324 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



that it is the intensity of the light and not its direction which acts as the 

 orienting agency. 



Zoospores. Various groupings were observed by Colomb and by Olivi *, as well 

 as by Nageli, Cohn, and Famintzin 2 , but up to the time of Stahl 3 and Strasburger 

 insufficient attention was paid to the mechanical influence of currents in the water. 

 Sachs 4 has, in fact, shown that the slight warming due to unilateral illumination 

 causes currents sufficient to produce special grouping of non-motile drops of oil sus- 

 pended in a mixture of alcohol and water. The movements are, however, largely 

 due to the changes of surface-tension as the alcohol evaporates, and if pure water is 

 used and evaporation checked the streaming in the liquid is much feebler 6 , and is 

 insufficient to prevent the normal phototactic orientation 6 . Thus, in a mixture of 

 dissimilar zoospores, one kind may show a negative, the other a positive action, while 

 non-reacting or dead forms become uniformly distributed, or at least not definitely 

 grouped. Active living zoospores, however, assume a suitable position in one or more 

 minutes. 



Most chlorophyllous zoospores such as those of Ulothrix zonata^ Ulva, 

 Enteromorpha, Bryopsis plumosa, Scytosiphon lomentarius, as well as Euglena 

 and other green Flagellatae, and the unicellular and colonial Volvocineae, show 

 various grades of phototactic irritability 7 . The zoospores of Vaucheria 8 , as 

 well as the small yellow zoospores of Bryopsis plumosa, but not the large 

 green ones, are irresponsive to light. According to Thuret, the zoospores of 

 Codium tomentosum and Ectocarpus firmus hardly show any phototactic 

 irritability, although the colourless zoospores of Chytridium vorax and Poly- 

 phagus euglenae are strongly phototactic 9 , and the same applies to one 

 species at least of Bodo 10 . A phototactic irritability will obviously aid the 

 zoospores of parasites in seeking out regions where their hosts live, but 

 chemotactic stimuli are even more effective, and hence the zoospores of 



1 Usteri, Annal. d. Botanik, 1793, Stuck VI, p. 30. 



8 For the literature see Strasburger, Wirkung d. Lichts u. d. Warme auf Schwarmsporen, 1878, 

 p. i. 



3 Stahl, Bot. Ztg., 1878, p. 715 ; Verhandlg. d. physik.-med. Ges. zuWUrzburg, 1879, Bd - XIV 

 p. 7. 



4 Sachs, Flora, 1876, p. 241. 



6 Berthold, Protoplasmamechanik, 1886, p. 113. 

 8 Cf. Strasburger, 1. c., pp. 6-8. 



7 Strasburger, I.e.; Stahl,!. c.; Famintzin, Jahrb. f. wiss. Bot., 1867-8, Bd. vi, p. i; Woronin, 

 Bot. Ztg., 1880, 629 (Chromophytori) ; Berthold, Fauna u. Flora des Golfs von Neapel, 1882, p. n; 

 Pfeffer, Unters. a. d. bot Inst. zu Tubingen, 1884, Bd. i, p. 443; Overton, Bot. Centralbl., 1889, 

 Bd. xxxix, p. 68; Oltmanns, Flora, 1892, p. 187 (Volvox]\ Kolkwitz, Bot. Centralbl., 1897, Bd. 

 LXX, p. 187; Holmes, ibid., 1903, Bd. xcm, p. 18 (Volvox). According to Borzi (Bot. Jahresb., 

 1883, Bd. i, p. 26), the zoospores of Enteromorpha compressa lose their phototactic irritability on 

 copulation. 



8 Thuret, Ann. sci. nat., 1850, 3 se>., T. xiv, p. 246; Woronin, Bot. Ztg., 1869, p. 139; 

 Strasburger, 1. c., p. 42. 



9 Strasburger, 1. c., p. 18. Cf. also Kolkwitz, 1. c., p. 187. 

 10 Rothert, Flora, 1901, p. 372. 



