146 



TROPISMS 



a neutral medium. In this way a collection of the organ- 

 isms at the surface of a drop of acid may be brought 

 about. This phenomenon is not tropistic in the strict 

 sense of the word, and as a matter of fact Paramcecium is 

 not positively chemotropic to acid of any strength. 



Barratt 2 ^ investigated the chemotropism of Para- 

 mcecia for varying concentrations of different acids with 



Distilled Water 



HCL 

 O.OOOln 



Hay Infusion 



FIG. 42. Method of proving that Paramacia are not positive to acids of low concentration. 



(After Barratt.) 



Pfeffer's method of capillary tubes, counting the number 

 of individuals going into the tube containing acid and 

 comparing it with the number going simultaneously into 

 a control tube containing only distilled water free from 

 C0 2 (Fig. 42). b The acids used varied from 0.001 N to 

 0.0001 N. The results were unequivocal. Toward solu- 

 tions of 0.001 N the Paramcecia are negative and possibly 



b In addition two other controls accompanied the test, namely, one tube 

 containing hay infusion (the natural medium of the organisms) and one 

 alkali. 



