26 



MORPHOLOGY 



that are smaller than zoospores and larger than gametes, and which have 

 two or four cilia, germinate slowly in producing filaments, but are evi- 

 dently zoospores in behavior. Under certain conditions, also, some o f 

 the sm aller sw imming; cells that ordinarily function as gametes produce 

 s mall filaments, thus functioning as zoospore s. These fac ts have suggested 

 that gametes are derived from zoospores; that is, that the sexual cells 



are sim ply very small zoospores, which fuse in pairs before germinatioru 

 Ulva. — This is the well-known sea lettuce, a marine form, with a membranous 

 flat thallus, like a leaf in appearance but not at all in structure. It is interesting 

 in being a flat sheet of cells rather than a filament, but it shows the same repro- 

 ductive level as Ulothrix, the zoospores and gametes being of the same general 



kind. The zygospore, 

 a however, instead of giv- 



ing rise to zoospores, as 

 in Ulothrix, produces a 

 new thallus directly. 



Chaetophora. — This 

 form illustrates a freely 

 branching filamentous 

 body, in this case the 

 branches bearing hairlike 

 terminations, a. character 

 indicated by the name. 

 The zoospores and ga- 

 metes are as in Ulothrix 

 and Ulva. 



Stigeodonium. — The 

 body of this form is of 

 the same general type as 

 that of Chaetophora, and 

 with the same methods of 

 reproduction. It displays 

 in a striking way a feature 

 more or less character- 

 istic of all filamentous 

 Confervales, being re- 

 markable for passing 

 long periods as a one- 

 celled plant. Under cer- 

 tain conditions the cells 

 of a filament, when they 

 divide, round off and separate, this one-celled stage being called 'the palmella 

 form.' Under different conditions the filamentous form is resumed (figs. 858-860). 



' The name Palmella was given to the one-celled form before its relation to Stigeodonium 

 and other Confervales was known. It is now retained to indicate the one-celled stage. 



Figs. 63-65. — Cladophora: 63, portion of branching 

 filament; 64, enlarged cells, showing several nuclei in each 

 cell and numerous pyrenoids (after Chamberlain); 65, 

 portion of tip cell forming biciliate zoospores (after Stras- 



BURGEE). 



