PITS

• Depressions in secondary cell wall is called pit.
• A pit present on the free cell wall surface without its partner is called Blind pit.
• It consists of 2 parts - Pit chamber and Pit membrane.
• Pit chamber / pit cavity is depression in wall where secondary wall is absent.
• Opening of pit chamber into cell is called pit pore / aperture.
• Pit membrane is formed of (PCW + middle lamella + PCW) it is "trilaminar".
• Pit membrane also called closing membrane.
• Pit membrane is permeable due to permeability of primary cell wall.
• Pits are of 2 types (depending upon the shape of the pit chamber.)

(i) Simple pit

• Pit chamber has uniform width.
• Diameter of lumen remains the same.
• Found in vessels, tracheids & sclerenchyma.

(ii) Bordered pit

• Pit chamber is flask-shaped.
• Diameter of lumen is increases from inner to outer side.
• A disc-shaped thickening called Torus is present in the middle of pit chamber on pit membrane.
• It controls diffusion, it prevents rupturing of pit membrane when unequal pressure is present on its both sides.