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



of the important problems referred to above. The studies in ques- 

 tion will attempt to shed light on transpiration problems, nutrition 

 problems, and others equally important. The paper which is sub- 

 mitted herewith forms an introductory contribution to the subject 

 under investigation. 



The writer is not unaware of the essential similarity between the 

 physiological problems presented by citrus and other fruit trees. He 

 has chosen, however, to study the physiology of the citrus tree as a 

 separate entity because of the reasons given above, and the further 

 one that the peculiar climatic conditions under which this tree is 

 frequently placed in the arid southwest, demand a special treatment. 

 Doubtless much may be gained from these studies which will apply 

 to physiological problems connected with other trees. 



The data here presented were obtained during an investigation of 

 one of the so-called physiological diseases above mentioned, namely, 

 the June drop. 1 Ever since the Washington Navel orange has been 

 grown in the dry interior valleys of Arizona and California, this 

 variety has been subject to excessive dropping of the young fruits. 

 This has came to be known popularly as the June drop although the 

 fall of the fruits is by no means confined to June but may occur at 

 any time from petal fall, in April, until the fruit reaches several 

 inches in diameter in August. The prevalence and amount of this 

 dropping seems to be influenced to a marked degree by certain 

 environmental factors to which the trees are subject. The regular 

 annual shedding of the young fruits is most serious in regions where 

 the annual precipitation is lowest, the mean summer temperature 

 highest, atmospheric humidity lowest, solar radiation most intense, 

 and air movement greatest during the growing season. That the 

 excessive drop of young fruit is in some way intimately connected 

 with extreme climatic conditions is indicated by the fact that in some 

 parts of southern California, where the drop is ordinarily not excess- 

 ive, the hot wave of June 15-17, 1917, during which a temperature 

 of 118 F was experienced in the Riverside and Redlands districts, 

 was immediately followed by a drop so severe that practically the 

 entire young crop of navel oranges was lost. 



The experimental work from which the data were obtained was 

 carried on at Edison, Kern County, California. Edison comprises 



i This investigation, which is now in progress, was carried on in collabora- 

 tion with Professor J. Eliot Coit who planned the first series of experiments 

 and began the work in February, 1916. A joint-authorship paper correlating 

 this and other aspects of the June drop phenomenon is in course of prepara- 

 tion. 



