Leaf: Definition, Diagram, Types, Parts and Functions

All of us have seen leaves in different plants and trees. But do we know the function of the parts of leaves? A leaf is a flattened green growth from a vascular plant’s stem that help prepare food for the plants. 

But, what makes a leaf capable or eligible for doing so? Are there any other functions of the leaf apart from photosynthesis? Yes, leaf holds numerous other functions apart from just making food. Let us know more about leaf in this article.

Definition of Leaf

Leaf is a flat, lateral, green growth developing from the nodal region of the stem. There is always a bud in its axil (the angle between the leaf and the stem) that later develops into a branch. The process of photosynthesis takes place in leaves as chlorophyll and stomata are mainly found in these.

Leaf does not grow continuously like a stem, it stops growing on attaining full size.

Parts of Leaves

A typical leaf has three main parts that are mentioned in detail below:

1. Leaf Base (Hypopodium)
2. Petiole (Mesopodium)
3. Lamina (Epipodium)

Leaf Base

The part of the leaf attached to the stem or branch is known as the leaf base. The leaf base protects the axillary bud.

1. In monocotyledons, the leaf base usually expands into a sheath that partially or wholly encloses the stem called a sheathing leaf base.
2. In some plants, the leaf base becomes broad, flat and winged and ensheaths a part of the internode, then it is known as decurrent. Decurrent are seen in plants like Crotolaria, Symphytum, etc.

Stipules

In many dicotyledonous plants, two small lateral appendages are found at the juncture of the petiole with the stem. These appendages are known as stipules. A leaf with stipules is called stipulate, and the one without stipules is called exstipulate.

Based on their position, shape, colour and size stipules are of different types:

Petiole

Part of the leaf that connects the lamina with the stem is called a petiole. The leaf that does not possess petioles is called sessile such as Calotropis, Gloriosa, etc. Leaf with petiole is called petiolate, for example, Hibiscus rosa-sinensis, mango, guava, etc.

1. Petiole helps to hold the leaf blade towards the light. Petiole raises the lamina high to the level of the stem to provide maximum required exposure to light and air.
2. The petiole is usually cylindrical with a groove on its upper surface.
3. In some plants like Citrus, the petiole becomes flattened and leaf-like, it is said to be a winged petiole.
4.  In some plants like Australian acacia, the petiole is modified into a flattened sickle-shaped structure called phyllode.
5. In some climbers such as Clematis and Smilax, the petiole is modified into tendrils and helps in climbing the plant.
6. In a few water plants like Eichhornia, the petiole swells up into a spongy bulb-like structure and helps the plant to float.

Lamina

Lamina is also known as leaf blade. It is the flat, thin, broad, green and expanded portion of the leaf. Veins and veinlets configure the external framework of the lamina. It has a prominent median vein called the midrib or midvein.

It runs in the centre through the leaf blade from its base to the apex. Midrib gives out many thin lateral veins which produce thinner veins or veinlets and form a net-like pattern. Lamina is the most important part of the leaf as it is the site for photosynthesis, transpiration, gaseous exchange and other metabolic activities. It shows great variation in its shape, colour, margin, surface, venation, texture, incision, etc.

Functions of a Leaf

Leaves are the most important part of the plant as many metabolic activities like preparation of food, exchanges of gases, and transpiration occur in these. Let’s discuss these functions in a little more detail:

1. Photosynthesis: It is the process of making food in plants with the help of carbon dioxide and water in the presence of sunlight. Leaves have a green pigment called chlorophyll that traps the energy of sunlight for this process. Food is prepared in the form of sugar and stored in the form of starch.

Chlorophylls are found in a tiny structure called a chloroplast. These are found in the guard cells located in the leaf epidermis and in stomatal pores.

2. Exchange of Gases: Leaves of a plant contain many tiny pores called stomata. These are the sites for entry and exit of gases like carbon dioxide and oxygen during respiration and photosynthesis in plants.

3. Transpiration: It is the process of removal of excess water by evaporation. Transpiration helps in the conduction of water upwards in the stem and keeps the leaf surface cool.

Some other functions of leaf:

1. Leaves of some succulent plants like aloe store an excess amount of water in their cells.
2. In plants like Salvinia, one leaf of each node is modified into roots and helps plants to float.
3. Plants like Euphorbia, the young leaves are brightly coloured to attract insects for pollination.
4. Leaves in plants like onions store food as in the leaf base.
5. Some Leaves change into phyllodes to provide protection against transpiration.
6. In plants like Bryophyllum, vegetative reproduction takes place through leaves. These leaves have marginal buds that form plantlets while they are attached to the plant.

Types of Leaves

Leaves are of two types based on incision of the lamina:

1. Simple Leaves
2. Compound Leaves

Difference Between Simple and Compound Leaves:

Below are some differences between these two types of leaves:

Types of Compound Leaf

Compound leaves are further divided into two types:

1. Pinnately Compound Leaf
2. Palmately Compound Leaf

1. In Pinnately Compound Leaf, incisions of the lamina are directed towards the mid-rib, known as rachis. Leaflets are arranged on both sides of the rachis alternately or in an opposite manner. Pinnately compound leaves are further divided into four types:

2. In Palmately Compound Leaf all leaflets are connected at the tip of the petiole, like fingers of a palm. There are five types of palmately compound leaves:

Venation

The arrangement of veins and veinlets in the lamina of the leaf is called venation. Midrib, vein and veinlets contain vascular tissues like xylem and phloem for conduction of water, mineral salts and food. There are mainly two types of venation seen in leaves:

1. Parallel Venation: When veins run parallel to each other within a lamina, it is called parallel venation. Parallel venation is a characteristic feature of monocotyledons, but exceptionally seen in some dicots such as Calophyllum and Eryngium. Plants like banana, ginger, Canna, etc., have pinnate (unicostate) parallel venation. Bamboos and fan-palm are some palmate (multicostate) parallel venation.
2. Reticulate Venation: When veins and veinlets are irregularly distributed without a definite pattern, it is called reticulate venation. Reticulate venation is a characteristic feature of dicotyledons, but it can also be seen in some monocots like Smilax and Dioscorea. Peepal, mango, etc., are some examples of pinnate (unicostate) reticulate venation and cinnamon (Cinnamomum), jujube (Zizyphus jujuba) are some examples of palmate (multicostate)reticulate venation.

Phyllotaxy

The arrangement of leaves on a stem or branch is called phyllotaxy. It helps to avoid overcrowding of leaves and allows leaves to receive the maximum amount of sunlight for photosynthesis. There are three types of phyllotaxy:

Modifications in a Leaf

Leaves of plants are modified in various ways to perform additional functions in addition to their main function like photosynthesis. Some of them are mentioned below:

1. Phyllode: Phyllode is a flat, green and short-lived petiole or rachis of a leaf that looks like a leaf and performs the function of photosynthesis, such as in Australian acacia. It also possesses fewer stomata and helps to reduce the transpiration rate.
2. Leaf Tendrils: These are leaves modified into thread-like structures, they coil around a near object as support and help plant in climbing. Such modification is seen in weak plants like Lathyrus aphaca (wild pea), Pisum sativum (sweet pea), etc.
3. Protective leaves (Spines): In some plants like Opuntia, leaves are modified into spines or thorns to protect themselves from grazing animals. This modification also helps reduce the rate of transpiration in plants growing in hot habitats like the desert.
4. Leaf Scales: These are thin needle-like leaves found at the nodal region of the stem. Each scaly leaf has an axillary bud in its axil. These leaves protect axillary buds. For example, Zingiber officinale (ginger)
5. Food Storing Leaf: In some plants like onions, the leaf becomes scaly and stores food.
6. Leaf Hooks: In plants like Bignonia, Asparagus, the terminal leaflets of leaves get modified into hook-like structures and help the plant to climb other trees or tall objects.
7. Insectivorous plants: These are plants that depend on insects for nutrition. Their leaves are modified to attract and attack insects towards the plant. Following are certain modifications seen in insectivorous plants:
   (i) In the Pitcher plant (Nepenthes) petiole is modified into a tendril to hold the pitcher upright. The leaf base is enlarged to carry out photosynthesis and the leaf apex is changed into a lid.
   (ii) In some water plants like bladderwort (Utricularia) segments of the leaf modify into bladder-like structures that trap small insects in the water. The inner wall of this structure contains digestive glands that help in digesting the trapped insect. 
   (iii) Venus Fly Trap (Dionaea muscipula) is also an insectivorous plant that has a winged, green petiole that also performs photosynthesis. The lamina of this plant is bilobed and the midrib acts like a link between the two lobes of the lamina.
   Each of these lobes has trigger hairs that are very sensitive to nitrogenous substances(insects). When an insect sits on the leaf and touches the sensitive hairs, the two lobes of the lamina fold along the midrib and trap the insect between the lobes.

Significance of Leaf

Leaf holds a very important role in the growth and development of a plant. Some significance of leaf are mentioned below in brief:

1. As we already know that it is the site of food manufacture, it is not possible for a plant to survive without leaves.
2. Chlorophylls that give green colour to the plant are found in the leaf and help in photosynthesis.
3. Stomata present in a leaf helps in transpiration to keep plants cooling during hot weather.
4. In some plants, leaves help in reproduction, food storage, act as support, provide protection, etc.

Summary

Leaf is the lateral outgrowth of the stem that arises at the nodes of the stem. They are green in colour due to the presence of chlorophyll pigments and help in photosynthesis. Leaves are sites for transpiration, gaseous exchange, guttation, etc. Lamina, petiole and leaf base are the three main parts of the leaf. Stipules are a pair of leaf-like appendages borne at the base of the petiole. Leaves are classified as simple and compound based on the incision of the lamina.

Phyllotaxy is the arrangement of leaves on a stem or branch. Venation is the arrangement of veins and the veinlets of a leaf within a lamina. Leaves of various plants show modification to perform secondary functions like protection, storage, reproduction, etc.

FAQs

Q.1. What are the 4 functions of a leaf?
Ans: The four main functions of a leaf are:
1. Photosynthesis
2. Transpiration
3. Storage of food
4. Defense 

Q.2. What is a leaf in biology?
Ans: Leaf is a flat, lateral, green growth developing from the nodal region of the stem. Metabolic activities like photosynthesis, transpiration, and gaseous exchange take place in a leaf.

Q.3. What is a leaf and its function?
Ans: Leaf can be defined as a flattened, green structure growing from the nodal region of the stem responsible for performing photosynthesis in most of the higher plants. Photosynthesis, gaseous exchange and transpiration are three important functions of the leaf. It contains chlorophyll pigments that absorb sunlight during photosynthesis. It also contains stomata that help in respiration and transpiration.

Q.4. What are stipules?
Ans: A pair of small lateral appendages that are found at the juncture of the petiole with the stem are known as stipules. A leaf with stipules is called stipulate, and the one without stipules is called exstipulate. 

Q.5. What is chlorophyll?
Ans: Chlorophyll is the green pigment present in the leaves of a plant. It helps to absorb sunlight and convert it into energy required for the process of photosynthesis.

Q.6. What are the 4 parts of a leaf?
Ans: The four main parts of a leaf are:
1. Lamina
2. Midrib
3. Margin, and
4. Stipule

Now that you have got all the information on leaves and their functions, types, and parts, if you have any queries or doubts, you can ask us in the comments. We will be more than happy to clear all your queries.