184 Dr. R. D. Tno^rsoN on Parietin, a Yellow Colmring Matter. 



almost herbaceous appearance of some of the lichens which may be 

 observed in such situations. The subject, however, of the nutrition 

 of lichens is in its infancy, and will require a searching investigation. 

 It has been already stated that, according to the opinion of botanists 

 (Hookers English Flora), lichens derive no nourishment from the 

 rocks, stones, or trees on which they grow. The roots or fibres with 

 which they are often supplied, it is conceived, are only useful in fixing 

 the plant to its place of growth, its nutriment being derived from the 

 air. One of the most common of our lichens, the Peltidea canina, 

 possesses fibres on its under surface so closely resembling those of 

 shrubs, that one would be inclined to attribute to them similar func- 

 tions. The circumstance, as stated in chemical works, of the absence 

 of any considerable quantity of inorganic matter in the composition 

 of lichens, would appear to lend countenance to the view, that gases 

 constitute the only food of lichens. But the fact of oxalate of lime 

 having been obtained from many lichens, seemed to call in question 

 the validity of the conclusion. The detection, also, of small por- 

 tions of bitartrate of potash and phosphate of lime in some lichens, 

 added still further evidence against the opinion of botanists. So far 

 as I am aware, no other substance of an inorganic nature has been 

 hitherto detected in lichens, except in such minute proportion that it 

 might have been derived perhaps from extraneous sources. I was not 

 therefore prepared to expect the remarkable results which the analysis 

 of the yellow parmelia afforded. In one experiment, 50 grains, obtained 

 from mica slate rocks at Dunoon, on the west coast of Scotland, when 

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

 riment, 40 grains, to which, as in the preceding trial, no earthy matter 

 was attached, afforded, by burning, a residue of 2-7 grains. In a 

 third experiment, 7 grains of the carefully selected upper parts of 

 fronds, which had never been in contact with rock, and therefore were 

 free from the suspicion of having extraneous particles mixed with 

 them, after washing, as in the previous trials, yielded, by incin- 

 eration, 0*47 grains of a skeleton, answering to the form of the 

 lichen, and consisting of silica and phosphates, &c. These three 

 experiments, therefore, give a per centage respectively of ashes, amount- 

 ing to 6-8, 6*75, and 67 1.* In all these trials the colouring matter was 

 volatilized before the lichen caught fire. Another specimen, very care- 

 fully washed, and consisting of the upper parts of fronds, yielded 5 per 

 cent, of ash, in which phosphate of alumina formed a prominent ingre- 

 dient. In proof of the fact that the ash is in no degree connected 

 with the rock, a specimen of Parmelia omphalodes, taken from the stem 

 of an ash tree, ten feet from the ground, was ignited, and found to 

 yield 7 per cent of ash, consisting of silica, phosphates of lime, iron, 

 and alumina. The Cladonia pixidata, taken from a wall, and free 



* These determinations were made in conjunction with Mr. James Murdoch. 



