Required Practical 8: Investigating Plant Responses

🧪 Required Practical 8: Investigating Plant Responses

Aim: Investigate the effect of the direction of light (and gravity) on the growth of newly germinated seedlings.

This practical links directly to the auxin mechanism you have just studied — you are observing the real-world result of unequal auxin distribution causing differential cell elongation.

Equipment

• Cress or mustard seeds (fast-germinating, consistent response)
• Cotton wool or damp paper towel (to germinate seeds on)
• Petri dishes or small containers (identical for each condition)
• Cardboard boxes — one with a single small hole cut in one side (for unidirectional light), one sealed completely (for dark condition)
• A bench lamp or window providing a consistent light source
• Ruler and protractor (to measure angle of bending)
• Sticky labels and a pen (to identify conditions)

Method

1. Place equal numbers of seeds (at least 10 per condition) on damp cotton wool in identical containers. This controls the number of seeds and germination conditions.
2. Allow seeds to germinate in the dark for 2-3 days until shoots are approximately 1 cm tall and growing vertically.
3. Set up three experimental conditions:
   • Unidirectional light: Place container inside a cardboard box with a single hole cut in one side. Position a lamp so light enters only through the hole.
   • All-round light (control): Place container in the open, with light from all directions equally (e.g., on a window sill or under a lamp directly overhead).
   • Dark (control): Place container inside a completely sealed, lightproof cardboard box.
4. Keep all other variables constant: same room temperature, same volume of water on cotton wool, same number of seeds, same seed batch, same container size.
5. Leave for 5-7 days without disturbing the boxes. Check water levels every 2 days and add water if the cotton wool dries out.
6. After the observation period, carefully open the boxes and measure the angle of bending of shoots from vertical using a protractor. Record results for all seedlings in each condition and calculate the mean angle.

Variables

Variable Type	Variable
Independent	Direction or presence of light (unidirectional / all-round / dark)
Dependent	Direction of shoot growth / angle of bending from vertical (measured in degrees)
Control	Temperature, volume of water, seed type, number of seeds per container, container size, age of seedlings at start of experiment

Expected Results

• Unidirectional light: Shoots bend towards the light source (positive phototropism). The shaded side grows faster due to higher auxin concentration, pushing the shoot towards the hole.
• All-round light (control): Shoots grow straight upward. Auxin distributes evenly around the shoot, so all sides elongate equally — no bending occurs.
• Dark (control): Shoots grow tall and thin (etiolated) in a straight line, with pale yellow leaves. No phototropism occurs as there is no light stimulus. The plant grows rapidly upward, searching for light.

Improving Reliability

• Use at least 10 seedlings per condition — individual variation between seeds means one seedling is not representative
• Calculate a mean angle of bending across all seedlings in each condition to reduce the effect of outliers
• Repeat the whole experiment on a different day with a new batch of seeds — this tests reproducibility
• Use a protractor to measure bending angle precisely rather than estimating direction by eye
• Ensure the hole in the cardboard box is the only light source entering — seal any gaps with tape

Safety

• Wash hands after handling seeds (some seeds are treated with fungicides)
• Take care with scissors or a craft knife when cutting holes in cardboard boxes
• Keep the lamp away from flammable materials and do not leave it unattended for extended periods