Transpiration and the Ascent of Sap. 57 



When the pots are replaced by a leafy branch the wheel is kept 

 in lively rotation by the evaporation from the leaves. Variations in 

 the speed of rotation of the wheel mark the variations in the amount 

 of evaporation from the leaves. Thus, exposure to a draught or to 

 a hig-her temperature, as in the case of the pots, accelerates the rate 

 of rotation. In addition to this the influence of light upon the stomata 

 may be observed in the increased rate by day; while conversely 

 darkness diminishes the rate of rotation. 



It is interesting- to trace in this model how the molar work of 

 maintaining the wheel in rotation is derived from the molecular 

 action at the evaporating surface. The molecules of the liquid with 

 most vis viva emerge from the superficial layers of the liquid and, 

 escaping by reason of their momentum from the attractions of their 

 fellows, dash into the adjacent unsaturated space. Those with less 

 energy cannot free themselves from the bonds of their neighbours 

 and, if they get beyond the surface at all, they must needs fall back 

 again into the body of the liquid. In this manner from the surface 

 of an evaporating liquid there is a constant sorting out of those 

 molecules which possess the greatest amount of energy, while those 

 with less remain behind. In consequence of this selective action the 

 unevaporated liquid, being composed of those which possess least 

 energy, is maintained at a lower temperature, and therefore heat 

 continues to flow into it from its surroundings. It becomes in fact 

 a sink of energy. The heat, which is continually entering at the 

 evaporating surface prevents the liquid under ordinary conditions 

 falling to a temperature much below that of the surrounding objects 

 and so increases the vis viva of the remaining molecules and enables 

 evaporation to continue. This inflow of heat at the seat of evaporation 

 is the ultimate source of the energy which raises the water to turn 

 the wheel. This is true both for the evaporating surfaces of the 

 porous pots- and those of the leaves. 



In the model the evaporating menisci do work raising the water 

 which in its passage turns the wheel; in an intact plant the work 

 is done against the weight of the tensile transpiration stream and 

 the resistance of the conducting tracts. The capillary forces of the 

 menisci serve to hold the upper surface of the water in position 

 whilst the inflowing heat, by disconnecting and removing molecules 

 in these menisci, tends to shorten the water columns. The bonds 

 cast off by the escaping molecules are transferred to others within 

 the liquid which are thus drawn to the menisci, and the tension is 

 generated and maintained in the liquid. It is clear that the capillary 

 forces of the menisci must be sufficiently great to support the tension 

 needed to raise the water, and hence the fine grain of the walls of 

 the transpiring cells forms an essential link in the mechanism which 



