stoma

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Botany One of the minute pores in the epidermis of a leaf or stem through which gases and water vapor pass. Also called stomate.
2. n. Anatomy A small aperture in the surface of a membrane.
3. n. A surgically constructed opening, especially one in the abdominal wall that permits the passage of waste after a colostomy or ileostomy.
4. n. Zoology A mouthlike opening, such as the oral cavity of a nematode.

Century Dictionary and Cyclopedia

1. n. In zoology, a mouth or ingestive opening; an oral orifice; an ostium or ostiole: chiefly used of small or simple apertures, as a cytostome; hence, also, a small opening of any kind through which something may pass in or out; a pore. Specifically— An opening of a lymphatic vessel; a lymphatic pore or orifice, as an interstice between the cells of a serous membrane.
2. n. In botany, a minute orifice or slit in the epidermis of leaves, etc., which opens directly into air-cavities or intercellular spaces that pervade the interior, and through which free ingress and egress of air take place; a breathing-pore. The apparatus of the stoma consists usually of a pair of cells (there are several in the Equisetaceæ, Hepaticeæ, etc.), called guard-cells or guardian-cells, between the opposed concave sides of which lies the slit or opening, which extends through the whole height of the epidermis and permits free communication between the intercellular spaces and the external air. According to Van Tieghem, the stomata are always open in sunlight and closed in darkness. These cells are strongly thickened on the upper and under walls of their opposed faces, while elsewhere their walls are relatively thin. The opening and closing of a stoma depend upon the difference in thickness of the parts of the walls. When the turgescence of the guard-cells increases, they curve more strongly, and consequently the cleft widens; but with decreased turgescence the cleft becomes narrower. See also cut under Iris.
3. n. In Sweden borg's philosophy, a cubical figure with hollowed surfaces, being the figure of the interstices of spheres arranged in what Swedenborg calls the fixed quadrilateral pyramidal position, supposed to be that natural to the spherical particles of water.

Wiktionary

1. n. botany One of the tiny pores in the epidermis of a leaf or stem through which gases and water vapor pass.
2. n. A small opening in a membrane; a surgically constructed opening, especially one in the abdominal wall that permits the passage of waste after a colostomy or ileostomy.
3. n. zoology A mouthlike opening, such as the oral cavity of a nematode.
4. n. An artificial anus.

GNU Webster's 1913

1. n. (Anat.) One of the minute apertures between the cells in many serous membranes.
2. n. The minute breathing pores of leaves or other organs opening into the intercellular spaces, and usually bordered by two contractile cells.
3. n. The line of dehiscence of the sporangium of a fern. It is usually marked by two transversely elongated cells. See Illust. of Sporangium.
4. n. (Zoöl.) A stigma. See Stigma, n., 6 (a) & (b).

WordNet 3.0

1. n. a minute epidermal pore in a leaf or stem through which gases and water vapor can pass
2. n. a mouth or mouthlike opening (especially one created by surgery on the surface of the body to create an opening to an internal organ)