Mast, Orientation in Euylena with some Remarks on Tropisms. 653 



euglenae give the shock-reaction provided the light is so weak 

 that they do not orient; but if the illumination is increased until 

 they orient they no longer respond to contact with the particles. 

 He holds that the change of intensity theory can not account for 

 this, since, according to this theory, the orienting stimulus is 

 supposed to cease after the organisms is oriented. Consequently 

 he thinks, if this theory is valid, the euglenae ought to respond 

 to contact with particles after orientation quite as freely as 

 before. 



These results would be very serious indeed for the change-of- 

 intensity theory if this theory maintained that light acts only in 

 producing shock-reactions. I am, however, not aware that anyone 

 ever held such a view. Engelmann (1882 and 1883), more than 

 thirty years ago showed very clearly that the activity of Euylena, 

 and particularly of Bacterium photometricum, depends upon the 

 amount of light energy received, and that this is in all probability 

 not dependent upon the time rate of change of energy. And long 

 before this it was fairly well known that photosynthesis is related 

 to light in the same way. The change in the sign of orientation 

 also appears to be independent of the time rate of change of energy. 

 Light, as I have repeatedly stated (1907 and 1911), in all probability 

 has an effect on physiological processes (activity, etc.) in organisms, 

 an effect which bears a definite relation to the amount of energy 

 received, which is somewhat like the relation between such pro- 

 cesses and heat energy. This the advocates of the change-of-inten- 

 sity theory do not deny. They merely hold that orientation is 

 not due to such effects of light. 



The fact that Bancroft obtained reactions to contact with 

 particles in light of low intensity and none in light of high inten- 

 sity was probably due to changes in the physiological state of the 

 organism and not to differences in orientation, as he assumed. 

 This phenomenon is in full accord with the wellknown fact that 

 when organisms are simultaneously subjected to stimuli of different 

 sorts they may respond to any given one very differently than 

 when they are acted upon by that stimulus alone. The reactions 

 in question, consequently do not militate against the change-of- 

 intensity theory. 



4. In his very interesting experiments on galvanic reactions 

 Bancroft states that he found that orientation in an electric cur- 

 rent occurs precisely as does orientation in light as described by 

 the present writer; and he holds that there is no contrast between 

 these reactions in Euglena such as J en n ing's maintains there is 

 for other infusoria. 



I agree with him in this conclusion. He assumes however 

 that orientation in the electric current is regulated by continuous 

 XXXIV. 42 



