238 TROPIC MOVEMENTS 



zones. The radicle of Vicia Faba may, however, develop a pressure of 13 grams at 

 its apex when in perpendicular contact, and one of 1-5 to 2-2 grams when the con- 

 tact is oblique l . The maximal pressure is naturally only obtained after a certain 

 length of time, and if the resistance is suddenly removed a rapid curvature due to 

 the released strains is produced. This is more pronounced in the case of variation 

 than of growth movements, since in the case of the latter the plasticity of the tissues 

 and the regulation of growth prevent the attainment of any pronounced strain, so that 

 the attempted curvature is only completed some time after the removal of the resistance 

 to it 2 . Hofmeister supposed that negative heliotropism required high, and positive 

 heliotropism low, tissue-strains, but these conclusions are based upon incorrect ideas 

 as to the importance of the strains in the tissues for tropic curvature, and there is no 

 evidence in support of his conclusions 8 . 



SECTION 53. The Mechanism of Curvature. 



Since the rigidity of the pulvinus of Phaseolus remains constant 

 when a negatively geotropic curvature is performed as the result of the 

 reversal of the plant, it follows that the expansive energy of the compressed 

 ventral side which is now uppermost must decrease in exactly the same 

 degree that the expansive energy of the under side increases 4 . If this were 

 not the case a pronounced decrease of rigidity must ensue, since the force 

 of curvature may amount to a pressure of one to three atmospheres. 

 The plasmolytic experiments of Hilburg 5 showed in fact that the osmotic 

 pressure does actually fall in the upper side of a reversed pulvinus and rises 

 in the under half, the observed differences approximating to I per cent, of 

 potassium nitrate, which is amply sufficient to produce the required energy 

 of movement. The same takes place, according to Hilburg, during the 

 heliotropic curvature of the pulvinus of Phaseolns. 



Thigmotropic growth-curvatures involve a pronounced transitory 

 acceleration of the average rate of growth, whereas, according to Sachs 

 and M tiller, the mean growth appears in many cases to be somewhat 

 retarded during heliotropic and geotropic curvature. In the case of the 

 nodes of grasses and of other plants geotropic curvature involves a pronounced 

 acceleration of the mean rate of growth, but it has not been determined 

 whether the growth is also more rapid than in the case of nodes in which 

 growth but not curvature has been excited by rotation on a klinostat. 

 The same question has also to be answered in the case of those nodes 

 which retain the power of slow growth when the stem is vertical. 



1 Cf. Pfeffer, 1. c., p. 270. 



* Sachs, Flora, 1873, p. 207; de Vries, Sur les causes des mouvements auxotoniqnes, 1880, 

 p. 14 (reprint from the Archives Neerlandaises, Vol. xv) ; Pfeffer, Druck- und Arbeitsleistungen, 

 1893, p. 402. 



s Cf. Pfeffer, 1. c., 1893, p. 426. 



4 Pfeffer, Periodische Bewegungen, 1875, pp. 140, 145. 



5 Hilburg, Unters. a. d. hot. Inst. zu Tiibingen, 1881, Bd. I, p. 31. 



