Question

Define reflex action. How do plants respond to external stimuli?

Answer 1

Hi mate !!

• Reflex action :- It is the simplest form of response in the nervous system.
It is automatic response to a stimulus which is not under the voluntary control of the brain. That's y it is said to be involuntary action.


Eg. :- Moving of our hand when we see something tasty .

It happens as follow :-

The receptor cell present in our eye send the message to the brain through sensory neurones in the form of electrical impulses.

Brain read the message and suggest the act against it to the effector cell through motor neurones in the form of electrical impulses.

And effector does the work [ pick the glass ] [ fig . 01 ]


• How do plants respond to external stimuli?

ans :- Since plants haven't nervous system hence they respond through the action of harmones !!


They have harmones which act against the Stimulus !!

Plants have 4 main harmones they are :-

° Auxin

° Gibberellin

° Cytokinins

° Abscisic acid

Auxin , Gibberellin and cytokinins are harmone which promote growth of plant whereas , Abscisic acid is a harmone which inhibits the growth of plant !!

• There are two types of response from which plants respond !!

( i ) Tropic movement :- It is growth movement of plant part in response to an external stimuli. It is a directional movement.

It is mainly of 5 types :-

° Phototropism :- response to light
Eg. :- Bending of shoot towards sun !


° Geotropism :- response to gravity
Eg. Root grows towards gravity!!


° Chemotropism :- Response to Chemical
Eg. During fertilisation the growth of pollen grains !


° Hydrotropism :- Response to water
EG. growth of root towards water.

° Thigmotropism :- Response to touch
Eg. Growth and currls made by tendrils.


( ii ) Nastic Movement :- The movement of a plant part in response to an external stimuli. It is non directional movement !!

It is mainly of 2 types :-

° Thigmonasty :- Response to touch
[ fig. 02 ]

° Photonasty :- Response to light !!
Eg. Sun flower opens in day
Dandelion opens in night !!