40 Dr. R. D. Thomson vn Parietin and on the 



species of plants, only three of which, however, can be reck- 

 oned true lichens, the remainder being Hepaticae and Algae, 

 closely allied tribes. In Hooker's British Flora, published in 

 1833, there are described 4-20 species, included under 39 

 genera of lichens, as inhabiting the three kingdoms. It is well 

 known that many of these are capable of affording powerful 

 colouring matters. 



The lichen from which the colouring matter to be described, 

 which I propose to term Parietin^ is derived, is of very fre- 

 quent occurrence on walls and trees. It is the Parmelia pa- 

 rietina (yellow wall Parmelia), described by Hooker as having 

 the frond " orbicular, bright yellow ; the lobes marginal, ra- 

 diating, appressed, rounded, crenate and crisped, granulated 

 in the centre; beneath paler and fibrillose, the repositories 

 being deep orange, concave, with an entire border." The 

 bright yellow colour of the lichen is a sufficient indication of 

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

 the colour could scarcely be anticipated merely by an inspec- 

 tion of the plant. 



Inorganic constituents of Lichens. 



It is stated by Sir William Hooker, that among the lichens 

 *' imperfect roots are sometimes formed, but more for the pur- 

 pose of fixing the plant to its place of growth than of deriving 

 nutriment, which appears to be afforded solely by the air" 

 (English Flora, vol. v. part 1. p. 129). And if we examine 

 the writings of other botanists, from the circumstance of the 

 occurrence of inorganic matter in lichens being particularized 

 in very few instances, and as being rather singular, the im- 

 pression produced is, that inorganic matter is by no means a 

 necessary ingi'edient in the constitution of lichens ; oxalate of 

 lime, it is true, had been observed in several species of lichens, 

 but these were considered peculiar instances. Small portions 

 of bitartrate of potash and phosphate of lime had also been 

 detected in one or two species, but these examples do not ap- 

 pear to have led to any generalization, or even to the suspi- 

 cion that inorganic matter was of very frequent occurrence in 

 the tribe of lichens. I was not, therefore, prepared to expect 

 the remarkable results which the analysis of the yellow Par- 

 melia afforded. In one experiment 50 grains of the plant ob- 

 tained from the mica slate rocks of Dunson, on the west coast 

 of Scodand, when carefully washed, dried, and then ignited, 

 yielded 3-4 grains of inorganic matter; and in another expe- 

 riment 40 grains, carefully washed, as in the preceding trial, 

 afforded by burning a residue of 2*7 grains; in a third expe- 

 riment 7 grains of the carefully selected upper parts of fronds, 



