Fink: SomE Common Types oF LICHEN Formations 413 
lems are solved, we can hardly hope to discuss very intelligently 
chemical composition of substrata as an edaphic factor. 
On the other hand the anatomical structure of lichen thalli is fairly 
well known, so that structural adaptations may be more intelligently 
considered. Moreover, a large amount of observation has shown 
conclusively that many lichens commonly occur upon substrata of 
quite varied chemical composition and physical structure provided 
the conditions of light and moisture are favorable. Therefore, all 
things considered, the structural adaptations of lichens to these con- 
ditions must for the present receive chief attention in any considera- 
tion of lichen formations. 
With this much of preliminary statement, I shall follow out 
the consideration of four or five of the commoner kinds of lichen 
formations. Doubtless the first to present themselves to the 
average collector would be what I have, in my studies of the Min- 
nesota lichens, seen fit to call the Parmele: formations of trees with 
rough bark and the Pyrenula formations of trees with smooth bark. 
The lichens composing these formations are the ones best known 
and most collected, and I shall not reproduce here the rather 
laborious list of plants most commonly found in them, such lists 
having already appeared in the Minnesota Botanical Studies. 
However, the lichens growing more or less commonly on rough 
bark vary widely as to structural adaptations. The typical mem- 
bers of such formations are the species of Parmetia and their near 
relatives in Zheloschistes and Physcia. ‘The lichens of these three 
genera commonly have foliose thalli, not very closely adnate, 
and possessing rhizoids for attachment and support and secondarily 
for absorption of moisture. These thalli have good cellular cortices 
on all sides, serving both for support and for protection against ex-. 
cessive evaporation. Scarcely less at home in these formations are 
the fruticose species of Ramalina and Usnea. These plants also 
have well-developed cortices, which serve for protection against 
excessive evaporation and also give mechanical support so that 
the plants are able to rise from the substrata or to hang in pendu- 
lous fashion. The foliose species of Leptogzwm with thin upper 
cellular cortices only, seek damp places in the formations and the 
species of Co//ema devoid of cellular cortex, yet more moist habitats. 
Of the lichens of the smooth bark formations, the species of Py- 
