Pits as natural irregularities in softwood fibers

Parameters for pit size, shape, orientation, and density, and their correlations with fiber dimensions in Scots pine and Norway spruce fibers were determined. Bordered pits were smaller with greater cross-sectional fiber area, and more circular the wider the fibers. Angular pit orientation became more transverse in pine fibers, and pit density increased in both species with fiber width. Normalized pit size and pit density increased towards fiber tips, especially in thin-walled pine fibers. These observations can be explained by the developing process and the functional role of tracheids in wood. The appearance of pits as stress-enhancing irregularities in fiber structure should be considered in fiber-network theories predicting paper properties, as well as in the measurement of the mechanical properties of fibers.