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Published In: The Gardeners Dictionary...Abridged...fourth edition vol. 3. 1754. (28 Jan 1754) (Gard. Dict. Abr. (ed. 4)) Name publication detail

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3. Toxicodendron Mill. (poison ivy, poison oak)

Plants shrubs or lianas, dioecious, climbing by masses of aerial roots, often with rhizomes. Leaves trifoliate (pinnately compound elsewhere). Leaflets with the margins entire or coarsely toothed to irregularly lobed. Inflorescences axillary panicles, loose and drooping, developing after the leaves expand. Flowers all fertile, the stalks not plumose-hairy. Sepals 5, united at the base, greenish to cream-colored. Petals 5, glabrous, cream-colored. Staminate flowers with the stamens 5, the anthers ovoid to more or less oblong in outline. Pistillate flowers with the styles free, appearing terminal, equal in length or nearly so, short, sometimes fused toward the base. Fruits globose to subglobose drupes, glabrous or pubescent with inconspicuous nonglandular hairs, sometimes with minute tubercles, the thin outer layer often splitting open at maturity to expose the waxy middle layer, this appearing white or nearly so, often spotted with black resin ducts, the stone with a few longitudinal ridges. About 15 species, mostly temperate North America and eastern Asia, less commonly South America.

Toxicodendron, which means poison tree, has a well-deserved reputation as a plant to be avoided. All parts of the plant contain a resinous oil, urushiol, that causes an irritating rash in many people. The oil contains a mixture of catechols with long alkyl side-chains that penetrate the skin, interact with proteins, and trigger an immune response. The rash usually appears 24–48 hours after contact with the oil, and until washed off, the oil may spread to other parts of the body. Affected skin reddens, itches, and tiny blisters often appear. The rash usually fades away in 2–3 weeks. Symptoms may develop after merely brushing the plant, or by coming into contact with clothing, tools, or pets that have touched the plant. Some people seem to be more susceptible than others, and sensitivity often increases with repeated exposures. Although people do not develop symptoms without direct contact with the plant, burning poison ivy leaves can release droplets of the oil, which can then be carried by the smoke to the eyes, throat, and lungs. If contact with the plant is suspected, immediate washing with cold water may help remove some of the oils. Cleaning the exposed area with isopropyl alcohol may also be helpful. Washing with soap and hot water may actually spread the oils. Once a rash has started, there is no proven cure, but the discomfort can be soothed with cooling compresses or calamine lotion. Scratching the blisters will not spread the rash, but it can lead to infection and should be resisted. A doctor should be consulted in extreme cases, if the rash spreads over much of the body, causes significant swelling, or affects the eyes, throat, or internal organs. Many folk cures and myths about poison ivy exist. Most are harmless, but so-called preventative measures such as eating small pieces of the leaves to induce immunity are dangerous and not recommended.

The best medicine is prevention, learning to recognize poison ivy, and avoiding contact with it. The old adage “leaflets three, let it be” is useful, although there are a few plants with trifoliate leaves that are harmless. Toxicodendron radicans and T. pubescens are distinguished by the distinctive petiole-like stalk of the terminal leaflet. The whitish berries are another distinguishing character, although they are green while maturing and are not produced on the staminate plants. Toxicodendron radicans often grows straight up a tree trunk, adhering by dense aerial roots (these produced in response to contact of the stem with a substrate), but it takes on a variety of growth forms in different habitats. Among the plants frequently mistaken for poison ivy, fragrant sumac (Rhus aromatica) has trifoliate leaves, but the terminal leaflet lacks the distinct stalk found in poison ivy, and the berries are reddish. The leaves of box elder (Acer negundo) have 3–7 leaflets, but the leaves are opposite on the stem rather than alternate as in poison ivy. Virginia creeper (Parthenocissus quinquefolia) climbs like T. radicans but usually has 5 leaflets instead of three (except in seedlings) and has blue berries.

The name poison sumac sometimes has been attributed to Missouri plants, but in fact this species does not occur in the state. True poison sumac, Toxicodendron vernix (L.) Kuntze, has pinnately compound leaves with 7–13 entire-margined leaflets and occurs in swamps and bogs to the east and north of the state.


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1 1.Plants low shrubs, not climbing, not producing aerial roots; leaflets usually with 3–7 deep, more or less rounded lobes; fruits inconspicuously hairy and/or with minute papillae ... 1. T. PUBESCENS

Toxicodendron pubescens
2 1.Plants often climbing (but can appear shrubby in open habitats), usually producing aerial roots; leaflets entire or coarsely and bluntly saw-toothed, sometimes irregularly lobed; fruits glabrous, rarely with minute papillae ... 2. T. RADICANS Toxicodendron radicans
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