418 Fink: Some Common Types oF LICHEN FORMATIONS 
Ls 
Verrucaria and Lecanora privigna (Ach.) Nyl., which has a very 
evanescent as well as rudimentary thallus, have more or less de- 
veloped cellular or pseudo-cellular cortices above. These upper 
cortices give the small thalli sufficient protection against evapora- 
tion, thus adapting them to their xerophytic habitat. 
The occurrence of the fatty secretions in some of the lichens 
of the last two formations should be noted as an adaptation to 
their calcareous substrata. Also it may be stated that all but one 
of the types of lichen formations recorded in this paper are essen- 
tially xerophytic, for I suppose that the lichen formations on rough 
bark in mesophytic woods may be regarded as xerophytic at least 
as regards the more typical foliose and fruticose members. The 
one exception is the formations of the smooth bark, at least when 
borne hypophloeodally on trees with living bark so that moisture 
passes readily from the trees to the lichens growing upon them. 
The five types of lichen formations considered in this paper 
have been selected from some twenty-five that I have observed in 
the field. Ecologic distribution of lichens is an extremely inter- 
esting field of botanical study, and my object has been to indicate, 
by preliminary statement followed by application to some com- 
mon types of formations, what may be accomplished in this line by 
careful investigation, 
FAYETTE, Iowa, 
