Formation of the seed and fruit

The seed: the ovule becomes the seed:

• the zygote → embryo (plumule, radicle, cotyledons);
• the fertilised central cell → endosperm;
• the integuments → seed coat (testa), hard and protective;
• the micropyle stays as a small pore through which water enters at germination;
• the synergids and antipodals disappear.

Types of seeds, according to the endosperm:

1. Endospermic (albuminous) seeds: the embryo does not use up the endosperm while the seed develops, so it stays in the seed and feeds the embryo at germination; the cotyledons do not store food. Examples: monocots such as wheat, maize (the grain), date; and dicots such as castor.
   • In wheat and maize the ovary wall fuses with the integuments of the single ovule, so the fruit is a one-seeded grain (caryopsis). In dates the seed is not fused with the fruit wall.
2. Non-endospermic (exalbuminous) seeds: the embryo takes in all the endosperm during seed development, and the food is stored in the cotyledons, which become fleshy. Examples: bean, pea (dicots).

The fruit:

• after fertilisation the calyx, corolla, stamens, style and stigma wither and drop; only the ovary remains, which grows, accumulates food and ripens into the fruit, under the effect of auxins secreted by the developing seeds/ovary.
• The ovary wall becomes the pericarp and the ovule wall (integuments) becomes the testa.

• Some fruits keep parts of the flower: calyx and stamens in pomegranate; calyx in eggplant (and date); corolla remains in marrow (courgette).
• False (accessory) fruit: a fruit in which a part of the flower other than the ovary enlarges and stores food — e.g. the receptacle in apple.

If pollination and fertilisation do not occur, the flower withers and falls off without forming a fruit.

Role of pollination in fruit formation: pollination provides the male nuclei for fertilisation; it also stimulates the formation of auxins necessary for the ovary to grow into a fruit, even without fertilisation.

Parthenocarpy:

• natural formation of seedless fruits because fertilisation has not occurred (banana, pineapple, some oranges).
• It can be induced artificially by spraying the stigmas with extract of pollen grains (pollen powder in ether) or with synthetic auxins (indole acetic acid or naphthalene acetic acid), which stimulate the ovary to develop into a fruit with no seeds.

The ripening of fruit and seeds often stops the growth of the plant and may lead to its death, especially in annual plants, because of the consumption of stored food and the inhibiting hormones.