186 Db. R. D. Thomson on Parietin, a Yellmv Colouring Matter, 



To ascertain if the great abundance of inorganic matter was pecu- 

 liar to this species, the Parmelia omphalodes was incinerated, the spe- 

 cimen being taken from a portion collected by a Highlander on the 

 borders of Loch Venachar, where it is extensively used, as well as 

 generally in the Highlands, with an alum mordant, to impart a fine 

 purple to woollen cloths. Its habitat had been a rock, and portions 

 were selected free from any appearance of suspended earthy particles 

 among their roots ; 200 grains gave a residue of 7*8 grains, consisting 

 of substances similar to those already enumerated in the analysis of 

 the yellow parmelia. Part of these, however, may have been foreign. 

 When we compare the amount of these inorganic constituents with 

 those found in trees, the balance appears in favour of the lichens, as 

 shown by the analyses of the ashes of genuine specimens of lima, 

 sapan, and logwoods. The results are, in 1000 parts — 



Lima Wood. Sapan Wood. Logwood. 



1000- 1000- 1000 000* 



Both of these classes of plants alluded to, however, appear but insig- 

 nificantly supplied with inorganic matter, when contrasted with some 

 of the gigantic sea-weeds from Cape Horn. 490 grains of one of these 

 enormous inhabitants of the deep, supplied me by Dr. Joseph Hooker, 

 yielded, by incineration, 116-7 of ashes, equivalent to a per centage of 

 23-8. 



The introduction of inorganic matter into the substance of trees 

 and lichens, can only be effected by the inferior extremity and surface 

 of those portions which are in contact with the source of this peculiar 

 pabulum of vegetable life ; while it would appear that the connexion 

 which we always find to exist between sea-weeds and some fixed rocky 

 position, even in the case of these immense inhabitants of the southern 

 seas, according to some physiologists, only serves the purpose of retaining 

 them stationary in one locality, their food being derived from the fluid 

 in which they are immersed. But whether this be true or not, it is 

 certain that the waters of the ocean are capable of affording nearly, 

 if not all, the inorganic ingredients with which these plants are sup- 

 plied. Trees and lichens have no such atmosphere, rich in salts, from 



* In these analjrscs I was assisted by Mr. John Aitken. 



