CAUSE OF THE ASCENT OF THE SAP. 87 



Stances the fluid will reach the height of several feet, and will flow out 

 or run over at the open end of the tube. At the same lime tlie water in 

 the vessel will become sweet, indicating that while so much liquid has 

 passed through the membrane inwards, a quantity has also passed out- 

 wards, carrying sugar along with it.* To these opposite effects Dutro- 

 chet, who first drew attention to the fact, gave the names of Endosmose, 

 denoting the inward progress, and Exosmose,{he outward progress of the 

 fluid. He supposed them to be due to the action of two opposite cur- 

 rents of electricity, and he likens the phenomena observed during the 

 circulation of the sap in plants, to the appearances presented during the 

 above experiment. 



Without discussing the degree of probability which exists as to the in- 

 fluence of electricity in producing the phenomena of endosmose and ex- 

 osmose, it must be admitted that the appearances themselves bear a 

 strong resemblance to those presented in the absorption and excretion of 

 fluids by the roots of ])lanls — and point very distinctly to at least a 

 kindred cause. 



Thus, if the spongy termination of the root represent the thin porous 

 membrane in the above experiment — the sap with which the tubes of 

 the wood are filled, the artificial solution introduced into the experimen- 

 tal tube — and the water in the soil, the water or aqueous solution into 

 which the closed extremity of the tube is introduced, — we have a series 

 of conditions precisely similar to those in the experiment. Fluids ought 

 consequently to enter from the soil into the roots, and thence to ascend 

 into the stem, as in nature they appear to do. 



This ascent, we have said, will continue till the fluid in the tubes of 

 the wood (the sap) is reduced to a density as low as that of the liquid 

 entering the roots from the soil. But in a growing tree, clothed with 

 foliage, this will never happen. The leaves are continually exhaling 

 aqueous vapour, as one of their constant functions, and sometimes in 

 very large (juantily. The sap, therefore, when it reaches the leaves, is 

 concentrated or thickened, and rendered more dense by the separation 

 of the water, so that when it descends to the root, and again begins its 

 upward course, it will admit of large dilution before its density can be 

 so far diminished as to a|)proach that of the comparatively pure water 

 which is absorbed from the soil. And this illustration of the ascent of 

 the sap appears the more correct from the obvious purpose it points out 

 — (in addition to others long recognised) — as served by the evaporation 

 which is constantly taking place from the surface of the leaf. 



Still the cause of the ascent of the sap is not the more clear that we 

 can imitate it in some measure by an artificial experiment. But it will 

 be conceded by the strictest reasoners on physical phenomena, that to 

 have obtained the command, or even a partial control, over a natural 



* Instead of sugar, common salt, gum, or other soluble substances may be dissolved in 

 the water introduced at first into the tube, and tlie denser this solution the larger the quantity 

 of water which will enter by the membrane, and the greater the height to which the column 

 will rise. It ceases in all cases to rise only when the portions of liquid within and without 

 the membrane attain nearly to the same density [i. e. contain nearly the same weight of solid 

 matter in solution.] Instead of pure water the vessel into which the extremity of the tube 

 is phjnged may also contain a weak solution of some soluble substance — such as lime or soda 

 — in which casa while the sugar, or salt, or gum, will pass outwards, in smaller quantity, the 

 lime or soda will pass inwards, along with the currents of water in which they are severally 

 dissolved. 



