(P) Plant growth responses

• You can investigate the effect of light on the growth of cress seeds

  
• 1)Put 10 cress seeds into 3 different Petri dishes, each lined with moist filter paper. (Remember to label the dishes, e.g. A, B, C)

  
• 2) Shine a light onto one of the dishes from above and 2 of the dishes from different directions

  
• 3) Leave the cress seeds alone for 1 week until you can observe their responses - you'll find seedlings grow towards the light

  
• 4) You know that the growth response of the cress seeds is due to light only, if you control all other variables

  
• You can also investigate the effect of gravity on plant growth. Just place 4 seedlings on damp cotton wool is a Petri dish, each with their roots pointing in a different direction, and store the Petri dish vertically for a few days in the dark. You should find that the roots of each seedling grow downwards
• Examples of variables to control are:

  • 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 (i.e. away from heat sources and draughts)
  • Water - use a measuring cylinder to add the same amount of water
  • Light intensity - keep the distance between the buld and dish the same