320 



FUNGI 



consisting of one or more algal cells surrounded by li\[)lKe which separate 

 from the parent-thallus. As a rule one species of alga furnishes all the 

 algal cells of a lichen ; more rarely two, and then one prevails in abundance 



(T 



^M 



>?^?rN 





a 



d 



Fig. 282. — Usnca barbata Fr. Development of soredes. a, group of eight algal cells attached to 

 hypha ; b. similar group with Ijranching hj-pha ; c, sorede with algal cell in optical section ; d, sorede 

 with algal cells divided ; c,f, germinating soredes ( x 500 —700). (After Schwendener.) 



over the other. The same species of alga, however, may be found in 

 consortism with different species of fungus, and taking part in the com- 

 position, therefore, of differently formed thalli — different lichens in short. 









Fig. 283. — Cetraria islatidica Ach., a fruticose lichen (natural size). 



Stahl experimentally proved this in his successful attempts at lichen 

 synthesis. The alg^ which furnish the hosts belong to different groups, 

 and both unicellular and filamentous forms occur. 



The thallus of lichens is of two sorts, the hetei'oniei^oiis and the 



