74 PHOTOSYNTHESIS 



gradient in diffusion through apertures are possible by the use of inter- 

 acting substances such as sulphates and chromates into gels of agar con- 

 taining barium chloride. The Liesegang phenomena can also be used in 

 such a manner that the reacting substances produce the rhythmical series of. 

 zones which follow the contours of surfaces of equal density. 



Brown and Escombe's investigations thus established that the effect 

 of interposing a diaphragm pierced by a single circular aperture on the 

 diffusion of a gas was quite significant. The velocity of the flow through 

 unit area of such an aperture varies inversely as the diameter. The 

 original object of their study was to determine the mechanism of gas 

 interchange through the minute apertures in the surface of a leaf and to 

 develop experimental conditions which simulated such a gas interchange. 

 The discovery of the "diameter law" warranted the anticipation that 

 under proper circumstances a thin septum might be pierced by the cor- 

 rect number, size and distribution of apertures so that the septum would 

 cause but little obstruction to the diffusive flow of the gas while the 

 aggregate area of the apertures would represent but a small percentage 

 of the entire area of the septum. That a leaf l^ehaves as such a system 

 had been established by their earlier studies. 



Experimental studies with multi-perforated septa showed that these 

 conditions could be realized. Brown and Escombe used celluloid dia- 

 phragms fixed over short glass tubes. The latter were provided with a 

 side tube and stop-cock for running in the sodium hydroxide solution to 

 any desired height, as well as another tube for drawing off the solution 

 for analysis. In the multi-perforate septa each hole was 0.38 mm. in 

 diameter and the septum itself was 0.1 mm. in thickness. The factor 

 which was varied in the experiments was the distance the holes were 

 apart, i.e., the number of holes per square centimeter. The experiments 

 were carried out in still air, and the amounts of carbon dioxide which 

 had diffused through the multi-perforate septa were compared with the 

 amounts which would have diffused down the open tube if there had 

 been no obstructing septum present. The latter amount was calculated 

 on the basis of previous results of (1) the diffusion coefficient of carbon 

 dioxide in air, (2) the dimensions of the tube and (3) the mean density 

 of carbon dioxide in the air during the experiment. 



In the last column are given the calculated values of the relation of 

 the carbon dioxide diffusion through the septum to the rate of diffusion 

 without the septum. It becomes evident that the multi-perforate septum 

 exerts but a slight effect on the rate of diffusion. The slight increase in 

 No. 1 is due to the fact that in this experiment the air was not per- 

 fectly still. In No. 2, for example, 43.2 per cent of the open tube diffusion 

 has taken place although the aggregate area through which the gas could 

 pass was only 2.82 per cent of the cross-section of the tube. In the second 

 portion of the table it will be seen that when the distance between the 

 septum and the absorbing surface is decreased, the actual amount of 

 carbon dioxide absorbed is increased, but the septum diffusion is a smaller 



