350 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



a particular substance might unavoidably bring about a sensitivity to more 

 or less closely allied substances. In this way we can understand how it is 

 that, for instance, Bacteria are able to react to salts of rubidium or to 

 aniline blue, and the spermatozoa of Ferns 1 to salts of rubidium and 

 caesium, substances to which under natural conditions they are never called 

 upon to respond. 



The physiological action of a substance is of course dependent upon its 

 chemical properties, but we are unfortunately unable to deduce from these 

 chemical properties why one substance should be especially active but 

 another less so. In the case of compounds which dissociate in watery solu- 

 tions it needs to be determined how far the stimulation is due to the free ions, 

 and how far to the undissociated molecules. The antherozoids of Ferns 

 respond equally well to free malic acid and to its salts, whereas sodium and 

 ammonium chlorides as well as the non-dissociating diethylester of malic 

 acid are inactive. Hence the stimulating action is due to the malic acid 

 ions. In the same way it can be determined that the repelling action of 

 acids is due to the hydrogen ions, and that the intensity of action is pro- 

 portional to the degree of dissociation 2 . 



SECTION 75. Chemotactic and Osmotactic Repulsion. 



Chemotactic attraction is due to the chemical properties of the 

 stimulating substance, but it is also possible that certain organisms may 

 possess a power of positive osmotactic response to differences of osmotic 

 concentration in the surrounding medium. Massart ascribes to this cause 

 the passage of certain marine Bacteria and Flagellatae from very dilute solu- 

 tions to sea-water, and similarly Stahl has observed that the plasmodia of 

 Myxomycetes may creep from a dilute to a more concentrated solution 

 of sugar 3 . 



Many organisms show negative osmotaxis with high concentrations, 

 and hence whenever increasing concentration produces repulsion it needs 

 to be determined whether this is due to negative osmotaxis or chemotaxis, 

 or to their conjoint action. In some cases no repulsion appears to occur, 

 as for example in the case of Bacterium termo 4 , a marine Spirillum 5 , 

 Polytoma uvella, Euglena viridis, and various flagellate and ciliate In- 

 fusoria 6 . In all such cases the organisms swim without any check into 



1 Buller, Annals of Botany, 1900, Vol. xiv, pp. 571 and 572. 



2 Cf. Buller, Annals of Botany, 1900, Vol. XIV, p. 543. 



3 Massart, Bull, de 1'Acad. royale de Belgique, 1891, 3 ser., T. xxii, p. 152; Stahl, Bot. Ztg., 

 1884, *p. 166. The proof of the absence of chemotaxis is by no means sure in either case. 



4 Pfeffer, Unters. a. d. bot. Inst. zu Tubingen, 1888, Bd. II, p. 626. 



5 Massart, 1. c., p. 153. 



6 Massart, Archives de Biologic, 1889, T. IX, p. 560. The power of reaction is not always fully 



