mesophyll

Mesophyll is the inner tissue of a leaf, located between the upper and lower epidermis. In most vascular plants, mesophyll is specialized for photosynthesis and consists of chlorophyll-containing cells with spaces that facilitate gas exchange.

Typically, dicot leaves exhibit two distinct layers: the palisade mesophyll, a layer of elongated, densely packed cells just below the upper epidermis rich in chloroplasts; and the spongy mesophyll, a more loosely arranged region beneath it with many intercellular air spaces that increase the leaf’s surface area for gas diffusion.

Chloroplasts in mesophyll cells capture light energy and drive the Calvin cycle. The intercellular air spaces in the spongy mesophyll allow CO2 to diffuse to photosynthetic cells and enable rapid movement of oxygen out of the leaf. Stomata in the epidermis regulate gas exchange and water loss, linking mesophyll activity to transpiration.

In some plants, such as C4 species, the mesophyll participates in initial CO2 fixation to form four-carbon compounds, which are transported to bundle-sheath cells where CO2 is released for the Calvin cycle; this arrangement is part of Kranz anatomy and is a key feature of C4 photosynthesis. In other leaves, the palisade and spongy tissues may be arranged differently or less clearly defined.

Etymology: the term derives from Greek meso "middle" and phyllon "leaf."