Resin canals can occur in xylem, phloem, or bark of either type, but are more common to softwoods. Gymnosperms produce softwood . The xylem consists only of tracheids , which are long cells that conduct water through openings in the side walls. Angiosperms produce hardwood. The xylem has both tracheids and vessels .
Vessels are tube-like cells that conduct water through perforations in their end walls.
In spring or the wet season, cambium makes many large xylem cells which appear lighter. In summer or the dry season, only a few small cells are made and the wood appears darker. In winter, cell production ceases. In the tropics where there is little variation in climate, the trees do not produce rings. Rings may be present if there is a marked wet and dry season.