170 



TROPIC MOVEMENTS 



the above instrument in accuracy and convenience. A simple form suitable for 

 demonstration can easily be made from an ordinary American clock, either by fixing 

 a cork plate to the finger-axis so as to get horizontal rotation, or by attaching a glass 

 rod to it by means of stout rubber tubing, and so obtaining a horizontally-rotating 

 rod on which seedlings may be fixed with the aid of cork rings. If the clock is 

 fixed in a heavy frame so that it may be canted at various angles, the direction of 

 rotation may be given varying degrees of obliquity 1 . 



Any rapidly rotating wheel may be used to demonstrate the action of centrifugal 

 force 2 , and slight centrifugal actions may be obtained by means of a klinostat 3 . In 

 this way the intensity of stimulation required to produce a geotropic curvature can be 

 determined, as well as the relationship between the intensity of stimulation and the 

 response. It must, however, be remembered that under the action of high centrifugal 

 forces purely physical mass-actions come in and cause the attempted curvatures to be 

 more or less overcome and replaced by mechanical bending. 



SECTION 36. Heliotropism. 



Under this heading we may conveniently include all orienting move- 

 ments produced by unilateral illumination ; but since variations in the 

 intensity of diffuse daylight may produce photonastic movements, or 

 may cause the tone of the organ to alter, it is not always easy to say 

 whether a particular curvature is heliotropic in character, or results from 

 a dissimilar form of stimulatory response, or is due to a combination of 

 factors. It must also be remembered that the position of heliotropic 

 equilibrium may vary according to the intensity of the light, and may in 

 some cases alter to such an extent that the direction of the curvature is 

 reversed. 



A reversal of this kind is especially well shown by those swarm-spores 

 which react positively phototactically in weak light, but negatively photo- 



1 Cf. F. Darwin, Linnean Soc., 1881, p. 449; Wortmann, Ber. d. hot. Ges., 1886, p. 245; 

 Klemm, Flora, 1893, p. 472; Hansen, Flora, 1897, Erg.-Bd., p. 352; W. Oels, Pflanzenphysiol. 

 Unters., 1893, p. 50. The mechanism used for rotating tables in shop-windows is easily made into 

 a klinostat by changing the escapement so that the rotation is slower. Where a room at constant 

 temperature is not available, the arrangement employed by Czapek (Ber. d. bot. Ges., 1900, p. 131) 

 may be used to avoid effects due to changes of temperature. 



3 An apparatus driven by a water-motor, and which enables centrifugal forces up to 40 g. to be 

 produced is described in Unters. a. d. bot. Inst. zu Tiibingen, 1881, Bd. I, p. 57. At Leipzig the 

 apparatus used was driven by a one-horse-power gas-motor, and varying velocities obtained by the 

 use of axes of different sizes, and of conical axes. Cf. Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 304. 



2 - 3 



The centrifugal force is determined by the formula - , where - =a constant (4-024); 



6 0> 



r = radius in cms. ; / = time of a rotation in seconds. On a large wheel the centring need not be 

 so accurately performed as on a small one. On simpler forms of apparatus cf. Oels, 1. c., p. 51 ; 

 Detmer, Pflanzenphysiol. Practicum, 1895, 2. AutL, p. 384; Hansen, Flora, 1893, Erg.-Bd., p. 352. 

 Mottier, Annals of Botany, 1899, v l- XIII, p. 326. Pt'effer has more recently used a specially 

 constructed milk centrifuge to obtain centrifugal forces up to 4,000 g. 

 3 Cf. Czapek, 1. c., p. 305. 



