The movement in plants is mostly seen as bending, turning, twisting or as elongation of plant parts.Plants are fixed in a place with their roots in the ground.They lack the power of locomotion,but they can move individual parts or organs when subjected to external stimuli like light,water, chemical substances and touch.
These plants movements made in response to external stimuli fall into two main categories namely Tropic movements and Nastic movements.
These are directional movements and are in response to stimulus,which comes mostly from one direction.The growth may be towards the stimulus or at the specific angle to the stimulus.Movement in which the direction of stimulus determines the direction of response is called tropism.
Depending on the types of the stimulus the tropic movement is classified as follows:
If the growth or movement of a plant part is towards the stimulus,it is called positive tropism.If the growth or movement of a plant part is away from the stimulus,it is called negative tropism.
The movement of a plant part in response to light is called Phototropism.If the plant part moves towards the sunlight,it is called positive phototropism.If the plant part moves away from the sunlight, it is called negative phototropism.The stem always grows towards light and the root always grows away from sunlight.
The movement of plant part in response to gravity is called Geotropism.If the plant part moves in the direction of gravity,it is called positive geotropism.If the plant part moves against the pull of gravity,it is called negative geotropism.Roots of a plant always grow downwards in the direction of gravity and the stem always grows upwards against gravity.
The movement of a plant part in response to a chemical stimulus is called Chemotropism.If the plant part moves towards the chemical,it is called positive chemotropism.On the other hand if the plant part moves away from the chemical,then it is called negative chemotropism.
The movement of a plant part in response to water is called Hydrotropism.If the plant part moves towards water,it is called positive hydrotropism.On the other hand if the part moves away from water it is called negative hydrotropism.The roots of a plant always go towards water and they are positively hydrotropic.
Climbing plants have weak stems.They cannot stand erect.They use thin thread like climbing organs called tendrils.Tendrils are sensitive to touch or contact of other objects,then the side of the tendril in contact with the object grows slower than its other side.This causes the tendril to bend towards the object by growing towards it,wind around the object and cling to it.The winding movement of the tendril of a climbing plant is an example for Thigmotropism.
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.
Some of the Nastic movements are as follows:
The non directional movement of a plant part in response to the touch of an object is called Thigmonasty.The best example for thigmonasty is mimosa pudica.If we touch the leaves of the sensitive plant,then its leaves fold up and droop immediately.
The non directional movement of a plant in response to light is called Photonasty.The opening of leaves and flowers during the day and their closure at night is an example.A dandelion flower opens up in the morning in bright light but closes in the evening when the light fades and it gets dark.
The non directional movement of a plant in response to temperature is called Thermonasty.In crocus, the flowers open at high temperature and close at low temperature.