B1.2.3 Control in Plants

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Plant Responses

Plants are sensitive to:

  • Light
  • Moisture
  • Gravity

Phototropism- growth in response to light

Geotropism/gravitropism- growth in response to gravity

Positive tropism- growing towards the stimulus

Negative tropism- growing away from the stimulus

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Auxin in Plants

Roots - towards moisture
          -towards gravity = positive geotropism

Shoots - towards light = positive phototropism
           - away from gravity = negative geotropism

Auxin - plant hormone that controls tropisms 

Auxin is produced in the shoot or root tip, then travels to the target cells. It causes shoot cells to elongate, and inhibits elongation in root cells.
The unequal distribution of auxin causes unequal growth rates.

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Plant Hormones

Plant growth hormones can be used in:

- Agriculture (growing crop plants in fields)
- Horticulture (growing flowers, fruit, and vegetable plants)


Plant hormones used in selective weedkillers. These kill weeds but don't harm the grass.

Rooting Hormones

Plant hormones used in rooting powder- plant stems dipped in the powder develop roots more quickly.

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