Dicotyledonous leaf structure for photosynthesis

A leaf’s broad, flat shape exposes a large surface area to light. Its thin shape gives carbon dioxide a short distance to diffuse from the air to the cells that carry out photosynthesis.

In a cross-section of a dicotyledonous leaf, the tissues are arranged in layers. This arrangement allows light, carbon dioxide and water to reach photosynthesising cells, while sugars can be transported away.

Light absorption in the upper leaf

The cuticle is a thin, transparent outer layer that limits water loss. Beneath it, the upper epidermis forms a layer of cells that allows light to pass through to the palisade mesophyll.

The palisade mesophyll lies just below the upper epidermis. Its cells are elongated and closely packed, and they contain many chloroplasts. Chlorophyll in the chloroplasts absorbs light for photosynthesis. The position of the palisade layer near the upper surface helps its cells receive light.

Gas exchange through the leaf

Below the palisade layer, the spongy mesophyll consists of more loosely arranged, irregularly shaped cells. The air spaces between them allow carbon dioxide to diffuse between the stomata and photosynthesising cells.

Stomata are pores in the epidermis, commonly found in the lower epidermis. Each pore is bordered by a pair of guard cells, which control its opening. Opening a stoma allows gas exchange; closing it helps limit the loss of water vapour from the leaf.

Transport in vascular bundles

Vascular bundles form the leaf veins. They contain two transport tissues:

• Xylem supplies water to the leaf for photosynthesis.
• Phloem transports sugars made by photosynthesis away from the leaf.

Identifying structures in a leaf cross-section

The cuticle appears as a thin outer layer, with the upper epidermis directly beneath it. The palisade layer is recognisable by its closely packed, elongated cells; the spongy layer below has larger spaces between irregular cells. The lower epidermis forms the lower surface, where a stoma can be seen as a pore between two guard cells.

Vascular bundles appear within the leaf as grouped transport tissues. In a typical cross-section, the xylem is towards the upper surface and the phloem is beneath it. Xylem vessels can often be recognised by their thick walls.