188 Dr Thomson on Parte tin, 



called by some persons hryon^ and was useful in the cure of 

 sanguineous fluxes and inflammations. Pliny likewise uses the 

 term lichen ; but from his describing it as growing on rocks, 

 with one leaf from a broad root, and with one small stem, 

 it is obvious he refers to a species of the hepaticse.* Galen 

 likewise enumerates lichens among the instruments of cure, in 

 the treatment of impetiginous or cutaneous diseases. Modern 

 botanists, up to a comparatively recent period, appear to have 

 overlooked this class of plants, if we may draw this conclusion 

 from the catalogue of English plants, by John Ray, the second 

 edition of which was published in 1677. In this work, the 

 celebrated author describes, under the title of lichen, eight 

 species of plants, only three of which, however, can be reckoned 

 true lichens, the remainder being hepaticse and algaB. In 

 Hooker's Flora, published in 18rJ3, there are enumerated and 

 described thirty genera and 420 species of lichens. It is well 

 known that many of these are capable of supplying powerful 

 dyes. 



The lichen from which the colouring matter to be described 

 is derived, is of very frequent occurrence on walls and trees. 

 It is the Parmelia parietina, (yellow wall parmelia), described 

 by Hooker as possessing a rounded bright yellow frond, with 

 lobes radiating, marginal, appressed, rounded, crenate, crisped, 

 and granulated in the centre. The repositories, or apothecia, 

 are deep orange, concave, with an entire border. The bright 

 yellow colour of the lichen is a sufficient indication of the pre- 

 sence of a colouring matter ; but the real intensity of the 

 colour could scarcely be anticipated merely by an inspection 

 of the plant. 



FOOD OF LICHENS. 

 The most luxuriant samples of the parmelia grow in the 

 neighbourhood of the sea, from what cause, unless it be the 

 moistness of the air, it is not easy to determine. Botanists 

 consider that this race of plants derive no nourishment from 

 the rocks upon which they grow, although the circumstance 

 of many of them containing oxalate of lime would appear to 

 aff'ord a demonstration of their being enabled to suck up inor- 



* Nat. Hist, xxvii. c. iv. 



