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



action and concludes that orientation in light must consequently 

 also be a function of its continuous action. I do not agree with 

 him in this conclusion, for I hold that the assumption upon which 

 it rests is unwarranted. In the case of Euglena, contrary to what 

 holds under certain conditions for some other organisms, the gal- 

 vanic orienting stimuli may be due for all that is known to the 

 contrary, to the time rate of change of electric energy received 

 by the sensitive tissue as the unoriented organism revolves on its 

 axis, and if this is true it does not support Bancroft's conclusion 

 regarding the action of light. 



5. As a final argument in favor of Loeb's continuous-action 

 theory, Bancroft attempts to controvert the evidence which I 

 have presented to demonstrate that this theory will not account 

 for orientation in fire-flies. I found (1912, pp. 270, 271) that the 

 flashes of light produced by certain fire-flies serve as signals by 

 means of which the opposite sexes are brought together for the 

 purpose of copulation. If a male is anywhere between one and 

 six hundred centimeters from a female when she produces as flash 

 of light, he usually turns and flies directly toward her. This flash 

 of light is very short and it usually if not always disappears entirely 

 before the male starts to turn. Thus in total darkness he turns 

 through the proper angle and proceeds sometimes as far as several 

 meters, directly toward the point w 7 here the female produced 

 the glow. 



There are three points in connection with this process of orien- 

 tation that I wish to emphasize. (1) The male fire-fly will not 

 orient in continuous illumination. To induce orientation it is 

 necessary to produce a light, leave it a certain definite period of 

 time, and then extinguish it. If it is not extinguished or if it is 

 left too long there is no response. (2) The length of the period 

 of illumination necessary to induce orientation is approximately 

 the same regardless of the distance between the male and the 

 female. (H) The male fire-fly not only orients in total darkness, 

 but after orientation he continues on a direct course. 



In agreement with Loeb, Bancroft holds that the Bun sen- 

 Rose oe law affords the best criterion for testing the validity of 

 the continuous-action theory. He holds that the results of my 

 experiments on fire-flies are in harmony with this law, and he con- 

 sequently maintains that they do not militate against the continuous- 

 action theory. 



It has been found that the plumules of certain plants (Avena, etc.) 



orient if exposed to a strong light for only a very short period 



1/800 of a second or even less, and that these reactions are at 



least superficially in accord with the Bunsen-Roscoe law, i. e., 



