tylose A balloon-like extension of a parenchyma cell that protrudes into the lumen of a neighbouring xylem vessel or tracheid through a pit in the cell wall. Tyloses form most commonly in older woody tissue, possibly in response to injury; they may eventually block the vessels and thus help prevent the spread of fungi and other pathogens within the plant. Tyloses may become filled with tannins, gums, pigments, etc., giving heartwood its dark colour, and their walls can remain thin or become lignified.

tyloses In older wood, and sometimes in the vessels of herbs, hollow ingrowths developed from adjacent parenchyma cells that eventually cause blockage, Tyloses often fill with resins, gums, tannins, or other pigmented materials (some of which are used as dyes), giving the wood a characteristic colour.