Part 2
single ‘bay’ (McNICOL) The Sexuality of the Ascomycetes (FRASER and WELSsFORD) . Three alternative theories of past history of Gnetales (ARBER and PARKIN) Diagrammatic relative plans of Gnetalean strobili (ARBER and PARKIN) The microsporangia of Gnetales (ARBER and PARKIN) Elevation of control and heating baths (BALLS) Apparatus (BALLS) Elevation of microscope and chamber (BALLS) General plan of apparatus (BALLS) . Sympodial structure (WoRSDELL) . Petiolar vascular structure in Dicotyledons (WorsDELL) Petiolar vascular structure of Paeonia and Magnolia (WORSDELL) . . Petiolar vascular structure of Calycanthacene (WORSDELL) Bensonites fusiformis (SCOTT) . Bensonites fusiformis (SCOTT) . Normal seedling of Arisarum (HILL) Abnormal seedling (HILL) . Types of hair in the Wallflower (BoopLz) Seedling Structure of Gymnosperms (HILL and DE FRAINE) Temperature and Growth (BALLS) . . oe Vil PAGE 7 84 98 99 163 164 170 170 170 199 . 202 - 203 261 . » 262 . . 263 264 271 - 2972 274 « 277 ° - 402 : - 404 404 406 . . - 406 47! 49! 499 503 561 F - §62 © : . 563 . . 564 . 653 676 677 678 684 . 685 - 713 713 715 . . 691-707 871-588 Spore Formation in Derbesia. BY BRADLEY MOORE DAVIS. With Plates I and II. HE genus Deréesia described by Solier (’47) is chiefly remarkable for its zoospores. These are very large, and each is provided with a circle of numerous long cilia at its forward end. The general mor- phology of Derbesia is that of the Siphonales, with many points of resemblance to Bryopsis, except that the filaments are sparingly and ir- regularly branched. The zoospores of the Siphonales, however, are biciliate, Vaucheria excepted, and small. The peculiarities of the zoospores of Derbesia have led Blackman and Tansley (’02) to suggest that its affinities are widely different from Bryopsis, a point which will be con- sidered later in the paper. The zoospores of Derbesia offer, then, a very interesting subject for , cytological study, since their large size and the peculiar arrangement of the cilia give promise of interesting details in the form and development of the blepharoplast. This interest is further enhanced by Berthold’s account of their development. The zoospores are almost invariably uninucleate at maturity, but they are developed in relatively small numbers in sporangia which contain many thousands of nuclei when they are first formed from the parent filaments. Berthold (’81) reported ‘that the larger nuclei, finally present one in each zoospore, are formed by the successive fusions of the very numerous nuclei within the spor- angium. This is, I think, the last account of nuclear fusions of this character which has not been disproved by detailed cytological investiga- tion, for similar accounts of nuclear fusions during oogenesis in the Saprolegniales, Peronosporales, and Vaucheria have been shown by later research to be incorrect, and such reductions in the number of nuclei within multinucleate reproductive cells have proved to be due in every case to nuclear degeneration rather than to nuclear fusions. {Annals of Botany, Vol. XXII. No. LEXXV. January, 1908.) B 2 Davis.— Spore Formation in Derbesta. This paper will describe the development and germination of the zoospores of Derbesia Lamourouxti (Agardh), Solier, with special reference to the structure and behaviour of the nuclei, and the development and fate of the blepharoplast. The material was gathered in the spring of 1904 at Naples, where, at the Zoological Station, I occupied a table of the’ Carnegie Institution. The most satisfactory fixation was obtained with a weak chrom- osmo-acetic formula as follows:—1 per cent. chromic acid 25c.c., I per cent. acetic acid Ioc.c., I per cent. osmic acid 5c.c., sea water 60 c.c. A similar formula, with the omission of osmic acid, was also satisfactory. Sections were generally cut 3 thick. The best stain proved to be iron- alum haematoxylin with congo red, and haematoxylin alone for the details of nuclear structure. It proved somewhat difficult to use safranin and gentian violet because of the numerous plastids which take these stains with avidity. THE HABITS OF THE ZOOSPORES. The zoospores are formed slowly, and several days may elapse after the development of the sporangium before the segmentation of the protoplasm begins. The first large crop generally appeared two or three days after material of Derdesta was brought into the laboratory and left in sea water, after which zoospores continued to be formed for several days. Attempts to hasten their development by raising the temperature of the water or by diluting it resulted in the death of the plants. The zoospores in the Naples material were remarkably variable in size and in number, ranging from 30-50 in the smaller sporangia to perhaps 200-300 in the larger. They are spherical or oval, and swim | slowly through the water with the circle of cilia forward (Pl. I, Fig. 1). They contain very large numbers of small disk-shaped chloroplasts dis- tributed throughout the protoplast, so that the zoospores are uniformly green and the ciliated region is not conspicuously lighter in colour ; there are no pigment spots. The zoospores come to rest on the region of the protoplast within the circle of cilia, and the latter become distributed in a radiating arrangement (Fig. 2) over the surface to which the zoospores become attached. THE DEVELOPMENT OF THE SPORANGIUM. The sporangia are globular structures arising from the sides of the filaments. It must require a number of days for them to reach maturity. The contents of the developing sporangium remain connected with the parent filament by. a broad strand of protoplasm until the structure has attained full size (Fig. 3). The strand then becomes narrower by the Davis.—Spore Formation in Der bestia. 3 gradual formation of a thick ring at the base of the sporangium. ‘Sec- tions of this ring (Fig. 4) show that its substance (evidently cellulose in character) is laid down in concentric layers on the inside of the wall of the filament at the base of the sporangium. The protoplasm of the sporangium is thus gradually pinched off from that of the filament and is not separated by thin cleavage furrows as in the sporangia of the moulds. The ring-like deposit thickens and finally becomes a heavy plate of cellulose (Fig. 5), which presents a laminate structure, showing that its substance is deposited in successive layers as the protoplasm in the sporangium and in the filament withdraw from one another. The wall shown in Fig. 5 is from a sporangium in which the protoplasmic cleavage to form the spores had already begun. THE DIFFERENTIATION OF THE NUCLEI IN THE SPORANGIUM AND THE DEGENERATION OF THE SMALLER NUCLEI. The nuclei which enter the developing sporangium are similar and all of about the same size and a little larger than the plastids (Fig. 6). Each contains a small deeply-staining nucleolus which frequently lies within a clearer circle. The remainder of the nuclear cavity is almost wholly occupied by a large body .which stains lightly but is readily distinguish- able. This body is probably chromatic in character as shown by its’ later history, but at this stage it is homogeneous and gives no indica- tion of granular or fibrillar structure. There must be several thousand of these nuclei which enter the developing sporangium. The nuclei begin to show a differentiation in size very shortly after the protoplasm of the sporangium becomes separated from that of its parent filament or even before this time. Certain of them increase in size and become conspicuous in the sporangium, finally reaching a diameter 4-6 times the size of the plastids (Fig. 8). The large nuclei may have smaller nuclei very near them, singly or in groups, to which they appear in sharp contrast (Fig. 7). The internal structure of the nucleus does not apparently change during its growth. The large homogeneous chromatin body, however, increases in size. There are, however, marked changes in the appearance of the proto- plasm around the enlarging nuclei. A granular cytoplasm forms an envelope so that the plastids come to lie at some distance from the nuclear membrane (Fig. 9), and delicate protoplasmic strands radiate out from the membrane between the plastids into the surrounding protoplasm (Fig. 9). These strands end in deeply-staining granules just outside of the nuclear. membrane. The large nuclei are thus very prominent in the sporangia because of their size and the radiating strands from the enveloping zone of granular protoplasm bordered by the plastids. B 2 4 Davis.—Spore Formation in Derbesia. The larger nuclei are distributed rather uniformly throughout the protoplasm within the sporangium, and they lie generally at some distance from one another. The protoplasm between is vacuolate and filled with plastids among which the smaller nuclei lie singly or in groups (Fig. 7). These nuclei at this time are of about the same size as when the protoplasm entered the developing sporangium, that is, about the size of the plastids, but some of them are a little larger and some smaller. They show a marked tendency to become massed in groups (Fig. 10) in regions of granular cytoplasm free from plastids. There is no evidence that the smaller nuclei fuse with one another to form the larger. This point was studied with great care. These nuclei decrease in size, and shortly become much smaller than the plastids (Fig. 10). Although they may lie so close together as to be in actual contact, they never unite. The small nuclei are not easily found when they have become half the size of the plastids, but careful search will reveal them in larger and smaller groups and also singly, and the fully mature sporangium, shortly before spore formation, has probably just as many nuclei as when first formed. The small nuclei finally break down and are generally completely © lost at the time when the spores are formed, except for deeply-staining globules, which are probably the remains of the nucleoli. It is difficult to understand the reasons for this rapid differen- ‘tiation of the nuclei in size. I was not able to discover any cytoplasmic structures, such as coenocentra, which could be interpreted as centres of dynamic or metabolic activity affecting the nourishment of nuclei in their vicinity, as appears to be the case during oogenesis in the Perisporiales and Saprolegniales. However, the arrangement of the protoplasm around the larger nuclei is very suggestive of important dynamic or metabolic relations between them and the cytoplasm in the immediate vicinity. The envelope of granular cytoplasm, which has the appearance of kinoplasm, and more especially the radiating protoplasmic strands, give to the nuclei the appear- ance of being themselves important centres of dynamic and metabolic activity. Their distribution also at relatively wide intervals throughout the protoplasm indicates that each is really the centre of a definite region of protoplasm in the sporangium. It is probable that the radiating strands are the paths of protoplasmic streams which pass to and fro between the large nucleiand the surrounding protoplasm. They are, therefore, probably the paths of dynamic and metabolic activities between the large nuclei and the regions of the protoplasm which they dominate. It would seem as though there were an actual struggle for existence among the nuclei in the sporangium to control a limited amount of proto- plasm with limited metabolic and dynamic possibilities, and that relatively few of the nuclei were successful while the majority perished. I have already suggested this explanation to account for the degeneration of the Davis.— Spore Formation in Derbesia. 5 nuclei in the oogonia of the Saprolegniales, Peronosporales, and Vaucheria (Davis, 03, p. 341, and ’04). The physiological conditions within the oogonia of these groups and the sporangium of Derdesia are very similar. These reproductive organs are overstocked with nuclei in the beginning of their development, for reasons that are probably phylogenetic, and related to times when the organs developed many more reproductive cells than they do at present. After the reproductive organs are separated from the parent filaments the nuclei find themselves under conditions where there is not sufficient nourishment for all, and a struggle for existence develops with the subsequent degeneration of all the nuclei except the favoured few. In the Saprolegniales and Peronosporales the survival of certain nuclei is determined by their favourable position near those metabolic centres in the cytoplasm, the coenocentra. In Vaucheria (Davis, ’04) the surviving nucleus lies near the centre of the oogonium in an accumulation of cyto- plasm which is probably the region of the cell most favourable for nuclear metabolism. The sporangium of Derdesia is so large a ccenocyte that its cytoplasm would not be expected to become uninucleate. However, regions become differentiated and dominated by certain nuclei which find there more favourable conditions for growth, and consequently obtain a lead over all others, finally forcing their degeneration. The positions of these favoured centres of metabolic activity are marked by the accumulation of granular cytoplasm around the large nuclei and the protoplasmic strands radiating out among the plastids. The last stages in the degeneration of the small nuclei are more easily followed in Derbesta than in Saprolegnia, Albugo, or Vaucheria, because the nuclei are not so minute as in the latter forms. The nuclei decrease in size as shown in Fig. 10 until they are less than half the diameter of the plastids. The nucleolus still stains distinctly, but the chromatin body becomes faint and is finally quite lost, so that the nucleolus alone comes to lie in a vacuole which is the remains of the nuclear cavity. The boundary of the vacuole finally breaks down, and the nucleolus then passes into the cytoplasm as a deeply-staining globule. Fig. 11 shows a group of de- generating nuclei in a region of granular cytoplasm surrounded by plastids. This group is in an advanced state of degeneration. Some of the nuclei still have the nuclear membrane, but the globules lying freely in the cyto- plasm are from those that have become disorganized. These nucleoli fragment into smaller globules, which finally disappear. THE SEGMENTATION OF THE PROTOPLASM. The segmentation of the protoplasm does not begin until the process of nuclear degeneration is practically ended. Traces of the smaller nuclei may still be found, but they are very difficult torecognize. The sporangium then contains only the larger nuclei quite uniformly distributed through the