THALLOPHYTA: ALGAE 9 



would be fatal to almost all other forms of life. For this reason, because 

 of their simplicity in cellular organization, and because they are auto- 

 trophic, members of this group may have lived on the earth before con- 

 ditions were favorable for the existence of other organisms, with the 

 possible exception of bacteria. Their great antiquity is indicated by the 

 presence, in rocks of Proterozoic age, of what seem to be fossil Cyano- 

 phyceae, as well as numerous calcareous deposits resembling those made 

 by blue-green algae now living in hot springs. There is more certain 



V-/ Vi/ J t^ 



'o'iG 0,000 OqX/OJ 



c-N /^ /'^ /~^ f^ /^ rrs 



B 





>■ 



'ii- 



i. 



■I. 





'1:! 



}': 



'• 'V. 





D 



Fig. 1. Some simple colonial blue-green algae. A, Gloeocapsa, X750; B, Merismopedia, 

 X750; C, Nostoc, X 1,000; D, Oscillatoria, X600. Except in Oscillatoria, the cells are 

 embedded in a mucilaginous matrix. 



evidence of their existence in the Paleozoic era, particularly in the 

 Cambrian, Silurian, and Devonian periods. 



Plant Body. All the Cyanophyceae are unicellular and in nearly all 

 of them the cells are organized to form colonies (Fig. 1). None has a 

 truly multicellular body, although this condition is approached by the 

 higher members of the group. In some species of Chroococcus the cells 

 are solitary, while in Gloeocapsa they form small irregular colonies loosely 

 held together in a gelatinous matrix. In Merismopedia the colonies are 

 plate-hke, the cells being arranged in regular rows. In Coelosphaerium 

 the colonies are globular and hollow, in Nostoc they resemble a string of 

 beads, while in Oscillatoria they form a compact filament. The fila- 

 mentous type of colony is most common. 



