EVAPORATION 159 



reduces transpiration directly through the walls of the epidermal cells to a 

 magnitude much less than it would be, were there no such layer present. The 

 thickness of the cutin layer varies with the species of the plant, and the en- 

 vironmental conditions under which the leaves have developed. The layer 

 of cutin is usually thicker on leaves which have developed in bright sunlight, 

 for example, than on leaves of the same species which have developed in shade. 

 Even in leaves which are heavily coated with cutin, some cuticular transpira- 

 tion occurs, possibly largely through tiny rifts in the cutin layer. In most 

 species of plants of the temperate zone less than 10 per cent of the foliar 

 transpiration occurs through the cuticle, the remainder being stomatal tran- 

 spiration. 



Evaporation. — The fact that transpiration may be regarded as essen- 

 tially a modified form of the process of evaporation makes desirable a fuller 

 consideration of the dynamics of this process. When an open pan of water is 

 exposed to the atmosphere, the level of the water in the pan slowly drops. 

 Evidently water molecules are being slowly lost into the atmosphere. This 

 is an example of the well known process of evaporation. All of the molecules 

 in a mass of water are not travelling with the same velocity, although for any 

 given temperature the average speed of all of the molecules in the liquid mass 

 is a constant. Some of the molecules in the liquid water attain sufficient mo- 

 mentum to entirely overcome the attractive forces holding them in the liquid, 

 and escape into the surrounding atmosphere in the form of vapor. The swift- 

 est molecules present are most likely to be able to overcome the attraction 

 of the other water molecules, therefore they are the most likely to be lost from 

 the liquid. During evaporation any body of water is thus slowly being 

 depleted of its more rapidly moving molecules. The residual water becomes 

 progressively richer in relatively sluggish molecules ; in other words it becomes 

 cooler. This cooling effect is more or less completely offset, however, by 

 physical transfer of heat into the water from its surroundings as soon as its 

 temperature drops below that of the environment. The proportion of "high 

 speed" molecules in the molecular population of the pan of water is thus 

 maintained at close to its original value and the rate of evaporation continues 

 with very little diminution. 



In the preceding paragraph we have focussed our attention only on the 

 water molecules which escape from the liquid. Water-vapor molecules are 

 also returning to the liquid from the atmosphere during the process of evapora- 

 tion. If a water-vapor molecule in the atmosphere above the evaporating 

 surface, particularly one of the more sluggish ones, strikes the surface of the 

 water at the proper angle and with not too great a velocity, it will be held 

 there by the attractive forces exerted by the liquid water molecules, and again 



