56 RESPONSE IN THE LIVING AND NON-LIVING 



a common effect of magnetisation is to produce an 

 elongation of an iron rod. But Bidwell finds that as 

 the magnetising force is pushed to an extreme, at a 

 certain point elongation ceases and is succeeded, with 

 further increase of magnetising force, by an actual con- 

 traction. Again a photographic plate, when exposed 

 continuously to light, gives at first a negative image. 

 Still longer exposure produces a positive. Then again 

 we have a negative. There is thus produced a series of 

 recurrent reversals. In photographic prints of flashes 

 of lightning, two kinds of images are observed, one, the 

 positive when the lightning discharge is moderately 

 intense and the other, negative, the so-called ' dark 

 lightning ' due to the reversal action of an intensely 

 strong discharge. 



In studying the changes of conductivity produced in 

 metallic particles by the stimulus of Hertzian, radiation, 

 I have often noticed that whereas feeble radiation pro- 

 duces one effect, strong radiation produces the opposite. 

 Again, under the continuous action of electric radiation, 

 I have frequently found recurrent reversals. 1 



Diminution of response under strong stimulus traced 

 to fatigue. But there are instances in plant response 

 where the diminution effect can be definitely traced 

 to fatigue. The records of these cases are extremely 

 suggestive as to the manner in which the diminu- 

 tion is brought about. The accompanying figures 

 (fig. 34) give records of responses to increasing stimulus. 

 They were made with specimens of cauliflower-stalks, one 

 of which (a) showed little fatigue, while in the other (b) 



1 See ' On Electric Touch,' Proc. Roy. Soc. Aug. 1900. 



