In botany, a stoma (also stomate; plural stomata) is a tiny opening or pore, found mostly on the undersurface of a plant leaf, and used for gas exchange. Air containing carbon dioxide and oxygen enters the plant through these openings where it gets used in photosynthesis and respiration. Waste oxygen produced by photosynthesis in the chlorenchyma cells of the leaf interior exits through these same openings. Also, water vapor is released into the atmosphere through these pores in a process called transpiration. The opening and closing of a stoma is controlled by guard cells that surround the opening and involves cellular sodium-potassium pumps.

Dicotyledons usually have more stomata on the lower epidermis than the upper epidermis. As these leaves are held horizontally, upper epidermis is directly illuminated. Less number of stomata on the upper epidermis can then prevent water loss.

Monocotyledons are different. For their leaves are held vertically, they will have the same number of stomata on the two epidermis.

If the plant has floating leaves, there will be no stomata on the lower epidermis as it can absorb gases directly from water through cuticle. If it is submerged leaf, no stomata will be present on both sides of it.

Stoma in Greek means "mouth".

Stoma Paradox

As the key reactant in photosynthesis, carbon dioxide, is found in the atmosphere, most plants require the stoma to be open during daytime. The problem is that during hot weather, this causes plants to lose huge amounts of water through transpiration. This problematic situation is known as the stoma paradox. A small group of plants evolved with a solution to the paradox; they open the stoma at night and store the carbon dioxide for use during the following day, allowing the stoma to close at dawn. These plants are known as CAM plants.

Viewing Stoma

The easiest way to view stomata on a leaf is to take a nail varnish impression of it.

1. Paint about one square centimeter of the underside of the leaf with transparent nail varnish.(or thin layer of PVA glue)
2. Allow to dry out thoroughly (takes a good 30 minutes).
3. Peel off and place on a microscope slide.

The stomata leave clearly visible impressions in the nail varnish. A graticule slide allows for the counting of how many stomata (per unit area) are on the leaf surface, a characteristic of physiological significance.