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For thousands of years mankind has used the resources of flora in the treatment of various diseases. There are reports, for example, of the use of plants for therapeutic purposes dating back to 3000 BC in the Chinese work Pen Ts'ao by Shen Nung (Tyler, 1996; Ko, 1999). In 78 AD, the Greek botanist Pedanios Dioscorides described about 600 medicinal plants and this treaty remained as a reference source for more than fourteen centuries (Robbers et al., 1996; Tyler, 1996). The therapeutic properties of certain plants were discovered and propagated from generation to generation as a part of the popular culture through observation and experimentation by primitive people (Turolla & Nascimento, 2006). In the sixteenth century, the Swiss physician Philippus Aureolus Theophrastus Bombastus von Hohenheim, known as Paracelsus (1493 1541), formulated the "Theory of Signatures" based on the latin proverb similia similibus curantur, which means "alike cures alike". According to this theory it was believed that the shape, color, flavor and odor of the plants were related to their therapeutical properties, giving clues to their clinical use. Some of these plants have become part of the allopathic and homeopathic pharmacopoeias since the nineteenth century, when their therapeutic basis started to be investigated (Elvin-Lewis, 2001). The isolation of morphine from Papaver somniferum in 1803 by the pharmacist Friedrich Wilhelm Adam Serturmer marked the beginning of the extraction process of active ingredients from plants. Since then, other substances have been isolated, such as quinine and quinidine obtained from Cinchona sp in 1819, and atropine from Atropa belladonna, in 1831, which started to be used instead of plant extracts (Tyler 1996; Schulz, 2001). The production of drugs by chemical synthesis, the growing of the economic power of pharmaceutical industries and the lack of scientific evidence on the efficacy of substances from plant origin combined with the difficulty of chemical, physico-chemical, pharmacological and toxicological control of plant extracts used so far, boosted their replacement by synthetic drugs (Rates, 2001). After the 1960s, there was a lack of interest from pharmaceutical companies and research institutes in the search for new substances from plant origin. This fact was due to the belief that the main active ingredients from known herbal drugs had already been isolated and all possible chemical modifications in these substances had