Plant tissues - epidermis, palisade mesophyll and spongy mesophyll

The stucture of a leaf

Plant leaves are adapted for photosynthesis and the exchange of gases required for the process. The xylem transports water and mineral ions to the leaves from the roots and the phloem transports sugars made in photosynthesis away from the leaves to other parts of the plant.

The structure of the tissues is related to their functions in the plant.

Diagram showing the process of diffusion in a leaf

Absorbing light energy

The palisade mesophyll layer of the leaf is adapted to absorb light efficiently. The cells:

  • are packed with many chloroplasts
  • are column-shaped and arranged closely together
  • are towards the upper surface of the leaf

Gas exchange

Spongy mesophyll tissue is packed loosely for efficient gas exchange. The spongy mesophyll cells are covered by a thin layer of water. Gases dissolve in this water as they move into and out of the cells.

When the plant is photosynthesising during the day, these features allow carbon dioxide to diffuse into the spongy mesophyll cells and oxygen to diffuse out of them.

To enter the leaf, gases diffuse through small pores called stomata. As the stomata opens, water is lost by the process of transpiration. Closing the stomata helps to control water loss.