34 
Ingvar Jorgensen and Walter Stiles. 
of sunlight measured. The leaf was interposed above the coils of 
the instrument for a few minutes and the intensity of radiation 
again measured. The leaf was then withdrawn when the value of 
the full intensity of radiation was again recorded on the drum of 
the self recorder. The ratio of the middle reading to the mean of 
the first and third readings gives the coefficient of transmission, 
and the difference between unity and the coefficient of transmission 
is the coefficient of absorption. The following table shows the 
coefficients of absorption and transmission found by Brown and 
Escombe for various species. 
Table XL1I. 
Coefficients of Absorption and Transmission of Radiant Energy 
of Sunlight. 
Species. 
Coefficient, 
of Absorption. 
Coefficient 
of Transmission. 
Helianthus annuus ... ..- . 
0-686 
0-314 
Polygonum Weyrichii . 
0-647 
0-353 
,, Sacchalinense 
0-691 
0-309 
Petasites officinalis 
0-728 
0-272 
Silphium terebinthaceum 
0-699 
0-301 
Arctium majus 
0-728 
0-272 
Verbascum olympicum 
0-758 
0-242 
Senecio grandifolius 
0-774 
0-226 
No considerable difference in the coefficient of absorption was 
found between leaves of the same species of different ages. 
Of course, in these determinations the part of the energy 
reflected from the surface of the leaf is neglected. Brown and 
Escombe regard the reflected energy as forming a very small 
fraction of the total incident energy, but having regard to the 
information available from pure physics it is unlikely to be negligible, 
as a black cloth, for instance, may reflect 1% of the radiant energy 
incident upon it. 
The difference between the total incident energy absorbed on 
the one hand, and that used in assimilation and transpiration on the 
other hand, gives that part of the energy lost by re-radiation, 
conduction and convection, i.e., that lost by emission. 
The following numbers show the results of a typical experiment. 
