Organisation of the Cell Walls of Tracheids and Wood Fibres

Abstract

DURING the past thirteen years the finer detail of structure in conifer tracheids has been the subject of some controversy^1,2,3,4. This has centred round the fact that when transverse sections, of conifer wood are examined between crossed Nicols three layers can be distinguished in the secondary wall of a tracheid, an outer and an inner layer which are bright, and a central layer, much thicker in late wood than the other two, which is dark. A similar behaviour is observed in cross-sections of wood fibres and is shown in the case of two isolated fibres of Nothofagus cunninghami in Fig. 1. On the basis of observations such as the run of striations in surface view, the orientation of crystals grown in the wall and the features associated with fungal attack, Bailey and his collaborators have claimed that the optical heterogeneity is due to different orientation of the cellulose ‘micelles’ in the three layers; in that while the micelles in the dark layer are orientated in a steep spiral, those in the outer and inner layers lie more or less transversely1 or, in later papers, at angles between 45° and 90° to the length of the cell^3,5. The case where the micelles in the outer layer are orientated transversely and those in the middle layer longitudinally is shown diagrammatically in Fig. 2 (a). On the other hand, Preston^2,4,6,7, criticizing the validity of the use of evidence obtained from swollen or otherwise distorted walls and basing his conclusions only on X-ray and optical examination of undistorted material, has maintained that the optical heterogeneity is to be explained largely by a pronounced difference in angular dispersion between the layers (Fig. 2b); and that, further, any wall layers with transverse orientation can only be present to an extent so small as to give no record on the X-ray diagrams of conifer wood7 and to cause no disturbance in the optical properties of walls in surface view⁷. The apparently contradictory X-ray evidence brought forward by Bailey and Berkeley³ was completely disposed of as an error in interpretation⁶, and it seemed at that time that the case was at last clear so far as the physical evidence went.