l82 



THE STOMATAL MECHANISM 



steep than they would be were there only one pore in the septum. As a 

 result the rate of diffusion through each pore is less than it would be through 

 a pore of equal dimensions in a uniperforate septum. 



The loss of water-vapor by diffusion per- septum decreases with increase 

 in the distance between the openings, i.e. with decrease in the number of 

 pores per septum (Fig. 46). The decrease in diffusive capacity is not, how- 

 ever, in proportion to the reduction in the aggregate area of the pores in the 

 septum. For example, when the pores are spaced 5 diameters apart their 

 aggregate area is only 3.38 per cent of the septum area, yet diffusion through 



PER CENT OF SEPTUM AREA AS PORES 

 338 0.79 0.18 



5 10 15 20 



DISTANCE APART OF PORES IN DIAMETERS 



Fig. 45. Relation benveen 

 water loss per pore and distance 

 in diameters between pores. Data 

 of Weishaupt (1935). 



5 10 15 20 



DISTANCE APART OF PORES IN DIAMETERS 



Fig. 46. Relation between 

 water loss per septum and dis- 

 tance in diameters between pores. 

 Data of Weishaupt (i935)- 



them was 62 per cent of that through an open bottle with a mouth of the 

 same area as the septum (see Table t>z). 



The dimensions of the pores (0.3 mm. diameter, 0.0707 mm.2 area) used 

 in the experiments just discussed are much greater than those of the average 

 stomate. What about the diffusive capacity of still smaller pores? Huber 

 (1930) has shown that the smaller and more numerous the pores, aggregate 

 pore area bemg constant, the greater the diffusion per septum.^ For example, 

 when pores 0.05 mm. in diameter were used, occupying 3.2 per cent of the 



Mf equal-sized pores are spaced equidistantly, and represent the same pro- 

 portionate area of the septum, they will be the same distance apart in terms of 

 their diameters, regardless of pore size. 



