Description:
<jats:sec><jats:title>Premise of the Study</jats:title><jats:p>Xylem vessels transition through different stages during their functional lifespan, including expansion and development of vessel elements, transition to vessel hydraulic functionality, and eventual transition to post‐functionality. We used information on vessel development and function to develop a model of vessel lifespan for woody plants.</jats:p></jats:sec><jats:sec><jats:title>Methods</jats:title><jats:p>We examined vessel functional lifespan using repeated anatomical sampling throughout the growing season, combined with active‐xylem staining to evaluate vessel hydraulic transport functionality. These data were combined with a literature review. The transitions between vessel functional lifespans for several species are illustrated, including grapevine (<jats:italic>Vitis vinifera</jats:italic> L., Vitaceae), English oak (<jats:italic>Quercus robur</jats:italic> L., Fagaceae), American chestnut [<jats:italic>Castanea dentata</jats:italic> (Marshall) Borkh.; Fagaceae], and several arid and semi‐arid shrub species.</jats:p></jats:sec><jats:sec><jats:title>Key Results</jats:title><jats:p>In intact woody plants, development and maturation of vessel elements may be gradual. Once hydraulically functional, vessel elements connect to form a vessel network that is responsible for bulk hydraulic flow through the xylem. Vessels become nonfunctional due to the formation of gas emboli. In some species and under some conditions, vessel functionality of embolized conduits may be restored through refilling. Blockages, such as tyloses, gels, or gums, indicate permanent losses in hydraulic functional capacity; however, there may be some interesting exceptions to permanent loss of functionality for gel‐based blockages.</jats:p></jats:sec><jats:sec><jats:title>Conclusions</jats:title><jats:p>The gradual development and maturation of vessel elements in woody plants, variation in the onset of functionality between different populations of vessels throughout the growing season, and differences in the timing of vessel transitions to post‐functionality are important aspects of plant hydraulic function.</jats:p></jats:sec>