62 



TKOPISMS 



3. HELIOTROPISM OF UNICELLULAR ORGANISMS 



In unicellular organisms, where cilia act as locomotor 

 organs, it can easily be shown that the orientation by 

 light is of the nature of changes in the position of cilia; 

 this is for instance the case in respect to V'cfyox. 

 Holmes 226 states for the heliotropic reactions of this 

 organism, that they are due to differences in the activity 

 of the cilia on both sides of the organism and this ex- 

 planation agrees with the actual observations of Bancroft 

 on the galvanotropic reactions of Volvox. 



In flagellates, the mechanism of locomotion is very 

 complicated and does not consist in an oar-like action of 

 a flagellum as was formerly assumed. Bancroft has shown 

 that in Euglena, as already stated, the flagellum inserted 

 at the anterior end of the organism is bent backward in 

 the form of an inverted U, and that locomotion is brought 



about by the formation of a 

 loop which travels from the 

 base of the flagellum toward the 

 free end (Fig. 23). The path 

 of the organism which results 

 from this action is a spiral with 

 continual rotation of the organ- 

 ism around its longitudinal 

 axis. Bancroft has shown that 

 the behavior of the organism 

 under the influence of light is 

 identical with that in a constant 

 galvanic field. 21 One-sided illu- 

 mination as well as a current going transversally through 

 such an organism cause changes in the position of cilia 

 comparable with those observed in the legs of crustaceans, 

 insects, and vertebrates. 



V 



FIG. 23. Diagram showing the 

 position of the flagellum as seen in a 

 viscid medium, a, when Euglena is 

 swimming forward in a narrow spiral; 

 6, when swerving sharply toward the 

 dorsal side; c, when moving backward. 

 (After Bancroft.) 



