GROWTH SUBSTANCES FOR GEOTROPISM 189 



for curvature. More precise evidence for the transmission of 

 the geotropic stimuhis in stems is lacking. 



In BeUis, geotropic sensitivity extends over the greater part of 

 the flower stalk but is not uniformly distributed. An apical zone 

 of 7 mm. is more sensitive than the remaining portion. 



Negatively Geotropic Curvature in Stems. — Sachs (1872, 1888) 

 measured the upward curvature of various stems in order to 

 discover the course of negatively geotropic curvatures. The zone 

 of growth and curvature of the stem was found to be as long as 

 15 to 40 cm. At the beginning, the strongest bending was in the 

 vicinity of the tip. As the bend moved downward, the upper 

 originally curved zones straightened; finally, the basal, slowly 

 growing zone often showed a sharp curvature. Sometimes the 

 tip curved back and forth several times before a final position of 

 equilibrium was reached. Sachs showed that negatively geo- 

 tropic curvatures in stems arise by an increased rate of growth on 

 the lower and an accompanying retardation of growth on the 

 upper side. Sachs did not determine whether the average rate of 

 growth changed during bending. Cholodny (19296), using a 

 micropotometer, concluded that the hypocotyls of Lupinus and 

 Helianthus grow at the same rate, whether in the horizontal or 

 in the erect position. 



The Growth-substance Explanation. — In order to show that 

 the geotropic curvature of an organ is produced by the function- 

 ing of growth substance, it is necessary to prove (1) that a growth 

 substance is present in the plant organ in question and (2) that 

 the organ responds to the application of the growth substance, 

 the rate of growth being either increased or decreased. If proof 

 of these two points can be obtained, it must be shown, in addition, 

 that gravit}^ brings about an unequal distribution of growth 

 substance. This differential distribution can occur in two 

 difTerent ways: either growth substance may be displaced to the 

 lower side as in the Avena coleoptile under the unilateral effect of 

 gravity, or increased formation of growth substance may take 

 place upon the lower side. It is possible, also, that the unequal 

 distribution of another substance influences the action of the uni- 

 formly present growth substance. 



Experiments with Split Stems. -^Avaong the early investigations 

 concerning the role of growth hormones in negatively geotropic 

 curvature of stems, those of Loeb (1916, 1917) should be men- 



