658 LIGHT AND LIFE 



prolonged exposure again gives a positive curvature, as can be seen 

 above in Fig. 3. However, the curvature at very high energies is of 

 a somewhat chffeient type. While the res2:)onse to low dosages be- 

 gins slowly, is clearly centered close to the tip, and extends gradually 

 downwards, the high-energy curvature starts earlier and is distributed 

 along the whole length of the coleoptile, so that the plant shows no 

 localized zone of curving. After 120 minutes the high-energy curva- 

 ture appears mainly in the base. For this reason the second type 

 has been generally called the "base response," though "high thres- 

 hold response" would be a better term. It is this which mainly con- 

 stitutes the normal curvatures of plants in daylight, as shown for 

 instance by potted plants in a window. 



The same type of basal curvature residts when the Avena coleoptile 

 is unilaterally illuminated with ultraviolet light. This has made it 

 possible to determine an action spectrum for the base curvatiue in 

 ultraviolet light. If the light intensity is held constant and the dura- 

 tion of exposure varied, the resulting curvature shows a satisfactory 

 linear relationship with the log of the exposure. However, for the 

 data to be meaningful, tip curvatures must be excluded. The plants 

 were therefore exposed behind a mask which completely shaded the 

 most apical 2-4 mm. Using a grating monochromator and a high pres- 

 sure mercury vapor lamp, the whole region down to 235 m^ was ex- 

 plored. The results are shown in Fig. 8. There are apparently two 

 peaks close together, at 280 and 297 m^, and there is evidence of a 

 rise again at the shortest wavelengths. Sensitivity falls off very sharply 

 above 300 m^u. 



It is evident, therefore, that this response differs from the tip re- 

 sponse not only in its distribution within the jjlant but in the effec- 

 tive photoreceptor. It is notable, for instance, that at 265 and 365 m^n, 

 where flavins show major peaks, the sensitivity is low; at 365 myn, 

 indeed, it is extremely low. None of the colorless carotenoid pre- 

 cursors whose spectra have been recorded (e.g., phytofluene and 

 phytoene) shows absorption matching the spectrum of Fig. 8. An 

 attractive hypothesis (Curry, Thimann, and Ray, 1956) is that the 

 action spectrum is the absorption spectrum of indole acetic acid 

 (lAA) itself, but shifted 12 niyu, towards the visible. Both the short 

 wavelength end and the steep fall on the long-wave side agree with 

 this interpretation very well; there is a difference around the peak, 

 however. The spectrinn of indole acetic acid has the characteristic 

 "pip" at about 287 ny, shown by most indole compounds which do 

 not contain a side-chain double bond conjugated with those in the 



