682 MINNESOTA BOTANICAL STUDIES. 



similar formation at another. I have, consequently, seldom at- 

 tempted such explanations in this paper. 



In the present paper, after a more extended study of lichen 

 formations in the field, I have attempted a more minute discus- 

 sion of structural adaptations based upon careful study of thalli 

 in the laboratory. This analysis has not in most instances de- 

 tracted from the apparent genuineness of the formations, though 

 in some it has not been possible to show that every plant is 

 structurally adapted to the formation in which it occurs. 



Throughout this paper I have referred only once to the in- 

 fluence of physical structure and chemical composition of sub- 

 strata as influencing the distribution of lichens and the compo- 

 sition of lichen formations. In the beginning, doubtless, lichen 

 species were influenced in their choice of substrata by their 

 adaptations to light, shade, moisture and other conditions, 

 though of course such physical conditions of substrata as in- 

 fluence transfer of moisture are also to be considered as they 

 have been in my discussions, as have also roughness and 

 smoothness of surface. No doubt both physiological and an- 

 atomical changes frequently result in lichens from adoption of 

 certain substrata, but it is well known that the conditions of 

 life in lichens are such that they are not so much dependent 

 upon or influenced by their substrata as are most other plants. 

 Thus a large proportion of lichens occur commonly upon sub- 

 strata of the most varied chemical composition and physical 

 structure, provided the conditions of light, moisture, etc., named 

 above are favorable. Therefore these factors must, for the 

 present at least, receive chief attention in the study of ecologic 

 distribution of lichens, though the more difficult and less im- 

 portant subject of the influence of physical and chemical make 

 up of substrata is well worth attention. Doubtless in the 

 struggle of lichen species for possession of substrata, adapta- 

 tions to chemical composition of substrata sometimes play an 

 important part. For instances, the crustaceous lichens of the 

 calcareous rocks and earth produce a fat which is probably 

 utilized by the plants for purposes of nutrition. Hence, as 

 these plants can build up the fats from material obtained wholly 

 or in part from the calcareous substrata, they would have an 

 advantage over lichens which can not thus utilize the carbon 

 of the rocks, in the struggle for possession. Lichens produce 

 other chemical compounds, some of which are doubtless depen- 



