Previous analysis showed that, in its initial phase, the geotropic response of Lens culinaris L. roots cannot be explained by a simple action by sliding, pressure or contact of amyloplasts on a sensitive surface located along the longitudinal wall. In this study another mode of action is tested by considering the following parameters as functions of the roots inclination: (1) the distance (d) which the amyloplasts move; (2) their number of contacts (mean c) with parietal cytoplasm; (3) the variable (sin alpha) of the transversal component of the statolith weight (mean M x g sin alpha). It is shown that the initial rate of curvature (mean V), at the various angles, is related to the sedimentation of the amyloplasts by the equation mean V = a log b mean d mean c sin alpha (where a and b are constants). The results obtained prove that the geotropic stimulation is dependent upon the sine of the angle (alpha) of the root inclination and explain the sine rule deviation. The role of statoliths is discussed in the light of recent literature on growth inhibitors which are involved in the geotropic reaction.

When roots of lentil (Lens culinaris L., cv. Large blonde) were placed in horizontal position for 2 h, their upper side elongated faster than their lower side, and also faster than vertical controls. The length of the cortical cells was greater in the upper half than in the lower half of roots which had been horizontally stimulated for 2 h. The zone of curvature extended from the distal part of the meristem to the proximal part of the cell elongation zone. The curvature in the meristem was due to early differentiation of the cells of its upper part. In the proximal part of the cell elongation zone, bending took place due to inhibition of cell growth in the lower half of the root. The results obtained are in agreement with the hypothesis of lateral transport of an inhibitor in gravistimulated roots. This inhibitor should be present in greater amounts in the lower side of the stimulated root and in lower amounts in its upper side than in the vertical controls.