Q.Why do you think the zygote is dormant for sometime in a fertilised ovule?
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Seed Dormancy: A Survival Strategy in Plants
Imagine you buy a packet of seeds for your garden. You plant them, water them, and wait. Some sprout in a few days. Others sit in the soil for weeks or months before they finally wake up. That delay is not laziness — it is a deliberate, built-in pause called seed dormancy.
The Everyday Intuition
Think of a seed as a tiny, sleeping baby plant wrapped in a protective coat. If every seed sprouted the moment it fell from the parent plant, what would happen? A seed that lands in the middle of a dry summer would germinate, then die from lack of water. A seed that falls in autumn would sprout just before winter frost kills the tender shoot. Nature avoids this disaster by giving seeds a "pause button." That pause is seed dormancy.
The Precise Meaning
In biology, seed dormancy is a state in which a viable seed (one that is alive and capable of growing) does not germinate even when all external conditions — water, oxygen, temperature, light — appear favourable. The seed is not dead; it is simply refusing to wake up until some internal or external "lock" is released.
Dormancy is not the same as a dead seed. A dormant seed is alive but inactive. A dead seed will never germinate, no matter what you do.
Why Does Dormancy Exist?
The purpose is survival. A seed that germinates at the wrong time will die. Dormancy ensures that germination happens only when the environment is likely to support the seedling's growth. Here are the main reasons:
- To survive unfavourable seasons — Seeds of many temperate plants remain dormant through winter and germinate only in spring.
- To avoid competition — Some seeds lie dormant in the soil for years, waiting for a fire, flood, or clearing that removes competing plants.
- To spread germination over time — Not all seeds from the same plant germinate in the same year. This "bet-hedging" ensures that if one year is disastrous, some seeds remain for the next.
Types of Seed Dormancy (as per NCERT)
The NCERT textbook classifies dormancy into two broad categories:
1. Innate (Primary) Dormancy
The seed is born dormant. It will not germinate even under ideal conditions until a specific trigger occurs. This is further divided into:
- Physical dormancy — The seed coat is hard and impermeable to water or oxygen. The seed cannot absorb water to start germination. Example: seeds of many legumes (peas, beans).
- Physiological dormancy — The embryo itself is immature or contains chemical inhibitors that block germination. Example: seeds of apple, peach, which need a period of cold (stratification) before they can sprout.
2. Induced (Secondary) Dormancy
A seed that was initially capable of germination becomes dormant again because of unfavourable conditions. For example, if a seed starts to germinate but then faces extreme heat or drought, it may re-enter dormancy.
How Is Dormancy Broken?
Nature has evolved several mechanisms to "wake up" dormant seeds:
| Trigger | How It Works | Example |
|---|---|---|
| Scarification | Physical abrasion or cracking of the hard seed coat | Seeds passing through an animal's digestive tract |
| Stratification | Exposure to cold, moist conditions for weeks | Apple seeds need winter cold |
| Fire | Heat or smoke chemicals break dormancy | Many pine species |
| Light | Some seeds need a flash of red light to germinate | Lettuce seeds |
| Leaching | Rain washes away chemical inhibitors from the seed coat | Desert seeds |
The zygote in a fertilised ovule remains dormant for a period because the ovule itself needs time to develop into a seed with all the necessary protective and nutritive structures. Immediately after fertilisation, the ovule is not yet a mature seed — the integuments must harden into the seed coat, the endosperm must develop to store food, and the embryo sac must reorganise to support the growing embryo.
If the zygote began dividing right away, the embryo would start forming before these essential support systems are in place. The dormancy ensures that when the zygote does divide, the developing embryo has a ready supply of nutrients from the endosperm and is enclosed within a protective seed coat. This synchronisation between embryo development and seed maturation is crucial for the survival of the new p …
The zygote remains dormant in a fertilised ovule to allow the endosperm to develop first, ensuring that a nutritive tissue is ready to support the embryo once it begins active growth.
When fertilisation occurs in a flowering plant, two critical events happen almost simultaneously: the egg fuses with one sperm to form the diploid zygote, and the two polar nuclei fuse with the second sperm to form the triploid primary endosperm nucleus. Yet the zygote does not immediately leap into division. It pauses. This dormancy is not accidental — it reflects a carefully timed developmental strategy.
The endosperm is the tissue that will nourish the developing embryo. It forms from the primary endosperm nucleus and begins dividing well before the zygote does. This head start is essential. If the zygote were to start developing immediately, the embryo would quickly outpace the formation of its food supply. The result would be a growing embryo with nothing to sustain it — a developmental dead end.
By remaining dormant, the zygote allows the endosperm to establish itself. The endosperm divides rapidly, filling the embryo sac with nutrient-rich tissue. Only after this nutritive scaffold is in place does the zygote begin its own divisions, forming first the proembryo and then the mature embryo with its cotyledons, plumule, and radicle. The timing ensures that every stage of embryo development is supported by an adequate food reserve. …
- TG EAPCET 2025Set ap-2025-04-29-FN1 markMCQQ.Consider the following statements Assertion (A): Seed dormancy may be caused by hard seed coats in Fabaceae Reason (R): Such type of seed dormancy can be broken by stratification The correct answer is (A) Both (A) and (R) are true, (R) is the correct explanation of (A) (B) Both (A) and (R) are true, (R) is not the correct explanation of (A) (C) (A) is true, but (R) is false (D) (A) is false, but (R) is true
›Reveal solutionSolution
Seed dormancy can indeed be caused by hard seed coats, especially in plants like those in the Fabaceae family. However, this type of dormancy is primarily broken by scarification, not stratification. Therefore, the Assertion is true, but the Reason is false.
Seed dormancy is a crucial survival mechanism for plants, ensuring that seeds germinate only when conditions are most favorable for seedling establishment. It's a state where a viable seed fails to germinate even when provided with optimal environmental conditions (water, oxygen, suitable temperature, light). Understanding the causes of dormancy and the specific methods to break them is key.
There are several reasons why a seed might be dormant, and each type of dormancy often requires a specific treatment to overcome it.
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Evaluate Assertion (A): Seed dormancy may be caused by hard seed coats in Fabaceae.
- Concept: One of the most common causes of seed dormancy is a physically impermeable or very hard seed coat. Such a coat prevents water from imbibing into the seed (which is the first step of germination) and/or restricts the exchange of gases (like oxygen) required for respiration by the embryo.
- Fabaceae: The family Fabaceae (legumes, e.g., beans, peas, lentils) is well-known for producing seeds with exceptionally hard and impermeable seed coats. This characteristic contributes significantly to their longevity in soil seed banks but also makes them difficult to germinate without specific treatments.
- Conclusion for A: The statement is true. Hard seed coats are a recognized cause of seed dormancy, and this is particularly prevalent in the Fabaceae family.
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Evaluate Reason (R): Such type of seed dormancy can be broken by stratification.
- Concept of Dormancy Breaking Methods:
- Scarification: This is a process that involves weakening, breaking, or scratching the hard seed coat to make it permeable to water and gases. It can be done mechanically (e.g., rubbing with sandpaper, nicking with a knife) or chemically (e.g., treating with strong acids). Scarification is the primary method used to overcome physical dormancy caused by hard seed coats.
- Stratification: This typically refers to exposing seeds to cold, moist conditions for a specific period. This treatment mimics the natural conditions seeds experience during winter. Cold stratification is primarily used to break physiological dormancy, which might be due to an immature embryo, the presence of germination inhibitors, or a requirement for a chilling period to complete metabolic processes. …
- Concept of Dormancy Breaking Methods:
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- TG EAPCET 2025Set ap-2025-04-30-FN1 markMCQQ.Diplontic life cycle is shown by (A) Volvox (B) Polysiphonia (C) Laminaria (D) Fucus
›Reveal solutionSolution
The diplontic life cycle, where the dominant, free-living phase is diploid and the only haploid stage is the gamete, is shown by Fucus — a brown alga that lacks a free-living gametophyte.
The key to this question is understanding what a diplontic life cycle actually means. In biology, life cycles are classified based on which phase — haploid or diploid — is multicellular and dominant. In a diplontic cycle, the organism you see is diploid (2n). It produces haploid gametes by meiosis, and those gametes fuse immediately to form a zygote, which grows into the next diploid adult. There is no multicellular haploid stage at all — the only haploid cells are the gametes themselves.
This is the pattern we see in most animals, including humans. Among algae, it is relatively rare. Most algae show a haplo-diplontic cycle (alternation of generations) where both haploid and diploid phases are multicellular. But a few brown algae, especially in the genus Fucus, have evolved a strictly diplontic cycle.
Let’s examine each option:
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Volvox — This is a colonial green alga. Its life cycle is haplontic: the dominant, motile colony is haploid (n). The only diploid stage is the zygote, which undergoes meiosis to produce new haploid cells. So Volvox is not diplontic.
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Polysiphonia — This is a red alga with a complex triphasic haplo-diplontic life cycle. It has three distinct phases: a haploid gametophyte, a diploid carposporophyte (which grows on the gametophyte), and a free-living diploid tetrasporophyte. The dominant phase is the haploid gametophyte. Clearly not diplontic.
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Laminaria — This is a large brown alga (a kelp) with a haplo-diplontic life cycle. The familiar large blade is the diploid sporophyte, but it alternates with a microscopic, filamentous haploid gametophyte. Both phases are multicellular, so it is not purely diplontic. …
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- TG EAPCET 2022Set ap-2022-07-31-AN1 markMCQQ.Which of the following condition leads to development of more than one embryo in a seed? (A) Growth of thick walled nucellar cells into the embryosac (B) Fruit developed with parthenocarpy (C) Thick walled collenchymatous cells surrounding embryosac (D) Growth of thin walled parenchymatous cells around the embryosac into the embryosac
›Reveal solutionSolution
Polyembryony arises when the thin-walled parenchymatous nucellar cells surrounding the embryo sac proliferate and grow into it, each developing into an embryo. Option (D).
The concept first
Polyembryony = the occurrence of more than one embryo in a single seed. It was recorded as long ago as Leeuwenhoek's observation on orange seeds.
Where do the extra embryos come from? The dominant route in angiosperms is nucellar (adventive) polyembryony:
- The nucellus is the diploid, parenchymatous maternal tissue of the ovule that surrounds the embryo sac.
- In many citrus and mango varieties, some of these nucellar cells begin to divide, push into the embryo sac, and each develops into an embryo.
- Because they arise from maternal somatic tissue, these embryos are genetically identical to the mother plant — they are natural clones. This is horticulturally precious: it lets a desirable variety be propagated true-to-type through seed, without the genetic reshuffling of sexual reproduction.
- Alongside them, the normal zygotic embryo also develops. Hence one seed, several embryos.
Step-by-step
- What tissue surrounds the embryo sac? The nucellus — and nucellar cells are thin-walled parenchyma, not thick-walled and not collenchyma. This is the key descriptive fact the question is testing.
- What must these cells do to produce embryos? They must divide and grow into the embryo sac, where they then differentiate into embryos. Merely surrounding the sac achieves nothing.
- Evaluate each option.
- (A) "Growth of thick walled nucellar cells into the embryosac." The mechanism (growth into the sac) is right, but the tissue is misdescribed. Nucellar cells are not thick-walled. Rejected on the descriptor. …
- TG EAPCET 2021Set ap-2021-08-09-AN1 markMCQQ.Choose the correct statements among the following. A) The stage between two meiotic division is called interphase B) During zygotene pairing of homologous chromosomes and synapsis takes place C) Initiation of mitotic spindle takes place during metaphase D) In animals, mitotic cell division takes place in diploid somatic cells and not in haploid cells (A) A, B (B) B, C (C) C, D (D) B, D
›Reveal solutionSolution
Meiosis and mitosis have distinct phases and rules. The correct statements are B (synapsis occurs at zygotene) and D (animal mitosis is restricted to diploid somatic cells), making option (D) the answer.
The question tests your grasp of the precise vocabulary and sequence of cell division. Each statement must be checked against the standard definitions — one wrong word can flip a statement from true to false.
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Statement A: "The stage between two meiotic division is called interphase"
This is false. In meiosis, the two divisions (Meiosis I and Meiosis II) are separated by a brief stage called interkinesis, not interphase. Interkinesis lacks DNA replication (no S phase), whereas interphase includes G1, S, and G2. Calling it "interphase" is a common error — the two terms are not interchangeable.
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Statement B: "During zygotene pairing of homologous chromosomes and synapsis takes place"
This is true. Zygotene is the second stage of prophase I in meiosis. Its defining events are the pairing of homologous chromosomes (called synapsis) and the formation of the synaptonemal complex. The statement is accurate.
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Statement C: "Initiation of mitotic spindle takes place during metaphase"
This is false. The mitotic spindle begins to form during prophase (in animal cells, from the centrosomes). By metaphase, the spindle is already fully formed and attached to kinetochores; it does not "initiate" then. Initiation is a prophase event.
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Statement D: "In animals, mitotic cell division takes place in diploid somatic cells and not in haploid cells" …
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