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 cell
s", crescent shaped cell
s 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
being lost. Similarly, very cold
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 osmosi
s, the movement of water through a semi-permeable membrane
across a concentration gradient. When the concentration of solvent
s 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 ion
s 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 wall
s 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).