Thickening takes place in the cell wail during formation of secondary wall on the inner side of the primanry wail, This type of thickening depends upon the nature of the cell wall. This thickening takes place due to deposition of hard substances such as lignin. cellulose, hemicelluloses. suberin etc. The thickening is found in the cells of certain parts like pericycle, phloem. xylem etc.

Some of the cells (cg parenchvina, sclerenchvma. sieve plates, vessels. trachids cells) remain unthickened. The unthickened parts develop pits which are of two types –

• Simple pits – The are found in between the cell wall of adjacent cells of sieve tubes, parenchyma. companion cells etc. These pits arc 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.
• Bordered pit – The bordered pits are more complex than the simple pits. In this case. the secondar’ 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.

The liquid in cell put pressure on the pit membrane and the pit membrane pushes the torus. Thus the toruse closes the bordered pit. Hence, it acts as a valve.These are found in the vessles, tracheids and fibres of xylem.