TACTIC RESPONSE TO TROPIC STIMULI 309 



chemo-phobotaxis we may indicate an irritability by which an organism 

 is able to avoid or to remain in solutions of chemical substances owing to the 

 backward shock-movement produced on entering or leaving them as the 

 case may be *. In many cases the exact nature of the response is uncertain, 

 and in others tropic and phobic actions may co-operate in producing the 

 result observed. 



In the case of small and active organisms it is difficult to determine 

 whether a tactic or a phobic response is given, for during chemotactic 

 attraction the individuals do not all travel along straight paths to the 

 capillary containing the exciting substance, while at its mouth and within 

 it the forms move about in the same way as organisms attracted in a phobic 

 manner. Hence it was only after careful study and after using slowly 

 moving forms that Jennings and Crosby were able to show that the 

 attraction of Bacteria by chemical substances was the result of a phobic 

 action, although Engelmann had previously shown that the attraction of 

 Bacterium photometricum to illuminated areas was produced in this manner 2 . 

 The phobic reaction and accumulation of various Infusoria and Flagellatae 

 were demonstrated by Jennings 3 , and were confirmed by Garrey 4 before the 

 chemophobic responses of Bacteria were investigated. 



It is possible that in many cases the same agency may excite a feeble 

 phobic and a strong tactic, or a strong phobic and a feeble tactic response. 

 This may explain the backward movement of the strongly chemotactic 

 antherozoids of Ferns when they attempt to enter a capillary filled with 

 a solution of malic acid. The phototactic zoospores of Botrydium also 

 appear to be weakly photophobic 5 , and some species of Bacteria may 

 possess a strong power of chemotactic response in spite of Rothert's con- 

 clusions as to the general chemophobic reaction of Bacteria. 



If a chemophobic action is always exercised when the organism 



1 [There seems to be no reason for adopting the terms topotropism and topotaxis, as suggested 

 by Pfeffer, to indicate the typical orienting movements, since the term ' phobism ' put forward by 

 Massart, Centralbl., 1902, Bd. XXII, p. 49, suffices to distinguish these special forms of tropic and 

 tactic irritability from the more general case. It is still possible to use the term ' tropism ' in the 

 general sense (cf. Bot. Ztg., 1902, Referate, p. 17) instead of restricting it in the way that Massart 

 (1. c., p. 49) and Nagel (Bot. Ztg., 1902, Ref., p. 24) do. Rothert's term ' apobatic' (Flora, 1901, 

 P- 393) is b otn uncouth and unnecessary, nor can his term of ' strophotaxis ' be adopted, since 

 ' strophism ' has already been used in an equally superfluous way to indicate movements produced by 

 torsion. The error arises in supposing that a dissimilar response necessarily indicates a totally distinct 

 form of irritability, and hence needs a new term, or that phenomena are made simpler or more easy to 

 understand by giving them a classical terminology. The same applies to the use of the term ' argo- 

 taxis' (apyus, passive) to indicate purely physical, passive movements due to surface-tension, like those 

 of a drop of oil in a soap-solution. In any case Nagel (Bot. Ztg., 1901, p. 297 ; 1902, Ref., p. 24) 

 is in error in considering that phobic reactions alone arise from a special discriminatory sense.] 



3 Engelmann, Pfliiger's Archiv f. Physiologic, 1882, Bd. xxx, p. 95; Jennings and Crosby, 

 American Journal of Physiology, 1901, Vol. vi, p. 29; Rothert, Flora, 1901, Vol. VI, p. 29. 



s Jennings, American Journal of Physiology, 1899, Vol. II ; 1900, Vol. III. 



4 Garrey, Centralbl. f. Physiol., 1900, Bd. xiv, p. 105. 



5 See the literature quoted by Rothert, 1. c., p. 386. 



