Water transport through xylem. A, Global view. B, Segmented architecture of xylem, with both axial and radial flow of sap. C, Cross-sectional view of a pit and its membrane. D, Cross-sectional view of the BPM and its interconnected pore network through which sap flows. The effective hydraulic conductance of an individual pore () depends on its own properties and those of the sap. [See online article for color version of this figure.]