TRANSMITTANCE AND REFLECTANCE OF LEAVES 677 



whereas thick leaves with glossy surfaces may show a completely diffuse 

 transmission, but a marked specular reflection. 



Figure 22.2 shows the forward and back scattering of light by a small 

 vessel containing a suspension of Chlorella cells, as observed by Noddack 

 and Eichhoff (1939). The sharp peak (C) at 180° is caused by specular 

 reflection from the glass wall. 



In practical work, one can often di'op the distinction between T and R 

 and measure the total scattered flux .S' ^ {T -{- R) b\' means of some inte- 

 grating device: 



(22.8) A = I - S 



An example is the study by Rabideau, French and Holt (1946) of the ab- 

 sorption spectra of leaves and pigment extracts. 



2. Average Transmittance and Reflectance of Leaves and Thalli in 

 White Light. Intensity Adaptation and Movements of Chloroplasts 



The first measurements of the proportion of white light transmitted by 

 leaves were carried out by Sachs in 1861. Later this magnitude was meas- 

 ured by Detlefson (1888), Linsbauer (1901), Brown and Escombe (1905), 

 Purevich (Purjewitsch) (1914), Schanderl and Kaempfert (1933), Seybold 

 (1932i'2, 19331-2, 1943), Loomis, Carr and Randall (1941,1947,1949). The 

 transmittance oj algae was investigated by Reinke (1886), Wurmser (1921) 

 and Seybold and co-workers (1934, 1942). 



The first measurements of the reflectance of leaves were made by Co- 

 blentz in 1912, and were followed by those of Pokrovski (1925), Shull 

 (1929), Seybold and co-workers (19322,1933i'2,1942,1943) and Loomis, Carr 

 and Randall (1941,1949). The only data on the reflectance of algae are 

 those of Seybold and co-workers (1934, 1942, 1943). 



Brown and Escombe (1905) and Purevich (1914) found comparatively 

 high values — of the order of 20% — for the transmission of (infrared-free) 

 white light by average leaves. Seybold (1932) suggested that these re- 

 sults were falsified by the inclusion, in the measured transmitted flux, of 

 the thermal radiation of the leaves. In agreement with Pokrovski (1925), 

 Seybold found that an average fully green leaf transmits not more than 

 10% of infrared-free white light. Leaves are almost transparent in the 

 far red and near infrared (c/. figs. 22.30 and 31). Therefore, transmission 

 values obtained by means of thermopiles (or other infrared-sensitive in- 

 struments) are deceivingly large if the light used for the measurements 

 contains a large proportion of infrared radiations. According to Loomis, 

 and co-workers (1941,1949), an average leaf transmits 30% of total sun- 

 light, including the infrared. With artificial light sources of lower tempera- 



