236 



PKACTICAL BOTANY 



resulting in clecompositiou 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. 



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 ,, 

 • i^A 'I and. instrumental in!, , 



-/^P^^ building up soil 



" :?:&;;;ii*^'N5 where at first 











m 



■''Kl 



|C 





no other 



plants 



could 



live. 



Fic. 191. A hanging lichen {Usnea) which is 

 often called the "bearded moss." Also upon 

 the dead spruce twig which supports this licljen 

 there is another foliose lichen {Parmelia). Upon 

 the Usiica 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 /o/i'ose (Fig. 190) ; those 

 that form closely adhering, scale-like 

 growths are cnififniu'dvs forms; those 

 that branch and are partially free from 

 the substratum ea^e fi-Ktiivisc (Fig. 191); 

 while a few are mucilaginous or ge- 

 latinous forms. Foliose forms are com- 

 mon upon the rougher-barkiid trees, 

 fences, etc. ; crustaceous forms grow 

 upon smooth-barked trees and upon 



