JACQUES LOEB 703 



When we dip isolated leaves with their apices into water an enor- 

 mous root and shoot formation will occur in the notches of the apex 

 but nowhere else. The proof that the rapid growth of roots and 

 shoots in the apex inhibits the formation of roots and shoots in 

 the other notches was furnished by the following experiment. After 

 some time the leaves were taken out of the water and suspended in 

 moist air and the shoots formed in water were removed. The roots 

 formed in water dried out. Very soon roots and shoots began to 

 appear in the upper notches of the leaf thus proving that the growth 

 in the upper notches had originally been suppressed by the more 

 rapid growth of the roots and shoots ■ formed at the apex when the 

 latter was dipping into water. 



If we turn from these experiments to the observations on root for- 

 mation in whole stems suspended horizontally (Figs. 8 and 9), we reach 

 the conclusion that the upper side of such a stem (with the exception 

 of the extreme basal end) is free from roots for the reason that 

 water or solutes or both collect a little more freely on the lower side 

 of the stem, thus favoring root formation on that side. The greater 

 rapidity of growth of the roots on the lower side creates secondary 

 conditions by which the growth of roots on the upper side is inhibited. 

 When we remove the lower half of a horizontally suspended stem we 

 remove this inhibitory influence and now roots can grow out on the 

 upper side. 



Since in all these cases the horizontally suspended stems bent 

 geotropically, whereby the under side became convex, it might be 

 argued that this convexity was the cause of the greater abundance 

 of roots on the under side. A series of simple experiments showed 

 that this was not the case. Pieces of stem were bent passively and 

 tied in this position to sticks of wood (Fig. 15). They were sus- 

 pended in such a way that they turned their concave side downwards. 

 In all cases, without any exception, the roots developed on the lower 

 sides of the stems which were now concave (Fig. 15, after 35 days). 

 They also developed on the under side when the bending of the stem 

 was prevented by cork rings and when the stem was enclosed in a 

 glass tube, as in Fig. 16. 



