Xl] INSECTIVOROUS PLANTS 117 



one pulvinus to another in regular order. The pheno- 

 menon is an irritable one, and we must suppose that the 

 determining cause lies in the irritability to contact or 

 shock of the living protoplasm of the cells touched : the 

 excited protoplasm then expels water from the cells, on 

 the one hand, and transmits the stimulus, on the other, 

 but we are as yet totally ignorant of the details of the 

 mechanism by which either of the consequent phenomena 

 is brought about. 



The same is true of the sensitive movements, similar 

 in principle but differing in detail, which result when the 

 filaments on the leaves of Dioncea or Drosera are touched : 

 the effect of the contact is seen in the rapid snapping 

 together of the two halves of the lamina (Dioncea) or the 

 slower incurving of the capitate filaments (Drosera). 

 These latter examples, however, are evidently connected 

 with the special functions, as insect-traps, of the leaves of 

 these insectivorous plants ; whereas we do not know what 

 advantages are secured to the leaves of the Sensitive 

 Plant, &c., by the irritable leaf-movements. 



So far we have sketched the principal movements of 

 leaves, some of which result in placing the lamina in such 

 a position that its surface is fully exposed to the light 

 and air surrounding it. We have now to trace the 

 chief events which follow as it performs its principal 

 function. 



In the first place it must be understood that the 

 living leaf, thus exposed to the environment, affects ex- 

 changes with the latter ; and since its fluids are continuous 

 with the fluids in the rest of the plant, there exist at 

 least the conditions for exchanges with these also. As 

 matter of experiment, we know that both events are 

 accomplished, and it remains to show how this takes 

 place in view of the organisation of the leaf, and its 

 adaptations to this end. 



