What are the differences between Tropism and Nastic Movement?

A tropic movement is the type of  biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant  induced by some external stimuli. This response is dependent on the direction of the stimulus while as the nastic movement is the type of movement that is not governed by the direction of the stimulus. Whatever be the direction of stimulus, the movements always occur in a particular direction.

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ANS:Tropisms and nastic movements are both plant responses to external stimuli. however tropisms are depend on the direction of the stimulus nastic movements do not depend on the direction of a stimulu

In detail

  Nastic movements are rapid, reversible responses to non-directional stimuli (eg. Temperature, Humidity, Light irradiance). It is a plants response to touch. Nastic movement is caused by turgor pressure change due to movement of water in cells as opposed to tropic movement which is actual growth and therefore irreversible. The rate or frequency of these responses increases as intensity of the stimuli increases. An example of such a response is the opening and closing of flowers (photonastic response). Nastic responses are usually associated with plants. They are named with the suffix "-nasty" and have prefixes that depend on the stimuli:

Photo- response to light
Nycti- movements at night or in the dark
Chemo- response to chemicals or nutrients
Hydro- response to water
Thermo- response to temperature
Geo-/Gravi- response to gravity
Thigmo-/Hapto- response to touch

Tropisms are directional movement responses that occur in response to a directional stimulus. Plants are not able to relocated if they happen to start growing where conditions are suboptimal. However, plants can alter their growth so they can grow into more favorable conditions, To do so requires the ability to detect where the conditions are better and then alter their growth so they can "move" in the appropriate direction. One of the most commonly observed tropic responses in plants is phototropism, in which plant stems grow towards light. As anyone who has grown plants near a window knows, the plants tend to lean towards the window where the light is usually stronger than inside the room. Another commonly observed tropic responses is gravitropism, where a plant will grow so that it stays oriented relative to the source of gravity (the earth). Thus, if a plant is knocked down the shoot will grow faster on the lower side until the shoot is more-or-less standing up again.

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