Hormones (auxins and tropisms)
Plants can respond to gravity, moisture and light. These responses are called tropisms - growth movements caused by auxins or 'plant hormones'. Auxins make some parts of a plant grow faster than others; the result may be that the plant bends towards or away from the stimulus.
1. Etiolation
When a plant is left in the dark it puts all its energy into growing up to reach the light and makes very little chlorophyll. It will become etiolated. Eventually it should reach the light and then it can start to turn green by making chlorophyll for photosynthesis. Etiolation is the result of a tropism