22 MOVEMENTS OF CURVATURE 



scandens circumnutate as rapidly as the best twining plants 1 . In all cases, however, 

 the rapidity of movement is subject to pronounced variations even under constant 

 external conditions. 



Variation movements. Spontaneous variation movements appear to be 



shown by all motile pulvini. These are very 

 slight in the leaflets of Acacia lophantha> more 

 perceptible in those of Mimosa pudica and 

 Phaseolm vulgaris, and very pronounced in 

 those of Oxalis acetosella and Trifolium pra- 

 tense. The leaflets of Oxalis swing to and fro 

 through an angle of 20 to 70 in from 45 

 minutes to 2 hours, those of Trifolium through 

 an angle of 45 to 150 degrees in i^ to 4 hours 2 . 

 The basal leaflets of Desmodium gyrans de- 

 scribe an elliptic curve and require only 85 



FIG. 4. Leaf of Desmodium gyrans. to OO SCCOnds for a single revolution at 

 Nat. size, s = paired basal leaflets. p 3 



35 * 



The movements of Desmodium are very dependent upon temperature, for Kabsch 

 found that at 28 to 30 C. a revolution takes four minutes, and at 22C. the move- 

 ment is reduced to a minimum. Apparently also the excentricity of the elliptic path 

 alters with the speed. The fact that the ascent takes longer than the descent is the 

 natural result of the extra amount of work done in raising the leaf 4 . 



The column in the flower of Stylidium adnatum shows a distinct to-and-fro 

 movement. When it presses against the labellum a trigger arrangement on the latter 

 holds it until the attempted return movement has produced considerable strain, when 

 release is followed by sudden movement. Kabsch, observing that contact was able 

 to produce this movement, considered it to be a physiological response to stimulation,, 

 whereas Gad has shown that the rapid movement does not take place if the labellum 

 is removed, or if a piece of paper is laid upon it. The latter prevents the catch 

 arrangement from acting, so that the column leaves the labellum as soon as the 

 return movement begins. A similar rapid movement can be produced by retarding 



1 Darwin, The Power of Movement in Plants, 1880, pp. 106, 153. 



2 Pfeffer, Periodische Bewegungen, 1875, p. 133; Darwin, 1. c., p. 352. 



3 Kabsch, Bot. Ztg., 1861, p. 355 ; Hofmeister, Pflanzenzelle, 1867, p. 332 ; Meyen, Pflanzen- 

 physiol., 1839, Bd. in, p. 553; Treviranus, Physiologic, 1838, Bd. n, p. 766. The older literature 

 on these long-known movements is given by these authors. Cf. also Stahl, Bot. Ztg., 1897, p. 98. 



4 Cels, Sylvestre and Halle", Annal. d. Botanik von Usteri, 1796, Stuck 19, p. 63; Kabsch, 1. c., 



P. 355- 



5 Gad, Bot. Ztg., 1880, p. 216 ; Schilling, Der Einflnss der Bewegungshemmung auf d. Arbeits- 

 leist. d. Blattgelenke v. Mimosa pudica, Habilitationsschrift, 1895, p. 18. According to Burns 

 (Flora, 1900, p. 344) we are dealing with a growth-movement. Haberlandt, Sinnesorgane im 

 Pflanzenreich, 1901, p. 73. Whether the movements of the labellum of Megadinium falcatum and 

 si Pterostylis observed by Lindley and Morren (Ann. d. sci. nat., 1843, 2 e ser., T. xix, p. 91) are 

 growth or variation movements is not yet certain, or even whether these movements are really 

 spontaneous. Cf. the literature given by Hansgirg, Phycol. u. phytophysiol. Unters., 1893, p. 149. 



