EVOLUTIONARY VOYAGE OF ANGIOSPERM VESSEL STRUCTURE-FUNCTION AND ITS SIGNIFICANCE FOR EARLY ANGIOSPERM SUCCESS

A hypothesized advantage of the building block of the angiosperm vascular network, the vessel, is often cited as a critical innovation that elevated the competitive abilities of early angiosperms above nonangiosperms during the Cretaceous. Here we synthesize recent discoveries on the hydraulic functions of living basal angiosperm lineages with evidence from the fossil record to trace the early evolutionary significance of vessels in the early ecophysiological radiation of angiosperms. Evidence from extant comparative biology and the Early Cretaceous fossil record of leaves and wood do not support the hypotheses that vessels improved drought tolerance of angiosperms, increased angiosperm's photosynthetic abilities, or provided an immediate leap in hydraulic capacity. Instead, later tuning of vessel structure for increased flow efficiency-in particular, by the evolution of simple perforation plates-enabled major increases in xylem hydraulic efficiency.