Plants responding to a stimulus by growing towards or away from it is called tropism. Tropism caused by light is called phototropism. A tropism away from a stimulus is negative tropism. Plant roots are negatively phototrophic. Plant shoots are positively phototrophic, to get enough light for photosynthesis.
Plants produce hormones called plant growth substance to respond to stimuli.
An Auxin causes positive phototropism. They're produced in the tips of shoots and effects the growth and elongation of cells in plants. They cause shoots to grow towards the light.
Auxin's are also found in the tips of roots where they cause cells to stop elongating which causes positive gravitropism (or geotropism) which is growth towards gravity.
Once a seed germinates roots and shoots start to grow. The seed releases plant hormones called gibberellins. This causes starch in a seed to turn into sugars which the seed uses for energy to grow. It can also stimulate flower and fruits in some plants.
Key Words:
Tropism
Phototropism
Negative tropism
Photosynthesis
Plant growth substances
Auxin
Positive gravitropism
Geotropism
Germinates
Gibberellins
Questions:
1. What effects do auxin's have on cells in shoot tips?
2. What is tropism?
3. In the experiment above explain what's happened to each one
4. What does a gibberellin do?
What you should know:
How plant growth substances bring about:
a. Positive phototropism in shoots.
b. Positive gravitropism in roots.
How auxins bring about shoot curvature using cell elongation.
How to analyse, interpret and evaluate data from plant hormone experiments, including the action of auxins and gibberellins.
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