Reproduction · Classify
Seed dispersal
The worst place a seed can land is directly underneath the plant that made it. Everything on this page follows from that.
Start here
Parachutes, hooks, wings, explosives and bribes. All for a journey of a few metres.
A plant that has just spent a season building a fruit then spends more on getting rid of it. Whatever the seeds are being moved away from must be costing them something.
Why is it worth so much to a plant to move its seeds away?
Because the parent is the competition. A seedling germinating in its parent's shade is asking for light that is already taken, water from soil that is already being drained, and minerals from ground that has been stripped for years — against an opponent a thousand times its size that got there first. It loses. Spreading into new territory is a consequence of dispersal, not the reason for it; the immediate problem is much closer to home.
Sort the specimens · eight fruits and seeds
Classify by what you can see
0 of 8 checked
Read the description, choose a method, then check it. Judge each one on its structure alone — the plant's name tells you nothing, and three of these eight are wind-dispersed although only two look it.
Dandelion
A single tiny seed, under a milligram, carrying a spray of fine white hairs on a thin stalk above it.
Sycamore key
A hard seed at one end of a stiff papery blade about four centimetres long, curved along its length.
Poppy capsule
A dry pepper-pot capsule on a long thin stem, with a ring of small holes opening under the rim. Hundreds of dust-fine seeds inside.
Blackberry
Sweet dark flesh in clustered segments, each holding one small hard pip. Green, hard and sour a fortnight earlier.
Goosegrass, or cleavers
Small round green fruits in pairs, covered in tiny stiff hooked bristles. The whole plant sticks to clothing.
Burdock burr
A dense round burr, two centimetres across, made of many stiff bracts each ending in a backward-facing hook.
Coconut
A very large seed inside a thick fibrous husk, waterproof and full of air spaces, with its own store of liquid inside.
Gorse pod
A small dry pod that twists as it dries in hot sun and splits open with an audible crack.
How is it dispersed?
CorrectNot this one
Wind
The hairs give a large surface area against the air and almost no extra mass, so the whole thing falls very slowly. Falling slowly is the entire trick: the longer it takes to reach the ground, the longer the wind has to carry it sideways. On a dry breezy day some travel a kilometre.
Deciding featureA parachute of hairs on a seed light enough for it to matter.
CorrectNot this one
Wind
The off-centre wing makes it autorotate on the way down, like a helicopter rotor with the engine off, and that slows the fall by a factor of several. A heavier seed than a dandelion’s can then still be moved by wind — not a kilometre, but well clear of the parent tree’s shade.
Deciding featureOne asymmetric wing, which makes it spin rather than drop.
CorrectNot this one
Wind
This is the one that catches people, because there is no parachute and no wing. The stem sways in the wind and the seeds are shaken out through the holes a few at a time, then blown along the ground. The long stem and the small high holes are both essential: the capsule only empties when something is moving it, so the seeds leave on windy days and stay put on still ones.
Deciding featureHoles near the top of a capsule on a long springy stem.
CorrectNot this one
Inside an animal — eaten
The flesh is payment and the pip is the cargo. Its coat survives the whole passage through a bird or a mammal, and it is deposited some distance away in a small heap of fertiliser. The timing is part of the design: the fruit stays hard, sour and green until the seeds inside are ready, and only then turns black and sweet.
Deciding featureEdible flesh around a seed with a very tough coat.
CorrectNot this one
On an animal — hooked to fur
No flesh, no reward and no cooperation required — this one is stealing the ride. The hooks catch in fur or wool, the animal carries the fruit until it grooms or brushes past something, and the seed is dropped wherever that happens. Cheap for the plant, since hooks cost far less to build than sugar.
Deciding featureHooks on the outside, and nothing edible anywhere.
CorrectNot this one
On an animal — hooked to fur
Worth knowing for what it caused. In 1941 a Swiss engineer, Georges de Mestral, pulled burdock burrs out of his dog’s coat, looked at one under a microscope, saw hooks catching in loops of fur — and spent the next decade turning that into Velcro. A dispersal mechanism, copied exactly.
Deciding featureThe same hook principle, built at a larger scale.
CorrectNot this one
Water
Far too heavy for wind and far too big for an animal to carry, so the husk does the work: it floats, keeps salt water out, and can survive months at sea before washing up on a beach and germinating. Every coconut palm on a remote island arrived that way.
Deciding featureA buoyant, waterproof case around a seed too heavy for anything else to move.
CorrectNot this one
Flung by the plant itself
The two halves of the drying pod pull against each other until the seam gives way, and the sudden release flings the seeds several metres. On a hot still afternoon a gorse bank crackles continuously. This is the only method on the list that needs no wind, no water and no animal — the plant supplies the energy itself, stored in the way the pod dried.
Deciding featureA pod that dries, twists and tears itself apart.
One problem · five answers
Nothing here is free. Read what each method costs.
- WindA parachute of hairs, a stiff wing, or a shaker capsule on a long stem. Always a very light seed.Cheap per seed, but almost all of them land somewhere useless, so the plant makes very large numbers.
- WaterA buoyant, waterproof case — fibrous, corky or air-filled.Expensive to build and only available to plants growing by water, but it moves seeds no other method could shift at all.
- Inside an animalSweet, coloured flesh around a seed with a very tough coat.The most expensive of all — the plant gives away sugar it made itself — and the most reliable, because the courier travels a long way and leaves the seed with fertiliser.
- On an animalHooks, barbs or a sticky surface. No reward offered.Very cheap: hooks cost almost nothing compared with flesh. The trade-off is no control over where the animal goes or when it grooms.
- Flung by the plantA dry pod that twists and splits, often audibly.Cheap, and independent of weather, wind and animals — but the range is metres rather than kilometres.
Key fact
Every dispersal structure is a solution to one problem: a seed that germinates beneath its parent competes with a large established plant for light, water and minerals, and usually loses. Classify a specimen by the structure you can see, not by the plant it came from.
Think again
“Plants disperse their seeds so the species can spread to new places.”
Nothing in a plant intends anything, and the sentence quietly assumes a plan. What actually happened has no foresight in it at all: among the seeds of long-dead plants, some happened to have a slightly better wing, a stickier hook, sweeter flesh. Those seeds landed further from the parent more often, more of them survived, and more of them grew into plants carrying the same feature. Repeat for a few million generations and every plant around you has dispersal machinery — not because any of them wanted to travel, but because the ones whose seeds did not travel left fewer descendants. This is natural selection again, and the test of whether you have the idea is whether you can describe a dandelion parachute without using the words so that, wants or tries.
“Fruit is food the plant provides for animals.”
The flesh is a fee, and the seed is what is being smuggled. Look at how carefully the arrangement is engineered against the animal's interest: the seed inside has a coat tough enough to survive the whole journey through a gut and come out able to grow, in the middle of a small heap of fertiliser some distance from the parent. Notice also the timing. An unripe blackberry is green, hard and mouth-puckeringly sour, and it becomes sweet, soft and black only when the seeds inside are finished — the colour change is a signal that says now, and everything before it says not yet. And plenty of fruits are lethal to us while being perfectly good bird food, because we are not the customer they are advertising to. Payment, not generosity.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the structure
A fruit you have never seen before is covered in small stiff hooks and has no flesh on it. How is it dispersed?
Rung 2 · The one that catches people
Which of these is wind-dispersed?
Rung 3 · Explain the whole idea
Explain why seed dispersal matters to a plant, then describe three different methods, giving for each the structure that identifies it and why that structure works.
Rung 4 · Take it somewhere new
You find an unfamiliar seed: four millimetres across, hard, smooth, with no wings or hooks, and it came out of a bright red fleshy fruit. Say how it is most likely dispersed, state the evidence you used, and give one prediction you could test to check.
Key note
Seeds are dispersed because a seedling under its parent competes with it and loses. Wind dispersal uses parachutes, wings or a shaker capsule and needs a very light seed. Water dispersal needs a buoyant waterproof case. Animal dispersal comes in two forms: hooks that catch on fur, and edible flesh around a seed with a coat tough enough to survive being eaten. Some plants fling their own seeds from a pod that dries and splits. Classify by the structure in front of you, not by the plant's name.
Going further
How far does a seed actually go? You cannot follow one, so for most of the history of the subject the answer was a guess. The method that settled it borrows from DNA: take a plot of forest, take a small sample from every adult tree of one species and record each tree's genetic fingerprint, then fingerprint the seedlings on the ground. Because each seedling's DNA carries a combination that could only have come from particular parents, every seedling can be matched to the tree it came from — and the distance between them measured. What comes back is always the same shape of graph. Most seeds land within a few metres of the parent, the numbers fall away sharply with distance, and there is a long thin tail of seeds that travelled hundreds of metres or further. That tail is small and it matters more than all the rest: it is how a species reaches a new wood, recolonises after a fire, and shifts its range as the climate changes. The rare events are the ones doing the important work, which is a hard thing to see if you only measure the average.
Before this lesson
Connects to
At GCSE this becomes
- Adaptation, competition and the way populations spread through an environment.
Where to next
Ask Mr Badmus AI
Want to check how to classify a specimen you have never seen?
The specimens are typical British examples described from their structures. Several of them are strictly fruits rather than seeds — a dandelion “seed” and a sycamore “key” are both single-seeded fruits — which is the distinction drawn in the previous lesson.
Lesson content © MrBadmusAI.