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. Pfeffer 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, Pfeffer 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 
