236 



PEACTICAL BOTANY 



resulting in decomposition of the substratum. Examination 

 of almost any stone pile that is but a few years old will 

 show the presence of these forerunners of other plant life. 

 x v We have, therefore, a combination 



of alga and fungus, neither of 

 which alone could keep alive 

 in places of such great ex- 

 posure, living together 

 >\ and instrumental in 

 building up soil 

 where at first 

 no other 

 plants 

 could 

 live. 





FIG. 191. A hanging lichen (Usnea) which is 

 often called the "bearded moss." Also upon 

 the dead spruce twig which supports this lichen 

 there is another foliose lichen (Parmelia). Upon 

 the Usnea plant there are shown several of the 

 disk-like cups in which ascospores are formed 



228. Form, structure, and reproduc- 

 tion. Those lichens which adhere like 

 leaves to the material upon which they 

 grow are called foliose (Fig. 190) ; those 

 that form closely adhering, scale-like 

 growths are crustaceous forms ; those 

 that branch and are partially free from 

 the substratum are fruticose (Fig. 191); 

 while a few are mucilaginous or ge- 

 latinous forms. Foliose forms are com- 

 mon upon the rougher-barked trees, 

 fences, etc. ; crustaceous forms grow 

 upon smooth-barked trees and upon 



