Inside a Leaf: Labelled Leaf Cross-Section

If you cut very thinly across a leaf and look at the slice under a microscope, you see that it is not one solid sheet. It is a stack of different tissues, each a group of similar cells doing one job. Together they make the leaf an organ that is very good at one thing: photosynthesis.

From the top surface to the bottom, the layers are always in the same order: cuticle, upper epidermis, palisade mesophyll, spongy mesophyll (with the veins running through it), lower epidermis and a second, thinner cuticle. The tiny pores called stomata sit mostly in the lower epidermis.

The cuticle is a thin layer of wax made by the epidermis. It is waterproof, so it cuts down the water lost by evaporation, and it is transparent, so light passes straight through.

The upper epidermis is usually a single layer of cells. Its cells have no chloroplasts, so they do not block the light. Their job is to protect the softer cells underneath, a bit like a clear plastic cover over a solar panel.

Mesophyll means 'middle of the leaf', and it is where most chloroplasts are found.

• Palisade mesophyll cells are tall and column-shaped. They stand side by side in one to three tightly packed rows just under the upper surface, where the light is brightest, and each one is crammed with chloroplasts. Most of a leaf's photosynthesis happens here.
• Spongy mesophyll cells are rounder and irregular, with fewer chloroplasts. They are packed loosely, leaving air spaces between them.

The air spaces matter as much as the cells. Carbon dioxide that enters through the stomata spreads (diffuses) through these spaces until it reaches the damp surface of a mesophyll cell and dissolves. Oxygen and water vapour travel the opposite way and leave the leaf.

Each vein is a bundle of two transport tissues wrapped in a layer of cells:

• Xylem is a set of hollow tubes strengthened with a tough substance called lignin. It brings water and mineral ions up from the roots. In a leaf vein, the xylem is on the upper side.
• Phloem is made of living cells. It carries dissolved sugars made in the leaf to the rest of the plant, for example to roots, growing tips and fruit. This is called translocation. In a leaf vein, the phloem is on the lower side.

Veins also act like the ribs of an umbrella, holding the thin leaf out flat so it catches as much light as possible.

A stoma (plural stomata) is a tiny pore surrounded by two curved guard cells. Unlike other epidermis cells, guard cells contain chloroplasts. They change shape to open or close the pore, which lets the plant control two things at once:

1. Gas exchange: when the stomata are open, carbon dioxide can diffuse in for photosynthesis and oxygen can diffuse out.
2. Water loss: open stomata also let water vapour escape. When the plant is short of water, the guard cells close the stomata to save it.

That is a trade-off: a thirsty plant that shuts its stomata also cuts off its own supply of carbon dioxide.

Almost every feature of a leaf helps photosynthesis. It is broad and flat, so it has a large surface area to catch light. It is thin, so light reaches the inner cells and gases only have a short distance to travel. The light-catching cells are packed near the top, the air spaces sit next to the stomata, and the veins reach every part of the leaf. When you learn the layers, ask of each one: how does this help the leaf catch light, get carbon dioxide and water in, or send sugar out?