9.4 Sexual Reproduction in Plants

Short Introduction Sexual reproduction in plants, like in animals, involves the fertilization of gametes (egg and sperm) and the formation of a zygote and embryo. Different groups of plants, like bryophytes, pteridophytes, and seed plants (spermatophytes), have different mechanisms of gamete formation and fertilization. In flowering plants (angiosperms), the major plant body is the sporophyte, which may be a herb, shrub, or tree. Flowers are the reproductive parts of these plants.

Main Idea

Sexual reproduction in flowering plants involves the flower, which contains male and female reproductive structures. The male parts (stamens) produce pollen grains (microspores), and the female parts (carpels/pistils) contain ovules (megaspores). Pollination is the transfer of pollen from anther to stigma. Fertilization involves the fusion of a sperm nucleus with the egg to form a zygote, and another sperm nucleus with polar nuclei to form the endosperm nucleus (double fertilization). The ovule develops into a seed, and the ovary develops into a fruit. Seed germination is the process by which the embryo grows into a new plant.

Core Concept

Sexual Reproduction in Flowering Plants (Angiosperms) In flowering plants, the major plant body is the sporophyte (herb, shrub, or tree). Flowers are the reproductive parts of these plants. Structure of a Flower A flower is a part of the plant that has been changed and adapted for the job of reproduction. Pedicel: The stalk that supports the flower. Thalamus: The swollen part at the top of the pedicel, like the flower's platform. The thalamus possesses four kinds of parts called floral leaves, arranged in whorls (circles) on its upper surface: generated by DeepSeek to PDF

1. Calyx: The outermost circle, made up of green leaves called sepals. It acts as a protective cover for the inside parts of the flower. 2. Corolla: Just inside the calyx, this circle consists of variously colored floral leaves called petals. 3. Androecium: The male parts of the flower, called stamens. These are important for making pollen. 4. Gynoecium: The female parts, located in the center. This is where the female parts are, in something called the carpel or pistil. This is where new seeds start to form. The calyx and corolla are called accessory whorls (they protect the flower but do not take part in reproduction directly). The androecium and gynoecium are the necessary whorls (they are the ones that make reproduction happen). Formation of Microspores and Megaspores Microspores (Male): Each stamen consists of a long stalk, the filament, and a bilobed swollen tip called the anther. In the anther lobes, four pollen sacs (microsporangia) are present that produce haploid pollen grains (microspores) by meiosis. Megaspores (Female): Each carpel or pistil consists of a sac-like structure at the bottom called the ovary, a long tubular structure above called the style, and a swollen tip on the style called the stigma. The ovary contains one to numerous ovules (megasporangia). Each ovule possesses one spore mother cell that undergoes meiosis to produce four megaspores. Out of four, only one is capable of surviving further and becomes the functional megaspore. Pollination When microspores are formed, the wall of pollen sacs bursts, and the microspores are liberated. The microspores are transferred from the anther of the stamen to the stigma of the carpel by means of wind, insects, or other animals. This event is known as pollination, and the microspores during pollination are called pollen grains. If pollination occurs within the same flower or between two flowers of the same plant, it is called self- pollination. If it occurs between two flowers of separate plants, it is called cross-pollination, which ensures more genetic variability in coming generations. Development of Male and Female Gametophytes Development of Male Gametophyte: After pollination, the microspores (pollen grains) germinate on the surface of the stigma to produce a male gametophyte. The microspore nucleus first divides by mitosis to form two nuclei: the tube nucleus and the generative nucleus. The generative nucleus divides further to form two sperm nuclei. The tube nucleus controls the formation of a pollen tube which the microspore passes down through the style and ultimately penetrates the ovule. This whole structure (microspore body, pollen tube, tube nucleus, and two sperm nuclei) is referred to as the male gametophyte. generated by DeepSeek to PDF

Development of Female Gametophyte: The nucleus of the functional megaspore in the center of the ovule divides three times by mitosis to make 8 nuclei. At this stage: Three nuclei migrate to one end of the ovule; the central one becomes the egg, and the two side ones are called synergids. Three other nuclei migrate to the other end and act as antipodals. The remaining two daughter nuclei rest in the center of the ovule and are called polar nuclei, which in some cases fuse together to become a diploid secondary nucleus before the entry of sperm. This whole structure (body of megaspore, three antipodals, two synergids, two polar nuclei or secondary nucleus, and an egg) is referred to as the female gametophyte or embryo sac. Fertilization and Formation of Zygote and Endosperm Nucleus When the pollen tube penetrates the ovule, one sperm nucleus fuses with the egg to form a diploid zygote. The other sperm nucleus fuses with the polar nuclei or secondary nucleus to make a triploid endosperm nucleus. Since one male gametophyte contributes two sperm nuclei for fertilization, this event is called double fertilization. It is assumed that the rest of the nuclei in the embryo sac (synergids and antipodals) control the process of fertilization. Seed and Fruit Formation After fertilization, the ovule is transformed into a seed (hence defined as "ripened ovule"). The surrounding wall of the ovary is transformed into fruit (hence defined as "ripened ovary"). Parts of a Seed: 1. Embryo: Comes from the union of male and female cells (zygote). It is like a baby plant not fully grown. It consists of a radicle (embryonic root), a plumule (embryonic shoot), and one or two cotyledons (seed leaves). The radicle grows into a new root. The plumule grows into a new shoot. The embryonic stem above the point of attachment of the cotyledon(s) is called the epicotyl. The embryonic stem below the point of attachment is the hypocotyl. 2. Endosperm: A storage space for food, derived from the endosperm nucleus. It is rich in oil or starch and protein. In many seeds, the food of the endosperm is absorbed and stored by cotyledons. 3. Seed Coat: The seed's protective jacket, developed from the outer part that used to wrap around the seed. It can be thin like paper (peanuts) or thick and tough (coconuts). It keeps the baby plant safe and stops it from drying out. Hilum: A little mark on the seed coat where the seed was connected to the plant. Micropyle: A small opening near the hilum that the seed uses to get water. Seed Germination For seeds to start growing, they need the right conditions. This process is called seed germination. The seed absorbs water, which makes it swell and breaks open the seed coat. generated by DeepSeek to PDF

First, a root pops out from the radicle. It grows quickly and starts taking in water and nutrients from the soil. Then, a tiny shoot called the plumule begins to grow. It stretches up and out of the soil. Types of Germination: 1. Epigeal Germination: The hypocotyl stretches out and forms a hook, pulling the seed leaves (cotyledons) above the ground. Examples: beans, cotton, papaya. 2. Hypogeal Germination: The epicotyl stretches and forms a hook, but the seed leaves (cotyledons) stay underground. Examples: peas, maize, coconuts. Conditions for Seed Germination: Internal: The seed must have a living embryo and enough stored food. External: 1. Water (Moisture): Seeds need water to break down stored food and help parts of the baby plant grow. 2. Oxygen: Cells in the baby plant need oxygen for respiration. 3. Temperature: Most plants germinate best between 25–30°C.

Detailed Explanation

Sexual reproduction in flowering plants is a complex process involving specialized structures and precise timing. 1. Flower Structure: The flower is the reproductive organ of angiosperms. It consists of four whorls: calyx (sepals), corolla (petals), androecium (stamens), and gynoecium (carpels). The calyx and corolla are accessory whorls; the androecium and gynoecium are necessary whorls. 2. Microsporogenesis and Megasporogenesis: In the anther, microspore mother cells undergo meiosis to produce microspores (pollen grains). In the ovule, the megaspore mother cell undergoes meiosis to produce four megaspores, one of which survives as the functional megaspore. 3. Pollination: Pollen grains are transferred from the anther to the stigma by wind, insects, or other animals. Self-pollination occurs within the same flower or plant; cross-pollination occurs between different plants and promotes genetic diversity. 4. Gametophyte Development: The pollen grain germinates on the stigma, forming a pollen tube that carries two sperm nuclei. The functional megaspore undergoes mitosis to form the embryo sac, which contains the egg, synergids, antipodals, and polar nuclei. 5. Double Fertilization: One sperm nucleus fuses with the egg to form the zygote (2n). The other sperm nucleus fuses with the polar nuclei/secondary nucleus to form the endosperm nucleus (3n). This is a unique feature of angiosperms. 6. Seed and Fruit Formation: The ovule becomes the seed, containing the embryo, endosperm, and seed coat. The ovary becomes the fruit. 7. Seed Germination: The seed germinates when conditions are suitable (water, oxygen, appropriate temperature). The radicle emerges first, then the plumule. Germination can be epigeal (cotyledons above ground) or hypogeal (cotyledons remain underground).

Intermediate Level

Sexual reproduction in flowering plants involves the flower, which has four whorls: calyx (sepals), corolla (petals), androecium (stamens), and gynoecium (carpels). The anther produces microspores (pollen) by meiosis; the ovule produces megaspores by meiosis. Pollination is the transfer of pollen from anther to stigma (self or cross). The pollen grain germinates to form a male gametophyte with two sperm nuclei. The functional megaspore undergoes mitosis to form the female gametophyte (embryo sac) with 8 nuclei (egg, synergids, antipodals, polar nuclei). Double fertilization occurs: one sperm fuses with the egg (zygote, 2n), the other with polar nuclei (endosperm, 3n). The ovule becomes the seed (embryo + endosperm + seed coat), and the ovary becomes the fruit. Seed germination requires water, oxygen, and suitable temperature (25–30°C). Epigeal germination: cotyledons above ground (beans); hypogeal: cotyledons underground (peas).

Important Points

• 1. In flowering plants, the major plant body is the sporophyte.
• 2. Flowers are the reproductive parts.
• 3. Structure of a flower:
• Pedicel: Stalk.
• Thalamus: Swollen platform.
• Calyx (sepals): Outermost protective whorl (accessory).
• Corolla (petals): Colored whorl (accessory).
• Androecium (stamens): Male parts (necessary).
• Gynoecium (carpels/pistils): Female parts (necessary).
• 4. Microspores (male): Produced in pollen sacs (microsporangia) of the anther by meiosis.
• generated by DeepSeek to PDF
• 5. Megaspores (female): Produced in ovules (megasporangia) of the ovary by meiosis; only one functional
• megaspore survives.
• 6. Pollination: Transfer of pollen from anther to stigma by wind, insects, or other animals.
• Self-pollination: Within the same flower or same plant.
• Cross-pollination: Between different plants; ensures genetic variability.
• 7. Male gametophyte: Pollen grain germinates on stigma; microspore nucleus divides by mitosis to form
• tube nucleus and generative nucleus; generative nucleus divides to form two sperm nuclei; tube nucleus
• controls pollen tube formation.
• 8. Female gametophyte (embryo sac): Functional megaspore nucleus divides three times by mitosis to
• form 8 nuclei: 3 antipodals, 2 synergids, 2 polar nuclei (or secondary nucleus), and 1 egg.
• 9. Double fertilization: One sperm fuses with egg → zygote (2n); other sperm fuses with polar
• nuclei/secondary nucleus → endosperm nucleus (3n).
• 10. Seed: Ripened ovule; contains embryo (radicle, plumule, cotyledons), endosperm, and seed coat.
• 11. Fruit: Ripened ovary.
• 12. Seed germination: Requires water, oxygen, and temperature (25–30°C).
• Epigeal: Hypocotyl elongates, cotyledons above ground (beans, cotton, papaya).
• Hypogeal: Epicotyl elongates, cotyledons underground (peas, maize, coconuts).

Remember

• Flower whorls: Calyx (sepals), Corolla (petals), Androecium (stamens), Gynoecium (carpels).
• Accessory whorls: Calyx + Corolla. Necessary whorls: Androecium + Gynoecium.
• Microspores = pollen (anther); Megaspores = ovule (ovary).
• Pollination = pollen to stigma (self vs. cross).
• Male gametophyte = pollen tube + tube nucleus + 2 sperm nuclei.
• Female gametophyte = embryo sac (egg, synergids, antipodals, polar nuclei).
• Double fertilization: Egg + sperm → zygote (2n); Polar nuclei + sperm → endosperm (3n).
• Seed = ovule; Fruit = ovary.
• Germination: Water, oxygen, temperature (25–30°C).
• Epigeal = cotyledons above ground; Hypogeal = cotyledons underground.

Common Mistakes

• 1. Confusing accessory and necessary whorls: Calyx and corolla are accessory; androecium and
• gynoecium are necessary.
• 2. Thinking pollen grains are male gametes: Pollen grains are microspores that develop into the male
• gametophyte, which produces sperm.
• 3. Forgetting that only one megaspore survives: Four megaspores are produced by meiosis, but only one
• becomes functional.
• 4. Confusing self-pollination and cross-pollination: Self-pollination is within the same flower/plant; cross-
• pollination is between different plants.
• 5. Thinking fertilization and pollination are the same: Pollination is the transfer of pollen; fertilization is
• the fusion of gametes.
• 6. Forgetting double fertilization is unique to angiosperms: One sperm fertilizes the egg; the other
• fertilizes the polar nuclei.
• 7. Mixing up epigeal and hypogeal germination: Epigeal = cotyledons above ground (hypocotyl
• elongates); hypogeal = cotyledons underground (epicotyl elongates).
• generated by DeepSeek to PDF

Quick Revision

• Sexual reproduction in flowering plants involves the flower, which has four whorls: calyx (sepals), corolla (petals),
• androecium (stamens), and gynoecium (carpels). The anther produces microspores (pollen) by meiosis; the ovule
• produces megaspores by meiosis, with only one functional megaspore surviving. Pollination is the transfer of pollen
• from anther to stigma (self or cross). The pollen grain germinates to form a male gametophyte with two sperm
• nuclei. The functional megaspore undergoes mitosis to form the female gametophyte (embryo sac) with 8 nuclei
• (egg, synergids, antipodals, polar nuclei). Double fertilization occurs: one sperm fuses with the egg (zygote, 2n), the
• other with polar nuclei (endosperm, 3n). The ovule becomes the seed (embryo + endosperm + seed coat), and the
• ovary becomes the fruit. Seed germination requires water, oxygen, and suitable temperature (25–30°C). Epigeal
• germination: cotyledons above ground (beans); hypogeal: cotyledons underground (peas).

60-Second Revision

• Flower whorls: Calyx (sepals), Corolla (petals), Androecium (stamens), Gynoecium (carpels).
• Accessory whorls: Calyx + Corolla. Necessary whorls: Androecium + Gynoecium.
• Microspores = pollen (anther); Megaspores = ovule (ovary).
• Pollination = pollen to stigma (self vs. cross).
• Male gametophyte = pollen tube + tube nucleus + 2 sperm nuclei.
• Female gametophyte = embryo sac (egg, synergids, antipodals, polar nuclei).
• Double fertilization: Egg + sperm → zygote (2n); Polar nuclei + sperm → endosperm (3n).
• Seed = ovule; Fruit = ovary.
• Germination: Water, oxygen, temperature (25–30°C).
• Epigeal = cotyledons above ground; Hypogeal = cotyledons underground.
• Frequently Asked Questions (FAQs)
• Q1. What are the four whorls of a flower? Answer: The four whorls are the calyx (sepals), corolla (petals),
• androecium (stamens), and gynoecium (carpels/pistils).
• Q2. What are accessory and necessary whorls? Answer: Accessory whorls are the calyx and corolla (they protect
• the flower but do not take part in reproduction directly). Necessary whorls are the androecium and gynoecium (they
• are the reproductive parts).
• generated by DeepSeek to PDF

• Q3. How are microspores and megaspores formed? Answer: Microspores (pollen grains) are produced in the
• pollen sacs (microsporangia) of the anther by meiosis. Megaspores are produced in the ovules (megasporangia) of
• the ovary by meiosis. Out of four megaspores, only one survives as the functional megaspore.
• Q4. What is pollination? Answer: Pollination is the transfer of microspores (pollen grains) from the anther of the
• stamen to the stigma of the carpel by wind, insects, or other animals.
• Q5. What is the difference between self-pollination and cross-pollination? Answer: Self-pollination occurs
• within the same flower or between two flowers of the same plant. Cross-pollination occurs between two flowers of
• separate plants and ensures more genetic variability.
• Q6. Describe the development of the male gametophyte. Answer: The microspore nucleus divides by mitosis to
• form a tube nucleus and a generative nucleus. The generative nucleus divides to form two sperm nuclei. The tube
• nucleus controls the formation of the pollen tube. The whole structure (microspore body, pollen tube, tube nucleus,
• and two sperm nuclei) is the male gametophyte.
• Q7. Describe the development of the female gametophyte. Answer: The functional megaspore nucleus divides
• three times by mitosis to form 8 nuclei. Three nuclei migrate to one end (one becomes the egg, two become
• synergids). Three migrate to the other end (antipodals). Two remain in the center (polar nuclei, which may fuse to
• form a secondary nucleus). This whole structure is the female gametophyte or embryo sac.
• Q8. What is double fertilization? Answer: Double fertilization is the fusion of one sperm nucleus with the egg to
• form a diploid zygote, and the fusion of the other sperm nucleus with the polar nuclei (or secondary nucleus) to
• form a triploid endosperm nucleus.
• Q9. What do the ovule and ovary become after fertilization? Answer: The ovule becomes the seed (ripened
• ovule), and the ovary becomes the fruit (ripened ovary).
• Q10. What are the conditions for seed germination? Answer: Internal conditions: a living embryo and enough
• stored food. External conditions: water (moisture), oxygen, and suitable temperature (25–30°C for most plants).
• Multiple Choice Questions (MCQs)
• Q1. The male reproductive part of a flower is the: A. Calyx B. Corolla C. Androecium D. Gynoecium Correct Answer: C
• Explanation: The androecium (stamens) is the male reproductive part.
• Q2. The female reproductive part of a flower is the: A. Calyx B. Corolla C. Androecium D. Gynoecium Correct Answer:
• D Explanation: The gynoecium (carpels) is the female reproductive part.
• Q3. Pollen grains are produced in the: A. Ovule B. Anther C. Stigma D. Style Correct Answer: B Explanation: Pollen
• grains are produced in the anther.
• Q4. Megaspores are produced in the: A. Anther B. Ovule C. Stigma D. Filament Correct Answer: B Explanation:
• Megaspores are produced in the ovule.
• Q5. The transfer of pollen from anther to stigma is called: A. Fertilization B. Pollination C. Germination D. Double
• fertilization Correct Answer: B Explanation: Pollination is the transfer of pollen from anther to stigma.
• Q6. The fusion of one sperm nucleus with the egg forms the: A. Endosperm nucleus B. Zygote C. Polar nuclei D.
• Synergids Correct Answer: B Explanation: The fusion of one sperm nucleus with the egg forms the zygote.
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• Q7. Double fertilization is unique to: A. Bryophytes B. Pteridophytes C. Angiosperms D. Gymnosperms Correct
• Answer: C Explanation: Double fertilization is unique to angiosperms (flowering plants).
• Q8. After fertilization, the ovule becomes the: A. Fruit B. Seed C. Flower D. Embryo Correct Answer: B Explanation:
• After fertilization, the ovule becomes the seed.
• Q9. In epigeal germination, the cotyledons are: A. Underground B. Above ground C. Absent D. Photosynthetic
• Correct Answer: B Explanation: In epigeal germination, the hypocotyl elongates and pulls the cotyledons above
• ground.
• Q10. The best temperature for most seeds to germinate is: A. 10–15°C B. 25–30°C C. 35–40°C D. 0–5°C Correct
• Answer: B Explanation: For most plants, the best temperature for germination is between 25–30°C.
• Chapter 9: Plant Reproduction is now fully complete (all 4 subtopics covered: 9.1 through 9.4).
• Reply "next" if you would like to begin Chapter 10: Evolution, starting with Subtopic 1: 10.1 Variation and
• Evolution.