﻿i 9 16] LLOYD- ABSCISSION 217 



Evidence, however, will be furnished to show that a zone of 

 tissue, here designated the abscission zone, composed of a number 

 (in large internodes about 10-12) of tiers of cells below the sepa- 

 ration layer, but not above, shows enough visible alteration, quite 

 aside from cell divisions and distinguishable by optical and chemical 

 means, to warrant the conclusion that it constitutes a physiologi- 

 cally active zone concerned in abscission. It is not simply a second- 

 ary meristem, if by this term is implied a renewal of transverse 

 cell division, since this does not always occur, but a mass of cells 

 going through the initial steps, consisting most obviously of 

 chemical alteration of the cell wall, leading to abscission, and of 

 which only some cells finally conclude the process. These latter, 

 which are sufficiently active to go far toward and many of them 

 finally to conclude separation, constitute the separation layer. 

 A secondary meristem may or may not intervene. 



THE CENTERS OF ABSCISSION ACTIVITY AND THE DIRECTION OF ITS 

 PROPAGATION 



According to Hannig, abscission begins under the epidermis 

 (which soon tears) and travels around the cortex, finally passing 

 through the vascular bundles and into the pith. 



The fact appears to be that the first step in abscission (sepa- 

 ration) in internodes takes place in the innermost cells of the 

 cortex at two points in a plane normal to the plane of the opposed 

 leaves. From these points the process is propagated outwardly, 

 inwardly, and around the stem, but more rapidly toward the center 

 of the pith than toward the epidermis. Indeed, separation is 

 usually completed in the pith before the epidermis is ruptured 

 (pi. figs. 2, 4). Because of the longitudinal growth of the sepa- 

 ration cells, there is a considerable displacement (0.1-0.2 mm.) 

 of the parts to be separated, and there is a synchronous rupture 

 of the passive portions of the xylem.- We therefore find the vas- 

 cular tissues to be in an advanced state of disruption (text fig. 2d; 

 pi. fig. 14) before the epidermis is ready to break. When this 

 finally occurs, the fracture of the separation layer is most easily 

 accomplished. 



