Toxicodendron
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Toxicodendron is a small genus of woody trees, shrubs and vines, all of which produce the skin-irritating oil urushiol, which can cause a severe allergic reaction; hence the scientific name which means "poison tree".
Members of this genus are very often included in the genus Rhus. As genetic studies show that Rhus without Toxicodendron is paraphyletic, this is the better treatment to follow botanically, but there is some practical convenience in having the highly allergenic species listed separately.
They have pinnately compound, alternate leaves and whitish or grayish drupes. The best known member is poison-ivy, practically ubiquitous throughout much of North America.
The plants are quite variable in appearance. The leaves may have smooth, toothed or lobed edges, and all three types of leaf edge may be present in a single plant. The plants grow as creeping vines, climbing vines, shrubs, or, in the case of Lacquer Tree and Poison Sumac, as trees. While leaves of Poison-ivy and poison-oaks usually have three leaflets, sometimes there are five or, occasionally, even seven leaflets. Leaves of Poison Sumac have 7-13 leaflets, and of Lacquer Tree, 7-19 leaflets.
The common names are somewhat misleading. Technically, the plants do not contain a poison; they contain a potent allergen. Poison-oak is not an oak (Quercus, family Fagaceae) at all, although leaves in some plants bear a resemblance to oak leaves; nor is poison-ivy an ivy (Hedera, family Araliaceae) despite the superficially similar growth form. Both are members of the sumac family, Anacardiaceae.
Avoidance, treatment, and safety
For specific information on prevention and treatment of Toxicodendron rashes, see Urushiol-induced contact dermatitis.Species of Toxicodendron
- Western Poison-oak (Toxicodendron diversilobum or Rhus diversiloba) is found only on the Pacific Coast of North America, ranging from southern Canada to Baja California. It is extremely common in that region, where it is the predominant species of the genus. Extremely variable, it grows as a dense shrub in open sunlight, or as a climbing vine in shaded areas. Like poison-ivy, it reproduces by creeping rootstocks or by seeds. The leaves are divided into three leaflets, 35-100 mm long, with scalloped, toothed, or lobed edges. Californians learn to recognize it by the rhyme "leaves of three, let it be". The leaves may be red, yellow, green, or some combination of those colors, depending on various factors, such as the time of year.
- Asian Poison-ivy (Toxicodendron orientale or Rhus orientale) is very similar to the American Poison-ivy, and replaces it throughout east Asia (so similar that some texts treat it as just a variety of the American species).
- Potanin's Lacquer Tree or Chinese Varnish Tree (Toxicodendron potaninii or Rhus potaninii) from central China, is similar to T. vernicifluum but with (usually) fewer leaflets per leaf. Growing up to 20 m tall, like T. vernicifluum it is used for lacquer production. The leaves have 7-9 leaflets.
- Poison-ivy (Toxicodendron radicans or Rhus radicans) is extremely common in some areas of North America. In the United States it grows in all states except Alaska, Hawaii, California and Nebraska, but it is much less common than Poison-oak in western North America. It also grows in Central America. Appearing as a creeping vine, a climbing vine, or a shrub, it reproduces both by creeping rootstocks and by seeds. The appearance varies. Leaves, arranged in an alternate pattern, usually in groups of three, are from 20 to 50 mm long, pointed at the tip, and may be toothed, smooth, or lobed, but never serrated. Leaves may be shiny or dull, and the color varies with the season. Vines grow almost straight up rather than wrapping around their support, and can grow to 8-10 m in height. In some cases, Poison-ivy may entirely engulf the supporting structure, and vines may extend outward like limbs, so that it appears to be a Poison-ivy "tree".
- Western Poison-ivy (Toxicodendron rydbergii or Rhus rydbergii) is found in northern parts of the eastern United States. It also exists in the western United States and Canada, but is much less common than Poison-oak. It may grow as a vine or a shrub. It was once considered a subspecies of Poison-ivy. It does sometimes hybridize with the climbing species. Western Poison-ivy is found in much of western and central United States and Canada, although not on the West Coast. In the eastern United States it is rarely found south of New England.
- Wax Tree (Toxicodendron succedaneum or Rhus succedanea), a native of Asia, although it has been planted elsewhere, most notably Australia and New Zealand. It is a large shrub or tree, up to 8 m tall, somewhat similar to a sumac tree. Because of its beautiful autumn foliage, it has been planted outside of Asia as an ornamental plant, often by gardeners who were apparently unaware of the dangers of allergic reactions. It is now officially classified as a noxious weed in Australia and New Zealand.
- Atlantic Poison-oak (Toxicodendron pubescens or Rhus toxicarium) grows mostly in sandy soils in eastern parts of the United States. Growing as a shrub, its leaves are in groups of three. Leaves are typically rounded or lobed, and are densely haired. Poison-ivy shrubs are sometimes mistaken for, or simply called, Atlantic Poison-oak. (Atlantic Poison-oak has small clumps of hair on the veins on the underside of the leaves, while Poison-ivy does not).
- Lacquer Tree or Varnish Tree (Toxicodendron vernicifluum or Rhus verniciflua) grows in Asia, especially China and Japan. Growing up to 20 m tall, its sap produces an extremely durable lacquer. The leaves have 7-19 leaflets (most often 11-13). The sap contains the allergenic oil, urushiol. Urushiol gets its name from this species which in Japanese is called Urushi. Other names for this species include Japanese lacquer tree, Japanese Varnish Tree and Japanese Sumac (Note: the term "varnish tree" is also occasionally applied to the Candlenut, Aleurites moluccana, a southeast Asian tree unrelated to Toxicodendron).
- Poison Sumac (Toxicodendron vernix or Rhus vernix) grows exclusively in very wet or flooded soils, in swamps and peat bogs in parts of eastern North America, especially in the coastal plains and the Great Lakes region. It is a tall shrub or a small tree, from 2-7 m tall. It reproduces by seeds. The leaves have between 7-13 leaflets that resemble a feather.
See also
External links
- [Article about urushi lacquer made from the sap of the Urushi tree of Japan]
- [Article about poison-oak]
- [UVSC Herbarium - Toxicodendron]
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