Plant responses required practical

Aim: To investigate how the direction of light (or gravity) affects the growth of newly germinated seedlings.

Variables

• Independent: Direction of light: all-round light, light from one side, or complete darkness (or orientation relative to gravity).
• Dependent: The height and direction (angle) of growth of the seedlings.
• Control: Type and number of seeds; Volume of water added and the type of growing medium; Temperature; Number of days of growth

Equipment

• Cress or mustard seeds (or broad bean seedlings for gravity)
• Three Petri dishes
• Cotton wool or filter paper
• Water and dropping pipette
• Two cardboard boxes, one with a slit cut in one side
• Light source such as a windowsill or lamp
• Ruler and protractor
• Labels

Method

1. Place damp cotton wool in three Petri dishes, adding the same volume of water to each.
2. Sprinkle the same number of seeds (for example ten) evenly onto each dish and label the dishes.
3. Leave the dishes somewhere warm and light for a day or two until the seeds germinate.
4. Put dish A in all-round light, dish B inside a box with a slit so light enters from one side only, and dish C inside a closed box in the dark.
5. Keep all dishes at the same temperature and water them equally each day so they do not dry out.
6. After about five days, measure the height of each seedling with a ruler and record the direction of growth.
7. Use a protractor to measure the angle of curvature of the seedlings in dish B.
8. Calculate the mean height for each dish.

Safety

• Wash hands after handling seeds, as some are treated with chemicals.
• Keep water away from any electrical lamp used as a light source.
• Take care if using a scalpel to cut the slit in the box.

Results and calculations

Record the height of each seedling and calculate the mean for each condition, noting the direction of growth and any angle of curvature. Present the means in a table or bar chart; seedlings in one-sided light bend towards the light, while those in the dark grow tall, thin and pale.

Common mistakes and improvements

• Seedlings drying out and dying; water each dish daily with the same volume.
• Measuring curved seedlings inaccurately; gently straighten them along a ruler or use string to measure their full length.
• Too few seeds so one unusual seedling affects the mean; use more seeds and repeat the experiment.
• Light leaking into the dark box; seal the box and check it is fully light-proof.

Exam tips

• Explain the response using auxin: auxin moves to the shaded side of the shoot, making cells there elongate more, so the shoot bends towards the light (positive phototropism).
• For gravity, remember that auxin collects on the lower side; it speeds growth in shoots (negative gravitropism) but slows it in roots (positive gravitropism).
• Explain why seedlings grown in the dark are taller: auxin is spread evenly and they elongate in search of light, but they are yellow because chlorophyll is not made.
• Identify the control dish and describe how to make the results more valid, for example keeping temperature constant and using the same seed type.