The leaf

The structure of the leaf is adapted for gas exchange. The cells in the spongy mesophyll (lower layer) are loosely packed, and covered by a thin film of water. There are tiny pores, called stomata, in the surface of the leaf. Most of these are in the lower epidermis, away from the brightest sunlight.

Sunlight hits top level waxy cuticle, below this is upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis and another waxy cuticle. Guard cells allow exchange of gases through stoma.

The role of stomata

The stomata control gas exchange in the leaf. Each stoma can be open or closed, depending on how turgid its guard cells are.

In the light, the guard cells absorb water by osmosis, become turgid and the stoma opens.

In the dark, the guard cells lose water, become flaccid and the stoma closes.

Long, narrow opening flanked by guard cells with chloroplasts, surrounded by epidermal cells. Guard cells have a thick inner wall around stoma and thin outer wall where they border epidermal cells.

Diffusion of carbon dioxide, oxygen and water vapour into (or out of) the leaf is greatest when the stomata are open.