Geotropism (also referred to as gravitropism) is defined as the response of plants to gravity. Since 1869, research has been undertaken to prove that geotropism rotation promotes seed germination. Geotropism focuses on the movement of roots and stems in response to changes in the force of gravity. Research has shown that roots exhibit a positive geotropism (grows towards the gravitational pull) while stems exhibit negative geotropism (grows in the opposite direction, upwards).
Secondary branches and roots are said to be plageotropic. That is, they grow at right angles to the gravitational force. Lateral roots are referred to as Diageo tropic; they grow oblique to the gravitational force. Tertiary roots are not affected by the force of gravity and they are known as a geotropic.
Experiments such as rotating seedlings in a vertical plane to cancel gravity have contributed a lot to geotropism and some have even been adopted in science-fair projects. Experiments available are of varying degrees of complexity: for elementary grades, for middle school grades and for the high school grades. Rotating a seedling at a certain speed ensures that the force of gravity acts equally on all parts of the seedling.
It was found that a plant hormone known as auxin is responsible for the geotropism responses. The hormone accelerates cell elongation which is evident with the observed root curvature in the experiments. The greater the concentration of the hormone on the lower side of roots the lower the growth rate, the lower the concentration on the lower side the higher the growth hence the bending. It is opposite for the stems, a greater concentration on the lower side exhibited greater growth. A significant increase in the growth rate of upward and downward directed branches of weeping trees was noted.
