Leaf Cross Section: Labeled Diagram and Interactive 3D Model

A leaf cross section shows, from top to bottom: a waxy cuticle, the upper epidermis, a row of tall palisade mesophyll cells, loosely packed spongy mesophyll with air spaces, and the lower epidermis with stomata and their guard cells, covered by a thinner lower cuticle. Veins (vascular bundles) run between the palisade and spongy layers; each has xylem on the upper side, phloem on the lower side and a bundle sheath around them.

A cross section (also called a transverse section, T.S. or C.S.) is a thin slice cut at right angles to the surface of the blade and across the main vein. It shows all the tissue layers at once, which is why it is the standard view of a leaf in textbooks and exams. A slice cut parallel to the surface, through just one layer, is called a paradermal section.

The reference leaf on this page is common lilac, a dicot shrub. Lilac is the textbook example of a mesomorphic leaf, one adapted to a place with enough water in the soil: broad, flat and thin, with a single-layered epidermis, palisade above spongy mesophyll, and veins with xylem above phloem. Most broad-leaved trees, shrubs and garden plants have leaves built the same way. Further down, the page compares it with a monocot (maize), a C4 grass, a CAM succulent (aloe) and a conifer needle (pine).

In a labeled cross section of a dicot leaf, you should be able to find 15 to 20 structures. They belong to three tissue systems: the dermal tissue (cuticle, epidermis, stomata, guard cells, hairs), the ground tissue or mesophyll (palisade and spongy layers, air spaces), and the vascular tissue (veins with xylem and phloem).

Two quick ways to tell top from bottom in any slide: the palisade layer is always on the side facing the light (the upper, or adaxial, side), and in each vein the xylem faces the upper side while the phloem faces the lower (abaxial) side.

This is the text version of the 3D model: every labeled structure in the lilac leaf block, grouped by tissue system, with what it does. Use it as a study list or to check a labeling worksheet.

A leaf is an organ built to absorb light and exchange gases while losing as little water as possible. For GCSE and IGCSE biology, the key adaptations are: a large, thin, flat shape; a transparent epidermis and cuticle; palisade cells packed with chloroplasts near the top; air spaces in the spongy mesophyll; stomata controlled by guard cells; and a network of veins that supplies water and removes sugar.

Exams often ask why a leaf is a compromise. The same stomata and air spaces that let carbon dioxide in also let water vapour out (transpiration). The plant controls this trade-off with the cuticle, which blocks water loss everywhere except at the stomata, and with the guard cells, which close the pores when the plant is short of water. The tissue names GCSE specifications use are epidermal tissue, palisade mesophyll, spongy mesophyll, xylem and phloem.

Every vein in a leaf is a vascular bundle with xylem on the upper side and phloem on the lower side. The midrib is the largest vein, running down the middle of the blade; the minor veins branch from it into a net (reticulate venation) so that no mesophyll cell is more than a few cells away from a vein.

• Xylem (minor vein): Brings water and dissolved minerals from the roots into the leaf. The water is used in photosynthesis and replaces water lost by transpiration.
• Phloem (minor vein): Carries sugars made in photosynthesis (as sucrose) out of the leaf to the rest of the plant, a process called translocation.
• Bundle sheath: Controls what passes between the vein and the mesophyll and keeps the vein's conducting cells out of the air spaces. In C4 plants such as maize it is enlarged and does the Calvin cycle.
• Xylem (midrib): The main water supply line of the blade, which branches into the smaller veins.
• Phloem (midrib): Collects sugar from the smaller veins and carries it out of the leaf through the petiole.
• Collenchyma: Strengthens the midrib while letting it bend, so the leaf can flex in the wind without snapping.
• Midrib parenchyma: Fills and supports the midrib, stores water and starch, and passes substances between the vein and the blade.

Why is the xylem on top? A leaf vein is a branch of the vascular system of the stem. In the stem, the xylem lies towards the centre and the phloem towards the outside. Where a bundle bends out into a leaf, the side that faced the centre of the stem becomes the upper side of the leaf, so the xylem ends up on top. Some plants, such as oleander, have phloem on both sides of the xylem (a bicollateral bundle), but lilac has phloem below only.

In a cross section, a stoma looks like a narrow gap in the lower epidermis with a small, rounded guard cell on each side and a large air space, the substomatal chamber, just above it. Guard cells are green because they are the only epidermal cells with chloroplasts.

• Stoma (stomatal pore): Lets carbon dioxide diffuse into the leaf for photosynthesis and lets oxygen and water vapour out. It closes when the plant is short of water.
• Guard cells: When they take up water and swell, the pore opens; when they lose water and go limp, the pore closes. This balances carbon dioxide uptake against water loss.
• Substomatal chamber: Connects the stomatal pore to the air spaces of the mesophyll, so gases entering or leaving the stoma can spread through the leaf.

In the 3D model, the front stoma is cut through the middle, exactly as on a slide, and a second stoma sits further back on the underside, where you can see the two kidney-shaped guard cells around the pore from below. For how guard cells open and close the pore, the different stomatal types and stomatal density, see the stomata diagram. For the inside of a single chloroplast, see chloroplast structure.

A dicot leaf such as lilac is dorsiventral (bifacial): its two sides differ, with palisade on top, spongy mesophyll below, and most stomata on the underside. A monocot leaf such as maize or a grass is isobilateral: both sides look alike, the mesophyll is not divided into palisade and spongy layers, and stomata are found on both surfaces.

Bulliform cells are large, empty-looking, colourless cells in the upper epidermis of grass leaves, usually in groups between the veins. When they are full of water the leaf lies flat; when they lose water in a drought they shrink, and the leaf curls inward, which reduces water loss. The labeled monocot leaf cross section most courses use is maize (corn), which is also a C4 plant, so it shows Kranz anatomy as well. Do not assume every monocot is C4: wheat, rice and lilies are C3 monocots without Kranz anatomy.

C3, C4 and CAM are three ways of fixing carbon dioxide, and you can often tell them apart in a cross section. A C3 leaf (lilac, wheat, rice) has ordinary mesophyll and a small, pale bundle sheath. A C4 leaf (maize, sorghum, sugarcane) has Kranz anatomy: a wreath of large, chloroplast-rich bundle sheath cells around each vein, ringed by mesophyll cells. A CAM leaf (aloe, pineapple, many cacti and stonecrops) is usually thick and succulent, with large water-filled cells.

Kranz anatomy works as a carbon dioxide pump. Mesophyll cells use PEP carboxylase, which does not react with oxygen, to fix carbon dioxide into a 4-carbon acid. The acid moves into the bundle sheath, where carbon dioxide is released next to Rubisco and enters the Calvin cycle. Because the sheath is tightly packed and has thick walls, the carbon dioxide stays concentrated there, which limits photorespiration, the wasteful reaction of Rubisco with oxygen. In CAM plants the two steps are separated in time instead of space: stomata open at night, carbon dioxide is stored as malic acid in the large vacuoles, and it is released for photosynthesis during the day with the stomata closed.

A pine needle cross section shows a leaf built to survive cold, dry winters: a thick cuticle, an epidermis of thick-walled cells, a hypodermis below it, stomata sunk in pits, mesophyll cells whose walls fold inward, resin ducts, and a central cylinder in which an endodermis encloses transfusion tissue and one or two vascular bundles.

The half-moon outline of the section is a clue to the species group: pines with two needles per bundle have semicircular needles, pines with three have triangular ones. Sources describe the mesophyll differently: some call it spongy parenchyma, others arm-palisade cells, because each cell has ridges of wall folding into it. The number of vascular bundles is characteristic of the species.

An aloe vera leaf cross section shows a thin green rind around a thick, clear core. From the outside in: a thick cuticle and epidermis with sunken stomata, a band of green chlorenchyma (photosynthetic cells), a ring of vascular bundles at the edge of the core, and a large central mass of colourless, water-storing (aquiferous) parenchyma, which is the clear gel people use.

Aloe vera is a CAM plant, and its leaf shows the typical CAM traits: thick succulent tissue, large cells with big vacuoles that store water and, overnight, malic acid, and a thick waxy surface. The bitter yellow sap that seeps from a cut aloe leaf is stored near the vascular bundles, not in the gel. Unlike lilac, an aloe leaf has no palisade layer: in Aloe vera the chlorenchyma cells are rounded.

Under a light microscope, a leaf cross section shows a clear epidermis on each side, a dark green row of upright palisade cells under the upper epidermis, a paler, loose spongy layer with white air spaces, and veins of thick-walled, empty-looking xylem cells above a patch of small phloem cells.

• Low power (×40): find the midrib bulge and the thin blade on either side. In a lilac slide the midrib shows a crescent of xylem and phloem in a mass of colourless cells.
• Medium power (×100): move into the blade and count the layers from top to bottom. The palisade row is easy to spot because the cells stand on end like a fence.
• High power (×400): look for chloroplasts lining the walls of the palisade cells, a guard cell pair in the lower epidermis, and the substomatal chamber above it.

Colours depend on how the slide was made. In a fresh hand section, the only colour is the natural green of the chloroplasts. Commercial prepared slides are usually double-stained with safranin and fast green: safranin stains lignified walls (xylem, fibres) and the cutin of the cuticle red, and fast green stains ordinary cellulose walls green. So in a typical lilac slide the xylem and cuticle look red and the mesophyll, epidermis, phloem and collenchyma look green or blue-green. Chloroplasts in old, stained slides often look like dark dots rather than green discs.

You can make your own leaf cross section with a new razor blade and a piece of carrot or potato to hold the leaf. The aim is a slice thin enough to be almost transparent; you will need to cut many sections and keep only the best.

1. Cut a strip of leaf about 5 mm wide across the midrib. Soft leaves such as privet, lilac, spinach or a geranium work well.
2. Cut a small block of carrot or potato, split it in half, and cut a shallow groove in one half. Lay the leaf strip in the groove and press the other half on top, so the leaf is held upright in a sandwich.
3. Keep the blade, the tissue and your fingers wet with water. Rest your forearms on the bench, use your forefinger as a guide, and cut across the top of the sandwich in one smooth pull or push. Do not saw.
4. Cut many thin slices and float them all in water in a watch glass or Petri dish.
5. Look at them against a dark background or under a dissecting microscope, and pick the thinnest. They should be almost transparent.
6. Move the best sections with a fine brush into a small drop of water on a slide. Lower a coverslip slowly from one side with forceps to avoid air bubbles.
7. Examine under low power first. For waxy leaves that repel water, a very small amount of detergent in the water helps the section sink and wet.

Safety: razor blades are very sharp. Use a single-edged blade or a blade holder, always cut away from your fingers, and follow your teacher's instructions. Change the blade as soon as it starts to drag, because a blunt blade crushes the cells.

The number of palisade layers is not fixed for a species. A leaf that grows in strong light develops a palisade tissue several cells deep, while a leaf that grows in shade keeps a single palisade layer. You can see this on one tree: leaves from the sunny outside of the crown are thicker than leaves from the shaded inside. (Botany Online (University of Hamburg): the mesophyll, main assimilation tissue of leaves.)

The two mesophyll layers also differ in how much they photosynthesise. Palisade cells contain three to five times as many chloroplasts as spongy cells, and the amount of air space in the mesophyll varies widely between species and habitats, from about 70 to 700 parts per thousand of the leaf volume. (Botany Online (University of Hamburg): the mesophyll, main assimilation tissue of leaves.)

Lilac is called a mesomorphic leaf because it grows where soil moisture is adequate: a broad, flat blade, a one-cell-thick epidermis, palisade and spongy mesophyll, and veins with xylem above phloem. The same lab compares it with oleander (Nerium), a xeromorphic leaf from dry places, whose epidermis is several cells thick and whose stomata sit in pits called stomatal crypts on the underside, and with the water lily (Nymphaea), whose spongy tissue is full of large air spaces (aerenchyma), whose stomata are on the upper surface only, and whose veins have little xylem. (University of Wisconsin–Madison, Botany 130: leaf anatomy (lilac, oleander, water lily, maize).)

Leaves from harsh, dry or salty places show the same trends. An Acacia leaf has a thick cuticle, a mangrove (Rhizophora) leaf has more closely packed cells and a thicker cuticle, and a bottlebrush (Callistemon) leaf contains a lot of thick-walled sclerenchyma. In the floating water lily leaf the cuticle is much thinner, and the aerenchyma lets the leaf float. (Bellairs, Plant Anatomy and Physiology (LibreTexts), 3.2 Leaves.)

In maize, each vein is wrapped in a ring of bundle sheath cells, and one or more rings of mesophyll cells sit outside that. A proteomics study of the two cell types showed how the work is split: the Rubisco large subunit accumulates only in bundle sheath chloroplasts, while mesophyll chloroplasts hold photosystem II and the enzyme PPDK that regenerates the carbon dioxide acceptor. Bundle sheath cells take in malate and break it down to pyruvate, releasing carbon dioxide for the Calvin cycle. (Majeran et al. (2005): bundle sheath and mesophyll chloroplasts of maize, The Plant Cell.)

The two kinds of chloroplast also look different. Bundle sheath chloroplasts in maize sit towards the outer wall of the cell and contain large starch grains; mesophyll chloroplasts are arranged randomly and have stacked thylakoids (grana). (Majeran et al. (2005): bundle sheath and mesophyll chloroplasts of maize, The Plant Cell.)

Kranz means wreath. The bundle sheath cells of C4 plants have many chloroplasts, thick walls that gases pass through poorly, and no air spaces between them, which keeps the concentrated carbon dioxide inside. The first carbon dioxide-fixing enzyme in the mesophyll is PEP carboxylase, and the mesophyll cells lack Rubisco. (NCERT Biology Class XI, Chapter 11: Photosynthesis in Higher Plants (PDF).)

Bulliform cells are enlarged, colourless epidermal cells found only on the upper side of grass leaves, usually in strips two to five cells wide that run along the leaf. They are also called hinge or motor cells, and they are linked to the way grass leaves roll inward when the plant is short of water. Exactly how they work is still being debated. (Matschi et al. (2020): the maize bulliform cell cuticle and leaf rolling, Plant Direct.)

A 2020 study of maize found that bulliform cells shrink more than other epidermal cells when a leaf dries out, and that their cuticle is about four times thicker than the cuticle of ordinary epidermal cells but lets water through more easily. The authors suggest this uneven shrinking helps the leaf fold. (Matschi et al. (2020): the maize bulliform cell cuticle and leaf rolling, Plant Direct.)

A study of six Aloe species found that every leaf had the same basic layers: epidermis, chlorenchyma (the green photosynthetic layer), aquiferous (water-storing) tissue, and vascular bundles. All six showed xeromorphic features: thick epidermal walls, a thick cuticle, sunken stomata and well-developed water-storage tissue. In Aloe vera the chlorenchyma cells are rounded, not palisade-like as in Aloe arborescens. (Li et al. (2003): leaf structure and aloin content in six Aloe species, Journal of Integrative Plant Biology.)

Fluorescence microscopy showed that the bitter yellow compound aloin is stored in the large parenchyma cells of the vascular bundles, the bundle sheath and the sheath around the water-storage tissue, not in the green chlorenchyma. Aloe vera had less aloin than Aloe arborescens. (Li et al. (2003): leaf structure and aloin content in six Aloe species, Journal of Integrative Plant Biology.)

Test yourself: name each numbered structure on this unlabeled leaf cross section diagram. You can print it or copy the numbers onto paper. The answer key is below.

Extra questions:

1. Which two leaf cell types contain chloroplasts, and which layer has the most?
2. Why are most stomata on the lower surface of a lilac leaf?
3. In a vein, which tissue is on the upper side and which on the lower side?
4. Give two differences between a dicot and a monocot leaf cross section.
5. What is Kranz anatomy, and in which plant would you see it?
6. Name three features of a pine needle that reduce water loss.