A growth curve shows size against time. To describe one, name each phase, say whether the rate of growth is slow, rapid, slowing or zero, and back each with numbers.
This lesson follows interpreting hormonal-cycle graphs in the reproduction, development and growth chapter.
What are the phases of an S-shaped curve?
- Slow start: the organism is small and few cells divide.
- Rapid growth: the curve is steepest, because the rate of growth is highest.
- Slowing growth: the curve flattens as resources or genes limit growth.
- Plateau: size stays constant at the mature size.
Worked example: a sunflower over twelve weeks
A sunflower’s height is measured every two weeks.
| Week | 0 | 2 | 4 | 6 | 8 | 10 | 12 |
|---|---|---|---|---|---|---|---|
| Height (cm) | 2 | 3 | 8 | 22 | 40 | 48 | 50 |
| Growth in the period (cm) | 1 | 5 | 14 | 18 | 8 | 2 |
Slow start. Weeks 0 to 2: the height rises by only 1 cm.
Rapid growth. Weeks 4 to 8: the plant grows 14 cm then 18 cm. The fastest period is weeks 6 to 8, at 18 cm in two weeks, or 9 cm per week.
Slowing growth. Weeks 8 to 10: the increase falls to 8 cm.
Plateau. Weeks 10 to 12: the increase is only 2 cm, so the height is close to its mature value of about 50 cm.
Check the total: 1 + 5 + 14 + 18 + 8 + 2 = 48 cm, and 50 − 2 = 48 cm.
The mistake that costs marks
The common slip is to read the height as the rate. “Growth is fastest at week 12 because the height is highest” confuses size with rate.
| Wrong | Right | |
|---|---|---|
| Fastest growth | Week 12, the tallest | Weeks 6 to 8, where the curve is steepest |
| Plateau | Growth is fastest | Growth is nearly zero, size is constant |
| Evidence | The plant is tall | 18 cm gained in weeks 6 to 8 |
The rate of growth is the gradient of the curve, and the size is its height.
Check yourself
Between which two measurements did the sunflower reach half of its final height of 50 cm, and how do you know?
Answer
Half of 50 cm is 25 cm. The height is 22 cm at week 6 and 40 cm at week 8, so it crosses 25 cm between week 6 and week 8.
This is also the period of fastest growth, which is why the curve passes the halfway height there.
What to study next
Test the whole chapter with the reproduction, development and growth practice set. For a plant-specific version of the same skill, see reading plant-growth investigation data.
The graph evidence and fair-comparison lab offers more curves to describe, and the mistake log and paper-error review records where you confused size with rate. For a teacher to go through your graph descriptions, see online one-to-one Biology tuition.