Why Every Flowering Plant Needs Two Fertilisations to Make One Seed

A pollen grain on a stigma is only the beginning. It must grow a tube down to an ovule, deliver two male gametes, and set off a chain of changes that turns the ovule into a seed and the ovary into a fruit.

This lesson covers double fertilisation, seed and fruit formation with the types of endosperm, kinds of seeds and fruits, and apomixis, parthenocarpy, polyembryony and dispersal.

In double fertilisation, one male gamete fuses with the egg to form a diploid zygote (syngamy) and the other fuses with the two polar nuclei to form a triploid primary endosperm nucleus (triple fusion) — two fusions inside one embryo sac.

How it happens:

- The pollen grain germinates on the stigma and grows a pollen tube through the style
- The tube enters the ovule, usually through the micropyle, and then one of the synergids, guided by the filiform apparatus
- It releases two male gametes into the embryo sac
- Syngamy — a male gamete fuses with the egg cell, forming the zygote (2n)
- Triple fusion — the other male gamete fuses with the two polar nuclei, forming the primary endosperm nucleus (3n)

Significance:

- The zygote develops into the embryo
- The primary endosperm nucleus develops into the endosperm, which nourishes the embryo
- Double fertilisation is a hallmark of flowering plants

An everyday example. The white flesh of a coconut and the starchy part of a wheat grain are endosperm, built from the product of triple fusion.

The substance. Triple fusion involves three haploid nuclei but only two cells — one male gamete and the central cell with its two polar nuclei.

After fertilisation the primary endosperm nucleus forms the endosperm, the zygote forms the embryo, the ovule becomes the seed, the integuments become the seed coat, and the ovary becomes the fruit, while the sepals, petals and stamens usually fall away.

Types of endosperm development:

- Nuclear — the nucleus divides repeatedly without cell walls, forming many free nuclei that later become cellular
- Cellular — every nuclear division is followed by wall formation
- Helobial — the first division forms two cells, and further development is partly free-nuclear

Embryo development:

- The zygote divides to form a proembryo, then passes through globular and heart-shaped stages to a mature embryo
- A dicot embryo has an embryonal axis with two cotyledons, a plumule and a radicle
- A monocot embryo has one cotyledon, the scutellum, with the plumule enclosed in a coleoptile and the radicle in a coleorhiza

Seed and fruit:

- The ovule becomes the seed, the integuments become the seed coat, and the micropyle remains as a small pore
- The ovary becomes the fruit, and its wall becomes the pericarp
- The seed loses water and enters dormancy

An everyday example. Tender coconut water sold at roadside stalls is free-nuclear endosperm, while the white kernel around it is cellular endosperm.

The substance. Endosperm develops before the embryo — the embryo needs that food supply in place first.

Albuminous seeds keep part of their endosperm as stored food at maturity, while non-albuminous seeds use it all up during development; a true fruit develops from the ovary alone, while a false fruit also includes other floral parts such as the thalamus.

Albuminous versus non-albuminous seeds:

- Albuminous — endosperm persists in the mature seed; wheat, maize, barley, castor and coconut
- Non-albuminous — the embryo consumes all the endosperm, so food is stored in the cotyledons; pea, groundnut and beans

Perisperm. In some seeds, such as black pepper, part of the nucellus remains as a food-storing perisperm.

True versus false fruits:

- True fruits — develop only from the ovary; mango and tomato
- False fruits — the thalamus or other floral parts also form part of the fruit; apple, strawberry and cashew

An everyday example. The juicy part of an apple is the swollen thalamus — the true fruit is the core that holds the seeds.

The substance. A groundnut kernel splits into two halves because it is non-albuminous — those halves are the food-filled cotyledons, not endosperm.

Apomixis is the formation of seeds without fertilisation, parthenocarpy is the formation of fruits without fertilisation, and polyembryony is the presence of more than one embryo in a seed, while dispersal carries seeds and fruits away from the parent plant.

Apomixis:

- A diploid egg cell forms without meiosis and develops into an embryo, or nucellar cells grow directly into embryos
- The offspring are genetic copies of the parent
- Seen in some grasses and members of the Asteraceae
- If hybrid crops were apomictic, farmers could reuse their own seed without losing hybrid qualities

Parthenocarpy. The ovary develops into a seedless fruit without fertilisation, as in banana, and it can be induced with growth hormones.

Polyembryony. Extra embryos develop from nucellar cells alongside the normal embryo, as in many Citrus and mango varieties.

Seed and fruit dispersal:

- Wind — winged or hairy seeds, as in drumstick and cotton
- Water — fibrous, floating fruits, as in coconut
- Animals — hooked fruits, or fleshy fruits eaten by birds
- Dispersal reduces crowding and competition and helps plants reach new places

An everyday example. Seedless bananas in every fruit market are parthenocarpic — their fruit forms without fertilisation.

The substance. Apomixis and parthenocarpy are not the same — apomixis makes seeds without fertilisation, while parthenocarpy makes fruits, usually with no seeds at all.