UNIVERSITY OF CALIFORNIA PUBLICATIONS 



IN 



BOTANY 



Vol. 6, No. 15, pp. 417-428, 3 figs, in text 



February 1, 1918 



AN ACCOUNT OF THE MODE OF FOLIAR 

 ABSCISSION IN CITRUS 



BY 



EOBEET W. HODGSON 



The study wliich is reported on here represents one phase of an 

 investigation on the shedding of immature oranges of the Washington 

 Navel variety, begun by Professor J. Eliot Coit and the writer in the 

 spring of 1916, and reported on in part elsewhere (Hodgson, 1917). 

 It developed that the young fruits drop from the trees while still alive 

 and actively functioning; therefore the shedding is typical abscission 

 in contrast with exfoliation which involves the cutting off of dead or 

 dying plant parts as a result of the activity of a cork cambium. This 

 latter phenomenon has been rather thoroughly investigated wuth a 

 wide diversity of plant materials, but most of the studies on the former 

 have hitherto had to do with herbaceous or shrubby plants in which 

 the walls of the cells involved are relatively thin. In view of the 

 economic importance of this process of abscission in the genus Citrus, 

 causing as it does an annual loss of many thousands of dollars, and 

 because of the thickness of the cell walls concerned, it has seemed 

 desirable to make a rather intensive study of the abscission process in 

 the leaves as well as in the young fruits. The study which constitutes 

 the subject of this paper has to do with the former, a consideration 

 of the process as it occurs in the fruits being reserved for presentation 

 at a later time. A somewhat extensive histological and cytologieal 

 study of the abscission zone, the separation layer, and the process itself 

 as it takes place in the leaf has yielded results which have been so 

 definite and striking that they are deemed worthy of ])i'»sfiitation in 

 a preliminary paper at this time. 



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