A Controlled Experiment on Seed Germination
FoundationA seed is a coat around an embryo with food stored in its cotyledons. Germination needs water, air and a suitable temperature — light is not required. In a controlled experiment change only one condition at a time and keep the rest identical, and the conclusion will be reliable.
- A seed is a coat around an embryo; food sits in cotyledons (beans) or endosperm (grains).
- Germination needs water, air and a suitable temperature; light matters only once true leaves grow.
- A controlled experiment changes one variable at a time, keeping the rest identical to the control.
文體小知識：對照實驗報告以「實驗組與對照組」比較單一變數的影響，是生物探究最常用的方法。
§Text Passage
1Peel open a soaked soya bean: the outermost layer is the protective seed coat; inside is the embryo — a plumule that will grow into shoot and leaves, a radicle that will become the root, joined by the hypocotyl — flanked by two thick cotyledons storing food for germination. In grains like rice the food is stored in the endosperm. A dry seed lies dormant like a stopped alarm clock, waiting for the right conditions to wake and germinate.
2Germination needs three things. Water softens the coat and rouses enzymes that break down the cotyledons' food for the growing embryo. Oxygen from the air lets the seed respire and release energy for growth. A suitable temperature, roughly twenty to thirty degrees Celsius, keeps the enzymes working — too cold or too hot and they fail. Notably, light is needed only later, when the seedling grows true leaves and starts photosynthesis; germination itself can happen in the dark.
3To verify these conditions we design a controlled experiment. The control group receives every suitable condition and serves as the baseline; each experimental group lacks one thing: no watering (no water), sealed completely with cling film or fully submerged (no air), placed in the fridge (low temperature), or kept in a cardboard box (no light). Every group uses ten seeds of the same mung beans, the same cups and cotton, and starts the same day; only one condition changes at a time so any result can be traced to that factor.
4During the experiment, observe and record daily at the same time: how many seeds have germinated, shoot length in centimetres measured with a ruler, and colour changes, all organised into a table. The expected results: most control seeds germinate within a few days; the dry and cold groups do not germinate; fully submerged seeds rot for lack of oxygen; the dark group germinates as usual, proving light is not required. Only after true leaves appear does the seedling photosynthesise and need plenty of light. Controlling variables and recording honestly are the most important basics of scientific inquiry.
1Summary 重點摘要
A seed is a coat around an embryo with food stored in its cotyledons. Germination needs water, air and a suitable temperature — light is not required. In a controlled experiment change only one condition at a time and keep the rest identical, and the conclusion will be reliable.
2Vocabulary cards Vocabulary
The protective outer layer of a seed, which splits during germination.
The part inside a seed that grows into a new plant — plumule, hypocotyl and radicle.
The seed leaf storing food for germination; beans have two.
The process of a seed waking after absorbing water and putting out root and shoot.
The group given all suitable conditions, serving as the baseline for comparison.
A group deliberately missing or changing one condition, compared against the control.
The gas seeds need to respire while germinating; without it they rot.
The proportion of seeds in a group that germinate — 8 of 10 is 80 percent.
The resting state of a seed, with slowed life processes, awaiting suitable conditions.
3Step by step Steps
- Step 1: Learn the parts of a seedExample: soak a soya bean until soft, then carefully peel it open: outside is the seed coat that protects the inside, within are two thick cotyledons storing food, and tucked between them the tiny plumule and radicle of the embryo. The plumule will grow into stem and leaves, the radicle into roots, while the cotyledons act like a packed lunch supplying food for the first stage of growth. Knowing the parts helps us understand what each does during germination. Peel the seed under a magnifier, draw each part and label it; note the embryo's orientation too — the radicle always points the way roots will later grow and is the first part to emerge.
- Step 2: Watch what water doesExample: a dry bean is hard and small, but after a night in water it swells and softens — water at work: the coat softens so the embryo can break through, and enzymes break down the food in the cotyledons into usable substances for the growing embryo. Without water a seed simply stays dormant. That is why seeds are watered after sowing and why farmers irrigate their fields in a drought. Soaking has its limit too: left in water two or three days before sowing, seeds may rot for lack of air; that is why sprouters wrap beans in wet cloth — giving water and air at once, both conditions met.
- Step 3: Test the need for airExample: submerge ten mung beans completely in deep water and not a single one germinates; after several days they even smell rotten. Deep water lacks oxygen, so the seeds cannot respire to release energy and slowly rot. The experiment proves that air is essential for germination — which is why beans being sprouted are never drowned completely, and waterlogged fields must be drained. Think of the applications: when sprouting beans the water should just cover them, and sprouted ones move to airy spots; farmers sprout rice seeds before planting, and waterlogged paddies must be drained or roots go short of oxygen.
- Step 4: Test the effect of temperatureExample: at a room temperature of about twenty-five degrees Celsius, mung beans sprout within two or three days; beans kept in the fridge at about four degrees show no movement for ten days. Temperature affects how active the enzymes are — the cold slows the breaking down of food and the growth reactions, so the seeds stay ungerminated. The cold does not kill them, though: most sprout again once returned to room temperature, showing temperature is a key condition for speed. Use one calibrated thermometer and fix where you measure; if a high-temperature and a low-temperature group are tested together, never put them on different shelves of the same fridge, for shelves differ in temperature.
- Step 5: Test whether light is necessaryExample: keep one tray of mung beans in the sunshine and another in a light-proof cardboard box, giving both water and air — both trays sprout and grow. This proves light is not necessary for germination, because the food a seed needs is already stored in its cotyledons and photosynthesis is not yet involved. Note, however, that once seedlings have grown leaves they do need sunlight to keep growing healthily. Seedlings grown in the dark turn tall and yellow, for without light they cannot make chlorophyll; move a box-grown seedling into sunshine and it greens within days — a neat demonstration of how chlorophyll depends on light.
- Step 6: Set up control and experimental groupsExample: prepare four identical paper cups, each with ten beans from the same batch on well-soaked cotton. The control group is watered normally and kept at room temperature; the three experimental groups respectively get no water, are sealed completely with cling film, or are placed in the fridge. Changing only one condition at a time and keeping everything else identical to the control means any difference can be traced to the one changed condition — the conclusion then stands firm. Use many beans per group: with only one or two, chance decides the result; ten or more lets a germination rate stand for the batch — exactly why science values repetition.
- Step 7: Record daily and organise dataExample: observe every group at the same time each day, recording how many seeds have germinated and the length of each shoot, then organise the numbers into a table or line chart. If the control group reaches ninety percent germination within eight days while the dry, sealed and cold groups stay near zero, it clearly shows that water, air and a suitable temperature are all indispensable. Organising data makes the result plain at a glance and easy to report and compare with classmates. On the line chart, dates run along the bottom and the cumulative count of germinated beans up the side, one coloured line per group; missed observations stay blank with a note, never filled in with guesses — honest records come first.
4Diagrams Diagrams
5Worked example Example
Example: Yau wants to know whether temperature affects how fast mung beans germinate. Design a controlled experiment for her: write the hypothesis, materials, steps, how to control variables, and the expected results.
- Hypothesis: temperature affects germination speed; room temperature around twenty-five degrees is best, and cold delays or prevents germination.
- Materials: twenty mung beans from one packet, two identical paper cups, cotton, water, a room-temperature spot and a fridge compartment.
- Steps: put ten beans and water-soaked cotton in each cup; keep cup A at room temperature (about 25 degrees) and cup B in the fridge (about 4 degrees); observe and record germination and shoot length daily at the same time.
- Controlling variables: the beans' batch and number, the cotton's wetness and the watering time are identical in both cups — only temperature differs.
- Expected results: cup A sprouts in two or three days with fast growth; cup B hardly sprouts; returned to room temperature it sprouts slowly, showing cold suppresses rather than kills the seeds.
- Answer: Changing only the temperature with everything else identical shows that a suitable temperature speeds germination while cold suppresses it, and the seeds recover when warmed — temperature is a key condition.
6Common mistakes Mistakes
7Quick check Quick check
Q1What three conditions does a seed need to germinate?
Q2Is light necessary for germination?
Q3Where is a seed's food stored?
Q4What is a control group?
Q5Why observe and record over several days?
8Recap Recap
- A seed is a coat around an embryo; food sits in cotyledons (beans) or endosperm (grains).
- Germination needs water, air and a suitable temperature; light matters only once true leaves grow.
- A controlled experiment changes one variable at a time, keeping the rest identical to the control.
- Submerged seeds rot without oxygen; low temperature idles the enzymes and delays germination.
- Record germination rate and shoot length daily in a table for complete, reliable data.