656 Mast, Orientation in Euglena with some Remarks on Tropisms. 



of light energy received the less the deflection. Bancroft admits 

 that Euglena receives least energy when its dorsal and most when 

 its ventral surface is illuminated; therefore if the ventral surface 

 is illuminated after the direction of the rays is changed there 

 should be, according to this theory, an immediate response con- 

 sisting of a decrease in deflection. Failure to get such a response 

 can not be accounted for on the basis of time, for if the dorsal 

 surface becomes fully illuminated when the direction of the rays 

 is changed there is an immediate response. The same difficulty is 

 encountered in attempting to account for the orienting reactions 

 in Stentor (Mast, 1911, p. 117) on the basis of the continuous- 

 action theory. And in Volvox the discrepancy between the reac- 

 tions observed during the process of orientation and those demanded 

 by this theory is even more serious. In this organism in the 

 positive state it is found that an increase in light energy causes 

 an increase in the activity of the zooids, but in unoriented individuals 

 it causes, contrary to expectations on the basis of the theory in 

 question, the greater activity on the shaded side of the colonies. 

 This results in orientation. These phenomena together with others 

 seem to show that in Volvox activity is dependent upon the 

 absolute amount of light received, and orientation upon the time 

 rate of change of light, (See Mast, 1907, pp. 151 154; 1911, 

 pp. 140143.) 



I have elsewhere given the experimental results which 

 directly support the change-of-intensity theory of orientation in 

 Euglena. In my opinion if there is any evidence whatever which 

 militates against this theory it is found in certain cases presented 

 by Ban croft in regard to the correspondence between orientation 

 and shock-reactions; but as has been shown this evidence militates 

 against his own theory quite as seriously. If this evidence should 

 be confirmed under carefully controlled conditions, both theories 

 of orientation would have to be abandoned, and it might be 

 necessary to return to Sachs's very vague ray-direction theory. 



Let me state in concluding this section that I have never 

 maintained that the stimulating agent does not function in the 

 process of orientation in animals in accord with the continous- 

 action theory. In fact I believe that it does so function in some 

 animals. I have however persistently maintained and I still affirm 

 that there is no conclusive evidence in support of this conviction. 

 It has never been proved that this theory holds for the orientation 

 of any animals, with the possible exception of Eudendrium. Regard- 

 ing this organism, after making some very careful observations on 

 the process of orientation, I came to the following conclusion (1911, 

 p. 163): "In the orientation of Eudendrium it seems probable that 

 light acts as a constant directive stimulation." Loeb and Ewald 



