THE THEORY OF TROPISMS. IOI 



In the reactions of the ciliate infusoria to the constant electric current, 

 however, we have, if nowhere else, phenomena which show to a 

 certain extent clear-cut and undoubted agreement with the theory of 

 tropisms. To this agreement with the theory of tropisms much of the 

 widespread adherence to the tropism theory for reactions in general is 

 doubtless due. The reaction of infusoria to the electric current is con- 

 sidered a type for the other reactions of organisms. 



Yet, in deciding to what extent the theory of tropisms helps us to 

 understand the behavior of organisms, certain striking facts in regard 

 to the reaction to the electric current need to be taken into considera- 

 tion. These are the following : 



(1) The reaction to the electric current never takes place in nature. 

 As has been repeatedly pointed out, the electric reaction is a product 

 of the laboratory ; it is a reaction which the organism never gives 

 under normal conditions. This being true, it should not be made the 

 type for reactions in general unless it can be shown clearly that the 

 characteristic features in the effects of electricity on organisms are 

 present also in the effects of other agents. Otherwise we may fall into 

 the same error that would exist if we considered the contortions of a 

 person who had grasped the electrodes of a powerful battery as a type 

 of human behavior in general. 



(2) But examination shows that the characteristic features of the 

 effect of electricity on organisms are not present in the case of other 

 stimuli. The electric current, as the experiments of Kuhne (1864) 

 and Roux (1891) have shown, polarizes the cell. That is, it divides 

 it into halves, differing in chemical reaction. One half, in the case 

 described by Kuhne, had apparently an acid reaction, the other 

 half an alkaline reaction. In its effects on free-swimming organisms 

 a similar polarity is shown. In Paramecium, for example, the cilia 

 on one half of the body (where the current is entering) are caused to 

 take a certain position, while those on the other half (where the cur- 

 rent is leaving) take the opposite position. No other agent known 

 produces these polar effieCts, either chemically or in the effect on the 

 motor organs. Yet it is to exactly these effects that the orientation 

 which makes this reaction the type for the tropism theory is due. 



If other agents produce these effects why are they not known and 

 described ? There is no great difficulty in observing these effects with 

 the use of the electric current.^ Just as exact studies have been made 

 of the reactions to other stimuli ; yet, so far as I am aware, no one 

 has ever described any other stimulus as giving these characteristic 

 polar effects. On the contrary, the reactions to other stimuli are well 

 known not to show these characteristic phenomena. 



Since, therefore, the characteristic phenomena of the reaction to the 



