710 LIGHT ABSORPTION BY PIGMENTS IN VIVO CHAP. 22 



spectra, log (Tq/T), obtained by comparison of colored with colorless tis- 

 sues (or, in the case of cell suspensions, by comparison with pure water). 



Similar data for a variety of green, yellow and red leaves can be found 

 in Seybold and Weissweiler's paper (1943). 



The question arises: can the leveling off of the absorption peaks and 

 the filling in of the absorption valleys be attributed entirely to geometrical- 

 optical effects (scattering and "sieve effect") or do they indicate a genuine 

 deformation of the absorption curves of the pigments (which could be 

 caused by close packing of pigment molecules in the chloroplasts, as well as 

 by their association with proteins and lipides) ? 



The fact that the leveling off is much stronger in the spectra of leaves 

 than in those of algae indicates that geometrical-optical effects account for 

 a considerable part of the phenomenon. Tanada's (1951) observations 

 with glycerol (table 22.VII) support this interpretation. Clearly, scat- 

 tering and "sieve effect" must lead to an apparent decrease in the selec- 

 tivity of absorption, as far as plots of log {I/T) or of log (To/T) are con- 

 cerned, since, in these two representations, losses of the weakly absorbed 

 (green and extreme red) light by scattering obviously simulate absorption. 

 One could attempt to explain in this way the results under 4, 5, 7, and 8a- 

 8d in Table 22.VII, which were obtained by the use of blank cells with 

 pure water. On the other hand, the similarity of results of the experiments 

 listed under 6 and 8, in which the true absorption of Chlorella cell suspensions 

 was determined, with the results listed under 7, in which the transmission 

 of a similar suspension was measured, cannot be explained in this way. 



In the case of leaf spectra too, it is not immediately obvious why diffuse 

 scattering should lead to transmission curves, log (To/T) (obtained by com- 

 parison of green with white specimens), characterized by high optical den- 

 sity in the regions of weak pigment absorption (green and far red) . 



If one would assume, for example (c/. Meyer 1939), that the weakening of the trans- 

 mitted beam by passage through a green leaf can be represented by the equation: 



(22.9) T = I X 10 "('^"^+''^^ 



where K is a proportionality constant, equivalent to the product (concentration X thick- 

 ness of the absorbing layer) in Beer's law, and a a "scattering coefficient"; and assuming 

 also that the corresponding equation for the white leaf is: 



(22.9A) To = I X 10~''>^ 



then the "transmission curve" would be given by the equation: 



(22.10) log To/T = -Ka-^ 



In other words, the "transmission curve" would follow faithfully — 

 except for a proportionality factar K — the true absorption curve of the 



