THE FRESH-WATER ALGAE 137 



21 (20) An oval opening at the center between the transverse septa. 



Aptogonum Ralfs. 



A B Filaments often twisted, cells slightly 



longer than broad, with three or four 

 projections on each end which exactly 

 meet others on the adjoining cells, some- 

 times slightly constricted. Several py- 

 renoids in each cell, from which radiate 

 the plate-like chromatophores. 



The genus Aptogonum is included by 

 many under Desmidium, but the space 

 at the center between two adjoining cells, 

 the lack of the narrow central construc- 

 tion, and the greater length of the cells 

 would seem to distinguish it from Des- 

 midium, 



FIG. 116. Aptogonum baileyi Ralfs. a. side 

 view. b. end view. c. optical section. X 425. 

 C (Original.) 



22 (4) Cells not united into filaments 23 



23 (33) Cells not constricted at the center, or at the most only very slightly 



so 24 



24 (25) Cells crescent-shaped; tapering toward both ends. 



Closterium Nitzsch. 



Cells varying from short, thick cells swollen in the 

 middle to very slender cells sometimes bent in the 

 shape of an S. Membrane smooth, or longitudinally 

 striated, rarely with a yellow hue. Chromatophores 

 in each cell-half of several radially-placed plates, 

 including one or more rows of pyrenoids; at each 

 end a large vacuole containing moving granules. 



FIG. 117. Closterium moniliferum var. concavum 

 Ehrenberg. X about 200. (Original.) 



25 (24) Cells cylindrical or fusiform 26 



26 (27, 28) Chromatophore one or more parietal, spiral bands. 



Spirotaenia Brebisson. 



Cells straight, oblong, cylindrical, or fusiform, with rounded ends, 

 tophores one or several parietal bands with pyrenoids. 



Chroma- 



FIG. 118. Spirotaenia minuta Thuret. X 365. (After West.) 



27 (26, 28) Chromatophore star-shaped, one in each cell-half. 



Cylindrocystis de Bary. 



Cells with rounded ends, often oval in outline. Chroma- 

 tophores two, star-shaped, many rayed, each enclosing a 

 pyrenoid at the center. 



Fte. 119. Cylindrocystis diplospora Lundell. X 375- (Original) 



28(26,27) Chromatophore straight, simple, or multiple 



