gra·vit·ro·pism 
(grə-vĭtrə-pĭz′əm)
(grə-vĭtrə-pĭz′əm)n.
Growth or movement of a sessile organism in response to gravity, as the downward growth of plant roots. Also called geotropism.
[GRAVI(TY) + -TROPISM.]
grav′i·tropic (grăv′ĭ-trōpĭk) adj.