Plant tissues: epidermal tissues

The idea

Epidermal tissue is the thin outer layer of cells covering leaves, stems and roots. In leaves it is usually one cell thick and transparent so light passes through to the photosynthetic cells beneath. Its main roles are protection and reducing water loss. The upper epidermis is covered by a waxy cuticle that limits evaporation. The lower epidermis contains stomata, each pore surrounded by two guard cells. Guard cells change shape to open or close the pore, controlling gas exchange and water loss. In roots, epidermal cells may grow into root hair cells, increasing surface area for water and mineral ion uptake. Epidermal cells generally lack chloroplasts, so they do not photosynthesise significantly.

What earns the marks

• Epidermal tissue is a single layer of closely packed cells covering the surfaces of leaves, stems and roots.
• The waxy cuticle on the upper epidermis reduces water loss by evaporation.
• Epidermal cells are transparent and lack chloroplasts in most plants, allowing light to reach the palisade mesophyll.
• Stomata are pores in the lower epidermis that allow carbon dioxide to diffuse in and oxygen and water vapour to diffuse out.
• Guard cells surround each stoma and control its opening and closing by changing shape.
• Root epidermal cells can form root hair cells that increase the surface area for absorption of water and mineral ions.

Common mistakes and how to put them right

• Saying the epidermis is many cells thick.

  Put it right by stating 'a single layer of cells'.

• Believing stomata are always open.

  Put it right: Guard cells close stomata in hot or dry conditions to reduce water loss, so describe them as controlled pores.

• Thinking epidermal cells contain many chloroplasts.

  Put it right: Most epidermal cells lack chloroplasts, which is why they are transparent; only guard cells contain chloroplasts.

Exam tips

• State the location and function together, for example 'the lower epidermis contains stomata that allow gas exchange'.
• When explaining water loss, mention both the waxy cuticle and the closing of stomata by guard cells.
• Use the term 'diffusion' when describing movement of carbon dioxide, oxygen and water vapour through stomata.