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Accession No. 53457

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THE STRUCTURE OF ECONOMIC PLANTS

THE MACMILLAN COMPANY

NEW YORK • BOSTON • CHICAGO • DALLAS
ATLANTA - SAN FRANCISCO

MACMILLAN AND CO., Limited

LONDON • BOMBAY - CALCUTTA • MADRAS
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THE MACMILLAN COMPANY
OF CANADA, Limited

Photograph by Dorothea Lange

July Corn

THE STRUCTURE
OF ECONOMIC PLANTS

By

HERMAN E. HAYWARD

Professor of Botany
The University of Chicago

New York THE MACMILLAN COMPANY 1938

Copyright, 1938.
By the MACMILLAN COMPANY.

All rights reserved — no part of this book may be
reproduced in any form without permission in writing
from the publisher, except by a reviewer who wishes
to quote brief passages in connection with a review
written for inclusion in magazine or newspaper.

Published November, 1938.

SET UP AND ELECTROTYPED BY J, S. GUSHING CO.
PRINTED IN THE UNITED STATES OF AMERICA

To

My Mother and Father

PREFACE

THIS book is an outgrowth of courses in plant anatomy offered
at the University of Chicago since 19x7. Much valuable
anatomical w^ork has been done in the form of monographs
on specific plants, special studies of restricted scope, general
studies of organography, and in studies incidental to research in
physiology, pathology, pharmacology, and other fields of plant
science. Discussions w^ith those interested in applied botany,
research workers and students, suggested the desirability of pre-
paring a work which would bring together some of these investi-
gations relating to the structure and developmental anatomy of a
number of economic plants. In this book, the available data have
been freely drawn upon, and at the conclusion of each chapter
there is appended a selected list of citations from which material
has been obtained. There has been no attempt to compile a com-
plete bibliography, but the selections have been made that more
detailed information on special phases of plant anatomy may be
readily available. In many instances, the citations listed have
extensive bibliographies.

The book has been organized in two parts, the first dealing with
general plant anatomy, the second with the structure of a selected
number of economic plants. In Part I, the point of view of develop-
mental anatomy has been presented briefly, together with the
nomenclature which is used in Part II. Since nomenclature is
frequently inconsistent, a glossary defining the terms in the sense
in which they are used in this work is appended. The intro-
ductory chapters are intended to supplement other works on
anatomy and to make this work more generally useful.

In Part II, it was obviously impossible to consider a large number
of plants in as great detail as the amount of material available and
the usefulness of such material to investigators would warrant.
The problem of which to include and which to omit presented
a real difficulty. The principal criteria determining the selection
were the economic importance of the plant, its suitability as a
representative of the family to which it belongs, and the intricacy

Vll

viii PREFACE

of its anatomical and morphological detail. Important fruit
crops have been omitted because a second volume is contemplated
which will deal more especially with them.

ACKNOWLEDGMENTS

The preparation of this work has been greatly aided through
the generous assistance of fellow workers in the fields of botany
and agriculture. Many have contributed the results of their
researches and illustrative material, and the author expresses his
deep appreciation of this cooperative spirit which has made
possible the inclusion of valuable data. Individual acknowledg-
ment accompanies the text and illustrations in each case. Thanks
are due The University of Chicago Press for permission to use
illustrations from the Botanical Gazette. Acknowledgment to
other journals, publications, and publishing houses appears at the
point of insertion of the material used.

The preparation of the original plates was aided in part by a
grant to the University of Chicago from the Rockefeller Founda-
tion. The original line drawings and photomicrographs were
prepared by the author and associated students, especially Miss
Jean Port, who also aided in the preparation of the manuscript.
The author is deeply indebted to Professor E. J. Kraus, who read
the entire manuscript, for valuable suggestions and encouragement
from the inception of the idea of the book to its final completion.

Herman E. Hayward
Chicago, Illinois
September, 1938

CONTENTS

PART I. GENERAL ANATOMY

CHAPTER I

PAGE

Cells and Tissues and Their Development i

CHAPTER II
The Anatomy of the Root 39

CHAPTER III
The Anatomy of the Shoot 57

CHAPTER IV
The Anatomy of the Flower and Fruit ....... 88

PART II. ECONOMIC PLANTS

CHAPTER V
Gramineae. Zea (Corn) . . . . m

CHAPTER VI
Gramineae — Continued. Triticum (Wheat) 141

CHAPTER VII
LiLiACEAE. Allium (Onion) . . . i79

CHAPTER VIII
MoRACEAE. Cannabis (Hemp) 2.14

CHAPTER IX
Chenopodiaceae. Beta (Beet) ......... 2.4°

CHAPTER X
Cruciferae. Raphanus (Radish) . 2.83

CHAPTER XI
Leguminosae. Medicago (Alfalfa) . . . . . . . • 3°9

IX

58157

X CONTENTS

CHAPTER XII

PAGE

Leguminosae — Continued. Pi sum (Pea) . . . . . . -339

CHAPTER XIII

LiNACEAE. LiNUM (FlAx) 37I

CHAPTER XIV
Malvaceae. Gossypium (Cotton) 411

CHAPTER XV
Umbelliferae. Apium (Celery) . . . . . . . -451

CHAPTER XVI

CONVOLVULACEAE. IpOMOEA (SwEET PoTATo) ...... 485

CHAPTER XVII
Solan ACEAE. Solanum (White Potato) 514

CHAPTER XVIII
Solanaceae — Continued. Lycopersicum (Tomato) . . . . -55°

CHAPTER XIX
Cucurbit ACEAE. Cucurbita (Squash) . . . . . . . .580

CHAPTER XX
CoMPOsiTAE. Lactuca (Lettuce) 6ll

Glossary 653

Index . • • 665

^ L I •: i s- V -^

THE STRUCTURE OF ECONOMIC PLANTS

PART I. GENERAL ANATOMY
CHAPTER I

CELLS AND TISSUES AND THEIR
DEVELOPMENT

IN the description of the structure and development of the seed
plant, it is convenient to consider separately its major vegetative
organs, root and shoot Qsfem and leaves'); and, more or less intimately,
its structures associated with gametic reproduction, the fiower and
fruit. The organs may be resolved into their constituent tissues;
and these, in turn, into types and groups of cells, constituting a
simple tissue, that are essentially alike in structure and in the
functions they perform.

THE CELL

The term cell has been variously applied. Early microscopists
interpreted it as the box-like unit of structure of which plant and
animal tissues are comprised, obviously referring to the cell wall
rather than to any included matter. Later, emphasis shifted to the
cell contents, or protoplast, rather than the wall. On this basis,
a cell may be defined as a protoplast which usually consists of a
nucleus, cytoplasm, and various inclusions. (Fig. i.) The term cell
is still applied, however, in cases in which the protoplast is lost in
the process of differentiation and maturation. Thus tracheids, ves-
sel segments, and various types of sclerotic elements which have no
protoplast at maturity are frequently referred to as cells in descrip-
tive anatomy although it is preferable to use the term element in
such cases. The most significant investigations relative to the
detailed structure of the cell have been treated by Wilson (45) and
Sharp (40). For this reason, the following account is restricted
to those aspects of cytology which are most frequently associated
with the problems of developmental anatomy.

2. THE STRUCTURE OF ECONOMIC PLANTS

The Protoplast. — The protoplast is commonly differentiated
into several parts: cytoplasm, nucleus, plastids, and other cell
inclusions.

The cytoplasm constitutes the main mass of the protoplast, and
at its limiting outer periphery is the plasma membrane. Physically,
cytoplasm is a colloidal system, probably of the emulsion type,
with properties similar to those of other recognized colloids such as
viscosity, adsorptive power, semi-permeability, and its structural

«-

Fig. I. Meristematic cells from the growing point of the root of Liliuin Harrisii showing
character of the cytoplasm and nuclear activity.

reactions to external and internal factors both chemical and physi-
cal. Optically, cytoplasm appears to be a hyaline, homogeneous
semi-fluid with numerous inclusions (granules, globules of fat and
oil, chondriosomes, plastids, etc.) which occur in varying sizes and
frequencies. The physical appearance of the colloidal system may
be modified by the degree of viscosity of the cytoplasm as well as
by the extent and character of vacuolization. In a young meriste-
matic cell, the cytoplasm is relatively dense with numerous minute
vacuoles; but, as it matures, these become fewer and larger until
finally a large central vacuole or sap cavity is formed. In this case,
the protoplast is limited centripetally by a vacuolar membrane and

CELLS AND TISSUES 3

is restricted to a narrow peripheral layer within the cell wall. In
cells where the nucleus is centrally located, strands of cytoplasm
may extend to it from the periphery.

As Sharp has pointed out: "It cannot be maintained that any-
thing like an adequate chemical picture of living protoplasm has
been obtained." The protein content is high, consisting of
peptones, albumins, and globulins; but relatively little phosphorus
is present, except in the nucleoproteins of the nucleus. The fatty
substances most commonly found include true fats and other fatty
compounds such as cholesterol and lecithin. Carbohydrates occur
most frequently as pentoses, and inorganic salts have been recog-
nized in increasing numbers. The amount of water present is large,
varying with the type of tissue and its degree of maturity.

The Nucleus. — The nucleus is conspicuous in meristematic
cells; but, as demonstrated by Bailey (3) in cambial initials, there
is no constant relationship between the volume of the nucleus
and that of the cell as suggested by Strasburger and others. It
usually occupies a more or less central position in the cytoplasm,
but this may depend upon the degree of vacuolization of the cell,
since it always lies embedded in the cytoplasm. It is commonly
spherical or ellipsoidal and is bounded by a thin nuclear membrane
within which is a transparent substance known as the karyolymph or
nuclear sap. In this sap is a reticulum of threads which are associated
with or constituent parts of the filamentous portions of the chromo-
somes. The existence of the reticulum as a valid nuclear structure
has been questioned; but, on the basis of critical comparisons
between living nuclei and those to which various fixatives have
been applied, it seems to be established that it is in no sense an
artifact. The reticulum has been regarded as a dual structure con-
sisting of a framework of linin supporting granules of chromatin.,
but the more recent and probably more generally accepted view is
that it consists of a single complex substance, karyotin. One to
several nucleates may occur in the resting nucleus which appear as
viscous droplets and are composed of proteins and lipins.

No complete statement regarding the function of the nucleus
can be made, but it is associated with the process of cell division
and directly or indirectly with wall formation. It undoubtedly
plays an important role in the later development and differentiation
of the cell, and is intimately concerned in the cellular metabolism.
From the genetic standpoint, it may be regarded as carrying and.

4 THE STRUCTURE OF ECONOMIC PLANTS

transmitting the heritable characters of the organism; but some
instances of cytoplasmic or plastid inheritance have been demon-
strated.

Plastids. — Plastids are the most conspicuous structures in
the protoplast and occur in some of the tissues of practically all
plants with the exception of certain groups of Thallophytes
(Cyanophyceae, and fungi). Like the nucleus, they lie in the
cytoplasm and are not known to occupy vacuolar cavities. They
are variable in shape, being spherical, ovoid, or discoid; but the
size is relatively constant. Mobius Ql^^ found that 75 per cent of
the plastids in ii5 species of plants examined had a long axis rang-
ing from 4 to 6 ju. Among those which fall within the average
range are the plastids of Cucurbita, Gossypium, Lactuca, and
Phaseolus; in Beta they are larger, ranging from 7 to 10 /i.

The manner in which the plastids originate is not entirely clear;
but, in most instances, it appears that they arise by the division of
preexisting plastids or from proplastids. Randolph (35), working
with Zea, was able to observe the behavior and development of
proplastids in living cells and noted the formation of mature
plastids from minute granular proplastids. Early stages occur
in the tips of leaf primordia, transitional ones appear near the
apices of successively older seedling leaves not yet exposed to sun-
light, and later stages may be found in leaves that are about to
emerge from the surrounding sheath. In cells of the mesophyll of
well-developed seedling leaves, the chloroplasts have a diameter of
7 to 8 /i.

Whether or not the proplastids are always formed from other
cytoplasmic inclusions such as chondriosomes, or arise de novo, is
not established. Randolph has concluded that "the question
regarding the extent to which the plastids are to be considered
permanent cell organs with an unbroken genetic continuity
throughout the life cycle must remain an open one"; and Sharp
(40) has reasoned that "if plastids represent transformations of the
cytoplasm resulting from the localization of certain processes, they
may well be expected to differentiate anew as these processes begin,
and to preserve varying degrees of permanence depending upon the
processes carried on . " On the other hand , Bowen (9), working with
the root tissues of several plants including Pisum, Hordeum, Cucur-
bita, and Phaseolus, concludes that "it is perfectly clear now that
plastids are formed only by the division of preexisting plastids."

CELLS AND TISSUES 5

The principal types of plastids are chloroplasfs found in photo-
synthetic tissues (chlorenchyma), and colorless leucoplasts which
occur in meristematic tissues and in the cells of storage organs.
The latter are sometimes more specialized, and are known as
amyloplasts because of their relation to the formation of starch
reserves. Chromoplasts, other than chloroplasts, may contain
many of the color pigments found in flowers and fruits. The caro-
tene in the fruit of the tomato occurs inside the plastids; and, after
their decomposition, in free crystalline or globular form. The
yellow and a few of the red pigments of many flowers and vegetative
structures may be localized in plastids; while others are in solution
in the cell sap, such as the anthocyanins in the fleshy root and
hypocotyl of beet or radish.

Various cell inclusions may be sufficiently characteristic to
be helpful in anatomical diagnosis. These include crystals of cal-
cium oxalate, calcium carbonate, silica, inulin and protein crystals,
aleurone grains, starch, and semi-solid compounds classed as gum,
resin, fat, mucilage, or latex. The cell sap may contain salts,
carbohydrates, proteins, alkaloids, and pigments in solution which
can be used in micro-chemical diagnosis, and similar techniques
are applied in cell wall analysis.

The Cell Wall. — With few exceptions, the cells of seed plants
have walls that are formed by the activity of the protoplast. The
character of the cell wall is important in tissue analysis since
various cell types have been named on the basis of differences in
its structure. The mature wall is generally regarded as a non-living
membrane; but it is commonly penetrated by cytoplasmic strands,
plasmodesma, which may be either primary or secondary in origin.
The primary type is present during the formation of the primary
wall and persists thereafter, while the secondary type develops
later by penetration. The plasmodesma may be aggregated into
groups to form conspicuous strands, or sometimes occur as evenly
distributed single threads — in which case they are difficult to
demonstrate because of their extreme fineness. The penetration of
the wall by these cytoplasmic threads is thought to establish proto-
plasmic continuity in living tissues, and it is probably not inac-
curate to regard the young wall as consisting in part of living
matter. In any event, the interrelation between the development
of the wall and the protoplast is very intimate, since the growth of
the former is conditioned upon the activity of the latter.

THE STRUCTURE OF ECONOMIC PLANTS

The terminology used with reference to the constituent parts
of the cell wall has been confusing and contradictory, but Kerr and
Bailey (^5) have proposed a system of nomenclature with which
Frey-Wyssling (19), Anderson (i), and others working on cell
structure are in general agreement. Briefly summarized, they
suggest that the term primary wall "should no longer be applied to
the first formed layer of secondary thickening, but should be used

solely in designating the cambial
wall and its homologues in other
tissues." With reference to the
term middle lamella, they suggest
that "it should be used synony-
mously with intercellular substance
in referring to the truly isotropic
layer of intercellular material."
These recommendations are based
upon investigations which indi-
cate that cambial initials have
walls that are independent mor-
phological structures composed
chiefly of cellulose and pectic sub-
stances and that they are aniso-
tropic; while the intercellular
substance (middle lamella) be-
tween the walls of adjacent

Fig. 1. A, diagrammatic transection of initials is an amorphouS, isO-
one entire tracheid and parts of seven others; . n • i j i i

B, section of adjacent walls more highly trOplC COlloid, COmpOSed largely

magnified: a, truly isotropic intercellular of pCCtic SubstanceS deposited by
substance; b, cambial or primary wall; c, , ^„. . „ -^

outer layer of secondary wall; d, central the CytOplasm. (^hlg. X, A, D.J

layer of secondary wall; e, inner layer of An important characteristic of
secondary wail. (Redrawn from Kerr and , . n i „ „ ,u^^^^^^ '.c ;<-c

Bailey, Jour. Arnold Arb.) ^he intercellular substance is Its

plasticity, which permits the
intercellular movements and adjustments that are necessary in the
development of cambial or meristematic derivatives.

Recently, Bailey (4) has reemphasized the necessity of critical
cell wall analysis, stating that "in any discussion of the higher
plants, it is essential to differentiate accurately and consistently
between three distinct categories of structures." These are:
(i) the meristematic, embryonic, or cambial cells and derivatives of
them which have retained their capacity for growth and increase in

CELLS AND TISSUES 7

volume by expanding and increasing the surface area of the primary
wall, and are further characterized by their potentiality to undergo
reversible changes in thickness, etc.; Ql) the fibers, and other
highly specialized cells, which have lost the above potentialities
owing to irreversible changes; and, in addition 'to the primary
wall, which may be more or less specialized, have a supplementary
or secondary wall which is primarily mechanical in function and
incapable of growth or increase in surface area; and (3) the inter-
cellular layer referred to above as the intercellular substance or middle
lamella.

Primary Wall Formation. — The first step in wall formation
is the development of the cell plate, which occurs in the equatorial
plane of the spindle during the later phases of nuclear division.
One theory of cell plate formation is that it is derived from the
materials of the spindle, and that the plate subsequently splits to
form two plasma membranes, which in turn secrete the pectic sub-
stances of the middle lamella. An alternative theory is that the
cell plate zone may be a more fluid layer resulting from the forma-
tion of two dissociated protoplasmic phases, one of which forms
the cell plate region and the other the plasma membranes which
secrete the pectic materials into the fluid layer. More investiga-
tion is needed on this point, but the concept of a fluid cell plate
zone is not out of harmony with the process of cell division by
furrowing such as occurs in the formation of the microspores of
many of the higher plants (Nicotiana, Medicago, Melilotus,
Lactuca, etc.) as well as in lower plant forms. In the microspore
mother cell, the furrows are initiated at the periphery and develop
centripetally in contrast to the formation of the cell plate from the
spindle, which is usually centrifugal.

The primary wall, which consists of an anisotropic layer of
cellulose and pectic materials, keeps pace with the enlargement of
the protoplast by stretching and by the addition of new wall sub-
stances by intussusception or apposition. In the former, the addi-
tion of materials is carried on through a process of infiltration
in which additional substances are introduced among the older
ones; while in the latter there is a centripetal deposition of addi-
tional wall substance which forms thin plates or lamellae against
the original wall. It seems probable that in many instances, the
thickening of the wall is accomplished by the joint operation of
both processes.

THE STRUCTURE OF ECONOMIC PLANTS

Secondary Wall Thickening. — As the cell differentiates, its
walls may be variously thickened and additional layers are depos-
ited against the primary wall. Bailey and Kerr (5) have described
the secondary wall of normal tracheids, fiber-tracheids, and libri-
form fibers as consisting of three layers : narrow outer and inner
layers, and an intervening one of variable thickness. (Fig. i.)
The outer and inner layers differ from the middle one in the orien-
tation of the cellulose fibrils, which are arranged at approximate
right angles to the long axis of the cell, while those of the mid-

rw --'■' 7^^^ ^^^ layer are longitudinally or

^^^^fejUgl^^^^g diagonally parallel to it. They
' y^^^Sa^^^^^^SKL. ^ find, however, that deviations

from the "typical 3 -layered type
of secondary wall are not of
infrequent occurrence. ' ' In many
thick-walled fibers, there appears
to be no differentiation of the
inner layer; and the middle layer
may consist of a series of lamellae
or growth rings. Kerr (14) has
shown for Gossypium that each
growth ring is comprised of two
lamellae, one porous and the
other compact, and that one of
these double rings is deposited
daily. (Fig. 3.)
The Pits. — The pits are thin spots in the cell wall which occur
at points where no secondary wall materials have been deposited.
In the formation of the secondary walls of two adjacent cells, the
wall substance is laid down in such a way that these spots coincide,
being separated only by the pit membrane consisting of the two
primary walls and the intercellular substance. A simple pit is
comprised of two opposed cavities with straight sides, which are
separated by a closing membrane; but such pits may be variously
modified with respect to size and shape, and their frequency and
distribution depend upon the nature of the tissue of which the cell
is a component part. In some instances, the secondary walls bound-
ing the pit project inwardly without contacting the membrane and
overarch the cavity to form a bordered pit. In such types, the cen-
tral portion of the primary wall or pit membrane may be thickened

A

Fig. 3. Transection of a cotton fiber
grown under field conditions, swollen in
cuprammonia, and subjected to pressure to
show the conspicuous pattern of the growth
rings. X550. (Photomicrograph by Anderson
and Moore, Ind. and Eng. Chem.)

CELLS AND TISSUES 9

to form a torus. The bordered pit is found chiefly in the walls of
the vascular elements of secondary xylem; and, where these abut
parenchymatous cells, the pit may be bordered in the wall of the
vascular element and simple in the wall of the parenchymatous
cell, thus forming a half-bordered pit. Other types of secondary
thickening found in vascular elements, including the annular,
spiral, scalariform, and reticulate types, are described under xylem
tissue.

Structure of the Cell Wall. — The detailed structure of the
cell wall has been investigated by a number of methods, including
X-ray analysis, differential staining, chemical and ash analyses,
polarized light, microscopic observation of both treated and
untreated materials, and hydration and swelling of the wall with
various reagents. As a result, several concepts of the wall struc-
ture have been proposed, which are here summarized.

The oldest idea, and one which still receives widespread sup-
port, is the hypothesis proposed by Nageli (30). According to
this concept, the cellulose portion of the wall consists of elongated,
crystalline, submicroscopic units known as micelles., which are sepa-
rated from one another by colloidal materials. On the basis of
X-ray analysis, it is thought that the micelle consists of cellulose
groups made up of parallel chains of glucose residues to which vary-
ing lengths and numbers of glucose units have been assigned by dif-
ferent investigators.

Sponsler (41, 41) has proposed a second idea of the structure
of the wall in which the cellulose units form a three-dimensional
space lattice; and has demonstrated that the molecules are arranged
in parallel layers, some of which extend lengthwise of the fibers
while others are at right angles or otherwise oriented at definitely
determined angles. The X-ray patterns and other data indicate
"that the units are attached in chains of indefinite length"; and
the lattice concept of wall structure does not necessitate the assump-
tion of "the existence of micellae as structural units of the cell wall,
that is, large units made up of many molecules, such as Nageli
proposed."

A third theory presupposes the existence of larger, visible
cellulose units, which have been called by various names: fibrils,
cellulose units, ellipsoid bodies, dermatosomes, etc. This idea
has been supported by Liidtke (2.8), who described the units as
being enclosed by thin films of non-cellulose cementing material

lo THE STRUCTURE OF ECONOMIC PLANTS

which may vary in chemical constitution in different walls. Farr
and Eckerson (17) state that the ellipsoid bodies in Gossypium are
jacketed by a pectic cement, and Farr and Sisson (18) suggest that
these visible units satisfactorily account for the X-ray patterns
that have been ascribed to the existence of micellar units. This
view is not supported by Bailey and Kerr (5), who object to the
size of the ellipsoid bodies on the ground that they would exceed
the width of more than four lamellae in the cotton fiber. Anderson
and Kerr Ql) raise additional objections relative to the cementing
substances of pectic material which are described as surrounding
the ellipsoidal particles, the migration of these particles into the
wall through the selectivity of the permeable membrane, and the
orientation of the particles in the wall.

Chemistry of the Cell Wall. — The chemical composition of
the cell wall varies with the age of the cell and with the type into
which it ultimately differentiates. In the initial stages of develop-
ment, the middle lamella is made up chiefly of pectose with small
amounts of cellulose. As the cell attains its full size the pectose
rapidly forms an insoluble pectate, commonly calcium pectate,
thus making the wall more firm and resistant. The continued
deposition of pectose and increasing amounts of cellulose, as well
as some hemicelluloses, results in the formation of the primary
wall, while the secondary wall is at first composed almost entirely
of cellulose and some hemicelluloses, but no appreciable amount of
pectic compounds. In parenchymatous cells, this condition may
obtain throughout the life of the cell; but, in other tissues, the
wall may be further modified by lignification, cutinization, or
suberization.

In lignification, the lignin, which is an amorphous substance,
first appears in the middle lamella; and, although it has been sug-
gested that the pectic substances of this zone are converted into
lignin, the investigations of Kerr and Bailey (15) and of Harlow
(xx) indicate that they are still present even in heavily lignified
middle lamellae. The primary wall may be lignified and also the
secondary one, but it seems probable that there is more than one
type of lignin involved. The addition of lignin may increase the
mechanical strength of the cell wall without affecting its permea-
bility to any great extent; but, when the wall is infiltrated with
cutin or suberin, it becomes relatively impermeable to liquids or
gases.

CELLS AND TISSUES ii

Suberization occurs in connection with the maturation of the
walls of cork cells, phellem. Priestley and Woffenden (34) have
considered the causal factors in cork formation and report that
there is first a blocking of the parenchymatous surface, which
usually is accomplished by the deposition of suberin or cutin in the
presence of air. This is followed by an accumulation of sap at the
blocked parenchymatous surface and the development of an active
phellogen in the parenchyma.

In cutinization, the cutin may be deposited in all the walls
of the epidermal cell, or on the inner face of the outer epidermal
wall; but, most frequently, it is laid down on the outer surface of
this wall, forming a protective non-cellular membrane known as
the cuticle, which forms an impervious layer except where there are
stomata. The cuticle is variable in thickness, being well developed
on the epidermis of ripe fruits and the vegetative organs of many
xerophytic plants.

Other chemical changes may occur in the epidermal walls of
some seed coats. In Linum, they become mucilaginous, as a
result of modifications of the pectic substances and cellulose of the
wall; and their capacity to hold water is greatly increased. Under
certain circumstances, mineral substances, carbonates, oxalates, or
silicates are deposited in cell walls. Silicates occur in relatively
large quantities in the peripheral cells of the stems of the grasses,
and an extreme case of mineralization takes place in the develop-
ment of cystoliths in the epidermal cells of Cannabis.

TISSUES

Strictly defined, a tissue is a group of cells of common origin
having essentially the same structure and performing the same
functions. Usage has broadened the scope of the term, however,
so that now it is necessary to refer to simple tissues, which conform
to this definition; and complex tissues, aggregations of simple
tissues, that have come to be regarded as having structural or func-
tional unity. This extension of the term to include complex tis-
sues such as xylem and phloem is convenient in descriptive
anatomy, but it should be noted that they refer to vascular com-
plexes consisting, in some instances, of four or more distinct cell
types rather than a single one. The tissue concept is further com-
plicated by duplication and lack of uniformity in the usage of terms,
the nomenclature used depending upon whether the point of view is

12.

THE STRUCTURE OF ECONOMIC PLANTS

anatomical or physiological. For example, where the classifica-
tion is based upon function, conducting tissues have been called the
mestome and protective tissues the stereome. The mestome has been
further divided into the hadrome, conducting water and substances
in solution; and the leptome, comprised of food-conducting tissues.
Since these terms are not the morphological equivalents of such
designations as sclerenchyma, xylem, and phloem, their use in

^~^..

^^^£!U

Fig. 4. A quadrant of the root tip of corn illustrating the progressive differentiation of
meristematic cells to form vessel segments, parenchymatous cells, epidermis, and root cap.

anatomical descriptions may result in confusion, and they are not
used in the succeeding chapters.

Meristems. — All the potentialities for development which are
ultimately expressed in the mature seed plant reside in the fertil-
ized egg or Xjgote; and, in the embryo derived from it, these are
transmitted by the actively dividing nuclei of the cells which form
the terminal, intercalary , and lateral meristems of the axis. (Fig- 40
In developmental anatomy, tissues may be further classified as
being actively meristematic, potentially meristematic, or non-
meristematic. Active meristems are cell aggregates constituting

CELLS AND TISSUES 13

the genetit or formative tissue, in contrast to those which retain
this quality but seldom express it except under unusual circum-
stances (potential meristems), or which have no capacity for
further growth and differentiation (non-meristems). The cells in
meristematic regions are characterized by their approximate
isodiametric form, except for some of the cambial initials of
lateral meristems; the presence of a functional protoplast which is
capable of active nuclear and cellular division; and a primary wall
that may grow and enlarge or undergo reversible changes in thick-
ness. The wall is composed largely of cellulose and possesses
plasmodesma in varying degrees of frequency and aggregation.

Factors of Differentiation. — The tissues which form the
complex structure of the plant are derived from the meristems,
but the causes which underlie differentiation and the factors which
control the processes concerned in it are still incompletely under-
stood. Geneticists have provided a theoretical unit of heredity
in the gene^ and the transmission of heritable characters from one
cell generation to another through nuclear division has been shown
to be related to this remarkably regular and normally precise proc-
ess. This aids in the understanding of many of the phenomena of
vegetative reproduction, and variation in nuclear behavior may
explain some of the problems of mutation and variation in the
species; but it is yet to be determined why two adjacent cells of
common origin, and presumably with the same hereditary pattern,
should differentiate into elements that appear totally unlike each
other in structure and function. The factors which are responsible
for this differentiation may lie partly within the cell; but undoubt-
edly the mechanical, physical, and chemical stimuli of the sur-
rounding environment play an important part in the process.

In this connection, continued work with tissues and organs in
culture such as that initiated by Robbins (36), White (44), LaRue
(17), and Bonner (8) should increase our knowledge of the factors
involved in differentiation. Recent work by Went and Thimann
(43), Kraus and his coworkers (x6), Boysen-Jensen (10), and others
on phytohormones and their relation to growth and development
offers an additional avenue of information regarding this subject.

It seems probable that the positional relationships of cells
may play some part in the character of their differentiation.
Seeliger (39) points out in his study of sugar beets that "in the
development of supernumerary tissues it is not the morphological

14 THE STRUCTURE OF ECONOMIC PLANTS

origin of a cell but its topographic relation to the axis and the
neighboring tissues which determines its future." Priestley and
Swingle C33), in their work on vegetative propagation, reach some-
what the same conclusion.

Parenchyma — Potential Meristem. — The fundamental par-
enchyma occurring in various organs of the plant may be re-
garded as potentially meristematic in contrast to the active
meristems; and, as long as these cells retain their functional pro-
toplasts, they also have the capacity for further development and
differentiation which may or may not be expressed later. This
characteristic of parenchyma explains the origin of many of the
anomalous or adventitious structures that arise in the later develop-
ment of the plant body. Some of the parenchymatous cells may
achieve maturity without further differentiation except for an
increase in cell size and wall thickness. Such cells form the funda-
mental ground tissue which constitutes, at least in part, the pith,
cortex, and rays of the stem axis; the cortical region of the root
and the pith when present; and the mesophyll. In the leaf, the
parenchyma is often characterized by the presence of chloroplasts
and is referred to as chlorenchyma; but, in addition to the photo-
synthetic function, these cells store food for short or relatively
long periods.

Where no potentialities are expressed, the parenchymatous cell
may maintain itself as such throughout the major portion of the
life of the plant; and, in some woody perennials, the cells of the
ray tissue remain alive for many years. In other cases, they grad-
ually become necrotic; or they may disintegrate early in ontogeny
so that lysigenous or schizogenous cavities occur. The hollow
medullary cavity frequently found in mature herbaceous stems such
as Triticum and Cucurbita, the large lacunae in the vascular bundles
of Zea, and those in the cortex of the stem of Pisum are formed
in this manner.

The Pericycle. — The pericycle is the peripheral layer of the
stele, and may be regarded as a potential meristem. In the early
ontogeny of the axis, it commonly consists of a single layer of
parenchymatous cells lying immediately centrad to the endo-
dermis. The pericyclic cells have no morphological characteristics
which distinguish them from other parenchymatous cells; and
were it not for their special relation to the stele and the variety of
ultimate cell types that may be derived from them, they would not

CELLS AND TISSUES

15

require separate consideration. In root axes, a very common devel-
opment is for the pericyclic cells to continue active division in
both radial and tangential planes so that the pericyclic zone
increases in width and maintains its continuity circumferentially as
secondary thickening occurs in the stele. The individual cells also
tend to enlarge tangentially and frequently serve for storage. In
stem axes, the pericyclic zone may also be broad; but, in the
absence of a clearly marked endodermis, its outer limits are often
difficult to define since the pericyclic cells may resemble struc-
turally those of the cortex or phloem.

Other tissues that frequently develop from the pericycle are
the protective periderm and fibers which are described in succeed-
ing sections under their appropriate functional classifications.
Another important pericyclic function is its capacity to produce
lateral and adventitious roots and shoots. Other less common
potentialities include the production of secondary cambiums
(Beta), the development of oil ducts (Apium and Pastinaca), and
the formation of glands, lactiferous cells, and canals.

PROTECTIVE AND ABSORPTIVE TISSUES

The superficial protective or absorptive tissues of the plant
may be differentiated early in ontogeny, as is the epidermis; or
they may arise later in development, as in the formation of cork
tissue or phellem.

The Epidermis. — The epidermis serves for protection, or, in
some regions, for absorption and secretion; and the entire outer
surface, with the exception of certain protected growing points, is
covered by a single layer of epidermal cells which is continuous
except where it is interrupted by stomatal or lenticular openings.
Although it is usually one layer in thickness, the cells may divide
periclinally to form a double or multiple layer as in Solanum tu-
berosum, where an ephemeral periderm develops from the epidermis
early in the ontogeny of the tuber. The epidermal cells may vary
greatly in size and shape; but they are typically flattened parallel
to the surface, and square or rectangular in transection with the
free surface exhibiting more or less convexity. In surface view,
they may be tabular or platelike; but frequently the walls are
undulating or sinuous, with lobes or projecting angles.

The cells contain cytoplasm and a nucleus, the former usually
occupying the periphery of the cell surrounding a large central

i6 THE STRUCTURE OF ECONOMIC PLANTS

in

B

vacuole. Chloroplasts are commonly absent except in specialized
guard cells. The walls may be unequally thickened, the outer one
most cases being the thickest, while the radial walls are fre-
quently thicker than the inner one.

The Stomata. — The distribution and num-
ber of stomata vary with the species, and also
with the organ of the plant. In general, they
are more abundant on leaves than on stems;
and, in the former, more numerous on the
adaxial surface than on the abaxial; although,
in some instances, as in the grasses, they occur
in approximately equal frequency on the two
surfaces. The stoma is a slit-like opening in
the epidermal layer which permits movement
of gases between the outer atmosphere and the
intercellular spaces of the mesophyll. It is
bounded by a pair of guard cells with concave
facing surfaces and contiguous end walls; and
the size and shape of the stomatal aperture is
determined by the guard cells, and accessory cells
when present, varying with changes in their
turgidity. The guard cells may have charac-
teristic wall thickenings that are correlated
with their turgor movements. In some cases,
the walls are uniformly thickened and the
Fig. 5. /I, surface view guard Cell appears round or elliptical in tran-
of the stoma of wheat section; but, more frequently, each guard cell

showing guard cells and , n 1 ■ 1 • r

accessory cells; B, median has two parallel ridges on its concave surface

transection of same; c, which form protruding flange-like margins

S^rnglrcrrj^fi Ordering the outer and inneV limits of the

cell. (Redrawn from Per- aperture. The outcr ridge is generally more

civai. m Wheat Plant, pj-Q^^inent than the inner, which may be

Duckworth and Co.; n

lacking. (Fig. 5, 5.)
The position of the guard cells in relation to the outer surface
of the epidermis is variable. In Allium, the level of the guard
cells does not equal that of the epidermis; and they are somewhat
depressed, forming a cavity which may be overarched by the
adjacent epidermal cells. In Ipomoea, Solanum, and Zea the
guard cells, though smaller than the epidermal cells, are so oriented
that they project slightly above the epidermal surface; while

CELLS AND TISSUES 17

in Lactuca, the guard cells and epidermal cells are equal in
height.

In the development of a stoma, the dermatogen cell which is to
produce it usually divides into two unequal daughter cells, one of
which becomes the initial cell of the stoma and the other an epider-
mal cell. Then the initial cell, which may be designated as the
stomatal mother cell, is bisected by an anticlinal wall, the two
daughter cells becoming the guard cells. This usually occurs in
young epidermal cells shortly after they have been cut off by the
dermatogen; but the formation of stomata in a given region does
not occur simultaneously, and a progressive series of develop-
mental stages may frequently lie in close proximity to one another.

De Bary (7) has described three distinct types of stomatal
development, which are here briefly summarized. In type (i),
as in Allium, the initial cell is the stomatal mother cell which divides
to form the two guard cells, their opposing walls separating schi-
zogenously to form the stomatal opening. In type (x), as in the
first, the initial cell is the mother cell of the stoma; but, shortly
after it is differentiated, accessory cells are cut off from the adjacent
epidermal cells and run nearly parallel to the guard cells. This type
occurs in most Gramineae, including Zea and Triticum. (Fig. 5.)
In type (3), the initial cell is not the mother cell of the stoma, but
undergoes one to several successive divisions which result in the
formation of the mother cell and one or more accessory cells. The
manner in which the initial cell is divided and the new cell walls
are laid down is variable; but a common sequence is found in
Raphanus, where the new anticlinal walls are formed successively
in three directions and cut off a series of accessory cells which
surround the mother cell. Other examples of this type occur in
Apium, Ipomoea, and many of the Solanaceae. (Fig. 6.)

Epidermal Hairs. — The epidermal cells may form directly or
give rise by division to various types of absorptive, protective,
glandular, or non-glandular hairs. These may be conical and
pointed, globular and capitate, unicellular or multicellular, and
unbranched or variously branched.

Root Hairs. — In root tips, the epidermal cells immediately
above the zone of elongation develop as root hairs by the extension
of the outer wall to form an elongated tube. The hair first appears
as a rounded protuberance involving the whole or a part of the
outer wall; and, by rapid elongation, develops into a unicellular

i8

THE STRUCTURE OF ECONOMIC PLANTS

tubular structure with thin cellulose walls. The cytoplasm forms
a thin layer immediately within the wall, and the nucleus may
frequently be located in the extended portion of the cell. With
few exceptions, the hairs are short-lived, soon collapse, and the
basal portion may become suberized or lignified. New hairs are
formed continuously so that a definite zone of root hairs is main-
tained directly back of the elongating portion of the root axis.

Fig. 6. A, a portion of the epidermis of the celery petiole showing a stage in the develop-
ment of the stoma; B, the epidermis of the blade showing stomata, guard and accessory
cells: ac, accessory cells; wc, mother cell.

Shoot Hairs. — The numerous epidermal hairs produced on the
stem, leaf, and inflorescence are variable as to form and structure.
In many instances, they appear to have little functional significance,
but the abundant production of hairs on the exposed surfaces of
aerial organs does result in a pubescence which may be protective.
Characteristic glandular hairs are developed in Cannabis and
Lycopersicum ; and, in Lactuca, stigmatic hairs occur which are
related to the process of pollination. Many plants, including
Cucurbita and Solanum, have multicellular hairs which may or may
not be capitate, and unicellular hairs are common in Medicago and
Cannabis. A type of special economic interest is the fibrous hair
which develops from cells of the epidermis of the ovule in Gossyp-
ium. (Chapter XIV, Fig. liy.)

CELLS AND TISSUES 19

The Periderm. — In both root and stem, there may be derived
from the pericycle a phellogen or cork cambium whose derivatives
mature as a periderm that serves as a protective tissue following the
loss of the epidermis and cortex. Normally, the cells of the
phellogen, derived from the outer pericyclic cells, undergo tangen-
tial divisions similar to those of a true cambium, differentiating as
phellem (cork cells) centrifugally, and phellodermal cells, centripe-
tally; but the production of phellem and phelloderm is not neces-
sarily reciprocal. Frequently, the number of layers of cork cells
cut off greatly exceeds the production of phellodermal layers;
and, in some cases, the activity of the phellogen may be unilateral
so that only cork cells are produced. The phellodermal cells and
the parenchymatous cells of the pericycle centrad to them form a
zone which has unfortunately been referred to as a secondary cortex
because of its functional and structural similarity to the true cortex.
This terminology is misleading, since it implies a cortical rather
than a stelar origin of these tissues; and, where such a zone is
formed, it is less confusing to denote it as a pericyclic region, or as
secondary parenchyma in cases where the pericyclic and phloem
tissues cannot be accurately delimited.

Although the phellogen is commonly derived from pericyclic
tissue, this is not always the case. In the young tuber of Solanum
an ephemeral phellogen may arise in the epidermis which is soon
replaced by one of cortical origin. In herbaceous and young woody
stems, the initial phellogen very frequently forms in the subepider-
mal layers of cortical parenchyma; while, in older woody axes,
the cork cambium develops from secondary phloem parenchyma.

The Endodermis. — The functional significance of the endodermis
in roots is a matter involving much difference of opinion, and
several theories have been advanced which have some experi-
mental confirmation. The presence of the Casparian strip makes
the radial and end walls relatively impervious to the diffusion of
water. This suggests that the endodermis may act as a controlling
factor in the lateral diffusion of substances between the cortex
and stele, since it restricts the passage of material to the protoplast
with its semi-permeable and limiting membranes rather than
affording free diffusion through the walls. The presence oi passage
cells in cases where the endodermis develops to the tertiary stage
may be regarded as supporting evidence for this theory. (Fig. 7.)
Priestley and North (31) have proposed that the endodermis may

xo THE STRUCTURE OF ECONOMIC PLANTS

also serve to maintain pressure relations in the stele and prevent the
accumulation of air in the conductive tissues. Schwendener (37)
has proposed that the primary function is that of mechanical sup-
port and protection to the stele when the outer cortical cells are
lost. The presence of abundant quantities of starch in the endo-
dermis of the stem in some plants suggests that storage may be an
important function. In Linum, Cucurbita, and others, the endo-
dermal layer may be involved in the initial stages of lateral root
development, forming a part of the root cap.

The endodermis is a relatively restricted tissue constituting
the inner limit of the cortex and lying adjacent to the stele. It

Fig. 7. Transection of a young primary root of potato showing Casparian thickenings
on the endodermal walls: co, cortex; en, endodermis; mx, metaxylem; pel, pericycle; ph i,
primary phloem; px, protoxylem.

is usually a single layer in thickness, but an exception has long
been known in the root of Brassica oleracea, reported by Woronin
(46), in which a double endodermis may develop. Other excep-
tions occur in Lactuca, in which single cells or sectors of the
endodermis may undergo tangential division; and in Cynara Scoly-
mus, where it becomes two-layered early in ontogeny.

The endodermal cells form a compact layer which is uninterrupted
by intercellular spaces. In transection, they are four-sided, oval,
or elliptical, and more or less extended in the tangential direction;
while, in the vertical plane, they are elongated with end walls that
are usually transverse. The cells retain their protoplasts and
possess all the potentialities of typical parenchyma, including the
capacity for cell growth and division. The nucleus is prominent,
and the cytoplasm frequently contains starch grains in such large

CELLS AND TISSUES zi

numbers, as compared with the adjacent pericyclic and cortical
cells, that the term starch sheath has been applied to the layer. In
addition to starch, other substances such as tannin, mucilage, and
occasionally crystals may be present.

The structure of the cell wall is the outstanding feature of this
tissue in cases where Casparian thickenings are laid down. The
endodermal cells are at first uniformly thin-walled and paren-
chymatous with no characteristics which might distinguish them
from adjacent cells. In the early development of the root, thicken-
ings are laid down on the radial and end walls which consist of
strips of wall material deposited on the primary wall. The strips
are suberized or cutinized and as seen in transection (in this view
referred to as Casparian dots), may appear to consist of a central
core of cellulose and pectic substances covered by a layer of suberin
or cutin. It is also possible that the water-proofing substances
may infiltrate into the primary wall to some degree; but, in any
event, the effect is to allow water and solutes to pass through the
protoplasts and tangential walls of the endodermal cells, and to
greatly restrict the movement through the radial and end walls.
The width of the strip is variable, being very narrow in some cases
so that in transection it appears as a rounded or oval dot; while
in others it may equal the width of the whole radial wall, which
then appears in transectional view to be more or less spindle-
shaped. At this stage, according to Eames and MacDaniels (15),
the tangential walls consist of relatively thin cellulose with few,
if any, pits. Simple pits which extend through the Casparian strip
when it is wide occur in the radial walls and there are some pits
in the end walls.

The endodermis frequently remains in this secondary stage of
development without further modification; but there may be a
tertiary phase in which additional secondary wall thickening
occurs involving the inner tangential, radial, and end walls and
occasionally the outer tangential wall. These walls are sometimes
much thickened and lignified, the size of the cell cavity being very
greatly reduced. In roots where tertiary development occurs, cer-
tain cells usually opposite the protoxylem points may remain in the
secondary condition and are referred to as passage or transfusion
cells. In this stage, the endodermis has no further capacity for divi-
sion and growth; and, if the axis continues to enlarge, its cells are
ultimately ruptured, together with the structures outside it.

Z7. THE STRUCTURE OF ECONOMIC PLANTS

An endodermis of this type, in either the secondary or tertiary
stage, is frequently found in roots, underground stems, and the
stems of hydrophytic plants. In the aerial portions of the axis of
herbaceous plants, the endodermis generally remains in the primary
condition without developing Casparian strips; and, if it differs
from the adjacent tissues at all, it is in the greater starch content of
its cells, in their shape or size, and in the uninterrupted continuity
of the layer.

MECHANICAL TISSUES

The tissues which give strength, and those that provide tough-
ness combined with elasticity and flexibility, constitute the me-
chanical elements of the plant body. These include collenchyma,
non-sclerotic fibers, sclerenchyma, and the lignified elements of the
vascular system. The sclerenchyma may consist of elongated
sclerotic fibers or approximately isodiametric stone cells.

CoLLENCHYMA. — This simple tissue has many characteristics of
parenchyma and may be regarded as a derived form of it. The cells
retain their protoplasts at maturity, are capable of further division,
and differ from parenchyma chiefly in the manner in which the
walls are thickened. The cells are axially elongated, rounded, or
more frequently angular, and four- to six-sided in transection, with
end walls that are straight, oblique, or tapering. The elastic
walls have simple pits, are readily permeable to water, and are
composed chiefly of cellulose although there may be some lignifica-
tion in old cells. The wall thickenings are laid down in longi-
tudinal strips which are commonly located at the angles of the
cells, but the tangential walls, and less frequently the radial walls,
may be thickened. (Fig. X4i.)

In the stems and petioles of herbaceous plants, collenchyma
forms discrete zones or continuous bands of mechanical tissue
immediately beneath or near the epidermis. It also occurs in the
blade of the leaf, forming strands that parallel the midrib above
and sometimes below it. Differentiation of collenchyma begins
while the organs are still developing, and it is ordinarily the first
type of mechanical tissue to mature in the ontogeny of the stem
axis. This may be correlated with the fact that collenchymatous
cells retain their capacity to divide and grow which is an essential
characteristic for mechanical tissues located in regions that are
undergoing elongation.

CELLS AND TISSUES Z3

The Fibers. — Fibers occur in the xylem, phloem, pericyclic,
and cortical regions of the axis. On the basis of wall structure,
they may be divided into non-sclerotic and sclerenchymatous types,
the latter occurring chiefly in the xylem, although the fibers in
any zone may become lignified. The non-sclerotic fibers cannot
readily be distinguished from one another structurally, and are
therefore designated on the basis of their position in the axis and
their relation to other tissues. They are sometimes referred to as
"bast" fibers in contrast to the wood fibers of the xylem, but the
term has been so loosely used that it has little value in descriptive
anatomy. It is more accurate to indicate the fiber type by pre-
fixing the name of the region in which it is differentiated, i.e.,
cortical fiber, phloem fiber, and pericyclic fiber.

NoN-scLEROTic FiBERs. — Frequently in stems, and not uncom-
monly in roots, non-sclerotic fibers are differentiated in the peri-
cyclic zone. These may form a continuous cylinder of mechanical
tissue, a series of pericyclic masses lying outside the phloem of
the vascular bundles, or occur as strands in a discontinuous cylinder.
Scattered groups of fibers may also be differentiated in the phloem
region and less frequently in the cortex as in Pisum. In this type,
the walls are non-lignified, or very slightly so, and consist chiefly of
cellulose. Where the proportion of cellulose is high (Linum) the
fiber has remarkable tensile strength and durability coupled with
flexibility and elasticity; but, in fibers where some lignification
occurs (Cannabis) these latter properties are less pronounced and
they are more brittle.

In ontogeny, the fiber initial grows rapidly in the axial direction,
attaining a length which may be several to many times the original
dimension of the cell. In hemp, the average is 3.5 to 4 cm., and
fibers 10 cm. in length have been reported; while, in flax, the aver-
age is from 1.5 to 3 cm. The extreme length of the fiber is due to
the manner of its development. The cell grows without under-
going transverse division as frequently as the adjacent cells, and
elongation may continue without cross-wall formation as long as
the portion of the axis in which the cell is located continues to
grow in length. In addition to this method of fiber elongation,
there is probably some sliding action by which the tips of the elon-
gating cells push past one another, thus adding greatly to the
mechanical strength of the tissue. This has been termed " gliding
growth"; and, although the mechanism of the process is not

2-4

THE STRUCTURE OF ECONOMIC PLANTS

clearly understood, it seems likely that it may be accomplished by a
splitting or modification of the middle lamella (intercellular sub-
stance) which permits the ends of the separated elements to slip
past one another.

Wood Fibers. — The wood or xylem fiber is an elongated sclerotic
element with tapered ends, which gives mechanical strength and

. .. Jl'JiJ

Fig. 8. Tracheae, tracheids, wood-fibers, and xylem parenchyma of a dicotyledon with
transition forms between the various elements. Diagrammatic. (From Strasburger, Textbook
of Botany, Fifth English Ed.)

rigidity to the axis. They vary greatly in form, size, and the
chemical properties of the wall; but are usually angular in transec-
tion, with thick, heavily lignified walls in which there are bordered
pits that may become occluded as the wall thickens. (Fig. 8.)
In the development of the wood fiber, the thin-walled, paren-
chymatous fiber-element grows rapidly, attaining full size before
lignification and consequent hardening of the wall occurs. The
continued deposition of wall substance results in a reduction in the
size of the lumen of the cell, and the protoplast finally dies, leaving

CELLS AND TISSUES X5

a narrow canal which may be occupied in part by products resulting
from the disintegration of the protoplast.

There are intermediate types, representing intergrades between
parenchyma and sclerenchyma, in which the elements have rela-
tively thick walls, simple pits, and retain their protoplasts. Such
forms are designated as substitute fibers. Another intergrading form
is the fiber tracheid, which exhibits some of the characteristics of
the tracheid but functions primarily as a mechanical element.

Stone Cells. — A second type of sclerenchyma is the stone cell,
which is frequently short and approximately isodiametric but may
vary greatly in shape. The stone cells occur in compact or loosely
organized groups as in the fleshy parenchyma of the pear, Pyrus
communis. They are also common in the hard outer coats of seeds
and fruits, as well as in the bark and pericyclic regions of woody
stems. There is no sharp distinction between the fiber and the
stone cell, and intermediate types of all sizes occur, so that it would
be possible to arrange a complete series of sclerenchymatous ele-
ments ranging from the isodiametric to the elongated form. The
wall of the stone cell is thick, highly lignified, and may be much
stratified with clearly distinguishable lamellae. Canals, which
are usually branched, occur in the walls, and the continued addition
of materials thickens them until, finally, the lumen becomes very
small and may be almost completely occluded.

CONDUCTIVE TISSUES

The conductive elements of the vascular plant can be divided
into two principal categories, xylem and phloem, each of which is a
complex tissue consisting of an aggregate of four or more cell
types. There is also much general conduction through all paren-
chymatous tissues, especially that of the ray.

The Xylem. — The xylem may include tracheids, vessels (tracheae^,
fibers, and parenchyma; and frequently contains ducts or glands of
various types. Developmentally, it is classified as primary xylem,
derived directly from the primary meristem; and secondary xylem,
differentiated from the cambium. The primary xylem is further
divided into the protoxylem, which is the first to differentiate and
mature at a given locus, and the metaxylem, which usually matures
later. (Figs. 8, 9.)

The spatial relationship of the proto- and metaxylem is impor-
tant in anatomical diagnosis; and, in the development of the pri-

x6

THE STRUCTURE OF ECONOMIC PLANTS

mary xylem, this is indicated by the terms exarch, mesarch, or end-
arch. In exarch development, the protoxylem is differentiated at
or near the periphery of the stele, usually abutting the pericycle,
and the metaxylem is laid down centripetally with respect to it.
In the mesarch type, the differentiation of metaxylem proceeds both
centripetally and centrifugally from the protoxylem so that it forms
a zone on either side of the protoxylem and may completely sur-
round it. In the endarch relation, the protoxylem is differentiated

a^ b

/ g h

i j k I m n 0

'HI

Fig. 9. Longisection of the stem of the parsnip through a bundle: a, parenchyma of the
pith; i, epithelial cells of a medullary oil duct; c, mechanical tissue; ^, protoxylem; <?, meta-
xylem; /, secondary xylem vessels and parenchyma; g, cambium; /', sieve tubes, companion
cells, and parenchyma; /, pericyclic fibers; ;, endodermis; k, cortical parenchyma; /, cortical
oil duct and its epithelial cells; m, coUenchyma fibers; «, collenchymatous parenchyma;
0, epidermis.

at or near the center of the axis and the metaxylem is laid down cen-
trifugally. These terms are only used in reference to primary
xylem and are not applied to secondary xylem, which is usually
laid down centripetally by the cambium.

The Tracheids. — The tracheid is regarded as the basic and,
phylogenetically, the most primitive cell type in the xylem tissue.
It is angular or roughly four-sided in transection and much elon-
gated with tapered, pointed, or wedge-shaped ends. It is com-
monly small in transection with a reLitively large lumen that is
devoid of protoplasm at maturity; and the walls, which are

CELLS AND TISSUES -l-j

variously thickened and reinforced, are abundantly pitted, espe-
cially at the ends where the cells interlock with one another. The
pits are bordered, although in some cases the borders are so narrow
that the pits appear to be simple. Thus a vertical series of tra-
cheids forms a conducting strand through which water may move
freely.

The character of the secondary wall determines the type of the
conducting element. In addition to fitted forms, annular, spiral,
scalariform, and reticulate elements may be differentiated in the
primary xylem as a result of the deposition of secondary wall mate-
rials in various patterns. (Fig. 8.) The annular and spiral types
are most commonly found in the protoxylem. In the former, there
is a series of separate ring-like reinforcements, the spacing between
adjacent rings depending in part upon the rate and degree of matur-
ity of the element, the rings being farthest apart in the most rapidly
growing annular elements because of the vertical elongation of the
primary wall between them. In the spiral element, the bands of
the secondary wall are arranged in a compact or loosely coiled
spiral, the latter condition occurring in protoxylem elements which
are differentiated before the elongation of the axis has been com-
pleted. It is not uncommon to find transitional forms between
these two types in which both spiral and annular thickenings occur.

The scalariform and reticulate elements usually develop in the
metaxylem. In the former, the secondary thickenings are laid
down in a close series in a ladder-like pattern. The reticulate
element has more secondary wall thickening than the scalariform,
and the mesh of the network is much finer. Because the proportion
of secondary wall material to total wall area is much greater, the
scalariform and reticulate elements are more rigid than the annular
and spiral types. This affords a good example of structural and
functional correlation, since the annular and spiral elements of
the protoxylem are commonly differentiated before complete
elongation of the axis is accomplished; while the more rigid
scalariform and reticulate elements of the metaxylem are usually
formed after this has occurred.

The Vessels. — The vessel consists of a vertical series of cells
(vessel segments') in which the adjacent end walls are perforated
in various ways so that a continuous tube is formed. Frost (lo,
ii), in discussing the origin of the vessel, points out that it is
clearly "a series of individual cells, properly called vessel elements

i8 THE STRUCTURE OF ECONOMIC PLANTS

or vessel segments, which are joined end to end with perforate
division walls. " The vessel segment may be distinguished from a
tracheid in two important respects: (i) it has a definite end wall;
and Ql) this end wall is perforated in some manner. The perfora-
tions may be scalariform-porous, oblique-porous, or transverse-
porous; and this sequence represents in a general way the probable
phylogenetic order in which the perforations of the end walls of
the vessel segments have evolved. The situation in herbaceous
dicotyledons is in accord with this generalization; for, in most
cases, the vessel segments of the secondary xylem are relatively
short, with transverse end walls and perforations of the transverse-
porous type. In many angiosperms, the terminal perforation
consists of a single large opening; and the end wall may be so
completely resorbed that there is no evidence of it except for a
slightly projecting ring between adjacent vessel segments. (Fig.

lo, C.)

The vessel segment may be narrow and elongated, but frequently
the cells are broader than tall. The former condition is common in
the loose spiral and annular vessels of the protoxylem, while vessels
which are differentiated and matured after the elongation of the
axis is complete may have broad segments that are short, cylindri-
cal, and barrel-shaped. Bailey and Tupper (6), in a comprehensive
series of measurements of tracheids and vessel segments, found that
the former are usually considerably longer than the latter; and
that there is a definite correlation between vessel segment length
and the character of the perforations. In ±-/6 species of dicotyle-
dons the average vessel segment length was 0.57 mm., while the
average length of the tracheids in 146 species of conifers was 3.64
mm. In X74 species of dicotyledons, 168 had transverse-porous
perforations; and 138 were 0.3 to 0.6 mm. or less in length. They
found further that vessel segments are shorter in the more slowly
growing plants; but noted that length is also dependent upon the
length of the cambial mother cell and the amount of elongation
that the vessel segment undergoes during differentiation. The
lateral walls of the vessel segment correspond to those described
for the tracheid; and there may be spiral, annular, scalariform,
reticulate, and pitted vessel segments.

The Ontogeny of the Vessel. — There are several ways in
which a vessel may develop, the most common one being that
described by Fames and MacDaniels (15), in which the vessel

CELLS AND TISSUES

19

segments forming the vertical series enlarge in diameter without
much increase in length. Until the full size of the vessel is at-
tained, the segments are separated by definite end walls which may
lay down some secondary wall thickening. (Fig. lo.) When
the segments have attained full size and shape, the dissolution of
the end wall begins; and, in the transverse-porous type, the entire
end wall disappears, so that a continuous tube results. In Apium,
Esau (i6) found that the ontogeny of the vessel agrees in general

Fig. io. A-C, successive stages in the ontogeny of the vessel in Cucurbita showing the
disintegration of the end walls of the vessel segments.

detail with that given above, except that no deposition of second-
ary wall material occurs on the end walls. The wall thickening
appears to be primary and of a peculiar lenticular type somewhat
similar to the torus formed in some bordered pits. (See Fig. 2.44.)
Priestley and his coworkers (31) have described the perforation
of the end walls of several species of woody angiosperms as occur-
ring early in the ontogeny of the vessel segment and prior to the
initiation of secondary wall thickening; but note that "pectin
films are often left stretched across the perforated ends of the vessel
segments." They point out that because of these films there
is no sudden intermingling of the contents of adjacent vessel seg-
ments. Scott (38) has described a coenocytic development of

30 THE STRUCTURE OF ECONOMIC PLANTS

spiral elements in Ricinus communis, in which multinucleate cells
are formed from uninucleate procambial cells. After attaining a
maximum elongation, a fine spiral of cellulose is laid down
throughout the entire length of the vessel; and the development
of the spiral and its initial lignification proceed rapidly, keeping
pace with the deposition of cellulose. During lignification, the
protoplasm of the developing vessel does not appear to disintegrate.

In the later stages of axial development, the thin-walled pro-
toxylem elements may be ruptured owing to the stretching and
subsequent collapse of the walls. This frequently occurs with
annular elements as in the bundle of Zea, where the disintegration
of the protoxylem and adjacent parenchyma results in the formation
of a large lacuna. In reticulate and pitted vessels, there may be a
penetration of the lumen of the vessel segment by the cytoplasm of
adjacent parenchymatous cells. This frequently occurs in Cucur-
bita and Solanum and results in the formation of tyloses which may
plug the vessel passage and render it functionless except for the
support afforded by its walls. (Fig. 318.)

Xylem Fibers. — The structural details of fibers have been
discussed, but there is a special relationship existing between the
xylem fiber and the tracheid, since both are constituent elements of
the complex xylem tissue. The xylem fiber may be regarded as a
tracheid which has undergone changes in the course of phylogeny
so that it is non-functional as a conductive element, but has in-
creased properties for mechanical ^support. Because of this, no
sharp line can be drawn between xylem fibers and tracheids; and
there are intermediate types known as fiber-tracheids. Some of
these may have a limited conductive function in addition to their
mechanical qualities, since their pits permit some movement of
water and substances in solution.

Xylem Parenchyma. — The parenchymatous cells of the xylem
which form an important constituent in the xylem complex,
especially in many herbaceous angiosperms, do not differ in general
characteristics from those described in the section on parenchyma.
They are usually arranged in longitudinal rows and may be elon-
gated in the axial direction, except in the xylem rays, where their
radial dimension is the greatest. The walls are relatively thin at
first, but frequently become thickened and lignified later. The pits
are simple in the walls of adjacent parenchymatous cells, while
half-bordered pits occur where they abut vessels or tracheids.

CELLS AND TISSUES 31

In fleshy storage organs, the proportion of xylem parenchyma
may be very large as in the root and hypocotyl of Raphanus and
Beta; and, in such cases, the cells may remain active and greatly
increase in number. In woody axes, xylem parenchyma is limited
or may be totally lacking except in the xylem rays. Where second-
ary thickening occurs, parenchymatous cells cut off by the cambium
constitute the xylem rays, which may be one to several cell layers in
width. The ray appears in a radial longisection as a sheet of tissue,
and its height may vary from few to many cells. In herbaceous
plants, the xylem ray cells are parenchymatous; but, in woody
ones, they may become sclerenchymatous and lignified, eventually
forming the most resistant portions of the wood. Unlike the
medullary rays, the xylem rays may have no direct contact with the
pith, but the latter are in intimate contact with the vertically
oriented elements of the stele, functioning in transverse conduction,
and serving as a storage region, especially in semi-woody biennials
and perennials.

The Phloem. — Like the xylem, the phloem is a complex vas-
cular tissue of the stele. It may include sieve tubes, companion cells,
fibers, parenchyma, and secretory cells; but, frequently, one or more of
these elements are lacking.

Sieve Tubes. — The sieve tube is the principal conductive unit of
the phloem; but parenchyma is important in this respect, especially
in the protophloem, in which sieve tubes may not be differentiated
and the elements consist of slender, elongated, parenchymatous
cells. Structurally, the sieve tube is a vertical series of elongated,
thin-walled cells which are interconnected by perforations in their
walls occurring in areas known as sieve plates. The end walls may
be transverse, oblique, tapered, or beveled; and the sieve plates
occur in them, or the lateral walls, or both. Although many
angiosperms have lateral plates, there is commonly a single trans-
verse one which is perforated by numerous pores through which the
protoplasmic strands extend as continuations of the cytoplasm of
the sieve tube segments. It has no nucleus at maturity, but may
contain minute starch grains and plastids. Slime bodies have been
observed in the cytoplasm of the sieve tubes of many plants, in-
cluding Beta, Cucurbita, and Solanum; and from the proteinaceous
contents of the cytoplasm and cell sap, slime plugs may form which
penetrate the pores of the sieve plate.

Hill (x3) and others have described the protoplasmic strand as

31 THE STRUCTURE OF ECONOMIC PLANTS

enclosing a slime string of proteinaceous material and being itself
surrounded by a callus sheath lying between it and the margin of the
perforation. Crafts in a series of papers (li, 13, 14) reports that
the protoplasmic strands are probably solid, that they do not con-
tain pores through which slime strings extend; and suggests that
the "only continuous permeable phase throughout the plant is the
cell wall," which "is highly hydrated" in the phloem. He also
states that the permeability of the sieve tube increases as the ele-
ment matures ; and that "it seems possible that movement may take
place partly through the sieve-tube lumina and partly through the
phloem walls."

A development which usually occurs late in the growing season,
or late in the ontogeny of the individual sieve tube, is the formation
of callus on the sieve plate. This at first surrounds the perforation,
later may completely cover it; and, in some cases, continues to be
deposited until a large mass of callus is formed. Later, it may be
dissolved off with a consequent resumption of the activity of the
sieve tube; but, in many instances, it forms a permanent layer
which terminates the conductive function of the element.

Companion Cells. — The companion cell is so named because of
its intimate structural and probable functional relationship to the
sieve tube. In the ontogeny of the sieve tube and companion cell,
the two elements are derived from a common mother cell of the
procambial strand in primary phloem, or from a phloem mother cell
derived from the cambium in secondary phloem. A longitudinal
division of the phloem mother cell divides it into two daughter
cells of unequal size, the larger becoming the sieve tube and the
smaller the companion cell. The latter contains a very dense pro-
toplast with a distinct nucleus; and is thin-walled, elongated, and
triangular, square, or somewhat rounded in transection. The
walls adjacent to the sieve tube have numerous simple pits, and
there are a smaller number of them in the walls that are in contact
with parenchymatous cells. In many herbaceous angiosperms, the
length of the companion cell is equal to that of its sieve tube; but
the original companion cell may be divided transversely into several
shorter cells, so that each sieve tube lies adjacent to a vertical row
of two, three, or more short companion cells.

There is variation with respect to the arrangement of the sieve
tubes and companion cells. In some instances, the pattern is so
regular that the phloem appears in transection as an almost perfect

CELLS AND TISSUES 33

mosaic, and the relationship between sieve tube and companion
cell can be readily determined. On the other hand, there may be
much irregularity; and, where there is little difference in the size
and shape of the two elements, it is difficult to distinguish one from
the other except in longisections where the sieve plates can be
observed.

The function of the companion cell is not definitely known; but
the fact that the sieve tube and companion cell are intimately con-
nected by plasmodesma, and that the sieve tube lacks an organized
nucleus while the companion cell has a conspicuous one, suggests
that the companion cell is concerned in some way with the con-
ductive function of the sieve tube.

Phloem Parenchyma. — The parenchymatous cells of the
phloem do not differ histologically from those described under the
section devoted to parenchyma, but they may have an important
conductive function. This is especially true in protophloem in
which sieve tubes are frequently absent and the parenchymatous
cells constitute the chief conducting elements. These cells are
short-lived and are usually crushed and resorbed as the axis en-
larges. The metaphloem also contains some phloem parenchyma,
but sieve tubes and companion cells are differentiated.

In the secondary phloem, the proportionate number of parenchy-
matous cells, sieve tubes, and companion cells is variable, the
former being quite abundant in many Compositae and almost com-
pletely lacking in some species of Ranunculaceae. Where phloem
parenchyma is derived from the cambium, the resultant cells are
frequently elongated and tapered, and have been termed cambijorm
parenchyma. In some instances, transverse divisions of these
elongated elements result in the formation of a linear series of
relatively short isodiametric cells.

Ray parenchyma of the secondary phloem is also cut off by the
cambium and the phloem rays often lie on the same radii as the
secondary xylem rays, appearing as continuations of the latter out-
side the cambial ring. At its outer limit, the ray may be funnel-
shaped where it widens, owing to continued growth and division
of the parenchymatous cells comprising it. The ray cells are
commonly elongated in the radial direction, and may remain active
over long periods, sometimes through the life of the plant, provid-
ing a means of lateral conduction and also a place for storage of
food.

34

THE STRUCTURE OF ECONOMIC PLANTS

Phloem Fibers. — Both primary and secondary phloem fibers
occur, which may be non-sclerotic or sclerenchymatous. They
are usually arranged in groups or zones, frequently bounding the
periphery of the phloem. They also occur as bands which alternate
with sieve tubes and companion cells; and, in other cases, they are
scattered singly or in small groups throughout the tissue. The
thick walls may be somewhat lignified at maturity, differing from
those of the xylem fiber chiefly in the presence of simple rather
than bordered pits.

SECRETORY TISSUES

Glands. — The gland may consist of a single secretory cell, such
as the glandular hair in Cannabis and Solanum; or several cells
may form a glandular organ or nectary. These cell aggregates
differ in their method of formation and in degree of complexity.
Frequently, the gland has a central cavity in which the secreted
substance is stored. The cavity may be formed schizogenously by
the splitting apart of the walls of adjacent cells and the subsequent
formation of a duct by the growth and realignment of the bordering
cells. Ducts of this type are termed schizpgenous, and a common
example occurs in the primary oil ducts of the Umbelliferae (Apium
and Pastinaca). (Fig. ii.) In other cases, the formation of the
gland or duct may result from the destruction or solution of groups
of cells, thus forming a cavity or tube, surrounded by other cells

Fig. II. A, sector of the pericycle of the parsnip root showing an initial stage in the
formation of the oil ducts; B, a later stage in which the oil ducts have been formed; C, a
section of the primary phloem showing the phloem oil duct: c pri od, central primary oil duct;
en, endodermis; ept, epithelial cells; pel, pericycle; ph, phloem; ph od, phloem oil duct;
pri od, primary oil duct.

CELLS AND TISSUES 35

which may remain active or also become dissolved. This type is
referred to as lysigenous. Lysigenous glands are conspicuous in the
leaves and other structures of Gossypium, and form the oil sacs of
the citrus fruits.

Digestive glands occur in many plants as highly specialized
structures or as layers of secretory tissue, especially in fruits and
seeds which secrete enzymes that digest reserve foods. This is
the case in the grain of Zea, where the cotyledon (scutellum) of
the embryo is bounded by a layer of digestive cells which forms
epithelial glands; and comparable digestive regions occur in the
akene of Lactuca.

The Nectaries. — In plants depending upon insects for pollina-
tion, nectaries are commonly present. The secretion of nectar is a
function of somewhat papillate epidermal cells which vary in their
degree of organization and specialization. These cells are not al-
ways limited to specific regions constituting a nectary; and may be
unspecialized, in which case the secreting cells resemble other
epidermal cells except that they have no cuticle. Although necta-
ries are ordinarily associated with floral structures, extra-floral
nectaries may occur as in Gossypium, where they develop on the
midrib of the leaf and at the bases of the involucral bracts. (See
Fig. X14.)

Lactiferous Cells and Ducts. — A special type of secretory
tissue, characterized by the presence of latex, occurs in several
plant families. This substance is a complex emulsion that varies
greatly with respect to its chemical constituents. Among other
substances, it may contain sugars, oils, proteins, tannins, alka-
loids, and gums; and, in addition, starch grains and salts may be
present. The viscous fluid is usually milky white (Lactuca and
Ipomoea); but it may be golden brown (Cannabis), or less fre-
quently yellow (Argemone).

In the Euphorbiaceae, Asclepidaceae, and Apocynaceae, latex
may occur in cells which develop in the embryo and finally form a
much branched system of tubes. These are morphologically
equivalent to a single cell initial which keeps pace with the growth
of the apical tissue of the plant. A different type of lactiferous
cell is found in the Liliaceae; and, in Allium, these appear early
in ontogeny in the cortical region of the leaf. They are arranged
in longitudinal rows parallel to the epidermis and are intercon-
nected by pits. (See Fig. 91.) A unique form occurs in some spe-

36 THE STRUCTURE OF ECONOMIC PLANTS

cies of the Moraceae; and, in Cannabis, they develop as un-
branched, unicellular vessels which are multinucleate. The vessels
originate in young leaves at the tips of the vegetative axis, and
develop in the periphery of the primary phloem, attaining an
extraordinary length in some instances. Latex vessels of the
articulated type in which a longitudinal series of elongated cells
coalesce to form a continuous tube occur in Ipomoea and Lactuca.
In the latter, the ducts at first consist of a longitudinal series of
cells with definite end walls; but, later in ontogeny, they become
continuous non-septate passages, and frequent cross-anastomoses
result in a system of interconnecting canals. (See Fig. 334.) In
both Ipomoea and Lactuca, they develop primarily in the phloem
and pericyclic tissues; but, in the latter, they also occur in the
cortical region of the axis and the mesophyll of the leaf.

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38 THE STRUCTURE OF ECONOMIC PLANTS

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CHAPTER II

THE ANATOMY OF THE ROOT

THE root is one of the primary organs of the vascular plant.
It is typically subterranean, serving in the mechanical support
of the aerial portions of the axis, in the absorption and conduction
of water and mineral nutrients, and in the storage of reserve foods.
In addition to these primary functions, several additional ones may
be performed in specific cases, such as the development of adventi-
tious shoots from the fleshy root, the peculiar properties of sup-
port and absorption afforded by the aerial roots of certain orchids,
and the special nutritional functions performed by many of the
roots of the Leguminosae in the symbiotic relations which exist
between them and the nitrogen-fixing bacteria of the soil.

Upon germination of the seed, the primary root typically grows
directly downward, penetrates the soil rapidly, and becomes the
initial absorbing and anchoring organ. Other lateral roots may
develop in rapid succession. In some plants, especially the
Gramineae, embryonic or adventitious roots from the stem are
present in the embryo, so that the seminal root system may con-
sist of a main tap root and several laterals. When the primary
root continues to develop and forms a conspicuous structure from
which numerous laterals diverge, it is referred to as a tap root
whether slender or fleshy. If the primary root and its laterals
develop more or less in equal degree and are relatively numerous,
the root system \s fibrous. In some cases, the primary root system
persists only for a relatively short time, and the permanent roots
arise adventitiously in a succession of whorls from the lower nodes
of the stem.

Intergrades exist between the several types, and it is not uncom-
mon for a plant to develop a deeply penetrating tap root together
with a mass of fibrous surface laterals. Differences in soil texture
and water content, and variations in methods of cultivation often
affect to a very considerable degree the general configuration and

39

40 THE STRUCTURE OF ECONOMIC PLANTS

development of roots. Weaver (13) and Weaver and Bruner (14)
have described and discussed the roots of the principal field and
vegetable crops in connection v^ith these points.

Gross Morphology. — The root axis differs in certain anatomi-
cal respects from that of the stem so that generally it can be dis-
tinguished from the latter on the basis of its structure. A
representative root in which the differentiation of primary tissues is
complete has three w^ell-defined regions, epdermis, cortex, and
stele. The epidermis consists of a single layer of thin-walled cells
which are devoid of cuticle and have the potentialities to form
root hairs by a lateral extension of their outer walls. The number
and character of development of root hairs depend upon the
species involved and the soil conditions. Centrad to the epidermis
is the cortical region, consisting of several layers of parenchyma-
tous cells that are limited internally by a uniseriate endodermis
which is regarded as a part of the cortex. The endodermal cells
have Casparian thickenings that are usually more clearly defined
than those of the endodermis of most stems. At maturity, the
inner tangential, radial, and end walls of some of the endodermal
cells may be appreciably thickened; while others, which lie
approximately outside the protoxylem points, may have their
walls thickened to a lesser degree.

The primary root of dicotyledons typically has a radial proto-
stele; or, less frequently, a radial siphonostele. The former con-
sists of a central mass of primary xylem tissue with two to several
radiating arms extending centrifugally to the pericycle. The lat-
ter constitutes the outermost layer of the stele, and lies immedi-
ately adjacent to the endodermis. At the time of maturation of
the primary tissues of the stele, the pericycle is commonly a single
layer of cells which form a continuous cylinder; but, in some in-
stances, it may be interrupted by the protoxylem cells which di-
rectly abut the endodermis. In some roots, the pericyclic cells be-
come active and undergo division prior to the complete maturation
of the vascular tissues, and this frequently happens where there
is an early differentiation of lateral roots.

Between the radiating arms of xylem are located the two to
several groups of primary phloem; and, between the xylem and
phloem, there may be a zone of parenchymatous cells from which
a cambium may be differentiated. The number of protoxylem
strands is fairly constant for a given species, but some variation

THE ANATOMY OF THE ROOT

41

may occur, and it is not uncommon for a plant to have a different
number of points in the primary root as compared with its laterals.
The most common types are the diarch (two protoxylem points),
represented by the primary roots of Beta, Solanum, and Raphanus
(Fig. II.); the triarch, found in Pisum and Medicago (Fig. 13);
and the tetrach, in Cucurbita, Gossypium, and Ipomoea. (Fig.

Fig. IX. Transection of the primary root of Raphanus showing the diarch primary xylem
strand prior to secondary thickening of the axis. Outside the right-hand protoxylem point,
the pericycle has divided tangentially, initiating lateral root formation.

3 10.) Pentarch, hexarch, septarch, and octarch types occur; and, in the
Gramineae, there may be a larger number of arcs. In the radial
siphonostele, the number of xylem arcs is usually considerably
greater than found in radial protosteles; and they are commonly
designated as being polyarch. (Fig. 14.) The number of arcs in
the primary root of corn may be xo or more; and, in secondary
roots, twice that number frequently occur.

In some of the older literature dealing with the stelar anatomy
of the root, the term axile bundle has been used to designate the pro-

42-

THE STRUCTURE OF ECONOMIC PLANTS

tostele of the root, as well as the single vascular strand in the stem
of certain hydrophytic dicotyledons; and the term monostele has also
been applied as a synonym for protostele. The former term arose
prior to the development of the stelar theory, and the latter has
been largely superseded by the term -protostele. In radial siphono-
steles, the adjacent groups of xylem and phloem occurring on
separate radii have been termed radial bundles. Inasmuch as the

Fig. 13. Transection of the primary root of Pisum showing the triarch primary xylem
strand and the development of phloem fibers adjacent to the pericycle. Outside the upper-
most protoxylem point a pericyclic ceil has divided tangentially, initiating lateral root forma-
tion.

primary xylem and phloem tissues are definitely separated by non-
vascular tissue, it is probably more accurate to reserve the term
bundle for those cases in which the two tissues arise from a single
provascular strand and to designate the relation of the primary
xylem and the phloem in the root as a radial arrangement.

Ontogeny of the Primary Root. — The primary root, or at
least the apical growing point, is developed during the early stages
of embryogeny. The degree of development attained prior to
germination varies greatly, ranging from an undifferentiated zone
of meristem terminating a short conical hypocotyl (Cucurbita), to

THE ANATOMY OF THE ROOT

43

a well-organized root with a root cap and partially differentiated
stele (Zea and Lactuca). Even in the former instance, in the very
early stages of germination, the peripheral cells of the meristem
divide rapidly; and, before the emergence of the root tip from the
seed coat, a root cap (calyptra) is organized.

The root cap persists throughout the life of the root. As growth
and development proceed, it is reduced outwardly by the abrasion
and disintegration of the outer cells as the root penetrates the soil,
and is renewed from within by the addition of new cells from the
meristem. Thus, it maintains a relatively uniform size. In a

Fig. 14. A sector of the polyarch stele of the corn root showing the thick-walled endo-
dermis with passage cells, the pericycle, protoxylem points with thin-walled phloem elements
lying on alternate radii to them, the large metaxylem vessels surrounded by thick-walled
connective tissue, and the pith.

median longisection of a root tip, it appears as a more or less conical
cap several layers thick at its apex which decreases in thickness
laterally.

After considerable growth and elongation, the primary root is
divisible into several regions, in addition to the root cap and the
growing point it surrounds. Directly back of the growing point is
a zone known as the region of elongation, in which the newly formed
cells are increasing in size, chiefly by the elongation of the com-
ponent cells in the axial direction. This region is relatively short
and merges insensibly into what may be termed the Zpne of differ-
entiation, in which many of the cells may continue to enlarge and
some still divide, but the majority begin to develop the special
characteristics of the several tissues which constitute the primary

44 THE STRUCTURE OF ECONOMIC PLANTS

regions of the root axis. Further back is the region of maturation ,
in which the primary tissues attain the characteristics which they
possess at maturity. The designation of regions of elongation,
differentiation, maturation, and maturity in describing the ontog-
eny of the root axis is one largely of convenience, and there are
of course no sharp boundaries delimiting these zones. There may
be continued cell division in certain tissues of the developing axis,
while the adjacent cells of some other tissue are compensating for
this by cellular enlargement. In addition, some cells, even in the
mature region, retain meristematic potentialities which may or
may not later be expressed.

The Histogens. — The primary meristem at the tip of the root,
where cell division is going on most actively, may itself be dif-
ferentiated into regions. These are known as histogens; and, in
ontogeny, the cells derived directly from them can be specifically
related to the mature regions of the root. Hanstein (5) designated
three of these regions as the dermatogen, peribletn, and pleromej and,
later, Janczewski (6) added a fourth, the calyptrogen. The latter
term is applied when the root cap is derived from and maintained
by the activity of a definite layer of meristematic cells distal to the
other cells of the growing point. In cases where it is not possible
to separate the meristematic cells that give rise to the root cap from
others in the growing point, no specific designation is made except
when the epidermis and root cap arise from a common layer. In
this instance, the term dermatogen-calyptrogen has been applied.

The dermatogen, when distinct from the calyptrogen, consists
of a single layer of cells underlying the latter, which perpetuates
itself and produces epidermal cells by continued anticlinal divi-
sions. The periblem is a generative layer inside the dermatogen
which gives rise to the tissues of the cortex. In most cases, this
histogen is one layer in thickness at the apex of the root; and,
like the other histogens, it maintains itself by anticlinal divisions.
The lateral cells of the periblem divide periclinally, so that the
zone becomes several layered; and, subsequently, divisions of the
derivative cells in three planes result in the formation of the cortical
parenchyma and endodermis. The innermost histogen or plerome
produces the stelar elements many of which are elongated; and the
derivative cells divide transversely less frequently than do those of
the cortex. This difference in the mode of cell division in the two
adjacent regions derived from the periblem and plerome results in

THE ANATOMY OF THE ROOT

45

a blocking off of the cortex and stele on the basis of cell shape;
and, in many cases, it is possible to delimit them very early in
ontogeny.

The degree to which the meristem is specialized into the four
histogens is variable. Janczewski (6) has recognized four princi-
pal types of such meristematic differentiation in angiosperms. In
type (i), the meristem is definitely and sharply differentiated into

Fig. 15. Longisection of the apex of the corn root showing the histogens: a, the plerome;

b, periblem which gives rise by periclinal divisions to the epidermis, d, and the cortical cells, e;

c, the calyptrogen which produces the root cap.

the four histogens, plerome, periblem, dermatogen, and calyptro-
gen, from whose respective derivative cells the stele, cortex, epi-
dermis, and root cap are developed. This type is an exceptional
one, and does not occur in any of the economic plants later de-
scribed. It is reported for a few hydrophytic monocotyledons in
the family Hydrocharitaceae (Pistia and Hydrocharis).

At the apex of the root in type (x), the plerome and calyptrogen
are sharply defined, and there is an intermediate layer between them

46

THE STRUCTURE OF ECONOMIC PLANTS

which is one cell layer in thickness. The lateral members of this
intermediate zone divide periclinally, and the derivatives of the
inner cells function as a periblem producing the cortical tissue,
while the outer daughter cells become the dermatogen and form the
epidermis. This is the type found in many monocotyledons in-
cluding Zea and Triticum. (Fig. 15.) Treub (11) has described
a modification of type (i) that applies to monocotyledons in which

a definite calyptrogen is not
diff"erentiated. In this case,
the root cap, epidermis, and
cortex arise from a common
group of initials two cell
layers in thickness, and
within this zone is a sharply
defined plerome. In this
modified type (i) are mem-
bers of the Liliaceae, includ-
ing the genus Allium.

In type (3), the plerome
and periblem are sharply
defined; and outside the lat-
ter is a common initial layer
which produces the root cap
and epidermis. The cells of

Fig. 16. Longisection of the apex of the root ^uj^ i.^^- nerDetuate the his-

of Linum showing the character of the terminal ^"^^ ^^Y^^ perpetuate tne niS

meristem: a and b, root cap and epidermis re- tOgen by anticlinal divisionS,

spectively derived from the calyptrogen-der- ^^^ periclinal divisionS at
matogen layer; c, endodermis; a, stele, derived ^

from the plerome; e, pericycle limiting the itS apeX form SUCCeSsive

stele; /, cortex, derived from the two-layered j^yetS of the rOOt Cap, the

periblem; g, epidermis. (After Janczewski, Ann. . ^ 111 c

Set. Nat.) innermost cells always func-

tioning as the common in-
itial layer. The cells of the common layer, which form the
proximal margin, undergo a final periclinal division; and
the inner daughter cells function as a dermatogen producing
the epidermis by anticlinal divisions, while the outer ones are
added to the root cap. Thus, according to Janczewski (6), the
dermatogen and root cap are derived from the calyptrogen
layer; but Crooks (3) and others, recognizing the dual character
of the histogen, have designated it as the dermafogen-calyptrogen
layer. In most cases reported as representing this type, the

THE ANATOMY OF THE ROOT

47

periblem at the root apex consists of an initial cell or a single
layer of a few cells which later becomes several layered by peri-
clinal divisions. A variation occurs in Linum in which the peri-
blem consists of two layers of cells overlying the plerome. The
outermost layer divides only anticlinally to form the outer layer
of the cortex or hypodermis, and the inner layer divides in all planes
to form the remainder of the cortex. Type (3) is probably the most
common one in dicotyledons; and occurs, among others, in Rapha-
nus, Ipomoea, Lactuca,
and Linum. (Fig. 16.)

In type (4) there is a
common meristematic
zone which extends
across the apex of the
root, and sharply de-
limited histogens are
lacking. The peripheral
cells of this .terminal
meristem enlarge and
mature as the root cap.
The stele and cortex
arise from derivatives of
the centrally located
cells, and the epidermis
is differentiated from the
laterally placed ones.
This type is frequently
referred to as the open
type meristem. The Cu-
curbitaceae (Cucurbita)

and Leguminosae (Pisum, Phaseolus, and Medicago) are represen-
tative of this type. (Fig. 17.)

Irrespective of the type of meristematic organization, growth
and differentiation occur in varying degrees; and there is an early
blocking out of the epidermal, cortical, and stelar regions. In
ontogeny, the cells of each primary tissue soon reach maturity, and
the stelar pattern is laid down conforming to one of the protoste-
lic or siphonostelic types noted above.

The dynamic aspect of root development is clear when successive
transections of a root are studied through the regions of elongation

Fig. 17. Longisection of the apex of the root of Pisum
showing the character of the terminal meristem : a, epi-
dermis; b, pericycle limiting the stele; c, correx; d,
root cap; e, meristem. (After Janczewski, Ann. Set.

Nat.')

48 THE STRUCTURE OF ECONOMIC PLANTS

and maturation and up to the point where the primary tissues are
mature. On the basis of such an examination, it is evident that
the differentiation and maturation of the several types of cells do
not proceed with equal rapidity. Various stages of differentiation
of the xylem may be observed at a given level owing to the fact that
the development of the primary xylem is a progressive process in
which the protoxylem elements are the first to mature, followed
later by those of the metaxylem. Similarly, there is a progressive
differentiation and maturation of the primary phloem; and the
appearance of well-defined sieve tubes and companion cells follows
the initial differentiation of slender parenchymatous protophloem
elements. Like the stelar elements, the endodermal tissue under-
goes a succession of stages leading to maturation, passing through
primary, secondary, and tertiary phases of wall thickening as
described in the preceding chapter. The degree of epidermal dif-
ferentiation may be gauged by the origin of the root hairs, which
reach a maximum production in the region of maturation. Many
subsequently die and disintegrate as the root hair zone is extended
or additional ones develop.

Secondary Thickening. — In many roots, especially those
which are fleshy, increase in the diameter of the axis is accom-
plished by secondary thickening. This is usually initiated upon
the completion of the maturation of the primary stelar tissues, but
may begin before this process is entirely completed. The first
stages of secondary thickening occur in the zone of fundamental
parenchyma which lies between the primary xylem and phloem
strands. At points centrad to the primary phloem strands, the
parenchymatous cells begin to function as a cambium and divide
tangentially and radially. The derived cells differentiate, matur-
ing as secondary phloem elements centrifugally, and secondary
xylem elements centripetally.

From the points of initial cambial activity, there is a progressive
tangential development; and the zones of cambium are extended
laterally until they reach the points where the protoxylem strands
abut the pericycle. In this way, a continuous meristematic cylin-
der is formed, including the sectors of the pericycle from which the
broad parenchymatous rays are derived. (Fig. i8.) The manner
in which this occurs results in a rounding out of the xylem portion
of the stele. The first secondary xylem elements derived from the
cambium are laid down in the angles between the protoxylem

THE ANATOMY OF THE ROOT

49

points, and the continued differentiation and maturation of second-
ary xylem ultimately smooths out the curve. In some cases, not
all of the fundamental parenchyma lying between the primary
xylem and phloem is involved in this cambial activity and there is
a narrow zone of this tissue remaining between the primary and
secondary xylem.

The elements which constitute the secondary tissues of the stele
are derived from the cambium in such a way that at first they tend
to lie in regular radial rows; wx^
but these may become some-
what irregular as growth and
maturation proceed. This
occurs in part because of the
unequal growth rate of the
elements, some of which
attain larger size than others.
In addition, the subsequent
divisions and growth of some
of the parenchymatous ele-
ments derived from the
cambium may result in dis-
placement and reorientation
of the secondary tissues.
(Fig. 19.) The secondary
xylem consists of tracheids,
vessels (tracheae), paren-
chyma, and fibers. The
secondary phloem commonly
includes sieve tubes, compan-
ion cells, parenchyma, and
occasionally fibers and secre-
tory tissue. The relative proportions of the constituent elements
in the xylem and phloem, especially with reference to the paren-
chymatous cells, varies strikingly with the species of plant; and,
where there is considerable secondary thickening, the amount of
storage parenchyma may be large.

The continued enlargement of the stele results in definite changes
in the outer portion of the axis. The primary phloem groups are
forced outward, and this tissue is usually crushed or digested. The
pericycle remains active and by radial divisions of its cells increases

Fig. 18. Transection of a portion of the root of
Brassica showing the diarch primary xylem
strand, the broad pericyclic ray, and the secondary
xylem.

5°

THE STRUCTURE OF ECONOMIC PLANTS

xy par

in circumference, so that it persists as a continuous zone outside of
the secondary phloem. Commonly, the pericyclic region also
increases in width, owing to successive tangential cell divisions;

and this results in the for-
mation of a few to many-
layered zone of active
pericyclic tissue. The epi-
dermis, cortical paren-
chyma, and endodermis
become fractured and dis-
integrate. As this occurs,
the outermost layers of
derivatives of the pericycle
form a phellogen (cork
cambium) which develops
a protective periderm.

In many fleshy roots the
pericycle may form a zone
of storage parenchyma;
and, in some cases, the
term secondary cortex has
been applied to this region.
Since it is clearly of stelar
origin, this terminology is
apt to be misleading; and
it is more accurate and

Fig. 19. Transection of a sector of the stelar Convenient tO USe the term

portion of an old root of Pastinaca showing the perky elk Xpne. In Very old,

primary xylem strand and the development of -^ , ■'

secondary xylem. The enlargement and division WOody rOOtS, especially of

of the parenchymatous cells have resulted in a dis- t5erennial t)lantS the tDCri-
persion of the secondary xylem vessels : >nx, metaxy- . 1 ' u 1

lem; par, interstitial parenchyma which has become CyCle may alSO De lOSt,

radially elongated; px, protoxylem; ra, pericyclic and, when this OCCUrS, a
ray; xy par, xylem parenchyma; xy 1, secondary ■ • 1

xylem. ' ' ^ ' ■" •' new protective periderm is

formed from a phellogen
arising in the tissues of the secondary phloem. While the
primary phloem and other tissues outside the cambium may be
lost, the primary xylem remains in place at the center of the axis.
Frequently, there is a proliferation of the cells of fundamental
parenchyma adjacent to the primary xylem strand, or separating
the metaxylem from the protoxylem, and this results, in a spreading

mx

— par

THE ANATOMY OF THE ROOT 51

and dispersion of the primary xylem elements so that the original
configuration of the protostele does not persist in the mature root
(Pastinaca). (Fig. 19.)

In addition to the usual methods of secondary thickening out-
lined above, certain anomalous cases occur in which the process is
modified by the supplementary activity of both primary and second-
ary tissues. Among economic plants, this occurs in Beta and Ipo-
moea. In the former, further thickening is accomplished by the
differentiation of secondary cambiums. These arise in rapid suc-
cession in the pericycle and phloem parenchyma, producing a series
of concentric rings of vascular and parenchymatous tissue which lie
outside the secondary xylem and phloem tissues formed by the
primary cambium. (Chap. IX.) In Ipomoea, secondary thicken-
ing involves the formation of secondary cambiums which surround
the protoxylem points. These may develop throughout the
central portion of the axis, forming numerous zones adjacent to the
secondary xylem elements or even occasionally in the secondary
phloem. (Chap. XVI.)

Lateral Root Formation. — In angiosperms, lateral roots usu-
ally arise in the pericyclic tissue at loci directly outside the proto-
xylem points although there are exceptions to this generalization.
They are differentiated early in the ontogeny of the axis, usually
prior to the initiation of secondary thickening. The first evidence
of lateral root formation consists in the radial enlargement of two
to several pericyclic cells adjacent to a protoxylem point. This is
followed by tangential and radial divisions of the lateral root
initials, and a conical growing point is formed. As a result of
further growth, the lateral root primordium enlarges and elongates,
forcing the endodermis and overlying cortical tissues forward so
that eventually they are ruptured or laterally displaced.

In some roots, as in many cucurbits, legumes, and flax, the endo-
dermal cells adjacent to the active pericyclic cells also enter into
the early stages of lateral root formation, and may contribute to the
formation of the peripheral cells of the newly formed growing
point. In flax, the endodermis forms a single layer of meristematic
cells overlying the root cap, and this remains active until the lateral
root has pushed through the cortex of the primary root. In such
cases, the pericyclic region differentiates the stelar portion of the
lateral axis, and the endodermis and adjacent cortical cells of the
primary root form the outer zones of the root primordium. A

52-

THE STRUCTURE OF ECONOMIC PLANTS

slightly different situation occurs in Zea and other monocotyledons,
in which the entire lateral root is formed from derivatives of the
pericycle, except that the endodermis constitutes an exterior layer
over the root cap.

In the organization of the growing point, the histogens are
differentiated in the same manner as described for the primary root,
and the type of ontogeny is similar to that of the axis from which
the lateral root arises, although the number of points of protoxylem
differentiated is not necessarily the same. The elongation of the
lateral root primordium proceeds with varying degrees of rapidity,

CO--

latrt--

FiG. xo. Longisection of the primary root of Pastinaca showing the origin of a lateral
root: CO, cortex; e«, endodermis; to r/^, lateral root; mx, metzxylem.

depending upon the plant and the environmental conditions affect-
ing it; but, eventually, the cells of the endodermis, cortex, and
epidermis of the main axis are either mechanically pushed aside
and crushed, or are resorbed by the adjacent tissues. The point of
origin of the lateral root is such that the cells of its stele are dif-
ferentiated directly in contact with the vascular elements of the
main axis; and, thus, the xylem and phloem of the lateral root may
be continuous with the corresponding tissues of the main axis, but
this is not true of the cortex and epidermis. (Fig. xo.)

In cases where secondary thickening of both the lateral and
primary axes occurs, the cambium of the primary root and that of
the lateral root may be continuous. As a result, successive addi-
tions of secondary vascular tissue to the two axes proceed simul-
taneously, and continuity of the vascular regions is maintained.

THE ANATOMY OF THE ROOT

53

Because of the definite manner in which the lateral roots originate
with respect to the protoxylem points of the primary stele, the
number of rows of lateral roots is commonly equal to, or twice the
number of, the protoxylem points, so that in a tetrarch root there
are commonly four vertical rows of laterals, in a triarch root,
three, etc. In diarch roots, the laterals are frequently initiated
at a slight angle to the protoxylem point, and for this reason in
roots such as Apium, Solanum, and Lycopersicum four rows of
laterals arise. In many grasses, the lateral roots are initiated at
points in the pericycle outside the primary phloem groups.

Adventitious Roots. — In addition to the development of
lateral roots from young root axes, they arise from other organs of
the plant, especially the stem. Such roots are called adventitious or
adventive. In the most restricted sense, the term is not ordinarily
applied to roots arising from other roots, even though they are
produced late in ontogeny and in the same manner as those which
arise from the stem. The term is applied, in a broader sense, to all
roots which do not arise from the pericycle and adjacent tissues in
typical acropetal succession at about the time when secondary
thickening of the axis has been initiated. On this basis, roots
formed late in ontogeny from the peripheral tissues of woody or
fleshy roots and those arising as a result of injury to a mature root
axis are regarded as adventitious.

The organization of the root meristem and the ontogeny of the
adventitious root are similar in all respects to that described for
the lateral root; but the point of origin of such roots deserves
further consideration. In general, the origin of adventitious
structures, including both roots and shoots, requires the presence
at the point of origin of potentially meristematic tissue. For this
reason, adventitious roots frequently arise from intercalary meri-
stems at the bases of internodes, from the meristematic tissue
which commonly occurs at the axils of leaves, from lateral meri-
stems such as the cambium and pericycle, from ray parenchyma;
and, less frequently, from the parenchymatous tissue of other
regions.

In the case of roots arising from stems, Priestley and Swingle
(9) have noted that the tissue from which the root originates is
determined in part by the age of the stem from which the adventi-
tious root arises. In young stems, they are usually pericyclic in
origin, arising from a group of cells rather than from a single cell.

54 THE STRUCTURE OF ECONOMIC PLANTS

Less frequently, they originate in phloem and ray parenchyma.
The organization of the root meristem may involve several layers
of procambial tissue in cases where the point of origin is not an
internodal region, or in proximity to a node which is still in the
meristematic condition and undergoing elongation. In older
stems, the adventitious roots frequently arise in the parenchyma-
tous ray tissue adjacent to the youngest secondary xylem and
phloem elements; and the root primordium may be regarded as
being derived from the ray or the interfascicular cambium. Some
roots may arise from the persistent pericycle under these conditions;
but the origin of roots from ray and secondary parenchyma derived
from the cambium is more common.

Adventitious roots frequently occur at or near the nodal regions
where the tissues are usually meristematic, at least in young stems;
but the production of roots is not restricted to the nodal zone and
they may develop at any point along the internode. This is illus-
trated in the production of adventitious roots in the first internode
of Zea, and the formation of internodal roots in Lycopersicum.
(Fig. II.) A special case of adventitious root development occurs
in some crucifers in which the origin of the root is exogenous in-
stead of endogenous, involving epidermal and cortical tissues rather
than stelar ones. This was demonstrated by Hansen (4) for
Cardamine. In this case, the roots arise only in the axils of the
leaves and are related to the axillary bud, forming a lateral out-
growth from it.

Adventitious roots may also arise from leaves, but this is less
common than their origin from stems. In the former, the root ini-
tials originate from parenchymatous tissues in close proximity to
the cambium of the vascular bundles. This occurs in the leaves of
Begonia and Bryophyllum. Crooks (3) demonstrated, in the
excised cotyledons of Linum, that adventitious roots originate
from a layer of parenchymatous cells on the adaxial side of the
veins. LaRue (7, 8) observed the rooting of excised cotyledons
of 41 plants belonging to 19 different families.

The whole problem of vegetative reproduction, including the
production of adventitious roots, is associated with the retention
of the meristematic capacity on the part of certain cells of the plant
body. Where this occurs, the potentialities for further growth
and differentiation of structures are limited only by the environ-
mental conditions and the external and internal stimuli that may

THE ANATOMY OF THE ROOT

55

be operative in any given case. The individual differences which
exist in plants with respect to their capacity to respond to stimulus
in the production of adventitious structures undoubtedly lie deeper
than the matter of tissue relations, and may be hereditary.

The importance of the potentiality in economic plants to pro-
duce adventitious roots and shoots can hardly be overestimated.

~>A ■-

'<''<X f ~A,

^^ ' h^.^

Fig. 11. The origin of an adventitious root in Zea.

Investigations by Robbins and coworkers (lo, ii), Cooper (x),
Zimmerman and Wilcoxon (15), Bonner (i), and many others of
methods of artificial stimulation in the production of adventitious
roots through the use of phytohormones and other growth-stimu-
lating substances have been productive of most encouraging results.

LITERATURE CITED

I. Bonner, J., "Vitamin Bi: A growth factor for higher plants." Science 8j:

183-184, 1937.
X. Cooper, W. C, "Hormones in relation to root formation on stem cuttings."

Plant Phys. 10: 789-794,1935.

3. Crooks, D. M., "Histological and regenerative studies on the flax seedling."

Bot. Gaz- 95- -LOS-XT,^, 1933.

4. Hansen, A., "Vergleichende Untersuchungen iiber Adventivbildungen bei

den Pflanzen." Abhandl. Senckenberg. Nat. Gesell. 12: (i47)-i98, 1881.

56 THE STRUCTURE OF ECONOMIC PLANTS

5. Hanstein, J., "Die Scheitelzellgruppe im Vegetationspunkt der Phanero-

gamen." Abhandl. Geb. Nat. Math. u. Med., 109-134, 1868.

6. Janczewski, E., "Recherches sur raccroissemcnt terminal des racines, dans

les Phanerogames." Ann. Set. Nat., Bot. Ser. V, 20: i6i-ioi, 1874.

7. LaRue, C. D., "Regeneration in mutilated seedlings." Proc. Nat. Acad. Set.

i9- 53-63' 1933-

8. , "Regeneration in monocotyledonous seedlings." Am. Jour. Bot. 22:

486-492., 1935.

9. Priestley, J. H., and Swingle, C. F., "Vegetative propagation from the

standpoint of plant anatomy." U. S. Dept. Agr. Tech. Bull, iji, 1919.

10. RoBBiNS, W. J., "Cultivation of excised root tips and stem tips under sterile

conditions." Bot. Gaz- 73- 376-390, i^zt.

11. , Bartley, Mary, and White, Virginia B., "Growth of fragments

of excised root tips." Bot. Gaz- 97: 554-579' ^91^-

11. Treub, M., "Le meristeme primitif de la racine dan les monocotyledones."

Musk Bot. de Leide. 2, 1875.
13. Weaver, J. E., Root Development of Field Crops, McGraw-Hill Book Co., 1916.
14. , and Bruner, W. E., Root Development of Vegetable Crops, McGraw-Hill

Book Co., 192.7.
15. Zimmerman, P. W., and Wilcoxon, F., "Several chemical growth substances

which cause initiation of roots and other responses in plants." Cont.

Boyce Thomp. Inst, j: 109-1x9, 1935.

CHAPTER III

THE ANATOMY OF THE SHOOT

GENERAL MORPHOLOGY OF THE STEM

THE stem and its appendages, foliage leaves and inflorescences, con-
stitute the shoot. Tlie stem is typically an aerial structure;
but, in some cases, it may be wholly or in part subterranean, pro-
ducing underground branches some of which may be greatly spe-
cialized for storage.

In general form, the aerial stem may be erect and unbranched,
bearing the leaves and flowers on a single vertical axis, or much
branched with several axes of approximately equal size so that it
develops a bushy crown. In a vine type, the stem may be prostrate
as in Ipomoea, twining as in species of Phaseolus, or climbing by
means of tendrils, Pisum and Cucurbita. In some plants, the stem
does not elongate greatly during its early development, and forms
a short conical crown of leaves. This type of growth may result
in the formation of a bulb if the leaves are fleshy (Allium); a head
(some species of Brassica and Lactuca), or a simple rosette (Beta
and Raphanus). Later, the stem or a portion of it may elongate
rapidly, forming an erect axis bearing foliage leaves and the
inflorescence.

The principal types of underground stems are the rhizpme, tuber,
and corm. The rhizome consists of a more or less elongated under-
ground stem which arises from a lateral bud near the base of the
main stem axis and extends horizontally through the soil. Al-
though it may have a superficial resemblance to a root, it has the
anatomical characters of a stem with respect to the internal organi-
zation of its tissues. Externally, it may be recognized as such by
its nodes and internodes, although the underground habit results
in the reduction of the leaves diverged from it so that they are
small and scale-like. Functionally, such stems are often storage
organs, and a means of vegetative extension. Aerial shoots fre-
quently develop from lateral buds of the rhizome, and adventitious

57

58 THE STRUCTURE OF ECONOMIC PLANTS

roots may also be produced abundantly. In Solanum tuberosum,
the rhizomes extend outward in the soil for several inches and then
the terminal portion begins to thicken, developing as a fleshy stor-
age organ, the tuber. This, like the rhizome itself, is a true stem
and has buds (eyes) in the axils of reduced leaves.

Closely related to the rhizome in general habit, and often con-
fused with it, is the stolon or runner. Like the rhizome, this
arises from a lateral bud near the base of the stem axis; but it
remains on the surface of the ground rather than being subter-
ranean. It develops adventitious roots and is a means of vegetative
extension and propagation as illustrated in the case of certain
stoloniferous grasses and the strawberry.

The corm is a type of underground stem in which the axis is
short, vertical, and fleshy. There is a tuft of aerial leaves above,
and a mass of fibrous adventitious roots below the stem, which is
generally sheathed by the leaf bases. Typical examples of this
structure occur in Gladiolus and in many of the Orchidaceae.

ANATOMY OF THE STEM

The stem differs from the root externally in the divergence from
it of lateral members, the leaves, at more or less regular intervals.
The places of such divergence are nodes; the intervals between them,
the internodes. Histologically, it is composed of tissues similar to
those in the root, the chief differences being the manner of dis-
tribution of the vascular tissue and the character and specialization
of the epidermal layer.

The Herbaceous Dicotyledonous Stem. — In most herbaceous
dicotyledonous stems the vascular system constitutes a dictyostele, or
dissected siphonostele. In such a stelar type, the vascular bundles
which comprise it occur as a number of distinct longitudinal
strands, so that in transection there is a ring of vascular bundles
that are separated from one another radially by rays or zones of
parenchymatous tissue. (Fig. ix.) Great variation exists with
respect to the amount of parenchymatous tissue in proportion to
the vascular tissues, either as cortex, pith, or rays. In some cases,
even the vascular regions possess large quantities of parenchyma.
The fact that parenchymatous tissues may be present in such very
large proportions is one of the principal characteristics which may
render a plant of economic value as food for animals and man. This
is true because, in such tissues, large quantities of foods may be

THE ANATOMY OF THE SHOOT

59

stored for shorter or longer periods, and because these tissues are
comparatively easily crushed or broken and the foods they contain
readily released.

The primary bundles of the stem are typically common bundles.
This term was first applied by Hanstein as referring to those bundles
that are common to both stem and leaf, in contrast to cauline
bundles, which originate in the stem and have no direct connection
with the leaves. The common bundle originates at or near the
base of the leaf primordium, and one extension of it forms a vein of

lb

"m^'^'-

Fig. il. Transection of a sector of a stem of Raphanus showing the vascular cylinder with
the bundles forming a dictyostele. The phloem is subtended by groups of pericyclic fibers
and the xylem is surrounded by thick-walled connective tissue. The ray parenchyma has
also become thick-walled and there is a limited development of interfascicular cambium.

the leaf while the other follows an approximately perpendicular
course down the stem axis. After traversing one or more inter-
nodes, it usually anastomoses with the common bundles of lower
leaves. Cauline bundles may be cross-connected with common
bundles by branches or by the development of secondary vascular
tissue.

The bundles which comprise the dictyostele are usually collateral,
although bicollateral bundles are not uncommon in certain plant
families. (Fig. X3.) In the collateral bundle, the xylem and
phloem lie on the same radius with the phloem external to the

6o THE STRUCTURE OF ECONOMIC PLANTS

xylem. In the bicollateral type, the bundle is organized like the
collateral one; but it develops an inner phloem as well as an outer
one so that the xylem lies between two primary phloem groups.
This type occurs in the Cucurbitaceae, Solanaceae, and Con-
volvulaceae; but there is some difference of opinion as to the
interpretation of the bundle in certain members of the last two
families.

The pericycle which lies outside the outer phloem is a continuous
zone of cells one to several layers in width. In some instances,
it can be clearly distinguished from adjacent tissues because of the
differentiation of a complete ring of pericyclic fibers; but, in other
stems, the fibers may be formed only over the phloem portion of
the bundles where they serve as protective caps. The intervening

A B c J)

Fig. 13. Types of vascular bundles: A, collateral; B, bicollateral; C, amphivasal; D,
amphicribral. The stippled areas represent phloem, the black ones, xylem.

sectors of the pericycle may remain parenchymatous so that they
cannot be separated on any histological basis from ray tissue; or,
late in ontogeny, they may mature as stone cells. Because of the
relationship of the pericyclic fibers to the vascular elements in
many plants, the term fibro-vascular bundle has been employed; but,
since fibers are not a constant feature of the vascular unit and in
many cases often occur as distinct and isolated strands, the term
vascular bundle is more commonly used.

Except in some underground stems and those of certain woody
plants, the endodermis is generally not as clearly defined as it is in
roots, since it often lacks characteristic Casparian strips. It may
be distinguished in some cases by the size and regularity of its
component cells, which are oval or rectangular in transection, by
the presence of starch and other substances in the lumen of the
cells, or by its characteristic physiological reactions. The cortical
region consists mainly of thin-walled parenchymatous cells, the
outermost layers sometimes containing chlorophyll. Frequently,

THE ANATOMY OF THE SHOOT

6i

there is a peripheral zone of collenchyma which may form a con-
tinuous, several-layered cylinder immediately adjacent to the
epidermis. In some cases it may be restricted to limited regions,
as in angular stems where strands of collenchyma form reinforcing
ridges and add greatly to the mechanical strength and pliability of
the plant as a whole.

The epidermis constitutes the outer protective layer of the axis
and is typically uniseriate. The cells of this tissue are compactly
organized into a continuous layer which is uninterrupted except
for stomatal openings. The outer and radial walls are usually

Fig. 14. Transection of a portion of a stem of Triticum showing the arrangement of
the bundles, the distribution of the chlorenchyma and mechanical tissue, and the hollow
center.

thicker than the inner ones, being cutinized or suberized; and
they may be further water-proofed by the deposition of a non-cellu-
lar cuticle. Epidermal hairs, prickles, thorns, and other emer-
gences may occur as noted in Chapter I.

The Herbaceous Monocotyledonous Stem. — The stem of the
herbaceous monocotyledon differs from that of the dicotyledon
chiefly in respect to the components of the vascular bundles, which
in most cases do not possess a cambium. The arrangement of the
bundles may conform to the dictyostele type but meristeles are
common. In Triticum, the stem bundles comprise a dictyostele
with a central medullary region which may be hollow; while, in
Zea, there is a meristele and the bundles are scattered through the
fundamental parenchyma of the axis. (Figs. ^4, 2.5.)

62 THE STRUCTURE OF ECONOMIC PLANTS

There is great diversity and complexity among monocotyle-
donous stems in regard to tlie course of the bundles of the meristele.
In a large number of them, including the palms and some grasses,
the bundles conform to the plan which is briefly summarized below.
In these plants, the leaf base is very broad at its point of divergence
and commonly encloses the greater part of the stem or completely
encircles it at the node. Each leaf has a very large number of
bundles (it may exceed loo in Zea), and these enter the stem at
the node, extend downward through several successive internodes,

Fig. 2.5. Transection of a sector of a stem of Zea showing the distribution of the bundles

and mechanical tissues.

and finally are united with the bundles of lower leaves. Because
of the regularity with which the bundles are anastomosed with
lower ones, and the uniformity of the distance that they extend
down the axis, approximately the same number of bundles may be
observed in transections at successive internodes.

Some of the bundles extend down the outer periphery of the
vascular zone, while others lie nearer the center of the stem and

THE ANATOMY OF THE SHOOT 63

constitute the inner bundles of the meristele. The latter are usually
the median or larger bundles of the leaves, while the smaller,
marginal bundles are those which occupy the peripheral portion of
the stele. The more crowded distribution of bundles in the outer
zone of the vascular region is due to the fact that the bundles,
except for the peripheral ones, do not describe a perpendicular
course in the stem axis, but are differentiated in a radially oblique
direction until they finally anastomose at lower levels with bundles
of the outer zone. Since a single bundle may continue through
several internodes before being united with another one, the effect
of intercalary elongation of the axis is to straighten out the curva-
ture, so that each bundle apparently describes a perpendicular
course in the mature internode. The insertion and anastomosis of
bundles with those lower in the axis occurs chiefly at the nodes,
where a very complex nodal plate is formed consisting of a large
number of transversely and otherwise oriented bundles.

The individual bundles which constitute the meristele vary in
type. The most common is the collateral or half-amphivasal
bundle, and less frequently a true amphivasal bundle is differenti-
ated. A third type is the amphicribral bundle which commonly
occurs in pteridophytes (Polypodium), but is rarely developed in
angiosperms. In the collateral bundle of the meristele, the vascu-
lar tissues are usually oriented so that the xylem is centrad to the
phloem; and, in amphivasal types, it more or less completely
encircles the phloem with outwardly projecting arms of metaxylem,
the development of the primary xylem being endarch. The
mechanical tissue associated with the bundles consists of fibers or
sclerotic cells which form a more or less complete ring around the
bundle (Zea and Triticum). In addition to this mechanical tissue,
the stem may have a zone of sclerenchyma immediately inside the
epidermis, which is several layers in thickness and consists of
small, compact, elongated elements.

Ontogeny of the Stem Axis. — The apical meristem is pro-
tected from injury by the developing leaves and the bud scales when
present. An unelongated axis of this kind with its leaves is called a
bud, and its terminal portion consists of meristematic cells from
which lateral members are differentiated exogenously. (Fig. i6.)
The ontogeny of the stem axis is complicated by the manner in
which these lateral members are initiated and differentiated, since
the stelar organization of the stem depends upon the development

64

THE STRUCTURE OF ECONOMIC PLANTS

of the vascular bundles of the leaves and branches which in the
aggregate constitute the stele.

The meristematic regions of the root and shoot are similar with
respect to the characteristics of the cells which comprise them, but
they differ markedly in other ways. One obvious difference is that
the root meristem is surrounded and protected by cells differentiated
from it which constitute the root cap, while the stem meristem is
exposed except for the protection afforded by the leaves diverged

Fig. i6. Median longisection of the stem tip of Linum showing several stages in the ontog-
eny of the leaf: a, section of a very young leaf primordium; h and c, older primordia showing
early development of provascular strands; /»/', pith. (After Crooks.)

THE ANATOMY OF THE SHOOT 65

from it. Another fundamental difference exists in the manner of
organization of the histogens and the way in which these respective
layers undergo cell division. In the stem apex, the surface layer or
dermatogen, as well as the one or two layers underlying it, divide
anticlinally with the result that they increase in area without
adding to the thickness of the axis. On the other hand, the more
centrally located cells divide in all planes; but the rate of division
is essentially the same in both cases, as demonstrated by Schiiepp
(35, 36). As Priestley and Swingle (30) have pointed out, the
addition of cells to both the superficial and central regions at the
same rate, coupled with the fact that the divisions of the dermato-
gen and adjacent layers are only anticlinal, while those of the inner
layers occur in all planes, results in the formation of folds by the
surface tissues, which become the initials of the new leaves. The
further development of leaf primordia is discussed in a later
section.

Deferring, for convenience in description, the discussion of the
formation of the lateral members, the ontogeny of the stem axis
may best be understood by considering the progressive development
of the stem as represented at successive levels from its apex to a
point where secondary thickening is initiated. At the apex, the
meristematic growing point consists of thin-walled parenchyma-
tous cells which are relatively small with a dense cytoplasm and
prominent nuclei. These cells divide actively, and there is no
evident differentiation among them except for changes in the shape
of the cells of the outer layer (dermatogen) which, because of their
position and internal pressure, may become tangentially elongated.
It is in this region that the superficial folds occur which are the
forerunners of leaf primordia.

Somewhat below this level, the differentiation of the provascular
strands occurs. In herbaceous dicotyledons, these form a pro-
cambial cylinder in which the strands consist of groups of small
cells that continue to divide while the adjacent ones increase in
size and become more vacuolate. As progressive development oc-
curs, the vascular cylinder becomes more sharply delimited owing
to the increased growth and vacuolation of the parenchymatous
tissue of the pith, cortex, and rays; and the appearance of larger
intercellular spaces in these regions. The cells of the procambial
strand continue to divide, and each strand may become somewhat
extended tangentially as a result of such meristematic activity.

66 THE STRUCTURE OF ECONOMIC PLANTS

In longisections, these cells can be distinguished from adjacent
parenchyma by their smaller caliber, greater length, and the density
of their protoplasts.

At the level where the differentiation of the vascular elements
begins, the endarch character of the xylem is indicated by the
formation of protoxylem elements on the centrad faces of the pro-
vascular strands. Simultaneously, or even slightly in advance of
the development of the protoxylem, the first protophloem elements
are differentiated on the outer face of the strand. The strands form
common bundles which are direct continuations of those of the
leaf. The point of their initial differentiation is at the level of
divergence of the leaf primordium (at a node); and, from this
point of initiation, the development of the bundle proceeds in two
directions. Outwardly, it is continued into the developing leaf as
a part of its vascular system; and, downwardly, it becomes a
component part of the vascular system of the stem, where it ulti-
mately is anastomosed with the vascular bundles of leaves diverged
at a lower level.

At a level where the differentiation of the primary tissue is
practically complete, the ring of vascular bundles is clearly de-
limited from the pith and cortex, and adjacent bundles are separated
from one another by medullary rays. At this stage, the collateral
bundle has an inner zone of endarch primary xylem, and an outer
one of primary phloem. Between the two, a region of meri-
stematic cells may remain undifferentiated, instead of maturing as
primary vascular tissue; and this may function later as a fascicular
cambium.

Secondary Thickening of the Stem Axis. — When the vascular
bundle is collateral with an intervening zone of meristematic cells
between the primary xylem and primary phloem, it is an open type.
By continued activity, this meristem or cambium separating the pri-
mary vascular elements may give rise to cells which mature as
secondary tissues.

In the formation of secondary vascular elements, the cambial
cell divides tangentially to form two daughter cells. One of these
remains meristematic while the other differentiates into either a
secondary xylem or phloem element, either directly or after a sub-
sequent division of the xylem or phloem mother cell. Such second-
ary division commonly occurs in the phloem where a new wall
divides the mother cell into two unequal daughter cells, one of

THE ANATOMY OF THE SHOOT 67

which becomes the sieve tube, the other the companion cell. In
general, a series of adjacent cambial cells divides simultaneously,
and the daughter cells on one face differentiate into xylem or
phloem respectively. The remaining daughter cells continue as
cambial cells; and in this manner the cambium is perpetuated.

A further effect of the simultaneous division of adjacent cambial
cells is that the elements formed from them lie in radial rows which
may be very regular in their alignment. Variation in the subse-
quent development of the vascular elements, together with division
of some of the derived cells, may disturb the radial symmetry of
the secondary tissues, this being especially true in herbaceous stems.
In addition to the tangential division of the cambial cells, divisions
occur in other planes so that the cambium maintains itself as a
continuous layer. Thus radial divisions take place which com-
pensate for the increase in the circumference in the axis as secondary
thickening proceeds. As Bailey (8) has pointed out, there are two
general types of cambial activity which account for the increase in
girth of the cambium. In pteridophytes, gymnosperms, and some
of the dicotyledons that are less specialized structurally,

"the anticlinal divisions are more or less transverse and the products
of these divisions elongate and crowd by one another, producing
thereby an increase in the girth of the cambium and a non-stratified
arrangement of its cells. In certain of the more highly differentiated
dicotyledons, on the other hand, the anticlinal divisions are radio-
longitudinal and the products of these divisions expand laterally,
thereby increasing the circumference of the cambium, but they do
not elongate to any considerable extent, and thus become symmetrically
grouped in parallel, horizontal series."

The cambium initials are not all of the same size in a given plant,
and Bailey (6, 7) has noted that the elongated fusiform initials
may vary greatly in their nucleo-cytoplasmic ratio, while this is
relatively constant in the smaller ray initials. The latter are much
shorter than the fusiform initials whose derivatives mature as
elongated vascular elements. With respect to the rhythm or
sequence of formation of secondary xylem and phloem elements by
the cambium, no specific statement can be made, except that the
process is not necessarily a reciprocal one in which there is an
alternate development of such elements. In general, the amount of
secondary xylem produced exceeds the secondary phloem, which
would mean that several successive layers of xylem mother cells

68 THE STRUCTURE OF ECONOMIC PLANTS

might originate from the cambium without the formation of a
layer of phloem initials.

In secondary thickening, the dissected siphonostele may become
a solid cylinder through the development in the ray tissue of an
interfascicular cambium which is continuous with the fascicular
cambium. This does not involve any movement or invasion of cells
into the ray region, but is usually accomplished by a progressive
activation of the parenchymatous cells immediately adjacent to the
cambium in a zone which may be regarded as potentially pro-
cambial. This may proceed until there is a cambium completely
across the rays. In other cases, the interfascicular cambium may
occur first at a median point in the ray where a small bundle of
secondary tissue is formed; and, later, by a lateral activation from
this point, as well as from the fascicular cambium of the primary
bundles, the cambial cylinder becomes complete.

There are varying degrees of development of interfascicular
cambium, ranging from cases where none is formed and the vascular
tissue remains as a dissected siphonostele at maturity, to those
where the cambial cylinder becomes continuous and a solid zone of
secondary vascular tissue is formed. The secondary tissues derived
from the interfascicular cambium are not always vascular; and,
especially in semi-herbaceous or semi-woody stems, the major
portion of such tissue may consist of non-vascular conjunctive or
connective tissue which by subsequent lignification affords added
mechanical support.

In most monocotyledons, there is no secondary thickening, and
enlargement of the axis is limited to the capacity of the primary
cells to grow in size, although there may be a limited amount of
division in the cells of the medullary and cortical parenchyma. In
many instances, the tissue of the provascular strand is completely
differentiated into primary xylem and phloem elements and no meri-
stematic cells remain to form a fascicular cambium. This results
in a bundle of the closed type. Arber (i, x, 3, 4), Dauphine (11),
and Gatin (2.0) have reported intrafascicular cambium in monocot-
yledons, the first named reporting on 2.i families in which some
member has a cambium. There is no extensive development of
secondary tissues, and the interpretation of the meristematic
tissue of the bundle as a cambium in many of these cases is con-
troversial. In some of the woody monocotyledons, such as Yucca
and Dracaena, a type of anomalous secondary thickening occurs in

THE ANATOMY OF THE SHOOT 69

which the activation of pericyclic or cortical tissues, or both,
results in the formation of secondary bundles and interfascicular
parenchyma.

Secondary thickening is accompanied by changes in the cortical
and epidermal tissues of the stem. When the development of
secondary stelar tissue is not extensive, it may be compensated for
by the grov^^th and division of the cells of the outer regions of the
axis; but w^here there is considerable secondary thickening, it
usually involves a loss of the epidermis and often portions of the
cortex. In such cases, a periderm is formed following the develop-
ment of a phellogen which is initiated in the cortical or pericyclic
parenchyma; or, infrequently, in the epidermis. The phellogen
produces phellem (cork cells) centrifugally and phelloderm cen-
tripetally. (See Chapter I.)

Lentkels, which serve as aerating structures, are developed in
the periderm when that tissue becomes extensive. They are com-
monly initiated at points beneath stomata, and the first step in their
formation consists in the division of the cortical cells which form
a group of loosely organized cowplementary cells. Later, the true
phellogen arises and the growth of the peridermal layers causes an
outbulging of the mass of complementary cells rupturing the
epidermis at the stomatal opening. Continued activity of the
phellogen at this point, or the production of successive phellogens,
results in the formation of layers of complementary cells. More
compact closing layers are also developed which serve to hold the
spongy mass of complementary cells together. The closing layers
are progressively ruptured by growth and stretching so that the
margins of the lenticel appear irregular and fragmented. A typical
lenticel is elliptical, somewhat raised above the surface of the bark,
and is usually oriented with its greatest dimension in the vertical
plane. They may become tangentially stretched where there is a
persistent periderm as in Prunus and Betula.

Vascular Transition. — In describing the ontogeny of the root
and stem axes, no statement has been made with respect to the
interconnection between the vascular systems of the two. The
vascular organization is different in orientation and mode of
development in the stem and root which together constitute the
axis of the plant, and it follows that there must be a region of
vascular transition at some point where continuity of the radial
exarch protostele of the root with the endarch dictyostele of the

70 THE STRUCTURE OF ECONOMIC PLANTS

shoot is established. The transitional region may involve several
internodes (three or four in Pisum), or it may be very short, occur-
ring within a vertical distance of but a few millimeters in the
hypocotyl (Beta). In the majority of cases, the transition is
hypocotyledonary if the cotyledons are brought above ground in
germination; but it may involve the first nodes and internodes
above the cotyledonary node when the cotyledon or cotyledons are
hypogeal.

In the terminology of the seedling axis, the hypocotyl, where
vascular transition most frequently occurs, is regarded as that
portion of the axis immediately below the first node (cotyledonary
node), in contrast with the epkofyl, which is the whole portion of
the axis above that point. The two terms express axial relation-
ships with respect to the first node of the plant from which the
first leaves (cotyledons) are diverged; but they do not connote any-
thing with respect to the structural organization of the portion of
the seedling axis to which they are applied. Thus, the hypocotyl
may be almost entirely root-like in structure, as in many hypogeal
forms (Zea and Pisum). On the other hand, the upper portion of a
hypocotyl may be stem-like in structure as in many epigeal forms
where the transition occurs in the lower and middle hypocotyl.

The manner in which the vascular transition is accomplished is
constant for a given species, except for minor variations. It is
brought about in ontogeny by a progressive reorientation of the
vascular tissues which are differentiated in the embryo and by the
apical and intercalary meristems of the hypocotyl. In a represen-
tative case, the vascular elements at the upper limits of the transi-
tion region are differentiated as endarch collateral bundles; and, at
successively lower levels, the vascular strands are oriented in a
radial relationship with an exarch development of the primary
xylem.

While descriptions of specific vascular transitions tend to imply
motion, the cells are laid down and differentiated in place so that
the vascular reorientation is not accomplished by cell movement,
but by a gradual change in the vascular pattern of the stele at each
succeeding level of the transition region. In many cases, the
changes involved are described as proceeding from the root and
extending up the axis to the point where stem structure occurs, but
this is done for reasons of convenience and clarity of description.
In the actual ontogeny, the processes of growth and differentiation

THE ANATOMY OF THE SHOOT 71

proceed in the opposite direction, since the older portion of the
axis is that part of the hypocotyl adjacent to the cotyledonary
plate, and the tissues laid down distal to it are necessarily younger.
Supplementary intercalary growth of the hypocotyl also may serve
to complicate the description of transition in terms of progressive
development.

The type of root-stem transition depends upon the character of
the root axis, whether it is diarch, tetrarch, or polyarch; the
character of the stem bundles, whether they are collateral or
bicollateral; and the nature of the seedling development, hypogeal
or epigeal. There are several well-recognized types of transition,
and many of them are described in detail in connection with specific
plants in later chapters. Regardless of type, the essential point
of transition is that a reorientation of primary vascular tissues
is effected so that vascular continuity may be established and
maintained.

Sometimes actual continuity is attained only through the
development of secondary vascular structures, but usually they are
not involved. The cambial layer forms a lateral meristem which
is continuous through root, hypocotyl, and stem; and the
mechanism of secondary thickening is identical in all these regions.
For this reason, the secondary vascular tissues are not only con-
tinuous throughout the axial extent of the plant, but have the same
spatial relationships in the root, stem, and transition regions.
There is no reorientation of the epidermis, cortical parenchyma,
endodermis, and pericycle in the transition zone.

GENERAL MORPHOLOGY OF THE LEAF

The shoot has been defined as a stem axis with its divergent
members, lateral branches, and leaves of various types. The princi-
pal leaf categories are: foliage leaves, which perform the photo-
synthetic function; bracteal leaves, which are primarily protective;
scale leaves, or cataphylls, which may be vestigial or perform impor-
tant functions in storage and nutrition; and sporophylls, which con-
stitute the flower and are related to the function of gametic repro-
duction. Intergrades occur, a special case being that of the coty-
ledons, which may be related exclusively to the nutritive function,
or may be nutritive in the early ontogeny of the seedling, and later
carry on photosynthesis. Special leaf forms also occur, including
spines and tendrils.

71 THE STRUCTURE OF ECONOMIC PLANTS

The Foliage Leaf. — The foliage leaf may be regarded mor-
phologically as a lateral divergence from the axis, which, in the
majority of cases, is expanded and dorsi-ventrally flattened. Its
point of divergence is designated as a node, and usually one or
several buds develop in the axil of the leaf, which may be vegetative
or flower buds. In the former, the further development of the bud
results in a branch bearing foliage leaves in the same manner as the
primary axis from which it arises; while, in the latter, the bud
develops as an inflorescence. (See Chapter IV.) In other in-
stances, the bud may be dormant, developing later in the life of
the plant ; or, in the normal cycle of the plant, it may never develop
further. Less frequently, there is apparently no bud in the axil;
but it seems likely that the potentialities for bud development
reside in the tissue at this point, and under some circumstances
adventitious buds may be produced.

Phyllotaxy. — The leaf arrangement or phyllotaxy of the foliage
leaves is often remarkably constant for a given species; and, con-
sequently, constitutes a helpful diagnostic character. There are
three principal phyllotaxies : alternate, in which a single leaf is
diverged from each node (Fig. xy); opposite, in which two leaves
are so diverged; and whorled, or verticillate, where there are three or
more leaves at a node. In the alternate phyllotaxy, the leaves are
arranged on the axis in a spiral; and because of the regularity of
the angular divergence (the angle formed between two successive
leaves) they are arranged in a definite number of ranks or rows.
The relationship is commonly expressed fractionally. Thus, in the
alternate arrangement, the Yi divergence is the characteristic one
for the Gramineae; the 3^ phyllotaxy is found commonly in the
Cyperaceae, sedges; and the 2^ arrangement is perhaps the most
common among dicotyledons. In this fractional expression of
phyllotaxy, the denominator indicates the number of leaves or
ranks in a complete cycle; and the numerator expresses the number
of times the stem is encircled to complete a cycle. In a % diverg-
ence, the cycle consists of five leaves and passes twice around the
stem, so that there are five ranks or rows of leaves; and, starting
at any given leaf, the sixth leaf lies in the same vertical plane as
the first leaf. In the opposite arrangement, the successive pairs of
leaves may lie in the same plane or at right angles to each other,
and the latter type of phyllotaxy is often referred to as decussate.

Although the phyllotaxy of most plants is sufficiently constant

THE ANATOMY OF THE SHOOT

73

to indicate a genetic characteristic, variations often occur; and,
in a single axis, two or more types may develop. In Helianthus,
whorled, opposite, and alternate arrangements may develop on
one stem; and similar combination phyllotaxies occur in Linum,
Cannabis, and many other plants. Regardless of the system of
phyllotaxy, there is an adjustment of leaves in relation to light so
that the resultant leaf pattern or mosaic exposes a maximum
amount of leaf surface to it.

\^

Fig. vj. Types of phyllotaxy. A, B, C, various alternate arrangements; D, opposite
arrangement. The upper figure in each case shows the phyllotaxy as seen from above. (From
Smith et al., Textbook of General Botany.^

Gross Characteristics. — A representative vegetative leaf com-
monly consists of an expanded blade or lamina and a stalk or petiole.
The latter is variable in length and in the sharpness with which it
is delimited from the blade. Some leaves are non-petiolate and ses-
sile. A special type occurs in the grass leaf, in which a basal sheath
surrounds the axis; and, at its upper limit, there may be a ligule
or collar-like membrane which clasps the stem.

In other cases, stipules develop which are generally regarded as
divergences from the base of the leaf. Sinnott and Bailey (3 8) have

74 THE STRUCTURE OF ECONOMIC PLANTS

stated that the facts "favor the contention that stipules are integral
portions of the base of the leaf, a view which is well expressed by
Eichler when he says that stipules arise without exception as a
product of the leaf-base of the primordial leaf. ' ' Recent work by
Cross (lo) on Morus, which attacks the problem from the stand-
point of foliar ontogeny, confirms this point of view. In some
instances, the stipules are very prominent as in Pisum, where
they are large and perform the same functions as the blade. Some
stipules are protective in relation to the axillary bud, others
differentiate as spines.

The blade may be variously specialized; and differences in its
color, shape, marginal characteristics and venation result in a
great number of forms. Typically, the foliage leaf is green owing
to the presence of chlorophyll in the mesophyll; but this pigment
may be masked by the presence of accessory pigments, chiefly
anthocyanins; and, in variegated leaves, the chlorophyll may be
restricted to definite regions of the blade.

The shape of leaves may range from the slender, needle-like
leaves of many conifers and the linear or lanceolate types found in
grasses, to the more expanded ovate, cordate, or orbicular leaves
which are commonly developed in dicotyledons. The degree of
marginal modification varies from those which are smooth and
entire through a progressive series of marginal indentations that
include the serrate, dentate, and crenate forms. Where the cutting
of the margin is more pronounced, the leaf is lobed; and this may
be palmate (Gossypium and Cucurbita) or pinnate (Raphanus).
If the blade is divided into segments or leaflets, the leaf is
compound; and the arrangement of the leaflets may be palmate
(Cannabis) or pinnate (Pisum and Apium).

The venation of the blade may be parallel or netted. In the
former, the primary veins extend through the lamina parallel to one
another without conspicuous anastomoses; while in the netted sys-
tem the veins form a complicated reticulum. Studies of character-
istic systems of parallel venation (Zea and Allium) indicate that
there are many small cross-veins interconnecting the parallel mem-
bers of the system. In the net-veined types, the ultimate veinlets
commonly end blindly in a small area of chlorophyll parenchyma
which is termed a vein islet.

The foliage leaf may persist throughout the life of the plant,
or through several seasons in perennial plants. Evergreen leaves

THE ANATOMY OF THE SHOOT 75

occur on many conifers, and are also found on such trees as the
olive, orange, and others. Many perennials have a deciduous
habit, and a new set of foliage leaves is differentiated each year
which is shed at the end of the growing season, or at the beginning
of the ensuing period of vegetative extension. In annual plants, it
is not uncommon for the cotyledons and basal leaves to be shed
before the growth cycle is complete. In deciduous types, leaf fall
is commonly accomplished by means of an abscission layer, which is
usually located at the base of the petiole, but may sometimes be
developed at the base of the lamina or at an intermediate point in
the petiole. The separation of the leaf from the axis results from
the development of the cells of the abscission layer which round
off and separate from one another owing to the disintegration of
the intercellular substance of their middle lamellae. A detailed
account of this phenomenon is given for Lycopersicum. (See
Chapter XVIII.)

ANATOMY OF THE LEAF

The leaf may include all the types of tissues which are differenti-
ated in the stem (epidermis, parenchyma, mechanical, and vascular
tissues). As might be expected on the basis of the ontogeny of the
leaf, these tissues are continuous with those of the stem axis.
Thus, the epidermis of the adaxial and abaxial surfaces of a foliage
leaf forms an uninterrupted protective layer with the epidermis
of the stem; the vascular bundles of the leaf are continuous with
those of the stem axis at the node, and the parenchymatous regions
of the petiole are contiguous with parenchyma of the stem.

Structurally, the leaf consists of three major regions: (i) the
protective epidermis, which covers its entire surface, (i) the meso-
phyll, which is generally parenchymatous, and (3) the veins.
(Fig. i8.) The character of the epidermis, the specialized stomatal
apparatus, and the types of epidermal hairs which may be produced
by this tissue have been discussed. (Chapter I.) The epidermis
may be regarded as consisting in most cases of an impervious, non-
chlorophyllose (except for the guard cells) layer which may or
may not develop a non-cellular cuticle. Stomata occur in varying
frequencies and distributions, so that ready means of gas exchange
is provided. The number of stomata on the abaxial surface usually
exceeds that on the adaxial one when leaves are oriented dorsi-
ventrally. In those that assume a more or less vertical position

76

THE STRUCTURE OF ECONOMIC PLANTS

(many of the Gramineae), the stomatal frequency may be approxi-
mately the same for both surfaces. Stomatal counts are often at
variance, owing to the fact that the number of stomata per unit
area for a given species or even for a single plant may vary depend-
ing upon the age of the leaf and the environmental conditions under
which it has developed. Expressed in approximate ratios, Pisum
has about twice as many stomata on the abaxial surface as the
adaxial one; in Triticum, the ratio of adaxial to abaxial stomata is
about ID to 7; and in Zea, 3 to 5. In Olea, olive, and Ficus, rubber
plant, no stomata occur on the adaxial surface.

spo

Fig. 2.8. Transection through a portion of the blade of rhe tobacco leaf : g/^, epidermis; i>r,
epidermal hair; pal, palisade cells; ph, phloem; spo, spongy parenchyma; sto, stoma; xy,
xylem.

The mesophyll is usually differentiated into palisade and spongy
cells; but in some cases it consists of parenchymatous cells that are
more or less uniform in size, shape, and arrangement (Zea,
Lactuca). Where differentiation exists, the palisade zone lies
immediately below the adaxial epidermis, consisting of one, some-
times two, or infrequently more, layers of cells which are compactly
arranged with their long axes at right angles to the surface of the
blade. The palisade cells are more compactly arranged than are
the spongy cells, but there are more intercellular spaces in this
region than are ordinarily shown in transections of a leaf. The
cells of the spongy parenchyma are approximately isodiametric and

THE ANATOMY OF THE SHOOT 77

loosely arranged, with intercellular spaces that form a system of
air passages that are continuous with the substomatal cavities.
Both palisade and spongy cells are chlorenchymatous, the number
of plastids usually being greater in the former; and chloroplasts
also occur in the guard cells of the epidermis. The border paren-
chyma, adjacent to the vascular elements of the smaller bundles and
to single tracheids, may be lacking in chlorophyll.

The veins resemble the vascular bundles of the stem with respect
to their primary tissues; and, in most instances, consist of collateral
bundles that are oriented so that the xylem lies toward the adaxial
surface. In plants which have bicoUateral stem bundles, such
vascular organization may also occur in the leaf; but it is com-
monly wanting in the lateral veins and in the peripheral portions of
the main ones. (Fig. 19.) The secondary veins are progressively
smaller with fewer vascular elements; and, ordinarily, phloem ele-
ments do not occur in the ultimate veinlets which consist of one or
two xylem elements, usually tracheids, accompanied by non-chloro-
phyllose parenchymatous cells. Such cells presumably serve in
translocation as a connecting link between the photosynthetic cells
of the mesophyll and the phloem of the larger veins. The occur-
rence of a cambium is not uncommon in the main veins; and, in
some large leaves, there may be a considerable amount of secondary
vascular tissue. Associated with the main veins as a bundle
sheath, or occurring as strands which parallel the veins along the
adaxial and abaxial surface, are the mechanical tissues which
strengthen the framework of the petiole and blade. These scleren-
chymatous elements are similar to those described for the stem,
consisting of elongated cells with thickened walls.

In addition to the principal tissues noted, special glandular
structures may develop in the mesophyll (Gossypium), and lactif-
erous cells and ducts may be differentiated (Lactuca). Cysto-
liths (Cannabis) and cells containing crystalline substances and
mucilages also occur.

Ontogeny of the Leaf. — Ontogenetic studies of the leaf have
been few, and it is only within recent years that interest in this
neglected phase of developmental anatomy has been renewed.
Following the classical studies of Trecul (39), Eichler (13), Han-
stein (x5), and others of that period, little significant work on leaf
ontogeny was published until the first part of this century, except
for the contribution by Lund Qlj) to which Foster (18) recently

78

THE STRUCTURE OF ECONOMIC PLANTS

directed attention. The latter (19) has also reviewed the literature
on leaf ontogeny in the angiosperms and has accompanied it with an
extensive bibliography. On the basis of the work that has been

F,n . a Transection through the midrib of a tobacco leaf: ab ph, abaxial phloem; ad ph,
.^^U^7^^^^S^ /., epidermal hair; M P^Hsade cells; .,. spongy paren-
chyma; xy, xylem.

reported, it is clear that the development of the leaf in the angk,-
sperm does not conform to any one pattern. The specific details,
concerning a relatively large number of angiosperms, show no

THE ANATOMY OF THE SHOOT 79

general uniformity in the details of development, and there is a lack
of agreement as to the terminology applied in ontogenetic descrip-
tions. For this reason, it seems desirable to summarize briefly the
situation with respect to nomenclature before discussing the details
of leaf development.

Hanstein (x5) applied the same terminology to the meristematic
tissues of the stem tip as to the root tip, with the exception of
the calyptrogen ; and described the former as being organized into a
dermatogen, periblem, and plerome. According to his account of
leaf development, lateral members originate below the tip of the
growing point where definite groups of meristem are formed involv-
ing elements of both the dermatogen and periblem. Subsequent
divisions of the dermatogen are anticlinal only so that it forms a
continuous, single layer covering the leaf, which later develops as
the epidermis. The underlying cells of the periblem, by divisions
in all planes, develop the tissues of the mesophyll, including the
vascular bundles. The plerome of the stem axis does not enter into
the formation of the lateral structures, either leaf or branch.

More recent studies indicate that there are exceptions to this
type of ontogeny; and that the number of layers of meristematic
cells which may be involved in leaf development is variable, rang-
ing from one to several . Rosier (3 1) found that the leaf of Triticum
develops entirely from the dermatogen, and Priestley and his
associates (19) observed periclinal divisions in the dermatogen as
well as underlying layers in some of their studies. On the other
hand, subepidermal layers of the apical meristem of the shoot may
be responsible for the development of the leaf tissues with the
exception of the epidermis, and several different types of sub-
epidermal activity have been reported. In some instances, the
third layer of cells appears to be responsible for the development
of the midrib or a portion of it, while the remainder of the meso-
phyll arises from the subepidermal layer. In Linum, the leaf
primordium involves at least the three outermost layers, according
to Crooks (9); while in tobacco, Avery (5) states that

"the leaf arises as a lateral projection of the embryonic stem tip.
Its initial impetus for development comes from a few localized dividing
and enlarging cells in the outer layers of the promeristem, in the
portion which becomes the periblem, and still later cortex."

Because of the difficulty in following the ontogeny of the leaf
on the basis of the histogens as used by Hanstein (15), Schmidt (33)

8o

THE STRUCTURE OF ECONOMIC PLANTS

has proposed two more inclusive terms to be applied in describing
the early stages. According to his concept, the apex of the shoot
consists of one or more peripheral layers which perpetuate them-
selves by continued division, collectively termed the tunica. Within
the tunica is the inner core of the growing point which he has
designated the corpus. The essential distinction between the two
regions is based upon differences in their manner of cell division.
The cells of the tunica divide anticlinally to perpetuate its layers,
and in periclinal planes only during leaf or bud formation. The

cells of the corpus, on the other
hand, divide in all planes and
thus increase the volume of the
growing point. These terms have
been rather generally adopted in
recent literature on leaf develop-
ment, and their use appears to be
logical, since it is now established
that periclinal divisions may occur
in the peripheral or dermatogen
layer as well as in those which
underlie it. Under this system of
nomenclature, the number of layers
of the tunica is variable, depending
upon the plant under considera-
tion. It commonly consists of
one, two, or three layers. In
some plants, leaves may arise
exclusively from the tunica; in
others the internal tissues of the leaf are derived from the corpus;
and an intermediate condition may exist in which the cells of
both the tunica and corpus are involved in the differentiation of
the leaf.

In the development of the primordium, the first evidence of its
differentiation is associated with the formation of small aggregates
of procambial cells. The occurrence of these zones of meristematic
tissue precedes the development of any actual protuberance, and
results in a transverse expansion of the stem apex to form what
Louis (x6) has designated as foliar buttresses, confirming the theories
of Gregoire (^3, 14). (Fig. 30.) From these buttresses, the
primordia emerge, indicating the intimate relationship between

Fig. 30. The apical region of two pri-
mary shoots of Zea showing apex of a
very small vegetative cone from which the
leaves b, b', b", and b'" arise as multi-
cellular protuberances which soon sur-
round the stem. In the axil of the leaf,
b", is a rudimentary bud. (After Sachs,
Textbook of Botany, Clarendon Press.)

THE ANATOMY OF THE SHOOT 8i

the stem and leaf, and emphasizing the earlier statement of Sachs
(31), that

"the morphological conceptions of stem and leaf are correlative;
one cannot be conceived without the other ... in other words, the
expressions stem and leaf denote only certain relationships of the
parts of a whole — the shoot."

In connection with the actual extension of the leaf primordium
from the surface of the stem apex, Schiiepp (36) has pointed out
that the frequency of cell division is approximately the same in all
portions of the shoot apex. In other words, the number of cell
divisions of the cells of the tunica would be approximately the same
as those in the corpus. Since the divisions in the corpus occur in
all planes, while those of the tunica are anticlinal, except during
the formation of a leaf or bud primordium, the development of both
regions produces folds in the superficial tissues or tunica as a
necessary adjustment to the differences in the manner of cell divi-
sion of the two regions.

In the development of the leaf at the foliar buttress or node,
the first stages in ontogeny result in an elongation of the primor-
dium so that it frequently becomes a tapered cone that is somewhat
flattened on its adaxial surface. As Avery (5) and others have
pointed out, this growth in length is apical and results from con-
tinued periclinal and anticlinal divisions of a subepidermal cell
or cells. The apical growth is not limited to subepidermal cells
in all cases, and subapical growth has also been reported; so that,
again, it is clear that there is no strict uniformity in the ontogeny
of the leaf of angiosperms. In most cases, apical growth is
retarded or ceases altogether early in development, and the ensuing
extension of the primordium is due to intercalary activity in addi-
tion to some unlocalized cell division which may result in an
increase in the radial thickness of the primordium. It has been
found that the broadening of the base of the leaf and the early
development of the petiole in many dicotyledonous leaves is the
result of the activity of meristematic tissues lying on the adaxial
surface of the leaf base. The development of the lamina, which
follows the formation of the petiolar midrib region, is initiated
by the activity of marginal ridges of meristem that develop on the
terminal and adaxial portion of the primordium.

Variations occur with respect to the layers involved in the
production of the marginal meristems that form the young blade.

8z THE STRUCTURE OF ECONOMIC PLANTS

In some cases, it has been shown that the development of the blade
results from the activity of a band of marginal cells which are
located along the edge of the midrib; while, in others, additional
underlying layers, as well as the subepidermal ones, are involved
in the development of the lamina.

The characteristic thinness of the blade of foliage leaves can
be related to its ontogeny. There is a relatively constant number
of layers of meristematic cells in the embryonic blade, ranging from
five to eight; and, from these layers, the epidermis, mesophyll, and
vascular tissues of the mature blade are differentiated. Examples
of the meristematic layering in the young lamina include the bean
and sweet potato with six layers, and tobacco with six or seven.
From these layers, the mature tissues of the leaf are derived in a
definite cellular succession which is essentially constant for a given
species.

In general, the upper and lower layers function as dermatogen,
producing the upper and the lower epidermis respectively. The
adaxial subepidermal layer develops the palisade parenchyma; the
middle layer or layers produce the provascular strands, and ulti-
mately the vascular tissue; and the abaxial subepidermal layer
produces the major portion of the spongy parenchyma. Modifica-
tions in this sequence of cell lineage occur, depending upon the
number of initial cell layers in the embryonic blade. In some cases,
derivatives of the middle layers form a part of the spongy paren-
chyma, in addition to producing the vascular system; and, where
a double palisade occurs, they may form a portion of that
region.

The specificity of the initial cell layers of the young leaf with
respect to the tissues ultimately derived from them has long
been recognized and three generative layers corresponding to the
three histogens of the stem were outlined by Flot (14) and Gravis
(ii). On this basis, the formative layers of the lamina were
designated by Schiiepp (36) as protoderm, giving rise to epidermis;
ground meristem, producing the parenchymatous tissues of the meso-
phyll; and procambium, from which the vascular tissues are formed.
The growth of the blade in surface area is due to the activity of
what he has termed plate meristems. This activity follows the mar-
ginal growth of the lamina and continues long after the latter has
ceased. Marginal growth is not discontinued simultaneously in all
parts of the leaf; and, in the basipetal type of development, the

THE ANATOMY OF THE SHOOT 83

inhibition of the marginal meristem proceeds from the apex toward
the base. The activity of the plate meristems consists essentially
of anticlinal divisions in the respective layers so that no pro-
nounced thickening of the blade results except in the region of
the procambial strands. Differential rates of cell division and cell
enlargement occur in the various tissues of the developing leaf.
The palisade cells continue division after the cells of the spongy
parenchyma have stopped dividing, and cell enlargement usually
continues longest in the epidermal cells. These factors create
stresses v^hich account for the schizogenous formation of air spaces
in the mesophyll.

The significant stages in the ontogeny of a simple foliage leaf
may be summarized as follows :

(i) The development of a meristematic primordium at the foliar
buttress.

(i) The extension of the primordium by apical and intercalary
growth to form the petiolar and midrib region.

(3) The development of the lamina by marginal meristems.

(4) The differentiation of the tissues of the lamina from marginal
and submarginal initial layers — protoderm, ground meristem,
and procambium.

(5) The surface growth of the lamina owing to the activity of
plate meristems.

Ontogeny of Compound Leaves. — The foregoing account is
based upon the development of simple leaves. The situation with
respect to the differentiation of compound leaves has been less
thoroughly investigated. In general, the leaflets or lobes of leaves
arise in basipetal succession, from the apex toward the base; in
acropetal succession, from the base toward the apex; or in what
Troll (40) has termed a divergent sequence in which intermediate
leaflet primordia first develop, and additional leaflets form both
acropetally and basipetally from that point. An example of the
acropetal method of development has been described by Sachs (31)
for the Umbelliferae. In this instance, the initials of the leaflets
arise at the base of the cone-like primordium of the leaf, and grow
apically in the same manner in which the central portion of the
leaf primordium develops. Secondary leaflets are formed in a
similar way in cases where the leaf is bipinnately compound.
(Fig. 31.) Foster (16, 17) has made a study of the ontogeny of
the leaf of Carya in which the development is also acropetal.

84

THE STRUCTURE OF ECONOMIC PLANTS

Development of Stipules. — Stipules may be considered mor-
phologically as integral portions of the leaf, but relatively little
work has been done in regard to the details of their development.
Early studies on stipules w^ere concerned primarily with speculation
as to the relation of the stipule to the leaf; and, more particularly,
the consideration of any evidence based on the presence or absence
of stipules which might serve as an aid in phylogenetic inter-
pretations. Considerations of this nature have been reported by

•--si

— L-1

A B C

Fig. 31. Development of the pinnate leaves of the Umbel liferae. A, B, of Pastinaca sativa;
C, of Levisticum officinale. In A, the stem tip and youngest leaf L-i are shown. The leaf,
L-i, has commenced pinnation. In B, the primary pinnation of a single leaf is shown. In
C, the leaflets of the first order, /', have given rise to secondary leaflets, /"; st, stem tip.
(Redrawn and adapted from Sachs, Textbook of Botany, Clarendon Press.)

Domin (ix), Sinnott and Bailey (38), Schrodinger (34), and more
recently by Ponzo (xS).

Ontogenetic studies have been limited, and have received but
brief mention in connection with general foliar studies. Cross (10)
has described the development of the stipules in Morus alba and
states that they "undergo a decidedly different ontogenetic devel-
opment from that of the foliage leaf, but they exhibit certain fea-
tures of development which are strikingly suggestive of bud scale
histogenesis." Following expansion, stipules sometimes absciss
early, before the foliage leaf attains its full size. Abscission
is not a constant character of these structures; for, in many cases
as in Pisum, they are not only persistent, but perform an important
role in photosynthesis.

THE ANATOMY OF THE SHOOT 85

Development of Bud Scales. — Like the stipule, the bud scale
or cataphyll has received relatively little attention from the onto-
genetic point of view, the principal references to it being con-
cerned with its relation to phylogeny and speculation with respect
to theories of recapitulation and metamorphosis. Certain generali-
zations can be made regarding the principal anatomical differences
between the bud scale and the foliage leaf, (i) Stomata are
reduced in number or may be entirely lacking; and, when present,
are more numerous on the adaxial surface of the scale than on the
abaxial one, which usually has a heavy cuticle, (i) There is a
reduction in the amount of mechanical tissue of the collenchyma-
tous type. (3) There is usually a reduction or complete absence of
palisade tissue; and the parenchymatous cells are homogeneous
with fewer air spaces than are found in the spongy parenchyma of
the foliage leaf. (4) There is a definite reduction in the amount of
vascular tissue, the number of xylem and phloem elements is
reduced, and the former are less strongly lignified.

Considerable interest attaches to the question as to what factors
determine whether a foliar primordium will develop as a cata-
phyll or as a foliage leaf. Goebel (xi) has regarded the cataphyll
as developing from a foliar primordium which is arrested in its
developmental stages and as a result pursues a divergent ontogeny
leading to the differentiation of a bud scale. Objections to this
"transformation" theory have been raised by Schiiepp (37) and
Foster (15), the former showing that the character and position of
the meristematic tissue, and the rapidity of its maturation, are
important factors in determining the ultimate development of a
foliar primordium. Foster has suggested that there is a significant
periodic alternation of cataphylls and foliage leaves. He has also
noted differences in the character of the meristematic activity and
the development of the derivative cells of the bud scale as compared
with the foliage leaf.

LITERATURE CITED

I. Arber, Agnes, "On the occurrence of intrafascicular cambium in monocoty-
ledons." Ann. Bot. ^i: 41-45, 1917-
1. , "Further notes on intrafascicular cambium in monocotyledons."

Ann. Bot. ^2: 87-89, 1918.
3. , "Studies on intrafascicular cambium in monocotyledons. Ill, IV."

Ann. Bot. ^y 459-465, 191 9.

86 THE STRUCTURE OF ECONOMIC PLANTS

4. Arber, Agnes, "Studies on intrafascicular cambium in monocotyledons. V."

Ann. Bot. ^6: z'yi-z'>,6, 1911.

5. Avery, G. S., Jr., ■"Structure and development of the tobacco leaf." Am.

Jour. Bot. 20: 565-592., 1933.

6. Bailey, I. W., 'The significance of the cambium in the study of certain physio-

logical problems." Jour. Gen. Phys. 2: 519-533, 192.0.

■7. , "The cambium and its derivative tissues. II. Size variations of

cambial initials in gymnosperms and angiosperms." Am. Jour. Bot. y:

355-367. 192-0-
8. , "The cambium and its derivative tissues. IV. The increase in

girth of the cambium." Am. Jour. Bot. 10: 499-509, 192.3.
9. Crooks, D. M., "Histological and regenerative studies on the flax seedling."
Bot. Gaz.. 95: 2.09-2.39, 1933.

10. Cross, G. L., "The origin and development of the foliage leaves and stipules

of Morus alba." Bull. Torr. Bot. Cluh 64: 145-163, 1937.

11. Dauphine, a., "Sur la valeur des formations libero-ligneuses supplementaires

chez certaines Monocotyledones." Ann. Sci. Nat., Bot. Ser. IX, 20: 309-

314, 1917.

11. DoMiN, K., "Morphologische und phylogenetische Studien iiber die Stipular-
bildungen." Ann. Jard. Bot. Buiten^org. II, 9; 1x7-3x6, 1911.

13. Eichler, a. W., "Zur Entwickelungsgeschichte des Blattes mit besonderer

Beriicksichtigung der Nebenblatt-Bildung." Qlnaug.-Diss.'): 1-60, 1861.

14. Plot, L., "Recherches sur la naissance des feuilles et sur I'origine foliaire

de la tige." Rev. Gen. Bot. ij: 449-471; 519-535, 1905.

15. Foster, A. S., "Salient features of the problem of bud-scale morphology."

Biol. Rev. y 113-164, 1918.

16. , "A histogenetic study of foliar determination in Carya Buckleyi

var. arkansana." Atn. Jour. Bot. 22: 88-147, 1935.

17. , "Comparative histogenesis of foliar transition forms in Carya."

Univ. Calif. Pub. Bot. ip: 159-186, 1935.

18. , "A neglected monograph on foliar histogenesis." Madrono j: 311-

315, 1936.

19. , "Leaf differentiation in angiosperms." Bot. Rev. 2: 349-371, 1936.

10. Gatin, V. C, "Recherches anatomiques sur le pedoncule et la fleur

des Liliacees." Rev. Gen. Bot. ^2: 369-437; 460-518; 561-591, 1910.

11. GoEBEL, K., "Beitrage zur Morphologie und Physiologic des Blattes."

Bot. Zeit. ^8.- 753 et seq., 1880.
11. Gravis, A., "A propos de la genese des tissus de la feuille." Arch. Inst.
Bot. Univ. Liege 4: 1-8, 1907.

13. Gregoire, v., "Donnees nouvelles sur la morphogenese de I'axe feuille dans

les Dicotylees." Compt. Rend. Acad. Sci. Paris 200: 1117-1119, 1935.

14. , "Les liens morphogenetiques entre la feuille et la tige dans les

Dicotylees." Compt. Rend. Acad. Sci. Paris 200: 1349-1351, 1935.

15. Hanstein, J., "Die Scheitelzellgruppe im Vegetationspunkt der Phanero-

gamen." Festschr. derNied. Ges. Natur.- u. Heilkunde, 109-143, 1868.

16. Louis, J., "L'ontogenese du systeme conducteur dans la pousse feuillee des

Dicotylees et des Gymnospermes." La Cellule 44: 87-171,1935.

THE ANATOM,Y OF THE SHOOT S7

17. Lund, S., "Baegeret hos Kurvblomsterne, et histologisk fors^g pa at haevde

udviklingens enhed i planteriget." Bot. Tidsskrift Anden Raekke 2: i-iio,
1871. (Trans, into French as "Le calice des Composees. Essai sur I'unite
du developpement dans le regne vegetal." Ibid., 12.1-2.60.)

18. PoNZO, A., "Stipule e guaina." Nuovo Giorn. Ital. 41: i-X4, 1934.

19. Priestley, J. H., Scott, L. I., and Gillett, E. C, "The development of the

shoot in Alstroemeria and the unit of shoot growth in monocotyledons."

Ann. Bot. 4g: 161-179, 1935.
JO. , and Sw^iNGLE, C. F., "Vegetative propagation from the standpoint

of plant anatomy." U. S. Depf. Agr. Tech. Bull, ifi, 1919.
31. RosLER, P., "Histologische Studien am Vegetationspunkt von Triticum

vulgare." Planta j.- 18-69, 192-8-
32.. Sachs, J., Text-hook of Botany, Clarendon Press, Oxford, 1875.

33. Schmidt, A., "Histologische Studien an phanerogamen Vegetationspunkten."

Bot. Archiv. 8: 345-404, 1914.

34. ScHRODiNGER, R., "Phylogenetischc Ansichten iiber Scheiden- und Stipular-

bildungen." Verhandl. Zool.-Bot. Gesell. Wien. 6q: 161-193, 1919.

35. ScHiJEPP, O., "Wachstum und Formwechsel des Sprossvegetationspunktes

der Angiospermen." Ber. Deut. Bot. Gesell. }2: 318-339,1914.

36. , Meristeme. Handbuch der Pflanxenanatomie, IV. K. Linsbauer, Ed.,

Berlin, 1916.

37. , "Untersuchungen zur beschreibenden und experimentellen Entwick-

lungsgeschichte von Acer pseudoplatanus L." Jahrb. Wiss. Bot. yo: 743-
804, 1919.

38. SiNNOTT, E. W., and Bailey, I. W., "Investigations on the phylogeny of the

Angiosperms. III. Nodal anatomy and the morphology of stipules."
Am. Jour. Bot. i: 441-453, 1914.

39. Trecul, a., "Memoire sur la formation des feuilles." Ann. Sci. Nat., Bot.

Ser. Ill, 20: 135-314, 1853.

40. Troll, W., "Vergleichende Morphologie der Fiederblatter." Nova Acta

LeopodinaN. F. 2: 315-455, 1935.

CHAPTER IV

THE ANATOMY OF THE FLOWER
AND FRUIT

FUNCTIONALLY, the flower is an organ concerned with the
development of the structures that lead to gametic reproduction.
On this basis, the statement of Goebel (3) that a flower is "a shoot
beset with sporophylls" is in accord with the floral function,
since the stamens and carpels produce microspores and megaspores
from which the gametophytic structures arise. These, in turn,
produce gametes, gametic union ensues, and the resultant zygote
develops as a new sporophytic generation. It is evident from this
that the flower is a sporophytic structure, and that it is a mis-
conception to refer to the stamens and carpels as organs of gametic
or sexual reproduction.

Anatomically, the flower is a shoot. On this basis, the floral
axis is not unlike a vegetative one, as described in the preceding
chapter, which gives rise to successive leaf primordia from a ter-
minal meristem. The difference is not fundamentally in the man-
ner of origin of the foliar and floral parts respectively; but, rather,
in the character and function of the structures.

Floral Parts. — The typical, perfect flower of the angiosperms
has four types of floral leaves, sepals, petals, stamens, and carpels,
which are diverged from the receptacle or torus. The two first
named do not possess sporangia, and constitute the floral envelope
or perianth; the other two, stamens and carpels, normally bear
functional sporangia. In addition to these parts various types of
bracts may be associated with an individual flower, as in many of
the Rosaceae. The differences which distinguish the many floral
types are quantitative rather than qualitative variations, since the
same basic structures may be present in any given flower; but
their size, number, and degree or extent of development may vary
widely. There may be qualitative modification of such structures
as carpels and stamens; but, this is far less common than quantita-

88

THE FLOWER AND FRUIT 89

tive variations in the number, size, shape, and degree of divergence
of these parts. On the basis of this concept, it is possible to inter-
pret the great variety of floral structures, and to interrelate them
in a logical manner.

The sepals collectively constitute the calyx, forming the outer-
most spiral or cycle of the floral axis; and the sepal often resembles
a vegetative leaf in structure and venation. They are usually
chlorophyllose and photosynthetic; but their primary function,
at least in the early ontogeny of many flowers, is that of protec-
tion. In the bud stage, they form an enveloping structure which
overarches and encloses the other floral parts prior to anthesis.
Other specialized functions are attraction of insects and the dis-
semination of the fruit. In some instances, the calyx is entirely
lacking.

The petals, collectively termed the corolla, comprise the second
or inner set of the perianth. They may occur in one or more
spirals or cycles and are often white or some color other than
green. Their function is primarily that of attraction of insects,
but they also serve as a protection to the stamens and carpels. In
this connection, they may be specialized as to their shape, position,
or habit of opening and closing. In some floral types (Anemone),
the petals are lacking; and the sepals are brightly colored, or
there may be brilliant leafy bracts (Poinsettia). In wind-polli-
nated cereals, the petals, like the sepals, are either much reduced
or absent.

The stamen or microsporophyll consists of a stalk or filament
and a terminal anther comprised of one to several (usually four)
microsporangia or pollen sacs. During the development of the micro-
sporangia, and following reduction division, microspores and even-
tually pollen grains (microgametophytes) are formed. Com-
monly, the wall between adjacent microsporangia is broken; so
that, in the mature stamen, the anther consists of two locules or
pollen chambers instead of four separate microsporangia. These
pollen sacs dehisce in various ways at maturity, and the pollen
may be exposed or liberated through terminal chinks or pores, or
by means of a splitting of the anther wall.

The carpels or megasporophylls constitute the central or termi-
nal set of floral structures, and are referred to singly or collectively
as a pistil, or the pistils. The pistil may be simple consisting of
a single carpel, or compound if it is made up of two or more carpels.

90 THE STRUCTURE OF ECONOMIC PLANTS

Three distinct parts usually comprise the pistil, the basal ovary,
the more or less elongated style, and the stigma, which is terminal
or lateral in greater or lesser degree. The receptive surface of the
stigma, upon which the pollen grains lodge and initiate growth
of the pollen tube, may be papillate or plumose and feathery. In
many cases, glandular hairs provide a sticky secretion which
stimulates growth of the pollen. The entire carpel, or some por-
tion of it, constitutes the true fruit, and the seeds are developed
and contained within it. In many cases, other structures are
associated with it at maturity, and the assemblage is called the
fruit.

Floral Ontogeny. — In the ontogeny of the flower, the primor-
dium first appears as a rounded growing point of meristematic
tissue. From this, the primordia of the floral parts arise as lateral
structures in a spiral or series of cycles and develop much in the
manner of foliar primordia, expanding and developing vascular
traces. The subsequent development of the floral axis differs
from that of a vegetative one in that there is little internodal
elongation, so that the floral parts mature in a compact relation-
ship rather than being more or less widely separated from one
another.

In flowers that are regarded as relatively primitive, the ori-
gin of the floral primordia in an acropetal succession results in
the development of the hypogynous flower. (Fig. 3X.) In a cyclic
flower of this type, each successive cycle is definitely above and
centrad to the next outer cycle; and the level of divergence of the
stamens, petals, and sepals is below that of the carpels.

In many flowers, development involves some degree of zpnal
growth. Instead of each part differentiating as a separate entity,
there may be an activation of a whole zone of meristematic tissue
with varying degrees of subsequent divergence or differentiation of
individual members. Where this tendency toward zonal growth
is expressed perigynous or epigynous flowers are formed.

In perigyny, the elongation of the terminal portion of the floral
axis which gives rise to the carpels is inhibited, and the outer
cycles of the growing point develop as a zone so that the three
outer sets of floral parts grow conjointly at their bases and are
diverged from the rim of an urn or cup-like outgrowth as separate
structures. This makes it appear that the carpels arise from a
depression in the torus or receptacle; but it is actually the result

THE FLOWER AND FRUIT

91

of two factors, the differential growth rate of the cycles of floral
primordia, and zonal or partial conjoint growth of the outer cycles.

(Fig- 33-)

In epigyny, differential and zonal growth occur to a greater

degree and the apical growth of the morphological end of the

floral axis is inhibited. The zonal growth of the outer cycles forms

Fig. 31. Ranunculus ; development of the hypogynous flower. A, the floral axis with
terminal primordium and two lateral floral axes; B-D, progressive stages in the ontogeny of
the flower; £, four stages in the development of a carpel and the single ovule.

an enclosing structure which is non-diverged from the walls of the
ovary. The styles, stamens, petals, and sepals are then diverged
as separate structures from the top of this common zone of tissue;
or there may be further expression of non-divergence, a common
case being that in which the corolla is a tubular structure of non-
diverged petals. Many intermediate stages between hypogyny and
complete epigyny occur in floral development, and this again
points to the fact that differences in floral type are more of a quan-
titative nature than qualitative. (Fig. 34.)

9z

THE STRUCTURE OF ECONOMIC PLANTS

In many instances, the individual members of a given whorl do
not arise simultaneously but in succession. This is illustrated in
the papilionaceous Leguminosae (Pisum), where the abaxial sepal
arises first, followed by the two lateral ones; and, finally, the
two adaxial lobes. Similarly in the corolla, the two petal primor-
dia which form the keel are the first to arise, followed by the two
lateral primordia forming the wings; and, lastly, by the primor-
dium of the standard. In other cases, it has been demonstrated

A B

Fig. 33. A, the floral branch of Spiraea; B, a young flower showing perigyny.

that the sequence in a given cycle may be progressive in a counter-
clockwise or a clockwise spiral.

Many exceptions to the acropetal mode of development are
known. In certain Compositae, the much reduced or modified
sepals do not appear until after the differentiation of the stamens
and carpels; and in some of the Cruciferae, the petals are the last
primordia to appear. Instances have been observed in the Rosaceae
in which the succession of development is sepals, inner stamens,
carpels, outer stamens, and petals; while in other members of this
family, the primordia of the carpels appear before the full quota of
stamens has been determined. This indicates that there are excep-
tions to true acropetal succession and that the actual appearance
of the floral organs may be in part basipetal and in part acropetal
or irregular. In some cases, these apparent irregularities can be
interpreted as a delayed enlargement of potential primordia or a
non-divergence of parts of a cycle or adjacent cycles. For example,
in the Primulaceae the petals appear late in ontogeny after the

THE FLOWER AND FRUIT

93

stamens; and, as Coulter and Chamberlain (i) point out, this may
be interpreted "as a case in which the primordia of stamen and
petal have a common origin, entirely analogous to the sympetalous
corolla with stamens 'inserted on its tube,* but in which the

Fig. 34. A-H, stages in the development of the flower of the apple showing epigyny.
The stippled regions indicate the pith of the axis. /, a young flower bud showing epigynous
character and vascular system. /, diagrammatic representation of the vascular system of
the apple. (Redrawn and adapted from Kraus.)

separate primordia of the petals have been delayed in their appear-
ance."

The character of the symmetry of a flower also constitutes a
point of variation. In the actinomorphic forms, the parts of the
flower are arranged in a radial symmetry; while, in the zygomorphic
type, the flower is bilaterally symmetrical and can be divided in
only one plane of symmetry. The former occurs in the majority of
cases, while the latter more specialized type is represented in many
legumes, including Pisum and Medicago.

In contrast with the view that the vegetative and floral axes
are fundamentally alike, and that the foliar and floral organs are
similar in the initial stages of their development, Gregoire (7)

94

THE STRUCTURE OF ECONOMIC PLANTS

holds that the growing points of the two axes are different, and
that no homology between foliar and floral organs exists. He
concludes that the theory of metamorphosis does not apply in any
way to the flower, and that it is an autonomous structure. For
this reason, he dissents from the idea that the floral organs are
sporophylls, and suggests that the stamens be termed "micro-
sporangiophores," and the carpels "spermatophores."

The Inflorescence. — In addition to variations in the struc-
ture of individual flowers, the character of the inflorescence may

Fig. 35. Types of inflorescences. A, raceme (diagrammatic); B, corymb (diagrammatic);
C, umbel (diagrammatic); D, spike of plantain, after Bailey; E, catkin of willow; F, head
of clover, after Smalian. (From Smith, et al.. Textbook of General Botany.')

be distinctive. Flowers occur separately in a terminal or axillary
position; or, more commonly, form a flower cluster in which they
are more or less closely arranged with reference to one another.
The development of the inflorescence may be mono^odial or sym-
podial. In the monopodial development, the inflorescences are
indefinite, and result from the formation of a series of branches
which arise from the main axis in acropetal succession. Where
the lateral branches do not rebranch, the inflorescence may be a
spike or raceme if the main axis is elongated; or a head or simple
umbel if it is unelongated. (Fig. 35.) In the spike, the flowers are

THE FLOWER AND FRUIT

95

sessile on an elongated axis as in the carpellate flowers of Cannabis.
In the raceme, the flowers are pediceled on an elongated axis rather
than being sessile like the spike (Raphanus and Medicago). In
the head, the sessile flowers are closely aggregated on a shortened
axis (Lactuca and most Compositae). In the umbel, the lateral
axes bearing the flowers are approximately equal in length, each
one terminating in a flower.

Where the lateral branches rebranch and each axis is terminated
by a flower, a panicle is formed. (Fig. 36.) Included in this
category are the true panicle; the compound panicle, or spike made up
of spikelets (Triticum); and, where the main axis is unelongated,

Fig. j6. Types of inflorescences. A, cyme (diagrammatic); B, compound cyme of Sap-
onaria, after Rushby; C, panicle (diagrammatic). (From Smith, et al., Textbook of General
Botany.^

the compound umbel. In the compound umbel, each lateral axis is
terminated by a small umbellet bearing several flowers (Apium and
other members of the Umbelliferae). In the cyme, the branching is
sympodial and determinate. The inflorescence is usually broad
and flat, and the lateral axes terminate in flowers after producing
one or more branches of a second order which also bear flowers
CLinum").

Floral Types and Phylogeny. — The possibilities of variation
in floral types are almost unlimited. The most obvious of these
are variations in the color of the flower, and in the size, shape,
arrangement, and number of floral parts. Where the flowers are
cyclic, the most common floral plan is the pentacyclic one, in which
there are five sets of parts, these usually consisting of one cycle of
sepals, one of petals, two of stamens, and a central cycle of carpels.
In the tetracyclic arrangement, there is only one cycle of stamens.
Where the number of carpels is the same as the number of sepals and

96

THE STRUCTURE OF ECONOMIC PLANTS

petals, the flower is regarded as isocarpic, and when the number of

carpels is less than that of the other floral parts, it is said to be

anisocarfic.

In applying the facts of floral development and structure to

phylogenetic interpretation, certain characteristics have come to

be regarded as representing relatively high evolutionary position,

while others seem to be indicative of primitive rank. In a general

way, the following table indicates some of the high and low

characters.

TABLE I

Relatively Low or Primitive Characters

Spiral arrangement of floral parts

Distinct parts

Indefinite, usually large numbers of

parts
Pentacyclic flowers
Hypogyny
Isocarpy
Actinomorphy

Relatively High or Advanced Characters

Cyclic arrangement of floral parts
Undiverged parts
Definite numbers of parts

Tetracyclic flowers
Epigyny
Anisocarpy
Zygomorphy

Vascular Anatomy of the Flower. — The divergence or non-
divergence of the floral parts is commonly associated with a diver-
gence or non-divergence of the vascular bundles which supply these
parts; and, in the last analysis, the interpretation of the floral
structures may rest upon their vascular anatomy. In simple types,
with a regular acropetal succession, the floral parts diverge one
above the other, in the same manner as foliage leaves, and each
floral structure has a distinct vascular bundle or bundles. In
types where there is a non-divergence of floral parts, there may be
an accompanying non-divergence of the vascular system; and, in
some instances, this reaches a point where it is not possible to dis-
tinguish between the stem tissue and the conjoint floral structures.
This situation occurs in many perigynous and epigynous flowers.
In the apple, for example, where the receptacle grows up around the
carpels, there is no delimitation between the carpels and the stem.
Vascular reduction of this type is generally associated with
advanced floral types, and non-divergence of the vascular system is
considered an advanced evolutionary character.

In general, the primary vascular supply to a sepal consists of
the same number of bundles as supply the foliage leaf of the plant.

THE FLOWER AND FRUIT 97

Each petal and stamen is ordinarily supplied by a single bundle,
and the number of major bundles supplying a carpel is normally
three, one abaxial and two adaxial. These are frequently modified,
especially in compound pistils where adjacent adaxial bundles may
anastomose. In some instances, the abaxial bundle may appear to
be a double bundle (Pisum), owing to the development of a bisect-
ing parenchymatous ray.

MicROSPOROGENESis. — The initial protuberance of the primor-
dium of a stamen develops into two parts, the terminal anther and
the filament. In the anther, the cells are at first uniform, except
for the epidermis; but, a little later, the cells of the provascular
strand are differentiated, and a hypodermal cell or a group of them
appear which have a denser cytoplasm and can be distinguished
from the adjacent cells on this basis. These are the archesporial
cells, although the whole hypodermal layer may be regarded as
having archesporial potentialities. The number of cells which are
functional is ordinarily limited, and their subsequent development
results in the formation of the lobed regions of the anther. They
occur in longitudinal rows; and, in a transection of the anther, may
appear as a single cell or as groups of four to six or more. Cases
have been reported in which a single cell may constitute the whole
archesporium.

The archesporial cells enlarge and divide both radially and
tangentially. The tangential divisions result in the differentia-
tion of outer cells as the primary -parietal cells and the inner as
the primary sporogenous cells. From the former and their deriva-
tives, the walls of the embedded sporangia, of which there are typi-
cally four, develop. If the cells of the primary parietal layer
divide periclinally, a series of parietal wall layers of varying number
is developed. The outermost wall layer constitutes the endothecium.
It may become definitely specialized by the development of wall
thickenings which are related to the dehiscence of the anther.

The innermost parietal layer or sometimes the outer sporogenous
layers form the tapetum. Regardless of its morphological origin,
the tapetum serves as a nutritive layer for the developing microspore
mother cells and microspores., and it is more or less completely digested
during the progress of their growth and maturation. During the
development of the primary parietal cells, the primary sporogenous
cells may undergo successive divisions or function directly as micro-
spore mother cells. In either case, each microspore mother cell

98 THE STRUCTURE OF ECONOMIC PLANTS

undergoes the reduction divisions which result in a tetrad of
microspores (young pollen grains).

Coincident with the formation of the microspores, digestion of
the non-sporogenous tissues leads to the formation of the locules
of the four sporangia; and, frequently, the walls between adjacent
sporangial cavities disintegrate so that the mature anther contains
two locules or pollen sacs. Dehiscence of the anther occurs in ex-
tremely varied ways. In some cases, a longitudinal splitting occurs
along a definite stomtum. In others, there is a small apical fissure,
a terminal pore, or an irregular breaking of the tissues. The wall
tissues of the pollen sacs are frequently hygroscopic; and when
they become moist, the endothecial cells may close the stomia
again, in this manner covering any pollen which has not been shed.

At the time of shedding, the pollen grains have usually passed
beyond the microspore stage, their nuclei having divided to form a
ttihe and generative nucleus. In some instances, the latter may have
again divided so that at the time of shedding, the pollen grains are
trinucleate. Thus the mature pollen grain may be regarded as a
micro gametophyte, which consists either of a tube and generative
cell, or a tube cell and two micro gametes.

Development of the Carpel. — The carpels may develop singly
as in some of the Rosaceae and Leguminosae, or several of them
may develop conjointly or undiverged so that the resultant pistil
is compound (syncarpy). When a single simple carpel develops,
the primordium of the carpel, or megasporophyll, generally arises as a
more or less open structure whose margins, because of differential
growth, eventually meet, leaving a slight furrow or suture at their
junction which in some instances is complete. The basal portion
of the carpel constitutes the ovary, the terminal portion of the
primordium elongates to a greater or lesser degree to form the style
with the stigma at its tip or along its apical margins. The adaxial
suture represents the margin of the sporophyll, its abaxial suture
the midrib. There is often a prominent vascular bundle extending
the length of the abaxial suture, and one in each of the edges along
the adaxial suture.

Placentation. — The inner ridges of the carpellary margins in
the ovarian portion make up the placentae. It is from this region
that the ovule or ovules arise. In the simple carpel where the
initiation of the ovules is marginal, the placentation is parietal.
This type frequently occurs in syncarpous ovaries when the margins

THE FLOWER AND FRUIT

99

of adjacent carpels form peripheral ridges which protrude from the
inner surface of the wall, but do not extend centripetally to the
center of the ovarian chamber. (Fig. 37, A.')

In the axile type of placentation, the placental tissue lies at the
center of the ovary and may be foliar, or in part foliar and in part
cauline, as in the tomato. In some instances, the inturned margins
of the carpels extend to the center of the ovary and then are folded
back toward the periphery of each locule so that they appear as
projecting ridges extending
from a central axis in the
ovary (Cucurbita). (Fig.

37, 5.)

The less common type
is free-central placentation,
which occurs in the Caryo-
phyllaceae and Primu-
laceae. Dickson (x) has
described the placental de-
velopment for representa-
tives of each family. In
the former, the entire pla-
cental region consists of
the fused margins of five carpels which are separated from the
ovary wall by the disintegration of the intervening tissue. In
this manner, a form of free-central placentation is brought about
which is also referred to as "free-marginal" placentation. In the
Primulaceae, the free-central placenta is interpreted

"as the fused margins of the carpels (plus or minus residual axial
tissue), which are separated from the outer portions of the carpels
at their origin in the receptacle, instead of breaking away after the
definition of the individual carpels, as in the Caryophyllaceae."
(Fig. 37, C.)

Types of Ovules. — The ovule consists of a nucellus, sometimes
called the megasporangium, and the integuments which grow over and
around it. There are one or two integuments; or, infrequently,
there may be none, as in some of the Amaryllidaceae.

From the placenta, a dome-shaped mass arises which develops as
one of the several types of ovules . In the development of the ovule,
the initial protuberance is formed by the division of cells of the
hypodermal layer which form a mass of tissue, while the over-

FiG. 37. Types of placentation: A, parietal (dia-
grammatic); B, axile (diagrammatic); C, free central ,
longisection of young flower of Primula.

lOO

THE STRUCTURE OF ECONOMIC PLANTS

lying epidermal cells divide only anticlinally to compensate for
the enlargement of the nucellus. As this takes place, an annular
fold arises at the base of the nucellus which becomes the inner in-
tegument. This usually grows with greater rapidity than the
nucellar tissue; and, in most cases, eventually encloses it. When
there are two integuments, the outer one is initiated soon after the
inner is differentiated and later completely surrounds it.

In the orthotropous type, the ovule develops as a straight struc-
ture; and the integuments grow out and enclose the nucellus in
such a way that the microfyle is in a straight line with the chala%a
and hilum. This form of ovule is also termed atropous. Common
examples occur in the Polygonaceae (including Fagopyrum, buck-

Fig. 38. Types of ovules: A, orthotropous; B, campylotropous; C, hemitropous; D,
anatropous (all diagrammatic): cha, chalaza; /«, funiculus; hi, hilum; mic, micropyle; nls,
nucellus; r, raphe.

wheat, and Rheum, rhubarb), in the Juglandaceae, Urticaceae, and
several other families. (Fig. 38, A.^

Where the nucellus is not in a straight line with the funiculus
but is curved or bent over in such a way that the chalaza and micro-
pyle are in a plane at right angles to the funiculus, the type is
known as campylotropous. This is common in many of the Caryo-
phyllaceae and Berberidaceae. (Fig. 38, B.) Where the funiculus
is sharply curved just below the chalaza so that the nucellus and
integuments are inverted and lie alongside the stalk of the funiculus
with the micropyle directed toward the placenta, the ovule is
anatropous. (Fig. 38, D.)

A fourth type, hemitropous , which is intermediate between ortho-
tropous and anatropous, occurs when the ovule is half inverted
rather than completely so, as in anatropous ovules, so that the

THE FLOWER AND FRUIT loi

micropyle and chalaza are at right angles to the hilum. It differs
from the campylotropous type in that the nucellus is straight in
this type rather than curved. (Fig. 38, C)

Occasionally, in the development of the ovule there is a non-
divergence of its parts. This results in conjoint growth of the
nucellus and integuments, or conjoint growth of the funiculus and
the integument. Frequently, in anatropous ovules, this leads to
the development of a ridge-like structure, the raphe.

Megasporogenesis. — The nucellus begins its development as a
more or less localized activity of the hypodermal layers of the
sporophyll; and, as in microsporogenesis, the archesporial cells are
hypodermal. A hypodermal cell (or several of them) constitutes
the arches-porium, becoming larger than the adjacent cells. The
cells of the epidermal layer sometimes divide periclinally so that
several layers may develop from it, forming an epidermal cap. The
archesporium, which in most instances consists of a single cell,
may divide transversely, the outer daughter cell becoming the
primary parietal cell, and the inner one functioning as the primary
sporogenous cell. In other cases, the archesporial cell may function
directly as a megaspore mother cell. If a primary parietal cell is cut
off, it may undergo several periclinal divisions producing wall cells,
in addition to such as may have come from the epidermis. There
are many variations in the behavior of the primary parietal tissue,
from types in which there is a pronounced development to those in
which the parietal cells undergo few or no periclinal divisions.
Where there is no initial transverse division of the archesporial
cell, there is no development of parietal wall tissue.

The behavior of the megaspore mother cell is markedly uniform
in the angiosperms, despite the fact that several types are recorded.
A linear tetrad of megaspores is formed, accompanied by reduction
division. Of the megaspores, the chalazal one of the linear series
is usually functional; and, as a result of successive nuclear divi-
sions, an eight-celled megagametophyte is formed. At maturity,
it consists of a megagamete and two synergids, located at the micro-
pylar end; three antipodals at the chalazal end; and two polar
nuclei which are more or less centrally located. Commonly, the
polar nuclei unite just prior to or coincident with their union with
one of the microgametes to form the endosperm nucleus. This occurs
in the process of double fertilization at the time when the mega-
gamete and microgamete unite to form the zygote.

loi THE STRUCTURE OF ECONOMIC PLANTS

The Fruit. — The jruit of the angiosperms consists of the ovary
together with any closely associated parts. The ovary may be
simple, as in the akene (buttercup), pod (pea), and the drupe (plum);
or it may involve two or more carpels, as in the silique (radish),
the berry (tomato), and many others. Associated structures which
may be a part of the fruit in certain types include the receptacle,
sepals, basal portions of the stamens, and the axis itself. In some
special cases (artichoke), the floral bracts and all the floral parts
may constitute the fruit if the whole head is considered as
such.

There are several systems of classification of fruit. These vary,
depending upon the point of view upon which the classification is
based, and the characters which are regarded as determinative. On
the basis of the texture of the pericarp, and accessory parts when
present, fruits may be classified as being dry (papery, stony), fleshy,
or an intermediate combined type. In the dry types, the entire
pericarp or ovary wall becomes desiccated as the fruit matures.
Such fruits may be further subdivided into dehiscent and indehiscent
types, dependent upon whether they do or do not open at maturity.

In the completely fleshy types, the entire pericarp, together with
accessory structures in special cases, becomes succulent. In the
intermediate dry-fleshy forms, part of the pericarp becomes desic-
cated; or, in some cases, hardened; while other regions of the
ovary wall may remain fleshy. In such types (drupe) the pericarp
consists of an outer fleshy portion or exocarp and the endocarp,
which forms a stony pith. These categories may be further subdi-
vided on the basis of the number of carpels and accessory structures
involved in the development of the fruit. (Table II.)

Dry Fruits. — The principal indehiscent dry fruits are the cary-
opsis or grain, the akene, the schizpcarp, and the nut. The caryopsis
develops from a single carpel and contains a single seed. In its
development, the integuments may be partially disorganized and
the remainder of the seed coat becomes so closely appressed to the
pericarp that the two structures cannot be readily separated in the
mature grain. The akene may be the product of the development of
one or two carpels, the former being the case in the Ranunculaceae,
and the latter obtaining in the Compositae, in which the receptacle
is also a part of the fruit. (Fig. 39, A, B.) The fruit is one-seeded,
differing from the caryopsis in that the pericarp can be removed
from the mature seed.

THE FLOWER AND FRUIT

TABLE II
Classification of Fruits

to)

Structure

Classification

OF Fruits

One Carpel

Two or More
Carpels

Two or More
Carpels Plus

Carpels Plus
Stem Axis and

Stem Axis

Accessory Parts

Dry

Indehiscent

Caryopsis

(Grain)

Gramineae

(Zea)
Akene
Ranunculaceae

(Ranun-
culus)

Akene
Compositae
(Lactuca)

SCHIZOCARP

Umbelliferae
(Apium)

SiLIQUE

Cruciferae
(Raphanus)

Dry

Follicle

Capsule

Capsule

Dehiscent

Ranun-

Malvaceae

Iridaceae

culaceae

(Gossy-

(Iris)

(Delphin-

pium)

ium)

Liliaceae

Legume

(Allium)

Leguminosae

SiLIQUE

(Pisum)

Cruciferae
(Brassica)

Dry-Fleshy

Drupe

Drupaceous
Berry

Pome

Multiple

Rosaceae

Rosaceae

Moraceae

(Prunus)

Aquifoliaceae

(Pyrus)

(Ficus — Fig)

(Ilex)

Aggregate

Rosaceae
(Fragaria
and Rubus)

(Morus —
Mulberry)
Bromeliaceae
(Ananas —
Pineapple)

Fleshy

Berry
Berberidaceae

Berry

Solanaceae

Pepo (Infe-
rior Berry)

(Podo-

(Lycoper-

Cucurbitaceae

phyllum)

sicum)
Rutaceae
(Citrus)

(Cucurbita)

The schixpcarp consists of two or more carpels, each of which is
an indehiscent merkarp containing a single seed. At maturity, the

I04 THE STRUCTURE OF ECONOMIC PLANTS

mericarps split apart, each one resembling an akene in general
structure. This type is characteristic of the Umbelliferae and is
also found in the Geraniaceae. Where the mericarp is winged
(maple), the mericarp is called a samara or key. In the nut., the
structure resembles an akene, but the dry pericarp becomes thick
and hard. The term has been loosely applied to many fruits (al-
mond, walnut, cocoanut); seeds (Brazil-nut); and to legumes

Fig. 39. Fruit types; A, B, akene; C, capsule of Iris; D, pod or legume of pea; E, tran-
section of same; F, G, the drupe, plum.

(peanut); but among the true nuts are the acorn, chestnut, and
hazelnut.

The principal dehiscent dry fruits are the follicle, legume, capsule,
and silique. The follicle and legume fruit are developed from single
carpels, and differ from each other primarily in their manner of
dehiscence. In the follicle, there is a single line of dehiscence which
occurs along the suture on its adaxial surface; while, in the legume,
two sutures are formed and the fruit splits along both at maturity.
(Fig. 39, D, £.) The capsule develops from two or more carpels
and may involve a portion of the axis as well (Iris). (Fig. 39, C)
The dehiscence in the capsule may be loculicidal or septicidal or both;
and in the poppy, it occurs by means of pores. In the loculicidal
type, the lines of dehiscence bisect each carpel longitudinally in
the region of the abaxial bundle; in the septicidal type, the lines

THE FLOWER AND FRUIT

105

of dehiscence correspond to the points of union or non-divergence
of the adjacent carpels.

The silique is characteristic of the Cruciferae and consists of two
carpels which form a bilocular ovary with a longitudinal septum.
In most genera, the two valves of the fruit separate longitudinally;
but in a few instances (radish) the silique is indehiscent. Where
the fruit is short it is sometimes called a silicle.

Fleshy Fruits. — The principal fleshy fruits are the berry and the
pfo. The berry develops from the ovary of a hypogynous flower;
and produces a fleshy pericarp as it matures. In most cases, the

Fig. 40. Fruit types: A, the berry, a bilocular fruit of the tomato; B, pepo, cucumber;
C, pome, the apple in transection; D, the same in longisection. (B and C drawn by Kraus.)

berry is derived from two or more carpels (tomato); but it may
develop from a single carpel (Podophyllum, May apple). (Fig.
40, A.') The citrus fruit is also a berry of a specialized type, some-
times called a hesperidium, in which the dorsal walls of the carpels
develop into a thick rind and the central portion of the fruit in-
volves the axis as well as the carpels.

In the pepo, or inferior berry, the fruit develops from an epigynous
flower in which the tissue of the non-diverged receptacle surrounds
the pericarp and forms an outer rind which in squashes and pump-
kins may be very hard. The edible portion consists of the car-
pellary walls, often mainly the placentae; and the enveloping
receptacle. (Fig. 40, B.)

io6 THE STRUCTURE OF ECONOMIC PLANTS

Dry-Fleshy Fruits. — Several of the major fruit types are in
part dry and in part fleshy. In this intermediate group are the
drupe, pome, aggregate, and multiple types. The drupe or stone fruit
develops from a single carpel in v^^hich the pericarp is differentiated
into a thin epicarp or skin of the fruit, a fleshy mesocarp, and an
endocarp which is hard and stony (plum, apricot, peach). (Fig. 39,
F, G.) The mature fruit commonly contains a single seed, but two
ovules generally arise, one of them aborting as the fruit develops.
The term drupe in a less restricted sense is also applied to the fruits
of some members of the Caprifoliaceae (elder. Viburnum) which are
tricarpellate and may have one to three seeds.

In the pome (apple) the fleshy outer portion is formed by the
undiverged floral parts which surround the ovary, together with
the carpels in whole or in part. The pericarp consists of a papery
endocarp forming the inner limit of the core, and a fleshy mesocarp
and exocarp. (Fig. 40, C, D.) The aggregate fruit is composed of
several simple fruits developed from a flower having several simple
pistils. In some instances, the axis does not comprise a part of the
edible structure. The raspberry represents a form in which the re-
ceptacle is readily separated from the small edible drupes; while, in
the blackberry, it is not. In the strawberry, the edible tissue is the
fleshy receptacle on which a large number of small akenes are borne.

The multiple fruit differs from the aggregate in that it is the
product of the conjoint development of several flowers, rather than
being derived from several pistils of a single flower. While the
multiple types resemble one another in this principle of organiza-
tion, they may be quite variable with respect to the manner of
differentiation and the number of accessory parts that enter into
the fruit structure. In the fig, the fleshy receptacle is a hollow
chamber on the inner surface of which the flowers are borne. The
flower cluster may contain both staminate and carpellate flowers or
these may occur in separate inflorescences. This type of multiple
fruit is sometimes called a synconium. In the mulberry, the stami-
nate and carpellate flowers develop in separate inflorescences; and,
in the latter, each flower develops a fruit which is nut-like and
surrounded by a fleshy pericarp. As the fruits develop, they be-
come closely aggregated; and, together with the fleshy receptacle
or axis, form a multiple fruit or pseudocarp.

A third multiple type is the pineapple. Ananas, in which the
flowers are also borne in dense heads that are crowned by a tuft of

THE FLOWER AND FRUIT 107

leaves. The fruit involves the entire inflorescence, and individual
fruits, which are pomaceous and fleshy throughout, are surrounded
by a fleshy mass developed from the bracts, sepals, petals, and
axis of the inflorescence. A somewhat different multiple type oc-
curs in the globe artichoke, Cynara, in which the axis terminates
in a globular head that is surrounded by numerous involucral
bracts. The heads, together with the bracts, are harvested when
immature; and the bases of the involucral leaves and the apex of
the inflorescence constitute the edible portion.

The Seed. — In angiosperms, the seed consists of an embryo or
embryos surrounded by a covering which is generally integumentary.
Endosperm and nucellar tissue containing reserve foods may be
present; but, frequently, one or both of these tissues are lacking,
in which case the food reserves are stored in the cotyledons.

Following pollination, fertilization is effected by the union of
one of the two microgametes and the megagamete. The resultant
zygote undergoes divisions which initiate the embryo sporophyte.
The development of the endosperm precedes or parallels embry-
ogeny and begins following the union of the second microgamete
with the polar nuclei to form the endosperm nucleus. The forma-
tion of endosperm tissue may proceed more rapidly than the devel-
opment of the embryo, and food reserves are accumulated in the
endosperm cells. The event of double fertilization probably
represents the point at which the seed may be differentiated from
the ovule; since, from that time, subsequent developments are
concerned directly with the embryogeny of the new sporophyte
generation and its related nutritive tissues.

The Embryo. — The embryo in angiosperms consists of a hypo-
cotyledonary axis, one or two cotyledons (infrequently three or more
occur), and an epicotyl. The degree of epicotyledonary develop-
ment prior to the germination of the seed is variable. In some
cases (Pisum, Zea), the growing point of the epicotyl may differ-
entiate one or more leaf primordia above the cotyledonary node;
while, in others, it may consist of nothing more than a small mass
of meristematic cells. The growing point of the hypocotyl may
be organized as a well-defined primary root in which the histogens
and root cap are definitely determined; or it, too, may consist of
nothing more than a rudimentary terminal meristem.

Embryogeny is extremely varied with respect to the details of
ontogeny. The descriptions given by Hanstein (8) for Capsella;

io8 THE STRUCTURE OF ECONOMIC PLANTS

Schaffner (9) for Sagittaria; and Soueges (10) for Allium, outline
the general sequence of events for dicotyledons and monocoty-
ledons respectively; but the details should be described for each
plant individually.

In the dicotyledonous embryogeny, the zygote usually divides
transversely to form an apical and basal cell; and subsequent trans-
verse divisions, which may involve the apical cell or both daughter
cells, result in the formation of a proembryo consisting of a filament
of cells of varying length. In most cases, the derivatives of the
apical cell of the proembryo form the embryo; while the basal cell
or its derivatives and the remaining linear cells of the proembryo
form the suspensor. The first division of the apical cell of the
proembryo is usually longitudinal. This is followed by a second
division in the longitudinal plane, at right angles to the first divi-
sion, and a transverse division, or the order may be reversed. In
either case, the octant stage is reached and the transverse division
delimits the cotyledonary from the hypocotyledonary portion of
the embryo.

Following the octant stage, the histogens may be differentiated,
and the dermatogen is cut off by periclinal divisions which occur
first in the terminal portion of the embryo and progress toward the
hypocotyledonary region. The differentiation is continued rapidly
so that the dermatogen is soon complete except at the apex of the
primary root which is in contact with the adjacent cell of the sus-
pensor. This portion of the dermatogen is usually derived from the
uppermost cell of the suspensor, and the periblem and plerome then
differentiate within the dermatogen. The epicotyl and cotyledons
are derived from the four apical octants of the young embryo, while
the major portion of the hypocotyl is derived from the four basal
ones. The embryo rapidly develops its hypocotyledonary axis and
cotyledons, the degree of differentiation which it may attain
being determined largely on the basis of the speed with which the
fruit and seed ripen.

In a monocotyledonous embryogeny, as represented by Allium,
the first division of the zygote is transverse, producing an apical and
basal cell. This is followed by a second transverse division of the
basal cell so that a three-celled embryo is formed. The subsequent
divisions of the apical cell do not appear to be entirely regular;
but, as a result of oblique or vertical divisions of the apical cell,
quadrants of the embryo are formed which produce the cotyledon

THE FLOWER AND FRUIT 109

and growing point of the epicotyl. Subsequent divisions of the
intermediate cell produce the hypocotyledonary portions of the axis
as well as the root cap, and the inferior cell of the linear triad gives
rise to the suspensor by a series of divisions. As the hypocotyl
develops, there is a blocking off of a clearly defined plerome, peri-
blem, dermatogen, and root cap.

LITERATURE CITED

I. Coulter, J. M., and Chamberlain, C. J., Morphology of Angiosperms , D. Apple-
ton and Co., igix.

1. Dickson, Jean, "Studies in floral anatomy. III. An interpretation of the
gynaeceum in the Primulaceae." Am. Jour. Bot. zy 385-393, 1936.

3. GoEBEL, K., Organography of Plants, The Clarendon Press, Oxford, 1900.

4. Gregoire, v., "La valeur morphologique des carpelles dans les Angio-

spermes." Bull. Acad. Roy. Belg. V. 17.- 1186-1302., 1931.

5. , "Donnees nouvelles sur la morphogenese de I'axe feuille dans les

Dicotylees." Compt. Rend. Acad. Sci. Paris 200: 1117-1119, 1935.

6. , "Les liens morphogenetiques entre la feuille et la tige dans les Dicoty-
lees." Compt. Rend. Acad. Sci. Paris zoo: 1349-1351, 1935.

7. , "Sporophylles et organes floraux, tige et axe floral." Rec. Trav.

Bot. Neerl. ^z: 453-466, 1935.

8. Hanstein, J., "Entwickelungsgeschichte der Keime der Monocotyledonen

und Dicotyledonen." Bot. Abhandl. Bonn., 1870.

9. ScHAFFNER, J. H., "Contribution to the life history of Sagittaria variabilis."

Bot. Gaz- zj: z'^z-i.y^, 1897.
10. SouEGEs, R., "Embryogenie des Liliacees. Developpement de I'embryon
chez r Allium ursinum L." Compt. Rend. Acad. Sci. i8z: 1344-1346, 1916.

PART II. ECONOMIC PLANTS
CHAPTER V

GRAMINEAE

ZEA MAYS

THE grass family includes many important crop plants which are
used for forage and as cereals. Among the cereals grown in
the United States are: oats, Avena sativa L.; barley, Hordeum
vulgareL.; rye, Secale cereale L.; rice, Oryza sativa L.; wheat,
Triticum sp.; and corn, Zea Mays L. Sugar cane, Saccharum
officinarum L., is also an outstanding economic grass; and there
are numerous meadow and range grasses which constitute the
chief forage crops.

GENERAL MORPHOLOGY

The Root. — Corn is an herbaceous monocotyledon with an
annual cycle. The mature plant has an extensive fibrous root
system consisting of several whorls of adventitious roots some of
which become wide-spread laterals, others deeply penetrating
vertical roots. Weaver (14) reports,

"A lateral spread of 3.5 feet on all sides of the plant is not uncommon
even early in its development, and a depth of penetration of 5 to 6 feet
is usual. The degree of spreading as well as the depth varies some-
what with soil and other conditions, the root system probably reaching
its greatest development in deep mellow soils only moderately well
supplied with water." (Fig- 41)

The primary root may persist during the entire life of the plant,
although it frequently decays; but it has little functional impor-
tance after the establishment of the adventitious root system.

The Shoot. — The stem of the corn plant varies within a wide
range, some of the dwarf varieties being less than x feet tall while
certain types of field corn may attain a height of ix to 15 or even
lo feet. Northern varieties usually reach a height of 6 to 7 feet.

III

112. THE STRUCTURE OF ECONOMIC PLANTS

while the southern ones frequently attain the maximum noted
above. The main stem axis is sometimes unbranched, except for
lateral carpellate inflorescences or "ears" which are borne on short
branches; but tillers, suckers, or lateral branches may arise from
buds located in the axils of the lower leaves. The latter occa-
sionally develop as ears or they may form vegetative stems which
in turn develop adventitious roots and produce inflorescences.

Fig. 41 . The mature root system of corn showing its lateral and vertical spread.
(After Weaver, Root Development of Field Crops, McGraw-Hill Book Co.)

The stem is solid with the characteristic jointed appearance of all
grasses, and the internodes are alternately furrowed just above the
node on the side next to the leaf blade. This furrowing is related
to the formation of axillary buds, and, consequently, is more pro-
nounced in the internodes above each ear than in the upper inter-
nodes where the buds are not well developed.

The leaves are alternate and two-ranked as in other grasses.
Each leaf, except the scutellum and coleoptile, consists of a basal
sheath, ligule, and blade. The sheath surrounds the internode

ZEA MAYS 113

for some distance above the node from which it arises and its edges
overlap on the side of the axis distal to the blade. According to
Weatherwax (xi), the collar-like ligule develops from the adaxial
epidermis at the top of the sheath, fitting tightly around the stem
and the blades or sheaths of the included leaves. The long, flat
blade is auricled at the base and is somewhat fluted or wavy along
its edges at maturity. This results from inequalities in the growth
rate of the marginal and central cells of the leaf. There is a promi-
nent midrib with many smaller, parallel veins which do not anas-
tomose with one another directly, but may be cross-connected
at frequent intervals by transverse veinlets. The number of leaves
on a given axis is variable, ranging from 10 to 18 or more.

The Inflorescences. — Corn is monoecious, normally producing
staminate and carpellate flowers on separate inflorescences on the
same plant. The staminate flowers are borne on a terminal panicle
(tassel) consisting of a central spike and several lateral branches
or rachises arising from the central spike in a spiral arrangement.
The carpellate inflorescence (ear) originates in the axil of one of
the lower leaves and is borne on a short branch or shank. Leaves
arise from each node of the shank and form the husks of the ear.
The internodes of the shank are but slightly elongated, the upper
ones being progressively shorter than the lower so that the husks
overlap one another closely. In most cases, the leaves which form
the husks are only partially developed; but occasionally they may
have normal blades and ligules.

Variations in the monoecious condition are not uncommon.
Plants may develop which have staminate inflorescences only. In
other cases portions of the tassel, usually the central spike, pro-
duce carpellate spikelets bearing fruits. Another variation con-
sists in the development of staminate flowers on the ear, usually
near its apex; and functionally perfect flowers have been known to
occur. In suckers, the inflorescences formed may be normal ears
or normal tassels or there may be any of the variations noted above.

ANATOMY

The Mature Grain. — The fruit of maize is a caryopsis in which
the pericarp, remains of the integumentary tissue, and nucellar
membrane are so intimately connected at maturity as to appear
fused. The mature grain. consists of the hull which is made up of
the pericarp and remains of the integuments, which may or may not

114 THE STRUCTURE OF ECONOMIC PLANTS

be present, the nucellar membrane, the aleurone layer, the starchy
endosperm, and the embryo. The pericarp has been described by
Winton (15), Randolph (15), and others. It has several layers,
the outer zone or epicarp consisting of elongated cells with thick
porous walls and a well-developed cuticle covering the outer sur-
face. The mesocarp or central zone is several layers thick and the
cells resemble those of the epicarp, except that the walls are some-
what thicker. Underlying this is the spongy parenchyma, con-
sisting of cells with radiating arms which are joined in such a way
as to form relatively large intercellular spaces. This zone is much
more pronounced at the pointed end of the grain. Adjacent to the
parenchyma is a layer of tube cells. These are the inner epidermal
cells of the pericarp which are drawn apart laterally as the grain
enlarges and give the appearance of a network over its inner face.
Within the pericarp, there may or may not be some remnants of the
integuments. The outer integument, which never completely
encloses the nucellus, is crushed or resorbed early in development;
but, in some cases, portions of the inner integument persist. This
integument is two-layered except near the micropyle, where there
are three to five layers. Weatherwax (2.1) found crushed remains
of this tissue in the mature grain and referred to it as a testa. Ran-
dolph (15) made an ontogenetic study of the grain in the variety
Pride of Michigan and "concluded that the maize caryopsis has no
true seed coat."

The nucellus in the mature grain consists of non-cellular remains
of the disintegrated tissue and a suberized membrane lying between
the pericarp and the aleurone layer. The endosperm consists of
an aleurone layer and the starchy-parenchyma. The thick-walled,
cubical, aleurone cells lie directly within the nucellar membrane;
and the zone is usually one cell layer in thickness, although it may
be two-layered at some points owing to periclinal divisions. The
starchy-parenchyma consists of thin-walled storage cells. Those
immediately abutting the aleurone layer are smaller than the more
centrally located ones which constitute the horny portion of the
endosperm. They are higher in protein content than the starchy
endosperm and contain large numbers of small polygonal starch
grains. In the larger cells of the starchy endosperm, the grains are
for the most part rounded, and the larger ones have a distinct hilum.
The amount of horny and starchy endosperm varies greatly with
the variety of corn, as do the percentages of carbohydrates, pro-

ZEA MAYS

115

pi-o CO .•?//•

teins, and oils contained in the storage and embryonic tissues of the
grain. According to Randolph, the antipodal cells frequently
proliferate during the
development of the cary-
opsis, and may persist in
the mature grain, form-
ing an oval "mass of
haploid tissue between
the aleurone layer and
the pericarp at the tip
of the kernel."

The Embryo. — The
embryo lies embedded in
the endosperm at one
side and toward the base
of the caryopsis. (Fig.
41.) Its axis is oriented
in such a manner that
the primary root is di-
rected toward the at-
tached end of the grain.
The scutellum is a large
lateral structure lying
directly in contact with
the endosperm, and its
edges partially enclose
the embryonic axis. It
diverges at the coty-
ledonary plate or first
node of the seedling
axis. The tissue of the
scutellum consists of
parenchymatous cells ex-
cept for the abaxial sur-
face, which is covered by
a layer of epithelial cells
where the scutellum is
in contact with the en-
dosperm. The occur-

'Scu no

— coir

pri rt

re

rence of glands in this node. (After Avery.)

Fig. 4z. Longisection of the mature embryo of corn:
colp, coleoptile; coir, coleorhiza; epith gl, epithelial
gland; pri rt, primary root; pro ca str, procambial
strand; r f, root cap; sen, scutellum; scu no, scutellax

ii6 THE STRUCTURE OF ECONOMIC PLANTS

layer has been reported by Sargant and Robertson (17) as
follows :

"The glands are scattered over the whole surface of the epithelium
except where it covers the top of the scutellum." They "differ greatly
in size. Some of the smallest are funnel-shaped pits, others shallow
slits, and it is very difficult to draw the line between structures which
deserve the name of glands and mere depressions or wrinkles."

B

colp no
- mer reg

■ -.\-.-.-adv rt

- scu no

— coir

— pri rt

Fig. 43. Longitudinal diagram of the embryo showing vascular system and meristematic
regions: ^, before germination; B, after germination: ^^j; ^^, adventitious root; colp, coleop-
tile; colp no, coleoptilar node; coir, coleorhiza; mer reg, meristematic region; pri rt, primary
root; scu, scutellum; scu no, scutellar node. (After Avery.)

Their frequency is variable, but they are more abundant on the
upper portion of the abaxial surface and on the wings of the scutel-
lum than on the lower portion.

The primary root is enclosed by the coleorhiza; and the partially
disintegrated suspensor, which persists during the maturation of
the grain, may be seen in median longisections at its distal end.
The coleoptile is cone-shaped, enclosing the embryonic leaves, of
which about five develop, and the growing point of the epicotyl.

ZEA MAYS

117

It arises at the second node, the internodal region between the
scutellar node and the coleoptilar node forming a short, slightly
elongated neck in the embryo. (Fig. 43.) The other internodes
are unelongated so that the embryonic leaves enclosed by the cole-
optile closely surround the leaf primordia and the growing point
of the stem axis. Two or more adventitious roots originate just

level of scutellar
node on axis

A B

Fig. 44. A, face view diagram of scutellum showing its vascular anatomy; B, face view
diagram of the axis of the embryo showing the vascular system : adv rt, adventitious root; colp,
coleoptile; coir, coleorhiza; pri rt, primary root; pro ca, procambial strand; scu no, level of
scutellar node. (After Avery.)

above the level of the scutellar node and are at first directed later-
ally upward. (Fig. 44, B.) The primary root has a well-defined
root cap; and, in the stelar portion, the differentiation of vascular
elements is well advanced.

Avery (i) has described the vascular system of the embryo :

"The embryonic vascular system consists partly of a large procambial
strand laid down in the scutellum. This main procambial bundle
branches throughout its length. In the lower part of the scutellum
are several procambial strands which radiate out and downward
from the level where they diverge from the axis. In longitudinal
face section they appear much like the ribs of an inverted fan. (Fig.
44, A.') The procambial bundles from the upper and lower ends of the

ii8 THE STRUCTURE OF ECONOMIC PLANTS

scutellum are joined at the level of divergence of the scutellum from
the axis and within the embryonic axis are connected directly with the
stele. Continuing from the place of attachment of the scutellar bundle
to the stele it is easily possible to trace procambial elements to the first
leaf above the coleoptile. The vascular system of the coleoptile
consists usually of two vascular bundles one on either side. They
arise directly from the stele on the embryonic axis, at the upper end
of the neck-like interval in the axis already referred to."

Germination. — In germination, the grain begins to swell, the
coleorhiza grows through the pericarp, and is, in turn, penetrated

H.

\—pri rt—-\

Fig. 45. Stages in the development of the seedling: advrt, adventitious root; colp, coleop-
tile; pri rt, primary root. (After Avery.)

by the primary root, which elongates very rapidly. The coleoptile
is pushed upward by the elongation of the first internode, which
results from the activity of an intercalary meristem located just
below the second or coleoptilar node. This internode differs in
this respect from the internodes above it; since, in all others, the
intercalary meristems are located near the base. When the coleop-
tile has been raised until its base reaches the soil surface, its edges
spread apart at the tip and the enclosed foliage leaves begin to
emerge. The adventitious roots, arising above the cotyledonary

ZEA MAYS 119

node, grow upward until they emerge through the pericarp and
they then incline downward. Later a whorl of adventitious roots,
four or more in number, originates just above the level of the second
node. (Fig. 45.)

The Seedling Axis. — The seedling axis has been variously in-
terpreted by anatomists working with the grasses. Celakovsky
(4) considered the scutellum as homologous with the blade of a
leaf and the coleoptile as comparable to the closed ligule of the
foliage leaf of the grasses. He regarded the portion of the axis
between the scutellum and the coleoptile as an elongated node and
called it the mesocotyl. Worsdell (2.6) agreed in part with Cela-
kovsky and considered the mesocotyl an elongated primary node.
He interpreted the scutellum as the lamina of the cotyledon, and
the coleoptile as being a part of the cotyledon comparable to the
ligule of a foliage leaf. He also stated that "the position of the
cotyledon in all monocotyledons as shown by the facts of develop-
ment, there being no epicotyledonary axis present on its first forma-
tion, is always terminal, and is the natural continuation and
termination of the hypocotyl." Sargant and Arber (16) regarded
the scutellum as the sucking apex of the cotyledon, the coleoptile
as the cotyledonary sheath, and the mesocotyl as a unique structure
representing a fusion of the cotyledonary stalk with the hypocotyl.
Weatherwax (xi) pointed out that the embryogeny of maize in-
dicates "that the coleoptile is the homologue of a foliage leaf and
that the cotyledon is a lateral organ." Avery (i) stated that the
scutellum is the cotyledon, "the coleoptile is homologous with a
foliage leaf and is the second leaf of the plant. . . . The elongated
structure between the cotyledon and the coleoptile is the first inter-
node of the axis . ' ' This interpretation appears to be in accord with
the anatomy of the corn seedling; and, under it, the use of the
term mesocotyl is without point.

The Primary Root. — The mature primary root has an exarch,
radial siphonostele with a central pith composed of large paren-
chymatous cells. In the embryonic axis, three distinct histogens
are differentiated in the meristematic growing point of the primary
root. (Fig. 46.) The plerome, or innermost of these histogens,
produces the tissues of the stele and included pith. The periblem-
dermatogen consists of a single layer of meristematic cells overlying
the plerome. Periclinal divisions of the lateral cells of this his-
togen produce two daughter cells, the outer daughter cell becoming

lio THE STRUCTURE OF ECONOMIC PLANTS

an epidermal initial and the inner one a cortical initial. Subse-
quent anticlinal divisions of the epidermal initials produce the
epidermis of the root, while divisions of the cortical initials in all
three planes result in the formation of several parenchymatous
layers of the cortex. The calyptrogen lies immediately outside the

'^Jv^-v^V^

Fig. 46. Median longisection through the tip of the primary root of corn showing the

histogens. (See also Fig. 15.)

periblem-dermatogen layer and by successive periclinal divisions
produces the multi-layered root cap. Some anticlinal divisions
of root cap cells may follow the initial periclinal divisions.

In the development of the stelar tissue, the protoxylem is dif-
ferentiated early in ontogeny, consisting of slender elongated ele-
ments with spiral or annular thickenings. The vessel segments
of the metaxylem increase in diameter very rapidly and may be
broader than they are long, but final maturation is not completed

ZEA MAYS

III

, rt hr

exarch

until after that of the protoxylem. As they mature, the vessel
segments become somewhat longer, the end walls are resorbed, and
the secondary thickening of the lateral walls results in a narrowly
reticulate or pitted type. The phloem elements are elongated and
thin-walled.

The number of protoxylem points is variable, ranging from xo to
40, and there are commonly two or three protoxylem strands to
each large metaxylem vessel.
(Fig. 47.) The groups of
primary phloem are on radii
alternate with the protoxylem
and abut the pericycle. The
parenchymatous cells which
lie between the xylem points
and surround the phloem be-
come thickened and lignified,
forming a continuous zone of
thick-walled connective tis-
sue as the root matures. The
pericycle consists of a single
layer of thin-walled cells
which may become thick-
walled later in ontogeny; and
lateral roots originate in this
region. The endodermis is „ _ . , r ,. •

o Fig. 47. Transection or a sector or the pri-

alsO a single layer; and, at maryroot: «, cortex; g«, endodermis; ep,epideT-

maturity its cells have the U- "^'^' ""^^ metaxylem; pd, pericycle; ph, phloem;

•^ ' r 1 • 1 • • ?'' P'"^hi P^' protoxylem; rt hr, root hair.

shaped type of thickening in (After Avery.)
which the end, radial, and

inner tangential walls are reinforced. The passage cells have no
secondary thickening except the Casparian strip, and they usually
occur on the same radii as the protoxylem points. (Fig. 14.) The
cortex is comprised of several layers of parenchymatous cells
bounded externally by a more or less ephemeral epidermis. It pro-
duces an abundance of root hairs immediately back of the zone of
root elongation; but disintegrates later, and the outer cells of the
cortex form a lignified and suberized exodermis.

The ontogeny of the seminal roots arising above the scutellar
node, and of the adventitious roots that develop later in the basal
intercalary meristems of higher internodes, is essentially like that

ixL THE STRUCTURE OF ECONOMIC PLANTS

of the primary root. The seminal roots may be somewhat smaller
than the primary root with fewer protoxylem points. The princi-
pal adventitious or prop roots are much larger than the primary
root, and the number of xylem points may be double that found in
the primary stele. Another difference exists in the aerial portions
of the adventitious roots where the epidermis instead of disinte-
grating is persistent, developing a cuticle similar to that of the
stem.

Adventitious roots may arise in the pericyclic region of the first
internode and in the meristematic basal portions of any of the
higher internodes. A root primordium is differentiated from a
ring of meristematic tissue which is comparable in function and
location to a pericyclic zone although there is no definite endoder-
mis to determine its outer limit. The elongating root then forces
its way through the cortex and the basal portion of the leaf which
ensheathes the node. The emergence of the lateral is accomplished
by a splitting of the cells of the regions penetrated and by digestive
action. (Fig. xi.)

Hypocotyl and Stem. — The hypocotyl is very limited in extent
and strictly root-like in structure so that the description given
for the mature primary root applies to the axis up to the cotyle-
donary node. The vascular transition is epicotyledonary, occur-
ring chiefly in the vascular plate of the first node and in the first
internode. It is continued to some degree in the second internode
in which transition bundles may be found; and, occasionally, there
may be some transitional structures in the third and fourth inter-
nodes.

The First Internode. — The first internode has a central pith
with an annular zone of vascular elements bounded by a pericycle
and endodermis, both of which are one cell layer in thickness.
(Fig. 48.) The cortex, consisting of thin-walled parenchyma, is
several cells in thickness and the epidermal cells have a thin cuticle.
In the stelar region, there are groups of exarch xylem which usually
alternate with endarch collateral bundles. The primary phloem
is in a collateral position with respect to the endarch primary
xylem; and in an alternate, radial position in relation to the exarch
xylem groups. Thus, the transitional region exhibits an intermix-
ture of root-like and stem-like orientations of the vascular tissues.

The Coleoptile. — The coleoptile is the second leaf of the seed-
ling axis and arises at the second node. Its elevation to approxi-

ZEA MAYS 12.3

mately the soil level is accomplished by the activity of meristematic
cells immediately below the second node. (Fig. 49.) The pri-
mordium of the coleoptile is initiated as an open sheath; but, as
growth continues, the edges soon unite, forming a closed structure
although the line of union remains visible, especially near the top.

--en
--pel

~ ~P'^ I endarch
- -mx\ collateral
I bundle

Fig. 48. Transection of the first internode of the axis showing transitional character
with endarch collateral bundles, and radial arrangement of primary phloem and exarch
xylem: co, cortex; en, endodermis; mx, metaxylem; pel, pericycle; pf), phloem; pi, pith;
px, protoxylem. (After Avery.)

The structure of the coleoptile is simple, consisting of a relatively
compact chlorenchyma and two vascular bundles which are bilater-
ally placed. Each bundle is made up of several xylem strands
which partially surround the phloem so that it may be regarded as
half-amphi vasal. In exceptional cases, the number of vascular
bundles in the coleoptile may exceed two; and, in some instances,
four are formed by a division of the original bundles. Avery (i)
has reported "the presence of as many as five bundles. The extra
bundles have ordinarily an origin similar to that of the usual two,
although in the case of five, the fifth bundle usually arises de novo

124 THE STRUCTURE OF ECONOMIC PLANTS

in the cortex below the coleoptilar node." A bud is seldom dif-
ferentiated in the axil of the coleoptile. Stomata are found only
in the outer epidermis and usually occur in rows on either side of
the vascular bundles.

-0 ep

- - chl par

ph

'- -xy

Fig. 49. Transection of a portion of the coleoptile showing coleoptilar bundle: chl par,
chlorophyll parenchymai 0 ep, outer epidermis; ph, phloem; xj>, xylem. (After Avery.)

Second Internode. — The second internode is also transitional.
(Fig. 50.) It is less root-like than the first internode, but lacks the
definite meristelic structure of the higher internodes. The cortex
is relatively thick, and centrad to it there is a meristematic zone of
tissue which is probably of pericyclic origin. In this internode
there are transitional bundles of several types, as well as scattered
groups of xylem and phloem elements in various orientations.
The transition is practically complete at the third node, and the
internodes above this point have a meristele of collateral bundles.

Early in ontogeny, the final pattern of the plant is determined.
By the time the stem axis is an inch long or even less, all the plant
structures — leaves, nodes and internodes, terminal tassel, lateral

ZEA MAYS

ii5

ears, primary and adventitious root systems — have been suffi-
ciently differentiated that their topography can be readily deter-
mined.

The Stem. — The mature stem is a meristele with bundles scat-
tered throughout the fundamental parenchyma. The bundles near
the periphery of the stem are smaller and more numerous than those

tran bit

Fig. 50. Transection of the second internode showing transitional character: co, cortex;
ep, epidermis; pfi, phloem; tran hu, transition bundle; xy, xylem. (After Avery.)

toward the center of the axis. The centrally located parenchym-
atous cells are large and thin-walled, with intercellular spaces at
their angles. Those adjacent to the epidermis are smaller; and, in
the mature stem, become thick-walled and lignified, forming a
protective rind inside the epidermis which also lignifies at maturity.
The stomata, guard cells, and accessory cells are similar to those
of the leaf . (Fig. 15.)

The node is woody and rigid and its vascular anatomy is very
intricate owing to the fact that at each node there is a large number

12.6 THE STRUCTURE OF ECONOMIC PLANTS

of bundles entering the stem which penetrate the nodal zone hori-
zontally for various distances before they incline downward. In
addition to this, although many of the bundles of the internode
above pass directly through the node, a certain proportion of them
terminate at each node by anastomosing with other bundles.

Evans (6) investigated the vascular anatomy of the node using
a technique involving staining with methylene blue and retting.
He found that the vascular bundles "seldom pass through more
than two or three nodes without branching"; and points out that
the nodal complex results from numerous small branches that arise
from the vascular bundles as they enter the node, and from the
small peripheral bundles. The division, subdivision, and anas-
tomosing of these bundles account for the vascular meshwork
which constitutes the nodal plate.

The Stem Bundle. — The collateral or amphivasal bundle is en-
closed by a sheath of mechanical tissue, and is generally oriented
with the phloem toward the periphery of the stem. (Fig. 51.)
There are usually two protoxylem vessels in each bundle, one with
annular and the other with spiral wall thickenings. The two large
laterally placed metaxylem vessels are either narrowly reticulate or
pitted, the pits being of the simple unbordered type. Between the
two large metaxylem vessels is a connecting band of smaller pitted
tracheids. As the bundle matures, the parenchyma surrounding
the two protoxylem elements becomes separated from them, form-
ing an air space or lacuna; and at this time the unthickened portion
of the wall of the annular vessel is frequently crushed or destroyed.
The phloem, which lies in a collateral position with respect to the
xylem, consists of a small number of protophloem cells and meta-
phloem in which the sieve tubes and companion cells are very
regularly arranged. As the bundle matures, the protophloem
becomes non-functional and the crushed cells form a narrow band
just outside of the metaphloem. The bundle is of the closed type
and all the cells of the provascular strand become mature primary
vascular elements without the formation of a cambium.

Vascular Anatomy of Stem. — All the bundles of the stem are
common and their downward course is nearly vertical, although
there may be some lateral and tangential curvature. The course
of the bundles has been described by de Bary (x) following the
classic account of von Mohl. The bundles of the leaf trace enter
the nodal plate from the base of the leaf and pass downward

ZEA MAYS

1X7

through the internode. Upon entering the node, some pass directly
downward near the periphery of the stem and are almost radially
perpendicular; while others penetrate the stem more deeply, fol-
lowing a horizontal course. These may at first even incline
slightly upward and then curve downward toward the periphery,
where they become almost vertical. All of the bundles descend
through two or three or more internodes without branching or
anastomosing with other bundles.

Fig. 51. Section of the sixth internode of the stem : an v, annular vessel ; bu sh, bundle
sheath ; ep, epidermis ; lac, lacuna ; mph, metaphloem ; mx, metaxylem ; par, parenchyma ;
/I/'/', protophloem ; /)x, protoxylem ; j/» f, spiral vessel ; /r^, tracheids. (After Avery.)

As a result of the anastomoses of descending bundles with those
which enter the stem at lower nodes, the total number of bundles
in the lower internodes is approximately the same. Hershey and
Martin (9) investigated the development of the bundles in yellow
dent corn and found that 90 per cent of them were differentiated in
the lower internodes by the forty-fifth day. The number formed
at maturity was 680 for the first internode and 750 for the second.

Since the entering bundles incline toward the periphery of the
stem in their downward course, they are more crowded near the sur-
face than in the central portion of the axis. (Fig. i5.) The in-
clination of the bundles toward the periphery is not readily detected

ii8 THE STRUCTURE OF ECONOMIC PLANTS

in mature stems, since the internodes are very long and the outward
curvature of the bundles occurs in the internodes prior to inter-
nodal elongation by intercalary growth.

The Leaf. — The leaf consists of a sheath, a collar-like ligule,
and a blade. All the main veins are parallel and there are cross-
connecting veinlets. The mesophyll is comprised of relatively
compact chlorenchyma with few intercellular spaces, and there is
no differentiation into palisade and spongy tissue. The collateral
vascular bundles are of two sizes, the larger type resembling the
stem bundle except in two respects. The mechanical tissue, instead
of completely surrounding the bundle, consists of a zone of thick-
walled elements, above and below it. This tissue abuts the upper
and lower epidermis and, with the mechanical elements of the mid-
rib, serves as the chief support of the leaf. Surrounding the bundle
is a sheath of chlorophyllose cells in which the chloroplasts are
definitely larger than those found in the mesophyll. The smaller
type of leaf bundle has no adjacent mechanical tissue and consists
of a few xylem and phloem elements surrounded by a sheath of
parenchymatous cells containing large chloroplasts. The trans-
verse bundles which cross-connect the small parallel veins with one
another or with a larger bundle consist of one or a few xylem
elements with or without accompanying phloem elements. (Fig.

Randolph (14) investigated the cytology of the several types of
chloroplasts in corn and found them to be the same in their initial
structure. They originate as very minute proplastids and gradu-
ally enlarge, developing chlorophyll until they reach maturity.
He also observed that both partially developed and fully matured
plastids may divide, but was unable to determine the origin of the
proplastid because of its small size. The question as to whether
the plastids are permanent cell organs with an unbroken genetic
continuity or arise de novo is still undetermined.

Both epidermal surfaces are cutinized, the lower being more so
than the upper; and both contain stomata arranged in parallel
longitudinal rows. Varying stomatal counts have been made on
maize, but all are in agreement that the number in the lower sur-
face exceeds that in the upper. Moore (ii) has made a study of the
histology of the upper epidermis and has recognized three zones
that run lengthwise of the leaf. These are the zone of bulliform or
motor cells, the zone of narrow epidermal cells, and the stomatal

ZEA MAYS

119

zone. The bulliform zone is a band of epidermal cells two to five
or six rows in width which consists of motor cells, basal cells, and
the trichomes arising from the latter. The bulliform cells are
approximately hexagonal, protruding slightly above the surface of
the leaf, and extending deeper in the mesophyll than do the other
epidermal cells. The trichome, which arises from the center of a
group of basal cells, is a single thick-walled cell with a neck-like
constriction where it passes through the basal cells.

The zone of narrow epidermal cells is limited to those portions
of the surface which overlie the vascular bundles; and varies in
width depending upon the size of the bundle. The cells are narrow

ac c

I sp

Fig. 51. Transection of a portion of the leaf : ^cc, accessory cell ; ^«, bundle; e/^, epidermis ;
g c, guard cell ; i sp, intercellular space ; m c, motor cell ; ph, phloem ; scl, sclerenchyma ;
sto, stoma; xy, xylem. (After Avery.)

w^ith sinuous margins and may lie end to end in a series. Fre-
quently, they are separated from one another by short cells of two
kinds — silicified cells and crescent-shaped cells, the latter usually
occurring in pairs or in a series of pairs. The silicified cell is two-
to four-lobed and may contain crystalline inclusions. The cres-
cent-shaped cells are roughly semilunar, three to five times as wide
as long, with walls that are somewhat thinner than those of the
adjacent epidermal cells.

The stomatal zone ranges from three to fourteen cells in width
and contains epidermal cells with very undulate walls. They may
be joined end to end in series; but, more commonly, are separated
from one another by two-celled hairs, crescent-shaped cells, silici-
fied cells, horn cells, or the stomatal cells. The two-celled hair is
probably a modification of the crescent-shaped cell and is thin-
walled with dense cytoplasm. The horn cells are usually confined
to the border of the bulliform zone and are fairly thick-walled,
frequently being associated with silicified and crescent-shaped

I30

THE STRUCTURE OF ECONOMIC PLANTS

cells. The guard cells of the stomatal apparatus are dumb-bell
shaped, and each one is paired with a triangular accessory cell;
Campbell (3) has described the development of the stomata in
maize. A vertical w^all is formed across the end of an epidermal
cell in the stomatal zone, cutting off a daughter cell w^hich is very
short. (Fig. 53.) This mother cell lengthens rapidly until it is
approximately square in surface outline, and, while it is develop-
ing, two small cells at the sides are cut out from laterally adjacent
epidermal cells. The accessory cells at first barely keep pace with
the growth of the mother cell; but, finally, grow much more

Fig. 53. The development of stoma showing successive stages: ac c, accessory cells; g c,
guard cells ; m c, mother cell ; sto, stoma. (After Campbell, American Naturalist.')

rapidly and in so doing modify the shape of the guard cells. The
mother cell rounds up and divides longitudinally, forming the
two guard cells; and, as these lengthen, the accessory cells begin
to grow more rapidly and become sub-triangular. The guard cells
continue to elongate until they are rectangular and two or three
times as long as broad. Meanwhile, the stomatal aperture which
is formed schizogenously between the opposing faces of the guard
cells elongates, the guard cells attain their final shape, and the
accessory cells become larger and more triangular.

The Staminate Inflorescence. — The staminate inflorescence is
a broad panicle consisting of a central rachis, which is a continu-
ation of the central axis of the stem, and its lateral branches.
(Fig. 54.) The central rachis bears several rows of paired spike-
lets, while the lateral branches have only two rows of paired spike-

ZEA MAYS

131

lets. One of the spikelets of each pair is pedicillate, the other
sessile. The staminate spikelet may be regarded as the unit of the
inflorescence and is normally two-flowered, although one-flowered
spikelets have been observed in cases where the second flower is
abortive.

At the base of the spikelet are two glumes which subtend and
enclose the flowers. Up to the time of anthesis, these are firm

Fig. 54. Staminate inflorescence showing : A, a terminal panicle with its central spike and
lateral rachises; B, the paired spikelets; and C, habit of a partially opened spikelet.

and thickly covered with stiff bristle-like hairs with the lower
glume overlapping the edges of the upper one. Each flower is sub-
tended by a lemma and a palea, the paleas of the two flowers of the
spikelet lying back to back. At the base of each lemma are two
lodicules which become very much enlarged at anthesis and hold
the glumes open as the filaments elongate. The three stamens are
arranged so that two are adjacent to the palea and one is in a dorsal
position next to the lemma. The rudimentary carpel is centrally
located. The two flowers of the spikelet are identical in structure,
but the upper one is the first to mature. Weatherwax (2.6) has
pointed out that the early development of staminate and carpellate

132. THE STRUCTURE OF ECONOMIC PLANTS

flowers is very similar, "up to the time of differentiation of the
primordia of the stamen and pistil." As described by him,

"The spikelet primordium makes its appearance as a rounded protuber-
ance on the rachis. The first differentiation to appear is the formation
of the lower glume, and it is soon followed by the upper one. (Fig. 55.)
The lemmae arise almost simultaneously with the appearance of the
stamens of the upper flower. From the lower side of the undiffer-
entiated end of the spikelet now appears the primordium of the lower
flower and the palea of the upper flower soon follows. The palea of
the lower flower appears much later. The older flower seems to be
terminal and the younger one lateral on the rachilla, but it is probably
better to consider both flowers lateral branches of this axis, which
terminates between the two paleae. ... In the development of
the flower from its primordium the stamens are first differentiated;
these are followed by the lodicules and the part that is left is the
primordium of the pistil. ... In both flowers of the male spikelet
the stamens and lodicules are fully developed but the development
of the pistils is soon arrested, and they disorganize."

Fig. 55. Diagrams showing details of development of staminate spikelet: fl, flower pri-
mordium ; g, glume ; Im, lemma ; lo, lodicule; 0 w, ovary wall ; p, palea ; psl, pistil ; st, sta-
men. (Redrawn after Weatherwax, Bull. Ton. Bot. Club.^

The Carpellate Inflorescence. — The carpellate inflorescence
is a thickened spike which ordinarily develops into the ear. Sev-
eral different views obtain as to the evolutionary development of

ZEA MAYS 133

this type of inflorescence. One, advanced by Harshberger (8), sug-
gests that the lateral branches or rachids of a primitive inflorescence
were united or undiverged to form the ear. On the other hand,
Montgomery (11) maintains that only the central spike of the
primitive tassel has persisted to form the ear. An alternative
theory has been advanced by Collins (5) on the basis of his study
of seed hybrids between maize and Euchlaena. He suggests that
"the shortening and twisting of the axis of a single spike" may be
"a possible method of deriving a structure like the maize ear from
the inflorescence of Euchlaena." In all probability, none of these
explanations is complete and a final interpretation must await
further studies on the phylogeny of the grasses.

The characteristic two-ranked appearance of the mature grains
on the ear is due to the manner in which the spikelets and their
flowers develop. This has been fully described by Miller (10) and
Weatherwax (19, lo) and the essential details are here summarized.
Just back of the meristematic tip of the young axis of the cob,
numerous primordia arise on its periphery. Each of these rudi-
mentary primordia, by an equal division, forms a pair of primordial
lobes which lie side by side in the same transverse plane; and these
in turn develop into the spikelets. The arrangement of the spike-
lets in transverse pairs is thus the result of the common origin of
their primordia from a single rudimentary primordium.

The spikelet is two-flowered, but the lower flower is usually
abortive so that a single grain is produced in it. Since the spikelets
arise in pairs and a single grain is produced in each one, the grains
on the ear have the typical double-rowed arrangement mentioned
above. There are some notable exceptions to this arrangement, as
in Country Gentleman sweet corn, in which both flowers of the
spikelet regularly develop grains. This results in crowding and
an irregular arrangement. Other variations include cases in which
the grains are paired, connate grains where two kernels are more
or less completely united, and anomalous kernels with twin
embryos arranged side by side. Stratton (18) studied the develop-
ment of the double kernel in Zea Mays var. polysperma. In this
case, both flowers of the spikelet are functional and several types
of arrangement of the grains occur. In some instances, a pair of
separate kernels lie back to back; in others, connate seeds develop
with double kernels enclosed in a common pericarp, formed from
the coalesced ovaries of the two flowers. Semi-connate types

134 THE STRUCTURE OF ECONOMIC PLANTS

also occur where the pericarp extends only partially between the
kernels.

In the mature carpellate spikelet, like the staminate one, the
two flowers are enclosed by a pair of glumes and each flower is
subtended by a lemma and palea, these being much shorter than the
glumes. The two paleas are back to back and separate the flowers.
The sterile (lower) flower has a rudimentary carpel, three rudimen-
tary stamens, and two well-developed lodicules which lie outside of
and alternate with the stamens. The fertile (upper) flower has a
well-developed carpel, and rudimentary stamens can be distin-
guished in the early stages of ontogeny. The lodicules of the fertile
flower are not easily recognized when the spikelet is mature. The
palea and lemma of the lower abortive flower persist and with those
of the fertile flower constitute the chaff found at the base of the
mature grain.

The sequence of development of the parts of the carpellate
spikelet is much like that of the staminate. From the primordium,
the lower glume, upper glume, and lemmas of the two flowers are
developed in the order named. These are followed closely by the
rudiments of the sterile flower and then the rudimentary stamens of
the fertile flower appear. The palea of the fertile flower next
differentiates and the palea of the sterile flower arises somewhat
later.

Following the appearance of the palea and rudimentary stamens
of the fertile flower, the carpellary primordium of that flower
begins to develop. (Fig. 56.) This arises as a ring from the
rounded mass of meristematic tissue which remains after the
divergence of the staminal primordia. Growth of this ring is
uneven, the side adjacent to the lemma growing rapidly so that it
may extend one-third of the way around the young nucellus before
the opposite side has begun to develop. The more rapidly growing
portion of the ovary wall becomes thicker and more meristematic
at its top; and, from this region, the silk or style immediately
begins to develop. During the elongation of the style, the ovary
wall continues to grow until it completely encloses the ovule except
for a small opening at the top of the ovary and adaxial to the style.
This opening, which Guignard (7) termed the "stylar canal," later
closes as its edges come in contact. These do not unite, however,
and the canal can be recognized in the mature ovary.

The silk is solid, somewhat flattened, with two grooves running

ZEA MAYS

135

longitudinally throughout its length. Two vascular bundles ex-
tend from its unequally branched apex through the walls of the
ovary to its base, where they are continuous with other bundles
supplying the spikelet. These are surrounded by slender sheath
cells that are densely cytoplasmic with elongated nuclei. The
pollen passes down the silk between these dense cells until it
reaches the ovarian cavity. The surface of the silk is hairy; and,
throughout most of its length, is receptive to pollen so that the
major portion of it may be regarded as a stigma.

Fig. 56. I, longisection of young carpellate spikelet; i, the same, at the time the carpel
wall has begun to develop; t,, longisection of the fertile carpel showing nucellus still in erect
position ; 4, longisection of the carpellate spikelet at the time of pollination : c and c\ develop-
ing ovary walls, c' is the portion of the carpel from which the style or silk will develop;
CO, ovarian cavity; e s, embryo sac; g, lower glume; g', upper glume; hit, integuments;
/w, lemma of fertile flower ; /w', lemma of sterile flower ; mic, micropyle; mmc, megaspore
mother cell ; or, ovule; ova, owiry ; 0 w, ovary wall ; />, palea of fertile flower ; /?', palea of
sterile flower ; pp, primordium of carpel ; ra, rachilla ; s c, stylar canal ; s fl, sterile flower ;
j/^, silk or style ; jr, rudimentary stamen ; fi, vascular bundle ; vZij, one of the vascular bundles
of the silk; vsc, sheath cells of the silk. (Diagram 4 reduced to approximately one-sixth
the size of diagrams 1-3. Redrawn after Miller, Jour. Agr. Rw.)

Development of the Ovule. — Although the older interpreta-
tion that the pistil consists of a single carpel is accepted by some
morphologists, the more generally held view is that it is tri-car-
pellate. Randolph (15) has summarized this concept in describing
the formation of the ovary and the development of the ovule. The
ovary consists of three undi verged carpels, and a single ovule
arises near the base of the ovary on the side adjacent to the rachilla.

136 THE STRUCTURE OF ECONOMIC PLANTS

The midribs of the bundles of the lateral carpels extend into the
silk, their marginal bundles supplying the ovule. The third carpel,
which is regarded as rudimentary, has a vestigial median bundle
that is sometimes present at the base of the ovary opposite the point
of divergence of the ovule. The nucellus arises in an erect position;
and, shortly before the megaspore mother cell is differentiated,
there is an acceleration of cell division and growth on the side of
the nucellus adjacent to the palea. As a result of this differential
growth, the ovule finally becomes semianatropous or approximately
campylotropous. There is no well-defined funiculus at the broad
base of the ovule, and the two integuments which are initiated at
this point are free from each other. The inner integument com-
pletely surrounds the nucellus except at the micropyle; but the
outer one extends only partially around it, forming a wedge-
shaped angle where it projects into the funnel-like depression of
the stylar canal. (Fig. 56-^.)

The Megagametophyte. — Megasporogenesis occurs at about
the time that the ovule begins to curve. The megaspore mother
cell is then differentiated and subsequently four megaspores are
produced, the innermost one being functional. When the mega-
gametophyte reaches the eight-nucleate stage, two polar nuclei
move to the center of the embryo sac; but they do not completely
fuse prior to fertilization. The three antipodal nuclei begin to
divide at once, and the number increases until as many as 14 to 36
cells are formed which may be binucleate. Meanwhile, the
synergids become elongated or lunar-shaped with very dense con-
tents. In most instances, they do not remain intact after the time
for fertilization has arrived and usually disintegrate prior to that
event. The megagamete increases in size until its width is approxi-
mately half that of the embryo sac.

The Microgametophyte and Pollination. — The development
of the stamens is like that of other grasses; and, following reduc-
tion division, the pollen matures to form grains that are almost
spherical, with a minutely roughened exine and a prominent germ
pore. A thickened ring of the intine surrounds the pore and closes
it with a plate of tissue that resembles the rest of the wall in struc-
ture. The pollen grain, now a microgametophyte, has dense
protoplasm and contains a vegetative and a generative nucleus.
Before shedding, the latter divides to form two long, slender
microgametes that are crescent-shaped and pointed at the ends.

ZEA MAYS 137

The fine, light pollen is well adapted to wind pollination.
Gravity may play some part in the process, but it is unlikely that
many insects act as pollinating agents. A few hours after the pol-
len grain lodges on the hairs of the silk, the pollen tube pushes
through the germ pore and gains access to the sheath cells adjacent
to the vascular bundles of the silk. Practically all functional
pollen tubes develop from pollen grains that are lodged on the
multicellular hairs of the silk, although occasionally penetration
may be other than through a hair. Growth of the pollen tube in
the silk is very rapid and it may reach the embryo sac within twenty-
four hours after pollination. Upon the entrance of the pollen into
the embryo sac, fertilization is effected. This is double in charac-
ter, one microgamete uniting with the megagamete to form the
zygote, and the other with the polar nuclei to form the primary
endosperm nucleus. The latter undergoes division directly after
fusion; and, as a result of several free nuclear divisions, there are
usually four to eight endosperm nuclei formed before the division
of the zygote takes place.

Embryogeny. — The zygote divides into two unequal cells about
i8 to 34 hours after pollination; and further divisions of the smaller
apical cell form the embryo, while the basal cell divides to form a
massive suspensor. The subsequent development has been de-
scribed in detail by Randolph (15). He observed three- or four-
celled embryos 36 hours after pollination; and at the end of four
days, they usually were comprised of from 10 to 14 cells. Unlike
the ontogeny of many grass embryos, there is little regularity in
the sequence of cell divisions or arrangement of the cells in tiers,
so that the proembryo cannot be described as conforming to a regu-
lar pattern.

From the fourth to eighth days, growth is restricted chiefly
to the apical region, there being few divisions of the suspensor;
and, as a result of this differential growth, the embryo becomes
club-shaped. About eight days after pollination, the epidermis is
differentiated in the apical portion of the embryo and extends to-
ward the suspensor.

The axis of the more mature embryo can be determined in about
ten days, when a slight protuberance is formed on the anterior sur-
face of the proembryo. This represents the apex of the stem axis
of the embryo and is oriented at an oblique angle to the pro-
embryo. During the period from ten to fourteen days after pollina-

138 THE STRUCTURE OF ECONOMIC PLANTS

tion, the meristems of the primary root and epicotyl are differen-
tiated, the scutellum enlarges, and there is also considerable
growth of the suspensor. In the ensuing week, the primordia of
the coleoptile and first foliage leaves arise, which usually number
five by the time the embryo is mature. The suspensor ceases to
enlarge after about twenty days. (Fig. 57.)

The growth of the embryo is rapid during the next three weeks;
and, at the end of about forty-five days, it is morphologically
mature. The coleoptile is differentiated as a closed sheath about

cot (scu)

Fig. 57. Diagrams i-6, steps in the development of the embryo ; 7, longisection of nearly
mature embryo, reduced one-half : w/^, coleoptile ; co/*-, coleorhiza ; co/, cotyledon or scutel-
lum; /, leaf; r c, root cap; rt, root; su, suspensor. (Redrawn after Weatherwax, The
Story of Maize, Univ. of Chicago Press.)

the young foliage leaves; and the portion of the axis between the
suspensor and the cotyledonary node forms the primary root within
the surrounding tissue of the coleorhiza. The differentiation of
two or more seminal roots just above the cotyledonary node
completes the ontogeny of the embryo.

The Endosperm. — According to Randolph, the primary endo-
sperm nucleus undergoes division prior to that of the zygote, and
free nuclear divisions occur so that three days after pollination
approximately ii8 free nuclei may be present. Shortly after this
time, wall formation is initiated in the endosperm adjacent to the
embryo and it becomes almost completely cellular by the end of
four days, except in the antipodal region. The size and shape of
the endosperm changes rapidly thereafter, and the dimensions of the

ZEA MAYS 139

mature endosperm are approximately fifty times that of its initial
stage. This growth is accomplished by a general cell division of
the parenchymatous cells; but, later, there is a localization of this
activity to the peripheral region of the endosperm. The outermost
layer of the endosperm, which forms the aleurone zone, exhibits
certain epidermal characteristics; and, after about three weeks,
divides only anticlinally. The subepidermal cells may continue to
divide until the embryo is mature and are consequently smaller than
the aleurone cells.

The Antipodal Tissue. — The persistence of antipodal tissue
has been noted by Weatherwax (x}) and Randolph (15). In many
grasses, the antipodals are reported as disintegrating during the
formation of the endosperm; but in corn, this haploid tissue (10
chromosomes) persists until the caryopsis is mature. It occupies
a position near the micropyle during the early ontogeny; but as
the endosperm enlarges is displaced and comes to occupy a region
in the crown of the kernel. At maturity, the antipodal tissue con-
sists of hundreds of cells, which are packed with starch, surrounded
by an epidermis that is starch-free.

LITERATURE CITED

I. Avery, G. S., Jr., "Comparative anatomy and morphology of embryos and

seedlings of maize, oats and wheat." Bot. Gaz- Sp: 1-39, 1930.
z. DE Bary, a., Comparative Anatomy of the Vegetative Organs of Phanerogams and
Ferns, Clarendon Press, Oxford, 1884.

3. Campbell, D. H., "On the development of the stomata of Tradescantia and

Indian corn." Am. Nat. i;: 761-766,1881.

4. Celakovsky, L. J., "Uber die Homologien des Grasembryos." Bot. Zeit. //.•

141-174, 1897.

5. Collins, G. N., "Structure of the maize ear as indicated in Zea-Euchlaena

hybrids." Jour. Agr. Res. 17: 117-135, 1919.

6. Evans, A. T., "Vascularization of the node in Zea mays." Bot. Gaz. ■?/•'

97-103, 19x8.

7. GuiGNARD, L., "La double fecondation dans le mais." Jour, de Bot. //.•

37-50, 1901.

8. Harshberger, J. W., "Maize, a botanical and economic study. Contr. Bot.

Lab. Univ. Pa. i: 75-101, 1893.

9. Hershey, H. L., and Martin, J. N., "Development of the vascular system of

corn." Proc. Iowa Acad. Sci. }j: 115-116, 1930.
10. Miller, E. C, "The development of the pistillate spikelet and fertilization
in Zea mays L." Jour. Agr. Kes. 18: i.'^'^-i.Gj, 1919.

II. Montgomery, E. G., "Perfect flowers in maize." Pop. Sci. Monthly 79:

346-349, 1911.

I40 THE STRUCTURE OF ECONOMIC PLANTS

II. Moore, R. H., "Cell specialization in the epidermis of maize." JJnpubl. MS.
Thesis, Univ. of Okla., 192.9.

13. Randolph, Fannie R., "A cytological study of two types of variegated

pericarp in maize." Cornell Univ. Agr. Exp. Sta. Mem. 102, 1916.

14. Randolph, L. F., "Cytology of chlorophyll types of maize." Bat. Gaz- 73:

337-375. 192-2-

15. , "Developmental morphology of the caryopsis in maize." Jour.

Agr. Kes. /^.- 881-916, 1936.

16. Sargant, Ethel, and Arber, Agnes, "The comparative morphology of

the embryo and seedling in the Gramineae." Ann. Bat. 2p.- i6i-iii, 191 5.

17. , and Robertson, A., "The anatomy of the scutellum in Zea mays."

Ann. Bot. ig: 115-114, 1905.

18. Stratton, Mildred E., "The morphology of the double kernel in Zea mays

var. polysperma." Cornell Agr. Exp. Sta. Mem. 6g, 192.3.

19. Weatherwax, p., "Morphology of the flow^ers of Zea mays." Bull. Torr.

Bot. Club 4}.- 117-144, 1916.
2.0. , "The development of the spikelets of Zea mays." Bull. Torr. Bot.

Club 44: 483-496, 1917.
ii. , "Position of scutellum and homology in coleoptile in maize." Bot.

Gaz- 6g: 179-181, 1910.
11. , The Story of the Mai%e Plant, Univ. of Chicago Press, 1913.

13. , "Persistence of the antipodal tissue in the development of the seeds

of maize." Bull. Torr. Bot. Club $3: 381-384, 1916.

14. Weaver, J. E., Koot Development of Field Crops, McGraw-Hill Book Co., 1916.

15. Winton, a. L., and Winton, Kate B., The Structure and Composition of Foods,

Vol. I, Wiley and Sons, 1931.

16. Worsdell, W. C, "The morphology of the monocotyledonous embryo and

of that of the grass in particular." Ann. Bot. 30: 509-514, 1916.

CHAPTER VI

GRAMINEAE — Continued

TRITICUM spp.

WHEAT is grown in the temperate regions of every continent,
within the Arctic Circle, and in the highlands of Ecuador
and Colombia at the Equator. In the United States, it is a winter
or summer annual, being harvested in the southern states in June
and in the northern wheat areas in July and August.

There has been much discussion as to the origin of wheat; and,
although some botanists maintain that all cultivated forms are
derived from a single wild ancestor, the majority, according to
Percival (lo), "agree that the wheats are polyphyletic, and that
at least two species are concerned in their origin. ' ' Several systems
of classification have been proposed for the numerous species, races,
and varieties which are recognized at the present time. Hackel (6)
has divided the genus into two sections, Aegilops and Sitopyros;
and the latter, which includes the cultivated wheats, into three
main species. The first of these is T. monococcum L., commonly
known as Einkorn or one-kerneled wheat, which is cultivated in
Spain and probably grows wild in Greece and Mesopotamia. The
second, T. sativum Lam., is the commonly cultivated wheat, which
he further subdivides into three principal races : T. spelta L., which
includes the ordinary spelt wheats; T. dicoccum Schr., emmer
wheat; and T. tenax. The last named has four subraces: T. vul-
gare Vill., common wheat; T. compactum Host., club wheat;
T. turgidum L., poulard wheat; and T. durum Desf., durum wheat.
The third main species is T. polonicum L., Polish wheat, which is
not a native of Poland, but occurs in Italy and Abyssinia and is
cultivated to some extent in North America.

Percival recognizes two wild species, and classifies the cultivated
wheats in eleven natural groups or races of major rank. Of these,
the common bread wheat, Triticum vulgare, is the best known and
most generally used. This may be attributed to its physical

141

141 THE STRUCTURE OF ECONOMIC PLANTS

qualities, which are suited to bread-making, and to its adaptability
to a variety of climatic conditions, which has resulted in its being
the most widespread of all cultivated forms. For this reason, the
following account is based upon T. vulgare unless otherwise stated.

GENERAL MORPHOLOGY

The Root System. — The root system is dual, consisting of a
primary seminal system and a permanent adventitious one which

forms a deeply penetrat-
ing, widely spread, and
profusely branched system
of fibrous roots. The pri-
mary system typically con-
sists of five roots, the tap
root and two pairs of lat-
erals which arise in the
region of the vascular
plate of the scutellar node.
(Fig. 58.) When a sixth
root arises, it occurs at a
point near the base of the
coleoptile adjacent to the
epi blast; and a third pair
of rootlets sometimes de-
velops at the first inter-
node above the second pair
and in the same plane with
them. All the seminal
roots are slender, of uni-
form diameter, and pro-
duce fine lateral branches.
The primary roots do not form a large proportion of the total
root system; but, under favorable conditions, they penetrate the
soil to a depth of 8 to ii inches or more; and may persist through-
out the entire life of the plant. In fact, McCall (9) has pointed
out that the subcrown root system is capable of carrying the plant
to complete maturity, and Locke and Clark (8) have reported
instances under certain arid conditions where the root system con-
sisted entirely of seminal roots. In such cases, the plant does not
tiller to any extent; and only the main culm develops, which.

Fig. 58. Stages in the development of the seedling.
(After Avery.)

TRITICUM 143

though reduced in size, is normal but tends to lodge, owing to
the absence of the support usually afforded by crown roots.

The permanent root system is comprised of whorls of adventitious
roots which arise from the lower nodal regions of the main stem
and its branches near the soil level. The first adventitious roots
appear at the tillering node about an inch below the soil level, and
consist of a pair of roots arising at right angles to the plane of
phyllotaxy and parallel to the plane of the three pairs of seminal
roots. Another pair is formed at the second node in similar man-
ner; but, near the ground level, where the internodes are longer,
the whorls may consist of four to six roots arranged in pairs. Each
secondary axis produces its adventitious system in a manner similar
to that of the main culm, except that a single root instead of a pair
is usually produced at the base of each secondary or subsequent
lateral axis.

The rapidity and extent of development of the adventitious
roots vary with cultural conditions and practice. Weaver (15)
has investigated this point; and finds that, under favorable con-
ditions, root elongation is very rapid, being maintained in some
cases at the rate of half an inch a day for 60 or 70 days in winter
wheat. In describing the root system of spring wheat, he states,

"The roots of the secondary system ramify the soil near the surface
6 to 9 inches on all sides, and likewise fill the x to 3 feet below this
area with a network of well-branched roots. Winter varieties are
similar in general habit but more deeply rooted. Crops planted early
during seasons favorable for growth form a secondary root system
which rather thoroughly fills the surface 12. to 1.0 inches of soil, while
the primary roots extend well into the third and fourth foot. The
mature root system has a working level of 3.5 to 4 feet and a maximum
depth of 5 to 7 feet."

The degree of branching and depth of penetration are subject to
great modification, depending upon the type of the subsoil, the
amount of moisture and aeration, and the character of the fertilizer
applied to the soil. (Fig. 59.)

The Stem. — The jointed, cylindrical stems or culms are smooth
or scabrous, and contracted at the nodes, which are solid. The
internodal portions are hollow at maturity, except in some varieties
of macaroni and poulard wheats in which there is a persistent pith.
The total height may vary from x to 5 feet, being even shorter in
some of the dry land areas; and there are usually about six inter-

144

THE STRUCTURE OF ECONOMIC PLANTS

nodes, although five or seven are not uncommon. The basal
internode remains short, while the second one elongates so that

the first adventitious roots of the
secondary root system are about an
inch below the surface of the soil.
The internodes above the second
are successively longer, the upper-
most one which bears the terminal
spike being the longest. The di-
ameter of the stem is influenced by
various factors; but it increases up
to the fifth internode, while the
sixth is more slender than the rest.
Secondary shoots or tillers arise
from axillary buds at the basal
nodes of the main axis; and from
these lateral culms, still others
may develop. In this manner, the
process of tillering results in the
formation of several axes of the
second and third order until many
branches have been produced. The
behavior of the axillary buds in
forming tillers depends in part on
the depth of planting of the grain.
When the depth is less than 2.
inches, the bud in the axil of the
first foliage leaf develops immedi-
ately; at about 2. inches, the third
axillary bud is usually the first to
grow vigorously; and where plant-
ing is 3 or more inches deep, the
first three buds of the main axis
remain dormant or grow weakly,
while the fourth lateral bud pro-
duces a strong stem.

The Leaves. — The alternate,
Fig. 59. The root system at the time two-ranked leaves are of Several

of blossoming. (From Weaver R../D.- j^jj^jg. ^^^ ^^^ seedling leaveS
velofment of Field Crops. McGraw-Hill ^^ -inz-ni

Book Co.) (scutellum and coleoptile}; {2.) the

TRITICUM 145

prophylls of the lateral branches; (3) the foliage leaves; and
(4) the glumes of the inflorescence. The seedling leaves are de-
scribed in connection v^ith the structure of the embryo and the
germination of the seedling.

The first leaf or prophyll of a lateral shoot is binerved, resembling
the coleoptile in its conical form and in the apical opening through
which the succeeding foliage leaves emerge. It attains a length of
an inch or more, and its upper portion is green w^ith deflexed hairs
along the tw^o veins. The prophyll is oriented so that its concave
or flattened surface opposes the axis from which the shoot enclosed
by it arises; but the next leaf is diverged at a 90° angle from the
prophyll, and each succeeding one has a divergence of 180°, so that
the plane of phyllotaxy is at right angles to that of the axis from
which the shoot is diverged.

The foliage leaf consists of the sheath, blade, and ligule. The
sheath encloses the culm, being entire near its base; but its upper
part is open on the side distal to the blade, and the outer of the two
overlapping edges is slightly raised. The lamina is variable in
shape, being narrowly linear to linear-lanceolate, and frequently
twisted, the torsion being to the right. The venation is parallel,
and the bundles are reinforced by mechanical tissue which gives
support to the blade, while further stiffening may result from the
presence of silica in the epidermis. The curling or expanding of the
blade along its longitudinal axis is controlled by the presence of
bulliform or motor cells on the adaxial surface. Stomata occur on
both surfaces, but with greater frequency on the adaxial one in a
ratio of approximately 10 to 7. The claw-like auricles which
develop at the base of the blade, where they loosely clasp the sheath
and the stem, are somewhat larger than those of rye and not as
prominent as those of barley. In young wheat plants, they are
frequently somewhat pubescent; and at maturity the tips and
margins may have a few unicellular hairs. The ligule, which
arises at the junction of the blade and the sheath and surrounds the
stem, is a thin, colorless, membranous structure with an irregular
edge that is fringed with small hairs.

The Inflorescence. — The inflorescence is a compound spike
which is usually 3 to 4 inches in length, but may vary from x to
5 inches. (Fig. 60.) It may differ greatly in its shape and degree
of compactness, the density of the spike being determined in part
by the length of the internodes and the distribution of the spikelets.

146 THE STRUCTURE OF ECONOMIC PLANTS

Fig. 60. Habit ul tlie uiature spike of Triticum vulgare, Vill., Portage variety. (Photo-
graph by J. Horace McFarland Co.)

TRITICUM

147

In most cases, this is uniform, but where the spikelets are crowded
at the tip, the spike becomes clubbed. It consists of a zigzag cen-
tral axis or rachis, and lateral spikelets which arise in two rows
from each notch or node of the central stalk. (Fig. 61, A, B.)
There is a fertile terminal spikelet which is placed at right angles
to the others. Each joint of the rachis is flattened on one side,
somewhat concave on the other, and broader at the apex than at its
base. The sessile spikelets lie against the concave surface of each
joint of the rachis in an alternate arrangement.
Grantham and Frazier (5) report that

"the average spike of wheat (Triticum spp.) contains from fifteen to
twenty spikelets each of which under favorable conditions is capable
of producing two or more kernels. Ordinarily, however, the lower
two or three spikelets on the spike do not develop."

In the bearded varieties the percentage of sterile spikelets is
higher than in the beardless ones.

Percival (10) has described the structure of the rachis, which
resembles that of the internodes of the culm. Its upper internodes
are flattened and spindle-shaped in transection, while the lower
ones are somewhat semicircular. The epidermal cells are oblong
with sinuous, thickened walls which alternate with oval or kidney-
shaped "dwarf" cells; and at the edges of the rachis are unicellular
epidermal hairs. Stomata occur in rows and overlie longitudinal
zones of chlorenchyma which are found only on the convex surface
of the rachis; while, on the flattened surface, there is a band of
subepidermal mechanical tissue. The center of the rachis is
parenchymatous and the large vascular bundles are arranged in a
circle with the smaller ones disposed on the inner face of the
chlorenchyma. The rachilla is similar in structure to the rachis
but much more slender and usually contains three vascular bundles.

The Spikelet. — The spikelet is the unit of the inflorescence, and
consists of from two to nine flowers which arise from an
unelongated axis or rachilla in a distichous arrangement corre-
sponding to the phyllotaxy of the vegetative axis. (Fig. 61,^,5.)
They are subtended by overlapping bracts or glumes which resemble
the sheath of the foliage leaf in general structure, and the two
lowermost ones do not have flowers in their axils. The slightly
crowded flowers of each spikelet are completely enclosed by the
glumes until anthesis. One or more of the upper flowers are

148 THE STRUCTURE OF ECONOMIC PLANTS

usually sterile and imperfect, so that only two or three kernels
are matured except in some of the club wheats, in which the spike-
let may contain five or more. On the average, each spikelet has
four flowers and produces two mature grains.

Each bract, aside from the two sterile basal glumes, is known as a
lemma and bears a flower in its axis. The lemma may be beardless.

: — ra

roc

B

Fig. 61. A, the rachis of the inflorescence showing the rachilla, ra; B, a. single spikelet;
C, a flower with the lemma removed and the palea drawn back to expose the enclosed floral
parts: «, awn; rf«, anther; ^.glumes; /w, lemmas ; /o, lodicules ; ety, ovary; />, palea; rac,
rachis; stg, stigmas. (^A, redrawn from Percival, The Wheat Plant, Duckworth and Co.;
B and C, redrawn from Robbins, Botany of Crop Plants, P. Blakiston's Son and Co.)

although few wheats are strictly so; or bearded with a terminal
awn that tapers from base to tip and is sub-triangular in transection.
Its epidermis like that of the glume is comprised of elongated cells
with sinuous walls, and oval ones that are often papillate. The
fine pointed, thick-walled hairs are directed toward the apex of the
awn and give it a scabrous character. Stomata occur in rows on the
outer face of the awn; and underlying them are zones of chloren-
chyma, while the angles are reinforced by bands of mechanical

TRITICUM 149

tissue. One large and two smaller vascular bundles are present,
the former being continuous with the midrib of the glume.

Opposite to and enclosed by the lemma, with its abaxial surface
toward the rachilla, is the palea, which is diverged from a very
short pedicel or flower stalk. (Fig. 61, C) This is an awnless,
two-keeled bract of thinner texture than the lemma, having trans-
parent, infolded margins. Enclosed by it are the three stamens and
single pistil, as well as two small, ovate, membranous scales, called
the lodicules, which lie on the opposite side of the pedicel from the
palea and near the base of the ovary. The large anthers are sagit-
tate, and the cylindrical filaments have a single vascular strand.
The ovary is one-celled, covered with down at its apex, and sur-
mounted by two short styles which terminate in feathery, brush-
like, stigmatic branches.

At anthesis, the swelling of the lodicules results in the pushing
apart of the lemma and palea; and the filaments elongate rapidly,
carrying the anthers up and outside of the glume. Dehiscence
begins before the flowers are fully open, continuing until they again
close; and, in most wheats, close pollination occurs although there
may be a small percentage of natural crossing. The latter is
commonly the case in durum wheat.

ANATOMY

The Mature Grain. — The mature grain or fruit is a caryopsis.
The pericarp and the remains of the integuments of the single seed
are so closely adherent that they cannot be readily separated from
each other at maturity. The grain is ovate with a furrow or crease
on the surface facing the palea which is caused by an infolding of
the lateral portions as maturation and desiccation proceed. The
surface of the grain is smooth except at the stigmatic end, where
there is a brush or tuft of persistent hairs. These become thick-
walled and rigid surrounding the styles which may sometimes
persist. (Fig. 6i.) The embryo develops at the base of the fruit
on the side away from the groove, and may be located externally
by the wrinkled surface of that portion of the pericarp which
overlies it.

The histology of the grain has been investigated by Bessey (x),
Percival (10), Tschirch and Oesterle (14), Winton (16), and others.
It may be divided into three main regions: (i) the bran layer
including the pericarp, remains of the seed coat and nucellus;

I50 THE STRUCTURE OF ECONOMIC PLANTS

(i) the endosperm ; (3) the embryo. There are usually five layers
or zones of cells in the pericarp. These in order from the outer
surface are : (i) the epicarp or outer epidermis ; (x) the hypodermal
parenchyma; (3) the cross cells; (4) the intermediate cells; and
(5) the tube cells.

The cells of the epicarp are elongated parallel to the long axis of
the grain, and have pitted walls that are much thickened and
cutinized, appearing beaded in surface view. At the apex, the
cells are more polygonal; and some of them develop as epidermal
hairs which are thick-walled, pointed, and broad or bulbous at

Fig. 6i. The grain of wheat and early stages in germination.

the base. Beneath the epicarp are one or two layers of elongated
hypodermal cells which resemble those of the former. The cells
of both epicarp and hypodermis become much compressed, and their
walls are so heavily thickened at maturity that the cell cavities
are not easily distinguishable. Underlying this layer, there may
be some thin-walled intermediate cells associated with cross cells,
the latter being so named because they are oriented with their
long axes at right angles to the outer layers of the pericarp. The
cross cells have characteristic transverse pits and contain chloro-
phyll when the grain is young. The tube cells make up the inner
epidermis of the pericarp, and underlie the cross cells, forming a
spongy parenchyma in which there are numerous intercellular

TRITICUM

151

spaces. Their long axes are parallel to those of the cells of the
epicarp; and, because of their loose arrangement, they appear as
rings in transection. (Fig. 63.)

The seed coat is much crushed at maturity, and appears as a zone
of cells in which the walls can scarcely be distinguished. Its
ontogeny is described in connection with the development of the
ovule and embryogeny. The nucellar tissue or perisperm is, for
the most part, resorbed in the development of the grain, and may
be completely absent at maturity. When present, it consists of one

op£2='

-ecp

-hd
-i c

pep

—end

Fig. 63. Transection of a portion of a mature grain showing detail of pericarp, integuments,
nucellus and endosperm : «/, aleurone layer ; c>- c, cross cells ; ^cp, epicarp; ?«</, endosperm ;
hd, hypodermis; / c, intermediate cells; nls, nucellus; pep, pericarp; s c, seed coat; st,
starchy endosperm; t c, tube cells. (Redrawn from Tschirch and Oesterle, Anatomischer
Atlas der Pharmakognosie und Nahrungsmittelkunde, Herm. Tauchnitz.)

or two layers of cells with thickened walls which overlie the
aleurone cells; and, in the mature grain, they are commonly
crushed so as to form a bright colorless line, the hyaline layer.
The aleurone cells constitute the outer zone of the endosperm,
which is usually a single layer except in the region of the furrow;
and the thick-walled cells appear square or rectangular in a transec-
tion of the grain, being more or less rounded to polygonal in
surface view. They contain an abundance of protein granules and
some fat. The protein appears to be formed by condensation
processes during the desiccation of the wheat kernel.

i5i THE STRUCTURE OF ECONOMIC PLANTS

The starchy endosperm constitutes the main bulk of the
grain, and its cells are thin-walled, with their long axes at right
angles to the surface of the grain. They are filled with starch
and contain all of the gluten found in the endosperm. The
lenticular starch grains are intermediate in size between the
larger ones found in rye and the smaller ones which occur in
barley. The endosperm, including the aleurone layer, constitutes
approximately 85 to 90 per cent of the entire bulk of the ma-
ture grain, the embryo about 6 per cent, and the bran layer the
remainder.

The Embryo. — The mature embryo lies embedded in the endo-
sperm slightly to one side and at the base of the grain opposite the
groove. The embryonic axis consists of the primary root, enclosed
by the root sheath or coleorhiza; the epicotyledonary portion of
the axis, enclosed by the coleoptile; and two lateral divergences,
the scutellum and the epiblast. (Fig. 64.) The scutellum is the
large, lateral, shield-shaped cotyledon which slightly exceeds the
embryonic axis in length. Its convex, abaxial surface lies in close
contact with the starchy cells of the endosperm; and the epidermal
cells are elongated at right angles to the surface of the scutellum,
constituting an epithelial layer which is secretory in function. In
some varieties, invaginations occur in the surface, but this is not
as common as in corn. The scutellum partially surrounds the
coleoptile and extends slightly beyond it so that a projection is
formed, known as the ventral scale, which overhangs the coleop-
tile. The epiblast is a small non-vascular outgrowth which
arises in the region of the cotyledonary node on the side of the axis
distal to the scutellum.

The conical coleoptile is entirely closed except just below the
tip, where there is a slit-like pore away from the scutellum; and
enclosed by it are the primordia of two or three foliage leaves.
The first foliage leaf is diverged from the axis on the side opposite
the scutellum, and succeeding leaves have a similar angular
divergence of 180°. A lateral bud is developed in the axil of the
coleoptile on the side adjacent to the scutellum.

The Vascular System of the Embryo. — The vascular strands
of the embryo consist of elongated thin-walled conducting ele-
ments, and the scutellum is supplied with a broad strand which is
directed towards its tip, branching frequently at its upper limits
so that a fan-like system is formed. The small branches curve to

TRITICUM

153

the right and left, extending downward almost to the base of the
scutellum . The scutellar bundle enters the axis at the cotyledonary

Fig. 64. Median longisection of a wheat embryo (Turkey variety) : a, ventral scale of scu-
rrnwT; *'.^'^"^''^'; .^"<:/;. ^' coleoptile; d, first foliage leaf; ., second foliage leaf; /,
straTTfi'"''- ^' '/ ^'''^' '''^' ^' ^"'^ ^" ^'^^^ of coleoptile; .-, epiblast; j, procambial
Trr,^'-/'' " ''/''°'y'''^°""'y"°'^^' '".coleorhiza; «, primary root ; ..root cap;

P, remains of suspensor. (After McCall, >«r ^gr. Rw.)

154 THE STRUCTURE OF ECONOMIC PLANTS

colp

node and may extend up the first internode a short distance
before becoming a part of the stele. (Fig. 65.)

According to Avery (i), the median third of the scutellar bundle
becomes a part of the stele directly; and extends downward as a
part of it to the scutellar node, where it turns upward. At the
point of upward curvature, vascular strands from the root anasto-
mose with it, and the combined strand becomes the midrib of the
foliage leaf above the coleoptile. (Fig. 66.) Each of the outer

branches proceeds laterally
upward and again forks to
form two bundles, the outer
one of each pair being a cole-
optilar trace. In T. vulgare,
two unbranched bundles pass
from the base to the apex of
the coleoptile; but in some
level of scu no species of Triticum, as many
as six have been reported by
Percival (11). The inner
bundle of each pair, after
passing upward in the axis,
branches and gives rise to
one or more bundles which
constitute the trace of the
first foliage leaf above the
coleoptile.

Interpretation of the

lat sem rts

— -/ — coir

~ pri rt

Fig. 65. Diagrammatic face view of the embry-
onic axis showing the procambial vascular system
in black : colp, coleoptile; coir, coleorhiza ; lat sem
r.., lateral seminal roots, /,..>/, primary root ; scu £^^^^0 AND THE SeEDLING
no, scutellar node. QAiter Avery. j

Axis. — The structure of the
embryo and the seedling axis of the grasses has occupied the atten-
tion of morphologists for over a century; and there has been no
general agreement in the interpretation of the various parts of the
embryo. This has resulted in a voluminous literature which is
well summarized by Avery (i), McCall (9).

Percival (10) states that the view which "is most in agreement
with the development and structure of the embryo of wheat is
that which regards the scutellum, epiblast, coleoptile, and first
green leaf as the first four leaves of the plant." McCall (9)
interprets the epiblast "as a vestigial leaf," the scutellum as
"the functional cotyledon, divergent from the second node" and

TRITICUM

155

"the coleoptile as a third leaf." Avery (i) regards the scutellum
as the cotyledon or first leaf of the plant, the coleoptile as the
homologue of a foliage leaf or the second leaf, and the leaf distal
to it as the third leaf. He states that "the epiblast, when present,
cannot be considered a rudimentary cotyledon." More recently
Boyd and Avery (3) reaffirmed this view, stating that "the cole-
optile is the first leaf above the single cotyledon."

All of these investigators agree that the use of the term mesocotyl.

— mer reg

mer reg

Fig. 66. A, median side longisection of dormant embryo ; B, the same, 7 days old in which
slight elongation of the axis above the scutellar node is taking place ; C, the same, in a month-
old plant showing the elongated second internode sheathed by the coleoptile. Meristematic
regions of elongation are shown with dashes : colp, coleoptile ; colp no, coleoptilar node ;
coir, coleorhiza ; mer reg, meristematic region ; pri rt, primary root ; scu, scutellum ; scu no,
scutellar node. (After Avery.)

as originally applied to the first internode, is misleading, since
it implies that the first internode is a part of the cotyledon. While
differences in terminology do not modify the actual structure,
they do tend to create confusion in the literature; and it is desirable
that a uniform nomenclature be applied to the embryo and seedling
of the grasses.

Germination. — In germination, the wrinkled surface of the
pericarp over the embryo becomes smooth, and the entire wall is
stretched. Following the rupture of the pericarp, the coleorhiza
emerges, and the enlargement of the epiblast and other embryonic

156 THE STRUCTURE OF ECONOMIC PLANTS

structures extends the slit in the pericarp until the coleoptile
is exposed. After the coleorhiza has elongated about a millimeter,
the enclosed primary root digests and pushes its way through it.
Shortly thereafter, the first pair of seminal roots emerges from
the sides of the short hypocotyl, near the cotyledonary node;
and these three roots grow with about equal rapidity until, at
the end of two or three days, there is little difference in their
length or thickness. (Figs. 58, 6i.) At about this time, the second
pair of seminal roots emerges above the first pair; and, in some
cases, a sixth rootlet may develop behind the epiblast.

McCall (9) reports that he and Taylor observed that in most
cases only three roots developed; but in a fairly large number of
cases, four developed — the primary root, the first pair, and one
of the second pair of seminal roots. In a smaller number, five
roots developed; and, "as a rule, only relatively few seedlings
of most varieties of wheat have the full potential complement
of six seminal roots."

During the development of the seminal root system, the cole-
optile elongates and the enclosed foliage leaves enlarge, the
former protecting the leaf primordia and stem apex as they grow
through the soil. The degree of emergence of the coleoptile is
conditioned by depth of planting and environmental factors; and,
after the tip of the coleoptile is above the soil surface, the first
and second foliage leaves push through the small pore near its
apex. Where wheat is sown at a depth of i| to x^ inches, there
is little elongation of the first internode. After several days,
the second internode elongates until the third node, from which
the first foliage leaf is diverged, is raised to the level of the soil
surface or just below it. The bud in the axil of the coleoptile
develops, and adventitious roots may arise from the internode
above. At this time, elongation of the second internode ceases,
and the roots which constitute the first whorl of the adventitious
root system are formed.

Under average conditions the lower internodes, above the first
whorl of adventitious roots, do not elongate much; and the pro-
duction of roots at the nodes, together with the development of
the axillary buds, results in the formation of several secondary
axes or tillers and a fibrous secondary root system.

The Primary Root. — The primary root has an exarch, radial
protostele with from seven to eleven protoxylem points. At a

TRITICUM

157

level where maturation is complete, there are three well-defined
regions: (i) the epidermis; (x) the cortical zone limited cen-
tripetally by the endodermis; and (3) the polyarch stele limited
externally by the pericycle. (Fig. 67.)

pel

-I mx)

!> emrck

Fig. 67. Transection of primary root of the wheat seedling : co, cortex ; en, endodermis ;
e/>, epidermis ; »?x, metaxylem ; pc/, pericycle ; /»A, phloem; /ix, protoxylem. (After Avery.)

The epidermis consists of thin-walled cells which are elongated
in their axial dimension. Many of them develop as root hairs by
an extension of a portion of the outer tangential wall, usually
that part of it nearest the root tip; and they may reach a length
of I to i^ mm. Ultimately they become desiccated, and the epider-
mis, with much of the cortical tissue, may disintegrate and die.
The cortex consists of four to eight layers of rather compact

158 THE STRUCTURE OF ECONOMIC PLANTS

parenchymatous tissue with small intercellular spaces at the
angles of the cells. The endodermis is thin-walled, except for
the Casparian strips; but, later in ontogeny, the radial, end, and
inner tangential walls become much thickened, the latter ones
appearing to be stratified.

The pericycle is single-layered, with cells of almost uniform
size and dimension, except outside the protoxylem points, where
they are smaller. As the root matures, and the cortical cells
disintegrate, the walls of the pericyclic cells may become some-
what lignified and sclerotic. The protoxylem consists of a few
small annular or spiral vessels; and centrad to them is a single
large pitted metaxylem element. Alternate with the protoxylem
strands are small groups of phloem cells, usually two to four in
number; and between the phloem and xylem are fundamental
parenchyma cells. When the epidermis and cortical tissues are
finally shed, the stelar tissues remain functional, being protected
by the thick-walled endodermal and pericyclic cells, as well as by
the fundamental parenchyma, which also becomes thick-walled.
The character of the phloem and xylem cells remains unchanged.

The ontogeny of the root is similar to that of corn, barley
and other grasses in which the root meristem is differentiated into
three histogens. The outermost one is the calyptrogen, from
which the root cap is differentiated by successive periclinal divi-
sions; and, since the peripheral layers of the root cap are succes-
sively abraded, it never becomes very massive. (Fig. 68.) The
plerome gives rise to the stelar structures; and, between the
plerome and calyptrogen, a single layer of meristematic cells
produces the cortical and epidermal tissues. Periclinal divisions
of the cells of this intermediate layer produce daughter cells, the
outermost ones dividing only anticlinally to form the epidermis;
while the inner daughter cells, by subsequent divisions in all
planes, give rise to the cortical parenchyma and endodermis.
The other seminal roots resemble the primary one in the essential
details of structure and ontogeny.

The origin of lateral roots is pericyclic, occurring in sectors
of this layer directly outside of the phloem groups and between
two adjacent protoxylem strands. The pericycle becomes multi-
layered by successive periclinal divisions, a conical mass of meri-
stematic cells is formed, and the histogens develop which give
rise to tissues as described for the primary root.

TRITICUM

159

Adventitious Roots. — The roots of the adventitious system
arise in the intercalary meristems at the bases of the lower inter-
nodes of the culm, and differ from the seminal roots in certain

-c w

pbm-dgn

Fig 68 Median longisection of tip of seminal root : cal, calyptrogen ; co, cortex ; c w,
mucilaeinous cell wall; en, endodermis; ef, epidermis; pbm-dgn, periblem-dermatogen ;
pel, pericycle; pi, plerome; r c, root cap; stl, stele; ves, cells which become vessel seg-
ments. (Redrawn from Percival, The Wheat Plant, Duckworth and Co.)

i6o THE STRUCTURE OF ECONOMIC PLANTS

respects. The cortical cells are more persistent; and there may be
a hypodermal zone, two or three cells in thickness, which becomes
sclerotic and protects the parenchymatous cells centrad to it.
Percival (lo) reports that in the stronger adventitious roots
arising from aerial nodes, the cortical cells frequently have chloro-

FiG. 69. Transection of the lower portion of the first internode of a four weeks old plant :
CO, cortex ; co hu, cortical bundle ; end col bu, endarch collateral bundles ; ep, epidermis ; ph,
phloem; atj, xylem. (After Avery.)

plasts so that the stele appears to be surrounded by a zone of
chlorophyllose tissue. Other differences in the stele are the
development of several large metaxylem vessels instead of a
single one; and the presence of a correspondingly smaller amount
of stelar parenchyma which becomes lignified early in ontogeny so
that it is difficult to distinguish protoxylem from parenchyma
in transections.

TRITICUM

i6i

Anatomy of the Transition Region — First and Second
Internodes. — In the first internode, the arrangement of the
xylem, phloem, and parenchymatous tissue is transitional, lacking
the definite arrangement found in the upper internodes. (Fig. 69.)
There are two collateral bundles located on opposite sides of the
stele; and the remaining xylem and phloem elements are arranged

= midrib & 2 lateral bundles of 3rd leaf
= = •• • 6 ■ ' ■ 2nd "

m = coleoptile bundles, cotyledon bundle
and common bundle of the axis

- - ventral scale [ligule)

coleoptile {1st leaf above cotyledon)
2nd internode

upper limit of fig 71

1st internode {no elongation)

epiblast {portion of ligule)
nodal plate of the cotyledon
lower limit of fig 71

— primary root stele
coleorhiza

Fig. 70. Diagrammatic reconstruction of portion of seedling showing principal structures
present, and vascular inter-relationships of cotyledon with axis, coleoptile, and first foliage
leaf. Nodal plate of the cotyledon is main level at which vascular transition takes place and
the first internode is also in part a transition region. (After Boyd and Avery.)

in an irregular fashion, the xylem being exarch, endarch, or indefi-
nite in its orientation. The scutellar bundle occupies a cortical
position, but the stele and cortex are not limited by a distinct
pericycle and endodermis respectively.

The vascular system of the transition region has been described
by Boyd and Avery (3) as consisting of four groups: (i) A bundle
connecting the midrib of the second leaf to the cotyledonary
plate. Ql) The traces of the scutellum, coleoptile and second

i6x THE STRUCTURE OF ECONOMIC PLANTS

leaf. These are connected on the scutellar side of the axis with a
"common" bundle, that extends through the stem for some dis-

■ cotyledon

' = midrib & 2 lateral bundles of 3rd leaf

, = „ - ^ - . - 2nd •

> = coleoptile bundles

coleoptile

1st leaf above cotyledon

midrib of 2nd leaf
above the cotyledon

•- - level of divergence of coleoptile
- cotyledon

ventral scale {ligule)

endarch collateral bundles

^jprimordia of upper (younger)
pair of seminal roots

^^^^y*— H CN> tip of epiblast

primordium of seminal root

nodal plate of the cotyledon

steles of lower (older) pair of
seminal roots

■'-stele of primary root

Fig. 71. Detailed course of vascular bundles and scattered vascular elements in portion
of seedling, upper and lower limits of which are indicated in Fig. 70. (After Boyd and
Avery.)

tance and terminates as a leaf trace or traces. The median lateral
bundles of the second leaf are united with adjacent coleoptilar
bundles, and the resultant bundles anastomose with the scutellar

TRITICUM

163

bundle. In this manner, the common bundle is formed which
extends to the cotyledonary plate where it is united with the bundle
from the midrib of the second leaf. (3) The remaining four lateral
bundles of the second leaf and two lateral bundles of the third
leaf; and (4) the vascular elements of the seminal roots which
anastomose with the bundles of the second leaf and are connected

Fig. 71. A transection of the second internode of young whleat plant.

indirectly with the lateral bundles of the third leaf. (Figs. 70
and 71.)

The second internode, which extends from the point of diver-
gence of the coleoptile to the first foliage leaf above, is less transi-
tional in character than the first internode. It resembles the
higher internodes except that the bundles are not as clearly defined,
and there are some strands in which the differentiation of the
xylem is intermediate between the exarch and the endarch arrange-
ment. (Fig. 72..)

The Anatomy of the Stem. — In most wheats, the culm is sub-
terete, hollow in the internodal regions and solid at the constricted

[64

THE STRUCTURE OF ECONOMIC PLANTS

nodes where the vascular bundles form a lignified diaphragm.
There are five principal tissue systems: (i) epidermis, (i) mechani-
cal tissue, (3) chlorophyll parenchyma, (4) fundamental paren-
chyma, and (5) vascular bundles. (Fig. 73; see also Fig, 24.)
Aside from the guard cells of the stomata, there are two types
of epidermal cells, one of which is narrow and elongated, the
other short and isodiametric. The elongated cells are arranged

in longitudinal rows, sep-
arated at intervals by the
smaller cubical ones; and
the width of the cells va-
ries, those which adjoin
the mechanical tissue of
the hypodermis being nar-
rower than those outside
the chlorenchyma. The
walls are much thickened
and there is a well-
developed cuticle on the
portions which are not
covered by the leaf sheath.
The stomata occur in
single or double rows and
are similar to those found
in the leaf. In varieties
which the stem

in

IS

Fig. 73. A transection of a sector of the stem : cav
central cavity of the stem ; chl par, chlorophyll paren- j u U

chyma; ^ c, guard cells ; w^ec/i, mechanical tissue ; mx, grOOVecl, tnere may DC

metaxylem ; par, parenchyma ; ph, phloem ; px, proto- short SCabtOUS hairs dcvel-

^^^"' Oped along the furrows.

The mechanical tissue of the hypodermal region is made up
of elongated, fibrous cells with thick walls and narrow lumina,
which form a continuous zone of variable thickness within the
epidermis; and this encloses the longitudinal bands of chloren-
chyma, as well as the smaller peripheral bundles of the vascular
system. (Fig. 74, A.') The chlorenchyma abuts the epidermis
directly, consisting of thin-walled cells that are circular in tran-
section, elongated in the axial dimension with somewhat sinuous
longitudinal walls. The rows of stomata are so located that the
substomatal cavities open into the parallel bands of chlorenchyma.
These are frequently arranged in pairs and adjacent bands may

TRITICUM 165

be united, forming broader ones. Tliey are wide at the upper
limits of the internode, taper gradually, becoming much nar-
rower beneath, and finally disappear entirely near the base of
the internode. Percival (10) has noted that the width of the
individual bands of chlorenchyma is greatest in the upper inter-
nodes, and that they are also more closely grouped so that the
upper part of the culm has a uniform green tint. In the three
lowest internodes, there is less chlorenchyma, and it occurs chiefly
in the upper part of each, being reduced or entirely wanting at the
bases.

The fundamental parenchyma extends from the zone of mechani-
cal tissue to the central cavity. It consists of thin-walled cells
that are polygonal in transection and elongated in the longitudinal
dimension, with those adjacent to the mechanical tissue being
longer than the centrally located ones. The cells may become
lignified in the lower internodes, and parenchymatous cells which
have become thick-walled also constitute a part of the solid dia-
phragm at the nodes.

The vascular tissue in the internodal region consists of two
rings of bundles. The outer ring consists of small bundles that
alternate with bands of chlorenchyma, while the larger bundles
of the inner ring lie in the fundamental parenchyma. (Fig. 73.)
The endarch, collateral bundles are surrounded by a sheath of
mechanical tissue consisting of slender elongated fibers; but, in
the case of the smaller bundles which are embedded in the hypo-
dermis, the fibers of the sheath cannot be differentiated from those
of the adjacent tissue. The protoxylem consists of one or two
annular or spiral vessels; and, in some cases, the innermost element
is reinforced with a combination of rings and spiral thickenings.
The large, laterally placed, metaxylem vessels are pitted and
between them are several small pitted tracheids. The phloem
is comprised of slender thin-walled sieve tubes and companion
cells. (Fig. 74, A and B.)

The Course of the Bundles in the Stem. — At each node, some
i5 to 35 bundles enter the stem from the leaf sheath. About
half of these pass through the node into the outer hypodermal
ring, while the remaining somewhat larger bundles penetrate more
deeply and become a part of the inner vascular ring. The parallel
bundles follow a vertical course through the internode to the next
lower node, where there are anastomoses, branchings, and changes

i66 THE STRUCTURE OF ECONOMIC PLANTS

in the course of the bundles which, together with the entrance of
additional bundles from the leaf, result in the formation of the
nodal plate. Percival (lo) has observed that, in most cases,
the hypodermal bundles extend through a single internode, anasto-
mosing with other bundles at the first node below. The number
of bundles in the inner ring is reduced by anastomoses to about
one-half their original number as they pass through the second

a b e d e f g h i

Fig. 74. A, longisection of a portion of the stem from epidermis to lacuna. The plane of
the section with respect to the vascular bundle is indicated by the dotted line i-i in B. B,
transection of a mature stem bundle : a, the epidermis showing two types of epidermal cells ;
h, chlorenchyma ; c, mechanical tissue centrad to the chlorenchyma; d, parenchyma; e
and /', mechanical tissue of the bundle sheath ; /, phloem ; g, primary xylem elements consist-
ing of a pitted tracheid, spiral vessel, and an annular or annular-spiral vessel ; g', metaxylem
vessel, not shown in ^; h, xylem parenchyma surrounding protoxylem elements; ;, paren-
chyma; k, margin of the central lacuna.

internode. Thus, in any given internode, one-half of the large
bundles can be traced to the leaf that is diverged at the node
immediately above; and the remainder to the leaf diverged from
the second node.

In the nodal region, the bundles are surrounded by a sheath
of chlorenchyma, and differ from the internodal bundles in the
number and character of the xylem elements. In general, the
xylem elements are short, pitted or reticulate tracheids; and the
bundles may be amphi vasal rather than collateral.

TRITICUM

167

Anatomy of the Leaves — the Coleoptile. — The coleoptile,
or second leaf, consists of a parenchymatous cylinder, limited out-
wardly and inwardly by epidermal tissue, through which two
vascular bundles extend. The thin-walled cells of the outer
epidermis are elongated, and there are one or two rows of stomata,
lying parallel to the vascular strands, that pass from the base
to approximately the tip of the leaf where the stomata are more

chl par

Fig. 75. Transection of a small portion of the coleoptile including the vascular bundle and
surrounding tissue: c^/ /»^r, chlorenchyma ; /;A, phloem; xy, xylem. (After Avery.)

numerous. No hairs are developed on either epidermis, and the
inner one has no stomata. The parenchymatous cells abutting
the outer epidermis and the cells surrounding the vascular bundles
contain chloroplasts. (Fig. 75.) The vascular bundles are col-
lateral; and, as compared with the stem bundles, have relatively
more phloem than xylem.

The Prophylls. — The structure of the prophyll is not unlike
that of the coleoptile. The rectangular epidermal cells are elon-
gated and there are unicellular hairs along the margins of the
flat surface of the prophyll. At its apex, longitudinal rows of

i68 THE STRUCTURE OF ECONOMIC PLANTS

stomata lie parallel to, and on either side of, the bundles. The
parenchymatous cells of the mesophyll do not contain chloroplasts
except in the apical region and adjacent to the bundles. Two
large bundles are located in the angles of the prophyll where the
convex and flat sides meet, and the convex portion is traversed by
several smaller ones. The margins of the leaf are strengthened by
strands of mechanical tissue, and there may be a few supporting
cells abutting the smaller vascular bundles.

The Foliage Leaves. — The structure of the foliage leaf is
more complex than that of the coleoptile and prophylls. The
epidermal cells are arranged in rows which run parallel to the
long axis of the blade; and, as in the stem, some of these are
elongated while others are approximately square in surface view.
The elongated cells may form continuous rows, or they may be
separated at intervals by the short cells; and they also occur
between the longitudinal rows of stomata. Epidermal hairs, when
present, are of varying lengths and arise from the surface in a
more or less regular arrangement.

The adaxial surface is somewhat ridged, while the abaxial
one is nearly flat; and in the furrows of the former are single
or double lines of stomata with bands of motor or buUiform cells
between them. (Fig. 76.) Each band is three to seven cells in
width and the cells comprising it are thinner walled than the
adjacent epidermal cells. The motor cells are shorter than the
long cells of the epidermis; but, as seen in transection, are thicker
than any of the other epidermal cells. Since their outer surfaces
are not cutinized, they lose water under conditions which cause
high transpiration, and the decreased turgidity results in an incurv-
ing of the adaxial surface of the blade. The number of stomata is
greater on this surface than the abaxial one and the curling tends
to decrease the amount of water loss. The stomatal apparatus
consists of two narrow guard cells which surround the stoma, and
two accessory cells. (Fig. 5.) The development of the stoma is
similar to that described for Zea.

The abaxial epidermis is like the adaxial with respect to the
arrangement of the cells, but differs from it in the absence of ridges
and motor cells. In addition to this, the cell walls may be some-
what thicker, the number of hairs fewer, and the stomata more
commonly occur in single rows. The abaxial epidermis of the
sheath resembles that of the blade, with thick sinuous walls,

TRITICUM 169

while the cells of the adaxial epidermis are thin-walled with few
stomata.

The chlorenchyma of the mesophyll is very uniform, although
the cells abutting both epidermal surfaces may be somewhat
elongated in a manner suggesting palisade tissue. There are
large intercellular spaces subtending the stomata and projecting
into the central portion of the mesophyll. In the sheath, chloren-
chyma strands, three or four cell layers in thickness, occur between
the vascular bundles; and in the mature sheath, large lacunae
develop centrad to them.

Vascular bundles of two sizes extend parallel with the long
axis of the lamina, and small transverse veins interconnect the

I'-mech

^-bu sh

Fig. 76. Transection of small portion of foliage leaf: ac c, accessory cell; bush, bundle
sheath ; ep, epidermis ; g c, guard cell ; m c, motor cells ; meci>, mechanical tissue ; msl,
mesophyll; ^^, phloem; xj, xylem. (After Avery.)

longitudinal strands which may occasionally branch, or anasto-
mose toward the apex of the blade. All the bundles are collateral
and the larger ones resemble those of the stem in their organiza-
tion, having one or two annular or spiral elements and two large
pitted vessels. The small longitudinal bundles have no large ves-
sels, and the xylem of the transverse bundles usually consists of
short tracheids. In all cases, phloem is present, but in the trans-
verse bundles it is limited to a few somewhat elongated parenchym-
atous cells. The bundles are surrounded by a double sheath, the
inner layer consisting of elongated thick-walled cells, and the
outer of parenchymatous cells which are usually devoid of chloro-
plasts at maturity. In some cases, the outer layer of the sheath
does not completely encircle the bundle, being restricted to a
sector over its adaxial or xylem portion.

Longitudinal strands of thick-walled mechanical tissue are
located above and below the bundles, and there is also a strand
of this type extending along each margin of the leaf blade immedi-

lyo THE STRUCTURE OF ECONOMIC PLANTS

ately within the epidermis. Those adjacent to the smaller bundles
consist of a few fibers; but near the larger ones, the fibrous tissue is
much more extensive. It may occupy the entire space between the
bundle and the epidermal layers, so that a continuous band of
sclerenchyma is formed, consisting of the fiber strands and the
mechanical tissue of the inner layer of the bundle sheath. In
the leaf sheath, the sclerenchyma is usually restricted to zones
adjacent to the abaxial surface, the amount increasing toward the
node; but there are a few mechanical elements abutting the
adaxial one, except opposite the smaller bundles.

The ligule is a non-vascular emergence which arises at the
junction of the blade and sheath. It is parenchymatous and is
three or four cells in thickness at its base. The inner surface
of the ligule is continuous with the epidermis of the leaf sheath,
and both surfaces are without hairs or stomata. The free upper
edge may be somewhat fringed, due to the elongation of its mar-
ginal cells.

The Ontogeny of the Shoot. — The development of the grow-
ing point of the stem axis, and the differentiation of the leaves and
axillary buds, have been investigated by Rosier (ix). The periph-
eral layer of the bud meristem constitutes a dermatogen which
divides only anticlinally, except at points where a leaf primordium
is initiated. The leaf originates entirely from the dermatogen,
the first evidence of primordial development being the periclinal
division of a group of these cells . Rapid divisions of the derivative
cells follow, and there is an early blocking off of the mesophyll
region. The cells of the subepidermal layer, underlying the leaf
primordium, also divide tangentially; but they contribute only
to the development of the vascular system at the base of the leaf
and the internode.

The formation of new buds or growing points involves the
subepidermal tissues; and, according to Rosier, two different
types of ontogeny may occur. In young plants, the tissue of the
bud within the dermatogen may arise from a single subepidermal
apical cell; or in some cases, from a plate of several subepidermal
cells. This initial, or group of initials, forms the corpus of the
axis, and by periclinal divisions two layers are produced. These
are not exactly comparable to Hanstein's periblem and plerome,
since the outer one gives rise to the cortex and the peripheral
part of the stele, while the inner one differentiates the remainder

TRITICUM

171

of the central portion of the axis. In older plants, there are two
layers of initials within the dermatogen, one lying above the
other. The outer one consists of a group of cells, the inner one
of a single cell; and these differentiate the regions of the axis as
in the first case. This type of development is found especially
in the growing points of the floral region.

Floral Development. — In the development of the flowers of
a spikelet, the primordia of the bracts and the enclosed perfect,
hypogynous flowers arise in acropetal succession. (Fig. 77, A.')

B

Fig. 77. A, median longisection of a young spikelet ; B, longisection of the ovary showing
the ovule with outer and inner integuments and the embryo sac: g, glume; Im, lemma;
/tf, lodicule ; p, palea. The primordia of the stamens and carpels are indicated by the symbols
$ and 9 respectively. (Redrawn from Percival, The Wheat Plant, Duckworth and Co.)

Percival (10) has described the sequence and origin of the floral
parts. In each flower, the lemma or flowering glume is the
first structure to be differentiated, arising as a semicircular fold of
tissue on the abaxial side of the meristematic tip with reference
to the main axis of the spikelet. This is soon followed by three
papillate, staminal primordia; and, shortly thereafter, the carpel-
lary primordium arises as a crescentic ridge that partially surrounds
the apex of the floral axis. At about this time, the primordia of
the palea and lodicules appear, the latter arising from points on
the axis somewhat lower than the point of origin of the staminal
primordia.

172. THE STRUCTURE OF ECONOMIC PLANTS

In the differentiation of the carpel, the styles develop as two
blunt conical outgrowths; and, coincident with this, the upper
portion of the carpel grows over and encloses the rounded portion
of the floral axis. In so doing, it forms a chamber in which the
tip of the axis appears as a protuberance from the inner surface of
the ovary; and it is at this point that the single ovule later arises.

MiCROSPOROGENESis. — The staminal primordia elongate rapidly
and there is an early differentiation of a vascular strand in the
meristematic tissue. An archesporial cell is formed in each angle
of the anther, and this divides periclinally to form the primary
parietal and inner sporogenous cells. Radial divisions of the
parietal cell produce four cells; and these, with adjoining ones,
form a parietal layer which encloses the sporogenous cell. Suc-
cessive periclinal divisions of the cells of this layer result in three
concentric, parietal layers, the inner one becoming the tapetum,
and the outer, the endothecium. At first, the sporogenous cells
form a single longitudinal row; but, later, radial and longitudinal
divisions occur so that in transection there are about six radially
arranged cells which function as pollen-mother cells. As the
locules enlarge, the pollen-mother cells become spherical or oval
and separate from one another, forming a single layer lining the
anther chambers. Finally, meiotic divisions result in the forma-
tion of tetrads of microspores. The haploid number has been
reported as eight for most varieties of wheat.

The Development of the Ovule. — The primordium of the
ovule first appears as a hemispherical mass arising in a lateral
position on the inner adaxial surface of the ovary from a broad
placenta. (Fig. 77, 5.) Percival (10) regards it as being derived
"from the morphological apex of the floral axis," and states that
its lateral position is "due to rapid growth of one side of the axis
before the closure of the carpel." As the nucellar tissue increases,
two integuments arise at its base, the inner one being formed at
about the time that the archesporial cell is clearly difl^erentiated,
while the outer one begins to develop before the archesporial cell
divides. (Fig. 78, 5.) As growth of the nucellus and the integ-
uments proceeds, the more rapid development of the latter results in
the complete enclosure of the nucellus. (Fig. 78, C, D.) The
outer integument never completely reaches the micropylar opening
which is formed by the growth of the inner integument; and the
former disintegrates before the maturation of the seed. The

TRITICUM

173

unequal growth of the nucellus and its investing integuments
results in the formation of a completely anatropous ovule.

Megasporo GENESIS AND THE Megagametophyte. — The hypo-
dermal archesporial cell is wedge-shaped, and successive divisions
of it result in the formation of a linear tetrad of megaspore mother
cells. (Fig. 78, A-C) The three outer cells disintegrate, and
the innermost one enlarges greatly, functioning as the megaspore.
(Fig. 78, D.) The development of the megagametophyte proceeds

B

Fig. 78. A-D, median longisections of portions of young ovules : A, stage showing
primary archesporial cell ; B, after the first division of the archesporial cell ; C, the tetrad
stage; D, a later stage showing the collapse of the three outer cells. (Redrawn from
Percival, The Wheat Plant, Duckworth and Co.)

to an eight-celled stage, during which time it increases in size and
absorbs some of the nucellar tissue. The two synergids and the
megagamete occupy the micropylar end of the megagameto-
phyte; and, prior to gametic union, the polar nuclei fuse. The
three antipodals increase in size and divide, the number ultimately
produced varying from six to ten according to Percival (10),
although Kornicke (7) suggests that 36 or more may be formed.

Brenchley (4) states that double fertilization occurs between
2.4 and 48 hours after pollination; and, although there may be
some doubt as to a union of the primary endosperm nucleus with

174 THE STRUCTURE OF ECONOMIC PLANTS

the second microgamete in all cases in Triticum vulgare, Sax (13)
observed the phenomenon in Triticum durum. It is possible that
the fusion of the polar nuclei with the second microgamete may
take place so rapidly that this stage is difficult to observe. The
antipodal cells show signs of disintegration at the time of fertiliza-
tion; and, as the embryo and endosperm develop, the surrounding
nucellar tissue is also gradually resorbed. The outermost layer of
the nucellus persists as an actively dividing tissue for some time,
but ultimately its cells become disorganized and the radial walls
are broken down. In the mature grain, it may appear as a crushed
layer outside the aleurone cells of the endosperm.

Embryogeny. — In the development of the embryo, the first
division of the zygote is transverse, and the large, basal, daughter
cell functions as the suspensor which does not divide again. The
upper cell forms the embryo, dividing first in the transverse plane,
and then a vertical wall is formed in both the cells so that the pro-
embryo consists of four cells with a basal suspensor. Further
divisions in all planes result in a marked increase in the size of the
embryo; and, in a few days, the dermatogen can be distinguished
surrounding a central body of cells.

At this time, the embryo is club-shaped with an elongated base,
and the first form change occurs in the development of a lateral
notch indicating the position of the growing point of the axis
which appears on the side of the embryo away from the endosperm.
(Fig. 79, B.) The distal portion of the embryo becomes the coty-
ledon or scutellum, which Percival regards as a terminal structure.
A more generally accepted interpretation is that the growing point
is terminal and the cotyledon a lateral structure; which, because
of the slow growth of the apex and the rapid growth of the coty-
ledon, appears to occupy a terminal position. The position of the
suspensor at the base of the axis, rather than at the lower portion
of the cotyledon, supports the latter view.

Shortly after the apical meristem of the stem is defined, the pri-
mordium of the coleoptile arises as a fold of tissue surrounding the
stem tip and forms a cone-shaped structure which encloses the
primordia of the first foliage leaves. (Fig. 79, C, D.) At this
time, the epiblast arises as a lateral outgrowth in the region of the
cotyledonary node. There is intercalary growth at the base of the
upper portion of the scutellum which elongates the apex, resulting
in the formation of the overhanging ventral scale; and the develop-

TRITICUM

175

ment of the epithelial layer occurs after this elongation has taken
place. (Fig. 79, F, G.)

The differentiation of the primary root within the coleorhiza,
or root sheath, takes place at about the time that the coleoptile
begins to develop; and the histogens and root cap are clearly de-
fined before the embryo is a millimeter in length. A difference in
the growth rate of the cells of the root cap and those of the adjacent
tissues results in the formation of a schizogenous cavity around the

— e

Fig. 79. A-F, longisections of embryos in progressive stages of development; G, epi-
cotyledonary portion of embryo : a, upper portion of scutellum ; c, coleoptile ; e, epiblast ;
/, first foliage leaf ; f, ventral scale; at, region of intercalary elongation of scutellum. (Re-
drawn from Percival, The Wheat Plant, Duckworth and Co.)

root apex. As the elongation of the embryonic root proceeds, this
cavity is extended so that in the mature embryo the root is separated
from the coleorhiza by a cylindrical cavity.

The Endosperm. — After fertilization, the development of the
endosperm parallels that of the embryo; and, before the first divi-
sion of the zygote, the primary endosperm nucleus undergoes
division followed by a series of free nuclear divisions until con-
siderable numbers of nuclei are formed. These occupy a parietal
position in the embryo sac; and, by the time the embryo has
reached the 10- to 15-celled stage, there is a continuous layer of
endosperm cells lining the embryo sac. In six or seven days, the
sac is completely filled with endosperm tissue which has developed

176 THE STRUCTURE OF ECONOMIC PLANTS

centripetally. The initial development of the endosperm occurs
at the expense of the antipodal cells and the surrounding nucellar
tissue, some of which is absorbed.

Brenchley (4) has investigated the development of the endosperm
and points out that the differentiation of the cells on the ventral
side opposite the furrow results in the formation of a peripheral
layer of aleurone cells. The deposition of starch begins at about
the tenth to eleventh day, when the endosperm cells are completely
formed, and proceeds for five or six weeks until the conclusion of
the ripening period. Starch is deposited first on the flanks of the
endosperm, and none can be detected on the portion of the endo-
sperm beneath the furrow connecting the two flanks until later in
ontogeny. Finally, the cells become so filled with starch that the
nuclei and cytoplasm of the cells are affected, and in those which
contain large amounts of starch, the protoplasmic contents and the
nuclei become disorganized.

The Integuments. — While the embryo and endosperm are
undergoing progressive development and change, the two integu-
ments are also being modified. Initially, each integument consists
of two cell layers; but at about the time that the zygote divides,
the cells of the outer one begin to disintegrate. This is evidenced
in a loss of turgidity and a degeneration of the protoplasts; until,
finally, the cell contents disappear completely and the cell walls
are crushed and obliterated. (Fig. 80, A, B, C.) The inner integ-
ument retains its form, and its cells remain distinct, increasing in
size until about the milk-ripe stage, when there is a collapse of the
cells of the outer layer of the inner integument owing to a loss of
cell contents. The cells of the inner layer of the inner integument
persist for some time longer and in them are the pigments which
give color to the yellow and red grained wheats. Finally, as the
grain dries and shrinks in ripening, the radial walls of the inner
layer become crushed and partially dissolved, so that the collapsed
remains of the outer and inner walls represent all that is left of the
seed coat at maturity. (Fig. 80, D.)

The Pericarp. — The pericarp or ovary wall keeps pace with
the development of the enclosed ovule. It consists chiefly of
parenchymatous cells which lie between an outer and inner epi-
dermis. As maturation proceeds, there is some differentiation and
the cells of its outer epidermis begin to exhibit the pits and beaded
thickenings described for the mature grain. The parenchyma is

TRITICUM

177

colorless except for a chlorophyllose region one cell in thickness,
adjacent to the inner epidermis; but there may also be chloroplasts
in the subepidermal cells along the furrowed portion of the young
pericarp. As maturity approaches, the cells of the inner epidermis
fail to keep pace with the growth of the adjacent cells and become
separated, forming the characteristic tube cells of the ripe pericarp.

D

Fig. 80. A-D, transections showing the developmental changes occurring in the pericarp,
integuments, and nucellus during the maturation of the grain : a, the cross cells of the peri-
carp; b, inner epidermis of pericarp; c, outer integument of ovule; d, inner integument of
ovule ; e, outer layer of nucellus ; /, cells of nucellus ; g, aleurone layer. (Redrawn from Per-
cival. The Wheat Plant, Duckworth and Co.)

Shrinkage and desiccation cause a compression of the cells which
come into intimate contact with those of the testa and in this man-
ner the combined fruit-seed coat of the caryopsis is formed.

I.

1.

3-

LITERATURE CITED

Avery, G. S., Jr., "Comparative anatomy and morphology of embryos and

seedlings of maize, oats, and wheat." Bot. Gax_. 8p: 1-39, 1930-
Bessey, C. E., "The structure of the wheat grain." Neb. Agr. Exp. Sta. Bull.

}2: 100-114, 1894.
Boyd, Lucy, and Avery, G. S., Jr., "Grass seedling anatomy: The first

internode of Avena and Triticum." Bot. Gaz- 97- 765-779, 1936.
Brenchley, W. E., "On the strength and development of the grain of wheat

(Triticum vulgare)." Ann. Bot. z^: 117-139,1909.

178 THE STRUCTURE OF ECONOMIC PLANTS

5. Grantham, A. E., and Frazier, G., "Occurrence of sterile spikelets in

wheat." Jour. Agr. Res. 6: i35-i5o, 192.6.

6. Hackel, E., The True Grasses, Trans, by Scribner, F. L., and Southworth, E. A.,

Henry Holt and Co., 1890.

7. KoRNicKE, M., "Untersuchung iiber die Entwickelung der Sexualorgane von

Triticum mit besonderer Beriicksichtung der Kernteilungen." Verhandl.
Natur. Hist. Ver. die Preuss. Reinl. und Westf. ;^: 149, 1896.

8. Locke, L. F., and Clark, J. A., "Development of wheat plants from seminal

roots." Jour. Am. Soc. Agron. 16: 161-2.68, 192.4.

9. McCall, M. a., "Developmental anatomy and homologies in wheat."

Jour. Agr. Res. 48: 183-311, 1934.

10. Percival, J., The Wheat Plant, Duckworth and Co., London, 1911.

II. , "The coleoptile bundles of Indo-Abyssinian Emmer Wheat. (Trit-
icum dicoccum, Schiibl.) " Ann. Bot. 41: 101-105, 1917.

12.. RosLER, P., "Histologische Studien am Vegetationspunkt von Triticum
vulgare." Planta /.• 18-69, 1918.

13. Sax, K., "The behavior of the chromosomes in fertilization." Genetics

i- 309' 1918-

14. TscHiRCH, A., and Oesterle, O., Anatomischer Atlas der Pharmakognosie und

Nahrungsmittelkunde, Herm. Tauchnitz, Leipzig, 1900.

15. Weaver, J. E., Root Development of Field Crops, McGraw-Hill Book Co., 1916.

16. WiNTON, A. L., A Course in Food Analysis, Wiley and Sons, 1917.

CHAPTER VII

LILIACEAE

ALLIUM CEPA

ALLIUM and Asparagus are important genera in the lily family,
and there are many ornamental plants in this group, including
species of Lilium, Tulipa, and Hyacinthus. . In the genus Allium,
there are a number of strong-scented, pungent herbs characterized
by the development of a scapose stem which, with the leaf bases,
forms a bulb that may be small as in chives (Allium Schoenoprasum
L.), or very well developed as in the common onion (A. Cepa L.).
Other widely cultivated species are: A. sativum L., garlic; A. as-
calonicum L., shallot; and A. Porrum L., leek, in which a pro-
nounced bulb is nearly always absent.

GENERAL MORPHOLOGY

The Root System. — The root system is relatively shallow and
fibrous. New cycles of adventitious roots continue to arise from
the stem throughout the life of the plant, which radiate in all
directions but do not penetrate the soil to any great depth.
Sideris (i6) reports that

"onion plants grown in water or in soil cultures produce two sets
of roots from the time of their germination to the completion of their
life cycle. The first set is produced at the time of germination and
functions during the period between germination and formation of
the bulb. . . . The second set is produced at the time of formation
of the bulb and later, and functions during the period between the
formation of the bulb and the death of the plant."

Thompson (19) finds that the roots of onion seedlings attain a
depth of -L inches and a lateral spread of 3 inches within X5 days
after being planted; while in plants li inches tall with a base the
size of a lead pencil, the roots reach a depth of 4 inches. At
maturity, the majority of the roots are within 6 to 8 inches of the
soil surface and the greatest penetration does not exceed xo inches.

179

i8o THE STRUCTURE OF ECONOMIC PLANTS

The Shoot. — The plant has a biennial habit, although it may
persist vegetatively as a perennial by means of bulbs. The sub-
terranean stem is a short, subconical structure from which the
linear leaves are diverged in a |^ phyllotaxy. It produces a long,
hollow, leafless stem which arises from the terminal bud of the

Fig. 8i. Basal portion of onion showing the bulb, the two-ranked arrangement of leaves,
the manner in which the leaf sheaths enclose younger leaves, and the adventitious roots.
(Photograph by J. Horace McFarland Co.)

underground axis and bears the inflorescence at its apex. In some
instances, lateral buds from the axils of the fleshy leaves of the bulb
may develop into flower stalks and produce terminal inflorescences.
The leaves arise from the short crown stem in a compact series,
and the sheaths of the outermost, older leaves enclose the younger
ones. The sheathing base of each leaf completely encircles the
stem, and it is the development of the fleshy leaf bases, together
with the absence of internodal elongation, that results in the for-

ALLIUM CEPA i8i

mation of the bulb of commerce. (Fig. 8i.) The inner leaves
emerge through the outer ones by way of a lateral slit at the apex of
each leaf base. The linear blade is parallel-veined and cylindrical,
with a hollow center at maturity. This is not continuous with the
hollow formed by the circular leaf base, but is the result of a
splitting away and disintegration of the centrally located paren-
chymatous tissue.

The Inflorescence and Flower. — The inflorescence is a
terminal umbel that is borne on an elongated flower stalk which
reaches an average height of 3 or 4 feet, but may occasionally
approach 6 feet. It is subtended by a papery spathe consisting of
two or sometimes three bracts which enclose the umbel until it is
split by the development of the first flowers. (Fig. 82., A.") The
number of flowers in an umbel ranges from as few as 50 to over
1000. The pedicels which bear the individual flowers are usually
long and slender, but may be short and rigid. The flower buds do
not develop in regular centripetal or centrifugal order; and, con-
sequently, flowers in various stages of development occur through-
out the umbel.

The white or bluish flowers are regular and pentacyclic, consist-
ing of a perianth of six similar parts arranged in two whorls, six
stamens inserted at the base of the perianth segments in two cycles,
and a pistil of three undiverged carpels. (Fig. Sx, E, G.) The
nectaries occur in the axils of the three inner stamens. The ovary
is superior and there is a single, thin persistent style with a slightly
three-lobed stigma. The anthers of the inner whorl of stamens
dehisce before those of the outer, and both sets do so before the
stigma is receptive. For this reason, cross-pollination is the rule
and is effected through the agency of insects, although inter-
pollination between flowers of the same umbel is undoubtedly a
frequent occurrence.

ANATOMY

The Fruit and Seed. — The fruit is a three-lobed, three-celled
capsule, each valve or locule containing one or two black seeds at
maturity, and the dehiscence of the fruit is loculicidal. Sachs (15)
has described the seed, and the development of the seedling has
been outlined by Anderson (i) and Hoffman (7). Kondo (11) in-
vestigated the structure of the seed coats of several species of Allium
and found them to be similar in all major details.

l82.

THE STRUCTURE OF ECONOMIC PLANTS

In Allium Cepa, the thick-walled epidermal cells are filled with
a very dark brown pigment; and the parenchymatous layer is
similarly colored, consisting of six rows of thin-walled cells that

f

.^ri^wery

Fig 8i A the inflorescence ; B and C, the inner and outer stamens ; D, a single flower in
side section; £, the same, face view; f, flower with perianth removed; G, floral diagram
showing arrangement of parts.

are flattened tangentially, and roughly oblong in surface view.
(Fig. 84, A, B.) The layer of perisperm that underlies the paren-
chymatous 'layer is hyaline, and the seed is filled with a horny

ALLIUM CEPA

183

endosperm in which the coiled embryo lies embedded. (Fig. 83,
/4.) The endosperm cells are thick-walled, with a limited number
of interconnecting pits. Sachs has shown that the thickening on
the walls of the endosperm decreases during germination; and
Cooley (3) has confirmed this, pointing out that the reserve hemi-
cellulose, laid down on the wall as a secondary product, yields
mannose on hydrolysis. The endosperm cells, which retain their
protoplasmic contents, contain oil globules and protein, the latter
being the first food to be withdrawn during germination. These
cells, together with the cells of the haustorial portion of the coty-

FiG. 83. A, the seed, dotted lines indicate position of the embryo ; B, diagram of embryo ;
C, diagram of transection of embryo at level 3-3, in B; D, the same, at level 4-5, showing
the slit ; E, the same, showing variation in the slit ; F, the same, at level 6-6 ; G, diagram
of longisection of the lower portion of embryo ; H, longisection through haustorial tip of
cotyledon showing procambial cells extending to the epidermis which is without a cuticle :
cof, cotyledon ; h, hilum ; hyp, hypocotyl ; p, leaf primordium ; p c, procambial strand ;
ph, phloem ; s, slit in cotyledon ; st, stem ; v, cavity in hollow base of cotyledon ; x, xylem.
(After Hoffman.)

ledon, function in the absorption and transfer of food; and the
endosperm cells do not collapse until all of the reserve cellulose is
exhausted. (Fig. 83, /f.)

The Embryo. — The mature embryo is a curved cylinder about
6 mm. long and 0.4 mm. in diameter, the latter being uniform
throughout except that the hypocotyl is pointed and the haustorial
tip of the cotyledon somewhat rounded. (Fig. 83, B.) The cur-
vature of the axis varies from crescentic to more than a complete
circle, and, according to Hoffman, a 170° arc is the average. The
hypocotyl is small and short, and the major portion of the embryo
consists of the cotyledon which arises from the cotyledonary node

i84 THE STRUCTURE OF ECONOMIC PLANTS

of the short stem axis. The epicotyl is surrounded by the basal
sheath of the cotyledon and may bear a single, slightly developed
leaf primordium prior to germination. (Fig. 83, G.) Near the
base of the cotyledon there is a lateral slit leading into the cavity
which, according to Sachs (15), occurs with equal frequency on the
convex and the concave sides of the embryo. In his description of

Fig. 84. A, transection of a portion of seed showing principal regions ; B, surface view of
the epidermis and parenchymatous region : end, endosperm ; ep, epidermis ; par, paren-
chyma; p g, protein granules; psm, perisperm. (Redrawn and rearranged after Kondo,
Ohara Institute.)

the embryo, as seen in longisection, he recognizes five regions: (i)
The peripheral or epidermal layer of thin-walled, parenchymatous
cells. These are arranged in longitudinal rows covering the root
and cotyledon; and, in the latter, stomata with guard cells may be
found, (x) The parenchyma of the hypocotyl and cotyledons.

(3) The single provascular strand which can be recognized by the
long narrow cells that are filled with an albuminous material.

(4) The meristematic regions occurring at the root tip and at the
growing point of the epicotyl; and (5) the well-defined root cap.

ALLIUM CEPA

185

Development of the Seedling. — In germination, there is an
initial elongation and growth of the lower and middle parts of the
cotyledon resulting in the emergence of the root tip through the
seed coat at the micropyle. (Fig. 85.) Within 14 hours, the
hypocotyl has grown out through the seed coat; and the cotyledon

1

Fig. 85. The development of seedling: A, diagram of seed showing position of embryo;
B, the emergence of first foliage leaf through the cotyledonary slit.

soon emerges except for its tip, which remains embedded in the
endosperm and acts as a haustorium for the absorption of food.
The emerging root tip is usually directed upward at first, owing to
the shape of the seed; but after it has attained a length of from
3 to 5 mm., downward curvature takes place in the elongating
parts of the seedling which already lie outside the seed. This
results in the formation of a sharp bend in the cotyledon, which

i86 THE STRUCTURE OF ECONOMIC PLANTS

is known as the "knee." One limb extends downward to the coty-
ledonary plate and the other to the terminal portion of the cotyle-
don which remains embedded in the endosperm.

Continued elongation of the two limbs brings the sharp "knee"
above the soil surface; and then the growth of the two limbs
becomes unequal, the one that joins the cotyledonary plate growing
more rapidly than the one leading to the haustorial tip of the coty-
ledon. As a result of this unequal growth, the cotyledon assumes
the shape of a bow in which the descending limb is curved and the
ascending one is stretched like a bow string. The effect of this
tension is to draw the tip of the cotyledon out of the seed and above
the soil, so that it straightens, except for a slight kink which re-
mains at the original locus of the knee. In some cases, when the
soil is not too firmly packed, the seed may be lifted above the soil
by the tip of the cotyledon. By this time, the cotyledon is green
and photosynthetic. (Fig. 85.) The cotyledon attains a length
of about IX mm. ; and, at the end of a week, the primary root may
have grown downward from 1.5 to 5 cm. By the time seedling
development is complete, the root may reach a depth of about
10 cm.; but it does not branch, and all later roots are developed
adventitiously from the stem.

Root hairs are produced in abundance; and, just above the
piliferous region at the top of the hypocotyl, the first two or three
adventitious roots emerge through the cortex. According to
Hoffman (7), they originate from the meristematic parenchyma of
the inner cortical region. The first of these roots arises in this
cortical sector just below the cotyledonary slit, the mode of emer-
gence appearing to be both lysigenous and schizogenous. The
second one usually originates at about a 90° angle to the right or
left of the first; and if a third one develops, it is in a position dia-
metrically opposite to the second. Subsequent adventitious roots
originate like the earlier ones, but there is no regularity with
respect to the order or place of their appearance.

While the root and cotyledonary development is taking place,
the epicotyl is growing and the first foliage leaves elongate rapidly,
keeping pace with the enlargement of the basal sheath of the
cotyledon which surrounds them. Finally, the first foliage leaf
pushes its way through the small longitudinal slit in the cotyledon.
(Fig. 85, B.) By this time, the primordium of the second leaf has
developed, and emerges through a slit on the outer side of the

ALLIUM CEPA

187

sheath of the first foliage leaf which surrounds it. In this manner,
the successive leaves are produced and emerge in rapid sequence.
The stem axis from which the leaves arise elongates very slightly,
and this slow growth, together with the development of the basal

Fig. 86. A, transection through upper portion of primary root of six-day seedling; B,
same, at higher level than in A, showing protoxylem, pxs, consistingof a single cell ; C, same,
slightly above B, showing actively dividing cells of cortex (stippled) initiating first adven-
titious roots ; D, same, at higher level than C, showing second leaf with first procambial
strand (stippled) surrounded by cotyledon : co, cortex; s«, endodermis ; ^/», epidermis ; epi,
epidermis of cotyledon ; ^/)2, epidermis of second leaf ; wat, metaxylem ; /»f/, pericycle; ph,
phloem; />x, protoxylem ; r, root hair. (After Hoffman.)

portions of the foliage leaves as storage regions, leads to the forma-
tion of the bulb.

The Primary Root. — The primary root has an exarch, radial
protostele which is usually diarch, rarely triarch. At the level
of the root hair zone, there are two protoxylem points which al-
ternate with the two phloem groups and differentiate centripetally.
(Fig. 86, A.') The central portion of the stele is at first parenchym-
atous; but, later in ontogeny, metaxylem elements are differen-

i88 THE STRUCTURE OF ECONOMIC PLANTS

tiated consisting of one to five, usually two, large scalariform ves-
sels; and when maturation of the stele is complete, there are no
fundamental parenchyma cells remaining except the single-layered
pericycle. Casparian strips are laid down early on the radial and
end walls of the endodermal cells and are well developed in seed-
lings a week old. The cortical parenchyma consists of six or seven
layers of thin-walled cells with intercellular spaces at their angles,
and the epidermis is a single layer of cells, many of which elongate
radially to form hairs.

The development of the root is similar to "type x" as described
by Janczewski (8) and modified by Treub (2.1). In this type, the
meristem consists of two histogens, a well-defined plerome which
gives rise to the stele, and, overlying it, a group of common initial
cells two layers in thickness from which originate the cells of the
root cap, epidermis and cortex. The plerome consists of a layer of
a few cells from which the pericycle is first differentiated. Then
as the protoxylem is differentiating, the cells of a centrally located
axial row enlarge, especially in the longitudinal direction, without
further division; while the adjacent cells continue to divide.
These large cells finally mature as segments of a metaxylem vessel
or vessels. (Fig. 87, C, D.)

One or two periclinal divisions of the undifferentiated cells
enlarge the stele so that it is from five to ten, most commonly
seven, cells in diameter. The pericyclic cells may be somewhat
shorter than the adjacent endodermal and cortical cells, and can be
distinguished from the provascular cells which lie immediately
centrad to them by the small caliber of the latter.

Distal to the plerome is a meristematic region three to six cells
in diameter and two to four cells in depth. The cells of the inner
layer or layers of this region function as a periblem and produce the
cortex, which reaches its mature width as a result of one or more
periclinal divisions. The cells in this region divide in the trans-
verse plane more frequently than do those of the stele; and less so
than the epidermal cells, so that they are intermediate in length.
The lateral cells of this same meristematic region produce a single
layer of epidermal cells which divide only anticlinally. The outer-
most layer of the meristematic region cuts off the cells of the root
cap. This consists of several dome-shaped layers of cells overlying
one another, the outermost being the oldest and largest. At the
apex of each layer, the cells are larger, slightly more numerous, and

ALLIUM CEPA

189

axially elongated so that the cap is wedge-shaped in outline. In
the central portion of the root cap, where there is the least variation

Fig. 87. A and B, transections of mature portions of adventitious roots showing pentarch
and tetrarch types ; C, longisection of primary root grown in soil ; D, adventitious root
grown in water : co, cortex ; dgn, dermatogen ; en, endodermis ; ep, epidermis ; mx, metaxy-
lem ; pbm, periblem ; pel, pericycle ; ph, phloem ; pi, plerome ; px, protoxylem ; r c, root
cap. (After Hoffman.)

in the size of the cells, a longisection shows two to four irregularly
vertical rows of cells, each of which may be traced back to a single
cell of the histogen. (Fig. 87, D.)

I90 THE STRUCTURE OF ECONOMIC PLANTS

The Structure of the Cotyledon. — In the growth and devel-
opment of the cotyledon, Hoffman observed that there is "appar-
ently no localized meristematic region in any part of the cotyledon,
cell divisions occurring throughout its entire length, although the
cells at each end reach their full size later than the others." As
this general growth takes place, the cells become vacuolate, en-
larging to six or eight times their embryonic length; and, subse-
quently, they may divide two or three times. He found no mitotic
figures during the first five days of development, and concluded that
growth up to this point is mainly the result of cell enlargement. A
few days later, mitoses occur in large numbers in all tissues, the
last region to initiate nuclear and cell division being that part of
the cotyledon adjacent to the seed. At the end of two weeks,
mitotic figures are again lacking.

The cotyledon above the lateral opening has a single, centrally
located vascular strand extending to its extreme tip. This is
surrounded by several rows of cortical parenchyma which are in
turn bounded by the epidermis. The tip of the cotyledon is haus-
torial in function, and the epidermis at this point is without a
cuticle. (Fig. 83, H.)

The vascular anatomy of the cotyledon and its relation to that of
the root has been described by Chauveaud (x) and Hoffman (7), the
account of the latter differing from the former chiefly with respect
to the ultimate disposition of the protoxylem elements. The
stelar organization of the hypocotyl, up to the cotyledonary plate,
resembles that of the primary root, and the two protoxylem points
of the primary xylem strand are alternate with the two primary
phloem groups. In the center of the stele are a few large paren-
chymatous cells which later mature to form the two or more large
metaxylem vessels. (Fig. 86, /4.)

The differentiation of the xylem of the cotyledonary bundle
begins at a level slightly above the slit and proceeds from this point
upward and downward at about equal rates. As first noted by
Sachs (15), the xylem of the cotyledonary bundle is continuous
with one of the exarch xylem strands of the hypocotyl, and the
phloem of this bundle is joined with both of the phloem groups of
the hypocotyl. The difl^erentiation of the other primary xylem
strand of the stele is retarded at the point where the vascular system
of the first foliage leaf later differentiates and anastomoses with it.
At the base of the cotyledon, the vascular portion of the axis is

ALLIUM CEPA 191

slightly enlarged, the metaxylem vessels are somewhat smaller,
and the alternate, radial position of the xylem and phloem still
persists. (Figs. 83, C; 86, B.)

The development of the cotyledonary xylem is exarch through-
out. In most cases, the protoxylem consists of spiral elements,
although some annular elements may occur; and annular thick-
enings are frequently present in the tapering ends of the tracheids.
The cotyledonary bundle has one protoxylem point, and a band of
metaxylem is later differentiated on each side of the youngest or
most recently formed protoxylem. This results in the formation
of a Y-shaped xylem region with an accompanying zone of primary
phloem at the end of each arm of the Y. The orientation of the
xylem and phloem is shown diagrammaticallyat successively higher
levels in the cotyledon in Figure 83, C-F. In some cases, the xylem
is separated into three or four groups by parenchymatous cells;
while, in others, the Y-shaped configuration may be flattened so
that it appears like a short-stemmed T.

The maturation of the phloem occurs first at points farthest
removed from the protoxylem. Chauveaud (i), in describing the
development of the cotyledonary bundle in seedlings eight to ten
days old, reports a disintegration of the protoxylem as the meta-
xylem differentiates, attributing this to a complete resorption of
the protoxylem elements and their replacement in situ by meta-
xylem. Studies of this region by Hoffman, especially in longi-
section, indicate that there is not a complete resorption of these
elements; but that "the protoxylem vessels become so stretched
that the walls between thickenings collapse; and, in transection,
the vessels are seen as only a very small patch of cellulose, easily
overlooked." This stretched protoxylem is more difficult to find
as the cotyledon becomes older; but can be seen in longisections
of material less than four weeks old, at which time the second and
third leaves have appeared and the cotyledon has begun to wither
from its tip downward toward its base.

Adventitious Roots. — All the roots, except the primary one,
are adventitious in origin. During the development of the seed-
ling, the first of these arises from the meristematic region of the
inner cortex. (Fig. 86, C.) Subsequent roots originate in a similar
manner, being produced in great abundance so that a plant five
months old may have up to a hundred root primordia and roots.
The later-formed roots originate in the pericycle, rather than in the

i9i THE STRUCTURE OF ECONOMIC PLANTS

inner cortical region. Tliis is not a single layer, but consists of a
meristematic region adjacent to the cortex. (Fig. 88.) The
cortical parenchyma of the adventitious root is continuous with
that of the crown stem, and, as the root elongates, it pushes

j.>.i»»V-.-i^,

1 mm.

Fig. 88. Above, transection of stem five months old, slightly below the apical meristem,
showing origin of four adventitious roots, R, which are continuous with the outer pro-
cambial tissue. The seven cycles of bundles in the stelar portion extend to leaf blades and
sheaths, one cycle for each leaf. The vascular bundles of sheath, L, are shown in partial
longisection in cortex.

Below, longisection through apical meristem showing adventitious roots, Ri and R2,
apical meristem, X, and vascular bundle of a sheath, L. (After Hoffman.)

ALLIUM CEPA 193

through the outer cortical cells, finally penetrating the bases of
two or three fleshy leaf sheaths. In some instances, the root pene-
trates the inner epidermis of a leaf sheath but fails to pierce the
outer, and the root grows upward through the mesophyll, eventu-
ally dying when the sheath becomes desiccated. The emergent
adventitious roots may branch after attaining a length of 10 to 15
cm.; but, during the first year, the older roots, as well as the lower
part of the crown stem and outer leaves, shrivel and die so that the
base of the bulb appears somewhat flattened.

The adventitious roots are tetrarch, pentarch, or hexarch, the
pentarch being most common; and all three types may occur in the
same plant. (Fig. 87, A, B.) Their development resembles that
of the primary root, the chief difference being the larger histogenic
region and the correspondingly greater number of cells formed.

The Vascular Anatomy of the Crown Stem. — The structure
of the entire hypocotyl is root-like; and, near its top, the two
protoxylem strands separate, one continuing into the cotyledon,
while the other ends blindly beneath the point of divergence of
the primordium of the second leaf. (Fig. 86, B.) Above the
point at which the hypocotyledonary xylem terminates, the xylem
of the cotyledonary bundle and of the stem develops in one direc-
tion — away from the protoxylem of the cotyledon and toward the
center of the stem axis. About ten days after germination, the very
short stem contains no mature xylem, except that of the coty-
ledonary bundle; and approximately ten more days are required
before the xylem of the trace of the second leaf is completely dif-
ferentiated in the first internode. The elements of this trace are
reticulate cells about three times as long as wide.

All the bundles of the stem are common, representing the down-
ward divergence of leaf traces; and the vascular arrangement of the
stele is determined largely by the manner in which the leaves are
differentiated. As each leaf matures, the bundles in its cylindrical
sheath extend into the stem and anastomose with the ring of vas-
cular tissue. The bundles of each successively higher leaf trace
are vertically connected with those below, so that the central pith
is surrounded by a cylindrical stele consisting of a complex network
of vascular strands. The larger bundles of each leaf trace are con-
nected directly with the inner perimedullary portion of the vascular
cylinder, while the smaller ones are anastomosed with the periph-
eral bundles of the stele.

194 THE STRUCTURE OF ECONOMIC PLANTS

In longisection, the stem appears somewhat heart-shaped, since
the successively formed parts have increasing diameters and grow
more rapidly than the terminal meristem. The thick, parenchym-
atous cortex is traversed by numerous traces of adventitious roots
which form an almost continuous zone surrounding the stele;
and the leaf traces are separated from them by a layer of paren-
chymatous cells so that there is no direct connection between the
two vascular systems. The basal stem bundles are usually amphi-
vasal, with a layer or two of parenchymatous cells separating the
phloem from the surrounding xylem; but higher in the stem, the
amount of xylem on the outer face of the vascular strand diminishes
and the bundles are collateral.

Ontogeny of the Leaf. — Each leaf develops as an upright,
hollow cylinder, one side of which grows more rapidly than the
other forming the blade. The leaf primordium arises as a dome-
shaped mass of cells which grows more rapidly than the center of
the axis and begins to partially enclose the growing point. (Fig.
90, A-G.^ As its eccentric growth continues and one side becomes
higher, the entire periphery of the growing region forms a fold
which grows upward and around the axis to constitute the basal
sheath of the young leaf. While this is taking place, another
primordium arises within the first, and its blade grows at the same
rate as the more slowly developing portion of the periphery of the
enclosing leaf. (Fig. 90, H, /.) In this manner, the vertically
oriented, cylindrical base of each young leaf completely surrounds
younger leaves which in turn enclose the apical meristem.

As the differentiation of the sheath and blade proceeds, there
is a rapid increase in the diameter of the growing region of the axis,
and the base of the newly formed leaf is pushed farther away from
the center of the stem. By the time the next younger leaf is dif-
ferentiated, no more periclinal walls are formed, and further in-
creases in the thickness of the sheath, which becomes the fleshy
bulb scale, are due to cell growth and to the formation and enlarge-
ment of intercellular spaces.

The orifice of the sheath is surrounded by a thin membrane ex-
tending around its upper edge, including the side bearing the blade.
It is an outgrowth of the blade, five or six cells in thickness, which
is usually devoid of vascular tissue. In the first six or eight leaves,
elongation is greater than in the later formed ones; and the open-
ings become much stretched longitudinally so that their margins

ALLIUM CEPA

195

are in contact with one another, closing the cavity within until the
next younger leaf pushes through it. In the later leaves, the tip
of the blade may be pushed through the orifice of the next older

Fig. 89. Median longisection, except at the bottom, through the plane of phyllotaxy of
five-months old stem : B, transection through a perimedullary bundle ; C, cortex ; L, leaf
trace; P, pith; R, root trace, X-X, sheath and blade of one leaf. (After Hoffman.)

leaf while both are themselves enclosed within the sheath of a still
older one. (Fig. 89.)

Hoffman has found that in the growth of the first leaves, the
blade attains a considerable length before the sheath begins to
elongate appreciably. Growth takes place at about equal rates
in all parts of the blade, but the leaf tip is the first part to cease

196 THE STRUCTURE OF ECONOMIC PLANTS

elongation, while the base of the sheath remains meristematic
longer than any other part. Thus,

"in the earliest stage of development the entire leaf is meristematic;
at a later date, only the sheath and base of the blade; and still later,

Fig. 90. A, longisection through growing point of axis shown in B ; a and d are opposite
sides of same leaf which overgrow central region, bee ; B, earliest stage in leaf differentiation ;
C, cell detail of region, x in A and / (plane of cell division marks boundary between
regions e and c); ^ ^"d E, one side, c, has outgrown e, and the other side, b, is beginning to
overgrow it ; F and G, both sides, b and c, overgrowing axis, e, which is beginning to elon-
gate ; H and /, more rapidly growing side, c, is forming blade of leaf while b forms the sheath
or bulb scale ; /, diagram of vascular system in mature leaf at orifice : a, adaxial side of blade ;
^, branches of larger bundles ; c, cross-connections ; ^, edge of orifice ; »«, continuous bundle
of sheath and blade; /;, pith; j, outer side of sheath ; ^, tip of younger leaf ; i^, bundles end-
ing blindly under orifice. (The small letters in A-H correspond to A', B' , C , D' in
Z; while a and d in 1 represent blade and sheath sides of a younger leaf. (After Hoffman.)

ALLIUM CEPA 197

only the base of the sheath. The lowest portion of a half-grown blade
remains meristematic until the leaf is nearly grown, however, so that
this region gives rise to a large share of the blade. Any one leaf,
after it reaches a length of three centimeters, attains its mature length
in a few days."

In the development of the leaf, mitoses, indicating continued cell
divisions, may occur in all parts, even after intercellular spaces are
formed. Transverse divisions and growth of the surrounding
parenchymatous cells cause the intercellular spaces to become en-
larged and much elongated throughout the entire leaf. There is a
peripheral zone in which the cells enlarge radially rather than
longitudinally; and, in the mature leaf, these form two or three
subepidermal layers of columnar cells which are comparable to the
palisade region found in dicotyledonous leaves. (Fig. 91, B.)

The parenchymatous cells centrad to the chlorenchyma do not
keep pace with the growth of the latter, and a large central cavity
develops which extends through the entire length of the blade.
(Fig. 91, A.') Prior to the formation of the cavity, the inner cells
are alive, although the volume of the intercellular spaces formed
between them is probably greater than the volume occupied by the
cells themselves. Newcombe (ix) has described cavity formation
in onion and other plants, and regards the initial stages which
occur during the primary growth of the leaf as being schizogenous.
The final ones are lysigenous, and result from the death and collapse
of the centrally located parenchyma cells which by their disinte-
gration add to the size of the central cavity.

The Mature Leaf. — The blade of the mature leaf consists of
the epidermis, the palisade layers, the vascular bundles, and eight
or ten rows of spongy parenchyma which surround the central
cavity. (Fig. 91, B.) The development of the xylem in the pro-
cambial strand is endarch, and differentiation proceeds rapidly from
the base of the leaf to its tip so that the first protoxylem elements
are mature throughout the entire length of the leaf before any of
the metaxylem is completely developed. Nearly all of the primary
xylem elements have spiral thickenings.

The cotyledon has a single vascular bundle, and the number
increases in successive leaves, the second leaf having five bundles,
the third usually eight. Each of the older leaves has two more
bundles than the preceding one until, in the sheath of a mature
bulb scale, there may be 40 or more. The bundles are arranged

198

THE STRUCTURE OF ECONOMIC PLANTS

around the periphery of the leaf blade or base and are parallel
throughout its entire length. There are many small branches and
cross-connections between adjacent bundles which arise in the rows
of parenchymatous cells that surround and separate the bundles.

_3>^.^-X--lac

par

=^ lac

Fig. 91. A, diagrammatic transection of the lamina showing distribution of bundles and
large central lacuna; B, a section of A showing cellular detail : cti, cuticle; ep, epidermis;
/' sp, intercellular space; lac, lacuna (broken down parenchymatous cells are shown on the
margin of lacuna in B); pal, palisade cells; par, parenchyma; ph, phloem; sto, stoma;
xy, xylem.

There are relatively few transverse veinlets in the lower half of the
outer sheaths of the mature bulb, but many occur in the upper parts.
The unthickened sheaths of a young onion have numerous cross-
connections at all levels. Both large and small bundles may origi-
nate blindly and then anastomose with larger ones, while some of the

ALLIUM CEPA 199

smaller ones may end at the base of the blade without joining other
veins. In the outer sheaths, the bundles frequently appear green
because of a layer of chlorenchyma which partially surrounds them.

In the mature leaf, the orifice leading to its basal cavity is oval
in outline, and appears to be laterally placed owing to the greater
elongation of the sheath on the side bearing the blade. At the
base of the sheath, the distribution of the vascular bundles is fairly
uniform; but near its top, they become more closely aggregated
on the side of the sheath from which the blade extends. On the
opposite side, there are one or two smaller bundles which, after
branching several times and producing cross-connections, end
blindly under or around the margin of the sheath. On each side
of these smaller bundles is a larger one which parallels them up to
the orifice and then curves around its edge. (Fig. 90, /.)

Lactiferous Cells. — The cotyledon and all other leaves develop
longitudinal rows of lactiferous cells. They first appear early in
the ontogeny of the plant, and may be found in a longisection of the
cotyledon about 14 days after germination. In the leaf base, they
are almost invariably separated from the epidermis by two layers
of parenchyma, rarely by only one; but in the green tubular portion
of the blade, they may be more deeply located, lying just within
the chlorophyllose palisade tissue. (Fig. jl, D.) Hanstein (5)
described these structures in various species of Allium and called
them "vesicular-vessels." Later, Rendle (14) pointed out that
the term "vessel" is not appropriate in this case, since the latex-
containing cells are "not cell-fusions, and continuity between the
contents of adjacent members can very rarely be seen." He sug-
gested the term "laticiferous cells" for these structures, which has
been shortened to "lactiferous cells" by Hoffman.

Each row of lactiferous cells is parallel to the epidermis; and
in their early stages of development, the transverse septa are not
pitted; but become so as the leaf enlarges and the cells elongate.
(Fig. 9i, A, D, £.) In the succulent portion of the leaf, these cells
do not elongate much, and they are extremely short at the base,
where the longitudinal series may be irregularly connected by cross
unions. (Fig. 9X, F.) Both Hanstein (5) and Rendle (14) state
that row^s of lactiferous cells may occur side by side; and in such
cases, the longitudinal wall separating them is also pitted; but
where parenchyma abuts these cells, the dividing walls are
unpitted. (Fig. 9i, 5.)

loo THE STRUCTURE OF ECONOMIC PLANTS

Ic

_; --ep

B

par-

ep---j

The lactiferous cells end near the extreme base and apex of the
leaf respectively, and there is no evidence of any connection between

them and the vascular bun-
-^ J dies or the assimilating tis-
sue, nor do they follow the
general course of the former.
The contents of the cells is a
more or less granular turbid
fluid which appears on a cut
surface of the leaf bases as
a pale milky liquid, but is
clearer and more watery in
the green leaves . The latex
does not contain any food
substances, according to
Rendle, but seems to con-
sist of a compound that is
easily hydrolyzed to allyl
sulphide. Apparently it is
an excretion product, and
the function of these cells is
excretory or protective.

Owing to the superficial
resemblance between the
lactiferous cells and sieve
tubes, it has been suggested
that they may function in
conduction; but they appear
to be nothing more than
rows of excretory sacs that
are sealed up by the depo-
sition of callus very early
in ontogeny. Rendle has
observed that "even before
the transverse septa become
conspicuously pitted, cal-
lus-formation occurs upon
them; at first usually as
small plugs, afterwards often of a more irregular form and sometimes
spreading more or less over the whole plate." (Fig. 91, B, C)

• > iff®®?

H

G

Fig. 92.. The development of lactiferous cells in
Allium. A, longisection of base of young inner
leaf; B, tangential section of an inner succulent
leaf prior to elongation ; C, tangential sections of
base of an outer leaf showing callus formation on
transverse septa and occasionally on side walls of
cells; D, longisection of a portion of green blade
showing location of lactiferous cells ; E, tangential
sections of base of an outer succulent leaf showing
the pitting in swollen transverse septa; F, tangen-
tial section of unelongated leaf showing cross unions
between two rows of lactiferous cells ; G, transverse
septum from young leaf of the shoot enclosed in a
germinating onion : c p, callus plug ; ep, epidermis ;
/ c, lactiferous cell ; par, chlorophyll parenchyma.
(After Rendle, Ann. Bot.)

I

ALLIUM CEPA

101

Stomata. — Stomata are numerous in the epidermis of the coty-
ledon less than a week after germination. In the development of
the stoma, a mother cell is cut off from one end of an epidermal cell
so that it is approximately square in surface view. This cell by
one longitudinal division forms the guard cells which subtend the
stoma. In the mature leaf, the guard cells are sunken beneath the
surface of the adjacent epidermal cells which partially overgrow
them and develop a thick, lamellated cuticle which may exceed
half the radial dimension of the cells.

The Bulb. — The bulb consists of a short stem which bears a
series of thickened leaves and leaf bases, together with the cycles
of adventitious roots which have
been described as arising from the
cortical and pericyclic regions.
(Fig. 93.) The arrangement of
the leaves is concentric so that
the older ones surround the
younger, and the apical meristem
is located at the center of the
bulb. The blades of the leaves
comprising the bulb may be
photosynthetic or they may dry
down entirely. In a full-sized
mature bulb, some of the inner
leaves do not develop an elongated
blade, and the functional blades
of the outermost six or seven leaves become dry and break off so
that only the sheaths remain. These protect the fleshy storage
leaves which surround three or four leaves with short undevel-
oped blades; and centrad to the latter are successively younger
leaves with longer blades.

In the spring, the youngest leaves continue development. The
storage leaves outside them also elongate, especially at the base,
but their green blades remain relatively small. As the inner leaves
are growing, the outer storage leaves dry progressively from the
top of the sheath to its base because of the withdrawal of food re-
serves and moisture. Preceding and accompanying this develop-
ment, numerous adventitious roots arise from the periphery of the
stem and penetrate the soil. These remain functional throughout
the life of the plant.

Fig. 93. Diagrammatic transection of
young bulb showing relation of leaves to
each other : / b, leaf bases ; / s, leaf sheath ;
/ /, leaf tips.

loi THE STRUCTURE OF ECONOMIC PLANTS

The Floral Axis. — The flowers are borne in simple umbels
at the apex of a floral stem which is commonly hollow when
mature, and somewhat swollen at its middle or near the base.
(Fig. 94.) The number of floral stems per plant may vary from

Vrs. .

Fig. 94. Field of Yellow Globe onions. (Qjurtesy of Ferry-Morse Seed Co.)

one to twenty or more, depending upon the variety and the size
of the mother bulb. In the development of the floral axis, the first
structure to arise is the bract which encloses it. The primordium
of this bract originates at a point opposite the youngest leaf blade,
and it is not until the central axis begins to elongate to form the
flower stalk that it can be determined whether the leaf-like struc-
ture is a bract or another vegetative leaf. The bract encloses the

ALLIUM CEPA

103

meristematic growing point from which the flowers arise; so that,
during early ontogeny, the flower cluster is protected by the invo-
lucre as well as by a series of bracts which lie within. Each bract
surrounds the primordia of several individual flowers.

c par

mx

Fig. 95. A sector of a transection of flower stalk near base : c par, chlorophyll parenchyma;
ep, epidermis ; g c, guard cell ; mx, metaxylem ; par, parenchyma ; ph, phloem ; pi, pith ;
px, protoxylem; xj, xylem.

In the mature flower stalk, the epidermis is protected by a thick
lamellated cuticle; and the cells are radially elongated with thin
radial walls and somewhat thickened inner tangential ones. The
sunken stomata are numerous and open into substomatal cavities
in the palisade tissue. (Fig. 95.) The chlorophyllose region
consists of one or more rows of palisade cells and a row or two of
underlying spongy parenchyma in which lactiferous cells, such as
are found in the leaf, are located. The parenchymatous cells
centrad to the chlorenchyma are large and surround the outer cycle

2.04 THE STRUCTURE OF ECONOMIC PLANTS

of vascular bundles. This region is bounded centripetally by a
band of small, compact, parenchymatous cells which form a com-
plete ring. Within this zone are the large, loosely organized cells
which surround the inner cycle of vascular bundles. At maturity,
the central region breaks down schizogenously, and a large lacuna
is formed similar to that in the blade of the leaf. The collateral
bundles consist of primary sieve tubes and companion cells, and
primary xylem elements that are annular, spiral, and reticulate.

Ontogeny of the Flower. — Jones and Boswell (9) have in-
vestigated the formation of flower primordia and the relation of
the time of planting onion sets or young bulbs to their differentia-
tion. Examination of the bulbs for the development of flower
primordia indicates that in general the flower primordia are dif-
ferentiated early in the spring under conditions prevailing in Col-
lege Park, Maryland. This occurs regardless of "whether or not
the bulbs are planted in the fall or early in the following spring,
having been kept in storage over winter." It was also observed
that "fall planted bulbs, which made a luxuriant foliage develop-
ment before the differentiation of flower primordia, produced uni-
formly taller and heavier seed-stalks with a heavier set of blossoms,
than was obtained from the spring planted bulbs." Under Cali-
fornia conditions, bulbs planted in December differentiate the floral
axes in February.

Jones and Emsweller (10) have worked out the floral development,
and the following account is based in part upon their investiga-
tions. After the differentiation of the involucral bract, numerous
membranous bracts arise on the meristematic surface of the tip of
the floral axis, and these enclose the clusters of flower primordia
in their early stages. (Fig. 96, H, Z.) Each of these areas is
somewhat elevated and kidney-shaped, and it is from them that
the individual flowers arise.

The flower primordium develops as a slight projection on the
meristematic surface, which becomes globose and later elongates
slightly so that it is circular in transection and convex at its apex.
(Fig. 97, B.) As growth proceeds, the primordium becomes tri-
angular in transection as a result of the differentiation of the outer
perianth segments and the outer stamen whorl which is the first
to be formed. (Fig. 97, C) At each angle of the triangular pri-
mordium, there is differentiated a perianth segment; and in the
axil of each is a primordium of one of the stamens of the outer

ALLIUM CEPA

i05

whorL (Fig. 97, E, G.) There is sometimes a lag in the develop-
ment of the primordia of the outer stamens; so that the first lobe
of the perianth may be well developed before the primordium of the

Fig. 96. Early stages in floral development. A, longisection through growing point of
onion bulb. 1, i ; l, 1; 3, 3 respectively are opposite portions of leaves enclosing axis a;
B, primordium of axis of the inflorescence ia with involucral bract just arising ; i, i are oppo-
site sides of the leaf enclosing it ; C, the elongating peduncle of axis of the inflorescence show-
ing involucral bract ib which completely covers the central region ; D, apex of axis of the
inflorescence somewhat flattened with additional bracts arising within involucre; E, axis
of the inflorescence at about the stage of development as in B, showing unilateral development
of involucre ; F, G, developing flower stalks. At the base of peduncle at p another shcwt has
differentiated ; H, within young bracts b are raised kidney-shaped areas from which the
flowers develop ; /, primordia of the first flowers are shown : i, bract ; //>, flower primordia.
The bract covering four kidney-shaped areas has been cut away. (After Jones and Emswel-
ler, Hilgardia.')

xo6 THE STRUCTURE OF ECONOMIC PLANTS

stamen, which it subtends, becomes evident. In most instances,
the two appear to be differentiated simultaneously from a common
meristematic region, the outer portion of the primordium forming

opl
J

Fig. 97. A, a. slightly older stage in floral development than in preceding figure, /; fp,
flower primordia ; B, side view of three flower primordia ; C, top view of a young flower
showing counter-clockwise diff'erentiation of outer perianth lobes, opl, and outer whorl of
stamens. The youngest primordium VI will appear opposite the oldest perianth segment I;
D, longisection of a portion of inflorescence at about the time that floral organs of the oldest
flowers are beginning to diff'erentiate ; E, top view of a young flower showing outer whorls
of perianth and stamens differentiated. The inner whorls are just beginning to differentiate.
Roman numerals indicate sequence of origin of different segments ; F, side view of flower
slightly younger than one shown in E; G, top view of young flower in which primordia of all
perianth segments have been differentiated : ipl, inner perianth lobe ; ist, inner stamen ; ost,
outer stamen. (After Jones and Emsweller, Hilgardia.^

the perianth segment and the inner part, one of the outer stamens.
The outer perianth segments arise in a counter-clockwise direction,
and the stamens follow the same order. (Fig. 97, C.)

ALLIUM CEPA 107

The inner cycle of perianth segments, and the inner stamens
which lie centrad to them, are differentiated following the forma-
tion of the outer whorls; and the first of the inner perianth seg-
ments with its stamen is formed between the oldest and second
oldest of the outer whorl. (Fig. 97, £.) The second and third
members of the inner cycles of perianth segments and stamens arise
in sequence in a clockwise direction, although in some instances
the direction is reversed. In most cases, the last segments to dif-
ferentiate are opposite the oldest but "somet mes segments of the
inner whorls lying both clockwise and counter-clockwise to the
oldest segments of the flower appear to arise simultaneously."
Although the inner stamens are differentiated later in ontogeny
than the outer ones, they are the first to shed their pollen.

The carpellary primordia develop as the outer perianth segments
are beginning to overarch the stamens and appear as three cres-
centic outgrowths which arise on the flattened meristematic surface
alternate to the three inner stamens. (Fig. 98, A.') The young
carpels elongate, and owing to lateral and basipetal growth, their
inturned edges meet to form the three-loculed ovary of the pistil.
The style is differentiated by the apical growth of the three carpels;
and while the elongation of the style is taking place, the ovules
arise along placentae on the inner edges of the carpels, two being
formed in each locule. (Fig. 98, £-G.) Two integuments develop
which surround the nucellar tissue; and by the time of anthesis,
the ovule has become almost completely anatropous.

Vascular Anatomy of the Flower. — At the upper limits of the
flower stalk, the vascular bundles of the outer cycle anastomose so
that they form a nearly continuous ring. Slightly higher, just
below the divergence of the bracts which constitute the spathe,
the entire vascular supply forms a complicated network of branch-
ing and anastomosing bundles from which the traces to the spathe
and to the pedicels of the individual flowers diverge. Each pedicel
has six vascular bundles arranged in two cycles as in the main
flower stalk of the umbel. At higher levels, the pedicel becomes
subtriangular, and the three outer bundles occupy positions centrad
from the angles, while the three inner bundles lie slightly within
the outer cycle and alternate to them. (Fig. 99, D, £.)

Each outer bundle branches, one trace supplying a segment of
the outer perianth, the other the outer stamen which is subtended
by this perianth segment. The inner bundles supply the inner

io8 THE STRUCTURE OF ECONOMIC PLANTS

segments of the perianth and the inner cycle of stamens by branch-
ing in a similar manner. Each carpel has a single vascular bundle
which arises as a branch from the trace that supplies the outer
perianth segment and outer stamen. The point of divergence of
the carpellary bundles is slightly above the level at which the outer

■•■'^i;*«,i«#Si^<-'

Fig. 98. A, top view of young flower in which outer perianth lobes almost cover stamens.
At about this time, the carpels are differentiated ; B, side view of flower slightly younger
than A; C, early differentiation of the three carpels; D, carpels have grown upward until
they nearly meet at center ; E, carpels with the style just beginning to form ; F, inturned
edges of single carpel showing origin of two ovules ; G, a slightly older carpel than F. (After
Jones and Emsweller, Htlgardia.^

cycle of bundles of the pedicel branch to supply the outer perianth
segments and the outer stamens respectively. (Fig. 99, C.) As a
result of this method of branching, each perianth segment, stamen,
and carpel is supplied by one main vascular bundle. (Fig. 99,

F, GO

Development of the Megagametophyte. — Megasporogenesis
and the development of the megagametophyte have been inves-
tigated for several species of Allium, including A. Cepa, by Weber

ALLIUM CEPA

109

(ix), Strasburger (18), and Jones and Emsweller (10). In mega-
sporogenesis, the subepidermal, archesporial cell functions as the
megaspore mother cell. Jones and Emsweller observed that the
nucleus of the megaspore mother cell is in the prophase at about

Fig. 99. A, longisection of young flower primordium ; B, the same, more fully developed
in which the primordia of outer perianth and stamen and inner perianth and stamen are shown ;
C, a later stage in which inner and outer stamens are well differentiated and the carpels have
begun to overarch primordia of the ovules ; D, transection of pedicel of flower showing six
vascular bundles; E, transection at a higher level, slightly above the divergence of outer
perianth segments; f, a transection showing outer perianth segments subtending outer
stamens, inner perianth segments and undiverged inner stamens, and the three crescentic
carpels; G, transection at a higher level showing the inner and outer perianth segments : car,
carpel ; i pth, inner perianth segment ; / sta, inner stamen ; o pth, outer perianth segment ;
0 sta, outer stamen ; pcd, pedicel .

the time that the inner integument begins to differentiate. Fol-
lowing the first nuclear division, which is meiotic, there is a
gradual disorganization of the micropylar daughter cell. This is
characteristic of many of the species of Allium, but the formation
of the megaspores and their subsequent development appears to be
variable. Porter (13) has described this stage for A. mutabile

iio THE STRUCTURE OF ECONOMIC PLANTS

Michx. and reports that the chalazal daughter cell undergoes one
more division, forming two megaspores, the inner one being func-
tional and the micropylar one disintegrating. In A. Cepa, accord-
ing to Jones and Emsweller, the chalazal daughter cell functions
directly as the megaspore rather than undergoing further division,
and a similar situation has been described by Strasburger (i8) and
Weber (ix) for other species of Allium. In species of Allium where
both bulblets and flowers occur in the inflorescences, there may be
some variations from this type of development.

In the formation of the megagametophyte, the megaspore
undergoes successive nuclear divisions to form an eight-nucleate
gametophyte. The two synergids are of unequal size, the volume
of the larger one being several times that of the smaller, but both
are well supplied with food. In some species, the synergids disap-
pear immediately following fertilization while in others they are
more or less enlarged; and, according to Weber (xx), probably act
as nutritive cells until the embryo has developed to the two- or, in
some instances, a several-celled stage. In Allium Cepa, they are
probably nutritive for a short time.

Embryogeny. — The development of the embryo in Allium has
been investigated by Soueges (17) for A. ursinum L. and by Porter
(13) for A. mutabile Michx. The two accounts agree in general
details, and the mature embryos in each case are much like the one
described by Hoffman for A. Cepa. The first division of the zygote
is transverse, producing an apical and a basal cell. (Fig. 100, A.^
This is followed by another transverse division of the basal cell, so
that the three-celled embryo consists of an apical cell which is the
product of the first segmentation of the egg, and an intermediate
and terminal cell which result from the subsequent transverse divi-
sion of the basal cell. (Fig. 100, B.) Later, oblique or vertical
divisions of the apical cell give rise to the quadrants of the embryo
which by further division and differentiation produce the cotyledon
and growing point of the epicotyl. (Fig. 100, H, Z.) Divisions
of the intermediate cell produce the hypocotyledonary portions
of the embryo, including the root cap, and the basal cell of the
linear triad forms the suspensor by a series of divisions. As the
development of the hypocotyl proceeds, a blocking off of the his-
togens occurs, and in the embryo there is a clearly defined plerome
outside of which lies a group of meristematic cells that produce
the periblem, dermatogen and the root cap. (Fig. 100, /, /.)

ALLIUM CEPA

III

Polyembryony has been observed in this genus, in A. odorum L.
Tretjakow (xo), Hegelmaier (6), and Haberlandt (4) report
the formation of polyembryos in this species and describe their
origin from synergids, antipodal cells, and even from the inner
integument of the ovule. Tretjakow found from one to three
embryos derived from antipodal cells, with the zygote and some-

3P

re- — •-

Fig. icxd. A-I, principal stages in the embryogeny of Allium ursinum L., showing origin
of histogens; /, schematic representation of embryo showing regions figured in detail in /:
a c, apical cell ; b c, basal cell ; m and / c, superior and inferior cells derived from b c ; n and
«', basal cells derived from i c; o and p, cells derived from n'; q, a quadrant of embryo ; dgn,
dermatogen ; ^ /», growing point of stem ; /»^w, periblem ; ^/, plerome; r c, root cap. (Re-
drawn after Soueges, Compt. Rend. Acad. Sci.^

times the synergid forming additional embryos. Hegelmaier
found as many as five embryos in a single embryo sac, one being
derived from the zygote, one from the synergid, two from antipodal
cells, and one from the inner integument. If the usual sequence
of reduction divisions had occurred, this would result in haploid
embryos, derived from the synergid and the antipodal cells; and
diploid embryos from the zygote and inner integument. It appears
that the embryos in investigated cases are diploid (16 chromo-
somes); and Haberlandt has suggested that this is the result of a

tii THE STRUCTURE OF ECONOMIC PLANTS

parthenogenetic development of embryos in an ovule in which no
reduction division of the megaspore mother cell occurred. If fer-
tilization had been effected, there would be triploid embryos; and
tetraploids could be formed in cases where there was also a failure
of reduction division in microsporogenesis. Experiments with
A. odorum, in which the anthers were removed and the flowers
protected against fertilization, indicate that parthenogenetic
development, following failure of the reduction division in the
formation of the megaspore, does occur. Under such experimental
conditions, diploid embryos as described above were obtained.

LITERATURE CITED

1. Anderson, P. J., "Development and pathogenesis of the onion smut fungus."

Mass. Agr. Exp. Sta. Tech. Bull. 4, 1911.
1. Chauveaud, G., "Passage de la position alterne a la position superposee de

I'appareil conducteur, avec destruction des vaisseaux centripetes primitifs,

dans le cotyledon de I'Oignon (Allium cepa)." Bull. Museum d'Hist. Nat.

8: 51-59, 1901.

3. CooLEY, Grace E., "On the reserve cellulose of the seeds of Liliaceae and of

some related orders." Boston Soc. Nat. Hist. Mem. j.- 1-19, 1895.

4. Haberlandt, G., "Zur embryologie von Allium odorum L." Ber. Deut.

Bat. Gesell. 41: 174-179, 192.3.

5. Hanstein, J., "Ein noch nicht bekanntes System schlauchformiger Gefasse

im Parenchym der Blatter und des Stengels vieler Monocotylen vor. Monats-
bcrichter der Koniglichen Preufs." Akad. Wiss. Berlin, 705-713, 1859.

6. Hegelmaier, p., "Zur Kenntniss der Polyembryonie von Allium odorum."

Bot. Zeit. //.• 133-140, 1897.

7. Hoffman, C. A., "Developmental morphology of Allium cepa." Bot. Ga^.

9S: 179-2.99. 1933-

8. Janczewski, E., "Recherches sur I'accroissement terminal des racines dans

les phanero games." Ann. Sci. Nat., Bot. Ser. V, 20: 161-101, 1874.

9. Jones, H. A., and Boswell, V. R., "Time of flower primordia formation in

the onion (Allium cepa, L.)." Proc. Am. Soc. Hort. Sci. ig: 144-147, 1911.
ID. , and Emsweller, S. L., "Development of the flower and macrogame-

tophyte of Allium cepa." Hilgardia 10: 415-418, 1936.
II. Kondo, M., "iJber die in der Landwirtschaft Japans gebrauchten Samen."

Ber. Ohara Inst, fiir landw. Forsch. i: 405-407, 1919.
II. Newcombe, F. C, "The cause and conditions of lysigenous cavity-formation."

Ann. Bot. 8: 403-411, 1894.

13. Porter, T. R., "Development of the megagametophyte and embryo of Allium

mutabile." Bot. Gaz- 98: 317-317, 1936.

14. Rendle, a. B., "On the vesicular vessels of the onion." Ann. Bot. y 169-

i77> 1889.

15. Sachs, J., "Ueber die Keimung des Saamens von Allium cepa." Bot. Zeit.

21: 57-70, 1863.

ALLIUM CEPA

ZI3

1 6. SiDERis, C. P., "Observations on the development of the root system of

Allium cepa L." Am. Jour. Bot. 12: 155-158, 1915.

17. Sou^GES, R., "Embryogenie des Liliacees. Developpement de I'embryon

chez TAllium ursinum." Compt. Kend. Acad. Sci. 182: 1344-1346, 1916.

18. Strasburger, E., Die Angiospermen und die Gymnospermen, Gustav Fischer,

Jena, 1879.

19. Thompson, H. C, "Effects of cultivation on soil moisture and on yields of

certain vegetables." Proc. Am. Soc. Hort. Sci. ij: 155-161, 1910.

10. Tretjakow, S., "Die Betheiligung der Antipoden in Fallen der Polyembryonie

hie Allium odorum L." Ber. Dent. Bot. Gesell. i^: 13-17, 1895.

11. Treub, M., "Le meristeme primitif de la racine dans les monocotyledones."

Musee Bot. Leide. II, 1875.
11. Weber, Erna, "Entwicklungsgeschichtliche Untersuchungen iiber die
Gattung Allium." Bot. Archiv. 2s: 1-44, 192-9-

CHAPTER VIII

MORACEAE

CANNABIS SATIVA

THE true hemp plant, Cannabis sativa L., is cultivated chiefly
for its fiber; but the seed is used for medicinal purposes, and
certain varieties are grown occasionally as ornamentals. The term
hemp has been used in connection with many plants producing bast
fibers that are somewhat alike in appearance and quality, and
Dodge (13) has listed 31 plants to which this common name is
applied. Among these are Manila hemp, derived from the leaf
stalks of Musa textilis Nee; Sisal hemp, secured from the leaves
of species of Agave; Mauritius hemp, obtained from the green
aloe, Furcraea gigantea (D. Dietr.) Vent.; and Sunn hemp, pro-
cured from Crotalaria juncea L.

The genus Cannabis is generally regarded as monotypic; but
since the plant exhibits several different growth forms, it has been
described under several names, and numerous varieties are recog-
nized. It now is widely distributed, but de Candolle (8) suggested
that its original habitat might have been in some region east of
the Caucasus. Dewey (ii) states that it was one of the earliest
plants cultivated for fiber, and that its original home was probably
somewhere in central Asia.

Cultivation of hemp has resulted in the development of at least
three distinct types, each of which is further subdivided into varie-
ties. One is grown in Europe, Central Asia, and the Americas
specifically for the fiber; a second type is cultivated for the fruit,
which is utilized as a source of oil and as a food; while the third,
Cannabis sativa, var. Indica (sometimes regarded as a separate
species. Cannabis Indica), is grown in India, Arabia, and Northern
Africa for its medicinal and narcotic products. These are derived
from the dried inflorescences and upper leaves of the carpellate
plant, and produce alkaloids of a narcotic character which are
variously known as "hashish," "ganja," and "bhang" in the

2-14

CANNABIS SATIVA 2.15

Orient. In the Western Hemisphere, especially in Mexico and
southwestern United States, the drug is known as "marihuana" and
is smoked in cigarettes. In many places, the species has become a
most troublesome weed.

Hemp is grown for fiber in Russia and throughout Europe,
especially in Italy, as well as in Africa, India, China, Japan, and
to a lesser degree in Brazil and the United States. Russia produces
more hemp for export purposes than all other countries, but the
fiber of best quality is produced in Italy, owing in part to the
manner in which the fiber is retted. The production in this coun-
try is confined chiefly to Wisconsin and Kentucky, and much of
the seed used elsewhere is raised in the latter state. Hemp is also
grown in Nebraska, California, Indiana, and New York.

GENERAL MORPHOLOGY

The plant is an annual and at maturity develops a rigid, woody
stem ranging in height from 3 to 16 feet. The tallest forms are
produced in China and Japan, where the slender, sparsely branched
stalks have internodes 8 to 10 inches long. The European varieties
are usually shorter with more rigid stems, and require 10 to 15 days
less time to mature than do the Asiatic forms.

The Shoot. — The young stem is succulent, but it lignifies
rapidly. At maturity, it is obtusely hexagonal, more or less
grooved or furrowed; and the most common varieties in this
country have a hollow stem. As Heuser (15) has pointed out, cul-
tural conditions exert a great influence on the form of the plant,
especially with respect to length of stem and degree of branching.
Where the crop is not crowded, a single plant may occupy a large
area, becoming much branched and attaining a bushy habit, and
cases have been reported in which the main axis attained a diameter
of 6 cm. or more. Such plants are undesirable for commercial fiber,
and the best results are obtained when the crop is seeded thickly
so that they are slender and essentially unbranched except at the
tip. Under these conditions, the diameter of the stem may range
from 6 to lo mm. It is not uncommon for the lower portion of the
stem to exhibit a definite curvature known as "wind-bending,"
which results from the pressure of prevailing winds on young
plants. This and other mechanical stresses may have a detrimental
effect upon the quality of the fiber produced.

The stem is leafy, but the leaves fall from the lower portions of

ii6 THE STRUCTURE OF ECONOMIC PLANTS

the axis as the plant matures. (Fig. loi.) The phyllotaxy is
both opposite and alternate; and, ordinarily, the lower leaves are
arranged in opposite pairs while the upper ones are alternate.
Schaffner (15) has reported changes of opposite to alternate phyllo-
taxy as a result of rejuvenation
by means of variation in the
photoperiod.

The leaves are palmately com-
pound, with from 5 to 11
leaflets, 7 to 9 being the usual
number. These are rough, dark
green above, somewhat lighter
on the abaxial surface, linear to
lanceolate and tapered at both
ends, with serrate or dentate
margins. They vary in length
from 5 to 15 cm., are i to i cm.
in width, and the petiole is 4
to 6 cm. long with persistent,
pointed stipules at its base. In
the leaves which subtend the
inflorescences, the number of
leaflets is reduced to three or
even to a single large leaflet
with very small awl-shaped
laterals.

The Root. — The root system
consists of many radiating lat-
erals arising from a primary root
that extends vertically down-
ward to a variable depth depend-

(Reproduced from Yearbook of U. S. Dept. of ■ ^ ^j^^ character of the

Agrtculture, K^iT,.) ", tt ^ ^ i u i

sou. Heuser Q15J has observed
that roots may penetrate to a depth of i meters in well-cultivated
soil with a permeable subsoil; but where the plants grow in a
heavy humus soil, the main tap root seldom reaches a depth in
excess of 30 to 40 cm. The horizontal spread of the lateral roots
also depends to a large degree upon the soil type. In mineral
soils, where the primary root penetrates deeply, the laterals arise
chiefly from the upper 10 to 40 cm. of the axis, and the largest of

Fig. ioi. Left, carpellate hemp plant;
right, slaminate plant held up for comparison.

CANNABIS SATIVA

2-17

them may have a lateral spread of 80 cm. In the richer humus
soils where the main tap root penetrates less deeply, numerous lat-
eral roots are formed in the upper 10 to lo cm. of the soil; and the
root system is much more compact.

The Inflorescences. — Hemp is dioecious, and the number of
staminate and pistillate plants is relatively constant under normal
conditions, the number of the former being somev^hat less than that
of the latter. Associated with the dioecious condition is a distinct
vegetative dimorphism which has been described by McPhee (xi).
The staminate plants are taller
and more slender than the car-
pellate, and the terminal stam-
inate inflorescences have few
leaves. The carpellate plants
are shorter and more stocky,
with a broad crown of leaves
associated with the terminal
inflorescence. (Figs. lox and
103.)

Although hemp is dioe-
cious, it is not uncommon for
an individual plant to bear
both staminate and carpel-
late flowers. According to F.g. lox. Habit of staminate flowers of hemp.

^ (Reproduced from Yearbook of U. S. Dept. of Agn-

McPhee (xx), the occurrence „,/,^,,^ ^^.^ )
of monoecious plants is more

common when the daily exposure to light is short; and under
experimental conditions, it has been possible to modify and control
to some degree the production of staminate and carpellate flowers.
This does not seem to affect the vegetative characters, which
remain the same regardless of whether the plant is strictly
dioecious, or later becomes monoecious through the production of
both staminate and carpellate flowers. Schaffner (x6), as a result
of rejuvenation experiments, has reported that "sex reversal can
be successfully brought about during the second ontogenetic
differentiation cycle of rejuvenated plants in individuals that were
of pure sex expression in the first or natural differentiation cycle";
and has suggested that "speculations as to the nature and fixity of
sex, especially in relation to heredity, are of questionable value, and
formulae based arbitrarily on the behavior of individuals during

ii8 THE STRUCTURE OF ECONOMIC PLANTS

their first differentiation cycle are not to be accepted as an explana-
tion of the real nature of such individuals." On the other hand,
McPhee (x}) notes that "sex determination in hemp can be ex-
plained on a genetic basis, and that the inheritance is of the XY
type."

The Staminate Inflorescence and Flower. — The staminate
flowers develop in small, drooping, branched panicles, which

Fig. 103. Habit of carpellate flowers of hemp. (Reproduced from Yearbook of U. S. Dept.

of Agriculture , 1913.)

arise in the axils of foliage leaves. (Figs. loi. and 104, y4.) The
flowers of the panicle may occur singly on slender pedicels or in
groups, and usually the terminal branches bear three flowers, a
median one and two laterals which are subtended by bracts or
stipules. (Fig. 104, 5.) The individual flowers are apetalous
with a deeply parted calyx having five greenish-yellow or red
lobes that are widespread at maturity. There are five stamens, and
the anthers are suspended from long thread-like filaments. (Fig.
104, C.) The plants are wind pollinated, and large numbers of

CANNABIS SATIVA

XI9

white, spherical, papillate pollen grains are liberated through
terminal pores.

The oval sepals are acuminate, the outer surface and margins
being covered with multicellular glands and slender, pointed,
unicellular hairs with crystals of calcium oxalate deposited in
their swollen bases. The inner epidermis is practically devoid
of hairs and stomata which are present in the outer epidermis.
Except in the region of the median vein, the mesophyll is spongy

Fig. 104. A, diagrammatic representation of staminate inflorescence; B, diagram of
branch of staminate inflorescence showing three flowers ; C, habit of staminate flower ; D,
face view of young flower, showing arrangement of parts ; E, F, and G, successive stages in
development of staminate flower. The numerals indicate the sequence of origin of sepals :
fl ax, floral axis ; ped, pedicel ; se, sepal ; sta, stamen ; stp, stipule. (Redrawn and adapted
from Briosi and Tognini, Istituto Botanico di Pavia.^

and one to two cells in thickness; but at the margins, mesophyll
is lacking and the sepal consists of only the epidermal layers. As
the sepal matures, the inner epidermis becomes much stretched and
crushed.

In the ontogeny of the staminate flower, the conical floral
primordium which commonly develops in the axil of a stipule first
produces the five cone-shaped initials of the sepals. These arise
in a definite succession; the first two are adjacent to the stipule,
the third one develops opposite to the stipule, and is followed by
two laterally placed ones. (Fig. 104, E, F, G.) After the differ-

zio THE STRUCTURE OF ECONOMIC PLANTS

entiation of the sepal primordia, the five staminal primordia arise,
each one being opposite a sepal. (Fig. 104, D.)

The Carpellate Inflorescence and Flower. — The carpellate
flowers are borne in short spikes on a more or less compact bushy
and leafy shoot. (Fig. 103.) The flowers are closely aggregated
at the apex of the inflorescence and are produced in pairs in the axil
of a leaf, each flower being subtended by a stipule. (Fig. 105, B.)
Commonly one of each pair of flowers is abortive. A short branch

I--

---sti

0 w

D

\- stp

Fig. 105. A, transection of apex of carpellate inflorescence, showing arrangement of
leaves, stipules, bracts, and flowers ; B, diagram of branch of carpellate inflorescence, showing
a pair of flowers ; C, habit of single carpellate flower ; D, longitudinal diagram of carpellate
flower, showing floral parts : <7x, floral axis ; if/, floral bract ; /, leaf ; ovy, ovary; our, ova.ry
wall ; pth, perianth ; sta p, staminal primordium ; sri, stigma ; stp, stipule ; sty, style. (Re-
drawn and adapted from Briosi and Tognini, Istituto Botanico di Pavia.^

which arises in the axil of the leaf subtending the pair of flowers
bears another leaf with stipules and flowers, and this in turn pro-
duces an axillary branch of the third order. (Fig. 105, A.') Since
the branches are short, this type of branching results in the forma-
tion of the compact carpellate inflorescence. The leaves which sub-
tend the paired flowers tend to be in a % phyllotaxy rather than
the decussate arrangement of the lower portion of the vegetative
axis.

The carpellate flower is relatively simple in structure, consisting
of a pistil surrounded by a transparent, more or less papery perianth.
(Fig. 105, C.) In its normal position, the green scabrous floral

CANNABIS SATIVA rn

bract with its overlapping edges completely surrounds the mature
flower except for the exserted stigmas. It is conical, open at the
top, terminated by a pointed beak; and, when uncurled and ex-
panded, heart-shaped and acuminate. The abaxial surface of the
bract is thickly covered with glands and glandular emergences that
are intermixed with large, stout, conical hairs. (Fig. 105, D.)
The adaxial surface is without glands, and hairs are seldom present
except at the margin and near the apex of the bract where the inner
surface is more exposed. The bracts are several cells in thickness
at the median line, and the spongy mesophyll is chlorophyllose.
The projecting emergences of the abaxial surface which are termi-
nated by glands contain compact chlorenchymatous cells at their
bases. The multicellular glands are derived from epidermal cells
and consist of a stalk and a terminal cap or plate. The stalk may
be a single or multicellular row of cells, and the expanded terminal
portion consists of several cells over which a very thick cuticle is
formed .

The perianth is continuous and entire, forming a cup-like struc-
ture with a smooth or slightly fringed margin which at maturity
covers about two-thirds of the ovary. (Fig. 106, H.) It develops
as a definitely diverged structure free from the ovary but becomes
closely adherent as the ovary enlarges. (Fig. 106, D.) The
perianth is a simple hyaline membrane consisting of the two
epidermal layers between which are a few spongy mesophyll cells;
and at its base, a network of small vascular bundles extends through
the mesophyll. The cells of the abaxial epidermis are elongated
in the direction of the long axis of the ovary; and, rarely, long uni-
cellular hairs may be produced. The adaxial surface resembles the
abaxial but is devoid of hairs, and neither epidermis contains
stomata. The mesophyll is comprised of transparent, thin-walled
cells that are homogeneous and non-chlorophyllose, and it seldom
exceeds three or four cell layers in thickness.

The pistil consists of an oval, unilocular ovary, which is
slightly flattened and somewhat depressed at its stylar end, and
two stylar branches and stigmas. The styles are cylindrical, and
the papillate stigmas are divaricate with their apices curved out-
ward and downward. (Fig. 105, C)

According to Briosi and Tognini (6) the primordium of the car-
pellate flower originates as a dome-shaped structure in the axil
of a foliar bract or stipule. (Fig. 106, A, B.) At the base of the

7.2.Z THE STRUCTURE OF ECONOMIC PLANTS

flower, there is a floral bract which later encloses the other floral
parts. The ovary wall arises as a ring-like fold at the base of the
floral primordium, and gradually encloses its apex. (Fig. io6, C.)
When the ovary wall has completely overgrown the floral axis,
two opposing points on its margin begin to grow more rapidly,
initiating the two styles and stigmas. (Fig. io6, E, P".) The

ax yflv
A B

Fig. io6. A-M, stages in floral development of carpellate flower : A, C, E, F, G, and H,
habit drawings of successive stages ; B, D, I, J, and L, diagrammatic longisections showing
relation of floral parts at successive stages ; K, face view of a stage somewhat more developed
than /; M, face view, at stage about the age of L ; N-P, stages in development of embryo :
«x, vegetative axis; ^cr, floral bract; cha, cha.la.za.; cot; cotyledons; ec/, epicotyl ; emb,
embryo ; emb s, embryo sac ; end, endosperm ; fl p, floral primordium ; int, integuments ;
/■«'«/, inner integument ; «/j, nucellus ; o i«r, outer integument ; sf, ovule; « w, ovary wall ;
pbm, periblem ; pi, plerome ; ptf>, perianth ; sta p, staminal primordium ; sti, stigma ; stp,
stipule; sty, style; sus, suspensor. (Redrawn and adapted from Briosi and Tognini, Isti-
tuto Botanko di Pavia.')

Styles elongate unequally, and one is always somewhat longer than
the other even at maturity. (Figs. io6, G, H, and 105, C) Within
the ovary, the remaining tissue of the primordium forms the
nucellus and finally a single ovule is developed which becomes
completely campylotropous with a curved megagametophyte.
(Fig. 106, /, M, N.)

Shortly after the initiation of the carpellary wall, a second fold
of tissue arises at the base of the primordium, growing up and

CANNABIS SATIVA 113

around the carpel until it encloses approximately two-thirds of
it. (Fig. 106, D-H.^ This becomes the membranous perianth
described above. Briosi and Tognini observed knob-like projec-
tions at the base of the ovary and above the perianth, and inter-
preted them as being rudimentary primordia of stamens that become
arrested in development. (Figs. 105, D, and 106, M.)

Zinger's (31) account of the ontogeny of the carpellate flower is
at variance with the above in respect to the development and
character of the perianth. In most of the varieties of hemp which
he examined, there were two distinct perianth primordia. The
first primordium appears at the base of the floral axis on the side
toward the floral bract, while the second develops on the dorsal
side of the floral axis at a higher level than the first. Both may
appear somewhat late in ontogeny, so that they are not perceptible
until the carpellary primordia are differentiated. In most in-
stances, the primordium on the side toward the floral bract persists
as a small protuberance without further development; and this is
sometimes the case with the posterior primordium, so that there
may be no perianth when the fruit is mature. When the posterior
primordium develops, it forms a thin, round or elliptical scale-like
structure which lies on the dorsal side of the ovary. Zinger found
a perianth completely enclosing the basal portion of the ovary in
two horticultural varieties which he designated as Cannabis
gigantea Hort. and C. himalayana Hort.

At the base of the nucellus, the inner and outer integuments
arise in that order, the inner one at first exceeding the outer in rate
of growth, but ultimately being overgrown and enclosed by it.
(Fig. 106, K, L, M.) As noted by Zinger, the massive inner integu-
ment becomes completely coalescent with the thick outer one, and
covers the apex of the nucellus in such a way that the micropylar
canal becomes entirely closed by the integumentary tissue. Fol-
lowing pollination, the pollen tube either grows through the tissue
which fills the micropyle or pierces the two integuments; and,
upon reaching the nucellus, it branches about its apex, finally
sending a single branch into the megagametophyte.

Embryogeny. — Following fertilization, the transverse seg-
mentation of the zygote produces the suspensor and embryo proper.
The suspensor never becomes elongate, remaining as a short stalk
of one or two cells. (Fig. 106, 0.) The embryo is at first globular
but there is an early differentiation of the cotyledons which develop

114 THE STRUCTURE OF ECONOMIC PLANTS

as lateral lobes on either side of the growing point of the epicotyl.

(Fig. io6, P.)

Concurrent with the development of the embryo, an abundant
endosperm is formed which fills the embryo sac. This is gradually
utilized by the embryo, which develops a curvature corresponding
to the contour of the embryo sac. Some of the endosperm still
remains in the seed at maturity, surrounding the embryo and
occupying the region between the primary root and the cotyledons.
(Fig. io8, /4.) Twin embryos have been reported by Savelli (14).

Histology of the Fruit and Seed. — The fruit is an akene
which is light brown to dark gray, or in some cases mottled. It
is smooth, somewhat compressed, and orbicular or oval in form;
but varies in this respect and in size, as does the enclosed seed.
Heuser (15) reports the average length of the pointed types as
4.3 mm., with diameters ranging from 1.75 to 3.3 mm.; and Dewey
(ii) records the length of the fruit as from 3 to 6 mm. with diame-
ters of from 1.5 to 4 mm.

The "seed" of commerce is actually the fruit which in some cases
comes into the market enclosed within its hooded floral bract.
(Fig. 107, A.') The outer epidermis of the floral bract is charac-
terized by its hairs and glands. The hairs attain a length of 0.5 to
i.o mm. and taper abruptly or gradually to a pointed apex from
spherical bases which contain cystoliths of calcium oxalate. The
globular gland consists of eight or more cells radiating from two
central ones which may be sessile; but, in most cases, they are
borne on many-celled stalks. (Fig. 107, D.) The secretory cavity
is formed by the separation of the outer cuticle from the terminal
cells.

The pericarp of the fruit, which is comprised of five distinct
regions, has been described by Winton (19). The outermost
zone, or epicarp, is a layer of sclerenchymatous cells which are
sinuous in outline as seen in surface view. Their radial walls are
only slightly thickened in some regions; while, in others, they
are so thick that the lumen of the cell is much reduced. All the
walls of this layer are porous. The second region of spongy paren-
chyma consists of one or more layers of colorless cells between
which there are many intercellular spaces. Numerous anastomos-
ing bundles, that can be seen through the epicarp, extend through-
out this layer, which is thicker along the two keels of the fruit.
(Fig. 107, B.) Underlying the hypodermal layers are the brown

CANNABIS SATIVA

2-2-5

cells which form the third zone. These derive their name from
the brown contents, and can be distinguished from the hypodermal
cells by the thick walls and color. The fourth layer consists of
the dwarf cells, which are small, colorless, and porous, with

pep-

pmi —

I ep

Fig. 107. A, hemp fruit enclosed by persistent floral bract; B, outer surface of fruit with
bract removed ; C, transection through section of fruit coat, seed coat and cotyledon ; D, sur-
face view of portion of floral bract showing two types of epidermal hairs arising from it : al,
aleurone grains ; br, brown cells ; cot, cotyledon ; cy, cystolith ; ecp, epicarp ; end, endo-
sperm ; g/ hr, glandular hair ; hd, hypodermis ; hr, non-glandular hair ; i ep, inner (adaxial)
epidermis of cotyledon ; 0 ep (left), outer (abaxial) epidermis of cotyledon ; 0 ep (right), outer
epidermis of floral bract; pal, palisade cells of pericarp; pep, pericarp; psm, perisperm ;
f f, seed coat ; j/j, spongy parenchyma of seed coat ; / f, tube cells ; m', drawf cells. (Redrawn
and adapted by permission from Structure and Composition of Foods, by Winton and Winton,
John Wiley and Sons, Inc.)

sinuous radial walls. Because of their extreme thinness, they can
be seen in transections only in carefully prepared material. The
innermost zone of the pericarp is the conspicuous palisade layer.
The outer and radial walls of the palisade cells are so greatly thick-
ened that the lumen is reduced to a narrow line for nearly two-

1x6 THE STRUCTURE OF ECONOMIC PLANTS

thirds of the length of the cell; but, near the inner tangential wall,
the radial walls are abruptly narrowed so that the lumen is wide.
The inner wall is porous and moderately thickened. (Fig. 107, C)

The green seed coat consists of two layers. The outermost one
is comprised of tube cells which are distinct from the inner layer
owing to their elongated form and the extended intercellular
spaces. The inner layer consists of spongy parenchymatous cells
that are somewhat irregular in outline. In the mature fruit, the
perisperm and endosperm are each one cell layer in thickness and
adhere to each other; but, in a soaked fruit, they may be readily
separated from the seed coat and the embryo. The endosperm also
extends between the cotyledons and hypocotyl as a membrane
several layers in thickness. The cells of the endosperm contain
protein grains resembling the aleurone grains of cereals.

The embryo, which is curved so that its axis is U-shaped, consists
of two plano-convex cotyledons enclosing a well-developed
epicotyl, a subcylindrical hypocotyl, and primary root. The
epidermal layers of the cotyledons are composed of small cells
containing aleurone grains. Beneath the abaxial epidermis are
several layers of isodiametric cells, and underlying the adaxial one
are two or more layers of palisade cells. (Fig. 107, C) The orien-
tation of the embryo is such that the tip of the primary root and
and the apices of the cotyledons are directed toward the micropylar
(chalazal) end of the seed, and the stylar end of the ovary. (Fig.
108, /4.)

Development of the Seedling. — Upon germination, the
primary root emerges from the fruit coat at the stylar end, splitting
the coat into halves which remain united at the base, enclosing the
cotyledons until they are pulled above the ground by hypocotyle-
donary growth. (Fig. 109.) The primary tap root grows very
rapidly; and, under favorable conditions, may reach a length of
7.5 to 10. o cm. within a period of 48 hours after emergence. This
rapid rate of elongation is then retarded; and, as early as the third
or fourth day, there is a marked development of lateral roots which
arise just below the soil surface. At this time, the hypocotyl is
also elongating and becomes erect. The epicotyl is pubescent and
oval in transection, increasing slightly in diameter toward the
cotyledonary node. The expansion of the cotyledons is rapid after
they become free from the fruit coat; and, according to Lubbock
(2.0), they are approximately equal in width but differ in length.

CANNABIS SATIVA

2.17

the longer one being 1.4 to 1.6 cm., the shorter i.o to 1.3 cm. long.
The cotyledons function both as photosynthetic and storage organs
for 15 to xo days, when they become discolored, shrivel, and finally
fall.

The cotyledons are obovate to spatulate, taper toward their
bases, and the terminal portions are somewhat recurved. They are
sessile and connate at the base, forming a cotyledonary collar.
The venation consists of a prominent midrib with two lateral veins
arising from its base which unite with two others that originate

'--y

Fig. 108. A, median longisection of fruit cut perpendicular to cotyledonary plane; B,
diagram of cotyledonary collar showing venation. The collar has been cut open in inter-
cotyledonary plane and the cotyledons spread with abaxial surface up : coll v, collar vein ;
cot, cotyledon ; cot coll, cotyledonary collar ; eel, epicotyl ; end, endodermis ; fr c, fruit coat ;
/■«/, integument ; /.leaf; /t', primary lateral vein ; /r', secondary lateral vein; wi', mid vein;
pr rt, primary root ; sty e, stylar end. (After Berkman.)

from the midvein at about the midpoint of the cotyledon and
extend to the apex. (Fig. 108, B.) The lamina of the cotyledon is
thicker and more spongy than that of the first foliage leaves. The
mesophyll is chlorophyllose, consisting of two or three layers of
palisade cells which occupy the adaxial half of the leaf, and as
many as ten or more layers of spongy parenchyma may develop
in the abaxial portion. Stomata occur with about equal frequency
in the upper and lower epidermal layers, and both surfaces produce
unicellular hairs. The cells of the adaxial epidermis have sinuous
walls, except where they overlie the median vein, while those of
the abaxial surface are rectangular with straight walls.

zx8 THE STRUCTURE OF ECONOMIC PLANTS

During the first week or ten days of seedling development, while
the hypocotyl is obtaining its maximum length of 5 to 6 cm., the
epicotyl elongates rather slowly. Later, the rate of epicotyle-
donary growth is accelerated; and, according to Berkman (3), the
first internode reaches its normal length of 3 to 5 cm. by the twelfth
or thirteenth day. The first pair of foliage leaves above the cotyle-
dons are simple and ovate with serrate margins, and are functional
for a short time only. In most instances, the second and third
pairs are digitate, the former usually having three leaflets and the

latter four or five. The leaf-

Is

■©

ff?)

\

Fig

109.

lets above the third pair are
as described under general
morphology.

The Primary Root. — The
root is diarch, with two
groups of primary phloem
flanking the primary xylem
strand. The protoxylem abuts
the pericycle, which is multi-
seriate, consisting of four or
more layers lying centrad to
the endodermis. In the inter-
cotyledonary plane, it is usu-
ally only two layers in width.
The endodermis is uniseriate
and Casparian strips begin to
differentiate at about the time that the metaxylem matures, but
they do not become well developed until secondary thickening is
initiated. The cortex is composed of five to seven concentric lay-
ers of parenchymatous cells with prominent intercellular spaces.
The outermost layer forms a distinct hypodermis which is similar
in structure to the epidermis except that it produces no root hairs.
(Fig. no, A.^ The root has a blunt cone-shaped root cap.

There are several accounts of the structure and ontogeny of the
root, and these do not agree in all respects. Van Tieghem and
Douliot (i8) described the pericycle as a single layer of cells, and
also interpreted the periblem as one layer of initials. Flahault
(14), Briosi and Tognini (7), and Berkman (3) report the periblem
as consisting of two layers of initials, and the pericyclic layer as
soon becoming multiseriate as a result of periclinal divisions.

The fruit and stages in development
of seedling.

CANNABIS SATIVA

12-9

In the ontogeny of the root, the primary structures are derived
from three well-defined histogens which are similar to those
described by Janczewski (i6) and Crooks (9) for the primary root
of Linum usitatissimum. The root cap and epidermis are derived
from a common layer of initials designated as the calyptrogen-
dermatogen. The cortical tissue originates from the two-layered
periblem, and the stelar tissues are derived from the plerome.

The development of the root cap and epidermis from the calyptro-
gen-dermatogen layer results from the differential character of the

B

Fig. 1 10. A, transection of primary root 0.6 mm. above root tip; B, median longisection
of root apex showing histogens in detail : cal, calyptrogen ; co, cortex ; dgn, dermatogen ;
en, endodermis; ep, epidermis; hd, hypodermis; mx, metaxylem; pbm, periblem; pel,
pericycle; ph, phloem; />/, plerome; />x, protoxylem ; >-c, root cap. (After Berk man.)

division of its cells. The cells of this layer which immediately
overlie the periblem at the tip of the root divide periclinally to
form the root cap cells. The more laterally placed ones divide
anticlinally, and function as a true dermatogen in producing the
epidermis. The manner in which the root cap and dermatogen
arise from the dual histogen has been described by de Bary (i):

"The divisions of the initial layer, which are parallel to the surface
of the bluntly conical apex, add on the one hand new cells to the root-
cap above the apex, and on the other hand renew the initial layer
itself. As the distance increases from the apex of the periblem, which
by its growth in length is constantly advancing, the divisions become
rarer, and at last cease. The last of these separates the initial cell

2.30 THE STRUCTURE OF ECONOMIC PLANTS

into two, one of which is added to the root-cap, the other to the
dermatogen as a permanent member of it. . . . The cells of the der-
matogen and root-cap, which owe their origin to the division just
described, divide further by walls perpendicular to the surface; from
each therefore is produced a section of a layer consisting of several or
many cells."

The cells of the root cap, which later become oriented in a
lateral position because of the elongation of the root axis, may
divide anticlinally. In this manner, they compensate for the
growth in length of the root axis, and each series of root cap
cells keeps pace with the elongation of the root axis until the
layer of which they are a part becomes the outermost one and dis-
integrates. The cells of the calyptra which form the terminal
vertical rows of the cap do not usually undergo further division,
except for one or two cells which are proximal to the calyptrogen;
but they enlarge in all three dimensions until they are two or three
times the size of the calyptrogen initials. As a result of the anti-
clinal divisions of the lateral rows of the calyptrogen and the
concurrent enlargement of the cells in the vertical rows, the root
cap consists of a very regular series of cell layers which overlie
one another. (Fig. no, B.)

The cortex is derived from the two-layered periblem which forms
a terminal cap over the small group of plerome cells at the root
apex. The outer layer of the periblem divides only in anticlinal
planes, in this manner giving rise to the outermost cortical layer
which becomes the hypodermis. The initials of the inner layer
divide in all planes to produce the four- to seven-layered paren-
chymatous portion of the cortex and the endodermis.

The pericycle is the first tissue of the stele to differentiate and is
derived from the outermost layer of the plerome. The pericyclic
cells can be identified early in ontogeny by their dense contents and
greater axial dimension as compared with adjacent cortical cells.
Following the differentiation of the pericycle, the primary phloem
and xylem develop simultaneously. The protophloem is comprised
of elongated parenchymatous cells with relatively small diameters.
The primary phloem ducts are formed by the breakdown of a
linear row of cells adjacent to the pericycle, as many as three or
four occurring in each phloem group. They are completely dif-
ferentiated at a level approximately 1.5 cm. above the root tip,
and the number may increase until there are seven or more in each

CANNABIS SATIVA Z3i

primary phloem group by the time that secondary growth is
initiated. The ducts are ephemeral; and, together with other
elements of the protophloem, become crushed against the pericycle
as secondary growth proceeds.

Of the primary xylem, three to five thin-walled annular and
spiral elements constitute each protoxylem point. The outermost
and smallest of these abuts the inner layer of the pericycle, which
is three or four cells in width by the time the first annular vessel is
mature. The other protoxylem elements are bounded laterally by
regions of fundamental parenchyma in which cambial activity
is later initiated. The metaxylem elements continue to mature
centripetally so that the primary xylem strand is complete at a
level about 0.6 mm. above the root tip. The metaxylem vessels
have distinctly bordered pits in which the borders are narrow and
the torus thin. Scalariform vessels may rarely occur.

A rather unusual case of tetraploidy in the primary root has
been reported by de Litardiere (19) and Breslawetz (5). The
diploid number of chromosomes in hemp is 1.0 and this obtains
for the cells in the plerome, but both investigators found the
tetraploid number of 40 in the cells of the periblem. This situation
was observed by de Litardiere in specimens selected from two
varieties of hemp grown in different localities; and, coupled with
the confirmatory report of Breslawetz (5), would indicate that
the condition is general rather than aberrant. Several explanations
have been advanced to account for it. Breslawetz attributes it to
the occurrence of nuclear divisions in the cells of the periblem
which are not followed by cytogenesis, but, instead, by the enlarge-
ment of the cells involved and by the fusion of the two nuclei in
each cell. On the other hand, de Litardiere did not encounter
any binucleate cells, and suggests that the situation may arise
from a normal metaphase followed by a monocentric telophase
which would produce a single tetraploid daughter nucleus. This
same phenomenon has also been reported by de Litardiere (18)
and Langlet (17) for Spinacia. The latter, however, did not
agree with de Litardiere in respect to Cannabis sativa, as he found
tetraploid cells abundant in all tissues of its root tip.

The Formation of Lateral Roots. — The primordia of lateral
roots arise early in the ontogeny of the primary root at about the
time the protoxylem begins to mature. As a result of divisions of
the pericyclic cells in three planes, conical growing points are

132. THE STRUCTURE OF ECONOMIC PLANTS

differentiated in roots that are less than forty-eight hours old. The
endodermis of the primary axis persists as a continuous layer over
the lateral root primordium until it is well differentiated and
has reached the outer limits of the cortex. The point of origin
of the lateral roots is approximately on the same radius as the
protoxylem strands; and, according to van Tieghem and Douliot
(x8), the axis of the new root usually deviates 15° to xo° from this
plane. The ontogeny of the lateral root is similar to that of the
primary root, and this is also true of adventitious roots which may
arise in the hypocotyl.

Vascular Transition. — The structure of the hypocotyl is root-
like from the ground level to a point about 1.5 cm. below the
cotyledonary node, except that the central portion of the axis
becomes parenchymatous. This condition begins about a milli-
meter above the ground level, where the central portion of the
axis is comprised of sclerenchymatous cells rather than metaxylem.
At successively higher levels, the cells of the central parenchyma
have thinner walls, and immediately below the cotyledonary node
there may be a complete disintegration of the pith in plants that
are not more than two weeks old.

The character of the metaxylem elements also changes, these
being differentiated with spiral thickenings in the upper hypocotyl;
and vessels with bordered pits, such as occur in the metaxylem of
the root are rarely found. The pericycle is multilayered as in the
root, the number of layers ranging from three to five outside the
protoxylem points, with two layers in the intercotyledonary plane.
The endodermis is not clearly defined, and the cortex of seven or
more layers is limited outwardly by a definite hypodermis.

The first modification of the root-like organization of the stelar
tissue occurs about 1.5 cm. below the cotyledonary node where
the metaxylem is differentiated tangentially with reference to
the protoxylem points. This results in the formation of two
V-shaped groups of primary xylem in which the protoxylem
occupies the apex of the V and the metaxylem the extended arms.
Each phloem group lies adjacent to one of the lateral metaxylem
groups, and this relationship is maintained throughout the entire
transition region.

As the reorientation of the metaxylem proceeds, a vascular
strand is diverged from each arm of metaxylem toward the inter-
cotyledonary plane; and the two strands on either side of the inter-

CANNABIS SATIVA 2.33

cotyledonary plane so derived converge at a slightly higher level,
forming the vascular bundles that supply the first epicotyledonary
leaves. As maturation of the tangentially oriented metaxylem
and the epicotyledonary bundles proceeds, the pericycle becomes
active, resulting in a slight increase in the diameter of the hypo-
cotyl from this point up to the cotyledonary node. At the same
time, there is a differentiation of a large number of resin or latex
ducts in the phloem of the epicotyledonary bundles. As the
metaxylem is differentiated in a more and more tangential position,
it finally assumes a position with relation to the protoxylem so
that the two arms of the metaxylem are extended at right angles
to the cotyledonary plane. This orientation occurs slightly below
or at the cotyledonary node, the protoxylem lying between two
groups of metaxylem and in a line with them. In this manner, the
double bundle of the mid vein of the cotyledon is formed.

In most instances, the primary xylem is definitely endarch at
the base of the cotyledon, but the protoxylem is separated from
the metaxylem by parenchymatous tissue. This relationship
persists to a point just above the middle of the cotyledon, where
the protoxylem and two metaxylem groups converge to form a
single bundle. At the base of the cotyledon, the midvein branches,
giving off a lateral endarch bundle on each side, one being diverged
at a slightly higher level than the other; and these lateral strands
in turn are branched to form two principal veins. In each case,
the divergent branch which is directed toward the apex of the
cotyledon becomes a primary lateral vein. (Fig. iii.) The other
branch descends to meet a similar one of the opposite cotyledon,
and the two meet at a point somewhat lower than their point
of divergence. This results in the formation of a continuous
V-shaped strand which lies in the intercotyledonary plane of the
cotyledonary collar. (Fig. iii.)

The Stem. — The fiber of commerce is derived from the stem;
and, in consequence, its structure has been studied extensively
by textile engineers. Dewey (11) has described the ideal plant
as one which attains a height of 10 to lifeet, is 34 to % of an inch
in diameter at the base, and develops internodes 10 inches or more in
length. The stem should be prominently fluted, with a com-
paratively large central pith cavity so that it forms a hollow
cylinder that is more easily broken by retting, since in this type,
the fiber is generally tougher.

X34 THE STRUCTURE OF ECONOMIC PLANTS

The young stem is more or less grooved and ridged, owing to
the development of the principal bundles and the longitudinal
strands of collenchyma. (Fig. 113.) These are arranged in a ring

. — coll V , — Iv

/-■u ep

■ r~hP-cot

/--ep hr

r-Ph )
I — mx > mv

Fig. III. Transection of cotyledons near their bases and enclosed apex of shoot: coll v
collar vein; cot, cotyledon; ep hr, epidermal hair; / i-/ i', first pair of foliage leaves; / z-
/ i', second pair ; / 3-/ 3', third pair ; / ep, lower epidermis ; Iv, lateral vein ; mv, mid vein ;
mx, metaxylem ; ph, phloem; px, protoxylem ; s, stem tip; st, stoma; u ep, upper epi-
dermis. (After Berkman.)

and are separated from one another by medullary rays which are
later occluded when an interfascicular cambium develops and
secondary thickening is begun.

The epidermis is somewhat cutinized, producing numerous hairs
and glands. The pointed non-glandular hairs are unicellular and

CANNABIS SATIVA

2-35

develop from a raised base so that when they break off, a warty
circular scar remains. Cystoliths are formed in their bases as
in the epidermal hairs produced on the leaves and floral bracts.
The glandular hairs are multicellular and resemble those described
for the floral bract. Stomata are not produced in large numbers,

imv

Fig. III. Schematic diagram of vascular system of hypocotyl showing transition : coll v,
collar vein; cot, cotyledon; /, leaf; / v, lateral veins;) / t, leaf trace; mv, midvein; mx,
metaxylem ; px, protoxylem. (After Berkman.)

136 THE STRUCTURE OF ECONOMIC PLANTS

and, according to Heuser (15), their frequency is only ii per square
centimeter as compared with approximately 3000 per square centi-
meter in flax. Briosi and Tognini (7) reported a frequency of 5 per
square millimeter in a young stem 2. mm. in diameter, pointing out
that the frequency per unit area diminishes greatly as the stem
matures. The radial and tangential diameters of the epidermal

Fig. 113. Transection of portion of young stem: ca, cambium; co, cortex; col, collen-
chyma ; ep, epidermis ; hi; epidermal hair ; la d, latex or resin duct ; m r, medullary ray ;
pel, pericycle; ph, phloem; pi, pith; xy i, primary xylem; xy 2., secondary xylem.

cells are approximately equal, and they are three to four times as
long as broad.

Adjacent to the epidermis is a zone of chlorenchyma, two or three
cells in width, which extends completely around the periphery of
the stem including the ridges where it separates the collenchyma
from the epidermis. The chlorenchymatous cells form a compact
zone at first; but, later in ontogeny, they become considerably
stretched tangentially and large intercellular spaces develop. The
cells of the hypodermal layer are larger than the others, and also
become thicker-walled as maturation proceeds. Within the zone
of chlorenchyma, strands of thick-walled, compact collenchyma-
tous cells form a part of the ridges of the stem. The cortical zone
is not wide; and, aside from the chlorenchyma and collenchyma,
there are only a few layers of parenchymatous cells. (Fig. 114.)

CANNABIS SATIVA

2-37

As the stem develops, the pericycle forms a multilayered region in
which the fibers are developed. These cells can be recognized in

pel fib

Fig. 114. A sector of transection of nearly mature stem showing development of peri-
cyclic and phloem fibers : ca, cambium ; cM, chlorenchyma ; en, endodermis ; ep, epidermis ;
hyp, hypodermis ; la d, latex duct ; pel fib, pericyclic fibers ; ph fib, phloem fibers ; ph t.,
secondary phloem; ves, vessel; xy ra, xylem ray; xy 2., secondary xylem. The primary
xylem, pith, and central cavity are not shown.

i38 THE STRUCTURE OF ECONOMIC PLANTS

the transection of a relatively young stem, since they form a definite
zone of angular cells lying between the phloem and the cortical
region. Their development and characteristics are described in a
succeeding section.

The collateral vascular bundles are separated from one another
by medullary rays. The number of primary bundles varies at
different stem levels, owing to branching and anastomoses, and to
variation from a % spiral to a decussate phyllotaxy. Briosi and
Tognini (7) have discussed the course and number of bundles of
the stem axis beginning in the hypocotyl. By successive branch-
ings, the hypocotyledonary bundles are increased from t to 4, 8,
and IX, then reduced by anastomoses to 10. Of these, four com-
prise the cotyledonary traces and enter the cotyledons at the cotyle-
donary node. In succeeding internodes, the number of primary
bundles ranges from it to 16, depending upon the location of the
transection examined in relation to a node. Three bundles supply
each leaf.

The primary xylem consists of annular, spiral, reticulate, or
pitted vessels. These are differentiated in radial rows, each usu-
ally consisting of a single series of progressively larger protoxylem
and metaxylem elements that are separated from those of adjacent
rows by parenchymatous ray tissue. As secondary thickening
proceeds, the development of interfascicular cambium forms a
continuous cylinder, and there is an extensive production of xylem
fibers, parenchyma, large vessels, and ray parenchyma. Where
two vessels lie in direct contact with each other, they are intercon-
nected with bordered pits, and simple ones occur in walls separating
the vessels from xylem parenchyma and ray cells. The xylem rays
are one or two cells in width, consisting of thin-walled parenchym-
atous cells whose radial dimensions slightly exceed the tangential
ones, and which are approximately twice as high as broad. The
elongated, tapered xylem fibers do not attain great length and they
are not pitted. A single fiber usually does not exceed 0.5 mm., and
the uniformity in length is very striking.

The phloem consists of sieve tubes, companion cells, parenchyma,
fibers, and latex or resin ducts. The sieve tubes have clearly de-
fined sieve plates that are either transverse or oblique, and they
can be readily distinguished from the ducts by this characteristic.
The companion cells are slender and taper to a point where the
sieve plate of the adjacent sieve tube is somewhat larger in diameter

CANNABIS SATIVA 2.39

than the caliber of the remainder of the tube. The phloem fibers
are smaller, shorter, thinner-walled, and more brittle than the
pericyclic fibers. They occur in zones in the phloem, either singly
or in groups. In the older portions of the stem, continued second-
ary thickening may result in the formation of successive zones of
fibers, giving the phloem a banded appearance.

The ducts are apparently unique for the Moraceae, as they differ
from the unsegmented types found in the Asclepiadaceae and
Euphorbiaceae in being unbranched, and in their manner and point
of origin. They are also unlike the latex vessels of Papavaraceae,
Cariaceae, and Musaceae, since they are not formed from a longi-
tudinal series of cells by the absorption of their end walls. Zander
(30) has described the structure and development of the ducts in
detail, and notes that they are unbranched, unicellular, and multi-
nucleate. They do not occur in the root, cotyledons, or greater
part of the hypocotyl; and are not similar in their ontogeny to the
primary phloem ducts found in the primary root.

In ontogeny, the ducts first appear at the time of the differentia-
tion of the first leaves above the cotyledons. They occur in the
primary phloem of the median bundle of the leaf, and are differen-
tiated downward through the cotyledonary node and into the upper
portion of the hypocotyledonary axis. Additional latex ducts
arise in the same manner at the tips of the vegetative axes as the
subsequent leaves are differentiated. (Fig. 115.) The initials of
the latex ducts are round or oval when first distinguishable in
the growing point and in the leaf primordia. The lumen of the
duct is larger than that of the adjacent cells and the cell wall is
somewhat thicker. They contain a golden-brown granular sub-
stance and have spindle-shaped nuclei that are larger than the sub-
spherical nuclei of the adjacent parenchymatous cells.

As the leaf and axis develop, the latex ducts become long, sinu-
ous, unsegmented canals which may attain an extraordinary
length. Zander observed ducts which extended through more than
one internode of a mature plant exceeding 30 cm. in length. In
no instance was there any indication of either branching or anas-
tomosing, and he concluded that each one was an independent struc-
ture. They occur in the outer limits of the primary phloem, and
the number of ducts in transections of stems, varying in age from
very young seedlings up to mature plants over 3 meters in height,
reveal no significant differences. Zander concluded from this that

i4o THE STRUCTURE OF ECONOMIC PLANTS

there is no production of additional ducts as a result of secondary
thickening.

The pith consists of large thin-walled cells with intercellular
spaces, and prior to the disintegration of the centrally located ones,
they contain numerous crystals of calcium oxalate. Similar
crystals occur in the parenchymatous cells of the cortex, pericycle,
and phloem.

-^ - ql hr

I

4

I

Fig. 115. Median longisection of apex of vegetative axis showing origin of latex ducts in
young leaves — diagrammatic : «x i, axillary bud ; ^/ z&r, glandular hair; i-r, hair; lad,
latex duct; /)/»/', protophloem ; p 1^ j, provascular strand ; /^ a:, protoxylem.

CANNABIS SATIVA 2.41

Pericyclic Fibers. — Bredemann (4) and Heuser (15) have
pointed out that staminate plants produce fibers of better quality
than the carpellate ones; and, despite the fact that the number of
carpellate plants slightly exceeds the staminate in a given crop, the
percentage of fiber content is greater for the staminate plants than
for the carpellate ones.

Early in ontogeny, the pericyclic region becomes multilayered,
and the cells which comprise it are noticeably larger than those of
the adjacent cortical parenchyma. The pericyclic cells enlarge,
elongate, become conspicuously vacuolate, and the cytoplasm
finally forms a thin layer against the primary wall. As the process
of secondary wall formation continues, the lumen of each fiber
becomes increasingly smaller, the cytoplasm is concentrated into a
smaller space; and, in some instances, the cell of the nearly mature
fiber is wholly filled with a granular cytoplasm. When the fiber is
completely mature, the cytoplasm disintegrates so that only
traces of desiccated material remain in the elongated oval lumen
which occupies about one-third of the transection of the cell.

The wall of the fiber consists chiefly of pure cellulose, but the
proportion does not run as high as in the flax fiber. The amount
of cellulose in one of the best types of Italian hemp was found to
be 77.77 per cent. The cell walls frequently exhibit stratification,
resembling those of flax in this respect, but the fibers are less trans-
parent, and the lumen is less easy to distinguish because of surface
striations.

In transection, a single fiber is three- to seven-angled; but owing
to secondary growth of the stem, it may be much compressed so
that the angles become somewhat rounded or flattened. The
average diameter is ix ju; but there is great variation between
individual fibers, extremes of 16 to 50 )u being reported by Matthews
(ii), and Cross and Bevan (10). The diameter for a given fiber
is variable, since the fiber is tapered at both ends, and there is also
a wide range in the length, which may vary from i to 10 cm. The
average is between 3.5 and 4 cm., according to Heuser (15), while
Matthews (xi) gives a somewhat lower figure, 2. cm. The tips of
the fiber cells usually are blunt or rounded, but some are forked,
this being regarded as a differentiating characteristic between the
northern and southern varieties of hemp, branching being more
frequent in the latter. Schilling (17) has pointed out that the
bifurcation of hemp fibers can be experimentally induced during

2.41 THE STRUCTURE OF ECONOMIC PLANTS

development by a mechanical breaking of the stem over a five- to
ten-day period, and that it also occurs in nature due to wind and
other mechanical factors. In small healthy plants, he found the
fibers free from bifurcation and other abnormal form changes.

The Preparation of the Fiber. — In the preparation of the
fiber, the plants are harvested, shocked and dried before retting is
undertaken. This is accomplished by dew-retting or water-retting,
the former being more common in the United States and the latter
in Europe and Asia. In dew-retting, the stalks are spread out on
the ground and subjected to moisture or to freezing and thawing;
while in water-retting, they are placed directly in streams, ponds,
or tanks. During this period, bacteria and fungi act upon the
middle lamellae of the fibers and on the tissues surrounding them.
This is followed by breaking and skutching to remove the disinte-
grated tissues from the fiber, which is then hackled by hand.

It should be noted that the technical hemp fiber of commerce is
not a single fiber cell, but consists of a group of cells which are
held together by their middle lamellae. One of the problems of
retting to obtain desirable commercial fiber is to insure the dissolu-
tion of the middle lamellae which connect the fiber strands with
adjacent tissue, and at the same time avoid a complete dissolution
of the middle lamellae between adjacent fiber cells.

The Leaf. — The blade of the leaf is relatively thin and the
principal veins form prominent ridges on the abaxial surface while
the adaxial surface is depressed into a groove above each v^in.
(Fig. ii6.) The cells of the upper epidermis are considerably
larger than those of the lower and much more heavily cutinized.
Stomata are infrequent or lacking on the upper epidermis and very
numerous in the lower one. Briosi and Tognini (7) made several
stomatal counts for the cotyledons, foliage leaves, stipules, and
bracts, which are here summarized:

TABLE III

Type of Leaf

No. OF Stomata per Sq. Mm.

Lower Epidermis

Upper Epidermis

Cotyledons

Central leaflet

Foliar bract

Stipule

Sepal of staminate flower . . .
Floral bract

100

-joo

53

100

43

100
13

ZI

2-4

CANNABIS SATIVA

2-43

Epidermal hairs are produced on both surfaces, being more
numerous on the lower, and the large persistent hairs produce
basal cystoliths. Glandular hairs also occur in large numbers,
these being especially noticeable in the young leaf and less so in
the mature blade, since they tend to break off with age.

The mesophyll, except adjacent to the principal veins, consists
of a single layer of slender, elongated palisade cells which occupy

Fig. ii6. Transection of a portion of mature leaf through midvein : ca, cambium; cy,
cystolith ; e^, epidermis; gl hr, glandular hair; hr, hair; mech, mechanical tissue; fal,
palisade; ph, phloem; spo, sponge cells; sto, stoma; xy i, primary xylem; xy x, secondary
xylem. The dark spots in phloem indicate location of latex ducts.

slightly more than half the thickness of the leaf. The spongy
parenchyma is very loosely organized, and there are large inter-
cellular spaces leading to the substomatal cavities. The midvein
of the leaflet is reinforced by a group of mechanical cells which are
located below the groove on the adaxial surface. There is also a
zone of mechanical tissue three to four, cells in width immediately
inside the abaxial epidermis. The vascular bundle is collateral,
resembling that described for the stem; and, in the midvein, there

144 THE STRUCTURE OF ECONOMIC PLANTS

is some development of secondary vascular tissues. Latex ducts
occur in the primary phloem and extend into the stem without
branching. As in the stem, numerous crystals of calcium oxalate
are found in the parenchymatous tissue of the mesophyll and in the
phloem parenchyma of the veins.

The petiole is subtriangular in transection with a groove extend-
ing along its adaxial surface which is shallow near the base of the
petiole and becomes gradually deeper toward the point of diver-
gence of the leaflets. It is strengthened by a zone of collenchyma
extending completely around the periphery adjacent to the epider-
mis, which is somewhat thicker along the abaxial angle and lateral
margins. The remaining tissue of the petiole consists of chloro-
phyll parenchyma, except for the veins. There are three at the
base of the petiole, but these usually anastomose in the upper por-
tion of the petiole, forming a continuous crescentic zone from which
branches arise that extend into the leaflets.

A situation has been reported regarding the xylem of the petiolar
bundles which may be noted here. Alexandrov and Alexandrova
(i), in investigating the question as to whether or not lignification
of cell walls is an irreversible process, have pointed out that in
hemp the oldest xylem vessels in the lower petioles are regularly
delignified. According to their account, the walls of the vessels
are dissolved completely, even the cellulose, and the vessels are
replaced by proliferating parenchymatous cells. In their opinion,
this delignification may occur normally to a great extent and is not
necessarily connected with pathological or traumatic conditions.

LITERATURE CITED

I. Alexandrov, W. G., and Alexandrova, O. G., "1st die Verholzung ein
reversibler oder irreversibler Vorgang?" Planta y: 340-346, 192.9.

1. DE Bary, a., Comparative Anatomy of the Vegetative Organs of the Phanerogams
and Ferns, Clarendon Press, Oxford, 1884.

3. Berkman, a. H., "Seedling anatomy of Cannabis sativa L." Unpubl. Ph.D.

Thesis, Univ. of Chicago, 1936.

4. Bredemann, G., "Beitrage zur Hanfziichtung. III. Weitere Versuche iiber

Ziichtung und Fasergehalt." Zeitschr. Pflanzensiicht . 12: 159-2.68, 1916.

5. Breslawetz, L., "Polyploide Mitosen bei Cannabis sativa L." Ber. Deut.

Bot. Gesell. 44: 498-502., 1916.

6. Briosi, G., and Tognini, F., "Intorno alia anatomia della canapa (Cannabis

sativa L.). Parte prima: Organi sessuali." Atti. 1st. Bot. Pavia, Ser. II,
}.- 91-109, 1894.

CANNABIS SATIVA 145

7. Briosi, G., and Tocnini, F., "Iiitonio alia aiiatomia della canapa (Cannabis

sativa L.). Parte seconda: Organi vegetativi." Atti. 1st. Bot. Pavia,
Ser. II, 4: 155-32.9, 1897.

8. DE Candolle, a., Origin of Cultivated Plants, D. Appleton and Co., 1885.

9. Crooks, D. M., "Histological and regenerative studies on the flax seedling."

Bot. Gaz- 95: 109-139, 1933.

10. Cross, C. F., and Bevan, E. J., Researches on Cellulose II, Longmans, Green

and Co., London, 1913.

11. Dewey, L. H., "The hemp industry in the United States." U. S. Dept. Agr.

Yearbook, 541-554, 1901.
II. , "Hemp." U. S. Dept. Agr. Yearbook, 2.83-346, 1913.

13. Dodge, C. R., "A descriptive catalog of the useful fiber plants of the world."

U. S. Dept. Agr. Fiber Investigations Report p, 361, 1897.

14. Flahault, C, "L'accroissement terminal de la racine." Ann. Sci. Nat.,

Bot. Ser. VI, 6: 1-168, 1878.

15. Hevser, O., Die Hanfpflanze. In "Technologie der Textilfasern. V. Part x.

Hanf und Hartfasern." Herzog, R. O., Ed. Julius Springer, Berlin, 1917.

16. Janczewski, E., "Recherches sur l'accroissement terminal des racines, dans

les Phanerogames." Ann. Sci. Nat., Bot. Ser. V, 20: i6i-ioi, 1874.

17. Langlet, O. F., "Zur Kenntnis der polysomatischen Zellkerne im Wurzel-

meristem." Svensk. Bot. Tidskr. 21: 397-411, 1917.

18. DE LiTARDiERE, R., "Lcs anomalies de la caryocinese somatique chez le

Spinacia oleraceae L." Rev. Gen. Bot. ^/.- 369-380, 1913.

15. , "Sur I'exisience de figures didiploides dans le meristeme radiculaire

du Cannabis sativa L." La Cellule ^j: 11-15, 1915-

10. Lubbock, Sir John, Seedlings, Vol. 1, D. Appleton and Co., 1891.
LI. Matthews, J. M., The Textile Fibres, John Wiley and Sons, 191 1.

11. McPhee, H. C, "The Influence of environment on sex in hemp, Cannabis

sativa L." Jour. Agr. Res. 28: 1067-1080, 1914.
13. , "The genetics of sex in hemp." Jour. Agr. Res. ^i: 935-943, 1915-

14. Savelli, R., "Poliembrionia in Cannabis sativa L." Arch. Bot. Sist. Fitogeogr.

e Genet. 4 (2): 118-137, 1918.

15. Schaffner, J. H., "The change of opposite to alternate phyllotaxy and

repeated rejuvenations in hemp by means of changed photoperiodicity."
Ecology j: 315-315, 1916.

16. , "Further experiments in repeated rejuvenations in hemp and their

bearing on the general problem of sex." Am. Jour. Bot. //.• 77-85, 1918.

17. Schilling, E., "Zur Morphologie, Physio logie und diagnostischen Bewertung

der Bastfasern von Cannabis sativa." Ber. Deut. Bot. Gesell. 41: 111-117,

192-3-

18. VAN Tieghem, p., and Douliot, H., "Recherches comparative sur I'origine

des membres endogenes." Ann. Sci. Nat., Bot. Ser. VII, 8: 1-656, 1888.

19. WiNTON, A. L., The Microscopy of Vegetable Foods, John Wiley & Sons, 1906.

30. Zander, A., "Uber Verlauf und Entstehung der Milchrohren des Hanfes

(Cannabis sativa)." Flora N. S. 2^: 191-118, 1918.

31. ZiNGER, N., "Beitrage zur Kenntnis der weiblichen Bliithen und Inflorescenzen

bei Cannabineen." Flora Sj: 189-153, 1898.

CHAPTER IX

CHENOPODIACEAE

BETA VULGARIS

THERE are several varieties of Beta vulgaris which Bailey
(4) has divided into five cultural sections: garden beets,
mangels, sugar beets, chard, and foliage beets. ^ The cultivated
beet is an herbaceous dicotyledon which normally completes its
life cycle in two years; but like its wild ancestor, Beta maritima
Linn., the biennial habit is variable and under certain conditions
the plant may function as an annual or even as a perennial. When
growing as a biennial, the beet produces a large, succulent, fleshy
tap root with a short crown stem and a rosette of leaves during the
first season. In this phase of the vegetative cycle, large food
reserves are accumulated in the storage regions of the root which
are utilized in the second year; and, after the formation of a second
rosette of leaves, the plant develops a bushy shoot and produces
its flowering branches. These are stout and angular, and, in some
of the seed fields, may attain a height of 5 to 6 feet or more.

GENERAL MORPHOLOGY

The Fleshy Axis — Root, Hypocotyl and Stem. — The mature
beet of commerce is a pear-shaped or globular fleshy axis made up
of a crown, neck, and a more or less elongated tap root. The crown
is an unelongated stem from which the rosette of leaves arises,
while the smooth hypocotyledonary neck below it, which is
devoid of lateral roots, forms the upper portion of the thickened
axis. The lower root-like part of the axis is described by Weaver
(xo) as follows,

"The sugar beet has a strong, deep, very fleshy taproot, which in
moist soil grows rapidly and almost vertically downward, reaching

1 Much of the anatomical work reported in this chapter, especially that of Artschwager,
was done in connection with the sugar beet group, but some of the investigations were con-
cerned with table varieties of the Detroit Dark Red type.

146

BETA VULGARIS

2-47

depths of 5 to 6 feet. Beginning just below the soil surface and extend-
ing to a depth of 6 to lo inches, laterals occur in two opposite rows
on the sides of the roots. These begin to appear when the plants
are 6 to 8 weeks old and finally occur in great numbers, 6o to 70 per
linear inch. Running horizontally 6 to 18 inches or more on all sides
of the plant and branching profusely, they form an excellent absorbing
system in the surface foot of soil. Numerous larger and longer branches
arise usually at depths of 8 inches to 4 feet. They spread from 6 inches
to i feet laterally and penetrate deeply into the subsoil. . . . This root
habit, however, is greatly modified by variations in soil conditions.
In dry soil, the taproot is smaller, pursues a more tortuous course,
does not penetrate so deeply, and is branched more nearly to the tip.
The larger, deeper seated branches turn downward rather abruptly,
reaching depths of 3 to 4 feet. Branching is more profuse throughout.
Development of the surface absorbing system may be greatly delayed,
although it branches more profusely and may extend even more widely
when the soil becomes moist." (Fig. 117.)

Fig. 117. Diagrams showing habit and extent of root system of the sugar beet grown (^)
in dry land ; and (B) in fully irrigated soil. (After Weaver, Root Development of Field Crops,
McGraw-Hill Book Co.)

The two rows of laterals, referred to by Weaver, are usually
double rows and the grooves from which they arise are oriented
in the same vertical plane as the primary xylem strand of the root.
The furrowing and transectional plan of the fleshy axis can be
correlated with the mode of secondary thickening of the axis, and

2.48 THE STRUCTURE OF ECONOMIC PLANTS

the manner in which the secondary or lateral roots arise. Typi-
cally, the beet has a single tap root, but it may branch to form
several fleshy members.

The Leaves. — The large succulent leaves of the basal rosette
are arranged in a close spiral in a ^{3 phyllotaxy, and are extremely
variable as to size, shape, and color. In general, the petiole
is somewhat triangular in transection and more or less flattened
at the base. The lamina is prominently veined and the lateral
branches which arise from the main veins form a netted system in
which some of the small veinlets anastomose, while others end
blindly in the mesophyll. The blade is elongated, somewhat
oblong or triangular with a cordate base, its margins may be
straight or undulate, and the surface is smooth or crinkly. There
is an equally wide variation with respect to color, which ranges
from dark red to a light green. The rosette of leaves produced
the second year resembles that of the first but they are progressively
smaller.

The Inflorescences and Flowers. — The stem elongates
rapidly to form the floral axis, and the development of numerous
branches which arise from the axillary buds of the basal leaves
results in the bushy habit of the mature plant. The terminal
portions of the main axes and lateral branches produce inflores-
cences which are paniculate or spicate in character. (Fig. 118.)
The flowers are sessile and occur singly or more commonly in
clusters of two or three. They arise from a very short floral branch
subtended by a large median bract and two lateral ones which
bear flowers in their axils. The flowers are perfect and consist
of five narrow sepals, five stamens, and a tricarpellate pistil. The
ovary of the mature pistil forms a fruit which is embedded in the
base of the perianth of the flower, and these are enclosed by the
common receptacle of the flower cluster so that a multiple fruit or
"seed ball" is formed by the cohesion of two or three such enclosed
fruits. The fruit is hard, containing a single seed and, where
flowers occur singly, forms what is known in commercial practice
as a "single germ beet seed." When the "seed ball" is formed by
the aggregation of two or three flowers, the so-called "multiple
beet seed" is produced. These terms, of course, refer to the fruits,
as the true seeds are small, dark, kidney-shaped structures.

Seed Production. — The annual method of producing sugar
beet seed was developed by Overpeck (13) in New Mexico about ten

BETA VULGARIS

2-49

years ago. The seed is planted about September first, and the
plants are allowed to remain unthinned so that they develop tap
roots about Yi to 13=-^ inches in diameter by the time cold weather
stops further development. Early in March, the plants begin to
produce seed stalks; by the latter part of June, these have reached

Fig. ii8. A, general habic of inflorescences; E, flower, showing arrangement of floral
parts ; C, longisection through floral axis showing relation of parts ; D, single inflorescence
showing axillary position of sessile flowers.

a height of from 4 to 8 feet, depending upon soil fertility, and bear
clusters of seed-containing fruits. This method requires a climate
characterized by winters warm enough to allow the plants to live
over without injury, but cold enough to cause the physiological
changes which are necessary to throw the plant from the vegetative
into the reproductive phase of its cycle. Such conditions exist
in southern California, Arizona, New Mexico and southern Utah

X50 THE STRUCTURE OF ECONOMIC PLANTS

and may be easily duplicated in controlled environments in green-
houses.

In Europe, the biennial method is used. It consists in planting
the seed in July or August and digging the small beets before frost.
The roots are then stored in out-of-door pits over the w^inter where
the lovv^ temperatures condition the roots for seed stalk production.
In the spring, the roots are replanted, seed stalks develop, and the
seed is harvested in July.

The Seed. — The mature seed is a flat, shiny, reddish-brow^n
structure which ranges from 1.5 to 3 mm. in diameter, and is
approximately 1.5 mm. in thickness. The seed coat has two
integuments, the curved embryo within completely surrounds the
perisperm, and a single layer of endosperm encloses the distal
portionof the primary root. (Fig. 119.) Each of the integuments
consists initially of two cell layers, except near the chalaza where
they are more numerous; but in the mature seed coat, only three
remain, since the outermost layer of the inner integument dis-
integrates during the development of the seed. The cells of the
outer integument are at first undifferentiated, but later the outer
wall becomes thickened and is covered with an extensive cuticle.
The second layer of the outer integument is comprised of small
cells with dense contents, and the individual cells are separated
from one another by small intercellular spaces. The layers of the
inner integument are at first meristematic; but, as maturation
of the seed proceeds, the outer layer remains small, the nuclei dis-
integrate and the cells are obliterated. Meanwhile, the cells of
the inner layer of the inner integument enlarge and the walls
become delicately sculptured. A thick cuticle is laid down
between the inner integument and the nucellus which, according
to Bennett and Esau (6), may be recognized by microchemical
tests when the embryo consists of approximately nine cells. They
point out that the cuticle is lacking in the chalazal region where
the phloem approaches the perisperm, so that a "passage region"
is formed through which materials can move into the latter.
When the seed has reached maturity, the walls of the cells in this
zone also become cutinized or suberized, sealing off the embryo
and perisperm.

Artschwager (x) reports that "the cells of the outer integument
contain starch and there is an extensive tannin deposit in both
integumentary layers." The embryo is filled with albuminous

BETA VULGARIS

2-51

and oily materials and the perisperm is packed with large starch
grains except at the chalazal region. Starch is not found in the
resting embryo but may appear there during germination. With
respect to the movement of reserve foods into the storage tissues
of the seed, Bennett and Esau find that the vascular strand which
passes through the funiculus is collateral; but after it has entered

perisperm.

t. endosperm.

embryo -

nucellus -
i.cuticle-
i.integument-

o.integument -

0. cuticle •

p. endosperm

funiculus

Fig. 119. Section through ovule and young embryo showing structure of seed and seed
coat. (After Artschwager, Jour. Agr. Kes.')

the chalazal region, it tends to be amphicribral, and the phloem
extends farther than the xylem. (Fig. iio, A-I.') No direct
vascular connection exists between the embryo and the storage
tissues adjacent to it, but protoxylem elements are completely
differentiated in the cotyledons before germination, and proto-
phloem elements are also present.

Development of the Seedling. — Under favorable conditions
for germination, the primary root pushes through the seed and

2.51 THE STRUCTURE OF ECONOMIC PLANTS

fruit coats of the "seed ball" and grows directly downward with-
out branching. The hypocotyledonary arch elongates, lifting the
cotyledons above the soil; and, finally, the hypocotyl straightens

h -

funiculus

^ cotyledons

endosperm

seed coat

Fig. iio. A-B, diagrams of sugar beet seed showing distribution of vascular tissues ; C,
section of mature seed with embryo in longisection ; D, transection made along axis a-a in
C\ E-I, successive transections made parallel to axis b-b in C; £ is the lowest section. In
figures C-I, xylem is shown in solid black, phloem by heavy stipples, and tannin-containing
cells by circles. (After Bennett and Esau, Jour. Agr. R«.r.)

BETA VULGARIS

2-53

and the cotyledons expand laterally. (Fig. iii.) The young
seedling consists of three regions, root, hypocotyl, and cotyledons;
and the point of transition between root and hypocotyl can be
determined with a fair degree of accuracy by the abrupt tapering
of the axis and by the location of the lateral roots. Epicotyledon-
ary development is slow; but after the complete expansion of the

#

Fig. 12.1. Habit of beet fruit and stages in development of seedling.

cotyledons, the first pair of foliage leaves begins to grow rapidly.
This occurs about eight to ten days after germination and is fol-
lowed by the differentiation and growth of the foliage leaves
which form the primary rosette.

ANATOMY

The Cotyledons. — The fleshy cotyledons are single-nerved
and elongated with short petioles or tapering bases. Their
structure is simple and the mesophyll consists of chlorenchyma
in which there is little differentiation into palisade and spongy
tissues. The small vascular bundles are collateral and run longi-
tudinally between the spongy parenchyma and the palisade.
The epidermal cells are irregularly polygonal in outline with

i54 THE STRUCTURE OF ECONOMIC PLANTS

sinuous walls, and numerous stomata occur with equal frequency
in the upper and lower surfaces.

The Primary Root. — The primary root has an exarch, diarch
protostele; and, at the time of complete maturation of its primary
tissues, consists of an epidermis, cortex, and stele. In the apical
region, many of the epidermal cells develop as root hairs; while
in the transition zone above the piliferous area, the surface
is glabrous and cutinized. The cortical region is comprised of
several rows (usually three to seven) of parenchymatous cells
with large intercellular spaces, the innermost ones being smaller

and more regular in shape
A than those in the peripheral

rows. (Fig. I2.X.) The cor-
tex is limited inwardly by the
endodermis, which is single-
layered and without inter-
pif-par cellular spaces.

px The development of Cas-
parian strips and the later
suberization of the walls of
the endodermal cells has been
investigated by Plaut (15) and

Fig. 12.1. Transection of primary root at the Ruggeberg (l6), whose find-

maturation of primary tissues: co, cortex, ■ j^^^^ ^^^^ Confirmed by

en, endodermis; ep, epidermis; mx, metaxylem; o j

par, interstitial parenchyma; pel, pericycle; Artschwaget (l). Except for

/>/-, phloem; /-at, protoxylem. (Redrawn after ^^^ ^^.^j. f^^ millimeters of
Artschwager, Jour. Agr. Res.) .

the root tip, the endodermal
cells are characterized by Casparian strips on the radial and
end walls which are detectable with the use of stain techniques
at about the time that the metaxylem vessels mature. This
secondary phase of the endodermis extends axially for approxi-
mately 3 cm., at which level the cells are in the tertiary stage
of wall development characterized by the deposition of a suberin
layer over the entire surface. This occurs later in the cells
lying opposite the protoxylem points than in those in other
sectors of the endodermis. In the lower hypocotyl, the endo-
dermal wall-thickenings are in the secondary phase; and, at
its upper limits, no well-defined Casparian strips are laid down.

The development of the root axis and the differentiation of
the primary vascular tissues has been investigated by Lyle (11)

BETA VULGARIS

155

r c

and Artsch wager (i). Their results are here summarized and
quoted in part. The ontogeny follows Janczewski's (10) third
type, in which the plerome and periblem give rise to the stele
and cortex respectively, while the surrounding calyptrogen-
dermatogen layer initiates
the epidermis and the root
cap. According to Lyle

(II),

"this calyptrogen-dermato-
gen layer of cells, overlying
the periblem, divides peri-
clinally adding a layer of
cells to the root cap and
renewing the initial layer
itself. The marginal cells
of this new calyptrogen-
dermatogen layer farthest
from the root apex divide
only anticlinally forming
epidermal cells. This distal
portion of the dual layer is
thus a dermatogen while
the remaining apical section
is the calyptrogen. This
method of formation of root
cap and epidermis accounts
for the stair-step arrange-
ment of the latter. (Fig.
1x3.) Cells of the layers
of the root cap also divide
anticlinally thus compen-
sating for the increase in
length of the underlying
zones of the root axis. Seg-
ments of a few rows of the
root cap may also divide peri-
clinally forming additional
short layers in the root cap.

"The cortex is derived from the periblem, which consists of a single
layer of cells directly inside the calyptrogen-dermatogen. These cells
divide both periclinally and anticlinally resulting in three to seven
rows of typical cortical cells and an inner layer of axially elongated
cells, the endodermis.

' The stele is differentiated from a group of plerome cells that divide
in all planes. The outer layer forms the pericycle which is composed

-- cal-dgn

Fig. 12.3. Longisection of root tip showing
histogens : cal-dgn, calyptrogen-dermatogen ; co,
cortex ; en, endodermis ; ep, epidermis ; pbm, peri-
blem ; pel, pericycle ; pi, plerome ; r c, root cap ;
stl, stele. (After Lyle.)

2.^6 THE STRUCTURE OF ECONOMIC PLANTS

of cells that are more or less rectangular in longitudinal section. About
two millimeters above the plerome initials, the primary tissues of the
stele begin to differentiate from the elongated procambial cells. Seeliger
(17) found the first differentiation to occur in two cells adjacent to
the pericycle and 180° from each other. These are the initials of the
two groups of primary phloem cells. Protoxylem elements are later
differentiated in a plane at right angles to these phloem groups. Differ-
entiation begins in the cells bordering the pericycle and progresses
toward the center of the axis. The three or four outermost protoxylem
elements have annular or spiral thickenings. The metaxylem vessels
in the center of the plate are larger in diameter and have reticulate
or scalariform thickenings. As soon as the plate is completely differ-
entiated, cells on the lateral faces of the same mature as metaxylem
vessels or parenchyma."

Artschwager's account of the primary xylem formation is
essentially like that of Lyle, but he regards the protoxylem as
being continuous across the entire primary xylem strand and the
metaxylem as being lateral to this continuous zone:

"Differentiation in the protoxylem progresses centripetally until
the two protoxylem points meet in the center to form the primary
xylem plates. From now on xylem cells mature to the right and
left of the xylem plate until all the cells of the primary wood have
been formed."

Further differentiation of the primary phloem is not easy to
describe because of the small size of the elements, but Seeliger (17)
has reported that the metaphloem differentiates from the pro-
cambium one or more cells to the inside of the pericycle and that
it consists of sieve tubes, companion cells, and phloem parenchyma.

The Vascular Transition. — As Artschwager has pointed out,
"the change from the exarch condition in the root to the endarch
condition in the upper hypocotyl is very abrupt, with the transi-
tion region extending over only a few millimeters"; but complete
transition of the primary . vascular elements to the collateral
endarch arrangement is only attained in the blade of the cotyledon.
Lyle (11) described and figured the transition in the Detroit Dark
Red variety which occurs chiefly in the region that later forms the
fleshy portion of the axis:

"The lower portion of the hypocotyl is root-like in the arrangement
of the vascular tissues, resembling the primary root previously described
in this respect. (Fig. 115, A.^ The first indication of a change in
the vascular arrangement occurs in the upper third of the hypocotyl
where there is a gradual reorientation of the primary vascular tissues

BETA VULGARIS

2-57

at successively higher levels until the endarch condition is attained
in the blade of the cotyledon. Since the tissues of the hypocotyl
differentiate at varying rates, maturing progressively towards the
cotyledonary plate, the lower portion in a young seedling may show

D

B protoxylem
g metaxylem
[g phloem

Fig. 114. Diagrams showing vascular transition : A, root with diarch xylem strand of
proto- and metaxylem; B, lower hypocotyl, lateral differentiation of metaxylem; C, two-
thirds distance between root and epicotyl, reorientation of xylem, circumferential extension
of phloem; D E, upper third of hypocotyl, primary xylem separated into two groups by
parenchyma, lateral orientation of metaxylem; F, cotyledonary node; G-H, cotyledonary
petiole; /, base of cotyledonary blade, adaxial differentiation of protoxylem and abaxial
development of metaxylem ; /, blade with endarch primary xylem. (After Lyle.)

primary tissues completely differentiated and the beginning of secondary
thickening while in the upper part the primary tissues are just beginning
to mature. The meristematic condition of the region near the coty-
ledonary plate accounts for the continued elongation of the upper
hypocotyl. •

X58 THE STRUCTURE OF ECONOMIC PLANTS

"As indicated in Figure 1x4, C, and Figure 1x5, B, the primary
xylem plate is separated into two distinct units by parenchyma. These
parenchymatous cells are thin-walled, rectangular in longitudinal
section, and polygonal or nearly round in cross section. The radially

Fig. 115. A, transection of lower hypocotyl, diarch primary xylem strand ; B, transec-
tion of upper hypocotyl about two-thirds distance between root and epicotyl ; C, tran-
section at slightly higher level in hypocotyl ; large pith ; lateral differentiation of
metaxylem ; xylem of bundles of first and second foliage leaves ; D, transection of coty-
ledonary plate region ; continuous cylinder of vascular tissue composed of cotyledonary
bundles and leaf bundles : w, cambium ; w, endodermis ; wat, metaxylem ; />c/, pericycle;
ph, phloem; pi, pith; px, protoxylem ; xy 1, secondary xylem; xji l-i, xylem of first
foliage leaf. (After Lyle.)

arranged phloem groups on either side of the xylem plate have enlarged
circumferentially.

"The diameter of the stele becomes greater at successively higher
levels due to an increase in the number of parenchymatous cells at the
center of the axis. The units of primary xylem appear to be farther

BETA VULGARIS 159

apart although the protoxylem points are approximately in the same
position with reference to the diameter of the axis. The metaxylem
vessels occupy a more and more lateral position with respect to the
protoxylem, and the phloem masses show a slight division into two
units, being separated from each other by cells of the fundamental
parenchyma. (Fig. 1x4, D.)

"The metaxylem continues to differentiate more and more tan-
gentially with reference to the protoxylem in the upper hypocotyl.
With the protoxylem point as the common apex, it forms two sub-
triangular groups which occupy a collateral position with respect
to the phloem groups. (Fig. 114, E and Fig. 1x5, C.) The vascular
system of the hypocotyl in the region of the cotyledonary plate is a
dissected siphonostele made up of two cotyledonary bundles and the
bundles of the foliage leaves. (Fig. ii6, /4.) The cotyledonary traces
diverge from the stele quite abruptly. In the cotyledonary bundle
the primary xylem is still approximately exarch, but the metaxylem
vessels have differentiated more laterally forming the vascular strands
which Thomas (18) designates as 'double bundles.' (Fig. 1x4, F
and Fig. ixy, B.)

"At increasing distances from the cotyledonary node, in the coty-
ledonary petiole and blade, the protoxylem occupies a more and more
adaxial position and the metaxylem a more abaxial one. (Fig. 114,
G, H, I, and Fig. 117, C, D.) Thus, the endarch condition of the
primary xylem is finally attained in the blade of the cotyledon. (Fig.
1x4, /.) The phloem groups in the 'double bundles' are separated
from each other by parenchyma, but at the level at which the primary
xylem is endarch they differentiate as one group in an abaxial position
to the metaxylem."

The Epicotyl. — The first two foliage leaves develop from
the growing point of the unelongated epicotyledonary axis which
is surrounded by the bases of the cotyledons. The pro vascular
strands develop early in the leaf primordia, and all the primary
bundles of the leaf and epicotyledonary axis are collateral and
endarch. The trace of the first foliage leaf above the cotyledons
consists of a large central bundle and two smaller lateral ones which
pass into the cortex independently, but the laterals anastomose
with a vascular strand which continues into the internode above.
Fron (8) has designated this combined strand as a "cauline"
bundle. The leaf trace is differentiated downward into the
nodal region of the cotyledon where the two "cauline bundles"
and the central bundle of the trace anastomose. This endarch
bundle gradually becomes smaller in the upper hypocotyl and
finally ends blindly in the fundamental parenchyma between the
cotyledonary bundles. (Fig. 1x5, C.) The maturation of the

i6o THE STRUCTURE OF ECONOMIC PLANTS

primary tissues is accompanied by the formation of fascicular
and interfascicular cambiums which produce a continuous cyl-
inder of secondary tissues, and these form the only connection

Fig. 12.6 /J, transection of portion of upper hypocotyl of older seedling showing cotyle-
donary bundle and those of first and second foliage leaves; B, same axis at slightly higher
level showing further differentiation of leaf bundles : iu /-i, bundle of first foliage leaf ca
cambium; ca x secondary cambium; en, endodermis; wx, metaxylem ; W pericycle' p/j
phloem; />/, pith; ;,.v,protoxylem; xj 2., secondary xylem. (After Lyle )

BETA VULGARIS

i6i

between the vascular systems of the hypocotyl and epicotyl.
(Fig. 115, D.)

The Ontogeny of the Fleshy Axis. — The enlarged fleshy
portion of the axis is chiefly root-like, but the upper limits of
the hypocotyl are transitional with respect to the arrangement of
the primary vascular tissues. In the center of the fleshy portion
of the root, there is a small star-shaped region of vascular tissue
which consists of the elements of the primary xylem strand and

Fig. 117. A, cellular drawing of cotyledonary bundle, below level of B, the primary xylem
still approximately exarch ; B, transverse section of cotyledon showing protoxylem in a more
adaxial position and metaxylem more abaxial than in A. The two phloem groups are still
separated from each other by parenchyma ; C, bundle of cotyledon showing further reorien-
tation of protoxylem and metaxylem; D, bundle of cotyledon showing endarch primary
xylem ; the phloem groups have differentiated in a single unit abaxial to the xylem : co,
cortex; ?/;, epidermis ; /><?r, parenchyma; ph, phloem; /ix, protoxylem. (After Lyle.)

the secondary xylem formed from the primary cambium. Outside
this central region is a series of concentric rings of vascular tissue
which alternate with bands of storage parenchyma. The number
of vascular rings may vary from four to ten or twelve or more,
but only four or five of the inner rings contain many mature
vascular elements, the outermost ones being very narrow and
remaining in a more or less meristematic condition. (Fig. ii8.)
A reduction in the number of rings occurs toward the hypocotyle-
donary portion of the axis adjacent to the crown stem; and, at this
point, many anastomoses of vascular elements occur in connection

7.6z THE STRUCTURE OF ECONOMIC PLANTS

with the insertion of the bundles of the leaf traces. There may
also be a reduction in the number of rings in the narrow tapering
portion of the lower root. Chiefly because of differences in
cultural conditions, there is considerable variation in the develop-
ment of the beet with respect to the number of rings, their width,
and the amount of parenchymatous storage tissue, but the funda-
mental organization of the fleshy axis is identical in all cases.

At the time when the primary tissues are completely matured,
the root has a single-layered epidermis, a cortical region consist-
ing of a few layers of parenchyma limited centripetally by the

Fig. 12.8. Transection of fleshy portion of the axis showing central zone of primary and
secondary vascular tissues and concentric rings of tertiary tissues produced by the secondary
cambiums. (After Artschwager, /oar. Agr. Res.^

endodermis, and the stele. The stele is bounded by a uniseriate
pericycle enclosing the primary xylem and phloem, and these
are separated from each other by a zone of interstitial parenchyma
which increases in amount during the maturation of the primary
vascular tissues. This takes about lo toiz days and is complete
when the axis is only i or x mm. in diameter in contrast to a final
one of 15 cm. or more which may be attained in the mature sugar
beet.

When the primary tissues are nearly mature, the first lateral
roots originate in the pericycle approximately outside the proto-
xylem points. They occur in two double longitudinal rows,
since the root primordia are initiated slightly to the right and
left of radii passing through the two protoxylem points. The

BETA VULGARIS

2.63

lateral roots are slender and differ from the fleshy, primary root
chiefly in having a larger proportion of xylem elements, especially
vessels, and in the absence of any considerable amount of ray
parenchyma cells. Artschwager (i) has also observed in this
connection that the primary xylem strand of the lateral roots is
occasionally "triarch instead of diarch, which is always the case
in the taproot."

Secondary Thickening. — The great increase in the size of
the axis is due to the activity of primary and secondary cambiums.
The cells of the interstitial parenchyma begin to elongate axially
and divide tangentially, forming the primary cambium which cuts
off secondary vascular elements that are centripetally placed in
relation to the primary
phloem groups and abut
the metaxylem. (Fig.
119.) In some instances,
there may be a layer of
interstitial parenchyma
remaining between the
primary xylem and the
first-formed secondary xy-
lem. The production of

secondary xylem and p,,, ^^^ ^ portion of stele of primary root in

phloem proceeds simul- transection showing early stages in secondary thicken-

1 A tVi ' '"S • ^"^ cambium ; en, endodermis ; mx, metaxylem ;

taneOUSly ana tnere is ^^^^ pericyde ; ph, phloem ; px, protoxylem ; xy z,

also a progressive lateral secondary xylem.

extension of the two in-
itial regions of cambial activity toward the protoxylem points,
so that finally the primary cambium forms a complete cylinder
involving sectors of the pericycle outside the protoxylem. The
cambium in these sectors does not produce vascular elements, but
cuts off successive layers of parenchymatous cells so that two
wedge-shaped xylem rays are formed. These rays may be re-
garded as pericyclic in cases where the production of the ray
parenchyma is the result of a general division of the pericyclic
cells, compensating for the growth of the adjacent tissue, rather
than the product of a definite cambial layer that cuts off paren-
chymatous cells reciprocally.

The development of the secondary tissues of the stele results
in modifications of the primary tissues lying outside this region.

1.64 THE STRUCTURE OF ECONOMIC PLANTS

The primary phloem keeps pace with the stelar growth by con-
tinued divisions of the parenchyma, and the effect of this is to
scatter and stretch the sieve tubes until they are finally obliterated.
The pericycle also persists, undergoing continued radial divisions
of its cells; and, by the time the epidermis and cortical regions
are ruptured, it is several-layered as a result of successive periclinal
divisions. In the outermost of these layers, periderm formation
is initiated at about the time that the first secondary cambiums
, are beginning to form, this coinciding roughly with cortical dis-
integration. The phellogen cuts off phellem or cork cells centrif-
ugally and phellodermal cells centripetally, but the divisions do not
take place in regular alternation and more cork cells are cut off than
phellodermal cells. The outer cork cells are shed continuously
as the axis enlarges so that the phellem maintains a relatively
constant thickness of from five to eight cell layers. The cork
cells are flattened and the thin walls are suberized except for the
lignified middle lamella.

The Secondary Cambium. — The first of the rings ^ of
the fleshy axis is formed by the activity of the primary cam-
bium. The remaining rings are produced by supernumerary or
secondary cambiums, the first of which arises while the
formation of the first ring by the primary cambium is still in
progress.

Tertiary Thickening. — The origin of the supernumerary
cambiums and the mechanism of secondary and tertiary thickening
has been described by van Tieghem (19), de Bary (5), and Art-
schwager (i). According to the latter, the point of origin of these
cambiums varies with the level of the axis concerned. In the
root and lower hypocotyl, the first supernumerary cambium
originates in a zone of primary phloem parenchyma between the
pericycle and the secondary phloem. (Fig. 130.) In the upper
hypocotyl, it arises from the pericycle; and in the intermediate
region, from either the pericycle or phloem parenchyma. Outside
the protoxylem points, all secondary cambiums are of pericyclic
origin. The formation of additional cambiums then takes place
in the following manner:

^ The use of the term ring in the description of the secondary and tertiary thickening of
the fleshy axis in this and succeeding sections is one of convenience and applies to the tran-
sectional aspect of the axis. Actually, each ring should be regarded as a conical layer of
tissue; and the secondary and tertiary rings of the axis as consisting of a series of successively
larger cones each of which encloses the one centrad to it.

BETA VULGARIS

x65

"When the cambium initial undergoes the first division, the outer
of the two daughter cells becomes the initial of a new supernumerary
cambium while the inner daughter cell divides further and produces
xylem, phloem and medullary ray tissue. This process is repeated
until all supernumerary cambiums have been formed. However, there
is no uniform method governing the formation of the supernumerary
cambiums. Often sections of two supernumerary cambiums originate
simultaneously, one from an inner, the other from a more peripheral
phloem parenchyma cell. Since most of the supernumerary cambiums
of the beet are initiated in quick

succession, a beet no thicker y^x'X:^7S-iXJL?JViS<>..J^~^r=^ en

than a pencil contains practi- :^^^^^^^^^^^^^^r^i>-— -pc7

cally all annular zones of growth ^^^^^^^^^^^^^^^V^~ 'P^ 3

developing simultaneously." ^^^^^^^^^^^^^^^^^- — ca 2

An alternative explanation
of the mechanism of tertiary
thickening accounts for the
origin of the secondary cam-
biums as a result of the con-
tinued activity of the pericycle.
Early in ontogeny, the pericycle
becomes an actively dividing
multi-layered zone which keeps
pace with the enlargement of
the axis. It seems probable

that the secondary cambiums p^^ ^^^ Transection of portion of stele

arise in fairly rapid succession of primary root showing primary xylem,

from the pericyclic parenchyma; ^^^""'^^[y ^hickening and initiation of ter-

ir ■' r J ^ nary thickening : ca, primary cambium ;

but there may be some time ca-L, secondary cambium; en, endodermis;

elapsing between the formation M pericycle; ^h i., secondary phloem;

f. . . ,. ^>!) 3, tertiary phloem; />«.vj, primary xylem;

of the successive cambiums, ^^i, secondary xylem; x> 3, tertiary xylem.

During this interval, those al-
ready formed function actively in producing the tissues of their
respective rings while the remaining pericyclic tissue is increased
by continued radial and tangential divisions. This would har-
monize with the fact that the innermost rings are the widest ones.
(Fig. 118.) In either case, the pericycle perpetuates itself as the
outer zone of the axis and produces a phellogen which forms
cork and phellodermal cells.

Regardless of what explanation is advanced for the mode
and sequence of origin of the secondary cambiums, there is

2.66 THE STRUCTURE OF ECONOMIC PLANTS

a general agreement as to the subsequent development of tissues
derived from them. There are commonly five or six relatively v^ide

-^ca 2

>xy 3

- — int par

--ph3

I

ph2-^

ca — -

xy 2— I

Fig. 131. A-E, sectors of successive secondary cambiums from peripheral one, A, to one
of innermost rings, E, showing progressive development of rings ; F, sector of central portion
of stele showing primary cambium and secondary tissues derived from it : ca, primary cam-
bium ; ca 2., secondary cambium ; int par, interzonal parenchyma ; pf> i, secondary phloem ;
ph 3, tertiary phloem ; xy t., secondary xylem , xj 3, tertiary xylem.

BETA VULGARIS

^67

concentric rings, outside of which there may be several narrower
ones; but the inner rings are not equal in width, each being nar-
rower than the one centrad to it. This indicates that, in ring
formation, the activation of the successively formed secondary
cambiums is centrifugally progressive; and that several of them
are functional at one time.
(Fig. 131.)

Because of this mode of de-
velopment, it is possible to
determine the ontogeny of one
ring by studying successive
rings in a centripetal direc-
tion. The outermost one,
lying immediately within the
periderm, is entirely meriste-
matic, consisting of cambial
cells derived from the peri-
cycle, parenchyma, and undif-
ferentiated vascular elements.
The first of these to differ-
entiate are sieve tubes and
companion cells which may
generally be found in the
second youngest ring. (Fig.
131, J5.) Following this,
there is a differentiation of
a large number of phloem
parenchyma cells which by
their later divisions and en-
largement separate the sieve
tubes and companion cells
from the cambium ring that
produced them. Ultimately,
these first-formed phloem ele-
ments are obliterated and crushed. (Fig. 131, F.) In most cases,
the xylem elements are arranged in narrow radial bands and are
separated from each other tangentially by zones of ray parenchyma.
(Fig. 133.) In general, the production of phloem cells precedes
the differentiation of xylem cells in any given ring; but^ occasion-
ally, the production of vascular elements-is reciprocal.

Fig. 132.. Ontogeny of phloem: A, most
peripheral ring ; B, second ring ; C, third ring ;
D, fourth ring; E, fifth ring; F, fifth ring but
from a different part of the beet. The phloem
shows a very marked degree of development
compared with the xylem. The outermost
phloem groups in F are already obliterated :
ca 2., secondary cambium , obi fh, obliterated
phloem; phi,, tertiary phloem; xj 3, tertiary
xylem. (After Artschwager, Jour. Agr. Rw.)

i68

THE STRUCTURE OF ECONOMIC PLANTS

The manner in which the vascular elements are distributed
indicates that the secondary cambiums do not occur as complete
rings, but consist of discrete sectors derived from the pericycle.
The parenchymatous cells which intervene between the radial rows
of vascular elements are of pericyclic origin and may be regarded
as pericyclic rays. Each cambium produces a zone of paren-
chymatous cells centripetally before the tertiary xylem elements
are differentiated; and further growth and division of these
cells results in a radial or, occasionally, a tangential displacement

of the xylem elements.
mtpar j£ ^.j^^ successive second-
ary cambiums are formed
from a persistent peri-
cyclic zone, the develop-
ment of the interzonal
parenchyma can be ex-
plained as the result of
the proliferation of the
increments of the pericycle
which remain centrad to
each secondary cambium.
To this zone of pericyclic
parenchyma are added the
products of the activity of
,,,,,,. the secondary cambiums

Fig. 133. Sector of large bundle from fourth ring : ■'

ca 1, secondary cambium; int par, interzonal paren- On either Side OI It. ihe
chyma; obi ph 3, obliterated tertiary phloem; pel ra, cambium within produceS

phloem parenchyma which

constitutes the inner zone

of the parenchymatous band; and, in this region, the obliterated

remains of crushed sieve tubes and companion cells occur. The

cambium external to the parenchymatous band contributes an outer

zone of xylem parenchyma in which there are scattered vessels.

On the basis of this interpretation, the interzonal parenchyma

is made up of three regions: an outer zone of xylem parenchyma;

an intermediate one of pericyclic parenchyma; and an inner region

of phloem parenchyma. A somewhat different explanation is

given by Artschwager (i) in which he states that

"the interzonal parenchyma in its entirety is made up of three regions:
an outer zone containing scattered xylem cells, a central purely paren-

- -int par

pericyclic ray; ph t,, tertiary phloem; xjy 3, tertiary
xylem. (After Artschwager, Jour. Agr. Kes.')

BETA VULGARIS 169

chyma zone, and an inner zone containing obliterated phloem. The
first two zones have been formed by centripetal growth of the outer
ring, while the third zone is the product of centrifugal growth of the
older ring."

Tertiary thickening in the hypocotyl is like that described
for the fleshy portion of the root except that the initial secondary
cambium is pericyclic in origin rather than arising in the phloem
parenchyma. The central portion of the axis consists of a pith
which widens out toward the crown stem, and the hypocotyl
may become hollow at its upper limit. A union of rings occurs
in the upper hypocotyl which is related to the manner of the
insertion of the leaf traces in which the bundles of each leaf trace
extend inwardly for different distances, the median bundle of
each trace reaching the center of the axis while the lateral bundles
differentiate centripetally for shorter distances. This vascular
arrangement, together with the anastomosing of the annular
rings, results in an interconnection between all of the leaves of the
rosette and all of the rings of the fleshy axis.

The Floral Axis. - — Slightly below the growing point of the
floral axis, a circular zone of procambial tissue is formed which
separates the included pith from the cortex. In this zone, col-
lateral vascular bundles are differentiated, the first elements
formed being narrow, thin-walled phloem cells which are difficult
to distinguish until they are nearly mature. Protoxylem elements
of a loose spiral type are next formed and finally, in the inter-
vening zone of procambial tissue, a fascicular cambium arises
which produces secondary xylem and phloem. The cortex is paren-
chymatous, except for ridges of collenchyma near its periphery;
and the epidermis is single-layered.

At this stage, the floral axis is roughly polygonal in outline
and there are five large cauline bundles at its angles, with numerous
smaller bundles in the intervening sectors of the vascular ring.
After the fascicular cambium has functioned for some time in
the usual manner, an interfascicular cambium is differentiated
which is oriented in a way that is unlike the arrangement found
in most stems. One arm of the interfascicular cambium is con-
tinuous with the fascicular cambium, while the other may arch
around outside the phloem of the adjacent bundle so that a uni-
lateral connection with the fascicular cambium, rather than a
bilateral one, results. This method of development produces an

-L-jo THE STRUCTURE OF ECONOMIC PLANTS

undulating zone of cambium which connects some of the bundles
directly, and passes outside of others, and the latter may be sepa-
rated from the subtending cambium by several layers of paren-
chymatous cells.

As the activity of the anomalous interfascicular cambium pro-
ceeds, a secondary cambium arises in the pericycle. This at first
produces only xylem elements and parenchyma by centripetal
divisions; but, later, it forms groups of phloem and parenchyma-
tous cells centrifugally. Finally, at the points of phloem forma-
tion, the secondary cambium becomes inactive and is itself differ-
entiated into vascular tissue and parenchyma. This results in a
discontinuity of the extrafascicular or pericyclic cambium, but
new segments of secondary cambium appear on the outer face of the
extrafascicular phloem which become confluent with the older
persistent segments of pericyclic cambium, thus restoring the con-
tinuity of the cambial ring. This cambium may then undergo
reciprocal divisions producing xylem and phloem tissue.

As a result of this anomalous type of secondary thickening, sev-
eral systems of mechanical tissue are laid down, (i) The periph-
eral portions of the primary bundles are bounded by a zone of
thick-walled cells in which scattered groups of phloem are located.
The cells are not true xylem, except at points immediately centrad
to the phloem groups, but form connective or conjunctive tissue
consisting of cells which may be parenchymatous or prosenchyma-
tous. (i) The extrafascicular or secondary cambium forms an
irregular series of vascular bundles which are surrounded by thick-
walled mechanical tissue. (3) The outer zone of the pericycle,
from which the secondary cambium originates, is delimited by
sectors of lignified fibers. (Fig. 134.)

In summary, there are three phases of cambial activity with re-
spect to the secondary and tertiary thickening of the floral axis
and vegetative stem: (i) the production of a normal fascicular
cambium which produces phloem and xylem in the usual manner;
(2.) the formation of an interfascicular cambium which forms
unilateral connections with the fascicular cambium; and (3) the
development of a .secondary or pericyclic cambium, sometimes re-
ferred to as extrafascicular, which produces both xylem and phloem
elements. Xylem and parenchyma are first produced centripetally
by the pericyclic cambium, followed by the centrifugal formation
of phloem and parenchyma. Ultimately, sectors of the extra-

BETA VULGARIS

171

fascicular cambium become inactive and differentiate as vascular
tissue or parenchyma; but the continuity of the extrafascicular
cambial ring is reestablished by the formation of newr sectors of

Fig. 134. Transection of portion of stem showing development of secondary and tertiary-
tissues : CO, cortex ; col, coUenchyma ; cu, cuticle ; en, endodermis ; e^, epidermis ; jas ca,
fascicular cambium ; int ca, interfascicular cambium ; pel ca, pericyclic cambium ; pi fib,
pericyclic fiber; fh z, secondary phloem; ph 3, tertiary phloem; pi, pith; xy i, primary
xylem ; xy i, secondary xylem ; atj 3, tertiary xylem.

cambium from the pericyclic cells located on the outer face of the
last-formed phloem. This type of axial thickening continues as
long as the stem lives.

The Course of the Bundles in the STEM^-^^^^'-'^FhFphyllotaxy of
the floral axis is usuall^L^lternate with a % divergence, although

XTL THE STRUCTURE OF ECONOMIC PLANTS

the lower leaves may appear to be in approximate pairs. The
trace of each young leaf consists of a large median bundle and two
smaller lateral ones, but at maturity this number is considerably
increased so that there are several bundles in the semicircular base
of the petiole at its point of divergence from the stem. The course

of the bundles in the Chenopodiaceae
and in the genus Beta has been in-
vestigated by Fron (8), Wilson (xi),
Artschwager (x), and others. The
accounts are in agreement except for
differences in the course of the
median bundle as reported by Fron
who worked with Beta cycla. Art-
schwager's report, based on Beta
vulgaris, is here given in part:

"Occupying the corners of the ir-
regular polygon are five large cauline
bundles alternating with an equal
number of strands, usually somewhat
smaller in size, which represent the
median traces of the first five leaves.
Flanking the sides of the cauline
bundles are small strands of vascular
tissue which represent the lateral
traces." (Fig. 135.)

Considering only the three main
bundles of each trace and disregard-
FiG. 135. A portion of floral axis ing variations which occur,

five internodes in length showing vas-
cular anatomy. Cauline bundles are "the derivation of the traces accords
solid black, median traces of leaves are • u u r 11 • 1 r- u

■^ 1 I I , , I u ji f I With the rollowinff plan. Each new

stippled, and lateral bundles of leaves 1 1 r

are white. (Redrawn after Artschwa- median trace IS derived from a left
g£r,Jour. Agr. Res.') cauline bundle (viewed from the stem

apex) a little distance above the node,
it ascends without fusion or forking for five internodes and passes into
the leaf without branching. The left lateral trace is derived from the
cauline bundle next it, passes through three internodes, and usually
branches several times before entering the leaf. The right lateral
trace is also derived from its adjacent cauline bundle. It passes
through only two internodes, but otherwise conforms to the left
lateral trace. The length of the lateral trace is, however, subject
to variation since it often remains a part of the cauline bundles for
a longer or shorter portion of their ascent.

BETA VULGARIS 173

"In each node, then, the cauline bundles flanking the oldest median
trace undergo the following changes: A little distance below, or
sometimes above the insertion of the leaf each cauline bundle gives
off a small branch which becomes a new lateral trace; a little higher
up they approach each other and may even temporarily fuse. Upon
separation, the left cauline bundle branches once more and the new
strand which is segregated becomes a new median trace. . . . The
vascular supply of the axillary members is also derived from the cauline
bundles a short distance below the node."

The Leaf. — In transection, the petiole is roughly triangular
with a trough-like depression along its adaxial surface, but its
basipetal portion is wide and more or less flattened. At the base
of the lamina, the petiole broadens out and forms wing-like pro-
jections which are continuous with the blade. The number of
bundles in the petiole varies depending upon the development of
the leaf, and it decreases toward the blade as a result of anastomoses
of the smaller ones. The large bundles extend the entire length
of the petiole and are reinforced by crescentic strands of mechanical
tissue which lie immediately outside the phloem and to a lesser
degree adjacent to the xylem. The petiole is further strengthened
by zones of collenchyma located just within the epidermis which
are so distributed that they form projecting ridges on the abaxial
surface of the petiole and a band several cells in width in the hypo-
dermal region of the adaxial surface.

The epidermal cells are somewhat elongated and there are numer-
ous stomata except in regions overlying the coUenchymatous
strands. The hypodermal cells may contain anthocyanin, and
chloroplasts are present in the compact outer layers of parenchyma
and to some extent in the more spongy central parenchyma. The
bundles are collateral or occasionally half-amphivasal. The
phloem consists chiefly of sieve tubes and companion cells with
some parenchyma, while the xylem is comprised of primary and
secondary vessels with both thin- and thick-walled fibers.

The epidermal cells of the blade are polygonal in surface view
with sinuous walls and the stomata are simple without accessory
cells, each pore being surrounded by a pair of guard cells containing
several chloroplasts. The stomatal counts which have been
recorded indicate a great variation in the absolute number per unit
area, probably due to differences in variety and in the condition of
the leaf at the time the count was made. Sixteerucounts, made
by Artsch wager (i), of leaves^i^^arying size and color gave an

174 THE STRUCTURE OF ECONOMIC PLANTS

average of 99 per sq. mm. in the upper epidermis and ixi in the
lower.

The mesophyll is chlorenchymatous and rather compact with
no sharp differentiation between the palisade and spongy tissue.
There is apparently a correlation between the thickness of the leaf
and the character of the mesophyll cells; and Artsch wager found
that in thick leaves, practically all of the cells are of the elongated
palisade type, while in very thin leaves, the entire mesophyll con-
sists of relatively short, round cells. The main veins are like those
of the petiole and are reinforced by zones of coUenchyma which
abut both epidermal layers, the more extensive abaxial zone being
several layers in width.

In connection with investigations of the curly-top disease in the
sugar beet, Esau (7) has studied the ontogeny of the phloem in the
leaves. The sieve tube and its companion cell arise from the
mother cell by a longitudinal division. The daughter nuclei in
the two cells are unlike, that of the companion cell being more
granular and deeply staining. (Fig. 136, 4.') The cytoplasm is
vacuolate, the vacuoles being numerous and small in young primary
phloem cells, and large and relatively few in number in young
secondary phloem cells. (Fig. 136, 4 and 2.) As the sieve tube
develops, characteristic slime bodies form which are plastic and
shaped like irregular drops. These are regarded as inclusions of
proteinaceous material which later disintegrate and become a part
of the vacuolar substance in mature sieve tubes. A very charac-
teristic feature of the phloem is the development of plastids in the
sieve tube and companion cell. These are commonly disk-shaped,
but sometimes appear as thick rings with a clear area in the center.
The plastids persist as permanent cell organs and are at first scat-
tered throughout the cytoplasm, later accumulating in greater
numb&rs near the sieve plates. (Fig. 136, 2.) As the sieve tubes
mature and later become crushed, the plastids begin to disintegrate,
this process being initiated by swelling.

The nucleus of the sieve tube disintegrates during its maturation
at about the time that the slime bodies break down. The walls of
the tube thicken perceptibly, especially in the primary phloem, and
the areas where the sieve perforations develop can be detected at
about the time that the slime bodies are formed. (Fig. 136, /.)
Later, the plate thickens, the pores become distinct, the sieve ele-
ment attains its full length, and the tube may be regarded as mature.

BETA VULGARIS

2-75

Senility is indicated by the formation of definitive callus on the
sieve plates; and, eventually, the tube is crushed. (Fig. 136, 77.)

Fig. 136. Ontogeny of sieve tubes in leaf of sugar beet, i, young sieve tube with slime
bodies, plastids, and disintegrating nuclei; i and 8, younger; 3 and 9, older sieve tubes and
companion cells from secondary phloem; 4, young sieve tube and companion cell from pri-
mary phloem ; 5 and 6, portions of mature sieve tubes showing primary lamellae and second-
ary thickenings on sieve plate, slime accumulations, and plastids; 7 and 11, definitive callus
on sieve plates, plastids, and cytoplasmic network of old sieve tubes; 10, sieve tube and com-
panion cell in early stage of obliteration and two crushed cells; 11, crushed sieve tube and
companion cell; 13 and 14, two stages in obliteration; cai^csillusfcc, companion cell; nuc,
nucleus; 0^/, obliterated cells; /»/, plastid; j/, slime bodies; j^, sieve tube. (After Esau.)

i.-jG

THE STRUCTURE OF ECONOMIC PLANTS

The companion cells are frequently divided transversely, espe-
cially in the secondary phloem, so that there may be two to a sieve
tube. Another variation occurs when the mother cell divides more
than once longitudinally, forming two companion cells and one
sieve tube. The former may divide transversely so that four com-
panion cells abut one sieve element. Chloroplasts occur in the
cytoplasm which is vacuolate. The phloem parenchyma is not

specialized; and frequently exhibits
meristematic potentialities in general
cell division and in cambium forma-
tion, as it does in the root and stem.
Floral Development. — The gen-
eral habit of the floral axis and the
inflorescences has been described.
(Fig. ii8.) The sessile flowers which
occur either singly or more com-
monly in clusters of two or three
are small and greenish in color.
(Fig. 137, C) They are perfect and
have a five-parted calyx, five stamens,
and a pistil consisting of three car-
pels. The calyx is incurved, adherent
to the base of the ovary, and be-

^, single^mature flower ^^^^^ j^^^j -^^ ^.J^^ £j.^ij. -pj^^ ^^^_

mens are inserted at the base of the
calyx lobes and opposite them.
They are introrse, bilocular, and
with young embryo; F, flower with dehisce by means of longitudinal

mature embryo : g/.^^X glandular disc. ,■ Thp nkfil n<;nallv rnnsisrs nf

(After Artschwager, >«r. ^gr. R^j.) ^^^^^- ^^^ V^^^^^ USUally COnslStS Ot

three carpels, but occasionally there
may be two, four, or five. The half-inferior ovary is sunken in
a fleshy disk, and the very short style is terminated by two to
five awl-shaped stigmas. A single campylotropous ovule is at-
tached laterally to the ovary wall by a short funiculus.

Payer (14) studied the floral development of Beta maritima and
the development in the sugar beet as reported by Artschwager (i)
agrees with it in all essential details.

"The rudiments of the young flowers appear close behind the growing
points of branches of the inflorescence axis. Each rudiment is borne
in the axil of a large bract and is accompanied by two lateral bracts

sligma

Fig. 137

viewed from above ; B, flower with
stamens already dropped ; C, cluster
of flowers attached to branch of in-
florescence axis and subtended by bract ;
D, flower after fertilization ; £, flower

BETA VULGARIS 177

each of which has in its axil a flower. The flower primordium forms
a small protuberance on which are soon differentiated five narrow
ridges, the beginnings of the sepals. Next there appears within this
circle a whorl of papillae, the stamen rudiments, each opposite a
calyx lobe. Finally, three more ridges develop at first separately;
but later joining to inclose a more or less compressed roundish cavity
in which a single ovule develops.

"Once initiated, the primordia of the different floral parts develop
very rapidly. The young stamens become stalked early and attain
in cross section the characteristic quadrilocular form. The anther is
at first a homogeneous mass of tissue covered by an epidermis. As
soon as it becomes faintly 4-lobed in cross section the cells next to
the epidermis begin to elongate radially and divide periclinally, forming
two layers of narrow cells which by their size and staining reaction
appear distinctly set off from the central tissue which forms the arche-
sporium. These two layers of cells together with the epidermis
constitute the anther wall. The cells of the middle layer, in their
subsequent development, enlarge rapidly and finally develop thicken-
ing bands. This constitutes the endothecium and forms a continuous
mantle except in the region where the anther later dehisces. . . .
The cells remain small and as the anther enlarges they become stretched
tangentially, but they do not become obliterated before the endothecium
is fully developed and the pollen practically mature.

"The ovule arises near the base of the carpels. It is seen first as a
slight protuberance which enlarges to form the nucellus. In its sub-
sequent development the young ovule becomes distinct from the surface
of the carpel by a stalklike base or funiculus. Very early there appears
at the base of the nucellus an annular outgrowth followed soon by a
second one of a similar nature. The first outgrowth develops into
the inner, the second into the outer integument. The young ovule
is straight, but with the appearance of the integuments it curves and
becomes campylotropous. Finally, the ovule twists about its own
funiculus and comes to lie horizontally in the cavity of the ovary."

At the base of the receptacle, the vascular traces of the floral
organs form a short stele which is a somewhat irregular central
core rather than a dissected siphonostele. Five distinct groups of
vascular units diverge in the broader portion of the receptacle
occupying a peripheral position in the cortical zone, and the re-
mainder of the central core forms a single bundle which is con-
tinuous with the vascular supply to the ovule. (Fig. 138, A.^
The vascular strands which diverge from the cortical region pass
out into the five calyx lobes, and each of these bundles later gives
off branches to form the lateral traces of the lobes. (Fig- 138, C'.)
The pistil, which consists normally of three undiverged carpels,

i78 THE STRUCTURE OF ECONOMIC PLANTS

occupies the central portion of the flower; and, although carpels
are commonly supplied with three bundles (a central bundle and
two marginal ones), in this instance the marginal bundles have
been suppressed, and each carpel is supplied by a single vascular
strand. The carpellary bundles are derived from the cortical

bundles of the receptacle,
and are so differentiated
that two of them constitute
direct branches of two of
the cortical bundles which
lie opposite them while a
third carpellary bundle is
the product of a fusion of
two vascular strands de-
rived from adjacent cor-
tical bundles. In some
instances, these two bundles
remain distinct so that the
upper part of the pistil may
contain four bundles in-
stead of three. After the
divergence of the vascular
strands to the sepals and

Fig. 1^8. Diagrammatic series of sections through i r u • i

flower to illustrate origin and course of vascular Catpels from the COttlCal

supply; A, section through base of receptacle bundles, the distal SeCtionS
showing five peripheral bundles and sixth one (. 0 . ^^^ stamenS,

in close proximity to base of ovary cavity ; B, sec- r

tion taken at slightly higher level, bundle Qo 0 each being supplied by a

forms vascular supply of single ovule (W); C, sec- g-^jg bundle. (FlZ. 1^8,
tion at level where sepals are differentiating and & ■ \ b J '

peripheral bundles are branching. Four strands D.)

designated (<r 0 form vascular supply of three MiCROSPOROGENESIS. —
carpels; D, section through upper part of flower ■ r ai

showing distinct sepals dO- The filaments of MlCrOSporOgeneSlS follOWS

stamens are still part of glandular disk region Qgl dt). ^J^g general Sequence that
Each stamen is supplied with a single trace (st 0- • ^ „ ^„„;^

(After Artschwager.V- ^^r R...) OCCUtS in many anglO-

sperms, although Matthy-
sen (ii) has noted some differences in the early prophases; and
the sporogenous tissue of the young anther develops rapidly to the
mother cell stage. Each pollen grain contains nine chromosomes,
which is the haploid number reported for the genus and for many
other members of the Chenopodiaceae. Matthysen reports find-
ing eight chromosomes for the cultivated garden beet; but this

BETA VULGARIS X79

is the only notable exception to the findings of Winge (xi) and
Heel (9).

Since microsporogenesis proceeds at a more rapid rate than mega-
sporogenesis in any given flower, it is probable that the individual
flowers of a cluster are not ordinarily self-pollinated. The mature
pollen grain has a thick, sculptured outer wall with many thin
spots or germ pores, one of which serves as the point of emergence
of the pollen tube. The division of the microspore nucleus to
form the tube nucleus and the two microgametes occurs several
days before the liberation of the pollen. Prior to dehiscence, there
are also small irregular starch grains in the microspore which
disappear before the pollen is shed. There is a considerable amount
of pollen degeneration, but Artschwager and Starrett (3) find that,
in most cases, more than one pollen tube reaches the megagameto-
phyte. These grow down the stylar canal, and fertilization may
be eff"ected in lo hours or less.

Megasporogenesis. — In megasporogenesis, the archesporium
is formed from a single hypodermal cell of the nucellus; and sub-
sequent periclinal divisions of epidermal cells result in the embed-
ding of the megaspore mother cell more deeply in the nucellar
tissue. The meiotic divisions produce a linear tetrad of mega-
spores, the outer three of which degenerate while the inner one is
functional. The orientation of the megagametophyte is at first
straight; but changes in the position and content of the nucellar
cells, together with their unequal growth rate, result in curvature;
and further divisions of the nucellar cells produce the tissue which
ultimately forms the perisperm of the mature seed. (Fig. iio.)

The development of the megagametophyte follows the usual
program. There are three successive equational divisions of the
megaspore nucleus producing eight free nuclei. Of these, the egg,
or megagamete, and two synergids are located toward the micro-
pyle; the two polar nuclei unite to form the primary endosperm
nucleus, which is usually centrally located; and the three antipodal
cells are at the chalazal end. There may be more than three an-
tipodals which persist for several days after fertilization and then
disintegrate.

Embryogeny. — At the time of fertilization, the ovule is small;
but later it grows rapidly and attains mature size before the embryo
within is fully developed. During this period^of growth, the calyx
also enlarges and its lobes curve back to the position held at anthe-

2.80 THE STRUCTURE OF ECONOMIC PLANTS

sis. The zygote undergoes a period of rest, then divides trans-
versely, and the two daughter cells again divide in the same plane
so that a linear tetrad of cells is formed. Of these, the basal one
gives rise to the suspensor and the other three produce the embryo
proper. Artschwager and Starrett (3) have described the subse-
quent stages in the embryogeny and point out that "after the forma-
tion of the quadrant, no strict law governs the sequence of cell
division"; although "a certain balance seems to be maintained so
that the final product is remarkably uniform."

Fig. 139. A-0, series of diagrammatic drawings showing sequence of cell division in devel-
opment of young embryo. Cell a represents apical cell of tetrad and cell d basal one. (Re-
drawn after Artschwager and Starrett, Jour. Agr. Res.')

In most cases, each cell of the tetrad divides once to form an
eight-celled embryo. Cell d usually divides first, followed by cell
c, or they may divide simultaneously; and this is also true of cells
a and b. (Fig. 139) Cell a always divides vertically; but b may
divide either vertically or transversely, and this is the case with cell
c, so that the octant stage may be five, six, or seven tiered. Sub-
sequent divisions produce the i6-celled embryo which, like the
octant, varies in its number of tiers depending upon the plane of
the division walls. At this time, the embryo begins to be club-
shaped; and, in about five days, becomes globular. The cotyle-
dons are differentiated approximately two days later and the
embryo is usually mature by the twelfth to fourteenth day.

Concurrent with the development of the young embryo, there is
free nuclear division of the endosperm nuclei which occupy a

BETA VULGARIS z8i

peripheral position in the embryo sac. Wall formation follows,
and, finally, the entire sac is filled with storage cells which are
later resorbed by the growing embryo except for a single layer
surrounding the hypocotyledonary portion of the embryonic axis.
This persistent layer consists of small, very regular cells with dense
contents. At the time that the curved embryo sac has reached
its final form and size, the peripheral layer of nucellar tissue is four
to six cells in thickness; but as the embryo enlarges, the inner pe-
ripheral layers are gradually resorbed. The remaining cells of the
perisperm ultimately become packed with large starch grains except
in the chalazal region.

LITERATURE CITED

I. Artschwager, E., "Anatomy of the vegetative organs of the sugar beet."

Jour. Agr. Res. )^: 143-176, 192.6.
1. , "Development of flowers and seed in the sugar beet." Jour. Agr. Res.

54- 1-2-5 > 192-7-

., and Starrett, Ruth C, "The time factor in fertilization and embryo

5

development in the sugar beet." Jour. Agr. Res. 47: 82.3-843, 1933.
Bailey, L. H., The Standard Cyclopedia of Horticulture, 1-468, The Macmillaii

Co., 1914.
DE Bary, a., a Comparative Anatomy of the Vegetative Organs of the Phanerogams
and Ferns, Clarendon Press, Oxford, 1884.

6. Bennett, C. W., and Esau, Katherine, "Further studies on the relation

of the curly top virus to plant tissues." Jour. Agr. Res. sh' 595-610, 1936.

7. Esau, Katherine, "Ontogeny of phloem in the sugar beet (Beta vulgaris L.\"

Am. Jour. Bot. 21: 631-644, 1934.

8. Fron, M. G., "Recherches anatomiques sur la racine et la tige des Cheno-

podiacees." Ann. Sci. Nat., Bot. Ser. VIII, p.- 157-140, 1899.

9. van Heel, J. P. D., "Onderzoekringen over de ontwikkeling van de anthere,

van den zaadknop en van het zaad bij Beta vulgaris L. Naarden." (Thesis,
Delft}, 1915.

10. Janczewski, E., "Recherches sur I'accroissement terminal des racines dans

les Phanerogames." Ann. Sci. Nat., Bot. Ser. V, 20: 161-101, 1874.

11. Lyle, Elizabeth A., "The structure and development of the seedling of Beta

vulgaris L." Unpub. M.S. Thesis, Univ. of Chicago, 1937.
II. Matthysen, J. O., "Cytologische und anatomische untersuchungen an Beta
vulgaris, nebst einigen Bemerkungen iiber die enzyme dieser pflanze."
Ztschr. Ver. Deut. Zuckerindus. (n. F. 49) 62Q2): 137-15 1, 1911.

13. OvERPECK, J. C, "Methods of seed production from sugar beets overwintered

in the field." U. S. Dept. Agr. Cir. 7/5, 1931.

14. Payer, J. B., Traite d' organogenic comparee de la fleur. Text and atlas, 2 v.

Paris, 1857. ^

15. Plaut, M., "Epiblem, Hypodermis und Endodermis der Zuckerriibe."

Kaiser-Wilhelm Inst, fiir Landw. Bromberg. j.- 63-68, 1910.

i8i THE STRUCTURE OF ECONOMIC PLANTS

i6. RiJGGEBERG, H., "Beitfage zur anatomic der Zuckerrube." Kaiser-Wilhelm
Inst, fiir Landw. Bromberg. 4: 399-415, i^i'i-

17. Seeliger, R., "Untersuchungen iiber das Dickenwachstum der Zuckerrube."

Arb. Biol. Keichsanst. fiir Land- und Forstw. 10: 149-194, 19x1.

18. Thomas, Ethel, "A theory of the double leaf-trace founded on seedling

structure." Nw Phytol. 6: 77-91, 1907.

19. VAN TiEGHEM, P., "Rechetches sur la symetrie de structure des plantes vascu-

laires." Ann. Sci. Nat., Bot. Ser. V, i^: 1-314, 1870-1871.
■Lo. Weaver, J. E., Root Development of Field Crops, McGraw-Hill Book Co., 1916.
II. Wilson, C. L., "Medullary bundle in relation to primary vascular system in

Chenopodiaceae and Amaranthaceae." Bot. Gax_. j8: 175-199, 1914.
Ti.. WiNGE, O., "The chromosomes. Their numbers and general importance."

Compt. Rend. Lab. Carlsberg, 13: 13 1-175 , ^9^7-

J

CHAPTER X

CRUCIFERAE

RAPHANUS SATIVUS

THE mustard family is large and its members widely distributed.
It includes several species of economic importance and numer-
ous common weeds. These are chiefly herbaceous with a pungent
watery juice, simple alternate leaves that are variously lobed and
dissected, and flowers that are regular, perfect, and cruciform. The
fruit is a pod-like structure Icnown as a silique which is usually
dehiscent. Among the more widely cultivated crucifers are radish,
Raphanus sativus L.; cabbage, Brassica oleracea, var. capitata L.;
cauliflower, B. oleracea, var. botrytis L.; Brussels sprouts, B.
oleracea, var. gemmifera, Zenk.; turnip, B. rapa, L.; mustard, B.
nigra (L.) Koch., and B. alba (L.) Boiss.; rape, B. Napus L.; and
horseradish, Radicula Armoracia (L.) Robinson.

GENERAL MORPHOLOGY

The radish is extensively cultivated as a garden crop and grows
as an herbaceous annual or a true biennial in northern climates.
It normally produces a fleshy tap root and a rosette of leaves the
first year; but, when planted early, may form the flower and fruit
the same season. (Fig. 140.) Radishes may be classified on the
basis of their time of maturation; and, with respect to seasonal
development, there are early or forcing varieties, summer, and win-
ter types. The early varieties are the most commonly planted and
are often grown in hot beds and greenhouses, frequently reaching
marketable size in from 10 to 30 days. The quickly maturing
types, including the Scarlet Globe variety upon which the present
studies are largely based, remain tender and-succulent for a short
time only. Summer radishes require six to eight weeks to mature
and are larger than the early types. The winter varieties, which

^83

i84 THE STRUCTURE OF ECONOMIC PLANTS

need several months to reach maturity and often attain a large size,
have a compact, firm flesh and can be successfully stored for some
time.

The Root. — The root and hypocotyl constitute the succulent
portion of the plant which is generally eaten in the fresh state.
This fleshy axis is variable in size, shape, and color depending upon
the variety. The shape may be spherical, bluntly cylindrical, or

Fig. 140. Radish plant in seed. (Courtesy Ferry-Morse Seed Co.)

conical and much elongated; and the color ranges from cream-
white, pink, red, and variously mottled combinations to gray or
even black.

Weaver and Bruner (lo) have pointed out that the development
of the root system is of an entirely different character in the round
or turnip-shaped varieties than in the half-long or long types. The
former are characterized by a rapidly growing tap root which may
penetrate the soil to a depth of x to 3 feet with a lateral spread of
IX to 16 inches. Most of the absorbing area lies in the upper 2. to
8 inches of surface soil; and, even in fully matured plants, only the
portion of the tap root occupying the first x to ix inches of surface

RAPHANUS SATIVUS z8^

soil produces large laterals. These may extend horizontally to
distances of 3 feet or more on all sides of the plant, branching pro-
fusely and forming a network of rootlets. In the Long Scarlet
type, by the time the upper portion of the root has reached market-
able size, it may have penetrated to depths of 3 feet with much-
branched laterals which spread radially to distances of 2. feet. The
mature plants are characterized by tap roots 43^2 to 5 feet long with
many major laterals in the surface foot of soil which frequently
extend outward to a maximum of 40 inches. They in turn may
give rise to branches which penetrate the soil deeply; and, as a
result, the surface soil is well filled with roots while the subsoil
also contains numerous branched laterals.

The Hypocotyl. — The upper portion of the fleshy primary axis
is practically devoid of lateral roots, consisting of a thickened
hypocotyl which is transitional in its vascular organization. The
relative proportion of the lower part of the axis which can be re-
garded as the root is greater in the case of the half-long or long
icicle varieties than in the globe types. Golinska (6) interprets
the fleshy portion as a transition region and regards it as being
more root-like than stem-like. (Fig. 141.) There is a reorienta-
tion of the vascular elements in the upper limits of the hypocotyl,
but the endarch relationship is not attained at the cotyledonary
node, and exarch strands extend into the cotyledons as in the
potato.

Although it has been demonstrated genetically by Malinowski
(10), Moldenhawer (li), and Sutton (19) that the development
of the fleshy axis is a hereditary characteristic, external factors,
especially length of day, have a marked influence upon the degree
of its development. Sinskaja (17) found that varieties brought
from China, Mongolia, Japan, and India produced no thickenings
and bloomed the first year when grown in greenhouses in Lenin-
grad; but when the photoperiod was reduced to an interval of 7
to li hours, they became fleshy and bloomed much later or not at
all. Garner and Allard (5) obtained fleshy root-hypocotyl axes
with a 7-hour day, and with a ii-hour day were able to produce
seeds in the winter months.

The Stem. — There are two phases in the development of the
stem. The first occupies the initial 4 to 6 weeks after germination
when a rosette of leaves develops from a short crown stem. The
second period is that during which the flower stalk is formed fol-

i86 THE STRUCTURE OF ECONOMIC PLANTS

lowing the vegetative period. The axis elongates at this time to
produce an erect, much-branched stem which may attain a height
of I to 3 or more feet. It is usually somewhat glaucous, sometimes
sparsely pubescent with a few stiff epidermal hairs on the lower
portions of the stem system; and, in rare instances, is essentially
glabrous throughout. (Fig. 141, A.')

Fig. 141. Development of fleshy axis of Raphanus in icicle, i, half long, 2., and globe
types, 3-5, showing region that becomes fleshy and origin of secondary roots; 6, abnormal
thickening of axis. (After Golinska, Die Gartenbauwissenschajt.')

The Leaves. — The simple petiolate leaves are glabrous or
sparingly pubescent and are spirally arranged in a % phyllotaxy.
They are variable in shape, the smaller upper ones being more or
less oblong while the lower ones are deeply lyrate-pinnatifid.

The Inflorescence and Flower. — The inflorescence is an
elongated raceme which is ordinarily devoid of bracts, and the
white or rose-lilac flowers are borne on slender pedicels. (Fig.
142., y4.) The pentacyclic flower is hypogynous. It consists of a
calyx of four sepals, a corolla with a similar number of petals, six
stamens in two cycles, and a pistil that has been interpreted as con-
sisting of four carpels by some investigators and of two by others.
(Fig. 143, ^.)

The four sepals are paired. The outer median pair slightly

RAPHANUS SATIVUS

2-87

overlap the inner pair in the bud stage, and are somewhat more
beaked at their apices, which commonly bear a few prominent
hairs. The four petals are alternately arranged with respect to the
sepals and their spreading limbs form a cross. The stamens are
arranged in two cycles, the outer consisting of two functional sta-
mens which are shorter than the inner ones, and two rudimentary
or vestigial stamens which develop as small knob-like, or slightly

Fig. 142.. A, habit of inflorescence ; B, single flower; C, same, in partial face view; D,
petal ; E, stamens and pistil with perianth removed ; F, receptacle with perianth and two
of inner stamens removed showing one rudimentary outer stamen and location of nectaries.

z88

THE STRUCTURE OF ECONOMIC PLANTS

elongated, protuberances. The members of this whorl lie alternate
to the petals and opposite the sepals. Adaxial to and at the bases

--ax

m se----^

Fig. 143. Floral development : /4, semi-diagrammatic transection of young flower show-
ing its relation to axis and arrangement of floral parts ; B, longisection of terminal portion
of axis of inflorescence showing origin of flower primordia ; C-H, longisections of stages in
development of flower ; D and E, sections which pass through median or outer pair of sepals
and include segments of inner pair (G and H are oriented in plane parallel to septum of ovary) ;
/, longisection of ovary at right angles to septum (points of divergence of ovules are not
shown in this plane) : ax, axis ; car, carpel ; fl p, flower primordium ; i se, inner sepal ; m se,
one of the median or outer sepals ; o j?, outer sepal ; w, ovule; /)?^ petal ; ;• .fr<», rudimentary
outer stamen; se, sepal; sep, septum; sta, stamen; sti, stigma.

RAPHANUS SATIVUS zS^

of the two functional stamens are the nectaries. (Fig. 142., F.)
There are four equal stamens in the inner whorl which lie opposite
the petals. The anthers of the functional stamens are two-loculed
at maturity and longitudinally dehiscent.

The pistil has been interpreted as consisting of four carpels by
Eames and Wilson Ql, 3). Based upon the character of the vascular
supply of the ovary, they state that

"it appears that there are four carpels in the Cruciferae instead of two,
the generally accepted number. Two of these carpels are valve-like
and sterile and are placed below two other carpels, nearly enclosing
them. This second set of carpels, the fertile or 'solid' carpels, is
reduced; the loculus has disappeared, and the ovules borne by these
carpels have been forced out of the loculus and lie in the loculus of
the valve, or sterile, carpels. This has taken place phylogenetically,
not ontogenetically."

Saunders (15, 16) also describes the gynoecium of the crucifers as
consisting of four carpels and presents the concept of carpel poly-
morphism to explain the occurrence of different combinations of
various carpel types, stating that "when the median carpels become
solid the characteristic form is a siliqua, when they become semi-
solid the result is a silicula."

A somewhat different interpretation of the relationship of the
floral parts is presented by Eichler (4), and this has been accepted
at least in part by Sachs (14) and Strasburger (18). In the Eichler
system, the typical cruciferous flower is described as consisting of
two lower median sepals, two upper lateral sepals, four diagonal
petals in one whorl, two lower lateral stamens, two upper median
stamens, and two lateral carpels. The cycle of apparently four
inner stamens is regarded as consisting in reality of but two sta-
mens, each of which is branched at its base. This concept of the
dimerous symmetry of the flowers of the Cruciferae was carried a
step further by some of the early botanists who regarded the four
petals as being derived from a single pair by a lateral doubling or
branching, in a manner similar to that described for the inner whorl
of stamens.

ANATOMY

Ontogeny of the Flower. — In the ontogeny of the flower, the
origin of the floral parts does not occur in acropetal succession;
and like Capsella, as described by Coulter and Chamberlain (i), the
petals are the last floral organs to be differentiated. The primor-

X90 THE STRUCTURE OF ECONOMIC PLANTS

dium of the flower is diverged from the main axis of the inflores-
cence as a conical growing point, and shortly thereafter the primor-
dia of the sepals are differentiated.^ (Fig- 143? -6.) The growth
of the sepal primordia is differential; and the median pair, which
become the outer sepals, grow more rapidly than the lateral ones
partly enclosing and overarching them. (Fig. 143, D.) The
staminal primordia are next differentiated, followed shortly by the
carpels, and lastly by the petals. (Fig. 143, E, F.')

As the carpels develop, a partition is formed which has been
termed the dissepiment or septum. In structure this is somewhat
variable. At the base of the ovary, it may be thick and solid,
while above, it is frequently hollow or double, consisting of two
thin walls between which there may be an empty space or some
spongy parenchyma. It is thought by some to originate as a
secondary placental outgrowth; while others, including Eames
and Wilson (x), regard the septum as "an expansion of the ventral
margin of the folded solid carpels."

Fruit and Seed. — The mature fruit is an elongated, conical
pod or silique, i to 3 inches in length. It has an extended beak
which may equal or exceed the pod itself; and, unlike the majority
of cruciferous fruits, is indehiscent. It produces two or three, or
sometimes several, seeds which are partially embedded in the
spongy tissue of the placentae and septum of the ovary. The ovules
are campylotropous, the micropyle being directed toward the
stylar end of the ovary. (Fig. 143, H.) The mature embryo is
curved and occupies the entire space within the seed coat except for
a very small amount of endosperm. The cotyledons are condupli-
cate, or folded upon themselves and around the embryonic axis,
with the outer, larger cotyledon clasping the inner one. (Fig.

145)
Winton (ii) has described several cruciferous seeds, and Kondo

(9) has investigated that of the radish in detail. Both agree that

the seed coat normally consists of four layers, but Winton states

that "in many species the first and second layers at maturity are

not distinctly cellular. ' ' Kondo's work included about 30 varieties

which he divided into three form groups: (i) heart-shaped, (x)

oval or egg-shaped, and (3) long-oval. (Fig. 144, A, B, C) Type

* The author is indebted to Mr. Charles H. Quibell, who kindly permitted the use of his
materials in the preparation of this section and the accompanying figures on floral develop-
ment.

RAPHANUS SATIVUS

xgi

two is the most common, including the Scarlet Globe and many
other leading varieties grown in the United States. The color may
be yellow, golden brown, cinnamon, russet, or walnut brown.
There is also considerable variation in size, the dimensional range
being 3.6 to 4.1 mm. in length, x.y to 3.3 mm. in breadth, and x.i
to 1.3 mm. in thickness.

Fig. 144. A-C, three common forms of seed ; A, heart-shaped ; B, oval or egg-shaped ; C,
long-oval ; D, transection of seed coat and endosperm showing principal layers ; E, surface
view of cells of same: i, epidermis; i, outer parenchyma; 3, palisade cells; 4, pigment
layer; 5, endosperm; and 6, hyaline layer. (Redrawn and rearranged after Kondo, Ohara
Institute.)

The outer surface of the seed coat is pitted, and the polygonal
reticulations are five or six-sided, varying in degree of prominence
and size depending upon the variety. The epidermal layer consists
of tabular cells which have thin or sometimes thick, lamellate
walls. Underlying them is a subepidermal parenchymatous zone
comprised of a series of tangentially oriented cells that are strongly
compressed and have thin golden-brown walls. The third zone
is made up of palisade cells which form the most conspicuous layer
of the seed coat owing to the fact that the inner walls, or at least
the inner portion of the lateral walls, are more or less strongly

i9i THE STRUCTURE OF ECONOMIC PLANTS

thickened. They are gold or brown and appear reticulate and
sharply polygonal in surface view. (Fig. 144, D, £.)

The fourth or pigmented zone is one or more cell layers in
thickness and contains the coloring matter of the coat. It con-
sists of much-compressed cells with extremely thin walls, which
in some varieties are small, but in others, including the Scarlet
Globe, are large and polygonal with an abundant dark-brown
pigment.

The remaining layers have been interpreted by some investigators
as a part of the inner integument, but Winton and Kondo have
demonstrated that they are portions of the endosperm. The aleu-
rone cells of this zone are somewhat similar to those found in the
outer portion of the endosperm of cereals. This layer is usually one
cell in thickness except in the region of the micropyle, where there
may be two or more layers, and adjacent to the hilum where it may
be entirely absent. The remainder of the endosperm consists of a
hyaline zone made up of several layers of tangentially stretched
and much crushed parenchymatous tissue.

Development of the Seedling. — The germination of the seed
is rapid. Seedlings have been observed with from six to eight
laterals on the upper inch of the tap root within four days of plant-
ing and before the cotyledons are completely unfolded. At the
end of nine days, when the first foliage leaves appear, secondary
laterals have been formed. The primary root pushes through the
seed coat near the micropyle and elongates rapidly, becoming
slightly arched at its upper limits. In the early stages of develop-
ment, elongation of the hypocotyl and root proceeds much more
quickly than in the epicotyl. The growth and straightening of
the hypocotyl lifts the conduplicate cotyledons above ground, and
they soon expand to form the first photosynthetic organs of the
seedling. (Fig. 145.)

Ontogeny of the Root. — The primary root has a diarch pro-
tostele. The cells of the two protoxylem points are differentiated
centripetally, and with the larger metaxylem vessels toward and at
the center, form the complete primary xylem strand. The two
groups of primary phloem alternate with the points of protoxylem
and are separated from the xylem by a zone of fundamental paren-
chyma. The pericycle consists of a single layer of cells lying imme-
diately within the endodermis in which Casparian thickenings are
evident by the time the primary xylem is completely matured.

RAPHANUS SATIVUS

2-93

(Fig. 147, /4.) The cortex is comprised of a few layers of large
parenchymatous cells, and the epidermal cells are also thin-walled,
many of them becoming root hairs.

The development of the primary root corresponds to type three
as described by Janczewski (8). The terminal meristem of the root
is differentiated into three histogens, the calyptrogen-dermatogen,
the periblem, and the ple-
rome, each consisting of a
single layer of cells. (Fig.
146.) In the ontogeny of
the axis, the root cap and
epidermis arise from the
calyptrogen-dermatogen
layer, the cortical cells
from the periblem, and
the stelar tissue from the
plerome.

The root cap consists
of several layers of cells
which form a series of
overlapping cones sur-
rounding the meristematic
region of the root tip.
As the calyptrogen cuts
off successive cell layers
to form the root cap, the
distal or marginal cells of
the conical histogen cease
periclinal division and,
by continued anticlinal
divisions, form the single
layer of epidermal cells.
This method of differen-
tiation results in an epidermis with a stair-step arrangement in
which each step of the epidermis abuts one layer of the root cap
cells, rather than an epidermis with a continuous smooth surface.
The steps become progressively shorter toward the apex of the
root and finally end in one which is a single cell in height, touching
the last-formed layer of the root cap. In consequence, the number
of steps corresponds to the number of layers of the calyptra, except

,M^e^

Fig. 145.

Srages in development of seedling, variety
Red Globe.

Z94 THE STRUCTURE OF ECONOMIC PLANTS

where subsequent periclinal divisions of the cells of the calyptra
may have occurred. (Fig. 146.)

The cortex originates from the periblem and is composed of
large parenchymatous cells whose radial dimensions at first much
exceed the axial ones. They decrease in size toward the stele and
are limited inwardly by the endodermal layer. This is formed by a

Fig. 146. Median longisection of root apex showing histogens at growing point : cal-
dgn, calyptrogen-dermatogen ; co, cortex ; en, endodermis ; ep, epidermis ; pbm, periblem ;
/>c/, pericycle; />/, plerome; rr, root cap; j//, stele. (After Grassley.)

final periclinal division of the innermost row of cortical cells which
takes place at about the level where the epidermis is no longer
enclosed by the root cap. The plerome produces the cells of the
stele which are at first slightly and later greatly elongated. The
median cell of the plerome gives rise to the metaxylem elements;
while the protoxylem, phloem, fundamental parenchyma, and
pericyclic cells are derived from the lateral ones.

As the maturation of the cells derived from the three histogens

RAPHANUS SATIVUS 195

proceeds, the diarch, radial protostele, cortex and epidermis may be
clearly distinguished approximately one millimeter from the tip of
the root. At this level, the epidermal cells are nearly isodiametric
and the cortical parenchyma consists of thin-walled cells which
have formed many intercellular spaces. The cells of the endoder-
mis are small and compactly arranged, with no intercellular spaces.
Before there is any differentiation of the secondary cell walls of
the primary xylem, the centrally located metaxylem vessels can be
distinguished by their larger caliber, as well as the elongated
parenchymatous elements of the protophloem.

The protoxylem develops centripetally and the first elements
differentiate from the procambial cells abutting the pericycle as
spiral vessels. The metaxylem vessels are scalariform or reticulate,
some of the latter approximating the pitted type. The primary
xylem and phloem are separated by a zone of parenchymatous
cells from which the cambium is later differentiated. The pericycle
is at first a uniseriate layer; but very early in ontogeny becomes
multi-layered as a result of periclinal divisions. At this stage,
the epidermal cells elongate slightly in their axial dimension, the
cortical cells enlarge in all three dimensions, keeping pace with
the increase in the size of the stele, and the cells of the endoder-
mis develop well-defined Casparian strips on their radial and end
walls.

Lateral Roots. — The initiation of lateral roots is coincident
with the maturation of the large, reticulate, metaxylem elements.
Their point of origin is definitely related to the diarch structure
of the stele since the root primordia arise in the pericyclic region
outside the protoxylem points or in some cases slightly tangent to
them. This results in the formation of two rows of lateral roots
which appear to arise from longitudinal furrows. These are not
present at the initiation of lateral root formation, but result from
the less rapid development of the tissues in the plane of the second-
ary roots. The number of secondary roots may be large, depending
upon cultural conditions, but they are limited for the most part to
the lower portion of the fleshy tap root, very few being formed in
the upper hypocotyl. The structure of the lateral root resembles
that of the non-fleshy portion of the primary root.

The Hypocotyl. — Except for a zone x or 3 mm. in length, the
hypocotyl elongates rapidly in the young seedling owing to con-
tinued division and enlargement of the cells. There is little struc-

i96 THE STRUCTURE OF ECONOMIC PLANTS

tural difference between the middle and lower hypocotyl, but the
basal portion of the latter resembles the primary root except that
the epidermal cells produce no root hairs and their outer walls are
slightly cutinized. In addition, two or three layers of subepider-
mal cortical cells may exhibit some cutinization. The point at
which these differences appear has been termed the collet.

Vascular Transition. — The first transitional changes occur
in the lower hypocotyl where the groups of phloem cells are
extended circumferentially to form two crescent-shaped sectors.
In the middle hypocotyl, there is a separation of each phloem sector
into two groups. Coincident with this change in the position of
the primary phloem, the metaxylem is differentiated in a lateral
direction in relation to the spiral protoxylem vessels rather than
in a centripetal one. Thus two broad wedges of scalariform and
reticulate metaxylem elements are formed extending from each
protoxylem strand, while the central region of the axis consists of
undifferentiated pith parenchyma. (Fig. 147, B, C)

Grassley (7) has described the hypocotyledonary transition in
detail.

"The change from a root structure to stem structure occurs in the
upper hypocotyl. The first indication of such a change is in the forma-
tion of a central pith which separates the differentiating primary xylem
into two distinct units. The primary phloem groups at this level form
two broad arcs in the inter-cotyledonary plane, and at a slightly higher
level these divide into two tangentially oriented groups, one on either
side of the bifurcated xylem units. Each xylem unit and the two groups
of phloem adjacent to it constitute a cotyledonary trace. (Fig. 148,
C, D.)

"Procambial strands of the foliar traces of the first two foliage leaves
appear in the intercotyledonary plane between the vascular elements
of the cotyledonary traces. (Fig. 147, C, L-i, L-z.) The vascular tissue
of the hypocotyl at this level forms a siphonostele made up of two coty-
ledonary traces and two leaf traces of the foliage leaves. Just above this
point, lateral veins are diverged from either side of the cotyledonary
traces. These with a second set of branches which diverge from the
midrib of the cotyledonary petioles at a higher level form the main
veins of the cotyledons.

' 'The branches of the cotyledonary traces which diverge outwardly
from the midrib above the base of the cotyledons are endarch bundles.
Complete transition to the endarch collateral arrangement does not
occur in the upper hypocotyl. The arrangement of the vascular tissues
at the base of the cotyledonary petioles is the same as in the upper hypo-
cotyl. (Fig. 147, D.) At a higher level in the petiole, the protoxylem

RAPHANUS SATIVUS

2-97

elements of the midrib are oriented more in the adaxial direction and
the metaxylem differentiates more in the abaxial direction so that the
endarch relationship is attained."

The Cotyledons. — There are five endarch collateral bundles in
the elongated petiole which enter the notched obovate blade. The

L-2

Fig. 147. Stages in vascular transition : A, transection of stelar portion of primary root;
B, lower hypocotyl, transection of stele showing lateral orientation of metaxylem, and cen-
tral parenchyma; C, upper hypocotyl, transection of portion of stele showing laterally
oriented metaxylem, and traces of first and second foliage leaves ; D, transection of cotyle-
donary bundle ; E, higher in cotyledonary petiole, showing lateral placement of metaxylem :
ca, cambium; en, endodermis; L-i and L-i, leaf traces of first and second foliage leaves;
mx, metaxylem; pel, pericycle; ph, phloem; pi, pith; px, protoxylem; xy r, secondary
xylem. (After Grassley.)

X98

THE STRUCTURE OF ECONOMIC PLANTS

mesophyll consists of three to five rows of palisade cells and the
spongy parenchyma occupies a zone as thick as the palisade region,
being made up of cells that are compactly arranged, so that the
intercellular spaces are very small. Stomata are more abundant on
the lower than on the upper surface. (Fig. 154, F.)

Secondary Thickening of Primary Root and Hypocotyl. —
Secondary thickening begins early in ontogeny; and approximately
at the time that the first foliage leaf appears, the hypocotyl and

upper portion of the root
begin to enlarge. This in-
crease results from the growth
of the stelar portion of the
axis, since the cortical and
epidermal cells do not divide
or enlarge to any great extent
after the primary tissues of
the stele are completely differ-
entiated. As a result, there
is a rupturing of the epidermis
and cortex, forming two lon-
gitudinal splits that arise in
the transitional region of the

Fig. 148. A-D, diagrams showing stages in hypOCOtyl below the COty-
vascular transition : A, at root level ; B, lower , , , , ,

hypocotyl; c, middle hypocotyl; D, upper ledonary plate and extend

hypocotyl. Stippled portions represent phloem; down tOW^atd the roOt. The
cross-hatched areas protoxylem; lined areas, ^ ,^ ^ ^^^ j. •

metaxylem. (After Grassley.) y

tical plane which is median
between the two cotyledons; and as enlargement continues, these
are extended nearly to the cotyledonary node. Two flap-like seg-
ments of partially split off cortical tissue may remain in the plane
of the cotyledons until the fleshy axis has reached its maximum
size. (Fig. 150.)

Except for the green upper portion, there is little axial elongation
of the hypocotyl after the maturation of the primary tissues unless
there are unfavorable environmental conditions, such as dryness
or light deficiency, which delay the initiation of secondary thicken-
ing. In such cases, the entire hypocotyl elongates considerably
and there may be little or no secondary thickening. Temperature
may also have a pronounced eff^ect upon the development of the
hypocotyl, and Plitt (13) found that under short day conditions

RAPHANUS SATIVUS

2-99

there were fewer thickened hypocotyls at 1.'^° C. than under long
day, while at 15.5° C. the thickening of the axis was well developed
in both cases.

At the time of initiation of secondary thickening, the differ-
entiation of the primary xylem is practically complete. (Fig.
149.) The cambium arises in the fundamental parenchyma lying
between the metaxylem and the primary phloem. The activity
begins in this zone at a mid-point on the inner face of the primary
phloem; and, as growth continues, there is a lateral extension of

Fig. 149. Transection of stelar portion of root at time when secondary thickening has
been initiated : ca, cambium ; co, cortex ; en, endodermis ; mx, metaxylem ; pel, pericycle ;
fh, phloem ; px, protoxylem ; xy x, secondary xylem, still immature.

the cambium which finally involves the pericyclic parenchyma
abutting the protoxylem points. The occurrence of the first
cambial activity in a plane at right angles to that of the cotyledons
accounts for the splitting of the cortex and epidermis described
above. The secondary phloem consists of sieve tubes, companion
cells and some parenchyma while the secondary xylem is made up
of large reticulate vessels surrounded by parenchyma.

As the root and hypocotyl thicken, they tend to become some-
what oval in transection, but subsequent secondary and tertiary
growth may result in an axis that is sub-terete. In the sectors of
the cambial ring which arise by an activation of the pericyclic
cells outside the protoxylem points, no vascular elements are

300

THE STRUCTURE OF ECONOMIC PLANTS

differentiated and two broad parenchymatous rays are formed
which may be regarded as either pericyclic or secondary xylem
rays. (Fig. 151, C)

A periderm arises in the persistent multi-layered pericycle form-
ing the outer surface of the radish. (Fig. 151, £.) The phellogen

-ph2

Fig. 150. Transection of young root axis in which secondary thickening has caused split-
ting of cortical zone : ca, cambium ; co, remains of ruptured cortex which forms persistent
flap in cotyledonary plane; eti, endodermis; mx, metaxylem ; pel, pericycle; ph z, second-
ary phloem ; px, protoxylem ; ra, ray ; xy x, secondary xylem.

which produces it is not continuous but consists of plates of tissue
that are more or less overlapping. As the axis increases in size,
there are frequently flaky plates of cork cells which are shed and
replaced by new phellem. In red radishes the pericyclic cells
contain anthocyanin.

The secondary xylem vessels are arranged in approximate radial
rows separated by parenchyma, but they may be somewhat dis-

t

RAPHANUS SATIVUS 301

placed tangentially and widely separated from one another radially
by the continued growth and division of the parenchyma. (Fig.
151.) The limited development of lignified vascular elements and
the differentiation of abundant thin-walled parenchyma from the
cambial derivatives accounts for the succulence of the edible root-
hypocotyledonary axis. The vessels are relatively large with walls
that are reticulately thickened or pitted. Reticulate thickening is
more common where the vessels adjoin parenchymatous cells, and
pits commonly occur between adjacent vessels. The vessel seg-
ments vary in length, ranging from two to five or six times as long
as broad; and the end walls, which are resorbed as the vessel
develops, are transverse or slightly oblique. There is great varia-
tion in the width of the vessel, some being much narrower than
others. Each vessel is surrounded by parenchyma consisting of
rows of thin-walled, elongated, prismatic cells which have pointed
or transverse end walls. There are also radially arranged, isodia-
metric, parenchymatous cells which are produced in progressively
larger numbers as the diameter of the axis increases. The vessels
immediately centrad to the cambial ring in a nearly mature radish
are widely separated tangentially by rays of parenchyma.

Further activity of the xylem parenchyma results in the develop-
ment of crescentic or circular zones of secondary cambium which
give rise to tertiary xylem and phloem elements. (Fig. 151.) In
these regions, vascular strands may be differentiated having a cen-
tral phloem, a type frequently found in the Cruciferae. Tertiary
thickening may be extensive, and this activity, together with a
general division of the secondary xylem parenchyma, supplements
the function of the primary cambium, and with the latter accounts
for the fleshiness of the axis. (Fig. 151, £.) The amount of
secondary phloem produced is relatively small as compared with
the xylem, and the pericyclic zone is narrow. The vascular strands
which are differentiated in interrupted radial rows are not vertical
but form a reticulate system by branching and anastomosing.

The secondary and tertiary thickening of the hypocotyl is
identical to that described for the root, except that there is a
central pith in the upper transitional portion of the former. The
development of secondary xylem proceeds as in the root and two
broad sectors develop separated by wedge-shaped parenchymatous
rays outside the protoxylem points. The cortex is shed as in the
root region, but this occurs later in ontogeny.

301 THE STRUCTURE OF ECONOMIC PLANTS

Minin (ii) and Golinska (6) suggest that the enlargement of the
axis does not always occur at the same level, and the former main-

—xy par

Fig. 151. Transection of sector of fleshy portion of hypocotyl showing secondary and
tertiary thickening. Secondary cambiums arising in secondary xylem parenchyma have
already produced tertiary phloem : ca, cambium ; ca r, secondary cambium ; co, cortex ; fh x,
secondary phloem; ph 3, tertiary phloem; ra, ray; xy i, secondary xylem vessels; xy par,
secondary xylem parenchyma.

RAPHANUS SATIVUS

303

tained that the root and hypocotyl can replace each other in con-
structive function, "konstruktiver Funktion." He experimented
with a Red Globe type, cutting away two-thirds of the hypocotyl
of the young seedling. The remaining portion developed roots
and produced a normal fleshy axis. Golinska repeated the experi-

-ep"^^^

Fig. 1 51. Diagrams illustrating progressive stages in secondary and tertiary thickening
of fleshy axis of Raphanus. A, root before primary xylem is completely differentiated ; B,
same, with primary xylem strand differentiated ; C, secondary thickening and development
of two broad rays outside protoxylem points (two sectors of crushed primary phloem are
also indicated); D, secondary thickening has resulted in splitting of cortical zone in a ver-
tical plane at right angles to primary xylem strand and cotyledons ; E, further splitting of
cortex, a periderm has been formed in pericyclic zone. Secondary cambiums are shown in
secondary xylem parenchyma : ca, primary cambium ; ca x, secondary cambium ; co, cortex ;
««, endodermis ; «/», epidermis ; wat, metaxylem; p^r, parenchyma; /if/, pericycle; /;</, peri-
derm; p/j I, primary phloem; />/) i, secondary phloem ; /)x, protoxylem; w, ray; xj i, sec-
ondary xylem.

ments of Minin, using large elongated radishes. He made cuttings
at a time when the seedling had developed its cotyledons and a few
small foliage leaves. In most of the plants, the upper portion of
the hypocotyl and even the epicotyl enlarged while the lower por-
tion remained slender, indicating that the potentialities for second-
ary thickening exist throughout the axis.

304 THE STRUCTURE OF ECONOMIC PLANTS

Food Reserves. — There is little stored food in the radish,
the bulk of the fleshy axis being water and cellulose. Some reserves
are accumulated in the thickened hypocotyl in the form of glucose
and there is a small amount of fructose. In certain Chinese types,
there may be a considerable amount of starch; but the French,
Japanese, and American varieties contain little if any under normal
conditions. In this connection, Plitt (13) found that slightly
thickened hypocotyls grown at high temperature with a long day
contained many small starch grains in the parenchyma.

Anatomy of the Mature Stem. — The mature stem is sub-terete
or somewhat lobed and fluted in transection. The vascular bundles
are collateral and form a dissected siphonostele, but adjacent
bundles may be connected by a weak interfascicular cambium at
maturity. In addition to this, the thickening of the parenchyma-
tous cells surrounding the bundles results in the formation of a
practically uninterrupted zone of connective tissue giving the
vascular ring the appearance of complete continuity. The bundles
vary in size with the larger ones occupying the arcs of the vascular
ring centrad to the lobed or ridged portions of the stem. (Fig. xi.)

Both of the tangential walls of the epidermal cells are thicker
than the radial and end walls, the outer one being slightly cuti-
nized. The stomata are subtended by small guard cells and are
continuous with substomatal cavities in the chlorenchyma. The
chlorenchyma consists of a compact subepidermal layer and several
adjacent layers of spongy cells which are smaller than the non-
chlorophyllose parenchyma centrad to this zone.

The phloem of each collateral bundle is capped by an arc of
pericyclic fibers. The sectors of fibers which subtend the larger
bundles are more fully developed than those outside the smaller
ones; and, together, they form a discontinuous cylinder of mechan-
ical tissue. Adjacent bundles are separated by medullary rays of
thin-walled parenchyma but later in ontogeny the walls thicken.
In some instances, an interfascicular cambium may form across the
ray and produce several radial rows of non-vascular tissue. (Fig.
153.) Except for the connective tissue which forms a thickened
sheath around the xylem portion of each collateral bundle, the
pith consists of parenchymatous cells; and the stem is commonly
hollow at maturity owing to the disintegration of the central cells.

The primary xylem consists of annular and spiral elements that
are separated from each other by thin-walled parenchyma. The

RAPHANUS SATIVUS

305

large, secondary xylem vessels are reticulate or pitted and are
surrounded by small thick-walled cells. The secondary phloem is
comprised of sieve tubes, companion cells, and parenchyma.

-par

Fig. 153. Sector of old stem in transection showing vascular bundle: ca, cambium; ep,
epidermis ; mech, mechanical tissue ; par, cortical parenchyma ; pel, pericyclic fibers ; ph 1,
primary phloem ; /?^ i, secondary phloem ; />/, pith ; r^, medullary ray ; atj i, primary xylem;
xy 1, secondary xylem.

3o6 THE STRUCTURE OF ECONOMIC PLANTS

The Leaf. — The petiolate basal leaves which form a rosette in
the first phase of vegetative development are variously incised and
deeply lobed or lyrately pinnatifid. The petiole encircles the
stem for about a third of its circumference, and its basal margins,
which are winged, continue as a ridge as far as the lobes of the
lamina. (Fig. 154, /4.) The vascular supply consists of five collat-
eral bundles which occupy positions corresponding to the lobing of
the petiole. Each bundle is made up of three to five units which
are separated from one another by parenchymatous rays. The
bundles are crescent-shaped with the open arc directed toward
the adaxial surface of the petiole, and transverse veinlets cross-
connect the main ones. There is a well-defined cambium in the
principal veins which gives rise to some secondary vascular tissue.
The phloem is bounded by a zone of mechanical tissue, comparable
to the pericyclic strands in the stem; and the parenchymatous cells
adaxial to the bundle and between the rows of the secondary xylem
may also be thick-walled.

The subepidermal layers are chlorophyllose and there are chloro-
plasts in the cells immediately outside the mechanical tissue of the
bundles, in the phloem parenchyma, and in the parenchymatous
rays separating the radial rows of xylem in each bundle. Antho-
cyanin may occur in the cells of the epidermal layers, but this con-
dition is a variable one.

The surfaces of the blade are much roughened, and the veins
form prominent abaxial ridges. The degree of pubescence is not
constant, but commonly there are epidermal hairs sparsely distrib-
uted over both surfaces, especially along the veins on the abaxial
one. The large, stiff, unicellular hairs extend from a basal group
of slightly raised epidermal cells. (Fig. 154, D.) The latter are
roughly pentagonal or hexagonal in surface view except over the
veins, where they are rectangular and much elongated, with their
long axes parallel to the vein. The mesophyll consists of a single-
layered palisade region and the spongy parenchyma which is several
cells in thickness. The air passages are large and some of the
palisade cells may be transversely divided so as to form a partial
double layer. (Fig. 154, C.) Chloroplasts occur in all the cells in
the mesophyll and the guard cells, but are absent in the tissue
surrounding the principal veins. At these points, the parenchyma-
tous cells are compact and there is a sheath of mechanical tissue
as in the petiole. The venation is pinnate-reticulate with fre-

RAPHANUS SATIVUS

307

quent lateral connections between the main bundles, and the ulti-
mate veinlets end blindly in the mesophyll.

Stomata are more abundant in the lower than the upper epider-
mis. They are formed by successive divisions of an initial epi-
dermal cell in which anticlinal walls are laid down so that the

Fig. 154. A, diagrammatic transection of young stem and petiole showing vascular bun-
dles ; B, same, of petiole ; C, transection of sector of blade ; D, epidermal hair with basal
cells (much enlarged); E, surface view of epidermis showing ontogeny of stoma : i-i, t.-i,
and 3-3 indicate successive anticlinal walls which divide initial cell into mother cell and
surrounding accessory cells. A final division of mother cell forms two guard cells ; F, tran-
section of sector of lamina of cotyledon : ^/>, epidermis; gc, guard cell ; /ir, epidermal hair;
/ St), intercellular space; I g, leaf gap; pal, palisade cells; spo, spongy cells; sto, stoma;
f, vein.

3o8 THE STRUCTURE OF ECONOMIC PLANTS

stomatal mother cell is surrounded by a few subsidiary or accessory
cells. A final division of the stomatal mother cell then produces
the two guard cells which subtend the stoma. (Fig. 154, £.)

LITERATURE CITED

I. Coulter, J. M., and Chamberlain, C. J., Morphology of Angiospertns,

D. Appleton and Co., 1911.
1. Eames, a. J., and Wilson, C. L., "Carpel morphology in the Cruciferae,"

Am. Jour. Bot. //.• 151-170, 1918.

3. , "Crucifer carpels." Am. Jour. Bot. ly: 638-656, 1930.

4. EiCHLER, A. W., Bliithendiagramme . Vol. 2, Wilhelm Engelmann, Leipzig, 1875.

5. Garner, W. W., and Allard, H. A., "Effect of relative length of day and

night and other factors of environment on growth and reproduction in
plants." Jour. Agr. Res. 18: 553-606, 1910.

6. Golinska, H., "Einige Beobachtungen iiber die Morphologie und Anatomie

der Radieschenknolle." Die Gartenbauwissenschaft, i: 488-499, 1918.

7. Grassley, Frances E., "The anatomy of the primary body of Raphanus

sativus, L." Unpubl. M.S. Thesis, Univ. of Chicago, 1932.-

8. Janczewski, E., "Recherches sur I'accroissement terminal des racines dans

les Phanerogames." Ann. Sci. Nat., Bot. Ser. V, 20: i6i-ioi, 1874.

9. KoNDo, M., "Uber die in der Landwirtschaft Japans gebrauchten Samen. I."

Ber. Ohara Inst, fiir landw. Forsch. i: 161-178, 1918.

10. Malinowski, E., "On the inheritance of some characters in the radishes."

Comp. Rend. Soc. Sci. Varsovie p.- H. 7. 1916.

11. MiNiN, I., "Zur Frage des Experimentalen Studiums der Normalformen der

Wurzelgemiise und ihrer Fehler." Jahrh. Landw. Wiss. j: Nr. 5/6, 1918.
II. Moldenhawer, K., "Uber Artkreuzungen der Raphanus und Brassica."
Ogrodnictwo Cracow. Nr. 4/7, 1915.

13. Plitt, Thora M., "Some photoperiodic and temperature responses of the

radish." Unpubl. Research, Hull Bot. Lab., Univ. of Chicago, 1931.

14. Sachs, J., Text-book of Botany, Clarendon Press, Oxford, 1875.

15. Saunders, Edith R., "On carpel polymorphism." Ann. Bot. _jp.- 113-167,

1915.

16. , "On a new view of the nature of the median carpels in the Cruciferae."

Am. Jour. Bot. 16: 111-137, 1919.

17. Sinskaja, E. N., "On the nature and the conditions of the formation of

esculent roots (preliminary report)." Bull, of Applied Bot. and PI. Breeding,
16: Nr. I, 1916.

18. Strasburger, E., Text-book of Botany, Fifth English ed.. The Macmillan Co.,

London, 1911.

19. Sutton, A. W., "Brassica Crosses." Jour. Linn. Soc. Bot. 1907-1909.

10. Weaver, J. E., and Bruner, W. E., Root Development of Vegetable Crops,

McGraw-Hill Book Co., 1917.

11. WiNTON, A, L., Microscopy of Vegetable Foods, Wiley and Sons, 1916.

CHAPTER XI

LEGUMINOSAE
MEDICAGO SATIVA

THE large pea family is widespread and contains a number of
species of economic plants, among which are Pisum sativum
L., Phaseolus vulgaris L., Trifolium pratense L., Medicago sativa
L., and many others grown for food or decorative purposes. The
clovers and medics are important forage plants, and alfalfa is an
outstanding representative of the group. A native of southwestern
Asia, where it was used as a forage crop centuries before the Chris-
tian era, it has spread successively to Greece, Italy, Spain, Mexico,
and South America. According to Bailey (i), it was introduced
into New York from Europe as early as 1791, and was cultivated
there in limited areas. It was brought into California from Chile
in 1854; and to Texas from Mexico in the early part of the nine-
teenth century. Since introduction, its production has extended
over the entire western half of the United States until it is now one
of the principal forage crops west of the Mississippi.

There are approximately 55 species of the genus Medicago, of
which seven are perennial and one biennial or perennial. Al-
though the commonly grown alfalfa is known as Medicago sativa
L., it seems probable that many of the commercial strains are
hybrid forms. It has been suggested by Westgate (34), and by
Garver (ii), that crossing of M. sativa with the yellow-flowered
M. falcata has occurred, and that this has contributed to the
hardiness of northern strains of alfalfa such as Grimm, which is
classified as M. media by some authorities. Fryer (11) has con-
firmed this point on the basis of cytological evidence, stating that
"Cytological study has shown the chromosomes of Medicago
sativa, M. falcata (3i-chromosome form) and M. media to be the
same in number and very similar morphologically"; and he con-
cludes that "there is therefore no reason, from cytological con-
siderations, to doubt the origin of M. media from interspecific
hybridization between M. sativa and M. falcata."

309

3IO THE STRUCTURE OF ECONOMIC PLANTS

GENERAL MORPHOLOGY

The alfalfa plant is a long-lived perennial, its average length
of life being from five to seven years, depending upon environmental
conditions. It is know^n as a deep feeder and grows best in soils
of a type in v^hich a deeply penetrating root system can develop.
Where this is not possible, there is an increase in the amount of
branching and in the lateral spread of the branches formed.

The Root and Shoot. — The young plant develops a tap root
that penetrates very deeply and rapidly, often reaching depths
of five to six feet the first season of growth. (Fig. 155) Garver
(ii) estimated the volume of soil occupied by a single three-year-
old plant to be 3 feet in diameter and 10 feet in depth. According
to Weaver (33), it may penetrate to 10 or ii feet by the end of the
second year; and, ultimately, may extend to depths of xo feet or
more. It develops a few branches in the first inches of surface soil;
but these, instead of spreading laterally, penetrate more deeply
and finally follow a course parallel to the tap root. Frequently,
branches are limited in number and the tap root is always the most
important part of the root system.

From the short stout crown stem, numerous erect branches
arise which attain a height of i to 4 feet. In hardy types, such
as Grimm alfalfa, the woody rhizome is branched, and shoots arise
from it below or at the soil surface forming a bushy crown. The
upper portions are herbaceous; and the slender, aerial stems are
much branched, bearing pinnately compound, trifoliate leaves in
an alternate phyllotaxy. The leaflets are linear, oblong or obo-
vate-oblong, and toothed toward their apices, with slender, awl-
shaped stipules that are adnate to the petiole. (Fig. 157, A.^

The Flower and Fruit. — The flowers are borne in short,
compact, axillary racemes; and the papilionaceous blossoms are
purple or rarely white. (Fig. 156.) In hybrid forms, yellow
flowers are occasionally produced; and the sand lucerne, M. media,
has flowers which vary from yellow to purple. The floral structure
and ontogeny are similar to that of the pea in all essential details.
(See Chapter XII.)

The calyx tube consists of five undiverged sepals terminated by
five lobes or teeth that exceed the length of the tube, and the five
petals form a corolla of the papilionaceous type in which the
standard is somewhat longer than the two lateral wings, these, in

MEDICAGO SATIVA

311

turn, exceeding the length of the keel. The stamens are diadel-
phous, nine of them being undiverged with the tenth or uppermost
stamen free. The stamens forming the staminal tube are held in a
state of tension by two projections of the keel which meet in front

1 /V ... T

/—jiff' ''■TwL

^
^

9 ?r

1^

•>

1

0

4

I

4

^M't

^%L

G

\ )*¥ ^

!•

7

^ i

0

1

A

t

A

Fig. 155. Root systems of alfalfa in July of second year of growth : A, grown in dry land;
B, in irrigated soil. (From Weaver, Root Development of Field Crops, McGraw-Hill Book Co.)

311 THE STRUCTURE OF ECONOMIC PLANTS

of the staminal column; and, in addition, tliere is a horn-like
projection on each wing which fits into the evaginations of the
keel. The release of tension in connection with pollination is
known as "tripping."

The pistil consists of a single carpel which develops a one-celled,
superior ovary, and a smooth, awl-shaped style. (Fig. 157, C.)

Fig. 156. Habit of leaves and inflorescence of Grimm alfalfa.

McFarland Co.)

(Photograph by J. Horace

In the ovary, ii to 18 ovules are formed, according to Martin (xo);
while Cooper (9) reports a smaller range of 10 to ii. From one to
six ovules begin to develop after fertilization, the others aborting;
and, in heavy seed-producing strains, there is an average of three
to four seeds per pod.

The mature fruit is a brown, curved or coiled, slightly pubescent,
indehiscent pod which may have two or three spirals. (Fig. 160,
A.') The fruit coat consists of three distinct regions, (i) The

MEDICAGO SATIVA

313

epicarp is a single layer of elongated cells, except where they form a
rosette around a stoma; and epidermal hairs are frequently formed
which tend to break off as the fruit dries, leaving a thickened scar.
(2.) In the mesocarp, the two important cellular elements are the
very thin-walled crystal cells, each of which contains a single
crystal; and the fibers which have thick pitted walls and blunt
ends. (3) The endocarp con-
sists of a single layer of epi-
dermal cells in which there are
no stomata.

Pollination. — The varia-
tion in the amount of seed
produced by a given strain of
alfalfa has resulted in many
investigations of pollination;
and this, in turn, has focused
attention on the mechanism of
the process. When tripping
occurs, due to any of the agen-
cies noted below, the staminal
column is released and snaps
forward until it strikes the
standard with considerable
force. This must be sufficient,
according to Armstrong and
White (i), to rupture the cu-
ticular membrane which con-
stitutes the limiting outer surface of the stigma. The presence of
such a limiting membrane was demonstrated byjost (16) in Lupi-
nus and Cytisus. Tripping of the stamens may be due to insect
visitation; to "automatic tripping" in which some other factor,'
possibly climatic, may release the tension; or it may be artificially
induced by various agencies. Piper and coworkers (2.5) state that
"Atmospheric or climatic conditions greatly affect automatic trip-
ping, so that it is not improbable that this factor alone accounts
for a great variation in seed production during different seasons."
Carlson and Stewart (7) determined that "artificial tripping resulted
in an increase of approximately 140 per cent in the percentage of
flowers forming buds as compared with natural development"; and,
also, that alfalfa may "be expected to seed best under desert or

Fig. 157. A, habit of inflorescence; B, fruit;
C, carpel with other floral parts removed ; D and
E, flower, lateral and face views ; F, stamens
with perianth removed; G, floral diagram.

314 THE STRUCTURE OF ECONOMIC PLANTS

semi-desert conditions, where warm or hot days, cool nights, and
a relatively dry air both night and day generally are encountered."

Although it is believed by many investigators that tripping is
essential to effective pollination. Brink and Cooper (4) point out
that "Under greenhouse conditions in the winter, tripping is prac-
tically indispensable to seed setting, but in the field many pods may
arise from untripped flowers." Carlson (5) noted in Utah that
only about 8 per cent of the flowers were tripped, while 14 per cent
set pods; and the semi-arid climate is undoubtedly an important
factor in the production of seed in that region. Factors which
may tend to decrease the amount of seed produced are variations in
the behavior of the stigmatic membrane as a block to effective
pollination; abortion of a relatively high percentage of the pollen;
and, less frequently, abnormal positional relationships between the
stigma and the anthers. An additional factor has been suggested
by Martin (xo), who attributes blasting of the seed, in some cases,
to an arrested development of the embryo, due to the inability of
the plant to furnish proper water and food supply; as well as to
pathological conditions to which the seed is more susceptible under
drought conditions.

MicROSPOROGENEsis. — In the development of the anther, there
is an early differentiation of a hypodermal column of archesporial
cells one or two layers in thickness. These in turn produce the
primary parietal cells and the primary sporogenous tissue by the
formation of periclinal walls. The parietal cells again divide
periclinally and the outermost layer redivides, so that the anther
wall consists of an epidermal layer, two middle layers, and the
tapetum. The primary sporogenous cells divide several times to
form a column of microspore mother cells. According to Reeves
(i6), the tapetum differentiates rather slowly and its cells remain
uninucleate, while the outer parietal cells become elongated
radially and form an endothecium. Later the tapetum and inner
layer of parietal tissue disintegrate and their contents are absorbed
by the developing microspores; so that, at anthesis, the walls of
the locules consist of only the epidermis and endothecium. Fol-
lowing reduction division and wall formation, which occurs by
furrowing rather than by the formation of cell plates, the micro-
spores increase in size until they are several times their original
diameter. The wall of the slightly three-lobed mature pollen
grain consists of an intine and exine layer with three germ pores,

MEDICAGO SATIVA 315

and, prior to dehiscence, tlie nucleus divides to form two daughter
nuclei of unequal size.

Dehiscence of the anthers occurs in the bud stage and the liber-
ated pollen is confined to the keel, closely packed around and
directly on the stigma. This method of pollen release leads to self-
fertilization in most instances, but there is some cross-pollination.
It has been noted that pollen frequently occurs on the standards of
untripped flowers in the field, and such grains, which have been
transmitted by insects or other agencies, provide an opportunity for
cross-fertilization. It has been demonstrated further that cross-
fertilization greatly enhances the production of seed, and Carlson
(6) reports "that 17 per cent of alfalfa flowers allowed to develop
naturally formed seed pods, while 44 per cent of those tripped arti-
ficially and 54 per cent of those artificially cross-pollinated formed
seed pods."

The Development of the Ovule and Megasporogenesis. — The
ovules arise from parietal placentae in two rows which are so
crowded that transections of the carpel may show two ovules
apparently side by side, but as the carpel lengthens rapidly, they
become separated and appear to be in one row. The young ovule
is erect, beginning to curve at about the time that it comes in
contact with the dorsal wall of the carpel. Reeves Qvf) observed
that the curvature is usually toward the basal portion of the ovary;
although, occasionally, one of the ovules at the stylar end may
curve toward its apex. (Fig. 158, A.^ At maturity, the ovule is
campylotropous with the micropyle resting against the funiculus.
Two integuments develop, each of which is two cell layers in thick-
ness, except near the micropyle where the outer integument is
considerably thicker. In the development of the ovule, the inner
integument arises first, but the outer one grows more rapidly and
soon encloses it. Since the inner integument does not usually
completely surround the nucellus, a considerable part of the
nucellus adjacent to the micropyle is covered only by the outer
integument. (Fig. 158, B and C.) As the ovary develops, the
carpel itself becomes coiled, the curvature being greater at its apical
end.

The number of archesporial cells may vary. Martin (19)
observed a multiple archesporium from which primary parietal and
primary sporogenous cells are derived; and both Reeves (xy) and
Cooper (9) have noted that there are usually two or three primary

3i6 THE STRUCTURE OF ECONOMIC PLANTS

sporogenous cells, which are commonly arranged side by side, but
may occur in a linear row. These cells function directly as mega-
spore mother cells producing tetrads of megaspores; and, fre-
quently, two or three tetrads are formed in a single ovule. In most
instances, only a single chalazal megaspore of one of the tetrads
develops to form a mature megagametophyte; but Cooper reports
that an ovule may contain two well-developed gametophytes.
The megagametophyte reaches the eight-nucleate stage as a result
of three equational nuclear divisions, and cell plates are formed in

Fig. 158. A, longisection of young carpel showing initial stages in curvature of campylo-
tropous ovules ; one row of ovules has been removed ; B, longisection of portion of an ovule
showing two-nucleate megagametophyte and two integuments, the outer one completely
enclosing nucellus ; C, longisection of campylotropous ovule showing mature megagameto-
phyte ready for fertilization. (Redrawn after Reeves.)

such a way that three nuclei at each end of the embryo sac are
enclosed by walls while the fourth nucleus of each group remains
in the central region. In this manner, a seven-celled gametophyte
is developed with a megagamete and two synergids at the micro-
pylar end, three antipodals at the chalazal end, and an elongated
endosperm mother cell in the central region.

Fertilization and Embryogeny. — Fertilization occurs approx-
imately 2.4 hours after pollination, one microgamete uniting with
the megagamete while the other fuses with the two polar nuclei
to form the endosperm nucleus. The zygote divides transversely
to form a two-celled proembryo, and further transverse divisions
of the apical cell result in the formation of a proembryo consisting

MEDICAGO SATIVA

317

of a linear row of six cells. (Fig. 159, A, B.) The embryo is
developed from the apical cell of the six-celled proembryo, while
the remaining cells produce the suspensor.

The first division of the apical cell to form the embryo is, in most
instances, parallel to the long axis of the proembryo; and two
additional divisions, one vertical and the other transverse, result
in the formation of an eight-celled embryo. (Fig. 159, C, D.)

Fig. 159. Stages in embryogeny : A, formation of four-celled proembryo; B, six -celled
proembryo ; C, two-celled embryo with five-celled suspensor ; D, eight-celled embryo with
eight-celled suspiensor; E, initial differentiation of cotyledons, epicotyl, and hypocotyl
with suspensor at stage of greatest development ; F, initiation of curvature of embryo in
region of epicotyl, the suspensor is disintegrating; G, mature embryo with portion of upper
cotyledon removed to show stem tip and leaf primordia : cof, cotyledon ; eel, epicotyl ; hyp,
hypocotyl; /.leaf; st, stem tip, j-«j, suspensor. (Redrawn and adapted from Cooper, /w/r.
Agr. Res.^

Subsequent periclinal divisions cut off the dermatogen layer, and
continued growth results in the formation of an embryo which is
at first spherical and later elongates and broadens at its apex.
In the development of the hypocotyl and cotyledons, the former
elongates, and this is followed by a curvature of the embryo so
that the hypocotyl and cotyledons lie approximately parallel at
maturity. (Fig. 159, E, F, G.) There is an early differentiation
of the provascular strands, and the advanced condition of the mature
embryo is evidenced by the presence of the foliage leaves.

3i8 THE STRUCTURE OF ECONOMIC PLANTS

ANATOMY

The Seed. — The seeds are somewhat kidney-shaped, i to 3 mm.
in length, about twice as long as broad, and the seed coat is rela-
tively smooth, varying from a dull yellow, sometimes tinged with
green, to a reddish brown. (Fig. 160, B.) Its structure, which
has been investigated by Pammel (14), Winton (37), and Lute (18),
appears to have a significant relation to the problem of germina-
tion. The outermost layer of the coat consists of a row of palisade

cu

s c

end-

B

cot-

^joo&pononmoO^^

Fig. 160. A, fruit; B, seed; C, transection of portion of seed coat and cotyledon: cot,
cotyledon; cu, cuticle; end, endosperm; ep, epidermis of cotyledon; / /, the light line;
pal, palisade cells; par, parenchyma; s c, seed coat; sub ep, sub-epidermal layer of "hour-
glass" cells. (/4 and C redrawn after Winton.)

cells, called Malpighian cells by Pammel, which are from 35 to
4x /i high and 8 to 10 ju broad. Their outer ends are rounded, and
the exposed surfaces are covered by a thin cuticle which forms a
continuous layer of unequal thickness and extends between the
conical projections of the palisade cells. (Fig. 160, C.) The
"light line" is situated below the cuticle at the upper limits of the
palisade layer, and can be readily observed in transections of the
seed coat. The lumen of the palisade cell is large at the base and
tapers toward the top, being very narrow in the upper part of the
cell owing to pronounced thickening of the cell walls. The
radiating character of the cavity at the top of the cell can be seen

MEDICAGO SATIVA 319

in surface view, while at tlie base the cell is thinner walled and the
broad lumen is circular or oval in outline.

Beneath the palisade cells is a subepidermal layer of cells that
are usually broad at the base with intercellular spaces at their outer
limits. They are about 6 fx high in most parts of the seed coat and
very broad, ranging up to 30 n. These hour-glass or I-shaped cells
are characteristic of many legumes and are sometimes referred to as
"osteosclerids." Their basal portion is somewhat broader than
the outer part, and the thick walls have deep pits and longitudinal
striations so that the surface view presents a radiating pattern.

Under the subepidermal cells is a parenchymatous zone which
consists of several layers of much compressed cells that are peri-
clinally elongated and thin-walled. The outermost layers are
without intercellular spaces while the inner ones are commonly
spongy in their arrangement. The endosperm has a distinct outer
layer of aleurone cells, and underlying it is a zone of large thin-
walled parenchymatous cells which become mucilaginous follow-
ing the intake of water. The epidermal cells of the cotyledon are
small, and the mesophyll usually consists of three layers of palisade
cells and several rows of spongy cells. The cotyledons are high in
protein content, and fat and starch may also be present.

The occurrence of impermeable or "hard" seeds is not uncom-
mon. The percentage of such seed in commercial lots may be
high enough to constitute a real problem in alfalfa production, as
in Colorado, where, over a period of years, it was found that ap-
proximately IX per cent of the seed was of that type. The
impermeable seeds fail to imbibe water and germinate within a
reasonable length of time, and various methods have been used to
overcome this condition. These include treatments with hot
water, sulphuric acid, scarification, and the application of dry heat.
In relating this problem to the structure of the seed coat. Lute (18)
determined that it is not the cuticle which is the impermeable
layer, but that this characteristic resides in some peculiarity of the
outer portion of the palisade cells, although no structural or
mechanical differences between permeable and impermeable seeds
were discovered.

Development of the Seedling. — In germination, the primary
root emerges near the hilum and penetrates the soil rapidly, forming
a slender unbranched tap root. The seedling is epigeal, in contrast
to the hypogeal pea; and the oval cotyledons are raised above the

3^0

THE STRUCTURE OF ECONOMIC PLANTS

ground as the hypocotyl elongates and straightens. The first
epicotyledonary leaf is simple and obcordate or orbicular with a
very slender petiole, while the second, third, and subsequent
leaves are trifoliate. (Fig. i6i.)

The seedling axis is slender with a short first internode, but the
second one may elongate very greatly as the plant matures. There
are buds in the axils of the cotyledon as well as in the axil of each
foliage leaf, and these develop coincident with the growth of the

C3P

7

Fig. i6i. Stages in development of seedling in Grimm alfalfa.

primary axis to form a "crown." This term has been used by
Stewart (^9) to include the perennial portions of the stems of the
plant.

Very commonly, there are three or sometimes four branches, in
addition to the primary axis, which form this characteristic crown.
When four develop, the fourth arises from the axillary bud of the
first trifoliate leaf, following the development of the other three
from the cotyledonary buds and that of the unifoliate leaf. (Fig.
i6i. A.') In the case of vigorously growing plants, additional
branches may arise from adventitious buds which occur near the
bases of the axillary branches, the first one usually developing

MEDICAGO SATIVA

3ZI

between the branches arising from the buds in the axils of the
cotyledons. Several buds may develop from any point in the
circumference of the crown stem at this level, or even higher up on
the axis when the plants have become embedded in the soil. (Fig.
i6i, 5.)

In very large crowns which have many small stems, it appears
that these arise in part adventitiously and in part from buds in the
axils of bracts and basal leaves. Jones (i 5) has pointed out that the
behavior of the plant in regard to the development of the crown

A B

Fig. 162.. A, symmetrical crown of a hardy alfalfa plant at end of its first winter : a, first
seedling stem; b and b' , stems from a.xils of cotyledons; c, stem from axil of unifoliate leaf;
d, stem from axil of first trifoliate leaf; e, first bud from base of crown, arising between stems
from axils of cotyledons and opposite the stem from the lowest seedling bud : B, crown and
portion of tap root of an uninjured alfalfa plant from a 4-year-old field at Madison, Wisconsin.
(After Jones, ]our. Agr. Rw.)

is variable, depending upon the variety, the vigor of the plant, and
environmental conditions. Where the seedling is buried deeply,
the first branches of the crown may arise from axillary buds higher
up on the primary axis; and where a plant after forming a crown is
buried more deeply, a new crown may form higher up. No buds
arise from the root portion of the axis in cases where it may become
exposed.

The IPrimary Root. — The stele of the primary root is triarch
or rarely tetrarch. The protoxylem abuts the pericycle and the
primary xylem is differentiated centripetally until a continuous
xylem strand is formed. The number of protoxylem elements in
each arm is usually limited to three or four annular and spiral

311 THE STRUCTURE OF ECONOMIC PLANTS

vessels; and, in most cases, there is a large, centrally located,
reticulate metaxylem vessel which may or may not be in direct
contact with the protoxylem at all points. Three primary phloem
groups develop between the xylem arms, and the peripheral phloem
cells differentiate as fibers adjacent to the pericycle. (Fig. 163.)
The pericycle is uniseriate at first; but early in ontogeny, there
may be tangential divisions of the cells outside the protoxylem
points, and later the entire pericyclic region becomes multiseriate.

Fig. 163. Transection of young primary root showing triarch xylem: co, cortex; en,
endodermis ; ep, epidermis ; fib, phloem fibers ; tnx, metaxylem ; pel, pericycle ; ph, phloem ;
px, protoxylem ; r h, root hair.

The cortex consists of an endodermis and four or five layers of large
parenchymatous cells which are limited outwardly by the epider-
mis. Lateral roots originate in the pericycle on the same radii as
the protoxylem points so that they are arranged in three longitudi-
nal rows, except in cases where the root is tetrarch.

Vascular Transition. — The vascular system of the seedling
axis has been investigated by van Tieghem (31), Compton (8),
Winter (36), and others. Unlike the pea, the transition is entirely
hypocotyledonary, and the reorientation of the vascular tissues

MEDICAGO SATIVA 32.3

from the exarch radial to the endarch collateral arrangement is
completed in the veins of the cotyledons. Both van Tieghem and
Gerard (13), in describing the vascular anatomy of the primary
root and hypocotyl, state that two of the xylem groups of the
triarch root are continuous with the cotyledonary bundles, while
the third strand passes through the cotyledonary plate and con-
tinues as a trace to the first epicotyledonary leaf. On the other
hand, Compton and Winter find no direct continuity between the
primary vascular tissues of the root and hypocotyl and those of
the epicotyledonary system. They ascribe van Tieghem's interpre-
tation to a study of older seedlings in which the differentiation of
the vascular tissue is advanced to a point where the downwardly
diverging foliar traces are anastomosed with the hypocotyledonary
portions of the cotyledonary traces.

The lower portion of the primary root is commonly triarch,
but a tetrarch arrangement is evident beginning approximately
at the region of the collar. (Fig. 164, A, B.) At about this point,
one of the three xylem arms appears somewhat smaller than the
other two and the angular divergence of the latter pair increases
until it approximates 180°. These two rays, which can be desig-
nated as the polar xylem, lie in the cotyledonary plane. The third
ray occupies a position at right angles to the cotyledonary plane,
and a fourth protoxylem group is differentiated which lies in the
intercotyledonary plane opposite the third ray. Thus, at the base
of the hypocotyl, the stele is commonly tetrarch with two large
xylem points in the cotyledonary plane and two smaller ones occu-
pying the intercotyledonary one. (Fig. 164, C, D.) During this
development, a fourth phloem group is differentiated, and the four
primary phloem strands occupy positions alternate with the xylem.

At higher levels in the hypocotyl, the four phloem groups
are divided into eight groups, and these are oriented in a more
nearly collateral position with respect to the xylem. (Fig. 164,
E, F.) A few millimeters below the cotyledonary plate, the polar
metaxylem elements are reoriented to form two V-shaped groups
which, with the polar protoxylem, constitute the triad structure of
each cotyledonary bundle. (Fig. 164, G, H.) Coincident with
the reorientation of the pattern of the vascular tissues in the upper
hypocotyl, a central pith is differentiated; and cell enlargement
and division result in an increase in the diameter of the hypo-
cotyledonary axis. It has been suggested by Jones (15) and Rim-

3X4 THE STRUCTURE OF ECONOMIC PLANTS

bach (i8) that this radial enlargement may result in a marked
shortening of the hypocotyl.

According to Winter (36), the polar protoxylem elements,

Fig. 164. A, triarch root of alfalfa showing lateral root formation; B, tetrarch stage;
C, D, E, bifurcation of metaxylem and formation of pith ; F, cotyledonary node below point
of divergence of cotyledons; G, transection of cotyledons and epicotyl; H, triad bundle of
cotyledon; cot, cotyledonary plane; int, intercotyledonary plane; a, h, c, bundles of first
foliage leaf; d, median bundle of second foliage leaf. (After Winter.)

MEDICAGO SATIVA

32-5

although much stretched by the rapid elongation of the hypocotyl,
retain their identity and position in the cotyledonary plane,
while those in the intercotyledonary plane eventually become dis-
organized and disappear. With the exception of the intercotyle-
donary protoxylem, all the primary xylem of the root forms a
continuous system with that of the cotyledonary traces. The
situation is somewhat complex because of an early initiation of
cambial activity in the hypocotyl with the consequent formation of
secondary xylem elements. In older seedlings, there is a further
complication owing to the downward differentiation of the
foliar traces of the epicotyledonary leaves which anastomose
with the primary and secondary vascular tissues of the hypocotyl.

The Development of the Epicotyl. — The growth of the
epicotyl is usually retarded, so that the vascular system of the
cotyledons and hypocotyl is well developed and secondary thicken-
ing is in progress by the time the primary vascular strands of the
first foliage leaves have differentiated downward to the level of the
cotyledonary node. The median bundle of the three which supply
the first leaf is located in the intercotyledonary plane and con-
sists of collaterally oriented phloem and xylem, the latter
continuing as the primary xylem of the fourth ray of the hypo-
cotyledonary axis. The two lateral bundles are located on either
side of the stele, and anastomose with the secondary tissues of
the hypocotyl. Branches of the lateral traces supply the stipules,
and the main laterals continue up the petiole, anastomosing
with the median bundle above the point where the stipules are
diverged from the petiole. Nearer the lamina, the bundles sepa-
rate again and continue in the petiole as three parallel and distinct
veins.

In the first, unifoliate leaf, the three bundles anastomose and
enter the blade as a single bundle. In the second, trifoliate leaf,
and in subsequent leaves, the terminal leaflet is supplied by the
median bundle and the lateral bundles enter the lateral leaflets.
There are cross-branches between the median and lateral bundles,
each of the latter receiving a branch from the median bundle, and
giving off a strand which joins the median bundle, after penetrating
for a distance into the terminal leaflet. (Fig. 170, A.") The
median bundle of the second leaf lies opposite that of the first
leaf; and, like the latter, anastomoses with the vascular system
of the hypocotyl at or slightly below the cotyledonary node.

32.6 THE STRUCTURE OF ECONOMIC PLANTS

Secondary Thickening of the Root. — Secondary thickening
follows maturation of the primary tissues; and, in the main
roots which may become several years old, a massive woody axis
is formed. Cambial activity results in the formation of three, or
four, wedge-shaped sectors of secondary xylem which are separated
by broad parenchymatous rays that extend radially from each
protoxylem point toward the periphery. These main primary
rays are pericyclic in origin, but there are also broad secondary
xylem rays in each of the major sectors. (Fig. 165.)

Jones (15) has described the annual rings formed in the develop-
ment of the tap root in connection with studies of winter injury,
and points out that they may be distinguished readily in most
instances. Exceptions occur in plants grown under dry farming
conditions, such as those of western Kansas, where the annual
increments of secondary growth are small and of uniform charac-
ter; and, also, in regions like southern California, where the
plants do not become dormant in the winter. The demarcation
of the rings is evident because the late summer and autumn
growth of xylem is characterized by vessels of small diameter and
small uniform parenchymatous cells w^ith relatively few or no
interspersed fibers, while the spring wood has larger vessels, less
parenchyma, and numerous fibers. The fibers are arranged in
groups separated by zones of vessels and tangential bands of
parenchyma, so that these layers may sometimes be mistaken for
annual rings. (Fig. 165.) In old roots, the enlargement and
division of the parenchymatous cells located near the center of
the axis result in the separation of the primary xylem elements
from the adjacent secondary xylem and from each other.

The annual increments of secondary phloem are not as large as
those of xylem, but they are well defined. This is because the
first elements to be differentiated each growing season are phloem
fibers, while, later in the summer, fiber formation is followed by
the development of sieve tubes and parenchyma. With the produc-
tion of each succeeding group of phloem fibers, there is a tendency
for the sieve tubes and parenchyma of the preceding year to be
crushed; and although each annual increment of phloem consists
of a group of fibers, sieve tubes, and parenchyma, the last two
may be somewhat obliterated. Jones (15) has noted that the
secondary xylem of the first year has fewer fibers than occur in
succeeding years, and this is true also of the secondary phloem of

MEDICAGO SATIVA

32-7

the first year which consists of a few scattered strands produced
early in the year and a larger group of fibers formed later in the
same season.

The cortical and epidermal tissues do not persist for any great
length of time, since the cells of these zones are ruptured and
disintegrated by secondary activity. For this reason, the outer-
most portion of the mature root consists of a pericyclic region

pd
pel

-fib
■ph2

ca

ra

xy 2

xy par

— xy fib

xy 1

Fig. 165. Sectors of four-year-old tap root: at left, showing the outer portion; and at
right, the inner portion of root. Each annual addition in growth may be distinguished in
both xylem and phloem. In xylem, the spring wood has relatively large vessels and abundant
fibers. The late autumn wood has few or no fibers and small vessels surrounded by xylem
parenchyma. The end of each annual cycle is marked by an abrupt transition from small to
large vessels. In phloem, a bundle of fibers is laid down in spring, when the xylem is also
producing fibers. Sieve tubes and parenchyma are produced in late summer and fall, only to
be crushed as soon as the next fibers are produced in spring : ca, cambium ; fih, phloem fibers ;
;)ir/, pericycle ; pd, periderm ; />A i, secondary phloem ; r^, xylem ray; xjy i, primary xylem ;
xy -L, secondary xylem; xy fib, xylem fibers; xy par, xylem parenchyma. (After Jones, Jour.
Agr. Km.)

3i8 THE STRUCTURE OF ECONOMIC PLANTS

in which the cells are much elongated tangentially, and the
external layers of the pericycle function as an active phellogen
producing a well-defined periderm of regularly arranged and
flattened cells. Scattered lenticels may occur over the surface of
the root.

In connection with a study of the plugging of alfalfa roots
caused by wilt or root rot, LeClerg and Durrell (17) have described
the tissues of the root. In the secondary xylem, the vessel seg-
ments are very short and the walls are densely beset with bordered
pits. These are transversely elongated, in some instances to such
an extent that the wall appears to have scalariform or reticulate
thickenings while in others the pits are more nearly circular in
outline. The end walls of adjacent vessel segments are perforated
with many minute pores. The walls of mature fibers are so thick
that the lumen of the cell is nearly occluded.

Graber and his coworkers (14) have pointed out that, "The
alfalfa plant has a high metabolic activity, with great ability
for regeneration; and its reserves are rapidly reduced by frequent
cuttings." There is a marked difference in alfalfa roots grown
under environmental conditions and cultural practices which pro-
duce high reserves as compared with those which result in low
reserves. In general, roots of high reserve plants have a greater
proportion of parenchyma in relation to the fibrous tissue; and
the parenchymatous cells are packed with reserve foods, principally
starch.

Root Nodules. — The general considerations regarding the
root nodules of the Leguminosae, including method of infection,
the penetration of the bacteria into the tissues, and the growth
of the nodule, are discussed in the succeeding chapter on Pisum,
but certain points specifically dealing with the nodules in Medicago
deserve mention.

Thornton (30) has investigated the early development of the
nodule in alfalfa and on the basis of Milovidov's (ii) classifica-
tion of types of infection, places Medicago in type I in which
"most, if not all the cells of the infected region receive their
bacteria by separate intrusions of the infection thread." The
infecting organism in this case is Rhizobium meliloti which,
according to Fred, Baldwin and McCoy (10), is apparently specific
for the alfalfa group, including six species of Medicago, three
of Melilotus, and one of Trigonella. The bacteria usually enter

MEDICAGO SATIVA 319

the root hair at its distal extremity, although occasionally penetra-
tion may be effected through other epidermal cells. Within
the root hair, they form infection threads and these penetrate
to the inner layers of the cortex, but Thornton states that they
do not enter the endodermis.

As a result of bacterial invasion, the cells become more densely
protoplasmic, their nuclei enlarge, and cell division ensues. The
actively dividing area involves two or three layers of cells on
either side of the infected cells, and this suggests that division
is induced by some substance diffused from the infection thread.
Cell divisions occur not only in the cortex but to some degree
in the endodermis and pericycle. The infection thread continues
to grow and penetrates the newly formed cells or it may form
zoogloeal masses from which the infection thread continues to
grow. There is a rapid multiplication of the bacteria in the
bacteroidal tissue and certain form changes occur in them so that
they increase in size and become banded. It is thought that the
banded rods represent a stage which is significant in nitrogen
fixation.

When the infected cells cease to divide, they enlarge to approxi-
mately twice their original size, become vacuolated, and ultimately
form a large central vacuole. While cell enlargement is being
initiated in the central and basal regions of the nodule, a meristem
persists at its distal end which causes it to increase in length and
to assume a cylindrical form. Thornton (31) has measured nod-
ules at various ages and finds that those two weeks old aver-
age I mm. in diameter with a length of 1.3 mm. Increase in
diameter is rapid for the first two days, and the nodule reaches
a relatively constant diameter at the end of the first week, when
its length begins to exceed the breadth. The development
of vascular strands in the nodule is similar to that described by
Brenchley and Thornton (3) for Vicia Faba. (See Pisum,
Chapter XII.)

Fred, Baldwin, and McCoy (10) have pointed out that the
nodule is

"distinctly not a modified lateral root, for it has no central cylinder,
root cap, nor epidermis. Furthermore it does not digest its way out
from the cortex of the main root but remains covered with a consider-
able layer of cortical parenchyma. Anatomically then, it differs from
non-leguminous nodules, many of which are clearly modified roots."

330 THE STRUCTURE OF ECONOMIC PLANTS

--ep

xy 2

Probably the correct interpretation of the nodule is to consider it
as a parenchymatous outgrowth of the root that is connected with
it by a system of vascular bundles which provide for the trans-
location of material. Its development results from a meristematic
activity of the cells which is induced by and is a response to the
stimulation of the invading bacteria.

The Young Stem. — The young stem is approximately square
in transection with a pith consisting of large, compactly arranged,
parenchymatous cells that are continuous with those of the cortex
through the medullary rays. The major bundles are located at

the angles of the axis -with the
smaller ones occupying the inter-
vals between. All of the bundles
are collateral, the phloem being
limited outwardly by strands of
elongated, thick-walled peri-
cyclic fibers which, with the
lignified xylem elements and the
coUenchyma, constitute the me-
chanical system of the stem.
^^ ^ The pericyclic fibers do not form
a continuous zone around the
stele, but constitute a series of
strands or tangentially extended
sectors abutting the endodermis.
The sieve tubes are long and
narrow with simple sieve plates
in their end walls; and with
them are companion cells and
parenchyma. The primary xylem elements have spiral thicken-
ings, the secondary xylem consists of radial rows of vessels with
oval bordered pits, and 'between the rows are fibers and wood
parenchyma. (Fig. i66.)

The cortex is limited centripetally by endodermal cells that
usually lack Casparian thickenings. They can be recognized by
their size and starch content, as well as by the occurrence of crys-
tals of calcium oxalate which frequently form late in ontogeny. In
transection, they appear as a continuous row of rectangular or
oval, non-chlorophyllose cells that are conspicuously larger than
the pericyclic cells centrad to them. Outside the endodermis

Fig. i66. Transection of sector of young
stem showing vascular bundle : ca, cambium ;
col, coUenchyma ; en, endodermis ; ep, epi-
dermis ; pel, pericycle ; ph, phloem ; pi, pith ;
xy I, primary xylem; xy i, secondary xylem.

MEDICAGO SATIVA 331

is a zone of chlorenchyma; and this is limited peripherally by
collenchyma which is one layer in width, except at the angles of
the stem. A strand of collenchyma reinforces the stem at these
points, and may project to form a more or less pronounced longi-
tudinal ridge. In some instances, it extends from the epidermis
to the endodermis so that the continuity of the chlorenchyma is
interrupted. The collenchymatous cells are thickened at their
angles and about as long as the epidermal cells.

The epidermal cells are elongated in the axial direction and
are somewhat irregular in shape. There are numerous stomata,
Wilson (35) reporting approximately 300 to the square millimeter.
Two types of epidermal hairs may develop, one being glandular
and two or more celled, while the other is an elongated, slender,
unicellular trichome similar to that found on the leaves. The
glandular hairs, which are not commonly found among other
genera of this tribe, occur in a frequency of approximately 1.^
per square millimeter, and the unicellular ones are somewhat
more numerous.

The Mature Stem. — As secondary thickening proceeds, an
interfascicular cambium develops, and a cambial cylinder is
formed which produces a continuous zone of lignified xylem. The
connective tissue cut off by the interfascicular cambium consists
chiefly of wood parenchyma, while the fascicular cambium con-
tinues to produce vessels, fibers, and xylem parenchyma. (Fig.
167.) As maturation continues, some of the smaller parenchyma-
tous cells on the inner face of the bundles may become lignified,
forming an uninterrupted zone of mechanical tissue in conjunction
with the connective tissue produced by the interfascicular cam-
bium. This band of lignified parenchyma frequently encloses a
zone of thin-walled parenchyma which surrounds the protoxylem
elements of each bundle. The parenchymatous cells of the pith
may disintegrate or become ruptured so that the mature stem
becomes hollow. The inner tangential, radial, and end walls of
the endodermis may become lignified, forming an almost con-
tinuous sclerenchymatous sheath. In the aerial portion of the
stem, the amount of secondary thickening is not usually sufficient
to cause any splitting of the epidermis and cortex; but, when
this does occur, a cortical phellogen develops.

The Underground Stem or Rhizome. — The extent to which
the underground portion of the crown stem may develop is vari-

332.

THE STRUCTURE OF ECONOMIC PLANTS

able, depending upon the variety of alfalfa and, to some degree,
upon cultural practices and climatic conditions. Garver (ix) has

— xy 2

-^^xy 1

Fig. 167. Transection of sector of old stem: ca, cambium; col, coUenchyma; en, endo-
dermis ; ep, epidermis ; par, cortical parenchyma ; pel, pericycle ; ph, phloem ; pi, pith ;
xy I, primary xylem ; xy i, secondary xylem.

pointed out that most of the southern-grown forms have relatively
few rooting rhizomes; while in the northern-grown, variegated
alfalfas, especially Grimm, the rhizomes are often well devel-

MEDICAGO SATIVA

333

oped, although not as much so as in the yellow-flowered, M.
falcata.

The structure of the rhizome resembles that of the aerial portions
in the general arrangement of the tissues, but there are certain
definite specializations which may be related to the soil environ-
ment, and to the greater age that the underground portions usually
attain. One important difference lies in the formation of well-
defined Casparian thickenings

in the endodermal layer of the
rhizome, followed later by a
pronounced lignification of the
radial and inner tangential walls.
Another development that
does not commonly occur in the
aerial stem takes place in the
pericyclic zone, where several
layers of parenchymatous cells
develop between the endodermis
and the sectors of pericyclic
fibers. As the rhizome increases
in diameter, the cells in this zone
adjacent to the endodermis func-
tion as a phellogen, producing
a periderm which serves as the
protective layer when the epi-
dermis and cortex are destroyed.
(Fig. i68.) Jones (15) has noted
this development in the endo-
dermis of the perennial portion

— ep

—CO

— en

P9

-pel

-ph

— ca

xy

—pi

Fig. 168. Sector of rhizome showing
detail of vascular ring : ca, cambium ;
CO, cortex ; en, endodermis ; ep, epidermis ;
pel, pericyclic fibers ; pg, phellogen ; ph,
of the stem, and attributes the phloem; pi, pith; xy,xy\em.

origin of the phellogen to this

layer rather than to the pericyclic cells centrad to it. He states

that

"When the cambium begins to produce secondary growth the endo-
dermis becomes a phellogen. The portions of the endodermal cell
walls inside the thickenings, if any have been laid down, expand con-
spicuously, and almost simultaneously two or three cross walls are
laid down interior to these thickenings. Hereupon the epidermis
dies, separates from the cortex, and forms the brown membranous
covering often seen on young stem bases early in spring. The phellem

334 THE STRUCTURE OF ECONOMIC PLANTS

originating from the endodermis becomes the effective outer covering,
both in the large crown branches from axillary buds and in the small
stems."

The Vascular Anatomy of the Stem. — Nageli (x}) has classi-
fied the genus Medicago, on the basis of its phyllotaxy and vascular
anatomy, in the type in which the leaves alternate in two rows
and the leaf trace consists of three bundles which pectinate with
the traces of both rows. The median bundle divides at the base
of the leaf into three branches of which the smallest central branch
enters the petiole, while the lateral ones anastomose with the
two lateral bundles of the trace forming the bundles which supply
the stipules.

Upon entering the stem, the median bundle extends down
through two internodes without branching. It then curves
slightly to one side from its original position in an angle of the
stem; and, just below this point of curvature, forks into two
branches. These anastomose with the lateral bundles of the next
lower leaf trace. The two lateral bundles of each leaf trace extend
downward in the corners through their own internode; and, at the
next node, enter the vascular ring alternating with the lateral
bundles of the next lower trace. Each of them then unites with a
branch of the median bundle of the next higher trace, and extends
with it through two internodes. In the third internode, this
bundle finally anastomoses with a lateral bundle of the second
lower trace. Where there is an axillary bud, it is supplied by
two bundles which extend separately through one internode, and
anastomose at the next node with the lateral bundles supplying the
next higher leaf. (Fig. 169.)

The Leaf. — The general structure of the stipulate, pinnately
compound leaf, and the course of its vascular bundles have been
described. Above the stipules, the three petiolar bundles extend
to the point of divergence of the short petiolules of the leaflets
where the terminal leaflet receives the median bundle, and the
lateral bundles become the midveins of the lateral leaflets. There
are branch veins between the three major bundles at this point
so that the three major veins of the leaflets are cross-connected.
(Fig. 170, y4.) In each elliptical or oval leaflet, the midvein
extends throughout its entire length, and the lateral branches
are pinnately arranged. These rebranch to form a net-veined
system in which the ultimate veinlets end in the mesophyll, but

MEDICAGO SATIVA

335

some of the major lateral veins extend directly to the margin of
the leaflet, terminating in the prominent teeth which occur at its

Fig. 169. Schematic representation of bundles near apex of stem showing vascular anatomy
of five nodes and internodes in Medicago-Lathyrus type. Each leaf is supplied by a median
bundle and two laterals, and the alternate phyllotaxy of leaves is indicated by departure of
traces ABC, DEF, GHI, LMN, OPQ. The bundles R, S, T, U supply axillary buds. (Dia-
gram based on Nageli.)

336

THE STRUCTURE OF ECONOMIC PLANTS

tip. Wilson (35) observed beads of water accumulating on the
teeth under some conditions, and has suggested that these termina-
tions might be hydathodes serving as drip tips.

The midvein of the leaflet forms a prominent ridge on its abaxial
surface. The branches from it lie much closer to the lower epi-
dermis than the upper, and, in some instances, the mechanical
tissue surrounding the secondary veins is in contact with the lower
epidermis. The midvein is almost completely surrounded by a
bundle sheath of thick-walled mechanical cells which become

pal ■;

Fig. 170. A, habit of foliage leaf showing stipules and character of venation; B, tran-
section of portion of leaflet cut through midvein showing distribution of chloroplasts and
intercellular spaces: hr, hair; mech, mechanical tissue; pa/, palisade region; ph, phloem;
spo, spongy tissue; stp, stipule; xy, xylem.

sclerotic in the abaxial sector, and crystals of calcium oxalate are
numerous in the cells of this region. (Fig. 170, B.) Toward the
distal portion of the midvein and in the lateral veins, the degree
of lignification of the bundle sheaths is less pronounced, and the
smaller veins are surrounded by parenchymatous cells. The
vascular tissues of the midvein are similar to those of the stem,
and some secondary thickening occurs in the larger veins.

The mesophyll usually consists of a palisade zone of two rows
of cells approximately twice as high as broad which together
constitute about half of the thickness of the leaf. The spongy
parenchyma is loosely arranged and air spaces communicate freely

MEDICAGO SATIVA 337

with stomata on both surfaces. The cells of the upper epidermis
are somewhat papillate with sinuous radial walls. Stomata are
numerous in the lower epidermis. Westgate (34) has reported an
average of 191 per sq. mm. for ordinary alfalfa, 185 for M. falcata,
and III for variegated alfalfa; while Wilson (35) found consider-
ably over twice that number, reporting up to 700 per sq. mm. The
lower epidermis is similar to the upper except that there are greater
numbers of hairs produced, especially along the veins. These are
of two kinds, the more numerous type being a long (0.7 to 1.5 mm.)
unicellular hair which is somewhat curved and arises from a
small thick-walled, basal cell around which the epidermal cells
form a rosette pattern. The capitate hairs are scattered and
much less abundant, consisting of a stalk of two or three cells and
a terminal multicellular head.

LITERATURE CITED

I. Armstrong, J. M., and White, W. J., "Factors influencing seed-setting in

alfalfa." Jour. Agr. Sci. 2$: 161-179, 1935.
X. Bailey, L. H., Cyclopedia of American Agriculture, The Macmillan Co., 1907.

3. Brenchley, W. E., and Thornton, H. G., "The relation between the develop-

ment, structure and functioning of the nodules on Vicia faba, as influenced
by the presence or absence of boron in the nutrient medium." Proc. Koy.
Soc. London, Ser. B, 98: 373-399, 192-5.

4. Brink, R. A., and Cooper, D. C, "The mechanism of pollination in alfalfa

(Medicago sativa)." Am. Jour. Bot. zy 678-683, 1936.

5. Carlson, J. W., "Artificial tripping of flowers in alfalfa in relation to seed

production." Jour. Am. Soc. Agron. zz: 780-786, 193P.
6. , "Alfalfa-seed investigations in Utah." Utah Agr. Exf. Sta. Bull.

2s8, 1935.
7. , and Stewart, G., "Alfalfa-seed production." Ufah Agr. Exp. Sta.

Bull. zz6, 1 93 1.

8. Compton,R. H., "An investigation of seedling structure in the Leguminosae."

Linn. Jour. Bot. 41: i-iii, 1911.

9. Cooper, D. C, "Macrosporogenesis and embryology of Medicago." Jour.

Agr. Res. //.• 47i-477> i935-

10. Fred, E. B., Baldwin, I. L., and McCoy, Elizabeth, "Root nodule bacteria

and leguminous plants." Univ. Wise. Studies Sci. /, 1931.

11. Fryer, J. R., "Cytological studies in Medicago, Melilotus and Trigonella."

Can. Jour. Res. }: 3-50, 1930.
iz. Garver, S., "Alfalfa root studies." U. S. Dept. Agr. Bull. 1087,

1911.
13. Gerard, R., "Recherches sur le passage de la racine a la tige." Ann. Sci.

Nat., Bot. Ser. VI, 11: 2.79-430, 1881.

338 THE STRUCTURE OF ECONOMIC PLANTS

14. Graber, L. F., Nelson, N. T., Leukel, W. A., and Albert, W. B., "Organic

food reserves in relation to the growth of alfalfa and other perennial herba-
ceous plants." Wise. Agr. Exp. Sta. Res. Bull. 80, 192.7.

15. Jones, F. R., "Winter injury of alfalfa." Jour. Agr. Res. 57.- 189-111, 19x8.

16. JosT, L., "Uberdie Selbststerilitat einiger Bliiten." Bot. Zeit. 6j.- 77-117, 1907.

17. LeClerg, E. L., and Durrell, L. W., "Vascular structure and plugging of

alfalfa roots." Colo. Agr. Exp. Sta. Bull, j^g, 1918.

18. Lute, Anna M., "Impermeable seed of alfalfa." Colo. Agr. Exp. Sta. Bull.

}26, 19Z8.

19. Martin, J. N., "Comparative morphology of some Leguminosae." Bot.

Gaz- s8: 154-167, 1914.
io. , "Relation of moisture to seed production in alfalfa." Iowa Agr.

Exp. Sta. Res. Bull. 2_j, 191 5.
■Li. MiLOviDov, P., "Uber einige neue Beobachtungen an den Lupinenknoll-

chen." Centhl. Bakt. (jtc.'), 2 Abt., 68: 333-345, 1916.
ii. , "Recherches sur les tubercles du lupin." Rev. Gen. Bot. 40: 1-13,

192.8.
13. Nageli, C, Beitrdge %ur Wissenschaftlkhen Botanik, Part I, Engelmann, Leipzig,

1858.
24. Pammel, L. H., "Anatomical characters of seeds of Leguminosae, chiefly

genera of Gray's Manual." Trans. Acad. Set. St. Louis, q: 91-174, 1899.
15. Piper, C. V., Evans, M. W., McKee, R., and Morse, W. J., "Alfalfa seed

production; pollination studies." U. S. Dept. Agr. Dept. Bull. 7/, 1914.
z6. Reeves, R. G., "Nuclear and cytoplasmic divisions in the microsporogenesis

of alfalfa." Am. Jour. Bot. ly: 19-40, 1930.
2.J. , "Development of the ovule and embryo sac of alfalfa." Am. Jour.

Bot. ij: 139-146, 1930.
i8. Rimbach, a., "Die Wirkung der Wurzelverkiirzung bei einigen Nutz- und

Zierpflanzen." Angew. Bot. 4: 81-90, 1911.
19. Stewart, G., Alfalfa Growing in the United States and Canada, The Macmillan

Co., 1916.

30. Thornton, H. G., "The early development of the root nodule of lucerne

(Medicago sativa L.)." Ann. Bot. 44: 385-391, 1930.

31. , "The influence of the host plant in inducing parasitism in lucerne and

clover nodules." Proc. Roy. Soc. London, Ser. B, 106: iio-iii, 1930.

31. VAN Tieghem, p., "Recherches sur la symctrie de structure des plantes vascu-
laires." Ann. Sci. Nat., Bot. Ser. V, i}: 5-314, 1871.

33. Weaver, J. E., Root Development of Field Crops, McGraw-Hill Book Co., 1916.

34. Westgate, J. M., "Variegated alfalfa." U. S. Dept. Agr. Bur. Plant Ind.

Bull. 169: 1-63, 1910.

35. Wilson, O. T., "Studies on the anatomy of alfalfa (Medicago sativa L.)."

Kan. Univ. Sci. Bull, j: 191-199, 1913.

36. Winter, Clara W., "Vascular system of young plants of Medicago sativa."

Bot. Gaz. 94: 151-167, 1931.

37. WiNTON, Kate B., "Comparative histology of alfalfa and clovers." Bot.

Gaz- 57: 53^63, 1914.

CHAPTER XII

LEGUMINOSAE — Continued

PISUM SATIVUM

GENERAL MORPHOLOGY

T

HE pea is an herbaceous annual with a trailing, climbing,
or bushy habit; and

the mature stem is dwarf
or tall depending upon the
variety. The pinnately
compound leaves are alter-
nately arranged, and the
glaucous, oval to ovate
leaflets are in one to three
pairs with the terminal
leaflet, and occasionally
the upper lateral ones,
differentiated as prehensile
tendrils which serve to
support the plant. (Fig.
171.) At the base of the
petiole are two stipules
which frequently equal the
leaflets in size. The leaves
at the first and second
nodes above the cotyledon-
ary node develop as small
trifid bracts. (Fig. lyx.)

The root system consists
of a main tap root with
numerous laterals which
may penetrate to a depth
of 4 or more feet depending
upon the character of the

Fig. 171.
teristics.

339

Habit of plant showing leaf charac-
(Courtesy of the Ferry-Morse Seed Co.)

34°

THE STRUCTURE OF ECONOMIC PLANTS

soil. Root tubercles induced by the invasion of a soil organism,
Rhizobium leguminosarum, are often present on the root.

The inflorescence is a few-flowered axillary raceme with an
elongated peduncle. In most garden peas, the flowers are white,
while in field types, they are purplish, light lilac, or dull white
with purple wings and a greenish keel. The calyx tube is oblique,
consisting of five sepals which are undiverged at their bases, with
four to five somewhat unequal marginal lobes. (Fig. 173.) The
papilionaceous corolla is made up of five petals: a large broad

upper one (the standard)
which encloses the others
in the bud; two lateral
ones (wings) which ex-
tend outward obliquely;
and two lower petals (the
keel) which are more or
less united along their
ventral edges and surround
the stamens and pistil.
The diadelphous stamens
are ten in number, the un-
diverged filaments of nine
of them forming an open
tube which is cleft on the
upper side, while the
tenth, uppermost stamen
is separate. (Fig. 173, £.)
The pistil consists of a
single carpel, and the one-
celled, superior ovary contains tw^o rows of ovules that arise on
adjacent parallel parietal placentae so that they may appear to be
in a single row. The slender style is oriented at an approximate
right angle with the ovary, and the stigma is bearded on its inner
or lower surface. (Fig. 173, C) The plant is entirely self-fertile,
and in nature there are probably very few instances of natural
cross-pollination. Wellensiek (19) found but one case of spon-
taneous crossing in a period of five years, and states that "it
appears that the pea is preeminently a self-fecundating plant,
which is to be ascribed partly to its being scarcely visited by
insects, partly to the early opening of the anthers followed by

J>^%

Fig. 171. Stages in development of pea seedling.

PISUM SATIVUM

341

self-fecundation, even before the flower-bud opens." In the
process of early self-pollination, the anthers dehisce prior to the
opening of the flower bud, and the conical point of the keel is
filled with pollen as the anthers are withdrawn into its base,
so that prior to anthesis the stigma is already covered with
pollen.

The fruit is a legume or pod which opens along both abaxial
and adaxial sutures, and the pericarp differs in the several varieties,

Fig. 173. /!, habit of leaves and inflorescence; B, single flower ; C, mature fruit ; D, parts
of corolla showing standard, keel, and wing; E, stamens, showing position of free stamen;
f , flower with corolla removed ; G, young carpel ; H, floral diagram.

especially with respect to the character of the endocarp or inner
surface of the ovary wall. In the edible-podded or sugar pea,
the endocarp remains soft and succulent, and the ripe pods do not
dehisce, while in the shelling peas, it becomes hard and dry and
the pod splits open at maturity. Other differences with respect
to the fruit are variations in the thickness of the ovary wall and
in the shape and size of the pod, which may be curved or straight
with a blunt or acute apex. The pods of some of the larger vari-
eties attain a length of 18 cm. and a width of i.5 cm.

342- THE STRUCTURE OF ECONOMIC PLANTS

ANATOMY

The Seed. — The mature seeds are large and globular with a
thin seed coat which encloses an embryo consisting of two fleshy,
hemispherical cotyledons, a well-developed hypocotyl, and an
epicotyl with the first foliage leaves, or trifid bracts, already
differentiated. The seeds vary greatly with respect to color,
degree of smoothness, and the character of the reserve foods con-
tained in the cotyledons.

The structural details have been described by Pammel (2.^),
Winton (34), Tschirch and Oesterle (^8); and other investigators

--73 ep

- cot

s ep

Fig. 174. A, section through seed coat and portion of cotyledon; B, section through seed
coat in region of hilum ; C, enlarged detail of porous sclerenchyma beneath hilum : cot, coty-
ledon; cot ep, cotyledonary epidermis; hi, hilum; par, parenchyma; p ep, palisaded epi-
dermis of seed coat ; p epi and p ep-i, double layer of palisaded epidermis surrounding hilum ;
scl, porous sclerenchyma; s ep, subepidermal cells which are more extensive beneath hilum,
forming cushion in which sclerenchyma cells are embedded. (^ and C, redrawn from
Tschirch and Oesterle, Anatomtscher Atlas der Pharmakognosie und Nahrungsmittelkunde , Herm.
Tauchnitz; B, redrawn from Winton and Winton, Structure and Composition of Foods, John
Wiley and Sons, Inc.)

have added more or less complete accounts in connection with
studies of germination, food storage, and related subjects. In
the mature seed, the coat is thin and somewhat brittle, except at
the hilum, consisting of three layers, two of which are but one cell
in thickness while the third may be several cells thick. (Fig. 174,
y4.) All the layers are a part of the outer integument, since the
inner integument is completely resorbed within three or four days
after fertilization.

PISUM SATIVUM 343

The outermost layer of palisade cells, sometimes referred to
as the Malpighian layer, consists of elongated cells which may
be from 60 to 100 /x long, and are further characterized by the
presence of a narrow light line immediately below the thin cuticle.
The cells are thick-walled and the lumen is much broader at the
inner end of each cell than at the outer where it is very narrow.
The second layer consists of the column cells or osteosclerids,
which have a characteristic I-shaped or hour-glass form. They
vary from i.'y to 40 /x in height and are conspicuous in cross-sectional
view. The spongy parenchyma forms the third layer, consisting
of 10 to 1.0 rows of thin-walled cells which are very much com-
pressed and crushed at maturity. At the hilum, two layers of
palisade cells are present; and, immediately beneath the slit of the
hilum, there is a group of sclerenchymatous cells with reticulate
walls which, according to Tschirch and Oesterle, may serve a pro-
tective function against the invasion of fungi. The subepidermal
layer of column cells is expanded beneath the hilum into a cushion
in which the porous sclerenchymatous cells are embedded. (Fig.

174, 5)

According to Pultz (x5), the seed coat is the only part of the pea

seed that contains starch during the first eight or ten days of growth
following fertilization, but it does not at any time contain enough
to affect the edibility of the seed . The development of the palisade
layer, which may become very hard, is an important factor in
tenderness and succulence. Nucellar tissue is absent in the mature
seed, and this is also true of the endosperm, which is entirely ab-
sorbed by the growing embryo at the end of 11 or ix days. Prior
to this time, the endosperm plays an important part in the nutrition
of the developing embryo and contributes to the sweetness and
tenderness of the pea until it is more than half grown.

The epidermal cells of the cotyledons are small, rectangular,
arranged end to end in rows, and contain protein and fat but no
starch. The subepidermal layer consists of a single row of cells
containing some starch; and underlying these are numerous large,
thin-walled cells which are more loosely arranged with intercel-
lular spaces. The cells of the mesophyll contain starch grains
which are variable in shape, being ellipsoid, reniform, or globular
with irregular, rounded protuberances.

Development of the Seedling. — The development of the pea
seedling is hypogeal. The imbibition of water results in a soft-

344 THE STRUCTURE OF ECONOMIC PLANTS

ening and wrinkling of the seed coat, followed by a rapid swelling
of the cotyledons, and a penetration of the seed coat at the micro-
pylar zone by the tip of the conical hypocotyl. The short hypo-
cotyledonary portion of the axis is, for the most part, root-like;
and a few millimeters beneath the cotyledonary node, the epidermis
is of the root type producing root hairs. In some cases, the epi-
dermal cells of the first and second internodes are elongated so that
they resemble root hairs in appearance.

The cotyledons are diverged at right angles to the seedling axis at
an angle of about 11.0° from each other. Throughout germination,
they remain in the testa; but the cotyledonary petioles elongate
sufficiently to draw the epicotyledonary portion of the axis out
from its original position between the cotyledons. The epicotyl,
which has differentiated the first leaves, or trifid bracts, prior to
germination, pushes up through the soil, and the differentiation
of the true foliage leaves follows. The stem is quadrangular with
comparatively short internodes, and the phyllotaxy is distichous.
The main shoot may die after a short period of growth, and the
development of the plant is continued by branches arising from buds
in the axils of the lower bracts or leaves. There are buds in the
axils of the cotyledons which usually remain dormant; but, when
the epicotyl is severed just above the cotyledons, as is done in
connection with some of the experimental work on growth hor-
mones, these may develop rapidly into lateral shoots.

The Primary Root. — The ontogeny of the primary root corre-
sponds to the fourth angiospermous type as described by Janczewski
(17), in which growth is accomplished by means of a general
meristem that forms a transverse zone extending across the apex of
the root. (Fig. 17.) From this generative zone, cell divisions
on its distal surface produce the successive layers which form the
conical central portion of the calyptra, and divisions of a lateral
continuation of this transverse meristem contribute to the marginal
portions of the root cap. At a higher level, this lateral meristem
functions as a dermatogen and its ultimate divisions produce the
epidermis. The proximal face of the transverse meristem gives
rise to a massive plerome cylinder, and to a periblem which is
several layers in thickness.

In the primary root, the stele is triarch (Fig. 175), rarely te-
trarch, and the protoxylem elements abut the pericycle, which occa-
sionally may be two or three cells in width at those points. The

PISUM SATIVUM

345

-ep

'- -CO

- -en

centripetal differentiation of the metaxylem results in the formation
of a solid primary xylem strand; and, early in the ontogeny of the
primary phloem, three groups of fibers are differentiated adjacent
to the single-layered pericycle. A broad zone of interstitial paren-
chyma remains between the phloem and the metaxylem regions;
and, later in development, a cambium arises in this zone. The
cambium is relatively inactive, however; and there is little lateral
extension of cambial activity beyond the initial points of origin.
Consequently, the production of
secondary tissue is limited to a
few large xylem vessels; and a
small number of phloem elements.
(Fig. 176.) The cortex is rela-
tively large, consisting of paren-
chymatous cells with intercellular
spaces at their angles. It is
limited outwardly by the epi-
dermis, and the innermost layer
differentiates as an endodermis
with well-defined Casparian thick-
enings.

Root Nodules. — The structure
of the nodule has been investi-
gated in several species of legumi-
nous plants. These include the

work of Pierce (X3) on MedicagO p,^, ,^^ Transection of sector of young
denticulata. Whiting (31) chiefly root of pea: co, cortex; en, endodermis;

with Soia and Vigna, Brenchley '^\'P''*'ri' ' ,7' ""T^'""!' ^'^' ^"'

' . . cycle ; ph fib, phloem nbers ; px, protoxy-

and Thornton (i) on Vicia faba, lem.

Prazmowski (14), who worked

with species of Trifolium, Medicago and Phaseolus, and Nemec

(ii) on Ornithopus.

The bacillus causing nodule formation in the pea is Rhizobium
leguminosarum which, according to Fred, Baldwin and McCoy (9),
can induce this reaction in 18 species of legumes. The genus is
distinguished from other genera of bacteria by its ability to form
nodules on legumes; and, while there seems to be a group specific-
ity, with respect to species of Rhizobium, Wilson (33) has pointed
out that the question of cross-inoculation groups is still a con-
troversial one.

— px

- - pel

— mx

ph fib

346 THE STRUCTURE OF ECONOMIC PLANTS

Invasion of the root by the tubercle bacillus is effected through
the root hairs. The bacteria have very little, if any, motility.
In nature, they are probably disseminated mainly by the disinte-
gration of the nodules of the host plant, the liberation of the bac-
teria into the soil and their subsequent spread by such agencies as
wind, rain, soil organisms, or by tillage of the soil. When a root

Fig. 176. Transection of stelar portion of nearly mature root : en, endodermis ; mx, meta-
xylem ; pel, pericycle ; ph, phloem ; ph fib, phloem fibers ; px, protoxylem ; xy x, secondary
xylem.

hair does contact the organisms, they aggregate at its tip and pene-
trate it by dissolving the cell wall. Thornton and McCoy (16)
report that

"there is evidence that ... a substance is secreted from the roots which
assists the entry of the bacteria. Infection of the root hairs is always
accompanied by a characteristic curling or deformation of the tip of
the hair, which is caused by a filterable secretion of the bacteria. There
is some evidence that the root hairs on seedlings are not curled until
after the opening of the first true leaves, suggesting that the root secre-
tion also plays a part in producing the curling. The composition
of the cell wall in the deformed region appears to be modified and it
is perhaps due to this that the organisms can enter at this spot.

"The bacteria penetrate the tissues of the root cortex in the form

PISUM SATIVUM - 347

of a thread of bacterial zoogloea from which irregular masses of zoogloea
grow out within the cells. The thread itself becomes encased in a
sheath which is chemically similar to the walls of the host cortical
cells and is probably a secretion of the plant. Release of bacteria
from the ensheathed infection thread into the host cytoplasm takes
place by the formation of blister-like swellings of the sheath which
eventually bursts."

The tubercle bacilli multiply rapidly; the thread-like infection
strand extends through the length of the hair to its basal portion;
and, from that point, continues the invasion through the under-
lying cortical parenchyma, the endodermis, and finally into the
pericycle. As a result of the penetration of this strand, the cells
of the pericycle proliferate and form a conical mass of meristematic
tissue which in its initial stages resembles the meristem of the
primordium of a lateral root. Because of this similarity, the
nodule has been regarded as the morphological equivalent of the
lateral root, but the resemblance is purely a superficial one.

The dome-like mass of pericyclic tissue pushes out into the cor-
tex, producing a nodular swelling of the root axis at that point.
At first, the surrounding epidermal and cortical cells keep pace
with the growth of nodular tissue; but later they may become dis-
integrated as a result of mechanical stretching and abrasion. The
central parenchymatous and vascular tissue of the nodule is sur-
rounded by a well-defined endodermal layer, except at the tip, and
apical growth of the nodule by means of meristematic activity may
proceed for some time. The bacterial tissue occupies the central
portion of the nodule, and between this zone and the endodermis are
several layers of parenchyma in which two or three vascular strands
are differentiated. These abut the protoxylem points of the stele
of the root and extend the full length of the nodule to the meri-
stematic zone.

The vascularity of the nodule in Pisum is similar in all essential
details to that described for Vicia Faba by Brenchley and Thornton
(i), who state that under conditions of normal nutrition,

"The strands run the full length of the nodule, and merge into the
meristematic tissue, the smaller protoplasmic cells being distinguishable
for a rather greater distance than are the lignified elements. The
strands follow a sinuous course and often branch, and in cross section
as many as ten may appear, some of which in reality represent the
same strand cut through more than once owing to the curvature. No
evidence of anastomosis has been obtained, even in the large nodules

348 THE STRUCTURE OF ECONOMIC PLANTS

with an abundant vascular supply. Each strand is surrounded by an
endodermis which is continuous with that of the root, and which is
not evident quite so far as the point at which the tissues merge into
the meristem."

There is no cambial activity and secondary growth in the devel-
opment of the nodule, and the primary growth is limited so that
it soon attains full size. Division may take place in the large cen-
trally located cells which contain the bacteria, and intercellular
spaces usually are formed. Under normal conditions, the relation
of the bacillus to the host plant is a symbiotic one and there is no
destruction of the central cells until late in the ontogeny of the host
plant; but Nemec, as well as Brenchley and Thornton, points out
that under unfavorable conditions there may be a complete disin-
tegration of the central parenchymatous cells, resulting in large
intercellular spaces and a general degeneration of the bacteria-
containing tissue. Where there is a nutritional deficiency in the
soil, such as the absence of boron, there may be a development of
incipient nodular tissue in the cortex only, and an absence of peri-
cyclic activity or the development of tracheids in the nodules.

In the symbiotic relationship, the nitrogen bacteria receive car-
bohydrate material from the host plant and contribute nitrogenous
compounds to the host, but the bacteria may become parasitic when
the carbohydrate supply to the nodule is suspended. Under these
conditions, the bacteria attack and destroy the cytoplasm of the
cells in which they are located, ultimately destroying the cell walls
as well; and, similarly, a destruction of the nodule tissue by bac-
teria occurs in old nodules at the end of the growing season.

Vascular Transition in the Axis. — The root-stem transition
is not completed in the short hypocotyl, but also involves the first
three internodes of the stem so that the stele is not an endarch
dictyostele until the fourth internode is reached. This situation has
been investigated by Dangeard (8), Herail (15), Tourneux (2.7),
Compton (5), Gourley (11), and the following account is based in
part on these reports, supplemented by further investigation of a
confirmatory nature.^

' A recent memoir (MuUer, C, '"La tige feuillee et les cotyledons des Viciees a germination
hypogee." LaCellule 46: 195-354, 1937) has just been received. This includes a comparative
study of the vascular anatomy of the seedlings of five representatives of the tribe Vicieae:
Pisum sativum, Vicia sativa, V. Faba, Lens esculenta, and Cicer arietinum. In this extensive
study, Muller describes the vascular anatomy of the seedling in each instance. The author
presents a somewhat different interpretation of the relation of the vascular structures than

PISUM SATIVUM 349

In the lower hypocotyl, at a level slightly above the triarch root
described above (Fig. 175 and Fig. 177, A), definite changes occur
in the arrangement of the stelar elements. The xylem is not triarch
and the radial primary xylem strand is organized into groups that
are tangentially mesarch in their orientation. A central pith is
differentiated just below the level of divergence of the cotyledonary
traces and the first leaf trace from the hypocotyledonary stele.
Tangential bands of metaxylem connect these three potential traces,
thus forming with them a ring of vascular tissue which surrounds
the medullary parenchyma. (Fig. 177, B.)

Slightly above this point, the three phloem groups are each
separated into two distinct phloem strands. (Fig. 177, C) At
succeedingly higher levels, the groups of primary xylem, which are
directly continuous with the cotyledonary traces, increase their
angular divergence from one another and incline outward. (Fig.
177, D, c-i and c-x.) Associated with the xylem of each cotyledon-
ary strand are two groups of phloem which originate as branches
from the adjacent strands of the root phloems, and the remaining
halves of the original phloem groups of the root anastomose out-
side the xylem of the first leaf trace. (Fig. 177, C, ph I i.) Each
group of cotyledonary phloem is branched, and the resulting strands
that lie in the sector between the cotyledons are again divided into
several groups, the two median ones anastomosing to form the
phloem of the second leaf trace. (Fig. 177, D, I 2..) The two
outermost groups of phloem in the intercotyledonary plane, to-
gether with small strands of xylem which are diverged from the
adjacent metaxylem, continue into the epicotyl as cortical fibro-
vascular bundles. (Fig. i-yy, D, cortfv b.^ The remaining phloem
groups increase in size and form the four lateral phloem strands of
the epicotyl.

At the cotyledonary node, the protoxylem of the second leaf trace
is differentiated and lies in the sector between the cotyledonary
traces, although Compton notes that the position of the xylem of
this trace may vary. (Fig. 177, D, / i.) In some instances, its
initial position is external to the intercotyledonary band of meta-

that given by Gerard, Compton, Gourley and other earlier authors whose work is reviewed.
He concludes that the special structure of the central portion of the axis of the first four
internodes does not show any relation to the vascular transition of the root and stem, ascribing
the characteristic organization of the lower internodes to the peculiar orientation of the
vascular bundles that supply the first bract-like leaves and to the manner in which they become
confluent.

350 THE STRUCTURE OF ECONOMIC PLANTS

xylem, and it diverges centripetally until it is continuous with the
inner face of this band; in other cases, the trace may occupy this
final position from the outset.

cort fv h

cort fv b

cort f b M

cort fv b

M
coHfb M

coHfb 1-2

M fv b

Fig. 177. A-H, diagrams showing vascular transition. A, primary root ; B and C, hypo-
cotyl; D, cotyledonary node; E, first internode immediately above divergence of cotyle-
dons ; F and G, middle and upper regions of first internode ; H, second node : c-i and c-i,
cotyledons ; cort f b l-i, cortical fiber bundle to first leaf; cort / b 1-l, cortical fiber bundle to
second leaf; cort fv b, cortical fibrovascular bundle; end, endodermis ; /-i, /-i, vascular
strands to first and second leaves ; l-i fv b, fibrovascular bundle of first leaf; mx, metaxylem ;
pel, pericycle ; ph, phloem ; ph c-i and ph c-t, phloem to cotyledons ; ph fib, phloem fibers ;
ph /-I, phloem to first leaf; pi, pith; pri ph, primary phloem; px, protoxylem ; x c-i and
X c-T., xylem to cotyledons ; .v /-i and x /-x, xylem to first and second leaves. QB-H adapted
after Compton, Linn. Jour. Bot.')

PISUM SATIVUM 351

At a level just above the point of divergence of the cotyledons,
the metaxylem bands undergo further rearrangement, and the band
which lies in the intercotyledonary plane on the side of the cotyle-
donary divergence forms an ellipse. (Fig. 177, £.) This band
later divides to form two bundles which anastomose with the
lateral groups so that two parallel metaxylem strands are formed.
(Fig. 177, F.) In some cases, these anastomose along their inner
faces, forming a central zone of metaxylem that occupies the pith
region a few millimeters above the cotyledonary node. (Fig.

177, <^-)

At this level, the vascular strand, which extends to the first leaf
as its median bundle, gives off a fiber strand which is diverged
outward into the cortex; and, a short distance above this point,
the second leaf trace branches in the same manner. (Fig. 177, F,
G.) At the middle of the first internode, as a result of these reorien-
tations, there are four cortical bundles, two of which consist of
fibers only, while the other two are fibrovascular. Within the
endodermis, there are two polar fibrovascular bundles, a central,
dumbbell-shaped zone of metaxylem, and four laterally placed
groups of phloem in which fibers are differentiated adjacent to the
pericycle. (Fig. 177, G.)

The first trifid bract diverges at the second node, and its median
bundle passes out and joins the cortical fiber bundle located on that
side of the axis. At the same time, the laterally placed cortical
fibrovascular bundles branch, and a lateral bundle from each
becomes a lateral vein of the bract. (Fig. 177, H.) Thus, the
vascular system of the bract consists of a median and two lateral
fibrovascular bundles which later anastomose. The centrally
located metaxylem divides along its minor axis into two groups,
and the group adjacent to the bract separates longitudinally so
that two bundles are produced, which lie centrad to the lateral
phloems. (Fig. 177, H.) The two lateral bundles toward the
apex of the ellipse give off branches which anastomose to form the
median trace of the third leaf. In the third internode, the medul-
lary xylem remaining at the other end of the ellipse divides longi-
tudinally, duplicating the process which took place at the opposite
pole in the internode below. As a result of these vascular rear-
rangements, the fourth internode exhibits a typical stem structure
with a central pith and an endarch dictyostele which persists
through all higher internodes. (Fig. 178.)

35i THE STRUCTURE OF ECONOMIC PLANTS

Some variations have been noted with respect to the arrangement
of the vascular strands in the epicotyl. In some cases, the medul-
lary xylem does not anastomose to form a solid core; but, after the

'6ct2

First Internode

Third Internode

Second Internode

Fourth Internode

Fig. 178. Diagrammatic transections of first four internodes. Protoxylem is repre-
sented in solid black, metaxylem lined, phloem stippled, and phloem and cortical fibers cross-
hatched : bet 1 and bet 1, first and second trifid bracts; co bu, cortical fibrovascular bundles;
/-3, /-4, /-J, traces to foliage leaves; /ac, lacunae; ttt bu, stipular bundle.

first tangential fusion into two lateral bands, may break up into
eight groups again. In other instances, the lateral phloem groups
reunite so that there may be only two or three strands within the
endodermis instead of four.

PISUM SATIVUM 353

The Stem. — Since the vascular transition involves the first
three internodes of the axis, as well as the hypocotyl, it is desirable
to summarize the vascular anatomy of these internodes in order to
compare them with the stem pattern which occurs in the fourth
internode.

The First Internode. — The first internode has a wide cortical
region limited outwardly by an epidermis which has a thin cuticle
and numerous stomata, and inwardly by an endodermis with well-
defined Casparian thickenings. As maturation proceeds, large
schizogenous lacunae develop in the cortical parenchyma so that
there are usually four such cavities located between the cortical
bundles. The cortical bundles consist of two fibrous strands lying
in the plane of the polar bundles of the stele and two fibrovascular
bundles which lie in a plane at right angles to them. (Fig. 178.)

The stele is bounded by a single-layered pericycle; and abutting
this layer are six well-defined groups of phloem fibers, two subtend-
ing the polar bundles, the other four being arranged in pairs lateral
to the metaxylem. The sieve tubes, companion cells, and phloem
parenchyma lie centrad to the groups of phloem fibers. The pri-
mary xylem is organized in two distinct regions which differ in
the manner in which the proto- and metaxylem are differentiated.
The centrally located primary xylem consists of two crescentic
bands whose convex surfaces may be in contact at the center of
the axis, with the four tips of the crescents directed toward the
polar bundles of the stele. (Fig. 179, A.) The development of
the primary xylem in this region is exarch, the protoxylem cells
constituting the four points of the two bands, while the metaxylem
comprises the central primary xylem strand. Parenchyma occupies
the space between the protoxylem arms at each pole of the plate.
Some variation may occur with respect to the degree of confluence
of the two metaxylem bands and there may be a narrow zone of
centrally located parenchyma cells between them.

The two polar bundles are endarch and collateral in their orienta-
tion, constituting the median bundles of leaf traces. One is the
median bundle of the first trifid bract which diverges at the second
node, and the other is the median bundle of the second bract-like
leaf which arises at the third node. At the upper limits of the first
internode, the metaxylem bands described above are somewhat
separated laterally, so that there is a narrow central pith parallel to
the long axis of the primary xylem strand which is continuous

354 THE STRUCTURE OF ECONOMIC PLANTS

with the leaf gap resulting from the outward divergence of the polar
bundle leading to the trifid bract.

The Second Internode. — The epidermal and cortical regions
in the second internode are like those of the first, except that,
occasionally, one of the cortical fiber bundles may be divided to
form two groups which later converge to form a single strand. In
the stele, slightly above the second node, only one polar bundle is
completely organized, and at the other pole there is a leaf gap of
parenchymatous tissue resulting from the divergence of the median
trace to the first trifid bract. (Fig. 179, 5.) At a higher level,
divergences from the two bands of exarch xylem result in the
separation of the central zone of primary xylem into two exarch
groups lying adjacent to the leaf gap; and two lateral groups of
metaxylem forming the central mass which are separated from each
other by a narrow band of parenchyma. Progressive reorientation
of the two exarch groups adjacent to the leaf gap results in the
formation of a single endarch bundle which becomes the median
trace of the third foliage leaf. Near the upper limit of the second
internode, which is a relatively short one in most varieties of
Pisum, the polar bundle supplying the second bract-like leaf is
diverged centrifugally through the cortex.

The Third Internode. — The third internode is somewhat more
quadrangular in transection than the ones below it, but the cortical
organization is identical to that described for internodes one and
two. In the stele, each lateral group of phloem fibers may consist
of three, rather than two, strands and the laterally oriented bands
of xylem are more widely separated by the pith. (Fig. 178.)
After the divergence of the polar trace to the second trifid bract at
the third node, there is a reorientation of the exarch groups of
primary xylem to form a new polar bundle, as described for the
opposite pole in the second internode, resulting in the formation
of the median bundle for the fourth foliage leaf. The remaining
lateral bands of primary xylem may be regarded as tangentially
mesarch; that is, the protoxylem of each of the bands is centrally
located on its inner face and between flanking groups of metaxylem
elements. (Fig. 178 and Fig. 179, C)

One of the quantitative tests for auxin devised by Went (30)
utilizes the third internode of etiolated pea seedlings. When the
seedlings are about seven days old, and 10 to ix cm. in length, the
top is cut off 5 mm. below the terminal bud, and the elongated

PISUM SATIVUM

355

third internode is bisected longitudinally for about 3 cm. The
split halves would curve outward if placed in water because of

^end bu
(1-3)

end bu
(l-i)

end bu
(1-5)

px
mx

end bu
(l-i)

Fig. 179. Transections of axis at various levels to illustrate significant changes in vascular
transition. In each figure, the stele and a sector of cortex including one cortical fiber strand
are shown. The laterally placed cortical fibrovascular bundles do not appear in the figure.
/4, first internode ; B, second internode; C, third internode; D, fourth internode ; w, cortex;
CO fi bu, cortical fiber bundle; en, endodermis ; end bu, endarch bundle; ep, epidermis; l-i,
l-r, /-3, /-4, 7-5, first, second, third, etc., leaves; mx, metaxylem ; pel, pericycle; ph fib,
phloem fibers ; pi, pith ; px, protoxylem ; sti bu, stipular bundle.

356 THE STRUCTURE OF ECONOMIC PLANTS

tissue tension, since the epidermal cells are under tension and the
pith cells under pressure; but, when placed in auxin solution, there
is an inward curvature of the two halves which can be measured to
determine the auxin concentration.

The Fourth Internode. — In the fourth internode, a definite
stem pattern is laid down, and the transitional situation existing
in the lower internodes, in which both exarch and endarch primary
xylem groups are found, no longer occurs. The stem is subquad-
rangular and becomes definitely four-sided at internodes above.
The endodermis is not as well defined as in lower internodes and
in the stele there are two polar bundles which supply the leaves,
and, at right angles to them, two bundles which supply the
stipules. (Fig. 178 and Fig. 179, D.) Higher in the stem the
number of vascular bundles in the ring is increased because of the
downward divergence of vascular traces from the vegetative and
floral axes which develop in the axils of the upper foliage
leaves.

Gourley (11) in his study of the course of the cortical fibro-
vascular bundles showed that they supply the lateral lobes of the
trifid bracts, which may be interpreted as homologous to stipules,
and the stipules of the vegetative leaves. The polar bundles of the
stele provide the chief vascular supply for the central lobe of the
trifid bract, and for the lamina of the compound leaf. The cortical
fiber bundles which are in the same vertical plane as the polar
bundles become a part of the mechanical tissue of the latter when
they diverge outward in the nodal region.

The course of the cortical fibrovascular bundles which are located
at approximate right angles to the plane of phyllotaxy is as follows :
beginning at the cotyledonary node, they diverge from the margins
of the vascular bundles which supply the cotyledons. In some
instances, two may arise on each side, but only one of these persists.
They extend upward in the cortex to the second node, where a
branch from each supplies the lateral lobes of the first trifid bract.
Within the bract these bundles anastomose with the median stelar
bundle so that the bract appears to have a single vascular strand.
The main portion of each cortical vascular bundle is continued up
through the second internode; and, at the third node, each bundle
again branches to supply lateral veins to the lobes of the second
bract. The main cortical vascular bundles continue through the
third internode; and, at the fourth node, branches from each follow

PISUM SATIVUM 357

a spiral course around the stem and enter the stipules at the base of
the petiole.

The stipular bundles of the leaf at the fifth node anastomose with
bundles of the stele a short distance below the fourth node. These
bundles occupy a position in the cortex similar to that of the bun-
dles in the lower internodes, and they can be traced as strands which
unite with the vascular bundles of the stele that are located in a
plane at right angles to the leaf trace. At the fifth node, a part of
each stipular bundle joins with the vascular elements of the blade
of the leaf and with them forms its vascular supply. A repetition
of this vascular plan occurs at each internode above this point, in
this manner providing the vascular supply for each successive set
of stipules.

The Upper Internodes. — Above the fourth internode, the
number of bundles increases until there may be twenty or more
constituting the vascular cylinder. The upper internodes are
subtriangular to quadrangular in outline and the bundles form a
dictyostele in which the larger ones are located at the angles of the
stem.

The epidermis is well cutinized, and there are numerous stomata
surrounded by small guard cells that are about one-half the radial
dimension of the adjacent cells. The cortex consists of several
layers of chlorenchyma, except outside the larger bundles at the
angles of the stem where three or four layers of mechanical tissue
may be differentiated. (Fig. i8o.) The endodermis is a single
layer of large oval parenchymatous cells lying immediately outside
the groups of pericyclic fibers that cap the vascular bundles. The
fibers are small, thick-walled, and very compactly arranged; but
between the fibers and the phloem, there are two or more layers of
larger, thin-walled pericyclic cells.

The bundle is collateral with a relatively active fascicular cam-
bium, and there are several layers of secondary phloem and xylem
elements formed in the larger bundles. There are few fibers in the
phloem as compared with the number found in the transition re-
gion. The radial rows of secondary xylem vessels are separated by
ray tissue which is thin-walled and parenchymatous.

As the stem matures, the parenchymatous cells of the medullary
rays and those abutting the inner face of the bundle become thick-
walled, forming a more or less continuous zone of connective tissue.
There is some development of a somewhat inactive interfascicular

358 THE STRUCTURE OF ECONOMIC PLANTS

cambium, and small groups of secondary xylem and phloem are
formed between the major bundles of the stele. The large, thin-
walled parenchymatous cells of the pith centrad to the connective

^ sto

€\Xy--<^\SQ(^^ chlpar

^^^>--^;^3-£'~T^^> en

pclfijj

par

p/i2

Tj(jvWtr\HD<^^=w^^ TCI

Fig. i8o. Transection of sector of upper internode showing vascular bundle : w, cambium;
chl par, chlorophyll parenchyma; col, collenchyma ; con, connective tissue of pith; cu, cu-
ticle; e«, endodermis ; ^/7, epidermis ; /»«r, parenchyma of pericycle ; /)c/ /^, pericyclic fibers ;
fh -L, secondary phloem ; ft, pith ; ra, ray ; sto, stoma ; xy i, primary xylem ; xy x, secondary
xylem.

tissue usually disintegrate so that the mature stem is commonly
hollow.

Anatomy of the Leaf — the Blade. — The blades of the paired
leaflets are very thin, and the mesophyll, except in the region of

PISUM SATIVUM

359

' — spo

/.— ff c

the principal veins, does not exceed more than six or eight cells in
thickness. (Fig. i8i, A.') Both of the somewhat papillate epi-
dermal surfaces are thinly cutinized, and the cells are sinuous in
outline, except for those lying above the veins, which are elongated
parallel to the long axis of the leaf.

Stomata are approximately twice as numerous on the lower sur-
face as the upper, Holman and Robbins (i6) reporting counts of
■L16 and loi per sq. mm.
respectively. They are
formed by the interposition
of a wall which cuts off
one of the lobed portions
of an epidermal cell to form
the stomatal mother cell.
Subsequent division of this
mother cell produces the
two guard cells, which are
somewhat depressed below
the surface of the adjacent
epidermal cells.

The palisade layer con-
sists of a single row of
slender cells and comprises
about one-third of the
thickness of the mesophyll.
The cells are loosely organ-
ized so that numerous in-
tercellular spaces occur be-
tween adjacent groups and
this is also true of the
spongy tissue, which con-
sists of from four to six
layers of parenchymatous cells. (Fig. 181, A.^ The principal
veins are collateral, and the abaxial surface of the leaflet is ridged
along the line of the midvein because of the development of a
strand of mechanical tissue which parallels it. Supporting tissue
is also developed on the adaxial face of the large bundles and
there may be a few phloem fibers at the abaxial margin of the
phloem region of the bundle. The larger veins have a cambium,
and a limited amount of secondary vascular tissue is formed, the

Fig. 181. A, transection of marginal portion of
blade of leaflet ; B, transection through midvein :
cu, cuticle ; ep, epidermis ; g c, guard cells ; mech,
mechanical tissue; pal, palisade; ph, phloem;
spo, sponge cells; sto, stoma; xy, xylem.

36o THE STRUCTURE OF ECONOMIC PLANTS

smaller ones consist of a few primary xylem and phloem elements,
and the ultimate veinlets end blindly in the mesophyll. (Fig.
i8i, B.)

The Petiole. — The petiole is terete to subtriangular and con-
tains three main bundles. The largest one is located on the abaxial
side of the petiole, the other two lie toward the adaxial surface,
occupying points about equidistant from the abaxial bundle; and,
between the main bundles, two or more smaller vascular strands
may occur. (Fig. i8x. A.') The epidermis resembles that of the

Fig. 182.. A, diagrammatic transection of petiole showing arrangement of bundles. Cross-
hatched portions indicate fibers; stippled portions, phloem; lined regions, xylem. B,
transection of leaf tendril showing arrangement of bundles ; C, detail of a sector of petiole
showing bundle : «^, epidermis ; /^.fibers; /»A, phloem; atj, xylem.

blade with a thin cuticle and stomata similar to those described
for that organ. The subepidermal region consists of several layers
of chlorenchyma, and the central portion is made up of large thin-
walled parenchymatous cells which ultimately disintegrate so that
thp mature petiole is usually hollow. The main bundles resemble
those of the stem. (Fig. i8i, C.)

The Tendril. — The leaf tendril is subtriangular in transection,
its adaxial and lateral faces being somewhat grooved. Like the
petiole, it has three main bundles, the abaxial one which frequently
has a subtending cap of mechanical tissue being larger than the two
adaxial bundles. (Fig. iSi, B.) The remainder of the tissue is
chlorenchymatous.

PISUM SATIVUM

361

Floral Development. — Floral development in the Legumino-
sae has been investigated for several genera, including the work of
Goebel (10) on Phaseolus and other legumes; Gregoire (ix) on
Lathyrus; Bugnon (3) on Lathyrus, Trifolium and Lupinus; West-
gate, Coe, and others (31) on Trifolium; Coe and Martin (4) on
Melilotus; and Guard (13) on Soja. The floral development in
these genera is in agreement with that of Pisum, the only important
difference of opinion being in regard to the character of the car-

ad bii

Fig. 183. A-E, diagrammatic longisections of flower bud showing stages in development ;
F-I, diagrammatic transections of carpel in E, showing vascular anatomy and origin of ovules :
ab bu, abaxial bundle; ad hu, adaxial bundle; ad su; adaxial suture; bet, bract; car, carpel ;
Of, ovule; ovy, ovzry; /k?^, pedicel ; p^r, petal ; j?, sepal ; j/^, stamen.

pellary development in which the interpretation of Gregoire is
not in complete accord with the findings of other investigators.
The flower primordium first appears as a knob-like growing
point of meristematic tissue in the axil of a bract. (Fig. 183, /4.)
The sepals, petals, outer cycle of stamens, inner cycle of stamens,
and carpel then appear in the order named, but the subsequent
development and maturation of the floral parts does not follow this
succession. The first of the five sepal primordia arises from the
growing point on the abaxial side, followed by the development
of the two lateral lobes of the calyx, and, finally, the two adaxial
lobes. (Fig. 183, B.) Shortly after the initiation of the pri-

362. THE STRUCTURE OF ECONOMIC PLANTS

mordia of the sepals, zonal growth takes place at their bases, and
the continued growth of this zone and the tips results in a cylin-
drical tube with five acuminate lobes at its outer margin. This
forms a protective covering over the growing point and other
floral parts. (Fig. 183, £.)

As the growth of the calyx proceeds, the five petal primordia
arise from the growing point alternate with the sepal lobes, the
two primordia which form the keel of the corolla being formed
first, followed by those of the two lateral wings and the standard.
The primordia of the keel are at first distinct structures; as growth
proceeds, their adjacent edges become more or less completely
united along the abaxial margin of the keel; and, at maturity,
they form a hood-like structure which partially surrounds the
stamens and carpel. (Fig. 184, F, G, H.^ Although the petal
primordia are the second cycle of organs to be differentiated, they
grow very slowly at first, remaining small during the early develop-
ment of the stamens.

In the mature flower, the stamens are diadelphous and appear to
be arranged in one whorl in which nine stamens are "united" by
their filaments, while the tenth, adaxial stamen is free. This is
due to the perigynous development of the flower, in which there is
non-divergence of the basal portions of the stamen primordia, and
the early ontogeny indicates that the flower is actually pentacyclic
with two whorls of stamens, each consisting of five primordia.
(Fig. 184, H.) The development of two cycles of stamens as noted
above has also been reported for Trifolium, Melilotus, Phaseolus
and other legumes.

As a result of the growth of the tissue basal to the whorls of
stamen primordia, the bi-cyclic character of the androecium is soon
obscured, and nine of the stamens develop with a common basal
collar of non-diverged tissue, leaving the tenth stamen free. (Fig.
184, G.) This stamen is a member of the inner whorl, and the
elongation of its free filament maintains the anther at the same
level as those of the other members of the whorl. The maturation
of the filaments and anthers of the outer whorl is more rapid than
that of the inner, and the outer stamens produce mature pollen
grains earlier. (Fig. 183, £.)

Immediately following the differentiation of the inner cycle of
stamen primordia, the carpel primordium is differentiated, and
grows so rapidly that it soon becomes larger than any of the other

PISUM SATIVUM 363

primordia except the enclosing sepals. (Fig. 183, C, D.) It
arises as an open crescentic structure with its abaxial surface
directed toward the keel of the flower. (Fig. 183, G, H.) Thus,
the young carpel develops as an open sporophyll whose edges later
coalesce, becoming united so intimately that the original suture
can only be detected with difficulty in the mature ovary, and may
be completely obliterated. (Fig. 183, Z.) This type of develop-
ment has been described by all the investigators cited above with
the exception of Gregoire who has stated that in Lathyrus there is
no distinct line between the carpellary margins at any time. It
seems doubtful, in the light of other work, that this can be the
case; and Bugnon, studying the same genus, has found it to be in
agreement with the development reported above.

Vascular Anatomy of the Flower. — Moore (19, 2.6) has in-
vestigated the vascular anatomy of the flower in a number of the
papilionaceous legumes, dividing the species studied into two
major series based upon the number of cycles of traces which supply
the perianth and the androecium. He places Pisum in the "di-
hiate" series, under the Lathyrus type in which two cycles of
traces supply the perianth and androecium, and "the gaps of the
perianth traces extend into the departing stamen traces making
the stamen traces appear double in origin." Moore's account of
this type agrees in its major details with that given below, the
chief difference being in the ultimate branching of the traces sup-
plying the petals.

In a transection of the pedicel of a very young flower slightly
below the receptacle, the vascular tissue forms a partially dissected
siphonostele in which several distinct vascular strands may be
differentiated in the provascular ring. (Fig. 184, A.') Slightly
above this level and below the floral node from which the calyx
primordia arise, 10 perianth traces diverge obliquely from the
vascular ring. (Fig. 184, B.) This is followed by the divergence
of the stamen traces from the vascular ring, and three primary
carpellary traces are finally differentiated from it. (Fig. 184, C, D.)
The branching of the perianth traces occurs just below the nodes
from which the petals diverge, and alternate perianth bundles
incline inward, forming the principal veins of the five petals. At
this point of inward differentiation, each petal trace gives off two
lateral branches which continue upward and become the right and
left lateral calyx bundles of the adjacent sepals. The median bun-

364

THE STRUCTURE OF ECONOMIC PLANTS

die of each sepal lobe is one of the unbranched perianth bundles,
and branches from the petal traces form the main laterals. (Fig.
184, £.) Additional branching of the mid vein and laterals results
in the formation of a number of small veins which extend through-
out the undiverged portion of the calyx. (Fig. 184, F, G, H.)
Finally, each ultimate calyx lobe is supplied with three principal
bundles, each petal and stamen has one main bundle, and the vas-

sta r

ri-OV

Fig. 184. A-H, diagrammatic transections of flower bud from pedicel upward showing
arrangement of vascular traces and floral parts: car, carpel; cur tr, carpellary trace; c/x,
calyx tube; / sta, inner stamen; ke, keel; o sta, filament of outer stamen; ov, ovule; pet
/r, petal trace; /»r;6 /^ perianth trace; /)?^r, provascular ring; .rf /^r, sepal trace ; sta r, stzmiml
ring; j/^ /;-, staminal trace ; j^</, standard ; w, vascular strand ; /W, wing.

cular system of the carpel consists of a large abaxial and two smaller
adaxial bundles. (Fig. 183, /.)

MicROSPOROGENESis. — Sporogenesis in the garden pea has been
investigated by Cooper (7), who used two horticultural varieties,
Little Marvel and Asgrow's Pride. The microspore mother cells
have larger nuclei than the tapetal cells which ultimately become
binucleate. The somatic number of chromosomes is fourteen and
seven pairs are present at diakinesis. Following the second meiotic
division, each microspore mother cell has four nuclei. The division
spindles persist for a time and cell plates are formed, after which
the walls of the four microspores are deposited. The pollen grains
are soon released from the microspore mother cell wall; and, subse-
quently, the walls of the former thicken except for two germ pores.

PISUM SATIVUM 365

The nucleus of the microspore divides to form a tube cell and a
smaller generative cell, the former disintegrating early so that it
may disappear completely by the time of pollination. The genera-
tive nucleus divides during the growth of the pollen tube, and the
two microgametes which are surrounded by a narrow layer of
cytoplasm are separated from the cytoplasm of the tube by a thin
membrane.

Development of the Ovule and Megasporogenesis. — As the
young carpel develops, the ovules arise on the adaxial margins of
the carpel along two parietal placentae. (Fig. 183, Z.) At this
time, the adaxial suture is still visible; but in the later development
of the carpel, it becomes less and less distinct. At maturity, the
carpellary margins are intimately fused and the ovules appear to be
arranged in a single row. This appearance is further emphasized
by the formation of an abaxial suture and the development of pa-
renchymatous ray tissue which bisects the large abaxial bundle.
The suture becomes more and more pronounced and finally con-
stitutes one of the lines of dehiscence in the mature fruit.

The ovule first appears as a dome-shaped primordium of nucellar
tissue and at its base two integuments arise in close succession.
The outer integument precedes the inner one and by its rapid
growth soon encloses the latter. As the integuments are differen-
tiated around the nucellus, the ovule curves toward the stylar end
of the carpel and gradually becomes campylotropous.

In megasporogenesis, a hypodermal cell at the apex of the ovule
becomes the archesporial cell, and divides to form a primary
parietal and a primary sporogenous cell. The latter functions as
the megaspore mother cell, enlarging and developing a finely vacu-
olated cytoplasm preceding diakinesis. Cooper (7) noted several
cases in which two or even three sporogenous cells developed in
the ovule.

Four megaspores result from the meiotic divisions, and the one
nearest the chalaza is functional, enlarging and elongating prior
to further division. The usual sequence follows, three successive
nuclear divisions producing an eight-celled megagametophyte.
(Fig. 185.) Three of the nuclei remain at the chalazal end and
become the antipodals, three form the megagamete and synergids
toward the micropyle, and the other two serve as the polar nuclei
uniting at about the time of fertilization. The mature megagamet-
ophyte is elongated and curved, eight nucleate, and seven celled.

366 THE STRUCTURE OF ECONOMIC PLANTS

Embryogeny. — Studies of embryogeny in the Leguminosae
indicate that there are many variations in the details of embryo
development. Guignard (14) has pointed out that these are due in
part to significant differences in the character of the suspensor
and the shape of the proembryo in the early stages of embryogeny.
However, Martin (18), working with Trifolium (three species),
Medicago, and Vicia, found that all five were alike in the develop-
ment of the integument of the ovule, in the production of four
megaspores, and in the rapid destruction of the nucellar tissue in

/— a6 V

'^HW

;-

0 '"'~7"/7 /^
i in\-4-\j//

?)nV-t-- Y_^

vv^^Vx^

\

■ ' \

— -0 w

^--ad V

Fig. 185. Longisection of portion of carpel showing orientation of developing ovules:
ab V, abaxial vein ; ad v, adaxial vein ; emb sac, embryo sac ; / int, inner integument ; mk,
micropyle; o int, outer integument; o w, ovary wall.

embryogeny, indicating that certain groups of legumes may be
very similar in their developmental behavior.

According to Cooper (6), fertilization occurs in the late bud
stage about ix to i4 hours before the open flower stage. Following
fertilization, the first division of the zygote is transverse, and at
this two-celled stage the endosperm may be two or four nucleate.
This occurs in the open flower stage and by the time the flower has
withered, the proembryo is four-celled, the division of the basal cell
being longitudinal. The true embryo develops from the apical
cell of the four-celled proembryo while the two basal cells form
much elongated, multinucleate structures which, with the deriva-
tives of the middle cell, constitute the suspensor. The middle cell
also divides longitudinally, and the daughter cells likewise become
multinucleate, much enlarged and globular. These four multi-
nucleate cells disintegrate as the cotyledons develop.

PISUM SATIVUM

367

Pultz (15) in his study of the deposition of starch in the seeds of
Pisum has followed the embryogeny from early stages to maturity.
Twenty-four hours after pollination, when the embryo is just
beginning to develop, there is an abundant accumulation of starch
which is strictly localized in the cells of the inner integument and
in those of the outer integument at the chalaza near the end of the
embryo sac. (Fig. 186, A.') At the end of thirty-six hours, the
young embryo is developing in the embryo sac near its micropylar
end, and by this time the endosperm occupies a peripheral position.

0 int

mxc-

D

Fig. 186. A-D, stages in development of ovule showing progressive deposition of starch,
represented by dots. A, ovule, 14 hours after pollination; B, same, 48 hours after pollina-
tion in which digestion of inner integument is beginning and a more general distribution of
starch is evident; C, young seed three days after pollination, showing complete digestion of
inner integument and presence of starch in outer integument ; D, young seed four and one-
half days after pollination, showing change in position of embryo and increased deposition
of starch; £, diagrammatic drawing of young embryo and suspensor : cha, chalaza; emb,
embryo ; end, endosperm ; i int, inner integument ; mk, micropyle ; 0 int, outer integument ;
sus, suspensor. (Figures A-D after Pultz; E after Guignard, Ann. Sci. Nat.')

Both the embryo and endosperm are devoid of starch at this stage,
but there is an increase in the starch in the inner and outer integ-
uments. (Fig. 187.)

Between two and three days after pollination, continued growth
of the embryo results in the complete digestion of the nucellar
tissue and the beginning of digestion of the inner integument and
the starch which it contained. (Fig. i86, B.) At this stage, the
suspensor begins to elongate; and the embryo is pushed toward the
curved portion of the embryo sac where it ultimately completes its
development. Three to four days following pollination, the inner
integument is completely resorbed and the embryo sac is enlarged

368

THE STRUCTURE OF ECONOMIC PLANTS

,^-i'S^!53!^

,-,*"***'^*si*..

and lined with a peripheral layer of endosperm. The starch is still
restricted to the outer integument, but it is increased in amount
near the micropyle and at the chalaza. (Fig. i86, C) At about
five days, the embryo occupies a position in the curved portion of
the embryo sac, and the endosperm constitutes a considerable pro-
portion of the seed, since the embryo sac is enlarged and the em-
bryo is relatively small. (Fig. i86, D.) The endosperm nuclei are
peripherally located, the central cavity of the embryo sac contains
a watery cytoplasm; and, although starch is present in increased

amounts, it is still restricted
to the outer integument.

About six days after pollina-
tion, the embryo differentiates
cotyledons which enlarge as a
result of an increase in the
number and size of cells, divi-
sions taking place somewhat
more rapidly near the pe-
riphery. Later, this meriste-
matic condition is restricted
to the tip and outer surface of
each cotyledon, and the cells
, at the center and toward the

Fig. 187. Ovule thirty-six hours after pol- j • 1 r l J

lination, showing development of embryo sac adaxial SUtiace enlarge and

and peripheral position of endosperm. Dark cease dividing. It is at this

areas indicate regions of starch deposition. , , , . .

(Photomicrograph by Pultz.) Stage that starch deposition

is initiated in the non-meriste-
matic regions of the cotyledons; and when the seeds are eight
or nine days old, starch appears in the middle region of the
cotyledons and at the cotyledonary plate. The peripheral cells
on the abaxial surface and tips of the cotyledons continue to divide
and contain little or no starch, as is also the case in the hypocotyl
and epicotyl.

The developing embryo occupies a small part of the embryo sac,
the remainder being filled with a more or less watery, sweet-tasting
endosperm. Starch is not present in the endosperm at this or at
any other time during its development. The cotyledons continue
to grow actively by means of a peripheral band of meristematic
cells, and this is followed by a progressive deposition of starch
which explains the greater amounts in the middle of the cotyledons

PISUM SATIVUM 369

and at the cotyledonary plate, than along the edges of the cotyle-
don which are practically without starch, at least in the meriste-
matic zone.

The endosperm never becomes definitely cellular, although tem-
porary walls may form around the nuclei adjacent to the embryo
as described for Phaseolus by Brown (i). By the time the seed is
eleven to twelve days old, the endosperm has been entirely digested.
The mature seed consists of the embryo and the seed coat, the latter
being developed from the outer integument only, the inner integu-
ment being completely digested during the growth of the embryo.

LITERATURE CITED

I. Brenchley, W. E., and Thornton, H. G., "The relation between the develop-
ment, structure and functioning of the nodules on Vicia faba, as influenced
by the presence or absence of boron in the nutrient medium." Proc. Koy.
Soc. London, Ser. B, qS: 373-399, 192-5.

2.. Brown, M. M., "The development of the embryo-sac and of the embryo in
Phaseolus vulgaris." B////. Ton. Bot. Club, 44: 535-544, 1917.

3. BuGNON, P., "Organogenese et dehiscence de la gousse des Papilionacees."

Bull. Soc. Bot. France, jz: 445-448, 192.5.

4. CoE, H. S., and Martin, J. N., "Sweet-clover seed. Part I. Pollination

studies of seed production." IJ . S. Dept. Agr. Bull. 844, 1910.

5. CoMPTON, R. H., "An investigation of the seedling structure in the Legumi-

nosae. ' Linn. Jour. Bot. 41: i-iii, 1911-1913.

6. Cooper, D. C, Unpublished results, Univ. of Wise, 1937.

7. Cooper, G. O., "Cytological investigations of Pisum sativum." Bot. Gaz.. gg:

584-591, 1938.

8. Dangeard, p. a., "Recherches sur le mode d'union de la tige et de la racinc

chez les Dicotyledones." Le Bot. Ser. I: 75-115, 1889.

9. Fred, E. B., Baldwin, I. L., and McCoy, Elizabeth, "Root nodule bacteria

and leguminous plants." Univ. of Wise. Studies, Sci. f, 1931.

10. Goebel, K., Outlines of classification and special morphology of plants, Eng. trans.,

Clarendon Press, Oxford, 1887.

11. GouRLEY, J. H., "Anatomy of the transition region of Pisum sativum."

Bot. Gaz- 92: 367-383, 1 93 1.
It. Gregoire, v., "L'Organogenese de I'ovaire et la dehiscence du fruit." Bull.
Soc. Roy. Bot. Belgique, j6: 134-140, 192.4.

13. Guard, A. T., "Development of floral organs of the soy bean." Bot. Gaz.

91: 97-101, 193 1.

14. GuiGNARD, L., "Recherches d'embryogenie vegetale comparee." Ann. Sci.

Nat., Bot. Ser. VI, 12: 5-166, 1881.

15. Herail, J., "Recherches sur I'anatomie comparee de la tige des Dicoty-

ledones." Ann. Sci. Nat., Bot. Ser. VII, z: 103-314, 1885.

370 THE STRUCTURE OF ECONOMIC PLANTS

i6. HoLMAN, R. M., and Robbins, W. W., Textbook of General Botany, 3d ed., John
Wiley and Sons, 1934.

17. Janczewski, E., "Recherches sur I'accroissement terminal des racines dans

les Phanerogames." Ann. Set. Nat., Bot. Ser. V, 20: 161-101, 1874.

18. Martin, J.N. , "Comparative morphology of some Leguminosae." Bot.Gai-

S8: 154-167, 1914.

19. Moore, J. A., "The vascular anatomy of the flower in the papilionaceous

Leguminosae. L" Am. Jour. Bot. z^: 179-190, 1936.

2.0. , "The vascular anatomy of the flower in the papilionaceous Legumi-
nosae. IL" Am. Jour. Bot. 2}: 349-355,1936.

zi. Nemec, B., "Uber die Bakterienknollchen von Ornithopus sativus." Bull.
Internat. Acad. Set. Boheme, i-ii, 1915.

11. Pammel, L. H., "Anatomical characters of seeds of Leguminosae, chiefly
genera of Gray's Manual." Trans. Acad. Set. St. Louis, 9: 91-174, 1899.

13. Pierce, G.J., "The root-tubercles of bur clover (Medicago denticulata Willd.)

and of some other Leguminous plants." Proc. Calif. Acad. Sci. Ser., Ill, 2:
195-318, 1900-1904.

14. Prazmowski, a., "Die Wurzelknollchen der Erbse." Landw. Vers. -Stat.

Berlin, ^y: 161-138, 1890.

15. PuLTz, L. M., "A study of starch deposition in the seeds of Pisum sativum."

Unpubl. Ph.D. Thesis, Univ. of Chicago, 1919.

16. Thornton, H. G., and McCoy, E. P., "The relation of the nodule organism

(Bact. radicicola) to its host plant." Fifth Internat. Bot. Cong. Rep. Proc,
Cambridge Univ. Press, 193 1.

17. TouRNEux, C, "Recherches sur la structure des plantules chez les viciees."

Le Bot. 11: 313-330, 1910.
iS. TscHiRCH, A., and Oesterle, O., Anatomischer Atlas der Pharmakognosie und

Nahrungsmittelkunde, Leipzig, Herm. Tauchnitz, 1900.
19. Wellensiek, S. J., "Genetic monograph on Pisum." (Abridged digest.)

Bibl. Genet. 2: 343-473, 1915.

30. Went, F. W., and Thimann, K. V., Phytohormones, The Macmillan Co., 1937.

31. Westgate, J. M., Coe, H. S., et al., "Red-clover seed production: pollination

studies." U. S. Dept. Agr. Bull. 289, 1915.
31. Whiting, A. L., "A biochemical study of nitrogen in certain legumes."
Univ. of III. Exp. Sta. Bull, ijq, 1915.

33. Wilson, P. W., "Symbiotic nitrogen-fixation by the Leguminosae." Bot.

Rev. }.- 365-399, 1937.

34. WiNTON, A. L., The Microscopy of Vegetable Foods, John Wiley & Sons, 1916.

CHAPTER XIII

LINACEAE

LINUM USITATISSIMUM

THE flax family is not a large one, and common flax is the only
member of economic importance, except for a few species that
are grown for decorative purposes. In addition to commercial
fiber, the plant produces oil which is extracted from the seed, the
residue being pressed into cakes that are used as food for livestock.

There is evidence that flax was grown during the Stone Age,
and its cultivation and preparation for fiber constituted one of the
earliest textile industries. It has been cultivated so long that it
is not now known in its wild state, although an annual form grows
as an escape in the vicinity of the Persian Gulf and the Caspian and
Black seas. The first cultivated form was a biennial type with
small, narrow leaves, Linum angustifolium Huds.; but the annual
or common flax, Linum usitatissimum L., has been grown in Meso-
potamia for at least 4000 years.

The major portion of the crop is produced in Russia, but fiber
of the finest quality is grown in Belgium and Holland. Other
countries which raise flax commercially include : Austria, Hungary,
France, Ireland; and, to a lesser degree, Germany, Italy, Rumania,
and Japan. In Argentina, India, and the United States, the crop
is grown chiefly for the linseed oil, rather than for the fiber, and is
known as seed-flax. This is produced in North and South Dakota,
Wyoming, and Montana, where the climate is too dry for fiber flax.
Robinson (19) has pointed out, however, that it is possible to grow
fiber flax of excellent quality in the United States, and this is being
done in the Willamette Valley in Oregon, as well as the Puget Sound
region in Washington, where cool, moist weather obtains from
March to June, followed by a warm, dry climate in July.

All the varieties used in the United States are regarded as be-
longing to one species, L. usitatissimum; and, although much of
the seed is imported from Europe and Russia, that of several vari-

371

372. THE STRUCTURE OF ECONOMIC PLANTS

eties, including the widely grown Bison, is produced in this coun-
try. For high quality fiber, most growers regard seed from the
Netherlands and Belgium, which has been derived from Russian
strains, as most desirable.

GENERAL MORPHOLOGY

The Root. — The root system consists of a slender tap root 3 or
4 feet in length with lateral branches that arise mainly from its
upper limits. Ten Eyck (14), in describing it, states that

"flax has a different system of rooting from that of wheat or oats.
Its roots do not go so deep . . . but it makes a much greater fibrous
root growth in the upper two feet of the soil. Each plant sends down
a single, small taproot, which gives off many small side roots or
branches, and these in turn give off numerous fibrous roots or feeders.
The upper branches soon curve downward along with the main root
which grows rapidly slender and thread-like, until it can scarcely be
distinguished from its branches. ... By its intricate system of rooting
flax occupies the soil very completely."

Arny and Johnson (3) studied the progressive development of the
Winona variety in Minnesota. They found that the tap root
reached a depth of approximately 3 feet, when the plants were
beginning to bloom and the stems were 2. feet tall, there being a
lateral spread of about a foot. The shallow main lateral roots grew
horizontally for about 6 inches, then turned sharply downward,
and ultimately equaled the main tap root in length. When the
plant was in full bloom, after nearly two months' growth, the tap
root was about 33^^ feet long; and another 6 inches was added in
the ensuing five weeks which brought it to full maturity.

The Shoot. — The stem is slender and erect, branching at some
point above the middle, usually nearer the top, to form the few-
flowered inflorescence which may be a panicle, corymb, or cyme.
(Fig. 188.) In flax grown for fiber, the length of the stalk from the
soil surface to the lowest branches of the inflorescence is an im-
portant factor in determining quality, as only the unbranched
portion of the axis has commercial value since the fiber in the
branches is broken in the processes of fiber preparation.

The simple leaves are linear to lanceolate, sessile, entire, and
without stipules. The leaf arrangement is variable, but the basal
leaves are commonly in alternate pairs, while those above the

LINUM USITATISSIMUM

373

fourth node are spirally arranged. (Fig. 189.) Tognini (15)
reported the phyllotaxy as %, but noted that there were frequent
exceptions to this plan even on one plant. Crooks (7), working
with the Bison variety, occasionally found three leaves in a whorl

Fig. 188. Panicles of Bison flax showing flowers and arrangement of leaves.
(Courtesy Bureau Plant Industry.)

at one of the lower nodes and observed cases in which leaves at the
sixth, seventh, or eighth nodes were bilobed. (Fig. 199, A.') In
some of these, the leaf was broad and cleft nearly to the base of the
lamina; in others, it was narrow with a very small cleft at the
apex. Ontogenetic studies indicate that such double leaves are
the result of the non-divergence in varying degree of two leaf pri-
mordia.

374 THE STRUCTURE OF ECONOMIC PLANTS

The Flower. — The flowers are terminal and blue or white in
commercial varieties, the more common blue-flowered types fre-
quently having a streak of dark blue in the throat of the corolla.
The white-blossomed types require 7 to 10 days longer for bloom-
ing and are grown very little in
the United States since they pro-
duce a relatively coarse fiber.
The flowers are hypogynous and
tetracyclic with five sepals, five
petals, five stamens, and a com-
pound pistil of five carpels in a
radially symmetrical arrange-
ment. (Fig. 190, A-F.') The
persistent sepals are much im-
bricated in the bud and are un-
equal in size, the two outermost
ones being smaller than the other
three, the third sepal intermedi-
ate in size, and the two innermost
ones the largest. The blade of
the sepal is oval, acuminate and
glabrous, except for the margins
which may be scarious, and the
tip is ciliated. The ephemeral
petals are wedge-shaped, about
twice the length of the sepals,
and their margins overlap one
another in either a right- or left-
handed spiral. In most instances,
the flowers open in early morning
and the petals begin to fall at
about nine or ten o'clock when
the sun becomes hot; but, if it
is cloudy, the corolla may remain in place throughout the day.

The androecium is slightly monodelphous at the base; and,
on the margin of the undiverged staminal ring, there are usually
five tooth-like staminodia which alternate with the filaments of
the five fertile stamens. These are small processes, not exceeding
a millimeter in length; but Tammes (xx) has reported cases in
which varying degrees of development were found, some even

Fig. 189. — Habit of flax plant showing
character of stem and inflorescence.
(Photograph by Copson.)

LINUM USITATISSIMUM

375

producing anthers containing pollen. The anthers and pollen
in both the blue and white varieties of fiber flax are blue; but the
pollen is sometimes yellow in the oil flaxes, especially in the
Indian varieties; and the anther walls may be yellow or white.
On the outer side and at the base of each filament, there is a nectary

Fig. 190. A, inflorescence; B, longitudinal view of flower with portion of perianth
removed ; C, stamens and pistil with perianth removed ; D, stamen ; E, mature fruit ;
F, floral diagram.

at the lower end of a longitudinal groove. The anthers are introrse
and when the flowers close toward noon, the filaments are bent
inward so that the anthers contact the stigmatic surfaces, resulting
in self-pollination. The pistil has five distinct styles and stigmas,
but the carpels are undi verged at their bases. The ovary is five-
loculed, although it often looks ten-loculed because of the forma-
tion of false septa which develop on a line with the dorsal suture,

376 THE STRUCTURE OF ECONOMIC PLANTS

growing centripetally until they partially or completely divide
each locule. The septa are non-ciliate in some forms and pilose
in others, the latter condition existing in most non-dehiscent
types.

The Fruit. — The fruit is a spherical or egg-shaped capsule
ID to li mm. in diameter and 8 to 15 mm. in height. It is dry
at maturity and the partial dehiscence is septicidal, but each locule
may also separate into two parts along the line of the dorsal
septum so that the ripe fruit seems to be ten-parted. Tammes (ii)
reports a variation with respect to dehiscence in flax, it being the
general condition in wild species and in a form of cultivated flax,
L. usitatissimum crepitans, Boningh., which is still grown in
some regions of the Ukraine. In other cultivated forms, the
capsule is either non-dehiscent, or only slightly so at the apex
of the fruit, and the seeds cannot escape. Two anatropous ovules
arise from axillary placentae toward the summit of each locule
so that they appear to be suspended. (Fig. 191, A.^

The Seed. — The somewhat beaked, oval seeds are flattened or
lens-shaped in transection and variable in size, being designated
as small or large in commercial practice. In general, the seed
of oil flax is the larger, averaging about 5 to 6 mm. in length
and X.5 to 3.5 mm. in diameter, while that of fiber flax approxi-
mates 3 to 5 mm. and 2. to 1.5 mm. respectively. The smooth
surface is highly polished and the seed coat may vary in color from
white to various shades of yellow or brown, some being reported
as variegated.

Development of the Seedling. — The germination of the
seed is rapid at temperatures of 65-75° F., and the primary tap
root emerges in one or two days, constituting the main axis of the
root system. The seedling emerges above the ground level in
four or five days and the hypocotyl elongates at its basal point,
curving until it takes the shape of an inverted U. (Fig. 191.)
It pulls the cotyledons and seed coats above the ground, and in
about five days, the hypocotyl straightens and the remains of the
seed coats are shed. After a period of general growth, there is a
zonal elongation of the hypocotyl which is initiated at the ground
level and extends progressively upward to the cotyledonary plate
until growth in length ceases at the end of 15 to xo days. The
first lateral roots appear in five to six days, usually emerging within
3 cm. of the soil surface, and most of the subsequent ones originate

LINUM USITATISSIMUM

377

along the upper 8 cm. of the primary root. When the cotyledons
are liberated from the seed coats, they separate and expand for
several days prior to the elongation of the epicotyl, which differ-
entiates very slowly at first, appearing as a compact bud with
several small leaves at the end of ii to 15 days.

After the seedlings are about an inch above the ground, they
grow slowly for two or three weeks and then there is rapid increase

O

01

Fig. 191. The seed and stages in development of seedling.

in height which continues until the time of blossoming if condi-
tions are favorable. The first internode always remains short,
not exceeding i mm. in length at maturity, the second internode
is longer than the others, attaining a length of about 15 mm.,
and the remaining ones vary from 5 to 10 mm.

ANATOMY

Anatomy of the Seed. — The mature seed coat consists of
several distinct layers. The outer one is made up of cells which
are polygonal in surface view and rectangular in longisection.

378 THE STRUCTURE OF ECONOMIC PLANTS

They are covered by a finely granular cuticle; and, as the cells
develop, stratified layers of material are laid down against the
thick outer walls until they nearly fill the cell cavities with a

0 int~

i int

,0 int

i int\ —

Fig. 191. A, median longisection of anatropous ovule; B, C, and D, progressive stages
in development of seed coats. The numbers 1-6 represent corresponding cell layers or their
derivatives in the four figures : car, caruncle ; emb s, outline of embryo sac ; end, endosperm ;
fun, funiculus; i int, inner integument; mk, micropyle; 0 int, outer integument. (Re-
drawn after Tschirch and Oesterle, Anatomischer Atlas der Pharmakognosie und Nahnmgsmittle-
kunde, Herm. Tauchnitz.)

mucilaginous substance that is related to the medicinal properties
of the seed. (Fig. 191, D.)

Haberlandt (9) has described the structure and behavior of this
layer when water is gradually imbibed. Each cell is separated
from those adjacent by a thin, sharply defined middle lamella

LINUM USITATISSIMUM 379

which is limited externally in the cutinized outer wall. As the
secondary layers gradually absorb water, their stratification
becomes very evident and they begin to swell. The middle
lamellae are relatively inelastic; and, as a result of the pressure
exerted by the swelling layers against the comparatively tough
cutinized outer wall, they give way and the outer walls are lifted
up and fissured by the prismatic masses of swollen mucilage.

Underlying the epidermis are one or two layers of "round cells"
with well-developed intercellular spaces, which are so called
because of the circular appearance of the cell cavities in surface
view. The third layer consists of longitudinally oriented fiber
cells which are thick-walled and porous. According to Winton
(i8), their length may vary considerably, ranging up to 150 ix
with radial dimensions that greatly exceed their breadth. The
fourth and fifth zones are made up of several layers of very thin-
walled, colorless "cross cells" which are so named because their
long axes are oriented at right angles to the fiber cells. Tschirch
and Oesterle (x6) found that the cells of this zone contain large
deposits of starch; but, as the seed develops, this is utilized and
the cells become crushed and more or less obliterated.

The uniseriate pigment layer is the innermost one of the seed
coat, and is made up of cells that are square or polygonal with
thick porous walls and contents which determine the color of
the seed. The straight embryo has a short hypocotyl and two
long fleshy cotyledons that are thicker than the surrounding endo-
sperm. This varies in thickness, consisting of from two to six
layers of parenchymatous cells which are thicker walled than
those of the cotyledons and contain aleurone grains and fat.

Embryogeny. — The early stages in the development of the
embryo of the Linaceae have been described by Soueges (lo), who
used Linum catharticum L. as a type form. Following fertiliza-
tion, the first division of the zygote occurs in a transverse plane
producing an apical and a basal cell, and subsequent transverse
divisions of these two cells result in the formation of a linear row
of four cells, /, /', m and ci. (Fig. 193, x-6.) Of this linear series,
the apical cell, /, always divides in a vertical plane; and the basal
cell, d, always divides transversely. The division of cell /' is
usually either longitudinal or transverse (occasionally oblique);
and cell m may divide transversely or longitudinally. (Fig. 193,
7-2-1 •)

38o THE STRUCTURE OF ECONOMIC PLANTS

Because of the variation in the divisions of the two median cells
of the linear tetrad and the consequent variation in the orientation
of the derivative cells, three distinct types of eight-celled pro-
embryos occur. The most common one is an octant which has
seven linear layers of cells; but other possibilities include ones
with six or with five layers. (Fig. 193, 13, 15.) All later stages
in the embryonic development can be referred to one of these three
tvpes, the derivatives of the more common seven-layered type are

Fig. 19}. Early stages in development of embryo of Linum catharticum L. : ac and be,
apical and basal cells of the proembryo ; m and ci, cells derived from basal cell ; / and /', cells
derived from apical cell ; « and «', cells derived from ci; d and /, cells derived from m: cal-
dgn, calyptrogen-dermatogen ; h, hypophysis; ^hm, periblem ; p/, plerome; sus, suspensor.
(Redrawn and adapted from Soueges, Compt. Rend. Acad. Sci.^

shown in Figure 193, 19, lo, ii, and those of the six-layered type
in Figure 193, 18, 2.1.

In the subsequent development of the embryo, the cell /, or the
upper daughter cell derived from it, undergoes periclinal division,
and further development of the resultant cells leads to the differ-
entiation of the cotyledonary portion of the embryo. The behavior
of the cells /' and m of the original tetrad varies with the type of
eight-celled proembryo; but in the most common case (the seven-
layered proembryo) the cell /' divides into two superposed cells,
the upper one producing the hypocotyl, while the lower one
divides again transversely, forming two cells. The upper of these

LINUM USITATISSIMUM 381

two cells becomes the hypophysis and the lower one contributes
to the development of the suspensor. In the case of the six-layered
octant, derivatives of the cell /' form the hypocotyl; and the upper
daughter cell, d, derived from the cell m, produces the hypophysis
after one or two subsequent transverse segmentations while the
lower daughter cell, /, enters into the structure of the suspensor.
In the least frequent case, in which the octant consists of five
layers (Fig. 193, 13), the layer /' produces the hypocotyl and the
layer m, the hypophysis. This divides transversely to form two
cells which enter into the development of the two outer histogens
of the growing point of the root tip. The upper cell contributes
initials to the periblem, while the lower one becomes a part of
the calyptrogen-dermatogen layer that produces the root cap
terminally and the epidermis laterally. (Fig. 193, 19-30.)

The Primary Root. — The primary root has a diarch protostele
with two rather large groups of primary phloem flanking the
primary xylem strand. (Fig. 197, A.') Tognini (15) has reported a
special case in which seedlings differentiated three cotyledons,
and the six downwardly diverging cotyledonary traces were united
in pairs in the lower hypocotyl so that the primary root had a
triarch stele.

There are three to five elements in each protoxylem group, and
the metaxylem vessels are successively larger toward the center.
The pericycle and endodermis are single-layered and the Casparian
strips of the latter are laid down on the radial and end walls before
the metaxylem is mature. The cortical cells are isodiametric with
large intercellular spaces, except for the compact hypodermal layer
in which the cells are somewhat radially elongated and resemble
the epidermal cells. The cortex persists for some time and when
disintegration occurs, as a result of secondary thickening, it begins
in a midregion between endodermis and hypodermis.

The ontogeny of the primary root has been investigated by
Janczewski (13), who used it to illustrate a special variation in his
third type of angiospermous roots in which the plerome and peri-
blem are sharply defined, and are covered by a common initial
layer which gives rise to the epidermis and root cap. In most
roots with this type of development, the periblem consists of a
single layer at its apical point and becomes wider laterally by sub-
sequent periclinal divisions; but, in flax, the apical portion of the
periblem consists of two initial layers. In his studies of the devel-

38i THE STRUCTURE OF ECONOMIC PLANTS

B

Fig. 194. A, median longisection of primary root ; B, transection of primary root 0.7
mm. from tip; C, schematic diagram of hypocotyl showing vascular transition from root to
cotyledons ; a, h, bundles which anastomose and form double bundle of cotyledon ; a' , h ,
lateral metaxy'lem elements which form lateral bundles of cotyledons : cal, calyptrogen ;
w, cortex; co/, cotyledons ; J^«, dermatogen ; ^c/, epicotyl ; ««, endodermis ; ^p, epidermis;
hd, hypodermis ; lb, lateral bundle to cotyledon ; mh, median double bundle of cotyledon ;
mx, metaxylem; fbm, periblem ; pel, pericycle; pd, primary phloem duct; p, pith; pi,
plerome; /)x, protoxylem ; re, root cap. (After Crooks.)

LINUM USITATISSIMUM

383

opment and regeneration in the flax seedling, Crooks (7) has con-
firmed and elaborated Janczewski's work, and his detailed account
of the root ontogeny is quoted.

"The calyptrogen-dermatogen layer of cells overlying the periblem
multiplies by periclinal divisions around the tip and forms a root cap
of regular, radial rows of cells. In the lateral portion of the same
layer the epidermis is formed by anticlinal divisions. Laterally,
where the divisions are anticlinal the histogen is strictly a dermatogen,
while the layer at the tip is a calyptrogen. The root cap at the tip
is very regular because the cell divisions in the calyptrogen are periclinal
only. Occasionally, some of the
cells in the lateral portion of the
root cap divide anticlinally and
distort the radial rows. This
occurs only in lateral portions of
the root cap and not at the tip,
where the rows of cells are regular.

"The cortex is derived from the
periblem, which consists of two
layers of cells overlying the
plerome. The outermost layer of
the periblem divides only anti-
clinally and forms the outer layer
of the cortex, the hypodermis.
The inner layer divides in all
planes and forms the remainder of
the cortex, which at maturity is
about 6-9 cells in thickness. (Fig.
194, /4.)

' 'The stele is differentiated from
a small group of plerome cells
which divide in all planes at the
tip, while divisions at a higher
level are chiefly in a transverse
plane. The outer layer of this
group produces the pericycle, which can be distinguished rather early
by the relatively dense cytoplasm and larger size of the cells. The first
evidence of differentiation of the vascular system appears about 0.4 mm.
above the tip of the plerome. Two primary phloem ducts are first dif-
ferentiated opposite each other by the elongation and breakdown of a
single row of cells lying next to the pericycle, and alternate with the
two protoxylem points which are differentiated later. Figure 195 shows
how these ducts are formed and stretched by the elongation of neigh-
boring cells. These ducts collapse after becoming greatly stretched
but usually persist until the protoxylem elements are well differentiated.
Elongation of the root often causes the protoxylem to collapse. The

Fig. 195. A, B, and C, longisections of
portions of primary root showing forma-
tion of primary phloem duct at 0.61, i.o,
and 1.5 mm. from tip respectively. (After
Crooks.)

384 THE STRUCTURE OF ECONOMIC PLANTS

metaxylem elements differentiate centripetally until all the elements
of the xylem plate are lignified and no central parenchyma remains.
The outermost metaxylem elements have scalariform wall thickenings
while those centrally located have pitted wall thickenings."

The Origin of Lateral Roots. — The lateral roots are initiated
at about the time that the secondary wall thickenings of the
metaxylem elements are being laid down. The first secondary
root primordia occur just below the soil level and are oriented in
the plane of the protoxylem strands or more frequently slightly
to one side of them. The cells of the pericycle adjacent to the
protoxylem points undergo tangential divisions which result in the
production of two distinct layers. (Fig. 196, A, a and ^.) The
cells of each of these layers divide tangentially so that the root pri-
mordium is four layers in width as seen in a transection of the
primary root. (Fig. 196, B.) The divisions of the first two
layers do not take place simultaneously, and ordinarily the inner
layer divides somewhat in advance of the outer one. In order to
compensate for the increasing length of the overlapping layers, a
few radial divisions occur in the root primordium but these take
place in such a way that they cause no distortion of the tangential
alignment of the rows of cells. The four layers, derived from the
pericycle, function as histogens: the outer one a' becomes the
calyptrogen-dermatogeA layer; the next one a" functions as the
periblem; and the two inner ones b' and b" give rise to the plerome.
While the growing point of the lateral root is differentiated
from pericyclic derivatives, the adjacent cells of the endodermis
divide radially to compensate for its enlargement. This anticlinal
division of the endodermal cells continues so that it persists as a
single layer of cells about the tip of the lateral root. (Fig. 196,
£-H.) It never becomes more than one cell layer in thickness, but
maintains its position until the lateral root penetrates through the
cortex of the primary root and into the soil, when it disintegrates,
together with the outermost layer of the root cap. As the lateral
root penetrates the cortex, there is little accumulation of debris or
distortion of the cortical cells, which indicates that the emergence
of the lateral root involves some digestion as well as mechanical
crushing of the cortical tissues.

In tracing the subsequent development of the histogens, Crooks
has pointed out that the two inner layers constituting the plerome
continue to divide in all planes and produce a conical group of cells

LINUM USITATISSIMUM

385

which differentiate stelar tissue in the same manner as occurs in the
primary root. (Fig. 196, D.) The stele is diarch and the primary

Fig. 196. A-H, transections of young primary roots showing origin of lateral roots; a,
outer layer of first two layers derived from pericycle; a', outer layer derived from a; a",
inner layer derived from a; a" Qphni), periblem ; b, inner layer of first two layers derived
from pericycle; b' , outer layer derived from b; b" , inner layer derived from b; c, cortex;
cal, calyptrogen ; dgn, dermatogen ; e, endodermis ; hd, hypodermis ; pel, pericycle ; pi,
plerome; px, protoxylem. Derivatives of layer a' become calpytrogen and dermatogen;
those from layer a" become periblem ; and those from layer b, plerome. (After Crooks.)

386 THE STRUCTURE OF ECONOMIC PLANTS

xylem strand of the lateral root lies in the same plane as the strand
of the root axis from which it originates.

As the plerome increases in size, anticlinal divisions occur in the
two outer layers of the pericycle (<«', a"^ to compensate for this
growth, and the innermost of these two layers (a"^ begins peri-
clinal divisions at a point farthest removed from the tip. These
periclinal divisions occur progressively in this layer toward the tip
of the primordium; and, subsequently, their innermost derivatives
also undergo periclinal divisions. This stage is illustrated at the
right in Figure 196, £, where the basal portion of the layer a" has
divided once and one of the innermost derivatives is undergoing
another division as indicated by the mitotic figures. In this man-
ner, the number of cell layers is increased by successive periclinal
divisions from the newly formed layers of the periblem. (Fig.
196, F.) In each instance, the divisions of the inner layers are
continued progressively toward the tip of the root; and, finally,
the remaining cells of layer a" are all divided once periclinally and
the basic plan of the periblem is established which is similar to that
described for the primary root. (Fig. 196, H.) The outermost
layer of the periblem divides only anticlinally, producing a single
layer which differentiates into the hypodermis. At the tip of the
root, the innermost layer of the periblem continues to divide and
produces the other six to eight layers of cortical cells, the innermost
becoming the endodermis.

By the time that the periblem has formed about three cell layers
around the base of the conical plerome, the outermost one, a' ,
undergoes periclinal divisions at its tip. (Fig. 196, F.) Of the
two layers thus formed, the outer one becomes the first layer of the
root cap, while the inner one remains meristematic and by further
divisions produces additional layers of the root cap. As in the
primary root, the laterally oriented cells of the layer a' continue
anticlinal divisions and function as a dermatogen which gives rise
to the epidermis. Following the establishment of the histogens,
subsequent stages in the ontogeny of the lateral root are similar to
those of the primary root.

Vascular Transition. — Vascular transition takes place in the
hypocotyl although the stelar tissue is not completely collateral
and endarch until the bases of the cotyledons are reached. At
approximately the ground level, the stele of the lower hypocotyl is
root-like with a diarch primary xylem strand, laterally placed

LINUM USITATISSIMUM

387

phloem groups, and no central pith; but above this point, the
center of the axis is parenchymatous and there is a gradual reorien-
tation of the vascular elements. (Fig. 197, /I.) Each of the
phloem groups divides to form four phloem regions in the low^er
one-fourth of the hypocotyl; and, at a higher level, they become

Fig. 197. A-D, transections of six-day seedling showing vascular transition : A, about
2. mm. below ground level ; B, middle hypocotyl showing parenchymatous pith and disin-
tegration of protoxylem ; C, upper region of hypocotyl showing eight metaxylem groups
with associated phloem ; D, at base of cotyledons and epicotyl showing vascular bundles
of cotyledon : a, b, bundles which anastomose to form double bundle of cotyledon ; a', b',
lateral bundles of cotyledon : en, endodermis ; mx, metaxylem ; pel, pericycle ; ph, phloem ;
fx, protoxylem; vb, vascular bundle of first foliage leaf. (After Crooks.)

equally spaced at about 90° intervals from one another. (Fig.
197, B.) The primary phloem of the leaf traces of the first two
foliage leaves is dow^nwardly differentiated into the hypocotyl
and anastomoses with the phloem of the cotyledonary bundles at
about the point where the phloem groups of the lower hypocotyl
are branched.

388 THE STRUCTURE OF ECONOMIC PLANTS

Coincident with the reorientation and branching of the primary
phloem, the metaxylem elements are differentiated in a more and
more tangential position in relation to their respective protoxylem
points. There are four metaxylem groups, each of which is
adjacent to a phloem strand and centrad to it. At about the mid-
point in the hypocotyl, the four metaxylem groups and their
collateral phloem strands are each bifurcated so that eight
vascular bundles are formed which constitute the cotyledonary
traces. At higher levels, two groups, each of four metaxylem
strands, are formed; and these become oriented in a more nearly
lateral position with respect to the protoxylem poles. As this
reorientation of the two groups of cotyledonary bundles proceeds,
they become widely separated, each group of four bundles con-
stituting the vascular supply to a cotyledon. (Fig. 197, C.)

At the cotyledonary node, the two lateral bundles of metaxylem
and the accompanying phloem in each group of four, become more
widely separated from the two centrally located ones, and continue
without branching or anastomosing into the cotyledons forming
the lateral veins. At higher levels, the two centrally located
metaxylem groups are oriented in a more centrad position with
reference to the protoxylem; and, at the point of divergence of the
cotyledons, the primary xylem is endarch. The two metaxylem
groups anastomose, forming the median double bundle of each
cotyledon; but the two phloem groups of each double bundle
are distinct at this level, anastomosing x or 3 mm. farther up in the
cotyledon. (Fig. 197, D.)

The vascular system of the epicotyl is differentiated somewhat
later than that of the cotyledons and hypocotyl. The bundles
which constitute the traces of the first foliage leaves above the
cotyledonary plate are differentiated as endarch, collateral bundles
and extend downward into the hypocotyl where they may anasto-
mose with groups of metaxylem and phloem; but, in some in-
stances, end blindly in the parenchymatous tissue.

There is disintegration and a partial or complete resorption of the
primary xylem in the seedling axis, similar to that described for
Allium by Chauveaud (5) and Hoffman (12.)' ^^^ because of this
ephemeral character of the primary xylem. Crooks used plants
about six days old. In older axes, the protoxylem is first crushed
and stretched; later, in the upper and middle hypocotyl, the first-
formed metaxylem disintegrates; and, ultimately, nearly all of the

LINUM USITATISSIMUM 389

primary xylem disappears. The stretching of the spiral elements
results in their collapse; and, in plants two weeks old, practically
all of them in the upper hypocotyl are obliterated. (Fig. 198.)
However, fragments of the primary xylem may remain for longer
periods, or throughout the life of the plant, without being com-

FiG. 198. Transection of portion of vascular ring in upper hypocotyl showing early dis-
integration and resorption of primary xylem : ca, cambium ; mx, metaxylem ; ^h, phloem ;
/>/, pith; /).v, protoxylem; atj i, secondary xylem. (After Crooks.)

pletely resorbed; and may be seen, especially in longisection,
among the parenchymatous cells.

The Cotyledons. — The obovate cotyledons are subsessile, and,
in young seedlings, are somewhat broader than the hypocotyle-
donary axis at their point of divergence. The mesophyll is made
up of very compact storage cells, two or three of the adaxial layers
forming a palisade region while the lower four or five layers are
more rounded and compact. There is an early differentiation of the
protoxylem elements in the median bundle; and under favorable
conditions they are completely matured twenty-four hours after
germination. Cases have been observed where some of the proto-
xylem is differentiated in the mature seed, and scattered phloem
elements occur in the provascular strands of the immature cotyle-

390 THE STRUCTURE OF ECONOMIC PLANTS

dons before wall thickenings can be observed in the primary
xylem.

After emergence above the ground, the cotyledons enlarge and
thicken for about two weeks, persisting for a longer period than
do many seedlings; but, in about thirty days, they become yellow
and die. The mesophyll of the mature cotyledon resembles that of
a foliage leaf, except that it is thicker and the intercellular spaces
are smaller. Stomata occur in considerable numbers in both the
upper and the lower epidermis and are subtended by large air spaces.
The venation is closed, resembling that of the leaf; and, except at

Fig. 199. A-C, diagrams showing venation: A, double leaf; B, cotyledon; C, typical

foliage leaf. (After Crooks.)

the base of the median vein, all of the bundles are collateral. (Fig.
199, B.)

Ontogeny of the Leaf. — The epicotyledonary axis develops
very slowly for the first eight or ten days and is surrounded by
the bases of the cotyledons. The leaf primordium involves at
least three of the outermost cell layers at its point of origin, arising
from the growing point as a conical mass of cells which elongates
and broadens, due to general meristematic activity, and becomes
a somewhat flattened subconical projection. (Fig. x6.) Con-
tinued general cell division increases the length of the flattened
primordium, until it is about one-fifth the length of the mature
leaf; and then a localization of meristematic activity to the lateral

LINUM USITATISSIMUM 391

and adaxial portions of the primordium results in the formation of
the lamina. As this proceeds, the cells of the mesophyll undergo
enlargement, and the procambial strands, which were determined
early in ontogeny, differentiate as vascular bundles.

The meristematic activity does not cease at one time throughout
the developing leaf, but is discontinued first at the tip of the leaf,
then at its base, and finally in the midbasal portion of the lamina.
(Fig. 1.00, A, B, C.) Following the cessation of cell division,
which occurs when the leaf has attained about one-fifth its mature
size, further enlargement is accomplished by the expansion, stretch-
ing, and separation of the cells already formed, and, at maturity,
the mesophyll is very loose and spongy with large intercellular
spaces. (Fig. 1.00, D.) It consists of about four cell layers which
are not differentiated into a well-defined palisade and spongy paren-
chyma, and the cells are all somewhat elongated and essentially
alike. A network of veins extends throughout the mesophyll,
forming a complicated system made up of bundles of various sizes.
The midvein and occasionally the two larger lateral veins may
develop some secondary tissue. The epidermal cells are irregular
in size with a thin cuticle, and numerous stomata occur in about
equal numbers on both surfaces of the leaf. The guard cells
are subtended by accessory cells in which, as pointed out by
Haberlandt (9), the thin walls act as hinges in the stomatal
mechanism.

The successive leaf primordia are at first very close to one another
so that there may be ten to twenty nodes and internodes in a grow-
ing point which is only o.i to 0.15 mm. in length. The provascular
tissue of the epicotyl appears to be differentiated simultaneously
in the leaf primordia and meristematic tissue of the growing point,
and each strand extends downward through the axis for several
internodes, keeping pace with internodal elongation so that there
is no change in the fundamental plan of the vascular system as
differentiation proceeds.

All the bundles of the mature vascular system are collateral and
common. The vascular supply to each leaf consists of three large
bundles which pass through the cortex separately, anastomosing
with one another at the point where they form a part of the stele
of the stem, and gradually becoming smaller at lower levels until
they end blindly after extending through fifteen to twenty nodes.
There are usually eight to ten foliar bundles which extend through

392-

THE STRUCTURE OF ECONOMIC PLANTS

the cotyledonary node and into the hypocotyl; and, of these, the
bundles from the first pair of leaves above the cotyledons may

Fig. ioo. A, transection of first leaf 0.9 mm. from tip where cell divisions have ceased;
B, transection of same leaf 0.18 mm. from tip showing meristematic activity in lamina; C,
transection o.i mm. from base of same leaf showing level where cell divisions have ceased;
D, transection through median vein of mature leaf from middle region of stem : /b, lateral
bundle; /^p, lower epidermis ; a«(^, median bundle; w.f/, mesophyll ; ph, phloem; r/o, stoma ;
uep, upper epidermis; xj i, primary xylem; xj i, secondary xylem. (After Crooks.)

LINUM USITATISSIMUM 393

anastomose with the lateral arches of metaxylem in the root-like
portion of the lower hypocotyl while the others end blindly.
Interconnections between the bundles of the stem are accomplished,
following the maturation of the primary tissues, by the develop-
ment of fascicular and interfascicular cambiums which produce a
continuous cylinder of secondary vascular tissue.

The Stem. — In the United States, relatively little investigation
of the structure of the stem and its fibers has been carried on; but in
Europe, where fiber production is more important, extensive studies
have been made. The most desirable stems for fiber purposes are
those which, in addition to having the genetic qualities necessary
for good fiber production, have been grown closely together so that
the stems are long, slender, and unbranched. In such cases, the
diameter of the mature stem of fiber flax may not exceed i or 3 mm. ;
but it may be considerably greater under unfavorable cultural
conditions, or in the more branched, stocky types of flax that are
grown for seed.

The young stem is round or subterete in transection; and, as it
matures, it becomes hollow due to a progressive disintegration of
the centrally located parenchymatous cells. The relatively large,
thick-walled epidermal cells have a well-defined cuticle; their
tangential dimension is about twice the radial one; and they are
elongated in the axial direction, being several times as long as wide.
The stomata which occur in longitudinal rows are somewhat
depressed, and the guard cells are subtended by crescentic acces-
sory cells that form a part of the motor mechanism as in the leaf.
Tammes (xi) reports a stomatal frequency of 30-40 per sq. mm.,
while Herzog (11) records counts of 2.5-35.

Within the epidermis is a single-layered hypodermis in which
the cells resemble those of the former, but are much smaller and
thinner walled. The hypodermal cells contain chlorophyll, and
are compact rather than having the spongy organization of the
other four or five layers of chlorenchyma lying between the hypo-
dermis and the endodermis. The spongy cells are approximately
isodiametric and arranged in longitudinal rows that may be more
or less separated depending upon the age of the stem. As the stem
becomes older, the epidermis persists, its component cells under-
going radial divisions without rupturing, while the cortical cells
first become tangentially elongated and then much crushed and
radially compressed. The large, oval endodermal cells can be

394 THE STRUCTURE OF ECONOMIC PLANTS

readily distinguished in the young stem by their dense cytoplasmic
contents.

The pericycle at first consists of a single layer of thin-walled
cells, but later forms a multiseriate zone between the endodermis

Fig. 7.01. A, transection of sector of mature flax stem; B, transection of comparable sec-
tor of young stem ; ca, cambium ; chl, chlorenchyma ; con, connective tissue ; en, endodermis ;
ep, epidermis ; hd, hypodermis ; pel, pericyclic fibers ; ph i, primary phloem ; ph 2., secondary
phloem; pi, pith; xy i, primary xylem; xy 2., secondary xylem.

and the phloem, and in this region groups of fibers develop which
constitute the fiber bundles of commerce. The development of the
pericyclic fibers is centripetal, the first ones differentiated being
adjacent to the endodermis, while the later formed ones abut the
phloem. (Fig. xoi.)

The vascular bundles form a dissected siphonostele in which

LINUM USITATISSIMUM 395

adjacent bundles are separated from one another by medullary rays
one to several cells in width. As the stem matures, the activity
of the fascicular cambium, together with the initiation of an inter-
fascicular cambium, results in the formation of a continuous
cylinder of secondary xylem. In addition to this, there is a thick-
ening of the walls of the ray parenchyma, as well as of the medul-
lary parenchyma surrounding the primary xylem points, so that
connective tissue also contributes to the continuity of the mechani-
cal elements of the stele. The remaining cells of the medulla, aside
from the centrally located ones which disintegrate, are large and
thin-walled.

The number of primary xylem elements in a single bundle is not
large, the protoxylem consisting of a few annular, annular-spiral,
and spiral types. As the stem matures, the rings in the annular
elements are usually distorted or partially obliterated, and the
spirals in the other types become much stretched as the inter-
nodes elongate. The metaxylem elements are scalariform and
reticulate, while the secondary xylem vessels are pitted with
vessel segments that commonly have oblique end walls. The
cells of the xylem parenchyma are much elongated axially and
have transverse end walls. The phloem consists of slender
sieve tubes, companion cells, parenchyma; and occasionally
phloem fibers, but the latter have no commercial importance.
(Fig. loi.)

The Ontogeny of the Fiber — Initial Stages. — The fibers are
differentiated in the pericyclic region, and the initial stages in their
ontogeny take place just below the meristematic growing point of
the stem axis. In longisection, the pericyclic cells can be dis-
tinguished from the endodermal cells which lie immediately outside
them, since the former do not divide transversely as frequently as
the latter, the pericyclic cells compensating for the increase in the
length of the axis by cellular elongation rather than by division.
Early in ontogeny, however, the initial pericyclic cells do divide
periclinally several times so that this zone becomes a multilayered
region of large, thin-walled, elongated cells.

Anderson Ql) has pointed out that the

"enlargement phase of fiber development involves two types of increase:
an early, rapid, and extensive increase in diameter and length; and a
slow but perceptible increase in diameter, limited to certain cells,
that continues during the life of the plant."

396 THE STRUCTURE OF ECONOMIC PLANTS

While some investigators have expressed doubt regarding the
second type of growth, critical experiments by Tammes (ii) indi-
cate that as the stem increases in length, there is also an increase
in fiber diameter. This increase does not occur at a uniform rate,
being much more rapid in the initial stages of fiber development
than during the later phases of maturation. She found that as the
stem increased from i mm. up to a h:^ight of 77 cm., the average
diameter of the fiber increased from 11.6 ^ to 31.0 /-i. Anderson (x)

a b c d e f g h i j k

Fig. 2.02.. Semi-diagrammatic longisection of portion of nearly mature flax stem : a, pith
cells including thick-walled connective cells adjacent to xylem; b, protoxylem elements and
xylem parenchyma ; c, metaxylem ; d, secondary xylem ; e, cambium ; /, phloem zone in-
cluding secondary and primary phloem ; g, pericyclic fibers ; h, endodermis ; /, chloren-
chyma ; /, hypodermis ; k, epidermis.

compared measurements of young and mature fibers and concluded
that developmental increases in fiber size were not common for all
fiber cells, occurring more conspicuously in some than in others.
He states that there is no doubt but that fibers do undergo "a slow
increase in diameter during their development until the time that
deposition of cell-wall material ceases."

The length and diameter which the fiber attains are variable,
depending upon variety and cultural conditions. Diameters range
from IX to X5 m- Matthews (15) and Cross and Bevan (8) have
reported average lengths of 15 to 30 mm., Haberlandt (9) lo to
40 mm., Wiesner (2.7) xo to 50 mm., Herzog (10) 1.7 to 53.9 mm..

LINUM USITATISSIMUM 397

and Tammes (ii), extremes of i to ixo mm. with averages approxi-
mating those of other investigators. The extreme lengths of the
individual fiber cells are based upon their continued independent
growth. Tammes (ii) points out that the pericyclic cells which
differentiate into fibers do not undergo any transverse division,
or at least not as frequently as adjacent cells, and continue to
elongate without cross wall formation. This general growth
proceeds until the basal portion of the cell is below the region of
axial elongation of the meristematic portion of the stem tip,
and then that part of the fiber ceases to elongate, further increase
in length being restricted to the upper portion of the fiber. Theo-
retically, the pericyclic cell may continue to elongate as long as the
adjacent cells are dividing transversely and the axis in that locus
is undergoing internodal elongation; but, actually, there are
occasional transverse divisions. In connection with secondary
wall thickening, there may be a peculiar separation of portions of
the protoplast by secondary layers of cellulose which results in local
dilations in the fibers. (Fig. 2.03, D, 1-4.^

In addition to the method of fiber elongation described by
Tammes (2.1), Anderson (2.) believes that there is a sliding action
by which the tips of the elongating pericyclic cells push past one
another. It seems likely that this phase of elongation accompanies
general cellular enlargement. The tips of the fibers are not trans-
verse, but taper gradually to a very acute point, and the length of
the tapered portion of the fiber may indicate the approximate
amount of sliding that has occurred. (Fig. 2.03, D, /.)

Growth and Wall Formation of the Fiber. — The wall
structure of the flax fiber has been the subject of intermittent inves-
tigation since Nageli (16) observed the concentric lamellae and
alternating light and dark bands in its secondary walls. Subse-
quently, Krabbe (14), Correns (6), Tammes (ii), Nodder (17),
Aldaba (i), Anderson (i), and others have each added something
to an understanding of fiber structure and development.

The middle lamella consists of pectic materials deposited by the
split halves of the cell plate when wall formation is initiated. Her-
20g (11) and others interpret it as being intercellular substance,
applying the term primary wall to the first deposits of material
against the middle lamella and using the term secondary wall in
referring to the lamellated structures deposited later. (Fig. 103,
A-C.^ There has been some confusion in cell wall terminology

398

THE STRUCTURE OF ECONOMIC PLANTS

(Chapter I), but it is clear that the first phase of growth involves a
rapid elongation and enlargement of the fiber cell and a slight
increase in the thickness of the cell wall resulting from additions of
cellulose to the middle lamella. Adjacent to the middle lamella

D

\

1 2 3

p w
s w

Fig. 2.03. Structure of fiber: A, transection of group of fibers showing middle lamellae
and concentric layers of secondary wall with delicate reticulated structure; B, transection
of single fiber showing primary wall and two layers of secondary wall, innermost of which is
divided into seven lamellae; C, diagram of fiber in transection showing lumen, lu, middle
lamella, m Im, primary wall, p w, and secondary wall, s w; D, longitudinal views of ends
of fibers (fibers 1-4 illustrate abnormal forms from basal portion of stem, while those in
group 5 are normal types) ; E, F, and G, transections of progressive stages in development
of fiber ; E, transections of young fibers showing, a, fiber with infolded inner wall layer ;
b, point of fiber in transection, and c, fiber cells still unthickened ; F, transection of two
mature fibers from lower part of stem showing infolded inner wall layers ; G, transection of
group of fibers from uppermost part of mature stem showing thickened primary wall, very
thick secondary wall, and formation of some intercellular spaces, i sp. (/I, B, C, D, redrawn
from Herzog ; A after Reimers ; B after Krabbe ; Technologie des Textilfasern, Julius Springer ;
E, F, and G, after Tammes, Der Flacksstengel.^

there may be more or less pectic material which does not form a
definite layer but is intermixed with the cellulose.

Cell wall formation is usually the result of an intussusception or
apposition of wall substances, and it is by means of such processes
that the primary and secondary walls of the fiber are probably laid
down. However, it has been suggested by Aldaba (i) and Ander-
son Ql) that these processes can hardly account for the formation of
the successive lamellae which constitute the thickenings of the

LINUM USITATISSIMUM 399

flax fiber. Aldaba states that these may be formed by transforma-
tions of successive telescoping hyaline membranes which later
differentiate into cell wall lamellae. According to this explana-
tion, the lamellae are initially formed as a series of tubular mem-
branes which overlie one another. Each successively formed mem-
brane arises at the base of and centripetal to the previous one,
growing upward toward the tip of the elongating fiber. These
hyaline structures resemble plasma membranes, and are gradually
transformed into lamellae which become thickened by a process
suggestive of intussusception.

Anderson's explanation of the formation of the secondary layers
agrees in many respects with that of Aldaba (i). He notes, that
the first evidences of deposition of the lamellae occur at the basal
portion of the fiber following the period of rapid elongation, that
these layers are in contact with the remainder of the wall at only a
few points or not at all, that the first cellulose deposited is much
hydrated and in a state of viscosity that seems almost gelatinous,
and that there is evidence to indicate that the wall formation
actually occurs in the living cell free from adjacent walls of the
same cell, later becoming coherent with them.

The process of secondary wall thickening is not a uniform one in
which there is a steady increase in the thickness of the wall, but
takes place as a series of periodic additions of new layers of cellu-
lose. Each additional increment is deposited by the protoplast and
appears in a transectional view of the fiber as a much involved and
wrinkled layer which is apparently in contact with the previously
formed secondary wall only at widely separated points or in some
instances not at all. Later, it is pushed closely against this wall,
but it does not adhere to it nor is there any welding material
between them. The lamellae can be separated from one another,
which indicates that they are not cemented together and empha-
sizes their independent origin. The force which causes the flatten-
ing out of the infolded layers is probably turgor pressure; and as a
result of its action, the secondary wall loses its wrinkled appear-
ance and becomes firm and more rigid. (Fig. i03, E-G.') Several
lamellae are added in this manner, but they are not laid down with
regularity and the number varies in different fibers.

The secondary wall is usually divided into two to five major
layers, and these in turn may consist of many minute lamellae.
The lines marking the limits of the major layers, as well as those

400 THE STRUCTURE OF ECONOMIC PLANTS

between the minute lamellae, have been described by Anderson as
being the result of the periodicity of the deposition of wall
materials.

"A series of lamellae is deposited by the protoplast, and then an
interval of time intervenes before the next series of fine lamellae is
laid down. This interval of time makes possible in the different
layer divisions some changes in physical condition, and these differ-
ences are evidenced by slight variations in shades of color between
them."

The layers of the secondary wall system are composed of unmodified
cellulose which may vary in its physical condition depending upon
its relative stage of development.

At maturity, the fiber cell consists of its constituent part of the
middle lamella, the primary wall, and an extensive secondary wall.
The middle lamella is originally composed of pectic substances,
but undergoes changes as the fiber develops, the chief one being a
tendency to become more or less lignified. The primary wall is
largely cellulose, modified to some extent by pectose; and the
secondary wall is made up of several (usually two to five) major
layers of pure cellulose, which are in turn composed of thin layers
that have been deposited in a manner that gives a definite striated
appearance to the cell wall.

Fiber Structure. — Various details with respect to the charac-
teristics of the fiber have been reported, including banding, which is
a transverse marking or thickening of the fiber wall. This may be
attributed to a wrinkling or folding of the cell wall; or, more
frequently, the bands are the remains of primary thickenings which
adhere to the secondary wall after retting. In addition to these
fragments, there also may be particles of the walls of adjacent
parenchymatous cells adhering to the fiber wall. Local displace-
ments and distortions of the fibers also occur which appear to
result from stresses involved in the manipulation of the fibers in
retting or cutting rather than to any developmental factors. The
minute spiral striations which can be observed on the surface of
the fiber are related to the manner of wall formation, since the
lamellae are made up of fibrils that are spirally arranged in the
wall. The individual fibrils are below the resolving power of the
microscope and the visible striations consist of groups of fibrils
rather than of individual ones. Another feature of the physical
structure of the secondary cell wall is that the direction of the spiral

LINUM USITATISSIMUM 401

striations in successive lamellae is alternately right- and left-
handed.

Fiber Lignification. — Lignification is not necessarily uniform
in all the fibers in a given pericyclic area nor even in an individual
fiber, and one fiber of a group may be strongly lignified while
adjacent ones show little or no deposition of lignin. In general,
the peripheral fibers in a group tend to be more strongly lignified
than others; and it has been noted that this is also true of fibers
located immediately centrad to a lenticel. In regard to the rela-
tion of lignification to the position of the fiber in the stem, it has
been demonstrated that it is most extensive in the fibers at the base
of the stem, and increases as the stem matures. This fact is of great
importance in fiber flax because of its bearing on the process of ret-
ting and the final texture of the fibers. In instances where the flax
becomes too mature, retting, which involves the decomposition of
the middle lamella and the primary wall, is less complete and the
fiber is coarse and harsh in texture.

Lignification is usually restricted to the middle lamella and
primary wall; and there is little, if any, lignin in the secondary
wall. This explains the fact that investigations, carried on to
determine the degree of lignification, show the presence of more
lignin in unretted fibers than in those which have been isolated by
retting. The middle lamella is the first region to become lignified
as the fiber matures, and lignification may stop at this point. In
some cases, however, it involves portions of the primary wall, and
may finally extend more or less completely through it. This
decreases the commercial value of the fiber, as it is less easily pre-
pared for market, and the fiber is of inferior quality, tending to be
harsh and brittle instead of exhibiting the desirable qualities of
smoothness, strength, and flexibility.

The chemistry of lignification is not entirely clear; but it has
been suggested by Anderson (x) that it "is not a conversion of the
cellulose directly into a lignified wall, but that the cellulose first
undergoes a transformation to pectin-like substances."

Vegetative Regeneration of the Plant. — As early as 1857,
Reichardt (18) observed certain phases of vegetative regeneration
in the shoots of flax, and these have been verified more recently
by Tammes (xi) in Holland. Beals (4), in describing the phenome-
non of regeneration in flax, states, "first the epidermis divides and
then the innermost row of those cells and the stimulated cells of

401 THE STRUCTURE OF ECONOMIC PLANTS

the region just beneath form the regenerated part, root or shoot."
Crooks (7), in a detailed study, found specific differences in the
ontogeny of adventitious roots and adventitious shoots.

Adventitious Buds. — When the growing points of young
plants are injured or removed, the axis readily produces shoots;
and when the injury occurs above the cotyledonary node, an
axillary bud becomes active and produces a shoot. In his experi-
mental work. Crooks removed all the organized buds by severing
seedlings of various ages at different levels below the cotyledonary
node, and in all plants which were not more than ten days old, from
twenty to fifty adventitious buds developed on the remaining lower
portion of each hypocotyl. This occurred even when the top of the
plant had been removed to within a few millimeters of the ground
level. In cases where plants two months old were similarly
treated, approximately 60 per cent died without forming buds.

In seedlings less than ten days old, the origin of adventitious buds
continues for six to eight days after the severing of the axis; and,
since they are not all initiated at the same time, the hypocotyl
may at one time have some adventitious buds which are large
enough to show small leaves while others are in earlier stages of
development. By the time that eight to ten of the buds have
differentiated leaves, one of them usually outgrows the others.
There is no spatial relationship of the dominant bud to any of the
others, or to any special point on the hypocotyl, nor is the more
rapidly growing bud necessarily the first one to be formed. Occa-
sionally, more than one bud may continue development. The
shoot which is formed by the growth of the adventitious bud
develops an unbranched axis until the formation of the inflores-
cence; and it matures at about the same time as do plants of the
same age which are uncut, but it attains only about one-half their
height.

In the ontogeny of the adventitious bud, the first evidence of
differentiation, following the abscission of the upper portion of the
hypocotyl, is the formation of large intercellular spaces, resembling
those of the mesophyll, in the cortical parenchyma of the hypo-
cotyl. This occurs at the point of bud origin, and the bud is then
initiated by the divison of a single epidermal cell, quickly fol-
lowed by a second division so that a four-celled primordium is
formed. (Fig. xo4. A, 5.) Five or six of the epidermal cells
adjoining the derivatives of the first meristematic epidermal cell

LINUM USITATISSIMUM

403

may also divide; and, after one or two more epidermal cells have
become active, the underlying cortical cells lose their chlorophyll,
become less vacuolate, and undergo cell division. (Fig. 104, B,
C) These cells continue division and growth so that the large
intercellular spaces become occluded. Adjacent cortical cells

Fig. 104. A-E, transections of hypocotyl showing origin and development of adven-
titious buds after plant has been severed below cotyledons : a, four derivatives of a single
epidermal cell; co, cortex; ep, epidermis of hypocotyl; Ip, leaf primordia of adventitious
bud. (After Crooks.)

are also activated, and this continues progressively in a centripetal
direction until it has reached the endodermis. In this manner, a
meristematic zone of compact cells is formed, extending from the
epidermal bud primordium to the endodermis. Continued divi-
sions of the cells of the primordium, derived from the original
epidermal cells, produce a dome-shaped growing point, and this is
followed by the rapid formation of two or three leaf primordia.
(Fig. io4, D, Ej and Fig. i05.)

404 THE STRUCTURE OF ECONOMIC PLANTS

The development of the first leaves of the adventitious bud is
accompanied by a differentiation of their vascular elements so that
protoxylem is present in them before cell division has begun in the
inner layers of the cortex of the hypocotyl. (Fig. xo6.) Follow-
ing the proliferation of the parenchymatous cells of the inner
portion of the cortex, the vascular tissue of the new bud is differen-
tiated centripetally to the endodermis from some of the derivative
cells thus formed. After several tracheids have been differentiated

Fig. 105. Longisection of hypocotyl showing development of adventitious bud after
plant has been severed below cotyledons. (After Crooks.)

in this manner in the cortical parenchyma, a number of tangential
and radial divisions occur in the endodermis. (Fig. io6.) Some
of these endodermal derivatives differentiate as tracheids and estab-
lish continuity between the vascular system of the bud and tra-
cheids which have differentiated from pericyclic cells and phloem
parenchyma. The phloem associated with these newly differenti-
ated tracheids consists chiefly of elongated parenchymatous cells,
and by the time the vascular connection described above is com-
pleted, a layer of cells surrounding the tracheids begins to function
as an active cambium. Within a few weeks, the secondary tissues

LINUM USITATISSIMUM

405

produced by this cambium, and the cambium of the hypocotyl
proper, form a new plant axis which is almost straight.

Fig. zo6. Longisection of hypocotyl showing development of adventitious bud after
plant has been severed below cotyledons : en, endodermis. (After Crooks.)

Adventitious Roots. — When seedlings six to ten days old are
severed at the mid-hypocotyledonary region, and the upper part
of the axis is set in moist soil with the cotyledons just above the
soil level, the elongation of the hypocotyl lifts the cotyledons from
I to 3 cm. above the ground level, depending upon the age of the
plant at the time of cutting. Cuttings so treated produce from two
to five adventitious roots at the lower limits of the hypocotyl, even
when it is severed very close to the cotyledons. Cuttings of the
epicotyledonary internodes, treated in a similar manner, also pro-
duce adventitious roots.

In the hypocotyl, the roots are usually initiated by a general
activation of the cells of the V-shaped rays of parenchymatous

4o6

THE STRUCTURE OF ECONOMIC PLANTS

Fig. -loj. A-F, stages in development of adventitious roots from cuttings of hypocotyl.
A, B, and D, initiation of roots in ray parenchyma on same radius as protoxylem; C, initia-
tion of root in pericycle and phloem parenchyma not on radius with protoxylem ; E, median
longisection of adventitious root with histogens established; F, transection of hypocotyl,
median through an adventitious root, showing cell activity throughout pith region : cal, calyp-
trogen ; co, cortex ; dgn, dermatogen ; en, endodermis ; fib, fiber ; phrn, periblem ; ph, phloem ;
pi, pith; pi, plerome; atj i, primary xylem; xji ±, secondary xylem. (After Crooks.)

LINUM USITATISSIMUM 407

tissue which extend outward from each protoxylem strand, and the
roots arise in two rows, each of which lies in the vertical plane of
the cotyledons. In some cases, the meristematic activity extends
from the outermost layer of the pericycle centrad to the parenchym-
atous cells adjacent to the primary xylem. (Fig. 107, A.^ Most
of the primary xylem elements are crushed and disintegrate as this
activity proceeds, and the pith parenchyma may be involved in the
process of adventitious root formation. Cell divisions occur in all
planes, and a dome-shaped root primordium is formed which
pushes outward into the cortical region. The endodermis becomes
active, undergoing radial cell divisions, keeping pace with the
subsequent growth of the underlying cells. (Fig. 107, 5.) It
maintains itself as a single layer, and persists over the root cap of
the adventitious root until it has grown i or x mm. into the soil.

No organized histogens can be observed until about the time
that the root emerges from the cortex, when the cell layer adjacent
to the endodermis begins to divide periclinally, and functions as a
calyptrogen. (Fig. ioy, D.) The periblem and plerome are
clearly defined by the time the root has grown 2. or 3 mm. in length,
and the dermatogen is derived from the calyptrogen which func-
tions as a dual layer in the manner described for the primary root.
(Fig. to-/, £.) Coincident with root formation, cell divisions fre-
quently occur throughout the medullary region of the hypocotyl,
forming a continuous cylinder of parenchymatous cells which may
remain active for a month or more after the new roots develop.
(Fig. i07, F.) This activity of the pith is more pronounced when
two adventitious roots develop at the same level, and some tra-
cheids may differentiate in the meristematic region.

Less frequently, adventitious roots may form as a result of gen-
eral cell activity in the pericycle and phloem parenchyma, in which
case they are not in the same vertical plane as the cotyledon.
(Fig. X07, C.) Crooks has shown that adventitious roots may also
arise from severed cotyledons when they are placed on the surface
of moist soil. About 90 per cent of the cotyledons taken from
plants 10 days old developed from three to six adventitious roots,
and a few cotyledons from xo-day-old plants also produced roots
under similar conditions. The cotyledons develop an extensive
root system and enlarge somewhat, being thicker than those which
are not removed from the plant axis; but they do not produce
adventitious buds, although they live for 60 days or more as com-

4o8

THE STRUCTURE OF ECONOMIC PLANTS

pared with a longevity of approximately 30 days when they remain
on the plant axis. The roots usually originate adjacent to each of
the three veins of the cotyledon and as many as three roots may
develop near one vein. They push through the adaxial portion of

r-tnes

y--sto

Fig. 2.08. A, longisection through median vein of cotyledon showing origin of adven-
titous root on adaxial side of bundle; B, transection of cotyledon showing development of
adventitious root; C, transection of cotyledon showing later stage in development of adven-
titious root : /w.r, mesophyll ; /^A, phloem; j/o, stoma; //f/?, upper epidermis ; .vy i, primary
xylem; xj z, secondary xyleni. (After Crooks.)

the mesophyll of the cotyledon; but may emerge from the cut sur-
face 3 to 4 mm. distant. The primordia are initiated by an activa-
tion of a layer of parenchymatous cells surrounding the vascular
bundle. (Fig. xo8, B.) The meristematic region usually extends
a millimeter or more along the bundle, producing a dome-shaped
group of cells on the adaxial side of the vein, and the subsequent

LINUM USITATISSIMUM 409

development of the roots is similar to the adventitious roots of
the hypocotyl. (Fig. 108, A, C)

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18. Reichardt, H. W., "Beitrage zur Kenntniss hypokotylischer Adventivknospen

und Wurzelsprosse bei krautigen Dikotylen." Wien, 1857.

19. Robinson, B. B., Flax fiber production." U. S. Dept. Agr. Farm. Bull. 1J28,

1934-

10. Soueges, R., "Embryogenie des Linacees. Developpement de I'embryon

chez Linum catharticum L." Compt. Rend. Acad. Sci. 178.- 1307-13 10, 1914.

11. Tammes, Tine, Der Flachsstengel, Natuurkundige Verhandelingen van de

Hollandsche Maatschappij der Wetenschappen. Haarlem, Ser. Ill, 6, 1907.

1

4IO THE STRUCTURE OF ECONOMIC PLANTS

11. , "Die Flachsbliite." Rec. des travaux hot. neerlandais . i;: 185-117,

1918.
13. , "The genetics of the genus Linum." Bibl. Genet. 4: 1-36, 1918.

14. Ten Eyck, A. M., "A study of the root systems of cultivated plants grown

as farm crops." N. Dak. Agr. Exp. Sta. Bull. 4}, 1900.

15. ToGNiNi, F., "Sopra il percorso dei fasci libro-legnosi primari negli organi

vegetativi del Lino (Linum usitatissimum L.)." Atti 1st. Bot. Pavia,
Ser. II, 2: 153-173, 1891.

16. TscHiRCH, A., and Oesterle, O., Anatomischer Atlas der Pharmakognosie und

Nahrungsmittelkunde, Herm. Tauchnitz, Leipzig, 1900.

17. WiESNER, J., Die Rohstojfe des Pflanxenrekhes, id. ed., Leipzig, 1903.

18. WiNTON, A. L., The Microscopy of Vegetable Foods, John Wiley & Sons, 1906.

CHAPTER XIV

MALVACEAE

GOSSYPIUM spp.

COTTON, Gossypium spp., is the chief fiber crop of the world.
There are several commercial species in the genus, the number
ranging from 19 to 40, depending upon the classification followed.
These have been divided into two main groups, the New World
cottons which include the Upland and Sea Island varieties, char-
acterized by a % phyllotaxy; and the Old World cottons, among
which are G. herbaceum, G. arboreum, and others having a 3^
phyllotaxy.

According to Brown (9), the three important species represented
in the United States are G. barbadense, G. peruvianum, and
G. hirsutum. From these, and very probably from others, many
varieties have been derived by selection and hybridization. The
number of varieties in cultivation fluctuates since new hybrids
are constantly being developed on the one hand, while the use of
others is discontinued for some reason or other. In 192.7, Brown
listed 75 varieties of Upland cotton, stating that the list was not
to be considered complete or exhaustive, as he had included in
it only the "most prominent old-time varieties, some varieties from
which common everyday varieties have sprung, and the leading
varieties being grown at present." Duggar (13), some years
earlier, stated: "It is probable that the number of distinct vari-
eties, each differing from the other in one or more items of botanical
importance, exceeds one hundred . ' ' He simplified the classification
greatly by dividing the American Upland, short-staple cotton into
six classes, to which he added a seventh class with intermediate
staple, and an eighth group including all the long-staple varieties.

GENERAL MORPHOLOGY

The Shoot. — In the wild state, cotton is a perennial with a
shrubby or tree-like habit; but, in cultivation, it grows as an
herbaceous annual or biennial. In Upland varieties, the main

411

411 THE STRUCTURE OF ECONOMIC PLANTS

stems are erect or somewhat branching, and may attain a height
of 1 to 6 feet. (Fig. 109.) In the native, tropical home of cotton,
some of the tree-cottons reach a height of 13 to lo feet. The

Fig. Z09. Cotton plant showing branching and inflorescence. (Photograph by Harrold.)

leaves are petiolate, roughly cordate, three to nine lobed, and
usually palmately veined; but there are great differences in size,
shape, texture, and degree of pubescence. The basal stipules are
for the most part deciduous. The plants are conspicuously glan-
dular, large glands occurring on the abaxial surface of the leaves, on

GOSSYPIUM

413

the bracts, petioles, stems, and the embryo, especially its cotyle-
dons. Floral nectaries occur at the base of the inner surface of
the calyx, and there are three sets of extra-floral nectaries located
on the leaf, and on the outer and inner surfaces of the involucral
bracts.

The primary axis consists of two types of branches. The mam
stem is indeterminate with a terminal growing point, while the
lateral branches arising in the axils of the leaves of the main
stem are of two kinds, one type being vegetative and the other
fruiting. The vegetative branches are similar in function and
morphology to the primary axis, but the fruiting branches are
distinctive in form, function, and mode of development. In the
axil of each leaf there are two buds, one of which may be rudi-
mentary. The true axillary bud is centrally located above the
middle of the base of the petiole, while the lateral or extra-axillary
bud is to the right or left of the axillary bud. These two buds
differ with respect to their further development, the axillary bud
producing only vegetative branches or "limbs," while the branches
which bear flowers and fruit are developed from the extra-axillary
one. (Fig. 110.)

There is some variation with respect to the axillary and extra-
axillary or accessory bud "development in the several varieties
of cotton, and in some instances, a single bud may occur in the
axil of the leaf. According to Templeton (34), there is a bud
in the axil of each cotyledon in Egyptian cotton which does not
develop, and each of the other leaves has a true axillary and an
accessory bud. In the usual development of the plant, only one of
the two buds in a given axil develops into a branch. Normally, at
the first three or four nodes above the cotyledons, the axillary
buds develop and the extra-axillary buds remain dormant, so that
a zone of vegetative branches is formed in which the branches
resemble the main axis in the development of nodes, leaves and
axillary buds. Above the fourth or fifth node, only the extra-
axillary buds usually develop, forming a zone of fruiting branches.
There may be a third or transition zone between the vegetative and
fruiting zones in which the buds are irregular in their behavior.
In this zone, both buds may become active, both may abort or
remain dormant, or either one of the two may develop. When the
vegetative branch develops, the phyllotaxy is the same as that of
the main axis; and the axillary buds may give rise to fruiting or to

414 THE STRUCTURE OF ECONOMIC PLANTS

new secondary branches. Near the top of the main stem, both
buds may develop; and when this occurs, the axillary buds pro-
duce new sympodia or single flowers; and the extra-axillary ones
fruiting branches.

The fruiting branches that develop from the extra-axillary
buds assume a nearly horizontal position, rather than the vertical
or ascending one of the vegetative bud. The first internode of

Fig. -lio. Sympodial branching in cotton. The plant was pruned in such a way that
both axillary and extra-axillary buds developed. (Photograph by Doak.)

GOSSYPIUM 415

the fruiting branch is long; and, at the second node, a leaf arises
with two basal stipules which almost completely encircle the node.
The floral bud appears to arise approximately opposite the point
of divergence of this leaf rather than from its axil. Gore (18)
describes this situation,

"Examination of a fruiting branch shows that the flowers are not
in the axil of a leaf, but appear to stand opposite the adjacent leaf.
The internodes of this branch are slightly zigzag and the leaves appear
as if alternately arranged. . . . Such a sympodium is developed
more or less as follows: from the axil of a leaf on a given axis, there
arises a new axis which by its growth pushes the terminal portion of
the axis from which it arose to one side so that this once terminal
portion may come to appear as if lateral in origin."

The axillary bud may develop in the usual fashion, producing
a vegetative branch, or it may produce a short vegetative branch
which will in turn produce fruiting branches. The development
of a short vegetative branch producing fruiting branches gives it
the appearance of a fruiting branch bearing several flowers or
bolls; but, actually, only one is borne on each branch. The leaves
on the fruiting branches do not have the same arrangement as those
on the main axis owing to the sympodial method of branching and
the twisting of the joints of the fruiting branches, which orients
the flower buds in an upright position and aligns the alternate
leaves in two rows.

The Root. — The plant develops a tap root which penetrates
the soil for 2. feet or more under favorable conditions. With
irrigation, Balls (6) records a penetration of about 7 feet in one
of the Egyptian varieties, and King (13) reports a tap root of
the Pima variety grown in Arizona that reached a depth of nearly
II feet, noting that "a considerable number of secondary roots
extended to great depths and apparently shared the function of
the tap root." He also observed a relation between extent of the
root system and "water stress"; stating that

"Plants which had produced the greatest quantity of vegetative
growth appeared to suffer most frequently from 'water stress' remaining
longer in a wilted condition between irrigations and showing an
earlier recurrence of wilting after irrigation. The fact that there
was no appreciable difference in size or distribution between roots of
large plants and small plants seems to indicate that a limiting root
system may have an important bearing on the water-stress behavior
of the largest plants."

4i6 THE STRUCTURE OF ECONOMIC PLANTS

The depth and extent of the lateral root system depend upon
soil moisture; but, in general, it is shallow, and the laterals
originate x to 6 inches below the soil surface, although they may
be even more shallow under very moist conditions. The radial
extent of the laterals may approximate 4 feet, and they branch
and rebranch so that the first few inches of surface soil is well
filled with roots. In some instances, a second system of laterals
may develop at lower levels when the tap root reaches saturated
soil.

The secondary roots arise in four or five shallow longitudinal
grooves that may be somewhat spiral owing to the torsion of the
root, and subterranean shoots sometimes arise in the depressions
adjacent to the roots. According to Cook and Meade (10),

"the underground shoots have at first a rounded or irregular form,
Hke root nodules or galls, and may represent modified root primordia.
The nodules grow to various sizes, sometimes attaining a diameter
of nearly an inch before showing the leafy bud that develops into a
vegetative branch. . . . Subterranean shoots seem to be developed
much more freely in the Egyptian cotton than in Upland varieties."

The Flower and Fruit. — The flowers are convolute in the bud,
and are usually white or yellowish although they may be deep
yellow at anthesis or even a brilliant red in some foreign varieties.
In some of the white-colored forms, there may be a spot of purple on
the claw of the petal. The first day after anthesis, the corolla
becomes pink and turns red the second day. At the end of the
third day, it withers and falls off, together with the staminal
column, the stamens, and the stigmas, while the ovary remains,
subtended by the calyx and covered by the persistent involucre.

The somewhat bell-shaped flowers are large, regular, hypogy-
nous, and tetracyclic. The persistent calyx consists of five undi-
verged sepals that form a shallow cup around the base of the
petals, and its lobes are variable in size, being short and broad,
or somewhat long and pointed. As the boll develops, the calyx
becomes tightly adherent to its base. The sepals have a large
number of globular subepidermal glands that occur in irregular
rows in the parenchyma between the vascular bundles. Within
the calyx and alternate with its lobes, greenish spatulate or obovate
organs may be frequently found. They are usually five in number,
and may be very small and rudimentary, or, in some instances, large
enough to extend to the margin of the calyx. Cook and Meade

GOSSYPIUM

4^7

(lo) have interpreted them as "ingrown margins of tiie calyx
lobes" or "stipular elements." Centrad to and at the base of the
sepals is a ring of nectaries made up of multicellular glands which
are surrounded by stiff hairs.

The tubular corolla consists of five obcordate petals with lobes
that alternate with those of the calyx, and each glandular petal
overlaps the next one in the series in a convolute manner. The
stamens are numerous, there frequently being as many as 90 or
100. These are diverged from a tubular sheath of tissue, or
staminal column, which com-
pletely surrounds the pistil except
for the projecting stigmas and a
short portion of the style. (Fig.
III.) The filaments appear to
arise in five fascicles or groups,
but close examination shows
that there are actually ten fairly
well defined groups of stamens
arranged in more or less vertical
rows. The anthers are bilocular,
and dehisce along a single line
running over the crest of the
anther, liberating large, spinose
pollen grains.

The pistil consists of from
three to five undiverged carpels,

and the three- to five-loculed or corolla tube; C, ovary ;D bract of invo-
... , ,, II- lucre. (Photograph by Doak after Balls.)

locked boll is a dehiscent cap-
sule that splits along the dorsal sutures. In a given variety, the
number of carpels may vary within the limits noted above even
on an individual plant. In Upland cotton, the four- to five-locked
bolls are more common than the three-locked ones, and two-locked
types occur in Egyptian cotton. Abnormal cases have been
reported in which six or more carpels may occur. The boll is
thick and leathery at maturity and is usually subglobose or oval
in shape. (Fig. xix.) In each locule, eight to ten ovules arise
in two parallel rows, and ultimately the seeds produce the com-
mercial fiber.

The flower is subtended by an involucre composed of three,
or occasionally four, unequal, leaf-like bracts. These are ridged,

Fig. 111. Median longisectioii through
flower. Ay staminal column; B, base of

4i8

THE STRUCTURE OF ECONOMIC PLANTS

deeply notched with about ten teeth which correspond to the
principal veins. On their outer surfaces, characteristic stellate
hairs are developed along the ridges formed by the vascular bundles.
Bracteoles may occur inside of the involucre alternating with the

Fig. xiz. Habit of mature bolls. (Photograph by J. Horace McFarland Co.)

bracts. Frequently, only two are present; and, in this case, they
are located on either side of the small bract.

ANATOMY

The Seed. — The seeds are irregularly pear-shaped, ranging
in length from K to 3^ inch, and they are usually covered with
fibers of two kinds, the commercial fiber or "lint" and the "fuzz"

GOSSYPIUM 419

or short hairs. When the lint and fuzz are completely removed, the
naked seed is black or dark brown. In some varieties, no fuzz is
produced and the seed is left naked after ginning. In Upland
cotton, the two kinds of hairs are mixed together over the whole
surface of the seed; but, in some varieties, the lint tends to have
longer fiber at the rounded end than at the pointed one. The
lint is usually white while the fuzz is frequently greenish, brown-
ish, or tawny.

The seed coats are developed from two integuments and have
been described in detail by Winton (36). The outer epidermis,
aside from the fiber cells or hairs, consists of cells which are
irregular in shape. In surface view, they are somewhat stratified,
with yellow walls and dark brown contents. They form rosette-
like groups around the hairs and vary in size from 10 to 60 ix.
Stomata with thin, translucent-walled guard cells occur either
singly or in pairs. The hypodermal layer is two to three cells
in thickness, except in the region of the raphe where it is thicker,
and consists of thin-walled cells with irregular contour and brown
pigmentation that may become more or less compressed or crushed.
Inside this zone is the innermost layer of the outer integument
comprised of rather thick-walled, colorless cells. (Fig. xi}.)

The palisade cells constitute approximately one-half of the
thickness of the seed coat. These elongated cells are very char-
acteristic in structure, consisting of an outer portion occupying
about one-third of the length of the cell with nearly colorless
walls, and an inner portion with yellowish-brown walls. In
the outer portion of each cell, the lumen is very narrow except
for a globular enlargement at the inner end which contains a dark-
colored substance. The inner portion appears to have no lumen
when viewed in cross section; and, in the tangential view, radiat-
ing lines may be seen which probably result from the arrangement
of the lamellae of the cell wall. The inner brown coat consists of
several compressed layers of more or less spongy polygonal cells
which contain a brown coloring matter such as is found in the outer
brown coat.

Within the inner brown coat is a delicate white sheath two
cell-layers in thickness which encloses the embryo. The cells of
the outer layer were designated as "fringe cells" by Hanausek
(xo), who regarded them as the remains of the perisperm or nucel-
lus, and a similar interpretation was made by Winton (36) and

410 THE STRUCTURE OF ECONOMIC PLANTS

/-ep hr

— ep
—hr c

— — CO c

— lu

Balls (4). Reeves (xy) has reached a different conclusion with
respect to this layer, on the basis of comparative studies of young
ovules; and states that "the fringe tissue originates as the inner
epidermis of the inner integument." The cells are polyhedral,
and the walls are irregularly thickened or fringed so that they
appear to be nodular or wavy. The second layer of the sheath
consists of thick-walled cells containing aleurone grains which

are the remains of the endosperm.
The cotyledons are thrown into
complicated folds in their devel-
opment; but, when straightened
out, are broad and kidney-shaped.
p(,l I They contain very prominent resin
cavities and reserve foods, includ-
ing aleurone and fat from which
cotton seed oil is derived.

Development of the Seedling.
— The cotton seedling develops
rapidly and the primary root pushes
its way through the micropyle of
the seed and grows without branch-
don: <?ir c, outer brown coat ; w c, layer ing for Several days. Measure-

of colorless cells, r./, cotyledon; .«^, ^^^^^ . g^jj^ ^g>) indicated a

endosperm ; ep, epidermis ; ep hi; epider- •' ^ -^

mal hair; g/, gland; ? ^r r, layers of inner grOWth in length of 6 Cm. in the

brown coat; / .^ lower epidermis of coty- ''^^.^^ fQ^j. days,"and an additional 8

ledon ; ///, lumen of palisade layer of seed . . ,

coat; />.// I, palisade cells of seed coat; cm. in the ensuing three days.
pa/ 1., palisade cells of cotyledon; psm. During this time, there is a prolific

perisperm consisting of fringe cells ; ra, , , '' <- ....

raphe; ., seed coat ; «./,, upper epider- development of root hairs in the
mis of cotyledon. (After Winton and region above the zone of elonga-

tion. The straightening oi the
arched hypocotyl lifts the much
convoluted cotyledons out of the ground; and after being freed
from the seed coats, they expand rapidly. Coincident with this
development, lateral root formation occurs, and subsequent growth
of the epicotyl results in the elongation of the axis and the pro-
duction of the first foliage leaves. (Fig. 114.)

The Primary Root. — The primary root has an exarch, radial
protostele which is generally tetrarch, although pentarch primary
roots may occur. (Figs. 115 A^ 116.) The protoxylem is com-
posed of spiral and annular elements that become much elongated

pal 2

u ep

Fig. 113. Section of seed including seed
coat, perisperm, endosperm, and cotyle

Winton, Structure and Composition oj Foods,
John Wiley and Sons, Inc.)

GOSSYPIUM

411

as the axis matures, with the result that the spiral thickenings
become longitudinally stretched, and the thin walls frequently
collapse. The metaxylem develops centripetally, consisting of
scalariform and scalariform-reticulate elements. At the center of
the stele, there are four to eight large tracheae which are some-
times separated by smaller xylem elements with scalariform thicken-
ings. Complete maturation of the centrally located elements does

Fig. 2.14. Stages in development of seedling.

not occur until about the time that secondary thickening is ini-
tiated.

On alternate radii to the protoxylem are four groups of primary
phloem that are separated from the central metaxylem elements
by parenchyma. The protophloem is parenchymatous and is soon
crushed, while the metaphloem consists of sieve tubes, companion
cells, and parenchyma. As the maturation of the primary axis pro-
ceeds, some of the phloem cells adjacent to the cambium initials
become thick-walled, and sieve tubes and companion cells are
located at the lateral edges of these groups. (Fig. ii5, A.^ The
thin-walled, elongated cells of the pericycle form a single layer
external to the phloem region, but this region may be two or three
cells in thickness outside the protoxylem elements.

The endodermal cells form a single layer with well-defined

4XX THE STRUCTURE OF ECONOMIC PLANTS

Casparian thickenings, and those abutting the phloem groups are
filled with a mucilaginous substance that is lacking in those
adjacent to the protoxylem. The cortical parenchyma is spongy,

Fig. 115. A, transection of primary root; B and C, of lower hypocotyl ; D, of middle
hypocotyl : ca, cambium ; co, cortex ; en, endodermis ; ep, epidermis ; mx, metaxylem ; pel,
pericycle; ^^, phloem; pi, pith; />x, protoxylem. (After Spieth.)

consisting of eight to twelve concentric layers of thin-walled,
isodiametric cells except for the two or three rows immediately
within the epidermis, which are axially elongated and more com-
pact. The epidermis consists of a single layer of small cells.

GOSSYPIUM

42-3

Secondary Thickening of the Root. — Secondary thickening
begins early in ontogeny and the first cambial activity occurs
in the zone of fundamental parenchyma lying between the central
metaxylem vessels and the primary phloem. The secondary xylem
consists of large vessels, smaller conductive elements, fibers and
parenchyma. The parenchyma forms rays that are one to three
or more cells in width, and the thin-walled ray-cells are radially
elongated. The large ves-
sels are laid down in
radial rows, but become
somewhat irregular in ar-
rangement owing to the
increase in their size. Fre-
quently, two or more of
these large vessels are clus-
tered together, but the ma-
jor portion of the xylem
consists of smaller vascular
elements, fibers, and the '^^^^>^^^^^^^>'^^^~r^^ TrTOMSSrr'Tf^-?'

thin-walled parenchyma -'-<):Ai;i^cJXK?v">K^tAA^^#KTv^

which surrounds the large

vessels.

Outside of the cambial
zone, the secondary phloem r~A ywyWrSKYyrKh~\ ^--par

has a characteristic layered / -y v^ VXY'Y'^N-^ ^ csp

appearance which results

from the differentiation of p.G. 116. Transection of young pentarch root .- co,

zones of small, thick- cortex; csp, Casparian strip; en, endodermis ; ep,

^11 J *»U1„ ™ i:u ^i- ^ epidermis; mx, metaxylem; par, parencliyma; pel,

walled phloem fibers that ^ • 1 ' t ui ^1

^ pencycle ; ph, phloem; px, protoxylem.

alternate with regions of

sieve tubes and companion cells. In some instances, the fibers
occur in compact groups rather than in layers. The wedge-
shaped segments of fibers and sieve tubes are separated from each
other by funnel-shaped zones of ray cells. In most cases, these
are secondary phloem rays that are continuous centripetally with
the secondary xylem rays; but in less frequent instances where
they are not produced by cambial activity, they may be regarded
as being pericyclic in origin. As the root increases in size, the
cortical and epidermal cells are stretched and ultimately disinte-
grate; but the pericycle remains active during the secondary

---ma;

414 THE STRUCTURE OF ECONOMIC PLANTS

phases of growth, producing a multilayered periderm that serves as
a protective zone for the mature root.

The Vascular Transition. — The vascular transition in the
seedling axis has been investigated by Spieth (31), who used
five-day-old seedlings. In a hypocotyl of this age, the primary
vascular tissues of the lower portion are completely differentiated
and secondary thickening initiated; but, in the upper limits of
the same hypocotyl, the maturation of the primary tissues is just
beginning, there being little differentiation of vascular elements
at the cotyledonary node. The immature condition of the upper
hypocotyl and the cotyledonary node as compared with the middle
and lower hypocotyl is due to the fact that the former is a region
of continued axial elongation resulting from the activity of an
intercalary meristem.

The first evidence of change in the vascular plan occurs in the
lower hypocotyl, about one centimeter below the soil surface,
in connection with the differentiation of the metaxylem. Instead
of forming a central strand, it differentiates in a ring surrounding
centrally located parenchymatous cells which are circular in
transection and vertically elongated with intercellular spaces
at their angles. (Figs. 115, B; 119, 5.) The phloem groups are
more extensive at this level and one or two rows of phloem cells
immediately inside of the pericycle have thick walls. Sieve tubes
and companion cells differentiate laterally in relation to the
thick-walled phloem cells, and adjacent to the protoxylem points.
Just above this level, the stele enlarges within a very short
vertical distance, and four triangular groups of primary xylem cells
are differentiated. (Figs. 115,0 119, C) Each group consists of
protoxylem at the outer apex of the triangle, and two tangentially
oriented arms of metaxylem which lie in an approximate collateral
position with reference to the primary phloem. At a higher level,
parenchyma is differentiated between the metaxylem arms so that
the four distinct groups of primary xylem are separated from each
other by medullary rays. Further differentiation results in the
formation of tangential bands of metaxylem that alternate with the
four transition bundles. (Figs. 115 D; 2.19, D.) The position of
the four principal phloem groups remains unchanged, except that
there is a large number of thick-walled phloem cells adjacent to the
pericycle, and small groups of sieve tubes and companion cells are
scattered among these thick-walled elements.

GOSSYPIUM

4^5

About 2. cm. above the soil level, the xylem of each of the four
original transition bundles consists of two rows of spiral elements
and two lateral metaxylem arms. (Fig. iiy, A.') The cambium
is active and there is considerable secondary thickening which
results in the differentiation of secondary xylem adjacent to the

Fig. 117. A-D, successive stages showing transition in upper hypocotyl : en, endodermis ;
wx, metaxylem ; /)f/, pericycle; /^A, phloem; />/, pith ; /;.v, protoxylem. (After Spieth.)

tangential rows of metaxylem. Secondary phloem is laid down
centrifugally, the endodermis and the pericycle maintaining their
continuity by anticlinal divisions. The cortex is composed of
fifteen or more concentric layers of cells, and lysigenous canals may
develop in the outer portion of this region.

At approximately 4 cm. above the level where transition is
initiated, the separation of each bundle into two units is com-
pleted. Except over the protoxylem points, where the tissue

4i6 THE STRUCTURE OF ECONOMIC PLANTS

remains parenchymatous, thick-walled pericyclic cells abut the
endodermis, which becomes discontinuous, retaining its identity
only over the transition bundles. (Figs, xiy B; 119, F.) Second-
ary xylem and phloem are formed except outside the protoxylem
points where parenchymatous rays occur. The protoxylem points
of the divided transition bundles occupy a position more in line
with the metaxylem cells so that the primary xylem arms are tan-
gentially oriented.

Somewhat higher in the hypocotyledonary axis, there is a
definite decrease in cambial activity, and the phloem is arranged
in smaller groups separated from one another by parenchymatous
cells. (Fig. XI 8, A.^ At successively higher levels, the progres-
sive differentiation of the protoxylem in a more centripetal posi-
tion, and of the metaxylem in a centrifugal one, results in an
endarch orientation of the primary xylem tissues. (Fig. ii8, B.)
This reorientation is gradual, but eventually the endarch arrange-
ment is reached in all bundles. The parenchymatous rays, separat-
ing the halves of the original transition bundles, become progres-
sively wider; and there is evidence of the meristematic character
of the hypocotyl at this level in the relatively small number of
mature metaxylem elements. The sieve tubes and companion
cells lie in a collateral position external to the four pairs of transi-
tion bundles, but there are no thick-walled cells in the phloem.

About 8 cm. above the soil surface, the rays that bisect the
transition bundles lying in the intercotyledonary plane widen to
such an extent that one-half of each bundle lies on either side
of the hypocotyledonary axis. (Fig. 119, G.) Thus, four units of
the original transition bundles are located on each side of the
axis. Each group consists of one original transition bundle in
which the two units are separated by a narrow ray, and one unit
from each of the transition bundles of the intercotyledonary plane,
the bundles ad'dc' and a'bb'c constituting the left and right
cotyledonary traces respectively. (Fig. 119, H, 7.) In the inter-
cotyledonary plane, there are from one to three endarch bundles
which in this stage of development consist of a few primary xylem
and phloem elements. (Fig. ±1^, /.)

At the cotyledonary plate, the four bundles of each trace diverge
into their respective cotyledons. The endarch bundles are the
downwardly diverging strands of the traces of the foliage leaves
above. Ultimately they become continuous with the cotyle-

GOSSYPIUM

42-7

denary vascular system by anastomoses, and by the development of
secondary vascular tissues as the node and upper portions of the
hypocotyl mature.

Fig. 2.18. A and B, transections of upper hypocotyl showing stages in transition at level
where endarch condition is attained: mx, metaxylem; pb, phloem; pi, pith; px, protoxy-
lem. (After Spieth.)

Ontogeny of the Fruiting Branch. — The development of the
main stem and its vegetative branches results from the continued
activity of the terminal meristem of each axis. This monopodial
growth is in contrast to the sympodial development of the fruiting
branch. (Fig. 1.1.0, A, B.) Gore (18) has described the ontogeny
of the fruiting axis in detail.

42.8

THE STRUCTURE OF ECONOMIC PLANTS

"A single segment of a fruiting branch consists of an internode, a
leaf with stipules, the flower bud, and two axillary buds one of which
remains dormant and becomes the axillary bud of the fruiting branch
while the other continues the secondary axis. The fruiting branch
of cotton is made up of a series of these segments which gives to the
entire structure a jointed appearance. (Fig- 2.2.1, A.^ . . . The
first evidence of the development of a fruiting branch is a primordium

D

fa oT

fd

y

b\
b'i

\d

\d

op Hi

'» "

a'

b^i

6^'L

G

^cndodcnnis
^ptricyde
IM phloem
(HO cambium

■ •^•i/fem

Q] parenchyma

Fig. ■Li.'). Diagrams illustrating vascular transition : A, transection at root level; B and
C, lower hypocotyl ; D and E. middle hypocotyl ; F-l, upper hypocotyl ; /, cotyledonary
node. (After Spieth.)

GOSSYPIUM

42-9

which occurs in the axil of a leaf on the main stem at the second or
third node back from the apical meristem. (Fig. ixo, B.) This
axial primordium is early differentiated into two separate growing
points: one grows faster and becomes the fruiting branch primordium;
the other grows slowly and develops the axillary bud on the main axis,
which is homologous with the axillary bud of the fruiting branch.
The fruiting branch primordium becomes raised at three points where
the first leaf and stipules are to arise. At this stage a single primordium
exists which resembles the apical primordium of the main stem. After
the leaf and stipule primordia are definitely differentiated it begins
to grow away from the subtending leaf and is directed away at an
increasing angle. It becomes bluntly conical in shape and projects
beyond the subtending stipules continuing its development into a

Fig. 12-0. A, longisection of apex of main axis; 6, same of fruiting axis; ax bud, axil-
lary fruiting branch bud; / /7r, flower primordium; //;>-, leaf primordium; />r, procambium;
ta- fr bi\ terminal fruiting branch; ter mer, terminal meristem. (After Gore.)

flower bud. At this same time a zone of tissue becomes active between
the leaf and the primordium of a flower. It soon develops two unequal-
sized growing points, the smaller of which gives rise to the axillary
bud; the larger one continues the fruiting branch by developing a
leaf and stipules for the next axis. By a repetition of this process
the fruiting branch comes to consist of a series of axes, each axis made
up of a single internode and each terminated by a flower. (Fig. 2.2.1,
B, C) Cottons in which a sympodium consists of more than one
internode are exceptional. . . . Early in the development of the fruit-
ing branch, when the first fruiting axis is being formed, it is pushed
to one side so that the developing structures are projected to one
side of the leaf on the main stem. Thus a fruiting branch is more
nearly horizontal with the primary axis and grows out either to the
right or left of the subtending leaf on the main stem."

Development of the Axillary Bud of the Fruiting Branch. —
The development of the bud which occurs in the axil of the leaf on a
fruiting sympodium has been reported by King, Cook and Meade,
and Gore. Cook and Meade (lo) state that

430 THE STRUCTURE OF ECONOMIC PLANTS

"In addition to the bud that continues the growth, there are buds
in the axils of the leaves of the fruiting branches, and if these develop
they may produce vegetative branches of the usual form. In other
cases the axillary buds of the fruiting branch may produce very short
vegetative branches, and these may give rise in turn to very short
fruiting branches, so that one joint of a fruiting branch may appear
to bear two or three bolls in an exceptionally fertile plant. Careful
examination will show that only one boll is borne directly on the
joint and that the others come from branches of the short axillary."

- -ax 2

—pe 1

stip 1

-ax b 2
-ped 1

Fig. 12.1. A, portion of fruiting branch showing parts of two sympodia. The termi-
nation of axis I is the flower whose pedicel is shown, together with leaf and stipules : ax i,
■L, axes I and r; ax b z, axillary bud developed from axis i; pe i, petiole; ped i, pedicel of
flower; stip i, stipule : B, outline of transection of terminal bud of fruiting branch showing
three sympodia and terminal of fourth axis (axillary fruiting branches not shown): fi i,
i, 3 ; /, I, 2., 3 ; st i ; and t, flowers, leaves, stipule, and terminal for respective sympodia :
C, lower level on sympodia i and i with greater enlargement: ab, axillary bud of axis 3,
ax 3. (After Gore.)

King (14) points out that

"The axillary buds and branches on the fertile branches of Egyptian
cotton differ in morphology and behavior from those on upland cotton.
In the Pima variety of Egyptian cotton rudimentary axillary branches
begin development with the advent of minute triangular buds in the
axil of each leaf, but ordinarily this development is cut short by the
shedding or drying up of the buds, which maintain active growth
for only a few days. In upland varieties the axillary buds usually
remain dormant, but they can be stimulated to growth at any time
later in the season."

GOSSYPIUM 431

Gore (18) investigated the axillary buds of Pima, Sea Island,
and Mebane varieties and describes the development of the latter
as follows:

"The first divergence from the axillary bud primordium is a bract-
like structure, unattended by stipules, possibly representing a reduced
leaf and stipules. There next arise a leaf and stipules for the second
node, followed by the turning aside of the growing point and the
development of a sympodium. The new terminal primordium develops
a monopodial axis at the next node, however, and at a succeeding
node another sympodium may be developed. This alternate production
of a sympodial axis and a monopodial axis may continue until three
or four flower primordia are produced."

Anatomy of the Stem. — The young stem is irregularly three to
five lobed or ridged with large bundles lying on the same radii as
the lobes. Numerous smaller ones are located in the vascular
cylinder between the larger bundles, being separated from them by
narrow parenchymatous rays which are one or two cells in width.
The pith consists of thin-walled parenchymatous cells with small
intercellular spaces at their angles. The stelar tissue is so compact
that it gives the appearance of having a continuous cambial zone;
but, in the young stem, no interfascicular cambium crosses the rays,
and the ray cells keep pace with the enlargement of the axis by
radial elongation and periclinal divisions. Where the bundles are
separated by wider rays, an interfascicular cambium may cut off
connective tissue; and, occasionally, secondary xylem and phloem
elements are differentiated.

The primary xylem is endarch and it usually lies in direct radial
alignment with the secondary xylem vessels. (Fig. xxi.) The
phloem is differentiated collaterally in relation to the xylem; and
consists of sieve tubes and companion cells, bounded by a pericyclic
zone in which the cells become thicker walled as the stem matures.
The inner cortical region is comprised of several layers of thin-
walled cells, which compensate for the increasing size of the stele
by tangential enlargement and radial divisions. Outside this
region is an intermediate zone of smaller, more compactly organ-
ized, coUenchymatous cells which may extend to the epidermis at
the angles of the stem. The thin-walled cells in the outermost
zone are chlorophyllose, and a number of lysigenous glands develop
in this tissue close to the epidermis.

This type of gland is also found in the embryo, primary root.

43'

THE STRUCTURE OF ECONOMIC PLANTS

— gl hr

leaves, floral parts, and in the pericyclic and secondary ray tissues
of the stem. Stanford and Viehoever (33) have investigated them,
as well as the nectaries, and state that the former are lysigenous in
their development, although some of them may be formed by the

enlargement of a single
cell. With respect to their
contents, they report that

"The glands in portions
of the plant which are
exposed to light are sur-
rounded by an anthocyan-
bearing envelope of flat-
tened cells, and contain
quercitin, . . . ethereal oil,
resins, and perhaps tan-
nins. The glands not nor-
mally exposed to the light
are surrounded by a layer
of flattened cells contain-
ing no anthocyans; they
contain gossypol."

-y:^ CO par

The epidermis consists

of a single layer of com-
pact cells which are uni-
form in size and shape with
thick outer walls overlaid
by a cuticle. There are
numerous stomata, each
bounded by two small
guard cells, which are
raised slightly above the

Fig. 12.1. Transection of portion of young stem: SUtface of the epidermal
ca, cambium; col, collenchyma; co par, cortical pa- ]^ygj. TwO tVpCS of epi-
dermal hairs are produced,
one of which is long,
slightly pointed, unicellu-
lar and subtended by a group of basal cells. The other is smaller,
capitate, multicellular, and usually consists of a stalk of three cells,
the outer, broadly wedge-shaped one bearing a terminal group of
four cells that are probably glandular. As maturation of the
stem proceeds, the epidermal and cortical regions become fissured.

renchyma; cu, cuticle; ep, epidermis; gl, gland; gl hr,
glandular hair; hr, hair; mx, metaxylem; ph i, pri-
mary phloem; ph z, secondary phloem; pi, pith; px,
protoxylem; ra, medullary ray; xy z, secondary xylem.

GOSSYPIUM 433

and phellogens are produced which form a protective layer of
phellem. The first phellogens are of cortical origin, while later
ones are differentiated in the pericyclic parenchyma. The outer
layers of the axis may become quite corky, accompanied by the
development of lenticels. As secondary thickening proceeds, a
large number of thick-walled fibers are produced in the secondary
phloem. The secondary xylem is not heavily lignified, and the
soft woody tissue disintegrates rapidly.

Ontogeny of the Leaf. — The early development of the leaf
and the stipules has been reported by Gore (i8) as follows:

"The leaf primordium and stipule primordia arise practically simul-
taneously from the apical meristem of the primary axis. (Fig. 1x3,
A, B.) The leaf primordium is rounded and collar-like at first, later
tapering somewhat up to the rounded tip. As the embryonic leaf
grows upward it becomes bluntly pointed. . . . There is little evidence
of differentiation into petiole and blade at this stage, except the slight
beginning of the plicate folding characteristic of the lamina of a young
leaf. No epidermal hairs are present at this stage, although they
begin their development early. The third young leaf from the tip
is completely covered with them.

"The tip of this first protuberance becomes the median lobe of the
palmate leaf; its base develops into the petiolar region. There soon
develops on the slightly incurved margins of this primordium two
pointed protuberances located about halfway between the tip and
its base. These are the primordia of the two lateral lobes of the leaf.
The young leaf, consisting of three lobes, now proceeds to develop
in length; continued inward plicate folding, as the lobes increase
laterally in size, forms a cuplike portion. (Fig. 113, B, C.) If addi-
tional lobes occur they arise in the same way and are similar in all
respects. (Fig. xxt,, D, £.) Seven lobes is the usual number for
leaves on the main axis, while on the fruiting branches of Pima and
Sea Island varieties the leaves usually have three. In Mebane five-
lobed leaves are commonly found on the fruiting branches. Soon
the tips of the individual lobes meet and the lamina develops rapidly.
The main veins appear as definite ridges on the abaxial surface of the
leaf and are very thick in proportion to the rest of the leaf. The
venation is palmate.

"Simultaneously with or shortly after the initial leaf primordium
becomes distinct, two stipular flanking protuberances arise. (Fig.
2-Z3, B.) These are pointed and concave at first, but as they grow
they widen out from the base somewhat, later become toothed, fold
inward very slightly, and keep pace with the growth of the leaf.
They are not folded or plicate as is the young leaf but remain more or
less flattened, their interlocking hairs holding them tightly appressed
to the leaves."

434 THE STRUCTURE OF ECONOMIC PLANTS

The Mature Leaf. — The leaf has an upper epidermis of brick-
shaped cells with outer walls that are cutinized. (Fig. 1x4, A.^
Stomata occur in the upper epidermis but are more numerous in the
lower. Stomatal counts, per square millimeter of surface, reported
by Balls (6) for the vegetative organs are: leaf, upper epidermis,
44-97, lower epidermis, 1 16-176; cotyledons, upper epidermis, xoo,

Fig. L13. A, growing point of cotton stem showing early stages in ontogeny of leaf,
stipules, axillary bud, lobes of young leaf, and general topography of tip of mrtin axis, drawn
from above and slightly to one side, Pima ; B, later stage in leaf ontogeny, one stipule cut
away, Mebane ; C, general topography of fruiting branch terminal, origin of bracts on floral
primordium and new axillary primordium. Sea Island ; D, later stage in ontogeny of sym-
podial axis showing slightly older floral bracts, Pima; E, later stage than figure D, showing
origin of new fruiting branch terminal between leaf and floral primordium, Mebane; F,
axillary bud of fruiting branch showing bractlike structure, growing point, leaf and stipule
primordia of second node, Mebane; G, early ontogenetic stage of staminal column, petal
primordia well differentiated and showing alternate staminal column lobes. Sea Island; H,
later stage in ontogeny of staminal column showing stamen primordia in two rows on each
ridge, Pima; /, one of five lobes of developing staminal column with stamen primordia,
under greater magnification than figure H, Pima ; /, young calyx with bracts removed,
Mebane; K, developing petals, Pima; L, young pistil, Mebane. (After Gore.)

GOSSYPIUM 435

lower epidermis, 175; hypocotyl and stem, io. The stoma is
surrounded by two small guard cells with conspicuously thickened
outer, inner, and radial walls. In the palisade region, the cells are
very much elongated so that this layer comprises approximately
one-third to one-half the thickness of the leaf. The underlying
spongy cells are more loosely arranged with numerous intercellular
spaces; and, adjacent to the enlarged midrib, lysigenous glands
occur similar to those in the cortex of the stem. In some instances,
these are very large and extend from the lower epidermis into the
palisade layer. The lower epidermis resembles the upper except
that the cells are more irregular.

The main veins have rib-like projections on the adaxial surface
consisting of strands of thick-walled mechanical cells which are
separated from them by several layers of thin-walled parenchyma.
The vascular bundle is collateral, resembling that of the stem in the
seriate arrangement of its xylem cells, in the well-defined cambium,
and the rather regularly arranged groups of phloem elements. On
the abaxial surface, there is a zone of compact parenchyma forming
a prominent ridge beneath each of the larger veins, and this is
usually reinforced by two or three layers of thick-walled cells.

Nectaries occur on the abaxial surface of the main ribs of the
leaf in all cultivated varieties grown in the United States with the
possible exception of Gossypium tomentosum Nutt. Tyler (35)
reports that the number varies from one to five with one to three
being most common. They appear as small, shallow, oval, pear-
shaped, or sagittate pits that may be naked, although, in many
varieties, there are stellate hairs protruding from the surrounding
surface. Club-shaped papillae arise from the floor of the pit, each
consisting of several glandular cells. (Fig. xi4. A, B.) Reed (16)
has described the development of the nectaries in Gossypium
hirsutum, and reports their occurrence on the cotyledons as well
as on the other foliage leaves. The papillae are epidermal in ori-
gin, and the first walls that come in are transverse so that short
papillae are formed consisting of two to four cells. This is fol-
lowed by a vertical division of the terminal cell, and each resulting
daughter cell again divides, the new wall lying at right angles to
the first cross wall. Periclinal divisions of these four terminal
cells result in the formation of four central and four peripheral cells.
Finally, each of the external cells divides so that a peripheral layer
of eight cells is formed.

436 THE STRUCTURE OF ECONOMIC PLANTS

The petiole of the leaf is terete or oval in transection, and the
vascular bundles are arranged in a ring with four or five large
bundles and several smaller ones constituting the leaf trace. These
are surrounded by thin-w^alled parenchyma, and there is an outer
zone of small, compact cells in which glands occur similar to those
found in the lamina.

--^^ stoc

Fig. 114. A, transection of portion of mature leaf through main vein ; B, detail of papillae
in nectary: c^, cambium; c«, cuticle; ^/), epidermis ; ^ c, guard cell ; ^/, gland ; wr, mechani-
cal cells; ne, leaf nectary; pal, palisade cells; ph, phloem ; spo, sponge cells; sto c, stomatal
cavity; xy 1, primary xylem ; xy 2., secondary xylem.

GOSSYPIUM 437

Floral Development. — The ontogeny of the flower has been
investigated by Doak (ii, ii) and Gore (i8). In the development
of the floral organs from the meristem of the terminal bud, the
primordia of the involucre, calyx, corolla, androecium, and gynoe-
cium arise in acropetal succession. The three bracts of the involu-
cre appear very early in ontogeny as crescentic primordia, the first
one arising opposite the leaf, and the other two following in suc-
cession. At first, the margins of the bracts are entire, but early in
their development they become serrate. They increase rapidly in
size; and, at maturity, form a triangular bud within which the
floral organs deveJop until anthesis, when the unfolding petals
force the bracts apart. Following anthesis and the shedding of all
the flower parts, except the pistil and calyx, the bracts return to
their original position so that a young fruit may have the out-
ward appearance of a large bud.

The extra-floral nectaries of the involucre are formed compara-
tively late in its development. The three outer ones, which are
located at the base and on the outside of the involucral bracts, occur
in all American species of cotton, although they are not necessarily
present in every flower. They are somewhat larger than the leaf
glands, forming rounded or pear-shaped pits from the floor of which
arise round secreting cells. The three inner involucral nectaries
which are present in most species of cotton are located between the
calyx and involucre and alternate with the outer ones. The inner
nectaries are shallow and usually naked in the case of American
species, but in Old World cottons may be protected by a covering
of short stellate hairs.

The primordium of the calyx arises next; and, at first, forms an
undulating ring of meristematic tissue. As this grows upward,
five crescentic protuberances are diff^erentiated which ultimately
become the tips of the five-lobed gamosepalous calyx. The lobes
of the calyx incline over the floral axis, and, for a time, enclose
the primordia of the other floral parts. Further development of
the calyx results in the formation of a shallow cup with a con-
stricted throat and an undulating rim. Inequalities in the rate of
growth at the apex and base of the calyx result in the dilatation of
the basal portion of the tube while the throat of the calyx fits
tightly over the structures within. Later, there is a reversal in
the growth rate of the upper and basal regions; and, at blossoming
time, the throat of the tube is greatly enlarged. The floral nee-

438 THE STRUCTURE OF ECONOMIC PLANTS

taries arise comparatively late in ontogeny, and consist of a ring
of papillate cells which are a part of the adaxial epidermis at the
base of the calyx. These are bordered by hairs that are stiff and
greatly elongated.

After the divergence of the primordia of the sepals, a common
stamen-petal primordium is formed from the rounded mass of
meristematic tissue remaining toward the center of the axis. This
first develops as a collar-like ring which grows uniformly at all
points, but more rapidly at the margin, so that a saucer-like depres-
sion is formed surrounded by a thick marginal rim. Five narrow
crescentic points, which ultimately differentiate into the petals,
arise on this margin alternate with the sepal lobes. Each petal
primordium soon develops an apical notch; and, later in develop-
ment, the two halves grow at unequal rates. The longer half
becomes hairy on both surfaces while the shorter one remains
glabrous. It is the interlocking of the hairs of the longer halves
that holds them tightly together in the convolute bud. Continued
growth of the corolla results in its projection beyond the sepals,
and it forms a cone-shaped structure about which the calyx is
closely appressed.

While the petals are forming, the inner margin of the meriste-
matic ring, which is the common point of origin of the petals and
stamens, continues to grow upward and forms a ridge with five
prominent protuberances. This structure is the forerunner of the
staminal column, each protuberance producing a fascicle of stamen-
primordia; and further development of the five primordial lobes
results in the formation of ten paired ridges. The lobes of the
staminal column are alternate with the petals, but the growth of
the column and the torsion of the petals make it appear that the
lobes are opposite them. Gore (i8) has described the subsequent
development of the staminal column:

"soon after the first spherical stamen primordia arise, other primordia
appear on top and down the outside face of each lobe of the staminal
column, until ten definite rows are formed. Each of these ten rows
of stamen primordia, except the bottom members of each row, divides
one or more times. Multiplication of stamens by division of existing
primordia produces a large number of stamens, from 50 to 1x5 or more
in the varieties studied."

As growth proceeds, the anthers become crowded and closely
packed, so that their natural orientation is soon lost, and transec-

GOSSYPIUM 439

tions of the buds show the loculi cut at every possible angle. The
young anthers are capitate, but finally become reniform and grooved
along the line of future dehiscence. At first, there is little elonga-
tion of the filament; but just prior to blossoming, it lengthens
rapidly and raises the anther several millimeters.

Doak and Gore are in general agreement with respect to the
ontogeny of the pistil, which may consist of from three to five
carpels. The primordial points of the carpels arise from a dome-
shaped growing point surrounded by the staminal column, and
these ultimately become the stylar and stigmatic portions of the
pistil. At first, they grow slowly as compared with the other
parts; but the upward growth of the ovary results in the projection
of the young stylar structures into the staminal tube at an early
age. As the undiverged bases of the carpels increase in size, in-
ternal ridges are produced which develop into the true septa of
the ovary. These may be regarded as the free inturned margins of
the undiverged carpels which grow centripetally and upwardly
until the original cavity is divided into as many locules as there are
carpels. Continued growth of the reflexed margins of each carpel
forms the placentae from which the four to six ovules arise. In
describing the formation of the placentae. Gore states that "from
the two vertical edges of each septum the rounded placental ridges
appear. The edges of these placental masses grow outward at an
increasing angle toward the external carpel wall, finally becoming
a flattened mass of meristematic tissue." The style and stigmas
remain within the throat of the staminal cylinder until one or two
days before blossoming, when an increase in the rate of growth of
the style carries the stigmas beyond the staminal column and above
the anthers.

The Vascular Anatomy of the Flower. — The vascular system
of the floral axis is similar to that of the stem forming a dictyostele
of from io to 30 bundles. At the involucral node, the pedicel is
triangular in transection, and the two main bundles of each bract
are diverged from the vascular ring at this point. They pass into
the bracts at a higher level, after being further branched so that
several bundles supply each bract. Slightly above this point, there
is an inward divergence of an adaxial series of vascular strands
which ultimately become the placental bundles. At a higher level,
there is a divergence of a series of abaxial bundles which supply the
outer wall of the boll, the number varying with the number of

440 THE STRUCTURE OF ECONOMIC PLANTS

carpels, each of which is usually supplied by ten or more bundles.
The anastomosed sepal-petal-stamen group of vascular strands con-
sists of five or more bundles supplying each sepal, three or more to
each petal, and ten main bundles for the staminal column. These
are successively diverged in the order named. (Fig. xi5.)

-br b

V ped

Fig. 12.5. Longisection of flower bud showing vascular supply to various organs. The
variety is Mebane : ab car, abaxial carpellary bundle; hr b, bract bundle; pet, petal; se,
sepal; sta, stamen; st col, staminal column; sub car, subcarpellary complex; v ped, vascular
supply to pedicel. (After Gore.)

MicROSPOROGENESis. — Beal (8) investigated microsporogenesis
in two forms of Gossypium barbadense L., and three Upland vari-
eties belonging to G. hirsutum L. and found no observable differ-
ences in their meiotic behavior. In the development of the anthers,
the outer layer of cells is differentiated as an epidermis which per-
sists in the mature anther. The "archesporium proper arises from
localized areas of the cell layer immediately inside the epidermis,
the cells of which divide first by periclinal walls. The outer layer
resulting from this division again divides, giving rise to two layers
of tissue (vegetative) cells, while the inner layer, in due time, gives
rise to the pollen mother cells." The haploid number of chromo-

GOSSYPIUM 441

somes for cultivated species of cotton is 16; but Skovsted (30, 31)
has reported several species with a haploid number of 13 chromo-
somes, as well as a triploid hybrid in Asiatic cotton with a somatic
chromosome count of 39, and tetraploids with 5x.

Development of the Ovule. — The primordia of the ovules
arise on the placentae as rounded masses of meristematic cells at
about the time that the locules become closed. During the devel-
opment of the nucellus, a basal fold appears growing upward to
form the inner integument; and as the apex of the nucellus begins
to curve, a second fold initiates the outer integument, which soon
overgrows the inner one. Both continue their growth until they
completely cover the nucellus, except for a small micropylar open-
ing around which the outer integument forms a lip-like ring. The
inner integument is 10 to 11. layers in thickness at the time of flower-
ing, and the outer one from six to eight layers. While the integu-
ments develop, the ovule continues its curving growth until it
becomes completely anatropous; and as the ovule assumes this
position, the portion of the funiculus which is undiverged from
the integument elongates greatly, forming the raphe. (Fig. 1.16,
B-D.')

Megasporogenesis and the Megagametophyte. — The early
development of the archesporial cell is similar to that found in most
dicotyledons. According to Gore (17),

"it has its origin from a subepidermal cell of the nucellus and arises
before the differentiation of the integuments. From its division a
primary parietal cell and a primary sporogenous cell are formed. . . .
While this sporogenous cell increases in size and becomes the megaspore
mother cell, the primary parietal cell divides many times. The other
cells of the nucellus also divide periclinally and anticlinally, thus
embedding the megaspore mother cell in several layers of nucellar
tissue. The mature megaspore mother cell is somewhat elongated
and lies quite near the chalazal end."

Reduction divisions result in the formation of a linear tetrad of
megaspores at the micropylar end of the ovule. The chalazal
megaspore functions; and the outer three abort, although Balls (3)
does report an anomalous condition in which the outermost or
micropylar megaspore may function. In the development of the
megagametophyte, three successive nuclear divisions result in an
eight-nucleate stage, and it becomes an elongated narrow structure
which extends close to the chalaza. (Fig. xi6, D.) Just prior to

442- THE STRUCTURE OF ECONOMIC PLANTS

fertilization, there are three antipodal nuclei at the chalazal end
of the gametophyte, two centrally located polar nuclei, and the
megagamete with two synergids at the micropylar end. At this
time, the megagametophyte is surrounded by nucellar tissue which
is 2.0 to 15 layers in thickness except at the chalaza. The antip-
odal nuclei are ephemeral and disappear by the time the flower
opens.

Fertilization. — Various estimates have been made as to the
length of time required between pollination and fertilization.
These range from approximately 15 to 36 or 40 hours. The insect-
borne pollen grains reach
the stigmatic surface of
the pistil which is covered
with unicellular hairs, and
the pollen tubes rapidly
penetrate the tissue of the
stigma and style. Once
inside the ovary, the pollen
tube follows the placenta,
growing within the tissue
or on the surface; and,
upon reaching the base of
an ovule, travels up the
funiculus toward the mi-
cropyle. The path of the
pollen tube through the
micropyle and into the
nucellar tissue is variable, the tendency being for the tube to pene-
trate for a distance between the integuments, or between the inner
integument and the nucellus, before entering the latter. This
results in the tube having a more or less branched or haustorial
appearance.

Following the penetration of the gametophyte by the pollen
tube, double fertilization is effected. One microgamete fuses with
the megagamete to form the zygote, while the second microgamete
undergoes a triple fusion with the polar nuclei to form the primary
endosperm nucleus. Variation in the details of the triple fusion
has been reported, but most frequently the polar nuclei fuse before
the union with the second microgamete. In other instances, the
fusion of the two polar nuclei and the second microgamete seems

Fig. zx6. A, diagram of transection of young
four-locked ovary showing placentae and develop-
ment of young ovules ; B, C, and D, successive stages
in development of ovule. (B and C, after Gore.)

GOSSYPIUM 443

to be simultaneous. After the formation of the endosperm nucleus,
free nuclear divisions occur until there may be from twenty to
twenty-five endosperm nuclei in the embryo sac. Wall formation
is initiated at the micropylar end of the embryo sac, and finally a
massive endosperm is produced, which is later resorbed by the
embryo until only one layer remains.

Embryogeny. — The development of the cotton embryo has been
described by Reeves and Beasley (x8). The first division of the
zygote, which usually occurs the second day after fertilization,
is transverse; and this is followed by a vertical division of the
apical cell and occasionally of the basal cell. A short suspensor is
formed, but it apparently disintegrates early in ontogeny; and
Balls' (3) statement that there is no suspensor may be explained by
this fact. The two apical cells divide to form a quadrant, and from
this the eight-celled embryo arises about the fourth day. Divisions
of each of the eight cells result in a sixteen-celled embryo that is
usually five-tiered.

The initial stages occur before there is any appreciable increase
in the size of the embryo; and in about nine days, the cotyledons
and hypocotyl are differentiated. This is followed by a period of
rapid growth so that in twelve to fifteen days provascular strands
develop, and the palisade cells differentiate in the cotyledons. At
about this time, the resin glands appear, being formed lysigenously
from two or three cells. By the eighteenth day or before, oil,
starch, pentosans, gossypol and proteins are present in the embryo.
During the closing periods of growth, the embryo continues to
increase in weight until about the thirty-fifth day, when it begins
to lose weight by desiccation, reaching an approximate equilibrium
by the forty-third to forty-seventh day.

Development of the Fiber. — The development of the fiber has
been described by Balls (5), Hawkins and Serviss (ii), Gulati (19),
Singh (19), and Farr (14, 15, 16); and, recently, its structure has
been investigated by Kerr (xi), Anderson and Moore (i), and
Anderson and Kerr (i).

The ontogeny of the fiber may be divided into two phases, the
first dealing with its initiation and elongation; and the second
with the thickening of its wall. The epidermal cells nearest the
chalazal end of the ovule are the first to develop and produce the
longest fibers, while those at the micropylar end arise later or may
fail to develop at all. On the day of flowering, the individual

444

THE STRUCTURE OF ECONOMIC PLANTS

epidermal cells that are to become fibers begin to swell slightly;
and at the end of twelve hours, they have a bulbous appearance and
the cells are more vacuolate. (Fig. 12.7, A.^ After twenty-four
hours, the tubular fiber cells are much elongated and have usually
attained their full diameter. (Fig. xxy, B.) By the second day,
there is a differentiation of the fibers so that some are longer with
pointed tips, while others are short and rounded. At the end of
three days, they appear definitely tapered for approximately a
quarter of their total length. At this time, the cells have large

:i^ f%

(^1

miMmm

Fig. L17. A, transection of epidermis of ovule on day of flowering showing origin of
cotton hairs ; B, same, 14 hours after opening showing elongation of hairs. The variety is
Mexican Big Boll. (Photomicrographs by Lang.)

vacuoles and the walls are still thin, since during the entire period
of elongation the protoplast of the hair is enclosed only by a pri-
mary wall containing pectic substances, cellulose and probably
waxes. Anderson and Kerr state that there is no evidence of cutin
at this time, but that there is "a coherent skeleton of cellulose from
the first day of its appearance." This is in contrast to reports by
other investigators [Farr and Eckerson; Hess, Trogus and Wergin;
Sakostschikoff, Korsheniovsky, and Rutikoly — reviewed in
Anderson and Kerr (i)] that cellulose does not appear in the
walls until from five to as many as thirty-six days after flowering.
This difference may be accounted for on the basis of the small
amount of cellulose present, and its effective insulation by pectic
substances.

GOSSYPIUM 445

At this stage, the lint can be differentiated from the fuzz by the
relative size of the two types of epidermal outgrowths. The devel-
opment of fibers from the epidermal cells may continue for several
days after flowering, and Anderson and Kerr suggest that those
produced during the first two or three days become the lint while
the hairs that are formed later become the fuzz. Brown (9) at-
tributes the differentiation of some of the epidermal cells into lint
and others into fuzz, to hereditary characters; pointing out that,
regardless of cultural conditions, certain varieties always have a
low percentage of lint in comparison with the amount of fuzz,
while others have a relatively high percentage of long hairs.
While the two types of hair initials are at first similar in structure,
they become very unlike at maturity. The lint may vary in length
from % to 1. inches, while the fuzz remains very short.

In connection with the origin of the later formed fibers, Farr (16)
has shown that they do not all arise from the original epidermal
cells, but from daughter cells which have been derived from the
former by cell divisions that occur subsequent to flowering. There
is a thirty-two-fold increase in the surface area of the ovule during
the first twenty days of its development, and only a two-fold in-
crease in the surface dimensions of the epidermal cells. The
increasing surface area cannot be accounted for entirely on the basis
of cell enlargement, and continued cell divisions of epidermal cells
must occur, to compensate for the growth of the maturing ovule.
This fact is in harmony with the recent findings relative to con-
tinued fiber formation.

The nutrition of the develop ng fiber was attributed by Barritt
(7) to liquid in the intercellular spaces of the boll cavity, but Farr
(14) regarded this as unlikely and failed to find any evidence of
"a liquid or semi-liquid substance in the boll cavity." It seems
probable that nutrition during the development and maturation of
the fiber takes place only through its basal connection with the
adjacent tissues of the integument.

The lint usually attains its full length by the end of the fifteenth
to twenty-fifth day of growth, the period of elongation apparently
depending upon several factors, including environmental condi-
tions, the time of the year that the flower opens, as well as the
variety. Following this, the second phase of fiber development
begins involving secondary wall thickening. Earlier investiga-
tions by Balls (5) have been confirmed by Kerr (xx) with respect

446 THE STRUCTURE OF ECONOMIC PLANTS

to the periodic deposition of the lamellated secondary wall. In
this connection, Balls stated that

"the secondary thickening of the wall proceeds intermittently under
normal Egyptian field crop conditions, being arrested each afternoon;
that the cellulose of the hair consequently consists of a number of
concentric shells, layers, or 'growth-rings,' each one representing one
day's growth, with the exception of that of the primary wall; and
that the so-called fuzz-hairs are analogous with the lint-hairs, though
their growth-rings are coarser and more sharply demarcated."

In describing the structure of the walls he found that the

"hairs are covered, outside the cellulose of the primary wall, by a cuti-
cle, bearing wax, which is structurally . . . identical with the cuticle
of the testa, while it is structurally and chemically distinct from the
cellulose. The secondary wall, but not the primary, is traversed
obliquely to the hair axis by simple pits which are rarely visible except
in the living hair, and to these pits is due the twisting of the hair and
its characteristic convolutions after death."

The death of the cell occurs following the opening of the boll, with
the resultant loss of water which causes the fiber cells to collapse,
forming flattened tubes with many twists.

Kerr found that during the period of secondary wall formation
there are two lamellae laid down each twenty-four hours which
together constitute a daily growth ring. These two zones differ
in their porosity, the compact zones being deposited during the
day and the porous ones at night. The differences in porosity,
which result in the lamellated appearance of swollen fibers, are
correlated with temperature fluctuations, and the contrast between
adjacent porous and compact lamellae is greater when the night
temperature falls below io° C, while there is less differentiation
between the zones when it does not fall below 1.2.° C. Thus, while
Kerr confirmed Balls' observations regarding the diurnal deposition
of the growth rings, they are not in agreement with respect to the
method by which this is accomplished. Balls suggested that the
ring was formed in the night and early morning, and that growth
stopped entirely during the hot afternoons; while Kerr attributes
the rings to the formation of a continuous matrix of cellulose laid
down in different densities.

Anderson and Moore (x) investigated the effect of continuous
light on the structure of the walls of fibers and coUenchyma and
found that the reaction of the two tissues to this factor is quite

GOSSYPIUM

447

different. In fibers grown under continuous artificial light there
are no lamellations, such as appear in fibers grown under field con-
ditions and then swollen in cuprammonia and subjected to pressure.
(Fig. 3.) On the other hand, continuous illumination has no
effect upon the conspicuous lamellations of the collenchymatous
cells . Although the light effect
may be correlated with the tem-
perature relations determined
by Kerr, it is suggested that
light may have an effect inde-
pendent of temperature. The
fibers grown under continuous
illumination have the same
convolutions and reversals as
those grown in the field, and
the spirally wound strands of
cellulose exhibit the same struc-
tural pattern. Limited tests
also indicate that the tensile
strength of the unlamellated
fibers grown under continuous
illumination is not significantly
different from that of lamel-
lated ones grown in the field.

The structure of the fiber has
been represented diagrammati-
cally by Anderson and Kerr (i);
and they have described it as
follows: (Fig. ii8.)

Fig. 118. Diagram of mature cotton fiber.
The apparently structureless primary wall of
cellulose and pectic substances is shown at base.
The next layer represents the cellulose structure
of primary wall with its three systems of cel-
lulose threads. Zone 3 shows first layer of
secondary wall at point of a reversal. Zones
4 and 5 represent second and third layers of
secondary wall. The growth rings formed by
the alternation of zones of dense with zones
of less dense cellulose are shown at top of fig-
ure. (After Anderson and Kerr, Ind. and Eng.
Chem?)

"i. A primary wall contain-
ing cellulose and pectic sub-
stances. The cellulose micelles

in this wall are grouped into delicate anastomosing threads which have
at least two systems of orientation: {a) a flat right-hand spiral, (Jj) a
flat left-hand spiral, and probably also (c) a transverse position. All
three systems seem uniform over the entire surface of the fiber cell.

"i. A secondary wall composed of many lamellae of cellulose.
The lamellae are not separated from one another by non-cellulosic
substances but represent dense and less dense areas of cellulose. The
layers are formed of systems of spirally wound branching threads, and
the direction of the spiral is reversed at frequent intervals.

448 THE STRUCTURE OF ECONOMIC PLANTS

"3. Frequently the pattern of spirals first appearing in the secondary
wall is not similar to that in subsequent layers of the wall. Most of
the layers of the wall, however, follow a pattern that is established
soon after secondary thickening has begun."

The strength of a fiber is dependent primarily upon the thickness
of the cell wall, and not necessarily upon the diameter of the cell
since some large fibers may have relatively thin walls. Strength
and length are greatly affected by variations in the environmental
factors, as well as by the genetic constitution of the strain of cotton
grown. Fineness in quality is conditioned upon the diameter of
the fibers together with the thickness of their walls; and the silky
texture of Sea Island cotton is related in part to the fact that it has
fibers of a somewhat smaller caliber than a variety such as the
Indian wool cotton. Color and luster may be associated to some
extent with cultivation and selection, the majority of wild cottons
being brownish in shade, while the cultivated varieties tend to be
white, cream-colored, or less frequently brown. The luster of the
l.r.t seems to be associated with the reflection of light from the
cutinized surface of the hair; but according to Balls (4), it also
results from the translucence of the fibers and the refraction of a
certain proportion of the light in addition to its reflection from the
concave surfaces within the fiber.

LITERATURE CITED

Anderson, D. B., and Kerr, T., "Growth and structure of cotton fiber."

Itjd. and Eng. Chem. ^0: 48-54, 1938.
, and MooRE, J. H., 'The influence of constant light and temperature

upon the structure of the walls of cotton fibers and collenchymatous cells."'

Am. Jour. Bot. 24: 503-507, 1937.
Balls, W. L., "The sexuality of cotton." Year Book, Khed. Agr. Sac, Cairo,

1905.
, The Development and Properties of Raw Cotton, A. & C. Black Ltd.,

London, 1915.
, "The existence of daily growth rings in the cell walls of cotton hairs."

Proc. Roy. Soc. B. qo: 541-554, 1919.

_, The Cotton Plant in Egypt, The Macmillan Co., 1919.

Barritt, N. W., "The structure of the seed coat of Gossypium and its relation
to growth and nutrition of the lint." Ann. Bot. 43: 482.-489, 192.9.

Beal, J. M., "A study of the heterotypic prophases in the microsporogenesis
of cotton." La Cellule, }8: 147-168,192.8.

Brown, H. B., Cotton, McGraw-Hill Book Co., 192.7.

GOSSYPIUM

449

10. Cook, O. F., and Meade, R. M., "Arrangement of parts in the cotton plant."

U. S. Dept. Agr., Bur. Plant Ind. Bull. 222, 191 1.

11. DoAK, C. C, "The ontogeny of the floral organs of cotton." Unpubl. M.S.

Thesis, Agr. and Mech. College of Texas, 192.8.

12.. , "The pistil anatomy of cotton as related to experimental control of

fertilization under varied conditions of pollination." Am. Jour. Bot. 24:
187-194, 1937.

13. DuGGAR, J. F., Southern Field Crops, The Macmillan Co., 19x2..

14. Farr, Wanda K., "Cotton Fibers. I. Origin and early stages of elongation."

Contr. Boyce Thompson Inst. }: 454-458, 1931.

15. , and Clark, G. L., "Cotton Fibers. II. Structural features of the

wall suggested by X-ray diffraction analyses and observations in ordinary
and plane-polarized light." Contr. Boyce Thompson Inst. 4: xy^-z^^, 1932..

16. , "Cotton Fibers. III. Cell divisions in the epidermal layer of the

ovule subsequent to fertilization." Contr. Boyce Thompson Inst, j: 167-171,

1933-

17. Gore, U. R., "Development of the female gametophyte and embryo in cotton."

Am. Jour. Bot. ip.- 795-807, 1932..
18. , "Morphogenetic studies on the inflorescence of cotton." Bot. Gaz-

Qj: 118-138, 1935.
19. GuLATi, A. N., "A note on the differentiation of hairs on the epidermis of

cottonseeds." Agr. Jour. India 2 j: 313-316,1930.

10. Hanausek, T. R., Lehrhuch der technischen Mikroskopie, Stuttgart, 1901.

11. Hawkins, R. S., and Serviss, G. H., "Development of cotton fibers in the

Pima and Acala varieties." Jour. Agr. Res. 40: 1017-101}, 1930.
11. Kerr, T., "The structure of the growth rings in the seco-Jary wall of the

cotton hair." Protoplasma 2j: 119-141, 1937.
13. King, C. J., "Water stress behavior of Pima cotton in Arizona." U. S.

Dept. Agr., Bur. Plant Ind. Bull. 1018, 1911.
14. , "Development of axillary buds on fruiting branches of Pima and

Upland cotton." Jour. Agr. Res. 41: 697-714, 1930.

15. Levine, B. S., "The structure of the cotton fiber. Science, N. S. 40: 906, 1914.
i6. Reed, E. L., "Leaf nectaries of Gossypium." Bot. Gax.- 6^: 119-131, 1917.
17. Reeves, R. G., "Origin of the fringe tissue of the cotton seed." Bot.Ga^- 97:

179-184, 1937.
i8. , and Beasley,J. O., "The development of the cotton embryo." Jour.

Agr. Res. si: 935-944, 1935.
19. Singh, T. C. N., "Notes on the early stages in the development of the

cotton-fibre and the structure of the boll and seed." Ann. Bot. 4$: 378-

380, 1931.

30. Skovsted, a., "Cytological studies in cotton. I. The mitosis and the

meiosis in diploid and triploid Asiatic cotton." Ann. Bot. 4j: 117-151,

1933-

31. , "Chromosome numbers in the Malvaceae." Jour, of Genetics ^i: 163-

196, 1935.
31. Spieth, Alda M., "Anatomy of the transition region in Gossypium." Bot.
Gaz- 95: 338-347. 1933-

450 THE STRUCTURE OF ECONOMIC PLANTS

33. Stanford, E. E., and Viehoever, A., "Chemistry and histology of the glands

of the cotton plant, with notes on the occurrence of similar glands in
related plants." Jour. Agr. Res. i^: 419-436, 1918.

34. Templeton, J., "The branching of Egyptian cotton plants." Min. Agric.

Egypt. Tech. and Set. Sen. Bot. Sec. Bull. 8y, 192.9.

35. Tyler, F. J., "The nectaries of cotton." U. S. Dept. Agr., Bur. Plant Ind.

Bull, iji: 45-54, 1908.

36. WiNTON, A. L., The Microscopy of Vegetable Foods, John Wiley & Sons, 1916.

CHAPTER XV

UMBELLIFERAE

APIUM GRAVEOLENS

THE parsley family is a large one consisting chiefly of herbaceous
plants which may be annuals, biennials, or perennials.
Among the more important economic representatives are celery.

Fig. 119. Habit of celery plant at time of harvest. (Photograph by J. Horace McFarland Co.)

Apium graveolens L. ; parsnip, Pastinaca sativa L. ; carrot, Daucus
carota L. ; and parsley, Petroselinum hortense Hoffm. The family
name is derived from the character of the inflorescence, which is

451

452. THE STRUCTURE OF ECONOMIC PLANTS

usually a compound umbel with secondary divisions known as
umbellets. The fruit is also characteristic and is called a schizo-
carp. It is composed of two dry carpels or mericarps which cohere

Fig. 2.30. Mature plant at time ol flowering showing floral axes and umbels. (Courtesy

of Ferry-Morse Seed Co.)

by their inner faces during maturation, but separate when ripe, each
being suspended from its summit by a branch of the slender forked
carpophore.

Celery is an important salad crop and is grown throughout the
United States, especially in New York, Michigan, Oregon, Califor-

APIUM GRAVEOLENS 453

nia, and Florida. The several varieties are classed as self-blanch-
ing, and green or winter types; and in the former group, Golden
Self7Blanching represents about one-half of the total crop according
to Jones and Rosa (11). A form known as celeriac, A. graveolens,
var. rapaceum, D. C., is grown to a limited extent. In this type,
i to 4 inches of the fleshy root is edible.

The celery plant, like the parsnip, is normally a biennial, al-
though under certain conditions it develops as an annual. In the
course of the usual biennial cycle, a well-developed root system,
a short crown stem, and a rosette of leaves are produced the first
year. (Fig. 119.) The second year, the much branched stems or
seed stalks elongate, producing a shrubby plant i to 3 feet in height.
(Fig. 2.30.) These bear compound umbels of small white flowers
which produce flattened fruits toward the end of the second season.

A common practice in growing celery is to plant the seed in beds
from which the seedlings are transplanted three to four months
later; and, at the end of the first season, the crop is ready for mar-
keting. Occasionally, premature seeding occurs and the plant
develops flowers and fruits during the first season. There is con-
siderable variation in this regard, and in general, the vigorously
growing early varieties are most likely to go to seed the first season.
Thompson (15) has pointed out that "the environment seems to
control the expression of the hereditary factor or factors for annual
flowering and for seed." The environmental factors which may
inhibit seed stalk development are those which involve any serious
check in growth. These include freezing, stunting of plants by
overcrowding in the transplanting flats, and water deficiency.
High temperature, 70° F. or above, tends to prevent premature
seeding unless the seed stalks have already started prior to being
subjected to this temperature. On the other hand, plants sub-
jected to relatively low temperatures, 40° to 50° F., for two weeks
or longer are likely to seed prematurely, and this may occur when
plants are hardened in a cold frame in early spring.

GENERAL MORPHOLOGY

The Root. — The primary root system, consisting of a tap root
and its laterals, becomes very well developed in regions where
the plants are grown from seed sown in the open ground. When
transplanting is practiced, the tap root is destroyed and the fibrous
system is comprised of a large number of adventitious roots that

454 THE STRUCTURE OF ECONOMIC PLANTS

grow out from the base of the plant. Thompson (x4) states that
the major portion of the roots occupies the upper 6 inches of soil,
many of them being within i or 3 inches of the surface while a few
penetrate to a depth of 2. feet or more.

The Shoot. — The subconical, fleshy stem remains short during
the first year and bears a spiral rosette of leaves. The broadened

bases of their petioles over-
lap one another and these,
with a variable portion of
the fleshy stem, constitute
the edible portion of the
plant. The number of
leaves is not constant, but
commonly there are xo to
15 in the rosette, including
the small, immature, cen-
trally located ones. The
outer 11. or 15 leaves are
large, with somewhat erect
petioles that range from 4
to 10 inches in length. The
petiole is glabrous, crescen-
tic in transection with a
prominently ribbed abaxial
surface, and a relatively
smooth concave adaxial
one. The blade is odd-pinnately compound, and usually there are
two or three pairs of stalked leaflets. These are pinnately lobed
or compounded, and the ultimate divisions are broadly wedge-
shaped with coarsely dentate or crenate margins.

The Inflorescence and Flower. — The flowers are borne in
compound umbels which may or may not be subtended by an in-
volucre, and involucels may or may not subtend the umbellets.
The flowers are perfect and epigynous, with calyx lobes that are
rudimentary or lacking. The small white or greenish-white petals
are incurved at their tips. Alternate with them are five stamens
with very slender filaments and versatile anthers which are diverged
at a level above the top of the ovary. The pistil consists of two
carpels which cohere during early development to form a two-
loculed ovary terminated by two distinct styles. (Fig. 2.31.) The

Fig. 131. A, habit of inflorescence ; B, lateral view
of flower ; C, face view of same; D, fruit.

APIUM GRAVEOLENS

455

stylopodium, which is characteristic of the pistils of most umbel-
lifers, is inconspicuous in celery. In each locule, a single seed is
produced, and the schizocarp separates at maturity to produce two
one-seeded mericarps.

ANATOMY

The Fruit and Seed. — The schizocarp is flattened laterally and
the mericarps are pentagonal in transection. (Fig. i.t,i.. A, B.)
Winton (31) and Kondo (13) have investigated the histology of the

Fig. x}!. a, diagrammatic side view of schizocarp; B, transection of mericarp; C, sec-
tion of fruit and seed coat through rib ; D, transection of fruit and seed coat through oil duct :
al g, aleurone granules; cm s, commissural surface; emb, embryo; end, endosperm; ep, outer
epidermis of fruit coat ; ?/»', inner epidermis of fruit coat; /r c, fruit coat ; o</, oil duct; rb,
rib; j c, seed coat ; sty, style; t^^j ^«, vascular bundle. (Redrawn and adapted from Kondo,
Ohara Institute.)

fruit, which conforms in its general characters to the umbelliferous
type. The fruits are minute, somewhat broader than long, and the
mericarps have five rather pronounced primary corky ribs, each of
which is traversed by a fibro vascular bundle. In the intervals
between the ribs are one to three oil ducts, and there are two on the
commissural side of the mericarp. Kondo reports that he found
one oil duct in each interval in seven of the varieties investigated,
and in only two European types were two to three present in each
furrow. The pericarp, which consists of from seven to ten rows of
parenchymatous cells, is marked with delicate striations and papil-

456 THE STRUCTURE OF ECONOMIC PLANTS

lae that are more or less evident in the form of warty protuberances.
The cells of the epicarp or outer epidermis are small and isodia-
metric, and in surface view, are sinuous in outline. The oil ducts
in the mesocarp are surrounded by a layer of polygonal cells which
become brownish at maturity, and the bundles located in the ribs
are accompanied by groups of sclerenchymatous cells. The some-
what elongated cells of the inner layer of the mesocarp are broader
than those of the endocarp. Winton states that they are usually
transversely oriented, but that it is not uncommon for them to be
arranged with their long axes in other directions. The endocarp
or inner epidermis is composed of narrower cross cells, which are
transversely elongated or occasionally develop in a parqueted or
mosaic arrangement.

The mature mericarps, each the product of the development of
one of the carpels, separate along the commissural line and are
suspended from a branched carpophore. This is a unique structure
peculiar to the fruit of the UmbelLferae and has been interpreted
by some investigators as being axial — that is, as a prolongation
of the receptacle between the carpels. Others regard it as being
appendicular and derived from portions of the two carpels. Jack-
son (9) has studied the structure on the basis of its development in
the flower and fruit and has concluded that

"a small basal portion is usually receptacular or axial; by far the
greater part is appendicular. ... It represents an innermost, ventral
portion of the two carpels and consists chiefly of the ventral traces
of these carpels. Associated with the traces in the formation of this
structure is a greater or lesser amount of adjacent non-vascular tissue."

In the mature fruit, the point of separation of the mericarps from
the carpophore begins at the level at: which the stele of the recep-
tacle branches to form the peripheral and central bundles. The
peripheral bundles constitute the vascular supply for the two
mericarps, while the two central bundles form the carpophore.
This consists of the two bundles, the large lignified parenchymatous
cells which lie between them, and some non-lignified parenchyma
which dies as the carpophore and fruit mature. The break which
separates the carpophore into two branches occurs in the lignified
parenchyma between the central bundles so that each carpophore-
half consists of one central bundle.

The seed has a thin coat consisting of an outer layer of large
cells and an inner zone of several rows of crushed cells. The cells

APIUM GRAVEOLENS

457

of the abundant endosperm, which completely surrounds the small
embryo, are thick-walled and contain fat and aleurone grains.
(Fig. i3X, D.) No starch is present and Kondo reports the absence
of crystals of calcium oxalate which are frequently found in the
endosperm of some umbelliferous seeds. The embryo is straight
and lies in the endosperm with the tip of the minute primary root
inclined upward.

Development of the Seedling. — The germination and growth
of the seedling are relatively slow. In commercial practice, the
seed is either sown in outdoor seed beds, which is the custom in the

Fig. 2.33. Stages in development of celery seedling.

South; or grown in hotbeds or cold frames, as is the common
method in the North.

The primary tap root develops laterals which are arranged in two
double rows; and, following the emergence of the narrowly ovate
cotyledons, the first two foliage leaves expand. The petioles are
slender, and the simple, three-lobed blades have serrate to dentate
margins. (Fig. 2.33.) When the crown of the plant has reached
a diameter of 3^ to ^ of an inch, it is transplanted.

Anatomy of the Root. — The slender primary root has a diarch
protostele, and resembles the primary root of parsnip, as described
by Warning (18), in the principal details of its structure. Its
ontogeny follows Janczewski's (10) third type in which there is a
definite plerome and periblem, and a combined calyptrogen-dermat-

458 THE STRUCTURE OF ECONOMIC PLANTS

ogen layer. The plerome produces the stele; the periblem, the
cortical tissues; and the calyptrogen-dermatogen layer, the root
cap and epidermis.

The cortex of the young root consists of several layers of paren-
chymatous cells limited outwardly by the epidermis which pro-
duces a large number of root hairs. The endodermis is not defined
very early in ontogeny by the development of Casparian strips,
but its position may be readily determined with reference to the
pericycle which lies immediately centrad to it. The pericyclic
cells become enlarged and some of them undergo a series of oblique

xrf pri

Fig. 134. Left, transection of young primary root showing stele; right, development of
lateral root from pericyclic tissue : co, cortex ; en, endodermis ; ep, epidermis ; mx, meta-
xylem; 0 d, oil duct; pel, pericycle ; ph, phloem; /ia-, protoxylem ; >-r/?r/, root primordium.

longitudinal divisions which result in the formation of the charac-
teristic primary oil ducts found in this family. These lie in two
arcs, each of which contains about nine ducts. The central one,
which lies directly outside the protoxylem point, is quadrangular
in transection while those lateral to it are usually triangular.
(Fig. 134.)

Trecul (xy) and van Tieghem (i6) have described the develop-
ment of these ducts in detail, and de Bary (3) has summarized their
work. The first two oil ducts to be cut off in the pericycle are
quadrangular in transection and located 180° apart. The two
protoxylem points of the diarch xylem plate which later differen-
tiate in the plerome lie immediately centrad to them. The forma-
tion of the two centrally located primary oil ducts is schizogenous,

APIUM GRAVEOLENS 459

and is initiated by the radial elongation of two adjacent rectangular
pericyclic cells. Then an oblique wall comes in at about a 45°
angle to the radial wall between the two pericyclic cells involved
in the formation of the duct. (Fig. 11, A.^ This results in the
cutting of each pericyclic cell into two daughter cells, one of which
is large and five- or six-angled, the other small and quadrangular
in transection. The two quadrangular cells lie at the outer limit
of the pericycle while the five-angled ones extend through the
entire width of the pericycle. These four cells have a common
angle at a midpoint on the radial wall between the original peri-
cyclic cells, and the oil passage is formed by a splitting of the walls
along their middle lamellae beginning at this point. This results
in a quadrangular passage which is limited externally by two small
pentagonal cells and internally by two hexagonal ones. (Fig.
11,5.)

At the time that the pericyclic ducts are being formed, the pri-
mary phloem is differentiating; and its position can be determined
early in ontogeny by the formation of a primary duct in each
phloem group adjacent to the pericycle. These appear to be formed
schizogenously in a manner similar to the pericyclic oil ducts except
that they are bounded externally by two pericyclic cells and in-
ternally by three or more phloem cells so that they are pentagonal
in transection. (Fig. 11, C) Before the metaphloem is com-
pletely differentiated, the protoxylem begins to mature. This
proceeds centripetally from two points centrad to the central
pericyclic ducts and continues until a solid diarch xylem strand is
formed. The strand is narrow and from five to ten cells from one
pole to the other. The protoxylem elements are annular and spiral,
while the wall thickenings of the metaxylem are spiral-reticulate
and reticulate. The primary xylem and phloem are separated by
a zone of interstitial parenchyma.

Lateral Roots. — The first lateral roots arise from the pericycle
at approximately the time that the maturation of the primary
xylem has been completed. The root primordia do not form
directly outside the protoxylem points, as is frequently the case
in lateral root formation; but, instead, are developed in arcs of the
pericycle to the right and left of the diarch xylem strand. (Fig.
X34.) This may be related to the fact that the arcs of pericyclic
cells abutting the protoxylem points are involved in the formation
of the primary oil ducts. The origin of the lateral roots at these

46o THE STRUCTURE OF ECONOMIC PLANTS

loci results in their alignment in four longitudinal rows which tend
to be paired, although the torsion of the root may make the orien-
tation appear to be spiral.

The ontogeny of the lateral roots is similar to that of the primary
root, and its histogens follow the same developmental sequence
as in the main axis. As the growing point is formed by tangential

and radial divisions of the per-
—pd icyclic cells, the endodermis is
forced outward; and the emer-
gence of the lateral root through
the cortical and epidermal tissue
is accomplished by a rupturing
and resorption of the cells.

Secondary Thickening of
THE Primary Root. — The first
cambial activity occurs in the
interstitial parenchyma immedi-
ately centrad to the phloem.
There is a progressive lateral
activation toward the primary
xylem poles which may involve
the pericyclic cells over those
points. In most instances, the
cells produced by the active
tissue outside the protoxylem
points are parenchymatous so
that two well-defined rays are
formed.

The secondary xylem vessels
are laid down in radial rows, one

Fig. 135. Transection of sector of old
root showing secondary thickening : ca, cam-
bium ; mx, metaxylem ; pc/, pericyclic paren-

chyma; pd, periderm; ph, phloem; px, pro- ^q j^q qj. ^J^j-^g ^^^^ ^^ width,
toxylem; .vj x, secondary xylem. ^ ^ _,

(Fig. ^35.) This approximate
radial alignment may be maintained, except that there is also some
radial and tangential separation of the vessels owing to the activity
of the adjacent parenchymatous cells. The vessels are large with
loosely reticulate or reticulate-pitted walls. The end walls of the
vessel segments are commonly transverse and the segments are
two to five times as long as broad. The cells of the secondary
xylem parenchyma are thin-walled, isodiametric in transection,
and approximately the length of the vessel segments.

APIUM GRAVEOLENS 461

There is an extensive development of secondary phloem which
forms a broad zone consisting of groups of sieve tubes, companion
cells and fibers, separated by rays of parenchyma. Numerous oil
ducts occur in the phloem region which resemble the primary ones
except that the number of epithelial cells is ordinarily greater.

The rapid development of secondary stelar tissue results in an
early disorganization and rupturing of the cortical and epidermal
regions. The endodermis is broken; and, as disintegration pro-
ceeds, the pericyclic cells divide both anticlinally and periclinally
so that a zone of parenchymatous tissue is formed. The peripheral
portion becomes a periderm, and the phellem is several cells in
thickness, being renewed by a phellogen that also cuts off some
phellodermal cells on its inner face.

Vascular Transition. — The transition from the radial exarch
arrangement of the stele of the root to the collateral endarch
orientation of the vascular tissues in the cotyledons takes place in
the upper hypocotyl. The change is abrupt, being accomplished
within the space of a few millimeters. The first stage consists
of the segmentation of each of the primary phloem groups into
two separate groups. At the level at which this segmentation
occurs, there is a reorientation of the centrally located metaxylem
elements which, instead of forming a continuous spindle-shaped
zone from one protoxylem pole to the other, differentiate laterally
so that parenchymatous cells occupy the center of the stele.

At higher levels, there is further lateral differentiation of the
metaxylem. The polar sectors of protoxylem are oriented in such
a way that the later-formed metaxylem cells are tangentially
placed and separated from the former by parenchymatous cells.
The phloem groups, each of which has an oil duct, lie in an ap-
proximately collateral position with respect to the protoxylem,
and lateral branches occupy a similar relation to the primary xylem
that with them forms the lateral traces of each cotyledon. The
vascular supply to each cotyledon consists of three bundles. The
large median bundle, sometimes called a "double bundle" or
"triad," is comprised of a protoxylem strand, a portion of the
metaxylem lying tangential to it, and two groups of phloem which
are collateral with the metaxylem. The two smaller lateral
bundles are collateral.

At the point of divergence of the cotyledons, the bases of the
petioles form a continuous collar around the epicotyl enclosing

46i THE STRUCTURE OF ECONOMIC PLANTS

the growing point and the primordia of the first foliage leaves.
(Fig. i36.) When the petiolar bases diverge, each is crescentic
in transection. In the lamina of the cotyledon, the mesophyll is
differentiated into palisade and spongy parenchyma. The palisade
cells are not as elongated as in the foliage leaf, but they do form a
distinct layer beneath the adaxial surface that is more compact
and has fewer intercellular spaces than occur in the five or six
layers of spongy tissue.

Anatomy of the Stem. — The fleshy crown stem of the first
year constitutes the so-called "heart" of celery. It has a large

Fig. 136. A, transection of upper hypocotyl showing divergence of six cotyledonary
bundles. Two procambial strands of foliar bundles appear in the intercotyledonary plane;
B, transection of seedling axis showing cotyledonary collar surrounding growing point of
epicotyl and base of first foliage leaf. The xylem of median cotyledonary bundle is tangen-
tially oriented rather than endarch at this level.

central pith, and oil ducts occur in the peripheral portion. Associ-
ated with them are numerous medullary bundles which lie centrad
to the vascular ring. The occurrence of medullary bundles in the
Umbelliferae is not constant, but has been reported by Solereder
(ii) for sixteen genera, including Apium. They do not occur
in Pastinaca or Daucus.

At the periphery of the pith is an almost continuous cylinder of
vascular tissue comprised of bundles of the leaves which form
the rosette of the first year. The leaf traces constitute a ramifying
network of vascular strands that continue through the cortex into
the bases of the petioles. Because internodal elongation is very

APIUM GRAVEOLENS

463

limited, the leaf bases overlap one another in such a way that there
is little exposed epidermal surface. Buds occur in the axils of the

m but.

'-xy \

Fig. 137. Transection of mature stem showing sector of vascular cylinder and medullary-
bundles : ca, cambium; col, collenchyma; conj, conjunctive tissue; ep, epidermis; m hu,
medullary bundles in various stages of development ; mech, mechanical tissue ; 0 gl, oil gland ;
far, parenchyma; ph, phloem; pi, pith; xy, xylem; xy 1, primary xylem; xy x, secondary
xylem.

464 THE STRUCTURE OF ECONOMIC PLANTS

leaves and their subsequent development results in the formation
of the much branched shoot of the second year.

Anatomy of the Floral Axis. — The mature floral axis is
strongly ridged and fluted and may or may not become hollow^ at
maturity. The outer walls of the epidermis are thickened and
covered with a somewhat roughened cuticle. Stomata occur in
the portions of the epidermis overlying the chlorenchyma, and
Nestel (16) has indicated their frequency at about 70 per square
millimeter. The zone of chlorenchyma which lies within the
epidermis, except at the angles, is four or five layers in width, and
the round or elliptical cells are slightly longer than broad. The
non-chlorophyllose parenchyma of the cortex consists of large
cells that are more loosely organized than those of the chloren-
chyma. (Fig. 137. ) They occupy the region between the chlo-
renchyma and vascular cylinder, also separating the collenchyma
from the large bundles located at the angles of the stem.

The strands of collenchyma constitute the chief mechanical
tissue of the axis and form the ridges, being separated from the
epidermis by one or two layers of parenchyma which later become
thick-walled. These cells are larger in transection than the
coUenchymatous elements; and, in longisection, can be dis-
tinguished from them since they are only two to three times as
long as broad with transverse end walls, while the coUenchymatous
cells are much elongated and sharply pointed or oblique. Addi-
tional strands of collenchyma also may be located in the grooves
between the ridges, and these are separated from the epidermis by
chlorenchyma rather than by thick-walled parenchyma. The
coUenchymatous strands at the ridges are oval or kidney-shaped
in transection, while those in the intervals are more or less circular.
Centrad to each strand is an oil gland or duct, and similar ones
commonly occur outside the phloem of each vascular bundle.

The vascular bundles are collateral, the larger ones lying on
radii which pass through the angles of the stem. Nestel, in a
comparative study of the stems of the Umbelliferae, points out
that celery belongs to the type in which there is no interfascicular
cambium. Because of this, the adjacent bundles are distinct from
one another, except that the medullary rays are occluded on the
xylem side of the ring. This results from the differentiation of
the ray parenchyma to form connective or conjunctive tissue by
lignification and wall thickening as the stem matures. The

APIUM GRAVEOLENS 465

libriform or connective type of cell also surrounds the inner face
of the xylem of each bundle to some extent. The portion of the
ray adjacent to the phloem does not mature as connective tissue,
but the outer face of the phloem is capped by cells which tend to
become thick-walled.

The xylem region consists of numerous vessels separated from
one another by parenchymatous cells. The first-formed proto-
xylem elements are of small caliber with open spiral thickenings,
while those that mature later are larger with much closer spirals.
A rather striking feature of the spiral wall thickenings is that in
many cases the bands are double. The vessel segments of the
metaxylem have very close spirals or a reticulum.

One or more ducts are developed in each primary phloem group,
and there are three in most of the larger bundles that subtend the
angles of the stem. Two or three occur in bundles of intermediate
size and there is a single duct in the small bundles. The smaller
secretory canals located in the peripheral portion of the pith are
bounded by five or six epithelial cells. The fascicular cambium
is relatively inactive, producing a limited amount of secondary
xylem and phloem; but at the nodes, there is more activity and
the nodal plate may become considerably lignified owing to the
development of secondary xylem consisting of pitted vessels,
fibers and parenchyma.

Medullary Bundles. — Medullary bundles are distributed in
the peripheral portion of the pith, and in a given stem, may be
found in several stages of development. In most cases, the phloem
is well developed, while the extent of xylem formation is variable.
The bundle may be completely amphivasal, half-amphivasal, or
collateral, and types are found in which the bundle appears to be
bicollateral with xylem on two sides of the intervening phloem.
The more completely developed bundles, regardless of type, are
surrounded by sheaths of wood parenchyma. (Fig. 138.)

In most instances, an oil duct is centrally located in the medullary
bundle and the phloem is arranged in concentric layers around it.
Outside the phloem, the xylem is variously distributed or may be
entirely lacking. Preliminary investigations indicate that the
ontogeny of the medullary bundle is related to the secretory canals,
and it appears that they are initiated by an activation of cells
adjacent to the epithelial cells which line the duct. (Fig. 1^,^, A.^
These cells form a meristematic strand by a series of periclinal

466 THE STRUCTURE OF ECONOMIC PLANTS

divisions wliich proceed until there is a strand several cell layers
in width surrounding the duct. (Fig. i.t,S, D.) They later
differentiate as the xylem and phloem elements of the mature
medullary bundle.

Medullary bundles are not restricted to the Umbelliferae, but
as noted by Wilson (31) are found "in a large number of unrelated

Fig. 2.38. Medullary bundles: A-C, stages in development of bundle showing central
oil duct and amphivasal arrangement of vascular tissues ; D, longisection of portion of oil
duct showing formation of provascular strand adjacent to epithelial cells of duct: bu s,
bundle sheath ; ept, epithelial cells ; o d, oil duct ; ph, phloem ; p v s, provascular strand ;
xy, xylem.

families, perhaps thirty or more." The term "medullary bundle"
has been variously applied by those who have described these
structures. They were considered by Weiss (19) and Col (5) to be
common rather than cauline in most instances, and were regarded
as the extensions of leaf traces instead of an independent system of
stem bundles. Westermaier (30), working with species of Begonia
which have tubers or rhizomes, suggested that the medullary
bundles are supplementary to the usual ring of vascular bundles.
In Apium, it appears that the medullary bundles arise early in
ontogeny of the stem and may be traced to the ultimate branches
of the umbel. It seems probable that they are cauline, supple-
menting the common bundles in the translocation of materials
during the period of flowering and fruit development. They may

APIUM GRAVEOLENS 467

compensate for the absence of an interfascicular cambium wliich,
when present, serves the purpose of adding secondary vascular
elements to the conducting system of an axis. Nestel (16) has
reported rare instances in which there were small isolated bundles
in the cortical parenchyma. These were amphicribral in contrast
with the amphivasal condition found in medullary bundles.

Anatomy of the Petiole. — The petioles of the leaves of the
first year constitute the principal edible part of the plant, and the
marketability of celery depends upon their quality. Sayre (lo)
has pointed out that the desirable characteristics. are crispness,
tenderness, the absence of strings, and a pleasant, sweet, nut-like
flavor. Undesirable qualities which decrease marketability are
pithiness, toughness, stringiness, and a rank or pungent flavor.

Several of these characters are definitely related to structure,
and this has focused attention upon the developmental anatomy
of the petiole. Norton (17) has stated that "Quality in celery
is primarily a question of the proportional relationships of the
so-called parenchymatous tissue to the fibrous tissue," and he
attributes the differences in quality, aside from those that are
hereditary, to external factors including soil moisture, plant food,
especially nitrates, rate of growth, light, and temperature. Mills
(14) regards high quality as being determined by flavor, which he
thinks is associated with the chlorophyll content in the stalks,
and by degree of crispness, which he relates to the water content
in the plant. He notes that lack of crispness may be the result of
excessive temperature, too much or too little moisture, nitrogen
deficiency, and excessive retardation or acceleration of the growth
rate. Like other investigators, he attributes part of the pithiness
to heredity, pointing out that some varieties of celery develop
this undesirable characteristic more frequently than others.

Thompson (13) has noted that storage conditions may affect
crispness, especially insufficient moisture and too high temperature,
which result in over-rapid maturation and pithiness. Sayre (1.0)
investigated eight varieties of celery to determine the anatomical
factors which might have a bearing upon stringiness and tough-
ness, concluding that

"The only celery tissue that seemed to have a very definite relation
to stringiness was the collenchyma," and "pithiness is evidently
correlated with a breaking down of the parenchyma cells which leaves
large open spaces through the center of the stalk."

468

THE STRUCTURE OF ECONOMIC PLANTS

More recently Esau (7), who attacked the problem by com-
paring the relative mechanical properties of the collenchyma and
vascular tissues, concluded that "Mechanically the collenchyma

■^"■-rrf^'

" ' I III iri"'"ri '

;.A.

Fig. 2.39. Upper, transection through mature petiole showing distribution of collenchyma
and vascular bundles. Schizogenous lacunae have developed in parenchyma. Lower,
transection of somewhat less mature petiole showing same features except for absence of
lacunae. In both figures, the abaxial surface is upward. (Photomicrographs by Esau,
Hilgardia.^ .

is much stronger than the vascular tissue. The breaking load of
collenchyma may be two to four times that of the entire vascular
bundles or the bundle cap."

In transection, the crescentic petiole is broadly expanded at its
base and more subcylindrical above with an adaxial groove extend-

APIUM GRAVEOLENS 469

ing to the point of divergence of the lower leaflets. The central
axis of the compound lamina and the stalks of the lateral leaflets
are also subterete with a grooved adaxial surface. The abaxial
surface is prominently ridged, owing to the underlying strands of
collenchyma, while the adaxial one is smooth and without re-
inforcing strands of collenchyma although there are usually two
or more layers of thick-walled subepidermal parenchyma. The
winged margins of the petiole are also strengthened with mechani-
cal tissue.

The mesophyll is parenchymatous with large intercellular
spaces. In plants that tend to be pithy, the parenchyma breaks
down schizogenously at various points, leaving irregular internal
cavities. (Fig. 2.39.) The number and distribution of the chloro-
plasts depend upon the variety and cultural methods practiced
with respect to blanching. In most types, there is a zone of
chlorenchyma immediately within the epidermis, except at the
ridges and on the adaxial surface, and it also frequently surrounds
the vascular bundles.

The principal vascular bundles lie in a semicircle immediately
inside the coUenchymatous strands with their phloem caps directed
toward the abaxial surface. Between the major bundles, there
may be smaller ones, their number and orientation depending upon
the variety of celery and the level at which the transection is
taken. A row of small adaxial bundles may occur, these being
more evident at the base of the petiole than at its upper limits.
(Fig. 139.) At intermediate levels, the smaller bundles occupy a
more nearly central position in the mesophyll; and, above, they
come to lie approximately in the abaxial ring. As reported by
Esau (7), they may consist of phloem only; in some instances
solitary xylem cells have been noted; and, in others, there are
bundles in reverse orientation in which the phloem of the adaxial
bundles is directed toward the adaxial surface.

Numerous oil ducts occur in the phloem, prominent ones lying
centrad to each coUenchymatous strand and outside the phloem
caps of the larger bundles. They are also generally distributed
throughout the parenchymatous tissue adaxial to the main vascular
ring. (Fig. x4o.)

The walls of the epidermal cells are thickened, especially the
outer one over which there is a well-defined cuticle. There is
considerable irregularity in the shape of these cells, as seen in

470 THE STRUCTURE OF ECONOMIC PLANTS

surface view; but, in general, they are quadrangular and their
greatest dimension is parallel to the long axis of the petiole.
Stomata occur on both surfaces, being more numerous on the
abaxial one where they are arranged in approximate rows in the
furrows. The stoma is of the type in which the initial epidermal

mech

y-xy 2

-xy 1

Fig. 140. Transection of sector of petiole showing vascular bundle and location of mechan-
ical tissue underlying abaxial epidermis: ca, cambium; col, collenchyma; cu, cuticle; ep,
epidermis; mech, mechanical tissue; 0 gl, oil gland; par, parenchyma; ph, phloem; xy i,
primary xylem; xy t, secondary xylem.

cell is not the mother cell. In its development, the initial cell is
divided successively so that two or three accessory cells are formed
surrounding the mother cell. A final division of the mother cell
results in the two guard cells. (Fig. 6.)

Ontogeny of the Leaf. — The ontogeny of the leaf is similar
to that described by Sachs (19) for Pastinaca and other Umbel-
liferae. It first appears as a cone-shaped primordium which

APIUM GRAVEOLENS 471

"quickly broadens into a shell-like form, and grows vigorously at
its apex. Beneath the apex, protuberances arise at the right and left
angles in acropetal order; these also grow in the same manner at their
apex, and produce again lateral protuberances of the second order;
and according to the extent to which the surface of the leaf is developed,
these protuberances become lobes of a simple leaf or distinctly separated
leaflets." (Fig. 31, A, B, C.)

Esau (7) has described in detail the histogenesis of the petiole
in which there is an early differentiation into three meristematic
regions, the protoderm or dermatogen which gives rise to the
epidermis, the procambium which produces the vascular strand,
and the ground meristem which becomes the fundamental paren-
chyma. The cells of the three meristems can be distinguished
early in ontogeny by their size, degree of vacuolation, and the
character of the cell divisions which they undergo. The cells of
the ground meristem are the largest and the first to develop promi-
nent vacuoles and intercellular spaces. The small protodermal
cells are more dense than those of the ground meristem ; and the
procambial cells are the least vacuolate and smallest in transection
of the three types, but are longer than the others in their axial
dimension. The cells of the protoderm divide only anticlinally,
producing the epidermis and stomatal cells. The ground meristem
divides in three planes, but the divisions are less frequent than in
either of the other histogens, and they compensate in part for this
by more rapid cell enlargement.

The procambial tissue arises in the ground meristem through
a series of rapid periclinal longitudinal divisions, and the strands
which are the forerunners of the larger bundles are differentiated
in this manner earlier than those which form the smaller interven-
ing bundles and the adaxial ones.

According to Ambronn (i) the strands of collenchyma differen-
tiate from the procambial tissue, later becoming separated from
it through the differentiation of an intervening layer of paren-
chyma. This is not in agreement with the work of Esau (7),
who finds that the collenchyma and the vascular bundle have
independent origins, and that they are clearly set apart from
each other. Their separate origin is emphasized by the early
differentiation of an oil duct which lies in the parenchyma be-
tween the collenchyma strand and the bundle cap outside the
phloem. (Fig. x4i.)

472. THE STRUCTURE OF ECONOMIC PLANTS

Development of the Collenchyma. — The mature collen-
chymatous cells are small in transection and very much elongated

Fio. 2.41. A~D, transections of collenchyma in successive stages of development; E,
stage showing late initiation of collenchyma; F, longisection of young collenchyma.
Stippled areas represent oil ducts. (After Esau, Hilgardia.')

axially with oblique end walls. The walls are thickened chiefly
at the angles, although there are some additional thickenings on
the lateral surfaces. (Fig. X4i.) The development of the young
strand at first resembles that of a procambial one; and longitudinal,

APIUM GRAVEOLENS 473

periclinal, and later anticlinal divisions result in the formation of
a large number of much elongated cells with small diameters.
The strand continues to increase in size through repeated divisions
of the cells of the strand itself, and through the addition of pe-
ripheral cells derived from the adjacent fundamental parenchyma.
The divisions occur so rapidly in the early ontogeny of the col-
lenchyma that the resultant cells are much smaller than those of
the surrounding tissue.

Cell enlargement does not begin until cell division is slowed
down, at which time the deposition of the characteristic wall
thickenings is initiated. In their meristematic condition, the
young collenchymatous cells are longer than wide with transverse
or slightly oblique end walls which become more tapering as
growth proceeds. In later stages of development, thin transverse
walls may be laid down. The mature cells retain their protoplasts
and their potentiality for growth and enlargement. The walls are
chiefly cellulose with a high percentage of water, but they also
contain pectic substances which are more abundant near the middle
lamella.

Anderson (x) has found that the cellulose and pectic substances
of the collenchyma occur in alternate layers and has suggested
that this alternation may help to explain the fact that contraction
in the transverse plane is much more pronounced than in the
longitudinal one. He points out that this also may account for
the high percentage of water found in the walls of the collenchyma,
which greatly exceeds that found in those of lignified cells.

Ontogeny of the Bundle. — The vascular bundles are endarch
and collateral. At maturity each consists of a xylem region which
is oriented toward the adaxial surface and a phloem zone which
partially surrounds the abaxial face of the xylem so that the bundle
may be half-amphicribral. The walls of the outermost phloem
cells thicken to form a strengthening cap on the abaxial face of the
bundle. A weak cambium may be formed, but the amount of
secondary tissues cut off by it is relatively small so that the major
portion of the vascular tissue can be regarded as primary.

In the ontogeny of the bundle, the procambial strand gives rise
to protoxylem and protophloem cells which are located rather
near to each other; but, according to Esau,

"continued division of procambial cells in the center of the bundle
increases the distance between them. The procambial cells formed

474 THE STRUCTURE OF ECONOMIC PLANTS

centrifugally from the xylem differentiate into new elements of this
tissue, while new phloem cells arise on the opposite side of the bundle.
Since, however, the differentiation of elements progresses faster than
the division of procambial cells the xylem and phloem gradually
approach each other." (Fig. 142..)

Fig. 2.4X. A-C, transections through portions of petioles showing differentiation of pro-
cambial strand ; E, G, I, transections showing early differentiation of vascular tissue ; D,
F, H, diagrams of entire bundles from which drawings £, G, I were made : obi, obliterated
phloem ; 0 d, oil duct ; ^r, procambium. (Afcer Esau, Hilgardia.')

APIUM GRAVEOLENS

475

The narrow layer of procambial cells which finally separates the
primary xylem and phloem is the region which may form the
cambial-like zone referred to above.

During elongation, only protoxylem and protophloem elements
are matured; but when the petiole ceases to elongate, the pro-
cambium gives rise to metaxylem and metaphloem elements. In
most bundles, this completes the maturation, since little or no
secondary tissue is present in the petioles.

Ontogeny of the Xylem. — The xylem tissue of the bundle
consists only of vessels and parenchyma, no fibers being formed.

(I b c de f g hi j k I

Fig. 143. Median longisection through bundle and adjacent tissue of petiole: a, pith;
b, protoxylem, including spiral vessels and parenchyma; c, metaxylem; d, pitted secondary
xylem vessel ; e, cambium ; /, phloem ; g, bundle sheath ; h, cortical parenchyma ; i, cortical
oil duct with epithelial cells ; j, collenchyma ; k, thick-walled parenchymatous cells ; /,
epidermis with cuticle.

The first-formed protoxylem elements are small in diameter, and
the majority of them have secondary thickenings of an open spiral
type, although infrequently there may be a limited production of
vessels with annular bands. As the bundle elongates, the spirals
become more and more stretched and some of the protoxylem
elements are finally obliterated. The progressively larger meta-
xylem elements are scalariform and reticulate. There is a large
amount of parenchyma adjacent to the protoxylem, and a limited
number of parenchymatous cells surround the metaxylem vessels.
(Fig. X43.)

In Esau's (8) study of vessel development, it is pointed out that
the vessel segments expand very rapidly; and, in so doing, affect
the spatial relationships of adjacent cells. These are not only
flattened and distorted; but, in some cases, actual tearing of tissue
may occur so that "new contacts are made by the expanding

476 THE STRUCTURE OF ECONOMIC PLANTS

vessel." When the vessel segment reaches its full diameter, it
still contains its protoplast, the longitudinal walls are thin, and

!-■■

H -1

;'

1

!»»

m

it
It

I

^

•

«

G

E

Fig. 144. Stages in development and disintegration of vessel end wall : A-D, stages
showing thickened end wall and formation of spiral secondary thickenings ; E, H, rim be-
tween two mature vessel elements shown at two different focal planes; F, G, I, stages in
disintegration of end wall; /, end wall of vessel elements which in mature state communicate
through small perforation ; K, median line in thickened end wall ; L, end wall in face view ;
M, end wall broken in preparation. (After Esau, Hilgardia.^

APIUM GRAVEOLENS 477

the end walls are characteristically thickened. (Fig. 144, A-C.^
These thickenings are lenticular, occurring only on the portion of
the wall which is later removed during the maturation of the ves-
sel, while the remainder of the end wall is thin. After the end
wall has thickened, the vessel segments develop secondary wall
thickenings of the spiral, scalariform, or reticulate types which
are deposited on the longitudinal walls and also on the thin
portion of the end walls. The disintegration of the end wall of
the vessel segment occurs when the longitudinal secondary wall is
mature and the protoplast is beginning to disappear. (Fig. 144,
F, G.) At this time, the lenticular portion of the end wall becomes
thinner until finally it completely disappears, indicating that the
process is one of dissolution.

Ontogeny of the Phloem. — The protophloem consists of
sieve tubes and companion cells, each tube having one of the
latter. (Fig. 2.45.) As development proceeds, some of the first-
formed protophloem is obliterated by crushing and additional
sieve tubes are formed centripetal to them. The metaphloem has
less parenchymatous tissue; and the sieve tubes, though similar
in structure to those of the protophloem, are larger. Each tube
usually has one companion cell, but sometimes there are two short
ones. (Fig. 145, G.)

In the formation of the protophloem, the procambial mother
cell divides several times. The next to final division forms a
parenchymatous cell and the mother cell, after which a longitudinal
division of the latter produces a sieve tube and companion cell.
In the development of the metaphloem, some of the parenchy-
matous cells may develop directly from procambial cells; but, in
most instances, a procambial cell divides longitudinally. One
daughter cell becomes a parenchymatous cell and the other, the
mother cell of a sieve tube and companion cell. In the division of
the mother cell, the larger of the daughter cells forms the sieve tube.

In maturation, the sieve tube first develops thickenings on the
longitudinal walls, and the sieve plate thickenings occur later
at about the time that the nucleus disintegrates. The wall thicken-
ing appears to be uniform on all longitudinal walls, but the one
adjacent to the companion cell may or may not be thickened. In
old age, prior to obliteration, the walls of the sieve tube lose
their thickening, and this is followed by crushing as a result of
the growth and enlargement of adjacent parenchymatous cells.

478 THE STRUCTURE OF ECONOMIC PLANTS

The bundle cap subtending the outer, abaxial face of the phloem
is composed of enlarged parenchymatous cells whose walls have

Fig. 2.45. Details of phloem structure : A, B, two transections of a sieve tube with sieve
plate in A; C-G, stages of sieve tube differentiation shown in longisection ; H-M, stages of
phloem development shown in transection: cc, companion cell; obi, obliterated tissue;
0^, oil duct; J/, sieve tube ; f, vessel. (^Aker Esa.u, Htlgardia.^

APIUM GRAVEOLENS 479

thickened. These cells retain their protoplasts; and, as they
mature, the end walls, which are at first transverse or slightly-
oblique, become more tapering. The wall thickening consists of
cellulose, and its distribution resembles that of collenchymatous
cells. (Fig. 2.43.) The obliteration of sieve tubes and companion
cells is the result of active growth of the phloem parenchyma, and
the same displacement of cells may occur which has been noted
in the growth of the xylem vessels.

Oil Ducts. — Oil ducts occur in both the protophloem and
metaphloem, and their formation in this region, as well as in the
pith and cortex, is schizogenous. They are commonly formed by
the development of an intercellular space at a point where three
cells are in contact. The schizogenous splitting of the adjacent
cell walls results in the formation of the duct, which is at first
bounded by the three original cells but successive divisions of
these cells may produce several epithelial cells. In the phloem,
the schizogenous formation of intercellular spaces may occur so
rapidly that additional cells are brought into contact with the
duct by the separation of pairs of cells. The epithelial cells are
densely cytoplasmic with large nuclei, and secrete an ethereal oil
which accumulates in the duct. The duct is a continuous canal,
and the epithelial cells surrounding it are arranged end to end in
rows. (Fig. X43, L)

Mechanical Tissue of the Petiole. — The only tissues which
contribute to the strength, and possible toughness or stringiness
of the petiole, are the cells of the collenchymatous strands, the
phloem elements which constitute the bundle cap, and the lignified
elements of the xylem.

Esau (7) determined the relative breaking load of strands of
these three tissues. In all cases, it was found that the collenchym-
atous cells were much tougher than those of either the xylem or
the bundle cap; and that they also exceeded the combined strength
of the last two tissues. This was not because of the size of the
strands of collenchyma, for in many cases they were smaller than
the entire vascular bundle, in some instances being thinner than
the bundle caps alone. Differences in the strength of collenchyma
were found in comparing varieties of celery; and tests indicated
that the collenchymatous strands of Tall Golden Self-Blanching
were weaker than those of Golden Plume, while some of the strands
from an old leaf of the Utah variety were most tenacious. Varia-

4So

THE STRUCTURE OF ECONOMIC PLANTS

tion was also noted in the breaking load when young collenchym-
atous strands were compared with older ones, the marked increase
in toughness with increasing age of the tissue being correlated
with increase in thickness of the cell walls.

The Lamina. — The blade of the much incised and compounded
leaflet is relatively thin. The epidermal cells, as seen in surface

— ph 0 d

Fig. 146. A, transection of portion of lamina; B, transection through midrib of leaflet:
col, collenchyma; 0 d, oil duct; pal, palisade cells; ph 0 d, phloem oil duct; tpo, spongy
parenchyma.

view, are of two shapes; those subtending the collenchymatous
strands above the veins being rectangular with the elongated axis
parallel to the vein, while those which overlie the chlorenchyma
of the mesophyll have sinuous walls. The stomata occurring in
this type of epidermis are bounded by two guard cells containing
chloroplasts, and are surrounded by accessory cells as described
for the petiole. The stomata occur on both surfaces, but are
twice as numerous on the under one. Nestel (i6) reports a count

APIUM GRAVEOLENS

481

of ioo per square millimeter on the under surface and approximately
100 per square millimeter on the upper one.

The outer walls of the epidermal cells are slightly thickened
where the epidermis overlies the chlorophyll parenchyma, and very
much thickened over the veins and at the leaf margins. The
mesophyll consists of a single adaxial layer of palisade cells and
four to six layers of spongy parenchyma which are rather compactly
arranged. (Fig. 1.^6, A.^ The principal veins form projecting

Fig. 147. Floral development : A, transection of young tlovver showing arrangement of
parts ; B, transection of young ovary with two carpels and ovules ; C and D, early stages in
formation of umbellet showing centripetal development of flowers ; E-J, successive stages
in ontogeny of irtdividual flower (in /, filaments of stamens are not in plane of section) : bu,
vascular bundle; car, carpel; inv, rudimentary involucel of umbellet; o d, oil duct; ov,
ovule; /)«^, petal ; j-^^, stamen.

ridges on both surfaces of the blade owing to the strands of col-
lenchyma which lie immediately beneath the epidermis. The
abaxial and adaxial strands are separated from the bundle by several
rows of parenchymatous cells, and an oil duct lies above and below
the vein. (Fig. 146, B.)

The vascular bundle resembles that of the petiole and is oriented
with the xylem directed toward the adaxial surface. In the
larger veins, the phloem cap is well developed and there is also

48z THE STRUCTURE OF ECONOMIC PLANTS

some thickening of the xylem parenchyma on the adaxial face
of the bundle. The main bundles extend to the margin of the
leaflet, where they terminate in conical projections, and the
secondary veins form a reticulum with many cross-connecting
veinlets.

Floral Development. — The floral development of the Umbel-
liferae received early attention by many workers, including Payer
(i8), von Mohl (15), and Sieler (ii). More recently, the subject
has been investigated by Jurica (ix) and Beghtel (4), the latter
account dealing specifically with Pastinaca, which is similar to
Apium in its floral ontogeny. The umbellets of the compound
umbel develop centripetally so that the centrally located ones are
the last to mature. This is also true of the flowers in each umbellet,
where the peripheral ones are the first to develop. (Fig. i.^j, C, D.)
The flower primordia arise from the broadened apex of the umbellet
as club-shaped structures. The apex of each pedicel broadens and
the primordia of the petals, stamens, and carpels arise in acropetal
succession. (Fig. i47, £-/.) The involucel is rudimentary and
does not develop, being represented in the young umbellet by a
projecting fold of tissue. The flower is epigynous and the calyx
is represented by a ridge at the summit of the inferior ovary.

(Fig- M7, /•)

The primordia of the two carpels originate as independent

crescentic structures which unite to form the two-loculed ovary.

The pendulous, anatropous ovules are initiated before complete

coherence of the carpels has been effected. In the development

of the ovule, the nucellus arises from the placenta on the inturned

edge of the carpel as a club-shaped structure which is at first

straight. As growth continues, it begins to turn toward the outer

wall of the ovary until it becomes completely anatropous. The

funiculus is slender, and a single, thick, fleshy integument develops

which completely encloses the nucellus. On the side next to the

funiculus, the integument is scarcely recognizable because of the

complete anatropy of the ovule.

LITERATURE CITED

I. Ambronn, H., "Ueber die Entwickelungsgeschichte und die mechanischen
Eigenschaften des CoUenchyms. Ein Beitrag zur Kenntniss des mechan-
ischen Gewebesystems." Jahrb. Wiss. Bot. 12: 473-541, 1881.

z. Anderson, D., ■'Ueber die Struktur der Kollenchymzellwand auf Grund

APIUM GRAVEOLENS 483

mikrochemischer Untersuchungen." Sitxher. Akad. Wiss. Wien. Math.-
Naturw. Kl. i}6: 419-440, 1917.

3 . DE Bary, a. , Comparative Anatomy of the Phanerogams and Ferns, Clarendon Press,

Oxford, 1884.

4. Beghtel, F. E., "The embryogeny of Pastinaca sativa." Am. Jour. Bot. 12: ^

32-7-337. 192-5 •

5. Col, a., "Recherches sur la disposition des faisceaux dans la tige et les feuilles

de quelques dicotyledones." Ann. Sci. Nat., Bot. Ser. VIII, 20: i-i88,
1904.

6. Coulter, J. M., and Rose, J. N., "Notes on Umbelliferae of eastern United

States. III." Bot. Gaz. 12: 73-76, 1887.

7. Esau, Katherine, "Ontogeny and structure of the collenchyma and of vascular

tissues in celery petioles." Hilgardia 10: 431-476, 1936.

8. , "Vessel development in celery." Hilgardia 10: 479-488, 1936.

9. Jackson, Gemma, "A study of the carpophore of the Umbelliferae." Am.

Jour. Bot. 20: 111-144, 1933.

10. Janczewski, E., "Recherches sur le developpement des radicelles dans les

Phanerogames." Ann. Sci. Nat., Bot. Ser. V, 20: 108-133, 1874.

11. Jones, H. A., and Rosa, J. T., TruckCrop Plants, McGraw-Hill Book Co., 1918.

II. JuRiCA, H. S., "A morphological study of the Umbelliferae." Bot. Gaz.. 74- \

191-307, 1911.

13. KoNDO, M., "Uber die in der Landwirtschafte Japans gebrauchten Samen."

Ber. Ohara Inst, fiir landw. Forsch. i: 410-414, 1919.

14. Mills, H. S., "Quality in celery." Market Growers Jour., May, 1913.

15. VON MoHL, H., "Eine kurze Bemerkung iiber das Carpophorum der Umbelli-

ferenfrucht." Bot. Zeit. 21: 164-166, 1863.

16. Nestel, a., "Beitrage zur Kenntnis der Stengel- und Blattanatomie der Umbel-

liferen." Inaug. Diss. Mitt. Bot. Museum der Univ. Zurich. XXIV, 1905.

17. Norton, J. B., "Concerning quality in celery." Vt. Agr. Exp. Sta. Bull. 20},

1917.

18. Payer, J. B., "Organogenic des families: Ombelliferes." Ann. Sci. Nat.,

Bot. Ser. Ill, 20: III, 1853.

19. Sachs, J., Textbook of Botany, Clarendon Press, Oxford, 1875.

10. Sayre, C. B., "Quality in celery as related to structure." Univ. of III. Agr. I

Exp. Sta. Bull. ^^6: 559-588, 1919.

11. Sieler, T., "Beitrage zur Entv^^icklungsgeschichte des Bliitenstandes und

der BliitebeidenUmbelliferen." Bot. Zeit. 28: 360-369; 376-381,1870.
11. Solereder, H., Systematic Anatomy of the Dicotyledons, Eng. Trans. Boodle and
Fritsch I: 419-416; II: 941. Oxford, 1908.

13. Thompson, H. C, "Celery storage experiments." U. S. Dept. Agr. Bull. S79^

1917.

14. , Vegetable Crops., McGraw-Hill Book Co., New York, 1913.

15. , "Premature seeding of celery." Cornell Agr. Exp. Sta. Bull. 480, 1919.

16. VAN TiEGHEM, P., "Memoirc sur les canaux oleo resineaux." Ann. Sci. Nat.,

Bot. Ser. V, 16: 96-101, 1871.

17. Trecul, a., "Des vaisseaux propres dans les Ombelliferes." Compt. Rend.

Acad. Sci. 6}: 154-160; 101-109, 1866.

484 THE STRUCTURE OF ECONOMIC PLANTS

i8. Warning, Winifred C, "Anatomy of the vegetative organs of the parsnip."

Bot. Gaz- 9^: 44-72-, 1934.
Z9. Weiss, J. E., "Das markstandige Gefassbiindelsystem einiger Dikotyledonen

in seiner Beziehung zu den Blattspuren." Bot. Centr. ij: 180-2.95; 4°^"

415, 1883.

30. Westermaier, M., "iJber das markstandige Bundelsystem der Begoniaceen."

Flora 62: 177-188; 193-2.01, 1879.

31. Wilson, C. L., "Medullary bundle in relation to primary vascular system in

Chenopodiaceae and Amaranthaceae." Bot. Gaz. 78: 175-199, 192.4-
31. WiNTON, A. L., Microscopy of Vegetable Foods, ]dhn Wiley & Sons, London, 1916.

CHAPTER XVI

CONVOLVULACEAE

IPOMOEA BATATUS

THE sweet potato is the only representative of the Convolvulaceae
that is of major importance as a food plant. Originally a
native of tropical America and the West Indies, it is now cultivated
in the South Atlantic and Gulf States, South America, Africa,
Mediterranean Europe, India, Japan, the Malay Archipelago,
Australia, and New Zealand.

The numerous varieties produced in the United States may be
divided into two general groups. In one are the dry, mealy
types such as Big Stem Jersey, Yellow Jersey, Early Carolina, and
Triumph, which are much favored in the northern markets. The
other group consists of those varieties in which the flesh is soft,
moist, and sugary when cooked. These are preferred in the
southern markets and include Nancy Hall, Georgia, Pumpkin
Yam, and Porto Rico. The trade names used are subject to much
change, since breeding and selection result in constant fluctuation
in the number and names of commercial varieties.

GENERAL MORPHOLOGY

The Stem. — For the most part, the sweet potato is a vine-like
plant; but, according to Groth Ql), the length of the stem may
range from 2. feet in some bunch varieties to xo feet or more in
those of the liana type. In the latter, the stem may be strictly
twining or may not twine at all. It is purple-green, or a mottled
combination of purple, brown, and green ; and varies in diameter
from 3^ to more than 34 oi an inch at its largest part. Lenticels
are abundant and there may be some degree of pubescence, hairs
frequently being present on the young shoots which do not persist
on the older stem. In prostrate vines, adventitious roots occur
at each node.

485

486 THE STRUCTURE OF ECONOMIC PLANTS

The Leaf. — The simple, entire leaves are spirally arranged
with a y^ phyllotaxy. There is a wide diversity of leaf form, and
they may be cordate, hastate, slightly to deeply lobed, or cut.
The apex and the lobes are acute or obtuse, the base is cordate to

truncate; and, in some in-
stances, the midrib of the
leaf or leaflet is prolonged
beyond the blade to form
a short awn-like structure.
In general, the first leaves
are cordate, while those
formed later may be has-
tate, cut, or lobed. They
are equally variable with
respect to size, ranging
from 2.3^ to 6 inches in
their greatest dimension.
The venation is palmate,
the veins forming more or
less prominent projecting
ridges on both surfaces.

The petiole varies from
2-3^ to 9 inches in length.
Its base may twine about
a support to some extent,
depending upon the variety,
and it is commonly thick-
ened, forming a cushion-
like structure on the lower
surface where it bends
sharply upward. The
. . , . n u adaxial surface is grooved

Fig. 148. The origin of young sprouts from fleshy • 1 r i

root. and, on either side of the

petiole and just below the
blade, there are two small petiolar nectaries. Glandular hairs
occur on both surfaces but are more numerous on the abaxial one.
The Root. — The root system is adventitious (except in plants
grown from seed) and arises from fleshy roots, the stem, cuttings,
or young sprouts known as "draws" or sets. (Fig. 148.) Isbell
(5) has reported regeneration in leaf cuttings in which six different

IPOMOEA BATATUS

487

types of sweet potato leaves were rooted and maintained under
observation until the plants had formed shoots. The adventitious
roots form a fibrous root system and some of them thicken greatly,
the fleshy roots varying within wide limits from fusiform to
napiform or nearly spherical. (Fig. 149.) Some are smooth and
terete, others ribbed and more or less definitely four to six lobed
in transection. Laterals arise from the deeply lobed roots in rows
along the longitudinal grooves; and, in terete roots, each rootlet
lies in a shallow recess sur-
rounded by more or less
loosened scar tissue. Kamer-
ling (6) and Tuyihusa (14)
maintained that these fleshy
structures were stems rather
than roots, but it is clear
when the ontogeny of the
structure is critically inves-
tigated that this concept is
unwarranted.

The Flower. — The pro-
duction of flowers is rela-
tively rare in continental
United States, although
Hand and Cockerham (3)
report that the small,
"morning-glory-shaped
bloom, with a purple throat
and white margin, may be
noticed in commercial
fields." In 1914 Stout (12.)

made a summary of the published records regarding seed pro-
duction and concluded that seeds can be obtained from plants
which produce flowers if there is proper cross-pollination.
Blooming has been reported in all of the southern states where
sweet potatoes are grown, but seed is seldom produced and in
no instance has seed been matured in any quantity.

The plants flower profusely in the Virgin Islands, Puerto Rico,
Cuba, Santo Domingo, Barbados, St. Vincent, Hawaii, the Philip-
pine Islands, Java, and Australia. In many of these localities
sufficient seed has been produced either naturally or as a result of

Fig. 2.49. Yellow Jersey sweet potatoes show-
ing variation in shape and size. (Courtesy of the
Bur. Plant Industry.)

488 THE STRUCTURE OF ECONOMIC PLANTS

artificial cross-pollination to permit the growing of seedlings.
Experimental breeding has been carried on at the Virgin Islands
since 192.1, and a large number of seedlings have reached maturity.
In 192-5, the Virgin Islands Experiment Station reported that
approximately 140 varieties had been grown in the preceding
three-year period.

Thompson (13) describes the flower as follows:

"Usually, the flowers measure from ^^ to 13.2 inches in diameter, and
the tube i to 1^2 inches in length. The color varies somewhat in the
different varieties and, except in rare instances, is a shade of red,
turning darker in the throat of the blossom. In iioo seedlings examined
there were two exceptions to this rule . . . bearing pure white flowers.
Five stamens of varying lengths encircle the pistil and are attached
to the inner surface of the corolla tube, near its base. The two longer
ones either meet at or extend beyond the stigma, thus facilitating self-
pollination. The pollen grain is spherical and bears many minute
papillae which are symmetrically arranged upon its surface. The grain
is 0.09 to 0.1 mm. in diameter. . . . The seeds are borne in a seed
pod which is similar to that of the morning-glory, and range from
I to 4 in a pod."

Natural crossing occurs readily and is probably carried on by
hymenopterous insects, the most important of which is the honey
bee. According to Thompson:

"The sweet-potato blossom opens only once. The bud opens during
the night and remains open during the morning hours, then the corolla
closes and withers. The time of closing of the blossom varies some-
what with the character of the day and the season. The flowers may
remain open all day during cool, cloudy weather, especially in January
and February, but they ordinarily close about noon on bright, warm
days during this season. ... In the latter part of April, closing of
the flowers in the field on clear, warm mornings was observed to take
place between 9 and 10 o'clock."

The Fruit and Seed. — Under conditions in the United States,
the sweet potato rarely produces viable seeds. Stout (11) con-
cluded that sterility has been accentuated through the perpetua-
tion of varieties having sterile tendencies for more than 400 years
of vegetative propagation; that blooming requires a longer grow-
ing season than is necessary for the development of a crop of roots;
and that some type of self-sterility or incompatibility operates to
limit seed production even when environmental conditions favor
blooming.

IPOMOEA BATATUS

489

The fruit is a globose capsule usually consisting of a two-loculed
ovary with two ovules in each cell. False septa may divide the
locules so that the fruit appears to be a four-celled, four-valved
capsule with a seed in each locule. The anatropous ovules are
more or less basal and develop from axial placentae at the inner
angle of each cell. The seed is approximately 3 mm. in its greatest
dimension, black, somewhat flattened, with one surface angular
and the other roughly con-
vex. The seed coat is
tough and corneous, and
nearly impervious to water.
The internal structure is
similar to that described
by Lubbock (9) for other
species of Ipomoea and
Convolvulus. The micro-
pyle is deeply invaginated
so that a tubular cavity is
formed above the hilum
and the inner portion of
the seed coat is infolded
to such an extent that the
apical end of the seed is
two-chambered. This sep-
tum probably determines
the configuration of the ^ . j- ■ , j ■ u ■ c

•^ . Fig. 150. A, median view or seed with portion or

two deeply bilobed COty- cotyledons cut away to show septum; B, ventral

ledons TFiff i^O A B^ view; C, dorsal aspect of dissected embryo, after 14

u J ' 1' hours ; D, ventral aspect : an, auricle ; cot, cotyle-

As tne embryo develops, jon; /dr/', hypocotyl ; ;«/V, micropyle; ri?/?, septum.

the hypocotyledonary axis

occupies the tubular cavity formed by the invagination of the
micropyle. The development of the lateral lobes of the coty-
ledons, after the apex of each has come in contact with the in-
truded septum, results in the pushing of half of each cotyledon
into one of the lateral chambers and the formation of well-
defined auricles which lie on either side of the hypocotyl. As
the cotyledonary lobes continue to grow, they become transversely
folded. The dorsal and ventral aspects of the dissected embryo
are shown in Figure x5o, C and D. The seeds are albuminous
and a pulpy endosperm overlies the folded cotyledons. Upon

490 THE STRUCTURE OF ECONOMIC PLANTS

germination, this becomes mucilaginous and is absorbed by the
cotyledons.

Development of the Seedling. — Under greenhouse condi-
tions, scarified seeds germinate rapidly.^ The primary root
emerges by the end of the first or second day, and subsequently
there is a rapid elongation of the axis so that it may reach a length

Fig. 151. Stages in development of seedling; A, three days; B, five days; C, seven days;

D, three weeks ; E, first foliage leaf.

of i or 3 inches by the third or fourth day. There is an early
differentiation of lateral roots, and by the fifth day, small branch
roots can be seen at the upper root level. The hypocotyl elongates,
carrying the cotyledons forward with it; and as they emerge
from the soil and are liberated from the seed coat, the elongation
of the cotyledonary petioles separates them from each other. The
cotyledons are deeply bilobed, the lobes being obtuse or subacute,
and the rounded auricles are rather prominent. Following the

' The seedling studies described in this section are based upon material grown from seed
supplied by the Director of the Virgin Islands Experiment Station.

IPOMOEA BATATUS

491

expansion of the cotyledons, the epicotyl begins to elongate and
the first foliage leaves enlarge. In seedlings ii days old there
may be three or four foliage leaves which are simple, cordate at
the base, and acute or pointed at the tip. (Fig. 151, A-D.^

ANATOMY

The Primary Root. — The primary root has a tetrarch pro-
tostele, and four groups of small phloem cells alternate with the

Fig. 151. Transection of young primary root : co, cortex ; en, endodermis ; mx, meta-
xylem; pel, pericycle; pri ph, primary phloem; px, protoxylem. (After Artschwager,
Jour. Agr. Res.^

primary xylem strands, being separated from them by fundamental
parenchyma. (Fig. X52-.) Each point consists of five or six
protoxylem elements, the small outer ones abutting the pericycle
while the larger inner ones lie adjacent to the centrally located
metaxylem vessels. The cortex is limited centripetally by a

492. THE STRUCTURE OF ECONOMIC PLANTS

stl

COs

well-defined endodermis, and its outer surface is bounded by the
epidermis which produces root hairs.

In the ontogeny of the root, the meristem is differentiated into
three histogens. The plerome and periblem, which give rise to
the stele and cortex respectively, are clearly defined; and a layer

of cells, the calyptrogen-dermat-
ogen, overlying the periblem
produces the epidermis and the
root cap. (Fig. 2.53.) The root
cap consists of several layers of
cells, and those located at the tip
may divide again periclinally.
This differential activity results
in the formation of several ver-
tical rows of cells in the central
portion of the cap so that it is
thicker at its apex than at its
margin. Frequently, irregulari-
ties in the orientation of the
lateral cells result from these
divisions and subsequent cellular
enlargement.

The epidermis is produced by
the final periclinal divisions of
the most laterally placed cells of
the calyptrogen-dermatogen layer.
Each final division results in two
daughter cells, the inner one be-
coming an epidermal initial while
the outer forms a portion of the
In this manner, suc-
cessive cells become a part of the
epidermis as the root increases in
length; and, because of the manner in which this is accomplished,
the outer surface is not smooth near the apex of the root as is the
case when the epidermis is formed by anticlinal divisions of a
distinct dermatogen. Instead, it forms a series of steps each of
which is continuous with a layer of the root cap; and there are as
many steps to the epidermis as there are layers in the root cap, except
where subsequent periclinal divisions of the latter have occurred.

Fig. 2.53. Median longisection of primary
root : cal, calyptrogen-dermatogen ; co, cor-
tex ; en, endodermis ; ep, epidermis ; pbm, foOt Cap

periblem ; pel, pericycle ;
r c, root cap ; stl, stele.

pi, plerome ;

IPOMOEA BATATUS

493

The cortex is derived from the periblem, which consists of two
layers of cells overlying the plerome. The outer layer of this
histogen divides only anticlinally and forms the peripheral layer
of the cortex, while the inner layer divides in all planes to form
the remainder of this region, which is six to nine cells in thickness
at maturity. The stele is differentiated from a group of plerome
cells lying above the periblem. The lateral members of this group
produce the pericycle, which can be distinguished from the other
regions of the stele early in ontogeny since the cells composing it
are radially rather than vertically elongated. (Fig. Z53.) In the
development of the vascular portion of the stele, the primary

pri Ix ves

pri Ix ves
-lat rt

Fig. 154. A, transection of young primary root in region of differentiation; B, primary
root sliowing origin of lateral root: lat rt, lateral root; pel, pericycle; pri Ix ves, primary
latex vessel; pri ph, primary phloem.

phloem is first differentiated and four groups of elongated phloem
elements are formed by longitudinal divisions of plerome cells
abutting the pericycle. At the same time, a primary latex vessel
is differentiated adjacent to the pericycle by the enlargement of a
single vertical row of cells in each phloem group. (Fig. 154, A.')
The cells of these vessels retain their end walls and protoplasts.
The remainder of the primary phloem consists of long, very slender
sieve tubes with transverse or occasionally oblique sieve plates
and companion cells.

The primary xylem differentiates centripetally and the first
protoxylem cells to mature are thin-walled with annular or annular-
spiral thickenings. While the protoxylem elements are develop-
ing, a vertical row of parenchymatous cells at the center of the

494

THE STRUCTURE OF ECONOMIC PLANTS

stele becomes much enlarged and forms a single metaxylem vessel
which may be separated from the adjacent metaxylem and the
protoxylem points by a zone of fundamental parenchyma. In some
instances, two central metaxylem vessels are formed, one of which
is usually larger than the other. The outermost metaxylem ele-
ments have loosely reticulate secondary wall thickenings, while
the walls of the centrally located vessel are closely reticulate.

Lateral roots are formed very early in the ontogeny of the primary
root; and, before the protoxylem is completely differentiated, the
development of laterals may be initiated from the cells of the
pericycle lying directly outside the xylem points. (Fig. X54, B.)
Tangential and radial divisions of the pericyclic cells result in the
formation of a conical growing point of meristematic cells, and the
subsequent development of the lateral root is similar to that de-
scribed for the primary root. The laterals are usually tetrarch,
occasionally triarch. Pentarch, hexarch, and other types that
occur in adventitious roots were not observed in the seedling
material investigated.

Secondary Thickening of the Root. — The lateral or adven-
titious roots, which are destined to become fleshy, resemble the
primary root in the organization of their primary tissues except
that they are frequently pentarch or hexarch rather than tetrarch.
Likewise, the ontogeny of the fleshy root, up to the time of matura-
tion of the primary tissues, parallels that of the primary axis.
There is no question but that the development of the primary
xylem is exarch and that the arrangement of the primary xylem
and phloem is radial, so that it is inaccurate to interpret the fleshy
axis as a stem. Artschwager (i), in commenting upon this point,
states that

"the validity of this assumption . . . becomes untenable when young
material is studied. The exarch position of the protoxylem decides,
without further argument, in favor of the root-structure theory of
these organs."

At about the time that the primary xylem strand reaches
maturity, the parenchymatous zone separating the primary xylem
and phloem gives rise to a cambium which is irregular in outline.
The first-formed secondary elements, arising from this primary
cambium, are laid down in the angles between the primary xylem
points and centrad to the primary phloem groups. As these
angles are occupied by maturing secondary xylem elements, the

IPOMOEA BATATUS 495

lateral extension of the cambium from the points of initial activity
ultimately involves the pericyclic cells which lie outside the pri-
mary xylem points. This results in the formation of a complete
cambial cylinder which becomes more and more regular in outline
until it is finally symmetrical.

The relation of the activity of the centrally located parenchyma
to the differentiation of the primary xylem elements has been
differently interpreted by McCormick (10) and Artschwager (i).
The former states that "in the young root there is usually a solid
arrangement of vessels with no thin-walled parenchyma between
them"; and when a root starts to thicken, parenchyma increases in
amount and separates the protoxylem points from the centrally
located metaxylem until each primary xylem point consists of a
separate strand of one to several vessels. Artschwager, on the
other hand, finds that parenchymatous activity "always precedes
the differentiation of xylem on the inner face of the protoxylem
groups." The activity of the primary cambium and the conse-
quent enlargement of the axis results in a tangential stretching of
the endodermal cells, and the parenchymatous cells of the cortex
increase in number for a time, keeping pace with the growth of the
root. Later, the intercellular spaces become larger, the tissue of
the cortex disintegrates, and the epidermal cells are ruptured.

(Fig. 155.)

Tertiary Thickening of the Root. — Up to this point, the
development of the axis follows the usual sequence found in fleshy
roots which thicken secondarily; but at this time, certain anoma-
lous developments occur which complicate an interpretation of the
ontogeny unless successive stages are considered in order. The
first of these is the formation of secondary cambiums which origi-
nate in the parenchyma of the central portion of the axis surround-
ing each of the protoxylem points. (Fig. X56.) This is followed
by the origin of additional secondary cambiums which may develop
extensively throughout the central parenchymatous zone of the
axis. These cambiums frequently occur as cylinders which sur-
round groups of secondary xylem elements that have matured
centrad to the primary cambium. (Fig. i56.) Coincident with
the formation of the secondary cambiums, an active meriste-
matic zone forms around the central metaxylem vessel, and this
may increase in width until a region of considerable size is
formed.

496 THE STRUCTURE OF ECONOMIC PLANTS

Somewhat later, secondary cambiums may arise in the tissue on
the inner face of the secondary xylem which remains active for a
limited period. (Fig. 2.57.) Still later, secondary cambiums may
be formed in the parenchyma of the root which are not related
spatially to any vascular elements; and these, in turn, may pro-
duce xylem centripetally and phloem centrifugally. (Fig. ^58.)

Fig. 2.55. Transection of young fleshy root: ca, cambium, co, cortex; en, endodermis ;
ep, epidermis ; lac, lacuna ; rnx, metaxylem ; far, interstitial parenchyma ; pel, pericycle ;
/)/>, phloem; />x, protoxylem ; .vj i, secondary xylem.

Less frequently, secondary cambiums may develop in the paren-
chyma of the original phloem groups.

While the development of these anomalous secondary cambiums
is taking place, the primary cambium continues to function, form-
ing secondary xylem and phloem elements and a large amount of
storage parenchyma. The parenchyma around the secondary
xylem elements may in turn produce a secondary cambium which
forms new elements in the usual manner, and Artschwager (i)

IPOMOEA BATATUS

497

reports that "when the amount of tissue produced by the secondary
cambium is considerable, as can best be seen in the region of the
large central cell of the young root, a tertiary cambium develops

Fig. 156. Transection of portion of young fleshy root showing origin of cambiums. A
secondary cambium is being initiated around large metaxylem vessel at lower right-hand
corner of figure : w, primary cambium ; co, cortex ; en, endodermis ; pel, pericycle ; ph
phloem; px, protoxylem. (After Artschwager, /ow. A^r. Rw.)

around a number of the secondary elements and increases in the
same way as did the group from which it arose."

In roots which become ridged at maturity, the lobes are traversed
by vascular bundles which originate in the primary vascular ring
and are diverged into the cortex, continuing vertically through the
ridges. (Fig. 2.59.) Small bundles are occasionally present in the

498 THE STRUCTURE OF ECONOMIC PLANTS

peripheral portion of the cortex even in types where the ridges are
lacking.

As the development of the axis proceeds, the endodermis and
cortical parenchyma fail to keep pace with the growth of the stele,
although there is evidence of some anticlinal division of the epider-
mal cells which become very much stretched tangentially. An
ephemeral periderm may originate in the cortical cells, but they
soon disintegrate; and, as this occurs, a phellogen is formed in the
pericycle which produces the periderm of the mature root.

The Mature Fleshy Root. — In the mature fleshy root, the
epidermis, cortical parenchyma, and endodermis are no longer

\-ca2

Fig. 157. Transection of portion of cambial zone of larger fleshy root showing formation
of secondary cambium inside of group of secondary xylem elements : ca, cambium ; ca 2.,
secondary cambium ; ^^ 2, secondary phloem; Ary i, secondary xylem. (After Artschwager,
Jour. Agr. Rej.)

present and the protective "skin" consists of a pericyclic periderm
which has been maintained by an active phellogen throughout the
period of its growth and enlargement. Within the periderm, the
pericyclic zone is composed of large parenchymatous cells which
are filled with starch, and these abut projecting zones of phloem
consisting of radial rows of sieve tubes and companion cells which
are somewhat irregularly arranged because of the anomalous
method of axial growth. The large secondary sieve tubes have
transverse sieve plates, and in some regions two or three large
lateral sieve fields may be formed. The companion cells are smaller
in diameter than the sieve tubes but equal them in length.

The cambium also produces vertical rows of lactiferous cells
which form the latex vessels. The cells retain their transverse

IPOMOEA BATATUS

499

walls and differ in this respect from plants having a latex system
in which a continuous
tube is formed by the
resorption of the end
walls. (Fig. x6o.) The
prominent latex vessels
may be recognized in
young tissue by their
large size and by their
white viscid contents in
which oil droplets are
in suspension; but in
older tissue, they are
less readily identified
owing to the increased
size of the adjacent
cells. The elements of
the secondary xylem
consist of both wide
and narrow vessels in
which the vessel seg-
ments are intercon-
nected by a single large
pore. The walls are
pitted and tyloses fre-
quently develop in the
larger vessels.

Artschwager (i) clas-
sifies the storage paren-
chyma under two types :

"Qa) normal bundle
parenchyma which, like
the xylem and the
phloem, is a product of
the cambium; (b) in-
terstitial parenchyma —
a filler between the
groups of bundles. The
interstitial parenchyma
can be considered the
direct progeny of the

— ca 2

Fig. 2.58. Transection of portion of fleshy root about
10 mm. in diameter : ca, cambium ; ca 2., secondary
cambium ; co, cortex ; Ix c, latex cell ; Ix ves, latex
vessel ; pd, periderm ; ph z, secondary phloem ; xy 1,
secondary xylem. (After Artschwager, Jour. Agr. Res.^

500

THE STRUCTURE OF ECONOMIC PLANTS

cells of the parenchymatous sheath of the young rootlet. It forms
irregular areas and even broad zones, which are distinguishable from

the surrounding tissue by
their lighter color. The
cells composing it are
commonly irregular,
/v^-^T'^^r^: .■•;...;. Q. •....-■. y I clongatcd, and poor in

starch. The cells of the
bundle parenchyma are
polyhedral and fairly
uniform; they are very
rich in starch and possess
a well-developed nu-
cleus."

Vascular Transition.
— The lower portion of
the hypocotyl is root-

F.G. ^59. Diagrammatic transection of mature fleshy like in general plan. The
root. QAiter Artschwager, Jour. Agr. Res.) . "^ '■

primary xylem and
phloem are alternate and radial but the central portion of the
stele is parenchymatous. At successively higher levels, near the
middle of the hypocotyl,

d9^o'

^a

o

OOr

latex vessel

there is a gradual re-
orientation of the pri-
mary vascular tissue. The
four phloem regions are
divided into wide arcs
consisting of scattered
groups of phloem cells
which lie immediately
inside the pericycle and
alternate with the pri-
mary xylem points. The
metaxylem elements dif-
ferentiate in a tangential ^"^^^ss^^^yj^^'^^vM^
position with respect to T /o^l

the protoxylem so that 7;?'**'^'^^V___..-/^

each of the four xylem J-^^^ss-^^y

poles consists of the pro- -^1 >s

toxylem point with two
tangential arms of meta-

O

GO

PoOq

Op

ooo^lpoo^ ^^m

P%^oQ

o.

o

sieve
tube

O

Oa

.Or

O

O)

b //Roo

\Q

O

'd?-

O

Oi

80000

0

&

a

o,

OqO

^.

0^

_oi

^o

Fig. 2.60. Transection of radial row of secondary
sieve tubes with companion cells and adjacent latex
xylem. (Fig. z6l. A.) vessel. (After Artschwager, /«/;•. ^^r. R^i.)

IPOMOEA BATATUS

501

In the upper hypocotyl, the four arcs of scattered phloem groups
are extended tangentially so as to form a discontinuous ring of
phloem elements some of which are collateral with the metaxylem.
The first strands of inner phloem are differentiated in the pith and
lie between the two laterally divergent arms of the metaxylem of
each xylem pole. (Fig. i6i, B.) The inner phloem is not con-
tinuous with the primary phloem of the root, and arises later in
ontogeny than the outer phloem of the hypocotyl which is con-
tinuous with that of the root. It consists of strands that have

-iph

mx

A

B

Fig. i6i. A, transection of lower portion of hypocotyl of three-weeks-old seedling show-
ing tangentially oriented metaxylem arms ; B, transection through upper portion of same
showing further divisions of metaxylem and appearance of inner phloem : i ph, inner phloem ;
WZA-, metaxylem ; <? ^/>, outer phloem ; /)x, protoxylem.

differentiated downward from the cotyledonary plate as continu-
ations of the inner phloem of the bicollateral bundles of the cotyle-
donary traces.

There is usually no connection between the outer and inner
phloem, and the strands of the latter end blindly in the pith of
the hypocotyl. In the case illustrated in Figure 161, no inter-
connection between the outer and inner phloem occurred above or
below the level figured; but, in a few instances, they may be
transversely connected by the differentiation of parenchymatous
cells lying in the gap between the divergent and partially separated
arms of the primary xylem. The relationship of the inner phloem
to the outer has been variously interpreted by investigators who
have worked with other members of this family. Scott (11), in
his investigation of Ipomoea versicolor Meissn., states that

502. THE STRUCTURE OF ECONOMIC PLANTS

"the internal phloem extends downwards into the hypocotyl and passes
out between the converging protoxylem groups of each cotyledonary
pair of bundles, thus joining the external phloem of the root."

In his summary, he concludes that

"As regards the course of the internal phloem in the transition from
stem to root, Ipomoea versicolor may be taken as typical of plants
with bicollateral bundles generally."

Fig. 162.. A protoxylem point in lower hypocotyl of three-weeks-old seedling : ca, cam-
bium ; CO, cortex ; en, endodermis ; /' ^h, inner phloem ; mx, metaxylem ; 0 ph, outer phloem ;
/)£■/, pericycle; ^i, pith; /»x, protoxylem ; xj i, secondary xylem.

Lee (8) investigated the seedling anatomy of Convolvulus tri-
color L. , var. major, and also C. tricolor L. In regard to the former
he concludes that

"The phloem is extremely small in amount and at no point can
internal phloem be recognized with certainty — an observation which
emphasizes the late appearance of this tissue noticed by other
observers."

In Convolvulus tricolor, he notes frequent anastomoses of strands of
phloem connecting the bundles in the petiole of the cotyledon, and

IPOMOEA BATATUS 503

also calls attention to the abundance of inner phloem in the petiole
and hypocotyl. In this instance, he agrees with Scott, stating that

"in the upper part of the hypocotyl, during the rearrangements in the
cotyledonary strand so that the protoxylem becomes external, the
internal phloem masses of that bundle fuse, and the fused mass grad-
ually passes out between the metaxylem groups (before the latter
meet to form the root bundles) and joins on to the external phloem.
The same events take place at a lower level in connexion with the inter-
cotyledonary bundles, so that finally an ordinary tetrarch root is
produced."

Lamounette (7) investigated several of the Convolvulaceae,
including Ipomoea leucantha, and concluded that the inner phloem,
when present in the hypocotyl, is differentiated in the medullary
parenchyma and is not continuous with the outer phloem of the
root. In fact, he rejects the term bkollateral, supporting Herail (4)
in this regard.

In the sweet potato, anastomoses between inner and outer phloem
occur at the base of the cotyledonary petiole, but there is no evi-
dence to indicate that the inner phloem of the hypocotyl is regu-
larly connected with the outer phloem. The inner phloem may be
regarded as supplementary to the outer, and its appearance late
in the ontogeny of the seedling, at a time when it is beginning to
function photosynthetically, suggests a correlation between the
function of the phloem and its anatomica development.

In the upper hypocotyl, the cotyledonary plane may be dis-
tinguished from the intercotyledonary by the degree of separation
of the two metaxylem groups in each case. Slightly below the
cotyledonary node, the metaxylem groups of each intercotyle-
donary strand are more widely separated from each other than those
of the cotyledonary strand, each group inclining toward one of the
latter. At this level, the inner phloem is well developed, so that
there are two bicollateral polar bundles of the double bundle type,
and four smaller laterally placed bicollateral bundles derived from
the intercotyledonary strands.

At the cotyledonary node, the two lateral bundles of each cotyle-
donary trace abut the right and left flanks of the median double
bundle of the trace so that it appears to be a single large crescentic
unit. Slightly above the level of divergence of the cotyledonary
traces from the vascular ring, the lateral bundles of each trace
separate from the median bundle which is itself divided to form

504 THE STRUCTURE OF ECONOMIC PLANTS

two bicollateral bundles. The xylem of the median bundle is
not endarch in its development, but consists of a single protoxylem
strand and two tangentially oriented metaxylem groups which lie
between the outer and inner phloem.

The Cotyledons. — The slender cotyledonary petiole equals
or slightly exceeds the lamina in length, and its adaxial surface
is grooved so that it is crescentic in transection. There are four
bundles at its base; and in the two large median bicollateral
bundles, the outer phloem consists of several scattered groups of
cells while the inner phloem is composed of two or three strands.
In some instances, the xylem may be surrounded by a discontinuous
ring of phloem elements so that the bundle is essentially amphi-

cribral. The two smaller
lateral bundles are col-
■" ^^ lateral or half-amphicri-
bral. (Fig. 163.) The par-
enchymatous cells sur-
rounding the bundles are
thin-walled with large in-
^"--m bu tercellular spaces. Toward
the abaxial surface, the
outer cell layers are com-

FiG. 2.6^. Transection of cotyledonary petiole: j ,, 1

/ *«, lateral bundle ; ^ bu, median bundle. P^Ct and COllenchymatOUS

but contain chloroplasts.
The subepidermal cells of the adaxial surface are also chloro-
phyllose, but not so compactly arranged, and the more numerous
stomata open into large intercellular spaces.

The arrangement of the petiolar bundles remains unchanged up
to a point slightly below the lamina, where the two median bundles
anastomose to form a single broad one. (Fig. 164, A.^ At the
base of the lamina, the two small bundles (a, a'') curve abruptly
outward and supply the auricles. The median bundle cuts off two
lateral branches (b, b'^ which immediately give rise to two small
laterals (c, c') that supply the basal portion of the lamina. The
median bundle finally forks just below the notch of the blade, one
branch extending into each lobe (d, d'^, and these with the branches
Qb, b''), supply the lobes of the cotyledon.

The median bundle is still bicollateral at the base of the lamina,
but the inner (adaxial) phloem ends blindly in the mesophyll before
the strand forks. The other veins are collateral throughout their

IPOMOEA BATATUS

505

extent in the blade, and there is a progressive reduction in the
amount of phloem tissue in succeeding branches until the ultimate
veinlets consist of a single spiral tracheid.

The primary xylem, which is tangentially oriented in the petiole,
is differentiated in a manner equivalent to the endarch condition
in stems with the protoxylem elements toward the upper surface of
the cotyledon.

The cells of the upper epidermis of the lamina are larger than
those of the lower, irregular in size, and their outer walls are some-
what convex so that the surface is roughened. Stomata occur in

Fig. 2.64. /I, outline drawing of mature cotyledon showing venation of lamina and petiole;

B, same of foliage leaf.

both epidermal layers but are more abundant in the lower one. In
the mesophyll, the palisade parenchyma is loosely organized, and
usually consists of two rows of cells, but interspersed with them
are cells whose length equals that of the double palisade. The
spongy tissue is four or five cells in thickness, except at points
where the principal veins traverse it; and the isodiametric cells
are so arranged that large lacunae and substomatal cavities are
formed. In the region of the principal veins, the mesophyll is
more compact and the palisade tissue and spongy cells are replaced
by collenchymatous cells above and below the vein. (Fig. 165.)
Glandular scales or hairs occur on the lower epidermis and each
consists of a basal cell subtending several elongated ones that are

5o6 THE STRUCTURE OF ECONOMIC PLANTS

arranged in a disk to form the flattened or convex surface of the

scale.

The Anatomy of the Young Stem. — The epidermis of the
young stem is thin-walled and produces numerous glandular scales
and non-glandular hairs. The scale has a flat basal cell which
arises from a small somewhat sunken epidermal cell so that the
terminal portion of the scale appears to rest in a saucer-like depres-
sion in the epidermal surface. (Fig. 2.66, A.^ The distal part is
composed of several cells which are formed by vertical divisions of

Fig. 165 . Transection of portion of lamina of cotyledon through midvein : col, collen-
chyma; wa:, metaxylem ; o /)/>, outer phloem ; /?<«/, palisade; /?a:, protoxylem.

the terminal cell of the scale. The non-glandular hair has a basal
stalk cell which arises from an epidermal cell or a group of epider-
mal cells and a tubular terminal portion which is divided trans-
versely into two or more cells. (Fig. 1.66, B.) These hairs may
or may not persist on the mature stem, and their frequency appears
to be a varietal character. The stoma is somewhat raised above
the adjacent epidermal cells, due in part to the position of
the two accessory cells which lie parallel to the guard cells. (Fig.
i66,C.)

The cortical chlorenchyma is compact and there are numerous
vertical latex canals, usually bounded by four to six secreting cells,
located centrad to the two hypodermal layers. Crystals of calcium

IPOMOEA BATATUS

507

oxalate are abundant in the parenchymatous cells of both the
cortex and pith. The endodermal cells are distinct and can be
recognized by the relative density of their cell contents. Within
the endodermis, there are two or three layers of pericyclic cells
and abutting these are small scattered groups of outer phloem con-
sisting of narrow thin-walled elements.

Early in ontogeny, the procambial ring is well defined so that
it forms a continuous cylinder in the young stem. The vessels
of the primary xylem are spiral or variously reticulated. The
inner phloem, like the outer, is comprised of isolated groups of
sieve tubes and companion cells which are much smaller in diameter
than the adjacent parenchyma of the pith. They are not always in
radial alignment with the primary
xylem; and, for this reason, do not
exhibit a strictly bicollateral ar-
rangement such as occurs in the
Cucurbitaceae. The parenchyma-
tous cells of the pith are large with
intercellular spaces, and there are
numerous latex canals and crystal
inclusions similar to those found
in the cortex.

The Mature Stem. — In the
mature stem, the epidermis is some-
what thick-walled and CUtinized ^^cesso^y cells :.., accessory cells ; ^ .,

guard cells.

with relatively few glandular and

non-glandular hairs so that the surface is usually glabrous. Len-
ticels are numerous and the tabular thin-walled cork cells form-
ing them are produced by periclinal divisions of the epidermal cells
adjacent to a stomatal opening or by similar divisions of the hypo-
dermal layer. In cases where periderm formation has occurred,
the cells of the hypodermis are in radial alignment with the adja-
cent epidermal cells. Within the hypodermal layer are several
rows of compact parenchymatous or collenchymatous cells which
form the outer zone of the cortex. The inner zone consists of large
parenchymatous cells which undergo considerable tangential
elongation and some anticlinal division to compensate for the
secondary thickening of the stele. The latex canals persist but are
somewhat difficult to recognize because of the increased size of the
adjacent cells. The endodermis is well defined, and, in the under-

FiG. l66. ^, detail of young glandular
scale ; B, young non-glandular epidermal
hair ; C, stoma with guard cells and

5o8

THE STRUCTURE OF ECONOMIC PLANTS

ground portion of adventitious stems which arise from the fleshy-
roots, Casparian strips are well developed.

The pericycle is one to three cells in width and a discontinuous
cylinder of pericyclic fibers is differentiated in it, some of which
are very much thickened. (Fig. 167.) The outer phloem, in
addition to sieve tubes and companion cells, consists of parenchyma
which frequently contains crystals of calcium oxalate, and a few

Fig. 167. Transection of sector of stele of mature stem showing outer and inner cambiums :
i ca, inner cambium ; i ph, inner phloem ; Ix ves, latex vessel ; o ca, outer cambium ; o ph,
outer phloem; pel, pericyclic fiber; pi, pith; xy i, primary xylem; xj t, large secondary
xylem vessel.

latex canals may occur adjacent to the pericycle. The cambium
forms a continuous cylinder early in ontogeny as a result of the
development of an active interfascicular cambium. In the stele,
there are commonly two or three bundles which have larger vessels
than the intervening ones; and it is characteristic of the stem of
Ipomoea, as well as of many other representatives of the Convol-
vulaceae, that these bundles are located in two groups which lie
approximately opposite each other in the vascular cylinder. The

1

IPOMOEA BATATUS 509

large vessels have bordered pits and are surrounded by smaller ones
and by parenchyma that becomes thick-walled as the stem matures.
The pits of the wood parenchyma are simple, and half-bordered
ones occur where the vessels and parenchyma are adjacent to each
other. The interfascicular regions are characterized by radial
series of thick-walled elements, and groups two to five rows in
width are separated from adjoining ones by a row of thin-walled
ray-parenchyma cells.

Between the protoxylem and the inner phloem, there is a region
of thin-walled parenchyma; and, in this zone, an inner cambium
may arise which is relatively inactive and seldom forms a continu-
ous inner cambial ring. It usually produces additional phloem
centripetally; and, less frequently, may cut off a few secondary
xylem elements centrifugally, in w^hich case bundles with a reverse
orientation are formed. The inner phloem consists of scattered
groups of companion cells and sieve tubes, and the pith is as
described for the young stem.

The Development of the Foliage Leaves. — At the time of the
differentiation of the first foliage leaves, the epicotyledonary
axis is surrounded by the bases of the cotyledons which form a
partial sheath or cotyledonary collar. In the ontogeny of the
leaf, its primordium arises from the peripheral tissues of the grow-
ing point as a conical mass of cells. The primordium elongates
and broadens by general meristematic activity until it becomes
somewhat crescentic or triangular in transection at the base,
tapering to a rounded or slightly subtriangular apex. (Fig. 2.68,
/4.) In the basal portion of the primordium which later forms the
petiole of the leaf there is an early differentiation of procambial
strands. Growth in length of the primordium is accomplished
by general cell division, but there is an acceleration of meristematic
activity in tiie lateral portions of the adaxial surface. (Fig.
i68, B.) This results in the formation of the lamina which
develops with the upper surfaces of the two halves of the blade
opposing each other. They are thus oriented at right angles to
the midrib with their edges directed toward the axis of the epicotyl.
(Fig. x68, C.) Following differentiation of the lamina, the petiole
elongates by continued cell division and enlargement.

Glandular scales are formed in the epidermal tissue of the
developing leaf. These are more numerous on the exposed lower
epidermis than on the upper and resemble the scales found on the

5IO THE STRUCTURE OF ECONOMIC PLANTS

young stem. The mesophyll is very compact at first, and there is
little differentiation into spongy and palisade regions; but, later
in ontogeny, the spongy tissue becomes loosely organized and large

— g s

r- — SC

■msl

--mv

Fig. 168. Development of foliage leaf; A, transection of young leaf primordium near
base showing procambial strands and region of meristematic activity ; B, transection of
same primordium at slightly higher level showing development of lamina ; C, transection of
young leaf showing orientation of lamina with respect to midvein : g s, glandular scale;
msl, mesophyll ; 'fnv, midvein ; ps, procambial strand ; sc, secretory cell.

intercellular spaces are formed. The palisade region consists of
one to three layers of palisade parenchyma which are occasionally
interrupted by groups of transversely divided parenchyma. In
the young leaf, secretory cells develop between the palisade and
spongy tissue, but these are less conspicuous as maturation proceeds.

1

IPOMOEA BATATUS

511

The Mature Leaf. — At maturity, the mesophyll is much less
compact and there are numerous large intercellular spaces in the
spongy region and between the palisade cells. (Fig. 169.) The

—sto in

Fig. 2.69. Transection of base of foliage leaf through midvein : a c, accessory cell ; col,
collenchyma; g c, guard cell; g s, glandular scale; / ph, inner phloem; mx, metaxylem;
0 /i/j, outer phloem ; /)^/, palisade; /»a-, protoxylem.

secretory cells are not easily detected, resembling large air cavities;
but, unlike intercellular spaces, they are not directly connected
with adjacent air chambers owing to the retention of the original
cell wall.

The epidermal cells are irregular in size and have a definite
cuticle. Stomata are more numerous in the lower than in the upper
epidermis and are surrounded by

the guard cells and two or more . i/^ v XM .r-3 c

accessory cells. In the develop-
ment of the stoma, the epi-
dermal initial is divided by the
formation of successive walls
which are oriented in such a
way that several accessory cells
are formed which surround the
stomatal mother cell. (Fig.
2.-J0, A.^ The mother cell pro-
duces the two guard cells by a sto /«/stomatal Initid
final longitudinal division, and

the stoma is formed by a separation of their opposed primary
walls along the middle lamella. The number of accessory cells
which may be formed is commonly three or four, and two of these
abut the guard cells. (Fig. xyo, B.)

a c

Fig. 170. A, early stage in stomatal forma-
tion ; B, mature stoma with guard cells and
accessory cells. Numerals i, i ; 1,2.; 3,};
indicate sequence of wall formation : a c, ac-
cessory cells ; g c, guard cells ; sto, stoma ;

511 THE STRUCTURE OF ECONOMIC PLANTS

In the region of the midrib and the principal lateral veins, the
palisade is replaced by mechanical tissue above the vein which
forms a protecting ridge on the adaxial surface; and a zone of
collenchyma develops below the vein. The midvein is bicollateral
at the base of the leaf, but there is a reduction in the amount of
adaxial (inner) phloem toward its tip until finally the collateral
arrangement is attained. The lateral veins are collateral; but,
like the midvein, there is a reduction in the amount of phloem, in
this case the abaxial (outer) phloem, and the ultimate veinlets
terminate as single spiral tracheids.

At the base of the leaf, there are five principal veins which extend
for some distance down the petiole without anastomosing. (Fig.
2.6^, B.) The three median ones are bicollateral, while the two
laterals are half-amphicribral or collateral. At the base of the
petiole, the two lateral bundles anastomose with the right and left
median bundles, so that at this point there are two large lateral
bundles and a smaller median one. The leaf traces of the first,
second, and third foliage leaves constitute the primary vascular
tissue of the first internode, which consists of eight or nine endarch,
bicollateral bundles. Near the cotyledonary plate, these converge
to form two lateral zones of the stele which become confluent with
the lateral bundles of the cotyledonary traces in the intercotyle-
donary plane.

LITERATURE CITED

I. Artschwager, E., "On the anatomy of the sweet potato root, with notes
on the internal breakdown." Jour. Agr. Res. zj: 157-166, 1914.

1. Groth, B. H. A., "The sweet potato." Contr. Bot. Lab. Univ. Venn. 4: 1-104,
191 1.

3. Hand, T. E., and Cockerham, K. L., The Sweet Potato, The Macmillan Co.,

1911.

4. Herail, J., "Recherches sur I'anatomie comparee de la tige des Dicotyle-

dones." Ann. Set. Nat., Bot. Ser. VII, 2: 103-314, 1885.

5. IsBELL, C. L., "Regeneration in leaf cuttings of Ipomoea batatus." Bot. Gaz-

91: 411-415, 1 93 1.

6. Kamerling, Z., "Sind die Knollen von Batatas edulis Choisy Wurzeln oder

Stengel?" Ber. Deut. Bot. Gesell. jz: 351-360, 1914.

7. Lamounette, B., "Recherches sur I'origine morphologique du liber interne."

Ann. Set. Nat., Bot. Ser. VII, 11: 193-181, 1890.

8. Lee, E., "Observations on the seedling anatomy of certain Sympetalae.

I. Tubiflorae." Ann. Bot. z6: 717-746, 1911.

9. Lubbock, J., Seedlings, Vol. II: 167-190, D. Appleton and Co., 1891.

IPOMOEA BATATUS 513

10. McCoRMicK, F. A., "Notes on the anatomy of the young tuber of Ipomoea

batatas Lam." Bot. Gaz- 61: 388-398, 1916.

11. Scott, D. H., "On some points in the anatomy of Ipomoea versicolor Mcissn."

Ann. Bot. j: 173-180, 1891.
II. Stout, A. B., "The flowers and seed of sweet potatoes." Jour. N. Y. Bot.
Card. 2;: 153-168, 1914.

13. Thompson, J. B., "Production of sweet-potato seedlings at the Virgin Islands

Exp. Sta." Virgin Island Exp. Sta. Bull. ;, 1915.

14. TuYiHusA, v., "Are the bulbs of Batatas edulis choisy roots or stems?" Bot.

Mag. (Tokyo') 28: 475, 1914.

CHAPTER XVII

SOLANACEAE

SOLANUM TUBEROSUM

THE nightshade family, Solanaceae, contains many important
vegetable crops including: potato, Solanum tuberosum L.;
eggplant, S. Melongena L.; tobacco, Nicotiana Tabacum L.;
tomato, Lycopersicum esculentum Mill., and several varieties of
pepper belonging to the genus Capsicum. In addition, some mem-
bers of the family are grown as ornamentals, and a number of
them produce alkaloids that are used as drugs.

The potato plant is a native of the highlands of South and Central
America, and a wild tuber-pVoducing form, Solanum Jamesii, is
still found in the mountains of Arizona, Colorado, Texas, Utah,
and Mexico. The wild species resemble the cultivated Solanum
tuberosum, except that the latter has a larger vine and a more
pronounced development of tubers. Berthault (4) regards Sola-
num tuberosum as a distinct species which has not been derived
from native ones by selection or mutation, and points out that it
differs from the wild tuberous forms in its floral organization,
especially with respect to its rotate corolla and the sharp-pointed,
mucronate, calyx lobes.

GENERAL MORPHOLOGY

The potato is an annual, herbaceous dicotyledon as far as its
vegetative and flowering habits are concerned, but it may be
regarded as a potential perennial owing to its capacity for vegeta-
tive reproduction by means of tubers. These arise on underground
stems and from them new shoots are produced.

The Stem. — The aerial stem is herbaceous and erect in its early
development, but later may become spreading and semi-prostrate.
It reaches a height of i to 5 feet or more; and, after becoming pro-
cumbent, may produce several axillary branches. The stems are
green or purplish, and are round to subtriangular or quadrangular

514

SOLANUM TUBEROSUM

515

in transection. In some cases, the angular margins form ridges or
wings which are prominent in young plants. (Fig. xyi.)

The Leaf. — The leaves are alternate with a ^f 3 phyllotaxy,
and the spiral is usually counter-clockwise. The petioles are
semicircular in transection with a convex abaxial and a slightly
concave adaxial surface. The base
of the petiole is flattened some-
what and ensheathes about one-
third of the circumference of the
stem at the node. The wing-like
margins of the petiole are une-
qually decurrent, one extending
down the stem through a single
internode, the other through two.
In plants grown from seed, the
first leaves above the cotyledons
are simple, and this may be the
case with the first leaves arising
on stems grown from tubers. (Fig.
zyx.) The later leaves are com-
pound and irregularly odd-pinnate
with more or less petioled leaflets.

The number of leaflets varies
considerably with the variety,
and, in addition to the terminal
one, there are usually three or four
pairs of large, oval leaflets with en-
tire or serrate margins. Smaller,
secondary leaflets sometimes occur between the large primary
ones. The young leaflets are densely pubescent, bearing hairs
of several types, some of which are long ^nd straight consisting
of one to several cells, while others are short and glandular
with a spherical head of four cells which is borne upon a short
slender stalk cell. At maturity, the leaf is sparingly pubescent,
the persistent hairs occurring chiefly along the midrib and lateral
veins.

The leaflets are net-veined; and branches arise from the promi-
nent midrib, forming a dense reticulate system. The lateral veins
extend toward the apex and margins of the leaf, anastomosing
freely so that the marginal portions have a greater compactness

Fig. 171. Basal portion of potato plant
showing development of rhizomes and
young tubers.

5i6 THE STRUCTURE OF ECONOMIC PLANTS

than the central part of the lamina. Stomata are more numerous
on the abaxial surface.

The leaves that develop on the subterranean portions of the
stem are small and scale-like. Buds formed in the axils of these
leaves produce rhizomes which elongate rapidly and develop tubers

Fig. 2.71. Face and side view of potato seed and stages in development of seedling.

at their extremities. According to Clark (7), the rhizomes are
variable in length, ranging from less than an inch to 18 inches,
but they do not usually exceed 3 to 4 inches in cultivated varieties.
The tuber is morphologically a fleshy stem bearing buds or "eyes"
in the axils of small scale-like leaves which are soon shed, leaving
a ridge or leaf scar subtending the bud.

Vegetative propagation results from the development of axillary
buds which produce shoots, and these then give rise to adventitious

J

SOLANUM TUBEROSUM

517

roots. Rosa (xo) determined that the rapidity with which
potatoes sprout is related to the maturity of the tubers at the time
of harvesting, and noted that those harvested while immature
emerge from dormancy more slowly than mature tubers. He
also found that "The primordia of the vegetative sprouts develop
during the later stages of tuber growth, as well as during the

Fig. 2.73. The root of mature plant showing extent of one-half of total root system. (After
Weaver, Root Development of Field Crops, McGraw-Hill Book Co.)

dormant period." The bud primordia are present even in very
young tubers and there is a meristematic region, but the sprout
does not begin to form until the later stages of tuber growth.

The Root. — Plants grown from seed develop a slender tap root
from which lateral branches arise to form a more or less fibrous
system. (Fig. X73.) Those grown from tubers have a fibrous
system consisting of adventitious roots that arise in groups of three
at points just above the nodes of the underground stem. The root
has been described by Weaver (i8), who states,

5i8 THE STRUCTURE OF ECONOMIC PLANTS

"In its early growth, it is almost entirely confined to 8 inches of
surface soil. After extending horizontally to a distance of i to i feet
or more, the roots turn more or less abruptly downward and penetrate
the second and third foot of soil. Roots may also occur in the fourth
foot. Branching is very profuse throughout the root extent, and at
maturity, laterals occur to the root tips. Usually the branches are
relatively short but so numerous and well rebranched that the absorbing
system is very efficient. There is some evidence which indicates that
late-maturing varieties root deeper than early ones."

po

Fig. 174. A, median longisection of mature flower ; B, berry ; C, floral diagram : an,
anther; c/«, corolla; c/at, calyx; or^, ovules; po, terminal pore of anther. (After Robbins,
Botany of Crop Plants, P. Blakiston's Son and Co.)

The Inflorescence and Flower. — The inflorescence is cymose
and terminal although it appears to be lateral owing to the sym-
podial development of the axis. The five-merous flowers are
white, yellow, purple, or blue and are borne on bractless pedicels.
The calyx is tubular and lobed as is the gamopetalous corolla;
and, at anthesis, the latter may be i to i3'2 inches in diameter.
The stamens are alternate with lobes of the corolla and adnate to
its tube. They are straight or slightly curved with large, erect,
fleshy, yellow anthers that are longer than the filaments. Dehis-
cence of the anther is by means of two terminal pores. (Fig.
X74, A, C) The pistil consists of two undiverged carpels which

SOLANUM TUBEROSUM 519

form a two-loculed ovary with a single style and stigma. The
ovary is superior and the placentation axile.

The pollen is wind-borne, the flowers produce no nectar and are
not visited by insects to any great extent, although Miiller (15)
has reported that bumblebees do visit them, obtaining some pollen.
It seems probable that in most cases self-fertilization is natural
and cross-pollination rare. Several types of sterility exist and East
(11) has classified the varieties of potatoes into four groups with
reference to this condition : (i) those whose flower buds fall with-
out opening; (i) those in which a few flowers open but fall
immediately; (3) those in which flowers persist for several days
but lack viable pollen; and (4) varieties which flower freely,
produce viable pollen, and bear fruits. Differences in environmen-
tal conditions may also induce varying degrees of sterility.

The Fruit. — The fruit is often referred to as a potato "apple,"
potato "ball," or "seed ball"; and is brown or purplish-green
tinged with violet. (Fig. 174, 5.) It is a two-celled, many-
seeded berry, about 3^^ inch in diameter, globular to oval in shape,
and contains numerous small seeds that are attached to the thick
axile placenta and are embedded in the green pulp of the fruit.
Single fruits have been known to contain from xoo to 300 seeds;
while in other cases no seed at all is produced.

ANATOMY

The Seed. — The yellow to yellowish-brown seeds are small,
flat, and oval or kidney-shaped. During development, the ovule
arises from a massive placenta which is probably cauline, although
in later stages the placentation appears to be foliar and axile.
The orientation of the ovule is anatropous or partially campy-
lotropous. Young (i8) is of the opinion that the ovule does
not have the typical anatropous form, since the nucellus and
embryo are much curved, and has suggested that it represents
a transition stage in the direction of campylotropy.

A single massive integument develops and there is a long micro-
pyle. Soueges (ii) and Bhaduri (5) have investigated the histol-
ogy of the integument and are in general agreement. At first
the integument is only a few cells in thickness; but, as growth
proceeds, it becomes massive and exhibits a definite differentiation
of its tissues. Soueges has divided the fully mature integument
into three layers: (i) an external region consisting of a single

510 THE STRUCTURE OF ECONOMIC PLANTS

oo^

""---end

layer of epidermal cells; (i) an intermediate zone which he
divides into an external and internal region; and (3) an internal
or digestive zone which forms the innermost layer of cells
covering the endosperm. (Fig. uy^, B, C.)

As the seed develops
and approaches maturity,
definite changes occur in
the character of the in-
tegument. The inner
digestive layer acts as
an haustorial zone, and
there is a gradual diges-
tion of the inner region
of the middle layer
,'-0 1 which progresses centrif-
y-ml ugally until nothing is
left of the entire middle
zone except a narrow
band of much-crushed
tissue. While the diges-
tion of the intermediate
zone of the integument
is going on, a special
type of wall thickening
is taking place in the
cells of the external
layer. There is a dep-
, , osition of cellulose on

Fig. 175. A, median section through seed showing . ■ill

position of embryo; B, transection of portion of in- the inner tangential wall

tegument showing layers and part of endosperm; C, q( Qa.ch epidermal Cell

surface view of external layer of seed coat : cot, cotyle- J i f ' C

dons ; eW, endosperm ;/■/, inner digestive layer of seed ^^^ ^i"^^ lOrmatlOn OI

coat; m I, middle partially digested and crushed layer; longitudinal bands ofcel-

0 I, outer layer showing hair-like projections of cell i • i • i ^ „^ c .„

1 ru Ar A u <i - A c M,^ lulose which extend from

walls. {B and C, redrawn after Soueges, Ann. Set. Nat.)

the base of each cell
toward the surface. The cellulose bands are finally isolated
through a gelatinization of the intervening wall substance with
the result that hairs are formed on the surface of the mature seed.
These are not true epidermal hairs, but the thickened portions of
the epidermal cell walls which have been separated from one an-
other.

SOLANUM TUBEROSUM

511

Development of the Seedling. — The mature U-shaped embryo
lies embedded in the endosperm so that both the tip of the radicle
and those of the two cotyledons are directed toward the micro-
pyle. (Fig. 175, A.') Upon germination, the radicle emerges
from the micropylar end of the seed and grows rapidly to form a
tap root which soon produces numerous secondary roots. The
hypocotyl elongates, lifting the oval cotyledons above the ground;

B

Fig. 2.76. A, habit of young seedling showing development of rhizomes and young tubers
at cotyledonary node ; B, enlarged detail of base showing rhizomes and adventitious roots
at node.

and this is followed by the development of the epicotyl from which
the first simple, ovate, foliage leaves are differentiated. (Fig.
2.71..')

At this time, the first rhizomes arise in the axils of the cotyle-
donary leaves above the ground level. They are slender and
cylindrical, possessing small rudimentary leaves; and, after
elongating until their tips strike the ground, they penetrate the
soil and begin to swell and form tubers. (Fig. X76, A, 5.) Adven-
titious roots arise from the rhizomes and the cotyledonary node,
and additional rhizomes may develop in the axils of the first
foliage leaves. In general, the tubers which are produced the first

5XZ THE STRUCTURE OF ECONOMIC PLANTS

year from seedlings are relatively small. There are reported cases
in which large potatoes have been produced in one year from seed;
but, ordinarily, normal tuber size for a given variety is not attained
for about three years.

The Primary Root. — The young primary root has a diarch
radial protostele. (Fig. 7.) The two protoxylem points abut
the pericycle directly, and the metaxylem is differentiated cen-
tripetally to form a complete primary xylem strand early in the
ontogeny of the primary axis. In some cases, a few fundamental
parenchymatous cells may occur adjacent to the metaxylem; and,
in others, there may be a small parenchymatous pith region.
The first protoxylem elements to differentiate form annular or
loosely spiral thickenings while the later ones have scalariform
or scalariform-reticulate walls.

The primary phloem is radially arranged with respect to the
protoxylem points, and the protophloem differentiates adjacent
to the pericycle. Between the primary xylem strand and the two
phloem groups is a zone of fundamental parenchyma in which the
cambium is later differentiated. The stele is surrounded by an
endodermis which exhibits well-developed Casparian thickenings
by the time primary maturation has been completed. The cortex
consists of several rows of large parenchymatous cells with inter-
cellular spaces, and the cortical cells are limited externally by
an epidermis which bears root hairs.

Origin of Lateral Roots. — Lateral roots arise early in the
ontogeny of the primary root, and this is also true of laterals
developed from the adventitious roots. They originate before
or at about the time that the maturation of the primary xylem
has been completed and just prior to the initiation of cambial
activity. The first evidence of lateral root formation appears in
the radial enlargement of the pericyclic cells which lie directly
outside the protoxylem points or slightly to one side of them.
This is followed by tangential divisions of the pericyclic cells,
and further divisions in three planes result in the formation of the
conical growing point of the lateral root. This growth forces
the endodermis outward, and it finally ruptures, as do the cortical
cells in the line of lateral root extension. The emergence of the
root is accomplished in part by mechanical pressure and in part by
the digestion of the cortical parenchyma. By the time the lateral
root has elongated so as to reach the periphery of the cortex, a

SOLANUM TUBEROSUM

52-3

definite root cap has been organized and the stelar elements have
begun to differentiate against the primary xylem and phloem of
the main axis. Direct contact with the primary vascular elements
of the stele is possible owing to the point of origin of the lateral
root and its early initiation.

Origin of Adventitious Roots. — In commercial practice, the
propagation of the potato, except for experimental work in potato-
breeding, is accomplished by vegetative methods in which por-
tions of the tubers, known as "seed pieces" or "seed", are planted.
Vegetative growth results
in the development of
shoots, adventitious roots,
and tubers. In this case,
the entire root system is
adventitious; and, as pointed
out by Priestley and
Swingle (17), the roots do
not arise directly from the
tuber, but originate from
young sprouts which are
formed by the development
of the buds or "eyes" of the
seed piece. These roots fre-
quently occur in groups of
three, and their point of
origin is in the pericyclic
region of the subterranean
portion of the stele in close proximity to a nodal plate. As
in the origin of laterals, the first evidence of adventitious root
formation consists of an activation of the pericyclic cells, which
first elongate radially and then divide tangentially. The endo-
dermis keeps pace with the formation of the root primordium for
a time, and later is broken when the endodermal cells cease divi-
sion. Artschwager (i) states that the developing rootlet "pushes
its way mechanically through the cortex, and is aided by the
dissolving action of enzyms which are probably secreted by the
cells of the endodermis."

The ontogeny of the lateral and adventitious roots is similar
to that of the primary root except that they are not always diarch.
Lateral roots are usually diarch, but adventitious roots may fre-

'- — 'pel

Fig. 177. Transection of adventitious root : ca,
cambium ; co, cortex ; en, endodermis ; e^, epidermis ;
mx, metaxylem ; ^cl, pericycle ; ^h, phloem ; fx,
protoxylem ; xy 1, secondary xylem.

5M THE STRUCTURE OF ECONOMIC PLANTS

—CO

--phi

quently be triarch or pentarch. (Fig. 177.) In all cases, the
development of the primary xylem is centripetal and the arrange-
ment of the primary phloem radial.

Secondary Thickening of the Root. — At approximately the
time that the primary xylem is mature, cambial act vity is initiated
in the parenchymatous zone between the xylem and the phloem,
the first divisions occurring just outside the metaxylem portion
of the strand. (Fig. 178.) From these loci there is a lateral
extension of the cambial activity until it involves the pericyclic

cells that subtend the protoxylem
points. In this manner, a com-
plete cambial cylinder is formed
which produces secondary xylem
elements centripetally and second-
ary phloem centrifugally. The
cambium is relatively inactive and
none of the roots of the primary
or adventitious root systems attain
large size.

The secondary xylem consists
of vessels of two sizes, fibers,
and parenchyma, while typical

The large

initiation of secondary thickening: ca, j^ ^^^ arranged in approxi-

cambium; co, cortex; ^«, endodermis ; mx, o rr

metaxylem; pel, pericycle; ph i, primary mate radial rOWS, SUrroUnded by

piiioem; px, protoxylem; xy x, secondary parenchyma and vessels of smaller

xylem.

caliber. The tracheae are pitted,
and the larger ones frequently become filled with tyloses from
the adjacent parenchymatous cells. The few elongated fibers
occur as scattered elements between the vessels.

As a result of secondary thickening, the primary phloem cannot
be recognized in a transection of the mature root because of crush-
ing and partial digestion. The secondary phloem consists of
sieve tubes, companion cells, and parenchyma. No phloem fibers
are differentiated. The relative inactivity of the cambium and
the consequent slowness of secondary thickening result in a per-
sistence of the cortex through a considerable period of the later
ontogeny of the root. The parenchymatous cells of the cortex
compensate for the increasing size of the stele by enlarging tan-
gentially, but they do not divide to any great extent. In the

^ ca

Fig. 178. Young primary root after ttacheids are absent.

SOLANUM TUBEROSUM

52-5

final stages of development, the epidermis and cortex become
fissured and fragmented down to the endodermis. At such points,
the endodermis develops thickenings on the inner tangential walls
and the underlying pericyclic cells may divide tangentially to form
a periderm.

Vascular Transition. — The transition occurs in the hypocotyl
and cotyledonary petiole. It involves a reorientation of the stelar
elements, which is here described beginning at the root-like por-

FiG. 2.79. A-I, diagrams representing vascular transition. Dotted areas indicate primary
phloem; solid areas, protoxylem ; lined areas, metaxylem. (Adapted after Theil.)

tion of the lower hypocotyl and proceeding in an ascending direc-
tion to the cotyledons.

The first variation from the diarch radial protostele is the
division of the protoxylem strand into two sectors owing to the
fact that the cells of the central region remain parenchymatous
instead of differentiating into metaxylem. (Fig. 179, B.) At the
same level, each of the primary phloem regions subdivides into two
or three distinct groups. Above this point, there is a change
in the direction of differentiation of the later formed proto- and
metaxylem; which, instead of being laid down centripetally, on
the same radius as the protoxylem, is differentiated to the right

5z6 THE STRUCTURE OF ECONOMIC PLANTS

and left of the protoxylem points in a more or less tangential
direction so that each metaxylem group is bifurcated. (Fig.

2-79' <^' ^0

Artschwager (i) describes and figures this stage in the transi-
tion as a swinging of the two protoxylem groups in an outward
direction, "one group following a left, the other a right curve";
but Thiel (14) agrees with the description given above. In the
upper hypocotyl, the bifurcation of the metaxylem becomes more
pronounced until, at the level of the node, each cotyledonary
trace consists of a group of protoxylem cells with metaxylem
elements lying on either flank at an approximate right angle to the
plane of the protoxylem poles. (Fig. 179, £.)

While this reorientation of the primary xylem is taking place,
definite changes occur in the position of the primary phloem.
The central group of cells from each original phloem group diverges
gradually toward the center of the stele; and, ultimately, con-
stitutes the inner phloem of the bicollateral bundles of the cotyle-
dons. (Fig. 179, C, D, £.) The remaining groups of phloem
incline tangentially in the direction of the protoxylem points until
they occupy a position at the periphery of the stele and adjacent
to them. The number of phloem groups increases, and some of the
strands come to lie in a position outside the proto- and metaxylem.
The inner phloem groups also increase in number; and, as the
medullary region enlarges, they occupy a position on the inner face
of each double bundle. In this manner, the bicollateral bundle
of the cotyledonary trace is established. (Fig. 179, G.)

The position and direction of development of the proto- and
metaxylem at the cotyledonary node is not strictly endarch,
although the protoxylem may be nearer the periphery than the
metaxylem. (Fig. 2.y^, H.) At the base of the cotyledonary peti-
ole, the midvein consists of protoxylem and two groups of later-
ally placed metaxylem with adaxial (inner) phloem on one face
of the xylem and several strands of abaxial (outer) phloem on the
other. In the lamina of the cotyledon, the protoxylem is dif-
ferentiated in a more nearly adaxial position with reference to the
metaxylem until ultimately the endarch orientation is attained.
(Fig. 179, /.) The amount of adaxial phloem gradually decreases
toward the distal portion of the midvein; and, in the smaller
bundles, it may disappear completely so that they are collateral
instead of bicollateral.

SOLANUM TUBEROSUM 517

Ontogeny of the Stem. — Artschwager (i, x) has investigated
the ontogeny and vascular anatomy of the stem and tuber, and the
following account is based upon his report. The first differentia-
tion in the apical meristem is the demarcation of the dermatogen,
a peripheral layer of cells giving rise to the epidermis; and the
appearance of the procambial ring that separates the parenchyma-
tous tissue of the pith from that of the cortex. The procambial
ring IS the forerunner of the stele; and the small, elongated, thin-
walled cells which comprise it are filled with a dense cytoplasm.
They are readily distinguished from the adjacent parenchymatous
cells of the cortex and pith which are less dense, much larger, and
more nearly isodiametric.

As the leaf primordia arising from the growing point of the
stem undergo growth and development, the downwardly differen-
tiating bundles of their traces first appear in the procambial ring.
The first vascular elements to mature are the protoxylem cells
located in the median zone of the procambial cylinder. They are
long and slender and have annular wall thickenings in which the
rings are spaced at a considerable distance from one another.
Some of the protophloem cells may differentiate simultaneously
with the development of the protoxylem, but they are not structur-
ally different from the adjacent procambial cells and cannot be
distinguished from them. Shortly after the formation and devel-
opment of the protoxylem, the inner phloem is differentiated.

There is a centrifugal development of the succeeding primary
xylem elements so that they are endarch in orientation. The later-
formed protoxylem elements are somewhat larger than the initial
ones, and their secondary wall thickenings are characterized by
a series of close rings or loose spirals. As differentiation pro-
ceeds, groups of phloem initials are formed in the outer portion
of the procambial zone. These are similar to, but less numerous
than, those differentiated earlier in the procambial region adjacent
to the pith. The cells of the undifferentiated portion of the
procambial ring, between the primary xylem and the outer phloem,
appear very regular in arrangement owing to a series of tangential
divisions which mark the initiation of cambial activity. This
activity does not at first result in the formation of a continuous
ring, and at this stage there is a series of discontinuous sectors
of fascicular cambium lying on the same radii as the primary xylem
and phloem groups.

5z8

THE STRUCTURE OF ECONOMIC PLANTS

par

The slightly elongated cells of the inner phloem are rich in
cell contents, and the behavior of the cytoplasm in fixed tissue
suggests that there are cytoplasmic connections between adjacent
protophloem elements in the same vertical series, although actual
perforations cannot be detected. Following the development of

the inner protophloem,
further differentiation is
centrifugal and sieve
tubes occur in the later-
formed metaphloem
groups. Somewhat later,
sieve tubes also develop
in the outer phloem
groups. These are simi-
lar to those of the inner
pel fib (0) phloem, but they are laid
down centripetally in-
stead of centrifugally.

Coincident with the
progressive development
of the phloem groups,
the protoxylem becomes
more extensive and the
elements formed have
closely spaced rings and
flat spirals. Continued
development results in
the formation of larger
metaxylem elements with
scalariform or scalari-
form-reticulate walls.
As the primary xylem is
differentiating, the inner phloem groups increase in size and the
outer ones become larger and more distinct.

While the maturation of the primary tissues of the vascular
bundles is still taking place, there is an activation of the fas-
cicular cambium which forms a distinct region prior to the dif-
ferentiation of well-defined sieve tubes in the inner phloem.
Subsequent divisions of the cambial initials result in the formation
of two or three layers of cells between the primary xylem and the

-ph (0)

-xy 2

-ph(i)

^-—xy 1

- -pel fib (i)

- — pi

Fig. 2.80. Transection of sector of mature stem :
ca, cambium ; col, collenchyma ; en, endodermis ; ep,
epidermis; par, parenchyma; pel fib (i), inner pericyclic
fibers ; pel fib (0), outer pericyclic fibers ; ph (/), inner
phloem; ph (0), outer phloem; pi, pith; xy i, primary
xylem ; xy i, secondary xylem.

SOLANUM TUBEROSUM 519

outer phloem. In the differentiation of the primary vascular
system, six major bundles are formed which are separated by
medullary rays consisting of fundamental parenchyma; and, later,
an interfascicular cambium develops across them, although this
does not take place simultaneously at all points. While the inter-
fascicular cambium is being formed, there is also a lateral extension
of groups of outer and inner phloem. The outer groups are small
and rather closely arranged while the inner groups are more widely
separated from one another. (Fig. zSo.)

At this time, changes also occur in the epidermis and cortex.
The epidermal cells become cutinized and guard cells are formed.
Within the epidermis are one or two layers of vertically elongated
parenchymatous cells, and centrad to these is a zone of collen-
chymatous elements with thickenings at their angles and on the
radial and tangential walls. Centrad to the collenchyma, the
cells are large and there are numerous intercellular spaces. The
cortex is limited centripetally by an endodermis, and Casparian
strips develop at about the time that the collenchyma matures.

Among the last tissues to differentiate are the pericyclic fibers
which arise immediately adjacent to the endodermis, and fibers
also develop in the perimedullary zone inside the inner phloem
groups. These have been regarded by some investigators as
phloem fibers; but others, who interpret the stele as an amphi-
phloic one with an inner pericycle, have considered them to be
pericyclic in origin. The perimedullary zone is a band of paren-
chymatous tissue lying between the inner phloem and the primary
xylem which consists of small compact cells that are derived from
the procambial ring.

The Mature Stem. — The mature stem is subtriangular or quad-
rangular in transection. This results from the development of
three large vascular bundles and the wing-like projections of the
leaf which extend down the stem from each node because of the
decurrent habit of the petiole. Between each pair of major bundles
are three smaller ones; and, in the mature stem, a continuous
cylinder of vascular tissue is formed by the development of an
interfascicular cambium. The stelar region is limited outwardly
by the endodermis and inwardly by scattered groups of pericyclic
fibers, which become more or less irregularly oriented on its inner
margin owing to the inequalities of cell division and enlargement
in this region. (Fig. i8o.)

530 THE STRUCTURE OF ECONOMIC PLANTS

The histology of the proto- and metaxylem has been described.
The secondary xylem is laid down in radial rows, and consists of
large vessels, tracheids and parenchyma. The former are heavily
pitted with the pits arranged in transverse series, and each vessel
segment is two or three times longer than broad. Parenchymatous
cells surround the vessels, and are also laid down in radial rows
as xylem rays.

The mature sieve tubes are cylindrical, and a single sieve plate
comprises the entire transverse wall. Each plate is perforated
by a large number of circular pores, but these do not have sieve
fields such as occur in some phloem types. No plates occur on
the radial and tangential walls. The inner and outer phloem
elements are approximately equal in size, and the sieve tube seg-
ments do not vary much in length except that the segments are
short and relatively broad where anastomoses of bundles occur.

In general, the sieve tube arises from an initial which under-
goes longitudinal division to form two cells of unequal size, a
large sieve tube and a smaller companion cell, but there may be
variation in this respect. In some cases, the sieve tube initial
does not undergo longitudinal division so that there may be a
seHes of sieve tubes without adjacent companion cells. In addition
to the sieve tubes and companion cells, phloem parenchyma is
differentiated. These cells are elongated and rectangular, with
simple pits in their end walls unlike the sieve plates of the sieve
tubes.

The pericyclic fibers have pointed ends and are long and awl-
shaped with a small lumen and heavily thickened unpitted walls
that later become lignified. The endodermal cells are longer in
the tangential dimension than in the radial one and Casparian
strips are present but not well differentiated.

Vascular Anatomy of the Stem. — Each leaf trace consists
of five bundles and at the base of the semicircular petiole there
are three large centrally located bundles and two smaller ones
which lie at its outer edges. The course of the bundles in the
stem is illustrated in the accompanying figure. (Fig. x8i.) In
this figure, A, B, and C indicate the main bundles, while a and b
represent two of the three smaller bundles. The smaller inter-
mediate bundles, a and b, become median traces of the leaves.
Branches of the three large bundles A, B, and C form the four
lateral bundles of the leaf traces. The traces to the wings of the

SOLANUM TUBEROSUM

531

petiole are derived from further branching of the lateral groups,
E^ and A^, after these have become separated from the stem bundles.
Artschvsrager's summary of the course of the bundles follows:

"(i) The median trace ascends
without fusion or forking through
three internodes, and then passes
into the petiole without branch-
ing.

"(i) The lateral traces are given
off at the node from two of the three
large stem groups.

"(3) Each large stem bundle as-
cends without branching in turn for
two internodes or for one internode.

"(4) Where a large stem bundle
ascends for but one internode with-
out branching, it divides three times,
giving rise upon the first division
to a new stem bundle, upon the
second division to a new median
trace, and upon the third division
to lateral traces of the petiole.
Upon the division in the node, half
of the tissue is given off to unite
with that from the left (in a right
spiral) adjacent group, forming a
new large bundle directly above the
insertion of the leaf. Where the
bundle ascends for two internodes
without branching, it gives off vas-
cular tissue only for lateral traces of
the petiole.

"(5) Each leaf derives its supply
from two large bundles and the
smaller one lying between these.
The method of derivation is uniform
for all leaves, the bundles taking
part being each time a different pair
from those supplying the leaf be-
low. In a right spiral the right
member of the set supplying a leaf
supplies also the leaf above, becom-
ing there the left member of the
set. The median trace of a given

leaf is formed just below the third and vascular supply to leaves
node below the leaf it supplies." Artschwager, /o«r. Agr. Res.}

Fig. i8i. Diagrammatic longisection
of stem showing course of vascular bundles

(After

532.

THE STRUCTURE OF ECONOMIC PLANTS

The subterranean portion of the stem is similar to the aerial
in general plan, but differs from it in certain characteristics.
There is a decrease or complete absence of collenchyma in the
cortex; and immediately beneath the epidermal layer, the cells
tend to become more or less thickened at points where the epidermis
is ruptured. The epidermal cells are heavily suberized and Cas-
parian strips in the endodermis are clearly defined.

Anatomy of the Rhizome. — The rhizome is stem-like, and its
ontogeny resembles that of the aerial shoot. The meristem has
the three zones, dermatogen, procambium, and fundamental meri-
stem, that occur in the aerial stem. The dermatogen forms a
single-layered epidermis consisting of square or hexagonal cells
that are at first approximately isodiametric, but later become
elongated in the axial direction. The outer wall is somewhat
arched and is thicker than the inner one with a thin cuticle. Some
of the epidermal cells are specialized to form guard cells and hairs,
and both stomata and hairs are produced in limited numbers
but are ephemeral because of the early development of the
periderm.

The cells of the meristem enlarge without differentiation to
form the cortical zone external to the procambial ring and the
medullary zone within it. The cortex consists of a band of paren-
chymatous cells eight or nine layers in width which are rounded
and isodiametric or slightly elongated. Near the periphery, the
cells tend to be collenchymatous, but to a much less degree than in
the aerial stem. Small intercellular spaces are formed between the
cells in the middle cortical region. The cells adjacent to the
endodermis are smaller and approximately the size of the endo-
dermal cells in tangential dimension. The first visible storage
product, starch, can be detected in the cells adjacent to the pro-
cambial ring; and, later, larger deposits occur in the intermediate
cortical cells. The peripheral cells are usually devoid of starch,
but protein granules occur in increasing abundance as the matura-
tion of the tuber proceeds.

The pith of the young rhizome is small as compared with the
cortex, and the cells comprising it are axially elongated. Both
pith and cortical cells are profusely pitted with groups of simple
pits. The development of the vascular tissue from the procambium
is similar to that of the aerial stem, but there is proportionately
more phloem produced than xylem. The outer phloem groups

SOLANUM TUBEROSUM

533

form a nearly continuous band limited externally by pericyclic
parenchyma which in turn is bounded by the endodermis. The
inner phloem groups are separated from the xylem by thin-walled
cells of the perimedullary zone. Between the outer phloem and
the primary xylem, there is a single layer of procambial tissue
which later functions as a cambium
and gives rise to secondary xylem
and phloem.

In the young rhizome, the primary
xylem consists of a few scattered
annular and spiral elements sur-
rounded by parenchyma. (Fig. x8x.)
These initial elements occur very close
to the apex of the rhizome, and the
elongation of this portion of the axis
results in a stretching of the protoxy-
lem elements until the lumen of the
vessel may be closed owing to the
flattening of the lateral walls. The
later formed primary xylem consists
of vessels with close spiral thicken-
ings or shorter, scalariform elements.
The phloem cells are similar in form
and development to those described
for the aerial stem. The cambium
first appears in the vascular regions
of the procambial cylinder; but,
later, it forms a nearly continuous
zone which produces a limited amount
of secondary vascular tissue.

In that part of the rhizome which forms the connecting link
between the stem and the tuber, the endodermis is very well defined
with prominent Casparian strips, and there also may be a suberin
layer deposited over the inner tangential walls of its cells. Unlike
the portion which tuberizes, the epidermis persists, and its outer
walls as well as the epidermal hairs may become lignified. The
accompanying diagram (Fig. 183) indicates the proportionate
amount of tissue found in each zone of the mature rhizome, and
Artschwager (i) has computed the relative areas of each type of
tissue.

Fig. i8i. Transection of tip of
young rhizome showing a few annu-
lar and spiral elements of protoxylem
surrounded by parenchyma : co, cor-
tex ; en, endodermis ; e^, epidermis ;
/' ^h, inner phloem ; o ph, outer
phloem; fcl, pericycle; px, protoxy-
lem. (After Artschwager, Jour. Agr.
Kes.')

534 THE STRUCTURE OF ECONOMIC PLANTS

TABLE IV

Relative Areas Occupied by the Different Tissues of the Cross Section

OF A Mature Rhizome

Rhizome Tissues Measured

Area (Sq. Mm.)

Per Cent of
Total

Cross-sectional area of rhizome

Cross-sectional area of outer phloem ....
Cross-sectional area of inner phloem ....

Cross-sectional area of xylem

Cross-sectional area of pith

Cross-sectional area of cortex

1.130

•2-77

•2-39

.174

.350
1.090

100. 0

1 14.1

8.1
16.3
51.1

(After A rischwager)

It will be noted from this table that the total transectional area
of the outer phloem exceeds that of the inner, and that the two
combined are approximately three times the area of the xylem,

0 .1 .2 .3 .4 .5 mm.

Fig. 183. Diagrammatic transection of mature rhizome showing proportional amount of
tissue in the principal zones. (After Artschwager, Jour. Agr. Kes.')

SOLANUM TUBEROSUM 535

while the parenchymatous tissues of pith and cortex comprise
nearly two-thirds of the total area.

Crafts (9) has investigated the structure of the phloem in relation
to the problem of translocation. He rejects theories to account
for the rapid transport of carbohydrates to the tuber that are based
upon diffusion, protoplasmic streaming, or movement through
young sieve tubes, conducting parenchyma, and perforations in

Fig. 184. Basal portion of plant showing habit of mature tubers. (Photograph by J.

Horace McFarland Co.)

sieve plates. Instead, he proposes that "Diffusion along plas-
modesma of cross walls and acceleration by protoplasmic streaming
within non-vascular tissues, combined with pressure flow through
permeable sieve tubes and phloem walls within specialized con-
ducting organs, seem most satisfactorily to explain translocation
in the potato."

Anatomy of the Mature Tuber. — The mature tuber is mor-
phologically a short thickened stem bearing scale leaves that fall
off early in ontogeny, leaving prominent scars. (Fig. 184.) In

536 THE STRUCTURE OF ECONOMIC PLANTS

Fig. ^85. Stages in development of tuber: A, transection through periderm, cortex
pericyclic region, and vascular ring of mature tuber; B, same region as in A, ^"^er 1° mm in
diameter; C, same as in ^ and B, tuber ^.^ mm. in diameter: ^-.cambium; ^o^cone., en
endodermis; .p, epidermis ; .• />A, inner phloem ; . /,A, outer phloem; ^ pericycle. pa,
periderm; pi, pith; x^, xylem. (After Artschwager, 7o«r. /f^r R«.;

SOLANUM TUBEROSUM 537

the axil of each leaf is a compound bud consisting of an unelongated
branch bearing several rudimentary leaves and axillary buds. The
central bud, which is the most prominent, is the first to develop
when growth is resumed. The arrangement of the leaves is spiral
and agrees with the phyllotaxy of the vegetative stem.

Tuberization results from a cessation of the polar growth of the
axis of the rhizome and a marked lateral proliferation of storage
tissues which form the main bulk of the mature tuber. The
anatomy of the tuber has been variously interpreted. De Vries
(16), in his classic work on the potato, regarded the tissue of the
tuber as being chiefly vascular and derived from the cambium,
differing from the normal cambial product only in that the paren-
chyma produced was thin-walled storage tissue rather than woody
parenchyma. Esmarch-Bromberg (ii) attributed the principal
growth of the tuber to an enlargement of the medullary region,
and regarded the inner phloem groups as having an independent
origin unrelated to the vascular cylinder. Reed (18) ascribed
the growth of the tuber to the activity of three parenchymatous
regions, the medulla, phloem, and cortex; and Artschwager (x)
is in essential agreement with Reed except with reference to
the activity of the inner phloem parenchyma. In regard to this
region. Reed states that the cells between the inner phloem and
the protoxylem begin to divide very early in ontogeny, causing a
spreading out or dispersion of the phloem groups. He also holds
that the parenchyma of the inner phloem groups is very active,
contributing a considerable amount of tissue to the growing tuber.
Artschwager expresses doubt on this point, and attributes the
increased amount of storage parenchyma to the proliferation of
cells in the procambial zone, especially in the perimedullary re-
gion, rather than to an activity of the parenchyma of the inner
phloem.

The principal zones in the mature tuber from the periphery
inward are the periderm, cortex, vascular cylinder, perimedullary
zone, and central pith. (Fig. 185, A.^ The periderm is six to
ten cell layers in thickness and acts as a protective zone over the
entire tuber, being broken only by small lenticel-like structures.
These develop in the tissue underlying the stomata and are initiated
when the young tuber still retains its epidermis. There is con-
siderable variation in the width and character of the periderm in
different varieties of potatoes, some of which are sufficiently

538

THE STRUCTURE OF ECONOMIC PLANTS

constant to be of diagnostic value in describing types. In most
cases, however, individual variations under a wide range of cultural
conditions indicate that the character of the periderm is too plastic
to be of use in this connection.

The cortex consists of a narrow band of storage tissue within
the periderm, and its peripheral cells contain tannins, protein
crystals, and a limited amount of starch in addition to pigment
in the colored varieties. The pith forms a small central core from
which radiate arms of medullary parenchyma so that this region
appears irregularly stellate in transection. The cells are relatively

low in starch content,
have a greater propor-
tion of water, and
appear more translucent
than adjacent tissues.

The vascular cylinder
lying between the cortex
and the perimedullary
zone is narrow, contain-
ing secondary xylem and
phloem outside of which
the outer primary
phloem forms a rela-
tively limited storage
region. Gen trad to the
vascular ring is the
major region of storage

inner phloem; «.x.metaxylem; .;,^. outer phloem ;?x, parenchyma. This is
protoxylem ; xy i, secondary xylem. CArter Artsch- ^ . ■'

wager, Jour. Agr. Res.^ derived from the inner

portion of the procam-
bium and constitutes the perimedullary region in which numerous
groups of inner phloem are located. (Fig. 186.)

The relative activity of the tissues which enter into the develop-
ment of the tuber may be indicated in the following order; the
initial activity occurs in the pith and cortex, but the perimedullary
zone is the most active region and produces the largest proportion
of the tissue of the mature tuber. The parenchyma of the outer
phloem is relatively inactive, but divisions take place so that
this zone keeps pace with the enlargement of the tuber without
greatly increasing its width. The cambium, contrary to the

Fig. 2.86. Transection through portion of vascular
tissue of mature tuber: ca, cambium; co, cortex; / pf>.

SOLANUM TUBEROSUM 539

interpretation of de Vries, is relatively inactive and produces
little secondary tissue.

The Ontogeny of the Tuber. — Following the elongation of
the rhizome, which normally reaches a length of 3 to 4 inches,
the first change incident to tuber formation is the radial enlarge-
ment of its tip. The first region to grow actively is the pith; and,
as a result of its increasing size, the elements of the vascular
cylinder are inclined obliquely outward from the course which
they follow in the axis of the rhizome.

To compensate for medullary growth, tangential enlargement
and radial divisions of cells take place simultaneously in the
cortical, perimeduUary, and vascular regions. As tuberization
proceeds, the cortical cells become filled with starch and the storage
character of the organ is definitely indicated. Following cell
division in the pith and the coincident compensating growth in
the cortex, the pericyclic and perimeduUary zones become the
regions of greatest growth activity. The pericyclic cells surround-
ing the outer groups of primary phloem divide, enlarge rapidly,
and, like the cells of the cortex, become filled with starch. They
appear to be in direct contact with the cortical parenchyma and
also form radial wedges of tissue between the outer phloem groups.

In early ontogeny, the endodermis does constitute a line of
demarcation between cortex and outer pericyclic zone and this
helps to eliminate the possibility of misinterpreting the outer
pericyclic storage region as a centripetal extension of the cortical
parenchyma. (Fig. 185, B, C.) The failure of the endodermis
to form Casparian strips as the girth of the stele increases finally
results in its disappearance as a distinct layer. The activity of
the pericycle causes the original groups of outer phloem to become
widely separated, and additional ones are formed in the intervening
areas which consist of a few sieve tubes, companion cells, and
parenchyma.

Changes similar to those taking place in the pericycle occur in
the inner pericyclic and perimeduUary zone, where rapid and
continuous cell division results in the spreading and centripetal
deflection of the inner phloem groups. There is a limited amount
of cell division in the vascular ring which causes a tangential
dispersion of the xylem elements and a radial separation of the
protoxylem from the metaxylem. (Fig. t.S^, A.') The epidermis
of the tuber primordium is ephemeral because of the activity of

540

THE STRUCTURE OF ECONOMIC PLANTS

these zones in tuberization; but, for a short time, anticlinal walls
are laid down in some of the epidermal cells to compensate for
the increasing size of the tuber. This is followed by periclinal
divisions so that the initial periderm may be of epidermal origin
rather than hypodermal as indicated by Reed (i8).

While the epidermal cells are dividing periclinally, the hypo-
dermal layer begins to function as a phellogen, and tangential
divisions produce a periderm that is from six to fifteen layers in
width. The initial activity of the phellogen begins at the stem

Fig. i&j. Longisection through tip of tuber showing structure of buds and adjacent tissues.

(After Artschwager, Jour. Agr. Res.^

end; and by the time the tuber is the size of a pea, it has extended
over the entire tuber. The phellogen remains active throughout
the growth of the tuber and new phellem cells are formed to
replace those that disintegrate as a result of its enlargement. The
individual cork cells are brick-shaped with thin suberized walls
and form a compact layer without intercellular spaces. At
maturity they are devoid of contents, although in some varieties
they may contain pigments similar to those found in the peripheral
cells of the cortex. Coincident with the formation of the peri-
derm, the cells which underlie the stomata begin active division
and produce loose masses of rounded cells. Under favorable
conditions, these rupture the epidermis and proliferate, forming
small white dots, lenticels, on the surface of the tuber. During
tuberization, the buds or "eyes" are differentiated in the axils

SOLANUM TUBEROSUM 541

of the small scale leaves which occur in the same phyllotaxy as
the foliage leaves. Each "eye" may contain three or more buds.
(Fig. 2.87.) As the tuber enlarges, new buds are formed at its
apex, and the older ones develop to maturity, establishing vascular
connections with the stele.

Ontogeny of the Leaf. — The development of the leaf has been
described by de Vries (i6) and Artschwager (i).

The leaf primordia arise from the growing point of the stem
as small protuberances which elongate and curve adaxially so that
they finally arch over the growing point. While this growth and
curvature are taking place, the primordia undergo form changes,
the first being the production of lateral projections on both sides
of the primordium that later become the blades of the terminal
leaflets. Very early in ontogeny, the terminal leaflet consists of
a short stalk and blade with a well-developed midrib, and the
halves of the lamina are folded so that their adaxial faces oppose
each other. For some time, the terminal leaflet is far advanced
in its development over the other leaflets which diff"erentiate
gradually, and the slow elongation of the axis of the leaf provides
space for the formation of the primordia of the lateral leaflets.
Artschwager (i) states that "the diff"erence in the rate of develop-
ment is so great that the terminal leaflet is already one centimeter
in length and has become green long before the rest of the leaflets
appear." The development of the lateral leaflets occurs in basip-
etal succession and the uppermost leaflets are green and well
developed when the lower ones are still primordia.

The first epidermal hairs appear early on the abaxial surface
of the primordium, developing acropetally; and, later, hairs are
produced on the adaxial surface in the same order. These arise
in longitudinal rows adjacent to the veins, and the first glandular
ones are soon followed by hairs that are stiff" and unicellular.
Both kinds increase in number rapidly and mature before the
mesophyll is completely differentiated. While the leaf is small,
the hairs form a dense mat over the veins; but, since no new hairs
are formed, the density decreases as the leaf enlarges and they are
widely scattered at maturity.

In the development of the mesophyll, the cells adjacent to the
upper epidermis elongate to twice their original height and form
a compact palisade region; while the remaining parenchymatous
cells comprising the spongy region are approximately isodiametric.

542. THE STRUCTURE OF ECONOMIC PLANTS

In the mature blade, there is one layer of palisade cells and three
to five rows of spongy tissue. (Fig. x88, A.^

Differentiation of the vascular system is not completed until
the blade is fully developed. It is similar in general details to
that of the stem except for the position, number, and size of the
vascular elements. In the leaf primordium small crescentic groups

^. ep

Fig. i88. A, transection of margin of leaflet showing mesophyll and small veins ; B, transec-
tion through midvein of leaflet: ^^ p/i, abaxial phloem ; ^^/7/j, adaxial phloem ; w, cambium;
co/, collenchyma ; ?/», epidermis ; ^;-, hair; /«f, midvein; pa/, pnlisa.de; jpo, spongy tissue;
sto, stoma ; v, lateral vein ; xy, xylem.

of procambial cells occur, surrounded by the larger cells of the
fundamental parenchyma; and, at about the center of each group,
a few protoxylem elements are differentiated. This is followed
by the appearance of phloem initials which form a band of smaller
cells at the periphery of the procambial strand. The adaxial
(inner) phloem is the first to be differentiated, and these groups,
though few in number, make up the larger portion of the phloem

SOLANUM TUBEROSUM 543

tissue. The abaxial (outer) phloem forms a nearly continuous
band on the abaxial side of the procambium.

As the leaf matures, the cells of the procambial strand lying
between the abaxial phloem and the primary xylem form a cambium
which later gives rise to secondary vascular elements. (Fig.
i88, B.) In the mature leaf, both the abaxial and adaxial phloem
groups are distinct; but, in the principal veins, these may be
continuous around the flanks so that the bundle is essentially
amphicribral. The lateral veins resemble the midvein, but the
size and number of the phloem groups gradually decrease in the
smaller ones. No adaxial phloem is differentiated in the terminal
veinlets, which consist of one or two spiral protoxylem elements
and some adjacent conducting parenchyma located on the abaxial
side of the protoxylem.

Development and Vascular Anatomy of the Flower. — Floral
development has been studied in several genera of the Solanaceae
and appears to be uniform for the family. The reports of Young
(31) for Solanum, Augustin (3) on Capsicum, and Warner (xy)
for Lycopersicum are in general agreement. The flower cluster
is terminal, although it appears lateral in the later stages of its
development owing to the growth of the adjacent axillary bud
which forms the continuation of the vegetative axis of the plant.
The primordium of the flower appears as a dome-shaped enlarge-
ment which at first is directly in line with the main axis, and its
parts develop in acropetal succession. The ensuing details of
development are similar to those described for Lycopersicum.
(Chapter XVIII.)

In the pedicel of the flower the vascular tissue constitutes a
more or less continuous region; but as it broadens to form the
receptacle of the flower, five vascular strands diverge from this
cylinder and incline outwardly to occupy a peripheral position.
These form the five principal traces of the sepals. Slightly above
this point of divergence the parenchymatous gaps formed by them
are closed and the vascular cylinder again appears to be continuous.
This is followed by the divergence of five bundles constituting
the traces to the petals that alternate with the traces to the sepals.
At this same level, divisions of the original sepal bundles result
in the formation of lateral bundles so that each sepal has from
three to five vascular strands. The same situation prevails in
connection with the bundles to the corolla, and each petal ulti-

544 THE STRUCTURE OF ECONOMIC PLANTS

mately is supplied by one large median and four lateral bundles.
Above the point of departure of the traces to the petals, five
staminal traces diverge that alternate with those of the petals
and are opposite the main traces of the sepals.

Above the level at which the stamen traces diverge, the vascular
system is more or less elliptical in transection; and, at each end
of the ellipse, a small bundle passes outward and upward, one
opposite a petal and the other opposite a stamen. These are the
abaxial carpellary bundles which extend into the style for a
distance nearly equal to its length. The main vascular tissue
again becomes continuous, closing the gaps so that the stele
appears in transection as two crescentic masses broken by traces
extending outward and upward through the lateral walls of the
carpels. At a higher level, the vascular cylinder is divided into
many small bundles, some anastomosing to form a vascular net-
work while others extend into the ovules. Thus the vascular
supply to the ovules is apparently cauline rather than foliar.

Floral Abscission. — Some varieties habitually produce more
flowers than others; but in any given variety, flower production
may be extremely variable from year to year. The formation of
flowers does not necessarily mean that development will continue
to the point where the fruits or "seed balls" are produced con-
taining viable seed, and it commonly happens that abscission of
the flower may occur at some stage prior to the maturation of the
fruit and seed. Dorsey (lo) has investigated this matter, and
reports that the abscission of the flowers bears no essential relation
to abortion of pollen or failure to receive a stimulation from polli-
nation and fertilization. He regards abscission as being due to
"physiological influences operating independently of pollen or
pistil development."

Young (i9) has described the shedding of the blossoms in some
detail and points out that the abscission occurs at a definite point
about half an inch below the base of the blossom where there is a
joint in the pedicel of the flower. It is at this point that the
abscission layer is formed. This is marked at a very early stage
by a constriction in the pedicel beneath which the cells are smaller
than in the tissues immediately above or below. These cells
have dense protoplasmic contents with large nuclei, and form
a meristematic zone that is capable of growth and cell division
after the adjacent cells have become inactive. The abscission

SOLANUM TUBEROSUM

545

layer at first extends across the stem in a transverse direction;
but, later, the plane of the layer is curved downward, and the
stem becomes swollen below the constriction owing to the multi-
plication of the cells in the cortex.

No definite line of cleavage is formed before the shedding of
the flowers, nor is a true cork developed prior to that time. As
long as the blossoms appear to develop normally, the cells of the
abscission layer remain firmly joined; but, when blasting and
yellowing of the buds or blossoms occurs, a cleavage results in
the upper portion of the abscission region. The active tissue on
the pedicel then forms a callus over the broken surface. Abscission
may take place at any stage in the development of the buds or
blossoms.

MicROSPOROGENEsis. — The sporogenous tissue is differentiated
early in the development of the anther, and may first be recognized
as a mass of small dense cells with large nuclei which occupy
each of the four lobes of the anther. The archesporium consists
of a layer about two cells in thickness which is parallel to the
surface of the anther. It is covered by the epidermis and a variable
number of layers of parietal cells. The tissue of the archesporium
extends back the entire length of the anther, appearing in cross
section as a horseshoe-shaped structure. Since the liberation of
the pollen is through terminal pores, there is no modification of the
parietal cells to aid in dehiscence. The tapetal cells are large but
as the pollen grains mature the entire tapetum and the inner parietal
layers become disorganized. The archesporial cells divide several
times and the daughter cells are more or less angular in shape.
Finally, they lose their angularity and function as microspore
mother cells, producing the microspores as a result of two successive
divisions, the first of which is heterotypic.

Rees-Leonard (19) has observed several types of degeneration
during the floral development and sporo genesis of the potato.
A common type occurs in microsporogenesis in which the sporoge-
nous tissue completely disintegrates; and in some of the ovules of
such buds, the development of the megaspore mother cell seems
to be arrested. Degeneration may occur within the ovule at any
stage in development, but is less frequent in very early ones. In
some cases, it takes place following the meiotic divisions and all
four megaspores disintegrate; while, in others, the degeneration
occurs during the development of the megagametophyte. This

546 THE STRUCTURE OF ECONOMIC PLANTS

may happen during the early stages, or following the formation
of the eight-nucleate gametophyte. In commenting upon these
occurrences, Rees-Leonard states,

"These irregularities which occur during macrosporogenesis and
the development and maturation of the macrogametophyte may
partially account for the failure of seed and fruit development in the
potato. There seems to be some correlation, however, between the
irregularities occurring during microsporogenesis and the development
of the pollen grains on the one hand, and those taking place during
macrosporogenesis and macrogametophyte development in the ovules
of the same flower on the other hand."

Megasporogenesis and the Megagametophyte. — The forma-
tion of the partially campylotropous ovules from the massive
cauline placenta has been described in connection with the develop-
ment and structure of the seed. At the time that the ovules first
become slightly inclined the archesporial cell differentiates in the
hypodermis just over the bent tip of the ovule; and, according to
Bhaduri (5), this occurs prior to the differentiation of the integu-
ment. The archesporial cell, which is larger than the adjacent
cells of the hypodermis, functions directly as a megaspore mother
cell. It enlarges and becomes somewhat vacuolate prior to the
two successive divisions which result in the formation of a linear
tetrad of megaspores, each of which is invested with a distinct
cell wall. When first formed, the megaspores are alike in size
and content; but soon the one at the chalazal end of the nucellus
begins to enlarge and there is a gradual degeneration of the other
three, beginning at the micropylar end. The three disintegrated
megaspores form a cap over the functional one which by its growth
compresses the others into a compact mass.

In the development of the megagametophyte from the functional
megaspore, the two-, four-, and eight-nucleate stages occur in
rapid succession. The synergids and megagamete are located
toward the micropyle, and the position of the antipodals and
development of the fusion nucleus follow the common program
found in most angiosperms. The megagametophyte is surrounded
by a thin nucellar layer; but, at maturity, these cells degenerate
completely so that the fully developed gametophyte lies appressed
to the innermost layer of the integument.

Variations in the development of the gametophyte have been
reported in this family. Nanetti (16) and Young (31), investigat-

SOLANUM TUBEROSUM

547

ing Solanum muricatum and Solanum tuberosum respectively,
have described the development of the gametophyte as being
similar to what was once called the "Lilium" type of development
in which all four megaspores remain functional and divide to
produce the eight-nucleate gametophyte. Studies by Bhaduri (5),
Cooper (8), and Schnarf (ii) indicate that the Solanaceae have
the usual angiospermous type of gametogenesis.

According to Bhaduri and Soueges (xi) the presence of tapetal
tissue covering the embryo sac is a characteristic feature of the
Solanaceae, and the former found that it was derived from the

Fig. 189. A-F, stages in development of embryo ; G, diagrammatic outline of young
embryo at time of differentiation of cotyledons : ca/, calyptrogen ; dgn, dermatogen ; pbm,
periblem ; pi, plerome ; sus, suspensor. (After Tognini, Isf. Bot. Univ. Pavia.')

integument in all species investigated. This tissue serves the
dual function of secretion and absorption.

Young (30) has reported three instances in which the ovule
contained two megagametophytes. In these cases, the vascular
strands supplying the ovule branched in the funiculus and a branch
was directed toward each gametophyte.

Embryogeny. — Embryogeny in the Solanaceae has been studied
by several investigators including Guignard (14), Soueges (j-y),
Tognini (15), Ferguson (13), and Bhaduri (6). Guignard reports
that there is no early, free-nucleate stage in the formation of the
endosperm and describes double fertilization in the case of
Nicotiana and Datura with endosperm formation following triple
fusion. Ferguson, while agreeing with Guignard in respect to the
absence of a free nucleate stage, has demonstrated in Petunia that
the first divisions of the fusion nucleus occur prior to fertilization.

548 THE STRUCTURE OF ECONOMIC PLANTS

Following fertilization, the zygote undergoes a series of trans-
verse divisions which result in the formation of a linear series of
cells. (Fig. x89, A.^ The basal cells of the proembryo form the
suspensor, and vertical divisions of the terminal cells result in the
development of an enlarged spherical structure which becomes the
embryo. Early in its differentiation, there is a blocking off of
definite histogens so that the initials of the dermatogen, the
periblem, and the plerome can be determined; and development
of the epidermis, cortex, and stelar structures then follows. (Fig.
i.S^, D, E, F.^ The embryo soon becomes bilobed; and subsequent
growth of these lobes leads to the formation of the two cotyledons,
while elongation and differentiation of the axis of the embryo result
in the production of the hypocotyl and radicle. (Fig. 2.S<^, G.)
During ontogeny, the endosperm develops rapidly and at maturity
it completely surrounds the embryo. (Fig. 175, A.^

LITERATURE CITED

I. Artschwager, E., "Anatomy of the potato plant, with special reference to
the ontogeny of the vascular system." Jour. Agr. Res. 14: iii-z5i, 1918.
1. , "Studies on the potato tuber." Jour. Agr. Res. 2j: 809-836, 19x4.

3. AuGUSTiN, Bela, Historisch-kritische und anatomisch-entwicklungsgeschichtliche

Untersuchung iiber den Paprika, Adolph Rosner, Nemetbogsan, 1907.

4. Berthault, p., "Recherches botaniques sur les varietes cultivees du Solanum

tuberosum et les especes sauvages de Solanum tuberiferes voisins." Ann.
Set. Agron. 28: 1-59, 87-143, 173-116, 148-191, 1911.

5. Bhaduri, p. N., "Studies on the female gametophyte in Solanaceae." Jour.

Ind. Bat. Sac. 14: 133-150, 1935.
6. , "Studies on the embryogeny of the Solanaceae. I." Bot. Ga^. 98:

183-195, 1936.
Clark, C. F., "Development of tubers in the potato." U. S. Dept. Agr. Bull.

9^8, 1911.
Cooper, D. C, "Macrosporogenesis and the development of the macrogame-

tophyte of Lycopersicon esculentum." Am. Jour. Bot. 18: 739-748, 193 1.
Crafts, A. S., "Sieve-tube structure and translocation in the potato." Planf

Phys. 8: 81-104, 1933.
Dorsey, M. J., "A note on the dropping of flowers in the potato." Jour.

Heredity 10: 116-118, 1919.
East, E. M., "Some essential points in potato breeding." Conn. Agr. Exp.

Sta., Ann. Rep., 419-447, 1907-1908.
Esmarch-Bromberg, F., "Beitrage zur Anatomie der gesunden und kranken

Kartoffelpflanze." Landw. Jahrh. ^4: i6i-i66, 1919.
Ferguson, M. C, "A cytological and a genetical study of Petunia — I."

Bull. Torr. Bot. Club ^4: 657-664, 1917.

ID

II

II

SOLANUM TUBEROSUM 549

14. GuiGNARD, L., "La double fecondation chez les Solanees." Jour, de Bot. 16:

145-167, 1901.

15. MiJLLER, K. O., "Ein Beitrag zue Bliitenbiologie der Kartoffel." Angew.

Bot. /.• 146-153, 192.3.

16. Nanetti, a., "Sulla probabili cause della partenocarpia del Solanum murica-

tum. Ait." Nuova giorti. hot. Ital. N. S. ig: 91-111, 1911.

17. Priestley, J. H., and Swingle, C. F., "Vegetative propagation from the

standpoint of plant anatomy." U . S. Dept. Agr. Tech. Bull, i^i, 19x9.

18. Reed, T., "On the anatomy of some tubers." Ann. Bot. 24: 537-548, 1910.

19. Rees-Leonard, Olive L., "Macrosporogenesis and development of the macro-

gametophyte of Solanum tuberosum." Bot. GaZ- 96: 734-750, 1935.
lo. Rosa, J. T., "Relation of tuber maturity and of storage factors to potato

dormancy." Hilgardia ^: 99-1x4, 192.8.
XI. ScHNARF, K., Vergleichende Embryologie der Angwspermen, Gebriider Borntraeger,

Berlin, 193 1.
iz. SouEGEs, R., "Developpement et structure du tegument seminal chez les

Solanacees." Ann. Sci. Nat., Bot. Ser. IX, 6: 1-1x4, 1907-
X3. , "Recherches sur I'embryogenie des Solanacees." Bull. Sac. Bot.

France 6(): 555-585, 19XX.
X4. Thiel, A. F., "Vascularanatomy of the transition region of certain Solanaceous

plants." Bot. Gaz- 94: 598-604, 1933.
X5. ToGNiNi, F., "Suir embriogenia di alcune Solanacee da appunti lasciati."

Attj. 1st. Bot. Pavia, Ser. II, 6: 109-ixx, 1900.
x6. DE Vries, H., "Beitrage zur speciellen Physiologic landwirtschaftlicher

Kulturpflanzen. V. Wachstumsgeschichteder KartofFelpflanze." Landtv.

Jahrb. j: 591-68X, 1878.
X7. Warner, E. E., "The floral development and vascular anatomy of the flower

of Lycopersicon esculentum var. cerasiforme." Unpubl. M.S. Thesis,

Univ. of Chicago, 1933.
x8. Weaver, J. E., Root Development of Field Crops, McGraw-Hill Book Co., 19x6.
X9. Young, W. J., "Some phases of breeding work and seed production of Irish

potatoes." So. Car. Agr. Exp. Sta. Bull. 210, 191X.
30. , "Potato ovules with two embryo sacs." Am. Jour. Bot. g: X13-X14,

I9XX.
31. , "The formation and degeneration of germ cells in the potato." Am.

Jour. Bot. 10: 3x5-335, 19x3.

CHAPTER XVIII

SOLAN ACEAE — Continued

LYCOPERSICUM ESCULENTUM

THE tomato is a native of South and Central America and prob-
ably had been cultivated there by the native population long
before its introduction into southern Europe. It attained great
favor there and w^as introduced into the United States in the latter
part of the eighteenth century. At present, it is grown exten-
sively throughout the United States for home, market-garden, and
canning purposes. In addition to its commercial uses, the tomato
plant might be regarded as the "guinea pig" of the plant kingdom,
since it has been widely used for experimental purposes in problems
of plant physiology, pathology, and genetics, owing to the ease
with which it may be propagated, and its hardiness and adapt-
ability to a wide range of environmental conditions.

Bailey (x) recognizes two species, Lycopersicum esculentum
Mill., and L. pimpinellifolium Dunal; but the latter, known as
the currant tomato, is not widely used It has a spreading habit,
small leaves, flowers arranged in long racemes, and a small two-
loculed berry about ,?^ to 3^^ inch in diameter. Five botanical
varieties of L. esculentum are listed by Bailey (3), and there is a
large number of commercial varieties. Boswell and others (4), in
cooperation with members of the seed trade, canners, and market-
gardeners, have described the nine most widely used American
varieties in an attempt to eliminate some of the confusion that
exists with respect to their characteristics. They point out that
the vegetative habit is so variable and susceptible to modification
on the basis of response to environmental conditions that it is
difficult to give an exact description of any of the leading varieties
in terms of actual measurements. For this reason, the following
account is a generalized one designed to present the characters
that are important in connection with structural and anatomical
studies rather than to describe the specific characteristics of any
commercial type.

550

LYCOPERSICUM ESCULENTUM 551

GENERAL MORPHOLOGY

The tomato plant is grown in the United States as an annual,
and the length of time required for maturation of the fruit, after
transplanting into the field, ranges from 55 to 65 days for rapidly
maturing varieties, and from 85 to 100 days for the slowly growing
canning types grown in northern and central California. The
latter may not attain the peak of the harvest until 100 to 1x5 days
after transplanting. The variation in the size of the plant is
equally striking. In general, early varieties are small in stature
(16 to 18 inches in height) with a maximum spread of 4 to 4^^ feet,
while the slowly maturing forms are much larger, attaining a
height of 14 to 2.S inches and a maximum spread of 6 or 7 feet. These
figures refer to plants which have been grown without pruning.

The Root. — The character of the root system is greatly modi-
fied by cultural practices. When grown from seed, without
transplanting, a strong tap root is formed which, according to
Weaver and Bruner (36), may grow at a rate of an inch a day attain-
ing a depth of nearly 1. feet. In actual practice, the plants are
started in seed beds and transplanted once or twice before being set
in the field so that the primary root is injured and a dense fibrous
system of lateral and adventitious roots is formed. After the final
transplantation to the field, the adventitious roots develop rapidly
and, in mature plants, may have a lateral spread of 5 feet, with
numerous large branches. These occasionally penetrate 3 to 5
feet, although the major portion of the root system occupies the
first 8 or 10 inches of soil.

The Shoot. — The growth habit varies from the spreading vine
type through forms that are semi-erect to erect. (Fig. 190.) In
decumbent forms, there are relatively few short branches which
attain a length of 2. to i3^ feet and a diameter of approximately
an inch at the base. In large erect types, the main stems are very
thick, reaching a diameter of an inch and half, and the branches,
which range from 10 to 17 in number, may be 6 feet in length. The
branching is usually sympodial so that the successive principal axes
are developed from axillary buds and the terminal buds produce
inflorescences or abortive branches. (Fig. i9i.) Each axillary
bud continues the vegetative shoot, producing leaves at several
nodes, and then terminates in an inflorescence. This mode of
development may be repeated many times until the axis is several

552. THE STRUCTURE OF ECONOMIC PLANTS

feet long, bearing inflorescences and fruits throughout its length.
At the base of the plant and where no inflorescences are developed,
the branching may be monopodial. A case of true dichotomy of

V

Flu. i^o. The tomato plant in fruit, variety Marglobe. (Courtesy of the Ferry-Morse

Seed Co.)

the vegetative axis has been reported by Caldwell (6) but this is
not common except in the dichotomous branching of some inflores-
cences.

Adventitious roots develop readily from the stems, and for this
reason propagation by means of cuttings is sometimes practiced in

LYCOPERSICUM ESCULENTUM

553

greenhouse culture. This is especially helpful in experimental
work of a genetic character, such as that carried on by Lindstrom
and Koos (ii), where it is essential to perpetuate the plant without
modification of the genetic pattern. They were able to propagate
for five years without mutation or change a haploid (ii chromo-
somes) tomato plant which arose spontaneously in an Fo generation.
The arrangement of the leaves is usually alternate with a Jr^
phyllotaxy. They are odd-pinnately compound, the number of

Fig. 191. The plant showing sympodial branching.

larger leaflets ranging from seven to nine, rarely eleven; and,
between the adjacent pairs of major leaflets, a variable number of
smaller ones occur. In the smaller varieties, there may be from
four to six intermediate leaflets, and as many as nine to eleven occur
in the larger, more vigorously growing types. The margins of the
leaflets are variously incised or lobed except in Lycopersicum escu-
lentum, var. grandifolium, in which they are entire. Capitate
glandular hairs and large pointed non-glandular ones occur as on
the stem. (Fig. 191, E.)

The short petioles are relatively constant in length, ranging
from i}<t to 2.^2 inches, and the compound blade is approximately as

554 THE STRUCTURE OF ECONOMIC PLANTS

wide as long. The length in the early varieties ranges from 63>^ to
83^ inches, while in the larger types it may be slightly in excess
of li inches. In general, the leaf characters alone are of little
diagnostic value in separating the commercial varieties, as the
leaves exhibit great differences owing to variation in climate, soil
conditions, and cultural methods.

The Inflorescence. — The inflorescence is usually a two-
ranked raceme or a branched racemose-cyme, the former type

Fig. 2.92.. A, inflorescence showing unequal development of fruits, one flower has abscissed ;
B, flower with portion of perianth removed showing relation of stamens and pistil ; C, floral
diagram of hexamerous flower of Stone variety ; D, longisection of pistil with cauline pla-
centation; E, two types of hairs commonly found on calyx, pedicel, and vegetative parts.

occurring only in the currant tomato. (Fig. i9x, A.^ Bouquet
(5) made a study of the inflorescences of several varieties in order to
determine their characteristics and the fruiting habit in each in-
stance under greenhouse conditions. He described three types of
inflorescence, which sometimes occur on the same plant : (i) simple
raceme; (x) dichotomous or two-forked flower clusters; and
(3) the polychotomous inflorescence having more than two forks
or branches. The simple type occurs more frequently on the lower
portion of the plant, and the branched types are more common on
the upper parts.

LYCOPERSICUM ESCULENTUM 555

The number of flowers comprising an inflorescence may vary
from four to twelve or more. Bos well and his coworkers (4) found
that the more important commercial varieties bear an average of
four to five flowers per cluster; and that, under conditions where
the plants are not pruned or staked, from two to four flowers set
fruit. Ordinarily, it is unusual for more than two flowers of an
inflorescence to be open at one time; and, because of this pro-
gressive development, a single raceme may have small fruits, open
flowers, and buds. Under greenhouse conditions, some unproduc-
tive flowers occur. These may be classified into three categories:
(i) those producing small fruits that ripen prematurely before
attaining a marketable size; Ql) those having a persistent calyx,
but in which the fruit does not develop; and (3) flowers that
absciss.

Floral Abscission. — Kendall (17) has described the histology
of the pedicel of the flower and fruit with reference to the develop-
ment of the separation zone, the process of abscission, the time
involved, and experimental methods of inducing abscission. The
zone of separation in Lycopersicum is located at a mid-point in the
pedicel in contrast to Nicotiana where it occurs at the base. It is
externally visible because of enlargement at that point and owing
to the development of a circumferential groove which is fully half
the depth of the cortex. (Fig. 1.^1., A, and Fig. ^^t,, A.')

The vascular system of the pedicel forms a cylinder of separate
bundles which are surrounded by parenchyma. Outside the funda-
mental parenchyma, there is a layer of collenchymatous cells in
that portion of the pedicel which is proximal to the groove, but
this does not develop in the portion distal to it. Between the
collenchyma and the epidermis there is a zone of chlorenchyma.
(Fig. 193, B.) The separation layer consists of from three to six
tiers of cells which are located in the plane of the groove. In
buds in which the calyx is less than 1. mm. long, the groove is not
detectable, but it begins to appear when the corolla is about i mm.
in length, and is well developed in young buds in which the corolla
does not exceed 3 mm. The cells of the separation zone are smaller
than those on either side of it, and this difference in size becomes
more noticeable as the flowers mature, since the cells of this region
do not enlarge as the adjacent cortical cells increase in size. There
are apparently few cell divisions in the separation zone, but the
cells do retain their meristematic potentialities. The mechanical

556

THE STRUCTURE OF ECONOMIC PLANTS

tissues of the pedicel are not continuous through the separation
layer as in Nicotiana so that there is a break in a plane parallel
to the bottom of the groove. It is possible, therefore, for abscis-
sion to cause the fall of the fruit at any stage in its development,

but this rarely occurs ex-
cept at about two or three
days after anthesis.

On the basis of his in-
vestigation, Kendall agrees
with Kubart (lo) that the
separation layer represents
"merely a portion of the
primary meristem which
retains its original physio-
logical capacities." Dur-
ing the development of the
fruit, there is a very definite
increase in the size of the
pedicel which is restricted
almost entirely to the por-
tion distal to the groove;
and its diameter in the
fruit may be two or three
times greater than at the
time of the anthesis of the
flower. This results from
r- ... If the enlargement and divi-

FiG. 193. A, diagrammatic longisection or young o

flower just prior to anthesis, showing development sion of the Cells of the COt-
of cauline ovules and location of abscission layer; ^^^ ^^^ ^y^^ diameter of
B, diagrammatic longisection or portion of pedicel
in which abscission zone is located ; C, transection the
of ovary of bicarpellate pistil showing fleshy central
placentae which have grown around and embedded
developing ovules: a, abscission zone; b, vascular
strands; c, epidermis ; d, separation layer; ?, pith; separation and isolation of
/, chlorenchyma; e, collenchyma; ped, pedicel. , 11 • 1

(B, redrawn after Kendall, Univ of Calif Pub ) ^^e Cells in the Separation

zone. Frequently, the
process is initiated in the pith as well as the cortex, and proceeds
independently in the two regions until the final break occurs
through the vascular tissue and the epidermis. The separation
begins in two tiers of cells at the bottom of the groove, but may
later affect three or four layers. It takes place in such a way

pith remains practi-
cally constant.

Abscission involves the

LYCOPERSICUM ESCULENTUM 557

that the separation surface of the detached portion of the pedicel
may be convex, while that of the remaining part of the pedicel is
slightly concave.

The separation of the adjacent cells results from a dissolution of
the middle lamellae, and on the basis of Kendall's experiments, it
seems probable that turgor is not an initiating factor, but serves
mechanically to hasten the process. It appears from this work
that the middle lamellae of the cell walls in the abscission zone are
more easily hydrolyzed than are those in the more distal portions,
and the agent of hydrolysis is thought to be an enzyme. No cell
divisions or elongations were observed accompanying the process
of abscission.

In addition to this type of true abscission, individual plants may
lose their flowers by a process which involves no active cell separa-
tion. In such cases, the flower merely wilts and dries back to the
abscission groove, where it may hang until broken off by some
mechanical means. When this type of abscission occurs, it is first
indicated by a loss of chlorophyll in the pedicel, beginning at the
tip and extending to the separation zone. In many instances, both
types of abscission may occur in the same plant.

The Flower. — The pendent yellow flowers are perfect, hypog-
ynous, and regular; but in cultivated varieties, the number of
floral parts in each cycle is variable. The persistent calyx consists
of a very short tube terminated by five to ten lobes. The rotate
corolla also has a short tube which is expanded at the top into five
or more lobes that are at first greenish-yellow, becoming a brilliant
yellow later when the flower is fully developed and the lobes
reflexed. The five or more stamens are partially adnate or undi-
verged from the calyx tube. The filaments are short and the
connivant anthers are united laterally to form a hollow cone
around the pistil. The pistil consists of two to several carpels
and its elongated style and smooth, flattened stigma extend through
and somewhat beyond the encircling androecium. (Fig. 191, B.)

The solanaceous plants are usually pentamerous and the basic
floral plan is 5-5 -5-i. Warner (35) has described the ontogeny of
a flower of this type, while Cooper (7} and Smith Qirf) have pub-
lished accounts of the development of the hexamerous form which
is the most common floral plan in many commercial varieties.
(Fig. 191, C) In fact. Cooper found only one pentamerous blossom
in more than a thousand flowers examined. In addition to L. escu-

558 THE STRUCTURE OF ECONOMIC PLANTS

lentum, var. cerasiforme, the type investigated by Warner, the form
L. esculentum, var. pyriforme, is also pentamerous with a bilocular
ovary.

ANATOMY

Ontogeny of the Flower. — The floral primordium first
appears as a dome-shaped enlargement which is directly continuous
with the main axis, and its parts develop in acropetal succession.

Fig. Z94. A-I, stages in floral development : B, stage showing origin of sepal primordia ;
C, more advanced stage showing primordia of sepals and petals ; D, origin of stamen pri-
mordia ; E, origin of carpel primordia ; F, G, and H, longisections of flower buds showing
advanced stages in development of stem axis in relation to carpels ; I, longisection showing
united carpels and development of style : ax, stem axis ; car, carpel primordium ; ov, ovule ;
pet, petal primordium ; se, sepal primordium ; sfa, stamen primordium ; sty, style. (Redrawn
after Warner.)

(Fig. X94, A.^ Peripheral enlargement and rapid cell division of
this protuberance result in the formation of a somewhat raised
blunt ridge at the margin of the primordium. This is followed by
accelerated growth on the ridge at five or more equidistant points,
and the resultant outgrowths constitute the sepal primordia.
(Fig. 194, JB.) These arise in a clockwise direction; and by the
time the cycle is complete, the first sepal lobe is somewhat
advanced. The growth of the sepal primordia is more rapid on
their abaxial sides so that the free tips incline toward each other
as they elongate and partially enclose the cavity beneath them.

LYCOPERSICUM ESCULENTUM 559

After this overarching has been accomplished, there is little further
individual growth; but the entire basal portion of the calyx ring
grows uniformly to form a short cylindrical calyx tube. Each
sepal has three to five bundles with fine branches that extend
throughout all parts of the lobe. The structure of the calyx lobe
is leaf-like and the mesophyll consists of about fifteen layers of
spongy parenchyma. The epidermis is covered by a thin cuticle;
and numerous stomata occur on the abaxial surface, especially on
the tube and at the base of each lobe. The guard cells on the
abaxial surface protrude somewhat above the other epidermal cells,
while the smaller ones on the adaxial surface are not raised. The
calyx is pubescent, producing short, unicellular hairs and longer
multicellular ones with a group of basal cells. The glandular hairs
are stalked and terminated by a group of four or more cells. (Fig.
i9i, £.) Short-stalked hairs and papillae occur on the adaxial
surface of the calyx lobes.

Following the differentiation of the sepals, the continuous
circular ridge of the corolla develops from the growing point of the
flower primordium centrad to the sepals. On this, the primordia of
the petal lobes arise at points alternate with those of the calyx, and
their growth results in the formation of the five or more distinct
triangular teeth of the gamopetalous corolla. (Fig. r^i., C)
Just prior to anthesis, the corolla grows rapidly and pushes the
calyx open. (Fig. x^t,, A.^ The epidermis of the corolla is
composed of thin-walled cells; and on the adaxial surface near the
apex of the lobes, many of them protrude as papillae. Numerous
glandular hairs with short stalks occur on the abaxial surface.
Stomata are lacking. The corolla lobes are about eight to ten
layers in thickness and yellow chromoplasts occur in the spongy
mesophyll.

The stamen primordia arise as the corolla elongates and its
undiverged base becomes tubular. They are alternate with the
lobes of the petals, and are diverged near the base of the ridge from
which the petal primordia are developed. (Fig. 194, D.) This
is followed by the growth of the zone basal to the stamens and
petals, so that the lower portions of the filaments appear to be
undiverged from the tube of the corolla. (Fig. X93, A.') The
terminal portion of each stamen primordium develops inde-
pendently and at first they are distinct; but, later in ontogeny, the
anthers may be somewhat closely appressed laterally. The anther

560 THE STRUCTURE OF ECONOMIC PLANTS

is bilobed, the lobes being separated by connective tissue; and
each one develops two microsporangia which extend its full length.
The abaxial surface of the anther is papillate, while the adaxial one
has few hairs. No stomata are present.

In the bicarpellate types, the carpellary primordia arise soon
after the differentiation of the stamens. They appear centrad to the
joint petal-stamen zone, and are so arranged that one carpel is
opposite a stamen, the other being alternate with two stamens.
(Fig. X94, E, F.) In hexamerous forms, the six-carpellary pri-
mordia are alternate with the stamens. (Fig. ^^i, C) The early
development of the two carpels results in the formation of conical
hood-like structures whose concave faces oppose each other.
Within the carpel primordia, there remains a definite portion of
the axis which consists of a more or less flat or concave disk. This
part of the axis begins to elongate, and enlarges to form a conical
structure. (Fig. 194, G.) Later, growth is initiated at the base
of the elongating cone; and two septa develop involving a portion
of the axis and forming two locules. At this time, each carpel is
open at the top; and its cavity is a pit bordered by the elongated
central portion of the axis, the ridge-like septa, and the curved
walls of the carpel. (Fig. 1.^^, H.) Continued growth of the
carpels results in the tip of each being inclined toward the central
portion of the axis, and finally they become so closely appressed
to the elongated column of the axis that the two structures are no
longer recognizable as distinct from each other. Further elonga-
tion of the terminal portions of the carpels results in the formation
of a long narrow style. Continued enlargement and a bowing out
of the basal wall of each carpel forms two locules in which the
central axis develops as a columnar structure from which the
ovules arise. (Fig. 1.^1., D, and Fig. 194, Z.)

Although the placental structure is at this stage continuous
with the carpellary tissue, it is largely cauline in origin; and the
ovules may be regarded as arising from cauline, axile placentae
rather than from foliar ones. (Fig. 193, C) The epidermal cells
of the ovary as well as of the basal portion of the style are papillate;
and multicellular, glandular, and non-glandular hairs are also
produced. Cooper (7) observed a few large stomata in the stylar
epidermis, but found none in the epidermis of the ovary, agreeing
in this regard with Groth (n), Rosenbaum and Sando (i6), and
Gardner (10). Both Gardner and Cooper reported stomata on the

LYCOPERSICUM ESCULENTUM 561

pedicel, receptacle, calyx, and style; and Makemson (14) found
stomata and lenticels on the ovary.

MicROSPOROGENESis. — According to Smith (17) sporogenous
tissue develops early in the anthers, and a row of hypodermal
archesporial cells are differentiated in very young buds. Inside
the archesporial cells, a single row of primary sporogenous cells
is formed and these continue to divide. The outside row becomes
the outer tapetum; while, early in development, the vegetative
cells bordering the inner margin of the sporogenous tissue form
the inner tapetum. The tapetal cells are usually binucleate,
separating later as they undergo various form changes. In the
tomato, it appears to be typical that metaphases, anaphases, and
telophases of the second reduction division may occur simultane-
ously in a single locule. No wall formation takes place until both
nuclear divisions are completed. The microgametophytes are
binucleate, containing a generative and tube nucleus; and, follow-
ing pollination, the generative nucleus divides before the pollen
tube reaches the megagam^etophyte.

Pollination. — Dehiscence of the stamens begins X4 to 48 hours
after the opening of the corolla. Each locule contains several
hundred pollen grains, and the anthers split longitudinally in an
introrse manner so that the pollen may fall directly on the stigmatic
surface of the pistil. Namakawa (x5) has investigated dehiscence
in several solanaceous plants including tomato; and, although
there is some variation in detail, the general mechanism is rela-
tively constant. The epidermis at the point of abscission is
underlaid by a special disjunctive tissue that is from one to seven
layers in thickness. Anthesis may occur in one of three ways:
by the solution of the middle lamella of the epidermal cells along
the line of dehiscence; by the solution of the entire wall of such
epidermal cells; or by mechanical rupture of the epidermis due
to hygroscopic action of a fibrous layer of cells in the anther
wall.

The type of dehiscence, the pendent position of the flowers, and
the fact that the stigmas are receptive to pollen one or two days
before the anthers split, usually result in self-pollination. Some
crossing does occur, however, and as much as a 5 per cent cross-
pollination has been recorded in certain long-styled varieties in
which the style and stigma project beyond the staminal cone.
Where cross-pollination does occur, it is carried on through the

562. THE STRUCTURE OF ECONOMIC PLANTS

agency of insects, the bumblebee, according to Fink (9), being the
most common visitant.

The growth of the pollen tube is relatively slow even at optimum
temperatures. According to Smith and Cochran (19), these are
70° to 85° F. for the best germination and 70° for the maximum
growth of the pollen tube. Under favorable conditions, no case
was found by them where fertilization could be observed in less
than 50 hours after pollination. Germination and growth were
very poor on either side of the optimum temperatures at 50° and
100° F. respectively.

Megasporogenesis. — Many ovules are developed on the massive
fleshy cauline placentae, and they completely cover the free surfaces
at about the time that the microspore mother cells undergo the
first reduction division in the anthers. The ovules develop as
erect globular protuberances on the placental surface; but the
growth of the ovule is not uniform, and it becomes anatropous as
a result of a more rapid cell division on one side of the primordium.
(Fig. X93, A.^ It develops a single, massive integument that
completely encloses the nucellus, except for the micropylar opening,
by the time the four megaspores are formed. The cells divide in
all planes so that the integument increases in thickness as well as
in length. A single vascular strand is differentiated from the
placental bundles which passes through the funiculus terminating
in the chalazal region of the ovule.

According to Cooper (8), the hypodermal archesporial cell does
not divide to form a primary parietal and primary sporogenous
cell; but functions directly as the megaspore mother cell. By the
time young microspores have developed in the anther sacs, the
megaspore mother cell is undergoing its first or heterotypic divi-
sion; and, as a result of reduction divisions, a linear row of four
megaspores is formed. Of these, the megaspore at the chalazal
end forms the megagametophyte and the other three disintegrate.
The growth of the megagametophyte is rapid, and three suc-
cessive nuclear divisions result in the formation of an eight-
nucleate gametophyte, after which the two polar nuclei fuse so
that the mature megagametophyte is seven-nucleate. The two
synergids and the megagamete are located at the micropylar
end of the gametophyte, while the three more or less triangular
antipodal cells lie at the chalazal end and have a tendency to dis-
integrate rather early. As the nucellar tissue is resorbed, the

LYCOPERSICUM ESCULENTUM

563

inner cells of the integument elongate perpendicularly to the sur-
face of the gametophyte, forming a nutritive layer that has been
inaccurately designated as a "tapetum."

Embryogeny. — Soueges (31, 3i) and Tognini (34) have given
detailed accounts of the embryogeny of several species of Solana-
ceae, and Lycopersicum has been investigated recently by Smith
(yf). Following fertilization, the zygote divides transversely to
form a two-celled embryo; and transverse divisions of these two

Fig. 2.95. A, fertilized egg or zygote 54 hours after pollination; B, two-celled embryo;
C, four-celled embryo, no hours after pollination ; D, eight-nucleate embryo, 114 hours after
pollination; £, embryo in which dermatogen cells are cut off in tiers a and h, 130 hours
after pollination; F, embryo with dermatogen, periblem, and plerome differentiated, 2.2.4
hours after pollination (letters a, b, c, and d refer to four tiers or regions of development in
embryo, described in text); G, portion of longisection of ovule showing development of
embryo, endosperm, and inner layer of integument, same age as f ; H, longisection of basal
end of embryo when fruit is about 50 mm. in diameter, showing development of histogens :
dgn, dermatogen; emb, embryo; end, endosperm; /' int, inner layer of integument; pbtn,
periblem; pbm in, periblem initials; pi, plerome; r c, root cap. (Redrawn after Smith,
Cornell Agr. Exp. Sta. Mem?)

cells, a and b, result in the formation of a four-celled embryo with
its cells in linear arrangement. (Fig. 195, A, B, C) Smith found
two-celled embryos about 44 hours after fertilization, and the
four-celled stage after no hours. From this linear series of four
cells, the ontogeny of the embryo follows a regular and uniform
sequence in most of the Solanaceae. The apical cell, a, gives rise
to the cotyledons; cell b produces the hypocotyl and primary
root; the root cap and part of the suspensor come from cell c, and

564 THE STRUCTURE OF ECONOMIC PLANTS

the remainder of the suspensor from cell d. Thus, following the
formation of the linear series of cells of the four-celled embryo,
the cells a, b, c, and d divide to form tiers of cells from which the
parts of the embryo are differentiated as noted above. (Fig. X95,
D, E, F.) Smith observed ii-celled embryos after 130 hours, and
the development of the histogens (dermatogen, periblem, and
plerome) occurred after 114 hours. The formation of the coty-
ledons takes place in about 18 days and the primary root shows a
definite histogenic organization at that time. (Fig. 7.^^, H.)

The development of the endosperm precedes that of the embryo,
and the division of the primary endosperm nucleus begins in
advance of that of the zygote so that there may be an endosperm
of 8 or more cells before the two-celled embryo is formed. Between
94 and 130 hours after pollination, there are definite walls formed
in the endosperm; and at the end of 190 hours, it completely fills
the embryo sac surrounding the young embryo. (Fig. X95,G.) In
some cases, the adaxial faces of the cotyledons are closely appressed,
forming a single coil (Fig. 198, A), while in others they are curved
away from each other. (Fig. i^-y, A.^

The Fruit. — The fruit is a large berry which is globular or
oblate in the principal commercial varieties, but may be elongated
or pear-shaped in some of the special types. The very small cherry
tomatoes sometimes weigh but a fraction of an ounce, while the
table and market varieties weigh from 4!-^ to 6 or 7 ounces, and
some of the largest canning types, especially the Santa Clara, may
attain a weight of 9 to ix ounces. (Fig. X96.) As seen in transec-
tion, the fruit has from two to twenty or twenty-five locules, the
smaller numbers occurring in small-fruited forms and in the wild
types from which the cultivated varieties have been derived. The
principal commercial varieties have five to nine locules, except for
the large canning types which have the maximum number indi-
cated.

The quality of the fruit is dependent upon color, flavor, shape;
and, structurally, upon the relative amount of outer and inner
wall tissue. MacGillivray and Ford (13) divided the tomato into
five fractions (outer and inner wall, inner locule tissue, gelatinous
pulp, skin, and seed) and computed the percentage composition of
each in order to determine their relation to quality. On this
basis, they concluded that "the outer and inner wall region of a
tomato fruit is the most valuable in producing tomatoes of high

LYCOPERSICUM ESCULENTUM

565

quality" because it contains the highest percentage of dry material,
the highest percentage of insoluble solids, and possesses the
sweetest taste owing to its high percentage of reducing sugar.

The time elapsing from the setting of the fruit until its maturity
depends upon the variety as well as cultural and climatic condi-
tions; but from seven to nine weeks may be regarded as an average
period. Growth studies by Gustafson (12.) for two varieties
indicate that the growth curve for the fruit is very much like that
found in the vegetative parts; that is, "there is a period of slow
growth, gradually becoming more rapid until a point of maximum
growth is reached, and from that time there is a slowing up." The

Fig. 196. Fruits of Santa Clara Canner variety. (Courtesy of the Ferry-Morse Seed Co.)

moisture content increases throughout the growing period,
especially as the fruit passes from the green mature stage to the
ripe condition; but increase in the size of the fruit occurs without
an increase in the per cent of dry matter. The per cent of dry
weight is highest in the immature ovary, decreases rapidly during
the first two weeks of growth, and at no time is there an increase.
In one variety, Gustafson found the young ovary had 17 per cent
dry matter, 6.7. when two weeks old, and 5 per cent when mature,
six weeks later. MacDougal Q2.1.) reported a similar situation,
although the differences were not as great.

Pigmentation of the Fruit. — The color of the mature fruit
results from the presence of carotenoid pigments, lycopersicin
(lycopene) and carotin (carotene). Their distribution in the
tissues of the pericarp and the proportionate amounts of each

566 THE STRUCTURE OF ECONOMIC PLANTS

pigment determine the variation in the shade and intensity of
color.

There are differences of opinion in regard to pigmentation in
tomato fruits and two viewpoints are here reported. Jones and
Rosa (i6) state that

"In lemon-yellow fruit, carotin occurs in the pericarp, but the
epidermis is colorless; in orange-colored fruit, carotin occurs both in
pericarp and epidermis; in pink fruit, lycopersicin occurs in the peri-
carp, but the epidermis is colorless; in red fruit, lycopersicin occurs
in the pericarp and carotin in the epidermis and probably in the pericarp
as well. Albino, or white, fruit lacks all pigments."

Smith (x8) sums up the matter as follows:

"It has long been advocated that there are yellow and transparent
tomato skins which, combined with yellow or red fruit color, give
rise to light yellow, dark yellow, pink, and orange-red tomatoes.
These transparent skins have often been called colorless. This is erro-
neous. All skins of commercial red and pink tomato fruits are colored.
The cell walls of yellow skins are yellow or golden, the color being
due to an unidentified pigment. The cell walls of pink skins lack this
pigment and therefore are colorless. The color of the pink skin is
due to the presence of lycopene crystals, and often carotene also, as
a part of the cell contents. Light intensity and quality appear to have
an effect on the pigment content of the skins of tomato fruits by altering
the quantity of any or all of the three pigments which constitute
tomato-skin color."

In studies of fruit pigments, Howard (15) points out that
Willstatter and Escher (37) have demonstrated that lycopersicin
is identical in general composition and molecular weight with
carotin. Her work on tomato indicates that lycopersicin is the
principal pigment and that carotin is present in small amounts.
The lycopersicin crystals which occur in the ripe fruits are brownish
rose to carmen red and are usually in the form of needles or elon-
gated prisms. In some instances, crystal aggregates are formed,
while in others the crystals appear as long fine hair-like structures.

On the basis of Smith's investigations it appears that protection
of the fruit from intense light favors lycopene formation; but that

"the carotenoid content of both the skin and the flesh of fruits ripened
in the light is higher than in fruits ripened in the dark when the early
growth has occurred under the same light conditions. To attain
maximum carotenoid content, the fruits should be grown in complete
exposure to light and allowed to mature on the vine."

LYCOPERSICUM ESCULENTUM 567

There appears to be no direct relation between chlorophyll forma-
tion or decomposition and the lycopene content of the fruit, as
Smith was able to obtain lycopene from mature fruit which had
been grown in complete darkness. Furthermore, he found no
evidence of lycopene in plastids or of crystals formed from plastids
as a result of plastid decomposition; and he was able to demon-
strate that there are numerous lycopene crystals in the epidermal
cells which are plastid free. Carotene was found in granular form
inside plastids as well as after their decomposition, and it also
occurs in crystalline and globular forms.

Structure and Development of Fruit. — Structurally, the
fruit consists of the pericarp, placental tissue, and the seeds. The
skin of the pericarp has been studied by Groth (11). In all types,
it consists of an epidermal layer within which are three, occa-
sionally four, well-defined layers of collenchymatous tissue. The
epidermis is covered by a relatively thin cuticle; and the polyhedral
cells never have sinuous outlines as do those of the leaf epidermis.
The number of epidermal cells does not increase greatly with
growth of the fruit; and, in consequence, the epidermal cells in
mature fruit are much larger than those found in young ones.
Except in currant tomatoes, the epidermis develops hairs and
glands that are shed as the fruit matures. The hairs are un-
branched and consist of three to five cells, while the glands have a
unicellular basal stalk and a top of two to four cells. Rosenbaum
and Sando (i6) observed that the cuticular layer increased in
thickness as the fruit aged, and also noted a complete absence of
stomata in the epidermis of both young and old fruits.

The major portion of the pericarp is composed of large thin-
walled cells with numerous intercellular spaces; and, at the time
of differentiation of the megaspore mother cell, it is about 8 cells
in thickness. According to Smith and Cochran (x9), it continues
to thicken, being 14 layers in thickness 60 hours after pollination,
and xo layers thick 34 hours later. Two weeks after pollination,
when the fruit is about ix mm. in diameter, the pericarp is i8 to
30 cells thick. The cells enlarge enormously during development,
and this is more significant in the growth of the fruit than the
increase in number of cells. As the fruit matures, some of the
cells of the inner and central portion of the carpels may partially
disintegrate.

As the ovules develop, there is an outward growth of the paren-

568 THE STRUCTURE OF ECONOMIC PLANTS

chymatous cells of the placenta which surround their bases. This
can be observed within 60 hours after pollination, and the paren-
chyma increases until it completely encloses the developing seeds
in a homogeneous tissue of thin-walled cells. (Fig. 133, C.) The
cells do not unite with the carpellary walls, but press against them
and the surfaces of the seeds. At first the tissue is firm and com-
pact; but, as the fruit matures, the walls become thinner and the
cells partially collapse. Large numbers of round starch grains
are included in the gelatinous contents.

Parthenocarpy. — Parthenocarpy is not uncommon, but the
fruits produced are usually small or of poor quality. Hawthorn
(14) has reported a case of seedlessness in Texas in a cross between
a Large Cherry variety and Bonnie Best. In this instance, the
fruits contained seeds in June and usually in July; but were seedless
in midsummer, and again bore seed-containing fruits in November.
During the seedless period, the plants bore as profusely as at other
times and produced fruits of fine quality.

It is also possible to produce seedless tomatoes experimentally
by treating unpollinated flowers with various organic acids.
Gustafson (^3) applied organic acids in lanolin to the cut surfaces
of the styles of flowers from which the stamens had been removed.
Varying results were obtained with different acids, but mature
fruits with perfectly normal external appearance were developed
in several cases. In the larger fruits, there were occasionally well-
developed locules, but the smaller ones did not form them, and in
no instance were seeds produced. In general, the pericarp was
thinner and the fruits more fleshy, but the water content was
approximately the same as in those produced following pollination.

The Seed. — The mature seeds are oval in outline and very
much flattened laterally. (Fig. 197, A.^ They vary considerably
in size; and in a random sample of commercial seed averaged
3 to 5 mm. in length and i to 4 mm. in breadth. The surface of
the buff to straw-colored seed coat is covered with gray or silver
hairs and scales which are the remains of the lateral walls of the
outermost cell layer of the integument.

Soueges (30) has described the development of the integument
in the tomato and other solanaceous plants, including the manner
in which the hairs develop. He divides the massive integument
into four regions: an outer or epidermal layer which ultimately
produces the hairs, an intermediate parenchymatous region which

LYCOPERSICUM ESCULENTUM

569

-r / w \

G

o

■end

Fig. 197. A, longisection through mature seed showing position of embryo; B, tran-
section of outer layer of mature seed coat showing sinuous walls and character of thickening ;
C-G, successive stages in development of seed coat showing resorption of outer and inner
zones, and development of wall thickenings of outer layer which form hairs : cot, cotyledons ;
end, endosperm ; hr, hairs or longitudinal bands produced from outer layer of seed coat ;
hy^, hypocotyl ; i /, inner layer of seed coat ; i 1, inner zone of parenchyma ; / h, thickened
longitudinal bands of wall ; w /, membranous layer of mature seed coat ; 0 I, outer layer of
seed coat; 0 z, outer zone of parenchyma; s c, seed coat. QB-G redrawn after Soueges,
Ann. ScL Nat.^

570 THE STRUCTURE OF ECONOMIC PLANTS

is subdivided into an outer and inner zone, and the innermost
layer or epidermis which is highly pigmented, giving color to the
mature seed. (Fig. 197, C.) As the seed enlarges, the number
of parenchymatous cells of the outer zone is increased by many
cell divisions.

The walls of the outer epidermis develop thickenings which
are initiated on the inner walls and at the base of the lateral walls.
(Fig. 197, D.) While still in the fruit, the epidermal cells of the
partially mature seed show longitudinal bands or thickenings
that are more or less localized at the angles of the cells; and these
continue to the external wall, forming a large irregular network
or mesh. (Fig. X97, E, F.') During the final stages of maturation,
the portions of the lateral wall between the longitudinal bands
are split, and the external wall also disappears. In this manner
the bands are isolated from one another and form long "hairs"
or scales that cover the outer surface of the seed at maturity.
(Fig. X97, C) The basal portions of the bands are sinuous and
irregular, being thickest at the angles from which the "hairs"
arise. (Fig. ^97, B.)

While this development is taking place, there is a gradual
resorption of the intermediate zones. This is initiated in the
inner zone of parenchyma and continues progressively until
maturity. Nothing is left of the intermediate parenchyma except
a crushed membranous layer which with the inner layer gives
color to the seed. (Fig. x^-/, C-C) The haustorial or digestive
cells of the innermost layer are rectangular in transection and
polygonal as seen in surface view.

Development of the Seedling. — Although variation in the
size of the seed may not proportionately affect the yield of fruit,
the percentage of germination is much lower in lots of light seed
as compared with lots of larger, heavier seed. Under greenhouse
conditions, the seeds germinate rather rapidly; and, by the seventh
day, produce a slender tap root an inch or more in length with
numerous root hairs. The hypocotyledonary arch emerges from
the soil, and this is followed by liberation from the seed coat of
the slender linear or lanceolate cotyledons which function as the
first photosynthetic leaves. (Fig. X98, B-F.')

The Primary Root. — The primary root resembles other
solanaceous forms (potato, tobacco, eggplant) in all essential
details. It has a diarch protostele and the two groups of primary

LYCOPERSICUM ESCULENTUM

571

phloem lie on the flanks of the primary xylem strand, which con-
sists usually of six or eight xylem elements. The primary phloem
cells are very small, consisting chiefly of elongated parenchym-
atous elements. The pericycle is uniseriate and the protoxylem
cells abut it. The narrow cortex is three or four layers in width,
the innermost one forming an endodermis with prominent Cas-
parian thickenings.

Vascular Transition. — King (18) and Thiel (33) find that
the vascular transition agrees in all respects with that of the

Fig. X98. A, longisection through mature seed showing portion of embryo and seed coat;
B-F, successive stages in development of seedling; G, cotyledons and epicotyl, showing first
epicotyledonary leaves ; H, types of first foliage leaves.

potato and eggplant. The first stage in the reorientation from the
exarch, radial protostele of the root to the bicoUateral endarch
arrangement is the separation of the diarch xylem strand into two
units by parenchymatous cells. At the same time, each group
of primary phloem divides into three smaller strands; and, at
higher levels, the central one of each group is differentiated pro-
gressively toward the center, forming the inner phloem of the
hypocotyl.

The xylem units bifurcate, and the two metaxylem portions of
each divided bundle differentiate along two oppositely directed
curves. (Fig. 2.^^.') Above this point, the metaxylem is dif-

572-

THE STRUCTURE OF ECONOMIC PLANTS

ferentiated tangentially, approaching the periphery of the stele
more closely; and the protoxylem develops in a more centrad
position. The relation of the proto- and metaxylem is not com-
pletely endarch at the level of the cotyledonary plate. Coincident

px

~o ph
en

Fig. 199. A-F, successive stages in vascular transition from diarch, root-like, lower
hypocotyl to cotyledonary node. Completely endarch bicollateral bundles occur in cotyle-
donary petiole : en, endodermis ; / ph, inner phloem ; mx, metaxylem ; 0 ph, outer phloem ;
/»<*r, parenchyma ; pc/, pericycle ; ph, phloem; /ix, protoxylem. (After King.)

with the reorientation of the xylem, the strands of inner phloem
form two groups that are located on the inner faces of the primary
xylem groups, and the outer phloem is oriented outside them.
As a result of adaxial differentiation of the protoxylem and abaxial
orientation of the metaxylem in the cotyledon, the endarch condi-

LYCOPERSICUM ESCULENTUM

573

tion is definitely attained and a bicollateral cotyledonary bundle
is formed. (Fig. i99.) The traces of the first foliage leaves
above the CDtyledonary plate consist of bicollateral bundles that
are completely endarch. These anastomose with the vascular
elements of the hypocotyl at and slightly below^ the cotyledonary
node.

Secondary Roots. — The lateral and adventitious roots are
commonly tetrarch, occasionally pentarch. (Fig. 300.) Second-

FiG. }oo. Transection of adventitious root with tetrarch stele : ca, cambium ; co, cortex ;
csp, Casparian strip; en, endodermis; ep, epidermis; pel, pericycle; ph, phloem; ra, ray;
xy I, primary xylem; xy z, secondary xylem.

ary thickening is initiated in the usual
parenchyma centrad to the primary
radiating arms of protoxylem. As a
four wedge-shaped sectors of secondary
separated from one another by pericycl
region, the pericycle is uniseriate or
The endodermis and cortex persist for

manner in the fundamental
phloem and between the
result of cambial activity,
xylem are formed which are
ic rays. Except in the ray

occasionally two-layered.

some time after secondary

574 THE STRUCTURE OF ECONOMIC PLANTS

thickening has begun owing to tangential enlargement and radial
cell division. Lateral roots arise from sectors of pericyclic cells
on each side of the protoxylem points so that they come out at
oblique angles to the xylem strand between the protoxylem point
and the adjacent primary phloem. For this reason, the number of
rows of lateral roots is double the number of protoxylem points.

Adventitious roots originate in the parenchyma of the phloem
and pericycle of the stem. The root primordium is usually located
on the same radius as a medullary ray, and centripetal differentia-
tion of vascular tissue involves ray parenchyma and some phloem
parenchyma laterally. As the growing point of the adventitious
root increases in diameter, its lateral extension forces the peri-
cyclic fibers apart; and there is a displacement of the cortical
cells in addition to a resorption of tissue as the root penetrates the
cortex and epidermis. The rupture of the epidermis results in the
formation of wound tissue, and a phellogen produces cork cells
adjacent to the basal portion of the adventitious root.

The Stem. — The stem has a dissected amphiphloic siphonostele
in which the bicollateral bundles are at first separated by broad
medullary rays. Later in ontogeny, the fascicular and inter-
fascicular cambium form a continuous cylinder, and a solid zone
of secondary xylem is produced. The inner phloem is not re-
stricted to points directly centrad to the primary xylem of the
bicollateral bundles, but forms a ring of scattered phloem strands
that are separated from each other and from the xylem by paren-
chymatous cells. The outer phloem strands also form a dis-
continuous ring at first, but this zone becomes more or less
continuous as a result of the development of secondary phloem.
The secondary xylem formed by the fascicular cambium consists
of large vessels and wood parenchyma, while the interfascicular
cambium produces chiefly connective or conjunctive tissue. Cen-
trad to the inner phloem are groups of inner pericyclic fibers that
are less numerous than those of the outer pericyclic region and
parenchymatous cells separate the fiber groups. The endodermis
can be recognized by the presence of starch, and Casparian strips
may develop under certain conditions.

Immediately inside the epidermis is a zone of chlorenchyma
and centrad to it there is a band of collenchyma. The collen-
chymatous region is more strongly developed and the cells are
thicker angled in the more vegetative axes. Anderson (i) has

LYCOPERSICUM ESCULENTUM 575

made microchemical and physical tests of the stem at different
developmental stages, and finds that the thickened angles of the
walls of the collenchymatous cells are composed of a large number
of fine cellulose lamellae which alternate with layers of a pectic
compound. The epidermal cells may develop to form either of
the two types of hairs noted for the flower and leaf. The stomatal
frequency is about half that found in the upper epidermis of the
leaf.

The character and amount of secondary xylem formed is variable,
depending upon the degree of vegetativeness of the stem. As
Kraus and Kraybill (19) have pointed out, highly vegetative
plants grown with an abundant supply of available nitrogen have
stems that are much larger than those of feebly vegetative ones,
owing to the greater number and size of the pith cells. There
is a reduction in the amount of collenchyma and pith, accompanied
by a marked increase in the secondary xylem and a conspicuous
increase in the thickness of the walls of the pericyclic fibers in the
feebly vegetative stem. (Fig. 301.)

The Leaf. — The blade of the leaflet is thin, the mesophyll
consisting of a single row of palisade cells and a very loosely
organized spongy region of four or five layers of chlorenchyma.
The major portion of the vein projects prominently on the lower
surface, and there is a band of abaxial collenchyma reinforcing it
which with the adaxial strand and the mechanical elements of the
bundle gives support to the leaflet. (Fig. 301, A.") The bicol-
lateral bundle is like that of the stem, and there may be some
cambial activity in the larger veins. In the smaller veins, the
xylem consists chiefly of spiral elements. Both adaxial and
abaxial phloem are present in the main laterals, but adaxial phloem
is not found in the smaller branches. The walls of the cells of the
upper epidermis are much less sinuous than those of the lower
surface, and the stomata and hairs are less abundant on the latter.
(Fig. 301, C, D.) The two types of hairs are similar to those
described for the floral parts and the stem.

In the petiole, the epidermis is pubescent, and underlying it is
a continuous zone of chlorenchyma three or four cells in width.
Centrad to the chlorenchyma there is an uninterrupted band of
collenchyma which is three to six cells in width, and within this
is the parenchymatous tissue that surrounds the vascular strands.
The vascular strands are arranged in an approximate circle that is

576

THE STRUCTURE OF ECONOMIC PLANTS

open toward the adaxial surface, and the individual bundles are
separated from each other by parenchymatous rays of varying

Fig. 301. Transections of sectors of stems showing: left, feebly vegetative stem; and
right, highly vegetative one : ca, cambium ; col, collenchyma ; en, endodermis ; ef, epidermis ;
i fcly inner pericycle ; / ph, inner phloem ; 0 pel, outer pericycle ; 0 ph, outer phloem ; pi, pith ;
xy I, primary xylem ; xy 2., secondary xylem. (Redrawn from Kraus and Kraybill, Ore. A^r.
Exp. St a.')

LYCOPERSICUM ESCULENTUM

577

widths. In older petioles, some of the bundles may become inter-
connected by the development of an interfascicular cambium.

Fig. 301. A, transection of portion of leaflet including midvein; B, diagrammatic tran-
section of petiole of leaf; C, portion of upper epidermis showing distribution of stomata ;
D, same of lower epidermis. In B, zone of mechanical tissue inside chljrenchyma is lined,
outer and inner phloem are stippled, xylem is black, and developing interfascicular cambium
is indicated by dots : ab ph, abaxial phloem ; ad ph, adaxial phloem ; col, collenchyma ;
sto, stoma.

(Fig. 301, B.) The individual bicollateral bundles are similar in
organization to those of the stem.

LITERATURE CITED

I. Anderson, D. B., "Uber die Struktur der Kollenchymzellwand auf Grund
mikrochemischer Untersuchungen." Akad. Wiss. Wien. Math. -Nat. 1^6:
419-440, 1917.

L. Bailey, L. H., Standard Cyclopedia of Horticulture, id ed.. The Macmillan Co.,
1917.

3. , Manual of Cultivated Plants, The Macmillan Co., 1914.

4. BoswELL, V. R., et al., "Descriptions of types of principal American varieties

of tomatoes." U. S. Dept. Agr. Misc. Publ. 160, 1933.

5. Bouquet, A. G. B., "An analysis of the characters of the inflorescence and

the fruiting habit of some varieties of greenhouse tomatoes." Cornell Univ.
Agr. Exp. Sta. Met?/, i^q, 1931.

6. Caldwell, J., "A note on the dichotomous branching of the main stem of

the tomato (Lycopersicum esculentum)." Ann. Bot. 44: 495-498, 1930.

578 THE STRUCTURE OF ECONOMIC PLANTS

7. Cooper, D. C, "Anatomy and development of tomato flower." Bof. Gaz-

8}: 399-411, 1917.

., "Macrosporogenesis and the development of the macrogametophyte

of Lycopersicum esculentum." Am. Jour. Bof. 18: 739-748, 1931.
9. Fink, B., "Pollination and reproduction of Lycopersicum esculentum."
Minn. Hot. Studies i: 636-643 (1894-1898), 1896.

10. Gardner, M. W., "Cladosporium leaf mold of tomato; fruit invasion and

seed transmission." Jour. Agr. Kes. }i: 519-540, 19x5.

11. Groth, B. H. a., "Structure of tomato skins." N.J. Agr. Exp. Sta. Bull.

228, 1910.
II. GusTAFsON, F. G., "Growth studies on fruits." Plant Phys. i: 165-171, 1916.
15. , "Inducement of fruit development by growth-promoting chemicals."

Proc. Nat. Acad. Sci. 22: 618-636, 1936.

14. Hawthorn, L. R., "Seedlessness in tomatoes." Science 8;: 199, 1937.

15. Howard, Grace E., "Pigment studies with special reference to carotinoids

in fruits." Ann. Mo. Bot. Gard. 12: 145-111, 1915.

16. Jones, H. A., and Rosa, J. T., Truck Crop Plants, McGraw-Hill Book Co.,

1918.

17. Kendall, J. N., "Abscission of flowers and fruits in the Solanaceae with

special reference to Nicotiana." Univ. of Calif. Publ. Bot. ;: 347-418,
1918.

18. King, Effie, "Root-stem transition in the axis of Lycopersicum esculentum."

Unpuhl. M.S. Thesis, Univ. of Chicago, 1930.

19. Kraus, E. J., and Kraybill, H. R., "Vegetation and reproduction with special

reference to the tomato." Ore. Agr. Coll. Exp. Sta. Bull. 149, 1918.

10. Kubart, B., "Die organische Ablosung der Korollen nebst Bemerkung iiber

die Molsche Trennungschichte." S.-B. Akad. Wiss. Wien. Math. -Nat.
iij: 1 49 1, 1906.

11. LiNDSTROM, E. W., and Koos, Katherine, "Cyto-genetic investigations of a

haploid tomato and its diploid and tetraploid progeny." Am. Jour. Bot.
18: 398-410, 193 1.
11. MacDougal, D. T., "Hydration and growth." Carnegie Inst. Wash. Publ.

297, 1910.

13. MacGillivray, J. H., and Ford, O. W., "Tomato quality as influenced by

the relative amount of outer and inner wall region." Ind. Agr. Exp. Sta.
Bull. ^2j, 1918.

14. Makemson, W. K., "The leaf mold of tomatoes caused by Cladosporium

fulvum Cke." Mich. Acad. Sci. Kpt. 1918, 20: 311-348, 1919.

15. Namakawa, I., "Uber das Offnen der Antheren bei einigen Solanaceen."

Bot. Mag. (Tokyo) jj.- 61-69, 1919.

16. Rosenbaum, J., and Sando, C. E., "Correlation between size of the fruit and

the resistance of the tomato skin to puncture and its relation to infection
with Macrosporium tomato Cooke." Am. Jour. Bot. 7: 78-81, 1910.

17. Smith, O., "Pollination and life-history studies of the tomato (Lycopersicum

esculentum Mill.)." Cornell Univ. Agr. Exp. Sta. Mem. 184: 3-16, 1935.

xS. , "Effects of light on carotcnoid formation in tomato fruits." Cornell

Univ. Agr. Exp. Sta. Mem. i8j: 3-15, 1936.

LYCOPERSICUM ESCULENTUM 579

2.9. , and Cochran, H. L., "Effect of temperature on pollen germination

and tube growth in the tomato." Cornell Univ. Agr. Exp. Sta. Mem. i-j$:

3-"' 1935-

30. SouEGEs, R., "Developpement et structure du tegument seminal chez les

Solanacees." Ann. Set. Nat., Bot. Ser. IX, 6: 1-12.4, 1907.

31. , "Embryogenie des Solanacees. Developpement de I'embryon chez

les Nicotiana. ' Compt. Rend. Acad. Set. ijo: 1115-1117, 1910.

31. ^ , "Recherches sur I'embryogenie des Solanacees." Soc. Bot. France.

Bull. 6g: 163-178; 136-141; 351-365; 555-585, 1911.

33. Thiel, a. p., "Vascular anatomy of the transition region of certain Solana-

ceous plants." Bot. Gaz- 94: 598-604, 1933.

34. ToGNiNi, P., "Suir embriogenia di alcune Solancee da appunti lasciati."

Atti 1st. Bot. Pavia, Ser. II, 6: 109-111, 1900.

35. Warner, E. E., "The floral development and vascular anatomy of the flower

of Lycopersicon esculentum var. cerasiforme." Unpubl. M.S. Thesis, Univ.
of Chicago, 1933.

36. Weaver, J. E., and Bruner, W. E., Root Development of Vegetable Crops,

McGraw-Hill Book Co., 1917.

37. WiLLsTATTER, R., and EscHER, H. H., "Ueber den Parbstoff der Tomate."

Hoppe-Seyler's Zeitschr. f. physiol. Chem. 64: 47-61, 1910.

CHAPTER XIX

CUCURBITACEAE

CUCURBIT A spp.

THE gourd family, Cucurbitaceae, is widely distributed and
includes approximately 90 genera, most of which are tropical
or subtropical, although a few forms occur in the native flora of
the colder portions of the temperate zone. Three genera are
cultivated for their edible fruits: CitruUus, the watermelon;
Cucumis, the cucumber and melons; and Cucurbita, which includes
pumpkins, squashes, and gourds. In the genus Cucurbita three
species are extensively grown, C. Pepo L., C. maxima Duchesne,
and C. moschata Duchesne, commercial varieties of the first two
being the most generally used. Under cultivation the three
species grow as annuals; and, although sensitive to frost, they
develop with such rapidity that some varieties are grown in the
northern states.

GENERAL MORPHOLOGY

The Root. — The root system is characterized by a strong tap
root which, according to Weaver and Bruner (39), may penetrate
to a depth of 6 feet at maturity although branching is not extensive
below the two-foot level. The branches spread widely, occupying
a radius of nearly zo feet, and the main laterals have numerous
secondary branches i to 8 feet long which in turn branch until
there is a remarkable network of rootlets that completely occupies
the soil near the surface. In addition to the primary root system,
the adventitious or nodal roots may attain a length of 4 to 5 feet,
and they also branch so that the absorbing area is much increased.

The Shoot. — In general, the plants have prostrate vine-like
stems, but some varieties of C. Pepo are more or less bushy and
semi-erect. In the latter, the internodes are much shortened, as
compared with the trailing types, and tendrils are absent. In
the prostrate forms, three to several lateral branches arise from
nodes near the base of the stem axis; and extend outward for

580

CUCURBITA

5S1

many feet, having a tendency to produce adventitious roots at
the nodes. The growth of these branches is extremely vigorous,
and Holroyd (18) has stated that "members of this group may
form a growth of stem and leafage within the growing period of
four months that is probably unequaled by any other annual
herbaceous plant family." In C. Pepo, he has reported an instance
in which a single plant developed stem and branch growth exceed-
ing 140 feet in total length,
from which were diverged as
many as 450 leaves.

The stems are roughly five-
angled, hollow at maturity,
and bear multicellular hairs
or trichomes which are either
sharp-pointed and conical or
long-stalked, capitate, and
glandular. The branching is
of the sympodial type in which
a lateral branch continues the
stem axis; and, by its growth,
displaces the terminal branch
so that the latter occupies a
position on the opposite side
of the axis from the leaf which
arises at that node. The ter-
minal branches form the ten-
drils which may be simple or have three or four branches. (Fig.
303.) The tendril is generally interpreted as a shoot, the basal por-
tion being the branch, and the terminal portion a specialized leaf.

The large cordate leaves are petiolate and usually three- to five-
lobed, the prominence of the lobing varying with the species and
variety. In C. Pepo, the hairy leaves are commonly five-lobed,
sometimes three, and frequently have white spots at the angles of
the veins. In C. maxima, the leaves are usually without lobes or
with short, rounded ones; and in C. moschata, they are lobed but
more nearly orbicular in outline than in C. Pepo. The texture of
the leaves is variable, being somewhat harsh in C. Pepo and less
so in the other two species.

The Inflorescence. — The yellow flowers are monoecious and
occur singly in the axils of the leaves. In trailing varieties, the

Fig. 303. Cucurbita Pepo showing sympod-
ial branching and relative position of tendrils
and inflorescences, variety Table Queen.

58x THE STRUCTURE OF ECONOMIC PLANTS

staminate flowers are located near the center of the vine and are
borne on slender peduncles; while the pistillate flowers are borne
on short ridged stalks, distal to the staminate ones. In the bushy
types, the pistillate flowers occur near the base of the plant. The
character of the peduncle is sufficiently distinct in the three species
to be used as a diagnostic character. In C. Pepo, the fruit stalk is
deeply furrowed and five- to eight-ridged; in C. moschata, it is
five-ridged and enlarged next to the fruit, and in C. maxima, the
peduncle is cylindrical or claviform but never prominently ridged.

The Carpellate Flower. — The carpellate flowers are epigy-
nous. The calyx tube is terminated by five slender awl-shaped
lobes, and the companulate corolla is also deeply five-lobed, the
lobes being recurved at the tips. (Fig. 304, A, 5.) The pistil
consists of three carpels which form a three-loculed ovary, and the
thick, short style is terminated by three bilobed or divided papil-
late stigmas. Occasionally, the pistil may consist of four or five
carpels with a resultant four- or five-celled ovary. The stamens
are rudimentary, three staminodia being commonly present; and a
ring-like nectary is located between the base of the perianth tube
and the style. (Fig. 305, A.')

The development of the pistillate flower has been investigated
by Kirkwood (ix) for several genera of Cucurbitaceae; and by
Judson (xi) for Cucumis, as well as by several of the earlier mor-
phologists, including Payer (31) and Goebel (13). These accounts
are in general agreement, except for diff'erences in the interpretation
of the development of the ovary and the character of its placentation.

The floral axis first develops as a rounded protuberance which
elongates. The apex then becomes flattened, and a slight terminal
depression develops so that the surface is somewhat concave. The
primordia of the floral parts arise from this surface in acropetal
succession: sepals, petals, staminodia, and carpels. Because of
diff"erential growth at five points on the margin, outgrowths arise
which are the primordia of the sepal lobes. Growth of these
lobes, and of the underlying tissue at the outer margin of the
receptacle, results in an elevation of the outer border of the re-
ceptacle; and the inward curvature of the densely pubescent lobes
tends to cover the tip of the floral axis. At this time, the cen-
tral portion of the stem apex grows slowly; and the more rapid
growth of the outer portion forms a cup-shaped receptacle which
surrounds it.

CUCURBITA

583

The primordia of the petals appear shortly after those of the
sepals and alternate with them. They form a cycle of five small,
blunt protuberances which at first grow less rapidly than the
sepal primordia; but later accelerate their growth and, like the
sepal lobes, form an enclosing protective envelope over the tip of
the receptacle. These primordia are also pubescent but the hairs
are less rigid than those on the sepals. The development of these
two cycles of primordia,
which ultimately become
the lobes of the calyx and
corolla respectively, is ac-
companied by the growth
of the undiverged tissues
immediately below them,
which results in the for-
mation of a continuous
cylinder of tissue consti-
tuting the perianth tube.

One of the three rudi-
mentary stamens is smaller
than the other two, each
of which arises opposite a
petal lobe. The small one
is located in an interval
between a petal lobe and

1,1 J ^u^ ^*,„^ Fig. ^04. ^, habit of pistillate flower ; B, median

a sepal lobe and the spac- i^^^^J^-l^ 3howing epigynous character; C, longi-

ing is such that the three section of staminate flower with abortive pistil ; D,

are equidistant from one transection of tricarpellate ovary showing placen-

T- . ration.

another within the peri-
anth tube. They appear shortly after, or in some cases coincident
with, the origin of the petal primordia; the smallest staminodium
appealing first, and the second and third arising in a counter-clock-
wise direction. At first, the staminodia are conspicuous; but, as
the flower develops, their growth is arrested and their terminal
portions may become withered and dried. The continued growth
of the perianth tube, below the point of divergence of the stami-
nodia, gives them the appearance of diverging from a midpoint in
the perianth tube.

Following the appearance of the staminodia, three small lobes
arise equidistant from one another at the bottom of the cavity of

584

THE STRUCTURE OF ECONOMIC PLANTS

the receptacle, below and centrad to them. These carpellary
primordia grow in such a way that they form three crescentic
structures with edges extending toward the center of the ovary,
and with abaxial surfaces that are adnate to the receptacle except
at their distal ends. The growth of their inturned edges results
in the formation of three ridges, each of which represents the
edges of two adjacent carpellary margins. As centripetal growth
of the ridges proceeds, they reach the center of the ovary and

ultimately fill it except for
three narrow spaces which
form a triradiate cavity.
(Fig. 305, 5.) When the
infolded edges of the car-
pels reach the center of
the ovary, they curve out-
ward, grow centrifugally
until they approach the
outer wall of the locule,
and then again turn inward
toward the center of the
ovary. The infolded edges
of each carpel form the
placentae from which the
ovules arise; and in Cucur-

FiG. 305. ^, diagrammatic longisection of young ,. /-,>r„ll„^ .„J ^^ .

carpellate flower showing epigyny; B, transection ^Ita, ^^ItrullUS, aOQ tO a

of ovary with three locules and multiple placentae; leSSer degree in CuCUmis,

C diagram illustrating placenta, origin of ovules ^^^^ placenta beCOmeS a

and their comparative development: <;/<«, corolla ; clx, tr

calyx; w, nectary ; ovj, ovury; jf^, staminodia ; sti, multiple Structure W^lth

stigma. (C redrawn from Kirk wood, B«//. N^w Yar^ ^^q qj. (-^j-gg longitudinal

Bot. Card.') . , , ^, ,

ridges on each placental
margin. (Fig. 305, B, C.) In this manner, each carpel develops
a series of ovules of varying ages borne on parallel placental ridges.
Owing to the position in which the placental ridges are finally
oriented, the placentation has been described as parietal by some
investigators; but it is probably better to regard it as axile or
central placentation, plus a centrifugal overgrowth which carries
the placentae toward the periphery of each locule.

The most fully developed ovules are located on the placental
ridge farthest from the margin of the carpel, while the smallest
ovules are found on the placental fold nearest the margin. As the

CUCURBITA 585

flower reaches anthesis, however, the difference in the relative
development of the ovules is somewhat equalized so that the
chances for fertilization are about the same for all. The ovules
appear first as small papillae on the margins of the placentae and
become strictly anatropous at maturity.

The growth of the upper portions of the carpels results in the
formation of three undiverged styles, and the three stigmas are
distinct, appearing as bilobed or divided structures. The tissue
of the ring-like nectary is derived from the receptacle at the base
of the perianth tube, after the carpellary lobes have grown upward
to form the styles and stigmas. (Fig. 305, A.^

The Staminate Flower. — The number of staminate flowers
produced always exceeds that of the pistillate ones. They have a
campanulate corolla, which with the calyx forms an undiverged
basal perianth tube. The lobes of the latter are linear and alternate
with the five deeply lobed segments of the corolla. (Fig. 304, C)
The three stamens are not alike, as two of them are tetraspo-
rangiate, producing two locules at maturity, while the third
is bisporangiate and unilocular. The filaments are free, but
the stamens are more or less united by their anthers, which
grow vigorously in length, forming long vermiform coils. (Fig.
306.)

The sequence of development of the floral parts, according to
Heimlich (16), is perianth tube, stamens, pistillodium, calyx
lobes, and corolla lobes. As in the carpellate flower, the floral
axis is at first club-shaped, but its apex soon broadens and the
ring-like fold of the perianth tube is formed. This is followed
by the differentiation of the primordia of the stamens, which
arise in a close spiral arrangement, the first one to develop being a
unilocular stamen which is sometimes referred to as the "half
stamen." The second stamen develops in a position opposite the
leaf in the axil of which the flower arises, and the third is formed
at a point that is equidistant from the other two. The three
primordia of the pistillodium then develop in a close spiral, their
lobes alternating with the stamens.

Each stamen develops a wide filament with a broad connective
which supports the anther. The filaments are diverged from the
receptacle at the base of the inner surface of the perianth tube;
and, within the staminal cycle, a slightly-lobed, ring-like nectary
is developed which encircles the pistillodium. The rudimentary

586 THE STRUCTURE OF ECONOMIC PLANTS

carpels may attain varying degrees of development, occasionally
forming a mature pistil so that a perfect flower results.

As the stamens are developing, the calyx and corolla lobes arise
from the margin of the perianth tube in a spiral arrangement and
the broad thin corolla lobes overarch the androecium. Large
intercellular spaces develop in the parenchymatous tissue of the
perianth tube, and these schizo-lysigenous cavities finally extend
from near its base to the point of divergence of the corolla.

The morphological interpretation of the dissimilar stamens
which constitute the androecium of some of the cucurbits has
been a controversial matter. Heimlich (i6) has supported the

///

Fig. 306. Development of androecium ; the single stamen is to right in each figure, while
behind and to left are two double double ones. (From Sachs, after Payer, Textbook of Botany,
Clarendon Press.)

original interpretation of Naudin (30) that the two bilocular
stamens are complete; and that, in the third, one locule fails to
develop. Miller (xS), comparing Echinocystis with other genera,
has concluded that the bisporangiate, unilocular stamen is a
complete one and not a "half organ" in the developmental sense.
On this basis, he regards Cucumis and Cucurbita as having "five
stamens, four of which are united in two masses," and agrees
with Eichler (8) and van Tieghem (36), who regarded the tetra-
sporangiate, bilocular stamen as a double organ. Miller based
his interpretation upon the observation that the tetrasporangiate
stamen in Echinocystis arises from two separate primordia which
form a single structure owing to lateral coalescence and the growth
of intervening tissue between the original primordia.

A peculiarity of the pollen tube has been observed in the cucurbits
by Longo (2.7), and Kirkwood (13) has confirmed this point.
Longo found that the pollen tube expands into a large bulla at

CUCURBITA 587

the base of the neck of the nucellus. It then develops branches
which traverse the nucellus and the inner integument, also main-
taining an intimate connection with the internal layers of the
outer integument. This behavior of the pollen tube has a func-
tional significance, according to Longo, since the branches of the
tube act as haustoria in the translocation of nutritive material.

The Fruit. — The fruit of the cucurbits is one of the largest
found in the plant kingdom. It is indehiscent with the fleshy
receptacle adnate to the pericarp, and is classified as an inferior
berry or pepo. The varieties of squash and pumpkin are numerous.

Fig. 307. The fruit of Hubbard Squash. (Courtesy Bur. Plant Industry.)

and the list is constantly increasing owing to hybridization and
experimental breeding, so that there are wide variations in fruit
characters, especially with respect to shape, size, texture of the
skin, and color. Varieties of C. Pepo have fruits some of which
are round, flat, and scalloped at the edges, or elongated and crook-
necked; while others are short and club-shaped, or acorn-shaped
and variously grooved. Varieties of C. maxima include the
Hubbard squash which is roughly elliptical, tapering to a curved
stem end, and others that are turban-shaped owing to the fact
that the fleshy receptacle and the ovary are diverged for a longer
distance than is usually the case. (Fig. 307.) Those of the banana
type are elongated and somewhat pointed at the ends. In C.
moschata the fruits are also variable in shape, some having long
curved necks while others are more oval or spherical. The surface
may be smooth, warty, or covered with spines and other types of

588 THE STRUCTURE OF ECONOMIC PLANTS

emergences. The young fruits of practically all varieties bear
hairs of one or more kinds which may or may not persist until
maturity. The skin may be white, green, yellow, red or, in some
cases, variously spotted and striped.

ANATOMY

Anatomy of the Fruit. — Despite the pronounced variation in
shape, size, and color, the anatomical characters are fairly constant;
and Barber (z) has pointed out that a generalized description of
the fruit coat can be given for C. Pepo and C. maxima which
closely resemble each other in structural detail. At maturity,
the rind of the fruit is hard, while the central hollow portion
contains a mass of slimy fibers among which are the numerous
flattened seeds. The pericarp and undi verged tissues of the
receptacle constitute the bulk of the fruit, including the rind and
fibers, and the connecting parenchyma disintegrates before the
complete maturation of the fruit.

Barber (i) has divided the pericarp into six distinct regions.
The outermost zone (i) consists of prismatic cells that are polyg-
onal in surface view and form a palisade layer, having greatly
thickened outer and radial walls with a heavy striated cuticle.
The characteristic white spots on the surface, about which the
cells are elongated and curved, are stomata from which rows of
tangentially elongated cells radiate. The distribution of the
stomata is not uniform and occasionally two may be surrounded
by the same group of radiating cells. Yasuda (41) has reported a
frequency of 44 stomata per square millimeter for C. Pepo. Two
types of hairs are produced on the epidermal surface, both having
basal cells that are somewhat more rounded than the adjacent
epidermal cells. These hairs do not persist, but dry up while the
fruit is small, leaving the basal cell intact. The large, jointed
type is tapering, conical, and multicellular, frequently attaining a
length of i}'i to ^ mm The other is smaller and capitate, con-
sisting of a jointed structure of four or five cells, and a large globular
head of one or more cells. (Fig. 308, B.)

The hypodermal zone (x) consists of many layers of small,
thick-walled, isodiametric cells with small intercellular spaces.
The outer mesocarp (3) lies immediately within the hypodermis
and is comprised of cells of graded size, intermediate between those
of the hypodermal region and the larger ones of the middle meso-

CUCURBITA

589

carp. They are isodiametric with thick walls; and, in the variety
verrucosa, the region is characterized by many layers of polygonal
stone cells. The middle mesocarp (4) is made up of cells which
are progressively larger and more loosely arranged than those of
the outer mesocarp. The outermost cells contain numerous starch
grains, the number decreasing toward the inner limits of this zone.

A B

Fig. 308. A, transection of portion of seed of Cucurbita Pepo; B, surface view of portion
of epicarp of immature fruit sfiowing epidermal hairs : al, aieurone grains ; cot, cotyledon ;
end, endosperm ; ep, epidermis ; gl hr, glandular capitate hair ; hr, conical hair ; hy, pitted
subepidermal layer; w\ pitted parenchyma ; w^, reticulate spongy parenchyma; />', paren-
chyma; />^ spongy parenchyma; /'^ inner epidermis of seed coat; psm, perisperm; s c, seed
coat; jc/, sclerenchyma ; j- ^, starch grain ; j-fo, stoma. (Redrawn after Barber.)

The inner mesocarp (5) consists of large parenchymatous cells
without much cellular contents. Throughout the mesocarp
there are vascular bundles and isolated sieve tubes, as well as
latex tubes which anastomose with each other. The central
portion of the fruit is occupied by tough fibers surrounded by
disintegrated parenchymatous cells. The innermost zone or
endocarp (6) is made up of very small, thin-walled cells that are
tangentially elongated and form a thin membranous tissue which

590 THE STRUCTURE OF ECONOMIC PLANTS

adheres to the seeds. It becomes firmly attached to the seed coat,
but can be separated from dry seeds as a transparent colorless skin.

The Seed. — The large, flat, ovate seeds are pointed at one
end; and vary so definitely in shape, color, margin, and the scar
formed at the hilum that Russell (33) has devised a key to separate
the three species based on these characters.

The structure of the seed coat has been investigated by Barber (2.),
Kondo (2.4), Yasuda (42.), and Fickel (10). Kondo investigated
that of C. moschata and the other accounts described varieties of
C. Pepo. The seed coat is comprised of five distinct layers of tissue
which develop as parts of the two integuments of the anatropous
ovule. The outer integument produces the three outermost layers
of the seed coat, and a part of the fourth layer; while the remainder
of the coat is derived from the inner integument.

The outer epidermis (i) is made up of prismatic cells which
form a compact layer devoid of intercellular spaces. The cells
are elongated radially, and those lying at the border form a ridge
in which the cells are several times as long as those on the flattened
lateral surfaces of the seed. The outer walls of these cells are
thickened, but there is little or no cuticle. The radial walls are
very thin with characteristic strands of cellulose thickening which
extend from the base of the cell and branch frequently toward the
outer wall. (Fig. 308, A.")

The subepidermal layer Ql) is made up of small, thick-walled,
much pitted, polygonal cells that are somewhat elongated. There
are no intercellular spaces in this zone, which is three to five
layers in thickness except at the margin of the seed where the
number increases. The sclerenchymatous region (3) gives firmness
to the seed coat, and is one cell layer in thickness except at the
margin of the seed where it may be two or three. The longitudi-
nally elongated cells are arranged in rows end to end; and in surface
view, the very thick walls appear sinuous with infoldings that
overlap one another.

The parenchymatous zone (4) consists of three distinct layers.
The outer one is made up of small pitted cells which lie adjacent
to the sclerenchyma and have no intercellular spaces. The inter-
mediate region is two layers in thickness and is characteristic of
the genus. The intercellular spaces are very large, forming
cavities into which the cells project, and the cells are somewhat
stellate with walls that have reticulate thickenings. The inner

CUCURBITA

591

layers form a true spongy tissue of thin-walled cells, and the
innermost layers of this zone are chlorophyllose. The inner
epidermis (5) is a layer of small thin-walled cells. (Fig. 308, A.^

Underlying the seed coat is a zone of perisperm or nucellar tissue
about six layers in thickness. The cells are thin-walled, except
for the outermost one which is epidermal with a definite cuticle.
The endosperm consists of a layer of thick-walled cells which have
granular contents and large nuclei. The embryo has a short,
blunt hypocotyledonary axis from which two flat, somewhat
fleshy cotyledons are diverged. A very short conical growing
point constitutes the epicotyl.

Development of the Seedling. — Owing to the shape of the
seed, it lies flat in planting. Under favorable conditions, the
primary root protrudes between the halves of the seed coat at
the end of two days and extends downward at right angles to the
seed. The root grows very rapidly, and may be 3 cm. in length
by the end of the third day. At this time the peg begins to develop
as a small, lateral, parenchymatous outgrowth in the angle formed
by the base of the horizontal hypocotyl and the vertically oriented
root. By the fourth day, the peg is enlarged considerably, in
some varieties extending part way round the axis; and just below
it, the primary root produces many lateral roots. The hypocotyl
begins to elongate upward while the cotyledons still remain within
the seed coat. This soon splits, since its lower half is held in
place by contact with the lower surface of the peg while the upper
half is forced open by the growth of the arching hypocotyl.
(Fig. 309, A-C.^ In some instances, the lower half of the seed
coat slips from the peg, and the seed coat is carried above ground,
where it is later shed as the cotyledons enlarge.

Eventually, the cotyledons are freed from the seed coat, emerging
from the soil; and about the sixth day, the axis becomes erect.
The seedling phase is usually completed in seven or eight days,
when the primary structures are differentiated, and the reserve
foods stored in the cotyledons have been largely utilized. The
young plant is then independent, the outspread cotyledons are
photosynthetic, and there is a well-branched root system. The
growth of the epicotyl is relatively slow during germination;
but the first foliage leaf finally develops very close to the coty-
ledonary plate, since the first internode is extremely short. (Fig.
309, D, £.)

59^

THE STRUCTURE OF ECONOMIC PLANTS

The Primary Root. — The primary root has a tetrarch, exarch
protostele which is surrounded by a pericycle that is uniseriate
outside the four primary phloem groups, but may be four or more
cells in width at the protoxylem points. (Fig. 3 10.) The cortical
region is limited centripetally by endodermal cells that are verti-
cally elongated, being about equal in length to those of the peri-
cycle. The remainder of the cortex consists of seven to ten layers

-hyp cot-CZZ^j>^hp s c . /T\ — hyp

■ rt peg — A

^1 ^ C

A

Fig. 309. Stages in development of seedling: A, two-day seedling showing axis prior to
formation of peg (seed coat has been removed) ; B, at three days, showing development of
peg; C, same, at five days, illustrating function of peg in splitting seed coat and aiding in
withdrawal of cotyledons; D, at eight days, primary growth of seedling is complete; E,
older plant showing position of first foliage leaves and relative lengths of first and second
internodes: cot, cotyledon; eel, epicotyl ; hyp, hypocotyl ; ino i, first internode; ino i,
second internode; / i, first foliage leaf ; / z, second foliage leaf ; rr, primary root ; j c, seed
coat. (After Whiting.)

of large parenchymatous cells which are several times as long as
wide, and there are conspicuous intercellular spaces. The elon-
gated, tabular epidermal cells are small in radial dimension, and
each may form a root hair.

The ontogeny of the root follows Janczewski's (xo) fourth type,
which is characteristic for the Cucurbitaceae and Leguminosae.
In this type, the growing point is not differentiated into distinct
histogens; but, instead, the primary tissues of the root axis arise

CUCURBITA

593

--ep

from a common meristematic zone. Whiting (40), in a study of
Cucurbita maxima, confirms Janczewski's (xo) work and describes
this generative zone. The meristem consists of about seven layers
of cambiform cells, the number diminishing toward the periphery
of the growing point. Terminally, the outermost cells of the gen-
erative zone become the cells of the root cap, while the marginal
cells function as a dermatogen and produce the epidermis. The
joint production of root cap and epidermis by the peripheral
portions results in the charac-
teristic "stair-step" arrange-
ment referred to by Janczewski
in the description of his third
type of root ontogeny. (See
Linum, Chapter XIII.)

The remaining tissues are
derived from the inner face of
the generative zone. The cells
undergo several divisions,
some of which are irregularly
periclinal; and then the cen-
trally located cells differentiate
as stelar tissues, while adja-
cent cells produce the cortical
region. In this type, where
there is general meristematic
activity, it is difficult to fol-
low the differentiation of the
various tissues; and the situation is further complicated by the
rapid elongation of the root and the delay in the maturation of
the primary tissues. It is possible to determine the approximate
limits of the cortex and stele relatively early, since the cells of the
innermost layer of the cortex continue to divide tangentially for
a longer period of time than do the adjacent cells of the stele.

The first stelar tissue to differentiate is the protophloem, which
consists of four strands. No sieve tubes develop in the proto-
phloem, which is comprised only of parenchymatous elements
that are small in diameter and considerably elongated. In the
center of each protophloem group, and usually adjacent to two
large pericyclic cells, a single cell differentiates, forming a duct
which can be distinguished by its less dense contents. Other

Fig. 310. Transection of sector of young root
showing development of xylem prior to differ-
entiation of large central metaxylem : co, cortex ;
eti, endodermis ; ep, epidermis ; mx, metaxylem ;
pel, pericycle ; ph, phloem ; px, protoxylem.

594

THE STRUCTURE OF ECONOMIC PLANTS

protophloem ducts similar to the initial one may be differentiated
adjacent to the pericycle or one or two cell layers removed from it.

The metaphloem develops sieve tubes and companion cells in
addition to parenchyma. The sieve tubes are elongated, small in
diameter, and have comparatively small sieve plates. The com-
panion cells are nucleate, densely cytoplasmic, and shorter than
the sieve tubes so that there may be two or more of them abutting
one sieve tube. The major portion of the metaphloem is composed
of parenchymatous cells that are larger in diameter than the sieve
tubes and usually several times longer than their radial dimension.

The differentiation of the protoxylem parallels the protophloem.
The first elements which mature have relatively heavy annular
thickenings, and one vessel of this type develops at the outer
limit of each protoxylem point. The two or three protoxylem
elements centrad to the first formed, annular elements, differentiate
as annular or spiral vessels. The relative number of each type is
not constant, and occasionally intermediate spiral-annular elements
may be formed. The protoxylem vessels soon become stretched
and crushed, forming lacunae which may later become conspicuous.
The tetrarch pattern of the stele becomes more marked with the
progressive differentiation of the metaxylem elements. There are
intermediate types between the spiral, reticulate, and scalariform
patterns; so that it is difficult to distinguish definitely between
the protoxylem and metaxylem. In general, the metaxylem
vessels are conspicuously larger than the first formed protoxylem
elements, and, for the most part, have closely reticulated thicken-
ings.

Up to this point, the stelar development results in the formation
of four radial arms of primary xylem alternating with the four
zones of primary phloem, the central portion of the axis remaining
parenchymatous. (Fig. 310.) Possibly because of the delayed
differentiation of the central portion of the stele, the squash root
has been described as having a pith. Van Tieghem (35) and
Gerard (ix) both mention the presence of a pith; but in the latter
case the transection described was probably close to the transition
region where a pith is differentiated. Rutledge (34) and Whiting
point out that, in C. maxima, two or more cells at the center of
the stele finally develop to form conspicuously large pitted or
reticulate-pitted tracheae. These are surrounded by smaller,
isodiametric or horizontally elongated parenchymatous cells which

CUCURBITA 595

also have reticulately thickened or pitted walls. (Fig. 311, A.^
Tfie^centrally located metaxylem vessels with the adjacent thick-
walled parenchyma may abut the reticulate vessels of the four
primary xylem points; or they may be separated from them by
two or three layers of vertically elongated thin-walled xylem
parenchyma. (Fig. 312., B.) Between the primary xylem and
phloem, there is a zone of parenchyma which later gives rise to the
cambium.

As the stele matures, the cortical region is also developing.
The inner cortical cells divide periclinally, adding additional
layers, and radial divisions occur to compensate for the increased
diameter of the stele. Cell divisions cease at about the time that
the scalariform elements of the metaxylem develop, and the en-
dodermis then forms narrow Casparian strips.

Lateral Roots. — The lateral and adventitious roots resemble
the primary root in structure, but their steles may be triarch or
octarch as well as tetrarch. The origin of lateral roots occurs
very early in the ontogeny of the primary root when the proto-
phloem is differentiating, and prior to the development of the
protoxylem. Whiting observed several root tips in which lateral
primordia occurred within a millimeter of the apical meristem.
There is some disagreement in regard to the tissues involved in the
origin of secondary roots in Cucurbita. According to Janczewski
(19), in the ontogeny of lateral roots in Cucurbitaceae and Legumi-
nosae, the stele of the main root gives rise to the stele of the lateral
root; while the endodermis and adjacent cortical cells of the main
root form the cortex of the lateral root, at the distal surface of
which the meristematic growing point is later developed. Van
Tieghem and Douliot (38) rejected Janczewski 's explanation,
stating that in C. maxima and C. Pepo, there is definite evidence
that the lateral root arises from two pericyclic layers, and that
the endodermis and five or six of the inner cortical layers produce
a digestive pocket which assists in the emergence and the outward
growth of the young root. Whiting's studies tend to confirm
the observations made by Janczewski. However, the very early
initiation of the primordia of the lateral roots, prior to the matura-
tion of the protoxylem and before the endodermal layer of the
main root is clearly differentiated, makes it difficult to determine
with exactness the location of the tissues from which the primordia
arise.

596 THE STRUCTURE OF ECONOMIC PLANTS

According to Whiting, lateral root formation is first indicated
by radial divisions of cells in the two or three inner layers of
cortical parenchyma at points centripetal to the protoxylem.
(Fig. 311, A.^ The radial divisions of the regularly arranged
cells of the cortex occur while the innermost layers continue to
divide tangentially. At this time, the pericyclic cells begin to
divide; and those just centrad to the active cortical region also
divide tangentially, with the result that the exact identity of
the two adjacent layers is soon lost. Thus, divisions in all three
planes account for the formation of the lateral root primordium
from cortical and pericyclic tissue.

As additional layers of the cortex are involved, the lateral extent
of the root primordium is increased until its base reaches to the
phloem on either side of the protoxylem point. (Fig. 311, B, C)
The cortical cells remain meristematic and continue division, but
the outline of the mother cell can frequently be traced, even after
several divisions have ensued. The regular layered arrangement
of the cells in that part of the cortex which is involved in lateral
root formation tends to be distorted by the increased activity of
the tissues of the pericycle, which by successive divisions produces
the numerous small, radially elongated elements of the stele.
(Fig. 311, D.)

As the primordium elongates, the cortical cap maintains its
position over the stele by divisions of the cells at the base of the
primordium and adjacent to the phloem of the parent root. Finally,
when all but about three layers of the cortical tissue of the primary
root are involved in the root primordium, a meristematic zone
similar to that described for the primary root develops. (Fig. 311,
D, £.) This is formed by tangential divisions in the second, third,
or fourth outermost layers of the cortically derived tissues of the
root primordium. As a result of the activity of the apical meristem
of the lateral root, its tip is torn away from the cortical tissues
of the primary root; and it ultimately penetrates the remaining
tissues, emerging into the soil.

The Vascular Transition. — Investigations of the transition
by Rutledge (34) and Whiting (40) are in close agreement with
the early interpretations of Gerard (ix) and Dangeard (6), but
differ from that of Lamounette (2.5). An essential feature of the
transition is that, in relation to the number that occurs in the
primary root, there is a doubling of the xylem and phloem strands.

CUCURBITA

597

There is also a change in the spatial relationship of the proto-
and metaxylem, so that the primary xylem matures centrifugally
to form endarch strands in the upper limits of the transition, in

'ap mer

Fig. 311. Transections of portions of primary root showing origin of secondary roots:
A, first radial divisions in cortex and tangential divisions in endodermal and outermost
pericyclic layers (nucleated cells in region of primordium); B and C, further divisions in
cortex and pericycle ; D, formation of central cylinder and origin of apical meristem ; E,
secondary root which has nearly penetrated cortex of primary root with its stele and apical
meristem established : ap mer, apical meristem ; c cl, central cylinder ; co, cortex ; en, endo-
dermis; pel, pericycle; pph, protophloem; px, protoxylem; stl, stele. (After Whiting.)

598 THE STRUCTURE OF ECONOMIC PLANTS

contrast to the centripetal development and exarch orientation in
the root. The relation of the primary phloem and xylem also
shifts from the alternate, radial arrangement to the collateral,
which ultimately becomes bicollateral with the development
of the inner phloem. In this manner, eight bicollateral bundles
are formed, separated by narrow medullary rays. (Fig. 313.)

In C. maxima, the transition region extends from a point some-
what below the peg where the axis is exarch and protostelic to
a point just above the peg where the vascular tissues form a com-
pletely endarch dissected siphonostele. This involves a segment
of the axis which is about a centimeter in length in a week-old
seedling. The first transitional development is the increase in
the number of thickened, parenchymatous cells between the large
central vessels. As a result, the pitted vessels are separated into
two groups, and the number of large metaxylem vessels is increased
to four or more. The narrow band of parenchymatous cells which
develops in the intercotyledonary plane increases in width until
a definite pith region is formed. (Figs. 311, B, C, and 313, B.)

As the pith develops, the tangentially orientated metaxylem
vessels lie in four groups which occupy positions near the periphery
of the stele and alternate with the protoxylem points. They are
centrad to the four phloem groups and separated from them by
several parenchymatous cells which later function as cambial
initials. (Fig. 311, C, D.) This is followed by an increase in
the number of reticulate vessels, and a lateral development of large
pitted metaxylem elements, so that the primary xylem forms a
hollow diamond enclosing the pith. (Fig. 313, C.) At higher
levels, the reorientation of the stelar tissues is more abrupt; the
pith enlarges, and the first inner phloem is differentiated centrad
to the xylem triangles formed by the original proto- and metaxy-
lem. (Figs. 3ii, E, and 313, D.) The cells of the inner phloem
have end walls that are perforated, and they resemble the sieve
tubes of the outer phloem except that they are much larger.

A millimeter or two higher, each protoxylem point divides
and the halves are separated by parenchymatous rays. (Fig.
313, F.) Thus, a dissected siphonostele of four transition bundles
is formed, each consisting of a tangential band of metaxylem
terminated at both ends by annular and spiral protoxylem elements
with the primary phloem lying in a position collateral to it. The
metaxylem of the bundle consists of small reticulate tracheae, and

CUCURBITA

599

two or more larger, later-maturing, pitted vessels which lie on
the outer face of the tangential band. (Fig. 314, A.') The inner

■~i ca

Fig. 311. A-F, transections of sectors of root and hypocotyl showing stages in vascular
transition: A, upper level of primary root; B, lower hypocotyl showing pith; C, 4 mm.
above B; D, hypocotyl about i mm. above C in which inner cambium is forming; E, about
1.5 above D in which metaxylem groups are bifurcated and there is a definite inner cambium
and inner phloem ; F, one of transition bundles showing lateral position of protoxylem,
inner cambium, inner phloem, interfascicular phloem and some of connective phloem cells :
ca, cambium; en, endodermis ; i ca, inner cambium; i ph, inner phloem; mx, metaxylem;
pel, pericycle ; ph, phloem ; pi, pith ; px, protoxylem. (After Rutledge.)

6oo

THE STRUCTURE OF ECONOMIC PLANTS

Fig. 313. Vascular transition. Seriesof transections showing: ^,root; B, differentiation
of pith ; C, enlargement of pith area with metaxylem in peripheral position ; D, differentia-
tion of inner phloem; E, tangential divergence of each primary xylem strand; F, establish-
ment of dissected siphonostele of four, tangential, transition bundles; G, formation of eight
bundles, each with protoxylem in tangential position ; H, abrupt enlargement of axis at peg,

CUCURBITA

6oi

with primary xylem becoming endarch ; /, endarch bundles completing transition anastomos-
continue through hypocotyl; K-P, vascular anatomy of cotyledons. P showing epicotyl
xS SrVh- ;^g.7''°^"^^ '-' -^'-'^ n^etaxylem/and solid blac. l^'^^.

6o2. THE STRUCTURE OF ECONOMIC PLANTS

phloem forms a zone on the centrad face of each bundle, but is
interrupted by the four rays. The outer phloem is continuous
with that of the root, having maintained its original position.
There is often a differentiation of phloem across the rays connect-
ing the adjacent areas of outer phloem; also a development of con-
nective phloem in the ray, which establishes continuity between
the inner and outer phloem groups of each bundle. (Fig. 3ii, F.)
The diameter of the axis increases in the region of the peg,
the pith becomes larger, the bundles are separated by wider rays,
and their number is increased from four to eight. (Fig. 313, 6^, H.)
These are transitional, since the metaxylem is still tangentially
oriented in relation to the protoxylem, except that a few large
pitted vessels are located on the outer face of the first-formed
elements in an endarch relationship. (Fig. 314, A.^ The inner
phloem is separated from the xylem by one or two layers of paren-
chymatous cells which may or may not initiate cambial activity,
and this situation also obtains in the zone between the lateral
phloem and the vascular tissue. The pericyclic zone is irregularly
multi-layered, the endodermis persists as a continuous uniseriate
layer, and root hairs develop approximately to the peg.

In the upper portion of the peg, the primary xylem develops
in an endarch relationship rather than tangentially. (Fig. 313, /.)
This is accompanied by a change in the metaxylem elements,
which, instead of being scalariform and reticulate, as in the root,
are chiefly spiral or loosely scalariform and somewhat larger in
diameter. There is also a greater continuity of the phloem, the
outer strand forming a broad mass capping each bundle, while
the inner one maintains its centrad position.

The Peg. — Although the transition occurs in the axis at the
point where the peg develops, there seems to be no constant
relationship between the transition and the structure of the peg.
Whiting points out that in some cases the greatest dimension of
the peg occurs at the level where there are four tangential transition
bundles; while, in other instances, the maximum diameter of the
peg is at the point where there are eight or more endarch bundles.
The pattern of transition is determined in the embryo at a time
when there is no evidence of an enlargement to form the peg, and
this structure does not begin to develop as a lateral ridge until
about the third day after planting.

The anatomy of the peg is simple, and its growth results from

CUCURBITA

603

Fig. 314. The transition region: A, dissected siphonostele with four transition bundles,
each with a tangential band of metaxylem terminated at either end by protoxylem ; the
outer phloem masses are joined across rays by connective phloem, and inner phloem is contin-
uous with outer phloem ; B, complete endarch bundles in region just above peg. One of
anastomosed end bundles is branching. (Photomicrographs by Whiting.)

6o4 THE STRUCTURE OF ECONOMIC PLANTS

numerous cambial-like divisions which occur in a plane extending
tangentially across the axis. In the peg, the greatest dimension
of each parenchymatous cell is the radial one; while in other
parts of the hypocotyl, the long axes of the cells are in the vertical
plane. At the lower limits of the peg, this activity is usually
cortical; but in the upper part, the divisions may also involve
the stelar tissues which accounts for the abrupt outward divergence
in the course of the bundles at this level. The broad lower face
of the peg may bear root hairs and is oriented at right angles to
the axis; or, less frequently, at an acute angle to it. The upper
part of the peg merges gradually into the hypocotyl and has the
same smooth cutinized epidermis characteristic of the latter.
Above this point, the epidermis produces hairs and stomata, the
latter occurring in a frequency of about lo to 15 per sq. mm.

Crocker, Knight, and Roberts (5) state that the development
of the peg is related to external factors which are concerned with
the arching of the hypocotyl, the contact of the seed coats, and
gravity. They express the opinion that there is no evidence that
gravity acts as a direct stimulus to the lateral placement of the
peg; and that

"Contact of the coats is by far the more effective for it will induce
very sharp arching even against gravity. . . . Arching leads to an
increased development of the peg, as well as to its lateral placement,
and in many cases produces a peg where it would not otherwise appear
as in Big Tom. Contact likewise increases the size of the peg independ-
ent of its effect through arch production."

The Hypocotyl. — The peg is at the base of the oval hypocotyl
which continues this configuration to the cotyledonary node,
the long axis being in the vertical plane of the cotyledons. The
course and number of the bundles in the hypocotyl is variable.
The eight original bundles are arranged two at each end of the
oval and two along each side (Fig. 313, H); but, usually, the
number increases just above the level of the peg. This is accom-
panied by branching and anastomoses of the eight bundles, so
that they may vary from 10 to as many as 16, the most common
number being 10 or ix. (Fig. 314, B.)

In the upper portion of the transition zone, the two bundles at
each end of the oval anastomose, and each of the resulting large
bundles continues up the axis for a short distance and then is
divided to form • three bundles. (Fig. 313, I, /.) These six,

CUCURBITA 605

together with the two on each lateral face, make up the basic
number of 10 bundles. Frequently, an additional one is formed
on each side of the oval by the branching of one or the other of
the lateral bundles. (Fig. 315, A.') The bundles so derived
continue up the axis to the cotyledonary node; but any of them
may give rise to one or more additional bundles, this being espe-
cially true of those located at the ends of the oval. (Fig. 313,
/-M.)

The Cotyledonary Node. — At the cotyledonary node, the
divergence of the cotyledons results in the formation of a compli-
cated vascular pattern which, like that of the hypocotyl, may
show many variations. About 3 or 4 mm. below the node, the
middle bundle at each end of the oval divides into two parts which
separate and anastomose with adjacent lateral bundles. These
continue in a tangential course until each end bundle anastomoses
with its respective adjacent central bundle, reducing the number
of bundles to four, two in the center of each broad side of the
hypocotyl. (Fig. 313, N.) The upward continuations of these
four bundles connect with the traces of the cotyledons. The four
traces continue upward and outward into the broad bases of the
cotyledons; but before the divergence of the cotyledons is complete,
each trace branches laterally so that a single vascular bundle
differentiates toward the margin of the cotyledon. (Fig. 313, 0.)
From this lateral vein, four or more large veins diverge upward,
and the two median traces also continue into the cotyledon where
they may anastomose to form a single median vein or remain
distinct. Thus, the vascular system of each cotyledon consists
of at least nine principal vascular bundles. (Fig. 313, P.)

An inner or adaxial phloem is present in the veins; and, in the
larger ones, connective phloem is also differentiated. Along
the adaxial surface of the vascular bundles there are zones of
collenchyma, and the mesophyll is 15 or more cell layers in thick-
ness. The three adaxial ones form a closely arranged palisade
region, and the remaining ix or more comprise the spongy paren-
chyma, which has some intercellular spaces. The cells of the
upper epidermis are somewhat larger than those of the lower and
may develop multicellular hairs, while the lower surface is glabrous.
Stomata occur in both surfaces with about equal frequency, Yasuda
(41) reporting 114 per sq. mm. in the upper, and 153 in the lower
for C. Pepo.

6o6

THE STRUCTURE OF ECONOMIC PLANTS

The Epicotyl. — At the time of germination, the epicotyl con-
sists of a small growing point covered by the primordium of the

con ph

Fig. 315. A, transection in upper transition region showing two bundles formed by-
branching of one transition bundle. The bundle at right is essentially endarch, but protoxy-
lem of other bundle is at one side ; B, transection a short distance above transition showing
bundle with connecting phloem strands and absence of inner cambium : ca, cambium ; co, cor-
tex; w« />A, connective phloem ; e«, endodermis ; //)/), inner phloem ; w />>&, metaphloem ; mx,
metaxylem; /»<:/, pericycle ; />/, pith; /^Z-A, protophloem ; /)a-, protoxylem. (After Rutledge.)

CUCURBITA 607

first leaf. After a week, as many as six leaves may be differen-
tiated, but the entire structure is still small and enclosed by the
bases of the two cotyledons. (Fig. 309, D.) In about two weeks,
the first leaf expands, being diverged at right angles to the plane
of the cotyledons.

The bundle supplying the first leaf usually anastomoses with
one or two branches of the cotyledonary trace; and the other
epicotyledonary traces are later differentiated against the trans-
verse bundles of the cotyledonary plate. The number of bundles
and their arrangement in the first internode are variable; but in
the second and third internodes, the two cycles of bundles are
established which characterize the upper internodes.

The Bicollateral Bundle. — Since Hartig (15) described the
occurrence of inner phloem in C. Pepo in 1854, there have been
many investigations of the vascular relationships that exist in
the Cucurbitaceae. Because differences in the interpretation of
the phloem have arisen, a brief summation of the divergent views
is here included. De Bary (3) originated the term "bicollateral"
to apply to vascular bundles which have

"two groups of phloem one being situated ... on the outside of the
xylem and a second on its inner side."

He pointed out that

"both groups of phloem have the typical structure . . . and are espe-
cially remarkable for the size of their sieve tubes. They are frequently
connected by means of a narrow band, fringing the lateral edge of the
bundle, and containing some sieve tubes, so that in these cases the
bundle, strictly speaking, belongs to the concentric type." (Fi-.
315.5.)

Herail (17) agreed with de Bary, interpreting the bundle as
truly bicollateral, since he found the development of all three
parts — outer phloem, xylem, inner phloem — to be synchronous.
Von Faber (9) concluded that both the inner and outer phloem
were similar in structure, arising from a single procambial strand,
and regarded the term "bicollateral" as accurately describing the
bundle.

On the other hand, Lamounette (2.5) thought that the inner
phloem developed subsequently to the outer, being formed by the
activity of parenchymatous cells in the medulla. He observed no
inter-connection between the two and advocated the abandonment

6o8 THE STRUCTURE OF ECONOMIC PLANTS

of the designation "bicollateral." Baranetsky (i) regarded the
inner phloem as of independent origin, and Worsdell (41) stated
that "the medullary phloem represents, probably in all cases, a
vestigial structure, the remnant of a former system of medullary
vascular bundles in which the xylem has disappeared."

With respect to the interconnection of inner and outer phloem,
there has also been some difference of opinion. Gerard (ii) traced
the course of the bundles in the root and stem, finding that the
inner phloem consists of branches from the outer. Fischer (11)
stated that, in the transition region, the inner phloem ends blindly
at its lower extremity; but distinguished four kinds of phloem,
one called commissural because it connects the other types. The
three categories in addition to commissural phloem are: (i) hypo-
dermal or ectocyclic phloem, occurring between the epidermis and
the stele; (x) fascicular phloem, an integral part of the bundle;
and (3) entocyclic phloem, formed within the stele. Holroyd
(18) adopted the terminology of Fischer, attributing the origin
of the internal, or entocyclic, phloem to the development of what
he designated "perixylary" cambium, stating that "through imme-
diate activity of the inner cells of this cambium . . . patches of
internal phloem originate in front of each bundle."

Whiting has investigated the situation in C. maxima in connec-
tion with the ontogeny of the hypocotyledonary bundle. The
provascular strand at first consists of small elongated cells without
intercellular spaces. (Fig. 316, A.^ The annular or loosely
spiral protoxylem elements develop first; but they do not arise
at the extreme inner margin of the procambial strand, one or more
of the centrad layers remaining undifferentiated. The outer
protophloem is formed at the same time, followed by the meta-
phloem, which consists of sieve tubes and companion cells that
can be identified in seedlings five days old. (Fig. 316, B.) By
the sixth day, the small protophloem cells are crushed and
resorbed; and, in the meantime, the inner phloem differentiates
in the procambial tissue on the centrad margin of the provascular
strand, forming sieve tubes and companion cells similar to those
of the outer metaphloem.

The development of the inner phloem is centripetal, and a 2one
of undifferentiated parenchyma remains between it and the primary
xylem. In this region, the inner cambium arises later in ontogeny,
but it remains inactive during the primary growth. (Fig. 316, C.)

CUCURBITA

609

This and other investigations indicate that the vascular bundle
of the Cucurbitaceae should be regarded as strictly bicollateral,
if the criteria for that type as outlined by Herail (17) are accepted.
These are the differentiation of the inner phloem from the pro-

FiG. 316. Transections showing stages in differentiation of hypocotyledonary vascular
bundle: A, provascular strand in embryo just prior to germination; B, vascular bundle of
three-day seedling showing development of outer protophloem ; C, vascular bundle of seven-
day seedling in which primary differentiation is nearly complete, with crushed protophloem
elements, fibers, metaphloem, inner phloem, and connective phloem which is conspicuous
because of dark staining cells : cci, companion cells of inner phloem ; can, companion cells of
metaphloem; co, cortex; dsc, dark staining cells in connective phloem; en, endodermis;
/■ ph, inner phloem ; mx, metaxylem ; pel, pericycle ; pi, pith ; pph, protophloem ; pp//, paren-
chyma cells of protophloem ; px, protoxylem ; src, sieve tubes of connective phloem ; .m,
sieve tubes of inner phloem ; stm, sieve tubes of metaphloem. (After Whiting.)

cambial strand and the simultaneous development of the inner
and outer phloem.

Secondary Thickening of the Root. — Secondary thickening
of the root is initiated by the differentiation of a cambium in the
zone of fundamental parenchyma betw^een the primary xylem and

6io

THE STRUCTURE OF ECONQMIC PLANTS

phloem. The secondary tissues appear in transection as four large
wedge-shaped sectors that are separated from one another by
prominent rays formed by the continued activity of the pericyclic
cells. (Fig. 317.) These may be interpreted as pericyclic rather
than secondary xylem rays, since the cambium is confined to the
four sectors of vascular tissue. As secondary thickening proceeds,
the cortical and epidermal tissues are split; but the pericyclic
region continues to keep pace with cambial activity, maintaining
itself as a multi-layered zone outside of the secondary phloem

and producing a phellogen which
forms the protective periderm.

The secondary xylem consists
of large vessels, tracheids, fibers,
and small isodiametric paren-
chymatous cells which surround
the vessels and are connected with
them by simple or half-bordered
pits. The short vessel segments
are very wide, in some cases being
0.5 mm. in diameter; and in the
ontogeny of the vessel, the end
walls of a series of segments are
digested so as to form a continu-
ous trachea. (Fig. 10.) Tyloses
occur frequently in the older
vessels as a result of the intrusion of bladder-like projections of
adjacent parenchymatous cells through the pits. These sac-like
protrusions contain cytoplasm and usually a nucleus, and the intro-
duction of several such processes, together with their further
division, frequently results in the complete filling of the vessel
with tyloses that become flattened by reciprocal pressure. (Fig.
318.)

The large size attained by the vessels throws them out of a regular
radial alignment, so that they appear as scattered units distributed
through each of the xylem wedges. The fibers are long and slender
with tapering ends and their number and arrangement may vary.
In some cases, the secondary xylem consists largely of fibers and
tracheids surrounding the vessels; in others, the fibers occur
only in small groups; but in most instances, each vessel is nearly
enclosed by tracheids which are separated from it by a zone of

Fig. 317. Diagrammatic transection of
mature root : ca, cambium ; />c/, pericycle ;
fcl ra, pericyclic ray ; pd, periderm ; ph,
phloem; xy i, primary xylem strand; xy
1, secondary xylem.

CUCURBITA

6ii

parenchyma. The number and distribution of the parenchymatous
cells are also variable, but they are usually more numerous in the

xy par

pel ra

Fig. 318. Transection of sector of mature root showing ray and secondary tissues: ca,
cambium ; pel, pericycle ; pel ra, pericyclic ray ; pd, periderm ; ph, phloem ; ty, tyloses ; ves,
secondary xylem vessel ; xy par, xylem parenchyma.

6ix THE STRUCTURE OF ECONOMIC PLANTS

peripheral portion of the secondary xylem. They are small and
isodiametric except that those adjacent to the vessels are stretched
periclinally. Occasionally, the sectors of secondary xylem may
be partially segmented by xylem rays of thin-walled parenchyma
which extend about half way to the center of the axis.

The total amount of secondary phloem produced is not large as
compared with the secondary xylem. It occupies four zones out-
side the secondary xylem, the groups being separated from each
other and bounded externally by pericyclic tissue. The sieve
tubes are among the largest known, attaining an average diameter
of 0.05 mm., and Yasuda (41) has reported a width of 0.067 ^im.
for C. Pepo. The sieve plates have very large pores located in the
horizontal or slightly inclined end walls. The companion cells
are relatively large, and there may be three or four of them in a
linear series adjacent to a single sieve tube.

Secondary Thickening of the Hypocotyl. — Above the transi-
tion region where the bundles are strictly endarch, secondary
thickening is initiated in the usual manner, and the fascicular
cambium in each of the ten or more bundles produces secondary
xylem and phloem elements similar to those described for the root.
As the stele enlarges, the ray and pericyclic tissues keep pace so
that the bundles are separated from each other by parenchymatous
tissue. The outer phloem is limited outwardly by the persistent
pericycle, which forms a cap of fibers. The cortex and epidermis
persist longer than in the root but ultimately disintegrate and a
phellogen of pericyclic origin produces a periderm. The medullary
parenchyma also disintegrates so that the hypocotyl is hollow at
maturity.

The inner cambium becomes active soon after the outer one,
producing phloem elements centripetally and parenchymatous
cells centrifugally. (Fig. 319.) As Holroyd (18) has pointed
out, the outer cambium of each bundle may become continuous
with the inner cambium around the lateral and anterior faces of
each bundle, forming what he terms "perixylary" cambium. From
the inner cells of this cambium, strands of inner phloem originate
centrad to each bundle. The growth of the inner phloem is some-
what variable; and, since the inner cambium also cuts off paren-
chymatous cells in large numbers, the phloem strands in old hypo-
cotyls are frequently separated from the protoxylem of the bundle
by a wide zone of parenchyma. In some cases, this may become

CUCURBITA

613

active and produce strands of phloem adjacent to the primary
xylem. (Fig. 3 19.) The lateral sectors of the perixylary cambium
remain inactive for some time, but later produce groups of phloem
which extend tangentially from the flanks of the meta- and
secondary xylem of the bundle.

As the hypocotyl matures, the radial elongation of the paren-
chymatous cells separating the inner phloem from the protoxylem

-/ — m r

Fig. 319. Transection of centrad portion of hypocotyledonary bundle showing develop-
ment of inner and connective phloem : ca, inner cambium ; con ph, connective phloem ; i ph,
inner phloem; /«r, medullary ray ; /»/», phloem; />/, pith ; atj i, primary xylem; x)- 1, second-
ary xylem.

increases the interval between the two, and the inner cambium
may cut off some xylem elements centrifugally, forming an obcol-
lateral bundle. In addition to the phloem produced by the outer,
inner, and perixylary cambiums, a limited amount of intraxylary
phloem may be produced in the xylem parenchyma. This has been
noted by Holroyd (18) in C. Pepo, and by Rutledge (34) in
C. maxima. The situation with respect to the phloem is further
complicated by the fact that strands of ectocyclic phloem are
originated in the pericycle outside the bundles, and also in the

vas bu

-- -med cav

614 THE STRUCTURE OF ECONOMIC PLANTS

ray regions. All of these phloem types are interconnected by
strands of connective phloem, so that this diverse and complex
system may be regarded as having continuity although some of
the elements end blindly in parenchymatous tissue at various points

in the axis.

The Stem. — The vascular bundles of the stem are arranged in
two rings. The smaller ones of the outer ring are located at the
angles of the stem, and the larger bundles of the inner ring are
alternate with those of the outer. (Fig. 3x0.) The basic number

of bundles is ten, each cycle
consisting of five, although
occasionally additional
smaller ones may be pres-
ent. All the bundles are
common, and the bi-cyclic
character of the dictyostele
is related to the mode of
insertion and downward di-
vergence of the traces of the

Fig. 1,7.0. Diagrammatic transection of stem ^ , , ,

showing distribution of bundles : «/, coUenchyma ; leaf. Each vaSCukr Strand

ep, epidermis ; med cav, medullary cavity ; par, paren- eXtends downwatd thrOUgh

chyma; ../, sclerenchyma ; .^x ^ vascular bundles. ^^^ ^^^^ ^^^ ^^ average of

two internodes before anastomosing with another bundle. Within
the inner ring, the pith becomes disintegrated, and a large irregu-
larly shaped cavity is formed.

Outside the outer ring of bundles and continuous with the
medulla is a broad zone of parenchymatous tissue. This is limited
externally by an uninterrupted band of sclerenchyma five or more
cell layers in width; but the continuity may be broken in old
stems by secondary growth. (Fig. 311.) The parenchyma and the
adjacent fibers together constitute the pericyclic zone. There is
a narrow band of chlorenchyma outside the sclerenchymatous
ring; and between this and the epidermis is a discontinuous band
of collenchyma several cells in width which is interrupted by the
chlorenchyma at regular intervals. The epidermis is regular with
a thin, smooth cuticle, and stomata occur, where it is subtended
by chlorenchyma, in a frequency of about xo per sq. mm. Numer-
ous rigid multicellular hairs are produced on somewhat raised
bases; and of the four types of hairs occurring in the Cucurbi-
taceae, three are present on the stems of C. Pepo. These are sharp-

CUCURBITA

615

pointed, conical hairs; short-stalked, and long-stalked, glandular
ones.

The bundles are bicollateral but not as large as those in the hypo-
cotyl, and there is less cambial activity. The size of the secondary
xylem vessels is about the same as in the root, and the sieve tubes
may be larger, reaching a diameter of 0.088 mm. There is fre-
quently a well-defined inner cam-
bium in the larger bundles which
produces some phloem and pa-
renchyma, but a lateral perixylary
cambium is usually lacking. Iso-
lated strands of phloem occur in
the parenchymatous tissues of
the pericylic rays and medulla;
and, to a limited degree, in the
parenchyma between the epider-
mis and the sclerenchymatous
ring. Commissural or connective
sieve tubes serve to join all the
phloem elements into a continu-
ous system.

Crafts (4) has described the
sieve tubes as being of two types:
(i) the central tubes of the phloem
proper which consist of short,
broad elements; and (i) the pe-
ripheral tubes, including commis-
sural, entocyclic, and ectocyclic
phloem, which vary greatly in

cav

Fig. 311 . Transection of sector of mature
stem : ca, cambium ; cav, medullary cavity ;
«/, collenchyma; f/;, epidermis ; iph, inner
phloem; o ph, outer phloem; par, paren-

length, but tend to be of uniform ^hy'"^; /"• P"^; sd, sderenchyma ; xy I,
^ primary xylem ; xj i, secondary xylem.

diameter. Slime droplets occur

in the young sieve tubes, and protoplasmic connections, which
are continuous with the parietal cytoplasm, traverse the sieve
plates connecting successive elements in mature sieve tubes.
Callus develops around each cytoplasmic strand that passes through
the sieve plate, and the callus cylinders increase in diameter until
they fuse laterally to form a perforated plate which increases in
thickness. The callus may become definitive, and slime plugs
occur under certain conditions. In the peripheral phloem the
appearance of the definitive callus takes place late in the ontogeny

6i6 THE STRUCTURE OF ECONOMIC PLANTS

of the sieve tube in conjunction with its obliteration or partial
collapse.

On the basis of the structural characteristics of the sieve tubes,
Crafts proposed that the movement of substances might take place
"partly through sieve tube lumina and partly through phloem
walls," but the subject of translocation is a controversial one and
physiologists are by no means in agreement on this point. In
this connection, the role of the phloem parenchyma in transloca-
tion has been generally ignored, and it seems very probable that
a considerable proportion of the materials translocated in plants
is moved through parenchymatous tissues.

The Leaf. — The leaves are relatively thin as compared with
other cucurbits. The radial walls of the upper epidermal cells
are straight, while those of the lower epidermis are sinuous in
outline. The stomatal frequency in C. Pepo is about 150 per
sq. mm. for the upper surface as compared with about 350 on
the lower one. Hairs are also more abundant on the lower surface
than on the upper in a ratio of approximately four to one, there
being about 80 per sq. mm. on the former. According to Yasuda
(41), the lower epidermis in C. Pepo may be two or three layered
in some places; and at these points, the leaf has an etiolated appear-
ance. The palisade region comprises about three-fifths of the
thickness of the blade, consisting in some parts of a double row
of cells, and there are from two to six layers of loosely organized
isodiametric parenchymatous cells in the spongy region.

The subterete petiole is slightly grooved on its adaxial surface
and the center is hollow throughout. A limited number of
stomata occurs in the epidermis, the frequency being approximately
II per sq. mm.; and trichomes develop as in the blade. Strands
of collenchyma occur outside the ring of fibrovascular bundles
which are arranged in pairs, except for the largest one which
extends along the abaxial side of the petiole.

The Tendril. — As described by van Tieghem (37), the branched
tendril of the cucurbits is a shoot, the terminal portion being a
specialized leaf arising on a branch. This appears to be an axillary
branch, but it is actually a terminal structure that is laterally
displaced owing to the sympodial system of branching. Lisk (x6)
has pointed out that, although the physiology of the tendril has
received exhaustive study, there has been relatively little work
done on its anatomical structure. In C. Pepo, the proximal portion

CUCURBITA

617

of the tendril is subterete with a central hollow in its main
unbranched section. In the intermediate portions, it is more or
less polygonal, and the distal part is somewhat dorsiventrally
flattened. (Fig. 3xx, A, B.) Haberlandt (14) observed that
the tendrils are sensitive only on the abaxial side, which is usually
devoid of hairs. There are a few stomata which occur in other
sectors of the epidermis in a frequency of about 16 per sq. mm.

Fig. 3x2.. A, diagrammatic transection of proximal portion of tendril showing distribution
of vascular bundles and mechanical tissue ; B, transection of distal portion in region of curva-
ture. The diagrams are oriented with adaxial surface uppermost. The epidermal cells of
abaxial surface contain sensory spots shown in C. These spots are underlain by tactile pits as
shown in transection of similar cells in D. The pit (j) may contain a small crystal of calcium
oxalate. In A and B, collenchyma is indicated by cross-hatching; sclerenchyma is lined;
and chlorenchyma is designated with chloroplasts. The phloem of bicollateral bundles is
stippled and xylem is black. (C and D, redrawn after Strasburger, Textbook of Botany.')

The epidermis of the sensitive portion of the tendril is unlike
that of other regions in several respects. The cells are usually
smaller and the outer walls have little cuticle until after the
tendril is attached to a support, when they become thick and
heavily cutinized. Haberlandt found that the sense epithelia
on the sensitive side are located above a continuous wide band of
collenchyma. The sense cells are somewhat stretched; and, in
the center of each outer wall, there is a single sensitive spot or
papilla. (Fig. 32.1, C.) The interior of the papilla forms a

6i8 THE STRUCTURE OF ECONOMIC PLANTS

funnel or tactile pit, and a canal leading from it enters the lumen
of the cell which has a thick layer of cytoplasm containing chloro-
plasts and a large nucleus. (Fig. 3x1, D.) The papilla is com-
pletely filled with cytoplasm and a small crystal of calcium oxalate
usually lies in it, although sometimes the crystal may be located
in the canal leading to the cell. As in the cystoliths which occur
in the epidermal hairs of Cannabis, the function of these crystals
is not definitely known. They may be entirely non-functional
excretion products, but it is possible that they are related to the
irritability of the tactile cells.

Within the epidermis is a layer of collenchyma which is variously
distributed in different parts of the tendril. In C. maxima and
C. Pepo, Penhallow (3i) found the collenchyma interrupted on
the adaxial surface, and on the right and left flanks by chloren-
chyma in which intercellular spaces subtend the stomata. Within
the collenchyma, there are three or four rows of large parenchym-
atous cells. The sclerenchyma forms a continuous ring in the
proximal portion of the tendril, but occurs only on the abaxial
side in the distal part. At the base, the five to seven vascular
bundles are arranged in a ring around the hollow central region.

Miiller (p.^') found evidence of sclerenchyma in the tendrils of
cucurbits in the bud stage, and observed a progressive lignification
of this tissue and the xylem as the tendril matured. The portion
of the tendril which surrounds a support becomes swollen, hard,
and brittle owing to the enlargement and lignification of the cells
of the epidermis, parenchyma, and collenchyma. In contrast,
the portion which forms the spiral is not generally lignified except
the xylem and sclerenchyma; and, as a result, this part remains
more elastic.

Dastur and Kapadia (7), in investigating the mechanism of
curvature in the tendrils of cucurbits, found that the parenchym-
atous cells on the convex (adaxial) side of the tendril elongate
and produce the curvature, while the character of the mechanical
tissue on the concave (abaxial) side tends to prevent elongation
of this surface. The abaxial surface shows the effect of pressure
resulting from curvature since the transverse walls are thrown into
folds that project into the lumina of the cells. Actual measure-
ments of the cells of the epidermis, and of the underlying rows
of cells on the adaxial or convex side, indicate an average increase
in length of 69.1 per cent.

CUCURBITA 619

LITERATURE CITED

I. Baranetsky, J., "Recherches sur les Faisceaux bicollateraux." Ann. Set.
Nat., Bot. Ser. VIII, 12: 301-304, 1900.

I. Barber, Kate G., "Comparative histology of fruits and seeds of certain

species of Cucurbitaceae." Bot. Gaz- 47: z63-3io, 1909.

3. DE Bary, a., Comparative Anatomy of the Vegetative Organs of the Phanerogams

and Ferns, Clarendon Press, Oxford, 1884.

4. Crafts, A. S., "Phloem anatomy, exudation, and transport of organic nutrients

in Cucurbits." Plant Phys. y: 183-1x5,1931.

5. Crocker, W., Knight, L. I., and Roberts, Edith, "The peg of the Cucur-

bitaceae." Bot. Gal- jo: 311-339, 1910.

6. Dangeard, p. a., "Recherches sur la mode d'union de la tige et la racine

chez les dicotyledones." Le Bot. i: 75-115, 1889.

7. Dastur, R. H., and Kapadia, G. A., "Mechanism of curvature in the tendrils

of Cucurbitaceae." Ann. Bot. 4;: 179-301, 1931-

8. Eichler, a. W., Bliithendiagramtne, I; W. Englemann, Leipzig, 1875.

9. VON Faber, F. C, "Zur Entwicklungsgeschichte der bikollateralen Gefass-

biindel von Cucurbita Pepo." Ber. Deut. Bot. Gesell. 22: 196-303, 1904.

10. FiCKEL, J. F., "Ueber die Anatomie und Entwickelungsgeschichte der Samen-

schalen einiger Cucurbitaceen." Bot. Zeit. ^4: 737-744; 753-760; 785-
791, 1876.

11. Fischer, A., Untersuchungen uher die Siehrohrensystem der Cucurbitaceen, Berlin, 1884.
II. Gerard, R., "Recherches sur le passage de la racine a la tige." Ann. Sci.

Nat., Bot. Ser. VI, 11: 179-430, 1881.

13. GoEBEL, K., Organography of Plants, II, Clarendon Press, Oxford, 1905.

14. Haberlandt, G., Sinnesorgane in Pflanzenreich zur Perception Mechanischer Reize,

117-139, 1901.

15. Hartig, T., "Uber die Querscheidewande zwischen den einzelnen Gleidern

der Siebrohren in Cucurbita Pepo." Bot. Zeit. 12: 51-54, 1854.

16. Heimlich, L. F., "The development and anatomy of the staminate flower of

the cucumber." Am. Jour. Bot. 14: 117-137, 1917.

17. Herail, J., "Etude de la tige des Dicotyledones." Ann. Sci. Nat., Bot. Ser.

VII, 2: 101-314, 1885.

18. Holroyd, R., "Morphology and physiology of the axis in Cucurbitaceae."

Bot. Gaz. jS: 1-45, 192-4-

19. Janczewski, E., "Recherches sur le developpement des radicelles dans les

phanerogames." Ann. Sci. Nat., Bot. Ser. V, 20: 108-133, 1874.

lo. , "Recherches sur I'accroissement terminal dans les phanerogames."

Ann. Sci. Nat., Bot. Ser. V, 20: 161-101, 1874.

II. JuDsoN, J. E., "The morphology and vascular anatomy of the pistillate flower

of the cucumber." Am. Jour. Bot. 16: 69-86, 1919.
11. KiRKwooD, J. E., "The comparative embryology of the Cucurbitaceae."
Bull. N. Y. Bot. Gard. y 313-401, 1905.

13. , "The pollen-tube in some of the Cucurbitaceae." Bull. Torr. Bot.

Club 5j.- 317-341, 1906.

14. KoNDO, M., "iJber die in der Landwirtschaft Japans gebrauchten Samen."

Ber. Ohara Inst, fiir landw. Forsch. i: 161-314, 1918.

6io THE STRUCTURE OF ECONOMIC PLANTS

zy Lamounette, B., "Recherches sur I'origine morphologique du liber interne."
Ann. Set. Nat., Bot. Ser. VII, ii: i93-2.8i, 1890.

16. LisK, Henrietta, "Cellular structure of tendrils." Bot.Gai.78: 85-101,1914.

17. LoNGO, B., "Richerche sulle Cucurbitaceae e il significato del percorso inter-

cellulare (endotropico) del tubetto pollinico." Trans. R. Accad. Lined. V,

4- 52-3-547, 1903-
i8. Miller, W. L., "Staminate flower of Echinocystis lobata." Bot. Gaz_. 88:

161-184, 1919.
19. MiJLLER, O., "Untersuchungen iiber die Ranken der Cucurbitaceen." Cohn
Beit. Biol. Pflanien 4: 97-143, 1887.

30. Naudin, C, "Organographie vegetale observations relativs a la nature des

vrilles et la structure de la fleur chez le Cucurbitacees." Ann. Sci. Nat.,
Bot. Ser. IV, 4: 5-10, 1855.

31. Payer, J. B., Traite d' organogenic eomparee de la fleur. Ordre des Cucurbitaeees,

Paris, 1857.
31. Penhallow, D. p., "Tendril movements in Cucurbita maxima and C. Pepo."
Am. Jour. Sci. and Arts }i: 46-57; 100-114; 178-189,1886.

33. Russell, P., "Identification of the commonly cultivated species of Cucurbita

by means of seed characters." Jour. Wash. (D. C.) Aead. Sei. 14: 165-169,
1914.

34. RuTLEDGE, R. W., "The histological anatomy of the hypocotyl of Cucurbita

maxima, Duchesne." Unpubl. Ph.D. Thesis, Univ. of Chicago, 1930.

35. van Tieghem, p., "Recherches sur la symetrie de structure des plantes vascu-

laire. I. Memoire sur la racine." Ann. Sci. Nat., Bot. Ser. V, ij: 1-314,
1871.

36. , "Recherches sur la structure du pistil et sur I'anatomie eomparee

de la fleur." Mem. Acad. Sci. Inst. Imp. France 21: 1-161, 1875.

37. , and CosTANTiN, J., Elements de Botanique, I, Masson & Co., Paris, 1918.

38. , and DouLioT, H., "Origine des radicelles et des racines laterales chez

les Legumineuses et les Cucurbitacees." Bull. Soe. Bot. France, j_j.- 494-

501, 1886.

39. Weaver, J. E., and Bruner, W. E., Koot Development of Vegetable Crops,

McGraw-Hill Book Co., 1917.

40. Whiting, A. Geraldine, "Development and anatomy of primary structures

in the seedling of Cucurbita maxima Duchesne." Bot. Gaz- 99' 497-52-8,
1938.

41. Worsdell, W. C, "The origin and meaning of medullary (intraxylary)

phloem in the stems of Dicotyledons. I. Cucurbitaceae." Ann. Bot.
29: 567-590, 1 91 5.
41. Yasuda, a., "On the comparative anatomy of the Cucurbitaceae, wild and
cultivated, in Japan." Jour. Coll. Sci. Imp. Univ. Tokyo, Japan 18: 1-56,
1903.

CHAPTER XX

COMPOSITAE

LACTUCA SATIVA

ALTHOUGH the composite family is one of the largest in the
plant kingdom, it includes relatively few representatives of
economic importance. Of those grow^n as vegetable crops, but
two are outstanding: lettuce, Lactuca sativa L., and the globe
artichoke, Cynara Scolymus L. Another member of this group
is the Jerusalem artichoke, Helianthus tuberosus L.; and other less
important ones include endive, Cichorium endiva L.; chicory, C.
intybus L.; and salsify, Tragopogon porrifolius L. Of these
forms, lettuce is by far the most important, ranking as one of the
larger crops in Arizona and California.

The plant has enjoyed recent popularity and was also well
regarded by the ancients. According to de Candolle (4), it has
been in cultivation for more than xooo years and is said to have
been served on the tables of the Persian kings in 400 b.c. Because
of its long usage, the relationship of the cultivated form to the
numerous wild species is not definitely known. It is thought by
most authorities that it may be a descendant from the compass
plant, Lactuca scariola L., which was introduced into this country
from Europe and now grows throughout the United States as a
weed.

Bailey (i) recognizes four fairly distinct horticultural types:
(i) L. sativa, var. capitata, Hort., which is the common cabbage
type forming a head; (x) L. sativa, var. intybacea, Hort., or cut-
leaved lettuce which is loosely spread; (3) L. sativa, var. romana,
Hort., or Cos lettuce, whose leaves grow rapidly erect, forming a
conically shaped plant; and (4) L. sativa, var. augustana, Hort.,
known as asparagus lettuce, with long entire leaves that have a
slightly spreading habit. In commercial practice, the nomen-
clature has become extremely confusing owing to the large number
of trade names that have been given to the numerous variants

6ii

6zi THE STRUCTURE OF ECONOMIC PLANTS

of the four principal types. This is illustrated by Morse (i8),
who has compiled a list of approximately iioo names that have
been applied to the lettuce varieties in this country. This involves
tremendous duplication as there are probably less than 150 distinct
varieties even on the basis of minor differences. In order to
clarify the situation, Morse has described the forms w^hich he
regards as constituting the standard American varieties. Twenty-
two types are included, the most important being the New York
variety which, with its sub-type Imperial, is estimated to represent
approximately half of the commercial lettuce crop grown for
shipment.

GENERAL MORPHOLOGY

The lettuce plant is an annual; and, as the generic name implies,
one of its important characteristics is the presence of a milky juice
or latex. In head lettuce, the plant may produce heads in 65 to
90 days depending upon the strain used; while in California, where
a majority of the seed is grown, the crop is planted in late December
or January and the fruits begin to ripen early in August. (Fig.

32-3-)

The Root. — The plant has a strong fleshy tap root which

develops very rapidly. Weaver and Bruner (19) have described
the development of the root system and find that under favorable
conditions the tap root may elongate at the rate of an inch per
day, and, in some soils, may attain a maximum depth of over 6
feet by the time that the flower stalks are producing blossoms.
The average depth is approximately 5 feet at maturity, and the
major portion of the entire system occupies the first x feet of soil.
The principal lateral roots arise in two rows chiefly in the first
10 inches of soil, extending outward for 6 to 18 inches and then
downward. There are few laterals originating below the first
foot of soil and many of the lower ones are short and thread-
like. Those which arise from the upper portion of the tap root
may equal it in length, and like it attain a diameter of an inch or

more.

The Shoot. — At first, the plant develops a fleshy crown stem
from which basal leaves arise in an alternate spiral phyllotaxy,
the degree of compactness varying with the variety. (Fig. 3x4.)
In some instances, they form such a firm, solid head that in the
production of seed it must be partially cut open in the field in

LACTUCA SATIVA

613

order to permit elongation and emergence of the seed stalk. In
the non-heading types, the basal leaves are less compact and more
spreading; while, in the Cos or Romaine types, they are semi-erect

Fig. 313. Lettuce plant in seed. (Courtesy of the Ferry Morse Seed Co.)

624

THE STRUCTURE OF ECONOMIC PLANTS

to erect. The leaves vary in color from light to dark green;
and, in some forms, this is modified by a reddish or brown pigment.
This accessory pigment may be light, medium, or dark in intensity
and is a sufficiently constant character to be used in the descrip-
tions of commercial varieties.

Cabbage varieties, known as butter heads, are so called because
of their flavor and the oily appearance of the inner leaves, which

ifsseftr-f^^r* '^ ■

Fig. 314. Field of lettuce in head, variety New York. (Courtesy of the Ferry Morse

Seed Co.)

are thick and soft in texture with entire margins. The leaves
of the crisp or curled cabbage types are brittle with conspicuous
veins and margins that are crinkled or curled. Non-heading or
bunching varieties may have leaves of either the butter or the
crisp type. In cut-leafed lettuce, the loosely spreading leaves are
6 to 10 inches long and deeply incised on the edges. The Cos
variety has spatulate leaves that are long and entire or sometimes
sparingly dentate. They are upright in habit of growth and form

LACTUCA SATIVA

615

heads i or z feet in height. In the asparagus type, the lanceolate
leaves are 6 to li inches in length, i or x inches in width, usually
entire but sometimes lobed.

The Inflorescence and Flower. — The floral axes reach a
height of 2. to 4 feet or more and form a branched, bushy crown.
The branches bear leaves that are commonly sagittate and clasping
and are terminated by the inflorescences. The inflorescence con-
sists of a cymose cluster of heads each of which contains from 15
to X5 or more flowers, al-
though fewer may occur
in some instances. The
oldest head of the in-
florescence is the terminal
one, and the lateral heads
are axillary. (Fig. 3x5,
A.') The ligulate flowers
are all perfect and alike.
The flower primordia arise
simultaneously from a
naked receptacle which is
at first conical, then con-
vex, and finally flat and
broad. (Fig. 316, y4.) As
the individual primordium
enlarges, a marginal ring
forms, which marks the
initiation of the corolla
tube. (Fig. 316, B.) This
elongates and a fold of
meristematic tissue arises
at the base of its adaxial
surface from which the staminal rmg develops. At the same time,
a ring of tissue is formed outside and at the base of the corolla tube
which gives rise to the pappus. (Fig. 32.6, C, D.) The carpellary
primordia are the last to appear; and, by the time they begin to
differentiate, the corolla has started to incurve over the tip of the
floral axis. (Fig. 3x6, D, £.) The single cavity of the ovary is
formed by the upward growth of the carpellary tissue. The
growth of the undi verged tissue of the carpels, corolla tube, and
calyx tube results in the elevation of the anthers, the strap of the

Fig. 315. A, habit of flower; B, unopened head;
C, longisection through head ; D, single flower ;
E, flower with corolla removed ; F, akene with
pappus ; G, floral diagram.

e2.G THE STRUCTURE OF ECONOMIC PLANTS

ligulate corolla, and the pappus so that the flower is epigynous.
(Fig. 3^6, £, F.)

The mature pappus is white, fine, and soft, and ultimately
functions in the dissemination of the fruit. (Fig. 315, D, f.)
The sympetalous ligulate corolla is yellow or whitish yellow, and
the truncate, five-toothed ray consists of five undi verged petals.
Each ray inclines obliquely outward so that the expanded head is
about a centimeter and a half in diameter. (Fig. 3x5, C, D.) The

da,

Tpap

Fig. 3x6. Floral development : A, head, showing flower primordia at time of development
of corolla; B, same, showing development of corolla, stamens, and pappus; C, D, and E,
successive stages in development of individual flower ; F, longisection through flower showing
early and later stages in development; ^«, anther; wr, carpel ; c/^, corolla ; w, ovule; ovy,
ovarian cavity ; pap, pappus ; p m c, pollen mother cells ; sta, stamens. (After Jones, Hil-
gardia.^

filaments of the five stamens are diverged from the base of the
corolla tube as separate structures, but the anthers are united,
forming a cylinder which projects beyond the corolla tube and
surrounds the style. The ovary is unilocular and the slender
style is unbranched up to its point of emergence from the anther
cylinder where the two stylar branches terminate in a two-lobed
stigma. (Fig. 3x5, D, £.) The head is surrounded by a cylindri-
cal involucre consisting of a series of bracts that are closely im-
bricated in the bud, the outermost bracts being shorter than the
inner ones. (Fig. 3x5, B.)

LACTUCA SATIVA

617

m m c

^'-'in m c

—int

-m m c

Megasporogenesis. — In most cases, a single ovule arises at
the base of the ovary, but the occurrence of tw^o or more ovules is
not uncommon. There is an early differentiation of a single
hypodermal archesporial cell which, according to Jones (13),
functions as a megaspore mother cell. (Fig. 3x7, A.') The
nucellar tissue consists of
a single layer; and it never
becomes thicker, since its
cells divide only anti-
clinally as the megaspore
mother cell enlarges. The
one massive integument
begins to differentiate at
about the time the mega-
spore mother cell is clearly
differentiated, and its uni-
lateral growth results in a
gradual curvature of the
ovule until it becomes
completely anatropous.
(Fig. 3^7, B, C.) This is
accomplished by the time
the first or heterotypic division of the megaspore mother nucleus
occurs.

The formation of the linear tetrad of megaspores is similar to
that reported for related genera. The three micropylar megaspores
disintegrate and the innermost one is functional forming an eight-
nucleate megagametophyte by successive nuclear divisions. Jones
did not observe an early fusion of the polar nuclei as is reported for
some composites, and states that it occurs at the time of fertiliza-
tion. Although the number of antipodals has been reported as
variable for other members of this family, he was unable to observe
more than three in any instance. This seems to be the constant
number for most of the investigated Cichorieae. A conspicuous
integumentary layer, which persists until the seed is almost
mature, surrounds the megagametophyte and is regarded as having
a nutritive relation to the developing embryo.

MicROSPOROGENESis. — Microsporogenesis is initiated by the
development of a single row of hypodermal archesporial cells.
These divide periclinally to form a row of pollen mother cells and

QS''"nls

Fig. 317. A-C, stages in development of ovule:
inf, integument \ mm c, megaspore mother cell ; nh,
single-layered nucellus. (After Jones, Hilgardia?)

6i8 THE STRUCTURE OF ECONOMIC PLANTS

an outer layer of cells, the latter by subsequent divisions forming
the tapetum, the middle layer, and the endothecium. Gates and
Rees (lo) report that there are from 15 to lo pollen mother cells
producing a total of about 60 pollen grains. They observed that
the pollen mother cells frequently separate before synapsis and
this is confirmed by Jones. The walls of the microspores are
formed by furrowing which is initiated at the periphery of the
mother cell and extends centripetally until the four furrows meet.
Following the secretion of the walls of the microspores, the
original wall of the mother cell disintegrates, and at about this
time the tapetal cells become binucleate and later quadrinucleate.
The nuclei may be more or less completely disintegrated even
prior to the disorganization of the tapetal walls and finally a
Plasmodium is formed. The microspores continue their develop-
ment within the anther and undergo nuclear division so that at
anthesis each pollen grain possesses a vegetative nucleus and two
slender filamentous microgametes.

Pollination. — The floral buds elongate very rapidly in the
14 hours preceding anthesis, and the involucral bracts enclosing
the head begin to open at the summit because of the development
of the individual flowers within. The flowers open early in the
morning and remain so for about two hours, after which the corolla
closes tightly and does not again open. All the flowers of a
single head open at one time; and, two or three days later, the
corollas, stamens, and the wilted styles and stigmas are shed. In
California, where a large proportion of the lettuce for seed purposes
is grown, the crop is usually ready for harvest in July or early
August; but, in some places, the flowering peak may occur in
June and individual plants may produce flowers over a period of
two months or more.

The mechanism of pollination which has been described by
Knuth (15), Jones (13), Oliver (19), and Thompson (17) is such
that the flowers are almost entirely self-pollinated. Natural
cross-pollination has been observed, however; and, on the basis
of genetic experiments, Thompson found that natural crossing
occurred in zo different populations in percentages ranging from
1.33 to 6.1.-L, indicating that "the extent of natural crossing may
be greater than is generally believed." Insect visitations have
been observed in several instances, which also suggests that there
is opportunity for cross-pollination.

LACTUCA SATIVA 619

The method of pollination is related to the structure and on-
togeny of the flower. The stigma develops papillae on its inner
surface and these become hairs by the time the stigmatic lobes
expand. On the outer surfaces of the stigmas and style, brush
hairs are developed; and, since the anthers dehisce before the
flower head opens, these hairs gather masses of pollen as they are
pushed through the anther cylinder by the elongation of the style.
Knuth has stated that following the emergence of the stigmas,
"they ultimately roll back into a complete circle, so that automatic
self-pollination necessarily results from contact of the stigmatic
papillae with the pollen-grains clinging to the sweeping-hairs."
Jones found that, in some varieties at least, the backward revolu-
tion of the stigmatic lobes was not as complete as reported by
Knuth. There were a few pollen grains on the inner stigmatic
surface, but the edges of the stigmatic lobes were always covered
with pollen as well as the backs and he pointed out that "it is
not known definitely whether or not the pollen grains will germi-
nate elsewhere on the pistil than on the stigmatic papillae."

Fertilization and Embryogeny. — Fertilization follows soon
after pollination, and at the end of six hours, there may be some
two-celled embryos, while in practically all flowers fertilization
is completed. In double fertilization the second microgamete and
the two polar nuclei unite almost simultaneously to form the
endosperm nucleus.

Jones (13) has investigated the embryogeny in Lactuca and
finds that it agrees in general type with that of other Compositae
as reported by Carano (5) and Soueges (i6). Following fertiliza-
tion, the zygote elongates and develops a definite cell wall; and,
shortly thereafter, the first division of the zygote occurs. This is
transverse and cuts the zygote into a terminal cell, a, from which
the cotyledons and epicotyl develop, and a lower cell, h, which
forms the hypocotyl. (Fig. 3x8, A^) In the formation of the
four-celled embryo, the cell a forms two cells by a longitudinal
division, while cell h divides transversely to form two daughter
cells, c and d. (Fig. 3x8, B.)

In the formation of the eight-celled embryo, the initiation of
nuclear activity seems to be apical, and each tier of cells usually
divides slightly in advance of the cells below it. In the eight-
celled stage, tier a has four cells, tier c two cells, and e and / are
derived from the division of cell d. (Fig. 3x8, C.) The i6-celled

630 THE STRUCTURE OF ECONOMIC PLANTS

stage is reached about 15 hours after pollination; and, as in the
development of the latter, the cells of tier a are the first to divide
so that an octant is formed and the embryo is temporarily in a
li-celled stage. No further divisions occur in tier a until all the
lower tiers have undergone division. This occurs in sequence,
tier c dividing first, followed by tier e, and the transverse division
of cell / to form the daughter cells, g and /», thus completing the
formation of the i6-celled embryo. (Fig. 3x8, D.)

— 'phm

e - ^^ I ^/y phm in

'dgn

Fig. 318. Stages in embryogeny : A, two-celled embryo in process of division, 9 hours
after pollination ; B, four-celled embryo at ii hours ; C, eight-celled embryo at 2.0 hours ;
D, sixteen-celled embryo at i6 hours ; E, embryo showing differentiation of histogens at
46 hours ; F, initial lobing preceding formation of cotyledons at 3 days ; G, lower portion of
embryo showing differentiation of primary root. Letters a to h refer to the succession of cell
divisions (description in text) : dgn, dermatogen ; dgn in, dermatogen initial ; phm, periblem ;
/»^w /■«, periblem initial ; /ic/, pericycle; /^Z, plerome; />/ i«, plerome initial ; r c, cells of root
cap (this includes all shaded cells except those of suspensor, sus^. (After Jones, Hilgardia.')

Beyond the i6-celled stage, it is difficult to follow the sequence
of cell divisions; but the next development is the cutting off of
definite histogens (dermatogen, periblem, and plerome) by the
formation of tangential and vertical walls. (Fig. 3x8, £.) Three
days after anthesis, slight elevations can be observed in tier a
which later become the cotyledons. (Fig. 318, F.) Cell g under-
goes transverse and longitudinal division to form two layers of
cells, and those of the upper layer constitute the dermatogen initials
while the lower ones contribute to the root cap. The derivatives

LACTUCA SATIVA 631

of cell h produce the suspensor, which is variable in length and
in the number of its cells. (Fig. 318, G.)

Endosperm Formation. — The divisions of the primary endo-
sperm nucleus and the nuclei subsequently derived from it occur
slightly prior to those of the embryo; and by the time the latter
is four-celled, there usually have been four successive divisions
of the endosperm nuclei. About to hours after pollination when
the embryo is eight-celled, wall formation is initiated in the
endosperm; and its cells completely fill the embryo sac. They
continue rapid growth and division, keeping pace with the enlarg-
ing cavity; but the developing embryo grows more rapidly than
the endosperm which it digests, and a week after pollination the
endosperm is almost entirely resorbed. When the seed is mature,
only the two outermost layers of endosperm remain. These are
closely appressed to the cell walls of the innermost layer of the
integument, forming a membrane which completely surrounds
the embryo.

ANATOMY

Fruit and Seed. — During the preceding developments which
take place within the young akene, the bracts of the involucre
are tightly compressed about the developing fruits. The beak of
each fruit elongates and rapidly lifts the pappus upward, so that
within four or five days after anthesis it appears through the
bracts. In the New York variety, Jones found that approximately
li days elapsed from anthesis to fruit maturity, subject to variation
due to temperature and other factors. In this variety, the heads
contain about 18 flowers and produce a slightly smaller number of
normal akenes.

The beaked akene is spindle- or lance-shaped with a surface
that has a number of longitudinal ribs. Kondo (14) found a
variation of from 13 to 13 in 1.1. varieties. In the New York
variety, the average length of the akene is 4.1 mm.; the breadth
I.I mm. and thickness 0.4 mm. Cummings (8) has determined
that size of seed is a factor in production, stating that

"the merits of large seed in lettuce culture were shown in the production
of larger seedlings, an increased weight of edibly matured plants,
which displayed better heading-up capabilities, earliness and uniformity
in filling the heads; in short, augmented earliness and quality."

632.

THE STRUCTURE OF ECONOMIC PLANTS

Borthwick and Robbiiis (3) have investigated the anatomy of
the fruit and seed coat. In the young ovary, just prior to anthesis,
the nucellar tissue surrounding the megagametophyte is almost
entirely digested, and the massive integument of the single anatro-
pous ovule consists of from ix to 18 rows of parenchymatous cells.
The innermost layer is readily distinguishable from the others
because of the larger size of its cells. Even at this early stage,

-pep

-}--o ep

emb s-'"!

Fig. 3x9. A, transection of portion of ovary z hours after anthesis ; B, transection of akene
7 days after anthesis : emh, embryo ; emb s, embryo sac ; end, endosperm in which some wall
formation is still in progress ; i ep, inner epidermis of integument ; int, integument ; nls, disin-
tegrating cell of nucellus ; o ep, outer epidermis of integument ; pep, pericarp. (Redrawn
after Borthwick and Robbins, Hilgardia.')

the integumentary parenchyma may give evidence of some dis-
integration. The pericarp varies in thickness and its cells resemble
those of the integument except that they are somewhat smaller.
There is a distinct outer epidermis; but, by the time anthesis
occurs, lysigenous cavities have begun to develop in the inner
portion of the pericarp, and disintegration of all except the outer-
most layers of the ovary wall proceeds rapidly. (Fig. 3x9, A.^
A week after anthesis, there is further disintegration of the tissues
of pericarp and integument as the embryo enlarges and the endo-
sperm is formed. By the tenth day, the cells of the inner epidermis

LACTUCA SATIVA

633

of the integument are disorganized, and the inner walls of the
laterally placed epidermal cells may invest the endosperm so
closely that they appear to be a part of it. Borthwick and Robbins
determined that this translucent integumentary membrane is
continuous and semi-permeable.

As the akene matures, the ribs of the pericarp develop. (Fig.
330, A, JB.) These are rather equally spaced and consist of thick-
walled sclerenchymatous fibers which are pitted and lignified.
cot

■ S c

l-vas bu

fr c-^

^ al gr

Fig. }}o. a, transection of akene; B, transection of portion of fruit coat of light-colored
akene ; C, same of dark fruit : al gr, aleurone grain ; cot, cotyledon ; end, endosperm ; /;• c,
fruit coat; rb, rib; s c, seed coat; vas bu, vascular bundle. (Redrawn after Kondo, Ohara
Institute.)

The remainder of the pericarp in the mature akene is only one or
two cells in thickness since the other cells of the parenchymatous
portion of the fruit coat become disorganized. There is a wide
range in the color of the fruit from buff to dark brown and from
light gray to carbon gray or black. This difference is due in part
to the presence or absence of pigment in the epidermis of the
pericarp. In the dark-colored fruits, the outer epidermis is filled
with a dark brown pigment which is entirely lacking in the light-
colored fruits. (Fig. 330, B, C)

The cells of the mature integument are much crushed and par-
tially disintegrated, but the outer epidermis may persist as a

634 THE STRUCTURE OF ECONOMIC PLANTS

thick-walled layer. The suberized semi-permeable membrane
referred to above, which is a part of the wall of the inner epidermis,
lies adjacent to the endosperm. In most parts of the seed, the
endosperm is two cell layers in width; but, adjacent to the tip
of the primary root, it may be three or four cells in thickness. Its
cells are thick-walled; and, in some cases, projections from the
walls extend into the cell cavities which are filled with fatty
substances and protein granules.

Fig. 331. A, surface and side views of akene ; B-E, stages in development of seedling ; F,
outline of cotyledon showing pattern of main veins ; G, transection through akene showing
two cotyledons surrounded by endosperm, seed, and fruit coats, variety New York Regular.

Development of the Seedling. — The optimum germination of
lettuce seed is obtained where there is an adequate supply of
water, a temperature not in excess of i5° C, and good aeration.
Borthwick and Robbins secured the best results when the seed
was stored in moist folds of burlap at 4° C. for a period of four to
six days preceding planting. The percentage of germination falls
rapidly at temperatures above 15° C. and may be completely
inhibited at 30° C. in most varieties. The experimental evidence
indicates that this inhibition is due to the structures surrounding
the embryo, which consist of the two-layered endosperm and the
semi-permeable membrane of the integument. These are thought

LACTUCA SATIVA

635

to retard gas exchange; since, when all of the pericarp, seed coat,
and endosperm are removed from seeds, the naked embryos initiate
growth within 14 hours. In cases where only the pericarp and

— cot

--eel

-Ix d

Fig. 332.. Longisection through epicotyl and cotyledonary node of six-day seedling
showing development of first foliage leaf and presence of lactiferous ducts. Cortical lactifer-
ous ducts which occur in seedling axis are not shown in this plane, variety New York Regular :
cot, cotyledon; eel, growing point of epicotyl; hyp, hypocotyl; /, first foliage leaf; Ix d,
lactiferous duct.

outer portions of the seed coat were removed, so that the embryo
remained enclosed by the endosperm and the integumentary
membrane, Borthwick and Robbins found no germination at 30° C.

636

THE STRUCTURE OF ECONOMIC PLANTS

stl

CO

-en

The tip of the primary root emerges from the basal end of the
fruit coat and grows rapidly downward through the soil. This
is followed by an elongation of the hypocotyl which raises the
cotyledons and the partially enveloping remains of the akene. In
some instances, the cotyledons are freed from the combined fruit
and seed coats as they emerge from the soil. At the time of

emergence, the cotyledons
are in a horizontal posi-
tion; but as growth pro-
ceeds, they are reoriented
until they are nearly verti-
cal. (Fig. 331.) As the
two blades expand later-
--pcl ^22y^ j-j^g minute epicotyl

is exposed; and, within a
few days, the first foliage
leaf appears. (Fig. 332..)
The formation of lateral
roots is initiated early;
and in about six days,
secondary roots appear on
the first inch and a half
of the primary root . They
are usually diverged in
two opposite double rows
owing to the fact that

Fig. 333. Median longisection of tip of primary ^^ originate at a slight
root showing histogens : w, cortex ; dgn-cal , derimt- i j U A

ogen-calyptrogen ; en, endodermis ; ep, epidermis ; angle tO and On each Side

/?^«?, periblem ; />f/, pericycle ; /»/, plerome ; /■ <r, root q£ ^^^ j-^q prOtOXylem
cap; stl, stele.

points.

Ontogeny of the Primary Root. — The development of the
primary root and the vascular anatomy of the seedling axis have
been investigated by Port (ix), who used the New York variety.
The ontogeny of the root is similar to Janczewski's (11) third
type, in which three histogens are differentiated that produce the
stele, cortex, epidermis, and root cap. (Fig. 333.) As noted
earlier, the histogens originate in the embryo and are well defined
four days after pollination of the flower. (Fig. 318, E, F, G.)

The innermost histogen or plerome gives rise to a cylindrical
stele. The cells of this region divide in all three planes, but

— r c

LACTUCA SATIVA

637

divisions in vertical planes predominate in connection with the
differentiation of the vascular elements. This, coupled with
axial elongation, results in the development of slender, much-
elongated cells. The outermost layer of the plerome forms the
pericycle, which can be distinguished early by the larger size of
its cells. Centrad to the pericycle, activity can be observed at
two opposite poles where the cells of the primary phloem are

Ix d-

ph 2

B

Fig. 334. Lactiferous ducts : A, longisection through cotyledon showing lactiferous duct
on abaxial face of phloem ; B, transection of portion of stele and endodermis of primary root
of one-day-old seedling in which primary phloem and lactiferous ducts are formed and pro-
toxylem has just begun to differentiate; tangential divisions of endodermis are shown;
C, longisection through secondary phloem of old root showing lactiferous ducts with branches
and cross-anastomoses, variety New York : en, endodermis ; Ix d, lactiferous duct ; pel, peri-
cycle; ^/[> I, primary phloem; /)A 1, secondary phloem ; pAr, protoxylem.

formed. This occurs slightly in advance of the differentiation of
the primary xylem, which is initiated on radii at right angles to
the phloem. Abutting the pericycle, annular and spiral pro-
toxylem vessels develop, establishing the two points of the diarch
xylem strand. The maturation of the primary xylem elements
progresses centripetally, and the last ones to be differentiated are
the reticulate vessels that occupy the center of the axis and complete
the primary strand. (Fig. 335.) Port has noted the occurrence
of lactiferous ducts in the pericycle and primary phloem. These

638

THE STRUCTURE OF ECONOMIC PLANTS

are articulate and develop from a linear series of cells through the
dissolution of the end walls. They are located in the arcs of the
pericycle which lie outside the two groups of primary phloem, and
additional ducts may also be differentiated on the centrad surface
of the phloem. There are no ducts in the pericycle outside the
protoxylem. (Fig. 334, B.)

The cortical cells are derived from the periblem. The endo-
dermal layer adjacent to the pericycle consists of regularly arranged

Fig. 335. Transection of portion of primary root at initiation of cambial activity showing
lactiferous ducts in pericycle and tangential division of an endodermal cell, variety New York
Regular : ca, cambium ; co, cortex ; en, endodermis ; ep, epidermis ; Ix d, lactiferous duct ;
mx, metaxylem; pel, pericycle; ph i, primary phloem; px, protoxylem.

cells, and the differentiation of Casparian strips occurs at about the
time that secondary thickening of the root is initiated. A unique
development in the ontogeny of the endodermis is the tangential
division of single cells or sectors of cells. This happens first in
arcs lying outside the primary phloem, and finally the entire
endodermis may become biseriate. Phillips (xi) has described a
somewhat similar situation for Cynara, also a member of the
tribe Cichorieae, in which doubling of the endodermis occurs
very early in ontogeny, taking place prior to the differentiation
of the stelar elements. The remainder of the cortex consists of

LACTUCA SATIVA 639

six to eight layers of parenchymatous cells that are round in
transection and two or three times as long as broad, with regular
intercellular spaces.

The third histogen, the dermatogen-calyptrogen, gives rise to
both epidermis and root cap. The manner of its division has been
concisely described by Port,

"The cells of this layer by periclinal divisions perpetuate the histogen
and form successive layers of root cap cells which may subsequently
divide in one or more planes to keep pace with the growth of the
root axis. The cells which form the proximal margin of the dermat-
ogen-calyptrogen layer undergo a final periclinal division and each
inner daughter cell then functions as an epidermal initial thereafter
dividing only in anticlinal planes to form the epidermis. In this
manner a uniseriate epidermis develops and the initiating layer is
maintained."

At maturity, the primary root has a diarch radial protostele
in which the two primary phloem groups lie on the flanks of the
primary xylem strand and are separated from it by fundamental
parenchyma which later functions as a procambial zone. The
protoxylem points abut the pericycle which is surrounded by the
uniseriate or partially biseriate endodermis. The cortical paren-
chyma is limited externally by an epidermis and there are numerous
root hairs. (Fig. 335.)

Secondary Thickening. — Secondary thickening of the root is
initiated very early in ontogeny, in the zone of fundamental paren-
chyma lying between the primary phloem and xylem, and the
production of secondary vascular tissues may occur in seedlings
approximately eight days old. The secondary xylem consists of
parenchyma and large vessels that are laid down in single or
frequently double radial rows. The rows are separated by ray
parenchyma and the diarch character of the primary xylem is
evident even in older roots because of the development of two
prominent pericyclic rays that extend toward the periphery on the
same radii as the two protoxylem points. The vessel segments are
three to four times as long as broad, the walls being densely pitted
with oval or elliptical pits that have their longest dimension in
the transverse plane. The elongated xylem parenchyma cells
surrounding the vessels are at first thin-walled, but become lignified
in mature roots. The ray parenchyma cells are isodiametric or
sometimes elongated in the radial dimension.

640 THE STRUCTURE OF ECONOMIC PLANTS

The secondary phloem is comprised of groups of long, narrow
sieve tubes and companion cells surrounded by larger parenchyma-
tous cells. There are numerous latex tubes which consist of very
slender cells arranged in longitudinal series and forming a con-
tinuous system owing to numerous cross anastomoses. (Fig. 334,
C) The latex ducts are relatively infrequent in the pericyclic
tissue which surrounds the secondary phloem and forms the outer-
most zone of the root, and none occur in the secondary xylem.
Latex branches may ramify through the pericyclic tissue inter-
connecting the latex tubes of adjacent phloem strands, and they
also penetrate the pericycle together with the vascular tissue at
points where lateral roots are diverged.

There is an early loss of the cortex and epidermis as a result of
secondary thickening, and the outermost cells of the pericycle
function as a phellogen forming the periderm. The primary
pericyclic cells are rectangular in longisection, and may become
much extended tangentially as cellular growth compensates for
the increasing size of the axis.

Vascular Transition. — Among the studies of vascular transi-
tion in the Compositae are those of Chauveaud (6), Lee (17), who
investigated fifty species including Lactuca sagittata Waldst.,
Siler (i4) on Arctium minus Bernh., and Phillips (xi), who
studied Cynara Scolymus L.

In Lactuca sativa L., Port (ii) finds that the vascular transition
agrees in general plan with that reported by Lee (17) for L. sagit-
tata. The primary vascular system of the seedling axis, including
the primary root, hypocotyl and cotyledons, is a continuous one
which is distinct in its origin from that of the stem; but late in
ontogeny the downwardly extended traces of the epicotyledonary
leaves anastomose with the vascular strands of the hypocotyl.
The vascular transition involves a rather short portion of the
seedling axis. It begins in the upper third of the hypocotyl, and
the complete endarch condition is attained about halfway up the
midrib of the cotyledon.

The cotyledon has three main veins, a midrib and two small
laterals. (Fig. 336, H.) In the distal portion of the cotyledonary
blade, the bundles are endarch collateral, and the lateral ones
remain so even to a point below the cotyledonary node. In the
proximal portion of the cotyledon, the metaxylem of the midrib
is differentiated laterally in relation to the protoxylem, forming a

LACTUCA SATIVA

641

B

Fig. 336. A-H, successive levels in vascular transition in seedling axis, diagrammatic
sectors based upon condition in seedlings six days old (description in text). The solid areas
represent protoxylem, lined regions, metaxylem, and dots, primary phloem, variety New York
Regular.

64X THE STRUCTURE OF ECONOMIC PLANTS

triad, as Compton (7) has termed it, in which the two metaxylem
arms are tangentially oriented with respect to the protoxylem.
(Fig. 336, G.) At about the level of cotyledonary divergence, the
lateral orientation of the metaxylem is more marked; and, instead
of a single group of phloem elements lying on the abaxial side of
each bundle, there are two strands that are laterally placed and
separated from each other by parenchymatous cells. (Fig. 336,
E, F.)

Lower in the hypocotyl, the phloem is differentiated progres-
sively in a more lateral position, and may form a varying number
of distinct strands that are farther apart from one another than at
any point higher in the seedling axis. At this level, the bundles
occupy positions nearer the center of the hypocotyl, constituting
the primary stele of the axis. Below the cotyledonary node, the
opposed lateral bundles of the two cotyledons unite in pairs to
form two small bundles which lie at right angles to the larger
median bundles coming from the midribs of the cotyledons.
(Fig. 336, £.) These laterals end blindly somewhat lower in the
hypocotyl, but are later joined to the two large bundles by the
development of secondary vascular tissue.

At about the level at which the lateral bundles terminate, the
stelar pattern undergoes further change. The metaxylem arms of
each bundle are differentiated in a lateral position so that they
form a straight line with their respective protoxylem points.
(Fig. 336, D.) The phloem groups lie closer to each other, and
each bundle may be divided into two more or less distinct strands.
Slightly below this point, the metaxylem is differentiated laterally
and centripetally so that two tangentially exarch bundles are
formed. (Fig. 336, C.)

Near the middle of the hypocotyl, the relation of protoxylem
and metaxylem is strictly exarch. The diarch character of the
stele is evident although there are still parenchymatous cells at
the center of the axis separating the two halves of the xylem strand.
At this point, the phloem is differentiated laterally in relation
to the long axis of the xylem strand and forms two distinct groups.
(Fig. 336, B.) In the lower part of the hypocotyl, the stele is
similar to the primary root. (Fig. 336, A.')

The Cotyledons. — The fleshy cotyledons are alike in structure
but there may be considerable variation in their size and shape.
They have wide bases and the most common form is spatulate

LACTUCA SATIVA

643

with a broadened apex which is frequently notched at its extreme
tip. Lee (17) has observed that the venation of the cotyledons
in the composites is characterized by two facts: (i) the main
bundles never end blindly in the mesophyll, but anastomose with
one another in the terminal por-
tion of the blade; (i) the lateral
bundles on either side of the mid-
vein always unite with it at the
apex of the cotyledon. Lettuce
corresponds in this respect with
the Arctium type in which there
are two lateral veins and a midrib
at the base of the cotyledon.
The laterals follow the margins
of the blade, uniting with the
midrib at the apex of the cotyle-
don; and, in addition to them,
two major branches of the midrib
arise near the middle of the coty-
ledonary blade, pursue a course
parallel to the laterals for some
distance, and join them near the
tip of the cotyledon. (Fig. 331,
P.) All minor branches arising
in the basal portion of the blade
anastomose with one of the ma-
jor veins at some point so that
the venation constitutes a closed

■sz^lx d

Fig. 337. A, transection of main vein
near base of ten-day-old cotyledon showing
location of lactiferous ducts ; B, transec-
tion through portion of same cotyledon
nearer apex of blade, variety New York
Regular : ep, epidermis ; hr, hair ; Ix d,
lactiferous duct; pal, palisade; ph i, pri-
mary phloem; spo, spongy tissue; sto,
stoma; xy i, primary xylem.

system.

The mesophyll of the fleshy
blade consists of a palisade region
a single layer in thickness, and a spongy zone of several layers.
(Fig. 337, B.) The spongy cells adjacent to the palisade layer
are frequently elongated in the same plane as the palisade cells
so that they form a transitional type. As the cotyledon expands,
the intercellular spaces enlarge and the mesophyll becomes porous.
The vascular bundles are collateral, and neither Jean (ii) nor
Port (xi) found any evidence of an inner phloem associated with
them, such as is present in the stem and leaf. Lactiferous ducts
occur on the abaxial face of the phloem and occasionally in the

644 THE STRUCTURE OF ECONOMIC PLANTS

phloem strand. These parallel the course of the bundles and
are frequently interconnected by anastomoses and cross branches.
(Fig. 334, A.')

Fig. 338. Transection of portion of stem showing bundle and medullary phloem, a part of
cortex is not shown, variety New York Special: ca, cambium; col, collenchyma; en, endo-
dermis; ep, epidermis; Ix d, lactiferous duct; m ph, medullary phloem; par, parenchyma;
/)f/, pericycle; ph, phloem; a^, xylem.

Stomata occur in both epidermal surfaces, the frequency being
greater in the abaxial one. The epidermis is also characterized
by multicellular epidermal hairs which consist of a single or
double row of cells that are terminated by a capitate enlargement.

(

LACTUCA SATIVA

645

m ph

Ixd

The Stem. — In the early stages of its development, the stem
does not undergo internodal elongation, and the leaves, which are
diverged in a spiral phyllotaxy (commonly a % arrangement), are
crowded on the axis. As a result, either a head or a close rosette
of basal leaves surrounds the crown stem, which becomes large
and fleshy, owing to the pronounced increase in the parenchym-
atous tissue of the pith.
Later in ontogeny, the
seed stalk elongates and
the buds in the axils of the
upper leaves also expand
so that a branching crown
is formed which bears the
inflorescences.

The young stem is round
or oval and somewhat
ridged below each node
owing to the downwardly
extending bundles of the

. (. rpi -1 • Fig. 339. Diagrammatic transection of partly

leai trace. ine epiaermiS mature floral axis showing arrangement of bundles,

is glabrous except for medullary phloem and distribution of lactiferous

nrrasional sratrered hairs ducts, variety New York Special : f^, cambium; CO,
occasional SCatterea nairs ^^^^^^. ,„^ gndodermis ; ./;, epidermis ; /x^, lactif-

and contains numerous StO- erousduct; w ^^, medullary phloem ; /?c/, pericycle ;

mata in which the guard ^^^-phioem; xj, xylem.
cells are usually oriented with their long axes parallel with that
of the stem. The epidermal cells are irregular in outline, having
their long dimension in the vertical plane, and the walls are
curving as seen in surface view. In transection, they are square
or rectangular and their outer surfaces are moderately cutinized.
Within the epidermal layer are two or more rows of compact
collenchymatous cells. The large parenchymatous cells of the
cortex are loosely organized and are limited centripetally by the
endodermis. This layer is differentiated from the other cortical
tissues by the continuity of its cells, whose radial walls abut
without intercellular spaces. (Fig. 338.)

The vascular tissue of the immature stem constitutes a dissected
siphonostele, but the collateral bundles are soon connected,
through the development of interfascicular cambium, and a con-
tinuous vascular cylinder is formed. The pericycle becomes
multiseriate, and numerous lactiferous ducts develop in its inner

646 THE STRUCTURE OF ECONOMIC PLANTS

layer, especially in sectors adjacent to the phloem. (Fig. 339.)
In addition to the ducts occurring in the pericycle, smaller ones
are also found in the medullary phloem strands. They commonly
develop around the inner periphery, but may extend through the
central portion of the phloem, and similar ducts sometimes occur
in the outer phloem. The primary vascular elements of the
bundle are annular-spiral or spiral, the latter type frequently
forming walls w^ith double spiral thickenings. Slender, elongated
parenchymatous cells separate the vessels from each other. The
secondary vessels are large and angular with transversely elongated
pits which sometimes give the vessel segments the appearance
of being scalariform or reticulate.

Medullary Bundles. — Medullary phloem strands appear in
the pith centrad to the primary xylem so that the bundles may
appear to be bicollateral. However, the late development of the
medullary phloem strands, in addition to the fact that their
arrangement and the number of strands adjacent to a bundle may
vary, indicates that these units do not constitute an integral part
of the vascular bundle.

With respect to this situation Solereder (15) has pointed out
that in the Cichorieae there may be medullary vascular bundles
formed by the addition of xylem to the phloem. This was
observed by Petersen (lo), who noted intraxylary phloem in
Lactuca and described the occurrence of some xylem in connection
with this tissue. Weiss (30) also investigated this matter and
demonstrated that the chief bundles of the vascular system which
have inner phloem associated with them are not true bicollateral
bundles. Specifically referring to L. sativa, he stated that the
medullary phloem is directly continuous with the leaf trace in
some instances, and that the small lateral phloem strands adjacent
to the protoxylem groups are direct continuations of strands
arising on the adaxial face of the leaf bundle. Other phloem
strands were found to pass into the pith as branches from the outer
phloem of the stem bundle, so that they could be regarded as being
only indirectly connected with leaf traces. Kruch (16) also
investigated this group extensively, and concluded that the medul-
lary strands might be direct continuations from the phloem asso-
ciated with leaf or branch traces. In a few cases, including
Lactuca, there are both foliar and cauline strands.

Worsdell (31), having studied medullary phloem in the Com-

LACTUCA SATIVA 647

positae from the standpoint of its phylogenetic implications,
confirmed the work of previous investigators with respect to
the course of the strands in the stem of Lactuca sativa and L. virosa.
His summation follows :

"Medullary strands occur throughout the stem. In the higher part
of this organ phloem-strands only occur. In the basal region they
acquire xylem and become bundles. In this basal region, at a still
lower level, they lose the xylem, and eventually either die out in situ
or join the vascular ring. The amount of xylem in the medullary
strands varies with the thickness of the stem, and is always secondary
in origin. The fate of the strands, if traced upwards, is fourfold.
Some of them end blindly in the pith. Some pass into the vascular
ring. Others pass out to form the medullary system of a branch; but
in one and the same plant the branch-system may arise both in this
way and also de novo in its pith-tissue. The small peripheral phloem-
strands persisting in the higher part of the stem and peduncle become
part of the ordinary vascular system of the flowers."

The Leaf. — The gross morphology and the differences which
exist between the leaves of lettuce varieties have been pointed out.
In the New York variety, the sessile and somewhat auriculate,
clasping leaves of the floral axis have a central vascular region
which is thickened and concave on its adaxial surface. The
principal veins extend through this fleshy region parallel to one
another with occasional cross-connections occurring between the
lateral members and the central bundle. From the lateral veins of
this parallel series, branches extend outward to the periphery
of the blade in a pinnate arrangement. These in turn branch and
rebranch, forming a fine net-veined system in which some of the
ultimate veinlets end in small islands in the leaf blade while others
terminate in the marginal teeth. In the basal leaves of the heading
and bunching types, the principal veins are spreading and pal-
mately arranged ; but even the basal leaves of the Cos type have a
prominent midrib and a pinnate system of venation.

Except in the region of the midrib and of the major laterals,
the blade is thin. The epidermal cells of both upper and lower
surface are extremely sinuous as seen in surface view, except for
those which lie over the principal veins where the cells are elon-
gated with oblique or somewhat pointed ends. These regions
also differ from the remainder of the epidermal surface in that
there are few stomata. Elsewhere they occur in large num^bers
and in about equal frequency on both surfaces. As pointed out

648 THE STRUCTURE OF ECONOMIC PLANTS

by Eames and MacDaniels (9), the guard cells belong to the type
in which the wall is unevenly thickened and the exposed margins
of the two cells project as thickened ridges. (Fig. 340, B.)
Epidermal hairs occur at the margin of the leaf between the teeth,
and also along the veins. These are numerous in young leaves
but break up and wither as they mature. The hairs are multi-
cellular and usually capitate, the column consisting of either a
single or double row of cells arising from one or two short basal
epidermal cells.

Fig. 340. A, transections of segments of young foliage leaf ii days old showing character
of mesophyll, development of hairs and lactiferous ducts, Ix d; B, side section of stoma and
guard cells, variety New York Regular.

The mesophyll is uniform; and, in the young leaf, the paren-
chymatous cells which comprise it are approximately isodiametric
and rather compactly arranged. (Fig. 340, A.') As the leaf
enlarges, the mesophyll becomes more porous and large inter-
cellular spaces are formed. Even at maturity, there is little or
no differentiation of the mesophyll into a palisade and spongy
region, the chief difference between the adaxial and abaxial cells
being that the former are somewhat more compact, while the latter
tend to elongate in a plane parallel to the flat surface of the blade.
The distribution of chlorophyll is variable, depending upon the
variety as well as upon the position of the leaf. In the heading
types, many of the inner leaves are practically devoid of pigment,
while in leafy types and in the leaves arising on the floral axes,
chlorophyll is present in the guard cells and in all of the paren-
chyma of the mesophyll with the exception of the cells adjacent
to the veins. These usually have no chlorophyll although those

LACTUCA SATIVA 649

immediately abutting the vein may have a few plastids. There
may also be small plastids in the xylem parenchyma of the larger
veins.

The main bundles are collateral, but there are strands of adaxial
phloem which follow the course of the bundles and extend into
the medullary region of the stem. Lactiferous ducts occur around
the periphery of the abaxial phloem and there may be similar
ducts lying in the same relation to the adaxial phloem. The
primary xylem elements are chiefly of the spiral type. The
larger bundles develop a cambium and the secondary xylem vessels
are similar to those of the stem. The smaller veins do not have
strands of adaxial phloem associated with them and the ultimate
veinlets consist of single tracheids.

Lactiferous Ducts. — The presence of lactiferous ducts has
been noted in descriptions of the various organs of the plant, but
they are sufficiently characteristic to warrant a summary of their
occurrence and distribution. The development of articulated lac-
tiferous ducts in the Cichorieae has long been known and fre-
quently investigated. The early research on this point has been
summarized by de Bary (i) and Scott (2.3), and the latter has
described the origin and ontogeny of the ducts for a member of
this tribe. When the ducts occur in primary tissues, they arise
early in the ontogeny of the plant and are probably differentiated
during the embryogeny in many instances. Scott observed their
formation in the seedling during the initial stages in germination.
The ducts at first consist of a longitudinal series of cells which
have definite end walls. Later in ontogeny, they become con-
tinuous, non-septate passages owing to the perforation and resorp-
tion of the end walls, as well as to the occurrence of frequent cross
anastomoses between adjacent ducts which form connecting canals.
Solereder (2.'^') has pointed out that the

"tubes of the stem and leaf form a special system of their own, which
is distinct from the lactiferous system of the root," but notes that "the
primary lactiferous system of the root is however connected with the
primary system of the stem, for at a certain level both systems are
found to be present in a transverse section and united with one another
by anastomoses."

Summing up the situation for Lactuca, it has been noted that
the lactiferous ducts occur in the primary tissues of the root,
where they form an arc outside the primary phloem in the peri-

650 THE STRUCTURE OF ECONOMIC PLANTS

cycle; and they may also occur on the inner face of the primary
phloem. Both types may develop in young seedlings before the
protoxylem is differentiated. In the secondary tissues of the
root, the lactiferous ducts appear in the phloem parenchyma, and
branches frequently extend through the pericyclic parenchyma
adjacent to the phloem. (Fig. 334, C)

In the hypocotyl, ducts appear adjacent to the primary phloem
as in the root, and may occur also in the hypodermal region of
the cortex. In the cotyledons, they lie along the abaxial surface
of the phloem and extend into the mesophyll. In the stem, ducts
are present in the pericyclic arcs outside the outer phloem, and,
to a lesser degree, in the outer phloem. They are also found on
the centrad surface of the medullary phloem strands and are
reported by de Bary as ramifying into these strands. In the leaf,
the latex ducts occupy the same position with respect to the
phloem as in the cotyledon, and branches extend into the meso-
phyll. In the inflorescence, de Bary has noted that the latex tubes
accompany the bundles of the floral traces.

Solereder reports that lactiferous hairs have been observed on
involucral and other bracts in the genus Lactuca, and that there
is an excretion of latex when the involucral leaves are touched.
The lactiferous hairs together with two or more basal epidermal
cells become united with the free ends of branches of the lactiferous
system which accompany the vascular bundles. This is accom-
plished by the absorption of portions of the intervening walls
so that the hairs actually represent terminations of the lactiferous
system.

Oil Ducts. — Oil ducts have been reported by van Tieghem (i8)
for many species in the series Tubuliflorae, but they are usually
lacking in the Liguliflorae, which includes the tribe Cichorieae
and the genus Lactuca. Exceptions have been reported in the
roots of Scolymus by van Tieghem, and in Cynara by Phillips (xi),
where oil ducts are located between the two layers of a double
endodermis. These two forms are apparently exceptional cases
in which true oil passages as well as lactiferous ducts occur. Rudi-
mentary oil passages in the diarch primary roots of Cichorium
intybus and Lampsana communis have been reported by de Bary.
Port found a somewhat similar situation in Lactuca sativa where
intercellular spaces develop between adjacent layers of cortical
cells; but, as in Cichorium, no contents were observed.

LACTUCA SATIVA 651

LITERATURE CITED

I. Bailey, L. H., The Standard Cyclopedia of Horticulture , The Macmillan Co., 1919.
X. DE Bary, a., Comparative Anatomy of the Vegetative Organs of the Phanerogams
and Ferns, Clarendon Press, Oxford, 1884.

3. BoRTHwicK, H. A., and Robbins, W. W., "Lettuce seed and its germination."

Hilgardia j: 2.75-304, 1918.

4. DE Candolle, a.. Origin of Cultivated Plants, D. Appleton-Century Co., 1895.

5. Carano, E., "Richerche sull' embriogenesie delle Asteracee." Ann. di Bot.

13: 151-301, 191 5.

6. Chauveaud, G., "L'appareil conducteur des plantes vasculaires, et les phases

principales de son evolution." Ann. Sci. Nat., Bot. Ser. IX, i}: 113-438,
1911.

7. CoMPTON, R. H., "An investigation of the seedling structure in the Legu-

minosae." Jour. Linn. Soc. (London^ Bot. 41: i-iii, 1911.

8. CuMMiNGs, M. B., "Large seed a factor in plant production." Vt. Agr. Exp.

Sta. Bull, ijy, 1914.

9. Eames, a. J., and MacDaniels, L. H., An Introduction to Plant Anatomy,

McGraw-Hill Book Co., 192.5.

10. Gates, R. R., and Rees, E. M., "A cytological study of pollen development

in Lactuca." Ann. Bot. 5/.- 365-398, 192.1.

11. Janczewski, E., "Recherches sur I'accroissement terminal des racines, dans

les Phanerogames." Ann. Sci. Nat., Bot. Ser. V, 20: i6i-ioi, 1874.
IT.. Jean, M., "Essai sur I'anatomie comparee du liber interne dans quelques
families deDicotyledones: Etude des plantules." LeBot.ij: 115-364,1917.

13. Jones, H. A., "Pollination and life history studies of lettuce (Lactuca sativa

L.)." Hilgardia 2: 415-478, 1917.

14. KoNEK), M., "Uber die in der Landwirtschaft Japans gebrauchten Samen, II."

Ber. Ohara Inst, fiir landiv. Forsch. i: 433-440, 1919.

15. Knuth, P., Handbook of Flower Pollination II, Clarendon Press, Oxford, 1908.

16. Kruch, "Fasci midollari delle Cichoriaceae." Ann. R. 1st. bot. Kama. 4:

fasc. I, 1890.

17. Lee, E., "Observations on the seedling anatomy of certain Sympetalae. II.

Compositae." Ann. Bot. 28: 303-319, 1914.

18. Morse, L. L., Field Notes on Lettuce, Ferry-Morse Seed Co., San Francisco,

3d ed., 1930.

19. Oliver, G. W., "New methods of plant breeding." U . S. Dept. Agr. Bur.

of Plant Ind. Bull. iCj: 1-39, 1910.

10. Petersen, O. G., "Uber das Auftreten bicollateraler Gefassbiindel in

verschiedenen Pflanzenfamilien und iiber den Werth derselben fiir die
Systematik." Engler, Bot. Jahrb. f. syst. Pjlanxengesch. und Pf{an%engeogr. 3:
385-387, 1881.

11. Phillips, W. S., "Seedling anatomy of Cynara scolymus." Bot. Gaz_. qS:

711-714, 1937.
11. Port, Jean, "The anatomy of the seedling of Lactuca sativa L." Unpubl.

Research, Hull Bot. Lab., Univ. of Chicago, 1937.
13. Scott, D. H., "The development of articulated laticiferous vessels." Quar.

Jour. Micros. Sci. N. S. 22: 136-153, 1881.

6^x THE STRUCTURE OF ECONOMIC PLANTS

2.4. SiLER, Margaret B., "The transition from root to stem in Helianthus annuus

L. and Arctium minus Bernh." Am. Mid. Nat. 12: 415-487, 193 1.
15. SoLEREDER, H., Systematic Anatomy of the Dicotyledons, I, Clarendon Press,

Oxford, 1908.
i6. SouEGEs, R., "Embryogenie des Composees. Les premiers states du develop-

pement de I'embryon chez le Senecio vulgaris L." Cotfipt. Rend. Acad. Sci.

Paris lyi: 154-2.56, 1310.

17. Thompson, R. C, "Natural cross-pollination in lettuce. Proc. Am. Sac.

Hart. Sci. jo: 545-547, 1933.

18. VAN TiEGHEM, P., "Mcmoite sur les canaux secretuers des plantes." Ann.

Sci. Nat., Bot. Ser. V, 16: 96-101, 1871.

19. Weaver, J. E., and Bruner, W. E., Koot Development of Vegetable Crops,

McGraw-Hill Book Co., 1917.

30. Weiss, J. E., "Das markstandige Gefassbiindelsystem einiger Dikotyledonen

in seiner Beziehung zu den Blattspuren." Bot. Centr. ij: 390-415, 1883.

31. WoRSDELL, W. C, "The origin and meaning of medullary (intraxylary)

phloem in the stems of dicotyledons. II. Compositae." Ann. Bot. j^.-
411-458, 1919.

GLOSSARY

Abaxial, the side of a lateral organ
away from the axis.

AcHENE, see Akene.

AcROPETAL, applied to structures that
are produced in succession toward the
apex.

AcTiNOMORPHic, (of flower) arrange-
ment of floral parts in a regular pat-
tern capable of bisection into similar
halves in two or more planes.

Acuminate, with a tapering end.

Adaxial, the side of a lateral organ
next the axis.

Adnate, united, as an inferior ovary
with the calyx tube.

Adventitious, (of roots) those that
arise from any structure other than a
root ; (of buds) those that arise other
than as terminal or axillary struc-
tures.

Aggregate fruit, a collection of the
separate carpels of one flower, i.e.,
raspberry.

Akene, a small, dry, hard, one-celled,
one-seeded, indehiscent fruit consist-
ing of one carpel (buttercup), or more
than one with adnate calyx (com-
posites).

Amphicribral, (of bundles) with the
phloem completely surrounding the
xylem.

Amphiphloic, (of steles) with the
phloem outside of and also centrad
to the xylem cylinder.

Amphitropous, see Hemitropous.

Amphivasal, (of bundles) with the

Anatropous, (of ovules) one in which
the nucellus is inverted and straight,
with the micropyle adjacent to the
hilum.

Androecium, the stamens considered
collectively.

Anisocarpic, (of flowers) with the num-
ber of carpels less than the number
of parts in the other floral sets.

Ani^ual, with a life cycle of one year's
duration.

Annular, (of conductive elements)
with secondary wall thickenings in
the form of rings.

Anterior, placed in the front; remote
from the axis of an inflorescence.

Anther, the portion of the stamen con-
taining the pollen, usually bilocular.

Anthesis, the time of floral expansion.

Anticlinal, at right angles to the
surface; (of walls) those which cut
the surface or periclinal walls at right
angles.

Antipodal, (cells or nuclei) those
which occupy the chalazal end of the
megagametophyte.

Apet ALDUS, without petals.

Apical, at the point or summit of a
structure.

Archesporium, the cell or cells from
which wall tissue, if any, and the
sporogenous tissue are derived in
the micro- or megasporangium.

Atropous, see Orthotropous .

Auriculate, possessing ear-shaped ap-
pendages (auricles).

xylem completely surrounding the Awn, a bristle-shaped appendage, as on
phloem. the glumes of many grasses.

Anastomosis, union of one vein or Axil, the angle subtended by the stem
bundle with another. axis and a leaf or branch.

653

654

GLOSSARY

AxiLE, located on the axis, i.e., axile

placentation.
Axis, the central line of any organ or

the support of a group of organs; a

stem, root, etc.

Basipetal, applied to structures that

are produced in succession toward the

base.
Bast, phloem, sometimes the inner

fibrous portion of the bark.
Berry, a fleshy fruit derived from a

superior ovary consisting of one or,

more often, two or more carpels.
BicoLLATERAL, (of bundlcs) having

phloem on the outer and inner faces

of the xylem.
Biennial, with a life cycle of two years'

duration.
Blade, the expanded portion of the leaf,

lamina.
Bordered pit, a pit in which the margin

projects over the closing membrane.
Bract, a specialized leaf type commonly

subtending a flower or an inflores-
cence, sometimes cauline.
Bud, an unelongated stem tip with its

lateral members.
Bulb, a bud with fleshy leaves, usually

subterranean.
BuLLiFORM CELLS, large, thin-walled

epidermal cells which occur in certain

grasses, i.e., corn.

Caducous, falling off early.

Callus, (of sieve tube) a deposit on the
sieve plates.

Calyptra, (of root) the root cap.

Calyptrogen, the layer or histogen
from which the root cap is derived.

Calyx, the outermost cycle of floral
parts, the sepals considered collec-
tively.

Cambium, a lateral meristem which pro-
duces secondary xylem and phloem.

Campylotropous, (of ovules) one in
which the nucellus is curved and

the chalaza and micropyle are in a
plane at right angles to the funiculus.

Capillary, slender, hairlike.

Capitate, shaped like a head.

Capsule, a dry, dehiscent fruit com-
posed of more than one carpel and
sometimes involving a portion of the
axis.

Carpel, a simple pistil, or a member of
a compound pistil; a megasporo-
phyll.

Carpophore, (in Umbelliferae) a slender
extension of the floral axis which
supports the two pendulous meri-
carps.

Caryopsis, a fruit developing from a
carpel in which the pericarp is adnate
to the single seed.

Casparian strip, a secondary thicken-
ing which develops on the radial and
end walls of some endodermal cells.

Cataphyll, bud scales, also applied to
the scales on rhizomes and cotyledons.

Cauline, pertaining to the stem; (of
bundles) those which have no direct
connection with the common bundles
that pass into the leaves.

Cell, a protoplast which usually con-
sists of a nucleus, cytoplasm, and
various inclusions; also applied to
the cell wall.

Central cylinder, the vascular system
of an axis; the stele.

Centrifugal growth, from the center
outwards.

Centripetal growth, from the pe-
riphery towards the center.

Chalaza, the point in an ovule or seed
where the integuments diverge from
the nucellus.

Chloroplast, a plastid containing
chlorophyll.

Chondriosomes, minute bodies in the
cytoplasm having the form of gran-
ules, rods, or threads.

Chromoplast, a plastid containing pig-
ment.

GLOSSARY

655

CiLiATE, fringed with hairs.

CoLEOPTiLE, (in Gramineae) the first
leaf above the cotyledon which en-
closes the stem tip.

CoLEORHizA, the sheath which sur-
rounds the primary root in the embryo
of the grasses.

Collateral, (of bundles) having the
xylem and phloem side by side on the
same radius.

Collenchyma, elongated, parenchym-
atous cells with variously thickened
walls, commonly at the angles.

Collet, applied to the imaginary
boundary between the aboveground
and subterranean portions of a seed-
ling axis.

Commissure, (in Umbelliferae) the ad-
herent faces of the two carpels.

Common bundle, one that is common
to both stem and leaf and continuous
from one to the other.

Companion cell, a phloem element
associated with a sieve tube and
usually derived from a common
mother cell.

Complementary cells, loosely ar-
ranged cells which occur in the
lenticel.

Compound, (of leaf) one in which the
blade is divided into separate leaflets.

Conjunctive tissue, non-vascular tissue
adjacent to bundles which consists of
lignified, usually elongated, cells.

Connate, united.

Connective, that portion of the stamen
which joins the two cells of the
anther.

CoNNivANT, convergent or coming into
contact.

Convolute, with one part rolled up
longitudinally in another, as the
petals of cotton.

Cordate, heart-shaped.

CoRM, an enlarged, solid fleshy base of
a stem, i.e.. Cyclamen or Indian
Turnip.

Corolla, the inner cycle of the peri-
anth; the petals considered collec-
tively.

Corpus, the inner core of the growing
point of the apex of the shoot.

Cortex, that portion of an axis between
the epidermis and the stele.

Corymb, a flat-topped flower cluster,
arranged as a raceme, in which the
flowers develop centripetally.

Cotyledons, the first leaves of the
embryo.

Crenate, (of leaves) dentate with
rounded teeth.

Culm, the stem of the grasses.

Cuticle, a non-cellular deposit of cutin
on the surfaces of epidermal cells, oc-
casionally on the subepidermal ones.

Cyme, a determinate flower cluster in
which the central or terminal flowers
bloom first.

Cystolith, a concretion of calcium car-
bonate which occurs on an outgrowth
of the cellulose wall, i.e., some epi-
dermal cells of Cannabis.

Cytoplasm, the more or less transparent,
viscous, colorless substance which
with the nucleus constitutes the pro-
toplast of a cell.

Deciduous, (of leaves) not persistent,
falling in season.

Decompound, several times divided or
compounded.

Decumbent, reclining with an ascend-
ing summit.

Decurrent, (of a leaf) extending down
the stem below the node.

Decussate, (of leaves) an opposite
arrangement with the pairs alternat-
ing at right angles.

Dehiscent, opening by means of pores,
valves, slits, etc., as in a capsule or an
anther.

Dentate, toothed.

Dermatogen, the layer or histogen
from which the epidermis is derived.

656

GLOSSARY

DiADELPHOus, (of stamcns) arranged or
combined into two sets.

DiARCH, (of root) a stele with two pro-
toxylem groups.

DicHOTOMOus, forking regularly by
pairs.

Dicotyledonous, having two cotyle-
dons.

DicTYOsTELE, a vasculat cylinder or
stele consisting of distinct bundles; a
dissected siphonostele.

Digitate, (of leaves) with the leaflets
arising at the apex of the petiole.

Dimerous, having all the parts in twos.

Dioecious, with staminate and carpel-
late flowers on separate plants.

Distichous, in two vertical ranks, as the
leaves of grasses.

Dorsal, relating to the back or outer
surface of an organ.

Drupe, a fruit developed from a single
carpel with a fleshy exocarp and a
hard stony endocarp or pit usually
containing a single seed. Less ex-
actly applied to stone fruits that are
multi-carpellate with one to several
seeds.

Duct, an elongated cell or a multi-
cellular cavity; a gland.

Ectophloic siphonostele, a stele in
which there is a zone of phloem out-
side the xylem but no inner phloem.

Embryo, a rudimentary plant.

Emergence, a surface outgrowth arising
from a few superficial layers, i.e.,
prickles.

Endarch, applied to primary xylem in
which the progressive development
of the elements is centrifugal.

Endocarp, the inner layer of the peri-
carp or fruit coat.

Endodermis, the innermost layer of the
cortex which abuts the stele.

Endogenous, produced within from
deep-seated tissues instead of from
superficial layers.

Endosperm, the nutritive tissue occur-
ring within the seed which is de-
rived from the triploid endosperm
nucleus.

Endothecium, inner lining of the locule
of an anther.

Entire, (of leaves) without dentation
or division.

Ephemeral, short-lived, usually lasting
but a day.

Epiblast, a lateral outgrowth from the
embryonic axis of some grasses.

Epicarp, the outer layer of the pericarp
or fruit coat.

Epicotyl, that portion of the seedling
axis above the cotyledonary node.

Epidermis, the superficial layer of cells
covering a plant.

Epigeal, (of cotyledons) expanding
above the soil surface.

Epigynous, (of flowers) with all floral
parts conjoint and generally divergent
from the ovary at or near its summit.

Etiolate, deprived of color or length-
ened owing to the exclusion of light.

Exarch, applied to primary xylem in
which the progressive development of
the elements is centripetal.

ExocARP, the outer layer of the pericarp
or fruit coat.

Exogenous, arising from superficial
tissues.

Fascicular cambium, the cambium
which gives rise to the secondary
vascular tissues of a bundle.

Fiber, an elongated, thick-walled me-
chanical element.

Filament, the part of the stamen sup-
porting the anther.

Floral envelope, the perianth (calyx
and corolla).

Flower, a shoot beset with sporophylls
(stamens and carpels).

Follicle, a dry fruit consisting of a
single carpel which dehisces along
the ventral suture.

GLOSSARY

657

Fruit, (of angiosperms) the mature
ovary and any associated parts.

Funiculus, the stalk of an ovule or seed.

Fusiform, spindle-shaped, tapering at
both ends.

Gamete, a reproductive cell which by
union with another gamete produces
a zygote. Cf. Microgamete, Megagam-
ete.

Gamopetalous, having more or less un-
diverged or "united" petals.

Gamosepalous, having more or less un-
diverged or "united" sepals.

Glabrous, smooth, without hairs or
pubescence.

Gland, a secreting structure or surface.

Glaucous, covered with a bloom.

Glume, a chaffy bract occurring at the
base of the spikelet in grasses.

Grain, see Caryopsis.

Ground tissue, applied to the funda-
mental parenchyma — pith, medul-
lary rays, and cortex.

Gynoecium, the pistil or pistils of the
flower, the carpels considered collec-
tively. (Also gynaeceum.^

Habit, the general appearance of a
plant.

Head, an inflorescence of sessile or sub-
sessile flowers on a very short axis or
receptacle.

Hemitropous, (of ovules) applied when
the ovule is inverted with a straight
nucellus and the micropyle and
chalaza are at right angles to the
funiculus.

Hesperidium, a berry of the type found
in citrus fruits.

Hilum, the scar or point of attachment
of the seed.

HisTOGEN, regions or layers of meri-
stematic tissue from which primary
tissues are derived. Cf. Dertnatogen,
Periblem, etc.

Hyaline, translucent or transparent.

Hydathode, a pore or organ concerned
with the exudation of water.

Hypocotyl, the portion of the seedling
axis below the cotyledonary node.

Hypodermis, applied to the cells imme-
diately under the epidermis which
may be supportive or protective in
function.

Hypogeal, (of cotyledons) remaining
underground.

Hypogynous, (of flowers) with the
sepals, petals, and stamens free from
and divergent below the ovary.

Hypophysis, the cell or cells resulting
from the transverse division of the
cell next adjoining the suspensor
cell, and giving rise to the tip of
the root.

Imbricate, overlapping vertically or
spirally.

Inclusions, (of cell) applied to bodies
occurring in the cytoplasm; non-
protoplasmic substances in the form
of granules, droplets, crystals, etc.

Indehiscent, (of fruits) remaining
closed at maturity.

Inferior, (of ovary) applied when
there is conjoint growth of ovary
wall and other floral parts, i.e., in
epigynous flowers.

Inflorescence, the arrangement of the
flowers of a plant; more specifically,
the flower cluster itself.

Initial or initials, a cell or group of
cells from which a structure is de-
veloped or derived.

Integument, (of ovule) a covering
layer, later becoming a part of the
seed coat.

Intercalary, (of meristem) applied to
ifleristematic zones that are not
apical, basal, or lateral, i.e., the
intercalary meristem at the base of
an internode in many grasses.

Internode, the portion of the stem
between two nodes.

658

GLOSSARY

Introrse, turned inward or toward the

axis.
Involucel, a secondary involucre, as

that occurring on the umbellet in

some umbellifers.
Involucre, a cycle of bracts subtending

a flower cluster, head, or single

flower.
Irregular, (of flowers) with the parts

of a floral set of unequal size and

shape; zygomorphic.
IsocARPic, with the number of carpels

equal to that of the parts of other

floral sets.

Karyolymph, the hyaline fluid or sap

occurring in a nucleus.
Keel, the two anterior, united petals of

a papilionaceous flower.
Key fruit, see Samara.

Lacuna, an air-space in a tissue.

Lamella, a thin, flat plate or mem-
brane. Cf. Middle lamella.

Lamina, see Blade.

Lanceolate, narrow and tapering at
both ends, usually broadest above the
base and narrowed to the apex.

Latex, a white or yellow viscous emul-
sion.

Leaflet, one of the divisions of a
compound leaf.

Leaf trace, a bundle which extends
to a leaf; or, more inclusively, the
complex of all the bundles that supply
a single leaf.

Legume, the dry, dehiscent fruit of
leguminous plants developed from a
single carpel and usually opening
along both sutures.

Lemma, (in grasses) the lower of two
bracts enclosing the flower, some-
times called the flowering glume.

Lenticel, a corky area of loosely ar-
ranged cells in the periderm.

Leucoplast, a colorless plastid.

Liana, a climbing or twining plant.

Ligule, a thin, scarious projection

arising at the top of the leaf-sheath in

grasses.
Limb, the expanded portion of a gamo-

petalous corolla, or of a petal or leaf.
LiNiN, the substance comprising the

reticulum of the nucleus.
LocuLE, one of the chambers in an

anther or an ovary.
LocuLiciDAL, dehiscence of the locule of

an ovary along the dorsal suture.

Cf. Septicidal.
LoDicuLEs, the small scales outside the

stamens in the grass flower.
Lumen, (of cell) the cavity bounded by

the cell walls.
Lysigenous, applied to a cavity formed

by the disintegration and dissolution

of cells, a. Schiiogenous.

Medulla, the pith.

Megagamete, the larger of two uniting

cells or gametes; the egg.
Megasporangium, a sporangium in

which the megaspores are developed.
Megaspore, a tetraspore from which

the megagametophyte develops.
Megasporophyll, (in angiosperms) a

carpel.
Mericarp, one of the akene-like carpels

of the schizocarp of umbellifers.
Meristele, a stele in which the vascular

bundles are distributed throughout

the axis, i.e., corn.
Meristem, a group of embryonic cells

whose derivatives may differentiate

into various cell types and tissues.
Mesarch, applied to primary xylem in

which the protoxylem is centrally

located and the development of the

metaxylem is both centripetal and

centrifugal.
Mesocarp, the middle layer of the peri-
carp.
Mesophyll, the parenchymatous tissue

between the adaxial and abaxial

epidermis of the blade.

GLOSSARY

659

Metaphloem, the later-formed elements
of the primary phloem.

Metaxylem, the later-formed elements
of the primary xylem usually char-
acterized by reticulate or pitted walls.

Microgamete, the smaller of two unit-
ing cells or gametes, the sperm.

MicROPYLE, the orifice leading through
the integuments to the nucellus.
This frequently persists in the seed.

MiCROSPORANGiuM, a spotangium in
which microspores are developed.

Microspore, a tetraspore from which
the microgametophyte develops.

MicROSPOROPHYLL, (in angiospetms) a
stamen.

Middle lamella, the isotropic inter-
cellular substance between two pri-
mary walls.

MoNADELPHOus, (of stamcns) forming
a single cycle with undiverged fila-
ments.

Monoecious, with staminate and car-
pellate flowers on the same plant.

MoNOPODiAL, applied to a stem with a
single continuous axis.

Multiple fruit, one developed from a
cluster of flowers.

Napiform, turnip-shaped.

Nectary, an organ or surface where
nectar is secreted.

Node, the point on a stem from which
a leaf diverges.

Nucellus, the body of an ovule, distal
to its integuments.

Nucleolus, a small proteinaceous body
occurring in the nucleus.

Nucleus, a specialized often more or less
ovoid or spherical portion of the
protoplast surrounded by a delicate
membrane, consisting of chromatin
variously arranged, nuclear sap, and
nutritive substances. It is essential
in metabolism, growth, reproduction,
and in the transmission of the deter-
miners of hereditary characters.

Nut, a fruit resembling an akene but
with a thick hard pericarp, usually
developed from a compound ovary.

Obcordate, inverted heart-shaped.

Oblate, flattened at the poles.

Obovate, inverted ovate.

Ontogeny, the developmental history
of an individual as contrasted with a
race. Cf. Phytogeny.

Opposite, (of leaves) two at one node.

Orbicular, circular.

Orthotropous, (of ovule) one in which
the nucellus is erect with the micro-
pyle farthest from the hilum, in a
straight line with it and the chalaza;
also called atropous.

Ovary, the part of the carpel, or carpels
in a compound pistil, that contains
the ovules.

Ovate, egg-shaped with the broader
end downward.

Ovule, the nucellus and its integu-
ments.

Palea, or palet, (in grasses) the upper
bract which with the lemma encloses
the flower.

Palisade, the perpendicularly elongated
cells which commonly form the layer
of chlorenchyma beneath the adaxial
epidermis of a leaf.

Palmate, (of leaves) radiately lobed or
divided.

Panicle, a compound raceme.

Papilionaceous, (of corolla) with a
standard, keel, and wings as in many
leguminous plants.

Pappus, (in composites) the limb of the
calyx which may be capillary, bristle-
like, chaffy, or wanting.

Parenchyma, simple, non-specialized
vegetative tissue with more or less
isodiametric, thin-walled cells con-
taining functional protoplasts.

Parietal, (placentation) that type in
which the ovules are borne on the
ovary wall.

66o

GLOSSARY

Passage cells, the thin-walled endoder-
mal cells which commonly lie on the
same radii as the protoxylem strands.

Pectinate, (of a vascular system) ap-
plied where two series of vascular
bundles alternate with each other
as the teeth of two combs.

Pedicel, the supporting axis of a single
flower.

Peduncle, the supporting axis of a
single flower or a flower cluster.

Pentacyclic, (of flower) with five
whorls of floral parts.

Pentamerous, with parts in fives, as a
calyx of five sepals.

Pentarch, (of root) a stele with five
protoxylem groups.

Pepo, a fruit developed from an epigy-
nous flower in which the tissue of the
receptacle surrounds the pericarp,
forming a more or less hardened rind
as in cucurbits.

Perennial, with a life cycle of more
than two years' duration.

Perfect, (of flower) having both sta-
mens and carpels.

Perianth, the floral envelope comprised
of the calyx and corolla (if present)
regardless of their form.

Periblem, the layer or histogen from
which the cortex is derived.

Pericarp, the mature ovary wall.

Periclinal, curved in the same direc-
tion as the surface or circumference;
(of walls) those which are parallel
with the surface.

Pericycle, the outermost cell-layer of
the stele lying adjacent to the endo-
dermis, frequently becoming a multi-
layered zone.

Periderm, the outer protective layer
in older axes, consisting of the phel-
logen and its derivative tissues,
phellem and phelloderm.

Perigynous, (of flowers) applied when
there is conjoint growth of the outer
cycles of floral parts distinct from the

carpel or carpels and divergent below

them.
Peripheral, on or near the margin.
Petal, one of the members of the

corolla.
Petiole, the supporting stalk of a leaf.
Phellem, cork tissue.
Phelloderm, cells cut off centripetally

by the phellogen.
Phellogen, a lateral meristem that cuts

off phelloderm and phellem; the cork

cambium.
Phloem, a complex vascular tissue

which may include sieve tubes, com-
panion cells, fibers, parenchyma, and

secretory cells.
Phyllotaxy, the mode in which leaves

are arranged on the stem axis.
Phylogeny, the developmental history

of a race rather than an individual.

Cf. Ontogeny.
Pistil, a carpel or group of undiverged

carpels, commonly subdivided into

stigma, style, and ovary.
Pith, the central parenchymatous por-
tion of the axis; the medulla.
Placenta, the surface in an ovary that

bears the ovules.
Plasmodesma, delicate threads or fibrils

of cytoplasm that pass through the

walls of adjacent cells, establishing

protoplasmic continuity.
Plastid, a specialized body occurring

in the cytoplasm of a cell which may

be colorless or pigmented. Cf. Leuco-

plasf, Chloroplast.
Plerome, the layer or histogen from

which the stele is derived.
Plumule, the bud or growing point of

the embryo.
Pod, a dry, many-seeded, dehiscent

fruit; a legume, silique, etc.
Polyarch, (of root) a stele with several

to many protoxylem groups.
Pome, a fleshy fruit developed from an

epigynous flower in which the inner

portions of the carpel wall are some-

GLOSSARY

66 1

what thickened or, less commonly,
membranous.

Procambial strand, the meristematic
tissue that later develops as vascular
tissue; more exactly, a provascular
strand.

Procumbent, lying along the ground.

Proembryo, applied to the initial stages
in embryogeny.

Prosenchyma, a tissue in which the
cells are elongated and tapered.

Protoderm, the layer or histogen from
which the epidermis is derived; der-
matogen.

Protophloem, the first elements of
primary phloem to be differentiated.

Protoplast, the cytoplasm and nucleus
of a cell.

Protostele, a solid stele with phloem
surrounding a central core of xylem.

Protoxylem, the first elements of pri-
mary xylem to be differentiated, char-
acterized by annular, spiral, or retic-
ulate secondary walls.

Pubescent, covered with soft hairs.

Raceme, a simple indeterminate flower
cluster with a central axis that is more
or less elongated.

R ACHILLA, a secondary axis in the in-
florescence of grasses; the axis of a
spikelet.

Rachis, the axis of a spike.

Radicle, the hypocotyl and rudimen-
tary root of the embryo.

Raphe, the ridge of an ovule extending
from the hilum to the chalaza.

Ray, (of flowers) the branch of an
umbel; the marginal flowers in
radiate composites; (of axes) radially
oriented sheets of cells, i.e., medul-
lary, xylem, and phloem rays.

Receptacle, the portion of the axis
from which the floral parts arise; the
torus.

Reniform, kidney-shaped.

Reticulate, in the form of a network.

Rhaphe, see Raphe.

Rhizome, a subterranean stem from
which shoots and adventitious roots
may arise; a root-stalk.

Root cap, the protective structure
surrounding the terminal meristem
of the root, consisting of cells de-
rived from the calyptrogen; the
calyptra.

RooT-sTALK, see Rhizome.

Sagittate, arrow-shaped.

Samara, an indehiscent winged fruit; a
key fruit.

Scalariform, (of xylem elements) with
ladder-like secondary wall thicken-
ings.

Scape, a leafless floral axis arising from
the ground.

ScARious, thin, dry, and membrana-
ceous, not green.

ScHizocARP, a fruit derived from a com-
pound pistil in which the one-seeded
carpels separate from one another at
maturity.

ScHizoGENOus, applied to a cavity
formed by the splitting of cell walls
along the middle lamellae. Cf.
Lysigenous.

ScLERENCHYMA, Supporting or protective
tissue in which the cells have hard,
lignified walls and usually little pro-
toplasmic contents at maturity.

ScuTELLUM, the cotyledon of an embryo
of a grass.

Seed, (in angiosperms) an embryo or
embryos with or without an external
food reserve, surrounded by a covering
which is generally integumentary.

Seminal, relating to the seed.

Sepal, one of the members of the calyx.

Septate, divided by partitions.

Septicidal, dehiscence of the ovary
along the septa between the locules.
Cf. Loculicidal.

Serrate, with sharp teeth.

Sessile, without a stalk.

6€i.

GLOSSARY

Shoot, the stem and its appendages.

Sieve plate, the perforated area of a
sieve tube.

Sieve tube, an element of the phloem
with lateral or terminal sieve plates.

SiLiQUE, the dry, many-seeded, dehis-
cent (infrequently indehiscent) fruit
of the Cruciferae, consisting of two
carpels which form a bilocular ovary
with a longitudinal septum.

SiPHONOSTELE, a vascular cylinder sur-
rounding a central pith.

Spathe, a large bract or bracts enclosing
an inflorescence.

Spatulate, oblong with the basal end
attenuated.

Spike, a racemose inflorescence with
sessile flowers on an elongated axis.

Spikelet, a secondary spike; the unit
of the inflorescence in grasses.

Spongy tissue, parenchyma with many
intercellular spaces.

Sporophyll, a spore-bearing leaf. Cf.
Mega- and Microsporophyll.

Stamen, a pollen-bearing organ; a
microsporophyll .

Staminodium, an abortive, sterile, or
rudimentary stamen.

Standard, the large upper petal of a
papilionaceous corolla.

Stele, the vascular cylinder of an axis;
the central cylinder.

Stigma, the receptive portion of the
pistil.

Stipule, an appendage at the base of the
petiole.

Stolon, a runner or prostrate stem that
tends to root.

Stoma (pi. stomata), an opening in the
epidermis leading to an intercellular
space.

Stomium, an opening in an anther be-
tween lip-like cells through which
dehiscence occurs.

Stone cell, an approximately isodia-
metric sclerenchymatous cell.

Strict, straight, upright.

Style, the usually elongated portion
of the pistil connecting the stigma
and ovary.

Stylopodium, a disk-like enlargement
at the base of the style.

Superior, (of ovary) free from the other
floral parts which are hypogynous.

Suspensor, (of embryo) the part of the
proembryo at whose apex the embryo
proper develops.

Suture, a seam or line of union; fre-
quently a line of dehiscence in fruits.

Sympetalous, see Gamopetalous .

Sympodial, (of stem) made up of a
series of superposed branches so that
it resembles' a simple axis.

Syncarpous, of undiverged or "united"
carpels.

Synergid, the nuclei or cells adjacent
to the megagamete at the micropylar
end of the megagametophyte.

Tapetum, the layer or layers of cells
adjacent to the sporogenous tissue
in the micro- and megasporangium.

Terete, circular in transection; cylin-
drical and usually tapering.

Testa, the seed coat.

Tetracyclic, (of flower) applied to one
with four whorls of floral parts.

Tetrarch, (of root) a stele with four
protoxylem groups.

Tissue, a group of cells of common
origin having essentially the same
structure and performing the same
functions.

Torus, the receptacle of a flower.

Trachea, a xylem element consisting of
a linear series of cells which form a
continuous tube owing to the dis-
integration of their end walls; a
vessel.

Tracheid, an elongated, water-con-
ducting xylem cell with variously
thickened and pitted walls.

Triarch, (of root) a stele with three
protoxylem groups.

GLOSSARY

663

Tuber, a relatively short, thickened
rhizome with numerous buds.

Tunica, the peripheral layer or layers
of the apex of the shoot.

Tylose, the intrusion of a protoplast
into a vessel.

Umbel, a flower cluster in which the
pedicels arise at about the same level
on the peduncle.

Umbellet, a secondary umbel.

Undulate, with a wavy surface.

Vacuolar membrane, the limiting

membrane separating the cytoplasm

from an enclosed vacuole.
Vacuole, a cavity in the cytoplasm

containing cell sap.
Ventral, applied to the anterior or

inner face of an organ.

Vernation, the arrangement of leaves
in a bud.

Versatile, (of anther) attached near
the middle and turning freely on its
support.

Vessel, see Trachea.

Vessel segment, one of the cells com-
prising a vessel.

Whorl, an arrangement of leaves or
floral parts in a circle around the stem .

Xylem, a complex vascular tissue which
may include tracheids, vessels, fibers,
parenchyma, and frequently ducts or
glands.

Zygomorphic, (of flower) capable of di-
vision in only one plane of symmetry.

Zygote, the product of the union of
two gametes; a fertilized egg.

INDEX

(Figures in italics indicate pages on which illustrations occur.)

Abscission,

layer, 75; of stipules, 84
Absorptive tissue, 15
Accessory ceils, 16
Adventitious roots, 53, 54

artificial production of, 55
Agave (sisal hemp), ii4
Aggregate fruit, 103, 106
Akene, 102., 103, 104
Alfalfa (see Medkago')
Allium Cepa (onion), 179, 202

adventitious roots, 186, 187, i8g, 191,
192; anatomy, 181; bulb, iSo, loi, 20/;
cotyledon, 18}, 190, 197; crown stem,
193. 195' embryo, 183, 18}; embryog-
eny, 108, no, 211; floral axis, 102.,
zoj; floral ontogeny, 2.04, 20s, 206,
208, 2og; flower, 181, 182; fruit, 181;
guard cells, 16; inflorescence, 181, 182;
lactiferous cells, 35, 199, 200; leaf,
197, 198; leaf ontogeny, 194, 196;
megagametophyte, io8; morphology,
179; polyembryony, iii; primary root,
187, 187, i8g; related species, 179;
root ontogeny, 188; root system, 179;
seed, 181, 18}, 184; seedling, 185, /<?;;
shoot, 180; stomata, 17, loi; vascular
anatomy of flower, 107; vascular
transition, 190
Amaryllidaceae,

lack of integuments, 99
Ananas (pineapple),

fruit, iq6
Anemone,

flower, 89
Angiosperms,
embryo, 107; flower, 88; fruit, loi;
lateral roots, 51; leaf ontogeny, 78;
megasporogenesis, loi; root types, 45;
seed, 107
Anther, 89

microsporogenesis, 97
Antipodals, loi

Apium graveolens (celery), 451, 4}i, 452
anatomy, 455; carpophore, 456; col-
lenchyma, 471, 47Z, 472, 479; cotyledons,

462., 462; cultivation, 453; floral axis,
464; floral development, 481, 482.;
flower, 454, 4S4; fruit, 455, 4s 5; hypo-
cotyl, 462; inflorescence, 95, 454, 454;
lamina, 480, 480; lateral root, 459;
leaf ontogeny, 470; leaves, 454, 470,
480, 480; mechanical tissue, 479; medul-
lary bundles, 462., 46}, 465, 466; mericarp,
455; morphology, 453; oil ducts, 15,
458, 4s8, 465, 466, 469, 479; petiole,
467, 468, 470, 474, 47f, 479; phloem,
477, 478; root, 453, 457, 4s8, 459, 460,
460; schizocarp, 455, 4//; secondary
thickening, 460; seed, 455, 4ss; seed-
ling, 457, 4S7; shoot, 454; stem, 454,
462., 46}, 464; stomata, 17, 18, 464,
470, 480; stylopodium, 455; varieties,
453; vascular bundles, 473, 474, 475;
vascular transition, 461; vessel ontog-
eny, 19; xylem, 475, 476

Apocynaceae,
latex cells, 35

Apple (see Pyrus Malus)

Aquifoliaceae,
fruit, 103

Archesporial cells, 97

Arctium minus,

vascular transition, 640

Artichoke (see Cynara)

Asclepidaceae,
latex cells, 35, 139

Asparagus, 179

Avena sativa (oats), in

Axile bundle, 41

B

Bean (see Phaseolus^
Beet (see Beta')

_ onia,

adventitious roots, 54; medullary bundles,
466
Berberidaceae,

fruit, 103; ovule, 100
Berry, 102., 103

inferior, 105
Beta maritima, "Ljd
Beta vulgaris (beet), 2.46

anatomy, 2.53; cotyledons, 153, 159,

665

666

INDEX

261J embryo, 2//, zfi; embryogeny,
2.79, 280; endosperm, 2.80; epicotyl,
159; fleshy axis, 2.46, i6i, 262J floral
axis, 2.69, 271; floral ontogeny, 2.76;
flower, X48, 2^9, 276; fruit, 2.48, 2/5;
hypocotyl, 2.46, 2^8, 260; inflorescence,
148, 24g; lateral roots, 162.; leaves,
148,173; megasporogenesis, Z79; micro-
sporogenesis, 178; morphology, 2.46;
ontogeny of fleshy axis, 161; phloem
ontogeny, 167, 267, 2.74, 27/; plastids,
4; primary root, 154, 254, 2//; root
ontogeny, 2.55; root system, 146, 2^7;
secondary cambium, 15, 164, 26s, 266,
2.68; secondary thickening, 163, 26};
seed, 148, 150, 2//, 2}2; seedling, 151,
2Si; slime bodies, 31; stem, 146, 2.69,
271; stomata, 173, tertiary thickening,
164, 26s, 266, 268, z6^; vascular anatomy
of flower, 177, 278; vascular anatomy
of stem, 171, 272; vascular transition,
156, 2J7, 2^8, 260, 261; vegetative cycle,
146

Blade (of leaf) (see lamina)

Bracts, 71

floral, 88; of Poinsettia, 89

Brassica,

cultivated species, 183

Brassica oleracea (cabbage), 2.83
endodermis, io; root, ^p

Bromeliaceae, 103

Bryophyllum,

adventitious roots, 54

Bud, 63, 64
development of bud scales, 85

Bulb, 57

Bundles, 42.

amphicribral, 63; amphivasal, 63; axile,
41; bicollateral, 59 (see also under Cu-
curhita); cauline, 59; closed, 68; col-
lateral, 59; common, 59; diff'erentia-
tion of, 66; fibro-vascular, 60; half-
amphi vasal, 63; leaf, 75, 77; medul-
lary (see Apium and Lactuca'); open,
66; radial, 42.

Cabbage (see Brassica oleracea)

Calyptrogen, 44

Calyx, 89

Cambium,

activity, types of, 67; fascicular, 66;
initials, 67; interfascicular, 68; in root,
48; secondary, 51; in stems, 66; in
veins, 77

Cannabis Indica, 114

Cannabis sativa (hemp), 114, 216

carpellate flower, 95, 2.10, 222; carpellate
inflorescence, 218, lio, 220; cotyledons,
2.2.7, -2-27. ^i4: cultivation, 2.15; cysto-
liths, 77, 135, 24^; delignification, 144;
embryo, ii6; embryogeny, 222, 113;
fiber, 13, 2.33, 137; fruit, 114, 22/, 227;
history and distribution, 2.14; inflo-
rescences, 2.17; lateral root formation,
2.31; latex ducts, 36, 2.39, 240; leaf,
141, 24i: morphology, 2.15; pericyclic
fibers, 141; petiole, 144; phloem fibers,
139; preparation of fiber, 2.42.; primary
root, 2.18, 22^: root, ii6, X2.8, 22g;
root ontogeny, 2.2.8; seed, 12.4, 22$,
227; seedling, 2.2.6, 228: sex determi-
nation, 2.17; shoot, 115; staminate
flower, 2.18, 2/p; staminate inflorescence,
217, 118, 2/9; stem, 2.33, 2_jtf, 257; sto-
mata, 2.35, 142.; tetraploidy, 131; vas-
cular system, 2.38; vascular transition,

Caprifoliaceae,

fruit, 106
Capsella, 107, 189
Capsicum (pepper), 514

floral development, 543
Capsule, 103, 104, 104
Cardamine,

adventitious roots, 54
Cariaceae,

ducts, 139
Carpel, 88, 94, 98, loi et seq.

in Ranunculus, gi; vascular supply, 97
Carya,

ontogeny of leaf, 83
Caryophyllaceae,

ovule, 100; placentation, 99
Caryopsis, loi, 103
Casparian strip, 19, 20, ii
Cataphylls, 71, 85
Catkin, p^
Celery (see Apium)
Cell, I

accessory, 16; apical, 108; archesporial,
97; basal, 108; cambial, 66; col-
lenchyma, 2.2.; companion, 32.; cork, 19;
endodermal, 19; epidermal, 15; fiber,
13; guard, 16; inclusions,!; lactiferous,
35; megaspore mother, 101; meriste-
matic, 2, ix; microspore mother, 97;
parenchyma, 14, 30, 33; passage, 19, xi;
pericyclic, 14; phellodermal, 19; plate,
7; primary parietal, 97, loi; primary
sporogenous, 97, loi; secretory, 34;
sieve tube, 31; stomatal mother, 17;
stone, 15; tracheid, z6; xylem ray, 31

INDEX

667

Cell wall, 5, 6

chemistry, 10; constituent parts, 6; inter-
cellular substance, 6; middle lamella,
6, 7; nomenclature, 6; primary, 6;
secondary, 7, 8; structure, 9
Cellulose, 9, ID
Chalaza, 100
Chenopodiaceae, 2.46

chromosomes in, 178
Chloroplasts, 4, 5
Chromatin, 3
Chromosomes, 3
Cker arietinum,

vascular transition, 348
Cichorium endiva (endive}, 6ii
Cichorium tntyhus (chicory), 611

oil passages, 650
Citrullus (watermelon), 580, 584
Citrus,

fruit, 103, 105; oil ducts, 35
Collenchyma, 12., 61
Companion cell, 31, 33
Complex tissue, 11
Compositae, 62.1

akene, 102.; floral development, 92.; head

of, 95

Conductive tissue, 2.5

Convolvulaceae, 485
bundles, 60; stem, 508

Convolvulus,
seed, 489; seedling anatomy, 501

Cork cambium (see phellogen)

Cork cells (see phellem)

Corm, 58

Corn (see Zea)

Corolla, 89

Corpus, 80

Cortex,
of root, 40; secondary, 50; of stem, 60

Corymb, 94

Cotton (see Gossypiuni)

Cotyledons, 70, 71, 107

Crotalaria juncea (Sunn hemp), X14

Cruciferae, 183

carpels in, 189; cultivated crucifers, i&t,;
floral development, 92.; floral symmetry,
189; fruit, 103, 105; tertiary thickening,
301

Cucumis (cucumber and melon), 580
pistillate flower, 584; stamens, 586

Cucurbita spp. (squash, pumpkin), 580

anatomy, 588; bicollateral bundle, 606,
607, dop; carpellate flower, 58i, fS},
S84; cotyledonary node, 605; endoder-
mis, 2.0; epicotyl, 606; fruit, 587, /*/,
588; hairs, 588, jSg, 614, 616; hypocotyl,
604, 6ii, (f/^; inflorescence, 581; lateral

root, 595, /p7; leaves, 581, 616; medul-
lary cavity, 14; morphology, 580;
ovary, 581, s^3, S^4; peg, 591, 592, 6oi;
petiole, 616; phloem, 608, 615; placen-
tation, 99, 584, 584; plastids, 4; pollen
tube, 586; root, 580, 592., /p^, 609,
610, 611; root ontogeny, 592.; secondary
thickening, 609, 6ii,- seed, jSg, 590;
seedling, 591, /p^; shoot, 580; slime
bodies, 31; staminate flower, 581, s8},
585, /<?<?; stem, 580, 614, 614, 6is; sto-
mata, 588, 589, 614, 616, 617; tendril,
581, s8i, 616, 6ij; tyloses, 30, 610,
611; vascular transition, 596, 599, 600,
60), 606; vessel ontogeny, 29

Cucurbitaceae, 580

bundles, 60, 607; fruits, 103; root type, 47

Cuticle, II, 61, 75
of bud scales, 85

Cutin, II

Cyme, 95, p/

Cynara Scolymus (artichoke), 611
endodermis, xo, 638; fruit, 107; vascular
transition, 640

Cyperaceae,
phyllotaxy, 72.

Cystolith, II, 77

Cytisus,

pollination, 313

Cytoplasm, 2., 2, 3

D

Datura,

embryogeny, 547
Daucus carota (carrot), 45 1 , 462.
Dermatogen, 44, 65, 108

origin of leaf from, 79; of leaf, 8z
Dicotyledons,

embryogeny, 108; herbaceous stem, 58;
root types, 40, 46
Dictyostele (dissected siphonostele), 58

in stems, 68, 69
Differentiation,

factors, 13; histogens, 44, 108; of leaf
tissues, 8x; of root tissues, 47; of stem
tissues, 66; zone of, 43
Dissected siphonostele (see dictyostele)
Dracaena,

secondary thickening, 68
Drupe, lox, 103, 104, 106
Ducts (see glands)

E

Echinocystis,

stamens, 586
Element, i

Embryo, 107 (see also under specific plant
name)

668

INDEX

Endarch primary xylem, i6
Endocarp, loi, io6
Endodermis, 19

Casparian strip, 19; double, xo; passage
cells, 19, ri; in stems, Go; in transition,

71
Endosperm, 107

Endothecium, 97

Epicarp, 106

Epicotyl, 70, 107

Epidermis, 15

of leaf, 75, 82.; of root, 40; root hairs,
17; shoot hairs, 18; of stem, 61; in
transition zone, 71
Euphorbiaceae,

latex cells, 35, i39
Exarch primary xylem, 2.6
Exocarp, 101.

F

Fertilization, loi, 107

Fiber, 7, 13

phloem, 34; types, 2.3; wood, 24; xylem,
30 (see also Cannabis, Gossypium, Linum)

Ficus,

stomata, 76

Filament, 89, 97

Flax (see Linum)

Flower, 88

advanced and primitive characters, 96;
anatomy, 88; anisocarpic, 96; epigyny,
91, 95; floral types, 95; hypogyny, 90,
gi; inflorescences, 94; isocarpic, 96;
ontogeny, 90; pentacyclic, 95; perigyny
90, gz; phylogeny, 96; tetracyclic, 95;
vascular anatomy, 96; zonal growth,
90; zygomorphic, 93 (see also under
specific plant name)

Follicle, 103, 104

Fruit, 90, loL

anatomy, 88; classification, 103; dehis-
cent, 104; dry, 101, 103; dry-fleshy,
106; fleshy, loi, 105; indehiscent, 102.
(see also under specific plant name)

Funiculus, 100

Furcraea gigantea (Mauritius hemp), zi4

G

Gamete, 88

mega-, loi; micro-, 98
Gene, 13
Geraniaceae,

fruit, 104
Gladiolus,

corm, 58
Glands, 34

digestive, 35; in Gossypium,-jy; lysigenous,
35; in Pastinaca, J4; schizogenous, 34

Gliding growth, 13

Gossypium spp. (cotton), 411, 412

anatomy, 418; boll, 416, 417, 41^;
branches, fruiting, 413, 42:7, 42g;
branches, vegetative, 413; buds, acces-
sory, 413, 414; buds, axillary, 413, 414,
415,42.9; cotyledons, 42.0; embryogeny,
443; fertilization, 441; fiber, S, 10, 443,
444, 44j; fiber ontogeny, 443; floral
bracts, 417; floral ontogeny, 4^4, 437;
flower, vascular anatomy, 439, 440;
flowers, 413, 416, 417; fruit, 413, 416;
fuzz, 419, 445; glands, 412., 431, 431;
hypocotyl, 4L0, 422, 425, 427; inflo-
rescence, 412; leaf ontogeny, 433, 4^4;
leaves, 412., 434, 4^6; lint, 418, 445;
megagametophyte, 441; megasporo-
genesis, 441; microsporogenesis, 440;
morphology, 411; nectaries, 413, 432.,
435, 4)6, 437; ovary, 442; ovule, 441,
442; phyllotaxy, 411; pistil, 4^4, 439;
plastids, 4; root, 415, 42.0, 422, 42};
secondary thickening, 413; seed, 418,
420; seedling, 42.0, 421; shoot, 411,
42g; species, 411; staminal column,
4J4, 438; stem, 431, 4)2; stomata, 431,
434; sympodial branching, 414, 415,
42.7, 419, 4}o; vascular transition, 42.4,

4^S, 427, 42S
Gramineae, iii, 141

leaves, 76; phyllotaxy, 71; roots of, 39,
41; stomatal type, 17 (see also Triticum
and Zea)
Guard cells, 16, 17, 75
Gymnosperms,

cambial activity in, 67

H

Hadrome, 12.
Hairs, 17, 18
Head, g4, 95
Helianthus,

phyllotaxy, 73
Helianthus tuberosus (Jerusalem artichoke), 6zi
Hemicellulose, 10
Hemp (see Cannabis)
Hesperidium, 105
Hilum, 100
Histogens, 44, 4s, 107

of root, 44; of stem, 79; types of organ-
ization, 45
Hordeum (hurley'), in

plastid formation, 4
Hormones, 13
Hyacinthus, 179
Hydrocharis,

root type, 45

INDEX

669

Hypocotyl, 70, 107
Hypodermis, 47

I

Inflorescence, 94
Integuments, 99, 107
Intercalary,

growth, 71; meristem, ii
Intercellular substance, 6, 6, 7

in abscission, 75; chemistry, 10; ligni-
fication, 10
Ipomoea hatatus (sweet potato), 485

anatomy, 491; cotyledons, 48g, 504,
/o/, S06; embryo, 489; fleshy root, 486,

487, 496, 498, 499, soo; flower, 487; fruit,
488; hypocotyl, 489, 490; lateral roots,
494; latex vessels, 35, 498, soo, 507;
leaf, 486, jof, 509, 511, jii; leaf ontog-
eny, 509, iio; morphology, 485, root,
486, 491, 491, 492, 495^ root ontogeny,
492.; scales, 505, 506, /07, 509; second-
ary thickening, 494, 496, 497; seed,

488, 489J seedling, 490, 490J sprout,
486; stem, 485, 506, 507, joS; stomata,
16, 17, 505, 507, 511, ///; tertiary
thickening of root, 495, 498; varieties,
485; vascular transition, 500, joi

Ipomoea leucantha, 503
Ipomoea versicolor,

vascular transition, 501, 501
Iridaceae,

fruit, 103, 104

J

Juglandaceae,
ovule, 100

K

Karyolymph, 3

Lactiferous cells, 35

hactuca s a git tat a,

vascular transition, 640

hactuca sativa (lettuce), 62.1, 6zi, 624

anatomy, 631; cotyledons, 641, 64};
embryogeny, 619, 6^0; endodermis, 2.0,
638; endosperm, 631; epicotyl, tf_j/,
636; fertilization, 619; floral ontogeny,
62.5, 626; flower, 62.5, 62}; fruit, 62f,
631, 6}2, 6}}; hairs, 648, 650; inflo-
rescence, 95, 615; lactiferous ducts, 35,
635, 6i7, 6)8, 640, 643, 645, 649; leaves,
6ii, 647, 648; medullary bundles, 646;
megasporogenesis, 62.7; microsporo-
genesis, Gtj; morphology, 6ii; oil
ducts, 650; ovule, 62.7, 627; plastids,
4; pollination, 6i8; root, 6zi, 636,
6}6, 6}8; secondary thickening, 639;

seed, 631, 6}2, 6}}; seedling, 634, 634;
shoot, 62.2.; stem, 644, 645, 64s; stomata,
17, 644, 647; types, 62.1; vascular
transition, 640, 641
hactuca scariola (compass plant), 62.1
hactuca virosa,

medullary strands, 647
Lamina, 73
marginal growth, 81; meristems, 82.;
ontogeny, 81; venation, 74
hampsana communis,
oil passages, 650
Lateral roots,

origin of, 51; in Pastinaca, j2
Latex, 35 (see also Allium, Cannabis, Ipomoea,

and hactuca')
hathyrus,

floral development, 361, 363; vascular
anatomy of flower, 363
Leaf,

anatomy, 75; characteristics, 73; duration,
74; foliage, 72.; foliar buttresses, 80;
histogens, 79; marginal growth, 83;
morphology, 71; ontogeny of com-
pound, 83; ontogeny of simple, 77;
ontogeny, steps in, 83; phyllotaxy, 71;
primordium, 80; types, 71; venation,
74 (see also under specific plant name)
Legume, 103, 104, 104
Leguminosae, 309, 339

carpels, 98; embryogeny, 366; flowers,
91; fruit, 103, /o^; roots, 39, 47
hens esculenta,

vascular transition, 348
Lenticels, 69

in Betula and Prunus, 69
Leptome, 12.
Lettuce (see hactuca^
hevisticum officinale,
leaf ontogeny, 84
Lignin, 10
Ligule, 73
Liliaceae, 179
important genera, 179; lactiferous cells,

35

hilium, 179

hi Hum Harris it,

root, 2
Linaceae, 371

embryogeny, 379
Linin, 3

hinum angustifolium, 371
hinum catharticum,

embryogeny, 379, }8o
hinum usitatissimum (flax), 371

adventitious buds, 401, 403, 404, 40s; ad-
ventitious roots, 405, 406, 408; anat-

670

INDEX

omy, 377; cotyledons, 389, jpo, 408;
distribution, 371; embryo, 379; em-
bryogeny, 379, }8o: endodermis, 2.0;
fiber, 13, 395, 397; fiber lignification,
401; fiber ontogeny, 395, ^98; fiber
structure, 398, 400; fiber wall formation,
397, ^98; flower, 374, i7/; fruit, }75,
376; hypocotyl, }8g, 40}, 404, 40;, 406;
inflorescence, 95, 37i, izi. }74, 31 s:
lateral root, 384, }8y. leaf ontogeny,
390, 592; leaves, 79, yjT., 390, iP^'
morphology, 372.; ovule, ^8; phloem
duct, i8y. phyllotaxy, 373; primary
root, 381, i<f2, i8s; regeneration, 401;
root system, 371; root tip, 46; seed, 11,
376, 377, il^: seedling, 376, 577; shoot,
371; stem, 64, YJ-L, 374, 393. i94, }96;
stomata, 390, 393; vascular transition,
}8z, 386, }87

Lupinus

floral ontogeny, 361; pollination, 313

Lycoperskum esculentum (tomato), 514, 550,

adventitious roots, 54, 552., 51i, 574i
anatomy, 558; chromosomes, 553; em-
bryogeny, 563, /d^, $69, ;7/; floral
abscission, 555, //tf; floral ontogeny,
558, 5i8: flower, S54, 556, 557; fruit,
105, joj, 564, 56s, 567; fruit pigmenta-
tion, 565; hairs, S54, 559- 575; inflores-
cence, 554, 554: leaves, 553, //i, $71,
yi%,5n> megagametophyte, 562.; mega-
sporogenesis, 561; microsporogenesis,
561; morphology, 551; parthenocarpy,
568; petiole, 575, /77; pollination,
561; root, 551, 570, 573; seed, 568, $69,
///; seedling, 570, $7^: shoot, 551;
stem, 551, 574, 57^^ stomata, 559, 560,
575, /77' varieties, 550; vascular trans-
ition, 571, J7-2
Lycopersicum p imp inellifol turn, 550

M

Maize (see Zea)
Malvaceae, 411

fruit, 103
Mechanical tissue, zi
in bud scales, 85; collenchyma, xi; fibers,
2.3; fiber tracheids, 2.5; in leaf, 75, 77;
of monocot stem, 63; non-sclerotic
fibers, -LT,; sclerenchyma, 77; stone
cells, Z5; substitute fibers, 2.5
Medkago dentkulata,

nodules, 345
Medkago jalcata, 309, 333, 337
Medkago media, 309

Medicago sativa (alfalfa), 309
anatomy, 318; classification, 309; crown,
32.0 }2i; embryo, 317, ^17; embry-
ogeny, 316, 5/7; epicotyl, }24, 315;
flower, 93, 310, }i); fruit, 310, 311,
}i}, }i6, }i8; germination of hard seeds,
319; hairs, 337; inflorescence, 95, }i2,
}i}; introduction in U. S., 309; lateral
root, }24; leaves, 310, }i2, 334, i}6;
megasporogenesis, 315; microsporo-
genesis, 7, 314; morphology, 310;
ovule, 315, }i6; pollination, 313;
primary root, 311, }22; rhizome, 331,
}}); root, 310, }ii, 32.6, }27; root
nodules, 32.8; secondary thickening of
root, 32.6, ^27; seed, 318, }i8j seedling,
319, }2o; shoot, 310; stem, 310, 330,
}}o, }}2; stomata, 331, 337; tripping,
312., 313; vascular anatomy of stem,
334, }}}: vascular transition, 311, ^24

Megagamete, loi, 107

Megagametophyte, loi

Megaspore, 88, loi

Megasporogenesis, loi

Megasporophyll (see carpel)

Melilotus (sweet clover), 32.8

floral ontogeny, 361, 362.; microspore
formation, 7

Membrane,

plasma, z; vacuolar, 7.

Mericarp, 103

Meristele,
courseof bundles, 6i; in monocotyledons, 61

Meristem, 2, 12., 12

ground, 8l; intercalary, ii; lateral, ii;
of leaves, 82.; marginal, 83; non-, 13;
plate, 8l; potential, 13; specialization
of, 45; terminal, 12.; types of, li

Mesarch primary xylem, i6

Mesocarp, 106

Mesophyll, 75, 76, 8i

Mestome, iz

Micelles, 9

Microgamete, 98, 107

Microgametophyte, 98

Micropyle, 100

Microsporangia, 89, 97

Microspore, 88, 97, 98

formation by furrowing, 7; mother cell, 97

Microsporogenesis, 97

Microsfxjrophyll (see stamen)

Middle lamella (see intercellular substance)

Monocotyledons,
embryogeny, 108; herbaceous stem of, 61;
intrafascicular cambium, 68; lateral
roots, 5z; root types in, 45; secondary
thickening, 68

INDEX

671

Monostele, 41
Moraceae, 114

fruit, 103; latex vessels, 36, 139
Multiple fruit, 103, 106
Musa textilis (Manila hemp), 2.14
Musaceae,

ducts, ^39

N
Nectaries, 35
Nicotiana (tobacco), 514, 547

floral abscission, 556; formation of mi-
crospores, 7; leaf, 76, j8, 79
Nodules,

root (see Medicago and Visum)
Nucellus, 99, loi, 107
Nucleoles, 3
Nucleus, i, 3

chromosomes, 3; constituents of, 3; en-
dosperm, loi; generative, 98; polar, loi;
reticulum, 3; tube, 98
Nut, loi, 104

O

Oil ducts, 15

in citrus fruit, 35; of Pastinaca, }4
Oka (olive),

stomata, 76
Onion (see Allium)
Ontogeny,

bud scale, 85; carpel, p/, 98; compound
leaf, 83; floral, 90; ovule, 99; primary
root, 4z; simple leaf, 77; stem, 63;
stipules, 84
Organs, i
Ornithofus,

nodules, 345
Oryza sativa (rice), m
Ovary, 90, 98
Ovule, 99, 100

P

Palisade cells, 76, 81, 85
Panicle, 95, pj-
Papavaraceae,

ducts, i39
Parenchyma, 14
cambiform, 33; in herbaceous stem, 58;

in leaf, 75; phloem, 33; potentialities,

14; ray, 33; xylem, 30
Parsnip (see Pastinaca)
Passage cells, 19, ii
Pastinaca (parsnip), 451, 461
floral development, 482.; leaf ontogeny, 84,

470; oil ducts, 15, }4J origin of lateral

root, j2; root, 50,457; vascular bundle, ^tf
Pea (see Pisum)
Pectic substances, 10, 11
Pepo, 103, 105, 10$

Perianth, 88

Periblem, 44, 79, 108

Pericarp, 102., 105, 106

Pericycle, 14, 69

activity of, 49; fibers, 60; functions, 15

Periderm, 19
origin, 19; in roots, 50; in secondary
thickening, 69

Petal, 88

vascular supply, 97

Petiole, 73

Petroselinum hortense (parsley), 451

Petunia,
embryogeny, 547

Phaseolus (bean), 309
embryo, 369; floral ontogeny, 361, 361;
nodules, 345; plastids, 4; root type, 47

Phellem, 11, 15, 19, 69

Phelloderm, 19, 69

Phellogen, 19, 50
in stems, 69

Phloem, II, 31

companion cells, 31; fiber, 34; meta-
phloem, 33; parenchyma, 33; pro-
tophloem, 31, 33; secondary phloem,
33; sieve tubes, 31

Phyllotaxy, 71

leaf mosaic, 73; types of, 75

Phylogeny, 95

Phytohormones, 13, 55

Pistia,
root type, 45

Pistil, 89, 90 (see also carpel)

Pisum sativum (pea), 339

anatomy, 341; auxin test, 354; carpel,
}66; carpellary bundle, 97; cortical
fibers, 2.3; cortical lacunae, 14; coty-
ledons, 344; embryogeny, 366, }6jj
endosperm, 368; first internode, _j/2,
353' }ii' floral development, 361,
}6i; flower, 91, 93, 340, ^41; fourth
internode, jfz, m, 356; fruit, 103,
104, 341, }4i; inflorescence, 340, ^41;
leaves, 339, }}g, 358, ^/p; megasporo-
genesis, 365; microsporogenesis, 364;
morphology, 339; ovule, 365, }66, ^67,
}68; petiole, 360, }6o; plastid formation,
4; pollination, 340; primary root,
42, 47, 344, i4s; root, ^46; root nodules,
345; root system, 339; second inter-
node, isz, 354, m: seed, 342., ^42; seed-
ling, ^40, ii,y, stem, 339, 353, ^$8;
stomata, 76, 359; tendril, 360, _jtfo;
third internode, 5/2, 354, ^j/; upper
internodes, 357, ^8; vascular anatomy
of flower, 363, ^64; vascular transition,
348. iSO, )55

^1^

INDEX

Pits, 8, 9
Placenta, 98
Placentation, 98

types, 99
Plasma membrane, i
Plasmodesma, 5
Plastids, 1, 4

proplastids, 4; types, 5
Plerome, 44, 79, 108
Pod, 102. (see legume)
Foinsettia,

floral bracts, 89
Pollen, 89

dehiscence, sacs, stomium, 98
Pollination, 89
Polygonaceae,

ovule, 100
Polypodium,

bundle, 63
Pome, 103, lof, 106
Potato (see Solarium)
Primary wall (see wall)
Primary xylem (see xylem)
Primordia,

of adventitious root, 54; of compound
leaf, 83; of flower, 90, 91, 93; of lateral
root, 51; of stamen, 97
Primula,

flower, 99
Primulaceae,

floral development, 91; placentation,

99
Procambium, 81, 83

Proembryo, 108

Proplastids, 4

Protective tissues, 15

Protoderm, 8i, 83

Protoplast, X

Protostele, 41

radial, 40, 69
Provascular strand, 65, 68
Pteridophytes,

bundles, 63; cambial activity, 67
Pyrus communis (pear)

stone cells, 15
Pyrus Malus (apple)

floral development, 9^; fruit, /o/;
vascular system of flower, g}, 96

R

Raceme, 9^, 95

Radicula Armoracia (horseradish), 2.83

Radish (see Raphanus')

Ranunculaceae,

akene, 102.; follicle, 103
Ranunculus (buttercup)

floral development, gi

Raphanus sativus (radish), z83, 284

anatomy, 189; carpels, 2.89; cotyledons,
2.97, zgj, 507; fleshy axis, 185, 286, 2.98,
}02, }0}; floral ontogeny, 288, x&<j;
flower, 2.86, 28-;; food reserves, 304;
fruit, 103, 105, x9o; hairs, 306, 507;
hypocotyl, 2.85, 195, 198, 301, }02;
inflorescence, 95, i86, 287; lateral roots,
2.95; leaves, 2.86, 306, ;jo7; morphology,
183; primary root, 41, 191, 294, 2gj, 2.98,
■2pp. ioo; root ontogeny, i.'p., 294; root
system, 2.84; secondary thickening, 2.98,
^99i 300^ }<'2, }0}; seed, 2.90, 291; seed-
ling, 19Z, 2g^; stem, sg, x85, 304, 50/,
507; stomata, 17, 307, 507; tertiary
thickening, 301, }02, )0}; varieties,
2.83; vascular transition, 196, 297, 2g8

Raphe, loi

Rays, 14, 58

medullary, 31, S9, 66; pericyclic, 48, 49;
phloem, 33; in root, 48, so; in secon-
dary thickening, 68; xylem, 31

Receptacle, 88

Regeneration,

in Ipomoea, 486; in Linum, 401

Reproduction,
gametic, 88; vegetative, 14, 54

Reticulum, 3

Rheum (rhubarb),
ovule, 100

Rhizfibium, 345

leguminosarum, 340, 345; meliloti, 32.8

Rhizome, 57

Rhubarb (see Rheum)

Ricinus communis (castor bean)_
vessel, 30

Root, 39

adventitious, 53; cap, 43; fibrous, 39;
hairs, 17; histogens, 44; lateral, 51;
morphology, 40; number of arcs in, 41;
ontogeny, 41; tap, 39; tissue differenti-
ation, 47; secondary thickening, 48;
stelar types, 40; vascular (root-stem)
transition, 69 (see also under specific
plant name)

Root cap, 43

Root hairs, 17, 48

Root-stem transition (see vascular tran-
sition)

Rosaceae,

carpels, 98; floral bracts, 88; floral devel-
opment, 91; fruit, 103

Rutaceae (see Citrus)

Saccharum officinarum (sugar cane), 1 1 1
Sagittaria, 108

INDEX

673

Scale leaves, 71

bud, 85
Schizocarp, loz, 103 (see also Apium)
Sclerenchyma, 2.2., 13, 2.5
Scolymus,

oil ducts, 650
Secale cereale (rye), 1 1 1
Secondary thickening,

anomalous, 51; changes accompanying,
69; in Dracaena, 68; in monocotyledons,
68; of root, 48; of stem, 66; in transi-
tion region, 71 ; of walls, 8; in Yucca, 68
Secondary wall (see wall)
Secondary xylem (see xylem)
Secretory tissues,

ducts, glands, lactiferous cells, nectaries,

34. 35

Seed, 107

Seedling, 70

Sepal, 88

vascular supply, 96

Sheath, 73

Shoot, 57, 71,81,88 (see also under specific
plant name)

Sieve tubes, 31

callus, 32.; ontogeny, 32.

Silicle, 105

Silique, loi, 103

Simple tissue, 11

Siphonostele,

dissected, 58; radial, 40

Soja,

floral development, 361; nodules, 345

Solanaceae, 514, 550

bundles, 60; floral development, 543;
fruit, 103; stomatal type, 17

Solatium ]ame sit (wild potato), 514

Solarium M.elongena (eggplant), 514

Solarium murkatum,
gametophyte, 547

Solarium tuberosum (potato), 514

adventitious roots, 516, 513, iz^; anat-
omy, 519; cotyledons, 52.1; embryog-
eny, /20, 547, /^j; floral abscission,
544; floral development, 543; flower,
518, //(?, 543; fruit, //(?, 519; inflores-
cence, 518; lateral root, 52.1; leaf, 515,
541, S42; megagametophyte, 546; me-
gasporogenesis, 546; microsporogene-
sis, 545; morphology, 514; ovule,
519; periderm, 15; rhizome, ///,
516, 5Z1, 532., /55, ii4: root, 20, 517,
j/7, 512., 514, s^4i seed, //tf, 519, sio;
seedling, $16, 52.1, $21; stem, 514,
5x7, /2if, 52.9; sterility, 519; stomata,
16, 540; tuber, ///, 516, 535, sis,
S}6, }3^, 539, }4o; tuberization, 537;

tyloses, 30; vascular anatomy of stem,
530, j}i; vascular transition, 515, }2j;
vegetative propagation, 516, 52.3

Spike, 94, 94

Spines, 71

Spiraea,
flower, g2

Spongy tissue, 76, 82.

Sporangia, 88

Sporophylls, 71, 88, 94, 98 (see also micro-
and mega-)

Squash (see Cucurbita)

Stamen, 88, 89, 94

ontogeny and vascular supply, 97

Starch sheath, 2.1

Stele, 40
dictyostele, 58; meristele, 6i; monostele,
41; protostele, 41; radial siphonostele,
40; root types of, 40

Stem,

anatomy, 58; general morphology, 57; her-
baceous dicotyledonous, 58; herbaceous
monocotyledonous, 61; ontogeny, 63;
types, 57 (see also under specific plant
name)

Stereome, ii

Stigma, 90

Stipule, 73, 84

Stolon, 58

Stomata (sing, stoma), 16

in bud scales, 85; frequency and distri-
bution, 75; ontogeny, 17; types, 17

Stomium, 98

Stone cells, 15

Style, 90

Suberin, 11

Suspensor, 108

Sweet Potato (see Ipomoea)

Synergids, loi

Tapetum, 97

Tendrils, 71

Tissues,

absorptive, 15; collenchyma, ix; complex,
11; conjunctive and connective, 68;
culture, 13; epidermal, 15; mechanical,
12.; nonsclerotic, 13; palisade, 76;
phellem, 15; phloem, 31; protective,
15; simple, 11; spongy, 76; xylem, 15

Tobacco (see Nicotiana')

Tomato (see Ljcopersicum')

Torus (see receptacle)

Trachea (see vessel)

Tracheid, 24, i£

fiber tracheid, 15; types of, 17

Tragopogon porrifolius (salsify), 62.1

674

INDEX

Transfusion cells, zi

Trifolium,
embryogeny, 366; floral development, 361;
nodules, 345

Trifolium pratense, 309

Trigonella, 318

Triticum spp. (wheat), 141

adventitious roots, 143, 159; anatomy,
149; bundles, 165, 166, 169; classifica-
tion, 141; coleoptile, 167, 167; distri-
bution and origin, 141; embryo, 151,
IS}, 154; embryogeny, 174, 175:
endosperm, 151, 175; epiblast, 151;
first internode, 160, 161; floral develop-
ment, 171, 171; flower, 148, 149; ger-
mination, 155; grain, 149, //o, ///;
inflorescence, 95, 145, 146; integuments,
176, /;;; leaf, 79, 144, 167, i6g; mega-
gametophyte, 173; megasporogenesis,
173; microsporogenesis, ijx; morphol-
ogy, 142.; node, 166; ovary, 171; ovule,
171, 175; pericarp, 150, 176, 177;
pollination, 149; primary root, 141,
156, //;; prophyll, 167; rachis, 147,
148; root ontogeny, 158; root system,
142., 144; second internode, 163, 16};
seed coats, 149; seedling, 142, 154, ///;
seminal roots, 141, 156, 159; shoot
ontogeny, 170; spikelet, 147, 148, 171;
stem, 61, 143, 163, 164, 166; stomata,
16, 17, 76, 164, 168; tillers, 144; transi-
tion region, 161, 161, 161

Tuber, 58

Tulipa, 179

Tunica, 80

Tyloses, 30, 610, 611

U

Umbel, 94, 95

compound, 95; umbellet, 95
Umbelliferae, 451

floral development, 481; fruit, 103, 104;
inflorescence, 95; leaf ontogeny, 83, 84;
medullary bundles, 462., 466; oil ducts,
34; stems, 464
Urticaceae,
ovule, 100

V
Vascular anatomy,
of dicotyledonous stem, 58; of flower, 96;
of leaves, 74, 77; of monocotyledonous
stem, 61; of root, 40; of transition
region, 69
Vascular bundles (see bundles)
Vascular transition, 69

how accomplished, 70; types, 71 (see also
under specific plant names)

Vegetative propagation, 14, 54 (see also

Ipomoea, Linum, Solanurn)
Veins, 75, 77
Vessel, 24, 1.7
ontogeny, i8, 29; types, i8; vessel seg-
ments, 1.7
Vicia Faba, 319

nodules, 345, 347; vascular transition, 348
Vicia sativa,

vascular transition, 348
Vigna,
nodules, 345

W
Walls, 5, 6
cell plate, 7; chemistry of, 10; lignifica-
tion, 10; primary, 7; secondary, 7; sec-
ondary thickening, 8; structure, 9
Wheat (see Triticum)

Xylem, 11, 24, 15

fibers, 14, 24, 30; metaxylem, 2.5; paren-
chyma, 30; primary, 15; protoxylem,
Z5; rays, 31; secondary, 2.5; tracheids,
2.6; vessels, 17

Yucca,
secondary thickening, 68

Zea Mays (corn), in
adventitious root, //, 118, iiz; anatomy,
113; antipodal tissue, 139; bulliform
cells, 12.8; bundle, ii6, 127; carpellate
inflorescence, 132.; carpellate spikelet,
133, i)s: coleoptile, iii, 124; embryo,
115, ///, 116, 117: embryogeny, 137,
i}8; endosperm, 138; epidermis, 12.8;
first internode, 12.1, 12}; floral develop-
ment, i}i; germination, 118; glands,
35, 116; grain, 113; guard cells, 16,
130; hypocotyl, 12.2.; inflorescences,
113; lacunae, 14, 30; leaf, 118, 129;
megagametophyte, 136; microgameto-
phyte, 136; morphology, in; node,
12.5; ovule, 135; plastids, 4, 12.8; pol-
lination, 136; primary root, 119, 120,
izi; root, 12, 4i, 4S, m, 112; scutellum,
117; second internode, 114, 12s; seed-
ling, 118, 119; seminal root, 12.1;
shoot, 80, in; staminate inflorescence,

130, i}i, frontispiece; staminate spikelet,

131, 1)1, 1)2; stem, 62, 115, 1x6; sto-
mata, 17, 76, 118, 130, I}0

Zonal growth, 90, 91
Zygote, 12., 88, loi, 107

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