Plant hormones

Greg Foot explains how plants respond to light and gravity, and the role of auxins in controlling and coordinating plant growth

Just like animals, plants produce hormones. These chemical molecules act as messages from one part of the plant to another. Important plant hormones include:

  • auxins which control the rate of cell elongation
  • gibberellins also control cell elongation but also influence germination, flowering and dropping fruit and leaves
  • ethene, also known as ethylene, which controls fruit ripening


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

Shoots and roots respond differently to high concentrations of auxins:

  • cells in shoots grow more
  • cells in roots grow less


Phototropism is how plant shoots grow towards the light. In a shoot, the shaded side contains more auxin. This means that the shaded side grows longer, causing the shoot to bend towards the light.

Many famous scientists including Charles Darwin (1809-1882) completed experiments like this on phototropism.

SeedlingsResults and explanation
AThe tips have been removed. No auxin is produced and the shoots do not grow longer.
BThe tips have been covered so light cannot reach them. Auxin is in the same concentration on both sides of the shoots, so they grow evenly and longer on both sides.
COne side of the tips are in more light than the other side. Auxin is in a greater concentration on the shaded side, causing the cells there to grow longer than the cells on the lit side.
Diagram showing plants pointing towards the sunTypical growth of oat seedlings grown in a box with a light from one side