A stoma is a very small hole in the surface of a leaf, mainly found on the bottom layer. It is created by two "guard cells", crescent shaped cells which go together to form a gap. This hole can vary in size, and will change depending on the conditions the plant is under. In very hot conditions the stomata will be closed, or very small, in order to prevent large volumes of water being lost. Similarly, very cold and humid air will result in the stomata being widely open allowing enough transpiration to occur for the related functions to work.

The main function of transpiration is to transport water and minerals up through the plant to the leaves. Water absorbed into the plant by the roots moves up the plant due to the cohesive properties of water. As one particle is pulled from the top, all the particles below it are pulled up at the same time like a long piece of string, as they all stick together. The properties of the cells around the outside of the xylem, the tube that all of the water moves up through to get to the rest of the plant, prevent the water from cohesing to the sides of the xylem as far as possible.

The stomata open and close due to osmosis, the movement of water through a semi-permeable membrane across a concentration gradient. When the concentration of solvents in the water inside a cell are very high, there is much less water per volume. Water with a lower concentration of solvents on the outside of the cell will therefore move across the membrane into the cell. To open the stoma, solvents such as potassium ions are moved into the cell by active transport from the cells surrounding the guard cells. As a result, water moves into the guard cells by osmosis. This makes the cells bulge, and differences in the thicknesses of the cell walls at different positions on the cell make the cell bulge more at the ends of the cells than at the middle. This moves the two cells apart slightly in the middle, opening a hole. To close the stomata, active transport is used to move ions out of the cell, making water move out of the cell by osmosis, and hence removing the bulge in the cells, closing the hole.

The density of stomata can easily be counted using a normal light microscope magnifying at around 400 X. Similarly, the rate of transpiration is relatively easy to measure my attaching a tube filled with water to the bottom of a clipping of a plant stem. A very thin tubing can then be attached to the bottom of this tubing, again filled with water, and the rate of uptake of water can be measured by simply observing how quickly the water moves along the thin tubing.

Stomata are generally found on the bottom sides of leaves, and a count of the stomata on the top of a leaf will show very few, and often none. This is simply an effort to control the rate of transpiration, as the light and heat on the bottom of the leaf will be less than that on the top. It also leaves the top of the leaf to do all the photosynthesis, rather than using up this space with stomata.

Many stomata are indented into the leaf slightly in order to trap a small layer of water vapour (water comes out of the leaf in this form), and hence reduce the transpiration to some extent. The wind will blow this away, but it will have some effect. Cacti will have very indented stomata, as they want to lose as little water as possible (but they still need to transpire in order to transport minerals to all parts of the plant).

Stoma (Greek 'a mouth')is the medical term for an artificial opening between two body cavities or between a body cavity or passage and the surface of the body. This opening is created in a procedure called an -ostomy.

For example, a tracheal stoma is an opening between the trachea and the body surface that is created during a procedure called a tracheostomy. Quite often, the specific opening will also carry the name of the procedure, i.e. the stoma created through a tracheostomy is a tracheostomy. The stoma created through a colostomy (creating an opening between the colon and the body surface) is a colostomy.

Not all stomata (or stomas, either is correct) are permanent; for example, a tracheostomy may be used for better medical effect and comfort in a patient who needs to be intubated for a long time, and then, once the patient is extubated, the stoma may be obliterated, leaving only a scar. A colostomy may be used in a patient who needs intestinal surgery, but after the intestines heal an end-to-end anastomosis may be performed, allowing normal intestinal function again.

Stomata, being artificial openings, require special care specific to the type of stoma. Stomata of the excretory organs generally require some type of drainage pouch and skin barrier, other stomata, such as respiratory or gastric, need other types of care.

Sto"ma (?), n.; pl. Stomata (#). [NL., fr. Gr. , , a mouth.]

1. Anat.

One of the minute apertures between the cells in many serous membranes.

2. Bot. (a)

The minute breathing pores of leaves or other organs opening into the intercellular spaces, and usually bordered by two contractile cells.


The line of dehiscence of the sporangium of a fern. It is usually marked by two transversely elongated cells. See Illust. of Sporangium.

3. Zool.

A stigma. See Stigma, n., 6 (a) & (b).


© Webster 1913.

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