^^1^] Hodgson: Foliar Abscission in Citrus 419 



reduced to the vestigial structures known as the wings. Examination 

 of abscission in the trifoliate orange Citrus trifoliata, which possesses 

 a compound trifoliolate leaf, shows that separation occurs independ- 

 ently at the base of each leaflet, and also at the base of the petiole of 

 the leaf itself. In tlie ordinary citrus leaf it occurs both at the base 

 of the terminal leaflet and at the base of tlie petiole. Thus in either 

 case the abscission zone is situated at or near tlie base of an internode, 

 as is the case in most of the plants in which the process has been 

 investigated (cf. Goodspeed and Kendall, 1916; Lloyd, 1916; Kendall, 

 1917). 



The constriction which exists between the terminal leaflet and the 

 wings gives rise to a conspicuous grooved ring. Such a groove also 

 exists at the base of the petiole. These may be taken to represent the 

 node or simply motor tissue. Inasmuch as abscission takes place at 

 or near the base of the internode, and since such grooves are usually 

 found in this vicinity, some investigators have been led to believe that 

 they serve to delimit the location of the abscission zone. In the case 

 of Citrus, at least, it has been impossible to establish any such relation 

 (cf. Goodspeed and Kendall, 1916). Examination of very young 

 leaves shows no such grooved ring present at the base of petiole, and 

 leads to the conclusion that the grooves are formed because in tlie 

 development of the petiole certain cells do not increase in size as fast 

 as the neighboring cells on both proximal and distal sides. Since 

 abscission occurs in young leaves where no groove is present, the con- 

 clusion is forced that the grooved rings are formed subsequent to the 

 time at which the abscission zone becomes capable of activity. Indeed, 

 abscission was found to occur in leaves which were just developing, 

 a fact strongly indicating tliat the abscission zone is pre-formed and 

 ready to function at any time. 



In the case of the terminal leaflet, the abscission zone usually lies 

 roughly opposite the grooved ring, but at the base of the petiole ordi- 

 narily occurs eight to ten layers of cells distal to the groove. In the 

 latter case, however, considerable variation has been found in the 

 position of the abscission zone with respect to the grooved ring. Thus 

 it frequently happens that on the dorsal side tlie abscission zone and 

 the groove coincide, w'hereas on the ventral side the abscission zone is 

 located eight to ten tiers distal to the groove. The abscission zone 

 extends in a plate completely around the cortex and across the pith 

 both ill the case of the terminal leaflet and at the base of the entire 

 leaf. 



