666 LIGHT AND LIFE 



could be inactivated. There must therefore be an ampHfication factor 

 of at least several hundred and probably much more. 



As shown above, lAA can be inactivated by ultraviolet, though much 

 higher eneigies are required than those causing phototropic curvature. 

 However, even in the ultraviolet there is no evidence for a chain re- 

 action in the photo-inactivation of lAA; in solution or in agar the 

 quantum yield aj^proximates 0.2, and when sensitized by riboflavin 

 a value of 0.7 has been reported (Galston, 1949) . 



Recent experiments of von Guttenberg (1959) add qualitative 

 evidence against photo-destruction. Decapitated coleoptiles exposed 

 unilaterally to very high dosages of white light, around 500,000 

 ergs/cm^, give quite small curvatures; if, however, lAA in agar is 

 applied ajter the exposure the curvatures may be very large, reach- 

 ing 25° or more. The coleoptiles could even be placed in the dark 

 after illumination and the lAA applied as much as 8 hours later; 

 curvatures of 15° could still result. Thus direct photo-destruction 

 is excluded, and furthermore, the probability that photo-products 

 active enough to destroy lAA could survive in the plant for 8 hours 

 at 25 °C is obviously negligible. 



We conclude that light does not react with the auxin itself, but 

 that the photoreceptor brings about an effect (a) capable of surviving 

 some time in the plant, and (b) influencing many hundreds of auxin 

 molecules. Such an effect might well be exerted on an enzyme pro 

 ducing lAA. However, such an enzyme would be likely to produce 

 auxin molecules measured in millions rather than hundreds, and 

 in any case a modification of enzymic activity does not appear to offer 

 any basis for the influence of light on translocation of auxin. For 

 these reasons the following hypothesis is preferred. 



If the photoreceptor is indeed a carotenoid it would be expected 

 to be located in a plastid. Carotenoids in plants are almost invariably 

 present in plastids. The proplastids of etiolated Avena coleoptiles 

 appear small, nearly circular in optical section, and with a marked 

 "primary granum": they are found in good numbers in the lower 

 part of the coleoptile tip, mostly near the termination of the vascular 

 1)uik11cs. It may be assumed, therefore, that these bodies could con- 

 tain the photoreceptor. 



In the case of geotropism it has long been accepted that the recep- 

 tor of gravity must be a body which can fall, and attention has been 

 directed to a type of small starch grains termed "statoliths" (see e.g. 

 Haberlandt, 1911). In ilic vertical coleoptile these can be seen to 



