Leaf is a green expanded exogenous lateral outgrowth which arises from the node of a stem or its branches. Green leaves of the plant are collectively cold foliage leaves.
Leaves are of two types.
I: Simple Leaf – A life with undivided lamina is known as simple leaf. Depending upon the type of leaf and degree of incision, the leaves are called pinnatifid, palmatifid, pinnatipartite, palmatipartite, pinnatisect, palmatisect.
II: Compound Leaf – It is of different types. They are –
1. Pinnate compound leaf –
2. Palmate compound leaf – The compound leaf gives the appearance of a palm.types –
Venation – Venation is the arrangement of veins and veinletsin a lamina. They can be divided into reticulate venation and parallel or striate venation.
Reticulate Venation – Veins and veinlets of a lamina. They are of different types –
1. Pinnate or Unicostate reticulate venation- Peepal, Banyan, Mangoetc.
2. Palmate or Multifoliate reticulate venation- Zizyphus ,Luffa, Castor etc.
Parallel or Striate Venation – Veins run parallel without forming reticulation. Veinlets are not conspicuous. They are of different types-
1. Pinnate – Banana.
2. Palmate – Fan Palm.
Phyllotaxy – arrangement of leaf around stem or its branches are called phyllotaxy. It can be of different types. They are –
Spiral – A node bear only one leaf and successive leaves occur on opposite sides to form two alternate rows. Example - Grass.
Opposite - Two leaves occur on each node.
(i) Opposite and superposed – Opposite and successive nodes lie one above another forming only two rows (Syzygium, Quisqualis ).
(ii) Opposite and Decussate – Opposite leaf of successive nodes lie at right angles, forming four rows. Example - Guava, Calotropis.
Whorled or verticillate – more than two leaves occur at each branch. Example- Nerium.
Modifications of Leaves: Modifications of leaves can be of different types-
1. Leaf Tendrils - Leaf or leaf part are modified into tendril like structure for climbing. They are unbranched and naked. They are of different types-
2. Leaf Spines – Leaf is converted into prickles. Example- Opuntia , Asparagus, Barberry.
3. Leaflets Hooks - Terminal leaflets are modified into curved hooks for climbing. Example- Bignonia.
4. Leaf Roots – One leaf of each node is transformed into roots for balancing on water. Example- water fern.
5. Phyllodes – Phyllodes are flattened petioles and reaches which has taken over the function of lamina. Example - Australian Acacia.
6. Leaf Bladders – Some of the leaf segments of aquatic insectivore. Example-Cyclops, Utricularia.
7. Leaf Pitchers – Lamina or whole leaf is modified into pitcher to catch insects. Example- Nepenthes.
8. Hygroscopic Appendages – Leaves are modified into hygroscopic appendages. Example - Tamarix.