The Protective Layer as an Extension of the Apoplast

Abstract

The protective layer between the cell wall and plasmalemma of xylem parenchyma cells has variously been suggested to be involved in protection of the protoplast from attack by autolytic enzymes from neighbouring, dying cells, tylose formation, deep supercooling of xylem, and strengthening of the pit. None of these ideas has universal application to all species in which parenchyma cells possess a protective layer. It is proposed instead, that the protective layer is primarily laid down in order to preserve apoplastic continuity around the protoplast of a lignified cell, bringing the entire plasmalemma surface, and not just that part of it in contact with the porous pit membrane, into contact with the apoplast. If this is so, then other functions may be coincidental, or have arisen secondarily.