158 



THE LOSS OF WATER FROM PLANTS 



spaces with water-vapor. When the stomates are open, however, diffusion of 

 water-vapor may occur through them into the outside atmosphere. Such 

 outward diffusion will always take place unless the atmosphere has a vapor 

 pressure equal to or greater than that of the intercellular spaces, a condition 



guard cell 



intercellular 

 space 



chlorenohyma. 



Fig. 36. Longitudinal section (semi-diagrammatic) through a small portion of a leaf 



of white pine {Pinus strohus). 



which does not commonly exist during the daylight hours. The rate of such 

 diffusion will depend principally upon the excess of the vapor pressure in the 

 leaf over that of the atmosphere, although the "diffusive capacity" of the 

 stomates (Chap. XIII) is also an important factor. The process of stomatal 



transpiration therefore involves 

 — —cutin evaporation from the cell wall sur- 



faces bounding the intercellular 

 spaces and the diffusion of this 

 water-vapor from the intercellular 

 spaces into the atmosphere through 

 the stomates. 



One side of every epidermal 



cell on a leaf is also exposed to the 



atmosphere. Evaporation of water 



occurs into the atmosphere directly from these cell surfaces. The surfaces of 



practically all aerial leaves are covered with a layer of wax-like substance 



known as cutin (Fig. 37). This is not readily permeable to water and hence 



Fig. 37. 



Cutin layer on the upper epidermis 

 of a leaf of Cli'via nobU'ts. 



