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 of 

 the smaller swimming cells that ordinarily function as gametes produce 

 small filaments, thus functioning as zoospores. These facts have suggested 

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

 are simply very small zoospores, which fuse in pairs before germination. 

 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, 

 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. 



Stigeoclonium. The 

 body of this form is of 

 the same general type as 

 that of Chaetophora, and 

 with the same methods of 

 reproduction. It display" 

 in a striking way a feature 

 more or less characte r - 

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



1 The name Palmella was given to the one-celled form before its relation to Stigeoclonium 

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



65 



64 



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- 



BURGER). 



