460 LECTURE XVIII. 



tropic. In other words, if we resolve the force of gravity into 

 two forces, the one acting at right angles to the long axis of 

 the organ, and the other along it, it is only the former that is 

 active in producing a geotropic effect. It is clear that such a 

 resolution cannot be made when the organ is either horizontal 

 or vertical ; in the former position the force of gravity acts 

 wholly at right angles to the long axis of the organ, in the 

 latter, along it ; hence the geotropic effect is greatest in the 

 former position, and it is zero in the latter, and the greater the 

 angle of deviation of the long axis of the organ from the 

 vertical towards the horizontal, the more marked will be the 

 geotropic effect. Sachs bases this view upon the fact that 

 geotropic curvature is more rapidly produced when an organ 

 is horizontal than when it is in any other position. 



The correctness of this view is rendered somewhat doubtful 

 by the observations of Elfving. He found, namely, that when 

 primary roots, growing in moist air, were placed so that their 

 apices were directed upwards, that is, at an angle of 180 to 

 their normal direction, they curved downwards indeed, but 

 never so much as to assume their normal vertically downward 

 direction; that is, the apex in no case travelled through 180. 

 In some cases the angle described was only a small one ; in 

 others it was 90, so that the roots remained horizontal ; in 

 others it was as large as 130. The angle finally assumed 

 in each case is clearly the angle at which gravity ceased to 

 exert a geotropic influence, the differences in the size of the 

 angle being the expression of individual peculiarities of 

 irritability. In no case did a root grow vertically upwards, 

 but all curved downwards more or less. The conclusion which 

 Elfving draws from these observations is that the geotropic 

 effect of gravity is greatest when the angle of deviation is 

 1 80. 



It may be objected that the geotropic irritability of the 

 roots may have been interfered with in these experiments by 

 the abnormal conditions under which the roots were placed. 

 Sachs has, in fact, proved that this is the case. Elfving him- 

 self states that when roots growing in earth were placed in the 

 inverted position, they curved downwards so as nearly or 



