The Algae 



443 



Certain blue-greens constitute the principal vegetation of 

 hot springs, and are known to live in temperatures between 1 50° 



Fig. 273. Blue-green algae: A, Oscillator ia; B, Nostoc; C, Merismopedia; D, Ccelo- 

 sphcBrium. {B, D, after G. M. Smith; A, C, after J. E. Tilden.) 



and 185° F. These algae become encrusted with minerals from 

 the water, and in this way the brightly colored rock basins are 

 formed about such springs — as, for example, the Hot Springs 

 terraces in the Yellowstone National Park. When fresh-water 

 streams are polluted by sewage, by poisonous wastes from manu- 

 facturing processes, and by drainage from coal mines and oil 

 wells, the blue-green algae survive long after all other plants 

 except certain bacteria are killed. Many blue-greens grow more 

 luxuriantly when they have access to organic matter ; that is, 

 they are partial saprophytes. These forms aid in destroying 

 sewage in streams and are of great economic importance ; for 

 they, with the bacteria, constitute a first step in transforming such 

 waste materials into food for fishes and other aquatic animals. 



Blue-green forms in lichens. Many of the genera of lichens 

 (page 413) have blue-green algae as the food-manufacturing part 

 of the fungus-alga complex. When this host happens to be one 

 of the highly gelatinous algae like Nostoc, the lichen also forms a 

 jelly-like mass. 



Common genera. Some of the ^commonest genera among the 

 blue-greens are : Oscillatoria, filamentous forms with short 

 cylindrical or disk-shaped cells ; Glceocapsa, gelatinous unicellu- 

 lar forms ; Nostoc, gelatinous forms in which the cells are formed 



