226 PART III. PHYSIOLOGY. [ 53 



lost a portion of their cell-sap. The expanded state is regained by 

 the gradual expansion of the cells, tiu*gidity being restored by the 

 absorption of water. 



In the foregoing case, all the cells of the motile portion are 

 affected ; but in many cases some only of the cells are affected. 

 Thus, in the case of the leaf of the Sensitive Plant, the primary 

 petiole, when at rest, stands out nearly at right angles to the stem 

 (Fig. 126, p. 174) : on stimulation, it sinks downwards so as to 

 form an acute angle with the internode below its insertion. The 

 mechanism is this : when at rest, the cells of the pulvinus are all 

 turgid, and they support the petiole in its normal position : on 

 stimulation, the cells of the lower portion of the pulvinus lose 

 their turgidity, water escaping from them into the intercellular 

 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- 



