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Q8(a):
Define
(a) Juvenile phase,
Solution :
Biological Definition & Core Concept
In developmental biology and life history theory, the Juvenile Phase is defined as the period of active somatic growth and morphological development in an organism's life cycle that occurs immediately after birth (or hatching/germination) and continues until the organism attains sexual maturity.
Mathematically and chronologically, if $t_0$ represents the time of birth, $t_r$ represents the onset of reproductive capability, and $t_s$ represents the onset of senescence, the juvenile phase occupies the temporal interval $[t_0, t_r)$. During this interval, the organism directs entirely towards anabolism (building mass) and survival, rather than gametogenesis.
Key Physiological Characteristics
- Somatic Proliferation: There is a rapid rate of cell division (mitosis), leading to an exponential increase in biomass and dimensional growth.
- Resource Accumulation: Organisms accumulate the necessary energy reserves (lipids, carbohydrates) required to sustain the highly energetically demanding reproductive phase that follows.
- Absence of Reproductive Function: Reproductive organs (gonads in animals, floral meristems in plants) may be present in rudimentary forms but are strictly non-functional. Gametogenesis does not occur.
Terminology Variations Across Taxa
The nomenclature of this phase varies depending on the kingdom being studied:
| Kingdom/Group | Phase Name | Biological Marker of Termination |
|---|---|---|
| Animals (Zoology) | Juvenile Phase | Onset of puberty, development of secondary sexual characteristics, and functional gonads. |
| Plants (Botany) | Vegetative Phase | Transition of the shoot apical meristem into a floral meristem (onset of flowering). |
Visualizing the Phases of a Biological Life Cycle
The growth curve below illustrates the phase transitions in a standard organism's life cycle, demonstrating how the juvenile phase corresponds to the exponential growth portion of a sigmoidal growth curve.
Hormonal Regulation [Theoretical Justification]
The duration and termination of the juvenile phase are strictly regulated by endocrine mechanisms and genetic factors. For instance, in mammals, the release of Gonadotropin-Releasing Hormone (GnRH) from the hypothalamus marks the conclusion of the juvenile phase. In higher plants, the transition from the vegetative (juvenile) phase to the reproductive phase is triggered by environmental cues (such as photoperiodism and vernalization) which activate floral meristem identity genes (e.g., LEAFY, APETALA1).
Final Solution: The juvenile phase is the period of physical growth and somatic development in an organism's life cycle, spanning from birth (or germination) up to the onset of sexual maturity. During this phase, the organism cannot reproduce sexually. In plants, this specific pre-reproductive growth period is synonymously termed the vegetative phase.
More Questions from Class 12 Biology Reproduction in Organisms EXERCISES
- Q1: Why is reproduction essential for organisms?
- Q1: Why is reproduction essential for organisms?
- Q2: Which is a better mode of reproduction: sexual or asexual? Why?
- Q2: Which is a better mode of reproduction: sexual or asexual? Why?
- Q3: Why is the offspring formed by asexual reproduction referred to as clone?
- Q3: Why is the offspring formed by asexual reproduction referred to as clone?
- Q4: Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
- Q4: Offspring formed due to sexual reproduction have better chances of survival. Why? Is this statement always true?
- Q5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
- Q5: How does the progeny formed from asexual reproduction differ from those formed by sexual reproduction?
- Q6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
- Q6: Distinguish between asexual and sexual reproduction. Why is vegetative reproduction also considered as a type of asexual reproduction?
- Q7: What is vegetative propagation? Give two suitable examples.
- Q7: What is vegetative propagation? Give two suitable examples.
- Q8(a): Define (a) Juvenile phase,
- Q8(b): Define (b) Reproductive phase,
- Q8(b): Define (b) Reproductive phase,
- Q8(c): Define (c) Senescent phase.
- Q8(c): Define (c) Senescent phase.
- Q9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
- Q9: Higher organisms have resorted to sexual reproduction in spite of its complexity. Why?
- Q10: Explain why meiosis and gametogenesis are always interlinked?
- Q10: Explain why meiosis and gametogenesis are always interlinked?
- Q11(a): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (a) Ovary ———————————
- Q11(a): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (a) Ovary ———————————
- Q11(b): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (b) Anther ———————————
- Q11(b): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (b) Anther ———————————
- Q11(c): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (c) Egg ———————————
- Q11(c): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (c) Egg ———————————
- Q11(d): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (d) Pollen ———————————
- Q11(d): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (d) Pollen ———————————
- Q11(e): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (e) Male gamete ———————————
- Q11(e): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (e) Male gamete ———————————
- Q11(f): Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)). (f) Zygote ———————————
- Q11(f): Identify each part in a flowering plant and write whether it is haploid ((n)) or diploid ((2n)). (f ) Zygote ———————————
- Q12: Define external fertilisation. Mention its disadvantages.
- Q12: Define external fertilisation. Mention its disadvantages.
- Q13: Differentiate between a zoospore and a zygote.
- Q13: Differentiate between a zoospore and a zygote.
- Q14: Differentiate between gametogenesis from embryogenesis.
- Q14: Differentiate between gametogenesis from embryogenesis.
- Q15: Describe the post-fertilisation changes in a flower.
- Q15: Describe the post-fertilisation changes in a flower.
- Q16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
- Q16: What is a bisexual flower? Collect five bisexual flowers from your neighbourhood and with the help of your teacher find out their common and scientific names.
- Q17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q17: Examine a few flowers of any cucurbit plant and try to identify the staminate and pistillate flowers. Do you know any other plant that bears unisexual flowers?
- Q18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
- Q18: Why are offspring of oviparous animals at a greater risk as compared to offspring of viviparous animals?
CBSE Solutions for Class 12 Biology Reproduction in Organisms
Chapters in CBSE - Class 12 Biology
- Human Reproduction
- Reproductive Health
- Principles of Inheritance and Variation
- Molecular Basis of Inheritance
- Evolution
- Human Health and Disease
- Strategies for Enhancement in Food Production
- Microbes in Human Welfare
- Biotechnology: Principles and Processes
- Biotechnology and Its Applications
- Organisms and Populations
- Ecosystem
- Biodiversity and Conservation
- Environmental Issues
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