THE INTERNAL CAUSES OF MOVEMENT 245 



the thickness of the walls of the epidermal and collenchyma cells decreases \ 

 and often to a considerable extent, while the walls of the same cells on 

 the concave side frequently become distinctly thicker. According to 

 Wortmann 2 , the cell-walls become very much thicker on the upper sides 

 of shoots placed horizontally, and prevented from curving- upwards by an 

 attached weight. Elfving 3 found that a similar thickening was produced 

 in the cells of the convex side when a shoot was strongly bent and fixed in 

 this position. Since the same result is produced on a klinostat, it must 

 be the direct result of the altered strains, whereas in Wortmann's experi- 

 ment it probably results from the inductive action of gravity. Evenly 

 distributed longitudinal strains do not appear to produce any increased 

 thickening of the cell- walls 4 , but where the strains are always unevenly 

 distributed, as in the curved hooks of many tropical climbers, a pronounced 

 effect may be produced 5 . 



The fact that the changes in the thickness of the cell-wall only appear 

 during the curvature shows that they are the result and not the cause of it, 

 as Wortmann supposes 6 . Since normally the distension of the walls lies 

 within their limit of elasticity, the plastic growth of the cell-wall must 

 be preceded by a physiological diminution of the cohesion of the component 

 cellulose micellae. At the same time, the elasticity of the cell-walls on 

 the convex side appears to be so modified as to allow of an elastic 

 lengthening of the cells without any rise of turgor. The curvature produced 

 in this way is reversible by plasmolysis until it has been followed up and 

 fixed by growth. A combination of growth and variation movement is 

 also shown by the young growing pulvini of Phaseolus, which, when adult, 

 still remain capable of variation movements. A few days after a plant has 

 been inverted, and the pulvini have performed a geotropic variation 

 curvature, a certain amount of growth takes place in the inverted and 

 unusually elongated dorsal sides of the pulvini 7 . The result depends to 

 some extent, therefore, upon the nature and duration of the stimulus ; and, 

 according to Mobius 8 , the heliotropic curvature of the pulvinus of Maran- 

 taceae is rapidly rendered permanent by growth. 



Frank 9 showed that the persistence of a completed curvature when turgor was 



1 Noll, Arb. d. hot. Inst. in Wiirzburg, 1888, Bd. HI, p. 526; Flora, Ergzbd., 1895, p. 73; 

 Wortmann, Ber. d. hot. Ges., 1887, p. 463; Bot. Ztg., 1887, P- 8o8 > l888 > P- 4 6 9 5 Kohl, I.e., 

 p. 36 ; Macdougal, Botanical Gazette, 1897, Vol. XXIII, p. 364. 



3 Wortmann, Bot. Ztg., 1837, p. 824. 



* Elfving, Znr Kenntniss d. Kriimmungserscheinungen, 1888 (_Ofvertryck af Finska Vet. Soc. 

 Forhandlingar, Bd. XXX). 



* Ball, Jahrb. f. wiss. Bot., 1903, Bd. XXXIX, p. 305. 



5 Ewart, Ann. du Jard. bot. de Buitenzorg, 1898, Vol. XV, p. 190 seq. 



6 Cf. Noll, Flora, Ergzbd., 1895, p. 38. 



7 Pfeffer, Periodische Bewegungen, 1875, p. 139. 



8 Mobius, Festschrift fiir Schwendener, 1899, p. 60. 



9 Frank, Beitrage z. Pflanzenphysiol., 1868, p. 97. 



