Reproduction in Plants Chapter Notes | Biology Class 8 ICSE PDF Download

Introduction

Reproduction in plants is how plants make new plants of their own kind to keep their species alive. It is an important process for plants to grow and spread. Plants can reproduce in two main ways: asexually, where one parent plant produces a new plant, or sexually, where male and female parts work together to form seeds. Asexual reproduction happens through methods like budding or vegetative propagation, while sexual reproduction involves flowers, pollination, and seeds. In this chapter, we will learn about different types of reproduction, how flowers help in sexual reproduction, and how plants like potatoes and ginger grow new plants using their parts.

Reproduction

• All living organisms make new individuals of their own kind to keep their species alive.
• This process is called reproduction.
• In plants, reproduction means making new plants of their own kind.
• There are two main ways plants reproduce: asexual and vegetative, and sexual.
• In asexual and vegetative reproduction, lower plants like bacteria, algae, and fungi reproduce by methods like binary fission, fragmentation, or forming spores.
• Some plant parts like the stem, leaf, or root also help in producing new plants, which is called vegetative reproduction (also known as vegetative propagation).
• Most flowers you see have male and female reproductive parts that produce seeds, which is called sexual reproduction.
• Seeds germinate to form new plants through flowers, and this process is called sexual reproduction.

Asexual Reproduction in Plants

Asexual reproduction is very common among plants and can happen in different ways, which are described below.

1. Binary Fission

• Lower plants like bacteria reproduce asexually by binary fission.
• Binary fission means "two" and "splitting."
• In this method, the nucleus of the parent cell splits into two parts.
• Then the cell splits across the middle, forming two new identical cells called daughter cells.

2. Budding

• Budding is common in yeast.
• In this method, the parent cell produces a small outgrowth called a bud.
• The bud grows and then gets detached from the parent body to become an independent life.

3. Fragmentation

• Organisms like spirogyra, which are made of long, ribbon-like filaments, reproduce by fragmentation.
• The filament grows and breaks off into two or more parts called fragments.
• Each fragment then grows into an individual organism.

4. Spore Formation

• Spore formation is common in plants like mosses and ferns.
• These plants are ornamental and bear spores on the underside of their leaves.
• Spores are light and can be carried away by wind or insects to different places.
• Under suitable conditions for growth, spores give rise to new plants.

5. Vegetative Reproduction

• In this method, new plants are produced by the vegetative parts of the plant called propagules.
• Vegetative parts mean the leaf, stem, and root.
• Examples are potato, mint, ginger, sugarcane, rose, etc.
• The common grass and mint grow their stems horizontally parallel to the ground.

Reproduction by Stem

• In some organisms like chlorella and chlamydomonas, one cell divides into multiple daughter cells.
• New roots and shoots develop at their nodes.
• These roots grow downward into the soil, and the shoot grows upward to form new plants.
• Ginger is a modified stem with nodes and internodes, and it also has scaly leaves.
• You can also see some axillary buds growing out from the nodes.
• Similarly, potato is another example of a modified stem.
• Potato bears buds called "eyes."

Eyes

• A complete potato tuber showing buds in "eyes."
• A sprouted potato.
• Each new individual plant, together with some tuber parts, can be separated from the rest and grown separately.

Reproduction by Leaf

• Leaves of some plants, such as bryophyllum, produce buds in the notches in their margin.
• When such leaves fall in moist soil, their buds in the margin begin to grow as young tiny plants.

Reproduction by Root

• Plants like sweet potato and asparagus develop numerous large swollen fleshy roots.
• These roots contain a large quantity of food inside them.
• Each such single root is capable of giving rise to a new plant.
• Carrot, a taproot, is a biennial plant.
• Its roots grow vegetatively in the first year and store food.
• In the following year, its stem produces flowers and seeds, which die by the end of the year.
• Buds produced at the base of the old stem just above the taproot are meant for vegetative propagation.

Advantages of Vegetative or Asexual Reproduction

• Farmers prefer to grow certain crops by vegetative methods.
• The advantages in doing so are as follows:
• Reproduction by vegetative parts takes place in a shorter time.
• New plants, thus produced, spread very fast in a small area.
• It is a surer method.
• All the good characters of the mother plant are retained by the daughter plants.

Disadvantages of Vegetative Reproduction

• All plants developed by vegetative reproduction are identical, so they are likely to be affected simultaneously if a disease spreads in the farm.
• Dispersal of plants does not take place on its own.
• Daughter plants, so developed, tend to remain nearby and are restricted to a particular area, leading to competition for resources.

Artificial Vegetative Propagation

These days, farmers and horticulturists have developed certain artificial methods of vegetative propagation, such as cutting, layering, grafting, and tissue culture.

1. Cutting

• In this method, the stem is cut into small pieces with each bearing an axillary bud.
• The cut ends are planted in moist soil.
• After a few days, the new cells develop, and a new plant grows.

2. Layering

• In this method, a portion of one of the lower branches of the plant with an axillary bud at a node is bent down to the ground so that it touches the soil.
• A ring of bark is removed from this portion, which is then covered with soil.
• Some heavy object, such as a small piece of stone or a brick, is kept on the branch so that it does not come out of the soil.
• In a few days, when the branch gives out roots, it is cut off from the main plant.
• It then continues to grow as an independent plant.
• This method is used for the propagation of plants like mint, rose, jasmine, etc.

3. Grafting

• In some cases, such as rose, mango, guava, etc., a small shoot or bud of a desired variety of plant is intimately fixed on the stem of another plant of the same or related species.
• The plant receiving the bud or the shoot is called the stock, and the shoot fixed on it is called the scion.
• For a successful graft, it is important that the cambium layers of the stock and the scion must come into very close contact so that growth may continue.
• The grafted points are then bound together with a tape, and the joint is covered with wax to prevent dehydration and is protected from any bacterial infection.
• In a few days, the new cells develop, and a new plant grows.

4. Micro-propagation

• It is the propagation of plants by cell and tissue culture.
• If vegetative propagation is not possible in a crop, then buds, shoot apex, or any other part of the plant can be used as explant for micro-propagation.
• The explants are treated with sterilization chemicals to prevent microbial growth and then cultured in a particular nutrient medium.
• Cells grow and divide to form a cell mass called callus.
• Some growth regulators (plant hormones) are added.
• The callus differentiates into plant parts looking like a tiny plant (plantlet).
• After 4-6 weeks, the plantlets are transferred to the soil.
• This can be used to derive and multiply disease-free stock.
• This is a kind of tissue culture.

Benefits of Tissue Culture

• It provides rapid propagation of identical individuals.
• This technique is very productive for superior varieties.
• It can be applied to interspecific hybrids.
• It is very useful in cases where seeds are dormant.
• In these, the embryo can be cultured and micropropagated.

Limitations of Tissue Culture

• It cannot be applied to all cases.
• It is not easily applicable in remote agricultural areas.

Sexual Reproduction in Plants

• Flowers are the parts of plants that help in sexual reproduction.
• To understand how flowers work, we need to look at their different parts.

A Typical Flower with Its Internal Parts

Stalk

• The flower is attached to the plant by a part called the stalk or pedicel.
• The tip of the stalk is a bit bigger and flat, and this is called the thalamus.
• All other parts of the flower grow from the thalamus.

Sepals

• These are the green, leaf-like parts that form the outermost layer of the flower.
• Sepals are together called the calyx.
• They protect the inner parts of the flower when it is still a bud.

Petals

• These are the bright and colorful parts of the flower, also called the corolla.
• Petals are usually white or colored, but not green like sepals.
• They are the second layer of the flower, just inside the sepals.
• Petals attract insects because of their bright colors.

Stamens (The male parts)

• These are the male parts of the flower, found inside the petals.
• Each stamen has a long, thin part called the filament and a small sac-like part at the top called the anther.
• Anthers hold a powdery material called pollen grains.
• Stamens are the male reproductive parts of the flower.

Carpels (The female parts)

• These are the female parts of the flower, also called pistils.
• Carpels are found in the center of the flower.
• Each carpel has three parts: a swollen ovary at the bottom, a thin thread-like style in the middle, and a flat, sticky stigma at the top.
• Flowers that have both male (stamens) and female (carpels) parts are called bisexual flowers.
• Flowers that have only male parts (stamens) or only female parts (carpels) are called unisexual flowers.

Ovules (Future seeds)

• Inside the ovary, there are small, rounded structures called ovules.
• Ovules will become seeds after a process called fertilization.

Function of a flower

• A flower has both male parts (stamens) and female parts (carpels).
• The pollen grains from the stamens need to reach the stigma of the carpel for reproduction.
• This transfer of pollen grains from the anther to the stigma is called pollination.

Pollination

Pollination is the process where pollen grains from the anthers are moved to the stigma of a flower. Pollen grains are a fine powdery material that you can see if you crush the anthers of an old flower between your fingers.

Types of Pollination: Self-Pollination and Cross-Pollination

There are two types of pollination: self-pollination and cross-pollination.

1. Self-Pollination

• Self-pollination happens when pollen grains from the anthers fall on the stigma of the same flower.
• This can happen in a bisexual flower (which has both male and female parts).
• It can also happen when pollen grains from the anthers of one flower fall on the stigma of another flower on the same plant.

2. Cross-Pollination

• Cross-pollination happens when pollen grains from the anthers of a flower on one plant are transferred to the stigma of a flower on a different plant of the same species.

This type of pollination happens between two different plants.

1. Pollination by Insects

• Insects like butterflies, bees, and others help in pollination.
• They visit flowers to drink nectar (a sweet liquid) as their food.
• When an insect sits on a flower, pollen grains stick to its body, like on its wings, legs, or mouth parts.
• When the insect visits another flower, these pollen grains may fall on the stigma of that flower.
• This transfer of pollen grains by insects is called insect-pollination.
• Flowers like rose, marigold, dahlia, and salvia are pollinated by insects.

Specialties of Insect-Pollinated Flowers

• Insect-pollinated flowers have some special features:
• They have large and colorful petals to attract insects.
• They produce a sweet smell so insects can find them easily.
• They make nectar, which is food for insects.
• They make sticky pollen grains so the pollen can stick to the insect’s body.

Fun Fact: Insect-Pollinated Flowers

• When a honey bee visits a flower, its whole body gets covered with pollen grains.
• The bee then flies to other flowers of the same kind, and the pollen grains may fall on their stigma.
• This is how insect-pollinated flowers get pollinated.
• Flowers also have bright colors to attract insects, which helps in pollination.
• Some animals like birds, squirrels, and bats also help in cross-pollination in some flowers.

2. Pollination by Wind

• Some plants like maize, palm, pine, and others are pollinated by wind.
• These plants make a lot of dry pollen grains.
• When the flowers are ready, the pollen grains are blown away by the wind.
• The pollen grains may fall on the stigma of a flower of the same type on another plant.
• This transfer of pollen grains by wind is called wind-pollination.

Special Features of Wind-Pollinated Flowers

• Wind-pollinated flowers have some special features:
• They are usually small and have dull colors.
• They have long anthers that stick out of the flower so the pollen can be blown off easily.
• They make light pollen grains that can be easily carried away by the wind.
• They produce a lot of pollen grains.

3. Pollination by Water

• Some plants like Vallisneria are pollinated by water.
• These are aquatic plants (plants that live in water).
• Their male flowers come off when they are mature and float on the water surface.
• Female flowers of these plants also float on the water surface when they are ready.
• When the floating male flowers come in contact with the female flowers, pollination happens.

Fun Fact: Water-Pollinated Flowers

• Plants like lotus and trapa (Singhara) are aquatic plants, but their flowers are above the water.
• These flowers are pollinated by insects, not water.

Characteristics of Water-Pollinated Flowers

• Water-pollinated flowers are small and light so they can float on water easily.
• They make pollen grains in large numbers.
• Their male and female flowers are on separate plants.

Artificial Pollination

• Artificial pollination means transferring pollen grains to the stigma by humans, not naturally.
• In ancient times, this was common in “male flowers” and “female flowers” of palm trees.
• Nowadays, plant breeders use artificial pollination to create new varieties of plants.
• They remove the anthers from flowers and cover them with plastic bags.
• Then, they add pollen grains from the plants of the desired variety to the stigma.

Fertilisation (Fusion of Male and Female Sex Cells)

• After pollination, when pollen grains reach the stigma of a flower of the same type, they start to grow a tube.
• This tube is called the pollen tube.
• The pollen tube grows through the style and reaches the ovule inside the ovary.
• The male cells in the pollen grain move through the pollen tube and fuse with the ovule.
• This fusion of the male and female cells is called fertilisation.

Fruits and Seeds

• After fertilisation, the ovule turns into a seed.
• The ovary grows and becomes a fruit, which covers the seeds.
• The petals, sepals, and other parts of the flower fall off.

So-Called "Dry Fruits"

• Fruits like cashew, walnut, pistachio, and raisins are actually dried fruits, not dry fruits.
• Fruits like apple and strawberry are also dried fruits, even though they look juicy.

Fruits Containing Seeds

• Fruits like apple, strawberry, and pear are different because the main fleshy part of the fruit is not the ovary.
• The ovary in these fruits becomes the small central part that contains the seeds.
• These are called “false fruits” because the main edible part does not come from the ovary.

Dispersal of Seeds

• Seeds need to come out of the fruits so they can grow into new plants.
• In fleshy fruits, seeds come out when the fruit wall rots or when animals or humans eat the fruit and throw away the seeds.
• In dry fruits, the fruit wall bursts open, and the seeds come out.
• The process of seeds coming out and spreading to new places is called dispersal.

Dispersal of Seeds Through Different Means

Seeds can be spread by different ways:

• Man and Animals: Humans and animals eat fruits and throw away the seeds, or the seeds stick to their bodies and fall in new places.
• Wind: Some seeds are light and have wings or hair, so they can be carried away by the wind.
• Water: Some seeds float on water and are carried to new places by rivers or rain.

Seeds Through Human Food

• Fruits like canal (a type of tomato) are part of our food.
• We eat the ripe seeds, but we don’t eat the unripe seeds because they are not tasty.
• When we eat these fruits, the seeds pass through our body and come out in waste.
• These seeds can grow into new plants if they find the right conditions.