102 



POPULAR HISTORY OF LICHENS. 



Grapkidete. The rotten stumps of old trees are the favourite 

 haunt of many Caliciums and Cladonias ; while to wooden 

 palings a few Lecanoras and Parmelias are peculiar. Some 

 species are peculiar to certain trees. In the trees yield- 

 ing cinchona and other medicinal barks so much is this 

 the case, that a celebrated French author has proposed 

 classifying them according to the Lichens growing on their 

 surface. It has moreover been stated that the portions of 

 these valuable barks covered by Lichens abound in the pe- 

 culiar chemical principles on which their medicinal use de- 

 pends, while portions covered by Fungi are valueless, from 

 the tissue being destroyed by their ramifying mycelium. 

 This circumstance has been held corroborative of the belief 

 that Lichens are not destructive of the bark of trees on 

 which they grow, as Fungi undoubtedly are ; an opinion 

 which does not seem however fully borne out by other facts. 

 A few Lichens inhabit decayed herbaceous plants, as a form 

 of Lecanora tartarea, which sometimes incrusts common 

 heather ; others grow on moss, as various Lecideas and Col- 

 lemas ; the Strigula Babmgtonii coats the leaf of the com- 

 mon Box and Laurel ; and the genera Ahrothallus, Scutula, 

 and a few others, are parasitic on the thallus of various 

 common species. Of saxicolous Lichens, some are peculiar 



