304 



PLANT PHYSIOLOGY 



layer of epidermal cells, whose outer walls are coated with a 

 cuticle that is only slightly permeable to water or water vapor 

 (see Figs. 54 and 102). 



The cuticle, not being entirely impermeable to water, does 

 not completely stop evaporation. This can be demonstrated 

 by coating with vaseline the lower surface of leaves that have no 

 stomata on the upper surface ; there are many trees the leaves of 

 which are well suited to this purpose. These leaves will continue 

 to transpire, although at a reduced rate. This loss of water 



through the cuticle is called ''cuticular 

 transpiration," as distinguished from 

 ''stomatal transpiration," in which 

 water escapes through the stomata. In 

 fully developed leaves, stomatal trans- 

 piration is ten to twenty times as rapid 

 as cuticular transpiration. In young 

 leaves, with cuticle that is not com- 

 pletely developed, as well as in leaves 

 growing in the shade or in a moist 

 atmosphere, half of the water may be 

 transpired through the cuticle. 



Stomatal transpiration consists of 

 two phases: the evaporation from the 

 Fig. 96. — Diagrams show- surface of the water-saturated meso- 



Z^^'^t.:^::::^ ri:^ P^y" ^ells Iming the mterceUular spaces, 



and the diffusion of the vapor thus 

 formed through the stomata. The laws 

 governing diffusion through stomata have been discussed pre- 

 viously. It has been shown that the great number of these very 

 small openings in the cuticle strongly favors diffusion. This proc- 

 ess may go on through the stomatal openings, constituting only 

 1 to 2 per cent of the leaf area, almost at the same rate as if the 

 cuticle did not exist and the internal cavities were fully open to 

 the air. A diagram illustrating this is shown in Fig. 96. In 

 some very vigorously evaporating plants, the relative transpira- 

 tion may attain the magnitude of 0.8 to 0.9; while under favorable 

 conditions, 0.4 to 0.5 are average figures. If transpiration were 

 proportional to the area of the openings, then the values would 

 not exceed 0.01 to 0.02. Thus it is seen that, per stoma, the 

 diffusion rate of water vapor is very high. 



through a multiperforated 

 membrane B {after H. Walter) . 



