NO. II PHOTOTROPIC SENSITIVITY JOHNSTON 3 



About the same time Hurd (1919) showed wave-length effect on 

 young rhizoids by equalizing the intensity of the light coming through 

 a series of Wratten filters. Only the blue (4700 to 5200 A) and 

 violet (4000 to 4700 A) lights produced phototropism, negative in 

 direction. The other lights at the intensity of 1800 meter-candles had 

 no effect. However, with a greater intensity the green light (5200 to 

 5600 A) exerted a negative phototropic effect as well as the blue and 

 violet. 



For the purpose of investigating the wave-length effects of radia- 

 tion on phototropic bending of young plants, Johnston (1926) con- 

 structed and described a simple plant photometer. The apparatus 

 consisted of a long box divided into three compartments. Each end 

 compartment contained an electric lamp which could be moved toward 

 or away from the light-filter window in the partition separating it 

 from the central or plant compartment. Plants which easily respond 

 in their directional growth to differences in light intensities were em- 

 ployed in place of the adjustable indicator or photometer screen in 

 the ordinary Bunsen photometer. 



Sonne (1928-1929) determined the necessary amount of energy of 

 different wave lengths to produce a minimum phototropic response in 

 oats. The young plants were so placed that about i cm of their tips 

 were exposed at different distances from the light of a mono- 

 chromator for different exposure periods. The visible part of the 

 spectrum of a Hefner lamp was used as a standard of comparison. 

 Minimum response was obtained at 0.86 x io~^ g. cal. per cm^ in i 

 second. The energy was measured by a thermo-element. The results 

 are summarized in table i. 



Table i. — Sonne's Data showing Phototropic Sensitivity Determined from the 



Amount .of Energy Required to Produce a Minimum Response 



in Oats 



