Chapter 26: Response in the Flowering Plant

Plants have the ability to respond to their environment.

  • Stimulus: anything that causes a response in an organism.
  • Response: activity of an organism or part of an organism as a result of a stimulus.
  • Growth regulator: chemical that controls the growth of a plant.
  • Tropism: growth response of a plant to a stimulus.
  • Phototropism: growth of a plant in response to light.
  • Geotropism: growth response of a plant to gravity.
  • Thigmotropism: growth response of a plant to touch.
  • Hydrotropism: growth response of a plant to water.
  • Chemotropism: growth response of a plant to chemicals.

Auxin is a growth promoter. An example of an auxin is indole acetic acid (IAA).

Production sites:

  • Auxins are produced in the meristematic tissue of shoot tips and root tips.


  • stimulates cell elongation
  • stimulates cell division
  • differentiation of meristem cells into xylem and phloem
  • apical dominance
  • delaying of fruit ripening
  • phototropism and geotropism

Mechanism of a plant tropism – phototropism

  • IAA (auxin) is produced in the apical meristem of the shoot.
  • This diffuses down the shaded side of the stem.
  • This causes cell elongation on the shaded side.
  • The shaded side of the stem grows more quickly than the exposed side of the stem.
  • This causes the shoot to bend towards the source of light.

Uses of plant growth regulators

  • Naphthalene acetic acid (NAA) is used as a commercial rooting powder.
  • Ethene is used as a ripening agent for fruit.

Plant adaptations for protection:

Anatomical adaptations:

  • Epidermis – protects against pathogens entering the plant.
  • Guard cells – protect against excess water loss.
  • Some plants have bark – to protect against herbivores.
  • Cacti have evolved to have no leaves (to protect against water loss) and spikes (to protect against herbivores).

Chemical adaptations:

  • Corn lily produces a toxin called cyclopamine to protect itself against herbivores
  • Many plants produce alkaloids that protect against insects and herbivores.
  • Poison ivy produces a chemical called urushiol that protects against herbivores.
  • Conifers produce monoterpenes that protect against many insect herbivores.

Practical activity: to investigate the effect of IAA on the growth of plant tissue.

  • Set up 8 Petri dishes labelled A-H.
  • Make up a stock IAA solution (100mg/L) by first dissolving IAA in 2-3 ml of ethanol and then making up to 1 L using distilled water.
  • Add 10 ml of the stock IAA to dish A.

Serial dilution:

  • Take 1 ml from A and place in dish B.
  • Add 9 ml distilled water to dish B and mix.
  • Using a new pipette take 1 ml from dish B and place in dish C.
  • Add 9 ml distilled water to dish C and mix.
  • Repeat this procedure for dishes D to G.
  • Place 9 ml distilled water dish H (control).
  • In the lids of each dish place an acetate grid along with 5 radish seeds in a straight line.
  • Place a filler paper on top of each set of seeds in the lid of each dish.
  • Place cotton wool on top of each filter paper.
  • Pour the contents of each dish into the cotton wool in the lid of each dish.
  • Close each dish and tape shut.
  • Attach all dishes together with tape making sure the line of seeds in each dish are in the same orientation.

The following results tables should be filled in: