Hydraulic consequences of vessel evolution in angiosperms

We tested two hypotheses for how vessel evolution in angiosperms influenced xylem function. First, the transition to vessels decreased resistance to flow--often considered the driving force for their evolution. Second, the transition to vessels compromised safety from cavitation--a constraint emerging from the "pit area hypothesis" for vulnerability to cavitation. Data were obtained from branch wood of 17 basal taxa with vessels and two eudicots possessing "primitive" perforation plates. Results were compared with previous data from vesselless angiosperms and eudicots with simple perforation plates. Contrary to the first hypothesis, basal taxa did not have significantly lower sapwood-specific resistivity than vesselless angiosperms, despite vessels being wider than tracheids.