• The following parts of a leaf :
(i) Petiole : The parts of a leaf by which it is attached to the stem is called petiole.
(ii) Lamina : The broad green part of the leaf is called lamina
(iii) Veins : Lines on the leaf are called veins.
(iv) Midrib : A thick vein in the middle of the leaf called the midrib.
(v) Leaf venation : The design made by veins in a leaf is called the leaf venation. It is mainly two types :
(a) Reticulate venation : If leaf design made by veins in a leaf is net like on both sides of midrib the venation called Reticulate.e.g. Leaves of Mango, Pea etc.
(b) Parallel venation : If veins are parallel to one another in a leaf then venation called parallel venation. e.g. leaves of grass.
Functions of Leaf :
1. Manufacturing of Food : The leaf is called “the food factory” of a plant . Green plants make their own food by the process of photosynthesis.Leaves prepare their food in the presence of sunlight and green coloured substance (chlorophyll) present in them by utilizing carbondioxide and water by releasing oxygen.
2. Transpiration : The excess of water absorbed by the root hairs lost through the stomata.Process through which water comes out of leaves in the form of vapour.
Plants release a lot of water into the air through this process.
3. Gaseous exchange : Exchange of gases occurs during respiration and photosynthesis.
4. Vegetative propagation : In some plants, leaves bear buds in the marginal notches from which new plants grow and develop. eg. Bryophyllum, Begonia.
5. Leaves reduced to spines in a cactus plant. In a xerophytic plant, this modification helps to reduce water loss from the plant body.
Stem is fleshy and green in a cactus plant. Stem takes over the function of leaves as the leaves are reduced to spines.
Types of Leaf
• Simple and compound leaves :
(i) A simple leaf is one that consist of a single lamina, which is not divided into segments. Plants like mango, guava and money plant have simple leaves.
(ii) A compound leaf is one in which the lamina is divided into several small segments or leaflet each attached to the same petiole. The leaflet may be arranged on opposite sides of the petiole like in rose and neem or may arise from a common point at the petiole like in palm leaves.
Arrangement of Leaf
Leaves can be arranged along the stem in different ways.
(i) Alternate arrangement : In some plant, a single leaf grows from each point e.g. Mango, sunflower and mustard.
(ii) Opposite arrangement : In some plants two leaves grow opposite to each other from the same node. e.g. Guava and Tulsi.
(iii) Whorled arrangement : In some plans, three or more leaves grow from the same node.e.g. nerium and asparagus.
Modification of the Leaf
(i) Leaf tendrils : In plants like pea and sweet pea leaves are modified into long, slender, coiled structures called tendrils.
• These are used by the plants for climbing.
• Leaves are fully or partly modified to help the plant climb and support.
(ii) Leaf spines : In cacti and plants such as pineapple the leaves are modified into spines that are sharp, pointed structures.
• These structure protect the plant from grazing animals.
• In dry area, the leaves are reduced to spines to prevent loss of water by transpiration.
(iii) Leaves modified to eat insects : In some insectivorous plants, the leaves are modified into pitcher or bladder like structures to trap, hold and digest insect and small animals.
• These trapped insect are digested within these structures and serve as a source of nitrogen to the plant.
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Leaf is the most important part of the plant. Most leaves of plants are green in colour. There are different shapes and sizes of leaves.
Different parts of a Leaf :
Leaf has different parts:
- Midrib: It is the thick vein in the middle of the leaf.
- Types of venation: Two types of venation are found:
- Reticulate: If the venation is in a net-like appearance on both sides of midrib, it is called reticulate. For example, peepal.
- Parallel: In the leaves of grasses, the veins and veinlets are parallel to one another. Such a venation is called parallel.
- Node: Places where leaves and branches are joined to stem are called nodes.
- Internodes: The part of the stem between two nodes is called internode.
Function of Leaves :
- Transpiration: The loss of water in the form of vapours from the stomata on leaves is called transpiration.
- Photosynthesis: A process by which green plants make their food from sunlight, carbon dioxide and water is called photosynthesis.
The leaves of plants are majorly responsible for performing two essential functions for the plant’s survival and growth. These two functions are called transpiration and photosynthesis.
Transpiration: Transpiration is the process through which plants release the excess amount of water in the air. When the water travels via the stem to the leaves, some of it gets used up to prepare food, while the excess water is converted to water vapour due to the presence of the sun. Transpiration is important for the plants as it helps the plants to cool down. In the absence of transpiration, the temperature of the leaf becomes unregulated which may lead to the eventual death of the plant.
We can observe transpiration by enclosing a leafy part of a plant in a closed polythene bag and keeping it in the sun. After a while, we see tiny droplets of water on the inside of polythene bags, which are proof that the leaves have performed transpiration.
The process of Transpiration
Photosynthesis: Photosynthesis is defined as the process that helps leaves prepare food for the plant with the help of carbon dioxide and water. Photosynthesis occurs in the presence of sun and is aided by the presence of a green pigment in leaves called chlorophyll. Plants also release oxygen in the process. The food prepared is stored in various parts of the plant. In absence of photosynthesis, the plant is unable to utilise the water and minerals to prepare food for its nourishment and gradually dies.
In order to see if leaves really do perform photosynthesis, we take a leaf and immerse it in a test tube filled with spirit. Placing this test tube in a beaker filled with water, we heat the beaker. After the leaf loses its color, we wash it and pour iodine solution on it, as can be seen below, which shows the presence of starch thus disproving our doubts.
Parts of a Leaf
Petiole: This is the stalk via which the leaf is joined to the plant.
Lamina: This is the expanded part or the green portion of any leaf which is responsible for photosynthesis.
Veins: The many lines that run through the surface of the leaf are called veins and the design made by them is called leaf venation. They transport water and minerals.
Midrib: This is the central, prominent thick structure right in the middle of the leaf that helps support the leaf and prevent it from breaking.
We distinguish between two major types of leaf venation.
Reticulate venation is said to exist when the veins form a net-like shape on either side of the midrib. This type of venation is seen to exist in dicots like guava and mango.
Parallel venation is said to exist when the veins run parallel to one another. This type of venation is seen to exist in monocots like banana, wheat, coconut etc.