WHAT ARE LICHENS? 35 



brandling cliains of rather oblong irregular cells bear- 

 ing brownish substances known as pyrenoid bodies. 

 (Fig. 2.) 



3. Pleurococcus vulgaris. A single-celled alga of 

 rather irregular outline. It sometimes has a bluish 

 tinge. (Fig. 8.) 



4' Glceocapsa polydermatica. This is also a single- 

 celled alga. It differs from the preceding in that each 

 cell is enclosed by a thick, transparent, gelatinous mem- 

 brane. (Fig. 4.) 



5. Dactylococcus infusionum. A single-celled alga. 

 The cells are comparatively small and regularly ellipti- 

 cal ; they are usually associated in colonies of eight or 

 ten cells. (Fig. 7.) 



II. Cyanophyce^. — The alg* of this group differ 

 from the chlorophyce^e in their blue-green color. 



1. Nostoc lichenoides. This alga occurs in chains 

 of rather small cells. Each chain contains one, rarely 

 more, larger cell known as a heterocyst. Each chain 

 is enclosed by a gelatinous substance. (Fig. 3.) 



2. Rivularia nitida. This alga also occurs in 



o 



chains, but its cells are irregular in form. Gelatinous 

 substance present. (Fig. 5.) 



S. Poly coccus punctiformis. Tlie cells of this alga 

 are elliptical and occur in colonies enclosed by a com- 

 mon covering. (Fig. 6.) 



4. Sirosiphon puhinatus. This is a many-celled 

 branching alga of sufficient size to be seen by the 

 naked eye. It occurs in the lichen-genus Ephebe. 

 (Fig. 9.) 



