Question

Which one is present nearest to plasma membrane :

• A. Middle lamella
• B. Primary wall
• C. Secondary wall
• D. Tonoplast

Answer 1

The correct option is C Secondary wall

Secondary wall is present nearest to plasma membrane. It is produced in some mature cells when the latter have stopped growth, e.g., tracheids, vessel elements, fibres, collenchyma. Secondary wall is laid inner to the primary wall by accretion or deposition of materials over the surface of existing structure. It is thick (3-10 $\mu$m) and made up of at least three layers, sometimes more (e.g., latex tube of Euphorbia milli). Secondary wall may be absent, irregularly deposited or formed uniformally in the cells. This results in differentiation of cells- parenchyma, collenchyma, sclerenchyma, tracheids and vessels.