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Science

Control in plants

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Plants produce hormones and respond to external stimuli [stimuli: In drama, stimuli refer to the drama texts (photos, texts, video etc) you have been given to work with ], 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.

Sensitivity in plants

Plants need light and water for photosynthesis [photosynthesis: The chemical change that occurs in the leaves of green plants. It uses light energy to convert carbon dioxide and water into glucose. Oxygen is produced as a by-product of 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.

Summary of the different types of tropism

Part of plantLight stimulusGravity stimulus
shootpositive phototropism (grows towards the light)negative gravitropism (grows against the force of gravity)
rootnegative phototropism (grows away from the light)positive gravitropism (grows in the direction of the force of gravity)
Cilantro seedlings bending towards light

Positive phototropism in plant stems

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.

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