Reproduction in Plants

Lessons

Senior 4 · Biology

Reproduction in Plants

5 lessons

Lesson 5 of 5

Seeds, fruits and their dispersal

25 min

A market stall full of ovaries

Walk past a market stall in Kigali: tomatoes, avocados, passion fruit, bananas, mangoes, bean pods, maize cobs, sweet peppers. A cook sorts them into "fruits" and "vegetables". A botanist sees almost all of them as fruits, because each one developed from the ovary of a flower after fertilisation, and most contain seeds that developed from ovules. A maize grain is a whole fruit; an avocado is a berry with one seed; a banana is a berry that has lost its seeds.

This lesson follows a flower after double fertilisation: which parts become the seed and the fruit, how seeds and fruits are built and classified, and how their structure carries the next generation away from the parent plant.

Definition

Seed: a fertilised, mature ovule, containing an embryo and a food store inside a protective seed coat (testa).

Fruit: a mature, ripened ovary, containing the seed or seeds. Its wall, the pericarp, may have three layers: the outer exocarp, the middle mesocarp and the inner endocarp.

True fruit: develops from the ovary only. False (accessory) fruit: other parts, such as the receptacle, also form part of it (strawberry, apple).

Endospermic (albuminous) seed: food is stored in the endosperm at maturity (maize, wheat, castor). Non-endospermic (exalbuminous) seed: the endosperm has been absorbed and food is stored in the cotyledons (bean, groundnut).

Parthenocarpy: development of a fruit without fertilisation, giving a seedless fruit (cultivated banana).

Dispersal: the carrying of seeds or fruits away from the parent plant.

What becomes what after fertilisation

Part of the flower	What it becomes
Zygote	Embryo: radicle, plumule and one or two cotyledons
Endosperm nucleus (3n)	Endosperm, the food store (absorbed into the cotyledons in non-endospermic seeds)
Ovule	Seed
Integuments	Testa (seed coat)
Micropyle	Micropyle of the seed, through which water enters at germination
Funicle	Seed stalk, which falls away leaving a scar, the hilum
Ovary	Fruit
Ovary wall	Pericarp (fruit wall)
Petals, stamens, style and stigma	Usually wither and fall off
Sepals	Usually fall; in some plants they remain on the fruit (tomato, eggplant)

The ovary grows because the developing seeds release hormones, mainly auxins and gibberellins, which make the ovary wall cells divide and expand. This is why a fruit with only a few fertilised ovules often grows lopsided, and why spraying some plants with these hormones can produce seedless (parthenocarpic) fruit.

The structure of seeds

Feature	Bean (dicot)	Maize (monocot)
Cotyledons	Two, large and fleshy	One, the scutellum, thin and shield-shaped
Main food store	Cotyledons (non-endospermic)	Endosperm (endospermic)
Seed or fruit?	A seed, inside a pod (the fruit)	A one-seeded fruit (caryopsis): pericarp fused to the testa
Protection of the young shoot and root	None	Coleoptile around the plumule; coleorhiza around the radicle
External scars	Hilum and micropyle clearly visible	Not visible: covered by the pericarp

Types of fruits

Worked example 1: classifying fruits

Classify each fruit and give your reason: (a) mango, (b) avocado, (c) bean pod, (d) maize grain, (e) pineapple.

(a) Simple fleshy fruit, a drupe: the exocarp is a skin, the mesocarp is the juicy flesh, and the endocarp is the hard, fibrous "stone" around the single seed.
(b) Simple fleshy fruit, a berry: the whole pericarp is soft (skin and buttery flesh) and there is no stony endocarp; the large seed has its own thin seed coat.
(c) Simple dry dehiscent fruit, a legume: formed from one carpel, it splits along two seams when dry, releasing several seeds.
(d) Simple dry indehiscent fruit, a caryopsis: one seed whose testa is fused to the pericarp, so the "seed" we plant is really a whole fruit.
(e) Multiple fruit: the ovaries of many flowers on one stalk fuse into a single structure.

Dispersal of fruits and seeds

Seeds that fall straight under the parent must compete with it and with each other for light, water and minerals, and are easily found by the parent's pests and diseases. Dispersal reduces this competition and lets the species colonise new places.

Agent	Adaptations	Examples
Wind	Small, light seeds; hairy parachutes (pappus); wings that spin and slow the fall	Dandelion and Tridax (parachutes); Grevillea and jacaranda seeds (wings); tiny orchid seeds
Animals, eaten	Juicy, sweet, brightly coloured fruits; seeds with hard coats that survive the gut and are dropped in faeces far away	Tomato, guava, passion fruit, figs
Animals, carried outside	Hooks, barbs or sticky surfaces that catch on fur or clothing	Bidens (blackjack), Desmodium
Water	Fibrous or air-filled, waterproof fruits that float	Coconut, water lily
Self (explosive)	Pods or capsules that dry unevenly, build up tension and burst, flinging seeds out	Bean and other legume pods, castor, Impatiens

Worked example 2: how far can wind carry a seed?

A simple model treats a falling seed as dropping at a steady speed $v$ (its terminal velocity) while the wind carries it sideways at the wind speed $u$ . The time to fall a height $H$ is $\frac{H}{v}$ , so the horizontal distance travelled is:

d = u \times \frac{H}{v} = \frac{H\,u}{v}

(a) A winged seed is released from a branch 20 m high and falls at 1.0 m s⁻¹ in a wind of 3.0 m s⁻¹. How far does it travel? (b) A wingless seed of the same tree would fall at 5.0 m s⁻¹. How far would it travel? (c) What does this show about wings?

(a) $d = \frac{20 \times 3.0}{1.0} = 60\ \text{m}$ .
(b) $d = \frac{20 \times 3.0}{5.0} = 12\ \text{m}$ .
(c) Halving the fall speed doubles the distance: $d$ is inversely proportional to $v$ . Wings and parachutes increase air resistance and so reduce $v$ , here by a factor of 5, carrying the seed 5 times further. Releasing seeds high up (tall trees) and in windy weather also helps. The model ignores gusts and updraughts, which can carry seeds much further.

Free Animated Education: "How Plants Disperse Their Seeds". Watch for one example of each method of dispersal and the structure of the fruit or seed that makes it work.

Warning

Common mistakes

Saying the ovule becomes the fruit. The ovule becomes the seed; the ovary becomes the fruit.
Calling a maize grain a seed. It is a fruit (caryopsis) with a single seed fused to its wall.
Thinking all fruits are sweet. A bean pod, a maize grain and a dandelion "seed" are dry fruits.
Saying the testa comes from the ovary wall. The testa comes from the integuments of the ovule; the pericarp comes from the ovary wall.
Safety: some seeds are poisonous. Castor seeds contain ricin and must never be eaten; wash your hands after handling them in practical work.

Tip

Memory hook. "Ovule → seed, Ovary → fruit, Integuments → testa, Wall → pericarp": the small thing makes the small thing, the big thing makes the big thing. For fruit types: "Berry: soft all through; Drupe: a stone inside."

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Practice: seeds, fruits and dispersal

State what each of these becomes after fertilisation: (a) the integuments, (b) the ovary wall, (c) the funicle, (d) the endosperm nucleus.
Give three differences between a bean seed and a maize grain.
Explain how you would show that a bean pod is a fruit and not a seed.
Classify, with a reason: (a) passion fruit, (b) coffee cherry, (c) sunflower "seed", (d) strawberry.
For each, name the dispersal method and one adaptation: (a) Bidens (blackjack), (b) guava, (c) castor.
Using $d = \frac{H\,u}{v}$ , calculate how far a seed travels if released at 12 m, falling at 0.40 m s⁻¹, in a wind of 4.0 m s⁻¹.
Suggest why plants such as cultivated bananas, which make no viable seeds, do not need dispersal adaptations, and what they rely on instead.

Show answersHide answers

(a) The testa (seed coat). (b) The pericarp (fruit wall). (c) The seed stalk, which leaves the hilum scar. (d) The endosperm.
Any three: bean has two cotyledons, maize one (scutellum); bean stores food in its cotyledons, maize in the endosperm; the bean is a seed, the maize grain is a fruit with the pericarp fused to the testa; maize has a coleoptile and coleorhiza, bean has none; the hilum and micropyle are visible on the bean, not on the maize grain.
A fruit develops from an ovary, so it has two scars: one where it was attached to the flower stalk and one at the other end where the style and stigma were; and it contains seeds. The bean pod has both scars and contains seeds, so it is a fruit. A seed has only one scar (the hilum) and a micropyle.
(a) Berry: soft pericarp with many seeds and no stony layer. (b) Drupe: fleshy outer layers with a hard endocarp (the "parchment") around each seed. (c) Achene: a small, dry, one-seeded indehiscent fruit. (d) False (accessory) fruit: the red flesh is a swollen receptacle and the small "pips" on its surface are the true fruits (achenes).
(a) Animals (carried outside): barbed bristles hook onto fur and clothing. (b) Animals (eaten): sweet, scented, coloured flesh; small hard seeds pass through the gut unharmed. (c) Self (explosive): the dry capsule splits suddenly and flings the seeds out.
$d = \frac{12 \times 4.0}{0.40} = 120\ \text{m}$ .
Cultivated bananas are parthenocarpic: the fruit forms without fertilisation, so there are no viable seeds to disperse. They depend on people, who plant suckers or tissue-culture plantlets (vegetative propagation, Lessons 1 and 2) and so "disperse" the plant themselves.

Quick check

Match each part of the flower to what it becomes.

Tap one on the left, then its partner on the right.

Key points

After fertilisation: zygote → embryo; endosperm nucleus → endosperm; ovule → seed; integuments → testa; ovary → fruit; ovary wall → pericarp; most other floral parts wither.
Bean: dicot, non-endospermic seed, food in two cotyledons. Maize: monocot, endospermic, one cotyledon (scutellum), and the grain is a fruit.
Fruits: simple (fleshy: berry, drupe; dry: dehiscent, indehiscent), aggregate, multiple, and false fruits.
Dispersal by wind, animals, water and explosion reduces competition and spreads the species; wind distance $d = \frac{H\,u}{v}$ .

1A market stall full of ovaries

2What becomes what after fertilisation

3The structure of seeds

4Types of fruits

5Dispersal of fruits and seeds

A market stall full of ovaries

What becomes what after fertilisation

The structure of seeds

Types of fruits

Dispersal of fruits and seeds