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:
In a stem, the shaded side contains more auxin and grows longer – causing the stem to bend towards the light.
|Seedling A||Seedling B||Seedling C|
|Treatment||The tips have been removed||No light reaches the tips||More light reaches one side of the tips|
|Effect on auxin concentration||No auxin is produced||Equal concentration of auxin on both sides||Greater concentration of auxin on shaded side|
|Result||The stems do not grow longer||The stems grow evenly and longer on both sides||The 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.
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.