6 TRANSPIRATION AND ASCENT OF SAP ch. 



First the transpiring surface of the evaporating cells will 

 be diminished owing to the contraction of the cells when 

 they cease to be turgescent. Secondly, the diffusion of 

 the water-vapour from the inner tissues of the leaf will be 

 hindered by the collapse of the intercellular passages caused 

 by the drooping of the leaf ; and, finally, the evaporating 

 films which on the surface of the wall of the turgid cell 

 may be supposed to be but slightly concave, will under the 

 new conditions tend, by retreating inwards into the sub- 

 stance of the cell-walls, to form a series of more concave 

 menisci from which, as is known, evaporation will proceed 

 more slowly. However, as the walls of these cells are im- 

 bibed with water, and a continuous column of water 

 extends back from the outer evaporating walls down 

 through the conduits into the plant hanging from the 

 surface film formed on the outside of the evaporating cell- 

 wall, a slow movement upwards will take place of the 

 solution in the conduits even after the loss of turgor by 

 the osmotic cells. The supply is in most cases inadequate 

 to prevent the drying of the leaf. 



To the evaporation at the surface film formed on the 

 cell-walls must also be referred the ascent of water in 

 pieces of dead wood which have been soaked and injected 

 with water, as described by Strasburger ; for in this case, 

 of course, the intervention of osmotic forces is excluded. 

 It is to be noticed that in this experiment also the wood 

 dries downwards from above. 



From the foregoing considerations it seems quite feasible 

 to explain the process of transpiration as a purely physical 

 one in which the intervention of living matter, as such, is 

 not necessary ; for we might substitute a dead osmotic 

 cell in the plant for the living one without having reason 

 to expect a difference in the action of the mechanism. 

 This being so, it appears highly probable that transpira- 

 tion is often a purely physical phenomenon. 



However, we must not shut our eyes to the fact that 



