Lesson 2.12 · AQA Combined Science

Leaf tissues and stomata

A leaf is an organ

Leaves contain different tissues working together for photosynthesis and gas exchange. Roots, stems and leaves make up a plant organ system.

Illustrative leaf photograph; the veins contain transport tissues.

Look inside the leaf

Epidermal tissue covers and protects the leaf. The upper epidermis is transparent so light can reach the cells below. A waxy cuticle reduces water loss. Palisade mesophyll cells near the upper surface contain many chloroplasts to absorb light.

Spaces, vessels and pores

Spongy mesophyll has air spaces, allowing gases to diffuse through the leaf. Veins contain xylem to supply water and mineral ions and phloem to transport dissolved sugars. Stomata are pores, often most numerous on the lower surface.

Guard cells control stomata

Pairs of guard cells surround each stoma and control its opening. Open pores allow carbon dioxide to enter and oxygen and water vapour to leave. Closing stomata reduces water loss. The diagram shows two states; their timing is illustrative.

Growth at the tips

Meristem tissue is found at growing tips of roots and shoots. Its cells divide and can differentiate into other plant cell types.

Remember

Palisade: photosynthesis. Spongy: gas spaces. Xylem/phloem: transport. Guard cells: control pores.