[Chap. XLVII THE ALGAE 623 



ments. Common to all blue-greens is the presence of a mucilaginous 

 outer wall which may be so thick that the filaments appear to be em- 

 bedded in a mass of mucilage (Fig. 285). 



Fig. 285. Forms of blue-green algae. Genera represented: 1 Microcystis, 2 

 Aphanocapsa, 3 Coelosphaerium, 4 Chroococciis, 5 Merismopedia, 6 and 13 Lijng- 

 btja, 7 and 10 Anahaena, 8 Spindina, 9 Chroococcus, 11 and 14 Oscillatoria, 12 

 Phormidiiim, 15 Scijtonema, 16 Tolypothrix, 17 Glocotiichia, 18 Cylindrospermitm. 

 Figures from G. M. Smith, J. E. Tilden. 



Approximately 2500 species of blue-greens have been described and 

 named. Most of them grow in fresh water, but marine and other salt- 

 water species are not uncommon. They are able to grow in almost every 

 conceivable habitat. They are important constituents of fresh-water 

 plankton, and some of the water blooms of summer are due to their 

 abundance. The Red Sea was probably so named because of the presence 

 in large numbers of one of these plants (Trichodesmium) , which is pre- 

 dominantly red in color. Blue-greens that grow in hot springs live 



