Required Practical 8: The effect of light or gravity on the growth of newly germinated seedlings

Question 1

Apparatus use to investigate the effect of the direction of light on the growth of newly germinated seedlings:

Answer 1

• 20 germinating cress seedlings
• light source e.g. window
• ruler
• foil
• red, green and blue filters
• container for seedlings e.g. petri dish lid or base
• cotton wool
• scissors

Question 2

Method to investigate the effect of the distance from light on the growth of newly germinated seedlings:

Answer 2

1. Split your sample of seedlings into 5 groups of 4
2. Water the seedlings with 10cm³ of water at the beginning of each day
3. Place each group of 4 at set distances from the window e.g. next to the window (0cm), 20cm, 40cm, 60cm and 80cm
4. Place an aluminium cap over two of the seedlings in each set so it is an identical distance down the shoot of the seedling
5. Leave the other 2 seedlings without the cap
6. Leave the seedlings for a few days

OR

1. Cut a hole in a cardboard box to allow light in
2. Get 2 Petri dishes and line them with cotton wool
3. Scatter some cress seeds onto both sets of cotton wool and then wet the wools with a Pipette
4. Place 1 Petri dish with cress seeds on it inside the cardboard box, ensuring that the hole is near a light source
5. Place the other on top, where light can come from multiple angles
6. Water regularly for 1 week
7. Remove and record observations

Question 3

Independent variable when investigating the effect of the distance of light on the growth of newly germinated seedlings:

Answer 3

distance from light source e.g. window

Question 4

Dependent variable when investigating the effect of the direction of light on the growth of newly germinated seedlings:

Answer 4

angle of ‘bend’ from normal of plant

Question 5

Control variables when investigating the effect of the direction of light on the growth of newly germinated seedlings:

Answer 5

• time
• type of seed
• seed age
• seed number
• no other light sources

Question 6

Results for investigating the effect of the distance from light on the growth of newly germinated seedlings:

Answer 6

Question 7

Method to investigate the effect of the intensity of light on newly germinated seedling:

Answer 7

1. Place cotton wool in 3 petri dishes and then soak them with equal volumes of water (control variable)
2. Place 10 mustard seeds in each dish
3. Leave the dishes in a warm place and allow them to germinate
4. Water the seeds every day with the same volume of water
5. After a few days the seeds will germinate
6. Make sure that the three dishes have the same number of seedlings
7. Use a rule to measure the height of each seedling - hold the stems to make sure they’re straight - be careful not to damage the seedlings
8. Place each dish in a different condition e.g. one dish placed in full sunlight (very bright windowsill), another dish placed in partial light (back of a lab), final dish place in darkness (in a cupboard)
9. Measure the height of each seedlings very day for at least 5 consecutive days
10. Record the results in a table
11. Calculate a mean seedling height for each day
12. Height of the seedlings will be similar for full and partial sunlight as chlorophyll is very efficient at absorbing energy from sunlight so plants do not need full sunlight to grow. Seedlings grow towards the light source due to phototropism.
13. Seedlings in the dark have grown tallest as seeds usually germinate underground and grow rapidly to reach the sunlight. If we keep seedlings in the dark they will continue to grow rapidly to reach the light. The leaves are small and yellow as once the seedlings have used all their energy stores they cannot carry out photosynthesis on gravity

Question 8

Independent variable when investigating the effect of the intensity of light on newly germinated seedling:

Answer 8

the light intensity

Question 9

Dependent variable when investigating the effect of the intensity of light on newly germinated seedling:

Answer 9

height of the seedlings

Question 10

Control variables when investigating the effect of the intensity of light on newly germinated seedling:

Answer 10

• volume of water used to soak the cotton wool
• type of seed
• volume of water used to water the seeds
• number of seedlings

Question 11

Apparatus for investigating the effect of gravity on the growth of newly germinated seedlings:

Answer 11

• cress, white mustard or radish seedlings
• beaker
• black paper
• moist cotton wool
• sticky tape
• incubator at 25 degrees C

Question 12

Method to investigate the effect of gravity on the growth of newly germinated seedlings:

Answer 12

1. Place the moist but not wet cotton wool around the inside of the beaker
2. Carefully place 5 seedlings in a horizontal position, 5 in vertical position facing downwards and 5 in a reverse horizontal position
3. Seal the top of the beaker with black paper and surround the bottom and sides with black paper
4. Pierce the top of the beaker with a fine needle to allow gases in and out
5. Leave the seedlings in a warm place for a day and then observe the results

OR

1. Dish of seedlings is placed on its side in the dark
2. The shoots have grown upwards against the direction of gravity and the roots have grown downwards towards the direction of gravity