Although chloroplasts are found in the cells of young stems and immature fruits, leaves are the real photosynthetic factories of the plant.
A cross section through the blade of a typical dicot leaf reveals 4 distinct tissue layers.
This is a single layer of cells containing few or no chloroplasts. The cells are quite transparent and permit most of the light that strikes them to pass through to the underlying cells. The upper surface is covered with a waxy, waterproof cuticle, which serves to reduce water loss from the leaf.
This consists of one or more layers of cylindrical cells oriented with their long axis perpendicular to the plane of the leaf. The cells are filled with chloroplasts (usually several dozen of them) and carry on most of the photosynthesis in the leaf.
Lying beneath the palisade layer, its cells are irregular in shape and loosely packed. Although they contain a few chloroplasts, their main function seems to be the temporary storage of sugars and amino acids synthesized in the palisade layer. They also aid in the exchange of gases between the leaf and the environment. During the day, these cells give off oxygen and water vapor to the air spaces that surround them. They also pick up carbon dioxide from the air spaces. The air spaces are interconnected and eventually open to the outside through pores called stomata (sing., stoma).
Collectively, the palisade and spongy layers make up the mesophyll.
Typically, most of the stomata (thousands per square centimeter) are located in the lower epidermis. Although most of the cells of the lower epidermis resemble those of the upper epidermis, each stoma is flanked by two sausage-shaped cells called guard cells. These differ from the other cells of the lower epidermis not only in their shape but also in having chloroplasts. The guard cells regulate the opening and closing of the stomata. Thus they control the exchange of gases between the leaf and the surrounding atmosphere.
| Link to discussion of gas exchange in the leaf and the control of stomata. |
Not only must the cells of the mesophyll be close to their air supply
The photo shows the network of leaf veins in a maple leaf. Probably no cell in the spongy layer is more than two cells away from a vein.
The xylem and phloem of veins is often surrounded by layers of sclerenchyma cells. These impart strength to the vein providing a stiff framework to support the soft tissues of the leaf blade.
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