CONTROL BY LIGHT 223 



esses are not nearh as well understood. However, they certainly 

 cannot be lying around at random, dissolved in some relatively 

 homogeneous ground substance, particularly since recent electron 

 micrographs indicate that there is little if anything homogeneous 

 about a living svstem (e.g.,"Whaley et ah, 1960). In addition, ex- 

 periments with polarized light have suggested that photoreceptor 

 molecules are probablv higliK' oriented even in relatively simple 

 systems (see Jaffe, 1960). A recent hypothesis (Thimann and 

 Curry, 1960) on pliototropism suggests that the pigment molecules 

 are located in particles— proplastids, for example, in the higher plants 

 —and that the first event following light absorption may be a reorien- 

 tation of these particles. Much here remains to be demonstrated, 

 but it is interesting particularly since the mechanism suggested 

 essentially parallels that invoked in the "statolith" theory of geo- 

 tropism and thus might provide some unifying link between the two 

 phenomena. It will certainly be worthwhile, in view of such con- 

 siderations, to see what the electron microscope might show about 

 immediate structural changes, if any, following illumination. 



It should be added parentheticallv that the artificial distinction 

 made here between red- and blue-light effects is just that— artificial. 

 For example, there is evidence both for effects of blue light on the 

 red, far-red system (see Borthwick et al., 1956; Withrow, 1959b) and 

 for phototropic effects of red light under certain circumstances ( see 

 Galston, 1959). These observations pose further problems of the 

 same kind as those already discussed. A recent paper on diapause 

 induction in the cabbage-liutterfly, Pieris brassicae, seems even more 

 formidable. The effect of light appears to depend strongly on the 

 portion of the diurnal c\'cle in which it is given; but in addition, the 

 shorter wavelengths— red and yellow— inhibit diapause in the phase 

 during which blue and violet promote it, while they promote it dur- 

 ing the phase in which blue and violet inhibit. This may be due 

 to the presence of two separate pigment systems, both affecting dia- 

 pause, but the question requires much further investigation (Biin- 

 ning and Joerrens, 1960). 



In terms of the topic of this symposium— control mechanisms— the 

 basic mechanism for the control of many diverse processes bv light 

 is, of course, that manv biologically active compounds are pigments. 

 They absorb light and can thus be excited into states with proper- 

 ties different from their normal ground states. Whether or not this 

 is then in some way amplified to a physiological response depends 



