Biology XI:5 Morph of Flowering Plants(3 The Leaf)

The Leaf The leaf is a lateral, flattened photosynthetic structure originating from the shoot apical meristems. It bears a bud in its axil and axillary bud develops into branches later. Leaves are arranged in an acropetal order. Important for photosynthesis Parts of a leaf A typical leaf consists of: Leaf base It attaches the leaf with the base of stem. Bears two stipules (lateral, small leaf-like structure) In monocots, leaf base expands to form a sheath that covers the stem totally or partially. In some leguminous plants, leaf base is swollen and called pulvinus. Petiole Holds the leaf base to light Allows the leaves to flutter and thus, ringing fresh air to the leaf surface Lamina or leaf blade Green expanded part with midrib and veins Veins provide rigidity to the leaf and also help in the transport of water, minerals, and food. Venation Arrangement of veins and veinlets on the lamina of leaf Types of leaves Simple − When lamina is entire, or when incised, the incisions do not touch the midrib Compound − When incisions reach the midrib and break it into number of leaflets (bud is not present in the axil of leaflets)Two types of compound leaves may exist: Pinnately compound leaf − Leaflets are present on a common axis called rachis (equivalent to the midrib of leaf as in neem). Palmately compound leaf − Leaflets are attached at a common point (at tip of petiole as in cotton). Phyllotaxy Pattern of arrangement of leaves on stem or branch Modifications of Leaves To perform function other than photosynthesis Tendrils are used for support and climbing (eg. Peas). Spines are used for reducing water loss and defence. In insectivorous plants, leaves are modified to trap insects as in pitcher plant. Leaves are modified for storage of food in onion and garlic. In some plants such as Australian acacia, leaves are short-lived and petiole performs the function of leaves.