THE PROGRESS AND MODE OF MOVEMENT 



233 



ones, granted that the differences in the rate of growth on the opposed 

 sides are the same in both cases, while the weight of the organ will favour 

 or retard curvature according to the direction of the latter in regard to 

 gravitational attraction. 



When a radial shoot is placed in a horizontal position its negatively 

 geotropic upward curvature l begins first in the more actively growing zone, 

 so that the curvature does not exactly follow the arc of a circle. As the 

 apex curves upwards it is more and more withdrawn from the stimulating 

 action of gravity, but nevertheless it curves beyond the vertical, partly 

 owing to the persistence of the geotropic induction and partly because the 

 lower zones are still inclined to the perpendicular and hence continue to 

 curve. The apical region then performs a return curvature by which it 

 becomes straightened after one or more oscillations 2 . Ultimately only the 

 basal portion remains curved although the reaction began latest in this 

 region, and only pro- 

 gressed slowly in it. It e 

 is, however, by no means 

 surprising that in many 

 cases the excess curva- 

 ture and resultant oscilla- 

 tion should not in all 

 cases be perceptible. 



Changes of position 

 produced by torsion are 

 readjusted in a similar 

 manner. Thus when 

 young leaves of Fraxi- 

 nus, Robinia, and other 

 plants are turned upside 

 down, the orienting torsion begins first at the tip of the leaf and then 

 progresses basally, so that the apex passes beyond the appropriate 

 position and is caused to perform a return torsion 3 . Since the torsion 

 and retorsion progress basally, the twisting curvature is ultimately 

 restricted to the basal zone, as can easily be seen in the leaves on droop- 

 ing branches of the Ash and Weeping Willow, which must curve through 

 1 80 in order that the upper and under surfaces may gain their appropriate 



FIG. 45. Shoot of Impatiens glandiiligera showing phases of geo- 

 tropic curvature (a-e). From photographs. 



1 Cf. Sachs, Flora, 1873, p. 324 ; Arb. d. hot. Inst. in Wiirzburg, 1873, Bd. I, p. 453 ; Ed. Ill, 

 Plates; H. Mtiller, Flora, 1876, p. 88 ; Kohl, Mechanik d. Reizkriimmungen, 1894, p. n; Rothert, 

 Cohn's Beitrage z. Biologic, 1896, Bd. VH, pp. 161, 210; Meischke, Jahrb. f. wiss. Bot., 1899, 

 Bd. xxxm, p. 338. On the cinematographic representation of curvature see Pfeffer, Jahrb. f. wiss. 

 Bot., 1900, Bd. xxxv, p. 741. 



2 Baranetzsky, Flora, 1901, Ergzbd., pp. 145, 159. 



3 Schwendener and Krabbe, 1892 (Schwendener's gesammelte Abhandlungen, Bd. n, p. 288). 



