(a) Define reflex action. State its significance.
(b) How do plants respond to external stimuli ?

Explanation

(a) A reflex action is an automatic response to a stimulus. The simplest form of response in the nervous system is reflex action. This is a rapid, automatic response to a stimulus which is not under the voluntary control of brain. It is described as an involuntary action. The pathway taken by nerve impulses in a reflex action is called the reflex arc. Reflex actions are the actions which we perform without thinking to protect ourselves. For example, coughing is a reflex action which clears our windpipe. The pupils of our eyes get smaller in bright light. This reflex action protects the retina of our eyes from damage due to much light. The pupils of our eyes get bigger in dim light so as to help us see properly even in dim light.
(b) Plants respond to external stimuli such as light, touch, etc. A growth movement of a plant part in response to an external stimulus in which the direction of stimuli determines the direction of response in called tropism.
If the growth of a plant part is towards the stimuli, it is called positive tropism.
If the growth of a plant part is away from the stimulus, then it is called negative tropism.
Types of tropism:
The movement of a plant part in response to light is called phototropism.
The movement of a plant part in response to gravity is called geotropism.
The movement of a plant part in response to chemicals is called chemotropism.
The movement of a plant part in response to water is called hydrotropism.
The directional growth movement of a plant part in response to the touch of an object is called thigmotropism.
Nasties (or Nastic Movements). The movement of a plant part in response to an external stimulus in which the direction of response is not determined by the direction of stimulus is called nastic movement.
The folding up of a the leaves of a sensitive plant on touching is an example of nastic movement. Here the stimulus is touch.
The opening up of the petals of dandelion flowers in morning in bright light and closing in the evening when the light fades is an example of nastic movement. In this case the stimulus is light.