USES OF LICHENS. 



91 



The " mosses" are more irregularly designated, the specific 

 name in some being due to their geographical source, as 

 "Canary rock-moss;" in others, to their physical characters, 

 as " Tartareous, or Pustulatous, moss." 



We have spoken of colouring matters which are produced 

 by the metamorphosis of colourless organic acids; but some 

 Lichens possess brilliant yellow or greenish colouring matters, 

 also of an acid nature, which exist ready formed in, and 

 give the predominant tint to, the thallus of the plant. Such 

 colouring matters are the Vulpinic acid of Comicularia vul- 

 pina, and the Parietinic acid of the common yellow Wall 

 Lichen [Parmelia parietina) . Erom the purple colour which 

 it strikes with alkalies, the latter acid has been proposed as 

 a test for that class of bodies : it is also one of the colour- 

 ing matters of medicinal rhubarb,— an interesting instance 

 of the analogies, in composition and products, between the 

 Lichens and phanerogamic plants. We may here further 

 cite, as illustrations of these analogies, the occurrence of 

 fumaric or paramaleic acid, — which is moreover producible 

 artificially from malic or maleic acid, the sour principles of 

 the apple and other acid fruits, — equally in Cetraria Islan- 

 dica and Fumaria officinalis, the common Fumitory of our 

 fields ; of oxalic acid, in the form of various salts, equally 



