Auxins

Auxins are a family of plant hormones. They are mostly made in the tips of the growing stems and roots, and can diffuse to other parts of the stems or roots. Auxins change the rate of elongation in plant cells, controlling how long they become.

Stems and roots respond differently to high concentrations of auxins:

  • cells in stems grow more
  • cells in roots grow less

Phototropisms

In a stem, the shaded side contains more auxin and grows longer – causing the stem to bend towards the light.

Three sets of plants. A: furthest from light, pointing up, have grown about 2cm. B: grow straight, have grown twice as high as A. C: have grown highest, but are bending towards light.Typical results shown by oat seedlings grown in a box with a light source
Seedling ASeedling BSeedling C
TreatmentThe tips have been removedNo light reaches the tipsMore light reaches one side of the tips
Effect on auxin concentrationNo auxin is producedEqual concentration of auxin on both sidesGreater concentration of auxin on shaded side
ResultThe stems do not grow longerThe stems grow evenly and longer on both sidesThe cells on the darker side of the stems grow longer

Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin and grows less - causing the root to bend away from the light.

Geotropisms

In a root placed horizontally, the bottom side contains more auxin and grows less - causing the root to bend in the direction of the force of gravity.

The opposite happens in a stem. In a stem placed horizontally, the bottom side contains more auxin and grows more - causing the stem to bend upwards against the force of gravity.