Question

Define :
(a) Juvenile phase
(b) Reproductive phase
(c) Senescent phase

Answer 1

Hint: The lifetime of any individual can broadly be divided into three phases namely the Juvenile phase, the Reproductive phase, and the Senescent phase. They differ based on their metabolic needs and cell activities.

Complete answer:
The juvenile phase is defined as the phase of growth that begins with its birth and progresses till the reproductive organs are fully developed or sexual maturity is attained. It is known as the vegetative phase in plants.
The reproductive phase is defined as the phase of growth when an organism is sexually matured enough to reproduce or give rise to young ones.
The senescent phase is defined as the phase of growth when an organism loses its reproductive capacity and progressive degeneration of the cellular activities begins.

Additional Information: The reproductive phase in sexually reproducing organisms is further divided into three sequential events - fertilization, pre- fertilization, and post-fertilization events.
Pre-fertilization events: It includes all the events which lead up to the fusion of male and female gamete or fertilization.
- Gametogenesis: The process of formation of male and female gametes is defined as gametogenesis. The two gametes may be similar in appearance or they may be morphologically dissimilar. A gamete is formed by meiosis or reductional division in sex organs.
- Gamete transfer: The two gametes need to be physically brought together to facilitate fertilization. Mostly ova or female gamete is stationary while the male gamete is motile. But in some lower organisms like algae and few fungi, both the gametes are motile. In plants, pollen grains carry male gamete. The process of the landing of pollen grains on stigmas aided by agents like wind, water, and insects is known as pollination.
Fertilization
- Syngamy or the fusion of gametes produces a diploid cell ‘zygote’. If syngamy occurs outside the female’s body or in water, it is known as external fertilization while if it occurs inside the female’s body, then such type of syngamy is named as internal fertilization.
Post-fertilisation events:
- Zygote, thus formed, decides to divide or not after analyzing the environment it is exposed to. In algae and fungi, thick walls surround the zygote in unfavorable conditions. Germination begins once the favorable conditions are restored. It undergoes successive mitotic divisions and cell differentiation to produce a multicellular body inside or outside the female’s parent.

Note: Homogametes(isogametes): These are the type of which are morphologically similar to many algae.
Heterogametes: These are the type of gametes that are morphologically dissimilar as in humans or in angiosperms.
Oviparous animals: Animals like birds where the development of the embryo takes place outside the mother’s body.
Viviparous animals: Animals like humans where the development of the embryo takes place inside the female’s body.