The mesophyll cells in a plant leaf play a vital role in photosynthesis by enabling the gas exchange portion of the process, and through the actions of their specialized chlorophyll-containing organelles, called chloroplasts, in which the primary photosynthetic reactions take place. Because they are surrounded by intercellular air pockets, the mesophyll cells are able to perform a gas exchange when the stomatal pores in the leaf's epidermis open. The Greek term "mesophyll" means "middle leaf," and it reflects the mesophyll cells' location between the leaf's thinner and much stiffer upper and lower epidermal layers.
The mesophyll cells make up what is known as a plant's assimilation tissue, which refers to the primary location of photosynthetic reactions. In the majority of flowering plants and in ferns, the mesophyll tissue is comprised of two layers: the palisade layer and the spongy layer. The palisade layer lies directly beneath the upper, or adaxial, epidermal layer and contains vertically elongated mesophyll cells. The cells in this layer contain a greater number of chloroplasts. These mesophyll cells also have air spaces between them that make it possible for the cells to absorb carbon dioxide.
The spongy layer of mesophyll cells lies below the palisade layer and contains larger air spaces that enable carbon dioxide and oxygen to diffuse through the cell walls during photosynthesis and respiration. To enable the exchange of gases in and out of the plant, the stomatal pores in the epidermis lead into substomatal chambers, which connect to the intercellular air pockets between the mesophyll cells in the spongy layer.