Some common Types of Lichen Formations 
By BRucE FINK 
In a paper published about three years since by the Torrey 
Botanical Club, I referred to the lack of adequate consideration of 
the ecologic distribution of lichens and attempted to outline. briefly 
some of the interesting questions which may well claim the atten- 
tion of lichen collectors. It is my purpose in this paper to take 
up some of the ecologic factors there suggested and follow them 
out in considering four or five of the more common types of lichen 
formations that have come to my notice in field work. 
In attempting such a presentation I am well aware that there 
is much yet to be ascertained concerning the influence of physical 
structure and chemical composition of substrata upon lichen 
distribution. It is well known that those foliose lichens which 
have rhizoids are especially adapted to rough and loose surfaces, 
while the smoother and more dense surfaces usually bear lichens 
having more rudimentary crustose thalli. As to the influence 
of chemical composition of substrata in determining the composi- 
tion of lichen formations, M. Fiinfstiick has studied the fatty secre- 
tions of calcareous rock-lichens and finds that these fats are prob- 
ably utilized by the plants for nutrition. Hence, as these plants 
seem to build up fats from the carbonates of the rocks, they would 
doubtless have an advantage over other lichens in the struggle for 
possession of the calcareous rocks. That these and other rock- 
lichens secrete acids that cause the rocks to decay, rendering them 
easy of penetration, is well known, and G. Lindau finds that crus- 
tose bark-lichens act in much the same way in gaining posseere 
of the smooth bark on which they commonly grow. 
To what extent these lichens utilize the elaborated sap of the 
tree for nutrition we do not know, and definite knowledge in this: 
matter would assist greatly in ecologic studies. In general, lichens: 
are known to produce certain chemical compounds, varying more 
or less with the nature of the substratum. In what degree these 
compounds are of use to the plants and to what degree they are 
derived from the substratum are little known. Till these prob- 
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