What is meant by 'Tropisms'? Explain with an example.

Explanation

If the movement in a plant organ is induced by an external and directional stimulus, then it is called tropism or tropic movement.
The process induced by a stimulus is called a response or reaction. If the growth of the plant organ is towards the direction of the stimulus, it is called positive tropism and if the growth of the plant organ is in the opposite direction of the stimulus, it is called negative tropism.
Factors inducing stimulus: Light, gravitational force, water, chemical and physical touch are the common factors that induce stimuli.
Types of tropisms based on response shown by the plant organs:
(1) Phototropism:The reaction of plant organ in response to stimulus in the form of light is called phototropism or phototropic movement.
e.g., Root system of plant shows negative phototropism and the shoot system shows positive phototropism.
(2) Geotropism: The reaction of plant organ in response to the direction of the gravitational force is called geotropism.
e.g. Root system shows positive geotropism and the shoot system shows negative geotropism.
(3) Hydrotropism: The reaction or response of plant organ in response to the direction of water is called hydrotropism. e.g.,
The root system shows hydrotropism and the stem and leaves show negative hydrotropism.
( 4) Chemotropism: The reaction of plant positive organ in response to the direction of chemical substance is called chemotropism.
e.g., The elongation of the pollen tube in the direction of the micropyle of the ovule during fertilisation is the example of positive chemotropism.
( 5 ) Thigmotropism: The reaction of plant organ in response to the direction of touch is called thigmotropism.
e.g. The tendrils coil spirally along the supporting object that comes in contact with the tendril. It is an example of positive thigmotropism.