Pit (botany)

In botany, a pit refers to a thin or absent area in the secondary cell wall of a plant cell. Pits are typically found in the walls of xylem cells (tracheids and vessel elements), but can also occur in other cell types. They facilitate lateral transport of water and solutes between adjacent cells.

Pits appear as small depressions or pores in the cell wall when viewed under a microscope. They are formed where the secondary wall does not develop, leaving only the primary wall and middle lamella separating the protoplasts of adjacent cells.

Pits are typically arranged in pairs, with a pit in one cell wall aligning with a pit in the adjacent cell wall. These paired pits are known as pit-pairs. The thin region between the pit-pair, consisting of the primary cell walls and middle lamella, is called the pit membrane.

There are two main types of pits:

• Simple pits: These pits have a uniform diameter and lack any specialized structures. The pit membrane is relatively smooth and uniform.

• Bordered pits: These pits have an overarching border of secondary wall that extends beyond the pit membrane, creating a chamber-like structure. The pit membrane is often modified, for example, by the presence of a torus. Bordered pits are common in tracheids of gymnosperms and provide a means of regulating water flow and preventing air embolism.

The torus is a thickened, central portion of the pit membrane found in bordered pits, particularly in conifers. It can act as a valve, sealing the pit if pressure differences occur, preventing air bubbles from spreading in the xylem. This sealing mechanism is crucial for preventing water stress in plants.

Pits are essential for the efficient transport of water and nutrients throughout the plant. They provide pathways for lateral movement between cells, contributing to the overall vascular system's functionality. The structure and distribution of pits are important features used in wood identification and plant anatomy.