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



reactions (increase in swerving) to changes of intensity in the 

 field. 



Aside from that concerning the lack of correlation between 

 orientation and shock-movement, Bancroft presents five lines of 

 evidence, some against the change-of-intensity theory, others for 

 the continuous-action theory. 



1. Bancroft found that if the position of the source of 

 illumination is very gradually changed after euglenae are oriented, 

 they gradually change their direction of motion and he concludes 

 (p. 407) that "In this way the most convincing demonstrations of 

 gradual orientation without any shock-movements or widening of 

 the spiral were possible." Is this demonstration really convincing? 

 A shock-reaction consists of an increase in swerving toward the 

 dorsal surface dependent upon the time rate of change of energy 

 received by the sensitive tissue. Bancroft admits that this in- 

 crease in swerving may be very slight. He says (p. 395): "In the 

 weakest shock-movements merely a slight temporary widening of 

 the spiral is seen", i. e. a slight increase in swerving toward the 

 dorsal surface. But this is precisely what occurred in Bancroft's 

 experiments when the euglenae changed their direction of motion 

 (oriented) after the position of the light was changed; for as I 

 have previously shown, change in the axial direction of the spiral 

 course, no matter how gradual it may be, can not .occur to any 

 appreciable extent without increase in swerving toward the dorsal 

 surface. It is obvious then that the evidence presented by Ban- 

 croft in this connection has no bearing whatever on the question 

 as to whether or not orientation is the result of shock-reactions. 



2. Bancroft asserts (p. 408) that the time between increase 

 and decrease of illumination in the sensitive tissue in unoriented 

 euglenae (due to rotation on the long axis) is much shorter than 

 that required for a shock-movement, increase in swerving tow r ard 

 the dorsal side due to change of intensity in the field; and that 

 consequently orientation cannot be due to the time rate of change 

 of intensity, but must be due to the continuous action of light. 

 But his theory as well as ours demands change of illumination on 

 the sensitive tissue due to rotation, and increase in swerving owing 

 to this change. It likewise demands increase in swerving if the 

 intensity of light in the field is decreased. Thus it is evident that 

 if the time between increase and decrease of illumination on the 

 sensitive tissue due to rotation on the axis is too short to induce 

 increase in swerving resulting in orientation in accord with the 

 change of intensity theory, it is also too short to produce increase 

 in swerving in accord with the continuous action theory. 



3. It is maintained by Bancroft (p. 411) that if, under certain 

 conditions, there are particles of the proper size in suspension 



