782 BIOLOGICAL EFFECTS OF RADIATION 



tion around 6800 A, which corresponds to the maximum absorption band 

 of chlorophyll. 



Among the most comprehensive studies made to date on reflection, 

 transmission, and absorption of radiation by leaves are those of Seybold 

 (39 to 42) and of Schanderl and Kaempfert (36). 



Seybold (39) in his first paper presents the results of his determination 

 of transmission percentages of variegated and of green leaves in different 

 parts of the spectrum and also gives some data on the transmission of 

 chlorophyll. His determinations of spectral transmission were made with 

 an artificial hght source (a 500-watt "Osram Nitra-Kinoboxlampe ") and 

 a Linke actinometer, the effective part of which was a thermopile and 

 galvanometer connected with a photographic recorder. Transmission 

 in different parts of the spectrum was determined by means of a series 

 of Schott filters used in combination with a 6 per cent CUSO4 solution 

 of 1 cm. thickness to ehminate the infra-red. The transmission values 

 found by Seybold for various species of plants are presented in condensed 

 form in Table 5. A comparison of his figures for Tilia parviflora and 

 Fraxinus excelsior with those of Pokrowski for these species shows 

 Pokrowski's figures to be considerably higher, especially in the yellow. 

 This is probably to be attributed to the difference in methods used by the 

 two investigators. Seybold's figures also represent a much wider range 

 of the spectrum. From his studies of green leaves, Seybold concluded 

 that figures given for transmission by previous investigators have in 

 general been too high. 



Seybold also measured, by means of a sodium photoelectric cell, which 

 was sensitive only to visible and ultra-violet radiation, and with sunlight 

 as a source, the transmission by variegated and green leaves of the 

 region 3500 to 7400 A. With the total energy of sunlight equal to 1.2 

 cal./cm.Vmin., white portions of variegated leaves transmitted between 

 20 and 30 per cent, and green portions between 8 and 15 per cent of the 

 incident radiation, leaving a difference of about 10 to 20 per cent to be 

 attributed largely to chlorophyll. 



In his second paper Seybold (40) added reflection measurements 

 which, with his transmission measurements, enabled him to determine 

 absorption by the leaf. These measurements were made with the 

 photoelectric cell already mentioned. A concave mirror, elliptical 

 in form, was constructed out of brass and highly polished. The spherical 

 photocell was placed far enough into the concave mirror that a point on 

 its surface corresponded to the focal point of the mirror. At this point 

 a piece of black paper was fastened, to which was added a magnesium 

 carbonate layer. All reflections from leaves were compared with the 

 values obtained with this magnesium carbonate layer considered as unity. 

 Sections of leaves were fastened in the same place on the surface of the 

 photoelectric cell for measurement. The light passed through a hole in 



