192 Dr Thomson on Parietin, 



ashes of genuine specimens of lima, sapan, and logwoods. The 

 results are in 1000 parts : — 



1000. 1000. 1000.000* 



Both of these classes of plants alluded to, however, appear 

 but insignificantly supplied with inorganic matter, when con- 

 trasted 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 ex- 

 ist between sea-weeds and some fixed rocky position, even in 

 the case of these immense inhabitants of the southern seas, ac- 

 cording to some physiologists, only serves the purpose of re- 

 taining them stationary in one locality, their food being de- 

 rived 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 ingre- 

 dients with which these plants are supplied. Trees and lichens 

 have no such atmosphere, rich in salts, from which they can 

 derive their food. They must be indebted for the inorganic 

 matter which they contain to the soil upon which they grow. 

 Hence, since lichens do certainly contain inorganic matter of 

 various kinds, as appears by the facts detailed in this paper, 

 the inevitable conclusion is forced upon us, that these species 

 of plants are not only nourished by the atmosphere, to which 

 botanists have hitherto appeared to restrict their sources of 

 food, but that they are also capable of extracting inorganic 



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



