The Reversal of Tropisms 107 



these forms are not so much the direct effect of re- 

 actions to light as the result of the way in which 

 light alters responses to gravity. Daphnias swim 

 upward in the dark, but when illuminated they swim 

 vigorously downward regardless ordinarily of the 

 direction in which light falls upon them. 



In many Rhizopods the thigmotactic response is 

 positive to weak contact stimuli and negative to 

 strong ones. Similar relations are found in pla- 

 narians. Striking cases of the reversal of electro- 

 taxis have been described in a number of diverse 

 types. Volvox, which normally swims toward the ca- 

 thode, may be caused to swim to the anode by keep- 

 ing it for two or three days in the dark. Reexpo- 

 sure to light soon brings about the normal response 

 (Terry). Similar reversals of electrotaxis have 

 been observed by Moore and Goodspeed to be pro- 

 duced in Gonium by either acids or alkalies. And 

 Bancroft ( i ) has shown that Paramcecium, which 

 usually swims to the cathode, may be caused to swim 

 to the anode by various salts of sodium and potassi- 

 um. Certain salts were found to cause Paramoecium 

 to swim backwards to the anode, a reaction differing 

 from true positive or anodal electrotaxis in that the 

 organisms swim backward instead of forward. 



The reader who has had the patience to follow 

 the discussion thus far will appreciate not only how 

 wide spread is the reversal of tropic reaction, but 

 how varied are the causes by which reversal may 

 be produced. Moreover it is difficult to make any 



