HELIOTROPISM IN EUGLENA 415 



I think it may be said, then, that the main facts concerning 

 heliotropic orientation are: 



1. The locomotor mechanism of Euglena is of such a nature that 

 the organism always swims in a spiral and responds to all effective 

 stimuli by additional swerving towards the dorsal side. It cannot 

 turn in any other direction. 



2. The mechanism for the gradual heliotropic orientation is 

 different from and is independent of the mechanism that produces 

 the differential light reactions. 



3. This heliotropic mechanism, as we may call it, is of such a 

 nature that, in unoriented negatively heliotropic Euglenae, it 

 stimulates the flagellum when the dorsal side of the organism is 

 turned from the light. In other words, we must suppose that the 

 heliotropic mechanism like the galvanotropic mechanism is more 

 easily excited when^^ the light is coming from one direction. In 

 the case of negative Euglenae the stimulation takes place most 

 easily when the light shines on the ventral surface; in positive 

 individuals when the light shines on the dorsal surface. 



It is to be noted that in this method of heliotropic orientation 

 there is nothing of the nature of the selection of random move- 

 ments. When the spiral swimming brings the ventral surface 

 of negative Euglenae towards the light stimulation occurs and 

 the plant turns from the light. In other positions this response 

 cannot take place, hence the turning is always away from the 

 light and orientation is thus accomplished. The reaction is per- 

 fectly definite and stereotyped. It contains nothing of the nature 

 of 'trial and error' if by 'trial and error' is meant anything more 

 than the normal spiral swimming by means of which the ventral 

 surface is repeatedly brought to face the light. On the contrary, 

 it is perfectly clear that during the gradual heliotropic turning 



1^ Note that I do not say by light coming from one direction. It may be that 

 the difference between positive and negative heliotropism is that in one case the 

 light causes stimulation and in the other it prevents stimulation. There are many 

 other hypotheses possible wich are capable of experimental verification. Since the 

 experiments are lacking the description of the heliotropic mechanism must be 

 formulated in the most general way, and such a formulation does not mean any 

 more than it says. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 15, NO. 4 



