668 LIGHT AND LIFE 



Our own observations on the proplastids of Avena show that one 

 hour's exposure to red Hght, which reduces phototropic sensitivity by 

 a half or more, causes marked extension of the "primary granum" 

 which can be seen clearly with the optical microscope. 



That phototropism and geotropism should be caused by the move- 

 ments of comparable organelles would make understandable the fact 

 that both are accompanied by develoj^ment of an emf of 10-20 milli- 

 volts across the organ. This was first observed for geotropism by Bose 

 in 1907, and extended to the first positive phototropic curvature by 

 Backus and Schrank (1952) . In both tropisms the side which is to 

 grow the more becomes electropositive. 



An important difference between the two tropisms is that gravity 

 can be detected by the whole coleoptile, like the high light dosages 

 causing the second positive curvature. The first positive cvirvatures, 

 on the other hand, as well as the negative curvatures, are initiated in 

 the tip. This suggests that the plastids of the tip region differ from 

 those below — a possibility now under study in my laboratory. 



This theory could well provide for a moderate amplification of 

 the light effect so that one quantum could alter the transport of many 

 auxin molecules; and it would explain, as no other present theory 

 does, how light causes asymmetrical auxin movement. As stated above, 

 it accords with the known localization of carotenoids in plastids. 

 The theory is applicable to fungi, too, for in them also the carotenoid 

 is present in particles. These have been notably described in the light- 

 sensitive zone of Piloboliis by Buller, who noted their orange-red 

 color and referred to them as oil droplets. A study of these particles 

 would be of the greatest interest. 



In summary, phototropism is a light response of very wide distri- 

 bution throughout the plant kingdom. In the higher plants the 

 ultimate effector is asymmetric auxin distribution; in the fungi this 

 is most probably not the case. Yet careful study of a selected higher 

 plant and a fungus reveals that their photoreceptors for the visible 

 spectrum are virtually identical, while in the ultraviolet both have 

 a second type of response with, again, at least comparable photo- 

 receptors. The correlation between phototropic curvature and the 

 effects of symmetrical illumination on growth is far from complete, 

 and integrative factors like those in Avena may perhaps participate 

 in fungi also. A theory of the mechanism of phototropic curvature 

 is outlined which unifies this phenomenon with the geotropic re- 

 sponse and brings it into relation with the known movements of 

 chloroplasts. 



