A pit is a small, sharply defined area in the wall of a plant cell that remains thin while the rest of the wall becomes thickened. Pits appear as depressions in the cell wall. They coincide in position with pits in the walls of adjacent cells and are separated from them by a pit membrane consisting of middle lamella and a very thin layer of primary wall material on either side.
Simple pits, as described, occur in living cells and through them pass strands of cytoplasm – the plasmodesmata connecting one cell with another. Simple pits also occur in stone cells and some fibers. In bordered pits, characteristic of xylem vessels and tracheids, the pit cavity is partly enclosed by over-arching of the cell wall and the pit membrane may possess a central, thickened impermeable area (torus), which closes the aperture of a pit if the pit membrane is laterally displaced.