Stomata Diagram: Interactive 3D Guard Cells, Labels and How Stomata Open and Close

Stomata (singular stoma) are tiny pores in the epidermis of leaves and young stems. Each pore is surrounded by a pair of guard cells that open and close it. When the pore is open, carbon dioxide diffuses into the leaf for photosynthesis, and oxygen and water vapour diffuse out. When it is closed, the leaf saves water but cannot take in CO₂.

Stomata are microscopic. In broad bean each guard cell is about 40–50 µm long, and the open pore is roughly 8–16 µm wide, a tenth of the width of a human hair. All the pores together cover typically less than 3% of the leaf surface, yet some 98% of the CO₂ a leaf takes up and the water it loses passes through them. (Lawson & Blatt (2014))

Most eudicot leaves have more stomata on the lower surface than on the upper surface. In a typical dicot the guard cells are kidney-shaped (bean-shaped); in grasses they are dumbbell-shaped and flanked by subsidiary cells. (NCERT Biology Class XI, Chapter 6) For the whole leaf around them, see the leaf cross-section.

A labeled stomata diagram shows two kidney-shaped guard cells around a central pore, surrounded by epidermal cells. The diagram has two views, because textbooks and exams use both: the surface view you see when you peel the epidermis off a leaf, and a section (T.S.) cut across the middle of the stoma, which shows the walls, the cuticle and the air space underneath.

Describe the structure of stomata with a labelled diagram (model answer). A stoma is a pore in the epidermis bounded by two kidney-shaped guard cells (dumbbell-shaped in grasses). The guard cells are joined at their ends. Their inner walls, which line the pore, are thick and inelastic; their outer walls, next to the epidermal cells, are thin and elastic. Each guard cell is a living cell with a nucleus, cytoplasm, a vacuole and chloroplasts, unlike the surrounding epidermal cells, which have few or none. In some plants the guard cells are surrounded by subsidiary cells. The guard cells, the pore and the subsidiary cells together make up the stomatal apparatus. Below the pore is the substomatal cavity, an air space that connects with the air spaces of the spongy mesophyll. The whole surface, including the guard cells, is covered by a waxy cuticle. (NCERT Biology Class XI, Chapter 6)

Each part of a stoma has a job. The table lists what to label on a stomata diagram and what each part does.

Click any part in the 3D model for its full description. The stomatal apparatus (or stomatal complex) means the pore, the guard cells and any subsidiary cells together.

Stomata open when the guard cells take up solutes and water and swell, and close when they lose them and shrink. NCERT’s Class 10 summary is exactly this: the guard cells swell when water flows into them, causing the pore to open; the pore closes if the guard cells shrink. (NCERT Science Class X, Chapter 5) The rest of the story is how the guard cells move the solutes, and why swelling makes them bend apart.

1. Ions are pumped in: K⁺, Cl⁻ and malate

Opening starts at the plasma membrane. Blue light, red light and the hormone auxin activate a proton pump, the plasma-membrane H⁺-ATPase, which uses ATP to push protons (H⁺) out of the guard cell. This makes the inside of the cell strongly negative (hyperpolarised), which drives potassium ions in through inward K⁺ channels. Chloride ions enter with protons, and the guard cell makes malate, a negatively charged organic acid, partly from starch broken down in its chloroplasts. Later in the day sugars add to the solute load. (Schroeder et al. (2001))

The amounts are large. In broad bean guard cells, vacuolar K⁺ rises from about 38–92 mM when closed to 181–454 mM when open, and the total change in solute often exceeds 300–400 mosmol per litre. (Jezek & Blatt (2017)) Malate can supply 50–90% of the counter-ions for K⁺, depending on how much chloride is available. (Santelia & Lawson (2016))

2. Water follows: water potential and turgor

Dissolved solutes lower the water potential (Ψ) of the guard cells, making it more negative than in the cells around them. Water always moves from higher to lower water potential, so it flows into the guard cells by osmosis. The cell wall resists the swelling protoplast, and the pressure that builds up is turgor pressure. (OpenStax Biology 2e, 30.5 Transport of Water and Solutes in Plants) Guard-cell volume rises from about 2.7–3.7 pL to 4.0–7.5 pL as the stoma opens, and most of that is the vacuole, which makes up 80–90% of an open guard cell. (Jezek & Blatt (2017))

3. The wall makes the cells bend: microfibrils and stiff ends

A swelling cell would normally just get rounder. Guard cells bend instead because of their walls. Cellulose microfibrils wrap round each guard cell like hoops (in surface view they seem to radiate from the pore, which is called radial micellation), so the cell cannot get much fatter. The two cells are also joined and stiffened at their ends, so they cannot simply get longer. Pinned at both ends and unable to bulge, each cell bows outwards as it swells, and the gap between them opens.

The classic textbook explanation adds that the thick inner wall stretches less than the thin outer wall, so the outer side lengthens more and the cell curves away from the pore. Research suggests this is not the whole story. Atomic force microscopy found that young Arabidopsis guard cells open normally before their inner walls are stiffer, while the ends of the stoma are stiff in Arabidopsis, tomato and maize; the authors propose a ‘fix and flex’ model in which pectin-rich, stiff poles matter more than the thick inner wall. (Carter et al. (2017))

4. Closing: ions out, water out

Closing reverses the process. Abscisic acid (ABA), darkness, high CO₂ and pathogen signals raise the free calcium level in the guard-cell cytoplasm. Ca²⁺ blocks the proton pump and the inward K⁺ channels and opens anion channels, so Cl⁻ and malate leave. The membrane depolarises, which opens outward K⁺ channels, and K⁺ leaves too. Water follows the ions out, turgor falls, and the guard cells straighten and close the pore. (Schroeder et al. (2001))

What makes stomata open or close

The blue-light response saturates at a low light level, around 10 µmol m⁻² s⁻¹, so stomata start opening early in the morning. (Santelia & Lawson (2016)) The old starch–sugar hypothesis (that light makes guard cells turn starch into sugar, lowering their water potential) is no longer accepted as the main mechanism; potassium uptake is. Starch still matters, as a source of malate and sugar.

An open stoma has turgid, curved guard cells and a wide, oval pore; a closed stoma has flaccid, straighter guard cells pressed together, leaving a thin slit or no gap. Use the Open and Close buttons above the 3D model to see the change in 3D, or compare the two labeled diagrams.

Transpiration is the loss of water vapour from a plant, mostly through its stomata. Water evaporates from the wet walls of the mesophyll cells into the air spaces, collects in the substomatal cavity, and diffuses out of the open pore into the drier air outside. The cuticle is nearly waterproof, so when the stomata are open they are the main exit. Up to 90% of the water a plant's roots take up can be lost this way. (OpenStax Biology 2e, 30.5 Transport of Water and Solutes in Plants.)

Transpiration is not just a loss. Evaporation from the leaf creates a pull that draws water, and the minerals dissolved in it, up the xylem from the roots. It also cools the leaf. NCERT's Class 10 text puts it this way: during the day, when the stomata are open, the transpiration pull becomes the major driving force for water movement in the xylem; at night root pressure matters more. (NCERT Science Class X, Chapter 5: Life Processes (PDF).)

The plant controls transpiration mainly by changing the stomatal aperture. Wider pores let water out faster. Closing the stomata in drought, in the dark, or in very dry air saves water. Trichomes (leaf hairs), a thick cuticle and sunken stomata all slow transpiration further by keeping a layer of still, humid air over the pores. (OpenStax Biology 2e, 30.4 Leaves.)

Stomata are where a leaf exchanges gases with the air. In the light, CO₂ diffuses in through the pores to the mesophyll chloroplasts, where it is fixed into sugar; O₂ made in photosynthesis diffuses out by the same route. At night the leaf only respires, so it takes in a little O₂ and gives out CO₂. NCERT's Class 10 book describes stomata as the tiny pores through which massive amounts of gas exchange take place for photosynthesis. (NCERT Science Class X, Chapter 5: Life Processes (PDF).)

The pores cover typically less than 3% of the leaf surface, yet some 98% of all the CO₂ taken up and water lost passes through them. This is the central trade-off of plant life: every opening that lets CO₂ in also lets water out. Plants balance the two by opening stomata when light makes photosynthesis possible and closing them when water is short or CO₂ inside the leaf is high. (Lawson & Blatt (2014): Stomatal size, speed, and responsiveness impact on photosynthesis and water use efficiency, Plant Physiology.)

Stomatal speed matters too. Stomata respond in minutes, but light under a moving canopy changes in seconds, so slow stomata waste water when they lag behind shade and lose photosynthesis when they lag behind sun. Smaller stomata generally respond faster than larger ones, and stomatal density is negatively correlated with stomatal size: leaves tend to have either many small stomata or fewer large ones. (Lawson & Blatt (2014): Stomatal size, speed, and responsiveness impact on photosynthesis and water use efficiency, Plant Physiology.)

For where the CO₂ that enters through the stomata is actually fixed, see chloroplast structure.

Stomata are classified by the cells around the guard cells. Subsidiary cells are epidermal cells next to the guard cells that differ in shape or size from ordinary epidermal cells. The number and arrangement of subsidiary cells vary widely between plant groups, which makes stomatal type useful for identifying plants. (Gray, Liu & Facette (2020))

The grass type is a special paracytic stoma. Its guard cells are narrow, thick-walled rods with bulbous ends. When the ends swell they push the rods apart, so the pore opens as a parallel-sided slit. In maize, potassium shuttles back and forth: it is high in the guard cells when stomata are open and high in the subsidiary cells when they are closed. (Gray, Liu & Facette (2020)) Brachypodium mutants that lack subsidiary cells have slower stomata with a smaller range of apertures, which supports the idea that subsidiary cells make grass stomata better. (Raissig et al. (2017))

The textbook contrast is between a grass (the usual monocot example) and a typical eudicot. Grass stomata have dumbbell-shaped guard cells with subsidiary cells and lie in parallel rows; dicot stomata have kidney-shaped guard cells scattered over the leaf. NCERT also notes that an isobilateral (monocot) leaf usually has stomata on both surfaces, while a dorsiventral (dicot) leaf has more on the lower surface. (NCERT Biology Class XI, Chapter 6)

Two cautions. First, ‘monocot’ does not always mean dumbbell: lilies, onions and many other monocots have kidney-shaped guard cells. Second, the speed difference comes from a careful comparison of four species, in which wheat stomata moved about an order of magnitude faster than kidney-type stomata. (Franks & Farquhar (2007))

Leaves are classed by where their stomata are. A hypostomatic leaf has stomata only on the lower (abaxial) surface; an amphistomatic leaf has them on both surfaces; an epistomatic leaf has them only on the upper (adaxial) surface. NCERT notes that in a dorsiventral (typical dicot) leaf the lower epidermis generally bears more stomata than the upper, which may lack them, while in an isobilateral (monocot) leaf stomata are present on both surfaces. (NCERT Biology Class XI, Chapter 6: Anatomy of Flowering Plants (PDF).)

Floating leaves are the classic epistomatic case. In the water lily (Nymphaea) the lower surface lies on the water, so stomata are restricted to the upper surface, where they can reach the air. The leaf also has large air chambers that help it float. Submerged leaves often have few or no stomata. (UBC BIOL 210: Hydrophyte leaves.)

The two surfaces of one leaf are not independent copies. In Arabidopsis, stomatal patterning is regulated differently in the upper and lower epidermis, and the ratio of upper to lower stomata depends on genotype and growing conditions such as humidity. (Jalakas et al. (2024): Stomatal patterning is differently regulated in adaxial and abaxial epidermis in Arabidopsis, Journal of Experimental Botany.)

In xerophytes (plants of dry places) stomata are often sunken in pits, protected by hairs or wax, and covered by a thick cuticle. These features trap a layer of still, humid air over each pore.

Sunken stomata are stomata set in pits below the general leaf surface. They are a classic feature of xerophytes, plants adapted to dry habitats. Conifer needles such as pine have sunken stomata and a small surface area, two features OpenStax lists as reducing water loss. Xerophytes usually also have a much thicker cuticle and often a dense covering of trichomes. (OpenStax Biology 2e, 30.4 Leaves.)

Oleander (Nerium oleander) goes further: its stomata lie in stomatal crypts, large chambers in the leaf lined with epidermis that bears stomata and hairs. The leaf surface between the crypts has no stomata. The narrow crypt opening and the hairs keep the air inside still, so a water molecule leaving a pore has a good chance of diffusing back in rather than escaping. (Plant anatomy image notes: Stomatal crypts in a transverse section of an oleander leaf (Constantine the Philosopher University, Nitra).)

In some conifers the chamber above each sunken stoma is filled with a porous plug of wax. How much water these features save is still debated. A model of crypts found they cut transpiration by less than 15% when stomata are open, and a 2022 review points out that sunken stomata also occur in wet habitats and may protect the pore from water films, fog and fungi as much as from drought. (Šantrůček (2022): The why and how of sunken stomata, Annals of Botany.)

Stomatal density is the number of stomata per square millimetre of leaf surface. Stomatal index is the percentage of epidermal cells that are stomata:

Stomatal index (SI) = S ÷ (S + E) × 100

Here S is the number of stomata and E the number of ordinary epidermal (pavement) cells in the same area. Research papers often write it as SD ÷ (SD + PCD), where SD is stomatal density and PCD is pavement-cell density, and may leave out the × 100. (Jalakas et al. (2024)) Count each stoma once (one pair of guard cells = one stoma).

Worked example. On the upper surface of Arabidopsis rosette leaves, one study counted about 101 stomata and 243 pavement cells per mm². Stomatal density = 101 per mm². Stomatal index = 101 ÷ (101 + 243) × 100 ≈ 29%. (Inan et al. (2004))

Why have two measures? Stomatal density changes when leaf cells grow bigger or smaller (for example in shade or drought), even if the same proportion of cells became stomata. Stomatal index measures that proportion, so it is more stable and tells you whether the plant actually made more or fewer stomata. Both are used to study how plants respond to CO₂ and climate.

Method. Make an epidermal impression (see the next section), view it at a magnification where you can see individual cells, and count stomata and epidermal cells inside a known area: either the squares of an eyepiece graticule or the whole field of view. Calibrate that area with a stage micrometer. Count several fields or squares and take the mean. (Science & Plants for Schools (SAPS))

The quickest way to see stomata is to make a nail varnish impression of the lower surface of a leaf. Clear nail varnish copies the surface in fine detail, including the outlines of the guard cells and epidermal cells.

1. Choose a mature leaf with a smooth lower surface and paint a patch, about 1 cm², between the larger veins with clear (or pale) nail varnish.
2. Let it dry completely.
3. Press a strip of clear sticky tape over the patch and peel it off: the dried varnish comes away with it.
4. Stick the tape onto a clean glass slide.
5. Look at it under a light microscope, first at low power and then at about ×400. Count with a graticule, or count whole fields of view. (Rothamsted Research Bioimaging)

With some plants you can instead tear off a strip of real epidermis (an epidermal peel) and mount it in a drop of water. Broad bean, onion leaves and Tradescantia work well. A real peel shows living guard cells with green chloroplasts, which an impression cannot. Some leaves are damaged by the solvent in nail varnish, absorbing it and turning brown; test a leaf first, or use water-based varnish, which can even be applied to a leaf still on the plant. (Science & Plants for Schools (SAPS))

What you see: pairs of curved guard cells, each pair around a dark slit or oval, scattered among jigsaw-shaped or polygonal epidermal cells. In grass leaves the stomata lie in neat rows. Compare the lower and upper surfaces of the same leaf, or leaves from sun and shade, and calculate the stomatal density and index for each. Never eat or taste plant material in a lab, and wash your hands after handling nail varnish.

Almost all land plants have stomata, but where they sit differs. In mosses and hornworts, stomata occur only on the sporophyte capsule (the spore-producing part), not on the leafy plant. Liverworts are the only living land plants that lack stomata entirely; their thalloid forms have simple air pores instead. (Renzaglia, Browning & Merced (2020): With over 60 independent losses, stomata are expendable in mosses, Frontiers in Plant Science.)

Bryophyte stomata seem to work differently from those of flowering plants. Renzaglia and colleagues argue that moss and hornwort stomata do not open and close in response to light, water status or ABA, and that they mainly help the capsule dry out so spores can be released. Some laboratory studies have reported ABA responses in moss stomata, so this is an active debate. (Renzaglia, Browning & Merced (2020): With over 60 independent losses, stomata are expendable in mosses, Frontiers in Plant Science.)

Ferns and lycophytes have kidney-shaped guard cells without subsidiary cells. Many gymnosperms have sunken stomata; in Ginkgo the guard cells are recessed under a ring of subsidiary cells (a cyclocytic stoma). (Gray, Liu & Facette (2020): Flanking support: how subsidiary cells contribute to stomatal form and function, Frontiers in Plant Science.)

These are the key points school syllabuses expect, from simplest to most detailed.

Class 7 (nutrition in plants). Leaves have tiny pores called stomata, mostly on the lower surface. Each pore is surrounded by guard cells. Carbon dioxide for photosynthesis enters through the stomata, and oxygen and water vapour leave through them.

Class 9 (plant tissues). Stomata are pores in the epidermis, the outer protective tissue of the leaf. Each stoma is enclosed by two kidney-shaped guard cells. Stomata are needed for gas exchange with the atmosphere and for transpiration, the loss of water as vapour. Learn to draw guard cells and epidermal cells in surface view.

Class 10 (life processes). Gas exchange for photosynthesis happens through the stomata. Because a lot of water can be lost through them, the plant closes the pores when it does not need CO₂. Opening and closing is a function of the guard cells: they swell when water flows into them and the pore opens; they shrink and the pore closes. The diagram to know is ‘open and closed stomatal pore’. Transpiration pulls water up the xylem and is the main driving force during the day, when the stomata are open. (NCERT Science Class X, Chapter 5)

Class 11 (anatomy of flowering plants). Bean-shaped guard cells, dumbbell-shaped in grasses; thin outer walls and highly thickened inner walls; guard cells have chloroplasts; subsidiary cells; and the stomatal apparatus (pore + guard cells + subsidiary cells). (NCERT Biology Class XI, Chapter 6)

The text version of the 3D diagram: every structure you can click, grouped as in the viewer.

Test yourself with this unlabeled stomata diagram. Choose the correct name for each of the 12 numbers. It also works as a black and white printout: colour the guard cells green, the vacuoles blue, the nuclei purple and the cuticle yellow.

1. 1.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
2. 2.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
3. 3.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
4. 4.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
5. 5.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
6. 6.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
7. 7.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
8. 8.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
9. 9.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
10. 10.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
11. 11.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole
12. 12.Choose a structure…ChloroplastCuticleEpidermal (pavement) cellGuard cellNucleusOuter cuticular ledgeSpongy mesophyll cellStomatal poreSubstomatal cavityThick inner (ventral) wallThin outer (dorsal) wallVacuole

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Questions of the kind set in tests and exams, with answers.

1. Why are the inner walls of guard cells thick? To strengthen the guard cell where it bends and to resist stretching on the pore side, so the cell curves outwards when turgid (the classic explanation; stiff ends also matter).
2. Which ion moves into guard cells to open stomata? Potassium (K⁺), balanced by chloride and malate.
3. What does the proton pump do in stomatal opening? It pumps H⁺ out of the guard cell, making the inside negative so that K⁺ is drawn in.
4. Name the hormone that closes stomata in drought. Abscisic acid (ABA).
5. A leaf peel shows 24 stomata and 96 epidermal cells in one field. What is the stomatal index? 24 ÷ (24 + 96) × 100 = 20%.
6. Why does a water lily have stomata only on its upper surface? Its lower surface lies on the water, so only the upper surface is in contact with air.
7. How are grass stomata different from bean stomata? Dumbbell-shaped guard cells with subsidiary cells, in parallel rows, opening as a slit; bean stomata have kidney-shaped guard cells, usually without subsidiary cells, scattered over the leaf.
8. Why do guard cells have chloroplasts? To make ATP and sugars and store starch, which supply energy and solutes for stomatal movement.