408 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 1910. 



and the amount of water vapor in the stomatal chamber. At some 

 distance outside the stoma the crowding depends solely on the state of 

 saturation of the outside space. When the density outside is less than 

 that at the level of the stoma there will be a gradient of density estab- 

 lished extending outward from the stoma depending on the drift of 

 water molecules from the more crowded level at the stoma to the less 

 dense vapor outside. If we consider a point (a, fig. 1) immediately 

 over the middle of the stoma, the water vapor there will have a cer- 

 tain density intermediate between that of the outside space and that 

 in the stoma. All over the middle of the stoma places of the same 

 density of water vapor will be approximately equally removed from 

 the stoma, since these places lie in the general drift of water mole- 

 cules from the stoma outward. Toward the margin of the opening, 

 however, conditions are different. The molecules, jostling against 

 each other as they issue from the stoma, tend to travel laterally as 

 well as straight out from the stoma, so that the crow T ding at the mar- 

 gin is less intense than over the middle; hence a place (a') having 

 the same density as (a) will be closer to the stoma. By connecting 



up the points of the same den- 

 sity or crowding we get a 

 curve like a' a a', which rep- 

 resents the section of a layer 

 (or shell) of equal density 

 arching over the stoma. In 

 the same way at a distance 



somewhat more removed from 

 Fig. 1. . -nii 



the stoma, there will be a layer 



of less density, and this layer will be at a greater distance from the 

 middle of the stoma than it is from its margin. So we may imagine 

 a series of layers or shells of diminishing density overarching each 

 transpiring stoma, such are are represented in section in figure 1. Of 

 course, in reality the higher density within grades insensibly into the 

 lower density outside; this gradient of density, or crowding of the 

 water molecules, is steeper near the margin than over the middle of 

 the stoma. From this it follows that the flow of molecules outward 

 is less obstructed on the margins. Consequently greater numbers 

 escape there. In other words, the margin is more efficient in trans- 

 mitting water vapor than the middle region of the stoma. As the 

 size of an aperture is reduced the relation of its margin t© its area is 

 increased ; for the value of 2 ir r does not decrease as fast as -rrr - when r 

 is reduced. So, for a very small aperture like a stoma, the marginal 

 diffusion is very large compared to that over its cross section, and 

 hence the diffusion from a stoma is exceptionally efficient. 



It will be readily seen that in order to maintain the efficiency of 

 the marginal diffusion on the outside it is necessary that the diffu- 

 sion streams from adjoining stomata should not interfere with one 



