Juglone is a chemical produced by plants in the genus Juglans, which includes different species of walnut tree. Fig. 1.1 shows a black walnut tree. [Figure 1.1] Juglone produced by a black walnut tree can diffuse into the soil surrounding the tree. Groups of scientists have observed that juglone affects different species of plant growing close to black walnut trees. Juglone can reduce the percentage of seeds that germinate, delay the start of germination or reduce growth. In some studies, juglone has no effect on germination or growth. A student planned to investigate the effect of juglone on the cucumber plant, Cucumis sativus. In the investigation, the student: • placed some damp soil containing juglone in a tray • added some cucumber seeds to the soil • placed the container in a suitable environment for germination and growth. Fig. 1.2 shows cucumber seedlings (young plants) a few days after germination. [Figure 1.2] The student planned to compare the effect of exposing cucumber seeds to: • juglone solution with a concentration of 1.0 × 10⁻³mol dm⁻³, which is a concentration that has been measured in soils surrounding black walnut trees • a control treatment. The student planned to determine: • the percentage of cucumber seeds that germinated in soil • the growth of each cucumber seedling 10 days after the appearance of the seedling above the soil.