4 TRANSPIRATION AND ASCENT OF SAP ch. 



bibition in transpiration. The leaf trachea? contain- 

 ing the rising sap are separated from the intercellular 

 spaces by a layer of one or more thin-walled cells. The 

 thin wall, which is formed of cellulose, is permeable to 

 water and dissolved substances ; at the same time it 

 is tough and of considerable tensile strength. It is 

 lined by a layer of viscid protoplasm which in turn 

 surrounds, and is completely filled by, a solution the 

 vacuole. The latter contains various carbohydrates and 

 electrolytes in solution, and round it the protoplasmic 

 layer forms a fairly perfect semi-permeable membrane. 

 It is evident that, when sufficient water is available, 

 the solutes in the vacuole will exert an osmotic 

 pressure on the protoplasm which will be forced outwards 

 against the wall and distend the latter. Ultimately, 

 the tensile stress in the wall will balance the osmotic 

 pressure of the solutes of the vacuole and equilibrium will 

 be established and the cells will be tense and rigid. The 

 walls of the trachea? are also quite permeable, but they 

 are more rigid than those of the cells and they are further 

 prevented from collapsing by the presence of the internal 

 supports in the form of rings and spirals. The trachea? 

 do not enclose a semi-permeable membrane of protoplasm. 

 We may now consider how this mechanism will act in 

 the transference of water. The imbibitional or capillary 

 forces of the cell-walls in contact with the intercellular 

 spaces of the leaf will draw off water from their vacuoles 

 through the protoplasm until the concentration of solutes 

 in the vacuole is such that the vapour pressure of water 

 in it is equal to that obtaining in the cell-wall. Supposing 

 now that the vapour pressure of the water menisci in the 

 reseau of the cell-wall is greater than that obtaining in the 

 intercellular space, water will leave the wall and the menisci 

 will retreat into it. This will cause their curvature to 

 increase and will raise their capillary forces so that they 

 will endeavour to extract water from the solution in the 



