Leaf: Structure and Types

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The leaf is a flattened lateral outgrowth of the stem and bears a bud in its axil. It is green in colour due to the presence of chlorophyll and constitutes the main photosynthetic organ of the plants. They develop acropetally and exogenously. It originates from leaf primordium formed by the shoot meristem and bears a bud in its axil called axillary bud. It is the seat of very important physiological processes like photosynthesis, transpiration and respiration. Besides protecting axillary buds, leaf can get modified into structures for storing food and water, climbing, vegetative propagation etc.

Structure of Leaf

A typical angiospermic leaf consists of three parts; leaf base, petiole and lamina.

Leaf base: attach the leaf with the stem, expands into a sheath (in monocot) and in dicots it bears two lateral outgrowths called the stipules. In many plants the leaf base is swollen and is called pulvinus.

Petiole: is the stalk of the leaf. When the petiole is absent is called sessile (in monocot), when present, it is said to be petiolate or stalked (in dicot). Petiole may get modified and swell (e.g. water hyacinth) or develop wings (e.g. orange) or become flat like a leaf (e.g. Australian Acacia).

Lamina or leaf blade: is the expanded, green, thin generally prominent part of the leaf carried at the tip of the petiole. There is usually a middle prominent vein, which is known as the midrib. Veins provide rigidity to the lamina and act as channels of transport of the food materials in and out of the leaf.

Leaf shows a lot of variation in shapes of lamina, leaf apices and leaf margins.

Image showing Leaf and its parts.

Image Showing Leaf and Its Parts.

Image showing Leaf and its parts.

Image showing variations leaf shape, leaf apices and leaf margins.

Image Showing Variations Leaf Shape, Leaf Apices and Leaf Margins.

Image showing variations leaf shape, leaf apices and leaf margins.

Venation in Leaves

The arrangement of veins in the lamina is termed as venation, it is two types;

Reticulate: veins are irregularly distributed forming a network, e.g. dicotyledons

Parallel: veins are parallel and do not form a network, e.g. monocotyledons.

Reticulate and Parallel venation may be unicostate (Pinnate) with a single main midrib gives off lateral veins that proceed towards the margin or apex of the leaf and connected by smaller veins that pass in all directions forming a network or multicostate (palmate) with two or more costae or principal veins arising from a single point.

Image showing types of leaf venation; reticulate and parallel venation.

Image Showing Types of Leaf Venation.

Image showing types of leaf venation; reticulate and parallel venation.

Types of Leaves

There are two types of leaves; simple and compound.

Simple leaf: has single unit blade and lamina is not incised but incisions do not touch the midrib.

Compound leaf: incisions of the lamina touch the midrib and divide the leaf into segments.

Types of Compound Leaves

Table Showing Types of Compound Leaves.
Table showing types of compound leaves; Pinnate and palmate.

Pinnate

Palmate

Leaflets are attached to mid rib or rachis and are arranged laterally.

Leaflets radiate from the end of petiole like fingers of a palm.

Leaflets and mid rib may get further divided to form compound leaves that are unipinnate, bipinnate, tripinnate and decompound.

Depending upon the number of leaflets compound leaves are bifoliate, trifoliate quadrifoliate and multifoliate.

Leaflets occur in two rows.

All leaflets are clustered together.

A joint is not present between the leaflet and its axis.

A joint is usually found between the leaflet and its point of attachment.

Differences between Simple and Compound Leaf

Table Showing Differences between Simple and Compound Leaf.
Table showing Differences between Simple and Compound leaf.

Simple leaf

Compound leaf

The lamina is not divided into separate lobes or leaflets.

The lamina is divided into two or more leaflets which may either arise at the tip of petiole or on the sides of a rachis.

The simple leaves may be borne in one or more planes.

Leaflets of a compound leaf are always borne in one plane.

Simple leaves are produced on the stem in an acropetal succession

Leaflets of a compound leaf develop almost simultaneously.

Image showing different types of pinnately and palmately compound leaves.

Image Showing Different Types of Compound Leaves.

Image showing different types of pinnately and palmately compound leaves.