Seeds: dispersal, storage and germination
How seeds travel in space and in time, the difference between orthodox and recalcitrant seeds, and how seeds sense the right moment to germinate.
Key points
Summary
| Feature | What it means | Example |
|---|---|---|
| Quantity of seed | Many small (numbers) vs few large (food store) | Papaver rhoeas vs Quercus robur |
| Dispersal mechanism | Wind, animal, explosive, water | Taraxacum, Sorbus, Impatiens, Alnus |
| Physical dormancy | Hard, impermeable seed coat | Lathyrus odoratus |
| Physiological dormancy | Inhibitors, or embryo needs cold | Rosa, Crataegus monogyna |
| Seed coat | Protects embryo; controls water uptake | All seeds |
| Fats and oils | Food reserve for the embryo | Helianthus annuus |
| Orthodox | Tolerate drying and cold storage | Solanum lycopersicum, Lactuca sativa |
| Recalcitrant | Killed by drying; short-lived | Quercus robur, Aesculus hippocastanum |
Germination benefits (5.3): light (small seeds germinate near the surface), moisture (enough to swell the seed), temperature (warmth, or cold first, so seedlings appear in spring).
Test yourself
Which pair of features allows orthodox seeds to be kept in a seed bank?
- High oil content and a thin coat
- Tolerance of drying and of low temperatures
- Rapid germination and large size
- Needing light and warmth
Which of these has recalcitrant seed?
- Lactuca sativa
- Solanum lycopersicum
- Quercus robur
- Tropaeolum majus
How are the seeds of Himalayan balsam dispersed?
- By wind
- Explosively from the pod
- By birds
- They are not dispersed
Video transcript
A seed is a young plant packed with food and sent out into the world. This lesson covers how seeds travel, how they survive, and how they choose when to germinate.
Dispersal carries seeds away from the parent, reducing competition and colonising new ground. Wind carries dandelion parachutes and sycamore wings. Animals eat rowan berries and pass the seeds, or carry cleavers on their fur. Explosive pods fling Himalayan balsam seeds metres away. Water carries alder seeds downstream. Some plants make thousands of tiny seeds; others, like oaks, a few large ones with big food stores.
Orthodox seeds tolerate drying and storage at low temperature, so they stay viable for years, as in tomatoes and lettuce. Seed banks store them dried and frozen. Recalcitrant seeds are killed by drying, so they must be sown fresh, as with acorns and conkers. Many seeds store fats and oils as food for the embryo.
Seeds germinate only when conditions favour the seedling. Light: many small seeds need it, so they only germinate near the surface, where the seedling can reach daylight. Moisture: the seed must take up enough water to swell. Temperature: seeds wait until it is warm enough, and some need winter cold first, so they germinate in spring. Seed coats protect the embryo and control water uptake.
One exam tip. Dispersal happens in space and in time, and dormancy is dispersal in time. For example, dormant seeds germinate over several years. That's the point. So a single bad season can't kill all the offspring. That's the development.
This is one of 30 lessons in Unit 2
The full course adds sorting drills, flashcards, a written question with a model answer, and a printable sheet for every lesson.