Abstract
Inter-tracheid and cross-filed pit specifications in compression wood and opposite wood of Norway spruce (Picea abies) were determined. Fewer pits of a smaller size and a smaller aperture diameter were observed in compression wood. In contrast to the uniseriate arrangement of bordered pit pairs in compression wood, both uniseriate and biseriate pits were observed in opposite wood. In contrast to the circular view of the pit aperture in opposite wood, a slit-like pit aperture was often observed in compression wood. SEM images showed a number of helical fissures on the tracheid walls and bordered pits of compression wood along the microfibril angle in the S2 layer. The cross-field pits in compression wood were dominantly piceoid but sometimes cupressoid and occasionally taxodioid, whereas they were mostly piceoid and occasionally cupressoid in opposite wood. Overall, some significant differences in the inter-tracheid and cross-field pitting between the compression wood and opposite wood can give some explanations for their different air permeability and drying kinetics found in the previous studies.
Highlights
Small openings or recesses in the secondary cell wall known as pits are among the important wood microscopic characteristics of tracheids in softwoods
The results showed that the compression wood has a smaller pit diameter and pit aperture than the opposite wood
Uniseriate arrangement of bordered pits occurred on the radial walls of compression wood tracheids, whereas normally uniseriate and rarely biseriate pits were observed in opposite wood
Summary
Small openings or recesses in the secondary cell wall known as pits are among the important wood microscopic characteristics of tracheids in softwoods. Different types of pitting, such as the pits between two ray tracheids, the pits between a ray tracheid and a longitudinal tracheid, etc have been comprehensively studied in Pinus banksiana by Yang and Benson (1997) They indicated a significant variation in the size and structure of the pit pairs, occurring between the different wood cells.The wood permeability is affected by the size, frequency, and structure of pit pairs and it may be decreased as a result of pit aspiration, pit occlusion or pit incrustation (Siau, 1984). It is a key feature because of its contribution to the radial flow of fluids or moisture through wood (Siau, 1984; Usta and Hale, 2003)
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