24 



FUNDAMENTALS OF FRUIT PRODUCTION 



.'Pbf the leaves is an open question. The generally accepted opinion is 

 SSthat transpiration is a necessary evil rather than an advantage to the 

 ;^plant. 



. %t Cuticular and Stomatal Transpiration Compared. — Transpiration is 

 jgof two kinds; a small amount of water is lost from the cuticular surface 

 *^of the leaf, but by far the greater proportion is lost through the stomata. 

 ^Some measure of the proportion between these is furnished by the data 

 '^presented in Table 13. 



The amount of cuticular transpiration is relatively constant while the amount 

 of stomatal transpiration can be regulated by the activity of the stomatal guard 

 cells. In sunlight these cells manufacture sugars by means of the chloroplasts 

 which they contain and thereby increase their osmotic concentration so that they 

 absorb water from the surrounding tissue and increase in turgidity. The walls 

 of the guard cells are peculiarly thickened so that when the cells are turgid, the 

 stomatal aperture is open and when turgidity is lost the aperture is closed. The 

 guard cells are likewise sensitive to light stimuli, to which they respond by changes 

 in turgidity more rapid than those produced in the manner just described. 



The water lost by transpiration from the leaves first evaporates from the 

 surface of the mesophyll cells in the interior and collects as water vapor in the 

 intercellular spaces. This water vapor then passes from the intercellular spaces 

 of the leaf through the stomatal apertures to the outside. This is largely a 

 process of diffusion and follows Brown and Escombe's Law which states that 

 diffusion through an aperture is proportional to the radius and not to the area of 

 the aperture. Thus the diffusion which might take place through 10 small 

 apertures with a radius of 1 millimeter, would be equal to the diffusion which 

 could take place through one aperture with a radius of 1 centimeter. It is 

 evident that if the apertures are sufficiently small and are scattered over a surface 

 in such a way that the diffusion through one does not interfere with that through 

 another, then diffusion through such a perforated surface will take place as if no 

 surface were present. When the apertures are distributed over a surface so that 

 they are about 10 diameters distant from one another, the maximum amount of 

 diffusion is possible. This proportion holds roughly for the distribution of the 

 stomata in most leaves. It is evident that when the stomata are opened the 

 surface of the leaf offers little or no resistance to the diffusion of water vapor. 



