284 



SCIENCE 



[N. S. Vol. XXV. No. 634 



only complete account is that of Reinke on 

 M. hullosum, a fresh-water species showing 

 the nearest approach to Tetraspora, and not 

 infrequently included in the latter genus. 

 More extended investigation of the marine 

 species is desirable. 



In a Monostroma which is rather abun- 

 dant on rocks and woodwork in harbors 

 about New York (probably a small form of 

 M. crepidinum Farlow) a fairly complete 

 series of young plants has been obtained. 

 The earliest stage is not a hollow sphere as 

 described by Reinke for M. iullosum, but 

 a short, erect filament somewhat resembling 

 that of Vlothrix. Soon, by longitudinal 

 divisions in its upper cells, this filament be- 

 comes club-shaped, and by further divi- 

 sions, followed by separation of the cells, 

 balloon-shaped. The lower cells divide 

 little, but elongate to form the eharaeter- 

 istie rhizoidal cells found in most Ulvacese., 

 The balloon-shaped portion splits open at 

 the top by irregular rents, to form the 

 lobed, expanded thallus characteristic of 

 the mature plants. 



In two species of Enteromorpha found 

 with the Monostroma, I have discovered a 

 very similar early growth; beginning with 

 the formation of a shorter filament, the 

 thallus takes on a long cylindrical, club- 

 shaped form, which becomes tubular di- 

 rectly, and not after the formation and 

 subsequent splitting of a flat, two-layered 

 sheet, as usually described for this genus. 



The early growth of Viva is also said 

 to be filamentous. This similarity of 

 young stages shown by these three genera 

 appears to indicate a rather close relation- 

 ship to the UlotrichaccEe, so that the separa^ 

 tion by recent writers of the Ulvaceas to a 

 distinct order may not be sufSeiently war- 

 ranted. 



Spore Formation in Derhesia: Dr. B. M. 

 Davis, Cambridge, Mass. 

 Derhesia, whose general morphology and 



cytology is that of the Siphonales, is unlike 

 the other members of this group in having 

 very large zoospores, very unlike the small 

 biciliate zoospores and motile gametes char- 

 acteristic of this group of algse, since each 

 is provided with a large circle of cilia. 

 The zoospores are developed in relatively 

 small numbers (50-200), in sporangia 

 which contain many thousands of nuclei 

 when first formed upon the parent fila- 

 ments. 



A process of nuclear differentiation sets 

 in shortly after the sporangia are de- 

 veloped. Some of the nuclei increase to 

 four to six times their original size and 

 finally become the nuclei of the zoospores. 

 All others degenerate, decreasing in size, 

 losing their chromatin content and finally 

 breaking down in the cytoplasm. The 

 large surviving nuclei are rather uniformly 

 distributed throughout the cytoplasm, but 

 are not associated with any cytoplasmic 

 centers such as coenocentra. 



The spores are formed by cleavage fur- 

 rows which enter the protoplasm from the 

 periphery and by branching in different 

 planes cut out the protoplasm into approxi- 

 mately equal masses around the large 

 nuclei. The nuclei at this stage lie in the 

 centers of the spore origins and from them 

 many delicate fibrils radiate into the cyto- 

 plasm among the plastids. These radia- 

 ting cytoplasmic fibrils have small granules 

 at their bases lying against the nuclear 

 membrane. 



The nucleus of each spore origin comes 

 to lie near the periphery, the radiating 

 fibrils on that side (about one third of the 

 total number) becoming connected with the 

 plasma membrane of the spore. The fibrils 

 also take a funnel-shaped arrangement 

 from the nucleus outward. The granules 

 at the bases of these fibrils pass along them 

 to the periphery and finally lie in a circle 

 about twice the size of the nucleus, just 

 beneath the plasma membrane. 



