Leaves are specialised to capture light for photosynthesis. Mainly, it is observed that the environment determines the type of leaf plants possesses. A typical leaf has a thin, flat blade, and a stalk, or petiole which joins the leaf to the stem. There are two types of leaves; simple and compound leaves. A simple leaf has only one blade and one petiole while a compound leaf has leaflets that are attached to a petiole. Some leaves have no petiole and the leaf blades are directly attached to the stem such as lilies and corn. A network of veins runs through the leaf and the veins also contain the vascular tissue.
1. Internal structure of the leaf
Leaves are made of three types of tissue, namely protective, fundamental, and vascular tissue.
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2. Cuticle and epidermis
Both the upper and lower surface of a leaf has a clear, waxy cuticle. This layer protects the inner tissues and slows down transpiration. Under the cuticle is the epidermis, which also protects the inner tissues. The epidermal layer is only one cell thick. Most of the cells of the epidermis are clear because they have little or no pigment, thus allowing light to reach the photosynthetic tissues below. The epidermis has main stomata. There are more stomata on the lower surface than the upper surface of the leaf. The function of the stomata is to allow gaseous exchange between the inside of the leaf and the environment. Water also passes out of the leaf through the same structure. Each stoma is surrounded by a pair of specialised epidermal cells called guard cells whose role is to regulate the opening and closing of the stomata. This means that the stomata are not always open.
3. Mesophyll
Mesophyll layer comes between the upper and lower layers of epidermis. It is a layer of photosynthetic tissue. The mesophyll has two types of thin-walled cells in some plants. The palisade mesophyll is on the upper portion of the mesophyll. This layer is made of tall, closely packed cells with plenty of chloroplasts. Below the palisade layer is the spongy mesophyll made up of irregular shaped cells with plenty of air spaces between them. The spongy mesophyll cells have fewer chloroplasts compared to the palisade layer. Within the mesophyll layer is a network of veins which contain the vascular tissues. All mesophyll cells are near the veins. The xylem andphloem of the leaf veins are continuous with those of the stem and root. The arrangement of veins in a leaf is called its venation.
4. Meristematic tissues
Plants contain growth zones of unspecialized ‘cells called meristems, whose only function is to divide. Every time one of these cells divides, one of its daughter cells remains in the meristems, whereas the other differentiates into one of the three kinds of plant tissue and finally becomes part of the plant body.
In plants, primary growth is initiated at the tips by the apical meristems, regions of active cell division that occur at the tips of roots and shoots. The growth of these meristems results primarily in the extension of the plant body. As it elongates, it forms what is known as the primary plant body, which is made up of the primary tissues.
Growth in thickness, secondary growth, involves the activity of the lateral meristems, which are cylinders of meristematic tissue. The continued division of their cells results primarily in the thickening of the plant body. Meristematic tissues are made of cells that undergo mitosis and cell division frequently. There are two kinds of literal meristems: the vascular cambium, which gives rise to ultimately thick accumulations of secondary xylem and phloem; and the cork cambium, from which arise the outer layers of bark on both roots and shoot.
Most plants have three major tissue types: ground tissue in which vascular tissue is embedded and are used in the production, storage of food and in the support of the plant; dermal tissue or the epidermis which is the outer protective covering of the plant; and vascular tissue, which conducts water and dissolved minerals up the plant and also conducts the products of photosynthesis throughout. Xylem and phloem are the vascular or conducting tissues of the plant.
