2 Ingvar Jorgensen and Walter Stiles. 
Although numerous measurements of the radiant energy of the 
sun have been made by astrophysicists, yet such 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. 1 
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 energy, 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 that they 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. 
1 We do not discuss here the elaborate work 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. 
