(i) Due to continued secondary growth for many years, the older parts of the stem have a part of its secondary xylem becoming non-functional.
(ii) It is always the central portion of xylem from which the vessels and tracheids progressively become filled with resins and tannins produced by adjacent living cells.
(iii) The xylem elements of many woody trees are blocked by bladder like in-growths of neighbouring parenchymatous cells, thus blocking the continuity of these conducting elements. Such bladder like in growths are called tyloses. As a result, the conducting elements become non-functional.