Summary
Summary Plants response to the enviroment
- Course
- Life Sciences
- Institution
- 12th Grade
Summary and notes needed for grade 12 and, Relative to both IEB and NSC
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PLANT GROWTH SUBSTANCES: PHOTOTROPISM & GEOTROPISM:- Eg. Auxins, gibberellins & abscisic acid (considered to be hormones) - Tropism → growth movement / bending reaction of a plant (or part of a plant) in
- BUT they’re not true hormones because they may bring about an effect on the response to an external stimulus.
part of the plant where they’re produced. - Direction of bending depends on direction of stimulus
- Phototropism: growth movement of plant organs in response to the
stimulus of light.
o Produced at tips of roots & shoots : stems = positively phototropic (grow towards light)
o Well know eg. = indole-acetic acid (IAA) : roots = negatively phototropic (grow away from light)
o FUNCTIONS: - Geotropism: growth movement of plant organs in response to the
- Bring about bending in plants → tropism stimulus of gravity.
- Promote cell division & root development : roots = positively geotropic
- Responsible for cell enlargement & apical dominance : stems = negatively geotropic
- Apical dominance → auxins in apical bud promote vertical growth &
prevent lateral growth ∴ if apical bud is removed, so is apical
dominance ∴ allows growth of lateral branches.
> Auxins at tips of stem move downwards evenly
> Even distribution of auxins = equal growth on all sides ∴ stem grows straight
upwards
o FUNCTIONS > When stems are exposed to unilateral light (from one side), “lighter” side has
- Bring about elongation of internodes of stems shortage of auxins (either destroyed or moved to darker side)
- Stimulates root growth > High concentration of auxins promotes = growth ∴ uneven distribution of auxins
- Promotes development of flowers = uneven growth
- Increases fruit size > “Darker” side grows
- Aids in germination of seeds faster
- Promotes sprouting of dormant buds > ∴ stem bends
towards the light
(positively
o FUNCTIONS phototropic)
- Brings about dormancy of seeds (slows down germination) & apical
buds.
- Induces flowering
- Aids in closing of stomata during dry periods / drought
- Promotes ageing of leaves
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