Growing bean seedlings in light and in darkness · Concept Studio

The idea

Seedlings in darkness still germinate but grow tall, pale and weak on their seed reserves, while seedlings in light stay short, turn green and keep gaining dry mass by photosynthesis.

What you need

dwarf bean or pea seed, 27 seeds
9 identical 10 cm pots filled with the same seed-raising mix
light-proof cupboard or large lidded box in the same room as a sunny windowsill or light bank
watering can or measuring cylinder so each pot gets the same volume
ruler graduated in millimetres
balance reading to 0.01 g, paper bags and a drying oven set to 90–95 °C (the temperature in the SAPS radish dry-mass method)
gloves and P2 dust masks for handling potting mix

How to do it

Dampen the seed-raising mix before use, then fill 9 pots to the same level and sow 3 seeds per pot 2 cm deep.
Place 5 pots in the light-proof cupboard and 4 on the windowsill or under the light bank in the same room.
Give every pot the same volume of water whenever the light pots need it (open the cupboard only briefly).
After emergence, every 2 or 3 days record the height of each seedling from soil to growing tip, the number of leaves and the leaf colour.
On day 21, cut every shoot at soil level, weigh the fresh shoots from each pot, then dry them in paper bags at 90–95 °C until two weighings a day apart agree, and record the dry mass per plant.
On day 14 move one of the five dark pots into the light and record its colour each day until day 21; leave this pot out of the dry-mass comparison, so four pots stay in continuous darkness.
Calculate mean height and mean dry mass per plant for each treatment and compare them in a column graph.

What you should see

Seedlings in both treatments emerge. Dark-grown seedlings become taller and thinner, with small pale yellow leaves, and by day 21 they have less shoot dry mass per plant than the light-grown seedlings, which are shorter, sturdier and green. Dark seedlings do not grow on to maturity. The dark pot moved into the light turns green over the following days. The learner knows it worked when the dry-mass difference is in the same direction for all four replicate pots of each treatment.

What changes

What you change: light (light or continuous darkness)
What you measure: seedling height (mm), leaf number, leaf colour, shoot dry mass per plant (g)
What you keep the same: seed variety
pot size and mix
sowing depth
water volume
room temperature (both treatments in one room)

Common misconceptions

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

Plants in the dark grow taller, so darkness makes plants grow better.
Plants get their food from the soil.
A plant that has turned yellow in the dark has died.

Safety card

Low riskLearners carry it out

Hazards

dust and bacteria (including Legionella) in bagged potting mix
hot oven surfaces when drying samples

Controls

dampen potting mix before opening and handling; wear gloves and a dust mask (for example P2), as the Agricultural Technology syllabus advises mask and gloves for potting mixes
wash hands after handling mix
teacher loads and unloads the drying oven

Note

No chemicals beyond potting mix. Heat: the drying oven is covered by the general rules in NSW Department of Education, Chemical Safety in Schools (CSIS) 2021 Technical Update: Section 1 (general information for all staff) and Volume 2 Appendix D (generic risk assessment advice for each chemical). Complete a CSIS or RiskAssess site-specific risk assessment before the lesson.

Curriculum references

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

Agricultural Technology Years 7–10 Syllabus (2019)AG4-6 AG4-11
Technology 7–8 Syllabus (2023), focus area Food and agricultural practicesTE4-PPM-01
Australian Curriculum v9AC9TDE8K04AC9S7I02

Sources

The pages the author read to write this activity.