In order to survive, plants require light and water for photosynthesis. They have developed responses called tropisms to help ensure they grow towards adequate sources of light and water.
There are two main types of tropisms:
Phototropism is a response to the stimulus of light.
Auxins control the growth of plants by promoting cell division and causing elongation in plant cells (the cells get longer).
Stems and roots respond differently to high concentrations of auxins:
This is caused by an unequal distribution of auxin.
In a stem, the shaded side contains more auxin and grows longer, which causes the stem to grow towards the light. It is vital to note that the plant does NOT 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 the 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.