GEOTROPISM. 11 449 



of this kind a compensatory reaction afterwards occurs which tends to bring 

 about the original form. This compensatory reaction makes itself evident 

 also after a mechanically induced bending of the organ, and it begins, not 

 only after the cause of the bending has been removed, as in the case of geotro- 

 pism on the klinostat, but also when the organ is subjected to a weight acting 

 continuously and unilaterally. Its effect is, certainly, then only to flatten the 

 curve, not to remove it altogether. Autotropism must also play a part in 

 the straightening of the geotropic over-curvature, which (p. 434) we preferred 

 to regard as due to renewed and converse geotropic activity in the overcurved 

 organ. Autotropism is associated with the increased growth of the side 

 rendered concave by geotropism (Fig. 135). As BARANETZKY showed, an 

 organ, such as a shoot of A esculus, may, after suffering geotropic curvature ex- 

 hibit on the klinostat several pendulum-like movements, because, just as in 

 the case of geotropism, the autotropism overshoots the mark and is not at 

 once arrested on the attainment of the straight position but causes a new curva- 

 ture opposed to the first and with it autotropic growth on the opposite side. 



Returning to the discussion of branches of trees, BARANETZKY (1901) 

 found that during the evolution from the bud these frequently showed negative 

 geotropism (orthotropism) in a very striking manner, and afterwards took up 

 the oblique rest position. Apart from other factors, which we need not speak 

 of here, autotropism is especially responsible for this supplementary curvature 

 away from the vertical ; it causes the axis of the branch to lie in the direction 

 already determined by the relation of the bud to the stem. According to 

 WIESNER (1902) autotropism has nothing to do with this rest position ; he 

 believes it due to a special peculiarity of the lateral branch, which we have not 

 as yet referred to, acting in combination with geotropism. This peculiarity is 

 termed epinasty, and expresses itself in the effort on the part of the lateral organ 

 to grow more vigorously on the upper than the under side. It must not be 

 assumed, however, that this is an hereditary characteristic of the morpho- 

 logically upper side or that dorsiventrality of the branch has been established ; 

 on the other hand, epinasty must be acquired in the course of the life of the 

 individual, if we interpret WIESNER correctly, by the influence of the weight of 

 the branch. It is impossible to pass over WIESNER' s theory without mention, 

 yet the explanation discussed above appears to us to be more worthy of credence. 



A closely related problem, and one of great importance, is, what induces 

 the bud to take up a definite direction in space ? It may be easily shown, by 

 cultivation on the klinostat, that buds, like lateral roots, form definite angles 

 with their parent axes, due to correlations only, and not at all to external factors. 

 If NOLL'S klinostat theory be correct, then the lateral roots are not withdrawn 

 from the influence of gravity on the klinostat ; if the chief root be placed in the 

 axis of rotation of the instrument lessening of the limiting angle must take place 

 in geo-perception ; if placed at right angles to that axis the angle must be in- 

 creased. The essential experimental data on this subject are as yet non-existent. 

 As may be seen especially clearly in the root, the special angle induced in experi- 

 ments on the klinostat is, as a general rule, larger than the limiting angle formed 

 with the co-operation of gravity. In virtue of the internal directive force the 

 lateral roots would lie more horizontally ; their position as observed in nature 

 is a resultant. As a matter of fact, we can make the lateral roots approximate 

 to the chief roots and decrease the limiting angle by using a higher speed of rota- 

 tion (p. 451). The position of lateral shoots is also a resultant of geo- and 

 auto-tropism, and not infrequently it may be noted that as, in the course of 

 development, autotropism decreases negative geotropism predominates, so that 

 axes of inflorescence, for example (e.g.Aesculus), come to place themselves quite 

 vertically. Briefly, we may say that the influence of the chief axis on the lateral 

 organ expresses itself in the' special direction' and in the angle at which the lateral 



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