THE FRESH-WATER ALGAE 

 38 (37) Length of cells many times the breadth. . . 



139 

 39 



39 (40, 41) Cells before the middle constriction swollen, but without longitu- 

 dinal flutings; chromatophore of radially-placed plates, with 

 pyrenoids Pleurotaenium Lundell. 



Cells straight, cylindrical, somewhat taper- 

 ing toward the truncate ends. Membrane 

 smooth or with small warts; at each end a 

 FlO. 125. Pleurotaenium mdulosum Rabenhorst. colorless vacuole with dancing particles as in 



X 175- (Original.) 



Closterium. 



40 (39, 41) Cells before middle constriction swollen and with longitudinal 

 flutings; chromatophores of longitudinal radial plates. 



Docidium Lundell. 



Cells tapering somewhat towards the ends; no vacuoles with moving granules; membrane 

 either smooth or with minute protuberances and even with spines in certain regions. 



Fig. 126. Docidium baculum Brebisson. X 545. (Original.) 



41 (39, 40) Shape of cells much as in Pleurotaenium, but apices broadly cleft 

 or with bidentate processes Triploceras Bailey. 



Cells large, walls covered with rings of furcate processes or small, perpendicular longitudi- 

 nally-placed plates. Sometimes confused with Docidium. 



Fig. 127. Triploceras gracile Bailey. One-half of a cell. (After Cushman.) 



42 (34) Constrictions at the sides deep and abrupt 43 



43 (44) End views of cells 3 to several angled or rayed. 



Staurastrum Lundell. 



Side view hour-glass shaped; membrane smooth or with 

 warts or spines; chromatophores in each cell-half consisting 

 of radially-placed plates about a central pyrenoid, two 

 plates extendmg into each arm or angle. 



Fig. 1 28. Staurastrum crenulatum 'i^HgAi. X 600. (Original.) 



44 (43) End views of cells compressed or elliptical, often enlarged at the 

 center 4S 



4S (48) Cells at end with notches or linear incisions. 



46 



