CAUSES OF THE EXPULSION OF WATER. 653 



extended by the turgescent parenchyma, and even in the irritated state after the 

 contraction there still exists a similar though much feebler tension. 



Having now made ourselves acquainted with the facts of the movements due to 

 irritation so far as is accessible to direct observation, the question arises, in what does 

 the effect of stimulation essentially consist ? As in the previous lecture, where we 

 were concerned with the stimulus of light, so also here we must again keep in view 

 as the chief point, that the irritable organ, or we may say each of its irritable cells, is 

 turgescent in a high degree, and that in consequence of a touch or shake this turge- 

 scence is suddenly diminished, producing a sudden escape of water from the interior 

 of the cells. The question thus resolves itself into the following : — How is the 

 sudden expulsion of water from the cells produced in consequence of a stimulus ? 



According to all that we know as to the condition of turgescent cells, from 

 De Vries' researches on plasmolysis, Pfeifer's descriptions, and my own con- 

 siderations and experiments, it can scarcely be doubted that the cellulose walls 

 themselves are always in a high degree permeable to water, and that the con- 

 dition of turgescence of the cells depends upon the protoplasmic utricle opposing 

 the expulsion of the endosmotically absorbed water even under high pressure. 

 A sudden escape of water from turgescent cells can thus be rendered possible only 

 by this property of the protoplasmic utricle undergoing some change, or, in other 

 words, by the hitherto non-permeable protoplasm becoming permeable in conse- 

 quence of the stimulus, and thus letting water escape. 



It must at the same time be added that we can at present form no idea why 

 this change in the protoplasm occurs in consequence of a stimulus, and with what 

 molecular changes it is connected ; it must suffice for us meanwhile to know that 

 the externally perceptible effects of stimulations so far described are caused by the 

 change referred to in the protoplasm itself, and the question now is how the 

 mechanics of movements due to irritation are to be understood from this. 



It is to be observed, in the first place, that the escape of the water from the 

 tissues is connected with a proportional diminution of their volume. It follows 

 thence that the cellulose walls themselves must contract in the movement: in 

 proportion as the water at a high pressure in the cells filters out through the 

 irritated and therefore permeable protoplasmic utricle, it penetrates to the exterior 

 also through the cellulose walls, and these then contract elastically, whence follow 

 directly the movements described above. 



It will be noticed that in this mechanism the extensibility of the cellulose walls 

 plays an important part : true, neither the extensibility nor the elasticity of the cellu- 

 lose walls is altered directly by the influence of the stimulus, though both properties 

 are probably put in action. In order that a movement of the organ may come 

 to pass in consequence of the movement of the protoplasm, the cells concerned 

 must in the resting condition be strongly distended by turgescence, so that, when 

 the protoplasm suddenly becomes permeable, they can contract to a corresponding 

 extent, since only by this means is the movement itself called forth in consequence 

 of the stimulus. 



We might even suppose that the protoplasm forming a closed sac on the interior 

 of the cellulose wall, allows water to filter out in consequence of a stimulation, 

 and that, at the same time, it undergoes contraction, as occurs in the plasmolysis of 



