Lemna is a tiny green plant that floats on the water surface in ponds and lakes. It is made up of leaves that float and a root that trails down into the water.
Fig. 2.1 shows one plant with four leaves. D and E show the maximum length of two of the plant's leaves.
Fig. 2.1 labels: plant leaves on surface of water; root; magnification \times12.
This plant population increases because each plant splits into two smaller plants. These smaller plants then produce new leaves and divide again.
Some students chose to investigate Lemna growth. They put six plants in a small beaker containing nutrients in distilled water (nutrient solution). A lamp was used to give constant light.
The students decided to measure growth by counting the total number of leaves at the same time each day. At the beginning of the investigation there were 16 leaves in total on the plants.
Fig. 2.2 shows the beaker viewed from above on day 4.
Table 2.1:
Time/days and total number of leaves:
0 → 16
2 → 20
3 → 29
4 → [blank]
5 → 55
6 → 83
7 → 91
(a)[3]
On Fig. 2.1, draw a straight line joining D and E. Measure that line and note the result. Calculate the actual maximum length of two of the plant's leaves and give it to the nearest whole number.
(b(i))[1]
Suggest why nutrients were added to the distilled water.
(b(ii))[1]
Count the full number of leaves visible in Fig. 2.2 and record the number in Table 2.1.
(b(iii))[5]
On the grid, plot a line graph for the data in Table 2.1. Connect the points using ruled, straight lines.
(b(iv))[2]
Use your graph to estimate the total number of leaves that would have been present on day 1. Show your working on your graph.
(b(v))[2]
Predict the shape of the graph after day 7 if the investigation carries on for another six days. Explain your answer.
(b(vi))[1]
Suggest one other method that the students could use to measure the growth of Lemna.
(c)[6]
Plan an investigation to find out the effect of different concentrations of a nutrient solution on Lemna growth. Use the same method of counting the number of leaves that the students used in their investigation for measuring growth.