302 



GENERAL MORPHOLOGY OF PLANTS 



latter were merely painted. The minute fruit bodies are scat- 

 tered over the surface. The trunks of young beech or birch 

 trees are sometimes nearly covered with different species of 

 various colors. 



451. The gelatinous lichens. In the above three types of 

 lichens the fruit body is stratified, the algae being in a layer a short 

 distance below the upper surface. The algae present are usually 

 isolated cells resembling the alga Pleurococcus. In the gelatin- 



Lichen tundra, showing the 

 Harriman.) 



Fig. 252. 



reindeer moss" or lichen, Alaska. 



(Copyright by E. H. 



ous lichens the algal element is more evenly mixed with the fungus 

 element The algae here are often of the Nostoc type. Calluna 

 is an example. 



452. The work of lichens in soil building. The lichens are 

 believed to be among the most important early agencies in soil 

 making. Growing as many of them do on bare rocks where 

 scarcely any other vegetation can hold, especially on sloping sur- 

 faces, their disintegrating bodies mingle with the finely weathered 



