NO. II PHOTOTROPIC SENSITIVITY JOHNSTON 9 



curred in four hours. This indicated the approximate threshold region 

 of phototropism. In order to determine this point more accurately a 

 mercury arc in pyrex glass was substituted for the Mazda lamp of the 

 monochromator. and by passing this light through a number "jy 

 Wratten filter, a seedling was unilaterally illuminated by the 5461 

 mercury line. In five such tests, each lasting from two to several 

 hours, two gave positive bending and three no bending. With reason- 



Table 2. — Data from the Preliminary Experiment Shozving Phototropic 



Effectiveness of Restricted Regions of the Spectrum. That for 



Wave-length Region 3040-3160 A is Taken as Unity 



Filter " Relative 



Wave-length used with Light intensity ratio phototropic 



range (A) monochromator (Monochromator/standard) effectiveness 



7250-7700 W 88 



6900-7300 W 88 



6550-6950 CLF 2 



6250-6600 CLF 2 



5940-6270 CLE 3 



5670-5950 TR 



5430-5670 W 22; CLF 5.1 



5200-5430 W 77 



5040-5160 CLF 6.1 29.10 i.o 



4940-5070 CLF 6.1 2.49 1 1. 7 



4810-4930 CLF 6.1 0.68 42.8 



4670-4800 CLF 6.1 0.54 53.9 



4550-4670 CLF 7 0.29 100.3 



4450-4550 0.27'^ 107.8 



4410-4500 0.34" 85.6 



4360-4450 0.36" 80.8 



4280-4360 0.41" 71.0 



4210-4280 0.47 67.0 



4130-4220 0.84 34.6 



4070-4135 1.49 jg^ 



^W, Wratten; CLF, Corning line filter; TR, thermometer red. 

 With the standard lamp at a fixed position from the plant, the intensity of monochromator 

 light was varied by changing the resistance in its lamp circuit until a balance point was 

 obtained. 



able certainty it can be concluded that under these particular experi- 

 mental conditions the threshold wave-length efi'ect is at or very near 

 5461 A. 



When the phototropic efifectiveness is plotted against wave length, 

 a curve is obtained as shown in figure 3, with its maximum at about 

 wave length 4550 A. The horizontal lines represent the wave-length 

 ranges for which balance points were determined. Points where filters 

 were used in addition to the monochromator are represented as circles. 



