Guard cellOne of the two curved cells that flank the pore. Swollen with water, the pair bows apart and the pore opens.
StomaThe pore between the two guard cells. Carbon dioxide enters the leaf through it, and oxygen and water vapor leave. The name is also used for the pore and its guard cells together.
ChloroplastGuard cells have chloroplasts. The ordinary epidermal cells round them usually have none.
NucleusEach guard cell is a living cell with its own nucleus.
VacuoleA large space filled with cell sap. When the vacuoles take in water the guard cell swells, and when they lose it the cell goes limp.
Thick inner wallThe wall next to the pore is thicker than the rest. It stretches less, so a swelling guard cell bends away from the pore.
Thin outer wallThe wall on the side away from the pore is thinner. As the cell fills with water this side bulges more than the thick side.
Epidermal cellAn ordinary cell of the leaf's skin. Epidermal cells fit closely together, so the stomata are the only openings.
Closed stoma 3
Guard cellThe same cell after losing water. It is limp and lies straight against its partner.
StomaWith both guard cells limp the pore is shut to a slit. Far less water vapor can leave the leaf.
Epidermal cellThe same epidermal cells. Unlike the guard cells they have no chloroplasts.
How this list was compiled: Every name was checked on 2026-09-30 in two places, for facts only and with nothing copied. (1) J. N. Martin, Botany for Agricultural Students (1919), https://archive.org/details/botanyforagricul00mart. (2) J. M. Coulter, C. R. Barnes and H. C. Cowles, A Textbook of Botany for Colleges and Universities, volume 1 (1910), https://archive.org/details/textbookofbotany01couluoft, and volume 2, Ecology (1911), https://archive.org/details/textbookofbotany02coul, which says that the wall next to the slit is much thicker than the wall on the far side, and W. F. Ganong, A Textbook of Botany for Colleges (1920), https://archive.org/details/textbookofbotany00ganouoft; all three books are in the public domain. (3) Wikipedia, text only: Guard cell and Stoma, https://en.wikipedia.org/wiki/Guard_cell. (4) For the nucleus of a guard cell: two journal abstracts read through Europe PMC, https://europepmc.org/article/MED/38113262 and https://europepmc.org/article/MED/33327661. Licence: own work
The drawing: Drawn for this site on 2026-09-30 from plain geometry; nothing traced. Licence: own work
How it works
This guard cell diagram shows one stoma from the surface of a leaf, first open and then closed. Tap a name to read about the part, or press Quiz me.
Read the full guide
A stomata and guard cells diagram in two states
Open: the guard cells are swollen with water and bowed apart, so the pore between them is wide. Closed: they have lost water and lie straight, and the pore is a slit.
Why the pair bends
Each guard cell has a thick wall next to the pore and a thin wall on the far side. As water enters, the thin side bulges more than the thick side, so the cell curves away from its partner.
Guard cells have chloroplasts. The epidermal cells round them usually have none, which is a quick way to tell them apart.
Stomatal pore and stomate are accepted for stoma.
Questions and answers
What are the parts of a stomata?
One stoma is a pore with a guard cell on each side; stomata is the plural. Each guard cell has a nucleus, chloroplasts, vacuoles and a wall that is thicker next to the pore.
What do guard cells do?
They open and close the stoma. That controls how easily carbon dioxide enters the leaf and how much water vapor leaves it.
When do stomata open and close?
They tend to open in light and when water is plentiful, and to close in the dark and when water is scarce.
Why is the inner wall of a guard cell thicker?
A thick wall stretches less than a thin one. When the cell swells, its thin outer side bulges more, the cell bows outward and the pore opens.