Henry H. Dixon. 



In order to obtain a clearer idea of this remarkable result we 

 will consider in a general way the state of affairs round one stoma 

 so far as water vapour is concerned. At the level of the stoma the 

 water vapour has a certain densit}', i. e. the water molecules are 

 more or less crowded, depending on the state of saturation of the 

 external space and the amount of water vapour 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 established extending outwards from the 

 stoma depending on the drift of water molecules from the more 

 crowded level at the stoma to the less dense vapour outside. If we 

 consider a point (a, Fig. 1) immediately over the middle of the stoma 

 the water vapour there will have a certain 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 vapour 

 will be approximately equally 

 removed from the stoma, since 

 these places lie in the general 

 drift of water molecules from 

 the stoma outwards. Towards 

 the margin of the opening 

 however conditions are diffe- 

 rent. 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 crowding at the margin is less intense than 

 over the middle; hence a place («') having the same density as (a) will 

 be closer to the stoma. By connecting up the points of the same density 

 or crowding we get a curve like a' a a' which represents 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 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 as are represented in section 

 in Fig. 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 mole- 

 cules outwards is less obstructed on the margins. Consequently greater 

 numbers escape there. In other words the margin is more efficient 

 in transmitting water vapour than the middle region of the stoma. 



Fig. 1. Diagram to illustrate the gradients 



of the density of water vapour jouud a 



stoma. 



