LIGHT AND VEGETATION 49 



The same facts are required for the calculation of the 

 fraction of Hght transmitted or absorbed. The corresponding 

 factors must be known for each wave-length and their measure- 

 ment must be made in monochromatic light — in the ultra- 

 violet, as well as in the visible and the infra-red. This is the 

 first condition to be fulfilled. 



In the second place, care must be taken to include in the 

 measurement of the flux reflected (or transmitted) all the rays 

 difiused in various directions so that the total is measured. 

 If only a part is included, the proportion that it represents 

 remains unknown, for that proportion is certainly variable 

 with the wave-length and with the nature of the leaf studied. 



It has been observed that the Hght transmitted and collected 

 on the underside of the leaf is "completely diff'used". It is 

 therefore possible in this case, without too gross an exag- 

 geration, to compare the leaf to a "perfect difi*user". This 

 enables us to limit the measurement of the flux transmitted 

 to a measurement of brilUance (or some other equivalent), 

 but it will be only an approximation and we do not know 

 a priori its value. 



For reflection, such a comparison is generally impossible; 

 the distribution of the Hght diffused by reflection is too 

 variable with the nature of the surface of the leaves. This is 

 obvious, for example, if we compare the surface of an ivy 

 leaf, which is smooth and glossy and reflects rather hke a 

 mirror, to that of a hairy leaf, which is much more diffusive. 



It is therefore necessary, in measuring the reflected flux, 

 and preferable in measuring the transmitted flux, to be able 

 to measure the total of each of these fluxes. 



The following method is generally used for the measure- 

 ment of the diffused fluxes. They are made to penetrate into 

 an enclosed (or nearly enclosed) chamber the inside walls of 

 which are white and diffusive and which is preferably of 

 spherical shape. The illumination of the wall is then measured, 

 in a region protected against the direct flux by a small white 

 screen but receiving the diffused flux from the rest of the 

 walls (Fig. I, 11). 



