Pollination and Fertilisation

• Definition: A unique mode of sexual reproduction in angiosperms wherein both male gametes discharged by the pollen tube participate in two separate, simultaneous nuclear fusions inside the embryo sac:
1. Syngamy: One haploid male gamete ($n$) fuses with the haploid egg nucleus ($n$) to form a Diploid Zygote ($2n$), which develops into the future plant embryo.
2. Triple Fusion: The second haploid male gamete ($n$) fuses with the two haploid polar nuclei ($n + n$) in the central cell to form a Triploid Primary Endosperm Nucleus ($3n$), which divides to form the nutritive endosperm tissue.
• Why Unique: This double fertilization mechanism occurs strictly and exclusively in angiosperms; in gymnosperms, endosperm is haploid and formed before fertilization.

1. Immense, Light Pollen: Produce colossal numbers of extremely light, dry, smooth-surfaced, non-sticky pollen grains that easily float on gentle breezes.
   2. Large Feathery Stigmas: The stigmas are large, branched, and feathery (plumose), protruding outside the flower to trap passing airborne pollen grains.
   3. Versatile Anthers: Stamens possess long slender filaments with versatile anthers that swing and pivot freely in the wind.
   4. Inconspicuous Flowers: Flowers are small, dull greenish, without bright petals, fragrance, or nectaries (saving energy).

• (i) Protandry: A condition of dichogamy where the anthers of a bisexual flower mature and shed pollen before its stigma becomes receptive. Hence, the stigma cannot be fertilized by its own pollen. Example: Sunflower (Helianthus), Salvia, Sweet Pea.
• (ii) Herkogamy: A structural adaptation where a physical mechanical barrier separates the anther and stigma, physically preventing self-pollination. Example: In Iris, the petaloid stigma forms a hood high above the stamen.

• Cleistogamy: A condition in which bisexual flowers never open at all, remaining permanently closed throughout their life cycle.
• Example: Commelina benghalensis, Oxalis, Viola (Pansy).
• Biological Significance: Because the flower remains closed, it guarantees 100% self-pollination and seed set without depending on external insect pollinators or wind currents, ensuring species survival even in pollinator-scarce environments.

• Path:
1. The pollen tube emerges through a germ pore on the stigma surface.
2. It secretes enzymes to digest its way down through the solid transmitting tissue of the Style.
3. It enters the Ovary cavity and approaches the ovule.
4. It enters the ovule through the microscopic opening called the Micropyle (Porogamy).
5. It penetrates a synergid cell inside the embryo sac to discharge its two male gametes.
• Guiding Mechanism: Its directional growth is controlled by Chemotropism (growth guided by a concentration gradient of chemical attractants, specifically calcium, boron, and carbohydrates secreted by the synergids).

• (i) Ovary: Develops into the FRUIT.
• (ii) Ovule: Develops into the SEED.
• (iii) Outer Integument: Develops into the Testa (outer tough seed coat).
• (iv) Zygote: Develops into the Diploid Plant Embryo (consisting of radicle, plumule, and cotyledons).

• Salvia is an insect-pollinated flower with a bilabiate (two-lipped) corolla and two specialized stamens.
• Each stamen has a curved lever-like connective with a fertile upper anther lobe and a sterile lower plate.
• When a bumblebee lands on the lower lip and pushes its head into the floral throat to reach nectar, its head pushes against the sterile lower plate.
• This trips the lever, causing the fertile upper anther to swing downward and strike the bee's back, dusting it with pollen.
• In an older flower with a mature, downward-curved stigma, the bee's pollen-dusted back rubs against the stigma, accomplishing cross-pollination.

• Autogamy: The transfer of pollen grains from the anther to the stigma of the exact same individual flower.
• Geitonogamy: The transfer of pollen grains from the anther of one flower to the stigma of another flower on the same plant.
• Genetic Significance: Functionally, geitonogamy resembles cross-pollination because an external pollinating agent is involved, but genetically it is identical to self-pollination because both flowers share the identical genome of the parent plant.