Geotropism is a movement of plant organs in response to the stimulus of the Earth's gravitational force. Main roots exhibit positive geotropism, that is, they grow straight downward. Main stems and flower stalks exhibit negative geotropism, growing upward. Leaves, lateral roots, and many stem branches show diageotropism, i.e., assume a horizontal or diagonal position.
Geotropic curvatures may be most easily observed by placing plants on their sides. After several hours in this position, the growing tips of roots and stems show definite changes in the direction of their growth, the stems turning upward, the roots downward. That gravity is the stimulus initiating these movements may be demonstrated by placing a plant on a clinostat, an instrument that rotates the plant on a horizontal axis, equalizing the force of gravity on all sides of the plant. In this condition, the roots and stems continue their horizontal growth and do not show geotropic curvatures. When the clinostat is stopped, so that the plant is in an unchanging horizontal position, geotropic bending occurs.
The gravitational stimulus causes an unequal distribution of plant growth hormones (auxins) on the upper and lower sides of roots and stems, leading to a difference in the rates of tissue growth on the upper and lower sides. In main stems that are in a horizontal position, the rate of growth becomes more rapid on the lower side, thus causing an upward curvature. In main roots resting on their sides the rate of growth becomes more rapid on the upper side, thus causing a downward curvature.
Geotropism occurs in all higher plants and in such lower types as mosses, liverworts, ferns, many fungi, etc. Geotropism, like other tropisms, is a method by which plants adjust themselves to external stimuli and thus results in a definite advantage to plants.