CHAPTER V 



OBSERVATIONS ON UNICELLULAR FORMS IN THE PROCESS 

 OF ATTAINING AND RETAINING A DEFINITE AXLA.L 

 POSITION WITH REFERENCE TO THE 

 SOURCE OF LIGHT 



I. Myxomycetes and Rhizopods 



All the Rhizopods and the plasmodia of Myxomycetes 

 that are known to react to Hght are negative, as was shown 

 by Baranetzsky (1876, pp. 328, 340), Stahl (1884, p. 167), 

 Engelmann (1879, p. 3), Davenport (1897) and others. 

 The contention of Hofmeister (1867, p. 20) that plasmodia 

 are positive in Hght of very low intensity has not been 

 confirmed. 



Davenport (1897, p. 186) exposed specimens of Amoeba 

 proteus under a compound microscope in a small horizontal 

 beam of direct sunlight with all other light intercepted by 

 means of opaque screens and found that they orient directly. 

 They make no preliminary trial movements in this process. 

 If the direction of the rays is changed they always turn from 

 the source of light at once, never toward it. There is no 

 evidence of selection of random movements in these animals. 

 The same is probably true in case of other Rhizopods and 

 Myxomycetes, although there are no investigations which 

 bear directly on this point. Baranetzsky (1876) found that 

 even a slight increase in illumination causes a distinct 

 retardation in streaming movements of Myxomycetes. 

 Engelmann (1879) observed that light thrown upon a 

 pseudopod of Pelomyxa palustris causes it to be withdrawn 

 suddenly. Harrington and Leaming (1900) found that a 

 sudden increase in light intensity causes a retardation in 

 the movement of Amoeba. Ewart (1903, p. 69) says that 

 protoplasmic streaming in cells in general is retarded by 



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