CHEMOTACTIC AND OSMOTACTIC REPULSION 351 



the capillary filled with the chemotactic solution, however concentrated this 

 may be. On the other hand, Spirillum undula and Bodo saltans are 

 repelled by solutions of neutral salts having an osmotic concentration 

 equivalent to from 0-5 to i per cent, of potassium nitrate. To produce 

 the same repulsion in the case of Trepomonas agilis and Spirillum volutans 

 requires a somewhat higher concentration, the organisms being less 

 sensitive *. 



Repulsion produced by very dilute solutions, as for example by acids, 

 can only be due to negative chemotaxis. Thus the presence of o-i per 

 cent, of citric acid is sufficient to overcome the chemotactic attraction 

 exerted by 0-19 per cent, of potassium chloride upon Spirillum imdtda, 

 and 02 per cent, of citric acid neutralizes the attraction of o-oi per cent, 

 of malic acid upon the antherozoids of Ferns 2 . Similarly the repulsion 

 exerted by dilute solutions of potassium cyanide, and of calcium nitrate 

 are really chemotactic in character 3 . The attraction of Spirillum by low 

 partial pressures of oxygen and its repulsion by high ones is obviously 

 a chemotactic phenomenon, and also affords a good instance of a reversal 

 of the reaction by increasing concentration 4 . 



A repulsion of Fern antherozoids is produced only by increasing 

 concentrations of free malic acid, and not by its salts. The effect produced 

 is, therefore, the resultant of the attraction exercised by the molecules 

 of malic acid and the repulsion due to the free hydrogen ions. This is 

 coupled with the fact that the full attraction of malic acid is produced 

 by very dilute solutions, whereas that of the hydrogen ions increases pro- 

 gressively up to a high limit. It does not, however, follow that every 

 chemotactic substance should produce repulsion when concentrated, or 

 that every negatively chemotactic substance should produce attraction 

 when sufficiently diluted. Thus free citric and hydrochloric acids always 

 repel Spirillum undula, and the antherozoids of Ferns. Similarly, the 

 smallest pressure of oxygen appears to produce repulsion in certain motile 

 anaerobic Bacteria. On the other hand, even 15 per cent, solutions of 

 cane-sugar do not repel the antherozoids of Mosses 5 . Presumably in 

 cases where the positive chemotaxis persists, the repulsion is due to the 

 fact that the negative osmotaxis increases more rapidly with rising con- 

 centration. 



developed. Thus cultures of Spirillum undula are sometimes found to be almost non-sensitive, and 

 Pfeffer (1. c., p. 614) observed distinct repulsion in the case of Polytoma uvella, although Massart 

 found this organism to be non-sensitive. 



1 Cf. Pfeffer, l.'c., pp. 601, 614, 626 ; Massart, 1. c. 



2 Pfeffer, 1. c., 1888, p. 627 ; 1. c., 1884, p. 387. 



* Massart, 1. c., 1889, p. 525. 



* Cf. for this and the following, Pfeffer, 1. c., 1888, p. 621 seq. 

 5 Pfeffer, 1. c., 1884, p. 432. 



