Reproduction Part-2

Sexual Reproduction

  • The method of reproduction in which two individuals of different sexes, i.e., male and female, participate.
  • During sexual reproduction, male organism having male sex organs produces male gametes i.e. sperms which are small and motile and the female organism having female sex organs produces ova which are generally large and store food.
  • Male and female gametes fuse to form a zygote that grows into a new organism.

Significance of Sexual Reproduction:

  • Sexual reproduction involves the DNA and cellular structure of two separate organisms, which promotes character diversity in the progeny.
  • Since gametes are derived from two different organisms, it results in a new combination of genes which increases the chances of genetic variations.
  • Sexual reproduction results in the origin of new species.
  • Sexual reproduction requires the division of the sex organs, which divides the DNA matter in half so that the zygote created after fusion has the same amount of DNA as the parents. This process preserves the species' DNA.

Sexual Reproduction in Plants

  • Plants reproduce by both asexual and sexual methods.
  • Vegetative propagation is a type of asexual reproduction in plants. 
  • Sexual reproduction in plants occurs through flowers.
  • Important flower parts like the androecium and gynoecium aid in plants' sexual reproduction.


Non-Essential Parts of Flowers

  • Sepals and Petals are called non-essential parts of the plant as they do not directly take part in reproduction.
  • Sepals protect the inner delicate whorl during bud condition and also perform photosynthesis if they are green in colour.
  • When petals are coloured, pollinating insects are attracted to them.

Essential parts of Flowers

  • Androecium and gynoecium are called essential/reproductive whorls of a flower.
  • Gynoecium produces ovules, which are female gametes, while androecium produces pollen grains containing male gametes.
  • An anther and filament make up each individual androecium component, which is referred to as a stamen.
  • Anther produces haploid pollen grains.
  • Each individual member of the gynoecium is called a pistil and consists of a stigma, style and ovary.
  • The flowers may be bisexual i.e. having both stamens and carpels for example; Mustard China Rose (Hibiscus).
  • The flower may be unisexual i.e. paving either stamens or carpels for example; Papaya, Watermelon.

Pollination

  • The process of transfer of pollen grains from an anther to the stigma of the flower is pollination.
  • It is required for fertilization.
  • Pollination has two types, self-pollination (autogamy) and cross-pollination (allogamy).
  • Self-pollination-Transfer of pollen grains takes place from anthers to the stigma of the same flower or another flower of the same plant.
  • Cross Pollination - pollens are transferred from anthers to the stigma of another flower.
  • Many pollinating agents play their roles in cross-pollination. Examples: water, wind, insects, birds, bats, etc.

Fertilization: 

  • It is the process of fusion of male and female gamete to form a zygote during sexual reproduction. Pollination is followed by fertilisation in plants.
  • In flowering plants, after pollination, the pollens germinate on the stigma surface of the pistil and generate two male nuclei.
  • Ovule has an egg cell and two polar nuclei.
  • Pollen tube releases two male germ cells inside the ovule, one of them fuses with female germ cell and forms a zygote that gives rise to the embryo and future plant. The fusion is known as syngamy. 
  • Another male nucleus fuses with two polar nuclei and forms a triploid endosperm.

Post-fertilisation changes

  • Zygote divides many times and forms an embryo inside the ovule.
  • The ovule develops a tough coat and changes into the seed.
  • The ovary grows rapidly and ripens to form a fruit.
  • Petals, sepals, stamens, style and stigma shrivel and fall off.

Embryo

  • Embryo has two parts-Plumule and radicle.
  • Plumule develops into shoot and radicle develops into root.
  • The process of development of a seedling from the embryo under appropriate conditions is known as germination.