A stomate is a group of cells in the epidermis made up of guard cells and subsidiary cells.

Stomata are designed to allow gas exchange.

Stomata are most frequently found on the bottom (abaxial) side of leaves, but may occur on the top (adaxial) of leaves, stems and even fruit.

Illustration showing location of stomate in the cross section of a leaf.

A stomatal complex consists of:

Guard cells
Subsidiary cells.

Photo of a stomatal complex with guard and subsidiary cells pointed out.

Guard cells are a pair of kidney-shaped cells that form the opening of the stomate. When the guard cells are turgid (full of water), they are open and allow gas to enter the stomate.

However, this also exposes the leaf to potential water loss.

When the guard cells are flaccid (less water), they are closed to prevent air exchange and water loss.

Photo of a stomatal complex pointing out the guard cells.

Subsidiary cells - These are cells bordering guard cells of the stomate.

They do not directly participate in the opening and closing of the stomate, but may aid it in their functioning.

These cells vary in their arrangement and pattern depending on the plant species.

Photo of a stomatal complex pointing out the subsidiary cells.

When the plant is turgid (full of water) the guard cells are swollen and the stomate is open. This allows carbon dioxide to enter the leaf for photosynthesis, but it also allows water to leave the opening. Water leaving the leaf is called transpiration.

During times of water stress, the guard cells lose water and shrink. This closes the stomate. Most stomata are also closed at night, since most plants do not need to fix carbon dioxide in the dark and the plant does not need to lose water needlessly.

Photo of stomal complexes with open and closed stomates pointed out.

In side view, the stomatal cavity is evident.

The stomatal cavity helps with gas exchange across the leaf.

Photo showing a side view of plant material, with epidermis (cuticle layer), stomatal cavity, stomates, palisade mesophyll, and spongy mesophyll identifed.

This is a stomatal cavity in pine. This is an example of a sunken stomatal cavity.

Guard cells are not directly on the epidermal surface.

This arrangement is more efficient at preventing water loss.

Photo of pine plant cells showing stomal cavity, guard cell, and epidermis.

A photomicrograph and electron micrograph of the guard cells.

A photomicrograph of guard cells.

An electron micrograph of guard cells.

An electron micrograph of the guard cells reveals the large nucleus, relatively thick cell walls, numerous vacuoles and chloroplasts.

Electron micrograph of guard cells with cell wall, nucleus, chloroplast, vacuole, and ledge pointed out.

Surface view of a typical stomate by scanning electron microscopy.

Electron micrograph of surface view of stomates.

Close up electron micrograph of surface view of a stomate.

Electron micrograph of stomate in dogwood shows a wonderful surface pattern over subsidiary cells made by strands of wax.

Wax on the epidermal cells helps to reduce water evaporation from the surface.

Electron micrograph surfave view of dogwood stomate with wax strands extending out from it.