FECUNDATION IN PLANTS
DAVID M. MOTHER, PH. D.,
PROFESSOR OF BOTANY IN INDIANA UNIVERSITY
PUBLISHED BY THE CARNEGIE INSTITUTION
FECUNDATION IN PLANTS
DAVID M. MOTHER, PH. D.,
PROFESSOR OF BOTANY IN INDIANA UNIVERSITY
PUBLISHED BY THE CARNEGIE INSTITUTION
CARNEGIE INSTITUTION OF WASHINGTON,
PUBLICATION No. 15.
PRESS OF GIBSON BROS.,
WASHINGTON, D. C.
This volume presents the subject of fecundation in the vegetable
kingdom by the discussion of concrete cases, selecting from the great
groups of plants certain typical representatives in which the sexual
process seems to have been most thoroughly investigated. In the
introductory chapter I have discussed typical processes of nuclear
division and cell-formation, especially in spore mother-cells, together
with a few topics dealing with certain phenomena of the cell and the
significance of sexuality. This is considered necessary to a better
understanding of sexual reproduction, for problems of sexuality, like
problems of evolution, have in late years become reduced to problems
of the cell, and, since the nucleus plays by far the most important
part in fecundation, I am tempted to say to problems of the nucleus.
The processes leading to the development and differentiation of the
gametes have been regarded as of prime importance, and they have
therefore received emphasis. Whenever the subsequent history of the
fecundated egg has been followed to any extent this has been done, as
in the Ascomycetes and Floridece, to show the relation between the
real sexual process and the vegetative fusion of nuclei which has been
confused with the sexual act, and, as in the Desmids, for the sake of
pointing out certain nuclear phenomena that take place during the
germination of the zygote with similar phenomena just preceding the
sexual act in the Diatoms. Processes which are purely morphological
are assumed or dealt with very briefly.
In grouping the representative types into the several chapters I have
had in mind no particular theory of the evolution of sexuality, but
merely the idea of the evolution of the plant kingdom and the corre-
sponding differentiation of the sexual organs and cells accompanying
this evolution in the groups of plants themselves.
The chapters dealing with the lower plants in which the develop-
ment of the gametes is not known from a modern cytological standpoint,
and in which the behavior of the sexual nuclei in the fusion of the
gametes has not been followed have been made as brief as possible.
For a similar reason the mosses and liverworts have been omitted
No attempt has been made to discuss the numerous theories bearing
upon the subject. Whenever theoretical matters are touched upon the
object has been chiefly to suggest probable lines of investigation. I
have not hesitated, however, to express my own opinion in all cases
in which my special field of study has given me a first-hand knowledge
of the subject-matter.
To designate the sexual process which consists in the fusion of sex-
ually differentiated cells, or gametes, and especially the fusion of their
nuclei, the term fecundation has been used instead of fertilization
fecundation being the equivalent of the German Befruchtung and
the French fecondation.
It has been necessary, of course, to copy numerous figures from the
papers of other investigators, but in every case due credit is given.
In the citation of literature in the text the author is referred to by
the year in which his work was published. No attempt has been made
to give a complete bibliography, and no doubt many valuable refer-
ences have been omitted.
The author is indebted to Professors W. Belajeff, H. O. Juel,
F. Oltmanns, S. Ikeno, and to Dr. H. Klebahn, Dr. A. H. Trow,
Dr. H. Wager, Dr. S. Hirase, and Dr. V. H. Blackman, for re-
prints of their papers, from many of which illustrations have been
borrowed, and especially to Professor R. A. Harper for helpful
DAVID M. MOTTIER.
INDIANA UNIVERSITY, August, 1902.
CHAPTER I. INTRODUCTION.
Nuclear division, ........ 2-30
Karyokinesis in cells of the lower plants in which centrospheres are
developed, . . . . . . .2-10
Dictyota, ....... 2
Erysiphe, ....... 7
Mitosis in pollen mother-cells, ... . 11-30
The first or heterotypic mitosis, . . . . . 1 1-26
Resting nucleus and the development of the chromatin spirem 1 1
Development of the spindle, . . . . 15
Chromosomes, ....... 17
Metakinesis, ...... 20
The anaphase, ....... 22
The telophase, . . . ... . 23
The nucleolus, ....... 25
The second, or homotypic division, .... 27-31
Cell division, .... . 31-44
The type of the higher plants, . . . . . 31
Free cell-formation, ... 33
Cell-cleavage, ....... 36
Cell-division in Dictyota and Stypocaulon, . . .41
The centrosome and the blepharoplast, . . 44
The significance of the sexual process and the numerical reduction of
the chromosomes, . . 49~6
CHAPTER II. FECUNDATION; MOTILE ISOGAMETES.
Ulothrix and Hydrodictyon, .... . 61-65
Copulation of gametes, ....... 65
CHAPTER III. FECUNDATION ; NON-MOTILE ISOGAMETES.
Closterium and Cosmarium, ... 7*
Diatoms (Rhopalodia, Cocconeis), .
Basidiobolus, .... 76
CHAPTER IV. FECUNDATION; HETEROGAMETES.
Sphaeroplea, ........ 79
Fucaceae (Fucus, Halidrys), ...... 84
Volvox, ........ 88
(Edogonium, ........ 89
Coleochaete, ... ... 91
Vaucheria, ........ 94
Albugo (Cystopus), ... ... 96
Achlya and Saprolegnia, . .... 102
CHAPTER V. TYPE OF THE ASCOMYCETES AND RHODOPHYCE^E .
Sphaerotheca, . 108
Pyronema, . . in
Batrachospermum, . . 116-119
Collema, . 126-128
CHAPTER VI. ARCHEGONIATES.
Pteridophyta, ... I2 g
The spermatozoid, . 130-136
The egg-cell and fecundation, . 136-142
Gymnosperms, ... . r ^ 2
Cycas, Zamia, and Ginkgo, . l ^ 2
The male gametophyte and the development of the sperma-
tozoids, . . 142-155
The archegonium, . 156-158
Fecundation, . . . 158-163
Pinus, ... ... ^.j
The male and female gametoph) tes, . . . 163-164
Fecundation, ...... 165-168
CHAPTER VII. ANGIOSPERMS.
The embryo-sac, or female gametophyte, ... 169-174
The male gametophyte, ..... 174-176
The fusion of male and egg-nucleus, .... 176-177
The fate of the second male nucleus in the embryo-sac, . 177-180
Abies ........ 156
Achlya ......... 102-107
Adiantum ........ 136
Albugo ......... 96-100
Aspidium ........ 136
Basidiobolus ........ 76-78
Batrachospermum * . . . . . . 116-119
Callithamnion ........ 119-124
Cell-cleavage in Synchitrium discipens .... 36-38
Pilobolus crystallinus .... 38-41
Cell-division in higher plants ..... 3^33
Dictyota and Stypocaulon .... 4'~43
Cell-formation, free, in Erysiphe communis . . . 33~35
Lachnea scutellata .... 35
Centrosome, in Dictyota . . . . . . 3-7
Erysiphe ...... 8-10
Centrosome and Blepharoplast ..... 44~49
Cephalotaxis . . . . . ... . J57 1 *
Chara . . 135-136
Chromosomes in tetraspore mother-cell of Dictyota . . . 5-6
ascus of Erysiphe . 8-n
pollen mother-cells of Lilium . . . 17-31
Podophyllum . . 17-31
Tradescantia _ . . 1 T~Z 1
Significance of numerical reduction .... 49-60
Closterium ........ 71
Cocconeis ........ 75
Coleochaete ... . . . . . . 91-93
Collema ........ 126-128
Cosmarium . . . . . . . 71, 72
Cycas . 142-149, 156, 157, 163, 166
Cystopus ("see Albugo).
Dasya ......... 124
Diatoms ........ 73~76
Dictyota ........ 2-6, 26
Dudresnya ........ 119-125
Ectocarpus ........ 65, 66
Equisetum ........ 135
Erysiphe ........ 7-10
Fucus . . . . . . . . 84-88
Ginkgo . . 149-I55! 162, 163, 166
Gloecosiphonia ....... 124
Gnetum ........ 168, 173
Gymnogramme . . . . . . . 130-132
Halidrys ........ 85
Helleborus . . . . . .12, 158, 169-171, 173
Hydrodictyon ........ 63-65
Karyokinesis (see Mitosis).
Laboulbeniaceae ....... 126
Larix ........ 158, 170-171
Mitosis in pollen mother-cells .... 11-30
Development of mitotic spindle in pollen mother-cells . . 15-16
Behavior of chromosomes in pollen mother-cells . . 1 7~ 2 4
Nucleolus . . . . . . . 25
Second or homotypic mitosis in pollen mother-cells . . 2 7~3O
Embryo-sac and Fecundation ... . 169-177
Fate of second male nucleus in embryo-sac . . . 177-178
Marsilia . . .... 133, 134, 135
Mitosis in Dictyota ...... 2-7
Erysiphe . . . . . ^ . . 7-11
pollen mother-cells ..... 11-29
Monotropa ........ 177
Nemalion . . . . . . . . 119, 121
Nucleolus, discussion of v ..... 25, 26
CEdogonium . . . . . . . 89-91
Onoclea ....... IS -^, X 3 6 138-141
Peperomia ........ 173
Peronospora ........ 101
Picea ........ 163
Pinus ....*.... 156, 163-168
'Physcia ......... I2 8
Resting nucleus of pollen mother-cell . . . 11,12
Nature of nuclear membrane . . . . .13,24
Behavior of chromosomes in pollen mother-cell . . 18, 22
Pteridophyta ........ 129-142
Pyronema ........ iii-n6
Pythium ........ IO i
Rhopalodia gibba . . . . . . 73, 75. 76
Saprolegnia ........ 102/107
Sphaeroplea ........ 79 _ 84
Sphaerotheca . ...... 108-111
Spirogyra . . . . . . . . 2 6, 67-70, 168
Sporodinia . . . . . . 71
Synapsis ........ I3
Tradescantia virginica :
Behavior of chromosomes in pollen mother-cell . . . 18,19,22
Second or homotypic mitosis in pollen mother-cell . . 27 29
.... 163, 165, 166, '167
Vicia faba ....
H9-I55, i57-i6i, 163, 166
Zeama J' 8 25,178
FECUNDATION IN PLANTS.
CHAPTER I. INTRODUCTION.
The processes of nuclear division and cell-formation are so closely
associated with sexual cells and their development that an adequate
understanding of these cells is impossible without a definite and
thorough knowledge of the processes involved in their development.
Our interpretations of the significance of the sexual process and the
phenomena of heredity in all organisms will be more lasting and help-
ful as scientific knowledge if these interpretations or doctrines are
based upon a well-connected phylogenetic series of the most funda-
mental facts. Perhaps no other field of research has been more
helpful during the past quarter of a century in enabling the biologist
to gain a deeper and more far-reaching knowledge of the physical
basis of heredity than the study of mitosis, especially in reproductive
cells. The division of the nucleus naturally suggests the division of
the cell, or the process by which new cells are formed from a mother-
cell, and the study of cell-formation in very recent years, especially
among the lower plants, has not only wrought almost a revolution
in our knowledge of the processes here involved, but has also furnished
new criteria for determining relationships and probable lines of descent.
It is deemed necessary, therefore, to introduce the subject of sexual
reproduction in plants by a brief presentation of the typical processes
of nuclear and cell-division in both the lower and higher forms. In
doing so these processes will be described in a few of those forms
which have been subjected to a critical study by means of the most
improved methods and instruments. The processes described will be
confined largely, though not exclusively, to spore mother-cells.
The division of the nucleus and of the cell presents generally three
processes, the development of the karyokinetic spindle, the behavior
of the chromatin, and the formation of the cell-plate or new plasma
membrane. This division is made merely for the sake of convenience,
as it is not implied that three distinct or separate processes are
necessarily involved, although the development of the plasma mem-
brane in many cases has apparently little or no connection with the
division of the nucleus. The first two of these processes will be dis-
cussed under nuclear division, while the third will be dealt with in
connection with cell-formation.
KARYOKINESIS IN CELLS OF THE LOWER PLANTS IN WHICH
CENTROSOMES AND CENTROSPHERES ARE DEVELOPED.
At present there are recognized two types of development of the
karyokinetic spindle. In one the spindle arises through the instru-
mentality of individualized dynamic centers or centrospheres, as in
certain Thallophyta and Liverworts ; in the other, it is developed
wholly independently and in the absence of any such centers, as, for
example, in the higher plants. We speak of types of spindle develop-
ment in this connection also for the sake of convenience, since centro-
spheres have not been found in connection with the development of
the spindle in all Thallophytes ; but the author does maintain that
centrospheres have not been demonstrated to occur in any plant
above the Bryophytes, and that in the Angiosperms such structures
do not in all probability exist.
As illustrating the development of the spindle in which centro-
spheres are present, the tetraspore mother-cell in Dictyota dichotoma
will be selected from the algae and the mother-cell of the ascus in
Erysiphe from the fungi.
It is not considered necessary, nor conducive to any better under-
standing of the facts presented here, to enter into any lengthy dis-
cussion concerning the structure of the firmer framework of the
cytoplasm. The consensus of opinion now is that the firmer substance
of cytoplasm consists of either a reticulum of fibrillae or of an alveolar
or foam structure (Waben of German literature) and that, in many
cells, these two structures intergrade into one another.
The cytoplasm of the tetraspore mother-cell of Dictyota dichotoma
during the preparation for nuclear division presents two well-defined
portions, the kinoplasm, which is always associated with the nucleus
and plays the most important role in the karyokinetic process, and the
remaining alveolar portion. Numerous chloroplasts are also present.
The first indication of mitosis is the appearance, on opposite sides
of the nucleus, of two large sharply defined asters of kinoplasmic
fibers radiating from a rod-shaped body, which is often slightly bent,
lying either close to the nuclear membrane or at some little distance
from it (Fig. i, A). The rod-shaped body is the centrosome, which
together with the kinoplasmic radiations constitutes the centrosphere.
The planes of the longitudinal axes of the centrosomes may be parallel
or form various angles with each other. In Fig. i, B, the centrosome
at the upper side of the nucleus is seen from the side, the lower from
FIG. i. First mitosis in tetraspore mother-cell of Dictyota dickotoma.
A, B, early prophase ; the well-developed centrospheres are on diametrically opposite sides of nuclei.
C, the kinoplasmic fibers have begun to enter the nucleus to form the spindle and the chromosomes are
D, numerous spindle fibers have entered the nucleus, and the chromosomes are collected in the equa-
the end. Viewed from the pole, the centrosome is always rod-shaped.
The kinoplasmic fibers radiate in all directions into the cytoplasm
where they pass over into the framework of the same. On the side
next the nucleus they may run parallel with its wall for some dis-
tance. Near the nucleus the cytoplasm is more granular, with smaller
meshes. It is more nearly a thread-like net- work than alveolar in
structure, and appears with differential staining as kinoplasm. This
very fine granular thread-work often extends in among the radiations
of the centrosphere.
The resting nucleus shows a large vacuolated nucleolus and a fine
linin-reticulum with rather large meshes, upon which are arranged
small and nearly uniform granules, all of which do not react as
chromatin. With the advance of karyokinesis, the chromatin begins
to collect into larger and somewhat irregular masses that finally become
the chromosomes. There is not developed, as in vegetative cells of
this plant, a regular and uniform chromatin spirem or ribbon. The
nucleolus becomes more vacuolated and soon disappears. The nuclear
cavity presents a more granular appearance, the granules staining
The kinoplasmic fibers now penetrate the membrane of the nucleus
and enter its cavity, while at the same time the polar radiations seem
to diminish in number (Fig. i, C). On entering the cavity some of
the fibers proceed in advance of the others. Some pass straight to-
ward the center of the nucleus, while others diverge toward the sides.
As these fibers approach from opposite sides of the nucleus, they tend
to collect the chromosomes into an irregular mass in the equatorial
region, where they finally form the nuclear plate (Fig. i, D). Cer-
tain of these fibers coming from opposite sides seem to unite at their
ends to form the continuous spindle fibers which extend from pole to
pole ; others fasten themselves to the chromosomes, and still others
diverge toward the nuclear membrane in the equatorial region (Fig. 2,
E). In the mature spindle, therefore, the fibers present the following
orientation : those radiating from the poles, the continuous spindle
fibers extending uninterruptedly from pole to pole, those running from
the poles to the chromosomes, and the fibers which diverge from the
poles toward the equatorial region and end in the cytoplasm (Fig. 2, F) .
The nuclear membrane in the tetraspore mother-cell of Dictyota
disappears very gradually during the process of karyokinesis, often
persisting at the sides when the spindle is mature (Fig. 2, F). It begins
to disappear at the poles as soon as the fibers enter the nuclear cavity,
and by the time the anaphase is reached no part of the membrane can
be distinctly seen. Thus the spindle, with the exception of the polar
radiations, lies within the nuclear cavity, its fibers, however, being
largely of cytoplasmic origin. To what extent any nuclear substance
contributes to the formation of the spindle is difficult to determine.
On the disappearance of the nucleolus, numerous granules appear in
NUCLEAR DIVISION. 5
the nucleus, which stain deeply, closely resembling the chromatin
granules. In the meantime the chromosomes increase in size, and it
seems reasonable to suppose that the nucleolar substance contributes
materially to their growth. The development of the nucleolus in the
daughter nucleus and its behavior during the following, or second
mitosis, seem to strengthen this theory. The chromosomes, when
FIG. a. Spindle and telophase of first mitosis in the tetraspore mother-cell of Dictyota dickotoma.
E, spindle nearly mature ; nuclear membrane has disappeared at poles.
F, mature spindle ; the small lumpy chromosomes are regularly arranged in equatorial plate ; nuclear
membrane persists at sides.
G, daughter nuclei still connected by strand of connecting fibers ; at poles of each nucleus is a well-
arranged in the equatorial plate, appear, especially when crowded to-
gether a phenomenon of frequent occurrence as rounded lumps
(Fig. 2, E, F). A careful study in favorable cases shows clearly that
each chromosome is either in the shape of a ring, or so contracted as
to leave scarcely any central space, such, for example, as occurs in
some higher plants (Podophyllum, Helleborus} . In such cases each
segment or daughter chromosome forms one-half of the ring, or
each maybe in the form of a short, thick U (Fig. 2, F). Sixteen
chromosomes, the reduced number, are present in the first mitosis.
While on the way to the poles the daughter chromosomes sometimes
fuse with one another to form large masses. 1 This is especially so in
the second mitosis.
In the construction of the daughter nuclei, one or more larger masses
of chromatin are formed by the chromosomes ; a nucleolus appears
near the chromatin mass or masses, and a nuclear membrane is laid
down (Fig. 2, G). The membrane is unquestionably formed through
the agency of the kinoplasmic fibers. The centrosomes increase in
size, and the polar radiations are more distinct than in the spindle
stage. The connecting fibers usually persist until the nuclear mem-
brane is present, but a little later they disappear entirely. The chro-
matin mass, gradually becoming less dense, soon disintegrates, and
each daughter nucleus passes into the resting condition (Fig. 2, G).
From the preceding it will be seen that each daughter nucleus is
provided with one centrosome, but in the first mitosis the centrosomes
could not be made out until they were on opposite sides of the nucleus
and provided with radiations. The question naturally arises : Does
the centrosome divide to give rise to the two daughter centrosomes ?
Swingle ('97), who has traced the persistence of the centrosome
through several successive generations of vegetative cells in Stypo-
caulon, one of the Phceophycece, found that a division of the centro-
some takes place, and Strasburger ('97) arrives at the same conclusion
as regards Fucus. This is the generally accepted view.
We shall trace the early development of the spindle in the second
mitosis in the tetraspore mother-cell in order to see what evidence is
furnished by Dictyota toward the solution of this problem.
During the reconstruction of the daughter nucleus (Fig. 3, H)
two rod-shaped centrosomes, each with its radiations, were observed
close together, and in such a position as to form a wide V, giving the
impression that a longitudinal division of the single centrosome had
taken place. The manner in which a cluster of radiations is attached
to each daughter centrosome seems to lend weight to this conclusion.
The daughter centrosomes now separate, moving along the nuclear
membrane, but they do not, as in the first mitosis, traverse an angular
distance of 180 before the formation of the spindle begins (Fig. 3,
I, K). The development of the spindle is the same as in the first
mitosis, as Fig. 3, I, J, K, L, will clearly show.
In other brown algae, so far as known (Swingle '97, Strasburger '97) ,
1 This massing of the chromosomes may not occur in all cases.
the development of the karyokinetic spindle in both vegetative and
reproductive cells agrees essentially with that described for Dictyota.
In the diatoms the development of the spindle as described by
Lauterborn ('96) is singular and without parallel in the plant king-
dom. According to this author, the spindle develops directly from
the centrosome by a division of the same or by budding. We shall
refer to this phenomenon beyond in the section dealing especially with
the centrosome. In the red algae the development of the karyokinetic
figure is known somewhat in detail only in Corallina officinalis. In
this plant, Davis ('98) finds that the spindle arises through the agency
FIG. 3. Second mitosis in tetraspore mother-cell of Dictyota.
H-K, prophase, showing origin of spindle. L, a nearly mature spindle.
of centrospheres which undergo a great change in size during mitosis.
The persistence of these bodies was not followed from one cell genera-
tion to the next. The paucity of our knowledge of nuclear division
in the red algae precludes any further mention of the subject in this
group of plants. So far as is known to the author, no centrospheres
or centrosomes have been authentically observed in the green algae.
For the fungi, the most accurate and complete account of karyoki-
nesis is to be found in the classical work of Harper ('97) on certain
Ascomycetes. As an illustration of the process in this group of fungi,
which is probably best known cytologically, a brief account of mitosis
will be given as described by Harper in the ascus of Erysiphe
The ascus of this species offers unusually favorable material for the
study of mitosis on account of the clearness with which all details are
brought out, and because the three successive nuclear divisions follow
each other rapidly, making it possible to trace with unmistakable
clearness the persistence of the centrosome from one nuclear genera-
tion to the other. Since the spindles lie in different planes, it is pos-
sible also to observe, side by side, the same stages at different angles
in the same field of the microscope. The following refers especially
to the second mitosis in the ascus.
FIG. 4. Mitosis in ascus of Erysiphe communis. (After Harper.)