5° 



BOTANICAL GAZETTE 



[JANUARY 



latter becomes concave on the lower side). In such cases geo- 

 tropic curvature becomes possible only in stems with a leaf at the 

 apex, but not in stems with a leaf at the base. 



6. All these experiments become intelligible on the assumption 

 that each leaf has a tendency to send shoot-forming substances 

 toward the apex and root-forming substances toward the base of 



* 



the stem. If it could be proved that in Bryophyllum calycinum 

 a specific substance (hormone) is responsible for the geotropic 

 growth (in the cortex of the lower side of a horizontally suspended 

 stem), we might say that both substances show a tendency to 

 collect on the lower side of a horizontally placed stem, and that the 

 flow of both is influenced in the same way by the leaf. The apical 

 leaf sends both substances toward the base of a stem, while the 

 basal leaf acts as if it had a suction effect upon geotropic substances 

 contained in the apical region. Such an idea su 

 the fact that a leafless stem has the center of its geotropic curvature 

 in the middle (fig. 15), while a stem with a leaf at the base has 

 either no curvature (fig. 16) or has it only in the region of the leaf 

 While in Bryophyllum the hypothetical geotropic hormone is 



from 



forming hormone 



m 



be asso- 



would 



plants the hypothetical geotropic substance 



ciated with the shoot-forming hormone. This 



fact that in certain fir trees a horizontal branch next to the apex 



may suddenly become negatively geotropic when the apex is cut 



off. After the decanitation the 



he decapitation the (hypothetical) geotropic substance 

 e was flowing to the apex now can flow into the hori- 

 :hes next to the apex, and the one which by chance 

 more of the substance than the others will be the first 



to become vertical. 



;he mechanical advantage due to 

 continued flow of these substances 



becomes 



Rockefeller Institute for Medical Research 



New York City 





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