780 



BIOLOGICAL EFFECTS OF RADIATION 



Among the first to study the transmission of leaves in various parts 

 of the spectrum was Knuchel (20). By means of a spectrophotometer he 



Table 3. — Seybold's Light-energy Balance Sheet 



Incident light energy 



Absorption of colorless constituents of leaf 



Light reflection 



Pigment absorption 



Light transmission 



Leaf 



measured the transmission of sun leaves and shade-leaves of hazelnut, 

 linden, and beech, when diffused zenith skylight was the source. In the 

 blue region of the spectrum around 4400 A, shade leaves transmitted 

 only traces of the incident radiation and sun leaves only to 2 per cent. 

 At about 4720 A, shade leaves transmitted 5 per cent and sun leaves 



o 



2 to 5 per cent. In the green region (about 5200 A) shade leaves trans- 

 mitted 16 to 25 per cent and sun leaves 6 to 12 per cent. At 5890 A the 

 maximum transmission was 19 per cent and at 6520 A 12 per cent. His 

 figures thus indicated strong absorption by green leaves in the blue- 

 violet end of the spectrum, a fact which has been substantiated by later 

 investigators. Knuchel also found a greater percentage of green and of 

 yellow light in the forest as compared with sky light in the open. This 

 may be explained partly on the basis of the greater transmission of such 

 radiation by leaves and by the scattering of direct sunlight. 



Pokrowski (33), using a spectrophotometer, determined the reflection 

 of a number of tree leaves by a comparison with the reflection from 

 magnesium oxide. He also determined the transmission of several leaves 

 of Tilia parviflora and Fraxinus excelsior. By considering the total inci- 

 dent radiation as one, and subtracting from it the sum of the radiation 

 reflected and transmitted, he obtained figures for absorption at different 

 wave-lengths. In Table 4, we have averaged two series of his measure- 

 ments for each species and have brought them together into one table for 

 comparison. 



It will be seen from this table, and it is shown in Pokrowski's other 

 measurements, that maximum reflection occurred in the green at 5500 A. 

 This region is also characterized by maximum transmission and minimum 

 absorption, which might be expected from the green color of leaves and is 

 undoubtedly related to the absorptive properties of chlorophyll. On the 

 other hand, as Pokrowski himself points out, the maximum absorption 

 point of the leaves does not correspond to the maximum point of absorp- 

 tion of chlorophyll, which occurs at about 6600 A. Maximum absorption 



