Tylosis

Tylosis 

a blister-like protrusion of xylem parenchymal cells that obstruct the vessels or, less commonly, the tracheids. Tyloses form when the closing membrane of a pit grows into the lumen of a vessel. The cytoplasm and, sometimes, the nucleus of the cell move into the protrusion, which is not separated by a septum from the mother cell. Tyloses may be filled with starch, calcium salts, resin, or gum. Although they are most characteristic of woody plants, they also occur in herbaceous plants, such as pumpkin, purslane, and dock. Tyloses lower the water-retaining capacity of the heartwood and increase resistance to rotting and penetration by fungi. The formation of tyloses is sometimes a response to injury.