Studying the Growth Curve of a Plant

Aim

To study the pattern of growth of a plant seedling over time by measuring its height at regular intervals and plotting a growth curve.

Variables

• Manipulated variable: time, measured in days from the start of the investigation.
• Responding variable: the height of the seedling, measured from soil level to the tip of the shoot.
• Controlled variables: the species of plant used, the amount of water given at each watering, the light conditions the seedlings are exposed to, the surrounding temperature, and the type and amount of soil in each pot.

Materials and apparatus

• Germinating seeds or young seedlings of the same species, such as green bean (Vigna radiata)
• Several identical pots containing the same type and amount of soil
• A ruler or measuring tape
• Water
• A record sheet for tabulating measurements
• Graph paper
• A consistent light source, such as a sunny windowsill or a grow lamp

Procedure

1. Plant several seeds of the same species in identical pots containing the same type and amount of soil.
2. Water the pots and position them under the same light and temperature conditions.
3. Once the seedlings emerge, measure the height of each seedling from soil level to the tip of the shoot using a ruler.
4. Take measurements at the same time on the same days, for example every two days, over several weeks.
5. Record the height of each seedling in a table at every interval.
6. Calculate the mean height of the seedlings at each interval.
7. Plot a graph of mean height against time.

Expected results

The graph of mean height against time is typically S-shaped, also called a sigmoid growth curve. Growth is slow during the first few days after emergence (the lag phase), then speeds up over a period of rapid growth (the log or exponential phase), and finally slows down and levels off as the seedling approaches a stationary phase.

Conclusion

Plant growth over time follows a sigmoid growth curve, with a slow start, a rapid middle phase, and a levelling-off phase. This pattern reflects changes in the rate of cell division and cell elongation as the plant develops, from a slow initial establishment of roots and shoots to rapid extension growth and eventually a more stable rate of growth.

Paper 3-style questions

Question 1, hypothesis. Write a suitable hypothesis for this investigation.

Model answer. As time increases, the mean height of the seedling increases, until the rate of increase slows and the height levels off. A simple SPM form is: the longer the growth period, the greater the mean height of the seedling, up to a maximum.

Question 2, variables. State the manipulated, responding and two controlled variables, and explain why the controlled variables must be kept the same.

Model answer. Manipulated variable: time (days). Responding variable: mean height of the seedling (cm).

Controlled variables: species of plant, volume of water, light conditions, temperature and the type and amount of soil. They are kept the same so that any change in height is caused only by time and not by another factor, making the comparison fair.

Question 3, tabulation and graph. Describe how the readings should be tabulated and what graph should be drawn.

Model answer. Record time (days) in the first column and the mean height (cm) in the next, calculating the mean of the seedlings at each interval and giving values to a consistent number of decimal places. Plot mean height on the y-axis against time on the x-axis and join the points with a smooth curve, which should show the S-shaped (sigmoid) pattern.

Question 4, inference. Using the shape of the curve, infer during which phase the seedling is growing fastest and suggest why the curve later levels off.

Model answer. The seedling grows fastest during the steepest part of the curve (the log or exponential phase), when cell division and cell elongation are most rapid. The curve later levels off because growth slows as the plant matures and factors such as space and nutrients become limiting, so the rate of increase in height falls to almost zero.

Safety and good practice

• Wash your hands after handling soil and seeds to keep the work area clean.
• Keep the grow lamp away from water and switch it off when it is not needed, as it can become hot and is an electrical device.
• Handle the seedlings gently and measure without pulling them, so the plants are not damaged and the readings stay accurate.

Common mistakes

Source:SRC-DSKP-EN

Frequently asked questions