122 THALLOPHYTA. [CH. 



I. SCHIZOPHYCEAE (Cyanophyceae or Blue-green Algae). 



Chroococcaceae. Thallus of a single cell, the cells may be 

 either free, or more usually joined together in colonies enveloped 

 by a common gelatinous matrix, formed by the mucilaginous 

 degeneration of the outer portion of the cell-walls. Reproduc- 

 tion by means of simple division or resting cells. 



Nostocaceae. Thallus consists of simple or branched rows 

 of cells in which special cells known as heterocysts often occur. 

 Reproduction by means of germ-plants or hormogonia, or by 

 resting cells specially modified to resist unfavourable conditions. 



In both families the individuals are surrounded by a 

 gelatinous envelope, which in some genera assumes the form 

 of a conspicuous and comparatively resistant sheath. Marine, 

 freshwater, and aerial forms are represented among recent 

 genera. Several species occur as endophytes, living in the 

 tissues or mucilage-containing spaces in the bodies of higher 

 plants. In addition to the frequent occurrence of blue-green 

 algae in freshwater streams and on damp surfaces, certain forms 

 are particularly abundant in the open sea^, and in lakes or 

 meres^ where they are the cause of what is known in some 

 parts of the country as " the breaking of the meres " (" Fleurs 

 d'eau "). From the narrative of the cruise of the Challenger, 

 we learn that the Oscillariaceae are especially abundant in the 

 surface waters of the ocean. The "sea sawdust" so named by 

 Cook's sailors^, and the same floating scum collected by Darwin^ 

 affords an illustration of the abundance of some of these blue- 

 green algae in the sea. 



Another manner of occurrence of these plants has been 

 recorded by different writers, which is of special importance 

 from the point of view of fossil algae. On the shores of the 

 Great Salt Lake, Utah, there are found numerous small 

 oolitic calcareous bodies thrown up by the waves^ These 

 are coated with the cells of Glceocapsa and Gloeotheca, two 

 genera of the Chroococcaceae. Sections of the grains reveal 



1 Challenger (85) i?am'm. Schiitt (93). 2 Phillips W. (93). 



3 Kippis (78) p. 115. ^ Darwin (90) p. 13. ^ Rothpletz (92). 



