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. Inthe 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 discussicns, 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- 
