THE LICHENS 



237 



stones ; while fruticose forms grow upon the ground or hang 

 from branches of trees. Illustrations of the latter group are 

 the reindeer moss (Gladonia rangiferina) and other cladonias 

 (Fig. 193), and the bearded 

 moss (Usnea barbata). 



In sections or carefully 

 made dissections of a lichen 

 body usually the fungus is 

 seen to compose the outer 

 covering for the whole body. 

 The algae are within, and 

 often closely wound about 

 by the hyphse of the fungi 

 (Fig. 194), which absorb 

 food from the cells of the 

 algae. 



The fruiting cups usu- 

 ally resemble some of those 

 of the ascomycetes. Within 

 the base of the cup in most 

 lichens the fungal hyphse 

 form asci and ascospores, as 

 do many ascomycetes. These 

 spores belong to the fungus. 

 The algal part of the lichen 

 when it is a one-celled alga 

 like Pleurococcus reproduces 

 by division, as we have al- 

 ready found that it does in 

 the green algae. This repro- 



, ~: . The branches bear the fruiting cups, and 



auction 01 the alga OCCUrS branches may also grow from the cups, 



quite independently of the The open sides of the cups are shown in 6, 



* and the reverse surfaces in c 



reproduction of the fungus. 



229. Economic importance of lichens. Probably the greatest 

 economic importance of lichens is found in their relation to 

 formation of soils. Any freshly bared rock soon becomes the 



FIG. 192. A detail of a small piece of 

 Usnea barbata 



