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Why can only the outermost growth ring conduct water in some oak and ash trees?
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wood
Temperate softwoods (left column) and hardwoods (right column), selected to highlight natural variations in colour and figure: (A) Douglas fir, (B) sugar pine, (C) redwood, (D) white oak, (E) American sycamore, and (F) black cherry. Each image shows (from left to right) transverse, radial, and tangential surfaces. Click on an individual image for an enlarged view.
In some tree species with large vessels, such as certain oaks and ashes, only the outermost growth ring functions in water conduction because the vessels in older rings become occluded by tyloses.1
Tyloses are balloonlike outgrowths of surrounding living parenchyma cells that proliferate through pits in the cell walls to block the vessel lumens. This process can occur as early as the first year after the vessels are formed. While this occlusion renders the older wood non-functional for water transport, it also serves a practical purpose; for example, the presence of tyloses in white oak makes the wood watertight, which is why it is preferred for use in barrels and shipbuilding compared to red oak, which lacks these occlusions.21