Biology 11–12 · Years 11–12

Water balance in plants: water loss from cuttings with leaf surfaces sealed

Module 8: Non-infectious Disease and Disorders (Homeostasis: water balance in plants); also Module 2 (Transport) and Module 3 (Adaptations: structural adaptations for water conservation)

PracticalLow risk

The idea

Most water leaves a plant through stomata, so sealing the leaf surface that carries most stomata cuts water loss sharply, showing how stomata and cuticle together let a plant balance water uptake and loss.

What you need

Four similar cuttings about 8 cm long with leaves at the top, from privet, busy lizzie (Impatiens) or pot geranium (Pelargonium), the plants the source names
Four 10 or 25 mL measuring cylinders taped to a tray; tap water; cooking oil, about 1 mL per cylinder; teat pipettes
Petroleum jelly; balance reading to 0.01 g (the source uses 0.1 g); scissors; a bright, cool light such as a fluorescent or LED panel

How to do it

Coat the leaves of cutting 1 on the upper surface only, cutting 2 on the lower surface only and cutting 3 on both surfaces with a thin even layer of petroleum jelly; leave cutting 4 uncoated.
Stand each cutting in a measuring cylinder, adjust the water to an exact mark with the teat pipette and add about 1 mL of oil so it floats on the water.
Record the starting volume and weigh each cylinder with its cutting.
Leave the cylinders under the light and reweigh every 24 hours for 3 days.
Calculate the mass lost per hour for each cutting and, after tracing the leaves on squared paper, per square centimetre of leaf.
Relate the pattern to the stomatal density measured by nail-varnish impressions and explain how closing stomata and a waxy cuticle conserve water.

What you should see

The uncoated cutting loses the most water and the cutting sealed on both surfaces the least. For leaves with most stomata underneath, sealing the lower surface cuts loss far more than sealing the upper surface. A mass fall of 1.2 g over 48 hours is 25 mg per hour (computed). The learner knows it worked when the ranking of the four cuttings matches the distribution of stomata on the leaves used.

What changes

What you change: leaf surface sealed (none, upper, lower, both)
What you measure: mass of water lost per hour per square centimetre of leaf
What you keep the same: plant species and cutting size
light and temperature
oil layer
time between weighings

Common misconceptions

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

Water leaves a leaf evenly across its whole surface; the cuticle is nearly waterproof and stomata are the main exits.
Plants cannot control water loss; guard cells close stomata when water is short.
All the water a cutting takes up leaves through its leaves; a small fraction is used or stored in the plant.

Safety card

Low riskLearners carry it out

Hazards

Plant sap may irritate skin
Scissors

Controls

Wash hands after handling cuttings; anyone with sap sensitivity wears gloves
Cut away from the body

Note

No hazardous chemicals; RiskAssess (https://www.riskassess.com.au/).

Curriculum references

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

Biology Stage 6 Syllabus (2017), current: Year 11 taught to the end of 2026 and Year 12 to Term 3 2027BIO12-15 BIO11-9 BIO11-10 BIO11/12-4
Biology 11–12 Syllabus (2025), Year 11 focus area Cells to systems; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-02
Biology 11–12 Syllabus (2025), Year 11 focus area Evolution and ecosystems; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11-03
Biology 11–12 Syllabus (2025), Year 11 Working scientifically; new syllabus not yet taught: Year 11 from Term 1 2027, Year 12 from Term 4 2027BI-11WS-03 BI-11WS-04
Australian Curriculum v9No Australian Curriculum v9 code is listed.

Sources

The pages the author read to write this activity.

All Concept Studio activities