Mast, Orientation in Euglena with some Remarks on Tropisms. 641 



tretenen Aufienfaktoren: Alkali und Athylalkohol aufgefnnden 



werden. 



Zum Schluss will ich nicht versaumen, Herrn Geheimrai Klebs 

 meinen herzlichsten Dank auch hier auszusprechen. Bei einer 

 miindlichen Besprechung dieser Arbeit hat Herr Geheimrat Klebs 

 mir vielfach Anregung und Anhaltspunkte gegeben, die zum grofien 

 Teil in obiger Untersuchung verwertet und festgehalten worden sind. 



Orientation in Euglena with some Remarks on Tropisms. 



By S. 0. Mast. 



From the Zoological Laboratory of the .lohns Hopkins University. 



In two very interesting papers Ban croft (1913) and Torrey 

 have taken exception to some of the conclusions reached by Jen- 

 nings and myself regarding the process of orientation, especially 

 in Enylena. The points of controversy raised by these authors 

 refer however only to matters of interpretation. Bancroft says 

 (p. 414), "The facts of light reactions of Euglena described by 

 Jennings and Mast have been confirmed in all cases in which 

 they were reinvestigated. No differences of opinion exist as re- 

 gards these facts"; and Torrey does not question the accuracy of 

 our observations. We may then assume that the processes of 

 orientation in so far as they have been actually observed occur as 

 we have described them (Jennings 1904, p. 49 59; 1906, p. 134 

 141; Mast 1911, p. 80 112). 



The points at issue may be treated unter three heads : A. The 

 nature of the stimulating agent which induces orientation ; B. The 

 trial and error theory; C. The definition of tropism. 



A. The Nature of the Stimulating Agent which induces 



Orientation. 



Jennings and the writer maintain that the orienting stimulus 

 in Euglena is dependent upon the time ratte of change of the 

 intensity of light on the sensitive tissue. Bancroft asserts that 

 he has proved that this is not true, and he holds that all of the 

 evidence at hand favors the idea that the stimulus in question is 

 dependent upon the continuous action of light in accord with the 

 Bunsen-Roscoe law. I shall demonstrate that if the experimental 

 evidence which Bancroft brings forth against our theory is valid, 

 it completely overthrows his own theory in so far as it has any 

 bearing on the process of orientation in Euglena. Before entering 

 upon this demonstration it will be necessary, however, to present 

 the chief characteristics of the theories in question. 



