LIFE HABITS OF THE BLUE-GREEN ALG^E 177 



filaments may break apart on either side of the heterocyst, set- 

 ting free chains of cells which grow into new filaments. Certain 

 vegetative cells in Anabcena increase greatly in size and become 

 densely filled with protoplasm and food material and sur- 

 rounded by a thick protective wall (Fig. 174, B, s). Such cells 

 are called resting cells, or spores, and they are able to live through 

 seasons of drought or a winter's cold and with the return of 

 favorable conditions to germinate and form new filaments. The 

 filaments of Anabcena are held in a soft slime, but those of Nostoc 

 are surrounded by a stiff jelly, so that the mass of much-coiled 

 chains of cells has a firm boundary. Consequently, Nostoc colo- 

 nies (Fig. 174, C) may have a spherical form and become as 

 large as marbles. The slimy or jelly-like substance of Anabcena 

 and Nostoc is a modification of the delicate sheath around the 

 filaments and corresponds to the envelopes about the cells of 

 Crlceocapsa. 



Glceotrichia (Fig. 174, Z>) sometimes develops in such quan- 

 tities in ponds and lakes during the summer as to form a 

 brilliant green scum on the surface of the water. The fila- 

 ments have a radiate arrangement in a soft, gelatinous sub- 

 stance and end in long hairs, and a very large resting cell may 

 be formed at the base of each filament adjacent to the terminal 

 heterocyst. 



210. Life habits of the blue-green algae. The Cyanophycece 

 have some peculiar life habits of ecological interest. They are 

 generally found in warmish waters, both fresh and salt, and 

 many of the forms prefer those which are foul with decaying 

 organic matter. Thus open drains and reeking pools of stagnant 

 water present luxuriant growths of these algae. It is probable 

 that the plants actually use for food certain of the organic sub- 

 stances in such waters. Some of the most conspicuous green 

 scums on ponds and small lakes are composed of certain of these 

 algae (Ccelosphcerium, Clathrocystis, Anabcena, and GlceotricJiia } 

 etc.). Such scums may be called water blooms, after the German 

 term Wasserbluthe. The coloration of the Ked Sea is due to an 



