234 



LEE RAYMOND DICE 



negatively phototactic to any intensity of ordinary light at 

 these temperatures. 



It has been shown by Moore (1912) that Daphnia pulex may 

 be made negatively phototactic by the ultra-violet rays from a 

 mercury vapor arc lamp. The ultra-violet rays in sunlight are 

 nearly all absorbed in passing through the atmosphere. How- 

 ever, experiment 6 indicates that the quantity of these rays 

 reaching the surface of the earth at sea level (altitude of Berkeley 

 about 100 feet) is sufficient to have a slight effect in causing 

 negative phototaxis. In passing down through the water of a 

 lake the ultra-violet rays will be very rapidly absorbed. In 

 lakes at high altitudes, which receive more of these rays than 

 lakes at sea level, and in lakes whose water is very clear the 

 effects the negative phototaxis caused by these rays in the 

 bright part of the day might operate to keep the daphnids 

 down a short distance from the surface. 



Exp. 6 



Effect of ultra-violet rays on phototaxis 



April 13, 2 P.M., 13 Daphnia pulex from laboratory aquarium placed in an 18 

 cm. long glass dish marked off into five divisions. Temperature 21° to 22° C. 

 Exposed to sunlight with the ultra-violet rays cut off by one thickness of window 

 glass. 



