GEOTROPISM, je-6erifi-pizm, the influ ence which causes a tendency in plants and animals to grow toward the centre of the earth; it is defined by Dr. John Coulter as ((sensitive ness to gravity?' Geotropism in its simplest form, called "positive," causes growth directly downward, as in the tips of roots, which strike straight down into the earth. So strong is this influence upon roots that they will turn from any abnormal position in which they are placed and bend downward until they reach and pene trate the ground,— an adaptation for the preser vation of plants against dislodgment by wind or water. The reverse of this is negative geotro pism (apogeotropism), the influence which causes parts of plants, particularly stems, to grow away from the earth. A demonstration of these opposing influences in regulating plant growth is found in the fact that when seedlings are caused to revolve continuously and rapidly for a period, their stems point and increase to ward the centre of the centrifugal force, while the roots take the opposite line of growth. A third form of the tendency is called •diageotro pism," and leads parts subject to its influence, as runners and rhizomes, to horizontally; that is, in a direction at right angles to the plane of positive geotropism. This influence
is seen in the tendency of branches and foliage to assume a horizontal position. In all these cases, however, the result is modified and com plicated by influences of sunlight, moisture, etc., styled heliotropism, hydrotropism, chemotro pism, etc. (qq.v.) ; and sometimes, as in the case of twining plants, it is impossible at pres ent to extricate them and assign to each its part in the result.
Geotropism, or the influence of gravity, has had a great effect, also, upon the forms of ani mals, especially in determining proportions and strength of parts with reference to weight. As sociated with other influences it determines the •instincto which leads many of the lower forms of life to seek the earth whenever possible, or at the proper time, a striking example of which is found in those caterpillars whicii, born in tree-tops, migrate to the ground as soon as born or when ready to begin their metamorphoses.