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Q11(c):
Identify each part in a flowering plant and write whether it is haploid (\(n\)) or diploid (\(2n\)).
(c) Egg ———————————
Solution :
Biological Context & Initial Setup
In the reproductive morphology of flowering plants (angiosperms), the egg serves as the female gamete. It is situated within the embryo sac (the female gametophyte), which is typically housed inside the ovule of the flower's ovary. To determine its ploidy, we must trace its cytological lineage through the processes of megasporogenesis and megagametogenesis.
Step 1: Analyzing Megasporogenesis (Formation of the Spore)
The development of the female gamete begins in the diploid tissue of the nucellus inside the ovule. A specialized cell called the Megaspore Mother Cell (MMC) differentiates from this tissue.
- MMC Ploidy: The Megaspore Mother Cell is a part of the sporophyte generation and is therefore diploid ($2n$).
- Meiotic Division: The MMC undergoes meiosis [a reductional division] to produce a linear tetrad of four megaspores.
- Reduction of Ploidy: Because meiosis halves the chromosome number, each of the resulting four megaspores is haploid ($n$).
- Degeneration: In typical angiosperm development (Polygonum type), three of the megaspores degenerate, leaving a single functional haploid megaspore ($n$).
Step 2: Analyzing Megagametogenesis (Formation of the Embryo Sac)
The functional haploid megaspore ($n$) then undergoes three successive rounds of free-nuclear mitosis. Mitosis is an equational division, meaning the ploidy level remains strictly conserved.
- First mitotic division yields $2$ nuclei (both $n$).
- Second mitotic division yields $4$ nuclei (all $n$).
- Third mitotic division yields $8$ nuclei (all $n$).
Following nuclear division, cell wall formation occurs, organizing the $8$ haploid nuclei into a 7-celled, 8-nucleate embryo sac. The cellular distribution is as follows:
| Cell Type | Quantity | Ploidy Level | Role in Fertilization |
|---|---|---|---|
| Antipodal Cells | 3 | Haploid ($n$) | Nutritive function; degenerates post-fertilization. |
| Central Cell | 1 (with 2 polar nuclei) | Haploid + Haploid ($n + n$) | Fuses with the second male gamete to form the Primary Endosperm Nucleus ($3n$). |
| Synergids | 2 | Haploid ($n$) | Guides the pollen tube via the filiform apparatus. |
| Egg Cell | 1 | Haploid ($n$) | Fuses with the first male gamete to form the zygote ($2n$). |
Step 3: Visual Confirmation of the Embryo Sac Spatial Geometry
Below is a precise structural representation of the embryo sac inside a flowering plant, mapping the positional coordinates of the haploid components.
Step 4: Deduction of Ploidy
Because the egg cell is generated through the equational mitotic division of the haploid functional megaspore, there is no fusion of genetic material or duplication of the chromosomal set prior to its formation. Therefore, it possesses exactly one complete set of chromosomes, denoted mathematically as $n$. It remains haploid until the moment of syngamy (fertilization), where it fuses with the male gamete ($n$) to re-establish the diploid state ($2n$) in the zygote.
Final Solution: The egg in a flowering plant is Haploid ($n$).
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(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(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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