Do plants obey Murray's law

Vascular plants rely on water delivered by xylem conduits to maintain hydration and replace water lost to the atmosphere during carbon dioxide fixation. This dependence suggests that plants that maximize hydraulic conductance per investment in vascular volume would have a selective advantage. The law that describes the conduit taper necessary to maximize hydraulic conductance for a given branching structure and volume is known as Murray's law and was derived for animal cardiovascular systems and has not been systematically applied to xylem. This thesis extends Murray's law to plant xylem and tests agreement.;Although the plant and animal vascular systems are strikingly different, Murray's law predicts the optimal conduit taper occurs when volume flow rate (Q) remains proportional to the sum of the conduit radii cubed (Sigmar 3) across branch points for both groups of organisms. For networks with constant Q, Murray's law predicts Sigmar3 conservation throughout the plant. Comparison of Sigmar3 values across branch points in compound leaves, vines and young ring-porous wood, where the conduits do not provide structural support to the plant, shows agreement with Murray's law. These networks also exhibit increased numbers of conduits distally, which is hydraulically efficient.;When the xylem conduits provide structural support as well as water transport for the plant, such as in conifer and diffuse-porous wood, the conduits do not obey Murray's law. These networks maintain a nearly constant conduit number from the trunk to twigs, which is hydraulically inefficient, but mechanically stable.;To determine if compliance with Murray's law was restricted to seed plants, the xylem of the stem photosynthesizer Psilotum nudum, which is a seedless vascular plant, is tested. As water is lost from the stem, the Sigmar 3 diminishes distally in direct proportion with Q, as predicted by Murray's law. This suggests that accordance with Murray's law evolved early in vascular plant evolution.;Some of the observed patterns in hydraulic architecture, such as diminishing leaf-specific hydraulic conductance and conduit diameter distally, are consistent with complying with Murray's law and result in increased hydraulic efficiency. Species that rely on their xylem conduits for structural support cannot achieve the most hydraulically efficient networks because of the need for mechanical stability.