A * pit possessing an extension of the secondary cell wall, i.e. a border, arching over part of the pit cavity. In many gymnosperms, bordered pits also possess a thickening of the primary wall material, termed a * torus, in the central part of the pit membrane, the remaining unthickened part being termed the margo. Bordered pits mainly occur in vessel elements, tracheids, and fibres in the xylem, but may also occur in some extraxylary sclerenchyma cells. Pit pairs may be half-bordered, in which case one member of a pit pair is bordered and the other is not. This is sometimes the case in pits linking parenchyma cells and tracheary cells. Compare simple pit. |
