THE ORIENTATION OF FOLIAGE-LEAVES 257 



Vochting, and Krabbe have shown that leaves are not only plagio-heliotropic but also 

 plagio-geotropic. Naturally other factors may influence the position assumed, and 

 among these autogenic epinasty is included, which is often extremely pronounced. 

 Evidence of its existence is afforded by the fact that the leaves often curve strongly 

 backwards when the action of gravity is eliminated on the klinostat (Fig. 47)*. 

 When such a plant is inverted, plagio-geotropism and epinasty co-operate so that 

 a very rapid curvature ensues, but if a stronger curvature is produced than in 

 Fig. 47 B, on placing the plant on a klinostat a certain hyponastic lessening of the 

 curvature ensues, in place of the original epinasty. The epinasty of certain 

 leaves appears to be increased by a rise of the intensity of diffuse illumination, 

 and possibly a photonastic action of this kind may be responsible for the rising up of 

 leaves in weak light or in darkness. Further evidence is, however, required, for many 

 leaves curve downwards instead of upwards in darkness. The ' radical ' leaves of 

 many plants which become more or less erect in darkness press themselves against 

 the soil in strong light, and may even curve downwards when the plant is raised 

 above the level of the soil 2 . 



The leaf in many cases droops more or less owing to its own weight, but 

 nevertheless the plagiotropic orientation will take place under water, in which an 

 upthrust is exercised on the leaf 3 . In many cases complicated bending or actual 

 torsion is required to return the leaf to its proper position, but since this also is 

 produced under water, it cannot be due to the mechanical action of the weight of the 

 leaf, as de Vries supposed to be the case 4 . Any lateral curvature of the leaf may 

 tend to produce torsion, but nevertheless the energy of movement is sufficient to 

 overcome this action not only in the case of leaves but also of flower-stalks 5 . It is 

 also certain that some of the torsions shown by branches are not mechanical in 

 origin, as Baranetzsky 6 supposed them all to be. 



Orienting torsions are produced in darkness by gravity, but are still better shown 

 as the result of suitable lateral illumination 7 , although in many cases only under the 

 conjoint action of a gravitational stimulus. Thus, on a klinostat the leaves of Viola 

 and Dahlia no longer react to lateral illumination, while those of Phaseolus, Sofa, 

 and Acacia orient themselves to the light by pronounced curvature without torsion 8 . 

 The flowers of Viola orient themselves by torsion on a klinostat to lateral illumina- 

 tion, so that the co-operation of gravity is not always required for the production of 

 torsion '. It is, however, uncertain whether the orientation of the leaves of Malva 



1 F. Darwin, Linn. Soc. Journ., 1881, Vol. XVIII, p. 426; Vochting, I.e., 1888, p. 534; 

 Krabbe, 1. c., 1889, p. 248 ; Schwendener und Krabbe, 1. c., 1892, p. 340. 



8 Frank, Die natiirl. wagerechte Richtung von Pflanzentheilen, 1870, p. 45 ; Darwin, Insecti- 

 vorous Plants, 1876, p. 343; Wiesner, 1. c., 1880, p. 43; F. Darwin, I.e., 1881, p. 430; Vochting, 

 Bewegungen d. Bliithen u. Friichte, 1882, p. 179; Neger, Flora, 1903, p. 371. 



* Bonnet, 1. c., 1762, p. 61 ; Frank, Bot. Ztg., 1873, p. 55 ; Noll, I.e., p. 222. 

 4 De Vries, I.e., 1872, p. 266; Wiesner, I.e., 1882 ; O. Schmidt, I.e., 1883. 



6 VOchting, 1. c., 1888, p. 552 ; Noll, 1. c., 1885-7, PP- 220, 337. 



Baranetzsky, Flora, Ergzbd., 1901, pp. an, 194. 



7 [The suggested terms ' geotortism ' or ' geostrophism ' and * heliotortism * or ' heliostrophism ' 

 are as unnecessary as would be ' helioturgotropism ' or ' geoheterauxecism.'] 



8 Krabbe, 1. c., 1889, p. 244; Schwendener u. Krabbe, 1. c., 1892, p. 339. 



9 Schwendener u. Krabbe, 1. c., 1892, pp. 327, 335, 348. 



PFEFFER. Ill S 



