Sexual reproduction in flowering plants

Sexual reproduction combines DNA from two individuals, so new combinations of variations appear much faster than by copying mistakes alone.

"If each parent gave all its DNA, wouldn't the amount double every generation?" That's why the special reproductive cells, the germ-cells, carry only half the usual amount. In humans, body cells have 46 chromosomes; germ-cells have 23. When two germ-cells fuse, 23 + 23 = 46, and the normal amount is restored.

In flowering plants, the reproductive parts are in the flower:

Flowers with only stamens or only pistils are unisexual (papaya, watermelon); flowers with both are bisexual (Hibiscus, mustard).

Pollination is the moving of pollen to the stigma. In self-pollination it lands on a stigma of the same flower; in cross-pollination it reaches another flower, carried by wind, water or animals such as bees.

After pollination, a pollen tube grows from the pollen grain down through the style to the ovule. The male germ-cell travels down it and fuses with the female germ-cell in the ovule. This is fertilisation, and it makes a zygote.

Then the flower changes:

• the ovule becomes the seed;
• the ovary becomes the fruit (which is why fruits have seeds inside);
• the petals, sepals and stamens wither and fall.

The seed holds the future plant: the plumule grows into the shoot, the radicle into the root (R for radicle, R for root), and the cotyledons store food.