CHANGES PRODUCED BY NOURISHMENT. 105 



the same effect. The pruning of fruit trees is principally 

 designed to limit the production of fruit to the quantity 

 that can be properly nourished by the plant. The graft- 

 ing which is practised upon trees of analogous species, 

 only presents to the juices of the wild tree an organic 

 tissue different from its own ; in the cells of which the 

 juices receive a peculiar elaboration, which changes the 

 nature of their products. 



It is not by an analysis of plants, nor by the proportion 

 of their constituent principles, which can be extracted by 

 water, that we can judge of the nutritive quality of vege- 

 tables, or other alimentary substances. I have already 

 proved, that a nutritive substance, deprived of all its solu- 

 ble parts by water, is capable, in the progress of its de- 

 composition, of forming new and soluble compounds. It 

 is only by experiments, and by the effects of this or that 

 kind of food upon animals, that we can ascertain the 

 differences existing between various nutritive bodies. 



The digestive juices of the stomachs of animals and 

 the organs of plants animated by vital powers, of which 

 we are ignorant, have also their chymistry, with which 

 we are unacquainted, and of which we can understand 

 only the results. It is surely erroneous to pretend to 

 determine the quantity of nourishment, by that portion 

 I which can be extracted from any article of food by water ; 

 but upon this principle Davy has represented the nutritive 

 virtue of beets by the number of 136, and that of carrots 

 by 98 ; whilst M. Thayer has by his experiments estimated 

 that of the first to be 57, and of the last 98. Upon the 

 same principle Davy has valued the effects of linseed 

 cakes at 151, compared with those of beets as 136 ; 

 while it has been proved that 70 lb. of beets are hardly 

 equivalent in nourishment to 10 lb. of linseed cakes. 



In order to estimate the nutritive merits of any sub- 

 stance, it is necessary to have less regard to its chymical 

 character, than to the nature of the animal to be nourished 

 by it : one is disgusted by that which pleases another ; 

 and this will decompose what that will reject ; it is only 

 by observation that we can decide. 



These principles are still less applicable to the nourish- 

 ment of plants, than of animals ; because of the first it 

 is necessary that their food should be presented to them, 

 and in a state of solution or mixture ; whilst the last seek 

 theirs where it maybe found, and make choice of such 



