CH. II.] THE ROOT. - 93 



insoluble, and experiment has demonstrated that the 

 roots cannot absorb it in a solid state. Sennebier, 

 having obsened that water impregnated with car- 

 bonic acid, "svhen applied to the roots of plants, was 

 beneficial, concluded that the carbon of manures is 

 converted into carbonic acid, and is in that state 

 imbibed by them^. 



Thomson, in an early edition of his System of 

 Chemistry, gave a still more elaborate theory; which, 

 being in subsequent editions omitted, we have no 

 occasion to demonstrate absurd. I consider that the 

 facts of which we are in possession, if progressively 

 estimated, place the subject in a very clear light. 

 Saussure fomid that a soil deprived of its soluble 

 matters, by repeated decoctions with water, would not 

 support vegetation so well as that portion of the same 

 soil not so deprived of its soluble constituents^. The 

 extract thus obtained was e\'idently composed of 

 sacchaiine matter, mucilage, extractive principle, &c. 

 These we know are nutritive to plants, and are 

 elaborated and assimilated by them after introsuscep- 

 tion. Now, vegetable substances, as straw, &c., 

 gradually yield these soluble matters as they decay. 

 Straw, wood, leaves, &c., consist chiefly of woody 

 fibre : to convert this into saccharine and mucila- 

 ginous matters is the work of putrefaction ; to effect 



^ Phys. Yeg. v. iii. p. 55. 



*» Recherh. sur la Yeg. cv. sect. 11. p. 170. 



