Pits

The walls of tracheids and vessels are interrupted by numerous pits, which are a type of plasmodesmata. When the secondary wall is laid down inside the primary wall, it is not laid down over the area where the pits are located. The pit is not a hole in the wall, rather it is a small, circular area without secondary thickening. A pit in the wall of one cell usually lies opposite a pit in an adjacent cell. The middle lamella and the two primary walls between the two pits constitute the pit membrane. There are two basic types of pits, simple pits and bordered pits (See diagrams below). In bordered pits, the edges of the secondary wall overhang the pit chamber, forming pit borders. Water and dissolved substances move easily from cell to cell through the pits.