Leaf -Structure, Characteristics Notes CBSE Class-11

Leaf Structure and Characteristics – Class 11: The leaf is a peripheral component that is stem-borne and is usually flattened. It originates from the base of the node and produces an axillary bud. The axillary bud transforms into a branch in the near future. The leaves are organized in an acropetal pattern and come from the shoot apical meristems. They are the vegetative organs that produce the most oxygen through photosynthesis. Although a flowering plant’s morphology consists of its roots, stem, leaves, flowers, and fruits this article will specifically focus on the leaf types, structure, characteristics, arrangements, modifications, and functions.

What is Leaf?

A leaf is a plant’s lateral extension that is green and flat. They are typically dorso-ventrally flattened and thin, and they occur in a variety of sizes, colors, and shapes. They are the primary photosynthesis-related organs since they contain chlorophyll. It originates as primordia from the shoot meristem and eventually develops into a large leaf.

Types of Leaves

The two major kinds of leaves are simple and compound, which are further divided into other groups based on their size, shape, and placement on the stem.

Simple Leaf

A leaf is referred to as simple when only one lamina and the main stem are joined by a petiole. The midrib or petiole cannot be cut into while making an incision in a simple leaf, regardless of depth. Example: Guava leaves

Compound Leaf

A leaf with two or more leaflets is referred to as a compound leaf. A complex leaf’s several leaflets that branch off from the midrib are bound together by a single petiole. For example, palm leaves, peas, etc. The following leaf types are included within the category of compound leaves:

Palmately Compound Leaf

A palmately compound leaf’s leaflets are linked at the tip of the petiole like silk and cotton. These are again sub-divided into:

• Unifoliate: Because these leaves only have one leaflet, they are unifoliate. Example: Citrus.
• Bifoliate: Leaves have two leaflets each. Example: Balanites.
• Trifoliate: Three leaflets appear from the same location on the plant. Example: Oxalis.
• Quadrifoliate: This type of leaf has four leaflets that develop from a single point on the leaf as in Marsilea.
• Multifoliate: The leaves of this plant have several leaflets that grow from a single point as in Bombax.

Pinnately Compound Leaf

The midrib of a pinnately compound leaf, like Neem, is divided into plenty of leaflets that are each attached to each other by a single axis. These can be divided into groups as follows:

• Unipinnate: Similar to cassia, a unipinnate leaf has leaflets on either side of the axis.
• Bipinnate: In this situation, the principal axis generates a secondary axis that carries the leaflet, similar to the Acacia.
• Tripinnate: In this case, a tertiary axis carrying leaflets substitutes the secondary axis. Moringa is an example.
• Decompound: A leaf having more than three pinnate is said to be decompound. Old coriander leaves, for example.
• Paripinnate: A paripinnate leaf is one that does not possess a terminal leaflet. Example: Cassia.
• Imparipinnate: Its terminal leaflet is peculiar. Example: Pea.

Structure of Leaves

Simple leaf components include:

1. Lamina or Epipodium: The lamina, also known as the blade, is the flattened, enlarged portion of the leaf with a noticeable network of veins and veinlets constituting its outside framework. The midrib, a conspicuous vein in the middle of the lamina, frequently runs through it. Veins serve as a pathway for the transportation of food, water, and minerals and contain vascular tissue. They serve as lamina’s skeleton as well. The term “venation” refers to the complete network of veins, veinlets, and midribs in a leaf blade. The border and apex of a leaf are formed by the lamina’s side and tip, respectively.
2. Petiole or Mesopodium: The stalk of the lamina with which it is connected to the stem is represented by the petiole. A sessile leaf is a lamina without a petiole. An axillary bud can be found in the angle formed between a leaf and the stem, which is known as the axil. Petioles transmit food produced in the lamina or blade down into the stem as well as water and solutes from the stems into the leaf blades. For photosynthesis, they expose the lamina to sunlight.
3. Leaf Base or Hypopodium: The portion of the leaf that connects to the stem is referred to as the leaf base. When a monocot grows, the base of the leaf develops into a sheath that partially or entirely encircles the stem. The leaf base of several dicots has two lateral appendages known as stipules. Stipulate leaves are those that have stipules, whereas exstipulate leaves are those that do not. The stipules of Zizyphyus and Acacia are spiny. The base of the leaf in some legumes is inflated and is referred to as a pulvinus.

Characteristics of a Leaf

Characteristic features of a leaf include:

1. In the vicinity of a node, it is carried over the stem and its branch.
2. It is a different lateral stem appendage.
3. Its source is exogenous.
4. In its axil, there is a bud.
5. Stipules are two tiny lateral outgrowths that may be present near the base of a leaf.
6. Growth is constrained.
7. There isn’t a developing point.
8. The base, petiole, and lamina are the three distinct components of a leaf. The lamina, or leaf blade, is where photosynthesis takes place.
9. Stomata are present on the leaf blade to allow for gas exchange. Additionally, these are where water is lost through transpiration.
10. Veins serve as the skeleton that holds the leaf blade in place. The veins have a phloem and xylem for conducting tissues in addition to mechanical strength.

Leaf Arrangements

Phyllotaxy

The arrangement of leaves on a branch’s stem is commonly referred to as phyllotaxy. Phyllotaxy can be classified into three categories:

• Opposite Phyllotaxy: Two leaves develop from the stem at the same node, on opposite sides of the stem, in an opposite leaf arrangement, or opposite phyllotaxy. Guava, Calotropis, olive leaves, mint, etc. are a few examples.
• Alternative/Spiral Phyllotaxy: Each leaf develops at a distinct node on the stem in an alternative (spiral) pattern of phyllotaxy. Sunflower, peepal, mustard, etc. are a few examples.
• Whorled Phyllotaxy: When numerous leaves appear to emerge from the same node on a stem, this is known as whorled phyllotaxy. Examples include Tulsi, Sergula, Alstonia, etc.

Venation

In a leaf, venation refers to how the veins and veinlets are organized. Different plants display unique venational patterns. There are typically two categories of venation:

• Reticulate Venation: A venation reticulate is made up of an intricate arrangement of veinlets that are arranged haphazardly. A rose plant is an example of a dicotyledonous plant.
• Parallel Venation: A parallel venation is one in which the veinlets are parallel to adjacent ones. in monocotyledons, such as paddy.

Modifications of Leaves

Photosynthesis is a specific task carried out by leaves. They also provide other important functions, such as support, food storage, defense, etc. They have been altered into various shapes for each of these purposes.

Leaf Tendrils

Plants with fragile stems may create leaf tendrils. Tendril-like structures that imitate threads develop from the leaves. These tendrils assist the plant by holding it while they climb an adjacent stick or wall. For example, every single leaf in Lathyrus aphaca changes into tendrils. Pisum sativum produces tendrils that extend from its upper leaves.

Storage Leaves

The leaves of xerophytic plants including species of the Crassulaceae family are particularly thick and succulent, and store water in their tissues. Significant vacuoles filled with aqueous colloids can be observed within the cells that are parenchymatous of these leaves. This alteration renders the plant more resilient to desiccation.

Leaflet Hooks

Some plants produce hook-like attachments that attach to the terminal leaflets that assist them in climbing. Example: Bignonia unguiscati.

Leaf Roots

A few plants possess the capacity to convert one of the nodes’ leaves into adventitious roots, enabling them to float over the water. such as Salvinia.

Scale Leaves

These possess a brownish or colorless appearance and are thin, and have membranes without stalks. They safeguard an adjacent auxiliary bud in their axil. Onion scale leaves are succulent and thick, and they can store all food and water. In addition, asparagus and casuarina offer lucrative leaves.

Phyllode

There are some plants that possess a flattened petiole that mimics a leaf and turns green. Phyllode is an acronym for this. Australian Acacia, for example.

Insectivorous Leaves

Few plants absolutely require nitrogen to thrive. Such plants possess modified leaves that can capture and consume insects. Below are a few of the modifications:

• Leaf Pitcher: In a few plants, such as Nepenthes, the leaf lamina is altered to appear like a pitcher. The mite is broken down within the pitcher’s inner walls, which then discharge fluid from its stomach into the pitcher’s interior chamber.
• Leaf Bladder: In these plants, the leaf segments undergo conversion into bladders. In water, multiple plants are likely to be found. Digestive glands exist on the inner wall, which aids in digesting down the taken-in insect. Example: Utricularia.

Functions of Leaves

Guttation

Guttation is a means of eliminating additional fluid from the xylem at the leaf’s margins when its stomata are closed.

Photosynthesis

Leaf function depends primarily on photosynthesis. They employ the process of photosynthesis to transform carbon dioxide, water, and UV light into glucose.

Defense

To avoid damage or consumption by animals, some leaves have developed spines like opuntia.

Transpiration

Transpiration is a method by which plants discharge extra water into the atmosphere. The opening of the stomata that inhabit the leaves accomplishes this.

Storage

The photosynthetic process happens on leaves. They do this by retaining nutrients and water. The thick, succulent leaves are specially designed for retaining water.

FAQs on The Leaf

Q: Define the term Leaf.

Answer:

A leaf is a plant’s lateral extension that is green and flat. They are typically dorso-ventrally flattened and thin, and they occur in a variety of sizes, colors, and shapes.

Q: What is Phyllode?

Answer:

There are some plants that possess a flattened petiole that mimics a leaf and turns green. Phyllode is an acronym for this. Example: Australian Acacia.

Q: What are the types of leaves?

Answer:

The two major kinds of leaves are simple and compound, which are further divided into Palmately Compound and Pinnately Compound leaves.