To read plant-growth data, calculate the change, compare the groups with numbers, name the variables and keep the conclusion inside what was tested.
This lesson follows interpreting meristem activity in the plant organisation and growth chapter.
What is the routine?
- Describe: give two values and the change for each group.
- Compare: use the change or rate, not only the final value.
- Explain: link the difference to the variable that was changed.
- Conclude: answer the aim, and state the limits.
Worked example: seedlings with and without nitrate
A student grows two groups of ten seedlings. Group A receives a solution with nitrate, and Group B receives a solution without nitrate. The light, water, temperature and type of seed are the same for both. The mean height of each group is measured every 4 days.
| Day | 0 | 4 | 8 | 12 |
|---|---|---|---|---|
| Group A mean height (cm) | 3.0 | 6.2 | 10.4 | 15.0 |
| Group B mean height (cm) | 3.0 | 4.8 | 6.0 | 6.6 |
Describe. Group A grows from 3.0 cm to 15.0 cm, a rise of 12.0 cm. Group B grows from 3.0 cm to 6.6 cm, a rise of 3.6 cm.
Compare. Group A grew 12.0 ÷ 3.6, about 3.3 times as much as Group B. Group B’s growth also slows: +1.8 cm, then +1.2 cm, then +0.6 cm in each 4-day period.
Explain. Nitrate supplies nitrogen, which plants need to make proteins and other growth materials. Without it, the cells make fewer new materials, so growth is slower.
Conclude. Over these 12 days, seedlings given nitrate grew more than seedlings without it. The fixed variables were light, water, temperature and seed type.
The mistake that costs marks
The common slip is a conclusion that goes further than the data. “Nitrate makes all plants grow taller” claims more than one test on one type of seedling can show.
| Wrong | Right | |
|---|---|---|
| Conclusion | Nitrate makes all plants grow taller | In this test, seedlings with nitrate grew more in 12 days |
| Comparison | Group A is taller | Group A grew 12.0 cm, Group B grew 3.6 cm |
| Mean | One seedling chosen | Mean of ten seedlings |
A mean is worked out by adding the heights and dividing by the number of seedlings. For five seedlings at 14.2, 15.6, 14.8, 15.4 and 15.0 cm, the total is 75.0 cm and the mean is 15.0 cm.
Check yourself
In a different test, Group C ends at 9.0 cm and Group D ends at 8.0 cm, but Group C started at 6.0 cm and Group D started at 4.0 cm. Which group grew more?
Answer
Group C grew 9.0 − 6.0 = 3.0 cm. Group D grew 8.0 − 4.0 = 4.0 cm.
Group D grew more, even though Group C ended taller. The change in height is the fair comparison when the starting heights differ.
What to study next
Test the whole chapter with the plant organisation and growth practice set. To see what plants need in order to grow, read the plant nutrition chapter.
The graph evidence and fair-comparison lab lets you compare sets of results, and the mistake log and paper-error review records where your conclusions went too far. For a teacher to go through your conclusions, see online one-to-one Biology tuition.