An Investigation into Leaf Surface Temperature

This practical is designed to be used as the opening lesson of the transport in plants section of the A level specification. It aims to engage the students in an investigation-based introduction to this topic rather than one based on content. The simple investigation will ask, “What happens to the surface temperature of leaves if petroleum jelly is applied to their lower surfaces?”

Teaching notes

This practical aims to:

1. Provide an engaging introduction to the topic of transport in plants
2. Allow the students to plan an investigation to answer the question “What happens to the surface temperature of leaves if petroleum jelly is applied to their lower surfaces?”
3. Use hand-held infrared thermometers to produce quantitative data1 that will allow them to answer this question.
4. Develop their understanding of how science works.

As an introduction to plant physiology, and prior to any discussion of transpiration or stomatal structure, function and distribution, students will use hand-held infra-red thermometers to investigate the surface temperature of leaves. The students will measure the temperature of the upper side of each leaf before and after smearing the lower surface of the leaf with petroleum jelly. The investigation uses living plants with leaves still attached to the main stem and can either be carried out in the school grounds or can be a lab based investigation.

Before carrying out the practical, the students must plan their investigation (with guidance) and consider the variables in their investigation. Students can then carry out simple statistical analysis on their data to investigate the significance of the difference between the mean leaf surface temp without petroleum jelly and with petroleum jelly on the underside.

The next question to pose is “What is the likely cause of this difference?”
Hopefully, the students will make the connection between temperature change and evaporative cooling, and some will remember the role of stomata from GCSE.

Stomatal function and transpiration can then be introduced. Students can use the outcome of the statistical analysis to formulate a hypothesis to explain any observed trends in data (i.e. Evaporation of water from stomata [transpiration] cools the leaf).

Following this, the students outline how they could use the infrared thermometer to investigate differences in the stomatal density in the upper and lower epidermis of leaves. This exercise is a good opportunity for stretch and challenge as some students will need a lot of help to realise that this can be achieved by comparing the effect on leaf surface temperature of applying petroleum jelly to the lower epidermis (where there are lots of stomata) with the effect of applying petroleum jelly to the upper epidermis (where there are fewer, if any stomata).

The students can then be introduced to the procedure for investigating stomatal density2 on the upper and lower epidermis of the leaves, allowing them to collect data to further test their prediction that the leaf heating effect observed when the stomata are blocked will be greatest when the density of stomata in the epidermis is high.

This investigation and any extensions that the students devise provide opportunities for students to develop the skills outlined in AO2(application of knowledge) and AO3(how science works)3. Activity 8 should clarify the scientific method and hence how science works.

You may also wish to show your students our animation on Transport in Xylem and Phloem.