34 



POPULAR HISTORY OF LICHENS. 



the bare and weather-beaten rocky summits of our highest 

 mountains. When of greater tenuity — forming a scale-like 

 expansion on the surface of rocks or the bark of trees, as in 

 many Lecanoras and Lecideas — the crustaceous thallus is 

 denominated leprose. Leprose species are also exceedingly 

 common from our sea-coasts to our mountain summits ; 

 but they are much less valuable in the arts. When the 

 crustaceous thallus occurs in its greatest degree of simplifi- 

 cation, — when its constituent cellules exist in a very loose 

 state of aggregation, forming a mere powdery coating, of 

 varied colour, on a rock or tree, it is designated pulveru- 

 lent. This is the simplest form under w r hich lichenose vege- 

 tation occurs ; it is the rudimentary condition of many spe- 

 cies, and the abnormal or abortive form of others, depend- 

 ing upon or produced by an absence of the external condi- 

 tions necessary or favourable to development. By the older 

 lichenologists its true nature was not understood ; and Li- 

 chens having this form of thallus were by them constituted 

 into a distinct genus, under the name of Lepra or Lepraria, 

 the species being chiefly named according to their colour, 

 such as L. nigra, alba } jlava, c/ilorina, the black, white, 

 yellow, or green Lepraria. This form of thallus may be 

 examined by the student in any wood, on the moist rotten 



