THE CONDITIONS FOR STIMULATION AND ITS PROGRESS 219 



steadily with progressive upward or downward displacement, and attains 

 a maximal value when the upward displacement reaches about 90 C., 

 so that the angle with the perpendicular is one of 150 to i6oC, which 

 is about the same as that which produces the maximal geotropic excitation 

 of a parallelotropic main root. In vertical positions the lateral roots behave 

 similarly to diageotropic rhizomes, being in a condition of labile equilibrium 

 both when the apex points vertically upwards and when it is directed vertically 

 downwards. It does not, however, follow that all plagiotropic organs will 

 behave similarly. Dorsiventral organs also have only one position of 

 stable equilibrium, and it appears that the geotropic excitation does not 

 increase with equal rapidity when they are inclined upwardly and down- 

 wardly 1 . 



SECTION 50. Perception and Response. 



Even if the geotropic excitation proves to be due to the sinking of 

 the denser particles in the cells, we should only have found the internal 

 stimulus and should be as far as ever from understanding the mode of physio- 

 logical perception. The same applies when galvanotropism is found to be 

 due to the electrolytic action of the current producing the conditions for 

 chemotropic excitation 2 , or if the unilateral illumination were found to create 

 changes of surface-tension which acted as the immediate agencies in producing 

 a heliotropic curvature. Changes in the configuration of the protoplasm may 

 also be of importance in inducing a particular movement or in enabling it to 

 be performed, but they give no insight into the mode of perception. Local 

 accumulations of the protoplasm are also often merely the result of a 

 realized curvature, or are accessory to the reaction. 



Kohl and Wortmann have actually observed accumulations of the 

 protoplasm on the concave sides of organs performing geotropic, heliotropic, 

 and thigmotropic curvatures 3 . Elfving 4 has, however, shown that the 

 accumulation follows the curvature, and is also produced as the result of 

 forcible bending, so that it is possibly the mechanical result of the hindrance 

 interposed to the movement of the protoplasm. Wortmann 5 assumed 

 that in multicellular organs performing tropic curvatures the protoplasm 

 travelled to the concave side and largely accumulated there, but Noll and 

 Kohl G have shown that this is not the case. 



1 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. xxxil, p. 195. 



2 See Ewart and Bayliss, Phil. Trans., 1905. 



3 Kohl, Bot. Hefte von A. Wigand, 1885, Bd. I, p. 161; Wortmann, Bot. Ztg., 1887, p. 803; 

 1888, p. 469; 1889, p. 491. 



* Elfving, Zur Kenntniss d. Kriimmungserscheinungen, 1888, Sep. a. Ofversigt af Finska Vet. 

 Soc. Forhandlingar, Bd. XXX ; Bullot, Ann. de la Soc. belg. de Microscopic, 1897, Bd. xxxi, p. 71 ; 

 Mitschka, Ber. der bot. Ges., 1897, p. 164. Cf. also Noll, Flora, 1895, Ergzbd., p. 38; Haberlandt, 

 Oestreich. bot. Zeitschr., 1889, p. 5. 



5 Cf. Godlewski, Bot. Centralbl., 1888, Bd. xxxiv, p. 83. 



* Cf. Noll, 1. c., and Arb. d. bot. Inst. in Wiirzburg, 1888, Bd. I, p. 531 ; Kohl, Die Mechanik 

 der Reizkriimmungen, 1894, pp. 27, 35. 



