GROWTH SUBSTANCES FOR GEOTROPISM 199 



one horizontal, a curvature still appeared at the lower node. 

 Stimulus reception and response both appear exclusively in the 

 lower node. Schumacher (1923) objects to Miehe's experiments 

 because the vertical position of the upper node is essentially a 

 position of stimulation, since the shoots are plagiotropic. Accord- 

 ing to investigations by Uyldert (1931), the influence of the upper 

 node upon the lower one is brought about by the giving off of 

 growth substance. Actually, the presence of growth substance 

 was not demonstrated in the organs in question, but when growth 

 and geotropic curvature of the node was checked by removal of 

 the internode, an application of growth substance brought about 

 the usual response to gravity. 



ROOTS 



Roots are very sensitive to the force of gravity, but not all roots 

 exhibit the same type of response. Primary roots grow toward 

 the earth's center, i.e., they are ^positively geotropic; branch roots 

 frequently grow out at an angle and assume a position more or less 

 transverse to the direction of the earth's force, i.e., they are 

 diageotropic. Recent investigations indicate that growth hor- 

 mones, similar in nature to those occurring in leaves and stems, 

 control in a pecuUar way the processes of growth and tropic 

 curvature in roots. 



Stimulation and Response. Distribution of Geotropic Sensi- 

 tivity.— In roots of Vicia Faba, the geotropic curvature at first is 

 evenly distributed over the 3 mm. at the tip of the root (Sachs, 

 18826). After 23 hours, the curvature in the extreme tip has 

 disappeared, the region just behind is only sUghtly bent, and most 

 of the curvature is back about 3 mm. This means that the 

 response is confined to a very short region in contrast to the dis- 

 tribution of the negatively geotropic curvature of the stem. 



It was shown by Ciesielski (1872) and Darwin (1881) that the 

 ability of the root to curve geotropically could be checked by 

 removal of the root tip. From this, Darwin concluded that the 

 stimulus is perceived in the tip and transmitted from there to 

 the growing zone where the reaction takes place. Keeble and 

 Nelson (1935) found that amputation of the tip 1 mm. nearly 

 always removed the capacity for geotropic curvature even when 

 the root remained in a horizontal position for 24 hours. That this 

 decapitated root could still perceive the stimulus of gravity was 



