Recall the previous lesson, focusing on the plants found during the habitat survey and their leaf size and shape.

Brainstorm what students think they know about leaves and record it in the class science journal.

Referring to the data/graph from the previous lesson, discuss which type of plant was the most common in the school environment and why students think that was the case.

Show students an image of a prevalent local tree species or a tree on the school grounds, or use the image on the What’s in a tree? Resource sheet.

Ask students to name the main parts of the tree, including those that can be seen and not seen (trunk, branches, leaves, roots), and what they know about each part.

Focus students’ attention on the tree’s roots and discuss their function: to draw in water and nutrients to the tree.

Pose the question: Where does the water a tree absorbs go? Does it all stay in the tree?

The following investigation is best carried out on a sunny day, as it will yield results more quickly. However, it will still work in other conditions, though it may take a longer time to observe results. Consider the season/weather when planning this lesson. In colder months, or overcast weather, you may need to begin the investigation early in the week, and return to finish the lesson on a later day.

Explain to students how the investigation will be carried out: a plastic bag will be secured around the leaves of a healthy green tree for a few hours/days. Students will then make careful observations of what can be seen inside the bag to see if anything has changed. Use the image found on the Transpiration investigation Resource sheet (or prepare your own) to support students to visualise how the investigation is set up.

Ask students what claim about trees and water they think the investigation might be testing and why they think that. Students should have learned about evaporation and condensation in Year 4 Earth and space sciences, and may have already learned about the three states of matter as it appears in Year 5 Chemical sciences.

If required, present the claim to students that trees lose water through their leaves.

Discuss how the investigation might show the claim to be supported or incorrect.

Find a suitable tree/trees in the school ground and attach clear plastic bags as securely as possible, using either masking tape, zip ties (take care not to make them too tight and cut into the tree) or similar. Ensure that plenty of live leaves are inside the bag and that the bag is sealed around the stem/branch.

Place a “dead” stick (one no longer attached to a live tree and quite dried out), a pencil, or other piece of wood (that is approximately the same thickness as the stem/branch attached to the live leaves) into a plastic bag. Place the bag near the live leaves.

Ask students why the stick has been included in the investigation, and explain, if necessary, that this will allow students to compare the effects of live wood and leaves to those of dead wood. Any differences will be a result of this single variable.

Consider using labels or a sign to warn other members of the school community that a science investigation is in progress, to prevent anyone from “cleaning up” the bags.

Leave the plastic attached to the tree for long enough that condensation begins to form inside the plastic bag. The time frame required will depend on the weather. Periodically send students to make observations of the live and dead wood.

Once enough time has passed that condensation can be observed inside the bags wrapped around the leaves, the investigation can be concluded.

Take students to observe the bag/leaves and discuss their observations in situ.

After returning to the classroom, if required, introduce the term “condensation”: the process where water droplets are formed when water in its gaseous state (water vapour, steam, fog) contacts a cooler surface or cooler air and reverts to its liquid state.

Present (or re-present) the claim that trees lose water through their leaves and ask students if and how they think the investigation supports that claim. Students should conclude that the presence of condensate in the plastic bag around the leaves shows that water vapour must be escaping from the leaves of the live plant. The absence of any water vapour in the bag around the dead wood confirms that living trees, and indeed all other living plants, lose water.

Introduce the term “transpiration”: the process of trees and other plants losing water through their leaves.

Display a demonstration copy of the Desert and forest leaves Resource sheet.

Discuss the visible similarities and differences between the leaves of the two plants on display, the mulga (Acacia aneura) and the mountain ash (or giant ash, Eucalyptus regnans), and the habitat of each tree species.

Explain that describing the leaves as “bigger” or “smaller” can sometimes be ambiguous, vague and inaccurate, and that there is a better way to determine the size of something.

Introduce the term “surface area”: a measure of how much of an object is in contact with the outside world (often the air). Record an agreed definition of surface area in the class science journal.

Optional: You might demonstrate surface area by showing students two pieces of paper (with the same surface area, but don’t explain this yet), one long and thin (1 x 9 cm) and one short and wide (3 x 3 cm). Discuss what students think of the size of the pieces of paper, if one is larger than the other, and why/why not. Demonstrate that the surface area is the same by covering the paper in 1 cm x 1 cm cubes or 1 cm x 1 cm graph paper. Conclude that two items might appear to be different sizes because of their shape, but their surface area may be the same.

Pose the question: Now that we know that trees lose water through their leaves, does the size of the leaves affect how much water they lose? Or in simpler terms, Do big leaves lose more water than small leaves?

Explain to students that they will use folded or unfolded cloths as models for different sized leaves as a means of investigating if a leaf’s surface area has an impact on how much water it is able to retain/lose. They will wet the cloths and then collect data on how much water is lost over time, by weighing the cloths on digital scales a minimum of twice at appropriate intervals.

NOTE: The length of the intervals will be dictated by the climate and the weather conditions on the day of the investigation. In warmer weather the cloths can be expected to dry more quickly, so data should be collected every 15-30 minutes. In mild to cool conditions data can be collected at hourly intervals. It may be helpful to consult the weather forecast and conduct your own informal test investigation the day before to determine the right interval. Discuss/explain this to students if appropriate.

Explain that when scientists want to test, explore, or investigate phenomenon that require intricate equipment, sometimes they will build and use a model. For biologists (scientists that study living things), models provide insights without having to use (and potentially harm) real specimens.

In this case using absorbent cloths as a stand in for leaves is an efficient classroom model because the cloths hold water that is then evaporated at different rates depending on the conditions. Transpiration is one type of evaporation (the evaporation of water from leaves into the atmosphere).

Discuss with students the advantages and disadvantages of using models, and how the cloths replicate the two different leaf shapes looked at, the mulga (the wrapped cloth) and the mountain ash (the open cloth).

Pose the general question: What things might affect how much water a cloth loses over time?

Using a demonstration model of Variables grid Resource sheet, record what students will be measuring in the centre of the grid (water loss) and mark it with an “M”. Brainstorm and record possible variables for the investigation around this. These might include the type of cloth, the size of cloth, the temperature of the classroom, the location of the cloth (in the breeze or not), the exposed surface area of the cloth etc. Add or remove columns and rows as required.

Mark what is going to be changed (the surface area of the cloth) with a “C”, and what is going to remain the same (all the other variables) with an “S”. 
Model how to use the question stem to write a question for the investigation: What happens to the amount of water in the cloth when we change the exposed surface area of the cloth? 

Use a demonstration copy of the Surface area investigation planner Resource sheet to discuss in more detail how to plan the investigation, conduct the investigation, and record data, if required.

Discuss how to change the exposed surface area of the cloth while controlling the other variables, for example:

• fold the second cloth in half four times and fix it together with a binder clip. The first cloth will stay flat with a binder clip attached to the side for consistency.
• the amount of water in each cloth needs to be the same at the beginning. This can be achieved by squeezing water out until they weigh the same or by measuring how much water to put on them.

In collaborative teams, students plan and conduct their investigation, recording their results using pages 1 and 2 of the Surface area investigation planner Resource sheet. Ensure all students first complete the “predict” section of the planner before beginning the investigation process.

Students share the results of their investigations, specifically how the mass of each cloth changed over time.

Discuss the change and compare it to students’ initial predictions. Encourage students to probe deeply into groups’ results (and later in the discussion, their claims) using the science question starters.

Optional: If each group used the same brand/type of cloth in the same size and folded in the same manner (so their results are comparable), record the groups’ results of how the mass of the cloth changed over time in the class science journal and determine the average change. Alternatively, you might talk about the differences in groups’ results and how these factors contributed to these differences.

Discuss with and model for students how to construct a line graph to visually represent the data they recorded, including:

• the conventions of constructing a scientific graph: the vertical (or $y$) axis usually represents the variable we measure, and the horizontal (or $x$) axis represents the variable that we change.
• how to draw the two axes and what scale to use on the $y$ axis, for example, one box represents 10 g.
• how to draw two lines on their line graph in different colours, one for each cloth “shape”.
• how to record which cloth corresponds to which colour on the graphs using a key.
• how drawing two lines allows you to compare what is happening in each cloth.

If students are already experienced in constructing line graphs, they can do this independently. You might also provide them with exemplar line graphs and allow them to identify the key features before constructing one on their own.

Students analyse their graphs to look for patterns and relationships, and to make a scientific claim, supported by evidence, about leaf shape and its relationship to the habitat a tree might live in.

Explain that the claims about plants need to be conditional (using the word “could”) since the results are based on a model and not actually biological samples of plants in dry habitats. For example, “Having leaves with smaller surface area could help plants survive because there is less leaf surface for transpiration to take place.”

Further explain the reasons plants in drier habitats have smaller leaves, such as:

• Plants have openings in their leaves, called stomata, through which transpiration occurs.
• Having smaller and fewer leaves decreases the number of stomata and therefore the amount of water loss/transpiration.
• Some plants also have fewer and smaller stomata.
• The plants’ stomata close to reduce water loss.

Optional: Take an observation walk through the school grounds to consider and compare leaf surface area and the likelihood that the plant is adapted to dry conditions. Alternatively, leaves collected for a nature table or leaf gallery (from Lesson 1) could also be used.

Optional: Invite a botanist to attend the classroom (virtually or physically) to provide some local examples. Ask a horticulturalist or farmer to explain what they do to support their plants in hot weather.

Optional: Investigate other plant adaptations for dry conditions, for example:

• View the video David Attenborough—The fascinating life cycle of desert plants (5:41).
• Carry out a waxy leaves investigation: Discuss how wet cloths could also be used to investigate the claim that waxy coatings on leaves help to reduce water loss. For example, by comparing two wet folded cloths, with one wrapped in baking paper.

Reflect on the lesson

You might:

• add vocabulary related to surviving in dry environments to the class word wall or glossary.
• add to the W and H sections of the TWLH chart.
• revisit initial brainstorm and add to it—this represents what you learned.
• discuss how the learning from this lesson will be relevant for building a model of a plant or animal’s adaptations, including:
  • using alternate materials when modelling.
  • leaf adaptations that enable survival in habitats with limited water.
  • where else might models be useful in considering how things work?