Growth Substances in Relation to the Production 



of Tree Fruits 



FRANK E. GARDNER 



THE propensity of many of the plant growth substances to influence 

 abscission of flowers, fruits, and leaves has been used to considerable 

 practical advantage in horticulture, particularly in relation to tree fruits. 

 Strangely enough, the growth substances may act not only to delay 

 abscission but also to implement it, and both phenomena have found 

 horticultural application. The answer to this seeming paradox is by no 

 means clear, although it is perhaps to be explained by the mode of 

 abscission encountered, which differs in different kinds of plants and in 

 different organs of the same plant. This thought will be referred to in 

 more detail as this discussion develops. 



This review of accomplishments with plant hormones will be restricted 

 to tree fruits and will treat of control of mature fruit drop and of fruit 

 thinning and its opposite corollary, fruit set, with brief mention also of 

 several problems which call for investigation. A mere review of hterature 

 would seem inadequate to the occasion if it did not treat of the subject 

 objectively, attempting to trace the advances made to the present 

 status of accomplishment, pointing out the failures and the present Wind 

 spots in our understanding, and developing some rationale for the ap- 

 proach to new achievements in the light of our present knowledge, 

 limited as it is. 



Control of Preharvest Fruit Drop 



Apples. — The possibiUty of using growth substances to control the 

 drop of apple fruits was suggested by the observed effect of these 

 compounds in causing the persistence of leaf petiole stubs on mis- 



