3 i8 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



SECTION 68. The Influence of Illumination. 



Numerous plants and organs which develop in darkness also show 

 locomotion or streaming, while, even when normal development takes place 

 only in light, the power of movement is often retained for a long time, 

 or even until death ensues. This applies especially to streaming movements, 

 which usually appear in organs etiolated by development in darkness 1 . 

 The zoospores of Vaucheria and of other chlorophyllous plants are motile 

 even when formed in darkness 2 , and the period of swarming of asexual 

 zoospores is frequently prolonged in the absence of light. Thus Strasburger 3 

 found that when developed in darkness the zoospores of Ulothrix zonata 

 remained motile for over three days, and those of Haematococcus lacustris 

 for more than two weeks, whereas in favourable illumination the latter 

 more especially come to rest in a few minutes. This peculiarity is not 

 always so pronounced, but it aids in enabling the fixed form to be developed 

 where a suitable photic ration is assured. Many of the zoospores, in fact, 

 die in continued darkness without ever coming to rest and germinating. 



Apart from any transitory shock - effect, the activity of movement of 

 zoospores is not directly affected by the withdrawal of light, and the same 

 applies to streaming, when this is either normally present, or persists for 

 a long time when aroused by stimulation 4 . In all plants incapable of 

 indefinite existence in darkness, streaming is ultimately retarded more 

 or less, but only as the indirect result of the absence of light 5 , and the 

 same effect is shown among Oscillareae 6 and Volvocineae 7 . According to 

 Engelmann 8 , movement is excited in purple bacteria when they are 

 exposed to light, whereas they come to rest again in darkness or in constant 

 illumination. Winogradsky 9 observed, however, a continuance of the 

 movement in darkness, possibly as the result of racial or cultural peculi- 

 arities. According to Sorokin 10 , streaming ceases in the plasmodium of 

 Dictydium ambiguum in darkness, and is reawakened by illumination. 



1 Dutrochet, Ann. sci. nat, 1838, 2* sen, T. IX, p. 30 ; Nageli, Beitr. z. wiss. Bot., 1860, Heft ii, 

 p. 78; Sachs, Bot. Ztg., Beilage, 1863, p. 3; Hauptfleisch, Jahrb. f. wiss. Bot., 1892, Bd. xxiv, 

 p. aio ; Ewart, Journ. Linn. Soc., Vol. xxxi, 1896, pp. 564, 573 ; Josing, Jahrb. f. wiss. Bot., 1901, 

 Bd. xxxvi, pp. 198, 210. 



a Klebs, Die Bedingungen d. Fortpflanzung u. s. w., 1896, p. 19; Walz, Jahrb. f. wiss. Bot., 

 1866-7, Bd. v, p. 132. 



8 Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, pp. 27, 53. 



4 Nageli, 1. c., p. 102 ; Strasburger, 1. c., p. 27. On streaming cf. Hauptfleisch, 1. c. ; Josing, 

 1. c., p. 198. 



5 Ewart, Protoplasmic Streaming in Plants, 1903, p. 71. 



8 Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. vi, p. 31 ; Hansgirg, Bot. Centralbl., 1882, Bd. 

 XII, p. 361. 



7 Oltmanns, Flora, 1892, p. 196. 



8 Engelmann, Bot. Ztg., 1888, p. 663 ; Pfliiger's Archiv f. Physiologic, 1882, Bd. xxx, p. 103. 



9 Winogradsky, Beitr. z. Morphol. u. Physiol. d. Bact, 1888, p. 90. 

 10 Sorokin, Bot. Jahresb., 1878, p. 471. 



