188 GROWTH HORMONES IN PLANTS 



side, and this may be the reason for different rates of growth on 

 the two sides, although the supply of growth substance is equal. 

 Some observations of a geotropic curvature in the barley 

 coleoptile cannot be explained by the unequal distribution of 

 growth substance (Weber, 1931). When the coleoptile is placed 

 in a horizontal position, a difference in the growth rates of the 

 upper and lower sides arises throughout its whole length. If it is 

 placed in an upright position after it has been geotropically 

 stimulated for 30 minutes, there follow, after the first negative 

 curvature, several positive and negative curvatures which appear 

 to be due to alternating retardation and promotion of the rate of 

 growth in the upper zones. These may be identical with the 

 pendulum-like movements which appear in all negative geotropic 

 curvatures before the position of equilibrium is attained. No 

 curvatures appear in the basal portion of the coleoptile where 

 the vigorous growth is evenly distributed on both sides. Still 

 other cases are known where no growth-substance displacement 

 or geotropic curvature is observed, even though growth substance 

 is present and growth takes place. The young internodes of 

 grasses, for example, behave in this fashion. Further discussion 

 of these cases may be found elsewhere. 



DICOTYLEDONOUS HYPOCOTYLS, SHOOTS, ETC. 



The main stem of the higher plants is almost always negatively 

 geotropic. Experiments with hypocotyls and stems of dicoty- 

 ledons have resulted in substantial contributions to our knowl- 

 edge of the role of growth hormones in geotropism. 



Stimulation and Response. Distribution of Geotropic Sensi- 

 tivity. — Satisfactory data on the distribution of geotropic sensi- 

 tivity in seedling axes have been obtained by Herzog (1925) 

 with the Piccard (1904) method. The following hypocotyls 

 were investigated: Vicia sativa, Brassica napus, Linum usitatis- 

 simum, and Lepidium sativum. All of these were shown to be 

 geotropically sensitive only in the apical portion, and the length 

 of the zone in each species was 11, 18, 16, and 12 mm., respec- 

 tively. The sensitivity appears to be evenly distributed through- 

 out this region. The ability to respond extends beyond the 

 limits of the sensitive zone, which means that a part of the curva- 

 ture is induced by a conducted stimulus ; the length of this region 

 is about one-third of the entire portion which has the capacity 



