﻿BOTANICAL (,.!// /// 



cell wall cells gives way, partly by pulling away the entire thinned 

 walls (pi. figs. 5, 8) and partly by tearing them. The resulting 

 wound surfaces of the separated parts seem, both to sight and touch, 

 mucilaginous, as Hannig observes, but this is not due to the entire 

 dissolution of abscission cells, but to the fact that, on account of 

 the thinness of the walls, many of them break, allowing their 

 mucilaginous contents, accompanied by protoplasm, to ooze out 

 on to the exposed wound surface. The contrast, in this respect > 

 with the analogous surfaces of other plants (notably for example, 

 in Parthenium and Impatiens) cannot be adduced as evidence of 

 the nature of the process leading up to it. The only thing we 

 might say of it is that it would lead one to examine the antecedent 

 facts more closely, in order to detect essential differences, should 

 they obtain. In this case I believe there are none. 



Summary 



1 . Previous to abscission activity proper there is no antecedent 

 structural indication of the position of the abscission zone. In 

 young organs usually only one tier of cells is involved, while in old 

 ones (internodes) evidence of physiological activity is to be seen 

 in 10-12 tiers (the abscission zone) approximately. The greatest 

 activity is to be seen in 1-5 tiers of cells, constituting the sepa- 

 ration layer, at the upper limit of the abscission zone. Here new 

 transverse walls occur in varying numbers, giving rise to the "Fol- 

 gemeristem" of Von Mohl. The parenchyma at the base of a 

 mature internode shows many transverse walls, which, however, 

 have nothing to do with abscission, and do not constitute the 

 criterion of an abscission zone. Hannig's "Losungsschicht" and 

 the "separation layer" of this paper may be regarded as identical, 

 while his "Trennungsschicht" does not coincide with the "abscis- 

 sion zone" as here conceived. 



2. Abscission begins in the internode near its base at two points 

 which lie in a plane normal to that of the opposed leaves, and in 

 the innermost part of the cortex. From these two points it is 

 propagated outwardly toward the epidermis, and inwardly toward 

 and into the pith. When more than one tier of cells is engaged, 

 the changes which overtake them usually progress most rapidly 

 in the uppermost (most distal) tier. 



