Geotropism and the Growth Substance

1. When all growth substance is removed from the Avena coleoptile, growth comes to a standstill, and only recommences when growth substance is again present. 2. Unter the influence of the lateral action of gravity the total amount of growth substance produced in the coleoptile does not change. 3. The growth of the Avena coleoptile is not affected by rotating on the horizontal axis of the klinostat and subsequently returning to the vertical position. 4. The lateral action of gravity changes the direction of transport of the growth substance, so that more goes to the lower side of the coleoptile, and less to the upper. This was proven both for Avena sativa and for Zea Mays. 5- Shortly after fhe termination of the stimulus, the growth substance transport is again normal. 6. The deviation from normal transport is not limited to the tip, but occurs also in the more basal zones. 7. The geotropic sensitivity of the coleoptile decreases only slowly from the tip to the base, a zone about 10 mm long being sensitive. 8. The course of the curvature was registered with a cinematographic camera, and compared with the curvatures resulting from phototropism and from the unilateral application of growth substance. 9. The curvature of geotropism was thus found to move down the coleoptile more rapidly than that due to phototropism. The apical zones begin to straighten out soon after the stimulation. xo. The course of the curvatures can be explained qualitatively, on the theory of Went and C h o 1 o d n y, by the change in growth substance transport. ix. The presence of the tip is one of the factors controlling the autotropic straightening of the apical zones. 12. The geotropic curvature is due to the temporary deviation of the transport of growth substance to one side of the coleoptile over a considerable zone. This deviation leads to the using up of the other factors necessary for growth, and thus allows autotropic straightening to take place. 13. A possible mechanism whereby the perception of gravity results in a deviation from the normal growth substance transport is suggested. 14. The chain of reactions constituting geo-induction is influenced by the subsequent action of gravity in the longitudinal axis of the coleoptile. xj. The shorter is the geo-induction, the smaller is the subsequent influence of gravity acting longitudinally. This influence therefore does not appear in determinations of the presentation time.