Carbon Assimilation. 
37 
if a leaf is transpiring in the dark, its temperature will fall until the 
energy used in transpiration is equal to that which it receives from 
its surroundings in the same time. In some experiments made by 
Brown and Wilson (1905) it was found by measuring the temperature 
of leaves and the loss of water by transpiration, that when there 
is a temperature difference of 1°C between the leaf and its 
surroundings, the leaf of Tropceolum majus, for example, receives 
or loses, as the case may be, 0-01427 calories per square centimetre 
per minute in still air. This is the thermal emissivity. In Brown 
and Escombe’s experiments the total energy lost by radiation is 
obtained by difference, and the division of this number by the 
thermal emissivity calculated by Brown and Wilson is supposed to 
give the difference in temperature between the leaf and its 
surroundings. As the temperature of the latter is measured, that 
of the leaf is at once deduced. 
We do not propose to discuss Brown and Wilson’s method of 
finding the thermal emissivity, as the method of estimating the 
leaf temperature is crude, and other factors which may be important, 
such as respiration and temperature, are regarded as negligible. It 
is only to be expected that the temperatures so estimated should 
be far removed from the real temperatures. As Blackman and 
Matthaei (1905) point out, the temperatures given in Brown and 
Escombe’s tables “for leaves in the sun in the open air are never 
more than 2°C above the shade temperature of the air, while our 
few direct measurements with cherry-laurel leaves, brilliantly 
insolated, indicated 7° to 16°C above the thermometer in the shade.” 
We may add that differences similar to those observed by Blackman 
and Matthaei and of even greater magnitude have been observed by 
various workers, notably by Askenasy (1875), Ewart (1897) and 
Stahl (1909). 
Brown and Escombe’s results show that only a small proportion 
of the energy absorbed by the leaf is used in carbon assimilation, 
but that the actual percentage used for this purpose is a very 
variable quantity. This conclusion has also been reached by 
Puriewitsch (1914), who measured the total radiant energy incident 
on the leaf by means of the bolometer (see Kurlbaum, 1894). The 
energy used in assimilation was obtained in some cases by direct 
measurement of the increase of the heat of combustion per unit 
area of the leaf as described in the previous section of this chapter. 
Unfortunately, only a few such determinations were made, and the 
values for the remaining experiments calculated from them. 
