BLUE-GREEN ALGAE lOi 



mon plankton Crustacea, which themselves form the basis of the 

 food of many small fishes, depend to a great extent upon Aphani- 

 zomenon, Anabaena, and other blue-green algae for their own sus- 

 tenance. 



Some species of Cyanophyceae have become adapted to living in 

 hot springs; these organisms, in fact, together with certain sulphur 

 bacteria, constituting the sole organic hfe of thermal springs. 

 According to the careful observations of Setchell, the blue-green 

 algae grow in some abundance in thermal waters up to 68° C, 

 and scantily in springs showing a temperature as high as 75°- 

 77° C. 



The varied colors — shades of yellow, orange red, pink, blue, 

 and blue green — shown by the siliceous deposits around certain 

 hot springs of the Yellowstone Park, are due in great part to the 

 presence of brilliantly colored blue-green algae within the deposit. 

 Weed has discussed the part played by these algae in the formation 

 of carbonaceous and siliceous rocks about hot springs. 



Some of the Cyanophyceae, principally of the genera Scytonema, 

 Stigonema, and Nostoc, are found associated with certain fungi to 

 form lichens; while still others, notably Nostoc and Anabaena, occur 

 regularly endophytically in the roots of Cycads and in the leaves 

 of Azolla and other water plants. 



Like the bacteria, with which these algae are supposed to show 

 close relationship, most of the Cyanophyceae possess cell walls 

 which become much swollen and mucilaginous in their outer layers. 

 Thus most of the filamentous forms become invested in either a 

 thin mucous sheath or a tough, lamellose sheathing tube. Many 

 of the colonial forms consist of masses of cells embedded in a thick, 

 jelly-like matrix, the external surface of which is often covered 

 with a thin cuticle. 



Much dispute has arisen in recent years as to the nature of the 

 contents of the cells of these algae. On examination with the 

 compound microscope, one usually notes a number of granular 

 bodies, apparently of two kinds — numerous small granules and a 

 few larger, clear ones. In the shorter-celled species, the smaller 

 and more numerous granules frequently He in regular double rows, 

 on either side of the cross walls which separate the cells. In the 



