PARENT SESSION
Oral Session #63: Plant Ecology: Water Relations. Presiding: J. Cleverly.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Hall of Ideas H.

Do plants obey Murray's law?

McCulloh, Katherine¹, Sperry, John¹, Adler, Frederick¹, ¹

ABSTRACT- Recent interest in the allometry of plant vascular tissues has raised questions concerning what optimal design principles govern plant transport systems. Our research evaluated these principles in the xylem of vascular plants based on the predictions of a law developed to describe optimal animal vascular systems. This law, Murray's law, predicts that the most efficient transport system will maximize hydraulic conductance/ fixed vascular tissue volume by conserving the sum of the conduit radii cubed (r³ ) at all ranks in the system (e.g. ³ of the trunk= r³ of the petioles). This optimization criterion should be selected for in plants because it would maximize the efficiency of the transport system. Exceptions to Murray's law would occur where xylem conduits perform an additional, supporting function, as in the secondary xylem of conifers. Our first hypothesis is that Murray's law will apply to the xylem in primary growth and vines, because their vascular systems function only in transport. Our second hypothesis predicts that in the secondary xylem of trees, there will be a convergence on Murray's law as the fraction of wood occupied by conduits decreases, going from conifers to diffuse-porous to ring-porous angiosperms. Anatomical measurements have tested the degree of r^3 conservation across branch ranks and preliminary results support our hypotheses. Murray's law applies to the xylem of compound leaves, and the leaves and stems of two species of vine and a ring-porous tree, where structural support is provided by tissues other than the xylem conduits.

KEY WORDS: Murray's law, optimal vascular system, xylem