PHOTOTROPISM JOHNSTON 



321 



and by means of a galvanometer the two light intensities were 

 measured. 



From a number of such experiments the curve shown in figure 4 

 was constructed. This curve illustrates the sensitivity of the oat 

 seedling (plotted vertically) to the wave lengths of light (plotted 

 horizontally). The sensitivity increases rapidly from 4,100 A to 

 4,400 A, then falls off somewhat to about 4,575 A, and again rises to 

 a secondary maximum at about 4,750 A. From this point the sensi- 



4000 4i00 4200 4300 4400 



4600 4100 4S00 4900 5000 5100 5200 5300 S400 



FiGDEE 4. — Phototropic sensitivity curve of oat coleoptile for the wave-lengtli bauds 

 represented by the horizontal bars. 



tivity decreases rapidly to 6,000 A, from which point it gradually 

 tapers to 5,461 A, the threshold of sensitivity on the long wave-length 

 side. Briefly, it can be concluded that the region of greatest sensi- 

 tivity is in the blue. That is, growth is retarded the most in the blue 

 light. Orange and red light have no effect in retarding the growth 

 of these oat seedlings. 



It is interesting to compare the wave-length sensitivity of the oat 

 seedling to other light-sensitive organisms and to photoelectric sub- 

 stances. In figure 5 the relative reaction curve of the honeybee to 

 color, the relative visibility curve of the human eye, and the rela- 

 tive spectral response of a Weston photronic cell are plotted, to- 



