Chapter 1: Reproduction
in Organisms
The most complete, student-friendly guide — concepts, diagrams, MCQs, board questions, NCERT solutions & rapid revision, all in one place.
Chapter Introduction
Reproduction is one of the fundamental characteristics of all living organisms. It is the biological process by which organisms give rise to offspring of the same kind. Without reproduction, a species cannot persist over time. Chapter 1 of Class 12 Biology (NCERT) — Reproduction in Organisms — lays the foundation for understanding how life continues, explores asexual and sexual modes of reproduction, and explains key events like gametogenesis, fertilisation, and post-fertilisation developments.
Unicellular Organisms
The entire organism acts as a gamete or divides to reproduce — e.g., Amoeba, Paramecium, Bacteria, Yeast.
Multicellular Organisms
Specialised reproductive organs produce gametes — e.g., humans, higher plants, fungi, animals.
Lifespan vs. Reproduction
Organisms reproduce at least once in a lifetime. Lifespan ranges from hours (bacteria) to centuries (trees).
Key Terms & Definitions
Asexual Reproduction
Asexual reproduction involves a single parent. It does not involve the formation or fusion of gametes. The offspring produced are genetically identical to the parent, hence called clones. It is simpler, faster, and energetically less costly than sexual reproduction.
Methods of Asexual Reproduction
1. Binary Fission
The parent organism divides into two equal (or unequal) daughter cells, each developing into an adult. Common in Amoeba (irregular plane), Paramecium (transverse fission), and bacteria.
- Simple and rapid method
- In Amoeba — division plane is irregular (no fixed plane)
- In Paramecium — transverse binary fission occurs
- In bacteria — cell division (mostly by transverse fission)
2. Multiple Fission
The nucleus divides repeatedly to form many nuclei; each nucleus with a bit of cytoplasm becomes a daughter cell. E.g., Plasmodium (malarial parasite), Amoeba under unfavourable conditions.
3. Budding
A bud (outgrowth) arises from the parent body, grows, detaches and develops into a new individual.
- Yeast — budding in unicellular organisms
- Hydra — budding in multicellular animals
4. Fragmentation
Parent organism breaks into fragments; each fragment grows into a complete individual. E.g., Spirogyra (filamentous alga), Planaria (flatworm).
5. Regeneration
The ability of organisms to give rise to a whole organism from a part of the body. E.g., Planaria, Hydra, starfish. This is different from simple wound healing.
- Carried out by specialised cells
- Not possible in all organisms — e.g., not in humans (only wound healing possible)
- Planaria is a classic example of both fragmentation and regeneration
6. Spore Formation (Sporulation)
Special structures called sporangia produce spores, which germinate into new organisms. Common in fungi (Rhizopus, Penicillium), mosses, ferns (pteridophytes).
- Spores are surrounded by a tough coat to withstand unfavourable conditions
- In fungi: asexual spores called conidia (in Penicillium) or zoospores
7. Vegetative Propagation (in Plants)
New plants are produced from vegetative parts — roots, stems, leaves — without seeds. This is the most common type of asexual reproduction in plants.
| Vegetative Part | Example Plant | Structure |
|---|---|---|
| Underground stem (rhizome) | Ginger, Turmeric | Modified stem |
| Corm | Colocasia (arvi) | Condensed stem |
| Bulb | Onion, Garlic | Modified shoot |
| Offset | Water hyacinth, Pistia | Short horizontal stem |
| Runner | Grass, Strawberry | Horizontal stolon |
| Leaf | Bryophyllum | Notches on leaf margin |
| Eyes of tuber | Potato | Axillary buds on tuber |
8. Conidia
Non-motile spores produced exogenously (not inside sporangia) in fungi like Penicillium. They are formed at the tips of special hyphae called conidiophores.
9. Gemmules
Internal buds protected by a hard coating found in sponges (Porifera). They survive harsh conditions and germinate to form new sponges.
10. Zoospores
Microscopic motile asexual reproductive structures produced by algae like Chlamydomonas and fungi like Pythium. They possess flagella for movement.
| Method | Example Organisms | Key Feature |
|---|---|---|
| Binary Fission | Amoeba, Paramecium, Bacteria | 1 parent → 2 offspring |
| Multiple Fission | Plasmodium, Amoeba | 1 parent → many offspring |
| Budding | Yeast, Hydra | Bud detaches → new organism |
| Fragmentation | Spirogyra, Planaria | Body breaks → each piece grows |
| Regeneration | Planaria, Hydra, Starfish | Lost parts regrow |
| Spore Formation | Rhizopus, Penicillium, Ferns | Spores in sporangia |
| Vegetative Propagation | Ginger, Potato, Onion | Vegetative parts → new plant |
| Gemmules | Sponges | Internal protected buds |
| Zoospores | Chlamydomonas, Pythium | Motile asexual spores |
| Conidia | Penicillium | Exogenous asexual spores |
Sexual Reproduction
Sexual reproduction involves the fusion of two types of gametes — male and female. It results in offspring that are genetically distinct from each other and from the parents (due to recombination), providing variation essential for evolution and adaptation.
Why is Sexual Reproduction Preferred by Higher Organisms?
- Creates genetic variation through recombination and random assortment of chromosomes
- Variation helps in adaptation and evolution
- Offspring are not identical clones, allowing species to survive changing environments
- Though slower and energy-costly, the long-term survival advantage outweighs the cost
Characteristics of Sexual Reproduction
- Always involves two parents (or one parent producing both gametes)
- Involves formation of gametes (gametogenesis)
- Involves fertilisation (syngamy)
- Offspring have genetic material from both parents
- Leads to variation in offspring — raw material for natural selection
Bisexual vs. Unisexual Organisms
🌸 Bisexual (Hermaphrodite)
- Both male and female organs in same individual
- Plants: Hibiscus, Mustard (bisexual flowers)
- Animals: Earthworm, Leech, Tapeworm, Sponge
- Also called monoecious in plants
♂♀ Unisexual
- Male and female organs in separate individuals
- Plants: Papaya, Watermelon, Date palm
- Animals: Cockroach, Humans, Frogs
- Also called dioecious in plants
Monoecious = both sexes in one plant (e.g., Maize — staminate and pistillate flowers on same plant)
Dioecious = sexes in separate plants (e.g., Papaya — male and female plants separate)
Life Cycle & Reproductive Phases
All organisms have a definite life cycle — a sequence of phases from birth to death. The three main phases are:
🌱 Juvenile / Vegetative Phase
Growth phase before reproductive maturity. In plants: vegetative phase. In animals: juvenile phase. The organism grows and develops but does not reproduce.
🌸 Reproductive Phase
The organism attains reproductive maturity and is capable of producing offspring. In plants: marked by flowering. In animals: by puberty. This is the longest phase for most organisms.
🍂 Senescent Phase
Phase of deterioration, aging, and eventual death. Metabolic activities slow down. The reproductive capacity decreases or ceases.
Reproductive Cycle in Animals
- Seasonal breeders: Most mammals (deer, bears, frogs) reproduce only during a particular season of the year — the breeding season.
- Continuous breeders: Humans, apes — reproduce throughout the year.
- Oestrus cycle: Cyclic changes in non-primate female mammals during breeding season — e.g., cows, sheep, dogs, tigers.
- Menstrual cycle: Cyclic changes in primates (humans, apes, old-world monkeys) — approximately every 28 days.
Reproductive Phases in Plants
- Annual plants: Complete life cycle in one year/season — e.g., wheat, rice.
- Biennial plants: Take two years — vegetative growth in year 1, reproduction in year 2 — e.g., carrot, turnip.
- Perennial plants: Live for many years, reproduce several times — e.g., mango, banyan.
- Bamboo: Flowers only once in its lifetime (~50–100 years), then dies — example of semelparous reproduction.
- Strobilanthes kunthiana (Neelakurinji): Flowers once in 12 years.
Events in Sexual Reproduction
Sexual reproduction involves three broad events:
A. Pre-Fertilisation Events
These include all events that occur before fertilisation:
1. Gametogenesis
Formation of gametes by meiosis (mostly) or mitosis.
- Male gamete (antherozoid / sperm): Usually motile with flagella. Produced in large numbers.
- Female gamete (egg / ovum): Usually non-motile (exception: in algae and fungi, both gametes may be motile — isogametes).
| Type | Description | Example |
|---|---|---|
| Isogamy | Both gametes are morphologically similar | Cladophora (alga), Mucor |
| Anisogamy | Gametes are dissimilar in size/morphology | Frogs, Humans |
| Oogamy | Large non-motile egg + small motile sperm | Humans, Volvox, Humans |
2. Gamete Transfer
For fertilisation to occur, the male gamete must be brought to the female gamete.
- In plants: Pollination (transfer of pollen grains from anther to stigma) brings male gametes to the female reproductive organ.
- In algae and bryophytes: Male gametes (antherozoids) swim through water (require external water medium).
- In most animals: Through insemination (internal transfer) or via water (in amphibians, fish).
- In many organisms, male gametes are produced in enormous numbers to compensate for the low probability of reaching the female gamete.
B. Fertilisation (Syngamy)
Fertilisation is the fusion of male and female gametes to form a diploid zygote. This is the most critical event in sexual reproduction.
🌊 External Fertilisation
- Gametes fuse outside the body of organisms
- Requires external water medium
- Enormous number of gametes produced
- Examples: Most algae, bony fish, frogs, amphibians
- Offspring are vulnerable to predation
🔒 Internal Fertilisation
- Gametes fuse inside the body of the female
- Male gametes are transferred to female body
- Fewer gametes needed but offspring better protected
- Examples: Reptiles, birds, mammals (including humans), most insects
- Higher success rate per gamete
Special Case: Parthenogenesis
Development of a new individual from an unfertilised egg.
- Apis mellifera (Honey bee): Drone (male) develops from unfertilised eggs; queens and workers from fertilised eggs.
- Rotifers, some lizards, and turkeys also show parthenogenesis.
- Produces haploid or diploid offspring depending on species.
C. Post-Fertilisation Events
1. Zygote Formation
The zygote is the first cell of a new organism. It is diploid (2n) formed by fusion of two haploid (n) gametes.
- In organisms with external fertilisation (e.g., frogs) — zygote is formed in water
- In plants — zygote is formed inside the ovule
- The zygote undergoes dormancy in many organisms before development starts
2. Embryogenesis
The development of embryo from the zygote through repeated mitotic cell divisions and cell differentiation.
Zygote (2n)
Formed after fertilisation — starting point of embryo development.
Cleavage
Rapid mitotic divisions of zygote into a ball of cells (blastomeres) — cell number increases but overall size does not increase.
Blastula / Blastocyst (Animals)
Hollow ball of cells formed after cleavage. In plants, equivalent stage is pro-embryo.
Gastrulation
Cell movements lead to the formation of three germ layers — ectoderm, mesoderm, endoderm. This is the basis for all organs.
Organogenesis
Differentiation of germ layers into specific organs — heart, kidney, liver, brain, etc.
Oviparous vs. Viviparous
🥚 Oviparous
- Embryo develops inside an egg laid outside the body
- Young ones hatch from eggs
- Egg has stored food (yolk)
- Examples: All birds, reptiles, most fish, insects
👶 Viviparous
- Embryo develops inside the mother's uterus
- Young ones are born directly
- Better parental care; higher survival rate
- Examples: Mammals, including humans; some sharks
Important Diagrams
Fig 1. Binary Fission in Amoeba — Parent cell divides into two identical daughter cells
Fig 2. Budding in Hydra — An outgrowth (bud) arises, grows tentacles, and detaches to form a new organism
Fig 3. Sequence of events in sexual reproduction — from gametogenesis to embryogenesis
Fig 4. Types of gamete fusion — Isogamy, Anisogamy, and Oogamy
NCERT Exercise Solutions
Neither is universally "better" — each is advantageous in different contexts:
- Asexual reproduction is better when environmental conditions are stable and favourable. It is faster, requires only one parent, and efficiently produces large numbers of offspring (all of which are capable of reproduction).
- Sexual reproduction is better in changing/challenging environments. The genetic variation it produces enables natural selection, adaptation, and evolution. Offspring with new gene combinations may be better suited to survive environmental changes.
From an evolutionary perspective, sexual reproduction is considered a better long-term strategy because it drives evolution and adaptability.
Vegetative propagation is a type of asexual reproduction in plants where new individuals develop from vegetative parts of the plant — such as roots, stems, and leaves — without the involvement of seeds or gametes.
Examples:
- Potato (Solanum tuberosum): New plants grow from the "eyes" (axillary buds) of potato tubers.
- Bryophyllum: Adventitious buds develop from the notches on the leaf margins, and each bud grows into a new plant.
(a) Juvenile phase: The phase of growth that an organism goes through before attaining reproductive maturity. Also called the vegetative phase in plants. No reproduction occurs during this phase.
(b) Reproductive phase: The phase during which an organism is capable of reproduction. It is marked by the commencement of reproductive events. In plants, it is marked by the appearance of flowers; in animals, by puberty.
(c) Senescent phase: The phase of deterioration that follows the reproductive phase. Metabolic activities slow down, aging progresses, and the organism eventually dies.
- Meiosis — leads to recombination of genetic material
- Random assortment of chromosomes
- Crossing over between homologous chromosomes
This variation enables populations to adapt to changing environments, survive disease and predation, and drives evolution. Without sexual reproduction, populations would be genetically uniform (clones), making them highly vulnerable to extinction from a single disease or environmental change.
Meiosis and gametogenesis are interlinked because gametes must be haploid (n) to ensure that fertilisation produces a diploid (2n) offspring with the correct chromosome number.
Gametogenesis — the process of gamete formation — requires meiosis to halve the chromosome number. If gametes were formed by mitosis, they would be diploid (2n), and after fertilisation the zygote would be tetraploid (4n). Over generations, this would double the chromosome number each time, which is biologically unsustainable.
Therefore, gametogenesis always involves meiotic division to produce haploid gametes, so fertilisation restores the species-specific diploid number.
| Part | Ploidy | Reason |
|---|---|---|
| (a) Ovary | Diploid (2n) | Part of the sporophyte plant body |
| (b) Anther | Diploid (2n) | Part of the sporophyte plant body |
| (c) Egg | Haploid (n) | Female gamete formed by meiosis |
| (d) Pollen | Haploid (n) | Formed from microspore mother cells by meiosis |
| (e) Male gamete | Haploid (n) | Formed within the pollen grain |
| (f) Zygote | Diploid (2n) | Formed by fusion of egg (n) + male gamete (n) |
Parthenogenesis is the development of a new individual from an unfertilised egg (ovum) without fertilisation by a male gamete.
Examples:
- Honey bee (Apis mellifera): Drones (male honey bees) develop from unfertilised eggs by parthenogenesis (they are haploid). Queens and worker bees develop from fertilised eggs.
- Rotifers, some lizards (e.g., Whiptail lizard), Turkey: Also show parthenogenesis where females produce offspring without fertilisation.
Oviparous organisms lay eggs. The embryo develops outside the mother's body. The eggs usually contain stored food (yolk). Examples: birds, reptiles, frogs, most fish, insects, platypus.
Viviparous organisms give birth to young ones directly. The embryo develops inside the mother's uterus and receives nutrition from the mother via the placenta. Examples: humans, cows, horses, dogs, bats. Some sharks and snakes are also viviparous.
- (a) Pollen grains represent the male gametophyte generation.
- (b) A gamete is haploid.
- (c) A zygote is formed by the fusion (syngamy/fertilisation) of two gametes.
Multiple Choice Questions (MCQs)
Which of the following is NOT an asexual mode of reproduction?
Gemmules are the reproductive structures of:
Which of the following plants reproduces by the formation of adventitious buds in the leaf notches?
In honey bees, drones (males) are produced from:
The process of development of the embryo from the zygote is called:
Which of the following is an example of isogamy?
The term "Terror of Bengal" refers to:
Which of the following plants flowers only once in 50–100 years?
The ploidy level of the zygote formed after fertilisation in a diploid organism is:
Conidia are the asexual reproductive structures of:
Which of the following represents external fertilisation?
Strobilanthes kunthiana (Neelakurinji) blooms once every:
Monoecious plants bear:
Zoospores are found in:
Plasmodium reproduces asexually by:
Vegetative propagation through "eyes" is seen in:
Earthworms are:
The menstrual cycle is characteristic of:
Which of these is viviparous?
Asexual reproduction that involves the formation of internal hard-coated buds in Porifera is called:
Short Answer Questions (2–3 Marks)
| Feature | Binary Fission | Multiple Fission |
|---|---|---|
| Division | 1 parent → 2 daughter cells | 1 parent → many daughter cells |
| Nucleus division | Divides once | Nucleus divides repeatedly |
| Conditions | Favourable conditions | Often under unfavourable conditions |
| Example | Amoeba, Paramecium | Plasmodium, Amoeba (encystment) |
Monoecious plants: Both male (staminate) and female (pistillate) flowers are present on the same individual plant. E.g., Maize (Zea mays), Coconut.
Dioecious plants: Male and female flowers are present on separate individual plants. E.g., Papaya (Carica papaya), Date palm.
Parthenogenesis is the development of a new individual from an unfertilised egg without fertilisation. It is a special type of reproduction intermediate between sexual and asexual reproduction.
Significance: Allows reproduction even when mates are unavailable; some organisms use it to control the sex ratio.
Examples:
- Apis mellifera (Honey bee): Drones (haploid males) from unfertilised eggs; queens from fertilised eggs.
- Rotifers, some lizard species (Whiptail lizard), and turkeys also reproduce by parthenogenesis.
A clone is an individual that is morphologically and genetically identical to its parent. Clones are produced in asexual reproduction, where a single parent gives rise to offspring without the fusion of gametes. Since there is no recombination of genetic material, all offspring are exact copies of the parent.
- Binary fission: Single parent divides into two. E.g., Amoeba.
- Budding: A small outgrowth (bud) forms on the parent, detaches, and grows into a new individual. E.g., Hydra.
- Sporulation: Spores are produced in sporangia; on germination, they form new organisms. E.g., Rhizopus.
- Fragmentation: The organism breaks into fragments and each grows into a new individual. E.g., Spirogyra.
Long Answer Questions (5 Marks)
Sexual reproduction involves three broad sets of events:
1. Pre-Fertilisation Events:
- Gametogenesis: Formation of haploid male and female gametes by meiosis. Male gametes (sperm/antherozoids) are usually motile; female gametes (eggs/ovum) are non-motile and larger.
- Gamete transfer: Transfer of male gametes to the female gamete. In plants, this occurs through pollination; in animals, through insemination or external release into water.
2. Fertilisation (Syngamy):
- Fusion of male (n) and female (n) gametes to form a diploid (2n) zygote.
- External fertilisation occurs in water (frogs, fish); internal fertilisation occurs inside the female body (birds, mammals, reptiles).
3. Post-Fertilisation Events:
- Zygote formation: Diploid cell, starting point of the new individual.
- Embryogenesis: Development of the embryo from the zygote by repeated mitosis and cell differentiation, leading to gastrulation and organogenesis.
- Organisms are either oviparous (develop in eggs outside body) or viviparous (develop inside the mother's uterus).
Evolutionary Significance: Sexual reproduction generates genetic variation through meiosis (crossing over + independent assortment) and random fertilisation. This variation is the raw material for natural selection, driving evolution and enabling species to adapt to changing environments. Unlike asexual reproduction, sexual reproduction prevents accumulation of harmful mutations across generations.
Asexual reproduction involves a single parent, produces clones, and does not involve gametes.
- Binary Fission: Amoeba (irregular plane), Paramecium (transverse fission), Bacteria. Simple and fast.
- Multiple Fission: Plasmodium — under unfavourable conditions, the nucleus divides multiple times and each nucleus with cytoplasm becomes a daughter cell (merozoite).
- Budding: In Yeast (single-celled), a daughter bud pinches off. In Hydra (multicellular), a bud grows tentacles and detaches.
- Fragmentation: Spirogyra breaks into pieces; each piece grows into a new filament. Planaria also regenerates from fragments.
- Sporulation: Rhizopus (bread mould) produces non-motile spores in sporangia. Chlamydomonas (alga) produces motile zoospores.
- Vegetative propagation (Plants): Ginger (rhizome), Potato (stem tuber with eyes), Onion (bulb), Bryophyllum (leaf buds), Water hyacinth (offset).
- Gemmule formation: In sponges, internal protected buds (gemmules) survive harsh conditions and germinate later.
- Conidia: In Penicillium — non-motile spores formed exogenously at conidiophore tips.
Higher Order Thinking Skills (HOTS)
If gametes were produced by mitosis, they would be diploid (2n) instead of haploid (n). After fertilisation: 2n + 2n = 4n (tetraploid zygote). After the next generation: 4n + 4n = 8n. The chromosome number would double with each generation. Within a few generations, the organism would have an unmanageably large number of chromosomes, which would disrupt gene expression, cell division, and ultimately prove lethal to the species. This is why meiosis is essential for gametogenesis — it halves the chromosome number to maintain a constant species-specific chromosome count across generations.
Asexually reproducing organisms can produce massive numbers of offspring because:
- Only one parent is needed — no energy is spent on finding a mate or courtship.
- Reproduction is faster — bacterial binary fission can occur every 20 minutes.
- All offspring are clones and each can immediately reproduce again.
- Under favourable conditions, the population can expand exponentially.
However, this large number creates a genetically uniform population that is collectively vulnerable to any single pathogen or environmental change — the evolutionary "Achilles heel" of asexual reproduction.
Bamboo's strategy is called semelparous reproduction (big-bang reproduction). Benefits include:
- Mast seeding: All bamboo plants of a species flower simultaneously, producing an enormous quantity of seeds. This overwhelms seed predators (rodents, birds) — enough seeds survive to ensure successful reproduction.
- Resource concentration: All the plant's accumulated energy over decades is channelled into a single massive reproductive event, maximising reproductive success.
- Synchronous flowering ensures cross-pollination between individuals, promoting genetic diversity in offspring.
The death of the parent plant after seeding actually benefits the offspring by reducing competition for light, water, and nutrients in the same habitat.
Advantages of vegetative propagation:
- Preserves desirable traits exactly — seedless fruits (banana, seedless grapes) can only be propagated vegetatively.
- Faster than growing from seed — new plants mature earlier.
- Does not require pollination or seed germination.
- Allows propagation of plants that produce few or no seeds.
Disadvantages:
- No genetic variation — all offspring are clones; vulnerability to the same diseases/pests.
- Rapid spread of vegetative weeds like Eichhornia — can devastate ecosystems.
- Limited geographic spread compared to seed dispersal.
Previous Year Board Exam Questions
- Plants flower and fruit earlier (e.g., banana, sugarcane).
- Useful in plants that have lost the capacity to produce seeds (e.g., seedless banana, seedless grapes).
- Preserves identical/desirable traits — genetically identical offspring (clones).
- Can be used for rapid and large-scale propagation.
| Feature | Juvenile Phase | Reproductive Phase |
|---|---|---|
| Definition | Growth phase before reproductive maturity | Phase when organism can reproduce |
| Reproduction | Absent | Active |
| In plants | Vegetative phase | Flowering phase |
| In animals | Before puberty | After puberty |
Most Expected Board Exam Questions 2024–25
⭐ Very High Probability
1. Explain vegetative propagation with 3 examples.
2. Distinguish asexual from sexual reproduction (tabular).
3. What is parthenogenesis? Examples and significance.
4. Describe post-fertilisation events.
🔮 High Probability
5. What is embryogenesis?
6. Oviparous vs viviparous — examples.
7. What is syngamy? How does it ensure diploid offspring?
8. Why is meiosis essential in gametogenesis?
💡 Moderate Probability
9. Name and explain special asexual structures (gemmules, conidia, zoospores).
10. Explain binary and multiple fission.
11. Semelparous organisms — examples.
12. External vs internal fertilisation.
Expected 1-Mark Questions
- Name the type of asexual reproduction in Amoeba. → Binary fission
- What are clones? → Genetically identical individuals from asexual reproduction
- Give an example of an organism showing external fertilisation. → Frog
- Name the asexual structures formed in Penicillium. → Conidia
- What is the ploidy of a zygote? → Diploid (2n)
- Name a semelparous plant. → Bamboo
- What does dioecious mean? → Male and female flowers on separate plants
- Give one example of viviparous animal. → Humans / Cows
⚡ Quick Revision Notes
The most important points for last-minute revision before your exam:
🔬 Asexual Reproduction
Single parent · No gametes · Clones produced · Binary fission (Amoeba), Multiple fission (Plasmodium), Budding (Yeast, Hydra), Fragmentation (Spirogyra), Sporulation (Rhizopus), Vegetative propagation (plants)
🌸 Sexual Reproduction
Two parents (usually) · Gametes formed · Genetic variation · Three events: pre-fertilisation, fertilisation (syngamy), post-fertilisation
🧬 Gametogenesis
Gametes are haploid (n) · Formed by meiosis · Male = sperm/antherozoid · Female = ovum/egg · Types:
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