Write notes on-

Root pressure, Transpiration, Nerve cell, Human brain, Reflex action

a. Root pressure: Root pressure can be defined as a force that helps to drive fluids upward into the water conducting vessels. It is generated as a result of osmotic pressure in the cells of the roots and can be demonstrated by exudation of fluid when the stem is cut off just above ground. As, the stem is cut off, dops of solution begin to ooze out which is an effect of root pressure. The effect of root pressure is also visible at night and early morning. During this time the rate of evaporation is low, as a result of which water droplets can be seen the around special openings of veins near the tip of leaves of plants.

b. Transpiration is the loss of water in the form of vapours from the leaves of a plant. The rate of transpiration is affected by number of external and internal factors.

Internal factors:

(i) Leaf area: The greater the leaf area the higher will be the water loss due to transpiration.

(ii) Leaf structure: Presence of thick cuticle, wax layer, compact mesophyll cells reduces the rate of transpiration.

(iii) Root-shoot ratio: Low root-shoot ratio decreases rate of transpiration.

External factors:

(i) Atmospheric humidity: The higher the relative humidity lower will be the rate of transpiration.

(ii) Temperature: Rise in the atmospheric temperature increased the rate of transpiration.

(iii) Wind velocity: High wind velocity increases the transpiration rate.

(iv) Water supply: Deficiency of water in soil decreases the rate of transpiration.

Importance of transpiration:

(i) The rate of transpiration affects the rate of absorption of water by the roots.

(ii) Transpiration creates a suction pressure for the upward movement of water in tall trees.

(iii) It helps in the distribution of water throughout a plant.

(iv) Plants get rid of excess water by transpiration. It helps to keep the surface of the leaves cool.

c. Nerve cell- Nerve cells are also known as neurons.

A neuron consists of 3 parts.

(i) Cell body − It contains a nucleus and cytoplasm.

(ii) Axon − It is a long part arising from the cell body. It transmits impulses away from the cell body.

(iii) Dendrites − These are short, branched parts arising from the cell body. They receive the nerve impulses.


d. Human brain-

The human brain is the main coordinating centre of the body. It is a part of the central nervous system and receives information from the other parts of the body and interprets them. The human brain can be divided into three distinct regions.

(i) Forebrain: It consists of cerebrum, thalamus and hypothalamus. The forebrain is the main site of learning, intelligence and thinking. It also has sensory and motor sites which control all the voluntary movements. Hypothalamus is concerned with things like body temperature, urge to eat, drink, etc.

(ii) Midbrain: It consists of regions concerned with the sense of sight, hearing, etc. It also transmits motor impulses to the limbs.

(iii) Hindbrain: It controls most of the involuntary actions like heartbeat, blood pressure, etc. Cerebellum maintains the posture and equilibrium of the body.

e. Reflex action- The automatic action or response provoked by a stimulus is known as reflex action. The sensory nerves that detect the stimulus are connected to nerves that move the muscles. Such a connection is called a reflex arc. The reflex arc connections meet in a bundle in the spinal cord. The signal and the response that has taken place reach the brain but the brain does not coordinate the reflex response. For example, we withdraw our hands on touching any hot article.