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The name is derived from "urushi", Japanese name for lacquer made from the sap of the Japanese lacquer tree ("kiurushi" or "urushi ki").
Synonyms: Toxycodendron pubescens, Toxicodendron quercifolium, R. radicans, R. humile, R. pubescens, R. verrucosa,
English: Poison ivy; English Oak, Poison ash;
French: Arbre a poison, Sumac Veneneux;
German: Gift-sumach, Wurzel Sumach.
Tincture of fresh leaves gathered at sunset just before flowering time.
Plantae; Spermatophyta, Angiospermae - Flowering Plants; Dicotyledonae; Rosiflorae / Rosidae; Rutales; Anacardiaceae - Poison Ivy Family
(wichman natural History)
Introduced in Homoeopathic practice in 1816 by Hahnemann, under a common name Rhus. Allen's Encyclop. Mat. Med. Vol., VIII, 330.
Description of the substance
Poison oak is a widespread deciduous shrub throughout mountains and valleys of California, generally below 5,000 feet elevation. In shady canyons and riparian habitats it commonly grows as a climbing vine with aerial (adventitious) roots that adhere to the trunks of oaks and sycamores. Poison oak also forms dense thickets in chaparral and coastal sage scrub, particularly in central and northern California. It regenerates readily after disturbances such as fire and the clearing of land. Rocky Mountain poison oak (Toxicodendron rydbergii) occurs in canyons throughout the western United States and Canada. Because the two species of western poison oak often exhibit a viny growth form, they are listed as subspecies of eastern poison ivy by some authors.
The pinnately trifoliate leaves typically have three leaflets (sometimes five), the terminal one on a slender rachis (also called a stalk or petiolule). Eastern poison ivy often has a longer rachis and the leaflet margins tend to be less lobed and serrated (less "oak-like"). In the similar-appearing squaw bush (Rhus trilobata) the terminal leaflet is sessile (without a stalk). Like many members of the Sumac Family (Anacardiaceae) new foliage and autumn leaves often turn brilliant shades of pink and red due to anthocyanin pigments. In the eastern states poison ivy is often mistaken for another common native called Virginia creeper (Parthenocissus quinquefolia). Virginia creeper has a similar growth habit and beautiful autumn foliage, but typically has five leaflets rather than three. It belongs to the Grape familyFreshly cut stems exude a sticky, terpene oleoresin that oxidizes and polymerizes into a shiny black lacquer resembling pruning sealer. The resinous sap is produced in resin canals of the stems, roots, leaves and flowers. Cross sections of poison oak stems show distinct concentric annual rings (ring-porous wood). Numerous resin canals appear as tiny black dots and are confined to the phloem layer just inside the bark. [Caution: Cutting and sanding poison oak wood is extremely unwise and hazardous--even if you think you are immune to its dermatitis. This is how one of the authors (WPA) was rudely initiated into the ranks of poison oak sufferers, after tramping through it for decades with impunity.] Dark resin canals (appearing as black striations) also occur in the waxy mesocarp of the fruits just beneath the papery skin. Abundant resin canals is one of the reasons poison oak and poison ivy are now placed in the genus Toxicodendron rather than the older genus Rhus. Toxicodendron is also the updated generic name for poison sumac (T. vernix) and the Japanese lacquer tree (T. vernicifluum), the commercial source of natural lacquer. In fact, Pomo Indians of California used the natural lacquer of poison oak to dye their baskets. The resin canals also contain urushiol, the insidious allergen that gives poison oak its bad reputation. The name is derived from "urushi", Japanese name for lacquer made from the sap of the Japanese lacquer tree ("kiurushi" or "urushi ki").
Urushiol is a general term applied to the toxic substance in the sap causing allergic contact dermatitis in people. It is actually a mixture of phenolic compounds called catechols, potent benzene ring compounds with a long side-chain of 15 or 17 carbon atoms The side chain may be saturated or unsaturated with one, two, or three double bonds (Dawson, 1954, 1956). The remarkable immune reaction and specificity of the catechol molecule is determined by the long side-chain (Baer et al, 1967, 1968). Poison oak urushiol contains mostly catechols with 17 carbon side-chains (heptadecylcatechols), while poison ivy and poison sumac contain mostly 15 carbon side-chains (pentadecylcatechols).
Freshly cut stem of poison oak (Toxicodendron diversilobum) showing adventitious roots and black lacquer oozing from resin canals.