FIELD BOOK OF PONDS AND STREAMS 



Blue-green algae. — During midsummer blue-green algae 

 (Fig. 41) often become very abundant in lakes, especially in 

 reservoirs where they sometimes form the surface film known 

 as "water-bloom." They are microscopic, one- or many- 

 celled, but commonly form simple chains as in Anahcena 

 (Fig. 41). When light is thrown directly through them they 

 look blue-green, but when it merely falls upon them, as it does 

 in ponds, they make the water appear red or purplish. In 

 hot springs blue-green algae and sulphur bacteria are almost 

 the only living inhabitants and the blues, pinks, and yellows 

 of the silicious deposits around the hot springs of Yellowstone 

 Park are largely due to them. 



Fig. 41. — Blue-green algae: i, filaments of Ana- 

 hcena; 2, Oscillatoria; 3, Rivularia. 



Among the commonest of the blue-green algas is the free- 

 floating AnahcBna (Fig. 41), which forms the water-bloom on 

 ponds and lakes and is one of the principal foods of swimming 

 crustaceans. The purplish-black mats of Oscillatoria (Fig. 

 41) cling to the sides of ditches and pools, to rocks and to the 

 stone coping of reservoirs, often becoming detached and float- 

 ing free at the surface. Under the microscope its filaments 

 exhibit remarkable oscillations, twisting and writhing, coiling 

 and uncoiling, curving to one side and then to another. Gel- 

 atinous brown clumps of Rivularia (Fig. 41) are common upon 

 stones in rapids, on Chara, Myriophyllum, pondweeds and 

 broken water-soaked leaves of cat-tails, especially in the fall. 



Green filamentous algae, desmids. — In early spring there is 



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