Tropism 
An involuntary orienting response; positive or negative reaction to a stimulus source.  

Nastic 
Relates to the movement of a plant in response to a stimulus regardless of its direction, such as the opening of a flower in response to light or temperature, or the wilting of mimosa on being touched.  

Taxis 
A locomotor response toward or away from an external stimulus by a motile (and usually simple) organism
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