What Is a Seed?
A seed is a tiny package containing a plant embryo (baby plant), a food store, and a protective seed coat (testa). Seeds form after fertilisation — when pollen from one flower fertilises the egg cell in another.
Parts of a Seed
Testa (seed coat) — tough outer covering that protects the embryo from drying out, physical damage, and pathogens
Embryo — the tiny developing plant, consisting of:
- Radicle (embryonic root) — the first part to emerge during germination
- Plumule (embryonic shoot) — becomes the first shoot and leaves
- Cotyledons (seed leaves) — temporary "leaves" that store food for the seedling to use before it can photosynthesise
Germination
Germination is the process by which a seed sprouts and begins active growth. For most seeds, the three requirements are:
- Water — rehydrates dried cells; activates enzymes; softens the seed coat to let the root push through
- Warmth — enzymes work best at the right temperature (typically 10–25 °C for most plants)
- Oxygen — needed for respiration to release energy for growth
Light is only needed once the seedling reaches the surface and starts photosynthesis.
Many seeds have dormancy — they will not germinate immediately, even in ideal conditions. Dormancy can be broken by cold temperatures (winter), fire, passing through an animal's gut, or abrasion of the seed coat. This prevents seeds from germinating in autumn only to be killed by winter frost.
Stages of Germination
- Seed absorbs water (imbibition) — swells and softens
- Enzymes digest the food store in the cotyledons into simple sugars
- Radicle (root) breaks through the seed coat and grows downward
- Plumule (shoot) grows upward toward the light
- First true leaves emerge and begin photosynthesis
- Seedling becomes independent — no longer relying on the seed food store
Seed Dispersal
Before germinating, seeds must travel away from the parent plant to avoid competing with it for light, water, and nutrients. Seeds are dispersed by:
- Wind — dandelion parachutes, sycamore wings ("helicopters")
- Animals — seeds inside fruits are eaten and pass through the gut; others (burrs) cling to fur
- Water — coconuts can float thousands of kilometres
- Explosion — some pods (e.g. peas) burst and fling seeds out
Real world: Coffee seeds and storage
Coffee beans are actually the seeds of the Coffea plant. After harvesting, they are dried to reduce moisture content to around 11% — low enough to keep the embryo in dormancy. If stored at the right humidity and temperature, they remain viable (able to germinate) for years. Roasting destroys the embryo, so roasted coffee beans cannot germinate, but green (unroasted) coffee seeds can.
Real world: Growing mustard seeds on a damp cloth
A classic school experiment involves placing mustard or cress seeds on a wet paper towel at room temperature. Within 2–3 days the radicle (embryonic root) breaks through the seed coat and grows downward, followed by the plumule (shoot) growing upward. The seed contains enough stored food (in the cotyledons) to fuel the seedling until its first leaves unfurl and begin photosynthesis — no soil or light is needed at this early stage.
Real world: Conker dispersal in a park
Horse chestnut trees drop their large, heavy seeds (conkers) in autumn. The green spiky casing (the fruit) splits open on impact with the ground, releasing 1–3 seeds. The seeds need a cold winter (dormancy trigger) before they will germinate in spring. Children collect conkers in October — just as the seeds are being naturally dispersed and entering their dormancy period.