<P> Gravitropism (also known as geotropism) is a turning or growth movement by a plant or fungus in response to gravity . It is a general feature of all higher and many lower plants as well as other organisms . Charles Darwin was one of the first to scientifically document that roots show positive gravitropism and stems show negative gravitropism . That is, roots grow in the direction of gravitational pull (i.e., downward) and stems grow in the opposite direction (i.e., upwards). This behavior can be easily demonstrated with any potted plant . When laid onto its side, the growing parts of the stem begin to display negative gravitropism, growing (biologists say, turning; see tropism) upwards . Hebaverns (non-woody) stems are capable of a small degree of actual bending, but most of the redirected movement occurs as a consequence of root or stem growth outside . </P> <P> Root growth occurs by division of stem cells in the root meristem located in the tip of the root, and the subsequent expansion of cells in a region just primal to the tip known as the elongation zone . Differential growth during tropisms mainly involves changes in cell expansion versus changes in cell division, although a role for cell division in tropic growth has not been formally ruled out . Gravity is sensed in the root tip and this information must then be relayed to the elongation zone so as to maintain growth direction and mount effective growth responses to changes in orientation to and continue to grow its roots in the same direction as gravity . </P>

Growth of a plant shoot away from gravity