1. Toxicodendron pubescens Mill. (eastern poison oak)
Rhus
toxicodendron L.
T. toxicarium (Salisb.) Gillis
R.
toxicodendron var. quercifolium
Michx.
T.
quercifolium (Michx.)
Greene
Pl. 201 e; Map
835
Plants low
shrubs 0.3–0.5 m tall, spreading by rhizomes, not climbing, not producing
aerial roots. Leaves often clustered near the tip of the stem, the petiole 5–15
cm long. Leaflet stalks 1–3 mm long in lateral leaflets, 0.5–2.0 cm long in the
terminal leaflet. Leaflets 3–10 cm long, 3.0–7.5 cm wide, ovate to obovate,
angled at the base, rounded to broadly and bluntly pointed at the tip, the
margins with 3–7 coarse, deep, rounded lobes, those of the lateral leaflets
usually somewhat asymmetrical (deeper on the lower side than on the upper side)
and sometimes with a few blunt teeth, the upper surface sparsely hairy, the
undersurface sparsely to moderately hairy, especially along the veins.
Inflorescences 1–4 cm long. Sepals 1.0–1.2 mm long, narrowly ovate, green.
Petals 2.2–2.4 mm long, oblanceolate, cream-colored with dark veins. Ovary
densely hairy. Fruits 4.0–4.5 mm long, 4.0–4.5 mm wide, globose, greenish white
to tan, the outer layer separating at maturity to reveal a powdery white middle
layer, the stone kidney-shaped with a smooth surface. 2n=30. May–June.
Uncommon in the
southern portion of the Ozark Division and in the northern part of the
Mississippi Lowlands (southeastern U.S. west to Kansas and Texas). Glades,
openings of dry upland forests, sand prairies, and sand savannas; also
roadsides.
Toxicodendron
pubescens is recognized
by a combination of its shrubby, nonclimbing habit, deeply lobed leaves, hairy
fruits, and occurrence in low-nutrient, sandy or rocky soils. It produces aerial
stems from rhizomes, but it never climbs or produces aerial roots as in T.
radicans. In its typical form, the leaflets of T. pubescens are
deeply lobed and reminiscent of the leaves of white oak or blackjack oak, hence
the common name poison oak. Unfortunately, this common name is frequently
misapplied to lobed forms of T. radicans as well. According to Gillis
(1971), the lobes are deeper on staminate plants than on pistillate plants.
Although the fruits are usually sparsely hairy, occasionally they are nearly
glabrous. A few putative hybrid specimens between T. pubescens and T.
radicans have been identified, but the relative rarity of hybrids was a
factor in the recognition by Gillis (1971) of T. pubescens as a distinct
species.
The
nomenclatural history of T. pubescens is quite complex (Fernald, 1941a;
Gillis, 1971; Kartesz and Gandhi, 1991; Reveal, 1991a). The lobate-leaved,
pubescent forms of poison ivy originally were described as Rhus
toxicodendron L. However, when classified into the segregate genus Toxicodendron,
the name Toxicodendron toxicodendron (L.) Britton is not allowed under
the International Code of Botanical Nomenclature because it would form a
tautonym (a binomial in which the generic and specific epithets are identical).
Gillis (1971) recognized poison oak under the name T. toxicarium, which
was in turn an illegitimate combination, leaving the next available legitimate
name within Toxicodendron as T. pubescens.