I 9 2 TROPIC MOVEMENTS 



always a right angle, and considered that the directive action of the 

 substratum was due to the mass attraction of the latter. Van Tieghem l 

 supported this view, but its incorrectness was shown by Sachs 2 , and the 

 whole subject was discussed in a manner according with our present views 

 in the first editions of Pfeffer's Physiology. Various authors then brought 

 forward instances of the elimination of curvatures by autotropic action. 

 No precise determination is, however, possible at present of the complex 

 factors involved in all autotropic responses, for the same problems are 

 involved as in growth and formative activity in general. 



The fact that alterations in the .tissue-strains, as well as in the tension 

 of the plasmatic membranes, may affect growth affords no evidence as to 

 the origin of the autotropic curvatures, and hence it is impossible to follow 

 Noll 3 in his attempt to ascribe these curvatures to the result of the 

 changed strains in the tissues and plasmatic membranes. Klercker assumed 

 that the removal of the curvature was 'the mechanical result of the con- 

 tinuance of equal growth on the opposed sides, but Czapek 4 has shown 

 the insufficiency of this view. 



PART III 

 THE CONDITIONS FOR AND CHARACTER OF TROPIC STIMULATION 



SECTION 44. Instances of the Separate Localization of Perception 



and Response. 



Usually the effect of tropic stimulation is strictly localized and con- 

 ducted to only a short distance from the directly excited region 5 . In 

 addition, separated organs, or even fragments of organs, may still remain 

 capable of tropic response; and hence the existence of a power of transmitting 

 tropic stimuli from the percipient organs to the motory zones was overlooked 

 until Darwin's researches were made 6 . 



In all tropic action at a distance the intervening ductory processes are 

 such as to regulate the curvature to the direction of incidence of the exciting 

 agency upon the percipient organ. This is still the case when the motory 

 zone is not directly excitable, and can only be indirectly stimulated 



1 Van Tieghem, Bull, de la Soc. hot. de France, 1876, T. xxm, p. 56. 



2 Sachs, Arb. d. hot. Inst. in Wiirzburg, 1879, Bd. n, p. 224. 



3 Noll (Biol. Centralbl., 1903, Bd. xxm, p. 403). 



* Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 320. 



5 Cf. Czapek, Jahrb. f. wiss. Bot., 1895, Bd. xxvn, p. 263 ; 1898, Bd. xxxil, p. 248 ; Kohl, 

 Mechanik der Reizkriimmungen, 1 894. 



e Darwin, The Power of Movement in Plants, 1880, p. 523. 



