Cells of the mesophyll make up the bulk of internal leaf tissue and are the major site of photosynthesis in a plant by virtue of containing large populations of chloroplast organelles. The differentiation of the mesophyll and its coordinated expansion is important to leaf function because light interception by chloroplasts and gas exchange in the internal airspaces of the leaf are crucial to optimise rates of photosynthesis. In dicotyledonous leaves there are two types of mesophyll cell; palisade mesophyll and spongy mesophyll. Palisade mesophyll cells are elongate and form a layer beneath the upper epidermis, whereas spongy mesophyll cells are internal to the lower epidermis. Mesophyll cells in monocotyledonous leaves are often highly lobed. All mesophyll cells contain large populations of chloroplasts, which enable the leaf to carry out photosynthetic carbon assimilation.

Key Concepts:

• In dicotyledonous leaves there are two types of mesophyll cells, palisade mesophyll and spongy mesophyll.
  
• Palisade mesophyll cells form a layer beneath the upper epidermis whereas spongy mesophyll cells are internal to the lower epidermis.
  
• In monocotyledonous leaves, mesophyll cells are not differentiated into two types according to position in the leaf, although those cells circling the vascular tissue form a distinct cell‐type called the bundle sheath cell.
  
• Mesophyll cells in monocotyledonous leaves are often highly lobed.
  
• Mesophyll cells originate from the L2 and L3 layers of the shoot apical meristem.
  
• A major role of leaf mesophyll cells is to contain large populations of chloroplasts, which carry out photosynthetic carbon assimilation facilitating plant growth.