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Marine Biological Laboratory Library

Woods Hole, Massachusetts

AN INTRODUCTION

TO THE

EMBRYOLOGY OF ANGIOSPERMS

McGRAW-HILL PUBLICATIONS IN
THE BOTANICAL SCIENCES

Edmund W. Sinnott, Consulting Editor

Arnold An Introduction to Paleobotany

curtis and clark An Introduction to Plant Phvsiology

eames Morphology of the Angiosperms

eames Morphology of Vascular Plants: Lower Groups

eames and macdaniels An Introduction to Plant Anatomy

haupt An Introduction to Botany

haupt Laboratory Manual of Elementary Botany

haupt Plant Morphology

hill Economic Botanv

hill, overholts, popp, and grove Botany

johansen Plant Microtechnique

kramer Plant and Soil Water Relationships

kramer and kozlowski Physiology of Trees

lilly .and barnett Physiology of the Fungi

maheshwari An Introduction to the Embryology of the Angiosperms

miller Plant Phvsiology

pool Flowers and Flowering Plants

sharp Fundamentals of Cytology

sinnott Plant Morphogenesis

sinnott, dunn, and dobzhansky Principles of Genetics

sinnott and wilson Botany: Principles and Problems

smith Cryptogamic Botany

Vol. I. Algae and Fungi

Vol. II. Bryophytes and Pteridophvtes
smith The Fresh-water Algae of the United States
swingle Textbook of Systematic Botany
weaver and clements Plant Ecology

There are also the related series of McGraw-Hill Publications in the Zoological
Sciences, of which E. J. Boell is Consulting Editor, and in the Agricultural
Sciences, of which R. A. Brink is Consulting Editor.

AN INTRODUCTION M ^^,

TO THE (fi ^

EMBRYOLOGY
OF ANGIOSPERMS

P. MAHESHWAR1

Professor of Botany, University of Delhi
Delhi, India

McGRAW-HILL BOOK COMPANY, INC.

NEW YORK TORONTO LONDON
1950

AN INTRODUCTION TO THE EMBRYOLOGY OF ANGIOSPERMS

Copyright, 1950, by the McGraw-Hill Book Company, Inc. Printed in the
United States of America. All rights reserved. This book, or parts thereof,
may not be reproduced in any form without permission of the publishers.

VII
3962U

PREFACE

In these days of intense activity, when hundreds of papers are
being published in every field of botany in a steadily increasing
number of periodicals and in a multitude of languages, no apology
is needed for an attempt to summarize the existing state of our
knowledge in any branch of the subject and to point out the future
possibilities in it. Since the publication of Coulter and Chamber-
lain's "Morphology of Angiosperms" in 1903, no comprehensive
account of this aspect of botany has appeared in the English lan-
guage.

The original impetus for writing this work resulted from a course
of lectures which I gave on the subject in 1930 when I was teaching
at the Agra College. Several colleagues and pupils then suggested
that I should produce a book on the embryology of angiosperms.
This suggestion was repeated by Professor G. Tischler of the Uni-
versity of Kiel, whom I visited in 1936. Teaching and adminis-
trative duties and other difficulties made it impossible for me to
carry on this work in India at the speed I should have liked. Soon
after the war was over in 1945, therefore, I took the manuscript to
the United States in order to revise it and put it in shape for publi-
cation.

In a strict sense, embryology is confined to a study of the embryo,
but most botanists also include under it the events which lead on
to fertilization. I am in agreement with this wider comprehension
of the subject and have therefore included in this volume not only
an account of the embryo and endosperm, but also an account of
the development of the male and female gametophytes and fertiliza-
tion. To emphasize the recent trends of research in the subject,
two chapters of a general nature have been added, one dealing with
embryology in relation to taxonomy, and the other with experi-
mental embryology. In the former, an attempt has been made to
indicate the possibilities of the embryological method in the solu-
tion of problems of systematic botany. In the latter, emphasis
has been placed on the contacts between embryology, cytology,
genetics, and plant physiology.

vi PREFACE

In compiling my materials I must acknowledge the immense help
which I received from the writings of the late Professor K. Schnarf,
whom I came to know rather intimately during my stay in Vienna
in 1936. Without the existence of his books, entitled "Embryologie
der Angiospermen" (1929), "Vergleichende Embryologie der Angio-
spermen" (1931), and "Vergleichende Zytologie des Geschlecht-
sapparates des Kormophyten" (1941), my task would have been
appreciably greater. Mention must also be made of the numerous
and very valuable publications of Professor E. C. R. Soueges (Paris),
Professor K. V. O. Dahlgren (Uppsala), Dr. J. Mauritzon (Motala),
Dr. F. Fagerlind (Stockholm), Dr. A. Gustafsson (Svalof), and
Dr. H. Stenar (Sodertalje), upon which I drew rather freely. Pro-
fessor Dahlgren, Dr. Gustafsson, and Dr. Stenar also favored me
with their advice and criticisms whenever I applied to them for
help. In addition, a host of teachers and students in the United
States gave me every possible encouragement in the work. To
record my gratitude to all of them in any complete fashion would
fill several pages. I therefore content myself with naming a few
who took special interest in the project. To Professors R. H.
Wet more and I. W. Bailey I am heavily indebted for the free use of
their facilities and their assistance in other ways during my several
months' stay at Harvard. Professor A. F. Blakeslee and Mary E.
Sanders, Smith College, Northampton; Professor E. W. Sinnott,
Yale University; Professors A. J. Eames and L. W. Sharp, Cornell
University; Drs. D. C. Cooper, R. A. Brink, and C. L. Huskins,
University of Wisconsin; Dr. Th. Just, University of Notre Dame,
now at the Field Museum of Natural History, Chicago; Professor
J. T. Buchholz, University of Illinois; Professor A. S. Foster, Pro-
fessor G. L. Stebbins, Mrs. M. S. Cave, Drs. L. Constance, Katherine
Esau, and C. M. Rick, all of the University of California; Pro-
fessor G. M. Smith, Stanford University; Dr. D. A. Johansen, Po-
mona, and Professor A. W. Haupt, University of California at
Los Angeles, gave me the benefit of their suggestions and criti-
cisms. Last but not least, my colleagues and pupils, Dr. B. M.
Johri, Reayat Kahn, S. Narayanaswami, and J. S. Agrawal gave
me their fullest cooperation in the preparation of the bibliography
and revision of the proofs.

Only a few of the illustrations are original, most of them having
been borrowed from the works of other authors. Considerable
care has been exercised, however, in their selection not only that tht

PREFACE vii

text may be made as clear as possible but also that the student may-
acquire some familiarity with the names and contributions of the
better known embryologists, both past and present. While most
of the copying and redrawing was done by me personally, I am
glad to acknowledge the very able assistance I received from a few
friends. Figures 36, 68, 92, and 214 were drawn by Mrs. J. A.
Adams of Poughkeepsie, N. Y., daughter of my former teacher,
the late Dr. Winfield Dudgeon of Allahabad; Miss C. Pratt,
Harvard University, drew Figures 24, 41, 59, 65, 74, 89, 104, 118,
121, and 167; Dr. B. G. L. Swamy, Bangalore, drew Figures 19,
145, 149, 150, 153, and 163; Mrs. M. S. Cave, University of Cali-
fornia, drew Figure 14; Miss C. G. Nast, Wayne University, drew
Figure 111; and my former research assistant, Ashraful Haque,
University of Dacca, drew Figures 21, 43, 47, 54, 60, 61, 82, 85,
91, 147, 148, 152, 154, 157, 158, 159, 161, 162, 164, 165, 170, 172,
173, 175, 191, and 216. To all these I wish to tender my most
grateful thanks for the willingness with which they cooperated
with me.

A word about the citation of literature. No attempt has been
made to give a complete list of all that has been published on
angiosperm embryology, as this would make the volume too cum-
bersome, but it is hoped that the references which have been cited
will facilitate the task of the student who wishes to acquire fuller
information.

In a work of this nature it is unavoidable that there should be
some errors of judgment and also oversights and omissions. I
should appreciate the suggestions and criticisms of those who use

the book.

P. Maheshwari
University of Delhi, India
July, 1950

CONTENTS

Preface v

1 . Historical Sketch 1

Discovery of Pollen Tube — Schleiden's Theory of Origin of Embryo —
Discovery of True Relation between Pollen Tube and Embryo — Discovery

of Sexual Fusion in Lower Plants — Discovery of Nature and Development
of Male and Female Gametophytes — Embryo — Discovery of Syngamy —
Chalazogamy — Double Fertilization — Parthenogenesis — Twentieth Cen-
tury— References.

2. The Microsporangium 28

Wall Layers — Sporogenous Tissue — Cytomixis — Cytokinesis — Microspore
Tetrad — References.

3. The Megasporangium 54

Integuments — Micropyle — Nucellus — Integumentary Tapetum — Hypo-
stase— Epistase — Vascular Supply of Ovule— Arch esporium — Megasporo-
genesis — Functioning Megaspore — Failure of Wall Formation during
Meiosis — References.

4. The Female Gametophyte 84

Monosporic embryo sacs — Polygonum Type— Oenothera Type — Bisporic
embryo sacs — Allium Type — Tetrasporic embryo sacs — Peperomia Type —
Penaea Type — Drusa Type — Fritillaria Type — Plumbagella Type — Adoxa
Type — Plumbago Type— Aberrant and unclassified types — Limnanthes
douglasii — Bahamita vulgaris — Chrysan themum cinerariaefolium — Or-
ganization of mature embryo sac — Egg Apparatus — Antipodal Cells —
Polar Nuclei — Embryo Sacs with Disturbed Polarity — Food Reserves in
the Embryo Sac — Embryo Sac Haustoria — References.

5. The Male Gametophyte 154

Microspore — Formation of Vegetative and Generative Cells — Division of
Generative Cell— Male "Cells" or "Nuclei"— Vegetative Nucleus— De-
velopment of Pollen in Cyperaceae — Embryo-sac-like Pollen Grains —
References.

6. Fertilization 181

Germination of Pollen — Course of Pollen Tube — Entry of Pollen Tube
into Embryo Sac — Rate of Growth of Pollen Tube — Gametic Fusion —
Multiple Fusions and Polyspermy — Single Fertilization — Persistence and
Possible Haustorial Function of Pollen Tube — X-bodies — References.

7. The Endosperm 221

Types of Endosperm Formation — Free Nuclear Endosperm — Cellular
Endosperm — Helobial Endosperm — Relationships between Different
Types of Endosperm — Histology of Endosperm — Xenia — Mosaic Endo-
sperm— References.

x CONTENTS

8. The Embryo 268

Dicotyledons — Crucifer Type — Asterad Type — Solanad Type — Cheno-
podiad Type — Caryophyllad Type — Monocotyledons — Modifications of
suspensor — Unclassified and abnormal embryos — Unorganized and reduced
embryos — References .

9. Apomixis 313

Nonrecurrent apomixis — Recurrent apomixis — Generative Apospory — So-
matic Apospory — Unclassified Cases — Organization of Aposporic Embryo
Sacs — Development of Embryo in Aposporic Embryo Sacs — Adventive
embryony — References.

10. POLYEMBRYONY 343

Cleavage Polyembryony — Origin of Embryos from Cells of Embryo Sac
Other than Egg — Embryos Arising from Cells Outside Embryo Sac —
Embryos Originating from Other Embryo Sacs in Ovule — A Few Special
Cases — Twins and Triplets — Conclusion — References.

11. Embryology in Relation to Taxonomy 357

Empetraceae — Lennoaceae — Cactaceae — Garryaceae — Onagraceae — Calli-
trichaceae — Liliaceae-allioideae — Liliaceae-asphodeloideae — Liliaceae-lili-
oideae — References.

12. Experimental Embryology 375

Control of Fertilization — Embryo Culture — Induced Parthenogenesis —
Production of Adventive Embryos — Induced Parthenocarpy — Conclu-
sion— References.

13. Theoretical Conclusions 411

Male Gametophyte — Female Gametophyte — Fertilization — Endosperm —
Embryo — References.

Name Index 433

Subject and Plant Index 441

CHAPTER 1
HISTORICAL SKETCH

In tracing the history of a branch of natural science it is customary
to go back to the days of Aristotle. The greater part of his technical
writings is unfortunately lost to us, but it seems fairly certain that
he did not recognize the presence of sex in plants. He believed
instead that the male and female principles were so blended that
they generated of their own accord and the offspring arose from the
superfluous food in the plant.

Aristotle bequeathed his library and collections to his favorite
pupil Theophrastus. In his "Enquiry into Plants," written in the
third century B.C., the latter referred to the pollination of the date
palm, presumably on the basis of the account of Herodotus, who
had traveled in the East in the fifth century B.C. The Arabs and
Assyrians, Herodotus found, used to have a special ceremony at a
certain time of the year, in which a man climbed up a male tree,
brought down the inflorescence, and handed it over to the high
priest, who touched the female inflorescences with it, in order to
ensure a good supply of dates.

Approximately three hundred years after Theophrastus, Pliny
wrote an encyclopedia of natural history in which he mentioned the
male palm with its erect leaves as having somewhat of a military
bearing, while the females with their softer foliage and feminine ways
bent toward it, to save themselves as it were from the curse of
virginity or widowhood. However, Pliny did not make any ob-
servations of his own. His writings and ideas were based on other
people's reports and on the literature on the subject that existed in
those days.

After this the problem of sexuality in plants seems to have been
laid aside and forgotten for hundreds of years. Indeed, many scien-
tists of the fifteenth and sixteenth centuries totally denied the
occurrence of sex in plants and regarded even the mention of it as
inappropriate and obscene. Some thought the stamens to be ex-
cretory organs and the pollen to be a waste product.

2 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

It was only with the invention of the microscope that actual
observation of the sexual cells took the place of conjectures.
Leeuwenhoek (1677)1 discovered the sperms of some animals but
mistook them at first for "Wild animalcules" arising in the seminal
fluid by some sort of putrefaction.

In his "Anatomy of Plants," Grew (1682) made the first explicit
mention of the stamens as the male organs of the flower.
He thought that the pollen grains, by merely falling upon the stigma,
transmitted to the ovary a "vivifick effluvium" which prepared it
for the production of the fruit.

Rudolph Jakob Camerarius (1694), Director of the Botanical
Garden at Tubingen, approached the matter more scientifically. He
observed that in a female mulberry tree, which was growing without
any male plants in the vicinity, the fruits contained only abortive
seeds. Inspired by this discovery he next took some female plants
of Mercurialis annua and kept them in pots completely isolated from
the influence of male plants. Here too, he found that, although the
plants grew well, not one of the fruits contained a fertile seed. This
encouraged him to make further observations, which he summarized
in a famous treatise called "De sexu plantarum." He carefully de-
scribed the flower, anthers, pollen, and ovules. On removing the
male flowers (globuli) of Ricinus before the anthers had shed and
preventing the growth of the younger ones, he never obtained any
perfect seed but only empty fruits which withered and fell to the
ground. A similar lack of seed formation was noted in Zea mays
when the stigmas had been removed from the young ear. In con-
clusion he said: "In the plant kingdom, the production of seed,
which is the most perfect gift of nature and the general means of
maintenance of the species, does not take place unless the anthers
have previously prepared the young plant contained in the ovary."
To the anthers, in his opinion, was to be attributed, therefore, the
role of the male sexual organs just as the ovary with its style was
considered the female sexual organ.

We thus see that although Camerarius was not clear about the
exact manner in which the pollen functioned, he nevertheless made
a notable contribution to our knowledge by showing that some kind

1 Dates in parentheses refer to works listed in the bibliography at the end of
each chapter.

HISTORICAL SKETCH

of interaction between the stamens and carpels is necessary for
the production of seed-bearing fruits.

About sixty-five years later, Joseph Gottlieb Kolreuter (1761),
physician and professor of natural history at Wurtemberg, published
four parts of a treatise dealing with his experiments on sex in plants.
He fully confirmed the work of Camerarius and gave a detailed ac-
count of the importance of insects in flower pollination. He also

produced hybrids in Nicotiana,

Dianthus, Matthiola, and Hyo-
scyamus and showed that if the
stigma of a plant received its own
pollen and that of another species
at the same time, ordinarily the
former alone was effective. This,
he said, was the reason why hy-
brids were so rare in nature, al-
though they could be produced
artificially.

Discovery of the Pollen Tube.
After the role of the pollen began
to be understood, the next step
was to determine the exact man-
ner in which it influenced the
ovule. Accident supplied the
starting point of some important
discoveries. An Italian mathe-
matician and astronomer named
Giovanni Battista Amici (1824),

who was also a good microscope maker, found that the stigma of Portu-
laca oleracea was covered with hairs which contained some granules or
particles inside them. Curiosity prompted him to ascertain whether
they moved in the same way as the granules he had seen in the cells
of Char a. It pleased him to find that they did. While repeating
the observation, he accidentally saw a pollen grain attached to the
hair he had under observation. Suddenly the pollen grain split open
and sent out a kind of tube or "gut" which grew along the side of the
hair and entered the tissues of the stigma. For three hours he kept
it under observation and watched the cytoplasmic granules circulate

Fig. 1. Giovanni Battista Amici. {Pho-
tograph obtained through the courtesy oj
Br. E. Battaglia.)

4 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

inside it, but eventually he lost sight of them and could not say
whether they returned to the grain, entered the stigma, or dissolved
away in some manner.

Amici's discovery stimulated the young French botanist Brong-
niart (1827) to examine a large number of pollinated pistils with a
view to understanding the interaction between the pollen and the
stigma and the introduction of the fertilizing substance into the
ovule. He found the formation of the pollen tubes (he called them
"spermatic tubules") to be a very frequent occurrence but per-
suaded himself to believe that, after penetrating the stigma, the
tubes burst and discharged their granular contents, which he likened
to the spermatozoids of animals and considered to be the active part
of the pollen. He thought he saw these "spermatic granules" vi-
brating down the whole length of the style and entering the placenta
and ovule, and he drew a series of figures to illustrate the whole
process. In appreciation of this work, Brongniart was awarded a
prize by the Paris Academy of Sciences and recommended for ad-
mission to the Academy.

Amici (1830) applied himself once again to the problem, studying
Portulaca oleracea, Hibiscus syriacus, and other plants, and wrote a
letter to Mirbel in which he put the following question: "Is the
prolific humor passed out into the interstices of the transmitting
tissue of the style, as Brongniart has seen and drawn it, to be trans-
ported afterwards to the ovule, or is it that the pollen tubes elongate
bit by bit and finally come in contact with the ovules, one tube for
each ovule?" His observations completely ruled out the first
alternative, and he definitely concluded in favor of the second.

About the same time, Robert Brown (1831, 1833) saw pollen grains
on the stigmas and pollen tubes in the ovaries of certain orchids and
asclepiads but was uncertain as to whether the tubes were always
connected with the pollen grains. He thought instead that, at least
in some cases, the tubes arose within the style itself, although pos-
sibly they were stimulated to develop in consequence of the pollina-
tion of the stigma.2

Schleiden's Theory of the Origin of the Embryo. Meanwhile
other workers also became interested in the problem, and in 1837
Schleiden published some very detailed observations on the origin

2 It now seems that Brown was at times confusing pollen tubes with the elon-
gated cells of the transmitting tissue in the style.

HISTORICAL SKETCH

and development of the ovule. He confirmed Amici's statement
that the pollen tubes make their way from the stigma to the ovule,
entering the latter through the micropyle. His lively imagination
carried him too far, however, for he asserted that the extremity of the
pollen tube pushes the membrane of the embryo sac before it and
directly becomes the embryonal vesicle, which then undergoes a
number of divisions to produce the embryo. The cotyledons were
said to arise laterally, while the
original apical point remained pr" r
more or less free and formed
the plumule. To him the em-
bryo sac was, therefore, a sort
of nidus or incubator within
which the end of the pollen tube
was nourished to give rise to
the new plantlet. If this were
really the case, there would of
course be no sexuality in plants.
Nevertheless, with the influence
he commanded and the sharp
tongue with which he denounced
all opponents, Schleiden found
a number of warm supporters.
One of them, Schacht, sponsored
this absurd idea with special en-
thusiasm.

Amici boldly opposed the
views of Schleiden. In a meet-
ing of the Italian naturalists,

held at Padua in 1842, he tried to prove that the embryo did not
arise from the tip of the pollen tube but from a portion of the ovule
which was already in existence and was fertilized by the fluid in
the tube.

Schleiden (1845) gave a most spirited reply to this and said that
after his careful and thorough investigation of 1837 it was ridiculous
on the part of novices in the field to raise such meaningless objec-
tions. He described some fresh observations on Cucurbita and of-
fered to demonstrate the utter falsity of Amici's observations and
the complete truth of his own to anyone who visited him.

Fig. 2. Matthias Jakob Schleiden.
(Photograph obtained through the cour-
tesy of Prof. W. Troll.)

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Discovery of the True Relation between the Pollen Tube and the
Embryo. In spite of Schleiden's criticism, Amici continued further
work on the subject. In 1847 he produced decisive evidence (Fig. 3)
to show that in Orchis (which he found to be specially suited for such
studies), a body, the germinal vesicle, was already present inside the
embryo sac before the arrival of the pollen tube, and that it was
this vesicle which gave rise to the embryo, stimulated no doubt by
the presence of the pollen tube.

B C D E

Fig. 3. Development of ovule and embryo in Orchis. Note pollen tube in C
and D and suspensor haustorium in E. {After Amici, 1847.)

Support for Amici 's views now came forward from other quarters.
In a famous document, entitled "Die Entstehung des Embryo der
Phanerogamen," consisting of 89 quarto pages and 14 copper plates
with no fewer than 429 figures, Wilhelm Hofmeister (1849) published
his observations on 38 species belonging to 19 genera and showed that
in every case the embryo originated from a preexisting cell in the
embryo sac and not from the pollen tube. He described his obser-
vations in such a clear and dignified manner that the}' immediately
carried conviction and were soon confirmed by other workers from

HISTORICAL SKETCH

England, France, and Germany. In less than two years after the
publication of this memoir and in spite of his lack of a proper
university training, the University of Rostock conferred upon
Hofmeister the degree of Doctor of Philosophy honoris causa, thereby
giving formal recognition to his high position as a scientific investi-
gator. A few of Hofmeister's illustrations of the embryo sac and
the relation between the pollen tube and the egg are presented in
Fig. 5.

Schleiden and Schacht contin-
ued to hold their previous opin-
ion . Schacht brought out a large
monograph in 1850, with 26 plates
and a considerable number of
drawings. These were beauti-
fully executed, but in every case
he mistook the egg cell for the
tip of the pollen tube (Fig. 6) . In
conclusion he said: "The tend-
ency towards error is so inherent
in human nature that the work
of one's head, like that of his
hand, is never perfect, and con-
sequently I do not hold mine
to be free from error and mis-
conception, but I have tried to
minimize these as much as pos-
sible. ... In chief matter, i.e.,

the origin of the embryo from the pollen tube, no one can convince
me that there has been any mistake or misconception. . . . My
preparations are so conclusive on this point that I can confidently
look forward to answering any criticisms that may be directed
against it." The Imperial Institute of the Netherlands at Am-
sterdam accepted Schacht's essay and awarded him a prize for its
production.

However, the evidence against Schleiden and Schacht soon be-
came so overwhelming that eventually both of them had to retract
their opinions, and in 1856 Radlkofer published a comprehensive
review of the question accepting Hofmeister's conclusions in toto.
Schleiden soon gave up all botanical work and settled down in
Dresden as a private teacher of history and philosophy.

Fig. 4. Wilhelm Hofmeister.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

In this connection it is interesting to recall the words of the famous
anatomist Hugo von Mohl, who, in 1863, at the time of Amici's
death, wrote as follows: "Now that we know Schleiden's doctrine to
have been an illusion, it is instructive, although sad, to look back to

B C

Ovules and embryo sacs of Monotropa hypopitys, before and after fertili-

Fig. 5.

zation. A,B, embryo sacs at time of fertilization. C, same, showing pollen tube
about to enter micropyle. D,E, Fertilized embryo sacs, showing early stages in
formation of endosperm. (After Hofmeister, 1849.)

the past and see how readily the false was accepted for the true;
how some, renouncing all observation of their own, dressed up the
phantom in theoretical principles; how others, with microscope in
hand, but blinded by their preconceptions, believed that they saw
what they could not have seen and sought to establish the correct-
ness of Schleiden's notions with the aid of hundreds of figures which

HISTORICAL SKETCH 9

had anything but truth to recommend them; and how an academy
by rewarding such work gave fresh proof of the well-known experi-
ence that prize-essays are little adapted to contribute to the solution
of a doubtful question in science."

Discovery of Sexual Fusion in Lower Plants. During this interval
greater progress was being made with lower plants and animals.
Thuret, in 1854, showed that in Fucus the eggs must be activated by
sperms before they can germinate to give rise to new plants, and

A B C D E

Fig. 6. The so-called development of embryo from pollen tube in Martynia lutea
(tp = pollen tube; em = embryo ;edp = endosperm; is = integument; se = embryo
sac). A, l.s. ovule. B-D, stages in development of "pollen-tube embryo."
E, older embryo, together with a few of the surrounding endosperm cells. (After
Schacht, 1850.)

later he also obtained hybrids by associating the ova and sperms of
different forms. In 1855 Pringsheim observed spermatozoids in
the little horns (antheridia) of Vaucheria and showed that no further
development occurs unless the spermatozoids enter the ovum. The
decisive observation was made in 1856 in Oedogonium, where he saw
the moving spermatozoid come in contact with the egg and force
its way inside the latter. On the basis of these and similar dis-
coveries in lower animals, the German zoologist Oscar Hertwig
(1875) made a general statement that the essential feature of fertili-
zation is the union of two nuclei, one furnished by the male parent
and the other by the female.

In the phanerogams, where sex was supposed to be more apparent

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

than in cryptogams, the actual demonstration did not come until a
few years later, no doubt because of the technical difficulties in
making any direct observations on the embryo sac, which is sur-
rounded by the opaque tissues of the nucellus and the integuments.

Fig. 7. Stages in formation of microspore tetrads in Tradescantia. {After Hof-
meister, 1848; reproduced from Sharp, 1943.)

Discovery of the Nature and Development of Male and Female
Gametophytes. Among early students of the development of pollen,
Hofmeister (1848) presented some surprisingly good illustrations of

A B C D E F G

Fig. 8. Development of male gametophyte of Tradescantia virginica. (After
Elfving, 1879.)

the process of tetrad formation (Fig. 7), and Reichenbach, Hartig
and several other workers noted the presence of two nuclei in whole
mounts of the mature pollen grains of several angiosperms. Stras-
burger (1877) and his pupil Elfving (1879) extended these observa-
tions to cover several families and demonstrated the widespread
occurrence of the binucleate condition in pollen grains (Fig. 8).

HISTORICAL SKETCH

They further found that one of these nuclei originally lies in a
small cell cut off at the periphery of the pollen grain but later be-
comes free by a dissolution of the partition wall. Elfving also
germinated pollen grains in artificial media, and when this was un-
successful, he made preparations of pollen tubes from dissected
styles. Here he was able to find the three nuclei which we now know
to be the two male gametes and the tube or vegetative nucleus.
Unfortunately both Strasburger _________

and Elfving made the mistake
of interpreting the smaller cell
in the pollen grain as vegeta-
tive or prothallial and the larger
as generative. They further
thought that all the nuclei in
the pollen tube dissolved and
disappeared before fertilization.
These mistakes were, however,
rectified by Strasburger in a sub-
sequent paper (1884), which will
be referred to later.

For our knowledge of the or-
ganization of the embryo sac we
are indebted in the first instance
to the works of Hofmeister
(1847-1861). Working wholly
with cleared preparations and
freehand sections, he succeeded
in identifying the two groups of
cells at the opposite poles of the embryo sac. Those lying at the
micropylar end were designated as the "germinal" or "embryonal"
vesicles, all capable of giving rise to embryos and therefore to be
regarded as homologous with the corpuscula (archegonia) of the
gymnosperms. The cells at the chalazal end were considered to be
prothallial, and the embryo sac itself was interpreted as homologous
with the megaspore or female gametophyte of the heterosporous
pteridophytes and the gymnosperms.

Although Hofmeister's work was important, he failed to dis-
tinguish clearly between the synergids and the egg and regarded all
three of them as having the same function. Further, he was unable
to trace the mode of origin of the embryo sac, the general opinion

Fig. 9. Edward Strasburger. (Photo-
graph obtained through the courtesy of
Prof. A. W. Haupt.)

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

in those days being that it arose by the simple enlargement of a cell
of the nucellus.

Further knowledge of the development and organization of the
embryo sac became available as the result of a concentrated attack
on the problem made by several botanists during the years 1877 to
1881. To be named specially in this connection are Warming,

D E F G H

Fig. 10. Development of embryo sac in Polygonum divaricatum. A, megaspore
mother cell separated from nucellar epidermis by primary wall cell. B, dyad cells
in division. C, tetrad of megaspores with wall cells above. D, functioning mega-
spore. E-G, embryo sacs, showing two, four, and eight nuclei. H , l.s. ovule, show-
ing mature embryo sac. (After Strasburger, 1879.)

Vesque, Strasburger, Fischer, Ward, Jonsson, Treub and Mellink,
and Guignard. Strasburger (1879) demonstrated that at first one of
the nucellar cells becomes differentiated as the megaspore mother
cell (Fig. 10 A) and goes through two divisions to give rise to a row
of four cells (Fig. 10B,C). Of these, the three micropylar cells soon
degenerate and the chalazal alone enlarges and functions (Fig. 10D).
The nucleus of this cell (the functioning megaspore) divides thrice
to give rise to two groups of four nuclei, one at the micropylar end
and the other at the chalazal end of the cell (Fig. 10E-G). From

HISTORICAL SKETCH

the former arise the egg apparatus (consisting of an egg cell and two
synergids) and the upper polar nucleus; from the latter, the three
antipodal cells and the lower polar nucleus. The polar nuclei were
observed to fuse in the center to form a secondary nucleus, which

Fig. 11. Stages in development of embryo of Capsella bursa-pastoris (v = sus-
pensor; h-h' = hypophysis ; c-c = cotyledons ; s = stem tip ;w = radicle; the shaded
portions represent the dermatogen and plerome). (After Hanstein, 1870; repro-
duced from Sachs, 1874-)

gave rise to the endosperm (Fig. \0H). The synergids were re-
garded as modified structures assisting in the process of fertilization.
Treub and Mellink (1880) confirmed these observations but also
noted certain exceptions. In a few plants they found that the
megaspore mother cell divides into only two daughter cells, of which
either the upper (as in Agra-phis patula) or the lower (as in Narcissus

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

tazetta) can give rise to the embryo sac. In Lilium and Tulipa the
formation of daughter cells was found to be entirely omitted so that
the embryo sac arises directly from the megaspore mother cell.
. The Embryo. Hanstein (1870) was the first to follow the sequence
of early cell divisions in the development of the embryo. He gave a
detailed description of the embryogeny in Capsella and Alisma

Fig. 12. Stages in development of embryo of Alisma plantago (v = suspensor;
h = hypophysis; w = radicle; p = plumule; c = cotyledon; b = first leaf; the
shaded portions represent the derma togen). (After Hanstein, 1870; reproduced
rom Sachs, 187 4-)

(Figs. 11, 12). Famintzin confirmed these observations in 1879, and
in the same year Treub described the embryos of several orchids
with their remarkable suspensor haustoria. Two years later Guig-
nard (1881) gave a full account of the extremely massive suspensors
of the Leguminosae.

At about this time, detailed investigations were also made on the
peculiar phenomenon of polyembryony. Long ago Leeuwenhoek
(1719) had noted the occurrence of more than one embryo in certain

HISTORICAL SKETCH

orange seeds, and other instances of a similar nature were listed by-
Alexander Braun (1859). In no case, however, had the origin of
the abnormality been satisfactorily studied from the developmental
point of view. Strasburger, in 1878, demonstrated for the first time
that in Funkia (=Hosta) ovata, Coelebogyne ( = Alchornea) ilicifolia,
Nothoscordum fragrans, and Citrus aurantium, the nucellar cells lying
close to the apex of the embryo sac become richly protoplasmic and
divide to form small groups of cells which project into the cavity of
the embryo sac and grow into embryos (Fig. 13). Subsequent work

B D

Fig. 13. Development of ad ventive embryos in Funkia (= Hosta)ovata. A, upper
part of nucellus and embryo sac. B,C, enlargement and division of some of the
nucellar cells. D, more advanced stage, showing young zygotic embryo and several
nucellar embryos. (After Strasburger, 1878.)

by others revealed further possibilities, such as an origin of embryos
from the cells of the integument, or from those of the suspensor, or
from components of the embryo sac other than the egg. In Allium
odorum, Tretjakow (1895) and Hegelmaier (1897) showed that even
antipodal cells could give rise to embryos.

Discovery of Syngamy. These were all notable advances, but
the most important of all was Strasburger 's (1884) discovery of the
actual process of syngamy, or the fusion of the male and female
gametes. In a memorable paper, entitled "Neue Untersuchungen
uber den Befruchtungsvorgang bei den Phanerogamen," he cor-
rected some of the mistakes he had made in 1877 on the organization

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

of the male gametophyte. He now confessed that, owing to its
similarity both in position and origin with the prothallial cells of
gymnosperms, the small lenticular cell in the angiosperm pollen
grain had been formerly misinterpreted by him as being the "vegeta-
tive" cell. Also, in the earlier studies in his laboratory (which had
been based on whole mounts stained with iodine green) the nuclei
in the pollen grains were often quite indistinguishable and had there-
fore been supposed to have degenerated. His improved technique
(such as staining with picrocarmine and the cutting of the larger
pollen grains into thin sections), devised after 1877, had shown that

B C

E F H

Fig. 14. Fertilization in Monotropa hypopitys. A, pollen grain stained in iodine
green and acetic acid to show vegetative and generative nucleus. B,C, tips of
pollen tubes showing the two male nuclei; in B the vegetative nucleus is also visible.
D, upper part of embryo sac, showing egg apparatus. E-G, stages in union of male
and female nuclei. H, syngamy completed; primary endosperm nucleus dividing.
(After Strasburger, 1884-.)

this was really not the case. It was now clear that the 'first division
of the microspore gives rise to two cells, the smaller being the genera-
tive cell and the larger the vegetative. Further, the generative
cell loosens itself from the wall of the pollen grain and divides either
before or after the germination of the pollen grain, while the vegeta-

HISTORICAL SKETCH

tive or tube nucleus remains undivided. Thus, the pollen tube
eventually shows three nuclei, one vegetative and two generative3.

On the basis of his studies on the embryo sac of Monotropa and
some other plants, Strasburger further showed that the pollen tube
discharges its nuclei into the sac (previous to this it was believed
that fertilization occurred merely by the diffusion of the cell sap
from the tube) and that one of the two male nuclei fuses with the
nucleus of the egg, thus provid-
ing actual proof of the nature "™
of fertilization and its import-
ance in the life cycle of a plant
(Fig. 14).

In the concluding part of his
memoir, Strasburger made the
following generalizations, which
are now almost axiomatic with
us: (1) the process of fertilization
comprises the union of the nu-
cleus of the male gamete with
that of the egg; (2) the cyto-
plasm of the gametes is not con-
cerned in the process ; and (3) the
sperm nucleus and the egg nu-
cleus are true nuclei.

Chalazogamy. Strasburger's
work opened the way to a more
detailed study of the process
of fertilization in angiosperms.
Prior to 1891, it was believed that

the pollen tube always enters the ovule through the micropyle.
Treub, in that year, reported that in Casuarina it enters through the
chalaza (Fig. 16). This was thought to be so strange that he proposed
a new classification of the angiosperms into two classes : the chala-
zogams and theporogams, with Casuarina as the only representative
of the former. Later investigations showed, however, that there is no
uniformity in the mode of entry of the pollen tube into the embryo
sac, and in Ulmus (Nawaschin, 1898a) its behavior was found to be
particularly varied and irregular. The phenomenon of chalazogamy,

'These two are now called the male gametes.

Fig. 15. Melchior Treub. (Photo-
graph obtained through the courtesy of
Dr. F. Verdoorn)

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

therefore, lost the great phylogenetic importance which had been
attached to it by Treub. Today it is considered to be more of
physiological than of phylogenetic significance, although it does
have a certain taxonomic value in narrow circles of affinity.

Double Fertilization. The
fate of the second male gam-
ete discharged by the pollen
tube was not known so far.
In a study of Lilium marta-
gon and Fritillaria tenella,
S. G. Nawaschin (18986)
showed that in angiosperms
both male gametes are con-
cerned in fertilization, one fus-
ing with the egg (syngamy) and
the other with the two polar
nuclei (triple fusion). A few
months later L. Guignard
(1899) also reported the same
phenomenon in Lilium and
Fritillaria and presented a se-
ries of beautiful drawings to
illustrate it (Fig. 19). These
discoveries attracted wide-
spread attention and were fol-
lowed by a series of similar
investigations dealing with
other species of angiosperms.
Double fertilization was soon
demonstrated in several plants
and within a few years it began
to be considered as of universal occurrence in angiosperms. It is
interesting to note that a year earlier D. M. Mottier (1897) had seen
the second male nucleus in close proximity to one of the polar nuclei,
but that he had considered this proximity to be accidental and had
failed to realize its true significance. Of considerable interest in this
connection is Finn's (1931) report on a preparation of Scilla sibirica
( = S. cernua) made by a Russian botanist, W. Arnoldi, a number of
years before Nawaschin's announcement of 1898. Finn found both

Fig. 16. Casuarina suberosa, l.s. ovule,
showing chalazogamy (m = micropyle;
p = pollen tube;e = embryo sac). (After
Treub, 1891.)

HISTORICAL SKETCH

syngamy and triple fusion to be so clear in one of the sections on
this slide (Fig. 20) that it is surprising that the process could have been
missed at all. As it was, however, Arnoldi mistook the male gam-
etes for displaced nuclei (of the nucellus?) which had in some way
entered into the embryo sac during the process of sectioning, and
he therefore ignored them altogether.

«.V<*8«*}

J22L £^

Fig. 17.

Sergius Nawaschin. (Photograph ob-
tained through the courtesy of Dr. A. W.
Haupt.)

Fig. 18.
Leon Guignard.

One of the results of Nawaschin 's discovery was that it gave a
plausible explanation of "xenia." This term had been coined by
Focke (1881) to denote those cases in which the pollen produced a
visible influence on the hereditary characters of those parts of the
ovule which surround the embryo. It now became clear that just
as the fertilized egg gives rise to an embryo combining the charac-
ters of the two parents, so does the triple fusion nucleus give rise to
a tissue containing the potentialities of both the parents.

A controversy soon started, however, on the morphological nature
of the endosperm, which is neither n nor 2n but Sn. Some claimed
that it was a continuation of the old gametophytic tissue, while
others (Sargant, 1900) thought it to be a second embryo which took

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

a monstrous and highly abnormal shape because of the intrusion
of the lower polar nucleus. Strasburger (1900) suggested that only
the fusion of the male gamete with the egg was to be regarded as
true or "generative" fertilization, while the fusion of the polar nuclei

I J K L M N

Fig. 19. Double fertilization in Lilium martagon. A, mature embryo sac. B,
same, showing discharge of pollen tube. One male nucleus has entered the egg
and the other is in contact with the upper polar nucleus; the nucleus of one of the
synergids is in process of degeneration. C, one male nucleus in contact with the
egg nucleus and the other in contact with the two polar nuclei. D, same, slightly
more advanced stage. E-H , stages in fusion of egg nucleus and one male nucleus.
I-N, stages in triple fusion. {After Guignard, 1899.)

HISTORICAL SKETCH

with the second male nucleus was in the nature of a growth stimulus
and could therefore be called "vegetative" fertilization.

Parthenogenesis. About the same time that Nawaschin made
his discovery of double fertilization, two Swedish botanists, H. O.

Fig. 20. Double fertilization in Scilla sibirica. A, one male nucleus in contact
with egg nucleus, another in contact with the two polar nuclei. B, sperm and egg
nuclei, more highly magnified. C, sperm nucleus in contact with two polar nuclei,
more highly magnified. (After Finn, 1931.)

Juel (1898, 1900) and S. Murbeck (1897, 1901), were engaged in
studying the mechanism of parthenogenesis in Antennaria and Al-
chemilla. Some years earlier Kerner (1876) had noted that in An-
tennaria alpina male plants were extremely rare in nature but that
even unpollinated female plants were able to form seeds. Juel made
a thorough study of the development and showed that even when
staminate plants do occur, the pollen is either lacking or only feebly

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

developed. In the ovules the megaspore mother cell develops di-
rectly into the embryo sac without any reduction in the chromosome
number and the diploid egg produces an embryo without fertiliza-
tion (Fig. 21). Murbeck similarly showed that some species of

^Ol

0 O

.0,

'0/

'01

-^fe®

.01

! i

«;<€

^0
K)

B C

Fig. 21. Apomixis in Antennaria alpina. A, mature embryo sac. B, later
stage, showing enlargement of egg and increase in number of antipodal cells; polar
nuclei preparing to divide. C, embryo two-celled; polar nuclei in division. (Re-
drawn after Juel, 1900.)

HISTORICAL SKETCH

Alchemilla, belonging to the section Eualchemilla, develop partheno-
genetically without any chromosome change in the life cycle.

The Twentieth Century. The year 1900 marked the beginning
of a new era in angiosperm embryology. By this time most of the
facts on the development of the gametophytes and embryo had been
discovered, and an able summary of the literature was given by
Coulter and Chamberlain (1903) in their book entitled "Morphology
of Angiosperms." The stage was now set for more detailed inves-
tigations of special topics to clear
up previous obscurities, and for
studies of a comparative nature
on whole families and orders to
determine what light embryo-
logy could throw on problems
of taxonomy.

To outline the contributions
of the numerous individuals who
have been engaged in such studies
during recent years is out of place
in this brief and introductory
sketch but will be attempted in
the following chapters . Mention
may be made here of the names
of a few whose contributions
have been especially noteworthy.

Among modern students of the
subject, the name of the late
Karl Schnarf of Vienna stands
preeminent. His two works entitled "Embryologie der Angio-
spermen" (1929) and "Vergleichende Embryologie der Angiosper-
men" (1931) are the most important and exhaustive treatises
in this field, and still serve as valuable works of reference. E. C. R.
Soueges of France has distinguished himself by his painstaking
studies on the development of the embryo in several families and
genera of both dicotyledons and monocotyledons; and W. W. Finn
in the Ukraine has similarly engaged himself in a study of the
development and structure of the male gametophyte. In Swe-
den, Sv. Murbeck, 0. Rosenberg, and the late H. O. Juel in-
spired a flourishing school of research on all phases of embryology

Fig. 22. Karl Schnarf.

24 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

(especially gametogenesis and apomixis), work on which is being
continued at present by K. V. 0. Dahlgren, F. Fagerlind, A. Hakans-
son, H. Stenar, and A. Gustafsson. Among workers from other
countries may be mentioned the names of A. Chiarugi and the late
E. Carano from Italy; A. Ernst from Switzerland; H. D. Wulff
from Germany; and the late J. M. Coulter and D. S. Johnson from
the United States. During recent years there has also been con-
siderable activity in this field in India, Japan, and Australia.

Of particular interest is the origin of the new science of experi-
mental embryology, dealing with problems of storage and viability
of pollen, effect of environmental factors on pollen tube growth,
control of fertilization, production of seedless fruits, embryo culture,
and artificial induction of parthenogenesis and adventive embryony.
Here embryology stands in intimate relation with physiology and
genetics, and promises to offer many opportunities and openings
for the future.

References

Amici, G. B. 1824. Observations microscopiques sur diverses especes de plantes.

Ann. des Sci. Nat., Bot. 2: 41-70, 211-248.
. 1830. Note sur le mode d'action du pollen sur le stigmate. Extrait

d'une lettre d Amici a Mirbel. Ann. de? Sci. Nat., Bot. 21: 329-332.
. 1844. Quatrieme reunion des naturahstes italiens. Padua, 1843. Flora

1:359.
. 1847. Sur la fecondation des Orch'dees. Ann. des Sci. Nat., Bot.

7/8: 193-205.
Aristotle. "The Works of Aristotle." Engl, transl. by J. A. Smith and W. D.

Ross. Oxford, 1913.
Braun, A. 1859. tlber Polyembryonie und Keimung von Coelobogyne. Abh.

Konigl. Akad. Wiss. Berlin, phys. KL, pp. 107-263.
Brongniart, A. 1827. Memoire sur la generation et le developpement de l'em-

bryon dans les vegetaux phanerogamiques. Ann. des Sci. Nat., Bot. 12:

14-53, 145-172, 225-298.
Brown, R. 1831. Observations on the organs and mode of fecundation in Orchi-

deae and Asclepiadeae. Trans. Linn. Soc. London 16: 685-745.
. 1833. "The Miscellaneous Botanical Works of Robert Brown." Lon-
don, 1866-1868.
Camerarius, R. J. 1694. "De sexu plantarum epistola." Tubingen.
Coulter, J. M., and Chamberlain, C. J. 1903. "Morphology of Angiosperms."

New York.
Elfving, F. 1879. Studien iiber die Pollenkorner der Angiospermen. Jenaische

Ztschr. f. Naturw. 13: 1-28.

HISTORICAL SKETCH 25

Famintzin, A. 1879. Embryologische Studien. Mem. Acad. Imp. des Sci. St.

Petersburg VII, 26(10): 1-19.
Finn, W. W. 1931. Zur Geschichte der Entdeckung der doppelten Befruchtung.

Ber. deutsch. bot. Gesell. 49: 153-157.
Focke, W. 0. 1881. "Die Pflanzen-Mischlinge, ein Beitrag zur Biologie der

Gewachse." Berlin.
Grew, N. 1682. "The Anatomy of Plants." London.
Guignard, L. 1881. Recherches d'embryogenie vegetale comparee. I. Legu-

mineuses. Ann. des Sci. Nat., Bot. 12: 5-166.
. 1899. Sur les antherozoides et la double copulation sexuelle chez les

vegetaux angiospermes. Rev. Gen. de Bot. 11: 129-135.
Hanstein, J. 1870. Die Entwickelung des Keimes der Monocotylen und Dicoty-

len. Bot. Abhandl. Bonn 1: 1-112.
Hegelmaier, F. 1897. Zur Kenntnis der Polyembryonie von Allium odorum.

Bot. Ztg. 55: 133-140.
Herodotus, circa 484-425 B.C. "Historiae." Engl, transl. by A. D. Godley.

London, 1921.
Hertwig, O. 1918. "Dokumente zur Geschichte der Zeugungslehre. Eine his-

torische Studie." Bonn.
Hofmeister, W. 1847. Untersuchungen des Vorgangs bei der Befruchtung der

Oenotheren. Bot. Ztg. 5: 785-792.

-. 1848. tJber die Entwicklung des Pollens. Bot. Ztg. 6 : 425-434, 649-

658, 670-674.

. 1849. "Die Entstehung des Embryo der Phanerogamen." Leipzig.

. 1859. Neue Beitrage zur Kenntnis der Embryobildung der Phanero-
gamen. I. Dikotyledonen mit urspriinglich einzelligem, nur durch Zell-

theilung wachsendem Endosperm. Abh. Konigl. Sachs. Gesell. Wiss. 1859,

pp. 535-672.

1861. Neue Beitrage zur Kenntnis der Embryobildung der Phanero-

gamen. II. Monokotyledonen. Abh. Konigl. Sachs. Gesell. Wiss. 7:

629-760.
Juel, H. O. 1898. Parthenogenesis bei Antennaria alpina (L) R.Br. Vorlaufige

Mittheilung. Bot. Centbl. 74: 369-372.
. 1900. Vergleichende Untersuchungen iiber typische und partheno-

genetische Fortpflanzung bei der Gattung Antennaria. K. Svenska Vet.-

Akad. Handl. 33 (5) : 1-59.
Kerner, A. 1876. Parthenogenesis bei einer angiospermen Pflanze. Sitzber.

Math., Nat. Kl. Akad. der Wiss. Wien 1, 74: 469.
Kolreuter, J. C. 1761-1766. "Vorlaufige Nachricht von einigen das Geschlecht

der Pflanzen betreffenden Versuchen und Beobachtungen."
Leeuwenhoek, A. v. 1677. "Observations de natis e semine genitali animal-

culis."
. "The Secret Works of Antony van Leeuwenhoek, Containing his Micro-
scopical Discoveries in Many of the Works of Nature." Engl, transl. by

Samuel Hoole. London, 1800.

26 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Mottier, D. M. 1897. Tiber das Verhalten der Kerne bei der Entwicklung des

Embryosackes und die Vorgange bei der Befruchtung. Jahrb. f. wiss. Bot.

31: 125-158.
Murbeck, S. 1897. Om vegetativ embryobildning hos flertalet Alchemillar och

den fiorklaring ofver formbestandigheten inom slagtet, som densamma innebar.

Bot. Notiser 1897, pp. 273-277.
. 1901. Parthenogenetische Embryobildung in der Gattung Alchemilla.

Lunds Univ. Arsskr., Afd. II, 36(7): 1-41.
Nawaschin, S. G. 1898a. tlber das Verhalten des Pollenschlauches bei der Ulme.

Bui. Acad. Imp. des Sci. St. Petersburg 8: 345-357.
. 18986. Resultate einer Revision der Befruchtungs vorgange bei Lilium

martagon und Fritillaria tenella. Bui. Acad. Imp. des Sci. St. Petersburg 9 :

377-382.
Pliny (Plinius Secundus, Gaius). "The Historie of the World: Commonly Called

the Naturall Historie of G. Plinius Secundus." Engl. Transl. London, 1635.
Pringsheim, N. 1855. Uber die Befruchtung der Algen. Ber. Preuss. Akad. der

Wiss. Berlin 1855, pp. 133-165.
— . 1856. Uber die Befruchtung und der Generationswechsel der Algen.

Monatsber. Konigl. Preuss. Akad. der Wiss. Berlin 1856, pp. 225-237.
Radlkofer, L. 1856. "Die Befruchtung der Phanerogamen. Ein Beitrag zur

Entscheidung des dariiber bestehenden Streites." W. Engelmann, Leipzig.
Sachs, J. 1874. "Lehrbuch der Botanik." Leipzig.
Sargant, E. 1900. Recent work on the results of fertilization in angiosperms.

Ann. Bot. 22: 121-186.
Schacht, H. 1850. "Entwicklungsgeschichte der Pflanzenembryo." Amsterdam.
Schleiden, M. J. 1837. Einige Blicke auf die Entwicklungsgeschichte des vege-

tablischen Organismus bei den Phanerogamen. Arch. Bwl. Naturgeschichte

III, 1 : 289-320.

— . 1845. tlber Amicis letzten Beitrag zur Lehre von der Befruchtung der

Pflanzen. Flora: 593-600.
Schnarf, K. 1929. "Embryologie der Angiospermen." Berlin.

— . 1931. "Vergleichende Embryologie der Angiospermen." Berlin.
Sharp, L. W. 1943. "Fundamentals of Cytology." McGraw-Hill Book Com-
pany.
Strasburger, E. 1877. Uber Befruchtung und Zelltheilung. Jenaische Ztschr. f .

Naturw. 11: 435-536.
-. 1878. Uber Polyembryonie. Jenaische Ztschr. f. Naturw. 12 : 647-670.

. 1879. "Die Angiospermen und die Gymnospermen." Jena.

— . 1884. "Neue Untersuchungen liber den Befruchtungsvorgang bei den
Phanerogamen." Jena.
. 1900. Einige Bemerkungen zur Frage nach der "doppelten Befruchtung"

bei Angiospermen. Bot. Ztg. II, 58: 293-316.
Thuret, G. 1854. Recherches sur la fecondation des Fucacees, suivies d'observa-

tions sur les antheridies des algues. Ann. des Sci. Nat., Bot. 2: 196-214.
Tretjakow, S. 1895. Die Beteilung der Antipoden in Fallen der Polyembryonie

bei Allium odorum. Ber. deutsch bot. Gesell. 13: 13-17.

HISTORICAL SKETCH 27

Tieub, M. 1879. Notes sur l'embryogenie de quelques Orchidees. Natuurk.

Verh. Koninkl. Akad. Amsterdam 19: 1-50.
. 1891. Sur les Casuarinees et leur place dans le systeme naturel. Ann.

Jard. Bot. Buitenzorg 10: 145-231.

and Mellink, J. 1880. Notice sur le developpement du sac embryonnaire

dans quelques Angiospermes. Arch. Neerland. 15: 452-457.
Von Mohl, H. 1863. Giambattista Amici. Bot. Ztg. 21 (Beilage 34) : l-\

CHAPTER 2

THE MICROSPORANGIUM

In considering the course of events leading to the origin of the
embryo, we must first deal with the development of the micro- and
megasporangia. It is the microsporangium which produces the mi-
crospores and eventually the male gametophyte. Similarly, the
megasporangium, or ovule, is the place of formation of the mega-
spores and the female gametophyte. The latter, after fertilization,

Fig. 23. T.s. anther of Lilium philadelphicum, showing dissolution of cells sepa-
rating the two microsporangia on each side. Note the fibrous endothecium and
stomium s. The minute punctate markings lining the inner wall of the anther
probably represent remnants of tapetum. (After Coulter and Chamberlain, 1903.)

produces the embryo and endosperm, while the entire megasporan-
gium with its enclosed structures becomes the seed and the progen-
itor of the next generation.

A typical anther comprises four elongated microsporangia, but
at maturity the two sporangia of each side become confluent owing
to the breaking down of the partition between them (Fig. 23). A
cross section of a very young anther shows a mass of homogeneous
meristematic cells surrounded by the epidermis (Fig. 24 A, B). It

THE MICROSPORANGIUM

soon becomes slightly four-lobed, and rows of hypodermal cells
become differentiated in each lobe by their larger size, radial elon-
gation, and more conspicuous nuclei. These form the archespor-
ium. The extent of the archesporial tissue varies considerably both
lengthwise and breadthwise. Either a single archesporial cell may
be seen in each lobe in a cross section of the anther, as in Sanse-
vieria (Guerin, 1927), Dionaea (Smith, 1929), and Boerhaavia (Ma-
heshwari, 1929), or a plate of such cells, as in Ophiopogon (Mahesh-
wari, 1934), Urginea (Capoor, 1937a), and most other plants. In
longitudinal section also the row may comprise only one cell as in

Fig. 24. A-E, differentiation of parietal and sporogenous tissue in anthers of
Chrysanthemum leucanthemum (e = epidermis; end = endothecium ; m = middle
layer; t = tapetum; sp = sporogenous cell). (After Wanning, 1873.)

Enalus (Kausik, 1941), or two cells as in Boerhaavia (Maheshwari,
1929), or several cells as in Urginea (Capoor, 1937a).

Figure 24C-E shows the stages leading to the origin of the sporog-
enous tissue. The archesporial cells divide to form a primary
parietal layer toward the outside and a primary sporogenous layer
toward the inside. The cells of the former divide by periclinal and
anticlinal walls to give rise to a series of concentric layers, usually
three to five, composing the wall of the anther. The primary sporog-
enous cells either function directly as the spore mother cells or
undergo further divisions to form a larger number of cells.

30 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

In a few plants a hypodermal archesporium has not been clearly
distinguished and more deep-seated cells are said to give rise to the
sporogenous tissue. In Doryanthes (Newman, 1928), Pholisma
(Copeland, 1935), and Holoptelea (Capoor, 19376) it is stated that
there is no definite system of periclinal divisions separating the
parietal tissue from the archesporium and that the sporogenous
function is gradually taken over by a group of cells about three or
four layers below the epidermis. It is probable, however, that such
appearances are due to the difficulty of obtaining a sharp differen-
tiation between the cells during the early stages of development of
the anther, and further studies may reveal the hypodermal origin
of the archesporium in these plants also.

The Wall Layers. The epidermis, which is the outermost layer
of the anther, undergoes only anticlinal divisions. Its cells become
greatly stretched and flattened in order to keep pace with the en-
largement of the anther, and in many plants, especially those of dry
habitats, they eventually lose contact with each other so that only
their withering remains can be seen at maturity.

The layer of cells lying immediately beneath the epidermis is the
endothecium. Its maximum development is attained at the time
when the pollen grains are about to be shed (Fig. 23). The cells
become radially elongated, and from their inner tangential walls
fibrous bands run upward, ending near the outer wall of each cell.
In aquatics with aerial flowers like Utricularia (Kausik, 1938) and
even such reduced forms as Wolffia (Gupta, 1935) the fibrous thick-
enings occur as usual, but in several members of the Hydrocharita-
ceae (Ernst-Schwarzenbach, 1945; Maheshwari and Johri, 1950),
and in some cleistogamous forms whose flowers never open, they
fail to develop and there is no special mode of dehiscence. In those
plants, also, whose anthers open by apical pores, the endothecium
may not develop any fibrous thickenings and dehiscence takes place
here by the dissolution of certain cells at the apex of the anther.
In Erica, which is an example of this kind, there is a further pecu-
liarity in that the "apical" pores are in fact basal. Figure 25 shows
some stages in the curvature of the anther which bring about this
inversion (Matthews and Taylor, 1926).

Among other exceptions may be cited Musa (Juliano and Alcala,
1933), Sesamum (Nohara, 1934), Anona (Juliano, 1935a), Ipomoea,
(Juliano, 19356), Aeginetia (Juliano, 1935c), and Melastoma (Subra-

THE MICROSPORANGIUM

manyam, 1948) in which the fibrous thickening are absent but the
walls of the epidermal cells undergo a general cutinization and
lignification over the entire surface. Oryza (Juliano and Aldama,
1937), Ditepalanthus (Fagerlind, 1938), and Balanophora (Fager-
lind, 1945) are peculiar in that the parietal layers, one or two in
number, become crushed and disorganized during the development
of the anther so that a fibrous layer is absent and the epidermis
abuts directly on the tapetum.1

Fig. 25. Development of anther of Erica hirtiflora. A, B, l.s. young stamens,
showing gradual inversion of anther. C, l.s. stamen at spore mother cell stage,
showing almost complete inversion of anther, so that its lower end comes to lie
toward the upper side. (After Matthews and Taylor, 1926.)

Next to the endothecium there are usually one to three "middle"
layers. As a rule, all of them become flattened and crushed at the
time of the meiotic divisions in the microspore mother cells, but
there are a few exceptions. In Holoptelea (Capoor, 19376) there are
three to four middle layers, of which the outermost persists for a
long time. In Ranunculus (Singh, 1936) there are two middle
layers, of which the inner soon disappears but the outer persists;

1 In Styphelia (Brough, 1924), Arceuthobium (Pisek, 1924) and some members
of the Ericales it is the epidermis which is said to develop fibrous thickenings and
function as an endothecium, but this deserves confirmation.

32 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

occasionally its cells become densely protoplasmic and simulate
those of the tapetum. In Lilium there are several middle layers,
of which those lying adjacent to the endothecium persist for a long
time (Fig. 23), and in Gloriosa (Eunus, 1949) the outermost middle
layer develops fibrous thickenings similar to these of the endo-
thecium.

Rarely, a middle layer may be absent as in the anthers of Wolffia
(Gupta, 1935) and Vallisneria (Witmer, 1937), but some previous
reports of the absence of a middle layer have been shown to be mis-

A B

Fig. 26. Anthers, showing microspore mother cells and tapetum. A, Bougain-
villea, t.s. portion of anther, showing mitotic divisions in tapetal cells. (After
Cooper, 1931.) B, Salvia mellifera, t.s. portion of anther lobe. The tapetal cells
lying toward the connective are considerably larger than those on the outer side.
(After Carlson and Stuart, 1936.)

interpretations caused by its ephemeral nature and early disappear-
ance. Johri (1934) has demonstrated the presence of a middle layer
in Cuscuta where it was formerly reported to be absent (Peters,
1908).

The innermost wall layer or tapetum is of considerable physio-
logical significance, for all the food materials entering into the sporog-
enous cells must pass through it.2 Its cells are full of dense cyto-
plasm, and at the beginning of meiosis the tapetal nuclei may also
undergo some divisions (Fig. 26). 3 Because of these similarities of

2 Typically the tapetum is a single layer of cells but in Nicolaia and Costus
(Boehm, 1931) it is composed of several layers.

3 Rarely, tapetal nuclei may even pass through a condition resembling
the prophase of a meiotic division. Gates and Rees (1921) figure some tapetal

THE MICROSPORANGIUM

appearance and behavior between the cells of the tapetum and the
micrcsporogenous tissue, earlier botanists supposed that the former
is derived by a sterilization of the outer sporogenous cells. Develop-
mental studies of a precise nature have, however, nearly always
confirmed its parietal origin.4

The nuclear divisions in the tapetum were formerly believed to
be amitotic, but recent studies (Bonnet, 1912; Cooper, 1933; Wit-
kus, 1945) have shown that this is incorrect and that appearances
suggesting amitosis are really caused by mitotic irregularities and

G H I J K

Fig. 27. Nuclear divisions in tapetal cells of Zea mays (A-F), Lilium canadense
(G-H), and Podophyllum peltatum (I-K). (After Cooper, 1933.)

nuclear fusions. According to present conceptions, the nucleus of a
tapetal cell may divide in any of the following ways:5

1. By normal mitosis. The division takes place in the ordinary

nuclei of Lactuca in the synizesis stage, and Moissl (1941) reports a similar condi-
tion in some members of the Caprifoliaceae.

4 Recently, Capoor (19376) has reported that in Holoptelea the tapetal cells are
almost indistinguishable from the adjacent cells of the sporogenous tissue. He
cautiously adds, however, that this fact alone is insufficient to justify any inference
regarding the sporogenous origin of the tapetum.

6 It is to be noted that in a few families and orders, viz., Mimosaceae, Cras-
sulaceae, Gentianaceae, Boraginaceae, Hydrophyllaceae, Juncaceae, Orchidaceae,
and Helobiales, the tapetal cells usually remain uninucleate from the time of their
formation to their eventual disintegration.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

way, but no cell plate is laid down. The two daughter nuclei,
which are diploid, remain inside the cell (Fig. 27 A-F).

2. By a "sticky" type of division. Here the chromosomes behave
normally up to the early anaphase stage. After this, one or more of
them fail to separate, forming chromosome bridges which persist
during the telophase as well as the resting stage. As a result a
single dumbbell-shaped tetraploid nucleus is formed whose middle
portion may be broad or narrow depending on the number of
chromosome bridges present (Fig. 27 G-K).

3. By endomitosis.6 Here the nucleolus and the nuclear mem-
brane remain intact and there is no spindle formation. The chro-

Fig. 28. Diagrams showing "endomitosis" in tapetal cells of Spinacia oleracea.
A, endoprophase. B, endometaphase. C, endo-anaphase. D, endotelo phase.
(Drawing supplied by Dr. E. R. Witkus.)

mosomes contract and split longitudinally, but all of them remain
within the same nucleus, which becomes tetraploid (Fig. 28).

The first nuclear division in a tapetal cell is often followed by
further divisions. Some of the divisions may be accompanied by
nuclear fusions, resulting in one or more large polyploid nuclei.
The latter may, however, divide again and give rise to smaller nu-
clei. Since this type of behavior is very frequent in tapetal cells,
it is unnecessary to give specific instances.

An interesting condition has been reported in certain haploid and

6 This type of division was first postulated by Meyer (1925). In the tapetal
cells of Leontodon he found diploid nuclei in younger stages and polyploid nuclei in
older stages. Since no spindle fibers were observed, he concluded that there was an
"internal division" of the chromosomes without any nuclear division. See also
Brown (1949) who has recently given a detailed account of endomitosis in the
tapetal cells of tomato.

THE MICROSPORANGIUM 35

diploid plants of Oenothera rubricalyx (Gates and Goodwin, 1930).
In the former the tapetal cells are uninucleate and in the latter
they are binucleate — a fact which is no doubt related to the general
reduction of tissues in haploid individuals. More difficult to ex-
plain is the marked difference in shape and structure of the tapetal
cells belonging to the same anther. In Lathraea (Gates and Latter,
1927), Salvia (Carlson and Stuart, 1936) (Fig. 265), and Moringa
(Puri, 1941) the tapetal cells on the inner side of the loculus show a
marked radial elongation and are much larger than those on the
outer side. Further, in Lathraea the cells on the outer side are uni-
nucleate while those adjacent to the connective are binucleate.
In Lactuca sativa (Gates and Rees, 1921) the tapetal cells lying on
one side of the loculus may be quadrinucleate while those on the
other are binucleate. The binucleate cells are nearly always shorter
and broader than the quadrinucleate. Possibly these differences
are related to the varying amounts of nutritive materials passing
into the cells.

Toward the close of the meiotic divisions in the microspore mother
cells, the tapetal cells begin to lose contact with each other. Large
vacuoles appear in the cytoplasm and the nuclei begin to show signs
of degeneration.7 Finally the cells are entirely absorbed at the
time when the microspores begin to separate from one another.
This type of tapetum, in which the cells remain in situ, is called the
glandular or secretory tapetum and is of common occurrence in
angiosperms. However, there are several genera and families (see
Juel, 1915; Tischler, 1915; Mascre, 1919 a, b) in which the walls of
the tapetal cells break down but the protoplasts, which remain
intact, protrude and "wander" inside the loculus, where they may
coalesce to form a continuous mass called the tapetal periplasmodium
(Fig. 29). Clausen (1927), who has reviewed the previous literature
in this connection, classifies this kind of tapetum (often called the
"amoeboid" tapetum) into four subtypes:

1. Sagittaria type. The tapetal cells lose their walls by the time
the microspore tetrads have been formed, and their protoplasts
begin to project inward as soon as the microspores have separated.
Later the periplasmodium becomes continuous. Examples: Sagit-
taria, Alisma, Limnocharis, Hydrocharis.

7 At this stage the anther loculi frequently show a densely staining jelly-like
or mucilaginous fluid which disappears at maturity. As suggested by Nietsch
(1941), this is probably a secretion from the tapetal cells.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

2. Butomus type. In this case the formation of the periplasmo-
dium occurs a little earlier, when the microspores are still grouped
in tetrads. Examples: Butomus, Stratiotes, and Ouvirandra.

3. Sparganium type. Here also the fusion of protoplasts begins
at the tetrad stage but the tapetal cells are multinucleate. Ex-
amples: Sparganium, Typha, Tradescantia.

4. Triglochin type. In a few plants the tapetum begins its activ-
ity while the microspore mother cells are still undergoing the meio-
tic divisions. The tapetal protoplasts and nuclei protrude into the

\.:-v..i.

Fig. 29. Tapetal Plasmodium in Symphoricarpos racemosm (A) and Lonicera
pyrenaica (B). (After Moissl, 1941.)

spaces between the mother cells so that the periplasmodium is
formed at a very early stage. Examples: Triglochin, Potamogeton,
and several members of the Araceae.

Like the glandular or secretory tapetum, the amoeboid tapetum
also serves for the nutrition of the spores, and is probably more
effective for this purpose. As some authors (see Mezzetti-Bamba-
cioni, 1941) have suggested, it seems probable that the periplas-
modium contributes to the formation of the exine, but this point
deserves further study. A curious feature which has been observed
in several plants (see Ubisch, 1927; Kosmath, 1927; Kajale, 1940;
Puri, 1941; Singh, 1950) is the appearance of small granular mark-

THE MICROSPORANGIUM

ings on the inner surface of the tapetum (Fig. 30) and later on the
inner surface of the middle layers or the endothecium. They give
the same staining reactions as the exine of the pollen grains and prob-
ably contribute to the development of the latter. This seems to be
supported by Gorczynski's (1934) observations on Cardamine, ac-
cording to which the exine first begins to develop on that side of the
pollen grains which lies towards the tapetum.

Fig. 30. Tapetal cells, showing cutinization of inner walls. A, Magnolia youlan,
tapetal cell, showing prominent thickenings on inner surface. B, the thickenings
as seen in surface view. C, Lilium tigrinum, thickenings on inner walls of tapetal
cells. D, same in surface view. E, more highly magnified than D. (After Kos-
math, 1927.)

Sporogenous Tissue. The primary sporogenous cells give rise
to the microspore mother cells. In some plants the sporogenous
cells undergo several divisions, in others only a few divisions, and
rarely there are no divisions at all, so that the primary sporogenous
cells function directly as the microspore mother cells. Alangium,
Sansevieria, Knautia, and some members of the Malvaceae and
Cucurbitaceae are examples of the third kind, showing a single row
of microspore mother cells in each anther lobe.

38 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

A peculiar feature met with in some members of the Mimosaceae
is the development of transversely placed sterile septa in the anther
lobes (Fig. 31). In some members of the Loranthaceae also, viz.,
Dendrophthoe (Rauch, 1936), Elytranthe, and Amyema (Schaeppi

Fig. 31. Structure of anther in some members of the Mimosaceae. A, Parkia,
l.s. anther showing two rows of pollinia. B,C, pollinia dissected out from anther.
D, Dichrostachys, l.s. anther, showing pollinia; note stalked gland gl at apex of
anther. (After Engler, 1876.)

and Steindl, 1942), the microsporangia become vertically parti-
tioned by the formation of sterile septa, and in Viscum (Schaeppi
and Steindl, 1945) such partitions arise not only in the vertical plane
but also in the horizontal one so that each anther has as many as
50 loculi.

THE MICROSPORANGIUM 39

Formation of sterile septa is also known in a few other plants.
Caldwell (1899) reports that in Lemna the archesporial tissue orig-
inally comprises a single mass of cells. After the usual wall layers
have been cut off, a plate of sterile cells divides this mass into two
and then into four. In Limnophyton (Johri, 1935) and Ranunculus
(Singh, 1936) a cross section of the young anther shows an oval or
somewhat dumbbell-shaped outline with a plate of archesporial
cells on each side. Both of these become partitioned by the ap-
pearance of a sterile septum resulting in the usual tetralocular con-
dition. In Quamoclit (Fedortschuk, 1932) there is a single row of
sporogenous cells in each lobe of the anther but one or two of these
fail to keep pace with the others and become nonfunctional. These
give rise to sterile partitions separating the loculus into two or three
parts.

In some plants there are fewer than four groups of sporogenous
cells. In the family Malvaceae (Stenar, 1925) the anthers are uni-
formly bisporangiate and the two loculi eventually fuse to form a
single loculus. In Elodea (Wylie, 1904), Styphelia (Brough, 1924),
Circaeaster (Junell, 1931), Phoradendron (Billings, 1932), Wolffia
(Gupta, 1935), and Moringa (Puri, 1941) also, there are two micro-
sporangia which may later become confluent by the breaking down
of the intervening cell layers. The anthers of Naias (Campbell,
1897) are said to be unilocular, but the developmental stages have
not been traced satisfactorily. In Vallisneria (Witmer, 1937) there
are all gradations from a unilocular to a tetralocular condition.
Typically two loculi are formed, owing to the appearance of a sterile
septum in the sporogenous tissue, but sometimes the septum is
incomplete, resulting in a unilocular condition, and frequently each
of the two loculi becomes bisected so as to form four loculi.

The stamens of Piper betle (Johnson, 1910) are peculiar in that the
number of microsporangia in an anther may be four, three, two, or
one, and it remains constant from the time of initiation of the
sporangia to the maturation of the anther. There is no secondary
fusion of the sporogenous tissue.

In Korthalsella (Stevenson, 1934; Rutishauser, 1935) there are
three stamens, each of which consists of two microsporangia, but
since all the anthers fuse to form a synandrium, a cross section of
the flower shows six microsporangia arranged in a ring. At matur-

40 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

ity the partitions between the sporangia break down and the loculi
become continuous.

The genus Arceuthobium is unique in having a single annular
pollen sac forming a continuous ring around a central column of
sterile cells called "columella" (Stadtler, 1923; Fisek, 1924; Thoday
and Johnson, 1930; Dowding, 1931). Regarding the origin of this
condition there is, however, some difference of opinion. Stadtler
(1923) thinks that the anther is at first multilocular but the par-
titions break down at maturity. Pisek (1924), on the other hand,
contends that it is unilocular from the commencement, and this is
supported by Thoday and Johnson (1930) who state that even in
the youngest anthers there is a ring-shaped archesporium surround-
ing the central columella. Dowding (1931) agrees regarding the
continuity of the archesporium but finds that the columella exhibits
a considerable amount of variation. It frequently forms a sort of
flange dividing the anther into two halves; sometimes the first
flange tends to disappear, and a new one arises at right angles to it.
Rarely, the flanges give out branches extending outwards to the
anther wall. In Dowding 's opinion these flanges of the columella
are to be regarded as remnants of the septa which once separated
four distinct archesporia.

Although all the sporogenous cells in the anther are potentially
capable of giving rise to microspores, some of them frequently de-
generate and become absorbed by the remaining cells. In Ophiopo-
gon (Maheshwari, 1934) and Holoptelea (Capoor, 19376) some of the
sporogenous cells do not reach even the mother cell stage and prob-
ably serve to nourish the remaining cells. In Zoster a (Rosenberg,
1901) most of the sporogenous cells divide longitudinally to form the
numerous long microspore mother cells but others interspersed be-
tween them undergo transverse divisions and give rise to sterile
cells which are later crushed and used up by the functioning cells.
In certain members of the Gentianaceae (Guerin, 1926) which are
devoid of any well-formed tapetum, the nutritive function is taken
over by some of the sporogenous cells themselves. These become
sterile and do not go through the reduction divisions (Fig. 32).
In Kigelia (Venkatasubban, 1945) degeneration takes place at a
later stage; some of the microspores in a tretrad fail to develop fur-
ther and become functionless.

Cytomixis. While making a study of Oenothera gigas and 0.

THE MICROSPORANGIUM

biennis, Gates (1911) observed a frequent migration of chromatic
material from one microspore mother cell into another and called
it cytomixis. Since then it has been reported in several other plants,
and while it is most frequent between the synizesis and diakinesis
stages, it may sometimes occur even during the interkinesis stage,
i.e., after the first meiotic division has been completed. In Lathraea
(Gates and Latter, 1927), which is an instance of this kind, the
microspore mother cells do not round up but remain in close contact
with one another. During interkinesis the nuclei of the two dyad
cells occupy an eccentric position near the cell wall so that the

A B

Fig. 32. Sterilization of part of sporogenous tissue in anthers of Sivertia perennis.
A, anther lobe at microspore mother cell stage. B, same, at microspore tetrad
stage. {After Guerin, 1926.)

chances of cytomixis are increased. In Coreopsis tripteris (Gelin,
1934) cytomixis may also occur at the close of the meiotic divisions
but the multinucleate cells formed in this way again break up into
smaller units consisting of one or two nuclei.

In some plants individual chromosomes, or groups of chromo-
somes, or even whole spindles are said to be carried from one cell
into another. It is believed, however, that it is a pathological
phenomenon, or that such appearances are caused by faulty fixa-
tion. Woodworth (1931), who used smear preparations of anthers,
states that cytomixis was common when a little extra pressure was
used in squeezing out the microspore mother cells. Further, such
abnormalities were found to be more frequent in hybrids than in
other plants, and he attributes this to an "innate unbalance" in the

42 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

heterozygous cytoplasm which makes it more susceptible to pressures
and other similar treatments.

Mention may also be made here of fusions of entire cells of the
sporogenous tissue. Matsura (1935) reported that in Phacellan-
thus the separating walls between adjacent microspore mother cells
sometimes dissolve and fuse in pairs to form giant cells, which may
either give rise to polyploid gametes or degenerate without com-
pleting the meiotic divisions. In two haploid plants of Phleum
pratense, Levan (1941) observed the fusion of as many as 30 micro-
spore mother cells, giving rise to large plasmodia or "syncytes."
A similar behavior has also been reported by Stern (1946) in sugar
suspensions of the microspore mother cells of Trillium erectum.
Here the extent of the fusions appeared to be unlimited, although the
maximum number of nuclei actually observed in a cell was 32.

Cytokinesis. The divisions of the microspore mother cells may
be of the successive or the simultaneous type.8 In the former a
cell plate is laid down immediately after the first meiotic division
and another in each of the two daughter cells after the second meio-
tic division. In the simultaneous type, on the other hand, no wall
is laid down after the first division and the mother cell becomes
separated all at once into four parts after both the meiotic divisions
are over.

The investigations of C. H. Farr (1916) and others have shown
that there is also another difference in the mechanism of cytokine-
sis. In the successive type the cell plate is laid down in the center
and then extends centrifugally on both sides, dividing the cell into
two equal halves. In the simultaneous type, on the other hand, the
division usually occurs by centripetally advancing constriction fur-
rows, which meet in the center and divide the mother cell into four
parts.

Farr (1916) studied Nicotiana tdbacum in special detail. At first
there is an enlargement of the nucleus of the microspore mother
cell, accompanied by a thickening of the mother cell wall. No cell
plate is laid down after Meiosis I, and the spindle fibers of this divi-
sion disappear during the metaphases of Meiosis II. After the four
daughter nuclei have become organized, they assume a tetrahedral
arrangement and a spindle is re-formed between every two nuclei,

8 For an account of the nuclear changes in meiosis, see Sharp (1943) and other
vvorks on cytology

THE MICROSPORANGIUM 43

making a total of six spindles. However, these spindles have noth-
ing to do with the quadripartition of the mother cell, and there is
no laying down of centrifugally growing cell plates such as are
characteristic of other dividing cells. Instead, constriction furrows
now start at the periphery and proceed inward until they meet at
the center, so that there is a simultaneous division of the protoplast
into four cells, i.e., the microspores.

In Melilotus alba (Castetter, 1925) vacuoles seem to play a con-
spicuous part in cytokinesis (Fig. 33). After Meiosis II, hyaline
areas develop between the four nuclei, apparently as the result of
a migration of the denser cytoplasm toward the nuclei and an ex-
trusion of sap into the regions between them. The small vacuoles
arising in this manner soon fuse to form larger ones which virtually
split the cytoplasm into four masses. Furrows originating at the
surface now grow inward and soon meet the vacuoles. Meanwhile,
the mother cell rounds up and secretes a thick layer of callose or some
other gelatinous material, which extends inward with the cleavage
furrows and eventually completes the division of the cell into the
four microspores.9

Zea mays (Reeves, 1928) may be taken as an example of the suc-
cessive type of microspore formation (Fig. 34). At the end of Meiosis
I, thickenings are formed on the spindle fibers at the equatorial
region of the cell. They gradually increase in size, coming in con-
tact with each other and fusing to form the cell plate. Additional
spindle fibers continue to appear just beyond the periphery of the
plate so as to increase the diameter of the spindle. At the same time
the cell plate extends centrifugally and joins the wall of the mother
cell, so as to complete the division of the protoplast into two halves.
Now the second meiotic division follows, and a new partition wall
develops in each cell in the same way as after Meiosis I, resulting
in a tetrad showing the bilateral arrangement of microspores.

The question as to which of the two modes of tetrad formation
is primitive and which is the more advanced is difficult to decide.
It seems, however, that since a division by furrowing is common

9 The mode of origin of this gelatinous layer has been a subject of much discus-
sion. Beer (1906), Gates (1925), and Castetter (1925) have expressed the view
that it is secreted by the cytoplasm of the mother cell, while Farr (1922), Bowers
(1931), and Capoor (1937a) believe that it is the result of a swelling of the secondary
lamellae of the cell wall.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fig. 33. Cytokinesis in microspore mother cells of Melilotus alba. A, telophase
of Meiosis I. B, metaphase of Meiosis II. C, end of Meiosis II, showing three
of the four microspore nuclei. Note large extranuclear "centrosome-like bodies"
seen here and in A. D, microspore nuclei in resting stage. Spindles have almost
disappeared and protoplast has begun to invaginate at the periphery, at points
equidistant from the nuclei. Note origin of special wall, shown in black. E,F,
formation cf vacuoles in portions of cytoplasm lying between nuclei. G, special
wall entering furrows. //, special walls have met in center, forming partitions be-
tween microspore nuclei. I, fully formed microspores. (After Castetter, 1925.)

THE MICROSPORANGIUM

in the Thallophytes and other lower plants, the simultaneous type
is the more ancient and the successive type the derived. In gen-
eral, the former is prevalent in the majority of dicotyledons and the
latter in the majority of monocotyledons. There is no hard and
fast rule, however, and exceptions are frequent. Thus the suc-
cessive type is found in a few dicotyledonous families like the
Asclepiadaceae, Podostemonaceae, and Apocynaceae, and the simul-
taneous type in a few monocotyledonous families, viz., the Iridaceae,
Taccaceae, Juncaceae, and Dioscoreaceae, and in several genera
of the Liliaceae, Palmaceae, and Orchidaceae.

In Magnolia (Farr, 1918) there are isobilateral tetrads formed by
furrowing instead of by cell plates. A cleavage furrow starts after

A B C D E

Fig. 34. Cytokinesis in microspore mother cells of Zea mays. A, anaphase of

Meiosis I. B,C, laying down of partition wall after Meiosis I. D, telophase of
Meiosis II. E, isobilateral tetrad. {After Reeves, 1928.)

Meiosis I, but its development is arrested during the second meiotic
division. It resumes growth at the end of Meiosis II and forms a
partition through the equatorial region of the mother cell. At the
same time additional furrows originate at the periphery, and the
two dyad cells now become subdivided to give rise to the four
microspores. A similar condition occurs in Anona (Juliano, 1935a)
and Asimina (Locke, 1936).

The Microspore Tetrad. As mentioned above, the microspores
are usually arranged in a tetrahedral (Fig. 35 A) or isobilateral (Fig.
35B) fashion, but there are exceptions (Fig. 35C-E). A decussate
arrangement of the cells has been recorded in Magnolia (Farr, 1918),
Atriplex (Billings, 1934), Comas (D'Amato, 1946), and many other
plants. In some genera of the Asclepiadaceae (Gager, 1902) and
in the genus Halophila of the Hydrocharitaceae (Kausik and Rao,
1942) the mother cells divide transversely so as to give rise to

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

linear tetrads (Fig. 36A-D). T-shaped tetrads also occur some-
times as in Aristolochia (Samuelsson, 1914) and Butomopsis (Johri,
1936). In Zostera (Rosenberg, 1901) the elongated microspore
mother cells, measuring 5 by 60 microns at the time of meiosis,
divide in a plane parallel to the longitudinal axis of the cell, result-
ing in a group of four filiform cells which undergo further elongation
and become approximately 2000 microns long when mature.10 Of
considerable interest are Musa (Juliano and Alcala, 1933), Neottia
(Goebel, 1933), Agave (Vignoli, 1936, 1937), Nicolaia (Boehm, 1931),
Habenaria (Swamy, 1946), Laurus (Battaglia, 1947), and Ottelia
(Islam, 1950) in which two or three types of dispositions may be
found in one and the same species.

Occasionally there are either fewer than four spores resulting from
the divisions of the microspore mother cell, or more than four.

A 'B " — ^C ^-^D ^^E

Fig. 35. Diagram showing different types of microspore tetrads. A, tetrahedral.
B, isobilateral. C, decussate. Z),T-shaped. ^linear. (B-E, ajter\ Boehm, 1931.)

The former condition originates as the result of a failure of one
division, or the formation of a "restitution nucleus" after the first
division, or an irregular wall formation giving rise to one binucleate
and two uninucleate spores. The latter condition, i.e., the forma-
tion of more than four spores (polyspory), usually results from the
occurrence of lagging chromosomes which organize into micronuclei.
In general, however, such abnormalities in the number of micro-
spores are found only in hybrids characterized by a high degree of
sterility and the pollen grains arising in this way are nonfunctional.
Usually the microspores soon separate from one another but in
some plants they adhere in tetrads to form the so-called "com-
pound" pollen grains.11 As examples may be cited Drimys, Anona,

10 Filiform pollen grains also occur in Phyllospadix and Cymodocea, but the
method by which they arise does not seem to have been studied so far.

11 For detailed information on such variations of external form, see Wodehouse
(1936) and Erdtman (1943, 1945).

THE MICROSPORANGIUM

E F G H I

Fig. 36. Development of microspores and male gametophyte of Halophila ovata.
A, l.s. of young staminate flower. B, microspore mother cells with a few tapetal
cells t. C, chains of microspores; note vacant spaces x separating individual
tetrads. D, single microspore. E, microspore, showing tube and generative
cells. F, older stage, showing spindle-shaped generative cell lying inside vegeta-
tive cytoplasm. G, pollen grain, showing division of generative cell. H , same,
more advanced stage. Note formation of constriction furrow across generative
cell. I,J, formation of sperm cells completed. K,L, division of generative cell,
showing formation of transitory cell plate. (After Kausik and Rao, 194%.)

48 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Drosera, Elodea, Typha, Furcraea, and several members of the Eri-
caceae, Apocynaceae, Asclepiadaceae, Juncaceae, and Orchidaceae.
In the Mimosaceae there are larger units composed of 8 to 64 cells,
and in a number of genera belonging to the Asclepiadaceae all the
microspores in a sporangium remain together to form a single mass
called the pollinium. The family Orchidaceae is especially interest-
ing in this connection (see Swamy, 1948). In some genera, such as
Cypripedium and Vanilla, the microspores separate from one another
and become free. In Pogonia the four cells of a tetrad adhere and
form a compound pollen grain. In the tribes Ophrydeae and Neot-
tieae this tendency is carried further and the compound grains are
themselves held together in small units known as massulae. Fi-
nally, in Coelogyne and Pholidota all the microspore mother cells and
their derivatives remain together and continue their development
as a single unit.

References

Battaglia, E. 1947. Meiosi anormale nella microsporogenesi di Laurus nobilis L.

Atti d. Societa Toscana Sci. Nat. 54: 1-22.
Beer, R. 1906. On the development of the pollen grain and anther of some

Onagraceae. Beihefte bot. Centbl. 19A: 286-313.
Billings, F. H. 1932. Microsporogenesis in Phoradendron. Ann. Bot. 46: 979-

992.
. 1934. Male gametophyte of Atriplex hymenelytra. Bot. Gaz. 95:

477-484.
Boehm, K. 1931. Embryologische Untersuchungen an Zingiberaceen. Planta

14: 411-440.
Bonnet, F. 1912. Recherches sur revolution des cellules nourricieres du pollen

chez les Angiospermes. Arch. f. Zellforsch. 7: 605-722.
Bowers, C. G. 1931. The development of pollen and viscin strands in Rhododen-
dron catawbiense. Bui. Torrey Bot. Club 57: 285-314.
Brough, P. 1924. Studies in the Epacridaceae. I. The life history of Styphelia

longifolia. Proc. Linn. Soc. N. S. Wales 49: 162-178.
Brown, S. W. 1949. Endomitosis in the tapetum of tomato. Amer. Jour. Bot.

36: 703-716.
Caldwell, 0. W. 1899. On the life history of Lemna minor. Bot. Gaz. 27:

37-66.
Campbell, D. H. 1897. A morphological study of Naias and Zannichellia. Proc.

Calif. Acad. Sci. III. Bot, 1: 1-71.
Capoor, S. P. 1937a. Contributions to the morphology of some Indian Liliaceae.

II. The gametophytes of Urginea indica Kunth. Beihefte bot. Centbl. 57A:

156-170.

THE MICROSPORANGIUM 49

Capoor, S. P. 19376. The life history of Holoptelea integrifolia Planch. Beihefte

bot. Centlbl. 57A: 233-249.
Carlson, E. M., and Stuart, B. C. 1936. Development of spores and gameto-

phytes in certain New World species of Salvia. New Phytol. 35: 68-91.
Castetter, E. F. 1925. Studies on the comparative cytology of the annual and

biennial varieties of Melilotus alba. Amer. Jour. Bot. 12: 270-286.
Claussen, P. 1927. Uber das Verhalten des Antherentapetums bei einigen Mono-

cotylen und Ranales. Bot. Arch. 18 : 1-27.
Cooper, D. C. 1931. Microsporogenesis in Bougainvillaea glabra. Amer. Jour.

Bot. 18: 337-358.
. 1933. Nuclear divisions in the tapetal cells of certain angiosperms.

Amer. Jour. Bot. 20: 358-364.
Copeland, H. F. 1935. The structure of the flower of Pholisma arenarium.

Amer. Jour. Bot. 22: 366-383.
Coulter, J. M., and Chamberlain, C. J. 1903. "Morphology of Angiosperms."

New York.
D'Amato, F. 1946. Osservazioni cito-embryologiche su Cornus mas L. con parti-

colare riguardo alia sterilita di un biotipo triploide. Nuovo Gior. Bot. Ital.

N.S. 53: 170-210.
Dowding, E. S. 1931. Floral morphology of Arceuthobium americanum. Bot.

Gaz. 91 : 42-54.
Engler, A. 1876. Beitrage zur Kenntnis der Antherenbildung der Metaspermen.

Jahrb. f. wiss. Bot. 10: 275-316.
Erdtman, G. 1943. "An Introduction to Pollen Analysis." Chronica Botanica

Company.
. 1945. Pollen morphology and plant taxonomy. V. On the occurrence

of tetrads and dyads. Svensk Bot. Tidskr. 39:286-297.
Ernst-Schwarzenbach, M. 1945. Zur Blutenbiologie einiger Hydrocharitaceen.

Ber. schweiz. bot. Gesell. 55: 33-69.
Eunus, A. M. 1949. Contributions to the embryology of the Liliaceae (Gloriosa

superba). Proc. 36th Indian Sci. Cong. Allahabad, Bot. Sect. No. 25.
Fagerlind, F. 1938. Ditepalanthus, eine neue Balanophoraceen Gattung aus

Madagaskar. Arkiv for Bot. 29A: 1-15.
. 1945. Blute und Blutenstand der Gattung Balanophora. Bot. Notiser

1945, pp. 330-350.
Farr, C. H. 1916. Cytokinesis of the pollen mother cells of certain dicotyledons.

Mem. N. Y. Bot. Gard. 6: 253-317.
. 1918. Cell division by furrowing in Magnolia. Amer. Jour. Bot. 5:

379-395.
. 1922. Quadripartition by furrowing in Sisyrinchium. Bui. Torrey Bot.

Club 49: 51-61.
Fedortschuk, W. 1932. Entwicklung und Bau des mannlichen Gametophyten

bei den Artcn der Convolvulaceen-Gattung Qua?noclit. Planta 16: 554-574.
Gager, C. S. 1902. The development of the pollinium and sperm cells in

Asclepias cornuti Decne. Ann. Bot. 16: 123-148.

50 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Gates, R. R. 1911. Pollen formation in Oenothera gigas. Ann. Bot. 25 : 909-940
. 1925. Pollen tetrad wall formation in Lathraea. Cellule 35 : 47-60.

and Goodwin, K. M. 1930. A new haploid Oenothera, with some con-
siderations on haploidy in plants and animals. Jour. Genet. 23: 123-156.

and Latter, J. 1927. Observations on the pollen development of two

species of Lathraea. Jour. Roy. Micros. Soc. 1927, pp. 209-224.

and Rees, E. M. 1921. A cytological study of pollen development in

Lactuca. Ann. Bot. 16: 123-148.
Gelin, O. E. V. 1934. Embryologische und cytologische Studien in Heliantheae-

Coreopsidinae. Acta Horti Bergiani 11: 99-123.
Goebel, K. 1933. "Organographie der Pflanzen. III. Samenpflanzen." 3d ed.

Jena.
Gorczyhski, T. 1934. Zytologische Analyse einiger Pollenentwicklungs vorgange

bei der Apfelsorte "Schoner von Boskoop." Acta Soc. Bot. Poloniae 11: 103-

118.
Guerin, P. 1926. Le developpement de l'anthere chez les Gentianacees. Bui.

Soc. Bot. de France 73: 5-18.
. 1927. Le developpement de l'anthere et du pollen chez les Liliacees

(Sansevieria, Ophiopogon, Peliosanthes) . Bui. Soc. Bot. de France 74:

102-107.
Gupta, B. L. 1935. Studies in the development of the pollen grain and embryo

sac of Wolffia arrhiza. Current Sci. [India] 4: 104-105.
Islam, A. S. 1950. The embryology of Ottelia alismoides Pers. Jour. Indian

Bot. Soc. 29: 79-91.
Johnson, D. S. 1910. Studies in the development of Piperaceae. I. The sup-
pression and extension of sporogenous tissue in the flower of Piper betle L. var.

monoicum C. DC. Jour. Expt. Zool. 9: 715-749.
Johri, B. M. 1934. The development of the male and female gametophytes in

Cuscuta rejlexa Roxb. Proc. Indian Acad. Sci. Sect. B. 1 : 2S3-239.
. 1935. Studies in the family Alismaceae. I. Limnophyton obtusifolium

Miq. Jour. Indian Bot. Soc. 14: 49-66.

1936. The life history of Butomopsis lanceolata Kunth. Proc. Indian

Acad. Sci. Sect. B. 4: 139-162.
Juel, H. O. 1915. Untersuchung liber die Auflosung der Tapetenzellen in den

Pollensacken der Angiospermen. Jahrb. f. wiss. Bot. 58: 337-364.
Juliano, J. B. 1935a. Morphological contribution on the genus Anona. Philip-
pine Agr. 24: 528-541.
. 19356. Morphology of the sweet potato, Ipomoea batatus (Linn.) Poir.

Philippine Agr. 23: 833-858.
. 1935c. Anatomy and morphology of the Bunga Aeginetia indica L.

Philippine Jour. Sci. 56: 405-451.
and Alcala, P. E. 1933. Floral morphology of Musa errans (Blanco)

Teodoro var. Botoan Teodoro. Philippine Agr. 22: 91-126.

— and Aldama, M. J. 1937. Morphology of Oryza saliva L. Philippine

Agr. 26: 1-134.

THE MICROSPORANGIUM 51

Junell, S. 1931. Die Entwicklungsgeschichte von Circaeaster agrestis. Svensk

Bot. Tidskr. 25:238-270.
Kajale, L. B. 1940. A contribution to the embryology of the Amaranthaceae.

Proc. Natl. Inst. Sci. India 6: 597-625.
Kausik, S. B. 1938. Pollen development and seed formation in Utricularia

coerulea L. Beihefte bot, Centbl. 58A : 365-378.
. 1941. Structure and development of the staminate flower and male

gametophyte of Enalus acoroides (L. fil) Steud. Proc. Indian Acad. Sci.

Sect. B. 14: 1-16.
and Rao, P. V. K. 1942. The male gametophyte of Halophila ovata

Gaudich. Jour. Mysore Univ. Sect. B. 3: 43-49.
Kosmath, L. 1927. Studien liber das Antherentapetum. Osterr. bot. Ztschr.

76: 235-241.
Levan, A. 1941. Syncyte formation in the pollen mother cells of haploid Phleum

pratense. Hereditas 27: 243-252.
Locke, J. F. 1936. Microsporogenesis and cytokinesis in Asimina triloba. Bot.

Gaz. 98: 159-168.
Maheshwari, P. 1929. Contributions to the morphology of Boerhaavia diffusa.

I. Jour. Indian Bot. Soc. 8: 219-234.
. 1934. Contributions to the morphology of some Indian Liliaceae. I.

The gametophytes of Ophiopogon wallichianus Hook. f. Proc. Indian Acad.

Sci. Sect. B. 1: 197-204.

and Johri, B. M. 1950. The embryology of Hydrilla verticillata (in press).

Mascre, M. 1919a. Sur le role de l'assise nourriciere du pollen. Compt. Rend.

Acad, des Sci. Paris 168: 1120-1122.
. 19196. Nouvelles remarques sur le role de l'assise nourriciere du pollen.

Compt. Rend. Acad, des Sci. Paris 168: 1214-1216.
Matthews, J. R., and Taylor, G. 1926. The structure and development of

the stamen in Erica hirtiUora. Trans, and Proc. Bot. Soc. Edinb. 29:

235-242.
Meyer, K. 1925. tlber die Entwicklung des Pollens bei Leontodon autumnalis

L. Ber. deutsch. bot. Gesell. 43: 108-114.
Mezzetti-Bambacioni, V. 1941. Ricerche morfologiche sulle Lauraceae. Embri-

ologia della Umbellularia calif ornica Nutt. Ann. di Bot. 22: 99-116.
Moissl, E. 1941. Vergleichende embryologische Studien liber die Familie der

Caprifoliaceae. Osterr. bot. Ztschr. 90: 153-212.
Newman, I. V. 1928. The life history of Doryanthes excelsa. I. Some ecological

and vegetative features and spore production. Proc. Linn. Soc. N. S. Wales

53: 499-538.
Nietsch, H. 1941. Zur systematischen Stellung von Cyanastrum. Osterr. bot.

Ztschr. 90: 31-52.
Nohara, S. 1934. Gametogenesis and embryogeny of Sesamum indicum L.

Jour. Col. Agr. Tokyo Imp. Univ. 13: 19-25.
Peters, K. 1903. "Vergleichende Untersuchungen fiber die Ausbildung der

sexuellen Reproduktionsorgane bei Convolvulus und Cmcuta." Diss. Zurich.

52 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Pisek, A. 1924. Antherenentwicklung und meiotische Teilung bei der Wachol-
dermistel (Arceuthobium oxycedri [D.C.] M.B.); Antherenbau und Chromo-
somen zahlen von Loranthus europaeus Jacq. Sitzber. math., nat. Kl. Akad.
der Wiss. Wien 1 133: 1-15.

Puri, V. 1941. Life history of Moringa oleifera Lamk. Jour. Indian Bot. Soc.
20: 263-284.

Rauch, K. V. 1936. Cytologische-embryologische Untersuchungen an Scurrula
atropurpurea Dans, und Dendrophthoe pentandra Miq. Ber. schweiz. bot.
Gesell. 45:5-61.

Reeves, R. G. 1928. Partition wall formation in the pollen mother cells of Zea
mays. Amer. Jour. Bot. 15: 114-122.

Rosenberg, O. 1901. tJber die Embryologie von Zostera marina L. Bihang Till
K. Svenska Vet.-Akad. Handl, HI, 27(6) : 1-24.

Rutishauser, A. 1935. Entwicklungsgeschichtliche und zytologische Untersuch-
ungen an KorthaJsella dacrydii (Ridl.) Danser. Ber. schweiz. bot. Gesell.
44: 389-436.

Samuelsson, G. 1914. tlber die Pollenentwicklung von Anona und Aristolochia
und ihre systematische Bedeutung. Svensk Bot. Tidskr. 8: 181-189.

Schaeppi, H., and Steindl, F. 1942. Bliitenmorphologische und embryologische
Untersuchungen an Loranthoideen. Vrtljschr. naturf. Gesell. Zurich 87:

301-372.

and . 1945. Bliitenmorphologische und embryologische Unter-
suchungen an einigen Viscoideen. Vrtljschr. naturf. Gesell. Zurich 90: 1-46.

Sharp, L. W. 1943. "Fundamentals of Cytology." McGraw-Hill Book
Company.

Singh, Bahadur. 1936. The life history of Ranuncidus sceleratus Linn. Proc.
Indian Acad. Sci. Sect. B. 4: 75-91.

. 1950. The embryology of Dendrophthoe falcata (Linn, fil.) Ettingshausen.

Proc. 37th Indian Sci. Cong. Sect. Bot.

Smith, C. M. 1929. Development of Dionaea muscipula. I. Flower and seed.
Bot. Gaz. 87: 507-530.

Stadtler, G. 1923. tlber Reduktionserscheinungen im Bau der Antherenwand
von Angiospermen. Flora 16: 85-108.

Stenar, H. 1925. "Embryologische Studien. I. Zur Embryologie einiger
Columniferen. II. Die Embryologie der Amaryllidaceen." Diss. Uppsala.

Stern, H. 1946. The formation of polynucleated pollen mother cells. Jour.
Hered. 37: 47-50.

Stevenson, G. B. 1934. The life-history of the New Zealand species of the para-
sitic genus Korthahella. Trans, and Proc. Roy. Soc. New Zeal. 64: 175-190.

Subramanyam, K. 1948. An embryological study of Melastoma malabathricum
L. Jour. Indian Bot. Soc. 27: 1-9.

Swamy, B. G. L. 1946. Embryology of Habenaria. Proc. Natl. Inst. Sci. India

12: 413-426.
. 1948. Embryological studies in Orchidaceae. I. Gametophytes.

Amer. Midland Nat. 41: 184-201.

THE MICROSPORANGIUM 53

Thoday, D., and Johnson, E. T. 1930. On Arceuthobium pusillum Peck. II.

Flowers and fruit. Ann. Bot. 44: 813-824.
Tischler, G. 1915. Die Periplasmodienbildung in den Antheren der Commelina-

ceen und Ausblicke auf das Verhalten der Tapetenzellen bei den ubrigen Mono-

cotylen. Jahrb. f. wiss. Bot. 55: 52-90.
Ubisch, G. V. 1927. Zur Entwicklungsgeschichte der Antheren. Planta 3:

490-495.
Venkatasubban, K. R. 1945. Cytological studies in Bignoniaceae. The cytology

of Dolichandrone rheedii Seem, and allied genera. Proc. Indian Acad. Sci.

Sect. B. 21: 77-92.
Vignoli, L. 1936. Cariologia del genre Agave. I. Lavori, R. 1st. Bot. Palermo 7.

. 1937. Cariologia del genre Agave. II. Lavori f. R. 1st. Bot. Palermo 8.

Warming, E. 1873. Untersuchungen iiber Pollenbildende Phyllome und Kaul-

ome. Hanstein's Bot. Abhandl. 2: 1-90.
Witkus, E. R. 1945. Endomitotic tapetal cell divisions in Spinacia. Amer.

Jour. Bot. 32: 326-330.
Witmer, S. W. 1937. Morphology and cytology of Vallisneria. Amer. Midland

Nat. 18: 309-327.
Wodehouse, R. P. 1936. "Pollen Grains." McGraw-Hill Book Company.
Woodworth, R. H. 1931. Cytomixis. Jour. Arnold Arboretum 12: 23-25.
Wylie, R. B. 1904. The morphology of Elodea canadensis. Bot. Gaz. 37: 1-22.

CHAPTER 3

THE MEGASPORANGIUM

The megasporangium or ovule consists of the nucellus and one or
two integuments. It may have various forms, which sometimes
intergrade into one another, and very often the same ovule changes
its form during the course of its development. Mature ovules are
usually classed under five types. In the orthotropous or atropous
type the micropyle lies directly in line with the hilum and above
it (Fig. 37 A) as in Polygonaceae, Urticaceae, Cistaceae and Pipera-
ceae. In the anatropous type the body of the ovule becomes com-
pletely inverted so that the micropyle and hilum come to lie very
close to each other (Fig. 375). This form is universal in almost
all members of the Sympetalae and is also found in several other
families belonging to both dicotyledons and monocotyledons. When
the ovule is curved, as in some of the Resedaceae and Leguminosae,
it is called campylotropous (Fig. 37C); when the curvature is more
pronounced and also affects the embryo sac, so that the latter be-
comes bent like a horseshoe, as in the Alismaceae, Butomaceae,
and Centrospermales, the ovule is called amphitropous (Fig. 37 E);
and when the nucellus and integuments lie more or less at right
angles to the funiculus as in Ranunculus, Nothoscordum, and Tulbag-
hia, it is called hemianatropous or hemitropous (Fig. 37 D). Ovules
may also be designated as epitropous, apotropous, or pleurotropous,
according as the inversion or bending is directed towards the top,
bottom, or sides of the ovary.

A very peculiar type of ovule is seen in some members of the
Plumbaginaceae (Fig. 38). Here the nucellar protuberance is at
first in the same line as the axis, but the rapid growth on one side
causes it to become anatropous. The curvature does not stop but
continues until the ovule has turned over completely so that the
micropylar end again points upwards. It has been suggested that
this kind of ovule, also seen in Opuntia (Fig. 39), is distinctive enough
to merit a separate name, circinotropous (Archibald, 1939).

THE MEGASPORANGIUM

Integuments. Ordinarily the ovule has either one or two integu-
ments. The number is constant in most families, and only in rare
cases do unitegmic and bitegmic ovules occur in the same family.
In the Sympetalae a single massive integument is almost universal,

ABC D E

Fig. 37. Types of ovules as seen in vertical longitudinal section. A, atropous or
orthotropous. B, anatropous. C, campy] otropous. D, hemianatropous. E, am-
phitropous. (After Prantl.)

the Plumbaginales and Primulales being the only important excep-
tions. In the Archichlamydeae and the monocotyledons most gen-
era have two integuments but a few have only one. There is
evidence that in many cases the single integument has originated

C ~ v D E F

Fig. 38. Development of ovule of Plumbago capensis. (After Haupt, 1934-)

by a fusion of two separate primordia. Transitional types have
been observed in some members of the Ranunculaceae, Rosaceae,
Connaraceae, and Icacinaceae.

The unitegmic condition may also arise by an elimination of one

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

of the two integuments. In Cytinus, a member of the Ramesia-
ceae, the outer integument is arrested in its development. In the
Salicaceae (see Schnarf, 1929) Populus tremula has a single integu-
ment, while P. canadensis and P. candicans also possess a weakly
developed inner integument which is apparently on its way to ex-
tinction. In the Icacinaceae (Fagerlind, 1945c) Gomphandra and
Gonocaryum are unitegmic but Phytocrene shows two primordia.
In some plants there is also a third integument or aril.1 In Ulmus

Fig. 39. Development of ovule of Opuntia aurantiaca. A, front view of young
ovule. B-D, longitudinal sections of progressively older ovules (nu = nucellus;
it = inner integument; oi = outer integument; / = funiculus). (After Archibald,
1939.)

(Shattuck, 1905) it is said to originate by the splitting of the outer
integument, but in most other cases it is a new structure arising
from the base of the ovule. Good examples of this kind are seen in
Asphodelus (Fig. 40 A, B) and Trianthema (Fig. 40C). Of a differ-
ent origin is the "caruncle," found in several members of the Euphor-
biaceae, which arises by a proliferation of the integumentary cells
at the micropylar region (Landes, 1946). Sometimes this prolifera-

1 In Canangium, Mezzettia, and Xylopia Corner (1949) records the presence of
a "middle integument" arising between the outer and inner integuments. In
Canangium and Xylopia, which also have an aril, the middle integument becomes
the fourth integument of the seed.

THE MEGASPORANGIUM

tion becomes more pronounced and takes the form of a backwardly
directed process (Fig. 40D-G) which resembles an aril in later
stages. A very peculiar condition occurs in Opuntia (Archibald,
1939), where the extremely long funiculus completely surrounds the
ovule and looks like a third integument (Fig. 39).

D E F G

Fig. 40. Diagrams of ovules showing origin of aril or third integument (A-C) and
caruncle (D-G). A,B, Asphodelus fistulosus. (After Stenar, 1928.) C, Trian-
thema monogyna. (After Bhargava, 1935.) D, Brachychilum horsfieldii. (After
Mauritzon, 1936.) E, Burbidgea scMzocheila. (After Mauritzon, 1936.) F,G,
Careya arborea. (After Mauritzon, 1939.)

Whatever may be the condition of the integuments in the younger
stages, they often present a very different and a more complicated
aspect in the mature seed. Frequently several layers of cells are

58 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

completely absorbed and do not take any part in the formation of
the seed coat. In the Umbelliferae only two or three of the outer
layers persist at maturity; in the Compositae most of the cells dis-
appear, leaving only a thin layer of crushed and disorganized tissue ;
and in Circaeaster (Junell, 1931), Thesium (Rutishauser, 1937), and
Zea (Randolph, 1936) practically nothing remains of the seed coat.
In Symplocarpus (Rosendahl, 1909) both integuments and endo-
sperm are consumed so that the embryo lies naked inside the ovary
wall. So variable is the nature of the cell layers surrounding the
embryo that only a thorough study of the developmental stages can
reveal their true nature.

Mention must be made of a few records of the occurrence of
chlorophyll in the integuments. Hofmeister (1861) observed this
in Brunsvigia minor and Amaryllis belladonna, and Treub (1879)
in Sobralia micrantha. Later, Berg (1898) and Puri (1941) reported
the presence of chlorophyll in the outer integument and a portion
of the chalaza in Gladiolus communis, Lilium martagon, and Mor-
inga oleifera. Schlimbach (1924) observed the presence of stomata
on the outer integument of Nerine curvifolia, and Flint and More-
land (1943) have described the occurrence of an elaborate chloro-
phyllous tissue with stomata in Hymenocallis occidentalis. Stomata
have also been found on the outer integument of Gossypium, but
they are believed to be concerned with respiration rather than
transpiration or photosynthesis (Seshadri Ayyangar, 1948).u

Micropyle. When two integuments are present, the micropyle
may be formed either by the inner integument as in the Centro-
spermales and Plumbaginales (Fig. 38) or by both inner and outer
integuments as in the Pontederiaceae (Fig. 142). Less frequently,
as in the Podostemonaceae, Rhamnaceae, and Euphorbiaceae, it
may be formed by the outer integument alone (Fig. 67 A). When
both the integuments take part in the formation of the micropyle,
the passage formed by the outer integument (exostome) may not
be in line with that formed by the inner integument (endostome)
so that the micropylar canal has a somewhat zigzag outline. Good
examples of this kind are seen in the Resedaceae (Oksijuk, 1937)
and in some members of the Melastomaceae (Subramanyam, 1948).
In Leitneria (Pfeiffer, 1912) and Malpighia (Subba Rao, 1941) there

laSee Boursnell (1950) on the occurrence of a fungus in the funiculus and outer
integument of Helianthemum chamaecistus.

THE MEGASPORANGIUM 59

is an excessive development of the upper portion of the integuments
so that the micropylar canal lies in folds over the nucellus. Rarely,
as in Ficus (Condit, 1932), Fouquieria (Khan, 1943), and Cyno-
morium (Steindl, 1945), the integumentary cells come in such in-
timate contact with each other that the micropylar canal is ex-
tremely narrow and imperceptible.

Nucellus.16 Depending on the extent of development of the nucel-
lus, ovules are called crassinucellate or tenuinucellate.2 In the first
type, there is a well-developed parietal tissue and the megaspore
mother cell is separated from the nucellar epidermis by one or sev-
eral layers of cells. In the second type, parietal cells are absent
and the megaspore mother cell lies directly below the nucellar
epidermis.3

In the crassinucellate forms the nucellus may enlarge either by an
increase in the number of the parietal cells or by periclinal divisions
of the nucellar epidermis. In some plants like Zizyphus (Kajale,
1944) and Quisqualis (Fagerlind, 1941) (Fig. 41) both these processes
take place simultaneously.

Several members of the Salicaceae, Nyctaginaceae, Euphorbia-
ceae, Polygonaceae, and Cucurbitaceae are characterized by having
a beak-shaped nucellus which reaches out into the micropyle. In
one species, Polygonum persicaria (Soueges, 1919), the beak forms
a very conspicuous structure protruding upward to the base of the
style (Fig. 42).

The tenuinucellate forms are of two kinds: (1) those in which the
nucellus is short and the primordia of the integument or integu-

16 For more detailed information on the nucellus, see Dahlgren (1927).

2 It should be noted that the above distinction between crassinucellate and
tenuinucellate ovules, although convenient and useful, is not always sharp and
clear-cut and there are various intergradations between them. Further, both
types may sometimes occur in one and the same species. To mention only two
examples, in Butomus (Holmgren, 1913) and Ophiopogon (Maheshwari, 1934) in
some ovules the megaspore mother cell is situated directly below the nucellar epi-
dermis while in others it is separated from the latter by a wall cell.

3 Even in those plants in which the ovules are usually tenuinucellate and devoid
of parietal cells, some of the cells of the nucellar epidermis may undergo one or two
periclinal divisions. Svensson (1925) and Dahlgren (1927) have figured this in
Helioptropium and Cobaea. Here the epidermal cells just above the megaspore
tetrad undergo a radial elongation followed by a periclinal division which may
give the false impression of the cutting off of parietal cells.

60 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

ments arise near its apex (Fig. 43£), and (2) those in which the nucel-
lus is elongated and the integuments arise near its base (Fig. 43 A).
The Asclepiadaceae, Orobanchaceae, and Rubiaceae are good ex-
amples of the first condition, and the Orchidaceae of the second.
As the embryo sac matures, the nucellar cells gradually become

Fig. 41. L.s. ovules of Quisqualis indica showing progressively increasing amount
of parietal tissue, arising partly by divisions of the wall layers and partly by divi-
sions of cells of nucellar epidermis. A, B, megaspore mother cell stage. C, func-
tioning megaspore stage. In B and C, note enlarging cells of obturator. (After
Fagerlind, 19/, J.)

THE MEGASPORANGIUM

used up.4 In the tenuinucellate forms this takes place at such an
early stage (even before fertilization) that some workers have mis-
interpreted the integument as the nucellus. Schleiden (1837) wrote
long ago that in the Rubiaceae the ovules are naked. Lloyd (1902)
demonstrated the presence of an integument in all the genera studied
by him excepting Houstonia. Owing to its narrow and incon-

Fig. 42. Formation of nucellar beak in Polygonum persicaria. A, young nucellus,
showing megaspore mother cell and four wall cells; note periclinal division of a cell
of the nucellar epidermis. B, older stage, showing megaspore tetrad, wall cells,
and nucellar beak. C, mature embryo sac with part of nucellar beak; wall cells
have degenerated and disappeared. (After Soueges, 1919.)

spicuous micropyle, Schleiden mistook the integument for the nucel-
lus, while the latter escaped his notice altogether. More recently,
Fagerlind (1937) has shown that even in Houstonia an integument
is present as usual and it is really the nucellus which is on its way
to extinction. He presents a series of stages to show how this con-
dition has been derived (Fig. 44). In Phyllis, which is at the begin-

4 It is only in a few families like the Piperaceae and Scitamineae that the nucellus
persists in the seed; it is then known as the perisperm.

62 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

ning of the series, the nucellus comprises a single layer of cells (the
epidermis) surrounding the archesporium (Fig. 4:4 A). This is in
accordance with the general condition in the Sympetalae. In Bou-
vardia and Vaillantia, which represent the next stage, the nucellar
epidermis is reduced to a few cells lying immediately above the
sporogenous tissue (Fig. 44B,C). In Rubia olivieri there is further

A B

Fig. 43. Young ovules of Orchis maculatus (A) and Aeginetia indica (B). Note
that in Orchis the integuments arise near base of megaspore mother cell, while in
Aeginetia the single integument arises near apical end of nucellus. (A, after Hage-
rup, 1944,' B, afar Juliano, 1935.)

A B C D E F

Fig. 44. Diagram illustrating different types of nucelli found in the Rubiaceae.
A, Phyllis. B, Bouvardia. C, Vaillantia. D, Rubia. E, Oldenlandia. F, Hous-
tonia. {After Fagerlind, 1937.)

THE MEGASPORANGIUM 63

reduction in their number, although this is accompanied by a pro-
nounced radial elongation of the walls (Fig. 44Z)). In Oldenlandia
the micella r epidermis is represented by one or two cells only (Fig.
4AE), and in Houstonia, which is the last member of the series, there
is no distinguishable epidermis and the ovule consists of only the
sporogenous cells and the integument (Fig. 44i^).

Fagerlind's series is so clear and convincing that there is no longer
any doubt about the true relationships of the nucellus and integu-
ment in the Kubiaceae. Houk's (1938) statement that in Coffea
there is no distinction between the tissues of the integument and
nucellus is therefore incorrect (see also Mendes, 1941).

Woodcock's (1943) report that in Ipotnoea the ovule has no dis-
tinct integument and the micropyle is formed by an "invagination"
is also due to a misinterpretation. As in other members of the
Convolvulaceae (see Maheshwari, 1944), an integument is present
and it is the nucellus which soon disappears. The micropyle is
not an invagination but a continuous passage, which begins to be
more or less occluded in postfertilization stages and is therefore
difficult to demonstrate in nonmedian sections.

Formerly the Olacaceae were also believed to have naked ovules.
A recent study by Fagerlind (1947) has shown that an integument
is present as usual but the nucellus is extremely reduced and ephem-
eral and is represented by only a few epidermal cells lying just
above the megaspore mother cell.

A complete absence of the integuments is known only in some
members of the Loranthaceae and Balanophoraceae, but it seems
probable that this is a derived condition. Fagerlind (1945c?) has
given a series of illustrations showing the stages by which this may
have been brought about (Fig. 45). The case of Crinum (Amarylli-
daceae), in which the nucellus is ephemeral and the integuments are
said to be absent (Tomita, 1931), deserves further study.

Integumentary Tapetum. In those plants in which the nucellus
is soon disorganized, the embryo sac comes in direct contact with
the inner layer of the seed coat. The cells of this layer frequently
become specially differentiated from the rest by their form and con-
tents (Fig. 46). They show a pronounced radial elongation and
sometimes become binucleate. Owing to these similarities with the
cells of the anther tapetum, this layer of cells is known as the integu-
mentary tapetum or endothelium.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

There seems to be no doubt that the endothelium is a nutritive
layer whose chief function is to serve as an intermediary for the
transport of food materials from the integument to the embryo sac.

E F G H I

Fig. 45. Diagram illustrating derivation of the female flower and ovule of Bala-
nophora. A, l.s. hypothetical ovary showing two ovules. B, ovary of Thesium.
C, ovary, as in Osyris, Santalum and Myzodendron. D, as in Arceuthobium and
Helosis. E, as in Korthalsella. F, as in Viscum and Dendrophthoe. G,H, as in
Scurrula. I, as in Balanophora. {After Fagerlind, 1945d.)

Some writers also claim that it contains diastase and other enzymes
which convert the food into a suitable form for the use of the embryo
sac. In later stages, when the embryo is approaching maturity,

THE MEGASPORANGIUM

the inner surface of the endothelium becomes cutinized and this

layer seems to take up a protective instead of a nutritive function.

Hypostase.6 Just at the level of origin of the two integuments

and directly below the embryo sac, there is often a well-defined but

Fig. 46. Stages in the formation of integumentary tapetum in Lobelia trigona.
A, two-nucleate embryo sac with remains of degenerating megaspores; nucellar
epidermis still intact. B, Four-nucleate embryo sac, showing degeneration of
nucellar epidermis and formation of integumentary tapetum from inner layer of
integument. C, mature embryo sac bounded by cells of integumentary tapetum.
(After Kausik, 1935.)

irregularly outlined group of nucellar cells which are usually poor
in cytoplasmic contents but have partially lignified or suberized
walls composed of a highly refractive material. Van Tieghem (1901),

6 Dahlgren (1940) has reviewed the literature on the occurrence of the hypostase
in angiosperms and also recommended some changes of terminology. Reference
should be made to this paper for fuller information on the subject.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

who first called attention to this
patch of cells, gave it the name
hypostase, and believed that
it formed a sort of barrier or
boundary for the growing em-
bryo sac and prevented it from
pushing into the base of the
ovule. Goebel (1933) says,
however, that the peculiar po-
sition of this tissue — directly
above the termination of the
vascular supply of the ovule —
is indicative of its relation to
the water economy of the em-
bryo sac. While the function
of the hypostase is still in
doubt, morphologically it is a
very characteristic feature of
certain families and genera.
Zostera (Dahlgren, 1939) offers
an especially good instance of
a well-developed hypostase
(Fig. 47). The hypostase may
not always consist of thick-
walled cells. In Knautia (La-
vialle, 1925) it comprises a
group of small thin-walled cells
having a number of schizoge-
nous cavities which branch
and anastomose and become
filled with a yellowish sub-
stance, which also spreads into
the antipodal cells and other
adjacent tissue. In Dionaea
(Smith, 1929) some of the thin-
walled cells in the chalaza be-
come disorganized and replaced
Jig. 47. Zosteia marina, l.s. young seed, °

showing prominent hypostase and well-de- by airspaces . In A Ilium odorum
veloped embryo. (After Dahlgren, 1939.) (Haberlandt, 1923) the cells

THE MEGASPORANGIUM 67

become richly protoplasmic and the hypostase has an appearance
similar to that of the epithem of many hydathodes. Haberlandt
considers it to be a sort of glandular tissue secreting some hor-
mone or enzyme required for the growth of the embryo sac.

Epistase. Van Tieghem also reported the occasional presence
of a similar well-marked tissue in the micropylar part of the ovule
and called it the epistase. Usually it originates from the apical
cells of the nucellar epidermis, which show a marked radial elonga-
tion and become somewhat thickened or suberized. Occasionally
the cells undergo one or more periclinal divisions to form the so-
called nucellar cap, which persists as a hood over the apex of the
embryo sac even after the cells at the sides have disorganized and
disappeared.6 In Castalia (Cook, 1906) the epidermal cells lying
at the apex of the embryo sac show "a very pronounced sclerifica-
tion," and in Costus (Boehm, 1931) the inner tangential walls of
these cells become conspicuously thickened. In Nicolaia (Boehm,
1931) the walls surrounding the megaspore tetrad become cutinized
and form a firm covering, which becomes ruptured and separated
into two parts only with the continued enlargement of the embryo
sac. The thickenings at the micropylar end disappear but are seen
once again at the time of organization of the mature embryo sac.

In some plants the apical cells of the integuments give rise to a
proliferation usually called the "operculum." To mention a few
examples, in Lemna (Caldwell, 1899) the cells forming the micro-
pylar portion of the two integuments enlarge and divide to form a
compact tissue lying just above the nucellus (Fig. 48). In Dionaea
(Smith, 1929) a similar tissue is formed by the cells of the inner
integument. In Acorus (Buell, 1935) the cells become elongated
and coiled around one another, so as to form a plug in the lower
part of the micropyle.

Vascular Supply of Ovule. As a rule the vascular bundle entering
the ovule terminates at the chalaza but in some plants it gives out
branches, a few of which enter the integument. If two integuments
are present, the branches may enter only the outer integument or
both the outer and the inner integuments. Since integumentary
vascular bundles are common in gymnosperms, their presence is
usually considered to be a primitive feature and the loss of the con-

6 Dahlgren (1940) designates a persistent nucellar cap of this kind by the name
"petasus."

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

ducting tissue to be an advanced one. There is, however, no defi-
nite evidence in favor of this view. Integumentary vascular bun-
dles are now known to occur in a number of families, both primitive
(Betulaceae, Euphorbiaceae, Ranunculaceae, and Berberidaceae)
and specialized (Moringaceae, Leguminosae, Punicaceae, Rhamna-
ceae, Convolvulaceae, Cuscutaceae, Boraginaceae, Caprifoliaceae,
Compositae, and Cyanastraceae). In Zizyphus (Kajale, 1944) the
vascular strands extend far up into the tip of the outer integument.
In the large succulent seeds of Hymenocallis occidentalis (Whitehead

emb

ABC

Fig. 48. Development of "operculum" in Lemna minor. A, ovule showing two
integuments, ox and xi, nucellar cap nc, and young embryo sac. B, upper part of
ovule showing portion of embryo emb, nucellar cap, and two integuments; note
enlargement of cells of outer integument. C, embryo, nucellus, and thickened tips
of integuments which form the so-called "operculum." (After Caldwell, 1899.)

and Brown, 1940) four bundles enter the ovule and during their
upward course they freely branch and anastomose so that a cross
section of a large seed shows from 14 to 18 bundles in the seed coat.
The inner integument of Croton (Landes, 1946) shows a network of
tracheids which remain conspicuous even after the other cells of the
integument have become flattened and crushed.

The occurrence of vascular elements in the nucellus is much rarer.
Benson (1894), Frye (1902), and Benson, Sanday, and Berridge
(1906) identified some nucellar tracheids in Castanea, Asclepias,
and Carpinus respectively, but they showed no connection with the
vascular bundle of the funiculus. Guerin (1915) described the oc-
currence of connecting nucellar tracheids in some genera of the
Thymelaeaceae,7 and Orr (1921a, b) reported the same in a few mem-
bers of the Capparidaceae and Resedaceae.

7 According to Mauritzon (1939) all statements of the occurrence of xylem ele-

THE MEGASPORANGIUM 69

Among recent records, in Agave (Grove, 1941) and Strombosia
(Fagerlind, 1947), the vascular strand of the ovule is said to pene-
trate into the nucellus up to the base of the embryo sac, and in
Magnolia (Earle, 1938) it gives out short branches in the chalaza,
one of which is directed towards the embryo sac. In Acalypha
(Landes, 1946) the main bundle of the ovule proceeds up to the hy-
postase and forms a number of short branches whose ultimate rami-
fications extend into the nucellus up to a distance about one-fifth
of the length of the ovule. More striking still is the condition
recently reported in Casuarina (Swamy, 1948), where the funicular
strand extends up to the base of the sporogenous tissue, some of
whose cells elongate and themselves assume a conducting function
instead of giving rise to embryo sacs (Fig. 51).

The occurrence of vascular elements in the nucellus is of con-
siderable theoretical importance, as such a condition has been consid-
ered by some authors to be a relic of the highly developed "trachei-
dal envelope" found in some fossil gymnosperms. A few years ago
integumentary vascular bundles were considered to be very un-
common in angiosperms, but now they are known to occur in several
families. Possibly the occurrence of xylem elements in the nucellus
may also be found to be more frequent than the few reports just
mentioned may seem to indicate.

Archesporium. The archesporial tissue is of hypodermal origin.
In general, one cell of the nucellus, situated directly below the epi-
dermis, becomes more conspicuous than the others owing to its larger
size, denser cytoplasm, and more prominent nucleus. This is the
primary archesporial cell. Frequently the cells situated below it
lie in a row so that the archesporial cell appears as the terminal
member of a series of nucellar cells (Fig. 43 A).

The archesporial cell may divide to form a primary parietal
cell and a primary sporogenous cell (Fig. 49A-B), or it may func-
tion directly as the megaspore mother cell (Fig. 50H). The primary
parietal cell may remain undivided or it may undergo periclinal and
anticlinal divisions to form a variable number of wall layers. The

ments in the nucellus or inner integument of the Thymelaeaceae are due to mis-
interpretations. In his opinion these tracheids really belong to the chalazal tissue
which, by "vigorous growth," extends around the endosperm and thus forms a
part of the seed coat. Fuchs (1938) and Kausik (1940) also failed to observe any
nucellar tracheids in the species studied by them.

H G

/ig. 49. Hydrilla verticil lain, formation of megaspores. .4, hypodermal arche-
sporial cell. B, cutting off of primary parietal cell. C, anticlinal division of pri-
mary parietal cell. D, ovule at megaspore mother cell stage. E, megaspore
mother cell in prophase of Meiosis I ; two other cells of the nucellus lying below it
simulate sporogenous cells. F,G, first division of megaspore mother cell resulting
in formation of dyad cells. H, tetrad of megaspores.

THE MEGASPORANGIUM 71

primary sporogenous cell usually functions as the megaspore mother
cell without undergoing any further divisions.

The outline presented above is subject to many variations. In
some plants the archesporial cell is said to originate from the third
layer of cells in the nucellus, but this is probably a misinterpretation
caused by the difficulty in distinguishing the archesporial cell at
an earlier stage of development. Sometimes, as in the Onagraceae
(Khan, 1942), the archesporium may comprise a small group of
half a dozen cells or more (Fig. 5022). Of these usually the central
cell alone is functional, but frequently one or two of the other cells
also reach the megaspore mother cell stage. In the Malvaceae
(Stenar, 1925) the primary sporogenous cell divides to form a few
accessory cells in addition to the functional megaspore mother cell.
In some members of the Rubiaceae and Compositae there are sev-
eral sporogenous cells, all of which may go through the meiotic
divisions (Fig. 50B-C). In Scurrula (Rauch, 1936) and Dendroph-
thoe (Singh, 1950), which have a very massive archesporium, the
sporogenous cells undergo further division to give rise to a still
larger number of cells. These begin to elongate very actively and
become so closely interlocked that the whole tissue gives an appear-
ance suggestive of the hy menial layer of an ascomycete.

In Hydrilla there are sometimes two or three archesporial cells
in a single row (Fig. 50A). In Ruppia (Murbeck, 1902), Butomus
(Holmgren, 1913), and Urginea (Capoor, 1937) the primary parie-
tal cell may also assume a sporogenous function so that two mega-
spore tetrads are formed in the same row (Fig. 52D). In Oncidium
praetextum (Afzelius, 1916) a cell of the nucellar epidermis may func-
tion as a megaspore mother cell (Fig. 50F), and in Solanum (Bha-
duri, 1932) and Limnanthes (Fagerlind, 1939) some of the integu-
mentary cells may behave similarly (Fig. 50D,G).

In the Sympetalae a parietal cell is absent, the only important
exceptions being the Plumbaginales and some members of the fam-
ily Convolvulaceae. Since this is also the condition in some other
advanced families like the Umbelliferae and Orchidaceae, the pres-
ence of a massive parietal tissue is regarded as a primitive feature
and its absence as advanced. An objection to this view is that
parietal cells are often absent even in some admittedly primitive
families like the Ranunculaceae (Hafliger, 1943).

In the Casuarinaceae (Fig. 51), and some other families (see

72 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fig. 50. Variations in origin and extent of sporogenous tissue in ovule. A,
Hydrilla verticillata, young ovule, showing a row of three sporogenous cells. B,
Achillea millefolium, nucellus, showing a number of megaspore mother cells in

THE MEGASPORANGIUM 73

Schnarf, 1929) there is a multicellular archesporium and also an
extensive parietal tissue. In some members of the Rosaceae the
cells of the nucellar epidermis also divide periclinally and thus add
to the wall tissue (see also page 60).

Megasporogenesis. The megaspore mother cell undergoes the
usual meiotic divisions to form a tetrad of four cells. The first
division is always transverse and gives rise to two dyad cells (Fig.
4QF-G). Typically the second division is also transverse and re-
sults in a linear tetrad of four megaspores (Fig. 52 A). Frequently
the micropylar dyad cell divides in a plane at right angles to that
of the chalazal dyad cell. This results in a T-shaped tetrad in
which the two outer megaspores lie in contact with the third mega-
spore which separates them both from the chalazal megaspore (Fig.
52B). Since both linear and T-shaped tetrads may occur in ovules
of one and the same ovary, it is unnecessary to give specific ex-
amples. Tetrads of an intermediate type (Fig. 4QH), in which the
wall separating the two micropylar megaspores lies at an angle of
approximately 45° with respect to the chalazal megaspores, are also
not infrequent.

Rarely, the two upper megaspores of a tetrad lie in a line parallel
to the long axis of the ovule but the lower two lie at right angles
to it. Such 1-shaped tetrads are sometimes found in the Onagra-
ceae and have been reported in Zauschneria (Johansen, 1931a),
Anogra (Johansen, 19316), and Ludwigia (Maheshwari and Gupta,
1934). 8 Among other examples of a similar kind may be cited
Drimiopsis (Baranow, 1926), Tacca (Paetow, 1931), Styrax (Cope-

8 The occurrence of 1-shaped tetrads in the Onagraceae is probably related to
the fact that here the micropylar megaspore gives rise to the embryo sac, while
the three chalazal megaspores are nonfunctional (see Chap. 4).

prophase. (After Dahlgren, 1927.) C, Chrysanthemum corymbosum, nucellus, show-
ing megaspore mother cells each with four megaspore nuclei; one cell at the bottom
has lagged behind. (After Dahlgren, 1927.) D, Solanum melongena, ovule show-
ing hypodermal megaspore mother cell and two other such cells in the tissues of
the integument. (After Bhaduri, 1932.) E, Jussieua repens, nucellus, showing
multicellular archesporium. (After Khan, 1942.) F, Oncidium praetextum, ovule,
showing supernumerary archesporial cell arising from the nucellar epidermis.
(After Afzelius, 1916.) G, Limnanthes douglasii, normal archesporial cells in
nucellus and supernumerary archesporial cell in integument. (After Fagerlind,
1939.) H, Machaerocarpus californicus, megaspore mother cell in prophase. (Af-
ter Maheshwari and Singh, 1943.)

74 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

land, 1938), Cyathula (Kajale, 1940), Costus (Banerji, 1940), and
Desmodium (Pantulu, 1941).

An isobilateral or a tetrahedral arrangement of megaspores is
very rare and has been reported only as an abnormality (Fig. 52C).

Fig. 51. Part of ovule of Casuarina montana, showing multiple archesporium.
Some of the sporogenous cells are in prophase; others have gone through meiotic
divisions to form megaspore tetrads; and a few have formed two- and four-nucleate
embryo sacs. Note that in tetrad at upper end, all megaspores are binucleate.
{After Swamy, 1948.)

THE MEGASPORANGIUM

The genus Musa is of special interest, for here tetrads of four dif-
ferent kinds — linear, T-shaped, 1-shaped and isobilateral — may oc-
cur in the same species (Dodds, 1945). Tetrads of very variable
appearances have also been described in Poa alpina (Hakansson,
1943).

Frequently a row of only three cells is seen in place of the usual
four. This is due to an omission of the second meiotic division in
one of the two dyad cells, usually the upper.9 All intermediate
stages leading towards this condition have been seen. In some
plants the division in the upper dyad cell merely lags behind that

Fig. 52. Megaspore tetrads in Urginea indica. A, linear tetrad. 5,T-shaped
tetrad. C, tetrad showing decussate arrangement of megaspores. D, two tetrads
lying in same row. (After Capoor, 1937.)

in the lower dyad cell, and all four cells are formed as usual; in
others the nucleus divides normally, but a separating wall is not
laid down; in still others the division is abortive and merely gives
rise to two degenerating clumps of chromosomes; and in a few
cases the nucleus degenerates without undergoing any division.

Functioning Megaspore. Normally it is the chalazal megaspore
of the tetrad which functions and gives rise to the embryo sac, while
the remaining three megaspores degenerate and disappear. But

9 It is to be noted that sometimes one gets a false impression of the occurrence
of a row of three cells either because of the plane of the section or because of the
orientation of the wall between the two upper megaspores so that one of the cells
lies superposed over the other. In such cases more careful focusing, or a study of the
adjacent section, reveals the presence of the fourth megaspore (see Graves, 1908).

K L

Fig. 53. Formation of megaspore tetrads in various angiosperms. A, Balano-
phora elongata, megaspore mother cell. B, dyad stage; lower dyad cell is much
smaller than upper. C,D, both dyad cells dividing. E,F, tetrad stage; note that
uppermost megaspore functions, while the other three degenerate and disappear.
(After Fagerlind, 1945b.) G,H, Gloriosa virescens, megaspore tetrads in which
every cell is binucleate. /, one of the megaspores has developed to four-nucleate
stage and another to two-nucleate. (After Afzelius, 1916.) J, Aristotelia racemosa,
two tetrads with third megaspore functioning. (After Mauritzon, 1934.) K, Rosa,
two tetrads with micropylar megaspores functioning. L, same, megaspore tetrad
with both micropylar and submicropylar megaspores enlarging. (After Hurst,
1931.) M, Senecio abrotanifolius, megaspore tetrad, showing two middle mega-
spores lying side by side. (After Afzelius, 1924) N, Culcitium reflexum, mega-
spore tetrad, showing micropylar as well as chalazal megaspore enlarging. (After
Afzelius, 1924)

THE MEGASPORANGIUM

in Elytranthe (Schaeppi and Steindl, 1942), Langsdorffia (Fagerlind,
1945a), and Balanophora (Fagerlind, 19456) (Fig. 53 A-F) the micro-
pylar megaspore gives rise to the embryo sac and the other three
soon degenerate. A similar condition occurs in the Onagraceae
and in a few members of the Compositae, although here it is not
unusual to find both the terminal megaspores, micropylar and chala-
zal, growing concurrently (Fig. 53ilf,iV). In Rosa (Hurst, 1931)

Fig. 54. Formation of megaspore haustoria in Galium lucidum (A-C), Sedum
sempervivoides (D), and Rosularia pallida (E). (A-C after Fagerlind, 1987;
D-E after Mauritzon, 1933.)

it is usually the micropylar megaspore which functions (Fig. 532£)
but sometimes it is the second (Fig. 53L). In Aristotelia (Maurit-
zon, 1934), belonging to the Elaeocarpaceae, it is the third mega-
spore from the micropylar end which gives rise to the embryo sac
(Fig. 53 J"). Rarely, as in Gloriosa (Afzelius, 1918; Eunus, 1949)
(Fig. 53G-I), Ostrya (Finn, 1936), Poa (Hakansson, 1943), and
Casuarina (Swamy, 1948) (Fig. 51) all or any of the four megaspores
may begin to enlarge and divide. A peculiar condition occurs in
Rosularia, Sedum (Mauritzon, 1933), Laurus (Bambacioni-Mezzetti,

78 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

1935), Potentilla (Rutishauser, 1945), and some members of the Ru-
biaceae (Fagerlind, 1937), in which the megaspores give out lateral
tubes which subsequently begin to grow upward and are in a state
of competition with one another (Fig. 54). In Putoria and Galium
the tubes enter the tissues of the integument. In Rosularia two
to three megaspore tetrads may be formed, and as every megaspore
can give rise to a haustorium, the upper part of the nucellus often
shows quite a tangle of haustorial processes competing with one
another.

Failure of Wall Formation during Meiosis. The functioning of
only one megaspore out of four is the commonest condition in
angiosperms, but in several plants only the first of the two meiotic
divisions is accompanied by wall formation, so that after the meiotic
divisions are over each of the dyad cells is binucleate. In others
wall formation fails altogether, or if walls are laid down they soon
disappear, so that all the four megaspore nuclei lie within the same
cell. Such differences in the mode of origin of the megaspores
form the basis for a classification of the types of embryo sacs in
angiosperms.

References

Afzelius, K. 1916. Zur Embryosackentwicklung der Orchideen. Svensk Bot.

Tidskr. 10: 183-227.
. 1918. Zur Entwicklungsgeschichte der Gattung Gloriosa. Acta Horti

Bergiani 6(3): 3-12.
. 1924. Embryologische und zytologische Studien in Senecio und ver-

wandten Gattungen. Acta Horti Bergiani 8: 123-219.
Archibald, E. E. A. 1939. The development of the ovule and seed of jointed

cactus (Opuntia aurantiaca Lindley). South African Jour. Sci. 36: 195-211.
Bambacioni-Mezzetti, V. 1935. Ricerche morfologiche sulle Lauraceae. Lo

sviluppo dell'ovulo e dei sacchi pollinici nel Laurus nobilis L. Ann. di Bot.

21(1): 1-19.
Banerji, I. 1940. A contribution to the life history of Costm speciosus Smith.

Jour. Indian Bot. Soc. 19: 181-196.
Baranow, P. 1926. Zytologische und embryologische Untersuchungen an Drimi-

opsis maculata Lindl. Ztschr. f. Zellforsch. und Mikros. Anat. 3: 131-148.
Benson, M. 1894. Contribution to the embryology of the Amentiferae. I.

Trans. Linn. Soc. London, II. Bot. 3 : 409-424.
, Sanday, E., and Berridge, E. 1906. Contribution to the embryology of

the Amentiferae. II. Carpinus betulus. Trans. Linn. Soc. London, II. Bot.

7: 37-44.
Berg, 0. 1898. "Beitrag zur Kenntnis der Entwicklung des Embryosackes der

Angiospermen." Diss. Erlangen.

THE MEGASPORANGIUM 79

Bhargava, H. R. 1935. The life history of Trianthema monogyna L. Proc.

Indian Acad. Sci. Sect. B. 2: 49-58.
Boehm, K. 1931. Embryologische Untersuchungen an Zingiberaceen. Planta

14: 411-440.
Bhaduri, P. N. 1932. The development of ovule and embryo sac in Solanum

melongena L. Jour. Indian Bot. Soc. 11 : 202-224.
Boursnell, J. G. 1950. The symbiotic seed-borne fungus in the Cistaceae. Ann.

Bot. 14: 217-243.
Buell, M. F. 1935. Seed and seedling of Acorus calamus. Bot. Gaz. 96: 758-765.
Caldwell, 0. W. 1899. On the life history of Lemna minor. Bot. Gaz. 27:

37-66.
Capoor, S. P. 1937. Contributions to the morphology of some Indian Liliaceae.

II. The gametophytes of Urginea indica Kunth. Beihefte bot. Centbl.

56A: 156-170.
Condit, I. J. 1932. The structure and development of flowers in Ficus carica

L. Hilgardia 6: 443-481.
Cook, M. T. 1906. The embryology of some Cuban Nymphaeaceae. Bot. Gaz.

42: 376-392.
Copeland, H. F. 1938. The Styrax of Northern California and the relationships

of the Styracaceae. Amer. Jour. Bot. 25: 771-780.
Corner, E. J. H. 1949. The annonaceous seed and its four integuments. New

Phytol. 48: 332-364.
Dahlgren, K. V. O. 1927. Die Morphologie des Nuzellus mit besonderer Beruck-

sichtigung der deckzellosen Typen. Jahrb. f. wiss. Bot. 67: 347-426.
. 1939. Endosperm und Embryobildung bei Zostera marina. Bot. Notiser

1939, pp. 607-615.
— . 1940. Postamentbildungen in den Embryosacken der Angiospermen.

Bot. Notiser 1940, pp. 347-369.
Dodds, K. S. 1945. Genetical and cytological studies of Musa. VI. The

development of female cells of certain edible diploids. Jour. Genet. 46: 161—

179.
Earle, T. T. 1938. Embryology of certain Ranales. Bot. Gaz. 100: 257-275.
Eunus, A. M. 1949. Contributions to the embryology of the Liliaceae (Gloriosa

superba). Proc. 36th Indian Sci. Cong. Allahabad, Bot. Sect. No. 25.
Fagerlind, F. 1937. Embryologische, zytologische und Bestaubungs-experimen-

telle Studien in der Familie Rubiaceae nebst Bemerkungen liber einige Pol-

ploiditats-probleme. Acta Horti Bergiani 11: 195-470.
. 1939. Kritische und revidierende Untersuchungen iiber das Vorkommen

des Adoxa ("Lilium")-Typus. Acta Horti Bergiani 13: 1-49.
. 1941. Der Bau der Samenanlage und des Makrogametophyten bei

Quisqualis indica. Bot. Notiser 1941, pp. 217-222.
■ . 1945a. Bau der floralen Organe bei der Gattung Langsdorffia. Svensk

Bot. Tidskr. 39 : 197-210.
19456. Bildung und Entwicklung des Embryosacks bei sexuellen und

agamospermischen Balanophora-Arten. Svensk Bot. Tidskr. 39: 65-82.

80 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fagerlind, F. 1945c. Bau des Gynoceums, der Samenanlage und de? Embryo-
sackes bei einigen Reprasentaten der Familie Icacinaceae. Svensk Bot.
Tidskr. 39:346-364.

. 1945<i. Bliite und Bliitenstand der Gattung Balanophora. Bot. Notiser

1945, pp. 330-350.

1947. Gynoceummorphologische und embryologische Studien in der

Familie Olacaceae. Bot. Notiser 1947, pp. 207-230.
Finn, W. W. 1936. Zur Entwicklungsgeschichte der Chalazogamen, Ostrya

carpinifolia Scop. Jour. Inst. Bot. Acad. Sci. Ukraine 8: 15-25.
Flint, L. H., and Moreland, C. G. 1943. Notes on photosynthetic activity in

seeds of the spider lily. Amer. Jour. Bot. 30: 315-317.
Frye, T. C. 1902. A morphological study of certain Ascelpiadaceae. Bot.

Gaz. 34: 389-413.
Fuchs, A. 1938. Beitrage zur Embryologie der Thymelaeaceae. Osterr. bot.

Ztschr. 87: 1-41.
Goebel, K. 1933. "Organographie der Pflanzen. III. Samenpflanzen." 3d ed.

Jena.
Graves, A. H. 1908. The morphology of Ruppia maritima. Trans. Conn. Acad.

Arts and Sci. 14:59-170.
Grove, A. R. 1941. Morphological study of Agave lechuguilla. Bot. Gaz. 103:

354-365.
Gue>in, P. 1915. Reliquiae Treubinae. I. Recherches sur la structure ana-

tomique de l'ovule et de la graine des Thymeleacees. Ann. Jard. Bot. Buiten-

zorg 14: 1-35.
Haberlandt, G. 1923. Zur Embryologie von Allium odorum. Ber. deutsch. bot.

Gesell. 41: 174-179.
Hafliger, E. 1943. Zytologisch-embryologische Untersuchungen pseudogamer

Ranunkeln der Auricomus-Gruppe. Ber. schweiz. bot. Gesell. 53: 317-382.
Hagerup, O. 1944. On fertilisation, polyploidy and haploidy in Orchis maculatus

L. sens. lat. Dansk Bot. Arkiv 11(5): 1-26.
Hakansson, A. 1943. Die Entwicklung des Embryosackes und die Befruchtung

bei Poa alpina. Hereditas 28: 25-61.
Haupt. A. W. 1934. Ovule and embryo sac of Plumbago capensis. Bot. Gaz.

95: 649-659.
Hofmeister, W. 1861. Neue Beitrage zur Kenntnis der Embryobildung der

Phanerogamen. II. Monokotyledonen. Abh. Konigl. Sachs. Gesell. Wiss.

7: 629-760.
Holmgren, J. 1913. Zur Entwicklungsgeschichte von Butomus umbellatus L.

Svensk Bot. Tidskr. 7: 58-77.
Houk, W. G. 1938. Endosperm and perisperm of coffee with notes on the mor-
phology of the ovule and seed development. Amer. Jour. Bot. 25 : 56-61.
Hurst, C. C. 1931. Embryo sac formation in diploid and polyploid species of

Roseae. Proc. Roy. Soc. London Ser. B. 109: 126-148.
Johansen, D. A. 1931a. Studies on the morphology of the Onagraceae. V.

Zauschneria latifolia, typical of a genus characterized by irregular embryology.

Ann. N. Y. Acad. Sci. 33: 1-26.
. 19316. Studies on the morphology of the Onagraceae. VI. Anogra

vallida. Amer. Jour. Bot. 18: 854-863.

THE MEGASPORANGIUM 81

Juliano, J. B. 1935. Anatomy and morphology of the Bunga, Aeginetia indica

L. Philippine Jour. Sci. 56: 405-451.
Junell, S. 1931. Die Entwicklungsgeschichte von Circaeaster agrestis. Svensk

Bot. Tidskr. 25: 238-270.
Kajale, L. B. 1940. A contribution to the embryology of the Amarantaceae

Proc. Nat. Inst. Sci. India 6: 597-625.
. 1944. A contribution to the life history of Zizyphus jujuba Lamk. Proc

Nat. Inst. Sci. India 10: 387-391.
Kausik, S. B. 1935. The life history of Lobelia trigona Roxb. with special refer-
ence to the nutrition of the embryo sac. Proc. Indian Acad. Sci. Sect. B.

2: 410-418.
. 1940. Structure and development of the ovule and embryo sac of

Lasiosiphon eriocephalus Dene. Proc. Natl. Inst. Sci. India 6: 117-132.
Khan, R. 1942. A contribution to the embryology of Jussieua repens L. Jour.

Indian Bot. Soc. 21: 267-282.
. 1943. The ovule and embryo sac of Fouquieria. Proc. Natl. Inst. Sci.

India 9: 253-256.
Landes, M. 1946. Seed development in Acalypha rhomboidea and some other

Euphorbiaceae. Amer. Jour. Bot. 33: 562-568.
Lavialle, P. 1925. Sur la nutrition du sac embryonnaire chez Knautia arvensis.

Compt. Rend. Acad, des Sci. Paris 180: 2055-2056.
Lloyd, F. E. 1902. The comparative morphology of the Rubiaceae. Mem.

TorreyBot. Club 8: 1-112.
Maheshwari, P. 1934. Contributions to the morphology of some Indian Liliaceae.

I. The gametophytes of Ophiopogon wallichianus Hook f. Proc. Indian

Acad. Sci. Sect. B. 1: 197-204.
. 1944. The seed structure of Ipomoea, a criticism. Sci. and Culture 9 :

557.
and Gupta, B. L. 1934. The development of the female gametophyte

of Ludivigia parviflora Roxb. and Jussieua repens Linn. Current Sci. [India]

3: 107-108.

and Singh, Balwant. 1943. Studies in the family Alismaceae. V. The

embryology of Machaerocarpus californicus (Torr.) Small. Proc. Natl. Inst.

Sci. India 9: 311-322.
Mauritzon, J. 1933. "Studien iiber die Embryologie der Familien Crassulaceae

und Saxifragaceae." Diss. Lund.
. 1934. Zur Embryologie der Elaeocarpaceae. Arkiv for Bot. 26A:

1-8.
. 1936. Samenbau und Embroyologie einiger Scitameen. Lunds Univ.

Arsskr. N. F. Avd. II, 31(9): 1-31. '

1939. Contributions to the embryology of the orders Rosales and

Myrtales. Lunds Univ. Arsskr. N.F. Avd. II, 35(2): 1-120.
Mendes, A. J. T. 1941. Cytological observations in Coffea. VI. Embryo and

endosperm development in Coffea arabica L. Amer. Jour. Bot. 28: 784-789.
Murbeck, S. 1902. tJber die Embryologie von Ruppia rostellata. K. Svenska

Vet.-Akad. Handl. 36(5): 1-21.
Okeijuk, P. 1937. Vergleichende zytologisch-embryologische Untersuchung der

82 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Familie Resedaceae. I. Reseda und Astrocarpus. Jour. Inst. Bot. Acad.

Sci. Ukraine 12: 3-81.
Orr, M. Y. 1921a. The occurrence of tracheids in the nucellus of Steriphoma

cleomides Spreng. Notes Roy. Bot. Gard. Edinb. 12: 241-242.
. 19216. The occurrence of a tracheal tissue enveloping the embryo in

certain Capparidaceae. Notes Roy. Bot. Gard. Edinb. 12: 249-257.
Paetow, W. 1931. Embryologische Untersuchungen an Taccaceen, Meliaceen

und Dilleniaceen. Planta 14: 441-470.
Pantulu, J. V. 1941. Some unusual megaspore tetrads in the Leguminosae.

Current Sci. [India] 10: 175-176.
Pfeiffer, W. M. 1912. The morphology of Leitneria floridana. Bot. Gaz. 53:

189-203.
Puri, V. 1941. Life history of Moringa oleifera Lamk. Jour. Indian Bot. Soc.

20: 263-284.
Randolph, L. F. 1936. Developmental morphology of the caryopsis in maize.

Jour. Agr. Res. 53: 881-916.
Rauch, K. V. 1936. Cytologisch-embryologische Untersuchungen an Scurrula

atropurpurea Dans, und Dendrophthoe pentandra Miq. Ber. schweiz. bot.

Gesell. 45:5-61.
Rosendahl, C. O. 1909. Embryo sac development and embryology of Symplo-

carpus foetidus. Minn. Bot. Studies 4: 1-9.
Rutishauser, A. 1937. Entwicklungsgeschichtliche Untersuchungen an Thesium

rostratum. Mitt, naturf. Gesell. Schaffhausen 13: 25-47.
. 1945. Zur Embryologie amphimiktischer Potentillen. Ber. schweiz.

bot. Gesell 55: 19-32.
Schaeppi, H., and Steindl, F. 1942. Bliitenmorphologische und embryologische

Untersuchungen an Loranthoideen. Vtljschr. naturf. Gesell. Zurich 87:

301-372.
Schleiden, M. J. 1837. Einige Blicke auf die Entwicklungsgeschichte der vege-

tablischen Organismus bei den Phanerogamen. Wiegmanns Archiv 3:

289-320.
Schlimbach, H. 1924. Beitrage zur Kenntnis der Samenanlagen und Samen der

Amaryllidaceen mit Beriicksichtigung des Wassergehaltes der Samen. Flora

117:41-54.
Schnarf, K. 1929 "Embryologie der Angiospermen." Berlin.
Seshadri Ayyangar, G. 1948. Some observtions on stomata found on cotton

ovules. Indian Cotton Growing Rev. 2: 187-192.
Shattuck, C. H. 1905. A morphological study of Ulmus americana. Bot. Gaz.

40: 209-223.
Singh, Bahadur. 1950. The embryology of Dendrophthoe falcata (Linn, fil.)

Ettingshausen. Proc. 37th Indian Sci. Cong. Sect. Bot.
Smith, C. M. 1929. Development of Dionaea muscipula. I. Flower and seed.

Bot. Gaz. 87:508-530.
Soueges, E. C. R. 1919. Recherches sur l'embryogeme des Polygonacees. Bui.

Soc. Bot de France 66: 168-199; 67: 1-11, 75-85.

THE MEGASPORANGIUM 83

Steindl, F. 1945. Beitrag zur Pollen- und Embryobildung bei Cynomorium

coccinewn L. Arch. Julius Klaus-Stift. f. Vererbungsforsch. 20: 342-355.
Stenar, H. 1925. "Embryologische Studien. I. Zur Embryologie der Colum-

niferen. II. Die Embryologie der Amaryllideen." Diss. Uppsala.
. 1928. Zur Embryologie der Asphodeline-Gruppe. Ein Beitrag zur syste-

matischen Stellung der Gattungen Bulbine und Paradisia. Svensk. Bot.

Tidskr. 22: 145-159.
Subba Rao, A. M. 1941. Studies in the Malpighiaceae. II. Structure and de-
velopment of the ovules and embryo sacs of Malpighia coccifera Linn, and

Tristellateia australis. Proc. Natl. Inst. Sci. India 7: 393-404.
Subramanyam, K. 1948. An embryological study of Melastoma malabathricum

L. Jour. Indian Bot. Soc. 27: 1-9.
Svensson, H. G. 1925. "Zur Embryologie der Hydrophyllaceen, Borraginaceen

und Heliotropiaceen." Diss. Uppsala.
Swamy, B. G. L. 1948. A contribution to the life history of Casuarina. Proc.

Amer. Acad. Arts and Sci. 77: 1-32.
Tomita, K. 1931. Uber die Entwicklung des nackten Embryos von Crinum

latifolium L. Sci. Rpt. Tohoku Imp. Univ. 6: 163-169.
Treub, M. 1879. Notes sur l'embryogenie de quelques Orchidees. Natuurk.

Verb. Koninkl. Akad. Amsterdam 19: 1-50.
Van Tieghem, Ph. 1901. L'hypostase, sa structure et son role constantes, sa

position et sa forme variables. Bui. Mus. Hist. Nat. 7: 412-418.
Whitehead, M. R., and Brown, C. A. 1940. The seed of the spider lily, Hymeno-

callis occidentalis. Amer. Jour. Bot. 27: 199-203.
Woodcock, E. F. 1943. Seed development in morning-glory (Ipomoea rubro

caerulea Hook). Papers Mich. Acad. Sci. Arts and Letters 28: 209-212.

CHAPTER 4
THE FEMALE GAMETOPHYTE1

Depending on the number of megaspore nuclei taking part in
the development, the female gametophytes of angiosperms may be
classified into three main types: monosporic, bisporic, and tetra-
sporic. In the first, only one of the four megaspores takes part in
the development of the gametophyte. In the second, two mega-
spore nuclei take part in its formation; and in the third, all four of
them. A further subdivision is based on the number of nuclear
divisions intervening between the time of megaspore formation and
the time of differentiation of the egg, and the total number of nuclei
present in the gametophyte at the moment when such differentiation
takes place. A secondary increase in their number, which sometimes
takes place at a later stage, is not taken into account in this classi-
fication.

The monosporic female gametophytes or embryo sacs fall under
two types: 8-nucleate and 4-nucleate. In the development of the
8-nucleate embryo sacs, the first division of the functioning mega-
spore gives rise to 2 nuclei : the primary micropylar and the primary
chalazal. The second division produces one pair of nuclei at the
micropylar end and one at the chalazal, and the third results in two
groups of 4 nuclei lying at the opposite poles of the elongated embryo
sac. The micropylar quartet differentiates into a three-celled egg
apparatus and the upper polar nucleus, and the chalazal quartet
into a group of three antipodal cells (or nuclei) and the lower polar
nucleus. The 2 polar nuclei fuse to give rise to a secondary nucleus.

This type of embryo sac is the most common and is, therefore,
commonly designated as the "Normal type." However, since the
others are by no means so infrequent as was once supposed, it will
be designated here as the "Polygonum type," for it was in Polygo-
num divaricatum that Strasburger (1879) gave the first clear and well-

1 In writing this chapter the author has drawn freely upon some of his review
articles (Maheshwari, 1937, 1941, 1946a,6; 1947, 1948), to which reference may be
made for fuller information.

THE FEMALE GAMETOPHYTE 85

illustrated account of the development of a monosporic 8-nucleate
embryo sac.

In certain other monosporic embryo sacs, the megaspore nucleus
undergoes only two divisions and a micropylar quartet alone is
formed. This quartet gives rise to a normal egg apparatus and a
single polar nucleus. The lower polar nucleus and antipodal nuclei
are absent. This type of development is known as the "Oenothera
type" and has so far been reported only in the family Onagraceae.

The bisporic embryo sacs are typically 8-nucleate ("Allium type")
and arise from one of the two dyad cells formed after Meiosis I.
Since no wall is laid down after Meiosis II and both the megaspore
nuclei formed in the functional dyad cell take part in the develop-
ment of the embryo sac, only two further divisions are necessary to
give rise to the 8-nucleate stage. A doubtful 4-nucleate type ("Po-
dostemon type") has been reported in a few members of the Podo-
stemonaceae but this is questionable and will not receive detailed
consideration.

The tctrasporic embryo sacs present a great deal of variation. In
several cases 16 nuclei are formed as the result of two divisions fol-
lowing megasporogenesis. These are classified under the following
types, depending on the polarity and organization of the nuclei in
the sac: "Peperomia type," "Penaea type," "Drusa type."

In some plants, owing to a crowding of 3 of the megaspore nuclei
into the chalazal end of the cell (1 + 3 arrangement), there is a
fusion of their spindles in the next division, resulting in a secondary
4-nucleate stage with 2 haploicl nuclei at the micropylar end and
2 triploid ones at the chalazal. The next division results in 8 nuclei,
4 of which are haploid and 4 triploid. This mode of development
is known as the "Fritillaria type."

The "Plumbagella type," reported only in Plumbagella micrantha,
is similar to the Fritillaria type, except that here the development
stops at the secondary 4-nucleate stage, which is at once followed
by the organization of the embryo sac.

Finally, there are the "Adoxa" and "Plumbago" types, in both
of which the 4 megaspore nuclei divide once to give rise to 8 nuclei.
In Adoxa, however, the organization is bipolar and in Plumbago it is
tetrapolar.

All the variations of embryo sac development described above
are shown diagrammatically in Fig. 55.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Type

Megasporogenesis

Megaspore
mother cell

Division
I

Division
II

Megagametogenesis

Division
III

Division
IV

Division
V

Mature
embryo sac

Monosporic

8 -nucleate

Polygonum type

Monosporic

4 -nucleate

Oenothera type

Bisporic
8 -nucleate
Allium type

Tetrasporic

16 -nucleate

Peperomia type

Tetrasporic
16 -nucleate
Penaea type

Tetrasporic
16 -nucleate
Drusa type

Tetrasporic

8 -nucleate

Fritillaria type

Tetrasporic

8 -nucleate

Plumbagella type

Tetrasporic

8 -nucleate

Plumbago type

Tetrasporic
8 -nucleate
Adoxa type

nfetj

Fig. 55. Diagram showing important types of embryo sacs in angiosperms.

THE FEMALE GAMETOPHYTE 87

MONOSPORIC EMBRYO SACS

Polygonum type. The monosporic 8-nucleate embryo sac, formed
by three divisions of the functioning megaspore, occurs in at least
70 per cent of the angiosperms now known. The enlargement of
the megaspore is always accompanied by increased vacuolation, one
large vacuole usually appearing on either side of the nucleus in the
direction of the long axis of the cell (Fig. 56 A). After the first
division has taken place, the two daughter nuclei move apart to
opposite poles. Most of the cytoplasm is aggregated around them
and the rest forms a thin peripheral layer, the center being occupied
by a large vacuole (Fig. 565). The next division gives rise to a
4-nucleate stage (Fig. 56C) which is followed by the 8-nucleate
stage comprising a micropylar and a chalazal quartet.

Of the 8 nuclei arising in this manner, 3 at the micropylar end
give rise to the egg and two synergids; 3 at the chalazal end give
rise to antipodal cells;2 and the remaining 2, one from each pole,
fuse in the center to form a secondary nucleus (Fig. 56/)).

Occasionally embryo sacs are found with less than the normal
quota of 8 nuclei. This is usually because of an early degeneration
of the antipodals, which obscures the true nature of the embryo
sac. Even when the antipodals are present, they are sometimes
overlooked because of their being situated in the narrow chalazal end
of the embryo sac, which is seen only in median sections (Puri, 1939,
1941).

In some cases there is a genuine reduction in the number of nuclei.
In certain species of Phajus, Corallorhiza, Broughtonia (Sharp, 1912),
Chamaeorchis, Oncidium (Afzelius, 1916), Elatine (Frisendahl, 1927)
(Fig. 57 A), Thesium (Rutishauser, 1937a), Calypso (Stenar, 1940),
and Bulbophyllum and Geodorum (Swamy, 1949a) the embryo sacs
are 6-nucleate owing to a suppression of division of the two chalazal
nuclei of the 4-nucleate stage. In Orchis morio (Afzelius, 1916) the
primary chalazal nucleus of the 2-nucleate stage may degenerate
without undergoing any division, so as to result in a 5-nucleate
embryo sac.

A reduction in the number of nuclei may also be brought about in
a different way. In Epipactis pubescens (Brown and Sharp, 1911)

8 In several plants, like Thesiiwi rostratum (Rutishauser, 1937a), cell formation
does not occur at the chalazal end and the antipodal nuclei remain free.

D C

Fig. 56. Development of embryo sac in Hydrilla verticillat a. A, tetrad of mega-
spores with chalazal cell functioning. B,C, two-nucleate and four-nucleate embryo
sacs. D, mature embryo sac; synergids have degenerated.

THE FEMALE GAMETOPHYTE

and Paphiopedilum insigne (Afzelius, 1916) it has been noted that
sometimes the two chalazal spindles of the last division come to
lie more or less parallel and very close to each other and eventually
coalesce to form a single large spindle which produces 2 diploid
nuclei instead of the 4 haploid ones which would have been formed

B C

Fig. 57. Embryo sacs with fewer or more than eight nuclei. A, Elatine triandra,
six-nucleate embryo sac which has arisen by omission of last division at chalazal
end; the two black masses represent degenerated synergids. B, E. hydropiper,
fusion of two eight-nucleate embryo sacs, resulting in 16-nucleate compound embryo
sac. (After Frisendahl, 1927.) C, Sandoricum koetjape, embryo sac containing
cytoplasmic vesicle with several nuclei. (After Juliano, 1934-)

in the ordinary way. This results in a 6-nucleate embryo sac with
a haploid micropylar quartet, a diploid lower polar nucleus, and a
single diploid antipodal cell (Fig. 58).

The reverse condition, i.e., the occurrence of more than 8 nuclei
in the embryo sac, is less frequent and may arise in three ways:
(1) fusion of two embryo sacs (2) migration of the nuclei of nucellar
cells into the embryo sac ; and (3) occurrence of secondary divisions
of some of the first-formed 8 nuclei.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

In plants having a multicellular archesporium, several megaspore
tetrads are formed and a number of megaspores may begin to en-
large. As examples may be mentioned the Casuarinaceae, Loran-
thaceae, and Rosaceae, and some members of the Rhamnaceae,
Rubiaceae, and Compositae.3 Commonly most of the embryo sacs
become arrested in their development at a comparatively early
stage and only a few reach maturity. Rarely the separating walls
between the sacs may dissolve so that the contents become included

A B C D E

Fig. 58. Some stages in development of embryo sac of Epipactis pubescens. A,
telophase of last division in embryo sac; note that two chalazal spindles show tend-
ency to lie parallel to each other. B, metaphase of similar division; two chalazal
spindles have coalesced to form single large spindle. C,D, later stages of division.
E, mature embryo sac, showing egg apparatus, two polar nuclei, and single antip-
odal cell. (After Brown and Sharp, 1911.)

in a common cavity. A very good example of this kind has been
figured in Elatine hydropiper (Frisendahl, 1927), showing an embryo
sac with two egg apparatuses, two pairs of polar nuclei, and two
groups of three antipodal cells each (Fig. 57 B). This must clearly
have originated by a fusion of two normally growing sacs. Similar
"compound" sacs have been noted by Oksijuk (1937) in Reseda alba
and R. inodora. Sometimes he found less than 16 nuclei, which is

3 In Potentilla, heptaphylla (Rutishauser, 1945) as many as nine embryo sacs
were seen in one ovule.

THE FEMALE GAMETOPHYTE 91

quite possible if one of the fusing gametophytes is at a younger stage
of development than the other.

In a Musa variety known as "I.R. 53," Dodds (1945) has re-
cently described one compound embryo sac with three egg appa-
ratuses and two pairs of polar nuclei; and another with two egg
apparatuses, one pair of polar nuclei, and an additional group of
7 large "polar-like" nuclei at the chalazal end. Juliano (1934) has
figured a peculiar embryo sac in a fallen flower of Sandoricum
koetjape with a normal egg apparatus, two polar nuclei, and a large
cytoplasmic vesicle extending from the chalazal end of the sac to
its middle and containing more than a dozen nuclei (Fig. 57C).
Since the antipodals are very ephemeral in this species, it is con-
sidered probable that the embryo sac proper was formed from the
third megaspore and that the multinucleate vesicle arose as a result
of some free nuclear divisions in the fourth megaspore.4

In some plants there is a migration of the nucellar nuclei into the
embryo sac. This migration is due to the fact that during the
growth and enlargement of the latter, the adjacent cells of the
nucellus become flattened and crushed. Their walls, which are
very thin and delicate, get ruptured, and the contents — both cyto-
plasm and nuclei, or only the latter — may "wander" into the em-
bryo sac and become incorporated in it.5 Two instances of this
nature deserve special mention. In Hedychium gardnerianum
(Madge, 1934) the nuclei of the nucellar cells lying just below the
hypostase migrate "from cell to cell" through a small hole in the
walls until they reach the hypostase. Here their progress is stopped
for a time and groups of 20 or 30 nuclei collect together, surrounded
by the ragged cell walls of the ruptured cells. Some of the nuclei
now make their way around the hypostase into the cavity of the
embryo sac, where they are believed to serve a nutritive function.
In Pandanus (Fagerlind, 1940), which has no thick-walled hypos-
tase, the nucellar cells lying directly beneath and on the sides of
the young embryo sac show a marked tendency to enlarge. Their
nuclei become swollen and the plasma assumes an appearance simi-

4 Another possibility, not mentioned by Juliano, is that the embryo sac proper
arose normally from the chalazal megaspore and the vesicle was of aposporic
origin.

5 This is comparable to the condition in many gymnosperms in which the nuclei
of the jacket cells often make their way inside the egg.

92 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

lar to that of the embryo sac (Fig. 59A). The enlarged nuclei
eoon approach the embryo sac wall, which becomes perforated at
such points. Gradually the pores become wider and finally the
entire separating wall is absorbed. The embryo sac now encroaches
upon these areas and soon incorporates them, coming in contact
with newer cells which may also meet the same fate (Fig. 595,(7) .6
Even at the 4-nucleate stage, as many as 10 or more nucellar cells
may become included inside the embryo sac in this fashion. Their
nuclei divide synchronously with the sac nuclei, resulting in the
formation of 8 haploid and a variable number of diploid nuclei
(Fig. 59 D). The secondary nucleus attains varying degrees of poly-
ploidy, depending not only on the number of the nuclei which take
part in the fusion but also on their chromosome content. In the
mature embryo sac (Fig. 59F), the egg apparatus contains haploid
nuclei only; some of the antipodal cells contain haploid nuclei,
others diploid; most of the lateral cells (Fig. 59E) are diploid.

The third possibility, i.e., an increase in the number of nuclei
caused by further divisions of the original nuclei of the sac, is rare
except with regard to the antipodal nuclei or cells, for which see
page 134. To mention some examples from recent literature,
in Crassula schmidtii and Umbilicus intermedins (Mauritzon, 1933)
it is reported that occasionally there is a fourth division in the
embryo sac, resulting in the formation of 16 nuclei, which organize
to form four synergids, two eggs, six antipodal cells, and four polar
nuclei. In Crepis capillaris (Gerassimova, 1933) some supernu-
merary egg cells were occasionally seen in addition to the other
and usual components of the embryo sac, but their origin could
not be traced and eventually they were found to degenerate and
disappear. In Nicotiana, Goodspeed (1947) has recently reported
some embryo sacs having 9 to 16 nuclei, — "obviously the result of
division of from one to all of the normal eight nuclei." Here 3 to
5 nuclei were found to take part in polar fusion.

Special mention may be made of the development of the embryo
sac in Balanophora and Langsdorffia (Fagerlind, 1945a,6). In both
cases the micropylar megaspore functions and the three chalazal
mega spores degenerate at a very early stage, although their re-

6 Harling (1946) reports that in Carludovica the nucellar cells enlarge and push
against the wall of the embryo sac, but in this case their contents do not actually
enter the sac.

D E F

Fig. 59. Development of embryo sac in Fandanus. A, P. ornatus, four-nucleate
stage; note enlargement of nucellar cells in chalazal region. B, same, with some
nucellar cells incorporated inside embryo sac. C, P. dubius, four-nucleate stage,
showing nucellar nuclei entering into embryo sac. D, P. oleiocephalus, embryo sac,
showing several nuclei some of which are apparently derived from nucellus. E,
same, mature embryo sac, showing two lateral cells and supernumerary polar
nuclei. F, P. ornatus, mature embryo sac, showing large number of antipodal
cells. (After Fagerlind, 1940.)

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

mains can be recognized for a considerable time (Fig. 60B,C; Fig.
61A-C). In Balanophora even the dyad cells show a marked dif-
ference in size, the micropylar cell being much larger than the
chalazal (Fig. GO A). At the 2- or 4-nucleate stage a tubular out-

C ' — ' F v G

Fig. 60. Development of embryo sac in Balanophora elongata. A, dyad cells
undergoing second meiotic division. B, functioning megaspore, with remains of
three degenerating megaspores at its base. C,D, two-nucleate embryo sacs; note
lateral outgrowth from basal part of embryo sac in D. E, four-nucleate embryo
sac. F, eight-nucleate embryo sac; note that egg apparatus has organized in
morphologically lower end of embryo sac. G, older stage of same, showing fusion
of four nuclei at antipodal end. (After Fagerlind, 1945a.)

growth arises from the embryo sac and then grows upward. In
Balanophora it originates near the basal end of the sac (Fig. 60D)
and in Langsdorffia near its apical end (Fig. 61 A). In both cases
it grows very quickly and soon comes to lie at a higher level than
the originally upper end of the embryo sac (Fig. QOE, Fig. 61B,C).
The 4 nuclei of the sac now undergo the last division to form the

THE FEMALE GAMETOPHYTE

usual 8 nuclei, of which those belonging to the morphologically
basal end give rise to the egg apparatus and one polar nucleus
(Fig. 60 G) and those belonging to the upper end fuse to form an
irregularly lobed nucleus which usually degenerates in situ. A sim-
ilar fusion takes place in Langsdorffia (Fig. 61F-7) except that some-

^-'-•■■''•v.:-".Oa:\;
3 •v*.*:.r 5

B ^ C \2») D

Fig. 61. Development of embryo sac in Langsdorffia hypogaea. A, four-nucleate
stage; note three degenerating megaspores at lower end and formation of lateral
protuberance near upper end. B, older stage, showing entry of two basal nuclei of
sac into lateral arm. C, more advanced stage, showing pronounced upward growth
of lateral arm, which is now situated at a higher level than morphologically upper
end of sac; note three degenerating megaspores at lower end. D, two upper nuclei
of the sac, dividing. E, mature embryo sac. F-I, stages in fusion of four nuclei
belonging to antipodal end of embryo sac. (After Fagerlind, 1945b.)

96 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

times the last division fails to occur at this end of the embryo sac
(Fig. 6 ID), so that only 6 nuclei are formed (Fig. 61 E).

Oenothera Type. About a hundred years ago, Hofmeister (1847,
1849) published some remarkably accurate figures of the embryo
sac of a few members of the Onagraceae, but because of the crude
technique of those days he was unable to give a full account of the
development. Geerts, in 1908, found that in Oenothera lamarckiana
the embryo sac is usually formed by the micropylar megaspore of
the tetrad, which undergoes only two nuclear divisions instead of
the usual three occurring in the Polygonum type of embryo sac.
In this way, 4 nuclei are produced which organize into the two
synergids, the egg, and a single polar nucleus. Since the third
division is omitted and all the nuclei are situated in the micropylar
part of the developing embryo sac, there is neither a lower polar
nucleus nor any antipodal cells. Modilewski (1909) independently
studied species of Oenothera, Epilobium, and Circaea and confirmed
the observations of Geerts in all essential respects. These two in-
vestigations were soon followed by several others and this mode of
development, known as the Oenothera type, has been found to be a
characteristic and constant feature of the entire family Onagraceae,
having been demonstrated in more than 16 genera. The only ex-
ception is Trapa, which has an 8-nucleate embryo sac of the
Polygonum type, but this genus, as most systematists now agree,
is best assigned to a separate family, the Hydrocaryaceae or
Trapaceae.

A noteworthy feature in the development of the Oenothera type
of embryo sac is the concurrent growth of more than one cell of the
tetrad. Eventually it is the micropylar megaspore which func-
tions, but sometimes it may be the chalazal and occasionally both
grow simultaneously forming "twin" embryo sacs (Fig. 62).

Rarely, more than 4 nuclei may be seen in an embryo sac. Usu-
ally this condition results from the incorporation of an adjacent
megaspore and its contents, but it appears that sometimes there
may be further division or divisions of the nuclei of the embryo
sac. In Anogra pallida7 Johansen (1931a) reported repeated ami-

7 This plant is a native of the arid regions of southern Arizona and California.
It shows little or no seed production, and propagation occurs by means of offshoots
at the ends of subterranean stolons.

THE FEMALE GAMETOPHYTE

totic divisions of the polar nucleus, and in one embryo sac as many
as 140 nuclei were formed by this method. In a few instances he
found a synergid containing about 20 nuclei. In Zauschneria lati-
folia (Johansen, 19316) the nuclei of the nucellar cells are said to

B C D E F

Fig. 62. Development of embryo sac in Oenothera suaveolens. A , tetrad of mega-
spores; both micropylar and chalazal megaspores are enlarging. B, embryo sac
formed from micropylar megaspore; the three chalazal megaspores in process of
degeneration. C, embryo sac formed from micropylar megaspore; one of chalazal
megaspores has also developed up to two-nucleate stage. D-F, twin embryo sacs
formed by concurrent growth of two megaspores. (After Hoeppener and Renner,
1929.)

migrate into the embryo sac to form a variable number of bodies
looking like micronuclei of different sizes.

Embryo sacs with fewer than 4 nuclei are rare, but in Hartmannia
tetraptera (Johansen, 1929) and Jussieua repens, Khan (1942) saw
two 3-nucleate embryo sacs having a single synergid, an egg, and a
polar nucleus. Their origin is probably to be explained by a lack
of division of the primary synergid nucleus of the 2-nucleate stage.

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

BISPORIC EMBRYO SAC

Allium Type. A bisporic embryo sac was first described in Al-
lium fistulosum (Strasburger, 1879) and has since been confirmed in
several species of this genus (Weber, 1929; Messeri, 1931; Jones and
Emsweller, 1936; and others). The megaspore mother cell (Fig.
63 A) divides to form two dyad cells, of which the upper is much
smaller and soon degenerates (Fig. 63B). The nucleus of the lower
divides to form 2 (Fig. 63C), 4 (Fig. 63D) and then 8 nuclei, which
give rise to an embryo sac with the usual organization.

Fig. 63. Early stages in development of embryo sac of Allium cepa. A, mega-
spore mother cell. B, dyad cells, upper degenerating. C, two-nucleate embryo
sac. D, four-nucleate embryo sac. {After Jones and Emsweller, 1936.)

Treub and Mellink (1880) independently described the same type
of development in Agraphis patula (= Scilla hispanica), and this
has also been found to be true of other species of Scilla (see Hoare,
1934). The chief difference between Allium and Scilla lies in the
fact that while in Allium it is the lower dyad cell which gives rise
to the embryo sac, in Scilla it is usually the upper. The lower
does not degenerate at once, however, but often develops up to the
4-nucleate stage forming the so-called "antigone," which probably
serves for the nutrition of the functional embryo sac.

During the last seven decades the Allium type has been reported
in several plants belonging to diverse groups and it appears to be
quite characteristic of certain families, viz., Podostemonaceae, Bu-

THE FEMALE GAMETOPHYTE 99

tomaceae (except Butomus), Alismaceae, and the tribe Viscoideae
of the Loranthaceae. It is also found in several members of the
Balanophoraceae, Liliaceae, Amaryllidaceae, and Orchidaceae, but
in other groups its occurrence is more or less sporadic.

The chief variation in development is a tendency toward reduc-
tion in the number of nuclei at the chalazal end. This has been
very clearly demonstrated in the Alismaceae (Dahlgren, 19286, 1934;
Johri, 1935a,6,c, 1936a; Maheshwari and Singh, 1943), Butoma-
ceae (Johri, 19366, 1938a, 6), Podostemonaceae (Went, 1910, 1912,
1926), and some members of the Orchidaceae (see Swamy,
1949a). In the Alismaeae, of which Machaerocarpus calif ornicus
(Maheshwari and Singh, 1943) may be cited as an example (Fig.
64), the development usually proceeds normally up to the 4-nucleate
stage. After this only the 2 micropylar nuclei divide again, re-
sulting in a 6-nucleate stage comprising an egg apparatus, two
polar nuclei, and a single antipodal nucleus. In those plants in
which reduction has gone still further, only 5 nuclei are formed,
four at the upper end and the undivided primary chalazal nucleus
at the lower. The mature embryo sac therefore comprises an egg
apparatus, an upper polar nucleus, and a single antipodal nucleus;
a lower polar nucleus is absent.

Special mention may be made of a few plants following the
Allium type of development.

In the tribe Viscoideae, belonging to the Loranthaceae, this mode
of development seems to be of general occurrence and has recently
been described in some detail in Ginalloa (Rutishauser, 19376),
Korthalsella (Rutishauser, 1935, 19376 ; Schaeppi and Steindl, 1945),
and Viscum (Steindl, 1935; Schaeppi and Steindl, 1945). In all
these genera the central ovarian papilla has two or more arche-
sporial cells, each of which divides to form two dyad cells (Fig.
Q5A-C). Of these, the upper dyad cell is the larger and functions,
while the lower soon degenerates (Fig. 65 D). A peculiar feature
is that after the 4-nucleate stage there is a slow but steady curva-
ture of the embryo sac, which causes its lower end to bend out of
the papilla and proceed upward into the carpellary tissue (Fig.
65£"). Meanwhile, the 4 nuclei divide to form 8, one quartet being
situated at each pole of the embryo sac. The egg apparatus dif-
ferentiates in the originally lower pole, which is, however, now
situated at a higher level than the upper (Fig. 65F).

100

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

In Convallaria majalis (Stenar, 1941) the first division of the
megaspore mother cell results in the formation of the usual dyad
cells (Fig. QQA,B). Walls are also laid down after the second
division (Fig. 66C), but these soon break down so that the dyad
cells are again restored although each of them is now binucleate

Fig. 64. Development of embryo sac in Machaerocarpus calif ornicus. A, mega-
spore mother cell. B, dyad stage; nucleus of lower dyad cell dividing. C, two-
nucleate embryo sac with remains of degenerated upper dyad cell. D, four-
nucleate embryo sac. E, six-nucleate embryo sac; lower two nuclei of four-nucleate
stage have remained undivided. F, mature embryo sac, showing two synergids,
egg, two polar nuclei, and single antipodal nucleus. (After Maheshwari and Singh,
1948.)

THE FEMALE GAMETOPHYTE

101

(Fig. 66D). The micropylar dyad cell is at first the larger and
more vacuolated (Fig. QQE) but gradually the chalazal dyad cell
increases in size and plays the more dominant role (Fig. 66F). The

Fig. 65. Development of embryo sac in Korthahella dacrydii. A, Is. central
papilla. B, portion of older papilla, showing a megaspore mother cell. C, telo-
phase of Meiosis I. D, two-nucleate embryo sac formed from upper dyad cell;
note degenerating lower dyad cell. E, central papilla showing two four-nucleate
embryo sacs; note beginning of curvature in embryo sac on right. F, mature
embryo sac; egg apparatus has differentiated in originally basal end, which has now
penetrated upward into tissues of carpel. (After Rutishauser, 1935.)

2 nuclei of this cell divide to form 4 (Fig. ffiG-H) and then the 8
nuclei of the mature stage (Fig. 667). The interesting point in
the development is that it starts like that of a monosporic form

102 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

but is actually bisporic as a result of an early dissolution of the
cell walls laid down after the second meiotic division.

In 1907, Pace published an interesting paper on the develop-
ment of the embryo sac in four species of Cypripedium. According

G H

Fig. 66. Development of embryo sac in Convallaria majalis. A, l.s. nucellus, show-
ing megaspore mother cell. B, dyad stage. C, T-shaped tetrad. D, wall sepa-
rating the two megaspores of each dyad cell has disappeared. E, the two dyad
cells, each binucleate. F, upper dyad cell degenerating; lower enlarging. G,
four-nucleate embryo sac formed from lower dyad cell. H, same, more advanced
stage. /, mature embryo sac. (After Stenar, 1941.)

THE FEMALE GAMETOPHYTE 103

to her account, the megaspore mother cell divides to form two
dyad cells, of which the lower develops normally up to the 4-nu-
cleate stage. One of the micropylar nuclei is now said to form the
egg and the other a synergid; the second synergid is formed by one
of the chalazal nuclei which migrates upward; and the remaining
nucleus functions as the single polar. At the time of fertilization,
one of the synergid nuclei is said to become displaced by the in-
coming pollen tube and forced, as it were, to take part in triple
fusion. Owing to its unique and distinctive nature, this mode of
development was designated as the "Cypripedium type."

The reinvestigations made by Prosina (1930), Francini (1931),
Carlson (1945), and Swamy (1945) have, however, shown that the
development does not end at the 4-nucleate stage but continues
further. Occasionally all 8 nuclei may be formed, but in any case
at least the 2 micropylar nuclei go through the next division, so
that the embryo sacs are 6 nucleate.

The ovules and embryo sacs of the Podostemonaceae show several
interesting features to which a brief reference may be made here,
using Podostemon ceratophyllum (Hammond, 1937) as an example.
The outer integument appears first and forms the micropyle (Fig.
67 A). The megaspore mother cell (Fig. 67B,C), which is situated
directly below the epidermis, divides to form the two dyad cells
(Fig. 67D), of which the micropylar soon aborts although its nucleus
may occasionally divide (Fig. &7E). The nucleus of the chalazal
dyad cell divides to form 2 nuclei (Fig. 67F-H), of which the lower
promptly degenerates and disappears (Fig. 67 G). The remaining
nucleus undergoes two divisions, resulting in 4 nuclei (Fig. 677),
which organize to form two synergids, an egg, and a polar nucleus.
Occasionally the primary chalazal nucleus persists up to this stage
so that the 5-nucleate nature of the embryo sac is easily recognized.
More commonly, however, only 4 nuclei are seen and the fifth is
no longer recognizable at this stage (see also Razi, 1949).

The following members of the Podostemonaceae are reported to
have tetranucleate embryo sacs: Podostemon subulatus, Hydrobium
(= Zeylanidium) olivaceum, Farmeria metzgerioides (Magnus, 1913),
and Weddelina squamulosa (Chiarugi, 1933). Here the lower dyad
cell is said to undergo only two divisions, resulting in 4 nuclei
which organize into the egg apparatus and a single polar nucleus
(Fig. Q7J-K, QSA-F). This type of development, sometimes called

E F I J

Fig. 67. Development of embryo sac in Podostemon ceratophyllum (A-I) and
Weddelina squamulosa (J,K). A, Podostemon, l.s. ovule, diagrammatic. B, l.s.
young ovule, showing archesporial cell. C, older stage, showing formation of
psuedo embryo sac by disintegration of nucellar cells lying just below the mega-
spore mother cell. D, formation of dyad cells. E, degeneration of upper dyad
cell. F, two-nucleate embryo sac; note enlarging pseudo embryo sac. G, two-
nucleate embryo sac; primary chalazal nucleus disorganizing. H, primary micro-
pylar nucleus divided into two daughter nuclei; primary chalazal nucleus has disap-
peared. J, five-nucleate stage in which the primary chalazal nucleus has degenerated
and disappeared. (After Hammond, 1937.) J,K, Weddelina, stages corresponding
to I. (After Chiarugi, 1983.)

104

I J K

Fig. 68. Development of embryo sac in Podostemon subidatus (A-F) and Dicraea
elongata (G-K). A, Podostemon, megaspore mother cell. B, dyad cells. C,
lower dyad cell enlarging; upper in course of degeneration. D, two-nucleate embryo
sac formed from lower dyad cell. E, four-nucleate stage with accompanying wall
formation. F, mature embryo sac showing two synergids, egg, and a polar nucleus.
G, Dicraea, upper dyad cell degenerating; lower divided into two cells. H-J, upper
dyad cell crushed and disorganized; of the other two cells, upper has divided trans-
versely and lower has divided vertically. K, embryo sac after fertilization, show-
ing degeneration of all the cells except the zygote. (After Magnus, 1913.)

105

106 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the "Podostemon type", has, however, always been considered
doubtful, and the four plants named above deserve to be re-
investigated.

An even more doubtful case is that of Dicraea elongata (Magnus,
1913), in which the chalazal dyad cell is said to divide transversely
to form two cells (Fig. 68G). Of these the upper, which is larger,
again divides in the same plane (Fig. 68//,/) to produce one syn-
ergid and an egg cell, and the lower divides anticlinally to form
two antipodal cells (Fig. 68/). According to this interpretation
the polar nuclei are absent, and all the cells except the zygote
degenerate after fertilization (Fig. 68/v). These observations need
to be confirmed before they can be accepted.

TETRASPORIC EMBRYO SACS8

Peperomia Type. Campbell (1899«,o; 1901) and Johnson (1900)
reported that in Peperomia pellucida each of the 4 megaspore nuclei
divides twice, resulting in a total of 16 nuclei which become more
or less uniformly distributed in the rather thick layer of cytoplasm
lying at the periphery of the embryo sac. According to Johnson,
2 nuclei at the micropylar end now become organized to form the
egg and a synergid, 8 fuse to form the secondary nucleus, and the
remaining 6 are cut off at the periphery of the embryo sac. Ac-
cording to Campbell, on the other hand, 1 to 3 nuclei in the vicinity
of the egg show a more or less evident aggregation of cytoplasm
around them and are to be regarded as the equivalents of syn-
ergids; approximately 8 nuclei enter into the formation of the sec-
ondary nucleus; and the remaining 4 to 6 nuclei are cut off as
antipodal cells.

Subsequent studies, made by others on several species of
Peperomia, have confirmed Johnson's account. The chief varia-
tions concern the number of nuclei which fuse to form the secondary
nucleus, and the number left over to form the antipodals. In
every case only one synergid was observed.

A recent study of Peperomia pellucida (Fagerlind, 1939a) has
shown that after the meiotic divisions are over (Fig. 69 A-C), the
coenomegaspore9 may either retain its more or less spherical form
or become slightly pear-shaped with a little protuberance at the

8 See Fagerlind (1944) for fuller information on tetrasporic embryo sacs.

9 This term is used to denote the cell containing the four free megaspore nuclei.

THE FEMALE GAMETOPHYTE

107

micropylar end. The 4 mega spore nuclei are usually arranged tet-
rahedrally, but in the embryo sacs of the pear-shaped type one
nucleus projects rather conspicuously towards the upper papillate

Fig.

I K L

69. Development of embryo sac in Peperomia pellucida.

A, megaspore

mother cell. B, two-nucleate stage. C, four-nucleate stage. D,E, eight-nucleate
stage as seen in spherical and pear-shaped tj^pes of embryo sacs respectively.
F,G, 16-nucleate stage. H, mature embryo sac of spherical type, showing a single
synergid, six lateral cells, and eight polar nuclei (all nuclei of sac are not seen in
this section). 7, same, showing one lateral cell in close proximity to egg and there-
fore simulating a second synergid. J-M, successive sections through a pear-shaped
embryo sac, showing a three-celled egg apparatus. (After Fagerlind, 1939a.)

end. In the next stage, the 8 nuclei are either distributed more
or less symmetrically around the periphery of the embryo sac (Fig.
69D), or 2 nuclei may lie somewhat closer to each other at its upper
end (Fig. 692?) . The fourth and the last division now gives rise to

108 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

16 nuclei, which may form either eight groups of 2 nuclei each (in
the spherical embryo sacs) (Fig. 69F) or six groups of 2 and a micro-
pylar group of 4 nuclei (in the pear-shaped embryo sacs) (Fig. QQG).
In the former case the egg apparatus is usually two-celled (egg and
one synergid) ; 8 nuclei fuse in the center to form the secondary
nucleus; and 6 nuclei are cut off at the periphery (Fig. 69H). Only
occasionally, because of slight displacements and the small size of
the embryo sac, one may find another peripheral cell lying so close
to the egg that the egg apparatus may be said to comprise three cells
(Fig. 697). In the pear-shaped embryo sacs, however, a three-celled
egg apparatus is the rule, the fourth nucleus from the micropylar
end and one member from each of the six peripheral pairs form the
seven polars, and 6 nuclei are cut off to form the lateral cells (Fig.
69K-M).

Fagerlind's observations help us to understand the slight diver-
gence between the account of Johnson (1900) and that of Campbell
(1899a,6; 1901). The former saw only one synergid, while the
latter believed that there were more than one. Now it appears
that both these conditions are possible, depending on the form
which the embryo sac takes during its growth and development.
In the pear-shaped type there are invariably two synergids; in the
spherical type there is usually only one synergid unless another
peripheral cell accidentally happens to lie so close to the egg as to
look like a second synergid.

Johnson (1914) discovered a different type, however, in P. his-
pidula. At the 8-nucleate stage, 2 nuclei are seen at the micro-
pylar and 6 at the chalazal end (Fig. 70A); at the 16-nucleate
stage, 4 lie at the micropylar end and 12 at the chalazal (Fig. 70B).
Two nuclei of the micropylar group now form the egg and single
synergid, as in other species, but the remaining 2 nuclei of this
group and all the remaining 12 nuclei meet near the center and
fuse to form a single large secondary nucleus (Fig. 70C,D).

The embryo sac of Gunnera (Haloragidaceae) is essentially similar
to that of Peperomia pcllucida. Two species have been studied:
G. macrophylla (Ernst, 1908; Samuels, 1912) and G. chilensis
(Modilewski, 1908). After the 16-nucleate stage, 3 of the micro-
pylar nuclei form the egg apparatus, the fourth descends and fuses
with 6 other nuclei tu form a large secondary nucleus, and the re-
maining 6 are cut off as antipodal cells. It is possible that if the

THE FEMALE GAMETOPHYTE

109

C D

Fig. 70. Some stages in development of embryo sac of Peperomia hispidula.
A, eight-nucleate embryo sac; nuclei shown in dotted outline have been included
from adjacent sections of sac. B, l.s. ovule, showing 16-nucleate embryo sac. C,
embryo sac, showing egg, one synergid, and 12 polar nuclei. D, embryo sac with
egg, single synergid, and large lobed secondary nucleus. (After Johnson, 1914-)

110 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

embryo sac of Gunnera were to be studied again and a sufficient
quantity of material examined, it would show a range of variation
similar to that in Peperomia.

Penaea Type. Stephens (1909) described an interesting mode of
development in three genera of the Penaeaceae, viz., Penaea,
Br achy 'siphon, and Sarcocolla (Fig. 71). Here the 16 nuclei lie in
four distinct quarters which are arranged crosswise, one at each
end of the embryo sac and two at the sides. Now 3 nuclei of each
quartet become cut off as cells, while the fourth remains free and

^i&te

L^rMPi

Fig. 71. Development of embryo sac in Penaeaceae. A, Sarcocolla minor, four
megaspore nuclei at close of the second meiotic division. B, S. formosa, eight-
nucleate stage. C, S. squamosa, 16-nucleate stage, showing four groups of four
nuclei each. D, Penaea mucronata, mature embryo sac. {After Stephens, 1909.)

moves to the center. There are thus four "triads" and four polar
nuclei. As a rule, the egg cell of the micropylar "triad" alone is
functional, although the others often look very similar.

Embryo sacs of this type have since been described in several
members of the Malpighiaceae (see Stenar, 1937; Subba Rao, 1940,
1941) and Euphorbiaceae (Modilewski, 1910, 1911; Arnoldi, 1912;
Tateishi, 1927; and others) and in a few scattered genera belonging
to other families.

Special mention may be made of the embryo sac of Acalypha
indica (Maheshwari and Johri, 1941), which, although similar, does
not entirely fit into the type described above. Up to the 16-nu-

THE FEMALE GAMETOPHYTE

111

cleate stage (Fig. 72A-D) the development corresponds with that
of the Penaeaceae and other species of Acalypha, but the organiza-
tion of the mature embryo sac presents a great variation. The
commonest condition found was that 2 nuclei of each quartet re-
main free and migrate to the center of the embryo sac, while the
other two organize into cells. Thus there are four groups of two
cells each at the periphery and 8 free nuclei in the center (Fig.
72E).

Fig. 72. Development of embryo sac in Acalypha indica. A, megaspore mother
cell with four megaspore nuclei. B, megaspore nuclei in division. C, eight-
nucleate stage. D, sixteen-nucleate stage. E, mature embryo sac, showing four
peripheral pairs of cells and eight polar nuclei. (After Maheshwari and Johri,

mi.)

This was not the only kind of organization, however. Some
ovules showed a micropylar group of three cells and three other
groups of two cells each, leaving only 7 nuclei (instead of the usual
8) to fuse in the center. In one embryo sac, the chalazal group
had three cells and all the rest had two cells each. Another em-
bryo sac showed three two-celled groups, one lateral cell, and 9 free
nuclei meeting in the center. In a third and very peculiar embryo
sac, one lateral group was entirely missing, the second had only one
cell, the micropylar had three cells, and the chalazal had two cells,
leaving 10 free nuclei to take part in polar fusion.

A few cases were noted in which it seemed that fewer than 16
nuclei had been formed, and others with slightly more than this

112 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

number. These counts could not be regarded as certain, however,
since such embryo sacs ran into three or four sections and their
exact reconstruction was a matter of doubt.

Abnormalities of a somewhat similar nature have also been re-
corded in Combretum (Mauritzon, 1939), but it has to be seen how
far these are related to differences of environment.

Drusa Type. A 16-nucleate embryo sac of a different nature was
recorded by Hakansson (1923) in Drusa oppositifolia, a member of
the family Umbelliferae (Fig. 73 A). After the meiotic divisions
are over, three of the megaspore nuclei pass down to the basal end
of the embryo sac, and only one remains at the micropylar end.
This 1+3 arrangement is followed by a 2+6 and then a 4 + 12
stage. The four micropylar nuclei give rise to the egg apparatus
and upper polar nucleus, and the 12 chalazal nuclei to a lower polar
nucleus and 11 antipodal cells.

During recent years this type of development has been recorded
in Mallotus japonicus (Ventura, 1934), Maianthemum bifolium and
M. canadense (Stenar, 1934; Swamy, 194969a), Crucianella latifolia,
Rubia olivieri (Fagerlind, 1937), Tanacetum vulgare, Chrysanthemum
parthenium (Fagerlind, 1941), Ulmus (Ekdahl, 1941; Walker, 1950),
and a few other plants. A few of these deserve special mention
and are briefly discussed below.

Shattuck (1905) reported an 8-nucleate embryo sac of the Adoxa
type in Ulmus americana, but he observed that frequently there
seemed to be a further nuclear division. Several embryo sacs were
found to contain as many as 12 or more nuclei, rather evenly dis-

9a In M. canadense, according to Swamy (19496), in about 13 per cent of the
ovules the chalazal spindles of the last division fuse in pairs so that the mature
embryo sac comes to possess 4 haploid nuclei at the micropylar end and 6 diploid
nuclei at the chalazal end.

Pig. 73. Development of embryo sac in Drusa oppositifolia (A), Chrysanthemum
parthenium (B-II), and Crucianella latifolia (I-M). A, Drusa, 16-nucleate embryo
sac, showing four nuclei at micropylar end and twelve at chalazal. (After Hakans-
son, 1928.) B,C, Chrysanthemum, young embryo sacs showing varying arrange-
ments of the four megaspore nuclei. D-F, eight-nucleate stage; note degeneration
of basal nucleus in E. G, last division in embryo sac, basal nucleus degenerating.
//, mature embryo sac, showing 12 nuclei. (After Fagerlind, 1941.) I, Crucianella,
megaspore nuclei. J, same, in division. K, fourth division in embryo sac; some
of nuclei at chalazal end have failed to divide. L, embryo sac, showing 15 nuclei.
M, mature embryo sac. (After Fagerlind, 1937.)

THE FEMALE GAMETOPHYTE

113

& !

■> ■'

M^g

«3?^

'(.

■!■;-;;>'

114 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

tributed and very similar to one another in appearance. Some
other workers also obtained similar results, and noted that the
mature embryo sacs occasionally showed more than three antip-
odals. D'Amato (1940a), Ekdahl (1941), and Walker (1950), work-
ing on several species of Ulmus, have clarified the position by show-
ing that as a rule four divisions intervene between the megaspore
mother cell stage and the organization of the embryo sac, and not
three. The coenomegaspore shows a 1 + 3 arrangement of the
megaspore nuclei, each of which undergoes two further divisions,
resulting in the formation of 4 nuclei at the micropylar end and 12
at the chalazal end. Frequently, however, some of the chalazal
nuclei fail to undergo the fourth division, resulting in a total of 14,
12, or only 10 nuclei, of which 4 are at the micropylar end and the
rest at the chalazal. Several of the latter degenerate soon after
their formation, so that there is a further decrease in the number of
nuclei, and eventually only two to four antipodal cells may be differ-
entiated. Also, in certain cases the 4 megaspore nuclei divide only
once, so as to give rise to an 8-nucleate embryo sac of the Adoxa
type.

The embryo sac of Chrysanthemum parthenium presents a range
of variation which seems to indicate that there are several races of
this plant which behave somewhat differently from one another,
although possibly the differences are related to environmental con-
ditions. According to Palm (1915), who gave the first detailed
account of the embryo sac of this species, each of the 4 megaspore
nuclei divides twice. The 16 nuclei arising in this way organize
to form a three-celled egg apparatus, two polar nuclei, and eight
antipodal cells of which the basal cell is four-nucleate.

Fagerlind (1941) studied two specimens of the same species. In
specimen 1 the 4 megaspore nuclei were observed to take up the
most variable positions, and frequently the 3 basal nuclei were
seen to lie in close contact (Fig. 735). With the subsequent elon-
gation of the sac the nuclei became separated from one another by
vacuoles, the micropylar nucleus being larger than the rest (Fig.
73C). All the nuclei now divided simultaneously, resulting in 8
nuclei, of which the 2 basal were the smallest and soon began to
degenerate (Fig. 73D-F). When the next division (Fig. 73G) was
over, there were 14 nuclei in the sac, of which 3 organized into an
egg apparatus, 2 functioned as polar nuclei, and the rest formed

THE FEMALE GAMETOPHYTE 115

the antipodal cells (Fig. 73H). The basal antipodal cell contained
a variable number of nuclei, which subsequently fused to form 1
nucleus. Embryo sacs with fewer than 14 nuclei were also seen,
but this was due to a degeneration and disappearance of some of
the nuclei at the chalazal end.

In specimen 2, collected from a different locality in Sweden,
the megaspore mother cells as well as the developing embryo sacs
and their nuclei were found to lie of a larger size than in the first
plant. The chalazal megaspore nucleus degenerated soon after its
formation. The remaining 3 nuclei divided to form 6 and then 12
nuclei. In the mature embryo sac the basal antipodal cell was
observed to have more than one nucleus, while the remaining antip-
odal cells were uninucleate.

Material of the same species collected from the Brooklyn Bo-
tanical Gardens, New York (Maheshwari and Haque, 1949), showed
the usual 4- and 8-nucleate stages, after which all the nuclei were
found to divide again, resulting in 16 nuclei, 4 at the micropylar end
and 12 at the chalazal. These organize to form a three-celled egg-
apparatus, two polar nuclei, and eleven uninucleate antipodal cells.

The embryo sac of Tanacetum vulgar e (Fagerlind, 1941) is funda-
mentally similar to that of Chrysanthemum. The megaspore mother
cell (Fig. 74 A) undergoes the usual reduction divisions to produce
2 (Fig. 745) and then 4 nuclei (Fig. 74C) which become arranged
in a linear fashion (Fig. 74D). Vacuoles soon appear between the
nuclei, which now increase in size and prepare for the next division
(Fig. 74E), resulting in the formation of 8 nuclei (Fig. 74//"). In
many cases, however, the basal nucleus does not take part in the
division and soon begins to degenerate (Fig. 74G), and sometimes
the subbasal nucleus also remains undivided (Fig. 74F). At this
stage the embryo sac may, therefore, contain 8, 7, or only 6 nuclei.
If all of them take part in the next division, the mature embryo
sacs may be 16-, 14-, or 12-nucleate (Fig. 74/). But frequently
there is a further degeneration of one or two of the chalazal nuclei
so that embryo sacs with fewer than 12 nuclei are not uncommon
(Fig. 74/).

Crucianella laiifolia (Fagerlind, 1937), a member of the Rubiaceae,
also belongs to the Drusa type. After the reduction divisions are
over, the coenomegaspore shows a pronounced elongation, rupturing
the nucellar epidermis at its micropylar end (Fig. 737). The next

116 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

division proceeds normally (Fig. 73/), but of the 8 nuclei now
formed the basal nucleus remains undivided (Fig. 73 Jv) so that
the mature embryo sac shows only 15 nuclei (Fig. 73L) which be-
come organized to form a three-celled egg apparatus, two polar
nuclei, and 10 antipodals (Fig. 73M).

Fig. 74. Development of embryo sac in Tanacetum vulgare. A, mega spore mother
cell. B,C, end of first and second meiotic division, respectively. D, megaspore
nuclei. E, same, older stage, showing vacuolation. F-H, first postmeiotic divi-
sion; two megaspore nuclei dividing in F, three in G, and all four in H. I, J, mature
embryo sacs with varying number of nuclei. (After Fagerlind, 1941.)

In Maianthemum bifolium (Stenar, 1934) both the reduction divi-
sions are accompanied by the formation of cell plates (Fig. 75 A-C).
They soon become absorbed, however, resulting in a common tetra-
nucleate cell (Fig. 75D). The four megaspore nuclei take up a
1+3 arrangement, so that the next stage shows 2 nuclei at the
micropylar pole and 6 at the chalazal pole (Fig. 75E-F). There is
one more division, resulting in 16 nuclei (Fig. 75G). These organize
into an egg apparatus, two polar nuclei, and 11 antipodal cells.

THE FEMALE GAMETOPHYTE

117

Most of the antipodal cells soon degenerate, and only a few may be
seen in the mature embryo sac (Fig. 75H).

Fritillaria Type. Following the work of Treub and Mellink

E F G H

Fig. 75. Development of embryo sac in Maianthemum bifolium. A, l.s. nucellus,
showing megaspore mother cell. B, clyad stage. C, tetrad. D, four-nucleate
embryo sac formed by dissolution of walls separating megaspores. E, eight-
nucleate stage. F, same, nuclei in prophase of next division. G, embryo sac with
16 nuclei. H, mature embryo sac. (After Stenar, 1934)

(1880) on Lilium bulbiferum, several other investigators, notably
Strasburger, Mottier, Guignard, Coulter, and Sargant, studied a
number of species of this genus and repeatedly confirmed that the

118 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

4 megaspore nuclei undergo only one division to give rise to the 8
nuclei of the mature embryo sac. They no doubt observed certain
peculiarities and curious appearances which could not be explained
on this interpretation, but these were disregarded as abnormal or
even "pathological" conditions.

Bambacioni (1928a, 6) showed that in Fritillaria and Lilium the
formation of the 4 megaspore nuclei is not followed directly by the
8-nucleate stage but by a secondary I^-nucleate stage, in which the
2 chalazal nuclei are much larger than the micropylar. This comes
about in a very peculiar manner. At first there is a 1+3 arrange-
ment of the megaspore nuclei (Fig. 76A-D) so that the 3 chalazal
nuclei come to lie very close to each other. During the next stage
the micropylar nucleus divides normally, but the three chalazal
spindles fuse to form a single common spindle (Fig. 7QE-F), so
that at the close of the division there are two haploid nuclei at the
micropylar end and two triploid nuclei at the chalazal (Fig. 76
G-H). One more division occurs, resulting in 8 nuclei, of which
the 4 chalazal nuclei are triploid and the 4 micropylar are haploid
(Fig. 767). The mature embryo sac thus consists of three haploid
cells (the egg and two synergids), three triploid cells (the antip-
odals), and a tetraploid secondary nucleus formed by the fusion of
the two polar nuclei, one haploid and the other triploid (Fig. 76/).
Of the antipodals, the two lowest frequently show a flattened and
degenerated appearance — a condition originating from the fact that
the basal nucleus of the secondary 4-nucleate stage often divides in
a more or less abortive fashion.

Cooper (1935a) extended the observations of Bambacioni to sev-
eral other species of Lilium, and since then the Fritillaria type has
been demonstrated in a general way for the entire tribe Lilioideae and
several other genera belonging to diverse families; Piper, Heekeria,
Myricaria, Tamarix, Cornus (some spp.), Armeria, Statiee (most
spp.), Rudbeckia (most spp.), GaiUardia, Cardiocrinum, Gagea,
Erythronium (most spp.), Tulipa (some spp.), and Clintonia (see
Maheshwari, 19466, for detailed information).

It may be noted that the fusion of the 3 chalazal megaspore
nuclei may take place when they are either in the prophase stage
or in early metaphase. In the former case the secondary 4-nu-
cleate stage is preceded by a secondary 2-nucleate one, and the

THE FEMALE GAMETOPHYTE

119

sequence then is as follows: megaspore mother cell, primary 2-nu-
cleate stage, primary 4-nucleate, secondary 2-nucleate, secondary
4-nucleate, and last of all the S-nucleate stage.

Fig. 76. Development of embryo sac in Fritillaria persica. A, l.s. nucellus, show-
ing megaspore mother cell in prophase of Meiosis I. B, two-nucleate stage. C,
primary four-nucleate stage. D, megaspore nuclei, showing 1 +3 arrangement.
E, megaspore nuclei dividing. F, same, showing fusion of three chalazal spin-
dles. G, telophase of same division. H, secondary four-nucleate stage in which
the two micropylar nuclei are haploid and chalazal nuclei are triploid. 7, four
nuclei dividing to form eight. J, eight-nucleate embryo sac. (After Bambacioni,
1928a.)

Normally all the 4 megaspore nuclei are of the same size, but in
some plants the micropylar nucleus is the largest and the other 3
nuclei are considerably smaller. When this happens, the nuclei of

120 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the secondary 2-nucleate and secondary 4-nucleate stages show no
appreciable difference in size, and rarely the chalazal nuclei are
smaller than the micropylar in spite of the triploid nature of the
former.

Finally, the basal nucleus of the secondary 4-nucleate stage some-
times fails to divide, resulting in a 7-nucleate gametophyte with
two antipodal cells instead of three, as in some species of Gagea
(Romanov, 1936) ; or, both the basal as well as the subbasal nucleus
remain undivided and the embryo sac is 6-nucleate, as in Statice
(Fagerlind, 19396). In Tulipa maximovitii (Romanov, 1939) the
3 chalazal megaspore nuclei undergo an abnormal division in which
all the telophase chromosome groups become included in a common
membrane, so that the mature embryo sac is 5-nucleate. In one
genus, Clintonia (R. W. Smith, 1911; F. H. Smith, 1943; Walker,
1944), the chalazal megaspore nuclei degenerate as soon as they are
formed, without undergoing any division at all.

Plumbagella Type. In this type also, which has so far been re-
ported only in Plumbagella micrantha (Fagerlind, 19386; Boyes,
1939), the 4 megaspore nuclei take up a 1+3 arrangement (Fig.
77 A-C), and a large vacuole separates the 3 chalazal nuclei from the
micropylar nucleus (Fig. 77 D). The former gradually approach
one another and eventually fuse to give rise to a single triploid
nucleus (Fig. 77 E). This results in a secondary 2-nucleate stage,
followed by a secondary 4-nucleate one, in which the 2 micropylar
nuclei are haploid and the chalazal are triploid (Fig. 77 F-G).
There are no further divisions. The nucleus nearest the micro-
pylar end organizes into the egg; the triploid nucleus nearest the
chalazal end forms the single antipodal cell; and the remaining^
nuclei, one haploid and the other triploid, fuse to form a tetraploid
secondary nucleus (Fig. 77/7-7).

This mode of development shows an evident relationship with

Fig. 77. Development of embryo sac in Plumbagella micrantha. A, megaspore
mother cell. B, second meiotic division in megaspore mother cell. C, megaspore
nuclei showing 1+3 arrangement; the three chalazal nuclei are of a smaller size.
D, chalazal nuclei in process of fusion. E, fusion of the three chalazal nuclei is
completed, resulting in formation of secondary two-nucleate stage. F,G, forma-
tion of secondary four-nucleate sLage. H, wall formation in embryo sac. /,
mature embryo sac showing egg, secondary nucleus, and single antipodal cell.
(After Fagerlind, 1938b.)

THE FEMALE GAMETOPHYTE

121

122 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the Fritillaria type, the only difference being that in Plumbagella
the development is arrested at the secondary 4-nucleate stage and
the fourth division is omitted.

Adoxa Type. The Adoxa type, formerly known as "Lilium type,"
is characterized by all 4 megaspore nuclei undergoing just one more
division to form an 8-nucleate embryo sac having a normal egg
apparatus, three antipodal cells, and two polar nuclei (Fig. 78).
It was described for the first time by Jonsson (1879-1880) in Adoxa
moschatellina and later by Lagerberg (1909) and Fagerlind (1938a).

Until only a few years ago there was a long list of plants under
the Adoxa type. With the publication of Bambacioni's work and
the consequent reinvestigation of Lilium, Fritillaria, and several
other genera, its ranks have steadily diminished and there are now
only five genera in which its occurrence is a more or less regular
feature: Adoxa, Sambucus, and some species of Erythronium10,
Tulipa, and Ulmus.

An interesting variation has been reported in some species of
Tulipa. In T. sylvestris (Bambacioni-Mezzetti, 1931) vacuolation
frequently commences even at the megaspore mother cell stage,
and all the 4 megaspore nuclei gather at the micropylar end of the
cell, where they divide to give rise to a group of six cells (one of
which is to be interpreted as the egg) and 2 free nuclei. T. tet-
raphylla (Romanov, 1938) is essentially similar. After the meiotic
divisions are over, 3 nuclei go to the micropylar pole and one to
the chalazal (Fig. 79A-D). All of them divide again (Fig. 79 E),
so that there are 6 daughter nuclei in the upper part of the sac and
2 in the lower. Cell plates are laid down at the conclusion of the
division, resulting in the formation of five cells at the micropylar
end (one of these is to be regarded as the egg) and one cell at the
chalazal, leaving 2 free nuclei (the polars) in the center (Fig. 79F).

Since this peculiar mode of development occurs only in the
Eriostemones section of the genus Tulipa, it is known as the "Erio-
stemones form" of the Adoxa type. Other species of the genus
come under the Fritillaria or the Drusa type (see Maheshwari,
1948).

10 Haque's (1950) observations on E. americanum and Walker's (1950) on U.
fulva, U. racemosa, and U. glabra show that the development sometimes follows
the Adoxa type and sometimes the Fritillaria type.

THE FEMALE GAMETOPHYTE

123

Fig. 78. Development of embryo sac in Adoxa moschatellina. A, megaspore
mother cell. B, two-nucleate stage. C, two nuclei dividing. D-G, four-nucleate
embryo sacs. H, division of four nuclei. /, same, telophase. J,K, mature embryo
sacs. (After Fagerlind, 1938a.)

124

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

One species of Leontodon, L. hispidus, also deserves mention in
this connection (Bergman, 1935). Ordinarily a row of four mega-
spores is formed, and the embryo sac is of the Polygonum type.
But in more than 50 per cent of the ovules of one plant the sepa-
rating walls between the megaspore nuclei frequently dissolved and
disappeared, and all the 4 nuclei divided only once to give rise to
the 8-nucleate stage (Fig. 80). Since here only three divisions
intervened between the megaspore mother cell stage and the dif-
ferentiation of the egg, this mode of development comes under
the Adoxa type.

Fig. 79. Development of embryo sac in Tulipa tetraphylla. A-C, formation of
megaspore nuclei. D, 3+1 arrangement of megaspore nuclei. E, all four nuclei
F, mature embryo sac. (After Romanov, 1988.)

dividing

Plumbago Type. The embryo sac of Plumbago capensis,
described by Haupt (1934), may be presented as a representative
of the Plumbago type. The 2- and 4-nucleate stages (Fig. 81 A-B)
are normal, and the 4 megaspore nuclei, which are arranged in a
crosswise fashion, undergo a further division (Fig. 81C) resulting
in 8 free nuclei arranged in four pairs (Fig. 81 D), One nucleus of
the micropylar pair is now cut off to form the lenticular egg cell
(Fig. &IE). Of the remaining 7 nuclei, 4 (presumably one member
of each of the original four pairs) undergo a slight increase in size
and gradually approach one another, functioning as polar nuclei
(Fig. 81F). The remaining 3 nuclei degenerate at their original
places, but occasionally 1, 2, or all 3 of them are cut off at the
periphery to form cells which may persist and assume an egg-like
appearance; synergids are entirely absent (Fig. 81G-H).

THE FEMALE GAMETOPHYTE

125

The Plumbago type occurs not only in other species of the genus
Plumbago (Dahlgren, 1937; Fagerlind, 19386) but also in two other
genera of the Plumbaginaceae, viz., Ceratostigma (D'Amato, 19406)
and Vogelia (Mathur and Khan, 1941). It is so far unknown
outside this family.

Fig. 80. Development of embryo sac in Leontodon hispidus. A, tetrad of mega-
spores. B-E, dissolution of separating walls between megaspore nuclei. F,
mature eight-nucleate embryo sac. (After Bergman, 1935.)

ABERRANT AND UNCLASSIFIED TYPES

In addition to the above fairly distinct and well-established types
of embryo sac development, there are a few which appear to be
more or less isolated. The more important of them are mentioned
below.

Limnanthes douglasii. The embryo sac of this plant has been
investigated by three different workers but without any complete
agreement regarding the mode of development. Stenar (1925a) re-
ported an Adoxa type of embryo sac and called attention to the

126 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

reduced size of the nuclei at the chalazal end. Eysel (1937) con-
firmed this report but noted an occasional reduction in the number
of nuclei at the chalazal end of the embryo sac, owing to a failure

H G F

Fig. 81. Development of embryo sac in Plumbago capensis. A, two-nucleate
stage derived from first division of megaspore mother cell. B, four-nucleate stage.
C, all four nuclei dividing; one of the mitotic figures is oriented at right angles to
plane of sectioning. D, eight-nucleate stage, showing cutting off of egg cell. E,
differentiation of the four polar nuclei. F, fusion of polar nuclei. G,H, later
stages, showing egg at micropylar end and secondary nucleus in center. The two
lateral cells in G are derived from nuclei which ordinarily disappear in earlier stages.
(After Haupt, 1934.)

of the basal nucleus of the 4-nucleate stage to undergo the last
division. In other cases he observed a disappearance of the wall
separating the megaspore mother cell from the nucellar cell situated
directly below it and the consequent incorporation of the latter
into the embryo sac. One embryo sac showed 9 nuclei, of which

THE FEMALE GAMETOPHYTE

127

7 had organized into cells (four looking like synergids, two looking
like eggs, and one of an undecided nature) and 2 resembled polar
nuclei; antipodals were absent.

Fagerlind's (1939c) observations differ from those of both Stenar
and Eysel. The megaspore mother cell has a highly vacuolated
cytoplasm (Fig. 82 A). u As a result of the first division, 2 nuclei
are formed of which the lower promptly degenerates and is reduced

Fig. 82. Development of embryo sac in Limnanthes douglasii. A, megaspore
mother cell. B,C, two-nucleate embryo sacs ; note degeneration of primary chalazal
nucleus. D, three-nucleate stage originating by division of primary micropylar
nucleus. E, division of micropylar nucleus. F, embryo sac piercing the nucellar
epidermis; note two nuclei at micropylar end, one nucleus in middle, and degene-
rated nucleus at chalazal end. G, mature embryo sac showing egg apparatus,
upper polar nucleus, lower polar nucleus (?), and degenerating antipodal cell.
(After Fagerlind, 1939c.)

to a densely staining homogeneous blob which lies at the bottom
of the embryo sac and takes no further part in the development
(Fig. 82B-C). The upper nucleus divides to form 2 daughter nu-
clei, of which the lower is much smaller and usually incapable of
further division (Fig. 82D). Following meiosis, we thus have a
3-nucleate stage showing a micropylar nucleus, a middle nucleus,
and a chalazal nucleus. Of these the micropylar nucleus divides

11 In the majority of angiosperms vacuolation takes place only after the meiotic
divisions are over.

128 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

twice, to give rise to a group of 4 nuclei which form the egg ap-
paratus and the upper polar nucleus (Fig. S2E-G). Of the re-
maining 2 nuclei, one may be considered as an antipodal and the
other as the lower polar nucleus.

Several variations in the development and organization of the
embryo sac were found, however. Most of these seemed to have
their origin in the behavior of the middle nucleus. In some cases
it was found to take part in the third or fourth division, resulting
in a 7-nucleate embryo sac with two antipodal nuclei instead of one.
Less frequently it divided synchronously with the micropylar nu-
cleus, but only one of its daughter nuclei divided again, resulting
in an 8-nucleate embryo sac.

It is probable that at least some of the variations reported by
Stenar, Eysel, and Fagerlind are due to environmental influences,
and a more detailed study is necessary to decide the point.110

Balsamita vulgaris. A recent investigation of the embryo sac
of this plant (Fagerlind, 1939c) has revealed several interesting
features. As in other Compositae, the ovules are tenuinucellate.
The archesporium is usually two-celled (Fig. S3 A), but sometimes
three cells may be present and occasionally there is only one. After
the first meiotic division 2 nuclei are formed of which the upper soon
becomes larger than the lower (Fig. 831?). Both divide again with-
out wall formation and the resulting 4 nuclei take up a 1+3 ar-
rangement (Fig. 83C-D). Only the micropylar nucleus functions,
while the other 3 nuclei soon begin to degenerate. Vacuolation
takes place at this stage and is followed by the appearance of a
lateral vesicular outgrowth, which assumes a tubular form and
gradually makes its way upward into the micropyle (Fig. S3E-F).
The functioning megaspore nucleus, which has by this time moved

lla Mason (1949), who has made a recent study of Limnanthes, regards the
embryo sac as bisporic.

Fig. 83. Development of embryo sac in Balsamita vulgaris. A, l.s. nucellus
showing two-celled archesporium. B, mother cell on right has two nuclei (end of
Meiosis I); that on left has four nuclei (end of Meiosis II). C,D, megaspore nuclei
take up 1+3 position; micropylar nucleus has enlarged; smaller chalazal nuclei
are on way to degeneration. E, formation of vesicular outgrowth from chalazal
end of the cell. F, functional megaspore nucleus has entered vesicle. G-H, two-
and four-nucleate stages. /, eight-nucleate embryo sac; note three nonfunctioning
megaspore nuclei at base. J, mature embryo sac, showing egg apparatus, second-
ary nucleus, and multinucleate antipodal cells. (After Fagerlind, 1989c.)

THE FEMALE GAMETOPHYTE

129

Fig. 83.

130 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

into the apex of the tube, undergoes two divisions to form 4 nuclei,
which lie in two pairs, one at each end of a large vacuole (Fig.
SSG-H). The next division gives rise to 8 nuclei, of which the
upper 4 form the egg apparatus and the upper polar nucleus, and
the lower 4 give rise to the three antipodal cells and the lower
polar nucleus (Fig. 83 1-J). The lowest antipodal cell connects the
vesicular outgrowth with the body of the old megaspore mother
cell in which the three degenerated megaspore nuclei are sometimes
still distinguishable. The nuclei of the antipodal cells frequently
undergo a few divisions but the daughter nuclei fuse once again
to form a single lobed nucleus.

Chrysanthemum cinerariaefolium. Martinoli (1939) has dis-
covered a peculiar mode of development in this plant. The em-
bryo sac is tetrasporic and the megaspore nuclei take up a 1+2 + 1
arrangement so that there is 1 nucleus at each pole and 2 nuclei
lie in the middle (Fig. 84A-C). The two central nuclei become
separated from the terminal nuclei by vacuoles and may either
fuse to form a single diploid nucleus (Fig. 84#) or may merely re-
main close to one another without undergoing any fusion (Fig.
84D). The subsequent development differs, depending on which
of the two conditions is present.

In the first case the next division gives rise to 6 nuclei (a haploid
pair at either end and a diploid pair in the center) (Fig. 847) which
divide again to form three groups of 4 nuclei each (Fig. 84/). The
micropylar quartet now produces the egg apparatus and upper
polar nucleus, all haploid. The chalazal quartet gives rise to four
antipodal cells, also haploid. The central quartet is composed of
diploid nuclei; one of these functions as the lower polar nucleus and
the remaining 3 organize as additional antipodal cells (Fig. 84iv).
Sometimes less than 12 nuclei are formed (10 or 7), either because
of a failure of some divisions at the chalazal end or because the
central diploid nucleus of the 3 -nucleate stage undergoes only one
division instead of two.

In the second of the two previously mentioned alternatives, i.e.,
when the two central megaspore nuclei do not fuse but only lie in
contact with each other, neither undergoes any further divisions
and both function directly as the polar nuclei. Meanwhile the
micropylar megaspore nucleus divides twice, to give rise to the
micropylar quartet, but there is no regularity in the behavior of

THE FEMALE GAMETOPHYTE

131

the chalazal nucleus. The total number of nuclei in the mature
embryo sac may therefore be 10 or 9 or even as few as 6, depending
upon two divisions, or a single division, or a complete failure of
division, of this nucleus (Fig. 84E-G).

Fig. 84. Two modes of development of embryo sac of Chrysanthemum cinerariae-
folium. A-C, formation of the four megaspore nuclei. D-G, first type of develop-
ment, in which the two central megaspore nuclei remain undivided and function
directly as polar nuclei. H-K, second type of development, in which two central
megaspore nuclei fuse to form diploid nucleus which undergoes two divisions to
give rise to four nuclei; of these, one functions as polar nucleus and three form anti-
podal cells. For details, see text. (Adapted from Martinoli, 1939.)

ORGANIZATION OF MATURE EMBRYO SAC

Although the origin of the mature embryo sac may differ, its
eventual organization shows a surprisingly uniform pattern in the
majority of angiosperms. The Polygonum, Allium, Fritillaria, and
Adoxa types of embryo sacs all have a similar appearance at the
time of fertilization (three -celled egg apparatus, three antipodals,
and two polar nuclei). Even in the remaining types an egg ap-
paratus, at least, is almost always present and it is only in a few
genera like Peperomia, Plumbago, Plumbagella, and Acalypha indica
that we see a radical departure from the basic plan. Ignoring for

132 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the present the origin of the embryo sac (whether mono-, bi-, or
tetrasporic), we shall now confine our attention to the organiza-
tion of the mature stage only.

The Egg Apparatus. Typically

the egg apparatus is composed
of an egg and two synergids. As
a rule each of the synergids is
notched by an indentation result-
ing in the formation of a promi-
nent hook (Fig. 85). The upper
part of the cell is occupied by the
so-called "filiform apparatus"
which shows a number of stria-
tions converging towards the
apex. The nucleus lies in or just
below the region of the hook and
the lower part of the cell contains
a large vacuole (Dahlgren, 1928a,
1938). In the egg, on the other
hand, the nucleus and most of the
cytoplasm lie in the lower part of
the cell and the vacuole in the
upper. Hooks and indentations
are usually absent, having been
described only in Plumbagella,
Ditepalanthus, Hclosis, and a few
members of the Ulmaceae and
Urticaceae (Fagerlind, 1943).

Usually the synergids are eph-
emeral structures which degener-
ate and disappear soon after fer-
tilization or even before it. In
some cases, however, one or both
of them may persist for a time and

Fig. 85. Mature embryo sac of Oeno-
thera nutans, showing synergids with fili-
form apparatus and indentations. Note
that nucleus of synergids lies towards
upper end of cell, and vacuole towards
lower end. (After Ishikawa, 1918.)

show signs of considerable activ-
ity. In Allium unifolium and A. rotundum (Weber, 1929) this be-
havior is particularly pronounced, and one of the synergids begins
to degenerate only after the development of the embryo is well under
way. Nothoscordum (Stenar, 1932), Limnanthes (Fig. 86B) (Fager-

THE FEMALE GAMETOPHYTE

133

lind in 1939c), and Albuca (Eunus, 1950) are essentially similar; and
in some Cucurbitaceae (Fig. 86^4) both the synergids become large
and prominent and seem to play an important role in the nutrition of
the embryo sac.

Fig. 86. Modifications of synergids. A, Luffa acutangula, embryo sac, showing
extremely long synergids reaching down to level below middle of sac. {After
Kirkwood, 1905.) B, Limnanthes douglasii, embryo sac showing three-celled
embryo and persisting synergid. {After Fagerlind, 1939c.) C, Ursinia anthe-
moides, beak-shaped synergids protruding through micropyle. {After Dahlgren,
1924.)

In none of the plants cited above do the synergids extend beyond
the limits of the embryo sac wall. This condition has so far been
noted to a pronounced extent only in the Compositae. Dahlgren
(1924) found that in Ursinea (Fig. 86C) and Calendula the synergids
elongate so much that their tips project to a considerable distance
into the micropyle and outside it, sometimes reaching as far as the
funiculus.

Certain other reports of the occurrence of synergid haustoria,

134 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

as in Lathraea, Lobelia, and Angelonia, have, however, to be in-
terpreted differently, for there is now no doubt that the cells in
question are really endosperm derivatives. The confusion was
caused by the fact that the micropylar cells of the endosperm some-
times show an appearance identical with that of the synergids — a
vacuole lying in the lower part of the cell and the nucleus and cyto-
plasm in the tapering upper part (see Rosen, 1947). In Myriophyl-
lum (Stolt, 1928; Soueges, 1940) and Hypecoum (Soueges, 1943)
even suspensor cells are known to show a surprising resemblance
to synergids.

Antipodal Cells. Although usually short-lived, the antipodals
frequently show a considerable increase in size or number. In
some members of the Gentianaceae (Stolt, 1921) the three antipodal
cells divide to form about 10 to 12 cells (Fig. 87 D), and
in the Gramineae a still larger number of cells is produced (Fig.
87C). In Sasa paniculata (Yamaura, 1933), a member of the Bam-
busae, an many as 300 antipodal cells have been reported.

In several genera of the Rubiaceae, like Putoria (Fagerlind, 1936a)
and Galium (Fagerlind, 1937), the basal antipodal cell is often
greatly elongated and acts as an aggressive haustorium (Fig. 87
B, E). In Phyllis (Fagerlind, 19366) all three of the antipodal cells
are swollen; the basal becomes 8-nucleate and each of the upper
two becomes 4-nucleate (Fig. 87 G,H).

An increase in the number of antipodal cells and the number of
nuclei per antipodal cell is well known in the Compositae (Fig.
87 A, F). In Grindelia squarrosa, according to Howe (1926), only
two antipodal cells are formed, the one nearer the micropyle being
binucleate. One or both of these cells undergo further develop-
ment, growing laterally into the integument for a considerable dis-
tance. In Artemisia (Diettert, 1938) the number of antipodal cells
varies from three to six and each cell may have 2 or more nuclei.
The basal antipodal cell frequently elongates and penetrates
through the chalazal tissue, finally entering the ovarian chamber.
Rudbeckia bicolor (Maheshwari and Srinivasan, 1944), whose em-
bryo sac follows the Fritillaria type of development, has triploid
antipodal cells which attain a much larger size than the cells of the
egg apparatus (Fig. 88). The central antipodal cell, in particular,
persists for a long time, being recognizable even during embryonal
development.

THE FEMALE GAMETOPHYTE

135

Fig. 87. Embryo sacs showing abnormal behavior of antipodal cells. A, Ligu-
laria sibirica; embryo sac showing increase in number of antipodal cells, some of
which are binucleate. (After Afzelius, 1924-) B, Putoria calabrica, three embryo
sacs of which two are well organized; note extreme elongation of basal antipodal
cell. (After Fagerlind, 1936a.) C, Zea mays, embryo sac showing mass of antipodal
cells at lower end. (After Randolph, 1£83.) D, Gentiana campestris, embryo sac
showing increase in number of antipodal cells. (After Stolt, 1921.) E, Galium
moliugo, embryo sac showing elongation of basal antipodal cell. (After Fagerlind,
1987.) F, Aster novae-anglieae, several multinucleate antipodal cells, of which
basal has undergone considerable enlargement. (After Chamberlain, 1895.) G,
Phyllis nobla, l.s. ovule. H, embryo sac of same enlarged to show young embryo,
endosperm, and three haustorial antipodal cells. (After Fagerlind, 1936b.)

136 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

The antipodal cells of some members of the Ranunculaceae be-
come greatly enlarged and assume a glandular appearance (Fig.
89). Graft (1941) has shown that in Caltha palustris they attain a

high degree of polyploidy. At first each antipo-
dal cell becomes binucleate. The two nuclei
now divide again, but the spindles fuse dur-
ing this process so that there are again only
two nuclei which, however, possess the diploid
number of chromosomes. This process may be
repeated, leading to the formation of tetraploid
and even octoploid nuclei. It gives an indica-
tion of the high metabolic activity in these cells
and offers a close analogy with the behavior of
the anther tapetum.

Polar Nuclei. The central portion of the em-
bryo sac containing the polar nuclei eventually
gives rise to the endosperm and has therefore
been called the Endospermanlage or "endo-
sperm mother cell." Usually the two nuclei are
so similar to each other that once they have come
together it is difficult to distinguish the micro-
pylar from the chalazal. When there is a dif-
ference in size between the two, it is usually
the micropylar which is the larger. In embryo
sacs of the Fritillaria type, however, the chala-
zal polar nucleus is the larger (see page 118).

The fusion of the polar nuclei may occur either
before, or during, or sometimes after, the entry
of the pollen tube inside the embryo sac. The
secondary nucleus formed after fusion usually
lies just below the egg and is separated from
the antipodal cells by a large vacuole. In those
plants in which it lies near the center, it is con-
nected with the egg apparatus by a conspicuous
cytoplasmic strand. A chalazal position is less
frequent except in those plants which are characterized by a Helo-
bial type of endosperm (see page 245).

Embryo Sacs with Disturbed Polarity. Rarely, embryo sacs may
be found in which the usual polarity and organization are absent.

Fig. 88. Embryo sac
of Rudbeckia bicolor,
showing three large
antipodal cells which
are arranged like cells
of egg apparatus.
(After Maheshwari
and Srinivasan, 1944-)

THE FEMALE GAMETOPHYTE

137

Fig. 89. Embryo sac of Aconitum napellus showing three large antipodal cells.
Note young embryo at micropylar end and endosperm nuclei in various stages of
division. (After Osterwalder, 1898.)

138 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Sometimes one or all of the antipodal nuclei move up and function
as supernumerary polar nuclei; or the secondary nucleus fragments
to form a group of micronuclei of varying sizes. Supernumerary
egg cells and synergids have also been noted. Very rarely, the
embryo sac shows a reversed polarity, with the egg apparatus
differentiating at the chalazal end and the antipodals at the micro-
pylar. As examples may be cited Atamosco texana (Pace, 1913),
Fuchsia marinka (Tackholm, 1915), Lindelofia longiflora (Svens-
son, 1925), Saccharum officinarum (Dutt and Subba Rao, 1933;
Narayanaswami, 1940), Woodfordia floribunda (Joshi and Venka-
teswarlu, 1935), Eriodendron anfractuosum (Thirumalachar and
Khan, 1941), Heptapleurum venulosum (Gopinath, 1943), and
Crinum asiaticum (Swamy, 1946). In certain other plants, a nor-
mal egg apparatus is differentiated at the micropylar end, but two
of the antipodal cells also look like synergids and the third resembles
an egg, (Fig. 94A) so that the embryo sac apparently shows two
egg apparatuses, one at each end. Poa alpina (Hakansson, 1943)
sometimes shows the reverse condition, i.e., the occurrence of two
groups of antipodal cells, one at the micropylar end and the other
at the chalazal. Embryo sacs of the latter type are functionless,
however, and do not produce embryos.12

The embryo sacs of the Viscoideae (Fig. 65), some members of
the Balanophoraceae (Figs. 60, 61), and a few saprophytic genera of
the Gentianaceae (Fig. 90) also appear to be inverted. Oehler
(1927) has given the correct explanation when he says that the
ovules of Leiphaimos and Cotylanthera, although seemingly ortho-
tropous, are in fact anatropous, and that the inversion in the polar-
ity of the embryo sac is only apparent but not real.

Food Reserves in the Embryo Sac. It is usually taken for
granted that the angiosperm embryo sac is devoid of any appre-
ciable food reserves. While this is generally true, there are now
several records of the occurrence of starch in embryo sacs, and in
the families Aizoaceae, Cactaceae, Portulacaceae, Bruniaceae, Tilia-
ceae, Crassulaceae, and Asclepiadaceae this is a common phenome-
non. Dahlgren (1927, 1939) who has reviewed the subject in recent
years, states that the reason why starch grains have not been re-

12 A fertilization of antipodal cells seems to have been recorded only in Nigella
arvensis (Derschau, 1918), but it is quite likely that it also occurs sometimes in
Ulmus (Shattuck, 1905; Ekdahl, 1941).

THE FEMALE GAMETOPHYTE

139

ported more frequently in embryo sacs is that they are not very
distinct in the usual balsam mounts, and very few workers take the
trouble of removing the coverslip and testing the sections with
an iodine solution.

While reference must be made to Dahlgren's papers for fuller
information on the subject, a few noteworthy cases of the occur-
rence of starch grains in the embryo sac may be mentioned here.

Fig. 90. Development of ovule and embryo sac of Leiphaimos spectabilis.
Oehler, 1927.)

(After

In Arachis (Reed, 1924), Tilia (Stenar, 19256), Pentstemon (Evans,
1919), and Acacia (Newman, 1934) the embryo sacs are so full of
starch that it becomes difficult to study the nuclei inside them.
In Styphelia (Brough, 1924) starch grains are so abundant in the
vicinity of the egg that the latter is obscured by them. In Den-
drophthora (York, 1913) their crowding is said to cause a degenera-
tion and disappearance of the nuclei.

The stage at which the starch makes its appearance in the embryo
sac varies in different plants. In Loranthus pentandrus, Treub
(1883) saw starch grains even at the megaspore mother cell stage;

140 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

in Psychotria (Fagerlind, 1937) starch appears at the dyad cell
stage; in Castalia (Cook, 1902), Acacia (Guignard, 1881), Sedum
(D'Hubert, 1896), Pentas, Richardsonia, and Cephalantus (Fager-
lind, 1937) at the functioning megaspore stage; in Portulaca oleracea
(Cooper, 1940) at the binucleate stage; and in Corchorus trilocularis
(Stenar, 19256), Cynanchum acutum (Francini, 1927), and Medicago
saliva (Cooper, 19356) at the 4-nucleate stage. In the majority of

Fig. 91. Embryo sacs of Acacia baileyana, showing starch grains. A\,Ai, succes-
sive sections of unfertilized embryo sac. B, postfertilization stage. (After New-
man, 1934.)

plants, however, the starch appears when the embryo sac is mature
and reaches a maximum shortly after fertilization, gradually de-
creasing in postfertilization stages. Xyris indica (Weinzieher,
1914), Acacia baileyana (Newman, 1934) (Fig. 91), and Petunia
(Cooper, 1946) (Fig. 112) are peculiar in having large quantities
of starch even during endosperm formation.

A few cases are on record in which the starch occurs not merely
in the cavity of the embryo sac but also in the cells of the egg ap-

THE FEMALE GAMETOPHYTE

141

paratus and rarely even in the antipodal cells. The following are
some examples of the occurrence of starch grains in the egg: Astilbe
grandis (Dahlgren, 1930), Aspidistra elatior (Golaszewska, 1934),
Acacia baileyana (Newman, 1934), Medicago saliva (Cooper, 19356;
Cooper, Brink, and Albrecht, 1937), Korthalsella opuntia (Rutis-
hauser, 19376), Zea mays,
Euchlaena mexicana (Cooper,
1938), Portulaca oleracea
(Cooper, 1940), and Phryma
leptostachya (Cooper, 1941).
In Korthalsella (Rutishauser,
19376) starch is also found in
antipodal cells.

In Sonneratia (Venkates-
warlu, 1937; Mauritzon, 1939)
certain oily bodies of an un-
known nature persist from the
megaspore mother cell stage to
the formation of the mature
embryo sac, and in Aspidistra
(Fig. 92) (Golaszewska, 1934)
large raphides have been seen
in the mature stages. The
significance of these structures
in the economy of the embryo
sac has not been elucidated up
to this time.

Embryo Sac Haustoria. In
the majority of angiosperms
the entire surface of the em-
bryo sac serves an absorptive
function, demolishing the ad-
jacent cells of the nucellus and
even the inner layers of the
integument. In some plants,

however, more active growth

., , „ ,, Fig. 92. Embryo sac of Aspidistra elatior,

is seen at the ends of the sac. , • + , • , i WA

showing starch grams and a large raphide.

In Phaseolus (Fig. 93 A) (Wein- (After Golaszewska, 1984).

142

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

stein, 1926) and Melilotus, (Cooper, 1933) the embryo sac ruptures
the nucellar epidermis and grows beyond it, so that more than one-
third of it lies in direct contact with the cells lining the micropy-
lar canal. In Arechavaletaia (Ventura, 1937) and Kirengeshoma
(Mauritzon, 1939) it protrudes out of the endostome and comes to
lie in the exostome. In Philadelphus (Mauritzon, 1933), Thesium
(Schulle, 1933), Galium (Fagerlind, 1937), Utricularia (Kausik,

Fig. 93. Some instances of embryo sacs protruding into and beyond micropyle.
A, Phaseolus vulgaris, upward elongation of embryo sac, resulting in rupture of
nucellar tissue; nucellar epidermis is intact, however, at apex of embryo sac. (After
Weinstein, 1926.) B, Torenia hirsuta, embryo sac protruding out of micropyle.
(After Krishna Iyengar, 1941.) C, Philadelphus coronarius, embryo sac protruding
out of micropyle. (After Mauritzon, 1933.) D, Galium lucidum, one embryo sac
completely outside micropyle; another in position, but in process of degeneration.
(After Fagerlind, 1937.)

1938), and certain members of the Scrophulariaceae like Vandellia
and Torenia (Krishna Iyengar, 1940, 1941) the nucellus breaks
down completely at a rather early stage and the naked embryo sac
protrudes out of the ovule, establishing direct contact with the
placenta and digesting its way into the tissue of the latter (Fig.
93B-D). Strangest of all are some genera of the Loranthaceae,
like Scurrula and Dendrophthoe (Rauch, 1936; Singh, 1950), in which
ovules and integuments are absent in the usual sense and the embryo
sacs undergo a remarkable elongation toward both the top and the
bottom. At the lower end they are soon stopped by a pad of col-

THE FEMALE GAMETOPHYTE

143

lenchymatous cells, but the upper part continues to grow, sometimes
reaching a considerable distance into the style. Fertilization occurs
here by the incoming pollen tubes and the embryos are thrust down
again by the elongating suspensors. The observations of Schaeppi
and Steindl (1942), who have recently studied several other genera
of the family, show that in Macrosolen the upper end of the embryo

D E

Fig. 94. Formation of embryo sac caeca in Allium paniculatum (A-C), and Digera
arvensis (D, E). {A-C, after Modilewski, 192S; D-E, after Joshi, 1930.)

sac reaches up to the base of the style, in Elythranthe it grows beyond
its base, in Lepeostegeres it is at about one-fourth of the height of
the style, in Amyema somewhere near its middle region, in Taxillus,
slightly above the middle, and in HelixantheraUa just below the
papillate layer of the stigma.

In other plants it is a downward growth which is more striking.

Ua See also Johri and Maheshwari (1950).

144 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

In several members of the Centrospermales (Oksijuk, 1927; Art-
schwager and Starrett, 1933; Joshi, 1936; Cooper, 1949) the em-
bryo sac pushes forward at the chalazal end and digests its way
through the nucellus, while the antipodal cells remain in situ and
are left behind in a lateral position (Fig. 94E). This "caecum,"
which is also known in Allium (Modilewski, 1928) (Fig. 94A-C),
Elegia (Borwein, et al., 1949), Macrosolen (Maheshwari and Singh,
1950), and certain other plants (see Finn, 1936), seems to be a very
effective haustorial organ.

In Veltheimia (Stiffler, 1925; Buchner, 1948), Paradisia (Stenar,
1928), and Eucomis (Buchner, 1948) the haustorium arises laterally
rather than from the pole of the embryo sac. As a rule, however,
such a condition is met with only in postfertilization stages and will
therefore be considered in the chapter on the endosperm.

References

Afzelius, K. 1916. Zur Embryosackentwicklung der Orchideen. Svensk Bot.

Tidskr. 10: 183-227.
. 1924. Embryologische und zytologische Studien in Senecio und ver-

wandten Gattungen. Acta Horti Bergiani 8(7): 123-219.
Arnoldi, W. 1912. Zur Embryologie einiger Euphorbiaeeen. Trav. Mus. Bot.

Acad. St. Petersburg 9: 136-154.
Artschwager, E., and Starrett, R. C. 1933. The time factor in fertilization and

embryo development in the sugar beet. Jour. Agr. Res. 47: 823-847.
Bambacioni, V. 1928a. Ricerche sulla ecologia e sulla embriologia di Fritillaria

persica L. Ann. di Bot. 18: 7-37.
. 19286. Contribute alia embriologia di Lilium candidum L. R. C. Accad.

Naz. Lincei 8: 612-618.
Bambacioni-Mezzetti,V. 1931. Nouve ricerche suH'embriologia delle Gigliaceae.

Ann. di Bot. 19:365-382.
Bergman, B. 1935. Zytologische Studien tiber die Fortpflanzung bei den Gat-
tungen Leontodon und Picris. Svensk Bot. Tidskr. 29: 155-301.
Borwein, B., Coetsee, M. L., and Krupko, S. 1949. Development of the embryo

sac of Restio dodii and Elegia racemosa. Jour. South AM. Bot. 15: 1-11.
Boyes, J. W. 1939. Development of the embryo sac of Plumbagella micrantha.

Amer. Jour. Bot. 26: 539-547.
Brough, P. 1924. Studies in the Epacridaceae. I. The life history of Styphelia

longifolia (R.Br.). Proc. Linn. Soc. N. S. Wales 49: 162-178.
Brown, W. H., and Sharp, L. W. 1911. The embryo sac of Epipactis. Bot.

Gaz. 52: 439-452.
Buchner, L. 1949. Vergleichende embryologische Studien an Scilloideae. Osterr.

bot. Zeitschr. 95: 428-450.

THE FEMALE GAMETOPHYTE 145

Campbell, D. H. 1899a. Die Entwicklung des Embryosackes von Peperomia

pellucida. Ber. deutsch. bot. Gesell. 17: 452-456.
. 18996. A peculiar embryo sac in Peperomia pellucida. Ann. Bot. 13:

626.

1901. The embryo sac of Peperomia. Ann. Bot. 15: 103-117.

Carlson, M. C. 1945. Megasporogenesis and development of the embryo sac

of Cypripedium parviflorum. Bot. Gaz. 107: 107-114.
Chamberlain, C. J. 1895. Contributions to the embryology of Aster novae-

angliae. Bot. Gaz. 20: 205-212.
Chiarugi, A. 1933. Lo sviluppo del gametofito femmineo della Weddelina squam-

ulosa Tul. (Podostemonaceae) . R. C. Accad. Naz. Lincei 17: 1095-1100.
Cook, M. T. 1902. Development of the embryo sac and embryo of Castalia

odorata and Numphaea advena. Bui. Torrey Bot. Club 29: 211-220.
Cooper, D. C. 1933. Macrosporogenesis and embryology of Melilotus. Bot.

Gaz. 95: 143-155.
. 1935a. Macrosporogenesis and development of the embryo sac of

Lilium henryi. Bot. Gaz. 97: 364-355.
. 19356. Macrosporogenesis and embryology of Medicago. Jour. Agr.

Res. 51:471-477.
. 1938. Macrosporogenesis and embryo sac development in Euchlaena

mexicana and Zea mays. Jour. Agr. Res. 55: 539-551.
. 1940. Macrosporogenesis and embryo ogy of Portulaca oleracea. Amer.

Jour. Bot. 27: 326-330.
. 1941. Macrosporogenesis and the development of the seed of Phryma

leptostachya. Amer. Jour. Bot. 28: 755-761.

. 1946. Double fertilization in Petunia. Amer. Jour. Bot. 33: 53-57.

. 1949. Flower and seed development in Oxybaphus nyctagineus. Amer.

Jour. Bot. 36: 348-355.
-, Brink, R. A., and Albrecht, H. R. 1937. Embryo mortality in relation

to seed formation in alfalfa {Medicago sativa). Amer. Jour. Bot. 24: 203-213.
Dahlgren, K. V. O. 1924. Studien iiber die Endospermbildung der Kompositen.

Svensk Bot. Tidskr. 18: 177-203.
. 1927. tJber das Vorkommen von Starke in den Embryosacken der

Angiospermen. Ber deutsch. bot. Gesell. 45: 374-384.
. 1928a. Hakenformige Leistenbildungen bei Synergiden. Ber. deutsch.

bot. Gesell. 46: 434-443.
. 19286. Die Embryologie einiger Alismataceae. Svensk Bot. Tidskr.

22: 1-17.
. 1930. Zur Embryologie der Saxifragagoideen. Svensk Bot. Tidskr.

24: 429-448.
. 1934. Die Embryosackentwicklung von Echinodorus macrophyllus und

Sagittaria sagittifolia. Planta 21: 602-612.
. 1937. Die Entwicklung des Embryosackes bei Plumbago zeylanica.

Bot. Notiser 1937, pp. 487-498.

146 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Dahlgren, K.V.O. 193S. Hakenbildungen bei Synergiden. Zweite Mitteilung.

Svensk Bot. Tidskr. 52: 221-237.
. 1939. Sur la presence d'amidon dans h sac embryonnaire chez bs

angiospermes. Bot. Notiser 1939, pp. 437-49S.
D'Amato, F. 1940a. Embriologia di Ulmus campestris L. Nuovo Gior. Bot.

Ital. N.S. 47: 247-233.
. 19406. Contributo all'embriologia delle Plumbaginaceae. Nuovo Gior.

Bot. Ital. N.S. 47:349-382.
Derschau, M. v. 1918. Uber disperme Befruchtung der Antipoden von Nigella

arvensis. Ber. deutsch. bot. Gesell. 36: 260-262.
D 'Hubert, E. 1896. Recherches sur le sac embryonnaire des plantes grasses.

Ann. des Sci. Nat., Bot. 2: 37-128.
Diettert, R. A. 1938. The morphology of Artemisia tridentata Nutt. Lloydia

1 : 3-74.
Dodds, K. S. 1945. Genetical and cytological studies of Musa. VI. The develop-
ment of female cells of certain edible diploids. Jour. Genet. 46: 161-179.
Dutt, N. L., and Subba Rao, K. S. 1933. Observations on the cytology of the

sugarcane. Indian Jour. Agr. Sci. 3 : 37-56.
Ekdahl, I. 1941. Die Entwicklung von Embryosack und Embryo bei Ulmus

glabra Huds. Svensk Bot. Tidskr. 35: 143-156.
Ernst, A. 1903. Ergebnisse neurer Untersuchungen uber den Embryosack der

Angiospermen. Verh. schvveiz. naturf. Ges. 91, Jahresv. I: 230-263.
Eunus, A. M. 1950. Contributions to the embryology of the Liliaceae. X.

Albuca transvalensis Moss-Verdoorn. Jour. Indian Bot. Soc. 29: 68-78.
Evans, A. T. 1919. Embryo sac and embryo of Pentstemon secundiflorus. Bot.

Gaz. 67: 426-437.
Eysel, G. 1937. "Die Embryosackentwicklung von Limnanthes douglasii R.

Br." Diss. Marburg.
Fagerlind, F. 1936a. Die Embryologie von Putoria. Svensk Bot. Tidskr. 30:

362-372.
. 19366. Embryologische Beobachtungen liber die Gattung Phyllis. Bot.

Notiser 1936, pp. 577-584.
. 1937. Embryologische, zytologische und bestaubungsexperimentelle

Studien in der Familie Rubiaceae nebst Bemerkungen uber einige Polyploidi-

tatsprobleme. Acta Horti Bergiani 11: 195-470.
. 1938a. Wo kommen tetrasporische durch drei Teilungsschritte vollen-

entwickelte Embryosacke unter den Angiospermen vor? Bot. Notiser 1938,

pp. 461-498.
. 19386. Embryosack von Plumbagella und Plumbago. Arkiv. for Bot.

29B: 1-8.
. 1939a. Die Entwicklung des Embryosackes bei Peperomia pellucida.

Arkiv. for Bot. 29A: 1-15.
-. 19396. Drei Beispiele des Fritillaria-Typus. Svensk Bot. Tidskr. 33:

188-204.

THE FEMALE GAMETOPHYTE 147

Fagerlind, F. 1939c. Kritische und revidierende Untersuchungen uber das

Vorkommen des Adoxa ("Lilium")-Typus. Acta Horti Bergiani 13: 1-49.
. 1940. Stempelbau und Embryosackentwicklung bei einigen Pandana-

zeen. Ann. Jard. Bot. Buitenzorg 49 : 55-78.
, 1941. Die Embryosackentwicklung bei Tanacetum vulgare und einigen

Chrysanthemum-Arten. Svensk Bot. Tidskr. 35: 157-176.
. 1943. Vorkommen und Entstehung von Hakenleisten an Synergiden

und Eizellen. Svensk Bot. Tidskr. 37: 339-351.
. 1944. Der tetrasporische Angiospermen-Embryosack und dessen Bedeu-

tung fur das Verhaltnis der Entwicklungsmechanik und Phylogenie des

Embryosacks. Arkiv. for Bot. 31A: 1-71.
.. 1945a. Bildung und Entwicklung des Embryosacks bei sexuellen und

agamospermischen Balanophora-Arten. Svensk Bot. Tidskr. 39: 65-82.

19456. Bau der floralen Organe bei der Gattung Langsdorffia. Svensk

Bot. Tidskr. 39: 197-210.
Finn, W. W. 1936. Zur Entwicklungsgeschichte der chalazogamen Ostrya carpi-

nifolia Scop. Jour. Inst. Bot. Acad. Sci. Ukraine 8: 15-25.
Francini, D. E. 1927. L'embriologia del Cynanchum acutum L. Nuovo Gior.

Bot.' Ital. N.S. 34: 381-395.
. 1931. Ricercheembriologicb.ee cariologichesulgenere Cypripedium. Nu-
ovo Gior. Bot. Ital. N.S. 38: 155-212.
Frisendahl, A. 1927. tJber die Entwicklung chasmogamer und kleistogamer

Bluten bei der Gattung Elatine. Acta Hort. Gothoburg. 3 : 99-142.
Geerts, J. M. 1908. Beitrage zur Kenntnis der cytologischen Entwicklung von

Oenothera lamarckiana. Ber. deutsch. bot. Gesell. 26A: 608-614.
Gerassimova, H. 1933. Fertilisation in Crepis capillaris. Cellule 42: 103-148.
Golaszewska, Z. 1934. Die Entwicklung des Embryosackes bei Aspidistra ela-

tior. Acta Soc. Bot. Poloniae 11: 399-407.
Goodspeed, T. H. 1947. Maturation of the gametes and fertilization in Nico-

tiana. Madrono 9: 110-120.
Gopinath, D. M. 1943. Reversed polarity in the embryo sac of Heptapleurum

venulosum Seem. Current Sci. [India] 12 : 58.
Grafl, I. 1941. Uber das Wachstum der Antipodenkerne von Caltha palustris.

Chromosoma 2: 1-11.
Guignard, L. 1881. Recherches d'embryogenie vegetale compared. I. Legumi-

neuses. Ann. des Sci. Nat., Bot. 12: 5-166.
Hakansson, A. 1923. Studien iiber die Entwicklungsgeschichte der Umbellife-

ren. Acta Univ. Lund N.F. Avd. II, 18(7) : 1-120.
. 1943. Die Entwicklung des Embryosacks und die Bsfruchtung bei

Poa alpina. Hereditas 29: 25-61.
Hammond, B. L. 1937. Development of Podostemon ceratophyUum. Bui. Tor-

rey Bot. Club 64: 17-36.
Haque, A. 1950. The embryo sac of Erythronium amervanum. (In press.)
Harling, G. 1946. Studien uber den Blutenbau und die Embryologie der Familie

Cyclanthaceae. Svensk Bot. Tidskr. 40: 257-272.

148 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Haupt, A. W. 1934. Ovule and embryo sac of Plumbago capensis. Bot. Gaz.

95:649-659.
Hoare, G. 1934. Gametogenesis and fertilization in Scilla nonscripta. Cellule

42 : 269-292.
Hoeppener, E., and Renner, 0. 1929. Genetische und zytologische Oenotheren-

studien. II. Bot. Abh. Goebel 15: 1-66.
Hofmeister, W. 1847. Untersuchungen des Vorgangs bei der Befruchtung der

Oenotheren. Bot. Ztg. 5 : 785-792.

. 1849. "Die Entstehung des Embryo der Phanerogamen." Leipzig.

Howe, T. D. 1926. Development of embryo sac in Grindelia squarrosa. Bot.

Gaz. 81: 280-296.
Ishikawa, M. 1918. Studies on the embryo sac and fertilisation in Oenothera.

Ann. Bot. 32: 279-317.
Jonsson, B. 1879-80. Om embryosackens utveckling hos angiospermerna.

Lunds Univ. Arskr. 16 : 1-86.
Johansen, D. A. 1929. Studies on the morphology of the Onagraceae. I. The

megagametophyte of Hartmannia tetraptera. Bui. Torrey Bot. Club 56:

285-298.
. 1931a. Studies on the morphology of the Onagraceae. II. Anogra

pallida. Amer. Jour. Bot. 18: 854-863.

19316. Studies on the morphology of the Onagraceae. V. Zauschneria

latifolia, typical of a genus characterised by irregular embryology. Ann. N. Y.

Acad. Sci. 33: 1-26.
Johnson, D. S. 1900. On the endosperm and embryo of Peperomia pellucida.

Bot. Gaz. 80: 1-11.
. 1914. Studies in the development of the Piperaceae. II. The structure

and seed development of Peperomia hispidula. Amer. Jour. Bot. 1: 323-

339, 357-397.
Johri, B. M. 1935a. Studies in the family Alismaceae. I. Limnophyton obtusi-

folium Miq. Jour. Indian Bot. Soc. 14: 49-66.
. 19356. Studies in the family Alismaceae. II. Sagittaria sagittifolia L.

Proc. Indian Acad. Sci. Sect. B. 1: 340-348.
. 1935c. Studies in the family Alismaceae. III. Sagittaria guayanensis

H.B.K. and S. latifolia Willd. Proc. Indian Acad. Sci. Sect. B. 2: 33-48.
. 1936a. Studies in the family Alismaceae. IV. Alisma plantago L.;

A. plantago-aquatica L., and Sagittaria graminea Mich. Proc. Indian Acad.

Sci. Sect. B. 4: 128-138.
. 19366. The life history of Butomopsis lanceolata Kunth. Proc. Indian

Acad. Sci. Sect. B. 4: 139-162.

— . 1938a. The embryo sac of Limnocharis emarginata L. New Phytol.

37:279-285.

19386. The embryo sac of Hydrocleis nymphoides Buchen. Beihefte

bot. Centbl. 48A: 165-172.

Johri, B. M. and Maheshwari, P. 1950. The development of the embryo sac,

embryo, and endosperm in Helixanthera ligustrina (Wall.) Dans. (In press.)

THE FEMALE GAMETOPHYTE 149

Jones, H. A., and Emsweller, S. L. 1936. Development of the flower and macro-

gametophyte of Allium cepa. Hilgardia 10: 415-454.
Joshi, A. C. 1936. A note on the antipodals of Digera arvensis Forsk. Current

Sci. [India] 4: 741-742.
and Venkateswarlu, J. 1935. A case of reversed polarity in the embryo

sac. Ann. Bot. 49:841-843.
Juliano, J. B. 1934. Studies on the morphology of the Meliaceae. II. Sterility

in santol, Sandoricum kootjape (Burm. F.) Merill. Philippine Jour. Agr. 23:

253-266.
Kausik, S. B. 1938. Pollen development and seed formation in Utricularia

coerulea L. Beihefte bot. Centbl. 58A: 365-578.
Khan, R. 1942. A contribution to the embryology of Jussieua repens Linn.

Jour. Indian Bot. Soc. 21: 267-2S2.
Kirkwood, J. E. 1905. The comparative embryology of the Cucurbitaceae.

Bui. N. Y. Bot. Gard. 3: 313-402.
Krishna Iyengar, C. V. 1940. Development of embryo sac and endosperm-

haustoria in some members of Scrophularineae. IV. Vandellia hirsuta Ham.

and V. scabra Benth. Jour. Indian Bot. Soc. 18: 179-189.
. 1941. Development of the embryo sac and endosperm haustoria in

Torenia cordifolia Roxb. and T. hirsuta Benth. Proc. Natl. Inst. Sci. India

7:61-71.
Lagerberg, T. 1909. Studien iiber die Entwicklungsgeschichte und systematische

Stellung von Adoxa moschatellina L. Svenska Vet.-Akad. Handl. 44(4) : 1-86.
Madge, M. 1934. Nuclear migrations in Hcdychium. Proc. Linn. Soc. Lon-
don 146: 108-109.
Magnus, W. 1913. Die atypische Embryosackentwicklung der Podostomaceen.

Flora 105: 275-336.
Maheshwari, P. 1937. A critical review of the types of embryo sacs in angio-

sperms. New Phytol. 36: 359-417.
. 1941. Recent work on the types of embryo sacs in angiosperms — a

critical review. Jour. Indian Bot. Soc. 20: 229-261.
. 1946a. The Fritillaria type of embryo sac: a critical review. Jour.

Indian Bot. Soc, M. O. P. Iyengar Comm. Vol., pp. 101-119.
. 19466. The Adoxa type of embryo sac — a critical review. Lloydia 9:

73-113.

. 1947. Tetranucleate embryo sacs in angiosperms. Lloydia 10: 1-18.

. 1948. The angiosperm embryo sac. Bot. Rev. 14: 1-56.

and Haque, A. 1949. The embryo sac of Chrysanthemum parthenium L.

(Bernh). New Phytol. 48: 255-258.

and Johri, B. M. 1941. The embryo sac of Acalypha indica L. Beihefte

bot. Centbl. 61A: 125-136.
Maheshwari, P. and Singh, Bahadur. 1950. The embryology of Macrosolen

cochinchinensis Van Tieghem. (In press.)
and Singh, Balwant. 1943. Studies in the family Alismaceae. V. The

embryology of Machaerocarpus californicus (Torr.) Small. Proc. Natl. Inst.

Sci. India 9: 311-322.

150 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Maheshwari and Srinivasan, A. R. 1944. A contribution to the embryology of

Rudbeckia bicolor Nutt. New Phytol. 43: 135-142.
Martinoli, G. 1939. Contribute all'embriologia dalle Asteracaae. I— III. Nu-

ovo Gior. Bot. Ital. N.S. 45: 259-298.
Mason, C. T. 1949. Development of the embryo sac in the genus Limnanthes.

Amer. Jour. Bot. 36: 799.
Mathur, K. L., and Khan, R. 1941. The development of the embryo sac in

Vogelia indica Lamk. Proc. Indian Acad. Sci. Sect. B. 13: 330-388.
Mauritzon, J. 1933. "Studien liber die Embryologie der Familien Crassulaceae

und Saxifragaceae." Diss. Lund.
. 1939. Contributions to the embryology of the orders Rosales and

Myrtales. Lunds Univ. Arsskr. N.F. Avd. II, 35(2) : 1-120.
Messeri, A. 1931. Ricerche embriologiche e cariologiche sopra i gsnri Allium

e Nothoscordum. Nuovo Gior. Bot. Ital. N.S. 33: 409-441.
Modilewski, J. 1908. Zur Embryobildung von Gunnera chilensis. Ber. deutsch.

bot. Gesell. 26A: 550-556.
. 1909. Zur Embryobildung von einiger Onagraceen. Ber. deutsch. bot.

Gesell. 27: 287-291.
. 1910. Weitere Beitrage zur Embryobildung einiger Euphorbiaceen.

Ber. deutsch. bot. Gesell. 28: 414-418.
. 1911. Uber die abnormale Embryosackentwicklung bai Euphorbia p%-

lustris L. und anderen Euphorbiaceen. Bar. deutsch. bot. Gasall. 23: 433-436.
. 1918. [Cytological and embryological studies on Neollia nidus aris (L.)

Rich.] Verh. Kiewer Ges. Naturf. 23 : 1-55.

1928. Weitere Beitrage zur Embryologie und Cytologie von Allium-

Arten. Bui. Jard. Bot. de Kieff 7/8: 57-64.
Narayanaswami, S. 1940. Megasporogenesis and the origin of triploids in Szc-

charum. Indian Jour. Agr. Sci. 10: 531-551.
Newman, I. V. 1934. Studies in the Australian acacias. IV. The life history

of Acacia baileyana F.v.M. Proc. Linn. Soc. N. S. Wales 53: 277-313.
Oehler, E. 1927. Entwicklungsgeschichtlich-zytologische Untersuchungen an ei-

nigen saprophytischen Gentianaceen. Planta 3: 611-733.
Oksijuk, P. 1927. Entwicklungsgeschichte der Zuckerriibe (Beta vulgaris). Bui.

Jard. Bot. de Kieff 5/6: 145-164.
. 1937. Vergleichende zytologisch-embryologische Untersuchung dar Fa-

milie Resedaceae. I. Reseda und Astrocarpus. Jour. Inst. Bot. Acad. Sci.

Ukraine 12: 3-81.
Osterwalder, A. 1898. Beitrage zur Embryologie von Aconitum nipellus L.

Flora 85: 254-292.
Pace, L. 1907. Fertilization in Cypripedium. Bot. Gaz. 41: 353-374.

. 1913. Apogamy in Atamosco. Bot. Gaz. 53: 376-334.

Palm, B. 1915. "Studien liber Konstruktionstypen und Entwicklungswege des

Embryosackes der Angiospermen." Diss. Stockholm.
Prosina, M. N. 1930. Uber die vom Cypripedilum-Typus abweichende Embryo-
sackentwicklung von Cypripedium guttatum SW. Planta 12: 532-544.

THE FEMALE GAMETOPHYTE 151

Puri, V. 1939. Studies in the order Parietales. II. A contribution to the morph-
ology of Garcinia livingstonii T. Anders. Proc. Indian Acad. Sci. Sect. B. 9:

74-86.
. 1941. Life history of Moringa oleifera Lamk. Jour. Indian Bot. Soc.

20: 263-284.
Randolph, L. F. 1936. Developmental morphology of the caryopsis in Maize.

Jour. Agr. Res. 53: 881-916.
Rauch, K. V. 1936. Cytologisch-embryologische Untersuchungen an Scumda

atropurpurea Dans, und Dendrophthoe pentandra Miq. Ber. schweiz. bot.

Gesell. 45:5-61.
Razi, B. A. 1949. Embryological studies of two members of the Podostomaeeae.

Bot. Gaz. 111:211-218.
Reed, E. L. 1924. Anatomy, embryology and ecology of Arachis hypogea. Bot.

Gaz. 78: 289-310.
Romanov, I. D. 1936. Die Embryosackentwicklung in der Gattung Gagea

Salisb. Planta 25: 438-459.
. 1938. Eine neue Form des Embryosackes von Adoxa-Typus bei Tulipa

tetraphylla und T. ostrovskiana. Compt. Rend. (Dok.) Acad, des Sci. U.R.S.S.

19: 113-115.
. 1939. Two new forms of embryo sac in the genus Tidipa. Compt.

Rend. (Dok.) Acad, des Sci. U.R.S.S. 22: 139-141.

Ros6n, W. 1947. The female gametophyte in Nolana and endosperm develop-
ment in Tubiflorae. Bot. Notiser 1947, pp. 372-382.

Rutishauser, A. 1935. Entwicklungsgeschichtliche und zytologische Untersuch-
ungen an Korthalsella dacrydii (Ridl.) Danser. Ber. schweiz. bot. Gesell. 44:
389-436.

. 1937a. Entwicklungsgeschichtliche Untersuchungen an Thesium rostra-

tum M.u.K. Mitt. Naturf. Gesell. Schaffhausen 13: 25-47.

19376. Blutenmorphologische und cytologische Untersuchungen an den
Viscoideen Korthalsella opuntia Morr. und Ginalloa linearis Dans. Ber.
schweiz. bot. Gesell. 47: 5-28.

1945. Zur Embryologie amphimiktischer Potentillen. Ber. schweiz.

bot. Gesell. 55: 19-32.
Samuels, J. A. 1912. Etudes sur le deVeloppement du sac embryonnaire et sur

la f^condation du Gunnera macrophylla Bl. Arch. f. Zellforsch. 8: 52-120.
Schaeppi, H., and Steindl, F. 1942. Blutenmorphologische und embryologische

Untersuchungen an Loranthoideen. Vrtljschr. naturf. Gesell. Zurich 87:

301-372.
and . 1945. Blutenmorphologische und embryologische Unter-
suchungen an einigen Viscoideen. Vrtljschr. naturf. Gesell. Zurich 99: 1-46.
Schulle, H. 1933. Zur Entwicklungsgeschichte von Thesium montanum Ehrh.

Flora 27: 140-184.
Sharp, L. W. 1912. The orchid embryo sac. Bot. Gaz. 51: 373-3S5.
Shattuck, C. H. 1905. A morphological study of Ulmus americana. Bot. Gaz.

49:209-223.

152 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Singh, Bahadur. 1950. The embryology of Dendrophthoe fzlcata (Linn, fil.)

Ettingshausen. Proc. 37th Indian Sci. Cong. Sect. Bot.
Smith, R. W. 1911. The tetranucleate embryo sac of Clintonia. Bot. Gaz.

52:209-217.
Smith, F. H. 1943. Megagametophyte of Clintonia. Bot. Gaz. 105: 233-257.
Soueges, E. C. R. 1940. Embryog6nie des Haloragacees. Developpement de

l'embryon chez le Myriophyllum aUerniflorum DC. Compt. Rend. Acad, des

Sci. Paris 211: 185-188.
■ . 1943. Embryogenie des Fumariac6es. L'origine et les premieres divi-
sions de la cellule embryonnaire proprement dite chez YHypecoum procumbens

L. Compt. Rend. Acad, des Sci. Paris 216: 310-311.
Steindl, F. 1935. Pollen- und Embryosackentvvicklung bei Viscum album L. und

V. articulatum Burm. Ber. schweiz. bot. Gesell. 44: 343-388.
Stenar, H. 1925a. Embryologische und zytologische Studien liber Limnanthes

douglassi R.Br. Svensk Bot. Tidskr. 19 : 133-152.
. 19256. "Embryologische Studien. I. Zur Embryologie der Columni-

feren. II. Die Embryologie der Amaryllideen." Diss. Upsala.
. 1928. Zur Embryologie der Asphodeline-Gruppe. Ein Beitrag zur sys-

tematischen Stellung der Gattungen Bulbine und Paradisia. Svensk Bot.

Tidskr. 22: 145-159.
. 1932. Studien liber die Entwicklungsgeschichte von N othoscordum frag-

rans Kunth u. N. striatum Kunth. Svensk Bot. Tidskr. 26: 25-44.
. 1934. Embryologische und zytologische Beobachtungen liber Majan-

themum bifolium und Smilacina stellata. Arkiv. for Bot. 26: 1-20.
. 1937. Zur Embryosackentvvicklung einiger Malpighiazeen. Bot. Noti-

ser 1937, pp. 110-118.
. 1940. Biologiska och embryologiska notiser rorande Calypso bulbosa

(L) Oakes. Jamten Heimbygdas Forlag, Ostersund 1940, pp. 184-189.

1941. tlber die Entwicklung des Embryosackes bei Convallaria majalis

L. Bot. Notiser 1941, pp. 123-128.
Stephens, E. L. 1909. The embryo sac and embryo of certain Penaeaceae. Ann.

Bot. 23:363-378.
Stiffler, E. G. 1925. Development of embryo sac in Gasteria, Cyrtanthus and

Veltheimia. Bot. Gaz. 79: 207-216.
Stolt, K. A. H. 1921. Zur Embryologie der Gentianaceen und Menyanthaceen.

K. Svenska Vet.-Akad. Handl. 61(14): 1-56.
■ . 1928. Die Embryologie von Myriophyllum aUerniflorum DC. Svensk

Bot. Tidskr. 22: 305-319.
Strasburger, E. 1879. "Die Angiospermen und die Gymnospermen." Jena.
Subba Rao, A. M. 1940. Studies in the Malpighiaceae. I. Embryo sac develop-
ment and embryogeny in the genera Hiptage, Banisteria and Stigmatophyl-

lum. Jour. Indian Bot. Soc. 18: 145-156.
. 1941. Studies in the Malpighiaceae. II. Structure and development

of the ovules and embryo sacs of Malpighia coccigera Linn, and Tristellateia

australis. Proc. Natl. Inst. Sci. India 7: 393-404.
Svensson, H. G. 1925. "Zur Embryologie der Hydrophyllaceen, Borraginaceen

und Heliotropiaceen." Diss. Upsala

THE FEMALE GAMETOPHYTE 153

Swamy, B. G. L. 1945. Embryo sac and fertilization in Cypripedium spectabile.
Bot. Gaz. 107: 291-295.

. 1946. Inverted polarity of the embryo sac of angiosperms and its rela-
tion to the archegonium theory. Ann. Bot. 9: 171-183.

. 1949a. Embryological studies in the Orchidaceae. I. Gametophytes.

Amer. Midland Nat. 41: 184-201.

. 19496. A reinvestigation of the embryo sac of Maianthemum canadense.

Bui. Torrey Bot. Club 76: 17-23.
Tackholm, G. 1915. Beobachtungen liber die Samenentwicklung einiger Onagra-

ceen. Svensk Bot. Tidskr. 9 : 294-361 .
Tateishi, S. 1927. On the development of the embryo sac and fertilization of

Acalypha australis L. (preliminary note). Bot. Mag. [Tokyo] 51: 477-485.
Thirumalachar, M. J., and Khan, B. A. 1941. Megasporogenesis and endosperm

formation in Eriodendron anfractuosum DC. Proc. Indian Acad. Sci. Sect. B.

14:461-465.
Treub, M. 1883. Observations sur les Loranthacees. IV. Loranthus pentan-

drus L. Ann. Jard. Bot. Buitenzorg 3 : 184-189.
and Mellink, J. 1880. Notice sur le deVeloppement du sac embryonnaire

dans quelques angiospermes. Arch. N6erld. des Sci. 15: 452-457.
Venkateswarlu, J. 1937. A contribution to the embryology of the Sonneratia-

ceae. Proc. Indian Acad. Sci. Sect. B. 5: 206-223.
Ventura, M. 1934. Sulla poliembryonia di Mallotus japonicus Muell. Arg.

Ann. di Bot. 20: 568-578.
. 1937. Osservazioni embryologiche su Arechavalelaia uruguayensis Speg.

Ann. diBot. 21:527-533.
Walker, R. I. 1944. Chromosome number, megasporogenesis and development

of embryo sac of Clintonia. Bui. Torrey Bot. Club 71 : 529-535.
Walker, R. I. 1950. Megasporogenesis and development of megagametophyte

in Ulmus. Amer. Jour. Bot. 37: 47-52.
Weber, E. 1929. Entwicklungsgeschichtliche Untersuchungen liber die Gattung

Allium. Bot. Arch. 25: 1-44.
Weinstein, A. J. 1926. Cytological studies on Phaseolus vulgaris. Amer. Jour.

Bot. 13:248-263.
Weinzieher, S. 1914. Beitrage zur Entwicklungsgeschichte von Xyris indica

L. Flora 106: 393-432.
Went, F. A. F. C. 1910. Untersuchungen liber Podostomaceen. I. Verhandel.

K. Akad. van Wetensch. te Amsterdam II, 16(1).
. 1912. Untersuchungen liber Podostomaceen. II. Verhandel. K. Akad.

van Wetensch. te Amsterdam II, 17(2).

1926. Untersuchungen liber Podostomaceen. III. Verhandel. K. Akad.

van Wetensch. te Amsterdam II, 25(1).
Yamaura, A. 1933. Karyologische und embryologische Studien liber einige

Bambusa -Arten. (Vorlaufige Mitteilung). Bot. Mag. [Tokyo] 47: 551-555.
York, H. H. 1913. The origin and development of the embryo sac and embryo

of Dendrophthora opuntioides and D. gracile. I and II. Bot. Gaz. 56: 89-111,

200-216.

CHAPTER 5
THE MALE GAMETOPHYTE1

The development of the male gametophyte is remarkably uniform
in angiosperms. The microspore, which is the first cell of the
gametophyte generation, undergoes only two divisions. The first
division gives rise to a large vegetative cell and a small generative
cell (Fig. 95 A-F). The second, which concerns only the generative
cell, may take place either in the pollen grain (Fig. 95G-H) or in the
pollen tube (Fig. 95/-/) and gives rise to the two male gametes.
Details of the process may be considered under the following heads :
microspore, formation of the vegetative and generative cells, divi-
sion of the generative cell, male "cells" or "nuclei," and vegetative
nucleus.

Microspore. The newly formed microspore has a very dense
cytoplasm with a centrally situated nucleus, but the cell rapidly
increases in volume and the accompanying vacuolation is followed
by a displacement of the nucleus from the center to a place adjacent
to the wall. In most tropical plants the nucleus begins to divide
almost immediately, but in plants belonging to colder regions
there is often a resting stage lasting from a few days to several weeks.
To mention a few instances, in Tradescantia reflexa the resting
period of the microspore is about four days or less, in Styrax obassia
about a week, and in Himantoglossum hircinum between two to three
weeks. In Uvularia sessilifolia, Empetrum nigrum, and Betula
odorata, the microspores are said to pass the entire winter in the
uninucleate stage (for further information, see Dahlgren, 1915;
Finn, 1937a).

Formation of Vegetative and Generative Cells. The first division
of the microspore gives rise to the vegetative and generative cells.
Geitler (1935) noted that the metaphase spindle usually shows a

1 For more detailed information on the development and organization of the male
gametophyte, see Wulff and Maheshwari (1938) and Maheshwari (1949). The
technique for the study of the male gametophyte has been reviewed in another
paper by Maheshwari and Wulff (1937).

154

THE MALE GAMETOPHYTE

155

pronounced asymmetry, the wallward pole being blunt and the free
pole acute. More recent studies (Brumfield, 1941) seem to indicate
that this asymmetry is associated with the form of the prophase
nucleus. In Allium, where the nucleus is strongly flattened on the

Fig. 95. Diagram to illustrate the more important stages in development of male
gametophyte. A, newly formed microspore. B, older stage, showing vacuolation
and wallward position of microspore nucleus. C, microspore nucleus dividing.

D, division completed; two-celled stage, showing vegetative and generative cells.

E, generative cell losing contact with wall. F, generative cell lying free in cyto-
plasm of vegetative cell. G, H, division of generative cell in pollen grain. /, J,
division of generative cell in pollen tube. (After Maheshwari, 19/f9.)

wallward side, the asymmetry is extreme (Fig. 96); in Pancratium,
where it is only slightly flattened, the asymmetry is much less pro-
nounced; and Tradescantia shows an intermediate condition. The
direct cause of the asymmetry has been attributed to a difference in

156

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the time of development of the two spindle poles, the wallward or
generative pole developing more slowly than the vegetative, pre-
sumably because of the smaller amount of cytoplasm associated
with the former. With the onset of the anaphase, the asymmetry
becomes less pronounced. In the telophase the generative chromo-
somes are arranged in a plane surface parallel to the inner wall of
the microspore, while the vegetative ones form a somewhat hemi-
spherical pattern.

Symmetrical spindles have been observed only occasionally. To
mention a few examples, in Asclepias (Gager, 1902) and Anthericum
(Geitler, 1935) both the poles of the spindle are blunt; in Adoxa
(Lagerberg, 1909) both are more or less pointed; and in Podophyllum

Fig. 96. Allium cermium, first division of microspore,
phase. C, end of anaphase. {After Brumfield, 1941.)

meta-

(Darlington, 1936) both symmetrical and asymmetrical spindles are
said to occur in the same loculus. Further, in Adoxa (Lagerberg,
1909), Myricaria (Frisendahl, 1912), Sambucus (Schurhoff, 1921),
Colylanthera (Oehler, 1927), and Uvularia (Geitler, 1935) the spindle
is not situated near the wall of the pollen grain but occupies almost
the entire width of the latter. In any case the cells formed by the
division are always unequal, although the conditions which bring
about this result are not clearly understood. In Cuscuta (Fedort-
schuk, 1931) and Slrychnos (Mohrbutter, 1936), where the daughter
cells are sometimes of the same size, this is clearly an abnormality
leading to the formation of double microspores, each of which is des-
tined to divide again to give rise to the vegetative and generative cells
(Fig. 99F). Double pollen grains comprising two units, each with its
own generative and vegetative cells, have also been figured in Podos-
temon subulalus (Magnus, 1913) (Fig. 9QH, I). The separating wall
between the two pollen grains is pitted (Fig. 99/) and only one of them

THE MALE GAMETOPHYTE

157

produces a pollen tube, the other presumably serving as a source of
food material.

It may be noted that unlike the reduction divisions, which occur
more or less simultaneously in all the microspore mother cells of an
anther, the microspores usually divide without any such synchron-
ization, and the same loculus may show different although not
widely separated stages of division and development. In those
plants in which the microspores remain together in a tetrad, all

Fig. 97. A-I, Zostera marina, division of microspore to form vegetative and
generative cells. Pollen grains are so long that only a part of each is shown. (After
Rosenberg, 1901 .) J, Vaccinium vitis idaea, pollen tetrad showing generative cell
cut off toward outer side of each microspore. (After Samuelsson, 1918.) K, Xyris
indica, pollen tetrad, showing generative cell cut off toward inner side of each
microspore. (After Weinzieher, 1914.) L, Acacia baileyana, pollinium, showing
various stages in division of microspore. (After Newman, 1934.)

four cells in a tetrad are usually in the same stage of division, but not
all the tetrads of an anther. A complete synchronization may
perhaps be expected only where the microspores are united into
pollinia (Mimosaceae, Asclepiadaceae, and Orchidaceae), for here
the cells probably exercise some influence over one another through
the uncuticularized walls which lie between them (Barber, 1942).
Exceptions do occur, however, even in such cases. Figure 97 L
of the pollinium of Acacia baileyana (Newman, 1934) shows one of
the microspores in prophase, another with the tube and generative
cells already formed, and the rest in various intermediate stages.

158 INTRODUCTION TO EMBRYOLOGY OF ANG10SPERMS

Goebel (1933) thought that in the angiosperms the generative
cell is always cut off on the distal (i.e., ventral) side of the micro-
spore. Geitler (1935) showed, however, that there is no such uni-
formity and that the generative cell may be cut off either on the
outer side (Fig. 97 J, L), or on the inner side (Fig. 97 K, 98), or on a
radial wall (Fig. 97 A-I), or in a corner instead of the middle of the
radial wall. To cite a few examples, the first-named condition has

Fig. 98. Microsporogenesis and development of male gametophyte in Juncus
filiformis (A-F) and J. squarrosus (G-I). A, interkinesis after Meiosis I, showing
formation of ephemeral cell plate. B, microspore nuclei. C, microspore nuclei
in prophase; note intervening plasma membranes. D, microspore nuclei in ana-
phase. E, F, formation of vegetative and generative cells. G, one member of
tetrad, showing generative cell in late anaphase. H, same, division nearly com-
pleted. 7, older stage, showing vegetative nucleus and two sperm cells. (After
Wulff, 1939a.)

been reported in Elodea (Wylie, 1904), Vaccinium (Samuelsson'
1913), Albizzia (Maheshwari, 1931), Acacia (Newman, 1934),
Asimina (Locke, 1936), and most members of the Orchidaceae
(Swamy, 1949); the second in Symplocarpus (Duggar, 1900), Xyris
(Weinzieher, 1914), Erica (Geitler, 1935), Juncus (Wulff, 1939a),
Cyanastrum (Nietsch, 1941), and most members of the Cyperaceae
(Piech, 1928); the third in Allium (Geitler, 1935); and the fourth

THE MALE GAMETOPHYTE 159

in Lilium (Strasburger, 1908), Anthericwn, and Convallaria (Geitler,
1935).

Whatever the position may be, it is usually constant in individuals
of the same species and sometimes in all the species of a genus or
family and is thus a character of some systematic significance. Un-
fortunately, it can be recognized most clearly only in those plants
in which the pollen grains remain together in tetrads. In most
genera the microspores round up at such an early stage that it be-
comes impossible to distinguish one side from the other, although
even here the position of the germ pores and furrows often serves
as a useful guide. An important point to keep in mind, however,
is that very soon the generative cell loses contact with the wall of
the microspore, and after this has happened it may change its posi-
tion in the pollen grain and come to lie in almost any part of it.2

There is considerable variation in the form of the generative cell.
Usually it is elliptical, lenticular, or spindle-shaped, but in Cuscuta
(Finn, 19376) and Ottelia (Islam, 1950) it becomes long enough to
occupy the entire width of the pollen grain, coming quite close to
the inner wall of the latter on either side. In Monochoria (Banerji
and Haldar, 1942) it is one and a half times as long as the diameter
of the pollen grain and is accommodated in the latter only by the
incurving of its whip-like ends. In Campanula ranunculoides
(Schnarf, 1937) the two ends are dissimilar, one being pointed and
the other more or less blunt and swollen so as to look like a "head."
There are also occasional reports of changes in the form of the
generative cell. More frequently, however, such appearances are
merely due to the plane of sectioning. A spindle-shaped cell appears
round when cut across and oval when cut obliquely.

In fixed material the cytoplasm of the generative cell is usually
distinguishable from that of the vegetative by its hyaline appearance
and general lack of food materials. Plastids (Ruhland and Wetzel,
1924; Krupko, 1926) (Fig. 990) and chondriosomes have, however,
been demonstrated in a few cases, and some recent studies on living
pollen grains and pollen tubes (Benetskaia, 1939; Kostriukova,
1939a, b; Kostriukova and Benetskaia, 1939) have confirmed the

2 This gradual extension of the vegetative cytoplasm around the generative cell
and the consequent "engulfing" of the latter has been referred to by several workers,
viz., Friemann (1910), Wefelscheid (1911), Capoor (1937a), and others.

160 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

THE MALE GAMETOPHYTE 161

presence of a vacuome and mitochondria in the generative as well
as the sperm cells of Narcissus, Asclepias, Vinca, Crinum, and
Lilium. Mention may also be made of the "colored bodies" de-
scribed by Kostriukova (19396) in living pollen tubes of Lilium
martagon. He saw two structures of a pale greenish color, one at
each end of the generative nucleus. In older stages these bodies were
found to divide and occupy similar positions in the sperm cells
(Fig. 100). They were not recognizable in fixed material, but in
their places small areolae were seen which stained black with osmic
acid. The author concludes that they probably correspond with
the structures described as Golgi bodies, but a further study is of
course necessary to confirm this.3

Regarding the contents of the vegetative cell, starch and fat are
the most conspicuous substances. The distinction between starchy
and fatty pollen has been recognized for a long time and their pos-
sible ecological significance has been a subject of much interest (see
Tischler, 1917; Kuhlwein, 1937). Luxemburg (1927) traced the
origin of the starch grains and fat bodies from plastids in the pollen
grains of several members of the Malvaceae, and believes that the

3 In his book "The Cytoplasm of the Plant Cell" Guilliermond (1941) remarks
that "there is no Golgi apparatus in plants" and that all formations described as
Golgi apparatus are elements belonging either to the vacuolar system or to the
chondriome.

Fig. 99. Pollen grains of various angiosperms. A, Cuscuta epithymum, mature
pollen grain, showing vegetative nucleus and two male cells. B, pollen grain,
showing two vegetative nuclei and dividing generative nucleus. C, vegetative
nucleus and three sperms. D, two vegetative nuclei. E, vegetative nucleus,
two sperms, and prothallial cell. F, microspore has divided into two parts, of
which one on right shows vegetative as well as generative nucleus. (After Fedort-
schuk, 1931.) G, Atriplex hymenelytra, pollen grain showing prothallial cell (?),
vegetative nucleus, and two male nuclei. (After Billings, 1934.) H, Podostemon
subulatus, double pollen grain. /, older stage, in which microspore nucleus of
each has divided to form vegetative and generative nuclei; small bodies outside
nuclei are starch grains. /, partition wall between the two cells, showing pits.
(After Magnus, 1913.) K, Vinca herbacea, dumbbell-shaped pollen grain with
two pairs of sperm cells and two vegetative nuclei. (After Finn, 192S.) L,
Erythronium americanum, pollen grain, showing vegetative and generative cells.
M, same, showing amoeboid nature of generative cell. (After Schaffner, 1901.)
N, Wormia suffruticosa, pollen grain, showing large crystal. (After Paetow, 1931.)
0, Lupinus luteus, pollen grain, showing chloroplasts in generative cell as seen after
silver impregnation. (Ruhland and Wetzel, 1924.)

162 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

plastids in turn arise either from preexisting plastids or from chon-
driosomes. In very young pollen grains the reserve food consists
almost entirely of droplets of fat, and starch formation begins only
after the pollen grains have increased in size.

Fig. 100. Portions of pollen tubes of Lilium martagon showing behavior of "colored
bodies" inside generative cell and sperm cells, as seen in living condition. A,
generative cell in division, showing colored body at either end. B, enlarged view
of one end of generative cell, showing detail of colored body. C-E, stages in
division of generative cell. F, G, sperm cells, showing the colored bodies. (After
Kostriukova, 1939b.)

Certain proteinaceous bodies have also been reported in pollen
grains and pollen tubes (Fig. 101), but their exact origin remains
unknown. They probably arise in plastids but soon become liber-
ated in the general cytoplasm of the pollen grain and pollen tube.
Most remarkable of all are the large transparent protein crystals of
Wormia suffruticosa (Paetow, 1931) (Fig. 99N), although these are
of a transitory nature and disappear during the later stages in the
maturation of the pollen grain.

THE MALE GAMETOPHYTE

163

Division of Generative Cell. The generative cell may divide
either in the pollen grain (Fig. 95G-H) or in the pollen tube (Fig.
95I-J). Formerly the second condition was believed to be the
more frequent, but during recent years three-celled pollen grains

4aV

E F

Fig. 101. Stages in development of male gametophyte of Asclepias, showing-
protein granules inside cytoplasm of pollen grain and pollen tube. A, microspore
mother cells. B, tetrad of microspores. C-F, microspores. G, pollen grain,
showing vegetative nucleus and two sperm nuclei. H, terminal portion of pollen
tube. (After Guignard, 1922.)

have been reported in several genera (see Schnarf, 1939) and it
seems certain that many of the older records were based on a study
of immature pollen.4

4 As the pollen grain grows older, the vacuoles become smaller and more evenly
distributed and finally they disappear almost entirely so that with the usual methods
of fixing and staining the mature pollen grain, like the young microspore, again
shows a dense cytoplasm devoid of all conspicuous vacuolation. This is such a
constant feature in most angiosperms (excluding some aquatics) that it serves as a
useful check for judging whether a pollen grain is fully mature or not (see Schnarf,
1937).

164 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

There is also considerable evidence to indicate that even in those
plants in which the pollen grains are shed in the two-celled condi-
tion, the generative nucleus is already in the prophase stage and the
process of division is merely continued in the pollen tube. Some-
times the nucleus may even show a pro-metaphase stage which is
distinguishable from a typical metaphase only by the delay in the
dissolution of the nuclear membrane and the organization of the
spindle. This has been demonstrated very clearly in Impatiens
(Wulff, 1934; Heitz and Resende, 1936), Bulbine (Geitler, 1942)
and other plants.

Occasionally both two- and three-celled pollen grains have been
reported in the same plant, as in the cleistogamous flowers of Viola
(West, 1930), in Dionaea (Smith, 1929), Circaeaster (Junell, 1931),
Nicotiana (Poddubnaja-Arnoldi, 1936), Epimedium, and Iris
(Schnarf, 1937), but this is probably due to environmental influ-
ences. Poddubnaja-Arnoldi (1936) found that, in several kinds
of pollen grains which are normally two-celled, the generative
nucleus divided before germination if the grains were kept for some
time on a sugar-agar substrate. Eigsti (1941) was similarly able to
induce a precocious division of the generative cell in the pollen
grains of Polygonatum canaliculatum. In Holoptelea integrifolia
(Capoor, 19376) the pollen grains are shed at the two-celled stage,
but the generative cell divides on the surface of the stigma before
the pollen tube has started to grow.6

Details of the division of the generative cell vary depending on
whether it takes place in the pollen grain or in the pollen tube. In
the former case, spindle fibers and a normal metaphase plate have
been regularly observed, and the process does not seem to differ in
any essential way from a normal mitosis. Cytokinesis, resulting
in a bipartitioning of the cell, may take place either by a process of
furrowing as in Juncus (Wulff, 1939a) (Fig. 98//"), or by the laying
down of a cell plate as in Asclepias (Finn, 1925) and Portulaca
(D. C. Cooper, 1935). Witmer (1937), who observed both cell
plates and constriction furrows in Vallisneria, states that in his
material these two factors varied in importance.6 In some pollen

6 In Euphorbia terracina (D'Amato, 1947), which is at the other extreme, the
division occurs only after the pollen tube has entered the embryo sac and its tip
has come to lie by the side of the egg.

6 See also Kausik and Rao (1942).

THE MALE GAMETOPHYTE 165

grains a definite cell plate was laid down in the beginning, but it
soon faded away, leaving the final separation of the sperms to a
constriction furrow which arose soon afterwards. In others the cell
plate persisted, and the progress of the constriction furrow was
arrested in this region although evident on either side of it; here the
splitting of the cell plate divided the generative cell before the
constriction could make much progress.

It has proved more difficult to understand the mechanism of the
division when it occurs in the pollen tube. The chief points in
question are: (1) whether a regular metaphase plate is formed during
the division, (2) whether spindle fibers are present or absent, and
(3) whether cytokinesis takes place by constriction or by cell-plate
formation.

Nawaschin (1910), O'Mara (1933), Wulff and Raghavan (1937),
Raghavan et al. (1939), and several other workers failed to find any
regular metaphase plates in the plants studied by them, viz., Lilium
martagon, L. regale, Nemophila insignis, and Impatiens balsamina.
On the other hand, Cooper (1936) reported their occurrence to be
a regular feature in Lilium regale, L. auratum, and L. philippinense
(Fig. 102), and believes that O'Mara's (1933) figures of an "irregular
metaphase" really represent a late prophase, the true metaphase
having been missed by him. Upcott (1936) and Madge (1936)
also found a metaphase plate in Tulipa and Hedychium respectively,
the only important difference being its oblique orientation which
gives more space to the chromosomes for their proper alignment.
More recently, well-differentiated metaphase plates have been re-
corded in Eichhornia (Banerji and Gangulee, 1937), Tulipa, Amar-
yllis, Nicotiana, Forsythia, Camellia, Bryophyllum (Johnston, 1941),
and Eschscholtzia (Beatty, 1943) (Fig. 103).

Regarding the presence or absence of spindle fibers, Nawaschin
(1909) in Lilium martagon, Welsford (1914) in L. auratum and
L. martagon, O'Mara (1933) in L. regale, Trankowsky (1931) in
Convallaria majalis and Galanthus nivalis, Fuchs (1936) in Elaeag-
nus angustifolius , Wunderlich (1937) in Muscari racemosum and
M. comosum, Finn (1939) in Phlomis tuberosa, Raghavan et al.
(1939) in Impatiens balsamina, and several other workers failed to
find a spindle. On the other hand, Trankowsky (1931) in Hemero-
callis fulva, Cooper (1936) in Lilium auratum, L. regale, and L.
philippinense (Fig. 102), Madge (1936) in Hedychium gardnerianum,

166

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Upcott (1936) in several species of Tulipa, Eigsti (1939) in Lilium
canadense, L. speciosum, L. auratum, Polygonatum commutatum,
Convallaria majalis, and Tradescantia reflexa, Johnston (1941) in
Tulipa gesneriana, Amaryllis spp., Nicotiana tabacum, Forsythia
viridissima, Camellia japonica, and Bryophyllum pinnatum, and

Fig. 102. Division of generative cell of Lilium regale as seen in pollen tubes grown
in culture. A, prophase. B-C, metaphase chromosomes advancing to equatorial
plate. D-F, metaphase. G, three chromosomes at metaphase. II, two chromo-
somes at early anaphase. /, late anaphase showing cell-plate formation. J, two
male gametes; cell plate completed. {After Cooper, 1936.)

Beatty (1943) in Eschscholtzia calif ornica (Fig. 103) have emphasized
that spindle fibers are present and perform the same functions as in
normal mitosis.

Coming finally to the mode of cytokinesis, Raghavan ct al. (1939)
in Impaliens, Banerji and Gangulee (1937) in Eichhomia, and several
other authors have reported that the division of the generative cell
occurs by a constriction. Eigsti (1940) also states that cell plates

THE MALE GAMETOPHYTE

4S7

are difficult to find and are temporary structures without any special
significance. On the other hand, very distinct cell plates have been
figured in Lilium regale (Cooper, 1936) (Fig. 102/) and a number of
other plants, and their occurrence has also been confirmed from
studies on living pollen tubes of Crinum hildebrandtii (Kostriukova,
1939a), Lilium martagon (Kostriukova, 19396), and Narcissus
poeticus (Kostriukova and Benetskaia, 1939).

In an important and extensive work on pollen tubes, Johnston
(1941) suggests that the inability to see cell plates is to be attributed

Fig. 103. Division of generative cell in Eschscholtzia californica. {After Beatty,
1943.)

to the exclusive use of nuclear stains in most studies of this type.
The same explanation holds good for the frequently reported absence
of spindle fibers in the division of the generative cell. Delafield's
haematoxylin, he says, is much superior to Heidenhain's haemato-
xylin for such purposes and should always be used for comparison.
As far as present evidence goes, it may therefore be concluded
that the division of the generative cell, whether it takes place in
the pollen grain or in the pollen tube, occurs in a fairly regular
fashion. However, in cases in which the tube is very narrow and
the chromosomes are rather large, there may be some disturbance
of the metaphasic alignment, resulting in their crowding or buckling.
It is also probable that the metaphase stage, owing to its very short

168 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

duration, has been entirely missed in some plants, thereby creating
the false impression that the nucleus passes directly from the pro-
phase into the anaphase. Regarding the spindle, an intensity of
staining which is adequate or just right for the chromosomes often
fails to bring out the fibers, which are more clearly seen in over-
stained material. Finally, the division of the cell may take place
either by means of a constriction furrow or by the laying down of a
cell plate.

Male "Cells" or "Nuclei." Formerly it was believed that when-
ever the division of the generative cell occurs in the pollen grain it
is followed by the formation of sperm cells, but if it takes place in
the streaming cytoplasm of the pollen tube only nuclei are formed.
Recent work has shown, however, that in all cases the male gametes
are definite cells and the cytoplasmic sheath persists throughout
their course in the pollen tube (see Schnarf, 1941).

Considering first the case of Lilium, which has been the favorite
object for such studies, Guignard (1889) figured the male gametes of
L. martagon as cells, but Koernicke (1906), Strasburger (1908) and
Nawaschin (1910) believed that the cytoplasmic sheath is lost during
the division of the generative cell. As to the exact time of dis-
appearance of the sheath, however, these authors are not in agree-
ment with one another. According to Koernicke it is lost when
the generative cell is in prophase; according to Strasburger, at the
metaphase stage; and according to Nawaschin, only during the telo-
phase. Later, Welsford (1914) and O'Mara (1933) reported that
in L. martagon and L. auratum definite sperm cells are formed, al-
though eventually the cytoplasm dissolves away so that only the
naked nuclei enter the embryo sac. Cooper (1936) showed, how-
ever, that the male gametes persist as cells right up to the time they
enter the embryo sac. This has received further confirmation from
the work of Anderson (1939), who finds that the cytoplasmic sheath
around the male nuclei possesses all the inclusions normally present
in the vegetative cytoplasm. He explains that the failure of other
workers to see the sheath is due to their use of nuclear stains, which
are not suited for bringing out the cytoplasm to the best advantage.

Not only in Lilium but also in other plants, the cytoplasmic sheath
around the male nuclei has been followed up to the time of their
discharge in the embryo sac. Nawaschin and Finn (1913) figured
a clear space around the male nuclei of Juglans, which, as Finn
(1925) subsequently explained, represents a thin layer of cytoplasm

THE MALE GAMETOPHYTE 169

around them. Tschernojarow (1915), Dahlgren (1916) and Ishi-
kawa (1918) demonstrated the occurrence of male cells in Myosurus,
Plumbagella, and Oenothera respectively. Wulff (1933) and Finn
(1935, 1940, 1941), who are the most active workers in this field,
categorically state that the occurrence of male cells may be assumed
in all angiosperms, and assert that in those plants in which only
male nuclei have been reported, proper methods of fixing and staining
will eventually reveal the thin cytoplasmic sheath around them.

Vegetative Nucleus. Earlier authors took it for granted that the
vegetative nucleus (often called "tube" nucleus) had an important
role in directing the growth of the pollen tube. Present evidence
seems to indicate, however, that its functional importance had been
greatly exaggerated.

The vegetative nucleus is not always in the distal end of the
pollen tube (where it would be most expected if it had any important
function in directing the growth of the tube) but frequently lies
considerably behind the male gametes. When the tube becomes
branched as in Aconitum, Cucurbita, and Papaver (Poddubnaja-
Arnoldi, 1936), the individual branches continue their growth for an
appreciable period, although only one of them contains the vegeta-
tive nucleus. In Ulmus (Shattuck, 1905), Senecio, Crepis, and
Secale (Poddubnaja-Arnoldi, 1936) it degenerates even before the
pollen grains begin to germinate and does not enter the tube at all;
nevertheless the tube continues to function normally.7 In Cheno-
podium, Atriplex, and Salsola it seems to break up and diffuse into
the surrounding cytoplasm (G. O. Cooper, 1935), and in Musa
(Juliano and Alcala, 1933) and Senecio (Poddubnaja-Arnoldi, 1933)
it fragments into small bits which seem to be quite functionless.
In some other plants also the vegetative nucleus assumes a very
abnormal appearance. For instance, in the pollen tubes of Viola
odorata (Madge, 1929) it becomes 4 times, in Cymbidium bicolor
(Swamy, 1941) 18 times, and in Vallisneria americana (Wylie, 1923)
27 times longer than broad. In a few members of the Labiatae
(Finn, 1939) and in Nicotiana (Goodspeed, 1947) the elongation is
sufficiently pronounced to give it a filamentous outline.

On the basis of these and other data Poddubnaja-Arnoldi (1936)
regards the vegetative nucleus as a vestigial structure without any
important function in the growth of the pollen tube. This view is

7 See Hewitt (1939) for other examples of an early degeneration of the vegetative
nucleus.

170 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

supported by Suita (1936, 1937a, b), who studied the pollen grains
of Crinum with the Feulgen method. He states that soon after its
formation the vegetative nucleus increases in size and becomes
amoeboid. Later it begins to stain very faintly, indicating a de-
composition of the chromatin. He agrees, therefore, that it is a
degenerating structure without any important function in the life
of the pollen tube.

While further evidence would be welcome, it seems safe to con-
clude that the old view attributing a leading role to the vegetative
nucleus in the growth and direction of the pollen tube now needs
modification. It is likely that these functions are really discharged
by the nucleus of the generative cell itself and later by the nuclei
of the two male cells formed by its division.

Development of Pollen in the Cyperaceae. The course of de-
velopment described above is generally characteristic of all angio-
sperms, dicotyledons as well as monocotyledons, the family Cy-
peraceae being the only notable exception. Juel (1900), Stout
(1912), Piech (1928) and others have shown that, of the four micro-
spore nuclei produced after meiosis, only one develops further, while
the other three become pushed toward one end of the mother cell
(Fig. 104 A, J5). The functional nucleus, which lies in the center,
divides with its spindle oriented in the direction of the long axis of
the cell (Fig. 104(7, D). The cell plate, which is laid down between
the vegetative nucleus and generative nucleus, extends around the
latter so as to give rise to a continuous plasma membrane. The
generative cell (Fig. 104Z?) soon becomes spindle-shaped and divides
to form the two sperm cells (Fig. 104jP).

A few doubtful points, which need further clarification, are the
following: (1) whether the functioning microspore nucleus is sepa-
rated from the three nonfunctioning nuclei by a wall, (2) whether
the nonfunctioning nuclei are separated from one another by walls,
and (3) what the fate of the nonfunctioning nuclei may be. Tanaka
(1940, 1941), who has recently discussed these questions, believes
that normally a plasma membrane separates the functioning micro-
spore nucleus from the three nonfunctioning nuclei and that subse-
quently similar membranes arise between the latter. The non-
functioning nuclei sometimes undergo one division, resulting in a
pair of daughter nuclei in each of the three cells. No separating
wall is formed between them, however, and they are soon absorbed.

THE MALE GAMETOPHYTE

171

D E F

Fig. 104. Development of male gametophyte of Scirpvs paluster. A, telophase
of Meiosis II resulting in formation of four microspore nuclei. B, three of micro-
spore nuclei pushed to one end of pollen grain; functioning nucleus in center. C, D,
functioning nucleus dividing; remaining three nuclei in process of degeneration.
E, pollen grain, showing vegetative and generative cells. F, generative cell dividing
to form two male cells. (After Piech, 192S.)

Embryo-sac-like Pollen Grains. In 1898 Nemec noted that in
the petaloid anthers of Hyacinthus orientalis the pollen grains some-
times form large eight-nucleate structures showing a surprising
resemblance to embryo sacs. He believed that they arose as the

172 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

result of a degeneration of the generative nucleus and three divisions
of the vegetative nucleus.

De Mol (1923) observed this so-called "Nemec-phenomenon" in
the anthers of other varieties of Hyacinthus orientalis which had
been subjected to certain special conditions in order to obtain early
flowering. He attributed the origin of the abnormality to a dupli-
cation of the generative nuclei.

Stow (1930, 1934) found similar embryo-sac-like pollen grains or
"pollen-embryo sacs" in the anthers of a variety called "La Victor"
whose bulbs had been subjected to a temperature of 20°C. at the
time of meiosis and were further "forced" in a greenhouse. He
traced their development more fully than either Nemec or De Mol.
At first the microspores increase in size to form large sac-like bodies
(Fig. 105 A, B), after which the nucleus undergoes three successive
divisions (Fig. 105C-F) to form 8 daughter nuclei. Of these, 3 lie
at the end where the exine is still intact, 3 at the opposite end, and
2 in the middle. The 6 nuclei at the two poles organize into cells,
while the remaining two fuse in the center (Fig. 105(7). Since the
three cells at the exine end were found to remain healthy for a much
longer time than those at the opposite end, Stow regards the former
as corresponding to the egg and synergids, and the latter to the
antipodals. In addition certain abnormal pollen-embryo sacs were
also seen, showing the following types of organization: (1) 8 nuclei
forming an egg, two polars, and five antipodal cells; (2) 4 nuclei
forming an egg, two polars, and one antipodal cell; (3) 4 nuclei
forming a polar and three antipodal cells but no egg; (4) 16 nuclei
forming a 5- to 10-celled egg apparatus, one or two polars, and a
few antipodal cells; and (5) more than 16 nuclei without any definite
arrangement.

According to Stow, it is not the divisions of the vegetative or
generative nucleus which give rise to the pollen-embryo sacs but
those of the microspore nucleus itself. Once the vegetative and

Fig. 105. Development of pollen-embryo sacs in Hyacinthus orientalis. A,
microspore in metaphase of first division. B, microspore showing tendency toward
formation of pollen-embryo sac; nucleus is in metaphase. C, second nuclear divi-
sion in pollen-embryo sac; on right, young pollen-embryo sac with undivided
nucleus. D, four- and two-nucleate pollen-embryo sacs. E, division of four
nuclei; metaphase. F, same; anaphase. G, well-developed pollen-embryo sac.
(After Stow, 1930.)

THE MALE GAMETOPHYTE

173

Fig. 105.

174 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

generative cells have been differentiated, further development is
quite normal and no pollen-embryo sacs are formed. Further, the
pollen-embryo sacs were always accompanied by a large number of
dead pollen grains, leading Stow to suggest that the latter secrete a
"necrohormone" which causes an abnormal growth of the surviving
pollen grains.

Stow also observed that when the pollen-embryo sacs were placed
on an agar medium, together with some normal pollen grains of
another variety, the pollen tubes formed from the latter coiled

Fig. 106. Fertilization of pollen-embryo sacs in Hyacinthus orientalis. A, normal
pollen grain, showing vegetative and generative cells. B, pollen-embryo sac.

C, pollen-embryo sac affected by pollen tube from pollen grain of another variety.

D, fertilized pollen-embryo sac; the smaller nuclei are presumed to be products of
division of triple fusion nucleus. (After Stow, 198/,.)

around the former (Fig. 106). Once a sperm nucleus was observed
to be in process of entering the pollen-embryo sac; and in another
case the pollen-embryo sac showed 16 nuclei, believed to have been
derived from the divisions of a triple fusion nucleus.

In conclusion Stow says that all pollen grains are potentially
capable of assuming either the male or the female form. Under
normal conditions the "male potency" is dominant over the "female
potency" leading to the formation of the generative cell and the
male gametes; but under abnormal conditions, when there is a re-
lease of necrohormones, the female potency gets the upper hand
resulting in the formation of embryo-sac-like structures.

THE MALE GAMETOPHYTE 175

Shortly after the publication of Stow's papers, Naithani (1937)
found embryo-sac-like pollen grains in the variety "Yellow Hammer"
whose bulbs had been treated for early flowering. He confirms
Stow's observations regarding the mode of development of these
abnormal pollen grains, but believes their formation to be a tempera-
ture effect and not the result of a liberation of necrohormones.
According to him, the degeneration of the other pollen grains is not
the cause but the effect of a hypertrophied growth of the more
favored ones, which use up all the available food for their own
growth.

More recently, pollen -embryo sacs with 8 and 16 nuclei have also
been observed in another plant, Orniihogalum nutans (Geitler,
1941). Those with 8 nuclei showed the typical embryo-sac-like
organization, but, contrary to Stow, Geitler interprets the three
cells at the exine end of the pollen grain as the equivalents of anti-
podals and the other three as equivalents of the egg and synergids.

References

Anderson, L. E. 1939. Cytoplasmic inclusions in the male gametes of Lilium'

Amer. Jour. Bot. 26: 761-766.
Banerji, I., and Gangulee, H. C. 1937. Spermatogenesis in Eichhornia crassipes

Solms. Jour. Indian Bot. Soc. 16: 289-296.
and Haldar, S. 1942. A contribution to the morphology and cytology

of Monochoria hastaefolia Presl. Proc. Indian Acad. Sci. Sect. B. 16: 91-106.
Barber, H. N. 1942. The pollen grain division in the Orchidaceae. Jour. Genet.

43: 97-103.
Beatty, A. V. 1943. The division of the generative nucleus in Eschscholtzia.

Amer. Jour. Bot. 30: 378-382.
Benetskaia, G. K. 1939. Observations in vivo des gametes males dans les tubes

polliniques d'Asclepias cornutii Decsn., de Vinca major L. et de Vinca minor L.

Jour. Bot. de l'U.R.S.S. 24: 273-281.
Billings, F. H. 1934. Male gametophyte of Atriplex hymenelytra. Bot. Gaz.

95: 477-484.
Brumfield, R. T. 1941. Asymmetrical spindles in the first microspore division

of certain angiosperms. Amer. Jour. Bot. 28: 713-722.
Capoor, S. P. 1937a. Contributions to the morphology of some Indian Liliaceae.

II. The gametophytes of Urginea indica Kunth. Beihefte bot. Centbl. 56 A:

156-170.
. 19376. The life history of Holoptelea integrifolia Planch. Beihefte bot.

Centbl. 57A: 233-249.
Cooper, D. C. 1935. Microsporogenesis and the development of the male

gametes in Portulaca oleracea. Amer. Jour. Bot. 22: 453-459.
. 1936. Development of the male gametes in Lilium. Bot. Gaz. 98:

389-177.

176 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Cooper, G. 0. 1935. Cytological studies in the Chenopodiaceae. I. Micro-
sporogenesis and pollen development. Bot. Gaz. 97: 169-178.

Dahlgren, K. V. O. 1915. liber die Uberwinterungsstadien der Pollensacke und
der Samenanlagen bei einigen Angiospermen. Svensk Bot. Tidskr. 9: 1-12.

. 1916. Zytologische und embryologische Studien liber die Reihen Primu-

lales und Plumbaginales. K. Svenska Vet.-Akad. Handl. 56(4): 1-80.

D'Amato, F. 1947. Nuove ricerche embriologiche e cariologiche sul genere
Euphorbia. Nuovo Gior. Bot. Ital. N.S. 53: 405-436.

Darlington, C. D. 1936. The analysis of chromosome movements. I. Podo-
phyllum versipella. Cytologia 7: 242-247.

De Mol, W. E. 1923. Duplication of generative nuclei by means of physiological
stimuli and its significance. Genetica 5: 225-272.

Duggar, B. M. 1900. Studies in the development of the pollen grain in Symplo-
carpus foetidus and Peltandra undulata. Bot. Gaz. 29: 81-98.

Eigsti, O. J. 1939. The morphology of pollen tubes in angiosperms. Proc. Okla.
Acad. Sci. 19: 105-107.

. 1940. The effects of colchicine upon the division of the generative cell

in Polygonatum, Tradescantia and Lilium. Amer. Jour. Bot. 27: 512-524.
1941. The occurrence of a pollen tube with four sperms and two tube

nuclei in Polygonatum. Proc. Okla. Acad. Sci. 21: 134-136.
Fedortschuk, W. 1931. Embryologische Untersuchung von Cuscuta monogyna

Vahl. und Cuscuta epithymum L. Planta 14: 94-111.
Finn, W. W. 1925. Male cells in angiosperms. I. Spermatogenesis and fertil-
ization in Asclepias cornutii. Bot. Gaz. 80: 1-25.
. 1928. Spermazellen bei Vinca minor und V. herbacea. Ber. deutsch.

bot. Gesell. 46: 235-246.
. 1935. Einige Bemerkungen iiber den mannlichen Gametophyten der

Angiospermen. Ber. deutsch. bot. Gesell. 53: 679-686.
. 1937a. Entwicklungsgeschwindigkeit des mannlichen Gametophyten

bei den Angiospermen. Trav. Inst. Rech. Sci. Biol., Univ. Kiew 1937, pp.

71-86.
. 1937o. Vergleichende Embryologie und Karyologie einiger Cuscuta-

Arten. Jour. Inst. Bot. Acad. Sci. Ukraine 12: 83-99.
. 1939. On the history of the development of the male gametophyte in

Labiatae. Jour. Inst. Bot. Acad. Sci. Ukraine 20: 77-96.
. 1940. Spermazellen bei Angiospermen. Jour. Bot. de l'U.R.S.S. 25:

155-175.

1941. Mannlichen Gameten bei Angiospermen. Compt. Rend. (Dok.)

Acad, des Sci. U.R.S.S. 30: 451-458.
Friemann, W. 1910. "Uber die Entwicklung der generativen Zelle im Pollenkorn

der monocotylen Pflanzen." Diss. Bonn.
Frisendahl, A. 1912. Cytologische und Entwicklungsgeschichtliche Studien an

Myricaria germanica Desv. K. Svenska Vet.-Akad. Handl. 48(7): 1-62.
Fuchs, A. 1936. Untersuchungen iiber den mannlichen Gametophyten von

Elaeagnus anguMifolia. Osterr. bot. Ztschr. 85: 1-16.

THE MALE GAMETOPHYTE 177

Gager, C. S. 1902. The development of the pollinium and sperm cells in Asclepias

cornutii Decaine. Ann. Bot. 16: 123-148.
Geitler, L. 1935. Beobachtungen iiber die erste Teilung im Pollenkorn der

Angiospermen. Planta 24: 361-386.
. 1941. Embryosacke aus Pollenkornern bei Ornithogalum. Ber. deutsch.

bot. Gesell. 59: 419-423.
. 1942. Uber die Struktur des generativen Kerns im zweikernigen Angio-

spermenpollen. Planta 32: 187-195.
Goebel, K. 1933. "Organographie der Pflanzen. III. Samenpflanzen." 3d ed.

Jena.
Goodspeed, T. H. 1947. Maturation of the gametes and fertilization in Nico-

tiana. Madrono 9: 110-120.
Guignard, L. 1889. Etudes sur les phenomenes morphologiques de la teconda-

tion. Bui. Soc. Bot. de France 36: 100-146.

. 1922. Sur l'existence de corps proteiques dans le pollen de diverses

Angiospermes. Compt. Rend. Acad. Sci. Paris 175: 1015-1020.
Guilliermond, A. 1941. "The Cytoplasm of the Plant Cell." Chronica Botanica

Company.
Heitz, E., and RSsende, F. 1936. Zur Methodik der Pollenkorn- und Pollen-

schlauch-untersuchung. Bol. Soc. Broteriana, Coimbra II, 11: 5-15.
Hewitt, W. C. 1939. Seed development of Lobelia amoena. Jour. Elisha Mitchel

Sci. Soc. 55: 63-82.
Ishikawa, M. 1918. Studies on the embryo sac and fertilization in Oenothera.

Ann. Bot. 32: 279-317.
Islam, A. S. 1950. The embryology of Ottelia alismoides Pers. Jour. Indian

Bot. Soc. 28: 79-91.
Johnston, G. W. 1941. Cytological studies of male gamete formation in certain

angiosperms. Amer. Jour. Bot. 28: 306-319.
Juel, H. O. 1900. Beitrage zur Kenntnis der Tetradenteilung. Jahrb. f. wiss.

Bot. 35: 626-649.
Juliano, J. B., and Alcala, P. E. 1933. Floral morphology of Musa errans (Blanco)

Teodoro var. Botoan Teodoro. Philippine Agr. 22: 91-126.
Junell, S. 1931. Die Entwicklungsgeschichte von Circaeaster agrestis. Svensk

Bot. Tidskr. 40: 111-126.
Kausik, S. B., and Rao, P. V. K. 1942. The male gametophyte of Halophila

ovata Gaudich. Jour. Mysore Univ. B. 3: 43-49.
Koernicke, M. 1906. Zentrosomen bei Angiospermen? Zugleich ein Beitrag

zur Kenntnis der generativen Elemente im Pollenschlauch. Flora 96: 501-522.
Kostriukova, K. 1939a. Spermatogenesis in Crinum hildebrandtii Vatke. Ob-
servations in vivo. Jour. Inst. Bot. Acad. Sci. Ukraine 21/22: 157-164.
. 19396. Observations in vivo on the formation of the male sex cells in

Lilium martagon L. Compt. Rend. (Dok.) Acad, des Sci. U.R.S.S. 22: 444-447.
and Benetskaia, G. 1939. Spermatogenesis in Narcissus poeticus L.

Observations in vivo. Jour. Bot. de l'U.R.S.S. 24: 209-220.
Krupko, S. 1926. Les plastides et le chondriome pendant la gonogen&se dans le
Gagea lutea. Acta Soc. Bot. Poloniae 4: 77-86.

ITS INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Kuhlwein, H. 1937. Zur Physiologie der Pollenkeimung, insbesondere der Frage
nach dem Befruchtungsverzug bei Gymnospermen. Beihefte bot. Centbl.
57A.

Lagerberg, T. 1909. Studien liber die Entwicklungsgeschichte und systematische
Stellung von Adoxa moschatellina L. K. Svenska Vet.-Akad. Handl. 44(4):

1-86.
Locke, J. F. 1936. Microsporogenesis and cytokinesis in Asimina triloba. Bot.

Gaz. 98: 159-168.
Luxemburg, A. 1927. Recherches cytologiques sur les grains de pollen chez les

Malvaceaes. Bui. Internatl. Acad. Polonaise Sci. et Lettr.: CI. Sci. Math, et

Nat. B. 1927, pp. 363-396.
Madge, M. 1929. Spermatogenesis and fertilisation in the cleistogamous flowers

of Viola odorata var. praecox. Ann. Bot. 43: 545-577.
. 1936. Division of the generative cell in Hedychium gardnerianum.

Cellule 45: 171-176.
Magnus, W. 1913. Die atypische Embryonalentwicklung der Podostomaceen.

Flora 105: 275-336.
Maheshwari, P. 1931. Contribution to the morphology of A Ibizzia lebbek. Jour.

Indian Bot. Soc. 10: 241-264.

. 1949. The male gametophyte of angiosperms. Bot. Rev. 15: 1-75.

and Wulff, H. D. 1937 Recent advances in microtechnic. I. Methods

of studying the male gametophyte of angiosperms. Stain Technol. 12: 61-70.
Mohrbutter, C. 1936. Embryologische Studien an Loganiaceen. Planta 26:

64-80.
Naithani, S. P. 1937. Chromosome studies in Hyacinthus orientalis L. III.

Reversal of sexual state in the anthers of H. orientalis L. var. Yellow Hammer.

Ann. Bot. 1: 369-377.
Nawaschin, S. G. 1909. Uber dasselbstandige Bewegungsvermogen des Sper-

makerns bei einigen Angiospermen. Osterr. bot. Ztschr. 59: 457-467.
. 1910. Naheres liber die Bildung der Spermakerne bei Lilium martagon.

Ann. Jard. Bot. Buitenzorg, 3rd Suppl. 2: 871-904.

and Finn, W. W. 1913. Zur Entwicklungsgeschichte der Chalazogamen,

Juglans regia, und Juglans nigra. M&n. Acad. Imp. des Sci. St. Petersburg
VIII, 31: 1-59.

N6mec, B. 1898. Uber den Pollen der petaloiden Antheren von Hyacinthus
orientalis L. Rozpravy Ceske Akad. Prag II, 7(17).

Newman, I. V. 1934. Studies in the Australian acacias. IV. The life history
of Acacia baileyana F.V.M. Part 2. Gametophytes, fertilisation, seed produc-
tion and germination, and general conclusion. Proc. Linn. Soc. N. S. Wales
59: 277-313.

Nietsch, H. 1941. Zur systematische Stellung von Cyanastrum. Osterr. bot.
Ztschr. 90: 31-52.

Oehler, E. 1927. Entwicklungsgeschichtlich-cytologische Untersuchungen an
einigen saprophytischen Gentianaceen. Planta 3: 641-733.

O'Mara, J. 1933. Division of the generative nucleus in the pollen tube of Lilium.
Bot. Gaz. 94: 567-578.

THE MALE GAMETOPHYTE 179

Paetow, W. 1931. Embryologische Untersuchungen an Taccaceen, Meliaceen

unci Dilleniaceen. Planta 14: 441-470.
Piech, K. 1928. Zytologische Studien an der Gattung Scirpus. Bui. Internatl.

Acad. Polonaise Sci. et Lettr. 1928 (1/2): 1-43.
Poddubnaja-Arnoldi, V. A. 1933. Kunstliche Kultur und zytologische Unter-

suchung des Pollenschlauches vom Senecio platanifolius Benth. Planta 19:

299-304.

. 1936. Beobachtungen iiber die Keimung des Pollens einiger Pflanzen

auf kunstlichen Nahrboden. Planta 25: 502-529.
Raghavan, T. S., et al. 1939. Division of the generative cell in Impatiens balsa-

mina L. Cytologia 9: 389-392.
Rosenberg, 0. 1901. Uber die Pollenbildung von Zostera. Meddel. Stock-

holma Hogskolas Bot. Inst.: 1-21.
Ruhland, W., and Wetzel, K. 1924. Der Nachweis von Chloroplasten in den

generativen Zellen von Pollenschliiuchen. Ber. deutsch. bot. Gesell. 42: 3-14.
Samuelsson, G. 1913. Studien iiber die Entwicklungsgeschichte einiger Bicornes-

Typen. Svensk Bot. Ticlskr. 7: 97-188.
Schaffner, J. H. 1901. A contribution to the life history and cytology of

Erythronium. Bot. Gaz. 31: 369-387.
Schnarf, K. 1937. Studien iiber den Bau der Pollenkorner der Angiospermen.

Planta 27: 450-465.
. 1939. Variation im Bau des Pollenkornes der Angiospermen. Tab. Biol.

Periodicae 27: 72-89.
. 1941. "Vergleichende Zytologie des Geschlechtsapparates des Kormo-

phyten. Monographien zur vergleichenden Zytologie." Berlin.
Schiirhoff, P. N. 1921. Uber die Teilung des generativen Kernes vor der Kei-
mung des Pollenkorns. Arch. f. Zellforsch. 15: 145-159.
Shattuck, C. H. 1905. A morphological study of Ulmus americana. Bot. Gaz.

40: 209-223.
Smith, C. M. 1929. Development of Dionaea muscipula. I. Flower and seed.

Bot. Gaz. 87: 508-530.
Stout, A. B. 1912. The individuality of the chromosomes and their serial ar-
rangements in Carex aquatilis. Arch. f. Zellforsch. 9: 114-140.
Stow, I. 1930. Experimental studies on the formation of embryo sac-like giant

pollen grain in the anther of Hyacinthus orientalis. Cytologia 1: 417-439.
. 1934. On the female tendencies of the embryo sac -like giant pollen

grain of Hyacinthus orientalis. Cytologia 5: 88-108.
Strasburger, E. 1908. Chromosomenzahlen, Plasmastrukturen, Vererbungstrager

und Reduktionsteilung. Jahrb. f. wiss. Bot. 45: 477-570.
Suita, N. 1936. Studies on the male gametophyte in Angiosperms. I. Studies

in the degenerating process of pollen tube nucleus, with special reference to

Feulgen's reaction. Bot. and Zool. [Tokyo] 4: 2033-2044.
. 1937a. On the mature pollen grains in angiosperms. Bot. Mag. [Tokyo]

51: 524-530.
. 19376. Studies on the male gametophyte in angiosperms. II. Differ-

180 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

entiation and behavior of the vegetative and generative elements in the pollen

grains of Crinum. Cytologia, Fujii Jubl. Vol., pp. 920-933.
Swamy, B. G. L. 1941. The development of the male gametes in Cymbidium

bicolor Lindl. Proc. Indian Acad. Sci. Sect. B. 14: 454-460.
. 1949. Embryological studies in the Orchidaceae. I. Gametophytes.

Amer. Midland Nat. 41: 184-201.
Tanaka, N. 1940. Chromosome studies in Cyperaceae. VI. Pollen develop-
ment and additional evidence for the compound chromosome in Scirpus la-

custris L. Cytologia 10: 348-362.
. 1941. Chromosome studies in Cyperaceae. XII. Pollen development

in five genera, with special reference to Rhyncospora. Bot. Mag. [Tokyo] 55:

55-65.
Tischler, G. 1917. Pollenbiologische Studien. Ztschr. f. Bot. 9: 417-488.
Trankowsky, D. A. 1931. Zytologische Beobachtungen liber die Entwicklung

der Pollenschlauche einiger Angiospermen. Planta 12: 1-18.
Tschnernojarow, M. 1915. Les nouvelles donnees dans l'embryologie du Myo-

surus minimus L. M6m. Soc. Nat. Kiew 24: 95-170.
Upcott, M. 1936. The mechanism of mitosis in the pollen-tube of Tulipa. Proc.

Roy. Soc. London, Ser. B. 121: 207-220.
Wefelscheid, G. 1911. "Uber die Entwicklung der generativen Zelle im Pollenkorn

der dikotylen Angiospermen." Diss.1 Bonn.
Weinzieher, S. 1914. Beitrage zur Entwicklungsgeschichte von Xyris indica

L. Flora 106: 393-432.
Welsford, E. J. 1914. The genesis of the male nuclei in Lilium. Ann. Bot. 28:

265-270.
West, G. 1930. Cleistogamy in Viola riviniana, with especial reference to the

cytological aspects. Ann. Bot. 44: 87-109.
Witmer, S. W. 1937. Morphology and cytology, of Vallisneria spiralis L. Amer.

Midland Nat. 18: 309-327.
Wulff, H. D. 1933. Beitrage zur Kenntnis des mannlichen Gametophyten der

Angiospermen. Planta 21: 12-50.
. 1934. Untersuchungen an Pollenkornern und Pollenschlauchen von

Impatiens parviflora. Ber. deutsch. bot. Gesell. 52: 43-47.
. 1939a. Die Pollenentwicklung der Juncaceen. Jahrb. f. wiss. Bot. 87:

533-556.
. 19396. Die Entwicklung der Pollenkorner von Triglochin palustris L.

und die verschiedenen Typen der Pollenkornentwicklung der Angiospermen.

Jahrb. f. wiss. Bot. 88: 141-168.
and Maheshwari, P. 1938. The male gametophyte of angiosperms (a

critical review). Jour. Indian Bot. Soc. 17: 117-140.

and Raghavan, T. S. 1937. Beobachtungen an Pollenschlauchkulturen

von der Hydrophyllaceae Nemophila insignis. Planta 27: 466-473.
Wunderlich, R. 1937. Zur vergleichenden Embryologie der Liliaceae-Scilloideae.

Flora 32: 48-90.
Wylie, R. B. 1904. The morphology of Elodea canadensis. Bot. Gaz. 37: 1-22.
. 1923. Sperms of Vallisneria spiralis. Bot. Gaz. 75: 191-202.

CHAPTER 6
FERTILIZATION

In gymnosperms the pollen grains usually land directly on the
nucellus, while in angiosperms they are deposited on the stigma.
There are various agencies which serve to bring about this transfer
of pollen from the anthers to the stigma, but since this is primarily
an ecological subject and information on it is readily available else-
where, it need not be dealt with here. It is sufficient to say that,
in the condition in which they are discharged from the anther, the
pollen grains show considerable resistance to environmental changes.
Sometimes they retain their viability for several weeks, and with
proper methods of storage this period can be prolonged still further
(see Chap. 12).

Germination of Pollen. Exact information on the time taken by
pollen to germinate on the stigma is available for only a few plants,
but the following examples will illustrate the range that has been
observed: 2 days in Garrya elliptica (Hallock, 1930), 3 hours in
Reseda spp. (Eigsti, 1937) ; 2 hours in Beta vulgaris (Artschwager and
Starrett, 1933); and 5 minutes in Taraxacum kok-saghys (Poddub-
naja-Arnoldi and Dianowa, 1934), Zea mays (Randolph, 1936),
and Hordeum distichon (Pope, 1937). In Saccharum oflicinarum
(Artschwager et al, 1929) and Sorghum vulgare (Artschwager and
McGuire, 1949) germination takes place almost immediately.

The first step in germination is the expansion of the pollen grain
by the absorption of liquid from the moist surface of the stigma and
the protrusion of the intine through a germ pore. The small tubular
structure which arises in this way then continues to elongate, mak-
ing its way down the tissues of the stigma and style. Only the
distal part of the tube has living cytoplasm, and as the nuclei pass
forward callose plugs are left in the empty portions behind them.

Most pollen grains are monosiphonous, i.e., only a single pollen
tube emerges from each pollen grain; others, like those of the Mal-
vaceae, Cucurbitaceae and Campanulaceae, are polysiphonous.
In Althaea rosea 10 tubes, and in Malva neglecta even 14 tubes, are

181

182 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

known to come out from the same pollen grain (Stenar, 1925).
Eventually, however, only one of them makes further progress.
Sometimes the same pollen tube may divide into one or more
branches. Such a condition seems to be frequent in the Amenti-
ferae, where the branching tubes give the appearance of a ramifying
fungous mycelium (see Finn, 19286). In plants whose pollen grains
are united into tetrads or into pollinia, several pollen tubes are
produced at the same time (Fig. 107 A, B).

A B C

Fig. 107. Acacia baileyana, pollinium germinating on stigma (ct = cuticle; g =
generative cell ; In = tube nucleus; in = intine). (After Newman, 1984.) B, Cymbi-
dium bicolor, germination of pollen grains united in tetrads. (After Swamy, 1941.)
C, Elatine triandra, t.s. anther of cleistogamous flower, showing pollen grains ger-
minating in situ. (After Frisendahl, 1927.)

The stigma is believed to play an important part in the germina-
tion of pollen, but in many plants germination can also be induced
in a sugar solution of appropriate strength. Martin (1913) germi-
nated the pollen of Trifolium pratense on hog's bladder moistened
with distilled water and suggested that the only use of the stigma
lies in controlling the water supply. Katz (1926) agreed with this
view and said that the chief function of the stigmatic secretion is to
protect the pollen as well as the stigma from desiccation. In her
experiments the pollen germinated even on the cut surface of the
style, provided the stigmatic secretion was applied to the stump and
the latter was kept moist for some time.

FERTILIZATION 183

Pollen grains may also germinate on other parts of the flower be-
sides the stigma. In cleistogamous flowers (Frisendahl, 1927;
Madge, 1929; West, 1930; Maheshwari and Singh, 1934) germina-
tion takes place within the anther loculi (Fig. 107C), and in Aeginetia
indica (Juliano, 1935) even on the moist surface of the corolla tube.
Frequently pollen grains germinate on a foreign stigma, i.e., stigma
of a different species (see Eigsti, 1937; Sanz, 1945). If fertilization
takes place, it results in the formation of interspecific and inter-
generic hybrids.

Course of Pollen Tube. After the tube has emerged from the
pollen grain, it makes its way between the stigmatic papillae into
the tissues of the style. The latter is extremely variable in length.
In some plants it is so short that the stigma is described as sessile,
while in Zea mays the so-called "silk" may attain a length of 50 cm.

Depending on the presence or absence of the transmitting tissue
and on the extent of its development, styles have been classified into
three main types called open, half -closed, and closed (Hanf, 1935).
In the first type there is a wide stylar canal and the inner epidermis
itself assumes the function of the nutrition and conduction of the
pollen tube, as in the Papaveraceae, Aristolochiaceae, Ericaceae,
and many monocotyledons. In the second type the canal is sur-
rounded by a rudimentary transmitting tissue of two or three layers
of glandular cells, as in several members of the Cactaceae. In the
third or closed type, illustrated by Datura and Gossypium, there
is no open channel but instead a solid core of elongated and richly
protoplasmic cells through which the pollen tube grows downward
in order to reach the ovary. Finally, there are other plants like
Salix, Acacia, and many grasses in which the styles are solid but are
not provided with any specialized transmitting tissue.

In open styles the pollen tube grows on the surface of the cells
lining the stylar canal (often in the mucilage secreted by them);
and in solid styles through the intercellular spaces between the cells
of the transmitting tissue, enlarging the spaces by the hydrostatic
pressure of its contents and secreting some enzymatic substances
which bring about a dissolution of the middle lamellae. Only rarely
does the pollen tube pass through the cells themselves.

After arriving at the top of the ovary, the tube may enter the
ovule either through the micropyle or by some other route. The
former is the usual condition and is known as porogamy, but even

184

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

in plants ordinarily classed as porogamous there are several modi-
fications. To mention a few examples, in Acacia (Newman, 1934)
the integuments are still below the apex of the nucellus at the time
of fertilization so that a micropyle does not exist at this stage and in
Philadelphus , Utricularia, Vandellia, and Torenia the embryo sac

Fig. 108. Course of pollen tube in ovule of Casuarina equisetifolia. A, optical
longitudinal section of entire ovary; pollen tube is represented by heavy black
line. B, l.s. ovule reconstructed from several sections to show path of pollen tube.
(After Swamy, 1948.)

FERTILIZATION

185

protrudes out of the micropyle so that the pollen tube comes in
direct contact with it. In several members of the Loranthaceae
there is no integument and therefore nothing that can be called a
micropyle. Here the embryo sacs undergo a remarkable elongation
and meet the pollen tubes at some point in the stylar region (see
also p. 143).

In some plants the pollen tube enters the ovule through the
chalaza. This condition, known as chalazogamy (see page 17),
was first reported in Casuarina (Treub, 1891) and soon afterwards

ABC

Fig. 109. Development of obturator in Acalypha indica. A, l.s. terminal flower
of inflorescence, showing ovule at megaspore mother cell stage. Note beginning
of formation of obturator (shaded). B, l.s. lateral flower at more advanced stage.
C, ovule, enlarged to show hood-like obturator fitting over nucellus. (After
Maheshwari and Johri, 1941.)

in several members of the Amentiferae. Nevertheless, it is not
confined to them, being also known in Rhus (Grimm, 1912), Cir-
caeaster (Junell, 1931), and a few other genera. Recent studies have,
however, shown that even in such cases, where entry into the ovule
is effected through the chalaza, the tube usually continues its
growth over the surface of the embryo sac and penetrates it only
after arriving near the egg apparatus. As examples may be men-
tioned Ostrya carpinifolia (Finn, 1936), Juglans regia (Nast, 1941),
and Casuarina equisetifolia (Swamy, 1948) (Fig. 108).

In Alchemilla (Murbeck, 1901), Cucurbita (Longo, 1901; Kirk-

186 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

wood, 1906), and Circaeaster (Junell, 1931) the pollen tube enters
through the funiculus or the integument. This is known as mes-

ogamy.

Formerly considerable phylogenetic significance was attached
to the route taken by the pollen tube during its entry into the
ovule, but now this point is considered to be of physiological
rather than phylogenetic importance, for we sometimes find con-
siderable variation in this respect even in one and the same species.
In Brassica oleracea (Thompson, 1933) the tube normally enters

through the micropyle, but some-
times it may do so by way of
the chalaza. In Ulmus, Shat-
tuck (1905) speaks of its branch-
ing and apparently aimless wand-
ering through the funiculus, the
integuments, and occasionally
the nucellus. In Ep Hob turn
(Werner, 1914; Tackholm, 1915)
it may enter either through the
micropyle, through the integu-
ments, or by an intermediate
route. In Boerhaavia (Mahesh-
wari, 1929), although the tube
actually enters through the mi-
cropyle, it first makes a horizon-
tal crossing through the funicu-
lus. In Gossypium (Gore, 1932)
it often passes from the funiculus
to the base of the ovule and then
travels up along the wall of the
latter to enter the micropyle.

An organ of special signifi-
cance in facilitating the entry of
the pollen tube into the ovule is
the so-called obturator, to which
reference had already been made
by Hofmeister in the year 1849.

Fig. 110. Origin and structure of obtu-
rator in Myriocarpa longipes (A), Leu-
cosyke capitellata (B), and Quisqualis

indica (C, D). (After Fagerlind 19U, Usually it is a swelling of the
1941.) placenta which grows towards

FERTILIZATION

187

the micropyle and fits like a hood or canopy over the nucellus,
serving as a sort of bridge for the pollen tube (Fig. 109). Often
the cells of the obturator may be greatly elongated or may have
a glandular appearance (Fig. HOC, D).

Some other structures having a different origin but serving the
same function may also be included under the general term ob-
turator. In the Thymelaeaceae (Fuchs, 1938) the cells belonging
to the base of the stylar canal elongate and grow down as hairy

Fig. 111. Development of obturator in some members of Thymelaeaceae. A,
Daphne laureola, l.s. pistil. B, same, part of ovary with cells of obturator pro-
truding downward into micropyle. C, more advanced stage, showing path of
pollen tube. D, Passerina pectinata, l.s. part of ovary, showing obturator (mi =
micropyle). (After Fuchs, 19SS.)

processes approaching the nucellus (Fig. 111). In Pilea (Fagerlind,
1944) a tuft of papillate cells extends from the base of the style to
the apex of the ovule, coming in intimate contact with the latter.
In Myriocarpa and Leucosyke (Fagerlind, 1944), on the other hand,
it is the cells of the inner integument which elongate upward and
penetrate into the stylar canal (Fig. 110A, B), forming what may
be called an integumentary obturator.

Usually there are no special modifications in the cells lining the
micropylar canal, but sometimes, as in Berkheya (Gelin, 1936),

188 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Grevillea (Brough, 1933), and Cynomorium (Steindl, 1945), they
become mucilaginous or glandular and seem to contribute to the
nutrition of the pollen tube. In Cardiospermum (Kadry, 1946) not
only the cells belonging to the inner integument but also those
forming the apical portion of the nucellus give rise to a mucilaginous
mass which facilitates the entry of the pollen tube. In plants with
a many-layered nucellar tissue, like Beta (Artschwager and Starrett,
1933), those of its cells which are in continuity with the micropyle
become elongated and richly protoplasmic and give an impression
as though they were designed to lead the pollen tube through the
path of least resistance.

It is of interest to note that even during its passage through the
nucellus the pollen tube usually makes its way between the cells
and not through them. Normally it causes but little disturbance
in their position and they soon return to their original shape, but in
a few families like the Lythraceae, Sonneratiaceae, Onagraceae,
and Cucurbitaceae the tubes are so broad that they destroy the
cells which lie in their way and cause a permanent break in the

tissues.

Entry of Pollen Tube into Embryo Sac. After penetrating the
wall of the embryo sac, the pollen tube may either pass between
the egg and one synergid as in Fagopyrum (Mahony, 1935), or be-
tween the embryo sac wall and a synergid as in Cardiospermum
(Kadry, 1946), or directly into a synergid as in Oxalis (Krupko,
1944), Elodea (Ernst-Schwarzenbach, 1945), and Daphne (Ven-
kateswarlu, 1947). In Viola it not only enters a synergid but is
said to force its way through the base of the latter (Madge, 1929).

As a rule only one synergid is destroyed by the impact of the
pollen tube and the other remains intact until some time afterward,
but in Mimusops, Achras, and Bassia (Murthy, 1941) both are
destroyed and in Phryma (Cooper, 1941) and Tropaeolum (Walker,
1947) neither of them seems to be affected.

In some genera, such as Tacca, Wormia (Paetow, 1931), and
Nelumbo (Ohga, 1937), the synergids degenerate even before the
entry of the pollen tube, and in others like Plumbago, Vogelia, and
Plumbagella (see Maheshwari, 1948) they are not formed at all.
This seems to indicate that they are not essential for fertilization,
and the view that they secrete substances which exercise a chemo-
tactic influence over the pollen tube, or that they act as shock

FERTILIZATION

189

absorbers against its impact, does not rest on a sound basis (see
also Dahlgren, 1938). In Zauschneria latifolia (Johansen, 19316)
pollen tubes were found to enter even those ovules whose embryo
sacs had degenerated and virtually disappeared.

Detailed information regarding the exact manner of discharge of
the male gametes is lacking. In Crepis capillaris (Gerassimova,
1933) and Taraxacum kok-saghys (Warmke, 1943) the tip of the

ABC

Fig. 112. Stages in fertilization in Petunia; globular bodies scattered inside embryo
sac are starch grains. A, mature embryo sac surrounded by cells of integumentary
tapetum. B, upper portion of embryo sac at time of double fertilization; note two
branches of pollen tube, through which two male gametes have been discharged;
just behind bifurcation are two X-bodies. C, upper portion of embryo sac, showing
zygote and two endosperm nuclei with cell plate forming between them; X-bodies
are still visible inside tip of pollen tube. (After Cooper, 1946.)

pollen tube becomes "wedged in" between the egg and the polar
fusion nucleus, so that both the male gametes are discharged in close
proximity to their mates. Fagerlind (1939) noted some embryo
sacs of Peperomia in which the tip of the tube had divided into two
short branches, one of which was directed toward the egg. Cooper
(1940, 1941, 1946) refers to a similar bifurcation of the tip of the
pollen tube in Portulaca, Phryma, and Petunia (Fig. 112), one
branch becoming closely appressed to the egg and the other extend-
ing in the direction of the polar nuclei, and suggests that the two
male gametes reach their destinations by way of these separate

190 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

branches. In Coffea arabica (Mendes, 1941) the pollen tube does
not bifurcate but shows two subterminal openings through which
the two male gametes are discharged into the cavity of the embryo
sac.

Rate of growth of pollen tube.1 A considerable amount of in-
formation is available on the subject but most of the older records
are rather vague and perhaps only rough estimates. It must also
be kept in mind that the rate of growth of the pollen tube is affected
to an appreciable degree by the environmental conditions prevailing
at the time of observation, and most of the existing data are there-
fore to be considered as approximate only.

In certain members of the Fagaceae and Betulaceae several
months elapse between the time the pollen grains alight on the
stigma and the time the pollen tube enters the embryo sac. In
some species of Quercus (Bagda, 1948) this period may be as long
as 12 to 14 months. In Hamamelis virginiana (Shoemaker, 1905)
pollination occurs in late autumn, and at the beginning of winter
the tips of the tubes reach near the base of the funiculus. But here
they "hibernate" for the rest of the winter and growth is resumed
only in the spring. Fertilization takes place in May, about 5 to 7
months after pollination. In Alnus glutinosa and Corylus avellana
(Benson, 1894) 3 to 4 months elapse between pollination and fertil-
ization. Since such long periods are also found in several gymno-
sperms, it is tempting to infer that this is a primitive feature, but
long intervals are also known in the Orchidaceae where the ovules
are not even formed until after pollination has taken place. To
mention a few examples, in Paphiopedilum maudiae (Duncan and
Curtis, 19426) approximately 19 to 20 weeks elapse between pollina-
tion and fertilization; in P. villosum (Duncan and Curtis, 19426) the
period is 14 weeks; in Phalaenopsis pamala (Duncan and Curtis,
1942a) and Dendrobium annosum (Pastrana and Santos, 1931) about
10 weeks; in Cattleya spp. (Duncan and Curtis, 1943) about 6 weeks;
in Cypripedium parviflorum (Carlson, 1940) 26 to 33 days; in C.
pubescens (Duncan and Curtis, 19426) about 4 weeks; and in Orchis
maculatus (Hagerup, 1944) about 2 weeks.

Fairly long intervals between pollination and fertilization are
also known in plants belonging to other families. In Garry a elliptica

1 For a more detailed account of this topic, see Finn (1937a).

FERTILIZATION 191

(Hallock, 1930) the pollen tube takes 17 days to arrive at the apex
of the nucellus, and in Carica papaya (Foster, 1943) about 10 days;
fertilization occurs a few days later. In Colchicum autumnale
(Heimann-Winawer, 1919) there is an interval of about 10 to 11
days between pollination and fertilization, and in Carya illinoensis
(McKay, 1947) about 4 to 7 days.

In most plants, however, the period ranges from 24 to 48 hours,
and in some it is still shorter. In Oryza sativa (Juliano and Aldama,
1937), Coffea arabica (Mendes, 1941), and Oxybaphus nyctagineus
(Cooper, 1949) fertilization takes place in about 12 to 14 hours after
pollination, and in Lactuca muralis (Dahlgren, 1920) within 6 to 7
hours. In L. sativa (Jones, 1927, 1929) sperm nuclei have been seen
in the embryo sac 3 hours after pollination and a couple of hours
later most of the embryo sacs were already fertilized. In Portulaca
oleracea (Cooper, 1940) the period between pollination and fertiliza-
tion is 3 to 4 hours; in Impatiens sultani (Lebon, 1929) and Hordeum
distichon palmella (Pope, 1937) pollen tubes arrive inside the embryo
sac in less than an hour after pollination ; and in Parthenium argenta-
tum (Dianowa et al., 1935) and Crepis capillaris (Gerassimova, 1933)
fertilization is completed within 60 minutes after pollination. The
shortest period on record is in Taraxacum kok-saghys (Poddubnaja-
Arnoldi and Dianowa, 1934; Warmke, 1943), where fertilization
occurs within 15 to 45 minutes after pollination.

Several computations have been made of the average hourly
distance traversed by the pollen tube: 4 mm. in Iris versicolor
(Sawyer, 1917), 6.25 mm. in Zea mays (Miller, 1919), 15 mm. in
Crepis capillaris (Gerassimova, 1933), and 35 mm. in Taraxacum
kok-saghys (Poddubnaja-Arnoldi and Dianowa, 1934). It is likely,
however, that the actual rate of growth is still higher; for the path
of the pollen tube from the stigma to the ovule is not like a straight
line but is marked by many twists and convolutions.

Of the factors influencing the rate of growth, temperature is the
most important. As early as 1861, Hofmeister had observed that
in Crocus vernus, in warm moist air and bright sunshine, the pollen
tubes can be seen in the micropyle within 24 hours after pollination,
while in cooler and drier weather they take twice or thrice this time.
Working on Monotropa uniflora, Shibata (1902) found that in the
first week of May the pollen tubes took 10 days to reach the embryo

192 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

sac, but in June only 6 days were necessary. He further observed
that tube growth is adversely affected by abnormally low tempera-
tures (8 to 10°C.) as well as abnormally high ones (above 31°C.) and
he was able to lengthen or shorten the interval between pollination
and fertilization merely by altering the experimental conditions.
In plums (Dorsey, 1919) a low temperature (4.5 to 10.5°C.) renders
fertilization uncertain because of a retardation of pollen tube growth.

The first detailed and direct study of the problem was made by
Buchholz and Blakeslee (1927) who found that in Datura stramo-
nium the rate of growth of the pollen tube steadily increased when
the temperature was raised from 11 to 33 °C. At 33 °C. it was four
and a half times that at 11°C.

Since then a somewhat similar range has been reported in several
other plants. In Lycopersicum esculentum (Smith and Cochran,
1935) the maximum growth rate occurs at 21 °C, gradually declining
at both lower and higher temperatures. At 38°C. germination was
extremely poor; 84 hours after pollination only 3.9 per cent of the
pollen grains had germinated, none of the pollen tubes had grown
more than 2 mm. long, and even these had become abnormally
swollen and bulbous at the tips. In Hordeum vulgar e var. pallidum
(Pope, 1943) the optimum temperature is about 23°C. At this
temperature the male gametes were found to reach the embryo sac in
about 20 minutes after pollination, while at 5°C. 140 minutes were
required for the attainment of this stage.

It may be concluded that according to present records (based
mostly on observations made in temperate regions) pollen germina-
tion and tube growth are definitely inhibited at temperatures below
5°C, but occur freely above 10°C. and reach an optimum at about
25 to 30°C. Although the pollen grains themselves can withstand
higher as well as lower temperatures and escape serious damage,
such extremes are definitely harmful to the delicate tissues of the
stigma and style. This may be the reason why many European
vegetables fail to set seed in the tropics.

Another factor which markedly affects the rate of pollen tube
growth is the degree of compatibility between the male gametophyte
and the sporophytic tissues of the pistil. When their reciprocal
relations are correct, the pollen tubes travel down the full length
of the style and fertilization is accomplished before the formation
of an abscission layer at the base of the flower. In incompatible

FERTILIZATION 193

matings, on the other hand, the tubes grow very slowly, if at all,
and the flower withers away before they reach the embryo sac.

In Brassica pekinensis, which has been thoroughly studied by
Stout (1931), the pollen tubes grow rapidly in cross-pollinated
flowers. Self-pollinated flowers, on the other hand, exhibit the
following incompatibility reactions: (1) low percentage of germina-
tion of pollen on "own" stigma, (2) coiling of pollen tubes on the
stigmatic papillae, (3) feeble or limited growth of the tubes through
the style, and (4) coiling of tips of the tubes in the ovary or ovules.
In Petunia violacea (Yasuda, 1930) also the pollen tubes grow
rapidly and reach the base of the pistil in about 36 hours after cross-
pollination. In self-pollinated flowers, on the other hand, not only
is the initial growth rate much lower, but also it continues to decrease
and the tubes reach only about one-fifth of the length of the style,
forming irregular swellings at their tips. In sugar solutions to which
an extract of "own" stigma is added, the growth of the tubes is also
extremely slow, but on the addition of extract from a different strain
of the species the tubes grow normally. Anderson and Sax (1934)
report that in Tradescantia the growth of the incompatible pollen
tubes is much slower and the generative cell does not enter the tube
even after 24 hours, while in compatible matings this takes place
in only 40 minutes. In Linaria reticulata (Sears, 1937) compatible
tubes reach the base of the style in less than 25 hours, while in-
compatible tubes grow only about one-fourth of the distance even
in four days' time. In Nemesia strumosa (Sears, 1937) incompatible
tubes grow at approximately the same rate as compatible ones
through the first three-fourths or four-fifths of the style, but slow
down rather suddenly and finally come to a stop at the base of the
style. In Trifolium repens (Atwood, 1941) incompatible matings
are characterized by two interference zones, one on the stigma and
the other in the style. Germination of pollen is poor, and the few
pollen tubes which happen to be formed seldom travel more than
three-fourths of the way down the style. Those which do proceed
further grow so slowly that the flowers wither and fall off before
fertilization can take place.

An effect more or less similar to the above is seen in the so-called
"illegitimate pollinations" between plants showing heterostyly.
Working on Fagopyrum esculentum, Stevens (1912) found that in
legitimate pollinations, fertilization takes place in 18 hours, but in

194 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

illegitimate pollinations more than 72 hours are required and fre-
quently fertilization fails altogether.2

A point of considerable interest is that while compatible pollen
tubes grow faster at higher temperatures, the incompatible tubes
show a further decline in their growth rate. This seems to indicate
that incompatibility is probably due to a chemical reaction similar
to an antigen-antibody reaction in animals, and that its speed, like
that of all chemical reactions, is increased by a rise in temperature
(Lewis, 1948).

Gametic Fusion. After the pollen tube has discharged its con-
tents into the embryo sac, one male gamete fuses with the egg
(syngamy) and the other with the two polar nuclei (triple fusion).
Because of the technical difficulties encountered in studying the
process, very few detailed accounts of it have appeared up to this
time. The time between the beginning and the end of the gametic
fusions is so short that one rarely succeeds in "catching" the material
at the right stage. There is also an element of chance in obtaining
proper median sections, for the embryo sac is usually large enough
at this stage to run into several sections, and thick sections do not
stain satisfactorily. Besides, even if the material has been properly
selected and the desired stages are actually at hand, detailed ob-
servations may still prove difficult for the following reasons: (1)
the pollen tube discharges a deeply staining material into the embryo
sac which surrounds the egg and decreases visibility; (2) one or
both of the synergids disorganize at this time and their contents
become converted into a tenacious mucus-like material which stains
densely; and (3) the vegetative nucleus (or its fragments) and the
nuclei of the synergids "wander" into the upper part of the embryo
sac and are liable to be confused with the male gametes.

In view of these difficulties it is not surprising that our knowledge
of the events concerned with fertilization has not advanced very
far beyond where it stood during the early part of this century.
Several workers have confessed with a feeling of disappointment
that, in spite of repeated efforts and the study of hundreds of ovules,
they failed to find many of the critical stages in the process.

The present status of the subject may be dealt with in two parts :
(1) form and structure of the male gametes, and (2) details of the

2 For further information on this topic, see Ernst, 1936.

FERTILIZATION 195

two fusions — one between the egg and the first male gamete, and
the other between the polar nuclei and the second male gamete.

In form, the male gametes may be spherical as in Erigeron (Land,
1900), ellipsoidal as in Levisticum (Hakansson, 1923), rod-shaped
as in Urtica (Strasburger, 1910), or vermiform as in Lilium (Guig-
nard, 1899) and Fritillaria (Sax, 1916). Frequently they may
change their shape after their discharge into the embryo sac. Na-
waschin (1898) reported that in Fritillaria the sperms lose their
worm -like form just before fertilization. In Silphium (Land, 1900),.
Monotropa (Shibata, 1902), Taraxacum (Poddubnaja-Arnoldi and
Dianowa, 1934), Lacluca (Jones, 1927), and Nicotiana (Goodspeed,
1947) they are at first elongated or oval, but gradually become
shorter and more spherical as they approach the female nuclei. In
Juglans (Nawaschin, 1900), on the other hand, they are spherical
in the beginning but become curved afterward. The male cells
of Vallisneria (Wylie, 1923, 1941) are originally only slightly
longer than broad (Fig. 1 13 A-D) but become considerably elongated
as the pollen tube enters the ovarian cavity (Fig. USE, F). Fi-
nally they once again present a contracted appearance at the time
of their discharge into the embryo sac (Fig. 11SG-J). Gerassimova
(1933) found that the changes undergone by the male gametes of
Crepis are so rapid that it is difficult to follow them satisfactorily.
Eventually the sperms become more or less band-shaped and appear
to consist of two definite halves, folded along their entire length.
Sometimes they roll up into a ball-shaped body, but the two longi-
tudinal halves still remain distinguishable.

There are a few reports of differences in the size and shape of the
two male nuclei discharged by a pollen tube. Guignard (1899)
stated that in Scilla nonscripta the male nucleus destined to fertilize
the egg is smaller than the one fusing with the polar nuclei, and
Hoare (1934) has confirmed this although admitting that the size
difference is not a constant feature. In Lilium auratum (Blackman
and Welsford, 1913), Iris versicolor (Sawyer, 1917), Fritillaria
pudica (Sax, 1916, 1918), Trillium grandiflorum (Nothnagel, 1918),
Acacia bailey ana (Newman, 1934), and Camassia leichtlinii (Smith,
1942) also, the male gamete entering the egg is said to be somewhat
smaller than the one fusing with the polar nuclei and is sometimes
also not so vermiform.

In some other plants the reverse condition has been reported.

196 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Persidsky (1926) states that in Orobanche cumana the sperm nucleus
fertilizing the egg is shaped like a hemisphere and is larger than the
other, which has an oval outline. In 0. ramosa (Finn and Rudenko,
1930) also, the sperm nucleus fusing with the egg is looser and more

Fig. 113. Fertilization in Vallisneria americana. A-D, portions of pollen tubes
from stylar region, showing male cells; note also vegetative nucleus in A, B, and D.
E,F, elongated male cells from pollen tubes which have entered the ovarian cham-
ber. G, upper part of embryo sac, showing egg, one synergid, two male cells, and a
densely staining mass formed by tube and second synergid. H, first male nucleus
lying tangent to the egg nucleus within the egg cell; nucleus of second male cell
emerging from its sheath. /, nucleus of first sperm fusing with egg nucleus and
second sperm nucleus touching the upper polar. Note the two X-bodies at the
tip of the pollen tube. J, later stage, showing male and female nuclei in advanced
stage of fusion, and second sperm nucleus in contact with secondary nucleus. {After
Wylie, 1923,1941.)

FERTILIZATION 197

faintly staining than the other. In Vallisneria (Wylie, 1923) both
nuclei are oval at the time of their discharge from the pollen tube
but the second soon becomes spherical. Very recently Kadry
(1946) has reported that in Cardiospermum the male nucleus fertil-
izing the egg is swollen and rounded in front but tapering behind,
and that it is about four times as long as the other male nucleus,
which is more or less spherical. In Vinca minor (Finn, 1928a) also,
the sperm cells are unequal, one with a larger and the other with a
shorter plasma tail, but it could not be determined which fuses with
the egg and which with the polar nuclei.

Although slight differences in the size and shape of the two male
gametes are possible, the few examples cited above do not seem
adequate to justify any generalization. In the first place consider-
able care has to be taken to make sure that the observed differences
are not due to the plane of sectioning. Stages in fertilization are
infrequent, and in the same section one male gamete may be cut
across its longer diameter and the other in a plane at right angles
to it, so that the two appear to be of different sizes, although in fact
they are quite similar. Further, the male gametes often change
their form and, as Gerassimova (1933) has suggested, the reported
size differences may well be due to a disparity in the rates of their
transformation.

There has been a good deal of discussion about the mechanism of
movement of the generative cell and the male gametes. It is well
known that the antherozoids of the thallophytes, bryophytes, and
pteridophytes are actively motile. Even among the gymnosperms,
the Cycadales and Ginkgoales have ciliated sperms and their pollen
tube serves merely as a haustorial organ. In the Coniferales,
Gnetales, and angiosperms, on the other hand, cilia are absent and
the pollen tube becomes the agent for the transport of the male
gametes from the pollen chamber or the stigma to the embryo sac.
Long ago, Strasburger (1884, 1900, 1908) put forth the view that
the male gametes are carried passively by the streaming movements
of the cytoplasm inside the pollen tube. On the other hand, Na-
waschin (1898) and some other workers believed that the vermiform
appearance of the male gametes, observed in several members of the
Liliaceae and Compositae, is indicative of an independent power of
movement. Studies on living pollen tubes (Wulff, 1933) seem to
support Nawaschin's view. The cytoplasm in the tube shows
several plasma strands running in opposite directions, while the

198 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

male gametes move in the forward direction only. This would
hardly be possible unless the male gametes have the power to move
independently of the strands of cytoplasm.

Concerning the actual course of fusion of the gametic nuclei, only
a few observers have described it in sufficient detail. The most
careful account is that of Gerassimova (1933) on Crepis capillaris.
At the time of its approach to the egg nucleus, the male nucleus has
the appearance of a continuous thread rolled into a ball (Fig. 114A),
which soon begins to unwind and spread out with its entire surface
adjacent to the nuclear membrane of the egg (Fig. 114B). It then
gradually "immerses" itself within the egg nucleus, although still
remaining distinguishable for a long time (Fig. 114C-F). Mean-
while, a nucleolus arises from it "at first as a small, scarcely visible,
weakly stained drop, which gradually increases in size" (Fig.
114E, F). At the same time the body of the sperm "becomes more
porous and breaks up lengthwise, losing its continuity" (Fig. 114G-
H). Very soon the male chromatin becomes indistinguishable from
the female, and it is only the presence of the two nucleoli which
serves to distinguish the fertilized from the unfertilized egg (Fig.
114I-K). Finally, however, the male nucleolus increases in size
and fuses with the female nucleolus "thus closing the perceivable
phenomena of sexual union of the nuclei" (Fig. 114L).

The fusion of the second sperm with the secondary nucleus takes
place in very much the same way. Here also the sperm has the
shape of a rolled thread, which gradually unwinds itself and comes
in contact with the secondary nucleus all along its surface. This is
followed by the immersion of the sperm into the secondary nucleus
and the formation of a small nucleolus. Finally the two chromatins
merge into each other, followed by a fusion of the nucleoli.

From the less frequent occurrence of stages in the fertilization of
the secondary nucleus, Gerassimova concludes that triple fusion is
accomplished more quickly than syngamy. She states that in
triple fusion "the formation of the nucleolus takes place so rapidly
that it is almost impossible to catch the beginning of the process"
and that "although fertilization always begins first in the nucleus
of the egg-cell, the stage of a complete fusion of the nuclei is often
simultaneous and sometimes terminates even earlier in the nucleus
of the endosperm." In several of her preparations she found that
triple fusion had already ended (as judged by the fusion of the

FERTILIZATION

199

Fig. 114. Some stages in syngamy in Crepis capillaris. A, portion of egg nucleus
with sperm lying on nuclear membrance. B, uncoiling of sperm nucleus, showing
dual bead-like appearance at some places. C, D, entry of sperm nucleus inside
nuclear membrane of egg. E-I, a gradual disintegration of the sperm thread and
appearance of its nucleolus. J, K, intermingling of contents of male and female
nuclei. L, later stage, showing fusion of the two nucleoli. (After Gerassimova,
1933.)

200 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

nucleoli), while inside the egg nucleus the male chromatin was still
distinguishable from the female.3

In most plants gametic fusion takes place when the nuclei are
in a resting condition, but there are exceptions. In Lilium Mar-
tagon, L. auratum (Welsford, 1914), L. philadelphicum, L. longi-
florum (Weniger, 1918), Fritillaria pudica (Sax, 1918), and Viola
odorata (Madge, 1929) the fusing nuclei are in prophase. In the
cleistogamous flowers of Viola (Madge, 1929) the male nucleus is in
a "spireme," and in Malus (Wanscher, 1939) "both gametes are in
a spireme stage during their union." In Triticum (Sax, 1918) the
first male nucleus and the nucleus of the egg were both found to
be in the "spireme" stage, but the second male nucleus had already
progressed into the early metaphase stage at the time of its fusion
with the polar nuclei. According to some reports, fusion may also
take place at a stage even before the male nuclei have recovered
from their original telophasic condition. Nawaschin (1909) be-
lieved this to be the case in Lilium martagon, Frisendahl (1912) in
Myricaria germanica, and Newman (1934) in Acacia baileyana. In
Scilla nonscripta, according to Hoare (1934), the male nuclei formed
after the division of the generative cell "never pass into a complete
resting stage" but show a dense chromatin reticulum which is inter-
preted as "the previous early telophase."

Conflicting statements have been made regarding the presence
or absence of a nucleolus in the male nuclei. In certain plants like
Cuscula (Finn, 19376) and Camassia (Smith, 1942) no nucleolus has
been seen. In Orobanche (Finn and Rudenko, 1930), Crepis (Geras-
simova, 1933), Mains (Wanscher, 1939), and Oxybaphus (Cooper,
1949) a nucleolus was not distinguishable at the time of formation
of the sperm cells but could be seen shortly before or during fertil-
ization, and in Taraxacum kok-saghys (Warmke, 1943) it appeared
only after fertilization. In Viola riviniana (West, 1930), on the
other hand, the male nuclei always showed a distinct nucleolus, and
the one entering into triple fusion is said to be represented almost
entirely by its nucleolus. In Orchis maculatus (Hagerup, 1944) also,
the male nuclei have one or more distinct nucleoli.

3 In Tacca (Paetow, 1931) and Jussieua (Khan, 1942) syngamy is completed
only after 16 to 32 endosperm nuclei have been formed. In Oxybaphus nyctagineus
(Cooper, 1949), on the other hand, "nuclear fusion is completed in the zygote prior
to that in the endosperm mother cell."

FERTILIZATION 201

From the cytological as well as the genetical standpoint, it is of
considerable interest to know whether the cytoplasm of the male
gamete also enters the egg in addition to the nucleus. Finn (1925)
considered it probable in Asclepias. Johri (1936a, b) in Sagittaria
graminea and Butomopsis lanceolata, and Smith (1942) in Camassia
leichtlinii, traced the male gametes as distinct cells up to the time
of their discharge into the embryo sac, but were unable to follow
the succeeding events in sufficient detail.

Wylie's (1923, 1941) work on Vallisneria is an important con-
tribution to the subject. The male gametes of V. americana main-
tain their integrity as cells up to the time they approach the egg
(Fig. IIZA-G), and the several cases of physical contact observed
between the first male cell and the egg strongly suggest their union
as protoplasts rather than naked nuclei (Fig. 113H). Further, the
fact that no residue of the first male cell could be seen on the surface
of the zygote (as would be expected if only the naked nucleus entered
the egg), while the same embryo sac clearly showed the cytoplasmic
sheath left behind by the second male gamete, is cited in support of
the view that the sperm fusing with the egg functions as a cell and
the other as a naked nucleus.

Some other investigators have, however, denied any participation
of the male cytoplasm in fertilization. Considering only the more
recent literature, Gurgenova (1928), Gerassimova (1933), and Hoare
(1934) mention having observed almost all stages of fertilization in
Orobanche ( = Phelipaea) ramosa, Crepis capillaris, and Scilla non-
scripta but report only sperm nuclei. Madge (1929) saw male cells
in the pollen tubes of Viola odorata but believed that the sheath is
lost at the time of syngamy. Breslavetz (1930) studied Melan-
drium album, using mitochondrial fixatives, but failed to detect
"even the thinnest plasma layer" around the sperm nuclei. In
Tulipa (Botschanzeva, 1937) the male nuclei are said to slip out of
their sheaths at the time of fertilization; and Trankowsky (1938)
and Gershoy (1940) report the gradual disappearance of the male
cytoplasm in the pollen tubes of Drosera and Viola. In a recent
study of Petunia, Cooper (1946) also implies that the male gametes
lose their sheaths at the time of fertilization.

It seems difficult to set aside all these observations as based on
inadequate technique. Nevertheless, Finn and Rudenko (1930),
in contradistinction to Gurgenova (1928), were able to see the cyto-

202 INTRODUCTION TO EMBRYOLOGY OF ANOIOSPERMS

plasmic sheath around the male nuclei of Orobanche, although they
could not follow it up to the time of fertilization. The inability of
Breslavetz (1930) to find mitochondria and plastids around the
male nuclei also cannot be considered as an absolute proof of the
absence of the plasma layer; in fact her reference to certain lighter
areas around the male nuclei is a fairly good indication of the pres-
ence of the male cytoplasm. Gershoy's observations, of which
only a preliminary account has so far appeared, seem to be contra-
dicted by those of West (1930).

In conclusion, it may be well to emphasize that until a few years
ago the male gametes of angiosperms were usually considered to be
naked nuclei. Recent studies on the subject leave no doubt, how-
ever, that the cytoplasmic sheath remains intact at least for the
period during which the male gametes are in the pollen tube.4 It
would not be surprising, therefore, if with some further improvement
in technique it may be possible in future to trace the fate of the
male cytoplasm in a more precise manner than has been done up
to this time. Finn (1935, 1940, 1941) has suggested that in order
to decide the point with certainty the whole series of events should
be studied in living material, but this seems to be impracticable with
most plants, as the embryo sac is enclosed in several opaque layers
which interfere with a direct and detailed observation of its con-
tents.6 The only alternative is to look for some suitable material
in which (1) fertilization stages may be found abundantly, (2) the
process does not take place too rapidly, and (3) the gametic cells
not only are fairly large but also respond more favorably to our
staining methods. In addition every effort must of course be made
to develop new methods of fixing and staining which would be more
suitable for a study of the contents of the embryo sac at the time of
fertilization.

Multiple Fusions and Polyspermy. As is well known, usually
only one pollen tube enters an ovule. Compton (1912) saw an ovule
of Lychnis with two embryo sacs, each of which had been penetrated

4 See also p. 168.

5 In a few genera like Torenia and Utricularia, where the embryo sac protrudes
out of the micropyle, it may be possible to make direct observations on living
material. In certain others, like Monotropa and some members of the Orchidaceae,
the seed coat is thin and transparent. No recent studies have, however, been made
on the phenomenon of fertilization in any of these plants.

FERTILIZATION 203

by a pollen tube. Another ovule with a single embryo sac had also
received two pollen tubes, but only one of them had entered the sac,
the other remaining behind in the nucellus. He concluded that
there is a quantitative relation between embryo sacs and pollen
tubes, two embryo sacs secreting enough chemotactic material to
attract two pollen tubes, while one can attract only one tube.

Nemec (1931) considered that there are mechanical contrivances
which exclude other pollen tubes from entering an ovule after the
first had done so. He found that in Gagea lutea the micropyle is
originally in close contact with the glandular conducting tissue of
the placenta, but that after a pollen tube has entered the micropyle,
there is a slight elongation of the funiculus causing the ovule to
retract from its original position and thus make it difficult for other
tubes to gain entrance.

Beside the position effect noted by Nemec, there must no doubt
be some other factors also which bring about a similar result. In
the pistils of Phaseolus vulgaris (Weinstein, 1926) there are many
more pollen tubes than ovules, yet only one tube enters each ovule;
the superfluous tubes grow down to the basal end of the ovary and
eventually disintegrate. In Scurrula atropurpurea, Rauch (1936)
frequently saw several pollen tubes attached to the wall of the
embryo sac, but only one entered it, and as soon as this had been
accomplished the wall of the sac seemed to become firmer and more
resistant so as to exclude the others. Cooper (1938) states that in
Pisum sativum he frequently saw two or more pollen tubes at the
entrance to the micropyle, but only one actually entered it. More
recently, Pope (1946) saw an ovule of Hordeum with one pollen
tube inside the micropyle and four at its mouth, but how the embryo
sac admitted the first and excluded the others could not be deter-
mined.

Although one pollen tube to an embryo sac may thus be considered
as the usual condition, the entry of accessory tubes is not unknown.
To quote a few examples, two pollen tubes have been recorded in
Elodea (Wylie, 1904), Ulmus (Shattuck, 1905), Juglans (Langdon,
1934), Xyris (Weinzieher, 1914), Oenothera (Ishikawa, 1918),
Boerhaavia (Maheshwari, 1929), Beta (Artschwager and Starrett,
1933), Acacia (Newman, 1934), Fagopyrum (Mahony, 1935), Sagit-
taria (Johri, 19366), Cephalanthera, Platanihera (Hagerup, 1947),

204 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

and Nicotiana (Goodspeed, 1947); three in Statice (Dahlgren, 1916),
Gossypium (Iyengar, 1938), and Orchis (Hagerup, 1944); and as
many as five in Juglans (Nawaschin and Finn, 1913).

The entry of additional pollen tubes naturally results in the
release of supernumerary male gametes inside the embryo sac.
Rarely, one and the same pollen tube may also carry more than

two sperm s . This abnormality
may originate either in the pol-
len grain or in the pollen tube.
To mention a few examples,
three sperms were sometimes
seen in the pollen grains of Cus-
cuta epithymum (Fedortschuk,
1931) (Fig. 99C); and four in
Helosis cayennensis (Umiker,
1920), Vinca herbacea (Finn,
1928a) (Fig. 99K), Parthenium
argentatu?n, and P. incanum
(Dianowa, Sosnovetz, and Stes-
china, 1935). Four sperms
have also been seen in the
pollen tubes of Allium rotun-
dum, A. zebdanense (Weber,
1929), Galanthus nivalis (Tran-
kowsky, 1931), Crepis capil-
laris (Gerassimova, 1933) (Fig.
117.D), and Polygonatum canal-
iculatum (Eigsti, 1941) (Fig.
115C). Further, Gerassimova
(1933) saw two, three, and even
five pairs of sperms in the em-
bryo sacs of Crepis capillaris
and considers it probable that
Fig. 115. Pollen grain and pollen tubes in they originated through addi-
Polygonatumcanaliculatum. A, mature pol- tional divisions 0f the original
len gram, showing vegetative nucleus and mirofsDerms Warmke (1943)
two sperms. B, pollen tube, showing elon- K . v '

gated vegetative nucleus and two sperms. saw eight sperms in an em-
C, portion of pollen tube, showing four Dry° sac of Taraxacum kok-
sperms. {After Eigsti, 19 %1.) saghys, and in another there

FERTILIZATION 205

were eight in addition to the two which took part in fertiliza-
tion.

The presence of extra sperms inside the embryo sac, whether they
are derived from one or more than one pollen tube, may result in
two kinds of abnormalities. Either some of the supernumerary
sperms enter the egg, resulting in a polyploid offspring, or more
than one cell of the egg apparatus may be fertilized, resulting in
multiple embryos. In Monotropa hypopitys (Strasburger, 1884),
Iris sibirica (Dodel, 1891), and Gagea lutea (Nemec, 1912), and in
Oenothera nutans pollinated by 0. pycnocarpa (Ishikawa, 1918), two
sperms were sometimes observed to enter the egg. Michaelis
and Dellinghausen (1942), who obtained a triploid plant from a
cross between Epilobium hirsutum and E. luteum, considers it
probable that it resulted from the entry of an E. luteum sperm in-
to the egg after it had already been fertilized by an E. hirsutum
sperm.

Fertilization of more than one cell of the egg apparatus has been
reported in several plants6 of which Sagittaria graminea (Johri,
19366) and Crepis capillaris (Gerassimova, 1933) may be cited as
examples. In Sagittaria fertilization usually occurs normally, one
male gamete fusing with the egg and the other with the two polar
nuclei. But the synergids often assume an egg-like appearance
(Fig. 116 A) and sometimes a second pollen tube enters the embryo
sac, releasing two additional sperms (Fig. 1165). Although an
actual fertilization of the synergids was not seen, the presence of
two pollen tubes and three proembryos in the upper part of the
embryo sac (Fig. 1162?) leave no doubt that this may happen.

In Crepis capillaris, Gerassimova (1933) observed some embryo
sacs with two to five eggs in addition to the two synergids (Fig.
117A). Usually only one of the eggs gives rise to an embryo (Fig.
117J5) and the others eventually degenerate and disappear, but if a
pollen tube carrying more than two sperms enters the embryo sac
there is a possibility of the production of additional embryos.

6Tischler (1943) has recently given a complete list of such plants but in some
cases the inference is based merely on the presence of a second or third embryo
beside the zygotic embryo. It is now known that even unfertilized synergids can
undergo a few divisions and sometimes develop into fully mature haploid embryos,
or the zygotic embryo may itself give rise to additional embryos by a process of
cleavage or budding (see Chaps. 9 and 10).

206

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Figure 117C shows two fertilized eggs in division, one in prophase
and the other in metaphase.

A couple of instances of fertilization of antipodal cells are also on

i i

r->

jr*

*.".'g

7"'': 3

#7/

Fig. 116. Some abnormalities of fertilization in Sagittaria graminea. A, upper
part of embryo sac, showing egg-like synergids, egg, and upper polar nucleus. B,
embryo sac, showing double fertilization (spi = male gamete fusing with egg; sp2 =
male gamete fusing with polar nuclei); note second pollen tube with another pair
of male gametes (sps, sp4). C, first division of primary endosperm nucleus com-
pleted; nucleus of zygote in metaphase. D, part of fertilized embryo sac, showing
two-celled proembryo, and two male gametes attached to two endosperm nuclei.
E, three two-celled proembryos, two of which have probably arisen from fertilized
synergids; note two pollen tubes. (After Johri, 1936a.)

record. Derschau (1918) reported the fusion of two sperms with
an antipodal cell in Nigella arvensis, and judging from the state-
ments of Shattuck (1905) and Ekdahl (1941) it seems probable that

FERTILIZATION

207

sometimes the fertilization of an antipodal cell may also occur in
Ulmus.

Embryo sacs which have received more than two sperms may also
show other abnormalities. Frisendahl (1912) noted that in Myri-
caria germanica each of the polar nuclei may sometimes fuse with a

Fig. 117. Some abnormalities of fertilization in Crepis capillaris. A, upper part
of embryo sac, showing four eggs and two sjmergids. B, four egg-like cells and
two-celled embryo. C, supernumerary sperm near apex of embryo sac; two eggs,
one in prophase and the other in metaphase, and two dividing endosperm nuclei.
D, portion of pollen tube, showing two pairs of sperms and unidentified body at
upper end which may be one of a third pair of sperms. (After Gerassimova, 1933.)

separate male nucleus.7 In Acacia bailcijana, Newman (1934) ob-
served division figures of the primary endosperm nucleus showing
4:n, 7n, and 8n chromosomes.8 Since such embryo sacs seemed to
have received more than one pollen tube, it is inferred that extra

7 Since the upper polar nucleus in Myricaria is haploid and the lower triploid,
some of the endosperm nuclei would be diploid (n + n) and some tetraploid
(3 n + n).

8 Rarely he found only the haploid number of chromosomes, presumably due to
an independent division of either a polar nucleus or a male nucleus.

208 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

sperms fused with the secondary nucleus. A different type of
abnormality, reported sometimes, is that the triple fusion nucleus
undergoes one or two divisions and then the male nuclei discharged
from a second pollen tube fuse with some of the endosperm nuclei
(Fig. 116Z)).

When two or more pollen tubes are discharged inside an embryo
sac, it is also possible that the sperm fusing with the egg is derived
from one tube and the one fusing with the secondary nucleus from
another. This "heterofertilization" has not yet been cytologically
demonstrated, but Sprague (1932), has inferred it in Zea mays on
genetical grounds. In a study of the inheritance of scutellum color
he found several kernels with white aleurone but a colored scutellum,
and their progeny segregated for aleurone color in ratios similar
to those obtained from hybrid kernels with colored aleurone. He
therefore concludes that in such cases the egg and polar nuclei had
been fertilized by sperms of unlike genotypes.

Single Fertilization. Although double fertilization is the rule in
angiosperms, the question arises whether development can proceed
with only a single fertilization, i.e., if there is syngamy without
triple fusion or triple fusion without syngamy. Cooke and Shively
(1904) stated that in Epiphegus virginiana endosperm formation
begins before fertilization, and Anderson (1922) reported the same
in Martynia louisiana. In Ramondia nathaliae and R. serbica
(Glisic, 1924) syngamy occurs regularly, but triple fusion is said
to be "facultative" and is frequently omitted. Wiger (1935) stated
that in some members of the Buxaceae and Meliaceae, endosperm
formation is entirely independent of fertilization.
1 All these reports are, however, of a doubtful nature (see also
Mauritzon, 1935). Without going into details it may be said that
some of the above workers seem to have overlooked the pollen tube,
and others mistook the unfused polar nuclei for the first pair of
endosperm nuclei. It is only rarely that development can take
place without triple fusion. Guignard (1921) reported a case in
Vincetoxicum nigrum in which the zygote had divided several times
while the polar nuclei were still lying unfused and the second male
gamete had not yet been discharged from the pollen tube. More
recently, Dahlgren (1930, 1939) has figured embryo sacs of Mitella
pentandra and Zostcra marina (Fig. 118) in which a several-celled
embryo is associated with an undivided secondary nucleus, and

FERTILIZATION

209

Johansen (1931a, b) has reported similar occurrences in Taraxia
ovata and Zauschneria latifolia. Sooner or later, however, such
embryos are likely to stop growth so that no viable seeds are pro-
duced.

The second of the two alternatives — i.e., the
occurrence of triple fusion without an accom-
paniment of syngamy— has been reported in
several plants, but the ovules soon begin to de-
generate. If seeds are formed, they are with-
out embryos and therefore nonviable. Rarely,
however, the unfertilized egg may develop into
a haploid embryo. Such cases will be discussed
in connection with apomixis (see Chap. 9).

Persistence and Possible Haustorial Func-
tion of Pollen Tube. Usually the pollen tube
collapses soon after fertilization, and there is
little evidence of it after the embryo has com-
menced its development. There are a few cases
on record, however, in which it has been known
to persist for longer periods. In Galinsoga
ciliata (Popham, 1938) it is recognizable up to
the seven-celled stage of the embryo, and in
Ulmus americana (Shattuck, 1905) up to the
20-celled satge. In Hicoria pecan, according to
Woodroof (1928), it persists for two to three
weeks, and in one ovule he saw the dead end of
the tube beside the fertilized egg even seven
weeks after pollination.

Cook (1909) noted a very peculiar behavior
of the pollen tube in Passi flora adenophylla.
Although fertilization stages were frequent, in
the majority of ovules the pollen tube did not
discharge its contents but continued its growth
within the embryo sac, becoming greatly twisted
and tangled in the process. Its growth was
sometimes so vigorous that all the contents of
the sac, including the egg apparatus, were com-
pletely absorbed. A few years later, Cook

Fig. 118. L.s. embryo
sac of Zostera marina,
showing young em-
bryo and undivided
secondary nucleus.

(1924) noted a similar phenomenon in an ovule (After Dahlgren,1989.)

210 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

of Crotalaria sagittalis. Cook regarded these as examples of para-
sitization of the male gametophyte on the female.

It has also been suggested that the pollen tube may sometimes
serve as a haustorial organ, not for its own benefit but for that of the
embryo sac or embryo. Longo (1903) believes this to be the case
in Cucurbita. He found that owing to a cutinization of the walls
of the nucellar epidermis and the formation of a suberized hypostase,
the embryo sac becomes cut off from the usual sources of its food
supply. The pollen tube is said to compensate for this deficiency.
As it approaches the embryo sac, it expands into a large swelling or
"bulla," which gives out a number of branches. One of these pene-
trates the embryo sac and effects fertilization, but the others ramify
into the tissues of the nucellus and inner integument, absorbing food
materials from the adjacent cells and transmitting them to the
embryo.

A somewhat similar phenomenon has been reported in certain
members of the Onagraceae (Werner, 1914; Tackholm, 1915). The
pollen tube becomes greatly broadened in the micropyle9 and often
sends out branches into the outer integument and the nucellus, while
the tip continues to grow toward the embryo sac, destroying the
cells lying in its path. The tube is recognizable even when the
embryo has attained a fairly large size, and it probably serves to
absorb food materials from the surrounding tissues and transmit
them to the embryo.

A broad and massive pollen tube has also been seen in Carica
papaya (Foster, 1943). It persists for about eight weeks after
fertilization and probably helps in conveying food materials to the
embryo. In Ottelia alismoidcs, Hydrilla verticillata (Maheshwari
and Johri, 1950), and Oxybaphus nyctagineus (Cooper, 1949) the
pollen tube persists throughout the development of the seed. Ac-
cording to Cooper, it acts as a haustorium and transports nutritive
materials from the secretory cells of the funiculus to the embryo.

9 A broad and persistent pollen tube may sometimes be mistaken for a synergid
haustorium or a micropylar extension of the embryo sac. Karsten (1891) reported
that in Sonneratia the fertilized embryo sac bored a hole through the wall layers
and came in direct contact with the nucellar epidermis, but Venkateswarlu (1937)
and Mauritzon (1939) have shown that this is incorrect and that Karsten was
actually looking at the pollen tube.

FERTILIZATION 211

X-bodies. In his studies on fertilization Nawaschin observed
certain densely staining structures either in the tip of the pollen
tube or adjacent to it. Since their exact nature could not be deter-
mined, he called them X-bodies. They have been variously inter-
preted in different plants and by different authors. In Adoxa
they are believed to be the nuclei of the disorganized synergids
(Lagerberg, 1909); in Crepis as fragments of the vegetative nucleus
(Gerassimova, 1933); in Petunia as the degenerated cytoplasmic
sheaths of the male gametes (Cooper, 1946); and in Beta as super-
numerary male nuclei in process of disintegration (Artschwager and
Starrett, 1933). Some authors have also interpreted them as the
nuclei of the adjacent nucellar cells, which become pushed into the
embryo sac by the impact of the pollen tube. Tschernojarow
(1926) states that in Myosurus they represent the remains of the
degenerating megaspores and nucellar cells which lie over the em-
bryo sac and are presumably carried into it at the time of entry of
the pollen tube. In Datura (Satina and Blakeslee, 1935) there are
two X-bodies, one said to be the nucleus of a disorganized synergid
and the other the degenerating Vegetative nucleus.

Wylie (1923) devoted special attention to the nature of the
X-bodies in Vallisneria (Fig. 1137). Since the sperms enter the
embryo sac as complete cells, he rules out the possibility of the
X-bodies being the remains of their cytoplasmic sheaths. The
sheaths were also seen intact in the "cystoids" or "tuber-like en-
largements" formed by some pollen tubes which terminated in the
ovarian chamber without reaching the ovules. The cystoids showed
no X-bodies if the vegetative nucleus was still intact, but in those
cases in which this was not visible they showed structures which
were identical with X-bodies. It is therefore concluded by Wylie
that, whatever their origin in other plants, the X-bodies of Vallis-
neria are nothing other than the decomposition products of the
vegetative nucleus.

References

Anderson, E., and Sax, K 1934. A cytological study of self-sterility in Trade-

scantia. Bot. Gaz. 95: 609-621.
Anderson, F. 1922. The development of the flower and embryogeny of Martynia

louisiana. Bui. Torrey Bot. Club 49: 141-157.

212 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Artschwager, E., Brandes, E. W., and Starrett, R. C. 1929. Development of
flower and seed of some varieties of sugarcane. Jour. Agr. Res. 39: 1-30.

— and McGuire, R. C. 1949. Cytology of reproduction in Sorghum vulgare.

Jour. Agr. Res. 78: 659-673.

and Starrett, R. C. 1933. The time factor in fertilization and embryo

development in sugar beet. Jour. Agr. Res. 47: 823-843.
Atwood, S. S. 1941. Cytological basis for incompatibility in Trijolium repens.

Amer. Jour. Bot. 28: 551-557.
Bagda, H. 1948. Morphologische und biologische Untersuchungen uber Valonea

Eichen (Quercus macrolepis Ky.) im Haci-Kadin-Tal bei Ankara Comm.

Fac. Sci. de l'Univ. Ankara. 1: 89-125.
Benson, M. 1894. Contribution to the embryology of the Amentiferae. I.

Trans. Linn. Soc. London. III. Bot. 2: 409-424.
Blackman, V. H., and Welsford, E. J. 1913. Fertilisation in Lilium. Ann. Bot.

27: 111-114.
Botschanzeva, Z. P. 1937. Spermogenesis bei Tulipa una seine Erlerung zwecKs

Kariosystematik. Bui. Univ. Asie Centrale [Tashkent] 22: 329-338.
Breslavetz, L. 1930. Spermatogenesis and fertilization process in some plants

in connection with the question of heredity through the plasma. Proc.

U.S.S.R. Cong. Genetics 2: 181-186.
Brough, P. 1933. The life history of Grevillea robusta Cunn. Proc. Linn. Soc.

N. S. Wales 58: 33-73.
Buchholz, J. T., and Blakeslee, A. F. 1927. Pollen-tube growth at various tem-
peratures. Amer. Jour. Bot. 14: 358-369.
Carlson, M. C. 1940. Formation of the seed of Cypripedium parviflorum. Bot.

Gaz. 102: 295-301.
Compton, R. H. 1912. Note on a case of doubling of embryo sac, pollen tube

and embryo. Ann. Bot. 26: 243-244.
Cook, M. T. 1909. Notes on the embryo-sac of Passiflora adenophylla. Bui.

Torrey Bot. Club 36: 373-374.
— . 1924. Development of seed of Crotalaria sagittalis. Bot. Gaz. 77: 440-

445.
Cooke, E., and Shively, A. 1904. Observations on the structure and development

of Epiphegus virginiana. Contrib. Bot. Lab. Univ. Pa. 1904, pp. 352-398.
Cooper, D. C. 1938. Embryology of Pisum sativum. Bot. Gaz. 100: 123-132.
. 1940. Macrosporogenesis and embryology of Portulaca oleracea. Amer.

Jour. Bot. 27: 326-330.
. 1941. Macrosporogenesis and the development of the seed of Phryma

leptostachya. Amer. Jour. Bot. 28: 755-761.

. 1946. Double fertilization in Petunia. Amer. Jour. Bot. 33: 54-57.

. 1949. Flower and seed development in Oxybaphus nydagineus. Amer.

Jour. Bot. 36: 348-355.
Dahlgren, K. V. O. 1915. Uber die tlberwinterungsstadien der Pollensacke und

der Samenanlagen bei einigen Angiospermen. Svensk Bot. Tidskr. 9: 1-12.
. 1916. Zytologische und embryologische Studien liber die Reihen Primu-

lales und Plumbaginales. K. Svenska Vet.-Akad. Handl. 56(4) : 1-80.
. 1920. Zur Embryologie der Kompositen mit besonderer Berucksichti-

gung der Endospermbildung. Ztschr. f. Bot. 12: 481-516.

FERTILIZATION 213

Dahlgren, K. V. 0. 1930. Zur Embryologie der Saxifragoicleen. Svensk Bot.

Tidskr. 24: 429-448.
. 1938. Hakenbildungen bei Synergiden. Zweite Mitteilung. Svensk

Bot. Tidskr. 52: 221-237.

1939. Endosperm- und Embryobildung bei Zostera marina. Bot.

Notiser 1939, pp. 607-615.
Derschau, M. v. 1918. liber disperme Befruchtung der Antipoden von Nigella

arvensis. Ber. deutsch. bot. Gesell. 36: 260-262.
Dianowa, W. J., Sosnovetz, A. A., and Steschina, N. A. 1935. Vergleichende

zytoembryologische Untersuchung der Varietaten vom Parthenium argentatum

Gray und P. incanum Gray. Beihefte bot. Centbl. 53A: 293-339.
Dodel, A. 1891. Beitrage zur Kenntnis der Befruchtungserscheinungen bei

7m sibirica. Festschr. f. Niigeli u. Kolliker, Zurich 1891.
Dorsey, M. J. 1919. A study of sterility in the plum. Genetics 4: 417-488.
Duncan, R. E., and Curtis, J. T. 1942a. Intermittent growth of Phalaenopsis.

A correlation of the growth phases of an orchid fruit with internal development.

Bui. Torrey Bot. Club 69: 167-183.
and . 19426. Intermittent growth of fruits of Cypripedium and

Paphiopedilum. A correlation of the growth of orchid fruits with their internal

development. Bui. Torrey Bot. Club 69: 353-359.

and . 1943. Growth of fruits in Cattleya and allied genera in the

Orchidaceae. Bui. Torrey Bot. Club 70: 104-119.
Eigsti, O. J. 1937. Pollen tube behavior in self-fertile and interspecific pollinated

Resedaceae. Amer. Nat. 71: 520-521.
. 1941. The occurrence of a pollen tube with four sperms and two tube

nuclei in Polygonatum. Proc. Okla. Acad. Sci. 21: 134-136.
Ekdahl, I. 1941. Die Entwicklung von Embryosack und Embryo bei Ulmus

glabra Huds. Svensk Bot. Tidskr. 35: 143-156.
Ernst, A. 1936. Der heutige Stand der Heterostylieforschung. Proc. 6th In-
ternal. Bot. Cong. 35-53.
Ernst-Schwarzenbach, M. 1945. Kreuzungsversuche an Hydrocharitaceen.

Arch. Julius Klaus-Stift. f. Vererbungsforsch 20: 22-41.
Fagerlind, F. 1939. Die Entwicklung des Embryosackes bei Peperomia pellu-

cida. Arkiv for. Bot. 29A: 1-15.
. 1941. Der Bau der Samenanlage und des Makrogametophyten bei

Quisqualis indica. Bot. Notiser 1941, pp. 217-222.
. 1944. Die Samenbildung und die Zytologie bei agamospermischen und

sexuellen Arten von Elatostema und einigen nahestehenden Gattungen nebst
Beleuchtung einiger damit zusammenhangender Probleme. K. Svenska
Vet.-Akad. Handl. Ill 21(4): 1-130.

Fedortschuk, W. 1931. Embryologische Untersuchung von Cuscuta monogyna
Vahl. und Cuscuta epithymum L. Planta 14: 94-111.

Finn, W. W. 1925. Male cells in angiosperms. I. Spermatogenesis and fertil-
ization in Asclepias cornutii. Bot. Gaz. 80: 1-25.

. 1928a. Spermazellen bei Vinca minor und V. herbacea. Ber. deutsch. bot.

Gesell. 46: 235-246.

. 19286. liber den Pollenschlauch bei Fagus sylvatica. Nawaschin

Festschr., pp. 63-66.

214 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Finn, W. W. 1935. Einige Bemerkungen liber den mannlichen Gametophyten

der Angiospermen. Ber. deutsch. bot. Gesell. 53: 679-6S6.
. 1936. Zur Entwicklungsgeschichte der Chalazogamen, Ostrya car-

pinifolia Scop. Jour. Inst. Bot. Acad. Sci. Ukraine 8: 15-25.
. 1937a. Entwicklungsgeschwindigkeit des mannlichen Gametophyten

bei den Angiospermen. Trav. Inst. Rech. Sci. Biol., Univ. Kiew 1937, pp.

71-86.
. 19376. Vergleichende Embryologie und Karyologie einiger Cuscuta-

Arten. Jour. Inst. Bot. Acad. Sci. Ukraine 20(2S) : 77-96.
. 1940. Spermazellen bei Angiospermen. Jour. Bot. de l'U.R.S.S. 26:

155-175.
. 1941. Mannlichen Gameten bei Angiospermen. Compt. Rend. (Dok.)

Acad, des Sci. U.R.S.S. 30: 451-458.

and Rudenko, T. 1930. Spermatogenesis und Befruchtung bei einigen

Orobanchaceae. Bui. Jard. Bot. de Kieff 11: 69-82.
Foster, L. T. 1943. Morphological and cytological studies on Carica papaya.

Bot. Gaz. 105: 116-126.
Frisendahl, A. 1912. Cytologische und Entwicklungsgeschichtliche Studien an

Myricaria germanica Desv. K. Svenska Vet.-Akad. Handl. 48(7): 1-62.
. 1927. Uber die Entwicklung chasmogamer und kleistogamer Bliiten

bei der Gattung Elatine. Acta Hort. Gothoburg 3: 99-142.
Fuchs, A. 1936. Untersuchungen liber den mannlichen Gametophyten von

Elaeagnus angustijolia. Osterr. bot. Ztschr. 85: 1-16.
. 1938. Beitrage zur Embryologie der Thymelaeaceae. Osterr. bot.

Ztschr. 87: 1-41.
Gelin, 0. E. V. 1936. Zur Embryologie und Zytologie von Berkheya bergiana

Soderb. und B. aldami Hook. f. Svensk Bot. Tidskr. 30: 324-328.
Gerassimova, H. 1933. Fertilization in Crepis capillaris. Cellule 42: 103-148.
Gershoy, A. 1940. The male gametophyte in some species of violets. Amer.

Jour. Bot. 27: 4s.
Glisic, Lj. 1924. "Development of the X-generation and embryo in Ramondia."

Diss. Belgrade.
Goodspeed, T. H. 1947. Maturation of the gametes and fertilization in Nico-

tiana. Madrono 9: 110-120.
Gore, U. R. 1932. The development of the female gametophyte and embryo in

cotton. Amer. Jour. Bot. 19: 795-S07.
Grimm, J. 1912. Entwicklungsgeschichtliche Untersuchungen an Rims und

Coriaria. Flora 104: 309-334.
Guignard, L. 1899. Sur les antherozoides et la double copulation sexuelle chez

les vegetaux angiospermes. Compt. Rend. Acad, des Sci. Paris 128: 864-S71.
. 1921. La fecondation et la polyembryonie chez les Vincetoxicum.

Mem. Acad. Sci. Inst. France II, 57: 1-25.
Gurgenova, M. 1928. Fertilization in Phelipaea ramosa. Nawaschin Festschr.,

pp. 157-168.
Hagerup, O. 1944. On fertilisation, polyploidy and haploidy in Orchis maculatus

L. Dansk Bot. Arkiv. 11(5): 1-26.

FERTILIZATION 215

Hagerup, 0. 1947. The spontaneous formation of haploid, polyploid, and aneuploid

embryos in some orchids. K. Danske Vidensk. Selsk., Biol. Meddel. 20(9) : 1-22.

Hakansson, A. 1923. Studien liber die Entwicklungsgeschichte der Umbelliferen.

Lunds Univ. Arsskr. N.F. Avd. II, 18: 1-120.
Hallock, F. A. 1930. The relationship of Garrya. The development of the flower

and seeds of Garrya and its bearing on the phylogenetic position of the genus.

Ann. Bot. 44: 771-812.
Hanf, M. 1935. Vergleichende und entwicklungsgeschichtliche Untersuchungen

iiber Morphologie und Anatomie der Griffel und Griffelaste. Beihefte bot.

Centbl. 54A: 99-141.
Heimann-Winawer, P. 1919. Beitnige zur Embryologie von Colchicum autum-

nale L. Arb. Inst. allg. Bot. Pflanzenphysiol, Univ. Zurich 21.
Hoare, G. 1934. Gametogenesis and fertilization in Scilla nonscripta. Cellule

42: 269-292.
Hofmeister, W. 1849. "Die Entstehung des Embryo der Phanerogamen."

Leipzig.
. 1861. Neue Beitrage zur Kenntnis der Embryobildung der Phanero-
gamen. II. Monokotyledonen. Abh. Konigl. Sachs. Ges. Wiss. 7: 629-760.
Ishikawa, M. 191S. Studies on the embryo sac and fertilization in Oenothera.

Ann. Bot. 32: 279-317.
Iyengar, N. K. 1938. Pollen-tube studies in Gossypium. Jour. Genet. 37:

69-106.
Johansen, D. A. 1931a. Studies on the morphology of the Onagraceae. III.

Taraxia ovata (Nutt.) Small. Ann. Bot. 45: 111-124.
. 19316. Studies on the morphology of the Onagraceae. V. Zauschneria

latifolia, typical of a genus characterised by irregular embryology. Ann.

N. Y. Acad. Sci. 33: 1-26.
Johri, B. M. 1936a. Studies in the family Alismaceae. IV. Alisma plantago

L.; A. plantago-aquatica L.; and Sagittaria graminea Mich. Proc. Indian

Acad. Sci. Sect. B. 4: 12S-13S.
. 19366. The life history of Butomopsis lanceolata Kunth. Proc. Indian

Acad. Sci. Sect. B. 4: 139-162.
Jones, H. A. 1927. Pollination and life history studies of lettuce (Lactuca

sativa). Hilgardia 2: 425-442.
. 1929. Pollination and life history studies of lettuce (Lactuca sativa).

Proc. Internatl. Cong. Plant Sci., Ithaca N. Y. 2: 1045-1049.
Juliano, J. B. 1935. Anatomy and morphology of the Bunga, Aeginetia indica

L. Philippine Jour. Sci. 56: 405-451.
and Aldama, M. J. 1937. Morphology of Oryza sativa L. Philippine

Agr. 26: 1-134.
Junell, S. 1931. Die Entwicklungsgeschichte von Circaeaster agrestis. Svensk

Bot. Tidskr. 25: 238-270.
Kadry, A. E. R. 1946. Embryology of Cardiospermum halicacabum L. Svensk

Bot. Tidskr. 40: 111-126.
Karsten, G. 1891. Uber die Mangrove-Vegetation im malayischen Archipel.
Biblioth. Bot 22.

216 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Katz, E. 1926. tlber die Funktion der Narbe bei der Keimung des Pollens.

Flora 120: 243-273.
Khan, R. 1942. A contribution to the embryology of Jussieua repens L. Jour.

Indian Bot. Soc. 21: 267-282.
Kirkwood, J. E. 1906. The pollen tube in some of the Cucurbitaceae. Bui.

Torrey Bot. Club 33: 327-342.
Krupko, S. 1944. On the sterility of Oxalis cernua in the Mediterranean. Przy-

roda 1944, pp. 1-32.
Lagerberg, T. 1909. Studien uber die Entwicklungsgeschichte und systematische

Stellung von Adoxa moschatellina L. K. Svenska Vet.-Akad. Handl. 44(4):

1-86.
Land, W. J. G. 1900. Double fertilization in Compositae. Bot. Gaz. 30:

252-260.
Langdon, L. M. 1934. Embryogeny of Carya and Juglans, a comparative study.

Bot. Gaz. 96: 93-117.
Lebon, E. 1929. Sur la formation de l'albumen chez Impatiens sultani. Compt.

Rend. Soc. de Biol. Paris 101: 1168-1170.
Lewis, D. 1948. Pollen-tubes, incompatibility and osmotic pressure in plants.

School Sci. Rev. 108: 206-212.
Longo, B. 1901 . La mesogamia nella commune zucca (Cucurbita pepo L.). Rend.

R. Accad. Lincei Roma V. 10: 168-172.
. 1903. La nutrizione delPembrione delle Cucurbita operate per mezzo del

tubetto pollinico. Ann. di Bot. 1: 71-74.
Madge, M. 1929. Spermatogenesis and fertilisation in the cleistogamous flowers

of Viola odorata var. praecox. Ann. Bot. 43: 545-577.
Maheshwari, P. 1929. Contributions to the morphology of Boerhaavia diffusa.

I. Jour. Indian Bot. Soc. 8: 219-234.
. 1948. The angiosperm embryo sac. Bot. Rev. 14: 1-56.

1949. The male gametophyte of angiosperms. Bot. Rev. 15: 1-75.

and Johri, B. M. 1941. The embryo sac of Acalypha indica L. Beihefte

bot. Centbl. 61A: 125-136.

and . 1950. The occurrence of persistent pollen tubes in

Hydrilla, Ottelia and Boerhaavia, together with a discussion of the possible
significance of this phenomenon! in the life history of angiosperms. Jour.
Indian Bot. Soc. 29: 47-51.

and Singh, Bahadur. 1934. A preliminary note on the morphology of the

aerial and underground flowers of Commelina benghalensis Linn. Current

Sci. [India] 3: 158-160.
Mahony, K. L. 1935. Morphological and cytological studies on Fagopyrum

esculentum. Amer. Jour. Bot. 22: 460-475.
Martin, J. N. 1913. The physiology of pollen of Trifolium pratense. Bot. Gaz.

56: 112-126.
Mauritzon, J. 1935. Kritik von J. Wigers Abhandlung "Embryological studies

on the families Buxaceae, Meliaceae, Simarubaceae and Burseraceae." Bot.

Notiser 1935, pp. 490-502.
. 1939. Contributions to the embryology of the orders Rosales and

Myrtales. Lunds Univ. Arsskr. N.F. Avd. II, 35: 1-120.

FERTILIZATION 217

McKay, J. W. 1947. Embryology of pecan. Jour. Agr. Res. 74: 263-283.

Mendes, A. J. T. 1941. Cytological observations in Cojjea. VI. Embryo and
endosperm development in Coffea arabica L. Amer. Jour. Bot. 28: 784-789.

Michaelis, P., and Dellinghausen, M. v. 1942. tlber reziprok verschiedene
Sippenbastarde bei Epilobium hirsutum. IV. Weitere Untersuchungen iiber
die genetischen Grundlagen der extrem stark gestorten Bastarde der E. hirsu-
fatra-Sippe. Jenaische Ztschr. f. Vererbungslehre 80 (3): 373-429.

Miller, E. C. 1919. Development of the pistillate spikelet and fertilization in
Zea mays L. Jour. Agr. Res. 18: 255-266.

Murbeck, S. 1901. Uber das Verhalten des Pollenschlauches bei Alchemilla

arvensis und das Wesen der Chalazogamie. Lunds Univ. Arsskr. N. F. Avd.

II, 9: 1-46.
Murthy, S. N. 1941. Morphological studies on the Sapotaceae. I. Embryology

of Bassia latifolia Roxb. and related genera. Jour. Mysore Univ. Sect. B. 2:

67-80.
Nast, C. G. 1941. The embryogeny and seedling morphology of Juglans regia.

L. Lilloa 6: 163-205.
Nawaschin, S. G. 1898. Resultate einer Revision der Befruchtungsvorgange

bei Lilium martagon und Fritillaria tenella. Bui. Acad. Imp. des Sci. St.

Petersburg 9: 377-382.
. 1900. tlber die Befruchtungsvorgange bei einigen Dicotyledonen. Ber.

deutsch. bot. Gesell. 18: 224-230.
. 1909. Uber dasselbstandige Bewegungsvermogen des Spermakerns bei

einigen Angiospermen. Osterr. bot. Ztschr. 59: 457-467.

and Finn, W. W. 1913. Zur Entwicklungsgeschichte der Chalazogamen,

Juglans regia und Juglans nigra. M£m. Acad. Imp. des Sci. St. Petersburg

VIII, 31: 1-59.
Neniec, B. 1912. Uber die Befruchtung bei Gagea. Bui. Internatl. Acad. Sci.

Boheme 17.

. 1931. Fecundation in Gagea lutea. Preslia 10: 104-110.

Newman, I. V. 1934. Studies in the Australian acacias. IV. The life history

of Acacia baileyana F.V.M. Part 2. Gametophytes, fertilization, seed pro-
duction and germination, and general conclusions. Proc. Linn. Soc. N. S.

Wales 59: 277-313.
Nothnagel, M. 1918. Fecundation and formation of the primary endosperm

nucleus in certain Liliaceae. Bot. Gaz. 66: 141-161.
Ohga, I. 1937. On the fertilization of Nelmnbo nucifera. Cytologia, Fujii

Jubl. Vol., pp. 1033-1035.
Paetow, W. 1931. Embryologische Untersuchungen an Taccaceen, Meliaceen

und Dilleniaceen. Planta 14: 441-470.
Pastrana, M. D., and Santos, J. K. 1931. A contribution on the life history of

Dendrobium anosmum Lindley. Nat. and Appl. Sci. Bui., Philippines Univ.

1: 133-144.
Persidsky, D. 1926. Zur Embryologie der Orobanche cumana Wall, und der

0. ramosa L. Bui. Jard. Bot. de Kieff 4: 6-10.
Poddubnaja-Arnoldi, V. A. and Dianowa, V. 1934. Eine zytoembryologische

Untersuchung einiger Arten der Gattung Taraxacum. Planta 23: 19-46.

218 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Pope, M. N. 1937. The time factor in pollen tube growth and fertilization in

barley. Jour. Agr. Res. 54: 525-529.
. 1943. The temperature factor in fertilization and growth of barley.

Jour. Agr. Res. 66: 389-402.
— . 1946. The course of the pollen tube in cultivated barley. Jour. Amer.

Soc. Agron. 38: 432-440.
Popham, R. A. 1938. A contribution to the life history of Galinsoga ciliata.

Bot. Gaz. 97: 543-555.
Randolph, L. F. 1936. Developmental morphology of the caryopsis in maize.

Jour. Agr. Res. 53: 881-916.
Rauch, K. V. 1936. Cytologisch-embryclogische Untersuchungen an Scurrula

atropurpurea Dans, und Dendrophthoe pcntandra Miq. Ber. schweiz. bot.

Gesell. 45: 5-61.
Sanz, C. 1945. Pollen-tube growth in intergeneric pollinations on Datura stra-
monium. Proc. Natl. Acad. Sci. 31: 361-367.
Satina, S., and Blakeslee, A. F. 1935. Fertilization in the incompatible cross

Datura stramonium X D. metel. Bui. Torrey. Bot. Club 62: 301-312.
Sawyer, M. L. 1917. Pollen tubes and spermatogenesis in Iris. Bot. Gaz. 64:

159-164.
Sax, K. 1916. Fertilization in Fritillaria pudica. Bui. Torrey Bot. Club 43:

505-522.
. 1918. The behavior of the chromosomes in fertilization. Genetics 3:

309-327.
Sears, E. R. 1937. Cytological phenomena connected with self-sterility in flower-
ing plants. Genetics 22: 130-181.
Shattuck, C. H. 1905. A morphological study of Ulmus americana. Bot. Gaz.

40: 209-223.
Shibata, K. 1902. Die Doppelbefruchtung bei Monotropa uniflora. Flora 90:

61-66.
Shoemaker, D. N. 1905. On the development of Hamamelis virginiana. Bot.

Gaz. 39: 245-266.
Smith, F. H. 1942. Development of the gametophytes and fertilization in

Camassia. Amer. Jour. Bot. 29: 657-663.
Smith, O., and Cochran, H. L. 1935. Effect of temperature on pollen germina-
tion and tube growth in tomato. Cornell Univ. Agr. Expt. Sta. Mem. 175:

1-11.
Soueges, E. C. R. 1910. Recherches sur l'embryogenie des Ranunculacees-

Clematidees. I. L'embryon. Bui. Soc. Bot. de France 57: 242-250, 266-275.
Sprague, G. F. 1932. The nature and extent of hetero-fertilization in maize.

Genetics 17: 358-368.
Steindl, F. 1945. Beitrag zur Pollen- und Embryobildung bei Cynomorium

cocrineum L. Arch. Julius Klaus-Stift. f. Vererbungsforsch. 20: 342-355.
Stenar, H. 1925. "Embryologische Studien. I. Zur Embryologie der Columni-

feren. II. Die Embryologie der Amaryllideen." Diss. Uppsala.
Stevens, N. 1912. Observations on heterostylous plants. Bot. Gaz. 53: 277-308.

FERTILIZATION 219

Stout, A. B. 1931. Pollen-tube behavior in Brassica pekinensis with reference

to self-incompatibility in fertilization. Amer. Jour. Bot. 18: 686-695.
Strasburger, E. 1884. "Neuere Untersuchungen iiber den Befruchtungs-

vorgang bei den Phanerogamen als Grundlage fur eine Theorie der Zeugung."

Jena.
. 1900. Einige Bemerkungen zur Frage nach der doppelten Befruchtung

bei Angiospermen. Bot. Ztg. 58: 293-316.
. 1908. Chromosomenzahlen, Plasmastrukturen, Vererbungstrager und

Reduktionsteilung. Jahrb. f. wiss. Bot. 45: 477-570.
. 1910. Sexuelle und apogame Fortpflanzung bei der Urticaceen. Jahrb.

f. wiss. Bot. 47: 245-288.
Swamy, B. G. L. 1941. The development of the male gametes in Cymbidivm

bicolor Lindl. Proc. Indian Acad. Sci. Sect. B. 14: 454-460.
. 1948. A contribution to the life history of Casuarina. Proc. Amer.

Acad. Arts and Sci. 77: 1-32.
Tackholm, G. 1915. Beobachtungen iiber die Samenentwicklung einiger Ona-

graceen. Svensk Bot. Tidskr. 9: 294-361.
Thompson, R. C. 1933. A morphological study of flower and seed development

in cabbage. Jour. Agr. Res. 47: 215-232.
Tischler, G. 1943. "Allgemeine Pflanzenkaryologie." Vol. II. Berlin.
Trankowsky, D. A. 1931. Zytologische Beobachtungen iiber die Entwicklung

der Pollenschlauche einiger Angiospermen. Planta 12: 1-18.
. 1938. La spermogenese et la fecondation chez la Drosere (Drosera).

Bui. Soc. Nat. de Moscou, Sect. Biol. 47: 104-112.
Treub, M. 1891 . Sur les Casuarinees et leur place dans le systeme naturel. Ann.

Jard. Bot. Buitenzorg 10: 145-231.
Tschernojarow, M. W. 1926. Befruchtungserscheinungen bei Myosurus minimus.

Osterr. bot. Ztschr. 75: 197-206.
Umiker, O. 1920. "Entwicklungsgeschichtlich-zytologische Untersuchungen an

Helosis guayensis Rich." Diss. Zurich.
Venkateswarlu, J. 1937. A contribution to the embryology of Sonneratiaceae.

Proc. Indian Acad. Sci. Sect. B. 5: 206-223.
. 1947. Embryological studies in the Thymelaeaceae. II. Daphne

cannabina Wall, and Wikstroemia canescens Meissn. Jour. Indian Bot. Soc.

26: 13-39.
Walker, R. I. 1947. Megasporogenesis and embryo development in Tropaeolum

majus L. Bui. Torrey Bot. Club 74: 240-249.
Wanscher, J. H. 1939. Contribution to the cytology and life history of apple

and pear. Roy. Vet. and Agr. Coll. Copenhagen, Yearbook 1939, pp. 21-70.
Warmke, H. E. 1943. Macrosporogenesis, fertilization, and early embryology of

Taraxacum kok-saghys. Bui. Torrey Bot. Club 70: 164-173.
Weber, E. 1929. Entwicklungsgeschichtliche Untersuchungen iiber die Gattung

Allium. Bot. Arch. 25: 1-44.
Weinstein, A. J. 1926. Cytological studies on Phaseolus vulgaris. Amer. Jour.

Bot. 13: 248-268.

220 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Weinzieher, S. 1914. Beitrage zur Entwicklungsgeschichte von Xyris indica

L. Flora 106: 393-432.
Welsford, E. J. 1914. The genesis of the male nuclei in Lilium. Ann. Bot. 28:

265-270.
Weniger, W. 1918. Fertilization in Lilium. Bot. Gaz. 66: 259-268.
Werner, E. 1914. Zur Okologie atypischer Samenanlagen. Beihefte bot.

Centbl. 32: 1-14.
West, G. 1930. Cleistogamy in Viola riviniana, with especial reference to the

cytological aspects. Ann. Bot. 44: 87-109.
Wiger, J. 1935. "Embryological Studies on the Families Buxaceae, Meliaceae,

Simarubaceae and Burseraceae." Diss. Lund.
Woodroof, N. C. 1928. Development of the embryo sac and young embryo in

Hicoria pecan. Amer. Jour. Bot. 15: 416-421.
Wulff, H. D. 1933. Beitrage zur Kenntnis des mannlichen Gametophyten der

Angiospermen. Planta 21: 12-50.
Wylie, R. B. 1904. The morphology of Elodea canadensis. Bot. Gaz. 37: 1-22.

. 1923. Sperms of Vallisneria spiralis. Bot. Gaz. 75: 191-202.

. 1941. Some aspects of fertilization in Vallisneria. Amer. Jour. Bot.

28: 169-174.
Yasuda, S. 1930. Physiological researches. VIII. On the self-fertilizing ability

of flowers in buds of self-incompatible plants. Bot. Mag. [Tokyo] 44: 678-687.

CHAPTER 7
THE ENDOSPERM1

The endosperm is important because it is the main source of food
for the embryo. In gymnosperms it is haploid and forms a con-
tinuation of the female gametophyte. In angiosperms, on the other
hand, it is a new structure formed in most cases as the result of a
fusion of the two polar nuclei and one of the male gametes. Since
all three of the fusing nuclei are usually haploid, the endosperm
contains the triploid number of chromosomes (Fig. 119).la

Endosperm formation is suppressed in two families, the Orchi-
daceae and Podostemonaceae.2 In the former, triple fusion is
usually completed, but the fusion product either degenerates imme-
diately or undergoes only one or two divisions (Fig. 120). Only in
Calopogon (Pace, 1909), Vanilla (Swamy, 1947), and Cephalanthera
(Hagerup, 1947) a few free nuclei are produced, but even these soon
degenerate and disappear. In the Podostemonaceae there is formed
in the nucellus a large cavity or pseudoembryo sac (Fig. 67C,F) into
which the embryo is pushed down by the elongation of the sus-
pensor. The pseudoembryo sac seems to serve as a kind of substi-
tute for the endosperm, although it has an entirely different origin.

Types of Endosperm Formation. There are three general modes
of endosperm formation. In the Nuclear type, the first division
and usually several of the following ones are unaccompanied by wall
formation (Fig. 121). The nuclei may either remain free or in later
stages they may become separated by walls. In the Cellular type,
the first and most of the subsequent divisions are accompanied by
wall formation, so that the sac becomes divided into several cham-
bers, some of which may contain more than one nucleus (Figs. 127,
132). The third or Helobial type (so called because of its frequent
occurrence in the order Helobiales) is intermediate between the

lFor more detailed information on this topic see Brink and Cooper (1947).
10 The exceptions to this condition are discussed in Chap. 13.
2 In Crinum latifolium also, according to Tomita (1931), embryo development is
often completed without endosperm formation.

221

222

INTRODUCTION TO EMBRYOLOGY OF AN GIO SPERMS

Nuclear and the Cellular types. Here the first division is followed
by a transverse wall resulting in a micropylar and a chalazal cham-
ber. Subsequent divisions are usually free
nuclear and may take place in both chambers,
but as a rule the main body of the endo-
sperm is formed by the micropylar chamber
only (Fig. 139).

Nuclear Endosperm. Usually at least the
first few divisions are synchronous, but in
later stages some of the nuclei may be seen
in the prophase stage, others in metaphase,
and still others in anaphase or telophase.3
Thus the number of endosperm nuclei may
not always be a multiple of two. As divi-
sions progress, the nuclei become pushed more
and more towards the periphery, so that the
center is occupied by a large vacuole. Often
the nuclei are especially aggregated at the
micropylar and chalazal ends of the sac and
form only a thin layer at the sides. An in-
teresting condition has been reported in Musa
errans (Juliano and Alcala, 1933), where some
of the endosperm nuclei divide more actively
than others, forming isolated groups or ''nod-
ules." They become invested with a distinct
cytoplasmic wall and extend into the center of
the embryo sac, developing as separate endo-
sperm masses (Fig. 122). Similar nodules and
vesicles have been seen in a few other plants,
but their further fate and function have not
been clarified. In Isomeris arborea (Billings,
1937) they are said to give rise to embryos,
but this deserves confirmation.

Frequently the endosperm nuclei in the
chalazal part of the embryo sac have been

3 Nuclei in different stages of division are not indis-
criminately scattered, however. They often show a
remarkable gradation, presumably due to the influence
of some kind of slowly diffusing hormonal substance
(see Dixon, 1946).

Fig. 119. Embryo sac
of Crepis capil'aris, show-
ing diploid chromosome
complement (2n = 6) in
cells of embryo and trip-
loid chromosome comple-
ment (3n = 9) in cells of
endosperm. (After (Jeras-
simova, 1933.)

THE ENDOSPERM

223

observed to be larger than those in the micropylar part.4 This may
be due to an actual growth in their size as in Colutea (Nemec, 1910),
Ranunculus (Schurhoff, 1915), and some members of the Cistaceae
(Chiamgi, 1925), or to a fusion of adjacent nuclei as in Primula

ABC D E

Fig. 120. Endosperm in orchids. A, Corallorhiza maculata, six-nucleate embryo
sac; micropylar polar nucleus has migrated to the base of the sac. B, a stage in
fertilization, showing one male nucleus fusing with the egg nucleus and the second
male nucleus still in micropylar part of embryo sac. C, embryo sac, showing the
young proembryo and the second male nucleus fusing with the other free nuclei in
sac. D, Bletia shepherdii, young proembryo and triple fusion nucleus. E, same,
later stage of proembryo accompanied by degeneration of endosperm nucleus.
(After Sharp, 1912.)

(Dahlgren, 1916), Tilia (Stenar, 1925), Mains (Wanscher, 1939),
and some members of the Compositae (Poddubnaja-Arnoldi, 1931). 5
An especially interesting case of an increase in size of the endo-
sperm nuclei has recently been described in Echinocystis macrocarpu

i Rarely, as in Fagraca (Mohrbutter, 1936), the reverse happens and the micro-
pylar nuclei are the larger.

5 Although fusions may take place during the free nuclear stage, they are more
abundant after wall formation has taken place and the endosperm has become
chambered into several multinucleate protoplasts.

224 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

(Scott, 1944). At first the nuclei measure 7 to 10 microns in
diameter and have one to three nucleoli, but in later stages they
become 150 to 200 microns in diameter and their nucleoli present a
great variation in size, shape, and number. The following patterns

Fig. 121. Endosperm formation in Zostera marina. A, l.s. ovule, showing mature
embryo sac. B, two-celled embryo and four-nucleate endosperm; note persisting
antipodal cells. C, D, more advanced stages in endosperm formation; note larger
size of nuclei in lower part of endosperm. (After Dahlgren, 1939.)

were observed: (1) the nucleus has several nucleoli, all of a small
size (Fig. 123 A); (2) some of the nucleoli are small, and others are
of a medium size (Fig. 1235) ; and (3) most of the nucleoli are small,
but one to three are of a very large size and these may have a
spherical, elliptical, or highly erratic outline (Fig. 123C, D). In
later stages the largest nucleoli become subdivided into irregular
segments, as if in readiness to break up into pieces (Fig. 123#).
Since an increased number of nucleoli in a nucleus is generally
considered to be an index of chromosome duplication, it seems likely

THE ENDOSPERM

225

that the abnormalities described above are due to a high degree of
polyploidy in the endosperm.

There are occasional reports of atypical or irregular divisions of
the endosperm nuclei. In Zauschneria latifolia (Johansen, 19316),
a member of the Onagraceae, the endo-
sperm nuclei lying close to the embryo
divide amitotically. In another member
of the same family, Anogra pallida (Jo-
hansen, 1931c), the unfertilized polar
nucleus is said to undergo repeated
amitotic divisions to form more than
a hundred nuclei of various sizes.
There are also other records of amitotic

divisions in the endosperm, but most

. , r ,i • i • i r r _ Fig. 122. Embryo sac of Musa

statements of this kind are tar trom . /

. . errans, showing tree nuclear en-

dependable.6 To mention two examples, dosperm and veside containing

Longo (1909) failed to find any mitoses nve endosperm nuclei. (After
in the endosperm of Ficus carica and Juliano and Alcala, 1933.)

D E H I

Fig. 123. Echinocystis macrocarpa, endosperm nuclei, showing peculiar behavior
of nucleoli. Only the nucleolus is shown in F-I. (After Scott, W44-)

6 Appearances formerly accepted as evidences of amitosis are now considered to
be the results of deranged mitoses or of nuclear fusions.

226 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

concluded that the division took place by fragmentation, but
Condit (1932) demonstrated the presence of mitotic figures in both
F. carica and F. palmata. Similarly Langdon's (1934) statement
that in Juglans sl period of free nuclear divisions lasting for 4 to 5
days is "characterized by the total absence of achromatic figures,
suggesting an amitotic division of the nuclei at this time" has been
contradicted by Nast (1935, 1941).

The number of free nuclear divisions varies in different plants.
In Primula (Dahlgren, 1916), Malva (Stenar, 1925), Cochlospermum
(Schnarf, 1931), Brexia (Mauritzon, 1933), Mangifera (Maheshwari,
1934), Juglans (Nast, 1935), Malus (Wanscher, 1939), 7 Jussieua
(Khan, 1942), and Citrus (Bacchi, 1943) several hundred endosperm
nuclei may be seen lining the wall of the embryo sac. In some
genera, e.g., Lopezia (Tackholm, 1915), Stenosiphon (Johansen,
1931a), Cardiospermum (Kadry, 1946), Tropaeolum (Walker, 1947),
and Mclastoma (Subramanyam, 1948) wall formation does not take
place at all. In others like Asclepias (Frye, 1902), Rafflesia (Ernst
and Schmid, 1913), Leiphaimos, Cohjlanthera (Oehler, 1927), Calo-
tropis (Sabet, 1931), Xeranthemum (Poddubnaja-Arnoldi, 1931),
and Crepis (Gerassimova, 1933) it occurs at a very early stage when
only 8 or 16 nuclei have been formed, and in Coffca (Mendes, 1941)
at the 4-nucleate stage.

When wall formation occurs, it is usually by the laying down of
cell plates which progress from the periphery of the sac towards the
center or from its apex towards the base. Less commonly, wall
formation may take place simultaneously in all parts of the sac as
in Tacca (Paetow, 1931), or it may start from the base toward the
apex as in Elatinc (Frisendahl, 1927), Cimicijuga (Earle, 1938), and
Carya (McKay, 1947). Very little is known, however, about the
exact origin of the partition walls.8 In Asclepias (Frye, 1902),
Calotropis (Sabet, 1931), Ficus (Condit, 1932), and Gossypium
(Gore, 1932) minute vacuoles appear in the areas between the nu-
clei, and it seems that partitioning of the embryo sac takes place
by a process of "indentation." In some other plants the formation
of cell plates is preceded by the appearance of secondary spindle
fibers between the nuclei (Jungers, 1931). Whatever the precise
mode of cell formation, eventually either the entire embryo sac is

7 Between two and three thousand nuclei have been counted in some varieties
of Malus.

8 The difficulty in ascertaining the precise mechanism of wall formation is no
doubt due to the rapidity of the event and the poor fixation obtained at this stage.

THE ENDOSPERM 227

filled with cells; or there are one or two peripheral layers of cells
and the rest of the endosperm remains in the free nuclear state ; or
cell formation is restricted only to the micropylar part of the sac.
Sometimes all three types occur in one and the same family, e.g.,
the Caryophyllaceae (Rocen, 1927). Not infrequently several
nuclei become enclosed in a cell where they may subsequently fuse
to form a single nucleus; or an originally uninucleate cell may be-
come multinucleate by divisions of its nucleus. Such variations
are so common that it is unnecessary to cite specific examples.

Special mention must be made of the haustorial structures met
with in some members of the Proteaceae (Kausik, 1938a, b, 1942).
Here most of the endosperm nuclei are distributed in the upper
portion of the embryo sac. Cell formation is restricted to this
region, while the lower portion of the sac remains free nuclear. In
Macadamia ternifolia (Fig. 124A-C) this part forms several promi-
nent lobes or diverticulae which invade the nutritive tissue at the
chalazal end of the ovule and thus function as haustoria. It is only
in later stages, when the food material in the chalazal cells is com-
pletely used up, that the activity of these haustorial lobes comes
to an end.

In another member of the Proteaceae, Grevillea robusta (Fig.
124ZM7), the lower coenocytic part of the endosperm grows in the
form of a coiled and tubular worm-like structure, which has been
aptly designated as the "vermiform appendage." It serves as a
haustorium of a very aggressive type whose coils invade the cells
of the chalaza and bring about their virtual dissolution. Later, the
appendage becomes partitioned into several large chambers which
undergo further subdivisions into smaller units, so that the cells
formed in this way constitute a kind of secondary endosperm tissue.
The reason why the vermiform appendage had been missed by
earlier workers is that they used only sections, which fail to give any
clear or complete picture of this organ, while Kausik used both
sections and whole mounts and was therefore able to give a very
thorough account of its development and organization. Using a
similar technique, Anantaswamy Rau (1950) has found that in Cas-
sia tora wall formation takes place only in the micropylar part of
the embryo sac. The chalazal part forms a narrow tube with dense
cytoplasm and many free nuclei. The lower part of the tube be-
comes irregularly coiled and twisted to form a very efficient haus-
torium.

The lateral haustoria or "diverticulae" mentioned on page 144

228 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fig. 124.

THE ENDOSPERM 229

often show increased activity in postfertilization stages. In Agro-
stemma (Fig. 125) the primary endosperm nucleus, originally situated
just below the zygote, enters the base of the diverticulum and divides
to form two daughter nuclei, one remaining in the embryo sac and
giving rise to the bulk of the endosperm and the other passing into
the diverticulum. Similar diverticulae are seen in several other
members of the Caryophyllaceae, but the time of their appearance
is not the same. In Gypsophila the outgrowth originates even
before fertilization; in Saponaria, after the second nuclear division
in the endosperm; and in Melandrium, at a still later stage (Rocen,
1927).

Cellular Endosperm. In the Cellular type the division of the
primary endosperm nucleus is followed immediately by a chambering
of the sac. The first wall is usually transverse but sometimes verti-
cal or oblique, and in a few cases the plane of division is not constant.
On the basis of the orientation of the walls following the first two or
three divisions, this type of endosperm has been classified into several
subtypes (see Schnarf, 1929). For our purpose, however, it will
suffice to refer to a few concrete examples in order to illustrate the
range of variation that has been reported.

Adoxa (Lagerberg, 1909) is a well-known instance in which the
first as well as the second division of the endosperm mother cell is
vertical, resulting in the formation of four large cylindrical cells,
all similar to one another (Fig. 12QA-B). The third division is
transverse and results in eight cells arranged in two tiers (Fig. 126C).
The fourth division is also transverse but further divisions are
irregular.

A similar orientation of the first wall is known in Scabiosa (Doll,
1927) and Circaeaster (Junell, 1931). In Peperomia (Johnson, 1900),
Centranthus (Asplund, 1920) (Fig. 126D), and Helosis (Fagerlind,
1938) also, the first wall is longitudinal but sometimes it may be

Fig. 124. Stages in development of endosperm in Macadamia ternifolia (A-C)
and Grevillea robusta (D-G). A, Macadamia, embryo sac showing free endosperm
nuclei; lower end of sac is invading nutritive tissue in chalaza. B, l.s. young seed
showing embryo sac with upper cellular endosperm and lower free nuclear endo-
sperm. C, embryo sac from B, enlarged to show details of endosperm; two-celled
proembryo is seen at upper end of sac. D, Grevillea, l.s. ovule (diagrammatic).

E, embryo sac from D, enlarged to show young embryo and free nuclear endosperm.

F, l.s. young seed, showing "vermiform appendage" formed from basal part of
endosperm. G, whole mount of endosperm, showing vermiform appendage. (After
Kausik 193Sa,b.)

230 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

oblique, and in Senecio (Afzelius, 1924) there is no constancy about
its orientation. Frequently it is longitudinal, but it may also be
transverse or oblique.

A B C

Fig. 125. Formation of lateral haustorium or diverticulum in embryo sac of
Agrostemma githago. A, l.s. ovule, showing early stage in formation of diverticulum.
B, enlarged view of embryo sac from A. C, l.s. nearly mature seed, showing large
embryo and persisting diverticulum. (After Roccn, 1927.)

C D

Fig. 126. Early stages in development of endosperm in Adoxa moschatellina
(A-C) and Centranthus mncrosipJwn (D). In both cases, the first wall is vertical.
(A-C, after Lagerberg, 1900; D, after Asplund, 1920.)

Except in the few plants cited above, the first division of the endo-
sperm mother cell is generally transverse. In the Anonaceae,
Aristolochiaceae, Sarraceniaceae, Gentianaceae, Boraginaceae (see

THE ENDOSPERM

231

Schnarf, 1929), and Marcgraviaceae (Mauritzon, 1939) the second
division, and sometimes the third also, is transverse, resulting in a
row of four or more cells (Fig. 127). More commonly, however,
the second division is vertical, and subsequent walls are laid down in
variable planes.

Of special interest are several members of the Sympetalae and of
a few families belonging to the Archichlamydeae and the Mono-
cotyledons, in which there is a differentiation of prominent endo-
sperm haustoria. The haustoria may arise at the chalazal end, or

C D E F

Fig. 127. Development of endosperm in Yillarsia reniformis. A, two-celled
stage. B,C, four-celled stage. D-F, eight-celled stage. (After Stolt, 1921.)

at the micropylar end, or at both. Taking the simplest forms first,
in the Nymphaeaceae and Araceae the primary chalazal cell formed
after the first division of the endosperm mother cell undergoes no
further divisions and functions directly as a haustorium. A whole
mount of the endosperm of Peltandra (Goldberg, 1941), belonging
to the Araceae, shows several strands of streaming cytoplasm trav-
ersing the large vacuole of this cell. Its nucleus becomes lobed
and hypertrophied, and the nucleolus breaks down into a number of
highly vacuolated fragments.

A similar large chalazal cell is seen in Thesium (Schulle, 1933;
Rutishauser, 1937) and Balanophora (Zweifel, 1939). The first
division of the primary endosperm nucleus, which lies in the vicinity

232 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

of the zygote, results in the formation of a small micropylar and a
considerably larger chalazal chamber. Further divisions take place
in the micropylar chamber only (Fig. 128 A, B). In Balanophora,
where the sequence has been followed in greater detail, the micro-
pylar chamber (Fig. 128C) divides vertically to form two adjoining
cells enveloping the zygote (Fig. 128D). The second and third
divisions are transverse and result in four and eight cells respectively
(Fig. 12SE,F). The following divisions are less regular, but the
young embryo soon becomes surrounded by a number of small endo-
sperm cells. Later, owing to a further increase in the number of
endosperm cells and their enlargement, the large chalazal haus-
torium becomes squeezed and crushed.

A very well developed and aggressive micropylar haustorium
occurs in Impatiens (Dahlgren, 19346). The first division of the
primary endosperm nucleus gives rise to two chambers (Fig. 129 A).
The micropylar chamber, which is the smaller, divides transversely
into three cells (Fig. 1295). The uppermost of these, containing
the zygote, forms a giant haustorium whose branching arms extend
as far as the funiculus (Fig. 129C-F). The next gives rise to a group
of cells which lie in close proximity to the young embryo. The
third, in which the divisions are not accompanied by wall formation,
fuses with the large chalazal chamber, which also contains only free
nuclei. Eventually cell formation takes place here also, but some
rounded masses of plasma, often containing several nuclei, seem to
become detached from the wall of the embryo sac and swim as "free
plasma balls" in the liquid below the cotyledons.

Passing now to plants in which both micropylar and chalazal
haustoria are formed, we may first refer to the family Acanthaceae,
several members of which have been studied by Mauritzon (1934).
In Ruellia and most other genera the first division of the primary
endosperm nucleus cuts off a small chalazal haustorium with dense
cytoplasm, in which there are two nuclear divisions resulting in four
free nuclei (Fig. 1305). The micropylar cell divides transversely
to give rise to two daughter cells, of which the upper becomes bi-
nucleate and forms the micropylar haustorium (Fig. 130C). The
central cell, which is the largest, forms a sac-like outgrowth in which
there are several free nuclear divisions (Fig. 130D-G). A large
vacuole appears in the center of the cell, and the nuclei become dis-
tributed in the peripheral layer of cytoplasm, with a stronger

Fig. 128. Some stages in development of endosperm in Thesium montanum (A,B)
and Balanophora abbreviata (C-F). A, Thesium; l.s. ovule, showing cell formation
in primary micropylar chamber, while chalazal chamber remains undivided and
forms large haustorium. B, l.s. central placenta, showing extent of growth of
chalazal haustorium; somewhat diagrammatic and drawn at a much lower mag-
nification than A. (After Schulle, 1933.) C, Balanophora; upper part of embryo
sac, showing micropylar endosperm chamber with zygote, and a portion of chalazal
endosperm chamber. D-F, same, showing cell formation in micropylar endosperm
chamber, while large chalazal chamber (not shown in figures) remains undivided.
(After Zweifel, 1939.)

233

234

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

B D E

Fig. 129. Development of endosperm in Impatiens roylei. A, first division of
endosperm mother cell, resulting in formation of small micropylar and large chalazal
chambers. B, more advanced stage, showing large chalazal chamber and three
cells formed by transverse divisions of micropylar chamber. C, uppermost cell
of micropylar chamber has grown into micropyle; zygote nucleus in division. D,
haustorium has penetrated beyond tip of inner integument. Note two-celled
embryo and laying down of walls in second endosperm cell from above. E, further
growth of micropylar haustorium. F, upper part of ovule, showing hypha-like
ramifications of haustorium and their penetration into tissues of funiculus. {After
Dahlgren, 1934b.)

THE ENDOSPERM

235

J I H

Fig. 130. Development of endosperm in Ruellia. A, R. solitaria, embryo sac
showing fertilized egg and primary endosperm nucleus. B, same, later stage,
showing delimitation of chalazal haustorium. C,D, R. squarrosa, micropylar
haustorium, chalazal haustorium, and central endosperm chamber. E,F, R.
solitaria, increase in number of free nuclei in central chamber. G, R. pulcherrima,
similar stage. //, R. decaisniana, micropylar and chalazal haustoria and centra]
endosperm chamber with many free nuclei. /, R. solitaria, more advanced stage,
showing beginning of wall formation in upper part of central chamber. J, R.
pulcherrima, central chamber consisting of upper cellular and lower free nuclear
portion. (After Mauritzon, 1934.)

236 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

accumulation of them in the region adjacent to the embryo and the
two haustoria (Fig. 130/f). Cell formation commences only after
64 or more nuclei have been produced and is confined to the upper
part, the lower still remaining free nuclear (Fig. 1307). In later
stages, therefore, the endosperm shows four distinct regions: micro-
pylar haustorium, chalazal haustorium, cellular endosperm, and
free nuclear endosperm (Fig. 130/). This last, which Mauritzon
calls the "basal apparatus," seems to be an intermediary for con-

A B C ^^D E

Fig. 131. Development of endosperm in Crossandra nilotica. A, first division of
endosperm mother cell. B, chalazal chamber divided vertically into two cells.
C, delimitation of central chamber from micropylar and chalazal haustorium. D,E,
cell divisions in central chamber. F, more advanced stage, showing binucleate
micropylar haustorium, central chamber with cellular endosperm and embryo, and
four-celled chalazal haustorium. (After Mauritzon, 1984.)

veying food materials from the integument to the cellular portion
of the endosperm. An interesting point to be noted is that owing
to the continued curvature of the embryo sac, the micropylar and
chalazal haustoria come to lie in close proximity to each other.

Most genera of the family are essentially similar to Ruellia, but
a somewhat different condition prevails in Crossandra (Fig. 131),
where the divisions in the chalazal haustorium as well as the central
chamber are accompanied by wall formation. The former under-
goes two vertical divisions to form four cells. In the central cham-

THE ENDOSPERM 237

ber also the first two divisions are vertical, but these are followed by
further divisions in varying planes, resulting in a small mass of endo-
sperm tissue surrounding the embryo. In Crossandra, therefore,
the endosperm consists of only three regions: the micropylar haus-
torium, the chalazal haustorium, and the central endosperm tissue.
The genus Acanthus is more or less similar. Thunbergia, which
seems to differ in several respects from both Ruellia and Crossandra,
needs further study and will not be discussed here.

The genus Nemophila, belonging to the Hydrophyllaceae, presents
an interesting mode of development. Of the two approximately
equal cells formed by the first division of the endosperm mother cell,
the lower functions directly as the chalazal haustorium (Fig. 132^1).
The upper divides transversely to give rise to a central cell, which
is responsible for the origin of the main body of the endosperm, and
a terminal cell, which serves as the micropylar haustorium (Fig.
1325). Cell divisions are confined to the central cell only, the first
wall being transverse and the others more or less irregular (Fig.
1S2C-F). The chalazal haustorium sometimes gives out a promi-
nent lateral branch (Fig. 132D) which grows toward the funiculus
and penetrates it so as to come in direct contact with the starchy
cells of the placenta. In one species, N. aurita, the micropylar
haustorium, as well as the chalazal one, becomes large and aggres-
sive. Fig. 132G, H, reconstructed from several sections, shows the
central ball-shaped mass of endosperm and conspicuous haustoria.

Well-developed micropylar and chalazal haustoria also occur in
the Lobeliaceae (Hewitt, 1939; Subramanyam, 1949). In Lobelia
amoena (Hewitt, 1939) the first division of the endosperm mother
cell is transverse and the second is vertical (Fig. 133 A, B). Each
of the four cells now divides transversely, resulting in four tiers of
two cells each, i.e., eight cells in all (Fig. 133C). The two cells of
the micropylar tier do not divide again but extend around the zy-
gote, forcing their way into the micropyle and forming a large
haustorium; the two cells of the chalazal tier grow downward into
the basal end of the ovule, forming a large chalazal haustorium; and
the middle tiers give rise to the main body of the endosperm (Fig.
133 D, 22). Cross sections of the haustoria, both micropylar and
chalazal, show their two-celled nature quite clearly.

An essentially similar mode of development occurs in Utricularia
coerulea (Kausik, 1938c) (Fig. 134 A-C). The first division of the

238 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fig. 132. Development of endosperm in Nemophila. A, N. insignis; two-cellel
endosperm. B, three-celled stage. C, four-celled stage resulting from transverse
division of middle cell. D, more advanced stage, showing small mass of cells
formed by divisions of two central cells; chalazal haustorium has given out a lateral
branch. E, l.s. ovule, diagrammatic. F, embryo sac from E, magnified to show
two-celled embryo, micropylar and chalazal haustoria, and central mass of endo-
sperm cells. G, N. aurita, diagram of l.s. ovule. //, embryo sac of G, magnified
to show aggressive micropylar and chalazal haustoria and central globular mass of
endosperm cells. (After Svensson, 1925.)

THE ENDOSPERM

239

endosperm mother cell is transverse, resulting in the formation of
the two primary chambers, micropylar and chalazal. The second is
vertical, but in both chambers the walls are incomplete and do not
extend to the two poles of the embryo sac. The third division is

Fig. 133. Development of endosperm in Lobelia amoena. A, l.s. ovule, showing
four-celled stage of endosperm. B, transverse division in some of the endosperm
cells. C, laying down of micropjdar and chalazal haustoria. D,E, more advanced
stages, showing multiplication of cells of endosperm, the enlarging haustoria, and
the developing embryo. {After Hewitt, 1939.)

transverse and takes place simultaneously in both the upper and
the lower chambers. Thus the embryo sac now consists of a central
portion of four cells, two contributed by the upper and two by the
lower primary chamber, and two terminal portions with two nuclei

240 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fig. 134. Endosperm formation in Utricularia. A, U. coerulea; l.s. ovary, showing
relation of ovules to the swollen placenta. B, embryo sac showing sequence of
wall formation in endosperm, resulting in a delimitation of the micropylar and
chalazal haustoria. C, l.s. seed, showing embryo and remnants of haustoria. (After
Kausik, 193^c.) D, U. vulgaris americana; l.s. ovule, showing the embryo e, micro-
pylar haustorium mh, nuclei of micropylar haustorium mn, chalazal haustorium
ch, integument i, and placental tissue p. E, invasion of placental tissue p by
micropylar haustorium mh; note the two large nuclei mn of the micropylar haus-
torium. F, more advanced stage, showing disappearance of cell walls of placental
tissue and scattering of placental nuclei pn. The more densely staining nuclei
mn belong to micropylar haustorium mh. (After Wylie and Yocom, 1928.)

THE ENDOSPERM 241

each. In subsequent stages only the four central cells divide further
and give rise to the body of the endosperm, while the terminal cells
remain undivided and take up a haustorial function.

An interesting feature, described in special detail in Utricularia
vulgaris americana (Wylie and Yocom. 1923), is that, because of the
disappearance of the nucellar epidermis and the protrusion of the
embryo sac through the micropyle, the micropylar haustorium comes
to lie in direct contact with the nutritive tissue of the placenta (Fig.
134D, E). The walls of the placental cells frequently break down,
so that their nuclei become scattered in a common mass of cyto-
plasm along with the nuclei of the haustorium. The placental nuclei
enlarge, and some of them fuse to form "tuber-like" structures
many times their normal size. The nuclei of the haustorium, which
are usually still larger and more chromatic, become lobed and bud
out into separate masses. "These with the placental nuclei lying
in the fluids of the haustorium offer a most peculiar assemblage of
nuclear structures" (Fig. 134F). The chalazal haustorium, although
less massive, also digests its way through the intervening cells and
ultimately comes in contact with the cells of the epidermis, which
become perceptibly weakened at this point.

The occurrence of endosperm haustoria, both micropylar and
chalazal, is also a universal feature of the Scrophulariaceae. A
widely distributed mode of development is illustrated by Lathraea
(Glisic, 1932) (Fig. 135). Of the two primary endosperm chambers,
the chalazal functions directly as a haustorium after undergoing one
nuclear division. The micropylar divides vertically and then trans-
versely to give rise to two tiers of two cells each. Of these, the
uppermost tier gives rise to two binucleate haustorial cells which
later fuse to form a single tetranucleate structure, and the lower
gives rise to the endosperm proper.

There are several variations of this scheme, which have been
described by Krishna Iyengar (1937, 1939, 1940, 1941, 1942) in recent
publications. Although the chalazal haustorium usually comprises
a single binucleate cell, in certain plants like Celsia, Isoplexis, and
Verbascum it is tetranucleate, while Vandellia, Sopubia, and Alonsoa
(Fig. 136D) have two uninucleate prongs which may later fuse to
form a single binucleate cell. The micropylar haustorium is differ-
entiated somewhat later, but it is more aggressive and persists for
a longer time. In some plants there are four uninucleate cells,
which may remain as such as in Alonsoa (Fig. 136), Isoplexis,

242 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Celsia, and Ilysanihes, or fuse to form a common tetranucleate
structure as in Vandellia and Torenia. In a few plants like Pedi-
cularis the haustorium is tetranucleate from the commenceirent.

A B C

Fig. 135. Development of endosperm in Lathraea squamaria. A, first division of
endosperm mother cell. B, nuclear division in both endosperm cells. C, division
completed; basal cell binucleate and upper cell partitioned by longitudinal wall.
D, basal cell forms large haustorium; transverse division in upper cells to form
two-celled micropylar haustorium and two central cells, which are destined to give
rise to main body of endosperm. (After Glisic, 1932.)

In Centranthera neither the micropylar nor the chalazal haustorium
is very active, but there are formed instead some secondary haustoria
from the endosperm cells just beneath the micropylar haustorium
(Fig. 137). Similar secondary haustoria are seen in Veronica, but
here they arise from the chalazal region of the haustorium.

THE ENDOSPERM

243

In a few plants, well-developed and aggressive haustoria of all
three kinds — micropylar, chalazal, and secondary — occur together
and form a very efficient absorptive system. An interesting case of

Fig. 136. Development of the endosperm in Alonsoa. Small globules lying around
nuclei are starch grains. A, mature embryo sac. B, first division of endosperm
mother cell. C, transverse division in micropylar chamber resulting in a three-
celled stage. D, more advanced stage, showing four-celled micropylar haustorium
(only two cells are seen in section), two-celled chalazal haustorium (one of the
cells is lying over the other), and central cells destined to give rise to endosperm
proper. (After Krishna Iyengar, 1937.)

this kind has been described by Rosen (1940) in Globularia vulgaris,
belonging to the allied family Globulariaceae. The micropylar
haustorium consists of two to four separate cells but they soon
unite to form a single entity. This composite structure often grows

244 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

out of the micropyle, and its hypha-like ramifications extend along
the outer surface of the funiculus and placenta, later penetrating
even the wall of the ovary. The chalazal haustorium also branches

A B C

Fig. 137. Origin and development of secondary haustoria in Centranthera hispida.
Primary haustoria, both micropylar and chalazal, are very weakly developed in
this plant. (After Krishna Iyengar, 1942.)

profusely and "sucks" the contents of the integumentary cells,
destroying their walls and causing the formation of large lacunae.
In addition, the endosperm cells lying nearest to the chalazal haus-
torium also increase in size and send out secondary haustoria, so

THE ENDOSPERM 245

that the entire tissue of the chalaza and the integument becomes
riddled by a number of haustorial processes.

A very peculiar mode of development, which seems to bear no
relation to any of the others, occurs in several members of the Lo-
ranthoideae (Treub, 1885; Rauch, 1936; Schaeppi and Steindl, 1942;
Singh, 1950; Johri and Maheshwari 1950; Maheshwari and Singh,
1950). Here the primary endosperm nucleus migrates to the cha-
lazal end of the extremely long and tubular embryo sac. The
exact sequence of divisions has not been traced, but eventually the
whole of the comparatively broad lower portion of the embryo sac
becomes cellular. Cell formation gradually extends upward to the
point at which the embryo sac protrudes out of the ovary into the
style (Fig. 138). Frequently the development of the endosperm
and embryo is initiated in several embryo sacs. Since these lie
close to one another, the separating walls between them get dis-
solved and their endosperms fuse to form a single composite tis-
sue. Sometimes, the boundaries between the individual embryo
sacs are still distinguishable at their upper ends, although their
basal portions have already fused and show several embryos sur-
rounded by a common mass of endosperm.

Helobial Endosperm. The Helobial type of endosperm is inter-
mediate between the Nuclear and the Cellular. The first division
of the primary endosperm nucleus results in the partition of the
embryo sac into two chambers, of which the micropylar is usually
much larger than the chalazal. Several free nuclear divisions take
place in the former, but in the latter either the nucleus remains
undivided or undergoes only a small number of divisions. Earlier
workers often mistook the chalazal chamber for a hypertrophied
antipodal cell.

Eremurus (Stenar, 1928a) may be cited as an example of a typical
Helobial endosperm. The first division of the primary endosperm
nucleus results in the formation of two chambers, a large micropylar
and a small chalazal (Fig. 139 A). Free nuclear divisions occur in
both but are more rapid in the micropylar chamber (Fig. 139 B-D).
Thus, when there are four nuclei in the chalazal chamber, the micro-
pylar has eight; when there are eight in the chalazal, the micropylar
has 16; and when there are 30 to 32 nuclei in the chalazal chamber,
the micropylar has a considerably larger number. In older ovules
the chalazal chamber becomes depleted of its cytoplasm and begins

246

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

C E F G J 1

Fig. 138. Some stages in development of endosperm and embryo in Loranthus
sphaerocarpus Bl. A,B, young proembryos of same age viewed from two different
sides. C, lower end of young proembryo. D, upper part of embryo sac, showing
elongated proembryo. E, lower part of same embryo sac, showing early stage in
endosperm formation. F,G, upper and lower halves of embryo sac, showing embryo
and endosperm. H, embryo beginning to penetrate through endosperm. 7, same,
more advanced stage. /, terminal part of embryo, shown at a somewhat higher
magnification. (After Treub, 1885.)

Fig. 139. Helobial type of endosperm. A-D, Eremurus himalaicus. {After
Stenar, 1928a) E, Scheuehzeria palustris. (After Stenar, 1935.) F, G, Muscari
racemosum. (After Wunderlich, 1937.) In Scheuehzeria the chalazal chamber
is uninucleate, while in Muscari it becomes multinucleate; Eremurus is interme-
diate.

247

248 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

to show signs of degeneration. Finally, when cell formation takes
place in the micropylar chamber, the chalazal is almost crushed and
shows only a few disorganized nuclei.

Other plants may show a fewer or a larger number of divisions in
the chalazal chamber. In Scheuchzcria (Stenar, 1935) it gradually
enlarges and becomes highly vacuolated, but the nucleus does not
undergo any division (Fig. 13922). An essentially similar condition
occurs in Echinodorus (Dahlgren, 1934a), Vallisneria (Witmer, 1937),
and Enalus (Kausik, 1940). In Bulbine (Stenar, 1928a) and Sagit-
taria (Dahlgren, 1934a) there is one division, resulting in two
daughter nuclei; and in Asphodelus (Stenar, 1928a) and Limnophyton
(Johri, 1935) there are two divisions resulting in four nuclei. On
the other hand, Hypoxis (Stenar,80 1925), Ornithogalum (Schnarf,
1928a), Dianella (Schnarf and Wunderlich, 1939), and Zephyranthes
(Swamy, 1946a) show a much larger number of nuclei in the chalazal
chamber. In Muscari racemosum (Wunderlich, 1937) (Fig. 139F) the
chalazal chamber has 64 nuclei at the time when wall formation
commences in the micropylar. Yet another division takes place,
resulting in the formation of approximately 128 nuclei, which are
imbedded in a dense mass of cytoplasm. After this stage the nuclei
begin to degenerate, but the outline of the chalazal chamber remains
recognizable for a long time (Fig. 139(2).

In some plants the divisions in the chalazal chamber are accom-
panied by wall formation and result in a small mass of cells which
is quite conspicuous for a time. As examples may be mentioned
Saxifraga granulata (Juel, 1907), Boykinia occidentalis, Mitella
diphylla (Dahlgren, 1930) (Fig. 140), and Lyonothamnus floribundus
(Juliano, 1931a). Or, the first few divisions may be free nuclear, and
wall formation may occur at a slightly later stage, as in Heloniopsis
breviscapa (Ono, 1928) and Tofieldia japonica (Ono, 1929). In
Narthecium asiaticum (Ono, 1929) the free nuclear divisions are
followed by a transverse segmentation of the chalazal chamber into
a few large multinucleate cells.

In Ixiolirion montanum (Stenar, 1925) the primary endosperm
nucleus comes to lie laterally, although the antipodal cells, which

80 The recent work of De Vos (1948, 1949) shows that in Hypoxis (= Ianthe)
the endosperm does not follow the same type of development in all species. Some
species such as I. schlechteri come under the Helobial type, while others like I. alba,
I. aquatica and 7. minuta come under the Nuclear type.

THE ENDOSPERM

249

are large and conspicuous, remain in their usual position in the basal
part of the embryo sac. Two endosperm chambers are formed
after the first division, but the chalazal is situated toward one side
rather than directly over the antipodal cells (Fig. 141C). For a
time free nuclear divisions take place in both chambers. Later
cell formation occurs in the mi-
cropylar chamber, while the cha-
lazal one degenerates.

Svensson (1925) has reported
that in Echium plantagineum, a
member of the Boraginaceae, the
first division wall in the embryo
sac is oblique, separating a
small lateral chamber from a
large central chamber. The lat-
eral chamber, which has denser
cytoplasm, divides first and
gives rise to two cells which
later show several hypertrophied
nuclei (Fig. 141 D-F). The cen-
tral chamber is more vacuolate,
and the divisions are all free nu-
clear, wall formation taking place
at a much later stage.

In Anthericum ra?nosum
(Stenar, 19286; Schnarf, 19286)
the chalazal chamber is cut off in
its usual position just above the
small and ephemeral antipodals,
but the embryo sac soon gives fig. 140. Embryo sac of Mitella di-
out a lateral outgrowth which phylla, showing cell formation in chalazal
advances toward the funicular endosperm chamber. (After Dahlgren,
side of the ovule and becomes 19S0^
an extremely conspicuous structure (Fig. 141 A, B).

More remarkable still are the lateral haustoria of Monochoria,
a member of the Pontederiaceae (Ono, 1928; Juliano, 19316). The
early stages are similar to those in other plants having a typical
Helobial endosperm (Fig. U2G,H). The chalazal chamber remains
small and has only about half a dozen nuclei or less. But the

250

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

micropylar chamber shows active nuclear divisions and soon gives
rise to two tubular outgrowths (one on each side of the chalazal
chamber) which grow downward and invade the tissues of the
chalaza (Fig. 142 1-L). Subsequently the main body of the cham-
ber also elongates and fuses with the two haustoria to form a con-
tinuous mass of endosperm cells with the chalazal chamber still
recognizable at the base (Fig. 142 A-F).

Fig. 141. Modifications of Helobial endosperm as illustrated by Anthericum
ramosum, Ixolirion montanum, and Echium plantagineum. A, Anthericum, l.s.
ovule showing mature embryo sac; secondary nucleus lies in diverticulum. B,
same, l.s. nucellus, showing further development of diverticulum and formation
of endosperm of Helobial type. {After Schnarf, 1928b.) C, Ixolirion, embryo sac,
showing laterally situated chalazal chamber; only two of the antipodal cells are
seen in section. {After Stenar, 1925.) D, Echium, embryo sac, showing' laterally
placed chalazal chamber consisting of two binucleate cells. E,F, more advanced
stages, showing formation of several free nuclei in "central" as well as "lateral"
chambers. {After Svensson, 192o.)

THE ENDOSPERM

251

An interesting deviation, which is of a very different nature from
those described above, occurs in Hyoscyamus niger (Svensson, 1926).
Here the first division of the primary endosperm nucleus takes place
in the vicinity of the egg, so that the chalazal chamber is the larger.
In the micropylar chamber either four to eight free nuclei are pro-
duced before cell formation takes place, or at first there are two
longitudinal walls and then some irregular divisions resulting in a
small mass of cells. In the chalazal chamber there are repeated

Fig. 142. Development of endosperm in Monochoria. A-F, diagrams showing
stages in development of endosperm. G, M. vaginalis; chalazal chamber is uninu-
cleate and micropylar is binucleate. (After Ono, 192S.) H, more advanced stage,
showing four nuclei in micropylar chamber and two in chalazal. (After Julianj,
1981b.) I, M. korasakowii; initiation of lateral haustoria from micropylar cham-
ber. J, M. vaginalis, haustorium visible on one side only. K, M. korsakowii,
haustoria in advanced stage of development. L, cross section of ovule, passing
through haustoria. (After Ono, 192S.)

252 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

free nuclear divisions, resulting in a number of nuclei distributed in
the peripheral layer of cytoplasm. Eventually cell formation occurs
here also and it becomes virtually impossible to distinguish the two
portions of the endosperm, one derived from the micropylar and the
other from the chalazal chamber.

Relationships between Different Types of Endosperm. We are
still in the dark regarding the phylogenetic sequence of the evolution
of the various types of endosperm. There are no doubt transitional
forms which interconnect the Nuclear, Helobial, and Cellular types
so that the classification is one of convenience rather than of abso-
lute accuracy, but whether the series is to be read from the Nuclear
toward the Cellular type or vice versa is not clear.

In a discussion of some of the transitional forms, reference may
first be made to Hypericum (Stenar, 1938). The initial divisions
of the primary endosperm nucleus are free nuclear (Fig. 143 A-C),
but at the eight-nucleate stage the two uppermost nuclei come to
lie in a dense mass of cytoplasm adjacent to the zygote (Fig. 143D).
Following the next division, four nuclei are seen in the micropylar
part of the embryo sac, 11 are distributed in the peripheral layer of
cytoplasm, and one becomes delimited in a dense mass of cytoplasm
at the chalazal end (Fig. 14SE). With further divisions, the endo-
sperm may be said to comprise three more or less distinct regions —
micropylar, chalazal, and central. The first two have a denser
cytoplasm and seem to be separated from the third, which has a
large central vacuole, by thin plasma membranes (Fig. 143F-7).
At about the octant stage of the embryo, the distinction between
the micropylar and the central portions becomes less sharp, and
eventually the two merge into one another. The chalazal portion,
however, forms a coenocytic "cyst," whose upper surface "seems to
become hardened and delimited from the rest of the embryo sac"
(Swamy, 19466). At this stage the endosperm of Hypericum may,
therefore, be mistaken for one of the Helobial type (Palm, 1922),
although the developmental studies made by Stenar and Swamy
leave no doubt that it is Nuclear. It does illustrate, however, a
kind of transition between the Nuclear and the Helobial types.

Lappula echinata, a member of the Boraginaceae, also shows a
condition which may be regarded as intermediate between the
Nuclear and the Cellular type. As reported by Svensson (1923)
the first division of the primary endosperm nucleus is followed by

THE ENDOSPERM

253

the formation of a vertical wall, but since most of the cytoplasm of
the embryo sac is aggregated in its micropylar part and the chalazal
is occupied by a large vacuole, the wall ends blindly, reaching down
to the upper margin of the vacuole only. The next wall also, which
is vertical but at right angles to the first, ends similarly and results

Fig. 143. Development of endosperm in Hypericum acutum. A, embryo sac
showing fertilized egg and endosperm nuclei in division. B, embryo sac with
four endosperm nuclei. C, same, all four nuclei dividing. D, eight-nucleate stage
of endosperm ; two of the nuclei are at upper end and one at lower end of embryo
sac. E, 16-nucleate stage showing four nuclei at upper end, one at chalazal, and
the rest irregularly distributed in middle. F,G, chalazal end of embryo sac showing
two and four endosperm nuclei respectively. H, micropylar part of embryo sac,
showing two-celled embryo surrounded by mass of 16 endosperm nuclei; endosperm
nucleus on right belongs to central portion. /, chalazal part of embryo sac of same
age, showing mass of eight endosperm nuclei. (After Stenar, 19S8.)

in the formation of four cells, all open toward the base. In the fol-
lowing divisions some of the daughter nuclei remain free and pass
into the cytoplasmic layer lining the vacuole, while others divide
with the accompaniment of walls. Eventually, therefore, the upper
part of the embryo sac shows Cellular endosperm and the lower
shows Nuclear endosperm.

254 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Some botanists have attempted to relate the occurrence of the
Nuclear or the Cellular endosperm to the spatial conditions (Raum-
verhdltnisse) in the embryo sac. According to them, long and nar-
row embryo sacs generally have a Cellular endosperm, while broad
and short embryo sacs have a Nuclear endosperm. This difference
is attributed to the fact that in broad embryo sacs the phragmoplast
is unable to form a complete partition across the embryo sac and

Fig. 144. Development of endosperm in Lappida echinata. A ,B, first division of
primary endosperm nucleus; partition wall ends blindly, without reaching down to
base of embryo sac. C, more advanced stage, showing cell formation in upper part
and free nuclei in lower. (After Svensson, 1925.)

soon disappears. While such an explanation may seem plausible
at first sight, it fails to take account of those cases in which the first
wall is longitudinal and bisects the embryo sac from pole to pole
(Fig. 126A), nor can it apply to others in which the Nuclear as well
as the Cellular types are both found in the same species and some-
times even in the same ovary (see Svensson, 1925; Sabet, 1931).

V. S. Rao (1938) suggested a correlation between the type of endo-
sperm and the rate of growth of the embryo. According to his
survey, those plants in which growth and differentiation of the

THE ENDOSPERM 255

embryo take place rapidly have a free nuclear endosperm. Others,
in which the growth is slow or the mature seed contains only an
undifferentiated embryo, have a Cellular endosperm or show cell
formation at a very early stage. Numerous examples can, however,
be cited in which there is no such correlation. In Impatiens, cited
by Rao in support of this theory, the endosperm is not Nuclear as
stated by him but Cellular (Dahlgren, 19346).

Finally the question arises as to which of the two types, the Nu-
clear or the Cellular, is the more primitive and which is the more
advanced, but to this we have no definite answer. Coulter and
Chamberlain (1903) suggest that the Cellular type is the more
primitive, since "even when the endosperm begins with free nuclear
division, a rudimentary cell-plate often appears, suggesting deriva-
tion from an endosperm in which nuclear division was followed by
cell-formation." Schiirhoff (1926), Ono (1928), and Glisic (1928)
have supported this view. Most other authors, however, consider
the reverse derivation, i.e., from the Nuclear to the Cellular type,
to be the more plausible (Schnarf, 1929). However, this is still a
debatable question, since both types occur side by side in the most
primitive orders (e.g., Ranales) as well as the most advanced (e.g.,
Campanulales).86 Further, in the Rubiaceae and Orchidaceae also,
which are generally admitted to be among the most highly evolved
families, we have a free nuclear endosperm.

Histology of the Endosperm. The cells of the endosperm are
usually isodiametric and store large quantities of food materials
whose exact nature and proportions vary from one plant to another.
As a rule the walls are thin and devoid of pits, but when hemicellu-
lose is the chief food reserve they are greatly thickened and pitted.
In such cases they may be more or less homogeneous as in Phytele-
phas, or show a distinct stratification as in Fritillaria. The pit
canals are sometimes very long and their mouths dilated in a trum-
pet-shaped manner. Worthy of note also are the plasmodesma
strands so clearly seen in the endosperm cells of several plants
(Jungers, 1930).

In the grasses and some other plants, the peripheral layer of the
endosperm functions like a cambium and produces on its inside a
series of thin-walled cells which become packed with starch. As
the seed approaches maturity, the outermost layer ceases to divide,

86 See in this connection De Vos (1948, 1949) and Stenar (1950).

256 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

its cells become filled with aleurone grains, and the walls become
slightly thickened.9

There is much uncertainty regarding the possible function of this
"aleurone layer." According to Haberlandt (1914), its chief func-
tion is not that of storage but the secretion of diastase and other
enzymes so that the food materials stored in the endosperm may be
made available to the embryo in a soluble form. Arber (1934) men-
tions experiments in which isolated fragments of the aleurone layer
were placed on damp filter paper and then covered with a mixture
of starch and water. As a control, a similar mixture of starch and
water was laid upon damp filter paper without any aleurone layer.
After 24 hours the starch grains of the control experiment were still
intact while those kept on the aleurone layer had been corroded and
were about to fall into pieces.

A different kind of differentiation of the outer layers of the endo-
sperm is seen in Circaeaster (Junell, 1931). Here the integument
is completely used up during the maturation of the seed, and the
peripheral cells of the endosperm become suberized to form a pro-
tective layer. In Crinum (Tomita, 1931; Merry, 1937), where the
ovule is naked (see page 63) and the endosperm ruptures the
thin pericarp so as to become completely exposed to the air, the
suberization is still more marked, and on being wounded the cells
react like a phellogen by undergoing tangential divisions and form-
ing additional layers of cells.

In the Annonaceae, Myristicaceae, and some members of the
Palmaceae and Rubiaceae, there is a "ruminate" endosperm. It is
said to arise as the result of invaginations of the outer tissues, which
penetrate deeper and deeper and eventually appear as dark wavy
bands in the mature seeds.10 In Psychotria, recently studied by
Fagerlind (1937), it seems that the rumination is not a passive
phenomenon but is due to the activity of the endosperm itself, which
grows out and fills the ridges arising in the integument in post-
fertilization stages.

Several workers (see Arber, 1934) have commented on the peculiar

9 In some plants like Oryza (Juliano and Aldama, 1937) there are two or three
layers of aleurone cells.

10 In most books the invaginations are said to be derived from the perisperm, but
this needs to be verified by careful developmental studies. As stated in Chap. 3,
the nucellus is often so ephemeral that there is no perisperm in postfertilization
stagcfc

THE ENDOSPERM 257

appearance of the nuclei in the cells of the endosperm. In the
earlier stages of development they appear to be in full activity and
their nucleoli are clearly visible, but with the gradual deposition of
starch in the cells the nucleoli disappear and the nuclei become
"deformed and squeezed out into networks of varying degrees of
coarseness" (Brenchley, 1912). Eventually they become com-
pletely disorganized or reduced to "amorphous lumps" and in the
mature seed even their remains can be made out only with the
greatest difficulty. In two recent studies dealing with the endo-
sperm of Agropyrum11 and Triticum, Alexandra v and Alexandre va
(1938) have confirmed this degeneration of the nuclei and discussed
its implications in some detail. According to them, the disorgan-
ization of the nuclei occurs first and the deformation follows later,
owing to the pressure exerted on them by the surrounding starch
grains. They state that "cells with dying nuclei may continue to
live for some time," but the coordination between the activities of
the cytoplasm and plastids is disturbed. Further, "in fully ripe
grains the endosperm represents a physiologically dead tissue." Its
death is no disadvantage, however, for the embryo is supposed to
be able to secure the food materials necessary for its growth and
further development, more easily from a dead rather than a living
tissue, and the "dying off of the nuclei in the endosperm cells pro-
motes filling of the grain."

Regarding the final fate of the endosperm, in some plants (Rici-
nus, Phoenix, Triticum, etc.) it forms a permanent storage tissue
which persists until the germination of the seed, while in others
(Cucurbita, Pisum, Arachis, etc.) it is used up by the growing
embryo and is no longer seen in the mature seed.12 Of special
interest is Symplocarpus (Rosendahl, 1909) in which the embryo
"devours" not only the endosperm but also the two integuments,
so that it ultimately lies naked inside the wall of the ovary. An
even more extreme case is that of Melocanna bambusioides (Stapf,
1904), a member of the Bambuseae, in which the embryo dissolves
even the ovary wall so that it lies completely naked at maturity.1'"

11 In this case the place of the degenerated endosperm nuclei is eventually taken
by druses of calcium oxalate, one in each cell.

12 Many seeds, described in taxonomic literature as "exalbuminous," do have
small amounts of endosperm. Others, described as "albuminous," may have little
or no endosperm but a perisperm derived from the nucellus.

13 In Cyanastrum (Fries, 1919; Nietsch, 1941) the nucellus and endosperm dis-

258 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Xenia. In concluding this discussion of the development and
organization of the endosperm, it seems desirable to call attention
to the phenomenon known as xenia. This term was coined by
Focke (1881) to denote the immediate or direct effect of pollen on
the character of the seed or fruit. In practice it is now limited to
the appearance of the endosperm only, and the effect, if any, on the
somatic tissues lying outside the endosperm has begun to be desig-
nated as metaxenia.

To cite an example of xenia, it is well known that certain races of
Zea mays have yellow (dominant) endosperm, while others have a
white (recessive) endosperm. If pollen from the yellow endosperm
race is placed on the stigmas of the white endosperm race, one
might expect to obtain a hybrid embryo which would show the
dominant character of the yellow endosperm when it grows into a
mature plant and fruits in the following season. Actually, how-
ever, the yellow color appears in the endosperm of the same ovule.

This was very puzzling at first but the explanation became quite
evident after Nawaschin's (1898) discovery of double fertilization.
Briefly, if a white variety (yy) is pollinated with pollen from a yellow
variety (YY), one of the male gametes (Y) unites with the egg
(y) and produces a hybrid embryo (Yy) which will behave as a
heterozygote for yellow endosperm in the next generation. The
second male gamete (Y) unites with the two polar nuclei (y, y) and
produces the primary endosperm nucleus (Yyy). Since the latter
has a factor for yellowness, the endosperm will naturally show the
color, although the ovule belongs to the white parent. In the recip-
rocal cross, i.e., when pollen from a white-grained variety (yy) is
used on a yellow-grained variety (YY), the grains are not white like
those of the pollen parent, but yellow like those of the ovule parent.
Here xenia would seem to be absent, but this is merely due to the
fact that yellowness is dominant over whiteness and therefore the
male gamete with a factor for whiteness has no effect on the color.
It is thus evident that the same mechanism operates in all cases,
but owing to dominance xenia appears only in certain plants and

appear during the development of the seed, but the cells in the chalazal part of the
ovule, lying just above the vascular bundle, divide actively and form a very promi-
nent tissue which, although loose and possessed of many air spaces, soon becomes
filled with fat and starch and serves as a substitute for the endosperm. Fries, who
gave it the name "chalazosperm," suggests that it may also function as a sort of
food body designed to facilitate the distribution of the seed by animals.

THE ENDOSPERM 259

not in others. Indeed, these effects are now so clearly understood
that there is hardly any reason for continuing to refer to them by the
obscure and somewhat confusing term xenia.

More difficult to interpret is the so-called metaxenia, i.e., the
effect of pollen on the maternal structures (seed coat or pericarp)
lying outside the embryo sac. The most important work in this
connection is that of Swingle (1928) who finds that in the date palm
(Phoenix dactylifera) the time of maturity of the fruits as well as
their size can be made to vary according to the type of pollen used
in fertilization.14 Regarding the nature of the mechanism which
enables this to take place, he suggests that possibly the embryo or
endosperm or both secrete hormones, or substances analogous to
them, which diffuse out into the wall of the seed and fruit and exert
a specific influence on them, varying according to the particular
male parent used in the cross.

Although it is not inconceivable that pollen may sometimes
exercise such an influence on the tissues of the ovary and conse-
quently on the shape, color, or flavor of the fruit, it seems that other
factors should also be taken into consideration before arriving at
such a conclusion. Differences in size and shape of fruits may also
be caused by the number, state of maturity, and genetic constitution
of the seeds which develop inside them. To mention a single ex-
ample, in certain apples self-pollination gives seedless or nearly
seedless fruits which are ribbed towards the apex and have a greater
height than breadth. On the other hand, the fruits formed after
cross-pollination have many seeds, their shape is much more sym-
metrical, the height and breadth are almost equal, and the ribbing
is scarcely noticeable (Crane and Lawrence, 1947). It must, there-
fore, be concluded that while Swingle's explanation may be correct,
we do not yet have a sufficiently extensive or critical set of observa-
tions to afford a clear insight into the matter.

Mosaic Endosperm. A very interesting condition, occasionally
encountered in some plants, is the lack of uniformity in the tissues
of the endosperm. In Zea mays, patches of two different colors
have sometimes been observed, forming a sort of irregular mosaic
pattern, or part of the endosperm is starchy and part is sugary.
Webber (1900), who observed an intermingling of red and white
color in Zea mays, proposed an ingenious explanation to account for

14 Similar reports have also been made for a few other plants (see Harrison, 1931;
Nebel, 1936; Schreiner and Duffield, 1942).

260 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

it. To quote his words, "It is not improbable that in some cases the
second sperm nucleus enters the embryo sac, but fails to unite with
the two polar nuclei. In such cases it may be able to form a spindle
and divide separately, the unfecundated embryo sac nucleus formed
by the union of the two polar nuclei also dividing separately. If
this occurs, there would then be formed in the protoplasm of the
embryo sac, nuclei of two distinct characters, one group from the
division of the embryo -sac nucleus and the other from the division
of the sperm nucleus." Since the nuclei become interspersed during
the free nuclear stage of the endosperm, such a hypothesis would
account for the occurrence of variegated kernels. Yet another
possibility which suggested itself to Webber was that the second
sperm nucleus may fuse with only one of the two polar nuclei and
that "after their fusion takes place the other nucleus is repelled and
develops independently." As in the preceding case, there would
thus arise two groups of nuclei — one from a fertilized polar nucleus
containing both maternal and paternal elements, the other from an
unfertilized polar nucleus containing only the maternal elements.

None of Webber's postulates has found adequate microscopic
support up to this time. It is pertinent, however, to call attention
to three reports which bear upon this subject.

In a study of Petunia, Ferguson (1927) claimed that endosperm
formation is initiated independently of fertilization and that the
pollen tube discharges its contents into the embryo sac only after
the two- or four-celled stage of the endosperm. One sperm nucleus
then fuses with the egg and the other with the nucleus of the upper-
most of the endosperm cells. Consequently the endosperm tissue
derived from this cell is triploid and the rest is diploid. She claimed
to have counted 21 chromosomes in the micropylar cells of the endo-
sperm and 14 in the chalazal cells.

Bhaduri (1933) reported a similar condition in a strain of Lyco-
persicum esculentum. Here also the polar fusion nucleus is said to
divide before fertilization. Two cells result, a small micropylar
and a large chalazal. After the discharge of the pollen tube one
sperm fertilizes the egg and the other fuses with the nucleus of the
micropylar endosperm cell.

The third report concerns Acorus calamus (Buell, 1938), in which
the pollen tube is said to enter the embryo sac only after the second-
ary nucleus has divided to form two chambers, a large micropylar
and a small chalazal, "Syngamy takes place normally but there is

THE ENDOSPERM 261

an indication that the second sperm fuses with the nucleus in the
micropylar chamber." The endosperm formed by the chalazal
chamber would therefore be diploid and that formed by the micro-
pylar chamber would be triploid.

While the peculiar behavior reported by Ferguson, Bhaduri, and
Buell would easily account for the presence of an endosperm made
up of two different genetical constitutions, none of these reports
seems to be dependable. Levan (1937) has shown that what Fer-
guson believed to be the unopened pollen tube in Petunia was really
the fertilized egg. He demonstrated that double fertilization takes
place in a perfectly normal fashion, and that all the cells of the endo-
sperm, whether micropylar or chalazal, have the normal triploid
number of chromosomes. Cooper (1946) has confirmed this, and
with this complete refutation of Ferguson's work it seems likely that
similar misinterpretations were made by Bhaduri and Buell.

In conclusion it may be stated that although it is possible that
sometimes only one of the polar nuclei is fertilized while the other
divides independently, this has not so far been cytologically demon-
strated. At the same time Webber's first hypothesis of a series of
independent divisions of the second male nucleus also seems im-
probable (cf. East, 1913). More likely, endosperm nuclei with
deviating chromosome numbers arise because of disturbed mitoses.
The most reasonable explanation for mosaic endosperms would,
therefore, lie either in an aberrant behavior of the chromosomes or
possibly in somatic mutations (cf. Clark and Copeland, 1940).

References

Afzelius, K. 1924. Embryologische und zytologische Studien in Senecio und
verwandte Gattungen. Acta Horti Bergiani 8: 123-219.

Alexandrov, V. G., and Alexandrova, 0. G. 1938. On the endosperm nucleus and
its role in the filling and ripening of the grains of cereals. Compt. Rend.
(Dak.) Acad, des Sci. U.R.S.S. 20: 613-616.

Anantaswamy Rau, M. 1950. Endosperm in Cassia tora Linn. Nature (London)
165: 157.

Arber, A. 1934. "The Gramineae." Cambridge University Press.

Asplund, E. 1920. Studien liber die Entwicklungsgeschichte der Bluten einiger
Valerianaceen. K. Svenska Vet.-Akad. Handl. 61(3): 1-66.

Bacchi, O. 1943. Cytological observations in Citrus. III. Megasporogenesis,
fertilization, and polyembryony. Bot. Gaz. 105: 221-225.

Bhaduri, P. N. 1933. A note on the "new type of fertilization" in plants. Cur-
rent Sci. [India] 2: 95.

Billings, F. H. 1937. Some new features in the reproductive cytology of angio-
sperms, illustrated by Isomeris arborea. New Phytol. 36: 301-326.

262 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Brenchley, W. E. 1912. The development of the grain of barley. Ann. Bot. 26:

903-928.
Brink, R. A., and Cooper, D. C. 1947. The endosperm in seed development.

Bot. Rev. 13: 423-541.
Buell, M. F. 1938. Embryogeny of Acorus calamus. Bot. Gaz. 99: 556-568.
Chiarugi, A. 1925. Embriologia delle Cistaceae. Nuovo Gior. Bot. Ital. N.S.

32: 223-316.
Clark, F. J., and Copeland, F. C. 1940. Chromosome aberrations in the endo-
sperm of maize. Amer. Jour. Bot. 27: 247-251.
Condit, I. J. 1932. The structure and development of flowers in Ficus carica

L. Hilgardia 6: 443-481.
Cooper, D. C. 1946. Double fertilization in Petunia. Amer. Jour. Bot. 33:

54-57.
Coulter, J. M., and Chamberlain, C. J. 1903. "Morphology of Angiosperms."

New York.
Crane, M. B., and Lawrence, W. J. C. 1947. "The Genetics of Garden Plants."

London.
Dahlgren, K. V. 0. 1916. Zytologische und embryologisclie Studien iiber die

Reihen Primulales und Plumbaginales. K. Svenska Vet.-Akad. Handl.

56(4): 1-80.
. 1930. Zur Embryologie der Saxifragoideen. Svensk Bot. Tidskr. 24:

429-448.
. 1934a. Die Embryosackentwlcklung von Echinodorus macrophyllus und

Sagittaria sagittifolia. Planta 21: 602-612.
. 1934&. Die Embryologie von Impatiens roylei. Svensk Bot. Tidskr.

28: 103-125.
. 1939. Endosperm- und Embryobildung bei Zostera marina. Bot.

Notiser 1939, pp. 607-615.
De Vos, M. P. 1948. The development of the ovule and seed in the Hypoxideae.

I. Ianthe Salisb. Jour. South Afri. Bot. 14: 159-169.
. 1949. The development of the ovule and seed in the Hypoxideae. II.

The genera Pauridia Harv. and Forbesia Ecklon. Jour. South Afri. Bot. 15.
Dixon, H. H. 1946. Evidence for a mitotic hormone: observations on the mitoses

of the embryo-sac of Fritillaria imperialis. Sci. Proc. Roy. Dublin Soc. 24:

119-124.
Doll, W. 1927. Beitrage zur Kenntnis der Dipsaceen und Dipsaceen-ahnlicher

Pflanzen. Bot. Arch. 17: 107-146.
Earle, T. T. 1938. Embryology of certain Ranales. Bot. Gaz. 100: 257-275.
East, E. M. 1913. Xenia and the endosperm of angiosperms. Bot. Gaz. 56:

217-224.
Ernst, A., and Schmid, E. 1913. Uber Bllite und Frucht von Rafflesia. Ann.

Jard. Bot. Buitenzorg II, 12: 1-58.
Fagerlind, F. 1937. Embryologisclie, zytologische und bestaubungsexperimen-

telle Studien in der Familie Rubiaceae nebst Bemerkungen liber einige Poly-

ploiditatsprobleme. Acta Horti Bergiani 11: 195-470.

THE ENDOSPERM 263

Fagerlind, F. 1938. Bau und Entwicklung der floralen Organe von Helosis

cayennensis. Svensk. Bot. Tidskr. 32: 139-159.
Ferguson, M. C. 1927. A cytological and genetical study of Petunia. I. Bui.

Torrey Bot. Club 54: 657-664.
Focke, W. 0. 1881. "Die Pflanzenmischlinge, ein Beitrag zur Biologie der

Gewachse." Berlin.
Fries, Th. C. E. 1919. Der Samenbau bei Cyanastrum Oliv. Svensk Bot. Tidskr.

13: 295-304.
Frisendahl, A. 1927. Ubar die Entwicklung chasmogamer und kleistogamei

Bliiten bei der Gattung Elatine. Acta Hort. Gothoburg. 3: 99-142.
Frye, T. C. 1902. A morphological study of certain Asclepiadaceae. Bot. Gaz.

34: 389-413.
Gerassimova, H. 1933. Fertilization in Crepis capillaris. Cellule 42: 103-148.
Glisic, Lj. 192S. Development of the female gametophyte and endosperm in

Haberlea rhodopensis Friv. Bui. Inst. Jard. Bot. Univ. Belgrade 1: 1-13.
. 1932. Zur Entwicklungsgeschichte von Lathraea squamaria L. Bui.

Inst. Jard. Bot. Univ. Belgrade 2: 20-56.
Goldberg, B. 1941. Life history of Peltandra virginica. Bot. Gaz. 102: 641-662.
Gore, U. R. 1932. The development of the female gametophyte and embryo in

cotton. Amer. Jour. Bot. 19: 795-807.
Haberlandt, G. 1914. "Physiological Plant Anatomy." Engl. Transl. London.
Hagerup, O. 1947. The spontaneous formation of haploid, polyploid, and aneu-

ploid embryos in some orchids. K. Danske Vidensk. Selsk., Biol. Meddel.

20: 1-22.
Harrison, G. J. 1931. Metaxenia in cotton. Jour. Agr. Res. 42: 521-544.
Hewitt, W. C. 1939. Seed development of Lobelia amoena. Jour. Elisha Mitchell

Sci. Soc. 55: 63-82.
Johansen, D. A. 1931a, Studies on the morphology of the Onagraceae. IV

Stenosiphon linifolium. Bui. Torrey Bot. Club 57: 315-326.
— . 19316. Studies on the morphology of the Onagraceae. V. Zauschneria

latifolia, typical of a genus characterized by irregular embryology. Ann.

N. Y. Acad. Sci. 33: 1-26.
— . 1931c. Studies on the morphology of the Onagraceae. VI. Anogra

pallida. Amer. Jour. Bot. 18: 854-863.
Johnson, D. S. 1900. On the endosperm and embryo of Peperomia pellucida.

Bot. Gaz. 80: 1-11.
Johri, B. M. 1935. Studies in the family Alismaceae. I. Limnophyton obtusi-

folium Miq. Jour. Indian Bot. Soc. 14: 49-66.
. and Maheshwari, P. 1950. The development of the embryo sac, embryo

and endosperm in Helixanthera ligustrina (Wall.) Dans. (In press.)
Juel, H. O. 1907. Studien iiber die Entwicklungsgeschichte von Saxifraga

granulata. Nova Acta R. Soc. Sci. Uppsal. IV, 1(9): 1-41.
Juliano, J. B. 1931a. Floral morphology of Lyonothamnus floribundus. Bot.

Gaz. 91: 426-438.
. 19316. Morphological study of the flower of Monochoria vaginalis

(Burn f.) Presl. Philippine Agr. 20: 177-186.

264 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Juliano, J. B. and Alcala, P. E. 1933. Floral morphology of Musa errans (Blanc:>)

Teodoro var. Botoan Teodoro. Philippine Agr. 22: 91-126.
and Aldama, M. J. 1937. Morphology of Oryza sativa L. Philippine

Agr. 26: 1-134.
Junell. S. 1931. Die Entwicklungsgeschichte von Circaeaster agrestis. Svensk

Bot. Tidskr. 25: 238-270.
Jungers, V. 1930. Recherches sur les plasmodesmes chez les v£getaux. I.

Cellule 40: 1-82.
. 1931. Figures caryocinetiques et cloisonnement du protoplasme dans

l'endosperme d'Iris. Cellule 40: 293-354.
Kadry, A. E. R. 1946. Embryology of Cardiospermum halicacabum L. Svensk

Bot. Tidskr. 40: 111-126.
Kausik, S. B. 1938a. Studies in the Proteaceae. I. Cytology and floral mor-
phology of Grevillea robusta Cunn. Ann. Bot. N.S. 2: 899-910.
. 19386. Studies in the Proteaceae. II. Floral anatomy and morphology

of Macadamia ternifolia F. Muell. Proc. Indian Acad. Sci. Sect. B. 8: 45-62.
. 1938c. Pollen development and seed formation in Utricularia coerulea

L. Beihefte bot. Centbl. 58A: 365-378.
. 1940. A contribution to the embryology of Enalus acoroides (L. fil.)

Steud. Proc. Indian Acad. Sci. Sect. B. 11: 83-99.

1942. Studies in the Proteaceae. VII. The endosperm of Grevillea

robusta Cunn. with special reference to the structure and development of the
vermiform appendage. Proc. Indian Acad. Sci. Sect. B. 16: 121-140.

Khan, R. 1942. A contribution to the embryology of Jussieua repens Linn.
Jour. Indian Bot. Soc. 21: 267-2S2.

Krishna Iyengar, C. V. 1937. Development of embryo-sac and endosperm-
haustoria in some members of the Scrophularineae. I. An account of Sopubia
delphinifolia G. Don. and Alonsoa sp. Jour. Indian Bot. Soc. 16: 93-109.

. 1939. Development of the embryo sac and endosperm haustoria in

some members of Scrophularineae. II. Isoplexis canariensis Lindl. and
Celsia coromandeliana Vahl. Jour. Indian Bot. Soc. 18: 13-20.

. 1940. Development of the embryo sac and endosperm haustoria in some

members of the Scrophularineae. IV. Vaniellia hirsuta Ham. and V. scabra
Benth. Jour. Indian Bot. Soc. 18: 179-189.

1941. Development of the embryo sac and endosperm haustoria in

Torenia cordifolia Roxb. and T. hirsuta Benth. Proc. Natl. Inst. Sci. India

7: 61-71.
. 1942. Development of seed and its nutritional mechanism in Scrophulari-

aceae. I. Rhamphicarpx Imgiflora Banth., Csniran'Jiera hispiii Be, and

Pedicularis zeylanica Benth. Proc. Natl. Inst. Ssi. India 8: 219-251.
Lagerberg, T. 1909. Studien liber die Entwicklungsgeschichte und systemati-

schen Stellung von Adoxa moschatellina L. K. Svenska Vet.-Akad. Handl.

44(4): 1-86.
Langdon, L. M. 1934. Embryogeny of Carya and Juglans, a comparative study.

Bot. Gaz. 96: 93-117.
Levan, A. 1937. Eine erbliche Anomalie der Samenanlage bei Petunia. Bot.

Notiser. 1937, pp. 35-55.

THE ENDOSPERM 265

Longo, B. 1909. Osservazioni e ricerche sul Ficus carica. Ann. di Bot. 7:

235-256.
Maheshwari, P. 1934. The Indian mango. Current Sci. [India] 3: 97-98.
and Singh, Bahadur. 1950. The embryology of Macrosolen cochinchinen-

sis Van Tieghem. (In press.)
Mauritzon, J. 1933. "Studien liber die Embryologie der Familien Crassulaceae

und Saxifragaceae." Diss. Lund.
. 1934. Die Endosperm- und Embryoentwicklung einiger Acanthaceen.

Lunds Univ. Arsskr. N.F. Avd. II, 30(5) : 1-42.
. 1939. tiber die Embryologie von Marcgravia. Bot. Notiser 1939, pp.

249-255.
McKay, J. W. 1947. Embryology of pecan. Jour. Agr. Res. 74: 263-2S3.
Mendes, A. J. T. 1941. Cytological observations in Cojfea. VI. Embryo and

endosperm development in Cojfea arabica L. Amer. Jour. Bot. 28: 784-789.
Merry, J. 1937. Formation of periderm in the endosperm of Crinum asiaticum.

Papers Mich. Acad. Sci., Arts, and Letters 22: 159-164.
Mohrbutter, C. 1936. Embryologische Studien an Loganiaceen. Planta 26:

64-80.
Nast, C. G. 1935. Morphological development of the fruit of Juglans regia.

Hilgardia 9: 345-381.
. 1941. The embryogeny and seedling morphology of Juglans regia L.

Lilloa 6: 163-205.
Nawaschin, S. 1898. Resultate einer Revision der Befruchtungsvorgange bei

Lilium martagon und Fritillaria temlla. Bull. Acad. Imp. des Sci. St. Peters-
burg 9(4): 377-382.
Nebel, B. R. 1936. Metaxenia in apples. Jour. Hered. 27: 345-349.
Nemec, B. 1910. "Das Problem der Befruchtungsvorgange und andere zyto-

logischen Fragen." Berlin.
Nietsch, H. 1941. Zur systematischen Stellung von Cyanastrum. Osterr. Bot.

Ztschr. 90: 31-52.
Oehler, E. 1927. Entwicklungsgeschichtlich-cytologische Untersuchungen an

einigen saprophytischen Gentianaceen. Planta 3: 641-733.
Ono, T. 1928. Embryologische Studien an einigen Pontederiaceen. Sci. Rpt.

Tohoku Imp. Univ. IV. Biol. 3: 405-415.
. 1929. Embryologie der Liliaceae, mit besonderer Rlicksicht auf die

Endospermbildung. I. Melanthioideae und Aletroideae. Sci. Rpt. Tohoku

Imp. Univ. IV. Biol. 4: 381-393.
Pace, L. 1909. The gametophytes of Calopogon. Bot. Gaz. 48: 126-137.
Paetow, W. 1931. Embryologische Untersuchungen an Taccaceen, Meliaceen

und Dilleniaceen. Planta 14: 441-470.
Palm, B. 1922. Das Endosperm von Hypericum. Svensk Bot. Tidskr. 16:

60-68.
Poddubnaja-Arnoldi, W. 1931. Ein Versuch der Anwendung der embryologi-

schen Methode bei der Losung einiger systematischer Fragen. Beihefte. bot.

Centbl. 48B: 141-237.
Rao, V. S. 1938. The correlation between embryo type and endosperm type.

Ann. Bot. N.S. 2: 535-536.

266 INTORDUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Rauch, K. V. 1936. Cytologisch-embryologische Untersuchungen an Scurrula

atropurpurea Dans, unci Dendrophthoe pentandra Miq. Ber. schweiz. bot.

Gesell. 45: 5-61.
Roc6n, T. 1927. "Zur Embryologie der Centrospermen." Diss. Uppsala.
Ros6n, W. 1940. Notes on the embryology of Globularia vulgaris L. Bot.

Notiser 1940, pp. 253-261.
Rosendahl, C. 0. 1909. Embryo sac development and embryology of Symplo-

carpus foetidus. Minn. Bot. Studies 4: 1-9.
Rutishauser, A. 1937. Entwicklungsgeschichtliche Untersuchungen an Thesium

rostratum. Mitt, naturf. Gesell. Schaffhausen 13: 25-47.
Sabet, Y. S. 1931. Development of the embryo sac in Calotropis procera, with

special reference to endosperm formation. Ann. Bot. 45: 503-518.
Schaeppi, H., and Steindl, F. 1942. Blutenmorphologische und embryologische

Untersuchungen an Loranthoideen. Vrtljschr. naturf. Gesell. Zurich 87:

301-372.
Schnarf, K. 1928a. Uber die Endospermentwicklung bei Ornithogalum. Osterr.

bot. Ztschr. 77: 173-177.
. 19286. Uber das Embryosackhaustorium bei Anthericum. Osterr.

bot. Ztschr. 77: 287-291.

. 1929. "Embryologie der Angiospermen." Berlin.

. 1931. Ein Beitrag zur Kenntnis der Samenentwicklung der Gattung

Cochlospermum. Osterr. bot. Ztschr. 80: 45-50.
- and Wunderlich, R. 1939. Zur vergleichenden Embryologie der Liliaceae-

Asphodeloideae. Flora 33: 297-327.
Schreiner, E. J., and Duffield, J. W. 1942. Metaxenia in an oak species cross.

Jour. Hered. 33: 97-98.
Schulle, H. 1933. Zur Entwicklungsgeschichte von Thesium montanum Ehrh.

Flora 27: 140-184.
Schiirhoff, P. N. 1915. Amitosen von Riesenkernen im Endosperm von Ranuncu-
lus acer. Jahrb. f. wiss. Bot. 55: 499-519.

-. 1926. "Die Zytologie der Bliitenpnanzen." Stuttgart.
Scott, F. M. 1944. Cytology and microchemistry of nuclei in developing seed of

Echinocystis macrocarpa. Bot. Gaz. 105: 329-338.
Sharp, L. W. 1912. The orchid embryo sac. Bot. Gaz. 54: 373-335.
Singh, Bahadur. 1950. The embryology of Dendrophthoe falcata (Linn, fil.)

Ettingshausen. Proc. 37th. Indian Sci. Cong. Bot. Sect.
Stapf, O. 1904. On the fruit of Melocanna bambusoides Trim, an endospermless,

viviparous genus of Bambuseae. Trans. Linn. Soc. London II Bot., 6: 401.
Stenar, H. 1925. "Embryologische Studien I and II. I. Zur Embryologie der

Columniferen. II. Die Embryologie der Amaryllideen." Diss. Uppsala.

-. 1928a. Zur Embryologie der Asphodeline-Gruppe. Ein Beitrag zur

systematischen Stellung der Gattungen Bulbine und Paradisea. Svensk Bot.

Tidskr. 22: 145-159.

— . 19286. Zur Embryologie der Veratrum- und Anthericum-Gmppe. Bot.

Notiser 1928, pp. 357-378.
. 1935. Embryologische Beobachtungen liber Scheuchzeria palustris L.

Bot. Notiser 1935, pp. 78-86.

THE ENDOSPERM 267

Stenar, H. 1938. Das Endosperm bei Hypericum acutum Moench. Bot. Notiser

1938, pp. 515-527.
. 1950. Studien uber das Endosperm bei Galtonia candicans (Bak.)

Decne und anderen Scilloideen. Acta Horti Bergiani 15: 169-184.
Stolt, K. A. H. 1921. Zur Embryologie der Gentianaceen und Menyanthaceen.

K. Svenska Vet.-Akad. Handl. 61(14): 1-56.
Subramanyam, K. 1948. An embryological study of Melastoma malabathricum.

Jour. Indian Bot. Soc. 27: 1-9.
. 1949. An embryological study of Lobelia pyramidalis Wall., with special

reference to the mechanism of nutrition of the embryo in the family Lobeliaceae.

NewPhytol.:48: 365-374.
Svensson, H. G. 1923. Om endospermet hos Lappula. Svensk. Bot. Tidskr. 17i

387-388.
. 1925. "Zur Embryologie der Hydrophyllaceen, Borraginaceen und

Heliotropiaceen." Diss. Uppsala.
. 1926. Zytologische-embryologische Solanaceenstudien I. Uber die

Samenentwicklung von H yoscyamus niger L. Svensk Bot. Tidskr. 20: 420-434.
Swamy, B. G. 1946a. Development of endosperm in Zephyranthes andersonii

Baker. Proc. Natl. Inst. Sci. India 12: 187-190.
. 1946&. Endosperm in Hypericum mysorense Heyne. Ann. Bot. N.S.

9: 165-169.

1947. On the life history of Vanilla planifolia. Bot. Gaz. 108: 449-456.

Swingle, W. T. 1928. Metaxenia in the date palm. Jour. Hered. 19: 257-268.
Tackholm, G. 1915. Beobachtungen iiber die Samenentwicklung einiger Ona-

graceen. Svensk Bot. Tidskr. 9: 294-361.
Tomita, K. 1931. Uber die Entwicklung des nackten Embryos von Crinum

latifolium L. Sci. Rpt. Tohoku Imp. Univ. IV. Biol. 6: 163-169.
Treub, M. 1885. Observations sur les Loranthacees. I. Developpement dans

les sacs embryonnaires dans le Loranthus sphaerocarpus. II. Embryogenie

du Loranthus sphaerocarpus. Ann. Jard. Bot. Buitenzorg 2: 54-76.
Walker, R. I. 1947. Megasporogenesis and embryo development in Tropaeohim

majus L. Bui. Torrey Bot. Club 74: 240-249.
Wanscher, J. H. 1939. Contribution to the cytology and life history of apple

and pear. Roy. Vet. and Agr. Coll. Copenhagen Yearbook 1939, pp. 21-70.
Webber, H. J. 1900. Xenia, or the immediate effect of pollen in maize. U.S.

Dept. Agr. Div. Veg. Path, and Physiol. Bui. 22.
Witmer, S. W. 1937. Morphology and cytology of Vallisneria spiralis L. Amer.

Midland Nat. 18: 309-327.
Wunderlich, R. 1937. Zur vergleichenden Embryologie der Liliaceae-Scilloideae.

Flora 32: 48-90.
Wylie, R. B., and Yocom, A. E. 1923. The endosperm of Utricularia. Univ.

Iowa Studies Nat. Hist. 10(2) : 3-18.
Zweifel, R. 1939. "Cytologisch-embryologische Untersuchungen an Balanophora

abbreviata Blume und B. indica Wall." Diss. Zurich.

CHAPTER 8
THE EMBRYO1

After syngamy the zygote undergoes a period of rest during which
the large vacuoles originally present in the upper part of the cell
gradually disappear and the cytoplasm assumes a fairly homoge-
neous appearance. Vacuoles may appear once again when the cell
begins to grow in preparation for the first division, but these are
more or less uniformly distributed and are not restricted to any
special portion.

The resting period of the zygote varies with different species and
is to some extent dependent on environmental conditions. In gen-
eral the primary endosperm nucleus divides first and the zygote
divides shortly afterwards. In Theobroma cacao (Cheesman, 1927)
the primary endosperm nucleus divides 4 to 5 days after fertilization
and the zygote 14 to 15 days after fertilization. In the fertile
banana "Rodoe Clamp" (White, 1928), Carya illinoensis (McKay,
1947), and Epidendrum prismatocarpum (Swamy, 1948) the zygote
remains undivided for about 6 weeks, and in Viscum alburn (Pisek,
1923) for about 8 weeks after fertilization. In Colchicum autumnale
(Heiman-Winawer, 1919) fertilization takes place in autumn and
endosperm nuclei are formed soon after, but the zygote remains
dormant for a period of 4 to 5 months during the winter.

The shortest resting periods occur in the Compositae and Grami-
neae. In Crepis capillaris (Gerassimova, 1933), for example, the
primary endosperm nucleus undergoes its first division within 4 to 7
hours after pollination and its second division 1 to 3 hours later,
while the first division of the zygote takes place 5 to 10 hours after
pollination. In Oryza sativa (Noguchi, 1929) the first division of
the zygote occurs about 6 hours after fertilization and 18 hours later
the embryo consists of four to seven cells. After 4 days it begins to
differentiate into the various body regions and in 10 days it is
completely mature.

The data on Hordeum distichon palmella (Pope, 1937) give the

1 For a detailed treatment of the various modes of embryonal development met
with in the angiosperms, reference should be made to Soueges (1934a, 1937a, 1934-
1939).

268

THE EMBRYO

269

sequence of events from the time of pollination to the first division
of the zygote. These data are given in the accompanying table.
In some plants the first division of the zygote and that of the
primary endosperm nucleus occur at approximately the same time.
As examples may be cited Alisma, Damasonium (Dahlgren, 1928),

Hordeum distichon palmella

Time elapsed
after pollination

Growth of pollen
tube

Development of
embryo

Development of
endosperm

5 min

10 min

45 min

Pollen germi-
nated

Male gametes
inside pollen
tube

Entry of pollen
tube into em-
bryo sac

One male gamete in
contact with egg

Male nucleus form-
ing a sector of the
egg nucleus

Male sector of zygote
nucleus becoming
more and more
diffuse

Prophase of first di-
vision of zygote

First division of zy-
gote nearing com-
pletion

Other male gamete in

5hr

contact with polar
nuclei
Male nucleus and po-

6hr

lar nuclei in process
of fusion
First division of pri-
mary endosperm
nucleus

Second division of

10 hr

13 hr

primary endosperm
nucleus

Four endosperm nu-
clei

Eight endosperm nu-
clei

15 hr...:

Cuscuta (Smith, 1934), Echinodorus (Dahlgren, 1934) and Taraxacum
kok-saghys (Cooper and Brink, 1949). Less frequently the zygote
may divide before the primary endosperm nucleus as in Cynomorium
(Steindl, 1945), Restio (Borwein et ah, 1949), and some species of
Allium (Weber, 1929). In any case eventually the development of
the endosperm proceeds at a quicker rate and surpasses that of the
embryo.2

2 Oxybaphus nyctagineus (Cooper, 1949) is an exception to this. At first nuclear
divisions in the endosperm keep pace with cell divisions in the embryo, but after

270

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

The second table summarizes the time relations of the divisions
in the embryo and endosperm of Taraxacum kok-saghys (Poddub-
naja-Arnoldi and Dianowa, 1934), where at least for the first few
days both develop more or less concurrently and neither has any
appreciable advantage over the other.

In all cases the first division of the zygote is followed by wall
formation. This is in sharp contrast with the condition in the

Taraxacum kok-saghys

Time elapsed

Pollen tube growth

Development

Development of

after pollination

of embryo

endosperm

15 min

Entry of pollen tube

into embryo sac

45 min

Discharge of pollen

tube and approach

of male gametes

toward egg and

secondary nucleus

1 hr., 15 min

Syngamy

Triple fusion

3 hr., 50 min

First division of pri-
mary endosperm
nucleus

5 hr

First division
of zygote

6 hr., 15 min.

Two-celled pro-

Two-nucleate endo-

embryo

sperm

8 hr., 15 min.

Four-celled em-

Four-nucleate endo-

bryo

sperm

24 hr., 45 min.. . .

Several-celled

Multicellular endo-

proembryo

sperm

gymnosperms, where the first few divisions are almost always free
nuclear. The only reported instance of a free nuclear embryo in

the 64-nucleate stage the embryo becomes more aggressive and mitoses occur more
slowly in the endosperm.

Pope (1943) suggests that the extra number of chromosomes and genes in the
primary endosperm nucleus may be a factor contributing to the more rapid growth
of the endosperm as compared with that of the embryo. This is an interesting
suggestion, but even in the Onagraceae, where the zygote and the primary endo-
sperm nucleus have the same genetic constitution, the development of the endo-
sperm proceeds at a much quicker rate.

THE EMBRYO 271

angiosperms is that of Moringa oleifera, in which Rutgers (1923)
claimed that wall formation commenced only after the 16-nucleate
stage. Puri (1941) has shown, however, that the free nuclear em-
bryo of Rutgers was really a micropylar accumulation of endosperm
nuclei, while the real embryo, which is formed quite normally, was
entirely overlooked by him.3

In the earlier stages of development there are no fundamental
differences between the embryos of the dicotyledons and those of
the monocotyledons. However, since the mature embryos are so
markedly different in the two groups, they will be treated separately
in the following account.

DICOTYLEDONS

Except in a very few species, which will be considered later, the
first division of the zygote is almost always followed by the laying
down of a transverse wall. Of the two cells thus formed, the one
which lies towards the interior of the embryo sac is called the
terminal cell and the other the basal cell. In the next stage the
terminal cell may divide transversely or longitudinally. The basal
cell usually undergoes a transverse division, but in some plants it
remains undivided and becomes hypertrophied to form a large
vesicular structure.

The French embryologist Soueges, who is the chief authority on
the development of the embryo in angiosperms, considers the mode
of origin of the four-celled proembryo and the contribution made by
each of these cells to the body regions of the mature embryo as the
most important aids in a classification of the embryonal types. Fol-
lowing him, Schnarf (1929) and Johansen (1945) have recognized
five principal types of embryos among the dicotyledons. These
may be distinguished from one another as follows:
I. The terminal cell of the two-celled proembryo divides by a
longitudinal wall —

(i) The basal cell plays only a minor part or none in the sub-
sequent development of the embryo Crucifer type4

3 The free nuclear divisions of the zygote reported in Ficus (Tischler, 1913) and
Ruta (Cappaletti, 1929) are in the nature of abnormalities which need not be
considered here.

4 Johansen (1945) prefers the term "Onagrad type" as the embryo of the Ona-
graceae is simpler and more typical than that of Capsella (Cruciferae) , usually
described in textbooks.

272 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

(ii) The basal and terminal cells both contribute to the de-
velopment of the embryo Asterad type

II. The terminal cell of the two-celled proembryo divides by a
transverse wall —

1. The basal cell plays only a minor part or none in the subse-
quent development of the embryo —

(i) The basal cell usually forms a suspensor of two or more

cells Solanad type

(ii) The basal cell undergoes no further division, and the
suspensor, if present, is always derived from the terminal
cell Caryophyllad type

2. The basal and terminal cells both contribute to the develop-
ment of the embryo Chenopodiad type

Crucifer Type. As mentioned in Chap. 1, the embryo of Capsella
bursa-pastoris was among the first to receive detailed attention.
Subsequent to the classical researches of Hanstein (1870) and
Famintzin (1879), Soueges (1914, 1919) has made a still more
thorough study of its embryogeny. The first division of the zygote
is transverse resulting in a basal cell cb and a terminal cell ca (Fig.
145 A, B). The former divides transversely and the latter divides
longitudinally, resulting in a J_ -shaped proembryo composed of four
cells (Fig. 145C-E1). Each of the two terminal cells now divides
by a vertical wall lying at right angles to the first, so as to result in
a quadrant stage (Fig. 145/). The quadrant cells in turn become
partitioned by a transverse wall, so as to form octants (Fig. 145K,
L). Of these the lower four are destined to give rise to the stem
tip and cotyledons and the upper four to the hypocotyl. All the
eight cells divide periclinally (Fig. 145M,iV). The outer deriva-
tives form the dermatogen, while the inner ones undergo further
divisions to give rise to the periblem and plerome initials (Fig. 145
O-Q).

Meanwhile, the two upper cells ci and cm, of the four-celled pro-
embryo (Fig. 1452)) divide to form a row of 6 to 10 suspensor cells
(Fig. 145F-K) of which the uppermost cell v becomes swollen and
vesicular and probably serves a haustorial function. The lowest
cell h functions as the "hypophysis" (Fig. 145A0- Although at
first similar in shape to the other cells of the suspensor, it soon be-
comes somewhat rounded at the lower end and divides transversely
to form two daughter cells (Fig. 1450), each of which undergoes two

THE EMBRYO

273

divisions by walls which are oriented at right angles to one another.
Of the resulting eight cells, the lower four form the initials of the
root cortex and the upper four give rise to the root cap and the root
epidermis.

At the same time further divisions take place in the embryo
proper, especially at two points in the lower tier which are destined
to form the cotyledons. At this stage the embryo appears more or

Fig. 145. Development of embryo in Capsella bursa-pastoris.
1914, 1919.)

(After Soueges,

•i74 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

less cordate in longitudinal section (Fig. 146 A, B). The hypocotyl
as well as the cotyledons soon elongate in size, mostly by transverse
divisions of their cells (Fig. 146C,Z)). During further development
the ovule becomes curved like a horseshoe and the growing coty-
ledons also conform to this shape for spatial reasons (Fig. 14QE,F).

B ^ E F

Fig. 146. Older stages in development of embryo of Capsella bursa-pastoris.
(After Schaffner, 190G.)

Although Capsella has been used for over fifty years to illustrate
the embryogeny of the dicotyledons, the embryos of the Onagraceae
are of a simpler and more uniform type and Ludwigia palustris
(Soueges, 1935a) may be used as an illustration. The first division
of the zygote is transverse (Fig. 147 A) after which the terminal cell
ca divides longitudinally to form two juxtaposed cells and the basal
cell cb divides transversely to form the two cells ci and m (Fig.

THE EMBRYO

275

1475). This is the four-celled stage of the proembryo. At the
third cell generation, the two cells of the tier ca divide by a vertical
wall, at right angles to the first, to give rise to the quadrant stage
(Fig. 147C). The quadrants q divide transversely to produce a
group of eight cells called the octants (I and V) and ci gives rise to
the two suspensor cells n and n' (Fig. 147 D,E). The wall in the
middle cell m, which functions as the hypophysis, is curved and
joined on both sides to the transverse wall which originally sepa-
rated ca from cb, and now separates the tiers V and m (Fig. 147 F).

G^— l-^ h * ^ I

Fig. 147. Development of embryo in Ludwigia pdustris. (After Soueges, 19S5a.\

Of the two daughter cells formed from m, the lower, which is lentic-
ular in shape, gives rise to the initials of the root tip while the upper
gives rise to the root cap. The cells of the tier I and V give rise to
the cotyledons and the stem tip. Figure 147 G-I shows some of the
stages in the differentiation of the dermatogen de, periblem pc,
plerome pi, the cells pco which are destined to form the root cap,
and the cells iec, which are destined to form the root cortex.

Asterad Type. Lactuca saliva (Jones, 1927) may be used as an
illustration of the Asterad type which has been based on the studies
of Carano (1915) and Soueges (1920c) on various members of the
Compositae. The four-celled proembryo consists of two juxtaposed

276 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

cells derived from the terminal cell ca and two superposed cells ci
and m derived from the basal cell cb (Fig. 148 A-C). In the follow-
ing stage, each of the four cells divides again so that the terminal
tier now comprises the quadrant cells q, the middle tier comprises
the two juxtaposed cells at m, and ci divides transversely to form
the daughter cells n and n' (Fig. 148D). Thus the upper three tiers
of this stage owe their origin to the basal cell cb, and the lowest tier
of four cells to the terminal cell ca of the two-celled proembryo.
The four cells of the tier q divide to form the octant stage, the walls
segmenting the quadrant cells being oriented more or less diagonally ;
the two cells of the tier m undergo a vertical division to give rise to
four cells lying directly above the octants; n also divides by a
vertical wall ; and n' divides by a transverse wall to form o and p

Fig. 148. Development of embryo in Lactuca sativa. {After Jones, 1927.)

(Fig. 14&E-G). At the same time tangential walls are laid down
in the tiers q and m to cut off an outer layer of dermatogen cells from
the inner cells which undergo further divisions to give rise to the
periblem and plerome (Fig. 148G). Regarding further develop-
ment, the cell p gives rise to a suspensor consisting of a variable
number of cells; o to the root cap and dermatogen of the root; n to
the remaining part of the root tip; m to the hypocotyledonary
region; and q to the cotyledons and stem tip.

Geum urbanum (Soueges, 19236) offers a significant variation from
the above scheme in the early demarcation of a special cell called
the epiphysis initial.5 After the two-celled stage (Fig. 149A) the
first wall in ca is markedly oblique, resulting in two unequal cells
a and b (Fig. 149B-C). Of these, a divides to cut off a wedge-shaped
cell e, which is called the epiphysis (Fig. 149 D,E). At the same
time the middle cell m divides vertically and ci divides transversely,
so that there are now four tiers of cells in all, designated as q, m, ny

6 For further information on the epiphysis, see Soueges (19346).

THE EMBRYO

277

and n', lying directly above the epiphysis initial e (Fig. 149F). Of
these, q gives rise to the cotyledonary region, m to the hypocotyl,
n or one of its derivatives to the hypophysis and a part of the sus-
pensor, and n' to the greater portion of the suspensor. Some of the
stages in development are shown in Fig. 149G-L.

An epiphysis initial has also been reported in Fragaria and Viola
(Soueges, 19356, 1937c). The former is essentially similar to
Geum, but the latter shows some differences in that the derivatives

Fig. 149. Development of embryo in Geum urbanum. (After Soueges, 1928b.)

of the tier q, lying just above the epiphysis, produce not only the
cotyledonary region but also the lower portion of the hypocotyl,
while the tier m contributes to the upper portion of the hypocotyl
and the root. A suspensor is virtually absent, as the uppermost tier
ci is devoted to the formation of the root cap.

Solanad Type. Soueges (19206, 1922) and Bhaduri (1936) have
studied a number of species of the Solanaceae, of which Nicotiana
may be described here as an example. The first division of the

278 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

zygote is transverse (Fig. 150 A, B). The terminal cell ca and the
basal cell cb both divide transversely to give rise to a four-celled
proembryo (Fig. 150C-F). The four tiers may be designated from
below upward as I, V , m, and d. Now I and V divide by vertical

Fig. 150. Development of embryo in Nicotiana. {After Soueges, 1922.)

walls oriented at right angles to each other to give rise to octants,
while m and ci divide transversely to produce d, f, n, and n' (Fig.
150 G-H). By subsequent divisions the tier I gives rise to the
cotyledonary portion, V to the hypocotyl and to the periblem and

Fig. 151. Development of embryo in Lobelia arnoena. (After Hewitt, 1939.)

plerome of the root, and d to the root tip. The remaining cells/, n,
and n' produce the suspensor.

Hewitt (1939) has recently given a very full account of the em-
bryogeny of Lobelia amoena, which also belongs to the Solanad type.

THE EMBRYO 279

After the two-celled stage (Fig. 151 A) the basal cell cb divides trans-
versely to produce ci and m (Fig. 1515) and the terminal cell divides
to produce I and V (Fig. 151C). Following this four-celled stage,
m and ci divide to produce the cells d,f, n, and n' (Fig. 151D), each
of which may divide again, resulting in a suspensor which is about
8 to 12 cells long. At the same time the cell V segments into ph and
h (Fig. 151D) after which I and ph divide by longitudinal walls
oriented at right angles to each other, and h divides by a transverse
wall. Two juxtaposed cells are thus produced from I and ph and
two superposed cells (ha and Kb)' from h (Fig. 151E). The embryo
proper (excluding the suspensor cells) now consists of four tiers, I,
ph, ha, and Kb. In I and ph the next division is vertical and in a
plane at right angles to the first division. At the same time ha also
divides by a vertical wall, resulting in two tiers of four cells each at
I and ph, one tier of two cells at ha, and one of a single cell Kb (Fig.
151F). In the cells of the apical tier /, the next division walls are
diagonal, followed by periclinal divisions which cut off the dermat-
ogen. The cells of the tier ph divide periclinally; the outer cells
form the dermatogen, and the inner undergo further longitudinal
divisions to form the periblem and plerome initials of the stem
(Fig. 151 G). The two cells of the tier ha give rise to a single semi-
circular layer of cells which contributes to the periblem, the dermat-
ogen, and a part of the root cap (Fig. 151H-J). The cell Kb divides
only at a comparatively late stage, at first by a transverse wall and
then by a vertical wall in each of the daughter cells. The upper
derivatives form a part of the root cap, which is supplemented on all
sides by a cell from the tier ha and in some cases by extra cells cut
off from the dermatogen of tier ph. A peculiarity of the suspensor
is the widening and vertical divisions of two or three of its cells
lying just above the embryo proper (Fig. 151//).

In Sherardia (Soueges, 1925), a member of the Rubiaceae, the
four-celled proembryo arises in the same way as in Nicotiana and
Lobelia. The first division in the cells / and V (Fig. 152A) may be
either transverse or vertical, but m and ci always divide transversely
(Fig. 1525). The eight-celled stage may thus comprise six, seven
(Fig. 152D), or eight (Fig. 152C) tiers of cells. Normally the de-
rivatives of I form the cotyledonary region and those of V form the
hypocotyl; m and ci give rise to a massive suspensor, which is fila-
mentous towards the apical end but is composed of a number of
large vesicular cells at the basal end (Fig. 152E-G). . Presumably

280 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the last cell of the suspensor functions as a hypophysis and con-
tributes to a portion of the root tip.

Chenopodiad Type. Chenopodium bonus-henricus (Soueges,
1920a) is similar to Nicotiana in having a four-celled proembryo

Fig. 152. Development of embryo in Sherardia arvensis. (After Soueges, 1925.)

consisting of the cells /, V, m, and ci arising by a transverse division
of ca and cb (Fig. 153 A-C). During the course of further develop-
ment I, l', and m become segmented into four cells each by the

Fig. 153. Development of embryo in Chenopodium bonus-henricus. {After Soueges,
1920a.)

laying down of two vertical walls oriented at right angles to each
other, while ci divides transversely to form n and n' and then the
four cells h, k, o, and p (Fig. 153 D-H). During further develop-

THE EMBRYO

281

merit the tier I gives rise to the cotyledons, V to the lower part of the
hypocotyl, and m to the upper part of the hypocotyl. The cells
originating from ci form the suspensor except the last cell h, which
functions as the hypophysis and contributes to the root tip.

Beta vulgaris (Artschwager and Starrett, 1933), which is also a
member of the Chenopodiaceae, is similar to Chenopodium except
for a few minor details. Following the four-celled stage (Fig. 154 A),
each cell of the proembryo divides again. The cell I always divides
by a vertical wall (Fig. 154B-D) ; V may divide either transversely
(Fig. 154C) or vertically (Fig. 154B,D), the latter being the more
frequent condition; m may also divide either transversely (Fig.
154H-E) or vertically (Fig. 154F); and ci divides transversely (Fig.

Fig. 154. Development of embryo in Beta vulgaris. (After Artschwager and
Starrett, 1983.)

1545 ,C). Thus, depending on the direction of the partition walls
in V and m the eight-celled embryo may comprise five, six (Fig.
154D), or seven tiers (Fig. 154#). During the next division, which
gives rise to the 16 -celled stage, the walls are so arranged as to
produce about eight tiers of cells (Fig. 154F). The authors empha-
size that no strict law governs the sequence of cell divisions, al-
though "a certain balance seems to be maintained so that the final
product is remarkably uniform."

The four-celled proembryo of Myosotis hispida (Soueges, 1923a)
also originates in the same way as that of Chenopodium and Beta,
but the cell I divides by two oblique walls so as to form a wedge-
shaped epiphysis initial e as in Geum, already described under the
Asterad type (Fig. 155 A-C). The epiphysis gives rise to the stem
tip, while the other cells produced by the tier I produce the coty-
ledons. The tiers V and m form the hypocotyl; and ci gives rise

282 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

to the daughter cells n and n', of which the former functions as a
hypophysis and gives rise to the root cap and periblem initials of
the root, and the latter gives rise to a short suspensor (Fig. 155D,E).

Fig. 155. Development of embryo in Myosotis hispida. (After Soueges, 1923a.)

Caryophyllad Type. The embryo of Sagina procumbens (Soueges,
1924a), which is the most typical of all the forms described under
the Caryophyllad type, is characterized by the fact that here the
basal cell cb remains undivided and forms a large vesicular struc-

Fig. 156. Development of embryo in Sagina procumbens. (After Soueges, 1924a.)

ture which does not take any further part in the development of the
embryo. The terminal cell ca undergoes transverse divisions to
form a row of four cells designated as ci, m, V, and I (Fig. 156 A-D).
Of these, each of the three lower cells divides by a vertical wall and
the upper cell ci divides by a transverse wall (Fig. 15QE,F). The

THE EMBRYO

2*3

tmbryo now comprises five tiers (excluding cb), namely n' , n, m, V ,
and I. The next division is also vertical (at right angles to the first)
in I, V, and m, and results in the formation of three quadrants; n
also divides by a vertical wall; and n' divides by a transverse wall
to give rise to o and p (Fig. 156G,H). Of the six tiers formed in
this way, I is destined to give rise to the stem tip, V to the cotyledons,
m to the hypocotyl, n to the root cap, and o and p to a short sus-
pensor which abuts on the large cell cb.

Fig. 157. A-H, development of embryo in Sediwi acre; cells marked end belong
to endosperm and ch is the chalazal haustorium. (After Soueges, 1927.) I, l.s.
portion of ovule, showing haustorial processes formed from basal cell of proembryo.
(After Mauritzon, 1933.)

Sedum acre (Soueges, 1927, 1936c?) resembles Sagina in having a
large and undivided basal cell (cb), but in other respects the develop-
ment shows some differences. The terminal cell ca, which is small
and lenticular, divides transversely into two superposed cells, of
which the upper again divides transversely (Fig. 157 A, B). Of the
resulting four cells, cc undergoes two vertical divisions to form
quadrants (Fig. 157 D,E) and then a transverse division to form
octants (Fig. 157 F) ; m divides transversely into two flattened cells
h and/, the former of which constitutes the hypophysis (Fig. 157i?) ;
ci becomes partitioned by vertical walls to form four juxtaposed
cells which do not divide again but become greatly flattened and
form a short suspensor (Fig. 157C-H); cb forms an aggressive
haustorium whose branches penetrate the seed coat (Fig. 157/).

284 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

During further development, I gives rise to the cotyledons and stem
tip, V to the hypocotyl, h to the root tip, / and ci to the suspensor.
Saxifraga granulata (Soueges, 1936a) shows a variation in that
here the basal cell cb undergoes a few divisions to produce four to
eight large cells which form part of the suspensor. The terminal
cell ca first divides transversely to produce the two cells cc and cd
(Fig. 158 A), of which the latter divides into two daughter cells m
and ci (Fig. 1585). Of the resulting cells, ci divides vertically into
two juxtaposed cells which contribute to the middle portion of the
suspensor; and m divides transversely into/ and d (Fig. 15SD,E)

ABC D E

Fig. 158. Development of embryo in Saxifraga granulata. (After Soueges, 1936a.)

the former contributing to the suspensor and the latter functioning
as the hypophysis. Meanwhile the cell cc undergoes two vertical
divisions (Fig. 158£,C) and one transverse division (Fig. 158D) to
give rise to the tiers I and V, which eventually produce the coty-
ledonary and hypocotyledonary portions of the embryo.

The embryo of Androsaemum officinale (Soueges, 19366), belong-
ing to the family Hypericaceae, also deserves mention here. In
this case the terminal as well as the basal cell of the two-celled pro-
embryo divides transversely to give rise to a quartet of four super-
posed cells (Fig. 159 A, B). Of these the lowest cell cc divides
vertically into two juxtaposed cells (Fig. 159C), each of which under-
goes another vertical division to form the quadrant stage (Fig.
159D) and then a transverse division to give rise to the octants l-V

THE EMBRYO

285

(Fig. 159E). The cell cd, sister to cc, divides into two superposed
cells m and ci (Fig. 159C) of which the former again segments into
d and / (Fig. 159D). Subsequently I and V produce the cotyle-
donary and hypocotyledonary parts of the embryo; d becomes
differentiated as the hypophysis; and the cells /, ci, and cb con-
tribute to the formation of a filamentous suspensor. Occasionally
a few vertical divisions may also take place in the suspensor, but
these are sporadic and of no importance.

Fig. 159. Development of embryo in Androsaemum officinale.
Soueges, 198Gb.)

(Redrawn after

The embryogeny of Drosera rotundifolia (Soueges, 1936c) is pe-
culiar in that it shows points of resemblance with three or four
different types of embryonal development. The basal cell cb under-
goes only one or two divisions to give rise to a short suspensor. The
terminal cell ca divides transversely into two superposed cells cc
and cd (Fig. 160A-C). The former again divides transversely to
form two cells I and V (Fig. 160Z)) both of which become vertically
partitioned to form first quadrants (Fig. 1Q0E) and then octants
(Fig. 160F). Meanwhile, cd also undergoes a transverse division
into two superposed cells h and h', both of which become partitioned
by vertical walls (Fig. 160E,F). Regarding the further fate of these

286 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

tiers, I gives rise to the cotyledonary region, V to the hypocotyle-
donary region, h to the initials of the root periblem, and h' to the
root cap (Fig. 1WG-K).

Comparing the development of the embryo of Drosera with the
types previously described, it is clear that the filamentous form of
the proembryo corresponds with that in the Solanad type (Fig. 150) ;
the behavior of the tiers h and h' corresponds with that of the tiers
m and n in the Asterad type (Fig. 148) ; and the disposition of the

Fig. 160. Development of embryo in Drosera rotundifolia. (After Soneges, 1986c.)

quadrants resembles that under the Solanad and Chenopodiad
types (Fig. 153). However, the greatest resemblance is with the
Caryophyllad type (Fig. 156) as in both cases the embryo is pro-
duced almost entirely from the terminal cell of the proembryonal
quartet, while the basal cell cb of the two -celled stage takes practi-
cally no part in further development.

MONOCOTYLEDONS

As already mentioned there is no essential difference between the
monocotyledons and the dicotyledons regarding the early cell divi-
sions of the proembryo. However, since the mature embryo is so

THE EMBRYO

287

different in the two groups, a few illustrative examples are described
below to show the range of variation in the monocotyledons.

Luzula forsteri (Soueges, 1923c), a member of the family Junca-
ceae, is characterized by a very simple type of embryogeny. The
terminal cell ca of the two-celled proembryo divides by a longitudinal
wall to produce two juxtaposed cells (Fig. 161 A, B), and a little
later the basal cell cb divides by a transverse wall to produce the two
cells ci and m (Fig. 161(7). In the next stage the two cells of the
tier ca undergo another vertical division, at right angles to the first,
to give rise to the quadrants q; cell m also divides by a longitudinal

Fig. 161.
1923c.)

Development of embryo in Luzula forsteri. {Redrawn after Soueges,

wall to give rise to two juxtaposed cells; and ci divides transversely
to form two cells n and n' (Fig. 161 D). By further divisions the
quadrants become partitioned into two portions I and V, of which I
gives rise to the lower part of the single cotyledon and V to its upper
part and to the hypocotyl and plumule. Of the remaining tiers, m
gives rise to the periblem and part of the root cap ; n to the remaining
part of the root cap; and n' to the short suspensor. An important
point, worthy of note in the embryogeny of Luzula, is the extremely
precocious differentiation of the epidermal initials, which are cut
off immediately after the quadrant stage (Fig. 161E"), although in
other angiosperms this occurs only after the octants have been
formed.

The embryogeny of Muscari comosum (Soueges, 1932), a repre-

288 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

sentative of the family Liliaceae, may be described next. As in
Luzula, the basal cell cb divides transversely and the terminal cell
ca divides longitudinally (Fig. 162A,B). In the next stage, the
two cells at ca and the middle cell m divide longitudinally, while
the upper cell ci divides transversely into n and n' (Fig. 162C).
Of the eight cells formed in this way, n' divides to give rise to o and
p; n divides vertically; the two juxtaposed cells of the tier m also
divide vertically, although in a somewhat irregular fashion, to pro-
duce four cells; and the quadrants at q divide by diagonal walls to
produce the octants (Fig. 1Q2D,E). Finally, the tier q gives rise
to the cotyledon, m to the hypocotyl and stem tip, n to the initials
of the root, o to the root cap, and p to the suspensor (Fig. 162F,G).

Fig. 162. Development of embryo in Muscari comosum. {After Soueges, 1932.)

The proembryo of Sagittaria (Soueges, 1931) is similar in some
respects to that of Sagina (Fig. 156) described under the dicoty-
ledons. The zygote divides transversely into the terminal cell ca
and the basal cell cb (Fig. 163 A). The basal cell, which is the larger,
does not divide again but becomes transformed directly into a large
vesicular structure.6 The terminal cell undergoes a transverse
division to form the two cells c and d (Fig. 1635 ,C). Of these, the
lower cell c divides vertically to form a pair of juxtaposed cells, and
the middle cell d divides transversely into m and ci (Fig. 163D). In
the next stage the two cells at c undergo another vertical division to
form quadrants, m also divides vertically, and ci divides transversely
(Fig. 163#,F). The quadrant cells at c now divide transversely to
give rise to octants (I, I'), the two juxtaposed cells at m become
vertically partitioned to give rise to four cells, and the two daughter
cells of ci divide to form the cells n, o, h, and s (Fig. 163F-H). With
further cell divisions and growth the tiers I and V become trans-

8 A similar large basal cell occurs in almost all members of the Helobiales and
has recently been reported in Lloydia (Bianchi, 1946) belonging to the Liliaceae.

THE EMBRYO

2*9

formed into the single cotyledon, m gives rise to the stem tip, n to
the root tip, o to the periblem and a part of the root cap, h to the
uppermost layer of the root cap, and s to the suspensor composed of
three to six superposed cells.

The embryo of the Gramineae is so different from that of most
monocotyledons that it merits separate attention. In Poa annua
(Soueges, 19246), which is the most thoroughly investigated species,
the first division of the zygote is transverse and results in the forma-
tion of two cells ca and cb (Fig. 164 A). The next division is trans-
verse in the basal cell and vertical in the terminal cell (Fig. 164B,C).

Fig. 163. Development of embryo in Sagittaria sagittifolia. (After Soueges,
1931.)

Of the four cells thus formed, the two at the lower end undergo a
further longitudinal division, resulting in the quadrant stage (Fig.
164D); and each of the quadrant cells in turn becomes partitioned
by a more or less transverse wall to give rise to the two tiers I and
V (Fig. 1Q4E). At the same time the upper cell ci divides trans-
versely to form n and n' , followed by a vertical division in m and n
and a transverse division in n' . As further development proceeds,
the tiers I and V give rise to the scutellum and part of the coleoptile ;
m gives rise to the remaining part of the coleoptile cl-cl', the stem
tip pv, and the periblem and plerome of the root tip; n to the root
cap, coleorrhiza, and epiblast eUa; and oand p, daughter cells of n',to
the hypoblast and the suspensor (Fig. 164F-7).

The most conspicuous part of the grass embryo is the scutellum.

60 This is a small scale-like structure lying opposite to the scutellum (see p. 429) .

290 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Much has been written on its morphological nature. Arber (1934)
considers that the scutellum and coleoptile jointly constitute the
cotyledon, the coleoptile representing the cotyledonary sheath.
Others contend that the coleoptile is the true cotyledon and that
the scutellum is a lateral outgrowth of the young axis. Avery
(1930), who has made a thorough study of the comparative anatomy
and morphology of the embryos and seedlings of maize, oat, and
wheat, interprets the scutellum as the cotyledon; the coleoptile as
the second leaf; and the elongated structure between the cotyledon

Fig. 164. Development of embryo in Poa annua. (Redrawn after Soueges, 1924b.)

and the coleoptile, sometimes called the "mesocotyl," as the first
internode.

It is to be noted that, on the side which is in contact with the
endosperm, the cells of the scutellum frequently elongate to form
finger-like processes, which project into the endosperm and probably
serve for the absorption of food from the latter.

MODIFICATIONS OF SUSPENSOR

In the above account of the principal types of embryonal develop-
ment in the dicotyledons and monocotyledons, the main emphasis
was on the embryo proper and little attention was paid to the sus-
pensor. This is because in the majority of angiosperms the sus-
pensor has no special function except that of pushing the embryo

THE EMBRYO 291

into the endosperm, where it is surrounded by cells containing
abundant food materials. Especially striking examples of an
elongated proembryo are seen in the long and narrow embryo sacs
of the Sympetalae and have been fully described and illustrated in
Haberlea (Glisic, 1928), Gratiola (Glisic, 1933), Utricularia (Kausik,
1938), and several other plants. In later stages the narrow basal
portion of the proembryonal tube becomes crushed and obliterated,
while the broader terminal portion, which is destined to give rise to
the embryo proper, becomes favorably placed in the central mass of
endosperm. In Salvia splendens (Carlson and Stuart, 1936) not
only does the proembryonal filament elongate downward into the
endosperm but it also protrudes in the opposite direction, so that a
portion of it comes to lie inside the endosperm haustorium produced
at its micropylar end.

In several plants the suspensor cells show a pronounced increase
in size or give rise to prominent haustorial structures which pene-
trate between the cells of the endosperm and encroach upon the
surrounding tissues of the ovule. The range of variation in this
respect may be illustrated by referring to a few families and genera
which are of special interest from this point of view.

Guignard (1882) gave a very detailed account of the modifications
of the suspensor in the Leguminosae. In several members of the
subfamily Mimosaceae and of the tribe Hedysareae, a suspensor is
virtually absent and the proembryo forms a spherical or ovoid mass
of cells without any differentiation. Soja, Amphicarpaea, and Tri-
folium have a rudimentary suspensor consisting of three or four
cells. In Pisuw, and Orobus, which have a suspensor consisting of
two pairs of cells arranged in a crosswise fashion, the two micro-
pylar cells are elongated but the next two are more or less spherical ;
all four of the cells are multinucleate (Fig. 1Q5E,F). In Ocer
(Fig. 165G) and Lupinus (Fig. 165A-C) the suspensor is much
longer. In L. pilosus (Fig. 165A) some of the cells become detached
from one another and lie free in the micropylar part of the embryo
sac. In Ononis (Fig. 1657) the suspensor consists of a filament of
large and conspicuous cells which are packed with food materials.
In Medicago and Trigonella it is more massive, and Phaseolus (Fig.
165D) is similar except that here the distinction between the cells
of the suspensor and embryo proper is not very sharp. Finally, in

292

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fig. 165. Modifications of suspensor in Leguminosae. A, Lupinus pilosus, l.s.
upper part of embryo sac; suspensor consists of a row of several large flattened cells,
of which a few at upper end have become isolated. B, L. luteus; most of suspensor
cells are binucleate. C, L. subcarnosus: greatly elongated susoensor, composed of

THE EMBRYO 293

Cytisus (Fig. 16522) the suspensor cells are large and spherical and
appear like a bunch of grapes.

During recent years more detailed accounts have appeared of the
embryogeny of the Leguminosae,7 of which Pisum sativum (Cooper,
1938) may be referred to very briefly. The zygote divides trans-
versely into two cells, of which the basal divides vertically to form
two suspensor cells and the other divides transversely to form a
middle cell and a terminal cell (Fig. 166A-D). The nuclei of the
two basal cells now divide without wall formation and the middle
cell becomes vertically partitioned by a wall placed at right angles
to that separating the two basal cells. At the same time the ter-
minal cell divides by an oblique wall (Fig. 16QF,G). Hereafter
the basal cells undergo much elongation and their nuclei as well as
those of the middle or subbasal cells undergo a series of free nuclear
divisions (Fig. 16622-J"). Eventually the basal cells have as many
as 64 nuclei and the subbasal cells have 32 nuclei each. A longi-
tudinal section of the ovule cut at this stage shows a free nuclear
endosperm, a globular embryo, and the four multinucleate suspensor
cells (Fig. 16620.

The suspensor haustoria of some members of the Rubiaceae have
been well known since the days of Hofmeister (1858). Lloyd (1902)
and Soueges (1925) gave further details of their origin and structure.
At first the suspensor is merely a filament of cells, but later the
micropylar cells send out lateral protrusions which penetrate into
the endosperm and swell at their distal ends (Fig. 167). Llo}'d was
so impressed by their appearance that he remarked: "The function
of the suspensor in these forms is therefore not alone to bring the
embryo into favorable position with relation to the food supply in a
mechanical sense, but to act as a temporary embryonic root."
Fagerlind's (1937) work seems to indicate, however, that the sus-

7 See especially Soueges (1946c-o; 1947a, b; 1948).

about 20 pairs of cells; outside embryo are nuclei of endosperm. D, Phaseolus
multiflorus; large massive suspensor, whose cells grade imperceptibly into those of
embryo proper. E,F, Pisum sativum and Orobus angustifolius; suspensor composed
of large multinucleate cells. G, Cicer arietinum, biseriate suspensor. H, Cytisus
laburnum; suspensor appearing like bunch of grapes; embryo showing initiation of
two cotyledons. I, Ononis fruticosa, suspensor composed of a row of seven large
cells, each with a single prominent nucleus and many starch grains. (After Guig-
nard, 1SF2.)

294 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

I J K

Fig. 166. Development of suspensor in Pisum sativum. (After Cooper, 193S.)

THE EMBRYO

295

pensor cells soon lose their connection with the main body of this
organ and occur merely as islands within the endosperm. Further,
sometimes they degenerate even before the connection is lost, and if
this be so, they naturally cannot function effectively as absorbing
organs.

Fig. 167. Suspensor haustoria of Asperula. A, young embryo with prominent
suspensor haustoria; the surrounding cells belong to endosperm. B, older stage,
showing extreme development of haustoria. (After Lloyd, 1902.)

The suspensor haustoria of the Halorrhagidaceae bear a remarkable
resemblance to synergids (Fig. 168 A-G). Stolt (1928) and Soueges
(1940) have shown that in Myriophyllum the two-celled proembryo
consists of a large basal cell and a much smaller terminal cell. The
former divides longitudinally to form two daughter cells, which
enlarge to such an extent as to occupy the entire space in the micro-
pylar part of the embryo sac. They remain distinguishable even
up to the time of differentiation of the cotyledons.

Three members of the family Fumariaceae are also of much
interest in this connection. In Hypecoum (Soueges, 1943a, b) the

296

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

basal cell cb does not divide but becomes greatly swollen and vesicu-
lar. The terminal cell ca divides diagonally into two unequal cells,
of which the larger cell cd becomes swollen like the basal cell.
These two haustorial cells, cb and cd, persist for a long time and
appear like synergids (Fig. 169 A-F).

In Corydalis (Soueges, 1946a,o) the basal cell cb undergoes a num-
ber of free nuclear divisions and becomes multinucleate. The
terminal cell ca divides transversely to form a short filament, whose
upper cells (cd and cf) become large and multinucleate and con-
tribute to the suspensor. Only the apical cell takes part in the
development of the embryo proper (Fig. 1Q9G-L).

ABC

Fig. 168. Synergid-like suspensor cells of Myriophyllum altemiflorum.
after Soueges, 191,0; G, after Stolt, 1928.)

(A-F,

In the third genus Fumaria (Soueges, 1941a, b) the nuclear divi-
sions in cb are accompanied by wall formation. All the derivatives
of this cell, as well as the upper derivatives of ca, give rise to the
suspensor, the embryo proper being produced only from the lower
derivatives of ca. Thus, approximately three-fourths of the zygote
is given to the production of the suspensor (Fig. 1Q9M-R).

Since the publication of Treub's (1879) classical paper on the
embryogeny of the orchids there have been several other investiga-
tions on the family, among which those of Stenar (1937, 1940) and
Swamy (1942a,6, 1943, 1946, 1948, 1949) deserve special mention.
It is noteworthy that while some genera like Epipactis, Lister a,
and Zeuxine completely lack a suspensor, others show a remarkable
variation in the form and organization of this organ, which may be
illustrated by the following examples. In Spathoglottis, Goodyera,
and Achroanlhes the basal cell of the proembryo enlarges and some-
times grows out of the micropyle. In Gymnadenia, Phajus, Epiden-
drum, and Habenaria it divides to form a filament of cells which
may frequently become very long and twisted and grow out as far as

THE EMBRYO

297

the placenta. The suspensor cells of Eulophea (Fig. 195) form long
fluffy structures which extend in various directions, piercing the cells of
the inner integument and coining in contact with those of the outer

Corydalis

Fumaria

Fig. 169. Development of embryo in Hypecoum procumbens (A-F), Corydalis
lutea (G-L), and Fumaria officinalis (M-R). (After Soueges, 19/t3a,b; 194Ga,b;
1941a,b.)

one. In Phalacnopsis the suspensor cells send outgrowths both
above and below, the former protruding out of the micropyle and
the latter surrounding the embryo. In Stanhopea the spherical
proembryo consists of 10 to 12 cells, only one of which gives rise tc

298 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the embryo proper, while the rest grow out into long tubes, some
pushing their way between the cells of the integuments and others
extending into the micropyle.

A reference has already been made to the well-developed sus-
pensor haustoria of the Crassulaceae (Mauritzon, 1933) (Fig. 157/).
Sometimes their branching is so profuse that it is hardly possible
to get a correct idea of it from the study of single sections. In some
species of Sedum and Pistoria the haustorial processes pierce the
integuments and even extend outside the ovule.

Walker (1947) has recently given a detailed account of the origin
of the massive haustoria of Tropaeohim majus. Here the basal cells
of the proembryo divide more actively than the other cells. Such
of the cells of this mass as lie on the side away from the funiculus
give rise to a long haustorial process which pierces the micropylar
part of the integument and finally enters the pericarp. Slightly
later, a second protuberance arises from those cells of the mass which
lie on the side nearest the funiculus. This, the placental haustorium,
grows through the integument and funiculus and reaches up to the
point of entry of the vascular bundle of the raphe.

Perhaps the longest suspensors in angiosperms occur in the
Loranthaceae. As mentioned on page 143, in several genera of
this family the embryo sacs grow up into the style, and after fertiliza-
tion there is a remarkable elongation of the two-rowed suspensor,
pushing the terminal cells of the proembryo into the ovary.

UNCLASSIFIED OR ABNORMAL EMBRYOS

Although there are several plants whose embryos do not conform
to any of the types described previously, only a few need be men-
tioned here.

In all the angiosperms described so far, the first division of the
zygote is transverse, but Treub (1885) reported long ago that in
Macrosolen cochinchinensis the first wall in the zygote is not trans-
verse but vertical. Since then a vertical or nearly vertical division
has also been described in several other members of the family,
viz., Korthahella (Rutishauser, 1935), Scurrula, Dendrophthoe,
(Rauch, 1936; Singh, 1950) (Fig. 170), Lepeostegeres, Amyema,
Helixanthera, and Taxillus (Schaeppi and Steindl, 1942). In
Balanophora (Zweifel, 1939) the first as well as the second division
is vertical and this is probably also true of the third (Fig. 171). In

THE EMBRYO

299

Fig. 170. Development of embryo of Scurrula atropurpurea. A, first division of
zygote. B,C, formation of biseriate proembryo; in B, the two terminal cells of
proembryo happen to overlap each other. D, proembryo penetrating inside endo-
sperm. E, enlarged view of terminal portion of proembryo of approximately same
age as D ; note beginning of differentiation between cells of suspensor and those of
embryo proper. (After Rauch, 198'!.)

Fig. 171. Early stages in development of embryo of Balanophora abbreviata. A,
two-celled embryo formed by vertical division of zygote. B,C, more advanced
stages. (After Zweifel, 1939.)

300 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Leitneria (Pfeiffer, 1912) and Sassafras (Coy, 1928) the condition
is variable, and the first wall may be either transverse or longi-
tudinal.

In Scabiosa succisa, which has been described in detail by Soueges
(19376), the first wall in the zygote is diagonal, dividing it into two
somewhat unequal cells, a and b (Fig. 172^1). The former corre-
sponds to the terminal and the latter to the basal cell of the two-
celled stage. In the next division each cell is partitioned more or
less transversely, producing the daughter cells c, d, e, and / (Fig.
1725). The following divisions do not follow any definite sequence
(Fig. 172C-E), but derivatives of c and e give rise to the cotyledonary
zone, d and the lower part of / give rise to the hypocotyledonary

A B C D E

Fig. 172. Development of embryo of Scabiosa succisa. (After Soueges, 1987b.)

region, and the upper part of / to the root cap and a poorly differen-
tiated suspensor. The dermatogen is differentiated at an early
stage, but the periblem and plerome are distinguishable only after
the appearance of the cotyledons.

A well-differentiated suspensor is also lacking in Cimicifuga (Earle,
1938). Here the first division of the zygote is transverse. The
basal cell, which is considerably larger, divides vertically, and the
terminal divides obliquely. Further divisions are slow and irregular
and the proembryo soon becomes a club-shaped mass, about ten
cells long and two to four cells broad. There is no clear line of de-
marcation between the suspensor and the embryo, and the cells of
the former are distinguishable only by their position and vacuolated
cytoplasm.

In several members of the Gramineae also there is no regular

THE EMBRYO

301

pattern of cell divisions in the development of the embryo. The
two-celled stage consists of a small lenticular terminal cell and a
much larger basal cell (Fig. 173 A). The terminal cell may divide
vertically (Fig. 173C) or obliquely (Fig. 173B), and sometimes the
first oblique wall is followed by another wall of the same type,
resulting in a kind of apical cell (Fig. 173D). The following divi-

P G H I J

Fig. 173. Development of embryo of Zea mays. (After Randolph, 1936.)

sions are quite irregular (Fig. 17 SE). Further growth is limited
chiefly to the terminal region, and only a few cell divisions occur in
the basal or suspensor region (Fig. 173F-J).

Carya glabra, Juglans mandshurica (Langdon, 1934), and J. nigra
(Nast, 1941) are similar in that the first division wall may be hori-
zontal or slightly oblique. The terminal cell does not form any
quadrants but divides by two oblique walls and then by a transverse
wall to form a group of six cells. The cells on the sides give rise to

302 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the cotyledons and those in the center to the stem tip, offering some
resemblance in this respect to Geum (Fig. 149) and Myosotis (Fig.
155), which have already been described.

Attention may finally be called to the fact that although Soueges
lays much emphasis both on the constancy in the destiny of the
cells of the four-celled proembryo and on their significance in indi-
cating relationships between different families and genera, his con-
clusions have not been supported by other workers. For example,
Borthwick (1931) points out that in Daucus each cell of the four-
celled proembryo does not always give rise to the same part of the
mature embryo. Bhaduri (1936) also states that "the method of
origin of the different parts of the mature embryo varies in different
and sometimes even in the same species of Solanaceae."

UNORGANIZED AND REDUCED EMBRYOS

Whatever their mode of origin and development, as a rule all
embryos become differentiated at maturity into three main parts,
viz., root tip, stem tip, and cotyledons (or cotyledon). In some
plants, however, the embryo remains small and rudimentary even
until the shedding stage of the seed. This condition is especially
prevalent in the families Balanophoraceae, Rafflesiaceae, Gentia-
naceae, Pirolaceae, Orobanchaceae, Burmanniaceae, and Orchida-
ceae, and appears to be associated in some degree with a parasitic or
saprophytic mode of life.

In the Orchidaceae the embryo is almost always a simple or ovoid
mass of cells in which even the differentiation of the primary layers
takes place only after the seeds have been shed. Swamy (1949)
recognizes two fundamental types of development. In the first
type (Fig. 174A-0), which is the more frequent, the zygote divides
by a transverse wall to form two cells, of which the basal again
divides transversely so as to result in a proembryo of three cells.
The upper cell adjacent to the wall of the embryo sac frequently
gives rise to one or more suspensor haustoria, which become very
prominent and aggressive structures in some species. The terminal
cell divides vertically to form two daughter cells, which by further
divisions give rise to the greater part of the embryo, the rest being
contributed by the middle cell. In the second type of development
(Fig. 174P-C7), which has been reported only in a few genera like
Cymbidium, Eulophia, Geodorum, and Stanhopca, the zygote may

THE EMBRYO

303

divide by either a transverse or an oblique wall. Further divisions,
which do not follow any definite pattern, give rise to a mass of five
to ten cells, some of which begin to enlarge enormously and assume
a haustorial function. One or two cells divide transversely to form

Fig. 174. Development of embryo in Vanda parviflora (A-O) ana Cymbiiium
bicolor (P-U). A, Vanda, two-celled proembryo with basal cell dividing. B,
three-celled proembryo. C, same, upper cell dividing. D, vertical division of
primary suspensor cell. E-J, development of eight-celled suspensor by three verti-
cal divisions of primary suspensor cell. K, optical t.s. through suspensor region of
a stage similar to J. L-N, stages in elongation of suspensor cells. 0, mature
embryo with remnants of suspensor haustoria. P-R, Cymbidium, early stages in
development of embryo. S-U, later stages, showing maximum elongation of sus-
pensor cells. (After Swamy, 19^2a,b.)

Aeginetia

Fig. 175. Some stages in development of embryos of Aeginetia indica {A- J),
Burmannia coelestis (K-N), Cotylanthera tenuis (0), and Leiphaimos spectabilis
(P). A-I, Aeginetia, stages in development of proembryo. J, Is. nearly mature
seed (seed coat not included), showing spherical embryo (shaded cells) surrounded
by endosperm. (After Juliano, 1935.) K-N, Burmannia, l.s. upper part of seed,
showing endosperm and embryo; of the two embryos seen in M, one is probably
derived from a synergid. (After Ernst and Bernard, 1912.) 0, Cotylanthera, l.s.
seed showing embryo, endosperm, and seed coat. P, Leiphaimos, l.s. seed showing
five-celled embryo surrounded by thick-walled cells of endosperm and seed coat.
(After Oehler, 1927.)

304

THE EMBRYO

305

a filament of variable length whose lower portion gives rise to the
embryo proper.

Ernst and Schmid (1913) report that in Rafflesia patma the ter-
minal cell of the two-celled proembryo undergoes a transverse
division to form two cells, each of which divides longitudinally. At
the same time the basal cell divides vertically. At this stage the
proembryo enters into a period of rest although cell divisions con-
tinue to take place in the endosperm. When development is
resumed, only one or two more divisions take place in the embryo.

B C D E

Fig. 176. Development of embryo in Ranunculus ficaria up to time of shedding
of seed. (After Soueges, 1913.)

In Aeginetia indica, a member of the Orobanchaceae (Juliano,
1935), the zygote remains dormant for a considerable time after
endosperm formation has commenced. The first two divisions are
transverse, resulting in a short filament (Fig. 175A-C). The nar-
row suspensor cells soon become crushed and disorganized, and the
two terminal cells divide to give rise to a small globose embryo
without any differentiation into a radicle, plumule, or cotyledons
(Fig. 175D-I). Figure 175 J" shows a longitudinal section of the
mature seed with the spherical embryo surrounded by a layer of
endosperm cells.

Embryos of an extremely reduced type are also known in the
Burmanniaceae (Ernst and Bernard, 1912). In Burmannia coe-
lestis (Fig. 175K-N) the basal cell of the two-celled embryo divides
by a transverse wall and the terminal cell divides by two vertical
walls to produce a quadrant. Development seems to be arrested
at this six-celled stage.

306 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Four saprophytic members of the Gentianaceae, studied by Oehler
(1927) are also of much interest in this connection. In Voyriella
parviflora the first division of the zygote takes place when the endo-
sperm is composed of 10 to 12 cells. A filamentous proembryo of
six or seven cells is formed. Of these, the three or four terminal
cells divide further to give rise to a mass of 16 to 24 cells. Cotylan-
thera tenuis (Fig. 1750) and Voyria coerulca are similar but somewhat
more reduced. They have a suspensor of three or four cells and an
embryonal portion consisting of about a dozen cells. The most
reduced of the four is Leiphaimos spectabilis in which only the
terminal cell of the four-celled embryo divides again, so that at the
shedding stage of the seed the embryo consists of no more than five
cells arranged in four tiers (Fig. 175P).

Reduced embryos without the usual organization into a radicle,
plumule, and cotyledons have also been described in Ranunculus
ficaria (Soueges, 1913) (Fig. 176), Corydalis cava (Hegelmaier,
1878), and a few other plants, although these are not characterized
by a parasitic or saprophytic habit.

References

Arber, A. 1934. "The Gramineae." Cambridge University Press.
Artschwager, E., and Starrett, R. C. 1933. The time factor in fertilization and

embryo development in the sugar beet. Jour. Agr. Res. 47: 823-843.
Avery, G. S., Jr. 1930. Comparative anatomy and morphology of embryos and

seedlings of maize, oats and wheat. Bot. Gaz. 89: 1-39.
Bhaduri, P. N. 1936. Studies in the embryology of the Solanaceae. I. Bot. Gaz.

98: 283-295.
Bianchi, R. 1946. Untersuchungen liber die Fortpfianzungsverhaltnisse von

Gagea fistulosa (Ram.) unci Lloydia serotina (Rchb.). Diss. Zurich.
Borthwick, R. A. 1931. Development of the macrogametophyte and embryo of

Daucus carota. Bot. Gaz. 92: 23-44.
Borwein, B., Coetsee, M. L., and Krupko, S. 1949. Development of the embryo

sac of Restio dodii and Elegia racemosa. Jour. South AM. Bot. 15: 1-11.
Cappaletti, C. 1929. Sterilita di origine micotica nella Ruta patavina L. Ann.

di Bot. 18: 145-166.
Carano, E. 1915. Ricerche sull'embriogenesi delle Asteraceae. Ann. di Bot.

13: 251-301.
Carlson, E. M., and Stuart, B. C. 1936. Development of spores and gameto-

phytes in certain New World species of Salvia. New Phytol. 35: 65-81.
Cheesman, E. E. 1927. Fertilization and embryogeny in Tiieobroma cacao L.

Ann. Bot. 41: 107-126.

THE EMBRYO 307

Cooper, D. C. 1938. Embryology of Pisum sativum. Bot. Gaz. 100: 123-132.

. 1949. Flower and seed development in Oxybaphus nyctagineus. Amer.

Jour. Bot. 36: 348-355.

and Brink, R. A. 1949. The endosperm-embryo relationship in an au-

tonomous apomict, Taraxacum officinale. Bot. Gaz. Ill: 139-152.
Coy, G. V. 1928. Morphology of Sassafras in relation to phylogeny of angio-

sperms. Bot. Gaz. 86: 149-171.
Dahlgren, K. V. 0. 1928. Die Embryologie einiger Alismataceae. Svensk Bot.

Tidskr. 22: 1-17.
. 1934. Die Embryosackentwicklung von Echinodorus macrophyllus und

Sagittaria sagittifolia. Planta 21: 602-612.
Earle, T. T. 1938. Embryology of certain Ranales. Bot. Gaz. 100: 257-275.
Ernst, A., and Bernard, Ch. 1912. Entwicklungsgeschichte des Embryosackes

und des Embryos von Burmannia Candida Engl, und B. championii Thw. Ann.

Jard. Bot. Buitenzorg II, 10: 161-188.
and Schmid, E. 1913. Uber Bliite und Frucht von Rafflesia. Ann.

Jard. Bot. Buitenzorg II, 12: 1-58.
Fagerlind, F. 1937. Embryologische, zytologische und bestaubungsexperimen-

telle Studien in der Familie Rubiaceae nebst Bemerkungen iiber einige Poly-

ploiditatsprobleme. Acta Horti Bergiani 11: 195-470.
Famintzin, A. 1879. Embryologische Studien. Mem. Acad. Imp. des Sci. St.

Petersburg VII, 26(10).
Gerassimova, H. 1933. Fertilization in Crepis capillaris. Cellule 42: 103-148.
Glisic, Lj. 1928. Development of the female gametophyte and endosperm

in Haberlea rhodopensis Friv. Bull. Inst. Jard. Bot. Univ. Belgrade 1(1):

1-13.

— . 1933. Zur Entwicklungsgeschichte von Gratiola officinalis L. Bull.

Inst. Jard. Bot. Univ. Belgrade 2: 129-152.
Guignard, L. 1882. "Recherches anatomiques et physiologiques sur des 16gu-

mineuses." Diss. Paris.
Hanstein, J. 1870. Entwicklungsgeschichte der Keime der Monokotyle und

Dikotyle. Bot. Abhandl. Bonn. 1: 1-112.
Hegelmaier, F. 1878. "Vergleichende Untersuchungen iiber die Entwicklung

dikotyledoner Keime mit Beriicksichtigung der pseudo-monokotyledonen."

Stuttgart.
Heimann-Winawer, P. 1919. Beitrage zur Embryologie von Colchicum autum-

nale L. Arb. Inst. allg. Bot. Pflanzenphysiol., Univ. Zurich 21.
Hewitt, W. C. 1939. Seed development of Lobelia amoena. Jour. Elisha Mitchell

Sci. Soc. 55: 63-82.
Hofmeister, W. 1858. Neuere Beobachtungen iiber Embryobildung der Phanero-

gamen. Jahrb. f. wiss. Bot. 1: 82-186.
Johansen, D. A. 1945. A critical survey of the present status of plant embryology.

Bot. Rev. 11: 87-107.
Jones, H. A. 1927. Pollination and life history studies of lettuce (Lactuca sativa).

Hilgardia 2: 425-442.

308 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Juliano, J. B. 1935. Anatomy and morphology of the Bunga, Aeginetia indica

L. Philippine Jour. Sci. 56: 405-451.
Kausik, S. B. 1938. Pollen development and seed formation in Utricularia

coerulea L. Beihefte bot. Centbl. 58A: 365-378.
Langdon, L. M. 1934. Embryogeny of Carya and Juglans, a comparative study.

Bot. Gaz. 96: 93-117.
Lloyd, F. E. 1902. The comparative morphology of the Rubiaceae. Mem.

Torrey Bot. Club 8: 1-112.
Mauritzon, J. 1933. Studien fiber die Embryologie der Familien Crassulaceae

und Saxifragaceae. Diss. Lund.
McKay, J. W. 1947. Embryology of pecan. Jour. Agr. Res. 74: 263-283.
Nast, C. G. 1941. The embryogeny and seedling morphology of Juglans regia

L. Lilloa 6: 163-205.
Noguchi, Y. 1929. Zur Kenntnis der Befruchtung und Kornbildung bei den

Reispflanzen. Jap. Jour. Bot. 4: 385-403.
Oehler, E. 1927. Entwicklungsgeschichtlich-cytologische Untersuchungen an

einigen saprophytischen Gentianaceen. Planta 3: 641-733.
Pfeiffer, W. M. 1912. The morphology of Leitneria floridana. Bot. Gaz. 53:

189-203.
Pisek, A. 1923. Chromosomenverhaltnisse, Reduktionsteilung und Revision der

Keimentwicklung der Miste' (Viscum album). Jahrb. f. wiss. Bot. 62: 1-19.
Poddubnaja-Arnoldi, V., and Dianowa, V. 1934. Eine zytoambryologische

Untersuchung einiger Arten der Gattung Taraxacum. Planta 23: 19-46.
Pope, M. N. 1937. The time factor in pollen tube growth and fertilization in

barley. Jour. Agr. Res. 54: 525-529.
. 1943. The temperature factor in fertilization and growth of the barley

ovule. Jour. Agr. Res. 66: 389-402.
Puri, V. 1941. Life history of Moringa oleifera Lamk. Jour. Indian Bot. Soc.

20: 263-284.
Randolph, W. M. 1936. Developmental morphology of the caryopsis in maize.

Jour. Agri. Res. 53: 8S1-916.
Rauch, K. V. 1936. Cytologisch-embryologische Untersuchungen an Scurrula

atropurpurea Dans, and Dendrophthoe pentandra Miq. Ber. schweiz. bot.

Gesell. 45: 5-61.
Rutgers, F. L. 1923. Embryo sac and embryo of Moringa oleifera. The female

gametophyte of angiosperms. Ann. Jard. Bot. Buitenzorg 33: 1-66.
Rutishauser, A. 1935. Entwicklungsgeschichtliche und zytologische Unter-
suchungen an Korthalsella dacrydii (Ridl.) Danser. Ber. schweiz. bot.

Gesell. 44: 389-436.
Schaeppi, H., and Steindl, F. 1942. Blutenmorphologische und embryologische

Untersuchungen an Loranthoideen. Vrtljschr. naturf. Gesell. Zurich 87:

301-372.
Schaffner, M. 1906. The embryology of the shepherd's purse. Ohio Nat. 7: 1-8.
Schnarf, K. 1929. "Embryologie der Angiospermen." Berlin.
Singh, Bahadur. 1950. The embryology of Dendrophthoe falcata (Linn, fil.) Et-

tingshausen. Proc. 37th Indian Sci. Cong. Bot. Sect.

THE EMBRYO 309

Smith, B. E. 1934. A taxonomic and morphological study of the genus Cuscuta,
dodders, in North Carolina. Jour. Elisha Mitchell Sci. Soc. 50: 283-302.

Soueges, E. C. R. 1913. Recherches sur l'embryogenie des Renonculacees.
Renonculees (Ficaria ranunculoides Roth). II. L'embryon. Bui. Soc. Bot.
de France 60: 237-243, 283-2S9.

. 1914. Nouvelle recherches sur le deVeloppement de l'embryon chez les

Crucif feres. Ann. Sci. Nat. IX. Bot. 19: 311-339.

. 1919. Les premieres divisions de l'oeuf et les differentiations du sus-

penseur chez le Capsella bursa-pastoris. Ann. Sci. Nat. X. Bot. 1: 1-28.

■ . 1920a. DeVeloppement de l'embryon chez le Chenopodium bonus-henri-

cus L. Bui. Soc. Bot. de France 67: 233-257.

• . 19206. Embryogenie des Solanacees. DeVeloppement de l'embryon

chez les Nicotiana. Compt. Rend. Acad. Sci. Paris 170: 1125-1127.

. 1920c. Embryogenie des Composees. I. Les premiers stades du de-
Veloppement de l'embryon chez le Senecio vulgaris L. II. Les derniers stades
du deVeloppement de l'embryon chez le Senecio vulgaris L. Compt. Rend.
Acad, des Sci. Paris 171: 254-256; 356-357.

. 1922. Recherches sur l'embryogenie des Solanacees. Bui. Soc. Bot.

de France 69: 163-178, 236-241, 352-365, 555-585.

. 1923a. DeVeloppement de l'embryon chez le Myosotis hispida Schlecht.

Bui. Soc. Bot. de France 70: 385-401.

. 19236. DeVeloppement de l'embryon chez le Geum urbanum L. Bui.

Soc. Bot. de France 70: 645-660.
— . 1923c. Embryogenie des Joncacees. DeVeloppement de l'embryon

chez le Luzula forsteri DC. Compt. Rend. Acad, des Sci. Paris 177: 705-70S.
— . 1924a. DeVeloppement de l'embryon chez le Sagina procumbens. L.

Bui. Soc. Bot. de France 71: 590-614.
— . 19246. Embryogenie des Graminees. DeVeloppement de l'embryon

chez le Poa annua L. Compt. Rend. Acad, des Sci. Paris 178: 1307-1310.
— . 1925. DeVeloppement de l'embryon chez le Sherardia arvensis L. Bui.

Soc. Bot. de France 72: 546-565.
— . 1927. DeVeloppement de l'embryon chez le Sedum acre L. Bui. Soc.

Bot. de France 74: 234-251.
— . 1931. L'embryon chez le Sagittaria sagittifolia L. Le cone vfegetatif de

la tige et l'extremite radiculaire chez le monocotyledones. Ann. Sci. Nat. X.

Bot. 13: 353-402.
— . 1932. Recherches sur l'embryogenie des Liliacfees (Muscari comosurn L.) .

Bui. Soc. Bot. de France 79: 11-23.

— . 1934a. "Titres et travaux scientifiques." Saint-Dizier.
— . 19346. L'hypophyse et l'epiphyse; les problemes d'histogenese qui leur

sontlies. I and II. Bui. Soc. Bot. de France 81: 737-758.
— . 1934-1939. "Exposes d'embryologie et de morphologie vegetales."
Vols. I-X. Paris.

— . 1935a. Embryogenie des Oenothferacees. Les principaux termes du
deVeloppement de l'embryon chez le Ludwigia palustris Elliott. Compt.
Rend. Acad, des Sci. Paris 200: 1626.

310 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Soueges, E. C. R. 19356. Observations embryologiques sur quelques Fragaria de

culture. Bui. Soc. Bot. de France 82: 458-461.
. 1936a. Embryogenie des Saxifragac6es. Developpement de l'embryon

chez le Saxifraga granulata L. Compt. Rend. Acad, des Sci. Paris 202: 240-

242.
. 19366. Embryogenie des Hypericac6es. DeVeloppement de l'embryon

chez YAndrosaemwn officinale All. Compt. Rend. Acad, des Sci. Paris 202:

679-681.
. 1936c. Embryogenie des Droserac6es. DeVeloppement de l'embryon

chez le Drosera rotundijolia L. Compt. Rend. Acad, des Sci. Paris 202:

1457-1459.

1936d. Modifications au tableau r&japitulatif des lois du developpement

chez Sedum acre L. Bui. Soc. Bot. de France 83: 13-18

. 1937a. "Titres et travaux scientifiques. Deuxieme notice (1934-1937)."

Saint-Dizier.

. 19376. Embryogenie des Dipsacacees. DeVeloppement de l'embryon

chez le Scabiosa succisa L. Compt. Rend. Acad, des Sci. Paris 204: 292-294.

. 1937c. Embryogenie des Violates. DeVeloppement de l'embryon

chez le Viola tricolor L. Compt. Rend. Acad, des Sci. Paris 205: 169-171.

. 1940. Embryogenie des Haloragacees. DeVeloppement de l'embryon

he le Myriophyllum alterniflorum DC. Compt. Rend. Acad, des Sci. Paris
211: 185-187.

. 1941a. Embryogenie des Fumariacees. L'origine du corps de l'em-
bryon chez le Fumaria officinalis L. Compt. Rend. Acad, des Sci. Paris 213:
528-530.

. 19416. Embryogenie des Fumariacees. La differenciation des regions

fondamentales du corps chez le Fumaria officinalis L. Compt. Rend. Acad,
des Sci. Paris 213: 699-702.

. 1943a. Embryogenie des Fumariacees. L'origine et les premieres divi-
sions de la cellule embryonnaire proprement dite chez VHypecoum procumbens
L. Compt. Rend. Acad, des Sci. Paris 216: 310-311.

. 19436. Embryogenie des Fumariacees. Le differenciation des regions

fondamentales du corps chez VHypecoum procumbens L. Compt. Rend. Acad,
des Sci. Paris 216: 354-356.

. 1946a. Embryogenie des Fumariacees. Developpement de l'embry-
on chez le Corydalis lutea DC. Compt. Rend. Acad, des Sci. Paris 222: 161-

163.

19466. Embryogenie des Fumariacees. La differentiations des regions

fondamentales du corps chez le Corydalis lutea DC. Compt. Rend. Acad, des
Sci. Paris 222: 253-255.

— . 1946c. Embryogenie des Papilionacees. Developpement de l'embryon
chez Melilotus arvensis Vallr. Compt. Rend. Acad, des Sci. Paris 222: 1361-

1363.

— . 1946d. Embryogenie des Papilionacees. Developpement de l'embryon
chez YOrobus vernus L. (=Lathyrus vermis Bernh.). Compt. Rend. Acad, des
Sci. Paris 223: 60-62

THE EMBRYO 311

Souegues, E. C. R. 1946e. Embryog6nie des Papilionac6es. DeVeloppement de l'em-

bryon chez le Vicia sepium L. Compt. Rend. Acad, des Sci. Paris 223: 389-391.
. 1946/. Embryog6nie des Papilionacees. DeVeloppement de l'embryon

chez YOrobus tuberosus L. (=Lathyrus macrorrhizus Wimm.). Compt.

Rend. Acad, des Sci. Paris 223: 493-495.
. 1946gr. Embryog6nie des Papilionacees. D^veloppement de l'embryon

chez YErvum hirsutum L. ( = Vicia hirsuta S. F. Gray). Compt. Rend.

Acad, des Sci. Paris 223: 838-840.
. 1947a. Embryogenie des Papilionacees. DeVeloppement de l'embryon

chez le Genista tinctoria L. Compt. Rend. Acad, des Sci. Paris 224: 79-81.
. 19476. Embryogenie des Papilionacees. DeVeloppement de l'embryon

chez le Sarothamnus scoparius Koch. (=Cytisus scoparius Link.). Compt.

Rend. Acad, des Sci. Paris 225: 776-778.

1948. Embryogenie des Papilionacees. DeVeloppement de l'embryon

chez le Vicia f aba L. (= Faba vulgaris Moench). Compt. Rend. Acad, des

Sci. Paris 226: 2101-2103.
Steindl, F. 1945. Bietrag zur Pollen- unci Embryobildung bei Cynomorium

coccineum L. Arch. Julius Klaus-Stift. f. Vererbungsforsch. 20: 342-355.
Stenar, H. 1937. Om Achroanthes monophyllos (L.) Greene, dess geografiska

utbredning och embryologi. Heimbygdas Tidskr., Felskr. Erik Modin 1937/

1938, pp. 177-221.
. 1940. Biologiska och embryologiska notiser rorande Calypso bulbosa

(L.) Oakes. Jamten Heimbyglas Forlag, Ostersund 1940, pp. 184-189.
Stolt, K. A. H. 1928. Die Embryologie von Myriophyllum alterniflorum DC.

Svensk Bot. Tidskr. 22: 305-319.
Swamy, B. G. L. 1942a. Female gametophyte and embryogeny in Cymbidium

bicolor Lindl. Proc. Indian Acad. Sci. Sect. B 15: 194-201.
. 19426. Morphological studies in three species of Vanda. Current Sci.

[India] 11: 285.
. 1943. Gametogenesis and embryogeny of Eulophea epidendraea Fischer.

Proc. Natl. Inst. Sci. India 9: 59-65.
. 1946. The embryology of Zeuxine sulcata Lindl. New Phytol. 45;

132-136.
. 1948. The embryology of Epidendrum prisma'. ocarpum. Bui. Torres

Bot. Club 75: 245-249.

1949. Embryological studies in the Orchidaceae. II. Embryogeny

Amer. Midland Nat. 41: 202-232.
Tischler, G. 1913. tjber die Entwicklung der Samenanlagen in parthenocarpen

Angiospermenfriichten. Jahrb. f. wiss. Bot. 52: 1-84.
Treub, M. 1879. Notes sur Pembryog6nie des quelques Orchid6es. Natuurk.

Verh. Koninkl. Akad. Amsterdam 19: 1-50.
. 1885. Observations sur les Loranthacfes. I. DeVeloppement dans les

sacs embryonnaires dans le Loranthus sphaerocarpus. II. Embryogenie du

Loranthus sphaerocarpus. Ann. Jard. Bot. Buitenzorg 2: 54-76.
Walker, R. I. 1947. Megasporogenesis and embryo development in Tropaeolum

majus L. Bui. Torrey Bot. Club 74: 279-282.

312 INTRODUCTION TO EMBRYOLOGY OF ANG10SPERMS

Weber, E. 1929. Entwicklungsgeschichtliche Untersuchungen uber die Gattung
Allium. Bot. Arch. 25: 1-44.

White, P. R. 1928. Studies on the banana : an investigation of the floral morphol-
ogy and cytology of certain types of the genus Musa L. Zeitschr. f. Zellforsch.
u. mikros. Anat. 7: 673-733.

Zweifel, R. 1939. "Cytologisch-embryologische Untersuchungen an Balanophora
abbreviata Blume und B. indica Wall. Diss. Zurich.

CHAPTER 9
APOMIXIS1

Apomixis maybe defined (Winkler, 1908, 1934) as the substitution
for sexual reproduction (amphimixis) of an asexual process which
does not involve any nuclear fusion. For the sake of convenience
it may be subdivided into four classes. In the first, or nonrecurrent
apomixis, the megaspore mother cell undergoes the usual meiotic
divisions and a haploid embryo sac is formed. The new embryo
may then arise either from the egg (haploid parthenogenesis) or
from some other cell of the gametophyte (haploid apogamy).
Since the plants produced by this method contain only a single set
of chromosomes, they are usually sterile, and the process is not
repeated from one generation to another.

In the second or recurrent type of apomixis, the embryo sac may
arise either from a cell of the archesporium (generative apospory)2
or from some other part of the nucellus (somatic apospory) . There
is no reduction in the number of chromosomes, and all the nuclei
of the embryo sac are diploid. The embryo may arise either from
the egg (diploid parthenogenesis) or from some other cell of the
gametophyte (diploid apogamy).

In the third type, whatever the method by which the embryo sac
is formed and whether it is haploid or diploid, the embryos do not
arise from the cells of the gametophyte but from those of the nu-
cellus or the integument. This is called adventive embryony or
sporophytic budding.3 Here we have no alternation of generations,
as the diploid tissues of the parent sporophyte directly give rise to
the new embryo.

In the fourth type the flowers are replaced by bulbils or other
vegetative propagules which frequently germinate while still on

1 For a more detailed treatment of apomixis, see Gustafsson (1946, 1947 a, b).

2 Some authors (see Gustafsson, 1946) call it "diplospory."

3 Winkler (1934) regards this as only a special form of vegetative propagation,
but, as Gustafsson (1946) points out, the morphological and physiological char-
acters of adventive embryos do not support this view.

313

314 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the plant. But since this is merely a form of vegetative reproduc-
tion, it will not be considered in the present account.

NONRECURRENT APOMIXIS

The development of a haploid cell of the gametophyte into the
embryo is a comparatively infrequent occurrence, and as mentioned
before, the plants arising in this way are sterile. Several years ago,
Kusano (1915) and Haberlandt (1922) observed mitoses in the un-
fertilized egg cells of Gastrodia and Oenothera, but the development
was found to stop at a very early stage. Fully developed haploid
plants, first found in Datura (Blakeslee et at., 1922), have now been
recorded in several genera4 but their exact origin remains obscure
except in a very few cases.

For the first critical account of the origin of haploid embryos,
we are indebted to C. A. J0rgensen (1928). He treated 90 flowers of
Solarium nigrum with the pollen of S. luteum, resulting in 43 fruits
with 70 seeds. From these arose 35 S. nigrum seedlings of which
seven turned out to be haploid.5 A developmental study showed
that the pollen of S. luteum readily germinated on the stigmas of
S. nigrum and the pollen tubes reached the embryo sacs in the
normal way. One of the sperms also entered the egg but failed to
fuse with the female nucleus and soon degenerated. Its presence
sufficed, however, to stimulate the haploid egg to develop parth-
enogenetically and give rise to an embryo (Fig. 177).

In some species of Lilium (Cooper, 1943), in Bergenia delavayi
(Lebegue, 1949) and in Erythraea centaurium (Crete, 1949), one of
the synergids may divide in a small number of the ovules. Twin
embryos are thus produced, one diploid and the other haploid (Fig.
178). However, in most cases of this kind the synergid embryo
usually degenerates at an early stage, and only the zygotic embryo
is seen in the mature seed.

In some recent studies of Orchis mocidata, Epipactis latifolia,
Platanthera chlorantha, Cephalanthera damasonium, and Listera ovata,
Hagerup (1944, 1945, 1947) found a number of haploid embryos.
He observed that while some ovules received more than one pollen
tube (Fig. 179), others received none at all, or the tube arrived too

4 For detailed references see Kostoff (1941).

6 The remaining seedlings were diploid and are believed to have arisen by a
process of endoduplication or nuclear division without cell formation, followed by
a fusion of the spindles in the next division.

A CD

Fig. 177. Origin of haploid embryos in Solatium nigrum after pollination with
S. luteum. A, mature embryo sac of S. nigrum, showing egg, secondary nucleus,
one synergid, and the antipodals. Note sperm nucleus inside egg. B, egg with
two sperm nuclei. C, upper part of embryo sac, showing disintegrating sperm
nucleus inside egg. D, young embryo and part of endosperm. (After J^rgensen,
1928.)

Fig. 178. Development of synergid embryos in Lilium martagon. A, apical
portion of ovule, showing first division of zygote and synergid. B, two-celled
zygotic proembryo; division of synergid not yet completed. C, twin proembryos;
smaller proembryo, on left, has arisen from a synergid. D, more advanced stage;
proembryo on right has arisen from an unfertilized synergid and is haploid ; proem-
bryo on left has arisen from fertilized egg and is diploid. (After Cooper, 1943.)

315

316

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

late to be able to effect fertilization. Under the two last-named
conditions, the unfertilized egg was frequently found to divide and
give rise to a haploid embryo ("facultative parthenogenesis") (Fig.
180). 6 In Orchis, Listera, and Platanthera instances of a different
nature were also noted, in which the egg was fertilized and gave rise
to a normal embryo, but simultaneously an unfertilized synergid
also began to develop, so that two embryos were formed, one diploid

A B D

Fig. 179. Supernumerary pollen tubes in embryo sacs of Epipadis latifolia. A,B,
double fertilization, showing one sperm nucleus fusing with egg and the other with
secondary nucleus ; note presence of second undischarged pollen tube. C, micropy-
lar part of embryo sac, showing two pollen tubes. D, micropylar part of another
embryo sac, showing five sperm nuclei in proximity to egg. (After Hagerup, 194-5.)

and the other haploid. Further, in one ovule of Orchis in which the
pollen tube had not yet entered the embryo sac, the egg as well
as one of the synergids had begun to divide, thus indicating the
possibility of a production of twin haploid embryos (Fig. 181C).7

6 In some embryo sacs of another orchid, Spiranthes australis, the pollen tube
was found to have entered the embryo sac but the male gametes were still undis-
charged, while embryo formation had already commenced (Maheshwari and
Narayanaswami, 1950).

7 Rarely the egg cell of Orchis was found to receive two sperms (Fig. 181 A) and
give rise to a triploid embryo (Fig. 1815), or both the egg and one synergid were
fertilized to give rise to twin diploid embryos (Fig. 181 D).

APOMIXIS

317

Three cases of androgenic haploids, in which the male nucleus
alone is concerned in the development, are also on record, but no
developmental studies have been made and the evidence is entirely
genetical. Kostoff (1929) pollinated Nicotiana tabacum var. macro-
phylla, having 72 chromosomes, with N. langsdorffii, having 18
chromosomes. Out of about 1000 seedlings resulting frcm this

Fig. 180. Haploid parthenogenesis in Epipactis latifolia (s = synergid; o = egg;
t = pollen tube; p = secondary nucleus). A, B, embryo sacs showing division of
haploid egg; pollen tube t is still undischarged. C, embryo sac has not received
any pollen tube, but egg and secondary nucleus are ready to divide. D,E, meta-
phases in first division of haploid egg, showing 20 chromosomes. (After Hagerup,
1945.)

cross, one reached maturity. This showed none of the characters of
N. tabacum but strongly resembled a dwarf N. langsdorffii. The
number of chromosomes turned out to be 9, strongly suggesting its
origin from a male gamete of N. langsdorffii.

In the same year Clausen and Lammerts (1929) reported a parallel
case in N. tabacum. They crossed N. digluta 9 , an allohexaploid
having 72 chromosomes, with N. tabacum <?, having 48 chromosomes,
and obtained a plant with 24 chromosomes. This plant agreed
morphologically and cytologically with other haploid tabacum plants

318 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

and is therefore believed to have arisen from a male gamete of
N. tabacum.

The third androgenic haploid was reported in Crepis by Geras-
simova (1936). She castrated the flowers of plants of C. tectorum
having certain dominant characters and treated them with X-rays.
They were then crossed with plants of the same species having
recessive characters. One haploid plant of the recessive type was

Fig. 181. Presence of twin embryos and other abnormalities in Orchis maculatus.
A, four male nuclei in embryo sac; two of these are in close contact with the egg,
suggesting the possibility of both fusing with it. Note also secondary nucleus and
nucleus of a displaced synergid (?). B, embryo sac has received two pollen tubes;
presence of 60 chromosomes (n = 20) in embryo seems to indicate that egg nucleus
had fused with two male nuclei. C, unfertilized embryo sac, showing division of
haploid egg and a synergid. D, embryo sac with twin embryos. (After Hagerup,
19U-)

obtained. It is believed that the X-ray treatment killed the egg
nucleus and the haploid embryo arose from the sperm nucleus only.
Summing up, haploid embryos may result in various ways. The
first and most important source is the unfertilized egg. Failure
of fertilization may be due to any of the following causes: (1) absence
of a pollen tube, (2) inability of the tube to discharge its contents,

(3) an insufficient attraction between the male and female nuclei,

(4) an early degeneration of the sperms, and (5) a discordance in the

APOMIXIS 319

time of maturation of the egg and the entrance of the male gametes.
It is also possible that there may be two embryo sacs in the same
ovule. The egg of one of these may be fertilized and give rise to a
normal diploid embryo, while that of the neighboring sac is stimu-
lated to develop without fertilization. A second source of origin
cf haploid embryos may be some cell of the embryo sac other than
the egg. Usually this is a synergid, but rarely even antipodal cells
may give rise to embrycs. The third possibility is the origin of the
embryo from a male gamete. This may be due either to a degenera-
tion of the egg nucleus, so that the male nucleus alone is left to
function, or to failure of pollen tube to open, so that one or both of
the male gametes begin to develop in situ.

RECURRENT APOMIXIS

In the second or recurrent type of apomixis the embryo sac is
diploid and may arise either from a cell of the archesporium or from
some other cell of the nucellus. The distinction between the two
forms, known as generative and somatic apospory, is somewhat
artificial, as it is frequently difficult to say whether the cell in ques-
tion belongs to the archesporial tissue or to the "somatic" tissues
of the nucellus.

Since there is no reduction in the chromosome number, the first
division of the initial cell is mitotic, semiheterotypic, or pseudo-
homo typic. The mitotic division requires no explanation except
that in apomictic plants there is a retardation of the prophase and
the resting nucleus shows an intense hydration of the chromosomes.
They do not exhibit either the pairing or the marked contraction
characteristic of meiosis, and at anaphase the two halves of each
chromosome move apart to the poles (Fig. 182M-R). The semi-
heterotypic division begins like a meiotic prophase,8 but a normal
metaphase plate is not organized; instead the chromosomes are
widely scattered on the spindle. In the anaphase there are numer-
ous laggards, and eventually a nuclear membrane is laid down in
such a manner as to incorporate the entire chromosome complement
within a single "restitution nucleus" (Fig. 182A-G). This nucleus
is somewhat dumbbell-shaped in the beginning, but it soon becomes

8 A pairing of the chromosomes and the occurrence of "bivalent-like formations"
have been noticed in some cases of semiheterotypic division, but chiasmata are rare
or absent (see Gustafsson, 1946).

320 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

/////l\%

w///

Fig. 182. Diagrams illustrating formation of restitution nucleus (A-G); pseudo-
homotypic division (H-L) ; and somatic division (M-R) in megaspore mother cells
of apomictic angiosperms. A, nucleus of megaspore mother cell in prophase; note
absence of pairing. B, diakinesis. C, semiheterotypic metaphase. D,E, forma-
tion of nuclear membrane enclosing all the chromosomes. F, fully formed restitu-
tion nucleus. G, homotypic metaphase. H, nucleus of megaspore mother cell in
prophase. I-L, stages in formation of daughter nuclei by pseudohomotypic
division. M-R, somatic division of nucleus of megaspore mother cell. (After
Gustafsson, 1985.)

rounded and the following divisions are entirely mitotic. In the
pseudohomotypic division the chromosomes are short, thick, and
contracted, as in meiosis, but do not show any pairing. In the
beginning of the metaphase the univalents are scattered over the
spindle, but gradually they arrange themselves in an equatorial

APOMIXIS

321

plate and undergo the usual longitudinal splitting, followed by a
separation of the daughter chromosomes (Fig. 182H-L).

Generative apospory. Holmgren (1919) has made a very detailed
study of the embryology of several species of the genus Ewpatorium.
Some species are entirely normal and reproduce sexually, but E.

Fig. 183. Development of embryo sac in Eupatorium glandulosum. A-C, mega-
spore mother cell. D, nucleus of megaspore mother cell in division. E,F, two-
nucleate embryo sacs. G, four-nucleate embryo sac. H, eight-nucleate embryo
sac. (After Holmgren, 1919.)

glandulosum, which is a triploid, is apomictic (Fig. 183). Here the
division of the megaspore mother cell differs little from a somatic
mitosis except in the fact that the cell elongates considerably, de-
molishing the nucellar epidermis and becoming vacuolate even before
it is ready to divide. There is no synapsis or pairing of chromo-
somes. Three nuclear divisions take place, to give rise to an eight-
nucleate embryo sac with two or three antipodal cells which may

322 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fig. 184 A-F.

APOMIXIS 323

undergo a further division with or without the formation of a cell
wall. The embryo arises from the unfertilized but diploid egg cell.

An essentially similar condition occurs in the guayule, Parthenium
argentatum (Esau, 1946). This species comprises two races, one
with 36 chromosomes and the other with 72. The former is mainly
sexual and the latter mainly apomictic. Since the condition is not
absolutely fixed, the apomixis may be regarded as facultative. In
the 36-chromosome race there is usually an orderly sequence of
events in megasporogenesis (Fig. 184A-C) and gametophyte de-
velopment. Megaspore mother cells are seen in various phases of
meiosis, megaspore tetrads are abundant, and uninucleate embryo
sacs are associated with the crushed remnants of the nonfunctioning
megaspores. Views of pollen tubes in embryo sacs, followed by
syngamy and triple fusion, are common in artificially pollinated
flowers, and the development of the embryo and endosperm is
closely correlated. In the 72-chromosome plants, on the other
hand, there is no orderly sequence of stages in the development of
the embryo sac (Fig. 184D-F). Young stages tend to persist in
fairly large ovules; megaspore mother cells enlarge, become vacuo-
late, and directly assume the characteristics of uninucleate embryo
sacs without the intervention of the dyad and tetrad stages; em-
bryo and endosperm show little correlation in their development;
and frequently even impollinated flowers give rise to embryos.
Nevertheless pollination is highly beneficial or even necessary for
the continued development of the endosperm without which the
embryo does not grow to maturity.

A good example of a semiheterotypic division, followed by the
formation of a restitution nucleus, is seen in Ixeris (Okabe, 1932),
also belonging to the family Compositae. The basic chromosome
number in this genus is 7. Species with the diploid number (2n =
14) reproduce normally by the sexual method, but /. dentata, which

Fig. 184. Some stages in development of embryo sac in 36-chromosome and
72-chromosome races of Parthenium argentatum. A, 36-chromosome race; mega-
spore mother cell. B, dyad formation. C, tetrad of megaspores. D, 72-chromo-
some race; megaspore mother cell elongating and functioning directly as diploid
spore. Note vacuolation of its cytoplasm and disorganization of the nucellar
epidermis, accompanied by a differentiation of the integumentary tapetum. E,
binucleate embryo sac with both nuclei in division. F, four-nucleate embryo sac.
(After Esau, 1946.)

324

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

is a triploid (2n = 21), is an apomict (Fig. 185). During the divi-
sion of the megaspore mother cell there is no evidence of any pairing
of the chromosomes. Even at anaphase the chromosomes are

Fig. 185. Development of unreduced embryo sac in Ixeris dentata. A, prophase
of first division of megaspore mother cell. B, later stage, showing 21 univalent
chromosomes. C, restitution nucleus. D, metaphase. E, telophase. F,G, two-
nucleate embryo sacs. H, four-nucleate embryo sac. /, mature embryo sac.
J, later stage, showing two-celled proembryo. {After Okabe, 1932.)

irregularly scattered on the spindle, and instead of forming two
separate groups they become enclosed in a common nuclear mem-
brane, forming a restitution nucleus which contains the unreduced
chromosome number. Further divisions are entirely mitotic and

APOMIXIS

325

lead to the formation of an eight-nucleate embryo sac, organized
in the usual manner.

The diploid species of the genus Taraxacum go through the usual
meiotic divisions and tetrad formation, followed by syngamy, but
the polyploid species show a semiheterotypic division and dyad
formation, followed by the development of an unreduced egg cell
into the embryo (Osawa, 1913; Sears, 1922; Poddubnaja-Arnoldi
and Dianowa, 1934; Gustafsson, 1935; Fagerlind, 1947a). Usually
it is the chalazal dyad cell which functions (Fig. 186), but in T.
laevigatum (Sears, 1922) it is frequently the upper.

Fig. 186. Development of unreduced embryo sac in Taraxacum albidum. AUA2)
consecutive sections of megaspore mother cell. B, megaspore mother cell in divi-
sion. C, formation of dyad cells, of which the lower functions and gives rise to
embryo sac. D, two-nucleate embryo sac. E, mature embryo sac, showing
endosperm formation. {After Osawa, 1913.)

In Erigeron some species are sexual, others partially apomictic,
and still others almost entirely apomictic. In E. annum (Fager-
lind, 19476), which belongs to the third category, a restitution
nucleus is formed during the first meiotic division. This divides
without wall formation to give rise to the mature embryo sac, which
is usually eight-nucleate. Ordinarily the development of the em-
bryo and endosperm go hand in hand (Holmgren, 1919), but some-
times the endosperm may lag behind, so that occasionally a many-
celled embryo is associated with an undivided endosperm nucleus
(Tahara, 1921).

Bergman (1941) has recently reported a considerable range of

326

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

APOMIXIS 327

variation in several species of the genus Hieracium, subgenus
Archier actum. In some cases the division is of the mitotic type
and the megaspore mother cell develops directly into the embryo
sac (Fig. 187 A-H). In others both the meiotic divisions take place
but several chromosomes remain unpaired, resulting in disturbances
in tetrad formation and consequent sterility (Fig. 187 1-P). In
still other cases there is a production of unreduced dyad cells (Fig.
1S7Q-T) either by the formation of a restitution nucleus or by a
pseudohomotypic division. Rarely, the first meiotic division takes
place in the usual way, but this is followed by a prolonged inter-
kinetic stage in which the chromosomes divide longitudinally so
that the diploid number is restored. In all three cases one of the
dyad cells functions and gives rise to the embryo sac.

Somatic Apospory. Rosenberg (1907) described the occurrence
of somatic apospory in three species of the genus Hieracium (sub-
genus Pilosella), viz., H. excellens (Fig. 188 A-E), H.flagellare (Fig.
188F-H), and H. aurantiacum. The megaspore mother cell goes
through the usual meiotic divisions, but at just about this stage a
somatic cell situated in the chalazal region begins to enlarge and
becomes vacuolated. This cell gradually increases in volume, en-
croaching upon the megaspores and finally crushing them. The
aposporic embryo sac, arising from it, has the unreduced chromo-
some number and is able to function without fertilization. In H.
excellens the normal and reduced embryo sac, as well as the aposporic
and unreduced embryo sac, sometimes develops simultaneously but
this is rare in the other two species.9 H. aurantiacum is peculiar
in that the aposporic embryo sac usually originates from a cell of
the nucellar epidermis.

Aposporic embryo sacs have also been reported in several other
genera like Malus (Dermen, 1936), Crepis (Stebbins and Jenkins,
1939), Hypericum (Noack, 1939), Ranunculus (Hafliger, 1943), and
Poa (Hakansson, 1943; Nielsen, 1945, 1946). The cell giving rise
to the embryo sac may belong either to the integument or to some
part of the nucellar epidermis. In every case, however, the develop-
ment is characterized by two common features — an increase in the

9 As remarked by Rosenberg, it is because of this combination of typical as well
as unreduced embryo sacs that H. excellens can give rise to hybrids in spite of being
an apomict.

328 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Fig. 188. Development of aposporic embryo sacs in Hieracium excellens (A-E)
and H. flagellare (F-H). A, nucellus, showing tetrad of megaspores; note enlarge-
ment of cell lying just below chalazal megaspore. B, megaspore tetrad in process

APOMIXIS 329

size of the cell and a vacuolation of its cytoplasm, both taking place
prior to nuclear division.

Unclassified Cases. In several plants it is difficult to draw a
sharp distinction between generative and somatic apospory. Atra-
phaxis frutescens (Edman, 1931) may be cited as an example of this
kind. Here we have a multicellular archesporium. Usually only
one or two of its cells, those which have a more central position,
take the characters of megaspore mother cells, while the rest undergo
a series of mitotic divisions. The true mother cells or their deriva-
tives soon degenerate. The aposporic embryo sacs arise from either
of the following sources: (1) the derivatives of the potentially
sporogenous cells lying close to the megaspore mother cell and
greatly resembling it in appearance (Fig. 189), or (2) the purely
somatic cells of the chalaza (Fig. 190).

In Antennaria alpina and some other species of this genus (Juel,
1900; Stebbins, 1932; Bergman, 1941) the megaspore mother cell
may divide meiotically or mitotically. In the first or meiotic type,
the chromosomes become greatly contracted and lie scattered over
the spindle in a disorderly fashion. The separation of the daughter
chromosomes takes place irregularly, so that although the number
is reduced it is not exactly halved and several univalents are left
out altogether. The next division may result in a tetrad of four
megaspores, but more often five, six, or even seven cells may be
formed. The gametophytes arising from them are functionless and
soon degenerate. In the second or mitotic type, the dividing cell
grows vigorously and attains a large size even before the onset of the
prophase. There is no reduction in the number of chromosomes and
the embryo sacs derived in this way are diploid and functional.

In Alchemilla arvensis (Murbeck, 1901 ; Boos, 1924) the megaspore
mother cell enters a meiotic prophase, but the nucleus soon de-
generates. Meanwhile, the surrounding cells, which are potentially
sporogenous, undergo a number of mitotic divisions resulting in a
few parietal cells and two to six axial cells, which may divide either

of degeneration; chalazal cell showing increase in size and vacuolation. C, mega-
spore tetrad and large nucellar cell destined to give rise to embryo sac. D, normal
and aposporic embryo sacs growing simultaneously. E, two fully developed embryo
sacs; lower is probably of aposporic origin. F-H, some stages in development of
aposporic embryo sac; note progressive degeneration of megaspore tetrad. (After
Rosenberg, 1907.)

330

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

meiotically or mitotically. The reduced embryo sacs must be
fertilized before they can give rise to embryos. The unreduced
embryo sacs, on the other hand, can function without fertilization.
Fagerlind (1944) has recently made a detailed study of some
species belonging to the genus Elatostema. In E. acuminatum the

Fig. 189. Development of aposporic embryo sacs in Atraphaxis frutescens (striped
portion in ovule represents hypostase). A, B, l.s. nucellus, showing archesporial
tissue. C,D, degeneration of cells belonging to primary archesporial tissue and
appearance of secondary archesporium. E, a two-nucleate haploid embryo sac
accompanied by uninucleate diploid embryo sac. F, young aposporic embryo sacs.
G, nucellus, showing three embryo sacs — one diploid and two-nucleate, another
haploid and four-nucleate, and the third haploid and eight-nucleate. {After
Edman,1931.)

megaspore mother cell enters into a meiotic prophase, but because
of the occurrence of certain irregularities the derivative cells are
nonviable and soon degenerate. The adjacent cells of the nucellus
divide mitotically to give rise to unreduced embryo sacs (somatic
apospory), but these also usually degenerate, resulting in consider-
able sterility. In E. eurhynchum there is little or no tendency

APOMIXIS

331

towards meiosis but several unreduced embryo sacs arise by mitotic
divisions in the central cells of the archesporium (generative apos-
pory). The same condition also occurs in E. machaerophyllum
except that here the division may be mitotic or pseudohomotypic,
or there may be a semiheterotypic division followed by the forma-
tion of a restitution nucleus.

Fig. 190. Apospory and polyembryony in Atraphaxis frutescens. A, normal
embryo sac with embryo and endosperm; additional aposporic embryo sac at
chalazal end showing inverted polarity. BiB2, consecutive sections of ovule show-
ing well-developed embryo in upper embryo sac and aborted embryos in chalazal
region. C, twin embryo sacs — one on left with two overlapping embryos, that on
right with one embryo. (After Edman, 1931.)

In some species of Potentilla (Rutishauser, 1943) there is a multi-
cellular archesporium without any well-marked distinction between
the axial and the lateral cells of the sporogenous tissue. Embryo
sacs may arise either from such cells by ordinary mitotic division as
in P. verna (generative apospory), or from the chalazal cells of the
nucellus as in P. canescens, P. praecox, and P. argentea (somatic
apospory). Hakansson (1946) has confirmed this in P. argentea

332 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

and also reported the occurrence of aposporic embryo sacs in P.
crantzii. In the latter it is the nucellar cells adjacent to the arch-
esporium which give rise to embryo sacs.

Organization of Aposporic Embryo Sacs. By whichever method
the aposporic embryo sac may arise {i.e., by generative or by somatic
apospory), it is usually eight-nucleate. Sometimes fewer than
eight nuclei occur as in Ochna serrulata (Chiarugi and Francini,

1930) and Atraphaxis frutescens (Edman, 1931), or more than eight
as in Elatostema eurrhynchum (Fagerlind, 1944), but these are occa-
sional deviations without any special significance. A more common
feature is the disturbed polarity and lack of proper organization of
the various elements of the embryo sac. Frequently the egg and
synergids are indistinguishable from one another, and sometimes
there are more than two polar nuclei, while the remaining elements
of the embryo sac remain undifferentiated. In Atraphaxis (Edman,

1931) some embryo sacs were found to lack an egg apparatus and
some the antipodal cells; occasionally both egg apparatus and anti-
podal cells were found to be absent and all the nuclei were aggre-
gated in the center. However, such embryo sacs are probably
functionless.

Not only the number but also the behavior of the polar nuclei is
extremely variable. In Zephyr anthes texana (Pace, 1913) the two
polar nuclei fuse with a male nucleus to give rise to a pentaploid
endosperm, while the embryo is formed by the unfertilized but
diploid egg cell. In the apomictic forms of Chondrilla (Poddubnaja-
Arnoldi, 1933), Taraxacum (Poddubnaja-Arnoldi and Dianowa,
1934), Ranunculus (Hafliger, 1943), and Elatostema there is no
triple fusion and the endosperm is tetraploid. In Antennaria
alpina (Juel, 1900) and Alchemilla arvensis (Murbeck, 1901) the
polar nuclei are said to divide independently and the endosperm is
diploid. In Ewpatorium glandulosum (Holmgren, 1919) and Ixeris
dentata (Okabe, 1932) the condition is variable and the endosperm
may be diploid or tetraploid. In Balanophora globosa, according to
Ernst (1913), the lower polar nucleus degenerates and only the
upper forms the endosperm.

Development of Embryo in Aposporic Embryo Sacs. Theoreti-
cally the new sporophyte may arise from any cell or nucleus of the
diploid embryo sac, but usually it is only the egg which is capable
of such development (diploid parthenogenesis). Sometimes one or

APOMIXIS

333

Fig. 191. Semigamy in Rudbeckia speciosa. A, part of embryo sac, showing two-
celled endosperm and egg containing male and female nuclei. B, egg, showing
division of both male and female nuclei. C,D, two-celled proembryos; two small
nuclei in basal cell have arisen b.y division of sperm nucleus. E, three-celled pro-
embryo showing nuclei derived from sperm in terminal cell. F-H, more advanced
stages; nuclei derived from sperm occupy variable positions in proembryo. (After
Batiaglia, 1947.)

334 INTRODUCTION TO EMBRYOLOGY OF ANGJOSPERMS

both of the synergids, and less frequently the antipodal cells, may
give rise to embryos (diploid apogamy). In some plants such de-
velopment takes place without the stimulus of pollination (auton-
omous apomixis) and may begin even in the bud stage. More com-
monly, however, the stimulus of pollination is essential and even
triple fusion occurs more or less regularly. This condition is called
pseudogamy.10

There are a few reports of embryos arising from the cells or nuclei
of the endosperm. Rosenberg (1907), Schnarf (1919), and Gent-
scheff (1937) reported such an occurrence in some species of Hier-
acium. However, in a later paper Rosenberg (1930) withdrew this
interpretation. He now thinks that here several aposporic embryo
sacs develop in the same ovule and fuse with one another so that
the boundaries between them get lost, and the embryos, although
really arising from the egg cells of the different embryo sacs, become
included in a common mass of endosperm tissue. The reports of
Billings (1937) and of Jeffrey and Haertl (1939) about the origin of
endosperm embryos in Isomeris and Trillium are also open to criti-
cism.11 To date there is, therefore, no established case of the origin
of embryos from the endosperm.

Mention must finally be made of the peculiar phenomenon called
"semigamy" which has been discovered very recently by Battaglia
(1946, 1947). Here a sperm nucleus enters the diploid egg cell but
does not fuse with its nucleus and divides independently to form a
few daughter nuclei. Embryonal chimaeras are thus produced in
which most of the cells and nuclei are diploid but a few are haploid.
The endosperm, however, is pentaploid, being formed as the result
of a fertilization of the secondary nucleus by one male nucleus. So
far this condition has been reported only in two species of Rud-
beckia, R. laciniata and R. speciosa (Fig. 191), but it is possible that
it occurs in other apomicts and has been overlooked.

ADVENTIVE EMBRYONY

In adventive embryony there is no alternation of generations and
the embryos originate from the diploid cells of the ovule lying out-

10 For a more detailed discussion of pseudogamy, see Hafliger (1943) and Fager-
lind (1946).

11 Swamy (1948), who has recently studied the development of the embryo and
endosperm in Trillium undulalum, refutes the occurrence of endosperm embryos in
this species but states that adventive embryony is frequent.

APOMIXIS

335

side the embryo sac and belonging either to the nucellus or the
integument. A common feature of the process is that the cells
concerned in such development become richly protoplasmic and
actively divide to form small groups of cells, which eventually push
their way into the embryo sac and grow further to form true em-
bryos. Frequently the zygotic embryo also develops at the same
time and is distinguishable from the adventive embryos only by the
somewhat lateral position and lack of a suspensor in the latter.

A favorite and frequently quoted instance of adventive embryony
is that of Citrus (Strasburger, 1878; Osawa, 1912; Webber and
Batchelor, 1943), (Fig. 192) in which four or five embryos are com-

A B C

Fig. 192. Development of adventive embryos in Citrus trifoliata. A, micropylar
portion of embryo sac, showing fertilized egg, pollen tube, and endosperm nuclei;
some of the nucellar cells have enlarged and show prominent nucleus and dense
cytoplasm. B, same, more advanced stage. C, upper part of embryo sac, showing
several embryos lying in endosperm; only zygotic embryo has suspensor. (After
Osawa, 1912)

mon and sometimes as many as 13 viable embryos can be found in
the same seed. Among other examples may be cited Euphorbia
dulcis (Carano, 1926), Sarcococca ilicifolia (Wiger, 1930), Eugenia
spp. (Tiwary, 1926; Pijl, 1934), Capparis frondosa (Mauritzon,
1934), Mangifera indica (Juliano, 1937), and Hiptage madablota
(Subba Rao, 1940). The chief variation in development concerns
the place of origin of the embryos. Whenever the nucellus is in-
tact, the adventive embryos originate from the nucellar cells, but
when it becomes disorganized the cells of the integument may take
over this function. Also, sometimes a single cell may become the
progenitor of an embryo, while on other occasions it is a small group
of cells.

An especially interesting type of adventive embryony occurs in
the Scandinavian forms of the orchid Nigritella nigra (Afzelius,

336

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

1928, 1932). n As in other members of the family, the nucellus is
reduced to a single layer of cells surrounding the megaspore mother
cell (Fig. 193 A). The latter gives rise to three or four daughter
cells of which the chalazal functions and proceeds to form the em-
bryo sac (Fig. 19SB-E). Its development becomes arrested at the
four-nucleate stage, but meanwhile one or two cells of the nucellar
epidermis show a considerable increase in size and begin dividing to
give rise to adventive embryos which are very close to the apex of

E
Fig. 193. Adventive embryony in Nigritella nigra. A, l.s. young nucellus, shov-
ing megaspore mother cell. B, older stage, showing micropylar dyad cell in course
of degeneration. C, functioning megaspore with remains of degenerating mega-
spores; note enlargement of two cells of nucellar epidermis. D, two-nucleate
embryo sac with young adventive embryos arising from cells of nucellar epidermis.
E, large nucellar embryo lying at apex of four-nucleate embryo sac. (After Afzelius,
1928.)

the embryo sac and are enclosed by the integuments (Fig. 193B-E).
Zeuxine sulcata (Swamy, 1946) is very similar except that here,
owing to disturbed meiosis, there is no regular megaspore formation
and if any embryo sacs are produced they abort at a very early
stage. As in Nigritella, the cells of the nucellar epidermis possess a
remarkable capacity for growth and differentiation. One or two
of them elongate considerably to give rise to filamentous proem-
bryos (often four-celled and therefore looking like megaspore

12 It is interesting to note that those forms of this species which occur in the
Alps show normal sexual reproduction (see Gustafsson, 1947a).

APOMIXIS 337

tetrads) which may undergo a secondary increase in number by
further proliferation, budding, or cleavage.

Adventive embryony may be completely autonomous, i.e., inde-
pendent of pollination and fertilization, or it may be induced by one
or both of these factors. The former condition prevails in Al~
chornea ilicifolia (Strasburger, 1878), Euphorbia dulcis (Carano,
1926), and Sarcococca pruniformis (Wiger, 1930). In Nigritella
nigra (Afzelius, 1928) also, neither pollination nor fertilization are
essential, but the occurrence of pollen tubes in the ovary seems to
accelerate the tendency towards the production of adventive em-
bryos. In most other plants, either pollination, or pollination
followed by fertilization, is an important factor in stimulating the
development of adventive embryos, although their exact roles have
not been properly elucidated. In the orchid Zygopetalum mackayi
(Sussenguth, 1923) unpollinated flowers were found to degenerate
and fall off shortly after blooming, but on treating the stigmas with
pollen from Oncidium the adventive embryos developed to maturity
a.nd viable seeds were formed. Here the foreign pollen, although
quite incapable of effecting fertilization, exercised some kind of a
chemical influence which affected the growth of the embryos in a
favorable manner. Eugenia jambos (Pijl, 1934) seems to illustrate
the next step, for in this plant the adventive embryos may originate
quite independently of pollination but do not attain their full de-
velopment unless fertilization has taken place. In most varieties
of Citrus also (see Webber and Batchelor, 1943) fertilization is con-
sidered necessary for the maturation of the adventive embryos, and
a similar condition occurs in the "carabao" mango (Juliano, 1937).

In all cases of adventive embryony there is a formation of the
endosperm, whether it originates as the result of triple fusion or
without it. The only exception is Opuntia aurantiaca (Archibald,
1939). Here the egg, synergids, and antipodals are said to de-
generate, and later also the polar nuclei, so that an endosperm is not
formed at all and the whole embryo sac is reduced to an irregular
darkly staining cavity.13 The nucellar tissue becomes more mas-
sive, however, and certain cells lying below the nucellar cap and
bordering on the cavity of the embryo sac enlarge and become

13 It should be noted, however, that Ganong (1898), who found nucellar em-
bryony in another species— 0. vulgaris — records normal fertilization and the forma-
tion of an "abundant endosperm."

338 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

rounded. Their walls become thickened and each cell undergoes two
divisions at right angles to each other to form a four-celled pro-
embryo. With further divisions the proembryo increases in size
and finally ruptures the wall of the parent cell and enters into the
cavity of the sac. Several other embryos are formed similarly but
owing to mutual competition only one or two reach maturity.

An interesting feature, which has often been commented on, is
that although the adventive embryos have precisely the same
germinal constitution as sporophytic buds, their developmental
behavior is quite different. A sporophytic bud, whether terminal
or axillary, directly proceeds to the formation of a stem, leaves, and
flowers, while the adventive embryo recapitulates in a very striking
manner the morphological features of true seedlings, viz., presence
of cotyledons, radicle, plumule, epicotyl, and hypocotyl. In Citrus,
sporophytic buds produce virtually thornless plants, but the mi-
cellar embryos produce plants which are thorny, like the zygotic
seedlings with which they are associated.14

Swingle (1927) suggested that this extraordinary recapitulation
of a stage in ontogeny, already undergone by the generation which
produces the nucellar embryos, is probably due to some powerful
morphogenetic influence exercised upon the embryos by the "magic
bath" of the embryo sac. However, we have no knowledge so far
of the true nature of these influences, and it would be interesting to
know how the nucellar embryos would behave if they are removed
from the ovule at a comparatively early stage of development and
grown in artificial media.

REFERENCES
Afzelius, K. 1928. Die Embryobildung bei Nigritella nigra. Svensk Bot. Tidskr.

22: 82-91.

. 1932. Zur Kenntnis der Fortpflanzungsverhaltnisse und Chromosomen-

rzahlen bei Nigritella nigra. Svensk. Bot. Tidskr. 26: 365-369.
Archibald, E. E. A. 1939. The development of the ovule and seed of jointed

cactus (Opuntia aurantiaca Lindley). South African Jour. Sci. 36: 195-211.
Battaglia, E. 1946. Ricerche cariologiche e embriologiche sul genere Rudbeckia

(Asteraceae) . VIII. Semigamia in Rudbeckia laciniata L. Nuovo Gior. Bot.

Ital. N.S. 53: 483-511.
14 It has also been noted (see Cook, 1938; Hodgson and Cameron, 1938; Frost,
1938) that Citrus clones continually propagated by cuttings eventually become
weak and sterile. Horticulturists rejuvenate the clone by using nucellar seedlings,
as the plants obtained from them show greater vigor and a more upright growth
than those obtained from cuttings.

APOMIXIS 339

Battaglia, E. 1947. Ricerche cariologiche e embriologiche sul genere Rudbeckia

(Asteraceae) . XI. Semigamia in Rudbeckia speciosa Wender. Nuovo Gior.

Bot. Ital. N.S. 54: 1-29.
Bergman, B. 1937. Eine neue apomiktische Antennaria. Svensk Bot. Tidskr.

36: 429-443.
. 1941. Studies on the embryo sac mother cell and its development in

Hieracium subg. Archieracium. Svensk Bot. Tidskr. 35: 1-42.
Billings, F. H. 1937. Some new features in the reproductive cytology of angio-

sperms, illustrated by Isomeris arborea. New Phytol. 36: 301-326.
Blakeslee, A. F., Belling, J., Franham, M. E., and Bergner, A. D. 1922. A hap-

loid mutant in the Jimson weed, Datura stramonium. Science 55: 646-647.
Boos, G. 1924. Neue embryologische Studien liber Alche7nilla arvensis (L.) Scop.

Bot. Notiser 1924, pp. 209-250.
Carano, E. 1926. Ulteriori osservazioni su Euphorbia dulcis L. in rapporto col

suo comportamento apomittico. Ann. di Bot. 17: 50-79.
Chiarugi, A., and Francini, E. 1930. Apomissia in Ochna serrulata Walp. Nuovo

Gior. Bot. Ital. N.S. 37: 1-250.
Clausen, R. E., and Lammerts, W. E. 1929. Interspecific hybridization in

Nicotiana. X. Haploid and diploid merogony. Amer. Nat. 63: 279-322.
Cook, R. 1938. A note on embryo rejuvenation. Jour. Hered. 29: 419-422.
Cooper, D. C. 1943. Haploid-diploid twin embryos in Lilium and Nicotiana.

Amer. Jour. Bot. 30: 408-413.
Crete, P. 1949. Un cas de polyernbryonie chez une Gentianacee, VErythraea

centaurium Pers. Bull. Soc. Bot. de France 96: 113-115.
Dermen, H. 1936. Aposporic parthenogenesis in a triploid apple, Malus hupehen-

sis. Jour. Arnold Arboretum 17: 90-105.
Edman, G. 1931. Apomeiosis unci Apomixis bei Atraphaxis frutescens C. Koch.

Acta Horti Bergiani 11: 13-66.
Ernst, A. 1913. Zur Kenntnis von Parthenogenesis und Apogamie bei Angio-

spermen. Verh. schweiz. naturf. Jahresv. Ges. II, 96: 1-13.
Esau, K. 1946. Morphology of reproduction in guayule and certain other species

of Parthenium. Hilgardia 17: 61-101.
Fagerlind, F. 1944. Die Samenbildung und die Zytologie bei agamospermischen

und sexuellen Arten von Elatostema und einigen nahestehenden Gattungen

nebst Beleuchtung einiger damit zusammenhangender Probleme. K. Svenska

Vet.-Akad. Handl. Ill, 21(4) : 1-130.
. 1946. Sporogenesis, Embryosackentwicklung und pseudogame Samen-
bildung bei Rudbeckia laciniata L. Acta Horti Bergiani 14: 39-90.
. 1947a. Makrosporogenese und Embryosackbildung bei agamosper-
mischen T'ara.racwm-Biotypen. Svensk Bot. Tidskr. 41: 365-390.
. 19476. Macrogametophyte formation in two agamospermous Erigeron

species. Acta Horti Bergiani 14: 221-247.

Frost, H. B. 1938. Nucellar embryony and juvenile characters in clonal varieties
of Citrus. Jour. Hered. 29: 423-432.

Ganong, W. F. 1898. Upon polyembryony and its morphology in Opuntia vul-
garis. Bot. Gaz. 25: 221-228

340 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Gentscheff, G. 1937. Zytologische und embryologische Studien iiber einige

Hieracium-Aiten. Planta 27: 165-195.
Gerassimova, H. 1936. Experimentell erhaltene haploide Pflanze von Crepis

tectorum L. Planta 25: 696-702.
Gustafsson, A. 1935. Studies on the mechanism of parthenogenesis. Hereditas

21: 1-112.
. 19-46. Apomixis in higher plants. I. The mechanism of apomixis.

Lunds Univ. Arsskr. N.F. Avd. II, 42(3) : 1-67.

— . 1947a. Apomixis in higher plants. II. The causal agent of apomixis.

Lunds Univ. Arsskr. N.F. Avd. II, 43(2): 71-179.

19476. Apomixis in higher plants. III. Biotype and species formation.

Lunds Univ. Arsskr. N.F. Avcl. II, 43(12): 183-370.
Habsrlandt, G. 1922. liber Zellteilungshormone und ihre Beziehungen zur

Wundheilung, Befruchtung, Parthenogenesis und Adventivembryonie. Biol.

Zentbl. 42: 145-172.
Hafliger, E. 1943. Zytologisch-embryologische Untersuchungen pseudogamer

Ranunkulen der Auricomus Gruppe. Ber. schweiz. bot. Gesell. 53: 317-382.
Hagerup, 0. 1944. On fertilization, polyploidy and haploidy in Orchis macu-

latus L. Dansk Bot. Arkiv. 11(5): 1-26.
— . 1945. Facultative parthenogenesis and haploidy in Epipactis latifolia.

K. Danske Vidensk. Selsk., Biol. Meddel. 19(11): 1-13.

1947. The spontaneous formation of haploid, polyploid and aneuploid

embryos in some orchids. K. Danske Vidensk. Selsk., Biol. Meddel. 20(9):

1-22.
Hakansson, A. 1943. Die Entwicklung des Embryosacks und die Befruchtung

bei Poa alpina. Hereditas 29: 25-61.

— . 1946. Untersuchungen iiber die Embryologie einiger Pote7itilla-Fovmen.

Lunds Univ. Arsskr. N.F. Avd. II, 42(5): 1-70.
Hodgson, R. W., and Cameron, S. H. 1938. Effects of reproduction by nucellar

embryony on clonal characteristics in Citrus. Jour. Hered. 29: 417-419.
Holmgren, J. 1919. Zytologische Studien iiber die Fortpflanzung bei den Gat-

tungen Erigeron und Eupatorium. K. Svenska Vet.-Akad. Handl. 59(7):

1-118.
Jeffrey, E. C., and Haertl, E. J. 1939. Apomixis in Trillium. Cellule 48: 79-88.
J0rgensen, C. A. 1928. The experimental formation of heteroploid plants in the

genus Solatium. Jour. Genet. 19: 133-211.
Juel, H. O. 1900. Vergleichende Untersuchungen iiber typische und partheno-

genetische Fortpflanzung bei der Gattung Antennaria. K. Svenska Vet.-
Akad. Handl. 33(5): 1-59.
Juliano, J. B. 1934. Origin of embryos in the strawberry mango. Philippine

Jour. Sci. 54: 553-556.

— . 1937. Embryos of carabao mango, Mangifera indica L. Philippine

Agr. 25: 749-760.
Kostoff, D. 1929. An androgenic Nicotiana haploid. Ztschr. f. Zellforsch. u.

Mikros. Anat. 9: 640-642.

APOMIXIS 341

Kostoff, D. 1941. The problem of haploidy (cytogenetic studies on Nicotiana

haploids and their bearings to some other cytogenetic problems). Bibl.

Genetica 13: 1-148.
Kuhn, E. 1930. Pseudogamie unci Androgenesis bei Pflanzen. Ziichter 2:

124-136.
Kusano, S. 1915. Experimental studies on the embryonal development in an

angiosperm. Jour. Col. Agr. Tokyo Imp. Univ. 6: 7-120.
Lebegue, A. 1949. Embryologie des Saxifragacees. Polyembryonie chez le Ber-
genia delavayi Engl. Bull. Soc. Bot. de France 96: 38-39.
Maheshwari, P., and Narayanaswami, S. 1950. Parthenogenetic development of

the egg in Spiranthes australis Lindl. (In press.)
Mauritzon, J. 1934. Die Embryologie einiger Capparidaceen sowie von Tovaria

pendala. Arkiv. for Bot. 26A 1-14.
Murbeck, S. 1901. Parthenogenetische Embryobilclung in der Gattung Al-

chemilla. Lunds Univ. Arsskr. N.F. Avd. II, 36(7): 1-41.
Nielsen, E. L. 1945. Cytology and breeding behavior of selected plants of Poa

pratensis. Bot. Gaz. 106: 357-382.
. 1946. The origin of multiple macrogametophytes in Poa pratensis.

Bot. Gaz. 108: 41-50.
Noack, K. 1939. Uber Hypericum-Rreuzxmgen. VI. Fortpflanzungs-verhalt-

nisse und Bastarde von Hypericum perforatum L. Ztschr. f. Inductive Abstain.

u. Vererbungsleshre 76: 569-601.
Okabe, S. 1932. Parthenogenesis bei Ixeris dentata. Bot. Mag. [Tokyo] 45:

518-532.
Osawa, I. 1912. Cytological and experimental studies in Citrus. Jour. Col.

Agr. Imp. Univ. Tokyo 4: 83-116.
— — . 1913. Studies on the cytology of some species of Taraxacum. Arch. f.

Zellforsch. 10: 450-469.
Pace, L. 1913. Apogamy in Atamosco. Bot. Gaz. 56: 376-394.
Pijl, L. v. 1934. Uber die Polyembryonie bei Eugenia. Rec. des Trav. Bot.

Neerland. 31: 113-187.
Poddubnaja-Arnoldi, V. 1933. Geschlechtliche und ungeschlechtliche Fort-

pflanzung bei einigen Chondrilla Arten. Planta 19: 46-86.
and Dianowa, V. 1934. Eine zytoembryologische Untersuchung einiger

Arten der Gattung Taraxacum. Planta 23: 19-46.
Rosenberg, O. 1907. Cytological studies on the apogamy in Hieracium. Bot.

Tidskr. 28: 143-170.
. 1930. Apogamie und Parthenogenesis bei Pflanzen (From Baur, E.,

and Hartman, M. "Handbuch der Vererbungswissenschaft." Berlin, 12:

1-66).
Rutishauser, A. 1943. Uber die Entwicklungsgeschichte pseudogamer Poten-

tillen. Arch. Julius Klaus-Stift. f. Vererbungsforsch. 18: 687-691.
Schnarf, K. 1919. Beobachtungen uber die Endospermentwicklung von
Hieracium aurantiacum. Sitzber. Akad. der Wiss. math, natur. Wien. Kl. I,
128: 1-17.

342 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Sears, P. B. 1922. Variation in cytology and gross morphology of Taraxacum.

Bot. Gaz. 73: 308-326.
Stebbins, G. L. 1932. Cytology of Antennaria. II. Parthenogenetic species.

Bot. Gaz. 94: 322-344.
and Jenkins, J. A. 1939. Aposporic development in the North American

species of Crepis. Genetica 21: 1-34.
Strasburger, E. 1878. tlber Polyembryonie. Jenaische Ztschr. f. Naturw. 12:

647-670.
Subba Rao, A. M. 1940. Studies in the Malpighiaceae. I. Embryo sac develop-
ment and embryogeny in the genera Hiptage, Banisteria and Stigmatophyllum.

Jour. Indian Bot. Soc. 18: 145-156.
Siissenguth, K. 1923. Tiber Pseudogamie bei Zygopetalum mackayi Hook. Ber.

deutsch. bot. Gesell. 41: 16-23.
Swamy, B. G. L. 1946. The embryology of Zeuxine sulcata Lindl. New Phytol.

45: 132-136.
. 1948. Post-fertilization development of Trillium undulatum. Cellule

52: 7-14.
Swingle, W. T. 1927. Seed production in sterile Citrus hybrids — its scientific

explanation and practical significance. Mem. Hort. Soc. N. Y. 3: 19-21.
Tahara, M. 1921. Cytologische Untersuchungen an einigen Compositen. Jour.

Col. Sci. Imp. Univ. Tokyo 43(7): 1-53.
Tiwary, N. K. 1926. On the occurrence of polyembryony in the genus Eugenia.

Jour. Indian Bot. Soc. 5: 124-136.
Webber, H. J., and Batchelor, L. D. 1943. "The Citrus Industry." Vol. I.

Berkeley.
Wiger, J. 1930. Ein neuer Fall von autonomer Nuzellarembryonie. Bot.

Notiser 1930, pp. 368-370.
Winkler, H. 1908. "tJber Parthenogenesis und Apogamie im Pflanzenreiche."

Jena.
. 1934. "Fortpflanzung der Gewachse." VII. Apomixis (In Hand-

worterbuch der Naturwissenschaft," 2d. ed. Jena. Vol. 4, pp. 451-461).

CHAPTER 10
POLYEMBRYONY

The phenomenon of polyembryony,1 i.e., the occurrence of more
than one embryo in a seed, has attracted much attention ever since
its initial discovery in the orange by Leeuwenhoek (1719). Ernst
(1918) and Schnarf (1929), who have reviewed the older literature,
classify it into two types— "true" and "false" — depending on
whether the embryos arise in the same embryo sac or in different
embryo sacs in the ovule. This classification, although useful, is
open to some objections. It has been pointed out that while those
cases in which two or more embryos are formed as a consequence of
the development of aposporic embryo sacs are here classed under
the "false" type, others showing adventive embryos — which also
originate from tissues outside the embryo sac — are classed as
"true." To obviate this difficulty, Gustafsson (1946) has proposed
that the term false polyembryony should be restricted to those cases
only in which two or more nucelli, each with its own embryo sac,
fuse at an early stage. All others are included under true poly-
embryony. Since the first is only a teratological condition, we
may confine our attention to true polyembryony only.

Cleavage Polyembryony. The simplest method of an increase in
the number of embryos is a cleavage of the zygote or proembryo
into two or more units. Although common in gymnosperms, its
occurrence is only sporadic in the angiosperms. Jeffrey (1895)
gave a detailed account of cleavage polyembryony in Erythronium
americanum (Fig. 194). After fertilization the synergids degenerate
and disappear, and the zygote divides to form a small group of cells,
which do not show any definite order or arrangement. This group
continues to increase in volume, and outgrowths arise at its lower
end which eventually function as independent embryos. The pro-
duction of two or three embryos from such an "embryogenic mass"

1 For more detailed treatments of this topic see Webber (1940), Gustafsson
(1946), and Maheshwari (1950).

343

344

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

was found to be a common feature, but sometimes as many as four
were distinguishable.

Following Jeffrey's discovery, which has been confirmed by
Guerin (1930) for another species of Erythronium, a similar pro-
liferation of the embryonic cells was reported in Tulipa gesneriana
(Ernst, 1901) and Limnocharis emarginata (Hall, 1902). In the
latter the first division of the zygote is transverse and results in the
formation of a large basal cell and a small terminal cell. As a rule,
the former increases in size without undergoing any further divi-
sions. In some cases, however, it ''divides and subdivides to form
an embryogenic mass" from which several embryos bud forth as

ABC

Fig. 191. Cleavage polyembryony in Erythronium arnericanum. A, Upper part
of embryo sac, showing the "embryogenic mass" formed from zygote. B,C,
proliferation of embryogenic mass to give rise to embryos. (After Jeffrey, 1895.)

in Erythronium. No older stages were seen, however, and it could
not be ascertained whether these embryos grow to full maturity.

Most other cases of cleavage polyembryony reported since then
have been in the nature of abnormalities. Cook (1902) noted an
embryo sac of Nymphaea advena showing twin embryos which he
considered to have originated by the "splitting of a very young
embryo." Later, the same author (1924) observed one instance of
two embryos, and another of four, at the micropylar end of the
embryo sac of Crotalaria sagittalis. Since the synergids are quite
ephemeral in this plant, these embryos are believed to have arisen
by a splitting of the single zygotic embryo. Samuelsson (1913)
also reported a similar splitting of the proembryo in Empetrum
nigrum, Guignard (1922) in Vincetoxicum nigrum, and Johansen
(1931) in Zauschneria latifolia. In Lobelia syphilitica (Crete, 1938)
frequently one and sometimes two additional embryos develop at the

POLYEMBRYONY

345

expense of the suspensor. In Nicotiana rustica Cooper (1943) noted
an ovule with two embryos of which the smaller had apparently
arisen as an outgrowth from the apex of the primary embryo.
Kausik and Subramanyam (1946) figure an embryo sac of Isotoma
longiflora in which an additional embryo seems to have budded out
from a suspensor cell.10

It is only in the family Orchidaceae that cleavage polyembryony
seems to be of more frequent occurrence. In Eulophia epidendraea,

Fig. 195. Polyembryony in Eulophia epidendraea; drawings made from whole
mounts of ovules. A, zygote has given rise to group of cells, three of which have
divided to form independent embryos. B, "bud" arising from right side of embryo.
C, two embryos presumed to have arisen by splitting of a single embryo; large
vacuolate cells belong to suspensor. (After Swamy, 1948.)

which may be cited as an example, Swamy (1943) records the
following variations: (1) the zygote divides irregularly to form a
mass of cells, of which those lying towards the chalazal end grow
simultaneously and give rise to multiple embryos (Fig. 195A); (2)
the filamentous proembryo becomes branched and each of the
branches grows into an embryo (Fig. 195C) ; (3) the proembryo gives
out small buds or outgrowths which may themselves function as
embryos (Fig. 1955).

ln It may be noted that some of the above-mentioned examples of cleavage
polyembryony may equally well be interpreted as cases of an intimate juxtaposition
of two embryos (see Fagerlind, 1944). For some reason, this possibility has not
been taken into account by most workers.

346 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Origin of Embryos from Cells of the Embryo Sac Other than the
Egg. Besides the zygotic embryo produced from the egg, embryos
may also be produced from other parts of the embryo sac. The
most common source is the synergids which frequently become egg-
like and may be fertilized by sperms from an additional pollen tube
(Fig. 116E) or develop without fertilization (Fig. 178). Several
cases of both kinds are known and have already been discussed in
Chaps. 6 and 9.

Production of embryos from antipodal cells is much rarer. Shat-
tuck (1905) noted that the antipodal cells of Ulmus americana often
present an egg-like appearance and in some cases he actually found
embryos in this position. Ekdahl (1941) has confirmed this in
U. glabra (Fig. 196 A-C), and Modilewski (1931), Mauritzon (1933),
and Fagerlind (1944) have figured similar cases in Allium odorum,
(Fig. 196D), Sedum fabaria, and Elatostema sinuatum eusinuatum
(Fig. 196.Z?). The further fate of these antipodal embryos has not
been determined, however, and it is not known whether they are
viable. It is to be noted that the egg-like antipodal cells which
have been recorded in some other plants like Plumbagella (Dahlgren,
1916) and Rudbeckia (Maheshwari and Srinivasan, 1944) have not
been observed to give rise to embryos. Nor have the laterally
situated egg-like cells of Penaea (Stephens, 1909), Plumbago (Dahl-
gren, 1937), Vogelia (Mathur and Khan, 1941), and Acalypha
(Maheshwari and Johri, 1941) ever shown such a behavior.

Embryos Arising from Cells outside Embryo Sac. The develop-
ment of embryos from the cells of the nucellus and integument has
already been considered in Chap. 9. Citrus, Eugenia, and Mangif-
era are well-known examples of this type of polyembryony. The
embryos, although initiated outside the embryo sac, subsequently
come to lie inside it and are nourished by the endosperm (Fig. 192).

Recent work indicates that the occurrence of adventive embryony
may not be a constant feature of all the individuals of a species.
Swamy (1948) has found that the orchid Spiranthes cernua com-
prises three races. The first shows normal sexual reproduction and
a single zygotic embryo is produced in each seed. In the second
race, which is apomictic, the male and female gametophytes are
both functionless. Fertilization does not occur but the cells of the
inner layer of the inner integument give rise to adventive embryos of
which two to six may mature in a seed. In the third race, which is of

POLY EM BRYONY

347

an intermediate type, some ovules of an ovary follow the course
outlined for the first race and others for the apomictic race.

Embryos Originating from Other Embryo Sacs in the Ovule. As
mentioned in the introductory paragraph, the polyembryonate

Fig. 196. Origin of embryos from antipodal cells. A,B, Ulmus glabra; embryo
sacs with egg-like antipodal cells. C, U. glabra, embryo sac, showing z}'gotic
embryo at micropylar end and antipodal embryo at chalazal end. (After Ekdahl,
1941.) D, Allium odorum; embryo sac, showing two embryos, one derived from
egg and the other from antipodal cell. (After Modilewski, 1931.) E, Elatostema
sinuatum eusinuatum; embryo sac with three antipodal embryos, of which one is
several-celled, another two-celled, and the third undergoing first division; note egg
embryo at upper end. (After Fagerlind, 1944-)

condition is sometimes due to the occurrence of multiple embryo
sacs within the ovule. These may arise (1) either from the deriva-
tives of the same megaspore mother cell, or (2) from two or more
megaspore mother cells, or (3) from nucellar cells (apospory).

348 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Bacchi (1943) sometimes found more than one embryo sac in an
ovule of Citrus resulting in two zygotic embryos, and Nielsen (1946)
has recorded the same in Poa pratensis.2

A Few Special Cases. In some plants multiple embryos are pro-
duced by the simultaneous operation of more than one of the
methods named above. One of the most interesting of them is
Allium odorum. Many years ago, Tretjakow (1895) and Hegel-
maier (1897) recorded the occurrence of synergid and antipodal
embryos in as many as one-third to one-half of the ovules of this
species. Later, Haberlandt (1923, 1925) reported that even in
castrated flowers there is an increase in the size of the ovules, accom-
panied by the production of embryos from several sources — egg,
synergids, antipodals, and the cells of the inner integument. He
found the diploid number of chromosomes in all the embryos.
Modilewski (1925, 1930, 1931), who made a further study of the
plant, found that it forms two kinds of embryo sacs, some haploid
and others diploid. In the diploid embryo sacs only the polar nuclei
are fertilized, resulting in a pentaploid endosperm; embryos arise
from the unfertilized but diploid egg and antipodal cells. In hap-
loid embryo sacs, on the other hand, viable embryos are formed only
after fertilization. In conclusion, four possibilities are mentioned:

1. In a haploid and normally fertilized embryo sac, embryos may
begin to develop from all cells of the embryo sac and even from the
adjacent integumentary cells, but only the zygotic embryo survives
so that the mature seeds contain a single embryo.

2. Embryos may also begin to form from one or more cells of the
haploid and unfertilized embryo sac, but owing to the lack of an
endosperm, which can arise only after triple fusion, their growth is
soon arrested and they become nonviable.

3. In a diploid but unfertilized embryo sac, any of its cells (also
the cells of the inner integument) may begin to form an embryo, but
eventually they all degenerate owing to the absence of an endosperm.

4. In diploid embryo sacs, in which the secondary nucleus is
fertilized, endosperm formation proceeds actively and all the cells
of the sac are capable of giving rise to embryos, but only the egg
embryo usually attains maturity.

2 Species of the genus Poa show other abnormalities also, for which a reference
may be made to the works of Tinney (1940), Engelbert (1941), Akerberg (1939, 1943),
and Hakansson (1943, 1944).

POLY EM BRYONY 349

Woodworth (1930) has called attention to the frequent occurrence
of polyembryonate seeds in Alnus rugosa. Meiosis was found to be
disturbed and only 2 to 3 per cent of the pollen grains were viable.
Pollen tubes were not observed, and bagged catkins produced per-
fectly normal and viable seeds, similar to those obtained from un-
bagged catkins. Embryo sac formation was not preceded by meio-
sis, and more than 50 per cent of the seeds showed diploid egg em-
bryos at the micropylar end of the ovule. A few seeds had an
embryo oriented in the opposite direction, suggesting its origin from
an antipodal cell. One ovule showed three embryos at the micro-
pylar end, two of which are believed to have originated from syner-
gids and the third from the egg. Nucellar embryony was frequent.
Some of the embryos were found sunken in the endosperm and are
believed to have originated from the cells of the latter. Several
ovules showed more than one embryo sac, each giving rise to one or
more embryos. From the occurrence of four to seven cotyledonary
buds on certain embryos, it further appeared that originally sepa-
rate embryos could sometimes fuse to form a composite structure.

Woodworth 's work, although of much interest, lacks the early
stages in embryonal development, and therefore some of his con-
clusions about the origin of the polyembryonate condition need
confirmation. In particular, his inference that endosperm cells
may also give rise to embryos must be regarded as very doubtful
(see page 334).

In Atraphaxis frutescens, Edman (1931) has described some
interesting cases of polyembryony. Haploid embryo sacs are pro-
duced only rarely and must be fertilized before they can give rise to
embryos. More often, meiosis fails and the embryo sacs are there-
fore diploid. Frequently two or more occur in the same ovule, and
each of them may produce an embryo. Figure 190C shows two
embryo sacs lying side by side, the left with two overlapping em-
bryos and the right with one embryo at the micropylar end. Figure
190 A shows a different condition, in which the second embryo sac
has arisen from a chalazal cell and is inversely oriented in relation
to the normal embryo sac. A similar but more advanced condition
is seen in Fig. 190£>i,B2, where a well-developed and nearly mature
embryo is present in the upper embryo sac, but the chalazal embryo
sacs and embryos failed to keep pace and are in the process of
obliteration. In addition, embryos of nucellar and apogamous

350

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

origin are also found in Atraphaxis. Of the two embryos seen in
the embryo sac on the left in Fig. 190C, one has obviously originated
from a synergid.

Fagerlind's (1944) recent study of some apomictic species of
Elatostema has also revealed some interesting features of a similar
nature. Briefly, the polyembryonate condition is due to one or
more of the following causes: (1) the occurrence of multiple embryo

A * G

Fig. 197. Polyembryony in Elatostema. A, E. eurhynchum; embryo sac with two
embryos, one arising laterally. B, upper part of embryo sac, showing three
embryos, two of which are lying somewhat horizontally. C,D, older ovules, show-
ing two embryos. E, ovule, showing two embryos at micropylar and one at chalazal
end. F, E. acuminatum; compound embryo sac formed by fusion of two sacs.
Embryo sac on left shows two well-developed embryos, and that on right shows two
smaller embryos. G, E. pedunculosum; two adjacent embryo sacs, each with
normally developed embryo at micropylar end. (After Fagerlind, 1944-)

sacs in the same ovule (Fig. 197 G), (2) the formation of embryos
from synergids and antipodal cells (Fig. 197 B-E), and (3) nucellar
budding (Fig. 197jP). Sometimes the separating wall between two
embryo sacs dissolves so that they form a common cavity (Fig.
197F), and if adjacent embryos come in close contact they may fuse
to form masses of tissue which defy analysis in later stages.

Twins and Triplets. When multiple seedlings arise in a species
in such a low frequency that it is not practicable to make a develop-
mental study of the embryogeny, there is a good deal of speculation

POLY EM BRYONY 351

about their origin. Of special interest are the diploid -diploid, hap-
loid-diploid, diploid-triploid, and haploid-triploid twins.3

Considering the diploid-diploid twins first, Randolph (1936) saw
two embryos lying parallel to each other in some kernels of Zea
mays, and the twin plants resulting from such kernels were found to
be genetically identical even in heterozygous stocks. He also saw
occasional seedlings with two plumules and a single radicle. Skov-
sted (1939) reported twins in Trifolium pratense, with both members
having a chromosome fragment, and in Medicago sativa, with both
having an extra chromosome. In all these cases the seedlings are
interpreted as having originated by the cleavage of a single embryo.
While this is probably true, it must be noted that cytologically simi-
lar diploid seedlings may also arise in other ways, the most frequent
source being the fertilization of more than one cell of the embryo
sac.4 Diploid-diploid twins may also arise from embryos produced
in two separate embryo sacs in an ovule or by nucellar budding.
Also, it is possible that sometimes a haploid cell of the embryo sac
may give rise to a diploid embryo by a process of "endoduplication"
of the chromosomes. A monozygotic origin may, therefore, be
assumed only when the seedlings are completely identical in all
essential respects.6

Haploid-diploid twins were reported by Kappert (1933) in Linum
usitatissimum, and by Ramiah et al. (1933, 1935) in Oryza sativa.
Subsequently, they have been recorded in several other plants,
viz., Triticum durum (Kihara, 1936), Solarium tuberosum, Phleum
pratense (Muntzing, 1937), Triticum vulgare (Kasparayan, 1938),
Secale cereale (Kostoff, 1939), Capsicum annuum (Christensen and
Bamford, 1943), Dactylis glomerata (Muntzing, 1943), and Gossyp-
ium barbadense (Harland, 1936; Webber, 1938; Skovsted, 1939;
Silow and Stephens, 1944). Kappert (1933) explained his twins of
Linum on the basis that the diploid member of the complex was
derived from the fertilized egg and the haploid member from an
unfertilized cell of the same embryo sac. Ramiah et al. (1933) also

3 See in this connection Webber's (1940) review of "polyembryony."

4 Except in bi- and tetrasporic embryo sacs, all the nuclei of an embryo sac are
genetically identical.

5 Identical seedlings may also arise if the egg and one synergid are fertilized by
the two male gametes discharged from the same pollen tube. Possibilities of such
an occurrence seem to be indicated in some orchids (Hagerup, 1947).

352 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

made the same interpretation but later (1935) considered it more
probable that the development of more than one embryo sac within
an ovule could also account for the origin of the twins. Harland
(1936) agreed with this view, adding that the fertilization of the
egg in one embryo sac might stimulate a parthenogenetic develop-
ment of the egg in the second and adjacent embryo sac. The re-
maining authors mentioned above fall in line with one or the other
of these explanations. Briefly then, in a case of haploid-diploid
seedlings, the haploid member is derived from an unfertilized cell
belonging either to the same embryo sac or to an adjacent embryo
sac.

Diploid-triploid twins have been reported in Triticum vulgare
(Yamamoto, 1936), Secede cereale (Kostoff, 1939), and a few other
plants (see especially Skovsted, 1939). According to Kostoff and
Yamamoto, the triploids observed by them arose from a part of the
endosperm, but this is merely a supposition without any positive
evidence in its favor. It is more likely that the triploid embryo
originated by the fertilization of an unreduced (aposporic) embryo
sac or by the fusion of a cell of a haploid embryo sac with two male
gametes or one unreduced male gamete.

Haploid-triploid seedlings are of comparatively rarer occurrence.
Nissen (1937) recorded one such case in Phleum. It is probable
that the haploid member arose from an unfertilized cell of the
embryo sac and the triploid member by one of the methods men-
tioned in the preceding paragraph.

While these are the probable ways in which twins and triplets
arise, it is often impossible to be sure of the exact origin of the
aberrant member or members of the combination, and it is unsafe
to make any categorical statements without taking all the possi-
bilities into consideration.

A particularly careful cytogenetic study of the multiple seedlings
of Asparagus officinale has recently been made by Randall and Rick
(1945). Of 405 multiple seedlings, 97 per cent were twins, 11 were
triplets, and one was a quadruplet. Diploids (2n = 20) were the
most frequent, but a few showed other chromosome numbers: 30
(triploid), 21 (trisomic), 10 (haploid), and 40 (tetraploid) . In
haploid-diploid pairs the haploid member was always much smaller
than its diploid partner, but aside from this combination the degree
of difference in size seldom gave any clue to the chromosome number
or the origin of the polyembryonate condition.

POLY EM BRYONY 353

The authors critically analyze the possible origins of the multiple
seedlings from a study of their chromosome numbers, stem color,
and distribution of sexes, and conclude that about one-fourth of the
diploid-diploid twin seedlings must have originated by a process of
cleavage polyembryony The remaining three-fourths are believed
to have arisen from two cells belonging either to the same embryo
sac or to two embryo sacs in an ovule. In addition a number of
"conjoined" twins were found, which showed varying degrees of
attachment to each other but were capable of developing into inde-
pendent plants. To explain their origin the following alternatives
are envisaged: (1) a partial fusion of two adjacent embryos, and
(2) an incomplete cleavage of one embryo. From the complete
identity in chromosome number and genetic characters between the
members of the conjoined type, it is concluded, however, that they
originated by an incomplete cleavage of a single initial embryo.

Conclusion. It may be said that polyembryony, although fairly
widespread in angiosperms, is much less common in them than it is
in the gymnosperms. The reason for this is that in the latter there
are several archegonia, but in the former there is only one cell in
the ovule (the egg) which is normally capable of giving rise to an
embryo. Sometimes, however, the proembryo may become sepa-
rated into two or more portions (cleavage polyembryony), or more
than one cell of the embryo sac may develop into an embryo. Less
frequently there may be two or more embryo sacs in an ovule, each
of which may give rise to embryos. A fourth source of poly-
embryony is the "budding" or proliferation of the cells of the nu-
cellus or integument (adventive embryony). The adventive em-
bryos are diploid and similar to one another as well as to the plant
from which they arise. Embryos produced by the cleavage of a
single zygote are also identical in all essential respects. Embryos
arising from two or more cells of one or separate embryo sacs may,
however, have the same or different chromosome numbers. Even
when developmental stages in embryogeny are not available, it is
possible in some cases to infer the mode of origin of the polyem-
bryonate condition on genetical evidence.

REFERENCES

Akerberg, E. 1939. Apomictic and sexual seed formation in Poa pratensis.

Hereditas 25: 359-370.
. 1943. Further studies of the embryo and endosperm development in

Poa pratensis. Hereditas 29: 199-201.

354 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Bacchi, 0. 1943. Cytological observations in Citrus. III. Megasporogenesis,

fertilization, and polyembryony. Bot. Gaz. 105: 221-225.
Christensen, H. M., and Bamford, R. 1943. Haploids in twin seedlings of pepper

Jour. Hered. 34: 99-104.
Cook, M. T. 1902. Development of the embryo sac and embryo of Castalia

odorata and Nymphaea advena. Bui. Torrey Bot. Club 29: 211-220.
. 1924. Development of seed of Crotalaria sagittalis. Bot. Gaz. 77:

440-445.
Cooper, D. C. 1943. Haploid-diploid twin embryos in Lilium and Nicotiana.

Amer. Jour. Bot. 30: 408-413.
Crete, P. 1938. La polyembryonie chez le Lobelia syphilitica L. Bull. Soc.

Bot. de France 85: 580-583.
Dahlgren, K. V. O. 1916. Zytologische und embryologische Studien tiber die

Reihen Primulales und Plumbaginales. K. Svenska Vet.-Akad. Handl.

56(4): 1-80.
. 1937. Die Entwicklung des Embryosackes bei Plumbago zeylanica.

Bot. Notiser 1937, pp. 487-498.
Edman, G. 1931. Apomeiosis und Apomixis bei Atraphaxis frutescens C. Koch.

Acta Horti Bergiani 11: 13-66.
Ekdahl, I. 1941. Die Entwicklung von Embryosack und Embryo bei Ulmus

glabra Huds. Svensk Bot. Tidskr. 35: 143-156.
Engelbert, V. 1941. The development of twin embryo sacs, embryos and endo-
sperm in Poa arctica R. Br. Canad. Jour. Res., Sect. C, Bot. Sci. 19: 135-144.
Ernst, A. 1901. Beitrage zur Kenntnis der Entwicklung des Embryosackes und

des Embryos (Polyembryonie) von Tulipa gesneriana L. Flora 88: 37-77.
. 1918. "Bastardierung als Ursache der Apogamie im Pflanzenreiche."

Jena.
Fagerlind, F. 1944. Die Samenbildung und die Zytologie bei agamospermischen

und sexuellen Arten von Elatostema und einigen nahestehenden Gattungen

nebst Beleuchtung einiger damit zusammenhangender Probleme. K. Svenska

Vet.-Akad, Handl. Ill, 21(4): 1-130.
Guerin, P. 1930. Le developpement de l'oeuf et la polyembryonie chez VEry-

thronium dens canis L. Compt. Rend. Acad, des Sci. Paris 191: 1369-1372.
Guignard, L. 1922. La fecondation et la polyembryonie chez les Yincetoxicum.

M6m. Acad. Sci. Inst. France II, 57: 1-25.
Gustafsson, A. 1946. Apomixis in higher plants. I. The mechanism of apo-
mixis. Lunds Univ. Arsskr. N.F. Avd. II, 42(3) : 1-67.
Haberlandt, G. 1923. Zur Embryologie von Allium odorum. Ber. deutsch. bot.

Gesell. 41: 174-179.
. 1925. Zur Embryologie und Cytologie von Allium odorum. Ber.

deutsch. bot. Gesell. 43: 559-564.
Hagerup, O. 1947. The spontaneous formation of haploid, polyploid, and aneu-

ploid embryos in some orchids. K. Danske Vidensk. Selsk., Biol. Meddel.

20(9): 1-22.
Hakansson, A. 1943. Die Entwicklung des Embryosackes und die Befruchtung

bei Poa alpina. Hereditas 28: 25-61.

POLYEMBRYONY 355

Hakansson, A. 1944. Erganzende Beitrage zur Embryologie von Poa alpina.

Bot. Notiser 1944, pp. 299-311.
Hall, J. G. 1902. An embryological study of Limnocharis emarginata. Bot. Gaz.

33: 214-219.
Harland, S. C. 1936. Haploids in polyembryonic seeds of Sea Island cotton.

Jour. Hered. 27: 229-231.
Hegelmaier, F. 1897. Zur Kenntnis der Polyembryonie von Allium odorum.

Bot. Ztg. 55: 133-140.
Jeffrey, E. C. 1895. Polyembryony in Erythronium americanum. Ann. Bot.

9: 'o37-541.
Johansen, D. A. 1931. Studies on the morphology of the Onagraceae. V.

Zauschneria latifolia, typical of a genus characterized by irregular embryology.

Ann. N. Y. Acad. Sci. 33: 1-26.
Kappert, H. 1933. Erbliche Polyembryonie bei Linum usitatissimum. Biol.

Zentbl. 53: 276-307.
Kasparayan, A. S. 1938. Haploids and haplo-diploids among hybrid twin seed-
lings in wheat. Compt. Rend. (Dok.) Acad, des Sci. U.R.S.S. 20: 53-56.
Kausik, S. B., and Subramanyam, K. 1946. A case of polyembryony in Isotoma

longiflora Presl. Current Sci. [India] 15: 257-258.
Kihara, H. 1936. A diplo-haploid twin plant in Triticum durum. Agri. and

Hort. [Tokyo] 11: 1425-1434.
Kostoff, D. 1939. Frequency of polyembryony and chlorophyll deficiency in

rye. Compt. Rend. (Dok.) Acad, des Sci. U.R.S.S. 24: 479-482.
Maheshwari, P. 1950. Polyembryony. Palaeobotanica, Salmi Memorial. Vol.
and Johri, B. M. 1941. The embryo sac of Acalypha indica L. Beihefte

bot. Centbl. 61A: 125-136.

and Srinivasan, A. R. 1944. A contribution to the embryology of Rud-

beckia bicolor Nutt. New Phytol. 43: 135-142.
Mathur, K. L., and Khan, R. 1941. The development of the embryo sac in

Vogelia indica Lamk. Proc. Indian Acad. Sci. Sect. B. 13: 360-368.
Mauritzon, J. 1933. "Studien uber die Embryologie der Familien Crassulaceae

und Saxifragaceae." Diss. Lund.
Modilewski, J. 1925. Zur Kenntnis der Polyembryonie von Allium odorum L.

Bull. Jard. Bot. de Kieff 2: 9-19.
— . 1928. Weitere Beitrage zur Embryologie und Cytologie von Allium-

Arten. Bull. Jard. Bot. de Kieff 7/8: 57-64.
. 1930. Neue Beitrage zur Polyembryonie von Allium odorum L. Ber.

deutsch. bot. Gesell. 48: 285-295.
. 1931. Die Embryobildung bei Allium odorum L. Bull. Jard. Bot. de

Kieff 12/13: 27-48.
Mlintzing, A. 1937. Polyploidy from twin seedlings. Cytologia, Fujii Jubl.

Vol., pp. 211-227.

— . 1943. Characteristics of two haploid twins in Dactylis glomerata. Here-

ditas 29: 134-140.
Nielsen, E. L. 1946. The origin of multiple macrogametophytes in Poa pratensis.

Bot. Gaz. 108: 41-50.

356 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Nissen, 0. 1937. Spalteapringenes storrelse has tvillingplanter med ulike kromo-

somtall. Bot. Notiser 1937, pp. 2S-34.
Ramiah, K., Parthasarthi, N., and Ramanujam, S. 1933. Haploid plant in rice

(Oryza sativa). Current Sci. [India] 1: 277-278.
f , and . 1935. Polyembryony in rice (Oryza sativa) . Indian

Jour. Agr. Sci. 5: 119-124.
Randall, T. E., and Rick, C. M. 1945. A cytogenetic study of polyembryony

in Asparagus officinalis L. Amer. Jour. Bot. 32: 560-569.
Randolph, L. F. 1936. Developmental morphology of the caryopsis in maize.

Jour. Agr. Res. 53: 881-916.
Samuelsson, G. 1913. Studien iiber die Entwicklungsgeschichte einiger Bicornes-

Typen. Svensk. Bot. Tidskr. 7: 97-188.
Schnarf, K. 1929. "Embryologie der Angiospermen." Berlin.
Shattuck, C. H. 1905. A morphological study of Ulmus americana. Bot. Gaz.

40:209-223.
Silow, R. A., and Stephens, S. G. 1944. "Twinning" in cotton. Jour. Hered.

35:76-78.
Skovsted, A. 1939. Cytological studies in twin plants. Compt. Rend, des

Trav. Carlsberg Lab. Ser. Physiol. 22: 427-446.
Stephens, E. L. 1909. The embryo sac and embryo of certain Penaeaceae.

Ann. Bot. 23:363-378.
Swamy, B. G. L. 1943. Gametogenesis and embryogeny of Eulophea epidendraea

Fischer. Proc. Natl. Inst. Sci. India 9: 59-65.

. 1948. Agamospermy in Spiranthes cernua. Lloydia 11: 149-162.

Tinney, F. W. 1940. Cytology of parthenogenesis in Poa pratensis. Jour. Agr.

Res. 60:351-360.
Tretjakow, S. 1895. Die Beteilung der Antipoden in Fallen der Polyembryonie

bei Allium odorum. Ber. deutsch. bot. Gesell. 13: 13-17.
Webber, J. M. 1938. Cytology of twin cotton plants. Jour. Agr. Res. 57:

155-160.

. 1940. Polyembryony. Bot. Rev. 6: 575-598.

Woodworth, R. H. 1930. Cytological studies in the Betulaceae. III. Parthe-
nogenesis and polyembryony in Alnus rugosa. Bot. Gaz. 89: 402-409.
Yamamoto, Y. 1936. Uber das Vorkommen von triploiden Pflanzen bei Mehr-

lingskeimlingen von Triticum vidgare Vill. Cytologia 7: 431-436.

CHAPTER 11
EMBRYOLOGY IN RELATION TO TAXONOMY

Although the existing systems of classification of angiosperms are
based mainly on the external characters of the flowers, fruits, and
seeds, it is now generally accepted that cytology, anatomy, em-
bryology, and genetics can contribute results of considerable sig-
nificance in a number of doubtful cases where floral morphology
alone has not proved adequate.

Many years ago Hofmeister and Strasburger indicated the possi-
bility of using embryological characters in taxonomy, but until the
invention of the microtome such investigations were confined to a
few skilled workers, and even their observations were not always
free from errors and misinterpretations. With the commencement
of the twentieth century, much higher standards were set in de-
scriptive embryology. A number of workers, particularly in Ger-
many and Sweden, began to discuss the bearings of their data on the
interrelationships of the families and genera which were being
studied by them. Unfortunately the preparation of the material is
a very time-consuming task, which requires a great deal of patience
and skill. The embryology of several families is therefore quite
unknown, and even with regard to others the existing data are often
quite fragmentary and inadequate. Enough has been done, how-
ever, to indicate that the embryological method has great possibili-
ties for the future (see especially Schnarf, 1933, 1937; Mauritzon,
1939; Maheshwari, 1945a,b; Just, 1946).

It is difficult to enumerate all the embryological features which
are of taxonomic significance, for almost every structure has been
shown to yield results of importance. The following characters
are, however, considered to be of major value in delimiting the larger
plant groups:

1. Anther tapetum. Whether it is of the glandular or the amoe-
boid type.

2. Quadripartition of the microspore mother cells. Whether it takes

358 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

place by furrowing or by the formation of cell plates, and whether
the mode of division is successive or simultaneous.

3. Development and organization of the pollen grain. Number and
position of the germ pores and furrows; adornments of the exine;
place of formation of the generative cell; number and shape of the
nuclei in the pollen grain at the time of its discharge from the
anther.

4. Development and structure of the ovide. Number of integuments
and the alterations in structure which they undergo during the
formation of the seed; presence or absence of vascular bundles in
the integuments; shape of the micropyle, whether it is formed by the
inner integument, or the outer, or both; presence or absence of an
obturator.

5. Form and extent of the nucellus. Whether it is broad and
massive or thin and ephemeral ; presence or absence of a hypostase ;
place of origin of the integument or integuments, whether close to
the apex of the nucellus or near its base; persistence or gradual
disappearance of the nucellus during seed formation.

6. Origin and extent of the sporogenous tissue in the ovule. Nature
of archesporium, whether it is one-celled or many-celled; presence
or absence of wall layers ; presence or absence of periclinal divisions
in the cells of the nucellar epidermis.

7. Megasporogenesis and development of the embryo sac. Arrange-
ment of megaspores; position of functioning megaspore; whether
the embryo sac is monosporic, bisporic, or tetrasporic; number of
nuclear divisions intervening between the megaspore mother cell
stage and the differentiation of the egg.

8. Form and organization of the mature embryo sac. Shape of the
embryo sac and the number and distribution of its nuclei ; persistence
or early disappearance of the synergids and antipodal cells ; increase
in number of antipodal cells, if any; formation of embryo sac caeca
or haustoria.

9. Fertilization. Path of entry of the pollen tube; interval be-
tween pollination and fertilization; any tendency toward a branch-
ing of the pollen tubes during their course to the ovule.

10. Endosperm. Whether it is of the Nuclear, Cellular, or
Helobial type; orientation of the first wall in those cases in which
it is Cellular; presence or absence of endosperm haustoria and the
manner in which they formed if present; nature of food reserves in

EMBRYOLOGY IN RELATION TO TAXONOMY 359

endosperm cells; persistence or gradual disappearance of endo-
sperm in the mature seed.

11. Embryo. Relation of the proembryonal cells to the body
regions of the embryo ; form and organization of the mature embryo ;
presence or absence of suspensor haustoria.

12. Certain abnormalities of development. Parthenogenesis; apog-
amy, adventive embryony, polyembryony etc.

An evaluation of the characters mentioned above has been of
considerable service in the determination of the proper position of
several difficult groups and subgroups, and sometimes it has given
a new orientation to our ideas of their affinities. Without going
into details, for which a reference may be made to the work of
Mauritzon (1939), the following selection is offered as an illustration.

Empetraceae. Don (1827), who first erected the group "Empe-
treae," considered it to be so different from the Ericaceae that he
rejected any possibility of a close alliance between them. He be-
lieved instead that the Empetreae was more closely related to the
Euphorbiaceae and the Celastraceae. "The Euphorbiaceae and
Empetreae agree in the imbricate aestivation of the calyx, in the
stamens being opposite to the divisions of the calyx, and both of
these being of an equal and definite number; in having bilocular
anthers ; in their superior ovarium ; in the plurality of styles ; in their
divided stigmas; and lastly in the arrangement of the ovula, and the
presence of a copious albumen." He went so far as to say that
whether the Empetreae was to be considered as a section of the
Euphorbiaceae or a separate family allied to the latter was a matter
of individual taste.

Bentham and Hooker (1880) felt less sure about a relationship
between the Euphorbiaceae and Empetraceae and assigned the
latter to their or dines anomali under the Monochlamydeae. Shortly
afterwards, Pax (1896) also denied that the Empetraceae showed
any recognizable affinities with either the Ericaceae or the Euphor-
biaceae. The floral structure and in particular the structure of the
ovules left no doubt in his opinion that it was to be placed in the
order Sapindales close to the Celastraceae1 and Buxaceae.

However, Agardh (1858), Gray (1858), Baillon (1892), and Hallier
(1912) considered the Empetraceae to be related to the Ericaceae,

1 Among recent writers Hutchinson (1948) still thinks that the Empetraceae has
its nearest relatives in the Celastrales.

360

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

regarding it as a reduced apetalous and polygamous or dioecious
derivative of the latter.

That this last view is the correct one and that the Empetraceae
is to be classed under the Ericales have now been definitely estab-
lished on the basis of the embryological data brought forward by
Samuelsson (1913). This order is characterized by the following

Fig. 198. Development of endosperm in Empetrum nigrum. A, embryo sac at
time of fertilization. B, two-celled stage of endosperm. C, four-celled endosperm.
D, more advanced stage, showing differentiation of micropylar and chalazal haus-
toria. E, l.s. nearly mature seed. (After Samuelsson, 1913.)

well-marked embryological features: (1) absence of a fibrous layer
in the anthers ; (2) anther tapetum of the glandular type with multi-
nucleate cells; (3) microspores remaining together in tetrads (Fig.
97/) ; (4) pollen grains two-celled ; (5) unitegmic ovules with a thin
nucellus, which soon disappears so that the embryo sac comes in
direct contact with the integumentary tapetum; (6) absence of

EMBRYOLOGY IN RELATION TO TAXONOMY 361

parietal cells in the ovule; (7) embryo sac monosporic and eight-
nucleate, broader at the micropylar end and narrower at the chala-
zal; (8) a fluted hollow style, which connects the lumen of the ovary
with the outside and along which the pollen tubes make their way
into the ovary; (9) endosperm cellular; the first two divisions trans-
verse, giving rise to a row of four superposed cells (Fig. 198A-C);
(10) formation of endosperm haustoria at both ends of the embryo
sac, micropylar as well as chalazal (Fig. 198D, E); (11) a single-
layered seed coat formed from the outermost layer of the integu-
ment, the remaining layers becoming absorbed during the growth
of the endosperm and embryo; (12) seeds albuminous with fleshy
endosperm and straight embryo.

All these are perfectly standard stages in Ericean embryology,
and their combination is quite unknown in any other order. The
Empetraceae show a close correspondence in all respects, while the
Sapindales and Celastrales differ in so many ways that there is no
doubt as to the correctness of Samuelsson's view.10

Lennoaceae. On the basis of his morphological studies on the
Lennoaceae, Solms-Laubach (1870) felt convinced that it belonged
to the Ericales, and Hutchinson (1926) accepted this disposition of
the group. But there were certain points in Solms-Laubach 's own
descriptions which seemed to militate against this view, and conse-
quently Engler and Gilg (1924) removed the Lennoaceae to the
order Tubiflorae and placed it in the neighborhood of the Boragi-
naceae. This received support from Siissenguth's (1927) anatomi-
cal and morphological study of Lennoa, and more recently Copeland
(1935) has also expressed his agreement with it on the basis of his
work on Pholisma. As remarked by Copeland, at the very outset
the equality in number of their stamens and corolla lobes (con-
trasted with the obdiplostemony of the Ericales) , alternate arrange-
ment of the floral members, adnation of the filaments to the corolla,
and dehiscence of the anthers by longitudinal slits, form weighty
objections against an assignment of the Lennoaceae to the Ericales.
Further, certain other characters possessed by the Lennoaceae,
viz., their short and solid style, normally developed endothecium,
separate pollen grains, and multilayered seed coat, render its

10 See also Hagerup (1946) who has confirmed this opinion as the result of his
morphological studies on the Empetraceae.

362 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

assignment to the Bicornes completely untenable and make it seem
more probable that its correct place is under the Tubiflorae as a
separate suborder occupying a primitive position.

Cactaceae. The members of this family present a motley assem-
blage with every variation from the leafy Pereskia to the tall ribbed
columns of Pachycereus, the flat joints of Opuntia, the phylloclades
of Epiphyllum, and the tubercled spheres of Mammillaria. There
has been considerable divergence of opinion as to its relationships.
Wettstein (1935) assigns it to the Centrospermae ; Engler and Diels
(1936) to a separate order Opuntiales near the Passifloraceae ;
Warming (1904) to the order Cactales following the Centrosper-
males; and Hutchinson (1926) to the same order but next to the
Cucurbitales (Cucurbitaceae, Begoniaceae, Datiscaceae, and Cari-
caceae).

Practically no embryological work had been done on the Cacta-
ceae until the publication of the papers of Mauritzon (1934) and
Neumann (1935) on Rhipsalis and Pereskia respectively. Although
additional data are desirable, Wettstein 's views have received very
definite support from the observations of these authors.16 The
Cactaceae agree with the rest of the Centrospermae in possessing
the following embryological characters: (1) anther tapetum glandu-
lar and its cells two- to four-nucleate; periplasmodium absent; (2)
divisions of microspore mother cells simultaneous; (3) pollen grains
trinucleate; (4) ovules campylotropous with strongly curved and
massive nucelli; (5) micropyle formed by the swollen apex of the
inner integument which protrude out and approach the funiculus;
(6) a hypodermal archesporial cell which cuts off a wall cell; (7)
formation of a nucellar cap arising from periclinal divisions of the
cells of the nucellar epidermis (Fig. 199C); (8) functioning of the
chalazal megaspore of the tetrad; (9) formation of a monosporic
eight-nucleate embryo sac;2 (10) functioning of the perisperm as the
chief storage region.3

An additional point of considerable interest is the occurrence of a

16 See also Buxbaum (1944, 1948, 1949) who agrees that the Cactaceae are
closely allied to the Aizoaceae and should be placed under the Centrospermales.

2 Archibald (1939) reports an Allium type of embryo sac in Opuntia aurantiaca,
but her figures are not convincing and it seems probable that the development is
really of the Polygonum type as in the other Cactaceae.

3 The only exception in this respect is the family Thelygonaceae, in which the
endosperm forms the chief storage tissue (Woodcock, 1929).

EMBRYOLOGY IN RELATION TO TAXONOMY

363

minute air space between the two integuments in the chalazal region
of the ovules of Pereskia (Fig. 199.4) and Opuntia (Neumann, 1935;
Archibald, 1939). This has also been noted since then in several
members of the Centrospermae. A radial elongation of the terminal
cells of the nucellar epidermis, followed by some periclinal divisions
in later stages (Fig. 199(7), is common to the Aizoaceae and Cac-
taceae. Further, some significant similarities of an anatomical
nature have recently been recorded between the spiny or scaly

Fig. 199. Ovary and ovule of Pereskia amapola. A, l.s. young ovule, showing a
prominent air space I between inner and outer integuments. B, l.s. pistil showing
stylar canal. C, l.s. upper lart of ovule, showing nucellar cap and micropyle formed
by inner integument. (After Neumann, 1935.)

emergences of Pereskia and Rhipsalis (Cactaceae) and Anacamp-
seros (Portulacaceae).4 Present evidence, therefore, seems to be
entirely in favor of regarding the Cactaceae as a sort of bridge be-
tween the Aizoaceae and Portulacaceae. Among specific simi-
larities between the Portulacaceae and Cactaceae, Neumann (1935)
cites the following: (1) microspore mother cells forming one to two
(only rarely more than two) rows in the anther loculus; (2) micro-
pyle in close proximity to the funiculus; (3) T-shaped tetrad of

4 For details, see Chorinsky (1931).

364 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

megaspores; (4) long synergids; and (5) embryo sac constricted in
the middle portion. On the other hand, if we turn to the Passi-
floraceae, we find a number of striking differences, for here (1) the
nucellar epidermis does not undergo any periclinal divisions; (2) it
is usually the uppermost megaspore which functions, not the cha-
lazal; (3) the ovule is completely anatropous with a perfectly sym-
metrical nucellus ; and (4) the outer integument grows up to a higher
level than the inner and takes part in the formation of the micro-
pyle (see Schnarf, 1931).

Garryaceae. The systematic position of the Garryaceae has
been disputed for a long time. Engler and Gilg (1924) placed it
among the most primitive families of the dicotyledons, and the
same course has been adopted by Engler and Diels (1936) in the
latest edition of the "Syllabus der Pflanzenfamilien." Others, like
Bentham and Hooker (1880) and Wangerin (1910), have taken a
different view and assigned it to the highest of the Archichlamydeae
placing it close to the Cornaceae.

Hallock's (1930) work on the morphology and embryology of
Garry a elliptica necessitates a fresh appraisal of the situation. She
reports as follows:

1. The staminate flower, although apparently monochlamy-
deous, invariably shows the primordia of the sepals in the earlier
stages of its development.

2. The pistillate flower is not naked. The two whorls of alter-
nately arranged "bractlets" or "folioles" seen on the top of the
ovary represent the reduced perianth lobes (the members of the
outer whorl are thick and green, and those of the inner are more
delicate and petal-like). The presence of a vascular supply in these
structures and the definite position which they occupy with respect
to each other and to the pistil supports this view, and from this fact
it further follows that the ovary is inferior and the flower parts
epigynous and not hypogynous.

3. The integument, described as "complete or incomplete"
(Engler and Gilg, 1924), is a thick and massive structure. It is only
in later stages that it begins to be consumed by the embryo and
therefore appears to be "incomplete."

4. The nucellus is thin and ephemeral, disappearing completely
at the sides of the embryo sac.

5. There is a single archesporial cell which divides to form a wall
cell and the megaspore mother cell.

EMBRYOLOGY IN RELATION TO TAXONOMY 365

6. The mature embryo sac is eight-nucleate and arises from the
chalazal cell of a linear tetrad of megaspores.

7. The endosperm becomes cellular at a very early stage.

8. The fertilized egg forms a long, tubular, somewhat vermiform
structure designed to penetrate rather deeply into the endosperm.

From a consideration of the sum total of these characters, Hallock
concludes that the Garryaceae are not primitive but must be con-
sidered as the highest of the Umbelliflorae immediately preceding
the Sympetalae. She further suggests that the dioeciousness of
Garry a may also be a derived rather than a primitive feature.

One point which has not been satisfactorily settled by Hallock
but which is nevertheless of considerable importance, is the nature
of the rudimentary haustorial structures observed by her at both
ends of the endosperm. She considers them to be derived from the
synergids and the antipodal cells, but her illustrations do not seem
to prove this interpretation. It is more likely that they really
originate from the micropylar and chalazal cells of the endosperm.
If this interpretation turns out to be correct, it would form one of
the strongest arguments in favor of the advanced position of the
Garryaceae.

Onagraceae. The family Onagraceae affords one of the best
examples of the utility of embryological characters in taxonomic
considerations. An unfailing characteristic of this family, ocurring
in every genus and species so far investigaged, is the peculiar raono-
sporic four-nucleate embryo sac, consisting of an egg apparatus and
single polar nucleus. The only exception is the genus Trapa. This
has been placed variously by different systematists : (1) under the
Onagraceae; (2) as an appendix to the Onagraceae; (3) as an iso-
lated member of the Halorrhagidaceae ; (4) as the only genus of a
separate family, Hydrocaryaceae or Trapaceae.

Embryological evidence strongly favors the last view. In addi-
tion to its eight-nucleate embryo sac, Trapa has a well-developed
suspensor haustorium (Fig. 200), both these features being unknown
in any member of the Onagraceae. Further, in the Onagraceae the
ovary is inferior and tetralocular, with axile placentae bearing nu-
merous ovules, and the fruit is generally a loculicidal capsule. In
Trapa, on the other hand, the ovary is semi-inferior and bilocular,
with only one ovule in each chamber, and the fruit is a large one-
seeded5 drupe whose fleshy layer soon disappears, leaving only the

6 Of the two ovules in the ovary, one aborts at an early stage in its development.

366

INTRODUCTION TO EMBRYOLOGY OF ANGIOSFERMS

stony endocarp with two to four upwardly directed prongs repre-
senting the persistent sepals.

Callitrichaceae. The genus Callitriche comprises about 25 species,
which are extremely reduced in both vegetative and floral struc-
ture. The male flower consists of a single terminal stamen, and the

CD E G<

Fig. 200. Development of embryo sac and embryo in Trapa. A, megaspore
mother cell in synizesis. B, same, undergoing first meiotic division. C, tetrad
of megaspores. D, two-nucleate embryo sac with three nonfunctioning megaspores.
E, upper part of mature embryo sac. F,G, two consecutive sections through lower
part of embryo sac, showing hypertrophied and degenerating antipodal nuclei.
(After Ishikawa, 1918.) H, diagrammatic l.s. of young seed, showing two integu-
ments, massive suspensor, and embryo. (After Tison, 1919.)

female of a short-stalked bicarpellary ovary situated between a
pair of delicate bracteoles. Each cell of the ovary becomes divided
by a false septum, and there is a single pendulous anatropous ovule
in each of the four loculi. There is a pair of long styles placed
transversely like the carpels.

The exact position of Callitriche has always been considered
doubtful. According to Bentham and Hooker, R. Brown, De
Candolle, and Hegelmaier and Hutchinson, it is related to the
Halorrhagidaceae ; Clarke (1865) recommended that it should be
placed under the Caryophyllaceae ; and Baillon (1858) included i>

EMBRYOLOGY IN RELATION TO TAXONOMY

367

under the Euphorbiaceae. Pax and Hoffman (1931) are in general
agreement with Baillon's views but consider that the best course is
to assign it to a separate family Callitrichaceae, placed close to the
Euphorbiaceae.

The work of J0rgensen (1923, 1925) has, however, revealed a
combination of embryological characters in Callitriche which make

E B C D

Fig. 201. Some stages in development of embryo sac and endosperm of Callitriche.
A, l.s. young ovule, showing reduced nucellus, single integument, and row of three
cells comprising two megaspores and binucleate dyad cell. B, twelve-celled stage
of endosperm; only six cells seen in section. C, later stage, showing laying down of
micropylar and chalazal haustoria. D,E, portions of embryo sacs, showing rnicro-
pylar and chalazal haustoria. (After Jtfrgensen, 1923.)

all of these assignments seem very unlikely. The ovule is tenui-
nucellate and has a single massive integument (Fig. 201 A); the
endosperm is cellular (Fig. 201B) and forms well-developed haus-
toria (Fig. 201C-E). These features are so characteristic of the
Tubiflorae that they suggest a closer relationship of the Callitrich-
aceae with the Labiatae or Verbenaceae than with any of the families
named in the preceding paragraph.6 In the absence of adequate

• This view also receives support from the structure of the fruit and the gyno-
basic style of CaUiiriche.

368 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

anatomical and cytological data, J0rgensen wisely refrains from
committing himself further but concludes that in any case the
present assignment of the Callitrichaceae to the Geraniales is open
to serious objection.

Liliaceae-Allioideae. The extensive studies made on the family
Liliaceae by Schnarf, Stenar, and other embryologists have given
some new orientations to our ideas of the interrelationships between
the subfamilies and tribes included under it. Considering the sub-
family Allioideae, Krause (1930) lias divided it into four tribes,
viz., Agapantheae, Allieae, Gillesieae, and Miluleae. The embryol-
ogy of the last two tribes is relatively unknown and they will there-
fore be left out of consideration, but Stenar 's (1933) work on the
Agapantheae and Allieae indicates a much closer relationship be-
tween them than was previously anticipated. Indeed, as Stenar
says, Agapanthus and Tulbaghia are only South African Allieae
with a rhizome instead of a bulb, and in Tulbaghia the resemblance
extends even to the possession of the leek-like odor characteristic of
Allium. Regarding the relative positions of these two genera,
Agapanthus (ovule anatropous, parietal cells present, embryo sac
of Polygonum type) is to be regarded as the more primitive, and
Tulbaghia (ovule hemianatropous, parietal cells absent, embryo sac
of Allium type) as relatively advanced. The latter connects with
Nothoscordum which also has hemianatropous ovules devoid of
parietal tissue. Here the development of the embryo sac may be
of the Allium type (N. fragrans) or the Polygonum type (N.
striatum).

Gagea, which was believed to have an embryo sac of the Adoxa
type (Stenar, 1927), used to be considered as the most advanced
member of the Allieae, but further work done on this genus revealed
that the embryo sac is of the Fritillaria type (see Maheshwari,
1946). The question arises, therefore, as to whether it should be
retained in the Allieae or transferred to the Lilioideae, where the
Fritillaria type is of general occurrence. In support of the second
alternative it may be added that even on other grounds Baillon
(1894) considered Gagea to be closely allied to Tulipa, which is a
member of the Lilioideae, and this assignment has been accepted by
Hutchinson (1934, 1948). 7

7 See also Schnarf (1948).

EMBRYOLOGY IN RELATION TO TAXONOMY

369

Liliaceae-Asphodeloideae. According to Krause (1930) the sub-
family Asphodeloideae comprises the following tribes: Asphodeleae,
Hemerocallideae, Aloeae, Aphyllantheae, Johnsonieae, Dasypogo-
neae, Lomandreae, and Calectasieae. The Asphodeleae is further
subdivided into Asphodelinae, Anthericinae, Chlorogalinae, Odonto-
stominae, Eriosperminae, Xeroneminae, and Dianellinae. To the
Asphodelinae belong the genera Asphodelus, Asphodeline, Paradisia,
Diuranthera, and Eremurus; and to the Anthericinae belong Bul-
binella, Bulbine, Bulbinopsis, Anemarrhena, Terauchia, Simethis,
Debesia, Anthericum, Alectorurus, Chlorophytum, Verdickia, Eremo-
crinum, Thysanotus, Dichopogon, Arthropodium, and a few others.
Although only a few genera under these two subtribes have so far
been investigated, the information available at present may be
summarized as shown in the accompanying table.

Asphodelinae

Ovules orthotropus or hemitropous

Aril present

Division of microspore mother
cells simultaneous

Embryo sac does not produce any
haustorial outgrowth

Anthericinae

Ovules typically anatropous

Aril absent

Division of microspore mother
cells successive

Embryo sac produces a lateral
haustorium

Now, Paradisia (Stenar, 1928a), which has been placed by Krause
under the Asphodelinae, has no aril, the divisions of its microspore
mother cells are of the successive type, and its embryo sac forms a
haustorium similar to that of Anthericum (Schnarf, 1928). In Bul-
bine, on the other hand, which has been placed under the Antheri-
cinae, there is a clear and well -developed aril, the divisions of the
microspore mother cells are of the simultaneous type, and there is no
embryo sac haustorium. The case for an interchange of the posi-
tions of these two genera is therefore quite evident, and in a revised
classification of the Asphodeloideae, Paradisia should be placed
under the Anthericinae and Bulbine under the Asphodelinae.

In a more recent paper Schnarf and Wunderlich (1939) go still
further and emphasize that on embryological grounds the Aspho-

370 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

delinae and Anthericinae may not be placed even under a common
tribe but ought each to be given the status of an independent tribe.
In support of this view is cited the work of Bouvier (1915), who
reports several anatomical differences also between the Asphodelinae
and Anthericinae.

Fig. 202. Diagram showing comparison of the more important embryological
features of Asphodelinae and Aloineae (A-F) with those of Anthericinae (G—L).
A, Asphodelinae- Aloineae; simultaneous division of microspore mother cell. B,
nucellus, showing megaspore mother cell and parietal cell. C, tetrad of megaspores.
D, mature embryo sac. E, embryo sac, showing Helobial endosperm. F, l.s.
ovule, showing aril. G, Anthericinae; successive division of microspore mother
cell. H, nucellus, showing megaspore mother cell and parietal cell. /, tetrad of
megaspores. J, mature embryo sac, showing lateral haustorium. K, embryo sac,
showing Helobial endosperm. L, l.s. ovule; note absence of aril. (After Schnarf,
1944-)

On the other hand, embryology indicates such a close alliance
between the Asphodelinae (Asphodelus, Asphodeline, Eremurus,
Bulbine, Bulbinella, Bulbinopsis, and Alectorurus) and the Aloineae
(Haworthia, Gasteria, Aloe, Kniphofia, Apicra, and Lomatophyllum)
that both of them may well be united into the same subfamily or
tribe (Schnarf, 1944). The following characters are common to

EMBRYOLOGY IN RELATION TO TAXONOMY 371

both: (1) divisions of the microspore mother cells of the simul-
taneous type; (2) generative cell cut off exactly opposite to the
furrow in the pollen grain; (3) ovule crassinucellate with two integu-
ments and an aril; (4) megaspore mother cell separated from the
nucellar epidermis by parietal cells; (5) embryo sac of Polygonum
type; and (6) endosperm of the Helobial type (demonstrated in
Gasteria and Kniphofia and inferred in Aloe from the position of the
primary endosperm nucleus). The more important differences
between the Asphodelinae-Aloinae and the Anthericinae are pre-
sented in Fig. 202.

Liliaceae-Lilioideae. Of the genera included under the subfamily
Lilioideae (Krause, 1930), Lilium, Fritillaria, Tulipa, Lloydia, and
Erythronium have been shown to have a tetrasporic embryo sac,
usually of the Fritillaria type; Nomocharis is uninvestigated. Only
Calochortus has been found to have an embryo sac of the monosporic
eight-nucleate type (Cave, 1941). It is interesting to note that this
genus also differs from the remaining members of the Lilioideae in
some other respects. The fruit of Calochortus is a septicidal capsule,
while that of the other genera is loculicidal. In Calochortus the
chromosome numbers are 6, 7, 8, 9 and 10 (Newton, 1926; Beal,
1939; Owenby, 1940), while in the remaining genera the number is
usually 12. On the basis of these and some other differences in the
structure and germination of the seed, Buxbaum (1937a, o) proposed
that Calochortus should be transferred to an independent subfamily
under the Liliaceae. This opinion, which is strongly supported by
the embryological data brought forward by Cave (1941), may now
be accepted without reservation.8

Conclusion. There are several other notable examples of the aid
which embryology has rendered in the solution of taxonomical
problems, but it is unnecessary to cite all of them here. What
has already been written is sufficient evidence of the value of em-
bryological data" in an elucidation of the interrelationships of fam-
ilies and genera. Although it is not claimed that these data will
always prove important, they should form a part of any thorough
taxonomic analysis. It is worth while, therefore, for every student
of embryology to try to assess the bearing of his observations and
those of his predecessors on the taxonomic position of the group of
plants which he is studying. Many of his conclusions are bound to

"See also Schnarf (1948).

372 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

be of a tentative nature and will have to be stated with due caution,
but every contribution, however small, will be one step nearer
to the final goal, and occasionally there will emerge fresh ideas and
new viewpoints which will more than compensate for the effort
expended upon the work.

References

Agarclh, J. G. 1858. "Theoria Systematis Plantarum." Lund.

Archibald, E. E. A. 1939. The development of the ovule and seed of jointed

cactus (Opuntia aurantiaca Lindley). South African Jour. Sci. 36: 195-211.
Baillon, H. 1858. Recherches sur l'organogenie du Callitriche et sur ses rapports

naturels. Bui. Soc. Bot. de France 5: 337.

. 1892. "Histoire des plantes." Vol. 11. Paris.

. 1894. "Histoire des plantes." Vol. 12. Paris.

Beal, J. M. 1939. Cytological studies in relation to the classification of the

genus Calochortus. Bot. Gaz. 100: 528-547.
Bentham, G., and Hooker, J. D. 1862-1883. "Genera plantarum." London.
Bouvier, W. 1915. Beitrage zur vergleichenden Anatomie der Asphodeloideae.

Denkschr. Akad. der Wiss. Wien, Math.-Nat. Kl. 91: 539-577.
Brown, R. 1814. "General Remarks on the Botany of Terra Australis." London.
Buxbaum, F. 1937a. Die Entwicklungslinien der Lilioideae. I. Die Wurm-

baeaoideae. Bot. Arch. 38: 213-293.
. 19376. Die Entwicklungslinien der Lilioideae. II. Die systematische

Stellung der Gattung Gagea. Bot. Arch. 38: 305-398.
. 1944. Untersuchungen zur Morphologie der Kakteenbltite. I. Das Gy-

naecium. Bot. Arch. 45: 190-247.

— . 1948. Zur Klarung der phylogenetischen Stellung der Aizoaceae und

Cactaceae im Pflanzenreich. Jahrb. d. schweiz. kakt. Ges. 2: 3-16.
. 1949. Vorlaufer des Kakteen-Habitus bei den Phytolaccaceen. Osterr.

bot. Ztschr. 96: 5-14.
Cave, M. S. 1941. Megasporogenesis and embryo sac development in Calo-
chortus. Amer. Jour. Bot. 28: 390-394.
Chorinsky, F. 1931. Vergleichend-anatomische Untersuchung der Haargebilde

bei Portulacaceen und Cactaceen. Osterr. bot. Ztschr. 80: 308-327.
Clarke, B. 1865. On the structure and affinities of Callitrichaceae. Jour. Bot. 3.
Copeland, H. F. 1935. The structure of the flower of Pholisma arenarium.

Amer. Jour. Bot. 22: 366-383.
Don, D. 1827. On the affinities of the Empetreae, a natural group of plants.

Edinburgh New Phil. Jour. 2: 59-63.
Engler, A., and Diels, L. 1936. "Syllabus der Pflanzenfamilien." 11th ed.

Berlin.
- and Gilg, E. 1924. "Syllabus der Pflanzenfamilien." 9th and 10th eds.

Berlin.
Gray, A. 1858. "Manual of the Botany of the Northern United States." New

York.

EMBRYOLOGY IN RELATION TO TAXONOMY 373

Hagerup, 0. 1946. Studies on the Empetraceae. K. Danske Vidensk. Selsk.,

Biol. Meddel. 20(5): 1-49.
Hallier, H. 1912. L'origine et le systeme phyl^tique des Angiospermes ex-
poses a l'aide de leur arbre g£n6alogique. Arch. Neerland. Ill B. 1
Hallock, F. A. 1930. The relationship of Garrya. The development of the

flowers and seeds of Garrya and its bearing on the phylogenetic position of the

genus. Ann. Bot. 44: 771-812.
Hegelmaier, F. 1867. Zur systematik von Callitriche. Verhandl. Bot. Ver. f.

der Brandenb. 9: 1.
Hutchinson, J. 1926. "The Families of Flowering Plants. I. Dicotyledons."

London.
. 1934. "The Families of Flowering Plants. II. Monocotyledons."

London.

. 1948. "British Flowering Plants." London.

Ishikawa, M. 1918. Studies on the embryo sac and fertilisation in Oenothera.

Ann. Bot. 32: 279-317.
J0rgensen, C. A. 1923. Studies on Callitrichaceae. Bot. Tidskr. 38: 81-126.
. 1925. Zur Frage der systematischen Stellung der Callitrichaceen.

Jahrb. f. wiss. Bot. 64: 440-442.
Just, Th. 1946. The use of embryological formulas in plant taxonomy. Bui.

Torrey Bot. Club 73: 351-355.
Krause, K. 1930. Liliaceae (In Engler and Prantl's "Natiirlichen Pflanzen-

familien, 2d ed. 15A: 227-390).
Maheshwari, P. 1945a. The place of angiosperm embryology in research and

teaching. Jour. Indian Bot. Soc. 24: 25-41.
. 1945&. Embryology of angiosperms as a field for research. Nature

[London] 156: 354-355.
— ■ . 1946. The Fritillaria type of embryo sac: a critical review. Jour.

Indian Bot. Soc. M. O. P. Iyengar Comm. Vol., pp. 101-119.
Mauritzon, J. 1934. Ein Beitrag zur Embryologie der Phytolaccaceen und

Cactaceen. Bot. Notiser 1934, pp. 111-135.
. 1939. Die Bedeutung der embryologischen Forschung fiir das naturliche

System der Pflanzen. Lunds Univ. Arsskr. N.F. II, 35(15): 1-70.
Neumann, M. 1935. Die Entwicklung des Pollens, der Samenanlage und des

Embryosackes von Pereskia amapola var. argentina. Osterr. bot. Ztschr.

84: 1-30.
Newton, W. C. F. 1926. Chromosome studies in Tulipa and some related

genera. Jour. Linn. Soc. London, Bot. 47: 339-354.
Ownbey, M. 1940. A monograph of the genus Calochortus. Ann. Missouri

Bot. Gard. 27: 371-560.
Pax, F. 1896. Empetraceae (in Engler and Prantl's "Natiirlichen Pflanzen-

familien," 1st ed.).
and Hoffman, C. 1931. Callitrichaceae (in Engler and Prantl's Na-
tiirlichen Pflanzenfamilien, 2d ed).
Samuelsson, G. 1913. Studien iiber die Entwicklungsgeschichte einiger Bicornes-

Typen. Svensk Bot. Tidskr. 7: 97-188.

374 INTRODUCTION TO EMBRYOLOGY OF ANGI03PERMS

Schnarf, K. 1928. tlber das Embryosackhaustorium bei Anthericum. Osterr.

bot. Ztschr. 77: 287-291.

. 1931. "Vergleichende Embryologie der Angiospermen." Berlin.

■ . 1933. Die Bedeutung der embryologischen Forschung fur das naturliche

System der Pflanzen. Biol. Gen. [Vienna] 9: 271-288.
■ . 1937. Ziele und Wege der vergleichenden Embryologie der Blutenpflan-

zen. Verhandl. zool.-bot. Gesell. Wien 86/87: 140-147.
. 1944. Ein Beitrag zur Kenntnis der Verbreitung des Aloins und ihrer

systematischen Bedeutung. Osterr. bot. Ztschr. 93: 113-122.
_# 1948. Der Umfang der Lilioideae im naturlichen System. Osterr. bot.

Ztschr. 95:257-269.

and Wunderlich, R. 1939. Zur vergleichende Embryologie der Liliaceae-

Asphodeloideae. Flora 33: 297-327.
Solms-Laubach, H. G. 1870. Die Familie der Lennoaceen. Abh. naturf.

Gesell. Halle 11: 119-178.
Stenar, H. 1927. Uber die Entwicklung des siebenkernigen Embryosackes bei

Gagea lutea Ker. Svensk Bot. Tidskr. 21: 344-360.
. 192Sa. Zur Embryologie der Asphodeline-Gruppe. Ein Beitrag zur

systematischen Stellung der Gattungen Bulbine und Paradisia. Svensk Bot.

Tidskr. 22: 145-159.
. 192S6. Zur Embryologie der Veratrum- und Anthericum-Gruppe. Bot.

Notiser 1928, pp. 357-378.
. 1933. Zur Embryologie der Agapanthus-Gruppe. Bot. Notiser 1933,

pp. 520-530.
Sussenguth, K. 1927. Uber die Gattung Lennoa. Flora 122: 264-305.
Tison, A. 1919. Sur le suspenseur du Trapa natans L. Rev. Gen. Bot. 31:

219-228.
Wangerin, W. 1910. Cornaceae (In Engler, A. "Das Pflanzenreich." IV. 229:

1-110.
Warming, E. 1904. "A Handbook of Systematic Botany." Engl. Transl. London.
Wettstein, R. 1935. "Handbuch der systematischen Botanik." Vienna.
Woodcock, E. 1929. Seed development in Thelygonum cynocrambe L. Papers

Mich. Acad. Sci., Arts, and Letters 11: 341-345.

CHAPTER 12
EXPERIMENTAL EMBRYOLOGY

As mentioned in the first chapter, modern embryology seems to
comprise three main disciplines. The first, or descriptive embryol-
ogy, is a study of the various developmental processes that take
place in a plant from the initiation of the sex organs to the matura-
tion of the embryo. The second, or phylogenetic embryology, at-
tempts to evaluate these data in determining the interrelationships
of the different orders and families with a view to improving the
existing schemes of classification. The third, or experimental em-
bryology, is concerned with an imitation and a modification of the
course of nature, with a view to understanding the physics and
chemistry of the various processes underlying the development and
differentiation of the embryo, so as to bring them under human
control to the furthest extent possible.

In attempting to summarize the present position of the subject
of experimental embryology, it seems convenient to discuss it under
the following topics: control of fertilization; embryo culture; in-
duced parthenogenesis; production of adventive embryos; and in-
duced parthenocarpy.

Control of Fertilization.1 Ever since the rediscovery of Mendel's
laws in 1900, breeders have been increasingly active in crossing
different varieties, species, and genera with a view to producing
newer and more useful types. However, their attempts are often
thwarted by one or more of the following difficulties: (1) disharmony
in time of flowering of the two parents ; (2) failure of pollen to germi-
nate on the stigma; (3) slow growth of pollen tubes; (4) bursting or
dying of pollen tubes in the style ; and (5) inability of the sperms to
effect fertilization.

Of these, the first is largely a physiological problem. A dis-
harmony in the time of flowering of the two parents can be partially
overcome by altering the environmental conditions, chiefly tempera -

1 For further information on some aspects of this problem, see Blakeslee (1945)
and Maheshwari (1950).

375

376 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

ture and photoperiod. More effective, however, is the storage of
pollen from one season to another, for this has the advantage of
enabling the breeder to cross two varieties which are separated from
each other not only in time but also in space. With modern air
transport, pollen may be sent from one part of the world to another
in a very short time.

Important though the question of the storage and viability of
pollen is, it has attracted proper attention only in recent years.
Under natural conditions most pollens remain viable for only a few
days or weeks, but this is not universal. The range of variation in
this respect may be illustrated by a few examples from common
plants.2 InHordeum (Anthony and Harlan, 1920) and Oryza (Nagao
and Takano, 1938) fertilization cannot be secured with certainty
unless the pollen is transferred directly from the anther to the stigma.
In Sorghum (Stephens and Quinby, 1934) no seed could be obtained
when pollen was used 5 hours or more after collection from the
dehiscing anthers. In some experiments at Coimbatore 65 per cent
of the pollen grains of Gossypium were found to retain their viability
up to the twenty-fourth hour after collection but none after twice
this period (Banerji, 1929). At New Delhi the pollen of Solatium
melongena remains viable for only 1 day in summer and 2 to 3 days
in winter (Pal and Singh, 1943). At the other extreme, however,
is the pollen of Phoenix daciylifera, which is said to retain its viabil-
ity for a whole year2a and which used to be an important article of
commerce in the past.

Most pollens fall between the two extremes mentioned above,
but recent work shows that, whatever the viability might be under
natural conditions, it can almost always be prolonged to an appre-
ciable extent by storing the pollen under proper conditions. Hol-
man and Brubaker (1926), who have reviewed the older literature,
mention the extension of longevity of Cyclamen pollen from 18 to
185 days and that of Lister a ovata from 40 to 164 days. Samples
of Typha pollen which had been stored in a calcium chloride desic-
cator for 71, 94, 116, and 158 days, gave respectively 75, 70, 65, and
56 per cent germination, and 2 per cent of the pollen grains remained

2 For further information on this topic see Maheshwari (1944).

*" Stout (1924) contradicts this and says that he found no evidence of survival
of air-dry pollen for more than 77 daj^s, but as pointed out by Holman and Bru-
baker (1926), it is possible that under favorable conditions the period may be
much longer.

EXPERIMENTAL EMBRYOLOGY 377

viable even after 336 days. Air-dry pollen of Coffea stored in the
ordinary way loses its germination power within a week, but when
kept in a desiccator it retains that power for more than a month
(Ferwerda, 1937). Pfeiffer (1944), who made similar experiments
with Cinchona, found that 5 to 19 per cent of the pollen retained
its viability even after a year's storage in darkness at a temperature
of 10°C. and a humidity of 35 to 50 per cent. Nebel (1939) has
been able to preserve apple pollen for \y2 years and sour cherry
pollen for 5}i years at a temperature of 2 to 8°C. and a humidity
of 50 per cent. Even grass pollen, which is notoriously ephemeral,
has been kept alive for 15 to 30 times its natural period of viability.
To mention only two examples, the pollen of Saccharum spontaneum
(a wild grass used in crosses with sugarcane), spread out on a watch
glass in diffused light, remained viable for only about 6 hours; the
same pollen stored in vials plugged with cotton and kept at room
temperature, for 12 to 24 hours; and the same kept at 7°C, for more
than a week (Sartoris, 1942). Similarly, the pollen of Zea mays
stored in pollinating bags in direct sunlight at a maximum tempera-
ture of 46°C. remained viable for only 3 hours; that stored in shade
at a maximum temperature of 30°C, for 30 hours; and the same
kept in tassel at a temperature of 4.5°C. and a relative humidity of
90 per cent, for 8 to 9 days (Jones and Newell, 1948).

The data given above indicate that the most important factor in
pollen storage is temperature and the next is relative humidity.
We know very little about the effect of light; but strong light is
undoubtedly harmful, and diffused light or even complete darkness
seems to be more conducive to successful storage.

Equal in importance to the viability of pollen is the receptivity
of the stigma, but this is less amenable to control. In many plants
the stigma is receptive for only a short period, and if the pollen is
not transferred to it at the right time, it fails to germinate, or the
germination is so slow that the flower withers and falls off before the
pollen tubes can reach the ovules. Attempts to prolong the re-
ceptivity of the stigma have usually been unsuccessful, or they are
associated with secondary effects which make it difficult to derive
much benefit from such prolongation. For instance, although a
lowering of the temperature can lengthen the blooming period and
also extend the period of receptivity of the stigma to a certain extent,
it has an adverse effect on the rate of growth of the pollen tube, so

378 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

that the net result is the same and fertilization still fails to take
place.

However, the main difficulty is not so much in the initial germi-
nation of the pollen as in the subsequent growth of the pollen tube
in a foreign style. This may be clue either to the fact that the
maximum length attainable by the pollen tubes of the male parent
is inadequate for enabling them to reach the ovules, or that the un-
favorable medium through which they have to make their way
causes an excessive retardation of their growth.

When the failure of a cross is due to such causes, one obvious
remedy is to try the reciprocal cross, but frequently even this is
unsuccessful. An alternative method is to amputate the style and
reduce it to a suitable length. In a cross between Zea and Trip-
sacum, Manglesdorf and Reeves (1931) shortened the style of the
former to a length suitable for the pollen tubes of Tripsacum and
thereby obtained intergeneric hybrids. However, since the cut end
of the style is not always as suitable for pollen germination as the
stigma, sometimes it is desirable to use a different method in which
the middle portion of the style is removed and the upper and lower
portions are then joined together and held in place (Buchholz, Doak,
and Blakeslee, 1932).

The Buchholz method, based on experiments with flowers of
Datura, may be briefly described as follows. On the plant which is
to serve as the maternal parent, a nearly mature unopened bud is
selected. One operator now makes a transverse cut through the
bud at the point k (Fig. 203 A, B), where the fluted inner surface of
the corolla shows a marked constriction. A second operator places
the style on a previously prepared gauge and makes a clean square
cut at the desired distance from the stigma, discarding the lower
part of the style. The newly cut end of the stigma-bearing portion
of the style is now inserted in a closely fitting grass straw, which is
lowered over the stump of the basal part of the style. The moment
the two cut surfaces come in contact, further lowering of the straw
gives an upward thrust to the part bearing the stigma. This en-
ables the operator to know when contact is made and the process is
complete. If the operation (Fig. 203A-H) is carried out carefully,
the pollen tubes pass down the joint and are able to reach the ovules.
A Japanese worker, Yasuda (1931), has gone one step further
and attempted an actual graft of the style upon the ovary in Petunia

EXPERIMENTAL EMBRYOLOGY

379

violacea. He cut off the style of one flower and glued it with gelatin
to the ovary of another flower whose style had been previously
cut away. In order to prevent the falling apart of the grafted style,
he supported it against an iron wire, tying the support and the style
with a spider's thread (Fig. 2037). When the operation was suc-
cessful, the grafted style grew normally and attained its usual size.

<r?

■ w

k-l--

£k

Fig. 203. Diagrams showing technique of splicing (A-H) and grafting (/) of
styles, (kk = level at which transverse cut is made through flower bud; s =
grafted style; w = wire support; t = spider's thread; o = ovary; g = gelatin joint
between ovary and style; p = pedicel). (A-H, after Buchholz, Doak and Blakeslee,
1932; I, after Yasuda, 1931.)

On the other hand, if the tissues failed to unite, the style turned
brown and shrank.

Yasuda 's method requires a great deal of manipulative skill and
has apparently never been tried by any other worker. Although it
seems to be impracticable with plants having thin styles, it is
possible that it can be used successfully when the styles are reason-
ably thick. A grafting of the style of the species used as the pa-
ternal parent on to the ovary of the maternal parent certainly
seems to be a promising method of combating incompatibility fac-
tors between two species or varieties.

380 INTRODUCTION TO EMBRYOLOGY OF ANOIOSPERMS

Yet another method of overcoming the difficulty caused by an
extremely slow growth of the pollen tube would be a direct introduc-
tion of the pollen grains into the ovary. Somewhat reminiscent
of the artificial insemination practiced in animals, this technique
has not so far been perfected for plants. That it is entirely possible
to do so, however, is indicated by some experiments of Dahlgren
(1926), who succeeded in bringing about a fertilization of the ovules
of Codonopsis ovata by this method. More recently, Bosio (1940)
tried some intraovarial pollinations in Helleborus and Paeonia. He
emasculated the flowers and either removed the stigmas or painted
them with celloidin. Then an incision was made in the ovary and
the pollen grains introduced into it artificially. In Helleborus the
germination was inadequate to cause any fertilization, but in Paeo-
nia several ovules were fertilized yielding viable seeds. In explana-
tion of this difference in the behavior of the two genera, the author
says that the pollen of the Ranunculaceae requires for its germina-
tion a sugary medium, with a pH close to neutral. Within the
ovary of Helleborus there is no free liquid, and the pH of the cells
lining it is about 4; in Paeonia, on the other hand, the cells at the
base of the ovarian cavity secrete an abundant liquid, which has a
suitable concentration of sugar and a pH of about 6. Germination
of the pollen, therefore, fails in the former but is quite successful
in the latter. In nature it occurs in both cases, since the stigma
fulfils the required conditions.

In addition to these mechanical devices for bringing the pollen
grains or pollen tubes in close proximity to the ovules, it seems
possible that the same result may sometimes be achieved by the
application of suitable chemical substances to either pollen grains
or stigmas. From experiments in vitro, P. F. Smith (1942) has
shown that 3-indoleacetic acid and 3-indolebutyric acid, in concen-
trations of one in a million, appreciably stimulate the germination
of the pollen as well as the rate of elongation of the pollen tubes.
More recently, Addicott (1943) has reported that several substances
including vitamins, plant hormones, pyridines, and purines are able
to bring about similar effects. He also states that germination of
pollen and the subsequent growth of pollen tubes are not necessarily
related phenomena and that one can be stimulated independently
of the other. In his experiments, inositol increased the germina-
tion of Milla pollen up to 90 per cent over that of the controls

EXPERIMENTAL EMBRYOLOGY 381

without greatly affecting the length of the pollen tubes, while gua-
nine increased the length of the tubes up to 157 per cent over that
of the controls without significantly affecting the percentage of
germination. Two other substances, paraaminobenzoic acid and
acenaphthene, were found to affect both processes.

Eyster (1941) has recently reported that the self -incompatibility
in Petunia, Tagetes, Trifolium repens, and Brassica oleracea can be
counteracted by spraying the plants with a solution of a-naphtha-
leneacetamide. This chemical is said to "neutralize the effects of
an ovarian secretion which diffuses into the style and inhibits or
greatly retards the growth of pollen tubes." Lewis (1942), who
used it with Prunus avium, found it ineffective in increasing or
decreasing the rate of pollen tube growth, but it delayed the forma-
tion of an abscission layer at the base of the style and thereby
allowed a longer time for the incompatible tubes to reach the ovary.
In his opinion, therefore, the use of a-naphthaleneacetamide may
not be confined to the counteraction of self -sterility but may also
be used to combat interspecific sterility in certain plants.

That a change in chromosome number can also be of service in
the inactivation of incompatibilities has been shown recently by
the work of Stout and Chandler (1941) and Stout (1944). As the
result of a series of controlled pollinations they found that the po-
tentially fertile flowers of Petunia axillaris are entirely self -incom-
patible and produce no seeds or capsules on self-pollination. How-
ever, the self -pollinated flowers of tetraploid branches on the same
plants (produced by colchicine treatment) produced large well-filled
capsules. Flowers on the self -incompatible diploid branches also
produced capsules when pollinated from flowers of tetraploid
branches on the same plant, but tetraploid 9 X diploid cf combina-
tions on the same plant failed to yield any seeds.

Of interest in this connection are also some observations of Buch-
holz and Blakeslee (1929), Satina (1944), and Blakeslee (1945)
on Datura. When pollen from a 2n plant is applied to the stigma
of a 4n plant, fertilization takes place freely and some viable seeds
are also produced ; but in the reciprocal cross when pollen is applied
from a 4??. plant to a 2n stigma, seed formation is extremely rare.
Histological studies have shown that the pollen tubes derived from
the 4n male parent burst in the 2n styles and never reach the ovary.
It was possible, however, to overcome this difficulty and change

382

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the 2ft X 4ft cross from an incompatible to a compatible one in an
ingenious way. Treatment with colchicine usually results in peri-
clinical chimeras in Datura. Some of these have the doubled chro-
mosome number in the epidermal layer but not in the underlying
tissues. Since it is the epidermal layer which forms the trans-
mitting tissue in the style in Datura, a periclinal chimera with a
4n epidermis should react exactly like a pure 4n parent so far as its
relation to pollen tubes from a 4ft male parent is concerned. This

Fig. 204. Method of removing embryo from seed of 7ns with help of sterile needle.
(After Randolph, 1945.)

was actually found to be the case, and diploid pollen tubes from
4ft males grew readily without bursting in styles of a periclinal
chimera with a 4n epidermis.

Finally, there remains the difficulty that even after the pollen
tubes have reached the ovary and ovules, fertilization may fail to
occur for some obscure reason. To mention one example, when
the flowers of Ribes nigrum are self -pollinated, the pollen tubes
enter the ovules and "even lay themselves against the embryo-sac
nucleus" but the male and female nuclei do not fuse and hence no
seeds are formed (Ledeboer and Rietsema, 1940). Since there is

EXPERIMENTAL EMBRYOLOGY

383

no known method of overcoming this trouble, it is not necessary to
discuss the point further.

Embryo Culture. In the preceding section we have considered
the methods for overcoming some of the barriers to fertilization.
It has frequently been observed, however, that even after gametic
fusion has taken place something may arrest the growth of the
embryo so that the resulting seeds are nonviable (see Sachet, 1948).
Recent research has shown that in such cases it is frequently possible
to excise the young embryos from the ovules and grow them in
artificial media (Figs. 204-208) — a process not unlike the famous

Fig. 205. Excised embryo being transferred to sterile culture bottle.
Randolph, 1945.)

(After

Caesarean section in which an immature animal embryo is removed
from the body of the mother and grown in an incubator.

In tracing the development of this new technique of embryo
culture, we find that Hannig (1904) was the first to make a success-
ful attempt of this kind. Using certain crucifers (e.g., Raphanus
and Cochlearia) as the objects of his study, he tested a variety of
nutrient media containing sugars, mineral salts, plant decoctions,
certain amino acids, and gelatin. Mature plants were reared from
embryos that were only 1.2 mm. in length at the time of their
excision, but presumably the radicle, plumule, and cotyledons had
already been formed at this stage. Following Hannig, Stingl (1907)
grew embryos of several cereals, but instead of placing them in
culture media he transferred them to the endosperms of other genera

384

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

of the family. Dietrich (1924), who experimented some years later
with a larger variety of plants, found that Knop's solution with 2.5
to 5 per cent cane sugar and 1.5 per cent agar enabled prompt and
regular growth of embryos removed from immature seeds of several
species. He further observed that the cultured embryos tended to
skip the stages of development which had not been completed at the
time of excision and grew directly into seedlings. His efforts to

Fig. 206. Apparatus and glassware used for preparing culture medium. (After
Randolph, 194-5.)

cultivate embryos less than one-third of their mature size were,
however, unsuccessful.

The work of Hannig, Stingl, and Dietrich, although of great
importance, seems to have been prompted more by curiosity than
any other reason, and it remained for Laibach (1925, 1929) to show
the possibilities of using this method to economic advantage. While
making some interspecific crosses in the genus Linum, he found
that the cross L. perenne X L. austriacum yielded fruits of approxi-
mately normal size but that the seeds were greatly shrunken and

EXPERIMENTAL EM BR YOLOG Y

385

Fig. 207. Culture bottles, 6 to 10 days old. The first contains an undissectev I
Iris seed lying in almost unchanged condition; the rest show seedlings which have
arisen from excised embryos. (After Randolph, 1945.)

Fig. 20S. Four-month-old seedlings of 7m grown from excised embryos (left);
seedlings from undissected seeds sown at same time (right). (After Randolph,
1945.)

386 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

only about half as heavy as the normal ones. By dissecting out
the embryos and placing them on damp blotting paper, he was able
to induce their germination, and the resulting plants flowered and
fruited abundantly. The reciprocal cross L. austriacum X L. per-
enne was more difficult, for here the fruits shed prematurely at a
time when the seeds had only one-thirteenth of the weight of normal
seeds and were incapable of germination. However, by excising
the embryos when they were about a fortnight old and placing them
on cotton wadding in little tubes containing 10 to 15 per cent sugar,
he was able to promote their growth. A couple of weeks later they
were removed from the sugar solution and placed on moist blotting
paper, where they germinated within a few days and eventually
yielded vigorous plants which flowered and fruited normally. Lai-
bach, therefore, expressed the opinion that a similar method of
"artificial premature birth" could perhaps be used to obtain off-
spring from various other crosses which had so far proved unfruit-
ful. In conclusion he said: "In any case, I deem it advisable to
be cautious in declaring combinations between higher plants to be
unviable after fertilization has taken place and after they have
begun to develop. Experiments to bring the aborted seed to de-
velopment should always be undertaken, if it is desirable for theoreti-
cal or practical reasons. The experiments will not always be suc-
cessful, but many a result might be obtained by studying the condi-
tions of ripeness of the embryos and by finding out the right time
for the preparing out of the seeds."

Laibach's brilliant exposition gave the lead for more intensive
studies on the artificial culture of embryos, and during recent years
a number of papers have appeared on the subject. In several
crosses which were formerly unsuccessful, the hybrid embryos have
been successfully reared to maturity.

In mentioning some specific cases where this technique has been
employed with success, we may first refer to certain stone fruits,
such as Prunus avium (sweet cherry), P. domestica (plum), and P.
persica (peach). When crosses are made in which the early ripen-
ing varieties of these plants are used as female parents, the embryos
abort and the seeds are not viable.3 Tukey (1944) attempted arti-

3 Histological studies showed that although fertilization takes place normally,
the endosperm and embryo soon cease developing, followed also by a collapse of
the nucellus and integument (Tukey, 1933).

EXPERIMENTAL EMBRYOLOGY

387

ficial cultures of the embryos and succeeded in obtaining mature
plants from them. The procedure adopted was to split the stony
endocarp with a scalpel, cut carefully through the integuments,
nucellus, and endosperm, and drop the embryos under aseptic con-
ditions into bottles containing nutrient agar. The seedlings aris-
ing from them were first transplanted to sterile sand watered with
a nutrient solution, then to soil, and finally grown in the field.

Fig. 209. Inflorescences of Hordeum jubatum (A), Secale cereale (C), and hybrid
between them (B) raised by artificial culture of excised embryo. (After Brink,
Cooper, and Ausherman, 1944.)

In time they developed into vigorous fruiting trees. A similar
technique used for the embryos of 7m is illustrated in Figs. 204-208.
It has been observed that in crossing Hordeum jubatum and Secale
cereale fertilization takes place within 4 hours after pollination but
the hybrid seeds collapse and fail to germinate. Brink, Cooper,
and Ausherman (1944) dissected out the hybrid embryos from
9- to 12-day-old seeds and reared them in artificial culture. The
embryos attained considerable growth and one of the seedlings grew
into a mature flowering specimen (Fig. 209). No seeds were set
on this plant, but the fact that the hybrid was able to attain the

388 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

flowering stage proves that the intergeneric combination is not
lethal in itself and that causes of the death of the embryo have to
be sought elsewhere.

Similar instances of the artificial culture of hybrid embryos have
been recorded in various other plants. To mention a few examples,
J0rgensen (1928) used this method to obtain hybrids between So-
larium nigrum and S. luteum; Beasley (1940) between Gossypium
hirsutum and G. herbaceum; Skirm (1942) between some species of
Prunus and of Lilium; Smith (1944) between Lycopersicum esculen-
tum and L. peruvianum; and Sanders (1948) between several species
of Datura.

When the embryo is of a rather large size, it can often be dissected
out with a needle while the seed is held between the fingers. With
smaller embryos a dissecting microscope is necessary. The excised
embryos are transferred into previously prepared culture bottles
containing a nutrient medium. Since conditions favorable for the
growth of the embryos are also favorable for the growth of various
bacteria and fungi, even a slight carelessness may cause the cultures
to become contaminated, resulting in death of the embryos. Suit-
able precautions must therefore be taken both at the time of dis-
section of the embryos and during their transfer to the culture
medium. Various chemicals are available for sterilizing the seeds,
and contamination of the culture room from air-borne spores may
be reduced either by using air filters or by spraying the tables and
walls with a 1 per cent solution of carbolic acid. The dissecting
instruments are dipped in 70 per cent alcohol and flamed. The
embryos, being delicate, are not treated with any solution but are
dropped immediately after dissection into the sterile culture me-
dium.

The composition of the medium is naturally a most important
factor. Older embryos are largely autotrophic and usually present
little difficulty. The present problem, however, is to try to rear
embryos in younger stages of development. There are two objects
in view. One is that hybrid embryos frequently abort at a very
early stage before the differentiation of the cotyledons and, there-
fore, it is necessary to develop proper methods for taking care of
them before degeneration has commenced ; the other is the long-range
question of understanding the nutritional physiology of embryo
development, of which we have little or no knowledge up to this
time.

EXPERIMENTAL EMBRYOLOGY 389

All the work so far has been of an exploratory nature only, but
some of the attempts made in this direction deserve special mention.
In 1936 La Rue reported having grown embryos of Taraxacum,
Chrysanthemum, Lactuca, Coreopsis, Lycopersicum, Nicotiana, Bryo-
phyllum, Zea, and Vallota which were only 0.5 mm. in length. His
experiments, as well as those of some of his predecessors, made it
clear that inorganic media alone are not adequate for the culture of
young embryos, since they also require sugar and other heat-stable
factors present in such substances as yeast extract or fibrin digest.
In some of his culture media, La Rue substituted indoleacetic acid
for yeast extract.4

Since embryos are nourished inside the seed by the endosperm,
Van Overbeek et al. (1942) thought of using coconut milk as one of
the ingredients of the culture medium. By adding this they suc-
ceeded in growing embryos of Datura stramonium which were still
in the heart-shaped stage and measured only 0.15 mm. in length
(the mature embryo is approximately 6 mm. long). Further study
revealed, however, that, in addition to the "embryo factor" neces-
sary for the growth of the embryo, coconut milk also contains one
deleterious substance which inhibits root growth and another which
causes a callus-like growth but no differentiation.

Blakeslee and Satina (1944) found that powdered malt extract
shows embryo factor activity if sterilized by nitration instead of
by heat. At the same time Van Overbeek, Siu, and Haagen-Smit
(1944) discovered that extracts of Datura ovules, yeast, wheat germ,
and almond meal also possess this quality.6 Finally, they claim to
have obtained a purified embryo factor preparation which, on the
basis of dry weight, showed an activity 170 times that of coconut
milk.

Unfortunately neither coconut milk nor the purified "embryo
factor" preparations have given any uniform results in the hands of
other workers. Working with selfed and hybrid embryos involving
four species of Datura, Sanders (1948, 1950) has explained that
differences exist not only in the nutritive requirements of the differ-
ent species and their hybrids but also in those of embryos of the

4 La Rue was also able to culture small bits of embryonic tissues, only 0.5 mm.
in length, and raise complete plants from them in Lactuca canadensis, Taraxacum
officinale, Chrysanthemum leucanthemum, and Lycopersicum esculentum.

5 Kent and Brink (1947) report the presence of "embryo factor" in casein hy-
drolysate and in water extracts of date, banana, and tomato.

390 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

same species at different stages of their development. The pH
value of the culture medium is an additional factor that has to be
kept in mind, a pH of about 6.0 having been found most favorable
for Datura. Regarding the constituents of the medium, Sanders
reports that in her experiments Seitz -filtered malt extract gave a
definite increase in growth values, while the addition of diastase or
auxins was of no advantage.

In conclusion it may be said that the artificial culture of embryos
is important in several ways. It is the method par excellence for
understanding the nutritive requirements of the developing em-
bryo ; it gives us an insight into the factors that influence embryonic
differentiation; and it promises to be of great economic value as a
means of achieving a much wider range of hybrid combinations
than has been possible up to this time. Further, seeds which
normally remain dormant for several weeks or months can now be
made to germinate not only without giving the embryos a period
of rest but even before they have become mature.6 To give a
single example, Iris seeds, which normally germinate only after two
to several years, can now be made to yield seedlings in the same year,
so that the breeder saves himself the uncertainty and delay which
troubled him previously (Randolph, 1945).

There are two chief limitations in exploiting the embryo culture
method to the fullest advantage. First, young embryos, especially
of hybrids, often fail to grow in artificial media. Second, excised
embryos tend to germinate immediately to produce miniature seed-
lings rather than to continue their usual growth and attain full
differentiation. This premature germination results in curious
growth patterns and weaker seedlings.7 For a solution of these
difficulties we require a better knowledge of the nutritive require-
ments of the embryos in terms of known chemical substances. Fu-
ture research will no doubt help to clear some of the present obscuri-
ties in this connection and may well enable us not only to obtain an
uninterrupted growth of embryos in artificial media but also to fol-
low the entire process of fertilization and embryogeny in a petri dish.

6 Since excised embryos are able to skip this "after-ripening" or "resting" period
in culture, this method has recently been used to make quick tests of the germinabil-
ity of peach seeds used by nurserymen (Tukey, 1944).

7 See Kent and Brink (1947) for some suggestions for bringing about a continua-
tion of the embryonic type of growth.

EXPERIMENTAL EMBRYOLOGY ■ 391

Induced Parthenogenesis. In normal fertilization the sperm im-
parts not only the activating stimulus but also a set of genes em-
bodying the contribution of the male parent toward the make-up
of the new individual. The prime interest in induced parthenogene-
sis lies in the fact that if the stimulus can be provided without the
usually accompanying paternal genes, it would greatly facilitate the
task of the geneticist in producing a homozygous true-breeding
type, which otherwise requires a long and laborious process of self-
fertilization (East, 1930).

Ever since the initial discovery of a Datura haploid (Blakeslee
etal., 1922) a variety of physical and chemical treatments have been
tried to achieve this result. The chief of these are (1) exposure to
very high or very low temperatures soon after pollination; (2) use
of X-rayed pollen on stigma; (3) use of foreign pollen or of delayed
pollination; and (4) chemical treatment.

To give an exhaustive survey of the successes and failures that
have attended these efforts is beyond the scope of this book. A
few examples are mentioned, however, to indicate the nature of the
work that has been done.8

Concerning the effect of temperature, it is interesting to note that
Muntzing (1937) obtained a haploid plant of Secale cereale by ex-
posing the spikes to low temperatures (0.3°C.),9 while Nordengkiold
(1939) achieved the same result by exposing them to high teupera-
tures (41 to 42°C.).

Kihara and Katayama (1932) obtained three haploids of Triticum
monococcum from spikes which had been exposed to X-rays at the
time of meiosis. Later, Katayama (1934, 1935) pollinated the stig-
mas with X-rayed pollen, and out of 91 seedlings raised by him, 16
turned out to be haploids. In another strain, which normally pro-
duces about 0.5 per cent haploids, Kihara (1940) was able to increase
the percentage to 13.66 by using the same method. However,
other workers have been less successful, and Smith (1946) reports
that he failed to obtain any increase in the number of haploids by
X-ray treatment.

The use of foreign pollen for inducing haploidy was first brought

8 For further information on this topic, see reviews by Ivanov (1938) and Kostoff
(1941).

9 Muntzing had really attempted to produce a doubling of chromosomes but
obtained instead a semilethal haploid.

392 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

into prominence by J0rgensen (1928) whose observations on Solarium
nigrum have already been referred to on page 314. Following
interspecific or intergeneric crossings, similar results have been re-
ported in Brassica (Noguchi, 1929), Oenothera (Gates and Goodwin,
1930), Triticum (Nakajima, 1935), and a few other plants.

Kihara (1940) found that in Triticum monococcum the frequency
of haploids could also be made to increase by merely delaying the
time of pollination. By applying pollen on the sixth day after
emasculation, he obtained two haploids among 10 plants; on the
seventh day, four haploids among 44 plants; on the eighth day, five
haploids among 18 plants; and on the ninth day, three haploids
among 8 plants.

Yasuda (1940) injected aqueous solutions of Belvitan into the
ovaries of Petunia violacea. In ovules fixed and sectioned 3 days
after treatment, he observed some striking changes. In some cases
the nucellar cells10 were found to have enlarged as the result of such
stimulation; in others the egg had divided once or twice, forming a
small proembryo ; and in still others the antipodal cells had increased
in volume. In explanation of the fact that division occurred only
in the egg and the other cells merely enlarged, Yasuda says that in
embryonic cells Belvitan promotes cell division, while in mature
cells it causes only a growth of the cell wall.

Yasuda's observations, although interesting, are still in a pre-
liminary stage, for it is not clear whether the embryos he obtained
were haploid or diploid, nor does he mention if he succeeded in fol-
lowing them to later stages of development.

In conclusion, it might therefore be confessed that so far we
have not succeeded in finding a suitable method for inducing par-
thenogenesis in higher plants. Although exposure to temperature
extremes and other shocks and pollination with X-rayed or foreign
pollen have apparently given some positive results in special cases,
the number of parthenogenetic plants obtained by these methods is
too small to warrant the deduction of a definite causal relationship
between the treatment and results. What is really needed is an
agent for producing haploid plants which will give positive results
with approximately the same consistency as colchicine does in pro-
ducing polyploidy. A logical method of approaching the problem

10 These must really be the cells of the integumentary tapetum, for the nucellus
disorganizes at an early stage in all members of the Solanaceae.

EXPERIMENTAL EMBRYOLOGY 393

would be to know what changes take place inside the embryo sac
in known cases of haploid parthenogenesis and then attempt to
duplicate them artificially. From the very meager information
that we possess in this regard it seems that although the egg can
frequently be made to develop parthenogenetically, the real diffi-
culty lies in an initiation of endosperm formation. Thus, Katayama
(1932) found several-celled embryos in unpollinated ovaries of Tri-
ticum fixed on the ninth day after castration, but further develop-
ment stopped owing to lack of endosperm. On the other hand, in
those ovaries in which pollination is not unduly delayed, triple
fusion is presumed to take place normally, resulting in endosperm
formation which enables the maturation of the haploid embryos.
Similarly, Kihara and Yamashita (1938), who used X-rayed pollen,
believe that owing to the greatly reduced rate of growth of the pollen
tubes arising from such pollen grains, the egg begins to divide
parthenogenetically but the polar nuclei are eventually fertilized
in the usual way.

These ideas need to be verified by making proper developmental
studies, for the objection arises as to why, in cases of delayed pol-
lination or the use of foreign pollen or X-rayed pollen, the egg
should begin dividing first, when in most angiosperms the endo-
sperm nucleus is the first to divide and the zygote undergoes its
first division only after a few endosperm nuclei have been formed.

Production of Adventive Embryos. Since embryos arising asex-
ually from the cells of the nucellus or the integument carry the
full chromosome complement of the maternal parent, they are ge-
netically identical with the latter. This phenomenon is of great
importance in some of our cultivated plants, especially fruit trees.
In Citrus it is used to yield large numbers of uniform rootstocks,
since much of the variation in the size and productivity of the
trees is due to the variability of the stocks. In Mangifera, those
varieties which form adventive embryos can be as safely prop-
agated by seeds as by budding or grafting. In others, with only a
zygotic embryo, seed propagation does not give a type true to the
mother and one must resort to vegetative propagation. Since seed-
lings can be raised much more cheaply, a method of inducing the
formation of adventive embryos would obviously have a great
advantage (see Leroy, 1947).

Recognizing the importance of adventive embryony in horticul-

394 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

ture, many attempts have been made to produce it experimentally
but without any marked success up to this time.

Haberlandt (1921, 1922) made the observation that in natural
adventive embryony the proliferation of the embryo-initiating cells
is invariably preceded by a degeneration of some of the adjoining
cells. He was led by this to put forward the so-called "necrohor-
mone theory," according to which the stimulus for cell division
and proliferation is supplied by certain substances liberated from
the adjacent degenerating cells. Proceeding on this basis, he tried
to produce adventive embryos in Oenothera by pricking the ovules
with a fine needle or by gently squeezing the ovary so as to damage
the cells slightly. In one ovule he obtained two embryos, which
he considers to be of nucellar origin (Fig. 210).

In repeating Haberlandt 's technique, Hedemann (1931) obtained
a two-celled embryo and a free nuclear endosperm in an unpol-
linated ovary of Mirabilis uniflora which had been pricked with a
fine insect needle. No chromosome counts could be made, how-
ever, to ascertain whether the embryo was haploid or diploid, and
the mode of its origin (whether from the egg or the nucellus) does
not seem to have been conclusively established.

After Hedemann, no other worker has reported any success in
the artificial production of adventive embryos by Haberlandt 's
methods, and Beth (1938), who made several unsuccessful attempts
with Oenothera and other plants, denies the nucellar origin of the
embryos even in Haberlandt 's material.11 He considers that in
Haberlandt 's experiments emasculation either was incomplete or
had been performed too late, and that the embryos arose from an
accidental fertilization of twin embryo sacs.12

Recently, Van Overbeek, Conklin, and Blakeslee (1941) injected
several chemical substances into the ovary of Datura stramonium in
the hope of inducing parthenogenetic development of the egg cell.
This attempt was unsuccessful, but they obtained instead, on in-
jection of a 0.1 per cent solution or emulsion of the ammonium
salt of naphthaleneacetic acid or indolebutyric acid, several multi-
cellular warty outgrowths which filled the embryo sacs (Fig. 211).

11 See also criticism by Gustafsson (1947) who concludes that there is no evi-
dence, experimental or morphological, to show that adventive embryony is induced
by substances from dying cells.

12 As mentioned on pp.96,97 twin embryo sacs frequently occur in the Onagraceae.

EXPERIMENTAL EMBRYOLOGY

395

The shape and contents of the cells closely resembled those of the
integumentary tapetum and they also showed the diploid number
of chromosomes. It is concluded, therefore, that these structures
were derived by a proliferation of the cells of the integumentary
tapetum, as is the case with many adventive embryos. However,

A B

Fig. 210. Artificial production of adventive embryos in Oenothera lamarckiana
(s = wound caused by pin-prick; e = ovular epidermis; e.s. = embryo sac; i = inner
layer of inner integument; n = nucellus; ne = nucellar embryo). A, diagram of
ovule punctured on upper side by fine needle; note two embryos inside embryo sac.
B, enlarged view of two embryos. (After Haberlandt, 1921.)

in view of their undifferentiated nature and the obscurity regard-
ing their final fate or potentialities, Van Overbeek et al. wisely
refrain from calling them true embryos and designate them as
' 'pseudoembryos . ' '

Of considerable interest in this connection are also the recent
observations made by Fagerlind (1946) on Hosta. Adventive em-

396

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

3I3PSI * - <*** * : > JUBBmi *46K*wP .®wK^? ■ ' &^383

Fig. 211. Sections of ovules of ,$w Datura stramonium from unpollinated ovaries
injected with 0.1 per cent naphthaleneacetic acid. A, l.s. ovule 10 days after
treatment, note prominent integumentary tapetum. B, l.s. ovule of same age,
showing proliferation of cells of integumentary tapetum. C, l.s. ovule 15 days after
treatment; note groups of cells occupying cavity of embryo sac. D, l.s. ovule 21
days after treatment, showing "pseudoembryo" in embryo sac surrounded by
degenerating cells of integument. (After Van Overbeek, Conklin and Blakeslee,
1941.)

EXPERIMENTAL EMBRYOLOGY 397

bryony has been known in this genus for a long time, but fertiliza-
tion is essential for the production of the embryos. Unpollinated
flowers wither and fall away without forming seeds (Strasburger,
1878).

Fagerlind performed three sets of experiments. In the first he
pollinated some of the pistils with a large quantity of pollen and
others with a small quantity of it; in the second he used foreign
pollen; and in the third the ovaries were treated only with growth
hormones. Those pistils which had received an adequate quantity
of pollen set seeds normally, the embryos being of nucellar origin.
In others, where the amount of pollen was insufficient, some of
the ovules increased in size but others remained small. On micro-
scopic examination the former showed the remains of a pollen
tube, a more or less well developed endosperm, and a number of
adventive embryos which seemed to be fully capable of further
development. On the other hand, those ovules which had failed
to grow showed neither pollen tubes nor endosperm but only the
earliest stages in adventive embryony, characterized by the ap-
pearance of a few richly protoplasmic nucellar cells. In older
stages, such "unpollinated ovules" (i.e., ovules not penetrated by
a pollen tube) showed a progressive shrinkage and drying up of
their tissues, accompanied by a degeneration of the embryo sac as
well as the embryo initials.

In the second experiment, in which the pistils were treated with
pollen from other genera, viz., Hemerocallis, Lilium, Galtonia, and
Canna, all of them withered and fell off exactly like unpollinated
pistils.

In the third set of experiments, in which some of the pistils were
treated with 1 per cent heteroauxin in lanolin and the controls
with pure lanolin, the latter dried up within 4 or 5 days. The
auxin-treated pistils, on the other hand, continued to remain at-
tached to the plant and three weeks later they showed the presence
of young adventive embryos. No endosperm was formed, how-
ever, and the embryos seemed to lack the capacity of developing
further.

Although of a preliminary nature, Fagerlind 's observations seem
to indicate that in plants showing adventive embryony it is pos-
sible to prepare the ovule for the production of adventive embryos
by the application of suitable growth hormones, but the real diffi-

398 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERM.S

culty lies in bringing about endosperm formation. Unfortunately
the nature of the stimulus which may lead to a division of the un-
fertilized secondary nucleus remains unknown. Until this is dis-
covered, the only way of overcoming the difficulty would probably
be to perfect a technique for excising the adventive embryos and
growing them in artificial culture.

Finally, it may be added that although a method of inducing
adventive embryony has undoubted possibilities, there is also a
need sometimes for the elimination of adventive embryos. In Cit-
rus, for example, where a number of nucellar embryos may mature
simultaneously with the zygotic embryo, it is quite difficult to
distinguish the two kinds of seedlings in early stages. Further,
rue zygotic embryo sometimes becomes crowded out by the nucel-
lar embryos so that all the seedlings are asexual. It would be a
distinct advantage to the breeder if he could exercise some control
over the two processes, eliminating either zygotic or nucellar em-
bryos according to his requirements at the moment.

Induced Parthenocarpy.13 Some of the world's most important
fruits are seedless or have only abortive seeds. As examples may
be mentioned varieties of banana, cucumber, orange, pineapple,
grape, grapefruit, persimmon, and breadfruit. A good many of
these varieties are believed to have arisen by gene mutation, and
some have been obtained by hybridization.

Recently attempts have been made to produce seedless fruits
on seeded varieties by withholding pollination and applying certain
chemical substances to the pistil. As early as 1849 Gaertner ob-
tained seedless fruits in certain cucurbits whose stigmas had been
"pollinated" with the spores of Lycopodium. Millardet (1901) in-
duced fruit formation in certain varieties of the European grape by
pollinating the stigmas with pollen of Ampelopsis hederacea, and a
partial development of the ovary in certain cucurbits by treating
the stigmas with powdered pollen. A year later Massart placed
dead pollen upon the stigmas of an orchid and observed a slight
increase in the size of the ovary. Subsequently, Fitting (1909)
painted the stigmas with an extract of pollen and ascribed a hor-
monal action to the latter. Additional experiments of a similar
nature made by later workers (see especially Laibach, 1932, 1933)

13 For more detailed information on this topic, see Maheshwari (1940), Gustaf-
son (1942), and Swarbrick (1947).

EXPERIMENTAL EMBRYOLOGY 399

proved conclusively that pollen has a definite influence on the
growth of the ovary which is independent of fertilization or matu-
ration of seeds.

More extensive studies on the role of pollination in fruit growth
— without the accompanying fertilization — were made by Yasuda
(1930, 1933, 1934, 1939). Although all plants did not react favor-
ably, he achieved an appreciable measure of success with certain
members of the Solanaceae and Cucurbitaceae. His experiments
and observations may be summarized as follows :

1. Castrated flowers were treated with pollen of the same species
in different stages of maturity. As was to be expected, ovaries
treated with mature and viable pollen developed into normal fruits;
but immature or overmature pollen also, although incapable of
causing fertilization, frequently stimulated fruit formation, with
the difference that in this case the fruits were devoid of seeds.

2. In a second lot the same procedure was followed using foreign
pollen, i.e., pollen from another plant belonging to the same or a
different family. When fruits were produced, these were either
devoid of seeds or showed only abortive ones.

3. The styles of a number of flowers of Solanum melongena were
cut off at their junction with the ovary at different intervals after
pollination. When the operation was sufficiently delayed, the pol-
len tubes were able to reach the ovules, resulting in fruits with
viable seeds. But if it was performed at a time when the pollen
tubes were close to the base of the style but had not entered the
ovary, the fruits were seedless. In control experiments, in which
the styles were removed as before but the stigmas were left unpol-
linated, no fruits of any kind were produced.

4. In a fourth set of experiments, the styles were cut off as before
but regrafted on the ovaries with an intervening layer of gelatin.
The plants were then divided into two lots, one lot being self -pol-
linated and the other left unpollinated. The ovaries of the former
occasionally developed into seedless fruits, but the unpollinated
ovaries usually failed to grow further. When they did produce
fruits on some very rare occasions, these were much smaller than
those of the first lot.

5. Finally, pollination was entirely omitted and in its place aque-
ous extracts of pollen were injected into the ovary. Solanum me-
longena ovaries, injected with an extract of Petunia pollen, grew to

400 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

a size of 4.1 by 7.3 cm. Also, out of 50 cucumber ovaries injected
with extracts of cucumber pollen, three continued growth.
Of these last, one attained a size of 4.3 by 20.3 cm., comparing
favorably with a normal cucumber.

As a result of these experiments, supported by some microscopic
studies, Yasuda arrived at the following conclusions: (1) pollen
tubes secrete or carry some chemical substance which diffuses into
the tissues of the ovary and thereby induces fruit formation; (2)
if the pollen tubes are allowed to grow only up to the base of the
style, so as to permit chemical diffusion but not fertilization, the
resulting fruits are devoid of seeds; and (3) seedless fruits can also
be obtained by pollinating the stigmas with immature, overma-
ture, or incompatible pollen, or by using extracts of pollen.

The fact that extracts of pollen, like the pollen tubes themselves,
could also induce parthenocarpic growth left no room for doubt
that the stimulation is entirely chemical in nature. In 1934
Thimann showed that many pollens contain considerable quantities
of an auxin or growth substance, and this led some workers, notably
Gustafson (1936, 1938a, 6), to experiment with some synthetic
hormones to see if they could bring about fruit formation .

Several substances were tried, chiefly indoleacetic, indolepropi-
onic, indolebutyric, w-naphthaleneacetic, and phenylacetic acids.
These were made up into a lanolin paste of about 0.5 to 1 per cent
strength and smeared on the stigma. In some cases it was found
better to remove the style just above the ovary and apply the paste
to the cut surface of the latter; this facilitated the diffusion of the
chemical into the ovary. The flowers were subjected to three dif-
ferent kinds of treatments. Some were pollinated in the usual
way, others were treated with the substances named above, and
the rest were left untreated. The first lot produced normal fruits,
while the third withered and dropped without forming any fruits.
As a result of the second treatment, some parthenocarpic fruits
were formed in tomato, pepper (Fig. 212A), tobacco (Fig. 2125),
eggplant, crookneck squash, Petunia, and Salpiglossis , although the
average weight of these fruits was somewhat lower than that of
normal fruits.

Gardner and Marth (1937) employed a different technique.
They sprayed the ovaries with aqueous solutions of several sub-
stances, of which naphthaleneacetic acid was found to be the most

EXPERIMENTAL EMBRYOLOGY

401

effective. Ilex opaca was used for most of the experiments, partly
because of its dioecious nature and partly because of its broad
stigma, which tends to facilitate diffusion. Considerable success
was achieved in obtaining seedless fruits in this plant. Some posi-

Fig. 212. A, fruit production in Capsicum; left, from fertilized ovary; right, after
treatment of pistil with indolebutyric acid. B, fruits in Nicotiana; two on left,
from fertilized ovary; two on right, after treatment of pistil with potassium salt of
indoleacetic acid. An unfertilized and untreated pistil is shown in the middle.
(Photographs supplied by Dr. F. G. Gustajson.)

tive results were also obtained with strawberries but none with
apples and pears.

Similar experiments were soon undertaken by other workers, and
a few studies have also been made on the comparative histology
and vitamin content of normal fruits (resulting from pollination)
as well as parthenocarpic ones (resulting from hormone treatment).

402 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

As far as present evidence goes, there is no essential difference
between the two except that in parthenocarpic fruits the ovules
become shriveled and shrunken and no embryos are formed (Fig.
213). To explain why some plants are naturally parthenocarpic
(e.g., varieties of Musa, Ananas, Vitis, etc.) while others are not,
Gustafson (1939a, o) studied the auxin content of the ovary in a

Fig. 213. Fruit production in tomato; left, from fertilized ovary; right, induced
by hormone treatment. (Photograph supplied by Dr. A. W. Hitchcock.)

few forms of each kind. In every instance he found it to be higher
in the ovaries of the parthenocarpic varieties than in the ovaries
of the nonparthenocarpic ones. Obviously, then, the reason why
some plants produce parthenocarpic fruits is that their ovaries
contain enough auxin to promote growth without fertilization, while
those of other plants do not possess it in sufficient quantity and it
must therefore be augmented either by pollination and fertiliza-
tion, or by pollination alone, or by an external application of the
requisite growth-promoting substance. In a detailed review of the

EXPERIMENTAL EMBRYOLOGY 403

subject, Gustafson (1942) says: "Some plants under some condi-
tions produce enough growth hormone so that with or without
pollination, as the case may be, they are able to prevent the absciss
layer from being formed in the pedicel; and that under favorable
nutritive conditions and with a minimum of competition, they are
further able to transport the necessary food and bring about en-
largement of the cells in the ovary to produce mature fruits with-
out seeds, whereas other plants are unable to do this."

Although the hormonal method of producing seedless fruit is an
invention of only the last 10 to 15 years' research, it has already
begun to be applied on a commercial scale in some countries. In
the United States there are several states where tomatoes are grown
in greenhouses during the winter (January to March), but owing
to the short days and low light intensity, the pollen is frequently
defective, pollination inadequate, and pollen tube growth restricted.
Many flowers, therefore, fall away without producing any fruits.
Even the fruits that are set are frequently small in size or are not
well filled with the gelatinous pulp characteristic of fruits of good
quality. The economic losses resulting from these difficulties
prompted the use of hormones in stimulating fruit growth.

As the chief objective in tomato culture is mainly to make up
for the deficiency of good pollen, rather than to obtain fruits which
are wholly seedless, castration is unnecessary. The procedure is
merely to spray the chemical on the flowers in a suitable manner.
Several substances have been tried, viz., indolebutyric acid, indole-
acetic acid, naphthaleneacetic acid, methylindolebutyrate, a-naph-
thylthioacetamide, potassium naphthaleneacetate, 2,4-dichloro-
phenoxyacetic acid, etc. Of all these, indolebutyric acid has been
found to be one of the most effective, and the fruits resulting from
its application are as large as those produced after natural pollina-
tion, if not larger (Fig. 213). The most marked improvement in
size is seen during the period from January to February, when
pollination is especially deficient and much of the pollen is non-
viable. Indeed, the success achieved is so significant that treat-
ment of greenhouse tomatoes with synthetic hormones bids fair
to become a standard practice (Howlett, 1943, 1944; Mitchell and
Marth, 1947). "■ 15

14 Some very spectacular results have also been achieved in pineapples (Van
Overbeek, 1946). By using 2,4-dichlorophenoxyacetic acid and a-naphthalene-

404 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

The following problems need further study in this connection:
(1) selection of the most effective chemical substance for inducing
fruit set; (2) method and time of applying the substance; and (3)
prevention of some undesirable secondary effects such as blossom-
end rot and malformations in the fruit or the plant.

According to recent tests (Howlett, 1946) a combination of in-
dolebutyric acid and /3-naphthoxyacetic acid is superior to indole-
butyric acid used alone, with regard to both fruit set and the filling
of the loculi. It was also found that an aqueous solution of these
chemicals is as effective as an emulsion.

Regarding the method of application, the usual procedure is to
spray the inflorescences with an atomizer, but since this is laborious
in large-scale projects some workers have suggested the so-called
vapor method. The original procedure was to place the potted
plants under bell jars and expose them for various lengths of time
to vapors of the desired growth substances. Zimmerman and
Hitchcock (1939) found this to be entirely successful with Ilex
opaca. More recently another method has been tried, based on
the aerosol principle used by entomologists for killing insects (see
Zimmerman and Hitchcock, 1944; Hamner, Schomer, and Marth,
1944; Howlett, Freeman, and Marth, 1946). Briefly, the hormone
is first dissolved in some liquid in which it is readily soluble and then
added to a highly volatile liquefied gas; or, if the hormone is soluble
in the liquefied gas itself, it is added directly to the latter. The
mixture is then held under pressure in a container from which it can
be released as a very fine mist. The gas soon volatilizes, leaving
the hormone as a finely divided liquid or solid.

While the effectiveness of the aerosol method has been amply
demonstrated and there is no doubt as to its rapidity and sim-
plicity, the chief drawback is that it exposes not only the flowers

acetic acid, it has been possible to induce the flowering and fruiting of pineapples
at any time of the year, even in varieties which are normally difficult to get into
bearing. The procedure is simple. When the plants have produced sufficient
leaves to support and mature a good-sized fruit, a few drops of an aqueous solution
of one of the two chemicals are placed into the tip of each plant. One ounce of
the dry chemical is sufficient for treating 113,000 plants.

"Stewart and Condit (1949) report that by spraying aqueous solutions of
2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid they were
able to obtain seedless figs of a size and sugar concent comparable to that of caprified
fruits.

EXPERIMENTAL EMBRYOLOGY 405

but the whole plant to the chemical, resulting sometimes in mal-
formations of the leaves. It is possible, however, that certain sub-
stances may have a less toxic effect than others and with further
experience new formulae can be worked out which will induce a
satisfactory fruit set without causing injury to the rest of the
plant.16

Conclusion. Although experimental embryology is a new sub-
ject, it is already possible to recognize the fundamental problems
which confront it. Their solution will be difficult and will no
doubt demand many years of patient research, but this is pre-
cisely what makes the field so interesting and at the same time so
promising. While descriptive and phylogenetic embryology will
continue to stay, the future trend is clearly towards the experi-
mental side. Here we have the frontier state within whose borders
the geneticist, physiologist, cytologist, and embryologist all find a
common ground.

References

Addicott, F. T. 1943. Pollen germination and pollen tube growth as influenced

by pure growth substances. Plant Physiol. 18: 270-279.
Anthony, S., and Harlan, H. V. 1920. Germination of barley pollen. Jour.

Agr. Res. 18 : 525-536.
Banerji, I. 1929. Studies in cotton pollen. Agr. Jour. India 24: 332-340.
Beasley, J. C. 1940. Hybridization of American 26-chromosome and Asiatic

13-chromosome species of Gossypium. Jour. Agr. Res. 60: 175-182.
Beth, K. 1938. Untersuchungen uber die Auslosung von Adventivembryonie

durch Wundreiz. Planta 28: 296-343.
Blakeslee, A. F. 1945. Removing some barriers to crossability in plants. Proc.

Amer. Phil. Soc. 89: 561-574.
, Belling, J., Farnham, M. E., and Bergner, A. D. 1922. A haploid mutant

in the Jimson weed, Datura stra?nonium. Science 55 : 646-647.

and Satina, S. 1944. New hybrids from incompatible crosses in Datura

through culture of excised embryos on malt media. Science 99: 331-334.
Bosio, M. G. 1940. Richerche sulla fecondazione intraovarica in Helleborus e

Paeonia. Nuovo Gior. Bot. Ital. 47 : 591-598.
Brink, R. A., Cooper, D. C, and Ausherman, L. E. 1944. A hybrid between

Hordeum jubatum and Secale cereale reared from an artificially cultivated

embryo. Jour. Hered. 35: 67-75.

16 Howlett and Marth (1946) report that with greenhouse tomatoes a combina-
tion of /S-naphthoxyacetic acid, indolebutyric acid, and 4-chlorophenoxyacetic
acid is more satisfactory than other substances.

406 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Buchholz, J. T., and Blakeslee, A. F. 1929. Pollen-tube growth in crosses be-
tween balanced chromosomal types of Datura stramonium. Genetics 14:
538-568.

, Doak, C. C, and Blakeslee, A. F. 1932. Control of gametophytic

selection in Datura through shortening and splicing of styles. Bui. Torrey
Bot. Club 59: 109-118.

Dahlgren, K. V. 0. 1926. Svensk Bot. Tidskr. 20: 97.

Dietrich, K. 1924. Uber Kultur von Embryonen ausserhalb das Samens. Flora
17:379-417.

East, E. M. 1930. The production of homozygotes through reduced partheno-
genesis. Science 72 : 148-149.

Eyster, H. C. 1941. Pollen tube growth of a self-sterile strain of Petunia. Proc.
S. Dak. Acad. Sci. 21:56-57.

Fagerlind, F. 1946. Hormonale Substanzen als Ursache der Frucht- und Em-
bryobildung bei pseudogamen //osto-biotypen. Svensk Bot. Tidskr. 40 : 230-
234.

Ferwerda, F. P. 1937. Kiemkracht en Levensduur van Koffiestuifmeel. Arch.
Koffiecult. Ned.-Ind. 11: 135-150.

Fitting, H. 1909. Die Beeinflussung der Orchideenbltiten durch die Bestaubung
und durch andere Umstande. Ztschr. f. Bot. 1: 1-86.

Gartner, K. F. 1849. "Versuche und Beobachtungen uber die Bastardzeugung
im Pflanzenreich." Stuttgart.

Gardner, F. E., and Marth, P. C. 1937. Parthenocarpic fruits induced by spray-
ing with growth promoting substances. Bot. Gaz. 99: 184-195.

Gates, R. R., and Goodwin, K. M. 1930. A new haploid Oenothera, with some
observations on haploidy in plants and animals. Jour. Genetics 23: 123—
153.

Gustafson, F. G. 1936. Inducement of fruit development by growth-promoting
chemicals. Proc. Natl. Acad. Sci. [U.S.A.] 22: 628-636.

. 1937. Parthenocarpy induced by pollen extracts. Amer. Jour. Bot. 25:

237-244.

. 1938a. Induced parthenocarpy. Bot. Gaz. 99: 840-844.

. 19386. Further studies on artificial parthenocarpy. Amer. Jour. Bot.

25:237-244.

. 1939a. The cause of natural parthenocarpy. Amer. Jour. Bot. 26:

135-138.

. 19396. Auxin distribution in fruits and its significance in fruit develop-
ment. Amer. Jour. Bot. 26: 189-194.

1942. Parthenocarpy: natural and artificial. Bot. Rev. 8: 599-654.

Gustafsson, A. 1947. Apomixis in higher plants. II. The causal aspect of
apomixis. Lunds Univ. Arsskr. N.F. Avd. II, 43(2): 71-179.

Haberlandt, G. 1921. Uber experimentelle Erzeugung von Adventivembryonen
bei Oenothera lamarckiana. Sitzber. Preuss. Akad. der Wiss. [Berlin] 40:

695-725.

. 1922. Uber Zellteilungshormone und ihre Beziehungen zur Wundhei-

lung, Befruchtung, Parthenogenesis und Adventivembryonie. Biol. Zentbl.
42: 145-172.

EXPERIMENTAL EMBRYOLOGY 407

Hamner, C. L., Schomer, H. A., and Marth, P. C. 1944. Application of growth-
regulating substance in aerosol form with special reference to fruit-set in
tomato. Bot. Gaz. 106: 108-123.

Hannig, E. 1904. Uber die Kultur von Cruciferen Embryonen ausserhalb des
Embryosacks. Bot. Ztg. 62: 45-80.

Hedemann, E. 1931. tlber experimentelle Erzeugung von Adventivembryonen
bei Mirabilis uniflora und Mirabilis frdebelii. Biol. Zentbl. 51: 647-652.

Holman, R. M., and Brubaker, F. 1926. On the longevity of pollen. Calif.
Univ. Pub. Bot. 13: 179-204.

Howlett, F. S. 1943. Growth promoting chemicals improve greenhouse tomato
production. Ohio Agr. Expt. Sta. Bimo. Bui. 28: 17-27.

. 1944. Comparative value of growth regulating chemicals for greenhouse

tomato production. Proc. Ohio Veg. & Potato Growers Assoc. 29th Ann.
meeting 29: 162-180.

. 1946. Synthetic plant hormones in relation to greenhouse tomato pro-
duction. Ohio Veg. and Potato Growers Rep. 1946, pp. 223-236.
-, Freeman, S., and Marth, P. C. 1946. Aerosol applications of growth

regulating substances to greenhouse tomato. Proc. Amer. Soc. Hort. Sci. 48:

485-474.
Ivanov, M. A. 1938. Experimental production of haploids in Nicotiana rustica

L. Genetica 20: 295-397.
Jones, M. D., and Newell, L. C. 1948. Longevity of pollen and stigmas of

grasses: Buffalograss, Buchloe dactyloides (Nutt.)Engelm., and Corn, Zea mays

L. Jour. Amer. Soc. Agron. 40: 195-204.
JOrgensen, C. A. 1928. The experimental production of heteroploid plants in

the genus Solarium. Jour. Genet. 19: 133-211.
Katayama, Y. 1932. Crossing experiments in certain cereals with special refer-
ence to different compatibility between the reciprocal crosses. Mem. Col.

Agr. Kyoto Univ. 27 (Genetic Ser. 2) : 1-75.
. 1934. Haploid formation by X-rays in Triticum monococcum. Cyto-

logia 5: 235-237.

1935. Karyogenetic studies on X-rayed sex cells and their derivatives

in Triticum monococcum. Agr. Col. Jour. Tokyo Imp. Univ. 13: 333-362.
Kent, N., and Brink, R. A. 1947. Growth in vitro of immature Hordeum em-
bryos. Science 106: 547-548.
Kihara, H. 1940. [Formation of haploids by means of delayed pollination in

Triticum monococcum.] Bot. Mag. [Tokyo] 54: 178-185.
and Katayama, Y. 1932. [On the progeny of haploid plants of Triticum

monococcum.] Kwagau 2: 408-410.
and Yamashita, K. 1938. Kiinstliche Erzeugung haploides und triplo-

ides Einkornweizen durch Bestaubung mit rontgenbestrahlten Pollen. Comra.

Papers, 30th Anniv. N. Akemine, pp. 9-20.
Kostoff, D. 1941. The problem of haploidy (cytogenetic studies on Nicotiana

haploids and their bearings to some other cytogenetic problems). Bibl.

Genetica 13: 1-148.
Laibach, F. 1925. Das Taubwerden der Bastardsamen und die kiinstliche

Aufzucht fruh absterbender Bastardembryonen. Ztschr. f. Bot. 17: 417-459.

408 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Laibach, F. 1929. Ectogenesis in plants: methods and genetic possibilities of
propagating embryos otherwise dying in the seed. Jour. Hered. 20: 201-208.

. 1932. Pollenhormone und Wuchsstoff. Ber. deutsch. bot. Gesell. 50:

383-390.

— . 1933. Versuche mit Wuchsstoffpaste. Ber. deutsch. bot. Gesell. 51:

386-392.
La Rue, C. D. 1936. The growth of plant embryos in culture. Bui. Torrey

Bot. Club 63: 365-382.
Ledeboer, M., and Rietsema, I. 1940. Unfruitfulness in black currants. Jour.

Pomol. and Hort. Sci. 18: 177-180.
Leroy, J. F. 1947. La polyembryonie chez les "Citrus." Son interet dans la

culture et amelioration. Rev. Bot. Appl. et Agr. Trop. 301/302: 483-495.
Lewis, D. 1942. Breakdown of self-incompatibility by a-naphthalene acetamide.

Nature [London] 149: 610-611.
Maheshwari, P. 1940. The role of growth hormones in the production of seed-
less fruits. Sci. and Culture 6: 85-89.

. 1944. On the longevity of pollen. Indian Jour. Hort. 2: 82-87.

. 1950. Contacts between embryology, physiology and genetics. Proc.

37th Indian Sci. Cong., Bot. Sect., presidential address.
Mangelsdorf, P. C, and Reeves, R. G. 1931. Hybridization of maize, Tripsacum

and Euchlaena. Jour. Hered. 22: 329-343.
Massart, J. 1902. Sur la pollination sans f£condation. Bui. Jard. Bot. Brussels

de P'Etatl: 85-95.
Millardet, A. 1901. Rev. de Vitic. 16: 677-680.
Mitchell, J. W., and Marth, P. C. 1947. "Growth Regulators for Garden, Field

and Orchard." Chicago.
Muntzing, A. 1937. Note on haploid rye plant. Hereditas 23 : 401-404.
Nagao, S., and Takano, T. 1938. Duration of the preservation of the fertiliza-
tion possibility in pollen and stigma of rice plant. Comm. Papers, 30th

Anniv. N. Akemine, pp. 88-92.
Nakajima, G. 1935. Occurrence of a haploid in Triticum turgidum. Jap. Jour.

Genet. 11:246-247.
Nebel, B. R. 1939. Longevity of pollen in apple, pear, plum, peach, apricot,

and sour cherry. Proc. Amer. Soc. Hort. Sci. 37: 130-132.
Noguchi, Y. 1929. Zur Kenntnis der Befruchtung und Kornbildung bei den

Reispflanzen. Jap. Jour. Bot. 4: 385-503.
Nordenskiold, H. 1939. Studies of a haploid rye plant. Hereditas 25: 204-210.
Pal, B. P., and Singh, H. B. 1943. Floral characters and fruit formation in the

egg plant. Indian Jour. Genet, and Plant Breeding 3: 45-58.
Pfeiffer, N. 1944. Prolonging the life of Cinchona pollen by storage under con-
trolled conditions of temperature and humidity. Contrib. Boyce Thompson

Inst. 13:281-294.
Randolph, L. R. 1945. Embryo culture of Iris seed. Bui. Amer. Iris Soc.

97 : 33-45.
Sachet, M. 1948. Fertilization in six incompatible species crosses of Datura.

Amer. Jour. Bot. 35 : 302-309.

EXPERIMENTAL EMBRYOLOGY 409

Sanders, M. E. 1948. Embryo development in four Datura species following

self and hybrid pollinations. Amer. Jour. Bot. 35: 525-532.
. 1950. Development of self and hybrid embryos in artificial culture.

Amer. Jour. Bot. 37: 6-15.
Sartoris, G. B. 1942. Longevity of sugarcane and corn pollen — a method for

long distance shipment of sugarcane pollen by airplane. Amer. Jour. Bot.

29: 395-400.
Satina, S. 1944. Periclinal chimaeras in Datura in relation to development and

structure, (A) of the style and stigma, (B) of calyx and corolla. Amer. Jour.

Bot. 31 : 493-502.
Skirm, G. W. 1942. Embryo culturing as an aid to plant breeding. Jour. Hered.

33:210-215.
Smith, L. 1946. Haploidy in einkorn. Jour. Agr. Res. 73: 291-301.
Smith, P. F. 1942. Studies of the growth of pollen with respect to temperature,

auxins, colchicine and vitamin Bi. Amer. Jour. Bot. 29: 56-66.
Smith, P. G. 1944. Embryo culture of a tomato species hybrid. Proc. Amer.

Soc. Hort. Sci. 44: 413-416.
Stephens, J. C., and Quinby, J. C. 1934. Anthesis, pollination and fertilization

in Sorghum. Jour. Agr. Res. 49: 123-136.
Stewart, W. S., and Condit, I. J. 1949. The effect of 2 , 4-dichlorophenoxyacetic

acid and other plant growth regulators on the Calimyrna fig. Amer. Jour.

Bot. 36 : 332-335.
Stingl, G. 1907. Experimentelle Studie iiber die Entstehung von pflanzlichen

Embryonen. Flora 97 : 308-331 .
Stout, A. B. 1924. The viability of date pollen. Jour. N. Y. Bot. Gard. 25:

101-106.
. 1944. Inactivation of incompatibilities in tetraploid progenies of Petunia

axillaris. Torreya 44: 45-51.
and Chandler, C. 1941. Change from self-incompatibility to self-compat-
ibility accompanying change from diploidy to tetraploidy. Science 94: 118.
Strasburger, E. 1878. tlber Polyembryonie. Jena. Z. Naturw. 12: 647-670.
Swarbrick, T. 1947. Growth regulating substances in horticulture. Jour. Roy.

Hort. Soc. 72: 313-327; 342-359.
Thimann, K. V. 1934. Studies on the growth hormones of plants. VI. The

distribution of growth substance in plant tissues. Jour. Gen. Physiol. 18:

23-34.
Tukey, H. B. 1933. Artificial culture of sweet cherry embryos. Jour. Hered.

24:7-12.
. 1944. The excised-embryo method of testing the germinability of fruit

seed with particular reference to peach seed. Proc. Amer. Soc. Hort. Sci.

45:211-219.
Van Overbeek, J. 1946. Control of flower formation and fruit size in the pine-
apple. Bot. Gaz. 108: 64-73.
— , Conklin, M. E., and Blakeslee, A. F. 1941. Chemical stimulation of

ovule development and its possible relation to parthenogenesis. Amer. Jour.

Bot. 28:647-656.

410 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Van Overbeek, J., Conklin, M. E., and Blakeslee, A. F. 1942. Cultivation in

vitro of small Datura embryos. Amer. Jour. Bot. 29: 472-477.
, Siu, R., and Haagen-Smit, A. J. 1944. Factors affecting the growth of

Datura embryos in vitro. Amer. Jour. Bot. 31: 219-224.
Yasuda, S. 1930. Parthenocarpy caused by the stimulation of pollination in

some plants of Solanaceae. Agr. and Hort. [Tokyo] 5 : 287-294.
. 1931. An experiment to graft the style upon the ovary in Petunia

violacea. Proc. Imp. Acad. Japan 7: 72-75.
. 1933. On the behaviour of pollen tubes in the production of seedless

fruits, caused by interspecific pollination. Jap. Jour. Genet. 8: 239-244.
. 1934. Physiological research on self-incompatibility in Petunia violacea.

Bui. Imp. Col. Agr. and Forestry, Japan 20: 1-95.
. 1939. Parthenocarpy induced by stimulation of pollination in some higher

plants. Mem. Faculty Sci. and Agr. Taihoku Imp. Univ. 27: 1-51.

1940. A preliminary note on the artificial parthenogenesis induced by

application of growth promoting substance. Bot. Mag. [Tokyo] 54: 506-510.
Zimmerman, P. W., and Hitchcock, A. E. 1939. Experiments with vapors and
solutions of growth substances. Contrib. Boyce Thompson Inst. 10: 481-

508.
and . 1944. Substances effective for increasing fruit set and in-
ducing seedless tomatoes. Proc. Amer. Soc. Hort. Sci. 45: 353-361.

CHAPTER 13
THEORETICAL CONCLUSIONS

The phylogeny and interrelationships of angiosperms present
problems which have baffled botanists for many years. As shown
in Chap. 11, embryology has been of appreciable help in reorienting
our ideas on the interrelationships of several doubtful families and
genera. In the present chapter we shall consider some problems
of wider interest concerning the origin and homologies of the male
and female gametophytes, endosperm, etc.

Male Gametophyte. In discussing the homologies of the male
gametophyte of angiosperms, we must naturally turn to the condi-
tion in gymnosperms. The available evidence suggests that in the
fossil gymnosperms the pollen grains were multicellular structures
containing both prothallial and spermatogenous cells. Probably
there were no pollen tubes and the sperms made their way directly
to the archegonia. Swimming sperms are found even in some mod-
ern representatives of the group, viz., the living cycads and Ginkgo,
but in addition a pollen tube is also present. It is interesting to
note that the tube originates from the upper end of the pollen grain
and grows laterally into the nucellar tissues, acting as a haustorial
and not a sperm-carrying structure. The basal end of the pollen
grain hangs free in a cavity, which may be said to be composed
partly of the pollen chamber and partly of the archegonial cham-
ber. There are present, beside the two sperms, the prothallial
cells (one in the Cycadales and two in the Ginkgoales), the stalk
cell, and the tube nucleus. In Microcycas there are 16 to 22
sperms, which should probably be considered a primitive feature.

The Coniferales differ in two important respects: (1) the sperms
do not possess any cilia, and (2) the pollen tube does not arise from
the upper end of the pollen grain but from its lower end, penetrat-
ing through the nucellus and discharging its contents into
the archegonium. The contents of the pollen grain and the tube
vary in different genera. In Araucaria, Podocarpus, Dacrydium,
and Phyllocladus there are several prothallial cells; in Pinus and

411

412 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

some other genera there are two prothallial cells; and in the Taxa-
ceae, Cephalotaxaceae, and most genera of the Taxodiacea and
Cupressaceae the prothallial tissue is completely eliminated.
Another feature of considerable interest is the great disparity in the
size of the two sperm cells in Taxus, Torreya, and Cephalotaxus,
the smaller cell presumably being on the way to elimination. Some
species of Cupressus are exceptional in having multiple male cells,
all of which seem to be capable of functioning.

Coming to the Gnetales, Ephedra with its two prothallial cells, a
stalk cell, a tube nucleus, and two male gametes shows consider-
able resemblance to Ginkgo and Pinus. The inner integu-
ment forms a long micropylar canal, but the pollen grains are
drawn down into the pollen chamber formed by the disintegration
of the nucellar cells. Welwitschia and Gnetum are only imperfectly
known, but in both genera the pollen grains seem to have a prothal-
lial cell, a generative cell, and a tube cell. The stalk cell is elimi-
nated and the generative cell directly gives rise to the two male
gametes.

Briefly then, although most gymnosperms possess a prothallial
tissue, they show a tendency toward its elimination, and in several
genera the male gametophyte is reduced to having a prothallial
or a stalk cell, a tube nucleus, and two male gametes. Only the
Cycadales and Ginkgoales have ciliated sperms. In the remaining
members the cilia have been lost and it is the pollen tube which
becomes the channel for the transportation of the male gametes.
The male gametophyte of angiosperms may be assumed to have
been derived from that of some gymnospermous ancestor by further
simplification and elimination of the single prothallial or the stalk cell.1

Female Gametophyte. Schnarf (1936) put forward the view that
the monosporic 8-nucleate embryo sue is the most primitive and
that all the other types have been derived from it. This idea has
also been favored by several other writers, the chief argument in
its support being that this type is the most widely distributed in
angiosperms and that the female gametophyte of the pteridophytes

1 There have been occasional reports of the occurrence of a prothallial cell in
some angiosperms like Lilium, Eichhornia, Yucca, Sparganium, Atriplex, and
Stellaria (see Wulff and Maheshwari, 1938), but these are in the nature of freaks
and abnormalities of little or no significance. Up to the present the occurrence
of a prothallial cell is not known to be a regular feature in any angiosperm.

THEORETICAL CONCLUSIONS 413

and gymnosperms is also monosporic. An additional argument in
favor of the primitive nature of the monosporic 8-nucleate type is
that all the other types can be easily derived from it while the
reverse is almost impossible. The Oenothera type presents no diffi-
culty; here only two divisions intervene between the functioning
megaspore stage and the differentiation of the egg apparatus, and
all the 4 nuclei are restricted to the micropylar part of the embryo
sac. In the Allium type, wall formation does not occur after Meio-
sis II, and even if it does occur the cell plates soon dissolve, so that
each dyad cell (or at least the functional one) contains 2 megaspore
nuclei. Only two further divisions are now required to give rise
to the 8-nucleate stage. In the tetrasporic types no permanent
walls are laid down after any of the meiotic divisions. As a result
all the 4 megaspore nuclei lie in a common cavity and may take
up varying arrangements, one pair of nuclei lying at the micro-
pylar end and the other at the chalazal (2+2), or one nucleus at
the micropylar end and three nuclei at the chalazal (1+3), or one
nucleus at each end and two at the sides (1 + 1 + 1 + 1). The mega-
spore nuclei may undergo two divisions or only one. The 2+2
position apparently leads to the Peperomia and Adoxa types; the
1+3 position to the Drusa, Fritillaria, and Plumbagella types;
and the 1 + 1 + 1 + 1 position to the Penaea and Plumbago types.
Assuming then that the monosporic 8-nucleate embryo sac is
the fundamental type, there are three principal theories as to its
homologies :

1. The embryo sac of angiosperms is derived from a form like
Gnetum in which all the nuclei of the embryo sac possess the same
morphological value, and any of them can function as an egg and
give rise to an embryo. First put forward by Hofmeister and
Strasburger, this view may for convenience be called the Gnetalean
theory.

2. The embryo sac of angiosperms is derived by reduction from
the female gametophyte of some gymnosperm and consists of only
two archegonia without any prothallial tissue (Fig. 214). Accord-
ing to this view the micropylar quartet represents one archegonium
(the synergids are equivalent to neck cells and the polar nucleus
to the ventral canal nucleus), and the chalazal quartet represents
the second but nonfunctional archegonium (Porsch, 1907).

3. The micropylar quartet represents two archegonia and the

414 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

ez ek d bk

o bk

Fig. 214. Diagrams illustrating derivation of angiosperm embryo sac from that
of coniferous ancestor (bk = ventral canal nucleus ; d = archegonial jacket ;ek = egg
nucleus; ez = egg cell; hz = neck cells; i = nonfunctional archegonium; p = pro-
thallus; ps = pollen tube; sta = sterilized archegonia forming archegonia jacket).
A, female gametophyte of hypothetical gymnosperm, essentially similar to modern
Sequoia. There are several laterally situated archegonia, each with its own jacket.
As a rule each pollen tube fertilizes a single archegonium, but rarely one tube may
fertilize two archegonia (left bottom). B, female gametophyte of Cupressus type,
showing fewer archegonia arranged in a compact group and enclosed in common
jacket. C, female gametophyte of Ephedra, showing further reduction in number
of archegonia. Jacket is supposed to have arisen by the sterilization of some
archegonium initials. D, hypothetical transitional stage, showing reduction in
size of gametophyte and disappearance of arohegonial jacket. E, gametophyte

THEORETICAL CONCLUSIONS 415

remaining four cells or nuclei are prothallial. According to this
view one synergid and the egg constitute the first archegonium, the
synergid being equivalent to the ventral canal cell; and the other
synergid and the upper polar nucleus constitute the second arche-
gonium. Neck cells are absent and both the archegonia are
fertilized, one giving rise to the embryo and the other to the endo-
sperm (Schurhoff, 1919, 1928).

The third view, which may be considered first for convenience,
is based on the wholly erroneous assumption that one synergid is
sister to the egg and the second to the upper polar nucleus, and
that these two pairs of nuclei constitute two separate archegonia.
Langlet (1927) produced evidence to show that the synergids are
formed from one pair of sister nuclei and the egg and upper polar
nucleus from another pair. During recent years this has received
further confirmation and at present there is not a single authentic
instance where the contrary has been definitely established.
Schurhoff 's theory may, therefore, be rejected without further dis-
cussion.

The second view, put forward by Porsch, has attracted consider-
able attention and is still favored by some embryologists (see
especially Nilsson, 1941; Schnarf, 1942). In support of it have
also been cited certain abnormal embryo sacs showing reversed
polarity. It is suggested thereby that both the archegonia, micro-
pylar as well as chalazal, were originally quite similar and that
either of them was capable of functioning in the ancestral type of
embryo sac (Swamy, 1946).

There are, however, some serious difficulties in accepting Porsch's
view:

1. It assumes that the female gametophyte comprises only two
archegonia, the prothallial tissue having disappeared completely.
Even if this could happen, it is surprising that the archegonia

showing only two archegonia, each consisting of egg, ventral canal nucleus, and two
neck cells corresponding to synergids. This is regarded as essentially similar to
condition in Balanophora. F, as in E, but the two archegonia occupy opposite
poles of embryo sac; pollen tube enters chalazal end of embryo sac. G, as in F,
but pollen tube turns around embryo sac and enters it at micropylar end. H,
embryo sac, showing pollen tube entering directly from above, as in majority of
present-day angiosperms. /, embryo sac in which only upper archegonium is
functional and lower soon degenerates. (After Porsch, 1907.)

416 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

themselves should have escaped all reduction, for they still main-
tain the same essential structures seen in the gymno sperms, being
provided with neck cells as well as a ventral canal nucleus.2 In-
deed, from this point of view, Gnetum and Welwitschia must be
regarded as more advanced than the angiosperms, for they have
no differentiated archegonia nor any other structures which can
be interpreted as archegonia in terms of Porsch's hypothesis.

2. In all archegonia of the pteridophytes and gymnosperms the
ventral canal nucleus is situated directly above the egg. In angio-
sperms, on the contrary, the upper nucleus, which should have
formed the ventral canal cell, is supposed to have given rise to the
egg, while the lower, which should have organized into the egg, is
said to represent the ventral canal nucleus! The question also
arises as to why the ventral canal nuclei belonging to two different
archegonia, one micropylar and the other chalazal, should jointly
fuse with a male gamete to give rise to the endosperm, while the
egg cell (i.e., the central antipodal cell) belonging to the chalazal
archegonium ordinarily exercises no attraction whatsoever toward
the second sperm. Normally, the ventral canal nucleus is an inert
structure, which is already disorganized at the time the egg is ready
for fertilization, and even in those gymnosperms in which it is
incidentally fertilized, the fusion nucleus does not undergo more
than a very few abortive divisions.

3. In the angiosperms there are several known instances of em-
bryos arising from synergids, either as the result of fertilization
or even without it. There is no recorded instance, however, where
the neck cell of a true archegonium has behaved in a similar fash-
ion. Since the embryo sac of angiosperms is a more reduced struc-
ture than that of gymnosperms, it seems rather strange that the
"neck cells" of the archegonium, which are headed toward extinc-
tion (they are already extinct in the embryo sacs of Plumbago and
Plumbagella) , should become egg-like and produce rival embryos.

4. In some species of Peperomia and in Acalypha indica the egg
is associated with a single synergid. Must we then suppose that
here we have an archegonium in which one neck cell has disap-
peared, leaving the other to carry on the function of both?

5. A fifth objection is that the components of the chalazal quar-

2 Even in the conifers there are some genera which do not have a ventral canal
nucleus (Chamberlain, 1935).

THEORETICAL CONCLUSIONS 417

tet, i.e., the antipodal cells and lower polar nucleus, show a great
variation in their behavior which is quite unknown for archegonia.
Usually the antipodal cells are ephemeral and may disorganize
even before any wall formation has taken place between the nuclei.
In the Podostomaceae the primary chalazal nucleus remains un-
divided, and in the Oenotheraceae there is no nucleus at the chala-
zal end. These must be considered as instances of a tendency
towards the reduction and final elimination of the chalazal arche-
gonium. There are other plants, however, in which the antipodal
cells persist and become very active. Sometimes they show nu-
clear divisions inside them, followed by fusions resulting in a high
degree of polyploidy. In other cases, the divisions are accompanied
by wall formation, resulting in a massive tissue which persists for
a long time. Finally, in several plants, like Drusa, Tanacetum,
and Chrysanthemum, even the initial number of antipodal cells ex-
ceeds three, and then the so-called archegonial plan cannot be
recognized at all.

6. Some insuperable difficulties arise in applying Porsch's inter-
pretation to the bisporic and tetrasporic embryo sacs. In the bi-
sporic sacs the micropylar archegonium is derived from one mega-
spore nucleus and must correspond to one prothallus, while the
chalazal archegonium is derived from another megaspore nucleus
and must therefore correspond to a second prothallus, i.e., the em-
bryo sac is composed of two prothalli. Proceeding on the same
analogies, in tetrasporic forms, like Adoxa, each archegonium must
be supposed to represent two prothalli, for the synergids (i.e., neck
cells) are derived from one megaspore nucleus, and the egg and
upper polar nucleus (i.e., ventral canal nucleus) from a second
megaspore nucleus. In the Fritillaria type, the micropylar arche-
gonium represents one prothallus, but the chalazal will have to be
considered as the equivalent of three prothalli which fuse at the
megaspore stage. Strangest of all would be certain forms of
Tulipa, belonging to the section Eriostemones, for here the micro-
pylar archegonium, which consists of more than three cells, must
be regarded as having originated from three megaspores.

That a single archegonium should correspond to one, two, and
even three prothalli is incomprehensible, and it seems that the
very simplicity of Porsch's theory, which led to its adoption in the
past, must now be the ground for its final rejection (see also

418 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Edman, 1931; Fagerlind, 1941). It seems impossible to interpret
the female gametophyte of angiosperms, with all its varied modes
of development (several of which were unknown at the time when
Porsch enunciated his theory), in terms of archegonium formation.
As is known from our knowledge of the prothalli of pteridophytes
and gymnosperms, archegonia are initiated only in a cellular phase.
In the angiosperms, on the other hand, we are taken back to the
2-nucleate stage of the embryo sac as the point of origin of the
archegonium initials, which is pushing morphology into absurdity.
It seems far more likely instead that the angiosperms have long
passed the stage of archegonia or that they never had them at any
time in their fossil history.

Coming finally to the Gnetalean theory, the name which has
been given to it does not imply any direct derivation of the angio-
sperm embryo sac from that of the Gnetales. It assumes, however,
that in the reduction of the prothallial tissue of the female gameto-
phyte, the Gnetales and the angiosperms followed a more or less
parallel course, leading to a complete loss of archegonia and a con-
dition in which all the nuclei are to be considered as potential
gametes. Because of the similar value attached to all the com-
ponents of the embryo sac, it has also been called the Gleichwertig-
keitstheorie or "theory of equivalence." No single botanist can
be credited with its authorship, for it seems to have developed
slowly as the result of certain opinions expressed from time to
time by Hofmeister, Strasburger, and others. It has found sup-
port during recent years from further elucidations of the morphol-
ogy and embryology of the Gnetales in general and of the genus
Gnetum in particular (Thompson, 1916; Pearson and Thomson
1918; Fagerlind, 1941, 1946).

Before entering into a comparison of the embryo sac of Gnetum
with that of the angiosperms, it may be well to recall the main
facts in the development and organization of both.

Taking the angiosperms first :

1. The embryo sac may originate from 1, 2, or all 4 megaspore
nuclei.

2. Only a few nuclear divisions occur after megasporogenesis,
and there seems to be a tendency towards further reduction in
this number.

THEORETICAL CONCLUSIONS 419

3. The mature embryo sac may contain a maximum of 16 nuclei
and a minimum of 4, the commonest being the 8-nucleate condition.

4. The functions of the sac seem to be adequately performed,
whatever the number of nuclei entering into its composition and
whether it is derived from one, two, or four megaspores.

5. The gametic characters are not confined to the egg cell alone;
sometimes the synergids, and less often the antipodal cells also,
may give rise to embryos.

6. The polar nuclei, although usually two, frequently exceed this
number, and sometimes there is only one polar nucleus.

7. Endosperm formation is postponed until after fertilization and
the primary endosperm nucleus shows varying degrees of poly-
ploidy depending on the number of nuclei which have entered into
its composition; one of the fusing nuclei is a male gamete.

Turning now to Gnetum :

1. As in the angiosperms, the gametophyte may be monosporic,
bisporic, or tetrasporic.3

2. The number of divisions taking place after megasporogenesis
is considerably less than in most other gymnosperms. At the con-
clusion of the divisions there are usually about 512 nuclei, but
sometimes there are twice as many and rarely only half the number.

3. The nuclei become distributed at the periphery of the cell,
leaving a large vacuole in the center. No compact tissue is formed
except in the chalazal portion of the gametophyte.

4. Archegonia are absent and apparently every nucleus in the
upper part of the gametophyte is a potential gamete. One or a
few of the nuclei increase in volume and become surrounded by
dense cytoplasm to form the eggs.

5. Fertilization may occur either in the free nuclear stage or
after partial cell formation.

6. A certain amount of storage tissue (endosperm) is often pres-
ent at the time of fertilization, but the bulk of it is formed only

3 Lotsy (1899) states that in G. gnemon the mother cell divides into two cells,
each of which may give rise to an embryo sac, i.e., the development is bisporic;
Thompson (1916) reports that the development is monosporic; and Fagerlind (1941)
writes that in G. gnemon var. ovalifolium all four megaspore nuclei take part in
the development, i.e., the embryo sac is tetrasporic. The reports of Lotsy and
Fagerlind of course need confirmation

420 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

after the entry of the pollen tube. Its cells are multinucleate and
nuclear fusions are common, but no male nucleus has been observed
to take part.

If we now compare these two sets of observations, four points
seem worthy of note :

1. In both cases there is a variation in the number of megaspore
nuclei which take part in the development of the gametophyte.
It may be stated parenthetically that no other genus among the
gymnosperms resembles the angiosperms in this respect.

2. There is a tendency toward reduction in the number of nuclei
of the embryo sac, but this is far more pronounced in the angio-
sperms.

3. Archegonia are completely suppressed in both cases.

4. There is a tendency toward a postponement in the formation
of the storage tissue until after fertilization, and its development is
preceded by nuclear fusions.

In inviting attention to these and to certain other similarities in
the vegetative anatomy of the two groups, Thompson (1916) says:
"In regard to the angiospermic relationship almost every structure
described [in Gnetum]. . .shows some approach to the angiospermic
condition and. . .some structures show conditions almost completely
angiospermic. . . Such a body of evidence can scarcely be ignored
or put aside as the result of parallel development.. . . Accordingly
the sum of the evidence from all sides seems to lead to the conclu-
sion that angiosperms are phyletically related to Gnetales. This
does not mean that any modern member of the Gnetales represents
the type from which angiosperms were derived but that the an-
cestors of angiosperms were not far removed from the genus
Gnetum."

Fagerlind (1941) also expresses himself in favor of "a more or
less intimate genetical connection between the ancestral types of
angiosperms and gymnosperms" and draws the following con-
clusions :

1. The polar nuclei of the angiosperms are the last remnants of
the free nuclei seen in the female gametophyte of Gnetum and in
the earlier stages of development of the gametophytes of other
gymnosperms.

2. The central vacuole of the angiosperm embryo sac is homolo-

THEORETICAL CONCLUSIONS 421

gous with the similar temporary or permanent vacuole seen in
Gnetum and other gymnosperms.

3. The cells in the angiosperm embryo sac are homologous with
the peripheral cells in the gametophyte of Gnetum; the egg is a
fertile peripheral cell or an arrested archegonium, and the antipodal
cells correspond to the lower nutritive part of the gametophyte of
Gnetum.

4. The endosperm of angiosperms is arrested game tophy tic tissue
which is stimulated to further development through fusion with a
male gamete.

Both these views, although interesting, leave one point unex-
plained. In the angiosperms the endosperm is formed only after
the polar nuclei have fused with a male gamete. In Gnetum, on
the other hand, there is no such fusion. A further difficulty, al-
though less serious, is the presence of the synergids in one group
and their absence in the other. They no doubt seem to be unes-
sential elements, for embryo sacs without synergids (Plumbagella
and Plumbago) seem to function just as satisfactorily as those
with them; nevertheless the fact that they are the usual accom-
paniments of the angiosperm egg demands an explanation, which is
not yet available.

If the embryo sac of angiosperms were to consist of only an egg
and a variable number of free nuclei, irregularly placed at the
periphery, we could probably assume its derivation from a condi-
tion like that in Gnetum. As it is, however, it seems best to con-
clude that while the angiosperms have probably passed through
some such stages as are shown by Gnetum, we have no decisive
evidence in favor of this view. Considered in this light, therefore,
the Gnetalean view, although the most attractive of the three we
have considered, can only be regarded as a working hypothesis,
useful to stimulate further research, but entirely tentative for the
present. Regarding the other two theories, proposed by Porsch
and SchurhofT, there is now little to support them.

Fertilization. In gymnosperms the pollen grains land directly
on the nucellus and the pollen tube has to grow only a short dis-
tance in order to reach the archegonium. In Larix and Pseudo-
tsuga the apical portion of the integument becomes stigmatic, and
in Tsupa. Araucaria, and Agathis pollen may germinate even on

422 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

the ovuliferous scale.4 In Gnetum germination frequently takes
place in the inicropylar canal at some distance from the apex of
the nucellus.

The angiosperms differ from all known gymnosperms in having a
closed carpel whose upper portion becomes differentiated into a
style and stigma. Pollen grains never land directly on the nucel-
lus but a considerable distance away from it on the tissues of the
stigma, and the pollen tubes have to grow all the way down through
the style before they can reach the ovules.

This difference, which is very significant, seems to be bridged to a
certain extent by Johri's (1936) discovery of pollen grains in the
stylar canal and ovary of Butomopsis. Here the style is a hollow
structure which remains open at its upper end, so that the ovary
is in direct communication with the exterior. As a rule the pollen
grains germinate on the stigma as in other angiosperms and the
pollen tubes travel down the walls of the hollow stylar canal to
the ovary, but in one carpel a row of six pollen grains was found
within the stylar canal, five of them having been seen in a single
section (Fig. 215A,B). In another carpel eight pollen grains were
seen (Fig. 215C) and in a third there was a pollen grain at the junc-
tion of the stigma and style (Fig. 215D). In a fourth there were
two pollen grains in the stylar canal, the upper of which had ger-
minated in situ (Fig. 215E). Finally one case was seen in which a
pollen grain had germinated on the surface of an ovule (Fig.
215F,G).

Intracarpellary pollen grains have since been found in some other
angiosperms, notably Trillium, Ottelia, Fritillaria, Amianthium, and
Erythronium,5 although it is not known if any of them germinate
to form pollen tubes which take part in fertilization. In any case
this is a remarkable phenomenon, which is comparable only to the
condition in the Caytoniales, in which the carpel is closed at ma-
turity but pollen grains are nevertheless found in the micropyles
of many ovules (Harris, 1933, 1940). Presumably the carpel was
open at its upper end at the time of pollination and the micropyles
of the ovules were connected to the stigma by means of narrow
canals through which the pollen grains were drawn in by some
suction mechanism. In conclusion Harris says: "There is virtu-

4 For further information, see Doyle (1945).

6 Unpublished observations made bv the author's pupils.

THEORETICAL CONCLUSIONS

423

Fig. 215. Occurrence of pollen grains in stylar canal and ovary of Butomopsis
lanceolata. A, l.s. carpel, showing a row of five pollen grains in the stylar canal.
B, upper part of same, enlarged to show structure of pollen grains. C, upper
part of carpel, showing eight pollen grains in stylar canal and one near stigma. D,
one pollen grain near upper end of stylar canal. E, upper end of carpel, showing
stylar canal with irregularly cut pollen tubes and two pollen grains. F, l.s. carpel
showing pollen grain germinating directly on an ovule. G, pollen grain and part
of ovule of F, enlarged to show nuclear details. (After Johri, 1936.)

424 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

ally no evidence to show what was the pollination mechanism of
primitive angiosperms but it seems by no means unlikely that it
may have been similar to what is postulated for Caytonia. Possibly
the style was originally an open canal along which the pollen was
conveyed to the ovules; a later stage would be the germination of
the pollen grain before it reached the micropyle, at first no doubt
at the bottom of the stylar canal, then at its middle and then at its
top. The final change to occur — the closure of the stigma — makes
It impossible for the pollen to pass down the stylar canal."50

The condition in Butomopsis is exactly what Harris expects in
his primitive angiospermous types and it seems likely that more
comprehensive studies on other angiosperms with open styles may
give some clue to a solution of the problem. Of considerable in-
terest in this connection are also the carpels of the new Fijian genus
Degeneria in which Bailey and Smith (1942) report that the carpel
is a conduplicate structure whose margins are not coherent but
tend to flare apart externally (see also Swamy, 1949). In most
cases the cleft-like opening becomes more or less occluded, owing to
the presence of numerous loosely interlocking papillae, but it is
not impossible that sometimes the pollen may have direct access
to the ovules.

Endosperm. There has been a great deal of discussion regarding
the morphological nature of the endosperm of angiosperms, which
is commonly neither haploid nor diploid but triploid.

Hofmeister (1858, 1859, 1861), in whose days neither syngamy
nor triple fusion had yet been discovered, considered the endo-
sperm of angiosperms to be a gametophytic structure whose growth
and differentiation remained arrested until the entry of the pollen
tube into the embryo sac.

Following Strasburger's (1884) discovery of syngamy in angio-
sperms, Le Monnier (1887) put forth the view that the fusion of
the polar nuclei is also an act of fertilization, comparable to the
fusion of the egg and the sperm nucleus. He therefore regarded
the endosperm as a second embryo, modified to serve as food tis-
sue for the zygotic embryo.

With Nawaschin's (1898) announcement of double fertilization,
emphasis shifted from the fusion of the polar nuclei to the partici-
pation of the second male gamete in this event. Nawaschin re-

** See Baum (1949).

THEORETICAL CONCLUSIONS 425

garded triple fusion as an act of true fertilization, and this view
was strongly supported by Sargant (1900). She compared it to a
sexual union in that the fusion involved one normal male element
(twin structure to the male gamete fertilizing the egg cell) and one
normal female element (the upper polar nucleus, which is sister to
the egg cell). However, there entered into the process a third
nucleus from the chalazal end which "with its redundant chromo-
somes" upset the whole balance and brought about the degeneracy
of the resulting tissue. The second embryo was thus "maimed"
from the beginning and converted into a formless mass of tissue
or "monster," enabling the survival of the first without a struggle.

Strasburger (1900) gave a different analysis and suggested that
triple fusion is not true fertilization but only a growth stimulus.
He emphasized the extremely reduced nature of the female gameto-
phyte of angiosperms, with little or no reserves of food material.
Triple fusion served as a stimulus toward its growth, he thought,
and the endosperm was therefore to be regarded as belated gameto-
phytic tissue. This postponement of endosperm formation was
considered by him to be an advantage, for it avoided the waste of
material which would occur if this massive tissue were lost by the
plant with every unfertilized ovule.

The views presented above were based on the assumption that
the endosperm is always a product of triple fusion. Detailed stud-
ies during the present century have revealed, however, that there
is no such uniformity in its origin. In the entire family Onagraceae
the embryo sacs are 4-nucleate, comprising only an egg apparatus
and an upper polar nucleus. Here the primary endosperm nucleus
arises from the fusion of the male gamete and single polar nucleus
and is therefore diploid (Fig. 216 A). The same is true of some
reduced embryo sacs like those of Butomopsis, in which there is an
egg apparatus, a single polar nucleus, and one degenerating antip-
odal nucleus (Fig. 2165), and of some members of the Balanophora-
ceae in which the upper polar nucleus alone is fertilized and the
lower fuses with the 3 antipodal nuclei to form a degenerat-
ing structure which does not take part in further development. In
Ditepalanthus (Fagerlind, 1938), on the other hand, the primary
endosperm nucleus is tetraploid, being derived from a fusion of 3
polar nuclei and a male gamete. In Fritillaria and Plumbagella, it
is formed from a fusion of the haploid upper polar nucleus, the

426 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

triploicl lower polar nucleus, and the haploid male gamete, and is
therefore pentaploid (Fig. 216D,E). In Penaea and Plumbago
there are 4 polar nuclei so that here also the endosperm is penta-
ploid (Fig. 216F ,G). Still higher degrees of polyploidy are seen in
Acalypha indica and Peperomia (Fig. 216H ,1), and in one species
P. hispidula, as many as 14 nuclei fuse to form a secondary nucleus
which, after fusion with a male nucleus, gives rise to a 15n endo-
sperm (Fig. 216/). In Pandanus even the nuclei of nucellar cells

Butomopsis

Polygonum

Fritillaria

Plumbagella

Plumbago

Acalypha indica

D E

Peperomia Peperomia hispidula

F G H I J

Fig. 216. Diagrams of embryo sacs of various plants, showing variations in the
number of nuclei fusing to form primary endosperm nucleus. In all cases, male
nucleus entering into fusion is shown in solid black.

enter into the embryo sac and take part in the fusion, so that the
endosperm shows varying degrees of polyploidy.

It is clear from the above that there is no uniformity in the
origin of the endosperm, and that whatever the number of polar
nuclei taking part in the fusion, the result is always a formless mass
of cells without any semblance of a second embryo and designed
solely to serve as a source of nutriment to the zygote.

The participation of a male gamete in the fusion is, however, a

THEORETICAL CONCLUSIONS 427

remarkably regular feature, which has been commented on by vari-
ous authors. Long ago Thomas (quoted in Sargant, 1900) sug-
gested that a tissue resulting from a nuclear fusion which includes a
male element may perhaps be more suitable for the nourishment
of an embryo arising from the same mixed stock than one derived
from the maternal plant alone. A few years later Nemec (1910)
expressed a more or less similar opinion. He said that the fertiliza-
tion of the polar nuclei has a double function: (1) the stimulation
of endosperm development, and (2) the creation of a nutritive
tissue which is physiologically compatible with the embryo.

According to Thomas and Nemec, therefore, the hybridity of
the endosperm is a method of adjusting its composition to the
needs of the developing plant, for otherwise the hybrid embryo
would be forced to depend upon the kind of food made available to
it by the maternal parent alone.

Brink and Cooper (1940, 1947) have recently restated this view.
They point out that in the gymnosperms the female gametophyte
is packed with food materials which are readily available to the
egg both at the time of fertilization and during the maturation of
the embryo. In the angiosperms, on the other hand, the female
gametophyte is a greatly reduced structure in comparison to the
total mass of the ovule, and contains little reserve food at the time
of fertilization. Further, there is a competition for food between
the tissues within the embryo sac and those belonging to the nucel-
lus and integument. In order that the reproductive process may
be completed, it is necessary to have a mechanism which would
tip the scale in favor of the endosperm and enable it to maintain a
certain aggressiveness over the adjacent tissues of the ovule so that
it can act as an efficient intermediary for the nutrition of the em-
bryo. Brink and Cooper suggest that double fertilization is
a means of conferring upon the endosperm the physiological ad-
vantage of hybridity so that it has "two chances instead of one
(as in the gymnosperms) of receiving the genetic equipment neces-
sary to perform its function."

This is an interesting hypothesis. Some previous writers have
also expressed the view that the vigor of the endosperm and its
parasitic relation to the nucellus might be attributed to its triploid
chromosomal constitution. However, the question arises as to why
the endosperm with its higher chromosome numbers should then

428 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

succumb to the embryo, which also contains the diploid number
like the nucellus and the integuments.

Embryo. The most difficult problem in the embryogeny of an-
giosperms is the derivation of the monocotyledonous state from
the dicotyledonous. In the dicotyledons the two cotyledons are
lateral structures and the stem tip is terminal, while in the mono-
cotyledons the cotyledon occupies a terminal position and the stem
tip is lateral. One view is that the single cotyledon arose by a
fusion of two originally separate primordia; the other is that one of
the cotyledons became suppressed at an early stage of develop-
ment. Each of these views has been supported by a considerable
mass of evidence, morphological as well as anatomical. The opin-
ion has also been advanced that in certain plants the monocoty-
ledonous state has arisen by a division of labor between the two
cotyledons, one of which retained the cotyledonary position and
function, while the other became modified to form the first plumu-
lar leaf.

It is natural to turn to a study of the development of the embryo
as an aid in the solution of this problem. Ontogenetic studies
should enable us to find the primordia of the two cotyledons and
then to trace the development of one and the arrest of the other, or
their fusion into a single member. Not enough work has yet been
done to enable a final decision, but a few contributions bearing on
this point are briefly referred to here.

Coulter and Land (1914) found a seedling of Agapanthus umbel-
latus (Liliaceae) with two well-developed cotyledons. A study of
the embryogeny revealed that, as the proembryo increases in size,
its basal or root end remains narrow and pointed while the shoot
end widens and becomes broad and flat. Here the peripheral cells
begin to divide more actively than the central cells and form a
"cotyledonary zone" which assumes a tube-like form with two
primordia growing at its tip. Meanwhile the apex of the proem-
bryo is left in a depression. Subsequent to this stage, if both the
primordia continue to develop equally, two cotyledons are formed.
More frequently, however, the cells of one primordium lose their
meristematic activity, resulting in a single cotyledon. In other
words, both the primordia are present in the beginning, but later
the whole growth may be diverted into a single primordium.
Looking at the mature stage only, one naturally gets the impression
that there is a single terminal cotyledon and a lateral stem tip.

THEORETICAL CONCLUSIONS 429

In a slightly later publication, Coulter (1915) extended this view
to include the embryo of grasses. He interpreted the scutellum as
the functional cotyledon arising from the peripheral cotyledonary
ring and the epiblast6 as a second and greatly reduced cotyledon.
From Leersia and Zizania, where the epiblast is a very conspicuous
structure, he traces a gradation to the condition in Zea, in which
this organ is practically nonexistent.

Turning now to the dicotyledons, we find that the seedlings of a
number of genera and species show only a single cotyledon. The
best known of these are: Ranunculus ficaria, Corydalis cava,
Abronia, Carum bulbocastanum, Blumium elegans, Erigenia bulbosa,
and several members of the Gesneriaceae. The embryogeny of
these plants should be especially instructive in giving us an indica-
tion as to whether the monocotyledonous state is the more primi-
tive or the dicotyledonous. Unfortunately most of the informa-
tion available on them deals with the morphology and anatomy of
the seedling rather than with the actual development of the
embryo.

Mention may, however, be made of the work of Metcalfe (1936)
on Ranunculus ficaria. The embryo is only a small club-shaped
mass of cells embedded in the endosperm (Fig. 176). Further
development takes place after the seeds are shed. It is interesting
to note that during this process a small parenchymatous hump,
supplied with a procambial strand, arises in the position in which
the second cotyledon would be expected to originate if one were
present. The position and mode of origin of this hump strongly
suggest that it is the rudiment of the second cotyledon which fails
to develop further.

In conclusion it might be said that, although embryology does
not throw any light on the ancestry of the angiosperms, it indicates
that the group is probably monophyletic in origin. There are no
essential differences between the monocotyledons and dicotyledons
as regards the development and organization of the male and fe-
male gametophytes and the endosperm, and the process of fertiliza-
tion is the same in both the subgroups. Further, the differences in
the organization of the embryo are not fundamental, for there are
some dicotyledons in which only one cotyledon develops fully and
the other becomes arrested, and some monocotyledons in which

8 See p. 289.

430 INTRODUCTION TO EMBRYOLOGY OF ANOIOSPERMS

both cotyledons develop equally. Regarding the relationship of
the angiosperms with other groups, we are at present entirely in
the dark. It is possible that a study of morphology and embryol-
ogy of the Degeneriaceae, Winteraceae, Trochodendraceae, etc.,
may throw some light on the problem.

References

Bailey, I. W., and Smith, A. C. 1942. Degeneriaceae, a new family of flowering

plants from Fiji. Jour. Arnold Arboretum 23 : 355-365.
Baum, H. 1949. Das Zustandekommen "offener'' Angiospermengynozeen.

Osterr. bot. Ztschr. 96: 285-288.
Brink, R. A., and Cooper, D. C. 1940. Double fertilization and development of

the seed in angiosperms. Bot. Gaz. 102 : 1-25.

and . 1947. The endosperm in seed development. Bot. Rev.

13:423-541.

Chamberlain, C. J. 1935. "Gymnosperms, structure and evolution." Chicago.
Coulter, J. M. 1915. The origin of monocotyledony. II. Monocotyledony in
grasses. Ann. Mo. Bot. Gard. 2: 175-183.

and Land, W. J. G. 1914. The origin of monocotyledony. Bot. Gaz.

57:509-519.

Doyle, J. 1945. Developmental lines in pollination mechanisms in the Conifer-
ales. Sci. Proc. Roy. Dublin Soc. 24: 43-62.
Edman, G. 1931. Apomeiosis und Apomixis bei Atraphaxis frutescens C. Koch.

Acta Horti Bergiani 11 : 13-66.
Fagerlind, F. 1938. Ditepalanthus, eine neue Balanophoraceen Gattung aus

Madagaskar. Arkiv for Bot. 29A(7) : 1-15.
. 1941. Bau und Entwicklung der Gnefrum-Gametophyten. K. Svenska

Vet.-Akad. Handl. 19(8): 1-55.
— — . 1946. Strobilus und Bliite von Gnetum und die Moglichkeit, aus ihrer

Struktur den Bliitenbau der Angiospermen zu deuten. Arkiv for Bot. 33A(8) :

1-57.
Harris, T. M. 1933. A new member of the Caytoniales. New Phytol. 23 : 97-

114.

. 1940. On Caytonia thomas. Ann. Bot. 4: 713-734.

Hofmeister, W. 1858. Neuere Beobachtungen liber Embryobildung der Phanero-

gamen. Jahrb. f. wiss. Bot. 1 : 82-186.
— . 1859. Neue Beitrage zur Kenntnis der Embryobildung der Phanero-

gamen. I. Dikotyledonen mit urspriinglich einzelligem, nur durch Zelltheilung

wachsendem Endosperm. Abh. Konigl. Sachs. Gesell. Wiss. 1859, pp. 535-

672.

1861. Neue Beitrage zur Kenntnis der Embryobildung der Phanero-

gamen. II. Monokotyledonen. Abh. Konigl. Sachs. Gesell. Wiss. 7: 629-
760.
Johri, B. M. 1936. The life history of Butomopsis lanceoJata Kunth. Proc.
Indian Acad. Sci. Sect. B. 4: 139-162.

THEORETICAL CONCLUSIONS 431

Langlet, 0. 1927. Tiber die Entwicklung des Eiapparates im Embryosack der

Angiospermen. Svensk Bot. Tidskr. 21 : 478-485.
Le Monnier, G. 1887. Sur la valeur morphologique de l'albumen chez les angio-

spermes. Jour, de Bot. [Paris] 1: 140-142.
Lotsy, J. P. 1899. Contributions to the life history of the genus Gnetum. Ann.

Jard. Bot. Buitenzorg II, 1: 46-114.
Metcalfe, C. R. 1936. An interpretation of the morphology of the single cotyledon

of Ranunculus ficaria based on embryology and seedling anatomy. Ann.

Bot. 50: 103-120.
Nawaschin, S. G. 1898. Resultate einer Revision der Befruchtungsvorgange bei

Lilium martagon und Fritillaria tenella. Bui. Acad. Imp. des Sci. St. Peters-
burg 9 : 377-382.
N6mec, B. 1910. "Das Problem der Befruchtungsvorgange und andere zytolo-

gische Fragen." Berlin.
Nilsson, H. 1941. Die Homologie des angiospermen Embryosackes. Bot. Noti-

ser 1941, pp. 50-58.
Pearson, H. H. W., and Thomson, M. R. H. 1918. On some stages in the life

history of Gnetum. Trans. Roy. Soc. So. Africa, Cape Town 6: 231-267.
Porsch, O. 1907. "Versuch einer phylogenetischen Erklarung des Embryosackes

und der doppelten Befruchtung der Angiospermen." Jena.
Sargant, E. 1900. Recent work on the results of fertilization in angiosperms.

Ann. Bot. 14:689-712.
Schnarf, K. 1936. Contemporary understanding of embryo sac development

among angiosperms. Bot. Rev. 2: 565-585.
. 1942. Archegonium und Archegontheorie, Biol. Gen. Vienna 16: 198-

224.
Schurhoff, P. N. 1919. Zur Phylogenie des Angiospermen-Embryosackes. Ber.

deutsch. bot. Gesell. 37: 160-168.
. 1928. Uber die Entwicklung des Eiapparates der Angiospermen. Ber.

deutsch. bot. Gesell. 46: 560-572.
Strasburger, E. 1884. "Neue Untersuchungen iiber den Befruchtungsvorgang

bei den Phanerogamen als Grundlage fur eine Theorie der Zeugung." Jena.
. 1900. Einige Bemerkungen zur Frage nach der doppelten Befruchtung

bei Angiospermen. Bot. Ztg. II, 58 : 293-316.
Swamy, B. G. L. 1946. Inverted polarity of the embryo sac of angiosperms and

its relation to the archegonium theory. Ann. Bot. 9: 171-183.
. 1949. Further contributions to the morphology of the Degeneriaceae.

Jour. Arnold Arboretum 30: 10-38.
Thompson, W. P. 1916. The morphology and affinities of Gnetum. Amer.

Jour. Bot. 3 : 135-184.
Wulff, H. D., and Maheshwari, P. 1938. The male gametophyte of angiosperms

(a critical review). Jour. Indian Bot. Soc. 17: 117-140.

NAME INDEX

Addicott, 380

Afzelius, 71, 73, 76, 77, 87, 89, 135,

335-337
Agardh, 359
Akerberg, 348
Albrecht, 141

Alcala, 30, 46, 169, 222, 225
Aldama, 31, 191, 256
Alexandrov, 257
Alexandrova, 257
Amici, 3-6, 8
Anantaswamy Rau, 227
Anderson, 168, 193, 208
Anthony, 376
Arber, 256, 290

Archibald, 54, 56, 57, 337, 362, 363
Aristotle, 1
Arnoldi, 18, 19, 110
Artschwager, 143, 181, 188, 203,

281
Asplund, 229, 230
Atwood, 193
Ausherman, 387
Avery, 290

Bacchi, 226, 348

Bailey, 424

Baillon, 359, 366-368

Bambacioni, 118, 119, 122

Bambacioni-Mezzetti, 77, 122

Bamford, 351

Banerji, 74, 159, 165, 166, 376

Baranow, 73

Barber, 157

Batchelor, 335, 337

Battaglia, 46, 333, 334

Baum, 424

Beal, 371

Beasley, 388

Beatty, 165-167

Beer, 43

Benetskaia, 159, 167
230, Benson, 68, 190

Bentham, 359, 364, 366

Berg, 58

Bergman, 124, 125, 325, 326, 329

Bernard, 304, 305

Berridge, 68

Beth, 394

Bhaduri, 71, 73, 260, 261, 277, 302

Bhargava, 57

Bianchi, 288

Billings, 39, 45, 161, 222, 334

Blackman, 195

Blakeslee, 192, 211, 314, 375, 378, 379,
381, 389, 391, 394, 396

Boehm, 32; 46, 67

Boos, 329

Bonnet, 33
2ii Borthwick, 302

Borwein, 144, 269

Bosio, 380

Botschanzeva, 201

Boursnell, 58

Bouvier, 370

Bowers, 43"

Boyes, 120

Brubaker, 376

Braun, 15

Brenchley, 257

Breslavetz, 201, 202

Brink, 141, 221, 269, 387, 389, 390, 427

Brongniart, 4

Brough, 31, 39, 139, 188

Brown, C. A., 68

Brown, R., 4, 366

Brown, S. W., 34

Brown, W. H., 87, 90

Brumfield, 155, 156

Buchholz, 192, 378, 379, 381

Buchner, 144

Buell, 67, 261

Buxbaum, 362, 371

433

434 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Caldwell, 39, 67, 68

Camerarius, 2, 3

Cameron, 33S

Campbell, 39, 106, 108

Capoor, 29-31, 33, 40, 43, 71, 75, 159,

164
Cappaletti, 271
Carano, 24, 275, 335, 337
Carlson, 32, 35, 103, 190, 291
Castetter, 43, 44
Cave, 371

Chamberlain, 23, 28, 135, 255, 416
Chandler, 381
Cheesman, 268

Chiarugi, 24, 103, 104, 223, 332
Chorinsky, 363
Christensen, 351
Clark, 261
Clarke, 366
Clausen, R. E., 317
Claussen, P., 35
Cochran, 192
Compton, 202
Condit, 59, 226, 404
Conklin, 394, 396
Cook, 67, 140, 209, 210, 344, 388
Cooke, 208
Cooper, D. C, 32, 33, 118, 140-142, 144,

164-168, 188, 189, 191, 200, 201, 203,

210, 211, 221, 261, 269, 293, 294, 314,

315, 345, 387, 427
Cooper, G. O., 169
Copeland, 30, 73, 261, 361
Corner, 56

Coulter, 23, 24, 28, 117, 255, 428, 429
Coy, 300

Crane, M. B., 259
Crete, 314, 344
Curtis, 190

Dahlgren, 24, 59, 65-67, 73, 99, 125, 132,
133, 138, 139, 141, 154, 169, 189, 191,
204, 208, 209, 223, 224, 226, 232, 234,
248, 249, 255, 269, 346, 380

D'Amato, 45, 114, 125, 164

Darlington, 156

De Candolle, 366

Dellinghausen, 205

De Mol, 172

Derman, 327

Derschau, 138, 206

De Vos, 248, 255

D'Hubert, 140

Dianowa, 181, 191, 195, 204, 270, 325, 332

Diels, 362

Dietrich, 384

Diettert, 134

Dixon, 222

Doak, 378, 379

Dodds, 75, 91

Dodel, 205

Doll, 229

Don, 359

Dorsey, 192

Dowding, 40

Doyle, 422

Duffield, 259

Duggar, 158

Duncan, 190

Dutt, 138

Earle, 69, 226, 300

East, 261, 391

Edman, 329-332, 349, 418

Eigsti, 164, 166, 181, 183, 204

Ekdahl, 112, 114, 138, 206, 346, 347

Elfving, 10, 11

Emsweller, 98

Engelbert, 348

Engler, 38, 361, 362, 364

Erdtman, 46

Ernst, 108, 194, 226, 304, 305, 332, 343, 344

Ernst-Schwarzenbach, 30, 188

Esau, 323

Eunus, 32, 77, 133

Evans, 139

Eysel, 126-128

Eyster, 381

Fagerlind, 24, 31, 56, 59-64, 69, 71, 73, 76-
78, 91-95, 106-108, 112, 114-116, 120,
122, 123, 125, 127, 128, 132-135, 140
142, 186. 187. 189. 229. 256. 325. 330.

NAME INDEX

435

332, 334, 345-347, 350, 395, 397, 418-

420, 425
Famintzin, 14, 272
Farr, 42, 43, 45
Fedortschuk, 39, 156, 161
Ferguson, 260, 261
Ferwerda, 377
Finn, 18, 21, 23, 77, 144, 154, 159, 161, 164,

165, 168, 169, 182, 185, 190, 196, 197,

200-202, 204
Fischer, 12
Fitting, 398
Flint, 58
Focke, 19, 258
Foster, 191, 210
Francini, 103, 140, 332
Friemann, 159
Fries, 257, 258
Frisendahl, 87, 89, 90, 156, 182, 183, 200,

207, 226
Frost, 338
Frye, 68, 226
Fuchs, 69, 165, 187

Gaertner, 398
Gager, 45, 156
Gangulee, 165, 166
Ganong, 337
Gardner, 400

Gates, 32, 34, 35, 41, 43, 392
Geerts, 96

Geitler, 154, 156, 158, 159, 164, 175
Gelin, 41, 187
Gentscheff, 334

Gerassimova, 92, 189, 191, 195, 197-201,
204, 205, 207, 211, 222, 226, 268, 318
Gershoy, 201, 202
Gilg, 361, 364

Gli6i6, 208, 241, 242, 255, 291
Goebel, 46, 66, 158
Golaszewska, 141
Goldberg, 231

Goodspeed, 92, 169, 195, 204
Goodwin, 34, 392
Gopinath, 138
Gorczynski, 37
Gore, 186, 226
Gran, 136
Graves, 75

Gray, 359

Grew, 2

Grimm, 185

Grove, 69

Guenn, 29, 40, 41, 68, 344

Guignard, 12, 14, 18-20, 117, 140, 163, 168,

195, 208, 291, 293, 344
Guilliermond, 161
Gupta, 30, 32, 39, 73
Gurgenova, 201
Gustafson, 398, 400-403
Gustafsson, 24, 313, 319, 320, 325, 336, 343,

394

Haagen-Smit, 389

Haberlandt, 66, 67, 256, 314, 348, 394, 395

Hafliger, 71, 327, 332, 334

Haertl, 334

Hagerup, 62, 190, 200, 203, 204, 221, 314,

316-318, 351, 361
Hakansson, 24, 75, 77, 112, 138, 195, 327,

331, 348
Haldar, 159
Hall, 344
Hallier, 359
Hallock, 181, 191, 364
Hammond, 103, 104
Hamner, 404
Hanf, 183
Hannig, 383, 384
Hanstein, 13, 14, 272
Haque, 115, 122
Harlan, 376
Harland, 351, 352
Harling, 92
Harris, 422, 424
Harrison, 259
Hartig, 10
Haupt, 55, 124, 126
Hedemann, 394
Hegelmaier, 15, 306, 348, 366
Heimann-Winawer, 191, 268
Heitz, 164
Herodotus, 1
Hertwig, 9

Hewitt, 169, 237, 239, 278
Hitchcock, 402, 404
Hoare, 195, 200, 201
Hodgson, 338

436 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Hoeppener, 97

Hoffman, 367

Hofmeister, 6-8, 10, 11, 58, 96, 186, 191

293, 357, 413, 418, 424
Holman, 376

Holmgren, 59, 71, 321, 325, 332
Hooker, 359, 364, 366
Houk, 63
Howe, 134
Howlett, 403-405
Hurst, 76, 77
Hutchinson, 359, 361, 362, 366, 368

Ishikawa, 132, 169, 203, 205, 366
Islam, 46, 159
Ivanov, 391
Iyengar, 204

Kausik, 29, 30, 45, 47, 65, 69, 142, 164, 227,

229, 237, 240, 248, 291, 345
Kent, 389, 390
Kerner, 21
Khan, B. A., 138

Khan, R., 59, 71, 73, 97, 125, 200, 226, 346
Kihara, 351, 391, 392, 393
Kirkwood, 133, 185
Kolreuter, 3
Koernicke, 168
Kosmath, 36, 37
Kostoff, 314, 317, 351, 352, 391
Kostriukova, 159, 161, 162, 167
Krause, 368, 369, 371
Krishna Iyengar, 142, 241, 243, 244
Krupko, 159, 188
Kuhlwein, 161
Kusano, 314

Jeffrey, 334, 343, 344

Jenkins, 327

Jonsson, 12, 122

Johansen, 73, 96, 97, 189, 209, 225, 226,

271, 344
Johnson, 24, 39, 40, 106, 108, 109, 229
Johnston, 165-167
Johri, 30, 32, 39, 46, 99, 110, 111, 143, 185,

201, 203, 205, 206, 210, 245, 248, 346,

422, 423
Jones, 98, 191, 195, 275, 276, 377
J0rgensen, 314, 315, 367, 368, 392
Joshi, 138, 143, 144
Juel, 21-23, 35, 170, 248, 329, 332
Juliano, 30, 31, 45, 46, 62, 89, 91, 169, 183,

191, 222, 225, 248, 249, 251, 256, 304,

305, 335, 337
Junell, 39, 58, 164, 185, 186, 229, 256
Jungers, 227, 255
Just, 357

Kadry, 188, 197, 226
Kajale, 36, 59, 68, 74
Kappert, 351
Karsten, 210
Kasparayan, 351
Katayama, 391, 393
Katz, 182

Lagerberg, 122, 156, 211, 227, 230

Laibach, 384, 386, 398

Lammerts, 317

Land, 195, 428

Landes, 56, 68, 69, 194

Langdon, 203, 226, 301

Langlet, 415

La Rue, 389

Latter, 35, 41

Lavialle, 66

Lawrence, 259

Lebegue, 314

Lebon, 191

Ledeboer, 382

Leeuwenhoek, 2, 14, 343

Le Monnier, 424

Leroy, 393

Levan, 41, 261

Lewis, 194, 381

Lloyd, 61, 293, 295

Locke, 45, 158

Longo, 185, 210, 225

Lotsy, 419

Luxemburg, 161

McGuire, 181

McKay, 191, 226, 268

Madge, 91, 165, 169, 183, 188, 200, 201

NAME INDEX

437

Magnus, 103, 105, 106, 156, 161
Maheshwari, 29, 30, 40, 59, 63, 73, 84, 99,

100, 110, 111, 115, 118, 122, 134, 136,

143, 144, 154, 155, 158, 183, 185, 186,

188, 203, 210, 226, 245, 316, 343, 346,

357, 368, 375, 376, 398, 412
Mahony, 188, 203
Mangelsdorf, 378
Marth, 400, 403, 404, 405
Martin, 182
Martinoli, 130, 131
Mascre, 35
Mason, 128
Massart, 398
Mathur, 125, 346
Matsura, 42
Matthews, 30, 31
Mauritzon, 57, 68, 76, 77, 92, 112, 141, 142,

208, 210, 226, 231, 232, 235, 236, 283,

298, 335, 346, 357, 359, 362
Mellink, 12, 13, 98, 117
Mendel, 375

Mendes, 63, 190, 191, 226
Merry, 256
Messeri, 98
Metcalfe, 429
Meyer, 34

Mezzetti-Bambacioni, 36
Michaelis, 205
Millardet, 398
Miller, 191
Mirbel, 4
Mitchell, 403
Modilewski, 96, 108, 110, 143, 144, 346-

348
Mohrbutter, 156, 223
Moissl, 33, 36
Moreland, 58
Mottier, 18, 117
Muntzing, 351, 391
Murbeck, 21-23, 71, 185, 329, 332
Murthy, 188

Nagao, 376
Naithani, 175
Nakajima, 392
Narayanaswami, 138, 316
Nast, 185, 226, 301

Nawaschin, 17-19, 21, 165, 168, 195, 197,
200, 204, 211, 258, 424

Nebel, 259, 377

N6mec, 171, 203, 205, 223, 427

Neumann, 362, 363

Newell, 377

Newman, 30, 139-141, 157, 158, 182, 184,

195, 200, 203, 207
Newton, 371
Nielsen, 327, 348
Nietsch, 35, 158, 257
Nilsson, 415
Nissen, 352
Noack, 327
Noguchi, 268, 392
Nohara, 30
Nordenskiold, 391
Nothnagel, 195

Oehler, 138, 139, 156, 226, 304, 306

Ohga, 188

Okabe, 323, 324, 332

Oksijuk, 58, 90, 143

O'Mara, 165, 168

Ono, 248, 249, 251, 255

Orr, 68

Osawa, 325, 335

Osterwalder, 137

Ownbey, 371

Pace, 138, 221, 332

Paetow, 73, 161, 162, 188, 200, 226

Pal, 376

Palm, 114, 252

Pantulu, 74

Pastrana, 190

Pax, 359, 367

Pearson, 418

Persidsky, 196

Peters, 32

Pfeiffer, 58, 300, 377

Piech, 158, 170, 171

Pijl, 335, 337

Pisek, 31, 40, 268

Pliny, 1

Poddubnaja-Arnoldi, 164, 169, 181, 191,

195, 223, 226, 270, 325, 332
Pope, 181, 191, 192, 203, 269, 270
Popham, 209

438

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Porsch, 413, 415-418

Prantl, 55

Pringsheim, 9

Prosina, 103

Puri, 35, 36, 39, 58, 87, 271

Quinby, 376

Radlkofer, 7

Raghavan, 165, 166

Rami ah, 351

Randall, 352

Randolph, 58, 135, 181, 301, 351, 382-385,

390
Rao, P. V. K., 45, 47, 164
Rao, V. S., 254

Rauch, 38, 71, 142, 203, 245, 298, 299
Razi, 103
Reed, 139
Rees, 32, 35
Reeves, 43, 45, 378
Reichenbach, 10
Renner, 97
Resende, 164
Rick, 352
Rietsema, 382
Rocen, 227, 229, 230
Romanov, 120, 122, 124
Ros6n, 134, 243

Rosenberg, 23, 40, 46, 157, 327, 329, 334
Rosendahl, 58, 257
Rudenko, 196, 200, 201
Ruhland, 159, 161
Rutgers, 271
Rutishauser, 39, 58, 78, 87, 90, 99, 101,

141, 231, 298, 331

Sabet, 226, 254

Sachet, 383

Sachs, 13, 14

Samuels, 108

Samue'.sson, 46, 157, 158, 344, 360

Sanday, 68

Sanders, 388, 389

Santos, 190

Sanz, 183

Sargant, 19, 117, 425, 427

Sartoris, 377

Satina, 211, 381, 389

Sawyer, 191, 195

Sax, 193, 195, 200

Schacht, 5, 7

Schaeppi, 38, 77, 99, 143, 245, 298

Schaffner, J. H., 161

Schaffner, M., 274

Schleiden, 4-8, 61

Schlimbach, 58

Schmid, 226, 305

Schnarf, 23, 56, 73, 159, 163, 164, 168,

226, 229, 231, 248-250, 255, 271, 334,

343, 357, 364, 368-371, 412, 415
Schomer, 404
Schreiner, 259

Schurhoff, 156, 223, 255, 415
Schulle, 142, 231, 233
Scott, 224, 225
Sears, 193, 325
Seshadri Ayyangar, 58
Sharp, 10, 42, 87, 90, 223
Shattuck, 56, 112, 138, 169, 186, 203, 206,

209, 346
Shibata, 191, 195
Shively, 208
Shoemaker, 190
Silow, 351
Singh, Bahadur, 31, 36, 39, 71, 142, 144,

245, 298
Singh, Balwant, 73, 99, 100, 183
Singh, H. B., 376
Siu, 389
Skirm, 388
Skovsted, 351, 352
Smith, A. C, 424
Smith, B. E., 269
Smith, C. M., 29, 66, 67, 164
Smith, F. H., 120, 195, 200, 201
Smith, L., 391
Smith, O., 192
Smith, P. F., 380
Smith, P. G., 388
Smith, R. W., 120
Solms-Laubach, 361
Sosnovetz, 204
Soueges, 23, 59, 61, 134, 268, 271-290, 293,

295-297, 300, 305, 306
Sprague, 208
Srinivasan, 134, 136, 346

NAME INDEX

439

Stadtler, 40

Stapf, 257

Starrett, 144, 181, 188, 203, 211, 281

Stebbins, 327, 329

Steindl, 38, 59, 77, 99, 143, 188, 245, 269,

298
Stenar, 24, 39, 57, 71, 87, 100, 102, 110,

112, 116, 117, 125, 127, 128, 132, 139,

140, 144, 182, 223, 226, 245, 247-250,

252, 253, 255, 296, 368, 369
Stephens, 110, 346, 351, 376
Stern, 42
Steschina, 204
Stevens, 193
Stevenson, 39
Stewart, 404
Stiffler, 144
Stingl, 383, 384
Stolt, 134, 135, 231, 295, 296
Stout, 170, 193, 376, 381
Stow, 172, 174, 175
Strasburger, 10-12, 15-17, 20, 84, 98, 117,

159, 168, 195, 197, 205, 335, 337, 357,

397, 413, 418, 424, 425
Stuart, 32, 35, 291
Subba Rao, 58, 110, 138, 335
Subramanyam, 30, 58, 226, 237, 345
Sussenguth, 337, 361
Suita, 170

Svensson, 59, 138, 238, 249, 250, 252, 254
Swamy, 46, 48, 69, 74, 77, 87, 99, 103, 112,

138, 158, 169, 182, 184, 185, 221, 248,

252, 268, 296, 302-303, 334, 336, 345,

346, 415, 424
Swarbrick, 398
Swingle, 259, 338

Tackholm, 138, 186, 210, 226
Tahara, 325
Takano, 376
Tanaka, 170
Tateishi, 110
Taylor, 30, 31
Theophrastus, 1
Thimann, 400
Thirumalachar, 138
Thoday, 40
Thomas, 427
Thompson, 186, 418^20

Thomson, 418

Thuret, 9

Tinney, 348

Tischler, 35, 161, 205, 271

Tison, 366

Tiwary, 335

Tomita, 63, 221, 256

Trankowsky, 165, 201, 204

Tretjakow, 15, 348

Treub, 12-14, 17, 18, 58, 98, 117, 139, 185,

245, 246, 296, 298
Tschernojarow, 169, 211
Tukey, 386, 390

Ubisch, 36
Umiker, 206
Upcott, 165, 166

Van Overbeek, 389, 394-396, 403
Van Tieghem, 65, 67
Venkateswarlu, 138, 141, 188, 210
Venkatasubban, 40
Ventura, 112, 142
Vesque, 12
Vignoli, 46
Von Mohl, 8

Walker, 112, 114, 120, 122, 188, 226, 298

Wangerin, 364

Wanscher, 200, 223, 226

Ward, 12

Warming, 12, 29, 362

Warmke, 189, 191, 200, 204

Webber, 260, 335, 337, 343, 351

Weber, 98, 132, 204, 269

Wefelscheid, 159

Weinstein, 141, 142, 203

Weinzieher, 140, 157, 158, 203

Welsford, 165, 168, 195, 200

Weniger, 200

Went, 99

Werner, 186, 210

West, 164, 183, 200, 202

Wettstein, 362

Wetzel, 159, 161

440 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

White, 268
Whitehead, 68
Wiger, 208, 335, 337
Winkler, 313
Witkus, 33, 34
Witmer, 32, 39, 164, 248
Wodehouse, 46
Woodcock, 63, 362
Woodroof, 209
Wood worth, 41, 349

Wulff, 24, 154, 158, 164, 165, 169, 197, 412
Wunderlich, 165, 247, 248, 369
Wylie, 39, 158, 169, 195-197, 201, 203, 211,
240, 241

Yamashita, 393

Yamaura, 134

Yamamoto, 352

Yasuda, 193, 378, 379, 392, 399, 400

Yocom, 240, 241

York, 139

Zimmerman, 404
Zweifel, 231, 233, 298, 299

SUBJECT AND PLANT INDEX

Aberrant and unclassified embryo sacs,

125
Abnormal embryos, 298
Abnormalities of fertilization, 204-209
Abortive embryos, 386
Abronia, 429
Acacia, 139-141, 157, 158, 182-184, 195,

200, 203, 207
Acalypha, 69, 110, 111, 131, 185, 346, 416,

426
Acanthus, 237

Accessory pollen tubes, 203
Achillea, 72
Achras, 188
Achroanthes, 296
Aconitum, 137, 169
Acorus, 67, 261

Adoxa, 85, 122, 123, 156, 211, 229, 230, 417
Adoxa type of embryo sac, 122
Adventive embryony, 15, 313, 334, 346,

393
Aeginetia, 30, 62, 183, 304, 305
Agapanthus, 368, 428
A gat his, 421
Agave, 46, 69
Agraphis, 13, 98
Agropyrum, 257
Agrostemma, 230
Aizoaceae, 362, 363
Alangium, 37
Albizzia, 158
Albuca, 133

Alchemilla, 21, 23, 185, 329, 332
Alchornea, 337
Alectorurus, 369, 370
Aleurone layer, 256
Alisma, 14, 35, 269
Allioideae, 368
Allium, 15, 66, 98, 132, 144, 155, 158, 204,

269, 346-348, 368
Allium type of embryo sac, 98
Alnus, 190, 349

Aloe, 370, 371

Alonsoa, 241, 243

Althaea, 181

Amaryllis, 58, 165, 166

Amianthium, 422

Amitotic divisions in endosperm, 225

Amoeboid tapetum, 35, 36

Ampelopsis , 398

Amphicarpaea, 291

Amphimixis, 313

Amputation of style, 378

Amyema, 38, 143, 298

Anacampseros , 363

Ananas, 402

Androgenic haploids, 317

Androsaemum, 284, 285

Anemarrhena, 369

Angelonia, 134

Anogra, 73, 96, 225

Anona, 30, 45, 46

Antennaria, 21, 22, 329, 332

Anther, 28

Anther tapetum, 32

Anthericum, 156, 159, 249, 250,

369
Antipodal cells, 134
Antipodal embryos, 346, 350
Antipodal haustoria, 134, 135
Apicra, 370
Apogamy, 313
Apomixis, 22, 209, 313

facultative, 323

nonrecurrent, 314

recurrent, 319

unclassified, 329
Aposporic embryo sacs, 332
Apospory, generative, 313, 321

somatic, 313, 327
Arachis, 139, 257
Araucaria, 411, 421
Arceuthobium, 31, 40, 64
Archesporium, in anther, 29

in ovule, 69
Archieracium, 327

441

442

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Arechavaletaia, 142

Aril, 56, 57, 369

Aristolochia, 46

Aristotelia, 76, 77

Armeria, 118

Artemisia, 134

Arthropodium, 369

Artificial culture of embryos, 383

Asclepias, 68, 156, 161, 163, 164, 201, 226

Asimina, 45, 158

Asparagus, 352

Asperula, 295

Asphodeline, 369, 370

Asphodeloideae, 369

Asphodelus, 56, 57, 248, 369, 370

Aspidistra, 141

Aster, 135

Asterad type of embrj-o, 272, 275

Astilbe, 141

Asymmetrical spindle in microspore, 155,

156
Atamosco, 138

Atraphaxis, 329-332, 349, 350
Atriplex, 45, 161, 169, 412

Balanophora, 31, 64, 76, 77, 92, 94, 231-

233, 298, 299, 332, 415
Balsamita, 128
Banana, 398
Basal apparatus, 236
Bassia, 188
Belvitan, 392
Bergenia, 314
Berkheya, 187

Beta, 181, 188, 203, 211, 281
Betula, 154

Bisporic embryo sac, 98
Bletia, 223
Blumium, 429

Bodies in pollen tube, colored, 161
Boerhaavia, 29, 186, 203
Bougainvillaea, 32
Bouvardia, 62
Boykinia, 248
Brachychilum, 57
Br achy siphon, 110
Brassica, 186, 193, 381, 392
Breadfruit, 398
Brexia, 226

Broughtonia, 87
Brunsvigia, 58
Bryophyllum, 165, 166, 389
Bulbils, 313

Bulbine, 164, 248, 369, 370
Bulbinella, 369, 370
Bulbinopsis, 369, 370
Bulbophyllum, 87
Burbidgea, 57
Burmannia, 304, 305
Butomopsis, 46, 201, 422-425
Butomus, 36, 59, 71, 99

Cactaceae, 362

Caecum, 143, 144

Calendula, 133

Callitrichaceae, 366

Callitriche, 366, 367

Calochortus, 371

Calopogon, 221

Calotropis, 226

Caltha, 136

Calypso, 87

Camassia, 195, 200, 201

Camellia, 165, 166

Campanula, 159

Canangium, 56

Canna, 397

Capparis, 335

Capsella, 13, 14, 271-274

Capsicum, 351, 401

Cardamine, 37

Cardiocrinum, 118

Cardiospermum, 188, 197, 226

Coreya, 57, 191, 226, 268, 301

Carc'ca, 191, 210

Carludovica, 92

Carpinus, 68

Carum, 429

Caruncle, 56

Caryophyllad type of embryo, 272, 282

Cassia, 227

Castalia, 67, 140

Castanea, 68

Caswormo, 17, 18, 69, 74, 77, 184, 185

Cattleya, 190

Caylonia, 424

Caytoniales, 422

Cellular type of endosperm, 221, 229

SUBJECT AND PLANT INDEX

443

Celsia, 241, 242

Centranthera, 242, 244

Centranthus, 229, 230

Centrospermales, 362

Cephalanthera, 203, 221, 314

Cephalanthus, 140

Cephalotaxus, 412

Ceratostigma, 125

Chalazogamy, 17, 185

Chalazosperm, 258

Chamaeorchis , 87

Char a, 3

Chenopodiad type of embryo, 272, 280

Chenopodium, 169, 280

Chimera, 382

Chlorophyll in integuments, 58

Chlorophytum, 369

Chondrilla, 332

Chrysanthemum, 29, 73, 112, 114, 115, 131,

389, 417
Cicer, 291, 293
Cimicifuga, 226, 300
Cinchona, 377
Circaea, 96

Circaeaster, 39, 58, 164, 185, 186, 229,256
Citrus, 15, 226, 335, 337, 338, 346, 348, 393,

398
Cleavage polyembryony, 343
Cleistogamous flower, 182, 183
Clintonia, 118, 120
Cobaea, 59
Cochlearia, 383
Cochlospermutn, 226
Codonopsis, 380
Coelebogyne, 15
Coelogyne, 48
Coenomegaspore, 106
Co#ea, 63, 190, 191, 226, 377
Colchium, 191, 268
Coleoptile, 289, 290
Coleorrhiza, 289

Colored bodies in pollen tube, 161
Columella, 40
Colutia, 223
Combretum, 112

Compound embryo sac, 90, 91, 350
Compound pollen grains, 46, 48
Conjoined twins, 353
Control of fertilization, 375
Convallaria, 100, 102, 159, 165, 166
Corallorhiza, 87, 223
Corchorus, 140
Coreopsis, 41, 389

Cornus, 45, 118
Corydalis, 296, 297, 303, 429
Corylus, 190
Costus, 32, 67, 74

Cotylanthera, 138, 156, 226, 304, 306
Course of pollen tube, 183
Crassula, 92

Crepis, 92, 169, 189, 191, 195 198-201, 204,
205, 207, 211, 222, 226, 268, 318, 327
Crinum, 63, 138, 161, 167, 170, 221, 256
Crocus, 191
Crossandra, 236, 237
Crotalaria, 210, 344
Croton, 6S

Crucianella, 112, 115
Crucifer type of embryo, 271, 272
Crystals in embryo sac, 141
Crystals in pollen grain, 161, 162
Cucumber, 398, 400
Cucurbita, 5, 169, 185, 210, 257
Culcitium, 76
Cupressus, 412

Cuscuta, 32, 156, 159, 161, 200, 204, 269
Cyanastrum, 158, 257
Cyathula, 74
Cyclamen, 376

Cymbidium, 169, 182, 302, 303
Cymodocea, 46
Cynanchum, 140
Cynomorium, 59, 183, 269
Cyperaceae, 170
Cypripedium, 48, 102, 190
Cypripedium type of embryo sac, 103
Cytinus, 56
Cytisus, 293
Cytokinesis, in generative cell, 164-16S

in microspore mother cell, 42-45
Cytomixis, 40

Dacrydium, 411

Dactylis, 351

Damasonium, 269

Daphne, 187, 188

Date palm, 1

Datura, 183, 192, 211, 314, 378, 381, 382,

388-391, 394, 396
Daucus 302
Debesia, 369
Degeneria, 424
Dendrobium, 190

444

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Dendrophthoe, 38, 64, 71, 142, 298

Dendrophthora , 139

Desmodium, 74

Development of embryo in aposporic

embryo sacs, 332
Development of pollen in Cyperaceae, 170
Dianella, 248
Dianthus, 3
Dichopogon, 369
Dichrostachys, 38
Dicotyledonous embryo, 271, 429
Dicraea, 105, 106
Digera, 143

Dionaea, 29, 66, 67, 164
Diplospory, 313
Diploid-diploid twins, 351, 352
Diploid parthenogenesis, 313
Diploid-triploid twins, 352
Discharge of male gametes, 189
Discovery, of pollen tube, 3

of sexual fusion in lower plants, 9

of syngamy, 15

of true relation between pollen tube
and embryo, 5
Ditepalanthus, 31, 132, 425
Diuranthera, 369
Diverticulum, 227, 230
Division of microspore nucleus, 154
Divisions, of generative cell, 163

sticky type of, 34

of tapetal nuclei, 33-35
Doryanthes, 30
Double fertilization, 18, 20
Drimiopsis, 73
Drimys, 46

Drosera, 48, 201, 285, 286
Drusa, 112, 417
Drusa type of embryo sac, 112

Echinocystis, 223, 225
Echinodorus, 248, 269
Echium, 249, 250
Egg, 11

Egg apparatus, 132
Egg-like antipodals 346
Egg-like synergids, 205, 346
Eggplant, 400
Eichhornia, 165, 166, 412
Elatine, 87, 89, 90, 182, 226

Elatostema, 330-332, 346, 347, 350

Elegia, 144

Elodea, 39, 48, 158, 188, 203

Elytranthe, 38, 77, 143

Embryo culture, 383

Embryo factor, 389

Embryo sac haustoria, 141

Embryo-sac-like pollen grains, 171

Embryo sac types, Adoxa, 122

Allium, 98

Drusa, 112

Fritillaria, 117

Oenothera, 96

Penaea, 110

Peperomia, 106

Plumbagella, 120

Plumbago, 124

Polygonum, 87

unclassified, 125
Embryo sacs, of aposporic origin, 332

compound, 90, 91, 350

food reserves in, 138

formation of haustoria in, 141

mature, 131

morphological nature of, 412

showing disturbed polarity, 136, 415
Embryogenic mass, 344
Embryology in relation to taxonomy, 357
Embryos and modification of suspensor,
290

types of Asterad, 272, 275
Caryophyllad, 272, 282
Chenopodiad, 272, 280
Crucifer, 271, 272
dicotyledonous, 271, 429
monocotyledonous, 286, 428
Solanad, 272, 277

unclassified and abnormal, 298

unorganized and reduced, 302
Empetraceae, 359
Empetrum, 154, 344, 360
Enalus, 29, 248
Endomitosis, 34
Endosperm, 221

formation of haustoria in, 231

histology of, 255

morphological nature of, 19, 424

types of, cellular, 229
helobial, 221, 245
nuclear, 222
relationship between different, 252

SUBJECT AND PLANT INDEX

445

Endosperm embryos, 222, 334, 349

Endosperm haustoria, 231

Endosperm mother cell (see Endo-

spermanlage)
Endospermanlage, 136
Endostome, 58
Endothecium, 28, 30, 31
Endothelium, 63, 65
Entry of pollen tube into embryo sac,

188
Ephedra, 412, 414
Epiblast, 289, 429
Epidendrum, 268, 296
Epilobium, 96, 186, 205
Epimedium, 164

Epipactis, 87, 90, 296, 316, 316, 317
Epiphegus, 208
Epiphyllum, 362
Epiphysis, 276, 277, 281, 282
Epistase, 67
Eremocrinum, 369
Eremurus, 245, 247, 369, 370
Erica, 31, 158
Erigenia, 429
Erigeron, 195, 325
Eriodendron, 138
Erythraea, 314
Erythronium, 118, 122, 161, 343, 344, 371,

422
Eschscholtzia, 165-167
Euchlaena, 141
Eucomis, 144
Eugenia, 335, 337, 346
Eulophia, 297, 302, 345
Eupatorium, 321, 322
Euphorbia, 164, 335, 337
Exine, 36, 37
Exostome, 58

Experimental embryology, 375
Extra sperms in embryo sacs, 205

Facultative apomixis, 323

Facultative parthenogenesis, 316

Fagopyrum, 188, 193, 203

Fagraea, 223

False polyembryony, 343

Farmeria, 103

Female gametophyte. 10, 84, 412

Fertilization, 17, 181, 421
antipodal cells, 138, 206, 207
of synergids, 205, 206

Ficus, 59, 225, 226, 271

Fig, 404

Filiform apparatus, 132

Food reserves, in embryo sac, 138
in pollen grain, 162

Form and structure of male gametes, 195

Formation of megaspores, 70, 73

Formation of vegetative and generative
cells in pollen grain, 154

Forsythia, 165, 166

Fouquieria, 59

Fragaria, 277

Fritillaria, 18, 118, 119, 122, 195, 200,
255, 371, 422, 425

Fritillaria type of embryo sac, 117

Fuchsia, 138

Fucus, 9

Fumaria, 296, 297

Functioning megaspore, 12, 75

Funiculus, 57, 186, 203, 210

Funkia, 15

Furcraea, 48

Gagea, 120, 203, 205, 368
Gaillardia, 118
Galanthus, 165, 204
Galinsoga, 209
Galium, 77, 78, 134, 135, 142
Galtonia, 397
Gametic fusion, 194
Garrya, 181, 190, 364, 365
Garryaceae, 364
Gasleria, 370, 371
Gastrodia, 314

Generative apospory, 313, 321
Generative cell, division of, 163
Generative cell, division of, 163
form of, 159

movement of, 197

origin of, 11, 16, 158

structure of, 159
Gentiana, 135
Geodorum, 87, 302
Germ pore, 181
Germinal vesicle, 11

446 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Germination of pollen, 181, 380

Geum, 276, 277, 281, 302

Ginalloa, 99

Ginkgo, 411, 412

Gladiolus, 58

Glandular tapetum, 35, 36

Globular ia, 243

Gloriosa, 32, 76, 77

Gnetallan theory, 413, 418

Gnetum, 412, 413, 416, 418-422

Golgi body, 161

Gomphandra, 56

Gonocaryum, 56

Goody era, 296

Gossypium, 58, 183, 186, 204, 226, 351,

376, 388
Grafting of styles, 378, 379
Granular markings on tapetum, 36, 37
Grape, 398
Grapefruit, 398
Gratiola, 291
Grevillea, 188, 227, 229
Grindelia, 134
Gunner a, 108, 110
Gymnadenia, 296
Gypsophila, 229

Habenaria, 46, 296
Haberlea, 291
Halophila, 45, 47
Hamamelis, 190
Haploid apogamy, 313
Haploid-diploid twins, 351
Haploid embryos, 205, 314, 391
Haploid parthenogenesis, 205, 209, 313,

393
Haploid-triploid twins, 352
Hartmannia, 97
Haustoria, 77, 78, 133-135, 141, 209,

227-245, 249, 283, 291-298, 303
Haworthia, 370
Heckeria, 118
Hedychium, 91, 165
Helianthemum, 58
Heliotr opium, 59
Helixanthera, 143, 298
Helleborus, 380

Helobial type of endosperm, 221, 245

Heloniopsis, 248

Helosis, 64, 132, 204, 229

Hemerocallis, 165, 397

Heptapleurum, 138

Heterofertilization, 208

Heterostyly, 193

Hibiscus, 4

Hicoria, 209

Hieracium, 326-328, 334

Himantoglossum, 154

Hiptage, 335

Histology of endosperm, 255

Holopteiea, 30, 31, 33, 40, 164

Hordeum, 181, 191, 192, 203, 269, 376, 387

Hormone, 380, 400

Hosta, 395

Houstonia, 61-63

Hyacinthus, 171, 172, 174

Hybrids, 3, 41, 46, 183, 384, 386, 387

Hydrilla, 70-72, 88, 210

Hydrobium, 103

Hydrocharis, 35

Hydrophyllaceae, 237

H ymenocallis , 58, 68

Hyoscyamus, 3, 250

Hypecoum, 134, 295, 297

Hypericum, 252, 253, 327

Hypophysis, 272, 273

Hypostase, 65, 91, 210

Hypoxis, 248

Ianthe, 248
Identical twins, 351
Ilex, 401, 404
Ilysanthes, 242

Impatiens, 164-166, 191, 232, 234, 255
Incompatibility, 193, 194, 381
Induced parthenocarpy, 398
Induced parthenogenesis, 391
Integumentary obturator, 187
Integumentary tapetum, 63, 65, 395, 396
Integumentary vascular bundles, 67, 68
Integuments, 55, 187

Interval between pollination and fertil-
ization, 190, 191
Intine, 181
Lntracarpellary pollen grains, 422, 423

SUBJECT AND PLANT INDEX

447

Intraovarial pollination, 380
Ipomoea, 30, 63

Iris, 164, 191, 195, 205, 382, 385, 387, 390
Irregular division of endosperm nuclei,

225
Isomer is, 222, 334
Isoplexis, 241
Isototna, 345
Ixeris, 323, 324, 332
Ixiolirion, 248, 250

Juglans, 168, 185, 195, 203, 204, 226, 301
Juncus, 158, 164
Jussieua, 73, 97, 200, 226

Kigelia, 40

Kirengeshoma, 142

Knautia, 37, 66

Kniphofia, 370, 371

Korthalsella, 39, 64, 99, 101, 141, 298

Lactuca, 33, 35, 191, 195, 275, 276, 389

Langsdorffia, 77, 92, 94, 95

Lappula, 252, 254

Larix, 421

Lateral haustoria, 230, 249

Lalhraea, 35, 41, 134, 241, 242

Laurus, 46, 77

Leer si a, 429

Leiphaimos, 138, 139, 226, 304, 306

Leitneria, 58, 300

Lemna, 39, 67, 68

Lennoa, 361

Lennoaceae, 361

Leontodon, 34, 124, 125

Lepeostegeres, 143, 298

Leucosyke 186, 187

Levisticum, 195

Ligularia, 135

Lilioideae, 371

LiZwm, 14, 18, 20, 28, 32, 33, 37, 58, 117,
118, 122, 159, 161, 162, 165-168, 195,
200, 314, 315, 371, 388, 397, 412

Limnanthes, 71, 73, 127, 128, 132, 133

Limnocharis , 35, 344

Limnophyton, 39, 248

Linaria, 193

Lindelofia, 138

Linum, 351, 384, 386

Listera, 296, 314, 316, 376

Lloydia, 288, 371

Lobelia, 65, 134, 237, 239, 278, 279, 344

Lomatophyllum, 370

Lonicera, 36

Lopezia, 226

Loranthus, 139, 246

Ludwigia, 73, 274, 275

Lu#*a, 133

Lupinus, 161, 291, 292

Luzula, 287, 288

Lychnis, 202

Lycopersicum, 192, 260, 388, 389

Lycopodium, 398

Lyonothamnus , 248

Macadamia, 221 , 229
Machaerocarpus , 73, 99, 100
Macrosolen, 144, 246, 298
Magnolia, 37, 45, 69
Maianthemum, 112, 116, 117
Male cells or nuclei, 168, 201, 202
Male gametes, 190, 195, 197
Male gametophyte, 10, 154, 411
Mallotus, 112
Malpighia, 58
MaJus, 200, 223, 226, 327
Malva, 181, 226
Mammillaria, 362
Mangifera, 226, 335, 346, 393
Martynia, 9, 208
Massula, 48
Matthiola, 3

Mature embryo sac, 131
Medicago, 140, 141, 291, 351
Megasporangium, 54
Megaspore haustoria, 77, 78
Megaspore mother cell, 12
Megaspore tetrad, 73-75
Megasporogenesis, 70, 73
Meiosis, in anther, 42, 43
in ovule, 73-75

448

INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Melandrium, 201, 229

Melastoma, 30, 226

Melilotus, 43, 44, 142

Melocanna, 257

Mercurialis, 2

Mesocotyl, 290

Mesogamy, 186

Metaxenia, 259

Mezzettia, 56

Microcycas, 411

Micropyle, 58, 183, 184, 187, 188, 202

Microsporangium, 28

Microspore, division of, 154

formation of, 45

resting period of, 154
Microspore mother cell, 37
Microspore tetrad, 10, 45-48
Microsporogenesis, simultaneous, 42, 44

successive, 43, 45
Middle layers in anthers, 31, 32
Migration of nucellar nuclei into embryo

sac, 91
Milla, 380
Mimusops, 188
Mirabilis, 394
Mitella, 208, 248, 249
Modifications of suspensor, 290
Monochoria, 159, 249, 251
Monocotyledonous embryo, 286, 428
Monosiphonous pollen grains, 181
Monosporic embryo sacs, 87
Monotropa, 8, 16, 17, 191, 195, 202, 205
Moringa, 35, 39, 58, 271
Morphological nature, of embryo sac, 412

of endosperm, 19, 424
Mosaic endosperm, 259
Mulberry, 2

Multiple embryo sacs, 89-91, 347, 350
Multiple embryos, 205, 348
Multiple fusions and polyspermy, 202
Multiple seedlings, 350, 352
Musa, 30, 46, 75, 91, 169, 222, 225, 492
Muscari, 165, 247, 248, 287, 288
Myosotis, 281, 282, 302
Myosurus, 169, 211
Myricaria, 118, 156, 200, 207
Mijriocarpa, 186, 187
Myriophyllum, 134, 295, 296
Myzodendron, 64

Naias, 39
Naked ovule, 63
Narcissus, 13, 161, 167
Narthecium, 248
Necrohormone, 174, 394
Nelumbo, 188
Nemec-phenomenon, 172
Nemesia, 193
Nemophila, 165, 237, 238
Neottia, 46
Nerine, 58
Nicolaia, 32, 46, 67

Nicotiana, 3, 42, 92, 164-166, 169, 195,
204, 277-280, 317, 318, 345, 389, 401
Nigella, 138, 206
Nigritella, 335-337
Nomocharis, 371
Nonrecurrent apomixis, 314
Nothoscordum, 15, 54, 132, 368
Nucellar beak, 59, 61
Nucellar budding, 351
Nucellar cap, 67, 68
Nucellar tracheids, 68, 69
Nucellar embryony, 335, 349
Nucellus, 59, 188

Nuclear divisions in tapetum, 33-35
Nuclear fusions in tapetum, 34
Nuclear type of endosperm, 221, 222
Nymphaea, 344

Obturator, 60, 185-187

Ochna, 332

Oedoqonium, 9

Oenothera, 34, 40, 96, 97, 132, 169, 203,

205, 314, 392, 394, 395
Oenothera type of embryo sac, 96
Oldenlandia, 62, 63
Onagraceae, 365
Oncidium, 71, 73, 87, 337
Ononis, 291, 293
Operculum, 67
Ophiopogon, 29, 40, 59
Opuntia, 54, 56, 57, 337, 362, 363
Orange, 398
Orchis, 6, 62, 87, 190, 200, 204, 314, 316,

318

SUBJECT AND PLANT INDEX

449

Ornithogalum, 175, 248
Orobanche, 196, 200-202
Orobus, 291, 293

Oryza, 31, 191, 256, 268, 351, 376
Ostrya, 77, 185
Osyris, 64

Ottelia, 46, 159, 210, 422
Ouvirandra, 36

Ovule, component parts of, archespo-
rium, 69

aril, 56-57

caruncle, 56

epistase, 67

hypostase, 65, 91

integumentary tapetum, 63-65

integuments, 55

micropyle, 54, 58

nucellus, 59

obturator, 60, 185-187
form of, 54, 55

occurrence of chlorophyll in, 58
vascular supply of, 67
Oxalis, 188
Oxybaphus, 191, 200, 210, 269

Pachycereus, 362
Paeonia, 380
Pancratium, 155
Pandanus, 91, 93, 426
Pa-paver, 169
Paphiopedilum, 89, 190
Paradisia, 144, 369
Parkia, 38

Parthenium, 191, 204, 323
Parthenocarpy, 398
Parthenogenesis, 21

diploid, 313

facultative, 316

haploid, 205, 209, 313, 393

induced, 391
Passerina, 187
Passiflora, 209
Pedicularis, 242
Peltandra, 231
Penaea, 110, 346, 426
Penaea type of embryo sac, 110
Pentas. 140

Pentstemon, 139

Peperomia, 106-110, 131, 189, 229, 416, 426

Peperomia type of embryo sac, 106

Pepper, 400

Pereskia, 362, 363

Periplasmodium, 35, 36

Perisperm, 61, 256, 257

Persimmon, 398

Petasus, 67

Petunia, 140, 189, 193, 201, 211, 260, 261,

378, 381, 392, 399, 400
Phacellanthus , 42
Phajus, 87, 296
Phalaenopsis, 190, 297
Phaseolus, 141, 142, 203, 291, 293
Philadelphia, 142, 184
Phleum, 42, 351, 352
Phlomis, 165
Phoenix, 257, 259, 376
Pholidota, 48
Pholisma, 30, 361
Phoradendron , 39
Phryma, 141, 188, 189
Phyllis, 61, 62, 134, 135
Phyllocladus, 411
Phyllospadix, 46
Phytelephas, 255
Phytocrene, 56
Pilea, 187
Pineapple, 398
Pinus, 411, 412
Piper, 39, 118
Pistoria, 298

Pisum, 203, 257, 291, 293, 294
Plasmodium, 42
Platanthera, 203, 314, 316
Plumbagella, 85, 120, 131, 132, 169, 188,

346, 416, 421, 425
Plumbagella type of embryo sac, 120
Plumbago, 55, 85, 124-126, 131, 188, 346,

416, 421, 426
Plumbago type of embryo sac, 124
Poa, 75, 138, 289, 290, 327, 348
Podocarpus, 411
Podophyllum, 33, 156
Podostemon, 103-105, 156, 161
Podostemon type of embryo sac, 106
Pogonia, 48
Polar nuclei, 13, 136
Pollen, development of, 155

450 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

food reserves, 162

germination of, 181, 380

shedding stage of, 164

viability of, 24, 181, 376
Pollen -embryo sacs, 172, 174
Pollen tubes, branching of, 182, 189, 210

bursting of, 375

callose plugs in, 181

coiling of, 174, 193

colored bodies in 161

course of, 183

discharge of, into embryo sac, 189

discovery of, 3

entry of, into embryo sac, 188

function of, 197

haustorial function of, 209

number of, per embryo sac, 203

persistence of, 209

rate of growth of, 190
Pollinium, 48, 157, 182
Polyembryony, 14, 15, 343
Polygonatum, 164, 166, 204
Polygonum, 12, 59, 61, 84
Polygonum type of embryo sac, 87
Polyploidy, 205

Polysiphonous pollen grains, 181
Polyspermy, 202
Polyspory, 46
Populus, 56
Porogamy, 183
Porsch's theory, 414, 415
Portulaca, 3, 4, 140, 141, 164, 189, 191
Potamogeton, 36
Potentilla, 78, 90, 331
Primary endosperm nucleus, 207
Primula, 223, 226

Production of adventive embryos, 393
Prothallial cells, 412
Prunus, 381, 386, 388
Pseudoembryo sac, 104, 221
Pseudoembryos, 395, 396
Pseudogamy, 334
Pseudohomotypic division, 319, 320, 326,

327, 331
Pseudotsuga, 421
Psychotria, 140, 256
Putoria, 78, 134, 135

Quamoclit, 39

Quercus, 190
Quisgualis, 59, 60, 186

Rafflesia, 220, 305

Ramondia, 208

Ranunculus, 31, 39, 54, 223, 305, 306,

327, 332, 429
Raphanus, 3S3
Raphide in embryo sac, 141
Rate of growth of pollen tube, 190
Receptivity of stigma, 377
Recurrent apomixis, 319
Relationships between different types of

endosperm, 252
Reseda, 90, 181
Restio, 269
Restitution nucleus, 46, 319, 323, 324,

326, 327, 331
Rhipsalis, 362, 363
Rhus, 185
Ribes, 382
Richardsonia, 140
Ricinus, 2, 257
Rosa, 76, 77
Rosularia, 77, 78
Rubia, 62, 112

Rudbeckia, 118, 134, 136, 333, 334, 346
Ruellia, 232, 235-237
Ruppia, 71
Ruta, 271

Saccharum, 138, 181, 377

Sagina, 282, 283, 288

Sagiltaria, 35, 201, 203, 205, 248, 288, 289

Salix, 183

Salpiglossis, 400

Salsola, 169

Salvia, 32, 35, 291

Sambucus, 122, 156

Sandoricum, 89, 91

Sansevieria, 29, 37

Santalum, 64

Saponaria, 229

Sarcococca, 335, 337

Sarcocolla, 110

SUBJECT AND PLANT INDEX

451

Sasa, 134

Sassafras, 300

Saxifraga, 248, 284

Scabiosa, 229, 300

Scheuchzeria, 247, 248

Schleiden's theory of origin of embryo, 4

Schiirhoff's theoiy, 415

Scilla, 18, 21, 98, 195, 200, 201

Scirpus, 171

Scurrula, 64, 71, 142, 203, 298, 299

Scutellum, 289, 290

Secale, 169, 351, 352, 387, 391

Secondary nucleus, 13

Sedimi, 77, 140, 283, 298, 346

Seedless fruits, 398

Semigamy, 333, 334

Semiheterotypic division, 319, 320, 323,

325
Senecio, 76, 169, 230
Sequoia, 414
Sesamum, 30
Sex in plants, 1-3
Sherardia, 279, 280
Silphium, 195
Simethis, 369
Single fertilization, 208
Sobralia, 58
Soja, 291

Solanad type of embryo, 272, 277
Solarium, 71, 73, 314, 315, 351, 376, 392,

399
Somatic apospory, 313, 327
Sonneratia, 141, 210
Sopubia, 241
Sorghum, 181, 376
Sparganium, 36, 412
Spathoglottis, 296
Spermatic granules, 4
Spermatic tubules, 4
Spinacia, 34
Spiranthes, 316, 346
Splicing of styles, 378, 379
Sporogenous tissue, in anther, 29, 37

in ovule, 71
Sporophytic budding, 313, 338
Stanhopea, 297, 302
Starch in embryo sac, 139
Statice, 118, 120, 204
Stellaria, 412
Stenosiphon, 226

Sterile septa in anther, 38, 39

Stigma, 182

Stomata on integument, 58

Stomium, 28

Storage of pollen, 24, 376

Stratiotes, 36

Strombosia, 69

Strychnos, 156

Style, 183, 187, 378, 379

Styphelia, 31, 39, 139

Styrax, 73, 154

Supernumerary eggs, 205

Supernumerary pollen tubes, 203

Supernumerary sperms, 204, 205

Suspensor, 134, 271, 272, 290

Suspensor haustorium, 14, 283, 291-298,

303
Swertia, 41
Symphoricarpos, 36
Symplocarpus, 58, 158, 257
Synergid embryo, 350
Synergid haustoria, 133
Synergids, 11, 132, 188, 194, 205, 211, 296,

314
Syngamy, 15, 16, 18, 20, 194, 198-201,

269, 270
Syngamy without triple fusion, 208

Tacca, 73, 188, 200, 226

Tagetes, 381

Tamarix, 118

Tanacetum, 112, 115, 116, 417

Tapetum, amoeboid, 35, 36

in anther, 32

cutinization of inner wall of, 37

glandular, 35, 36

granular markings on, 36, 37

nuclear divisions of, 33, 34, 35

periplasmodium, 35, 36
Taraxacum, 181, 189, 191, 195, 200, 204,

269, 270, 325, 332, 389
Taraxia, 209
Taxillus, 143, 298
Taxus, 412
Terauchia, 369
Tetrad, of megaspores, 73-75

of microspores, 45-48

452 INTRODUCTION TO EMBRYOLOGY OF ANGIOSPERMS

Tetrasporic embryo sacs, 106
Theobroma, 268

Thesium, 58, 64, 87, 142, 231, 233
Thunbergia, 237
Thysa?wtus, 369
Tilia, 139, 223
Tobacco, 400, 401
Tofieldia, 248
Tomato, 34, 400, 402, 403
Torenia, 142, 184, 202, 242
Torreya, 412

Tradescantia, 10, 36, 154, 155, 166, 193
Transmitting tissue, 4, 183
Trapa, 96, 365, 366
Trapaceae, 365
Trianthema, 56, 57
Trifolium, 182, 193, 291, 351, 381
Triglochin, 36
Trigonella, 291
Trillium, 42, 195, 334, 422
Triple fusion, 18, 20, 194, 198, 221, 269,
270

without syngamy, 208
Tripsacum, 378

Triticum, 200, 257, 351, 352, 391-393
Tropaeolum, 188, 226, 298
Tsuga, 421

Tube nucleus, 169, 194, 211
Tulbaghia, 54, 368
Tulipa, 14, 118, 120, 122, 124, 165, 166,

201, 344, 368, 371, 417
Twins and triplets, 350
Types, of embryo sac, 84-87

of endosperm formation, 221
Typha, 36, 48, 376

Ulmus, 17, 56, 112, 114, 122, 138, 169, 186,

203, 207, 209, 346, 347
Umbilicus, 92

Unclassified and abnormal embyros, 298
Unorganized and reduced embryos, 302
Urginea, 29, 71, 75
Ursinea, 133
Urtica, 195
Utricularia, 30, 142, 184, 202, 237, 240,

241, 291
Uvularia, 154, 156

Vaccinium, 157, 158

Vaillanlia, 62

Vallisneria, 32, 39, 164, 169, 195-197,

201, 211, 248
Vallota, 389
Vanda, 303

Vandellia, 142, 184, 241, 242
Vanilla, 38, 221
Vascular supply of ovule, 67
Vaucheria, 9

Vegetative cell, 11, 16, 161
Vegetative nucleus, 169, 194, 211
Veltheimia, 144
Verbascum, 241
Verdickia, 369

Vermiform appendage, 227-229
Veronica, 242

Viability of pollen, 24, 181, 376
Villarsia, 231
Vinca, 161, 197, 204
Vinceloxicum, 208, 344
Viola, 169, 188, 200, 201, 277
Viscum, 38, 64, 99, 268
Vitis, 402

Vivifick effluvium, 2
Vogelia, 125, 188, 346
Voyria, 306
Voyriella, 306

Wall formation in endosperm, 226
Wall layers, in anther, 30

in ovule, 69-73
Weddelina, 103, 104
Welwitschia, 412, 416
Wolffia, 30, 32, 39
Woodfordia, 138
Wormia, 161, 162, 188

X-bodies, 211
Xenia, 19, 258
Xeranthemum, 226

SUBJECT AND PLANT INDEX

453

Xylopia, 56 191, 208, 258-260, 301, 351, 377, 378.

Xyris, 140, 157, 158, 203 389, 429

Zephyranlhes, 248, 332
Y Zeuxine, 296, 336

Yucca 412 Zeylanidium, 103

Zizania, 429
Z Zizyphus, 59, 68

Zostera, 40, 46, 66, 157, 208, 209, 224
Zauschneria, 73, 97, 189, 209, 225, 344 Zygopetalum, 337
Zea, 2, 33, 43, 45, 58, 135, 141, 181, 183, Zygote, 268

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