RP8: Required practical activity 8: Seedling growth
Not started yet — this one needs some love.
Aim: investigate the effect of light or gravity on the growth of newly germinated seedlings. Results must be recorded as both length measurements and as accurate, labelled biological drawings.
Independent variable: light condition — full sunlight / partial light / complete darkness (for phototropism investigation). For gravitropism: orientation of seedling relative to gravity.
Dependent variable: mean height of seedlings each day (mm); direction of growth shown in labelled biological drawings.
Control variables:
same species (white mustard seeds recommended)
same number of germinated seedlings per dish (equalise after germination)
same volume of water added to cotton wool
same temperature
measurements taken at the same time each day
same time period (at least one week)
Method (phototropism — effect of light)
Half-fill three petri dishes with cotton wool soaked in equal amounts of water.
Place 10 white mustard seeds evenly in each petri dish.
Put the dishes in a warm place, undisturbed. Add equal amounts of water to each dish if the cotton wool begins to dry out.
Allow seeds to germinate (approximately 1–2 days).
Once germinated, equalise the number of seedlings across the three dishes — remove excess seedlings so all three have the same number.
Move the three dishes to different light conditions:
Dish 1 — full sunlight (bright windowsill)
Dish 2 — partial light (alternate one day on windowsill, one day in dark cupboard)
Dish 3 — complete darkness (dark cupboard)
Every day for at least one week, measure the height of each seedling by resting a ruler on the cotton wool and gently holding each seedling upright against it. Take care not to damage the seedlings.
Record all heights in a table. Calculate the mean height per condition per day.
Produce labelled biological drawings showing the direction of shoot growth in each condition.
Plot a graph of mean height (mm) on the y-axis against day number on the x-axis, with all three conditions on the same graph.
Safety
Wash hands after handling seeds and seedlings — seeds can carry allergens and fungal spores.
The equipment and materials used in this experiment are otherwise low hazard.
Handle seedlings very gently when measuring — damaged seedlings will not give reliable results and will skew the data.
Common exam mistakes
Use the correct term: shoots growing towards light show positive phototropism. "Tropism" on its own is often too vague.
Do not explain phototropism as extra photosynthesis on the lit side. In shoots, auxin causes more elongation on the shaded side, so the shoot bends towards light.
This practical measures growth responses over time; it is not a photosynthesis-rate practical.
Use several seedlings and control species, age, water, temperature, light intensity and time because individual seedlings vary.