Q.Cleistogamous flowers are self-pollinated because (A) they are bisexual flowers which do not open at all. (B) they are bisexual and open flowers. (C) they are unisexual. (D) their stigma matures before the anthers dehisce.
Concept understanding — Self Incompatibility Genetics
Imagine you are a plant. You cannot walk over to a neighbour and exchange pollen. Instead, you rely on wind, bees, or butterflies to carry your pollen to another flower. But here is the problem: if that pollen lands on your own stigma (the female part of the same flower, or a flower on the same plant), you might end up fertilising yourself. That would be like marrying your sibling — it leads to weak, inbred offspring. Plants have evolved a clever way to prevent this: self-incompatibility.
At its core, self-incompatibility is a plant's built-in "no-self-marriage" rule. It is a genetic mechanism that allows the pistil (the female organ) to recognise and reject pollen from the same plant (or a genetically identical plant). The pollen grain may land on the stigma, but the plant actively blocks it — the pollen tube fails to grow, or the ovule is not fertilised. The result is that only pollen from a different, genetically distinct plant can successfully fertilise the ovules.
This is not a random failure. It is a precise, genetically controlled rejection system. The NCERT textbook (Class 12 Biology, Chapter 2) describes it as "the inability of a plant with a functional male and female reproductive system to produce seeds when self-pollinated." In other words, the plant is perfectly fertile — it just refuses to mate with itself.
Self-incompatibility is not the same as male sterility. In male sterility, the plant produces no functional pollen. In self-incompatibility, the pollen is perfectly viable — it just gets rejected by the plant's own stigma.
Why does this matter? Because it forces cross-pollination — pollen from one plant to another. This ensures genetic diversity, which is the raw material for evolution. A diverse population is more resilient to diseases, pests, and changing climates. For farmers and plant breeders, self-incompatibility is both a challenge and a tool. It makes it harder to produce pure inbred lines (since the plant refuses to self-pollinate), but it also makes hybrid seed production easier — you can plant two varieties together and let nature enforce cross-pollination.
The genetics behind it can be simple or complex, but the core idea is this:
- There is a single gene (called the S-locus) with many different versions (alleles) in the population.
- If the pollen carries an S-allele that matches either of the two S-alleles in the pistil, the pollen is rejected.
- If the pollen carries a different S-allele (one not present in the pistil), it is accepted and fertilisation proceeds.
So, a plant with S1 and S2 alleles will reject pollen carrying S1 or S2, but accept pollen carrying S3, S4, S5, etc. This is called gametophytic self-incompatibility (common in many fruits like apples, pears, and tomatoes). There is also sporophytic self-incompatibility (seen in cabbage, mustard, and sunflowers), where the rejection is determined by the pollen parent's genetics, not the pollen grain's own S-allele.
Self-incompatibility is not a failure of the plant. It is an active, evolved strategy to avoid inbreeding. The plant is saying, "I will only mate with someone who is genetically different from me."
For a commerce or humanities student, think of it like a company policy: "No internal marriages between employees from the same department." The company (the plant) is perfectly capable of producing offspring, but it deliberately blocks internal matches to keep the workforce (the gene pool) diverse and strong. The S-alleles are like employee ID numbers — if the pollen's ID matches the pistil's ID, the door is locked.
In summary, self-incompatibility is a genetic lock-and-key system that prevents self-fertilisation and promotes outcrossing. It is a beautiful example of how plants have evolved sophisticated mechanisms to ensure genetic health, without ever needing to move from where they are rooted.
This concept is a common exam-prep search query, appearing online as "Self Incompatibility Genetics: Definition, Diagram & Examples", "Self Incompatibility Genetics notes class 12 biology", or "Self Incompatibility Genetics class 12 biology". This concept is directly part of the Sexual Reproduction in Flowering Plants chapter in the NCERT/CBSE Class 12 Biology syllabus, and it is also an important topic for NEET and state medical/CET entrance exams, making it worth mastering for both board and competitive-exam preparation.
Part (a): Cleistogamous flowers self-pollinate because they are bisexual flowers that never open — option (A).
Part (b): Asexual reproduction by motile zoospores occurs in the green alga Chlamydomonas — option (D).
Cleistogamy (Greek: 'closed marriage') describes flowers that remain permanently closed. Examining the options:
- (A) Bisexual flowers which do not open at all — correct: they possess both stamens and pistil, and because the flower never opens, the anthers dehisce inside the closed bud and the pollen is deposited on the stigma of the same flower, guaranteeing self-pollination (autogamy) even without pollinators.
- (B) Bisexual and open flowers — these are chasmogamous (normal open) flowers, not cleistogamous.
- (C) Unisexual — wrong; a unisexual flower cannot self-pollinate at all. Cleistogamous flowers are always bisexual.
- (D) Stigma matures before the anthers dehisce — this is protogyny, a device that promotes cross-pollination.
Cleistogamy is a classic example of obligate self-pollination — the flower has no option but to self, ensuring seed set even when pollinators are absent.
Cleistogamous flowers are self-pollinated because (A) they are bisexual flowers which do not open at all.
Concept understanding — Meiosis And Fertilization
Let’s begin with something you already know: every living thing comes from other living things. A mango tree grows from a seed, and that seed came from a parent mango tree. A puppy is born from its mother. But how does a single cell — a fertilized egg — turn into a whole new person or plant, with the right number of chromosomes and a mix of traits from both parents?
That’s where meiosis and fertilization come in. They are the two halves of the same story: making a new individual while keeping the species stable.
The chromosome problem
Every cell in your body (except sperm and eggs) has 46 chromosomes — 23 from your mother and 23 from your father. That’s the diploid number (2n). If a sperm and an egg each had 46 chromosomes, their fusion would give the baby 92. That would double every generation — impossible. So nature has a neat solution: before sperm and egg meet, each must halve its chromosome count.
That halving is meiosis.
What meiosis does
Meiosis is a special kind of cell division that happens only in the reproductive organs (testes in males, ovaries in females). It takes one diploid cell (2n = 46) and produces four haploid cells (n = 23) — each with one complete set of chromosomes.
Think of it like splitting a deck of cards into two halves, then shuffling each half separately. You end up with four half-decks, each unique.
The process has two rounds of division (meiosis I and meiosis II), but the key point for you is this: meiosis reduces the chromosome number by half and shuffles the genetic material so that each sperm or egg is genetically different from every other.
Why does shuffling matter? Because it creates variation. No two siblings (except identical twins) are exactly alike, and that variety is what allows a species to adapt and survive.
What fertilization does
Fertilization is the opposite of meiosis. It’s the fusion of two haploid cells — a sperm (n = 23) and an egg (n = 23) — to form a single diploid cell called a zygote (2n = 46). That zygote is the first cell of the new individual.
Fertilization restores the full chromosome number. Without meiosis, fertilization would double the count. Without fertilization, meiosis would leave cells with only half the needed chromosomes. The two processes are a matched pair.
Why this matters for you
- Continuity of life: Every human being starts as a single fertilized egg. Meiosis and fertilization are the biological machinery that makes that possible.
- Genetic uniqueness: Because meiosis shuffles chromosomes and fertilization brings together two different sets, every person (except identical twins) has a unique combination of genes. That’s why you look like a blend of both parents but are not identical to either.
- Stability of species: The chromosome number stays constant generation after generation. Humans always have 46, cats always have 38, fruit flies always have 8 — because meiosis and fertilization work together to keep the number fixed.
A quick summary in plain terms
| Process | What it does | Result |
|---|---|---|
| Meiosis | Halves chromosome number (46 → 23) and shuffles genes | Four unique haploid cells (sperm or eggs) |
| Fertilization | Fuses two haploid cells (23 + 23) | One diploid zygote (46) |
NCERT textbooks (Class 12 Biology, Chapter 3) describe meiosis as “reduction division” and fertilization as “syngamy.” The two together are called sexual reproduction.
So the next time you see a baby or a seedling, remember: behind that new life is a perfectly balanced dance — meiosis cuts the number in half, fertilization puts it back together, and the species continues, unchanged in count but endlessly varied in form.
This is a well-searched revision topic, typically framed as "Meiosis And Fertilization important questions", "Meiosis And Fertilization class 12 biology", or "Meiosis And Fertilization notes class 12 biology". This concept is directly part of the Human Reproduction chapter in the NCERT/CBSE Class 12 Biology syllabus, and it is also an important topic for NEET and state medical/CET entrance exams, making it worth mastering for both board and competitive-exam preparation.
Part (a): Cleistogamous flowers self-pollinate because they are bisexual flowers that never open — option (A).
Part (b): Asexual reproduction by motile zoospores occurs in the green alga Chlamydomonas — option (D).
Zoospores are flagellated, motile spores used for asexual reproduction, typically in aquatic algae and some lower fungi. Checking the options:
- (A) Penicillium — a fungus reproducing asexually by non-motile conidia.
- (B) Hydra — an animal reproducing asexually by budding.
- (C) Sponge — reproduces asexually by gemmules and fragmentation.
- (D) Chlamydomonas — a freshwater green alga that reproduces asexually by producing biflagellate zoospores. ✓
Asexual reproduction by zoospores is observed in (D) Chlamydomonas.
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