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NT Biology Edexcel B1: Control in Plants
Transcript of NT Biology Edexcel B1: Control in Plants
GRAVITROPISM or GEOTROPISM is how shoots and roots respond to gravity.
HYDROTROPISM is how shoots and roots respond to water. If you think about it when a shoot grows towards the light it is because the side furthest from the light grows faster than the side being illuminated. So the shoot grows towards the light. You can see this in the diagram.
Experiments have shown that this is due to the uneven distribution of a chemical called AUXIN.
Auxin is made in the tip of the shoot and stimulates shoot growth.
When light shines on a shoot from one side the auxin moves away from the light source and so more accumulates away from the light.
This means that the 'dark' side will grow faster than the 'light' side and the shoot will bend towards the light! How do roots grow away from the light? Auxin is also made in the root tip but this time auxin INHIBITS root growth.
When light shines on one side of the root auxins move away to the opposite side.
This increased auxin concentration away from the light inhibits the growth on this side and so it grows more slowly.
This means the root bends away from the light as shown in the diagram. How do roots and shoots respond to gravity? Gravity pulls auxins down to the lower side.
Auxin STIMULATES shoot growth on the lower side and INHIBITS root growth on the lower side.
As a result the shoot grows upwards away from graviy and the root grows downwards towards the gravity The diagram shows a simple experiment to show that roots grow towards water i.e. they are positively hydrotropic. How do roots respond to water? Plant hormones, like auxins, regulate plant growth.
Rooting powders are used when taking shoot cuttings to promote root growth.
Some hormones are sprayed onto soil to act as weedkillers that kill broad leaved plants but allow crops like cereals to grow. Plant hormones are used in agriculture. Homework