B1 - Plant Hormones and Growth

Auxins are plant hormones. They control the growth of shoots and roots on a plant. They are not stationary, but move around the plant in solution.

Auxin is produced in the tips of shoots and roots, and is diffused backwards through the plant.

Auxin affects shoots and roots differently - while it encourages growth in the shoots, it inhibits growth in the roots.

Shoots are positively phototropic; this means that they grow towards light. Auxin grows on the shady side of the shoot, meaning that cell elongation on the shady side happens faster than on the side facing the light.

Shoots are also negatively geotropic; this means that they grow away from gravity. Therefore, the auxin grows on the side facing down in order to quicken the cell elongation and make it grow against gravity.

Roots are negatively phototropic; this means that they grow away from light. Auxins grow on the shady side, which actually inhibits growth on this side of the root. This is what makes it grow away from the light.

Roots are also positively geotropic; this means they grow towards gravity. The auxins grow on the lower side, inhibiting growth on this side of the root. This is what makes the root grow downwards, as the cell elongation is increased on the upper side.

Experiments have been carried out in which there are three shoots. The tip of one of them is covered to keep out the light. Another one has a cover around the middle of the shoot, leaving the tip and bottom uncovered. The last one is left completely uncovered.

They were left for a few days, and when next examined, only the one with the covered tip had not moved. The other two had bent towards the light. This shows us that auxin is produced in the tip, as the bending of the plant is only prevented when the tip is covered, as this experiment shows.

Auxin - plant hormones

Positively geotropic - grows towards gravity

Negatively geotropic - grows away from gravity

Positively phototropic - grows towards light

Negatively phototropic - grows away from light

Solution - dissolved in water