Root and Stem Growth

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 effect caused by this force was also observed when the wheel was in a horizontal position: at a fast rotation grew all roots outwards (in the direction of the centrifugal force), at a low rotation, they grew in an angle of about 45o (the result of both centrifugal force and gravity).

A first, the scientist thought that the root tips where pulled downward by their own weight, but this assumption was soon refuted, since the downward movement also occurs when the weight of the root tips was compensated for by an opposite weight.

The term geotropism was coined in 1868, and three types identified:
  • Positive geotropism - growth toward gravity
  • Negative geotropism - growth away from gravity
  • Transversal geotropism - horizontal growth
Positive and negative geotropism together cause the vertical orientation of the plant's axis. Shoot growth is mostly negatively geotropic since shoots grow upwards even in complete darkness. Phototropism can therefore be understood as a secondary process, usually of the same direction as the negative geotropism.

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.

  1. Plants begin to display geotropism
    1. when they emerge from the soil
    2. as soon as a seed begins germination
    3. when leaves begin to grow
    4. when exposed to water
     
  2. The term "geotropism" was first used in
    1. 1800
    2. 1806
    3. 1868
    4. 1879
     
  3. The experiment in 1800 eliminated these two causes for root and stem growth patterns.
    1. gravity and light
    2. gravity and soil water
    3. light and soil water
    4. weight and centrifugal force
     
  4. Which experiment above used a millwheel?
    1. 1806
    2. 1868
    3. 1879
    4. 1892
     
  5. Horizontal growth under the influence of gravity is called
    1. positive geotropism
    2. negative geotropism
    3. transversal geotropism
    4. vertical geotropism
     
  6. Positive and negative geotropism together cause the
    1. growth pattern of the root
    2. growth pattern of the shoot
    3. growth pattern of the lateral shoots and leaves
    4. vertical orientation of the plant's axis
     
  7. Which experiment concluded that elongation of one side of a root is caused by the elongation of the cells on that side.
    1. 1868
    2. 1879
    3. 1892
    4. 1961