HOW TO KNOW THE FRESH-WATER ALGAE 



378a Trichome unicellular. Fig. 270 SPIRULINA 



Figure 270 



Fig. 270. a, Spirulina laxissima G. S. West; b, S. princeps (West & 

 West) G. S. West; c, S. subsalsa Oersted. 



Although essentially unicellular, this genus is thread-like and is 

 included with the Oscillatoriaceae (a filamentous family of the Cyano- 

 phyta). Although some species are solitary they are often found in 

 masses either by themselves or intermingled with Oscillatoria (Fig. 

 277). Species are differentiated by size and by type of coiling of the 

 cell. They are usually actively in motion when viewed microscopically. 

 Movement in this and other blue-green genera is accomplished by the 

 extrusion of mucilage. 



378b Trichomes multicellular, i.e., with cross walls 379 



379a Trichomes composed of bead-like or barrel-shaped cells, with heter- 

 ocysts present (cells located here and there in the trichome which 

 are larger and sometimes different in shape from the vegetative 

 colls). Fig. 271 ANABAENA 



Fig. 271. a, Anabaena spiroides 

 var. crassa Lemm.; b, A. sub- 

 cylindrica Borge, showing 

 spores; c, A. sp., showing oval 

 akinete; d, A. subcylindrica 

 Borge, showing heterocysts. 



There are many species of this 

 genus, some solitary and some 

 forming colonial masses of indefi- 

 nite shape. When colonial they 

 are surrounded by a conspicuous 

 mucilage and sometimes are mis- 

 taken for Nostoc (Fig. 300). The 

 colonial mass is indefinite in shape 

 and the mucilage soft, however. 



Whereas some forms are truly 

 planktonic species, others occur intermingled with algae in shallow 

 water or on moist soil. The planktonic species may form a bloom in 

 lakes of northern latitudes during summer months, but seldom cause 

 disagreeable conditions in lakes or reservoirs because the plants re- 

 main suspended throughout the water and do not form surface scums. 

 Anabaena spp. are responsible for the death of cattle and other animals 

 drinking from infested water. 



Figure 271 



158 



