DIFFUSIVE CAPACITY OF STOMATES 183 



septum area, diffusion was 72 per cent as great as from an open evaporatmg 

 surface {cf. with the value given above for pores 0.3 mm. in diameter). 



Ruber's results indicate that the smaller the pores for a given aggregate 

 pore area the more nearl}' the diffusion through the septum will approach 

 that of a free evaporating surface. In the sunflower and undoubtedly also 

 in some other species the aggregate area of the stomatal pores is about 3 per 

 cent of the leaf area. If diffusion through pores O.05 mm. in diameter, 

 occupying 3.2 per cent of the area of the septum, is 72 per cent of that 

 from a free evaporating surface, diffusion through the much smaller stomates 

 of this species (Table 21) should be proportionately greater. In such species 

 it seems probable that the aggregate diffusive capacity of the stomates may 

 closely approach that of a free evaporating surface. Even in species in 

 which the total area of the stomatal pores is less than in the sunflower their 

 aggregate diffusive capacity is very large relative to their area. The relatively 

 high rates of water loss which occur from leaves in proportion to the area of 

 the stomatal pores are thus explicable in terms of the principles of diffusion 

 through multiperforate septa. 



The theoretical diffusive capacity of the stomates is more than adequate 

 to account for known rates of transpiration. This is shown by the calcula- 

 tions of Brown and Escombe (1900) for sunflower leaves. Assuming a leaf 

 temperature of 20° C, the intercellular spaces to be saturated with water- 

 vapor at that temperature (vapor pressure = 17-54 ^^- Hg), and the vapor 

 pressure of the atmosphere to be one-fourth of this value, their computations 

 show that transpiration would occur at the rate of 17 g. of water-vapor per 

 square decimeter of leaf surface per hour. This is several times greater than 

 the maximum rate of transpiration ever recorded for a sunflower plant. Evi- 

 dently some other factor than the diffusive capacity of the stomates has a 

 limiting effect upon transpiration whenever the stomates are widely open. 



The foregoing discussion of the diffusion of gases through small openings 

 has been based on the assumption that diffusion is occurring into a quiet 

 atmosphere. If an air current is blowing across the surface of a multiperforate 

 septum wnth a sufficient velocity to prevent the formation of diffusion shells, 

 the result is a steeper diffusion gradient and hence a greater rate of diffusion 

 of gas through the pores of the septum. Hence, when subjected to air move- 

 ments, the diffusion of water through multiperforate septa or the stomates 

 may be even greater than the preceding discussion has indicated. Within 

 limits increase in the velocity of the wind results in a progressive increase in 

 the rate of diffusion through a multiperforate septum (Deneke, 1931). 



The Mechanism of the Opening and Closing of the Stomates. — The 

 degree of stomatal opening is influenced both by changes in the turgor of the 



