Ch. Xl BLUE-GREEN ALGiE 401 



green color, sometimes almost green-black in mass, is due to 

 a special pigment (phycocyanin), present with the chloro- 

 phyll, with which, presumably, it is some way associated in 

 photosynthesis. Occasionally they exhibit other pigments 

 also, especially a red, of which the function is equally obscure. 

 The secretion of gelatinous matter by their walls provides 

 the plants with a firm attachment to the substratum, and 

 also has the advantage of checking excessive desiccation, 

 while permitting rapid re-absorption of water. Thus they 

 are enabled to exist in far dryer and more exposed situ- 

 ations than their naked cells alone could endure, which 

 explains their ability to grow as land plants even in some- 

 what dry places. They reproduce exclusively by division, 

 without any trace of fertilization or sex, and the filamentous 

 forms are very prone to fragmentation. They spread by 

 movements of water and perhaps wind, without any evident 

 adaptations, aside from the formation of resistant resting 

 cells. 



A notable feature of these plants as a whole is their 

 preference for sluggish murky waters, especially such as carry 

 organic matters. They can also endure warm waters, to such 

 a degree indeed that a number of kinds live in hot springs 

 at temperatures above 90° C, — a much greater heat than 

 any other organisms can endure. Some kinds also live 

 as endophytes in cavities of larger plants (page 500), and 

 others are associated with Fungi to form Lichens (page 461). 



The Cyanophyceae have no economic importance aside 

 from the fact that some common kinds occasionally impart 

 offensive odor to water supplies, though they are now readily 

 controlled by the regulated use of copper salts. One of this 

 group, Trichodesmium, forms an additional reddish pigment 

 and floats as colonies in salt water, thus giving the name to 

 the Red Sea. Other dark green forms which float in fresh 



I water are called in Europe " Water Blooms." 

 Ecologically the Cyanophyceae are chiefly hydrophytes, 

 though many can live on the land. Phylogenetically, their 

 2d 



