Stomata are present in the sporophyte generation of the vast majority of land plants, with the exception of liverworts, as well as some mosses and hornworts. In vascular plants the number, size and distribution of stomata varies widely. Dicotyledons usually have more stomata on the lower surface of the leaves than the upper surface. Monocotyledons such as onion, oat and maize may have about the same number of stomata on both leaf surfaces.[2]: 5  In plants with floating leaves, stomata may be found only on the upper epidermis and submerged leaves may lack stomata entirely. Most tree species have stomata only on the lower leaf surface.[3] Leaves with stomata on both the upper and lower leaf surfaces are called amphistomatous leaves; leaves with stomata only on the lower surface are hypostomatous, and leaves with stomata only on the upper surface are epistomatous or hyperstomatous.[3] Size varies across species, with end-to-end lengths ranging from 10 to 80 μm and width ranging from a few to 50 μm.[4]

Stomata are small openings or pores in plant tissue that allow for gas exchange. They are present in the sporophyte generation of most land plants, except for liverworts and some mosses and hornworts. The number, size, and distribution of stomata can vary greatly among vascular plants.

For example, dicotyledons (a type of flowering plant) usually have more stomata on the lower surface of their leaves than the upper surface. However, monocotyledons (another type of flowering plant), such as onions, oats, and maize, may have roughly the same number of stomata on both leaf surfaces.

In plants with leaves that float, stomata may only be present on the upper epidermis (the outer layer of cells covering a plant), and leaves that are submerged may not have stomata at all. Most tree species only have stomata on the lower surface of their leaves.

Leaves can be categorized based on where their stomata are located:

• Amphistomatous leaves have stomata on both the upper and lower leaf surfaces.
• Hypostomatous leaves have stomata only on the lower surface.
• Epistomatous or hyperstomatous leaves have stomata only on the upper surface.

The size of stomata can also vary across different species, with lengths ranging from 10 to 80 micrometers and widths ranging from a few to 50 micrometers.