The three essential conditions
For a seed to germinate, it requires three conditions:
Water is absorbed through the micropyle and seed coat. It activates enzymes that break down stored food reserves (such as starch into maltose), and it causes the seed to swell and the seed coat (testa) to split, allowing the radicle (young root) to emerge.
Oxygen is needed for aerobic respiration. The seed has no leaves for photosynthesis, so it relies entirely on respiration of its food stores to release the energy required for cell division and growth.
A suitable temperature is required because germination depends on enzyme-controlled reactions. Too cold and enzyme activity is too slow; too hot and enzymes may be denatured. The optimum temperature varies by species.
Why light is not required
A common misconception is that seeds need light to germinate. Most seeds germinate in the dark because they use stored food reserves, not photosynthesis, as their energy source. Light becomes important only after the shoot emerges and the seedling begins to photosynthesise.
Some small seeds do require light as a germination trigger, but this is not part of the IGCSE syllabus requirement. The standard answer is that light is not needed for germination.
Investigating germination experimentally
The classic experiment uses several sets of seeds, each deprived of one condition while the others are provided. For example: seeds on damp cotton wool in air at room temperature (control, all conditions met); seeds on dry cotton wool (no water); seeds submerged in boiled and cooled water covered with oil (no oxygen); seeds on damp cotton wool in a refrigerator (low temperature).
Only the control group germinates. This shows that all three conditions are necessary. The boiled water removes dissolved oxygen, and the oil layer prevents new oxygen dissolving. Students often forget to explain why the water is boiled and cooled rather than just boiled.
From germination to seedling growth
The radicle (embryonic root) emerges first, anchoring the seed and absorbing water and minerals. The plumule (embryonic shoot) then grows upward toward light. Until the first true leaves expand and begin photosynthesis, the seedling depends entirely on the food reserves stored in the cotyledons or endosperm.
Enzymes such as amylase convert stored starch into soluble sugars like maltose, which are then transported to growing regions and used in respiration or as building materials for new cells.