Fertilization Post Fertilization

1.4 Double Fertilisation

Once the pollen tube has delivered its two male gametes into the embryo sac, an event takes place that is unique to angiosperms in the entire plant kingdom — double fertilisation.

The pollen tube discharges its contents into one synergid, which degenerates. The two male gametes are released and take very different journeys:

• Syngamy — male gamete 1 fuses with the egg cell, producing a diploid zygote (2n). The zygote will become the embryo of the seed.
• Triple Fusion — male gamete 2 fuses with the two polar nuclei of the central cell. Three haploid nuclei come together, giving a triploid Primary Endosperm Nucleus (PEN, 3n). The central cell, now called the Primary Endosperm Cell (PEC), will develop into the endosperm.

Because two separate fusion events (syngamy and triple fusion) happen in the same embryo sac, the process is called double fertilisation.

1.5 Post-Fertilisation: Structures and Events

After double fertilisation, a cascade of transformations converts the fertilised ovule into a seed and the ovary into a fruit. Each floral part has a predetermined fate:

1.5.1 Endosperm

The endosperm is the embryo's food-bank — a tissue packed with starch, proteins and oils. It develops before the embryo, so that nutrients are waiting the moment the embryo needs them. Endosperm development follows two distinct phases:

• Free-nuclear stage: The PEN divides repeatedly without cell-wall formation, producing many free nuclei scattered through the central cell's cytoplasm. A familiar example is the watery stage of coconut water.
• Cellular stage: Cell walls form around each nucleus, turning the endosperm into a solid tissue — the coconut meat is cellular endosperm.

In some seeds the endosperm is consumed by the embryo during development — these mature seeds are non-albuminous/exalbuminous (e.g., pea, bean, groundnut). In others the endosperm persists and nourishes the seedling at germination — these are albuminous (e.g., wheat, maize, castor, coconut).

A few seeds also retain a remnant of nucellus, called perisperm — examples include black pepper and beet.

1.5.2 Embryo — Embryogeny

The zygote sits quiet while the endosperm gets going; only later does it start dividing. Most dicot embryos pass through characteristic morphological stages:

A mature dicot embryo consists of an embryonal axis with two fleshy cotyledons. The portion of the axis above the level of the cotyledons is the epicotyl, terminating in the plumule (future shoot). The portion below is the hypocotyl, terminating in the radicle (future root) — the radicle is covered by a root cap.

Monocot embryos (e.g., rice, maize) have one cotyledon called the scutellum, which lies to one side of the axis. The plumule is enclosed in a protective sheath, the coleoptile, and the radicle in another, the coleorhiza.

1.5.3 Seed

A seed is the final product of sexual reproduction in angiosperms — a fertilised, ripened ovule. Its parts are:

• Seed coat — develops from the two integuments. The outer testa is tough; the inner tegmen is papery. A scar (hilum) and a tiny pore (micropyle) persist on the coat.
• Embryo — with radicle, plumule and cotyledon(s).
• Endosperm — present (albuminous seeds) or absent (non-albuminous seeds).

Seeds are built to survive. Once mature, they dehydrate (to 10–15 % water) and enter a metabolically inactive phase called dormancy. Under favourable moisture, temperature and oxygen they resume activity and germinate. Seeds of some species remain viable for years — a 2000-year-old date seed from Masada (Israel) was successfully germinated in 2005.

1.5.4 Fruit

The ovary wall transforms, typically, into the pericarp of the fruit. The pericarp can be fleshy (as in mango) or dry (as in a coriander fruit).

• True fruit — develops from the ovary alone (mango, pea, tomato, rice).
• False fruit — other floral parts contribute to its bulk. In apple and pear, the fleshy thalamus forms most of the "fruit"; in strawberry, it is the enlarged receptacle.
• Parthenocarpic fruit — forms without fertilisation, hence without seeds. Banana is a classic example; seedless varieties of grape and watermelon are also parthenocarpic.

1.6 Apomixis and Polyembryony

Finally, a couple of curious departures from the standard script.

Apomixis

Apomixis is the formation of seeds without meiosis and without fertilisation. The embryo develops directly from a diploid cell — often the nucellus — and therefore carries only the mother's genes. Seed-set in several grasses and in some varieties of Citrus and mango proceeds this way.

Importance to plant breeders: Hybrid varieties show spectacular hybrid vigour (heterosis) in the first generation, but segregation in the next generation dilutes the advantage. If a hybrid can be made apomictic, every seed it produces is a genetic clone of the prized hybrid — hybrid vigour is locked in, and farmers need not buy fresh hybrid seed every year.

Polyembryony

Polyembryony is the occurrence of more than one embryo inside a single seed. Extra embryos may arise from nucellar cells, synergids, or splitting of the original embryo. When you open a mango seed (especially varieties like 'Chausa' or many Citrus fruits) and find several seedlings sprouting, you are looking at polyembryony.