CHAPTER III. SPECIAL PHYSIOLOGY OF MOVEMENT. 763 



spaces ; these cells, being flaccid, are unable to counteract the 

 downward pressure of the still turgid cells of the upper half of the 

 pulvinus, and to support the weight of the leaf ; consequently the 

 primary petiole sinks downwards. The same mechanism obtains 

 in the movements of the leaflets and of the secondary petioles ; the 

 only difference being that, in the pulvinus of a leaflet, it is the 

 cells of the upper half of the pulvinus which lose their turgidity 

 on stimulation, so the leaflet is raised upwards ; and, in the pul- 

 vinus of the secondary petiole, it is the cells of the inner half 

 which lose their turgidity, so the secondary petioles approach the 

 middle line. This account is also applicable to all side-to-side 

 movements, such as that of the leaf of Dionsea, and that of the 

 stamens of Berberis and Mahonia. 



The heliotropic or other curvatures taking place in the elonga- 

 ting region of growing cellular members, are due to the shortening 

 of the cells on the side becoming concave, and to the elongation of 

 the cells on the side becoming convex. The mechanism of the 

 curvature seems to depend in this case not so much upon a differ- 

 ence of turgidity between the cells of the two sides as upon a 

 difference in its effect : whereas turgidity induces the usual longi- 

 tudinal elongation of the cells of the convex side, it induces longi- 

 tudinal shortening in the cells of the concave side in consequence 

 of extension in the other dimensions. 



Turgidity is then the main factor in the mechanism of the move- 

 ments of cellular members ; its mechanical importance is further 

 strikingly illustrated by the great rigidity of turgid members, and 

 by the great force, equivalent in some cases to twenty times the 

 atmospheric pressure, which they develope in opposition to ex- 

 ternal resistance, as when the roots of trees cause the splitting of 

 walls or of pavements. Although one essential factor in turgidity 

 (see p. 668) is the purely physical osmotic activity of substances 

 in the cell-sap, it must not be forgotten that it also depends upon 

 the resistance offered by the protoplasm to nitration under pres- 

 sure : so that the maintenance of turgidity is after all a vital act. 

 The maintenance of turgidity appears, in fact, to depend upon a 

 certain state of molecular aggregation of the protoplasm lining 

 the cell-wall, in which it offers resistance to the escape of the 

 cell-sap ; whereas in the flaccid condition the state of molecular 

 aggregation of the protoplasm is such that it readily permits the 

 escape of the cell-sap under the elastic pressure of the cell- wall. 



Whilst the fundamental mechanism of the movement of mature 



