HOW TO KNOW THE FRESH-WATER ALGAE 



79a Cells globose or flatttened on some sides from mutual compres- 

 sion; forming green films on moist substrates. Fig. 66 



PHOTOCOCCUS 



Fig. 66. Protococcus viridis Ag. a, clump of 

 cells; b, filamentous tendency in cell ar- 

 rangement. 



This plant, also known the world over as 

 Pleurococcus vulgaris Menegh., forms the fa- 

 miliar green film on the moist side of trees, 

 rocks, wet boards, etc. Essentially unicellular, 

 it forms clumps from repeated cell division 

 and the occasional tendency to form filaments 

 has led students of the algae to classify it in 

 the filamentous order, Ulotrichales, and to con- 

 sider it as having been reduced to its present 

 simple morphological condition. It reproduces 

 only by cell division and is easily distributed by wind, water, and 

 insects so that it appears throughout the world almost everywhere 

 that subaerial life can exist. 



Figure 66 



79b Cells differently shaped, not producing films on aerial substrates 80 



80a Cells crescent-shaped or sharply acicular (needle-shaped) 



81 



80b Cells some other shape 



82 



81a Cells strongly crescent-shaped, closely clustered but not entangled. 

 Fig. 67 SELENASTRUM 



Fig. 67. Selenastrum gracile Reinsch. 



These gracefully curved cells occur 

 in clusters of from 4 to 32, with a ten- 

 dency to have the convex or 'outer' 

 walls approximated. The curvature of 

 the 'outer' and 'inner' walls of the 

 crescent are more nearly the same 

 than in the somewhat similarly shaped 

 Figure 67 cells of Kirchneriella (Fig. 56), a genus 



which has cells irregularly arranged within a gelatinous envelope. 

 Four species are commonly found in this country, mostly differentiated 

 by size and degree of curvature. Mixtures of algae from shallow water 

 situations often include Selenastrum but they may be found also in 

 the euplankton. 



53 



