CHEMOTAXIS AND OSMOTAXIS 355 



than the power of movement. Rothert l appears to have obtained this 

 result by means of ether, which suppresses first the osmotactic and then 

 the chemotactic reactions, when applied in increasing concentrations. The 

 fact that a rise of concentration may convert attraction into repulsion is 

 also an instance of change of tone produced by demand. If the repulsion 

 is the result of the antagonism of positive chemotaxis and negative 

 osmotaxis, or of two opposed chemotactic actions as when malic acid 

 acts on Fern antherozoids, we have in both cases instances of the same 

 substance exercising two different stimulatory actions, of which one in- 

 creases more rapidly with concentration than the other. 



The lessened effect of increasing stimuli follows in approximate 

 accordance with the so-called Weber's law both in the case of the typical 

 chemotactic and the phobo-chernotactic reactions of bacteria and of anthero- 

 zoids. Probably also similar relationships will hold good for negative 

 osmotaxis. Furthermore, in the presence of two chemotactic substances 

 an organism may either be affected by each separately, or the two stimuli 

 may fuse to a single perception. 



SECTION 76. The Influence of Water. 



GENERAL ACTIONS. The power of movement, like that of growth, is 

 dependent upon the supply of water, and organisms become immotile or 

 sluggish in concentrated solutions without necessarily being killed 2 . Thus 

 Bacteria grow and form cilia in concentrated solutions, but these develop 

 no power of movement 3 . Similarly there must be a certain optimal 

 concentration for those forms which are unable to exist in dilute solutions 4 . 

 Pure water is, indeed, injurious to many forms. On the other hand, 

 streaming may continue in plasmolysed cells although more or less 

 retarded 5 . 



Sudden transference from dilute to concentrated solutions usually 

 causes disturbances of the power of movement, as for instance a temporary 

 cessation of the motion of cilia, or a partial stoppage of amoeboid move- 

 ment 6 . Sudden plasmolysis may produce a temporary stoppage of 



1 Jahrb. f. wiss. Bot, 1903, Bd. XXXIX, p. I. 



2 It has already been mentioned that salt-solutions repel Paramaecium. 



8 On ciliated epithelium see Engelmann, in Hermann's Handbuch fur Physiologic, Bd. I, p. 398. 



* The statements of Velten (Bot. Ztg., 1872, p. 649) and Dehnecke (Flora, 1881, p. 8) on the 

 optimal turgor for streaming have no value, since the other factors at work were insufficiently 

 considered. 



5 Dutrochet, Ann. d. sci. nat, 1838, 2 e ser., T. IX, p. 73 ; A. Braun, Verhandlg. d. Berl. Akad., 

 1852, p. 225 ; Nageli, Beitrage z. wiss. Bot., 1860, Heft ii, p. 75 ; M. Schultze, Protoplasma d. 

 Rhizopoden u. Pflanzenzellen, 1863, p. 41 ; Hofmeister, Pflanzenzelle, 1867, p. 52 ; Ewart, Proto- 

 plasmic Streaming in Plants, 1903, pp. 8-9. 



8 Stahl, Bot. Ztg., 1884, p. 166. 



A a 2 



