THE NATURE OF PHOTOSYNTHESIS 



73 



thetical shells above the disc. In tlie case of the i^erforated septum, as 

 shown m C, Figure 5, the lines of flow are convergent as they approach 

 the septum, bend around their foci, situated in the edges of the aperture, 

 and form a divergent system on the other side. If the absorbing surface 

 below the septum is a i)erfect one, there will be formed in the chamber 

 between this surface and the septum (provided the chamber is sufficiently 

 large) a system of shells exactly similar to those on the outside. In 

 these inner shells the gradient of carbon dioxide-density will, of course, 

 decrease away from the septum. In the latter case diffusion on the basis 

 of linear dimensions of the aperture is not affected by movement of 



B 



Fig. 5. — Brown and Escombe's diffusion shells or gradients of CO2 density about 

 a single opening above an absorbing medium. 



the outer air. The inner shells will be protected from the action of air 

 movement (if the aperture is not large) and are quite indei>endent of the 

 outer shells. They will be as effective, therefore, in regulating relative 

 rates of inflow of carbon dioxide according to the linear dimensions of 

 the aperture as the outer shells. The absolute rates of diffusion through 

 an aperture are decidedly influenced by conditions permitting a single or 

 double system of shells. 



The experimental observations on the rates of diffusion through small 

 apertures are capable of explanation on the assumption that the con- 

 verging and diverging lines of flow to and from the aperture "result in a 

 system of shells of equal density, which locally alter the gradient in the 

 immediate neighborhood of the septum." 



Brown and Escoml^e attempted to demonstrate the formation of 

 ellipsoidal shells by allowing a weak solution of methylene blue in 5 per 

 cent gelatine to diffuse into colorless gelatine through an aperture. The 

 formation of a limiting surface of color, ellipsoidal in shape, is clearly 

 shown. Similar arrangements for demonstrating the zones of density 



