Positive gravitropism describes how plant roots grow downward in response to gravity. This characteristic is found in all higher plants, lower plants and other organisms.
Like animals, plants use gravity to orient themselves in space. A plant removed from soil and hung upside down demonstrates positive gravitropism by bending its roots downward. It also demonstrates negative gravitropism by bending its shoot upward. Negative gravitropism also happens when a potted plant is placed on its side. The shoot soon reorients in a vertical position and continues growing away from the earth.
Research shows that plants use specialized cells called statocytes to identify gravitation. Statocytes are made up of specific amyloplast plastids that function as statoliths. In animals, these statoliths are dense granules of calcium carbonate that weights downward. Statoliths stimulate the hair cells they are in contact with to orient animals in space.
In plants, statoliths are found in root caps and in stems. These statoliths are denser than their surrounding plant cells and their vertical sedimentation reveals the direction of gravity.
Plants have been studied in space where there is no gravity. At first the different plant parts are confused without the cues from gravity and grow in all directions. However, after a while, the plant readjusts and uses other external stimuli. The shoots, stems and leaves grow toward the light, which is called positive phototrophism, and the roots grow toward the water, which is called positive hydrotropism.