The fine structure of the wall of the conifer tracheid I. The X-ray diagram of conifer wood

Abstract

X-ray diagrams of the wood of a number of species of conifers are examined in the light of the geometrical principles underlying the spiral diagram. It is shown that the meridional arcs in the diagrams, which have been reported to represent transversely oriented cellulose chains in the tracheid wall, are spurious; they arise mainly from overlapping of the ends of the lateral arcs and a fusion with this overlapping region of arcs with nearly the same spacing. It is also suggested that the ray tissue contributes towards the meridional arcs. Measurements of layer thickness in the tracheid wall and of the double refraction of the outer layer in the tracheid, show that there is no correlation between the properties of the outer layer and the appearance of meridional arcs. On the contrary, this evidence suggests very strongly that the outer layer does not differ from the central layer in cellulose chain direction, and that the wall is therefore uniform in this respect. It is considered that the marked optical and swelling heterogeneity in the tracheid wall is therefore to be sought in variations of some property other than cellulose chain direction, and it is again pointed out that changes in angular dispersion of the cellulose matrix offer a possible solution.