one or two rows of cells [1]. If the scar faces are examined after fracture, separation seems to have oc-
curred along the line of the middle lamella, leaving the intact rounded cells covering the surface (Figure
2).
Light microscope and transmission electron microscope (TEM) observations implicate cell wall
breakdown as a major factor in the separation of these intact cells [3] (Figure 3). Degradation is particu-
larly prominent in the central region of the wall and involves not only the middle lamella but also the ad-
jacent areas of the primary wall [5]. The fracture bisects all the tissues in a stem or petiole, and studies
have shown that wall degradation occurs in all the different living cell classes along the fracture line, in-
cluding the epidermis and phloem [6].
206 SEXTONFigure 1 Scanning electron micrograph of a fracturing leaf abscission zone from bean. Note that the fracture
is confined to only one or two rows of cells. The tissue on the left is senescing and its diameter is contracting (note
folds), while that on the right is still turgid and is enlarging. This differential growth causes stresses at the ab-
scission zone interface which help separate the cells in the separation layer and rupture the stele. (From Ref. 1.)
Figure 2 Scanning electron micrograph of part of a bean abscission zone fracture surface. Note that the cells
on the exposed scar are round and turgid, having separated as a result of breakdown of the central areas of the
wall. (From Ref. 1.)