Simple pit pairs occur in parenchyma cells, in medullary rays, in phloem fibres, companion cells, and in tracheids of several flowering plants. In the simple pits, the pit cavity remains of the same diameter and the pit or closing membrane also remains simple and uniform in its structure.

What is a pit pair?

Definition of pit-pair : two pits occurring opposite one another in the walls of adjacent cells of many higher vascular plants and acting together as a structural and functional unit.

What is pit in Tracheid?

Tracheids have pits, which are concave depressions on the cell wall. In general, each of the pits has a complimentary pit on the neighboring tracheid cell. These pits that are complimentary are called pit pairs. Through them, water can move across from one tracheid cell to another.