Hormones (auxins and tropisms)

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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

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These two plants were grown from cuttings so they are genetically identical. They are both the same age, but one has been left in a dark place.

You may notice that your houseplant turns all its leaves towards the window. It does this because light coming from one side destroys the auxin in a leaf on that side. On the other side auxin makes the leaf grow faster. The result is that the leaves are turned towards the light for photosynthesis. We call this a
positive phototropism.

Diagram Of positive phototropism



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