BLUE-GREEN ALGAE 



299 



a tiny worm in its creeping and bending to one side and then the 

 other. This movement indicates that the cells of the colony of 

 Oscillatoria work together as a unit and thus the many-celled 

 colony takes on the character of a many-celled plant where the 

 cells are closely associated in the activities of the plant. 



Another filamentous form (Fig. 257) is Nostoc, which is common 

 in fresh water and on moist soil. In this plant the cells are 

 rounded and the filament re- 

 sembles a chain of beads. Nostoc 

 secretes an extraordinary amount 

 of gelatinous substance and forms 

 jelly-like lumps in which a large 

 number of the plants are held. 

 These jelly-like masses are often 

 more or less rounded, and are of 

 various sizes up to that of a 

 marble or even larger. When 

 growing on soil, they often swell 

 up and glisten after a rain, on 

 which account they have been 

 called " fallen stars." 



In Nostoc there is some differ- 

 entiation of cells. At intervals 

 in the filament ordinary working 

 cells enlarge, lose their contents, 

 and thicken their walls. Being 

 larger in size and almost colorless, 

 they are quite distinct from the 

 other cells of the filament, and 

 thus divide the filament into sec- 

 tions called hormogonia. These special cells, called heterocysts, 

 seem to be concerned with the multiplication of filaments, for 

 it has been observed that the harmogonia break loose at the 

 heterocysts, wriggle out through the jelly-like matrix, and de- 

 velop new filaments. 



Another special kind of cell formed in Nostoc is the resting cell, 

 which is formed when periods unfavorable for the growth of the 

 plant appear. In this case certain cells of the filament enlarge, 

 accumulate food, and thicken their walls. These cells are able 

 to endure cold, drought, and other conditions which are destruo- 



FIG. 257. Nostoc. At the 

 left are jelly-like lumps of Nostoc 

 consisting of numerous colonies. 

 About natural size. At the right 

 is a single colony, showing the 

 gelatinous sheath and the hetero- 

 cysts, the large cells shown empty, 

 which segment the filament into 

 hormogonia. X 540. 



