PROGRESSION OF THE SAP. 



131 



273. Liebig thinks that in the case of the vine, in which, according 

 to Briicke, the specific gravity of the sap in spring is very little 

 more than that of water, the rise of 



the sap does not at this season depend 

 on endosmose, but on the disengage- 

 ment of gas, which was shown by 

 Hales to be given off in large quanti- 

 ties, when the vessels were cut. The 

 gas is conjectured to be carbonic acid 

 gas, judging from the experiments of 

 Geiger and Proust, who showed that 

 the sap of the vine contains much of 

 this acid. 



274. The height to which the sap 

 rises hi the case of lofty trees, with 

 spreading roots, is very great. The 

 force with which it ascends has been 

 measured by Hales, and is found to 

 vary according to the state of the 

 weather and the vigour of the plant. 

 By fastening a bent tube, containing 

 mercury, on the stem of the vine, he 

 found in one of his experiments, that 

 the sap raised the mercury upwards of 

 thirty inches. The apparatus used 

 by Hales, is similar to that used 

 by Dutrochet, to measure endosmose, 

 as is represented at fig. 224, where 

 c is the stem of a vine cut, t is a 

 bent glass tube fitted to the cut ex- 

 tremity of the vine by a copper ring, 

 v, carefully luted and secured by a bit 

 of bladder, m; n n represents the level 

 of the mercury in the two branches 

 of the lower curvature, before the 

 experiment, and n' n' the level at the 

 conclusion of it. He calculated that 

 the force of the sap in the vine, in 

 some of his experiments, was five times 

 greater than that of the blood in the 

 crural artery of the horse. 



Fig. 224, 



Fig. 224. Experiment by Hales, to show the force of ascent of the sap. c, Stock of a vine cut. 

 t, A glass tube with a double curvature attached to the upper part of the vine-stock, by means 

 of a copper cap, r, which is secured by means of a lute and piece of a bladder, m. n n Level of 

 the column of mercury in the two branches of the tube at the commencement of the experiment. 

 ri n', Level at the conclusion of the experiment. 



