CHEMOTROPISM 147 



also slightly negative to acids as weak as 0.0001 N. In no 

 case, not even with the weakest acid, was it possible to 

 prove the existence of positive chemotropism for acid 

 (or base). The number of Paramcecia which went into a 

 tube containing, e.g., 0.00002 N acid, was on the average 

 not greater than that which went into the control tubes. 

 The tubes were sufficiently wide so that the Param&cia 

 could and did move into the tubes. Barratt, therefore, 

 concludes that acids have only a repelling action upon 

 Paramcecia which, however, diminishes or disappears 

 when the hydrogen ion concentration approaches that of 

 distilled water. 



The observations of Barratt contradict the statement 

 that Paramcecia are positive to weak acid. We have seen 

 that when spermatozoa or swarmspores are positive to 

 malates this can be elegantly shown by Barratt 's method. 

 The same method has shown that when even a trace of acid 

 is added to the neutral malates this positivity disappears. 

 By testing systematically all concentrations of different 

 acids within the range to be considered, Barratt found no 

 trace of any positivity to or any trap action by weak acid 

 for Paramcscia. It may be true, however, that when the 

 organisms are in very dilute acid neutral or faintly alka- 

 line water repels them in the way described by Jennings. 



Barratt states also that there is nothing to support Jen- 

 nings 's assertion that the C0 2 given off by the Paramcecia 

 causes the aggregation in their natural medium, since 

 they are not positive to low concentrations of hydrogen 

 ions. The natural aggregations of infusorians may be 

 due, as Pfeffer suggested, to transitory agglutinations 

 when Paramcecia impinge upon each other, and the sticki- 

 ness or tendency to agglutinate may possibly be increased 



