THE EVIDENCE FOR A GROWTH-INHIBITING 

 SUBSTANCE IN THE PEAR TREE 1 



H. S. REED and F. F. HALMA 



It is the purpose of this paper to call attention to the manner 

 of development of new shoots on the cultivated pear tree and to 

 discuss the evidence for a growth-inhibiting substance afforded 

 by the phenomenon. The observations were made on a block of 

 young Bartlett pear trees growing on the grounds of the Citrus 

 Experiment Station. 



Figures 1 and 2 show young branches of a Bartlett pear tree 

 and the effect of heading them back. The points at which the 

 original shoots were cut off are plainly seen. Figure 1, A, shows 

 that two buds developed after the original shoot was amputated 

 and that the shoot from the upper bud reached a size many 

 times greater than that arising from the next lower bud. Figure 

 1, B, likewise shows that two subapical buds developed into 

 shoots. The disparity in size is not so marked as in the former 

 example, but it is nevertheless apparent. Figure 2 shows a 

 vigorous shoot which was twice cut back. During the winter it 

 was cut at A and in July of the following summer three of the 

 new shoots were cut off at B. Following the first pruning, six 

 vigorous shoots were produced from buds lying close to the point 

 of amputation. The three upper shoots were plainly more 

 vigorous than the three lower, though the distance between the 

 buds from which they arose is not great. As a result of the July 

 pruning, whorls of new branches were produced below the points of 

 amputation in essential repetition of the previous development. 

 Had the three upper shoots been unpruned they would have 

 remained with few or no branches as did the three lower shoots. 

 Emphasis must be laid on two features shown by these examples : 



1 Paper 59, University of California, Graduate School of Tropical Agriculture 

 and Citrus Experiment Station, Riverside, California. 



239 



