Why does the zygote begin to divide only after the division of Primary endosperm cell (PEC)?

NCERT Exemplar · Q46

Q. Why does the zygote begin to divide only after the division of Primary endosperm cell (PEC)?

Andhra Pradesh BieapShort· 2mImportance★★★★★

91% · 82/90 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

Concept understanding — Seed Dormancy

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.

Important

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

Start your 14-day free trial to unlock the full solution →

The zygote delays its division until the Primary Endosperm Cell (PEC) has divided sufficiently because the developing endosperm provides essential nutrition and hormonal signals that the embryo needs to grow.

In flowering plants, after double fertilisation, two distinct fusion events occur simultaneously. One sperm cell fuses with the egg cell to form the zygote (which will become the embryo), while the other sperm cell fuses with the two polar nuclei to form the triploid Primary Endosperm Cell (PEC). What follows is a carefully orchestrated sequence: the PEC begins dividing immediately, but the zygote remains dormant for a period before it starts its own division.

This delay is not accidental — it is a critical adaptation that ensures the embryo's survival. The endosperm, which develops from the PEC, serves as a nutritive tissue that will nourish the growing embryo. If the zygote began dividing at the same time as the PEC, the young embryo would have no ready source of food. By waiting until the endosperm has built up a substantial store of nutrients, the zygote ensures that when it does start dividing, there is a rich supply of starch, proteins, and oils available to fuel its growth.

Note

In some plants like coconut, the endosperm remains liquid (coconut water) for a long time, while in cereals it becomes solid and starchy. In both cases, the zygote waits until enough endosperm has formed.

There is also a hormonal dimension to this delay. The developing endosperm produces growth regulators — particularly auxins and cytokinins — that are necessary to trigger the zygote's division. The zygote essentially "waits" for these chemical signals from the endosperm before it begins its own mitotic activity. This ensures that the embryo's development is synchronised with the availability of resources. …

Start your 14-day free trial to unlock the full solution →

Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

AP EAPCET 2026Set ap-2026-05-19-FN1 markMCQ
$Q. Germination of seeds while still attached to the mother plant is (A) Vivipary (B) Embryogenesis (C) Syngamy (D) Parthenogenesis$
›Reveal solutionSolution
Seeds germinating while still on the mother plant is called vivipary, a well-known adaptation seen in mangroves.

Concept and Intuition

Most seeds enter dormancy after maturing and germinate only after dispersal, under suitable external conditions. Vivipary is the exception: the embryo continues developing directly into a seedling while the fruit/seed is still attached to the parent — an adaptation especially useful in mangrove trees growing in unstable, saline, waterlogged soil, where a pre-formed seedling (propagule) has a much better chance of establishing once it drops.

Step-by-Step Solution

Recall the definition: vivipary = germination beginning before seed dispersal, i.e. while still attached to the mother plant.

Recognise the classic textbook example — mangrove species such as Rhizophora, where a spear-like seedling (hypocotyl) develops and elongates on the tree before falling and embedding itself in the mud. …
Start your 14-day free trial to see the full solution →
AP EAPCET 2023Set ap-2023-05-22-FN1 markMCQ
$\mathrm{O_2}$
›Reveal solutionSolution
Statement (a) misuses the term "quiescence" for what is actually embryo-immaturity dormancy; (b), (c) and (d) are all standard, correct facts about seed dormancy mechanisms.

Concept and Intuition

Seed dormancy can arise from several distinct causes: an impermeable, hard seed coat that physically blocks water or oxygen entry (common in Fabaceae/legume seeds); the presence of chemical germination inhibitors in the seed or surrounding fruit tissue (as documented for tomato); or a specific requirement for a period of low temperature (stratification/chilling) to break dormancy before germination can proceed (documented for various seeds including Polygonum). "Quiescence," by contrast, specifically describes a fully mature and otherwise viable seed that simply fails to germinate because external conditions (temperature, moisture, etc.) are unfavourable — not because the embryo itself is immature or underdeveloped. An immature embryo failing to germinate is a distinct phenomenon (morphological/embryo dormancy), not quiescence.

Step-by-Step Solution

(a) Claims immature-embryo-caused failure to germinate is called "quiescence" — this conflates two different concepts; quiescence is about external unfavourable conditions on a mature seed, not embryo immaturity. INCORRECT. …
Start your 14-day free trial to see the full solution →

←Previous questionQ45 — Are parthenocarpy and apomixis different phenomena? Discuss their bene…→Next questionQ47 — The generative cell of a two-celled pollen divides in the pollen tube …

Q.Why does the zygote begin to divide only after the division of Primary endosperm cell (PEC)?

🎓Unlock everything free for 14 days

Unlock everything free for 14 days