As soon as a seed begins germination, roots display a tendency to grow downwards, while shoots grow upwards. This behavior is an example of a tropism, a movement triggered by a stimulus. The question is: what is the stimulus?
An experiment in 1800 produced this report:
"It is not the soil's humidity that causes the direction of the root, since the root will grow downwards and the shoot will grow upwards even if the plant is placed in an earth-filled tube where the upper part is moist and the lower is dry. If the plant is put into a water-filled tube the lower part of which is exposed to light while the upper is darkened, the direction of growth will still not change: a proof that it is independent of light."An experiment in 1806 showed that the direction of root growth is controlled by gravity.
A scientist fixed seedlings on a vertically standing wheel that was turning around its axis due to a small millwheel fuelled by a fast-flowing stream. The rotation caused a centrifugal force.The term geotropism was coined in 1868, and three types identified:
Transversal geotropism is a direction of growth that is at 90o to the shoot axis. The direction of lateral shoots, side roots, leaves is described by this term.
An experiment in 1879 extended earlier experiments with the invention of the clinostat, a device that helped stop the bending using centrifugal force to compensation for gravity.
By marking individual areas of a root, the scientist showed that it is not the tip that elongates but the section just above the tip. (We how know this is the growth zone). He concluded that elongation of one side of a root or shoot is caused by the elongation of the cells on that side, causing the root or shoot to bend.Although the mechanism for producing a curve in a root or shoot had been identified, the questions of how the stimulus is perceived and how it is transmitted remained a mystery.
In 1892 a scientist postulated the existence of microscopically small, mobile components in the cells of the root exerting a pressure on the plasma of the bottom side of the cell. The stimulus produced by the local pressure would then have to be converted into a stretching of the cell and into an increased growth by the cell's plasma.
This hypothesis was a good one, but it did not explain one observation that had been made.
The relocation of these mobile components did not cause the tough cell walls to become thinner. This meant that the cells were not being stretched - the cells themselves were elongating. Eventually the area just above the root tip was identified as the zone in which cell growth actually occurs (meristem).It was not until 1961 that scientists made the final connection. Geotropic bending is caused by the distribution of plant hormones, causing one side of a root or shoot to grow faster than the other side.
Use the passage above to answer the following questions.