76 



PHOTOSYNTHESIS 



unit area as the size of the aperture is diminished. This is caused in the 

 main by the degree of convergence and divergence of the lines of flow. 

 With a smaller aperture the convergence and divergence increase, result- 

 ing in an increase in the gradient of density. When the apertures are 

 ten diameters or more apart each perforation behaves independently 

 without interfering vi^ith its neighbors and follows the "diameter law." 



On this basis Brown and Escombe explain the fact that they were 

 able to block out nearly 90 per cent of the cross-section of a column of 

 diffusing gas leaving 100 circular apertures, and still produce no sensible 

 effect in obstructing the diffusion of carbon dioxide through air. They 

 were able to obtain photographs of the zones of equal density produced 

 by intermittent diffusion of two reacting substances through apertures. 



Fig. 6. — Brown and Escombe's conception of the lines of flux in a diffusive 



column through septa. 



Thus this behavior, as would be expected, is characteristic not only for 

 gases but also for substances in solution. 



In applying these results to the gaseous exchange of leaves, Brown 

 and Escombe studied an herbaceous plant which has been extensively 

 used for photosynthesis investigations. The sunflower, HeliantJms an- 

 nuiis, has an arrangement of stomata in the surface of the leaves corre- 

 sponding to a multi-perforate diaphragm below which is a very large 

 absorptive surface on the sides of the intercellular spaces. In these 

 spaces is ample room for the formation of negative shells as described 

 above. If the stomata are regarded as circular apertures they are about 

 eight diameters apart, permitting nearly full efficiency of each opening. 

 When they are partly closed, full efficiency of each is very probably 

 attained. When the stomata are wide open they form straight-sided 

 tubes about 0.014 mm. in length. This opening corresponds to a circle 

 0.0107 mm. in diameter and an area 0.0000908 sq. mm. Assuming the 

 following most favorable conditions : wide-open stomata, constant partial 

 pressure of carbon dioxide at the mouth of the stomata by a moving 

 current of air and zero partial pressure of carbon dioxide in the inter- 



