Biology Fieldwork



Stinging nettle in dappled shade

3. Discussion

Although pupils can measure the length and width of leaves as a surrogate for leaf area, more meaningful results can be obtained by weighing the leaves. Photocopy the leaves from each group onto a sheet of graph paper (use a light toner setting so that the lines on the graph paper show). Ask the pupils to trace round the edge of their leaves. Ignore the toothed edge of the leaf. Alternatively, if a photocopier is not available, pupils can trace directly around the outline of the leaf, but plastic gloves should be worn.

Cut out the leaf shape and weigh this piece of paper. Then weigh `1"cm"^2`of graph paper. A balance with a sensitivity of at least tenths of a gram is needed. Use these two figures to calculate the area of the leaf.

`"The mass of the leaf outline"/("The mass of" 1"cm"^2"of graph paper")= "area of the leaf in cm"^2`

If a balance is not available students can count the 2 mm squares. Squares more than half covered should be counted as a whole square and those less than half covered not counted.

Assuming that `1"cm"^2` of graph paper contains 25 x 2 mm squares,

`"The number of squares"/25= "area of the leaf in cm"^2`

Pupils can use the prompt questions to link the data they have collected to what they know about photosynthesis, and to consider how appropriate the methods were.