Introduction

Life is sustained in all living organisms through the metabolism of universally distributed ‘primary’ biochemicals; sugars, amino acids, common enzyme cofactors, nucleic acids, proteins, etc. In addition to the primary chemicals, plants and microorganisms accumulate a wide variety of others which are restricted in their distribution, usually to taxonomically related groups, and which appeared to their nineteenth century discoverers to have no role in the life of the organisms in which they were found. These ‘secondary’ compounds are responsible for the wide chemical diversity seen in plants and microorganisms and those of higher plants in particular have played a crucial role in human cultural and economic development as medicines, pesticides, dyes, flavourings, building materials, etc. These compounds gave to plants properties which were known, before the rise of the chemical sciences, as their ‘virtues’ and helped make civilisation both possible and tolerable.

This review will consider the discovery, evolution, distribution, role and economic value of the secondary compounds of higher plants and the importance of preserving the heterogeneity still to be found in wild species. We will argue that the range of chemicals found in plants provides a unique resource for the chemical industry in its search for new drugs and pesticides which will not be rendered valueless by the ‘rational’ approaches of molecular gene technology and ‘designer’ drugs, nor by the screening of molecules of microbial or synthetic origin.