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A deciduous shrub, either climbing or loosely spreading in habit, the climbing form attaching itself to rocks, walls, trunks of trees, etc., by means of aerial roots like those of the ivy, and frequently reaching to a considerable height; the bushy form up to 8 or 9 ft high. Leaves always composed of three leaflets, the side ones very shortly stalked, the end one with a stalk 1⁄2 to 11⁄4 in. long, the common leaf-stalk 2 to 4 in. long. Leaflets very variable in size, shape, and toothing, broadly ovate to obovate, pointed, sometimes quite entire, often coarsely and irregularly notched at the margin, and either glabrous or slighdy downy beneath. The terminal leaflet is always the largest, and from 2 to 5 in. long, the lateral ones about two-thirds as large. Flowers dull white, 1⁄6 in. across, on slender panicles 11⁄2 to 21⁄2 in. long, often unisexual, the sexes sometimes separated. Fruit a round, whitish drupe, 1⁄4 in. wide, smooth or downy. Bot. Mag. t. 1806.
The poison ivy is very abundant in N. America and Mexico. As a garden plant its chief value is in the beautiful red tints of its autumn foliage. It was cultivated by Compton, Bishop of London, at Fulham, in the 17th century. The poisonous effects of the sap – a yellowish milk-like fluid which soon turns black on exposure – have long been known. As long ago as 1623, the author of the Historye of the Bermudaes referred to them. On the skin of many persons, but far from all, the sap produces blisters and eczema-like eruptions, which are exceedingly painful and persistent. The supposed active principle, ‘toxicodendrol’, is insoluble in water, and it is of no use to attempt to remove it from the skin by ordinary washing. The best-known remedy to apply is an alcoholic solution of sugar of lead (lead acetate), and the sooner it is used on the affected parts the more effective it is. So serious are the effects of the rhus poison on some people that the plant should never be grown where anyone unaware of its dangerous properties could come in contact with it. Even in England I know of a man who had been making cuttings of it for propagation, who was kept in hospital for several months through the almost corrosive effects of the sap. It is said that the symptoms are sometimes recurrent, and that on some persons the eruptions break out annually at the same time of year, but with decreasing virulency. This phenomenon, extraordinary if true, does not appear to have been conclusively established, although the testimony of patients is on record who aver that they have had second and third attacks, although they have never been near the plant after the first. In my experience mere contiguity to the plant without touching it will not induce skin poisoning, although when in flower the escaping pollen appears to have evil effects, especially on the eyes, in N. America.
There is one other property of this remarkable plant to which attention may be drawn. This is the indelibility of its juice when applied to linen. It produces a quite ineradicable stain, and is, in fact, one of the best possible marking inks available.
Toxicodendron orientale Greene
T. radicans subsp. orientale (Greene) Gillis
R. ambigua of some authors, not (?) Lav. ex Dipp.
Ampelopsis japonica Hort., in part
R. rydbergii Small