Inside a leaf
A leaf is often only a few cells thick, yet it has a waterproof coat, two kinds of food-making cells, air passages, pipes and adjustable doors.
The outside of a leaf
Pick a leaf from a tree and look at it. The flat, wide part is the blade, or lamina. Many leaves hang from the stem on a stalk called the petiole; leaves without one are called sessile. Down the middle runs the midrib, which branches into veins of vascular tissue. The pattern of veins tells you something about the plant. Monocots such as tulips and grasses have parallel veins, dicots such as the linden have a net of veins, and the ginkgo tree has veins that fork in two again and again.
Layer by layer
Cut a leaf across and look at it under a microscope and you see layers. On both faces is the epidermis, usually a single layer of cells. It is covered by the cuticle, a waxy layer that slows water loss from the surface; every land plant has one. Some leaves also carry small hairs called trichomes, which can put off insects or slow the air moving across the leaf.
Between the two epidermis layers is the mesophyll, the “middle leaf”. Its upper part is the palisade mesophyll: tall, column-shaped cells packed side by side in one, two or three layers, like a row of fence posts. They sit just under the top surface where light is strongest, and they are full of chloroplasts. Below them is the spongy mesophyll, irregular cells packed loosely with air spaces in between. Both layers contain chloroplasts. The air spaces connect to the outside through the stomata, so carbon dioxide can spread to every cell.
Running through the mesophyll are the veins. Each vascular bundle, no matter how small, contains both xylem and phloem. Xylem is made of tracheids and vessels that bring water and minerals in from the roots. Phloem carries the products of photosynthesis out of the leaf to the rest of the plant. In the lab, the xylem sits on the upper side of the bundle and the phloem on the lower side, as it does in a typical dicot leaf.
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Doors in the skin
The epidermis has openings called stomata (one is a stoma). Each stoma is flanked by two guard cells that swell to open the pore and shrink to close it. OpenStax notes that guard cells are the only epidermal cells that contain chloroplasts. In most leaves the stomata are mainly on the underside, where the surface is shaded and cooler, which helps reduce water loss.
Leaves built for different places
Leaf shape fits the habitat. Plants on a dark rainforest floor tend to have very broad leaves that catch every bit of light. Conifers such as spruce, fir and pine that live in cold places have needles with a small surface area and sunken stomata, both of which save water. Cactus leaves are reduced to spines, and the green stem does the photosynthesis. Floating water plants have wide blades with a thick waxy cuticle that sheds water. Carnivorous plants such as the Venus flytrap grow in bogs poor in nitrogen, and their leaves are modified to catch insects as an extra nitrogen supply.
| Cuticle | Waxy layer that slows water loss. |
| Palisade mesophyll | Tall packed cells under the top surface; most photosynthesis. |
| Spongy mesophyll | Loose cells with air spaces for gas exchange. |
| Xylem | Brings water and minerals from the roots. |
| Phloem | Carries sugars out of the leaf. |
| Guard cells | Pair of cells that open and close a stoma. |
| Petiole | Stalk that attaches a leaf to the stem. |
- Epidermis and cuticle protect and waterproof.
- Palisade then spongy mesophyll hold the chloroplasts.
- Every vascular bundle has xylem (water in) and phloem (sugar out).
- Guard cells open and close stomata, mostly on the underside.
- Which layer does most of a leaf’s photosynthesis?
- Palisade mesophyll
- Upper epidermis
- Cuticle
- Lower epidermis
- What does the cuticle do?
- Slows water loss from the surface
- Carries sugar
- Makes oxygen
- Opens the stomata
- Which epidermal cells contain chloroplasts?
- Only the guard cells
- All of them
- None
- Only the upper epidermis
- A vascular bundle in a leaf contains…
- both xylem and phloem
- only xylem
- only phloem
- only air
- A grass leaf with veins running side by side is…
- a monocot
- a dicot
- a ginkgo
- a fern
- Why are the air spaces of the spongy mesophyll useful?
- Gases can move between the stomata and the cells
- They store sugar
- They hold the chlorophyll
- They make the leaf heavier
- Where are stomata found in most leaves?
- Mainly on the underside
- Only on the top
- In the veins
- In the palisade layer
- Pine needles with sunken stomata are an adaptation to…
- save water
- catch insects
- float on water
- catch more light on a forest floor
- Why do Venus flytraps catch insects?
- To get extra nitrogen in nitrogen-poor bogs
- To get water
- To get carbon dioxide
- For defence only
- Put the layers in order from the top surface of the leaf downward.
Sources for this course (16)
- -fix5: gravity calculation corrected using rho*g*h = 1000*9.8*h Pa
- the cited textbook has a tenfold coefficient error. Sources (checked 2026-10-17): OpenStax Biology 2e 8.1 Overview of Photosynthesis (photoautotrophs, mesophyll, stomata on the underside, guard cells, chloroplast, thylakoid, granum, stroma, two stages; openstax.org/books/biology-2e/pages/8-1-overview-of-photosynthesis)
- 8.2 The Light-Dependent Reactions (400 to 700 nm, chlorophyll a absorbs blue and red, carotenoids, 300 to 400 chlorophylls per antenna, water split at PSII, about 10 percent of the oxygen used by the leaf's mitochondria; .../8-2-the-light-dependent-reactions-of-photosynthesis)
- 8.3 Using Light Energy to Make Organic Molecules (RuBisCO, 3-PGA, G3P, three turns per G3P and six per glucose, "dark reaction" outdated, cacti fix CO2 at night; .../8-3-using-light-energy-to-make-organic-molecules)
- 30.3 Roots (tap and fibrous roots, root cap, three zones in the first centimetre, root hairs, endodermis and Casparian strip; .../30-3-roots)
- 30.4 Leaves (lamina, petiole, midrib, parallel and reticulate venation, Ginkgo, cuticle, trichomes, palisade in one to three layers, spongy mesophyll, guard cells the only epidermal cells with chloroplasts, xylem and phloem, needles and spines; .../30-4-leaves)
- 30.5 Transport of Water and Solutes (redwoods near 116 m, turgor 0.6 to 0.8 MPa and up to 1.5 MPa = 210 psi vs tyres 30 to 34 psi, gravity -0.1 MPa per metre, -2 MPa at the leaf surface, cohesion-tension, up to 90 percent of the water lost, sources and sinks, phloem sap up to 30 percent sugar; .../30-5-transport-of-water-and-solutes-in-plants)
- 31.3 Nutritional Adaptations (rhizobia, nodules, nitrogenase, biological fixation 65 percent of nitrogen used in agriculture, mycorrhizae in more than 80 percent of land plants, up to 20 percent of the carbon; .../31-3-nutritional-adaptations-of-plants)
- 32.2 Pollination and Fertilization (self and cross pollination, bees cannot see red, nectar guides, bats, birds, wind, water, at least 25,000 orchid species, double fertilization 2n zygote and 3n endosperm, seed parts, scutellum, fruit types, dispersal, dormancy; .../32-2-pollination-and-fertilization)
- Britannica, Photosynthesis: history (Priestley 1771, Ingenhousz 1779, 1782: oxygen formed at the expense of fixed air, identified the year before as carbon dioxide) (britannica.com/science/photosynthesis/History-of-the-study-of-photosynthesis)
- Smithsonian Libraries, Ingenhousz, Experiments upon Vegetables, 1779 (library.si.edu/digital-library/book/experimentsuponv00inge)
- van Helmont's account (200 pounds of earth, 5 pound willow, 169 pounds and about 3 ounces after five years, soil short by about 2 ounces)
- quoted from Howe 1965 by S. G. Saupe, College of St. Benedict and St. John's University (employees.csbsju.edu/ssaupe/biol115/van_helmont.htm)
- F. F. Blackman, Optima and Limiting Factors, Annals of Botany 19:281-296, April 1905 (doi.org/10.1093/oxfordjournals.aob.a089000)
- NOAA Global Monitoring Laboratory, Trends in CO2 (gml.noaa.gov/ccgg/trends/: Mauna Loa September 2026 monthly mean 426.07 ppm)
- NOAA Climate.gov, Climate change: atmospheric carbon dioxide (280 ppm or less before the Industrial Revolution of the mid-1700s, from ice cores; climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide).
Checked: 2026-10-17