138 PLANT PHYSIOLOGY 



1874). This fact indicates clearly that the angle is the resultant of two forces. 

 The relationships of lateral branches are of the same nature, for they also 

 assume a special angle on the klinostat, and this is subject to alteration by 

 negative geotropism. Negative geotropism has the effect of changing this 

 special angle, so as to make it sometimes less, sometimes greater, according 

 to the stage of development of the branch. Thus one often finds the axis of 

 inflorescence directed quite perpendicularly upward (e.g. in Aesculus), while 

 in Pinus all the young shoots at first stand up vertically, just like the candles 

 on a Christmas tree, and then gradually assume an oblique or horizontal 

 position. 



Taking all these facts into account the question arises whether plagio- 

 tropism is, generally speaking, determined by correlation influences. This is 

 certainly not so, since we shall learn presently that the main shoots and primary 

 roots may become plagio tropic in consequence of external influences. On the 

 other hand, we must note at this point that not only do the relations between 

 axes and lateral branches affect geotropism, but that other correlations also 

 play a part. Thus MIEHE (1902) has shown that the power of geotropic response 

 of many shoots depends on the presence of the growing point. After that is 

 removed no geotropic curvature takes place, although geoperception does not 

 occur in the growing point. 



Although we have previously stated that correlations, as well as geo- 

 tropism, determined the lie of the lateral branches, we have not exactly in- 

 dicated the force that operates in opposition to geotropism. According to 

 BARANETSKY (1901) this force is that of autotropism, which attempts to 

 undo curvatures induced by geotropism. It can scarcely be doubted that we 

 should attribute a significance of this kind to autotropism, the nature of which 

 we have already defined. Still there is another phenomenon which plays an 

 important part the so-called ' epinasty ' (WIESNER, 1902 ; PFEFFER, Phys. 

 III. 253). Under the influence of gravity there gradually arises the peculiarity 

 that the upper side of the branch exhibits a more vigorous growth in length 

 than the under side. Should epinasty be the only active agent, the branch 

 must bend so that the upper side becomes convex, while, owing to negative 

 geotropism, the under side should become convex. As a matter of fact, the 

 former curvature takes place on the klinostat, for although epinasty arises 

 from the unilateral influence of gravity, yet it does not cease at once if the 

 influence of gravity is put out of account, but operates for long afterwards. 



In addition to autotropism and epinasty, we must note finally the purely 

 physical action of the weight of the branch itself, which must produce an 

 effect in bending the later shoots. To these mechanical bendings are certainly 

 often to be attributed the vigorous reactions on the part of the plant which 

 again induce a lifting up of the shoot. Such weight curvatures are very obvious 

 in certain ' weeping trees ', such as Fagus and Salix babylonica (VoCHTiNG, 

 1884). 



450, 1. 23, after plagiotropic. read (According to HABERLANDT (1903) a low 

 temperature should quite inhibit geotropism. Further research is required 

 to show whether, in VOCHTING'S and LIDFORS'S experiments, the shoots were 

 geotropic.) 



452, 1. 55, after downwards, read BARANETSKY (1901) has in many cases 

 arrived at results contrary to those of FRANK ; the whole question wants 

 thorough investigation. 



454, 1. 24, for stand read strand 



455, 1. i, for weight read gravity 



