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



Our conclusion as to the nature of the orienting stimulus is 

 founded upon the following facts concerning which there is no 

 contention: 1. A sudden decrease in illumination of the field induces 

 in positive euglenae an increase in swerving toward the eye-spot. 

 This may be marked or only very slight. (See Bancroft, p. 395.) 

 It is known to depend upon the time rate of change, for if the 

 decrease in illumination is gradual enough this phenomenon does 

 not occur. We shall refer to it as a shock-movement or reaction. 

 2. The orienting deflections are, as far as can be observed, of 

 precisely the same nature as weak shock-movements. 3. There is 

 no response under certain conditions if the ventral surface of a 

 Euglena faces the light after the direction of the rays has been 

 changed, until in the process of rotation the surface containing the 

 eye-spot comes to face the light; then there is a sudden turning 

 toward this surface, i. e , toward the source of light. In many 

 instances the turning is so sharp immediately after the dorsal sur- 

 face becomes illuminated that it may appropriately be designated 

 as a jerk or a twitch. This is in opposition to the demands of 

 the continuous-action theory, as we shall see later. 4. After orien- 

 tation fire-flies may continue for considerable distances on a direct 

 course in darkness. The same is true for Euglena, according to Ban- 

 croft (p. 411), and it is probably also true for other organisms. 

 It is therefore not necessary to postulate continuous action of the 

 orienting stimulus to account for the direct movement toward the 

 source of stimulation after orientation. 



If I understand the matter correctly our critics do not deny 

 that shock-reactions may sometimes function in orientation. Torrey 

 says (1914, p. Ill), "[In Euglena] the 'motor reflex' [shock-reaction] 

 plays an important part in its orientation to light". Bancroft 

 however maintains that he has proved that orientation is not in 

 all cases due to shock-reactions. But while he admits that he 

 was unable to prove that it is in such cases due to continuous 

 action of light in accord with the continuous-action theory of orien- 

 tation, he concludes (p. 425), "In the case of Euglena we have 

 seen that what evidence we have is all in favor of the view 

 advocated byLoeb that the heliotropism [orientation] is a function 

 of the continuous action of the light". 



Let us examine the evidence. This Bancroft has presented 

 under eight heads. According to our theory (change-of-intensity 

 theory) orientation in Euglena, as previously stated consists of a 

 series of shock-reactions. In positive individuals these reactions 

 are supposed to be due to sudden decrease and in negative ones 

 to sudden increase of light intensity caused in unoriented individuals 

 by rotation on the long axis. Bancroft therefore maintains that 

 if a positive Euglena orients in a given condition of illumination,, 



