2] 



GEOTROPISM 



393 



same direction ; but all additional growth is vertical so that 

 the radicle bends sharply downwards it is positively geotropic. 

 The curvature does not take place at the very tip of the root, 

 in the region of growth by assimilation, -- but immediately 

 behind in the region of stretching or of growth by imbibition. 

 The precise region at which the curvature occurs was deter- 

 mined by CIESIELSKI ('72), by means of a method illustrated 

 in Fig. 106. It is here seen that 

 the maximum bending occurs in this 

 radicle at between 3 and 4 mm. from 

 the tip. It is also plain that the 

 geotropic curvature is in some way 

 connected with growth could not 

 occur without growth. 



To show that the normal vertical 

 growth of a root is due to the press- 

 ure of gravity, KNIGHT, in 1806, 

 determined experimentally that the 

 direction of its growth can be deter- 

 mined by other pressures replacing 

 gravity, such as centrifugal force. 

 Thus when seedlings were attached 

 to the rim of a wheel and this was 

 made to rotate rapidly about a hori- 

 zontal axis the radicles grew straight 

 outward from the axis ; thus in the 

 sense in which the centrifugal press- 

 ure acted, as before they had grown 

 in the sense of the pull of gravity. 



A second method of proof, employed by SACHS ('79), consisted 

 in eliminating the effect of gravity by means of the klinostat 

 (Chapter V, 1), under which circumstances the root grew 

 irregularly. Such results leave no room for doubt that it is 

 gravity which determines the verticality of the root. 



The question now arises, in what way does gravity control 

 the direction of growth of this organ ? An early suggestion 

 was that the action was direct, due to the relatively great 

 specific gravity of the root tip ; but this idea was easily refuted 

 by a mass of evidence. Thus it is not, a priori, easy to see 



FIG. 100. An originally straight 

 radicle of the pea, graduated 

 in 0.5 nun. spaces and placed 

 nearly vertically with its apex 

 directed upwards in the direc- 

 tion of a, has turned down- 

 wards in the zone of greatest 

 growth at between 3.5 and 4 

 mm. from the tip. (From CIE- 

 SIELSKI, '72.) 



