50 



POPULAR HISTORY OF LICHENS. 



Before leaving the subject of the thallus, we have still a 

 few general remarks to make on the subject of its growth 

 and decay. In regard to their food, Lichens have generally 

 been described as aerial, drawing their sole nourishment 

 from the atmosphere. If however we are guided, as we 

 ought to be, in our determination of the nature of their 

 food, by their chemical composition as ascertained by an 

 analysis of their ash, we must come to the conclusion that 

 they derive no inconsiderable amount, at least of their 

 inorganic constitutive elements, from their bases of sup- 

 port. The latter term we employ, not from a common 

 belief with the earlier Lichenologists, that the trees, rocks, 

 or ground on which Lichens grow contribute nowise to 

 the building up of their thallus, but merely as a term of 

 convenience ; our own opinion is, that, while the atmosphere 

 supplies the chief organic elements of the thallus, the sub- 

 stance on which a Lichen grows furnishes the chief inor- 

 ganic constituents. Almost all Lichens, as we have seen, 

 are more or less intimately united to the bodies on which 

 they grow ; the surface of the latter is frequently pierced or 

 broken up by the tissues of the Lichen, — nay, the hardest 

 calcareous rock, the smoothest quartz, is corroded and dis- 

 integrated ; and deeply sunk in their substance we find the 



