236 TROPIC MOVEMENTS 



the same end 1 . The reaction begins at first slowly, then attains a maximum 

 rapidity, and slowly decreases again. At the same time the apex extends 

 regularly or in jerks, and describes a simple or complicated curve in space 

 according to circumstances 2 . 



A tendency to curvature can naturally only find expression when it is 

 able to overcome the internal and external resistance. A rise of the 

 internal resistance due to the production of wood or sclerenchyma will 

 render the active tissues capable of only feeble curvature or of none at 

 all. Similarly, by determining the exact external resistance required to 

 prevent curvature a measure is obtained of the energy of movement 3 . The 

 latter is considerable in all movements produced by heterauxesis, and hence 

 a horizontal shoot is able to overcome a considerable statical moment 

 in curving upwards. To prevent movement the statical moment due 

 to the organ's own weight usually needs to be increased from four to thirty 

 times 4 , so that under normal conditions the plant works with a considerable 

 margin of safety. Usually also the rapidity of curvature is not affected by 

 fractional increases of the normal statical moment or even by doubling it 5 . 

 Exactly the same applies to the influence of a resistance upon rectilinear 

 growth, and in both cases a relatively considerable increase of resistance is 

 required to lessen the rate of growth or curvature perceptibly. Similarly, 

 a man may climb a mountain as rapidly with a small load as with none 

 at all, whereas when heavily laden he must climb slowly in order to be 

 able to perform the greater work required. 



No upward geotropic curvature is possible when a shoot is unable to 

 support its own weight, but nevertheless, as in the case of the hanging free 

 ends of the stems of climbers, the basal part bends mechanically downwards, 

 while the apex turns upwards. That is the natural result of the tendency 

 to upward curvature coupled with the fact that the statical moment at any 

 point is proportional to the length of free stem beyond it. Hence such 

 shoots assume a double curvature or S shape, such as may also be produced 

 in normally erect stems, when they are subjected to sufficiently intense 

 centrifugal forces 6 . 



The statical moment in the case of the basal growing zone of the peduncle of 

 the Hyacinth may amount to 6 kilograms, in that of the lowest nodes of a ripe stem 



1 Cf. Sachs, Flora, 1873, p. 327 ; Darwin, The Power of Movement in Plants. On heliotropic 

 curvatures see H. Muller, Flora, 1876, p. SS, and Wiesner, Die heliotropischen Erscheinungen, 1878, 

 Bd. I, p. 68. 



2 Darwin, 1. c., pp. 495-512. 



3 On dynamometers see Pfeffer, Period. Bewegungen, 1875, p. 9; Druck- and Arbeitsleistungen, 

 1893, p. 251 ; Meischke, Jahrb. f. wiss. Bot., 1899, Bd. xxxill, p. 345. 



4 Meischke, 1. c., p. 362. G Id., p. 364. 



6 F. Schwarz, Unters. a. d. hot. Inst. zu Tubingen, 1881, Bd. I, p. 80. Cf. also Baranetzsky, 

 Flora, 1901, Ergzsbd., p. 186. 



