PRINCIPLES GOVERNING DIFFUSION OF GASES i8i 



sealed with melted vaseline in order to prevent leaks. A solution of potassium 

 permanganate, which is purple in color, is now introduced into the vessel on 

 top of the disk (Fig. 44). After a few hours the molecules of potassium 

 permanganate, after difiusing through the aperture, become distributed in 

 such a way as to occupy a hemispherical zone below the opening. The 

 hemispherical distribution assumed by molecules which are diffusing through 

 a small opening is often spoken of as a diffusion shell. 

 Gases also form diffusion shells above small pores, 

 but their configuration is often relatively unstable due 

 to their ready distortion by wind or convection currents. 



Actually when molecules are diffusing through a 

 small orifice there is formed not one, but two diffusion 

 shells, one on each side of the diaphragm which is 

 pierced with the opening. The experiment described 

 above permits discernment of the hemispherical pattern 

 of diffusion only on the receiving side of the system. 

 There is also a second shell, a mirror image of the first, 

 adjacent to the aperture on the side of the system from 

 which diffusion is occurring. The deep color of the 

 permanganate solution prevents visible detection of 

 this second shell in the experiment described above. 



Diffusion of gases from or into a leaf through the 

 stomates involves a much more complex system than is 

 represented by a septum pierced by a single aperture. 

 Diffusion is occurring, not through a single opening, 

 but simultaneously through thousands of minute aper- 

 tures which are relatively close together. Experiments 

 have been performed in analogous physical systems in 



which multiperforate instead of uniperforate diaphragms have been used. The 

 results of experiments on different spacings of the pores in a septum upon the 

 rate of diffusion of water-vapor per pore and per septum are depicted graph- 

 ically in Fig. 45 and Fig. 46 respectively. 



As shown in Fig. 45, the diffusion per pore increases with increase in the 

 distance apart of the apertures, although not proportionately. With pores 

 of this diameter (0.3 mm.) nearly the maximum diffusive capacity is attained 

 when they are spaced at intervals of 20 diameters. The closer together the 

 pores in a septum the greater the overlapping of the diffusion shells. The 

 molecules diffusing through each aperture invade in part the zones into which 

 the molecules passing through neighboring pores would diffuse were each 

 opening the only one in the septum. Hence the diffusion gradients are less 



Fig. 44. Forma- 

 tion of a diffusion shell 

 of potassium perman- 

 ganate upon diffusion 

 into gelatin. 



