The majority of photosynthesis takes place in the mesophyll between the upper and lower epidermis layers. Usually, the two layers of mesophyll can be distinguished from each other. The upper region is made of cells that look like short posts in two rows. These cells are parenchyma cells and make up the palisade mesophyll tissue. It is this tissue that contains more than 80% of the chloroplasts in the leaf. The lower layer of mesophyll, the spongy mesophyll tissue, is composed of loosely arranged parenchyma cells with abundant air space. The lower layer also contains many chloroplasts and its loose structure allows for movement of air in from the stomata. For future reference, parenchyma tissue containing numerous chloroplasts is called chlorenchyma tissue. It is also found in the outer parts of the cortex in the stems of herbaceous plants. However, in the leaf, the surfaces of the mesophyll that come into contact with the air are moist. The stomata will close if the internal moisture drops below a certain level in order to reduce drying inside the leaf.
The skeleton of a leaf is the veins (or vascular bundles). They are of various sizes and as described in the leaf arrangement section, are scattered throughout the leaf and are organized distinctly in different types of leaves. The veins are surrounded by a jacket of fibers called the bundle sheath. The sugars produced in the mesophyll are transported via the veins throughout the plant—specifically in solution in the phloem. In dicots, the veins run in all directions. In monocots, the veins are parallel and are not scattered. In addition, monocots do not have mesophyll differentiated into two layers. Instead, some will have large thin-walled buliform cells surrounding the main vein. The thin-walled cells are sensitive to water conditions and will collapse in dry conditions which causes the leaf blade to fold or roll which reduces transpiration (water loss).
