There are several different plant hormones. Those controlling the growth of roots and shoots are called auxins which stimulate cell growth. Auxins can diffuse from cell to cell in solution or travel greater distances in the plant in the vascular bundles.

If a plant has light falling on it from one side only, it will grow towards the source of the light. This is as a result of auxin accumulating on the side of the plant away from the light, causing extra growth in this region and so causing the plant to bend towards the light.

Roots are not sensitive to light but grow downwards in response to the stimulus of gravity. Auxins are again involved, although scientists think another plant hormone might actually cause slowing of the growth of cells on one side of the root causing it to bend downwards.

Plants hormones are used widely in agriculture in the following ways:

• As selective weedkillers which are not harmful to the surrounding plants or to animals.
• To produce new plants from cuttings. With the use of hormone rooting powder, cut stems can develop roots and grow into clones of the parent plant.
• To promote the ripening of fruit or induce flowering when required.
• To speed up or slow down the growth of plants.
• To produce fruit without seeds. Unpollinated flowers are treated so that they grow fruit without seeds.