320 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



A sufficient increase in the intensity of the light always produces 

 a retardation of locomotion and streaming 1 . By localized action local 

 retardations or interruptions of streaming may be caused 2 , as well as various 

 protoplasmic deformations or vacuolation 3 . Owing to the fact that the 

 plasmodia of Myxomycetes are killed by exposure to light of moderate 

 intensity, even feeble illumination distinctly retards the amoeboid activity of 

 these organisms, and hence also the streaming of the endoplasm 4 . Similarly, 

 after several hours' exposure to continuous direct sunlight, streaming ceases 

 or becomes extremely slow in Elodea and Chara^ but may become active 

 again in feeble light 5 . Other instances in which submaximal intensities 

 of illumination produce a retardation of movement will probably be 

 discovered. 



A striking instance of shock stimulation is afforded by Bacterium 

 photometricum and other purple bacteria, which perform a pronounced 

 backward movement when the illumination suddenly decreases, but not 

 when it increases. No fatigue is shown in this case, however rapidly the 

 stimulation is repeated 6 . According to Engelmann 7 , Bacterium photo- 

 metricum, which Winogradsky 8 considers to be a small Chromatium, also 

 performs a shock - movement when the percentage of carbon dioxide 

 suddenly alters, but not when the air is suddenly replaced by hydrogen. 

 In the case of the Rhizopod Pelomyxa palustris Engelmann 9 found that 

 sudden illumination causes the pseudopodia to be rapidly withdrawn, and the 

 plasmodia of Myxomycetes seem to react much in the same way although 

 more feebly. A shock-movement is, on the other hand, produced in many 

 lower animals by the sudden withdrawal of light 10 , and, to a slight extent, 

 also in the zoospores of Botrydium granulatum n . In the case of the zoo- 

 spores of Bryopsis plumosa, however, sudden illumination produces a tem- 

 porary irregularity of movement. In many other organisms and zoospores 

 Strasburger could detect no perceptible photic shock-effect, and sudden 



1 Cf. Ewart, Protoplasmic Streaming in Plants, 1903, p. 69; Annals of Botany, 1898, Vol. xil, 

 pp. 383-9- 



3 Pringsheim, Jahrb. f. wiss. Bot., 1879, Bd. XII, pp. 334, 367. 



3 Klemm, ibid., 1895, Bd. xxvni, p. 647. 



4 Baranetzsky, Me"m. de la Soc. d. sci. nat. de Cherbourg, 1876, T. xix, pp. 328, 340; Hof- 

 meister, Pflanzenzelle, 1867, p. 21. Cf. also Lister, Annals of Botany, 1888-9, Vol. ill, p. 13. 



5 Ewart, Protoplasmic Streaming in Plants, 1903, p. 70. 



6 Engelmann, Pfluger's Archiv f. Physiologic, 1882, Bd. XXX, p. 103 ; Bot. Ztg., 1888, p. 666 ; 

 Winogradsky, Beitrage z. Morphol. u. Physiol. d. Bacterien, 1888, p. 95. 



7 Engelmann, 1882, 1. c., p. 112 ; 1888, 1. c., p. 689. 



8 Winogradsky, Bot. Ztg., 1888, p. 90. 



9 Engelmann, Pfluger's Archiv f. Physiologic, 1878, Bd.xix,p. 3; Blochmann, Biol. Centralbl., 

 1894, Bd. xiv, p. 85. 



10 See Loeb, Pfluger's Archiv f. Physiol., 1897, Bd. LXVI, p. 459; Nagel, Bot. Ztg., 1901, 

 Ref., p. 289. 



11 Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, p. 25; Stahl, Bot. 

 Ztg., 1880, p. ^\o>\Eugknd}. 



