384 FRANK W. BANCROFT 



1. INTRODUCTION 



In 1904 Jennings published a paper in which he came to the 

 conclusion that Euglena, among other light sensitive protista, 

 did not react to light in accordance with the tropism theory, but 

 by the method of trial and error. This view has been criticised 

 by Holmes ('05), Torrey ('07) and Parker ('07, '11), none of whom, 

 however, investigated especially the behavior of the organisms 

 in question. On repeated occasions this view has been defended 

 by Jennings and also by Mast who has reinvestigated the reac- 

 tions of several Euglenae and of other protista studied by Jennings. 



For reasons which will become clear in later parts of this paper 

 it seemed important to compare galvanotropic and heliotropic^ 

 reactions in Euglenae. Accordingly cultures of Euglena were 

 prepared in a number of different media in order to detect galva- 

 notropism which has not yet been described for this genus. The 

 reactions to both light and electricity of the Euglenae from some 

 of these cultures were tested and a state of affairs was discovered, 

 which, so far as I can see, is entirely incompatible with Jennings's 

 ideas. For, instead of finding that, as Jennings supposed, the 

 heliotropic turning takes place by a series of motor reactions, or 

 that the heliotropic mechanism and the mechanism for the motor 

 reactions were fundamentally similar, it appeared that in Euglena 

 these two mechanisms are independent variables and can be modi- 



^Heliotropism will be used here as it is used by Loeb, and most authors who use 

 the word, to indicate a certain kind of reaction, entirely apart from the theory 

 which may be adopted to explain the way in which the reaction takes place. Loeb 

 makes his use of the term clear in many places. As an example may be quoted : 

 "Unter den Tropismen der Tiere verstehen wir die zwangsmassige Orientierung 

 gegen resp. die zwangsmassige Progressivbewegung zu oder von einer Energie- 

 quelle" ('10 p. 452). Jennings has finally ('09, p. 307) come to use the word in nearly 

 the same way that Loeb does, saying "The tropism includes those reactions in 

 which the organism takes and maintains a definite orientation." Mast, however, 

 ('11) has not distinguished between the descriptive use of the term and the various 

 theories and explanations that have been offered to account for it. Thus, in his 

 section on "Various Definitions of Tropisms" (p. 53) we find definitions of the 

 term and theories to account for the reaction inextricably mingled. In this paper, 

 then Heliotropism includes those reactions in which there is a compulsory orienta- 

 tion with respect to the light, no matter how that orientation may have been 

 brou.sht about. 



