Leaves are the essential organs of a plant. They are dorsoventrally flattened, green-colored parts of a vascular plant. It is a lateral appendage of the stem.
A leaf arises from the shoot apical meristem in the young plant.
In older plants, leaves develop at the node of the stem and bear a bud in its axil.
This axillary bud develops into a branch.
They contain chlorophyll and are responsible for the production of glucose using carbon dioxide and water in the presence of sunlight.
The stem is responsible for raising the leaves to a level such that they can receive sunlight and thus make glucose which is used to release energy.
Types of Leaves
When we take a look around us, we find a variety of trees with different types of leaves.
All these leaf types can be classified under various categories by different methods of classification.
Scientifically, they are broadly classified into two types as
- Simple leaves
- Compound leaves.
A simple leaf has an entire intact lamina or incised lamina such that the incisions do not touch the midrib. The bud is present in the axil of the petiole.
See leaf anatomy for an idea.
A compound is one with the incisions of the lamina going up to the midrib.
In a compound leaf, the leaf is broken up into a number of leaflets. Here bud is found in the axil of the petiole but not in the axil of leaflets of the compound leaf. Compound leaves may be of two types-
- Pinnately compound -number of leaflets are present on a common axis, which is the midrib of the leaf.
- Palmately compound – the leaflets are attached at a common point at the tip of the petiole.
Based on venation
Veins are vascular bundles present on the leaf.
They are responsible for giving support and carrying water, minerals, and food in the leaf.
Veins can be observed as lines running on the surface of the leaf.
Venation is the arrangement of veins and veinlets on the lamina or surface of the leaf.
- Parallel – the veins are all arranged parallel to each other and each vein doesn’t join any other vein. There is no branching among the veins. This type of venation is commonly seen in monocots.
- Reticulate- the veins all form a web-like pattern and are interconnected with each other. There is branching in this case. This type of venation is mostly seen in dicots.
- Pinnate – In this case, there exists one main vein extending from the tip of the leaf to its base. The other veins branch off from this main vein and are called secondary veins. Each pair of secondary veins arise from the same point but in opposite directions.
- Palmate- several primary veins diverge from one point. Each primary vein has many smaller secondary veins that arise from it.
- Rotate- usually present on peltate (shield-shaped) leaves. All the veins arise from a single point and radiate outwards in all directions.
- Longitudinal – all the veins arise from the base and run lengthwise across the leaf surface to the tip of the leaf. They might or might not connect to each other.
- Arcuate- a central main vein is presently called the muffin which runs lengthwise along with the leaf from base to tip. The secondary veins arise from this morning and all arc upwards towards the tip or apex.
- Cross-venulate/ transverse- in this case, small veins connect the larger veins (secondary veins) which do not intersect otherwise.
- Dichotomous –similar-sized veins branch symmetrically in pairs. They continue branching till they reach the laminar edge of the leaf.
Based on shape
- Linear-they have a high length to breadth ratio. They are very slender and long. Their margins are almost parallel.
- Obovate- egg-shaped leaves with the broader end of the leaf away from the petiole and the narrow end connected to the petiole.
- Ovate – shaped like an egg with the broader end of the leaf attached to the petiole.
- Peltate-shield or disc-shaped with the stalk attached to the underside of the leaf surface.
- Round-the leaf is round in shape but the stalk is attached to the edge.
- Spatulate- broadly rounded at the apex and gradually curve down toward the base.
- Cordate –heart-shaped leaves with the stem attached to the broader end.
- Obcordate-heart shaped with the stem attached to the narrower end.
- Elliptic-There is the broadest part around the middle. The leaf narrows equally at the two ends.
- Reniform-leaves are kidney-shaped with a roughly circular notch.
- Lanceolate – wider just below the middle, narrower towards the apex. The leaves are long.
- Oblanceolate -wider just above the middle, slowly widens towards the apex. The leaves are long.
- are modified to have a needle-like appearance to reduce surface area and thus reduce water loss.
- Sagittate – they are shaped like an arrowhead with one pointed end in one direction and 2 pointed ends in the other direction.
Based on margin
- Entire-The margin of the leaf is smooth around the entire leaf edge. No rough edges are present.
- Crenate-the leaf margin has teeth but the teethed ends are rounded rather than sharp.
- Undulate: the leaf has a wavy margin.
- Dentate- having a toothed edge.
- Serrate- having a sharp edge.
- Lobed- they have a lobed structure.
- Palmate- shaped like the palm of a hand with projections spreading apart like fingers.
- Digitate- the leaf has a finger-like a margin.
- Bipartite: the leaf is divided into two parts but not up-to the leaf margin.
- Tripartite: the leaf is divided into three parts but not up-to the leaf margin.
- Palmatipartite: divided almost to the leaf margin.
Based on the arrangement of leaves around the stem
The pattern of leaf arrangement on the stem or branch is known as phyllotaxy.
This is of three types as
Alternate type -a single leaf arises at each node alternately.
Opposite type-a, a pair of leaves arise at each node but in opposite directions.
Whorled type – more than two leaves arise at a node and form a whorl-like structure.