

NUCLEAR CYCLE IN CHLOROPHYCEAE 73 



in the Diatom immediately precedes conjugation, while in the Desmid 

 it im mediately follows on conjugation. This fact is important for comparison 

 with the order of events in other Thallophytes. 



The post- sexual phase in Ulothrix, Oedogonium^ and Coleochaete has 

 been frequently brought into comparison with that of the Bryophytes, 

 but the necessary details are still very imperfect as regards their nuclei. 

 In Ulothrix the zygote is formed by the fusion of two motile gametes, 

 and after encystment it undergoes a period of rest : parthenospores similar 

 to the zygotes are also produced, as shown by Klebs, 1 when the sexual 

 cells are exposed to a 0*5% culture solution : these also undergo a period 

 of rest. On germination it is found that in either case there is a division 

 of the contents into non-motile cells, which grow directly into new filaments. 

 But apparently there is this difference, that the parthenospores form only 

 two of these, while the zygotes form four. If this be constantly so, the 

 point has a special interest as indicating that the tetrad-division is a 

 consequence of sexual fusion, and need not happen unless the fusion be 

 carried out. In a similar way the zygote of Oedogonium divides on germina- 

 tion into four, a number which suggests reduction ; it is quite possible 

 that the exceptions to this noted by Pringsheim,' 2 may be explained on 

 the ground of parthenogenesis, as in Ulothrix. The cell-body formed on 

 germination of the zygote of Coleochaete is that which has most frequently 

 been compared with the simplest sporogonia of Bryophytes. In this plant 

 nuclear fusion has been observed in the ovum (Fig. 42. 6, 7), after which 

 the zygote undergoes a period of rest, as a unicellular, uninucleate body, 

 covered by a pseudoparenchymatous coat. On germination a transverse 

 wall is formed at right angles to the axis of the oogonium : then follow 

 longitudinal walls to form octants, and these usually divide further till 8-16 

 cells are formed in each hemisphere. The outer coat then bursts about 

 the region of the transverse wall, and a zoospore is formed from each 

 of the cells, which germinates like any ordinary zoospore. The question 

 of homology of this body with a simple sporophyte has recently been 

 decided by observations made by C. E. Allen 2 : he found that reduction 

 takes place, with characteristic synapsis, on the first nuclear division in 

 the germinating zygote. The cells of the " fruit-body " are then haploid ? 

 and correspond cytologically with the gametophyte, not with the 

 sporophyte as had been commonly assumed. But there is no need 

 as an alternative to hold this body as in any sense a derivative of the 

 ordinary Coleochaete thallus : it seems more natural to see in it merely 

 an extension of the usual tetrad ; for if the second division of a normal 

 tetrad were in this case repeated twice or thrice, a cell-group would 

 result just as is seen in Coleochaete^ and the biological advantage of increase 

 in number of the progeny would thus be secured. Morphologically such 



1 Bediiigungen Jer Fortpftanzung, p. 321. 

 * Gesammelte Abhandlungen t i., p. 251. 

 3 Ber. d. D. Bot. Ges., 1905, p. 285. 



