PHOTOTAXIS IN VOLVOX. 



S. J. HOLMES, 

 FROM THE ZOOLOGICAL LABORATORY OF THE UNIVERSITY OF MICHIGAN. 



The reactions of Volvoxto light have been studied in some 

 detail by Oltmanns 1 who came to the conclusion that the photo- 

 tacti'C movements of this organism are determined through the 

 effort to reach a region of a certain optimum intensity of illumi- 

 nation. The theory at which Sachs and Strasburger had arrived 

 from their studies of the phototactic reactions of plants, and which 

 Loeb had applied to the phototaxis of animals, namely, that the 

 direction of the movement is determined by the direction of the 

 rays of light is not accepted by Oltmanns, but the experiments 

 upon which this writer bases his confutation of this theory do 

 not, I believe, bear out his interpretation. Oltmanns studied the 

 reactions of Volvox to light of varying intensity by keeping 

 specimens in a vessel so illuminated that the intensity of light 

 gradually increased from one end to the other. This was accom- 

 plished by covering the vessel with a box the top and ends of 

 which were made of wood while the sides were formed of nar- 

 row, wedge-shaped, hollow glass prisms filled with a mixture of 

 India-ink and gelatine. The small ends of the prisms allowed 

 most of the light to pass through, while the thick ends absorbed 

 most of the rays. The light entering the enclosed vessel was 

 obviously greatest opposite the thin ends of the prisms and 

 gradually diminished toward the opposite end. The Volvox 

 were found to assemble in different places in the vessel according 

 to the intensity of light which fell upon the lateral prisms. 

 With a comparatively dim outside light they would gather in the 

 brightest end of the vessel; in light of very strong intensity they 

 would seek the darkest end ; while with moderate illumination 

 they would take up some intermediate position. These results 

 were held by Oltmanns to indicate that the Volvo, seek the 

 places of optimum intensity of illumination and remain there, 



1 Flora, 1892, p. 183. 



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