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ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 4 



gether with the relative phototropic curve of the oat coleoptile. 

 The maximum sensitivity of the human eye and of the bee falls at 

 practically the same wave length, about 5,600 A. The bee can detect 

 blues somewhat better than man, but the human eye is more sensi- 

 tive in the red end of the spectrum. The entire sensitivity curve of 

 the oat is shifted toward the blue in comparison with the other 

 curves, its maximum being at about 4,400 A. As mentioned above, 

 wave lengths longer than 5,000 A in the visible spectrum exert but 



Figure 5. — Relative phototropic sensitivity of the oat seedling (continuous curve with 

 open circles) compared to the relative sensitivity of the honeybee (dash curve with 

 solid circles), the human eye (continuous curve) and a Weston photronic cell (dash 

 curve) in the spectrum. Wave lengths are indicated in Angstrom units. 



little influence on the growth response of the plant. The photronic 

 cell curve has its peak farthest toward the red end of the spectrum 

 but approximates rather closely the peak of the human eye curve. 

 It has a much broader range than the other sensitivity curves. This 

 means that this particular cell can detect light in the red end and the 

 violet end of the spectral range which neither the human eye, the 

 honeybee, nor the oat plant can detect. 



The growth movement of plants known as phototropism is closely 

 related to auxin, which some investigators claim is a true hormone. 



