Mast, Orientation in Euglena with some Remarks on Tropisms. 659 



ing the use of the term in question? Is it practicable and ser- 

 viceable? 



Bancroft's definition of tropism is no doubt in harmony with 

 the ideas expressed by Loeb in the statement quoted above, but 

 this statement contains only a part of Loeb's ideas concerning the 

 use of this term and in presenting only this isolated selection it 

 seems to me that Bancroft has unjustly misrepresented Loeb's 

 views. Loeb says (1912, p. 38), "Two factors govern the pro- 

 gressive movements of animals (in light) . . .; one is the symmetrical 

 structure of the animal, and the second is the photochemical action 

 of light". If an animal is illuminated laterally, the light intensity 

 received by the two eyes differs. Thus it is supposed to cause, 

 through direct nervous connection, a difference in the tension of 

 the muscles of the locomotor appendages on the two sides resulting 

 in unequal rates of movement on these sides and eventually in 

 orientation. The stimulating agent is supposed to act continuously, 

 after orientation as well as during the process of orientation. Only 

 those processes of orientation \vhich are brought about in this way 

 are considered to be tropisms. Bancroft maintains that Loeb 

 does not exclude from tropisms orientations due to stimuli depen- 

 dent upon the time rate of change of energy received by the sensi- 

 tive tissue. He says (p. 418) "Loeb does not think, as has been 

 assumed by some writers, that if a tropism is shown to be due to 

 differential sensibility that by definition it ceases to be a tropism". 

 Loeb however has stated in unequivocal terms that he does exclude 

 such reactions. He says (1903, p. 135), "Heliotropism covers only 

 those cases where turning to the light is compulsory and irresistible, 

 and is brought about automatically or mechanically by the light 

 itself. On the other hand, there are compulsory and mechanical 

 reactions to light which are not cases of heliotropism; namely, the 

 reaction to sudden changes in the intensity of light ... In the 

 former case the results are a function of the constant intensity, 

 in the latter a function of the quotient of the change of intensity 

 over time". Thus, according to these statements, Loeb considers 

 as tropisms only those processes of orientation which are due to 

 the continuous action of the stimulating agent on the sensitive 

 tissue, symmetrically located on opposite sides of the body. He 

 excludes all those processes of orientation which are due to stimuli 

 dependent upon the time rate of change of energy in the sensitive 

 tissue and all those which may be due to differential response to 

 localised stimulation. Bancroft does not exclude these. His 

 definition is in harmony with only one of the various features 

 found in Loeb's. 



The definitions of both of these authors however appear to 

 me to involve, contrary to Bancroft's opinion, an explanation of 



