Mast, Orientation in Euglena with some Remarks on Tropisms. (J51 



the theory upon which he relies to explain them, i. e., the con- 

 tinuous-action theory. 



According to both of these theories, as I have previously 

 stated, orientation is accomplished by increase in deflection on the 

 side of the spiral course facing the light, i. e., toward the dorsal 

 surface of the organism, the surface containing the eye-spot. In 

 an unoriented Euylena, as it proceeds on the spiral course rotating 

 on its long axis, the dorsal and ventral surfaces alternately face 

 the light. According to both theories the sensitive tissue in the 

 organism receives least light energy when the dorsal surface, the 

 surface bearing the eye-spot, is fully illuminated. In case of posi- 

 tive orientation it is only when the organism is in this position 

 that the increase in swerving resulting in orientation occurs. 

 According to the change-of-in tensity theory the increase in swerving 

 is dependent upon the time rate of change in the energy received 

 by the sensitive tissue; and according to the continuous-action 

 theory it is dependent upon the absolute difference in the amount 

 of energy received in different positions of the spiral. A change 

 in the amount of light energy in the field which necessarily would 

 result in a change of light energy on the sensitive tissue should 

 then, in positive organisms, result in increase in swerving; i. e., 

 in a wider spiral, on the basis of Bancroft's theory as well as 

 on the basis of ours. 



Bancroft says (p. 421) that in negative euglenae the spiral 

 becomes narrower, the deflection toward the dorsal surface diminishes 

 when the light is decreased, and that it becomes wider when it is 

 increased, i. e., the deflection increases. According to Bancroft's 

 theory then there ought to be the same correspondence between 

 orientation and reactions to changes of intensity in the field as is 

 demanded by the change-of-intensity theory. In case of positive 

 orientation a decrease in the intensity of the field ought to result 

 in an increase in swerving toward the dorsal surface; and in case 

 of negative orientation an increase in the intensity ought to induce 

 the same response; and of course orientation ought never occur 

 under conditions in which a change of intensity in the field will 

 produce no increase in swerving. Moreover in positive organisms 

 an increase of intensity ought to cause a decrease in swerving, 

 and in negative organisms a decrease ought to produce the same 

 response. But this is precisely what Bancroft maintains he has 

 proved does not occur. Thus he has, if his results are valid, 

 overthrown his own theory. Even the assumption that the lack 

 of correspondence between orientation and shock-reactions is due 

 to the dominance of the latter over swerving due to "continuous 

 action" would not save his theory, for this would in no way answer 

 all those cases in which individuals that oriented gave no shock- 



