106 BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



theoretical deductions. This explanation is not altogether sat- 

 isfying and leads to the query whether the diameter law holds 

 when extrapolated to the degree necessary to apply to the 

 stomatal system. The smallest apertures used in the develop- 

 ment of the diameter relationship was about 2 mm., whereas the 

 diameter of the stomatal openings is of the order of 0.01 mm. 



The rate of diffusion of water vapor outward through the 

 stomatal opening affords a possible method for checking Brown 

 and Escombe's deductions, which they do not appear to have 

 employed. By observing the diffusion-rate of water vapor in- 

 stead of carbon-dioxide the assumption regarding the imperfect 

 absorption of carbon-dioxide would be avoided. The determi- 

 nation of the vapor pressure in the interior of the leaf would be 

 necessary, but this could be made with a fair degree of accuracy 

 by measuring the temperature of the leaf and the concentration 

 of the cell contents. 



It is of interest to consider in this connection the work of 

 Buckingham 8 on the diffusion of carbon-dioxide through soils. 

 He found experimentally that the rate of diffusion varied as the 

 square of the porosity, the latter term denoting the volume of 

 the interstitial space between the soil grains unoccupied by 

 water, expressed in per cent of the total volume. He also 

 tested this observed relationship by assuming all the soil 

 grains to be removed, so that the porosity would be 100 per 

 cent. This should lead to the velocity of free diffusion of car- 

 bon-dioxide in air, and Buckingham's empirical equation gave 

 a result well in accord with previous determinations of free 

 diffusion by other investigators. It will at once appear that this 

 deduction departs widely from the diameter law of Brown and 

 Escombe. If we consider a volume of soil of unit thickness, the 

 porosity will be proportional to the integrated area of the 

 interstitial spaces in any plane parallel to the surface. If we 

 assume as a first approximation that the pores are uniform in 

 cross-section, then the porosity would be equal to the product 

 of the number of pores n and the cross-section a, or the diffusion 



3 Buckingham, E. Contributions to our knowledge of the aeration of soils. 

 U. S. Dept. Agr., Bur. Soils Bulletin No. 25. 1904. 



