﻿LLOYD— ABSCISSION 



is considerable thickening of the walls of certain tissues (especially 

 the prosenchyma sheath and cortex), from one to four or five tiers 

 of cells (pi. figs. 3, 4, 15) may undergo some of the changes in form 

 leading to separation, although it is seldom that the cells of more 

 than one tier actually complete the process. This is usually the 

 uppermost, although occasionally one below this may conclude 

 separation, but in a restricted region only. In no case, however, 

 do all the superimposed separation tiers pass entirely across the 

 stem, for in the pith usually one tier only is engaged, only occa- 

 sionally more. The maximum number of tiers activated is to be 



found in the prosenchyma sheath, where the resistance against 

 longitudinal growth is greatest, due to the thickness of the cell 

 walls and to the presence of the vascular tissues. 



The mechanical resistances doubtless affect also the direction 

 taken by the separation layer. Although, broadly speaking, this 

 follows the direction of the tiers of cells, it tends to depart from it, 

 and, as the epidermis is approached, to pass into the cell tiers 

 below (pi. figs. 3, 4), as if the tendency to lie normal to the abscis- 

 sion cell axes was to some extent overcome by the tendency to lie 

 normal to the stem axis. The maximum irregularities in direction 



