Reproduction in Organisms - Notes | Class 12 | Part 2: Sexual Reproduction
Sexual Reproduction
It is the reproduction that involves formation of male and female gametes, either by the same individual or by different individuals of the opposite sex.
It results in offspring that are not identical to the parents or amongst themselves.
It is an elaborate, complex, and slow process compared to asexual reproduction.
The period of growth to reach maturity for sexual reproduction is called the juvenile phase. In plants, it is known as the vegetative phase.
In higher plants, flowering indicates the end of the vegetative phase (beginning of the reproductive phase).
Annual and biennial plants show clear-cut vegetative, reproductive, and senescent phases. In perennial plants, these phases are very difficult to identify.
Some plants exhibit unusual flowering. E.g.:
Bamboo species flower only once in their lifetime (after 50-100 years), produce a large number of fruits, and die.
Strobilanthus kunthiana flowers once in 12 years.
In animals, the juvenile phase is followed by morphological and physiological changes prior to reproductive behavior.
Birds living in nature lay eggs only seasonally. However, birds in captivity (e.g., poultry) can be made to lay eggs throughout the year.
The females of placental mammals exhibit cyclical changes in the ovaries, accessory ducts, and hormones during the reproductive phase. It is called the oestrus cycle in non-primates (cows, sheep, rat, deer, dog, tiger, etc.) and the menstrual cycle in primates (monkeys, apes, and humans).
Based on breeding season, mammals are two types:
Seasonal Breeders: Mammals (living in natural conditions) exhibiting reproductive cycles only during favorable seasons.
Continuous Breeders: Mammals that are reproductively active throughout their reproductive phase.
Senescence (Old Age)
It is the last phase of lifespan and the end of the reproductive phase.
During this phase, concomitant changes occur in the body, e.g., slowing of metabolism. It ultimately leads to death.
In plants and animals, hormones cause transitions between the juvenile, reproductive, and senescence phases. Interaction between hormones and environmental factors regulates the reproductive processes and associated behavioral expressions of organisms.
