Leaf is one of the important structures in the plant responsible for photosynthesis. Botanically, the leaf is an integral part of the stem system emerging out of small bud called an apical bud. Some of the plant parts resemble the structure of the leaf, for example, sharp spines of cacti, needles of pi and conifers. Leaf is divided into external and the internal leaf. The external structure is made up of a blade-like arrangement known as a leaf blade or lamina.The petiole help in attaching the stem with the leaf. Veins found in the leaf gives them a pattern called venation pattern assisting to carry the water and nutrients. The pattern of arrangement of leaves on a stem is called by phyllotaxy.
Types of leaves
Leaves vary in their structure, functions, and forms. They are classified based on the Pattern of the laminar arrangement, Shape of the Blade, Presence or absence of the petiole, serration on the edge of the leaf blade, arrangement of veins and on the arrangement of leaves on the stem.
1. Based on the pattern of laminar arrangement, the leaves are categorized as simple, compound and Palmately compound leaves. In a simple leaf, the blade is left undivided hence the leaves are formed by the lobes and the gaps between the lobes that do not reach the main veins. On the other hand, a compound leaf is a leaf consisting of several distinct parts (leaflets) joined to a single stem. The palmate leaf has leaflets radiating outwards the stem from the end of the petiole.
2. Leaves are also classified based on the Shape of the Blade as elliptical, lanceolate, Linear, Ovate and Cordate leaves,
3. Depending upon whether there is a presence or absence of the petiole (stalk), leaves are classified as Petiolated leaves and sessile leaves. Petiolated leaves have their stalk or petiole and Sessile leaves do not have a petiole hence they are directly attached to the stem.
4. Depending upon the serration on the edge of the leaf blade, leaves are classified as Smooth, Sinuate, Serrate and lobed leaves. The smoothleaf margin is smooth all around, sinuate leaf has a smooth curve along the margins. A dentate leaf has a teethed margin. A serrate leaf has a saw-teeth shaped margin and the lobed leaf has a well-divided lobes that doesn’t reach the midrib
5. Leaves are also classified based on the arrangement of veins as parallel leaves, palmate leaves and the pinnate leaves. In the parallel leaves, the veins on the leaf blade run parallel to each other while keeping the same distance throughout the blade. The palmate leaf is the one where the veins originate at a point and get deviated from a particular point simply resembling our palm of the hand. A pinnate leaf has a midrib present in the middle of the leaf blade that protrudes the lateral veins.
6. Based on the arrangement of leaves on the stem, they can be classified as an alternate leaf, opposite leaf, Whorled leaf, and the rosulate leaf. In an alternate leaf, each leaf originates out of separate nodes on the stem at different levels, the opposite leaf has a node that gives rise to two leaves- one on each side placed in an opposing manner. The whorled leaf hasseveral leaves found at the same level attached to the stem giving it a whorled appearance. In a Rosulate leaf, the leaves arranged themselves in a ring-like pattern surrounded by the stem.
Cross-section of the leaf- Image source Wikimedia
Cross-section of the leaf
On the cross-section of a leaf, we can see 4 main layers; the upper epidermis, middle mesophyll, vascular bundle and the last one called waxy cuticle. Let us discuss each one in detail.
1. The internal structure of the leaf is protected by the outermost (upper part) epidermis that continues with the stem epidermis. The upper part has stoma- an opening for gas exchange. The Guard cells located on either side of stoma aid in regulating the rate of transpiration by systematically opening and closing the stomata.
2. The middle layer is the mesophyll. Mesophyll cells are formed by soft-walled parenchymal cells. About 20% of the mesophyll contain chlorophylls. Within the chlorophyll, there are many chloroplasts helping to absorb the sunlight necessary for photosynthesis. The oxygen released out of the leaf after the photosynthesis is passed into the atmosphere through the tiny pores known as the stomata. Chlorophylls are so named because they contain green pigments. However, chlorophylls also made up of yellow colour (carotene), pale yellow (xanthophyll), red ( if the sap is slightly acidic), bluish ( if the sap is slightly alkaline). Chlorophyll is present in the upper part of the mesophyll called palisade parenchyma cells. On the lower bed of the mesophyll, there are many spongy mesophyll cells characterized by wide spaces between them. The spaces allow carbon dioxide to move around during photosynthesis.
3. The next layer is the vascular Tissue layer made up of many veins and the vascular bed has xylem and phloem responsible for the transportation of water and food respectively.
4. The last layer is the waxy layer called the cuticle. This is a shiny outer layer help in reducing water loss from the leaf surface. Some of the plants have small hairs (trichomes).
Major functions of leaves
Leaves assist in Photosynthesis mediated by chloroplasts. After the food is being prepared, it is transferred to (food pipes)called phloem tissue. Leaves helps the plant to breathe through the stoma where the plant absorbs carbon dioxide and releases oxygen. In a few plants, the modified leaves help in the storage of food. These plants generally have succulent leaves as seen in xerophytic plants.