A GROWTH-INHIBITING SUBSTANCE 247 



This persistent tendency of artificially bent branches to produce 

 shoots on the upperside, is well illustrated in espalier or hori- 

 zontal cordon trees. Unless the dorsal shoots are promptly re- 

 moved when still small, the horizontal arms will suffer and the 

 shape of the tree will be spoiled. In changing the vertical 

 shoot of such a tree to a horizontal arm, the bending has to be 

 gradual because if done at one operation the ventral side of the 

 arm will be bare ; if done gradually the buds which will eventually 

 be on the lower side have a chance to grow, and the spurs will 

 form fruit buds before the arm finally assumes a horizontal 

 position. 



SUMMARY 



1. Young vertical shoots of the pear tree tend to remain un- 

 branched because of the dormancy of the lateral buds. Ampu- 

 tation of a portion of the shoot is followed by a development 

 of lateral buds situated immediately back of the point of 

 amputation. 



2. The manner of growth affords evidence for the hypothesis 

 that a growth-inhibiting substance is produced in the apical por- 

 tion of the shoot and that it travels towards the base, perpetuat- 

 ing a condition of dormancy in the subapical buds. 



3. Horizontal shoots of the pear tree produce the most vigor- 

 ous growth from buds near the dorsal line of the shoot, little 

 growth from those on the ventral line, and intermediate growth 

 from those situated between the dorsal and ventral positions. 

 This behavior indicates that the growth-inhibiting substance ac- 

 cumulates on the ventral side of such a shoot, thereby freeing 

 the dorsal buds from the restraining factors. 



