Quantitative Measurement of Radiant Energy. \ 39 



Although numerous measurements of the radiant energy of the 

 sun have been made by astrophysicists, yet sucii quantitative 

 measurements of radiant energy as have been made in plant 

 physiological experiments are inadequate, and in plant ecological 

 studies where light may be an all-important factor, such measure- 

 ments have not even been attempted.' 



Detlefsen (1888) appears to be the first to attempt energy 

 measurements in regard to problems connected with carbon 

 assimilation. He showed that more energy is used when the leaf 

 is supplied with an atmosphere containing carbon dioxide, than in 

 an atmosphere devoid of this gas. Similar determinations have been 

 made by Mayer (1893) and Ursprung (1903) by the use of a 

 thermopile, but Brown and Escombe (1905) were the first to 

 make an extensive series of measurements of radiant energy in 

 connection with plant physiological problems. 



Brown and Escombe measured the intensity of radiation on the 

 leaf by means of a pair of differential platinum thermometers, one 

 bright and the other black, as devised by Callendar (1898). The 

 instrument was rendered self-recording by connecting it with a 

 Callendar's recorder (1899). 



The characteristic feature of Brown and Escombe's work on 

 the energy relations of the leaf is that they assume there are 

 certain fundamental properties of the leaf in regard to enei'gy, and 

 they attempt to determine certain physical quantities, such as 

 coefficient of absorption and emissivity, which they regard as 

 constant for all conditions of experiment. But even a superficial 

 consideration is sufficient to tell us that this cannot be the case 

 and the performance of a larger number of experiments would 

 probably have shown these authors what range of variations were 

 likely to be obtained. Thus Puriewitsch insists that the absorption 

 of energy depends on the concentration of carbon dioxide ; his 

 results are given in the table overleaf. 



The beauty of Brown and Escombe's work lies not in the 

 reliability of the results obtained by measurement or calculation, 

 but in the fact tliat tiiey are the first, and up to now, the only 

 investigators who have attempted to obtain a complete balance 

 sheet for the leaf in regard to energy. 



■ We do not discuss here the elaborate worlt of Wiesner (1907), which 

 although interesting in its conceptions of light in respect of plant physiological 

 and ecological problems, does not render much help to the problems under 

 review on account of the inadequate method of energy intensity measurement 

 by means of photographic paper. On the other hand many of the observations 

 recorded may become useful as a basis for future observations. 



