Geotropism:

• Positive: Growth towards gravity (e.g., roots).
• Negative: Growth away from gravity (e.g., shoots).
• Importance: Helps roots anchor in the soil and absorb water and minerals.

Experiment: Identifying the Region that Responds to Gravitational Stimulus in Bean Seedlings

Materials

• Two newly germinated bean seedlings with straight radicles
• Corks for mounting seedlings
• Two jars
• Rotating clinostat
• Moist cotton wool
• Pins
• Dark room (or covering to eliminate light)

Procedure

1. Prepare two newly germinated bean seedlings with straight radicles.
2. Pin the first seedling onto a cork, and place the cork at the mouth of a jar laid horizontally, ensuring the radicle is perpendicular to the direction of gravity.
3. Pin the second seedling onto a cork and place it in another jar, but mount this jar on a rotating clinostat to ensure the radicle is exposed to gravity equally on all sides.
4. Moisten both setups with cotton wool to keep the seedlings hydrated.
5. Place both setups in complete darkness to eliminate any effect of light on seedling growth.
6. Leave the setups for two days, allowing the seedlings to respond to the gravitational conditions.
7. After two days, observe and compare the direction of radicle growth in both seedlings.

Expected Results

1. The radicle of the seedling in the stationary jar will bend and grow downward, towards gravity (positive gravitropism).
2. The radicle of the seedling in the rotating clinostat will remain straight, showing no directional growth because gravity was acting equally on all sides.

Conclusion

1. The experiment shows that the radicle responds to gravitational stimuli by growing towards the direction of gravity (positive gravitropism).
2. The region responsible for detecting and responding to gravity in bean seedlings is located in the radicle, which directs its growth in response to gravitational forces.

Hydrotropism:

• Positive: Growth towards water (e.g., roots).
• Importance: Enables plants to absorb minerals dissolved in water.

Thigmonasty

• Definition: Response to touch or vibration.
• Examples:
  • Folding of leaves of Mimosa pudica when touched.
  • Trapping an insect by clamping up of an insectivorous plant (Venus flytrap). The plant secretes enzymes to kill the insect and then digests it. The nutrients are then absorbed through the leaf surface.

• Chemotropism:
  • Response to chemicals (e.g., pollen tube growth).
• Heliotropism:
  • Growth response to gradual turning of leaves or the entire plant towards light.
  • Leaves orient to maximize sunlight absorption and reduce overlap (leaf mosaic).