CHAPTER XVIII 
THE LICHENS 
Lichens are variously colored, usually dry and leathery 
plants, consisting of symbioses of algae and fungi. In each case 
the fungus derives its food from materials manufactured by the 
alga and in return extracts water from the substratum and shares 
it with the alga. The association is therefore mutually benefi- 
Fic. 307.—A foliaceous lichen, Physica stellaris (L.) Nyb., growing on a rock. 
The cup-shaped structures are the fruiting bodies (apothecia). At the left are 
seen two very young plants. (Gager.) 
cial. While Blue-green and Protococcus forms of Green Algz 
and Ascomycete Fungi are for the most part concerned in lichen 
formation, in some instances Basidiomycete Fungi are also 
involved with the algze. ‘Thus we have two distinct classes of 
Lichens depending upon the type of fungus associated with the 
alga, viz.: Asco-lichens and Basidiolichens. 
Lichens are found on the bark of trees, on rocks, logs, old 
fences, etc. 
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