HOW TO KNOW THE FRESH-WATER ALGAE 





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Figure 56 



69a Cells decidedly curved with the 2 poles nearly touching one an- 

 other, arranged in scattered groups of 4 with convex walls apposed. 

 Fig. 56 KIRCHNER1ELLA 



Fig. 56 a, Kirchneriella lunaiis (Kirch.) 

 Moebius. b, K. obesa var. major (Ber.) 

 G. M. Smith. 



The crescent-shaped or arcuate cells 

 of this genus are inclosed in a (some- 

 times indistinct) mucilage and are usu- 

 ally found in the open water plankton. 

 There are about 5 species recognized 

 in the United States, showing different 

 degrees of curvature and variations in 

 stoutness. The cells are mostly so 

 sharply curved that their apices nearly touch, whereas in Selenastrum 

 (Fig. 67) the cells are symmetrically crescent-shaped. They do not 

 occur in gelatinous envelopes whereas Kirchneriella is characterized 

 by the presence of a sheath. 



69b Cells symmetrically crescent-shaped, in groups of 4, 2 of which 

 face one another. Fig. 57 TETRALLANTOS 



Fig. 57. Tetrallantos Lagerheimii Teiling. 



This rare species (the only one in the 

 genus) is widely distributed. The charac- 

 teristic arrangement of the cells is deter- 

 mined at the time that they are formed in 

 groups of 4 within the mother-cell. After 

 the mother-cell wall breaks down to release 

 the daughter-cells fragments of the wall may 

 persist as interconnecting or radiating 

 threads within the colonial mucilage which 

 is often very thin and difficult of determin- 

 ation. 



70a Cells fusiform or spindle-shaped 72 



70b Cells ovate, bean-shaped, or oblong 71 



71a Cells oval, somewhat irregularly arranged in 4's, forming a flat 

 plate. Fig. 58 D1SPORA 



Fig. 58. ^Dispora crucigenioides Printz. 



Only 1 species in the United Stales, 

 and apparently rare. The cells are in 

 rather irregular rectilinear series and usu- 

 ally show a quartet arrangement within 

 a flat gelatinous sheath. It should be 

 compared with Crucigenia (Fig. 72), espe- 

 cially C. irregularis in which the cells are 

 more definitely arranged in 4's. 



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Figure 57 



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Figure 58 



49 



