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tracheidany of several structures found in XYLEM, consisting of a cell which is long, slender and tapered with heavily lignified walls surrounding an empty lumen, the protoplasm having died.
The wall thickening can be continuous with BORDERED PITS, or else arranged in a variety of patterns, annular, helical and ladder-like. See Fig. 299 . Tracheids form the water-conducting tissues of GYMNOSPERMS, but are found also amongst the xylem vessels of ANGIOSPERMS