Asclepios with his daughter Hygieia
Except in botanical usage, an herb is any plant that is valued for flavor, scent, medicinal, or other qualities. (Botanists use the same term differently, for any non-woody flowering plant, regardless of its flavor, scent or other properties, and thus include only grass-like plants and forbs.)
Herbs have a variety of uses including culinary, medicinal, and in some cases spiritual usage. General usage differs between culinary herbs and medicinal herbs. In medicinal or spiritual use any of the parts of the plant might be considered "herbs", including leaves, roots, flowers, seeds, resin, root bark, inner bark (cambium), berries and sometimes the pericarp or other portions of the plant. Culinary use of the term "herb" typically distinguishes between herbs, from the leafy green parts of a plant (either fresh or dried), and spices, from other parts of the plant (usually dried), including seeds, berries, bark, root and fruit.
Plants contain phytochemicals that have effects on the body.
There may be some effects when consumed in the small levels that typify culinary "spicing", and some herbs are toxic in larger quantities. For instance, some types of herbal extract, such as the extract of St. John's-wort (Hypericum perforatum) or of kava (Piper methysticum) can be used for medical purposes to relieve depression and stress. However, large amounts of these herbs may lead to toxic overload that may involve complications, some of a serious nature, and should be used with caution. One herb-like substance, called Shilajit, may actually help lower blood glucose levels which is especially important for those suffering from diabetes. Herbs have long been used as the basis of traditional Chinese herbal medicine, with usage dating as far back as the first century CE and far before.
Mimosa pudica (Sensitive Plant) is a creeping annual or perennial herb. Medicinal use of herbs in Western cultures has its roots in the Hippocratic (Greek) elemental healing system, based on a quaternary elemental healing metaphor. Famous herbalist of the Western tradition include Avicenna (Persian), Galen (Roman), Paracelsus (German Swiss), Culpepper (English) and the botanically inclined Eclectic physicians of 19th century/early 20th century America (John Milton Scudder, Harvey Wickes Felter, John Uri Lloyd). Modern pharmaceuticals had their origins in crude herbal medicines, and to this day, many drugs are still extracted as fractionate/isolate compounds from raw herbs and then purified to meet pharmaceutical standards.
Some herbs are used not only for culinary and medicinal purposes, but also for psychoactive and/or recreational purposes; one such herb is cannabis.
Herbs are also known amongst gardeners to be useful for pest control. Mint, spearmint, peppermint, and pennyroyal are a few such herbs. These herbs when planted around a house's foundation can help keep away unwanted creatures such as flies, mice, ants, fleas, moth and tick amongst others. They are not known to be harmful or dangerous to children or pets, or any of the house's fixtures. Herbs are often used to repel pests from vegetable and flower gardens.
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Source and References http://en.wikipedia.org/wiki/Herb