HOW TO KNOW THE FRESH-WATER ALGAE 



90a Cells ovoid, arranged side by side in one or two alternating rows; 

 spines short, mostly arising from the poles of the cells only. Fig. 74. 

 SCENEDESMUS 



Fig. 74. a, Scenedesmus quadricauda (Turp.) 

 de Breb.; b, S. bijuga var. alternans 

 (Reinsch) Hansg.; c. S. incrassatulus var. 

 mononae G. M. Smith (Redrawn from Tif- 

 fany); d, S. opoliensis P. Richter. 

 There are numerous species of this genus 

 occurring both in the euplankton and the 

 tychoplankton (intermingled, free-floating al- 

 gae in shallow water near shore). The cells 

 Figure 74 are oval, fusiform, or crescent-shaped ac- 



cording to species, and occur side by side in one series oi 4, or in a 

 double, alternating series of 8; rarely a single series of cells will have 

 8 or 12 cells. Certain species of Scenedesmus invariably appear in 

 laboratory cultures, often coloring the water green. Under unnatural 

 conditions the cells often appear singly rather than in colonies of 4. 

 Perhaps the most common species is S. quadricauda (Turp.) de Breb in 

 which the two outer cells of the series bear a long curved spine at 

 their poles. 



90b Cells spherical, in groups of 4 or in multiples of 4 to form com- 

 pound colonies, outer walls bearing long, slender spines 91 



91a Colony triangular, spines 1-7. Fig. 75 MICRACT1N1UM 



Fig. 75. Micractinium pusillum Fres. 



This rare alga occurs in the euplankton, 

 having clusters of 4 round cells arranged 

 in the form of a pyramid. Each cell bears 

 1 to several long, tapering spines. An- 

 other species which is also seldom seen is 

 M. guadrisefum (Lemm.) G. M. Smith, hav- 

 ing oval or elliptic cells. 



Figure 75 



91b Colony pyramidal but with outer wall bearing a single stout spine. 

 Fig. 76 ERRERELLA 



Fig. 76. Eneiella bornhemiensis Conrad. 



Cells of this plant, of which there is but a 

 single species, are arranged to form a 3-dimen- 

 sion pyramid. It is 'known only from the eu- 

 plankton and apparently is very rare. 



Figure 76 



57 



