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.