562 LECTURE XXI. 



movements of motile organs whatsoever. Thus, in the case 

 of the leaf of Dionaea, there is evidence that stimulation sets 

 up currents of water. It was mentioned, when we were dis- 

 cussing the electrical phenomena presented by plants, that 

 stimulation of the leaf of Dionaea is followed by a well- 

 marked positive variation of the normal electrical condition 

 of the upper and lower surfaces of the leaf, which indicates 

 that currents of water are travelling from the upper towards 

 the lower surface. It is worthy of note that this electrical 

 disturbance is not dependent upon the actual movement, but 

 only on stimulation : it is well marked when the stimulated 

 leaf is mechanically prevented from moving. 



Such being the effect on the motile organ of the action 

 of a stimulus, the process of recovery must consist in the 

 restoration of the turgidity of the flaccid cells by the absorp- 

 tion of water, accompanied by a gradual assumption of the 

 position occupied before stimulation. The process of re- 

 covery is slow and gradual, as may be easily observed by 

 stimulating a leaf of Mimosa and noting the time which 

 elapses before it regains its original position. In the case of 

 the primary petiole it will be observed that the upward move- 

 ment begins almost immediately after the fall on stimula- 

 tion, and that the time occupied in regaining the normal 

 position is about twenty or twenty-five minutes. 



In endeavouring to penetrate into the intimate mechanism 

 of the movements of variation, the question which at once 

 arises is as to how exactly the sudden abolition of the tur- 

 gidity of the irritable cells which stimulation induces is 

 brought about, and here we find ourselves face to face with 

 the real difficulty of the subject. Let us call to mind what 

 the factors of the turgid condition are, and then endeavour to 

 determine which of them is affected on stimulation. The 

 factors, as we learned in a previous lecture (p. 42), are three : 

 the osmotically active substances in the cell-sap : the elas- 

 ticity of the cell-wall : the resistance to the escape of the cell- 

 sap offered by the layer of protoplasm lining the cell-wall. 



Taking, now, these factors one by one, if we assume that 

 the stimulus affects the first of them, we must conceive, as 



