Class 9 Science Chapter 11: Reproduction: How Life Continues — Notes, Examples & Practice

Reproduction is the production of new individuals of the same kind. It is not needed for an individual's survival, but it is essential for the continuity of the species.

At its heart, reproduction involves copying DNA — the information that builds a body. Copying is very accurate but not perfect, so offspring show small variations. In sexual reproduction, DNA from two parents is combined, producing much more variation.

Variation is valuable: if the environment changes (a hotter climate, a new disease), some variants may survive better. Over many generations this helps populations adapt.

Asexual reproduction involves a single parent and no gametes; offspring are almost identical to the parent (clones). It is fast and needs no partner.

Methods: binary fission — the cell divides into two (Amoeba, bacteria); multiple fission — many daughter cells at once (malaria parasite Plasmodium); budding — a bud grows and detaches (yeast, Hydra); fragmentation — the body breaks into pieces, each growing into a new organism (Spirogyra); regeneration — rebuilding from a part (Planaria); spore formation — light, tough spores released from sporangia (bread mould Rhizopus, ferns).

Vegetative propagation uses roots, stems or leaves of plants: potato tubers (eyes are buds), ginger rhizomes, onion bulbs, Bryophyllum leaves (buds on the leaf margin), and artificial methods like cuttings (rose, sugarcane), layering (jasmine) and grafting (mango). Farmers use it because the new plants are identical to a good parent, and fruit comes sooner.

The flower is the reproductive part of a flowering plant. Its parts, from outside in: sepals (protect the bud), petals (attract pollinators), stamens — the male part, each with an anther (makes pollen grains, which contain the male gametes) on a filament — and the pistil (carpel), the female part, with a sticky stigma, a style and an ovary that contains ovules (each with a female gamete, the egg).

Flowers with both stamens and pistil are bisexual (hibiscus, mustard); those with only one are unisexual (papaya, watermelon).

Pollination is the transfer of pollen from an anther to a stigma. Self-pollination happens within the same flower or plant; cross-pollination between flowers of different plants of the same kind, carried by insects, birds, wind or water. Cross-pollination gives more variation. Declining numbers of pollinators like bees threaten many crops.

After landing on a suitable stigma, a pollen grain grows a pollen tube down through the style to an ovule. The male gamete travels down the tube and fuses with the egg inside the ovule — this is fertilisation, which forms a zygote.

The zygote divides to form an embryo. The ovule develops into the seed (embryo + stored food + protective seed coat), and the ovary develops into the fruit, which protects the seeds and helps disperse them. Petals, sepals, stamens and stigma usually wither away.

Seeds are dispersed by wind (winged seeds of drumstick, light cotton fibres), water (coconut), animals (sticky or hooked seeds, edible fruits) and explosion (balsam pods). Under suitable conditions — water, air (oxygen) and suitable temperature — the seed germinates and the embryo grows into a seedling.

Humans reproduce sexually. At puberty (about 10–14 years in girls, 12–16 in boys) hormones bring changes: growth spurts, body hair, voice changes in boys, breast development and the start of menstruation in girls. These changes are normal and happen at different ages in different people.

The male reproductive system includes the testes, which produce sperm and the hormone testosterone; sperm travel through the sperm ducts and leave through the urethra in a fluid called semen. The female reproductive system includes two ovaries, which release an egg (ovum) about once a month, the oviducts (fallopian tubes), the uterus (womb) and the vagina.

Fertilisation is internal: a sperm fuses with the egg in the oviduct to form a zygote, which divides into an embryo and implants in the thick, blood-rich uterine lining. The embryo receives food and oxygen through the placenta and develops for about nine months. If fertilisation does not happen, the lining breaks down and leaves the body as the menstrual flow. Good reproductive health means hygiene, a nutritious diet, accurate information, freedom from stigma, and protection from infections. Some animals, like frogs and fish, have external fertilisation in water. Medical help such as IVF (in-vitro fertilisation) can help couples who cannot conceive naturally.