Plant movement is the result of irritability protoplasm, which is sensitive and respond to internal external stimuli. Unicellular plants or lower plants like many algae can freely move in water. Higher plants are firmly fixed in the soil and therefore the movement in some organs only change their direction or position is called movement of curvature.
"The changes in the environment which induces a change in the plant is called stimulus."
"Reaction of the plant to the stimulus is called response."
"The fundamental property of protoplasm by which is meant its ability to appreciate and respond to change in their environments."
Types of Movement
These movements are caused by change in the cell volume due to turgor changes and are reversible. Example is rolling up of leaves in dry weather.
Growth movement are the changes in the position of plant organs due to enlargement of cells or due to increase in number of cells. The growth movement are irreversible, i.e. the plant parts cannot come back to the original position.
(i) Autonomic or Spontaneous
These movements are independent of any external stimuli for example, in twig plants, the tip of the stem grows at unequal rate in different segments and thus causes the twining around the support. These movements are also called Notation.
(ii) Paratonic Movements
They are caused by external stimuli. Depending on the direction and types of response, the Paratonic Movements are classified into following categories:
(a) Tropic Movements
These are directional movements and are in response to stimulus, which comes mostly from one direction. They growth may be towards the stimulus or at the specific angle to the stimulus. Depending on the types of the stimulus the tropic movement are classified as under:
Stimulus is light. For example shoot is positively phototropism and root is negative phototrophic.
Stimulus is gravity. Roots are positively and shoot are negatively geotropic.
Stimulus is come chemical substances. Growth of pollen tube towards ovary is the result of secretion of chemical substance by the ovule.
Stimulus is water. Roots are positively hydrotropic.
Stimulus is solid surface or touch. Tendrils are positively haptotropic.
(b) Nastic Movements
The Nastic Movements are non-directional, i.e. they are independent of the direction of the stimulus. The stimulus may act from any direction of response will always be same. Examples are opening and closing of buds to form shoot.
For example leaves of some plants like jewelweed become horizontal during day and drop at night. Flowers of some plant open during day and close in the morning.
Crocus flower opens when it is war and closes when it is cold.
Leaves of Mimosa Pudica (Touch-me-not) plant drop when they are touched.
If Mimosa Plant is given a sharp blow, its leaflets at the top fold and drop.
(c) Tactic Movement (Taxes)
Tactic movement is the movement of an entire cell or organism in response to an external stimulus. The direction of the movement is obtained by the direction of the stimulus.
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