Since time immemorial, man has depended on nature for his needs – from food to shelter, clothing, transport, fertilizers, flavours and fragrances and medicines (Cragg and Newman, 2005). Plants are the basis on which thousands of years old advanced systems of traditional medicine are based that still provide mankind with new remedies for many ailments. Most of the time, although some of these therapeutic properties are proven to be false, the entire basis of medicinal plant therapy is evidence derived from many hundreds and possibly thousands of years of empirical observations. The oldest records relating to medicinal plants date back to about 2600 BC in ancient Mesopotamia (Heinrich et al., 2004), written on clay tablets in cuneiform. They included oils from Cedrus species (cedar) and Cupressus sempervirens (cypress), Glycyrrhiza glabra (licorice), Commiphora species (myrrh) and Papaver somniferum (poppy juice), which are still in use today for a variety of ailments such as coughs and colds, parasites, and inflammatory treatments. In ancient Egypt, bishop's weed (Ammi majus) was used to treat the loss of pigmentation of vitiligo (Staniszewska et al., 2003; Beisert and Schwarz, 2002). More recently, this plant has been the source of a drug (β-methoxypsoralen) for treating psoriasis and other skin disorders as well as T-cell lymphoma (Beisert and Schwarz, 2002).
Nature still remains a promising source for potential chemotherapeutic agents. More than 50% of the drugs in clinical use in the world today are natural products and their derivatives (Farnsworth et al., 1985; Cragg and Newman, 2005). Those derived from higher plants contribute no less than 25% of the total. Over the past 40 years many flowering plants have yielded some very potent drugs. For example, Dioscorea species have been invoked as the source of diosgenin, the precursor from which all anovulatory contraceptive agents are derived. Examples include the antihypertensive and tranquilizing alkaloids originating from Rauwolfia species; pilocarpine and pilocarpine for glaucoma and 'dry mouth' respectively, derived from a group of South American trees in the citrus family, namely: Pilocarpus spp.; the two most potent anti-cancer agents from rosy periwinkle (Catharanthus roseus); agent laxative from Cassia sp. and a cardiotonic for heart failure from Digitalis species (Newman et al., 2000).
Tropical forests contain about half of the world's flowering plant species (125,000), making them rich in potential drug sources. These forests provide and will continue to provide a vast storehouse of natural products that provide thousands of natural product chemists with invaluable starting material for the development of new drugs. Very little work has been done so far towards the discovery of tropical species, only 1% of them have been studied for their pharmaceutical potential (Cragg and Newman, 2005). The figure is even lower with respect to species restricted to tropical rain forests. So far, about 50 drugs have been derived from tropical plants. This is commonly cited as one of the major arguments for protecting tropical forests, the potential undiscovered pharmaceuticals in modern medicine. The rate of extinctions every year is therefore alarming.
Somewhat accidentally through observational laboratory research, three major sources of marketable or clinical trial-completing anti-cancer drugs come from North American plants used medicinally by Native Americans: pawpaw (Asimina spp.); western yew (Taxus brevifolia), effective against ovarian cancer; and mayapple (Podophyllum peltatum) used against leukemia, lymphoma lung and testicular cancer (Guraib Fakim, 2006).
Traditional Medicine
For thousands of years, plants have been used as medicine (Samuelson, 2004). They assumed crude forms of drugs in tinctures, teas, poultices, powders and other similar medicines (Balik and Cox, 1997; Samuelson, 2004). The plants used and the applications for particular diseases were oral texts passed down from one generation to the next. Eventually, this data will be collected in herbal pharmacopoeias (Balunas, 2005).
Modern allopathic medicine has also evolved since ancient times and it is expected that many of the great and important new treatments of the future will be discovered and marketed using information from traditional knowledge and experience as their predecessors did. European traditions are particularly well-known and have greatly influenced modern Western pharmacognosy, but every society has well-established herbal traditions, some of which have rarely been studied. Thus, studying these traditions will not only shed light on the development of the field but also serve as a fascinating example of how creative our ability to adapt diverse cultural practices can be in developing new herbal traditions.
In some countries medicinal plants are sometimes associated with supernatural powers and witchcraft because people are unable to see scientifically that plants can produce cures and predictions. The theory of signatures is one of such irrational ideas; its elements are found in most of the world's healing cultures (Boehm, 1982). It stipulates that plants - and these are considered signs of God on that plant - can indicate their curative properties. Red sap and juice indicate blood and menstrual problems; yellow flowers with bile and jaundice; the human shape of some roots with the female form of fertility, etc. The theory sometimes - but not always - works: Chelidonium majus has yellow flowers and a yellow alkaloid latex, and has been successfully used to treat jaundice (Guraib-Fakim, 2006).
