716 LIGHT ABSOKrTION BY PIGMENTS IN VIVO CHAr. 22 



changing the average length of the hght path in the mediinn, could also 

 change the absorption spectrum (log I/S as function of wave length, which 

 is what fig. 22.21 represents). However, it appears impossible that the 

 light path in the Chlorella suspension could be so lengthened by scattering 

 as to replace the practically complete transparency of the pigment extract 

 at 800 mjLi, by an absorption of over 10%. If Noddack and Eichhoff's re- 

 sults can be relied upon (which is not certain) we arc led to consider the 

 spread of the chlorophjdl absoriition in living cells into the far red and in- 

 frared, a genuine change in the absorption curve of the green pigment. It 

 may be noted (cf. fig. 22.48B) that the absorption curve of Chroococcus 

 (a Cyanophycea) , calculated by superposition of the absorption curves of 

 all the extracted pigments, although it is very close to the transmissi(jn 

 curve of a living cell suspension (probably, because of the absence of chloro- 

 plasts and consequent reduction of scattering), nevertheless also shows 

 enhanced absorption in the far red. 



Increased absorption in the green, which is so striking a feature of leaf 

 spectra, may or may not indicate a genuine change of the chlorophyll spec- 

 trum. The above-mentioned Chroococcus curve shows no similar effect. 

 In interpreting the absorption in this region, one has to consider, in addi- 

 tion to scattering, also the possible presence in living cells of protochloro- 

 phyll, pheophytin or other relatives of chlorophyll that have absorption 

 bands in the middle of the visible spectrum. The possible extensive spread 

 toward the longer waves of the absorption bands of the carotenoids also 

 has to be taken into account. 



Decreased absorption by live cells in the maximum of the red band, shown 

 in figures 22.21 and 22.22, cannot be explained by a sieve effect (which is 

 negligible in cell suspensions); diffuse reflection, too, probably is insuf- 

 ficient to account for this effect. A certain flattening of the red band may 

 therefore also be characteristic of the true absorption curve of chlorophyll 

 in the living cell. 



Table 22.VII shows that (as first noticed by Wurmser in 1921) the 

 ratio of the apparent absorptions in peaks of the blue-violet band and of the 

 red band is low in algae (about 1.5) and particularly in leaves (about 1.0), 

 as compared with pigment extracts (1.75). The blue-violet rays are scat- 

 tered more strongly than red ones; but this could not explain a decrease 

 in their absorption. (Such an effect would only be understandable if the 

 diffuse reflection of blue- violet light by leaves were stronger than that of red 

 light; but no such effect is revealed by figures 22.12 or 22.32.) The 

 comparatively weak absorption of blue-violet light by plants is made even 

 less understandable by the fact that yellow, water-soluble pigments present 

 in leaves must enhance the absorption in this region. 



However, Seybold and Weissweiler (1943) opposed Noddack and Eich- 



