58 Carbon Assimilation. 



Brown and Escombe have shown that the leaf of Catalpa 

 bignonioides can absorb from ordinary air 0'07 c.c. of carbon dioxide 

 measured at N.T.P. per sq. cm. of leaf surface per hour. The area 

 of the stoinatal openings is only 0'09% of the total leaf surface. 

 Hence diffusion through them must take place at the rate of 7-77 c.c. 

 per sq. cm. per hour. 



Now experiments made by Brown and Escombe showed that 

 a normal solution of sodium hydroxide exposed to moderately still 

 air containing about 3 parts of carbon dioxide per 10,000 absorbs 

 this gas at ordinary temperatures at the rate of about (H2Q c.c. 

 per sq. cm. of absorbing surface per hour, and this is only increased 

 to a maximum value of 0-177 c.c. per sq. cm. per hour when the rate 

 at which the air is passed "over the absorbing solution is increased. 



Hence, if the diffusion of carbon dioxide into the Iqaf takes 

 place entirely through the stomata, this absorption of carbon dioxide 

 must take place about SO^jmes as fast as it would by a solution of 

 normal sodium hydroxide of which the exposed surface had the 

 same area as the stomata. 



Brown and Escombe were thus led to investigate the rate of 

 diffusion of gases through small apertures in a septum. Their method 

 of procedure was as follows : 200 c.c. of normal sodium hydroxide 

 were placed in a flat-bottomed flask which was left open in 

 comparatively still air containing the normal amount of carbon 

 dioxide. The surface of the liquid was about 10 cm. in diameter. 

 A very steady and uniform absorption then took place at the rate 

 of about 0'25 c.c. carbon dioxide per hour. 



In order to obtain a suitably perforate septum between the 

 absorbing liquid and the outer air, the neck of the flask was passed 

 through the bottom of a small glass cup to which it was cemented. 

 The annular space of the cup was then filled with mercury. A flat- 

 bottomed nickel crucible was inverted over the mouth of the flask so 

 that the edges dipped into the cup of mercury, and in this way a 

 perfect mercury seal was obtained. A hole of the desired size was 

 made in the bottom of the nickel crucible. 



A number of such pieces of apparatus with variously perforated 

 septa were prepared at the same time, and after displacing the air 

 in them with air freed from carbon dioxide they were exposed to 

 the atmosphere under the same conditions. As a result of these 

 experiments, Brown and Escombe came to the conclusion that with 

 small apertures the rates of diffusion are proportional, not to the 

 areas, but to the diameters of the opening. The following table 



