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 unlabeled stomata diagram is a patch of leaf surface round one pore and its guard cells. Eight lines lead off, six from the upper drawing and two from the lower.
Read the full guide
The eleven names, open stoma and closed
Eight belong to the open stoma: guard cell, stoma, chloroplast, nucleus, vacuole, thick inner wall, thin outer wall and epidermal cell. Three come again on the closed one: guard cell, stoma and epidermal cell.
Which line ends where
Open stoma, top row from the left: thin outer wall, vacuole, nucleus, chloroplast. Of the two lines underneath, the left is guard cell and the right thick inner wall. The dark pore has no line, so stoma is written beside it, and epidermal cell goes in any of the plain cells. Closed stoma: the upper line runs to the slit, which is the stoma, and the lower line to a guard cell.
Stomatal pore and stomatal aperture are other names for the opening, so a course may ask for either.
The dot inside each plain cell takes no name.
Questions and answers
Is it stoma or stomata?
Stoma is one and stomata is the plural, from the Greek word for mouth. Stomate, plural stomates, means the same thing. The answer key writes Stoma, because each drawing shows one.
What shape are guard cells?
Bean-shaped, as drawn here. Grasses are an exception: each of their guard cells is narrow in the middle and bulbous at both ends.
Which side of a leaf has more stomata?
Usually the lower side in dicots. Monocots, for instance onion, oat and maize, may have equal numbers on both surfaces, and a floating leaf may have stomata on its upper surface only.
What are subsidiary cells?
Cells beside the guard cells that differ in size, shape or arrangement from the rest of the epidermis. This sheet has none: its guard cells are ringed by ordinary epidermal cells, so there is nothing more to label.