CHAETOPHORALES 73 



basal part enlarges; branches arise from the underlying cells and 

 eventually surround the oospore where they form a red or reddish 

 brown wall, though in the disk forms this wall is only formed on the 

 side away from the substrate. At the same time the enclosed oospore 

 develops a thick brown wall and the cells of the outer envelope 

 then die. The oospore, or spermocarp, hibernates until spring when 

 it becomes green and divides into sixteen or thirty-two cells, and 

 these, when the wall bursts, each give rise to a single swarmer which 

 must be regarded as a zoospore. Meiosis takes place at the segmenta- 

 tion of the zygote so that there is only the haploid generation. On 

 the other hand, some observers have recorded the development of 

 dwarf asexual plants before the reappearance of new sexual ones, 

 but this is a phase of the life history that demands reinvestigation, 

 for if it is correct it may mean that there is an alternation of two 

 unlike generations, an unusual phenomenon in the Chlorophyceae. 

 Under certain conditions the cells will also produce aplanospores. 

 The relation of this genus, with its advanced oogamy, to the other 

 green algae is by no means clear, and although in many of its 

 features the sexual reproduction is akin to that of the Rhodo- 

 phyceae, it is commonly regarded as parallel evolution rather than 

 as indicating a more direct relationship (cf. p. 256). 



REFERENCES 



Trentepohlia. Brand, F. (1910). Ber. dtsch. hot. Ges. 28, 83. 

 Trentepohlia. Rowland, L. J. (1929). Ann. Bot., Lotid., 43, 173. 

 Stigeoclonium. Reich, K. (1926). Arch. Protistenk. 53, 435. 

 Draparnaldia. Uspenskaja, W. J. (1929-30). Z. Bot. 2,2,, 337. 

 General. Visher, W. (1933). Beih. hot. Zhl. 51, i. 

 Coleochaete. Wesley, O. C. (1928). Bot. Gaz. 86, i. 



SIPHONOCLADIALES 



Until 1935 this represented a well-established order, but in that 

 year Fritsch placed most of the genera in the Siphonales but re- 

 tained the Cladophoraceae as a separate order, the Cladophorales, 

 with affinities to the Ulotrichales. More recently Feldmann (1938), 

 in a survey of the group, has returned to the earlier idea of a 

 relationship with the Siphonales via Valonia and Halicystis, 

 though he also suggests relationships with Chaetophora and Ulo- 

 thrix. Whatever the relations may be, the present order is clearly 



