564 LECTURE XXI. 



have been impossible. There is, on the face of it, no reason 

 whatever to believe that when a portion of the cell-sap of a 

 cell is forced out of it by pressure, that that portion should 

 consist of pure water, and as to the fact which Pfeffer brings 

 forward in support of his view, it may be equally well ac- 

 counted for by the assumption that osmotically active sub- 

 stances may have been formed in sufficient quantity in the 

 cells to make good the loss and to restore the cells to their 

 turgid condition. In fact, there is some reason to believe 

 that one of the effects of stimulation is to lead to the forma- 

 tion of osmotically active substances. From these consider- 

 ations it is clear that the case has not been made out in 

 favour of the view that the effect of stimulation on a motile 

 organ is to destroy the osmotically active substances in the 

 sap of its cells, or to diminish their osmotic activity. 



If we assume, now, that the effect of stimulation is to 

 lead to an increase of the pressure of the cell-wall upon the 

 cell-contents, it is clear that can only be brought about by an 

 increase in the rigidity of the cell-wall. Such a suggestion is, 

 in itself, improbable, and it is at once put out of the question 

 by Pfeffer's observation that, in the case of the stamens of 

 the Cynareae, the extensibility of the cell-walls is "the same 

 in both the expanded and contracted states of the filament. 



Since the action of the stimulus is not to be traced either 

 to the liquid cell-contents or to the cell-wall, we can only 

 assume that it is the protoplasm which is affected, an as- 

 sumption which, considering that the irritability of proto- 

 plasm is a well-established fact, is by no means hazardous. 

 We have then to ascertain if any changes are induced in the 

 protoplasm of the cells of a motile organ by stimulation, 

 and, if so, what the nature of these changes may be. 



No visible changes have been described as taking place 

 in the protoplasm of the cells of any of the motile organs 

 which we have been considering, but such changes have 

 been described as taking place in the cells of the tentacles 

 of Drosera. Attention was first drawn to this subject by 

 Darwin, who designated by the term "aggregation" the 

 changes induced in these cells by mechanical or chemical 



