234 



TROPIC MOVEMENTS 



positions. A similar progress of geotropic torsion or curvature may be 

 shown by the stalks of flowers. 



Although the curvature usually begins first in the more actively 

 growing zones exceptions may occur. Thus, when the tip only is irritable, 

 as in the cotyledons of Avena, the curvature begins first in the regions 

 bordering upon it, and later in the further removed most actively growing 

 zones l . This is, however, the natural result of the slow transmission of 

 the tropic stimulus, and similarly geotropic curvature begins first just 

 behind the percipient apex 2 , although shortly afterwards the curvature 

 is most marked in the most actively growing zone a little further away 

 from the apex (Fig. 46, B}. Later still, the curvature is transmitted basally, 



while the zones 2 and 3 (Fig. 46, C] which 

 have elongated most have nearly become 

 straight again 3 . These facts were correctly 

 interpreted by Frank 4 , whereas Hofmeister 5 

 erroneously concluded that no curvature took 

 place in the most actively growing zones. 



Similar relationships were found by Sachs, 

 Muller, and Rothert to exist in the case of 

 heliotropic organs, for here also the whole 

 growing zone appears to be capable of curva- 

 ture. According to Wiesner, the basal grow- 

 ing portion of seedling-stems does not react 

 heliotropically, but merely shows a mechanical 

 bending due to the weight of the curving 

 portion above 6 . Rothert has, however, shown 



FIG. 46. Seedlings of Lupimis albus 



showing geotropic curvature. The hori- that this is not the case, and that all the grow- 



zontally-placed radicle in A has its ter- 

 minal ten millimetres marked, and after ing ZOnCS are Capable of heliotropic respOHSC. 

 three hours has curved as in B, and after 



eight hours as in c. Presumably the same applies to all forms of 



tropic curvature, although further investigation is needed in this direction 7 . 



The power of tropic reaction is, however, not always localized in the 

 most actively growing zones, as is shown by the existence of variation- 

 movements, and by those nodes in which the awakening of growth is due 

 to the tropic stimulus. In addition, the amount of the reaction depends 



1 Darwin, The Power of Movement in Plants, pp. 421, 477 ; Rothert, Cohn's Beitrage z. Biologic, 

 1896, Bd. vn, pp. 163, 211. 



2 Cf. Czapek, Jahrb. f. wiss. Bot., 1900, Bd. XXXV, p. 361. 



3 For details see Sachs, Arb. d. bot. Inst. in Wiirzburg, 1874, Bd. I, pp. 440, 454, 612; 

 Cisielski, Cohn's Beitrage z. Biologic, 1872, Bd. I, p, 4; N. J. C. Miiller, Bot. Ztg., 1869, p. 390. 



4 Frank, Beitrage z. Pflanzenphysiologie, 1868, p. 10. 



6 Hofmeister, Jahrb. f. wiss. Bot., 1863, Bd. in, p. 96. 



6 Wiesner, Das Bewegungsvermb'gen d. Pflanzen, 1881, p. 45. Cf. Rothert, 1. c., pp. 141, 152. 



7 On traumatropism cf. Pollock, Botanical Gazette, 1900, Vol. xxix, pp. 17, 50; on rheo- 

 tropism, Juel, Jahrb. f. wiss. Bot., 1900, Bd. xxxiv, p. 530. 



