Chemical diversity in the plant world: Why can't we find more plant derived drugs?

The natural world has long been a rich source of medicinally useful compounds, yet many large scale discovery efforts in the later part of the 20^th century yielded far fewer useful discoveries than had been anticipated. At the same time, only a small percentage of plant species have been evaluated against a limited number of possible therapeutic targets. The scope of plant diversity is reviewed and it is estimated that there are perhaps close to 400,000 species of plants. Study of a small percentage of these species have yielded more than 100 medically useful compounds. Extrapolating from the numbers of plant species that were likely studied to discover these compounds it is concluded that there are at least 500, and possibly many more, medically useful novel biochemical compounds that remain to be discovered in plants. The reasons that efforts of the late 20^th century were not more successful are reviewed and possible alternative approaches suggested.