SPECIAL CASES 255 



their own weight. The growing apices are, however, usually not only strong enough 

 to bear their own weight, but also to curve vigorously upwards. In some cases the 

 parts which have become woody and ceased to elongate may perform an upward 

 curvature and so counteract the mechanical drooping of the branch. The young 

 shoots of the Pinus are at first more erect and then spread horizontally, but this is 

 not due to the influence of their own weight, as Baranetzsky supposed, for Wiesner 

 has shown 1 that to produce such a curvature a load of fifteen to thirty times the weight 

 of the branch is required in the case ofPinus Laricio. Vochting and Baranetzsky have 

 shown, however, that in certain weeping varieties the branches droop owing to their 

 own weight, and the apices continue to grow in the same direction without attempting 

 to curve upwards. 



Frank found that the branches of various trees returned to their original position 

 in both light and darkness after forcible displacement, whereas Baranetzsky observed 

 no such return. Further researches must determine whether the apparent contra- 

 diction is due to the existence of varying powers of reaction. Frank 2 also observed 

 orienting torsions in twigs of Abies in which dorsiventrality had been previously 

 induced, and these can hardly be mechanical in origin as Baranetzsky 3 suggests. 



SECTION 56. The Orientation of Foliage-leaves 4 . 



The leaves of such plants as Erica t Dracophyllum, and Viscum orient 

 themselves in regard to the stem alone, and so may stand out at various 

 angles with the perpendicular. Dorsiventral photometric leaves, however, 

 strive usually to place their surfaces at right angles to the direction of the 

 strongest diffuse light, whereas certain other leaves place themselves parallel 

 to it. Other leaves, again, place themselves parallel to the light only when 

 it is so intense that protection against it is needed 5 . 



Many responsive leaves when displaced in darkness return approxi- 

 mately to their original position, and if necessary by the aid of torsion, so 

 that gravity as well as light may act as an orienting stimulus, and Dutrochet 6 



1 Vochting, Organbildung im Pflanzenreiche, 1884, Bd. n, p. 90; Bot. Ztg., 1880, p. 595; 

 Baranetzsky, I.e., p. 216. 



3 Frank, 1. c., p. 22. See also Czapek, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 267. 



3 L. c., p. 203. 



4 Bonnet, Unters. iiber d. Nutzen d. Blatter, 1762, p. 45 ; Dutrochet, Keen. anat. et physiol., 

 1824, p. 126; Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870; Bot. Ztg., 1873, 

 p. 72 ; de Vries, Arb. d. bot. Inst. in Wiirzburg, 1872, Bd. I, p. 223; Wiesner, Die heliotropischen 

 Erscheinungen, 1880, Bd. n, p. 39; Biol. Centralbl., 1899, Bd. xix, p. i ; Darwin, The Power of 

 Movement in Plants, 1880; F. Darwin, Linnean Society Journal, 1881, Vol. xvin, p. 420; Schmidt, 

 Das Zustaudekommen d. fixen Lichtlage blattartiger Organe, 1883 ; Noll, Arb. d. bot. Inst. in Wiirz- 

 burg, 1885-7, Bd. in, pp. 189, 315 ; Flora, 1892, Ergzbd., p. 265 ; Vochting, Bot. Ztg., 1888, p. 501 ; 

 Krabbe, Jahrb. f. wiss. Bot., 1889, Bd. XX, p. 211; Schwendener und Krabbe, 1892 (Gesammelte 

 Abhandl. von Schwendener, Bd. n, p. 255) ; Oltmanns, Flora, 1892, p. 231 ; Czapek, Jahrb. f. wiss. 

 Bot., 1898, Bd. xxxn, p. 269; Flora, 1898, p. 429; Wiesner, Biol. Centralbl., 1903, Bd. xxm, 

 p. 209; Ber. d. bot. Ges., 1902, Generalvers. (p. 84). 



6 Cf. Ewart, Annals of Botany, 1897, Vol. XI, p. 447; Wiesner, Biol. Centralbl., 1899, 

 Bd. xix, p. i. 



6 Dutrochet, Memoires, etc., Bruxelles, 1837, p. 312; Vochting, Bot. Ztg., 1888, p. 549. 



