A. E. MURNEEK 337 



the heavy setters. By using still higher concentrations it is possible to 

 remove the apple crop completely if that be required. This rather late 

 thinning by means of a spray is more desirable than spraying in full 

 bloom at which time it is difficult, often impossible, to tell what the 

 crop is apt to be. 



Here then we have another illustration that the same growth regulat- 

 ing substance may have opposite effects depending on the concentration 

 used. At a relatively weak concentration NAA may initiate flower pro- 

 duction, foster the formation and growth of fruit and prevent fruit 

 abscission, while at higher concentrations it can stop fruit development 

 and cause their drop. 



How can NAA accomplish these rather striking results in fruit thinning 

 without harming the tree or the remaining fruit? Some Hght may be 

 shed on this question by considering the natural sequences in embryo, 

 endosperm, and fruit development of the peach and apple. 



Embryo Development and Fruit Growth 



The peach and other stone fruits require the presence of an embryo 

 for their growth. The same is true of the apple and other pome fruit, 

 which usually contain several seeds. Parthenocarpy is a rare phenomenon 

 with the genera Primus and Malus. Tukey (85) found that the peach 

 fruit develops in three stages: i) There is a rapid increase of the pericarp, 

 including nucellus and integuments, for about forty to fifty days after 

 full bloom. During this stage growth of the embryo does not parallel 

 that of the pericarp. It remains embryonic, in a kind of arrested develop- 

 ment. Note should be taken of the fact that development of the endo- 

 sperm usually precedes that of the embryo (27). 2) During stage two 

 the embryo grows rapidly to a maximal size. The duration is five to 

 forty days depending on the variety, that is, how early the fruit ripens. 

 Increase of the pericarp is at a relatively slow rate. 3) The pericarp or 

 fruit increases rapidly in size to the time of fruit ripening. In very early 

 varieties stage three is initiated while the embryo is in a period of rapid 

 growth, with the result that it fails to reach full size (aborts), the nucellus 

 and integuments collapse, the fruit ripens rapidly and drops from the 

 tree. This cyclic type of growth seems to be characteristic of other 

 cultivated drupe fruit. 



Tukey (87) artificially destroyed embryos in the peach at various 

 periods. Killing in stage two of development resulted in an abrupt check 



