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| Plants produce hormones and respond to external stimuli, growing towards sources of water and light, which they need to survive. A tropism is a growth in response to a stimulus and an auxin is a plant hormone produced in the stem tips and roots, which controls the direction of growth. Plant hormones are used in weed killers, rooting powder and to control fruit ripening. | Plants need light and water for photosynthesis. Plant responses - called tropisms - help make sure that any growth is towards sources of light and water. There are two main types of tropism: positive tropism – the plant grows towards the stimulus negative tropism – the plant grows away from the stimulus. Phototropism is a tropism where light is the stimulus. A gravitropism (also called a 'geotropism') is a tropism where gravity is the stimulus. The roots and shoots of a plant respond differently to the same stimuli. |
| The tropisms of shoots mean that the shoots are likely to grow into the air, where there is light for photosynthesis. The tropisms of roots mean that the roots are likely to grow into the soil, where there is moisture. | Auxins are a family of hormones found in plants. Auxins are mostly made in the tips of the shoots and roots, and can diffuse to other parts of the shoots or roots. They change the rate of elongation in plant cells, controlling how long they become. Shoots and roots respond differently to high concentrations of auxins: cells in shoots grow more cells in roots grow less. |
| In a shoot, the shaded side contains more auxin. This means that the shaded side grows longer, causing the shoot to bend towards the light. |
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| Auxins have the opposite effect on root cells. In a root, the shaded side contains more auxin, but this time the shaded side grows less than the lit side. This causes the root to bend away from the light. | Auxins are also involved in gravitropisms. In a root placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow less than the top side, causing the root to bend in the direction of the force of gravity. In a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity. |
| Auxins are also involved in gravitropisms. In a root placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow less than the top side, causing the root to bend in the direction of the force of gravity. In a shoot placed horizontally, the bottom side contains more auxin than the top side. This makes the bottom side grow more than the top side, causing the shoot to bend and grow against the force of gravity. | Plant hormones can be used as weed killers and rooting hormones by farmers and plant growers. |
| Selective weed killers work on some plants but not others. This can be useful for getting rid of dandelions in a lawn without killing the grass, or getting rid of thistles in a field without killing the wheat. Selective weed killers contain growth hormone that cause the weeds to grow too quickly and then die. The weed killer is absorbed in larger quantities by the weeds than the beneficial plants, which stay healthy. | Rooting powder contains growth hormones to make stem cuttings quickly develop roots. |
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