THE MECHANISM OF MOVEMENT 13 



which if they existed on one side only would suffice to produce a curvature 

 of the entire stem, if its mechanical rigidity were not too great. 



In organs adapted for temporary or variation movements the structure 

 is such as to give a considerable freedom of movement. Thus in the 

 pulvini of Leguminosae and other plants the relatively rigid and inelastic 

 vascular bundle is curved and surrounded by active tissue in which, owing 

 to the elasticity of the walls and the changes of shape in the cells, 

 considerable shortening and lengthening is possible l . The vascular bundle 

 at the middle of the pulvinus (Fig. i) lies in the neutral zone, and is but 

 little affected by the curvature produced by a rise of turgidity on the 

 lengthening side of the pulvinus or by a fall on the shortening one. In 

 the first case the shortening of the concave side is due to the cells being 

 compressed by the expansion of those on the upper convex side 2 , just as 

 happens when a pulvinus is moderately bent by applying an external force. 



In the case of a nutation movement, however, the median axis undergoes 

 permanent elongation, and it de- 

 pends upon the mean activity of 

 growth, upon the degree of curva- 

 ture, and upon the thickness of the 

 organ whether the concave side 

 becomes longer, shorter, or retains 

 the same length as before 3 . The 

 amount of curvature is naturally 

 dependent not only upon the re- 

 lative growth Of the antagonistic FIG. i. Pulvinus of Phaseplus vulgaris (magnified), 

 - 1 c (a) longitudinal, and (6) transverse sections. 



connected tissues, but also upon 



the resistance offered by the vascular bundles and other inactive elements. 

 The importance of this resistance is shown by the fact that when a young 

 pulvinus still capable of growth is caused to curve the vascular cylinder 

 undergoes a slight permanent elongation. 



The realized curvature affords no evidence as to whether one or both 

 zones are active, or in the latter case whether the response is of similar 

 character but unequal amount on the two sides, or of dissimilar character. 

 As a matter of fact various combinations occur. Thus the variation 

 movement of Mimosa pudica is produced by a fall of turgidity on the 

 concave side, the expansive energy of the unstimulated convex side then 



1 Pfeffer, Die period. Bewegungen d. Blattorgane, 1875, pp. 3, 157. On the anatomy of pulvini 

 see also A. Rodrigue, Bull, de la Soc. bot. de France, 1894, T. 41, p. 128 ; Schwendener, Sitzungsb. 

 d. Berl. Akad., 1896, p. 535; 1897, p. 228; 1898, p. 176 ; M. Mobius, Festschrift fUr Schwendener, 

 1899, p. 37. E. Pantanelli, Studii d'anatomia e fisiologia sui pulvini motori di Robinia et Porliera, 

 1901 ; Haberlandt, Physiol. Anat., 2. Aufl., 1896, p. 475. 



2 Pfeffer, Physiol. Unters., 1873, p. 73. 



3 Pfeffer, Die period. Bewegungen d. Blattorgane, 1875, p. 17. 



