Some of the cells (eg parenchyma, sclerenchyma, sieve plates, vessels, trachids cells) remain unthickened. The unthickened parts develop pits which are of two types –
(A) Simple pits – They are found in between the cell wall of adjacent cells of sieve tubes, parenchyma, companion cells etc. These pits are round & cylindrical. They allow the exchange of water and dissolved substances across the membrane. The diameter of simple pits remain uniform and the pit cavity trpens in the lumen of the cell..
(B) Bordered pit – The bordered pits are more complex than the simple pits. In this case, the secondary wall border over the cavity of the pits. Hence, the pit opening become narrow than the pit diameter. In certain cases, the pit aperture are lenticular. The pit membrance of bordered pit develops an oval thickening in the middle. It is called torus. Pit membrane is flexible.
