TROPISMS AND RELATED PHENOMENA 



141 



2. GALVANOTROPISM 



When animals are exposed to a galvanic current, compulsory re- 

 actions may occur which agree with the compulsory reactions produced 

 by light, with the difference that we have to substitute the current 

 curves for the light rays. When parallel current curves strike a sym- 

 metrical organ or organism sidewise, the contractile elements, e.g. 

 muscles, on one side of the organ, or organism, undergo a higher degree 

 of tension than on the other side; the outcome is a bending or turning 

 of the organ or animal until its axis, or plane of symmetry, is in the 

 direction of the current curves. As soon as this occurs, the symmet- 

 rical elements of the surface of the body are struck at the same angle 

 by the current curves and the kind and acceleration of chemical re- 

 action is the same on both sides of the organism; consequently the 

 symmetrical muscle elements show the same state of contraction. But 

 the fact that the current curves penetrate throughout the whole animal 

 causes often complications which prevent an ideal orientation such as 

 we observe in the case of light. 



A most striking case of galvanotropism was found recently by Ban- 

 croft * in Polyorchis penicillata, a Medusa. "The method of experi- 

 mentation consisted in cutting the Medusa in various ways, and placing 

 the pieces in a trough of sea water through which the galvanic current 

 was conducted with non-polarizable electrodes. The current strength 

 varied from 25 to 2008. The responses were usually distinct with 

 258, but became more decided as the current was increased. 



"If a meridional strip (Fig. 28) passing from the edge on one side 

 through the center of the bell to the other edge be prepared and the 

 current passed through transversely, tenta- 

 cles and manubrium turn and point toward 

 the cathode (Fig. 28). A reversal of the 

 current initiates a turning of these organs 

 in the opposite direction, which is usually 

 completed in a few seconds. This can be 

 repeated many times and the tentacles 

 continue to respond after hours of activity. 

 The manubrium, however, tires sooner and 

 fails to respond. If the strip is placed 



with its subumbrella surface upward and extended in a straight line 

 parallel to the current lines (Fig. 29), the making of the current 

 causes the tentacles at the anode end to turn through an angle of 



* F. W. Bancroft, Jour. Exper. Zool., Vol. I, p. 289, 1904. 



FIG. 28. AFTER BANCROFT. 



