Chapter 26: Response in the Flowering Plant
- 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).
- 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:
- 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).
- 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.
- 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: