THE MECHANISM OF MOVEMENT 17 



general rise or fall of the internal hydrostatic pressure can only produce 

 curvature when the opposed sides are unequally extensible, or are made 

 so by the action of the protoplast. 



Even when a variation movement is found to be due to changes in 

 the osmotic pressure, the elasticity of the cell-walls remaining constant, 

 it still remains to be determined in which of a variety of ways the 

 alterations in the osmotic pressure are produced. For instance, a fall of 

 turgor may be produced by a precipitation of the dissolved osmotic 

 materials, by their conversion into larger molecules of less osmotic activity, 

 by the physiological combustion of the osmotic materials or by their 

 removal in other ways, as when they are allowed to diosmose out of the 

 cell. If in the latter case they remained dissolved in the imbibed fluid 

 saturating the cell-wall, the osmotic pressure against the cell-wall would be 

 diminished in exactly the same way as when a plasmolysing solution is 

 applied. By the reabsorption of the excreted materials, or by the produc- 

 tion of new ones, the original condition of turgor may be restored. Here, 

 as in other cases, the disturbance due to a reaction excites activities tending 

 towards the restoration of equilibrium. 



A fall of turgor causes the cell to contract with an escape of water, 

 until the concentration of the sap again balances the decreased tension 

 in the cell-wall. If the fall of turgor is sudden, the cell readily permeable 

 to water, and if the latter is able to escape into the intercellular spaces, 

 then rapid movements may occur, as in the leaves of Mimosa and the 

 staminal filaments of Cynareae. That the cells are capable of rapid 

 filtration under pressure is shown by the rapidity with which they contract 

 or become plasmolysed when placed suddenly in strong solutions of salt. 



In the case of nutation movements, we have primarily to determine whether 

 the curvature does or does not involve any change in the average rate of growth, 

 and whether the latter is accelerated on the concave side as well as on the convex 

 one. Even in the case of small objects this can be ascertained by the use of suitable 

 micrometers. In measuring short distances the chord of an arc may be taken as the 

 length of the curved surface of the arc *. 



A change of osmotic concentration can only be detected by plasmolytic methods 

 when it persists for some time, and is not as rapidly readjusted as it is in the pulvinus 

 of Mimosa. Furthermore, the contraction or compression of the cell will always 

 cause a rise of the internal osmotic pressure if only water escapes from it. In fact, 

 it does not follow that a rapid movement must always be produced by a change 

 of turgor affecting the elastic stretching of the cell-wall. 



Especially in the case of movements of variation, measurements of rigidity afford 



1 Pfeffer, Druck- und Arbeitsleistungen, 1893, p. 293; Perioclische Bewegungen, 1875, p. 15; 

 Physiol. Unters., 1873, p. 27. [If the object is strongly curved, the length of its curved surfaces can 

 be satisfactorily found by reconstructing the figure on paper from a series of measured chords, or by 

 measuring the curved surface by means of an opisometer.] 



PFEFFER. Ill C 



