
1905.]| On the Physiological Processes of Green Leaves. 95 
found, however, that under these conditions both the white and the green 
leaves acted as perfect screens, and completely prevented any sensible amount 
of solar radiation from reaching the sensitive part of the instrument. 
The same difficulty was also experienced in attempting to adapt the 
Angstrém’s pyrheliometer to the purpose, the failure in both cases being due 
to the fact that the absorbing leaf-lamina, which must necessarily be at some 
little distance from the sensitive thermo-electric junction, so far scatters the 
radiation which passes through the small apertures of the instruments, that 
the vertical component of the transmitted rays, which can alone affect the 
instrument, becomes very small. The aperture covered by the leaf becomes, 
in fact, a new focus of radiation which spreads the rays over an area much 
larger than that occupied by the receiving surface of the sensitive portion of 
the instrument. 
It is necessary, therefore, that the receiving surface forming the ther- 
mometer should have, as in the Callendar’s instrument, a considerable 
extension, and should be well overlapped by the leaf, which must admit of 
being brought close to the receiving surface. 
For leaves too small for the Callendar’s radiometer, a Rubens’ thermopile 
was found to be suitable, the leaf being placed between two plates of thin 
glass and brought within about a millimetre of the receiving end of the pile. 
The absorptivity of the leaf for bright sunshine was determined by observing 
the amount of radiation transmitted throngh the glass plates, both with and 
without the interposition of the leaf. 
A series of concordant experiments made in this manner with the white 
and green portions of a leaf of Megundo, gave the following mean results in 
bright sunshine :— 
Transmission. 
Through glass alone: .5)...treeValives: 1000 
White leaf interposed ..............+006 25°D 
Green _,, Ne Mr ce Sear Face 213 
These results lead to the following values for the coefficients of absorption 
and transmission for the white and green portions of the leaf respectively. 
Table VII.—Absorption and Transmission of the Radiant Energy of Sunlight 
by the White and Green portions of the Leaf of Negundo aceroides. 
Coefficient Coefficient 
of absorption. of transmission. 
White leaf lamina ......... 0-745 0°255 
Green nee are 0°787 0:213 
