BENTLEY CLASS 895 



12 ni/ti lowiiiil the visible. Pure lAA in aqueous solution is inac- 

 tivated l)\ ultraviolet, and the action spectrum lor the process agrees 

 with the absorption spectrum ol \Al\\ but the inactivation recjuires 

 very high doses. In the plant, therelore, the lAA, il it is the photo- 

 receptor, nuist be greatly sensitized to the action ot ultraviolet radia- 

 tion. Support lor this view was adduced by a demonstration that 

 applied lAA can reinstate the phototropic sensitivity lost through 

 exposure to ultraviolet. Hence it seems likely that lAA in some com- 

 bined form is the photoreceptor for the high-threshold response, as 

 well as being the effector of it. 



The phototropic response of the mold Phycomyces is particularly 

 interesting because of its sensitivity, because the response occurs in 

 a single cell, the spc^rangiophore, and because no mediation by a 

 growth hormone has been detected, in spite of the fact that the spor- 

 angiophores contain indole acetic acid. The general similarity of 

 the Phycomyces system to Avena is very striking. The action spectrum 

 for Phycomyces shows close agreement with that of Avena; and the 

 sporangiophore of Phycomyces contains a carotenoid. High doses of 

 light reverse the direction of the curvature. The action spectrum 

 for the negative curvature in the short ultraviolet is also like that of 

 Avena, with a peak near 280 m/x. 



Since symmetrically distributed light accelerates plant growth, one 

 may wonder about the reason for a positive curvature in the usual 

 phototropic response. The response to unilateral illumination con- 

 tinues in the dark for much longer than the response to symmetrical 

 illumination. Thimann therefore is disinclined to view the curvature 

 as simply the outcome of the difference in growth of the two sides 

 of a coleoptile, or, in the case of Phycomyces, of a single cell. Rather, 

 he finds support for the lens theory, which holds that the light is 

 focused by the clear cellular substance onto the opposite side from 

 that which is directly illuminated. Gallic acid present in the sporangio- 

 phore would render it virtually opacjue to short ultraviolet, and there- 

 fore accelerate growth only on the illuminated side, and consequently 

 produce a negative curvature. This interpretation was supported by 

 finding that synniietrical illumination with ultraviolet in fact pro- 

 duces a simple acceleration of growth of the sporangiophore. 



Evidence from experiments, and considerations of quantum yield, 

 now speak strongly against the destruction of indole acetic acid by 

 light as being the mechanism of the phototro]:)ic response. It is es- 

 pecially significant that lAA, applied as many as 8 hours after ex- 

 posure of decapitated coleoptiles to very high doses of white light, will 



