GENERAL 161 



always produces a positive curvature towards the light. This is, however, 

 the natural result of the altered orienting action, and fails to reveal the 

 distribution of irritability in the leaf, for the movement continues only 

 until the appropriate plagio-phototropic position is again assumed. 



Historical. Numerous facts concerning orienting movements were noted by 

 Bonnet *, while Knight and de Candolle investigated the geotropic and heliotropic 

 responses more intimately. Dutrochet 2 then pointed out that light and gravity acted 

 as inducing stimuli, and showed that natural orientation is the result of the varied 

 co-operation of geotropism, heliotropism, autotropism, weight, and so forth. Our 

 special knowledge of the different modes of orientation is due mainly to the labours 

 of Hofmeister, Frank, Sachs, de Vries, Darwin and F. Darwin, Pfeffer, Wiesner, 

 Krabbe, and Vochting. Subsequent authors are quoted in the text concerned with 

 their special studies. 



Frank 3 followed Dutrochet in his attempt to give a full account of the various 

 factors concerned in the orientation of the plant and its organs. Apart from a few 

 errors and certain hypotheses based on insufficient proof, such as the supposed 

 polarity of the cell-wall, Frank's work corresponds in its general outlines to our 

 modern views. This applies also to Frank's transverse heliotropism and geotropism, 

 although de Vries * erroneously concluded that the unilateral action of gravity and 

 light was only capable of inducing parallelotropic orientation, and hence considered 

 that all plagiotropic positions were due to the antagonism of parallelotropism with 

 other tendencies to curvature. The actual existence of a diaheliotropic irritability 

 has been shown by Darwin and F. Darwin, while Pfeffer on more general grounds 

 came to the same conclusion 5 . A variety of instances of plagiotropic orientation 

 due to the isolated action of a single tropic agency were then brought forward 6 . 

 Several authors have, however, unfortunately failed to distinguish clearly between 

 nastic and tropic curvatures. 



Sachs adopted de Vries's view, and applied it to dorsiventral organs, incidentally 

 discovering several important facts, and more especially showing that the same agency 

 might simultaneously excite more than one tendency to curvature. Sachs 7 sup- 

 posed that the thallus of Marchantia might be considered to consist of cylindrical 

 elements arranged at right angles to the surface, and showing parallelotropic orienta- 

 tion ; but the facts that unicellular organs may show various modes of orientation, 



1 Bonnet,' Unters. iiber den Nutzen der Blatter, 1762. 



7 Dutrochet, Recherches anatomiques et physiologiques, 1824, P- 9 2 - 



3 A. B. Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870 ; Bot. Ztg., 1873, p. 17. 



* De Vries, Arb. d. hot. Inst. in Wiirzburg, 1872, Bd. I, p. 223. The supposition of Wiesner 

 (Die heliotropischen Erscheinungen, 1880, n, p. 50), that the fixed light-position of leaves is due to 

 the antagonism of their negative geotropism and negative heliotropism comes under the same 

 category. 



5 Darwin, The Power of Movement in Plants, 1881, p. 374 ; F. Darwin, Linnean Society Journal, 

 1881, Vol. xvm, p. 420; Pfeffer, Pflanzenphysiologie, i. AufL, 1881, Bd. II, p. 291. 



6 Vochting, Bot. Ztg., 1888, p. 200; Krabbe, Jahrb. f. wiss. Bot, 1889, Bd. xx, p. an; 

 Schwendener und Krabbe, 1892, Gesammelte Abhandlg., Bd. II, pp. 255 u. s. w. ; Czapek, Jahrb. f. 

 wiss. Bot., 1898, Bd. xxxii, p. 271. 



7 Sachs, Arb. d. bot. Inst. in Wiirzburg, 1879, Bd. II, pp. 226, 254. 



