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Science

Control in plants

Auxins

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.

Phototropisms

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.

3 groups of seeds in a box with a hole cut at one end. Shoots A are short, shoots B are tall with foil hats, shoots C are curving towards the light.

The diagram shows the typical results shown by oat seedlings grown in a box with a light from one side

SeedlingsResults and explanations
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.

Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin, but this time the shaded side grows less than the lit side. This causes the root to bend away from the light.

Gravitropisms

Auxins are also involved in gravitropisms. In a root placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow less than the top side, causing the root to bend in the direction of the force of gravity.

In a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity.

Back to Nerves and hormones index

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