36 THE PHYSIOLOGY OF STOMATA. 



micra. a, therefore, equals 7.5 micra. The transverse diameters have been 

 taken in geometrical progression from 26 equals 2 micra up, increasing in the 

 ratio 1.30, the areas increasing in the same ratio. The length of the stomatal 

 tube (the curved sides of which can not be taken into account) is taken at 

 15 micra and is, then, a constant. When the air-currents are sufficient to 

 remove the outer density shells the diffusive capacity is increased. The 

 formula, then, is 



^4 



= constant - (i) 



1 + x 



and when the two, outer and inner, density shells are present, as would be 

 the case in perfectly still air, the formula would be modified to read 



A 



1 = constant - (2) 



/ + 2X 



where 



A = transverse area of stomatal tube. 

 / = length of stomatal tube. 



also 



x = " X diameter of a circle of area equal to A . 



nr 

 x = = 0.7854*- 



4 



where r = radius of a circle of area equal to that of the stomatal tube (A ) . 

 Since A is equal to Ttr 2 formulas (i) and (2) may be written 



r 2 



Q =Cj- -for one shell .... (3) 



/ + 0.7854*- 



r 2 



Q I =C- -for two shells .... (4) 



1 + i. 57o8r 



where C is a constant. 



Assuming the stomatal pores to be elliptical, 



A =7tab (5) 



where a and b are the semi-axes. Also, since A = nr* we have TC ab = Ttr 1 

 and r = ]/ab (6) 



Table 18 contains the numbers which show, for stomata of several different 

 dimensions, the ratios of diffusion capacities for these dimensions, when the 

 air is still (Q l ) and in motion (Q}. These ratios are smaller than the ratios 

 between the areas, which are proportional to the transverse diameters (6) 

 and larger than those (ratio of r) between the radii (r) of circles of areas 

 equal to the areas of the stomata. The effect of the wind upon the diffusion 



capacity of a stoma of any given dimension is shown in the ratio ~ t and it 



j& 



