Mast, Orientation in Euglena with some Remarks on Tropisms. (J45 



in the stimulating agent is the controlling factor, thus still retain- 

 ing the central feature of Sachs's theory, that is, that the 

 direction auf the rays of force is all important. This theory 

 has always been "pure theory"; it has never had any positive 

 experimental support. I mention it here because I wish to refer 

 to it later. 



(c) Change-of-Intensity Theory. The kernel of this 

 theory consists of the conception that the stimulus resulting in 

 orientation is dependent upon the time rate of change of energy 

 received by the sensitive tissue, not upon the absolute amount as 

 is true for Bancroft's continuous-action theory. That there are 

 reactions which are thus dependent upon the time rate of change 

 has long been known, but Darwin seems to have been the first 

 to hold that orientation may result from reactions thus produced. 

 He says (1880. p. 566), "We believe that this case [referring to an 

 experiment of Wiesner on orientation in plants], as well as our 

 own [observations on orientation], may be explained by the ex- 

 citement from light being due not so much to its actual amount, 

 as to the difference in amount from that previously received". As 

 to how in the process of orientation the stimulus, caused by change 

 of intensity, acts, and as to the nature of the mechanism involved, 

 Darwin does not express an opinion. 



Engelmann (1882, p. 395) foreshadows the application of this 

 principle to account for orientation in motile forms, especially 

 Euglena. Jennings (1904, p 43, 59 63) was however the first 

 to analyse thoroughly by direct observation the process of orien- 

 tation in unicellular organisms, and to present evidence which 

 seemed to prove that in certain cases this process is the result of 

 stimuli produced by change of intensity; that is, that orientation 

 is dependent upon the time rate of change of energy on the sensitive 

 tissue. Jennings also at the same time (1904) concluded that 

 orientation in these forms is indirect ; i. e. that it results from the 

 successive assumptions of several different axial positions and the 

 retention of one of these. 



The results of my extensive studies of the process of orien- 

 tation in unicellular organisms, especially Euyleita, colonial forms 

 and others, support Jennings's conclusions regarding the nature 

 of the process of orientation and the character of the stimulus - 

 particularly the latter. 



It is the two conclusions stated by Jennings that Bancroft 

 andTorrey claim to have overthrown in so far as they apply to 

 the process of orientation in Euglena. What is the evidence upon 

 which this is based? We shall concider the question as to the na- 

 ture of the stimulus first. 



