Double fertilisation is the defining reproductive feature of angiosperms. It was discovered by Nawaschin (1898) in Fritillaria and Lilium. Two sperm cells from one pollen grain participate: First fertilisation: sperm 1 (n) + egg cell (n) → zygote (2n) → develops into embryo (future plant). Second fertilisation: sperm 2 (n) + central cell with 2 polar nuclei (n+n = 2n) → primary endosperm nucleus (3n) → develops into endosperm (nutritive tissue). Result: two distinct fertilisation events in one seed. This is called syngamy (first) and triple fusion (second). The endosperm formed is triploid (3n) — this is more nutritious than haploid gymnosperm endosperm because of greater genetic dosage.

Standard Polygonum-type embryo sac has 7 cells and 8 nuclei. At micropylar end: 1 egg cell (n) + 2 synergids (n each) = egg apparatus. At chalazal end: 3 antipodal cells (n each). Centre: 1 large central cell with 2 polar nuclei (n+n = total 2n DNA). After fertilisation: egg + sperm → zygote (2n). Polar nuclei + sperm → primary endosperm nucleus (3n). Synergids degenerate. Antipodals degenerate. The embryo sac is the female gametophyte generation. Memory: 7 cells = 1(egg) + 2(synergids) + 3(antipodals) + 1(central cell). 8 nuclei = above + 2 polar nuclei counted separately.

Integuments (2n) → Testa/seed coat (2n). Egg cell (n). Zygote (2n) → Embryo (2n). Polar nuclei (n+n = 2n collectively). Primary endosperm nucleus (3n) → Endosperm (3n). Antipodal cells (n). Synergids (n). In gymnosperm comparison: single fertilisation only. Endosperm = haploid (n) female gametophyte tissue. Angiosperm triploid endosperm is more nutritive. Endosperm persistence: Albuminous/endospermic seeds: endosperm present at maturity (wheat, maize, castor, coconut water = liquid endosperm). Non-albuminous/exalbuminous seeds: endosperm absorbed by embryo during development, stored in cotyledons (pea, bean, groundnut).

The triploid (3n) nature of endosperm has important significance. Genetic: having 3 copies of each chromosome (2 from mother via polar nuclei + 1 from father via sperm) may confer expression advantages. Economic: endosperm is the most important agricultural product — rice, wheat, maize (corn), barley, oat endosperm = majority of human caloric intake. Coconut: liquid endosperm = coconut water (immature) → coconut meat (mature = solid endosperm). Castor: endosperm rich in oil (ricinoleic acid). Manipulation: triploid watermelons (seedless) produced by crossing tetraploid (4n) × diploid (2n) → 3n seeds → seedless fruits. This exploits triploid nature for horticulture.

A seed consists of: seed coat (testa = from outer integument, tegmen = from inner integument). Embryo: embryonal axis + cotyledon(s). Monocot: 1 cotyledon (scutellum), coleoptile (protects plumule), coleorhiza (protects radicle). Dicot: 2 cotyledons. Endosperm: triploid nutritive tissue. Micropyle: small pore in seed coat. Hilum: scar where seed was attached to funicle. Seed types: Albuminous (endospermic): endosperm present at maturity. Cereals (wheat, rice, maize), castor, coconut, onion. Non-albuminous (exalbuminous): cotyledons absorb endosperm during development. Legumes (pea, bean, gram), gourd family, sunflower, mustard. Hypogeal germination: cotyledons remain underground (pea, gram). Epigeal germination: cotyledons come above ground (bean, castor).

Apomixis: seed formation without fertilisation. Economic importance: apomictic seeds produce plants genetically identical to parent → maintain hybrid vigour without costly hybrid seed production each year. Types: Diplospory: unreduced egg (2n) develops into embryo without fertilisation. Apospory: somatic cells of nucellus form unreduced embryo. Adventive embryony: sporophytic cells directly form embryo (in Citrus, Mangifera — multiple embryos). Polyembryony: presence of more than one embryo in a seed. Example: Citrus (orange, lemon) — nucellar embryos + normal zygotic embryo → polyembryony is common. Onion, groundnut: also show polyembryony. In polyembryony, extra embryos arise from: synergids, antipodals, sporophytic cells of integuments/nucellus (adventive embryony).

After fertilisation: ovary → fruit (pericarp = fruit wall). Ovules → seeds. Simple fruits: develop from single ovary of single flower. True fruits: develop from ovary only. False/pseudocarps: accessory parts (thalamus, calyx) contribute. Apple, pear, strawberry = pseudocarp (thalamus forms the fleshy part). Banana, pineapple = parthenocarpic (no seeds — develop without fertilisation, seedless). Aggregate fruits: from many carpels of one flower (strawberry = apocarpous gynoecium). Multiple/composite fruits: from multiple flowers in inflorescence (mulberry = multiple simple drupes; pineapple = multiple berries fused with fleshy axis). Pericarp layers: epicarp (outermost skin), mesocarp (middle — fleshy part in mango = edible), endocarp (innermost — hard stone in mango/cherry). Drupe: mango, coconut, peach. Berry: tomato, banana, grapes. Capsule: poppy, cotton.

Parthenocarpy: development of fruit without fertilisation → seedless fruits. Natural parthenocarpy: banana (wild banana has seeds but cultivated is triploid sterile), some grapes, cucumber. Induced parthenocarpy: applying auxin (IAA, 2,4-D, NAA) to unfertilised flowers → fruit develops without seeds. Commercial applications: seedless tomatoes (hormone spray), seedless watermelons (triploid × diploid cross). Parthenocarpic fruits: economically valuable (consumers prefer seedless). However: some seeds are valuable (mango seed → mangiferin). Seedless grape cultivation: treated with gibberellins → both seedlessness and larger berries. Fertilised → seeds give flesh → in parthenocarpy, pericarp grows without seed stimulus → usually hollow or different texture.