32 GUIDE TO THE STUDY OF LICHENS. 



assimilation. The fungus gave protection to the algae 

 by forming a suitable covering to prevent sudden 

 evaporation of moisture ; it also served to conduct 

 moisture and soluble food substances and supplied suit- 

 able mechanical tissues to hold the plant firmly to the 

 substratum (trees, soil, rock, etc.). The alga, due to 

 its chlorophyll, assimilated the carbon dioxide (COg) 

 of the air and formed organic compounds for the 

 use of the fungus as well as for its own use. The 

 mutual adaptations became more and more highly 

 specialized, until, after many thousands and, perhaps^ 

 millions, of generations, a large foliose lichen was pro- 

 duced, as one of the higher Parmelias. This is, in 

 brief, the probable history of the j^hylogenesis of Par- 

 melia. At different periods other lower algae entered 

 into mutualistic associations with other sac-fungi. Each 

 prototype developed in different directions, producing 

 new species and finally, perhaps, distinct genera of 

 lichens. Lichens did, therefore, not spring from a 

 single ancestral alga and fungus, they are represented 

 by a poly-ancestry. 



Another remarkable thing is the fact that the agree- 

 ment formed by a given alga and fungus was by no 

 means permanent ; after a long phyletic history a given 

 lichen may have changed its alga, that is, the partner- 

 ship was dissolved and another compact entered with 

 a new alga. In many instances a new partner was 

 taken in and the old was retained, so that we find some 

 lichens with two algal symbionts. 



From the above, it is evident that lichens are 

 peculiar plants ; they are the result of the mutualistic 



