No. IO.] FRESH-WATER AIXLE. 53 



2. Producing barrel-shaped and fusiform resting spores 



PITHOPH0RACE.E 



Without the barrel-shaped resting spores 



CLADOPHORACE.E 

 FAMILY I. SPH^ROPLEACE^E. 



The thallus is nnbranched, composed of cells from one to 

 ninety times as long as wide, each with several nuclei, and 

 parietal chloroplasts in the form of rings. Any segment of the 

 filament may form an oogonium or a bright red antheridium. 

 These sometimes alternate in a filament. Many antherozoids 

 are formed on the breaking up of the antheridium, and they 

 penetrate the oogonium through the transverse walls. Bright 

 red oospores with thick walls are produced, which hibernate in 

 the oogonium, and on germination from two to eight zoospores 

 are set free, which produce young plants, simple, fusiform, at- 

 tenuated to a fine point on each end. 



Sphaeroplea Ag. Coextensive with the family. Its fila- 

 ments are 36-62 microns in diameter. 



FAMILY II. PITHOPHORACE.E. 



Thallus much branched, segments six to twelve or more 

 times longer than broad. Usually many nuclei in a segment, 

 chloroplast parietal; growth apical; attached below by a 

 rhizoid. Asexual, green resting spores with thick walls are 

 produced here and there; when intercalated, cask-shaped; 

 when terminal, ovoid or fusiform. They develop on germina- 

 tion at both ends. 



Pithophora Wittr. Coextensive with the family. It is 

 almost exclusively tropical. 



FAMILY III. CLADOPHORACE.E. 



Thallus large, filamentous, incompletely septate, branched 

 in Cladophora. Each segment contains several nuclei and one 

 reticulated parietal chloroplast or several smaller ones, each 

 with a pyrenoid. In Cladophora and Chcetomorpha asexual re- 

 production is by zoogonidia, formed in great numbers in the 

 mother-cell. Rhizoclonium produces thick-walled cysts. Clado- 

 phora has also an isogamous sexual reproduction. 



