Survival and Responses in Plants

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  • Survival and Responses in plants
    • Plants increase their chances of survival by responding to stimulus
      • Grow towards direction of light for maximum light absorption for photosynthesis
      • Sensing gravity, so roots and shoots grow in the right direction
      • Climbing plants have a sense of touch so they can find things to climb and reach sunlight
    • A tropism is a plant's growth response to an external stimulus
      • The response of a plant to a directional stimulus
      • Plants respond to stimulus by directing their growth
      • Positive tropism= growth towards the stimulus
        • Negative tropism= growth away from the stimulus
      • Phototropism is the growth of the plant in response to light
        • Shoots are positively phototropic
        • Roots are negatively phototropic
      • Geotropism is the growth of a plant in response to gravity
        • Shoots are negatively geotropic
        • Roots are positively geoptropic
    • Responses are brought about by growth factors
      • Growth factors are chemicals that speed up or slow down plant growth
        • Produced in the growing regions of the plant and move to where they are needed in the plant
      • A growth factor called Gillerellin stimulates flowering and seed germination
      • Auxins stimulate growth by cell elongation- cell walls become lose and stretchy so the cells get longer
        • High concentrations of auxins inhibit growth in roots though
        • IAA is an important auxin
          • Produced in tips and shoots of flowering plants
          • Moves by diffusion and active transport over short distances, via the phloem to control tropisms
            • Different parts of the plant have different amounts of IAA, uneven distribution leads to uneven growth of the plant
          • Phototropism: IAA moves to shaded parts of shoots and roots so there's uneven growth
            • Cells elongate on side of higher concentration so shoot bends towards light
          • Geotropism: IAA moves to underside of shoots and roots
            • Cells elongate so shoot grows upwards
            • Root grows downwards

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