Ch 01 - Sexual Reproduction in Flowering Plants12th Biology

The typical complete, bisexual, and hypogynous flower has the following arrangement:

• Calyx: Outermost whorl of sepals.
• Corolla: Whorl of petals inside the calyx.
• Androecium: Whorl of stamens (male reproductive organs).
• Gynoecium: Innermost whorl of pistils/carpels (female reproductive organs) at the center.

• Bilobed: The anther consists of two distinct lobes.
• Dithecous: Each lobe contains two theca (compartments), often separated by a longitudinal groove.
• Tetrasporangiate: Because there are 2 lobes with 2 theca each, the anther possesses four microsporangia located at its corners.

Outer Three Layers (Protection & Dehiscence):

• Epidermis: Outermost protective layer.
• Endothecium: Fibrous thickenings that contract and help in anther dehiscence.
• Middle Layers: Thin-walled layers aiding in protection and dehiscence.

Innermost Layer (Nutrition):

• Tapetum: Consists of large, often multinucleate cells with dense cytoplasm. It nourishes the developing pollen grains and provides precursors for the pollen wall.

1. Sporogenous Tissue (2n): Located at the center of the microsporangium.
2. Pollen Mother Cell (PMC) / Microspore Mother Cell (2n): Differentiated cells of the sporogenous tissue.
3. Meiosis: Each PMC undergoes meiotic division.
4. Microspore Tetrad (n): Results in a cluster of four haploid microspores.
5. Pollen Grains (n): As the anther matures and dehydrates, the tetrad dissociates into individual pollen grains (male gametophytes).

• Exine (Outer Layer):
  • Made of sporopollenin, one of the most resistant organic materials known (withstands high heat, strong acids/alkalis, and enzymes).
  • Allows pollen to be preserved as fossils.
  • Possesses prominent apertures called germ pores where sporopollenin is absent.
• Intine (Inner Layer):
  • A continuous, thin layer made of cellulose and pectin.
  • Emerges through the germ pore to form the pollen tube during germination.

Vegetative Cell:

• Size: Much larger.
• Structure: Has abundant food reserves and a large, irregularly shaped nucleus.
• Function: Responsible for developing the pollen tube.

Generative Cell:

• Size: Small and spindle-shaped.
• Structure: Floats within the cytoplasm of the vegetative cell.
• Function: Divides mitotically to give rise to the two male gametes.

• 2-Celled Stage (~60% of Angiosperms): Pollen is shed containing only the Vegetative Cell and Generative Cell.
• 3-Celled Stage (~40% of Angiosperms): The Generative Cell divides mitotically before shedding, so the pollen grain contains the Vegetative Cell and two male gametes.

When a gynoecium has more than one pistil (multicarpellary):

• Syncarpous: The carpels are fused together into a single structure.
  • Example: Papaver (Poppy).
• Apocarpous: The carpels remain free and separate from one another.
  • Example: Michelia.

1. Funicle: The stalk attaching the ovule to the placenta.
2. Hilum: The junction where the body of the ovule fuses with the funicle.
3. Integuments: One or two protective envelopes surrounding the ovule.
4. Micropyle: A small opening at the tip where integuments are absent (pollen tube entry).
5. Chalaza: The basal part of the ovule opposite the micropyle.
6. Nucellus: The central mass of cells enclosed by integuments, serving as a food reserve.
7. Embryo Sac (Female Gametophyte): Located inside the nucellus.

• Name: The Anatropous ovule.
• Structure: It is a completely inverted ovule. It turns 180 degrees during development so that the micropyle comes to lie close to the funicle (the stalk) and the hilum.

• Location: It generally differentiates in the micropylar region of the nucellus. It is a large cell with dense cytoplasm and a prominent nucleus.
• Fate: The diploid MMC (2n) undergoes meiosis to produce a linear tetrad of four haploid megaspores (n).

• Out of the four haploid megaspores formed during megasporogenesis, three degenerate.
• Only one functional megaspore survives and develops into the entire female gametophyte (embryo sac).
• Because the embryo sac originates from a single spore, it is termed monosporic development.

During the development of the functional megaspore into the embryo sac, the nucleus undergoes three sequential mitotic divisions (creating 2, 4, and then 8 nuclei).
These divisions are not immediately followed by cytokinesis (cell wall formation).
The 8 nuclei float freely in the cytoplasm until the final stage, when cell walls are finally laid down to organize the cells.

• Micropylar End (3 cells): Forms the Egg Apparatus, comprising two synergids and one egg cell.
• Chalazal End (3 cells): Forms the three Antipodal cells.
• Center (1 large cell): The Central Cell, which contains the remaining two polar nuclei.
  Result: 7 distinct cells containing a total of 8 nuclei.

• Structure: Synergids feature special cellular thickenings at their micropylar tip known as the filiform apparatus.
• Role: They act as chemical beacons that guide the entry of the pollen tube into the synergid, facilitating the successful discharge of male gametes for double fertilization.

• Chasmogamous Flowers: Have exposed anthers and stigmas (open flowers). They permit both self and cross-pollination.
• Cleistogamous Flowers: Never open. Anthers and stigmas lie very close together, forcing 100% autogamy (self-pollination).
  • Advantage: They produce an assured seed-set even in the complete absence of pollinators.
  • Examples: Viola (common pansy), Oxalis, Commelina.

Geitonogamy:

• Transfer of pollen to another flower on the same plant.
• Functionally cross-pollination because it requires a pollinating agent.
• Genetically self-pollination because the pollen comes from the same maternal genome.

Xenogamy:

• Transfer of pollen to the stigma of a different plant.
• This is the only type of pollination that brings true genetic variation to the offspring.

1. Pollen: Produced in enormous amounts; light and non-sticky for easy transport.
2. Stamens: Well-exposed so pollen disperses easily into wind currents.
3. Stigma: Large and feathery (e.g., corn tassels) to trap airborne pollen.
4. Ovary: Often contains a single ovule, with numerous flowers packed into an inflorescence.

• Vallisneria (Freshwater): The female flower has a long stalk that reaches the water surface. Male flowers release pollen onto the surface, which is carried passively by water currents to the stigma.
• Zostera (Marine Seagrass): Female flowers remain submerged. Pollen grains are elongated (ribbon-like) and lack an exine, floating passively entirely underwater to reach the stigma.

No, not all aquatic plants use water for pollination.
In many aquatic plants, the flowers emerge above the level of the water and are pollinated by insects or wind, exactly like most terrestrial plants.
Classic Exceptions:

• Water hyacinth
• Water lily

The Yucca & Moth Relationship:

• Neither species can complete its life cycle without the other.
• The moth deposits its eggs inside the locule of the ovary.
• In return, the moth pollinates the flower.
• Moth larvae emerge precisely when the seeds start developing.

Amorphophallus:

• It has the tallest flower (about 6 feet high).
• Its floral reward is providing a safe place to lay eggs for its specific pollinators.

1. Dichogamy: Asynchrony in maturation (pollen release and stigma receptivity happen at different times).
2. Spatial Separation: Anther and stigma are placed at different physical positions preventing contact.
3. Self-incompatibility: A genetic mechanism that chemically rejects self-pollen, preventing tube growth.
4. Dioecy: Producing strictly unisexual flowers on separate male and female plants (prevents both autogamy and geitonogamy).

• The triploid Primary Endosperm Cell (PEC) divides repeatedly to form endosperm tissue, which is densely packed with reserve food materials.
• The zygote lies dormant until some endosperm is formed.
• This is a critical adaptation to guarantee assured nutrition for the developing embryo as it grows.

• The pistil conducts a chemical dialogue with the pollen to determine compatibility.
• If compatible, it accepts the pollen, allowing hydration, germination, and pollen tube growth down the style.
• If incompatible (due to wrong species or self-incompatibility), it rejects the pollen by actively preventing germination or arresting the pollen tube's growth in the style.

• Emasculation: Removal of anthers from a bisexual flower bud before dehiscence to prevent unwanted self-pollination.
• Bagging: Covering the emasculated flower with butter paper to protect the stigma from contaminating stray pollen.
• Exception: Emasculation is unnecessary if the female parent produces unisexual (pistillate) flowers, though bagging is still required.

When the pollen tube releases two male gametes into the synergid:

1. Syngamy (True Fertilisation): One male gamete (n) fuses with the egg cell (n) to form the Zygote (2n).
2. Triple Fusion: The second male gamete (n) fuses with the two polar nuclei (n + n) in the Central Cell to form the Primary Endosperm Nucleus / PEN (3n).

Free-Nuclear Endosperm:

• The Primary Endosperm Nucleus undergoes successive karyokinesis (nuclear divisions) without cytokinesis.
• Example: The liquid 'coconut water' containing thousands of free nuclei.

Cellular Endosperm:

• Subsequently, cell walls are laid down from the periphery towards the center.
• Example: The solid white 'coconut meat/kernel'.

Most zygotes divide only after a certain amount of endosperm is formed. The zygote undergoes mitotic divisions to form the proembryo, which subsequently differentiates through the following stages:

1. Globular embryo (spherical mass of cells)
2. Heart-shaped embryo (bilateral symmetry sets in as cotyledons begin to form)
3. Mature embryo

A typical dicot embryo consists of an embryonal axis and two cotyledons:

• Epicotyl: The portion of the embryonal axis above the level of cotyledons, which terminates with the plumule (stem tip).
• Hypocotyl: The cylindrical portion below the level of cotyledons, which terminates at its lower end in the radicle (root tip).
• The root tip is covered by a protective root cap.

Monocot embryos possess only one cotyledon and have highly specialized sheaths:

• Scutellum: The single, large, shield-shaped cotyledon situated towards one side (lateral) of the embryonal axis.
• Coleoptile: A hollow foliar structure that encloses and protects the shoot apex and leaf primordia.
• Coleorrhiza: An undifferentiated sheath that encloses the radicle and root cap at the lower end.
• Epiblast: A small flap sometimes present, representing the rudiment of a second cotyledon.

Non-albuminous (Ex-albuminous) Seeds:

• The endosperm is completely consumed during embryo development before seed maturation.
• Examples: Pea, groundnut, beans.

Albuminous Seeds:

• They retain a part of the endosperm because it is not completely used up by the developing embryo.
• Examples: Wheat, maize, barley, castor, coconut.

Perisperm:

• The residual, persistent nucellus found in some seeds.
• Since it develops directly from maternal sporophytic tissue, it is diploid (2n).
• Examples: Black pepper, beet.

Endosperm:

• A nutritive tissue formed from the Primary Endosperm Nucleus (PEN) following the triple fusion of two polar nuclei and one male gamete.
• It is typically triploid (3n).

Maturation & Dormancy:

• As water content reduces (10-15% moisture by mass), the embryo's general metabolic activity slows down.
• The embryo enters a state of inactivity called dormancy to survive unfavorable conditions.

Role of Micropyle:

• The micropyle remains as a small pore in the hardened seed coat.
• It acts as the primary gateway for the entry of oxygen and water into the seed, which is critical for breaking dormancy and triggering germination.

• Polyembryony: The occurrence of more than one embryo in a single seed.
• Mechanism (Adventive Embryony): In plants like Citrus and Mango, some of the diploid maternal nucellar cells surrounding the embryo sac start actively dividing, protrude into the embryo sac, and directly develop into multiple embryos alongside the normal zygotic embryo.

1. Water Independence: Reproductive processes like pollination and fertilisation do not require water, making seed formation highly dependable.
2. Dispersal: Seeds possess adaptive mechanisms (e.g., wings, hooks, fleshy fruits) for dispersal to new habitats, helping the species colonise.
3. Nourishment: They contain sufficient food reserves (endosperm/cotyledons) to nourish seedlings until they can photosynthesize.
4. Protection: The hard seed coat (derived from integuments) protects the delicate embryo from harsh environments.
5. Genetic Variation: Being products of sexual reproduction, they generate new genetic combinations leading to variations.

• Lupinus arcticus (Arctic Lupine): Seeds excavated from the Arctic Tundra successfully germinated and flowered after an estimated 10,000 years of dormancy.
• Phoenix dactylifera (Date Palm): A viable seed discovered during an archaeological excavation at King Herod's palace near the Dead Sea germinated after 2,000 years.

Definition: The pericarp is the structural wall of the fruit (which can be dry or fleshy).
Origin: It differentiates directly from the wall of the ovary as the ovary ripens into a fruit post-fertilisation.

True Fruit: Develops exclusively from the ovary of the flower after fertilisation.
False Fruit: Develops when other floral parts, predominantly the thalamus, contribute to the fleshy fruit formation alongside the ovary.
Examples of false fruits: Apple, strawberry, cashew.

Parthenocarpy: The phenomenon where an ovary develops into a fruit without fertilisation.
Result: Because fertilisation does not occur, ovules do not become seeds, making these fruits naturally seedless (e.g., bananas).
Artificial Induction: It can be induced commercially through the application of specific growth hormones to produce seedless varieties of economic importance.

Definition: Apomixis is the production of seeds without fertilisation (common in Asteraceae and grasses).
Mechanism: It is a form of asexual reproduction that produces embryos. For example, a diploid egg cell may form without meiosis and develop into an embryo, or nucellar cells may divide and protrude into the embryo sac.
Mimicry: The offspring are genetically identical to the parent (clones), yet they are packaged into a typical seed structure, perfectly mimicking the end-product of sexual reproduction.

The Problem: Hybrid seeds yield highly, but if these seeds are sown to raise the next generation, their progeny will undergo genetic segregation, losing the specific hybrid vigour. Therefore, farmers must buy expensive new hybrid seeds every year.
The Apomictic Solution: If hybrids are made apomictic, they produce seeds asexually (exact clones of the hybrid). There is no segregation of characters, allowing farmers to use farm-saved seeds year after year while perfectly maintaining hybrid vigour.

• Definition: Many insects or floral visitors consume pollen or nectar from a flower without bringing about pollination.
• Because they steal the plant's reproductive rewards without providing the evolutionary service of pollen transfer, they are termed robbers.

The protective seed coats develop directly from the integuments of the ovule after fertilisation:

• Testa (Outer Seed Coat): Develops from the outer integument.
• Tegmen (Inner Seed Coat): Develops from the inner integument.

Clinical Significance: Parthenium (carrot grass), an imported contaminant, causes severe pollen allergies and chronic respiratory disorders like asthma and bronchitis.

Artificial Preservation: Pollen grains can be cryopreserved in liquid nitrogen (-196°C) to be used later in pollen banks for artificial hybridization and breeding programs.

Haploid (n):

• Synergids
• Generative Cell (and Vegetative Cell, Egg cell, Antipodals, Pollen).

Diploid (2n):

• Nucellus (maternal sporophyte tissue)
• Integuments (maternal sporophyte tissue)
• Zygote (fusion of n + n)
  (Note: Microspore/Megaspore Mother Cells are also 2n).

Triploid (3n):

• PEN (Primary Endosperm Nucleus) and the resulting Endosperm.

Cereals (e.g., Rice, Wheat): Pollen grains lose viability very quickly, typically within 30 minutes of their release.
Rosaceae, Leguminosae, & Solanaceae: Pollen viability is highly extended, lasting for several months.

Nutritional Value: Pollen grains are extremely rich in nutrients.
Commercial Exploitation: In recent years, it has become a trend to use pollen products (in the form of tablets and syrups) as food supplements. They are actively claimed to enhance the performance of athletes and racehorses.