62 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
As regards the exhalation of the carbon dioxide of respiration, Blackman’s 
experiments are quite conclusive, but his method does not appear to be quite 
so well adapted to investigations of the assimilatory intake where, owing to 
the restrictions of time and leaf area, the amounts of carbon dioxide which 
have to be dealt with seldom exceed about 0:1 c.c., with a possible experimental 
error of about one-tenth of this amount, and where the conditions of 
experiment are such as to require “much watchfulness, and a favourable 
concurrence of circumstances such as is rarely accorded.” 
Since the method which we have employed for determining the rate of 
assimilation in ordinary air is one which readily lends itself to investigations 
of this natme, we have considered it desirable to repeat these classical 
experiments of Blackman under conditions which admit of measuring the 
intake of relatively large amounts of carbon dioxide from ordinary air by the 
two sides of a leaf on which the distribution of stomata is known. 
For this purpose special leaf-cases were constructed which were similar in 
principle to those described by Blackman.* They consisted of a pair of 
shallow Petri-dishes fixed into light brass frames with flanges which could be 
pressed together by simple spring-clips. When the leaf was clipped between 
the two flanges and the junction was made tight with soft wax the lamina 
formed a diaphragm dividing the case into two compartments, one connected 
with the upper and the other with the lower side of the leaf. Through each 
of these compurtments a separate current of air could be drawn by means of 
small brass tubes fitted into the sides of the chambers. 
Two of these cases were used, one exposing a leaf-area of 28°27 sq. cm., 
and the other 59:44 sq. cm. 
Measured volumes of ordinary air of known carbon dioxide content were 
aspirated through the cases in the way already described at rates varying 
from 10 to 20 litres per hour, the absorption and determination of the carbon 
dioxide in the emergent air being effected as usual. 
The number of stomata on known areas of the upper and lower sides of the 
leaf was, in all cases determined under the microscope by actual counting, the 
mean of a large number of observations being taken. 
In order to make the comparison as strict a one as possible the results of 
each pair of experiments have been reduced to equal partial pressures of 
carbon dioxide in the air passing through the case. Such a correction is very 
necessary in experiments on respiration in which there is sometimes a 
considerable difference in the mean composition of the air in the two 
compartments. 7 
* Loc, cit., p. 521. 
