Da Vinci's mischief: xylem conduits in the stems of woody plants do not furcate.

The hydraulic architecture of plants is often modeled as a 'furcating' network, in which xylem conduits proliferate in number toward the stem apex, analogous to animal circulatory systems. Yet whether furcation actually occurs within woody stems remains untested, despite major implications for carbon costs and hydraulic efficiency. We measured the number and diameter of functional xylem conduits at the base and tip of unbranched stem segments in 112 woody species spanning 57 families and 31 orders. Standard and phylogenetically informed regressions were used to evaluate relationships between conduit number, stem length, and tip-to-base conduit widening. Conduit number remained constant between base and tip, showing no evidence of furcation, whereas conduit diameter increased predictably with distance from the apex, with a slope of 0.22, closely matching theoretical expectations for hydraulically optimal widening. These results support a 'Widened Pipe Model' of xylem architecture, in which conductance is maintained through conduit widening rather than multiplication. We suggest that the absence of furcation suggests that da Vinci's rule of area-preserving branching does not apply within stems, emphasizing the need to revisit core assumptions in models of tree hydraulic design.