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stomata

noun, singular: stoma

1. (botany) tiny pores in plant leaves used for gas exchange; each pore is surrounded by a pair of guard cells that regulate its opening and closure.

The guard cells swell to form a pore to allow CO₂ to enter into the leaf, which is a necessary reactant of photosynthesis. The water vapor and O₂ are also allowed to escape via the pore. In order to form a pore or stoma, osmotic pressure draws water to increase the cells volume; this in turn causes the guard cells to bow apart from each other because the inner wall of the pore is more rigid than the wall on the oppostie side of the cell

Stomata are present in all terrestrial plants (in sporophyte phase), except for the liverworts. Dicots usually have more stomata on the lower epidermis than the upper epidermis whereas monocots usually have the same number of stomata on both sides. Plants whose leaves float in water have stomata only on the upper epidermis whereas plants whose leaves are completely submerged may lack stomata entirely.

2. (zoology) mouthlike openings, such as the stomata (or the oral cavities) of nematodes

3. (anatomy) natural openings in the body, such as the mouth

4. (medicine) artificial openings in the body that have been surgically created, which connects a portion of the body cavity to the outside environment. Any hollow organ can be manipulated into an artificial stoma as necessary. This includes the esophagus, stomach, duodenum, ileum, colon, pleural cavity, ureters, etc. A widely known example is colostomy, which is a stoma created in the abdominal wall to permit the passage of waste.

Word origin: From Ancient Greek στόμα (stoma), “‘mouth’”.