ABSTRACT
In 1806, a British botanist by the name of Knight conducted the following experiment: he placed a plantule (a germinating seed) on the edge of a horizontal disc made to rotate by a stream of water which also kept the plant humid. When the disc was stationary, the plantule root was directed vertically downwards; when the disk rotated at high speed, the root oriented itself according to the centrifugal force and became horizontal. On the other hand, a lower rotation speed induced the root to take a slanted position as if another force was partially counteracting the centrifugal force. This was the first demonstration that gravity affects the orientation of plant organs. The phenomenon of plant sensitivity to gravity and its consequences were later termed geotropism. This is but one of a number of instances of the influence of external factors on plants. The influence of light or phototropism is another well-known phenomenon.
