2l8 PLANT GROWTH SUBSTANCES 



higher concentration of amide required. The absence of any deforming 

 epinastic effects from the amide recommends its use where tender new 

 growth is concerned. In the meantime, the apple industry is making 

 increased use of NAA and its sodium salt for apple thinning and will 

 probably continue to do so until more effective and more consistent 

 materials are found. 



In the case of peaches it is of interest to note that Southwick et al. 

 (40) found no thinning effect from either the sodium salt of NAA up to 

 40 ppm. or its methyl ester up to 20 ppm. in the sprays. Murneek and 

 Hibbard (33) also reported the use on peaches of the sodium salt as a 

 thinning spray in the range of 5 to 40 ppm., but the conditions of their 

 experiment were such as to make interpretation of the results somewhat 

 difficult. In one test there appeared to be an actual increase in set over 

 the controls, but the counts were made before drop was completed. 

 The data of another test appeared to show a slight reduction in set with 

 the use of hormone sprays although, without the benefit of any statistical 

 evaluation, it is questionable that the difference is significant. It should 

 be recalled that applications of the naphthalene derivatives have also 

 been shown to be without effect on the preharvest drop of peaches. 



Growth Substance Effect in Relation to Stage of Development 



From the previous discussion it would appear that the available 

 hormone chemicals offer a possible solution to problems involving a 

 delay of the preharvest abscission, whereas in the case of flower and 

 young fruit abscission the same compounds offer no hope of increasing 

 fruit set and in fact, in many instances, will promote fruit shedding. The 

 reason for this seemingly different action of growth substances at these 

 different stages of development is of fundamental importance in the 

 approach to any problem involving abscission. Admittedly the question 

 calls for more investigation, but with little information available some 

 speculation may be permissible. 



Type of abscission. — In studying the abscission of apple fruits both 

 MacDaniels (26) and McCown (30) found that the anatomical changes 

 accompanying the preharvest drop differ in character from those of the 

 early drop of young fruits and flowers. Early drop according to these 

 workers is associated with a definite preformed abscission layer resulting 

 from secondary cell division. In late drop the changes are characterized 



