Due to Earth's conditions, the roots of plants always grow "downwards" and the stems and leaves grow "upwards". Of the forces on Earth that effect plants growth, gravity is one of the greatest. When plants are grown without the aid of gravity, in space or by limiting gravity's effects here on earth, plants roots and stems can grow in any random direction, roots and stems can even grow in the same direction, as they receive no clue from gravity. The effects of gravity or the lack there of are known as geotropisms.
Many geotropism experiments with plants are conducted in space, generally in orbit around the earth. Since spacecrafts in orbit are theoretically freefalling, plants inside them experience zero gravity in relation to the spacecraft. Sometimes, in long-term experiments, artificial gravity is used, and created by either the rotation of a plant chamber or by changes in speed and direction.
Roots of plants show positive geotropism whereas shoots, stems, leaves, etc., will show negative geotropism. Positive geotropism is growing towards the source of gravity; negative geotropism is plants growth away from the source of gravity. In addition, although geotropism was thought to be simply gravity's effect on a plant hormone, Auxin, which is observed to affect many plants reactions to light and growth, also known as phototropism, several other theories are being tested. .
Geotropism is considered a nastic movement. A nastic movement is the movement of plant part(s) in response external stimuli, or internal growth stimuli. As nastic movements tend to be slow, only noticeable in generally more than hour time periods, which lend themselves to time lapse photography, the preferred way of observing such changes as the ones that geotropism causes.
Jacques Loeb (1859-1924), whose works include The Mechanistic Conception of Life, Artificial Parthenogenesis and Fertilization, and The Organism as a Whole, is best known for the creation of the tropism theory and experiments in inducing parthenogenesis and regeneration by chemical stimulus.