Question

why is nastic movement not a growth movement

Answer 1

because..Such growth independent movements are commonly termed as a nastic movement. Certain plant ... For example, rapid folding of touch-me-not leaves.

Answer 2

Nastic movements are non-directional responses to stimuli (e.g. temperature, humidity, light irradiance), and are usually associated with plants. The movement can be due to changes in turgor or changes in growth . Nastic movements differ from tropic movements in that the direction of tropic responses depends on the direction of the stimulus, whereas the direction of nastic movements is independent of the stimulus's position.The tropic movement is growth movement but nastic movement maybe or may not be growth movement. The rate or frequency of these responses increases as intensity of the stimulus increases. An example of such a response is the opening and closing of flowers (photonastic response), movement of euglena, chlamydomonas towards the source of light. They are named with the suffix "-nasty" and have prefixes that depend on the stimuli:

Epinasty: downward-bending from growth at the top, for example, the bending down of a heavy flower.[1]HyponastyPhotonasty: response to lightNyctinasty: movements at night or in the darkChemonasty: response to chemicals or nutrientsHydronasty: response to waterThermonasty: response to temperature[2]Geonasty/gravinasty: response to gravityThigmonasty/seismonasty/haptonasty: response to contact

The suffix may come from Greek νάσσω = "I press", ναστός = "pressed", ἐπιναστια = "the condition of being pressed upon".