9 6 



PHYSIOLOGY. 



distinct from that of either of the organisms forming the component parts. 

 This community of existence between two different organisms is called by 

 some mutualism, or symbiosis. While the alga inclosed within the meshes 

 of the fungus is not so free to develop, and probably does not attain the full 

 development which it would alone under favorable conditions, still it is 



Fig. 86. 



Section of fruit body or apothecium of lichen (parmelia), showing asci and spores 

 of the fungus. 



very likely that it is often preserved from destruction during very dry 

 periods, within the tough thallus, on the surface of bare rocks. 



