The xylem wedge as a functional unit – speculations on the consequences of flow in leaky tubes

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Abstract
Summary: Xylem, as a mixture of elements of large and small diameter, presents a paradox when viewed as a conductor of sap flow, since the quantity carried is determined by only the largest diameter elements. The paradox may be s resolved, and an important function for the smallest elements revealed, by consideration of flow in leaky tubes. It is shown how the balance between axial flow and leakage becomes critical at a certain diameter, called the critical diameter (D0). Below D0, no axial flow can occur. When a stack of connected tracheary elements of different diameters is assembled into a xylem wedge, as in the major veins of dicotyledon leaves, this wedge will act as a functional unit. Elements with D > D0 have axial flow, but those where DD0 have only radial flow. The mixture of elements then acts to concentrate solutes of the sap in the smallest elements at the apex of the wedge, left behind as the water leaves. The balance between radial and axial flow is also self‐adjusting to changes in the longitudinal gradient of pressure. Consideration is given to the functions of these graded stacks in the metaxylem of leaves, of stem vascular bundles, within secondary wood, and in roots. It is shown how some xylem fibres may be expected to mediate exchanges of water and solutes, and may act as regulators of these exchanges in space and time. Some experimental demonstrations are presented of the predictions of this hypothesis.