THE FRESH-WATER ALGAE 



159 



178 (179, 180, 181) Cells spherical Coelastrum Nageli. 



^*- Cells spherical or slightly angled; chromatophore a hollow sphere, open 



at one side, with a pyrenoid opposite the opening. 



Coelastrum occurs in all bodies of water, and is found in the plankton. 

 It is very resistant to unfavorable conditions, persisting long after most 

 other algae have perished. 



FIG. 201. Coelastrum sphaericum Nageli. X 620. (Original.) 



179 (178, 180, 181) Cells cordate or reniform Sorastrum Kiitzing. 



Chloroplast parietal, with a single pyrenoid. Cells on short stalks 

 radiating from a gelatinous center, but both center and stalks usually 

 hidden by the cells. A new coenobium from each cell. 



Sorastrum is of less frequent occurrence than most of the other 

 members of the Coelastraceae, but is found in most localities where 

 they are found, especially in the sediment at the bottom of ponds, 

 and occasionally in the plankton. 



FIG. 202. Sorastrum spinulosum Nageli. X 600. (Original.) 



1 80 (178, 179, 181) Cells crescent-shaped, points turned outward. 



Selenastrum Reinsch. 



Cells acutely pointed. Chromatophore parietal, with no pyrenoid. By many 

 this is placed near to Ankistrodesmus rather than with the Coelastraceae. 



FIG. 203. Selenastrum gracile Reinsch. X 565. (Original.) 



181 (178, 179, 180) Cells club-shaped or elongated, forming a star. 



Actinastrum Lagerheim. 



Rays pointed, each ray composed of a single cell, all of which unite at the center. 

 Chromatophore single, parietal, often not extending to the ends. 



FIG. 204. Actinastrum hantzschii Lagerheim. X 550. (Original.) 



182 (177, 186) Cells in one plane 183 



183 (184, 185) Cells four, eight, or sixteen in one or two parallel or zigzag 



rows Scenedesmus Meyen. 



The cells oval or pointed, the membrane either smooth or 

 furnished with distinct spines at the ends. Chromatophore 

 single, parietal, with an opening at one side and a pyrenoid. 



This is one of the most common and the best known of 

 all of the lower algae, it being found in almost all localities 

 where algae are ever found. Its adaptation to various 

 environments, and to conditions unfavorable to most other 

 algae, accounts for its wide distribution. 



FIG. 205. Scenedesmus quadricauda Breb. X 585. (Original.) 



184 (183, 185) Cells grouped in fours, forming a rectangular plate of sixteen 



or more cells Crucigenia Morren. 



Cells spherical or elongated. Chromatophore thin, parietal, with a circular 

 opening and one pyrenoid. This is regarded by Schmidle as Staurogenia. 



FIG. 206. Crucigenia apiculata Chodat. X 1000. (Original.) 



