LICHENS. 143 
existence, a green and healthy appearance. The 
species that grow upon trees, it must be observed, 
are generally very different from those which 
‘grow upon stones. There is a considerable pre- 
ponderance of foliaceous and filamentous over 
crustaceous forms, and these, owing to the loose- 
ness of their hold upon the bark, being generally 
attached only by small roots in their centre, or 
by a single knot at one of their extremities, do 
not close up the breathing pores of the tree, or 
prevent that free circulation of air which is neces- 
sary for the healthy performance of all its func- 
tions. Indeed, I am disposed to think that 
lichens are not only harmless, but greatly bene- 
ficial to trees; for those who have paid particular 
attention to pines which growin open and elevated 
situations, must have often, noticed that, not only 
is their bark thicker and more rugged on the side 
most exposed to the prevailing winds and rains, 
but also that it is more densely covered with 
shaggy lichens, so as to afford considerable 
warmth and protection. The colder the climate, 
and the farther north we proceed, the more 
densely clothed with this picturesque garment of 
nature’s providing do we find the trees and shrubs, 
on the same principle, one would imagine, as the 
hyperborean animals are covered with thick furs. 
Indeed, so universally are lichens and mosses pro- 
