Dr Davy's Agricultural Discourse. 293 



nutritive principles. The sap so impregnated passes from 

 the rootlets by ascending vessels to the leaves, undergoing 

 some change in its passage, but a greater change in the 

 leaves, w^here carbonic acid gas is decomposed under the in- 

 fluence of light, oxygen evolved, and woody fibre either formed 

 completely, or a substance formed about to become woody 

 fibre, and to be deposited by the sap in its descent through 

 another order of vessels. And as in the animal frame, very 

 different compounds are secreted by different glands, so too, 

 in the vegetable, a vast variety of compounds are produced 

 by an analogous function of secretion ; tubes and cells in the 

 latter corresponding to glands in the former, the ultimate 

 structure of which is also similar, the glands being congeries 

 of tubes or cells. 



Returning to the sap, it may be asked — and it is an import- 

 ant question — how are certain of the substances, which I have 

 mentioned as essential to this nutritive fluid, dissolved in the 

 water of the sap, such as phosphate of lime, carbonate of 

 lime, silica, — themselves insoluble in water I My belief is, 

 and it is founded upon experiments which I have made, that 

 their solution is effected by the carbonic acid in the sap. It 

 is well known how soluble carbonate of lime, and I may 

 add carbonate of magnesia, is in water containing carbonic 

 acid ; — it is quite certain that phosphate of lime is also so- 

 luble in the same, and that not in an inconsiderable degree ; 

 and the experiments which I have made on silica, to me are 

 convincing that it likewise is soluble in water impregnated 

 with carbonic acid, though in a degree very much less than 

 phosphate of lime. 



Taking this for granted, a certain simplicity is imparted 

 to the nutritive process of plants. A fluid medium, water, 

 holding a gaseous acid, carbonic acid, is the menstruum of 

 the inorganic substances derived from the soil, which the 

 plant requires for its healthy growth. This compound solu- 

 tion becomes exposed, in the leaves, to the action of light, 

 and to the evaporating agency of the winds ; the carbonic 

 acid undergoes decomposition, as already mentioned, carbon 

 being detained for the use of the plant, oxygen being ex- 

 haled ; a portion of the water is removed by evaporation. 



