| Popis: |
Wood cells provide the channels for water and mineral transport in the tree and, at the same time, have to provide its mechanical stability. To understand the unique mechanical properties of wood, better knowledge of the ultrastructure of the cell wall components is crucial. We use the technique of small-angle X-ray scattering which is uniquely well suited to study the fibrillar structure of wood within the nanometer range. We studied variations of the cell wall ultrastructure in sprucewood (Picea abies) by scanning through several annual rings of a given stem. In about three hundred individual measurements from 10 different trees at several locations, the diameter of cellulose fibrils stayed remarkably constant at 2.5 nm, with a standard deviation of 0.14 nm. The spiral angle of the fibrils with respect to the longitudinal direction was found to be ≈4.6 ± 0.6° in earlywood. In latewood, however, only about half of the fibrils had this spiral angle, the rest being at ≈19.8 ± 0.7°. The transition from earlywood to latewood is not continuous and only these two discrete states were observed. This points to the existence of two types of tracheid structures meeting different mechanical requirements. While longitudinally oriented fibrils mostly improve tensile strength, the addition of 20° tilted fibrils, as found only in latewood tracheids, provides also stability against compression. |
