IRRITABILITY. 563 



Pieffer suggests, that the osmotically active substances are 

 destroyed, or that they are converted into substances of lower 

 osmotic activity, and that the attraction by which the water 

 is held in the vacuole being thus diminished, a portion of the 

 water of the cell-sap is forced out by the pressure of the 

 elastic cell-wall. Such a conception is not, however, easy of 

 comprehension. The escape of liquid from a cell by filtra- 

 tion under pressure has surely nothing to do with the osmotic 

 properties of the liquid. Were the escape of the liquid from 

 the cell due to osmosis, were it attracted into other cells, then 

 such a change in the quantity or quality of the osmotically 

 active substances would render possible the removal of a 

 portion of it. But this is not so. The liquid, in most cases 

 at least, escapes by filtration under pressure into the inter- 

 cellular spaces which contain air. The retention of the liquid 

 in the vacuole, or its escape from it, whatever the osmotic 

 properties of the liquid may be, is simply dependent on the 

 relation between the pressure of the elastic cell- wall on the 

 one hand, and the resistance to filtration under pressure 

 offered by the primordial utricle on the other. It is easy to 

 understand that, owing to the high osmotic activity of sub- 

 stances in the cell-sap, so much water may be absorbed into 

 a cell that the pressure of the cell-wall may exceed the re- 

 sistance to filtration offered by the primordial utricle, and 

 so an escape of a portion of the cell-sap takes place ; this 

 occurs, as a matter of fact, in the parenchymatous cells of 

 roots when absorption is active (p. 94) ; but it is not possible 

 to realise how a diminution in the attraction of the cell-sap 

 for water can bring about this result. This conception in- 

 volves the further assumption that the liquid which escapes 

 from the cells of a stimulated motile organ is pure water, an 

 assumption which is not justified by the evidence. Pfeffer 

 makes it on the ground that a stamen injected with water 

 and lying in water regains its turgid irritable condition on 

 being stimulated, arguing that had any osmotically active 

 substances been given out by the cells on shortening they 

 must have been removed by the surrounding water, and that, 

 had they been so removed, the recovery of turgidity would 



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