352 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



Positive chemotaxis may so delay the osmotactic repulsion that it 

 can only become manifest with concentrated solutions, but when the 

 substance induces negative chemotaxis when dilute, repulsion may be 

 produced by solutions of less osmotic value than the cell-sap. Hence the 

 actual result may differ considerably from that which would be produced 

 by the osmotactic stimulus alone. It is, therefore, hardly surprising that 

 the concentrations of various substances required to produce perceptible 

 repulsion upon Bacteria and Flagellatae are not exactly isosmotic. That 

 other factors may come into play is shown by the fact that glycerine 

 usually exerts no action upon osmotactic organs 1 . Furthermore, the 

 power of osmotic response may itself be influenced indirectly by chemo- 

 tactic stimulation. 



After Engelmann 2 had recognized the repulsion exerted by oxygen Pfeffer found 

 that various substances were able to produce the same result 3 , and concluded that 

 the result was either due to negative chemotaxis or directly to the concentration. 

 Massart 4 then observed that a variety of substances in isosmotic concentration produce 

 about the same degree of repulsion. These results have only Jbeen obtained with 

 Spirillum undula and Bacillus megatherium 5 , but nevertheless they appear to apply to 

 other motile forms. The stronger repulsion produced by potassium cyanide, calcium 

 nitrate, &c., is due to their exerting in addition a strong negative chemotaxis. The 

 lessened repulsion exercised by saccharose and dextrose, and the inefficiency of gly- 

 cerine, are ascribed by Massart to their rapid penetration of the protoplasm preventing 

 the depression of turgor which operates as the exciting stimulus 6 . Although several 

 facts point to this conclusion, it remains to be seen whether all substances which 

 rapidly penetrate the protoplast are unable to exert any repulsive action. 



Experimental evidence is necessary to determine in what way the diminution or 

 cessation of repulsion is produced. Phobotactic reactions may in fact be excited and 



1 Pfeffer, 1. c., 1888, p. 626 ; Massart, 1. c., 1891, pp. 528, 559. 



* Engelmann, Pfliiger's Archiv f. Physiologic, 1881 ; Bd. xxvi, p. 541 ; Bot. Ztg., 1881, p. 442. 



3 Pfeffer, Ber. d. hot. Ges., 1883, p. 524 ; Unters. a. d. hot. Inst. zu Tubingen, 1884, pp. 385, 

 453; ibid., 1888, Bd. n, p. 621. Stahl (Bot. Ztg., 1884, p. 166) considers the repellent action of 

 sugar-solution upon the plasmodium of Atthalium to be directly due to the withdrawal of water. 



4 Massart, Arch, de Biologic, 1889, Bd. ix, p. 529. The Bacteria used responded chemotacti- 

 cally to most of the substances used, and hence would have shown the antagonism between attraction 

 and repulsion without the addition of the potassium carbonate used by Massart. 



5 Repulsion was attained by solutions isosmotic with a solution of from 0-005 to 0-006 of a gram- 

 molecule (I to | of a gram) of KNO 3 per litre. The Spirillum undula used by Massart is apparently 

 slightly different to that used by Pfeffer. Cf. Rothert, Flora, 1901, p. 413 footnote. 



6 Massart, 1. c., p. 528 ; Rothert, Flora, 1901, p. 409. According to.Miyoshi (Bot. Ztg., 1894, 

 P- I 7)> glycerine appears to exert no repulsion upon the hyphae of Fungi. [Assuming that the ecto- 

 plasmic membrane were the percipient organ for osmotactic stimuli, it could only be stimulated 

 when its inner and outer surfaces were exposed to differences of osmotic concentration, which could 

 only be maintained by non-penetrating or slowly-penetrating substances. It is difficult to see how 

 a general fall of turgor, operating equally on all sides could act as a directive stimulus. The neutral 

 action of glycerine is certainly not due to its exerting a positive chemotaxis and negative osmotaxis 

 which balance at all concentrations.] 



