288 University of California Publications in Agricultural Sciences [Vol. 3 



Another distinctive feature of the Kellogg orchard is that it is 

 protected on three sides by a fairly efficient windbreak. On the north, 

 from which direction the prevailing winds blow, this consists of a 

 double row of pepper trees (Schinus molle), and a single row of 

 poplars. On the other two protected sides, the east and the west, there 

 are rows of eucalyptus. 



The Nature of June Drop 



A cursory investigation of the problem at once established the fact 

 that the young oranges are shed while still alive and actively function- 

 ing and as such the shedding constitutes true abscission. It is of 

 course quite a different process from exfoliation, which involves the 

 formation and activity of a phellogen. Before proceeding to a dis- 

 cussion of the process of abscission as determined by us, it may be 

 well to discuss the amount of bloom, time of abscission, reaction time, 

 and other important features. 



Navel orange trees growing under the conditions studied always 

 bloom very heavily (pis. 27, 28, and 29). The blossoms are borne on 

 shoots of the current season 's growth, being preceded and accompanied 

 by new leaves. The old leaves do not fall until anthesis is well under 

 way or completed. It is evident, therefore, that during anthesis the 

 trees are under a heavy drain, inasmuch as they are called upon to 

 support a heavy bloom in addition to both the new and old crops of 

 leaves. Shedding of the unopened flower buds occurs to a small extent 

 only. The opened flowers exhibit a certain amount of dimorphism. 

 Those capable of setting fruit possess large, fully formed ovaries, with 

 plump styles and stigmas. In many of the flowers, however, the 

 pistils show a varying degree of degeneration and shedding of the 

 flowers is largely confined to such individuals, beginning with the least 

 robust and grading off during petal fall and including many of the 

 most robust after petal fall. The period of maximum shedding takes 

 place when the young fruits are from one-half to two centimeters in 

 diameter. At first the point of abscission is always at the base of the 

 pedicel (pi. 30), but after the diameter of the fruit has reached one 

 centimeter or thereabouts it is usually at the base of the ovary. It is 

 interesting to note that where the larger fruits absciss at the base of 

 the ovary, abscission usually occurs also in the cortex at the base of 

 the pedicel; but on account of the formation of strengthening tissue 

 the process is not completed through the vascular elements and although 

 the pedicel dies, it remains very firmly attached to the twig. This is 

 shown in plate 31. It often happens that a certain amount of strength- 



