270 CAPILLARITY AND ROOT-PRESSURE. 



bundles, traverse the stems and leaves of plants. But the vessels are closed in 

 above and below, and therefore it is impossible that capillarity should be sufficiently- 

 developed in them. At best it could only raise the sap a trifling distance, and 

 could never convey fluid to a height of many metres. It is a striking fact that in 

 many plants the ascent of the sap is most vigorous after the evaporation from the 

 superficial parts exposed to the air has been weakest. The so-called "weeping" of 

 vines, i.e. the outflow of sap from the flat surface of a cut vine-branch, does not 

 take place in summer and autumn, immediately after the branch has been fully 

 adorned with foliage, and when its extensive leaf-surfaces have given up large 

 quantities of moisture to the surrounding air; it occurs at the end of the winter 

 sleep of the plants, when the brown branches rising above the ground are still in 

 a bare and leafless condition. The (iause of the ascent, or at least of the ascent 

 in the lower leafless branches, must therefore be sought for in the absorbent roots, 

 and it may be assumed that here the same causes are at work which induce the 

 fluid food materials of the surrounding earth to enter the superficial cells at the 

 root-tips. 



It has already been shown that the contents of these cells suck up the water of 

 the nutritive ground with great force in consequence of the chemical affinity they 

 have for it, or in other words, that the fluid reaches the interior of plant-cells by 

 endosmosis; it has also been mentioned that in consequence of the taking in of 

 water the volume of the cell-contents increases, producing pressure from within 

 outwards on the cell-wall, and the cell swells and becomes turgid. From this one of 

 three cases might be deduced: — first, suppose that the cell-wall is so composed 

 throughout that it allows the entrance of water into the cell, but not its exit, and 

 that consequently the cell-contents absorb water, but that a filtration of the same 

 towards the exterior cannot take place. Granted this hypothesis, the cell-wall by 

 virtue of its elasticity would yield to the pressure of the cell-contents, but only 

 within the limits of that elasticity; hence a condition of tension would be produced, 

 in which the reciprocal pressures of the cell-wall and cell-contents would be in 

 equilibrium. In the second case, suppose that the pressure of the cell-contents is 

 greater than the force of cohesion between the molecules of the cell-wall, this 

 consequently ruptures, and the cell-contents issue from the rent which is formed. 

 This phenomenon is seen in certain pollen grains when placed in water. In half 

 a second the cells absorb so much water that they double their volume; the cell- 

 contents still absorb the fluid, and the cell- wall can at length no longer withstand 

 the pressure; it bursts, and the contents, from which the pressure is now removed, 

 pour through the opening, and are diftused in the surrounding water. 



There is a third case possible. Suppose that in a given cell the opposite walls 

 are not of identical structure; that the wall which is in contact with the damp earth 

 is so organized as to allow the entrance of water, but not its filtration to the 

 exterior, while the opposite wall ofifers only a slight resistance to such filtration; 

 then by the increasing pressure of the cell-contents fluid will be forced through 

 that wall which offers least resistance, and the greater the affinity of the cell- 



