Mesophyll
Most of the interior of the leaf between the upper and lower layers of epidermis is a parenchyma (ground tissue) called the mesophyll. This tissue is the location of photosynthesis in the plant. The products of photosynthesis are called "assimilates".
In ferns and most flowering plants, the mesophyll is divided into two layers:
In ferns and most flowering plants, the mesophyll is divided into two layers:
- An upper barrier layer of tightly packed, vertically extended cells, one to two cells thick, directly beneath the epidermis. Its cells contain many more chloroplasts than the spongy layer. These long cylindrical cells are regularly arranged in one to five rows. Cylindrical cells, with the chloroplasts close to the walls of the cell, can take advantage of light. The separation of the cells provides maximum absorption of carbon dioxide. This separation must be minimal to afford water distribution. In order to live in their different environment (such as sun or shade), plants have to adapt this structure. Sun leaves are multi-layered, while shade leaves or older leaves closer to the soil are single-layered.
- Beneath the defence layer is the spongy layer. The cells of the spongy layer are more rounded and not so tightly packed. There are large air spaces. These cells contain fewer chloroplasts than those of the defence layer. The stomata of the epidermis open into chambers, which are connected to the air spaces between the spongy layer cells.