232 TROPIC MOVEMENTS 



growing zone l , the lengths of the growing and curving zones corresponding. 

 It is, however, possible that the power of curvature may be temporarily 

 retained by the zones which have just ceased to grow, although Kohl's 2 

 experiments do not suffice to show that this is a common phenomenon. 

 Woody twigs of Aesculus^ Tilia^ and other plants of one or more years' 

 age, and which have long ceased to grow in length, may still remain 

 capable of slow geotropic curvature when displaced from their normal 

 position 3 . Apparently the geotropic excitation awakens a corresponding 

 tendency to growth in the cambium and younger tissues, the energy of 

 which is sufficient to produce a gradual bending of the inactive and woody 

 parts. The existence of any power of geotropic reaction in the adult 

 petioles of Hedera helix 4 " is disputed by Frank 5 , and doubt also attaches 

 to Hofmeister's statement 6 that the adult petioles of Hedera and adult 

 portions of the roots of Ranunculus aquatilis are capable of heliotropic 

 curvature. Errera's 7 statement that the trunks of large trees may perform 

 geotropic curvatures does not require discussion, since to produce the 

 required bending moment the cambium would need to develop pressures 

 of several hundred atmospheres to compress and extend the inactive tissues. 

 Every tropic curvature naturally depends, not only upon the nature of 

 the excitation, but also upon the plant's power of reaction. Hence etiolated 

 stems are usually capable of more rapid geotropic curvature than normal 

 ones, owing to their more rapid rate of growth 8 . Hence also growth- 

 curvatures appear earlier in the more rapidly growing zones than in the 

 older ones even when the same degree of excitation is assured in both cases. 

 Other factors come into play, however, in determining the further progress 

 and final character of the curvature. Among these are included the 

 mechanical resistance to curvature and the counteraction excited by its 

 realization, as well as the altered orientation of the organ in its new position 

 and the changes of the power of reaction and response with the progress of 

 development. Thin organs will naturally curve more rapidly than thick 



I Sachs, Flora, 1873, p. 324; H. Miiller, Flora, 1876, p. 65; Wiesner, Bewegungsvermogen der 

 Pflanzen, 1881, p. 45 ; Rothert, Cohn's Beitrage z. Biologic, 1896, Bd. VII, p. 152. In the case of 

 fungi and rhizoids cf. Haberlandt, Oesterr. bot. Zeitschr., 1889, p. 3 of reprint ; Zacharias, Ber. d. hot. 

 Ges., 1890, Generalvers., p. 57; Flora, 1891, p. 489; Oltmanns, Flora, 1897, p. 9; Steyer, Reiz- 

 kriimmungen bei Phy corny ces, 1901, pp. 6, 25. 



II Kohl, Mechanik d. Reizkriimmungen, 1894, p. 13. Cf. Rothert, Biol. Centralbl., 1895, 

 Bd. xv, p. 596. 



3 Vochting, Organbildung im Pflanzenreiche, 1884, Bd. n, p. 85 ; Frank, Lehrbuch d. Botanik, 

 1892, Bd. i, p. 470 ; Meischke, Jahrb. f. wiss. Bot., 1899, Bd. xxxm, p. 363, footnote ; Jost, Bot. 

 Ztg., 1901, p. 20 ; Baranetzsky, Flora, 190-1, Ergzbd., pp. 202, 213; Wiesner, Sitzungsb. d. Wien. 

 Akad., 1902, Bd. cxi, Abth. i, p. 796. 



4 Hofmeister, Pflanzenzelle, 1867, p. 285; Bot. Ztg., 1869, p. 95. 



6 Frank, Bot. Ztg., 1868, p. 644. e Hofmeister, Pflanzenzelle, 1867, P- 289. 



7 Report of British Association. Cambridge, 1904. 



8 Wiesner, Die heliotropischen Erscheinungen, 1880, Bd. II, p. 7 ; H. Miiller, Flora, 1876, p. 91 ; 

 Darwin, The Power of Movement in Plants, 1881, p. 493. 



