To survive, plants need to be able to respond to their environment. All plants need water and light for photosynthesis, so they have developed responses called tropisms.
Tropisms are plant growth away from or towards a stimulus and they make sure plants grow towards sources of light and water.
There are two types of tropism:
As a result of tropisms, the stems of plants grow upwards towards the sun (for light) and the roots grow downwards to absorb water in the soil.
Both phototropism and gravitropism play an important role in helping plants adapt to their environments.
Plants produce hormones, such as auxins, to coordinate and control growth
Auxins are produced in the shoot tips and root tips of plants by actively dividing cells. It regulates plant growth by stimulating the elongation of plant cells (the cells get longer) in the plant shoot. Shoots and roots have different responses to higher concentrations of auxins:
Phototropism is the growth of a plant in response to light.
Shoots and roots in plants have opposite responses to light.
The auxins produced in the tip are more concentrated on the shaded side of shoot tips (away from light), which causes the cells on that side to elongate. The elongated cells make the shaded side heavier, which causes it to bend towards the light.
Gravitropism is the growth of a plant in response to gravity. Shoots and roots have opposite responses to gravity.
If a root is growing horizontally (sideways), more auxins will accumulate on the side of the root facing downwards. This is because auxins inhibit growth in roots. So, there will be more growth on the side facing upwards. This causes the root tip to bend downwards, which is an example of positive gravitropism.