Earth. An organ like the primary root that is maintained vertically and grows
downwards towards the gravity vector has a GSA of 0
, an extreme called positive
gravitropism. An organ like the shoot that is maintained vertically but grows
upwards against the gravity vector has a GSA of 180
, an extreme termed negative
gravitropism (Frank 1868). Other organs do not show strict positive or negative
gravitropism but have a GSA between these two extremes (plagiotropism;
Fig. 6.1 Higher plant organs make use of the only constant environmental cue, the gravity vector of
the Earth, for precisely orienting their organs. By orienting positive gravitropic roots they grow
towards the center of gravity to anchor themselves in the soil and take up water and nutrients. Shoots
grow negatively gravitropically away from the center of the Earth towards light. Gravitropism allows
plants to arrange their different organs in a most beneficial way to exploit the space below, on and
above the surface of the Earth
76
6 Gravitropism in Higher Plants: Cellular Aspects