RESPONSIVE CURVATURES — NEGATIVE GEOTROPISM 503 



native hypotheses will lie in determining which factor is 

 actively concerned in the production of responsive curvature. 

 If the lower should prove to be the side actively concerned, 

 then the response will be one of expansion ; the activity of the 

 upper, on the other hand, would demonstrate the contractile 

 nature of the response. 



The principle of the mode of investigation which I adopted 

 in order to decide this question will be understood, if we 

 remember that motile response can be abolished temporarily 

 by application of cold. Thus, if we cool the pulvinus of 

 Mimosa, it ceases to exhibit any responsive contraction 

 under stimulation. Now, in a horizontally placed organ, if 

 the continued responsive curvature be due to the excitatory 

 contraction of the upper side, then local application of cold 

 on that side ought to arrest it. The application of cold 

 on the lower side, however, should produce little effect. But 

 if, on the other hand, the lower side should be actively con- 

 cerned in the production of response, then the application 

 of cold on that side would have the effect of arresting 

 the curvature, while its application on the upper would 

 have little or no effect. In connection with this, it should 

 be remembered that a twitch may sometimes be produced 

 by the transient excitation due to sudden variation of tem- 

 perature, but the effect will be short-lived. The permanent 

 arrest of hitherto continuous responsive movement due to 

 gravitational stimulus will only take place when the active 

 side has its power of response abolished by cold. 



An experiment carried out in this manner would thus 

 decide the question as to whether it is the upper or the lower 

 side that is actively concerned, and also the question as to 

 whether the gravitational response, like all other forms of 

 response to direct external stimulus, is, or is not, funda- 

 mentally one of contraction. 



In carrying out experiments on the principle described 

 above, I first took a record of the responsive curvature of a 

 Crinum Lily, which was lying horizontally. When a uniform 

 rate of upward movement had been attained, the tip of the 



