178 THE STOMATAL MECHANISM 



leaves of plants of the same species which have developed during different 

 seasons. 



As shown in Table 21, stomates occur in both the upper and lower 

 epidermis of many species of plants. In numerous others, especially woody 

 species, they are confined to the lower epidermis. Even in those species in 

 which stomates occur on both surfaces of the leaf they are commonly, but 

 not always, more abundant in the lower epidermis. In floating leaves such 

 as those of the water-lily, stomates occur only in the upper epidermis. Species 

 in which the stomates are relatively small usually have more per unit area 

 than species in which the stomates are relatively large. 



The number of stomates per unit area usually varies on different leaves 

 of the same plant, and even in different parts of the same leaf. On individual 

 leaves it appears to be generally true that the greatest number of stomates 

 per unit area is at the tip, the lowest towards the base, while the middle 

 portion of a leaf shows a frequency midway between these two extremes. As 

 a rule, the higher the point of attachment of a leaf on the stem of a plant, 

 the greater the number of stomates per unit area. 



In general no correlation has been found between transpiration rates and 

 either the size or distribution of stomates, other factors being much more 

 important in determining the rate of loss of water-vapor from the intercellular 

 spaces. 



Principles Governing Diffusion of Gases through the Stomates. — 

 Since virtually all gaseous exchanges between the intercellular spaces and the 

 atmosphere take place through the stomates, the problem of the diffusive 

 capacity of the stomatal pores is an important one. Although the aggregate 

 area of the fully open stomates on a single leaf is seldom more than 3 per 

 cent of the leaf area and is often as low as i per cent, the rate of water loss 

 from the leaves (i.e. loss per unit area in a unit time) in many species may 

 be, under favorable conditions, as much as 50 per cent of the evaporation 

 from an exposed water surface of comparable dimensions. Stalfelt (1932) 

 records that transpiration from a leaf of birch {Betula pubescens) under the 

 most favorable conditions may be 65 per cent of the evaporation from a com- 

 parable evaporating surface, but this is probably an unusually high value. It 

 is therefore evident that water-vapor often diffuses through the stomates at 

 rates ranging up to at least 50 times greater than it diffuses away from an 

 equal area of exposed evaporating surface. 



This unexpectedly high diffusive capacity of the stomates is intelligible 

 in terms of the results of studies upon the principles of diffusion through 

 small apertures. The classical investigation of this problem was made by 

 Brown and Escombe (1900) who studied the rate of diffusion of carbon 



