784 



BIOLOGICAL EFFECTS OF RADIATION 



it increases more markedly with decreasing wave-length in the white 

 than in the green portions. That the presence of chlorophyll is responsi- 

 ble for the greater absorption in the green is obvious. Seybold's results 

 with reflection in general approximated those of ShuU, Hibben, and 

 Pokrowski, obtained with a spectrophotometer, although the wider 

 spectral ranges used by Seybold tend to obscure the greater reflection of 

 green leaves between 5400 and 5600 A. 



Table 6. — Average Values of Transmission, Reflection, and Absorption 



Obtained by Seybold with 10 Species of Variegated Plants 



(In percentage of the incident radiation) 



Transmission 



Reflection 



Absorption 



Light absorption coefficient 



White leaf -sections 



644 

 niM 



33 



46 



21 



0.21 



578 

 mjtt 



33 



47 



20 



0.20 



509 



m/x 



31 



43 



26 



0.26 



436 



ni/i 



20 



27 



53 



0.53 



336 



mju 



8 

 18 

 74 

 0.74 



Green leaf -sections 



644 

 nip 



9 

 13 



78 

 0.78 



578 

 m;u 



10 



14 



76 



0.76 



509 



10 



14 



76 



0.76 



436 



m/i 



2 

 11 



87 

 0.87 



336 



m^ 





 9 



91 

 0.91 



By comparing observed values of absorption with calculated values 

 for green leaves and white portions of variegated leaves, Seybold found 

 that the Lambert-Beer absorption law held for both. 



In the third paper of the series, Seybold (41) presents evidence to 

 show that whether the incident radiation used is parallel or diffused makes 

 little difference in the values obtained for transmission or reflection at 

 different wave-lengths. The maximum difference he found for trans- 

 mission values, when the two types of radiation were used, was 2 per 

 cent. He attributed this partly to the fact that the upper epidermis 

 diffuses the radiation that strikes it and hence, even a parallel beam will 

 immediately be diffused on striking the leaf. It is also interesting to 

 note that when he compared the transmission of fresh-leaf sections with 

 those air-dried for 3 days, there was an increase in infra-red transmission 

 in the dried sections of only 5 per cent, which is less than one would 

 expect from the well-known fact that water absorbs infra-red. By 

 using a yellow filter in combination with CUSO4 to eliminate infra-red, 

 a similar small increase in transmission occurred in the yellow part of the 

 visible region. 



Schanderl and Kaempfert (36), using sunlight exclusively as a source, 

 measured by means of a Linke actinometer containing a modified Moll 

 thermopile, not only the transmission of whole leaves but also the trans- 

 mission of different leaf tissues, especially different types of epidermises 

 and leaf surfaces, and also determined the effect of the position or orienta- 

 tion of chloroplasts and the presence of assimilation products on trans- 



