Asexual and Sexual Reproduction in Plants and Microorganisms

Lessons

Senior 3 · Biology

Asexual and Sexual Reproduction in Plants and Microorganisms

5 lessons

Lesson 4 of 5

Double fertilisation, seeds, fruits and dispersal

25 min

A hundred seeds, a hundred pollen grains

Cut a ripe passion fruit in half and count the seeds in their juicy coats: there are often well over a hundred. Each seed was once an ovule, and each ovule needed its own pollen grain to land on the stigma and send a tube down to it. If a flower receives too little pollen, the fruit is small and half empty. What happens between the moment a pollen grain lands and the moment a fruit full of seeds is ripe is the subject of this lesson.

Definition

Fertilisation: the fusion of a male gamete (nucleus) with a female gamete to form a zygote.

Double fertilisation: in flowering plants, two fusions happen in each ovule: one male nucleus fuses with the egg cell to form the zygote, and the other fuses with the two polar nuclei to form the endosperm nucleus.

Endosperm: a food store in the seed that develops from the endosperm nucleus. Its cells have three sets of chromosomes (triploid, 3n).

Seed: a fertilised ovule, containing an embryo and a food store, inside a seed coat (testa). Fruit: a ripened ovary, containing one or more seeds.

Dispersal: the spreading of seeds and fruits away from the parent plant.

From pollen grain to zygote

A pollen grain lands on a stigma of the same species. It absorbs water and sugar from the stigma and grows a pollen tube.
The tube grows down through the style, digesting its way with enzymes and growing towards chemicals released by the ovule (chemotropism, Senior 1). The tube nucleus stays near the tip.
Another nucleus in the pollen grain divides to form two male nuclei, which travel down the tube.
The tube enters an ovule through the micropyle, a small gap in its coats, and bursts, releasing the two male nuclei into the embryo sac.
Double fertilisation: one male nucleus (n) fuses with the egg cell (n) to form the zygote (2n); the other male nucleus (n) fuses with the two polar nuclei (n + n) to form the endosperm nucleus (3n).

Figure 1: a pollen tube reaching an ovule

Figure 1: simplified; the pollen tube and ovule are drawn far larger than real size, and only one ovule is shown.

After fertilisation

Part before fertilisation	Becomes
Zygote	Embryo: plumule (young shoot), radicle (young root) and cotyledons (seed leaves)
Endosperm nucleus	Endosperm (food store); in beans it is absorbed into the cotyledons before the seed is ripe
Integuments (ovule coats)	Testa (seed coat)
Ovule	Seed
Ovary wall	Pericarp (fruit wall)
Ovary	Fruit
Petals, stamens, stigma and style	Wither and fall off

A fruit has two scars, one where the style was and one where its stalk joined the plant; a seed has one scar, the hilum, where it was attached inside the fruit, beside the micropyle. A bean pod is a fruit, and the beans are seeds. A maize grain is a fruit with a single seed, its pericarp joined to the testa.

Dispersal of fruits and seeds

Dispersal carries seeds away from the parent, so seedlings do not compete with it or with each other for light, water and minerals, and the species can colonise new places.

Agent	Features	Examples
Wind	Light; wings or hairy parachutes that slow their fall	Jacaranda and African tulip tree (winged seeds); sowthistle (parachutes)
Animals: eaten	Juicy, coloured, scented fruit; seeds with hard coats that pass unharmed through the gut	Guava, tomato, figs
Animals: hooks	Hooks or barbs that catch on fur and clothes	Black jack (Bidens pilosa), Desmodium
Water	Air spaces or a fibrous coat that floats; waterproof	Coconut, water lily
Self (explosive)	Pods that dry unevenly, then split and twist, flinging the seeds out	Beans, peas

Worked example 1: counting pollen grains and nuclei

A passion fruit contains 150 seeds. What is the smallest number of pollen grains that must have reached the stigma, and how many male nuclei took part in fertilisation?

One ovule, one pollen tube: each of the 150 seeds came from one ovule fertilised through one pollen tube, so at least 150 pollen grains germinated on the stigma.
Two male nuclei per tube: each tube delivered two male nuclei, one to the egg and one to the polar nuclei, so $150 \times 2 = 300$ male nuclei took part.

Worked example 2: chromosome numbers in maize

Maize body cells have 20 chromosomes (2n = 20). How many chromosomes are in (a) a male nucleus, (b) the egg cell, (c) the zygote, (d) an endosperm cell?

Gametes are made by meiosis, so they are haploid: (a) male nucleus: n = 10; (b) egg: n = 10.
(c) Zygote = male nucleus + egg: $10 + 10 = 20$ (2n), the same as the parent.
(d) Endosperm nucleus = male nucleus + two polar nuclei: $10 + 10 + 10 = 30$ (3n).

Warning

Common mistakes. (1) Saying the whole pollen grain travels down the style: only the tube grows, carrying the male nuclei. (2) Mixing up the two fusions: egg + male nucleus gives the zygote (2n); two polar nuclei + male nucleus gives the endosperm (3n). (3) Writing that the ovule becomes the fruit: the ovary becomes the fruit; the ovule becomes the seed.

Safety. In a dispersal survey, collect fruits and seeds with your hands protected and never taste them: some seeds, such as castor seeds, are very poisonous. Wash your hands afterwards.

Double Fertilization in Angiosperms, Peter Klappa. Watch the pollen tube enter the ovule and follow the two male nuclei; compare with Figure 1.

Tip

Memory hook: "ovARY → fruit; ovULE → seed", and "1 + 1 = embryo, 1 + 2 = endosperm": one male nucleus with one egg; one male nucleus with two polar nuclei.

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Practice: fertilisation, seeds and fruits

Describe the growth of the pollen tube from the stigma to the ovule.
What is meant by double fertilisation?
State what each becomes: (a) ovary, (b) ovule, (c) integuments, (d) zygote.
A bean plant has 2n = 22. How many chromosomes are in (a) its egg cell, (b) its endosperm nucleus?
How can you tell a fruit from a seed?
Give two reasons why seed dispersal is important.
Describe two features of an animal-dispersed fruit and explain how each helps.
A tomato contains 80 seeds. What is the minimum number of pollen grains that germinated on its stigma? Explain.

Show answersHide answers

The pollen grain absorbs water and sugar and grows a tube that digests its way down through the style, growing towards chemicals from the ovule; it enters the ovule through the micropyle.
Two fusions in one ovule: one male nucleus fuses with the egg to form the zygote; the other fuses with the two polar nuclei to form the endosperm nucleus.
(a) Fruit; (b) seed; (c) testa (seed coat); (d) embryo.
(a) 11; (b) 33.
A fruit has two scars (from the style and the stalk) and contains seeds; a seed has one scar (the hilum) and a micropyle.
To reduce competition with the parent and between seedlings; to colonise new areas.
E.g. juicy, brightly coloured fruit attracts animals to eat it; hard seed coats survive the gut and the seeds are left far away in droppings. Or: hooks catch on fur and are carried away.
80: each seed came from an ovule fertilised by its own pollen tube, so at least 80 pollen grains germinated.

Key points

The pollen tube carries two male nuclei down the style and into the ovule through the micropyle.
Double fertilisation: male nucleus + egg → zygote (2n); male nucleus + two polar nuclei → endosperm (3n).
Ovule → seed; ovary → fruit; integuments → testa; ovary wall → pericarp.
Dispersal by wind, animals, water or explosion reduces competition and colonises new places.

Sources and credits

Photo: Bidens pilosa seeds, Forest and Kim Starr, Wikimedia Commons, CC BY 3.0 US.
Video: Double Fertilization in Angiosperms, Peter Klappa, embedded from YouTube.

Written by Ganzaa: the text, Figure 1, the tables, the worked examples and the practice questions.

1A hundred seeds, a hundred pollen grains

2From pollen grain to zygote

3Figure 1: a pollen tube reaching an ovule

4After fertilisation

5Dispersal of fruits and seeds

A hundred seeds, a hundred pollen grains

From pollen grain to zygote

Figure 1: a pollen tube reaching an ovule

After fertilisation

Dispersal of fruits and seeds