Geotropism

Geotropism, a term applied in vegetable physiology to the effect of gravitation (q.v.) upon the direction of growth. Growth towards the centre of gravity, which is characteristic of roots, is termed positive; that away from the centre of gravity, characteristic of stems, negative geotropism. Leaves (q.v.), especially those that are flattened and differ in the structure of their upper and under surfaces, are commonly dorsiventral, placing themselves, that is, at right-angles to the direction of gravity by what is termed diageotropism. To demonstrate that the force directing the growth of roots and stems is gravitation, and to measure the intensity of this tendency to rectipetality, or growth in a straight line, an apparatus known from its inventor, Thomas Andrew Knight, as Knight's machine, is employed. It consists of a wheel, which can be rotated in a horizontal or in a vertical plane, or with an alternating or rolling motion, round the edge of which seedlings are planted, so that when it is rotated centrifugal force is substituted for gravity. On this machine, if moving vertically or horizontally, roots tend to grow outwards and stems inwards; but with the rolling or alternating movement they grow indiscriminately.