GEOTROPISM IN PLANTS (from Gk. -a, ge, earth + rporh, trope, a turning, from rperetv, trepein, to turn). The sensitiveness of plant organs to gravity. The attraction of the earth acts as a stimulus to which the organ responds in a man ner comparable to that exhibited in heliotropism, chemotropism, etc. Different organs respond to this stimulus in different ways. Primary roots (i.e. those originating from the embryo itself) are positively geotropic. They normally grow with their tips directed toward the centre of the earth. If placed in any other position they bend so that the axis of the growing portion regains its normal direction. In these organs the recep tive (or perceptive) region is in the extreme tip, while the active or curving region lies two or three millimeters back of the tip. The attrac tion of gravity sets up a disturbance (the nature of which is not yet certainly known) in the receptive region, and this is propagated back ward through the intervening cells to the region of curvature. Here the disturbance causes an alteration in growth such that -the side of the root directed upward grows more rapidly than the other side, thus producing a curvature which ultimately directs the tip downward again (Fig. 1). Many other plant organs besides primary roots are positivelygeotropic. Among these are the rhizoids (q.v.) of many lower plants, stalks of certain fruits and fruit clusters, many aerial roots, etc.
The primary shoots of most plants are apogeotropic (negatively geotropic) —i.e. they normally direct their tips away from the centre of the earth. This kind of sensitiveness is called apogeotropism or negative geotropism (Fig. 2). In such organs the receptive region is.
not so well marked off from that of bending as in roots. It may extend throughout the whole growing region. Also any region where growth is taking place has the power of curving under this influence. The response is similar to that in roots, but in shoots the region where growth is accelerated is on the under side when placed horizontal. In certain regions where growth has ceased it may be renewed under the influence of geotropic stimulation, and curvature may then ensue. Examples of this are found in the mature joints of grass-stems; also in those of the com mon wandering-Jew (Tradescautia). These bend sharply when placed horizontal, in which posi tion they are stimulated by gravity (Fig. 3).
Many organs, such as ordinary foliage leaves, lateral branches, lateral roots, rhizomes, runners, etc., usually show another form of response to this stimulus. Their normal position is hori
zontal, and, if displaced, they return to this position by bending. This tendency is dependent on diageotropism. The stalks of certain flow ers, such as those of Narcissus and pansy, are diageotropic, so that the flower faces lateral ly. In dorsiventral organs, e.g. many leaves, diageotropic response may consist of two move ments, a curvature which results in bringing the main axis into the horizontal plane, and a tor sion of the whole organ which brings its two sur faces into their normal relation to the surface of the earth.
Still another form of geotropic curvature is shown by the growing regions of twincrs, like the hop (Fig. 4), morning-glory, and bean. If the tip of the stein of such a plant be directed up ward, gravity will exert an influence upon it which results in the acceleration of growth along one side. This produces a lateral nodding. But as soon as the tip begins to nod, the region of accelerated growth migrates to the flank. The apex is thereby swung to the right or left, de scribing an irregular circle, clockwise or counter clockwise, according to the plant. What deter mines these directions is not known. It differs among species of the same family or the same genus, and in some cases even in the plants of the same species. The tendency to respond is termed lateral geotropism, and it is on account of this property that such plants are enabled to twine about a support.
All geotropical ly sensitive or gans may be con strained to grow in a horizontal di rection, in spite of the stimulus of gravity, if they are slowly rotated on a horizontal axis by means of a clinostat (q.v.).
In order to pro duce bending the stimulus must af fect the organ in a one-sided man ner. When a plant is rotated on the clinostat all parts a r e successively directed toward the centre of the earth, for equal periods of time.
Hence there is as much tendency to bend in one direc tion as in another, and the resultant growth is uni formly accelerated on all sides.
Geotropism only in part determines the final position of subterranean plant organs. Hydro tropism, chemotropism, traumotropism, etc., all have their effect, and the final position is the resultant of all these reactions. The position of aerial organs is determined largely by geotropism and heliotropism (q.v.). It is difficult to analyze any response and tell to what factor it is due. This can be done best by the use of the elinostat, varying the position of the axis to the needs of the experiment. See also APC7GEOTRO PISM ; DIAGEOTROPISM.