You firstly need to de-starch a plant by storing it in darkness for 24-48 hours. This obviously needs to be done well in advance of the lesson.
Geraniums work well for most plant practicals in biology, It is worth looking after several plants at school rather than bringing one in each time one is required.
Some of the leaves on the plant need to be covered in strips of opaque black card or paper or tin foil. Ensure the strips are tightly pressed against the surface of the leaf. Paper clips are a good way of fastening them securely.
The strips will stop sunlight from getting to the surface of the leaf.
Place the plant in direct sunlight for several hours and then remove one of the leaves that has been covered and test it for starch. For details on how to test for starch, click the link at the bottom of this page.
Students should notice that there is no starch present in the areas that have been covered. The rest of the leaf that has been exposed to sunlight should contain starch. This practical proves that photosynthesis needs sunlight for the reaction to take place.