326 LOCOMOTORY AND PROTOPLASMIC MOVEMENTS 



exist, for in certain cases clear and definite orienting responses seem to be 

 given. 



Myxomycetes. Stahl and Baranetzsky only observed negative photo- 

 taxis in the plasmodia of Myxomycetes, and it is possible that they respond 

 to the intensity of the light rather than to its direction. It is, however, 

 doubtful whether very feeble light induces positive phototaxis, as Hofmeister 

 states l . The plasmodia of Acrasieae appear also to be phototactic 2 . 

 Strasburger 3 suggests that a change of phototactic tone may be responsible 

 for the upward movement during fruiting, but since this movement is also 

 shown in darkness other directive factors must enter into play which are 

 able to overcome the negative phototaxis induced by exposure to light. 



Coloured light. Phototaxis, like phototropism, is mainly or entirely 

 excited by the more refrangible rays. Strasburger 4 was in fact unable to 

 detect any phototaxis in zoospores exposed to red or green light, whereas 

 the maximum action is shown in the indigo region of the spectrum. The 

 same applies to Englena according to Engelmann 5 , and to Diatoms accord- 

 ing to Verworn 6 , whereas all the visible rays of the spectrum appear to act 

 as phototactic stimuli to Oscillaria. Since similar peculiarities exist in the 

 case of colourless organisms 7 , it does not follow that the general response 

 of Oscillaria is due to the special absorptive activity of the phycocyanin. 

 Various instances are known among animals in which the more refrangible 

 rays are not the most active in phototaxis, and to our eyes the yellow rays 

 are brighter than the blue and red. 



In the case of purple bacteria, however, the most pronounced phobic 

 action is exercised by the ultra red rays 8 , and owing to the association of 

 bacterio-purpurin with the chlorophyll of these organisms, these are also 

 the rays which are most readily absorbed. On the other hand, the photo- 

 tactic movements of plasmodia 9 and of chloroplastids are mainly produced 

 by the more refrangible rays, and according to Josing it is the blue and 

 not the green or red rays which excite streaming in etherized cells 10 . 

 Similarly, the injurious action of intense light is mainly due to the more 

 refrangible rays, and the disorganization observed by Borsfow and Luerssen n 



1 Hofmeister, Pflanzenzelle, 1867, p. 20; Allgemeine Morphologic, 1868, p. 625; Baranetzsky, 

 Mem. de la Soc. d. sci. nat. de Cherbourg, 1876, T. xix, p. 328 ; Stahl, Bot. Ztg., 1884, p. 167. 



2 Olive, Proceedings of the Boston Society of Natural History, 1902, Vol. xxx, p. 485. 



3 Strasburger, Wirkung des Lichts u. d. Warme auf Schwarmsporen, 1878, p. 70. 



4 Strasburger, 1. c., p. 44. Cf. also Cohn, Bot. Ztg., 1867, p. 171. 



5 Engelmann, Pfliiger's Archiv f. Physiol., 1882, Bd. XXIX, p. 398. 



5 Verworn, Psycho-Physiologische Protistenstudien, 1889, p. 49. When oxygen is deficient the 

 organisms may collect in the red and yellow regions of the spectrum where photosynthesis is most 

 active. Cf. Engelmann, 1. c., p. 390. 



7 Cf. Nagel, Bot. Ztg., 1901, Ref., p. 293. 



8 Engelmann, Bot. Ztg., 1888, p. 677. 8 Baranetzsky, 1. c., p. 331. 

 Josing, Jahrb. f. wiss. Bot., 1901, Bd. XXXVI, p. 208. 



11 Borsdow, Bull, de 1'Acad. de St. Pe'tersbourg, 1868, T. XII, pp. 211, 230; Luerssen, Einfluss d. 



