KENNETH J'. THIMANN AND GEORGE M. CURRY 651 



I.^Orr 



a. 



E 1.20 



■J) 



S 1.00 



0.80 



E 

 :! 



c 

 o 



0.60 



0.40 



0.20 



-I 1 I 1 r 



I I 



OLl 



300 320 



340 



440 



460 



480 



500 



360 380 400 420 

 Wavelength in m ^l 



Fig. 4. Action spectrum for the positive curvature of Avena coleoptiles caused 

 by monochromatic light sources of low intensity. Average of three series of experi- 

 ments. 



The spectrum in Fig. 4 has five characteristics: the main peak 

 at 445 m^a, the secondary peak at about 472 m/x, the trough separating 

 them, the inflection at about 425 va,x, and the broad peak in the 

 near ukraviolet. The trough at about 460 m/x, has been confirmed by 

 separate experiments in which equal energies were applied at 448, 

 460, and 472 m/^; the curvatures resulting were 16.02 ± 0.40, 14.82 

 ± 0.41 and 16.19 ± 0.38 respectively. Expressed as degrees per unit 

 of energy the values are 1.31, 1.19, and 1.31 at 448, 460, and 472 

 m-fx. respectively. In another comparison using different doses to ob- 

 tain equal curvatures, the following values were obtained: 1.37, 

 1.19, and 1.27 for Avavelengths 443, 458, and 473 m^ respectively. 

 The relative heights of the bands at 365 and 445 vajx were also com- 

 pared in direct experiments (first perfomed by Conrad Yocum) and 

 ratios of 0.45-0.50 (on a quantum basis) found. These agree with 

 the general conclusion of Blaauw (1909) and Sonne (1929) that 

 the near ultraviolet is less than half as effective as the blue. Thus 

 the action spectrum is well established. In the meantime, and in- 

 dependently, Shropshire and Withrow (1958) have performed some- 

 what comparable experiments, and while the results differ in certain 

 details, the presence of peaks at 410-415, 440-445, and 470-475 m^ is 

 in agreement with Fig. 4. Their finding that the proportionality con- 



