Tyloses and the Maintenance of Transpiration

During a study of transpiration and embolism-formation in petiolesof sunflower, tyloses were frequently observed in early metaxylemvessels. Tyloses were confined to the inner ends of the xylemarcs, remote from the phloem. Vessels in this position are especiallyvulnerable to embolism. All stages of the invasion of vessellumens by xylem parenchyma cells were observed, from the earlyprotuberance of a cell through a pit to the complete occlusionof the lumen by one to several cells. The lumen space not occupiedby tyloses was seen both filled with xylem sap, or embolizedand gas-filled. Thus, during the early stages of tylosis formationthe vessel remained active in carrying the transpiration stream.Thin-walled vessels of the protoxylem or early metaxylem werenot tylosed, but were squashed and disappeared. These observationsare interpreted as evidence that vessels vulnerable to embolismare decommissioned and replaced by parenchyma tissue, whilenew and less vulnerable vessels are added to the xylem arcsat the cambial side. It is proposed that tylosis formation istriggered by the frequent embolization of the vulnerable vesselsto give, ultimately, an incompressible tissue. Then tyloseswould be necessary to preserve the tissue pressure which expresseswater to refill embolisms in the remaining vessels, and maintaintranspiration, as explained by the compensating pressure theoryof water transport. Compensating pressure theory; embolisms; starch sheath; tissue pressure; transpiration; tyloses; vessel diameter