See this on the BBC Four Botany: A Blooming History for a clearer picture when using with students.

This video clip from the BBC (from the TV series Botany: A Blooming History) looks at the way in which classification techniques are used by research scientists at Kew Gardens in their search for new medicines and other useful plants.

Researchers at Kew Gardens use a wide variety of techniques to establish relationships between species, including scanning electon microscopy and DNA analysis. A massive archive and a DNA database can be used to find related species. A real world use for this is demonstrated using a variety of sage plant with the ability to produce a chemical which may be effective in the treatment of malaria. The DNA from the plant is extracted and compared with those in the database, so that closely related plants can be identified. It is possible that these plants may produce greater quantities and more effective versions of the chemical. This sequence illustrates the importance of continued research into plant properties and classification.

The modern techniques which underpin our knowledge of the relationships between species such as DNA analysis could then be discussed as well as DNA fingerprinting which many students will only associate with humans. Students could then do practical work to extract DNA from plant material (Strawberries, kiwi, onion etc) and if possible do gel electrophoresis of DNA. Higher up the school, students could develop the ideas of the importance of plants to humans, particularly in the context of medicine. This could be the stimulus for student research into the discovery of other drugs and the development processes for medicinal compounds. The material could also be stimulus material for discussions about the importance of preserving species, both in situ and ex situ, such as seed banks.