Seed structure and function

A seed is a small, protected package containing a plant embryo and a store of food, formed from a fertilised ovule. Its structure allows it to survive until conditions are right to germinate.

The embryo of a seed is made up of the radicle and the plumule, and the food it will need before it can make its own is held in the cotyledons or, in some seeds, in the endosperm. Exam questions usually give a labelled diagram of a broad bean or maize seed and test whether you can name each part and state its precise role in germination.

Parts and functions

How structure suits function

A seed must survive a dormant period and then, when conditions are right, supply its embryo with everything needed to grow until the first leaves can photosynthesise. Its parts are adapted to both of these stages.

Learn each part as a feature linked to a reason, because the mark scheme rewards the link, not the label on its own.

• The tough, often waterproof testa protects the delicate embryo from mechanical damage, pathogens and water loss during dormancy.
• The micropyle lets water and oxygen enter when conditions are suitable, so water can be absorbed and germination can begin.
• The cotyledon (or endosperm) stores food that supplies energy and raw materials for growth before the seedling can photosynthesise.
• The radicle grows out first to anchor the seedling and begin absorbing water, before the plumule pushes upwards.
• The plumule is often bent into a hook as it grows, which protects its delicate tip as it is pushed up through the soil.

Related processes

Germination begins when the seed absorbs water through the micropyle. Water activates enzymes such as amylase, which digest the stored starch in the cotyledon into soluble sugars like maltose and glucose.

These sugars are then used in respiration to release the energy the growing radicle and plumule need, and to build new cell material.

This is why a germinating seed also needs oxygen and a suitable temperature: oxygen for aerobic respiration and warmth so the enzymes work at a good rate. Once the plumule reaches the light and opens its first leaves, the seedling starts photosynthesis and no longer depends on the stored food.

The seed itself was formed after fertilisation of the ovule, so this topic follows on from pollination and fertilisation and leads into growth in plants.

Common labelling errors

Worked question

Question (in the style of Paper 2 Section A): The diagram shows a broad bean seed. (a) Name the part that develops into the root. [1 mark] (b) Explain the role of the cotyledon during the early stages of germination. [2 marks] (c) A germinating seed is kept in a sealed jar with all the oxygen removed.

Explain why it fails to grow. [2 marks] (d) Suggest why the dry mass of the seedling falls during the first few days before it rises again. [2 marks]

Model answer: (a) The radicle. (b) The cotyledon stores food reserves such as starch, which are digested and used in respiration to supply energy and materials for the embryo to grow before it can photosynthesise.

(c) Without oxygen the seed cannot carry out aerobic respiration, so too little energy is released for the cells to grow and divide. (d) The stored food is used up in respiration faster than new material is made, so dry mass falls; once the leaves open and photosynthesis begins, the seedling makes more food than it uses and the dry mass rises again.

Marking note: parts (c) and (d) both test respiration, so name the process and link it to energy or dry mass rather than just describing the result.

Source:SRC-DSKP-EN

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