104 CRYPTOGAMIA LICHENES. 



base of which, in shaded situations, it often covers with a 

 light grey pulverulent coat ; and in this latter situation I 

 have found, lying on the crust, small spherical bodies of a 

 shining red colour, which are a species of Sphceria. 



3. L. alba, pure white, uniform, pulverulent. Eng. Bot. 1. 1349. 

 Lichen albus, WITH. iv. 1. Byssus lactea, LIGHT F. Scot. 1007- 

 Lecidea alba, HOOK. Scot. ii. 38. 



Hob. On hypna and lichens in shaded situations, common, 

 making them appear as if they had been white-washed. 



The Lepraria have but slight claims to be considered perfect 

 plants. They appear to be true lichens struggling for ex- 

 istence in places unsuited to their full development, and 

 altered in appearance by their situation. 



Besides those partial uses which have been mentioned under 

 particular species, lichens play a most important part in the esta- 

 blishment of vegetation at the surface of the globe. " When we 

 remark the hardness, the dryness, and the bareness of rocks, we 

 should scarcely imagine that their summit might one day be 

 crowned with forests ; and yet this great work is carried on under 

 our eyes, and even in the midst of our habitations. We observe 

 the walls covered with greenish spots, which grow from humidity, 

 and which the light and heat reduce to black and tenacious spots ; 

 these are so many byssi which have essayed to establish vegeta- 

 tion there, as well as upon the most polished ^statues and marbles ; 

 it is they which impress the seal of age upon our old castles and 

 gothic edifices. Elsewhere, particularly upon rough stones, we 

 see spreading out into broad plats those lichens of various colours, 

 like the ulcerous crusts which corrode the skin of animals; they 

 scoop out and corrode the surface of rocks, and deposit in the va- 

 cuities which they have formed, the portion of earth produced by 

 their destruction. Although in very small quantity, this earth 

 suffices to administer to the development of lichens of a higher 

 order. Their debris, added to those of the former, furnish a 

 small layer of earth sufficient for the existence of mosses of an 

 inferior order, to which, in like manner, succeed more vigorous 

 species." Edin. Phil. Journ. xvi. 66. See also the Quart. 

 vol. xxxviii. p. 438. 



