A leaf is actually called ‘the kitchen of the plant’. This is because they are the main organ responsible for photosynthesis, through which the plant produces its energy a.k.a. it’s food. They obtain their green color due to the presence of chlorophyll. Let us learn more about them.
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.
Parts of a Leaf
Leaves have two main parts: The leaf blade and the Stalk or the petiole.
- The leaf blade: It is also called the lamina. It’s generally broad and flat. It is in this layer that photosynthesis occurs. It contains a prominent midrib at the center of the leaf blade which is the main vein. From this midrib arise branches called veins. They are of different types depending upon the type of edges, the pattern of the veins and the number of blades per leaf.
- The petiole: It is the stalk-like structure which connects the leaf blade to the stem. The petiole has tiny tubes, that connect the veins on the leaf blade to the stem. Few of these enable water transport to the leaf while the other carry food away from the leaf to other parts of the plant.
Some plants also contain another part called stipules. These are small flap-like structures that grow at the base of the petioles. They are protective in some plants when they protect the growing petiole while in others, they fall off once the petiole starts growing.
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Types of Leaves
Leaves can be classified based on many anatomic and morphologic features:
Based on Blade
- 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.
- Compound Leaf- The leaf blade is divided from the midrib into two or more parts. Sometimes these divided parts function as separate leaves.
Based on Shape of the Blade
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Based on the presence or absence of the petiole (stalk)
- Petiolated- These leaves have a stalk or petiole which attaches them to the stem.
- Sessile- These leaves do not have a petiole and are directly attached to the stem.
Based on the serration on the edge of the leaf blade
- Smooth: This type of leaf margin is called ‘entire’ leaf margin and is smooth all around
- Sinuate: Have smooth curves along the margins
- Dentate: They have teethed margins
- Serrate: Have saw-teeth shaped margins
- Lobed: the leaf blade is divided but the division doesn’t reach the midrib
Based on the arrangement of veins
- Parallel: The veins on the leaf blade run parallel to each other maintaining the same distance throughout.
- Palmate: The veins originate at a point and diverge from the point similar to the palm of the hand
- Pinnate: There is a midrib which is present in the middle of the leaf blade. From this midrib arise the lateral veins.
Based on their arrangement on the stem
- Alternate: Each leaf arises from a separate node on the stem at different levels
- Opposite: Each node gives rise to two leaves, one on each side placed oppositely.
- Whorled: In this arrangement, several leaves are present at the same level around the stem giving it a whorled appearance.
- Rosulate: The leaves arrange themselves in a ring-like pattern around the stem.
Structure of a leaf
Each leaf consists of the following layers.
- 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.
- 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.
- 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.
Learn more to understand the concept of Tissue here in detail.
Functions of a Leaf
- Photosynthesis: This is the most important function of a leaf. They contain chloroplasts which have the pigment chlorophyll that is responsible for helping in photosynthesis. The prepared food is transported to the other parts of the plant via phloem tissue.
- Helps the plant breathe: The epidermis of the leaf contains guard cells that control and regulate the small pores on the undersurface of the leaves. These pores are called stomata. Stomata are responsible for regulating water in and out of the cell. It is also responsible for the exchange of gases across the epidermis.
- Storage of food: In some plants, the leaves are modified to store food. These plants generally have succulent leaves as seen in xerophytic plants.
Solved Example for You
Q: Which structure of the leaves helps in gaseous exchange?
- Spongy mesophyll
Sol: The correct option is (a) Stomata
Stomata are small pores that are found in the lower epidermal layer of the leaf blade. They are regulated by the guard cells. The stomata help in regulating water intake and output across the cells and help in exchange or gases across them too.