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 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. 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. 
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). 
