712 



PLANT GROWTH AND PLANT COMMUNITIES 



Any vapor movement is limited not only by the characteristics of 

 the transmission path but also by the availability of the energy required 

 to evaporate the water at its vaporization site. Thus the existence of 

 vapor transport in a given path is dependent upon the exact locations 

 at which the leaf absorbs radiant or sensible energy and upon the 

 thermal-conductivity characteristics of the leaf tissue. 



The transmission characteristics of the vapor path from the meso- 

 phyll surface through the substomatal chamber, the stomata, the micro 

 vapor cap, and the layer of still air over the leaf surface can be approxi- 

 mated for corn, as has been done by Bange ( 1953 ) for Zebrina leaves, 

 although the calculations for corn will be less accurate than those for 

 Zebrina, due to the greater eccentricity of the elliptical stomatal pore 

 of corn. The resistances of the various parts of the vapor path are shown 

 in Figure 13 as a function of the stomatal aperature. Under normal 

 conditions, the greatest potential drop of the soil-plant-atmosphere sys- 

 tem occurs in this vapor path, the difference in potential between the 

 nearly saturated air at the mesophyll surface and an outer atmosphere 

 at 50 per cent relative humidity being about 700 atmospheres. When 

 the stomata are fully open in still air, about 80 per cent of the total 

 path resistance comes not inside the leaf but in the air layers over the 



2.0 



/i Substomatal cavity 



B Stomatal pore 



C Micro vapor cup 



D Adhering air layer 



± 



^_ 



10 20 30 40 50 100 



PERCENT OF FULL STOMATAL APERTURE 



Figure 13. Calculated resistance to water-vapor flow from a transpiring leaf 

 as a function of stomatal aperture. 



