CONCERNING NEGATIVE PHOTOTROPISM IN 

 DAPHNIA PULEX 



A. R. MOORE 



From the Rudolph Spreckels Physiological Laboratory of the University of California, 



Berkeley 



ONE FIGURE 



It has been shown by Loeb^ that the rays of a Heraens mercury 

 arc cause Balanus larvae and Daphnia to become negatively 

 phototropic. This effect is due principally to the ultra-violet 

 rays given off by the mercury lamp, because if a plate of ordinary 

 window-glass be interposed between the light and the dish con- 

 taining the animals, thus cutting off the greater part of the ultra- 

 violet rays, the negative effect of the light is much diminished. 



Before making further experiments along this Une, I wished 

 to determine at what point in the spectrum one thickness of 

 window-glass cuts off the ultra-violet light effectively. To this 

 end spectrophotographs were taken with and without the glass 

 plate interposed, exposure twenty seconds. I am greatly in- 

 debted to Professor Minor, of the Department of Physics, for 

 making the spectrophotographs and identifying the lines. Figure 

 1 at the top shows a spectrophotograph of the mercury arc used; 

 and below a photograph of the same with the glass plate inter- 

 posed between the arc and spectroscope. 



It is apparent that a pair of lines in the ultra-violet (lines of 

 wave length 3341 A. u.^ and 3390 A. u.) pass through the glass 

 plate only slightly impaired. Therefore, the much stronger nega- 

 tive effect of the light which does not pass through a glass plate 

 must be due to the rays of wave length shorter than 3341 A. u. 



1 Loeb, J. Pfliiger's Archiv, Bd. 115, S. 576. 

 ^ Angstrom units. 



573 



