254 ESSENTIALS OF BOTANY 
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 (symbiosis) 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 
Fre.178. A Lichen independent of the fungus, the ger- 
(Xanthoria). (Natural minating fungus spores can grow only 
as 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 2 
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 alge 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- Fie. 179. A Lichen (Usnea). 
: (Natural size.) 
merly important as sources 
of dyes. “Iceland moss” is a lichen used for food, and 
a finely branching form, growing in extensive mats on 
