Seed and fruit

The seed is the final product of sexual reproduction in angiosperms, often described as a fertilised ovule; it forms inside the fruit and has seed coat(s), cotyledon(s) and an embryo axis. Cotyledons are usually thick and swollen with stored food, as in legumes.

Non-albuminous (ex-albuminous) seeds have no endosperm left because the embryo used it up (pea, groundnut). Albuminous seeds keep part of the endosperm (wheat, maize, barley, castor).

In a few seeds such as black pepper and beet, part of the nucellus persists; this residual nucellus is the perisperm.

The integuments harden into tough seed coats. The micropyle stays as a small pore that lets oxygen and water in at germination.

As the seed matures it dries to 10-15 per cent moisture by mass and its metabolism slows. The embryo may enter dormancy, or it germinates if moisture, oxygen and a suitable temperature are available.

Ovules becoming seeds and the ovary becoming a fruit happen together. The ovary wall becomes the fruit wall, the pericarp. Fruits may be fleshy (guava, orange, mango) or dry (groundnut, mustard).

Fruits from the ovary alone are true fruits. In apple, strawberry and cashew the thalamus also forms part of the fruit, so these are false fruits.

Parthenocarpic fruits develop without fertilisation and are seedless; banana is an example, and parthenocarpy can be induced with growth hormones.

Seeds make reproduction dependable because pollination and fertilisation do not need water. They disperse to new habitats, feed the seedling until it photosynthesises, protect the embryo with a hard coat, and carry new genetic combinations. Their dryness and dormancy let us store them, which is the basis of agriculture.

Viability ranges from a few months to hundreds of years. A lupine, Lupinus arcticus, from the Arctic tundra germinated after an estimated 10,000 years of dormancy, and a date palm seed, Phoenix dactylifera, about 2000 years old was found near the Dead Sea at King Herod's palace.

Some fruits hold thousands of tiny seeds, as in orchids and the parasites Orobanche and Striga, and a tiny Ficus seed grows into a huge tree.