Only green plants can convert the single carbon units of atmospheric carbon dioxide into the multi-carbon organic molecules on which all forms of life depend. Only green plants can provide the oxygen required by man and other aerobic organisms. In addition to his basic need for preformed organic molecules and oxygen, man also depends on plants to provide him, directly or indirectly, with an array of specific compounds such as vitamins and essential amino acids. Inadequate supplies of these may hinder growth and development or give rise to well defined deficiency diseases. At the present time information concerning the distribution and concentrations of such essential nutrients in plants is largely restricted to those plants that are already used as human foods. Nothing or virtually nothing is known about the chemical composition of approximately 250000 wild and little-used species. Amongst these there may be many that could provide us with cheap and plentiful new sources of essential nutrients that could be of enormous benefit to those suffering not only from full-blown deficiency diseases but also suffering sub-normal health due to partial deficiencies. The destruction of much of the world's wilderness areas has already deprived us of the opportunity to evaluate the contributions that a great many plant species might have made towards the elimination of deficiency diseases.
Many plants used as human foods contain compounds that are toxic to man. If intake is sufficiently high, these toxins may cause disease. Breeding programmes designed to eliminate toxins from crops species or reduce their concentrations to acceptable levels depend on genetic variability within the species or the possibility of producing hybrids with the desired characteristics. The motor neurone disease, lathyrism, which affects populations in the Indian sub-continent, Africa and China is caused by a toxin in the seeds of Lathyrus sativus. Surveys of cultivated plant populations have shown great variability in toxin levels and such genetic differences make it possible to select and breed toxin-low varieties. The existence of toxin-free species within the same genus has led to research aimed at producing toxin-free hybrids suitable for agricultural use. Approaches, designed to reduce or eliminate diet-related diseases, depend on the maintenance of the greatest possible diversity among both wild and cultivated plant populations. Such diversity is under threat.
Some 250 plant species are used as sources of drugs in western medicine. Most of these drugs are obtained from plants whose therapeutic value was recognized long before the compounds were isolated. In the developing countries of the world, it is estimated that 25000 plant species may be used in medicinal preparations. Few of these plants have been studied systematically to determine whether their reputations are justified and if so, the nature of the drugs they contain. If only 0.1 % were to yield useful drugs, they could make a major contribution to human health and welfare. The plants and the indigenous populations who understand their uses are both disappearing and it is a matter of great regret that much that could be of value has been and will be lost.