ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 391 



many other dry and sterile and unpromising situations. 

 The gray encrusting species are commonest, but many forms 

 occur. The well known "reindeer moss" is a lichen. For 

 the purpose of the following study, one of the gray Parme- 

 lias (fig. 227a) and one of the chimney lichens that grow on 

 decaying stumps in damp woods (fig. 2296) will answer our 

 needs. 



r"- 



a 



Fig. 227. Lichens, a, a common encrusting lichen, showing fruiting cup- 

 ules; 6, a "chimney lichen." whose "chimneys" are covered with powdery 

 white lichen buds (soredia). 



