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).