What are tropisms?

A tropism is a growth in a plant in response to a stimulus. The stimulus could be a change in the direction of light or a water source. To recap, a stimulus is a change in the environment of an organism that could cause a response. There are different types of tropisms. Phototropism is a growth in response to the direction of light, and geotropism is a growth in response to gravity. Tropisms can also be negative or positive, where negative tropism is growth away from the stimulus, and positive tropism is a growth towards the stimulus. The direction of growth is controlled by a plant hormone called 'auxin'. Auxins are produced in the tips of shoots or roots and change the rate of elongation in plant cells. During phototropism for example, if a plant is only receiving light on one side, auxins gather on the shaded side of the tip of the shoot. This causes the shaded side to grow longer, and so the shoot bends towards the light.