Reproduction

Male gametes develop inside pollen grains, and the egg develops inside an ovule in the ovary. Many flowers are hermaphrodite, yet reproduction is still sexual, because gametes fuse.

1. Pollination: pollen moves from an anther to a stigma.
2. The pollen germinates, and a pollen tube grows down the style to the ovule.
3. Fertilization: a male nucleus fuses with the egg, forming a zygote. The other male nucleus triggers the formation of a food store.
4. The zygote divides by mitosis to form an embryo. The ovule becomes the seed, and the ovary the fruit.

Watch out: pollination delivers pollen to the stigma, not to the ovary.

Sepals protect the bud. Colourful petals and sugary nectar attract pollinators. The stamen is an anther, which makes pollen, on a filament. The carpel is a stigma, where pollen lands, with a style and an ovary containing ovules.

Cross-pollination moves pollen between plants of the same species, often by animals or wind. It increases variation. Plants encourage it in these ways:

• Pollen and stigma mature at different times.
• Separate male and female flowers grow on one plant, as in birch.
• Separate male and female plants exist, as in holly.

Self-pollination leads to inbreeding, which decreases genetic diversity and vigour. In many species, genes let a plant recognise its own pollen. If the pollen carries the same self-incompatibility allele as the style, the pollen tube stops growing. So gametes that fuse come from different plants.

Dispersal moves seeds away from the parent by wind, water, animals or explosive pods. This reduces competition for space, light, nutrients and water.

Germination needs water to rehydrate the cells, oxygen for aerobic respiration, and a temperature suitable for enzymes.

1. Water enters through the micropyle.
2. The embryo produces gibberellin.
3. Gibberellin stimulates enzymes in the food store.
4. Starch is hydrolysed to maltose, and then glucose.
5. Glucose and amino acids move to the growing points, for respiration and growth.

Watch out: pollination and dispersal are different events. Pollination comes before fertilization, and dispersal after it.