VARATION IN MICROFIBERILLAR ANGLE OF TRACHEID IN PINUS TABULAEFORMIS AND ITS CORREATIONS TO STRUCTURE, TENSILE AND BENDING STRENGTH
Five trees of Pinus tabulaeformis were collected from a stand of Zhongtiao Mountains in Shanxi province. Microfibrillar angle within the S_2 layer of the secondary wall in tracheid decreases rapidly from pith outwards, and then levels off after 13 years old. Along the trunk, microfibrillar angle tends to be great at the base, decreases to minimum at 3.3 m height, then increases again near the trunk tip. Within a single growth ring, microfibrillar angle is small at the begining of spring-wood, increases slowly to maximum, then decreases to late wood. At the outside of 80% position of late wood, the microfibrillar angle is minimum. There is a significant negative linear correlation between tracheid length (Y) and microfibrillar angle (θ), and the equation Y=a+b·θ is superior to Y=a+b·cos θ. The microfibrillar angle is inversely proportional to the trachcid width, the length-width radio of tracheid, the cell wall thickness, etc. The effects of microfibrillar angle on the tensile and bending strength are singnificant.