Tyloses are balloon-shaped structures in xylem vessels developed by xylem parenchyma cells.
Malpighi (1686) for the first time reported the ‘balloon-shaped sacs’ in the lumen of vessels of heartwood and he named it as tyloses based on the Greek word ‘Thyllen’ meaning ‘bag’ or ‘container’. Tyloses development occurs through the pit-pairs connecting parenchyma and tracheids or vessels. Sometimes many tyloses originate from a single parenchymatous cell. The tyloses completely fill the lumen rich in the tracheids and vessels of heartwood than sapwood.