TRUE ABSORPTION SPECTRUM 



715 



ity that a beam of light passes between all these chloroplasts, distributed 

 at random, is between 2 X IQ-^ and 5 X IQ-^. Consequently, if the dis- 

 tribution of the chloroplasts were random, the sieve effect would be negli- 

 gible. However, the effect can become important if the alignment of 

 chloroplasts attains a high degree of regularity. The results of Schanderl 

 and Kaempfert (1933) (Table 22.1) point toward a measure of success 

 that nature has achieved in this regulation {cf. fig. 22.5). In very young 

 or thin tissues, the sieve effect is important even without an alignment of 

 the chloroplasts. Meyer (1939) mentions that he was unable to measure 

 the absorption spectra of the seedlings of Tradescantia, and of oat, because 

 "chlorophyll in these seedlings was so granulated that they appeared in 

 transmitted light not green but checkered, consisting of white and dark 

 spots." 



One way of viewing the sieve effect is to consider the mutual "shading" of molecules 

 in each colored particle, which prevents them from exercising their full absorbing capac- 

 ity; this interference obviously cannot become effective unless the light is markedly 

 weakened by the passage through a single particle. This condition is satisfied in the 

 chloroplasts, since in the peaks of the absorption bands of chlorophyll a single chloro- 

 plast absorbs more than 50% of incident light {cf. page 683). 



Until quantitative theories have found actual systematic application to 

 cell suspensions, if not to leaves and thalli, the question— what, if any, 

 changes in the true shape of the absorption bands can be deduced from the 

 spectra of live plants— will beg detailed answer. Several qualitative indices 

 that such changes do occur can be noted even now, but none of them is 

 entirely reliable. These indications are: enhanced absorption in the 

 far red and near infrared (to which we referred on page 654), decreased 

 absorption in the maximum of the red band, and the comparatively weak 

 absorption in the blue- violet region. 



Increased absorption in the far red is exhibited not only by leaves {cf., 

 for example, fig. 22.15) but also by Chlorella and Chroococcus suspensions 

 (figs. 22.21, 22.22 and 22.23) and by aqueous protein-pigment suspensions 

 (fig. 21.28A). In the experiments of Smith (1941), this excess absorption 

 was observed to disappear upon the addition of a detergent, digitonin 

 (compare figure 21.28A with B); he therefore attributed it to scattering. 

 A difference that may be explained in the same way was noted by Rabi- 

 deau, French and Holt (1946) between the transmission and the absorption 

 spectra of chloroplastin dispersed by ultrasonic waves (cf. fig. 22.15). 

 However, as mentioned before, a strongly enhanced absorption in the 

 far red by live Chlorella cells is recognizable also in Noddack and Eich- 

 hoff's figure (fig. 22.21), which (unless the integrating apparatus failed to 

 function as intended) could not be affected by scattering in the way as- 

 sumed by Smith. It is true that, as explained on page 711, scattering, by 



