Epidermal tissue system

Each stoma is bounded by two bean-shaped guard cells (dumb-bell shaped in grasses).
The guard cells possess chloroplasts and regulate the opening and closing of stomata.
The other epidermal cells are parenchymatous cells with a large vacuole and generally lack chloroplasts.
Cytoskeleton, mitochondria and endoplasmic reticulum are present in all living epidermal cells, so they are not distinguishing features.
Hence guard cells differ from other epidermal cells in having chloroplasts.

The stomatal apparatus consists of the stomatal aperture, the two guard cells and the surrounding subsidiary cells.
C is the pore in the centre: the stomatal aperture.
D is one of the two bean-shaped cells that enclose the pore and contain chloroplasts: the guard cell.
B is one of the specialised epidermal cells lying next to the guard cells: the subsidiary cell.
A is an ordinary cell of the epidermis outside the apparatus: the epidermal cell.
Hence A – epidermal cell, B – subsidiary cell, C – stomatal aperture, D – guard cell.