To a botanist, rosemary and sage are not herbs at all
In the kitchen a herb is any leafy plant used for flavour. In botany the word means something stricter: a soft-stemmed plant whose parts above ground die back each season. By that rule, woody favourites like rosemary, sage and lavender are disqualified, and the bay tree is certainly out.
Cooks separate herbs from spices by the part of the plant used. Herbs are generally the green leaves or flowers, fresh or dried, while spices come from seeds, bark, roots or fruit. Some plants straddle both, like coriander, whose leaves and seeds are used differently. What makes a herb different from a vegetable is quantity: it is used sparingly for flavour rather than bulk. In the seventeenth century, as breeding changed their size and taste, the old pot herbs such as onions came to be called vegetables.
Classifying plants is an old habit. The Greek philosopher Theophrastus sorted plants into three kinds (trees, shrubs and herbs), the Talmud mentions around fifteen seasoning herbs, and Charlemagne ordered 74 different herbs to be grown in his gardens. Medieval cooks worked under humoral theory, believing foods could rebalance a person's temperament; parsley and sage turned up together in chicken broth and in green sauce, a standard of the age. The English cookbook The Forme of Cury claimed its liberal use of herbs had the backing of the court's physicians.
Medicine and herbs have been entwined for millennia. Sumerian cuneiform from around 5000 BCE records herbal remedies, and in 162 CE the physician Galen mixed preparations with up to 100 ingredients. Traditional Chinese medicine and India's Ayurveda rest on herbs, and some modern drugs are still purified from plant extracts. The plants carry real chemistry, and some that help in small doses are toxic in large amounts or react with prescription drugs.
Herbs have served many other purposes. Coca leaves have been chewed in northern Peru for over 8,000 years. Indigenous Australians used river mint, wattle and eucalyptus for coughs and fevers. Medieval householders scattered fragrant plants such as meadowsweet across their floors to freshen rooms and deter fleas, and religions from Hinduism, with holy basil, to Anglo-Saxon paganism, with its nine herbs charm, gave particular plants sacred roles.
Source: Herb
