present chapter to set forth comprehensively the
main conclusions of anatomical paleobotany in their relation to the
interpretation of the affinities of the main groups of vascular plants
now in existence on the earth.

THE DOCTRINE OF RECAPITULATION

An important general doctrine developed in connection with
the evolutionary study of living beings is the hypothesis of recapitu-
234

THE CANONS OF COMPARATIVE ANATOMY 235

lation. It is assumed in connection with this doctrine that the
young of any species may in the course of its individual development
pass through the phases present in ancestral forms. As examples
of this principle we may take extreme types of vegetation, such as
the almost leafless cacti, forms with phylloclads, or those character-
ized by the presence of short-shoots. In the mass of Cactaceae the
leaves are abortive and are represented at most by spines. Inthe
seedlings, in contrast to the adult, _z,
the foliar organs are distinctly pres- .
ent and are clearly recognizable as
such. In coniferous species belong-
ing to the genus Phyllocladus the
branches in the mature state form
flattened expansions known as
phylloclads. If a seedling of any
species of Phyllocladus be exam-
ined, it becomes clear that a nor-
mal round and leafy axis is present
such as ordinarily characterizes the
conifer (Fig. 175). Likewise in
the pine the seedling shows the
primary leaves arranged on the
stem in the usual fashion for
coniferous gymnosperms and not clustered on short-shoots or
brachyblasts as in the adult branches of the genus. Further,
in a conifer like the larch, which is differentiated in habit from the
mass of the group by its deciduous foliage, we find in the seedling
that the leaves persist for several years, thus revealing the probable
ancestral condition for the genus. An additional example among
the dicotyledons is supplied by the oak. The adult in north-
ern oaks is characterized by deciduous leaves. Oak seedlings and
saplings, however, even in the case of typically northern species,
retain their leaves during the winter, thus recalling a situation
characteristic of the live oaks of warmer latitudes which have
evergreen foliage and represent anatomically the primitive type
of organization.

Fic. 175.—Seedlings and mature
branch of Phyllocladus species.

236 THE ANATOMY OF WOODY PLANTS

The phenomenon of recapitulation is not confined, however, to
external features of organization, for it is often equally well exempli-
fied by internal anatomical structure. A good illustration of the
principle of recapitulation is presented by the seedling of the
araucarian conifers. The adult stem of the kauri, or of any other
araucarian conifer, is characterized by two unique features. One
of these is the persistence of the traces belonging to the leaves long
after the foliar organs have fallen. The foliar fibrovascular strands
are continued for many years, amounting in some cases to centuries,
through the activity of the cambium, even when the surface of the
trunk has long ceased to show even the scars of the leaves of which
they were once the fibrovascular supply. Further, in araucarian
woods there is present a peculiar variety of tracheary structure
which clearly differentiates their ligneous organization from that of
all other living subtribes of conifers. The tracheids in the Arau-
carlineae have their bordered pits arranged in an alternating manner
and not disposed in an opposite fashion, as is the characteristic
condition in the rest of the living conifers. In araucarian woods of
the Mesozoic belonging to the genus Brachyoxylon the leaf traces
persist only for a short time and are no longer formed through the
instrumentality of the cambium after the leaves to which they
belong have fallen from the stem. Again, the pits do not manifest
the alternating and crowded condition presented by the wood of the
living genera. In the seedling of both Agathis and Araucaria
the leaf trace persists only so long as it is related to a functional
leaf, and does not continue to develop for many years after the fall
of the foliar organs, as is the case in the older trunk. Also in the
araucarian seedling the pitting is like that found in the Cretaceous
araucarian genus Brachyoxylon. In this instance we have a striking
exemplification of the principle of recapitulation.

The law or principle under consideration has many illustrations
in the vascular plants, but on the whole it cannot be said to have
so great a validity as among the higher animals. It is further
necessary to note in the present connection that the absence of a
given structure in young individuals is by no means evidence
of its absence in the ancestral forms from which they have been
derived. For example, there is good evidence that the cycadean

THE CANONS OF COMPARATIVE ANATOMY 237

gymnosperms have come from ancestors possessing concentric
bundles and centripetal wood, yet the seedlings of cycads in general
do not support this conclusion by their anatomical organization.
The doctrine of recapitulation is of value, accordingly, when it
presents positive evidence from the seedling for the ancestral
occurrence of a given feature of organization; but negative testi-
mony from this standpoint must be estimated as having little or
no value. A failure to realize this situation is responsible for much
fallacious biological reasoning.

A very important exemplification of recapitulation is frequently
supplied by the first annual ring of the older stem of arboreal forms.
Often in groups which have suffered considerable reduction, such
as, for example, the gymnosperms in general, the phenomenon of
recapitulation, although absent in the seedling, may be clearly
illustrated by the first annual increment of woody growth in the
older regions of the stem. An illustration of this principle is
supplied by the living araucarian conifers. Taking as an example
the genus Agathis, the kauri of Australasia and the East Indian
region, we find in the first annual ring an organization distinctly
different from that in the subsequent annual increments of the
wood. More or less abundant wood parenchyma is present,
although longitudinal storage elements are conspicuous by their
absence in the adult wood of the stem. ‘This situation is of great
interest in view of the fact that the fossil wood of the kauri from
American Cretaceous deposits is characterized by the presence of
parenchymatous cells, not only in the first annual ring, but in all
subsequent zones of ligneous growth. The persistence of the
structure of Mesozoic forms in the first annual ring of living species
of the genus Agathis is a feature most appropriately falling under
the principle of recapitulation. The situation here indicated is of
great value and wide validity, not only for the gymnosperms, but
also for the dicotyledons. It might readily be much more abun-
dantly exemplified in the present connection, but the instance
supplied above will serve to make the situation clear. Many other
cases will present themselves in later chapters in connection with
the discussion of the evolution of the different groups as inferred
from their anatomical organization.

238 THE ANATOMY OF WOODY PLANTS

THE DOCTRINE OF CONSERVATIVE ORGANS

This doctrine has received a great impetus from the study of
Mesozoic conifers, but was first put forward, naturally enough, in
connection with comparisons between the older existing gymno-
sperms and their Paleozoic ancestors. The leaf first came into
prominence in relation to the hypothesis of conservative organs.
It has been known for many years, particularly since the investiga-
tions of Mettenius, that the foliar organs of the Cycadales present
remarkable features of anatomical structure. Here the fibro-
vascular bundles of the leaves are distinguished by the presence of
centripetal or cryptogamic wood, a detail of organization conspicu-
ously absent in the stem of the genera of the living Cycadales. In
the Cycadofilicales of the Paleozoic the bundles of the stem were
always characterized by the presence of centripetal xylem and
sometimes by concentric organization as well. The Cycado-
filicales are by common and competent consent regarded as the
ancestral types from which the living cycads have been derived.
The clear and universal presence of centripetal wood in the foliar
fibrovascular bundles of living genera of the Cycadales is good
evidence at once of the relationship of these gymnosperms to the
Cycadofilicales and of the validity of the doctrine of conservative
organs so far as it applies to the anatomy of leaves. Many other
illustrations of the prevalence of this principle might be supplied
in foliar organs, but the one described above will serve appropri-
ately to elucidate the situation and is particularly apposite in the
present connection because it is probably the first case to be cited
in evolutionary anatomy.

The foliar organs are not, however, the only parts of the adult
plant which present illustrations of the principle of conservative
organs. The stem in that region devoted to the function of repro-
duction has also figured strikingly in this connection. To Scott
belongs the credit of having drawn attention to the fact that the
peduncle or base of the cone in certain Cycadales furnishes a clear
example of the persistence of an ancestral structure in the
reproductive axis which has quite disappeared in the ordinary
vegetative stem. In Stangeria, Zamia, and other genera of the
Cycadales he noted that the fibrovascular strands of the axes

THE CANONS OF COMPARATIVE ANATOMY 239

of the cones frequently manifested the presence of vestiges of
centripetal wood, although xylem belonging to this category has
wholly disappeared in the vegetative axis. The value of this
generalization can scarcely be overestimated. During the interval
of nearly twenty years dating from Scott’s brilliant discovery,
much additional evidence has been supplied in support of the
conservatism of the anatomical structure of the reproductive axes.
For example, it has been shown by comparison of the vegetative
organization of Mesozoic conifers with the anatomical structures
found in the ovuliferous cones of living forms that the latter fre-
quently perpetuate features which have vanished in the vegetative
parts. This is manifestly the case in the abietineous and arauca-
rian conifers which compete with each other for the claim of being
the most ancient representative of the coniferous stock. The cone
in both Pinus and Agathis presents numerous Mesozoic features
which have disappeared elsewhere in the stem organs of existing
species belonging to these genera.

The value of the anatomy of reproductive axes cannot be
estimated so highly in the case of the angiosperms, since the
relatively slight development of fibrovascular structures in flowers
and inflorescences leaves less scope for the appearance of phylo-
genetically significant structures. This situation needs to be
particularly emphasized in view of some recent highly fallacious
attempts to utilize the anatomy of reproductive axes in working
out evolutionary sequences in the woody dicotyledons. Clearly in
this instance only structures are significant which find adequate
development in the somewhat slender annual woody cylinder of the
flowering parts of perennial dicotyledons. In the monocotyledons
the restrictions on interpretation are still greater on account of the
usual absence of secondary growth in this great division of the
angiosperms. In the application of the doctrine of conservative
parts to the reproductive axes of the angiosperms it must be
recognized that a little knowledge is a dangerous thing.

It has been made clear in the preceding paragraphs that the
doctrine of conservative parts is well exemplified by the leaf and
is manifested in the stem by the more archaic axes connected with
reproduction. The situation in the organs cited has for a long time

240 THE ANATOMY OF WOODY PLANTS

been apparent. Of more recent origin is the realization that, in
modern plants at any rate, the root is the most valuable of all
organs from the standpoint of evolutionary anatomy. In the
case of comparisons between Paleozoic and Mesozoic groups the
root, in fact, has proved to be in general too conservative to furnish
significant examples of the retention of ancestral characters. The
search for centripetal or cryptogamic wood has been in the fore-
ground in investigations bearing on the relationship between
Paleozoic and Mesozoic groups. Since in all forms of every geo-
logical age the root has centripetal primary wood, in regard to this
crucial feature of more ancient types it obviously does not supply
distinctive evidence in connection with the doctrine of descent.
In other respects, however, and particularly with reference to the
organization of the tissues of the secondary wood, the root has
proved itself to be of greater significance than any other organ of
the higher plants. Of course the most striking evidence of con-
servatism in the root has resulted from the comparison of Mesozoic
and modern forms. It has, for example, been shown that the root
in modern conifers clearly and persistently perpetuates the features
of organization which characterized the structure of the stem in
the Cretaceous and earlier periods of the Mesozoic age. In Pinus
and Agathis the root presents in many of its earlier annual rings
the distinctive organization found in the stem of these types
in the Mesozoic. Nor is the inherent conservatism of the root of
value in the study of the gymnosperms alone. Although our
knowledge of the anatomical structures of the angiosperms in
Mesozoic times is as yet extremely inadequate, we are able in
many instances by the application of the doctrine of the con-
servatism of the anatomical organization of the root to infer the
ancestral type of stem in the highest vascular plants.

It will be evident from the foregoing paragraphs that conserva-
tism is particularly inherent in the leaf and root of vascular plants
and that the highly progressive stem presents only features which
are of interest from the standpoint of the doctrine of descent, either
in its first annual ring or in axes specially allocated to the function
of reproduction. The sporangium is a fourth structure recognized
as a primitive organ of vascular plants in an earlier chapter of the

THE CANONS OF COMPARATIVE ANATOMY 241

present work. Here the lack of complexity of organization rather
militates against the presence of phylogenetically important
structures. It must nevertheless be noted that the sporangium is
an extremely conservative organ and, as far as the relative simplicity
of its organization supplies points of comparison, is of very great
significance for the doctrine of evolution. It has been demonstrated
in an earlier chapter that the sporangium perpetuates the protean
centripetal xylem in the form of its opening mechanism to a higher
point than any other organ but the root. The value of the spore
sac in phylogeny, although limited by the relative simplicity of
the organ,-must consequently be estimated as great.

The doctrine of conservative organs is of the greatest signifi-
cance in cqnnection with the study of the evolutionary history
of plants, because of the abundance and reliability of the evidence
which the various parts furnish in this connection. Obviously, if
leaf, reproductive axis, root, and sporangium all supply consonant
and harmonious testimony in the same direction, a sound con-
clusion must inevitably be reached. There can be little question
that the doctrine of conservative organs is the most important one
which modern inductive anatomy has supplied as a tool of evolu-
tionary investigation. In fact, the general principles included
under this head are of such great significance that the present
volume may be considered as written only for those whose anatom-
ical training has progressed to such a point that they are able to
appreciate the universal value and validity of the doctrine here
discussed.

THE DOCTRINE OF REVERSION

This doctrine is well shown in the case of plants with a consider-
able amount of secondary growth—namely, the gymnosperms and
the dicotyledonous angiosperms. It is of little value in herba-
ceous forms, whether cryptogamic or phanerogamic, since in these
the effects of injury are usually in the direction of degeneracy
only. In plants with secondary growth and consequently more
massive organs the effect of injuries frequently is to recall ances-
tral features of organization. This phenomenon is called reversion.
Only in certain conifers can we observe the effect of injury in
recalling in living forms features which are known to have been

242 THE ANATOMY OF WOODY PLANTS

present in their Mesozoic forebears. For example, in the arau-
carian conifers, cited earlier in another connection, the Mesozoic
organization of the wood is recalled in the stem of the existing
species as a consequence of injury. The wood formed subsequent
to the infliction of the injury shows a type of structure charac-
teristic of the group in an earlier geological epoch. Although
only in rare instances can structures which are actually known
to have been present in the past be recalled by injuries, in many
other cases we find the infliction of wounds followed by the for-
mation of features which are normally present in the conserva-
tive organs. Consequently the doctrine of reversion finds support,
not only in the facts of paleobotanical anatomy, but also in the
much more richly exemplified doctrine of conservative parts. It
is further clear that we cannot interpret all structural peculiarities
which result from injury as reversions to a former condition of
organization, but only those which are definitely paralleled by
known conditions in fossil forms or are illustrated in the anatomy
of the conservative organs—the leaf, reproductive axis, and stem,
or at least some of these.

Carefully as the doctrines of recapitulation and of conservative
parts must be applied to elucidate the course of evolution, the
principle of reversion must be invoked in phylogenetic studies
with even greater precautions. A wide knowledge of fossil forms
as well as an extensive acquaintance with the facts of comparative
anatomy are necessary for a successful application of this doctrine
to the data derived from the investigation of injuries. In this
connection it is further necessary to distinguish clearly between
the phenomena of hypertrophy and those of actual reversion. For
example, when the stem of a dicotyledon or of a conifer is injured, a
local damming of food substances results as a consequence of the
elimination of the conducting tissues of the phloem in the immediate
vicinity of the wound. Overdevelopment consequent on over-
nutrition accordingly is locally most conspicuous. Very often
true reversionary changes resulting from the impulse supplied by
the injury present themselves only at some distance from the
wound. This situation is found in the conifers in connection with
reversionary conditions following injuries in the case of the rays

THE CANONS OF COMPARATIVE ANATOMY 243

which ordinarily manifest themselves, not in the actual wound cap
or hypertrophied mass of wood formed after injury, but on the
opposite side of the stem. Among the dicotyledons the birch
exemplifies the same condition in regard to the formation of aggre-
gate rays. The diffuse structure of radial parenchyma which
characterizes the normal organization of the wood of the genus
Betula gives way to a condition of aggregation following injury, not
on the edges of the wound (that is, in the actual wound cap), but
in the region of the axis diametrically opposite to the injury. In
accordance with the general reversionary principles here stated,
the wound cap as often as not presents structures in an advanced
or accelerated, and not in a reversionary, condition. Some recent
publications on the anatomy of the dicotyledons reveal a failure to
realize this fundamental principle, and therefore it is well that it
should be emphasized in the present connection.

The foregoing paragraphs elucidate the most important canons
or principles of evolutionary anatomy. It cannot be too strongly
urged that all the evidence available under the various principles
here described and exemplified should be brought into considera-
tion, as a failure to check up one kind of evidence against another
often results in a fallacious and ephemeral deduction. The most
cogent testimony to the validity of any evolutionary conclusion is
naturally supplied by the conditions actually realized in ancestral
fossil forms. Since, however, by reason of the incompleteness of
the geological record and our consequent ignorance of the organiza-
tion of older forms we are frequently not in the position to avail
ourselves of the actual past history of given plant types, we must of
necessity have recourse to the valuable aid furnished by the
important general principles described in the earlier paragraphs
of the present chapter. Where the canons of evolutionary anatomy
are judiciously employed, the result is usually so clear and convin-
cing as to commend itself to the unprejudiced mind.

CHAPTER XVIII

THE LYCOPSIDA AND PTEROPSIDA

The discussion of tissues and organs and the elucidation of the
general principles applicable to these in the earlier chapters of the
present work bring us to the point where the higher plants may
profitably be discussed in regard to their general anatomy and
evolutionary affinities. Obviously, if it is possible to compass a
general grouping which will at the outset indicate the main lines
of evolution, the consideration of the particular groups will be
much facilitated. Some years ago the writer put forward a general
classification of vascular plants, based on the cardinal features of
the reproductive structures and the salient anatomical characters.
This attempt to group the higher plants in accordance with their
more important reproductive and anatomical features has met with
approval among competent judges and as a consequence may be
conveniently utilized in the present connection.

A general survey of vascular plants, existing and extinct,
reveals the fact that there are certain features correlated in a
significant way. For example, a large number are characterized
by the possession of ventral or adaxial sporangia together with
usually small leaves. Another large assemblage presents relatively
large leaves and more numerous sporangia which are dorsal or
abaxial in position. The former group is characteristically repre-
sented by the lycopods or club mosses and their allies, while the
latter includes the ferns and the forms more nearly related to them,
the gymnosperms, and the angiosperms. The first aggregation
of types, known as the Lycopsida, is now practically extinct, but
played a large réle in the Paleozoic and furnished a considerable
proportion of the raw material of that extremely important com-
bustible called coal, together with its derivative products, petroleum
and natural gas. The second alliance, to which the appellation
Pteropsida is appropriately given, although abundantly present
in the remote past, still prevails and has given rise to the seed plants

244

THE LYCOPSIDA AND PTEROPSIDA 245

and to the forests of Mesozoic and Cenozoic times, as well as to
the herbaceous types which under modern conditions more and
more predominate in the plant population of our earth.

A more detailed characterization of the Lycopsida is now desir-
able. Fig. 176 represents diagrammatically the organization of
an axis of the lycopsid type together with its appendages. The
latter consist of leaf, branch, and root. Root and branch are
intimately associated, the former usually proceeding from the base
of the latter. The upper end of the main axis is represented in
transverse section to show the anatomical relations of the organs.
The fibrovascular cylinder in the particular case illustrated is
siphonostelic, although it might equally well be protostelic, espe-
cially in the lower representatives of the group. The tubular
stele or central cylinder constituted by the fibrovascular sys-
tem is interrupted only at one point—where the fibrovascular
supply of a branch takes its origin from the main cylinder. The
gap thus appearing is known as the branch gap or ramular
lacuna. On the margin of the stem appear certain projections, the
leaf bases, within which are included the fibrovascular strands
destined for the leaves or foliar traces. These subtend projections
on the surface of the central cylinder occupied by groups of proto-
xylem. The projections in question are the starting-points ‘of the
foliar traces, and it is clear that in no instance is there an inter-
ruption in the continuity of the central cylinder corresponding to
the departing leaf traces. The central cylinder of the Lycopsida
is said on this account to be without foliar gaps. The absence of
leaf gaps or foliar lacunae is characteristic of all the Lycopsida and
is a diagnostic feature of importance for the great group or phylum
which they represent. Even in those Lycopsida in which the
leaves are, superficially at least, relatively large (for example, the
Sigillariae, in which the foliar organs were sometimes over a meter
in length) there were still no foliar gaps. The absence of foliar gaps
is an anciently inherited or palingenetic feature of the Lycopsida,
and they may therefore be technically characterized as palin-
genetically microphyllous (small-leaved).

Not only has this large group universally small leaves, but it
also possesses another important, salient, and constant general

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THE ANATOMY OF WOODY PLANTS

Fic. 176.—Diagrammatic representation of the Lycopsi

THE LYCOPSIDA AND PTEROPSIDA 247

feature of organization—in this instance related to reproduction.
In the phylum under discussion the sporangia or spore sacs are
invariably on the upper or adaxial (ventral) surface of the sporo-
phyll or reproductive leaf. The sporangia are single or at most
relatively few in number and are invariably ectokinetic in their
mode of dehiscence. ‘True seeds seem never to have made their
appearance in the Lycopsida, although, as has been pointed out
in an earlier chapter, structures somewhat simulating seeds have
been found in certain of the Paleozoic representatives of the
group. The organs in question, however, lacked a true integu-
ment and, so far as is known, were without arrangements for
receiving the microspores or pollen grains, a universal equipment
in the case of pteridospermous and gymnospermous seeds. Possibly
the failure to achieve true seminal structures was the cause of the
decline of the Lycopsida, which at the present time constitute an
insignificant proportion of the vegetation of the world. The group
is very ancient and goes back to the beginning of the geological
record. The primitive forms representing the Lycopsida were
often arboreal in their habit, and the alliance reached its culmina-
tion in the carboniferous forests of the Paleozoic. It became largely
reduced in the Mesozoic, and the Cenozoic saw its virtual extinction.

In the Pteropsida we have to do with forms in which the leaves
are relatively large in comparison to the stem and often extremely
complicated in structure. As is shown in the diagram (Fig. 177),
the transverse section of the stem reveals a central cylinder
which, when siphonostelic, as in the illustration, is characterized
by gaps corresponding to the traces departing to the leaves. This
feature of its anatomy is in marked contrast to that found univer-
sally in the Lycopsida, in which the leaves are not related to foliar
gaps. The traces of the branches in the Pteropsida likewise sub-
tend gaps in the central cylinder (which may be designated as
ramular gaps). The fibrovascular supply of the root makes its
exit from the stele or fibrovascular cylinder without causing any
gap or interruption. The roots are often, but not invariably,
related to the bases of the leaves. The foliar organs, usually much
larger in size than in the Lycopsida and often of great dimensions
and lobed in a complicated manner, are to be regarded as palin-

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THE LYCOPSIDA AND PTEROPSIDA 249

genetically megaphyllous, since their anatomical relations are
always characterized by the presence of foliar gaps. The repro-
ductive organs or sporangia in the case of the Pteropsida are on the
lower or abaxial (dorsal) surface of the leaf, and are often numerous
and complicated in structure. In the lower forms the dehiscence
of the spore sacs is ectokinetic, but in the higher representatives
of the phylum the opening mechanism is derived from modified
transfusion tissue (in turn derived from the centripetal wood of
the ectokinetic and lower forms). Those Pteropsida characterized
by an internal reproductive mechanism derived from transfusion
tissue are appropriately designated as endokinetic. The higher
members of the series have developed true seeds provided with an
integument and equipped with an apparatus either related to the
seed itself or to the seed leaf (or megasporophyll) for the reception
of microspores. or pollen. Although developed in very early
geological times in forms resembling ferns, the Pteropsida are still
in full vigor; and in their highest manifestation, the angiosperms,
they constitute an overwhelmingly large proportion of the present
vegetable population of the earth. They have reached their
zenith of efficiency in the herbaceous angiosperms, which in all
probability will supplant arboreal angiospermous types in the not
very remote future. The gymnosperms, and in particular the
conifers, were the prevailing Pteropsida of the Mesozoic, while in
the Paleozoic age pteridosperms (Cycadofilicales) and other lower
gymnosperms and fern allies represented the group.

The Pteropsida and Lycopsida are distinct as far back as the
geological record can be perused, and there seems to be little doubt
that they constitute two primitive stocks of vascular plants.
They are so clearly diverse even in their earliest manifestations
that it is difficult to picture how they may have been formerly
connected. It is obvious, however, that the lycopsid type has not
been able to cope with the changing conditions of environment and,
comparatively early in the periods recorded in the rocks, was
relegated to a position of relative inferiority. Whether this situa-
tion was the result of the failure to achieve true seeds or is to be
explained as the basis of some fundamental defect of internal
organization incapacitating the Lycopsida to succeed in competition

250 THE ANATOMY OF WOODY PLANTS

with the large-leaved Pteropsida must, for the present at any rate,
be left an open question. Although the Lycopsida were the pre-
dominant constituent of the Paleozoic forests, the Pteropsida in
many cases have entered largely into the composition of the more
ancient coals and can often be clearly recognized, particularly in the
carboniferous coals of the Middle Western states (Illinois, Ohio,
etc.), as charred remains of the axes and even as pinnae or smaller
segments of the leaves in the form of so-called ‘“‘ Mother of Coal.”

After having described the general characteristics of the Lycop-
sida and Pteropsida, we find it in order to indicate the main groups
which come under these two great divisions of the vascular plants.
The Lycopsida include two principal subdivisions—the Lycopo-
diales and Equisetales. The Lycopodiales are characterized by
the alternating nature of their foliage, while the Equisetales have
their leaves disposed in whorls on a stem presenting marked ridges
and furrows. The Lycopodiales are again subdivided into isos-
porous and heterosporous families. Of the former there are two—
the Lycopodiaceae and the Psilotaceae. The Lycopodiaceae are
characterized by the possession of well-developed roots and un-
divided sporangia, while in the Psilotaceae the sporangial struc-
tures are septate, and organs of the nature of roots are entirely
absent. ‘The heterosporous Lycopodiales have in common a foliar
appendage, known as a ligule, which is present on both vegetative
and reproductive leaves, and in the case of the latter occurs just
above the insertion of the sporangium. Of the three families
presenting the phenomenon of heterospory the first, the Selaginel-
laceae, are terrestrial forms included under a single genus, Selaginella,
in which the megasporangia never produce more than four spores.
In the second and usually aquatic family, the Isoetaceae, there is
likewise a single genus, but the sporangia are provided with trans-
verse processes known as trabeculae, and the megaspores are
numerous in each sack. Lastly, the Lepidodendraceae, including
the Sigillariae and their allies, are terrestrial extinct forms often of
arboreal habit and of somewhat diverse megasporangial structures.

The Equisetales, as has been indicated at the beginning of the
foregoing paragraph, are distinguished by the whorled arrangement
of their leaves. Another feature which they possess in common is

THE LYCOPSIDA 'AND PTEROPSIDA 251

the usual exhibition of a high degree of multiplication of the
sporangia which are often disposed on common stalks known as
sporangiophores. The branches instead of being truly axillary are
borne alternately with the leaves at the nodes. The Equisetales
may conveniently be divided into three families—the Sphenophyl-
laceae, the Calamitaceae, and the Equisetaceae—which are in all
probability related to one another in the order indicated by their
enumeration. The Sphenophyllaceae were forms in which the
central cylinder of the stem was protostelic. The cones consisted
of sporophylls presenting various degrees of complication. In the
simplest forms the sporangia were inserted singly on the stalks or
sporangiophores and were numerous for each sporophyll. In more
advanced types the sporangia became two or more for each spo-
rangiophore. The Calamitaceae, like the Sphenophyllaceae, are
organisms entirely extinct. They usually possessed the arboreal
habit and were invariably characterized by a siphonostelic central
cylinder. The sporangiophores bore four sporangia and were
variously related to the sporophylls. In the Equisetaceae are
included a number of genera, living and extinct, of herbaceous
habit and possessing so far as is known a simplified type of cone in
which sporophylls are represented by the sporangiophores alone.

In the Pteropsida, marked by the general features enumerated
in earlier paragraphs of the present chapter, there are three large
subdivisions—the Filicales, the Gymnospermae, and the Angio-
spermae. ‘The first of these, as the name indicates, include the
fernlike forms—that is, those in which the reproduction takes place
through unicellular bodies known as spores. In the Gymnospermae
true seeds are present which in every case are equipped to receive
the microspores; and these after germination effect fertilization
either by means of antherozoids (Archigymnospermae) or through
the agency of pollen tubes (Metagymnospermae). In the last and
(in the present epoch) most important subdivision of the Pterop-
sida, the Angiospermae, the pollen is received on the apex of the
closed megasporophyll and no longer falls upon the seed. Fertiliza-
tion is invariably by means of a pollen tube. The habit of the
angiosperms is either arboreal or herbaceous, and the fibrovascular
tissues show a high degree of specialization.

CHAPTER XIX
THE LYCOPODIALES

This group, as has been indicated in the last chapter, has spiral
phyllotaxy. It includes both isosporous and _heterosporous
families. The latter are distinguished by the presence of a ligule,
while in the former this structure is lacking. In the genus Lyco-
podium the central cylinder of the stem is radial and protostelic.
Asa consequence of the radial organization of the stele, the masses of
phloem lie in the intervals between the generally radially directed
bands of xylem. The sieve tubes are separated from the tracheids
by several rows of parenchyma on either side. In the vertically
directed reproductive axes of Lycopodium the organization of the
fibrovascular tissues is typically radial, while in the creeping stems
or rootstocks the arrangement of the xylem and phloem is somewhat
dorsiventral. The upright axes of Lycopodium, aside from the
fact that they bear leaves, are scarcely distinguishable in structure
from the roots. This resemblance in organization between root
and shoot is an indication of the antiquity of the lycopodineous
stock, since in the higher groups the differentiation between the
axial and radical organs becomes more and more marked.

The monotypic genus P/ylloglossum possesses a siphonostelic
stem. In this form the lower region of the axis is tuberous and
contains the most massive development of the fibrovascular system.
In the tuber also, as is shown in Fig. 178, there is both internal
phloem and internal endodermis. As the stele of the inferior
region passes upward, it gives off a branch into the peduncle of
the tuber which is to perpetuate the plant in a subsequent season.
The trace of this appendage in departing from the central cylinder
leaves a well-marked branch gap. The foliar organs indicated as
swellings on the outline of the section cause no interruptions in the
continuity of the central cylinder by their departure from its surface.
In this respect Phylloglossum shows itself to be a veritable repre-
sentative of the Lycopsida. In the higher and aérial region of the

252

THE LYCOPODIALES 253

axis the fibrovascular tissues become so much reduced that internal
phloem is no longer developed, and the continuity of the cylinder
is interrupted by gaps which are not related to organs, but merely
indicate the incom-
plete development
Otitine xylem:
Where a trace is
given off, as is
shown in Fig. 179,
it takes its origin
opposite a strand
and does not sub-
tend an interval be-
tween the bundles,
clearly showing
the lycopsid condition, even in the state of stelar reduction pre-
sented in the evan-
escent aérial axis.

The Psilotaceae are
anatomically distin-
guished from the Lyco-
podiaceae by the
absence of true roots.
Here the aérial stem,
unless it be of very
small size, is siphono-
stelic in its organiza-
tion. A thick-walled
medulla is often pres-
ent, but no internal
phloem has been ob-

Fic. 179.—Diagram of exit of leaf traces in the served. The organiza-
aérial stem of Phylloglossum (after Bertrand).

Fic. 178.—Diagram of the lower region of the stem
in Phylloglossum (after Bertrand).

dx@2s
By G'e%e:
f, () 5c
Ia gto ee 5)

tion of the conducting
tissues is radial and exarch; the leaf traces, as in Lycopodium, take
their origin from the angles of the stele. In smaller aérial shoots and
in the subterranean ones the central cylinder is usually protostelic.
Sometimes gaps are present in the walls of the tubular cylinder of

254 THE ANATOMY OF WOODY PLANTS

the larger stems, but these in no case are related to outgoing foliar
traces. The general topography of a stem of the type found in the
Psilotaceae is presented in Fig. 180.

In the Lycopodiaceae
in general the leaf traces
are ordinarily mesarch in
their organization, a
condition more or less
characteristic of the
Lycopsida as a whole.
An endodermis can
usually be distinguished
about the foliar strands,
although this limiting
layer is ordinarily con-
spicuous by its absence

Fic. 180.—Transverse section of the stem of in the stem in most spe-
Psilotum. cies of Lycopodium. In

other representatives of the
two families under discus-
sion an external endoder-
mis is usually found in the
stem, and, as has been
shown above, an internal
endodermal zone is seen in
the tuberous subterranean
stem of Phylloglossum.

In the genus Selaginella
the fibrovascular tissues of
the axis are distinguished
by considerable variety in
topography. In some spe- Fic. 181.—Transverse section of stem of
cies the stele is a single Selaginella laevigata, showing a siphonostelic
mass, separated from the central cyan:
cortical tissues by an air-containing region representing the endo-
dermis. In other species the fibrovascular system becomes divided,
and the protostelic condition as a result passes into that known as

THE LYCOPODIALES 255

polystelic. In still another modification, presented by S. /aevigata
from Madagascar (Fig. 181), a true siphonostele is exemplified which
is complicated by the presence of medullary strands joining up with
the walls of the tube in the regions where branches are given off.
In the species under discussion the traces of the leaves illustrate
the condition typical for the Lycopsida and pass off from the
cylinder without causing any
gaps in its continuity.

7

@ TOL SAS jak : ;
Ne BAGG O7.- Pm.
Tsoetes has a protostelic stem We ay oy
> S — oe, y
NS eee ee/

which is remarkable among ex- &. ~.

isting Lycopsida in manifesting .,-“\dc Ser,

well-marked secondary growth fi Wei v7
PS),

(Fig. 182). The external prod- SE

uct of cambial activity is a eee

radially disposed storage paren- as oS REE

chyma, while internally the di- oN (Sa
viding layer originates additions EWS
to the fibrovascular tissues Cat ae. <

which are most commonly inter- SRN —
preted as consisting of alternate ay wa 9 mee a
zones of xylem and phloem. Fs OG Be
The situation here, however, is Y 5S GS)
disputed, and uncertainty per- LOSS SSL) =
sists as a result of the indifferent ” “8G Vo fe) ax
development of the tissues re-
sulting from the characteristic-
ally aquatic habit of the plant. The roots in /soetes are distin-
guished, in common with the smaller radical organs of a number
of the lower Lycopsida, by the fact that they develop a single
mass of xylem in proximity to a single strand of phloem. The
leaf is not worthy of special note.

In the Lepidodendraceae the stem manifests great diversity
of structure, as would naturally be expected in a group which
in Paleozoic times displayed numerous types with generally
marked secondary growth. The primary structures of the stem
were either protostelic or siphonostelic. In the former condition
a considerable amount of parenchymatous tissue was developed

Fic. 182.—Cambial activity in Isoetes

256 THE ANATOMY OF WOODY PLANTS

among the tracheids, particularly toward the central region of
the stele. This peculiar organization of the median area of the
stele in protostelic lepidodendrids is responsible for a hypothesis
as to the origin of the medulla or pith. Quite generally it is con-
sidered that by continuing the process of transformation of
tracheids, first into short tracheary elements and then into paren-
chymatous cells, there is formed in the center of the stele a pith
of stelar origin. In
accordance with this
view the central re-
gion of the stele in
many protostelic
lepidodendrids is
called a ‘‘ partial
pith.”” There is no
conclusive evidence,
however, that the so-
called “ partial pith”
in reality gives rise
to the true medulla
in those lepidoden-
droid types which
possess it. More-
over, the evidence in
thevea sie ots thie
Pteropsida, which are very much better displayed in the period
of time which we are able to investigate, is distinctly against the
validity of the stelar origin of the pith, since the medulla in the
large-leaved vascular cryptogams shows very marked indications
of derivation from the fundamental system. It seems on the
whole more likely that the medulla in the lepidodendrids, where
such a structure is found, is an inclusion of fundamental tissues
on the part of the stele. This conclusion is particularly favored
by conditions found in lepidodendroid stems in which there is no
indication of secondary growth, as, for example, in Lepidodendron
Spenceri, shown in Figs. 183 and 184. Here the medulla is
largely occupied by dark-brown sclerenchymatous tissues similar

Fic. 183.—Transverse section of the stem of Lepido-
dendron S penceri.

THE LYCOPODIALES 257

to those appearing in the cortical region. It may accordingly
be stated that even the imperfect evidence supplied by the stem
of the lepidodendrids in a condition of obvious degeneracy of
the primary stelar tissues when they are first presented on the pages
of the geological record does not definitely justify the conclusion
that the pith is of stelar origin. In a later chapter it will be made
clear that the evidence supplied by the lower Pteropsida, which
is at once more
abundant and
more decisive, dis-
tinctly vouches for
the extra-stelar
derivation of the
medulla.

The degeneracy
of the tracheary
elements which has
already been noted
in the case of the
protostelic type of
cylinder in the
lepidodendrids
makes itself par- is
ticularly obvious Fic. 184.—Part of the same, more highly magnified
im” the higher
siphonostelic representatives of the group and especially in the
stems included under the Sigillariaceae. The reduction in amount
of the primary tissues results in the appearance of gaps in the wall
of the stelar tube which are not related to appendages. The
central cylinder consequently becomes discontinuous, as has
been shown in Fig. 122, page 170. Since the later-developed
secondary tissues naturally first appear opposite the framework
outlined in the primary wood, the secondary xylem is likewise at
the beginning of its formation discontinuous, and continuity
appears only after cambial activity has resulted in the formation
of a woody cylinder of some thickness. Where the primary struc-
ture of the xylem has undergone the extreme degree of reduction

258 THE ANATOMY OF WOODY PLANTS

found in the stem of many of the Equisetales as well as of all
but the very lowest seed plants, the initial discontinuity of the
secondary wood often becomes very marked. The situation
presented by these extreme types unco-ordinated with the eluci-
dative anatomical features presented by more primitive forms has
been the cause of serious misunderstanding as regards the origin
of the so-called medullary rays. The case of the siphonostelic
lepidodendrids and Sigillariae, as diagrammatically represented in
Fig. 122, page 170, seems to indicate definitely that parenchymatous
interruptions in the secondary cylinder resulting from the dis-
continuity of the primary wood cannot be interpreted as true rays
any more than are the gaps related to the departing traces of
appendages to be brought into the category of rays. Much
confusion of definition has resulted from the failure to interpret
the conditions found in higher forms in terms of the structures
presented in earlier and more primitive types.

It has been made clear in an earlier chapter that the lepido-
dendrids proper, which beyond question represent the more
primitive state of organization for the Lepidodendraceae as a whole,
supply evidence for the derivation of radial parenchyma as the
result of the transformation partial or complete of radial tra-
cheary strands into storage parenchyma. The older representa-
tives of the Lepidodendraceae show themselves in this respect the
most archaic of all the vascular plants with secondary growth.
Not only is the origin of radial storage cells in the secondary xylem
elucidated by the lepidodendrids, but, as has been pointed out in
an earlier chapter, the parenchymatous elements of the primary
wood have their origin illustrated in the conditions found in the
primary region of the stem in this ancient group of vascular plants.
It has been indicated in Figs. 29 and 30 that the living cells occurring
in the wood of the primary region of the axis in protostelic lepido-
dendrids have been derived by septation from elements belonging
to the category of tracheids. Some of the resultant elements
persist as short tracheids with thickly reticulated walls, while
others maintain a thinner wall and in all probability in life were
occupied by living protoplasm. In the siphonostelic Lepidoden-
draceae and in the sigillarian forms as a whole there is no evi-

THE LYCOPODIALES 259

dence as to the origin of parenchymatous elements in the primary
xylem. In the later (Permian) representatives of the Sigillariae the
primary cylinder became so much reduced that it was no longer
continuous. This topographical condition of the primary wood
was responsible, as indicated above, for a resultant discontinuity
of the secondary xylem. The processes of the pith extending
between the primary bundles and a short distance into the second-
ary cylinder are in a certain sense medullary rays, since they take
their origin from the medulla; but they have nothing in common
with the radial masses of storage tissue resulting from cambial
activity which characterize the organization of the secondary
cylinder. Further, they should not in any way be confused with
foliar gaps, since in the Lycopsida interruptions of this nature in
the fibrovascular cylinder do not occur. In the particular case
under consideration the leaf traces originate opposite the strands
of primary xylem and do not subtend the intervals between
them. It is clear that the Lepidodendraceae, although entirely
extinct, furnish extremely valuable data for the elucidation of the
origin of the parenchyma in the primary wood and for that of the
radial storage devices of the secondary xylem. Further, they
throw a very clear light on the general morphology of the secondary
woody cylinder in vascular plants, since the comparative study of
their stems from lower to higher geological levels makes it obvious
that the radial parenchymatous bands of the secondary wood
cannot appropriately be called medullary rays. They should be
called wood rays, as the inward relation to the medulla is neither
a primitive nor an essential condition.

Not only in regard to the parenchymatous structures of their
primary and secondary wood, as well as by their great geological
age and early culmination, do the lepidodendrids in the large sense
show themselves to be primitive representatives of vascular plants,
but also by the organization of the fibrous elements of the secondary
wood. It has been pointed out in an earlier chapter that a typical
element of the primary wood in all plants is the scalariform tracheid.
In the secondary xylem of plants in general the scalariform element
has given place to the pitted tracheid, which is universal for
the various groups of gymnosperms and for the angiosperms with

260 THE ANATOMY OF WOODY PLANTS

secondary growth. In the Lepidodendraceae in the narrower sense
the secondary wood is distinguished from the primary struc-
tures only by the presence, exclusively, of radial parenchyma
and by the radial seriation of its tracheary elements (Fig. 185).
The organization of the tracheids of the secondary wood is, in fact,
identical with that found in the primary region. In the higher
lepidodendroid forms assembled under the appellation Sigillariae
very frequently,
2a ee particularly in the
Pee regvon, of “ihe
secondary wood
more remote from
the pith, the tra-
cheids cease to be
scalariform and
assume the pitted
type characteristic
of the gymno-
sperms and other
higher representa-
tives of the Vascu-
lares.
The vascular
Fic. 185.—Longitudinal section through primary and strands of leaves in
secondary wood of Lepidodendron species. the lepidodendrids
are characterized,
as are those of the Lycopodiales in general, by mesarch organiza-
tion. This condition is clearly shown in Fig..186. It is appar-
ent that the foliar trace is surrounded by secondary wood.
It has been stated by Scott that true transfusion tissue is present
in the leaf of the lepidodendrids, but this statement, in view of the
situation present in the Lycopodiales in general and in the Lepido-
dendraceae in particular, seems open to some question, and certainly
the subject seems to require further investigation. The foliar organs
of the lepidodendroid stock were characterized by the presence of
two aérating strands on either side of the foliar trace; these were
in communication below with the external air through the agency

THE LYCOPODIALES 261

of stomata, ordinarily accommodated in furrows on the lower
surface of the leaf. The aériferous structures of the blades of the
leaves were continuous with air-containing radial structures in the
outer and inner bark known as parichni. ‘These are a noteworthy
feature of structure in the lepidodendrids and have attracted a
large amount of attention from students of the group. It should
be pointed out, however, tt

that they are by no means
a unique structure, since
special aérating devices
are likewise found present
in relation to the leaves
of the Coniferales.

The roots of the lepido-
dendrids have been the
subject of much discus-
sion. Their ultimate
divisions, the so-called
stigmarian rootlets
(Fig. 187), are character-
ized by a very simple
organization, since only a
single group of protoxy-
lem is present. The root- mrp oma
lets of this order divided Fic. 186.—Leaf trace of a lepidodendrid
dichotomously, as is often Sch G
the case with those of the living Jsoetes and Lycopodium. The
main roots of the lepidodendroid forms are in all probability only
partially known to us and present a curious type of structure. To
begin with, there is a large pithlike mass around which is developed
an extremely small amount of primary wood, at times so rudimen-
tary as to be scarcely recognizable. The small degree of develop-
ment of the primary structures and the quincuncial arrangement of
the lateral rootlets of Stigmaria have led to a great deal of doubt as
to their morphological nature. They have often been regarded as
creeping stems or rootstocks, and this view of their nature is found
even in recent literature on the subject. The mass of anatomical

262 THE ANATOMY OF WOODY PLANTS

opinion now, however, is united in favor of the view that they
represent the larger or main roots of lepidodendroid forms. They
are, in fact, to some extent comparable to the anomalous roots
found in the genus Selaginella known as rhizophores. It seems
highly improbable that the type of organization presented by
Stigmaria could have belonged to lepidodendrids with protostelic
central cylinders. In general, the subterranean organs of plants
are less frequently preserved with structural organization, since
they are from the nature
of things less likely to
find their resting-place
in open bodies of water,
and this condition must
usually be realized in
order to insure petrifac-
tion. It is accordingly
probable that the genus
Stigmaria, as at present
defined, represents only
to a limited extent the
main radical organs of
lepidodendroid forms.
The Lycopodiales as
a whole are a group
which reached its culmination in the Paleozoic age and from the
richness of its display in earlier geological times must be regarded
as extremely ancient. Although the group is almost extinct, its
interest from the evolutionary standpoint is great by reason of
its antiquity, which supplies valuable data for the elucidation
of some of the most important problems of primitive organiza-
tion in vascular plants. Clearly the group as a whole displays
a reduction series in which the few types which survive under
modern conditions represent, not primitive states, but the final
results of a process of simplification extending through almost
countless ages. Consequently it is highly inadvisable, in attempt-
ing to arrive at a conception of the evolutionary significance of
the group, to turn exclusive attention to modern simple forms,

Fic. 187.—Rootlet of Stigmaria (after Scott)

THE LYCOPODIALES 263

such as Lycopodium or Selaginella. On the contrary, the most
valuable results from the standpoint of the doctrine of descent
can be derived from the study of the complicated arboreal extinct
types known as lepidodendrids and sigillarians. It has been made
clear in the preceding paragraphs that study of the last-mentioned
types throws extremely important light on the origin of storage
devices in the primary and secondary wood and makes clear the
status of the so-called medullary rays. The Lycopodiales, although
largely extinct, cannot accordingly be neglected by students of
the data of evolution, and they supply valuable evidence, if any
were needed, for the necessity of a knowledge of extinct forms
as an indispensable basis for the understanding of organisms now
living.

CHAPTER XX _

THE EQUISETALES (INCLUDING SPHENOPHYLLALES)

The forms to be discussed in the present chapter are contrasted
with the Lycopodiales by the general fact that the appendages of
the stem are arranged in a whorled or verticillate manner instead
of in the spiral fashion characteristic of the group considered in
the preceding chapter. Not only, however, are they characterized
by the verticillate arrangement of the appendages, but also by the
longitudinal ridges and furrows which mark the surface of their
stems. Contrary to the conditions found in the furrowed or angular
stems of certain woody dicotyledons, the traces or fibrovascular
strands of the leaves correspond in position to the salient regions
of the stem and not to its depressions. The branches, moreover,
are not truly axillary as is characteristic of the higher forms, but
occur at the node in alternation with the foliar organs. This
' situation is very striking and characteristic. The older forms in
the Equisetales usually possessed dichotomously divided leaves
or at least foliar organs in which the veins repeatedly forked.
Another important characteristic of the older representatives of
the Equisetales was the continuity of the ridges and furrows at
the nodes. In more modern types this condition gives place to
distinct and finally universal alternation of the ridges and furrows
in the nodal region.

The Equisetales may be conveniently divided into three groups
—the Sphenophyllaceae, the Calamitaceae, and the Equisetaceae.
Of these the first-named may now be discussed. The Sphenophyl-
laceae are Paleozoic forms with slender stems marked by the pres-
ence of a relatively small amount of secondary growth. Their
slender conformation has led to the suggestion that they were
either vines or aquatics. There is, however, no convincing evidence
of the correctness of either of these views in regard to their habit.
Fig. 188 illustrates the organization of the stem in Sphenophyllum
insigne. ‘The center of the cylinder is occupied by the primary

264

THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 265

wood, which is triangular in configuration, with the small elements
of the protoxylem at the angles. The primary wood presents no
features of special interest as viewed in transverse section beyond
the fact that it is protostelic in its organization and consequently
lacks a parenchymatous medulla or pith. The secondary wood
which surrounds the primary structure is characterized by the
radial seriation of its elements and by the presence of medullary
rays. There are no
other parenchyma-
tous structures in the
secondary wood ex-
cept the rays. The
radial parenchyma of
the Sphenophyllaceae
is peculiar in the fact
that its cells, instead
of being strictly elon-
gated in the radial
direction and at right
angles to the longer
axis of the tracheids,
‘frequently have their :
greatest dimensions OE Sas es

5 : Fic. 188.—Transverse section of the stem of
in the vertical 2g lane. Sphenophyllum insigne.

This situation leads

to the extension of the cells of the rays along the edges upward and
downward among the tracheids in a manner simulating true wood
parenchyma. This is, however, merely an appearance, for longitu-
dinal storage cells of the type ordinarily known as wood parenchyma
have not yet been found in any Paleozoic wood of secondary origin.
Wood parenchyma, indeed, as has been indicated in an earlier chap-
ter, was primitively intimately associated with the phenomenon of
annual rings which appeared for the first time in the Mesozoic age.
It is clear from the description of the wood of the Sphenophyllaceae
supplied in the present connection that it shows, as indeed might
be expected, a general resemblance to that of the more ancient
representatives of the Lycopodiales. The tracheids were somewhat

NS .
\Y)

\
‘

4

Cri~
8

266 THE ANATOMY OF WOODY PLANTS

scalariform in their sculpture, but tend, like those of the arboreal
Lycopodiales, to develop the pitted condition. The pits, whether
scalariform or rounded, were found equally on radial and tangential
surfaces of the elements of the secondary wood, a condition par-
alleled in the ancient treelike representatives of the Lycopodiales.
The outer region of the figure shows the soft tissues in a condition
of relative disorganization, which does not make their discussion
profitable.

The most interesting general features presented by the anatomy
of the stem in
Sphenophyllum are
its essentially pro-
tostelic character,
thie: spect an
organization of the
rays, and the tan-
gential as well as
radial pitting of
the tracheids. The
leaves and roots
are not well known
as to their ana-
tomical organization and in the actual state of our ignorance mani-
fest no features of unusual interest.

The Calamitaceae are distinguished from the Sphenophyllaceae
anatomically by the siphonostelic organization of their central
cylinder. In the more ancient types of Calamites the ridges and
furrows of the stem were continuous at the nodes, precisely as is
the case in Sphenophyllum. Moreover, in the Archaeocalamitaceae
the leaves divided dichotomously. In more modern calamitean
types the alternation of the ridges and furrows in the nodal region
began to become a marked feature of organization except in the
cones or reproductive axes, which adhered to the more ancient
topography with non-alternation at the nodes.

Fig. 189 illustrates the organization of a younger stem in a
calamite. The outer region of the axis has generally disappeared
as a result of fossilization, but the woody and medullary regions are

Fic. 189.—Transverse section of a small stem of
Calamites.

THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 267

clearly shown. The wood is apparently entirely secondary in its
origin, but it is distinguished by certain lacunae or cavities which
occupy the apices of the wedgelike masses constituting the rather
slender cylinder under discussion. These cavities indicate the posi-
tion of the poorly differentiated and evanescent primary wood. So
far as is known, the primary xylem of the calamites of the later
Paleozoic was entirely centrifugal or peripheral in its development.
The great reduction in the primary structures of the calamites has
led to a discontinuity of the cylinder comparable to that found in
the higher and more modern members of the Sigillariae described in
the preceding chapter. The lack of continuous development in the
cylinder of primary wood brings with it a corresponding organiza-
tion of the secondary xylem. The general result of the situation
portrayed is the organization of the secondary cylinder in the first
instance as separate wedges which finally become united by their
increasing breadth. In the group under consideration we have
one of numerous examples of an interrupted secondary cylinder
resulting from the meager and sporadic development of the primary
wood when the latter presents an extreme degree of reduction.
The pointed, outward excursions of the pith in calamitean forms
are in marked contrast to the true medullary rays, which in this
case are narrow structures. The rays in Calamites are characterized
by the often vertical elongation of their elements, a situation which
parallels that described above in Sphenophyllum.

The slight development and the entirely centrifugal origin of
the primary xylem in the true calamitean forms is in marked
contrast to the conditions presented by the genus Sphenophyllum,
where the primary structure is not only massive, but also entirely
centrad or centripetal in its development. There is, of course,
a very wide gap between the organization of the axis in Spheno-
phyllum and that found in Calamites. This gap is for the most
part still unbridged by the discovery of intermediate forms, but
an interesting condition is found in a stem from the lower Car-
boniferous which is described by Scott. In Fig. 190 is repro-
duced a somewhat oblique section of the primary region of one of
the woody wedges of a calamitean stem. The lacuna or cavity
representing the position of the ephemeral primary wood has

268 THE ANATOMY OF WOODY PLANTS

tracheary elements, not only on the side which lies toward the
secondary wood, but also on that in juxtaposition to the medulla
or pith. In other
words, this calami-
tean stem, to which
its discoverer has
applied the name
Protocalamites,
somewhat clearly
presents xylem of
the centrad or cen-
tripetal type.

The secondary
wood of calamitean
forms was in its
early organization

Fic. 190.—Transverse section through the primary larg ely composed
wood of Protocalamites (after Scott). of scalariform ele-

@ ‘ m Y
Py 8
A it; mY te

ments which in the
later development
gave place more or
less completely to
the pitted tracheids.
Fig. 191 illustrates
the structure of the
stem of a calamite
in proximity to the
primary region. It
is clear that the tra-
cheids are still very
largely scalariform.
The pitting of the
tracheary elements
inCalamites,whether
scalariform or rounded, was confined to the radial walls of the ele-
ments as in the lower gymnosperms, and was not present on the

ae

vee
> 3

=e
FS

ss aie

nee

Fic. 191.—Longitudinal section of the wood of Calamites

THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 269

tangential aspects. True wood parenchyma did not occur in the
secondary wood of Calamites. Although the primary xylem of
calamitean forms has departed far from the primitive condition,
the secondary ligneous organization of the group is characterized
by features which are only less primitive than those presented by
the lepidodendrids.

The organization of the
leaf in Calamites is not well
known as regards those
features which are of inter-
est from the comparative
anatomical standpoint, but
this situation is fortunately
relieved by certain data
observed in the case of the
foliar organs of the living
genus Equisetum, which
will be described in a sub-
sequent paragraph. The
root of calamitean forms
was not recognized at first
as belonging to the forms
included under this appellation and was called Asiromyelon. Its
organization can best be discussed in connection with that of the
root of the living genus Equisetum.

In Fig. 192 is portrayed the structure of a transverse section of
the stem of the genus Equisetum. It is evident that the center of
the figure is occupied by a large air space shared by the calamitean
forms and indicative both for the Equisetaceae and their fossil
forebears, the Calamitaceae, of a primitively amphibious habitat.
This central cavity is often called the medullary fistula and in the
case of the ancient representatives of the Equisetales was often
molded in stone as pith casts resulting from mud entering the
central spaces of the fallen trunks rotting in the shallow waters of
Paleozoic lakes. Surrounding the large medullary space are the
fibrovascular bundles, which are of small size and somewhat remote
from one another. The strands are marked by cavities in their

Fic. 192.—Transverse section of stem of
Equisetum variegatum var. Jesupi.

270

THE ANATOMY OF WOODY PLANTS

inner region which from their topographical relation to the ridges

of the stem are known as carinal air spaces.

In alternation with

these are larger spaces in the cortex situated beneath the furrows
of the stem and designated consequently as vallecular lacunae.

Fic. 193.—Transverse section of bundle of
rootstock of Equisetum arvense.

true scalariform or reticulate elements
which are laid down after the elonga-
tion of the internodes has come to an
end. The two masses of metaxylem
inclose between them the tissues of
the phloem, consisting of larger sieve
tubes and smaller parenchymatous
cells. In Fig. 194 is shown a longi-
tudinal view of the fibrovascular
bundles taken a little to one side of
the central region. To the left may
be seen the carinal cavity containing

The cortex is largely com-
posed in life of green cells
and performs the assimila-
tive and transpiratory
functions inadequately sub-
served by the minute leaves.

The fibrovascular bundle
must now receive further
consideration (Fig 193).
The tracheary elements are
scantily present on the mar-
gins of the so-called carinal
lacunae. This region is the
protoxylem. Outwardly on
either flank is seen a row of
tracheids which constitute
the metaxylem. These are

Fic. 194.—Longitudinal sec-
tion of bundle of rootstock of
Equisetum silvaticum.

remains of ringed and spiral protoxylem. ‘To the right appear the
reticulate elements of the metaxylem which in the transverse view

flank the phloem.

The organization of the pith and the distribution of the endo-
dermal structures in the genus Eguisetum are of considerable

THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 271

evolutionary interest. In Eguisetum arvense (Fig. 1950), in all
probability one of the most highly specialized members of the
group, an external endodermis alone is for the most part present,
an internal boundary of
this nature being found
only in certain instances
in the region of the nodes.
In E. silvaticum, which may
be considered a somewhat
less specialized type, only
an external endodermis ap-
pears in the aérial stem,
which accordingly presents
a condition very similar to
that found in E. arvense.
In the subterranean stems,
however, and in primitive

Fic. 195 a, b, and c.—Diagrams illustrating the distribution of the endodermis
in the genus Eguisetum.

regions of the aérial axis an internal endodermis is likewise seen.
In other species such as E. hiemale (Fig. 195a), etc., both internal
and external limiting layers are developed throughout, while in
E. limosum (Fig. 195c) each bundle is surrounded by an individual

272 THE ANATOMY OF WOODY PLANTS

endodermis except in the region of the nodes where continuous
internal and external endodermal layers are seen. The pith of
the genus Equisetum, particularly in the region of the nodes, is
frequently characterized by the.presence of nests of dark-brown
sclerotic cells, resembling similar structures found in the cortical
tissues of both stem and leaf. On the grounds of comparative
anatomy we shall accordingly be compelled to regard the pith
of Equisetum as of cortical origin.

The arrangement of the fibrovascular strands at the nodes in
living and fossil representatives of the Equisetales (Equisetaceae
and Calamitaceae) must now be considered. The situation present
is best revealed by means of diagrams (Fig. 196). In A is depicted
the arrangement of the fibrovascular structures at the node in the
vegetative stem of the living Equisetum. Across the center of the
diagram passes a heavy transverse band, the so-called nodal wood.
In this the strands of the upper and lower internodes end in such a
manner that they alternate with one another. ‘The traces of the
leaves originate from the strands of the lower internode and thus
subtend the intervals between the strands which are joined with
the nodal wood from above. A superficial view of the topograph-
ical conditions represented here would result in the conclusion that
the Equisetaceae are provided with foliar gaps precisely as is the
case in the Pteropsida. A consideration of B makes this view of
the matter difficult to sustain. In the figure the foliar strand is
represented in radial aspect as it comes off from the fibrovascular
tissues of the axis. It is clear that the trace of the leaf takes its
origin below the so-called nodal wood and passes out without
showing any foliar gap above it. It is true that, as is indicated in
both A and B, a gap is present subtending the foliar trace above
the continuous zone of the wood at the node; but a consideration
of the historical and comparative anatomical data makes it difficult
indeed to regard the gap in question as a foliar one. In C is shown
the arrangement of the strands of the primary wood in an ancient
calamitean form (Archaeocalamites). It is evident in this case
that the strands of the upper and lower internodes, instead of
meeting the nodal wood in alternation as they typically do in
Equisetum, exactly coincide with one another and are not subject

THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 273

to alternation. As a result of this condition the leaf traces, which
here, as in the modern type, originate from the lower internodal
strand, subtend, not a gap above the nodal wood, but the strand
of the upper internode. It is thus clear that the older condition

mein
LAL
a a it

Fic. 196 a, b, c, and d.—Diagrams illustrating the relations of the bundles in
Equisetum and Archeocalamites. Explanation in the text.

for the Equisetales is one in which there cannot possibly be a gap
corresponding to a leaf, even above the so-called nodal wood. A
radial view of the situation in Archaeocalamites makes the topog-
raphy still more clear, and this is furnished in D. Obviously on
historical grounds the Equisetales are without foliar gaps and

274 THE ANATOMY OF WOODY PLANTS

hence are to be regarded as Lycopsida. The comparative anatom-
ical evidence on this point is equally unequivocal. In the repro-
ductive axes or cones of both Calamitaceae and Equisetaceae the
strands typically fail to alternate at the nodes, and the traces of
the sporophylls are consequently quite without corresponding gaps.
There apparently can be no question on anatomical grounds that
the Equisetales are justly included in the Lycopsida.

Ifthe evidence jas to the
relationship of the Equisetales to
the phylum Lycopsida is clear on
anatomical grounds, it is equally
definite from a consideration of
the features of organization of the

Fic. 197.—Longitudinal section of bundle in vegetative and reproductive leaves
of Equisetum (after Eames).

cones. In the Sphenophyllaceae and Calamitaceae the sporangia
or sporangiophores are known to be ventral appendages of the
sporophylls and thus present the condition characteristic of the
Lycopsida. On the basis of both reproductive and anatomical
features the group under discussion clearly belongs under the large
heading of Lycopsida.

The leaf in the genus Equisetum is of considerable interest in
view of the fact that it. displays the presence of centripetal or
cryptogamic xylem which has entirely disappeared in the stem.
Fig. 197a illustrates the organization of the trace of the vegetative
leaf of E. maximum. It is obvious that the xylem includes a central

THE EQUISETALES (INCLUDING SPHENOPHYLLALES) 275

spiral protoxylem region flanked inwardly and outwardly by
reticulate metaxylem. The condition present is, in fact, mesarch
and strikingly resembles the common anatomical situation in the
leaf of the Lycopodiales as described in an earlier chapter. In
the sporophyll of the living representatives of the Equisetales the
mesarch structure of the trace is even more conspicuous than in
the vegetative leaf. The situation in this respect is shown for
E. palustre (b). But the reproductive leaf not only manifests cen-
tripetal wood in its trace, but
it also presents an equally
significant condition in the rela-
tion of the phloem to the xylem.
In c is shown a transverse sec-
tion of the trace of the sporo-
phyll in E. hiemale. Sieve
tubes can plainly be seen sur-
rounding the xylem elements,
while the latter have in their
midst a more or less obvious
lacuna representing the evanes-
cent protoxylem. On the
grounds of comparative anat-
Saeed it is clear that the Fic. 198.—Transverse section of the
Equisetaceae once possessed oot of Equisetum hiemale.
centripetal wood in the stem.
That this was the former situation for the stock as a whole is clearly
indicated by the anatomy of Protocalamites shown in Fig. 190. It
is further rendered highly probable by the concentric as well as the
mesarch organization of the trace of the sporophyll in the living
genus that the bundles of the axis were formerly concentric in their
organization. This condition must, however, have been realized in
the extremely remote past, as no indication of concentric structure
has been found in the stems of even the most ancient anatomically
investigated remains of calamitean forms.

The root in the genus Equisetum (Fig. 198) shows the presence
of four protoxylem groups alternating with as many clusters of
elements of the phloem. The metaxylem consists ordinarily of a

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276 THE ANATOMY OF WOODY PLANTS

single large central tracheid in which the four masses of protoxylem
unite. In the smaller roots of Calamites the same general organi-
zation is found as presents itself in Equisetum. The radical organs
of this extinct group of the Equisetales are known as Astromyelon,
a name given before their connection with calamitean stems was
known.

In the Equisetales as a whole, represented by the Sphenophy!l-
laceae, Calamitaceae, and Equisetaceae, very marked features of
organization are present. In fact, the group shows characteristics
which may well be denominated unique. It is clear that the group
is of very ancient origin, since when it first comes into view it is
distinctly set off from the other large alliance of the Lycopsida, the
Lycopodiales, by the whorled character of its appendages, the
ridges and furrows of the stem, and the sporangiophoric manner of
reproduction. Scott has regarded these features as sufficiently
distinctive to warrant the establishment of a third great phylum
of vascular plants, the Sphenopsida. This group he considers as
on the whole more nearly allied to the Pteropsida than to the Lycop-
sida. In view of the absence of foliar gaps in the series under
discussion in the present chapter, particularly clear when both fossil
and living forms are brought into consideration, there does not
seem to be any adequate anatomical evidence to support the sepa-
ration of the Equisetales under the heading of Sphenopsida. The
reproductive characters of the equisetal series are likewise most
easily reconciled with an affinity to the Lycopsida in general and
to the Lycopodiales in particular. Scott regards the Psilotaceae as
more nearly related to the Sphenophyllaceae than to the lycopo-
dineous forms. The evidence in favor of this view does not seem,
however, to be of a compelling character.

CHAPTER XXI
THE FILICALES

This group of vascular plants presents the features of the
Pteropsida in their most primitive and least modified condition.
Large leaves are consequently the rule, and these normally, when
functioning as sporophylls, bear numerous sporangia on the lower
or abaxial (dorsal) surface. When the central cylinder of the
stem is siphonostelic, as is most frequently the case, the traces of
the leaves take their departure from the wall of the stelar tube
with the formation of foliar gaps subtending the departing strands.
In the Filicales reproduction is always by means of spores, which
are in general isosporous, but which in a few instances represent the
heterosporous condition. The Filicales constitute a remarkably
clearly defined group, at least so far as their modern representatives
are concerned; and the only family which has had its affinities
with the Pteropsida brought into question is the Ophioglossaceae,
regarded in some quarters as derived from lyéopsid ancestry.
This attribution of affinity, however, is not now considered justi-
fied. The Filicales constitute the largest element composed of
vascular cryptogams in the existing flora of our earth and are
on that account of great importance from the evolutionary stand-
point. Anatomical problems which, in the Lycopsida, are difficult
of elucidation by reason of the large degree to which the group has
suffered extinction in the existing flora are much more advanta-
geously approached, in the group under consideration, as a conse-
quence of the large number of forms which are offered for study
by the existing plant population of the earth. The value of
the filicinean Pteropsida is particularly great in respect to the
fibrovascular structures, and an attempt will be made in the
present connection to utilize these to the full. Naturally, in a
group surviving in relatively large numbers only the more salient
and significant facts can be brought into prominence in an
elementary treatise like this.

277

278 THE ANATOMY OF WOODY PLANTS

The stem as the most plastic of the organs in vascular plants
presents the greatest variety of structure in the Filicales. The
leaf, and particularly the root, offer little diversity of organization
and may consequently be dismissed with relatively slight con-
sideration. Anatomically the stem presents itself in the case
of the Filicales under two main conditions: the protostelic, in
which there is no medulla present in the fibrovascular system,
and the siphonostelic,
characterized by the
existence of a central
mass of parenchyma
known as the medulla
or pith. The first con-
dition is represented
in Fig. 199, portray-
ing the transverse
section of the stem of
a species of Glei-
chenia. The siphon-
ostelic modification is
delineated in Fig. 200,
reproducingthe

3y transverse aspect of
Fic. 199.—Transverse section of the stem of the stem of the
Gleichenia species.

maidenhair fern,
Adiantum pedatum. In the second figure we find the fibrovascular
tissues organized in the form of a tube, limited both internally and
externally by an endodermal boundary which becomes continuous
around the margins of the gaps caused by the exit of the traces of the
lateral branches and leaves. Not only is the tubular central cylin-
der bounded continuously by an endodermal layer, but it is likewise
characterized in the particular case under discussion by an inner
and outer lining of phloem. In the walls of the stelar tube so
organized there are gaps formed in connection with the exit of
the fibrovascular strands leading to both leaves and lateral branches.
In addition to these there may be interruptions in the continuity
of the fibrovascular hollow cylinder which are not related to

THE FILICALES 279

departing strands of any organs. The root is not responsible for
the appearance of a gap in the wall of the stelar tube unless it
happens, as is sometimes the case, to be closely related to a foliar
organ. Under these conditions the apparent gap is naturally
foliar and not radical.

Before passing to the discussion of modifications of the siphon-
ostelic central cylinder as presented by the stem in the Filicales
it will be well to consider the general topography of the fibrovascular
system in the various organs. The account of the stelar system

Fic. 200.—Transverse section of the stem of Adiantum pedatum

of the stem outlined in the preceding paragraph will suffice for
the cauline organ, so that it is possible to turn at once to the
discussion of the leaf and the root. As has been indicated above,
the foliar trace departs from the tubular central cylinder of the
axis in topographical relation to a lacuna in the stelar wall
which is known as the foliar gap. The outgoing foliar trace in
the lower region where it runs in the stipe or rachis or even
sometimes in the subdivisions in the main veins of the flattened
region or lamina, is concentric or bicollateral in its organization.
This characterization means that the xylem is either completely
surrounded by phloem or at least has phloem on its two
opposite sides. As the fibrovascular strands which innervate
the blade of the leaf become more finely divided they lose

280 THE ANATOMY OF WOODY PLANTS

their original concentric or bicollateral structure and develop
a collateral organization. This condition of the finer fibrovascular
structure of the foliar organ is clearly correlated with the dorsi-
ventral structure of the leaf as a whole, and favors its function-
ing in relation to photosynthesis and transpiration. The upper
region of the foliar strands is consequently largely under the sway
of physiological conditions, while in their lower course, and espe-
cially before they have suffered much in magnitude as a result of
subdivision, the ancestral conditions may as a rule be more readily
observed. . Further, the more aberrant the anatomical structures
are in any given case the less will be the degree of development
of the ancestral conditions in the foliar traces. In argument
regarding the interpretation of the fibrovascular structures in
stems the organization of the foliar trace after it has left the
stele of the leaf is of great importance. In the root of such
relatively low types as the Filicales little evidence is supplied
which is of value in the interpretation of the primitive organiza-
tion of the tubular stele of the stem. This statement holds,
not only for the true ferns, but also for those lower and ancient
gymnosperms which have the most marked filicinean affinities.
It will be apparent, however, when the discussion of the anatomical
organization of the axis in higher gymnosperms and angiosperms
is reached, that the root assumes an evolutionary significance
which is not observed in the lower groups of the Vasculares. With
these preliminary remarks it is possible to pass with advantage
to the consideration of the organization and evolution of the
tubular central cylinder of the axis. »

First will be discussed modifications of the tubular stele which
are in the direction of greater complexity. In many ferns the
central cylinder or stele in the adult stem is represented by a
complex grouping of strands. In the bracken fern (Pleris aquilina),
as is shown in Fig. 201, the older stem presents in transverse
section two series of bundles—two large central ones and a ring
of usually much smaller ones forming a circle outside these. The
significance of the conditions present in the stem of this most
commonly studied fern have been very generally misunderstood.
It has been maintained by the distinguished French anatomist

THE FILICALES 281

Van Tieghem that the vascular system of the bracken arises by
the continued forking of an originally simple (protostelic) strand.
According to his view the two large central bundles are the first
to appear as the result of the process of forking, and the smaller
circle of strands lying outside these is formed later. On account
of the supposed origin of the strands in the stem of Pleris aquilina
by repeated division, the name polystelic was given by Van Tieghem
to this and similar
conditions of ana-
tomical organization
in the ferns and their
allies. As a matter
of fact the situation
is very different
indeed from that
indicated by the
term polystelic. In
Fig. 202a is shown
the transverse sec-
tion of a very young
stem of the bracken
when it is still in the
upright condition
and has not given rise to the subterranean horizontal branches
which come into existence at a comparatively early stage. The
stem at this age is obviously siphonostelic and is marked by a gap
correlated to an outgoing leaf trace. In 0 a later stage of
development, characteristic of the young horizontal stem, is
shown. Here the tubular central cylinder is giving off internally
and toward the center a medullary strand. Later the single
medullary strand becomes double and the condition attained in
the adult is reached. It is clear from an examination of the
actual course of development in the stem of the bracken fern
that the medullary strands are not formed first, as is assumed
by Van Tieghem, nor does the stelar system of the adult result
from repeated bifurcation of an originally single strand. On the
contrary, the course of development presents first a protostelic

Fic. 201.—Complicated bundle system of Pteris aquilina

282 THE ANATOMY OF WOODY PLANTS

condition in which no medullary tissues are present, a phase
followed by the siphonostelic one, which in turn develops medullary
strands from the inner surface of its walls. The outer series of
bundles is consequently antecedent to the medullary bundles, a
situation which may be readily inferred without the study of
the young plant from the fact that the roots are attached to the
external strands. Not only does the bracken manifest the general

Fic. 202.—Diagram to illustrate the development of the stem in Péeris aquilina.
Explanation in the text.

conditions described above, but all ferns with complicated arrange-
ments of conducting strands in the stem can readily be included
under the same general statement. This, for example, is true
of the complex stem of the Cyathaceae or tree-ferns and also of
that of the large tropical ferns known as the Marattiaceae. It is
apparent from the account supplied in the present connection
that the tubular condition is both typical and primitive for the
ferns in general, with the sole exception of those forms in which
the organization of the stele maintains the original protostelic
condition. This type of structure is, however, comparatively
rare in existing ferns, although it has been found, as would

THE FILICALES 283

naturally be expected, much more generally in Paleozoic types
belonging to the Filicales.

Before proceeding further with the question of the organization
of the tubular or siphonostelic type of central cylinder, it will be
well to discuss the important problem of the origin of the pith
or medulla in the Filicales. The situation in this group is much
more favorable than it is in the Lycopsida, living and extinct,
because the facts at our disposal are much fuller and as a conse-
quence put the whole subject on a more satisfactory footing.
There are two views in regard to the origin of the medulla in
stems of the type here designated siphonostelic. One hypothesis
considers it as originating from the central region of the xylem
of the stele by the transformation of tracheary elements into
parenchyma. It has been shown in earlier chapters that there
can be no question that parenchymatous elements may be derived
_from tracheids in the primary wood, and the evidence in this
direction clearly points to a possible origin of the medullary tissues
from transformed tracheids. Indeed, in the case of the lepidoden-
drids the facts which favor the tracheary origin of the medulla
are not without importance. Therefore, if we are to regard the
medulla as derived from tracheary tissue on the basis of the exist-
ence of transitional stages between tracheids and parenchyma,
it will be only fair on the other side to assume that where elements
clearly of a cortical nature occur in the region of the pith they
afford evidence for the other hypothesis of the origin of medullary
parenchyma, namely, its derivation from the included cortex.
It is, of course, illogical to admit evidence for a derivation of
medullary structures from tracheids and at the same time to
reject equally cogent data as to the origin of the pith from cortical
tissues. Even in the lepidodendrids the evidence for the trache-
ary derivation of the medullary structures is not complete,
for transitions from tracheids to parenchymatous cells are pre-
sented only by protostelic stems and are entirely absent in
tubular cylinders. Indeed, in cases where secondary growth
is not characteristic of the siphonostele in this group, the medullary
region of the stele is frequently occupied by sclerenchymatous
elements similar to those which occur in the cortex. The hypothesis

284 THE ANATOMY OF WOODY PLANTS

of the tracheary origin of the pith is also open to question in other
representatives of the Lycopsida. In Phylloglossum the medulla
in the region of the tuberous portion of the stem is actually sur-
rounded by an endodermal layer, a structure peculiar to the cortex.
In Selaginella laevigata, also, the tubular stele is bounded internally
as well as externally by a well-defined endodermal layer, and the
tissues of the pith, further, clearly resemble those of the cortex.
In both Psilotum and Tmesipteris the medullary tissues are fre-
quently composed to a large extent of brown sclerenchymatous
elements resembling similar structures found in the external
fundamental tissues. In the genus Egquisetum dark-brown scle-
renchyma is present in the medulla and the cortex. It is clear
on the basis of data derived from resemblances between the tissues
of the pith and the cortex, on the one hand, and the tracheary
tissue, on the other, that the preponderance of evidence weighs
heavily on the side of the fundamental or cortical origin of the
medulla. And heavily as the scale seems to incline on the side
of the fundamental derivation of the pith in the Lycopsida, it
seems entirely overwhelming in the case of the Filicales.

There are two general arguments which have been invoked
against the origin of the pith from the fundamental tissues in
the Filicales. One is the denial that it is possible for the stele
to include the tissues of the cortex. This reasoning cannot be
given very serious weight in view of the fact that in certain fili-
cinean steles—for example, that of the polypodiaceous genus
Onoclea and the schizeaceous genus Anemia—the possibility of
the inclusion of fundamental tissues within the tubular stele must
apparently be granted, because the epidermis and chaffy ramentum
as well as the outside air are actually included within the medullary
region of the central cylinder. Moreover, the phloem, a tissue
primitively occurring on the outer surface of the central cylinder,
is frequently included in its interior. If any further evidence
were needed as to the possibility of the xylem including tissues
of another morphological category, it is furnished by the case
of certain filicinean foliar traces within which it is universally
admitted, even by the most convinced adherents of the hypothesis
of the tracheary origin of the pith, fundamental tissues may not

THE FILICALES 285

only be included, but in some instances may be entirely shut off
from the similar tissues outside. Good illustrations of this con-
dition are provided by species of the Gleicheniaceae, Polypodiaceae
(notably the common bracken fern), and Marattiaceae. The
possibility, then, of the inclusion of the pith within the stele must
apparently be granted.

The second argument against the cortical origin of the medulla
is directed against the evidence based on the similar histological
nature of pith and cortex in many cases, a resemblance which
is the more marked the more the medullary tissues are in histo-
logical continuity with those lying outside the tubular stele. It
is assumed by the advocates of the tracheary origin of the pith
that the striking histological similarity which often exists
between the medulla and the cortex is merely ‘‘physiological.”’
This argument would be worth more if its exponents did not admit
all histological evidence in favor of the tracheary origin of the
medullary tissues while denying that indicating their derivation
from the fundamental system. Such argument is clearly fallacious,
and the logical procedure is to admit to equal consideration evi-
dence for the tracheary hypothesis of the appearance of the medulla
on the one hand and that for its derivation by the inclusion of
cortical or fundamental tissues on the other. The advocates of
the cortical origin of the central parenchyma of the tubular stele
are apparently advantageously situated in this respect, for they
can equally well emphasize the extremely abundant data for
the origin of the pith from the fundamental system and at the
same time explain away the meager evidence for the tracheary
derivation of the medullary tissues. So far as the plentiful evi-
dence in the case of the Filicales is concerned, it seems beyond
reasonable doubt that the median parenchyma of the tubular
or siphonostelic central cylinder has come from the outside and
is not the result of internal differentiation within the stele.

The question of the origin of the pith is a very important one
from the phylogenetic standpoint, since correct views in regard
to this matter are necessary for the interpretation of the evolution
of the tubular cylinder from lower to higher forms. It would
be going beyond the range of the present somewhat elementary

286 THE ANATOMY OF WOODY PLANTS

treatise upon the anatomy of the vascular plants to elaborate at
any length the question of the phylogeny of the tubular central
cylinder, important as this is for the proper understanding of
the higher plants in the light of the doctrine of descent. The
whole situation may conveniently be illustrated by reference to
the Osmundaceae. In discussing the evolutionary principles
involved in the development of the siphonostele in this group, we

Fic. 203.—Transverse section of the stem of Osmundites skidegatensis

shall appropriately start with types from earlier geological periods.
In Fig. 203 is shown a photograph of the central cylinder of a
fossil osmundaceous stem from the Lower Cretaceous of Western
Canada. In the stem under discussion the magnitude of the
stelar system is much greater than that found in any living species
belonging to the tribe and, in fact, more clearly resembles the
conditions presented in a large stem of a fern of the ordinary type.
The pith as well as the cortex is occupied by numerous dark masses
of brown sclerenchyma—a condition closely resembling the state
found in siphonostelic stems of ordinary ferns. An inspection
of the periphery of the stele shows the presence of numerous

THE FILICALES 287

foliar gaps, through some of which the cortex and the pith are
actually continuous, as is made clear by Fig. 204, representing
a small segment of the fibrovascular cylinder. In Fig. 205 is
shown a still more magnified view of the marginal region of the
stele, and it is here apparent that phloem is present both on the
inside and on the outside of the stelar tube. The situation re-
vealed by the three figures of the stem of Osmundites skidegatensis
shown above
makes it evident
that in this ancient
representative of
the group a condi-
tion of organiza-
tion more nearly
resembling that
found in our ordi-
nary polypodiace-
ous ferns was
present. The only
important differ-
ence is furnished
by the extremely
numerous foliar
gaps correspond-

: Fic. 204.—Part of the last, more highly magnified to
ing to the more _ show the leaf gaps.

crowded condition

of the leaves. This, of course, is not a morphologically important
distinction.

After the description of an osmundaceous fern from the Meso-
zoic, the stele of living species of the Osmundaceae may con-
veniently be considered. Fig. 206 illustrates the organization
of the central cylinder of Osmunda cinnamomea, the cinnamon
fern. The fibrovascular tissues in this case are characterized by
the same numerous foliar gaps as are found in O. skidegatensis,
but the cortex and the pith do not communicate through them
as in the Mesozoic type. Further, the phloem in the specimen
under discussion, unlike that present in the cretaceous species,

288 THE ANATOMY OF WOODY PLANTS

is confined to the outside of the xylem. Endodermal layers,
both internal and external, are present, and the pith often contains
patches of brown sclerotic cells similar to those found in the cortex.
In spite of the fact that the bundles of the central cylinder of the
stem are collateral, those of the leaves are concentric in organiza-
tion. Applying the principles of comparative anatomy to the
situation, we find that in O. cinnamomea the pith and cortex
are of common
origin and that the
bundles of the
stele, on the evi-
dence furnished by
the foliar strands,
were formerly con-
centric instructure.
The conclusion
which is reached
from the considera-

aes
208%

eo Beem ie Oege ae ee tion of the stem of

08> .
the species under

discussion, in the
light of universally
valid principles of

Fic. 205.—Part of the stem of O. skidegatensis, still comparative anat-
more highly magnified to show the presence of both
internal and external phloem.

omy, is that its
tubular stele was
formerly concentric and that the foliar gaps were once large
enough to permit of the joining of cortical and medullary tis-
sues with one another. The inferences drawn from the data
indicated above are, moreover, entirely justified by the considera-
tion of the anatomy of the Lower Cretaceous species described
in the preceding paragraph. In particularly vigorous specimens
of the stem in O. cinnamomea the ancestral condition, moreover,
frequently returns, for both open foliar gaps and internal phloem
are often seen in such axes. In O. regalis among living repre-
sentatives of the Osmundaceae medullary brown sclerenchyma
is occasionally found in the pith, although an internal endo-

THE FILICALES 289

dermis has never been observed. In O. claytoniana the pith
never resembles the cortical tissues, and neither internal phloem
nor endodermis is known to exist even in the most vigorous speci-
mens. In the three living species cited in the present chapter
we find a reduction series in which O. cinnamomea represents the
most primitive condition and O. claytoniana the most aberrant.

A quite different interpretation is put on the anatomical facts
cited in the two preceding paragraphs by those who adhere to

Fic. 206.—Photographs of the fibrovascular region of the stem in Osmunda
cinnamomea, showing internal endodermis and medullary sclerenchyma.

the hypothesis of the tracheary origin of the pith. To those who
adopt the view that the medulla is derived from the substance of
the stele, the type of Osmunda without either internal phloem or
endodermis is the more primitive, while that in which both these
structures occur is more modern. Those who regard the anatomi-
cal facts from this standpoint are in a position that offers many
difficulties. First of all, they have to assume that an old type
like Osmunda skidegatensis has gained a high degree of development
and that the modern forms without open foliar gaps, sclerotic
pith, internal endodermis, and internal phloem represent in reality
a more primitive condition of organization than does the Mesozoic
type cited above. This view is not only not in harmony with

290 THE ANATOMY OF WOODY PLANTS

the paleontological facts, but is likewise at variance with the
fundamental principles of comparative anatomy. It presents
the further serious difficulty of supposing that the simpler type
of stele characteristic of the more modern representatives of the
Osmundaceae, although primitive, has nevertheless come from a
more complicated condition in the past. Finally, the hypothesis
of the greater primitiveness of tubular steles without internal
phloem supplies no valid explanation of the frequent occurrence
of internal phloem, internal endodermis, and cortical sclerenchyma
in the medullary region of various existing species of the Osmun-
daceae. The view that such structures where they occur are
“physiological” has little to commend it and is, moreover, a
hypothesis which cuts both ways; for if we are to interpret cortical
structures occurring in the medulla as merely physiological, we
must likewise consider the possibility of a similar explanation
of the rare and quite exceptional occurrence of a so-called ‘‘ mixed
pith,” consisting partially of tracheary tissue and partially of
parenchyma.

The Osmundaceae on the whole present the evidence in regard
to the evolution of the tubular stele in later geological times more
clearly than does any other group of ferns. It should, however,
be emphasized that in practically all cases where the siphonostelic
central cylinder without internal phloem or endodermis is present
there are found elements in the region of the medulla which
present the characteristics of cortical tissues. It seems illogical
to interpret these structures as possessing in every case merely a
physiological significance, particularly since, in view of their
imperfect and sporadic development, it is difficult to attribute
to them any functional importance whatever. The structures
in question possess, in fact, all the criteria of vestigial features
persisting from an earlier more complicated condition of organiza-
tion. Our knowledge of groups which have their climax of devel-
opment in the past justifies the view that evolution in decadent
series proceeds by simplification. A final general objection to
the tracheary hypothesis of the origin of the pith is the outstanding
fact that the more modern groups of plants, and especially the
seed plants, are the ones which particularly and universally represent

THE FILICALES 291

the condition which the supporters of this hypothesis regard as
primitive—that of the tubular stele without internal phloem and
endodermis. On the other hand, the Filicales, in:which the sipho-
nostele commonly incloses a pith strikingly resembling the tissues
of the cortex and quite generally lined inwardly with endodermis
and phloem, must be regarded as more modern. The improbability
of this general hypothesis seems very great.

It may be assumed on the basis of the considerations advanced
in earlier paragraphs that the siphonostelic condition in the Filicales
is susceptible of complication through the development of medullary
strands on the one hand, and, on the other, of simplification
through the loss of internal phloem and endodermis and the
progressive narrowing of the leaf gaps resulting in sequestration
of the pith. The view that the latter simpler condition is more
primitive not only runs counter to the conditions shown in the
general sequence of types in geological time, but is also at variance
with the general principles of comparative anatomy detailed in a
former chapter of this work. The hypothesis that the simpler
condition of the tubular stele is more primitive than the more
complex marks, moreover, an evolutionary attitude which is
becoming generally obsolete as our knowledge of the actual organ-
ization of extinct forms, particularly of those types which mani-
fested their greatest luxuriance in the past, becomes fuller and
more complete. It is now clear from evidence of this kind that
the lower gymnosperms have come from the Filicales as a result
of the simplification and reduction of the primary structures
of the stele of the stem on the one hand, accompanied by the
marked development of secondary fibrovascular tissues on the
other. In the gymnospermous Pteropsida this anatomical progress
has been associated with the attainment of the seed habit. In
the Lycopsida, by contrast, a similar anatomical progress culmi-
nated in much earlier geological times and was not accompanied
by the evolution of true seeds.

CHAPTER XXII

THE ARCHIGYMNOSPERMAE: CYCADOFILICALES
AND CYCADALES

The earlier seed plants of Paleozoic and Mesozoic time were
characterized by the possession of naked seeds upon which the
microspores or pollen grains were directly deposited. The free
exposure of the seminal structures to the air has gained for the
types marked by this feature the appellation of gymnosperms.
In the large group thus characterized there are two main modifi-
cations. In the earlier and more primitive gymnosperms the
pollen grains or microspores were accommodated in a chamber in
the apex of the megasporangium known as the pollen chamber.
This cavity, filled with fluid derived either from the surrounding
parenchymatous cells or in some cases from a special fibrovascular
system present in the walls of the megasporangium, provided
for the germination of the microspores and the subsequent fertil-
- ization, effected in every instance by swimming sperms. These
forms in general present a marked resemblance to the Filicales,
often in external habit and always in internal anatomical structure.
In contrast to them are the higher gymnosperms, in which there
is no true pollen chamber. Here the act of fertilization is effected,
not by swimming sperms, but by means of a pollen tube developed
as an outgrowth of the microspore and directed toward the egg.
The external habit of these gymnosperms is never fernlike, and
their anatomical organization shows only the slightest cryptogamic
features in living forms. The types possessing pollen chamber,
swimming sperms, frequent filicinean habit, and cryptogamic
organization of the fibrovascular structures are conveniently
designated as the Archigymnospermae or more primitive gymno-
sperms. ‘Those forms which are contrasted to these by the absence
of pollen chamber and the presence of pollen tube in connection
with fertilization, as well as by the disappearance of filicinean
habit and anatomical characteristics, may be, on the other hand,
designated as the Metagymnospermae or higher gymnosperms.

292

CYCADOFILICALES AND CYCADALES 293

The present and the following chapter will deal with the repre-
sentatives of the Archigymnospermae.

Our knowledge of the Archigymnospermae, from the fact
that many of the group are extinct, is necessarily incomplete.
It will accordingly be convenient in the present chapter to focus
attention on the best-known fossil group and the nearly related
family which possesses living representatives. The extinct aggrega-
tion of forms which
here present the
strongest claim to
attention are the

Cycadofilicales,

characterized by a _
habit so closely re- { “ae
sembling that of the ‘

true ferns that it is 8 er tt
only since the very | é
end of the last cen-
tury that they have
been recognized as
seedplants. The
type of seed con- :
nected with the veg- to tt. x

etative structures of Fic. 207.—Transverse section of the stem of Heter-

fee Gre ealieaics angium (after Scott).

has been diagrammatically indicated in an earlier chapter, to which
the reader is here referred. In the present connection only ana-
tomical features of the group need be discussed. The antiquity
of the Cycadofilicales is vouched for, not only by their Paleozoic
occurrence, but also by the occasional presence of protostelic
stems, axes of this type not being known for any other group
of seed plants living or extinct. Fig. 207 illustrates the organiza-
tion of the protostelic axis of the genus Heterangiwm from the
English Carboniferous. The transverse section shows the presence
of an external ribbing of sclerenchymatous strands, a feature com-
mon to many older representatives of the Gymnospermae. The
fibrovascular apparatus consists of protostelic primary wood sur-
rounded by a thin layer of secondary xylem.

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204 THE ANATOMY OF WOODY PLANTS

We may now turn our attention to more complicated stems
belonging to the general group of the Cycadofilicales. The genus
Medullosa is of particular importance, since it possesses a type of
stelar organization presented by several or many concentric strands
which are best considered as belonging to a siphonostelic cylinder.
The general organization of the axis in the genus under discussion
is well shown in Fig. 208, which portrays in a somewhat diagram-
matic fashion the structures involved. There are three large
concentric fibrovascular strands present which consist of a central
core of primary wood indicated by cross-hatching, surrounded

Fic. 208.—Transverse section of the stem of Medullosa anglica (after Scott)

with a cordon of secondary xylem, represented by radiating lines.
The outline of the stem is irregular, the salients being due to the
presence of the bases of large leaves. The surface is covered with
the reticulated sclerenchymatous strands already described in the
case of Heterangium. The fundamental tissues are occupied by
secretory canals, which probably contained mucilage, as do those
of the living Cycadales. In the cortical region are accommodated
larger and smaller strands ordinarily lacking secondary growth.
The bundles of greater dimensions are mostly the larger foliar
bundles and possess concentric organization. The small strands
which are found characteristically in the actual leaf bases are
collateral in their organization and exarch in the structure of their
xylem. Fig. 209 illustrates two of the smaller strands together
with fundamental tissues and contained mucilage- canals. The
general situation in the genus Medullosa is complicated frequently

CYCADOFILICALES AND CYCADALES 2905

by the presence of more numerous large concentric fibrovascular
strands than those shown in Fig. 208. In such instances the con-
centric strands often show a more or less complete degeneracy of
the secondary xylem on their inner surfaces. This situation has
been regarded with reason as of significance in foreshadowing the
type of organization found in the axes of the Cycadales, living
and extinct. It will be seen from the account of Medullosa here
supplied that the genus presents some marked features of ana-
tomical resemblance to
ferns, the main contrasts
in organization being due
to the appearance of
secondary growth in con-
nection with the strands
belonging to the stem
proper. In accordance
with the canons of anat-
omy formulated in an
earlier chapter of the pres-
ent work, the secondary
activity has not yet pene-
trated into the traces which
pass out into the more con-
servative foliar structures. The Medulloseae are undoubtedly of
great interest from the standpoint of the evolution of cycadean
forms, and there can be little question that these types so common
as charred remains in the Carboniferous coals of Europe and
America came very near to being the actual ancestors of our
living Cycadales.

As a third illustration of the Cycadofilicales we may take the
genus Lyginodendron, the organization of which has been so admir-
ably described by English anatomists. Fig. 210 illustrates the
structure of the stem in this genus. The same sclerified ribbing
is observed as in the other two genera discussed above. The
central cylinder, however, presents a marked contrast to that of
either Heterangium or Medullosa, for it consists of collateral strands
arranged in a closed circle. The pith is occupied by sclerotic

Fic. 209.—Foliar bundles of Medullosa anglica

2096 THE ANATOMY OF WOODY PLANTS

nests of cells which are duplicated by similar structures in the
cortex. The inner surface of the cylinder of xylem shows clusters
of primary wood which sharply contrast with the secondary
region by the irregularity of the arrangement of the elements and
the absence of rays. Fig. 211 illustrates a portion of the cylinder
more highly magnified, and with the greater enlargement the
distinct and mesarch character of the primary region becomes
apparent. The secondary wood is characterized both by its
regularly radial arrangement and by the presence of rays which

Fic. 210.—Transverse section of the stem of Lyginodendron oldhamtum (after
Scott).

are often two or three cells in width. The tracheids of secondary
origin are marked by crowded pits which alternate in position
and are often angular from mutual contact. These pores are
confined strictly to the radial walls of the elements, no tangential
pitting occurring in the secondary tracheids of any Paleozoic
gymnosperms with which we are at the present time acquainted.
The foliar traces in Lyginodendron are at first single, but bifurcate
shortly after leaving the primary region of the stem, and lose
their secondary xylem, becoming concentric instead of collateral
in their organization. It is evident on the basis of the general
canons earlier elucidated that the bundles of the stem in the
case of Lyginodendron must formerly have been concentric in
their structure, since this condition persists in the traces of the
leaves. Further, the presence of secondary wood in the stem

CYCADOFILICALES AND CYCADALES 207

and its absence in the foliar traces show that secondary growth
is a comparatively recently acquired feature in the genus under
discussion. The sclerotic nests in the pith appear to vouch for
the extra-stelar origin of the medullary region. The situation
as a whole is like that found in the case of the Osmundaceae,
except for the complication introduced by the presence of the
secondary growth. In the genus Botrychium among the Ophio-
glossaceae secondary tissues, however, are often well developed,
and it has long
been realized that
the occurrence of
secondary activity
in the fibrovascular
tissues is not an
extremely impor-
tant criterion from
the evolutionary
standpoint. It
was at one time
thought that the
genus Lyginoden-
dron was the proto-
Eye 7 ot, thie
Cycadales, but this
opinion has been

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given up m favor ary wood in Lyginodendron.
of a derivation of

the cycadean type from forms like the Medulloseae, for which the
evidence is extremely strong.

We may now turn our attention to the anatomical organization
of that group of gymnosperms which still persists as the Cycadales.
This family was well developed in Mesozoic times, but there is
no evidence that it was represented in the Paleozoic age. The
stem of the cycadean gymnosperms, in its modern representatives
at any rate, is soft and parenchymatous and is characterized by a
very large medulla. Both cortex and medulla contain numerous
mucilage canals. The woody cylinder is ordinarily very thin

2098 THE ANATOMY OF WOODY PLANTS

and in the vegetative axes does not manifest the presence of
cryptogamic or centripetal wood. In certain genera of the Cyca-
dales, both living and extinct, the woody cylinder often shows a
curious reduplication, resulting from the appearance of new circles
of fibrovascular tissue in the pericycle. The explanation of these
structures is somewhat difficult. One view is that they represent
a persistence of the numerous cycles of fibrovascular strands in
certain types of Medulloseae. This view, however, is rendered
difficult by a situation which often presents itself in the Cycadales.
In the region of the stem where a cone is attached the medulla is
frequently occupied by numerous strands which are absent else-
where. It is natural to regard such structures as representing a
return of the medullosan organization in relation to the attachment
of conservative reproductive axes. If this explanation be adopted
for the medullary bundles, the interpretation of the supernumer-
ary zones of fibrovascular tissue formed successively in the peri-
cycle of the original cylinder cannot be accepted. A more probable
elucidation of the situation is furnished by the habit of certain
Mesozoic representatives of the group which are contrasted to
modern forms by their slender and freely branching habit. This
condition, for example, is found in the genus Anomozamites. It is
not improbable that the genus mentioned was a climbing plant.
The formation of supernumerary zones of fibrovascular tissue is
a common feature of organization of the stem in climbing plants
of widely diverse gymnospermous and dicotyledonous affinities.
It is consequently not unlikely that the Mesozoic forebears of living
Cycadales developed successive zones of fibrovascular tissues in
their stems as a result of a climbing habit. After this feature had
become thoroughly fixed by the lapse of long geological time, a
desert habit was superimposed on it with a resulting anatomical
organization such as is found in the living representatives of the
group. A similar hypothesis has to be invoked, as will be shown
in a subsequent chapter, to explain the remarkable anatomical
resemblance between the desert-inhabiting gnetalian genus Welwit-
schia and the vinelike Gnetum.

Although there is no instance of the presence of centripetal
or cryptogamic wood in the vegetative axis of the Cycadales,

CYCADOFILICALES AND CYCADALES 299

not a few examples of the existence of xylem of this type have
been described by Scott in the reproductive axes or cones. In
Fig. 212 is shown a transverse section of part of the peduncle of
the female cone of Stangeria paradoxa. Although the wood as a
whole is strongly centrifugal or peripheral in its development, a

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Fic. 212.—Transverse section of one of the strands of the peduncle of the cone
in Stangeria paradoxa.

small amount is formed over against the medullary region. This
condition is a persistence of the centripetal wood of the Paleozoic
Cycadofilicales, from which the Cycadales have in all probability
been derived. The anatomical principle here involved is one of
great importance and was first clearly emphasized by Scott.
Subsequent to his observations on the Cycadales, it has been

300° THE ANATOMY OF WOODY PLANTS

discovered to be exemplified in a remarkable manner by the
observation of the wood in the axes of members of the Conifer-
ales, as will be noted at a later stage.

The leaves of the Cycadales are of great importance from the
standpoint of evolutionary anatomy. Here the centripetal wood,
which is poorly developed at best in reproductive axes, is abundant,
well defined, and universal in its occurrence. Fig. 213 illustrates
the transverse section of a leaflet of Cycas revoluta. It is apparent

ee, ae rae: ege. Sey
Ee SOY FEED
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Fic. 213.—Transverse section of a foliar strand in Cycas revoluta

that the mass of the xylem is of primary origin and spreads out
in a fanlike fashion toward the upper surface of the leaf. It is
therefore centripetal in its development. Below the mass of
centripetal xylem lies a much more scanty development in the
region of the regularly arranged phloem. This is the centrifugal
xylem, which is partially primary and partially secondary in its
origin. Fig. 214 illustrates the longitudinal section of a leaf
trace in Cycas. The phloem, easily recognized by its characteristic
sieve tubes, is seen toward the right. To the left is situated the
centripetal wood, composed from left to right of pitted, scalariform,
reticulate, and spiral elements, the latter constituting the protoxy-

CYCADOFILICALES AND CYCADALES 301

lem. After a narrow parenchymatous interval the pitted elements
of the centrifugal metaxylem are seen in close proximity to the
phloem. The general situation in the transverse and longitudinal
sections of the foliar strands shown in the two figures is universal
for the Cycadales, any minor divergences which have been ob-
served being mainly the result of the smaller size of the strands in

the genera other than Cycas.

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Fic. 214.—Longitudinal section of a foliar strand in Cycas revoluta

The foliar traces in the genus under consideration show other
interesting features of organization in the lower part of their
course, where they pass from the base of the petiole of the leaf
into the cortex of the axis. These features have to do both with
the arrangement of the traces and with their anatomical organiza-
tion. In the vegetative axes of living genera of the Cycadales
the foliar strands, instead of passing directly outward to their
respective leaves, pursue a meandering course through the cortex
and are known as ‘“‘girdles.”” This condition is confined to the
modern genera, for in the Mesozoic representatives of the group
united under the caption of Bennettitales the leaf traces pass
directly to their corresponding leaves. Interestingly enough this

302 THE ANATOMY OF WOODY PLANTS

condition is paralleled in the reproductive axes of the cones of
the living Cycadales and in the seedling of their most primitive
genus, Cycas.

As regards anatomical organization the foliar traces in their
cortical course, and even sometimes in the bases of the petioles,
show an interesting resemblance to the bundles of the stem in
Medullosa. Fig. 215 illustrates the structure of one of the leaf
traces of Cycas re-
voluta as it passes
through the cortex.
The bun dleers
clearly concentric
and consists of a
central, core sor
primary wood sur-
rounded by regu-
larly seriate
secondary xylem
and traversed by
wood rays. If the
figure under discus-
sion be compared
with that of a stem
bundle of Medul-
losa (Fig. 208), it
becomes evident that the only striking difference is presented by
the much smaller size of the strand of the living genus.

The root in the Cycadales presents little that is of interest,
since the feature of the presence of centripetal wood is equally
exemplified by all roots as a characteristic connected with their
extremely conservative organization. It is only between Mesozoic
and modern types in which the evolutionary importance of the
centripetal wood has passed into the background that the anatom-
ical structure of the root comes into the phylogenetic foreground
in connection with the doctrine of descent. A feature not of
evolutionary interest presented by the cycadean root is the frequent
presence of root tubercles.

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Fic. 215.—Cortical strand of Cycas revoluta

CYCADOFILICALES AND CYCADALES 303

The Mesozoic Cycadales are ordinarily grouped under the
heading Bennettitales on account of the remarkable features pre-
sented by their reproductive structures, which are very different
from those exemplified by any living Cycadales. The uniting of
both microsporophylls and megasporophylls in the same strobilus
is a frequent feature of this group and has been considered by some
to indicate an affinity with the angiosperms, particularly as the
ovuliferous sporophylls are shut in by a panoply provided by the
swollen apices of so-called interseminal scales—sterile structures
interposed among the fertile ones. The question of the possible
descent of the angiosperms from the Bennettitales is one which
can scarcely be considered seriously from the anatomical standpoint,
since there is practically nothing in common in the anatomical
organization of the two groups, either in the reproductive or in
the vegetative features. The fructifications of the Bennettitales
before their real affinities were known were attributed by the
distinguished French paleobotanist Saporta to a group which he
designated as proangiosperms. Now that their actual relation-
ships are known, there seems scarcely any reason for regarding
them as allied to the angiosperms. The Bennettitales supply an
excellent illustration of the relative conservatism of anatomical
structures, for, although they present marked differences in repro-
ductive features from the Cycadales, their vegetative anatomy
does not differ in any important particular from that of the modern
group.

If the anatomical situation for the Cycadales in the large
sense be summarized, it becomes clear on the basis of the general
canons of comparative anatomy elucidated in earlier pages of
the present volume that they exhibit strong filicinean features.
The anatomical examination of the foliar strands reveals crypto-
gamic or centripetal wood and in some cases concentric organiza-
tion. The evidence derived from the consideration of the anatomy
of the leaf is confirmed in a number of instances by the organiza-
tion of the strands of xylem in the reproductive axis. The testi-
mony of the conservative parts leads to the assumption of the
former presence of centripetal xylem and concentric fibrovascular
strands in the vegetative stem. The general result reached on

304 THE ANATOMY OF WOODY PLANTS

the basis of comparative anatomy receives strong support from
the organization of certain Paleozoic Cycadofilicales, notably the
Medulloseae. There seems to be little doubt that the genera of
Cycadales in the broadest sense of the term have been derived
from ancestors closely resembling vegetatively the Medullosa
type. It follows that so far as their primary fibrovascular struc-
tures are concerned they present a condition of reduction from
the anatomical organization found in the oldest and most fernlike
gymnosperms.

CHAPTER XXIII

THE ARCHIGYMNOSPERMAE: CORDAITALES
AND GINKGOALES

The two tribes of gymnosperms discussed in the present chapter
- no longer possess the fernlike habit of the Archigymnospermae
which have just been considered, but still maintain the pollen
chamber and the cryptogamic wood so characteristic of the Cyca-
dofilicales and Cycadales. Of the two groups here discussed the
Cordaitales became entirely extinct at the end of the Paleozoic
or at the beginning of the Mesozoic and have a geological range
extending far into the past. On the other hand, the Ginkgoales,
although present somewhat doubtfully in the Paleozoic, had their
greatest development in the Mesozoic and persist at the present
time in a monotypic genus of Eastern Asia.

The Cordaitales were freely branched trees resembling in
general habit our existing conifers, to which they are usually
considered to have given origin. The trunks often reached a
considerable size, and the woody cylinder included a large pith,
in some instances septate and occupied by air spaces, which,
however, unlike the medullary fistulae of Calamites, were much
more numerous than the internodes. The cavernous character
of the pith as well as its large size is responsible for the existence
of medullary casts in the case of the Cordaitales, and these are
assembled under the form-genus Artisia or Sternbergia. The
surface of the stem was often provided with the sclerenchymatous
armor which has been noted as a feature of organization of the
axes of the Cycadofilicales. Fig. 216 illustrates the general
topography of a segment of the stem in a cordaitean form. The
woody cylinder was without annual rings, except in the later
Paleozoic and in higher latitudes. This situation is very well
shown by Figs. 217 and 218, illustrating the structure of cordaitean
woods from Northern England (Lancashire) and Southern Canada
(Prince Edward Island). In the first specimen annual increments
can be somewhat clearly distinguished, while in the second they

395

306 THE ANATOMY OF WOODY PLANTS

are conspicuous by their absence. Although some indication of
the existence of annual diversities of temperature is supplied
by certain cordaitean stems, the organization of the annual ring
in such cases is extremely simple, and even the differentiation
involved in the presence of terminal tangential pitting is not seen,
as may be ascertained by reference to Fig. 219, representing a
specimen from Lancashire, England, in stereoscopic view under
a high magnification. It is apparent from the figures that the

Fic. 216.—Part of a transverse section of a cordaitean stem (after Scott)

secondary xylem of Cordaitales was of very simple structure and
that the rays, unlike those of the two tribes considered in a
previous chapter, were usually a single row of cells in width.
This narrowness of the strands of radial parenchyma is shared
by the Cordaitales with the Ginkgoales and the conifers. The
wood, like that of other Paleozoic gymnosperms, was entirely
without longitudinal parenchymatous elements, and the _bor-
dered pits were strictly confined to the radial walls of the
tracheids.

Another interesting feature of the organization of the xylem
in cordaitean forms was the extremely long region of transition
between the spiral elements of the protoxylem and the first pitted

CORDAITALES AND GINKGOALES 307

elements of the secondary wood. This is well shown in Fig. 220.
Among the living conifers this condition is most nearly paralleled
by the Abietineae.
The foliar trace
is of course of great
importance in this
as in other ancient
gymnosperms.
Fig. 221 illustrates
the organization of
one of the bundles
of a broad cordai-
tean leaf (Cordaites
principalis) in both
transverse and
longitudinal section.
In a is shown the
transverse topog-
raphy of the strand,
and it is clear that the centripetal wood is well developed, ending
upwardly in large
elements which are
pitted in their char-
acter. In the par-
ticular instance
figured there hap-
pens to be no devel-
Opment. of the
centrifugal xylem, so
that the phloem
abuts immediately
on the protoxylem.
With the flanks of
the metaxylem is
connected a zone of
narrow thick-walled
Fic. 218.—Cordaitean wood from Prince Edward Island elements which form

sy
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ait

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2.

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Gey
reth

~~

Fic. 217.—Cordaitean wood from England

308 THE ANATOMY OF WOODY PLANTS

a cordon completely around the phloem. Outside the cells of nar-
row lumen is found a second zone, composed of elements larger in
diameter. Both outer and inner zones consist of tracheary elements

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and constitute the so-called double transfusion sheath. In 8, the
longitudinal view, the various elements of the bundle are shown
in the median section. The protoxylem is continuous with tra-
cheids which by the usual transitions gradually pass into the

CORDAITALES AND GINKGOALES 309

pitted elements of the last-formed metaxylem. Below the pro-
toxylem lies the phloem, and still farther down the more elon-
gated and narrower elements of the inner transfusion sheath, which
in turn abut on the short, broad tracheary elements of the outer
transfusion sheath. At the very top lie other short transfusion
cells, and the inner elongated sheath in this region is absent as a
result of conditions which can readily be inferred from the in-
spection of the transverse view in a. There is some variety in
the development of the foliar bundles of the Cordaitales, but
all are characterized by the presence of well-marked centripetal
wood and a cordon of short tracheary elements, known as trans-

p pe 8p 8c bt

Fic. 220.—Longitudinal view of cordaitean wood near the pith (after Scott)

fusion cells, which are closely related to the centripetal or crypto-
gamic wood.

The root in cordaitean forms, for reasons applying equally
to all Paleozoic gymnosperms, presents no features of special
interest beyond illustrating the general cordaitean organization
modified to the needs of root organs.

It will be obvious from the statements made in the foregoing
paragraphs that there is clear evidence in the organization of
the foliar structures in the Cordaitales for their close affinity with
the Filicales, although naturally the degree of relationship is
much less intimate than that which characterizes the Cycadofili-
cales and even the Cycadales. Concerning the organization of
the microsporangia and seeds of the Cordaitales our knowledge
is unfortunately somewhat meager. The evidence in regard to
the microsporangium is not sufficiently definite to warrant an
opinion as to whether it was ectokinetic or endokinetic in its

310 THE ANATOMY OF WOODY PLANTS

mode of dehiscence; but, in view of the strong development of
transfusion tissue in the foliar organs of the group under discus-
sion, a clear feature of distinction from the Cycadales and Cyca-
dofilicales (in both of which the microsporangia are ectokinetic),
it is somewhat probable that the pollen sacs owed their dehiscence
to a layer of tracheary origin. The seeds of Cordaites have been

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Fic. 221.—Transverse and longitudinal sections of a leaf bundle in Cordaites
principalis.

anatomically investigated by Renault and they possessed a well-
marked pollen chamber. A more complete knowledge of the
reproductive structures of cordaitean forms and of the types
which connected them in the more remote Paleozoic with filicinean
ancestors is much to be desired.

The Ginkgoales are represented by a single living genus, but
were extremely abundant in the Mesozoic and are thought to
have been continued into the Paleozoic by the somewhat problem-
atical genus Whittleseya. Unfortunately our anatomical knowledge
of the group beyond that supplied by the investigation of the

CORDAITALES AND GINKGOALES ail

living genus is extremely meager and in fact is confined to the
structure of woods which have been referred to the group.

The stem in Ginkgo is characterized by the presence of clear
annual rings which terminate with tracheids provided with tan-
gential pits and in this respect reveal a marked contrast to the
tracheary elements constituting the remainder of the annual
increment. It is obvious that as regards the organization of the
annual ring the group under discussion is distinct from those rare
cordaitean stems in which yearly zones of growth can be dis-
tinguished, by the presence of tangential terminal pitting. In
other respects, however, the structure of the wood is clearly archaic,
for there are no parenchymatous elements present other than those
related to the rays. The pith and cortex in the group possess
secretory canals which are comparable to those found in certain
Abietineae.

The longitudinal aspect of the secondary xylem in Ginkgo
is very different from that of the Cordaitales. In the more ancient
group the radial pits are often extremely numerous and they are then
angular by mutual contact. In Ginkgo the pores of the tracheids
are not so abundant as to be described as crowded and, moreover,
instead of being alternating and angular as in the older tribe
are round and opposite. Another equally striking feature offers
itself in the presence of transverse bars of pectic cellulose in the
walls of the tracheids between the pairs of opposite pits. These
may conveniently be designated bars of Sanio, to distinguish
them from the trabeculae of Sanio, structures which are found
not uncommonly in all woods of secondary origin from (and
including) those of the Cycadales to those of the dicotyledons.
The latter structures consist of ligneous processes crossing the
cavity of the tracheid, possibly due to the activity of parasitic
fungi, while the true bars of Sanio are concealed in the wall itself
and consist of pectic cellulose. Bars of Sanio are found only in
the walls of tracheids of secondary origin, and statements as to
their occurrence in any elements of the primary wood are erroneous.

An interesting condition appears in the organization of the
secondary wood of the peduncle of the seed. In Fig. 222 is
shown the transition region in the xylem. It will be observed

gi

THE ANATOMY OF WOODY PLANTS

that the pitted tracheids nearest to the primary wood are entirely
without bars of Sanio, which make their appearance only at an
interval from the protoxylem. The pitting to a large extent is

alternate.

It is very generally admitted by competent judges

at the present time that the Ginkgoales are derived from cordaitean

ancestry,

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Fic. 222.—Longitudinal view of the tracheids in the peduncle of a seed in Ginkgo.
To the right is shown the arrangement of the tracheids in the mature wood.

organization of vegetative and reproductive axes evidence based
on the pitting and distribution of the bars of Sanio favorable to

such an

opinion. Farther away from the primary wood the

secondary tracheids quickly develop the opposite pitting and
bars of Sanio characteristic of the mature wood. It is clear from
the figure, moreover, that the tracheids of the primary wood are
quite devoid of structures of the nature of the bars of Sanio. It
is well to emphasize the conditions found in the organization of

the root

and reproductive axis of Ginkgo, because there prevails

CORDAITALES AND GINKGOALES 313

at the present time an almost inexcusable ignorance in regard to
the nature and distribution of the structures here designated
bars of Sanio. They are clearly correlated with opposite pitting
and are a feature of the secondary wood, not appearing in the
organization of the tracheids of the primary xylem. Evidently
the structures in question are of considerable value in the identi-
fication of gymnospermous woods and consequently must rank
high as a diagnostic criterion among competent anatomists.
The mature vegetative leaf in Ginkgo supplies very little evidence

Fic. 223.—Foliar bundle of Ginkgo, showing transfusion tissue (after Sprecher)

of the presence of centripetal elements in the strict sense of the
term. In the terminal region of the blade of the leaf a well-
marked zone of transfusion tissue manifests itself, as is shown in
Fig. 223; but typical centripetal tracheids are usually conspicuous
by their absence. In the cotyledon, however, the centripetal or
cryptogamic wood is present in a much clearer manner in accord-
ance with the principle of recapitulation discussed in an earlier
chapter. The reproductive leaves, both ovuliferous and staminate,
also show the centripetal elements in a good condition of develop-
ment, although even here they more nearly resemble transfusion
tissue. In the stalk which supports the pair of ovules centrip-
etal elements and ordinary transfusion cells are seen in the upper
region in great abundance and are likewise found in the collar sur-
rounding the base of the seeds. The situation presented by the

314 THE ANATOMY OF WOODY PLANTS

microsporophyll is, however, of greater interest in the present
connection. In the petiole of the bisporangiate microsporophyll
tracheary elements of a centripetal character occur on the upper
side of the protoxylem (Fig. 224). These elements can scarcely
be said to constitute typical centripetal
tracheids, since they are often of wide
lumen and are correspondingly abbrevi-
ated in length. As the foliar traces
ascend into proximity to the sporangia,
they separate from one another and the
xylem of each rotates so as to occupy a
position near the middle line of the

Fic. 224.—(a) longitudinal, (b) transverse, section of wood of bundle in micro-
sporophyll of Ginkgo.

sporophyll, while the strands of phloem turn outward, to end in the
bases of the sporangia. Meanwhile the transfusion elements occur-
ring on the upper side of the tracheary strands in their upward
course pass imperceptibly into the fibrously thickened mechanical
elements which are responsible for the dehiscence of the sporangium.
Further, the apex of the tracheary strands passes gradually by
means of short transfusion tracheids into the mechanical elements
which lie along the median sides of the sporangia. In this fashion
there is established an intimate relation between the tracheary
tissues of the bundles of the reticulate cells which constitute the

CORDAITALES AND GINKGOALES 315

opening mechanism of the microsporangia. The microsporophyll
of Ginkgo accordingly has a double interest from the evolutionary
standpoint, for it not only shows the centripetal or cryptogamic
wood more clearly than it is exhibited by the vegetative leaves,
but at the same time manifests its transition by imperceptible
gradations into the mechanical tissues of the sporangium wall.
As has been indicated in an earlier chapter, Ginkgo is the lowest
type in which the dehiscence of the microsporangium no longer
depends on an annulus derived from the epidermis but is effected
by an internal mechanism derived from the old centripetal or
cryptogamic wood of the fibrovascular bundle.

In the stalk of the ovule centripetal elements and transfusion
tissue are also well developed, but they apparently do not at any
time penetrate into the substance of the megasporangium. It is
not unlikely that tracheary tissues of a transfusionary nature were
formerly present in the megasporangial structures of the Gink-
goales, but that in the course of time they have suffered abortion.
The organization of a number of seeds of Paleozoic age of unascer-
tained affinities is good evidence in favor of the probability of this
view. Moreover, in one of these, Stephanospermum, characterized
by a tracheary mantle in the wall of the nucellus ending in the
pollen chamber, pollen grains are present, winged all around and
strongly resembling those of Ginkgo. It is accordingly not im-
possible that Stephanospermum was the seed of some Paleozoic
representative of the Ginkgoales.

The importance of the sole surviving and monotypic genus
Ginkgo from the standpoint of the evolutionary transition from
the ancient to the modern gymnosperms cannot be overestimated.
It constitutes virtually a link between the Archigymnospermae
and the Metagymnospermae, since it presents to so large a degree
the characteristics of both. Its affinities on the lower side are
clearly with the Cordaitales, as has been recognized by all com-
petent investigators in recent years. Its relationship with the
Abietineae among the Coniferales is equally well indicated by
comparative anatomical and paleobotanical data, as will be shown
in the following chapter.

The indications of relationship with the Cordaitales are pre-
sented in connection with the organization of the wood in primitive

316 THE ANATOMY OF WOODY PLANTS

organs and regions. It has been pointed out in the foregoing
paragraphs that, although centripetal wood of the cryptogamic
type is represented almost exclusively by transfusion tissues in
the mature vegetative leaf of Ginkgo, it is present in a clearly
recognizable form in the cotyledon, in the microsporophylls, and
in the peduncle of the ovuliferous apparatus. In the case of the
microsporophyll the xylem, and more particularly the vestigial
centripetal xylem and transfusion tissue, are in clear relation to
the reticulately thickened opening mechanism of the microspo-
rangia. ‘This feature is of value, not only as indicating the filiation
of the Ginkgoales with lower groups, but also as indicating the
morphological nature of the arrangements for dehiscence of the
spore sac in the seed plants above the Cycadales. The absence
-of longitudinal parenchyma in the secondary wood is another
criterion of the relationship of the Ginkgoales with lower groups,
while the presence of tangential pitting in the terminal region of
the summer wood clearly relates the group with modern gymno-
spermous types. The radial pitting of the tracheids and associated
structures is also of importance as indicating the phylogenetic
position of the genus. As has been shown above, the radial pits of
Ginkgo are opposite in the mature wood, and often in the inter-
vals between them, particularly toward the ends of the tracheids,
have transverse bars of pectic cellulose imbedded in the tracheary
wall, and these are conveniently designated bars of Sanio.
The opposite pitting and the occurrence of bars of Sanio are
features which clearly co-ordinate the wood of the Ginkgoales
with that of the higher gymnosperms. However, in the primitive
regions and organs of the living Ginkgo we find both the pitting
of the Cordaitales and the absence of bars of Sanio which are
universally characteristic of the older gymnosperms from the
Cycadales downward. It seems quite obvious that Ginkgo is a
genus of the utmost importance from the standpoint of evolution-
ary anatomy, since it summarizes in such a remarkable manner
the anatomical characteristics of both Archigymnospermae and
Metagymnospermae. Its significance in the direction indicated
will be fully realized only after the next tribe, the Coniferales,
have been anatomically considered in the following chapter.

CHAPTER XXIV
THE METAGYMNOSPERMAE: CONIFERALES

As has been indicated at an earlier stage, the gymnosperms
are somewhat clearly divisible into two large groups: the Archi-
gymnospermae, which are often fernlike in habit and always
cryptogamic in the anatomical organization of their primary
wood and in their mode of fertilization by antherozoids, and the
Metagymnospermae, which present no external resemblance to
the members of the fern series, and in which the centripetal or
cryptogamic primary xylem has given place, in living forms at
any rate, to transfusion tissue, and in which, also, fertilization
by means of a pollen tube is a universal feature. The Coniferales
are the largest and the most important group under the Meta-
gymnospermae. Their significance from the evolutionary stand-
point can scarcely be overestimated, not only because they are
more abundantly represented in the living plant population of
the earth than are any other gymnosperms, but also because
they are copiously preserved as fossils as far back as the Paleozoic
age. They thus supply the most important document for the
inductive study of general principles of evolution presented by
any group of living organisms, vegetable or animal, living or
extinct. The paleobotanical and anatomical investigation of
the Coniferales has greatly changed our views in regard to their
phylogenetic sequence in recent years. The older students of
the group restricted to a knowledge of living forms naturally
assumed that the conifers, which are the simplest in organization
of their vegetative and reproductive parts, are most primitive.
By those entertaining this view the Taxineae are considered the
most ancient conifers, and with them have been connected, not
only the Cordaitales, but also the living genus Ginkgo. In the
most recent systematic treatment of the coniferous tribe as a
whole we find this attitude maintained, for Ginkgo and Taxus
are regarded as closely related. It is needless to state that there

317

318 THE ANATOMY OF WOODY PLANTS

is nothing in common between the anatomical structures of vege-
tative and reproductive parts in Ginkgo and the Taxineae. A
later, but apparently equally erroneous, tendency is to interpret
the evolutionary sequence of the Coniferales entirely in the light
of the data derived from the study of Paleozoic gymnosperms.
This attitude is of course found strongly in evidence in those
countries which have contributed notably to the elucidation of
the organization of the seed plants of the Paleozoic. By those
who are affected by the Paleozoic bias the araucarian subtribe of
the Coniferales is considered the oldest representative of the
group. There are those again who attempt to reconcile the
taxinean and araucarian hypotheses of derivation by the assump-
tion that the Araucariineae have come from the Cordaitales,
while the remaining coniferous subtribes have been derived from
the Taxineae. The hypothesis of a lycopodineous origin of the
group has been put forward at various times, but need only be
mentioned here, as it has few advocates and does not appear to
derive any support from paleobotanical or anatomical facts.
The views in regard to the phylogeny and evolution of the Co-
niferales adopted in the present work represent an attempt to
interpret this large and important group of gymnosperms in the
light supplied by the anatomy of Mesozoic forms as compared
with representatives of the group still living. The method of
treatment adopted will be, so far as the limits of space in an ele-
mentary textbook permit, purely inductive. As has been em-
phasized in an earlier paragraph, the conifers, on account of their
abundant presence in the floras past and present, supply a most
valuable document for the interpretation of the fundamental
principles of evolution.

Since the araucarian conifers are quite generally regarded
at the present time as the primitive representatives of the group,
it will serve a useful purpose to consider these first. A transverse
section of the wood of the stem in this subtribe (Fig. 225) generally
reveals the presence of annual rings, unless the particular species
under investigation happens to be of lowland tropical origin. The
autumnal tracheids are marked by tangential pitting, a general
feature of organization of the more modern gymnosperms. The

CONIFERALES

319

wood normally shows the presence of only radial parenchyma, lon-
gitudinal storage elements being absent. The longitudinal radial

section of the wood
(Fig. 226) shows
a condition of pit-

2eees8eeen

ting resembling that . 6 86 6 6 0@ 6 eee

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found in the Cordai-
tales, namely, one
which is alternating
and _ characterized
by the absence of
the bars of Sanio.
The absence of wood
parenchyma and the
alternating char-
acter of the pitting
are features which at
first sight would

seem to justify the Fic. 225.—Transverse section of the wood of A gathis

assumption of gq wstralis.

Troe

SORE Re

A
a
&
al

ae

wan

Fic. 226.—Longitudinal section of the wood of A ga-
this australis.

close degree of rela-
tionship between
the araucarian coni-
fers and the Cordai-
tales, and this view
of their affinities has
been the one almost
universally adopted.
Before we inquire as
to its validity it is
necessary to exam-
ine the organization
of Mesozoic repre-
sentatives of the
group. In Fig. 227
is shown in trans-
verse section the

320 THE ANATOMY OF WOODY PLANTS

structure of a Cretaceous araucarian wood of the type designated
Araucarioxylon. The annual rings are much less clearly developed

Fic. 227.—Transverse section of Araucarioxylon from
the Cretaceous of the eastern United States.

rived as an exuda-
tion from the rays.
This is a condition
often present in both
living and extinct
woods of araucarian
affinities. We may
now turn our atten-
tion to another
araucarian type
commonly present
in the Mesozoic—
the genus Brachy-
oxylon. In this type
the wood in trans-
verse section shows
annual rings and the

than in the wood of
Agathis. The rays
are uniseriate, as in
the living type, but
a marked contrast is
presented by the
presence of wood
parenchyma in the
fossil. The longitu-
dinal section por-
trayed in Fig. 228
shows both alter-
nating pitting and
the presence of
parenchyma. Cer-
tain of the tracheids
are also filled with
dark contents de-

228.—Longitudinal section of the same
Araucarioxylon.

CONIFERALES 321

absence of longitudinal parenchyma. In longitudinal radial aspect
it manifests a kind of pitting which is only partially araucarian,
for more often than

not the pores are
separated by con-
siderable intervals
and fail to alternate
(Big..-220:)! (It) is
only occasionally
that the typical ar-
aucarian crowding
and alternation are
present. More-
over, in woods of
the Brachyoxylon
type wounding
brings about the
formation of trau-
matic resin canals
(Fig. 230) such as
appear after injury in certain of the Abietineae and in the

! genus Sequoia.

After the consideration
of the conditions in the
mature wood of living and
extinct representatives of
the araucarian conifers, we
may now turn our atten-
tion to the organization of
the xylem in the conserv-
ative organs of the exist-
ing araucarian conifers.
Fig. 231 illustrates the
structure of the wood in
the root of A gathis australis

Fic. 230.—Transverse section of the wood ina region not very remote
of Brachyoxylon formed after wounding. from the primary wood.

Fic. 229.—Longitudinal section of the wood of
Brachyoxylon.

322 THE ANATOMY OF WOODY PLANTS

It is evident that, in the secondary xylem of the root, parenchyma,
conspicuous by its absence in the mature wood of the stem, is
abundantly present. Not only is this the case with the root,
but the same situation is found in the first annual ring of the
vegetative stem and also very strikingly in the woody axis of the
ovuliferous cone. The facts here mentioned are of particular
significance when correlated with the organization of the Cretace-
ous Araucarioxylon
shown in Fig. 227.
Obviously the
parenchyma pres-
ent in the older
type of araucarian
wood is perpet-
uated in those re-
gions of the living
form which we
have learned in an
earlier chapter to
regard as conserv-
ative: lt may
accordingly be
logically assumed

Fic. 231.—Longitudinal section of the wood of the that woods of the
root in Agathis australis.

eH eee %

eek

Oaane

o

ce,

te
‘ae,

type of the living
araucarian conifers formerly possessed longitudinal parenchyma
and in this respect are at variance in organization with the ligne-
ous structure of the Paleozoic Cordaitales. This conclusion is
reinforced by an examination of the effects of injury in the living
genera, for the infliction of wounds results frequently in the recall
of the lost parenchyma even in the adult axis.

We may now pass to the consideration of other features which
are supposed to indicate a close degree of relationship between
the Araucariineae and the Cordaitales. The most important
of these are the crowded pitting and the absence of bars of
Sanio. In Fig. 232 is shown the organization of the wood in
the stem of the seedling of A gathis australis as viewed in longitudinal

CONIFERALES 323

radial section. The pits are neither crowded nor alternating as
in the wood of the adult. An examination of the organization
of the seedling in the living representatives of the araucarian
conifers therefore justifies the view that the ancestral forms did
not possess crowded pitting. Precisely similar conditions are
found in the cone, for here the pits in the tracheids nearer the
primary wood lack the crowded and alternating disposition of
the mature vegeta- . a

tive wood of the

genus. But a still
more important
feature is pre-
sented by the
organization of the
wood of the ovu-
liferous cones of
the living A gathis
and Araucaria.
Fig. 233 shows: a
longitudinal radial
view of the second-
ary wood of Arau-
caria Bidwillit in
the vicinity of the
protoxylem. The
pits show a very strong tendency to opposition in arrangement,
and are certainly not angular by mutual contact, as is often the
case in cordaitean woods. The most interesting feature shown
by the figure, however, is the presence of bars of Sanio such as
are entirely lacking in the adult vegetative wood of existing species
of the araucarian conifers. As a consequence of the situation
revealed in the conservative reproductive axis of the araucarian
conifers, we are justified in assuming that the absence of bars of
Sanio and the presence of alternating pitting are not primitive
features of the organization of the wood of the subtribe, and
consequently cannot be brought into court to prove its cor-
daitean affinities. The evidence, in fact, must be read in exactly

Fic. 232.—Longitudinal section of the wood of the
seedling in A gathis australis.

324 THE ANATOMY OF WOODY PLANTS

the opposite sense from that in the Ginkgoales; for, as has been
shown in the preceding chapter, the anatomical facts there justify
the assumption of the original presence of cordaitean structure,
characterized by alternation of pitting and absence of bars of
Sanio. In the case of the Araucariineae, on the contrary, we
must assume on the basis of the structure of primitive regions
that bars of Sanio and opposite pitting were an older feature of

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Fic. 233.—Longitudinal section of the wood of the cone axis in Araucaria Bid-
willit in the region of the pith.

organization of the wood and that they are as a consequence not
nearly related to the Cordaitales.

It becomes clear, when we consider the arguments derived
from comparative anatomical data and those furnished by the
study of extinct forms, that the organization of the mature wood
in the living Araucariineae cannot be accepted as sufficient evidence
of their relationship with the Cordaitales. The persistent foliar
traces of the two living araucarian genera have been regarded
in some quarters as an important indication of their primitive
character. Here again the comparative anatomical situation

CONIFERALES 325

as well as the conditions found in allied fossil conifers do not
justify the conclusion reached. The seedlings of both Araucaria
and Agathis show the earlier leaf traces as evanescent structures
which cease to be formed by the cambium after the leaves to which
they belonged have disappeared. It is only in the older trunk
that the formation of foliar strands is perpetuated for many
years, amounting even to centuries, after their corresponding
leaves have disappeared. The persistent leaf traces which con-
stitute so remarkable a feature of the organization of the mature
trunk of the existing Araucariineae cannot therefore be regarded
as anything but a bizarre and freakish feature which has no evo-
lutionary importance. If any remaining doubt can be considered
to exist on the subject, it is set aside by the organization of the
Mesozoic araucarian woods, which (with the exception of the
type known as Araucarioxylon), are distinguished by the absence of
persistent foliar traces. Comparative anatomy lends very little
support to the inference of cordaitean affinities for the araucarian
conifers, and the evidence against this widely cherished view
becomes quite overwhelming when the anatomical situation in
the other subtribes of the Coniferales is considered. The further
estimation of the claims of the Araucariineae to the pre-eminence
of being the oldest’ conifers may appropriately be delayed until
the anatomical features of other important subtribes have been
examined.

The Abietineae have in recent years made progress as claimants
to the primitive position among Coniferales and to that of nearest
proximity to the Cordaitales. At first sight the extremely com-
plicated organization of both vegetative and reproductive struc-
tures in the abietineous conifers appears to stand in the way of
any such conclusion; but the results of the comparative investiga-
tion of the living and fossil representatives of groups which have
passed the zenith of their development has taught us not to consider
complexity of organization as necessarily a criterion of modernity.
It will be shown in subsequent paragraphs that the Abietineae
are in a very strong position as regards primitiveness, both because
they are apparently related, on good anatomical evidence, to such
ancient groups as the Cordaitales and Ginkgoales and because

326 THE ANATOMY OF WOODY PLANTS

they are clearly antecedent to the mass of other living coniferous
subtribes.

The mature wood of Pinus, illustrated in Fig. 234, is char-
acterized in transverse section by the presence of resin canals and
by rays of complex organization. The longitudinal structure of
the wood, as is shown in Fig. 235, is characterized by the presence
of bars of Sanio. The
rays are of complicated
structure, even when uni-
seriate, and are composed
of central storage cells
and marginal elements
resembling tracheids. If
a primitive region such as
that provided by the
wood of the cone axis or
root be investigated, it
becomes clear that the
bars of Sanio are not an
ancestral feature of
organization of the wood,
since they are absent in
the inner region of the
wood of the cone and are
also often lacking in the tracheary elements of the root, especially
in proximity to the primary wood. This situation is portrayed
for the cone of the Italian nut pine (Pinus pinea) in Fig. 236.
It is obvious that the tracheids retain for some time the spiral
markings of the primitive region. The walls of the tracheary
elements show not the slightest indication of the presence of bars
of Sanio until a region remote from the pith has been reached.
The rays also are without the marginal tracheids which manifest
themselves at an early stage in the organization of the wood of
the vegetative branches. Clearly, so far as the structure of the
wood in the reproductive axis is concerned, the Abietineae as
represented by Pinus are derived from ancestors possessing the
structure of the wood of the Cordaitales. Another interesting

PNK 5a)

Fic. 234.—Longitudinal view of the wood in
Pinus resinosa.

CONIFERALES 327

feature of resemblance to cordaitean forms is the absence of:a
torus in the membranes of the bordered pits of the tracheids lying
nearer to the primary wood. The situation in this respect is the
exact opposite of that found in the case of the araucarian conifers,

sl

Fic. 235.—Highly magnified view of the tracheids of a species of pine, showing
the bars of Sanio (after Gerry).

in which the torus is sometimes present in the region near the
primary xylem of the wood of the cone axis and is entirely absent
elsewhere. The organization of the wood alone in primitive
organs and regions justifies the conclusion of a filiation between
the Abietineae and the Cordaitales rather than between the
Araucariineae and the Cordaitales.

328 THE ANATOMY OF WOODY PLANTS

We may now turn to the brief consideration of evidence derived
from the organization of the wood of fossil forms and bearing on
the respective antiquity of the Abietineae and the Araucariineae.
The investigations of recent years have brought to light in the
Jurassic and Cretaceous numerous coniferous woods which to a
large degree possess characteristics intermediate between those

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Fic. 236.—Transitional region from the xylem of the cone of Pinus pinea

of the Abietineae and Araucariineae. The conclusion naturally
follows that the two subtribes were less remote from one another
in Mesozoic time than they are in the present epoch. The question
of interpretation is strongly debated in the case of these woods.
The mass of paleobotanists, obsessed by the araucarian hypothesis
of the derivation of the Coniferales from their cordaitean ancestors
and little concerned with the fundamental principles of comparative

CONIFERALES 320

anatomy, have assumed that the transitional woods in question
are those of Abietineae which are losing their primitive araucarian
characters. A fatal objection to this point of view, however, is
the fact that none of these transitional woods shows the presence
of bars of Sanio. In other words, they must clearly be diagnosed
as belonging to the araucarian side on the basis of the most reliable
of diagnostics of coniferous woods—bars of Sanio. Attempts
in the direction of proving
the woods in question
abietineous rather than
araucarian have chiefly
taken the form of discus-
sions as to the value of ray
structure in the diagnosis
of coniferous woods. It is
beyond the range of the
present volume to discuss
details of the organization
of the radial structures in
the Coniferales; but it may
be stated in a general way
and on the basis of com-
parative anatomy that no
feature is more subject to variability within the limits of a single
subtribe and hence is less available for comprehensive conclusions
in regard to evolutionary sequence. A final argument against the
araucarian descent of the Coniferales from the Cordaitales is
supplied by the extremely abundant Mesozoic araucarian type
of wood known as Brachyoxylon. In wounded specimens of wood
of this genus traumatic resin canals are formed (Fig. 237),
resembling those of the normal wood of the pinelike conifers.
The occurrence of resin canals as a consequence of injury in
Brachyoxylon, in view of the fact that this genus is admitted
by competent paleobotanists to be of unquestionable araucarian
affinities, is of great significance. This being the case, we are
justified in interpreting the canals formed after wounding as a
reversionary phenomenon, indicating relationship to the pinelike

ti
a '
canta

ditt

Fic. 237.—Wood of Brachyoxylon formed
after wounding.

330 THE ANATOMY OF WOODY PLANTS

Abietineae. This interpretation of the situation is vindicated
by the recent discovery of normal resin canals in the wood of
the axis of the ovuliferous cone of a Javanese species of Agathis,
A. Bidwillii (Fig. 238).

Having discussed, so far as the limits of the present volume
permit, the organization of the wood in conservative axes and in
fossil forms, we must now turn to the discussion of that extremely
important organ, the leaf. It has been made clear in an earlier
chapter that the foliar
organ of Pinus is char-
acterized by the remark-
able structure of its
fibrovascular tissues. In
the genus under discussion
and to a less extent in
allied genera the foliar
conducting strand is sur-
rounded by a cordon of
transfusion tissue. The
situation in this respect
may be clearly ascertained
by reference to Fig. 239.
eevee immer es

are distinguished by the
absence of protoplasmic contents and by the occurrence of
bordered pits in their walls. It is obvious that the tissues of this
nature become joined with the xylem of the foliar bundles on its
flanks. The transfusion tissue in modern representatives of the
genus Pinus is not a continuous mass of tracheary cells, but has
interspersed throughout its substance a considerable number of
living cells provided with protoplasm and a nucleus. The investi-
gation of the Cretaceous deposits at Kreischerville, Staten Island,
has provided us with,extremely valuable data for the determina-
tion of the organization of the leaf in Pinus and allied forms in
the later Mesozoic. In some of the numerous species of Pinus
which flourished in the American Cretaceous, transfusion tissue
was present in large amount and contained little or no admixture

CONIFERALES 331

of parenchymatous elements. In still other species the short
tracheary elements ordinarily called transfusion cells were much
less well developed. It is the remarkable genus Prepinus, however,
which provides the most important evidence for estimating the
bearing of the anatomical organization of the leaf on the problem
of evolution of the genus Pinus in particular and that of the
Abietineae in general. In Fig. 240 is shown the transverse section

KX

Fy Ss.

Fic. 239.—Leaf bundle of Pinus Strobus

of the leaf in P. statenensis. The outline is angular because of
mutual contact with other and surrounding leaves of the fascicle.
In Prepinus the growing point of the short-shoots persisted as it
does in the living Ginkgo, and the fascicular leaves, instead of being
few and definite in their number, were indefinitely numerous.
It is interesting to note in this connection, however, that, although
in the true Pinus of the Cretaceous the number of leaves in the
fascicle was few and fixed as in modern forms, nevertheless the
growing point of the short-shoot persisted indefinitely and did
not disappear at an early stage, as in the living representative of

222 THE ANATOMY OF WOODY PLANTS

the genus. Within the angular outline of the leaf in Prepinus
is seen, beneath the epidermis, the ribbed hypodermal tissues,
recalling those of the older gymnosperms. The cortical region
of the leaf terminates in a not very clearly marked endodermis
which doubtless owes its loss of definiteness to the changes resulting
from fossilization. Within the endodermal boundary lies the
entirely tracheary and strongly pitted transfusion tissue of the

Fic. 240.—Leaf of Prepinus statenensis

leaf. The elements of this category are differentiated into two
zones—an outer one composed of short, broad, pitted elements,
and an inner one consisting of thick-walled cells of narrow lumen.
The former are known as the outer transfusion sheath and the
latter as the inner transfusion sheath. Both structures have
their counterpart in the leaf of certain of the Cordaitales. The
double transfusion sheath was also frequently present in the foliar
organs of true pines of the American Cretaceous. Another most
interesting feature of the organization of Prepinus was the struc-
ture of the xylem. As may be seen from Fig. 241, the wood
presents two regions, an upper and a lower. In the former the

CONIFERALES 333
elements are arranged in radial rows and usually increase in size
toward the upper surface of the leaf. In the xylem directed down-
ward are seen indications of wood rays, and the inspection of the
longitudinal aspect reveals the fact that it is made up largely of
tracheids with bordered pits. This is the centrifugal xylem and
corresponds to the mass of wood in the foliar trace of the fascicular

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Fic. 241.—Portion of leaf of Prepinus statenensis

leaves of living pines. The upwardly developing wood is the
cryptogamic xylem and confirms the conclusion as to the affinity
of Prepinus, already suggested by the organization of its trans-
fusion sheath, namely, that the genus is allied to the Cordaitales.
It is thus apparent that the details of organization of the leaf in
Prepinus, which in turn is clearly the ancestor of Pinus, justify
an attribution of cordaitean ancestry to the Abietineae. This
conclusion as to relationship is supported by the primitively cor-
daitean character of the pitting which so strikingly indicates a

334 THE ANATOMY OF WOODY PLANTS

relationship of the Abietineae rather than the Araucariineae with
the Paleozoic gymnosperms known as Cordaitales.

Not only do the Abietineae as a result of their anatomical
organization and paleobotanical history present a strong claim
to direct relationship with the older gymnosperms, but they
supply equally compelling evidence that they are ancestral to
other prominent coniferous subtribes. It will be well in this
connection to
begin with the in-
ternal situation in
the abietineous
subtrrbe. it 1s
readily subdivided
into two series on
the basis of ana-
tomical structure—
the Pineae and the
Abieteae. - Ihe
former are char-
acterized by the
possession of well-
developed resin

Fic. 242.—Transverse section of the root of Abies canals in the wood
balsamea, showing the presence of a resin canal in the jn both vertical
primary wood.

and horizontal
planes. In contrast to these, in the second series the ligneous resin
canals of the secondary wood are notably absent. It is only in
regions recognized as conservative that they make their appear-
ance—in the primary structures of the xylem of the root (Fig. 242),
in the secondary wood of the axis of the ovuliferous cone, and some-
times in the first annual ring of the vegetative branches. Further,
resin canals are found in the wood of the Abieteae as a result of in-
jury. Both comparative anatomical and experimental evidence,
as a consequence, vouch for the derivation of the Abieteae from
ancestral forms possessing well-developed ligneous resin canals. It
is of significance to note in this connection that the genus Cedrus,
for the great antiquity of which the geological record supplies clear

CONIFERALES 335

testimony, not only from American, but also from European de-
posits, is strikingly distinguished by the fact that it produces
both horizontal and vertical resin canals resulting from injury
(Fig. 243). This condition is in contrast to that manifested by the
other genera of the Abieteae, in which only vertical resin canals
make their appearance in the secondary wood after wounding. It
is now generally admitted by competent anatomists that there is
strong evidence for the
derivation of the Abieteae
from the Pineae as a re-
sult of reductionary
modification. This con-

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only on the testimony
supplied by the resin
canals as described above,
but also from the com-
parative anatomical
consideration of the
organization of the rays
and the parenchyma of

the secondary wood. It Fic. 243.—Transverse section of the wood of
is apparent in regard to Cedrus deodara formed after injury, showing

: reversionary appearance of resin canals in both
these particular struc-

vertical and horizontal planes.
tures that the Pineae are
more primitive than are the Abieteae. The ray of the Abieteae
is often characterized by the loss of the marginal tracheids so dis-
tinctively developed in the radial parenchymatous strands of
Pinus and its living allies.

The internal conditions in the Pineae may now claim our
attention. Here we find a striking separation between Pinus on
the one hand and Picea, Larix, and Pseudotsuga on the other,
resulting from a consideration of the lining of the resin canals
in the wood. In the first-named genus the secretory canals are
lined by thin-walled parenchyma which, in the transformation
of heartwood into sapwood, develops processes known as tyloses,
which more or less completely occlude the resin canals. In the

336 THE ANATOMY OF WOODY PLANTS

three remaining genera of the Pineae the lining of the secretory
space is composed mostly of thick-walled more or less lignified
cells. The resin canals in these forms do not accordingly contain
well-developed tyloses in the heartwood. Another important
distinction between Pinus and allied genera is the complete absence
of wood parenchyma in the former. It has been shown in an
earlier chapter that longitudinal storage parenchyma is formed
in the secondary wood as the result of the modification of elements

Ls
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U

mt js SO tS
(\ ci | A Jit KO) O
; @E ; SS : ‘
RE Gy PO

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JOOS.
NOR

Fic. 244.—Wood of Picea canadensis, showing terminal parenchyma

destined to be tracheids. In Picea, Larix, and Pseudotsuga storage
parenchyma is present, but at the end of the annual ring only
(Fig. 244). In this position, particularly in the case of the root, it
manifests convincing evidence as to its derivation by the occurrence
of transitional stages between merely septate tracheids and rows of
parenchymatous elements, resembling in their general configuration
tracheary elements. In Pinus, therefore, there is no true paren-
chyma of the wood, since such storage cells are found only in the
three other genera of the Pineae. Where wood parenchyma is
present, moreover, it is confined to the end of the annual ring and
is clearly in a condition of derivation from tracheids, a state found
normally in no other living representative of the Coniferales. It

CONIFERALES S37

has been demonstrated in earlier pages that Paleozoic gymno-
sperms are characterized by the complete absence of parenchyma-
tous elements in the wood and at the same time by the general
absence of annual rings in the stem. Pinus, as regards the organ-
ization of the storage devices of the wood, is therefore clearly
allied with Paleozoic types such as the Cordaitales.

There are other conditions, however, which indicate for Pinus
a primitive position among the Abietineae. First of all there is
the possession of short-shoots. Pinus in this feature of organiza-
tion presents a marked resemblance to the Ginkgoales, which

Fic. 245.—Microspores of Ginkgo and Abies

also bear their foliar organs on special spurs or short-shoots.
Nor is the common possession of short-shoots unparalleled by other
significant characteristics. Ginkgo and the Abietineae strongly
resemble one another in the possession of bisporangiate sporo-
phylls. In the two groups there are two microsporangia and
two megasporangia or seeds on the reproductive foliar organs.
The view sometimes advanced that the ovuliferous scales in the
Abietineae consist of a fused pair of foliar structures has apparently
no evidence in its favor. It is as clearly a single leaf as is the
microsporophyll. ‘The microspores in the Abietineae and in the
Ginkgoales also present striking points of resemblance which
have only. recently been completely realized. In the monotypic
Ginkgo the pollen is winged as in the more primitive Abietineae
and resembles in its internal organization the structures found
in the microspores of that subtribe of conifers. Fig. 245 illustrates
the numerous features of internal and external resemblance between

338 THE ANATOMY OF WOODY PLANTS

the pollen of the Abietineae and that of the Ginkgoales. Nor
is the similarity confined to the structure of the microspores.
It has been pointed out in an earlier chapter that the organ-
ization of the wall of the sporangium in relation to the opening
mechanism and to its derivation from the fibrovascular structures
is practically identical in the Ginkgoales and Abietineae. Finally,
the organization of the tracheids of the wood is similar in
the case of the two groups under consideration. Pinus seems
beyond question, by the possession of short-shoots, the number
and organization of its microsporangia and megasporangia, as
well as by the structure of its microspores, the general organization
of the wood, and, finally, by the absence of true wood parenchyma,
clearly allied to the sole surviving genus, Ginkgo.

Pinus, lastly, presents a very strong claim to primitiveness
among the Coniferales by reason of the general presence of short-
shoots, such as are usually regarded as the prototypes of the
ovuliferous scales of the female cone of the Coniferales as a whole.
It follows, on the general principles of anatomy laid down in an
earlier chapter, that the genus which still shows vegetatively the
structures known as short-shoots is in an excellent position
to claim a primitive position among the Coniferales. The absence
of short-shoots in the seedling of Pinus is obviously no ground for
an argument of any significance against the primitive presence of
short-shoots in the genus. Negative evidence furnished by seed-
lings is of no value, since only positive testimony in connection
with the hypothesis of recapitulation can be accepted as valid
in evolutionary argument. We may therefore assume that the
presence of short-shoots in Pinus and Ginkgo, as well as many
other features of resemblance between the two genera, is an unmis-
takable indication of affinity. Further, since short-shoots are
very generally assumed to have been the prototype of the ovulif-
erous scale in the female cones of the Coniferales throughout, we
may infer that the coniferous genus which has manifested these
structures as a normal vegetative feature from remote geological
times must be a very ancient representative of the Coniferales.

But we are not by any means limited to a consideration of
the general organization of the female cone in inferences regarding

CONIFERALES 339

the relationship of Pinus to the other Coniferales. An excellent
illustration of the value of anatomical evidence in the case of
this problem is furnished by the interesting taxodineous genus
Sequoia. The Taxodineae as well as the nearly allied Cupres-
sineae are characterized anatomically by the organization of the
female cone and the structure of the wood. The scales of the
cone are superficially single, but in section they show the presence
of a double series of oppositely orientated fibrovascular bundles,
thus indicating the origin of the seed scales from the externally

Fic. 246.—Transverse section of the cone scale of Sequoia gigantea, showing a
double system of bundles with opposite orientation.

double structures of the ovuliferous cone of the Abietineae
(Fig. 246). In the organization of their wood the Taxodineae differ
from the Abietineae in the absence of resin canals. There is, how-
ever, a resiniferous secretion produced by scattered parenchymatous
elements of the wood. In the structure of the radial parenchyma
a condition of simplicity contrasting with that found in the Abie-
tineae is manifested, for the marginal tracheids of the rays of the
Abietineae are conspicuously absent in the normal wood of the
Taxodineae in general and of Sequoia in particular. If we con-
sider the genus Sequoia in the light of the canons of anatomy
formulated above, very interesting results are reached. First, if a
transverse section of the axis of the cone or of the ovuliferous
scale of Sequoia gigantea be examined, resin canals reveal them-
selves in the wood in proximity to the primary xylem (Fig. 247).

J

340 THE ANATOMY OF WOODY PLANTS

Further, an investigation of the first annual ring of the stem in
trees which have attained such vigor of development as to pro-

Fic. 247.—Transverse section of reproductive axis
of Sequoia gigantea, showing resin canals in the wood.

gions indicated
above is good evi-
dence of the origi-
nal presence of
such structures in
the woody tissues
of Sequoia. Addi-
tional information
on this subject is
furnished by the
wound reactions of
the eenus. En
either of the two
species, S. gigantea
or S. sempervirens,
the infliction of
wounds may. be

fed
erry

eee me,

4
*
Ld
*
‘
*
t
e

he OR wy

duce seed often
shows the pres-
ence of resin canals
such as are not
normally found in
the subsequent an-
nual increments of
growth (Fig. 248).
Finally, the fibro-
vascular strand of
the leaf frequently
contains in the
region of the xylem
a single resin canal.
The occurrence of
ligneousresin
canals in the vari-
ous primitive re-

Fic. 248.—Twig of Sequoia gigantea, showing pres-
ence of resin canals in the first annual ring.

CONIFERALES 341

followed by the appearance of traumatic or wound resin canals as
areversionary feature. In S. sempervirens, the redwood, this is the
only mode of occurrence of these structures. It may be added in
this connection that the normal seedling of Sequoia (either species)
shows no resin canals in the wood. On the fallacious logic that
structures absent in the seedling are not ancestral it could be
argued that resin canals in the secondary wood are not an ances-
tral feature of the genus Sequoia. This genus is the only one in
any coniferous subtribe, other than the Abietineae, characterized
in living species by the formation of traumatic resin canals. In
Sequoia a sound inference based on the principle of conservative
organs is that its ancestral forms were provided with resin canals
throughout their structure.

Although only the genus considered in the foregoing paragraph
presents the abietineous feature of resin canals in the secondary
wood as a result of injury, many genera of both taxodineous and
cupressineous affinities revert to the abietineous type of ray as
a response to wounding. This is notably the situation in Sequoia
and is illustrated in Fig. 53. In another figure (Fig. 52) the occur-
rence of marginal tracheids is indicated for the cupressineous species
Chamaecyparis nootkatensis. The investigations of Miss Holden
have made it clear that the presence of traumatically recalled
ray-tracheids is a common feature of the Cupressineae and the
Taxodineae. In neither of these tribes are such structures known
to occur normally in conservative organs; hence their former
presence is revealed only by reversionary phenomena. It is
worth while to note that the recurrence of marginal tracheids
as the result of injury is usually exemplified, not in the immediate
region of the wound where hypertrophy alone prevails, but in a
region of the stem more or less remote from the actual injury.
This situation is of interest because it is paralleled by conditions
found in connection with certain other wound reactions, notably
those presented in the case of the rays in certain angiosperms. It
is obvious from what has been stated above that ray-tracheids,
although of much wider occurrence among the Cupressineae and
Taxodineae, probably appear only as the consequence of experi-
mental conditions and are no longer a normal feature of structure.

342 THE ANATOMY OF WOODY PLANTS

Normal and traumatic resin canals and traumatic ray-tracheids
are abietineous structures occurring in certain Taxodineae and
Cupressineae, facts which, in accordance with the general prin-
ciples of comparative anatomy already elucidated, may be regarded
as indicating the abietineous origin of the two coniferous subtribes
in which they occur. This conclusion is now somewhat generally
accepted by those whose anatomical knowledge of the conifers
makes their opinion of weight.

Before we take leave of the two subtribes considered in the
foregoing paragraphs it will be well to direct attention to the
distribution and origin of wood parenchyma in woods of this
type. The secondary xylem is characterized by the presence
of usually abundant wood parenchyma, not confined to the end
of the annual rings, but scattered throughout. The parenchyma-
tous elements secrete a generally highly antiseptic essential oil.
As a result of the presence of essential oils, and sometimes also
by the infiltration of the tracheary walls with tannin, woods of
taxodineous and cupressineous origin are frequently resistant to
decay. The oil-secreting cells in the subtribes under discussion
do not under normal conditions betray their derivation from
tracheids except by the fact that they are grouped in series which
have the fusiform shape of tracheary elements. In injured woods
it is often possible to observe transitions between septate tracheids
and rows of parenchymatous elements. It may accordingly be
assumed, independently of the evidence of abietineous affinities
supplied in the previous paragraphs, that the storage elements
in the woods of the Cupressineae and Taxodineae are of tracheary
origin. Another feature must be considered in this connection.
In the minds of those who regard the Coniferales as an ascending
series and not one of reduction, the resin canals which characterize
the wood structure of the older Abietineae owe their origin to
the clustering of the resin cells of the cupressineous or taxodineous
type. This view of the origin of secretory canals in the coniferous
series has been particularly emphasized by Penhallow. It meets
with numerous difficulties, the chief of which is that the cells
which surround the resin canals are not resin cells. The latter
possess dark-brown contents and produce their secretion in an

CONIFERALES 343

intracellular manner. The epithelial cells of the resin canals in
the abietineous conifers, on the contrary, do not manifest the
dark-brown (so-called) ‘‘resin,’’ but pour their secretion at once
into the secretory space. The secretory canals found in the case
of Sequoia gigantea are surrounded by cells which correspond only
to a very limited extent to the resin cells of cupressineous woods.
Most of the secretory elements are devoid of the so-called ‘‘resin”’
or dark-brown contents, and
those containing this substance |
are present in about the same
proportion near the canal as
they are in the structure of the
adjacent wood. This situation
is clearly revealed in Figs. 249
and 250.

In addition to the Taxineae,
Araucariineae, Abietineae, Cu-
pressineae, and Taxodineae—
subtribes of the Coniferales
which have been discussed to
a greater or less extent in the
earlier paragraphs of the pres-
ent chapter—there remain the
Podocarpineae, a group which
in the present period is con-
fined almost entirely to the FAG 240: peavetre secre One

: traumatic or wound resin canal of
Southern Hemisphere. The — sSeguoia sempervirens.
podocarpineous forms are
generally regarded, and with a strong degree of probability, as
somewhat closely allied to the Taxineae or yews, which have their
main distribution in the Northern Hemisphere. They are char-
acterized, however, by a less degree of simplification in their
ovuliferous cones and frequently by the possession of winged
pollen of the abietineous type, produced always in bisporangiate
microsporophylls. The organization of the female cone in the
Podocarpineae as a whole, and particularly in the genus Podocar pus,
recalls by the presence of a bract and a subtending ovuliferous

344 THE ANATOMY OF WOODY PLANTS

scale the conditions found in the double scales of the ovuliferous

strobilus of the Abietineae.

The reproductive features conse-

quently supply some evidence for the association of the podocarps

GAGS
L_

SW

i

oe

Ss SIDA
SS, CESS.
WOES
KCOYa2D}

Fic. 250.—Longitudinal section
of a traumatic resin canal in the same
species.

with the abietineous conifers.
The organization of the wood in
the group under consideration is
very similar to that found in the
Cupressineae and Taxodineae.
The tracheids possess opposite
pitting and bars of Sanio. The
parenchyma is abundant and
scattered throughout the annual
ring. A clear difference from the
Cupressineae and Taxodineae is
supplied, however, by experi-
mental evidence, since neither
traumatic resin canals nor margi-
nal ray-tracheids have been found
as yet in any of the genera of the
Podocarpineae. We shall prob-
ably not make further advance
in the final determination of the
phylogenetic or evolutionary
position of this subtribe of the
Coniferales until our present ex-
tremely meager knowledge of the -
fossil conifers of the Southern
Hemisphere has been notably in-
creased. It seems highly prob-
able, on the basis of the organiza-
tion of the scales of the female
cone and of the sporophylls and

spores of the male cone, that the Podocarpineae have abietineous
affinities. Their final position will ultimately be determined by a
better knowledge of the anatomy of extinct forms.

Of the coniferous subtribes enumerated in the preceding
pages the Taxineae have not as yet been considered anatomically.

CONIFERALES ‘345

The reproductive structures in this group are extremely simple,
and the ovuliferous apparatus produces in maturity a single ovule.
In the genus Cephalotaxus of the Eastern Hemisphere an ovuliferous
strobilus is present in the early stage of development; this is
composed of scales each producing a pair of ovules—a condition
comparable with that found in the Abietineae. As the develop-
ment of the seed progresses all but a single seminal structure

abort, so that in the
end no vestige of the
presence of a true fe-
male cone can ordi-
narily be observed.
In accordance with
the principle of re-
capitulation, we must
regard the young cone
of Cephalotaxus as in-
dicating the more
primitive condition
for the ovuliferous
apparatus in the
Cenmws 2 exws pre-
sents a marked con-
trast to that genus by
the fact that at no

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SAIPSS SIKH Re
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SSSI 8 ISI ocd ray Set
A O it

SSS TISS IRS bss eSeleseceet

Fic. 251.—Semidiagrammatic view of the longitudi-
nal organization of the wood in the Taxineae.

time is the presence of a female or ovuliferous cone indicated. From
the first there is but a single ovule, and this is not related at any time
to a visible ovuliferous scale. The organization of the male cones in
the Taxineae is naturally less reduced than that of the female, in
accordance with the general principle of the conservatism of the
microsporangial apparatus in heterosporous groups. In the case of
- the Taxineae the reproductive structures furnish convincing evidence
for the origin of the subtribe as the result of a process of reduction.
The structure of the wood in the Taxineae is quite characteristic.
The tracheids are marked by internal spiral bands which are of
late origin and are frequently for that reason designated as tertiary
thickenings(Fig. 251). Opposite pitting is present when the pores are

346 THE ANATOMY OF WOODY PLANTS

numerous, and bars of Sanio are clearly developed. The state-
ment is often made that the ligneous structure of the Taxineae
is free from parenchymatous elements (Fig. 252). This is certainly
true of the mature wood of the stem in both Taxus and Torreya.
If, however, the roots in the two genera be examined, a varying
amount of storage parenchyma is discovered which is ordinarily
better developed in proximity to the region of the primary wood
(Fig. 253). In species common to Europe and America, namely,
Taxus baccata and its vari-
eties, very little paren-
chyma is found even in the
root; but in oriental species
of the genus storage ele-
ments are somewhat abun-
dant in this organ. The
young stem, and particu-
larly the root, of the genus
Torreya show clearly
developed longitudinal
parenchyma in the wood.
In Cephalotaxus, which by
reason of the presence of a
well-developed female cone
at an early stage must be
regarded as a primitive genus of the subtribe, parenchyma is
markedly abundant in the organization of the wood. Injuries,
also, frequently result in the recall of parenchyma in those taxineous
woods which are normally without it. The generally uniseriate
rays of the subtribe do not show the presence of traumatic ray-
tracheids as a result of injury, and in this respect they present a
marked resemblance to the woods of the Podocarpineae. Resin
canals are conspicuously absent in the ligneous structures of the
Taxineae, and not the slightest evidence of their former occurrence
can be produced by experimental data. The genus Taxus is
entirely without resin canals, even in its leaves, thus providing
the only example of a conifer completely lacking these structures.
An interesting parallel is presented by the hemlock (Tsuga) among

Fic. 252.—Transverse section of the wood
of the stem in Taxus cus pidata.

CONIFERALES 347

the Abietineae. Here resin canals are normally absent, not only
in the structure of the wood, but also in the other tissues of the
stem. In the root of the genus they occur in the primary wood;
and in the cone and leaves, in the tissues of the cortex. This
distribution of resin canals is of course entirely in accord with
the general principles of evolutionary anatomy. TJsuga, however,
differs markedly from Taxus in the degree of obliteration of resin
canals, since these structures here persist in conservative regions,
while in the latter genus
they have entirely disap-
peared. In Cephalotaxus
resin canals are well devel-
oped except in the wood,
but are apparently not
susceptible of reversionary
recall in ligneous struc-
tures.

It will be obvious from
the description of the re-
productive and vegetative
organization supplied in
the two preceding para-
graphs that so simple a
type as the genus Taxus
cannot, according to the well-established principles of anatomy, be
regarded as a primitive form. Its acceptance in this capacity by
the earlier and philosophical taxonomy is shown on inductive evi-
dence to be entirely unjustified. It appears on the basis of the
facts at present available that the Taxineae are a reduction series
in which the oriental Cephalotaxus among existing genera occupies
the lowest position, while Taxus represents the summit. It is
further obvious that the absence of wood parenchyma is not a
primitive feature of organization of the group. If it be admitted
that longitudinal parenchymatous elements primitively character-
ized the organization of the wood, a derivation from the same
stock as the Podocarpineae is plainly indicated. This conclusion
is justified by the consideration of the young ovuliferous cone in

as Ge
x

Fic. 253.—Transverse section of the wood
of the root in Taxus cus pidata (after Bliss).

348 THE ANATOMY OF WOODY PLANTS

the genus Cephalotaxus. The anatomical evidence points very
clearly to the Taxineae as a reduction series, taking its origin from
the same general group or plexus as the Podocarpineae. The
somewhat meager fossil data at our disposal do not warrant
us in assuming a very ancient occurrence for the subtribe Taxineae,
since woods of this type are not found earlier than the Tertiary
or Cenozoic. It is true that cones and leafy twigs from various
levels of the Mesozoic have from time to time been referred to
taxineous affinities, but there is no anatomical evidence that they
belonged here. So far as any data derived from anatomical |
structure are concerned, we are not justified in attributing to
the Taxineae a great geological age. The external habit as a
diagnostic criterion in the Coniferales has been shown in recent
years to be almost as misleading as it has proved to be in the
Cycadofilicales and other Paleozoic groups.

Having considered the anatomical organization of the various
subtribes of the Coniferales in such detail as comports with an
elementary work like the present, before summing up the situation °
from the evolutionary standpoint we may well make some refer-
ence to the anatomical characteristics of coniferous woods which
are utilized by paleobotanical investigators. The genera of fossil
woods logically increase in number as our knowledge of the ligneous
organization of extinct plants becomes fuller and more complete.
For the present purpose only a few of the more important genera
of fossil woods need be considered. By reason of its relative
resistance to decay, wood naturally often becomes isolated from
its accompanying tissues and frequently is the sole surviving evi-
dence of the former existence of gymnospermous groups. The
situation thus presented is a difficult one, and the earlier charac-
terizations of fossil wood were naturally largely empirical and
made without reference to the facts or principles of comparative
anatomy.

In deference to the prevailing view that the araucarian conifers
are the most ancient and form the connecting link between the
Coniferales and the Cordaitales, we may consider them first.
It has been indicated in a previous paragraph that the mature
secondary wood of the two living genera of the Araucariineae is

CONIFERALES 349

characterized by alternating crowded pitting and the absence of
bars of Sanio as well as of parenchymatous storage elements. To
this type of wood occurring as a fossil the name Araucarioxylon
is given. A second genus of fossil woods is diagnosed by the
presence of spirals on the inner walls of the tracheids and by the
absence of wood parenchyma, under the generic designation
Taxoxylon. Those woods characterized by the possession’ of
ligneous resin canals in the horizontal and vertical planes are
included under the generic appellation Pityoxylon. In coniferous
wood without either resin canals or conspicuous storage paren-
chyma, the name Cedroxylon is adopted. Woods which, on the
contrary, are provided with abundant storage parenchyma are
designated Cupressinoxylon. In all genera of fossil woods except
the first the pitting is characteristically opposite. In the
Araucarioxylon type the pitting is frequently crowded and
alternation and bars of Sanio, universally present in other existing
coniferous woods, are conspicuous by their absence.

Clearly the use of even the small number of genera of fossil
woods indicated in the preceding paragraph in connection with
evolutionary inferences must, in the light of the conditions de-
scribed for the various living conifers in the earlier part of the
present chapter, be a matter of great difficulty. This arises from
the fact that the organization of the mature wood in a given
conifer is by no means necessarily an indication of its true system-
atic position. The interpretation of the significance of wood
structure in fossil and existing conifers can be successfully attacked
only with a knowledge of the general principles of comparative
anatomy. A failure to realize this situation has led to very many
erroneous interpretations, both anatomical and _ paleobotanical.
For example, it is quite clear from the data supplied at an earlier
stage that the primitive condition of both the Avraucarioxylon
and Taxoxylon type was a Cupressinoxylon, since abundant wood
parenchyma diffused throughout the annual ring was formerly a
feature of organization of these ligneous types, as is shown by
a consideration of conservative organs, experimental results, and
fossil evidence. Further, in some instances the Cupressinoxylon
type has been clearly derived from the complicated structure

350 THE ANATOMY OF WOODY PLANTS

found in Pityoxylon. This is, for example, true in the case of
the genus Sequoia, which, on the basis of general anatomical
principles, obviously formerly possessed both the ligneous resin
canals and the marginal ray-tracheids of the older Abietineae.
Examples might be indefinitely multiplied to show that the use
of the mature structure of the wood in the Coniferales, without
recourse to comparative anatomical and experimental data, is
almost certain to lead in a given case to fallacious conclusions.

It is now possible to sum up the situation from the standpoint
of evolutionary anatomy for the phylogenetic sequence. It will
be convenient to indicate preliminarily that the most generally
accepted hypothesis of the morphological nature of the ovuliferous
cone in the Coniferales furnishes a prima facie argument in favor
of the primitive position of the Abietineae. In this subtribe the
female cone consists of pairs of scales, the upper of which is ovulif-
erous and the lower sterile. The ovuliferous scale is with a strong
degree of probability to be regarded as a persistent single sporophyll
bearing two seeds or megasporangia on its morphologically lower
but physically upper surface. The megasporophyll is in relation
to the abortive axis of the reproductive short-shoot. Aside from
the morphological interpretation of the structures concerned,
however, the fact remains that the units of structure in the ovulif-
erous cone of the Abietineae are double and separate in their
nature. In the female cones of the remaining coniferous sub-
tribes there is clear evidence from comparative anatomy of the
presence of fused pairs of scales in the ovuliferous structures.
In accordance with general principles of morphology and without
necessarily accepting the hypothesis of the short-shoot nature
of the vertically paired scales of the abietineous cones, it is probable
that the Abietineae are an older group than are the remaining
coniferous subtribes.

An additional argument for the antiquity of the Abietineae
is furnished by their obviously close relationship to the Ginkgoales,
which are admitted on every hand to be a primitive group of
gymnosperms. The affinity with the Ginkgoales shows itself
in the common possession of vegetative short-shoots by both the
sole surviving Ginkgo and by the ancient but still prolific genus

CONIFERALES 351

Pinus. Further, both megasporophylls and microsporophylls
in the two cases produce paired sporangia. The microsporangia
of Ginkgo present a common difference with those of the Abie-
tineae from lower forms in owing their dehiscence to a mechanical
layer derived, not from the epidermal, but from the fibrovascular,
tissues. An additional feature of affinity is supplied by. the
winged character and the internal organization of the microspores
in the two groups, for they are practically identical. The structure
of the tracheids of the wood in the Abietineae and Ginkgoales is
significantly similar in the presence of opposite pitting and bars
of Sanio, a common feature which distinguishes them from the
Cordaitales and other older gymnospermous groups. Further,
although the pitting and other details of organization of the
tracheids in the two groups is of the modern gymnospermous
type, the structure of the xylem in primitive regions clearly shows
a filiation with the conditions characteristically presented by the
Paleozoic gymnosperms.

If a strong argument for the primitive position of the Abie-
tineae is supplied by a comparison with the structural features of
the Ginkgoales, an even more cogent one is furnished by their
resemblance in important anatomical characteristics to the Cor-
daitales. It is the leaf of the extinct genus Prepinus which mani-
fests, as has been indicated in earlier pages, the most categorical
and distinct similarity to the foliar organization of certain Cor-
daitales. Not only is Prepinus the only known representative
of the Coniferales to show distinct and unmistakable centripetal
wood as distinguished from transfusion tissue, but it manifests
its affinity to cordaitean forms by the presence of a double trans-
fusion sheath in relation to the centripetal wood of the foliar bundle.
It is further impossible to deny for Prepinus a close degree of
relationship with Cretaceous species of Pinus which, like Prepinus,
possess a well-marked double transfusion sheath and differ from
the more primitive genus only in the absence of true centripetal
wood. The agreement in foliar organization between Prepinus
and cretaceous species of Pinus on the one hand and the Cordaitales
on the other cannot be overlooked in any discussion of the evo-
lution and affinities of coniferous subtribes. Although the wood

352 THE ANATOMY OF WOODY PLANTS

of Pinus and Prepinus resembles that of Ginkgo in possessing
opposite pitting and bars of Sanio, this organization contrasting
with the tracheary structure of older types like the Cordaitales
and the Cycadales in reality presents no difficulty. For in prim-
itive regions and organs the alternating pitting without bars of
Sanio characteristic of the Cordaitales is present in the Abietineae
(particularly in Pinus and Prepinus), and passes by gradual
transitions into the opposite pitting with bars of Sanio exemplified
in the structure of the mature wood both in Ginkgo and in the
Abietineae. The reproductive structures of the Cordaitales are
too little known to us to supply appropriate points of comparison.
It may be stated in summary that the Abietineae present as good
anatomical evidence for derivation from the Cordaitales as do
the Ginkgoales; and, moreover, they have an additional claim to
such affinity by their clear relationship with the Ginkgoales.

But it is not only by reason of resemblances to the Ginkgoales
and the Cordaitales, neither few nor unimportant, that the Abie-
tineae show themselves to be a primitive subtribe of the Coniferales.
When the anatomical organization of the remaining coniferous
subtribes is investigated in the light of the general principles so
often emphasized in the present volume, it supplies in many cases
distinct evidence that the Abietineae are the primitive stock from
which all have taken their origin. This situation may be illus-
trated first in the case of the Araucariineae, which are somewhat
generally regarded as the primitive conifers and those most nearly
connected with the Cordaitales. The strongest evidence for this
claim is supplied by the organization of the tracheids, presenting
the same alternating pitting and the absence of bars of Sanio as
are manifested in the cordaitean and cycadalian gymnosperms.
If alternating pitting alone were a sufficient criterion of relation-
ship, many angiosperms by the possession of this feature could
establish a claim to relationship with that ancient group of gymno-
sperms. The evidence here as in other cases should be read in
the light of the general principles of comparative anatomy. The
longitudinal radial section of the cone axis, and to a lesser extent
of the leaf trace, of either Araucaria or Agathis at once reveals
the true situation. In these cases one always finds bars of Sanio

CONIFERALES 353

in the region of the secondary wood near the primary xylem, and,
where the pitting is abundant, frequent opposition of the pores.
In other words, a situation presents itself which is the exact con-
verse of that found, for example, in the Ginkgoales, in which the
primitive region of the secondary wood shows alternating pitting
and no bars of Sanio. In the later-formed wood the pits begin
to appear in opposition and are separated by bars of Sanio. If
we are justified in regarding the type of tracheid found in Ginkgo
as originating from that typical of Cordaitales and other ancient
gymnosperms, we are similarly warranted on the same evidenceé
in viewing the alternation of pitting and the absence of bars of
Sanio in the adult wood of the existing araucarian conifers as
derived from a state in which both opposition of pitting and bars
of Sanio were present. The evidence supplied by fossil forms,
moreover, justifies the derivation of the Araucariineae from abie-
tineous forbears, since many distinctly araucarian woods have
been discovered in recent years in the Mesozoic which clearly
present either normal or traumatic features uniting them with
the Abietineae. In some instances, for example, thick-walled,
heavily pitted ray cells, similar to those commonly characteristic
of radial parenchyma in living and fossil Abietineae, are found in
Mesozoic araucarian woods. This feature is significantly paral-
leled in the rays of the cones of living araucarians. Traumatic
resin canals are also found commonly in woods which are transi-
tional from the abietineous to the araucarian type. This is notably
the case in Brachyoxylon, a very common Mesozoic type of wood,
and also in the much rarer Araucariopitys. Normal resin canals
are, moreover, found in the wood of the ovuliferous cone of the
living Agathis Bidwillii, from Java. It has been stated in earlier
pages that there is distinct evidence from the organization of
actual fossils and from the structure of conservative. regions in
the existing Araucariineae that abundant wood parenchyma was
an original feature of organization of the araucarian as contrasted
with the abietineous Coniferales and the Ginkgoales, in which
tangential storage elements are conspicuously absent.

Not only, however, have abietineous forms obviously given
rise to the araucarian stock on the basis of the general principles of

354 THE ANATOMY OF WOODY PLANTS

comparative anatomy and the organization of fossil forms, but the
same derivation is likewise indicated by two other series—the
Taxodineae-Cupressineae on the one hand and the Podocarpineae-
Taxineae on the other. The first series, on the basis of com-
parative anatomical evidence, formerly possessed the ligneous
resin canals and the marginal ray-tracheids of the later Mesozoic
Abietineae. The supposed species of Seguota of the Mesozoic
are not representative of the living genus, but have the organiza-
tion of araucarian conifers, as has been shown by recent anatomical
investigation of the forms from the American Cretaceous. They
accordingly have no bearing on the question of the origin of the
Taxodineae-Cupressineae. In the case of the series which has
been described above as the Podocarpineae-Taxineae, the evidence
from fossil forms is practically non-existent in the present state
of our ignorance in regard to the organization of the Mesozoic
conifers of the Southern Hemisphere. The situation must there-
fore be judged on the basis of the living forms. The reproductive
structures of the Podocarpineae, particularly those of the genus
Podocarpus, are strikingly abietineous and sufficiently clearly
indicate the affinities of the subtribe. The primitive Taxineae,
obviously in accordance with the established principles of com-
parative anatomy, formerly possessed the abundant wood paren-
chyma of the Podocarpineae, and their systematic position is
therefore elucidated. If the statements in the present paragraph
are well founded, evidently both the Taxodineae-Cupressineae
and the Podocarpineae-Taxineae are of abietineous origin.

The time has now come to summarize the phylogenetic affini-
ties of the Coniferales, both as regards the general relationship
of the tribe and as regards the affinities of its particular subtribes
with one another. It seems clear that the Abietineae have on
all counts the strongest claim to be considered as primitive repre-
sentatives of the Coniferales. These may be summarized as
follows: filiation with the Cordaitales and co-ordination with the
Ginkgoales; precedence to the Araucariineae, to the Taxodineae-
Cupressineae, and to the Podocarpineae-Taxineae. The evidence
for the ancestral character of the abietineous conifers may in
the future be fuller, but scarcely any stronger, than it is at the

CONIFERALES 355

present time. The accompanying genealogical tree (Fig. 254)
will make clear the views as to sequence and affinities developed
in the previous paragraphs of the present chapter. The Cordaitales
of the Paleozoic serve as the starting-point, and from them were
derived two cognate stocks—the Coniferales and the Ginkgoales.
The latter have suffered much extinction and end in the present
epoch in the sole surviving genus, Ginkgo. The Abietineae in
earlier Mesozoic time
Raves hice lOrmt ae
Araucariineae, which
flourished greatly in

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the later epochs of } J _noainece
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ligneous resin canals,
but before marginal
ray-tracheids had
made their appearance. The Podocarpineae-Taxineae may have
originated at a still earlier epoch and before the abietineous stock
had developed the resin canals or the marginal ray-tracheids which
distinguish it from the Cretaceous onward. Unlike the Arau-
cariineae the Podocarps and their allies, the yews, did not lose the
opposite pitting and bars of Sanio common to the parent stock of
both Abietineae and Ginkgoales. The Taxodineae are a still later
offshoot of the strong abietineous line and come into existence sub-
sequent to the appearance of marginal ray-tracheids and ligneous
resin canals. Certain conifers of the earlier and later Mesozoic, such
as Voltzia, Brachyphyllum, Geinitzia (sometimes erroneously desig-
nated Sequoia), etc., which have been referred to taxodineous

Fic. 254.—Genealogical tree of the Coniferales,
showing their proximity to the Ginkgoales.

356 THE ANATOMY OF WOODY PLANTS

affinities, in reality have nothing to do with that group, but are
araucarian or pre-araucarian in their relationships. There is at the
present time no trustworthy evidence that the Taxodineae were in
existence before Tertiary times, although it is quite possible in view
of the general situation that they made their appearance in the
later Cretaceous. The Cupressineae must be regarded as a con-
tinuation of the taxodineous line and as having a similar relation
to the abietineous ancestral forms.

In conclusion, it may be remarked that, whatever may be
the differences of opinion in regard to the reading of the evolution-
ary document supplied by the Coniferales as they now present
themselves to our gaze or are preserved for us as fossils from
earlier geological epochs, there can be no doubt that they con-
stitute the most important of all documents for the develop-
ment of general evolutionary principles as the result of inductive
reasoning. The treatment of the group in the present and
preceding chapters is intended to clear the way for their
further study by the development of general situations in relation
to particular anatomical and paleobotanical facts. A continued
investigation of the group, for which our American Mesozoic
deposits have already yielded so much material of crucial impor-
tance, is likely to result in the firm establishment of extremely
valuable general principles for that type of biological research
which bases its conclusions on inductive reasoning rather than
on any purely philosophical attitude, either mechanistic or vital-
istic.

CHAPTER XXV
THE METAGYMNOSPERMAE: GNETALES

The aggregation of forms included in the present chapter is
very small, but is of great importance from the phylogenetic
standpoint. The Gnetales are represented in the existing flora
of our earth by three genera. Of these the genus Ephedra occurs
throughout the Northern Hemisphere, being somewhat abundant
in the American and Asiatic continents and rare in Europe. Gnetum
is a characteristically vinelike form occurring in the eastern and
western tropics. The third genus, Welwitschia, is monotypic
and is confined to the southwestern region of the African continent.
Of the three generic types enumerated, Ephedra must be consid-
ered on account'of both its reproductive and its vegetative features
as on the whole the most primitive, although naturally its desert
habitat has exerted more or less influence on its internal organiza-
tion. Gnetum in general represents the highest stage of develop-
ment attained in the group, and this statement is particularly
true of the features presented by the organization of its gameto-
phytes. Anatomically, however, Gnetum seems to occupy a
somewhat less specialized position than does the extremely xero-
phytic Welwitschia. The South African genus just named in its
gametophytic organization is in a general way intermediate
between Ephedra and Gnetum. The consideration of the repro-
ductive structures proper, using that designation to cover both
the floral organization and the gametophytes, need not be cov-
ered more than incidentally in the present volume, since the
morphology of the parts related to sex has recently been fully
discussed in Coulter and Chamberlain’s Morphology of Gymno-
Sperms.

A transverse section of a stem of Ephedra (Fig. 255) reveals an
anatomical situation not found in any of the Coniferales. Here the
woody cylinder is characterized by rays which become very broad in
the outer region of the wood but are quite narrow in the vicinity

357

358 THE ANATOMY OF WOODY PLANTS

of the pith. Not only is the structure of the wood in the genus
under discussion contrasted in the nature of its radial parenchyma
with that of the Coniferales, but it also presents a striking resem-
blance to the dicotyledons in the possession of true vessels, albeit
of a primitive type. The correlation of large rays and true vessels
in the organization of the wood in both Gnetales and dicotyledons
is a feature which is clearly not without evolutionary significance,
as will be shown in subsequent pages. The pith is of moderate
size and the phloem
and cortex constitute
a rather thin layer
on the surface of the
strongly convoluted
woody cylinder. The
depressions on the
face of the wood
plainly correspond to
the position of the
large rays, as is the
case with identical
conditions in certain
dicotyledons.

In Fig. 256 as
shown a_ transverse
view of the structure
of the wood some-
what highly magnified. The features presented by the vessels
and rays stand out very clearly. It is obvious that annual rings
are present, although not conspicuously developed. The paren-
chyma is of diffuse distribution as in the higher conifers. The
large rays, however, illustrate interesting and phylogenetically
important features. Even in the transverse view they are clearly
composite structures and do not consist homogeneously of paren-
chyma, as in the corresponding radial bands in our northern
oaks and in the genus Gnetum. Vessels are seen intimately
incorporated in the organization of the ray. The vascular are
not the only elements of the longitudinal structure of the wood

Fic. 255.—Transverse section of the stem of
Ephedra gerardiana.

GNETALES 350

incorporated in the large ray, for fibers are also present, although
they cannot be so well recognized in the transverse section. The
constitution and
origin of the large
rays in Ephedra are
of great signifi-
cance, not only for
the Gnetales them-
selves, but also in
connection with the
problem of the evo-
lution of the higher
forms known as
dicotyledonous an-
giosperms. It will
accordingly be con-
sidered somewhat
in detail.

Fig. 257 illus- Fic. 256.—Transverse section of the wood of Ephedra
trates the organiza- species.

tion of the large ray
in an early stage of
development when
it is still rather nar-
row and close to the
medullary region of
thevstem*, “Aniin-=
spection of the illus-
tration makes it
clear that the ray is ©
bay) oO) ime amis. a
homogeneous struc-
tue) composed
entirely of storage
parenchyma. Fi-
brous elements

Fic. 257.—Longitudinal section of a large ray of i
Ephedra in proximity to the pith. necessarily enter

360 THE ANATOMY OF WOODY PLANTS

largely into its composition, and these are often in the condition
of septation. The ray, in fact, is a composite structure, organized
only partially from true radial parenchyma and also consisting
largely of transformed longitudinal fibrous elements of the wood.
These first become septate, and, particularly in the more external
regions of the wood, their divisions become progressively more and
more like the ordinary storage elements of the ray. Fig. 258 por-
trays the longitu-
dinal organization
of the wood in the
outer region of a
rather thick stem of
Ephedra californica.
The large rays are
here conspicuous
and numerous, but
not of equal size.
In general, those of
greater dimensions
have originated in
the region of the
medulla, while
those less conspicu-
Fic. 258.—Tangential view of a large ray of Ephedra one by their Sac

in its external region. have come into ex-
istence more re-

cently. The small degree of magnification employed in the figure
does not make it possible to discern clearly the composite character
of the radial parenchyma. The next illustration, Fig. 129, which
reproduces one of the smaller radial masses under a higher magnifica-
tion, makes the organization of these structures apparent. Obvi-
ously not only ordinary radial parenchyma is concerned in the
constitution of the rays, but also numerous fibers and even vessels.
It may here be stated, although that situation is not clearly shown
in the illustration, that fibers are seen in such rays in all conditions
of transformation into elements resembling the ordinary radial
parenchyma. In the genus Ephedra we have the wedding, as

GNETALES 361

it were, of radial and longitudinal storage devices to constitute a
new and, from the evolutionary standpoint, an extremely important
type of radial organization. A fact not without significance in
the present connection is the correlation of vessels with the more
abundant storage devices present in the wood of the Gnetales.
The large ray in the Gnetales is evidently a composite derived.
from the fusion in certain radii of the wood of the original radial
storage paren-
chyma with paren-
chymatous ele-
ments derived from
the copious trans-
formation of the
longitudinal tra-
cheary elements of
the wood by septa-
tion. As has been
indicated in earlier
pages, wood paren-
chyma made its
appearance in the
earlier and lower
Coniferales first in
relation to the end Fic. 259.—Another view of the same

of the annual ring

and in Mesozoic times, when well-marked annual periods of vege-
tative inactivity had become established. Subsequently the wood
parenchyma, in its inception clearly derived from the septation of
tracheids, became diffused throughout the annual ring. When it
had become abundant and well established in this condition, a
situation favorable to the appearance of large rays was attained.
The final impetus to the appearance of these structures was
supplied by the origin of vessels which, by providing a greater
supply of food substances and water from the soil, rendered
possible larger and more efficient leaves. These in turn pro-
duced larger quantities of assimilates which by the appearance
of the large rays (co-ordinate in their origin with vessels and

Mee
M #84
3 se peek
¢; ak F

362 THE ANATOMY OF WOODY PLANTS

diffuse wood parenchyma) found storage in the woody tissues
of the axis.

It will be obvious from the facts brought forward in the pre-
ceding paragraph that diffuse and abundant wood parenchyma,
vessels, and large rays are features which are intimately corre-
lated in the organization of the woods of the Gnetales and the
dicotyledons. That the situation portrayed is correct from the
evolutionary stand-
point is rendered
clear by a condi-
tion presented not
infrequently in the
wood of our living
species of Pinus.
Fig. 260 illustrates
an interesting ab-
normal feature
which is often found
in the wood of the
white pine. Dhe
annual rings are
strongly depressed
. locally, and in these

Fic. 260.—Aggregate ray from Pinus Strobus regions the rays of

the wood are

unusually abundant, or, in other words, are clustered or aggregated.
Since in the genus Pinus wood parenchyma is absent—a feature,
as has been indicated in earlier pages, definitely correlated with
its primitive phylogenetic position—the clustering of the rays,
particularly in the absence of vessels, is of no evolutionary signifi-
cance. This, of course, is primarily the result of the absence of
the later acquired capacity of producing the longitudinal paren-
chyma so necessary for the fusion of the aggregation of rays into
large and homogeneous storage units. It may further be remarked
in a general way that there is no evidence to show that the large
masses of storage parenchyma which are so striking a feature
of the organization of the woody cylinder of both the Gnetales

GNETALES 363

and the dicotyledons were in the first instance related to the
appendages, whether branch, leaf, or root. While, however,
the prominent masses of radial storage parenchyma which dis-
tinguish the groups under consideration are not primarily related
to the appendages, they become somewhat definitely limited to
this position in many forms, and even in the Gnetales themselves
are more strongly developed in connection with lateral organs.

We may next turn
to the anatomical
organization of the
genus Gnetum. Fig. 261
illustrates the general
structure of the stem
in this genus as exem-
plified by a young stem
of Gnetum scandens.
The appearance is very
similar to that of a
dicotyledonous vine
like Clematis or Vitis.
Extremely prominent

large rays separate the
woody cylinder into Fic. 261.—Transverse section of young stem of
Gnetum scandens.

distinct fibrovascular
strands. The large rays of Gnetum are distinguished from the corre-
sponding features of organization in Ephedra by two important
details. In the first place, like those of the dicotyledonous climbers
cited above, they extend broadly to the pith and do not appear first
as narrow rudiments which are widened as they pass toward the out-
side of the woody cylinder. Secondly, the broad rays of Gnetum
are in the stem usually entirely homogeneous; that is, they contain
no distinct vestiges of fibrous and vascular structures such as
appear in the broad radial storage bands of Ephedra, although
in types like G. scandens the broad rays of the stem are not obviously
derived from compounding of aggregations of rays and longitudinal
elements of the wood. Investigations on the part of Professor
W. P. Thompson as yet unpublished seem to make it clear, how-

364 THE ANATOMY OF WOODY PLANTS

ever, on the basis of the organization of primitive organs and
regions that the condition of aggregation presented by the genus
Ephedra was once present in Gnetum. We may therefore regard
the large rays of the genus as comparable with the similar struc-
tures in the mature wood of the stem of our northern oaks and
derived in a similar manner as the result of aggregation and fusion.
The minute organization of the secondary xylem in Guetum reveals

she Bes. ly in addition to the
aaa prominent rays,
which have been dis-
cussed in the pre-
ceding paragraph,
other radial struc-
tures which are of the
uniseriate type and
correspond with the
similar rays in the
Coniferales and with
the linear rays of the
wood of the oak and
allied dicotyledonous

types. Parenchyma

Fic. 262.—Transverse section of an older stem of js likewise abundantly
the same.

present in the diffuse
condition. Last and by no means least, vessels which in most
species of the genus present the large caliber characteristic of
climbers are present. These will be considered later in a special
paragraph.

In Fig. 262 is shown an older stem of G. scandens. Here the
woody cylinder instead of consisting of a single circle of bundles
has become polydesmic. This condition cannot be regarded as
having any large evolutionary significance, as it is commonly
found in the stems of climbers of remote systematic affinities.
Its chief significance is in connection with the origin of the type
of cylinder presented by the genus Welwitschia and that found
in certain cycadean types, living and extinct. Gnetum shows,
not only the polydesmic stem often found in woody climbers,

GNETALES 365

but also the extreme condition found in strap-shaped lianae.
Fig. 263 portrays this condition in the stem of G. latifolium. The
woody strands of the polycyclic cylinder fail to develop on two
opposite sides of the axis, and this organization is correlated with
the flattened transverse section in the stem. It is obvious that
the genus under discussion has advanced to a condition of
organization such as is paralleled by perennial climbers among
the dicotyledons, and it must therefore be considered as having
reached a high evolutionary position.

The broad short axis of the remarkable genus Welwitschia may
now claim our attention. Here the stem never attains a height

Fic. 263.—Transverse section of the flattened stem of Gnetum latifolium

of more than a half-meter and bears two large perennial leaves
which, according to the investigations of Bower, are not the
persistent cotyledons, but a subsequent pair of foliar organs. The
perennial leaves of Welwitschia are inclosed at their bases in hollow
spaces resulting from the outgrowth of the stem. Within these
cavities, which function as moist chambers, are situated the basal
growing regions of the leaves; they are thus preserved from fatal
desiccation under the extremely arid conditions connected with
the desert habitat of the genus. Professor W. P. Thompson has
been able, through the co-operation of the Sheldon Foundation
of Harvard University, to secure an abundant supply of material
illustrating the anatomical organization of Welwitschia. When
the results of his investigations have been published, our knowledge
of this interesting and aberrant South African genus will be largely
increased beyond that supplied in Hooker’s well-known memoir.
The general organization of the axis in the genus is well illustrated

366 THE ANATOMY OF WOODY PLANTS

by the accompanying figure (Fig. 264) of the stem of a seedling
collected by Professor Thompson in Southwest Africa. It is clear
that the same polydesmic organization of the axis is present as
is found in the older stem of species of Gnetum. In the South
African genus, however, the polydesmic condition extends to the
roots and is accordingly to be regarded as a more innate feature
of organization than in Guetum. The structure of Welwitschia
suggests a climbing
ancestry. Although
we do not know the
explanation of the
polydesmic condition
in climbers, it is
clearly co-ordinated
with the vine habit.
We may suppose that
the forbears of the
genus were originally
forest climbers and
that the surviving
strongly truncated
desert species has per-
sisted in its present

Fic. 264.—Transverse section of young stem of mee
Welwitschia mirabilis. habitat with the re-

tention of the ances-

tral polydesmy. A similar suggestion has been made in an earlier
chapter as a possible explanation of the phenomenon of polydesmy
in the cycadean forms, living and extinct.

Having considered the general topography of the stem in the
Gnetales with particular reference to the presence of large rays
of the dicotyledonous type and the phenomenon of polydesmy, we
may now profitably turn our attention to the more minute organ-
ization of the wood. The ligneous tissues consist of rays and
longitudinal elements. The former have already been sufficiently
discussed in previous paragraphs. The longitudinal structures
of the xylem consist of tracheids, vessels, and storage parenchyma.
The tracheids do not need any extended reference, as they have

GNETALES 367

been described in an earlier page of the present work. They
possess clearly marked bordered pits larger than those of the
dicotyledons and provided with a distinct torus. The vessels
of the tribe are of considerable interest from the standpoint of
the doctrine of descent, as they clearly indicate the derivation of
vascular structures from tracheids. This takes place by the
modification of the terminal regions of the incipient vessel from
gradually tapering to distinctly inclined walls at angles with the
sides of the element. These differentiated terminal aspects of
the vessels are provided with very much larger pits than are found
in the lateral walls. These pits, however, generally in Ephedra,
and apparently universally in the two higher genera, lose their
membranes at an early stage, and free intercommunication is
thus established. In the higher genera there is a marked tendency
for the terminal walls of the vessel to develop a single huge bordered
pit in which the membrane is lacking. In Ephedra, on the other
hand, the terminal pits are numerous, and in a few cases are
found to fuse with one another horizontally with the resultant ap-
pearance of slits comparable to those in the vessels of the lower
dicotyledons. The type of vessel found in Ephedra has been re-
cently stated to persist in the reproductive axes of the genus Gnetum.
The parenchymatous structures of the Gnetales need no extended
reference, for on the whole they resemble those of the higher Coni-
ferales, both in their distribution in the annual ring (and this is
diffuse) and in their configuration. Sometimes structures occur in
the woods of the Gnetales more or less resembling substitute fibers,
since with pointed elongated configuration they unite a persistence
of protoplasmic contents. It is clear from the brief summary of
the organization of the wood here supplied that the Gnetales are
of great importance from the evolutionary standpoint, particularly
in connection with the important problems presented by the
evolution of large rays and vessels. They furnish a valuable
criterion for the estimation of primitive anatomical characteristics
in the organization of the wood in that huge heterogeneous aggre-
gation of forms assembled under the caption of dicotyledons.
Their value in this respect will appear, at a later stage. Although
the Gnetales clearly indicate conditions of anatomical organization

368 THE ANATOMY OF WOODY PLANTS

which are of a primitive nature, we are unfortunately even less
well informed as to their fossil representatives than in the case
of the angiosperms. There are, in fact, no well-authenticated
gnetalian remains, even from the later period of the Mesozoic,
in which the dicotyledons had become well established as an
important component of the plant population of the earth. The
wide geographical distribution of the three living’ genera may
perhaps be regarded, in conjunction with their anatomical organ-
ization, as a definite indication of their earlier more abundant
occurrence.

The root in the Gnetales needs no special consideration in the
present connection. In the genus Guetum it furnishes some
evidence as to the original organization of the large rays, but in
Welwitschia it is polydesmic like the stem and is of little value from
the standpoint of evolutionary anatomy. The structure of the
root in Ephedra closely resembles that in the stem, except as
regards those general features which distinguish root from axis.

The foliar organs of the group which forms the subject of the
present chapter are distinctly gymnospermous in their anatomy
and frequently exhibit the copious development of transfusion
tissues of the type characteristic of the leaves of the Ginkgoales
and Coniferales. Centripetal wood is conspicuously absent in the
leaves of the Gnetales, unless it be assumed that the transfusion
elements are actually representatives and not merely derivatives
of the old centripetal or cryptogamic wood. On account of the
small size of the leaves in Ephedra the transfusion tissues are
relatively poorly developed. In Gnetum a higher systematic
position is strongly vouched for by the organization of the leaf,
in which transfusion elements are not particularly well developed.
The foliar organ of Welwitschia supplies us with transfusionary
features most strongly manifested. The truth of this statement
may be verified by reference to Fig. 265, which illustrates a part
of a transverse section of the leaf of the South African genus.
Transfusion elements originating on the flanks of the xylem extend,
as in certain Coniferales, above and below outside the sclerenchyma-
tous sheath which surrounds the fibrovascular bundles of the
leaves.

GNETALES 369

The anatomy of the Gnetales is of particular importance at
the present time when they have come to the front once more,
either as a cognate stock with the dicotyledons or as their actual

ancestors.

The study of the internal organization of the group

in comparison with the dicotyledonous angiosperms reveals many

features of marked
resemblance. Both
are provided with
large rays which
are clearly fusion
products; and in
both the wood
shows conducting
elements belonging
to the category of
vessels. The rays
apparently supply
a very cogent argu-
ment for the close
affinity of the
Gnetales and the
angiosperms. In
the case of the
vascular struc-
tures, however, it
is not so clear that
a morphological
identity of the ele-
ments present in

the two groups can be successfully maintained.

=

oe "ia oe J, ‘f -
oad eo wall? Tips Y
ye IY g
ae * Satan gr
a AA ETS

12

IPS
@5
Oe)

S3¢ cS)

Sse fears
= See

-
See Seis

, gb os .23
4
at

4 4 (
bis
TL
7]
4,
VG G44

UT

SEES
SS NY Ss SS rs 2
WHYS SSF SS =
PRINS

‘
if
WG,

Fic. 265.—Transverse section of a leaf bundle in
Welwitschia mirabilis.

In the dicotyledons

the pits present on the terminal walls of the vascular elements are
not larger than those which appear laterally, and the perforation
of the vessel takes place as the result of fusions of opposite pits, as

has been shown in an earlier chapter.

A very different situation

manifests itself, as indicated above, in the genera of the Gnetales.
Here the vascular elements have exceedingly large pits on their
terminal walls, and in Ephedra these are usually without membranes,

370 THE ANATOMY OF WOODY PLANTS

while in the other two genera they are invariably so characterized.
The phenomenon of fusion of the pits to form the scalariform
perforations present in the lowest type of vessel of the dicoty-
ledons is rare in the woods of the Gnetales. The general
anatomical organization of the fibrovascular tissues in the Gnetales,
however, may be said distinctly to favor the hypothesis of dicoty-
ledonous affinities, or, at any rate, to be more in harmony with
such relationship than with that of any other group of seed plants.
The affinities of the Gnetales on the gymnospermous side are
much more difficult to discover. This situation is due no doubt
in large part to the fact that as yet no properly authenticated
fossil representatives of the group have been found. In recent
years there has been a tendency to associate this group with the
Cycadales, but it is extremely difficult to discover any valid
reasons from the anatomical side to justify such a relationship.
There seem, in fact, to be no anatomical features of wide validity
which can be invoked in favor of an affinity between the highest
living gymnosperms and the lowest. It seems on the whole
more probable that the Gnetales are cognate with, or derived
from, the Coniferales, since there are a number of features which
make such a connection likely. The transfusion tissue of the
highest gymnosperms is clearly of the coniferous type, and this
feature must count strongly against any close relationship with
the Cycadales, in which, owing to the strong persistence of the
centripetal wood, true transfusion tissues have not yet made their
appearance. The uniseriate rays of Ephedra and Gnetum also
point to coniferous affinities and not toward a connection with
the Cycadales, in which the rays are universally multiseriate.
The presence of bars of Sanio in the vessels of Ephedra supplies
likewise an argument for the coniferous rather than the cycadalian
relationship. :

The main support for the cycadean origin of the gymnospermous
group which forms the subject of the present chapter has been
derived from the reproductive structures of the living and extinct
representatives of the cycadean stock. The investigations of
Wieland have brought to our knowledge the complete organization
of the reproductive parts in the Bennettitales. These consist

GNETALES 371

of cones involved in the young condition in rather large sheltering,
bracts. Immediately above the series of bracts microsporophylls
(usually of large size and pinnate structure) are attached to the
axis of the strobilus. The main part of the cone is, however,
made up of the seminal organs, consisting of pedicels to which
single seeds are attached in an orthotropous manner. The seeds
are somewhat sheltered by the swollen ends of sterile appendages
inserted among the seminal organs on the axis. The protected
condition of the seeds has given rise to the suggestion of angio-
spermous organization. This apparently cannot be regarded as
more than the merest analogy, since the seeds are not shel-
tered within a closed megasporophyll and the pollen is deposited
directly upon them after the typical gymnospermous manner.
Further, the cycadean fertilization is not even siphonogamous,
as would be expected of a group presenting valid claims to be
regarded as ancestral to the highest seed plants. Nor is the
analogy with the flower of the angiosperms to be given greater
weight, for the arrangement alone and not the intimate organiza-
tion of the reproductive parts shows any real resemblance to
the conditions found in the floral structures of the angiosperms.
An androgynous cone such as is frequently present in the Coniferales
supplies an equally good basis for comparison; for here the bracts
below correspond to the floral envelopes of the flower, while the
lower whorls of microsporophylls simulate the anthers, and a
plausible resemblance to the carpellary whorls is presented by the
ovuliferous scales of the upper region of the modified cone. The
most striking objection and the same as occurs in the case of the
Bennettitales is the fact that the ovuliferous scales are not angio-
spermous. The siphonogamic fertilization of the Araucarian coni-
fers, on the other hand, reveals a nearer degree of resemblance to
the conditions found in the flower of the angiosperms than does
the zoidogamy of the archigymnospermous Bennettitales. In the
present state of our ignorance of the fossil ancestors of the Angio-
spermae it seems impossible to fix on any group of gymnosperms,
living or extinct, except the Gnetales, which can be regarded with
any degree of probability as having been either ancestral to the
highest of the seed plants or even cognate with them.

Bo THE ANATOMY OF WOODY PLANTS

In accordance with the hypothesis of the derivation of the
angiosperms from the Bennettitales, a relationship between this
cycadean group and the Gnetales has been suggested. There
seem to be very slight grounds for this assumption of affinities.
The so-called “pollen chamber” of Ephedra is not morphologically
equivalent to the pollen chamber of the lower gymnosperms
which is derived by the breaking down of cells in the lysigenous
manner, and in this respect is in marked contrast to the depression
on the apex of the nucellus of Ephedra, which is continuously
covered with the epidermis. Moreover, Ephedra and the other
Gnetales are siphonogamous in contrast to the zoidogamous
condition of fertilization in the Cycadales. The details of organ-
ization of such a flower-like structure as is found in Welwitschia
apparently supplies no adequate basis for comparison with the
bisporangiate strobilus of the Bennettitales. The original error of
Saporta in designating the impressions of the fructifications of
bennettitean types as belonging to hypothetical proangiosperms
or primitive angiosperms has manifested a considerable degree of
vitality in the rather long interval since it was first advanced;
but there seems little reason to accept it at the present time, in
view of our increased knowledge of the organization of both vege-
tative and reproductive parts in the various groups of seed plants,
living or extinct.

The Gnetales are clearly gymnosperms which in certain features
of anatomical structure and reproductive organization have made
a marked advance in the direction of the characteristics of the
angiosperms, and it is not improbable that they are at least a
cognate group. They present, however, no valid resemblances
in either their reproductive or their vegetative parts which justify
an assumption of relationship, even remote, with the extinct
bennettitean Cycadales. It therefore appears highly unlikely
that either the angiosperms or the Gnetales have any close degree
of relationship with any archigymnospermous group, although
it seems not at all improbable that they are somewhat closely
related to one another.

CHARTER XcxvVi
THE ANGIOSPERMS

The angiosperms as a whole possess features which separate
them sharply both reproductively and vegetatively from gymno-
spermous groups. The first of these in importance is the phenom-
enon of angiospermy itself. In the group the microspores or
pollen grains no longer reach the micropylar canal at the apex of
the ovule, but are accommodated on the tip of the megasporophyll,
which becomes modified as the receptive prominence or stigma.
The reception of the microspores on the terminal region of the
carpel or sporophyll is not, however, an exclusive characteristic
of the angiosperms, since the same feature is presented by the
surviving araucarian conifers. The megasporophylls in this
largest and most important group of the seed plants are either
folded upwardly upon themselves in such a manner as to inclose
the young seeds or ovules, or undergo protective fusions with
other similar structures in the same flower. The angiosperms
are characterized anatomically throughout by the possession of
vessels, these structures being absent only in certain aberrant
representatives of the Cactaceae, Crassulaceae, and Trochoden-
draceae. In all the exceptional cases mentioned there is clear
evidence on comparative anatomical and experimental grounds
that vessels were formerly present. The possession of histological
structures in the wood known as vessels may accordingly be set
down as a primitive feature of organization of the angiosperms.
Associated with the vascular structures just indicated, there is a
general improvement in the storage devices of the wood which,
in an extreme form, leads to the appearance of the herbaceous type.
The gametophytes, particularly the male gametophytes, of the
angiosperms present a very marked degree of uniformity in the
two great divisions of the group. The microspore or pollen grain
undergoes part of its germination in the microsporangium and
develops normally a tube nucleus and a generative cell. The

373

374 THE ANATOMY OF WOODY PLANTS

latter, unlike the similar structure in the microspores of the higher
Coniferales, does not give rise to stalk and body cells, but originates
directly by division the two generative nuclei which function in
fertilization. The microspores are sheltered in sporangia, which
are typically four in number, on each sporophyll. The sporangia
owe their dehiscence to a mechanical layer within the wall, resem-
bling in structure that found in Ginkgo and certain Coniferales, but
no longer related, as in these, to the fibrovascular system of the
sporophyll. The megasporangium is much modified and, as in
other known seed plants, is without any opening mechanism.
In certain of the lower and amentiferous dicotyledons, however,
tracheary structures have been discovered in the inferior region
of the megasporangium (Casuarina, Corylus, Castanea). The
germinated megaspore normally gives rise to a gametophyte
containing originally eight naked nuclei which result from three
successive divisions. Of these, six become organized as cells by
the development of inclosing protoplasmic bodies, while the two
remaining nuclei, one from each pole of the elongated sac, unite
to constitute the endosperm nucleus, a structure characteristic
for the angiosperms and not occurring in any lower forms. ‘Three
cells in the micropylar region of the gametophyte become organized
as the single egg and the two synergidae. Three others constitute
the antipodals in the base of the gametophyte. After the egg has
been fertilized by one of the sperm nuclei of the pollen tube and
the endosperm nucleus has contracted a union with the remaining
male element, the egg develops as the embryo and the endosperm
nucleus gives rise to a mass of tissue which usually quite supplants
the original gametophyte or embryo sac as nourishing substance
for the developing embryo. The seed of the angiosperms conse-
quently typically contains, in addition to the gametophyte and
sporophyte of lower seed plants, a third generation. This is
known as the trophophyte or endosperm, and it is distinguished
from gametophyte and sporophyte, not only by its peculiar mode
of origin, but also by the fact that in its nuclear divisions three
times as many chromosomes are present as in the gametophyte
and one-half more than in the sporophyte. This cytological
condition is doubtless due to the three nuclei which are fused to

THE ANGIOSPERMS S15)

constitute the original endosperm nucleus from which the cells
of the endosperm or trophophyte take their origin. The high
degree of constancy in the essential features of organization of
both sporophyte and gametophyte in the angiosperms mark them
as a monophyletic group in which the two great divisions, dicoty-
ledons and monocotyledons, have had a common origin.

There seems to be no reasonable doubt that both divisions of
the angiosperms—the dicotyledons and monocotyledons—have
originated from gymnospermous ancestors and not directly from
any existing or extinct group of vascular cryptogams, since they
entirely lack cryptogamic features of organization in all their
organs with the sole exception of the root. The radical organ,
however, is without significance as indicating cryptogamic deriva-
tion, for cryptogamic or centripetal primary wood is present in
all roots without exception. It further seems obvious that the
angiosperms in neither of their two divisions can have originated
from the Archigymnospermae, since they present the siphonoga-
mous mode of fertilization in contrast to the zoidogamy or fertiliza-
tion by antherozoids present in the lower gymnospermous tribes
which are more nearly related to the Filicales.

The general features of the angiosperms indicated above
characterize the two great divisions, dicotyledons and monocotyle-
dons, in common; and it is now necessary to specify the distinguish-
ing structures which separate these from one another. Since the
dicotyledons are with the greater probability the older and more
primitive of the two main groups, they will first be considered.
In the dicotyledonous angiosperms the seed is distinguished by
an embryo possessing paired cotyledons or seed leaves. This
feature is perhaps the most constant characteristic of the group.
In the fibrovascular structures the wood is distinguished by well-
marked secondary growth which becomes feeble only in forms in
which the herbaceous habit has become distinctly developed. The
tissues belonging to the conductive category are, moreover, typi-
cally arranged in the form of a cylinder which is continuous in
woody forms but becomes broken up into separate strands in
stems with herbaceous texture. When the fibrovascular organiza-
tion consists of isolated bundles, these are ordinarily arranged in

376 THE ANATOMY OF WOODY PLANTS

a circular fashion, but occasionally in stems with large leaves
provided with numerous foliar traces the periphery of the cylinder
is no longer capable of accommodating the bundles, so they have
to be disposed of in a position either medullary or cortical. The
leaves of the dicotyledons are usually distinguished by skeletal
structures or veins ending freely toward the margins. The main
veins may be arranged either in a radiating or in a palmate fashion,
or may take their origin in an alternating manner from opposite
sides of a main vein or midrib, in which case the venation is said
to be pinnate. The free venation of dicotyledonous angiosperms
gives them a considerable advantage over the monocotyledons
in the possibility of submerged existence or a shaded habitat, when
their leaves often become finely dissected. The root of dicotyledons
presents no special features worthy of note in a general statement.
The floral parts are ordinarily present in multiples of five, and
the floral envelopes show themselves on the whole less likely to
vary from the pentamerous condition than do the essential or
strictly reproductive whorls, the stamens and pistils. Pollination
is sometimes effected through the agency of currents in the air,
but more commonly in the higher families by insects. Fertiliza-
tion results from the penetration of the pollen tube from the stigma
to the micropyle of the ovule. The course of the pollen tube
after it leaves the region of the style may either be direct through
the cavity of the ovary to the micropyle or, avoiding the leap
across the ovarial air space, it may make its way round through
the basal or chalazal region of the ovule. The last method of
fertilization is characteristic of the genus Casuarina, the Betu-
laceae, the Juglandaceae, certain Urticaceae, etc., and is known
as breech fertilization or chalazogamy. It has been suggested
by Treub and Nawaschin that this is a primitive method of pene-
tration for the angiosperms and marks a transition from the
siphonogamous and higher gymnosperms in which the pollen,
being deposited directly upon the ovule, has not become accustomed
to leaping an air space. In the lower representatives of the
dicotyledons the pollen, although no longer deposited on the
micropyle, still maintains its primitive inability to cross an air
space. There is much to be said for the hypothesis of the primitive

THE ANGIOSPERMS ite |

significance of chalazogamy from the standpoint of fibrovascular
anatomy. It seems, however, a feature too likely to be modified
somewhat rapidly by conditions to rank as a criterion of the first
order for the distinguishing of the most primitive dicotyledons.
The monocotyledons are distinguished, as their name indicates,
by a seed containing an embryo with a single cotyledon or seed
leaf. This feature is very constant, but there are indications of
the presence of a second cotyledon in certain of the grasses, such
as Zizania, Avena, etc. The fibrovascular strands of this group
are ordinarily closed; that is, they do not possess the capacity to
increase in thickness through the activity of a cambial layer.
The arrangement of the strands in the monocotyledonous angio-
sperms is also distinctive, since the bundles, instead of being
disposed in a circular fashion as in the mass of herbaceous dicotyle-
dons, are scattered throughout the transverse section of the cylinder
and sometimes even occur in the cortex. This peculiar disposition
is, beyond any reasonable doubt, the result of the entrance of
numerous leaf traces into the axis at each node, a consequence of
the high assimilative efficiency of the foliar organs. The veins
of monocotyledonous leaves are distinguished primitively by a
closed arrangement; that is, starting out at the base of the leaf
as a closed system, they reunite at the apex of the leaf. This
disposition of the skeletal tissues of the leaf makes it immune
from tearing action. The lateral veins, in consequence of this
situation, are largely abortive. In many palms, aroids, and
Scitamineae the venation of the leaf becomes open as a result of
changes in the apex. In such cases the venation of the early-
formed leaves of seedlings is closed, showing that this condition
is the primitive one for the group. The root in monocotyledons
is distinguished, as is the stem, by the absence of secondary growth.
The bundles are usually distributed in a circular and radial fashion,
but in certain palms and orchids they may be scattered throughout
the transverse section of the organ as they are in the stem. The
floral parts occur in multiples of three, and the floral envelopes,
as in the dicotyledons, show less numerical variability than do
the essential whorls consisting of stamens and carpels. Pollination
is effected usually through the agency of insects, but may be

378 THE ANATOMY OF WOODY PLANTS

brought about by currents of air in some of the probably more
primitive groups. Fertilization is always porogamous (through
the micropyle), and chalazogamy is at the present time quite
unknown in the monocotyledons. The members of this group
are extremely important as food plants on account of their high
efficiency in the elaboration of assimilates. The proportion of
seed produced by some of the cereals in a vegetation period of
three or four months is often over 30 per cent of the total weight—
a relative productiveness seldom realized in other plants.

CHAPTER XXVII
THE WOODY DICOTYLEDONS

As a matter of convenience the anatomical organization of the
woody dicotyledons will be considered in the present chapter.
It must not be supposed, however, that such a mode of treatment
implies that the woody texture of the stem has any value from the
phylogenetic or taxonomic standpoint. Perennial dicotyledons
are distinguished by the possession of a thick woody cylinder
resulting from the activity of a cambial layer situated between
phloem and xylem; this adds largely to the wood and less copiously
to the inner bark during the periods of growth. The woods of the
dicotyledons offer a great range of structural organization, and
their identification on the basis of anatomical features is corre-
spondingly difficult.

In all except a few instances dicotyledonous woods are pro-
vided with vessels. These belong to two main types, namely,
those with scalariform and those with porous perforations. It
has been made clear in an earlier chapter dealing with the structure
and organization of the vessel that the type with scalariform per-
foration of the inclined terminal walls results from the fusion of
rows of opposite pits which gives rise to elongated horizontal
pores from which the membranes quickly disappear. This process
repeated in successive rows of pits results in the appearance of
lattice-like or scalariform perforations in the ends of the vessels,
and these permit a ready passage of water. On general evolution-
ary principles the vessel with scalariform perforations is to be
regarded as more primitive than the porous type presently to
be discussed. It is not surprising for that reason that it is a
characteristic feature of groups which are considered on good
grounds to be low in the dicotyledonous scale. The vessel with
the porous type of perforation is clearly derived, as has been dem-
onstrated in an earlier chapter, from the scalariform condition,
in the first instance at any rate, by the loss of the bars of

379

380 THE ANATOMY OF WOODY PLANTS

lattice-work through mucilaginous degeneration. The vessel of the
second type is found in the woods of the higher groups and indi-
cates a more advanced condition of evolution, other things being
equal, than does the vascular type with scalariform terminal
perforations. Frequently woods with the porously perforated
type of vessel in their mature structure show the scalariform con-
dition in the region of the primary wood, thus providing confirma-
tion of the conclusion that the latticed terminal wall of the vessel
is phylogenetically older than that in which large pores are present.

In many instances vessels in dicotyledonous woods become
more or less degenerate and are then easily recognized by their
inclined end walls and in any case by a lateral pitting and internal
sculpture which clearly distinguishes them from tracheids or fibers.
Such vessels are often present in woods in which the fibrous ele-
ments are of the nature of libriform, substitute, or septate fibers,
and are frequently inappropriately designated as tracheids. True
tracheids have always tapering or fusiform ends and are not
provided with the lateral pitting and sculpture of vessels. It is
important to diagnose degenerate vessels as such, because a failure
to make this distinction may result in quite erroneous views as
to the relationship of woody dicotyledonous forms. In some
cases typical vessels may disappear altogether from the mature
structure, either generally or locally. Cases of the general dis-
appearance of vascular elements are supplied by certain Cactaceae
and Crassulaceae. In the genus Drimys among the Magnoliaceae
and in certain genera of the allied Trochodendraceae vessels have
also entirely disappeared from the normal wood. In Drimys,
interestingly enough, wounds, especially wounds of the root,
recall elements which have the lateral sculpture of normal vessels
of the Magnoliaceae, without manifesting, however, their charac-
teristic scalariform perforations. This defect is easily explained
as a result of the comparatively small size of the reversionary
elements simulating vessels. Local absence of vessels is frequently
found in connection with the development of the large rays in
dicotyledonous woods. This is particularly well illustrated in
the wood of Alnus and in the root-wood of certain of our northern
oaks. In the region of the aggregate ray, as has been indicated

THE WOODY DICOTYLEDONS 381

in earlier pages, the organization includes only tracheary elements.
It is thus apparent that vessels, a characteristic feature of structure
in dicotyledonous woods, may in certain instances be absent.
There is no reason based on the general principles of comparative
anatomy for regarding their absence as a primitive feature. In
woods of temperate climates the vessels in the spring wood are
often much larger than in the later growth, and the organization
in such cases is described as ring-porous. The ring-porous condi-
tion is not, however, universal in trees of higher latitudes and is
not ordinarily found in tropical woods which in general have their
annual zones indistinctly marked. :

The tracheary elements in dicotyledonous wood, as has been
indicated previously, undergo very numerous modifications. In
the lower forms they resemble the similar structures in the gym-
nosperms, but quite generally they lose to a large extent their
water-conducting function and become mechanical or storage
elements. The least advanced condition in the mechanical direc-
tion is designated the fiber-tracheid, distinguished by the re-
duction in size and decrease in number of the bordered pits as
well as by increase in length and by thickening of the walls. The
libriform fiber, a further modification, has lost or nearly lost the
bordered pits, these being replaced by simple pores. In the sub-
stitute fiber, which retains its protoplasmic contents, and in the
septate fiber, which is divided by delicate partitions of pectic cel-
lulose into a number of separate units, are seen storage modifica-
tions of the tracheids present in the higher types of dicotyledonous
woods. .

The parenchymatous elements of dicotyledons are primitively
scattered throughout the annual ring. Although rarely and only
under experimental conditions revealing by actual transitions
derivation from tracheids, the storage parenchyma is generally
grouped in longitudinal fusiform or pointed series with robust
and lignified partitions which clearly indicate its origin. In the
systematically higher dicotyledons the parenchyma is character-
istically grouped in clusters around the vessels. This condition is
known as vasicentric and is a common feature of woods in which the
tracheary elements have become partially or entirely mechanical.

382 THE ANATOMY OF WOODY PLANTS

It must not be supposed, however, that the relation between me-
chanical fibrous elements and vasicentric parenchyma is abso-
lute, for in groups characterized by this mode of parenchymatous
distribution it is present even in genera with tracheary mechanical
cells. In other words, the grouping of parenchyma about the
vessels has a deeper significance than that of mere convenience
to water supply. In not a few instances the storage cells may be
confined to the end of the annual ring. This is, for example, the
situation found in the Salicales, and it also frequently characterizes
genera of the Magnoliaceae occurring in temperate climates. In
these instances an examination of the primitive regions, together
with experimental investigation, reveals as the original condition
either the vasicentric or the diffuse distribution of parenchyma.
It may be stated summarily that diffuse storage elements constitute
the primitive conditions in the woods of the dicotyledons, and
that a later modification is the vasicentric. By reduction either
of the two types mentioned may give rise to the terminal condi-
tion. Terminal parenchyma is accordingly a phenomenon of
reduction in the dicotyledonous series, while in the Coniferales,
as has been elucidated at an earlier stage, it represents the primitive
state in which all transitions between tracheary and storage ele-
ments are frequently and normally found.

It is in the organization of their wood rays or radial
storage tissues that the dicotyledons manifest the most distinct
differences from the mass of the gymnosperms. It has been made
clear in previous pages that the primitive type of ray organization
for the group was the linear or uniseriate ray. In the earliest
conditions presented to our investigation, however, that is ac-
companied by the aggregate ray, consisting of more or less fused
congeries or clusters of rays separated by fibrous elements. This
type we must regard as an original one for rays other than unise-
riate in the dicotyledons, because it is clearly found in Ephedra,
by common consent the most primitive representative of the
Gnetales, and because it is extremely persistent in primitive
organs and regions in the dicotyledons themselves. The facility
with which fibrous elements are transformed into storage cells
in the group under consideration has led to the metamorphosis

THE WOODY DICOTYLEDONS 383

of the aggregate ray, described above, into large homogeneous
masses of radial parenchyma as a consequence of the parenchyma-
tous modification of the separating fibers. This condition, known
as the compound ray, is found in relatively few dicotyledonous
woods, and these are ordinarily regarded as low in the systematic
scale. In woody types, moreover, it very readily passes into the
antecedent aggregate condition. A third and the commonest
condition of organization of the radial parenchyma in dicoty-
ledonous woods is presented by the diffuse condition. Here the
original aggregation, instead of retaining its identity or passing
into the compound state last described, spreads out in the manner
of a fan. This procedure results in the diffusion of the original
aggregations of rays evenly throughout the structure of the wood.
As a consequence of this phenomenon the organization of the
wood in the mass of dicotyledons is characterized by the presence
of abundance of rays which are of mediocre width. Sometimes
the rays of this type are nearly equal in size, but very generally
they range from extremely small to moderately large. Now and
then, however, as, for example, in the wood of beeches of the North-
ern Hemisphere and in the genus Platanus, extremely large rays
are found, readily distinguishable from those of the oak type
by the fact that they grade almost imperceptibly into radial
parenchymatous bands of mediocre dimensions. In the typical
compound ray such as occurs in certain species of Quercus, Cas-
uarina, and the Ericaceae the large ray is in sharpest contrast to
the primitive uniseriate condition.

In the foregoing paragraphs a general account has been sup-
plied of the various anatomical features of dicotyledonous woods,
and an attempt has been made to indicate the primitive condition
in connection with each category of structures. It must not,
however, be supposed that a primitive condition of organization
in regard to any one of the categories described in the preceding
pages necessarily indicates for a given group a low position in the
evolutionary scale. Taken altogether, nevertheless, they supply
extremely valuable testimony from the standpoint of the doctrine
of descent and on the whole the best available in the present state
of our ignorance regarding fossil ancestors of the angiosperms.

384 THE ANATOMY OF WOODY PLANTS

It will accordingly be of interest in the present connection to
summarize the evidence in regard to the primitive type of dicotyle-
don supplied by anatomical data.

A tendency of long standing is to consider the amentiferous
forms as representing a low condition among the dicotyledonous
angiosperms. In discussions in this connection it is well to dis-
tinguish the true Amentiferae from types which simulate them.
The Salicales, for exdmple, on the grounds both of their anatomical
organization and of important details of floral structure, cannot
be regarded as allied in any but the remotest way with types like
the alder and the oak. Further, on anatomical ground types
with more or less well-organized floral structure must be admitted
to affinity with the Betulaceae, Fagaceae, etc. This is true of
the Casuarinaceae and Ericaceae. On the basis of the co-ordinate
occurrence of vessels with scalariform perforation, tracheary
fibrous elements, diffuse wood parenchyma, and aggregate rays
we must accord to the genus Casuarina a primitive position among
the dicotyledons. This designation of affinity on the basis of the
organization of the wood is supported by the fact that it alone
among the dicotyledons possesses transfusion tissue of the flanking
gymnospermous type. A further indication of its primitive
position is furnished by the presence within the ovules of excep-
tionally large amounts of sporogenous tissue and also of tracheary
elements. Finally, we have the phenomenon of chalazogamy,
the significance of which is still much disputed. On anatomical
grounds there could scarcely be stronger reasons for regarding
Casuarina as the most primitive representative of the dicotyledons.
The only objection that has been seriously urged against this
position is the fusion of parts in the ovuliferous floral structures.
This objection, however, must weigh lightly in the balance in
view of our knowledge of the extreme conservatism of anatomical
structures in the case of living and extinct gymnosperms. It has,
for example, been pointed out that the living cycads are practically
identical in anatomical organization with the extinct bennettitean
forms, although their reproductive structures differ very widely.
Fusion of floral parts does not furnish a sufficient argument for
a high systematic position, as on that ground the genus Welwitschia

THE WOODY DICOTYLEDONS 385

would be put in a higher taxonomic position than Ranunculus,
because its reproductive structures present a condition of cohesion
not found in the flower of the buttercup. The suggestion that
Casuarina owes its anatomical organization to its xerophytic
habit must be definitely rejected because of the extremely general-
ized type presented by the organization of its woody structures.
In the single genus under consideration all the types of rays found
in dicotyledons, as has been shown in an earlier chapter, are
presented in the different species. Since all the species are equally
xerophytic, it is quite impossible to connect any type of radial
parenchymatous organization with the xerophilous habit. Physio-
logical or ecological explanations of anatomical facts are always
to be welcomed when they have any logical probability, but when
they fail in this respect they only obscure the evolutionary situa-
tion. The general anatomical evidence in the case of the interest-
ing genus Casuarina seems, in the present state of our knowledge
at any rate, entirely to justify the primitive position assigned
to it in the great systematic work of Engler and Prantl, in which
it is placed at the very base of the dicotyledons.

If on anatomical and other grounds Casuarina must be regarded
as a primitive representative of the dicotyledonous angiosperms,
it is equally clear on the same evidence that with it must be joined
the Betulaceae and Fagaceae and in all probability the Ericaceae.
The usual fusion of parts in the flowers of the groups enumerated
cannot apparently, in view of the overwhelming anatomical
evidence for their primitive position, be regarded as a very im-
portant systematic criterion. Wind pollination and the general
absence of herbaceous forms further supply striking features of
conformity to the conditions found in the higher gymnosperms
from which it is very likely the dicotyledons have taken their
origin.

Another assemblage of forms which has been pushed into the
foreground, particularly in recent years, as the primitive repre-
sentatives of the dicotyledonous angiosperms are the Ranales.
Without discussing whether the families included under this.
general heading all properly belong here, we may point out that
on anatomical grounds the claims of that group to affinity with

386 THE ANATOMY OF WOODY PLANTS

the Gnetales and other gymnospermous types must be regarded
as somewhat doubtful. The general anatomical organization of
the Ranales presents some range of variety; but they do not
strikingly exhibit either vessels, fibers, parenchymatous distribu-
tion, or organization of the rays such as would be expected from
the study of the higher gymnospermous types and the investigation
of primitive regions and organs in the dicotyledons themselves
in primitive representatives of the angiosperms. The only strong
argument which can be advanced for the low systematic position
of the Ranales is based on their floral organization. The fact
that this has been deliberately disregarded in the WNatiirliche
Pflanzenfamilien of Engler and Prantl shows that from the sys-
tematic standpoint it is a criterion which cannot be considered
as of overwhelming importance. With the ranalian origin of the
angiosperms is tied up the whole hypothesis of their derivation
from the Bennettitales. It has been pointed out that, from the
point of view of anatomy, there seems to be little reason to enter-
tain the hypothesis that the proangiosperms were of cycadean
origin, even if the earlier discovered impressions of the reproductive
remains of Mesozoic cycadean forms were originally referred to
angiospermous affinities by so distinguished a paleobotanist as
Saporta. Anatomically speaking, there seems accordingly to be
equally slight grounds for the derivation of the angiosperms from
zoidogamous gymnosperms as from ranalian dicotyledons. The
whole question, however, must await final and satisfactory solu-
tion until we are actually acquainted with the earlier angiospermous
types which will doubtless be sooner or later brought to light from
the Jurassic or from even earlier epochs of the Mesozoic. In
the meantime such knowledge of the general anatomical principles
as has been gained from the comparison of extinct and living
gymnosperms appears to point toward types like Casuarina and
its allies as representing, at any rate anatomically, the most
primitive conditions.

CHAPTER XXVIII
THE HERBACEOUS DICOTYLEDONS

The herbaceous type in the dicotyledons, although not of
any definite systematic value, has a considerable significance
from the anatomical and evolutionary standpoints. It has been
pointed out in earlier pages that discontinuity of the woody cylinder
in siphonostelic forms simulating that found in herbaceous dicoty-
ledons is frequently present in older groups. In such cases, how-
ever, the situation has a very different anatomical explanation
and a diverse evolutionary interpretation from that presented
by the dicotyledonous angiosperms. In many of the lower groups
of plants belonging to both Lycopsida and Pteropsida we find the
surviving representatives distinguished from those characteristic
of the period of greatest luxuriance by a herbaceous, as opposed
to an arboreal, habit. Such herbaceous types owe their origin
to degenerative changes and present a distinct contrast to the
situation exemplified in the herbaceous dicotyledons, where the
modification is the result of differentiation and specialization and
not the consequence of mere degeneracy.

It is necessary to illustrate clearly the mode of appearance of
the herbaceous type in the older groups before passing on to the
consideration of the situation manifest in the herbaceous angio-
sperms. Fig. 266a illustrates the organization of a siphonostelic
stem in a lepidodendrid. Here the primary wood is well developed
and constitutes a continuous cylinder. The secondary xylem,
which is the result of cambial activity on the inside of the primary
phloem, forms a continuous layer interrupted only in the region
of exit of a strand destined to a branch, which causes a gap in the
primary cylinder that is naturally perpetuated in the earlier
organization of the secondary cylinder. In 0 is shown the condi-
tion presented by a sigillarian of Permian age. In this instance
the siphonostelic primary cylinder has become degenerate and
consequently is only well developed in the regions facing departing

387

388 THE ANATOMY OF WOODY PLANTS

foliar traces. Asa direct result of this discontinuity of the primary
cylinder, the secondary cylinder becomes united only by the
gradual broadening of the originally separate secondary segments.
In c is portrayed the condition found in the stem of a third rep-
resentative of the Lycopsida—namely, a calamite. Here the
primary wood is extremely degenerate, and the secondary segments
as a consequence are very narrow and quite widely separated.
In the herbaceous living
Equisetum, the stem of
which is depicted on an
earlier page, secondary
growth has disappeared,
being present only as a
vestige in conservative
organs and regions.

The series indicated
above illustrates the origin
of the herbaceous type by
degeneracy. The Lycop-
sida have been chosen
because they reveal the
situation in the clearest

Fic. 266.—Diagram showing the effect of Manner and with the
degeneracy of the primary wood on the develop- fewest complications. In
cara the secondary cylinder. Explanationin po Pteropsida the topog-

“ raphy is rendered less easy
of comprehension by the presence of the often numerous foliar
gaps which characterize the anatomical organization of the
siphonostelic central cylinder of that phylum of vascular plants.
The principles involved are, however, the same. It may accord-
ingly be stated that discontinuity in the primary siphonostelic
central cylinder is due either to gaps related to the exits of traces
supplying the appendages or to the local degeneracy of the cylinder
itself. These interruptions are perpetuated in the early organiza-
tion of the secondary cylinder. If the secondary tissues are also
degenerate, as in the existing survivors of cryptogamic groups, a
pronounced herbaceous condition is the result. This is, however,

THE HERBACEOUS DICOTYLEDONS 389

pre-eminently the consequence of degeneracy and has no dynamic
evolutionary significance in contrast to herbaceous modifications
presented by the stems of dicotyledonous angiosperms.

With the foregoing preliminary statement in regard to the
appearance of the herbaceous type in primary and secondary
cylinders (or in the secondary cylinder alone) in lower forms we
are in a position to attack the origin of the stems of herbaceous
texture in the highest seed plants. It has been noted in the
preceding paragraphs that the degenerate herb is derived from
ancestral forms characterized by woody stems. The same general
condition is found in dynamic herbaceous types among dicoty-
ledons; and it is extremely important to keep this situation
clearly in view, as a failure to do so results in an anatomical fallacy..
It cannot accordingly be too strongly emphasized that a proper
understanding of herbaceous axes in the angiosperms can be reached
only by the comparison of nearly related stems of woody and
herbaceous texture. Any other procedure leads to erroneous
results.

In former chapters the various types of rays in the Gnetales
and the dicotyledons have been discussed. In the present con-
nection only one of these need seriously be considered—the com-
pound ray. The large or compound type of ray is characteristic
of only a few arboreal types of probably primitive antecedents, but
occurs in a more or less modified form in many herbaceous and
climbing stems. It will be convenient to discuss first the condi-
tions present in vines, as these are usually more woody in their
character and consequently serve as an appropriate transition to
axes of soft or herbaceous texture. As a preliminary to the con-
sideration of the anatomy of the vine, it will be well to devote
some attention to the organization of an exotic member of the
Vitaceae from the tropics. Fig. 267 reproduces the woody cylinder
of the shrubby genus Leea from the eastern tropics. To the right
are to be seen:a number of large rays which clearly belong to the
category designated in an earlier chapter as compound. Above
and below, and particularly to the left of the illustration, the com-
pound rays become much smaller in size. Those which are of
greater dimensions are in relation to leaf traces passing out in the

390 THE ANATOMY OF WOODY PLANTS

region of the node. Fig. 268a@ shows one of these on a higher scale
of magnification. In the large ray and toward the bottom of the
figure lies the leaf trace, which is both subtended and flanked by
storage parenchyma. The wood on either side of the large ray
shows the presence of the primitive rays, which, as has been
pointed out on an earlier page, may be considered in a general
way as equivalent to the narrow rays of the conifers and similar
forms. In Fig. 2680 is
seen a vertical tangen-
tial section through one
of the large leaf rays of
the same genus. It is
evident that the foliar
trace runs in a much
enlarged ray and is sur-
rounded on all sides in
its horizontal course by
ray parenchyma. It is
very clear from the con-
sideration of the genus
Leea that there are pres-
ent large rays in storage
relation to the leaf
traces, precisely as in
the oak, Casuarina, and similar forms. We may now pass to
the situation in Vitis, where the conditions are not so manifest,
but are quite intelligible by comparison with the shrubby member
of the Vitaceae just described. Fig. 269 shows the general topog-
raphy of the stem of the Concord grape in the region of the node.
Seven leaf traces are to be seen as darker masses. Of these only
one is passing into the cortex and the rest are still contained within
the woody cylinder. Fig. 270a illustrates one of the foliar seg-
ments of the stem with a higher degree of magnification. The
segment in question is separated on either hand from the rest of
the woody cylinder by broad radial bands of parenchyma. In the
leaf trace itself may be found radial dark stripes, the primitive
rays. These are absent in the segments of the cylinder which

Fic. 267.—Transverse section of woody cylin-
der of Leea. Explanation in the text.

THE HERBACEOUS DICOTYLEDONS 391

lie on either side of the trace. The leaf trace thus illustrates in
the genus Vitis the conservative character which has been asserted
for it on earlier pages. The most interesting feature presented
by Fig. 270a is, however, the fact that the leaf trace is subtended
externally by a broad mass of parenchyma which on its flanks
passes inwardly into the broad rays, separating the foliar segment
from its neighbors on either hand. The situation in a general

Fic. 268.—a, transverse section of leaf ray in Leea; b, vertical section of leaf ray
in Leea.

way, in fact, duplicates that found in the case of Leea, except that
primitive rays are confined to the traces proper in Vitis and the
broad ray subtending the trace is very much shortened in its
radial dimension. Fig. 270b shows the same trace in a lower
section or, in other words, considerably below the node. Here
the broad ray facing or confronting the leaf trace has disappeared,
having been gradually replaced by typical woody tissues, con-
sisting of septate fibers and vessels. As a consequence of this
situation the flanking rays are now separately continuous to the
outside of the cylinder and are not united by a broad tangential

392 THE ANATOMY OF WOODY PLANTS

mass of storage parenchyma, as is the case in the region shown
in Fig. 270a. Vertical tangential sections make the situation
more apparent. Fig. 271a@ shows a plane of section near the surface
of the cylinder. The leaf trace (seen as a dark mass) is completely
surrounded by storage parenchyma, which, however, is less well
developed below than on the upper side of the strand. It is
particularly obvious in this plane that the trace runs in a mass of
storage parenchyma
precisely as in Leea,
described above. The
tangential sections
taken nearer the cen-
ter of the*cylinder
present a very differ-
ent appearance. In
this region the trace
will still be pursuing
“its vertical course in
the stem and as a
consequence will be
flanked by storage
Fic. 269.—Transverse section of the node of the parenchyma laterally.
Concord grape. Explanation in the text. Above the trace lies
the parenchyma of

the foliar gap. This statement may be verified by reference to
Fig. 271. The trace distinctly contains numerous primitive rays,
conspicuously absent in the segments on either side, the segment
on the right showing the presence of some vasicentric paren-
chyma, such as is characteristic of the Vitaceae in general. It
will now be convenient to consider the topography of the foliar
trace in its relation to the woody cylinder of the stem in a
slender upper node of the vine. Fig. 272 reproduces the general
relations exhibited by such a thin axis. Obviously, as the cyl-
inder of the wood becomes more attenuated, the trace of the leaf
will increase in size relatively to surrounding parts. This situa-
tion is clearly shown in the illustration, which represents a plane
of section immediately below the node and corresponding in

THE HERBACEOUS DICOTYLEDONS 393

level to that in Fig. 270a. It is thus evident that, if the woody
cylinder is thin enough, the leaf trace will correspond to it in
thickness, and that as a result the subtending or confronting
parenchyma, so prominent a feature of the topography of the leaf
trace in the cylinder of Leea, Casuarina, the oak, and other woody
stems with compound rays, is plainly absent. The comparison
of very slender and thicker annual stems of the vine with those

Fic. 270.—a, transverse section of foliar ray in Vzizs in the region of the node
b, transverse section of foliar ray below the node.

of the shrubby Leea makes it clear that an extreme thinning of
the axis brings with it a condition of organization in which the
storage parenchyma no longer surrounds the foliar trace, but
merely flanks it on either hand. In climbing or herbaceous axes,
in which this slenderness has become a fixed feature, some com-
pensation for the loss of subtending or confronting storage tissues
is provided by the great lengthening of the flanking rays, which
often extend, as was pointed out by Strasburger over a quarter
of a century ago, through one or more internodes. It will be
obvious from the facts introduced in the present paragraph that

304 THE ANATOMY OF WOODY PLANTS

woody axes with the compound type of foliar ray, if they become
sufficiently slender, as in stems of herbaceous texture, give rise
to a condition in which the storage parenchyma is entirely lateral
to the leaf traces.

The subject of the organization of rays in typical vines will be
made clearer by diagrammatic representation. Fig. 273 illustrates
the appearance of decorticated stems of Leea and Vitis in three

Fic. 271.—a, tangential section showing leaf trace surrounded by storage paren-
chyma; ), section of leaf trace and ray near the pith. Explanation in the text.

dimensions. In a is seen a total view of the nodal region of Leea.
The traces originate from the axis along a crescentic line and are
shown in face view on the surface of the cylinder and in section
on its cut end. Obviously the leaf strands are imbedded in foliar
rays when viewed from either the superficial or the transverse
aspect. In addition to the foliar rays, which in this case are
both broad and shallow, there are other narrower rays, which are
usually of greater depth. There are also uniseriate rays, diffused
throughout the structure of the wood and presenting a rather
marked contrast to the two sorts of broad rays. In 0 is shown a

THE HERBACEOUS DICOTYLEDONS 395

magnified image of a part of the transverse aspect of the stem,
making clearer the relations of the leaf trace to the large foliar
ray. It is apparent that the ray is similar to that found in oaks
in cooler climates and that it has a like relation to the leaf trace.
In c is shown a diagram of Vitis, illustrating the relations of large
rays and foliar traces. The cylinder in this instance shows only
a single annual ring, and the rays subtending the leaf trace in
transverse section
are consequently less
deep in the radial
direction than those
of Leea. Further,
in the facial aspect
of the cylinder the
leaf rays are seen to
be much elongated
below and separated
by a median process
of wood into pairs of
rays. This situation
is strikingly unlike
that in the shallow
foliar rays of Leea.
Still another con- Fic. 272.—Transverse section of part of a thin axis
trast to the exotic of Vitis. Explanation in the text.

genus is offered by

the absence of uniseriate rays except in the actual leaf trace, an
interesting exemplification of the persistence of primitive characters
in foliar organs. A more highly magnified view (d) of a segment
of the transverse aspect of the cylinder makes clear the presence of
uniseriate rays in the leaf trace and their absence in the adjoining
parts.

We may now pass conveniently to the diagrammatic comparison
of the topographical relations presented by thicker and thinner
annual stems of Vitis. In Fig. 274a is shown a view of a thicker
axis from the surface of attachment of a leaf at the median node.
The upper part of the figure shows a transverse view of the stem

306 THE ANATOMY OF WOODY PLANTS

just below the next superior node. In the latter the foliar traces,
seven in number, are clearly subtended by a greater or less depth
of storage parenchyma, precisely as is the case in the oak. In b
the foliar trace is shown in its radial and tangential aspects. In
the radial view the trace is confronted in its upper region by
storage parenchyma (black), while below it is subtended by ordi-
nary wood. Above the trace lies the storage tissue of the leaf gap.
In the tangential figure the trace appears as a light spot in a mass

ee
(iM

Ss ety I
Si!

|

C d

Fic. 273.—Diagram of decorticated stems of Leea and Vitis in three dimensions.
Explanation in the text.

Poo 0 ON
O06 5 OF

of darkly rendered storage parenchyma. ‘The latter, farther down,
is divided into two by a median mass of unmodified wood. In ¢
is shown a view of a slender stem. In this instance, for variety,
the opposite or tendril side of the stem is exposed to observation.
On account of the shallow diameter of the woody cylinder the
trace is of a radial dimension so great that it equals that of the
cylinder from which it is derived. As a consequence there is no
parenchyma facing the leaf trace, and storage tissue is entirely
flanking in its distribution. This is a natural and necessary
geometrical result of the thinning of the cylinder of wood in the
more slender axes of the vine type. In d are shown the radial
and tangential relations of the foliar traces and the adjacent storage

THE HERBACEOUS DICOTYLEDONS 397

tissues. It is clear in the radial view that there is in thin axes
no parenchyma subtending the leaf traces immediately below the
node. This situation necessitates the tangential aspect shown
below, where the leaf trace appears flanked by storage tissues
on either side. Plainly, therefore, by reason of the slenderness
of the woody cylinder and the great lengthening of the rays related
to the leaf traces, which may be regarded as in some measure a

a b C d

Fic. 274.—Diagram of thicker and thinner axes of Vitis. Explanation in the
LEXIE:

compensation for their small radial dimension, axes of the vine
type approximate very closely those found in herbs.

The discussion of typical herbs may now be advantageously
taken up; and it may here be remarked that, although necessary
brevity makes it impossible to consider more than a few instances
of the origin of the herbaceous stem, a wide investigation of the
situation in many groups of herbaceous dicotyledons has made
it clear that the same general conditions are present in every
instance. Fig. 275 reproduces part of a transverse section through
the upper slender region of the stem of the common stinging

308 THE ANATOMY OF WOODY PLANTS

nettle, chosen on account of its chalazogamous affinities for com-
parison with Casuarina, Betula, Quercus, etc. In the illustration
a leaf trace with its flanking broad rays is shown, as well as the
adjacent segments of the cylinder. No primitive rays are to be
seen in the trace, which is composed of vessels and wood fibers.
In order properly to interpret the slender axis of the nettle, the
stouter and more woody regions of the stem should be examined.
Fig. 276a repro-
duces the appear-
ance of a part of a
section through the
thick stem of the
species under con-
sideration, taken
immediately below
the: Snvodess duke
trace appears as a
highly vascularized
nF radially directed
ee . Piste: :@: mass on the mar-
poe oy, gin of the pith.
; On either side of it

are radial bands of
parenchyma, which

¥
=

Fic. 275.—Transverse section through part of a slen-
der stem of the nettle. Explanation in the text. correspond to
those shown lateral

to the leaf traces in Fig. 275. In addition to flanking storage
tissue the woody region of the stem of Urtica shows a very mas-
sive band of radial storage tissue confronting the leaf trace, com-
parable to the similar structures found in the case of the oak
and Leea. The banded appearance of the broad foliar ray in
this instance is due to the presence of alternating stripes of true
parenchyma and substitute fibers. A region of the axis farther
below the node may now be considered. Fig. 2765 shows a part
of a transverse section of the stem some distance below the
node. The mass of confronting storage tissue in the region of
the node at the lower level has become transformed centrally

THE HERBACEOUS DICOTYLEDONS 309

into typical wood, precisely as has been shown above to occur
in the vine. A careless study of the facts and, above all, a neglect
to examine the conditions present in the region of the node, might
lead to the conclusion that the leaf trace in the more primitive
and lower woody portion of the stem in Urtica is flanked but not
confronted by storage parenchyma.

Although the Urticaceae have been chosen to illustrate the
herbaceous condition in a comparatively low order of dicotyledons,

Fic. 276.—a, part of the thick stem of the nettle immediately below the node;
b, section of the same some distance below the node. Explanation in the text.

the Ranunculaceae, as represented by the woody and herbaceous
species of Clematis or the more slender and thicker region of the
same woody species, yield like results. A group in an approxi-
mately intermediate position systematically is the Rosaceae.
Herbaceous representatives when compared with woody forms
of allied organization yield similar conclusions. For example,
in Rubus we find well-marked compound rays, which both flank
and subtend the foliar traces. In many herbaceous types of the
genus Potentilla broad rays like those of the oak are found in the

400 THE ANATOMY OF WOODY PLANTS

lower regions of the aérial axis, while in the upper parts the con-
fronting parenchyma is eliminated from the cylinder as a result
of its progressive reduction in thickness, precisely as in Vitis
and Urtica.

The Compositae, so generally conceded a very high systematic
position among the dicotyledons, exemplify exactly the same
principles as have been noted above. Fig. 277 represents a low
region of a somewhat
woody axis of Helianthus
hirsutus. The leaves,
opposite as in Urtica,
have three traces each.
The foliar strands corre-
spond to depressed seg-
ments of the stem a
situation which, as has
been noted in an earlier
chapter, 1s commonly
found in the vicinity of
large storage rays and
which results from the

Fic. 277.—Thick stem of Helianthus hirsutus greater amount of vege-
table substance present

in such segments and a consequent retarded rate of growth as com-
pared with the more woody regions of the cylinder. In H. tuberosus
the fluting of the lower part of the stem in the region of the storage
rays related to the leaves becomes extremely marked. As there are
six leaf traces at a given node, it follows that there are six cor-
responding furrows in the internode below. Further, since the
traces of successive internodes alternate, the furrows of necessity
show a similar alternation. The narrow bundles of greater diam-
eter in the figure represent the foliar traces of the next higher
node, and the masses of sclerenchyma subtending the median
region of the phloem of the broadest bundles correspond to the
now fused traces of a still higher node of the axis. Fig. 278
reproduces a high and herbaceous region of the aérial axis of
H. hirsutus. The figure is equivalent in nodal relations to the fore-

THE HERBACEOUS DICOTYLEDONS 401

going, but the topography is strikingly different. Here the foliar
traces, instead of merely standing flush with the surface of the

cylinder, as in the
upper region of the
more woody herbace-
ous types, are actu-
ally outstanding, or
salient. As a conse-
quence of this situa-
tion the leaf traces no
longer correspond to
depressions of the
stem, but actually
underlie ridges on its
surface. This condi-
tion is, in fact, highly
characteristic of ex-
treme herbs, in which

Fic. 278.—Slender

upper region of the stem of

the leaf trace has become the dominant factor in the organization

Fic. 279.—Section showing the leaf trace in the
herbaceous region of Helianthus.

of the cylinder of the
axis. This ana-
tomical situation
corresponds to a
very high degree of
assimilative effi-
ciency on the part of
foliar organs, result-
ine ion ae lado
amount of food
storage or seed pro-
duction, as the case
may be.

In: Fig. 27oas
shown the trace of
the herbaceous re-
gion more highly
magnified. The

402 THE ANATOMY OF WOODY PLANTS

foliar strand projects beyond the surface of the cylinder and is
merely flanked and not subtended by storage parenchyma, as in
similar regions in other herbaceous stems of the most varied
affinities. In contrast, in Fig. 280, which is from a lower and
more woody node, a large amount of confronting storage tissue is
seen, as well as that present on the flanks of the trace.

The topographi-
cal conditions in the
stem of Helianthus,
as representing a
high and typical
herbaceous condi-
tion, may now
advantageously be
depicted in stereo-
diagram. Fig. 2814
reproduces the
lower portion of
the stem of Helian-
thus. The scar of
the leaf of one node

Of TS BGL

¢.

<<
Pe.
-

af faces the observer,
Fic. 280.—Section of trace in thick woody region of while the region

Helianthus. 1
é wus just below the next

higher node is shown in transverse section. The six traces of
the two opposite leaves are clearly seen, and it can be readily
observed that they are both flanked and faced by storage tissue
(black). The very narrow deep bundles in the cross-section
represent leaf traces of the next higher node, while the light masses
on the periphery of the broad remaining strands indicate the
position of the fused foliar traces of a still higher node. In 6
is shown (above) a radial and (below) a tangential view of the
topographical relations of the leaf traces in these planes. In
the upper figure of 6 there is obviously much storage tissue con-
fronting the trace. In the lower item the trace in tangential
section is seen entirely surrounded by storage tissue. Fig. 281¢
is a picture of the solid relations of a higher part of the stem in

THE HERBACEOUS DICOTYLEDONS 403

the sunflower. In this region the foliar traces, instead of being
depressed below the level of the woody cylinder, are outstanding,
a condition very commonly found in extreme herbs, which have
largely lost their woody texture. In the case of the vine the radial
diameter of the trace in the slender region of the stem merely
equals that of the cylinder, while in strongly herbaceous types
the leaf trace is outstanding or salient. In the high part of the
sunflower stem the trace is only flanked by storage tissue in contrast

Fic. 281.—a, diagram of lower region of stem of Helianthus hirsutus; 6, radial
and tangential view of the topographical relations of leaf traces in a; c, diagram of the
upper region of the stem in the sunflower; d, topography of the trace in radial and
tangential aspects. Full explanation in the text.

to the both flanking and facing topography of the storage paren-
chyma in the lower and more woody part of the axis. It will be
noted that in the high portion of the axis the foliar traces cor-
respond to elevations or ridges on the surface of the stem, while
in the regions lower down they subtend longitudinal furrows. In
Fig. 281d the topography of the foliar trace is represented in
radial and tangential aspect. In the upper figure the absence
of subtending parenchyma can be clearly discerned, while in the
lower one the flanking disposition of the storage tissues is apparent.

404 THE ANATOMY OF WOODY PLANTS

It will be advantageous at this stage to compare the woody
region of the herbaceous stem of Helianthus with the axis of Ca-
suarina. Fig. 282a represents diagrammatically a Casuarina with
compound foliar rays. The best developed of the rays are di-
rectly related to leaf traces, while the intervening less pronounced
ones belong to the foliar strands of another node. In order that
the vertical relations of the foliar bundles to storage rays may

a

Fic. 282.—Stems of Casuarina and Helianihus. Explanation in the text

be seen, the bark of the stem is removed on the side facing the
reader. The distant aspect still retains its bark as well as its
leaf bases, which are represented as pyramidal elevations on the
surface. In Fig. 282b is seen a corresponding diagram of the
lower region of a stem in Helianthus. The topographical condi-
tions are virtually the same, but for the fact that there are three
leaf traces to each leaf in the sunflower, instead of a single one, as
in Casuarina.

In the oak the longitudinal depressions of the woody cylinder
do not correspond to rays belonging to a single leaf trace, but

THE HERBACEOUS DICOTYLEDONS 405

to pairs of these related to the foliar strands of diverse leaves at
different nodes. This phenomenon is rather rare in the herbaceous
type, but is occasionally found in the genera Aster and Solidago,
among the Compositae. Fig. 283 represents an axis of this type
in Aster multiflorus. There are five depressed segments correspond-
ing to five pairs of foliar traces, to be seen in the woody cylinder.
Fig. 284a is a diagram of the topography of the axis in the oak.
The bark is represented as removed on the side toward the reader
and as still present on the
opposite side. To the
right on the distant
aspect of the stem is
shown a leaf base. Into
this run three traces.
Since the median one
passes out at a different
level from the other two
and exercises no impor-
tant influence on the
topography of the stem,
it is represented bya
dotted outline. It is to .
be noted that the lateral Fic. 283.—Transverse section of the stem of
a ree Aster multiflorus.

traces take their origin

from the foliar rays which are most remote from the median trace.
On the left of the posterior aspect of the axis is shown by a dotted
outline a leaf base of a succeeding node. ‘The traces of this leaf
are plotted in by broken outlines, so as to indicate their relation
to the foliar rays of the stem. It is clear from the illustration
that the depressed segment lying uppermost in the diagram is
flanked by two foliar rays which belong to the lateral traces of
two successive leaves. The depressed segment of the woody
cylinder in this instance owes its position to the retarding effect
on growth of the two closely approximated foliar rays which bound
it on either side. In the case of stems in which the large rays are
equidistant there are no depressions, showing clearly that the
depressed segment is the result of growth mechanics. This is

406 THE ANATOMY OF WOODY PLANTS

clearly exemplified in certain species of Clematis. In the numerous
species in which the large rays are approximated in pairs there are
depressed segments in the stem; but in Clematis paniculata, in
which the large compound rays are equidistant, the segments of
the woody cylinder are all on a level and there are no depressed
regions. It is not possible to follow this subject further in the
present connection, but it is enough to point out that the facts

ne 2
ite

A 0
\"

SINS FO

nt, RE Ae

ES

a b

Fic. 284.—Diagram of the topography of the cylinder in Quercus and Aster.
Explanation in the text.

entirely invalidate the interpretation of depressed segments ad-
vanced by Sanio, Sachs, and De Bary and now classic in botanical
textbooks. The view of these authors is that the depressed seg-
ments, where they occur, owe their topography to a later forma-
tion than the rest of the cylinder. This is distinctly not the case.
The classic view just mentioned further involves the equally
erroneous hypothesis that the continuous cylinder of woody forms
has been derived from the discontinuous one of herbaceous types,
a conclusion which, on the evidence furnished in the present and
previous chapters, is exactly opposite to the true situation. In

THE HERBACEOUS DICOTYLEDONS 407

Fig. 284) is shown a diagrammatic view of the stem in the genus
Aster. Obviously exactly the same general conditions are found
as in the axis of the oak.

The result of all the evidence considered in the previous para-
graphs of the present chapter is to show that the herbaceous type
of stem is distinctly derived from the woody one in the dicotyledons.
Its earlier expression is in the rather thick stem of herbaceous
texture, in which the storage parenchyma especially related to
the foliar traces both subtends and flanks the fibrovascular bundles
of the leaf in that part of their course which lies within the cylinder
of the stem. With subsequent thinning of the cylinder, clearly
connected with greater efficiency in the production of seed and
coupled with a more strictly annual duration, the subtending
parenchyma is wiped out, and only the flanking storage tissues
continue to exist. Obviously a very high potency in the produc-
tion of seed, correlated with high assimilative power, ultimately
makes for a type of annual stem in which storage is effected mainly
in the seeds and no longer in the axis itself. Under these circum-
stances we have the herbaceous type reaching its extreme sim-
plification. The situation just pictured involves a final marked
reduction in the organization of the fibrovascular strands which
is expressed in its completest form in the bundle system of the
axes of the monocotyledons. This group, however, will be con-
sidered in the following chapter.

The result of the appearance of the herbaceous type in the
angiosperms has been momentous for the development of higher
organisms. It is a fact of very obvious significance that the
highest vertebrates and the highest seed plants have had a nearly
contemporaneous existence. The warm-blooded mammal is in
reality rendered possible by the appearance of the herbaceous
type in the angiosperms, which directly or indirectly supply the
most important part of the food of all the higher animals. It
should further be emphasized in the present connection that the
dicotyledonous herbs are very different indeed in their mode of
origin from those exemplified by lower types and in particular
by the vascular cryptogams. Here the herbaceous condition is
the result of mere degeneracy in the tissues of the xylem. In

408 THE ANATOMY OF WOODY PLANTS

the herbaceous dicotyledons, on the contrary, we find the origin
of herbaceous texture closely associated with special modifications
in the organization of the secondary cylinder, and this involves
a high degree of specialization on the part of the plants in which
it is manifested. The local transformation of the tissues of the
woody cylinder, in relation to improved storage in proximity to
the leaf trace, had its early expression in the phenomenon of
aggregation of rays in proximity to this in its course through the
woody cylinder. The phenomenon of aggregation is succeeded
in turn by that of compounding, the direct consequence of the
facility with which the longitudinal elements of the xylem become
transformed into storage elements in the case of the dicotyledons.
The final result achieved in the compound ray is the wedding of
longitudinal and radial parenchyma in the complex, which becomes
of such marked significance in the herbaceous dicotyledons.

CHAPTER XXTX

THE MONOCOTYLEDONS

This group of plants, although only about one-fifth as numer-
ous as the dicotyledons, is nevertheless of great importance on
account of its significance in supplying extremely valuable food
plants and also by reason of its remarkable anatomical organiza-
tion, which has been the cause of much speculation and dispute.
In this large group of the angiosperms we have exemplified a
practically complete absence of secondary growth coupled with
a complicated arrangement of the numerous though slender
fibrovascular strands. The absence of secondary growth has led
to the association of the monocotyledonous angiosperms with
the ferns and their allies. This view of their affinities has been
reinforced at various times by the discovery of leaves and even
inflorescences in Paleozoic strata which have been referred to
monocotyledonous affinities. The foliar and reproductive parts
from Paleozoic deposits have, however, in more recent times
been clearly recognized as the parts either of cryptogamous or
of gymnospermous types not related even remotely to the angio-
sperms. There is accordingly no reason based either on the
possession of cryptogamic characters or on very ancient occurrence
as fossils which justifies the view that the monocotyledons are
the more primitive group of the angiosperms. The evidence,
in fact, when considered in the light of the general principles of
comparative anatomy, points in quite the opposite direction and
seems to indicate that the group under consideration is derived
from herbaceous representatives of the dicotyledons.

A characteristic feature of organization of the axis in the
monocotyledons is the scattered distribution of the fibrovascular
bundles. These, instead of being arranged in the circular fashion
which usually distinguishes the structure of the herbaceous stem
in the dicotyledons, are disposed through the transverse section
of the organ. An examination of the seedlings and reproductive

409

410 THE ANATOMY OF WOODY PLANTS

axes in the group supplies convincing evidence that the peculiar
arrangement of the fibrovascular bundles in the monocotyledons
is not a primitive one. The original manner of distribution of
the conducting strands of the stem was in all probability that
found in the dicotyledons, characterized by a prevailing circular
arrangement of the bundles.

The root in monocotyledons has the usual radial organization,
and is distinguished from that of the mass of dicotyledons by the
absence of secondary growth. Another feature which is often
present in monocotyledonous roots is the origin of lateral roots,
not opposite the groups of protoxylem, as is the general situation
in the roots of the remaining vascular plants, but in the interval
between two protoxylem clusters. This peculiarity has gained
for such roots the not very appropriate designation of ‘‘double
roots.” Another feature which has been described in the roots
of the group under consideration is found in the abnormal order
of development of the elements of the xylem. In vascular plants
in general the protoxylem occupying the outside of the xylem
star of the root is differentiated first and the successively more
central elements in later order. In a number of monocotyledonous
roots a remarkable exception to this well-nigh universal seriation
of development has been observed, for the more central elements
belonging to the metaxylem are differentiated first and the tra-
cheids of the protoxylem are the last to manifest the sculptural
features of maturity.

The leaf in the monocotyledons is cracasterOed in general by
the closed disposition of the nerves or fibrovascular strands. These
usually come together at the tip of the foliar organ and sometimes
in this region are in relation to rifts or pores in the epidermis,
which allow fluid water to escape in the form of drops of dew. ‘In
certain tropical plants with large leaves—such, for example, as
the Agave—the loss of water during the night through the rifts
in the tips of the leaves where the foliar bundles converge is so
great that a constant dripping is heard, often so pronounced as
to disturb slumber. As a consequence of the closed disposition
of the fibrovascular bundles in monocotyledonous leaves, only
the longitudinal veins are, as a rule, well developed, and the

THE MONOCOTYLEDONS 411

lateral ones are weak and degenerate. The fibrovascular bundles
of the leaf in monocotyledons are ordinarily very numerous and
_ consequently enter the stem in large numbers at the nodes. The
large number of foliar bundles passing from the base of the mono-
cotyledonous leaf into the stem is correlated with a high degree
of assimilative efficiency which finds expression in a proportion
of seed production which has scarcely ever been reached in her-
baceous dicotyledons. In many of the cereals, for example, the
relative weight of the seed to
that of the whole plant very
frequently reaches over 30 per
cent. The high efficiency of the
group, both from the standpoint
of production of assimilates and
from that of the formation of
seeds, naturally puts it in a
unique position in supplying
important food plants.

In many cases, particularly
in the grasses and sedges, cam-
bial activity, absent in the stem Fic. 285.—Transverse section of a

5 : bundle of Avena barbata, showing cambial
and root, is often retained in actinty iter Chaalen!
the basal or sheath region of the
leaf or sometimes in relation to the node in the stem. ‘The capacity
which grasses manifest for erecting their stems after “lodging” is to
some extent the result of the presence of a persistent cambium in
the nodal region, either in the base of the leaf or in relation ‘to the
stem itself. Fig. 285 illustrates such cambial activity in the case

of Avena barbata. It is permissible to view this cambial activity
as a persistence of an ancestral character, particularly as it is often
found to be present in monocotyledonous seedlings in the lower
region of the epicotyl or primitive stem.

The organization of the closed fibrovascular bundles of the
monocotyledons is in many cases collateral and, as the descriptive
term implies, exhibits no indication of cambial activity. The
collateral type of fibrovascular strand is characteristic of the leaf,
since that organ, here as elsewhere, is conservative in its structure.

412 THE ANATOMY OF WOODY PLANTS

In the stem, however, particularly in the subterranean axis, the
collateral type gives place to a concentric condition in which
phloem is completely surrounded by xylem. This modification
is known as amphivasal, to distinguish it from the amphicribral
concentric strands of the Filicales and certain lower gymnosperms.
The concentric strands of the monocotyledons present themselves
in a very interesting fashion in the grasses and sedges. Here, in
the reproductive axes, amphivasal bundles are very numerous
in the nodal regions, where the entering of many foliar traces
produces a marked degree of crowding and disturbance. The
amphivasal bundle, in fact, seems to have originated as a con-
sequence of the multiplication of foliar traces in the nodal
regions of monocotyledonous stems. This hypothesis of the ori-
gin of the amphivasal strand is justified by a consideration of
parallel conditions exemplified in the organization of the axis
in certain dicotyledons. In many instances where the foliar
traces are numerous in the nodal region of herbaceous dicotyledons,
these become amphivasal in their structure. This organization,
for example, is frequently seen in the Araliaceae and Umbelliferae.
In the annual stems of the mass of monocotyledons, whether
leafy or scapose, amphivasal bundles are ordinarily absent. When
the anatomical structure of the perennial subterranean axis which
is often found in the monocotyledons is examined, it very generally
presents amphivasal bundles in great abundance and not by any
means confined to the nodal regions, even when—as less rarely
happens—the nodes are not closely approximated. The common
occurrence of amphivasal strands in the rhizomes of monocoty-
ledonous stems is no doubt primarily related to the crowded and
tufted character of the leaves which results in the strong approxi-
mation of the nodes with the consequent multiplication of amphi-
vasal strands.

It is highly probable that the glumaceous representatives of
the monocotyledons represent somewhat primitive conditions in
the stock, for here both reproductive and anatomical data seem
to harmonize in indicating a low systematic position for both
grasses and sedges. It is, moreover, probable that the Juncaceae,
which in anatomical organization agree very closely with the

THE MONOCOTYLEDONS 413

glumaceous monocotyledons, are nearly related to these forms.
A striking contrast is presented in the anatomy of the true palms
and the Scitamineae. In these groups amphivasal strands seem
to be entirely lacking. In the Principes, or true palms, we find a
marked difference in anatomical structure from the western
tropical Cyclanthaceae, which have on floral grounds often been
considered to form a systematic link between the palmlike mono-
cotyledons and the aroids. The anatomical structure of Carlu-
dovica and allied Cyclanthaceae is rather that of the aroids than
of the true palms, since amphivasal strands are conspicuously
present. Our knowledge of the development and comparative
anatomy of the groups which appear to lack amphivasal bundles
is still too meager to warrant any hypothetical conclusions as
to the phylogenetic significance of the apparent lack of amphivasal
fibrovascular strands in the conspicuously large-leaved forms,
which are united systematically under the headings of Principes
and Scitamineae. It may well be that the two large groups
above indicated, by the anatomical peculiarities revealed as a
result of an examination which is as yet only preliminary, occupy
a position high among the monocotyledonous orders.

It will be convenient to elucidate in a general way by means
of diagrams the main anatomical conditions presented by the
monocotyledons. Fig. 286a illustrates the distribution of amphi-
vasal regions in the stem of a sedge. In the reproductive axis
the amphivasal segments are remote and are clearly in the nodal
regions. In proximity to the substratum the nodes become more
approximated, and in the subterranean axis they are frequently
so closely disposed that amphivasal organization is often contin-
uous. This condition is well illustrated by the sedges and rushes
and less distinctly by the grasses, since the last have well-spaced
nodes. The next diagram (b) shows the situation commonest
among the monocotyledons. In this type of anatomical organ-
izations the amphivasal structures are found exclusively in the
perennial subterranean axis and do not appear in the annual
stem. In the last type (c), which portrays the conditions apparently
characteristic of the true palms and the Scitamineae: (bananas,
cannas, ginger, etc.), amphivasal regions are absent both in the

THE ANATOMY OF WOODY PLANTS

414

subterranean and in the aérial stem. Only continued investiga-

1 situation

1ca

ficance of the anatomi

igni

ll show the real s
exemplified in these cohorts or orders.

ion wi

t

286.—Diagrams to illustrate the distribution of amphivasal bundles in the

Fic.
monocotyledons.

The general anatomical configuration of the monocotyledons

the pre

sent condition of our knowledge warrants the conclusion

is impor

mM

tant class or division of the angiosperms formerly

that th

possessed bundles arranged in a circular fashion and characterized
by secondary growth. This condition is indicated by the study
of conservative organs and regions and seems to justify the inference

THE MONOCOTYLEDONS 415

that the monocotyledonous angiosperms have been derived from
a dicotyledonous ancestry. The justice of this hypothesis, how-
ever, will be finally established only when we shall have at our
disposal for anatomical investigation remains of monocotyledons
from Mesozoic deposits. Whether or not it is ultimately estab-
lished that the large group at present under discussion has actually
been derived from the dicotyledons, it will doubtless in any case
be clear that they cannot in any way be regarded as primitive
representatives of the angiosperms. The monocotyledons in
fact represent the herbaceous type in its extremest form. In
the group the fibrovascular tissues are released from the rigid
confinement of the tubular stele in the ancestral forms with sec-
ondary growth by the development of large parenchymatous
storage devices in relation to the foliar traces. Correlated with
this release is the possibility of accommodation of the numerous
foliar strands which characterize the basal regions of monocotyledo-
nous leaves throughout the transverse section of the stem. There
is doubtless some correlation also between the extreme multipli-
cation and consequent displacement of the strands in the stem
of monocotyledons and the disappearance of secondary growth.
Possibly an aquatic or amphibious habitat long maintained may
likewise have acted as a contributory cause in bringing about the
obliteration of cambial activity, since in the dicotyledonous
Nymphaceae, in which there is diffuse distribution of the bundles
and also the absence of cambial activity, we find these features
correlated with an aquatic habitat. It has, indeed, often been
suggested that the Nymphaceae or Ranunculaceae are the dicoty-
ledonous ancestors of the monocotyledons. The interesting inves-
tigations of Sargent on the fusion of the cotyledonary structures
in Ranunculus ficaria, etc., are of importance in indicating how
monocotyledony may have arisen as a result of the union of two
originally separate seed leaves. Another possibility, of course, is
the origin of the monocotyledonous embryo as a consequence of
the abortion of one of the two original cotyledons, and this view
is perhaps supported by the conditions found in certain grasses,
such as Zizania, Avena, etc., in which the vestige of a second
cotyledon is considered to be present. The problem of the origin

416 THE ANATOMY OF WOODY PLANTS

of the monocotyledons cannot yet be regarded as by any means
settled, and a much fuller knowledge of the anatomy of extinct
and living representatives of this extremely important group is
necessary before any final results can be reached. If we attempt
to picture to ourselves the probable future course of evolution in
the angiosperms, it is difficult to concede to the monocotyledons
a prominent position. This group seems to have reached such an
extreme degree of specialization that it will probably in the long
run entirely disappear and be replaced by new derivatives of the
still plastic dicotyledons.

CHAPTER XXX
ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION —

General views in regard to the ancient climatic conditions
must first occupy attention in the present connection. It is
commonly conceded, on the basis both of the nature of ancient
organisms and of the evidence supplied by geologic strata, that
the earth was formerly much warmer than it is in the present
epoch. It is further clear that, other things being equal, the
greatest degree of warmth existed in the most remote past. This
general situation, however, does not exclude the recurrence at
long intervals of periods of refrigeration or glaciation. An age of
ice is known to have occurred, not only at the end of the Cenozoic
as originally established by Agassiz, but glacial periods also termi-
nated both the Mesozoic and the Paleozoic as ordinarily defined.
Evidence of still earlier glacial epochs which exercised a devastating
influence on the most ancient animal and plant populations of
our earth is not lacking.

Evidence in regard to glaciation in former epochs is both direct
and indirect. Direct testimony concerning former ages of ice
is supplied by the comparative study of deposits formed in con-
nection with the existing glaciers of high latitudes or of high
altitudes. Intimately connected with glacial phenomena are the
formations of clays, till, and coarse morainal matter resulting
from the movement and melting of ice. Indications of the kinds
just enumerated in earlier geological strata supply direct testimony
as to former glacial action. Indirect information in the same
direction is often furnished by the wholesale extinction of impor-
tant groups of plants and animals. For example, in the Permian
glaciation which marked the close of the Paleozoic the treelike
cryptogams, which have contributed so largely to the formation
of the older deposits of combustible minerals, disappeared entirely
as an important constituent of the plant population of our earth.
Glacial epochs are, however, not of direct importance in relation

417

418 THE ANATOMY OF WOODY PLANTS

to the evolution of plants in response to climatic influences, since
their action is mainly negative. It is true, however, that by
bringing about the obliteration of important groups of plants or
animals greater opportunity is supplied for the surviving and more
adaptable forms to develop in the following warmer epochs.

It is the more gradual and not the spasmodic refrigeration
which has produced perhaps the greatest effect on the organization
of the successive plant populations of the earth. As a preliminary
to the discussion of the anatomical modifications which are more
or less definitely correlated with climatic changes in the successive
geological ages, the evidence furnished by plants in regard to
progressive climatic cooling must be considered. This evidence
is of two kinds. Perhaps the most important and trustworthy
is derived from the organization of the secondary woods in trees
of the various geological periods. This testimony can hardly
be estimated too highly in arriving at any conclusions in regard
to plants as reliable indicators of climatic change. Another
kind of evidence is afforded by the character of the plants them-
selves. At the present time there are large groups of plants which
are of more or less definite tropical occurrence and others which
prevail characteristically in cooler regions. The advance of
tropical types toward the poles or the progress of polar plants
in the direction of the equator in earlier geological eras must,
other things being equal, indicate variations in climatic conditions
in the direction either of greater or of less warmth, as the case
may be. Unfortunately, in earlier geologic times the differen-
tiation between polar and equatorial types was not nearly so
marked as it is at present, and this general situation militates
more or less strongly against reliable conclusions in regard to
climate in all but the latest geological eras. Since the present
work deals with anatomy, the subject of plant geography in rela-
tion to the climatic changes which have marked the successive
ages of the earth is obviously of less importance, aside from the
limitations indicated above.

As has been already shown, the organization of the secondary
wood in extinct plants furnishes the most reliable evidence as
to the climatic conditions which prevailed in earlier geological

ise present in

kewi

hi

1S

t

1

but

The information derived from this source is not only

ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 419

on the whole the most trustworthy,

epochs.

eoue
eeree

oe
we

*
rei

227d ROK
enerey

&
Ly

m
e

AN)

LEAS)

Bade

O80see

Cope ee ee

au

Aveciaccens
‘ere

SVC aveene’
Veale ceedamnaes:

a

Sees
Jéeee bus

name ve
SESS

ay igo Or ay a ti aia. (sla
sof SS Preah Similan
SSG oy, ea eS) o)
q a oan
(0 seed a fs ee o n N
=>} cS 3 tol 8) iS) m oN q ie)
Ceieree AOn re ree Lo
o oO bp O Lal Ss oo aH
= dt (oF 11) | n fae)
Cc oO 5 wn aw
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Bee seas 2
DH EG) fas} = OQ, or)

of
which

indicate any perio-
dicity in annual con-

Pet

erty

eer ee
ee.

rei
enere

f the wood of Cor-

Sant seeette

woeteos

modifications
StELUIC TUNE

tS)

ditions of growth.
The truth of this
statement is well
illustrated by the ac-

10n oO

G. 287.—Transverse sect

FT
ites from Prince Edward Island.

the
no da

organization of the
eran!

i toma

Southern
re

d by the
ica
of
a

oO

o
formity

eolo
unl

trunks which are

supplie
wood shows great

tions
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there

o
5

Mes ar colina ace a
= my OS to S
i ee ie 72 AS Cm = ao)
aosks aera’
eee Sr one
; Lael fs)
ee 8 SE) So
Se Oa O ele
ma Ea ™ si I o
S OS SG © ea O 6 SS
Bie, ea eae
ee er = Ce aen a Go
Ge eg cons g
Or ors ao ae) Gu ©

aS
OS

Fic. 288.—Transverse section of the wood of Cor-

daites from the north of England.

420 THE ANATOMY OF WOODY PLANTS

of periodic growth is, however, frequently found in regions of higher
latitude than that portrayed in Fig. 288. The next illustration
shows the organization of a Carboniferous cordaitean wood (Mesoxy-
Jon) from the northern part of England and consequently of consider-

= = IL.
BZ Cee See

= ‘Cy Zp-Ce, eT Ze See
2A | Cl Sey “ip CJ iaZ
epee meee San

Ze

EES SSSSS-S——————

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xe, BE
GED ; J

eo) ‘ , So
Zz $ 2 zs : LF
ZZ (_3@ 2
SSE OE OO ee «
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a ee a ae ae Zz Fy = |
sy, Sa So Ze > EB 3) “|
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= Gee wu = = —<eowW |

"
|

i : me

Fic. 289.—Details of wood structure of Mesoxylon (Cordaites) with annual rings.
Tangential pitting is absent.

ably higher latitude (54° N. in contrast to the 46° N., the latitude of
Prince Edward Island). The annual rings in the wood from the
English Carboniferous are clearly marked. The next illustration
(Fig. 289) reproduces the minute organization of the same wood

ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 421

under a sufficient degree of magnification to show the essential fea-
tures of structure. The tracheids are so long that only a small
portion of them can be shown in the figure. Their walls are
covered with crowded pits arranged in an alternating fashion with
no intervening bars of Sanio. The only parenchyma present is
radial, and the end of the annual ring is marked by the presence
of tracheids which are slightly narrower in radial dimensions but
do not otherwise
differ from those
found in the rest of
the annual ring.
For comparison
with the situation
revealed by the
cordaitean wood
from Northern
England, a trunk
from the Triassic
of the southwest
Ree1on..ol/-phe
United States is
shown. In Fig. 290
appears a trans-
verse section of the
wood of a tree from
the Triassic forest
of Arizona. The annual rings are not so distinct in the photo-
micrograph as they appear on the weathered end of the actual
petrified specimen. It will be clear from the information sup-
plied in this case that as far south as Arizona in the Triassic
annual rings were more or less clearly marked. A noteworthy
variation in the annual temperature in that somewhat remote
epoch is thus indicated. This situation presents an interesting
contrast to the climatic conditions which prevailed in the region
of Prince Edward Island toward the end of the Paleozoic. If the
situation be summarized, it is clear that in the later Paleozoic
the difference between 46° N. and 54° N. means the presence in the

Fic. 290.—Transverse section of a coniferous wood
from the Trias of Arizona.

422 THE ANATOMY OF WOODY PLANTS

higher latitude of annual rings and their absence in the lower one.
On the other hand, in the beginning of the Mesozoic (the Triassic)
even at a distance of to degrees south of the latitude of Prince
Edward Island annual rings were quite clearly developed.

Without considering the Jurassic we may pass at once to
the end of the Mesozoic period, namely, the Upper Cretaceous.
Fig. 291 reproduces the organization of the wood of a Cretaceous
araucarian conifer
closely allied to, if
not identical with,
thes living kaurs
(Agathis). A per-
sistent leaf trace
appears running
through the figure
transversely. The
wood is considerably
over half a century
(as recorded in
annual rings) from
the center of the
stem, and the foliar
strand still persists
in ‘the manner
characteristic of re-
lated living arau-
carian genera. The yearly increments of growth are distinctly
indicated, although not very strongly marked. For comparison
with the above illustration a photograph of the wood of Agathis
australis from New Zealand is shown in Fig. 292. Here annual
rings are clearly apparent, although the latitude in the Southern
Hemisphere is nearly the same as the place of origin of the
Cretaceous araucarian wood above, namely, 40 degrees. It is
thus apparent that the Cretaceous of Staten Island was marked
by a much less distinct annual variation of temperature than
is the South Island of New Zealand of today, although the two
localities are of almost identical latitude. Without pursuing the

Fic. 291.—Transverse section of an Araucarioxylon
from the Cretaceous of Staten Island.

ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 423

subject further we may point out that the organization of the
trunks of trees in succeeding geological epochs indicates a gradual
cooling of terrestrial climates which, when it became accentuated,
gave rise to a marked variation in annual temperature, recorded
ever more clearly in the progressively greater definition of the
annual rings in later geological times.

It is now necessary to correlate the climatic changes more in
detail with the
internal organiza-
tion of the plastic
stem organs of
vascular plants.
It has been shown
in Fig. 289, repre-
senting the wood
of the cordaitean
Mesoxylon of Car-
boniferous age
from the north of
England, that the
only differentiation
which marked
annual increments
of growth in this Fic. 292.—Transverse section of the wood of A gathis
early age was a _ australis from New Zealand.
slight narrowing
in radial diameter of the tracheids formed toward the end of the
period of growth. No other structural feature related to the
phenomenon of annual rings has been recorded as yet from either
the late Paleozoic or from the early Mesozoic (Triassic). In the
Jurassic, however, and practically universally from this epoch
onward, a marked modification in the organization of the annual
rings presents itself. This is well shown in the structure of the
wood of Ginkgo (Fig. 293), a survivor of a stock which attained its
zenith of development in the Mesozoic age. Clearly the tracheids
which mark the termination of the annual increment differ from
those previously formed in the character of their pitting. The

424 THE ANATOMY OF WOODY PLANTS

pores of the tracheary elements in the first-formed region of the
annual ring are exclusively radial. The terminal tracheids in con-
trast are distinguished by the presence of tangential pitting. Itis
in the gymnosperms of the middle and later Mesozoic that this

Al) ote

S PC ene

(h7—) Sel

Se es

ok |
pee
foe aL |
Bak eel
ee Cla

ZA Ace PLE
A BSE EB E Zp 38 GL
CO O Z LEX

SS
=

Ww

SS

S

LY

SS

UN

NC
we
mon
NAY

———

@
(

SSS

SOF

CN CS

WS (

Fic. 293.—Details of organization of wood of Ginkgo. Tangential pitting
appears at the end of the annual rings.

feature is first clearly expressed. It has been suggested by Stras-
burger that tangential pitting is a device for supplying water
rapidly and abundantly in the new period of growth to the reawak-
ening cambium. ‘This seems to be a highly probable explanation
of the anatomical conditions present and may be accepted in the

ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 425

absence of any evidence against its validity. The gymnosperms
of the Paleozoic had just begun to develop annual rings toward the
end of the epoch, and the phenomenon of tangential pitting had not
yet madeits appearance. As has been illustrated above, this feature
did not become marked until the Mesozoic was well advanced.

©©!|©© Ge Og

mE
ae .
ss
oo
(STS)

(OROMOMOMOMLY

Fic. 294.—Details of organization of the wood in Picea canadensis

The situation in gymnospermous woods did not, however,
rest with the appearance of tangential pitting at the end of the
annual ring. Not long after this feature had become well estab-
lished it was accompanied by another even more important ana-
tomical phenomenon. In Fig. 294 is shown a slightly diagrammatic
view of the wood of the root of Picea canadensis in three dimen-
sions. The illustration includes the transition from one annual
ring to the next, and the terminal tracheids manifest the tangential

426 THE ANATOMY OF WOODY PLANTS

pitting referred to above. Not only is the end of the annual
increment marked by tangential pitting, but it is also distinguished
by the presence of longitudinal tangential storage elements known
as wood parenchyma. In the figure these living cells are strictly
confined to the face or termination of the annual ring. They are
further present as derivatives from tracheary structures, for
stages between them and transversely divided tracheids are plainly
seen. Both the mode of occurrence and the manner of origin of
the first parenchymatous elements in coniferous woods are of
equal interest from the climatological and evolutionary standpoints.
The position of the tangential storage elements at the end of the
annual ring corresponds in an apparently highly significant manner
with the appearance of tangential pitting in those woods of later
geological time which have developed the phenomenon of annual
rings. It has been suggested with a strong degree of probability
that tangential pitting in the gymnosperms is an adaptation for
supplying abundance of water to the cambium when it renews its
activity after the annual period of rest. It is extremely likely
that the terminal parenchyma which, so far as our present knowl-
edge goes, first made its appearance in the Jurassic is likewise
a device favoring the activity of the cambium in a new period
of growth. If it is probable that the tangential pitting of the
terminal tracheids is for the purpose of supplying the cambium
with water, it seems not less likely that terminal parenchyma
provides a convenient supply of food for the initial cells of the
cambial layer. The conditions present strongly suggest that the
first appearance of tangential storage elements was in relation to
the nutritive demands of the cambial layer. Whether that con-
clusion is accepted or not, there can be no doubt of the correlation
of the phenomenon of annual rings and the first appearance of
longitudinal parenchyma in the tissues of the wood. This view
of the matter is strongly supported by the conditions observed
in Paleozoic woods, for these are equally characterized by the
absence of annual rings (except of course at the end of the period
and in higher latitudes) and of storage elements belonging to the
category of wood parenchyma.

Although at first terminal in position in gymnospermous woods,
parenchyma did not long remain restricted to this situation. In

ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 427

the Jurassic and Cretaceous, storage elements are already found
distributed throughout the annual ring. Among the living conif-
erous subtribes all but the Abietineae have or formerly possessed
diffuse parenchyma or storage tissue scattered throughout the
annual ring. This condition, for example, actually holds for the
Taxodineae, Cupressineae, and Podocarpineae, and there is good
evidence either from fossil forms or conservative organs or from
both together that the Taxineae and Araucariineae formerly
possessed abundant, diffuse storage parenchyma. In the Abie-
tineae a very interesting condition exists which is quite in
harmony with the ancestral position assigned to them in an earlier
chapter. In the group Pineae parenchymatous elements are
absent in the ancient Pinus and Prepinus; while in Picea, Larix,
and Pseudotsuga they occur in an exclusively terminal position
and present, particularly in Picea (notably in the root of the various
species), all transitionary stages of derivation from septate tra-
cheids. It may be assumed that in the Pineae we have the most
primitive conditions found in the conifers as regards the presence,
position, and mode of origin of parenchymatous elements. In the
Abieteae, comprising Abies, Tsuga, Cedrus, and Pseudolarix, the
longitudinal storage parenchyma is characteristically terminal,
but no longer shows normal evidence of derivation from tracheary
elements. In wood formed after injury, however, transitions
from tracheids to parenchyma cells can readily be observed. The
genera Abies and Tsuga are of particular interest among the
Abieteae on account of the fact that some of their species show
a transition toward the diffuse condition of distribution of wood
parenchyma. In A. webbiana and A. cephalonica storage elements
scattered: throughout the annual ring have been observed, and a
similar statement holds for the mature wood of T. mertensiana
and the branches of T. canadensis. In the remaining subtribes
of conifers, as shown above, the parenchyma is or has been diffuse
in its mode of occurrence. Further, it presents no normal tran-
sitions from tracheary elements, although such conditions can
frequently be observed in injured material.

The conifers have been chosen to illustrate the correlation
between the organization of wood and climatic conditions,
because their great geological age and the abundance of their

428 THE ANATOMY OF WOODY PLANTS

structurally preserved fossil woods make them a particularly
significant document in this connection. In the higher living
gymnosperms and in the dicotyledonous angiosperms the diffuse
distribution of the longitudinal storage elements is that typically
present. It is reasonable to suppose, in the absence of the abun-
dant evidence presented in the case of the Coniferales, that in
the remaining living seed plants wood parenchyma originally
appeared much in the same manner as in the coniferous gymno-
sperms.

It has been noted in a previous chapter that, although in the
mass of conifers tangential pitting is characteristic of the terminal
tracheids of the annual ring alone, in the higher groups of seed
plants it is diffused throughout the annual ring precisely as is
the case with the wood parenchyma. It is thus apparent that
the phenomenon of annual rings correlated to the recurrence of
yearly periods unfavorable to vegetative activity is in turn cor-
related to the appearance of tangential pitting and wood paren-
chyma in the series presented by the more modern seed plants.
It thus becomes evident that climatic conditions, so far as these
two extremely important features of organization of the wood are
concerned, have had a potent influence on the organization of
structure.

The next impetus to the upward evolution is provided by the
appearance of vessels. It is highly improbable that these impor-
tant structural features of the wood in higher plants owe their
origin in any way to climatic conditions. Their appearance,
although a phenomenon of the highest importance from the stand-
point of the doctrine of descent, is not obviously connected with
any known causal conditions. There is no question, however,
that the histological element known as the vessel has come into
existence as a consequence of the modification of tracheids of
the secondary wood. This is the situation in the case of the
Gnetales and is only less distinctly manifested by the lower dicot-
yledons. Although the appearance of vascular elements has,
no relation to climatic conditions, vessels once established as a
feature of ligneous organization have played an important part
in the evolutionary history of the higher vascular plants in rela-

ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 429

tion to conditions of temperature. The secondary wood of the
higher plants is much more efficient in conducting water when
vessels are present. It has been estimated by Pfeffer, for example,
that the woody cylinder of the birch has about twenty times the
water-conducting capacity of that of the pine, and the second-
ary xylem of the Betulaceae is probably less viable to water, on
account of the universal presence of vessels with scalariform per-
forations, than are many dicotyledonous woods of higher organiza-
tion with porously perforated vessels. The greater capacity for
conducting water which characterizes the secondary wood of
seed plants from the Gnetales upward provides, other things being
equal, for a much greater leaf surface. The greater superficial
development of foliar organs naturally results in a much increased
assimilative capacity. The greater accumulation of the products
of photosynthesis directly consequent on increase of foliar surface,
and indirectly on the appearance of vessels as an important feature
of structure in the wood, naturally involves an increase of storage
capacity. Since the secondary wood is the most important storage
tissue of the higher plants, increased accommodation for reserve
materials appropriately originates as a result of modification of
its structure. As has been pointed out in earlier chapters, the
Gnetales present, not only the lowest occurrence of vessels in
secondary wood, but at the same time the first appearance of
the typical aggregate ray. This type of radial storage device
results from the clustering and partial fusion of the original narrow
rays. Aggregate rays were at first not specially related to the
appendages, but in higher types tend to become somewhat definitely
connected with leaves, roots, and other lateral organs. The
aggregate ray passes by further changes into the compound ray,
characterized by a homogeneous organization and no longer includ-
‘ing fibers and other features of longitudinal structure of the wood.
As an alternative to the compound ray which is the result of fusion,
we have the diffuse condition of radial parenchyma resulting from
, the divergence of the original clusters or aggregations of rays in
the outer regions of the woody cylinder. We have accordingly
to do with two derivatives of the aggregate ray: the compound
condition resulting from fusion and the diffuse modification which

430 THE ANATOMY OF WOODY PLANTS

is the consequence of the separation of the original components of
the congeries or aggregation of rays.

The diffuse type of radial organization appears to have been
more in accordance with the needs of arboreal dicotyledons in
later geological times in which climatic refrigeration, annual or
secular, has become more and more marked. It is practically
of universal occurrence in the forest trees of the present epoch.
The compound type of ray is limited to a very few species of trees,
of which the oak is a notable example.

Although large rays of the compound type are usually entirely
absent in arboreal forms, they are extremely common in herbaceous
stems belonging to diverse and not nearly allied families of dicoty-
ledons. The accumulation of large quantities of storage tissues
about the foliar traces, a disadvantageous condition in deciduous
trees, is very frequent in typical herbs and vines of herbaceous
texture. There is an interesting correlation between climatic
conditions and the presence of herbaceous types which is not
only geographical, but likewise geological. Taking the geographical
or climatological conditions first, we know that it is a notable fact
that arboreal forms are much more prevalent within the tropics
than they are in temperate regions. This statement naturally
applies to the higher rather than to the lower families of the
dicotyledons. In the true amentiferous types, such as the Betu-
laceae, Fagaceae, Juglandaceae, etc., there are no herbaceous
representatives at all, although these families are characteristically
distributed in temperate climates. The absence of herbaceous
forms in this instance presents an interesting resemblance to the
conditions found among the Coniferales, which are also without
species of herbaceous texture, even in the extreme polar limits
of their range. The facts in the two parallel cases seem to have
a common and significant explanation, which is that both are
relatively primitive groups. It is in the higher orders, such as
the Compositae, Solanaceae, Leguminosae, etc., that the principle
of the occurrence of herbaceous types in temperate, and of arboreal
forms in tropical, climates is well illustrated. There are, for
example, very few leguminous trees in north-temperate regions,
while herbaceous forms are extremely common. Contrariwise,

ANATOMICAL STRUCTURE AND CLIMATIC EVOLUTION 431

tropical arboreal Leguminosae are common, while herbs belonging
to this family are much less abundant in equatorial latitudes.

The geological conditions so far as they are clearly displayed
seem to correspond closely with those indicated geographically in
the present era. For example, the Paleozoic, which was a period
of average high temperature compared with later geological eras,
was characterized by the prevalence of arboreal cryptogams belong-
ing to the groups whose survivors are practically entirely herba-
ceous. In the cooler but still warm Mesozoic the prevailing type
of seed plant vegetation was gymnospermous and in particular
coniferous, and was consequently arboreal and not herbaceous in
habit. Concerning the proportion of herbs and trees in the
angiospermous vegetation of the later Mesozoic and of the warmer
part of the Tertiary we are unfortunately not well informed. It
is certain from still unpublished investigations that herbaceous
types were present, although not abundant, in the Upper Cretaceous
of the Eastern United States. As the data in this case are sup-
plied by remains preserved from obliteration by charring and
subsequent sweeping into open water, it is not unlikely that they
indicate, approximately at any rate, the proportion of herbaceous
and woody forms in the floras of the later Cretaceous.

The general evidence points conclusively toward the herb as a
product of cooler climatic conditions and of later geological times.
It should not be forgotten in this connection that the appearance
of wood parenchyma, with which in the last analysis the evolution
of herbaceous angiosperms is clearly linked, is definitely related
to the phenomenon of annual rings, in turn the result of climatic
refrigeration in later geological epochs. The impulse toward the
formation of tangential parenchyma was thus obviously supplied
by climatic cooling. Tangential parenchyma united with radial
storage tissues furnishes the explanation of the broad or compound
ray as exemplified by the oak on the one hand and by vines and
herbs on the other. It will be apparent from the conditions
elucidated above that climate has had a paramount influence in
molding the organization of plants and that refrigeration has
always favored the appearance of herbaceous types. A careful
distinction must, however, be made between degenerate herbs

432 THE ANATOMY OF WOODY PLANTS

and dynamic ones. Exemplifications of the former are supplied
by the few surviving vascular cryptogams. The latter are rep-
resented by the huge aggregation of angiospermous herbs, both
dicotyledonous and monocotyledonous, which constitute such a
preponderant and aggressive element in the present plant popula-
tion of our earth. It seems not unlikely that should present
climatic conditions remain for a long period either unchanged or
even accentuated, the dynamic herbaceous type will definitely
supplant the arboreal, at least in temperate regions. If this view
is correct, we must regard the strikingly herbaceous flora of exist-
ing prairies, steppes, and pampas of higher latitudes as a fore-
runner of a condition which will ultimately become universal.
In any case the appearance of the dynamic herb of angiospermous
affinities, which is primarily the result of the differentiation and
not the degeneracy of the woody cylinder, is an evolutionary
phenomenon of the first order.

CHAPTER XXXI |
EVOLUTIONARY PRINCIPLES EXHIBITED BY THE COMPOSITAE

In earlier pages the principles derived from the comparative
anatomical study of existing and extinct plants, particularly
vascular cryptogams and gymnosperms, have been emphasized.
In the present chapter it will be shown that these principles are
as applicable to forms the past of which is unknown as to those
historically recorded. It will be advantageous in this connection
to consider a very high group among the dicotyledons, namely,
the Compositae. This family is commonly divided into two sec-
tions, the Tubuliflorae and the Liguliflorae. The former are
characterized by the fact that the axial florets of the heads are
tubular and not ligulate in their organization. Anatomically
they are distinguished by the presence of oil canals, which usually
occur in the more conservative organs even when they are lacking’
elsewhere. In the Liguliflorae, on the other hand, the axial flowers
of the inflorescence are invariably ligulate. Histologically this
group is characterized by a milky juice which is present in the
pericycle and the phloem. Indeed, it often happens that the
laticiferous ducts are actual sieve tubes or are at least continuous
with elements of this nature. It is an interesting fact that in
the higher representatives of the Compositae, marked by the
presence of a milky or laticiferous secretory system, frequently
oil canals such as occur normally in the Tubuliflorae are present
under the same conditions as the ancestral features surviving in
vascular cryptogams and gymnosperms. This situation is of
great general interest as illustrating the wide validity of the general
principles elucidated in chapter xvii. Since the Compositae are
readily obtainable by reason of their great abundance in the
present flora, either in a wild or in a cultivated state, they may
serve advantageously to illustrate pedagogically the fundamental
conceptions of comparative anatomy.

In Fig. 295 is shown a transverse section of part of the
stem of the Jerusalem artichoke (Helianthus tuberosus) moderately

433

4

4

THE ANATOMY OF WOODY PLANTS

Fic. 296.—Section of part of the root of a species of Aster

7

435

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These are the oil canals of the tubuliflorous section of the

EVOLUTIONARY PRINCIPLES OF THE COMPOSITAE

magnified.

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Fig. 296 illustrates the

occurrence of similar canals in the root

other regions.

Fic. 298.—Laticiferous system of

Scorzonera (after Sachs).

the stem and leaf, but appear never to

In this genus the canals are

of Aster.
Fic. 297.—Transverse section of a small root of the

dandelion.

The phloem of the fibrovascular

h better developed than the xylem, and in it occur

Fig. 297 illustrates the organization of a small root of the

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436 THE ANATOMY OF WOODY PLANTS

belongs. The circular zones of darker elements mentioned above
represent not only the distribution of the laticiferous tissue, but
that of the sieve tubes as well, which cannot be clearly distinguished
from the latter. The subject of the relation between characteristic
elements producing latex and sieve tubes needs further investiga-
tion with the improved methods now in vogue. Former statements
that the development of the milk system is in inverse proportion
to that of the sieve tubes, which have been both denied and
affirmed, may advantageously be controlled by examination of
more perfect sections. In Fig. 298 is seen a magnification of the
laticiferous system of the black salsify (Scorzonera). It is here
obvious that the secretory system with dark contents no longer
shows the identity of its originally separate elements, so com-
pletely has cellular fusion taken place.

A consideration of the subfamily Cynareae of the Tubuliflorae
will next occupy our attention. Here we have to do with a group
which is transitional anatomically from the Tubuliflorae to the
Liguliflorae, since it possesses partially the oil canals of the lower
Compositae and likewise the laticiferous system which is a feature
of organization of the higher members of this important group.
Fig. 299 illustrates a portion of a transverse section of a root of
the so-called French artichoke, Cynara Scolymus. Three oil
canals are to be seen in the upper region of the section. We may
now pass advantageously to a consideration of the common bur-
dock, Arctium minor. In this species the root resembles that of
the artichoke, figured above, in the possession of oil canals. The
stem of the burdock in its lower and first-formed region presents
some interesting features of organization, which are illustrated
in Fig. 300, showing part of a transverse section through the
axis. In the lower region of the figure appear the outer parts of a
number of fibrovascular bundles belonging to the stem. It is
clear that none of these shows the presence of any oil canals.
In the phloem may be seen dark dots, which indicate laticiferous
elements. Above the zone of stem bundles lies a single leaf trace
in the cortex. Along the outer margin of this appears a row of
secretory canals. The nature of these is more clearly seen in
Fig. 301, representing the transverse view of the foliar trace under

EVOLUTIONARY PRINCIPLES OF THE COMPOSITAE 437

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Fic. 2909.—Part of a transverse section o
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Transverse section of the

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438

THE ANATOMY OF WOODY PLANTS

Fic. 302.—Section through the upper aérial stem of Arctium

EVOLUTIONARY PRINCIPLES OF THE COMPOSITAE 439

a higher degree of magnification. External to the fibrovascular
tissues, on the upper side, is a row of somewhat imperfect and
vestigial oil canals. The interesting fact in the present connection
is that the oil canals appear in relation to the leaf trace, although
absent in the ordinary bundles of the stem. Sections lower down
toward the hypocotyledonary region show the secretory canals
somewhat better developed. In no case do they occur in bundles
which belong distinctly to the stem, although they may run a
short distance down on the foliar traces as they pass into the
cylinder of the axis. In the upper regions of the stem a different
situation presents itself. Fig. 302 shows a section through the
aérial stem of the burdock in transverse section. Here neither
the numerous bundles of the axis nor any of the three foliar strands
show the presence of oil canals, although small dark dots in the
region of the phloem sufficiently vouch for the presence of laticifer-
ous elements. Fig. 303 reproduces a magnified view of one of
the leaf traces, demonstrating the entire absence of oil canals
in the foliar strands given off from the upper part of the axis.
If the branches are followed to their tips, where the flowering
burs are produced, anatomical investigation reveals the continued
absence of oil canals.

Similar observations may readily be made on the Scotch thistle,
Onopordon, or on the Canada thistle, Cirsiwm, as well as on a
number of other representatives of the Cynareae. The burdock
is on the whole much more favorable than any of the common
thistles and has accordingly been chosen for illustration. It will
serve a useful purpose to return now to Cynara Scolymus, the
French artichoke. As is well known, the vegetable so designated
is a variety of the common Mediterranean thistle in which
the scales of the involucre surrounding the head have become
hypertrophied at the expense of the floral organs proper. This
overgrown involucral covering is eaten as a prized vegetable in
Southern Europe. Fig. 304 shows a section through the lower
region of the flowering axis of Cynara Scolymus. The bundles on
the lower side of the figure either belong to the axis or have very
recently taken their departure from the region of the pith. Those
yoward the upper side are passing into the basal portion of the

440 THE ANATOMY OF WOODY PLANTS

Fic. 304.—Section through the lower region of the flowering axis of Cynara
Scolymus.

EVOLUTIONARY PRINCIPLES OF THE COMPOSITAE 441

involucral scales. Certain dark dots on the superior margin of
the upper bundles indicate the presence of oil canals. Structures
of this nature are entirely lacking in the axial bundles and in the
traces which have only recently left the cylinder of the stem.
Fig. 305 will serve to demonstrate the truth of the statement
just made. In a is shown a strand near the axial region which
obviously is not characterized by the presence of oil canals. In
6, on the contrary, representing a trace destined to an involucral

Fic. 305.—a, axial strand of infloresence of C. Scolymus; b, a strand of an invo-
lucral scale in the same.

scale, a series of somewhat imperfectly developed oil canals appears
along the upper margin.

The Cynareae represent a transition anatomically from the
Tubuliflorae to the Liguliflorae. In the Cichoriae, on the other
hand, we have to do with very typical representatives of the
Liguliflorae, since in this subfamily all the florets are strap-shaped
and the secretion is milky in its nature and contained in the region
of the phloem. Interestingly enough in the oyster plant (Trago-
pogon) and in the salsifies (Scorzonera and Scolymus) vestiges of

442 THE ANATOMY OF WOODY PLANTS

an oil-secreting system are occasionally found in the root only.
Even in the chicory itself some slight indications of oil canals are
sometimes discovered in the roots, although these are never func-
tional as in the salsifies.

It will be apparent from the various illustrations and state-
ments of the foregoing paragraphs that in the Compositae the
oil canals of the lower forms tend to perpetuate themselves in a
vestigial fashion in certain conservative parts and organs of the
higher types belonging to the Cynareae and the Cichorieae. The
general situation can best be visualized by means of a diagram
which is made in accordance with the data supplied by the French
artichoke, Cynara Scolymus, since this form on the whole reveals
the anatomical situation in the fullest manner. In the center
of Fig. 306 appears a somewhat conventionalized external view
of the species under discussion. The regions which are of critical
importance are indicated by dotted lines, marked by letters from
Ato D. At the sides are shown diagrammatic transverse sections
of the regions indicated by the letters. In A is seen a view of
the root, conforming to the general structure of such organs and
at the same time showing a series of oil canals just external to an
outer circle indicating the endodermis. In B appears a view of
the lower region of the axis showing leaf traces, either present in
the cylinder of the stem or lying externally in the cortex. In
every case the foliar strand is accompanied by oil canals, which
are entirely absent in the bundles of the stem. In C is represented
a section through the high aérial portion of the stem. Here the
leaf traces, whether still within the cylinder or passing outward
in the cortex, are conspicuously without accompanying oil canals,
as are also those of the axis proper. In D, which illustrates a
region of section through the base of the reproductive axis, the
oil canals appear again in.relation to the traces of the involucral
appendages, but are as clearly absent in the axial bundles as they
are in the lower parts of the stem. It is accordingly obvious that
the various canons of comparative anatomy stated in chapter xvii
are as well illustrated by that very high dicotyledonous group,
the Compositae, as they are by the conifers and calamites. More-
over, while it is true that in the case of the high dicotyledonous

EVOLUTIONARY PRINCIPLES OF THE COMPOSITAE 443

group which forms the subject of the present chapter the only
data for regarding the Tubuliflorae and the Liguliflorae as respec-

Fic. 306.—Diagram of external habit and distribution of oil canals in the arti-
choke (C. Scolymus).

tively lower and higher are morphological in their nature, the lines
of evidence in this connection are so numerous and so generally
admitted that there can be no reasonable doubt as to their validity.

CHAPTER XXXII
ANATOMICAL TECHNIQUE

As a result of the fact that it has been mainly the soft and repro-
ductive parts of plants which in the past have been investigated
in connection with the hypothesis of evolution, the methods of
examining tissues belonging to these categories have naturally
made greater progress than those applicable to the study of harder
and vegetative structures. The reawakened interest in fossil
plants, which is an important feature of the present phase of the
development of morphology, has, however, brought into promi-
nence methods for the anatomical investigation of the hard tissues,
since in general it is only the harder and consequently more resist-
ant parts of extinct plants which furnish a basis for comparison
with modern types. ‘This general situation is, moreover, a some-
what fortunate one, for it is precisely the enduring woody struc-
tures which combine a greater immunity to decay with marked
conservatism of organization. Methods of investigating hard
tissues in plants must obviously, from the conditions outlined above,
include, not only those applicable to relatively unaltered living
plants, but also those which may be employed with success in the
case of carbonized or petrified vegetable remains. Since the pres-
ent volume deals with anatomy alone, it will be possible to confine
the description of technique to those processes which are of value
in the investigation of the hard parts of plants.

THE PRESERVATION OF MATERIAL

In many cases the preserving of material of hard structures
in plants does not present a problem of difficulty. For example,
petrified or carbonized stems which have survived the destroying
influences of often extended time need not be preserved in order
to be studied successfully. In such cases the problem of the prepa-
ration of the tissues rather than of their preservation is of impor-
tance. A similar situation presents itself in the study of woods.

444

ANATOMICAL TECHNIQUE 445

Here the material available is frequently in a dried condition; and
although its protoplasmic structures have been obliterated, it is
still by reason of the durability of its essential organization emi-
nently worthy of investigation. Preservation of hard structures,
although not in general as necessary as in the case of the softer
tissues, is often, however, a matter of prime importance.

The skeletal structures of plants consist of more or less thick-
walled cells united by means of a cement substance which is pectic
in its nature. The process of preservation must be such as will
not bring about maceration of the hard elements by the dissolution
of the bonding material or middle lamella. Further, preservatives
which increase the resistance of the thick cell wall to sectioning
are in general undesirable. As a result of the conditions just indi-
cated, chromic acid and the salts of chromium cannot in general be
advantageously employed for the fixation of hard tissues in plants,
valuable as these reagents are in the study of protoplasmic struc-
tures. Where as perfect a preservation of the protoplasmic organi-

zation as possible is essential, and at the same time the avoidance
of undesirable changes in the thick cell wall and the cement sub-
stance are sought, alcoholic fluids are to be chosen. Excellent re-
agents in this connection are solutions of corrosive sublimate or picric
acid in alcohol of from 100 to 30 per cent strength. The strength of
the alcohol will depend on the nature of the material. In general,
those parts which by reason of the presence of a large proportion
of water are subject to shrinkage in strong alcohol should be fixed
in solutions containing a lower percentage, while harder and more
impenetrable tissues may be treated with a higher grade of alcohol.
In many instances it is advisable to use the alcohol without any
added reagent. This is quite generally the case with wood; for
the addition of corrosive sublimate, for example, is not of sufficient
advantage in connection with fixation to warrant its employment.
Corrosive sublimate, picric acid, or whatever killing reagent may
be employed in conjunction with the alcohol can conveniently be
used in saturated solution in whatever grade of alcohol is found most
advantageous. Where material is fixed in alcoholic fluids it must
be washed in alcohol of corresponding or greater strength to remove
the excess of reagent. Corrosive sublimate forms a black precipitate

446 THE ANATOMY OF WOODY PLANTS

in the protoplasm of the cells which is to be removed, after the
material has been kept in strong alcohol for some time, by the addi-
tion of crystals of metallic iodine until the alcohol ceases to lose
the brown color imparted by the iodine. In all cases the preserved
material should finally be brought into strong alcohol as a prelimi-
nary to further treatment. Where the parts are delicate, such as
small roots, slender herbaceous stems, or the organs of aquatics,
alcohol of full strength should be reached by gradual stages, through
30, 50, 70, and go per cent grades.

THE MACERATION OF MATERIAL

It is often important, particularly in the case of the more
resistant tissues, to separate the cells from one another by the
dissolution of the cement substance or middle lamella. This end is
sometimes attained by the use of nitric acid and a chlorate. Sucha
procedure, however, is unnecessarily violent and causes a consid-
erable amount of injury to the cells themselves. A better method
is to use chromic acid in conjunction with nitric acid, and to warm
in a paraffin bath, if it is desirable to hasten the process. Macera-
tion in the cold, however, often gives much better results. The
strength of chromic acid varies according to the material. From
5 to 10 per cent strength of the two acids ordinarily suffices to
bring about the necessary degree of maceration. The material
after soaking for some time in the macerating fluid may be washed
in repeated changes of water and then gently scraped with the point
of a needle or scalpel. As a result of this procedure masses of ele-
ments may be removed which are readily teased apart with needles
or separated by tapping on the cover glass.

In the case of carbonized material, such as, for example, charred
wood in coal, commonly but not very aptly designated ‘mother
of coal,” boiling in nitric acid often yields quantities of isolated
tracheids and other elements of the wood showing all the details
of structure. The action of the hot nitric acid in this case is a
double one, since it not only affects the isolation of the elements,
but also bleaches the black hue of the coal to a light brown. In
this manner details of structure are made apparent. Spores are
likewise isolated from coal by this method. A more powerful action

ANATOMICAL TECHNIQUE 447

is naturally reached by the use of aqua regia (nitro-hydrochloric
acid) or of nitric acid and a chlorate.

THE SOFTENING OF MATERIAL

In the case of the hard tissues of plants it is usually necessary
to employ some method of softening in order to facilitate the sub-
sequent cutting of sections. It has been discovered that the hard-
ness of the wall of the cell in plants, other things being equal,
depends upon the amount of mineral matter present. Lime,
aluminum, and silicon are elements often found in the wall, par-
ticularly where it is considerably thickened. The removal of
these substances depends, of course, upon the use of an appropriate
acid, and hydrofluoric acid has proved most useful in this connec-
tion. This acid has the advantage of not attacking the middle
lamella—an advantage which is of considerable importance where
good sections have to be secured. For most purposes strong,
commercial hydrofluoric acid answers sufficiently well, but in
cases where the tissues are delicate the chemically pure reagent
is best employed. Hydrofluoric acid is best purchased in gallon
leads containing about ten pounds in weight. The leads should be
massive, as thin ones are sometimes eaten through, with disastrous
results, before the acid is used up. The material is prepared for
treatment in different ways, depending on its nature. In the case
of the shell of a hickory nut or a piece of ebony or live oak, the dry
material is first boiled for some time in water. The boiling should
be continued after the sinking of the material so as to insure the
driving out of all air and-the complete imbibition of the mem-
branes. It is then allowed to cool and is transferred to hydro-
fluoric acid of full strength. After sojourn in the reagent for a
week or two a piece is washed for a short time in running water
and is then tested with a knife to discover if it cuts readily. If
not, a longer stay in acid is indicated, and in case of very hard sub-
stances the time of sojourn may be as long as five or six weeks with
occasional renewal of the reagent in extreme instances. The hard-
est and most refractory tissues of existing plants may be mastered .
in this manner. Wood is usually cut into cubes, which may advan-
tageously be about one centimeter in each of the three dimensions

448 THE ANATOMY OF WOODY PLANTS

and should have their faces corresponding with the transverse,
radial, and tangential planes of structure of the wood.

When the wood is softer, after the preliminary boiling, which
should not be shortened, the pieces of larger size, on account of the
greater ease of manipulation, are transferred to acid which has
either been diluted with water or been used once before on harder
material. In this they are left for a shorter time than is necessary
to effect the softening of very hard tissues, such as heavy tropical
woods or the shells of nuts. For poplar or fir wood a week is quite
long enough to bring the tissues into good condition for sectioning.

When complete organs are to be sectioned, such as stems, roots,
leaves, etc., a preliminary fixation in some preserving fluid is
necessary. After the material has been freed of the fixing agent,
in case this is any fluid other than alcohol, it is run up into strong
alcohol and then freed of air by means of a good air pump. The
air must be removed as completely as possible so that the pieces
will sink even in water. After the pumping has been completed,
the specimens are transferred to water and left until they sink.
If the parts are rather delicate, it is well to carry on the process
of demineralization in weak alcohol instead of water to avoid
maceration. In most cases, however, if the material has been
well fixed in a suitable reagent, no appreciable injury is caused
either to the cell wall or to the protoplasm by the use of hydro-
- fluoric acid.

Naturally the demineralization of plant tissues of whatever
category through the agency of hydrofluoric acid must be carried
on in receptacles made of wax (either hard paraffin or beeswax
will serve); or, in case glass bottles are used, these must be coated
both internally and externally with wax. This is necessary to
avoid the destruction of the container. Gutta-percha or hard-
rubber bottles are sometimes sold for the purposes indicated, but
they are not so resistant to acid as wax and are unnecessarily
expensive. Further, they cannot, like wax, be used with chlorates
in certain procedures to be later described. Whatever be the
nature of the material to be softened, it must, after remaining
sufficiently long in hydrofluoric acid, be washed entirely free of this
reagent in running water. The washing is best effected in the case

ANATOMICAL TECHNIQUE 449

of small pieces in a vessel covered with gauze or cheesecloth to
prevent the loss of material. Larger pieces of wood, particularly
when they are heavy, can be washed in open bottles.

The next stage in the process of preparation for sectioning
depends on the nature of the material. If the objects are very
small, and especially if they lack homogeneity on account of the
presence of soft as well as hard tissues, they must be subjected
to imbedding in nitrocellulose or paraffin, and the former process
is ordinarily preferable. In the case of larger objects one of two
conditions may present itself. Either the structure is quite homo-
geneous, as in peachstones, cubes of oak wood, etc., or else it con-
sists of tissues both hard and soft, as, for example, segments of
stems or roots. In blocks of uniform texture, after washing is
complete, a transfer is effected to a fluid consisting of equal parts
of alcohol of 30 per cent strength and glycerin. Immersion for
some days in this reagent fits the material for cutting. When the
objects are of some size and are not of homogeneous organization
they are run up through alcohols after washing to a strong or 95
per cent solution. This is changed twice and the material is then
placed in equal parts of strong alcohol (g5 per cent) and glycerin.
A week or more is needed under these circumstances to bring about
a consistency suitable for sectioning.

IMBEDDING IN NITROCELLULOSE

When the material is unhomogeneous and is in small pieces,
it must be imbedded in nitrocellulose. There are a number of
different types of nitrocelluloses which may be employed. On
the whole, the best for the purpose is Schering’s celloidin. This
is a special preparation of particular toughness, solubility, and
purity. When extreme transparency is a desideratum, the nitro-
body known as photoxylin may be used. In general, however,
a nitrocellulose which is entirely transparent is quite unnecessary,
since all are sufficiently translucent to permit the ready orienta-
tion of the object in cutting. The cheapest and most readily pro-
curable nitrocellulose is photographic guncotton prepared for
making the so-called ‘‘wet-plates” used in certain photomechani-
cal processes. This is inexpensive and may be purchased at any

450 THE ANATOMY OF WOODY PLANTS

photographic supply store. Whatever nitrocellulose is used, it
must be first washed in clean water and then carefully dried. The

stock should always be kept under water both to delay changes in
_ chemical composition which result in insolubility and to avoid
the risk of explosion. The dry nitrocellulose is dissolved in a
mixture of equal parts of good ether and absolute methyl or ethyl
alcohol. The first is more advantageous because it forms a better
solvent of the nitro-body. A number of solutions grading from
2 to 16 per cent must ordinarily be prepared and kept in labeled
bottles with wide mouths and good corks. As a preliminary to
imbedding, the material is freed of all gases which may have gath-
ered in its intercellular spaces as the result of the treatment with
hydrofluoric acid by means of the air pump. It is then freed of
water by two or three successive changes of absolute alcohol. This
reagent is best purchased in gallon bottles from a reliable manu-
facturer, since in containers of one pound capacity it is extremely
expensive in the United States. The dehydrated material is now
ready for infiltration with nitrocellulose. Small strong bottles are
prepared by twisting tough wire about their necks in such a manner
as to provide two short and diametrically opposite loops. The
wire used for this purpose should be tenacious and not too slender.
The bottles must have rather wide mouths and be stopped with
corks of the very best quality. One of the bottles prepared as
described is partially filled with nitrocellulose of the 2 per cent
strength, and the objects are dropped quickly into it from the
absolute alcohol so as not to absorb either air or moisture. There
should be some space intervening between the cork and the solu-
tion of nitrocellulose to allow for expansion in heating. The cork
is finally forced in and a piece of soft tough brass or copper wire is
passed through the loops on the sides of the neck of the bottle de-
scribed above. The wire is then drawn tight with the fingers or a pair
of pliers and twisted over the top of the cork so as to be held securely
in place even when the bottle is exposed to the heat of the paraffin
bath. After the material has become thoroughly heated the con-
tainers are examined for the purpose of detecting any leak through
the cork. Additional security against escape is provided by keep-
ing the bottles in the warm bath either upside down or on their

ANATOMICAL TECHNIQUE 451

sides. It is usually best to allow the material to remain for twenty-
four hours in the first grade of collodion or celloidin. The bottle is
then cooled in the air or more quickly by cold water. The fluid is
thrown away because of the accumulation of extractives from the
material. Four per cent of celloidin is then poured on, and the
bottle wired again for a twelve-hour immersion in the warm bath.
The second solution is returned after cooling to its proper stock
bottle, and the successive grades of dissolved nitrocellulose are used
respectively until 16 per cent is reached. At this stage chips of
nitrocellulose previously dissolved and then dried are used at inter-
vals and in a small amount at a time further to thicken the highest
grade of solution. The bottle is returned to the bath with each
addition of dry nitrocellulose. If the process has been carried
on with proper precautions, the material has becone infiltrated
without shrinkage. In the case of soft material which is at the
same time rather impervious it is often an advantage thoroughly
to prick the pieces with a very fine needle. The pricks are not
usually near enough to injure the appearance of the sections which
are cut later, if all are made in the same plane, naturally not that
of greatest importance from the structural standpoint.

After a sufficient degree of thickening has been attained in the
manner described in the foregoing paragraph, the objects, each
surrounded by a coating of nitrocellulose, are removed to chloro-
form for hardening. The chloroform should be liberal in amount
to insure thorough induration of the nitrocellulose, and the mate-
rial should be kept in the reagent for at least twelve hours. Sub-
sequently it is transferred to a mixture of equal parts of alcohol
and glycerin, in which it may remain until needed for cutting.
Objects stored in this way seem to maintain their properties indefi-
nitely and are certainly still useful after as long an interval as twenty
years. The alcohol should be prevented from evaporating by
occasional renewals.

SECTIONING OF MATERIAL

Only in the rarest instances is it desirable to cut material free-
hand with a sectioning razor. If uniformly good results are sought,
the microtome should be used. There are many types of micro-
tomes, but those with the simplest mechanisms and the greatest

452 THE ANATOMY OF WOODY PLANTS

degree of rigidity are most useful for the sectioning of hard mate-
rials of vegetable origin. Microtomes in general which provide
for the raising of the object by a vertical screw and elaborate
ratchet devices should be avoided, as they are very difficult to
keep in order when the sections are cut wet in alcohol, as is invari-
ably the case with the materials under consideration. A slicing
instrument which has proved itself to be of the greatest value for
the purposes here enumerated is the Thoma microtome, manufac-
tured by the Jung firm of Heidelberg. The so-called Naples
model is the best size and type and has the advantage of being
immune from rust by reason of its phosphor-bronze construction.
In this microtome the object is raised by sliding up an inclined
plane through the action of a micrometer screw which is accurately
gauged by means of an adjustable clicking ratchet device. The
horizontal position of this mechanism and its manipulation by
hand are great advantages, since by reason of these features it
escapes the corrosion which rapidly impairs vertical screws for
regulating the thickness of the sections. For cutting small objects
the carrier provided with the instrument which can be orientated
in two planes is suitable. The object-carrier is provided with
small cylinders which are primarily intended for use with paraffin.
The hollow of the cylinder, instead of being filled with paraffin as
in its ordinary mode of employment, is blocked by a well-seasoned
and accurately rounded piece of wood. This should be dicoty-
ledonous and rather coarse-grained. ‘The smooth upper surface of
the block is varnished thoroughly (it is generally best to dip the
end of the block into the solution) with a 4 per cent solution of
nitrocellulose. If the blocks are being used for the first time, they
should receive a second coat of celloidin (or collodion) after the
first has had time to dry. The objects imbedded in celloidin (or
collodion) are now removed and examined with a lens to ascertain
the plane in which they are to be sectioned. When decision is
reached on this point, the surface of the hardened nitrocellulose
is sliced with a sharp knife in a plane parallel to that of the desired
sections. The smoothed surface should be perfectly flat. After
wiping, this surface, which should never actually expose the object,
is dipped into a 4 or 6 per cent solution of nitrocellulose and then

ANATOMICAL TECHNIQUE 453

firmly pressed for a moment on the prepared surface of one of the
wood-blocked cylinders. After resting for ten or fifteen minutes
in a warm place it becomes securely fixed and is ready for cutting.

The knives for cutting hard vegetable materials should not be
too thinly ground on the edges. The type of edge supplied by
the Jung firm under the designation of c answers very well for the
purpose. The knives are best not too large in size and should not
exceed eight inches in length. They are sharpened by the aid of a
cylindrical appliance slipped over the back, which gives the edge
a less acute angle to the sharpening stone, with a corresponding
advantage in saving of time. The stone or hone for putting the
edge of the knife in condition should be preferably a yellow Belgian
one, such as is ordinarily employed by barbers, but of considerably
larger size and of as fine texture as can be procured. It is better,
in fact, to have two stones, one coarser for preliminary use and a
finer one for finishing. Carborundum hones, although often sup-
plied for the purpose of sharpening microtome knives, are not
advantageous. In renewing the edge of the knife care should at
first be taken to remove all hacks or gaps by grinding on a coarse
hone. In the sharpening process the edge of the knife should be
pushed forward on the stone and not backward, as the latter pro-
cedure results in a so-called ‘‘wire edge.”’ After the removal of
the nicks the edge is finished on a finer hone. If, as a result of
frequent sharpening, the knife has been ground away to a very
thick edge and consequently sharpens very slowly, it is necessary
to grind it on an alundum or carborundum wheel of very fine tex-
ture. The knife is held on a support known as a knife-grinder and
brought against the revolving wheel at such an angle that the edge
is slightly hollow-ground. It requires a little skill to grind a knife
on the wheel, but this is quickly acquired and the frequent sending
of the knives to the cutler is thus avoided. A further advantage of
grinding the knives in the laboratory is the avoidance of drawing
of the temper, which is often the unfortunate result of sending
them to a professional grinder. The wheel should be revolved at
a high rate of speed and the edge of the knife held gently against
it. In this manner overheating is avoided. The grinding machines
furnished by the Carborundum Company of Niagara Falls and the

454 THE ANATOMY OF WOODY PLANTS

Luther Company of Milwaukee are excellent, particularly the
latter.

After the edges have been sharpened on the grinding wheel
and hone the knives are finished by means of a strop. The best
type has four sides and a solid wooden center. The four sides are
covered with leather surfaced in various manners. The first face
provides a coarse polish and the two following ones successively
finer degrees. The last face is of smooth leather and imparts the
final smoothness to the edge. The knife should be carefully wiped
with a cloth after being passed on the successive surfaces to avoid
the impairment of the finer ones by material from the coarser
grades. In stropping, the edge should be drawn backward and
not pushed forward as in honing, since failure to observe this pre-
caution results in injury to the leather surfaces. Strops consisting
of unsupported leather are not desirable for use with microtome
knives.

The knives are held in place on the microtome by the knife-
holder, which in turn is fastened to a heavy block of phosphor-
bronze running in a channel on the right side of the microtome.
The holder may be altered in position on the block, forward and
backward, inward and outward, by means of appropriate screw
holes. It may be raised by plates provided for insertion beneath
its shaft. The best type of holder is provided with a tilting mech-
anism which makes it possible to vary the horizontal angle of the
knife to the object. In cutting all hard tissues but coal the micro-
tome knife should be in an oblique position to the object, and in
general the inclination should be somewhat acute. In the case of
large objects and of those which are likely to curl in cutting, a
more oblique position is sometimes advantageous. In making
sections the surface of the knife should be kept wet with alcohol
of 95 per cent strength. This is ordinarily effected by means of a
camel’s-hair brush. This is also used for preventing the curling
of the section as it comes up on the knife. The proper touch for
flattening the section without either rolling or dragging it is acquired
by experience only. Sections may be cut from two or three micra
to many times that thickness, depending on the particular condi-
tions involved. If the sections curl after being placed in alcohol

ANATOMICAL TECHNIQUE 455

subsequent to removal from the edge of the knife, this disadvantage
may be largely obviated by allowing them nearly to dry on the
flat surface of the knife at some distance from the part of the edge
which is actually being used for cutting.

In the case of hard and homogeneous material, such as wood,
pieces of nutshell, coal, etc., a special type of object-holder must
be used, in which the material is firmly held in a rigid clamp. Two
varieties of these have been devised for the Thoma microtome,
recommended for the purposes here described. One, contrived
by Professor R. B. Thomson, is made in Toronto, and particulars
in regard to it may be obtained by writing to the Botanical Depart-
ment of the University of Toronto. This is an admirable device
and is particularly useful in cutting large pieces of wood which
have not been sufficiently softened. A second type of holder for
hard objects has been devised by the writer and is manufactured
to order by the Jung Company of Heidelberg. In these holders
the position of the object can be varied so as to obtain the proper
inclination to the edge of the knife. For example, in cutting radial
sections of wood the slices obviously must be accurately parallel
to the rays, or else a very confused condition is presented under
the microscope. The object-carriers in both these holders are
very heavy in order to insure the necessary rigidity and inertia.
A complete Thoma microtome with the additional holder for
cutting hard objects costs about one hundred dollars free of
American duty. With the Thomson devices the cost is consider-
ably greater.

As has been pointed out in an earlier paragraph, wood and
similar tissues need not be infiltrated with nitrocellulose to secure
the best results. If, however, the woody tissues for any reason
have become unduly softened, either from too prolonged immer-
sion in hydrofluoric acid or through the ravages of fungi (this is,
of course, particularly the case where diseased or rotten wood is
being studied), imbedding in celloidin or collodion is necessary.
As will be shown in a later paragraph, this procedure is absolutely
necessary with most coals. The cutting of sections of very hard
tissues involves the same principles as exemplified in the technique
of smaller and less homogeneous objects, with the exception only

456 THE ANATOMY OF WOODY PLANTS

of coal, which demands a transversely placed instead of an oblique
knife. In cutting more resistant plant substances the edge of the
knife must be very smooth and sharp and must. be much more
frequently renewed than in the case of softer materials.

It is also possible in certain cases to section unhomogeneous
materials, consisting of pieces of root, stem, etc., by adopting the
following procedure: When the sections are transverse, the pieces
may be clamped directly in the jaws of the wood-holder described
above. In the case of longitudinal sections, either tangential or
radial, the objects are removed from the mixture of equal parts
of strong alcohol and glycerin and accurately smoothed on one
surface by means of a sharp knife. The plane surface is dried
carefully with a cloth and then painted with a 6 per cent solution
of celloidin. After the film of nitrocellulose has thoroughly dried
the object is quickly dipped in 4 per cent solution and applied to
the surface of a block of wood which has previously been varnished
with nitrocellulose in the manner described in a former paragraph.
By clamping the block in the wood-holder it is possible to secure
extremely thin longitudinal sections without the labor involved
in imbedding in nitrocellulose. It should be emphasized, how-
ever, that this method is not available in the case of organs which
show a considerable diversity of texture. For example, the
branches of Ginkgo and Tsuga must be imbedded to secure success-
ful sections, while those of Pinus and Quercus provide good trans-
verse preparations without previous infiltration.

Other vegetable materials can best be prepared as thin sections
without any previous treatment whatever. This is true of peri-
derm, as illustrated by common bottle cork and “birch bark.”
These tissues need merely to be clamped in the microtome to fur-
nish sections as good as may be secured by the most elaborate
processes of softening and imbedding. Fresh leaves also provide
very thin sections when treated in the following manner: Rather
thick leaves are best in the case of the dicotyledons and mono-
cotyledons, for example, in Rhododendron and Yucca. These are
held between two smooth pieces of pine wood in the jaws of the
wood-holder. Sections are made until the knife has cut down
sufficiently near to the pieces of pine to secure rigidity in the leaf

ANATOMICAL TECHNIQUE 457

tissues. The sections are then cut thin by appropriate manipu-
lation of the micrometer screw and floated instantaneously on the
edge of the knife in alcohol. Thence they are transferred to a dish
of water and show all their natural organization and color prac- .
tically unchanged. Sections as thin as five and ten micra are
easily obtained in this manner and are very striking. The leaves
of gymnosperms such as cycads and conifers lend themselves to
the same treatment.

METHODS OF STAINING

The thin sections prepared by the methods above indicated
are frequently so tenuous as to present insufficient detail on micro-
scopic examination. This inconvenience is ordinarily overcome
by the use of stains to bring out contrasts in structure. The sub-
ject of staining has been so recently and admirably discussed for
American students in Professor Chamberlain’s Methods in Plant
Histology (3d ed., The University of Chicago Press, 1915) that it
need only be elucidated in the present connection in regard to the
special conditions presented by the hard tissues in plants.

A few stains give the best results in most anatomical investi-
gations, since the protoplasmic structures are of less importance
and the cellulosic and lignified conditions of the cell wall are of the
greatest significance. In most instances staining with hematoxy-
lin and counterstaining with the anilin dye known as safranin give
the best results. In the case of hematoxylin the Haidenhain
method on the whole answers best. The sections are washed in
distilled water and then immersed in a 3 per cent solution of
ammonia-iron alum. The alum should be bluish in color and free
from efflorescence. Ten to fifteen minutes in the alum solution
are sufficient. Careful washing in distilled water follows. The
first change of washing water should remain only a short time and
is followed by a second and a third change, so as to remove all
alum which has not become fixed in the sections. In material
containing a great deal of tannin special precautions must be taken
to insure thorough washing by repeated changes of distilled water.
When the sections are cleared of excess of alum, they are ready
for the hematoxylin solution, which in some cases is employed in

458 THE ANATOMY OF WOODY PLANTS

the strength of one-half of 1 per cent in distilled water. In some
instances this is too great a concentration and leads to overstaining.
This may be remedied by the transfer of the sections to distilled,
water with a few drops of iron-alum solution in which bleaching
slowly takes place. When a sufficient degree of decoloration has
been attained, the sections must be washed in repeated changes
of distilled water as in the first procedure, otherwise the blue colora-
tion will fade in a short time. A better method is to stain more
gradually, using only a few drops of the solution of hematoxylin
described above. Slow staining in general gives better results
than rapid. The blue-tinted sections when properly colored are
transferred to distilled water and thence to distilled water to which
a drop or two of safranin has been added. This stain is prepared
by adding equal parts of water-soluble and alcohol-soluble safranin
to strong alcohol until a saturated solution has been reached.
The coloration with safranin is best carried on slowly and in dilute
solution, as in this way both clear detail and strong contrast are
best secured. Very often the best results follow from leaving the
sections overnight in the dilute safranin. In this instance, how-
ever, precaution should be taken to wash the sections very care-
fully and repeatedly after the use of iron alum and hematoxylin,
as otherwise disagreeable precipitates are likely to make their
appearance.

Some plant tissues and remains show to best advantage
without staining. This is, for example, frequently true of darkly
colored heartwood and of fossil plants and coal. In the case of
the two latter it is often expedient to reduce the color by bleaching
agents. The most convenient method in this connection is sup-
plied by the use of chlorine water. Hydrogen peroxide, so often
recommended for bleaching vegetable tissues, is of no value in the
case of carbonized and fossil plant remains. When the dark colora-
tion is very intense, a more vigorous bleaching action results from
the use of nascent chlorine, which can be conveniently secured by
dissolving a few crystals of a chlorate in distilled water, with the
subsequent addition of a small quantity of nitric acid. This com-
bination gives sufficiently vigorous bleaching action without accom-
panying maceration.

ANATOMICAL TECHNIQUE 459

METHODS OF MOUNTING

In the case of the great mass of vegetable tissues it is frequently
an advantage to bring about the sharpest possible accentuation
of the color contrasts resulting from staining by mounting in media
of high refractive index. For this purpose the sections must be
usually entirely freed from water. This end is effected by means
of absolute alcohol. The sections are transferred on a section-
lifter from the solution of safranin to a watch-glass of absolute
alcohol, excess of fluid having been removed by touching the lifter
to a piece of blotter or filter paper. From the first absolute
alcohol they are lifted to a second, which effects the final removal
of water. Too long immersion in the first absolute alcohol is
likely to extract too much of the safranin stain, but a longer stay
is advantageous if the sections have received an excessive colora-
tion in red. In case the sections have been cut from material
infiltrated with nitrocellulose, the latter may often be retained
with advantage. This object is attained by adding a little chlo-
roform to the two changes of absolute alcohol, this procedure hav-
ing the effect of preventing the softening of the nitrocellulose.
Should it appear desirable to remove the infiltrating substance
completely, the sections are transferred to ether after the second
absolute alcohol. In any case, after dehydration is effected
in any of the ways described, the sections are finally cleared
with a reagent of high refractive index. The most convenient
of these for vegetable preparations is chemically pure and an-
hydrous benzene or benzole. After clarification the preparations
are ready for mounting. This is effected in hard Canada balsam,
dissolved in whatever agent has been used to clear the sec-
tions. Benzole, xylol, chloroform, etc., are employed for this
purpose.

The solution of balsam should not be too dilute, as the evapora-
tion of the solvent may under this condition result in the disas-
trous desiccation of the sections. Even where the section is not
actually left bare by the drying out of the mounting balsam, the
access of air is likely to promote a rapid fading of the colors. It is
usually necessary to flatten the sections because the balsam sets
by the evaporation of the solvent. This procedure is essential

460 THE ANATOMY OF WOODY PLANTS

in cases where the structures present are to be reproduced by pho-
tographic methods. After the preparations have dried for a day
or two in a horizontal position at laboratory temperature, they are
weighted with lead in the form of rolls. For photomicrographic
purposes the final degree of flattening is secured by the pressure
of clip clothespins which are prevented from injuring the thin
covers by the interposition of thin slices of cork. In the processes
of flattening described above, the preparations are subjected to
continually increasing heat. This is conveniently secured by
laying the slides upon a board on top of a steam radiator. The
end of the board nearer the supply pipe of the radiator is naturally
much hotter than that nearer the return. The final flattening and
setting of the sections is best carried on in the interior of the par-
afin bath, which is an adjunct of every botanical laboratory.
It is unnecessary, of course, to resort to such extreme measures
to secure flatness unless the preparation is to be reproduced by
photomicrographic methods. |

In a number of instances, especially when the sculpture of the
cell wall in elements of the wood is a particular feature of interest,
mounting in Canada balsam brings about too great a degree of
homogeneity. This inconvenience can be avoided by using other
media for mounting. Gum dammar dissolved in chloroform sup-
plies a mountant which presents many of the advantages of Canada
balsam without the extreme degree of clarification which makes
that reagent in some cases undesirable. Glycerin jelly is an even
better mounting medium, but has the disadvantage of tending to
extract the safranin. This can be largely overcome, however, by
transferring the sections after staining to glycerin containing a
large amount of safranin. Under these circumstances the safranin
is not extracted from the sections. In mounting, the sections are
placed upon the slide and warmed over a burner to make the gly-
cerin quite fluid. This is then allowed to drain off as much as pos-
sible, and any remaining trace is removed by wiping round the
section with a piece of clean cotton fabric. A drop of melted gly-
cerin jelly is then applied and the cover is placed in position.
Weighting is advisable during setting and cooling of the jelly to
secure flattening of the sections.

ANATOMICAL TECHNIQUE 401

METHODS OF SECTIONING COAL

This mineral is now universally recognized to be of vegetable
origin. Its practical importance and the interesting remains
which it contains make it an object not without interest both from
the general botanical and from the anatomical standpoints. All
except the most highly carbonized coals may be prepared. for
sectioning on the microtome without great difficulty in accord-
ance with the summary method here described. The mineral is
split in conformity to the layers by means of a stout knife and a
hammer. The thin slabs thus secured are broken transversely
by the aid of pliers or a chisel into pieces which vary in size accord-
ing to the resistance of the coal. The fragments are put into
melted phenol or carbolic acid and kept in corked bottles in the
warm bath for about a week. They are then transferred after
washing in warm water to strong hydrofluoric acid for a second
week. Except in the case of certain lignitic coals further treat-
ment with phenol after washing with water is necessary. This
is followed by a second immersion in hydrofluoric acid and subse-
quent washing. After a third return to phenol (if this is necessary)
the material is washed in water and then run up into absolute
alcohol which must be changed two or three times. During the
process of dehydration the coal is kept in or on the paraffin bath.
Imbedding in nitrocellulose follows. In this process only 2, 4,
and 6 per cent solutions of celloidin are used, and the last of these
is followed at once by nitrocellulose of 16 per cent strength. It
is an advantage in imbedding coal, not only to pump all air out of
its substance, but also, when in the 6 per cent celloidin, to subject
to a positive pressure of between two and three hundred pounds.
This is easily effected by inclosing in a metal cylinder and raising
the internal pressure by means of an automobile pump. After a
rapid course of thickening rendered possible by the resistant char-
acter of the material, the pieces are dropped into chloroform in the
usual manner and later transferred to glycerin and strong alcohol.
In sectioning, the fragments of coal are clamped securely in the
wood-carrier of the microtome and cut with a transversely placed
knife. Only sections which are five micra or less in thickness are
of value in the case of this extremely opaque material. The

462 THE ANATOMY OF WOODY PLANTS

utilization of botanical technique in the study of coal has appar-
ently made it necessary to revise the accepted views as to the origin
of this invaluable mineral. The hypothesis of the derivation of
coals from peat formed in place, as in the northern bogs of our
epoch, must be given up in favor of the conception of an accumu-
lation in open water by transport either aérial or aquatic. Coal
from its internal organization obviously is comparable to the muck
in the bottoms of modern lakes rather than to the surface accumu-
lations of peat which often surround them. The technique of
mounting sections of coal is the same as for imbedded material in
which the matrix of nitrocellulose is retained. No staining is
necessary.
PHOTOMICROGRAPHIC METHODS

Photography with the microscope has become in recent years
an extremely important aid in microscopical investigations of the
structure of plants. To such perfection have photographic lenses
and photographic plates attained that it is possible to reproduce
photographically most of the structural features of tissues and cells
in plants. The advantage of photomicrographic reproduction is
that it is at once less laborious and more accurate than represen-
tation by drawings.

The lenses used for photomicrographic purposes do not differ,
except in special cases, from those employed in general microscopic
investigation. For very low magnifications such as are ordinarily
not available with the compound microscope special lenses of great
perfection are now manufactured by the leading makers of micro-
scopical appliances. These are used without eyepieces and can
be applied as a rule only to special photomicrographic stands.
For moderate and higher magnifications, however, the same optical
outfit is used as for observation with the eye. The better grades of
lenses naturally give better photographic results than those which
are of inferior quality. In the case of very high magnifications
special condensers, insuring a great degree of concentration of
light and freedom from chromatic aberration, are employed for
illumination.

Microscopic lenses have certain general defects which militate
against their employment for photographic purposes. The most

ANATOMICAL TECHNIQUE 463

serious of these is the discrepancy between the chemical and visual
focus. As a result of this shortcoming, a picture sharply focused
to the eye appears dull and indistinct upon the photographic plate.
In the case of apochromatic objectives and compensation oculars
this optical defect is less apparent than in ordinary so-called achro-
matic lenses and Huygenian eyepieces. The lack of correspond-
ence of visual and chemical foci becomes practically negligible
where certain color screens are used in securing the photographic
image. In some instances, particularly in the photography of
unstained material, lenses of rock crystal are of value, since they
shut out less actinic light than do lenses of glass. The great cost,
however, of lenses of this construction renders them unavailable
for most laboratories, and in any case their value is by no means
proportionate to their price.

Since microscopic sections are generally colored either natu-
rally or by means of special stains, photographic plates which are
sensitive to color are necessary in photomicrography. There are
various types of such plates which are available for different pur-
poses. It has been found that certain chemical substances, par-
ticularly anilin dyes, possess the valuable property of greatly
increasing the sensitiveness of the photographic plate to colors.
The photographic negative is ordinarily produced on a gelatin
surface mounted on glass. The gelatin covering is known as the
emulsion and contains bromide of silver precipitated in its sub-
stance. The silver bromide is rendered more or less sensitive to
light by boiling in the presence of ammonia, on the one hand, or
by adding an excess of bromide of potash, on the other. It is most
sensitive to the radiations of the violet and ultra-violet region of
the spectrum. Consequently the common photographic plate will
not give good results in the case of sections stained with red, green,
or even very dark blue dyes. This original defect of the photo-
graphic plate has been almost entirely removed in recent years by
the use of anilin sensitizers. For example, erythrosin possesses
the property of rendering plates soaked in a weak solution sensi-
tive to greens. Plates which are so treated are ordinarily known
as isochromatic. The appellation is, however, obviously a mis-
nomer, since they are only in a very slight degree, if at all, sensitive

464 THE ANATOMY OF WOODY PLANTS

to reds. The most valuable type of plate for general photomicro-
graphic purposes is that treated with a blue anilin dye, isocyanin.
This treatment makes the emulsion sensitive to the red end of the
spectrum to a very large degree. Such plates are called “‘pan-
chromatic’? or “spectrum” plates and are of the greatest value
in the photomicrographic reproduction of extremely difficult micro-
scopic objects, such as coal, highly tanniferous stems, roots, etc.

Further aids to the microscope in the objective reproduction of
microscopic images are the so-called color screens. These are of
various hues, but need not be considered in detail, since only a few
of them are practically useful for photomicrographic purposes.
When a yellow screen is employed, for example, it shuts off to a
large extent the violet and ultra-violet radiations and as a conse-
quence gives the photographic plate greater efficiency in the repro-
duction of colors other than light blue. A further extremely
valuable result following the use of a yellow screen is the elimina-
tion of the contrast between chemical and visual foci mentioned
above as a disadvantageous feature distinguishing microscopic
lenses from those employed in ordinary photographic cameras.
This is a property of the utmost practical importance and makes
available lenses which would otherwise be quite useless. The
general effect of a yellow screen, then, is to make a plate relatively
more sensitive to radiations other than the violet ones and at the
same time to favor a more brilliant microscopic image as a result
of the elimination of the ordinary chemical or actinic rays. A
green screen is in certain cases highly useful, but the numerous
other hues which distinguish screens made for the appropriate
rendering of color are in general of less value in the practice of
photomicrography.

The photomicrographic apparatus may advantageously con-
sist of an ordinary camera with long bellows, supported in a ver-
tical position on a counterpoised sliding back. The back may be
fixed at any height by means of window fasteners. The camera
is connected with the microscope by a collar which engages with
a corresponding collar on the tube of the microscope in such a man-
ner as to break the path of the rays of light. Reflections are pre-
vented by the blackening of the collar rings with matt drop black.

ANATOMICAL TECHNIQUE 465

A shutter which fits on the front of the photographic camera is
most convenient, and an arrangement which will admit of exposures
of from three seconds to a hundredth part of a second is desirable.
The Thornton-Pickard shutter, of English manufacture, but used
a great deal in the United States, has proved itself of value in this
connection. When a color screen is used, it may be laid horizon-
tally just above the shutter in view of the vertical position of the
camera. A special photomicrographic microscope is desirable,
although not absolutely necessary, as any ordinary microscope of
good size and weight with adequate optical equipment may be
employed for photomicrographic work. The microscope is sup-
ported on a bench provided with three legs so that it may be always
steady. The upright stand to which the camera is attached should
likewise have three brass knobs screwed into its base to insure
stability. The best source of light is an arc lamp with hand feed.
The ordinary projection lantern answers very well for this pur-
pose if it is not too cumbersome. A condenser should be placed
in front of the arc to collect the light, and a water chamber should
likewise be interposed between the condenser and the microscope
to eliminate heat rays. The use of alum in the water chamber to
accentuate the exclusion of the heat radiations is unnecessary, and
it is inadvisable on account of the evil effects produced on the
apparatus by alum. The lantern should be so mounted that it
can readily be lowered and raised, and also inclined at any angle.
A fixed lantern may be employed, but its utility is much restricted.
The carbons are best inclined at an angle of ninety degrees. The
light is taken from the end of the positive carbon, which should be
in a horizontal position. The current should be direct and not
alternating, although the latter may be used with some degree of
success. The advantage of the direct current is the freedom from
noise and the intense light emanating from the positive carbon.
The amperage should be controlled by a rheostat which will per-
mit the passage of from seven or eight to fifteen amperes of current.
The high amperages are used in the case of dark-colored or opti-
cally opaque objects. In the case of photographs under low magni-
fication a ground-glass screen may often be inserted with advantage
between the mirror of the microscope and the water chamber.

466 THE ANATOMY OF WOODY PLANTS

The photographic dark room should be of moderate size and
provided if possible with electric lights. In addition to a free
light there may be one shaded by a ground-glass cover for examin-
ing negatives and exposing lantern slides. Two safe lights for
developing are likewise necessary. One of these should be screened
with orange and deep-ruby glasses, one behind the other. This
light is available for the development of the less sensitive chro-
matic plates, known as iso-plates, and for all ordinary photo-
graphic plates. A second safe light is necessary for use with plates
sensitized by isocyanin. (so-called spectrum and panchromatic
plates), which must ordinarily be developed in total darkness.
The Wratten & Wainwright Company, of Croyden, England, has
patented a light which may be used for this purpose, and this is
on sale in the better photographic supply houses in the larger
American cities. It is indispensable for all highly color-sensitive
plates. A sink of good length and with two taps should be provided.
To the left there should be a gently sloping draining-board with
shelves beneath and above for developing- and fixing-trays of
hard rubber, as well as developers and glassware. . Ventilation of
the dark room is desirable, and is absolutely essential in warm
weather. It is best effected by means of an ordinary electric fan
placed near the door of the room in such a position as to cause
strong currents into and out of the dark room. It may, of course, be
set in a special light-tight chamber communicating with the out-
side, but this fixed position considerably restricts its usefulness.

The plates employed for making negatives depend on the par-
ticular needs in special cases. For very exacting work in which
the object is either extremely dark-colored or opaque the use of
panchromatic or spectrum plates is indicated. In case the con-
trasts are very slight, and particularly in weakly stained material
or objects which present only a slight natural coloration, the so-
called iso-process plate is valuable. For the mass of colored prepa-
rations the ordinary isochromatic plate answers every purpose.
A common need in connection with teaching is the copying of
illustrations. When these consist of line or half-tone engravings,
process plates giving a high degree of contrast are employed. It
was formerly necessary to import color-sensitive plates from

ANATOMICAL TECHNIQUE 467

Europe, but these are now made in good variety and of excellent
quality by the Cramer Dry Plate Company, of St. Louis, Missouri.
All the types of plates described above may be purchased from the
Cramer Company. Since ordinary photographic supply houses
often do not carry in stock the special plates needed for photo-
micrographic and other scientific purposes, the Cramer Company
undertakes to furnish these direct from its factory in St. Louis.

The making of lantern slides is an activity which may often be
pursued profitably in larger botanical establishments in univer-
sities. The most convenient method of making such slides is by
placing the lantern plate directly behind a negative of suitable
size, namely, 34 by 4} inches. A few seconds’ expostre to a light
sheltered by ground glass in the dark room (the time depending
on the character of the negative) suffices. Development of lan-
tern slides is effected by means of a special developer to be men-
tioned in a subsequent paragraph. The slides must be covered
with a mask which can be made by cutting out black paper by
means of a wheel paper-cutter and appropriate metal forms, obtain-
able at photographic supply stores. Masks may also be purchased
with the various forms and sizes of opening necessitated by the
different kinds of pictures. After the mask is applied the lantern
slide is protected by a thin cover-glass held in position by black
paper binding strips which may be procured in any photographic
establishment. American and Continental European lantern slides
are 31 by 4 inches in dimension. The English slide is 3+ inches
square. The storing of lantern slides is always a problem when
their number becomes large, and many filing cabinets for this pur-
pose are on the market. The essential thing, however, is to have
numbers on the slides and a catalogue, so that they can readily
be selected for use and easily returned to their places. The com-
plexity of the catalogue will of course depend upon the tastes and
needs of the individual and the extent of his budget.

The development of the various plates described in the fore-
going paragraph is effected by reagents which reduce to metallic
silver the parts of the plate exposed to the light. Developers are
legion, but two or three seem to answer every practical purpose.
For making negatives pyrogallol developer is on the whole most

468 THE ANATOMY OF WOODY PLANTS

advantageous. This is made by dissolving one ounce of Mallinck-
rodt or Schering pyrogallol in 150 cubic centimeters of tap water,
acidulated beforehand with 20 drops of nitric acid. This solution
should be kept in the dark room and is known as “stock pyro.”
A second solution is made by adding to tap water to per cent of
carbonate of soda and 1o per cent of sulphite of soda, together
with a quarter of 1 per cent of bromide of potash. In this solution
the carbonate of soda is used (with the pyrogallol) for the purpose
of reducing the silver in the exposed plate. The sulphite of soda
prevents staining, and the bromide of potash restrains a too rapid
development and insures a clear image. To make the develop-
ing solution, dilute one volume of the ‘stock pyro” with nine
parts of water. This is solution No. I. Solution No. IT is the one
described above containing carbonate, sulphite, and bromide.
Of No. I and No. II take equal parts. The developer should be
poured quickly and evenly over the plate and air bubbles should
be avoided. The time of development depends upon the nature
of the plate, but ordinarily the image should begin to appear in
from thirty to sixty seconds. Development is continued until
the image begins to disappear. Lantern slides and bromide prints
are best developed with metol, which is an organic developer of
German origin used almost exclusively in the manufacture of
moving-picture films. Its employment in this connection is suffi-
cient proof of its value. The developer is made by adding together
in the proportion of one to three the solutions described below
under the denominations A and B. Solution A is made by adding
ro per cent of caustic soda or caustic potash to distilled water.
Solution B consists of 10 per cent sulphite of soda, 1 per cent metol
(Metol-Hauff), and a quarter of 1 per cent bromide of potash in
distilled water. After the two solutions are mixed they may be
diluted with advantage by adding a third- to a half-volume of
tap water. Development of lantern plates and bromide paper is
ordinarily complete in from fifty to sixty seconds. The lantern
plates are best exposed in the dark room, while it is often advan-
tageous from the standpoint of time-saving to expose printing
papers to daylight. Velox papers, and in particular Glossy Velox
(ordinary or special depending on the vigor of the negative), may

ANATOMICAL TECHNIQUE 469

be used for making prints for reproduction in scientific journals.
The smooth, glossy surface renders this type of paper more suit-
able for purposes of reproduction than the more artistic papers
with matt or rough surfaces.

Photomicrography is an art which can be learned only in part
from books, and the beginner is advised either to visit some lab-
oratory where it is successfully carried on, or, failing that, to seek
direction from a professional photographer, having regard in that
case to the special conditions involved in photomicrographic tech-
nique. The account given above is from limitation of space neces-
sarily incomplete and has been written only to supply some answer
to numerous inquiries as to the methods pursued in the laboratories
of plant morphology in Harvard University.

INDEX

INDEX

A

Abies, root of, 135, 149, 334

Abieteae, 334

Abietineae, 325; antiquity of, 325; bars
of Sanio in, 326; cone axis of, 328; re-
lation of, to Ginkgoales, 337; relation
of, to other Coniferales, 334; resin
canals of, 334; wood of, 326

Adiantum, stem of, 279

Agathis: leaf trace of, 324; seedling of,
323; wood of, 319; A. Bidwillii, 330,
353

Air spaces, 3

Alburnum or sap wood, 55, 56, 57

Alcohol, 445

Alnus: leaf of, 212; rays in, 153, 177

Amphivasal bundles, distribution of, in
monocotyledons, 414

Anatomical technique, 444
Angiosperms: general characters of, 373;

reproductive structures of, 374; vessels
of, 95; wood of, 373
Annual rings, 15, 16, 421

Annulus in Pteridophyta, 216

Araucaria Bidwillii, 323

Araucariineae, 318; alternating pitting
in, 319; bars of Sanio in, 323; per-
sistent leaf trace of, 324; wood of, 318

Araucariopitys, 353

Araucarioxylon, 340;
structure of, 320

Archaeocalamitaceae, 266
Archigymnospermae, 292, 305
Arctium, distribution of oil canals in, 437

Arrangement of bundles: in Equisetales,
273; 1m monocotyledons, 192

Aster: oil canals of, 434; resemblance of
stem to that of the oak, 406; root of,
434; stem of, 405

Astromyelon, 276

Avena, bundle of, 411

B

Barberry, substitute fibers of, 34

Bars of Sanio, 68; diagnostic value of, in
Coniferales, 323

pitting of, 320;

473

Bast fibers, 118
Bast, hard and soft, 118

Bennettitales, 303; bisporangiate cone of,
303; course of leaf traces in, 301

Betula, wood rays of, 155
Bleaching, 446
Bocoa provacensis, tracheids of, 33

Brachyoxylon, wood of, 320;
reactions of, 321, 320

Bundle in monocotyledons, 194, 195

wound

Cc

Calamitaceae, 251; course of bundles in,
273

Calamites, 266; pith casts of, 260;
primary wood of, 266, 268; secondary
wood in, 268; young stem in, 266

Callus: definitive, 123; in phloem, 110,
114; seasonal, 123

Cambial activity in monocotyledons, 195,
4II

Camera, photomicrographic, 464

Canada balsam, 459

Carbonized material, bleaching of, 446

Casuarina: leafy twig of, 211; rays of,
77, 82, 404; C. equisetifolia, rays of,
78, 88, 90; C. Fraseri, rays of, 77, 86;
C. torulosa, rays of, 77; systematic
position of, 384; transfusion tissue of,

384
Cedrus, resin canals of, 74, 335
Cedroxylon, structure of, 349
Cell, 1-5
Celloidin, 449
Centripetal wood in Eguisetum, 274
Chalazogamy in Amentiferae, 230
Chamaecyparis, marginal tracheids of, 72
Chicory, degenerate oil canals in root of,
442
Chlorates, 446, 458
Chlorine, 458
Chloroplastids, 2
Chromic acid, 445, 446
Cirsium, oil canals of, 439
Clematis, wood rays of, 179, 191

474

Climate in relation to evolution of plants,
417, 423

Coal, sectioning of, 461

Collateral protostele, 165

Collateral siphonostele, 165

Collodion, 440

Color screens, 464

Color-sensitive plates, 463

Companion cells in angiosperms, 118

Comparative anatomy, canons of, 234

Compositae: illustrative of general prin-
ciples of anatomy, 433; organization of
stem in, 400, 402, 404

Compound ray, origin of, 82, 181

Concentric bundles in monocotyledons,
195

Concentric protostele, 162

Concentric siphonostele, 163

Coniferales, 317; genealogical tree of,
355; subtribes of, 355

Conifers, phylogeny of, 355

Conservative organs, 238; leaf, 238; re-
productive axis, 238; root, 240

Cordaitales, 305; leaf in, 203, 309; rays
of, 66; stem in, 306; transfusion tissue
of, 208, 308; transition region in, 309;
wood of, 15, 28, 66, 305, 308, 419

Corrosive sublimate, 445

Cortex, Io

Crystallogenous cells, 118

Cupressineae, 339; marginal tracheids of,
341

Cupressinoxylon, 349

Cuticle, 128

Cycadales, 297; centripetal wood of leaf,
300, of cone axis, 299; supernumerary
zones in, 298

Cycadofilicales, 292

Cycas: cortical bundles in, 201;
bundle of, 199; leaflet of, 200

Cynara Scolymus, distribution of oil
canals in, 436, 439, 443

Cystoliths, 127

leaf

D
Dammar, 460
Dandelion, root of, 435

Depressed segments: in Aster, 405; in
Clematis, 179; in Quercus, 180

Developing reagents, 468
Diagram of distribution of oil canals in
artichoke, 443

THE ANATOMY OF WOODY PLANTS

Dicotyledons: characteristics of, 375;
degenerate vessels of, 373, 380; modifi-
cations of tracheids in, 381; parenchym-
atous elements of, 381; phylogeny of,
384; wood rays of, 382

Diffuse ray, origin of, 82, 89

Distribution of amphivasal bundles in
monocotyledons, 412

Duramen or heart wood, 56, 57, 58

E
Ectokinetic sporangium, 216
Electrical current for photomicrography,

465

Endarch, 23, 167

Endodermis, 10, 11, 12

Endokinetic sporangium, 217

Ephedra, 357; rays of, 174, 358; stem of,
173, 358; tracheids of, 31; wood of, 94

Epidermis, 126; multiple, 127; water
tissue, 127

Equisetaceae, 269; course of bundles in,
273

Equisetales, 250, 264; stem of, 272

Equisetum, 269; bundles of, 270; cone of,
274; leaf trace of, 274; medulla of, 272;
primary wood of, 270; root of, 275;
stem of, 269; types of stem in, 271

Erianthus, bundle of, 195

Ersatzfasern, 34

Exarch, 23, 167

F

Fagus, tracheids of, 32

Ferns, 277; root of, 150; stem of, 278

Fiber tracheids, 31; tylosis of, 106

Fibers: libriform origin of, 32; mucilagi-
nous, 33; septate, 34; substitute, 34

Filicales, 277; leaf of, 280; stem of, 278;
root of, 20, 150

Fixing reagents, 445

Fundamental tissues, 132

Fusiform rays: in Cedrus, 74; in Pinus,

etc., 69
G

Geinitzia, 355

Ginkgo: absence of parenchyma in, 424;
alternate pitting in, 312; leaf of, 313;
opposite pitting in, 311; secondary
wood of, 311; sporangia and sporo-
phylls of, 314; tangential pitting of,
423

Ginkgoales, 310;
tineae, 337

relation of, to Abie-

INDEX

Gleichenia, stem of, 278

Gnetales, 357; and Bennettitales, 370;
parenchyma in, 358; vessels in, 358;
vine habit in, 364

Gnetum, 363; stem of, 363; vessels of, 367

Grape vine, septate fibers of, 34

Grinding wheels, 453

Guard cells, 129; number of, 131

H
Hairs, 130
Hard tissues, preparation of, for sec-
tioning, 447
Heartwood: condition of parenchyma
in, 56, 57; heartwood or duramen, 55

Helianthus: compared with Casuarina,
404; diagram to illustrate herbaceous
type, 403; H. hirsutus, ray situation in,
400, 402, 404; H. tuberosus, oil canals
of, 434

Hematoxylin, 457

Herbaceous and arboreal types in rela-
tion to climatic conditions, 430

Herbaceous dicotyledons, 186

Herbaceous type: geographical distribu-
tion of, 430; in geological time, 431;
origin of, in cryptogams, 387, in the
dicotyledons, 186, 387, in Helianthus,
400, 402, 404, in Potentilla, 187, in
Salvia, 189, in Urtica, 397

Herbs, substitute fibers of, 35

Heterangium, stem of, 293

Hones, types of, 453

Hydrofluoric acid, uses of, 447

Hypoderma, 128

I
Imbedding in nitrocellulose, 450

Inclusion of pith: within leaf traces, 285;
within stele of stem, 284

Intercellular spaces, 2
Jodine, uses of, 446
Tris, pericarp of, 2
Tron alum, 457
Tsoetes, 255
K
Kinds of plates useful for photomicrog-
raphy, 463, 466
Knives, 453; position of, 453

1b
Labiates, origin of herbaceous type in, 189
Lamium, origin of herbaceous type in, 190

475

Lantern slides, 467

Larch, root of, 22, 145

Large storage rays in relation to climatic
conditions, 430

Laticiferous system in the Compositae,
435

Lattices, 122

Leaf: centripetal wood of, in Prepinus,
208; concentric bundles of, in Cycas,
201, 302; cortical bundles of, in Cycas,
201, 302; of Cycadales, 199, 300; defi-
nition of, 141; gap of, in Casuarina,
81, in conifers, 80, in Cordaitales, 203;
organization of, in Alnus, 213, in
Casuarina, 211, in dicotyledons, 213;
in Gnetales. 211, in monocotyledons,
213, in Pinus, 205, 330, in Prepinus,
207, 331; rachis of Cycadales, 199;
transfusion tissue in Casuarina, 311,
in Cordaitales, 203, in Pinus, 206, in
Prepinus, 207; vegetative, 199

Leea, organization of stem in, 184, 389,
396; origin of woody cylinder of, 184,
389

Leguminosae: fibers and tracheids of, 33;
origin of herbaceous type in, 191

Lenses, photomicrographic, 462

Lepidocar pon, 224

Lepidodendraceae, 250, 255; secondary
wood of, 16, 260

Lepidodendron, 256; primary wood of,
170; rays in, 258; septate tracheids of,
38

Libriform fibers, 32

Lights for photomicrographic purposes,
466

Ligulifloreae, laticiferous system of, 435

Ligquidambar, fibers of, 106

Liriodendron, parenchyma of, 51; vessels
of, 97, 100

Lycopodiales, 250, 252

Lycopodium, 19, 252; root and stem of,
143

Lycopods, wood of, 252

Lycopsida, geological
microphylly in, 245

Lyginodendron, 296

range of, 244;

M
Maceration, 446
Magnolia, fiber tracheids of, 32; paren-
chyma of, 51
Marginal tracheids: of Cupressineae, 73,
341; of Taxodineae, 72, 339

476

Medulla: cortical origin of, 283; in Os-
mundaceae, 286; origin of, in the
Filicales, 284, in Lycopsida, 283

Medullary rays: so called in gymno-
sperms, 172; in Lycopsida, 170

Meduilosa, 294; origin of cycads from,
205

Megasporangium: in Pteridophyta, 223;
in Selaginella, 216

Mesarch, 23, 167

Mesophyll, 213

Metagymnospermae, 317, 357

Metaxylem, 109

Methods of sectioning coal, 461

Miadesmia, 225

Microscope in photomicrography, 465

Microscopic lenses, defects of, 462

Microsporangium: in Conifers, 219; in
Cycadales, 216; in Ginkgoales, 218;
in Gnetales, 219; in liverworts, 214; in
Lycopsida, 216; in phanerogams, 220;
in Pteropsida, 216

Microtome, 452

Middle lamella, 3, 445

Monocotyledons, 409; amphivasal bun-
dles of, 196; arrangement of bundles
in, 192; bundle of, 193; cambial ac-
tivity in, 195, 411; characteristics of,
377; geological age of, 409; leaf struc-
ture in, 213, 410; phylogeny of, 412;
stem of, 192, 411; root of, 158, 410

Mounting of sections, 450

Mucilaginous fibers, 33

Mycorrhiza, 138

Nitrocellulose, 449

Nothofagus, parenchyma of, 53

Nucleus, 2

O

Oak, wood of, 14, 30, 55, 57

Objects, clamping of, 455

Oil canals of Compositae, 433, 436

Onopordon, canals of, 439

Organs, definitions of, 136

Origin of the herbaceous type: in Helian-
thus, 400, 402, 404; in Potentilla, 187;
in Salvia, 189; in Solanum, 190; in
Urtica, 398

Origin of the woody cylinder in Vitis, 391,
395

Osmunda cinnamomea, 287, 289; O. clay-
toniana, 289; O. regalis, 288

THE ANATOMY OF WOODY PLANTS

Osmundaceae: internal phloem in, 287;
medulla in, 286; root of, 20, 150

Osmundites skidegatensis, 286

P

Palms, anatomy of, 196, 413

Parenchyma. 37; absence of, in second-
ary woods of the Paleozoic, 40, in Pinus,
41; diffuse in conifers, 48, in dicotyle-
dons, 50; distribution of, in Abies
and Tsuga, 47, in dicotyledons, 50, 52,
in primary wood, 37, 38, 39; origin
of, at end of annual ring, 42, 425;
origin of, in Alder, 54, in dicotyledons,
54, in Liquidambar, 54, in Mesozoic, 40,
in Picea, 42, 425, in primary wood, 37,
39, in Prunus, 54, in secondary wood,
42, 54, 425; origin of, from tracheids,
38, 42, 425; relation of, to annual ring,
45, 425, to climatic evolution, 41, 425;
terminal in dicotyledons, 52; transition
to, from short tracheids, 43; vasicentric
in dicotyledons, 52

Parichni, 261

Peach stone, 4

Pericycle in Pteris, 108

Phloem: central ray cells in, 113, 115;
companion cells of, in angiosperms,
118; in Pinus, 110; in Pieris, 108; in
Tilia, 117; marginal cells in, 113;
parenchyma in, 114; rays in, 114;
sieve tubes in, 114; in Vitis, 122

Phloeoterma, 10, 12

Photographic dark room, 466

Photographic plates, 463, 467

Photomicrography, 462

Phylloglossum, 252

Picea: stem of, 85; terminal parenchyma
of, 336, 425; wood of, 42, 44, 425

Picric acid, 445

Pileorhiza, 138

Piliferous layer, 146, 150, 158

Pineae, 335

Pinus: absence of parenchyma in, 336;
aggregate rays in, 362; leat of, 11, 330;
needle of, 11; phloem ‘in, 110; short
shoots of, 337; structure of wood in
cone of, 326; tracheids of, 26; tyloses
of, 106; wood of, 24, 326

Pinus and Ginkgo: microspores of 3377
short shoots of, 337; tracheids of, 338

Pinus Strobus, rays in phloem, 110

Pit membrane, 3, 6

Pith, origin of, in the Filicales, 283

INDEX

Pits: absence of tangential, in Cordaitales
and other ancient gymnosperms, 27;
bordered, 6, 7; condition of, in heart-
wood, 56; half bordered, 7; radial, 27;
simple, 3, 5, 6, 7; tangential, 27

Pitting, distribution of, 27, 428

Pityoxylon, structure of, 349

Plastid, 2

Plates, photographic, 463, 466

Podocarpineae, 343

Pollen chamber, 226, 227

Polydesmy in Cycadales and Gnetales, 364

Porous perforation in vessels, origin of, 101

Potentilla, origin of herbaceous type in,
187

Prepinus: leaf of, 207, 331; relation of,
to Cordaitales, 332

Preservation of material, 444

Primary wood, degeneracy of, 169

Principes, anatomy of, 196, 413

Proangiosperms, 303

Protocalamites, 268

Protoplasm, 1

Protostele, 162, 278; collateral, 165; con-
centric, 162; in Filicales, 278

Protoxylem, 18, 19, 20, 21

Psilotum, 253

Pieris: bundle of, 21, 108; development
of stem in, 280, 282; phloem of, 109;
rhizome of, 9, 10, 132; vessels of, 92

Pteropsida, 244, 247; geological range of,
249; megaphylly of, 249

Q

Quercus: fiber tracheids of, 31; stem of,
179, 404; tracheids of, 31; vessels of,
99; wood of, 15; wood rays of, 76,
178, 181.

R

Ranales, characteristics of, 386

Rays: aggregate, 78, 82; compound, 78,
82; fusiform, 69, 74; linear, 69;
marginal cells of, 69; medullary, so
called, 61; origin of, in Coniferales, 68,
72, in Lepidodendron, 62, 63, 64; rela-
tion of, to resin canals, 70; tracheids
of, in Lepidodendron, 62, 64; types of,
in Casuarina, 77; uniseriate, 67, 69;
width of, in Cordaitales, 66, in Cyca-
dales 75, in Cycadofilicales, 65

Recapitulation: in Coniferales, 235; in
first annual ring, 237; in oaks, 235; in
Phyllocladus, 235

Resin canals, relation of, to resin cells, 343

477

Reversion, 72, 73, 74, 75, 241
Rhizophore, 262
Rhodotypus, fibers and tracheids of, 33

Robinia Pseudacacia, mucilaginous fibers
of, 34

Root: absence of medulla in, 173; defini-
tion of, 137; -development of, 145;
exarch ‘primary structure of, 145;
exodermis of, 158; in Coniferales, 145;
in dicotyledons, 151; in herbaceous
dicotyledons, 156; in Lycopodium, 143;
in monocotyledons, 159; in Osmunda,
150; piliferous layer of, 158; primary
and secondary structure of, 138; pri-
mary wood of, 145, 150, I51, 157,
159; radial organization of, 143; rays
in, 147; ‘spiral tracheids in, 161;
velamen of, 159

Root cap or pileorhiza, 138

Root hairs, 146, 150, 158

S

Sachs, erroneous views of, as to origin
of stem, 191, 406

Safe lights, 466

Safranin, 458

Salvia, origin of herbaceous type in, 189

Sapwood or alburnum, 55

Scalariform perforation of vessels, origin
of, 97

Scolymus, oil canals in root of, 441

Scorzonera: laticiferous system of, 435;
oil canals of, in root, 441

Secondary wood as indicator of climatic
conditions, 418

Sectioning of material, 454

Seed: in angiosperms, chalazogamous,
230; in Archigymnospermae, 226; in
Casuarina, 231; in dicotyledons, 232;
in monocotyledons, 231; in Pinus,
233; of Coniferales, 229; of Cycadales,
226, of Ginkgo, 228; of porogamous
angiosperms, 230

Seedlike megasporangia, 223, 225

Selaginella, 216, 225, 254

Sequoia: anatomy of, 339; cone axis of,
340; marginal tracheids of, 73, 341;
parenchyma of, 339; tracheids of, 25;
wound canals of, 341

Sharpening of knives, 453

Shutter, photomicrographic, 465

Sieve plates: in the angiosperms, 120,
123; Im gymnosperms, 114; in _ her-
baceous angiosperms, 125

478

Sieve tubes and vessels, comparison of,
123

Sigillaria, primary wood of, 171, 259

Sigillarians, 259

Siphonostele, 163; collateral, 165; con-
centric, 163; in Filicales, 278; lacunae
of, 164, 278; reduction of, 165

Smilax, root of, 12, 138; stem of, 192

Solanum, origin of herbaceous type in,
190

Solidago, resemblance of stem of certain
species to that of the oak, 405

Sphenophyllaceae, 251, 264

Sphenophyllum insigne, 265

Sporangia, 142; in Abietineae, 219; in
angiosperms, 220; in Coniferales, 219;
in Cycadales, 216; in Ginkgo, 217; in
Gnetales, 219; in Polypodium, 216; in
Selaginella, 216

Sporangia and trichomes, 130

Staining methods, 457

Stangeria, reproductive axis of, 299

Starch: in phloem of Pinus, 111; in
phloem of Tilia, 118

Stem: definition of, 140; development
of, in Pteris, 281; herbaceous, 186, 387;
organization of, in Casuarina, 82, in
Cupressus, 80, in dicotyledons, 81, in
monocotyledons, 192, in Selaginella,
254; primary tissues in, 169; second-
ary tissues in, 169; structure of, in
Cyathaceae, 282, in Marattiaceae, 282,
in Pteris, 281.

Stephanospermum, 315

Stigmaria, 262

Stomata, 129

Strops, types of, 454

Substitute fibers, 35

T
Tangential pitting in relation to climatic
evolution, 424, 425
Taraxacum, laticiferous system of, 435
Taxineae, 344; parenchyma in, 346; re-

productive structures of, 345; resin
canals in, 346; tracheids of, 345

Taxodineae, 339, marginal tracheids of,
341; wood parenchyma of, 342

Taxoxylon, structure of, 349

Teak, septate fibers of, 35

Terminal parenchyma, relation of, to
climatic evolution, 426

Tissue systems, 8, I0, 12, 13

THE ANATOMY OF WOODY PLANTS

Tissues, 9; epidermal, 10; fibrovascular,
10; fundamental, 10

Torus, 6, 68; position of, in heartwood, 56

Trabeculae, 68, 250

Tracheids, 25, 27, 29, 31; marginal, in
Coniferales, 60, 73, 341; reticulated,
17; ringed, 17; scalariform, 17;
spiral, 17; spring, 24, 26; summer, 24,
26; tylosis of, 106; types of, 25, 26, 31

Tragopogon, oil canals in root of, 441

Tree nails, 34

Trias, woods of, 421

Tsuga: parenchyma of, 46; resin canals
in, 346

Tubiflorae, oil canals of, 434

Tyloses: in fibers of Liguidambar, 106; in
resin canals of Coniferales, 56; in
vessels of dicotyledons, 57

U
Urtica, organization of the stem in, 397

V

Vacuole, 1

Vasicentric parenchyma, 51

Velamen, 160

Vessels: absence of, in certain angio-
sperms, 373, and sieve tubes, 123; in
Ephedra, 94; in Gnetum, 95; in Pleris,
92; in Welwitschia, 95; lateral walls of,
103; scalariform, 103; terminal walls
of, 93, 95; tylosis of, 104; types of,
94, 95, 100, tor; with pitted perfora-
tion, 94; with porous perforation, 96,
99; with scalariform perforation, 96

Vine type, origin of, 184, 390, 396, 397
Vitis, organization of stem in, 185, 391;
thick and thin annual stem of, 397

Volizia, 355

W

Welwitschia, 366; leaf of, 368; stem of,
366; vessels of, 366

Wood: cryptogamic, 222; holders for,
Asss primary, 610) 16, 20,5 22) tO
tangential pits of, 425

Wood parenchyma: origin of, in
Abietineae, 41, 43; in Araucariineae,
320, 322; in Podocarpineae, 344; in
Taxineae, 346; in Taxodineae, 339, 342

Woody dicotyledons, 379.

Woody tissues, softening of, 447

x
Xylem: endarch, 21, 167;
167; mesarch, 20, 167

exarch, 19,

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