Plant Growth

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Plant Growth

Like all of us, plants start life very small. They need the right kinds of things to grow properly and to respond in the right way.

Nutrients are chemicals that plants need in small amounts but which are essential to keep them healthy. They are similar to the vitamins that we need.

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Gardeners and farmers add fertilisers to plants in either organic material or as chemicals. Either way they contain the nutrients required. The best known of these nutrients are nitrates, phosphates and potassium. The relative amounts of these are often shown on bags of fertiliser as an "NPK" ratio.


Sometimes plants get ill because one particular nutrient is missing or has been lost from the soil perhaps using the field for one type of crop for year after year.
Nutrient Use in the plant Deficiency symptoms
Nitrates Make proteins and genetic material (DNA) Stunted growth and yellow older leaves
Phosphates Make DNA, cell membranes and enzymes for photosynthesis Poor roots and purple younger leaves
Potassium Help enzymes to work in photosynthesis and respiration Yellowing leaves with dead spots
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flower nutrient

Some people talk to plants. It is harmless, legal and they believe it helps them grow - although others disagree.

But plants are sensitive things. They do respond to certain stimuli. A stimulus is anything that could cause a response.

For example: a stimulus could be changing the direction of the light a plant receives. The plant's response would be to grow towards the light.

Such a growth response shown by a plant is called a "tropism". It can involve all of the plant or just a small part of it.

Such a response involving light is phototropism.

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However things are a bit more complicated than this - aren't they always! A growth response that is towards the stimulus is called a "positive" tropism, whereas one that is away from it is described as a "negative" tropism.

Decide for each of the following examples whether they are positive or negative tropisms...

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A hormone is a chemical messenger, it causes changes in how a plant or the body of an animal works.

Plants release chemicals which control the tropisms. The best known of these 'plant hormones' are the "auxins".

Auxins can make the shoot of a plant grow towards the sun. For example, by getting the cells on one side to grow larger and this pushes the whole shoot around. The auxins are released from the very tips of the shoots or roots.

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Plant hormones work in the same way to get roots to bend down towards gravity or water in the soil.

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However we can use these plant hormones for many uses:

  1. For example as a weedkiller to selectively kill weeds by affecting the way they grow.
  2. As a "rooting powder" to get roots to grow more quickly on plant cuttings.
  3. Another use to spray a plant hormone called ethene on fruit so that they ripen more quickly.
  4. Plant hormones are used in producing seedless fruit for us fussy eaters.
  5. A chemical called "Cutless" contains an auxin which stops the side shoots of hedges growing and so saves you having to cut them. Nice idea!

So plant hormones are more useful than you might at first think!