Dn. E. D. Thomson on Parietiriy a Yellow Colouring Matter, 183 



various parts of phenogamous plants, principally the roots and flowers. 

 The subject of the present paper is procured from a totally different 

 tribe — the lichens— but one to which we are indebted for some impor- 

 tant djes. The Greeks gave the name ■Kux.r^v to a disease of the skin, 

 and likewise to certain plants possessing the power of healing these 

 cutaneous eruptions. Dioscorides * tells us that the lichen, which is 

 familiarly known from its growing on stones, and attaching itself to 

 tlie rough parts of rocks, like a moss, was called by some persons 

 bryon, and was useful in the cure of sanguineous fluxes and inflam- 

 mations. 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 hepatica.t Galen 

 likewise enumerates lichens among the instruments of cure, in the treat- 

 ment 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, tlie 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 hepaticsD and alga). In 

 Hooker's Flora, published in 1833, there are enumerated and de- 

 scribed 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 briglit yellow frond, with lobes radiating, marginal, 

 appressed, rounded, crenate, crisped, and granulated in the centre. The 

 repositorieSy or apothecia, are deep orange, concave, with an entire 

 border. The bright yellow colour of the lichen is a sufficient indica- 

 tion of the presence of a colouring matter, but the real intensity of 

 the colour could scarcely be anticipated merely by an inspection of 

 the plant 



POOD OF LICHENS. 



The most luxuriant samples of the parmelia, grow in the neigh- 

 bourhood 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 aflford a demonstration of their being enabled 

 to suck up inorganic substances in the same manner as other plants. 

 Viewed in this light, the moistening and decomposing effect of a 

 humid atmosphere on the rocks on the sea coast., may explain the 



• Mat. Med. B. IV., Cap. 48. f Nat. Hist., xxvii. c. 4. 



