^MVvf-

'M&''vSp

'l^.'JV-' ' 7'

R D

Marine Biological Laboratory

Received FGP.IP., 19

^i:

Accession No. ooi>^2

Given By D^« A. 3. Foster and

D. van No strand Co.

Place, ^rr^' Tori: f'-^.t-r \

^^= -0

^^^ nj
^^= ru

CD'

- O

— — m

0

Practical
Plant Anatomy

//

By
ADRIANCE S. FOSTER

Associate Professor of Botany
University of California

NEW YORK

D. VAN NOSTRAND COMPANY, Inc.

250 FOURTH AVENUE

1942

Copyright, 1942

l.y

D. VAN XOSTRAXl) COMPANY, Inc.

AU Ixif/Jils- Fmrrvt'd

Tills })iiiik or iiiiij pint Ihereaf mai/ not
he reprodiired in nni/ form fiHiniit
vritti'ii iii'riiiixsinn iriiiii tlif piililixliers.

PRINTED TX U.S.A.

Preux of

George S. Fkucu-sox Co.
Philadelphi;!. Pa.

To my wife

PREFACE

Since a realistic foundation in plant anatomy depends upon
thorough laboratory practice, there appears to be a definite need
for a guide which will both direct as w^ell as orient the student
in his individual studies. The present book has been written
from this standpoint and is therefore intended for use in the
laboratory. Each exercise contains an introductory section in
which an effort is made to summarize briefly but clearly the
present status of knowledge of the subject for study. This
"Introduction" is in no sense to be regarded as a substitute for
collateral reading in the standard texts in plant anatomy and in
the selected modern literature which are appended at the end
of each exercise. But the author's experience has led to the
conviction that a wholly unnatural and artificial gap may easily
occur between ''theory" and "practice" in the teaching of plant
anatomy. To quote from De Bary's classic of 1884, "On the
anatomy of plants such an indescribable amount has been written
that, in a' comprehensive treatise, one or many authors might
be cited in reference to every word. ' ' The truth of this statement
is of course self-evident today and the beginner in anatomy is
often confused as well as discouraged by the wealth of detail
and maze of controversy presented in many anatomical texts. In
the present book, therefore, the aim has been to articulate as far
as possible the practical study of laboratory material with the
best of modern interpretation and theory. By this means the
student, through his own work in the laboratory should be able
gradually to acquire a practical basis for the critical evaluation
of theory.

The material suggested for study under each exercise has
been selected, as far as possible, from types of plants readily
available to most teachers. An effort has been made to avoid
rare or unusual plants and frequent reference is made to forms
of economic importance to man. Wherever it seemed desirable,
alternative material has been listed.

y

vu

Vlll PREFACE

111 view of the existence of several excellent texts in plant
microtechnique, special methods for the preparation of macerated
tissue and permanent mounts, as well as the use of microchemical
reagents, receive only brief attention in this book. However, a
few notes on these topics which may prove valuable to both the
teacher and student in the use of this book are included under
the "Appendix."

Since teaching methods vary, especially with respect to the
nature of the record which the student is required to make of
his laboratory work, each exercise contains a list of suggested
drawings and special topical reports. This, it is hoped, will per-
mit of selection on the part of the teacher in accordance with the
time and emphasis placed on a given topic.

Whatever practical merits the present volume may possess are

due to a large degree to the constructive criticisms of numerous

students who used the book in its previous planographed form.

The exercise on sieve-tube elements has been read and criticized

by Dr. Katherine Esau and Dr. A. S. Crafts for whose assistance

the author expresses his thanks. I am also grateful for the many

helpful suggestions made by Dr. Ernest Ball who served as my

laboratory assistant for the past three years. For all errors

in fact or interpretation, however, the writer assumes full

responsibility. a o xn

A. o. F.

Berkelcv, Talif.
Oct., 1941

Special Acknowledgments

Quotations from various texts are acknowledged as to pagina-
tion and autiior at appropriate points in this book. For special
ponnission to rei)i'o(luce these quotations, the author expresses his
thanks to the foUowiiig: Professor T. E. Kawlins and John Wiley
and Sons, for the quotation from Rawlins' Phytopathological and
Botanical Research MetJwds; University of Chicago Press, for
the quotation from Jeffrey's The Anatomy of Woody Plants;
]\lcfiraw-llill IJook Company, for the (piotations from Sliarp's
Introduction to Cj/toloc/y and Eames and ^IcDaniel's Introduc-
tion to Plant Anatomy : Longman's Green and Company, for the
(piotations from Priestley and Scott's Introduction to Botany;
The Mac.Milhiii Company, for the (|Uotations from Ilabei-Iandt's
Physif)lo(jic(d Plant Aiiafomy. Strasl)urger's Tc.rfhooli of Botany
and Hay ward's The Structure of Economic Phnifs.

TABLE OF CONTENTS

PAGE

Preface vii

EXERCISE

I. The Protoplast 1

II. The Cell Wall 7

III. Meristems 1^

IV. Problems in the Classification of Cell Types,

Tissues and Tissue Systems in Vascular
Plants (Including Tabular Summary of Main

Cell Types in Seed Plants) 32

V. The Epidermis 45

VI. Parenchyma Cells 57

VII. Collenchyma Cells 62

VIII. Sclereides 67

IX. Fibers 73

X. Tracheary Elements 80

XI. Sieve-tube Elements 93

XTT. The Stem 101

XIIT. The Leaf 123

XIV. The Root 130

Appendix 139

Index 143

IX

56399

General References

The books listed below constitute the most important general
references in plant anatomy and are cited by the author, wher-
ever useful, in the specific reference lists at the end of each exer-
cise. Additional references including recent papers and compre-
hensive review articles will be found at the end of most of the
exercises.

Bower, F. 0., Size and Form in Plants. London, Macmillan and Company,

Ltd., 1930.
Chamberlain, C. J., Gymnosperms, Structure and Evolution. Chicago,

University of Chicago Press, 193.1.
, , Methods in Plant Histology. Chicago, University of

Chicago Press, 1932.
Committee on Nomenclature, International Association of Wood Anatomists,

Glossary of Terms Used in Describing Woods. Tropical Woods 36:1-12.

1933.
De Bary, A., Comparative Anatomy of the Vegetative Organs of the

Phanerogams and Ferns. Oxford, Clarendon Press, 1884.
Fames, A. J., Morphology of Vascular Plants, Lower Groups. New York,

McGraw Hill Book Co., 1936.
, , and MacDaniels, L. H., An Introduction to Plant Anatomy.

New York, McGraw Hill Book Co., 1925.
Haberlandt, G., Physiological Plant Anatomy. London, Macmillan and

Company, Ltd., 1914.
Hayward, H. E., The Structure of Economic Plants. New York, The

Macmillan Company, 1938.
.Teffrey, E. C, The Anatomy of Woody Plants. Chicago, LTniversity of

Cliicago Press, 1917.
Johansen, D. A., Plant Microtechnique, New York, McGraw Hill Book

Co., 1940.
Mansfield, W., Histology of Medicinal Plants. New York, John Wiley and

Sons, Inc., 1916.
Penhallow, D. P., A Manual of the North American Gymnosperms. Boston,

(Jinn and Co., 1907.
Priestley, J. II., and Scott, L. I., An Introduction to Botany. London,

Longmans Green and Co., 1938.
Rawlins, T. E., Phytopathological and Botanical Research Methods. New

York, John Wiley and Sons, Inc., 1933.
Sachs, J., Te.xtbook o'f Botany. Oxford, Clarendon Press, 1882.
Sass, J. E., Elements of Botanical Microtechnique. New York, AIcGraw

Hill Book Co., 1940.
Sharp, L. W., Introduction to Cytology. Srd Ed. New Ycuk, McGraw Hill

Book Co., I!t34.
Smith, G. M., Cryptogamic Botany, Vol. II. Bryophytes and Pteridophytes.

New York, McGraw Hill Book Co., 1938.
Solereder, H., Systematic Anatomy of the Dicotyledons. Oxford, Clarendon

Press, 1908.'
, , and Meyer, F. J., Systematische Anatomic dor Mono-

kotyledonen. Berlin, (icl)riider Borntraeger (apparently as yet incom-
plete. So far have appeared 1 Heft, 1 Teil, 1933; Heft III, 1928;

Heft IV, 1929; Heft \l, 1930).
Strasburger, E., Ilandlxiok of I'ractical Botany (trans, by W. Hillhouse).

7th Ed., New York, The MacmiUan Company, 1911.
, , Textbook of Botany, 5th Eug. Ed. New York, The

Macmillan Company, 1921.

Exercise I

THE PROTOPLAST

I. Introduction. — It is now a well-known fact that protoplasm,
which constitutes the physical basis of all life, is organized
and "subdivided" in the higher plants into small microscopic
units which are termed cells. Usually each living cell, reg-ardless
of its position, form or function, consists of a wall which encloses
a uninucleate protoplast. Exceptions to this typical condition
are furnished by the multinucleate protoplasts of certain fibers,
vessels and lactiferous elements. Such coenocytes are of con-
siderable theoretical interest with reference to the problem of
the origin and significance of the multicellular plant. The so-
called "Cell Theory," propounded over a century ago, regards
the organism both ontogenetically and phylogenetically as a
"cell republic" which has arisen by "the aggregation of a vast
number of elementary individuals" or cells (cf. Sharp 1934, pp.
20-24). In contrast, the "Organismal Theory" attaches less
importance to the septate condition and regards cellular struc-
ture as the result of the growth of the organism as a whole. The
definitiveness of the cell wall in the tissues of all higher plants
doubtless has encouraged the continued wide acceptance of the
"Cell Theory" as the more useful concept, at least in any analyti-
cal study of plant anatomy.

The term "cell" was originally applied in 1665 by Robert
Hooke to each of the numerous "cavities" observed by him in
such material as charcoal and cork. Later, with the discovery
of protoplasm, the major emphasis was placed upon the living
protoplasmic body and the cell wall was regarded as a "lifeless"
secretion of the protoplast. At the present time, however, it
seems necessary and justifiable to include hoth the protoplast as
well as its wall under the general term of "cell." Evidence in
support of this viewpoint is furnished (1) by the apparently

Z THE PROTOPLAST

iiitiinato relationship existing between llie eell wall and the proto-
l)Ia.sm (luring eell differentiation, (2) by the very common occur-
rence of protoplasmic connections or plasmodesmuta which
penetrate the wall at various points, and (3) by the peculiar
behavior of the wall in certain algae (cf. Anderson 1935, pp.
71-72). ]\fany important cell types in vascular plants such as
fibers, tracheids, vessel elements, and sclereides consist only of
the wall at maturity. However, it is entirely appropriate to
designate them as "cells" since the loss of their jirotoplasts
occurs during the later stages of differentiation.

II. The Cells of Plant Hairs. — The cells of many plant hairs
furnish very useful material for a study of the protoplast. Be-
cause such cells are usually highly vacuolated and hence semi-
transparent, they may be easily studied without special prepara-
tion or staining. Indeed, in the staminal hairs of Tnidcscantia,
the brilliantly colored cell sap in the vacuome provides a splendid
optical contrast for the gray cytoplasm and nucleus.

Obtain a transverse section near the tij) of the stem of petunia
or squash, mount it carefully in water and examine the prepara-
tion under lov^ mugnification. A number of semi-transparent
hairs, variable in size and in the form of their terminal cells,
will be seen radiating from the edge of the section. Thin sec-
tions will show the mode of attachment of the base or "foot" of
the hair to the epidermis of the stem. Selecting an uninjured
and straight haii-, examine its component cells under hi<ih magni-
fication. Fre(|upntly the lower or basal cells of the hair will prove
most suitable I'oi- this study. Wy carefully regulating the light
and constantly using the fine adjustment on the microscope, the
nucleus, with its nucleolus will be visible. In color, the nucleiis
will aj)j)ear gray and slightly opaque. Often the nucleus may
appear to be imbedded in tlic thin layer of cutoplasni lining the
wall of the cell, ("onnnonly. however, the nucleus is suspended in
various positions by a delicate and complex network of cyto-
plasmic sfrauds which extend from the jieripheral cytoplasm
tlu'ough the clear watei-y cell-sap of the prominent vacuome of
the cell. In many of llic cells of the hair, small plasiids may be
seen in the pei-iphei-al cytoplasm and sometimes in the larger cyto-
plasmic strands. Well-mounted uninjured sections which have

PL AST IDS d

not been allowed to dry out are suitable for a study of the stream-
ing movement of the cytoplasm as well as changes in the relative
position of the nucleus in the hair-cells.

III. Plastids. — Specialized cytoplasmic bodies known as plas-
tids are commonly found in many types of plant cells. Recent
studies (cf. Weier 1938) emphasize the many unsolved problems
concerning the origin, fundamental structure and activity of
plastids, in particular of the photosynthetic chloroplast.

Obtain a leaf from the outer region of the terminal bud of
Elodea and mount it carefully in a drop of water. Examine this
leaf first under low magnification, noting that the thin lateral
flaps at either side of the "midrib" are composed of only two
layers of essentially similar cells. In short, there is no mesophyll
as distinct from an epidermis. Study a number of cells in this
leaf at varying depths of focus under high magnification. Note
that all cells contain numerous small discoid chloroplasts which
typically are in a iieripheral position. Many cells, at least in
healthy leaves, will exhibit cytoplasmic streaming. Careful ex-
amination will show that while the chloroplasts are non-motile in
themselves, they are passively carried in a clockwise or counter-
clockwise direction by the circulating cytoplasm. Often the
chloroplasts are so numerous in a cell that they mask the nucleus.
The latter can be more easily observed in the more transparent
teeth-like cells which occur irregularly at the margins of the
leaf.

Another extremely common type of plastid is the chromoplast
which frequently produces the red, orange, or yellow color of
petals, fruits and certain roots. {Note: Since similar colors may
be produced by anthocyanin pigments in the vacuome, the basis
for color in each instance can only be determined by first hand
investigation.) The role of chromoplasts in general is obscure
and demands further study.

Prepare water mounts of tomato fruit, carrot root, asparagus
and rose "hips," and examine them under low magnification.
Note the wide variation in size and shape of the chromoplasts.
According to Eames and MacDaniels (1925, p. l.S) the angular
forms of the chromoplasts in the carrot (Daucus) "are largely
due to the presence of crystals of coloring matter."

4 THE PROTOPLAST

IV. Ergastic Substances. — All living cells in plants contain va-
riable amounts of "lifeless" materials -svhicli may be collectively
designated as crgasiic substances. They include storage products,
waste material, or by-products of protoplasmic activity and in
elementary botanical texts are often termed "inclusions." One
of the most common examples of ergastic substances is the plant
vacuome which consists of a dilute aqueous solution of a Avide
range of inorganic as well as organic materials.

In addition to the vacuome, many types of plant cells contain
further ergastic material in the form of reserve food such as
starch, proteins and fats or oils. Perhaps the most common type
of non-transitory food is starch, Avhicli occurs as grains, the size
and form of which are highly specific. Obtain a thin, transverse
section of the stem of PcUionia and after mounting it in water,
examine the large parenchyma cells of the cortex under law and
high magnification. Most of these cells contain starch grains
which have developed within the chloroplasts. Often, "frag-
ments" of the chloroplast may be seen at the broad end of the
pear-shaped starch grains. The addition of dilute iodine to the
section will give the blue color reaction typical for starch. In
the tissue of such storage organs as tubers, fleshy roots and
cotyledons, starch grains ai-e formed by the activity of amifJo-
plasts. These starch-forming i)lastids hick chlorophyll and are
to be regarded as a specialized type of hucoplasf. Secure a small
amount of fresh potato tissue and after gently teasing it with
dissecting needles, mount it in water, jidd diluto iodine and ex-
amine under low magnification. Note the numerous obovoid starch
grains in every cell. Plxamine a single starch gi-ain under high
magnification and observe, near the smaller end of the grain, the
minute refi'active i)()iut Avhicli is termed the hit inn. Careful regu-
lation of the light and patient use of the fine adjustment will
usually reveal a nninbci- of more or less distinct rcc< )ityic lagers
arranged abdut the hilniu. For coniiiai-ative pui'poses, examine
the storage parenchyma cells in the ])can cotyledon, noting the
difference in the foi-ni and [lositidii of the hiluin in tlie ovoid
starch grains.

A very common type of ergastic su])stance in many kinds of
])lant cells is calcium oxalate which api)ears usually in the form

ERGASTIC SUBSTANCE.S O

of conspicuous well-defined crystals. It is generally held that
such crystals represent an excretory product of the protoplast,
being formed by the union of calcium with oxalic acid. The
recent monograph by Netolitzky (1929), however, reveals that
substances other than oxalic acid may combine with calcium to
produce crystalline bodies. Comparatively little is known con-
cerning the factors, chemical and biological, which regulate the
rate and mode of crystallization, and which hence determine the
form of the adult crystal. Netolitzky (1929, p. 47) concludes
that in the final analysis it is ''the nucleus which determines
which form of crystal will be produced, perhaps by regulating
the velocity of crystallization wnthin the cell itself." Examples
of the three main forms of plant crystals may now be studied.

1. Druses or sphaerraphides are compound and consist of
more or less spherical aggregates of sharp pointed angular crys-
tals, the whole mass often suggesting in form the mace-head of
medieval warfare. Examine the prepared section of the stem of
geranium {Pelargonium), noting the presence of druses in many
of the cortical cells. Transverse sections would suggest that the
crystal containing cells are solitary and isolated from one another.
But in longisection, it will be seen that frequently the crystal
containing cells or crystal sacs (cf. Haberlandt) are in short
supposed series, each cell of which may contain a druse.

2. Raphides are long slender needle-shaped crystals which
typically are arranged parallel to one another in definite bundles.
Such crystals appear to be most common in the monocotyledons.
According to Netolitzky (1929, p. 48) raphides constitute vir-
tually a family characteristic in the Oenotheraceae. Obtain a
single living plant of duckweed {Lemna sp.) and mount it in
water. Note under high magnification that many of the trans-
parent cells at the margins of the "thallus lobes" contain promi-
nent bundles of raphides. For comparison, examine under lovr
magnification freshly-cut longi-sections of the stem of Trades-
cant ia noting the much larger raphides, many of which may be
pulled from the cells during the process of sectioning.

3. Prismatic crystals are common in vascular tissue but may
also occur singly, or in association wdth other crystal types in
thin-walled cortical parenchyma cells (cf. Exercise VI). Ex-
amine prepared slides of the stem of Tilia or some similar woody

6 THE PROTOPLAST

dicutyldedoii, noting the solitary crystals in tlie phloem paren-
chyma cells.

V. Suggested Drawings and Notes. —

1. Prepare an enlarged drawing of a single hair of either
squash or petunia as seen under low magnification showing ac-
curately the number and form of the cells of which it is com-
posed, and its mode of attachment to the stem.

2. Draw a' single living cell of a hair as it appears under high
magnification. This drawing should portray a "median optical
view" and should include the following: cell wall, nucleus (and
its visible parts), cytoplasm, vacuole, plastids. Eccord, as lab-
oratory notes, all observations made on cytoplasmic streaming
and nuclear "movement" in the material studied.

3. Select a cell from the "midrib" region of the Elodca leaf
and prepare drawings to show its appearance aud contents as
seen in surface and median optical views. Describe concisely
the variations in the rate and direction of cytoplasmic streaming
in cells at different regions of the leaf. What may be the physio-
logical significance of cytoplasmic streaming? Summarize the
evidence indicating that plastids do not arise dc novo m the
cytoplasm of plant cells [cf. Sharp (1934, pp. 69-72)].

4. Pi'epai-e drawings to illustrate the form and the ari-auge-
ment of chromoplasts in the material studied.

5. Draw cells from the cortex of Pcllionia, the jxttato tuber
aud the bean cotyledon showing the size and form of the included
starch gr;iiiis.

6. Prepcirc drawings lo show the foi'iii hiuI position of the
various types of crystals studied.

RKFEREXCKS

1. AiHl.Tsoii, T). n., TIic ShMicliuv of tlic Walls of the Ili-hcr Plants.

Bol. K.'vi.'w 1 :r)-!-7(). l!);55.
'2. Kaiiics and MacDanicls, Cli. II, p. (5-1!).

;{. iiahriiaiKit. cii. X. pp. r);5()-r).%.

4. .Xctolitzky, F.. Die Kicsclkorper. Die Kalksal/.c als Zolliiihalts-
korjx'i'. llandhucli. d. Pflanzcnanatomio III. Berlin, 1929.

f). . Das Iropliisclic I'arciicliyiii. ('. Spciclicvgcwcbc.

IhirJ.. TV. Berlin. Pt.T).

(i. Scliiiihdir, r. X. Die I'lastidcn. Ihi,l.. I. B.-rlin. 1!)24.

7. Sliai')), L. W., Inlro(hicti()n to Cytoloj^y. .'Jd. cd. New York, 1934.

8. W.'icr, E., The Striu-ture of tiieCldoroplast. Jiot. Kcv. 4:41)7-530.
1!)38.

Exercise II

THE CELL WALL

L Introduction.— Sperms, eggs and certain other cells of the
gametophyte of aiigiosperms are naked protoplasts and show no
evidence of a definite cell wall. Save for these particular ex-
ceptions, the zygote and all of the succeeding generations of
sporophytic cells which are derived from it are provided from
the beginning with a cell wall. Indeed the rigid and often mas-
sively-thickened cell Avail is frequently cited as a significant dis-
tinction between plants and animals; the cells of the latter are
either naked protoplasts or else possess thin and less clearly
demarcated walls, features which are obviously related to the
marked pliancy of many animal tissues.

The form, relative thickness and chemical nature of the cell
wall, particularly the secondary wall, provide important and
definite characters for separating and classif.ying cell types in
higher plants. Consequently, a preliminary study of wall struc-
ture, together with a brief summary of the results of modern
research constitute a natural and necessary introduction to the
general problem of tissues and cell-types, which will be critically
discussed in the following exercise.

IL Plasmodesmata.^ — The protoplasts of adjacent cells of a
wide variety of plant tissues have been shown to be intercon-
nected by delicate threads of cytoplasm, which are termed plas-
modcsmata (for a complete review, ef. Meeuse 1941). Protoplas-
mic connections seem to support the idea that the cell wall, at
least at certain stages in its growth, is an integral part of the
living system and not simply a "lifeless" excretion of the proto-
plast. Knowledge as to the origin and development of plasmo-
desmata is meagre but there is some evidence that additional or
' ' secondarv ' ' connections may arise at diflPerent stages in ontogeny.

1 For specijil technique to demonstrate plasmodosniata, of. Crafts (1931).

7

8 THE CELL WALL

This problem assumes increased interest with reference to the fate
and possible renewed-formation of the plasmodesmata of certain
plant cells which are believed to "slide" or push past their neigh-
bors during dift'erentiation (cf. iSharp 1934, pp. 15-18). The
function of plasmodesmata is not entirely clear but Haberlandt
(pp. 635-638) suggests (1) that they may be essential in trans-
mitting both external and internal stimuli through plant tissues,
and (2) that in storage tissue, such as endosperm, they may
be "principally concerned with trans-location." Furthermore,
considerable evidence has accumulated within recent years which
indicates that the plasmodesmata, especially those of sieve-
tubes, are the channels through which certain economically-im-
portant viruses may travel. Lasth', it is probable that the high
degree of correlation between the cells, tissues and organs of the
plant ma.y depend upon the presence of plasmodesmata. Living-
ston, (1935) who maintains that plasmodesmata occur in all the
living tissues of the tobacco plant, concludes that "the evidence
presented by numerous investigators indicates that actual proto-
plasmic connections between cells, or plasmodesmata, are gen-
erally present throughout all living tissues of higher plants, thus
establishing the organism as a definitely correlated entity of inter-
connecting protoplasts, instead of a community of separate cells."
Endosperm tissue of certain seeds provides useful material
for a study of the general features of plasmodesmata. Obtain a
prepared slide of the endosperm tissue of the persimmon (Dio-
spyros) and examine the section under hif/h magnification. Note
that the greatly thickened walls are traversed by solitary or
spindle-shaped groups of very delicate "lines" which are the
])lasmodesmata. According to Qnisumbing (1925). the plas-
modesmata are ninnerous in the walls of the endosperm of D.
discolor and D. Ahcrnii while in 7). haki and Z). ehrnastcr "they
are few, restrict cd ;md gi-ouped at the walls. They occur single
or in groups (if two, three, foui', five oi- six, and are thicker when
single and usually thinner in groups.

) )

III. The Gross Layers of the Cell Wall. — Differentiation and
maturation of most phiiit cells produce marked changes in the
Mi'ca and lliickncss of the cell wall. Young cells, such as those
of embryos and terminal meristems, possess relatively thin walls

THE GROSS LAYERS OF THE CELL WALL i)

which, unless examined with special technique, appear more or
less homogeneous in structure. As a cell of this kind enlarges
and becomes mature, its wall naturally increases in surface. In
addition, the wall may become much thicker, apparently as the
result of (1) deposition of new particles of wall substance among
those present, a process termed ^^ intussusception^' by Nageli,
and/or (2) the deposition of successive plates or lamellae of wall
material, a process termed "apposition" by von Mohl. The
process of ai)position usually occurs in a centripetal direction
with reference to the first or original wall and may result in the
almost complete obliteration or occlusion of the cell lumen, as is
typical of certain fibers and sclereides (cf. Exercises VIII and
IX).

Much confusion exists in textbooks and in the specialized
research literature on cell walls with respect to the suhdivision of
the wall into major regions or layers. Distinctions between the
successive layers are based upon such criteria as (1) origin, (2)
appearance when viewed under polarized light, and (3) chemical
and physical structure. Very recently Kerr and Bailey (1934),
as a result of an extensive study of material with modern tech-
nique as well as a critical survey of the literature, proposed a
terminology which has been adopted by Anderson (1935). Ilay-
ward and others and which will be followed throughout this
book. According to Kerr and Bailey, three main categories of
wall layers exist :

1. The intercellular "layer" or substance which is composed
largely of prjlyuronides and is isotropic (i.e. dark or non-re-
fringent when viewed under polarized light). Further in-
formation on the precise origin of the intercellular layer is ur-
gently needed but provisionally it may be regarded as being
derived from the cell plate which is produced following mitosis.
In trachearA' elements, the intercellular layer may be more or less
lignified.

2. The primary wall which consists largely of cellulose and
polyuronides and is anisotropic (i.e., bright or refringent when
viewed under polarized light). Kerr and Bailey emphasize that
the term "primary wall" should be restricted to the original
cambial wall (in the cells of xylem and phloem) and its homo-

10 THE TELL WALL

logues in terminal meristems and other thin-walled tissue (e.g.
parenchyma ) . In contrast to the amorphous and ' ' structureless ' '
intercellular substance, the primary wall "is also characterized
by possessing plasmodesmata which may be uniformly distributed
or aggregated in more or less conspicuous primary pit fields."
Primary walls are capable of readjustments and reversible
changes in thickness during tissue development, lailess special
technique is adopted (e.g. polarized light) the intercellular sub-
stance and its adjoining primary walls usually appear as a single
non-lamellated partition, especially in tracheary elements. Un-
der such circumstances, the term "compound tniddlc lamella"
may be applied to this complex of lignified layers.

3. The sccoHdary wall, which is often extremely complex, both
chemicallv and phvsicallv, and which is normally the most mas-
sive of all the main layers of the cell Avail. Bailey and Kerr
(1935) use the term "secondary wall" to designate "the strongly
anisotropic layers of secondary thickening which are formed after
a cell has attained its final size and sliape." These investigators
strongly emphasize that in conti-ast to tiie primary or "cambial"
wall, the true secondary wall is incapable of undergoing rever-
sible changes in thickness. This is very often the case because of
the ultimate disintegration of the protoi)last of many cell types
which possess definite secondary walls, e.g. tracheids and fibers.
According to Bailey and Kerr, most tracheids, fiber-tracheids, and
libriform fibers in gymnosperms and angiosperms possess a three-
layered type of secondary wall. The inner and outer layers,
M'hich are of relatively constant thickness "exhibit strong double
refraction and are brilliant" when examined in transverse sec-
tion in j)olai-ized light l)etween crossed nicols. The middle layer,
on the conti-ary. "is dai-k or noticeably less bii-efi-ingent" and
fluctuates very widely in tiiickness. Not all secondary walls,
however, possess the above type of stratification, exceptions being-
furnished by cei-tain fibers and by the wall of the cotton hair.
Prom a physical-chemical standpoint, Bailey and Kerr conclude
1li;it the cen1f;il hiyei- of the secondary wall consists of an "ex-
tremely eomph^x and firmly cohei"ent matrix of cellulose" within
which "lignin" and a wide variety of other organic and inor-
ganic substances ina>- l)e deposited. The secondary wall of plant

PITS 11

cells is rarely coiiliiiuous (wcv ilie entire surface of tlie adjacent
})riinai'y wail. In certain Iraciieary elements of llie i)riniary
xyleni for example, the secondai-y wall is developed as discrete
rings, si)irals, bars or as a complex network oi- mesh, while in
other cell types, well-defiiied thin areas or pits occur.

Secure prepared slides of transverse sections of the stem of
basswood (Tilia) and geranium {Pelargonium) and examine the
parenchyma tissue of pith and cortex under high magni^cation.
Note carefully the thin, apparently unstratified "compound mid-
dle lamella" of each cell and the prominent intercellular air
spa-ces. Tlie very slightly thickened "walls" of these cells appear
to represent the original intercellular substance and the two
adjacent primary walls of the terminal meristem cells from which
they originated.

Examine under both low and high tnagnification the ex-
tremely thick-walled hast fibers of Tibia noting the very thin,
continuous compound middle lamella, the thick, obscurely strati-
fied secondary wall and the much reduced lumen of each cell.
Occasionally, in both transverse as Avell as longisections, very
small canal-like pits will be visible in the secondary w-all.

IV. Pits. — With few exceptions, the secondary wall of plant
cells is interrupted by small cavities or recesses which are termed
pits. These thin areas in the secondary wall vary widely in size,
structure and arrangement and, since they exhibit some con-
stancy depending upon the type of cell, they provide significant
criteria in comparative studies, especially of xylem cells. Pits
typically occur in pairs; i.e., a thin area in the secondary wall
of a given cell normally lies opposite a similar recess in the ad-
jacent cell. Hence the term "pit-pair" designates the usual
condition and is contrasted in meaning with "blind pit" which
is a pit "without a complement opposite to an intercellular
space" (cf. Glossary of Terms Used in Describing Woods, p. 5).
Each member of a simple pit-pair consists of (1) the pit cavity,
which is the actual space within the secondary wall, and (2) the
2nt aperture or opening into the cavity. The members of a pit-
pair are separated from one another by a common piit membrane
w^hich represents a discrete portion of the presumably modified
intercellular substance and the two primary walls. Compara-

12 THE CELL WALL

lively little detailed iiit'ormatiuii is available regarding the
ontogeny of pits. Possibly their usual paired character is asso-
ciated with the fact that at least in living cells, the pit membrane
is penetrated by plasmodesmata which thus may determine the
opposite position of the pits. At any event it is clear that pit-pairs
arise on the primary pit fields of meristematic cells. These pit
fields which are defined (cf. Glossary of Terms Used in Describ-
ing Woods, p. 4) as thinner areas of "the intercellular layer and
primary walls" are observable in cambial initials as well as in the
so-called i)rimordial meristem of the shoot apex of certain seed
plants (cf. Foster, 1938, 1939a, in "References" to Exercise III).
From the standpoint of function, pits are believed to facilitate
the process of diffusion between adjacent cells.

Pit-pairs may be conveniently classified under four major
types, viz. :

1. Simple pit-pairs, which are typical of cells which retain a
protoplast throughout their functional life, are particularly well
developed in parenchyma cells. In face view, the aperture ap-
pears as a circular, elliptical or even irregular area. In macer-
ated tissue,^ simple pit-pairs in this view appear as refractive red
points of light. A recognition of this optical characteristic will
help to distinguish simple i)its from jiarticles of i)rot<)plasm or
other substances lying free in the cell lumen. In sectional view,
the cavity of each member of the pit-pair is usually of equal
diameter throughout and there is no overarching rim or border
produced by the adjoining secondary wall.

Ohtaiii ii pi-eparation of macerated secondary xylem of the
.stem of the trumpet-creeper (Tccrnna radicdiis) and examine it
undei- l(/)r magnification noting the numerous wood parenchyma
and xylem-ray parenchyma cells. These cells are box-like in
form and occur singly or in groups depeiuliiig upon the extent
to which the xylem has been macerated. Study a coiniected
group of parenchyma cells under lii(/Jt niannification and investi-
gate the size, structure and position of the simple pit-i)airs as seen
in surface and sectional vieAvs. In nil studies of this kind, it is
e.ssentijil to sccni-e ci-itical illuuiination and to use the fine adjust-
ment of the mici-oscope constantly.

' Cf. Appoiulix, J)]). 140-141 for tlie t('(lmi(|ue of niacoiatiiiK i)iant tissue.

PITS 13

2, Bordered pit-pairs are typical of dead water-conducting
cells, notably tracheids and vessel elements. In contrast to the
previous type, the cavity of each member of the pit-pair is over-
arched by a rim-like development of the secondary wall which is
termed the border. As seen in face view, the pit aperture is cir-
cular or broadly elliptical. In median section view, the border
over-arching' each pit member is apparent and, an additional
structural peculiarity is observable, namely the to7-us. The latter
is a discoid, central, thicker portion of the pit membrane which
is slightly wider than the diameter of the pit aperture. The re-
mainder of the membrane is much thinner and sufficiently pliable
so that under certain conditions the torus may be pressed against
one or the other of the two pit apertures. To understand clearly
the structure of a bordered pit-pair, it must be visualized in both
face and sectional views. Reference to Eames and MacDaniels
(1925, pp. 27-30, Figs. 15, 16, 17 and 21) and to Jeffrey (1917,
pp. 5-6, Figs. 4 and 5) will prove helpful.

Obtain a preparation of macerated xylem of the stem of Pinus
and examine it under low magnification. Bordered pit-pairs are
large and obvious in the tracheids, which are elongated cells with
acute or blunt tips. Select a suitable tracheid and study the ap-
pearance of the bordered pits in face view under high magnifica-
tion. Observe that each pit appears as three concentric outlines.
The outermost circle demarcates the edge of the pit cavity, the
intermediate circle represents the edge of the torus and the some-
what refractive innermost circle is the pit aperture. In many
tracheids, the successive bordered pits are more or less clearly
set apart from each other by "eye-brow" or rim-like ridges,
termed crassulae. These are interpreted as ** thicker portions of
the intercellular layer and primary walls betw^een primary pit
fields" and in the past have been designated as "Bars of Sanio"
and "Rims of Sanio" (cf. Glossary of Terms Used in Describing
Woods, p. 4). The structure of bordered pit-pairs in sectional
view can be effectively studied in macerated material if the edge
of the pitted walls are turned towards the observer. In order to
study critically the pit membrane and the torus, it is neces-
sary to examine thin, properly-stained radial sections of pine
xvlem.

34 THE CELL WALL

3. Half-bordered pit-pairs represent "an intercellular pairing
of a simple and a bordered pit" (Glossary of Terms I'sed in
Describing Woods, p. 5) and occur when living parenchymatous
cells develop in contact with dead tracheary elements. According
to a frequent opinion expressed in texts, the pit-member on the
side of the living cell is simple while its mate on the side of the
tracheid or vessel is bordered. Frost (1929), however, in a study
of the nature of pitting between tracheary and parenchymatous
cells in angiosperm xylem. has found that this conception has no
general validity. He concludes that "fully bordered, half-
bordered and simple pits are characteristic features between
tracheary cells and vascular parenchyma" and that "the type of
pitting on the wall of the parenchyma cell is controlled largely
by the degree of specialization of the vessel or fiber which lies
next to it." Obviously the whole question of pitting in plant
cells demands further investigation, from both a comparative as
well as an ontogenetic point of view.

Secure prepared slides of conifer and dicotyledonous sec-
ondary xylem and investigate under high magnification the na-
ture of the pitting between wood parenchyma or wood ray cells
and the connected tracheary elements.

4. Vestigial pit-pairs are typical of thick-walled wood and
bast fibers. Tn these cells, the secondary wall is greatly thickened
and the pits are often so reduced in size and number as to appear
truly "vestigial" or functionless. The vestigial pit-pairs of
typical wood fibers are usually interpreted morphologically as
reduced and highly modified bordered pit-pairs. This conclusion
is based (1) on the belief that the wood fiber has developed phylo-
genetically from the tracheid, and (2) on the fact that a closely
graded series of intermediate conditions between typical "bord-
dered pit-i)airs" and "vestigial pit-pairs" can be seen in com-
paring the tracheids, fibcr-tracheids and fibers in the xylem of
the same plant. In typical vestigial pit-pairs of wood fibers, the
pit-cavities although circular are relatively small, the border is
greatly reduced in size or absent and a torus is frequently lacking.
The most distinctive feature of tliis type of pitting, however,
consists in the elongated slit-like apertures which instead of being
opposite fas is true of the circular apertures of the members of

SUGGESTED DRAWINGS AND NOTES 15

a pair of bordered pits) are crossed. Furthermore, each slit-
shaped aperture is connected with the pit cavity by a channel hav-
ing the form of a flattened funnel (cf. Eames and MacDaniels,
1925, p. 33, Figs. 21-22). The vestigial pit-pairs of bast fibers
are frequently less complex, and consist of small circular aper-
tures and very narrow tubular cavities. Such pits are often
regarded morphologically as specialized simple pits.

Ohtain a preparation of macerated secondary xylem of the
sycamore (Platanus) and examine it under low magnification,
noting the numerous long, acuminate wood fibers. Careful study
of these fibers under high magnification will reveal the character-
istic arrangement of tlie vestigial pits with their narrow apertures
and inconspicuous "halo-like" cavities. Occasional fibers may
be turned in such a way that the vestigial pits or pit-pairs may be
visible in sectional view. A study should also be made of pit
structure as revealed in prepared and stained longisections
through the xylem.

Make a comparative study of the vestigial pit-pairs of the bast
fibers of Tilia or Platanus with the use of macerated as well as
stained and sectioned material.

V. Suggested Drawings and Notes. —

1. Prepare drawings showing the arrangement and approxi-
mate number of plasmoclesmata in a small group of connected
endosperm cells of Diospyros. Briefl}- summarize the possible
importance of the presence of plasmodesmata with reference to
the trans-location of organic materials in plants.

2. Draw a group of pith or cortical parenchyma cells from
the stem section of Tilia or Pelargonium showing and labeling
the following: compound middle lamella, protoplast, intercellular
air spaces. In what kinds of plant tissue are intercellular air
spaces likely to be most prominent ? Explain, from a physio-
logical viewpoint (cf. Ilaberlandt, Ch. IX).

3. Draw a group of bast fibers of Tilia as seen in the trans-
section of the stem, showing and labeling the foHowing: com-
pound middle lamella, secondary wall, and lumen. What might
be the "cause" of the deeply-stained thickenings frequently vis-
ible at the common point of contact between several bast fibers?

16 THE CELL WALL

4. Draw several connected wood-parenchyma or wood-ray
cells from the macerated xylem of Tecoma radicans, showing
clearly the size, structure and arrangement of the simple pit-pairs
as seen in both face and sectional view. Label the following :
compotmd middle lamella, secondary wall, pit aperture, pit cav-
ity and pit membrane.

5. Draw, on a large scale, a single tracheid from macerated
pine xylem, showing the form and arrangement of all pits (bor-
dered and simple) as seen in face view. Label carefully. Pre-
pare drawings based on the study of the longisections of pine
wood showing sectioned vieivs of bordered pit-pairs as well as the
type of pitting between tracheids and wood rays. In these draw-
ings label the following: compounel middle lamella, secondary
wall, border of pit, aperture of pit, cavity of pit, pit membrane,
torus and crassidae.

6. Prepare drawings, based upon the study of macerated and
sectioned wood and bast fibers of Platanus and Tiliu, shoAving the
arrangement and structure of vestigial pit-pairs in both face and
sectional views.

REFERENCES

1. Anderson. D. B., The Structure of the Walls of tlio Higher
Plants. Bot. Rev. 1 :52-76. 1935.

2. Bailey, I. W., and Kerr, T., The Visil)lo Structure of the Sec-
ondary Wall and Its Significance in Physical and Chemical
Investigations of Tracheary Cells and Fibers. Jour. Arnold.
Arb. 16:273-800. 1935.

3. Bonner, J., The Chomistrv and IMn'siolotrv of the Pectins.
Bot. Rev. 2:475-497. 1936".

4. Crafts, A. S. A Tecimic for dciuonstrating plasmodesmata.
Stain Technology 4:127-129. 1931.

5. Eames and Mac Daniels, Ch. IT. i^p. 19-38.

6. Frost, F. II., Histology of the Wood of Angiosperms, I. The
nature of the jutting between trachearv and parenchvmatous
cIciiuMils. r.iill. T(invy P.ot. Club. 56:259-264. 19i29.

7. liaberhindl, Ch. I. pp. 43-50; Ch. XIII, pp. 635-638.

8. llavward, Ch. 1, pp. 5-11.

9. Jeffrey, Ch. I.

REFERENCES 17

10. Kerr, T., and Bailey, I. W., The Cambium and Its Derivative
Tissues, X. Structure, optical properties and chemical com-
position of the so-called middle lamella. Jour. Arnold Arb.
15:327-349. 1934.

11. Livingston, L. G.. The Nature and Distribution of Plasmodes-
mata in the Tobacco Plant. Amer. Jour. Bot. 22 :75-87. 1935.

12. Meeuse, A. D. J.. Plasmodesmata. Bot. Pvev. 7 :249-262. 1941.

13. Quisumbing', E.. Continuity of Protoplasm in Endosperm
Cells of Diospyros. Bot. Gaz. 80:439-449. 1925.

14. Sharp, L. W., Introduction to Cvtology. 3rd ed. New York.
1934.

15. van Wisselingh. C, Die Zellmembran. Handbuch d. Pflan-
zenanatomie. III. Berlin, 1924.

Exercise 111

MERISTEMS

I. Introduction. — A meristem may be defined as a specific re-
gion in the plant body where cells are engaged chiefly in division
and enlargement. Meristems thus represent emhrifonic areas and
can be conveniently classified according to position as apical
meristems and lateral meristems. The former type is illustrated
by the apex or "growing point" of the root and the shoot, the
latter by the vascular and cork eambia. In addition, many
authors recognize "intercalary meristems" which are said to
occur hetween areas of permanent or mature tissue, as for exam-
ple at the base of the leaf in certain monocotjdedons. A more
critical study of the so-called "intercalary meristems" is urgently
needed, however, and attention will be again directed to this
problem in the exercise dealing with the leaf (Exercise XIII).

The maintenance of meristems at certain restricted regions
of root and shoot is responsible for the distinctive "open system"
of growth characteristic of all higher plants. This consists in the
continuous formation of new tissues and new organs throughout
the life of the individual. The magnitude of this type of "un-
limited" ontogeny is most impressively shown in long-lived
woody perennials in which each season's growth is accomplished
by the formation of uew shoots, reproductive structures, and
roots, as well as by an increase in diameter of the older roots and
stems. When the open system of growth is further examined,
it becomes clear that there are varying degrees of "permanence"
in the various meristems of a plant. Thus the apical meristems
of the shoot and root in many ]ierennials appear capable of indefi-
nite life and activity. Actually, of course, various factors such
as malnutrition, insufficient water, injury, etc., may result in the
death of the shoot or root apex. Furtliermore, the phenomenon
of correlation, in this case involving the relative growth of main
stem or main root as compared with its laterals, becomes a com-

18

INTRODUCTION 19

plicatiiifr factor. Another example of a theoretically ' ' indefinite
or "})ei-nianent" nieristem is furnished by the vascular camhium,
which may continue to produce annual increments of phloem and
xylem for hundreds or, in the ^'enus Sequoia, thousands, of years.
Obviously, the maintenance of an indeterminate type of meristem,
such as a shoot apex or the cambium, requires that there shall be
a continuous new formation or " reii'eneration " of the meristem
as development takes place. In other words, a certain restricted
portion of such meristems remains indefinitely in the embryonic
state and does not pass into the zone of maturation. In contrast,
the meristems of determinate organs, such as leaves and fruits,
function for only a comparatively restricted period, and even-
tually all embryonic tissue passes into a state of maturity. Evi-
dently these differences in the functional life of meristems are
of fundamental morphological importance, but the nature of the
factors, genetical and physiological, which control them are very
poorly understood at present.

From an histological standpoint, a meristem is a "tissue"
composed of "undifferentiated" or meristematic cells. Accord-
ing to the classical viewpoint, which is still retained in many
textbooks, the tissue composing meristems possesses certain dis-
tinctive juvenile characteristics which seem to demarcate it from
the various types of functionally-mature "permanent" tissues.
Among the "negative" characters usually assigned to meri-
stematic tissue are: (1) the absence of intercellular spaces; (2)
the absence of thickened or pitted walls; and (3) the absence of
prominent ergastic materials in the cytoplasm. If, however,
undue emphasis is placed upon such morphological features, a
narrow and rather arbitrary concept of "meristematic tissue"
inevitably results. A good example of the restricted concept of
meristem is found in Priestley and Scott's recent (1938, p. 208)
"An Introduction to Botany." They state: "It is customary to
speak indifferently of any dividing tissues of the shoot apex as
meristematic, but in view of the fundamental character of the dif-
ferences in cell behavior, it is proposed in this book to restrict the
terms meristem and meristematic to the dense cells which are
devoid of obvious water vacuoles and have no intercellular spaces
between them, whilst the vacuolating cells will be spoken of as

20 ■ MERIRTEMS

exhibiting vacuolating cell growtli and division." Priestley's
effort to demarcate meristems on the basis of the absence or lack
of prominence of vacuoles is not supported by comparative
studies. For example, the apical cell and its most recent segments
in many lower vascular plants are highly vacuolate in character
and it is only at some distance from the summit of the apex that
small "dense" cells are found to occur (Eames and MacDaniels,
1925, Fig. 28; and Zirkle, 1932, taf III, Fig. 16). Likewise, in
the shoot apices of Ginkgo hiloha and Cycos rcvoluta, Zamia, and
Dioon cdule a more or less w'ell-defined central group of enlarg-
ing vacuolated cells, surrounded by smaller and more densely-
cytoplasmic cells is present (cf. Foster, 1938, 1939a, 1940, 1941b).
These examples clearly show that the relative position and extent
of "zones" characterized by the predominance of cell division or
cell enlargement are variable in the shoot apex of vascular plants
(Foster, 1941a; Boke, 1941). In short, cell division and con-
spicuous vacuolation are not processes confined in Priestley's
sense respectively to the summit and lower portion of a shoot
apex. On the contrary, these processes may overlap at the same
level in a growing apex. Doubtless the most significant evidence
of the vacuolated character of meristems has been secured by
recent studies on living tissue. The work of Bailey (1930) and
Zirkle (1932) on the vascular cambium and primary meristems
respectively has indicated that all meristems are vacuolated, and
furthermore, that the form of the "vacuome" varies within wide
limits at different seasons of the year and at dif!'ereiit stages of
growth ill the same type of meristem.

From the preceding brief critique it should be evident that
it is imj)ossible in the light of present knowledge to frame an
adequate "definition" of meristematic tissue. On the contrary,
it seems increasingly clear as investigation jiroceeds, that we have
to deal with various and i)ossibIy distinct types of "meristem,"
at least from a i)liysiological viewpoint. How the organization
and growth of meristems is related to the orderly progressive
differentiation of tissues from apical or latcM-iJ meristems consti-
tutes one of the most challenging prol)lems in Tiiodcrn botany.
Further insight will come when tlio results of comj^arative obser-
vation ai-c checked by experimental studies. It seems likelv that

APICAL MERISTEMS

21

the complex phenomena of regeneraiion or regressive differentia-
tion, when they are better understood, may be expected to shed
important light on the fundamental nature of meristems. (Cf.
Bloch, VMl; Sinnott and Bloch, 1941a, 1941b.)

In the present exercise, a preliminary study will be made of
the apical meristems of root and shoot, and of the vascular cam-
bium. Further experience with these meristems as well as with
the cork cambium will be gained particularly in the exercises
devoted to the anatomy of root, stem, and leaf.

II. Apical Meristems. —

1. The shoot a per. The classical investigations of C. F.
Wolff (1759) on bud development showed that new leaves and
new stem tissues are traceable in origin to the delicate tip of the
shoot. Wolff designated this region as the "punctum vegeta-
tionis," a term which has been rather freely translated as the
"growing point." Despite the widespread adoption by anatom-
ists of the expression "growing point," this term carries an inac-
curate implication and in the present book will be replaced by the
more appropriate and non-committal designation of "shoot
apex." This decision is based upon the fact that the chief signifi-
cance of the so-called growing point is that it represents the
region of initiation of the primary organization of the shoot,
rather than a localized area or "point" of "grow^th." As a
matter of fact, if ' ' growth ' ' is regarded as an increase in size of
cells, tissues, and organs, this process is obviously at a minimum
in the "growing point."

Great variation obtains with respect to the form and dimen-
sions of the shoot apex of seed plants. As seen in median longi-
seetion view% the apex commonly has the form of a mound or low
dome. In Elodea, Myriophyllum, and Hippuris, however, the
shape of the shoot apex is that of a slender, blunt-tipped cone
(cf. Louis, 1935, pp. 126-130 and PI. IX, Figs. 77-78). The apex
of dicotyledons with decussate phyllotaxis (e.g., Syringa, Loni-
cera, Ligustrum, etc.) is particularly suitable for developmental
studies because the initiation of each pair of foliar structures is
preceded by a notable and symmetrical expansion of the terminal
meristem. Since this process is repeated each time a pair of
leaves is produced, the apex exhibits a rhythmical alternation of

22 MERISTEa^IS

-what Schmidt (1924) called "minimal" and "maximal" areas
(cf. also Louis, 1935, PI. II, Figs. 20-21; and Cross, 1937, Figs.
10-11). This situation emphasizes the fact that the form and
the dimensions of the shoot apex are likely to vary depending
upon whether an active or dormant apex is measured as well as
upon the particular phase in shoot development which is under
examination. Extremely few careful measurements have been
made of the shoot apex of seed plants, and no generalizations are
possible at present. Apparently, however, the angiosperms typi-
cally possess rather small apices which range in diameter from
90^ in certain grasses to 500,a in some of the palms. Possibly
130-200/1 may prove to represent a frequent range in diameter
of the apex in dicotyledons (Boke, 1940). The width of the
shoot apex of some conifers, of Ginl-go hiloha, and of Zamia
slightly exceeds that of "typical" angiosperms. But in the Sago
Palm {Cycas revoluta Thunb.), the shoot apex may attain the
relatively enormous diameter of 3.5 millimeters, a dimension
gi'eatly exceeding that recorded for any vascular ])laiit (cf. Fos-
ter, 1940). The nature of the relationship between size and form
of the apex, on the one hand, and the morphology and primary
anatomy of the shoot, on the other, is obviously complex and
awaits further comparative studies for its solution (Bower, ]9:)0,
Ch. XII; Foster, 1939b, 1940, 1941a, 1941b).

When a thin median longi-section of the shoot apex of (di
angiosperni is examined under the microscope, two pi'incipal
zones or regions are usually distinguishable, viz.: (1) the tunica,
which consists of one or more discrete superficial layers of cells,
and (2) the corpus, which is a "core" occupying the center of
the apex and exhibiting an irregular or "i-andom" arrangement
of cells.' The differences in cell arrangement in tunica and corpus

^ In both of these zones, the cells are relatively siiiall and in sectional
view appear " isodianietric " in form. Jjittle is known, iiowever, al)out tlie
shape of such cells when regarded as three-dimensional structures. Ac-
cording to I'riestley and Scott (li)3S pp. 201-L'Oi2) macerated cells of the
apex "appear as rather irregular, many-sided figures, the facets of which
are mainly hexagonal or s(|iiare. " By compressing sidieres of plasticine,
these investigators ohtained 12-sided liodics wliidi thi'y assume are simi-
lar in form to nicristem cells. They conclude that "the shape of the cells
is tlius exjdained as the natural result of the growth and division of plas-
tic hodics miller mutual pressure." (For further information on the prob-
lem of form in isoilianu't ric plant cells cf. Kx. \'l, i)p. 57-.i8.

APICAL MERISTEMS

23

result from ditt'ereiiees in the direction of growth and plane of
cell division in these zones. In the tunica, surface growth accom-
panied by repeated antivlimd divisions predominates, resulting
in the maintenance at the summit of the apex of a more or less
regular and constant series of shell-lilve layers. On the sides or
flanks of the apex, however, the distinctness of the inner tunica
layers is somewhat lost, chiefly because of the periclinal and
oblique divisions which appear in them during the initiation of
foliar structures and lateral buds. In contrast to the tunica,
growth in volume is characteristic of the corpus, and the sequence
in the successive planes of cell division is variable and usually
very irregular. Tunica and corpus thus represent two interdepend-
ent zones in the shoot apex, and their extent and behavior may
be expected to fluctuate, depending upon the systematic position
of the plant in question as well as upon the phase of development
of the plant itself. Modern studies have shown that the number
of tunic layers varies from one in grasses and Scrophularia
nodosa to as many as five or six in Hippuris. Unfortunately, no
detailed survey along broad systematic lines has yet been at-
tempted, so that the phyletic significance, if any, of differences
in the number of tunic layers is quite obscure at present. The
classical "Histogen Theory" of Ilanstein (1868) attempted to
assign specific destinies or "prospective values" to the various
layers and to the central core of the shoot apex. In contrast, the
concept of tunica and corpus, which originated with Schmidt
(1924), is non-committal wdth respect to the nature of the tissues
produced by these two zones. Recent studies justify Schmidt's
cautious viewpoint. In certain angiosperms [e.g.. Viburnum
rufidulum. Cross (1937)], the corpus is exclusively concerned
with the production of the pith, while in other plants Carya
Bnckletji var. arkansana, Foster (1935) ; Morus alba, Cross
(1936), the provascular tissue and inner region of the cortex,
as well as the pith, originate from the corpus zone. In Hippuris
and Myriophyllum, the corpus gives rise to the central pith-less
stele of the axis (Louis, 1935, pp. 128-130, PI. IX, Figs. 77-78),
simulating in this respect the histogenesis characteristic of many
roots. The ' * prospective significance ' ' of the various layers of the
tunica also varies, particularly with respect to their role in the

24 MERISTEMS

initiation of leal: and bud priniordia (Foster, 1936). While the
outermost tunic layer very eonnnonly behaves as a "dermatogen"
and produces exclusively the epidermal system of leaf and stem,
the apices of Triticuin and AL'cna furnish interesting- exceptions.
In these grasses, the foliage leaf originates largely if not exclu-
sively from the single tunic layer which exhibits both perielinal
and anticlinal divisions at the early phases of foliar develop-
ment. Doubtless similar conditions will be discovered in other
angiosperms.

A proper study of the form and structure of the shoot apex
in seed plants and of the origin of primary stem tissues and
leaves is only possible if both longitudiiidl and iniusversc serial
sections are available. .Since the choice of bud material will
depend upon many factors, no detailed description of a specific
shoot apex will be made in this book. Instead, suggestions as
to the advantages and special features of several available types
of apices will be given. With the information presented in llie
earlier portions of this exercise and in the literature cited, the
student should have no difficulty in interpreting the general
organization of any angiospermous shoot apex. The mound- or
dome-shaped form of apex, with several tunic layers, is well illus-
trated in such genera as Carya, Morns, Rhododcndroru Acacia,
Syringa, Rosa, Sanibucus, and Hch'anthus. Aside from minor
variations, the origin of leaves and the differentiation of provas-
cular strands ("pro-cambium") and "i-ib meristem" are simi-
larly shown in all of these genera. "/>'//> iiKrishiii/' a coiu-ept
developed by Schiiepi) (1926), is a type of primary mei-istem
which in a hmgi-seetional view of a shoot apex appears as a tissue
composed of vertical filamentous gi-oujjs of vacuolating-dividiug
cells. This mci'istem typically differentiates into the paiHMU'hyiu;i
tissue of cortex and pith. The slender cone-shaped apices of
Elodea or Hippuris are instructive, providing median longi-sec-
tions are examined. Apices of these genei-a are jiarticnlarly use-
ful in demonstrating the mode of origin of the small leaf ]n-i-
mordia fi-om the tunica zone, as well as showing the early demar-
cation between cortex and the pith-less stele. Preparations of
the shoot apices of monocotyledons should also be studied. The
apex of Tradcscaiilia is of intei'cst since the (leinarcation betAveen

Al'ICAL MERISTEMS 25

tunica and corpus zones is not always clear. Furthermore, the
relation of rib meristem and provascular areas to the young-
nodes and internodes is clearly shown in this genus (cf. Kudiger,
1939, and Ball, 1941). For comparative purposes, a study should
also be made of the shoot apices of various gymnosperms. Here
the choice of material is often very limited and hence specific
recommendations may be of little value. But the apices of vigor-
ous growing shoots of Picca, Abies, or Cedrus are readily sec-
tioned and all agree in the absence of the tunica-corpus type of
zunation characteristic of angiosperms. Instead, a small group
of initials is situated at the summit, from which arise two major
tissue-areas or zones, viz.: (1) an outer peripheral zone, which
produces the leaves, epidermis, cortex, and provascular tissue,
and (2) an inner or central tissue zone which produces exclu-
sively the pith. The possible phylogenetic significance of this
type of apex is discussed in several recent papers (Foster, 1939b,
1941a; Cross, 1939, 1941). Apices of Ginl-cjo (Foster, 1938) and
of some type of cycad (Zamia, Johnson, 1939; Cycas revoluta,
Foster, 1939a, 1940; Bioon, Foster, 1941b) are also worthy of the
student's time, particularly because of the interesting phylo-
genetic as well as morphogenetic problems which are raised by
their unique growth and structure.

2. The Boot Apex. The apex of the root differs funda-
mentally from that of the shoot in the presence of a root cap.
The latter is a thimble-shaped or conical structure which occupies
the true physical apex of the root and which acts as a "buffer"
for the delicate meristematic tissue which is thus suhtcrminal
ill position. Great variation exists with respect to the histo-
genetic relationships between the root cap and the subtermiiial
meristem. Indeed, the differences are sufficiently evident to make
necessary the designation of a number of "types" of root apices
which are distinguished (1) by the mode of origin of the cap,
and (2) tlie relation of the various so-called "histogens" to the
origin of the primary tissue regions in the root proper. (Haber-
landt, 1914, pp. 89-94, and Hay ward. 1938. pp. 44-48.) It is
an interesting fact that while the highly deterministic scheme of
Hanstein (1868) has been largely abandoned for the shoot apex,
the structure and growth of the root apex is still generally inter-

26 MERISTEJVIS

preted in terms of the "histogeii theory" (Hayward, 1938, pp.
44-48; vou Guttenberg, 1940). While it is true tliat the absence
of foliar structures in roots makes it relatively easy to determine
the point of origin of a given tissue, it may well be questioned
whether Hanstein's concepts are any more justified for the root
than for the shoot. It seems evident, at any event, that a broad
systematic survey of the structure and behavior of the root apex
in angiosperms and gymnosperms would remove the problem from
the highly fortnalizcd i)o<sition which it now occupies.

Secure serial longitudinal and transverse sections of some of
the principal "types" of root apices, viz.: (1) the "grass type,"
characterized by the possession of a discrete meristem termed the
calyptrogeii which exclusively propagates the root cap, and by
the origin of "dermatogen" and "periblem" from a common
initial group; (2) the "Allium type" in which, according to
Ilayward (1988, p. 46) "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;" (3) the
" Helianthus" type which is believed to represent the most com-
mon type in dicotyledons, and in which "the plerome and
])eriblem are sharply defined; and outside the latter is a common
initial layer which produces the root cap and the epidermis"
(Ilayward, 1938, p. 46, and Ilaberlandt, pp. 89-90, Fig. 19) ;
and (4) the "Pisum" type in which a transverse initiation zone
is the common point of origin of root cap as well as the primary
tissues of the root. In this type, which is found in Cucurhita
and many Leguminosae, well-defined "histogens" are not recog-
nized (Ilayward, p. 47, Fig. 17). Regardless of "type," careful
inspection of median loiigi-sections under low and high magnifi-
cation will reveal the successive "zones" of cell origin, cell elon-
gation, and cell maturation. The "rih meristem" of the outer
peripheral region of the root tip is particularly useful in study-
ing these zones, because of the extremely regular arrangement
of the cell rows and the gradual changes in size, shape, wall thick-
ness, and degree of vacuolat ioii of Ilu> coihijoiumiI cells. Note in
contrast the relatively short "zone of transition" from the
"calyptrogen" (or its equivalent) to the outer senescent cells of
the root cap. The marked differences in the rate and duration of

THE VASCULAR CAMBIUM 27

cell division respectively in root cap and the body of the
root, present an important but entirely obscure morphogenetic
l)rttbleni.

III. The Vascular Cambium.— The term "vascular cambium"
is applied to vertical strips or narrow cylinders of enlargino-
and dividing cells which are lateral in position and which give
rise to the secondary phloem and secondary xylem tissue-systems.
The vascular cambium is properly regarded as a " secondary meri-
stem" since its activity is responsible for the addition, at some
distance from the apex of root or shoot, of new or secondary
vascular tissues to the original or "primary" conducting system
which in turn had its origin in the provascular meristem or
"procambium." In many herbaceous angiosperms, especially
many of the monocotyledons, and in most of the lower vascular
plants, cambial activity is reduced or absent and the vascular
system is therefore largely "primary" in character. But in
woody angiosperms and in the gymnosperms, the primary tissues
of stem and root are short-lived and become destroyed or buried
by the more massive secondary vascular system formed by the
cambium.

The most significant of modern studies on the structure and
growth of the cambium have been made by Bailey (1920, 1923,
1930), who has studied both fixed as well as living cells in a
wide range of gymnosperms and angiosperms. From a morpho-
logical standpoint, the cambium may be regarded as a single layer
of cells in which tangential (i.e., periclinal) divisions predomi-
nate during the propagation of phloem or xylem. Two prin-
cipal types of initials occur in the cambium, viz.: (1) the
fusiform initial, which as seen in tangential longi-sectional view
is prosenchymatous in form and in certain plants, according to
Bailey, may attain the enormous length of 5,000//, and (2) the
vascidar-ray initial, which is a much smaller cell and is more or
less isodiametric in form. The fusiform initials form such ele-
ments as tracheids, vessels, fibers, wood-parenchyma, and sieve-
tubes, while the ray initials are points of origin and propagation
of the radially-disposed phloem and xylem rays (cf. Barghoorn,
1940). One of the many interesting features of cambial cells is
their highly vacuolate character, which is only evident when liv-

28 MERISTEMS

ing- tissue is critically studied ^\ itli the aid of such vital stains as
"Neutral Red." Bailey (1930, p. 677) states: " Xormal ram-
bial initials are conspicuously vacuohited. Indeed certain of Ihcni
are as highly vacuolated as plant hairs, which are commonly cited
as illustrations of extreme specialization of the protoplast in fully
differentiated cells. The classical conception of non-vaeuolated
meristems. and the various physiological generalizations that have
been deduced therefrom should be abandoned." Just how the
form, Avail sti-ucture, vacuome, and peculiar methods of cytokine-
sis in cambial initials are related to the derivation in opposite
directions of such heterogeneous tissue syste7vs as i)hloem and
xylem is not yet clear. Tt would seem evident, however, that here,
as with comparable problems at the root ami shoot apex, experi-
mental studies (e.g., tissue cultures and transplantation) may
ultimately illuminate much of the o])scurity of this important
problem.

The most instructive and realistic views of the vascular cam-
bium are secured from a study of living material whit-li may be
stained with neutral red (cf. Appendix, j). 142). With the aid
of a sharp, heavy knife and a sliding microtome, it is possible to
obtain useful tangential, radial, and transverse sections of the
cambium and its recent phloem and xylem dei-ivatives. The cam-
bium of Piniis is a good gymnospermous type with gi'eatly elon-
gated non-strati fie el fusiform initials while Rohiuia illustrates
a dicotyledonous type with shorter, rather evidently sfr<itifie<]
fusiform initials.

IV. Suggested Drawings and Notes. —

1. l'rei)are outline drawings of various availal)le types of
angiospei-mous and gymnospermous shoot apices. In each out-
line, indicate diagraiiunatically by means of legends, the jiosition
and extent of each of the following when it occnrs : tunica, cori)Us,
provascnlar strand, rib meristem, i)ith, cortex, leaf primordium,
axillary bud primordium, i)eripheral tissue zone (in apex of
gymnosperms), central tissue zone (in apex of gymnosperms),
initial zone.

2. Prepare an outline di'awing of an angiospermous shoot
apex, filling in all the cells of the luuica and corpus zones and

REFERENCES 29

of the youngest leaf priinorclia. Also show the form and arrange-
ment of the cells (1) in a small portion of a provaseular strand,
and (2) at several successively older regions of the rib meristem.
Label carefully all important structures.

3. Prepare an outline drawing of one of the available types
of angiospermous root apices, showing diagrammatically and la-
belling the following : root cap, subtermina! meristem, region of
elongation, region of maturation, epidermis, cortex, stele.

4. Select one of the types of root apices and fill in the cellular
details of the root cap, the calyptrogen (if present) and the so-
called 'Miistogens" of the root proper, i.e.,: "dermatogen,"
"periblem," "plerome." Label all parts of the drawing.

5. Prepare drawings of the cambium of Pinus and Rohinia
showing its appearance as seen in the transverse, radial and tan-
gential planes of section. In the drawings of transverse and
radial sections, a small portion (i.e., several cell layers) of the
adjacent phloem and xylem tissue should be shown.

REFERENCES

1. Bailey, T. W., The Cambium and Its Derivative Tissues, III.

A reconnaissance of cvtological phenomena in the cambium.

Amer. Jour. Bot. 7 :4i7-434. 1920.
2. , , The Cambium and Its Derivative Tissues,

IV. The increase in girth of the cambium. Amer. Jour. Bot.
10:499-509. 1923.

3. ^ , The Cambium and Its Derivative Tissues,

V. A reconnaissance of tlie vacuome in living cells. Zeitsch.
fiir Zellforch. u. mik. Anatomic. 10:651-682. 1930.

4. Ball, E., Micrntechni(|ne for the KShoot Apex. Amer. Jour.
Bot. 28 :233-243. 1941.

5. Barghoorn, E. J., Jr., Origin and Development of the 1 'ui-
seriate Pxav in the Coniferae. Bull. Torr. Bot. Club 67:303-
328. 1940.

6. Bloch, R., Wound Healing in Higher Plants. Bot. Rev.
7:110-146. 1941.

7. Boke, N. II., Histogenesis and Morphology of the Phyllode in
Certain Species of Acacia. Amer. Jour. Bot. 27 :73-90. 1940.

8. , , Zonation in the shoot apices of TricJwcercus

spacliianus and Opunila cyUndrka. Amer. Jour. Bot. 28:
656-664. 1941.

9. Bower, F. 0., Size and Form in Plants. London, 1930.

30 MERISTKMS

10. Cross, G. L., The ]\Iorphology of the Bud and the Develop-
ment of the Leaves of Viburnum rufidulum. Amer. Jour.
Bot. 24 :266-276. 1937.

11. , , The Structure of the Growing Point and

the Development of the Bud Scales of Mar us alba. Bull.
Torr. Bot. Club 63 :151-465. 1936.

12. , , The Structure and development of the

Apical Meristem in the Shoots of Taxodium disiichnm. Bull.
Torr Bot. Club 66:431-452. 1939.

13. , , Some Ilistogenetic Features of the Shoot of

Crypiomeria japonica. Amer. Jour. Bot. 28:573-582. 1941.

14. Eames and MacDaniels, Ch. III.

15. Foster, A. S., A Ilistogenetic Study of Foliar Determination
in Ciin/d Bucklciji var. (irkansana. Amer. Jour. Bot. 22:88-
147. 3935.

16. , , Leaf Differentiation in Angiosperms. Bot.

Kev. 2 :349-372. 1936.

17. , , Structure and Growth of the Shoot Apex

in Ghtkgo bilobu. Bull. Torr. Bot. Club 65:531-556. 1938.

18. , , Structure and Growth of the Shoot Apex

of Cxjcas revoluta. Amer. Jour. Bot. 26 :372-385. 1939a.

19. , , Problems of Structure, Growth and Evo-
lution in the Shoot Apex of Seed Plants. Bot. Bev. 5 :454-
470. 1939b.

20. , , Further Studies on Zonal Structure and

Growth of the Shoot Apex of Cycas rcvolufa. Amer. Jour.
Bot. 27 :487-501. 1940.

21. , , Comparative Studies on the Structure of

the Shoot Apex in Seed Plants. Bull. Torr. Bot. Club. 68:
339-350. 1941a.

22. , , Zonal Structure of the Sliool Ai)ex of

Dioon cdulc. Amer. Jour. Bot. 28 :557-564. .1941b.

23. JIaberlandt, Ch. II.

24. Ilanstein, J., Die Scheitelzellgruppe im Vegetationspuiikt der
Phanerogamen. Festschr. Niederrhein. Ges. Natur. Ileli-
kunde: 109-143. 1868.

25. JIayward, pp. 12-15; 44-47; 63-67; 77-85.

26. Johnson, i\I. A., Structure of the Shoot Apex of Zainia. Bot.
Gaz. 101:189-203. 1939.

27. Louis, J., L'ontogencse du systeme conducteur dans la pousse
fcuillcc dcs Dicto.vh'cs et des Gymii()s])(MMnes. La Cellule 44:
87-172. 1935.

28. Priestley, J. II., and Scott, L. I., An Iiilroductiou to Botany.
London, 1938.

REFERENCES 51

29. Riidiyer, W., Die Sprossvegetatioiispuiikte eiiiiger Mouokoty-
len. Heitr. IHol. rtl. 26:401-443. 1939.

30. Schmidt, A., lli.stologische Studien am phancrogamen Vegeta-
tionspnnkten. P.ot. Areliiv. 8 ::Ur)-404. 1924.

31. Sclmepp, O., Meristeme llandbuch d. Pflanzenanatcmiie. IV,
1926.

32. Sinnott, E. W.. and Bloeh, E., Division in Vacnolate Plant
Cells. Amer. Jour. Bot. 28:225-232. 1941a.

33. , , , The Relative Position of Cell

Walls in developing Plant Tissues. Amer. Jour. Bot. 28 :607-
617. 1941b.

34. Von Guttenberg, H., Der primare Ban der Angiospermen-
wurzel. Ilandb. d. Pflanzenanatomie. VIII. Berlin. 1940.

35. , , Der primare Ban der Gvmnospermen-

wnrzel. Ihicl. Berlin, 1941.

36. Wolff, C. F. Theoria generationis. Halae, 1759.

37. Zirkle, C. Vacuoles in jirimary meristems. Zeitsch. fiir
Zellforsch. u. mik. Anatomic. 16:26-47. 1932.

Exercise IV

PROBLEMS IN THE CLASSIFICATION OF CELL TYPES,
TISSUES, AND TISSUE SYSTEMS IN VASCULAR

PLANTS

I. Introduction. — Despite the vast amount of information
which has aeeiimiilated during the present century regarding- the
cellular structure of higher plants, the nomenclature and classi-
fication of cell types and "tissues" is still in a confused and
uncertain state. Doubtless much of the difficulty arises from
the absence in plants of the high degree of structural and physio-
logical individuality which characterizes the tissues of higher
animals. For example, certain so-called ' ' permanent " or " adult ' '
tissues, such as parenchyma, collenehyma, and epidermis, may
"secondarily" revert to a meristematic state and produce tissues
or structures quite different from themselves. Furthermore,
it is difficult or even impossible to draw a clear morphological
demarcation between adjacent "])ermanent" tissues in many
instances. A good example is furnished by the gradual inter-
gradation between collenehyma' and parenchyma tissue in the
cortex of many stems. As a consequence of the.se and other
difficulties, the term "tissue" in Plant Histology has been used,
sometimes in a broad sense, sometimes in a restricted sense,
(lei)ending upon the relative importance attributed to position,
origin, structure, or function. Sachs (1875, p. 68) adopted in
the first place a broad concept by stating that "in the widest
sense every aggregate of cells which obeys a common law of
growth (usually, however, not luiiform in its action) may be
termed a tissue." A similar idea is found in Strasburger's Text-
book (1921, ]). 41), where a tissue is defined as a "continuous
aggregation of cells in intimate union." Other definitions of
"tissue" are less general in character and introduce, in various
ways, the idea's of origin, specific cell structure, and function.
For example, Eames and IMacDaniels (1925, p. 50) define a tissue

32

SYSTEMS OF TISSUE CLASSIFICATION 33

as "a continuous organized mass of cells, usually similar in oriyin,
and essentially alike in form and general function." Hay ward
(1938, p. 11) adopts a similar attitude by contending that
"strictly defined, a tissue is a group of cells of common origin
having essentially the same structure and performing the same
functions. " In these definitions, community of origin, continuity
and similarity in structure and function are essential attributes
of the cells of any "tissue." Ilaberlandt (1914, pp. 56-72), in
contrast, has approached the problem from quite a' different point
of view. He assumes that the differentiation of specific cell-
aggregates or "tissues" in plants is "mainly the outcome of divi-
sion of labor, and that consequently the most characteristic
features of each tissue are those which are most intimately con-
nected with its physiological activity." Haberlandt's concept is
thus essentially physiological or functional, and the ontogenetic
and phylogenetic aspects of tissues are more or less completely
disregarded.

It is necessary to emphasize that the wide differences in the
concepts of "tissues" outlined above have much more than an
historical significance. On the contrary, these differences under-
lie the varied schemes of tissue classification found in modern
botanical texts and continue to influence the study of comparative
histology and anatomy. For these reasons, the following critical
resume of the most important early as well as recent schemes of
tissue classification is offered as a guide for the student in the
interpretation and evaluation of modern histological literature.
In addition, it is hoped that this critique may serve to emphasize
the need for a complete re-examination of the fundamental
assumptions upon which these attempted classifications inevitably
rest.

II. Systems of Tissue Classification. —

1. Sack's classificaiion. Sachs assumed that, in the phylo-
genetic development of the higher plants from simple multi-
cellular forms, a distinction gradually arose between the outer
layers of cells or "tissue" and the internal mass. The latter
finally differentiated strands of cells surrounded by "funda-
mental" tissue. The final result is seen in the "primary" struc-
ture of the leaf, stem, and root, which consist, according to Sachs

34 CLASSIFICATION OF CELL TYPES, TISSUES, AND TISSUE SYSTEMS

(1875, pp. 77-78), of three principal Sij.sicms of Tissues, viz.:
(1) the Epidermal ISystem, under Avliicli are grouped the epi-
dermal and cork laj'ers; (2) the Fascicular System, Avhich con-
sists of the variously-arranged xylem and phloem; and (3) the
Funclamental Tissue System, which Sachs defined as "those
masses of tissue of a plant or of an organ which still remain after
the formation and development of the epidermal tissue and the
fibro-vascular bundles." Sachs (op. cit., p. 103) emphasized that
his classification was not concerned basically with the forms of
cells "but with the contrast of different systems of tissue, each
of which may contain the most various cell-forms." It is lastly
of interest to note that Sachs did not attempt to demarcate
rigidly "permanent" and "meristenuitic" tissue but on the
contrary emphasized the ability, especially of the epidermal and
fundamental systems, to regress to the state of a " formative ' ' or
dividing tissue.

Sachs' classification of the adult vegetative tissues of higher
plants into three main groups is appealing in its apparent sim-
plicity and practicability. That it does indeed possess consider-
able pedagogical value is shown by its adoption (with or without
minor changes) in such modern texts as Sinnott (1935) and
Molisch (1[)36). From a more technical standpoint, Sachs' classi-
fication is also used by Jeffrey (1917, pp. 8-13), who emphasizes,
however, that the structural boundaries between the three tissue
systems are more evident in lower than in higher vascular plants.
Furthermore, the limits of the systems appear less sharply defined
in tlic siciii than in the "more conservative" leaf and root. In
organs whei-e secondary growtli is jU'ominent, the boundaries
betAveen the vascular and fundamental systems may disappear
and "in such cases the limits of the tissues can only be inferred
from comparative and developmental anatomy." The chief
objection which has been repeatedly raised against Sachs' classi-
fication is concerned with the indefinite physiological as well as
.structural characterstics of the "Fundamental Tissue System."
In some organs, this system may consist largely of parenchyma,
but many other cell tyi>es or tissues may be present. The funda-
mental tissue system, as Ilabei-landt (1914. p. 712) states, in-
cludes "green photosynthetic parenchyma, colorless water-tissue,

SYSTEMS OP TISSUE CLASSIFICATION 35

storage-parenchyma, meehaiiieal strands and cell-masses, endo-
dermal layers, and the mnltifarions tissues which make up peri-
carps and seed coats. No one, therefore, will venture to maintain
that "ground-tissue" constitutes a "whole of definite physio-
logical character."

2. Haherlandt's classification. Probably no scheme for classi-
fying plant tissues has been carried out so consistently from a
single point of view or in such detail as Ilaberlandt's "Ana-
tomico-Physiological Classification." According to Ilaberlandt's
viewpoint, the "principal function" should be the sole guide in
the designation of any specific tissue "system." The "principal
function" of a tissue is defined as "that form of physiological
activity with which its most obvious and important anatomical
features are correlated." The application of this idea resulted
in the distinction by Haberlandt (1914, pp. 71-72) of twelve
"anatomico-physiological tissue systems," each of which is typi-
fied by one major or "principal" function: e.g., absorption, con-
duction, protection, support, etc. With reference to the merits
of his scheme of classification, he contends that "the anatomico-
physiological definition and arrangement of tissues provides the
broadest and most natural of all systems of tissue classification,
since from this point of view the plant-body is regarded not
merely as a more or less complex aggregate of formal elements,
but also as a living organism, composed of a number of functional
units and engaged in a corresponding number of physiological
activities, whch all contribute to the safety and welfare of the
whole."

Haherlandt's high estimate of the value of his method for
classifying tissues has been amply justified by its wide adoption
in elementary as w^ell as more advanced treatises on plant his-
tology. Tsehirch (1889) and Palladin (1914), for example,
follow Ilaberlandt's system with little modification and Molisch
(1936) champions its merits for the advanced student. In this
country, the anatomico-physiological classification has likewise
proved popular and is utilized, in a somewhat simplified form,
in such a recent compendium as Hayward (1938). But one of
the most significant illustrations of the prestige and influence of
Haherlandt's ideas and classification is furnished by the ambi-

36 CLASSIFICATION OF CELL TYPES, TISSUES, AND TISSUE SYSTEMS

tious "llaiiclbucb der PHaiizenaiiatomie" which treats of the
varied phases of anatomy in monographic fornL Linsbaiier, as
the original editor, states in the first volume of this encyclopaedic
Avork, that, aside from certain disagreement in details, the prin-
ciples of Haberlandt's physiological anatomy will be adopted.
In this same volume, an able and penetrating discussion of the
various concepts and classifications of tissues is given by Lunde-
gardh (1922). This author, while agreeing in principle witli
the anatomieo-physiological method of classification, emphasizes
the need for a cantions and critical api)roach to the problem,
since "the physiological-anatomical systems only indicate the
uoi-mal combination of structure and function and obviously do
not permit of any teleological conclusions as to the method of
their origin." Lnndegardh {op. cif., p. 175) i)roposes the fol-
lowing anatomieo-physiological conspectus of tissue systems, viz. :

1. The Coherent Tissue Systems (comjiosed of c(Hi1i uncus
cell aggregates).

A. The Formative or IMeristematic Tissues.
P.. The Mature Tissues.

1. Systems with dynamic j'um'tions, e.g., assimilation,
respiration, storage, absorption, etc.

2. Systems with static functions, e.g., protection, nie-
ciianical support, etc.

II. The Disperse Systems (composed of isoJuial cells or cell-
groups distributed as "islands'" in the midst of various
' ' coherent systems ") .

A. Stomata (i.e., guard and accessory cells).
15. Organs of Perce])tion.
('. Peproductive Apparati.

1). Idioblasts (e.g., isolated stone cells found in lli(> nieso-
pliyll of certain foliage leaves such as Camellia).

Two |)i-in('ipal objections have been advanced against Ilaber-
landl 's sclicnie of classification and the fundamental assumptions
upon which any anatomieo-physiological system is based. First
of all, Haberlandt's system is constructed with respect to the
nature of the "principal function" of each tissue system. IIow-

SYSTEMS OF TISSUE CLASSIFICATION 37

ever, many tissues or cell types carry on more than a single func-
tion. In such instances, a distinction between "principal" and
"subsidiary" function appears somewhat arbitrary. In other
words, certain types of cells might with equal justification be
classed in more than one of the anatomico-physiological "sys-
tems." Furthermore, as Luiidegardh admits, the principal func-
tion of a tissue (e.g., storage of reserve starch) can only rarely
be deduced from observation only. Hence, any anatomico-physio-
logical classification has a provisional character and is destined to
be changed or modified in the light of new experimental data.

In the second place, the objection is raised that in such a
scheme as Ilaberlandt proposes, confusion results because of the
disregard of the origin of cells and tissues. For example, in
Ilaberlandt 's classification, epidermal and cork cells, although
differing fundamentally in origin, are grouped for physio-topo-
graphical reasons under the "Dermal" or protective system.
Conversely, guard cells and root hairs, while having a common
origin from the embryonic surface cell-layer or "protoderm,"
are classified because of functional differences in the "Ventilat-
ing ' ' and ' ' Absorbing ' ' systems respectively. In short, as Ilaber-
landt (op. cit., p. 70) emphasizes, to the physiological anatomist
"the homologies of tissues are of no interest . . ." in defining
and classifying the various tissues of the plant body; "... his
concern is solely with analogy. " Whether such a viewpoint leads
to a "natural" insight into the evolutionary development of plant
tissues is open to serious question. Jeffrey (1917, p. 8) says in
this regard that "from the point of view of the doctrine of
descent, functional features are of less significance, since it is
precisely these which are the most readily modified and as a con-
sequence furnish the least valuable indications of the course of
evolutionary development in any given large group."

3. Fames' and MacDaniels' classification. In contrast to the
schemes of Sachs and Ilaberlandt, Fames and MacDaniels base
their classification of tissues on method of development. From
this standpoint, tissues "whicli are developed directly or indi-
rectly at the growing points by cell division in several or many
planes" are termed })rimary tissues. On the other hand, tissues
which "are formed largely by cell division in a single plane.

38 CLASSIFICATION OF CELL TYPES, TISSUES, AND TISSUE SYSTEMS

individual cells consecutively forming many neAv cells, -which
because of this method of formation lie in definite rows," are
designated as secondary tissues. They originate from canibia
of various types (e.g., the vascular cambium and the cork cam-
bium). Tliis ontogenetic viewpoint is based on the idea that
since "parench^'ma" is phjdogenetically the primitive tissue,
meristem, which is likewise "unspecialized" and "parenchyma-
like," constitutes the natural foundation upon which to base a
classification of adult specialized tissues. This ontogenetic scheme
of classification is very useful in emphasizing the difference
between the "primary" and "secondary" growth and structure
of the stem and root in gymnosperms and many dicotyledons.
But, from the point of view of cell structure, there is often little
or no morphological difference between certain cell types com-
mon to both primary as well as secondary tissues. Thus, for
example, fibers which differ little in form or structure occur in
the cortex and pericycle ("primary tissue" regions) and in the
secondary phloem.

Eames and MacDaniels further attempt to subdivide "perma-
nent" tissues into two main groups, viz.: (1) simple tissius,
such as parenchyma and collenchyma, which consist of a single
cell type and are thus structurally homogeneous; and (2) com-
plex tissues, such as xylem and i)hloem. which consist of several
distinct types of cells and hence are structurally heterogeneous.
Such a distinction appears to have a very restricted practical
value, although it may be theoretically justifiable on phylogenetic
grounds. First of all, very few of the cell types present in higher
vascular plants occur as "simple tissues." Parenchyma, it is
true, is often "homogeneous," but not infrecpiently idiohlasts,
in the form of branciied sclereides, are scattered among this "tis-
sue." From Lundegardh's standpoint, these idiobhtsts would
collectively compose a separate "diffuse tissue system." Further-
more, the elements of a "simple tissue" (e.g., fibers or par-
enchyma cells) may likewise be present as components of a "com-
j)lex tissue" (e.g., "phloem jiarenciiyma," "phloem fibers,"
"xylem parenchyma," "xylem fibers").

The preceding critical resume has attempted to point out
briefly the advantages as well as the apparent defects of certain

SYSTEMS OF TISSUE CLASSII'H ATION 39

outstanding schemes of tissue classification. Future progress
may be expected when our insight into the developmental and
functional potentialities of the various types of cells has been
increased. It seems clear that the fields of pathological and ex-
perimental plant anatomy are destined to contribute largely to
a more natural grouping of cells and cell aggregates. Weber
(1929), who has characterized all previous anatomy as "cell wall
anatomy," contends that a firm basis for the distinction of cell
types and tissues must depend upon a better knowledge of proto-
plasm, with less attention to the morphology of dead and fixed cell
walls. In his view, this requires the careful observational and
experimental investigation of living protoplasts, which may be
"physiologically" distinct although "morphologically" identi-
cal. This new approach, which Weber terms "Protoplasmic
Plant Anatomy, ' ' is still in its infancy, but undoubtedly a better
knowledge of structure will appear as our knowledge of the
behavior and potentialities of living cells and cell groups increases
[cf. the reviews by Bloch (1941) and White (1941)].

Since all methods for classifying plant tissues are open to
objection, the writer has adopted a non-committal and "practi-
cal ' ' attitude in this book. Instead of following any one scheme of
classification, the emphasis is placed first of all upon the salient
morphological features of the principal types of plant cells.
These cell types recur in various regions, "tissues" and organs
of the higher plants, and a thorough knowledge of their form,
structure, development, and presumable function (s) must con-
stitute the necessary analytical approach to anatomy. If such
knowledge is gained through practical laboratory studies, the
student should be in a position to study with some degree of
independence the comparative anatomy of such organs as the
stem, root, and leaf.

In the appended table an effort is made to summarize the
important features of the main types of plant cells. Reproduc-
tive cells (spores and gametes) as well as specialized secretory
or sensitory cells have been deliberately omitted. No pretense of
"completeness" is therefore made, but it is hoped that the table
may serve its purpose as a basis for the analytical study of plant
structure.

40 CLASSIFICATIOX OF (ELL TYPES. TISSUES, AND TISSUE SYSTEMS

CO

<

Ah

Q
H
H

02

Ph

a

o

<

•Si s ^

i - S -r

-£ i t i ii-

M+j M -J;

X " a- ^ c3

2

"-111

5^

^^ .S c

a^ i; :^ V

s

■|_ .S " i

— ^ "S

'w ^ —

^ :^ S ^

c *> ^- a a

— =^ 3 t- fc.

2 c I; c =

CL, o ^ n -_

Ah-^ bi S -S

^

r.

*7~*

Oj - O O '^

"k c JJ •

<_. o

— _^ ^ -4-

• ^ +-*

■^

*^ ^ ^ '"' ^

-2 i - "i 1

S C3

.;;^

c ^ ^

.2'*-'^ o 2;

.2 S

s

'""' C » ?*

Niv'

I' o 2 S S

■£•5 o^-^

.S • C3 o

~

.2--

^

i

i 3 r br G

•s *■"

£

^"iJ ^ ~ -M

£9

^

IK -- ,^ •" -M i:

_r m C 3
O 5 "" K "5

"S s

t«

+|2 = i^i

^^■^+iii.

+|S

11= pi

2 g S o i s

=*j

O 1^ - O

X ^ >j — 'i;

^

a.

-tf ^'^ ? L. 2

"; +r C3 C3 5c "S

5:

^ C p. tit p

o

=*^ -t^ cs._2

^ O -^ K .-

a.

•^ O _ - > ^-'

— o 2 O K ^

~ ;:: -= ~ ?^ i' -

CO O

^

<^=ss>.

^ x-Ei^^^z-

;^'H.

i2 6

1^ —
c 3

C i rt 54

u a £ "t;

s 0-3 ?

11:1

^

OJ Z

-L o O -

c -*

.§"•

<^ 2 - ,■

CO ij p _ _x

.1^.2 :: ?

r^.

-*-• -^ - *"**

^^

!B

" ^ .,

r* r— • "-^ £^

_ -So

— -— X

-= & ^- s

« "o

0 « J3 a

r C X r-

.- -5 £ .2

P 2 >.

£

<;>

3

a.

11

C8 be

t^'

.2 -

t3 ^

^ S

,*o

1- 0^

"^ a

u^

Cel

00 2

3 "E.
0^

SUMMARY OF MAIN CELL TYPES IN SEED PLANTS

41

^

protec-
tion of

; aeia-
ans of
rage of

? "^ S S O ex
- ^ ? .= ^ £

ij z ID o
-r "^ -2 S ^

•;= « 2 ^. 1

<^ -4— +- -♦— X

? c .^^ '+_

111! Ill

Ct tM ^ O

^ S rt
>^ 1^. M s

o; -^ "S 1^

— -a 7^ 0^ o

g 5 ^ -^ S

= ^ o =

'ro > c: n 'x

^ ^ " S ^ o o ,^

^

rt — TT « i»

^* '"'■*"' O ?^

C

o o .:: _2 .t;

£■ = = 5.15 -•

.i; .s S ;: "I' •-'*-' °

£r- a? 5|I3 O^

^^ a« ° p- .
j.S.5^ =^"-.2

it p t> C
'^ c 5 2'5

■K^

-:ti-| =

>"
C

^

rv
IB

^►/^■X: ? - o

^ c s

s cs c t: i:-5

.n -*-■ "^ o i

C3 ™ — ^'^

'~^ X S

■^ & ^ '=^ o P-

1 5^

- a? ==M

5:

o M »3

g o ^ ■" o

o

o bt

•: «=-"'r 03

t^

« '-' o ,„•
" — . o

0/ :: ^ ■ oj ■^

• rl ^ — ,^^ ,^j O —

2 ^ = ! ^

MI ■+- o -

1> X; ^ ^ & a c:

lii' ^ ^ 5 'I

•^ 'w (» 03 re

S ;.

„

O —

Gj ^

-^- SC

i. 0;

03 :^

03 "t:;

■rH

.rH 03

^

^ ;-,

0*

•"^

S "i

S "^

3

-■ fl

^

s

Cw

J-.

8

■13 w

T5

'-V

>H

— to

Nw

V

S «

S 4-4

f^

r^ c:

o

■^

"^ QJ

-*j

O P

O '^

p

- "^ F^

?H

o §'i

o^S

^

^

ri

S

:5^

^^

p

S

E-i'

s

J5

■fl

j^

^s

"u

u

«i

«

a

a

o

■«

«

«

•l-t

^

»— 1

A

CS

"o

H

^

o

42

CLASSIFICATION OF CELL TYPFS. TISSUES, AND TISSUE SYSTEMS

2

s

ft*

i ^ ^ "n

-. »- 5^ -^

J -^ ^ 1 .

CO 'm '- o
O X c! .-H

3 t< "^ ai ii

-o =- - S ^

2 § £.2 2

£/ aa
*-. o

:; •*-*

•5 ^
=t^ ®

o "»

a a
.2«

^ S 5

5
v.

"o ^ CO

o = « *^
K 5 2 ^

j^ O ^ -"^

= 5^2
= " 5 P ^
, . - ^ -^

'T, a^ .. 2
•C -^ ■- .£ r

rr ^ > O +j
O »^. •-' ^*-i fi

"1111

"^ a ■' "" ~ = >> n5
Tl-i-^-.^ — — — ^i:

5 ?-« -5 . sr^

5 ^.s o S " «

l^U'llill

1 i 'i^

^ « 1 ■;^ 5

? "B ~ £" a

O ^ T.

K o - = t:
" j; x ■- -

a.

o

= »- .« ~ IS "^ -r^
■- ^ ° i g " - s ^

1— 1 s.a.~+-+-*^c «c

t. >i CO t:

i C3 O

O & a »: K =

.5j

=4-1 V ' ^ =t-t

i- g '-^ ^ »3 g

Cj _ C ^ ,-,

g ^ a o g, a
a a -^ ._r r- -^

"+i O d ^^ . , —
- o -^ -' o

3 o 2J " a

i = - >■ S

c o '^ -r t. c3
1— I X ^ s ca a

s «

p .
isi

O tn i.

—1

a a

Ol

u

12

SUMMARY OK MAIN CELL TYPES IN SEED I'LANTS

4;]

_

Ol CO

o

2^.2

■>-> 0)

■*^ Z'

TL -*-•

P o s

^B.

=t-i

S-'S o

^

o "

o .

CO

^

fl ri

r- -J-j

15 ^ -

2-S

•r- 1^3 OJ

i; -. -►-'

O CO

^

5 Oj j;
la §

« ci cs

o a-

^ --t_i ■- 5 ^ O

g s M S

., o S ""^S
? S -s 1? -^ f ^■

oj -r ? 3 i i-
o " !S o :: a<

a1 ^ ^ - ?«

^■■S £ " ^ ^

£ =4^ S- o £
o cj o .S -t^ n

^ a CO r*"i rj

S ^^ 5 2

a 00 ti

-g C3 CO
c3 iS
&a a a
S «'a -2
S a 2

"5

5 -r; -M &t 02 c3

§§1^£^

O ^ 03 -r^ s

jh ^j Oi a< oj !»

03 _N __ ., _.

"«—

■*-' t- '^ ij -'-'

S r » > .=; -r i^

c^

s ^iic2^^ S

1 1 ;i = •^'

- 52 S ^ — ^^ aa

o -; o Qi — r- ^

£- a g o 3

C"* m P' O -4-*

c ■::; =^ =s

.S •" o o s a* o '-
M -x: .2 •- ^ -^s .Si

- >- ^ S *-. k ""
== :5 i ci s - "

5:

o o.2i oj

•So P ^ , -s ^ <„ ^

Si.

t- •-'

r^ ■:; fi '^ (u c

O C •■" S ""• '"^

©

-" ^ ^

a CO « CS q; - ;„

— =^ .2 b a "^

&H

c S ■" ^ ^
ri ««-t o > >

O C ^H „ K, 5P, O .

":: a- -1^ .t- S ^H ^

n S oi a P--S &^
ajcMo&osaao-o^

Ch O O O b£.S =*H o o

'T3

'd

^* ^^

a rH

c3 3

« §

—t

-2

a

S:

5 rt

^ Cu

i^

•^ "^

.^ V

s s

o

03 3

P CO

£2

'iii

fe.

'

:s^

©

fri'

^

-*-:>

--. -*-^

-d !=1

f^

T, ^

; '^

O' rH

0^ rH

t>

> a

a; 0*

O

>\A

i^W

O

44 CLASSIFICATION OF CELL TYPES, TISSUES, AND TISSUE SYSTEMS

REFERENCES

1. Bloeh, R.. Wound Healing in Higher Plants. Bot. Rev.
7:110-146. 1941.

2. Eames, A. J., and MacDaniels, L. U., An Introduction to
Plant Anatomy. New York, 1925.

3. Ilaberlandt, G., Phj'siologieal Plant Anatomy. Trans, of 4th
German Ed. London, 1914.

4. Havward, H. E., The Structure of Economic Plants. New
York, 1938.

5. Jetfrev, E. C, The Anatomy of Woody Plants. Chicago,
1917."^

6. Lundegardh, II. Zelle und Cytoplasma. Ilandb. d. Pflanzen-
anatomie. I. Berlin, 1922.

7. Molisch, II., Anatomic der Pflanze. Jena. 1936.

8. Palladia, W. I., Pflanzenanatomie. Trans, of 5th Russian
Edition. Leipzig u. Berlin, 1914.

9. Sachs, J., Text-Book of Botany. Oxford, Clarendon Press,
1875.

10. Sinnott, E. W., Botany, Principles and Problems. New York,
1935.

11. Strasburger's Textbook of Botany. 5th English Ed. Lon-
don, MacMillan and Co., Ltd., 1921.

12. Tschirch, A., Angewandte Pflanzenanatomie. Wieii u. Lci))-
zig, 1889.

13. Weber, F., Proto])lasmatisclie Pflanzenanatomie. Proto-
plasma 8 :291-306. 1929.

14. White, P. R., Plant Tissue Cultures. Biol. Rev. 16:34-48.
1941.

Exercise V

THE EPIDERMIS

I. Introduction. — From a purely topographical viewpoint, the
term epidermis may be applied to tiie superficial layer of cells
in young- stems and roots, and in foliar structures. Since the
epidermis represents, in this sense, the point of clireet eontaet
between the plant and its external environment, it is not surpris-
ing that this "tissue" exhibits considerable diversity in its
structure and functions. Haberlandt (p. 102) has proposed a
restricted physiological definition of the epidermis which would
include only "those superficial cells or cell-layers, the histologi-
cal features of which clearly indicate that their principal function
is that of a primary tegumentary or dermal system." Accord-
ing to this viewpoint, absorbing hairs and stomata would be
excluded on physiological grounds from the epidermis. But as
Linsbauer (1930, pp. 4-5) has clearly pointed out, it seems hardly
justifiable to place the chief emphasis on the function of "pro-
tection" in the definition of the term epidermis. On the con-
trary, the cells which are morphologically a part of the epidermis
may perform varied functions, important among which are me-
chanical protection, restriction of transpiration, water storage,
aeration, storage of various metabolic products, absorption and
photosynthesis. To subdivide such a "continuous" layer as the
epidermis into various "anatomico-physiological systems" is
more likely to result in confusion than is the retention of the
broader topographical -morphological concept expressed above.
The ontogenetic development of the epidermis likewise justifies its
interpretation as a "morphological unit," since its origin is
traceable to an external embryonic layer or "protoderm." In
lower vascular plants and in many gymnosperms, the protoderm
of the shoot appears some distance from the summit of the apex
as a superficial layer derived from the periclinal division of the
segments of the apical initial or initials. But in many angio-

4.5

V

/

46 THE EPIDERMIS

sperms the protoderm is directly continuous with the outermost
tunica layer or "dermatogen" of the shoot apex (for further
tletails, ef. Exercise III). In roots, the ])rotoderm likewise is
demarcated from the internal meristems in the vicinity of the
apex. Hay ward (pp. 45-47) has recently summarized the more
important "types" of root apices from the standpoint of the way
in which the protoderm ("dermatogen") is related to the devel-
opment of root cap, cortex and stele.

In the majority of seed plants, the epidermis is a uniseriate
layer of cells which clothes the "primary body." Aside from
the epidermal appendages or trichom.es, which will be studied
later in the exercise, the common cell types composing this layer
are epidermal cells and the guard cells. The epidermal cells,
although exhibiting considerable variation in size, shape and
arrangement, are usually closely joined with one another, thus
forming a sheet of cells which is pierced only by the intercellular
spaces or pores found between the guard cells. An exceptional
type of epidermis occurs in petals, however, since here inter-
cellular spaces are found between ordinary epidermal cells.
According to Eames and MacDaniels (pp. 284-28")) these "spaces
do not open to the outer air, however, since they are covered in
all cases by the cuticle." Epidermal cells are roughly "tabular"
in form and especially in the laminae of dicotyledonous foliage
leaves, have a characteristic undulate contour when seen in sur-
face view. A protoplast is normally retained and a great variety
of ergastic substances such as anthocyanin pigments, tannins
and oils occur in the cells. Epidermal cells exhibit to a notable
extent an a])ility for regressive diffen idiaiion. This is shown
not merely by the origin in certain plants of the jihellogen in this
layer (Eames and IMacDaniels, p. 210) but es])ecially by the
important role of epidermal cells in the ])roducti()n of adventi-
tious bud-primordia (cf. Crooks, 1933, Naylor and Johnson.
19.37, McVeigh, 1938, and Naylor, 1940).

With the exception of roots and the submersed portions of
aquatic plants, the outer walls of the epidermis are covered by a
sheet of waxy material which is termed the cuticle. This waxy
layer is continuous, except for (lie stomatal openings, and serves
to restrict the loss of water fi-om plant organs. The thickness of

INTRODUCTION'

47

the cuticle is highly variable, in some organs being a hardly per-
ceptible "film" while in other instances (e.g., fruits and certain
types of leaves) it is extremely prominent (Eames and Mac-
Daniels p. 37, Fig. 24). The imtUs of epidermal cells vary in
their structure and chemical composition. Typically, the outer
tangential vxill directly beneath the cuticle, is the most heavily
thickened of all the walls and the irmer-tmigential wall the thin-
nest. Often the radial tmill tapers in thickness towards the inner
tangential wall. Simple pits are common in the radial and inner
walls of epidermal cells. According to Ilaberlandt (p. 102) the
innermost zone of the outer wall usually consists of "unaltered
cellulose" and is followed externally by layers of wall substances
which contain varying amounts of cutin. The recent work of
Anderson (1934), however, has shown that in the leaf of CAivia
nobilis, the thick outer wall of the epidermal cells shows "two
distinct zones of cutinization." The outermost zone is devoid
of cellulose or pectin while "the inner zone of cutinized wall
consists of a series of cellulose lamellae separated by layers of
pectic material, both of which are impregnated with cutin. The
inner cutinized zone may be in direct contact with the protoplasm
of the cell or may be separated from the protoplasm by a second
zone of cellulose and pectic materials." It is clear from this
Avork that a thorough study of the process of cutinization in epi-
dermal walls of various plants is urgently needed.

The continuity of the epidermis, especially of foliage leaves
and young stems, is interrupted by minute openings or pores
which are termed stomata.. Each stoma represents an intercellu-
lar space between a pair of highly specialized epidermal cells
known as guard cells. As seen in surface view, guard cells are
very frequently crescent-shaped with their concave surfaces
adjacent to the slit-like pore. In contrast to ordinary epidermal
cells, the walls of guard cells are uneven in thickness, often with
ridge- or flange-like extensions at the edges of the pore. Further-
more, guard cells usually contain prominent chloroplasts. Since
stomata play such an important role in the processes of respira-
tion, photosynthesis and tranpiration, much attention has been
devoted to the "mechanism" by which the stomata are "opened"
and "closed." In general, changes in the width of the stoma

48 EPIDERMIS

are regulated by the relative degree of turgor in the guard cells,
which in turn causes slight alterations in their shape. When the
guard cells are turgid, the width of the pore is at a maximum
while closing of the aperture occurs when the turgor of the cells
decreases. A discussion of the variation in the construction of
the walls of guard cells and of the physiological factors influenc-
ing the turgor movements of these cells, however, is beyond the
scope of this book (cf. Ilaberlandt pp. 445-477). Guard cells
originate by the anticlinal division of certain protoderm cells
into two dissimilar daughter cells. In the simplest condition, one
of these cells functions as an initial cell and by an anticlinal
division directly produces the two guard cells; the other daughter
cell meanwhile differentiates into an ordinary epidermal cell.
But many deviations from this simple type of stomatal develop-
ment occur, especially with reference to the formation of the
subsidiary cells. The latter differ in form and arrangement from
the neighboring epidermal cells and are believed to cooperate
physiologically with the guard cells in regulating the width of
the stoma (for information regarding the various modes of
stomatal development cf. De Bary, pp. 39-45, Porterfield, 1!).'}7,
and Yarbrough, 1934).

A study will be made first of the structure of the uniseriate
epidermis. Later in this exercise a brief explanation and direc-
tions for study of the multiple epidermis and of t)ichomes will
be given.

II. Material for the Study of the Uniseriate Epidermis—

1. The bull)-sc{ile of the onion (Allium Cepa). Keniove, with
foi'ccps. a small strip of epidermis from the outer or abaxial
surface of the bulb-scjile and mount it carefully in w.ilci-. rnder
low magnification, note the rather orderly arrangement of the
"rectangular" or tabular epidermal cells. Stomata. which may
be abortive or "abnormaT' in appearance, are occasionally pres-
ent in this mntei-ial. Fndei- high magnification, it will be seen
that the radi<d walls of the epidermal cells are i)rovided with
nuinei'ons small simple pits. Because of the comparatively small
i-adial depth of the epidermal cells, nuclei are i-eatlily seen. Tiie
cytoi)lasm is highly vacuolat(^ and often is actively streaming or
moving within many of the cells. Small greenish-yellow bodies,

THE MULTIPLE EPIDERMIS

49

which may represent citlier IcKcophisis or ergastic material are
also observable. To study the cuticle and the structure of the
walls of the epidermal cells, cut thin trans-sections of the bulb-
scale, and mount them in a .1% solution of neutral red.^ If these
sections are not overstained, the neutral red will be confined
largely to the protoplasts and the walls will be clearly demar-
cated. Similar trans-sections, when gently heated in a solution
of Sudan TV,^ are particularly suitable for a study of the cuticle
which will appear under high magnification as a brilliant red
layer continuous over the outer walls of the epidermis.

2. The leaf blade of the geranium {Pelargonium) or the
"German Ivy" {Senicio miknnoidcs). Remove small strips of
the loAver {aha rial) epidermis and mount some in water, others
in a .IVr solution of neutral red. Note especially the charac-
teristic undulate contours of the epidermal cells and the numer-
ous stomata. If the geranium epidermis is used, the relationship
of the numerous multicellular hairs can be readily studied.

3. The leaf of Iris sp. IMount strips of the epidermis in water
as well as in neutral red and examine carefully under low magni-
fication, noting the regularly-arranged stomata and the elongate
form of the epidermal cells. Careful focusing will show that the
guard cells with their stomata are slightly suidvcn beneath the
surface and are overarched by the ends of the epidermal cells
adjacent to them. Thin trans-sections of the epidermis should
also be secured and treated with neutral red and Sudan IV.
These sections will reveal the thick cuticle and the prominently
thickened outer tangential walls of the epidermal cells.

4. Prepared slides of trans-sections of the leaf-blades of lilac
(Si/ringa vulgaris) and corn {Zea Mays). Trans-sections cut
by hand are often too thick to permit of an accurate examination
of the structure of the walls of guard cells. For this reason,
a supplementary study should be made of the guard cells as seen
in thin stained sections of the forms listed above.

III. The Multiple Epidermis. — In the leaves of certain angio-
sperms, particularly in members of the families Piperaeeae.
Begoniaceae and Moraccac, some or all of the cells of the original

1 Cf. Appendix, p. 142.
~Cf. Apppndix, p. 142.

50 THE EPIDERMIS

uiiiseriate epidermis may experience tangential divisions which
result in the formation of a uiuUiplc or muUiseriatc epidermis.
According to tlie resume of the literature given by Linsbauer
(1930, pp. ;58-4;i), the first tangential divisions leading to the
development of tiie multiple epidermis occur at a relatively late
stage in leaf ontogeny ; in Ficus, for example, as the leaf is
expanding and after its stipules have fallen away. Such divi-
sions apparently occur first near the middle portions of the
lamina and progress towards its margins. Often the tangential
divisions are so regular as to produce a tissue composed of thin-
walled cells arranged in radial rows. But anticlinal divisions
may also occur in the cells thus obscuring the point of origin
of the tissue. It should be apparent from these facts that the
concept of the "multiple epidermis" is based fundamentally
ujion its direct origin from the surface cell layer and not upon
its histology or functions. This is important because much con-
fusion has been produced, especially in the literature of physio-
logical anatomy, by failing to distinguish between a true multiple
epidermis and the various types of "hypodermal" tis.sues which
originate and develop independently of the epidermis. The chief
fimctiem of the multiple epidermis is water storage and a brief
discussion of its activities in this direction are summarized by
Linsbauer (1930, pp. 41-43). According to the earlier work of
l*fitzer which is outlined by Linsbauer, water-storing tissue of
epidermal origin may in Peperouiia pereskiaefolia attain a thick-
ness of 14-1") layers and thus represents about 7 times the thick-
ness of the otiier leaf tissues.

IV. Material for the Study of the Multiple Epidermis.— A

typical example of a multiple ei)ldermis is found in the leaf
blade of the common "rubber ])lant" {Ficus elastica). As early
as 1827, the (Jerman botanist Meyen observed that during the
formation of the multiple epidermis in this species, certain of
the original epidermal cells fail to divide tangentially but instead
become enormously distended inwardly and finally protrude into
the underlying palisade parenchyma. During this process of
distention, a cui-ious peg-like invagination develops from the
outer tangential wall into the cell cavity and eventuallv becomes

MATERIAL FOK STUDY OF MULTIPLE EPIDERMIS 51

coated with a crystalline mass of calcium carbonate. This curious
ingrowth of the cell wall, together with its covering of calcium
carbonate is termed a cystolith and the cell in which it occurs is
known as a lithacyst. (Cf. the recent work of Ajello, 1941.)
Ohtain a trans-section of a portion of a living leaf-blade of Ficus
elastica and examine it under low and high magnification. The
multiple epidermis on the upper (i.e., adaxial) surface consists
of several layers of cells the outermost of which are small com-
pact and overlaid by a prominent cuticle. These cells thus
exhibit the features of a normal "uniseriate epidermis."

In contrast, the inner eells of the multiple epidermis are rela-
tively large and because of their shape, their cellulose walls and
the presence of intercellular spaces suggest resemblance to corti-
cal parenchyma tissue. Notice that the cells are not aligned in
radial rows because, during their formation, both anticlinal as
well as periclinal divisions have occurred. A careful inspection
of the walls of these cells under high magnification will reveal
numerous simple pits. At intervals large sac-like litho>cysts will
be seen, protruding into the adjacent palisade parenchyma.
These distended cells have arisen directly from the original sur-
face cells of the leaf blade. Each lithocyst, unless injured in
sectioning the leaf, contains a cystolith with its knob-like end
covered by a crystalline mass of calcium carbonate. Introduce
a few drops of hydrochloric acid under the cover-glass and
observe the rapid dissolution of the calcium carbonate. This is
accompanied by the evolution of small bubbles of carbon dioxide.

V. Trichomes. — This term may be used in a collective sense
to designate the diversified types of epidermal appendages such
as hairs, scales, colleters and water vesicles. Despite the "end-
less" variation in the form and structure of trichomes (cf.
De Bary, pp. 54-66, and Netolitzky, 1932), these structures
originate from the extension or subdivision of protoderm cells.
Trichomes are therefore, morphologically, a part of the epi-
dermis in contrast to emergences (e.g., the prickles on the stem
of Rosa, Rihes, etc.) which consist of cells derived not only from
the protoderm but also from deeper hypodermal layers (cf,
De Bary, p. 58, and Eames and MacDaniels, p. 1 and p. 2, Fig. 1).

52 THE EPIDERMIS

The inurphological distim-tioii between trielioiiie>s and emergences
is of further interest in those eases where hairs or scales are
borne upon an emergence (Cooper, 1932).

Trichomes furnish a rich fiehl for morphogeiietic investiga-
tions because of their great diversity and because their super-
ficial position and relatively simple structure facilitate onto-
genetic studies with living material. As an introduction to the
problems in this field, a brief characterization of the four com-
monest "types" of trichomes is now given, viz. :

1. Hairs. In form, hairs are thread-like in appearance and
are either uniceUular (e.g., root hairs) or muliiceUulay. The
latter type of hair may consist of a single series of cells, terminat-
ing in an acute terminal cell or a (jlandular cell; or the hair may
be branched in various ways. In some jilants, the hairs are
composed of several layers of cells and are termed shag-haivs.
Such multiseriate hairs are often borne upon an emergence
(De Bary, pp. 64-6."), and Pig. 21c). Two general regions may
be distinguished in a hair, viz.: (1) the foot, which is the por-
tion lying within the epidermal surface and Avhich is often dif-
ferent in form from the adjacent epidermal cells, and (2) the
ho(h) which is the portion extending away from the epidermal
surface. Occasionally, a given epidermal surface develops but a
single type of hair (e.g., root hairs). ]More commonly, especially
in leaves, several different morphological types of hairs occur
side by side on the same ei:)idermal area.

2. Scales. These trichomes consist of a plale of cells and arc
eithei- pelf ale (as in certain angiosi)ernis ) or are attached to llic
epidei'mis only a1 one edge (e.g., tlic chaffy scales or /xthai so
characterisi ic of numy ferns). Scales in the gcnci-a SJieplu rdia
and ElaeagnuH ai'e so closely crowded on the surface of the young
stem and 1h(> leaves tlial Ihcy i)ro(]ucc a lypical "scurfy"
appearance.

3. Collefers. On many foliar oi-gans, ])ailicnlai-ly on bnd
scales and stipules and on Ihc foliage leaves of certain genera
(e.g.. Rho(l(Hl( n(lro)i, Carifa) ghnidnlai- Iriclionics occur. These
structures W(>re oi-iginally termed collcters by Ilanstein (1868).
Colleters consist of a short, often niulticellnhir stalk bearing an
expanded disc oi- knob of secretory cells. The characteristic

MATERIAL FOR STUDY OF TRICHOMES 53

sticky exudation found on certain foliar structures is secreted
from colleters (cf. Foster, 1929. pp. 457-458).

4. Water vesicles or bladders. Triehonies of this type con-
sist of greatly distended epidermal cells which presumably are of
physiological importance as reservoirs for water (cf. Ilaber-
landt, p. 116). In the so-called "Ice Plant" {Mesemhrijanthe-
mum crystallhium) , the water vesicles are so large and so numer-
ous that the leaves and young stems appear to be covered with
minute translucent beads of "ice."

VI. Material for the Study of Trichomes.—

1. Hairs. Obtain thin trans-sections of the petiole of the ger-
anium (Pelargonium) and after mounting them in water/ study
carefully the structure of the unicellular and the multicellular
unhranched hairs. Note the relatively thick outer walls of the
body of these hairs. The foot, especially of the multicellular hair,
consists of an enlarged bulbous cell, separated by a transverse
wall from the body, and surrounded by a circular group of more
or less elevated subsidiary cells. The true relationship of th&
subsidiary cells to the foot of the hair is clearly seen in thick
trans-sections of the petiole as well as in strips of epidermis
removed from the lower surface of the lamina. Note that cer-
tain of the hairs are (jlandular, terminating in a single large
secretory cell filled with dense, yellowish-brown ergastic mate-
rial. Excellent material for a study of multicellular branched
hairs is atforded by the leaves of various members of the Ma\-
vaceae, where typical stellate hairs occur. Each hair of this type
consists of a number of radiating unicellular "branches" which
have arisen from the subdivision of a single epidermal cell. Fur-
ther illustrations of multicellular branched hairs are provided
by the leaves of mullein (Verbascum Thapsu.'^). Scrape a small
amount of hairs from the leaf into water, carefully tease them
apart with dissecting needles and examine under low magnifica-
tion. Note that these complex hairs are "dendroid" in form;
i.e., each hair consists of a main "axis" (made of a vertical series
of cells) and whorls of radiating unicellular or bicellular
"branches." Trans-sections of very young mullein leaves are

1 Cf. Appendix, pp. 130140.

54 THE EPIDERMIS

very instructive in showinji' various stages in tlie ontogeny of
these hairs. Tlie Jeaf-blade of the sycamore or buttonball tree
{Platanus) likewise will provide examples of dendroid multi-
cellular hairs.

2. Scales. Scrape a small (juantity of trichomes from the leaf
of Sheplwrdia or Elaeagnus into a drop of water on a slide and
examine under low magnification. In Shepherdia two extreme
types of peltate trichomes will be seen, viz.: (1) Stellate hairs,
consisting of a delicate stalk bearing ten or more distinct radiat-
ing unicellular "branches," and (2) Scales which are gray-yel-
lowish brown in color, lobate and consist of a plate of many cells.
Note under high magnification that not all of the cells composing
the disc-like scale radiate from a common center. According to
the recent ontogenetic studies of Cooper (1932) this condition
results from niiequal arid oblique divisions which may occur near
the outei" end of certain of the cells early in the development of
the scale.

3. Colleters. The bud scales of the horse-chestnut (Aesculus
Hippocastaniim) ])rovide excellent material for a study of typical
colleters. Cut thin trans-sections of the inner scales of a winter
bud and mount them in water. Under low magnification note
that the abaxial surface of the scale in particular is densely
covered with colleters. Additional or alternative material for
a study of colleters is furnished by the bud scales of various
species of Bhododendron.

4. Water vesicles. Obtain several thin trans-sections of the
petiole of the "Ice-Plant" {Mesemhryanthemuin crystallinum)
and examine them in water under low magnification. The adult
water vesicle appears as a large clear hemisjiherical cell which
projects outwardly from the genei-al eindermal surface. Under
high nuignification, a nucleus, scanty cytoplasm and small plas-
tids may be detectable in the bladders. If the concave adaxial
surface of the petiole of immature leaves is examined, various
stages in the origin and expansion of the water vesicle may be
seen.

VII. Suggested Drawings and Notes, —

]. The nniseriate cpid(riiiis. Prepai-e carefully labeled draw-
injis to show the sti'ucturc. in both surface and trans-sectional

REFERENCES 55

views, of the epidermis of Alliitiii, Pclargoniuin (or Senecio) and
Iris. Ill drawings of the surface view, emphasize the shape and
arrangement of the stomata with reference to the epidermal cells.
In drawings of trans-sections, indicate clearly the extent and
relative thickness of the cuticle, the thickness and pitting of the
walls of the epidermal cells and the relation of the epidermal cells
to the underlying tissues of the leaf. Also draiv a' single stoma
from stained trans-sections of the leaf of Syriviga or Zea to show
the structure of the guard cells and the form and extent of the
substomatal cavity.

2. Multiple epidermis. Prepare a large drawing to show in
detail the structure of the multiple epidermis of the leaf of Ficus
elastica. Include in this drawing a lithocj'st with its cystolith.
Label all essential structures.

3. TricJiomes. Prepare drawings, to illustrate the structure
and relationship to the epidermal layer of various types of hairs.
If time permits, also prepare drawings of a scale from the leaf
of Shepherdia or Eloeagnns, of colleters as seen in trans-sections
of a bud scale, and of the water vesicle of the "Ice-Plant."

4. Describe, by means of diagrammatic drawings, the ontog-
eny of a water vesicle in the petiole of the "Ice-Plant." Sum-
marize, from information given in Ilaberlandt, the most
important functions performed by the various types of trichomes.

REFERENCES

1. Ajello, L., Cytology and Cellular Interrelationship of Cysto-
lith Formation in Ficus elastica. Amer. Jour. Bot. 28:589-
594. 1941.

2. Anderson, D. B., The Distribution of Cutin in the Outer Epi-
dermal Wall of Clivia nohilis. Ohio Jour. Science 34 :9-18.
1934.

3. Cooper, D. C, The Development of the Peltate Hairs of
Shepherdia canadensis. Amer. Jour. Bot. 19:423-428. 1932.

4. Crooks, D. ]\I., Histological and Regenerative Studies on the
Flax Seedling. Bot. Gaz. 95 :209-239. 1933.

5. De Bary, pp. 29-107.

6. Barnes and MacDaniels, pp. 107-112.

7. Foster, A. S., Investigations on the Morphology and Com-
parative History of Development of Foliar Organs, I. The
foliage leaves and cataphyllary structures in the horse-chest-

56 THE EPIDERMIS

nut {Aescidus Hiiypocasianum h.) . Amer. Jour. Bot. 16:
■441-501. 1929.

8. Haberlandt, pp. 101-133.

9. Hansteiii, J., Uber die Orgaiie der Harz-und Schleimabson-
derung' in den Laubknospen. Bot. Zeit. 26: 697-713 et seq.
1868.

10. Hayward, pp. 15-18.

11. Linsbaiier, K., Die Epidermis. Handbiieh d. Pflanzenanato-
mie, IV, 2. 1930.

12. McVeigh, I., Regeneration in Crassula multicava. Amer.
Jour. Bot. 25 :7-ll. 1938.

13. Naylor, E. E., and Joinison, B., A Histological Study of Vege-
tative Reproduction in Saintpanlia ionaniha.. Amer. Jour.
Bot. 24 :673-678. 1937.

14. Naylor, E., Propagation of Hyacinthus by Leaf Cuttings.
Bull. Torr. Bot. Club 67 :602-666. 1940.

15. Netolitzky, F., Die Pflanzenhaare. Handbuch d. Pfi«nzen-
anatomie, IV. 1932.

16. Porterfield, W. M. Jr., Histogenesis in the Bamboo witii Spe-
cial Reference to the Epidermis. Bull. Torr. Bot. Club
64:421-432. 1937.

17. Yarbi'ough. J. A., History of Leaf Development in Bri/o-
phyllum calycinuin. Amer. Jour. Bot. 21 :467-484. 1934.

Exercise VI
PARENCHYMA CELLS

I. Introduction. — The term "parenchyma" is used in a rather
abstract or loose sense to designate a wide variety of living
cells -which occur in many dilt'erent regions of the plant body.
Parenchyma cells may appear in groups scattered among highly
specialized conducting elements, as for example the cells of vas-
cular rays and the vertical files of phloem and xylem parenchyma
cells. Often, however, parenchyma cells form homogeneous or
"simple" tissues which may constitute a large part of the softer
regions of leaves, stems, roots and fruits. From these illustra-
tions it should be clearly evident that under the concept of
"parenchyma" are included cells which differ markedly in their
position, origin and functions. For this reason, "parenchyma"
is merely a convenient and long-established anatomical category
within which are included cell types which are not necessarily
either homologous or analogous.

In an effort to characterize parenchyma tissue more precisely,
it is commonly described as being composed of cells essentially
isodiametric in form, separated by more or less conspicuous inter-
cellular air spaces, and with thin walls and active protoplasts.
To appreciate the merits as well as the limitations of this set of
characteristics, the following critique will be found useful.

1. Form of cell. The recent work of Marvin (1939) and of
Marvin and Matzke (1939) has shown that the form of paren-
chyma cells in some regions (e.g., the pith of Enpatorium) is
approximately isodiametric and that with respect to the number
of faces of contact made with neighboring cells, "the cells show
an economy of surface to volume approaching, and in some cases
equaling, that of an orthic tetrakaidecahedron or a rhombic
dodecahedron of equal volume." This conclusion is particularly
interesting in view of Tupper-Carey and Priestley's (1924) state-
ment that the cells of the apical meristem approach the form

57

58 PARENCHYMA CELLS

of twelve- or fourteen-sided polyhedra. But, not all cells desig-
nated as "parenchyma" are isodiametric in shape. As exam-
ples, the following may be cited: the narrowly "cylindrical"
form of palisade parenchyma cells, the lobed form of "arm-pali-
sade" cells (cf. Meyer, 1923, p. 16) and the elongated shape of
the cells in vascular rays.

2. Structure and chemistry of the wall. During the differ-
entiation of the parenchyma cells in the cortex and pith of stems
and in the mesophyll of leaves, little or no appreciable increase in
Avail thickness occurs and a true secondary wall, clearly defined
from the original primary wall, may be absent. In such cells,
the thin primary wall seems to consist largely of cellulose. Sim-
ple pits are present but they are often restricted to certain local
regions of the wall (cf. De Bary, p. 117, Fig. 46). In contrast,
Avood-parenchyma cells and the cells of xylem rays are often pro-
vided with relatively thick walls, which are abundantly pitted.
According to Eames and MacDaniels (p. 68), the parenchyma
cells of secondary wood often have "thick, more or less strongly
lignified walls." Whether the thick areas of the walls of these
cells are "secondary" or "primary" in nature apparently con-
stitutes an open problem at present.

3. The protoplast . The retention at maturity of an active
protoplast represents one of the most important characteristics
of parenchyma cells. Indeed, because of this fact, parenchyma
cells perform many of the most fundamental physiological proc-
esses, notably photosynthesis, food and water storage and secre-
tion (Meyer, 1923; Netolitzky. 1935; Sperlich, 1939). In addi-
tion to their important metabolic activity, however, parenchyma
cells possess to an exceptional degree the ability to revert to a
meristematic state (Ilayward. 1938, p. 14) . This is clearly shown
by the i-apid response of parenchyma tissue to tlic^ ])liysical and
chemical effects of artificial or natural wounding. The nature of
such responses is highly variable and complex, ranging from the
production of "callus" or of cork tissue, to the regressive forma-
tion of root or bud primordia [cf. Priestley and Swingle (1929)
and Bloch (1941)]. It may be argued that the ease with which
parenchyma cells can be induced to divide and to produce new
tissues and organ primordia is evidence of their "primitive" or

MATERIAIv FOR STUDY OF PARENCHYMA 59

''unspecialized" nature. ])ut it is evident that many factors,
hereditary as well as environmental, influence the process of "re-
gressive differentiation" in parenchyma and that our knowledge
of the "potentialities" of such tissue is still in an exploratory
stage.

II. Material for the Study of Parenchyma.— At this point
suggested material for a preliminary study of storage and photo-
synthetic parenchyma is described. Additional examples of
parenchyma are given in several of the later exercises in this book.

1. Storage parenchyma. As noted above, parenchyma tissue
often serves as a region for storage of many different substances,
particularly starch. The cotyledons of bean embryos, during the
early stages of seed germination, provide useful material.
Examine thin sections as well as partially macerated tissue, not-
ing the closely-packed starch grains in the parenchyma cells.
Under high magnification, the type and distribution of the simple
pits on the various faces of the wall may be clearly studied.
Additional examples of storage parenchyma are furnished by the
cortical parenchyma of the young root (e.g., Ranunculus) and
the parenchyma tissue of the potato tuber (Solannm tuberosum).

2. Photosynthetic parenchyma. In the subepidermal region
of young stems and in the mesophyll of leaves, the thin-walled
parenchyma cells contain chloroplasts and perform the function
of photosynthesis. Obtain a thin transverse section of the stem
of Begonia, and, after mounting it in water, examine the prepara-
tion at low magnification. Notice that the cortex (i.e., the region
between the epidermis and the cylinder of vascular bundles) and
the pith are composed of large, thin-walled "isodiametric" cells.
(Note: Several layers of small collenchyma cells are found at the
outer edge of the cortex and may be disregarded in this study.)
Under low magnification observe that large, solitary prismatic
crystals as well as druses occur in many of the parenchyma cells.
Frequently, the form of the crystals is highly irregular. Under
high magnification, the cytoplasm, vacuole and small chloroplasts
can be readily studied especially if the sections are mounted in
.1% solution of neutral red.^ Because of the large size of the
cells, the nucleus is only seen occasionally in trans-sections of the

1 Cf . Appendix, p. 142,

60 PARENCHYMA CELLS

parenchyma tissue. Small simple pits may be visible in certain
walls of the cells. In order to understand the shape and propor-
tions of the parenchyma cells, as well as the intercommunication
between the prominent intercellular spaces, longi-sections of the
stem should be studied under low and high magnification. Ob-
serve that the intercellular space system in the longi-section
appears "black" because of the included air. This optical effect
is of great assistance in distinguishing between walls and longi-
tudinally exteuded air spaces.

III. Suggested Drawings and Notes. —

1. Prepare an enlarged drawing of a single cell from the
cotyledon of the bean seedling, showing its shape, wall structure
and the included starch grains. For comparison, draw several
cells from the cortex of the root of Bn'}n(nc}dus and the potato
tuber. Prepare notes on the differences in the form and structure
of the starch grains in these different examples.

2. Prepare drawings, from both transverse and longi-sections,
showing-ihe structure of the photosynthetic parenchyma of the
cortex of the Begonia stem. Include cells which show crystals,
cytoplasm and chloroplasts. What conclusion do you reach as to
the shape of the parenchyma cells? Summarize, from the refer-
ences you have read, the method of origin and functions of inter-
cellular spaces in parenchyma tissue (cf. especially Turrell,
1936).

REFERENCES

1. Rloch, P., "Wound Healing in Higher Plants. Pot. Peview
7:110-146. 1!)41.

2. De Bary, pp. 115-119.

8. Eames and MacDaniels, pp. 52-54.

4. Hay ward, p. 14.

5. Marvin, J. W., Cell Shape Studies in the Pith of Eupatorium
purpurcum. Amer. Jour. Bot. 26 :487-504, 1939.

6. Marvin, J. W., and Matzke, E. B., A New IMethod for the Con-
struction of Three-dimensional Cell IModels. Amer. Jour.
Bot. 26:101-103, 1939.

7. Meyer, F. J., Das trophische Parenchym. A. Assimilations-
gewebe. IIandl)ucli d. Pflanzenanatomie. IV. 1923.

8. Netolitzky, P"., Das trophische Parenchym. C. Speieher-
gewebe. Ilandbuch d. Pflanzenanatomie. IV. 1935.

REFERENCES 61

9. Priestley, J. H., and Swingle, C. F., Vegetative Propagation
from the Standpoint of Plant Anatomy. U. S. D. A. Tech.
Bulletin No. 151. 1929.

10. Sperlich, A., Das trophische Parenchym. B. Exkretions-
gewebe. Handbuch d. Pflanzenanatomie. IV. 1939.

11. Tnpper-Carey, R. M., and Priestley, J. H., The Cell Wall in
the Radicle of Vicia faha and the Shape of the Meristematic
Cells. New Phytol. 23:156-159. 1924.

12. Tnrrell, F. IM., The Area of the Internal Exposed Surface
of Dicotyledon Leaves. Amer. Jour. Bot. 23 :255-264. 1936.

/

Exercise VII

COLLEXCIIYMA CELLS

I. Introduction. — The subepidermal region of many stems, peti-
oles and ribs of leaves is occupied by a more or less "simple"
or homogeneous tissue which is termed collenchyma. Frequently
this tissue occurs as a continuous hypodermal cylinder but in
some stems and in the petioles of leaves, distinct strands of col-
lenchyma may be present. An interesting example of the strand-
like arrangement of collenchyma is furnished by the familiar
"strings" present in the abaxial ribs of celery petioles (cf. Esau,
1936, pi. 2 and 6).

In certain respects, collenchyma is remarkably similar to
cortical parenchyma. Indeed Hay ward (p. 22) regards collen-
chyma as a derived form of parenchyma, a viewpoint also ex-
pressed by De Bary (pp. 119-120) who stated that "it is then
to a great extent a matter of taste how far one Avill extend the
term Collenchyma." The parenchymatous character of collen-
chyma is shown by the fact that it is a tissue composed of living
cells, the protoplasts of which, like parenchyma, are able to
revert to a meristematic state. This feature is illustrated by the
origin of the phellogen or cork cambium in the outermost collen-
chyma cells in many stems. Furthermore, collenchyma. although
primarily a "mechanical tissue" in its function, presumably car-
ries on some photosynthesis as evidenced by the frequent occur-
rence of chloroplasts in its cells. The shape of collenchyma cells,
especially those adjacent to parenchyma elements in the cortex
or petiole, may be more or less "isodiametric." Typically, how-
ever, collenchyma cells are elongated, prismatic elements Avith
obtuse, pointed or oblique ends.

The most definitive characteristic of collenchyma cells is
found in the irr< (juJar and often mnssii'e fhirkeninr/a of the cell
wall. These wall thickenings, which are deposited by the proto-
plast in the form of extensive longitudinal .strips, vary in their

62

INTRODUCTION 63

})ositioii according to several definable types of patterns when the
cells are examined in trans-section. In perhaps the commonest
type, the thickened areas of the wall are largely restricted to the
corners where the cells meet. Collenchyma of this type is termed
"angular." In certain plants, the thickenings are confined to
the tangential walls of the cells, resnlting in the so-called lamellar
collenchyma. Ordinarily, intercellnlar spaces are small or even
absent in collenchyma tissne but in certain Compositae the thick-
ened areas of the walls of the collenchyma cells border upon large
and prominent air-spaces. The term "tubular'" has been applied
to this type of collenchyma. These "types" of collenchyma,
however, are not rigidly distinct, because in some instances transi-
tions from one to the other may occur in successive radial portions
of the same zone of collenchyma.

The structure and chemical composition of the wall-thicken-
ings in collenchyma cells have recently been studied by several
investigators. Anderson (1927), in his studies on the angular
collenchyma of tomato {Solanum lycopersicum), concluded that
the thickenings, which have a high content of water, consist of
alternating lamellae of cellulose and pectin. When viewed under
crossed Nicols, the walls appeared bi-refringent. Esau (1936)
found that the walls of collenchyma cells in celery petioles "are
chiefly of cellulose and contain a high percentage of water. ' ' She
interprets the wall thickenings as representing a special develop-
ment of the primary w^all. Simple pits are found in the walls
of collenchyma cells but are not necessarily restricted to either
the thin or thickened areas.

The thorough study of Esau (1936) has shown that the
ontogeny of collenchyma tissue in celery petioles exhibits many
interesting and important features. In the mature petiole, the
collenchyma occurs in the form of distinct strands which corre-
spond in position to the abaxial ribs. The origin of these collen-
chyma bundles is traceable to the localized periclinal and anti-
clinal subdivision of ground meristem cells in the young petiole.
Procambial-like strands of somewhat elongated, thin walled cells
are thereby produced and it is from such cells that the adult
collenchyma eventually differentiates. At first the young collen-

64 COLLENCHYMA CELLS

chyma cells are relatively small in diameter but as the rate of cell
division slows down, the cells expand and gradually acquire the
characteristic angular wall thickenings. The relative prominence
of intercellular air spaces in the mature collenchyma strand de-
pends in general upon the time of origin of the tissue. Air spaces
are only present when the divisions leading to collenchyma devel-
opment occur in loosely-arranged ground meristem cells.

Functionally, collenchj-ma tissue provides considerable
strength as well as elasticity to young stems and to leaves. Ac-
cording to Esau (1936), the collenchyma of celery petioles
mechanically "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."

II. Material for the Study of Collenchyma. — The clearly de-
fined angular collenchyma in the petiole of Datura stramonium
provides excellent material and its general structure will now be
described. (Note: If Datura is not available, the angular collen-
chyma in the cortex of the stems of Solanum lycopersicum,
Cucurhita, or Begonia, or the collenchyma strands in celery peti-
oles, may be used.)

Obtain a trans-section of the petiole of Datura and after
mounting it either in water or a .1% solution of neutral red,
examine under low magnification. The epidermis possesses char-
acteristically thickened outer walls covered by a prominent cuti-
cle; a protoplast and small scattered ehloroplasts should be vis-
ible in some of the epidermal cells. Note the })rominent multi-
cellular mihranched hairs and study carefully the structure of
their component cells. Beneath the epidermis are found six or
more "layers" of typical angular collenchyma cells, the irregu-
hirly thickened walls of which exhibit, when viewed in water, a
characteristic pearly-white lustre. Tn sections mounted in dilute
neutral red solution,^ the wvalls are brilliantly stained and their
relationships more readily investigated. Select a Ihin well-cut
area in the collenchyma and study llie cells under hiizh magnifica-
tion. The cell cavities have a more or less undulate outline which
is the i-esnlt of the alternation of thin and greatlv thickened

1 Cf. Appendix, p. 142.

REFERENCES 65

areas of the primary wall. These thickened areas will be seen
to be restricted largely to the cell-corners, hence the term "angu-
lar" collenchyma. Especially in sections stained with neutral
red, the wall thickenings exhibit evidence of a lamellated struc-
ture. Observe that "spaces," varying greatly in size and dis-
tribution, occur throughout the collenchyma tissue. Some of
these apparent "spaces" represent the trans-sectional appear-
ance of the narrow tapering ends of collenchyma cells ; others are
true intercellular air spaces. The distinction between these two
conditions is best appreciated, however, by a study of longi-
sections of the tissue. Note the highly vacuolate cytoplasm and
the chloroplasts in the collenchyma cells.

To understand thoroughly the shape of collenchyma cells and
the distribution of the thickened areas of their primary walls,
examine under low and high magnification longi-sections of the
Datura petiole. Study carefully the band-like thickenings of the
wall and note their relationship in adjacent cells. The inter-
cellular spaces should now be more evident. Simple pits may be
seen in both face and section views. Observe that individual
cells are often subdivided by thin transverse or sloping walls.

III. Suggested Drawings and Notes. —

1. Draw on a large scale a sector of the trans-section of the
petiole of Datura (or of the substitute material previously listed)
about 5 to 6 cells in width, extending from the epidermis to the
cortical parenchyma tissue. Label carefully all important
structures.

2. Prepare a drawing of a small portion of the collenchyma
tissue as seen in longi-sectional view. Show clearly the distribu-
tion of the thickened areas of the wall, the pitting and the inter-
cellular spaces. Fill in the contents of a single collenchyma cell.

3. Summarize, in the form of laboratory notes, the evidence
discussed by Esau (1936) which shows that the walls of collen-
chyma cells are rich in water.

REFERENCES

1. Anderson, D., Ueber die Struktur der Kollenchymzellwand
auf Grund mikrochemischer Untersuehuiigen. Sitzber.
Akad. Wiss. Wien. Math.— Natur. Kl. 136 .-429-440. 1927.

66 COLLENCnYMA CELLS

2. De Baiy, pp. ll'J-12U.

3. Eames and ^lacUaiiiels, pp. 54-56.

4. Esau, K., Ontogeny and Structure of Collenchyma and of
Vascular Tissues in Celery Petioles. Ililgardia 10:431-467.
1936.

5. Ilaberlandt. i^p. 155-158.

6. Hay ward, p. 22.

Exercise VIII

SCLEREIDES

I. Introduction. — From tlie standpoint of function, two gen-
eral types of strengthening or "mechanical" tissues are conven-
tionally distinguished, viz.: (1) collenchyma, which is composed
of living cells and which is the first strengthening tissue to appear
in the development of the stem and leaf, and (2) sclerenchyma
which is made up of thick-walled cells, usually lacking a proto-
plast at maturity and which represents the "permanent" and
more important of the supporting tissues in the older portions of
the plant body. From a morphological standpoint, many authors
(e.g., Eames and INIacDaniels, Ilayward) adopt the viewpoint of
De Bary (pp. 126-184) and recognize two main "forms" of
sclerenchyma; namely, "stone cells" which are short, more or
less isodiametric elements and fihers which are prosenchymatous
cells, often extremely long and with pointed or oblique ends.
Despite the convenience of this morphological subdivision of
sclerenchyma it seems hardly justified when the numerous struc-
tural and ontogenetic differences between so-called stone cells
and fibers are clearly understood. For this reason, the term
"sclerenchyma" appears vague and highly abstract and, as will
appear in the resume to follow, has been used to designate cell
types which are definitely unrelated in wall structure and method
of development (cf. Ilaberlandt, p. 721, note 92).

The expression "sclereide" was originally proposed by
Tschirch (1885, p. 308) for the variously formed thick-walled
cells which occur so commonly in the bark of woody seed plants,
in the hard shell of fruits, and in seed coats. In contrast to
typical "bast fibers," the thick walls of sclereides often appear
yellow in color, are usually highly lignified and possess tubular
pits which may branch in a complex fashion. On the basis of
form and structure, Tschirch (1889, pp. 301-302) distinguished
four principal types of sclereides which he named and described

67

68 SCLEREIDES

as follows: (1) Brachysclcreides, or "stone-cells" which are
roughly isodiametric in form and which occur in the fleshy por-
tion of the fruits of Pijnis and Cyclonia and in the bark of numer-
ous woody dicotyledons; (2) Macrosclereides, or "rod-cells"
which are columnar in form and often constitute an outer con-
tinuous palisade-like layer in the coat of seeds, especially in the
family Leguminosae. Here they are also referred to as "Mal-
pigliian Cells"; (3) Osteosdereides, or "bone-cells" which are
likewise columnar in form but possess dilated or knob-like ends.
Such cells occur within the palisade parenchyma in certain leaves
(cf. Haberlandt, p. 160, Fig. 52) ; and (4) Asirosclereides or
"branched sclereides," w^hich are highly irregular in form and
size with pointed "arms." Astrosclereides are well developed in
the leaves of certain dicotyledons (e.g., Thca, Camellia) and also
occur in the bark of Abies and Larix and in certain fruits (e.g.,
Carya).

The ontogeny of sclereides presents many distinctive and
peculiar features. Aside from the macrosclereides, which are
traceable in origin to the surface cells or "protoderm" of the
integument of the ovule (cf. Zimmerman, 1936), sclereides usu-
ally develop by the "secondary sclerosis" of parenchymatous
cells (cf. De Bary, pp. 539-544). This curious process involves
the marked centripetal increase in thickness of the cell wall, the
deposition of lignin within the cellulosic matrix and the produc-
tion of the characteristic "pit-canals" or "ramiform pits." The
physiological factors which induce these changes in a living par-
enchyma cell or cell group in the cortex or the bark of stems or
roots are obscure. Tschirch (1889, p. 303) states that in woody
dicotyledons, the process of secondary sclerosis occurs to such
an extent that sclereides may eventually constitute the major
porticm of the bark. A further interesting aspect of the process
of secondary sclerosis is exhibited by the development in many
stems of the so-called "composite cylinder" formed of both
brachysclcreides and bast fibers. In this case, an originally
continuous cylinder of bast fibers becomes ruptured at various
points as a result of the increase in thickness of the stem. Neigh-
boring parenchyma cells then intrude into the gaps, divide and
eventuallv become transformed into sclereides thus "repairing"

INTRODUCTION 69

the broken cj'linder (for details cf. Tschirch, 1885, p. 323 et seq.,
and llaberlandt, p. 159). With respect to the ontogeny of osteo-
selereides and astroselereides, little information appears to exist.
In the husk of the fruit of hickory (Carya) a comparison of
young and old material clearly suggests that the irregular astro-
selereides arise much like brachysclereides from the secondary
sclerosis of parenchymatous elements. But the details of the
development of the huge astroselereides occurring in the leaves
of such plants as Camellia (Ilaberlandt, p. 162, Fig. 54) deserve
careful investigation.

The statement is frequently made that stone-cells are devoid
of a protoplast at maturity. This idea requires further proof
because Alexandrov and Djaparidze (1927) contend that it is
possible to demonstrate, by staining with safranin and methyl
green, the presence of nuclei in the mature brachysclereides of
the fruits of quince (Cydonia) and pear (Pyrus). These inves-
tigators further maintain that during the ripening of the fruit
in Cydonia, the sclereides experience a process of " delignifica-
tion" consisting in the reduction in thickness of the wall, the dis-
appearance of lignin, and the obliteration of the ramiform pits.
This reversible change suggests enzymatic activity on the part of
the protoplasm within the stone cells. However, Crist and Batjer
(1931) reached a different conclusion in their detailed study of
the stone-cells of Pyrus. They state that the delignification re-
ported by Alexandrov and Djaparidze for Cydonia does not occur
in Kieff'er and Bartlett pears . . . ''without exception, the
downward trend of the cellulose curve is strictly parallel to that
of lignin and each one of the two is parallel to the ligno-cellulose
trend." Further study is obviously needed to determine more
precisely the chemical relations between the sclereides and the
neighboring parenchyma tissue during fruit ripening.

From a functional standpoint, sclereides undoubtedly impart
hardness to the organ in which they occur. Haberlandt (p. 158)
states that brachysclereides "serve to increase the incompressi-
bility of the bark ; their action may be compared to that of the
sand which a mason uses to increase the tenacity of his mortar,
or to that of the powdered glass which is added to gutta-percha
in order to render it less compressible." The functional signifi-

70 SCLEKEIDES

eaiice of the groups or ' ' nests ' ' of brachysclereides in fleshy fruits
is less evident. It has been suggested that phylogenetieally they
may represent the remains of a former continuous shell of stone
cells.

II. Material for the Study of Sclereides. —

1. Bruchysdereides or "stone-cells" in the fruit and fruit
stalk of Ptjrus. Obtain a small fragment of the fruit of pear and
mount it in water under a cover glass. Imbedded in the thin-
walled parenchymatous tissue will be found small groups or
"nests" of stone-cells which appear yellowish-brown in color.
Examining one of these groups of stone-cells under high magnifi-
cation note the form of tlie cells, their greatly thickened walls
and the characteristic ramiform piis. After this preliminary
examination, remove the cover-glass, add several large drops of
a saturated solution of pliloroglucinol ^ followed by a drop or two
of hydrochloric acid. Note the brilliant red color assumed by the
walls of the sclereides. Often this color change, which occurs
when lignin is present in the walls, aids in the study of the
branching and the relationship of the ramiform pits. Study
carefully the form of the pits in both sectional as well as face

view.

Secure a trans-section of the fruit stalk of Pyrus and treat
it with phloroglucinol and hydrochloric acid as described above.
Add a cover slip and examine the sectiou under both high and
low magnification. The edge of the section is formed of several
layers of cork the innermost cells of which are in contact with
the phcllogen or cork cambium. Internal to the phellogen occurs
the cortex which is composed of thick-walled parenchyma tissue
in which are imbedded nests of stone-cells. Progressing inwards,
there next occurs a "ring" of fihrovascular hundUs. Each of
these bundles consists of an external ca]) of phcrs (which are
usually less brilliantly stained than the stone-cells of the cortex),
a strand of phloem, and a strand of xylem. The pith of the fruit
stalk is composed of botli i);irone]iyma as well as groups of stone

cells.

2. Astrosclerridrs. Obtain a pi-eiiaration of nuicerated "husk"
of the fruit of (Uirya or Juylans and examine it under low mag-

1 Of. Appendix, p. 141.

SUGGESTED UKAWINCS AND AOTES 71

iiifieatioii. The two most abundant cell ty})e8 are (1) parenchyma
cells which vary from nearly isodiametric elements to irregular
forms, and (2) astrosclereides which exhibit remarkable varia-
tion in the form and proportion of the "arms." The thick
stratified wall and branched pits show very clearly in these
sclereides. It is instructive to note the frequent similarity be-
tween certain of the parenchyma cells and the sclereides, indicat-
ing the origin of the latter from parenchyma by the process of
secondary sclerosis w^hich has been described in the Introduction
of this exercise.

For comparative purposes, examine trans-sections of the
petiole of the leaf of Camellia after staining them in phloroglu-
cinol and hydrochloric acid. Study the sections under high
magnification and note the elaborately branched areas of astro-
sclereides. Sections through the lamina of the leaf should also
be stained and examined for the huge branched sclereides which
occur in the midst of the mesophyll.

3. Macrosclereides. Obtained a small amount of macerated
bean testa and examine it under low magnification noting the
small, tightly packed groups of columnar macrosclereides. Note
that the lumen of each macrosclereide is widest near the base of
the cell, being reduced to a narrow% virtually occluded channel
above. For illustrations of macrosclereides in situ in the testa
of seeds of the Leguminosae refer to Eames and MacDaniels (p.
294, Fig. 134c), Hay ward (p. 342, Fig. 174), and Netolitzky
(1926, p. 159).

III. Suggested Drawings and Notes.

1. Prepare drawings to show the form and pit relationships
of a small group of stone-cells in the fruit of the pear. How may
the fact be explained that some of the ramiform pits or their
branches fail to terminate at the "edge" of a stone-cell at a given
level of focus?

2. Make a diagrammatic drawing of the trans-section of the
fruit stalk of the pear, showdng and labeling all the important
tissues and regions. Summarize, in the form of notes, the posi-
tions and mechanical significance of the stone-cells and fibers in
this organ.

72 SCLEREIDES

3. Draw several types of astrosclereides from the macerated
' ' husk ' ' of the fruit of Carya or Juglans. For comparative pur-
poses, draw a parenchyma cell resembling in its form one of these
sclereides.

4. Prepare a diagrammatic drawing of the trans-section of
the petiole of Camellia, showing the position of the astrosclereides
with reference to other tissues. Draw a single astrosclereide, as
seen under high magnification, from the petiole and from the
lamina of the leaf.

5. Draw a few connected macrosclereides from the testa of
the bean seed. Show carefully the wall structure and the form
of tlie lumen.

REFERENCES

1. Alexandrov, W. G., and Djaparidze, L. I., Uber das Enthol-
zen und Verholzen der Zellhaut. Planta 4 :467-47o. 1927.

2. Crist, J. W. and Batjer, L. P., Tlie Stone Cells of Pear Fruits,
Especially the Kieffer Pear. Ag. Expt. Station Michigan
State College. Tech. Bulletin 113. 1931.

3. De Bary, pp. 126-128.

4. Eames and MacDaniels, pp. 57-59.

5. Haberlandt, pp. 158-161.

6. Hayward, p. 25.

7. Netolitzky, F., Anatomic der Angiospermen Samen. Iland-
buch d. Pflanzenanatomie. X. 1926.

8. Tobler, F., Die mechanischen Elemente und das mechanische
System. Handbucli d. Pflanzenanatomie. IV. 1939.

9. Tschirch, A., Beitrjige zur Kenntniss des mechanischen
Gewebesvstems der Pflanzon. Jahrb. Wiss. Bot. 16:303-335.
1885.

10. , Angewandte Pflanzenanatomie. Wien und

Leipzig. ]889.

11. Zimmermann, K.. Zur })hysiol()gisc]ier Anatomic der Legu-
minosentesta. Landw. Versuchs-stat. 127 :l-56. 1936.

Exercise IX

FIBERS

I. Introduction.— In the strict sense, the term "fiber" shonld
be applied only to certain prosenchymatons cells found in the
inner tissues or tissue systems of the plant body. Cells of this
type are not to be confused with the so-called cotton "fibers"
which morphologically represent epidermal hairs of the seed
coat (ef. Anderson and Kerr, 1938). The term "fiber" is em-
ployed in the present book in the above restricted anatomical

sense.

Fibers are the most important type of mechanical cell which
occur in higher plants where their great tensile strength, flexi-
bility and elasticity serve to enable plant organs successfully to
withstand a variety of strains and tensions resulting from the
action of gravity, wind. etc. (Cf. Haberlandt, pp. 161-164.) From
a commercial standpoint, many plants are cultivated largely or
exclusively for the fibers which they produce. Among the more
prominent of such textile plants may be mentioned Agave sp.,
the source of "Sisal Hemp," Musa textilis from which "Manila
Hemp" is derived, Cannabis sativa or the "true" hemp plant,
and Limnn usitatissimnni which furnishes the commercial flax
from which linen is derived. According to Hayward (p. 371),
"there is evidence that flax was grown during the Stone Age"
and that the annual form of Linum usitatissimum "has been
grown in Mesopotamia for at least 4000 years."

Because of the considerable economic importance of fibers, a
very extensive literature has developed. The limited scope of
this book however precludes any effort to discuss in detail the
many involved problems of wall structure and methods of de-
velopment of fibers. Instead, a brief resume is given now of the
salient features of fibers which may serve as an introduction to

73

74 FIBERS

the subject. For students wishing cidditional technical informa-
tion, reference should be made to the literature cited by Hayward
under Caiinahis (pp. 244-245) and Linuni (pp. 409-410).

1. Classification. Fibers, either singly or more commonly in
the form of strands or cylinders, are widely distributed in the
plant body. In stems and roots, fibers are commonly found in
the cortex, pericycle, phloem and xylem. In the leaves of many
monocotyledons (e.g., Miisa, palms, Agave, etc.), fibers are very
prominently developed, occurring as strands or sheaths which
accompany the vascular bundles; they may also appear inde-
pendently of the vascular strands, either as distinct bundles or
as massive hypodermal cylinders. (Cf. Ilaberlandt, pp. 168-184,
and Meeuse, 1938.) From a iopographical standpoint, two prin-
cipal "types" of fibers may be recognized, at least in stems and
roots which experience secondary growth in thickness, viz.: (1)
Bast or extracambial fibers, and (2) Wood fibers or intracambial
fibers. As Ilaberlandt (p. 155) has clearly emphasized, such a
distinction is quite arbitrary since bast fibers, as a class, cannot be
distinguished on a structural basis from wood fibers. Fames and
MacDaniels (p. 57) and Hay ward (p. 23) suggest that fiber types
should be more specifically designated according to the tissue or
tissue region in which they occur, e.g., cortical fibers, pericyclic
fibers, phloem fibers, etc. A classification of this kind, however,
necessarily depends upon accurate information on the origin and
development of the fibers in each particular case. This is very
clearly shown l)y the recent investigations of Esau (1938b, pp.
367-369) on the origin and development of the fibers in the stem
of tobacco. In this plant, Esau interprets the fibers morpho-
logically as part of the primary phloem rather than as "peri-
cyclic fibers," as has been done by certain workers. It is evident
from Esau's discussion of the literature that there is a great need
for a complete re-examination of the concept of "pericycle" from
an histogenetic point of view. Under such circumstances, the
classifications of fibers into "bast fibers" and "wood fibers" will
be followed in this book largely for the sake of simplicity and
convenience.

2. Form and length of fihcrs. Fibers are classical examples
of typical prosenchymatous cells. Tlie ends are either acute or

STRUCTURE AND CHEMISTRY OK CELL WALL 75

acuminate or, as in certain bast fibers, variously "lobed" or
"branched" (Mansfield, p. 22, Fig. 2). In short fibers, the
ratio of the diameter of the cell to its length may average 1 :10
to 1 :20 while in extreme cases (e.g., in the Urticaceae) the ratio
may reach or exceed 1:4000. These data, taken from De Bary
(p. 13), emphasize the fact that certain bast fibers may represent
the largest of all cells in higher plants. According to Hay ward's
(p. 241) discussion of the literature, hemp (Cannabis sativa)
fibers vary in length from 1-10 em. In flax (Linnm) the length
of the fibers likewise varies from 2.5 cm. to as high as 12 cm.
Apparently the longest bast fibers which have been accurately
measured occur in the stem of Boehmeria nivea, a member of
the Urticaceae. In this species, Aldaba (1927) succeeded, by
means of a special maceration technique, in isolating individual
fibers, the five longest of M'hich measured ' ' respectively 400, 500,
520, 540, and 550 mm."

3. Structure and chemistry of the cell wall. Mature fibers
characteristically possess a well-defined secondary wall which
is often so thick that the cell lumen may be almost or entirely
occluded at various points. The thick secondary wall exhibits
typically slit-like vestigial pits which in bast fibers are disposed
obliquely in a left-handed spiral series. Haberlandt (p. 154)
contends that this arrangement of pits indicates a "corresponding
arrangement of the micellar rows" and that "an obliquely pitted
bast-fiber may therefore be regarded as an aggregate of exceed-
ingly numerous and delicate fibrillae twisted together into a spiral
coil of many turns which surrounds a longitudinal canal consist-
ing of the cell cavity. ' ' Because of the great economic importance
of certain fibers, many studies have been devoted to the chemical
composition of their walls. Ilayward (p. 23), while admitting
that the degree of lignification of the cell wall may vary even
within the same zone of fibers, distinguishes between (1) non-
sclerotic fibers, which occur commonly in the pericycle of stems
and w'hich possess secondary walls with a relativelj^ high propor-
tion of cellulose (e.g., Linum), and (2) sclcrenchymatoiis fibers,
which are part of the xylem and which exhibit highly lignified
secondary walls. According to Ilayward, lignification tends to
render fibers rather brittle while the high cellulose content of

76 FIBERS

the walls of certain bast fibers is related to the greater tensile
strength and durability of such elements.

4. Ontogeny of fibers. Regardless of their location in the
plant, fibers arise from initial cells which are very short as com-
pared with the length of the mature element. An impressive
example of this fact is furnished by Aldaba's (1927) work on
fiber development in Bochmeria. In this plant, the fiber initials
"are approximately 20 microns in length" and "the increase in
the longitudinal dimension of the longer bast fibers is of the order
of 2,500,000 per cent, but the process of elongation is gradual
and extends over a number of months." The mechanics of the
process of elongation in fibers and the accompanying development
of a thick secondary wall has attracted much attention as well
as speculation. The investigations of Aldaba (1927) and An-
derson (1927) on flax fibers have revealed many peculiarities
but our knowledge of fiber development in other forms is still
meagre. It is apparent that in certain bast fibers, the upper
end of the element remains delicate and active during the phase
of cell elongation. Whether the necessary adjustment between
such greatly extending cells and tiieir neighbors is achieved by
"sliding" growth or by "differential" growth, however, is not
clear. (Cf. Anderson 1927, Meeuse 1938 and Ilayward 1938, pp.
395-400.) The behavior of the protoplast during the growth
and differentiation of certain types of fibers offers a number of
points of interest. In a recent study, Esau (1938a) has shown
that during the elongation of the primary phloem fibers in tobacco,
the protoplasts become multinucleate as a result of repeated mi-
totic divisions of the nuclei. Cell plates, howevei-, do not form
at the end of the successive nuclear divisions and "the spindle
fibers are less persistent than in ordinary division figures." At
the final stages in fiber ontogeny, usually after secondary walls
have developed, the nuclei appear to fuse or clumii and in nearly
mature fibers "the nuclear material frequently occurs as one
large degenerating mass." The physiological significance of this
multinucleate condition in young phloem fillers is quite obscure.
Ilaberlandt (p. 154) wiio has observed a multinucleate protoplast
in the bast fibers of TAnmn and certain members of the Legu-
minosae maintains that "the presence of several nuclei appears

MATERIAL FOR STUDY OF FIBERS 77

advantageous when the very considerable length of many bast-cells
and their active growth in length and thickness are taken into
account." In certain types of w'ood fibers, however, mitosis is
followed by cytokinesis, resulting in a chambered or septate fiber.
This condition has been observed and described by Vestal and
Vestal (19-40) in a recent study of the septate fiber-tracheids
of Hypericum Androsaeminn. In this species, the fiber-tracheid
retains its protoplast after the thick secondary wall has been
laid down. Mitosis may then occur in such a cell, the division
figure being oriented parallel to the long axis of the cell. Cell
plate formation then occurs in the normal manner and a thin
transverse septum is formed across the lumen, intersecting the
inner edge of the secondary wall of the "mother cell." Because
of the delicacy of this septum it Avas not possible to determine
whether it is "formed only of intercellular cement substance
or whether it consists of the intercellular substance and two
adjacent primary walls."

II. Material for the Study of Fibers. —

1. Bast fibers. Examine, under low and high magnification,
macerated bark of the twig of the basswood or linden tree (Tilia
sp.). The numerous prosenchymatous cells present are bast fibers
from the phloem region. Select an unbroken fiber and study
carefully its form and wall structure. Note especially the
channel-like lumen and the small vestigial pits. According to
Eames and MacDaniels (p. 57), the pits in bast fibers represent
modified simple pits. To appreciate fully the arrangement and
mechanical significance of the fibers in the phloem of Tilia, strip
off a small portion of the bark from a twig or young branch and
scrape off the outer tissues (i.e. epidermis, periderm and cortex)
with a scalpel. Then mount the fibrous tissue which has been
exposed in water and examine it under low magnification, noting
the closely-joined strands of grayish-white bast fibers. In order
to determine the degree of lignifieation of the secondary walls,
treat separate portions of the fibrous network with (1) IKI and
sulphuric acid,^ and (2) phloroglucinol and hydrochloric acid.-

1 Cf. Appendix, p. 141.

2 Cf. Appendix, p. 141.

78

FIBERS

If time permits, make similar studies and microehemical tests
of the bast fibers of various economically important textile plants,
such as "hemp" (Agave sp. and Ca7inabis sativa) and flax
(Linum u.sitatissimuni).

2. Wood fibers.

(a) Lihriform fibers. The fibers present in the secondary
xylem of woody dicotyledons often show massively thickened
secondary walls provided with scattered and rather small vestigial
pits. In such cells, which were termed "libriform" because of
their structural resemblance to phloem fibers, the lumen varies
in width and may be entirely occluded at certain points (cf.
Eames and MacDaniels, p. 63, Fig. 34, c, f, g). Study the form
and structure of the libriform fibers as shown in macerated wood
of oak (Qucrcns sp.).

(&) Septate fibers. This type of wood fiber is characterized
by the subdivision of the lumen into a series of compartments
which are separated from each other by transverse walls or septa.
The work of Vestal and Vestal (1940) discussed in the Introduc-
tion of this exercise has shown that in Hypericum, the septa of
the fiber-tracheids arise, after the formation of the lateral second-
ary wall, as a result of repeated mitoses accompanied by cyto-
kinesis. Doubtless the septate fibers in other genera of the
angiosperms pursue a similar ontogeny. Secure a small amount
of macerated xylem from the stem of the grape-vine {Vitis sp.)
and study under high magnification the form and structure of
the numerous septate fibers. Note that the septa in these cells
extend to the inner edge of the secondary wall but are independent
of the compound middle lamella of the "mother cell." If the
septa are examined with the aid of an oil-immersion lens, it is
apparent that they have a laminated as well as a "pitted" struc-
ture.

III. Suggested Drawings and Notes. —

1. Prepare careful drawings to illustrate the form, character
of the lumeu and the type and distribution of pits in the bast
fiber of Tilid and the libriform fiber of Qucrcus. Label all im-
portant structures.

2. Prepare a diagram to illustrate the arrangement of the
strands of bast fibers in the stem of Tilia.

REFERENCES 79

3. Draw a single septate fiber from the secondary xylem of
Vitis showing its form, pitting and the position and structure of
the septa.

4. Prepare a brief resume of the commercial process of "ret-
ting" fibers in hemp or flax (for literature and information ef.
Hayward, pp. 242, 244-245 and 409-410, and Anderson, 1927).

REFERENCES

1. Aldaba, V. C, The Structure and Development of the Cell
Wall in Plants, I. Bast fibers of Boehmeria and Linum.. Amer.
Jour. Bot. 14:16-24. 1927.

2. Anderson, D. B., A Microchemical Study of the Structure
and Development of Flax Fibers. Amer. Jour. Bot. 14 -187-
211. 1927.

3- , and Kerr, T., Growth and Structure of Cotton

Fiber. Indus, and Eng. Chemistry 30 :48-54. 1938

4. De Bary, pp. 128-134.

5. Fames and MacDaniels, pp. 56-57, 62-64 and p. 75.

6. Esau, K., The Multinucleate Condition in Fibers of Tobacco.
Hilgardia 11 :427-434. 1938a.

^- , . Ontogeny and Structure of the Phloem of

Tobacco. 7i/f/., 11:343-424. 1938b.

8. Haberlandt, pp. 152-155.

9. Hayward, pp. 23-25.

10. Mansfield, pp. 89-106.

11. Meeuse, A. D. J., Development and Growth of the Scleren-
chyma Fibers and Some Remarks on the Development of the
Tracheids in Some ^lonocotvledons. Rec. trav bot neer-
landais 35 : 288-321. 1938.

12. Vestal, P. A. and Vestal, M. P., The Formation of Septa in
the Fiber-tracheids of Hypericum Androsaemuin L. Harvard
Bot. Mus. Leaflets 8 :169-180. 1940.

Exercise X

TRACHEARY ELEMENTS

I. Introduction. — One of the most characteristic features of
the sporophyte of the "Tracheophyta" is the presence of a well-
defined conductive or vascular system. This system is his-
tologically complex in that its two component "tissues," i.e.,
phloem and .rylem, are formed of a variety of cell types differing-
in their arrangement, form, protoplasts and wall structure. While
the functions of storage and mechanical support are performed
to varying degrees by both the phloem and the xylem, these
"complex tissues" are concerned first of all in the trans-location
of water and solutes between root and shoot. This conductive
function is possible not only because of the structural character-
istics of the cells themselves but also because the vascular tissues
in the root, stem and leaves are interconnected and form a con-
tinuous system. From this standpoint, the vascular system may
be visualized as an internal "skeletal" framework to which new
increments are added during growth by the activity of the apical
meristems and, in plants with secondary growth, the vascular
cambium. This exercise is devoted to an introductory study of
the xylem, with particular emphasis upon the structure and
development of its definitive tracheary elements, viz. : the tracheid
and the vessel. In the exercise dealing with the anatomy of the
root, stem and leaf, additional information regarding xylem aiul
directions for studying it will be given.

1. Structure and morphology of tracheary elements. The ex-
pression "tracheary elements" is used in this book to designate
collectively the tracheid and the vessel element which represent
the two chief types of water-conducting cells present in the
xylem of vascular plants. The following characteristics are com-
mon to both tracheids and vessel elements, viz.: (1) they are
typically prosenchymatous in form with oblique or pointed ends.
An exception to this is furnished by tiie cylindi-ical form of vessel

80

INTRODUCTION 81

elements in certain angiosperms. (2) The secondary wall of ma-
ture traclieary elements consists of lignified cellulose and is
deposited as rings, spiral bands, bars, a reticulum or as pitted
layers upon the thin primary wall; (3) at maturity, traeheary
elements lack a protoplast and the lumen is occupied by gas
or fluids. The principal distinction between the two types of
traeheary elements consists in the fact that the tracheid is an
imperforate cell with a continuous primary wall, while a vessel
element is provided with distinct openings or perforations which
are usually located in the end-walls of the cell. When longi-
sections of the xylem are examined, it is evident that inter-
communication between adjacent tracheids is possible by means
of the bordered pit-pairs on their lateral walls. In contrast,
vessel elements occur in more or less distinct vertical series in
which the perforations of adjacent elements exactly coincide.
Thus, collectively regarded, a series of vessel elements constitutes
an open "pipe-like" structure which is termed a vessel. The
literature devoted to the structure and pitting of traeheary ele-
ments is so extensive that it will only be possible to outline below
some of the most salient features of tracheids and vessel elements.
(a) The tracheid. From a phylogenetic standpoint, the
tracheid is usually regarded as the "fundamental" cell type in
the xylem of vascular plants. According to Eames and Mac-
Daniels (p. 62), "tracheids alone probably made up the xylem
of very ancient plants." Among living plants, tracheids con-
stitute the only type of traeheary element in the xylem of most
lower vascular plants and, except for the Gnetales, are the
dominant cell type in the wood of gymnosperms. Tracheids are
also characteristic of angiospermous xylem where together with
vessel elements, fibers and parenchyma they contribute to pro-
duce the great histological complexity typical of the wood in this
group. Structurally, the tracheid is an elongate cell, the second-
ary wall of which is laid down in a variety of patterns. In
primary xylem, i.e., the xylem which develops first in the ontogeny
of the root, stem and leaf, the secondary wall has the form of
rings, spiral bands, bars, a network or else is provided with dis-
tinct pits. A more detailed discussion of the "fibrous" types of
secondary wall-thickening in the traeheary elements of primary

82 TBACHEARY ELEMENTS

xylem will be jiiven later in this Iiitrocluetioii. In secondary
xylem, i.e., the wood produced by the vascular cambium, the
walls of the tracheid are provided either with transverse, slit-like
scalariform bordered pits (as in ferns, and club mosses) or with
circular or oval bordered pits (as in most gymnosperms and
angiosperms) . The type and arrangement of the pits of tracheids
seem to be determined in part by the nature of the cell or cells
bordering the tracheid. Thus when two tracheids are in contact,
bordered pit-pairs occur while, if a wood-parenchyma or wood-
ray cell crosses the tracheid, half-bordered pit-pairs are developed.
Frost (1929) in a study of angiosperm xylem, however, has called
attention to the fact that the type of pitting in a given tracheary
element depends to a large extent upon the degree of phylogenetic
specialization of the cell itself rather than upon the type of
neighboring cells. Frost finds that the pit-pairs between tracheary
elements and parenchyma cells may be either bordered, half-
bordered, or simple, according to the species and that this situa-
tion constitutes a reliable criterion for distinguishing the xylem
of various plants. It seems clear that the whole question of
tracheary pitting demands more study both from a comparative
as well as an ontogenetic point of view. Indeed, considerable
diversity of opinion prevails as to the nature of the most primitive
type of pitting in seed-plants (cf. Jeffrey Ch. IV and VII, Brown
1918 and Bliss 1921).

(6 ) The vessel clement. This type of tracheary element is gen-
erally interpreted as having evolved, phylogenetically, from some
primitive type of tracheid. In the genus Ephedra for example,
the sloping end walls of certain of the tracheary elements are
provided with circular bordered pits, the membranes of some of
which disappear during ontogeny. Cells of this type may typify
one of the ways in which the perforations, distinctive of vessel
elements, may have orginated (cf. Jeffrey, pp. 94-95, Figs. 72-
73). Further evidence of the derived nature of vessel elements
is afforded by their distribution in extinct and living vascular
plants. According to Jeffrey (p. 93), vessels are absent from
the secondary wood of Paleozoic cryptogams. Among existing
lower vascular plants, vessels are known to occur in certain species
of Sclaginella (Duerden 1934) and in two species of Pteridiitm

ONTOGENY OF TRACHEARY ELEiVlENTS 8')

(Bliss 1939). Save for these examples, and the Gnetales, vessel
elements seem to be restricted in their occurrence to the xylem
of angiosperms. The recent survey of Cheadle ( 1939 ) has brought
out the interesting fact that while vessels occur consistently in
the roots of monocotyledons, this type of tracheary element is
comparatively infrequent in the xylem of the stems and leaves
of this class of the angiosperms. The significance of this condi-
tion remains to be explained. In the dicotyledons, vessels appar-
ently are of widespread occurrence in both primary and second-
ary xylem, and have only been reported absent in Drimys,
Trochodcudron, Tetraccntron (cf. Bailey and Thompson 1918)
and certain members of the Crassulaeeae and Cactaceae.

Two principal types of perforations occur in vessel elements,
viz.: (1) the simple perforation, which appears as a single large
oval or circular hole in each of the end-walls of the cell, a;nd
which is interpreted as the more advanced condition, and (2)
the sealariform perforation or perhaps more accurately the
scalariform perforation plate, which appears as a series of elon-
gated parallel openings separated by transverse bars, and which
is usually regarded as the more primitive type of perforation.
Simple perforations occur in vessel elements with either sloping
or transverse end-walls while scalariform perforations are typical
of elements with oblique or strongly-inclined end-walls (cf.
Eames and MacDaniels p. 65, Fig. 35). According to the in-
A-estigations of Cheadle (1939), in the majority of monocoty-
ledons examined the vessel elements possess the scalariform type
of perforation plate. A full discussion of the possible evolution-
ary history of the perforation-plate in vessel elements of the seed
plants is given by Jeffrey (pp. 92-102) and Bliss (1921).
2. Ontogeny of tracheary elements.

(a) The formation of perforations in vessel elements. Re-
cent studies of this problem have centered about two important
questions, viz. : (1) the exact period or time in vessel differentia-
tion when the dissolution of the end-walls occurs, and (2) the
physical and chemical nature of the portion of the end-wall which
becomes perforated. According to Eames and IMacDaniels (p.
151), the perforation of the transverse end-walls of the vessel ele-
ments in the secondary xylem of the black locust {Rohinia

84 TRACHEARY ELEMENTS

Pseudo-Acacia) occurs after the cells have reached their full size
and have developed secondary walls over all portions of the ele-
ment. Perforation in this species therefore involves the dissolu-
tion of a portion of hoth the secondary as well as the primary
layers from the central region of the end-walls. Esau (1936),
however, in her study of vessel development in the primary
xylem of celery, found that the secondary thickening', in the form
of spiral bands, is restricted to the lateral walls and to a thin
peripheral region of the transverse end-walls. The perforation in
this case involves the breakdown of a distinct central region of
the end wall which is interpreted as primary in nature. Tliis area
in section view appears as a lenticular thickening and is "similar
to the torus tliickening in bordered pits." In a later publication.
Esau and Hewitt (1940) investigated the nature of the end-walls
and the development of perforations in the vessel elements of
Cucurhita pepo, Zea mays, Nicotiana tahaciim, Daucus carota and
Beta vulgaris. The end-walls of Beta, Daucus and Nicotiana
agree with those in celery in possessing a conspicuous lens-shaped
thickening which breaks down to form the simple perforation
during the final stages of vessel development, after secondary
walls have been formed. Perforation of tlie vessel elements in
Cucurhita occurs at a similar period in ontogeny but in this form
the portion of the end-wall to be dissolved is not lens-shaped in
sectional view. As a' result of careful microchemical and optical
tests of the end-walls. Esau and Hewitt conclude that "two
superimposed vessel elements are separated from each other by
two cellulose layers — the two pi-iniary walls — cemented together
by isotropic intercellular substance."

(h) The nature and origin of "fibrous" thicl-eni)igs in pri-
mary xylem tracheae. The secondary wall of the tracheary ele-
ments in ])i"imai'y xylem is deposited ujion the delicate ju-imary
wall in a number of well-defined patterns. In the proto.rylem or
first-formed jiortion of the ]U'imary xylem, the secondary wall ap-
pears in the form of separate rings (annular elements), one or
more spiral bands (spiral elements) or as transverse, intercon-
nected bars (scalariform elements). These distinctive types of
tracheary elements usually originate successively in provascular
tissue ("procambium") in the order named above, although the

ONTOGETSTY OF TRACHEABY ELEMENTS 85

proportion of each t.vi)e varies within wide limits in different or-
gans and diff'erent plants. The assnmption is frequently made
that the significance of these various wall-patterns in protoxylem
tracheae is to permit the elements to "accommodate themselves"
to stretching. In the metaxylein or last-formed portion of the
primary xylcm, there is a marked increase in the relative extent
of secondary wall deposition, and elements with net-like thick-
enings {reticulate elements) and with pitted walls {pitted ele-
ments) are formed. A sharp transition does not exist, however,
between these varied wall-patterns and consequently the limits
between protoxylem and metaxylem can only be rather arbitrarily
established. Indeed, it is common to find tracheary elements with
several types of intergrading wall-patterns. Such transitional
types were early recognized and termed "vasa mixta." (Cf.
De Bar}', p. 156.)

Apparently very little intensive study has been devoted to the
origin and mode of development of the fibrous type of secondary
wall thickening in primary xylem tracheae. Stover (1924) con-
tends that in Calamovilfa "the annular and spiral thickenings
are the direct result of elongation." He finds that the "first
thickening is laid down in the pitted form" and that "the division
and enlargement of the surrounding cells tears apart this wall
thickening and the cell becomes annular, spiral or reticulate,
depending upon the amount of stretching." This interesting
mechanical interpretation, however, could not be confirmed by
the work of Barkley (1927) on the differentiation of tracheary
elements in Trichosanthes. According to Barkley. the future pat-
tern of the secondary wall is determined early in ontogeny by a
peculiar distribution of vacuoles in the peripheral cytoplasm of
the procambial initial. She states that "the spiral vessel of the
protoxylem in its early stages has bands of peripheral cytoplasm
which precede the spiral markings and have the same arrange-
ment, and become the basis of the lignified spiral. The position
of the cytoplasmic bands is determined by rows of vacuoles in the
cytoplasm immediately preceding and during the formation of
the cytoplasmic bands." In a similar way, the annular and
reticulate types of thickenings are predetermined by the pattern
of vacuolation in the cytoplasm. It is evident, however, that a

86 TRACIIEARY ELEMENTS

careful eytologieal .study of this problem from an extensive as
Avell as an intensive standpoint is needed in advance of any
generalizations. With the aid of recent improvements in plant
microtechnique, it should be possible to investigate successfully
the precise relationships between the cytoplasm and the develop-
ment of specific wall-patterns in tracheary elements.

3. The distinction between primary and secondary vascular
tissues. In practice, great difficulty is experienced in attempting
to distinguish the boundaries between the primary and secondary
vascular tissues especially in leaves and young stems. Protoxy-
lem, because of the definitive characters of the secondary wall
patterns, is usually readily demarcated but the limits between
metaphloem and secondary phloem on the one hand, and meta-
xylem and secondary xylem on the other hand, are often difficult
or indeed impossible to draw on the basis of adult structure. One
of the criteria often employed is based on the idea that the con-
ducting elements of secondary vascular tissues, since they orig-
inate from periclinal derivatives of the cambium, are arranged
in more or less regular radial rows in contrast to their irregular
arrangement in primary phloem and primary xylem. In a recent
review of this whole problem, Esau (1938, pp. 356-361) has shown,
however, that according to many ontogenetic studies, the first-
formed or "primary" vascular tissues "may be arranged in an
orderly manner." A good illustration is furnished by the radial
alignment of both "primary" and "secondary" tracheary ele-
ments in the vascular bundle of Trifolium (Eames and Mac-
Daniels, p. 255, Fig. 117A). Evidently then, the method of ar-
rangement of tracheary cells does not provide a consistent basis
for demarcating pi-imary and secondary xylem. However, when
certain differences are analysed between the so-called procam-
bium, wliich produces the primary vascular tissue and the "cam-
bium" which forms secondary vascular tissues, it appears tiiat
the distinction between the two tissues may have some justifica-
tion ontogenetically.

According to Esau, four significant differences can be recog-
nized, viz.: (1) in organs with well-defined secondary growth,
two kinds of initials, the /v/// and fusiform types, are character-
istic of the cambium. These initials produce respectively the

STUDY OF TKACIIEARY ELEMENTS IN PRIMARY XYLEM 87

vascular rays, aiiel the sieve-tubes, vertical i)areiichyma, fibers
and traclieary elements. Usually procanibial cells are very much
alike in form and structure. (2) As seen in trans-section, pro-
canibial cells tend to be polygonal in form in contrast to the
rectangular or "box-like" shape of cambial cells. (3) The
maturation, especially of tracheary elements, from the cambium
is more abrupt than is the case in the procambium, and (4) the
radial walls of cambial cells are often noticeably thicker than
the tangential walls, a distinction which is not apparently char-
acteristic of procambial cells. In view of the above differences
and because of the exploratory state of the problem, the terms
"primary" and "secondary" will be utilized in this book as
convenient designations for the vascular tissues derived re-
spectively from the procambium and the cambium. But the
author is in full agreement with Esau's statement that procam-
bium and cambium are to be regarded "not as two distinct
meristems, but as two developmental stages of the vascular
meristem."

II. Material for the Study of Tracheary Elements in Pri-
mary Xylem. —

1. Transverse and lo7igi-sections of the hypocotyl of hean
seedlings. Treat the sections with phloroglucinol and hydro-
chloric acid,^ mount in water and examine under low and high
magnification. Note carefully the irregular arrangement of the
tracheary elements in the differentiating primary xylem as shown
in the trans-sections. The longi-sections, if cut in the radial plane
with respect to the primary xylem, will show the order of appear-
ance and the types of secondary wall-patterns in the tracheary
elements of the protoxylem and metaxylem.

2. Prepared and stained transverse and longi-sections of the
stem of Trifolinm,. Examine the trans-sections noting partic-
ularly the regular arrangement of the tracheary elements in the
primary xylem of the collateral vascular bundles. A study of
the longi-sections will reveal not only the various types of wall-
patterns in the successive tracheary elements, but will also show
the effects of stretching on the primary and secondary walls of
the annular and spiral elements of the protoxylem.

1 Cf. Appendix, p. 141.

88 TRACIIEARY ELEMENTS

3. Maceruicd primary .rtjlein of ihc liypocotyl of hean ami
the ston of Trifolium. Ol)taiii small quantities of the macerated
tissue and study the form and wall-patterns of tlie various types
of isolated tracheary elements. Although many of the elements
may be broken or injured in the process of maceration, it is
possible to find intact tracheids or vessel elements. Note especially
the intergradations in certain elements between several different
types of secondary wall thickening. It is also instructive to
contrast the appearance of the rings and spiral bands in short
and elongated protoxylem elements.

4. Macerated primary xyleyn of the rhizome of the bracken
fern {Ptericlium latiusculum) . Obtain a small amount of macer-
ated primary xylem of Pteridium, mount it in water and add a
cover-glass. The individual tracheary elements are large cells,
clearly visible to the naked eye, and vary in form from broadly-
fusiform or obovate to narroAvly-acuminate. Examine these cells
under low and high magnification, noting that one or both end-
walls are oblique with reference to the lateral walls. The latter
are provided with vertical series of typical scalariform bordered
pits. The presence of well-defined scalariform perforation plates
on the sloping end-walls of certain of these cells indicates that
they represent vessel elements. According to the recent work of
Bliss (1939, p. 620) "there are many cells that may be inter-
preted as transitional between the tracheid and the vessel ele-
ment."

III. Material for the Study of Tracheary Elements in Sec-
ondary Xylem. —

1. Tracheids of (jymuosperms. Obtain a small quantity of
macerated wood of Finns and study the form and pitting of the
tracheids. Three types of elements are present, viz. : (1) tracheids
from the "spring wood," characterized by their relatively wide
lumina and by the restriction of pits to the radial walls; (2)
tracheids from the "sununer wood," distinguished by their much
narrower lumiiui and by having the pits confined to the tangential
walls, and (3) fiber-tracheids which in their thickened walls and
reduced pits are intermediate in character between "typical"
fibers and tracheids. Note, especially in the spring tracheids,
that at certain points in the cell there occur groups of large, in-

STUDY OF TRACIIEARY ELEMENTS IX SECONDARY XYLEM 89

distinctly bordered pits whieli mark tiie point of contact between
the tracheid and the living cells of a wood-raj'. As seen in face
view, the large circular bordered pits of a spring tracheid are
separated from one another by bars of wall substance (cf. Eames
and MacDaniels, p. 29, Fig. 17A). These wall-sculpterings are
termed "Bars of Sanio" and have occasioned much speculation
as to their significance. The Committee on Nomenclature of the
International Association of Wood Anatomists, however, sug-
gests that the term "Bars of Sanio" should be replaced by the
term "crassulae" which are defined as "thicker portions of the
intercellular layer and primary walls between primary pit
fields." To appreciate fully the form and distribution of pits in
conifer tracheids, a study should also be made of stained trans-
verse, radial and tangential sections of pine wood.

2. Traclieids of angiosperms. INIount a small quantity of
macerated oak wood in water and study carefully the form and
pitting of the tracheids. These cells are distinguished from the
very abundant wood fibers by their somewhat shorter length,
wider lumina and more obviously bordered pits. Many of the
tracheids will appear very irregular in contour with forked or
lobed ends. (Cf. Eames and MacDaniels, p. 60, Fig. 33D).
Under high magnification, note that the bordered pits are oval
in form, often crowded and somewhat smaller than the larger
circular bordered pits characteristic of the spring tracheids of
Pin us.

3. Vessel elements with sealariform perforations. Macerated
wood of birch (Betula) and of the tulip-tree {TAriocUndron) will
provide instructive examples of this primitive type of vessel
element. Note the variation in the number and relative width
of the slit-like openings in the oblique perforation plates of these
two plants, and the variety in type and arrangement of pits on
the lateral walls.

4. Vessel elements with simple perforaiions. As stated in the
Introduction of this exercise, the simple type of perforation may
occur in vessel elements with either oblique or more or less trans-
verse end-walls. The first condition is shown by certain of the
vessel elements in macerated oak wood. Note in this material that
the pointed ends of the vessel element extend beyond each of the

90 TRACIIEAKY ELEMENTS

large, oval terminal perforations. As in the vessel elements of
Bctula and Liricxlendron, various types and patterns of pits are
characteristic of the lateral walls. A study of the short cylindrical
type of vessel element, with greatly-enlarged simple perforations
in the transverse end-walls, may be made Avitli the aid of stem-
sections and macerated xylem of the pumpkin {Cucurbit a) . As
seen in macerated tissue, the individual vessel elements are some-
what drum-shaped with densely-pitted lateral walls. The
enormous diameter of these vessel elements and their union to
produce vessels may readily be studied in hand-sections stained
with phloroglucinol and hydrochloric acid. For a description
and illustrations of the development and structure of vessels in
Cucurhita reference should be made to the work of Esau and
Hewitt (1940).

5. The cellular composition of paper. A large proportion of
paper is obtained from the wood of certain gymnosperms (e.g.,
Abies, Picea) and angiosperms (e.g., Populus, Betula). The
first stage in the manufacture of pai)er from wood consists in
the mechanical and cliemical maceration of the x.ylem which re-
sults in the partial dissociation of its component cells. This
macerated xylem is commercially known as tvood pulp and after
being bleached, colored or "sized," according to the required use,
is compressed under great pressure into paper sheets (cf. Kellog,
1923 for further details). It is interesting to note, however,
that despite the drastic treatments involved in the production of
wood pulp, many of the tracheary elements are well preserved
and their structure and pitting is recognizable if small pieces of
soaked paper are carefully teased apart in water and examined
under the microscope. ]\Iake a study of the various types of cells
found in newspaper, blotting-paper and some cheap grade of
writing-paper.

IV. Suggested Drawings and Notes. —

1. Prepare drawings to show llie shape, structure, and ar-
rangement of the ])riniary xylem tracheae in the vascular bundle
of the bean hypocotyl or the stem of Trifoliuw. as seen in trans-
and longi-sectional view. These diawings shouUl be supple-
mented by showing, on a l;irge scab', the details of small jjortions
of the secondary wall-patterns of isolated elements of the proto-

REFERENCES 91

xylem and metaxylem drawn from macerated primary xylem of
the liypoeotyl of the bean seedling.

2. Outline, on a large scale, the form of a single vessel ele-
ment from the macerated primary xylem of Pteridmm, indicating
by horizontal lines the pattern of scalariform bordered pits on the
lateral walls. Draw, as seen under high magnification, a scalari-
form perforation plate of one of these tracheary elements.

3. Prepare drawings of spring and summer tracheids of
macerated pine wood, showing the type and arrangement of the
pits as seen in face and sectional view. For comparison, draw
a single tracheid from macerated oak wood.

4. Make drawings of vessel elements of macerated wood of
Liriodendron (or Bctida), QnercKs and Ciicnrbita, to illustrate
differences in the form of the elements and the type of perfora-
tion. Fill in the pits on a small portion of the lateral wall of each
vessel element.

5. Summarize, in tabular form, the various types of cells
observed in the specimens of paper studied. Indicate whether
differences in the kinds of tracheary elements observed can be
used to deduce the source of the paper in each case, i.e., from
conifer or angiosperm wood.

6. Prepare a' resume, based upon the conclusions of Jeffrey,
Brown (1918) and Bliss (1921) of the evolutionary development
of the vessel in seed plants.

REFERENCES

1. Bailey, I. W., and Thompson, W. P., Additional Notes upon
the Angiosperms, Tetracentron, Trochodcndron and Drimys
in which Vessels Are Absent from the Wood. Ann. Bot.
32:503-512. 1918.

2. Barkley, G., Differentiation of Vascular Bundle of Tricho-
santhes anguina. Bot. Gaz. 83 :173-184. 1927.

3. Bliss, M. C, The Vessel in Seed Plants. Bot. Gaz. 71 :314-
326. 1921.

4. , , The Tracheal Elements in the Ferns. Am.

Jour. Bot. 26 :620-624. 1939.

5. Brown, F. B. II., Scalariform Pitting a Primitive Feature in
Angiospermous Secondary Wood. Science 48 :16-18. 1918.

6. Cheadle, V. I., The Occurrence of Vessels in the Monocotyl-
edonae, Amer, Jour. Bot. 26. Supplement p. 9s. 1939.

92 TRACHEARY ELEMENTS

7. De Baiy, pp. 155-171.

8. Duerde'ii, 11., On the Occurrence of Vessels in Selaginclla.
Ann. Bot. 48 :459-465. 1984.

9. Eames and MacDaniels, pp. 59-62, 64-67, 86-97, 150-153,
162-188.

10. Esan, K., Vessel Development in Celery. Hilgardia 10 :479-
488. 1936.

11. , , Ontoo-env and Structure of the Phloem of

' Tobacco. liilcjardia 11 :343-424. 1938.

12. Esau, K., and Hewitt, W. B., Structure of End Walls in Dif-
ferentiatintj Vessels. Hilgardia 13:229-244. 1940.

13. Frost, F. II., Histology of the Wood of Angiosperms, I. The
nature of tlie pitting between tracheary and parenchymatous
elements. Bull. Torr. Bot. Club 56 :259-264. 1929.

14. Haberlandt, pp. 302-325.

15. Jeffrey, Ch. IV and VII.

16. Kellog, E. S., Pulpwood and Woodpulp in North America.
N. Y., 1923.

17. Stover, E. L., The Vascular Anatomy of Cahnnovilfa longi-
folia. Ohio -Jour. Sci. 24:169-179. 1924.

Exercise XI

SIEVE-TUBE ELEMENTS

I. Introduction. — The phloem of vascular plants, like the xylem,
is a "complex tissue" which may consist of four or five dif-
ferent types of cells. Nevertheless, the phloem is morphologi-
cally well defined by the consistent presence of a highly special-
ized kind of cell know^n as a sieve-tube element or sieve-cell. This
definitive cell type lacks a nucleus at maturity and in addition
possesses other structural and physiological properties which are
apparently unique. In many angiosperms, the sieve-tube ele-
ments are clearly arranged in definite vertical series, to each of
which the collective term "sieve-tube" may be applied. The
term sieve-tube has also been given to the individual enucleate
cells typical of the phloem in gymnosperms. Such cells, how-
ever, are not arranged in linear series. Abbe and Crafts (1939)
refer to these structures in conifers as "sieve elements." As
Esau (1939) has pointed out in a thorough review of the litera-
ture on phloem structure, the word "sieve-tube in its original
meaning had reference to a series of superposed cells with trans-
verse or somewhat inclined end walls bearing sieve-plates."
Under such circumstances, the term "sieve-tube" will be reserved
in this book for definable vertical cell-series and the term "sieve-
tube element" used for the individual members of such a series.
Similar cells of the phloem of gymnosperms, not arranged in
vertical superposed series, will be designated as ' ' sieve-cells.

In angiosperms, the sieve-tube elements are usually accom-
panied on one of their lateral walls by small prismatic or tubular
cells. These cells, which are intimately connected with the sieve-
tubes, are termed companion cells and differ from the sieve-tube
elements in possessing nuclei and in lacking definite sieve plates.
Companion cells are to be regarded as sister cells of the sieve-
tube elements since both types of cells originate by the division
of a common mother cell (cf. Esau, 1939, pp. 409-410). Sieve-

93

94 SIEVE-TUBE ELEMENTS

cells ill the gymnosperms lack companion cells in the above sense,
but specialized "albuminous cells" occur and have been regarded
as equivalent to the companion cells in angiosperms. However,
Abbe and Crafts (1939, p. 710) have questioned this analogy since
the albuminous cells "do not arise from a common fusiform
initial with the sieve-tube but from separate ray initials." The
primary phloem is usually relatively simple in structure, con-
sisting of sieve-tubes, companion cells and phloem parenchyma
as in Cucurbit a or only the first two types of cells may be present
(e.g., Zca Mays). But secondary phloem may be very complex
because of the presence of fibers, stone cells, vertical phloem
parenchyma and rays in addition to sieve-tubes and companion
cells. (Cf. Eames and MacDaniels, Ch. VIII, and Esau, 1939,
pp. 411-413.)

Aside from the secondary functions of food storage and sup-
port, the chief physiological role of phloem is the conduction of
various organic solutes. Experimental studies on translocation
(cf. Crafts, 1939a and 1939b) seem to indicate clearly that the
main channels of transport for organic materials, and also of
certain viruses are the sieve-tube elements. As to the mechanism
of this movement in sieve-tubes there is, however, considerable
disagreement. Crafts (1939a) on the basis of a wide series of
recent studies on this problem has developed a "pressure flow"
theory which he summarizes as follows: "In the pressure flow
mechanism, solvent and solute are assumed to flow together as
a solution thi-ough elements of specialized structure or per-
meability, the i)rotop]asm of which plays an entirely passive role
in the process. The sources of energy in this mechanism lie in
the osmotic activity of the products of assimilation in green por-
tions of the plant and in the accumulative ability of cells in grow-
ing and storing tissues." Other explanations of the mechanism
of transport in sieve-tube elements assume that protoplasmic
streaming or highly-active protoplasm is concerned in some way
with the process. One result of the lively interest in the function
of phloem has been a series of exploratory studies on the structure
and deveh)pment of sieve-tube elements (cf. Esau, 1938). The
recent literature in this field, as well as tlic historical background
of the various problems, have been discussed recently by Esau

PROTOPLAST OF SIEVE-TUBE ELEMENTS 95

(1939) and tlie following resume on sieve-tube elements is in-
tended simply as an introductory guide.

1. The protoplast of sieve-tube elements. One of the most
typical characters of the mature sieve-tube element is the absence
of a nucleus. Numerous developmental studies (cf. Esau, 1939,
p. 375 and 1941, pp. 452-454, PL 7) have shown that while the
yoking element possesses a normal nucleated protoplast, matura-
tion is accompanied by the eventual disintegration of the nucleus.
Prior to its breakdown, the nucleus has been observed to increase
significantly in size and to lovse its chromaticity. Crafts (1939a,
p. 176), in particular, has stressed the physiological significance
of the enucleate condition in the sieve-tube element as follows :
"The whole history of the sieve-tube portrays the intimate rela-
tion of the nucleus to the structure and function of the elements.
No student of ontogeny can fail to sense the influence that the
loss of nucleus has upon subsequent activity. From the begin-
ning of its functioning period to its death, the sieve-tube element
is doomed to a passive role, conditioned by its lack of nucleus
and consequent permeability." A peculiarity of the cytoplasm
of mature sieve-tube elements consists, according to the investi-
gations of Crafts, in its highly permeable nature. Following the
disappearance of the nucleus, Crafts finds that cytoplasmic
streaming ceases and the cytoplasm "fails to plasmolyse in hyper-
tonic solutions." Furthermore, the cytoplasm in the mature ele-
ment loses its former ability to accumulate neutral red, a vital
stain. Crafts (1939a, p. 175) interprets these facts as indicating
the highly permeable character of adult sieve-tube elements. As
Esau (1939, p. 403) has pointed out, the enucleate and permeable
cytoplasm of mature sieve-tube elements is not to be regarded
as "dead." This appears to be demonstrated by the continued
deposition of callus on the sieve-plates during late stages in
ontogeny. The contents of mature sieve-tube elements of certain
species (e.g., Cucurhita) consist of slimy proteinaceous material
which, in sections of phloem treated with heat or alcohol, may
coagulate on the sieve-plates and in the lumen to form funnel-
shaped structures known as slime-plugs. (Cf. Eames and Mac-
Daniels, pp. 193-194, Figs. 90C and 91C ; and Esau, 1939, pp.
379-383). The most recent evidence supports the belief that

96 SIEVE-TUBE ELEMENTS

slime-plugs are artifacts rather than structures peculiar to nor-
mal uninjured sieve-tube elements. The slime in sieve-tube ele-
ments originates from the disintegration of the slime-drops which
are commonly present in angiosperms as inclusions in the cyto-
plasm of 3'oung cells ; the disintegrated nucleus and the sieve-tube
sap also become part of the slimy contents of the sieve tubes. In
many plants, Icucoplasts and starch grains may be observed in the
sieve-tube elements (cf. Esau, 1939, pp. 384-386).

2. Sieve-plates and sieve-fields. In addition to the ultimate
loss of the nucleus, the mature sieve-tube element is characterized
by the presence of sieve-plates which occur in various regions of
the cell wall. The origin of the term "sieve-tube" rests upon
the erroneous idea that, in such a plant as Cucurhita, the end-
walls of the elements are perforated like a sieve. But modern
studies agree in showing that sieve-plates are not literally open,
perforated areas in the wall. On the contrary, the so-called pores
in the sieve-plate appear to be penetrated by either delicate or
rather coarse plasmodesmata. Crafts (1939a) has concluded that
these cytoplasmic strands, which thus connect the protoplasts of
adjacent sieve-tube elements, are solid rather than tubular in
structure as was maintained by certain earlier workers. Con-
siderable variation occurs with reference to the distribution of
sieve-plates in sieve-tube elements. In highly specialized sieve-
tubes (e.g., in Cucurhita), the transverse end-wall is occupied by
a single large plate with rather coarse plasmodesmata while the
lateral walls are provided with smaller, less distinct plate-like
areas. But several sieve areas forming a "compound sieve-plate"
may be present in end-walls which are inclined or sloping. In
the gymnosperms, very numerous small sieve-i)lates or "sieve-
pits" occur on the radial lateral walls (Abbe and Crafts, 1939).
The term ^'sieve-field" was originally applied by Niigeli to the
apparently reduced sieve-plates present on the lateral walls of
angiospermons sieve-tube elements. However, as Esau (1939,
p. 395) has clearly indicated, sieve-plates and sieve-fields may
intergrade in structui-e so that oidy an arbitrary distinction can
be made between them. The morphologiad nature of the sieve-
plate is still, to some extent, an unsolved problem. This is largely
the case because of the many gaps in our knowledge as to the

LATERAL WALLS OF SIEVE-TUBE ELEMENTS 97

method of development of tliese stractures. P'urthermore, the
terminology used in describing the adult sieve-plate and its
homologues is, as Esau (1939) has shown, in a confused state.
It does seem evident, however, that in some respects, sieve-plates
are fundamentally similar to simple pits (Esau, 1939, pp. 397-
399). In conifers, the sieve-plates arise directly from the large
primordial pits which are present on the radial walls of the young
sieve-cells. Also in many angiosperms, it is possible to trace the
origin of the sieve-plate to a primordial pit of a meristematic cell.
In some plants, however, such as Rohmia and Cucurhita, it does
not appear possible to refer the large solitary sieve-plates on the
end-walls to development from a single primary pit area. Esau
suggests that in Rohinia "one might assume that several shallow
primordial pits together form one sieve-plate, the single or
numerous plasmodesmata of one pit giving rise to one connecting
strand of a sieve-plate." During the development of the sieve-
plate, each plasmodesma or in gymnosperms each group of plas-
modesmata becomes surrounded by a cylinder of callus. The
chemical nature of this substance is still in question but callus-
cylinders are readily stained and differentiated by treating sec-
tions with aniline hluc. As the maturation of the sieve-tube ele-
ment progresses, the amount of callus on the sieve-plates increases
so that the originally separate cylinders become confluent or
fused and the plate becomes coated on both sides with callus.
According to Esau (1939, p. 391) this final accumulation is
" depiitive-c alius" and indicates "the approach of a functionless
state of the sieve-tube." There appears to exist no conclusive
evidence that definitive-callus blocks up the pores of the sieve-
plate. On the contrary, the plasmodesmata are noticeably
stretched and eventually die. In many plants, the definitive-
callus becomes dissolved away from the plate prior to the death
of the sieve-tube elements and their companion cells which even-
tually become obliterated or crushed by neighboring cells.

3. Lateral walls of sieve-tuhe elements. The lateral walls of
recently differentiated sieve-tube elements are frequently thick
and glistening in untreated sections. They have been termed
"nacre" because of this appearance. Chemically, these walls
appear to consist of cellulose and probably are, morphologically.

yS SIEVE-TUBE ELEMENTS

primary walls. In various members of the Abietineae, however,
Abbe and Crafts (1939) reported true secondary walls in the
sieve-elements of the secondary phloem. According to Esau's
review, the nature of the pits or protoplasmic connections of
sieve-tube elements with companion cells and phloem parenchyma
cells is not yet definitely established. In some cases at least, the
wall between the sieve-tube element and its companion cell is
penetrated by numerous scattered plasmodesmata.

II. Material for the Study of Sieve-Tube Elements. —

1. The phloem of Cucurbita. Obtain a thin transverse sec-
tion of the stem of pumpkin and examine its structure under low
magnification. In progressing from the edge of the section
toM'ards the center the following tissues may be observed, viz. :
(1) a typical uniseriate epidermis, certain cells of which have
developed into hairs; (2) a rather narrow eortex, consisting of
an outer, discontinuous zone of angular eollenchyma followed by
a region of parenchyma the innermost layer of which may contain
abundant starch grains and appear as a starch-sheath if the sec-
tion is stained in iodine; (3) the stele, the outer boundary of
which is clearly indicated by a continuous cylinder of thick-
walled pericyclic fibers. Internal to the fibers occurs a' broad
parenchymatous zone in tlie innei- portion of which are found
two "rings" of vascular bundles. Each bundle consists of a
median strand of xylem (characterized b}^ its large vessels)
flanked on both sides by a strand of phloem. A bundle of this
type is designated as a bicoUateral vascular bundle. Clear evi-
dence of a cambial zone may be seen between the xylem and each
of the phloem strands, especially in the larger inner bundles.
The center of the stem is occupied by an irregular cavity which
was produced by the collapse and disintegration of the pith. To
investigate the styurture of th( phloem, remove the cover-gla'ss
mid mount the section in a .1% aqueous solution of aniline blue.^
This dye "will stain the callus depositions on any of the sieve-
plates which may be present in the section. A careful study,
vnder high magnification, of the phloem of the various bi-col-
lateral bundles will usually reveal a number of large sieve-plates.
These structures in Cucurbita occupy virtually the entire end-

1 Cf. Appendix, p. 142.

SUGGESTED DRAWINGS AND NOTES 99

wall of the cylindrical sieve-tube elements. In critically stained
sieve-plates, each "pore" is occupied by a dark central spot,
representing- the large plasmodesma and is surrounded by a dis-
tinct callus-cylinder stained a light blue. If definitive-callm has
not yet appeared, the portions of the plate between the callus-
cylinders should appear unstained. Note that in addition to the
sieve-tubes, smaller companion cells appear and are distinguished
by their nucleated protoplasts and their triangular or quad-
rangular form as seen in trans-section. Phloem parenchyma is
also present but is difficult to distinguish from sieve-tube ele-
ments unless the latter exhibit sieve-plates. This study of phloem
should be continued with the aid of longi-sections which likewise
should be stained in aniline blue. Note carefully the appearance
and structure of the sieve-plates as seen in sectional view, and the
relation of the sieve-tube elements to the companion cells and the
phloem parenchyma. Often slime-plugs will appear in certain
of the sieve-tube elements. They can readily be induced by treat-
ing the section with 709^ alcohol.

2. The phloem of Rohinia and Finns. Mount thin radial and
tangential sections cut from living twigs of these genera in a .1%
aqueous solution of aniline blue and study under high magnifica-
tion the structure and distribution of the sieve-plates and the
form and relationship of the sieve-tube elements and sieve-cells.

III. Suggested Drawings and Notes. —

1. Diagram the general structure of a bicollateral bundle from
the stem of Cucurbit a as seen in trans-section. Label all essential
parts and indicate by circles the position of the largest vessels of
the xylem.

2. Draw small portions of the phloem tissue of Cucurhita as
seen in both trans- and longi-sectional views. Show carefully the
structure of at least one sieve-plate in each drawing. Label all
cell tj'pes and structures.

3. Prepare drawings of small portions of the phloem of
Eolinia and Pinus to show the form and structure of the sieve-
tube elements and sieve-cells.

4. Summarize the evidence which indicates that plant viruses
may move and be transmitted through phloem tissue (cf. Crafts,
1939b, and Esau, 1941).

100 SIEVE-TUBE ELEMENTS

5. Outline briefl}' the experimental evidence as to the role of
the phloem in the trans-location of organic solutes (cf. Crafts,
1939b).

REFERENCES

1. Abbe, L. B., and Crafts, A. S., Phloem of White Pine and
Other Coniferous Species. Bot. Gaz. 100 :69r)-722. 1939.

2. Crafts, A. S., The Relation between Structure and Function
of the Phloem. Amer. Jour. Bot. 26 :172-177. 1939a.

3. , . iMovement of Viruses, Auxins, and Chemi-
cal Indicators in Plauts. Bot. Rev. 5 :471-504. 1939b.

4. Eames and MacDaniels, pp. 69-76 and Ch. VIII.

5. Esau, K., Ontoojenv aud Structure of the Phloem of Tobacco.
Hilgardia 11 :343-424. 193S.

6. , , Development and Structure of the Phloem

Tissue. Bot. Rev. 5 :373-432. 1939.

7. , , Phloem Anatomy of Tobacco Affected with

Curly Top aud I^Iosaic. Ililo-ardia 13 :437-490. 1941.

8. Hayward, pp. 614-616.

Exercise XII

THE STEM

I. Introduction. — In this and the two following exercises, a
brief study will be made of the comparative anatomy of the three
principal vegetative "organs" of the sporophyte of seed plants,
viz. : the stem, leaf, and root. Paleobotanical evidence shows
clearly, however, that this conventional subdivision of the plant
body cannot be applied to the Psilophytales which are generally
regarded as the most primitive of all tracheophytes. Indeed, in
the Psilotales, which are the living representatives of this ancient
group, roots are absent and the aerial portion of the sporophyte
is not clearly demarcated into stem and leaves. Furthermore, it
is clear that even in seed plants the boundary between stem and
leaf can only be made rather arbitrarily. Both of these "organs"
arise from a common terminal meristem (i.e., the shoot apex) and
their further differentiation and growth is reciprocal and inter-
dependent to a large extent. For these reasons, it seems prefer-
able, from a morphological standpoint, to include both the axis
(i.e.. the stem) and its foliar appendages under the broader con-
cept of the shaot. This concept has found application not only
to the vegetative region but has also been widely adopted in the
anatomical interpretation of the flower of angiosperms (cf.
Eames, 1931, and Foster, 1939). Hence, in this book, the sepa-
rate treatment given to the stem and the leaf is largely a matter
of practical convenience and its limitations on morphological
grounds should be constantly borne in mind. (Cf. Arber, 1941,
for a penetrating discussion of the problem.) The root, which is
axis-like in form, clearly deserves separate discussion and study
and its anatomical features will be outlined briefly in Exer-
cise XIV.

In the followng resume of the basic aspects of stem anatomy,
the histology of the iniernodal regions of this structure is the
principal consideration. The nodal regions of stems, because of

101

102 THE STEM

the complications in vascular anatonn- resulting from the devel-
opment of leaves and buds, offer a series of problems which are
bej'ond the scope of the present book. In general, however, it
may be stated that in gymnosperms and dicotyledons, the cylin-
der of vascular tissue is interrupted at or near the node by the
development (early in ontogeny ) of leaf gaps, which are paren-
chymatous areas in the siphonostele situated above the point of
divergence of the leaf traces. Depending upon the nature of the
foliar structure as well as upon the plant, the anatomy of the
node is described as unilaeunar (one gap), trUacunar (three
gaps) or multilacunar (more than three gaps). There appears
to be some evidence that in angiosperms the trilacunar node is
the primitive condition. (For further details cf. Eames and
MacDaniels, pp. 114-120.) The development of the axillary bud
results in additional complications in the vascular anatomy of
the node. The earliest vascular bundles of the bud are known
as branch traces and their "divergence" from the main axis
likewise is associated with parenchymatous areas in the stele
which are termed branch gaps. Wide application of the prin-
cipals of nodal anatomy has been made in the study of the vas-
cular anatomy of the flower (cf. Eames, 1931, and Wilson and
Just, 1939).

1. The primary structure of the stem. In man}' of the k)wer
vascular plants and in certain herbaceous angiosperms (particu-
larly monocotyledons), all the stem tissues are primary, i.e., they
originate directly from the progressive differentiation of cells
derived from the apical meristem of the shoot. Despite the great
variation in the kinds and patterns of tissues, a common plan of
primary structural organization is found in the stems of most
gymnosperms and angiosperms. This consists in the existence of
three more or less well-demarcated zones or regions which now'
may be described briefly as follows :

{a) The epidermis. Stems possess a well-defined epidermis in
which stomata and various types of trichomes may be present in
addition to typical epidermal cells (cf. Exercise V).

(b) The cortex. Beneath the e]ndermis of stems is found a
cylindrical zone, variable in its radinl dimension and in the Idnds
of cells which occur. This region is the cortex and in the simplest

THE STELE 103

condition is formed of thin-walled parenchyma tissue. Often,
however, the cortex is more complex histologically and exhibits
an outer, subepidermal area of collenchyma (which occurs as a
continuous c^dinder or as separate strands) and an inner region
of parenchyma. Other cell types may also appear in the cortex,
particularly sclereides, fibers and secretory cells.

{c) The stele. This region of the stem includes the primary
vascular tissues as well as variable amounts of parenchyma.

Since the original formulation of the "Stelar Theory" by
Van Tieghem and Douliot in 1886. comparative studies on the
stem have largely centered upon the organization aiul phylogeny
of the stele, particularly with reference to the distribution of
phloem and xylem and the morphological nature of the pith. It
is now rather generally held that two principal types of steles
occur in the sporophyte of vascular plants, viz.: (1) the proto-
stele, which consists of a central core of xylem ensheathed by
phloem, and (2) the sipJionostele which is characterized by the
presence, internal to the protoxylem, of a central mass of paren-
chyma known as the pith. The evidence from comparative anat-
omy, including the facts of paleobotany, supports the idea that
the protostele is the primitive type. This type of stele is found
in both the stem as well as the root of many lower tracheophytes
but is restricted to the root of seed plants. The order of matura-
tion of the tracheary elements of the xylem in a protostele is
centripetal and primary xylem of this kind is termed exarch.
In contrast, the protoxylem in a siphonostele is situated at the
outer edge of the pith and the order of maturation of the xylem
is centrifngal. Primary xylem in the stems of seed plants is thus
endarch. A discussion of the involved controversy as to the way
in which the siphonostele may have originated from the proto-
stele is beyond the scope of this book and the student is referred
to the excellent resumes given by Eames and MacDaniels (pp.
112-114 and 337-340) and Smith (1938, pp. 124-131). In the
writer's opinion, however, the evidence from ontogeny appears
to favor the idea that at least in seed plants the pith region is
morphologically a part of the stele and represents the paren-
ehymatization of potential vascular tissue. As seen in trans-
section, the vascular tissue of the siphonostele appears either as

104 THE STEM

a continuous cylinder, broken only by the short leaf gaps in nodal
regions, or as a "ring" of vascular bundles. A stele of the latter
type should be visualized as a tubular network, the "meshes"
representing the long vertical leaf gaps or parench^-matous rays.
This kind of stele is termed a dictyostele and its component bun-
dles are either collateral or hicollateral. In gymnosperms and
dicotyledons, the siphonostele is specifically designated as ecto-
pliloic if there is only an external area of phloem, or as ainphi-
phloic if both internal as well as external phloem occurs. This
latter t^'pe of siphonostele is restricted to certain families in the
dicotyledons and also appears in the stems of some ferns. The
nature of the houndary between stele and cortex has produced
much discussion among anatomists. A common view is that the
cndodermis represents the innermost layer of the cortex and that
hence all tissues internal to it, including the peincycle, vascidar
tissues and pith, constitute the stele. Such a demarcation is
practical in roots which typically develop a well-defined cndo-
dermis, and the evidence of ontogeny indicates that at least in
some plants, the endodermis is morphologically a part of the cor-
tex (cf. Esau, 1941). In the stems of seed plants an endodermis
or its equivalent (i.e., the so-called ^'starch sheath'^) may be
present. Under such circumstances it should be clear that until
further ontogenetic evidence becomes available, only an approxi-
mate and somewhat arbitrary demarcation can be made in many
stems between the innermost region of the cortex and the adja-
cent pericycle.

2. The ontogeny of the srele in vascular plants. Within recent
years there lias been a inai-ked revival of interest in the origin and
differentiation of the ])rimary vascular system. Among the more
important contributions may be cited the work of Helm (1932),
I>ai'thelmess (1935), Louis (1935), Kaplan (1937), and Gregoire
(1938). One of the principal objectives in these investigations
has been to determine how the jn-ovaseular tissue or procambium
is produced from the tissue of the shoot apex. In view of the
involved aspects of this process, it will only be possible to outline
certain important steps.

Ill a iium])er of dicotyledons, the first stage in tlie determina-
tion of the position of the provascular tissue consists in the early

ONTOGENY OF STELE IN VASCULAR PLANTS 105

differentiation of the jiifJi. This sets apart, near the base of the
shoot apex, a peripheral zone of tissue which shows little or no
evidence of specific differentiation. The next step, as one pro-
ceeds away from the summit of the apex, is concerned with the
effects produced by the developing leaf primordia. These struc-
tures early assume a dorsiventral character and their abaxial
regions begin early to develop as parenchymatous tissue. As a
result, at least in many dicotyledons, trans-sections below the
base of the shoot apex, show a more or less distinct "ring" of
embryonic tissue demarcated on the outside by the united bases
of the leaves and on the inside by the developing pith. This tis-
sue zone has been designated by Helm as the "meristem ring"
and by Louis as the "prodesmogen." From it there is produced
the provascular or procambial cells which ultimately give rise
to the primary vascular system of the stem.

Divergent ideas are held as to the way in which procambium
arises from the meristem ring or prodesmogen. According to one
view, the procambium arises as isolated strands in the bases of
the young leaf primordia, from which points its further differ-
entiation proceeds acropetally towards each leaf apex and basi-
petally towards the prodesmogen tissue in the axis. However,
a number of recent studies (e.g., Boke, 1940, 1941) suggest that
this may be an erroneous interpretation and that the develop-
ment of provascular tissue in the region of the shoot apex may be
exclusively acropetal.

One of the important aspects of recent studies has been to
emphasize the interrelationship which exists between stem and
leaf in the building up of the stele. This is shown by the fact
that in conifers and many dicotyledons, the first vascular bundles
in the young siphonostele are leaf traces or so-called "common
bundles. ' ' The interfascicular areas in the ' ' meristem ring ' ' may
either produce additional procambial tissue from which addi-
tional primary xylem and phloem arise, or as in dictyosteles,
progress towards the formation of parenchyma. In the latter
case, the interfascicular strips either mature as typical medullary
rays or give rise to an interfascicular camhium.

The question of the differentiation of primary phloem and
primary xylem in leaf traces has apparently received only meagre

106 THE STEM

stud}'. According to the work of P]sau (1938, p. 396) on tobacco,
the primary xylem differentiates both upwardly into the leaf and
downwardly into the axis. But "the phloem of a leaf trace fol-
lows a different course of development from that of xylem. It
differentiates, at least in the species considered in these studies,
acropetally from the stem into the leaf."

3. The secondary structure of the stem. In gymnosperms and
many angiosperms, the so-called primary tissue regions are rela-
tively short-lived and eventually are destroyed or embedded by
the development of secondary tissues. As pointed out in Exer-
cise X, the distinction between primary and secondary vascular
tissues is rather arbitrary and depends fundamentally upon the
way in which "procambium" is distinguished from the "vascu-
lar cambium." The process of secondary growth in stems is
extremely complex and the following outline is intended merely
as an introductory guide. (For a detailed treatment cf. Eames
and MacDaniels, Ch. VI.)

The vascular cambium, theoretically regarded, is a uniseriate
meristem composed of ray initials, which produce the vascular
rays, and fusiform initials which give rise to the "vertical" cell
types in the secondary phloem and secondary xylem. In examin-
ing developing siphonosteles in trans-section, iiowever, it proves
difficult or impossible to distinguish the cambial initials from
their most recent derivatives. For this reason, the term "cambial
zone" may be used collectively to designate the cambial initials
and their adjacent undifferentiated phloem and xylem mother
cells. The vascular cambium may be thouglit of as the direct
continuation of the undifferentiated procambial tissue situated
between the metaxylem and the metaphloeni. If one is concerned
with a dictyostele, a distinction is made between the caml)ium
witliin each bundle (the fascicular camhium) and the cambium
Avhich arises fi-om i)arenchyma — like tissue l)etween the bundles
(interfascicular camhium). The latter may pi-oduce additional
phloem and xylem or as in certain vines, broad rays of "sec-
ondary" pai-enchyma. The effects of sustained cainl)!;!! activity
upon the ])riinai-y tissue i-egioiis is jirofound. All cxt i-a-cambial
tissues, i.e., ]n-imary plilocin, pcricycle, coi-tex and epidermis, are
affected and eventually, through the added activity of the

SECONDARY STRUCTURE OF THE STEM 107

phellogen or cork cambium, are sloughed away. Their place is
gradually taken by the bark which consists in many gymnosperms
and dicotyledons of dead or dying secondary phloem and of areas
of i^eriderm. The intra-cambial primary tissues, i.e., primary
xylem and pith, are completely buried within the cylinder of
secondary xylem and, as a result of stretching and compression,
may be crushed or even destroj'ed.

The early phases in development of secondary vascular tis-
sues in woody stems are usually accompanied by the formation
of a periderm or corky tissue beneath the epidermis. Func-
tionally, the periderm acts as a protective layer, replacing in
this respect the epidermis which is eventually killed and sloughed
away. Structurally, the term periderm is applied to the phello-
gen or cork cambium and its two derivative tissues, viz. : cork or
phellem, and phclloderm. The first-formed phellogen in the stem
appears to arise as a result of the regressive differentiation of
epidermal, cortical or pericyclic parenchyma cells. Its initiation
is indicated by the tangential division of certain cells. In some
species, these first tangential divisions appear in the epidermis.
Most commonly, perhaps, the phellogen originates in the outer-
most cells of the cortex. There is evidence that in some stems,
the cortical phellogen first makes its appearance beneath the sto-
mata, at which point lenticels are produced. Continued spread
of the phellogen from these structures may result in the forma-
tion of a cylinder of cork cambium. As a result of the repeated
tangential division of the phellogen cells, the derivative tissues
exhibit alignment of the cells in radial rows. Cells differentiat-
ing towards the outside of the phellogen lose their protoplasts,
acquire suherin in their unpitted walls, and finally mature as cork
cells. The phelloderm tissue, which is usually much less in extent
than the cork, originates from the inner derivatives of the phello-
gen. Phelloderm eells are described as being parenchyma-like in
retaining their protoplasts and in having simple pits in their
cellulose walls. The functional life of the first-formed phellogen
in woody stems is short and new layers of cork cambium arise
successively from deeper regions of the cortex and pericycle
until, finally, the living eells of the secondary phloem participate
in periderm formation. The ultimate result is the production

108 THE STEM

in many species of shell-shaped layers of periderm which enclose
masses of dead or dying phloem tissue (cf. Eames and Mac-
Daniels, p. 212, Pig. 96).

The first-formed as well as later developed periderm layers
in stems are usually provided with aerating structures termed
lenticels. As stated above, lenticels are usually initiated by the
appearance of a phellogen beneath a stoma (cf. Eames and ]\Iac-
Daniels, p. 219, Fig. 100). In the development of a lenticel, the
phellogen, instead of producing typical cork, forms a mass of
loosely-arranged cells with unsuberized walls which make up the
complementary tissue. This tissue in many lenticels may be sub-
divided by layers of smaller more compact cells which are termed
closing layers. The pressure exerted by the outwardly-developed
mass of complementary tissue is sufficient to rupture the epi-
dermis which, together with the underlying layers of adjacent
cork, curls back from the edges of the lenticel as flaps of broken
tissue. In many plants (e.g., SamJ)ucus), the extruded comple-
mentary tissue is very prominent. According to De Bary (p. 561)
the i)uffy swelling of lenticels in trees during wet weather may
be the result of the "hygroscopicity " of the complementary
tissue.

II. Material for the Comparative Study of the Stem. — The

choice of material for the study of stem anatomy will depend
upon the forms available as well as upon the points to be illus-
trated. The following stem types have proved useful and are
recommended. Free-hand sections of stems stained with phloro-
glucinol and hydrochloric acid are of considerable use. l>ut for
the finer details of structure and development, permanent mounts
of critically stained sections are necessary.^

1. The si (III of the gcr<niii(ni {P<'](n-g(nilinu sp.). Examine a
transverse section of the stem, and sliulx" the following tissues
and regions from the edge of the section inwardly, viz.:

{a) The epidermis, a uniseriate layer of small oval or elliptical
cells with thick inner and outer walls (the latter covered with a
thin cuticle) and somewhat thinner radial walls. A i)rotoplast,
which may appear somewhat collapsed, should be evident in most
cells. Certain of the epidermal cells have given rise to stomata

1 Cf. Appendix, p. 140.

COMPARATIVE STUDY OF THE STEM 109

while others have clevelu])e(l lo xarioiis tyj)es of tricliomes, includ-
ing glandular capitate hairs and multicellular unbranched hairs,
the latter similar to the hairs already studied on the geranium
leaf. (Refer to Exercise V.)

(h) Immediately within the epidermis is found the cortex, a
region composed of twelve or more layers of rather typical "iso-
diametrie" parenchyma cells which are separated from one an-
otlier by prominent intercellular air spaces. A protoplast is
present in many of these cortical cells, the main functions of
which are photosynthesis and food storage, as is evidenced by the
frequency of starch grains in these cells ; druses of calcium oxylate
are found in many of the cortical cells, occupying nearly the
whole cavity of the cell.

Depending largely on the distance from the shoot apex at
which the sections on your slide were taken, j^ou will find the
early or later stages in the development of cork or phellem. In
studying the phellem in the stem of the geranium, notice that
druses are occasionally found in some of the inner cork cells. If
your sections show a phellem four or five layers in thickness, you
will find that the tangential (and to some extent the radial walls)
are somewhat wavy or irregular. This condition, which is very
commonly found in corky tissue, results from the constant pres-
sure of the successively developing and enlarging cork cells upon
tlie older cells of the phellem. Ti/pical phelloderm, Jiowever, is
rarely developed in the young stem of the geranium.

(c) Directly inside the innermost layer of cortical cells occurs
the stele, the outer region of which is indicated by the pericycle
which in the geranium stem consists of an outer continuous ring
of thick-walled pericyclic fibers (elongate tightly joined cells, as
seen in longitudinal section, with characteristic slit-like spirally-
arranged pits) followed by several layers of small thin-walled
parenchyma cells. The vascular tissue of the stele lies directly
within the pericycle and consists of a ring of typical collateral
bundles (i.e., bundles in which the phloem lies radially external to
the xylem) which are separated from one another, at least in
certain regions, by strips or bands of parenchymatous tissue
which because of their direct connection with the pith (medulla)
may be termed medullary rays.

110 THE STEM

111 view of the obvious variation in tlie size and degree of
development of the vascular bundles in any given section, the
following description is only general in nature ; and all essential
variations and details must be indivdually interpreted.

A well-developed vascular bundle in the geranium stem is
somewhat wedge-shaped or triangular in cross-section and con-
sists of an external region of phloem tissue separated from the
internal region of xylem tissue by the camhial zone in which cell
divisions occur predominately in the tangential plane.

Beginning with the phloem tissue of the bundle first of all, you
should be able to find the somewhat crushed primary phloem lying
directly against the innermost cells of the pericycle; structurally,
primary phloem in the geranium stem appears to consist of small
sieve-tubes, companion cells and phloem parenchyma. Lying
directly inside of the primary phloem occurs the secondary
phloem which is composed of (1) sieve-tubes, rather large poly-
gonal cells, apparently devoid of contents; (2) companion cells,
extremely small, more or less triangular cells (in cross-section
of course) closely joined to the sieve-tubes and usually containing
a definite nucleated protoplast; and (3) large thin-walled
parenchyma cells.

Separated from the secondary phloem by the "cambial zone"
occurs the xylem tissue of the vascular bundle. In all of the
larger bundles, the xylem is of two kinds, viz.: (1) secondary
xylem, an external layer of thick-walled isndiametric, tightly
joined cells which are arranged in more or less definite radial
rows because of their origin from the fascicular cambium: and
(2) the primary xylem, an internal group of ratlier large, more
or less polygonal cells, irregularly arranged and imbedded among
isodiametric thin-walled parencliyma cells. The smallest cells
of the primary xylem (wliich has differentiated centrifugally
from the procambial strand) are found nearest the inner edge
of the vascular bundle and represent the first formed elements of
the protoxylem. Notice particularly that in many protoxylem
cells, portions of the spiral thickening have been torn from the
wall in the process of sectioning; in other cells, a ]neee of the
spiral band may be seen projecting into the lumen of the cell.

THE STEM OF THE BASS WOOD OR LINDEN 111

From what has already been said of the structural characteristics
of the tracheary cells of the xylem, it is almost unnecessary to
state that the limits between secondary xylem and metaxylem are
virtually impossible to determine when considering only trans-
verse sections of a vascular bundle. In the interval between each
of the individual bundles in the ring, there occurs a zone of small
thin-walled, obviously meristematic cells which are continuous
with the cambial zone in the bundles themselves and represent
what is termed the interfascicular cambium. The interfascicular
cambium originates by the tangential division of cells in the
medullary rays adjacent to the strips of fascicular cambium. In
certain plants, the interfascicular cambium only forms paren-
chyma (e.g., Clcmatic, Aristolochia) but in Pelargonium, as in
many herbaceous plants, the interfascicular cambium gives rise
to additional "bundles" composed entirely of secondary phloem
and secondary xylem ; the extensive development of vascular
tissue from the interfascicular cambium usually results in the
formation of a complete cylinder of secondary xylem and phloem.
In studying the sections note that phloem is the first vascular
tissue to differentiate from the interfascicular cambium and is
later followed by the centripetal formation of secondary xylem ;
whether this is an exceptional condition can only be determined
by the investigation of a large number of herbaceous stems.

The center of the stele is occupied by the pith, which is com-
posed exclusively of large isodiametric thin-walled parenchyma,
cells separated from one another by conspicuous intercellular air-
spaces. The extreme abundance of starch grains in the cells of
the pith indicates that this region has as its function the storing
of reserve food material.

2. The stem of the hasswood or linden (Tilia sp.). Obtain a
stained slide with transverse, radial and tangential sections of the
stem. Examine first the trans-section and study the following
tissues and regions from the periphery of the stem to its center,
viz. :

(a) The epidermis, a uniseriate layer of cells which appears
broken and cracked in numerous places due to the development
of a prominent periderm beneath it. Structurally the cells of the
epidermis are rather small, are oval in shape (in section view)

112

THE STEM

and possess thick, slightly stratified outer walls overlaid by a
cuticle; in most instances, inclusions and the remains of the dis-
integrated protoplast are present in the epidermal cells.

(6) Immediately within the epidermis occurs the periderm
which is composed of three layers of tissue, viz. : (1) the phellem
or cork which is differentiated centrifugally from the phellogen
and consists of a varying number of layers of narrow, laterally
compressed cells which are arranged in definite radial rows and
are densely packed (except the two outermost layers) with dark
brown material which consists probably of substances classed un-
der the general head of tannins; (2) the phdlo<jen which is a
uniseriate layer of meristematic cells found next to the innermost
layer of phellem cells. In Tilia, as in so many woody stems, the
phellogen is initiated by the tangential division of the outermost
layer of cortical cells. (3) Internal to, and directly next to the
phellogen occurs a single laj^er of cells characterized in this in-
stance by their rectangular form and obvious protoplasts. This
layer of cells is known as the phcUodcrm and differentiates centri-
petally from the phellogen. Note that the corky layer of the
stem is definitely broken at certain points which appear as some-
what lens-shaped areas; these regions are known as Icnticels. In
studying the structure of the lenticels of the basswood stem, ob-
serve particularly the continuity between the phellogen and
phelloderm of the lenticel and these tissues as they occur at either
side of the lenticel. In this particular developmental stage of
the stem in Tilia the lenticels have been formed from the first or
primary cork cambium ; as the stem increases in diameter from
year to year, new lenticels are formed at various points on the
surface of the bark by the activity of subsequently formed
phellogen layers; thus even in relatively old branches, lenticels
develop and represent the necessary areas through which exchange
of gases between the living tissues and the afmosphere can take
place in the essential process of respiration.

(r) Directly internal to the periderm occurs the cortex, which
consists of two rather definite regions, viz. :

(?) An outer region composed of four or more laj^ers of
coUenchyma cells, whicli in the position of their thickened pri-
mar}^ walls appear intermediate in character between "angular"

THE STELE 113

and "lamellar" collenchyma. Notice that intercellular air spaces
are extremely small and difficult to distinguish.

(//) An inner region composed of typical "isodiamet-
ric" parenchyma cells with active protoplasts and cells containing
druses of calcium oxalate. The latter type of cells tend to occur
in groups (refer to the radial section of the stem) and their
specialized character has earned for them the term of "crystal
sacs." It is important to notice the crushed appearance of many
of the parenchyma cells in the cortex ; this condition has been
caused by the pressure of the secondary phloem which is con-
stantly pushed toward the periphery of the stem.

(d) The stele. Tilia possesses an ectaphloic sij^honostele but
the arrangement of the tissues of the secondary phloem show^s a
number of features distinctive of this genus. The phloem occurs
just Avithin the innermost layer of the cortex and consists of a
cylinder made up of wedges of tissue showing a characteristic
"banded" appearance which alternate with triangular sectors
which are "homogeneous" in structure; the wedges showing the
band-like structure are broadest next to the cambium while just
the reverse relationship obtains in the case of the homogeneous
sectors.

Examine in detail, first of all, the structure of one of the large
'^lianded" sectors. You will observe that the characteristic
"banded" appearance of such a sector is due to the alternation
of tangential layers of extremely thick-walled fibers with layers
of thinner walled cells. These thick-walled phloem fibers, which
provide mechanical support and flexibility to the stem, have long
been known as "bast fibers" and their prominent development
in the genus Tilia is responsible for the old English name of this
tree, i.e., * ' bast-wood ' ' which in modern language has been changed
to "basswood. " Under high power, the phloem or "bast fibers"
appear irregularly polyhedral, are closely joined without evident
intercellular air spaces, and possess extremely small lumina ; under
especially favorable conditions of magnification and illumination
you may be able to see the infrequent canal-like pits between ad-
jacent cells.

The layers of thinner-walled cells alternating with the bands
of "bast fibers" are composed of the conducting and storage cells

114 THE STEM

of the phloem, viz.: radial rows (one or occasionally two cells
in thickness) of living parenchj'ma cells formed from the cam-
bium and extending out to the cortex through the phloem ; these
radial rows of cells (which of course are actually sheets of tissue)
are the phloem rays and are directly continuous across the cam-
bium with corresponding rays of the secondary xylem. The
phloem rays function both in radial transportation as well as in
the storage of food materials. Starch grains are often present
in these cells. The sicve-tuhes, which are the important vertical
conducting elements of the phloem, appear in transverse section
as large somewhat irregular thin-walled cells which appear devoid
of protoplasts and are closely joined on their smaller wall-facets
with the companion cells. The latter are very small, appear more
or less triangular in transverse section and in most cases possess
definite nucleated protoplasts. Interspersed among the sieve-
tubes are the small isodiametric phloem parenchyma cells which
possess a living protoplast.

Turning now to the wedges of ^'homogeneous tissue," it will
be found that these sectors are entirely composed of somewhat
"rectangular" parenchyma cells, most of which are provided
with a definite protoplast. The characteristic form of these sec-
tors is due to a process of dilatation of the multiseriate phloem
rays lying between the "banded" sectors of the stem. In other
words, certain of the phloem rays (usually those which are two
or three cells in width at the cambium) increase continuously in
width by the radial and tangential division of the parenchyma
cells; the intervening areas of phloem (i.e., those containing the
bast fibers and the sieve-tubes) are thus separated from each
other and correspond in position to the phloem portions of the
original vascular bundles. As the stem increases in age, a similar
process of dilatation occurs in the smaller rays of the ])hloem
with the result that the "banded" sectors become successively
subdivided into smaller groups (for complete details, cf. De Bary
pp. 536-537). It should be noted that these dilated phloem rays
are directly continuous across the cambium with hi- or triseriate
xylem rays. Druses occasionally appear in the parenchyma cells
of the dilated phloem rays.

TRACIIEIDS 115

The "camhial zone" occurs directly between the secondary
xylem and the secondary phloem and consists of the uniseriate
eamhium itself and a varying- number of layers of "maturing"
vascular elements. A careful study of the "cambial zone" will
lielp in understanding the direction of formation of secondary
xylem and secondary phloem and will furthermore shed some
light on the nature of the early developmental stages of the cells
making up these tissues.

Within the ' ' cambial zone ' ' occurs the cylinder of secondary
xylem which is composed of two or three more or less concentric
layers or cylinders of xylem tissue, each of which is known as an
annual ring. In all woody stems of north temperate plants, an
annual ring represents the amount of secondary xylem formed
during a single growing season. The presence of annual rings
in the secondary xylem of perennial w^oody plants seems to be de-
termined to some extent by seasonal conditions, since the xylem
tissue formed in the spring (the so-called ''spring wood") dif-
fers somewhat in respect to the size, type and arrangement of its
cells from that formed in the summer (the so-called "summer
ivood"). It is due to this structural difference between "spring"
and "summer" wood that an annual ring appears distinct. In
many cases, the cells formed during late summer tend to be some-
what smaller and thicker-walled than those arising in the spring;
often the difference is emphasized by the localization of the ma-
jority of the vessels in the spring wood.

The secondary xylem of Tilia is, like that of many woody
dicotyledons, a very complex "tissue system," consisting of
the following tj^pes of cells :

(a) Vessel elements, which are large prominent cells, rather
polj'gonal in transverse section, and possessing large empty
lumina. In especially thin regions of your section you may be
able to see small bordered pits in cross-sectional view. Careful
focusing should reveal the "compound middle lamella."

(&) Tracheids, which are smaller in size than the vessels and
are often more or less rectangular in shape as seen in transverse
section. In contrast to the vessels, the tracheids are frequently
arranged in definite radial rows.

116 THE STEM

(c) Fibers, which are sununvhat irregular in shape and are
usually much smaller than either the vessels or tracheids and are
usually provided with thicker walls.

(d) Scattered among the vessels, tracheids and fibers, occur
the wood parenchyma cells which are small in size, isodiametric
in form, and possess a definite protoplast. It should be realized
that wood parenchyma cells occur in "vertical chains" or rows
and function primarily in the storage of certain carbohydrates,
particularly starch; the function of wood parenchyma in "assist-
ing" the trans-location of substances in the xylem is imperfectly
understood. In Tilia, the wood-parenchyma has no definite dis-
tribution in the xylem and is hence termed "diffuse."

Extending radially through the secondary xylem are the rays
which are of two types, viz. :

{a) Large rays, which represent the xylem "extension" of
the large si^ecialized dilated rays of the phloem discussed pre-
viously. In the second annual ring, these large rays may be
two or three cells in width but, at least in the inner portion of
the first annual ring, they become a single cell in width and
finally terminate directly in the pith. Such rays have been termed
"primary medullary rays."

{h) Small rays, one cell in width which are the xylem ex-
tension of the rays extending through the "banded sectors" of
the phloem. These rays have been termed "secondary medullary
rays" but in some instances at least have no direct connection
with the pith.

The pri))i<try xylem occurs next to the pith and is completely
surrounded externally by the cylinder of secondar}- xylem. It
is naturally very difficult to distinguish between the metaxylcm
and the first-formed elements of tlie secondary xylem in a trans-
verse section of the stem of Tilia. The protoxylem, however,
is quite distinct and appears as solitary or grouped tliick-walled
cells which are imbedded among the small parenchyma cells of
the primary xylem at the periphery of the pith. The pith is a
solid rod of tissue occupying the center of the stem. In contrast
to tlie condition in the geranium stem, the pith of Tilia is not
homogeneous but shows a certain amount of tissue specialization
as follows :

SECONDARY XYLEM 117

The periphery of flu pith is formed of several layers of
rather thick-walled more or less isodiametric parenchyma cells
which are provided with protoplasts and in addition contain very
small starch grains; other of these peripheral cells are filled with
dark-staining granular material, the exact nature of which is ob-
scure at present. The bulk of the pith is composed of large
"isodiametric" parenchyma cells which are separated by definite
intercellular air-spaces ; some of these cells likewise contain small
starch grains and protoplasts. Interspersed among these large
pith cells occur much smaller thicker-walled parenchyma cells
which are filled with granular dark-staining bodies ; these latter
cells are found singly or in groups, but in longi-section appear in
scattered vertical series.

Attention must finally be directed to the mucilage-containing
cells which are arranged in a more or less definite ring near the
periphery of the pith. These rnucilage -containing cells may be
identified by their large size, by the disorganized reddish or purple
material found in them, and by the jacket of starch-containing
parenchyma cells which surrounds each of them. The function
of the mucilage formed in these cells is unknown.

3. The stem of Gymnosperms (e.g., conifers) is similar to that
of many woody dicotyledons in that the activity of a cambium
forms a cylinder of secondary phloem centrifugally and a cylin-
der of secondary xylem centripetally. A transverse section of a
young pine stem shows a number of interesting anatomical fea-
tures, viz. :

(a) The large cavities or resin canals which appear in the
cortex and in the secondary xylem (in the latter region the canals
are smaller than those in the cortex). Notice that the resin canal
(which has arisen by the pulling apart of groups of cells in certain
regions and is hence of the schizogenous type) is bordered by a
ring of small, densely protoplasmic secretory cells which exude
resinous material into the canal. In certain species of pine, the
resin is of great commercial importance and is obtained by "tap-
ing" the trees; the resin obtained in this way is the basis of tur-
pentine and allied products.

(&) The secondary xylem is relatively simple in structure and
consists of tracheids, fiber tracheids and uniseriate xylem rays. A

118 THE STEM

study of the secondary phloem has already been made in Exercise
XI. '

4. The stem of monocotyledons. The stem in many members
of this group is characterized by the fact that the stele consists
of collateral bundles which are more or less scattered through the
axis, i.e., they are not arranged in the form of a single "ring" as
in dicotyledons. As a consequence, the limits of cortex, pericycle
and pith are usually impossible to determine with exactness.
However, the recent work of Stover (1934) clearly shows that the
disposition of vascular bundles in the stems of grasses does not
conform to a single type. Indeed, some genera {Lcerzia, Agro-
pyron) possess a single series of bundles arranged in a cylinder
between the cortex and pith. In most monocotyledonous stems,
cambial activity is vestigial or usually absent from the vascular
bundles which are thus wholly primary in structure. Such
bundles are often termed "closed" bundles in contrast to the
"open" bundles of dicotyledons which possess secondary growth.

Obtain a transverse section of a corn stem, and note under low
power the characteristic arrangement of the collateral vascular
hnndles which are more numerous near the periphery of the stem
than in the center. Study one of the large bundles under high
power, noting that the phloem (composed of sieve-tubes and com-
panion cells) is directed towards the periphery of the stem while
the xylem is nearest the center of the stem. The x.ylem consists
of four large primary xylem vessels (which are arranged like the
eyes, nose and mouth of a face) and a number of smaller tracheae ;
a prominent air-lacuna is usually present at the inner edge of the
innermost large vessel. The bundle is more or less completely
surrounded by fibers, a feature very commonly found in the
bundles of monocotyledonous stems.

5. The vine type of stem in dicotyledons. This type of stem
often has its vascular system in the form of a "ring" of bundles
which are separated from each other by broad medullary rays
composed of parenchyma cells. Those radially directed sheets of
parenchyma cells may extend vertically tlie length of an internode
or more and in many cases continue to grow (by means of an
interfascicular cambium) at the same rate as the fascicular cam-
bium Avhich is increasing the size of the vasculai- bundles.

VINE TYPE OF STEM IN DICOTYLEDONS 119

Obtain a transverse seetioii of a one-year-old stem of the
" Dutchman's l^ipe" [Ayistoluchia) and examine it under low
power. The following tissues and regions can be seen in pro-
gressing from the edge of the section to its center, viz. :

(fl ) A typical uniseriate epidermis composed of rather densely
protoplasmic cells. Note the extremely thick cuticle which covers
the epidermis.

(h) Internal to the epidermis occurs the cortex, formed of an
outer zone of rather thin-walled "angular" collenchyma cells and
an inner zone of large "isodiametric" parenchyma cells; note
the presence of large druses in many of the parenchyma cells of
the cortex.

(c) The stele is sharply delimited by a broad cylinder of
closely joined pericyclic fibers, the secondary walls of which are
still increasing in thickness. Internal to the peric.yclic fibers oc-
curs the parenchynia of the pericycle which is quite similar in
general structure to the cortical parenchyma.

(d) The vascular system of the stele consists of a ring of
typical collateral hundles in each of which the clearly distinct
phloem and xylem is separated by a cambial zone (the fascicular
cambium). Notice that an interfascicular cambium has arisen as
the result of the tangential division of certain of the parenchyma
cells of the broad medullary rays which separate the bundles.

The pith of the stem is large and is composed of ' ' isodiametric ' '
parenchyma cells in many of which druses are evident.

Next obtain a transverse section of a two- or three-year-old
stem and notice the profound changes in structure which have
been occasioned by secondary growth, viz. :

(a) A discontinuous and rather thick layer of phellem (cork)
has appeared as the result of the activity of the phellogen layer.
Notice particularly how the epidermis has been forcibly broken
away from the cortex ; strips of epidermal tissue are visible on
the outer surface of the corky tissue. A comparatively extensive
development of phelloderm can also be seen internal to the phello-
gen. Lenticels are w^ell-developed at certain points.

(b) The previously continuous cylinder of collenchyma has
been broken as the result of the pressure of secondary growth.

120 THE STEM

Notice that parenchyma cells of the cortex have intruded into the
gaps between the strips of collenchyma cells.

(r) The pnvionslij continuous cylinder of ijcricjjclir filxrs
has likewise been ruptured and the gaps filled up by the paren-
chyma cells from the cortex. Notice the beginnings of wall-
thickness and lignification in some of the parenchyma cells be-
tween the strips of fibers; these parenchyma cells will finally
become thick-walled sfone-ceUs (by a process of secondary
sclerosis) and will thus effectively "repair" the broken me-
chanical cylinder. (For further details of this phenomenon, cf.
the Introduction of Exercise VIII.)

(d) The medullary rai/s between the vascular bundles have
become broad and long and their component par(Mi('hynia cells
are arranged in more or less definite radial raws as the result of
the continued activity of the intrrfascicular canihiuin.

(e) Each va.scular bundle has increased enormously in size
as the result of the continued activity of the fascicular cambium.
Notice the crushed condition of the cells in the outer portion of the
phloem: this has resulted from the centrifugal development of
additional secondary phloem. The xylem portion of each vascular
bundle shows three definite annual rings; notice that the largest
vessels occur at the edge (i.e., in the "spring wood") of each
annual ring.

(/) The irregularly-shaped 29?7/( is now much reduced in ex-
tent and shows clear indications of crushing. Aristolochia repre-
sents one of the exceptional cases where the jiitli is actually com-
pressed as the result of secondai-y growth.

III. Suggested Drawings and Notes. —

1. Pre])are a large diagrammatic drawing of the trans-section
of the stem of Pelargo)iiu)n indicating by legends or by labels
the position and relative extent of all th(> tissues and regions.

2. Prepare a drawing similar t(^ 1lie jibovo of the trans-section
of the stem of Tilia.

3. Di-aw a lenticel (of Samhucus. or Aristolocliia) as seen in
median longi-sectional view showing carefully the complementary
tissue and the closing layers and the relation of Ihese tissues to
the adjacent periderm of the stem. Label all important struc-
tures.

REFERENCES 121

4. Prepare enlarged drawings of the periderm of the stem of
Pelaryoniuin. Tilia or Smnbucu!^ showing the arrangement and
structure of the ceils of tlie phellem, phellogen and phelloderm.
Examine macerated bottle-corlv (i.e. the phellem of the cork oak,
Quercus siiber) and sketch several of the individual cells.

5. Draw in detail the resin canals as seen in trans-sections of
the stem of Pitius.

6. Show, by a diagram, the arrangement of the vascular
bundles as seen in the trans-section of the corn stem. Draw in
detail a: single vascular bundle showing the fibrous bundle-sheath
and the structure of the primary phloem and primary xylem.

7. Prepare diagrams of the trans-sections of the stem of
Aristolochia. These diagrams should illustrate the effects of
secondary growth on the primary structure of the stem.

8. Examine the demonstrations of various types of nodal
anatomy and prepare diagrams to illustrate them.

9. Outline the processes of mitosis and cytokinesis as they
occur in the fusiform initials of the cambium in gymnosperms
(cf. Bailey 1920 and 1928).

10. Outline the methods of secondary growth in the stem of
woody monocotyledons (cf. Chamberlain 1921 and Cheadle 1937).

REFERENCES

1. Arber, A., The Interpretation of Leaf and Pioot in the Augio-
sperms. Biol. Rev. 16 :81-105. 1941.

2. Bailey, T. W., The Cambium and Its Derivative Tissues, III.
A reconiuiissance of evtological phenomena in the cambium.
Amer. -lour. Bot. 7:417-434. 1920.

3. , , The cambium and Its Derivative Tissues,

IV. The increase in girth of the cambium. Ibid. 10:499-
509. 1923.

4. Barthelmess, A. TIeber den Zusammenhang zwischen Blatts-
tellung und Stelenbau unter besonderer Beriichsichtigung
der Konifereu. Bot. Archiv. 37 :207-260. 1935.

5. Boke, N. II., Histogenesis and Morphology of the Phyllode in
Certain Species of Acacia. Amer. Jour. Bot. 27 :73-90. 1940.

6. Boke, N. II. Zonation in the shoot apices of Trichocereus
spachianm and Opuntia cyUndrica. Amer. Jour. Bot. 28 :656-
664. 1941.

122 THE STEM

7. Clieadle, V. I., Secondary Growtli by Means of a Thickening
Ring' in Certain Monocotyledons. Bot. Gaz. 98 :535-555. 1937.

8. Chamberlain, C. J., Growth Rings in a Monocotyl. Bot. Gaz.
72:293-304. 1921.

9. De Bary, Ch. XIV, XV.

10. Eames, A. J., The Vascular Anatomy* of the Flower with Ref-
utation of the Theory of Carpel Polymorphism. Amer. Jour.
Bot. 18:147-188. 1931.

11. Eames and MacDaniels, Clis. V, VI, IX. XI.

12. Esau, K., Ililgardia, Ontogeny and Structure of tiie Phloem
of Tobacco. Ibid., 11 :343-424. 1938.

13. , . Phloem Anatomy of Tobacco Affected with

(^urly Top and Mosaic. Ililgardia 13:437-490. 1941.

14. Foster, A. S., Problems of Structure, Growth and Eyolution
in the Shoot Apex of Seed Plants. Bot. Rey. 5 :454-470. 1939.

15. Gregoire, V., La morphogenese et 1 'autonomic morphologique
de I'appareil floral. La Cellule 47:287-452. 1938.

16. Hay ward, Ch. III.

17. llehn, J., IJntersuchungen iiber die Differenzierung der
Sprossscheitelmeristeme yon Dicotylen unter besonderer
Beriichsichtigung des Procambiums. Planta 15:105-191.
1932.

18. Jeffrey, Ch. XIII.

19. Kaplan, R., Ueber die Bildung der Stele aus dem TTrmeristem
von Pteridophyten und Spermatophyten. Planta 27 :224-268.
1937.

20. Kiister, E., Sekundares Dickenwa'chstum ; IIolz und Rindc.
Allgemeine Einletung. Handbuch d. Pflanzenanatomie. IX.
Berlin. 1939.

21. Louis, J., L'ontogenese du systeme conducteur dans la pousse
feuilh'e des Dicotylees et des Gynniosiiermes. La Cellule 44:
87-172. 1935.

22. Smith, G. M., Cryptogamic Botany. Vol. IT. Ch. V. Xew
York. 1938.

23. Sperlich, A., Das trophische Parenchym. B. ?]xcretionsge-
webe. Ilandb. d. Pflanzenanatomie. I\'. Berlin. 1939.

24 Stover, E. L., Development and Differentiation of Tissues in
the Stem Ti])s of Grnsses. Ohio Jour. Sci. 34:150-160. 1934.

25. Wilson, C. L. and -Just, T., The Morphology of the Flower.
Bot. Rev. 5 :97-131. 1939.

Exercise XIII

THE LEAF

I. Introduction.— As stated in the previous exercise, it is diffi-
cult, on both theoretical as well as practical grounds, to de-
marcate rigidly the leaf from the stem. If, as. several mor-
phologists have suggested, the "leaves" of higher plants arose
phylogenetically from determinate branch systems, this difficulty
at once becomes understandable. Indeed, perhaps the most useful
character which distinguishes the leaf from the stem, apart from
its origin at the shoot apex, is the early cessation of apical growth.
The leaves of ferns retain an apical meristem for a relatively long
period in their development, but in seed plants the final size and
form of the leaf is largely determined by intercalary growth.
Leaves are without much question the most diversified of all the
"organs" produced by higher tracheophytes (cf. Troll 1938-
1939. and Arber 1941). The foliage leaf, which is the most
familiar type, varies from the small scale-like structures found in
certain gymnosperms and angiosperms to the enormous and com-
plex leaves of palms. In addition to foliage leaves, other types
of foliar organs must be considered under the morphological con-
cept of "leaf." As illustrations may be mentioned cotyledons,
bud scales, bracts, and according to classical theory, the appen-
dages of the flower. In view of such morphological and functional
diversity, it is obviously impossible to generalize with respect
to the histological structure of "leaves." From an anatomical
standpoint, the leaf may be regarded as an "expansion" of the
axis in which all the fundamental primary tissue regions (i.e.,
epidermis, cortex and stele) may be recognized. But the ar-
rangement and structure of the photosynthetic parenchyma (i.e.,
the 7nesophyll), the vascular system (i.e., the major and minor
veins) and the mechanical tissues (e.g.. collenchyma, sclereides
and fibers) vary within extremely wide limits.

123

124 THE LEAF

In llii.s exercise, a bi-iet' study will l)e made of a few represen-
tative leaf tyi)es, with particular emphasis upon the anatomy of
the leaf-blade or lamina. It is beyond the scope of this book to
discuss the process of leaf origin and the differentiation of the
various leaf tissues in seed plants. For information on these
matters, reference should be made to Foster (1936), Ilaywa'rd
(Ch. Ill, pp. 77-8.-)), Troll (1938), and Cross (194{). 1941).

II. Material for the Study of Leaf Anatomy. —

1. Tliv himhia of the foliage leaf of lilac {Syriiuja viilgari.'^).
Obtain a stained trans-section of the lamina and study its histology
mider low magnification. Note first of all the clear distinction
between tlie midrib and the two thin latei'al flaps of tissue. An
examination of the midrib reveals a large collateral vascidar
bundle in which the phloem is directed towards the ahaxial or
lower leaf-surface while the xylem is situated beneath the adaxial
or upper leaf surface. With this orientation in mind, it will now
be clear that the lamina exhibits a typical dorsiventral character,
shown not only by the relative positions of xylem and jihloem in
the larger veins but also by the differentiation of the mesophyll,
into palisade and spongy parenchyma. Since the anatomy of the
lamina in the region of the veins differs somewhat from the iiiter-
veinal areas, it will be more convenient to describe briefly the
various tissues and then to point out their topographical varia-
tions. In the lamina of this leaf, three principal tissues are
present, viz. :

{a) The epidermis. The adaxial epidermis consists of some-
what oval cells, the outer walls of which are covered by a thin
cuticle. Although exact measurements are lacking, there seems
to be relatively little difference in the thickness of the inner, outer
and radial walls of the epidermis. Observe that many of the
epidermal cells possess a protoplast which is peripheral in i)osi-
tion. Stoma fa ai'e not unconnnon in the adaxial epidermis. Note
partieularl.N' the I'elatively small size of the guard cells and the air
chamber pi-esent beneath each stomate. Trichomes, represented
by capitate <nid simple hairs, occasionally devc-lop but these struc-
tures are more abundant on the abaxial surface of the lamina.
The abaxial epidermis is fundamentally similai- to the adaxial
epidermis except that the cells are somewhat smaller and thinner-

MATERIAL FOR STUDY OF LEAF ANATOMY 125

walled. IStomata are more abundant in this layer of the lamina,
a connnon situation in many angiosperms. "IStalked" stomata
are frequently present in the midrib region. Multicellular capi-
tate halt's, lying within shallow depressions, are relatively com-
mon and consist of a terminal group of densely protoplasmic
secretory cells (with rather thick outer walls) which are seated
upon a small unicellular stalk. Haberlandt (p. 240 ) suggests that
the capitate hairs of Syringa may absorb thin films of water from
the leaf surface but this supposed function requires further in-
vestigation.

(b) The mcsophyll. This tissue region is composed of two
types of parenchyma, viz.: (1) the palisade parenckyma which
is found directly beneath the adaxial epidermis and consists of
rather narrow, thin-walled, somewhat "rectangular" cells, the
long axes of which are perpendicular to the epidermis. Notice
that intercellular air spaces are prominently developed in the
palisade parenchyma. In addition to a prominent nucleus, each
palisade parenchyma cell contains a large number of peripheral
cJiloroplasts in which the process of photosynthesis is carried on.
Directly internal to the abaxial epidermis occurs (2) the spongy
parc7ichyma which is composed of thin-walled irregular cells
which have no definite orientation and are very loosely arranged.
Notice that in many instances adjacent cells of the spongy par-
enchyma touch each other at their narrowest points so that the
maximum of wall surface borders upon the large air spaces.
The cells contain a peripheral protoplast and a smaller number
of chloroplasts than occurs in the cells of the palisade parenchyma.
The spongy parenchyma has at least two important functions.
First, because of its loosely arranged cells, it acts as a "ventilat-
ing tissue" of the leaf, i.e., diffusion of COi.. w^ater vapor and O2
between the air lacunae and the cells can take place with relative
ease. Second, the spongy parenchyma carries on some photo-
synthesis although this function is more efficiently performed by
the palisade parenchyma. According to Haberlandt (p. 287) no
trans-location takes place from one palisade-cell to another but
"the stream of synthetic products" follows the long ajrcs of these
elements. Some anatomical evidence in support of this view is
furnished by the fact that small groups of 2-10 palisade cells in

126 THE LEAF

certain plants converge at their lower ends and rest upon the
upper dilated end of a cell of the spongy parenchyma. Ilaber-
landt regards these specialized cells of the spongy parenchyma as
collecting cells which receive the products of photosynthesis from
the palisade cells and transmit them directly or indirectly to the
main vascular channels.

(c) The vascular system. Investigate first of all the general
structure of the midrib, and note particularly the absence of
photosynthetic parenchyma from this portion of the blade. In-
stead of the usual spongy and palisade parenchyma, the median
vascular bundle is covered on both sides by a number of layers
of isodiametric thick-walled cells, the outermost of which are
thickened in such a manner as to resemble coUenchyma. (Note:
This ''replacement" of photosynthetic parenchyma by col-
lenchynia and thick-walled parenchyma occurs to a less extent in
connection with the smaller vascular bundles of the Made. In
other types of leaves sclerenchyma may be present — mechanically
a ' ' girder effect ' ' is produced. ) Several smaller vascular bundles
with the phloem towards the adaxial surface are usually seen in
the upper part of the midrib; these ''accessory hioidles" unite
basally with the single leaf trace. A certain amount of secondary
growth has taken place in the median vascular bundle, a pheno-
menon which is of rather general occurrence in the larger veins
of dicotyledonous leaves. The conspicuous secondary xylem is
composed of conducting elements (probably both tradieids and
vessels) and fibers arranged in more or less definite radial rows
and uniseriate xylem rays which extend across the "cambial
zone" into the phloem. Tlie primary xylem (which lies above tlie
secondary xylem) consists of cells which show more or less of a
radial arrangement and which are imbedded among xylem paran-
chyma. The secondary phloem consists of polygonal thin-walled
sieve-tubes which are associated with small somewhat triangular
densely protojilasmic companion cells, phloem parenchyma, and
uniseriate phloem rays, each of which frequently terminates in a
large parenchyma cell. The primary phloem is indefinite and dif-
ficult to distinguish. The "cambial zone" can be distinguished but
is not nearly as i)i-oinineut or distinct as in llie case of a stem.
Nevertheless, three or more i-adial rows of differentiating cells

MATERIAL FOB STUDY OF LEAF ANATOMY

127

call be rather clearly seen. In general, cambial activity is never
prolonged in the veins of most leaves. In the smaller veins of
the lamina, the vascular tissue is considerably reduced in amount
and secondary growth may be entirely lacking. In certain regions
of the blade, the diverging bundles may be cut more or less
obliquely so that the characteristic type of primary xylem ele-
ments may be recognized. The very small leaf veins may consist
of several parenchyma cells and a few primary xylem elements ;
a bundle of this character is usually surrounded by a jacket of
parenchyma cells containing chloroplasts (i.e., the so-called "bor-
der parenchyma").

2. The leaf blade of corn {Zea Mays) . Obtain a transverse sec-
tion of a corn leaf and examine it under low power. The adaxial
epidermis is readily identified by the presence of groups (3-5
cells) of somewhat lens-shaped, apparently empty cells. These
cells are known as bulliform cells and by changes in their turgor
allow the leaf-blade to curl or uncurl, a phenomenon which may
be advantageous in restricting the loss of water from the leaf
under arid conditions. The typical epidermal cells of both the
abaxial and adaxial epidermis are somewhat oval in transverse
section (actually they are rather elongated cells) and are provided
with a definite cuticle. Stomafa, with conspicuous air-chambers
beneath them, are present in both epidermal layers. Occasional
unicellular, sharp-pointed hairs occur on the adaxial epidermis.
The mesophyll tissue of the leaf shows no clear differentiation
into palisade and spongy parenchyma but instead is composed of
several layers of rather compact parenchyma cells.

The vascular system of the leaf consists of a parallel series of
collateral bundles. The majority of the bundles are rather small ;
at intervals, fairly large bundles occur. Examining one of the
small bundles under high power, note that it is completely sur-
rounded by a bundle sheath of rather large "isodiametric" par-
enchyma cells which contain chloroplasts; the bundle sheath,
sometimes termed the "mestome sheath,'' may act as a conducting
layer which presumably transports the products of photosynthe-
sis directly to the vascular system, i.e., the phloem. The xylem of
each bundle is directed towards the adaxial surface of the leaf
and consists only of small tracheae. The phloem of the bundle

128 THE LEAF

is nearest tlie abaxial surface of the leaf and at most is formed
of a few small sieve-tubes and companion cells; in verj- small
bundles, the phloem may be represented by parenchyma cells.
The structure of a large vascular bundle in the leaf is quite simi-
lar to the anatomy of a stem bundle (ef. Exercise XII). Notice
particularly the clear distinction between the sieve-tubes and
companion cells of the phloem and tlie presence of an air lacuna
at the adaxial edge of the xylem. Fibers are present on both
edges of the bundle and may even partially surround it ; notice
the thick-walled character of the epidermal cells directly adjacent
to the fibers. This arrangement of fillers on either side of the
vascular bundle is quite characteristic of grass leaves and is re-
garded as a very efficient "plan" for securing mechanical
strength in the leaf.

3. The bud scede. In general, bud scales are distinguished
anatomically from foliage leaves by (1) their greatly reduced
vascular system, which may consist of a series of parallel or
dichotomizing veins, and (2) by a simple type of undifferentiated
mesophyll. The outer bud scales of certain trees may produce a
well-developed peridenn beneath the outer epidermis (e.g.,
Aesculus). Mechanical cells, such as fd)crs and sclereides, are
often prominent for example in Camellia, Fagus, Querrus, and
Populus. (For further details, consult Foster, 1928, i)p. 137-
146.) Study prepared trans-sections of the bud scales of several
of the forms listed above.

4. The leaves of giimnO'Sperms. Examine stained trans-sec-
1 ions of the needles of riinis noting especially the sunken stomata,
tile invaginatcd wails of the mesophyll cells nnd the vascular
bundle (in some s])ecies two bundles are present) embedded in
transfusion tissue. (Tf. Fames and MacDaniels, pp. 307-308.)
Foi- comparison, study trans-sections of the fan-shaped lamina of
the foliage leaf of Ginkgo bdoba. (A discussi(m of the histology
of this leaf is given by (liamberlain, 1935, pp. 191-193, and Fig.
210, p. 190.)

III. Suggested Drawings and Notes. —

1. Prepare a diagrannnatic drawing of the trans-section of
the lilac leaf, indicatinii- the position and extent of all important

REFERENCES 12*J

tissues. Draw, showing eellular detail, a narrow sector throngh
tlie tliin portion of the blade. This drawing should include a
vein. Label all cells and important structures.

2. Draw in detail the cellular structure of a small sector
through the corn leaf. This drawing should include a group of
bulliform cells and at least one well-developed vein.

3. Prepare diagrams to illustrate the structure and the
arrangement of tissues in the bud scale of some angiosperm and
in the foliage leaves of Piniis.

4. Outline the differentiation of a "typical" dicotyledonous
leaf, such as Nicotiana or PcJar(j(nnnm (cf. Foster, 1936).

5. Explain what is meant by tlie expressions "sun leaves"
and "shade leaves," (Cf. Ilaberlandt, pp. 295-297.)

REFERENCES

1. Arber, A.. The Interpretation of Leaf and Root in the Angio-
sperms. l^,iol. Kev. 16 :8l-l()r). 1941.

2. Chamberlain, C. J., Gymnosperms, Structure and Evolution.
Chicago, 1935.

3. Cross, G. L. Development of the foliage leaves of Tajochum
distichum. Amer. Jour. Dot. 27 :471-482. 1940.

4. . Some histogenetic features of the shoot of

Crypiomcria japonica. Amer.' Jour. Bot. 28:573-582. 1941.

5. Eames and MacDaniels, Ch. XII.

6 Foster, A. S., Salient Features of the Problem of Bud-scale
Morphology. Biol. Rev. 3 :r23-164. 1928.

7. , , Leaf Differentiation in Angiosperms. Bot.

Pvev. 2:349-'372. 1936.

8. Ilaberlandt, Ch. VI.

9. Ilavward, Ch. Ill, pp. 75-87.

10. Jeffrey, Ch. XIV.

11. Meyer, F. J., Das trophische Parencbym. A. Assimilations-
gewebe. Ilandb. d. Pflanzenanatomie. IV. Berlin, 1923.

12. Pfeiffer, II. Die pflanzlichen Trennungsgewebe. Ilandb. d.
Pflanzenanatomie. V. 1928.

13. Troll, W., Vergleichende Morphologic der hoheren Pflanzen.
Bd. i. Zweiter Teil. 1-4. Lieferung. (Morphologic des
Blattes) Berlin, 1938-39.

Exercise XIV

THE ROOT

I. Introduction. — Except in the Psilotales, Salvinia and a few
specialized parasitic forms, roots are a tj'pical feature of the
sporophyte of vascular plants. In many of the lower tracheo-
phytes, the primary root is short-lived and numerous adventi-
tious roots soon begin development from various portions of the
shoot system. In seed plants, however, the radicle often reaches
great prominence and in such cases is termed a tai) root. Addi-
tional fibrous roots in seed plants arise by successive branchings
beginning with the first root but are also commonly formed adven-
titiously from the stem. Roots perform a number of important
physiological functions. Primarily, they serve as organs which
absorb water and solutes from the soil solution. In addition,
they are very important as structures which anchor the plant
firmly in the soil. The storage of reserve food material also
occurs in most roots to some extent and is very obviously dis-
played in the fleshy "roots" of carrot, beet, turnip and similar
econonnc plants. Although the usual environment of roots is
the soil, aerial roots are produced in certain vines which serve
to attach the shoot firmly to the surface upon which it may be

grownig.

1. Origin and structure of the primarif tissues of the root.
The primary structure of the root differs from that of the stem
in a number of respects. The following brief resume of these
differences will emphasize the salient histological eluiracteristies
of the root, viz. :

(a) The structure and growth of the root apex. In marked
contrast to the superficial position of the terminal meristem of the
shoot, the root apex consists externally of a root cap which acts
as a protective buffer to the delicate meristem beneath it. A
wide variety of "types" of apical structure have been described

130

PRIMARY TISSUE REGIONS 131

for the roots of various seed plants (cf. Ilayward, pp. 44-47),
but it is evident that this region of the root deserves further
intensive study with the aid of modern botanical microtechnique
(cf. von Guttenberg, 1940, 1941). The essential point here is
that the activity of the terminal raeristem of the root is funda-
mentally different from that of the shoot apex. In the latter,
exogenous leaf primordia arise from the flanks or base of the
apex. But in the root, two dissimilar patterns of differentia-
tion originate from the terminal meristem, one leading to the
outward addition of new cells to the root cap, the other con-
tributing new cells to the main but unsegmented body of the
root. These differences are at present impossible to explain but
it is evident that they determine the fundamental morphological
differences between root and shoot (cf. Arber, 1941).

(b) Primary tissue regions. Young roots, prior to secondary
growth, resemble stems in the presence of epidermal, cortical and
stelar regions. The epidermis of roots is usually devoid of a
cuticle and stomata and its chief role appears to be that of
absorption, a process which is favored by the development, behind
the region of elongation, of a zone of root hairs. The cortex of
roots is often entirely composed of thin-walled storage paren-
chyma and is soon destroyed if secondary growth occurs. Appar-
ently the endodermis, which is commonly regarded as the inner-
most layer of the cortex, is a consistent feature of roots, in con-
trast to its variable distribution in the stem. But without doubt
one of the most fundamental characteristics of the young root
is shown by the arrangement and development of the primary
vascnlar tissues of the stele. In striking contrast to the collateral
position of xylem and phloem in the siphonostele of typical stems,
these tissues are arranged in a radial and alternate pattern in the
root. Thus, in the anatomical sense, there are no true primary
vascular bundles in the stele of the root. As seen in trans-sec-
tional view, the primary xylem and phloem of the root appear
as separate and alternating strands of tissue. Very frequently,
the xylem "plates" as they are often termed, meet in the center
to form a solid core. In many roots, however, particularly in
monocotyledons, the center of the stele is occupied by a core of

132 THE ROOT

pai-eiR'hyma Avhicli resembles the pilh region of tlie stem. Al-
though the number of phloem and xylem groups in a given root
is equal, variations occur between the roots of the same i)lant as
well as between different species (ef. p]sau, 1941, p. 452, Table I).
There appears to be no satisfactory explanation for the incon-
stancy in the number of phloem and xj'lem strands so character-
istic of the roots of some angiosperms. Depending upon the
number of primary xylem groups, the stele is described as diarrh
(2), triarch (3), tetrarch (4), pcntarch (5), etc. The stele in
monocotyledons usually consists of many alternating xylem and
phloem groups and hence is designated as pohjarch.

In further contrast to the stem of seed plants, the primary
xylem of the root is exarch. This means that the radial matura-
tion of the tracheary elements from jirovascular tissue occurs in
the centripetal direction. Ilence the protoxylem lies at the outer-
most edge of each xylem strand, next to the pericycle, while the
metaxylem is situated towards the center of the stele. Since
exarch primary xylem is only found in the stems of the lower vas-
cular plants (e.g., the Psilopsida and Lycopsida), the root of seed
plants has been regarded by some anatomists as a "conservative"
or "primitive" organ (cf. Jeffrey, Ch. XII). Much work needs to
be done on the comparative histogenesis of the primary tissues in
the root. In several recent studies, Esau (1940, 1941) has made
important contributions to our knowledge in this direction. It is
interesting to note that her observations indicate that the phloem
begins to differentiate nearer the apical meristem than does the
xylem. In the protaphloem of carrot, for example, "the sieve-
tubes complete their differentiation about 300 microns from the
apex of the root" whereas the first protoxylem elements "show
secondary walls approximately 1 millimeter from the root apex,
but do not lose Iheir jirotoplasts through another millimeter of
the root."

(c) Origin of lateral root>i. Branching of the stem normally
originates from lateral buds which develop at or near the shoot
apex from superficial cells or cell layers. In marked contrast,
the hranchinej of fhe root 7.s stricthf endogenous. The origin of
lateral root primordia usually occurs through the renewed growth
and division of certain cells in the pericych of the stele distal to

iSKCOXDAUV GUOWl'll IX liOOTS 133

tlie zone of root hairs. According to Ilayward (p. 51), in some
plants the adjacent cells of the endoderinis may contribute to the
formation of the lateral root primordinm. Arnold (li)40) has
shown that in the water hyacinth {Eichhornia crassipes) lateral
roots arise in the "innnature i^ericycle" and "at the forward end
of the region of elongation." Because of its internal origin, the
further develoi^ment of the lateral root involves its penetration
through the endodermis, cortex and epidermis of tlie mother root
to the outside. Just how this occurs is not entirely clear. The
suggestion has been made that the mechanical pressure exerted
by the emerging lateral root may also be accompanied by some
kind of chemical dissolution of the tissues interposed in its path.
In roots with three or more xylem plates, lateral root primordia
typically a])pear opposite each of the protoxylem points. Con-
sequently, ludess injuries or abnormalities occur, the lateral roots
tend to emerge in vertical rows which are equal in number to the
xylem groups. T>ut in diai-ch steles, lateral roots may appear
at each side of tlie two phloem groups. In this case there w^ould
be four vertical rows of lateral roots (cf. Esau, 1940, pp.
190-194).

2. Secondarji (jroirth in roofs. The roots of many herbaceous
dicotyledons and of all woody plants exhibit secondary growth in
thickness. However, because of the radial, alternate arrangement
of the primary vascular tissues, the cambium first appears as sepa-
rate bands of periclinally-dividing cells which originate from
])arenchyma cells internal to each phloem group. At these points
formation of secondary phloem outwardly, and secondly xylem
inwardly, occurs as in a typical stem. In woody plants, the orig-
inally separate strips of cambium finally become united laterally
as a result of tangential divisions in the pericycle external to
each xylem group. Thus at an early stage, the vascular cambium
in this type appears lobed in trans-section. Ultimately, by the
formation of secondary phloem and xylem external to the xylem
plates, the contour of the cambium becomes cylindrical. In roots
of this type, the primary xylem eventually becomes completely
surrounded by a cylinder of secondary xylem. In certain her-
haceous plants, in contrast, the cambium, at points opposite the
protoxylem points, forms broad parenchymatous rays so that a

134 THE ROOT

dissected type of secondary vascular cylinder results (Jeffrey,
pp. 156-157 and liayward, pp. 48-51). When secondary growth
is pronounced in a root, the primary phloem, cortex and epi-
dermis soon become crushed and slough away. In trees, a
typical "bark" is produced and, save for the exarch primary
xylem in the center, all structural resemblance with a root is lost.
The first periderm layer of the root arises by the formation of a
phellogen in the pericycle. Later-formed phellogen layers may
subsequently appear, as in the stem, from living cells in the sec-
ondary phloem.

II. Material for the Study of the Root.

1. The root of buttercup (Ranunculus sp.). Obtain a stained
trans-section of the root and study the following tissues and
regions beginning at the edge of the section :

(a) The epidermis, a nniseriate but broken layer of collapsed
and partially destroyed cells. The imperfect condition of the
epidermis presumably is due to the abrasive effect of the soil on
the root. Notice that a more or less disorganized protoplast is
visible in some of the epidermal cells.

(h) Within the epidermis occurs the rather broad, homo-
geneous cortex which is composed entirely of rather thin-w^alled,
"isodiametric" parenchyma cells most of which are separated
from one another by prominent intercellular air spaces. Observe
that while the outer layers of the cortex are composed of rather
tightly joined empty cells (forming a **hypodermis"), the inner
cortical cells contain prominent starch grains. Large, somewhat
irregidar simple pits are visible on the end walls of the cortical
parenchyma cells.

(r) The center of the root is occupied by tiie stele which is
externally separated from the cortex by a uniseriate cylinder
of cells, the endodcrmis. Study the endodermis under high
power, noting the presence of a protoi)last in many of the cells.
The salient feature of tlie endodermis (in llic primary condi-
tion) is the presence of a suberized or cutinized band which
extends comi)le1cly ;il)()ii1 1h(> inner surface of tlie radial and
end walls of each cell. These band-like thickenings of the wall
are known as Casparlan strips and in i-ecent years have received

MATERIAL FOR STUDY OF THE ROOT 135

a great deal of attention because of the apparent physiological
importance of the endodermis as a cellular layer which regulates
the entry of water and mineral salts into the vascular system of
the stele. The assumption is made that the suberized nature of
the Casparian strip renders it impermeable to water so that
diffusion must take place through the tangential walls and the
protoplasts of the endodermal cells, i.e., through a semi-perme-
able membrane. However, the exact function and significance
of the endodermis is in need of much further investigation.

In the endodermis of Ranunculus, the presence of the Cas-
parian strip is indicated by the red color of the short radial walls.
Individual cells of the endodermis may have uniformly and
heavily thickened walls, a phenomenon previously recorded by
Caspary in the case of Ranunculus Ficaria (ef. De Bary, p. 123).
In the roots of certain plants, all of the cells of the endodermis
may be thick- walled in character (cf. Eames and MacDaniels,
p. 102, Fig. 51). Within the endodermis occurs a single layer
of living cells which is known as the pericycle. The most con-
spicuous portion of the stele is represented by the primary vascu-
lar tissue which shows the characteristic radial arra7igement of
the xylem and phloem goups. It will be seen that four radial
plates of xylem (which join in the center of the root, thus form-
ing a protostele) are present; the stele is therefore designated
as tetrarch. The protoxylem (which presumably consists of
annular and spiral elements) is represented by several very small
"polygonal," thick-walled cells found at the outer edge of each
xylem plate. The metaxylem (which probably is composed of
pitted elements) consists of much larger cells and as stated previ-
ously forms a homogeneous tissue in the center of the root ; a pith
is thus absent. Laterally adjacent to each xylem plate near its
outer edge, you will find one or two rather thick- walled cells,
which are approximately hexagonal as seen in transverse section
and which appear distinct from the xylem because of their
lighter-staining walls and the possession of a more or less disin-
tegrated protoplast. The shape and structure of these cells sug-
gest that they are sieve-tubes ; their exact morphology is difficult
to interpret but they seem to belong to the phloem. Each of the
four phloem groups is sepai-ated laterally and internally from

136 THE ROOT

the xylem by one or more layers of parenchyma. The phloem
consists of thin-walled living cells which, as in many plants, are
distinguished with difficulty from the surrounding i)arenchyma.

2. The root of Smilax, Zea or some other monocotyledonous
type. Examine earefulh' the trans-section, noting particular!}'
the polyarch stele and the central pith-like region.

3. The origin of lateral roots. Study transverse and longi-
sections of the root of the water hyacinth (EichJwrnia crassipes)
and observe the method of origin and early ontogeny of the lat-
eral root primordia. For comparative jnirposes. make a similar
study of lateral root development in bean or willow.

4. Secotidary growth in roots. Study a series of trans-sections
of the root of a woody oi- herbaceous plant cut at levels suc-
cessively distal to the region of maturation of the primary vascu-
lar system. Observe the method of origin of the cambium and
the formation of secondary vascular tissues and the periderm.

III. Suggested Drawings and Notes. —

1. Prepare diagrammatic drawings of trans-sections of the
root of Banunculus and of some monocotyledonous type showing
the position and extent of all the primary tissues. Draw in detail
a portion of the stele in each type to illustrate the structure of
the primary phloem and xylem.

2. Prepare diagrams based upon both transverse and longi-
sections to illu.strate the origin of lateral roots and their emer-
gence to the surface of the mother root.

3. Prepare a series of diagrams based upon material studied
in the laboratory to illustrate the origin of the cambium and the
effects of secondary growth upon the primary tissues of the root.

REFERENCES

1. Arber. A., The Interpretation of Tioaf and Root in the Angio-
sperms. Biol. Kev. K; :81-10:). 1941.

2. Arnold. C. A.. A Note on the Origin of the Lateral Kootlets
of Eichhornia crassipes (]\Iart.) Solms. Amer. Jour. Bot.
27 :728-73(). 1940.

3. De Bary, pp. 348-360.

4. Eames and MacDaniels, Ch. X.

REFERENCES 137

5. Esau, K., Developmental Anatomy of the Fleshy Storage
ih'ij^-dn ot DaucHs carota. llilgardia 13:175-226. 1940.

6. , , Phloem Anatomy of Tobacco Affected with

CuHv Top and .Alosaic. //;/>/., 13 :437-49(). 1041.

7. Ilavward. Ch. J I.

8. Jeffrey. Ch. XII.

9. Von Gnttenbero', H. Der primare Ran der Ano'iospermen-
wnrzel. Ilandb. d. Pflanzenanatomie. VIII. P>pi-lin. 1940.

10. , Der primai'e Ban der Gymnospermenwnrzel.

Ibid.: Berlin, 1941.

APPENDIX

The following brief notes on certain phases of microtechnique
are given here to facilitate the use of this book by the teacher and
student. For full information on the various procedures used
in preparing tissue for microscopic study, reference should be
made to the publications of Chamberlain (1932), Rawlins (1933),
Johansen (1940), and Sass (1940), cited under "General
References. ' '

Free-hand Sections

In many of the exercises in this book, directions are given for
the study of sections cut by hand from living stems, leaves or
other plant structures. To prepare such material requires only
simple technique and in addition provides a realistic picture of
cells and tissues which should precede the examination of micro-
tomed and permanently-stained preparations. In the laboratory
the student can acquire the necessary skill with a sectioning-
razor to enable him to explore the structure of such tissues as the
epidermis, parenchyma and collenchyma. Sections cut by hand
should be carefully mounted on a clean slide either in distilled
water or in the various reagents designated and the cover-glass
lowered gently into place. For more resistant cells, such as
sclereides or fibers and for the critical study of the sieve-plates
in phloem elements, the nse of the carbon dioxide freezing micro-
tome is highly desirable. With the aid of this instrument, a large
number of thin sections may be jirepared by the instructor in
advance of class use. The student must learn to check free-hand
preparations at frequent intervals so that the sections are not
allowed to dry out. Cells immersed in fluid are not only easier
to study from an optical point of view but they also retain a
more or less normal structure over a relatively long period of
observation. Sections of hairy objects, such as many leaves or
stems, are often dii^icult to mount in water without the formation
of numerous air-bubbles. This difficulty may be removed by

139

140 APPENDIX

inuiuitiiig .sueli sections in a weak .sululion of aleuliol. This may
act as a Ivilling reagent for the protoplasm but it does mal^e pos-
sible the accurate study of the shape, arrangement and character
of the walls of cells.

Prepared Slides

The use of permanent slides is essential in the study of many
of the topics outlined in this book. This is particularly ti'iie for
the work to be done under Exercises III, X, XII. XIII. and
XIV. Suitable preparations as a basis for class study are obtain-
able from commercial supply houses or may be prepared for the
student directly. With reference to the latter possibility, detailed
suggestions for the collection, fixation, sectioning, and staining
of tissues and organs are presented systematically in the I'eeent
manuals on microtechnique by Johansen (1940) and Sass (1940).

Macerated Tissue

One of the most important skills which the student must
develop in laboratory practice is the ability to visualize cells as
three-dimensional bodies. This is often extremely difficult on the
basis of the examination of sections which tend to create a two-
dimensional concept. Furthermore, many definitive features of
cells, particularly the structure and arrangement of pits and
fibrous thickenings in traeheary elements, and the character of
perforations in vessel elements, can best be studied in isolated
cells. For these reasons, a study of macerated tissue is recom-
mended for many topics in this Ijook and is especially d(»sirable
in connection Avith Exercises II, VIII. IX, and X. The macera-
tion of plant tissue is most effectively accomplished by the use
of certain reagents which dissolve the intercellular substance and
thus cause the sei)aration of a piece of tissue into its comp(ment
cells. Jeffrey's method is usually satisfactory. Sm<dl i)ieces of
the material, no thicker than a match, are placed in a glass vial
containing a mixture of equal parts of lO'^f clu'omic jicid and
10% niti-ic acid. The vial is then coiked and ])Iac('d in an elec-
tric oven at a teini)erature of .■}0°-40° C. until the material
becomes soft or *'mushv" in tcxinro. llai-d material, such as

APPENDIX 141

wood and the sliells of nuts may require several days in the oven,
during' which time it is advisable to change the macerating fluid
once or twice. Boiling small slivers of wood before placing them
in the acids drives out the air and accelerates the maceration
process. The macerated tissue is carefully washed in distilled
water to remove as much of the acid as possible and can then
be transferred to 50% alcohol for future study. Often effective
results may be secured by staining the isolated cells in safranin.
For class use, it is only necessary to agitate the alcoholic sus-
pension of cells and to add a drop witli a pipette to a slide in
order to secure a fairly representative "sample" of the desired
cell types. Permanent preparations of macerated tissue are
easily made by placing small quantities of cells in water on a
slide, evaporating the excess water on an electric hot-plate and
mounting in glycerine-jelly. Circular cover-glasses should be
used, the edges of which can be sealed with some type of cement
which prevents drying out and the entrance of air.

Special Keagents

1. Phloroglucin and Hydrochloric Acid. — The addition of
these reagents produces a red color in the walls of sclereides,
lignified fibers and tracheary elements (cf. Exercises VIII, IX,
and X). The stain is not permanent but nevertheless is ex-
tremely useful in demarcating the thick walls of certain types of
cells. A saturated solution of phloroglucinol should be pre-
pared in 95% alcohol. IMount the section or tissue fragment
directly in a drop of this reagent on the slide and add a cover
slip. Then introduce a drop of concentrated hydrochloric acid
at the edge of the cover slip. Great care should be taken to carry
out this procedure some distance away from the microscope.

2. Potassium Iodide (IKI) and Sulphuric Acid. — This is a
specific test for cellulose. Mount the sections in the potassium
iodide (1 gr. iodine and 3 g. potassium iodide in 300 cc. of
distilled water, according to Rawlins, 1933) and add a cover slip.
The introduction of a drop of 65% sulphuric acid will cause
cellulose walls to turn blue in color.

142 APPENDIX

3. Aniline Blue. — This is a specific stain for callus depositions
on sieve-plates and is essential for the procedure outlined in
Exercise XI, Sections should be immersed for a short time in a
.1% aqueous solution, and then transferred, after gentle washing,
to a drop of water. The callus on the sieve-plates is stained blue.
Dr. A. S. Crafts has suggested to the writer the following
improvement: Place the sections in IKI, wash in water, stain in
aniline blue for about five minutes, then wash briefly again with
IKI and mount for study in tap water or glycerine.

4. Neutral Red. — This vital stain is very useful in staining the
vacuome of plant cells and is recommended for use in Exercises
V, VI, and VII. Mount the sections directly in a .1% aqueous
solution.

5. Sudan IV. — This reagent is specific for the cuticle, and for
cutinized and suberized cell walls. Place the sections in a drop
of alcoholic solution of Sudan IV (.5 g. in 100 cc. of 80% alcohol)
on a slide and warm gently over an alcohol flame. Add a cover-
glass and examine under the microscope. The cuticle, as well
as waxy materials present in walls are stained red. This reagent
is very desirable for use with Exercise V.

INDEX

ABIES, structure of shoot apex,
25

Accessory vasoulai- bundles, in

leaf of Si/riiiga, 126
Aesculus, colletei's on bud scales

of, 54
Albuminous cells, of gynmo-

sperinous pliloem, 94
AUiiu)!, study of epidoniiis of,

48-49
Aniyloplasts, in cells of storage

organs, 4
Aniline blue, stain for callus, 07,

142
Anisotropic, use of term, 9
Annual ring, structure of, 115
Annular elements of protoxvlem,

84
Anthocyanin pigments, in vac-

uome of cells, 3
Apex (see shoot apex, root apex,

apical meristems)
Ajiical cell, highly vacuolated

character of, 20
meristems, 21-27
Appendix, 139-142
Apposition, method of Avall-thick-

ening, 9
Aristolocliia, studv of stem of,

119-120
Astrosclereides, in fruit of Cariju

and Juglans, 70-71
in leaf of Camellia, 71
structure and occurrence of, 68
Arena, origin of leaf in, 24

BARK, general comjiosition of,
107

Bars of Sanio (see Crassulae),

13, 89
Basswood, bast fibers of, 77-78,
113
cell walls in pith and cortex, 11
study of stem of, 111-117
Bast fibers, of TUia, 11, 77-78, 113

wall structure in Tilia, 11
Bean, macrosclereides in testa of,
71
primary xylem in hyi^ocotvl of,
87^ 88
Begonia, photosynthetic paren-
chyma of, 59-60
Beta, vessel development in, 84
Betula, vessel elements of, 89
Bicollateral vascular bundle, in

stem of Cucurhita, 98
Birch, vessel elements of, 89
Black locust, ontogeny of vessels

in, 83-84
Blind pit, use of term, 11
Boehmeria, bast fibers in, 75

fiber elongation in, 76
Bordered i^its, of tracheids, 82
pit-pairs, in ti'acheids of Pinus,
13, 88-89
study of, 13
Brachysclereides, in fruit of Pn-
rws, 70
structui-e and occurrence of, 68
Bracken fern, primary xylem

tracheae of, 88
Branch gaps, use of term, 102
Branch traces, use of term, 102
Bud scale, anatomical character-
istics of, 128
study of anatomy of, 128

143

144

INDEX

Bullifonn cells, in leaf epidermis

of Zea, 127
Buttercup, study of root of, 134-

136

CALAMOVILFA, ontogeny of

tracheae in, 85
Calcium oxalate, in crystals, 4-5
Callus, definitive, 97

cylinders, in sieve-plates, 97
Calyptrogen, jjroduces root cap,

26
Cambial activity, effects on pri-
mary tissues, 106-107
initials, 27-28, 106
wall, use of term, 9-10
zone, use of term, 106
Cambium (see also vascular or
cork cambium)
characteristics of, 86-87
origin and activity of in roots,
133-134
Camellia, astrosclereides in leaf

of, 71
Carrot, chromoplasts of, 3
Cariia, astrosclereides of, 70-71
Casparian strips, of endodermis

of root, 134-135
Cedriis, structure of shoot ajiex,

25
Celery, structure and ontogeny of
eoUenchyma, 63-64
vessel development in, 84
Cell, application of term, 2
ergastic substances in, 4-6
Hooke's use of term, 1
modern use of term, 1
jilastids of, 3
])rotopIast of, 1-6
plate, relation to intercellular

layer, 9
saj"), in hail- cells, 2
theory, 1

types, tabulated sununary of,
40-43

Cell wall, absence in certain
cells, 7
criteria for layers of, 9
gross layers of, 8-11
in sieve-tube elements, 97-98
reversible changes in, 10
use in classifying cell types, 7
Cellulose, in secondary wall, 10, 75
Chlorojilasts, in guard cells, 47
in leaf cells of Elodea, 3
in mesophyll of leaf, 125
of Pellionia, 4
Chromoplasts, in Da tic us, 3

obscure role of, 3
Closing layers, of lenticels, 108
Coenocytes, relation to "Cell

Theory," 1
Collateral vascular bundle, in corn

stem, 118
Collenchyma, air spaces in, 64
functions of, 62, 64
material for study of, 64-65
occurrence of, 62
ontogeny of, 63-64
regressive differentiation of, 62
shape of cells in, 62
structure and composition of

walls of, 62-63
types of wall-thickening of, 62-

63
use of term, 62
Colleters, structure and occurrence

of, 52-53
Companion cells, in ])hIo(Mii of
Ci(ci(rhif(i, 9!)
origin aiul sti'ucfui'e of, 93
Comi)lementarv tisssue, of lenti-
cels, 108
Composite cylinder, fornialion of

by selereides, 68-69, 120
Coiii])onnd middle lamella, studv
of, 11
use of term, 10
sieve-plate, use of term, 96
Cork (see also 2)liclleiii)

INDEX

145

Cork, origin iuid structure of, 107
canibium (sec also jjliel logon)
origin and tuiK-tions of, 107-108
Corn, guard cells of leaf, 49
study of leaf of, 127-128
study of stem of, 118
Corpus, method of growth, 23

role in tissue differentiation. 23
Cortex, of roots, 131
of stems, 102-103
origin of, 23
Cotton, "fibers" of, 73
Crassulae, definition of, 13, 8!)
Crystals, in photosynthetic paren-
chyma of Begonia, 59
natui'e of, 5
role of nucleus in formation

of, 5
types of, 5-6
Crystal sacs, arrangement of, 5, 113
Cucurbita, sieve-tube elements of,
98-99
vessel development in, 84
vessel elements of, 84, 90
Cuticle, of epidermal cells, 46-47
Cutin, in walls of epidermal cells,

47
Cifcas revoluta, diameter of shoot
apex, 22
shoot apex, 20
Cystolith, development in F/ntf^,
50-51
use of term, 51
Cytoplasm, in hair cells, 2
Cytoplasmic strands, in hair cells,
2

streaming, in hair cells, 2-3
in leaf cells of Elodea, 3

DATURA, angular collenclnnna

of, 64-65
DaiiCHS, chrouioplasts of, 3
vessel development in, 84
Definitive — callus, on sieve-plates,

97

Delignification, of walls of sclere-

ides, 69
Dermatogen (see also protoderm,

tunica)
in shoot apex, 24
origin of epidermis from, 46
Dictyostele, general structure of,

104
Dilatation, of i^hloem rays, 114
Dioon eclule, shoot apex, 20
Diospyros, endosperm of, 8
Druses, form and structure of, 5
Duckweed, rajjhides in, 5
Dutchman's Pipe, study of stem

of, 119-120

EL AE AGNUS, scales of, 54
Elodea, chloroplasts in leaf of, 3

structure of shoot apex, 24
Emergences, origin and morphol-
ogy of, 51-52
Endarch xylem, in siphonostele of

stems, 103
Endodermis, functions of, 134-135

morphology' of, 104

of roots, 131

study of in Ranunculus, 134-135
Endosperm, plasmodesmata in, 8
Ephedra, vessel elements of, 82
Epidermal cells, form and struc-
ture of, 46-47

IHts in walls of, 47

wall-structure of, 47
Epidermal system, as defined by
Sachs," 34

origin of, 24
Epidermis, a uniseriate la^'er, 46

cell tyi^es in, 46

functions of, 45

Haberlandt's definition of, 45

material for study of, 48-49

multiple type of, 49-51

of Allium, 48-49

of geranium leaf, 49

of German-Ivy leaf, 49

146

INDEX

Epidermis, of Iris leaf, 49

of onion bulb-scale, 48-49

of Pelargonium, 49

of petals, 46

of roots, 131

of Senicio, 49

of stems, 102

of Syringa, 124-12.'5

origin from jirotoderm, 45-4()

use of term, 45
Ergastic substances, general na-
ture of, 4
Exarch xylem, in protostelo of
stems, 103

in stele of roots, 132

FASCICULAR cambium, use of
term, 106

system, as defined by Sachs, 34
Fibers, classification of, 74

form and length of, 74-75

functions of, 73

initials of, 76

material for study of, 77-78

microchemical tests for walls of,

multinucleate protoplasts of,

76-77
non-sclerotic types of, 75
occluded hunen of, 75
of flax, 73
of hemp, 73
of textile i)lants, 73
ontogeny of, 76-77
positions in plant, 74
sderenchymatous types of, 75-

76
sliding-growth of, 76
structure and composition of

Avails, 75
use of term, 73
vestigial pits in, 14-15, 75
Fiber-tracheids, develojiment of

septa in, 77
of Villus, 88

Ficits, cystolith of, 51

multiple epidermis of, 50-51

Flax, fibers of, 73-74, 75-76

Flower, anatomical interpretation
of, 101, 102

Free-hand sections, preparation
of, 139-140

Fundamental tissue system, as de-
fined by Sachs, 34

Fusiform initials, 27, 106

CERAXIUM, cell Avails in pith
and cortex, 11
druses in, 5
multicellular nnbranched hairs

of, 53
study of epidermis of, 49
study of stem of, 108-111
Ginkgo, shoot apex, 20
study of leaf of, 128
Grape-vine, septate fibers of, 78
(rround uieristem, origin of col-

lenchyiiia from, 63
GroAving point (see also shoot
apex), critique of term, 21
Guard cells, structure of, 47

HAIRS (see also Trichomes),

staminal hairs of Trades-

cantia, 2
types and structure of, 52
TIalf-bordered pit-pairs, stricture

of, 14
study of, 14
Ilanstein, histogen tiieory «)f, 23,

25-26
Hemp, fibers of, 73-74, 75
Hiium, in starch-grains, 4
Hi])f)iiiis, structui-e of shoot apex,

24
Histogen theory, as applied to

roots, 26
contrasted Avitli tunica-corpus

tlu'ory. 23-24
Hoisc-clicslinit, colletei's on bud

scales of, .54

INDEX

147

Hypericum, septate tiber-traeheids

of, 77
HyiDocotyl, study of primary

xylein of, 87, 88

ICE-PLANT, water vesicles of, 54
Idioblasts, illustration of, 36
in parenchyma tissues, 38
Inclusions (see Ergastie sub-
stances), 4
Intercalary growth, of leaf, 123

meristems, 18
Intercellular layer, structure and
origin of, 9
spaces, in collenchyma of celery,
64
in mesophyll of Stjringa, 125
in photosynthetic pai'en-
chyma of Begonia, 60
Interfascicular cambium, origin
of, 105
use of term, 106
Intussusception, method of wall-
thickening, 9
Iodine, test for starch in Pellionia,

4
Iris, study of epidermis of, 49
Isotropic, use of term, 9

JUGLANS. astrosclereides of,
70-71

LATERAL meristems (see also
vascular and cork cam-
bium), 18
roots, origin and development
of, 132-133
study of origin of, 130
Leaf, apical growth in, 123
evolution of, 123
from anatomical standpoint,

123
intercalary growth in, 123
origin of in grasses, 24
study of in Ginkgo, 128

Leaf, study of in I'iiius, 128
study of in Syringa, 124-127
study of in Zea, 127-128
Leaf anatomy, material for studv
of, 124-128
gaps, nature of, 102
traces, origin and development
of, 105-106
relation to leaf gaps, 102
use of term, 102
Leaves, variations and tvpes of,

123
Leiiina, raphides in, 5
Lentieels, function of, 108, 112
structure and development of,
108
Leucoplasts, function of, 4
Libriform fibers, of oak, 78
Lignin, in secondary wall, 10, 75-

76
Lilac, guard cells of leaf, 49

study of leaf of, 124-127
Linden, bast fibers of, 77-78, 113

study of stem of, 111-117
Linum, multinucleate fibers of, 76-

77
Liriodendron, study of vessel-ele-
ments of, 89
Lithocyst, use of term, 51
Lumen, occlusion of, 9
occlusion in fibers, 75
in libriform fibers, 78
in macrosclereides, 71

MACERATED tissue, of bean

cotyledon, 59
of bean hyiiocotyl, 88
of Betula, 89
of Cucurbita, 90
of fruit of Vary a and Juglans,

70-71
of Liriodendron, 89
of Pinus, 13, 88-89
of Platanns, 15
of Quercus, 78, 89
of rhizome of Pteriditim, 88

148

INDEX

Macerated tissue, of Tecomu, 12
of Tilia, 15, 77
of Trifolium, 88
of Vitis, 78

preparation of, 140-141
Macrosclereides, in testa of bean,
71
structure and occurrence of, 68
Maljiighian cells, in seed coat of

legumes, 68
Medullary rays, in stem of Arif<-
toiochia, 119-120
in stem of Pelargoiiaiii. 10!)
in stem of Tilia, 116
origin of, 105
Meristem ring, use of term, 105
Meristenuitic tissue, classical con-
cept of, 10, 28
difficulty of defining, 20-21
restricted concept of, 19-20
vacuome in cells of, 20, 28
Meristems, and open system of
growth, 18-19
apical, 21-27
definition of, 18
intercalary, 18
lateral, 27-28

of determinate organs, 19
types according to position, 18
Metse7)ib)\ita)itliei)i U)ii , water vesi-
cles of, 53, 54
Mesophyll. of leaf of Piints. 128
of leaf of Z('<(, 127
structure in Slj/ringa, 125-12()
^[estome sheath, in leaf of Zra.

127
^Ietai)hi()('iii, 86
:\ret:ixyiem. 85, 86, 132
Monocotyledons, study of stem of,

118
^lullciii, dendroid li;iii's of. 53
Midtiplc cpidcniiis, contrnslcd
with hypoderniis, 50
function of, 50
material for studv of. 50-51

Multiple epidermis, occurrence of
in angiosperms, 49-50
of Ficus elastica, 50-51
ontogeny of, 50
use of term, 50

NACRE, use of term, 97
Neutral red, for staining vacuome,
142
use for cambium, 28
use for collenchyma. 64
use for epidermis, 49
use for parenchyma, 59
Nicotiana, vessel developmeul in,

84
Nodal anatomy, types of, 102
Nucleolus in hair cells, 2
Nucleus, disintegration in sieve-
tube elements, 95
in hair cells, 2

OAK, libriform fibers of, 78

tracheids of, 89

vessel-elements of, 89-90
Onion, study of ei)i<lennis of. 4S-

49
Ontogeny, of collenchyma tissue,
63-64

of companion cell, 93

of epidermis, 45-46

of fibers, 76-77

of fibrous thickcniims in I ra-
cheae, 84-86

of leaf, 124

of leaf traces. 105-106

(.f Iciiticels, 108

of multiple epidermis, 49-50

(d' perforations in vessels, 83-84

of jx'i'idei'ni, 107-108

of pits, 12

of i)rimai'y \as<-u]ar tissues in
root, i32

of scales in SJicjilwrdia, 54

of sclereides, 68-69

of sieve-plates, 97

INDEX

149

Ontogeny, of sieve-tube elements,
!)4-i)7
of stele, 104-l(»(i, 131-132
of stoma, 48

of trac'hearv elements, 83-80
of vessel elements, 83-84
Organismal theorj', 1
Osteosclereides, structure and oc-
currence of, 68

PALISADE parenchyma, in leaf

of Syringu, 125
Pajier, cellular composition of, 90
Parenchyma, critique of concept
of, 57
form of cells of, 57-58
functions of, 58
material for study of, 59-60
photosynthetic, 59-60
j>r(jtoiilasts of, 58-59
regressive differentiation of, 58-

59
storage, 59

structure and chemistry of Avails
of, 58
Pear, stone-cells of, 70
Pelargonium,, cell walls in pith
and cortex, 11
druses in, 5
multicellular unbranched hairs

of, 53
study of epidermis of, 49
study of stem of, 108-111
J^eJIioiiia, starch-containing- cldor-

oplasts of, 4
Tepcromia, nmltiple epidermis of,

50
l-'erforation-plate, in vessel ele-
ments of Pferidiuni, 88
types and evolution of, 83
Perforations, distinctive of vessel
elements, 81
evolution of, 82-83
ontogeny of, 83-84
types of, 83

Pericycle, critique of, 74

functions of in root, 132-134
Pericyclic fibers, in stem of Ciicur-
^ bit a, 98
in stem of Pelargonium, 109
Periderm, function of, 107
origin of in roots, 134
structure and development of,
107-108
Persimmon, endosperm of, 8
Petunia, study of stem hairs of,

2-3
Phellem (see also Cork), origin

and structure of, 107
Phelloderm, origin and structure

of, 107
Phellogen (see also Cork cam-
bium), origin and func-
tions of, 107-108
origin of in roots, 134
Phloem, complex structure of, 93
fibers in, 74, 76, 94, 113
functions of, 94
of Cucurbita, 98-99
jiarenchyma, of Cucurbita, 99
rays, in stem of Tilia, 114

origin of, 27
structure of, 94
Phloroglucin and hydrochloric

acid, use as stain, 141
Phloroglucinol, use for bast fibers,
77
use for sclereides, 70-71
use for tracheary elements, 87
Picea, structure of shoot apex, 25
Pine, resin canals of, 117
secondary xylem of, 117
Piniis, bordered pit-pairs in, 13,
88-89
resin canals of, 117
study of cambium of, 28
study of leaf of, 128
study of phloem of, 99
study of tracheids of, 88-89
Pit aperture, use of term, 11

150

INDEX

Pit cavity, use of term, 11

membrane, general structure of,
11
Pith, origin of, 23

phylogeny of, 103
Pit-pairs, between tracheae and
living cells, 14

bordered, 13, 88-89

general structure, 11

half -bordered, 14

simple, 12

types of, 12-15

use of term, 11

vestigial, 14-15, 75, 77
Pits, function of, 12

in epidermal cells, 47

ontogeny of, 12

ramiform type in selereides, 68,
70

use in comparative study, 11

use of term, 11
Pitted elements, of metaxylem, 85
Plasmodesmata, functions of, 8

in pit membranes, 12

in sieve-plates, 9(5

in tobacco, 8

in walls of jihloem cells, 98

nature of, 7

relation to "Cell Theory," 2, 8

secondary, 7-8

study of in Diospyros, 8
Plastids, meaning of term, 3

types of, 3-4
Plataniis, dendroid hairs of, 54

vestigial pit-pairs in, 15
Polarized light, appearaiu'c of

walls under, 9-10
Pores, in sieve-plates, 96
Potassium iodide and sulphuric
acid, use as stain, 141

use for bast fibers, 77
Potato, storage parenchvma of,

59
Prepared slides, importance of,
140

Primary jihloem, cell types in, 94
differentiation of in leaf traces,

105-106
radial arrangement in roots,

131-132
pit fields, definition of, 12
in cambial initials, 12
in primordial meristem, 12
tissues, in roots, 130-133

in stems, 102-104
vascular system, ontogeny of,
104-106
tissues, arrangement in roots,
131-132
arrangement in stems, 103-

104
contrasted with secondary.
86-87
Avail, of collenchyma cells, 63
of primary xylem tracheae,

81, 84, 87
of sieve-tube elements, 97-98
structure and origin of, 9-10
xylem, differentiation of in leaf
traces, 105-106
endarch type of, 103
exarch in roots, 132
exarch type of, 103, 132
material for study of, 87-88
radial arrangement in roots,

131-132
radial alignment of cells in

stem, 86
secondary wall-tliickenings of,

84-86 '
vessels of in celery, 84
Primordial meristem, primary ])it
fields ill, 12
l)i(s, origin of sieve-jilates from,
97
Prismatic crystals, in Begoiiia, ."i9
in Tilid, 5-6
occurrence of, 5, 59
Procjuiibium (see also j^rovascular
meristem)

INDEX

151

Procaiubium, characteristics of,
86-87
origin and development of, 23,

104-106
origin of primary 2)hloem from,

105-106, 132
origin of primary xylem from,

84-86, 105-106, 132
produces primary vascular sys-
tem, 27
Prodesmogen, use of term, 105
Progressive differentiation, from
meristems, 20
of primar}^ stem-tissues, 102
Protoderm (see also dermatogen),
origin of epidermis from,
45-46
origin of tridiomes from, 51
Protophloem, in carrot, 132
Protoidasmic plant anatomy, 3!)
Protoplast, 1-6
Protostele, general structure of,

103
Protoxylem, traclieary elements
of, 84-85
use of term, 84
Pro vascular meristem (see also
procambium), produces

primary vascular system,
27
tissue, origin and development
of, 23, 104-106
origin of primary i)hloem

from, 105-106, 132
origin of primary xylem
from, 84-86, 105-io6, i32
Psilotales, sporophyte of, 101
Pteridiuui, primary xylem tra-
cheae of, 88
Pi/rKs, brachysclereides of, 70
stone-cells of, 70

QUEECrS, libriform tibers of, 78
tracheids of, 89

Quercus, vessel-elements of, 80-90

RANUNCULUS, storage paren-
chyma of, 59
study of root of, 134-136
Raphides, form and structure of, 5
in Lemna, 5
in Tradescantia, 5
Ray initials, 27, 106
Rays, origin of from cambium, 27,
106
origin of in roots, 133-134
Regeneration, with reference to

meristems, 21
Regressive differentiation, and
concept of meristematic
tissue, 21
and origin of phellogen, 107
and "i^ermanent" tissues, 32
of collenchyma, 62
of epidermis, 46
of parenchyma, 58-59
Reserve food, examples of, 4
Resin canals, origin and structure

in pine, 117
Reticulate elements, of metaxylem,

85
Rib meristem, concept of, 24
in roots, 26
origin of cortex and ])ith from,

24
structure and growth of, 24
Rims of Sanio (see Crassulae),

13, 89
Jxuhiiiia, ontogeny of vessels in,
83-84
study of cambium of, 28
study of phloem of, 99
Root, branching of, 132-133
cortex of, 131
endodermis of, 131
epidermis of, 131
general features of, 130
material for study of, 134-136

152

INDEX

Root, ontogeny of primarj^ vascu-
lar tissues in, 132
primary tissue regions in, 131-

133
secondary growth in, 133-134
study of in Banuncidus, 134-13G
study of in Smila^r, 136
Root apex, contrasted witli shoot
apex, 131
histogens in, 25-26
structure and gi'owth of, 25-27,

130-131
study of types of, 26
suhterniinal nieristeni of, 25,

130-131
types of, 26, 130-131
zones in. 26
Root cap, function of, 25

methods of origin, 26
Root hairs, position of, 131
Rubber jilarit, multiple epidermis
of, 50-51

SCALARIFORM elements, of
protoxylem, 84

perforation, 83, 88, 89
Scales, ontogeny of, 54

tyix's and structure of, 52
Sclereides, dcliunification of Avails
of, 6!)

function of, 60-70

material for study of, 70-71

ontogeny of, ()8-69

origin and use of term, 67

protophists of. ()!)

types of, 67-()8

wall slructure of, 67
Sclereiicliymn, ci'iticiue of term, 6^
Secondary growth, in roots, 133
134

in stems, 106-108

study of in roots, 136

studv of in stems, 110-111, 113-
116, 119-120

Secondary meristem, illustrated
by cambium, 27
phloem, cell types in, 94
sclerosis, descrijjtion of. 68-6i)

in stem of Aristulocliia, 120
vascular tissues, contrasted with

prinuiry, 86-87
wall, discontinuous character of,
10-11
in sieve-elements, 98
of primary xylem tracheae,

84-86
origin and structure of, 10-11
study of, 11

three-layered type of. 10
use of term, 10
xylem (see tracheid and vessel)
cell-arrangement in, 86
nuiterial for study of, 88-90
Secretorv cells, of resin canals,

ii7

Sclaginelld, vessels of, 82
Senrcio, study of epidermis of, 49
SejDtate fibers, structure of in

VUix, 78
Sliepherdia, scales of, 54
Shoot, morphology of, 101
Shoot apex, central vacuolated
zone in. 20
diameter in angiosperms, 22
diameter in gynniospei'ins, 22
form of cells in, 22
in species with decussate phyllo-

taxis, 21-22
of angiosperms, 21-25
of Ci/ciis rrrohitd, 20, 22
of I)i()o)i fdiilp. 20
of Giiil:(/<i hiJohii. 20
of gynniosperms, 20, 22, 25
of Zaniid, 20
study of angiospermous ty]ies,

24-25
studv of gvmnospermous types,
25

INDEX

153

Shoot apex, term replacing grow-
ing point, 21
tunica-corpus zones of, 22-24
variation in form of, 21-22
Sieve-cell, of gynniospernious

l)hIoeni, 1)3
Sieve-field, origin and use of term,

m

Sieve-plate, mori^hology of, 9C-07
origin and structure of, 96-!)7
position of, 06
Sieve-tube, solute-movement in,
94, 95
use of term, 93
elements, contents of, 9.'i-96
definitive cells of phloem, 93
disintegration of nuclei of, 95
in Ciicurhita, 98-99
lateral wails of, 97-98
material for study of, 98-99
obliteration of, 97
protoplasts of, 95-90
Simple perforation, 83, 89-90
liit-pairs, in xylem parenchyma
of Tccoma, 12
study of, 12
Siphonostele, ami)hiphloic type
of, 104
ectojihloic type of, 104
general structure of, 103
problems of evolution of, 103
Sliding gi'owth, of fibers, 7(i
Sliine-droi)s, in sieve-tube ele-
ments, 96
Slime-plugs, nature of, 95-90
Sviilux, study of root of, 136
SoJiiinii)!, collenchyma of, 03
storage parenchyma of, 5!)
Special reagents, 141-142
Sphaerraphides (see Druses), 5
Spiral elements, of protoxylem, 84
Spongy parenchyma, functions of,
125-126
in leaf ot Sifringa, 125

Spring tracheids, of Pinus, 88-8i)

wood, of annual ring, 115
Squash (.see also Cucnrhita), study

of stem hairs of, 2-3
Starch, in chlorophists of Pcl-
1i(>)iia, 4
in storage tissues, 4, 109, 111,

117, 134
sheath, relation to stele, 104
Stelar theory, application of, 103
Stele, boundary of, 104

morpliology of in roots, 132
ontogeny of in roots, 131-132
ontogeny of in stems, 104-106
principal types of, 103
structure and development of,

103-106, 131-132
structure of in roots, 131-132
structure of in stems, 103-104
types of in roots, 132
Stem, anatomy of in Cnciirhifa, 08
cortex of, 102-103
epidermis of, 102
material for study of. 108-120
morphology of, 101
nodal anatomy of, 101-102
])rinuiry structui'e of, 102-104
secondary structure of, 106-108
stele of, 103-106
structure of in gvnuiosperms,

117-118
study of in A n'sfolnrJn'ti, 119-

120
study of in corn, 118
study of in monocotyledons, 118
study of in PcJurgoniinn, 108-

111
study of ill pine, 117-118
study of in Tilia. 111-117
study of viiu^-type of, 118-120
Stoma, action of guard cells of,
47-48
functions of, 47
ontogeny of, 48
use of term, 47

154

INDEX

Stomate (see Stoma)
Stone cells, structure and occur-
rence of, 68, 70
Suherin, in walls of cork cells, 107
Subsidiaiy cells, of hairs, 53

of stoma, 48
Sudan IV, use for e]»id('rinis, 4!)

use of, 142
Suuaner tracheids, of 7^';;»n, 88

wood, of annual ring-, 115
Sycamore, dendroid hairs of, 54

vestigial pit-pairs in, 15
Si/ringa, guard cells of leaf, 4!)

study of leaf of, 124-127

TECOMA raflicans, simple pit-
pairs of, 12
Textile ])lants, fibers of, 73
Tilia, bast fibers of, 77-78, 113
cell walls in pith and cortex, 11
study of stem of, 111-117
walls of bast fibers in, 11
Tissue, various definitions of, 32-

33
Tissues, complex, 38

difficulty of classifying, 32, 3!)
Eames and MacDaniel's classifi-
cation, 37-38
Haberlandt's classification, 35-
37
T.undcgardh's classification, 30
permanent, 32
primary, 37, 86-87
Sach's classification, 33-35
secondary, 38, 86-87
simjile, 38

systems of classification, '.V.]-[V.)
Tobacco, multinncleate fil)ers of,

76
Tomato, coliencliyma of, 63
Torus, in bordered pit-pairs, 13
Tracheae, ontogeny of fibrous

thickenings of, 84-8()
Traclx'ary elements, ontogeny of,
83-86

Tracheary elements, structure and
morphology of, 80-83
study of in paper, 90
use of term, 80
Tracheid, bordered pits of, 13, 82,
88-89
disti'ibution of, 81
fil)r()us tliickenings in, 81, 84-86
general characters of, 80-81
imperforate character of, 81
in secondary xylem of oak, 89-

90
in secondary xylem of l'i)ius,

88-89
phylogenetic nature of, 81
study of, 88-89
types of ])itting in, 82
Tradescantia, raphides in, 5
staminal hairs of, 2
structure of shoot apex, 24-25
Ti'ansfusion tissue, in leaf of

Pinns, 128
Ti'ans-loeation. in vnsculnr system,
80
mechanism of in phloem, 94-95
Trichomes, material for study of,
53-54
origin of, 51
types of, 52-53
use of term, 51
Trichosantlies, ontogeny of tra-
cheae in, 85
Tn'foJiiim, primary xylem of. 86,
87, 88
vascular bnndle of. 87
Triticum, origin of leaf in. 21
Ti'um])et-creeper, simple pit-pairs

of, 12
Tnlip-tree, vessel elements of, 8!)
Tunica, method of growth. 23
role in leaf and bud initiation,

23-24
role in tissue differentiation, 23-

24
variation in nnm})er of layers
of, 23-24

INDEX

155

VACUOLES, and wall-thicken-
ings of tracheae, 85
in cells of shoot apex, 20
Vacuome (see also orgastic sub-
stances)
in cambial initials, 20, 27-28
in hair cells, 2
in meristems, 20, 27-28
Vasa mixta, use of term, 85
Vascular bundle (see also leaf
traces), 86, 87, 08, 110-111,
118-120
of AristoJovhia, 110-120
of corn, 118
of Cncurbita, 98
of Pelargnw'iini , 110-111
of Trij'oliiiDi, 8(J, 87
Vascular cambium, concept of,
106
fusiform initials of, 27, KXi
initials of, 27-28, 106
study of living cells in Pi)iiist, 28
study of living cells in h'i>hiiii(i,

28
vacuolated cells of, 27-28
vascular-ray initials of, 27, 106
use of term, 27
Vascular system, functions of, 80
general structure and develop-
ment, 80
in bud-scales, 128
in leaf of P/»»,s, 128
in leaf of Si/riiigii, 126-127
in leaf of Zea, 127-128
Vascular tissues, primary and sec-
ondary, 86-87
Vcrbascum, dendroid hairs of, 53
Vessel, use of term, 81
Vessel eli'iiieiit, evolution and dis-
tribution of, 82-83
formation of perforations in,

83-84
general characters of, 80-81
of Betula, 80
of Cucnrhita, 90
of Liriodendron, 89

Vessel elements, of oak, 89-90

of Pteridimn, 88

ontogeny of, 83-84

pitting of, 82, 88, 89-90

structure of end walls of, 84

study of, 89-90
Vessels, distribution in vascular
plants, 82-83

in ferns, 82, 88
Vestigial pit-pairs, in fibers, 14-
15, 75, 77-78

in wood fibers of Plat anus, 15

morphology of, 14

morphology of in bast fibers, 77

study of, 14-15
Vitis, septate fibers of, 78

WATP]R, in walls of collenchyma
cells, 63

vesicles, structure of in Mesem-
brij(nithenni)n, 53, 54
Wood fibers, of oak, 78

of PJatanits, 15

of Vitis, 78
AVood parenchyma, of Tilia, 116

simple pit-pairs in, 12
Wood pulji, production of, 90

XYLEM, definitive cells of, 80

plates, use of term, 131
Xylem-ray parenchynm, simple

pit- pairs in, 12
Xylem rays, of Tilia, 116

origin of, 27

ZA3IIA, shoot apex, 20
Zra, guard cells of leaf, 49
study of leaf of, 127-128
Zonal structure, of angiospei'mous
shoot apices, 22-25
of gymnospermous shoot apices,
20, 25
Zonation, of angiospermous shoot
apices, 22-25
of gymnospermous shoot apices,

20,

OP,