42 8 TUKEY 



chemical is adjusted to the variety, the season, and the vigor of fruit buds; 

 that is, stronger concentrations are used with varieties which characteristically 

 set heavily, with trees with vigorous fruit buds, and in seasons where weather 

 at blossom time is highly favorable to fruit set. Weaker concentrations are 

 used under the opposite conditions. Blossom thinning has proved one of the 

 most effective agents in maintaining annual production in American apple 

 orchards. Yet there is a feeling in some quarters that the fruit industry will be 

 better served by the use of solid blocks of a single variety of small controlled 

 trees upon which the blossoms are caused to set where desired, rather than 

 removed by somewhat uncertain means after they have set. 



To this end, studies with growth-regulating chemicals have shown some 

 possibilities. For example, the Kadota and Mission varieties of fig commonly 

 set fruit parthenocarpically, whereas the Calimyrna variety requires pollina- 

 tion and fertilization. Yet this last-named variety can be caused to set fruit 

 parthenocarpically by the use of certain growth regulators and without the 

 aid of the caprifying wasp. It now appears that the varieties which set fruit 

 without resort to pollination have a higher content of native hormone than 

 does the Calimyrna. Research shows that fruit set and fruit development are 

 related to liberation of specific hormones by pollen, by endosperm, by 

 embryo, and probably by other parts. Extracts of corn pollen and of corn 

 embryo will set tomato fruits. It is not too much to expect that the research 

 worker will in time appear with methods of controlling fruit set more exactly. 



Research in the field of plant regulators has made, and is still making, 

 contributions to the horticultural industry equal to, if not greater than, any 

 other field of endeavor at the moment. Not only are blossoms and fruits 

 both set and thinned, but the time of blossoming may be delayed. Maleic 

 hydrazide has proved suited to the raspberry and related brambles. No 

 practicable method of delaying fruit blossoming of tree fruits has yet been 

 devised, but the possibilities are there. Fall applications of such materials as 

 2,4-dichlorophenoxyacetic acid and naphthaleneacetic acid to cherry trees, for 

 example, have resulted in a 7- to 10-day delay in blossoming, but winter injury 

 to the tree is not uncommon and the developing fruit is often misshapen and 

 unmarketable. 



Sprays of certain plant regulators, as indolebutyric acid, applied to develop- 

 ing iigs at the proper time, cause them to ripen 14 days after treatment as 

 compared with 75 days for typical development of non-treated fruit. The 

 strawberry fruit develops because of the diffusion of plant-regulating ma- 

 terials from the achenes which dot its surface. About the sixteenth day after 

 bloom, the supply of regulator is low. Additional amounts of synthetic regula- 

 tors, as beta phenoxyacetic acid, applied at about this time, apparently induce 

 continued growth and increased size. Improved size of blackberries has 

 similarly been secured by similar treatments. 



The application of plant regulators to prevent pre-harvest drop of fruit 



