650 



LIGHT AND LIFE 



1000 



ENERGY IN ERGS / CM^ 



Fig. 3. The response of Avena coleoptiles to a series of light dosages at two 

 wavelengths. Each point is the mean of 10-14 individual curvatures, measured 

 after 100 minutes. Solid symbols, 436 ni/u; open symbols, 365 m^. Circles, tip 

 curvature; squares, base curvature. 



erg/cm-, which is very well compatible with Fig. 3. Curvature be- 

 comes maximal at about 27°, and at higher light dosages it de- 

 creases again, finally reaching a small negative value. However, the 

 negative curvatures occur only over limited ranges of rather high 

 intensity and will not be discussed further here. 



Fig. 3 shows that light of wavelength 436 m^u, is almost three times 

 as effective as that of 365 m/x. With the same procedure a great num- 

 ber of wavelengths have been compared over the range of linearity. 

 I.e., at low energies. The mercury lines 492, 436, 408, 405, 365, and 

 313 ni/A gave the best data, and additional wavelengths were obtained 

 by using narrow bands of the continuous spectrum from a tinigsten 

 lamp. The average of three series of experiments performed under 

 as nearly identical conditions as possible is shown in Fig. 4.- This, 

 then, is the action spectrum for the positive phototropic curvature 

 of Avena coleoptiles in the low-energy range. 



''The data of three groups of experiments, in each of wliicli tlie sensiti\ity of 

 the plants dMlcrcd sligiuly. were combinctl by making the scnsiti\itics at 436 niju 

 identical, and calculating the relative sensitivities at all other wavelengths there- 

 from. 



