254 



ESSENTIALS OF BOTANY 



^iltUkUl{»*i 



Fic. 178. A Lichen 

 (Xanthoria). (Natural 

 size.) 



sufficiently distinct so that lichens are best considered 

 by themselves for purposes of study and classification. 

 The relation of the fungus and its 

 algal host is not that of destructive' 

 parasitism, but rather a mutual rela- 

 tion {syynbiosis) in which both fungus 

 and alga may have a vigorous growth. 

 The relationship has been investi- 

 gated in various ways, and it has been 

 found that, while the alga may thrive 

 independent of the fungus, the ger- 

 minating fungus spores can grow only 

 to a limited extent if deprived of the 

 algal host ; but if supplied naturally or artificially with 

 the proper alga they make a 

 normal growth. The same 

 alga may serve as gonidia to 

 a number of lichens, often 

 of very different form, and 

 while the number of lichens 

 reaches into the thousands, 

 the number of algee known 

 to serve as gonidia is quite 

 small. 



Lichens are widely distrib- 

 uted in all zones but flourish 

 particularly in northern re- 

 gions where other vegetation 

 is scanty. Some were for- 

 merly important as sources 

 of dyes. " Iceland moss " is a lichen used for food, and 

 a finely branching form, growing in extensive mats on 



Fig. 179. A Lichen {TJsnea). 

 (Natural size.) 



