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Bladder-like ingrowths that protrude into the tracheary elements of older wood eventually causing blockage. They originate from adjacent parenchyma cells via paired pits in the cell walls. Tyloses often become filled with tannins, resins, gums, or various pigments, so giving the heartwood its characteristic darker colour. These substances also help to preserve and strengthen the wood, while some of the pigments are important commercially as dyes (e.g. haematoxylin). Tyloses are sometimes found in the vessels of herbaceous plants, in which their function is unclear. They may act to seal off damaged vessels.
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