The Leaf

The organ of plant that generates the major lateral appendage on the stem of vascular plants is referred to as a leaf. The leaf is a lateral, flattened structure that grows from the stem. It grows at the node and produces a bud in the axil. A branch arises from the axillary bud. Shoot apical meristems give rise to leaves, which are organized in an acropetal order. They are the most essential photosynthetic vegetative organs. It's also known as the plant's kitchen or food factory. This is due to the fact that they are the primary organ responsible for photosynthesis, which is how the plant obtains its energy or food. The presence of chlorophyll gives them their green color.

Figure 7: Parts of a leaf

The leaf base, petiole, and lamina are the three main sections of a normal leaf. The leaf base connects the leaf to the stem, while stipules are two lateral tiny leaf-like structures. In monocotyledons, the leaf base develops into a sheath that partially or completely covers the stem. The pulvinus occurs when the leaf base of some leguminous plants becomes inflated. The petiole aids in keeping the blade lit. Leaf-blades flap in the breeze, cooling the leaf and delivering fresh air to the surface, thanks to long thin flexible petioles. The leaf blade, or lamina, is the extended green section of the leaf with veins and veinlets. There is frequently a conspicuous middle vein, referred to as the midrib.Veins give the leaf blade structure and serve as routes for water, minerals, and food components. The shape, edge, apex, surface, and extent of lamina incision vary amongst leaves.

Venation: Venation is the arrangement of veins and veinlets in the lamina of a leaf. The venation is called reticulate when the veinlets create a network. Examples: Hibiscus, papaya, leaves of Tulsi, Coriander, China Rose, and Mangifera. A type of reticulate venation pattern in which the secondary veins run parallel to each other from the midrib toward the margin is called pinnate venation. Examples include Alder, Chestnut, and Oak. Palmate is a venation pattern in which several main veins radiate outward from the base of the leaf. Examples are, Maple, Tulip etc. The venation is called parallel when the veins inside a lamina run parallel to one other. Example: banana, grass, and wheat. Most dicotyledonous plants have reticulate venation on their leaves, but most monocotyledons have parallel venation.A vascular arrangement in leaves such that the veins are forked, with each vein dividing at intervals into smaller veins of approximately equal size is called Dichotomous venation, for example, Ginkgo biloba.

 Figure 8: Types of venation in leaves

Types of leaves:

 Figure 9: Classification of leaves.

 Different types of leaves are described below:

a) Simple leaf:  When a leaf's lamina is complete or when the incisions do not touch the midrib, it is considered to be simple.

b) Compound Leaf: The leaf is considered compound when the incisions of the lamina go up to the midrib, breaking it into a number of leaflets. There are two types of compound leaves. As in neem, a pinnatelycompound leaf has a number of leaflets on a common axis, the rachis, which represents the leaf's midrib. The leaflets of palmately compound leaves are joined at a common point, the tip of the petiole, as seen in the silk cotton plant.

In both simple and compound leaves, a bud can be found in the axil of the petiole, but not in the axil of the leaflets.

Phyllotaxy

The pattern of leaf arrangement on the stem or branch is known as Phyllotaxy. There are three varieties of this: alternate, opposite, and whorled. In alternate phyllotaxy, a single leaf emerges alternately at each node, as seen in China rose, mustard, and sunflower plants. As in Calotropis and guava plants, a pair of leaves emerges at each node and lies opposite each other. This is the opposite phyllotaxy. It is called whorled when more than two leaves emerge from a node and form a whorl, as in Alstonia.

 Figure 10: Different types of Phyllotaxy.

Modifications of a leaf:

Additional to photosynthesis, leaves are frequently adapted to fulfill other purposes. They are transformed into tendrils in peas for climbing and spines in cacti for defence. Onion and garlic leaves have fleshy leaves that store nourishment. The leaves of some plants, such as Australian acacia, are tiny and short-lived. These plants' petioles swell, turn green, and synthesize nourishment. Certain insectivorous plants, such as pitcher plants and venus fly traps, have modified leaves as well.

       Figure11: Leaf modifications.