GENERAL REMARKS ON LIGHT ABSORPTION BY PLANTS 675 



A suitable statistical theory (c/. page 713) may permit the calculation 

 of A from measurements of light transmission in one direction, made with 

 two or more different optical densities of the scattering material (e. g., 

 with a series of several leaves, or with several cell suspensions of different 

 concentration or layer thickness) . However, it is better not to rely on such 

 theoretical equations, but, particularly in working with leaves or thalli, 

 actually to measure the light fluxes transmitted and reflected in all direc- 

 tions. Having determined experimentally both T and R, one can use the 

 exact equation (22.4) for the evaluation of A. The time to use theoretical 

 equations for combined absorption and scattering comes when one is not 

 satisfied with the knowledge of the amount of absorbed energy, but wants 

 also to know the absorption coefficients, e. g., as indicators of the molecular 

 state of the pigment in the living cell. 



Attempts have been made to use "blanks," for example, white parts of variegated 

 leaves (cf. Linsbaur, 1901, Brown and Escombe 1905, Weigert 1911, Meyer 1939, 

 and Seybold 1932i'2, 1933i, 1934), or algal thalli from which the pigments had been ex- 

 tracted {cf. Reinke 1886), or tissues bleached by long exposure to light (c/. Wurmser 

 1926), and to imitate in this way the method usually applied to transparent media. In 

 the latter case, the blanks provide an automatic correction for reflection (cf. page 673) ; 

 in the case of plants, they were intended to provide a correction also for scattering. 

 However, the approximation (22.2b), which is generally satisfactory in work with trans- 

 parent media, may give entirely erroneous results when applied to optically inhomogene- 

 ous systems. This was pointed out by Willstatter and Stoll (1918) and Warburg (1925) 

 when they criticized the absorption calculations of Weigert (1911). The error is caused 

 by the large difference between the fluxes R and Ra {cf. equation 22.6) reflected by the 

 green and the colorless leaf. A green leaf may transnut about 10% and reflect another 

 10% of incident white light, while a similar, pigment-free leaf may transmit 50% and 

 reflect the other 50%. If the absorption of the green leaf is calculated from these figures 

 by means of equation (22.2b), the result is A = 40%, which is only one half the correct 

 value (80%)! 



Therefore, if one wants to determine absorption, A, by comparison of a green leaf 

 with a pigment-free leaf, one has to use the complete equation (22.6), i. e., to measure 

 the four quantities To, Ro, T and R, while measurement of only three quantities, /, T and 

 R, is sufficient to make the same determination with a single leaf, according to equation 

 (22.4). Furthermore, in plant work, one is never certain whether the "blank" is entirely 

 free of pigments: Accordmg to Seybold and co-workers (1933S 1942), so-called "white" 

 leaves of Acer negundo absorb 10-20% of incident white light; this absorption may be 

 caused by nonplastid pigments, or by a small quantity of residual chlorophyll or caro- 

 tenoids. 



The transmitted light flux (T) and the reflected light flux (R) can both 

 contain a coUimated component, T, or R^ (light transmitted in the direc- 

 tion of the incident beam, or reflected according to the laws of specular re- 

 flection), and a diffuse component, T^ or Ra, so that equation (22.4) can 

 be written more explicitly as follows : 

 (22.7) A = I - {r. + Ti) - {R. + Ri) 



