KENNETH V. I HI MANN AND r.EORGE M. CURRY 059 



235 240 250 260 270 280 290 300 310 320 330 340 350 360 365 



Wavelength in m^ 



Fig. 8. Action spectrum for the Avena colcoptile "base" curvature towards iiltra- 

 \iolet light (solid line). The dotted line shows the absorption spectrum of pure 

 indole acetic acid (L\.\). The black squares are points on the action spectrum for 

 tlie inac ti\aii()n of pure I AA in \ilro. (From Curry, Thimann, and Ray. 1956). 



ring (Kaper, 1957) . The apparent accentuation of this "pip" in 

 the action spectrum, together with the 12 ni/x shift to the right, 

 suggests that the lAA may be in chemical combination. The hypo- 

 thetical compound must still be a growth-substance, since its reaction 

 with light modifies the growth. Furthermore, pure lAA in aqueous 

 solution is inactivated by very high dosages of ultraviolet, and the 

 action spectrum for this process follows closely the absorption spec- 

 trum of lAA. In particular, the value at 297 ni/x is about one-fourth 

 the value at 287 mjx, and the peak lies near 280 m^x (the black 

 squares in Fig. 8) . However, the amounts of energy required for 

 inactivation in vitro are several thotisand times as great as those 

 needed for jjhototropic curvature. Therefore in the postulated com- 

 pound the indole acetic acid would have to be sensitized to the action 

 of ultraviolet. 



Some support for this view was obtained as follows. If the plants 

 are thrice decapitated 90 minutes apart so as to minimize their con- 

 tent of endogenous hormone, then they no longer curve toward 

 ultraviolet light, even at a dosage of 2000 ergs/cm^. In intact plants 

 this dose Avould give a maximum curvature of about 17°. However, 

 if agar containing lAA (0.5 mg per liter) is applied between the 

 second and third decajjitations, curvatures of good magnitude can 



