44 



TROPISMS 



A similar mechanism of galvanotropic conduct exists 

 in Volvox a spherical, unicellular organism which is sur- 

 rounded by cilia on its whole surface. A definite pole 

 of the organism is always foremost in all locomotions. 

 This organism usually swims to the anode when in a 

 galvanic field. Bancroft made the action of the cilia of 

 Volvox visible with the aid of india ink and was able to 

 show that the current made the cilia on the anode side 

 stop, while those on the cathode side continue to beat. 20 



Fig. 15. — -a, current going from left to right through Paramecium, the position of 

 cilia on the cathode side is now reversed, their free ends pointing forward. The animal 

 when swimming is automatically turned with its oral end toward the cathode, b, current 

 going through Paramecium from aboral to oral end. Cilia symmetrical on both sides but 

 pointing forward at oral end and backward at aboral end. 



Since the backward stroke is always the effective one the 

 organism is thus carried automatically toward the anode. 

 Terry 478 found that Volvox can be made to move 

 toward the anode as well as toward the cathode. It moves 

 to the anode after having been kept in the dark for two 

 or three days, while after exposure to light it swims to 

 the cathode. Volvox contains chlorophyll and the change 

 in the sense of reaction is therefore connected with the 

 formation of a product of chlorophyll activity. Bancroft 

 found that when Volvox was made cathodic by exposure 

 to sunlight, the cilia stop on the cathode side. 



