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Biology · Plant Adaptations and Hormones

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Biology · Edexcel Paper 2

Plant Adaptations and Hormones

Plants in Extreme Environments (Separate only)

A cactus in the desert, marram grass on a windy dune and a water lily on a pond all face very different problems.

  • Adaptation A feature of structure or behaviour that helps an organism to survive in its environment.
  • Xerophyte A plant that is adapted to live in a very dry habitat.
  • Cuticle A waxy waterproof layer on the surface of a leaf or stem.
Four plants in extreme environments with their adaptations: a cactus in a desert, marram grass on a dune, a water lily on a pond and a cushion plant on a cold mountain.
The leaf size and shape, the cuticle and the stomata are adapted to each environment.

Dry-environment plants and water plants

  • A plant from a dry place Small leaves, spines or rolled leaves; Very thick waxy cuticle; Few stomata, often sunken, or open at night
  • A floating water plant Large, flat leaves; Thin cuticle; Stomata only on the upper surface

Plant Hormones and Tropisms (Separate only)

A plant has no nerves or muscles, but it still grows towards the light and sends its roots down.

  • Plant hormone A chemical made in a plant which controls and coordinates growth and development.
  • Auxin A plant hormone, made in the tip of the shoot and the root, which controls cell elongation.
  • Tropism A growth response of a plant to a stimulus from one direction.
Shoots in light from above, light from one side, and with the tip covered, showing how auxin moves to the shaded side and makes the shoot bend towards the light.
Auxin moves to the shaded side, and makes the cells there grow longer, so the shoot bends towards the light.

Shoots and roots

  • A shoot Grows towards light (positive phototropism); Grows away from gravity (negative gravitropism); Auxin makes the cells on that side grow longer
  • A root Grows away from light, or is not affected by it; Grows towards gravity (positive gravitropism); Auxin slows the growth on that side

Investigating Tropic Responses (Separate only)

You can see a tropism happening in a few days with a pot of cress seedlings.

  • Clinostat A device that turns plants slowly so that gravity acts on all sides equally.
  • Seedling A very young plant that has just grown from a seed.
  • Control A test in which the factor being investigated is not changed, for comparison.
Set-ups to investigate the response of seedlings to light, using a lamp at the side, a lamp above and the dark, and to gravity, using an upright dish, a dish on its side and a clinostat.
Changing the direction of the light or of gravity shows that these are the stimuli for the bending.

Commercial Uses of Plant Hormones (Higher tier) (Separate only)

Farmers and growers use plant hormones to control how their crops grow and ripen.

  • Weedkiller A chemical that kills weeds, which are unwanted plants.
  • Selective weedkiller A weedkiller that kills some kinds of plant but not others.
  • Rooting powder A powder that contains auxin, used to make cuttings grow roots.
Six commercial uses of plant hormones: auxin as a weedkiller and in rooting powder, gibberellin for germination, seedless fruit and flowers, and ethene for ripening fruit.
Auxins, gibberellins and ethene are used to control the growth, flowering and ripening of crops.

The three groups of plant hormone and their uses

  • Auxins and gibberellins Auxins: selective weedkillers; Auxins: rooting powders for cuttings; Gibberellins: germination of seeds
  • Ethene A gas made by plants; Makes fruit ripen; Fruit is picked green and ripened later