FRANK E. GARDNER 219 



chiefly by alterations in the cell walls in an abscission zone. It must be 

 assumed that the action of the hormones in the late drop is to delay the 

 weakening of the cell-wall material so that separation of the cells is 

 slowed, but that these compounds do not prevent the secondary cell 

 division which results in the formation of an abscission layer observed 

 in the case of early drop. The fact should not be overlooked that growth 

 substances may also delay apple leaf abscission, particularly in instances 

 where the leaf blades have been removed as in the petiole test for hor- 

 mone activity. Delay of leaf fall in apple orchards has also been observed 

 in cases where 2,4-D was used. No description has been found in the 

 literature of the mode of abscission of apple leaves, but in certain other 

 plants the process appears to be a combination of cell-wall changes in 

 certain tissues and abscission layer formation in other tissues. 



Both early and late abscission delayed. — The possibility exists that there 

 is actually no fundamental difference in the response of the tissues to 

 growth substances in early and late abscission. Persistence of flowers, 

 or at least of their petals, for an appreciable period following a hormone 

 spray is a common observation. The usual occurrence following the 

 spraying of young fruits, particularly with a rather high concentration, 

 is a noticeable delay in their abscission, even though the subsequent drop 

 is increased. Thus growth substances may have a direct eifect in delaying 

 the weakening of the cell walls regardless of the type of abscission in- 

 volved. The increased drop of young fruits when abscission is resumed 

 might be explained on the basis of competitive antagonism between the 

 apphed compounds and the natural auxins produced by the developing 

 fruit which provides a steady supply to prevent abscission just as the 

 leaf blade provides its petiole with an anti-abscission auxin. This is 

 highly speculative but not without some basis of experimental evidence. 



Problems Needing Solution 



To date the record of the research worker in solving many of the 

 fruitgrowers' problems by means of growth substances is not outstanding 

 in relation to the seeming possibilities. It is true that control of pre- 

 harvest drop has been rather thoroughly studied and the fruit industry 

 has adopted these sprays as a regular practice and to great advantage. 

 The use of hormone sprays for fruit thinning, an unexpected outgrowth 

 from the attempts to improve set, is also gaining in orchard use. There 



