NO. II PHOTOTROPIC SENSITIVITY JOHNSTON II 



corresponding phototropic effectiveness are given in table 3 and the 

 midpoint of each band plotted in figure 3. Here a distinct break in 

 ascent of the curve is shown. 



Table 3. — Data from the Second Experiment Showing Phototrofyic 



Effectiveness in the Spectral Region Indicating the Presence 



of a Double Maxinnint 



Relative 

 Wave-length Light intensity ratio phototropic 



range (A) (nionochromator/standard) effectiveriess 



4460-4560 .29 100.3 



4558-4662 .42 69.3 



4685-4805 .41 71-0 



IMPROVED EXPERIMENTATION AND RESULTS 



Another experiment was planned and carried out in which the 

 technic was further improved. A motor generator was installed 

 wherein the current used for the light sources was automatically con- 

 trolled. Both the monochromator lamp and the standard lamp were 

 connected in series and replaced at the same time when one burned out. 

 These lamps were the Mazda projection type rated at 200 watts, 50 

 volts, with an average life of 50 hours. They were burned at four 

 amperes. The water jacket around the standard lamp was removed 

 and the filter cooled by a thermosiphon method in which distilled 

 water was used. In the longer-wave-length regions the light from the 

 monochromator was passed through suitable glass filters to reduce 

 the effect of scattered light of shorter wave lengths affecting the seed- 

 lings. Unfortunately no filters which transmitted an adequate per- 

 centage of light were available for wave lengths of 4500 A or shorter 

 when used in connection with these projection lamps. 



The data from this more accurately controlled experiment are pre- 

 sented in table 4 and shown graphically in figure 4. The maximum 

 phototropic effect occurs at 4400 A, a region about 150 A shorter 

 than the maximum found in the earlier experiment. A secondary 

 maximum occurs at approximately wave length 4750 A with the 

 intervening minimum at about 4575 A. From this double maximum 

 the sensitivity of Avena falls off rapidly to 5000 A on the long-wave- 

 length side, and to 4100 A on the short-wave-length side. It would 

 be interesting to determine if the limit of sensitivity in the case of 

 Avena continues to fall off on the short-wave-length end of the spec- 

 trum, as some previous work would indicate. At some future date 

 it may be possible to extend this curve into the violet and ultraviolet 

 regions. 



