Required Practical 8: Plant Responses

1⃣ Investigating Light on Seedlings (Phototropism)

Aim: See how light intensity affects the growth and direction of seedlings.

Variables:

• Independent variable: Light intensity (full light, partial light, darkness).

• Dependent variable: Height of seedlings.

• Control variables:

  • Volume of water

  • Type of seed (e.g., mustard)

  • Number of seeds per dish

  • Temperature / warm place

Method:

1. Place cotton wool in 3 petri dishes and soak with equal water.

2. Add 10 mustard seeds per dish.

3. Let seeds germinate in a warm place, watering daily with the same volume.

4. Ensure all dishes have the same number of seedlings (adjust if needed).

5. Measure height of each seedling using a ruler, holding stems straight.

6. Place dishes in:

   • Full sunlight

   • Partial light

   • Darkness

7. Measure height daily for at least 5 days.

8. Record results in a table and calculate mean seedling height.

9. Draw diagrams to show effects.

Expected Results:

• Seedlings grow towards light (phototropism) because auxin accumulates on the shaded side.

• Seedlings in darkness grow tallest (etiolation) but have small, yellow leaves due to lack of photosynthesis.

• Height in full vs partial light is similar because chlorophyll efficiently absorbs light.

2⃣ Investigating Gravity on Seedlings (Gravitropism / Geotropism)

Method:

1. Place a dish of seedlings on its side in the dark.

2. Observe growth direction over a few days.

Expected Results:

• Shoots grow upwards (against gravity).

• Roots grow downwards (toward gravity).

• Reason: Auxin inhibits cell growth in roots → cells on the lower side grow slower → root bends down.

💡 Key Concepts for Both Experiments:

• Auxin controls growth direction.

• Phototropism: auxin accumulates on the shaded side → shoot bends towards light.

• Gravitropism: auxin inhibits root growth on the lower side → root grows down.

• Always control variables carefully for valid results.