[Chap. XXIV 



TRANSPIRATION 



223 



true of the evaporation of water that is dispersed among the particles 

 of a cell wall. 



In the diagram (Fig. 78) of the diffusion of water from the vessels of 

 the stem to the veins and cells of the leaf, and finally into the atmos- 



CUTICLE 



UPPER EPIDERMIS 



PALISADE 



XYLEM- 



•PHLOEK 



SPONGY MESOPHYLL 



AIR SPACE 



LOWER EPIDERMIS 



Fig. 78. Diagrammatic section of a stem and leaf. Arrows indicate paths of move- 

 ment of water molecules. 



phere, the paths of the diffusion of liquid water and of water vapor are 

 indicated by arrows. From this diagram it should be clear that there 

 are two surfaces where evaporation of water occurs: first, the outer sur- 

 face of the epidermis, and, second, the cell wall surfaces of the mesoph\'ll. 

 The diffusion of water vapor from the outer surface is comparatively 

 simple and is discussed under "cuticular transpiration"; that from the 

 internal surfaces of the leaf is complicated bv certain structural and 

 physical factors and will be discussed under "stomatal transpiration." 



Cuticular transpiration. Let us now have another look at a leaf — first 

 the outside. The epidermis of the leaf is composed of living cells all con- 

 taining water and having more or less saturated cell walls. Some of the 

 water molecules continuallv acquire sufficient energv to break the 

 mutual bonds of cohesion and also of adhesion of the water to the wall 

 molecules, and pass into the atmosphere. The cuticle, which is the outer 

 layer of the epidermal walls, reduces this evaporation to an extent which 

 depends upon its thickness and its fat or wax content. The cutin simply 

 decreases the number of water molecules that reach the epidermal 

 surface since water molecules do not pass readilv into fat-like and wax- 

 like substances. Nevertheless the cuticle is not wholly impervious and 



