Leaf: Meaning, Types and Functions

Meaning of Leaf

A leaf is the green, flat lateral outgrowth in plants. They come in different shapes, sizes, and colors, and are generally dorso-ventrally flattened and thin. They are the main organ responsible for photosynthesis as they contain chlorophyll.

A leaf is the flattened outgrowth from the nodes of stems and their branches. A complete leaf bears a bud on its axil. A leaf is usually greenish in colour, in many dicot plants, the leaves are attached to the stem or branch by a stalk or petiole which continues in the lamina as mid-rib, branching into veins.

A dicot leaf has a net-work or reticulate venation. Many monocot leaves have no petioles. The leaves are attached to the stem by sheathing leaf-base almost encycling the entire stem. Monocot leaves have parallel venation along the sides of the midrib.

Yam is however an exception. Yam has reticulate venation like dicot leaves. In banana (a monocot) the veins branch from the mid-rib almost at right angles to it, but are parallel to each other.

Types of Leaves

Leaves can be classified based on many anatomic and morphologic features:

Type of leaves Based on Blade

1. Simple Leaf- the lamina or the leaf blade is undivided. Even if there are small divisions, they do not reach the midrib and divide the lamina.

2. Compound Leaf- The leaf blade is divided from the midrib into two or more parts. Sometimes these divided parts function as separate leaves.

Type of leaf Based on Shape of the Blade

1.     Elliptical

2.     Lanceolate

3.     Linear

4.     Ovate

5.     Cordate

Types of leave Based on the presence or absence of the petiole (stalk)

1. Petiolated- These leaves have a stalk or petiole which attaches them to the stem.

2. Sessile- These leaves do not have a petiole and are directly attached to the stem.

Type of leave Based on the serration on the edge of the leaf blade

1. Smooth: This type of leaf margin is called ‘entire’ leaf margin and is smooth all around

2. Sinuate: Have smooth curves along the margins

3. Dentate: They have teethed margins

4. Serrate: Have saw-teeth shaped margins

5. Lobed: the leaf blade is divided but the division doesn’t reach the midrib

Types of leaves Based on the arrangement of veins

1. Parallel: The veins on the leaf blade run parallel to each other maintaining the same distance throughout.

2. Palmate: The veins originate at a point and diverge from the point similar to the palm of the hand

3. Pinnate: There is a midrib which is present in the middle of the leaf blade. From this midrib arise the lateral veins.

Type of leaves Based on their arrangement on the stem

1. Alternate: Each leaf arises from a separate node on the stem at different levels

2. Opposite: Each node gives rise to two leaves, one on each side placed oppositely.

3. Whorled: In this arrangement, several leaves are present at the same level around the stem giving it a whorled appearance.

4. Rosulate: The leaves arrange themselves in a ring-like pattern around the stem.

The Internal Anatomy of a Leaf

The transverse section of a dicot leaf under the low power of a microscope shows the following tissues: The outer upper and lower epidermis, the middle mesophyll and the vascular bundles are at the core. The upper epidermis has thin layer of cuticle for protection while the lower epidermis has pores or stomata. The mesophyll is made up of two types of tissue – the palisade tissue above and the spongy tissue below. Each contains chlorophyll but there is more in the palisade than in the spongy tissue. The mid-rib contains the vascular bundles responsible for water transport.

Functions of Leaves

The normal functions of leaves are:

1. Photosynthesis

2. Transpiration

3. Respiration (gaseous exchange)

4. Transportation of water and food

Other special functions of leaves include:

1. Protection, such as the spines in palm fronds

2. Climbing, as in Gloriossa superba where the apex is modified into tendril.

3. Vegetable reproduction as in life plant (Bryophylum pinnatum).

4. Floating or supporting organ in water as in Nymphaea and water lettuce.

5. Food storage as in onion bulbs.

6. The leaves of some plants such as sun dew and Venus fly trap are modified for trapping insects for food.

Structure of a leaf

Each leaf consists of the following layers.

1. Epidermis: It is the outermost layer and secretes a waxy substance called the cuticle. The cuticle helps retain water inside the leaf cells. The epidermis houses the guard cells which regulate the movement of water into and outside the cell. Guard cells do so by controlling the size of the pores also called stomata.

2. Mesophyll: This forms the middle layer of the leaf. It is differentiated into two layers depending on the type of cells found: palisade and spongy mesophyll layers. It is in this layer that the chloroplasts are found. Chloroplasts are cell organelles that contain chlorophyll which is required for photosynthesis. The vascular tissues of the leaf are contained in the irregularly arranged spongy mesophyll cells.

3. Vascular Tissue: The vascular tissue is actually found in the veins of the leaf. The vascular tissues are composed of xylem and phloem which are responsible for the transport of water and food.