1166 THE LIGHT FACTOR. III. COLOR CHAP. 30 



sis, if other photochemical processes can interfere with this process. A 

 selective sensitizaton of oxidative processes in the photosynthetic apparatus 

 by the light absorbed by the carotenoids, or by chlorophyll in the blue-violet 

 band, offers one possibility of this type. However, in this case, only light 

 curves of the time-dependent "optimum" type (which have been observed, 

 e. g., in some umbrophilic plants; cf. page 994) should be affected, since the 

 maximum rate in these curves is determined by the (time-dependent) bal- 

 ance of photos:ynthesis and photoxidation. Light curves with true satura- 

 tion plateaus should remain unaffected by an earlier onset of "light inhibi- 

 tion" (except for a shorter extension of the saturation plateau). 



The effect of wave length on photoxidation has never been investigated 

 systematically. Franck and French (1941) found that photoxidation oc- 

 curs, in carbon dioxide-deprived leaves, in red as well as in blue light, 

 but this was merely a qualitative observation. A selective effect of blue 

 light on the respiration of Chlorella, observed by Emerson and Lewis (1943), 

 was mentioned in chapter 20 (page 568). Another specific function of 

 yellow pigments seems to be well established — the sensitization of photo- 

 tropic movements. As stated on page 681, the changes in the positions of 

 the chloroplasts in light are caused only by light absorbed by yellow pig- 

 ments (Voerkel 1933) ; and the same is true of the phototactic movements 

 of whole cells {of. Castle 1935). 



These pigments may be the carotenoids, although Galston attributed 

 this function to riboflavin (because the action spectrum showed only a 

 single peak). In purple bacteria, on the other hand, the action spectrum 

 of phototaxis coincides with that of photosynthesis {cf. p. 1188). 



Very extensive studies of the effects of light of different colors in photo- 

 synthesis and respiration were made by Danilov (1935, 1936), using green, 

 blue-green, and red algae. He reported very complicated results in which 

 the yield was found to depend not only on color (in monochromatic light), 

 but also on the combination of colors (in non-monochromatic light). Fol- 

 lowing the tendency of what Kostychev proclaimed as a new "physiological" 

 approach to photosynthesis (cf. chapter 26, p. 872) he discussed these phe- 

 nomena in vague terms of stimulation and inhibition of different protoplasmic 

 functions by hght of different wave length. Thus, yellow and green rays 

 were credited by him with increasing cell sensitivity to red light, and with 

 making it insensitive to infrared light; blue-violet light was said to assist 

 in the utilization of infrared light (supposedly for activation of the "dark" 

 reaction stages in photosynthesis). Blue-green rays were said to counter- 

 act the stimulation effects of yellow light, and enhance the stimulating 

 effects of blue rays, and, generally, to create in the cells a "regulator of the 



