Gravitaxis, gravikinesis, and gravitropism are different graviresponses fou
nd in protists and plants. The phenomena have been intensively studied unde
r variable stimulations ranging from microgravity to hypergravity. A huge a
mount of information is now available, e.g. about the time course of these
events, their adaptation capacity, thresholds, and interaction between grav
ity and other environmental stimuli. There is growing evidence that a pure
physical mechanism can be excluded for orientation of protists in the gravi
ty field. Similarly a physiological signal tranduction chain has been postu
lated in plants. Current investigations focus on the question whether gravi
ty is perceived by intracellular gravireceptors (e.g. the Muller organelle
of the ciliate Loxodes, barium sulfate vacuoles in Chara rhizoids or starch
statoliths in higher plants) or whether the whole cell acts as a sedimenti
ng body exerting pressure on the lower membrane. Behavioral studies in dens
ity adjusted media, effects of inhibitors of mechano-sensitive ion channels
or manipulations of the proposed gravireceptor structures revealed that bo
th mechanisms have been developed in protists and plants. The threshold val
ues for graviresponses indicate that even 10 % of the normal gravitational
field can be detected, which demands a focusing and amplifying system such
as the cytoskeleton and second messengers.