16 MOVEMENT 



stretched it may undergo a pronounced decrease in size before turgidity is 

 restored. In most cases the cell-wall is so little stretched that the shortening 

 of the cell on plasmolysis is slight or hardly measurable. In cells of the 

 staminal filaments of Cynareae, however, the walls are stretched to such 

 an extent that a slight fall of turgor produces a pronounced contraction 1 . 



A curvature may be produced in a tissue by a fall of turgor even when 

 the individual cells do not undergo any active contraction. For instance, 

 if the turgidity and hence also the rigidity of the cells in the stimulated 

 half of a pulvinus of Mimosa pudica diminishes, these cells will be 

 compressed by the tendency to expansion of the cells in the upper half 

 until equilibrium is reached. In other cases, as in the variation movements 

 due to light and gravity, the turgidity decreases on one side of the pulvinus 

 and increases on the other. 



Changes of turgor produced as physiological reactions act in exactly 

 the same way as changes due to plasmolytic action or to excessive 

 transpiration. The drooping movement of herbaceous parts is the direct 

 result of the diminished turgor with its correlated decrease of rigidity 

 in the stretched thin-walled cells. An artificial removal of turgor produces 

 no perceptible movement when the cells possess sufficiently thick and rigid 

 walls, and in such cases no fall of turgor resulting from stimuli can produce 

 any movement. 



If the turgidity remains constant, an active variation movement can 

 only be produced by a change in the properties of the cell-wall, a decreased 

 elasticity resulting in increased stretching, while an increase of elasticity 

 diminishes the stretching due to the osmotic pressure. In addition an 

 alteration in the power of imbibition may produce an active change of 

 shape in the cell. It must be admitted that the protoplast is able to 

 produce temporary or permanent changes of this kind in the cell-wall, 

 but hitherto no pronounced reversible movement has been traced to this 

 cause. 



Owing to the usual semi-fluid consistency of the protoplasm, the 

 pressure exercised upon the cell-wall is almost solely the result of the 

 osmotic concentration of the sap. Whenever the protoplasm attains 

 a high cohesion it may by its own changes of shape bring localized 

 pressure to bear against a resistance. This is evidenced by the movement 

 of cilia, and it is possible, especially in the case of minute organisms, that 

 the protoplast may be able to exert considerable pressure against the 

 cell-wall, or to antagonize a portion of the osmotic pressure exerted within 

 the cell. If the expansion or contraction were localized, curvature would 

 readily be produced in cells with equally distensible walls, whereas a 



1 Cf. Pfeffer, Zur Kenn trass d. Plasmahaut u. d. Vacuolen, 1890, p. 325 ; Studien zur Energetik, 

 )2, pp. 216, 221, &c. 



