176 THE STOAIATAL MECHANISM 



the individual stomates on any one plant. They are always very minute, 

 hovv^ever, their dimensions being expressed in terms of microns. Measure- 

 ments of the average length and breadth of the fully open stomatal pore on 

 a number of representative species of plants are listed in Table 21. 



Fig, 43. Perspective view of a stomate and adjacent cells (semi-diagrammatic). 



Minute as these openings appear to be from a human scale of values, 

 they are enormous when compared with the size of the gas molecules which 

 diffuse through them. The calculated diameter of a water molecule is 

 0.000454 {J- More than 2,000 water molecules would have to be placed side 

 by side to measure a distance of i /x. The molecules of both carbon dioxide 

 and oxygen are larger than water molecules. Since the stomatal diameters 

 usually are considerably in excess of i jx, it is evident that the stomates afford 

 relatively enormous portals to the gas molecules which diffuse through them. 



In general the number of stomates present in the epidermis of leaves may 

 range from a few thousand to over a hundred thousand per square centimeter, 

 the exact number depending upon the species and upon the environmental 

 conditions under which the leaf has developed. A single maize plant has 

 been estimated to bear from 140 to 240 million stomates, while the number 

 on a large tree could be expressed only by a figure of astronomical dimensions. 



The average numbers of stomates which have been found per square centi- 

 meter on the leaves of a number of representative species are listed in Table 

 21. However, marked deviations from such average values are possible 

 for any species, depending upon the environmental conditions under which 

 the leaves have developed. The number of stomates per unit area of leaf 

 surface may be quite different on leaves of two plants of the same species, 

 if one grew in a greenhouse, and the other grew in the open, or upon the 



