biology B1 (4)

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  • Created by: haidee
  • Created on: 29-04-12 11:39
  • Hormones and the control of plant growth
  • A tropism is a directional growth response of a plant
  • The direction of response is determined by the direction of the external stimulus
  • The stimulus light effects the shoots by making them grow towards the light called positive phototropism
  • The stimulus gravity affects the shoots by making them grow away from the force of gravity is called negative gravitropism
  • The stimulus gravity affects the roots by making them grow towards the force of gravity is called positive gravitropism
  • The stimulus moisture affects the roots by making them grow towards the moisture is called positive hydrotropism
  • Theses responses aid a plants survival:
  • -shoots grow towards light to allow maximum photosynthesis
  • -roots grow into the soil to provide anchorage
  • -roots grow towards water as it it vital for plant support, photosynthesis and a supply of dissolved mineral ions


  • The plant growth responses happen due to a plant hormone called auxin
  • Auxin is synthesised in cells at the tip of shoots
  • Auxin moves away from the tip by diffusion
  • Auxin causes the cell walls of the cells in a region behind the tip to become elastic
  • These cells fill with water, stretch, elongate and the plant increases in length
  • The stimuli of light and gravity cause auxin to be unevenly distributed in the shoot and root
  • This causes cells on one side to elongate more than cells on the other side, causing bending in the shoot or root
  • Shoot and roots respond differently to auxin:
  • -a high concentration of auxin in shoots stimulates cell elongation and hence growth
  • -a high concentration of auxin in roots inhibits cell elongation and reduces growth
  • Phototropism in shoots:
  • For a straight shoot:
  • -tip is receptor and auxin is synthesised
  • -auxin diffuses equally
  • -response area - all cells elongate equally
  • For a bend shoot:
  • -tip detects unequal light distribution and auxin diffuses sideways away from the light
  • -unequal amounts of auxin diffuse lack each side
  • -causing cell elongation to occur unequally on each side
  • Gravitropism:
  • A normal young bean plant is laid on it side
  • Auxin is equally spread through tissues resulting in straight shoot and root
  • Seedling placed horizontal in dark:
  • -gravity causes more auxin to gather on the lower sides of the root and shoot
  • -in the shoot the extra auxin increases cell elongation so it curves upwards
  • - in the root, raised auxin levels slows down the elongation of the cells
  • -so it curves downwards

Human use of plant hormone

  • Rooting powder:
  • -they are used to encourage root growth in cuttings so you can make many clones from fewer plants
  • -it is auxins and other types of plant hormone
  • -it may force roots to grow at the expense of the shoots
  • -it can help achieve many clones with the same desired characteristics successfully
  • Selective weed killer:
  • -it is artificial auxins that only effect broad leaf plants
  • -it is used to remove weeds which compete with crops and to make lawns look wee-free
  • -it works by the broad


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