Edexcel IGCSE Biology · Spec 3.4
Seeds and Fruits
How seeds and fruits form after fertilisation, and the structure of a seed.
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Seeds and Fruits
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Explained
From pollen grain to seed, one step at a time
Pollination gets the pollen onto the stigma. Fertilisation is what happens afterwards, and the two are different events separated by a journey. Confusing them is the commonest error in this topic, so keep the words apart from the start.
The journey
A pollen grain landing on the stigma germinates, growing a pollen tube down through the style towards the ovary.
The tube enters the ovule through a small opening called the micropyle. The male gamete nucleus travels down inside the tube and fuses with the female gamete, the ovum, inside the ovule. That fusion is fertilisation.
Note where it happens. Fertilisation takes place inside the ovule, not on the stigma and not in the style. The pollen tube exists precisely because the male gamete cannot move on its own.
What becomes what
After fertilisation, three changes follow, and they are worth learning as a set.
- The fertilised ovule becomes the seed.
- The ovary becomes the fruit.
- The petals, stamens and stigma wither and fall away, having done their job.
So a fruit is a developed ovary, which is why a tomato, a pea pod and a cucumber are all fruits in biological terms even though nobody puts them in a fruit salad. If it contains seeds, it developed from an ovary.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme sets this out as a four mark chain drawn from seven possible points: the pollen grain germinates, the pollen tube grows down the style, it enters the ovule via the micropyle, the male nucleus or gamete travels down the pollen tube, it fuses with the ovum or female gamete, the ovule becomes a seed, and the ovary becomes a fruit.
Every step is a separate mark, and there are seven of them for four marks, so you can afford to be unsure of one. What you cannot afford is to compress the chain. Writing that the pollen fertilises the ovule covers four of those points in a single clause and earns one.
The verb for what the pollen grain does is germinates, the same word used for a seed. The mark scheme uses it as the first point in the chain.
The seed itself
Inside the seed coat, or testa, are the embryo and its food store. The radicle will become the root, the plumule will become the shoot, and the cotyledon holds the food, usually as starch, until the seedling can photosynthesise.
The micropyle remains as a small hole in the testa. Having admitted the pollen tube, it later admits the water that starts germination, which is a neat piece of economy worth noticing.
Why fruits exist
A fruit is a dispersal mechanism. Fleshy fruits are eaten by animals, and the seeds pass through undamaged and are deposited some distance away. Others have hooks that catch on fur, wings that spin in the wind, or pods that dry and split explosively.
Dispersal matters because a seedling growing directly beneath its parent would compete with it for light, water and mineral ions. Naming the competition, rather than just saying the seeds spread out, is usually the second mark on this question.
Spec 3.4
What you need to know
- Describe how a pollen tube grows down to the ovule
- Know that fertilisation happens inside the ovule
- Understand that the ovule becomes a seed and the ovary becomes a fruit
Active recall
Quick check
Answer each question before opening the answer.
What do the ovule and ovary become after fertilisation?
The ovule becomes the seed and the ovary becomes the fruit.
Name the main parts of a seed.
The embryo (radicle and plumule), a food store (cotyledon) and the protective seed coat (testa).
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