The term leaf refers to the organ that forms the main lateral appendage on the stem of vascular plants. In general, leaves are thin, flat organs responsible for the photosynthesis of the plant. Although photosynthesis typically only occurs on the upper surface of the leaf, it can occur on both sides in some plant species.
Leaves are typically comprised of a distinct upper and lower surface, stomata for gas exchange, waxy coating, hairs, and venation. Each side of the leaf differs in regarding the level to which these features are expressed. Although leaves are typically located above ground. Some species have leaves that reside underground (e.g., bulb scales) or underwater (e.g., aquatic plant species). Moreover, the leaves of some plants may not be associated with photosynthesis (e.g., cataphylls). Leaves are typically oriented on a plant to avoid blocking the sunlight of the leaves situated underneath.
In this post, Pritish Kumar Halder takes a brief look at Leaf – Definition, structure and main functions.
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.
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.
Function of the Leaf
As one of the most important constituents of plants, leaves have several essential functions:
The primary function of the leaf is the conversion of carbon dioxide, water, and UV light into sugar (e.g., glucose) via photosynthesis (shown below). The simple sugars formed via photosynthesis are later processed into various macromolecules (e.g., cellulose) required for the formation of the plant cell wall and other structures. Therefore, the leaf must be highly specialized to combine the carbon dioxide, water, and UV light for this process. Carbon dioxide is diffused from the atmosphere through specialized pores, termed stomata, in the outer layer of the leaf.
Water is directed to the leaves via the plant’s vascular conducting system, termed the xylem. Leaves are orientated to ensure maximal exposure to sunlight, and are typically thin and flat in shape to allow sunlight to penetrate the leaf to reach the chloroplasts, which are specialized organelles that perform photosynthesis. Once sugar is formed from photosynthesis, the leaves function to transport it down the plant via specialized structures called the phloem, which run in parallel to the xylem. The sugar is typically transported to the roots and shoots of the plant, to support growth.
Transpiration refers to the movement of water through the plant, and subsequent evaporation via the leaves. When the stomata open to accommodate the diffusion of carbon dioxide into the plant for photosynthesis, water flows out. This process also serves to cool the plant via evaporation of the water from the leaf, as well as regulate the plant’s osmotic pressure.
Guttation refers to the excretion of xylem from the edges of leaves and other vascular plants due to increased levels of water in the soil at night, when the stomata are closed. The pressure caused at the roots results in the leakage of water from the xylem out of specialized water glands at the edges of leaves.
Leaves are a primary site of water and energy storage since they provide the site of photosynthesis. Succulents are particularly adept at water storage, as evidenced by the thick leaves. Due to the high levels of nutrients and water, many animal species ingest the leaves of plants as a source of food.
Some leaves have also evolved defense mechanisms to avoid being eaten or damaged. Some examples include the spines of cacti, cones of gymnosperms, respectively. In addition, hairs found on leaves prevent water loss in dry climates and sting animals that detour herbivores (e.g., Urticaceae). Moreover, the waxy coatings found on leaves serve to protect against water loss, rain, and forms of contamination. Oils and other secreted substances also detract from being consumed by herbivores.