There are two main types of tropisms:
Phototropism is the growth of plant shoots towards the light.
Auxins are a family of plant hormones. An example of a specific auxin is IAA, which is known as Indole acetic acid.
They are mostly made in the tips of the growing stems and roots, which are known as apical meristems, and can diffuse to other parts of the stems or roots.
Unequal distribution of auxin can cause unequal growth rates in plants and shoots. Auxins change the rates of elongation in plant cells and control growth of stems and roots.
Stems and roots respond differently to high concentrations of auxins:
In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. The plant does NOT bend towards the light. It grows because the auxin causes the cells to elongate on the shaded side, so this side grows more. This unequal growth of the two sides, results in the growth of the stem towards the light.
If lit from above, the plant will grow upwards.
|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.
Plants responses to gravity:
In a root placed horizontally, the bottom side contains more auxin and grows less - causing the root to grow in the direction of the force of gravity.
The opposite happens in a stem. When a stem is placed horizontally, the bottom side contains more auxin and grows more - causing the stem to grow upwards against the force of gravity.