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poisonous plant,any plant possessing a property injurious to man or animal. Plants may be poisonous to the touch (e.g., poison ivy, poison sumac), or orally toxic (e.g., poison hemlock, deadly amanita). Many poisonous plants are of great value medicinally, e.g., digitalis, belladonna, and aconite. Numerous plants have long been known and gathered (some from prehistoric times) for specific medicinal uses in controlled dosage. Some have been used for hunting poisons (e.g., strychnine) and for insecticides (e.g., pyrethrum). Some plants are poisonous in part and harmless otherwise (the leaf blades, not the stalks, of rhubarb are poisonous) or poisonous at one season and not at another (the very young poke, or pokeweed, shoot is sometimes cultivated as a green vegetable but the older plant is poisonous). Some plants contain properties that are poisonous only under certain conditions, such as those causing photosensitivity. While animals that feed on these plants (buckwheat and others) and are subsequently exposed to sunlight develop a serious skin disorder called photosensitization. A poisonous property (selenium) of some soils, particularly in parts of the West, is absorbed by some of the growing plants, not always in themselves poisonous, and transmitted to animals and sometimes to man. Since this poison is returned to the soil by the death of the plants and animals that have absorbed it, it is again available to other plants and may even be absorbed by crop plants. Locoweed is an example of a selenium-poisonous plant. Many of our ornamental plants are poisonous—larkspur, oleander, English ivy, and lily of the valley. Poisoning by ingestion of plants by human beings is usually a matter of mistaken identity, particularly with mushrooms. Poisonous plants are usually avoided by animals unless the pasture is overgrazed. Poisonous principles may be found throughout the plant kingdom from bacteria and fungi to ferns and flowering plants.
See W. C. Muenscher, Poisonous Plants of the United States (rev. ed. 1951); J. M. Kingsbury, Deadly Harvest: A Guide to Common Poisonous Plants (1965); K. F. Lampe and M. A. McAnn, AMA Handbook of Poisonous and Injurious Plants (1985); W. H. Blackwell, Poisonous and Medicinal Plants (1990).