GENERAL CHARACTERS OE BRITISH LICHENS. 51 



fructification of several species of Verrucaria and Lecidea. 

 This phenomenon has hitherto been unexplained; it pro- 

 bably depends on some chemical action exerted on the rock 

 by the Lichen. There are however exceptions apparently 

 to the rule that all Lichens are adherent; the Lecanora es- 

 culenta, which constitutes a kind of manna to the nomadic 

 tribes of the Asiatic steppes in the neighbourhood of the 

 Caucasus, is said to be free at every period of its growth. 

 The ash of Lichens constitutes about eight or ten per cent, 

 of their bulk, and consists of various earthy bases — such as 

 potash, soda, lime, magnesia, alumina, silica, manganese, and 

 peroxide of iron, in combination with various acids, such 

 as carbonic, phosphoric, sulphuric, and hydrochloric. As 

 illustrations of the composition of the ash of particular 

 species, Mr. Wallace Lindsay found in that of the common 

 " Iceland moss " of our druggists'' shops (Cetraria Islandica) 

 the bases lime, potash, soda, magnesia, and peroxide of iron, 

 in combination with sulphuric, hydrochloric, and phosphoric 

 acids ; and in that of a very sorediiferous form of Physcia 

 prunastri, from the woods of Floors Castle, Kelso, the bases 

 lime, soda, potash, alumina, magnesia, manganese, and per- 

 oxide of iron, in combination with the same acids. It is 

 probable that in the former case the inorganic matter was 



