Science and Technology K–6 · Year 6

Light or dark: how light changes the way seedlings grow

Science understanding: Biological sciences

PracticalLow risk

The idea

Light is a physical condition a plant needs to make its own food, so seedlings in darkness grow tall, thin and yellow and then collapse once the food stored in the seed is used up, while seedlings in light grow short, sturdy and green.

What you need

3 recycled cups with drainage holes, each holding 300 mL of the same potting mix
30 mung bean seeds (10 per cup), soaked in water overnight
a bright windowsill, a dark cupboard, and a shoebox with a 2 cm x 10 cm slit cut in one end
a 50 mL measuring cylinder for watering
a thermometer in each of the three places
a ruler in millimetres and a colour chart (dark green, light green, yellow, white)
gloves and a dust mask for handling the potting mix

How to do it

Plant 10 seeds 1 cm deep in each cup. Put one cup on the windowsill, one in the cupboard and one inside the shoebox with the slit facing the window.
Water each cup with 30 mL every second day, at the same time.
Every second day for 14 days, record for each cup: the number of seedlings up, the height of the 5 tallest (mean, in mm), colour from the chart, number of open leaves, and the temperature of each place.
On day 14, measure the stem thickness of 3 seedlings from each cup against the ruler and note which way the shoebox seedlings lean.
Graph mean height against day for the three cups on one set of axes.
Decide which measurements show healthy growth and write a conclusion that uses more than height alone.

What you should see

All three cups come up within a day or two of each other, because a seed germinates on its own stored food and needs water, warmth and air, not light (bean sprouts are grown in the dark). After that the dark seedlings grow tallest fastest, with long pale-yellow stems, small unopened leaves and thin stems; the windowsill seedlings are shorter, thicker and green with open leaves; the shoebox seedlings bend toward the slit. Kept in the dark long enough, the dark seedlings droop and wither once the seed's store is used up, because they cannot make more food without light; by day 14 they are pale and weak. Height alone would wrongly suggest darkness is best; colour, leaf opening, stem thickness and survival at day 14 show the effect of light. If the cupboard is more than a few degrees cooler than the sill, temperature is a second variable and the learner says so.

What changes

What you change: light condition (full light, darkness, light from one side)
What you measure: seedling height, colour, number of open leaves, stem thickness and survival
What you keep the same: seed type and number
potting mix volume
water volume and timing
temperature (measured in each place)
days of growth

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Seeds need light to germinate (these germinate in the dark).
The tallest seedling is the healthiest.
Plants get their food from the soil, so light only helps them see where to grow.

Safety card

Low riskLearners carry it out

Hazards

Legionella bacteria in potting mix dust
soaked seeds are not for eating

Controls

dampen the potting mix before use
an adult opens the bag away from faces
gloves and a dust mask while handling it
wash hands afterwards

Note

Risk assessment before the lesson using Primary RiskAssess or the school's own template.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science and Technology K-6 Syllabus (2017), current, taught until 2026; code read from the syllabus document on 22 September 2026ST3-4LW-S ST3-1WS-S
Science and Technology K-6 Syllabus (2024), implemented from 2027; NESA's timeline is 2026 plan and prepare and 2027 start teaching, and schools may choose to implement it during 2026; code read from the outcomes page on 22 September 2026ST3-PQU-01 ST3-DAT-01
Australian Curriculum v9AC9S6U01AC9S6I01AC9S6I02AC9S6I03AC9S6I04

Sources

The pages the author read to write this activity.

All Concept Studio activities