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Phycomyces, a fungus, exhibiting phototropism. A tropism (from Greek ÏÏÏÏÎ¿Ï, tropos, a turning) is a biological phenomenon, indicating growth or turning movement of a biological organism, usually a plant, in response to an environmental stimulus.
  A tropism is a growth response whose direction is determined by the direction of the stimulus.   Growth towards the stimulus is called a positive tropism. Growth away â¦ from the stimulus is called a negative tropism.  The response to light is called phototropism.Shoots are positively phototropic, roots are negatively phototropic.   The response to gravity is called gravitropism (or sometimes geotropism). Growth towards the stimulus is called a positive tropism. Growth away â¦ from the stimulus is called a negative tropism.   The response to light is called phototropism. Shoots are positively phototropic, roots are negatively phototropic.   The response to gravity is called gravitropism (or sometimes geotropism).