This video clip from the BBC Class Clips collection (from Botany: A Blooming History) demonstrates the classic experiment to show that light is needed for the production of starch, placing it in the context of pioneering 19th century scientist Julius von Sachs.

Julius von Sachs became Head of the botanical institute at Würzburg University in 1868. He wanted to know why plants need light. A repeat of an experiment he carried out is demonstrated. A leaf which has been in the light is decolourised and tested with iodine. It shows the presence of starch. A leaf which has not been exposed to light is also tested and does not show the presence of starch. A final test is demonstrated in which a leaf is partly covered and left in the sunlight. When this leaf is tested with iodine, only the illuminated regions show the presence of starch.

This is a classic demonstration done with students. This clip can be used to introduce the ideas that it illustrates. The process can be discussed in terms of what is being done to the leaf at each stage and why the demonstration is so elegant. Pupils can then do their own leaf shading and iodine tests. This can then be extended into tests on leaves which have also been deprived of carbon dioxide. At higher levels, this is a reminder for students that the light dependant and the light independent reactions are linked and interdependent.

See this on the BBC Four Botany: A Blooming History