GALVANOTROPISM 189 



regions of high electrical potential. Should these responses prove to be 

 tropic in character 1 it remains to be seen whether the action of electrical 

 waves corresponds to that of an electrical current 2 , and also whether the 

 varying magnetic permeabilities of the different constituents of the cells 

 and tissues 3 may render magnetotropic responses possible in a sufficiently 

 strong magnetic field. 



According to Brunchhorst 4 , the curvature produced by a strong current 

 is shown when the root is decapitated, but not the true galvanotropic 

 curvature in the opposite direction produced by a weak current. The 

 latter is shown when only the tip of the root is submerged in water or 

 touches a wet flannel 5 through which the current is passing. Hence 

 only the tip of the root seems capable of the perception of a negatively 

 galvanotropic stimulus 6 . 



SECTION 43. Autotropism and Somatotropism. 



It was long ago observed by Dutrochet 7 that the sporangiophores of 

 Mucor, Phycomyces, and Pilobohis, and of other fungi placed themselves 

 at right angles to the substratum from which they had emerged. The 

 phenomenon was further studied by Sachs and by Dietz 8 . The latter 

 author concluded that the escape from the substratum was regulated by 

 thigmotropic excitation whereas Steyer 9 denies the truth of this statement. 

 Sachs supposed that seedling-stems would, in the absence of any other 

 excitation, set themselves at right angles to a block of moist turf in which 

 they were germinated ; but Dietz has shown that this is not the case 10 . 

 The position assumed by the sporangiophores of the fungi mentioned is 

 primarily the result of their negative hydrotropism, since their position of 

 equilibrium is reached when they are parallel to the direction of diffusion of 

 the water-vapour from the substratum. Negatively heliotropic organs would 

 assume similar positions around a strong centre of illumination. This 

 apparent action of the substratum causes the young sporangiophores to grow 

 at first vertically outwards from the sides of the piece of bread, whereas 

 when they grow longer their geotropic stimulation becomes relatively 



1 [The true nature of these galvanogenic curvatures has been recently investigated by Ewart 

 and Bayliss, Proceedings of the Royal Society, Nov., 1905.] 



2 Induction-shocks act on tendrils like mechanical stimuli (Pfeffer, Unters. a. d. bot. Inst. zu 

 Tubingen, 1885, Bd. I, p. 504), and in much the same way upon protoplasmic streaming. (Cf. 

 Ewart, Protoplasmic Streaming in Plants, 1902, p. 88.) 



3 Ewart, 1. c., pp. 45-9. * Brunchhorst, Ber. d. bot. Ges., 1884, p. 204. 

 5 Muller-Hettlingen, Pfliiger's Archiv f. Physiologic, 1883, Bd. xxxi, p. 201. 



fi Cf. Rothert, Flora, 1894, Erg.-bd., p. 213. 

 7 Dutrochet, Rech. anat. et physiol., 1824, p. 100. 



s Sachs, Arb. d. bot. Inst. in \Viirzburg, 1879, Bd. II, p. 221. Dietz, Unters. a. d. bot. Inst. zu 

 Tubingen, 1888, Bd. in, p. 478. 



9 Steyer, Reizkriimmungen bei Phycotnyces, 1901, p. 27. 

 10 Dietz, 1. c., p. 480. 



