180 



GROWTH HORMONES IN PLANTS 



Koch (1934) found that when a lateral half of the tip of an 

 A vena coleoptile (1.5 mm. long) is cut away, making the growth- 

 substance supply unilateral, then both geo- and phototropic 

 effects can be compensated for, so that no curvature results 

 (Fig. 55). For example, when the side with the tip present was 

 oriented upw^ard in the horizontally placed coleoptiles, 18 out of 

 22 showed no negative geotropic bending. 



A 



y^ 



i 1 1 i i 



Gravdty 



IP 



(I 



J3 



be- 



^ 



ib^ 



D 



i 1 i 4 i 



Gravity 



B 



Fig. 55. — Diagrams showing that excision of half portions of the tips of Avena 

 coleoptiles affects their tropic response to light and gravity. A, when the half 

 tip is removed from the side nearest the light, strong phototropic curvature 

 takes place, due to downward movement of the growth hormone from the half 

 tip which remains on the back side. B, when the half tip is cut out from the 

 side away from the light, either no response takes place (due to radially sym- 

 metrical distribution of the hormone) or a small negative curvature results, 

 presumably from the slightly greater amount of growth hormone on the side 

 beneath the intact half tip. C, growth-hormone displacement due to gravity is 

 diminished when the half-tip portion is removed from the lower side. D, 

 negatively geotropic curvature occurs when the half tip is oriented below. 

 {After Koch, 1934.) 



The problem of how this unequal distribution of growth sub- 

 stance occurs was investigated by Dolk (19296). On the apical 

 end of each of several decapitated Avena coleoptiles was placed 

 a block of agar containing growth substance. The coleoptiles 

 then were placed in a horizontal position, and the growth sub- 

 stance exuding from the basal end was collected separately from 

 the upper and lower sides (Fig. 2). The results (Table 14) 

 indicate that the lower side gives off more growth substance than 

 the upper side. This difference might arise from unilateral 

 changes in permeability due to the action of gravity. According 

 to Nuernbergk (1933), wounding {e.g., by decapitation) retards 



