Current Literature 477 



through an aperture is proportional to its radius and not to its 

 area. The rate of diffusion at the margins is greater than over 

 the middle of the apertures. Therefore, an aperture having the 

 longest margin relatively to its area will be the most efficient. 

 The slit-like form of the average stomatal opening tends to pro- 

 duce this condition. The distances between the stomata are such 

 that the diffusion currents from one do not interfere with those of 

 another. Thus, the diffusion of gases through the stomata may 

 be explained on simple physical principles. This applies as well 

 to the diffusion of water vapor from the chambers, lying above the 

 stomata, into the adjacent atmosphere. 



The next step is to consider how water from the contiguous cells 

 is supplied to these chambers. The conducting tubes, containing 

 the rising sap, are separated from the intercellular spaces by a 

 layer of one or more thin, cellulose-walled cells, and these walls 

 are permeable to water and its dissolved substances. Behind the 

 wall of one of these cells is a layer of semi-permeable protoplasm, 

 which encloses the cell sap of the vacuole. Cells like these impinge 

 upon the conducting tubes whose walls are permeable to water, 

 but do not enclose a layer of protoplasm. The imbibitional forces 

 of the cells in contact with the intercellular spaces of the leaf will 

 draw off water from their vacuoles through the protoplasmic 

 layer until the vapor pressiire in the walls and in the vacuoles is 

 equal. Now, if the vapor pressure of the water menisci in the 

 minute interstices of the cell wall is greater than that obtaining in 

 the intercellular space, the water will leave the cell wall and the 

 menisci will retreat into it. This will cause their curvature to 

 increase and will raise their capillary forces so that they will 

 extract water from the solution in the vacuole. A concentration 

 of the solution in the vacuole results and consequently the osmotic 

 pull on the water in the adjacent conducting tubes is increased. 

 Hence, it follows that a transference of water from the conducting 

 tubes will take place so long as the vapor pressure of the water in 

 the conducting tubes is greater than that in the intercellular 

 spaces of the leaf. 



Based upon the above phenomena, the explanation of the pro- 

 cess of transpiration in plants may be made entirely physical. 

 None of the vital activities of living cells need to be called in to 

 aid. There are certain conditions, however, where it would seem 

 that the living cells do play a part in the process. For example. 



