42 Dr. R. D. Thomson oti Parietin and on the 



From these facts it is evident that this lichen requires the 

 same inorganic constituents for food as other plants, with the 

 addition that the amount of inorganic matter present in its 

 composition is greater than in higher orders of plants, but in 

 a proportion tending towards that existing in sea-weeds ; an- 

 other character, therefore, in addition to the general external 

 features, indicating an alliance between the alga and lichens. 



It became now a subject of interest to ascertain if other 

 lichens afforded analogous results. A number of experiments 

 was made upon different species, the results of which are ex- 

 hibited in the following Table: — 



These inorganic substances being tested, yielded the same 

 results as those described in reference to the constituents of 

 the yellow Parmelia. 



To ascertain if the inorganic matter was in any degree de- 

 pendent on the rocks from which most of these specimens were 

 procured, a specimen of Parmelia saxatilis, taken from the 

 stem of an ash-tree ten feet from the ground, was incinerated 

 and found to yield about 7 per cent, of ash, consisting of per- 

 oxide of iron and phosphates of iron, lime and alumina. 



A specimen of Parmelia saxatilis, from rocks on the banks 

 of Loch Venachar (where it is used, as well as generally in the 

 Highlands of Scotland, with an alum mordant to impart a fine 

 purple to woollen cloths), when ignited yielded only 3*9 per 

 cent, of ash, a fact which shows, as well as many others which 

 I have observed, that the amount of inorganic matter varies 

 considerably in the lichens; and perhaps the same observation 

 may without hesitation be considered applicable to all organic 

 bodies. 



It has been long remarked that lichens contribute essentially 

 to the process of the disintegration of rocks, as they are " the 

 first plants which clothe the bare rocks and form a humus for 

 others of a higher organization to live and flourish in " 

 (Hooker). The preceding experiments render it obvious 

 that the lichens are enabled to produce this soil by extracting 

 from the rocks upon which they are fixed, the ingredients re- 



