364 



THE NON-VASCULAR PLANTS 



Fig. 161. — Nostoc. The 

 small figure at the right 

 indicates the jelly-lilie 

 mass formed by a sort of 

 mucilage excreted by 

 the cells of this plant. 

 Such masses, brown in 

 color, are often found 

 on damp rocks in shady 

 places. Within ' them 

 the filaments of Nostoc 

 lie embedded as shown 

 in the larger figure. 

 Note four cells of the 

 filaments which are dif- 

 ferent from the others. 

 These are called hetero- 

 cysts. 



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 

 a substance which is 

 manufactured and ex- 

 creted by the cells. 

 Nostoc and Rivularia 

 are examples of such 

 forms. Nostoc is 

 found in jelly-like 

 lumps; Rivularia in 

 blue-green patches. 

 Both are found in 

 damp places. (See 

 Figures 161 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 ^'°, '62--^'"'/''™: 



•' A colonial alga found 



pressed together that they lose their spher- in running water. 

 ical shape. They have the shape of round 7"'^^ filament tapers 



mto a whip-like ex- 



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



