Sanyam Jain, Academic Content Creator at Edumarz

  • Plants, like animals, also use electro-chemical signals to communicate information but, do not have a specialised nervous system or muscular system.

  • They respond to environmental stimuli through movements which movements could be either i) growth-dependent or ii) growth-independent

  •   Growth-dependent movements (tropic movements)

    • Tropic movements are directional but slow. 

    • In tropic movements, a change in cellular growth induced by the stimulus occurs.

    • Phototropism

      • Movements of plant parts in response to sunlight 

      • Roots are negatively phototropic i.e. they bend away from the sunlight

      • The shoot is positively phototropic i.e. it bends towards the sunlight

    •  Hydrotropism

      • Bending of roots towards higher water (or moisture) content in the soil

    • Geotropism

      • Movement of plant parts in response to gravity

      • Roots are positively geotropic i.e. they bend in the direction of gravity

      • The shoot is negatively geotropic i.e. it bends in the direction opposite to gravity

    • Chemotropism

      • Movement of plant parts induced by some chemical

      • Example– Growth of pollen tube towards ovules inside carpel

    • Thigmotropism

      • Movement of plant parts in response to touch

      • Example– Growth of pea tendril upon contact with any support

  • Growth-independent movements (nastic movements)

    • These are non-directional, quick movements in plant parts caused by changing water content in the cells resulting in a change in the cell shape.

    • Example– Bending of leaves of Mimosa pudica (touch-me-not) on touch 

  • Phytohormones

    • Different plant hormones help to coordinate growth, development and responses to the environment. 

    • They are produced at sites different from where they act and simply diffuse to the area of action.

    • Auxin

      • Growth promoting hormone that helps the cells to grow longer

      • Produced in the actively growing shoot tip

      • Diffuses to the side opposite to that of sunlight causing increased cell growth on that side of stem resulting in bending of the shoot towards sunlight

    • Gibberellins

      • Cause stem elongation

    • Cytokinins

      • Enhance plant growth by promoting cell division 

      • Present in greater concentration in areas of rapid cell division, such as in fruits and seeds

    • Abscisic acid

      • Only growth inhibitor hormone

      • Cause leaf wilting & closing of stomata

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