222 EFFECTS ON TROPISTIC RESPONSE 



These curvatures are explained, not as responses to the direct in- 

 fluence of the radium rays, but as due to chemical ions produced by 

 the rays in the liquid. It is probable that the exciting negative 

 cations, travelling faster than the depressing, positive anions from the 

 neighborhood of the tube toward the roots produce the curvature 

 toward the tube by stimulating growth on one side. By this hypoth- 

 esis the results are brought into harmony with those of Elfving and 

 Plowman. It also follows that we have not here evidence of a true 

 radiotropism. If such a tropism, either positive or negative, is pos- 

 sible, it has yet to be demonstrated. Theoretically, as pointed out 

 above, we are led to expect its discovery. 



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