THE ELECTRO-MOTIVE RESPONSE OF PLANTS 21 



second to stimuli twice as strong. We may here see how 

 response is increased by increased intensity of stimulus. 

 One peculiarity to be noticed in this figure is the trend of 

 the base-line downwards, showing the increasing positivity 

 of the pulvinus. In order to obtain a photographic record 

 the experiment had to be carried out in a dark room, and 

 under these circumstances the pulvinus undergoes an increase, 

 or positive variation, of turgidity. And we shall see later 

 that a positive turgidity variation is associated with galvano- 

 metric positivity in the same way as the negative turgidity 

 variation is found to be accompanied by galvanometric 

 negativity. 



In consequence of the impression produced by the con- 

 spicuous movements of the leaf ofMtm0sa,it was assumed that 

 only plants showing such movements were to be regarded as 

 excitable. I have already shown elsewhere, however, that 

 this test of lateral motile responses, as a sign of sensitiveness, 

 is fallacious in the extreme. Such mechanical display is 

 possible only when the two halves of an organ are unequally 

 contractile, and there is consequently a greater expulsion of 

 water from one, in response to stimulus, than from the other. 

 If these conditions are not fulfilled, even the so-called 'sensi- 

 tive ' Mimosa would appear to be insensitive. Thus, when 

 we place a cut branch of Mimosa in water, the pulvini of the 

 leaves, on account of vigorous suction at the cut end, are 

 rendered over-turgid, and the leaves become highly erected. 

 On now applying stimulation, no responsive fall is found to 

 take place ; this is due to the difficulty encountered in the 

 expulsion of water from the gorged tissue. An intact plant, 

 again, which in the light has been found highly sensitive, 

 will often be found insensitive after a short time spent in a 

 dark room. It will then be difficult to be'lieve that the plant 

 is of the sensitive class, for the hardest blow will fail to 

 evoke any mechanical response. And not only does the 

 Mimosa cease to show responsive movement under these cir- 

 cumstances, which may perhaps be regarded as exceptional ; 

 but under perfectly normal condition? also, its sensitjvenes? 



