214 PLANT GROWTH SUBSTANCES 



variety. In general the hormone sprays are more effective on early- 

 maturing types than on late varieties, but there are exceptions to this 

 generahty. It is probable that in early varieties not only are tissues of 

 the stem and also the leaves in a more reactive condition, but also that 

 the sprays are appHed during periods of higher temperatures than in 

 the case of late varieties. 



Another factor which may account for the observed varietal differences 

 in response of apples is the variation in the mode of the abscission process. 

 McCown (30) stated that varieties differ in the order in which the 

 various tissues in the abscission zone begin their process of abscission 

 and that those varieties in which the pith abscission is delayed until 

 after the bark tissues show signs of separation respond most readily to 

 hormone sprays. In the case of Mcintosh, a variety on which hormone 

 sprays are generally reported as being effective for a period of only 10 

 days, he pointed out that abscission in the pith tissues begins very early, 

 by the time there is noticeable striping of the fruit, and he questioned 

 whether hormones could be very effective after this time. This observa- 

 tion may also explain the general lack of appreciably longer response of 

 Mcintosh to a second spraying, as has been reported by Batjer and Marth 

 (2), Murphy (34), and others, although with some varieties this pro- 

 cedure is effective. 



The existence of varietal differences in apples in their response to 

 2,4-D has already been noted, most varieties being quite indifferent to 

 this compound in both tree and fruit reactions. Edgerton (13) has pub- 

 lished some results comparing the effect of NAA and its methyl ester 

 and also 2,4-D and its methyl and amyl esters on apple petiole drop and 

 fruit drop in three varieties. The petiole drop test consists of cutting 

 uniform leafy shoots from the trees and placing their bases in water, 

 cUpping off the leaf blades, spraying the petioles with the growth- 

 substance solutions, and recording subsequent petiole abscission over a 

 period of days. Edgerton found that the petioles of Stayman Winesap 

 and Winesap varieties responded to 2,4-D and its esters, thus agreeing 

 with the observed response in fruit drop control with these two varieties. 

 Mcintosh, on the other hand, failed to respond to these compounds in 

 both fruit and petiole abscission. On Stayman Winesap and Winesap 

 the order of effectiveness of the various compounds on petiole drop 

 agreed quite well with the observed order of effectiveness in control 

 of fruit drop. It would appear from the limited evidence thus far 



