THE INFLUENCE OF TEMPERATURE 317 



to exercise a different effect to exposure to either extreme, but whether 

 this applies generally is uncertain. Similar reactions are, however, shown 

 by the plasmodia of Myxomycetes, in which moderate changes of tempera- 

 ture induce a temporary tendency to assume a spheroidal shape l . Possibly 

 also sudden changes of temperature may produce shock- movements in 

 many plant-zoospores. At least when suddenly exposed to high tempera- 

 tures they dart actively in all directions, like ants disturbed in their nest 2 . 



THERMOTAXIS. 



Paramoecium and other Infusoria are strongly thermotactic, being 

 positively so up to a certain temperature, beyond which they swim towards 

 the colder zones (negative thermotaxis) 3 . De Wildeman 4 ascribes posi- 

 tive thermotaxis to Euglena, not only in water, but also when on wet sand, 

 and this irritability may possibly be possessed by many free-swimming 

 plant- organisms, although the evidence brought forward by Schenk 5 is 

 unsatisfactory. Stahl 6 has, however, shown that the plasmodium of 

 Aethalium septicinn moves towards the warmer side, when resting on a strip 

 of wet filter-paper, one end of which lies in water at 30 C. and the other in 

 water at 7C. According to Wortmann 7 , the movement is reversed and 

 becomes negatively thermotactic when the temperature on one side rises 

 above 36 C. 



In creeping organisms a reaction of this kind may be of great utility, 

 whereas small free-swimming plants are likely to have their thermotactic 

 tendencies overcome by the convection currents set up by the difference of 

 temperature. This is, however, not the case where it is the surface layers 

 which become warmer, so that a thermotactic irritability is most likely 

 to occur in strongly motile surface organisms found in ponds exposed to 

 full insolation. It is evident that the slow response of plasmodia cannot 

 be phobic in character, but this does not necessarily apply to free-swimming 

 organisms, which may be capable of either thermotactic or thermophobic 

 responses. 



1 Kiihne, Unters. ti. d. Protoplasma, 1864, p. 87. 



2 On the influence of temperature on pulsating vacuoles and nuclear division cf. Matruchot et 

 Molliard, Rev. ge"n. de Bot, 1903, T. xv, p. 193. 



3 Mendelssohn, Pfliiger's Archiv f. Physiol., 1895, Bd. LX, p. i ; Zeitschrift f. allgem. Physiol., 

 1902, Bd. II, p. 38. 



4 De Wildeman, Bot. Centralbl., 1894, Bd. LX, p. 176. 



6 Schenk, Centralbl. f. Bact., 1893, Bd. xiv, p. 37. Beyerinck (ibid., 1894, Bd. XV, p. 799) 

 observed that Bacterium Zopfii spread on gelatine to the warmer side, because growth and repro- 

 duction are more rapid in that direction. [Zikes, Centralbl. f. Bact., 1903, Abth. ii, Bd. XI, p. 59.] 



6 Stahl, Bot. Ztg., 1884, p. 174. See also Clifford, Annals of Botany, 1897, Vol. xiv, p. 179. 



7 Wortmann, Ber. d. bot. Ges., 1885, p. 117. A negatively thermotactic reaction was observed 

 by Verworn~(Psycho-physiolog. Protistenstudien, 1889, p. 63) in the case of Amoeba. 



