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.

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.

Stomata and guard cells

A stoma is a small pore in the leaf epidermis, the outer layer of the leaf. Each stoma is surrounded by a pair of guard cells, which control how wide the pore is. The plural of stoma is stomata.

Stomata allow gases to diffuse between the air and the leaf. Carbon dioxide can enter for photosynthesis, while oxygen and water vapour can leave.

How guard cells control a stoma

When guard cells take in water, they become firm, or turgid. Their shape causes them to curve apart, opening the pore. When they lose water, they become less turgid and the pore closes.

Opening a stoma allows gas exchange, but it also gives water vapour more opportunity to escape from the leaf. Closing the pore reduces this opportunity, while also restricting gas exchange.

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.

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.

Plant epidermal tissue

Epidermal tissue forms a covering over plant surfaces, helping to protect the tissues beneath it.

In a leaf, epidermal cells generally form a closely packed layer. This provides a protective barrier while remaining transparent, so light can pass through to the photosynthesising cells beneath.

The outer surface of the leaf epidermis is covered by a waxy cuticle. This reduces water loss from the leaf by evaporation.

Xylem

Xylem transports water and dissolved mineral ions from the roots, through the plant, to the leaves. It also helps support the plant.

Xylem vessels are formed from dead cells joined end to end. Their end walls break down, leaving continuous hollow tubes through which water and dissolved mineral ions can flow.

The vessel walls contain lignin, a substance that strengthens them. Lignin helps prevent the vessels from collapsing and gives support to the plant.

Phloem

Phloem is plant tissue that transports dissolved sugars between different parts of a plant. This movement is called translocation.

Sugars move from a source, where they are produced or released from storage, to a region where they are used or stored. For example, sugars produced in a leaf can be transported to a growing region or to a storage region. A storage region can also become a source when it releases stored sugar. Translocation can occur in either direction through the plant, depending on where the source and receiving region are located.

Phloem structure

Phloem contains living cells joined end to end to form sieve tubes. The end walls between these cells are perforated and are called sieve plates. Their openings allow dissolved substances to move from one sieve-tube cell to the next.

Companion cells support the sieve-tube cells and provide energy for loading substances into the phloem and unloading them at their destination.