Sexual Reproduction in Plants - Class 7 Notes & Olympiad Questions

Sexual Reproduction in Plants

a) Sexual reproduction in plants is the process by which plants create new individuals with unique genetic traits. It involves the fusion of male and female reproductive cells or gametes to produce offspring.
b) In flowering plants, the flower is the reproductive part.

Reproductive Parts of a Flower

Plants have specialised structures that carry out the different roles in sexual reproduction:

Male Reproductive Structures

Stamens: These are the male reproductive organs of a flower. Each stamen consists of a slender stalk called a filament, which supports a pollen-producing structure called an anther. The anther produces and releases pollen grains or the male gametes.

Female Reproductive Structures

Pistil (or Carpels): The pistil is the female reproductive organ of a flower. It consists of three main parts:

a) Stigma: The stigma is the sticky tip of the pistil that captures pollen.
b) Style: The style is a slender tube-like structure that connects the stigma to the ovary.
c) Ovary: The ovary is the enlarged base of the pistil that contains one or more ovules or the female gametes.

Unisexual and Bisexual Flowers

Flowers can be divided into two categories: unisexual and bisexual.

Unisexual Flowers

a) Unisexual flowers are flowers that have either male or female reproductive parts, but not both.
b) It means that some flowers only have the male parts needed for reproduction, while others have only the female parts.
c) Watermelon, cucumber, and pine trees have separate male and female flowers.

Bisexual Flowers

a) Bisexual flowers, also known as perfect flowers, are flowers that have both male and female reproductive parts in the same flower.
b) It means that one flower contains both the male and female structures needed for reproduction.
c) Roses, sunflowers, and lilies have bisexual flowers.

Process of Sexual Reproduction in Plants

The process of sexual reproduction in plants involves the following steps:

l. Pollination: Bringing Pollen to the Stigma
ll. Fertilisation: Joining Male and Female Cells
lll. Seed Development: Creating New Plants

Pollination

Pollination is a vital step in the process of plant reproduction. It is how plants transfer pollen from one flower to another, enabling them to produce seeds and new plants.

1. Types of Pollination

Pollination can happen in two main ways:

Self-Pollination

a) Some flowers have both male and female parts within the same flower (bisexual flowers).
b) In self-pollination, pollen from the male part (anther) of a flower lands on the female part (stigma) of the same flower or a different flower on the same plant. This ensures that the plant can reproduce without the help of other plants.

Cross-Pollination

a) Other flowers have either male or female parts and need the help of different plants to reproduce.
b) In cross-pollination, pollen from the male part of one flower is transferred to the female part of a flower on a different plant of the same kind.
c) This transfer can occur through different agents like insects, wind, water, animals, or birds.

Pollination is crucial because it allows the transfer of male reproductive cells (pollen) to reach the female reproductive parts of a flower. When the pollen lands on the stigma, it can travel down a tube to reach the ovary, where the female reproductive cells (ovules) are located.

2. Agents of Pollination

Insects

a) Many flowers rely on insects like bees, butterflies, and beetles to help with pollination.
b) These insects are attracted to the bright colours and sweet nectar of flowers. As they move from flower to flower, pollen sticks to their bodies and gets transferred between flowers.
c) For example, bees collect pollen while gathering nectar from flowers, and in the process, they inadvertently transfer pollen from one flower to another.

Birds

a) Some flowers, especially those with vibrant colours and long, tubular shapes, are pollinated by birds, such as hummingbirds.
b) These birds visit the flowers to drink nectar and, while doing so, unintentionally pick up and deposit pollen on their bodies. As they move from flower to flower, the pollen is transferred between them.
c) A famous example is the hummingbird-pollinated flower called the trumpet creeper.

Wind

a) In certain plants like grasses and trees, pollination occurs with the help of the wind.
b) These plants produce lightweight pollen that is easily carried by the wind. The wind blows the pollen grains from the male flowers to the female flowers, allowing pollination to occur.
c) Examples of wind-pollinated plants include grasses, such as wheat and corn, as well as trees like pine and oak.

Water

a) Some aquatic plants, like water lilies, utilize water as an agent of pollination. Their pollen grains float on the water's surface.
b) When they come into contact with the female parts of other flowers floating on the water, pollination occurs.
c) Water hyacinth is an example of an aquatic plant that relies on water for pollination.

These different agents of pollination—such as insects, birds, wind, and water—help transfer pollen between flowers, enabling the process of fertilization and the production of seeds. Each agent plays a specific role in pollination, and the diverse methods of pollination have contributed to the remarkable variety of plants we see around us.

Fertilisation

a) Fertilisation is a crucial step in the process of plant reproduction. It involves the union of male and female reproductive cells, leading to the formation of a zygote and the development of new plants.
b) During fertilization, two main events take place:

1. Pollen Grain Arrival: Pollen grains, which contain the male reproductive cells, are transferred from the male part of a flower (anther) to the female part (stigma) of the same flower or another flower. This transfer can occur through agents like wind, insects, or other means of pollination.
2. Journey to the Ovule: Once the pollen grains reach the stigma, they start to grow a long tube called a pollen tube. This tube extends from the stigma, through the style, and reaches the ovary, which contains the female reproductive cells called ovules.

c) Inside the ovule, there is an egg cell, which is the female reproductive cell. The pollen tube acts as a pathway for the male reproductive cells to travel from the stigma to the egg cell.
d) When the pollen tube reaches the ovule, one of the male reproductive cells fuses with the egg cell in a process called fertilisation. This fusion creates a zygote, which is the first cell of the new plant. The zygote will eventually develop into an embryo.

Seed Development

a) After successful fertilisation, the ovule inside the flower undergoes transformation and develops into a seed. The fertilized ovule contains the zygote, which is the early stage of the embryo. The zygote starts to divide and grow, eventually forming the mature embryo. The embryo contains the potential to develop into a new plant.
b) Around the embryo, a protective covering called the seed coat forms. The seed coat acts as a shield, providing protection to the developing embryo from external factors like physical damage, dehydration, and pathogens. It also helps in maintaining the dormant state of the seed until conditions are favorable for germination and growth.
c) Simultaneously, the ovary of the flower, which surrounds and protects the ovule, undergoes changes and transforms into a fruit. The fruit is the mature ovary that has ripened and becomes fleshy or hard, depending on the plant species.
d) The fruit plays a vital role in seed dispersal. It helps in the distribution of seeds away from the parent plant, ensuring the survival and colonization of new areas.

Dispersal of Seeds

Seed dispersal is the process by which plants spread their seeds to different places. Since plants cannot move on their own, seed dispersal helps them grow in new areas where they can thrive.

There are several ways seeds can be dispersed:

Wind Dispersal

a) Some seeds are lightweight and have structures like hair or wings that allow them to be carried by the wind. These seeds are easily lifted and carried to new locations.
b) Examples include dandelions, whose fluffy seeds are carried by the wind, and maple trees, whose seeds have wing-like structures that help them float and travel through the air.

Water Dispersal

a) Seeds of certain plants near or in water bodies have adaptations that enable them to be dispersed through water. These seeds can float on water and be carried to different places.
b) The coconut is a well-known example of a seed dispersed by water, as its buoyant husk allows it to travel long distances through water.

Animal Dispersal

a) Some plants have fruits with structures that attach to the bodies of animals. Animals inadvertently carry these fruits and their seeds as they move.
b) For example, burrs and stickers can stick to animal fur or feathers and be carried to new locations.
c) Fruits like berries or fleshy fruits are often consumed by animals, and the seeds are dispersed through the animal's droppings.

Dispersal by Explosion

a) Certain plants have seed pods that burst open when they ripen, propelling the seeds in different directions. This explosive mechanism helps the seeds scatter away from the parent plant.
b) Examples include plants like peas, lady's finger (okra), and balsam.

These different methods of seed dispersal allow plants to colonize new areas, find suitable conditions for growth, and avoid competition with other plants of the same species. Seed dispersal helps plants spread their offspring far and wide, increasing their chances of survival.

Germination

a) Germination is the process by which a seed begins to grow into a new plant. It is an essential stage in the life cycle of plants and marks the beginning of their journey towards becoming mature plants.
b) Some key steps involved in germination:

Activation: Germination begins when a seed encounters favorable conditions, such as moisture, warmth, and sometimes light. These conditions trigger the seed to break its dormancy and start growing.

Water Absorption The seed soaks up water and swells up, which helps it start growing.

Root Growth A tiny root called the radicle comes out from the seed and grows down into the soil. It helps the plant get water and nutrients from the ground.

Shoot Development As the root grows, a little stem with leaves, called the shoot, pops up above the soil. The shoot reaches for the light so the plant can make food through a process called photosynthesis.

Leaf Growth The first leaves, called cotyledons, unfurl. They provide food for the young plant until it can make its own through photosynthesis.

Growth and Photosynthesis With the help of sunlight, carbon dioxide, and water, the plant can now make its own food through photosynthesis. It uses this food to keep growing and getting bigger.