Question

the stomatal Apparatus of plant possess guard cells. Why?

Answer 1

Stomata are small pores present in the epiderms of leaves. 
The stomatal aperture, guard cells and the subsidiary cells together are called the stomatal apparatus. 
Structure of stomata: Each stomata is composed of two bean-shaped structure called guard cells. The inner walls of guard cells are thick, while the outer walls are thin. The guard cells are surrounded by subsidiary cells. These are the specialised epidermal cells present around the guard cells called the subsidiary cells.