Leaf structure

A leaf is an organ made of different tissues that work together. Their arrangement brings light, water and gases to cells that carry out photosynthesis, and connects those cells to the plant’s transport tissues.

The epidermis covers the leaf. Beneath the upper epidermis is the palisade mesophyll, whose cells contain many chloroplasts. Light entering through the upper surface can reach these cells for photosynthesis.

Below the palisade mesophyll is the spongy mesophyll. Its cells are separated by air spaces, which connect with openings in the epidermis called stomata. Carbon dioxide enters through the stomata and moves through the air spaces to photosynthesising cells; oxygen produced by photosynthesis can leave by the same route. Each stoma is surrounded by guard cells, which control the size of its opening.

Veins run through the leaf and contain two transport tissues. Xylem brings water to the leaf for photosynthesis. Phloem carries sugars made by photosynthesis away from the leaf to other parts of the plant.

Plant epidermal tissue

Epidermal tissue covers and protects plant surfaces. In a leaf, it is usually a single layer of closely packed cells. The upper epidermis is transparent, allowing light to pass through to the mesophyll cells beneath it.

A waxy cuticle covers the leaf surface and reduces water loss from the leaf.

Stomata and guard cells

The epidermis contains pores called stomata. Each stoma is surrounded by a pair of guard cells. When guard cells take up water, they change shape and the pore opens. When they lose water, their shape changes and the pore closes.

Opening a stoma allows gases to move between the leaf and the air, but water vapour can also escape. Closing the pore reduces water loss, while also restricting gas exchange. By opening and closing stomata, guard cells regulate both processes.

Palisade mesophyll

Palisade mesophyll is a leaf tissue specialised for photosynthesis. It lies near the upper surface of the leaf, where it receives light passing through the epidermis, the outer layer of the leaf.

Palisade mesophyll cells are elongated and closely packed. Each cell contains many chloroplasts, which absorb light for photosynthesis. Their position near the upper surface helps the cells receive light, while their many chloroplasts help them absorb it.

Spongy mesophyll

Spongy mesophyll is a layer of loosely arranged cells inside a leaf. Large air spaces between the cells connect with the stomata, the pores through which gases enter and leave the leaf.

These air spaces provide routes for gases to diffuse between the stomata and photosynthesising cells. Carbon dioxide can diffuse from the stomata through the spaces to reach the cells. Oxygen and water vapour can diffuse through the spaces towards the stomata and leave the leaf.

Spongy mesophyll cells contain chloroplasts and carry out photosynthesis. They generally have fewer chloroplasts than palisade cells.

Xylem

Xylem transports water and dissolved mineral ions from the roots upwards through the plant. It consists of vessels that form continuous tubes, providing a path through which water can flow.

Xylem vessels are made from dead cells joined end to end. The cells form hollow tubes, so water can pass through them without living cell contents blocking its path.

The vessel walls are strengthened with lignin. This helps prevent the tubes from collapsing and also provides support to the plant.

Phloem

Phloem is plant tissue that transports dissolved sugars and other organic substances around the plant. This movement is called translocation.

Phloem structure

Phloem contains living sieve tube cells joined end to end. Their end walls form sieve plates, which have pores that allow dissolved substances to move from one cell to the next. Together, the cells form sieve tubes through which substances can travel.

Companion cells support the sieve tubes and supply energy for transport.

Direction of translocation

Substances move from regions where they are produced or released to regions where they are used or stored. For example, sugars produced in leaves can be transported to growing regions or storage organs. Stored substances can also be released and transported to regions where they are needed.

Because the locations that supply and receive substances can vary, transport in phloem can occur in either direction through the plant.

Plant meristem tissue

Plants grow at the tips of their roots and shoots because these regions contain meristem tissue. Meristem tissue contains unspecialised cells that can continue to divide by mitosis, producing new cells.

The new cells allow the root or shoot tip to grow. They can also differentiate: they develop the structures and functions of specialised plant cells. These specialised cells form the tissues needed as the plant develops.