Response:

• The reaction of an organism to a stimulus detected in its environment.
• Sensitivity/Irritability: The ability to detect and react to changes. Plants have limited sensitivity primarily related to their nutritional needs.

Types of Stimuli and Responses:

• Light
• Temperature
• Humidity
• Gravity
• Chemicals

Types of Responses:

1. Tropisms
   • Growth responses to the direction of a stimulus.
   • Examples:
     • Phototropism: Growth towards light.
     • Geotropism: Growth in response to gravity.
     • Hydrotropism: Growth towards water.
     • Chemotropism: Growth towards chemicals.
     • Thigmotropism: Response to touch.
     • Heliotropism: Gradual turning of leaves or entire plant towards the light.
   • Controlled by plant hormones called auxins.
2. Nasties
   • Non-directional movement responses not influenced by the direction of the stimulus.
   • Examples:
     • Photonasty: Quick response to light intensity changes (e.g., morning glory flowers opening).
     • Thigmonasty: Response to touch or vibration (e.g., Mimosa pudica leaves folding, Venus flytrap trapping insects).
     • Chemonasty: Response to chemical/nutrient concentration (e.g., sundew tentacles).
     • Hydronasty: Response to water (e.g., flowers opening/closing due to humidity).
3. Abscission
   • Shedding of leaves, flowers, or fruits in response to adverse environmental conditions (e.g., winter).
   • Cells near the base of the leaf dry out, leading to leaf fall.
4. Dormancy
   • A period of drastically reduced metabolic activity in seeds and buds, halting growth.
   • During dormancy, there is no growth in plant structures.
5. Taxes

• Definition: Locomotory response exhibited by unicellular organisms, determined by the direction of a stimulus. Can be positive or negative.
• Types of Taxes:
  1. Phototaxis: Movement in response to light.
     • Example: Chloroplasts in palisade mesophyll cells move to the upper surface of a leaf to capture more light, increasing the rate of photosynthesis.
  2. Chemotaxis: Movement towards or away from chemicals.
     • Example: Motile gametes move towards specific chemicals during fertilization.