A growth curve shows how size changes with time. To interpret it, compare the increase in each equal time interval, not the height itself.
This lesson belongs to Support movement and growth. Next, see how plant support structures relate to function.
What does an S-shaped curve show?
Most growth curves for a single organism rise slowly, climb steeply, then level off. Each part has a reason you can state.
- Slow start: the seedling is still using stored food and building its first leaves.
- Steep middle: the plant makes food quickly and cells divide and enlarge fast.
- Levelling off: growth slows as the plant reaches its mature size or as conditions limit it.
Keep the two ideas apart. Height tells you how big the plant is. Growth rate tells you how fast height is changing, and it is the increase per interval.
Worked example: a bean seedling
An original experiment measures the average height of ten bean seedlings grown in the same soil, with the same watering and the same window light. Height is measured from the soil surface to the tip.
| Day | 0 | 5 | 10 | 15 | 20 | 25 | 30 |
|---|---|---|---|---|---|---|---|
| Height (cm) | 2 | 3 | 6 | 12 | 19 | 23 | 24 |
Step 1: find the increase in each 5-day interval. Subtract each reading from the next: 1, 3, 6, 7, 4, 1 cm.
Step 2: read the pattern. The largest increase is 7 cm, between day 15 and day 20. So the fastest growth is in that interval, at 7 ÷ 5 = 1.4 cm per day.
Step 3: describe the plateau. From day 25 to day 30 the height rises by only 1 cm, so growth has nearly stopped.
Step 4: state the assumptions. The answer is fair only if water, light, soil and the measuring point stayed the same for all ten seedlings. Averaging ten seedlings also reduces the effect of one unusual plant.
A full-mark sentence reads: “The seedlings grew fastest between day 15 and day 20, with an increase of 7 cm, assuming water, light and soil were the same throughout.”
The mistake that costs marks
The common slip is to name the day with the greatest height as the day of fastest growth. Here that would be day 30 at 24 cm, yet the increase in that last interval is only 1 cm, the joint smallest.
| Question asks | Wrong reading | Right reading |
|---|---|---|
| When is growth fastest? | Day 30, tallest point | Day 15 to 20, largest increase |
| What happens at the end? | The plant stops living | Growth slows to 1 cm per interval |
| Is the result reliable? | Yes, the graph looks smooth | Yes, if conditions were kept the same |
If you want to practise judging whether a comparison is fair, the graph evidence and fair-comparison lab lets you test it on your own data.
Check yourself
Another set of plants is measured every 7 days. Average height in cm: day 0 is 1, day 7 is 5, day 14 is 11, day 21 is 19, day 28 is 23, day 35 is 24. In which interval is growth fastest, and what has happened after day 28?
Answer
The increases are 4, 6, 8, 4 and 1 cm. The largest is 8 cm, so growth is fastest between day 14 and day 21.
After day 28 the increase is only 1 cm over 7 days, so growth has nearly levelled off. Add an assumption such as “water and light were the same for all plants”.
What to study next
Work through the growth and support practice set, then log any slips with the mistake log and paper-error review. For the wider subject, see online one-to-one Science tuition.