A seed that lands at its parent’s feet is competing with a fully grown plant for light, water and minerals — a fight it will lose. So plants go to considerable lengths to throw their offspring somewhere else, and then keep them switched off until the conditions are worth waking up for.
📚 What you need to know
Seed dispersal distributes seeds away from the parent plant and reduces competition between offspring and parent.
Methods include wind or water, animals and explosive pods.
Dispersal is not pollination: pollination moves pollen from anther to stigma, dispersal moves mature seeds.
Once formed, seeds undergo a period of dormancy and germinate only when conditions become favourable.
Germination is the start of growth in the seed, and needs three things: water, oxygen and warmth.
Water is taken up by imbibition, which activates the biochemistry of the embryo and bursts the testa.
The radicle emerges first and grows downwards; the hypocotyl is the first structure above ground and grows upwards.
Food reserves are stored as endosperm and transferred to the embryo through the cotyledons.
Seed dispersal
The reason is competition. Seeds that germinate directly beneath the parent are shaded by it and are drawing on the same soil water and minerals. Moving them away gives them a genuine chance.
Method
How it works
Wind or water
Parachute-shaped or wing-shaped lightweight seeds travel on the wind or float in water
Animals (eaten)
Fleshy fruit is eaten and the seeds are distributed later through egestion
Animals (carried)
Sticky or hooked seeds catch on the fur or feathers of passing animals
Explosions
Some pods split open suddenly, propelling the seeds away from the parent plant
The classic confusion. Both pollination and seed dispersal can use wind, water or animals, which is exactly why students mix them up. The difference is what is being moved: pollen before fertilisation, seeds after it.
What a seed needs to germinate
Three factors, and each one has a reason behind it. Give the reason, not just the factor.
Factor
Why it is needed
Water
Allows the seed to swell up, bursting the seed coat (testa) so the growing embryo can exit; also lets the enzymes in the embryo start working, increasing metabolic activity
Oxygen
Required for respiration, so that energy can be released for germination
Warmth
Germination improves as temperature rises, up to a point, because the reactions are controlled by enzymes, which cannot function effectively when it is too cold
Notice that light is not on the list. A germinating seed lives on its own food reserves and is often underground, so it does not photosynthesise until the first leaves appear.
The process of germination
A seed contains a plant embryo and food reserves for its growth. Those reserves are endosperm tissue, transferred to the embryo through early leaf structures called cotyledons.
The seed first has to replenish the water it lost during dormancy, and it does so by imbibition, which activates the biochemistry of the embryo. The rate of respiration and protein synthesis increases and the embryo prepares to emerge through the seed coat.
The radicle is the first structure to emerge. It forms the initial root, responds to gravity and grows downwards into the soil. The first structure to appear above ground is the hypocotyl, a curved portion of the shoot found below the cotyledons, which grows upwards. As the shoot grows, the first leaves appear from the cotyledon and photosynthesis can begin. The root system establishes at the same time, and full plant growth can occur.
Until the leaves open, everything is paid for out of the seed’s own food reserves. That is why a seed cannot afford to germinate at the wrong time.
Worked examples
WORKED EXAMPLE
Explain why oxygen is needed for a seed to germinate. [2]
Point 1 — the process
Oxygen is required for aerobic respiration in the cells of the embryo.
Point 2 — what it is used for
Respiration releases energy, which is needed for the growth of the embryo and for the increased protein synthesis that accompanies germination.
2 marksSay “releases” energy, never “produces” it.
WORKED EXAMPLE
A student states that seed dispersal and pollination are the same process because both can use wind. Explain why the student is wrong. [3]
Point 1 — what pollination is
Pollination is the transfer of pollen from an anther to a stigma, and it happens before fertilisation.
Point 2 — what dispersal is
Seed dispersal is the distribution of mature seeds away from the parent plant, and it happens after fertilisation has produced a seed.
Point 3 — the shared agent
Both processes may use wind, water or animals, but the agent is only the method of transport; the material being moved and the stage of the life cycle are different.
3 marksQuestions like this reward you for naming the stage of the life cycle.
💡 Exam tip
Give the reason for each germination factor: water for swelling and enzymes, oxygen for respiration, warmth for enzyme activity.
Radicle is the root, hypocotyl is the shoot. Radicle sounds like radish, which grows down.
Always name competition as the advantage of dispersal.
Use testa for the seed coat and imbibition for the water uptake if you know the terms.
Endosperm is the food store; cotyledons transfer it to the embryo. Two different structures.
Dormancy is a survival strategy, not a failure to germinate.
⚠ Common mix-up
Light is not needed for germination. The seed uses stored food until leaves appear.
Seeds do not photosynthesise. They respire.
Dispersal is not pollination. Different material, different stage.
Higher temperature does not always mean faster germination. Beyond a point the enzymes are denatured.
The radicle is not the hypocotyl. One goes down, the other up.
Fruit is the ovary, seed is the ovule. A fruit can hold many seeds.
Up next: Hormones & Puberty — back to animals, and the hormone cascade that starts the whole reproductive system running.
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