Some students were asked to use leaf discs to investigate photosynthesis in leaves of different types. The students decided to compare the rate of photosynthesis in four different types of leaf in a room kept at a temperature of 20 °C. The leaves were taken from: • ivy, which has a thick waxy cuticle on the upper surface • geranium, which has a thin cuticle on the upper surface • spiderwort, which has purple and green stripes and a thin cuticle • sorghum, which has a thick waxy cuticle on the upper surface. The students prepared leaf discs of the same diameter from all four types of leaf and placed them onto damp paper in Petri dishes. Each Petri dish was covered by a lid. When first placed in a solution, the leaf discs sink. As photosynthesis occurs oxygen is released and the discs rise to the surface. The time taken for the discs to reach the surface allows the rate of photosynthesis to be determined. [Figure 1.1] shows leaf discs rising to the surface after being placed in a solution of sodium hydrogencarbonate.