Comprehensive Guide to Double Fertilization and Post-Fertilization Development in Flowering Plants

Entry Points into the Ovule: The pollen tube follows one of three specific paths to enter the ovule:
- Porogamy: The pollen tube enters through the micropyle. Example: Lily.
- Chalazogamy: The pollen tube enters through the chalaza. Example: Juglans, Casuarina, etc.
- Mesogamy: The pollen tube enters through the integuments. Example: Cucurbita, Populus, etc.
Entry into the Embryo Sac: Regardless of the initial entry point into the ovule (porogamy, chalazogamy, or mesogamy), the pollen tube invariably enters the embryo sac through the micropylar region.

Syngamy (Generative Fertilisation): One male gamete fuses with the egg cell to form a diploid zygote ( $2n$ ).
Triple Fusion (Vegetative Fertilisation): The second male gamete fuses with two polar nuclei (or a secondary nucleus) to form a triploid Primary Endosperm Nucleus ( $PEN$ ).
Double Fertilization Definition: The simultaneous occurrence of syngamy and triple fusion within a single embryo sac in angiosperms.
Scientific Discovery: Double fertilisation was first reported by S.G. Nawaschin in 1898 using the genera Fritillaria and Lilium.
Direct Products:
- The zygote develops into the embryo.
- The central cell, following triple fusion, becomes the Primary Endosperm Cell ( $PEC$ ) and develops into the endosperm.

Definition: Post-fertilisation events refer to the collection of processes following double fertilisation, including:
- Development of the endosperm.
- Development of the embryo.
- Maturation of ovules into seeds.
- Maturation of the ovary into a fruit.

Developmental Sequence: Endosperm development precedes embryo development as an adaptation to provide assured nutrition to the developing embryo.
Primary Endosperm Cell (PEC): This cell divides repeatedly to form a triploid endosperm tissue. Its cells are filled with reserve food materials.
Free-Nuclear Endosperm: This is the most common type of development where the Primary Endosperm Nucleus ( $PEN$ ) undergoes successive nuclear divisions without immediate cell wall formation, resulting in free nuclei.
Cellular Endosperm: Cell wall formation occurs subsequently, making the endosperm cellular. The number of free nuclei formed before cellularisation varies between species.
The Coconut Example:
- Coconut water from tender coconut is free-nuclear endosperm made up of $\text{thousands of nuclei}$ .
- The surrounding white kernel is the cellular endosperm.
Classification by Consumption:
- Non-endospermic / Exalbuminous Seeds: The endosperm is completely consumed by the developing embryo before the seed matures. Examples: Pea, groundnut, beans.
- Endospermic / Albuminous Seeds: The endosperm persists in the mature seed and is utilized during seed germination. Examples: Castor and coconut.

Site of Development: The embryo develops at the micropylar end of the embryo sac, where the zygote is located.
Timing: Most zygotes divide only after a certain amount of endosperm is formed to ensure food availability.
Stages of Embryogeny: While mature seeds differ, early development is similar in both monocotyledons and dicotyledons. The sequence is: Zygote $\rightarrow$ Proembryo $\rightarrow$ Globular Embryo $\rightarrow$ Heart-shaped Embryo $\rightarrow$ Mature Embryo.
Structure of a Typical Dicotyledonous Embryo:
- Embryonal Axis: The main central structure.
- Cotyledons: Two in number.
- Epicotyl: The portion of the embryonal axis above the level of cotyledons, terminating in the plumule (stem tip).
- Hypocotyl: The cylindrical portion below the level of cotyledons, terminating in the radicle (root tip).
- Root Cap (Calyptra): A protective covering for the root tip.
Structure of Monocotyledonous Embryos:
- Cotyledon: Single cotyledon, specifically called scutellum in the grass family, situated towards one side (lateral) of the axis.
- Radicle and Root Cap: Enclosed in an undifferentiated sheath called coleorhiza.
- Epicotyl: The portion above the level of attachment of the scutellum.
- Coleoptile: A hollow foliar structure enclosing the shoot apex and leaf primordia.

Definition: The final product of sexual reproduction in angiosperms; a fertilised ovule formed inside fruits.
Components: Seed coat(s), cotyledon(s), and embryo axis.
Cotyledons: Generally simple, thick, and swollen due to food storage (e.g., in legumes).
Perisperm: Occasional residual, persistent nucellus found in some seeds. Examples: Black pepper and beet.
Seed Coat: Formed from the hardening of ovule integuments. It contains a small pore called the micropyle which facilitates the entry of oxygen and water during germination.
Maturation and Dormancy: As seeds mature, water content is reduced to $10-15\%$ moisture by mass. Metabolic activity slows, and the embryo may enter dormancy (inactivity) until favourable conditions (moisture, oxygen, suitable temperature) allow germination.
Viability: The ability of seeds to retain the power of germination. It lasts from weeks to years.
- Oldest Record: Lupinus arcticus excavated from the Arctic Tundra.
Loss of Viability: Caused by the exhaustion of food around the embryo, damage to the embryo, or denaturation of enzymes.

Fruit Definition: A seed-containing part that develops from a fertilised ovary and surrounding tissues.
Pericarp: The wall covering the fruit, consisting of:
- Epicarp: The skin of the fruit.
- Mesocarp: The middle fleshy and pulpy portion.
- Endocarp: The innermost portion surrounding the seeds.
Fruit Classification by Origin:
- True Fruit (Eucarpic): Developed specifically and only from the ovary after fertilisation. Example: Mango.
- False Fruit (Pseudocarpic / Accessory Fruit): Flesh is derived from the ovary and other parts like the thalamus. Examples: Apple, strawberry.
Classification by Complexity:
- Simple Fruit: Develops from a monocarpellary ovary or a multicarpellary syncarpous ovary.
- Aggregate Fruit: Develops from a multicarpellary apocarpous (free) ovary.
- Composite (Multiple) Fruit: Develops from a complete inflorescence.

Parthenocarpy: The formation of fruit without fertilisation. The resulting fruits are seedless. The term was introduced by Noll in 1902.
- Natural Example: Banana.
- Induction: Can be induced through the application of growth hormones.
Pomology: The scientific study of fruits.
Seed Liberation:
- Dehiscent Fruits: Liberate seeds at maturity. Examples: Pea, bean.
- Indehiscent Fruits: Do not liberate seeds at maturity; seeds are only released after the disintegration of the fruit wall. Examples: Cashew nut, cereals.