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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 other hand, the bark that is 

 covered with the beautiful Hypochnus rubrocinctus 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 existence, a green and healthy ap- 

 pearance. The species that grow upon trees, it must be 

 observed, are generally very different from those which 

 grow upon stones. There is a considerable preponder- 

 ance of foliaceous and filamentous over crustaceous forms, 

 and these, owing to the looseness 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 extremi- 

 ties, do not close up the breathing pores of the tree, or 

 prevent that free circulation of air which is necessary 

 for the healthy performance of all its functions. Indeed, 

 I am disposed to think that lichens are not only harm- 

 less, but greatly beneficial to trees; for those who have 

 paid particular attention to pines which grow in 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 



