Geotropism

GEOTROPISM It is well known that the plant body takes up a definite posi tion in space. The main stem of a tree usually stands quite ver tically and its branches stand out more or less horizontally. The main or tap root also grows vertically downwards and the root lets at various angles. It is well known also that if a seedling is placed with its stem and its root horizontal the stem proceeds to curve upwards and the root downwards.

Geotropic Response.

This response is known as a geotropic response, that of the stem being negatively geotropic and the root positively geotropic. We note that the curvature is usually con fined to the tips of the organs, i.e., to the growing points of the stem and the root. That the curvature is due to the action of gravity was shown as long ago as 1806 by Knight, the English experimenter. He fixed a number of seedlings in different positions on the edge of a wheel rotating in a vertical plane, that is, on a horizontal axis. The wheel was rotated at a considerable speed so that at its edge there was considerable centrifugal force. The result he observed was that the roots, in whatever direction they had started growing, soon grew outwards at right angles to the direction of the axis. The stems, however, grew in the opposite

direction to that in which the centrifugal force was acting and they soon all met in the centre of the wheel. This experiment shows that plants do not distinguish between gravity and centrifugal force, and that the root grows in the direction of the force while the stem grows in the opposite direction. About 7o years later Sachs invented his klinostat, an apparatus by which plants are rotated on a disc in a vertical plane, but the speed of rotation is very slow, once in 10-20 minutes. As a result centrifugal force We see from the third column that the geotropic reaction obeys what is called the product law—the time of stimulation is in versely proportional to the intensity of the stimulus, so that the product of the two is constant. This product law is also clearly shown when the root or stem is placed at various angles with the horizontal. The horizontal position is found to give the most intense stimulus, the intensity being proportional to the sine of the angle with the vertical at which the organ is placed