LECT. III.] THE SAP. 105 



abled to discover, with a considerable degree of 

 accuracy, the real food of plants *. This, how- 

 ever, cannot be accomplished ; and as the sap, in 

 its progress, dissolves some ready-formed vege- 

 table matter, which had been deposited at the end 

 of the former autumn, in the upper part of the 

 root and at the base of the stem, its original pro- 

 perties are thus altered ; and the farther the part, 

 which is bored in order to procure the sap, is 

 from the root, the more vegetable matter this 

 fluid is found to contain. This fact was first no- 

 ticed by Mr. Knight, who ascertained that, owing 

 to this deposition, the wood of the stem, and the 

 large branches of trees, have a greater specific 

 gravity, and contain more soluble extractive mat- 

 ter when cut down in winter, than in spring, or 

 early in summer : on this, account, although there 

 is reason for believing that the food of almost all 

 vegetables is the same, yet, the sap, in the state in 

 which we can obtain it, differs in different species 

 of plants ; and, therefore, no just idea can be 

 formed of its nature from the most accurate 

 analysis of it, when procured from any single 

 plant. Were it possible, however, to obtain the 



* It has been supposed that the roots of plants absorb, in- 

 discriminatelyj all the soluble matter contained in the soil on 

 which they grow ; but were this the fact, many more sub- 

 stances would 5e found in the vegetable body, than have yet 

 been discovered in it. 



