104 MOVEMENTS OF CURVATURE 



movements, and it is possible that further research may reveal the existence 

 of such movements among vascular and even non-vascular Cryptogams. 



Variation movements are performed by comparatively short pulvini, whereas 

 nutation movements may be derived from growing zones of considerable length. 

 Indeed, in some cases the greater portion of the leaf may be capable of curvature, 

 which may in the case of the floral leaves of Silene nulans go so far as to cause a rolling 

 up of the leaf 1 . As growth dies out, the growing and curving zone gradually 

 decreases, so that in Malva, for instance, the nyctinastic movements are at first 

 carried out by the whole petiole, but ultimately only by the basal pulvinus 2 . 



The movement usually takes place in a vertical or oblique plane, but in many 

 cases a complicated curve is traced, and in others a twisting occurs which goes so far 

 in the leaves of Phyllanthus Niruri and Cassia 3 as to lead the downwardly curving 

 leaflets to apply their dorsal surfaces to one another. These twistings, as well as the 

 feebler ones of Mimosa pudica, are determined by the structure of the motile organs, 

 but the twisting of the leaflets of Mimosa, and possibly also of Phyllanthus Niruri, are 



not produced by a true torsion in the pulvini, but by 

 its curving along two intersecting planes 4 . As in 

 other cases, the movement may either be spasmodic 

 or regular, but the remarkable spasmodic movements 

 observed by Darwin on the leaves of Averrhoa 

 Bilimbimzy possibly be in part autonomic in origin 5 . 

 If the further movement of a leaf is prevented by 

 contact with the stem or with another leaf, the ten- 

 dency to curvature continues, so that the pressure 

 exerted reaches a maximum, and then dies away 

 again as the return movement begins. Hence a 



FlG. 30. ffieracium pilosella. A in \ a , c A -7*7 j? r if 



day position, B in night position. leaflet ot Acaaa loptiantha or of Mimosa curves to 



a greater extent when the opposite leaflet with its 



resistance to movement is removed. In this case, as with other free leaves, the return 

 movement begins soon after the attainment of the extreme night position, whereas 

 when the leaflets are in contact they remain for some time pressed together without 

 movement 6 . 



Not only are different leaves capable of varied movements, but in addition the 

 different parts of the same leaf may behave dissimilarly. Thus the petioles of the 

 leaves of Desmodium gyrans and of Phaseolus vulgaris rise up in the evening, whereas 

 the laminas sink downwards. Furthermore, the primary petiole of Mimosa pudica 

 and the leaflets move in opposite directions, while the palmate leaflets of some species 



1 Cf. Hansgirg, I.e., 1893, p. 13. 



* Vochting, Bot. Ztg., 1888, p. 519. 



3 Pfeffer, Periodische Bewegungen, 1875, p. 159; Darwin, The Power of Movement in Plants, 



1880, pp. 387-8- 



* Schwendener, Gesammelte bot. Mittheil., 1897, Bd. II, pp. 214, 242. 



5 Darwin, 1. c., p. 330. Cf. also Dewevre and Bordage, Rev. gen. de bot., 1892, T. iv, p. 77. 



6 Pfeffer, 1. c., pp. 48, 160. 



