572 LECTURE XXL 



We have now to consider the effect of various external 

 conditions in abolishing the irritability or in arresting the 

 movements of motile organs. It was pointed out in the last 

 lecture (p. 548) that motile organs which are irritable to touch 

 do not respond by a movement to stimulation when they have 

 been exposed to the vapour of chloroform or ether, or when 

 they have been fatigued by repeated stimulation. The con- 

 dition of the organs under these circumstances is probably the 

 same : their cells, as Pfeffer has shewn, are fully turgid ; so 

 that the reason why stimulation is not followed by movement 

 is probably the same in both. From Bert's observation that 

 the daily periodic movements, and probably also the spon- 

 taneous movements, of the leaf of Mimosa are not arrested by 

 anaesthetics, it is clear that the absence of movement on stimu- 

 lation is not to be attributed to any interference with the 

 motility of the protoplasm of the organs, but to an abolition 

 of their irritability. Now the manifestation of irritability 

 depends, as we have seen (p. 371), in the presence of readily 

 decomposable substance, and secondly, upon the decomposi- 

 tion of this substance. In the case of the fatigued organ, the 

 loss of irritability is to be attributed to the absence of this 

 readily decomposable substance ; in the case of the chloro- 

 formed organ, to an inhibition of its decomposition. This 

 mode of regarding the effect of anaesthetics is supported by 

 Darwin's observation, which was mentioned in the last lecture, 

 that, after exposure to the vapour of ether, the leaf of Dioncea 

 muscipula did not close when the sensitive hairs were touched, 

 but did so under the stronger stimulus of cutting off the tip of 

 the leaf. 



It would appear, though the data are scanty, that the loss 

 of the power of movement which, as we have seen, is induced 

 when the supply of free oxygen to the motile organ is cut off, 

 is due to the same cause as the effect of anaesthetics. Thus 

 Kabsch observed that a leaf of Mimosa in vacua, which did 

 not move when touched, moved under the stronger stimulus of 

 an electric shock. 



It was further stated in the last lecture that the movements 

 of motile organs are arrested by such conditions as drought, 



