A * tropism exhibited in response to gravity. Thus, when placed in a horizontal position with all-round illumination, shoots grow upwards and roots downwards, the former being negatively and the latter positively geotropic. A clinostat, consisting of a drum to which plant seedlings can be attached, revolves so as to cancel out the effects of gravity. Using it, one can demonstrate the lack of root and shoot curvature when no net gravitational forces are operating. Like * phototropism , the removal of the shoot tip destroys the geotropic response. Certain parts, such as tertiary roots, are naturally ageotropic. See also starch-statolith hypothesis. |
