MECHANISM OF THE MOVEMENTS. 893 



decrease in bulk. This decrease is due to an escape of water from the cells into the 

 intercellular spaces, from which it flows, as Pfeffer directly observed, when a filament 

 is cut across, just as is the case with the organs of Mimoseae. If the intercellular 

 spaces be injected with water, the filaments remain irritable and on stimulation the 

 escape of fluid from the cut surface is more evident. 



The filaments are very extensible and at the same time very perfectly elastic. 

 They may be stretched to twice their usual length, and, on being released, they return 

 to their original dimensions. 



When the filament is irritable, the axial bundle and the epidermis are stretched by 

 the turgid parenchyma, and even after stimulation a tension of the same kind but less 

 considerable still exists. 



The possible assumption that the movement is due to an increase of the elasticity of 

 the cell-walls under the influence of a stimulus, a contraction of all the cells and an 

 escape of water from them being the result, is shown by Pfeffer to be incorrect, for 

 the elasticity of the stimulated and of the unstimulated filament is the same. 



The assumption that the permeability of the cell-walls is suddenly increased by 

 a stimulus and that thus the escape of water is rendered possible, is also shown to be 

 very improbable. PfeflTer points out that the water filters through under high pressure, and 

 proves that the ordinary permeability of organic membranes is quite sufficiently great to 

 permit of the passage of the small quantity of water which escapes from stimulated cells. 

 It is therefore unnecessary to assume a sudden change in the properties of the cell-wall 

 which increases its permeability. 



Pfeffer overthrows Hofmeister's theory that the escaping water comes not from the 

 vacuole of the cell but from the cell-wall itself, by the fact that the lateral walls of the 

 parenchymatous cells become thicker on contraction, and he might have added that on 

 this theory a contraction of empty cells but not of full tense cells was possible. 



After having shown — though without absolute proof — how improbable it is that the 

 cell-wall undergoes a sudden change in consequence of stimulation, Pfeff'er goes on to 

 point out how probable it is that some change is produced in the protoplasm which lines 

 the cell-wall as a closed sac. For a complete discussion I refer the reader to his 

 exhaustive treatment of the subject; I will only append the following account for 

 the sake of clearness. It is evident that if permeability of the tense cell-wall remain 

 unaltered, the escape of water from the cell may depend upon the permeability of the 

 layer of protoplasm which lines the cell-wall. If it is not permeable, it becomes more 

 closely pressed to the cell-wall by the increased hydrostatic pressure effected by 

 endosmosis ; if now any force affects the protoplasm in such a way that the protoplasm 

 becomes permeable to water, an escape of fluid will take place not only through the 

 protoplasmic layer, but also through the cell-wall which has already been shown to 

 be sufficiently permeable. It has now to be shown that the occurrence of such a change 

 in the protoplasm is possible, and to be explained why it is that this suddenly increased 

 permeability of the protoplasm ceases after the movement, a fact which is essential to 

 the restoration of the irritability. On these points I would refer the reader to the 

 explanations given in PfefFer's work; I would only add that such changes in the 

 permeability of protoplasm as are here assumed are already known to occur. When the 

 protoplasm of a cell of Spirogyra contracts before conjugation, it must necessarily become 

 more permeable, for most of the water escapes from it ; this escape does not take place 

 when the cell is turgid and actively growing. If the cell-wall of the conjugating cell 

 were very tense and if it were at the same time very elastic, it would contract simul- 

 taneously with the protoplasm, and would permit of the escape of the water through it. 

 As a matter of fact the cell-wall of Spirogyra is not very tense, and it is rigid, so that 

 it does not materially alter its form when the protoplasm contracts ; the water which 

 escapes through the protoplasm therefore occupies the space between it and the cell-wall. 

 It may be objected that this contraction preparatory to the conjugation of the cell of 

 Spirogyra is not the result of the action of an external stimulus ; this is quite true, but 



