Inside a seed
A seed is a baby plant in a protective package. It contains an embryo (with an embryonic root, the radicle, and an embryonic shoot, the plumule), a food store (in the cotyledons or endosperm) and a tough seed coat (testa). Dry seeds are dormant: their metabolism is almost stopped, which lets them survive cold, drought and long journeys.
What happens during germination
- Imbibition: the seed absorbs water and swells, often splitting the seed coat.
- Activation: enzymes switch on and break down stored starch, proteins and fats into small molecules the embryo can use.
- Respiration: the embryo respires these foods to release energy for growth.
- Emergence: the radicle pushes out first, then the shoot grows toward the light. Once leaves open, the seedling can photosynthesise.
The conditions seeds need
| Condition | Why it is needed |
|---|---|
| Water | Rehydrates cells, activates enzymes and lets the seed swell |
| Oxygen | Needed for aerobic respiration to release energy from stored food |
| Suitable temperature | Enzymes work fastest at an optimum; too cold is slow, too hot denatures them |
Temperature and germination speed
Every species has a base temperature below which it will not germinate, anoptimum at which it is fastest, and a maximum above which it fails. Beans prefer warmth (an optimum near 25–30 °C), while lettuce prefers cool conditions and becomes dormant when it is too warm — which is why gardeners sow lettuce in spring and autumn.
Does light matter?
Most large seeds, like beans, germinate equally well in light or dark because they carry plenty of food. Some small seeds, including certain lettuce varieties, germinate much better in light. Their pigment phytochromedetects red light, signalling that the seed is close to the surface where the seedling will soon reach sunlight.
Designing a fair test
Change one independent variable at a time (for example water), keep all others the same, and measure the dependent variable (the percentage germinated). Use many seeds, because a few seeds are always dead (not viable) and individual seeds vary. Including a control dish with ideal conditions lets you compare results.
Interesting facts
- In 2005 scientists germinated a date palm seed about 2,000 years old, found at the ancient fortress of Masada in Israel. The seedling grew into a healthy tree.
- The Svalbard Global Seed Vault in Norway stores over a million seed samples at about −18 °C as a back-up for the world’s crops: cold, dry seeds stay dormant for decades.
- Some seeds of fire-prone habitats germinate only after a fire, triggered by heat or by chemicals in smoke.
- A seed’s food store (the endosperm or the cotyledons) feeds the seedling until its first leaves can photosynthesise.
Key terms
- Germination
- The process by which a seed begins to grow into a seedling.
- Dormancy
- A resting state in which a seed does not germinate even if conditions seem suitable.
- Radicle and plumule
- The embryonic root and embryonic shoot.
- Cotyledon
- A seed leaf that often stores food for the developing seedling.
- Viability
- Whether a seed is alive and able to germinate.
Real-world applications
- Farming and gardening: sowing at the right depth, soil temperature and moisture gives even, fast crop emergence.
- Seed testing: seed companies run germination tests to label the percentage of seeds that will grow.
- Seed banks: facilities such as the Svalbard Global Seed Vault store seeds dry and frozen to keep them dormant for decades.
- Brewing: malting deliberately germinates barley so its enzymes convert starch to sugar.
Common misconceptions
- “Seeds need soil to germinate.” Soil helps seedlings grow, but germination only needs water, oxygen and warmth.
- “Seeds need light to germinate.” Many germinate in the dark using stored food.
- “More water is always better.” Waterlogging starves seeds of oxygen.
Key takeaways
- Germination needs water, oxygen and a suitable temperature.
- Stored food supplies energy until the seedling can photosynthesise.
- Each species has its own temperature range; some small seeds also need light.
- Fair tests change one variable at a time and use many seeds.
Model note: germination times use a simple thermal-time model with natural variation; real values depend on variety and seed age.