TROPIC ACTION OF LIGHT ON FREELY MOTILE ORGANISMS 325 



Saprolegnia and of many colourless Flagellatae are not phototactic *. The 

 same applies to the antherozoids of Ferns 2 , whereas those of Sphaeroplea 3 

 and Fncus^ respond readily to light, although Bordet 5 obtained negative 

 results in the latter case. 



All chlorophyllous Diatoms appear to be phototactic 6 , but not the 

 colourless forms 7 . The orienting action is feeble, however, so that 

 the oscillating forms pursue irregular paths towards or from the light. 

 The negative phototaxis is shown with light of moderate intensity, and 

 commonly causes the Diatoms to creep into the mud. Chlorophyllous 

 Oscillariaceae place themselves parallel to the incident rays, and creep 

 towards the light even when it is moderately intense 8 . 



Desmids. Most motile forms show phototactic reactions, although 

 these are often feeble 9 , and the irritability may vary in different individuals 

 of the same species. Hence Aderhold and Stahl found that intense 

 illumination caused them in most cases to show negative phototaxis, whereas 

 Klebs could only detect positive phototaxis even on re-e-xamining the same 

 species. Such forms as Pleurotaenium, Micrasterias, and Penium respond 

 especially well and glide slowly to or from the light according to its inten- 

 sity. Such forms as Closterium moniliferum and other species of the same 

 genus which regularly turn over and attach the free end to the substratum 

 continue the same movement when exposed to light, but then progress to or 

 from the source of illumination as the case may be. 



According to Stahl and Aderhold the long axis is approximately parallel 

 to the direction of the light during positive, but at right angles to it during 

 negative phototaxis. Klebs, however, doubts the existence of any such 

 orientation, and, according to Braun 10 , the younger end of Penium cur turn is 

 always turned towards the light. Probably various grades of irritability 



1 Strasbnrger, 1. c., p. 18; Cohn, Bot. Ztg., 1867, p. 178; A. Fischer, Jahrb. f. wiss. Bot., 1882, 

 Bd. XIII, p. 297 ; Kolkwitz, 1. c. In regard to animals see J. J. Loeb, Der Heliotropismus d. Thiere, 

 1890; Verworn, Psycho-physiolog. Protistenstudien, 1889, p. 35; Herbst, Biol. Centralbl., 1894, 

 Bd. xiv, p. 659; Jourdan, Die Sinne u. die Sinnesorgane d. niederen Thiere, 1891; Nagel, Der 

 Lichtsinn augenloser Thiere, 1896. 



1 Pfeffer, 1. c., p. 372. 3 Cohn, Ann. sci. nat., 1856, 4" sen, T. v, p. 201. 



4 Thuret, Ann. sci. nat., 1854, 4* ser., T. n, p. 210. 



5 Bordet, Bull, de 1'Acad. royale de Belgique, 1894, 3 e ser., T. xxvn, p. 894. Cf. Winkler, 

 Ber. d. bot. Ges., 1900, p. 304. 



6 Cohn, Jahrb. d. schles. Ges. f. vaterl. Guitar, 1863, p. 102 ; Bot. Ztg., 1867, p. 171 ; Stahl, 

 Bot. Ztg., 1880, p. 400; Verworn, 1. c., p. 46. 



7 Benecke, Jahrb. f. wiss. Bot., 1900, Bd. XXXV, p. 554. 



8 Dutrochet, Mem. d. vegetanx et d. animaux, Bruxelles, 1837, p. 340; Famintzin, Jahrb. f. 

 wiss. Bot., 1867-8, Bd. vr, p. 27 ; Hansgirg, Bot. Centralbl., 1882, Bd. XII, p. 361 ; Verworn, 1. c. 

 p. 50. 



9 Stahl, Bot. Ztg., 1880, p. 392 ; Verhandlg. d. physik.-med. Ges. zu Wiirzburg, 1879, ^. F., 

 Bd. xiv ; Klebs, Biol. Centralbl., 1885, Bd. v, p. 353 ; Aderhold, Jenaische Zeitschrift f. Naturw. 

 1888, N. F., Bd. xv, p. 323. 



10 Braun, Verjiingung in d. Natur, 1851, p. 217. 



