borderedpit

Bordered pits are a type of pit pair found in the secondary cell walls of xylem elements in vascular plants. Each pit consists of a pit aperture formed by the primary walls of two adjacent cells and is flanked by a thickened border in the surrounding secondary wall. This border encloses the pit cavity and gives bordered pits their characteristic ringed appearance.

Much of the classic description of bordered pits comes from gymnosperms, where many pits exhibit a torus–margo structure. The central torus is a thickened region of the pit membrane, while the surrounding margo is a porous network. Under certain pressure differences, the torus can seal the pit by extending to block the aperture at the pit border, helping to prevent the spread of air embolisms between adjacent elements.

Functionally, bordered pits enable water transport between xylem cells while providing a hydraulic safety mechanism. The border reinforces the pit boundary, and the torus–margo complex can rapidly close in response to embolism, reducing hydraulic failure. This architectural variation influences the efficiency and safety of water transport in different species.

In wood anatomy, the presence and structure of bordered pits are diagnostic traits used to identify and classify plant species and to understand the hydraulic strategies of different lineages.