In conifers and diffuse-porous angiosperms, tracheary elements have an important role in structural support and water transport. In these plants it is expected that the conflict between the mechanic and hydraulic function will result in less efficient water transport at the whole plant level than the expected for plants whose conduits do not fulfil a mayor structural role (vines and ring-porous trees). Here we analyse the vascular architecture of Betula péndula saplings in order to examine predictions for diffuse-porous trees. Murray's law (conservation of Zr³) was not supported in basal tissues where the mechanical demands are be greater. Although distal portions did not depart significantly from Murray's law, lower exponents better described the relationship between consecutive levels. We found a moderate acropetal increase in conduit number, in disagreement with the model of West, Brown and Enquist, but loosely approximating Murray's law optimum. Conductive area decreased acropetally, giving a roughly conical area profile that is mechanically stable but has low transport efficiency, as predicted for diffuse-porous species. However, this trend in conductive area was uneven, the strongest reduction occurring between terminal branches and petioles. The vascular architecture of woody plants seems to respond to different constrains depending on the branching level and the mechanical functions associated with it.