Plant Responses – Tropisms

All living organisms are sensitive to changes in their surroundings.

Plants are sensitive to light, moisture and gravity.

The response to light is called phototropism.

The response to gravity is called geotropism.

The response to moisture is called hydrotropism.

• Shoots grow upwards – towards light and against the force of gravity
• Roots grow downwards – towards moisture and in the direction of the force of gravity

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The ways in which a plant responds to these stimuli is coordinated by hormones.

Hormones also control the growth of a plant.

One plant hormone is called auxin.

Auxin is made by cells in the tips of the roots and shoots.

It speeds up growth in shoots and slows down growth in roots.

Unequal distribution of hormones causes unequal growth rates in different tissues.

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Geotropism

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1. Rooting powder contains auxin. It is applied to cuttings to stimulate the growth of roots. Large numbers of plants can be produced quickly, so more money can be made.

2. Hormones control fruit ripening. Producers use chemicals to slow down the ripening of fruits, such as bananas, during transport. Just before sale, ripening can then be speeded up using different chemicals and a warmer temperature in the storerooms.

3. Weedkillers contain synthetic hormones that stimulate the plants to grow very rapidly. Their normal growth patterns are disrupted. They become tall and spindly, and eventually die.

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