170 GROWTH HORMONES IN PLANTS 



accumulation of food takes place on the lower side of the hori- 

 zontally placed stem owing to the effect of gravity and that this 

 causes more growth upon the lower than upon the upper side. A 

 negatively geotropic curvature results. 



The theory did not stand under experimental tests, for Johnson 

 in 1828 showed that during geotropic curvature the root can over- 

 come a resistance which is at least one hundred times the weight 

 of the root tip. Furthermore, Pinot found in 1829 that a root 

 can bend into mercury, in spite of the fact that the specific 

 gravity of the root tip is much less than that of the mercury. 

 It may be seen from these investigations that work is performed 

 in the positive geotropic curvature of a root and that this curva- 

 ture is active rather than passive. 



In spite of these investigations. Knight's theory was taken up 

 anew from the anatomical standpoint by Hofmeister more than 

 j&fty years later. No anatomical differences between the root and 

 the stem could be found as a basis for explaining these different 

 reactions. Hofmeister concluded that the cause must be some 

 fundamental difference or differences in regions where the curva- 

 tures actually occur. In the root, the zone in which the down- 

 ward bending takes place is made up of similar cells which 

 constitute "a region capable of curvature following the effect of 

 gravity like a drop of viscous liquid." On the other hand, the 

 portion of the stem which curves geotropically consists of differ- 

 entiated elements which are stretched taut next to each other. 

 Hofmeister (1867) concluded from a few experiments that the 

 upward geotropic curvature of the stem probably results from 

 the fact that "in the lower longitudinal half of the organ, the 

 extensibility of the cell membranes is increased." The explana- 

 tion of increased extensibility is that when the stem is in a 

 horizontal position, more water can penetrate the membranes 

 of the cells on the underside than those on the upper side. 



Frank (1868^.; see Rawitscher, 1932) and other investigators 

 opposed this hypothesis of Knight and Hofmeister, because they 

 were convinced that the downward curvature of the root is an 

 active process. The experiments and conclusions of Johnson 

 and Pinot formed the basis for a long controversy which was 

 finally decided in favor of Frank. The principal point of Frank's 

 theory is that positive as well as negative geotropic curvatures are 

 vital processes which result from changes in growth. 



