236 THE MOVEMENTS OF WATER 



Transpiration is influenced by the same external conditions as the 

 evaporation of water in general. It varies, however, very much in different 

 plants, and in different parts of the same plant under similar external 

 conditions. This is because the formation and escape of water-vapour is 

 largely dependent upon the specific peculiarities of the transpiring organs. 

 In the first place the plant employs relatively impermeable layers, such as 

 cork and cuticle, to restrict to a greater or less extent the evaporation of 

 water ; while, on the other hand, the loss of water by evaporation is 

 favoured by the existence of open channels to the exterior, of which the 

 stomata are the most important. Since, however, the stomata close in 

 time of need, the plant is able to regulate its transpiratory activity to 

 a greater or less extent. 



These structural arrangements and the adaptative modifications which 

 they may undergo have already been described in connexion with the 

 exchange of gases (Chap. V), where reasons have been given why any 

 check on transpiration also diminishes the rapidity of gaseous exchange. 

 A gaseous particle in order to escape from a cell must pass in dissolved 

 form to the outer surface of the cell-wall, and thence into the gaseous 

 condition if the partial pressure (or the degree of saturation) of the air 

 outside is sufficiently low, and the principle is the same whether the gas 

 or water-vapour is evolved from the free surface of a leaf or from the cells 

 which line its intercellular spaces. The former may be termed cuticular, 

 and the latter diastomatic or intracellular transpiration : , since in the 

 second case the water-vapour finds exit through the open stomata and 

 lenticels to the external air. When stomata are present, a certain specific 

 but variable relationship exists between the amounts of water lost by 

 diastomatic and cuticular transpiration. As the cuticle becomes more 

 and more impermeable, diastomatic transpiration increases in importance. 

 On the other hand, it diminishes as the apertures of the stomata narrow, 

 and ceases when they are entirely closed. 



How far the transpiration can be reduced to a very low ebb if 

 necessary, depends therefore upon the relative impermeability of the 

 cuticle. In members of the Cactaceae, Crassulaceae, and in certain Orchids, 

 the cuticle is extremely impermeable, so that when there is any deficiency 

 of water, or even when the plants are suspended in dry air, they may 

 remain comparatively fresh for weeks and shrivel but little if at all, 

 whereas when the sap is abundant the stomata open and are available 

 for rapid gaseous exchange. Less xerophilous plants, such as comprise 

 most of our native flora, do not possess so impermeable a cuticular layer, 

 and hence they continue to lose water even when the stomata are closed, 



1 [Either of these terms is preferable to that of ' stomatal transpiration,' since the water-vapour 

 merely escapes through the stomatal pore, and is not evolved from the stoma itself, as the latter 

 term would indicate.] 



