104 BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



stomata. Is the diffusion-rate through a multi-perforate sep- 

 tum of this kind sufficient to account for the rate at which carbon 

 dioxide is absorbed by the leaf? 



This problem was made the subject of an extensive investiga- 

 tion by Brown and Escombe 7 in 1900. The diffusion of carbon 

 dioxide in cylinders partially filled wih caustic soda solutions 

 was first studied. When such cylinders were exposed in still air 

 it was found, as might be expected, that the amount of carbon 

 dioxide diffusing down the cylinders in a given time varied di- 

 rectly as the cross-sectional area of the cylinders and inversely 

 as their length. If, however, the diffusing tubes were par- 

 tially closed at the top by 

 septa with single circular 

 openings of different diam- 

 eters, the diffusion was 

 found to be proportional, 

 not to the areas of the 

 apertures, but approxi- 

 mately to their radii. In 

 other words, the diffusion 

 through small isolated cir- 

 cular openings proceeds 

 much more rapidly than 

 would be indicated by the 

 area of the opening. Some 

 of the results obtained by 

 Brown and Escombe, from which they developed the so-called 

 diameter law controlling stomatal diffusion, are given in table 3. 

 Brown and Escombe call attention to the analogy between the 

 diffusion system which they investigated and that of an elec- 

 trified disk. The electrostatic capacity of such a disk may be 

 shown from theoretical considerations to be proportional, not 

 to the area of the disk, but to its diameter. The equipotential 

 surfaces about such a disk (fig. 5) are ellipsoids having the edges 



7 Brown, H. T., and Escombe, F. Static diffusion of gases and liquids in re- 

 lation to the assimilation of carbon and translocation in plants. Phil. Trans., 

 193b: 223-291. 1900. 



Fig. 5. Diffusion of carbon dioxide into a 

 stomatal opening of a leaf surrounded by still 

 air. The surfaces representing uniform par- 

 tial pressures of CO2 are ellipsoids, which are 

 cut at right angles by the diffusion stream 

 lines (hyperbolas). After Brown and Es- 

 combe. 



