CHEMOTROPISM 153 



in the opinion of the writer no connection with positive 

 chemotropism. d 



4. The method of Pfeffer cannot well be used for 

 larger organisms. Barrows 25 has devised an apparatus 

 which allowed him to test quantitatively the chemotropic 

 reactions of Drosophila. The flies which are positively 

 heliotropic were allowed to go to the light inside of a 

 narrow hollow groove. At a certain spot of the groove 

 two glass bottles were inserted with their openings oppo- 

 site each other, one of which contained the substance to 

 be tested for chemotropic efficiency, while the other served 

 as a control. The number of flies which on their path 

 were deviated by the bottle containing the substance to 

 be tested were counted and their number compared with 

 that going into the control bottle. The collection of odor- 

 ous matter in the groove was removed by suction. In this 

 way it was possible to ascertain that the flies are posi- 

 tively chemotropic to ethyl and amyl alcohol, acetic and 

 lactic acid, and to ether. The chemotropic effect of alcohol 

 was increased through the admixture of traces of an ester, 

 e.g., methyl acetate. 



In describing the manner of reaction of these flies, 

 Barrows makes the statement that when the odor is weak 

 the fruit fly ' ' attempts first to find the food by the method 

 of trial and error, but as the fly passes into an area of 

 greater stimulation, these movements give way to a direct 

 orientation. This orientation is a well defined tropism 

 response." A similar statement had been made by 



d Lillie also assumes that it is the intensity gradient which determines 

 the direction of motion in tropistic reactions. This is not correct, since posi- 

 tively heliotropic animals go to the light even if by so doing they have to go 

 from strong into weak light (see page 50). The direction of motion in 

 tropistic reactions is determined by differences in the mass of chemical 

 substances on both sides of a symmetrical animal. 



