Locate a clump of nettles at the edge of trees where one side is more shaded than the other. Using one clump of nettles spread by rhizomes from a single plant helps to control genetic variation and improves the reliability of the method.
Introducing the investigation to pupils
From KS3, pupils will have been introduced to the process of photosynthesis and have some knowledge and understanding of the structure of the leaf (including the role of palisade cells, chloroplasts and air holes). If there is time show pupils a nettle plant which has been dug up so that they can see the rhizome system and appreciate that the shoots all belong to one plant. Using live specimens of a sun shoot and a shade shoot, ask the pupils to list the differences that they notice and to suggest what the two shoots might look like if they are left in the sun for 15 minutes. Live specimens wilt very quickly, particularly of shade shoots, so you may need to rest the shoots in water. Write the differences on the board and discuss how these modifications might help the plant.
Divide the class into groups of 3–5 pupils. Each group lays a tape measure across the part of the nettle clump being sampled so that one end is in the shade and the other end is in the sun. Select the two largest shoots touching the first half metre of the tape (a shade shoot) and the two largest shoots touching the last half metre of the tape (a sun shoot). If possible all the shoots should be flowering or non-flowering as variation in leaf size can occur between flowering and non-flowering shoots. In total the class should sample a minimum of 10 shade and 10 sun shoots.
For each shoot sampled, pupils measure the length of the third and fourth internodes. The third internode is the distance between the third and fourth leaves down the stem, the fourth internode is the length between the fourth and fifth leaves. It is important that, when sampling to find leaf area, that all the leaves used are of the same maturity and are fully formed. Any side shoots should be avoided. Once the internodes have been measured, pupils remove the fifth and sixth leaves with scissors and place them into polythene bags. Each group should collect at least two shade leaves and at least two sun leaves.