1024 BIOLOGICAL EFFECTS OF RADIATION 



Table 1 are given mean relative values of the reflection of diffuse daylight 

 obtained by Seybold (1046). 



Table 1. — Relative Values of the Reflection of Diffuse White Light 



Per Cent 



Magnesium oxide 100 



A esculus Hippocaslanurn 



White-leaf surface 36 



Green-leaf surface 10 



Pelargonium zonule 



White-leaf surface 38 



Green-leaf surface 16 



Syringa vulgaris 14 



Polygonatuni sachalinense 14 



Acer j)seu(loplalanus 11 



Soot 4 



The epidermis of leaves, particularly if this is heavily cutinized or 

 covered with wax, as in xerophytes, affects the light which enters the 

 parenchyma. Such cuticular layers have an effect similar to a sheet of 

 parchment, so that the light reaching the chloroplasts is diffuse. Some 

 light is also absorbed by these layers; in shade plants this is very small, 

 but in alpine and desert plants the transmission may be as low as 15 to 

 25 per cent, according to Schanderl and Kaempfert (97). 



For almost 30 years the values for the amount of radiant energy which 

 is absorbed by green leaves as determined by Brown and Escombe (9) 

 have been used in investigations on the energy relations of photosyn- 

 thesis. They found that 65 to 77 per cent of the incident radiant energy 

 was absorbed by the various leaves which they studied. Recently these 

 determinations have been superseded by more accurate measurements 

 made possible through the development of modern light-measuring 

 apparatus and optical equipment, and in which some factors neglected 

 in the older determinations, chiefly reflection, have been considered. 

 The values for absorption have usually been obtained on the basis of the 

 transmission and reflection of light. These values were expressed as 

 absorption coefficients, but more recently in terms of per cent of the 

 incident light absorbed: 



Absorption = 100 — (transmission -|- reflection) 



Earlier determinations were made with sunlight, because of the desire 

 to approach "natural" conditions. They possess the uncertainties 

 associated with this source of illumination, owing to irregularities in 

 intensity and spectral composition. For obvious reasons it is desirable 

 to know the absorption by the leaf of different wave-lengths or portions 

 of the spectrum and this can be more easily obtained with artificial 

 sources than with sunlight. 



The determination of the absorption spectra of a variety of green 

 leaves has yielded results which are in general very much alike (4, 26, 26a, 



