42 Dr. H. T. Brown and Mr. F. Escombe. [Jan. 9, 
respiration going on in the leaf-cells, and the carbon dioxide due to this 
process is not evolved, but undergoes re-elaboration under the influence of the 
light. The intensity of the respiratory function increases, within certain 
limits, with the temperature. Under the conditions of the particular 
experiment just cited the re-synthesised carbon dioxide of respiration 
amounted to about 0°5 c.c. per square decimetre per hour, so that the total 
photosynthetic work accomplished is represented by 4°206+40°5 = 4°706 cc. 
of carbon dioxide per square decimetre per hour, being a further increase of 
about 11 per cent. 
In experiments where it is only required to know the effective assimilation, 
that is to say, that portion of the process which can alone contribute to an 
actual increase in weight of formative carbohydrate in the plant, this correction 
need not be applied. 
We may now consider the results of a series of experiments carried out on 
the lines indicated, and for the purpose of determining the rate of the effective 
assimilation of leaves in air containing the normal amount of carbon dioxide. 
The volumes of air and carbon dioxide are reduced to normal temperature 
and pressure (N.T.P.) and the carbon dioxide which has been fixed by the 
leaf is corrected to the partial pressure of that gas existing in the outer air at 
the time of the experiment. 
The comparatively large scale on which these experiments have been 
carried out is perhaps best realised by reference to Columns (3) and (6) in the 
following table, which give respectively the area of the leaf employed and the 
number of cubic centimetres of carbon dioxide absorbed and utilised by the 
leaf during the full time that the experiment lasted, amounting in some cases 
to upwards of 140 c.c. of the gas. 
Column (7) gives the volume of carbon dioxide (measured at normal 
temperature and pressure) which would have been assimilated by 1 square 
decimetre of leaf per hour, under a partial pressure equivalent to that of the 
CO, in the outer air at the time, and from this value we can readily deduce 
the actual weight of carbohydrate synthesised for unit-area and unit-time, 
provided we know the mean empirical formula of the carbohydrate. 
If the carbohydrate is a hexose (¢.9., glucose or fructose) or one with the 
general formula C,H,,O,, then the absorption by the leaf of one part by 
weight of carbon dioxide corresponds to the synthesis of 0°681 parts of the 
carbohydrate. If, on the other hand, we assume that it 1s cane-sugar which is 
synthesised, or any sugar of the C,,H,.0,, class, then the disappearance of one 
part by weight of carbon dioxide corresponds to the synthesis of 0°647 parts 
of the body in question. 
