254 TROPIC MOVEMENTS 



impressed upon them. According to Baranetzsky *, however, these 

 branches are negatively geotropic, but show no geotropic curvature because 

 at the same time an equal and opposed epinastic curvature is excited. 



Baranetzsky found that the apices of the branches of Prunus, Fraxinus, Tt'lia, 

 Ulmus, Philadelphus, and other woody plants always performed a curvature when 

 directed vertically upwards or downwards or when rotated on a klinostat, the original 

 upper side becoming convex. This epinasty is induced by the action of gravity in which- 

 ever side happens to be uppermost. The induction is transitory and reversible, the 

 curvature being automatically straightened again, while the shoots on a klinostat continue 

 to grow in any direction in which they may be placed. Since the epinastic curvature 

 is not shown in the normal plagiotropic position, it must be balanced by an opposed 

 tendency to negatively geotropic curvature *. The latter actually appears according 

 to Baranetzsky when a branch is laid flat, which was previously erect or had been 

 rotated on a klinostat for a long time. Hence the epinasty is apparently more 

 slowly induced, but persists longer when the exciting agency is removed than does 

 the hyponastic geotropic induction. Since the epinastic tendency is shown by 

 straight branches, it cannot result, as Baranetzsky supposed, from any realized 

 curvature, although the latter does actually awaken reactions directed towards its 

 removal. 



This suppressal of the geotropic reaction is only possible when the epinastic and 

 hyponastic tendencies alter correspondingly as the inclination varies. An autogenic 

 epinasty may aid in balancing the negative geotropism, but it is impossible to follow 

 de Vries in ascribing all plagiotropism to the antagonism of autogenic epinasty and 

 negative geotropism 3 . Wiesner considers that changes of position are due to varia- 

 tions of epinasty, the negatively geotropic action remaining constant. He also 

 concludes that the autogenic epinasty attains its maximal value with a medium rate 

 of growth, so that either a diminution or increase of the average rate of growth 

 increases the geotropic erection. Many of the objects in which Baranetzsky could 

 detect no autogenic epinasty appear to possess this power 4 , but it does not follow 

 that relationships of the kind described exist in all cases, nor does their discovery 

 reveal the causes producing them. 



Whatever its origin may be, we are dealing with a positively geotropic reaction 

 when a lateral shoot takes the place of the decapitated apex of a Pine, or when 

 without injury the shoots of certain other plants, as occasionally happens, assume an 

 erect position. The distribution of the buds and the factors which affect their 

 development naturally exercise a considerable influence on the type of branching 6 . 

 In addition, all long slender branches droop downwards more or less as the result of 



1 Baranetzsky, Flora, 1901, Ergzbd., p. 138; Frank, I.e.; de Vries, I.e.; Vochting, Organ- 

 bildung im Pflanzenreich, 1884, Bd. II, pp. 4, 93 ; Wiesner, Ber. d. bot. Ges., 1903, p. 321 ; Sitzungsb. 

 d. Wien. Akad., 1902, Bd. cxi, Abth. i, p. 733. 



a [If this is so, the growth of the under side should presumably be more rapid in the normal 

 position than it is on a klinostat.] 



8 Cf. Baranetzsky, 1. c., p. 141. 



4 Wiesner, Sitzungsb. d. Wien. Akad., 1902, Bd. cxi, Abth. i, p. 733. 



8 Cf. Goebel, Organography, 1900; Wiesner, I.e., p. 326. 



