A stimulus is anything in the environment that causes a response in an organism.
The response often involves movement either toward the stimulus or away from the stimulus.
Plants respond to stimuli such as touch, light, and gravity.
Some responses of a plant to an external stimuli are called tropisms
A tropism is generally termed “positive” if growth is toward the signal and
“negative” if it is away from the signal.

Response in plants to respond to gravity.
The stem/shoots show negative tropism. (Against gravity.)
The roots show
Positive (towards gravity.)

Picture from http://cbse.myindialist.com/biology-x-control-andcoordination-coordination-in-plants-movement-due-to-growth/ provide firm anchorage for roots.
ensure the plant can get adequate supply of water & mineral salts

Response to light.
When a plant grows toward light it is called a positive response to light, or positive phototropism

Picture from http://herbarium.desu.edu/pfk/page11/page
12/page13/page13.html
enables leaves to be in a position to receive as much sunlight as possible for photosynthesis

Response to water.
Example: plants moving to a higher humidity, roots moving to a greater concentration of water.

Picture from http://www.tutorvista.com/content/biology/biologyiv/plant-growth-movements/paratonic-movements.php
ensures that a plant gets enough water as a raw material for photosynthesis & enough mineral salts to maintain normal growth

Response to touch.
The stem bends and twists around any object it touches.

Picture from http://wikipedia.com
Gives the plant additional support.

Picture from http://www.sunnysidescience.com/plant_responses.htm
Rapid response of rapid movements of plant organs in response to a stimulus
Used as a defense.
Used to trap its prey in carnivorous plants.
Picture from http://cdnwrite.demandstudios.com/upl oad/1000/600/10/8/391618.jp
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