1918] Hodgson: Foliar Abscission in Citrus 427 



the process itself are now being carried on. Chemical studies as to 

 the nature of the process, the character of the cell walls involved, the 

 oxidations and reductions occurring, the enzymes present, and the 

 nature of the propagation of the stimulus are now under way and 

 w ill be reported on later. 



Summary 



1. The abscission zone arises at or ntnir the base of an internode, 

 abscission occurring at the base of the terminal leaflet and also at the 

 base of the petiole. 



2. The grooved ring present near the internode at the base of the 

 petiole and at the base of the terminal leaflet does not bear any definite 

 relation to the location of the abscission zone. 



3. At the base of the terminal leaflet the abscission zone lies 

 roughly opposite the grooved ring. At the base of the petiole it arises 

 eight to ten tiers of cells distal to the groove. 



4. The abscission zone is pre-formed and ready to function upon 

 proper stimulation. 



5. In the 3'oung material there are no visible histological differences 

 delimiting the abscission zone from the tissues on both sides. In older 

 material the cells of the abscission zone are smaller, isodiametric, and 

 have denser contents than those of adjoining tissues. 



6. The abscission zone consists of ten to eighteen layers of cells. 



7. The separation layer is located at the upper end of the abscission 

 zone. 



8. The process of abscission conforms to the usual type, involving 

 the separation of the cells along the plane of the middle lamellae. 



9. No cell divisions or elongations of the tertiary membranes have 

 been observed during the process. 



10. All tissues across the petiole except the tracheae and the cuticle 

 function in separation. 



11. The first stage in cell separation consists in a marked swelling 

 and gelatinization not only of the middle lamellae but of the entire 

 wall with the exception of the tertiary membranes. This is followed 

 by the dissolution of this gelatinized wall by hydrolysis. 



12. There is little evidence that turgor plays any considerable role 

 as a causal agent in separation. 



13. All of the cells in the abscission zone show marked swelling and 

 gelatinization and after separation resume growth and cell division 



