320 FO'A'GI 



consisting of one or more algal cells surrounded by hyphs 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 



Fig. 282. — l/snea barbata Fr. Development of soredes. «, group of eight algal cells attached to 

 hypha ; b^ similar group with branching hj-pha ; f, sorede with algal cell in optical section ; d^ sorede 

 with algal cells divided ; f, _^ germinating soredes ( x 500—700). (After Schwenderier.) 



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. 2S3. — Cetrttria islandica Ach. , a fruticose lichen (natural size). 



Stahl experimentally proved this in his successful attempts at lichen 

 sjmthesis. The algse which furnish the hosts belong to different groups, 

 and both unicellular and filamentous forms occur. 



The thallus of lichens is of two sorts, the heteromerous and the 



