258 TROPIC MOVEMENTS 



neglecta * on a klinostat, and the similar instances observed by Darwin 3 , afford true 

 cases of torsion. No orientation by torsion is produced by the action of light upon 

 most dorsiventral flowers, whereas gravity exercises this effect upon the flowers of 

 Aconitum, Delphinium, and Scrophularia*. 



Tropic orientation to a single agency may be performed by torsion as well as by 

 curvature, and since the former only requires the existence of a physiological dorsi- 

 ventrality, it is not surprising that Schwendener and Krabbe * should fail to detect in 

 the peduncle of Aconitum any visible signs of morphological dorsiventrality. On the 

 other hand, Vochting 6 found that the small flowers, which Impatient develops in dark- 

 ness, act like radial organs. Noll 6 assumed that the supporting axis radiated an 

 ' exotropic ' influence upon the orientation of dorsiventral flowers and leaves, but 

 there is no evidence of any such action in the case of leaves, while the stalks of the 

 dorsiventral flowers of Aconitum place themselves at a definite angle with the perpen- 

 dicular, and hence with the axis of the inflorescence, owing to their geotropic irritability. 

 According to Czapek 7 the pedicel of Aconitum performs its orienting torsion when the 

 flower is removed, but according to Meissner this is not the case 8 . 



It is probably owing to correlative influences that after the severance of the 

 inflorescence of Orchis the flower-buds near to the injury perform simple geotropic 

 curvatures in assuming their proper position instead of the normal torsion move- 

 ments 9 . In addition, a realized torsion excites a counter-action, which is sufficient to 

 remove the torsion of a pulvinus of Phaseolus when the agency inducing it is removed. 

 Autogenic torsions may also occur, as, for instance, when the leaves of Allium 

 ursinum and Alstromeria change from the inverted position to the normal one as 

 they expand from the bud. These leaves are also capable of aitiogenic torsion 10 . 



Although the detailed mode of production of torsion is unknown it certainly is 

 not necessarily always the result of growth movements, although these usually 

 accompany it. Noll u assumes that torsion is due to the co-operation of dissimilar 

 tendencies to curvature, which may possibly apply in certain cases in spite of 

 Schwendener and Krabbe's dictum to the contrary 12 . The fact that certain torsions 

 cease when the stimulating action of gravity is eliminated shows that the combined 

 action of more than one stimulus may be necessary to produce them. 



If a plant of Chenopodium, Coleus, or Helianthus is inverted and the curvature of 

 the main axis prevented, the leaves at first sink slightly owing to their own weight. 

 An upward curvature then begins, due to the co-operation of epinasty and geotropism 

 or heliotropism, which continues in active leaves until the dorsal side again faces 

 upwards. In many cases this curvature is not completed, owing to the early or late 



I Vochting, Bot. Ztg., 1888, p. 534. 2 F. Darwin, I.e., 1881, p. 426. 

 3 Noll, Arb. d. bot. Inst. in Wiirzburg, 1885-7 > Schwendener u. Krabbe, 1. c., 1892. 



* L. c., p. 317. 5 Vochting, Jahrb. f. wiss. Bot., 1893, Bd. xxv, p. 179. 



6 Noll, I.e., 1885-7, Bd. Ill, p. 367; Flora, 1892, Ergzbd., p. 273; Schwendener u. Krabbe, 

 1892, Gesammelte Abhandl., Bd. II, p. 255. 



7 Czapek, Jahrb. f. wiss. Bot., 1898, Bd. XXXII, p. 279. 



8 Meissner, Bot. Centralbl., 1894, p. 12. 9 Noll, I.e., p. 329. 

 10 Czapek, Flora, 1898, p. 249. Cf. Goebel, Organography, 1900, p. 250. 



II Noll, 1. c., 1885-7 and l8 9 2 - Cf. also Meissner, Bot. Centralbl., 1894, Bd. LX, p. i. 

 12 Schwendener u. Krabbe, 1. c., 1892. 



