Sexual Reproduction :
- Occurs with the help of two germ cells.
- Female gamete and male gamete are those two germ cells.
- Two main processes occur in the sexual reproduction -
- Gamete formation
- Fertilization
1. Gamete formation:
- Gametes are formed by the meiosis.
- In meiosis, chromosome number is reduced to half
- Hence haploid gametes are formed.
2. Fertilization:
- A diploid zygote is formed in this process by union of haploid male and female gametes.
- The zygote divides by mitosis and embryo is formed.
- The embryo develops to form new individual.
- Fusion of Gametes occurs.
- New individual always has the recombined genes of both the parents.
- Hence, the new individual shows similarities with the parents for some characters and has some characters different than both parents.
- Diversity in living organisms occurs due to genetic variation.
- Genetic variation helps the organisms to adjust with the changing environment and thereby to maintain their existence.
- Due to this, plants and animals can save themselves from being extinct.
A. Sexual reproduction in plants :
- Flower is structural unit of sexual reproduction in plants.
- It consists of four floral whorls as calyx, corolla, androecium and gynaecium; arranged in sequence from outside to inside.
- Androecium and gynoecium are called ‘essential whorls’ because they perform the function of reproduction.
- Calyx and corolla are called as ‘accessory whorls’ because they are responsible for protection of inner whorls.
- Members of calyx are called as ‘sepals’ and they are green coloured.
- Members of corolla are called as ‘petals’ and they are variously colored.
- A flower is called as ‘bisexual’ if both whorls i.e. androecium and gynoecium are present in the same flower. Ex. Hibiscus.
- A flower is called as ‘unisexual’ if any one of the abovementioned two whorls is present in the flower.
- If only androecium is present, it is ‘male flower’ and if only gynoecium is present, flower is ‘female flower’. Ex. Papaya.
- Many flowers have the stalk for support, called as ‘pedicel’ and such flowers are called as ‘pedicellate’.
- Flower without stalk is called as ‘sessile’.
- Androecium is male whorl and its members are called as stamens.
- Gynaecium is female whorl and its members are called as carpels.
- These may be separate or united.
- Ovary is present at the basal end of each carpel.
- A hollow ‘style’ comes up from the ovary.
- Stigma is present at the tip of style.
- Ovary contains one or many ovules.
- Embryo sac is formed in each ovule by meiosis.
- Each embryo sac consists of a haploid egg cell and two haploid polar nuclei.
- Pollen grains from anther are transferred to the stigma. This is called as pollination.
- Pollination occurs with the help of abiotic agents (wind, water) and biotic agents (insects and other animals).
- Stigma becomes sticky during pollination.
- Pollens germinate when they fall upon such sticky stigma i.e. a long pollen tube and two male gametes are formed.
- The pollen tube carries male gametes.
- Pollen tube reaches the embryo sac via style.
- Tip of the pollen tube bursts and two male gametes are released in embryo sac.
- One male gamete unites with the egg cell to form zygote. This is fertilization.
- Second Male gamete unites with two polar nuclei and endosperm is formed.
- As two male nuclei participate in this process, it is called as double fertilization.
- When pollination involves only one flower or two flowers borne on same plant, it is called as self-pollination.
- Whereas if it involves two flowers borne on two plants of same species, it is cross-pollination.
- Ovule develops into seed and ovary into fruit after fertilization.
- Seeds fall upon the ground when fruits break up and they germinate in the soil under favourable conditions.
- Zygote develops at the cost of food stored in endosperm of seed and thus a new plantlet is formed. This is called as seed germination.