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