SEPARATE LOCALIZATION OF PERCEPTION AND RESPONSE 197 



vertically downwards, but the rest of the root was horizontal. No curvature followed, 

 but when the root was placed as in Fig. 42, A, within twenty-four hours a curvature had 

 taken place as at B, so that the tip pointed downwards. It follows, therefore, that 

 the growing zones behind the apex which perform the curvature are incapable of 

 directly perceiving geotropic stimuli. 



To obtain successful results, the roots must be able to slip easily into the glass 

 tubes, since otherwise disturbances of growth ensue, such as prevented Wachtel 

 and Richter from obtaining any positive results *. The experiments when properly 

 performed are, however, fully satisfactory ; and Czapek 2 has shown that exactly the 

 same phenomena are shown after the removal of the tube if the apex of the root 

 remains permanently bent for a time. 



The special geotropic irritability of the root-tip is also shown by the fact 

 that the active zone curves beyond the vertical when the apical part is kept 

 permanently horizontal 3 . This method was used by F. Darwin 4 to show that the 

 cotyledon of a seedling of Panicum not only perceives heliotropic, but also geotropic 



B 



FlG. 42. Seedlings of Lufinus albus 

 (smaller size). The seedling (A) has been 

 removed from the klinostat after the apex 

 is fixed in the glass cap /, and after 

 twenty-four hours has curved so as to 

 place itself parallel with the perpendicular 

 line shown oy the arrow. 



FlG. 43. Seedlings of Setaria italica. The roots have been cut 

 away down to the rudiments w, the cotyledon fixed in the glass 

 tube a, and the seedling is then placed horizontally. In A the hypocotyl 

 has curved through 180, and at B has formed a complete coil. (Twice 

 enlarged.) 



stimuli (Fig. 43). This method is, however, unable to determine whether the power 

 of perception is totally absent from the responding zones, and it is not surprising that, 

 owing to the abnormal conditions, the plant is not always able to bring the irritable 

 region into the normal position of equilibrium 6 . 



The hydrotropic irritability was not conclusively shown by Darwin's 6 experiments 



1 Cf. Czapek, Jahrb. f. wiss. Bot., 1900, Bd. xxxv, p. 312 ; and the reference to Wachtel's 

 work in the Bot. Ztg., 1899, p. 227 ; Richter, Zur Frage nach der Function der Wurzelspitze, 1902. 



a Czapek, 1. c., p. 336. 



8 F. Darwin, Proceedings of the Cambridge Philosophical Society, 1901, Vol. XI, p. 133; 

 Linnean Soc. Journal, 1902, Vol. xxxv, p. 266. 



* F. Darwin, Annals of Botany, 1899, Vol. XIII, p. 568. The special geotropic irritability of 

 the tip of the cotyledon was suggested by certain observations of Rothert (Cohn's Beitrage z. Bio- 

 logic, 1896, Bd. vn, p. 189) and of Czapek (Jahrb. f. wiss. Bot., 1898, Bd. xxxn, p. 254). Massart 

 (Sur rirritabilite" d. plantes superieures), 1902, has applied this method to various roots and shoots. 



5 Cf. Miehe, Jahrb. f. wiss. Bot., 1902, Bd. xxxvn, p. 590. 



* Darwin, The Power of Movement in Plants, 1880, p. 180. Cf. Rothert, Flora, 1894, Ergzbd., 

 p. 208. 



