190 GROWTH HORMONES IN PLANTS 



tioned. In a series of experiments with Bryophylhim calycinum, 

 it was found that geotropic growth takes place on the lower side 

 of horizontally suspended stems and that the response is much 

 greater if a leaf is present on the stem (Fig. 1). From his 

 numerous experiments, Loeb postulated the presence of hormones 

 as a plausible explanation for the observed phenomenon. 



Gradmann (1925) tried to determine the presence of growth 

 substance during geotropic curvatures in different plants belong- 

 ing to the Labiatae and Scrophulariaceae. Whole intern odes of 

 Mentha and other plants were placed in a horizontal position, and 

 the epidermis was removed from the top and bottom sides. 

 Other internodes were split lengthwise, and the cut surfaces 

 placed in contact with the exposed surfaces of the whole inter- 

 nodes. After a time, the central internode curved toward the 

 upper contact surface. This and other similar experiments led 

 Gradmann to the conclusion that growth-promoting substances 

 are present in the lower half. However, the experiments were 

 complicated by the fact that traumatic effects were introduced 

 by the removal of the epidermis. 



The experiments were criticized by Cholodny (1927, 1929a, b, 

 1931c, /; see Gradmann, 1931), but the various hypotheses and 

 auxiliary hypotheses which were propounded scarcely warrant 

 discussion here, especially since no final decision concerning 

 the interpretation of the Gradmann experiments has seemed 

 possible. An improved method would be very desirable for 

 clarifying the situation. 



If Gradmann's experiments could be considered as conclusive 

 evidence, it would follow that the growth-substance content of the 

 lower halves is greater than that of the upper halves, even when 

 both are isolated. This difference cannot be attributed to 

 growth-substance displacement. Both halves would react sepa- 

 rately during the geotropic reaction, and a harmonic combined 

 effect would not result. It is of fundamental significance to know 

 whether this assumption is actually correct, and from this point 

 of view it might be profitable to study geotropic response in spHt 

 stems. 



When intact barley coleoptiles are placed in the horizontal 

 position, they show an increased growth rate on the lower side and 

 a retarded growth rate on the upper side (Weber, 19266). If the 

 coleoptiles are split lengthwise, an exchange of growth substance 



