JAMB - Biology (1978 - No. 43)
The number shown as 8 refers to
upper epidermise
cuticle
stoma
guard cell
lower epidermis
Explanation
The part labelled 8 is the guard cell. It's function is to regulate the opening and closing of stomata, which are small pores on the leaf surface. This regulation is crucial for controlling gas exchange (primarily CO\(_2\) uptake for photosynthesis and O\(_2\) release) and water loss (transpiration).
(See diagram above for better understanding)