Carbon Assimilation. 
125 
Brown and Escombe have shown that the leaf of Catalpci 
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 stomatal openings is only 0 09% of the total leaf surface. 
Hence diffusion through them must take place at the rate oil'll 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 0T20 c.c. 
per sq. cm. of absorbing surface per hour, and this is only increased 
to a maximum value of 0T77 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 leaf takes 
place entirely through the stomata, this absorption of carbon dioxide 
must take place about 50 times 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 other 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 
