(a) (i) Define a reflex arc. Why have reflex arcs evolved in animals? Trace the sequence of events which occur, when you suddenly touch a hot object. (ii) Name the part of nervous system which helps in communication between the central nervous system and other parts of the body. What are the two components of this system? OR (b) (i) Leaves of 'chhui-mui' plant begin to fold up and droop in response to a stimulus. Name the stimulus and write the cause for such a rapid movement. Is there any growth involved in the movement? (ii) Define geotropism in plants. What is meant by positive and negative geotropism? Give one example of each type.

PART (a)

(i) Reflex Arc and Sequence of Events

• Definition: A reflex arc is the shortest neural pathway that links a sensory receptor to an effector (muscle or gland) through the spinal cord, allowing for an immediate, automatic response to a stimulus without conscious thought.
• Evolutionary Significance: Reflex arcs evolved in animals because the thinking process of the brain is not fast enough to prevent injury in urgent and dangerous situations. Many animals have very little or no complex neural network for thinking, so reflex arcs serve as highly efficient survival mechanisms for quick responses.
• Sequence of Events on Touching a Hot Object:

$$\text{Stimulus (Heat)} \longrightarrow \text{Receptors (Skin)} \longrightarrow \text{Sensory Neuron} \longrightarrow \text{Spinal Cord (Relay Neuron)} \longrightarrow \text{Motor Neuron} \longrightarrow \text{Effector (Muscle)} \longrightarrow \text{Response (Hand withdrawn)}$$

1. Detection: Thermoreceptors in the skin of the hand detect the heat (stimulus).
2. Transmission: The sensory neuron carries this electrical impulse to the spinal cord.
3. Integration: In the spinal cord, the impulse is transmitted via a relay neuron directly to a motor neuron.
4. Action: The motor neuron carries the impulse to the effector muscle of the arm.
5. Response: The muscle contracts, pulling the hand away from the hot object instantly.

(ii) Peripheral Nervous System (PNS)

• The part of the nervous system that facilitates communication between the Central Nervous System (CNS) and the rest of the body is the Peripheral Nervous System (PNS).
• Two Components of PNS:

1. Cranial Nerves: Nerves that arise directly from the brain.
2. Spinal Nerves: Nerves that arise from the spinal cord.

---

PART (b) (OR)

(i) Movement in 'Chhui-Mui' (Sensitive Plant)

• Stimulus: Touch (thigmic stimulus).
• Cause of Rapid Movement: The plant uses electrical-chemical means to convey information from cell to cell. Unlike animals, plants do not have specialized nervous tissue. Instead, plant cells change shape by changing the amount of water inside them, leading to swelling or shrinking, which causes the leaves to fold up and droop.
• Growth Involvement: No, there is no growth involved in this movement. It is a growth-independent (nastic) movement.

(ii) Geotropism

• Definition: Geotropism is the directional growth or movement of plant parts in response to the pull of gravity.
• Positive Geotropism: Growth of a plant part towards the direction of gravity (downwards).
• Example: Roots of a plant growing downwards into the soil.
• Negative Geotropism: Growth of a plant part away from the direction of gravity (upwards).
• Example: Shoots of a plant growing upwards and away from the earth.