Tyloses and the Maintenance of Transpiration

During a study of transpiration and embolism-formation in petioles of sunflower, tyloses were frequently observed in early metaxylem vessels. Tyloses were confined to the inner ends of the xylem arcs, remote from the phloem. Vessels in this position are especially vulnerable to embolism. All stages of the invasion of vessel lumens by xylem parenchyma cells were observed, from the early protuberance of a cell through a pit to the complete occlusion of the lumen by one to several cells. The lumen space not occupied by tyloses was seen both filled with xylem sap, or embolized and gas-filled. Thus, during the early stages of tylosis formation the vessel remained active in carrying the transpiration stream. Thin-walled vessels of the protoxylem or early metaxylem were not tylosed, but were squashed and disappeared. These observations are interpreted as evidence that vessels vulnerable to embolism are decommissioned and replaced by parenchyma tissue, while new and less vulnerable vessels are added to the xylem arcs at the cambial side. It is proposed that tylosis formation is triggered by the frequent embolization of the vulnerable vessels to give, ultimately, an incompressible tissue. Then tyloses would be necessary to preserve the tissue pressure which expresses water to refill embolisms in the remaining vessels, and maintain transpiration, as explained by the compensating pressure theory of water transport.