Fecundation in plants

Front matter

WAM

waaay

a tanse dota

Preface

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 Florddee, 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 corresponding 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 development of the gametes is not known from a modern cytological standpoint, and in which the behavior of the sexual belek 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 entirely. iii iv PREFACE.

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 sexually differentiated cells, or gametes, and especially the fusion of their nuclei, the term fecuxdation has been used instead of fertzlization— fecundation being the equivalent of the German Befruchtung and the French fécondation.

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 references 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. 8S. Hirase, and Dr. V. H. Blackman, for reprints of their papers, from many of which illustrations have been borrowed, and especially to Professor R. A. Harper for helpful suggestions.

Davip M. Mortier.

Inpiana UNIVERSITY, August, 1902.

Contents

CuapTer I.—INTRODUCTION.

Pace.

Nuclear division, . . 2-30 Karyokinesis in cells of the ewe planta’ in qin seateceuhenee are developed, dis ‘ F : < ‘ . 2-10 Dictyota, . a a F Fi ‘ i 2 Erysiphe, 5 2 - ‘ ‘ F : 7 Mitosis in pollen mother-cells, : . : . 11-30 The first or heterotypic mitosis, ; . 11-26 Resting nucleus and the development wt the auvdtnasin spirem II Development of the spindle, é és : : 15 Chromosomes, . , ‘ ‘ : : : 17 Metakinesis, : ‘ F : e ¢ 20 The anaphase, . és : : : : : 22 The telophase, ‘ P 3 , F : 23 The nucleolus, . é : F - é 25 The second, or homotypic diviciois, : , : ‘ 27-31 Cell division, : F . - 31-44 The type of the higher ait: ‘i ‘ : ‘ 3 31 Free cell-formation, 3 , : . : . 33 Cell-cleavage, 36 Cell-division in Dictyota andl Si pakdulots : % 3 H 41 The centrosome and the blepharoplast, : 44 The significance of the sexual process and the ninnerieadl reduedion of the chromosomes, . 49-60 Cuaprer Il.—FeEcunpaTion ; Motitz IsoGAMETES. Ulothrix and Hydrodictyon, . 61-65 Copulation of gametes, - 5

Index

Abies Achlya Adiantum Albugo Aspidium Basidiobolus Batrachospermum Callithamnion Cell-cleavage in Synehitrium diccipens Pilobolus crystallinus Cell-division in higher plants Dictyota and Stypocaulon.

Cell-formation, free, in Erysiphe communis Lachnea scutellata Centrosome, in Dictyota . . .

Erysiphe Centrosome and Blepharoplast Cephalotaxis . , Chara Chromosomes in tetraspore mother-cell of Dictyota ascus of Erysiphe pollen mother-cells of Lilium Podophyllun Tradescantia Significance of numerical reduction Closterium : Cocconeis . . .

Coleochzte Collema Cosmarium Cycas Cystopus (see ¢ Albugo).

Dasya : Diatoms Dictyota Dudresnya Ectocarpus Equisetum Erysiphe Fucus Ginkgo Gleecosiphonia Gnetum Gymnogramme .

Halidrys Helleborus Hydrodictyon .

91-93 126-128

73, 72

124 73-76

2-6, 26

119-125

65, 66

7-10

84-88

9-155, 162, 163, 166

Vili INDEX.

Karyokinesis (see Mitosis).

Laboulbeniacee .

Larix . . s Lilium : Mitosis in pollen mother-cells

Development of mitotic spindle i in polien mother-cells Behavior of chromosomes in pollen mother-cells

Nucleolus

Second or honotyole mitoet in pollen miothen cells :

Embryo-sac and Fecundation Fate of second male nucleus in embryo-sac Marsilia . 7 : Mitosis in Dictyota . : Erysiphe . . . pollen mother-cells

Monotropa . . .

Nemalion . : , ‘ Nucleolus, dieviscion of CEdogonium . 2 A Onoclea . . . ‘ ‘ Peperomia 5 Peronospora F . .

Picea : 4 : P Pilularia . . . 7 Pinus : js . ‘ .

Physcia . ; : : . .

Podophyllum:

Resting nucleus of pollen mother-cell F

Nature of nuclear membrane .

Behavior of chromosomes in pollen motheyieell Pteridophyta - : Pyronema . . . .

Pythium A Rhopalodia eibba ‘ .

Saprolegnia : ‘ .

Spheroplea . : 5 7 Spherotheca . ¢ . .

Spirogyra .

Sporodinia Synapsis Tradescantia eiepinteas

Behavior of chromosomes in pollen mother-cell

Second or homotypic mitosis in pollen mother-cell

Tsuga . 7 Fi . : .

Tulipa F . . . . é Ulothrix :

Vaucheria . . :

Vicia faba y

Volvox . 5 j i

Zamia 4 : 7 7

Zeamays . e . : .

Pace.

126 158, 170-171

. . 11-30 15-16

17-24

247-30

169-177

177-178

133, 134: 135

2-7

4-11

11-29

119, 121

: 173 . 3 101

7 142

: 156, 163-168

: 128

18, 22

. Io1

73) 75, 76

+ 102, 107

: 79-84 + 108-111

. 13 « 18, 19, 22

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 helpful as scientific knowledge if these interpretations or doctrines are based upon a well-connected phylogenetic series of the most fundamental 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 membrane in many cases has apparently little or no connection with the division of the nucleus. The first two of these processes will be discussed under zuclear division, while the third will be dealt with in connection with cell-formation.

NUCLEAR DIVISION.

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 instrumentality 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 development in this connection also for the sake of convenience, since centrospheres 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 centrospheres are present, the tetraspore mother-cell in Dictyota dichotoma will be selected from the alge and the mother-cell of the ascus in Lrysiphe from the fungi.

It is not considered necessary, nor conducive to any better understanding of the facts presented here, to enter into any lengthy discussion 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 fibrillz or of an alveolar or foam structure (Waben of German literature) and that, in many cells, these two structures intergrade into one another.

DICTYOTA.

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 réle 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. 1, A). The rod-shaped body is the centrosome, which together with the kinoplasmic radiations constitutes the cextrosphere. The planes of the longitudinal axes of the centrosomes may be parallel or form various angles with each other. In Fig. 1, B, the centrosome at the upper side of the nucleus is seen from the side, the lower from

Fic. 1.—First mitosis in tetraspore mother-cell of Dictyota dichotoma.

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 being differentiated.

D, numerous spindle fibers have entered the nucleus, and the chromosomes are collected in the equatorial region.

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 distance. 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 more densely.

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. 1, C). On entering the cavity some of the fibers proceed in advance of the others. Some pass straight toward 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. 1, D). Certain 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 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

\ eae Si ies At bs,

#4/

Fic. 2.—Spindle and telophase of first mitosis in the tetraspore mother-cell of Dictyota dichotoma. 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-developed centrosphere, arranged in the equatorial plate, appear, especially when crowded together—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 may be 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.!_ 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 mémbrane is present, but a little later they disappear entirely. The chromatin 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 Stypocaulon, one of the Pheophycee, found that a division of the centrosome 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 Dzctyota toward thé 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 alge, 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 kingdom. 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 alge the development of the karyokinetic figure is known somewhat in detail only in Coral/ina offictnalis. In this plant, Davis (’98) finds that the spindle arises through the agency

Fic. 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 alge 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 algz. ERYSIPHE COMMUNIS.

For the fungi, the most accurate and complete account of karyokinesis is to be found in the classical work of Harper (’97) on certain Ascomycetes. Asan 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 Zryscphe communis.

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 generation to the other. Since the spindles lie in different planes, it is possible 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.

Fic. 4.—Mitosis in ascus of Erysiphe communis.—(After Harper.)

A, nucleus in resting stage of second nuclear generation in ascus, the flattened or disk-shaped centrosome closely applied to nuclear membrane.

B, early prophase ; the kinoplasmic radiations have been developed about the centrosome. C, D, E, F, successive steps in development of spindle. G, mature spindle, the nuclear membrane still persists at sides.

H, end of anaphase ; connecting fibers extend between the daughter nuclei, which are not yet provided with a nuclear membrane,

I, daughter nucleus provided with membrane, kinoplasmic radiations present. J, later stage in which the polar radiations have disappeared.

Between the successive nuclear divisions in the ascus, the chromatin of the daughter nuclei does not assume the complete resting condition. It consists (Fig. 4, A) of an irregular net with the angles of the meshes somewhat thickened. Generally the net lies tolerably free in the nuclear cavity, and a very distinct nucleolus is present. The centrosphere appears as a flattened disk closely applied to the nuclear membrane, giving the impression as if the two were grown together (Fig. 4, A). The chromatin net appears also attached at this place and frequently forms a dense mass. These phenomena indicate clearly that chromatin and centrosphere are in direct communication through the nuclear membrane. The first step in the division is characterized by the appearance of a well-developed aster or system of radiations about the centrosome. It seems very probable here that the radiations grow out into the cytoplasm from the centrosome as a center. In the development of the radiations the nucleus probably cooperates. At this stage the chromatin is contracted into a dense net toward the centrosphere and appears in close connection with it. From the chromatin mass several fine achromatic threads extend toward the nuclear membrane (Fig. 4, B).

In the next stage observed, the two poles of the spindle have been formed, which lie some distance apart on the nuclear membrane (Fig. 4, C). The polar radiations are well developed, and from each centrosome a cone of spindle fibers extends into the nuclear cavity. The diverging fibers seem to be inserted in the nuclear membrane at points opposite the centrosome. As in Dictyota the two systems of fibers cross each other at nearly right angles without in any way uniting. Whether the two centrospheres arose by a division of the primary centrosphere cannot be stated with absolute certainty, since the intermediate stages between B and C, Fig. 4, were not observed, yet from what is known in Stypocaulon and in Dictyota, it seems reasonable to suppose that the centrosphere may undergo a division in Zrystphe also.

The chromatin, at this stage, seems to be reduced in mass to that which will appear in the nuclear plate. It lies distributed in irregular lumps among the fibers opposite the two poles. The nucleolus has now disappeared, or, in some cases, it may remain in the form of a weakly staining residue. The spindle fibers within the nucleus become attached to the chromosomes and then contract strongly, bringing the chromosomes into the center of the nuclear cavity (Fig. 4, C, D, E, F). Some of the fibers of the bent spindle appear, at this stage, to extend uninterruptedly from pole to pole. The continuous fibers are, in all probability, formed by the union of those which are not attached to the chromosomes.

The polar radiations now undergo a marked change, becoming shorter and thicker, as if drawn in toward the poles. The majority of the radiations diverge only slightly. They are contracted into bundles or brush-like collections, which stand perpendicular to the surface of the nucleus. Some of these radiations, however, diverge somewhat from the central group, but all the polar radiations are not centered upon a single point. The pole of the spindle is exactly as broad as the base of the central group of polar radiations, and, as will be seen from Fig. 4, E, F, G, the impression is that the polar radiations and the spindle contain the same number of fibers, which are continued uninterruptedly through the poles. But the continuity of the fibers is sharply interrupted by an achromatic plane at the nuclear membrane, through which the deeply staining (violet, by the Flemming triple stain) fibers pass from nucleus to cytoplasm. Whether the spindle fibers actually end at the nuclear membrane, or whether their substance only stains less densely there, was not determined. However, the phenomenon leaves the impression that the central body consists merely of the bases of the polar radiations closely crowded together. If the centrosome is an individual organ here, it seems that it must consist of a very thin, flattened disk, equal in breadth to the blunt end of the spindle.

The poles of the spindle now separate farther from each other, whereby the spindle becomes straight. The individual chromosomes, eight in number, which are arranged in the equatorial plate, are sharply defined, and the nucleus has become somewhat elongated (Fig. 4, G). The polar radiations have again become fine elongated fibers, forming regular systems of sun-like radiations.

As soon as the daughter chromosomes have reached the poles of the spindle the nuclear membrane disappears (Fig. 4, H). The fibers of the central spindle become now less sharply defined and broken in different places. Their number is also gradually diminished, their substance soon being indistinguishable from the immediately surrounding cytoplasm. The polar radiations, however, form at this stage a more regular and sharply defined aster, owing to the outer rays bending somewhat backward round the chromosomes (Fig. 4, H). The latter form a dense mass in which the individual elements are no longer to be distinguished. The centrosome is likewise not to be distinguished from the chromatin mass near which it lies. A nuclear membrane is now formed about each daughter nucleus, which appears as a small vesicle with the chromatin mass at the polar side (Fig. 4, I). With the further development of the nuclear membrane the free cavity of the nucleus increases in size. The chromatin mass begins to swell, and is gradually transformed into threads and lumps which are arranged, at first, mostly along the nuclear membrane, but soon become distributed through the nuclear cavity. A nucleolus now appears, and with the further growth of the nucleus the chromatin passes over into the netlike framework like that in Fig. 4, J, A.

As soon as the nuclear membrane is formed, the polar radiations begin to disappear. In Hrysiphe they seem to be transformed into a granular mass (Fig. 4, J). Finally, when the daughter nucleus is

MITOSIS IN POLLEN MOTHER-CELLS, II mature, the centrosphere remains as a much flattened disc closely applied to the nuclear membrane.

From the foregoing it is clear that, although differing much in detail, the karyokinetic process in Hryszphe is, in general, similar to that in the brown alge. At our present state of knowledge, it is difficult to explain all the minor differences or to form an estimate of their relative importance.

MITOSIS IN POLLEN MOTHER-CELLS.

The spore mother-cells of certain Lz/éace@ and other monocotyledonous species, as well as a few dicotyledonous plants such as He/leborus and Podophyllum, have become classical objects for cytological study, and in these genera the mitotic process is now as well understood as in any other angiosperms. The following discussion of the first two nuclear divisions in the spore mother-cells of higher plants is based upon the author’s own investigations made upon Lilium martagon, L. candidum, Fritillaria persica, Tradescantia virginica, Helleborus fetidus and Podophyllum peltatum.

THE FIRST OR HETEROTYPIC MITOSIS.

RESTING NUCLEUS AND DEVELOPMENT OF CHROMATIN SPIREM.

Soon after the last nuclear division in the archesporium, or sporebearing tissue, which gives rise to the pollen mother-cells, the latter begin that period of growth so characteristic of spore mother-cells previously to the first mitosis. The nucleus is relatively large with a sharply defined membrane, and contains a fine linin network, in which the chromatin granules are held, and one or more nucleoli. The nucleolus may lie ina colorless, spherical cavity, which seems sharply circumscribed. The chromatin appears in larger and smaller granules, which are, as a rule, regularly distributed in the linin thread. The cytoplasm presents a uniform netlike structure (Fig. 5, A). This is the typical structure of a pollen mother-cell.

With further growth of the nucleus, the chromatin granules increase in size, probably through the union or aggregation of the smaller granules, while at the same time the linin thread contracts and shortens. In this stage the linin net consists of a complicated spirem or thread with short turns. The chromatin granules have attained a more uniform size, and lie more regularly distributed in the linin thread (Fig. 5, B). This contraction of the linin thread and fusion of the smaller chromatin granules continues, so that the nuclear thread, which later contains a row of larger granules or disks (the Chromatinscheiben of the German literature) of a tolerably uniform size, becomes a hollow spirem whose irregular turns traverse the nuclear cavity (Fig. 5, C).