Tropisms are directional growth responses of organisms like plants to specific stimuli.

Tropisms allow plants to respond to variations in environmental conditions, helping them make maximum use of the environment for survival.

Phototropism is the growth of an organism like a plant in response to light.

Chemotropism is the growth of an organism like a plant in response to chemicals.

Thigmotropism is the growth of an organism like a plant in response to touch.

Etiolation is the rapid upward growth that takes place in a plant grown in the dark. 

Plants grow upwards rapidly to reach the light for photosynthesis.

Etiolated plants are thin and pale due to a lack of chlorophyll.

Geotropism is the response of an organism like a plant to gravity.

Shoots are negatively geotropic as they need to obtain light for photosynthesis, which is usually upwards.

Roots are positively geotropic as they need to grow into the soil for support, and for minerals and water, which are usually downwards.

Gibberellins are responsible for the extreme elongation of the internodes when a plant is grown in the dark.

This helps seedlings grow out of soil to obtain light for photosynthesis. Once the plant is exposed to light, levels of gibberellin fall.

1. IAA is produced in cells in the tip of the plant shoot
2. IAA is transported downwards
3. Light stimulates IAA to move to the shaded side of the shoot
4. IAA becomes concentrated and stimulates more cell elongation on the shaded side of the shoot
5. The shoot bends towards the light

1. IAA is produced in cells in the tip of the plant root
2. IAA is transported along the root
3. Light stimulates IAA to move to the shaded side of the root
4. IAA becomes concentrated and inhibits cell elongation in the shaded side of the root
5. The root bends away from light