Reproduction, an important biological process occurring in living organisms, ensures continuity of life. Organisms reproduce either asexually or sexually. In the case of sexual reproduction, fusion of male and female gametes occurs. All organisms, go through three phases of life – the juvenile phase, reproductive phase and senescence phase.
The juvenile phase is a period of growth which is characterised by an increase in height in all animals including human beings. In plants, the juvenile phase is known as the vegetative phase. The duration of the juvenile varies in different organisms.
The juvenile phase is succeeded by the reproductive phase in both plants and animals. In the case of plants, this phase is marked by the appearance of flowers. In perennial plant, it is very difficult to define the various phases of life, because perennials, have a long reproductive phase, keep shedding their leaves throughout the year or once a year in winter. It is easy to identify the phases in the case of annuals and biennials as all three phases of an annual plant occur in one year whereas biennials have a two-year lifecycle in which the plants enter the vegetative phase in the first year and the reproductive followed by the senescent phase in the second year.
In human beings, the reproductive phase, starts with puberty, leads to the development of male and female secondary sexual characters such as facial hair in both male and females and breasts which is seen more prominently in females. Females of all placental mammals including primates and non-primates demonstrate cyclical changes in the activities of their ovaries and accessory ducts as well as hormones during the reproductive phase. In human beings, females experience the menstrual cycle and are active throughout their reproductive phase. They are therefore called continuous breeders.
The end of the reproductive phase in living organisms coincides with the beginning of the senescent phase during which we see changes such as senility, non-existence of the menstrual cycle and slowing of metabolism. Senescence ultimately leads to death.
The transition between the three stages is controlled by hormones as well as environmental factors. The release of the hormone oestrogen triggers the menstrual cycle in girls. Moreover, girls living in tropical regions experience menstruation at an earlier age than those living in temperate regions. In most organisms, the reproductive phase plays a crucial role as it ensures continuity of life.