Understanding Tree Growth

What you need

Three or four freshly cut dormant twigs of local shrubs or trees (about 30 cm long), two large glass jars, 50 mL of sugar, a 1 L beaker, a glass stirring rod, two large clear plastic bags, water, and a ruler marked in millimetres.

What it is for

To watch a tree's growth machinery working, on a twig, in the classroom.
To measure twig growth over two weeks, and to test what a sugar solution does to the buds.

The manual states these. What it does not state is not here.

How it runs

The teaching sequence as the manual gives it.

01

Open by asking how much a tree near home has grown since the students were small. Let them describe what they have noticed — a branch now touching a roof, a sapling now taller than a person, a root lifting the footpath. Then ask the sharper question: how would you tell whether a tree is growing right now? (Shoots extend from their tips, adding height and spread; the trunk and branches thicken.)
02

Ask where on a tree growth actually happens. (At the tips of shoots and roots, and in the cambium — the thin layer of living cells just inside the bark.)
03

Put each group to designing its own experiment: either to show where a tree grows from, or to test one thing that affects growth — water, light, position, temperature. Make them state three things before they touch anything: what they will change, what they will keep the same, and what they will measure. Have them write down a prediction. Then help them run it and reach a conclusion they can defend.
04

Groups short of an idea can run the experiment on the sheet below, or use it as a pattern for their own.
05

EXPERIMENT SHEET
06

Take one twig of each species you have collected. Sketch each twig and label the terminal bud, a lateral bud and the terminal bud scar.
07

Measure and record, for every twig: the total length of the twig; the length and diameter of the buds; the colour of the bud scales; and the distance from the terminal bud back to the terminal bud scar. That last measurement is last year's growth. Write it down before the experiment starts, because you will compare this year's growth against it at the end.
08

Make the sugar solution: dissolve 50 mL of sugar in water in the 1 L beaker, stirring with the glass rod until no grains are left. Fill both jars with plain water. Stand half the twigs in the first jar and add a few millilitres of the sugar solution to that jar. Stand the rest of the twigs in the second jar, in plain water only. The second jar is your control. Without it the experiment proves nothing.
09

Put both jars in a warm, sunny place in the room, side by side so they get the same light. Cover the twigs in each jar with a large clear plastic bag to hold the moisture in.
10

Every two or three days, measure the twigs and buds again and enter the readings in your table with the date beside them. Add a few millilitres more sugar solution to the first jar every second day, and keep the water in both jars topped up. Leaves and flowers usually appear after about two weeks — use them to identify the species.

Talking about it

Discussion points, worked examples and the handouts the manual prints with this session.

Points for discussion

Let students write their own questions and draw their own conclusions about what breaks a bud's dormancy. Use these only if they stall.
What does the sugar solution stand in for, in a real tree?
What does the plastic bag change for the twig?
Did any bud open into something other than a leaf? Could you have guessed from the way it looked?
What would happen to the twig if you snipped the terminal bud off?
Did the species grow at different rates? Which was quickest, which slowest?
How does this season's growth on your twig compare with last year's, which you measured on day one?

Preparation

Collect the twigs before the buds break — late in the dry season, ideally on a field trip, so students see the tree each twig came off. Cuttings keep in a freezer in plastic bags and can be thawed slowly when needed. Insist on permission before anyone cuts from a tree, on private or public land. Cut with sharp shears or a sharp blade. Take only twigs whose loss will not show, and never take several from the same spot or the same side of one tree.
Make copies of the experiment sheet and activity sheets, one per student or group.

Where this comes from

Bridge the Gap is a compilation. This is what the manual records for this session.

The manual prints no source for this session. It is published here as Swechha has used it, adapted over years of classroom work; if you recognise its origin, tell us and we will credit it.

Before you start sets out where the compendium as a whole comes from.

Where to go from here.

Next session

Sketching TreesTrees and Forests, 3 of 4

The theme

Trees and Forests4 sessions and the background reading

All eight themes

Teach43 sessions across 8 themes

Understand the data

The whole library · the A to Z