GEO TACTIC REACTIONS 337 



in capillaries filled with water and open at both ends. Schwarz also 

 observed an ascent through wet sand. Massart 1 found that Chromulina 

 Woroniniana behaves similarly at 15 to 20 C., but becomes positively 

 geotactic at 5 to 7 C. Changes of geotactic tone according to the 

 temperature may possibly be used by many motile organisms to enable 

 them to collect in zones at a suitable temperature, and changes of geotactic 

 tone do actually occur among Infusoria *. Aderhold found, however, that 

 Eiiglena viridis and Chlamydomonas pulvisculus remained negatively geo- 

 tactic at o C. 2 , while Schwarz could detect no geotactic reaction below 

 5 or 6 C. 3 



No geotaxis appears to be shown by Oscillariae and Diatomaceae, 

 although certain Desmids may possess this irritability to a feeble degree 4 . 

 Various Infusoria, including Polytoma uvella, afford instances of geotactic 

 non-chlorophyllous objects 5 . Massart found one species of Spirillum to be 

 negatively, and another species under the same conditions was found to be 

 positively geotactic 6 . Stahl 7 considers that the plasmodia of Myxomycetes 

 have no geotactic irritability, the creeping up to the surface of the sub- 

 stratum before fruiting being due to the change of the previous positive 

 into negative hydrotropism. Rosanoff and Baranetzsky 8 had previously 

 assumed the existence of negative geotaxis in plasmodia, but Strasburger 9 

 threw doubt upon this view. 



We can hardly speak of geotactic irritability when an organism rises 

 or sinks owing to autogenic changes of its specific gravity, or when the 

 position of the centre of gravity causes the axis of the organism and 

 the direction of movement to be parallel to the perpendicular 10 . That the 

 geotactic responses are not produced in this way is shown by the fact that 

 many zoospores with excentric centres of gravity show no geotaxis, while 

 Chromulina, according to its tone, is negatively or positively geotactic. 

 According to Jensen n , Euglena viridis, in virtue of the position of 

 its centre of gravity, would react positively, instead of being negatively 



1 Massart, Bull, de 1'Acad. royale de Belgique, 1891, 3 ser., T. xxil, p. 164; Sosnowsky, Bot. 

 Centralbl., 1901, Bd. LXXXVIII, p. 199. 



3 Aderhold, 1. c., p. 320. 



3 [In all cases the possibility of the existence of passive movements due to convection or 

 thermo-diffusion currents needs to be considered.] 



* Aderhold, 1. c., pp. 322, 359 ; Klebs, Biol. Centralbl., 1885, Bd. V, p. 360. 



5 Massart, 1. c., pp. 162, 166 ; Jensen, 1. c. ; Mendelsohn, Centralbl. f. Physiol., 1895, Bd. ix, 

 p. 374. In regard to other animals see Loeb, Centralbl. f. Physiol., 1891, p. 429 ; 1893, Bd. vil, 

 p. 304. 



6 Zikes, Centralbl. f. Bact., Abth. ii, 1903, Bd. XI, p. 59. 



7 Stahl, Bot. Ztg., 1884, p. 1 68. 



8 Rosanoff, Mem. de la Soc. de sci. nat. de Cherbourg, 1869, T. xiv, p. 149 ; Baranetzsky, ibid., 

 1876, Bd. xix, p. 322. 



9 Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, p. 71. 



10 Cf. Verworn, Psycho-physiol. Protistenstudien, 1889, p. 122. 



11 Jensen, Bot. Centralbl., 1893, Bd. LVI, p. 21. 



PFEFFER. Ill 



