GROWTH SUBSTANCES FOR GEOTROPISM 



201 



zontal position, and the size of the geotropic curvature was 

 measured in degrees. This value was corrected for the wound 

 curvatures caused by the incision. 



The following numbers were obtained: 



Incision on the upper side. . . (— 39.7°) + ( — 7.0°) = —46.7° 

 Incision on the lower side.... (-27.7°) - (-7.0°) = -20.7° 



Snow concluded that the stimulus can be transmitted on the 

 upper side as well as on the lower side, although greater trans- 

 mission of the stimulus takes place on the under side of the root. 



Keeble and Nelson (1935) showed that although the secretion 

 of a growth substance inhibiting root elongation must be localized 

 in the 1 mm. tip zone of Zea mays, the region capable of receiving 



t/ 



V. 



\J 



V 



B 



Fig. 61. — Transverse insertion of mica plates into roots of Zea mays, and their 

 influence upon resulting curvatures. A, when the plate is inserted near the tip, 

 bending occurs away from the wound, due presumably to hindrance of growth- 

 hormone transport from the tip into the elongation region. B, if the plate is 

 inserted farther back from the tip, curvature occurs toward the wound, due 

 presumably to the accumulation of hormones in the elongating region. {After 

 Keeble and Nelson, 1935.) 



the stimulus of gravity extends possibly into the whole elongating 

 region. By inserting small mica plates transversely halfway 

 through the root at different distances from the tip, it was possible 

 to demonstrate that (1) negative curvature takes place when the 

 semisection hinders growth substance from being transported to 

 the elongation region on the cut side and (2) positive curvature 

 (toward the wound) results when the section hinders growth 

 substance from escaping from the region of elongation on the cut 

 side (Fig. 61). 



The Quantity-of-stimulus Principle. — A definite amount of 

 stimulus is necessary for the attainment of a minimum reaction 

 in the geotropic curvature of the root. The geotropic stimulus 

 intensity can be changed by altering the direction of perception of 

 gravity. The effect may be computed in this case as g sine (p, 

 where (p is the angle between the root and a vertical axis. Another 



