144 Carbon Assimilation. 



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 measurin<f 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 Troposolum niajus, 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. 



