364 



THE NON-VASCULAR PLANTS 



B. Colonial Forms. \ 

 In algae of this 

 kind the cells are 

 individual plants, but 

 they occur in groups 

 called colonies. They 

 are held together by 

 FIG. 161. Nostoc. The a substance which is 



small figure at the right .. 



indicates the jelly-like manufactured and ex- 



mass formed by a sort of creted by the Cells, 

 mucilage excreted by ^ and R^^ 



the ceHs of this plant. 



Such masses, brown in are examples of such 



color, are often found f orms< NostOC is 

 on damp rocks in shady 



places. Within them found in jelly-hke 

 the filaments of Nostoc l um ps ; Rivularia in 



lie embedded as shown , 



in the larger figure, blue-green patches. 



Note four cells of the Both are found in 



filaments which are dif- j i /o 



ferent from the others. dam P P laCCS - ( See 

 These are called hetero- Figures l6l and 162.) 



It will be noted that 

 the cells are not all alike. In Rivularia 

 there is a difference between the cells at 

 the base and those at the apex of the 

 chain, while in Nostoc occasional cells are 

 much larger than their neighbors. The 

 reproduction of these plants is by cell divi- 

 sion only, as in Pleurococcus. Oscillatoria 

 is a blue-green form which is often found 

 on wet soil. In it the cells are so closely 

 pressed together that they lose their spher- 

 ical shape. They have the shape of round 



* * into a whip-like ex- 



pill-boxes. (See Figure 163.) Yet 'they tension. 



FIG. 162. Rivularia. 

 A colonial alga found 

 in running water. 

 The filament tapers 



