GROWTH SUBSTANCES FOR GEOTROPISM 191 



between the upper and lower sides cannot take place, and one 

 would expect that the halves would grow at the same rate at 

 which they do when in a vertical position. According to Weber, 

 this is not the case, for although the lower half grows at approxi- 

 mately the normal rate, growth is retarded in the upper half. In 

 both cases, the growth substances apparently accumulate on the 

 side that is downward. Since the cut surface of the upper half is 

 on the under side, it might be assumed that wounding influences 

 the effectiveness of the growth substance and decreases the rate 

 of growth, whereas no effect of this kind is present in the under 

 half. This agrees with some of Cholodny's (1931c) experiments, 

 in which the retarding influence of unilateral wounding upon the 

 rate of growth depended upon how the wound surface was oriented 

 with respect to the direction of gravity. The rate of growth 

 in Hehanthus hypocotyls, with the wound on the top, w^as 1.5 to 

 2 times as great as that in plants with the wound on the under 

 side. 



The circumstances that exist in split dicotyledonous stems 

 should be considered briefly. It is known that a median longi- 

 tudinal cut retards the rate of growth appreciably (Sachs, 1873 ; 

 Schtscherback, 1910) and that when a split stem is brought into 

 a horizontal- position, the under half grows more rapidly than the 

 upper half (and faster than a control half which is placed in a 

 vertical position). In an investigation of geotropism in many 

 plants, Ahrens (1934) found that when hypocotyls were halved 

 lengthwise, only the lower halves reacted geotropically. The 

 results of these experiments, however, are influenced so greatly by 

 traumatic effects that it is impossible to draw final conclusions 

 about the production of the geotropic curvature. 



Loeb (1917) marked portions of Bryophyllum stems into 

 1 cm. segments with India ink. Each stem portion possessed 

 one or more vigorous leaves and was placed in a horizontal 

 position. After several days, geotropic curvatures took place as 

 a result of stretching growth of the cortex in the lower half. In 

 other experiments, the stem was split lengthwise into two equal 

 halves, each having one vigorous leaf left at the apex. The stem 

 portions were suspended horizontally in moist air, one with the 

 cortex below, the other with the cortex above (Fig. 58). Only 

 that half stem with the cortex oriented downward showed 

 geotropic bending. It may be pointed out that the tissue into 



