142 FIRST FORMS OF VEGETATION. 
to the welfare of the forests in which they abound. 
Such individuals, however, it is evident, totally 
, misapprehend the nature of these plants, for they 
extract their nourishment principally from the 
medium with which they are surrounded, and 
not from the matrix on which they are developed, 
or to which they are attached. The fungi are 
the only plants that are produced from decay 
and corruption, and maintain their existence by 
exhausting the vital juices of other plants. That 
lichens are not injurious to the plants on which 
they grow, is clearly proved in the case of Peruvian 
bark ; for the specimens which are covered with 
healthy lichens abound more in the peculiar 
medicinal principle, and realize a larger price, 
than those which are bare and destitute of 
lichens ; while, on the ether hand, the bark that 
is covered with the beautiful Hypochnus rubro-cinc- 
zus and other fungi is utterly worthless, as these 
deadly parasites decompose all the substances 
upon which they fasten by the absorption of their 
nutritive matter. There is hardly a tree in the 
whole world which, at some stage or other of its 
existence, has not been covered with lichens. I 
have frequently observed the trees of a whole 
Highland forest, covered from head to foot with 
a dense shaggy garment of these plants, and yet 
maintaining, during the natural term of their 
