Plant Hormones
Plants do not have nervous systems but they do respond to changes in the environment by using hormones. The following diagrams show the experiments which were done to learn more about them. The experiments were done on the shoots of plants which are called coleoptiles.
If left alone the shoot grows straight upwards.
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If the tip is removed the shoot stops growing. If the tip is replaced then the shoot grows again. This shows that it is the tip which controls the growth.
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If the shoot is cut and an agar jely block is placed between the shoot and the tip, growth continues. If the tip is removed and placed on the agar block for some time and the block placed on the shoot without the tip, then growth continues. This shows that there is a chemical produced in the tip which can diffuse down through the agar to the shoot. If the tip is put on the agar block first, the chemical is passed on to the block so that when it is placed on the shoot it stimulates growth.
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If a piece of mica is placed between the shoot and the tip there is no growth. Mica prevents movement by diffusion and so helps confirm that there is a chemical which is made in the tip and it diffuses to the shoot.
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When a light is shone from one side of the shoot. The shoot grows towards the light.
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One kind of plant hormone is known as auxin. It is produced in the shoot tip and diffuses down the shoot. It stimulates the cells of the shoot, directly under the tip, to grow. If they do not get auxin the cells do not grow and divide and so the shoot does not grow.
Light damages auxin. If there is more light coming from one side than the other, then there will be an uneven distribution of auxin. There will be more auxin of the shaded side than the side in the light. The shaded side will grow faster resulting in the shoot growing towards the light. This helps plants receive as much light as possible for photosynthesis.
Tropisms
Growing in response to a stimuli is called a tropism. Phototropism is growth in response to light. This is an example of a positive tropism, growing towards the stimulus. Hydrotropism is a response to water whereby the roots grow towards it. Geotropism is a response to gravity. Roots show positive geotropism while shoots show negative geotropism (in that they grow away from gravity).
Uses of Plant Hormones
Plant hormones including auxins have been used in agriculture and by horticulturalists for a number purposes. Plant hormones are used in rooting powder to stimulate the development of roots from plant cuttings. They are also used to generally stimulate growth. This can be used to detrimental effect in some weedkillers which cause broad-leafed plants to grow too quickly and effectively burn themselves out while leaving narrow-leafed plants (such as crops like wheat) uneffected. They are also used in fruit ripening to make sure that all the fruit ripens at once to aid harvesting.
