THALLOPHYTA— LICHENE S 



95 



Class III.— LICHENES. 



The propriety of considering the Lichens as a separate class 

 is perhaps open to discussion. Before their true nature was 

 understood they ranked as such, but when they were ascertained 

 to be largely" fungal in their composition, and to contain algae in 

 their tissues, they were perhaps hastily relegated to the Fungi, 

 assuming them to be parasitic on the algae within them. 

 Further study has shown them to be symbiotic organisms, the 



Fig. 855. 



Fig. 855. A. I'snea hnrhata, a fruticose lichen (natural size), b. Stictd 2^ul- 

 zHowrtcm, a foliaceous lichen, rt. Apothecia. After Sachs. 



alga and the fungus living together to the mutual advantage 

 of both. The constituents may also sometimes vary, the same 

 algae growing with different fungi and vice versa. They may 

 consequently be restored to something of their old position. 



The lichens are thallophytes of various form, living on old 

 trees, rocks, &c., and presenting always a peculiar withered 

 appearance. According to their habit, they may be divided into 

 fruticose, foUaceous, and crustaceoiis lichens. 



