208 TRANSPIRATION AND ASCENT OF SAP ch. 



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 these 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 tem- 

 perature, 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 sur- 

 rounding objects, and so increases the vis viva of the re- 

 maining 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 when secretion 

 is not taking place. 



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 



