n8 MOVEMENTS OF CURVATURE 



changes of illumination and of temperature, and may still continue under 

 water, or in air saturated with moisture, in which the turgidity of the tissues 

 is maintained at the highest possible limit. The fact that such flowers and 

 leaves often perform sleep-movements when the sky becomes cloudy or 

 before the fall of rain is due to the induction of a photonastic or thermo- 

 nastic response, which is accelerated by the rise of turgor due to the 

 diminution of transpiration *. It has, however, yet to be shown that any 

 plants exist in which pronounced daily hydronastic sleep-movements are 

 produced by the normal daily changes of turgidity. 



Kraus, Wiesner, and Hansgirg have all shown that in many cases a moderate 

 change of turgidity produces a pronounced physical curvature, resulting in the 

 sinking of leaves or the closure of flowers, quite apart from the usual drooping 

 due to a pronounced fall of turgor. These movements often have considerable 

 biological importance by reducing the exposure, and in the same way the rolling-up 

 or folding of certain leaves by reducing the surface exposed aids in rendering them 

 resistant to desiccation 2 . The daily changes of turgor due to transpiration may 

 naturally cause the periodic repetition of the associated physical movements. Naturally 

 also, oscillations are bound to occur when the changes of turgor due to the sudden 

 withdrawal or absorption of water are produced with unequal rapidity in the tissues 

 on opposite sides of an organ 8 . 



Physical movements of this kind are possible, not only in growing organs, 

 but also in adult pulvini, although in most cases little or no effect is produced 

 by a moderate loss of water. A readier response is, however, given by a certain 

 form of Porliera hygrometrica, in which a deficiency of water causes a more or less 

 complete folding of the leaflets 4 . The contradictory observations upon the influence 

 of moisture upon the leaf movements of Porliera are partly due to the fact that all 

 forms are not equally sensitive, and that the removal of water was less pronounced 

 in some cases than in others. Paoletti and Pantanelli 5 have recently shown that the 

 daily sleep-movements of this plant are produced in the usual way by changes of 

 illumination. 



The continuance of the daily movements under water shows that they are not 

 of hydronastic origin, although in time the movement and power of reaction 

 disappear from the submerged plant. This is, in part, due to the injurious action 

 exercised by the insufficient supply of oxygen, owing to the diminution or almost 

 complete cessation of the gaseous exchanges, and by the cessation of transpiration. 



1 Cf. Pfeffer, Physiol. Unters., 1873, p. 188 ; Period. Bewegtmgen, 1875, p. 137, and the literature 

 quoted in these works. Kraus, 1. c., p. 35 ; F. W. Oliver, Bot. Centralbl., 1891, Bd. XLV, p. 52 ; 

 Hansgirg, 1. c., pp. 40, 122. 



3 See Ludwig, Biologic d. Pflanzen, 1895, p. 194; Tschirch, Jahrb. f. wiss. Bot., 1882, Bd. xiii, 

 P- 544- 



3 Cf. Pfeffer, Period. Bewegungen, 1875, p. 137. 



* Darwin, The Power of Movement in Plants. 



5 Paoletti, Nuovo giornale botanico italiano, 1892, T. xxiv, p. 65 ; Pantanelli, Studi d'anat. e 

 fisiolog. sui pulvini motori, 1901, p. 258. 



