The Formation and Structure of the Cell Wall in Fibres and Tracheids

Abstract

A study has been made of the changes in organization of the primary wall of xylem fibres and tracheids during the phase of surface growth using both direct electronmicroscopic observations and replica techniques. It has been demonstrated that the transverse microfibril orientation on the inner surface of the cell wall differs from that on the outer surface. It has been concluded that the observations made are consistent with some form of multi-net mechanism of growth in these cells. Autoradiographic studies were consistent with this conclusion. Measurements of the variation in path difference in fibres and tracheids undergoing secondary wall formation showed a maximum near the centre of the cells decreasing towards the cell tips. From these observations and parallel autoradaographic studies it has been concluded that secondary thickening begins near the centre of the cells and progresses towards the cell tips. Since all layers of the secondary wall are lamellated these observations imply that more lamellae are present near the centre of the cells than at their tips, so that as secondary thicken ing proceeds the concentric lamellae of microfibrils grow progressively towards the ends of the cells. The lamellae of the secondary wall appear to develop in close association with the limiting cytoplasmic membrane (the plasmalemma). No evidence was obtained of the inclusion of cytoplasmic components in the developing secondary wall. At the conclusion of secondary thickening the cytoplasmic membranes retract on to the inner surface of the secondary wall, forming the so-called ‘terminal lamella’ which shows some fibrillar texture. It has been concluded that this appearance reflects some intrinsic texture of the lamella itself, rather than the presence of cellulose microfibrils.