Investigating the effect of light

• Can investigate the effect of light on the growth of cress seeds like this:

  1. Put 10 cress seeds into three different petri dishes each lined with moist filter paper. Label your dishes
  2. Shine a light onto one of the dishes from above, and two of the dishes from different directions
  3. Leave cress seeds alone for one week until you can observe their responses- you'll find the seedlings grow towards the light
  4. You know that the growth response of the cress seedlings i due to light only, if you control all other variables
• Control variables:

  • Number of seeds- use the same number of seeds in each dish
  • Type of seed- Use seeds that all come from the same packet
  • Temperature- keep your petri dishes in a place where the temperature is stable
  • Water- use a measuring cylinder to add the same amount of water to each dish
  • Light intensity- keep the distance between the bulb and dish the same for each dish