4. Rhamnus japonica Maxim. (Japanese buckthorn)
Map 2399
Plants shrubs or
occasionally small trees, 1–5 m tall, dioecious, often incompletely so. Main
stems 1 to several, the main branches ascending to loosely ascending and
elongate, also with shorter, spreading to loosely ascending, relatively
straight branches, these mostly thorn-tipped. Bark gray to reddish brown,
sometimes with lighter blotches, relatively smooth, but with relatively
prominent, raised lenticels in cross lines and branch scars, somewhat peeling
on older, larger stems. Twigs relatively stout, slightly flattened, gray to
yellowish brown, with small, dark lenticels, initially often minutely hairy,
but soon glabrous or nearly so, the winter buds narrowly ovoid, with several
overlapping scales, these reddish brown, glabrous except for marginal hairs.
Leaves mostly opposite (occasionally subopposite on new growth, the petioles
5–18 mm long. Leaf blades 2–8 cm long, 1.5–3.0 times as long as wide, obovate to
elliptic-oblanceolate or broadly oblanceolate, mostly widest at or below the
midpoint, angled at the base, tapered to a sharply pointed tip or the smaller
leaves sometimes merely angled or rounded, the upper surface green to dark
green, minutely hairy, at least toward the tip and/or along the main veins,
slightly shiny, the undersurface light green, glabrous, the lateral veins
mostly 4 or 5 pairs, these strongly arched toward the blade tip. Inflorescences
axillary, small clusters of 2–6 staminate or pistillate flowers or reduced to
solitary flowers, the clusters sessile, the individual flower stalks 3–9 mm
long. Flowers imperfect. Sepals 4, 1.3–2.0 mm long. Petals 4, 1.0–1.3 mm long
in staminate flowers, 0.6–0.9 mm long in pistillate flowers, lanceolate, entire
at the tips. Style noticeably 2-branched toward the tip. Fruits 5–7 mm long,
more or less globose, with 2 stones, black at maturity. May–June.
Introduced,
uncommon, known thus far only from single specimens from Boone and Jackson
Counties (native of Japan; introduced sporadically in Illinois, Missouri).
Disturbed, mesic upland forests.
The first
collection of this species to date (Smith & McKenzie 3569 on 19 May
2000), which is in the Missouri Botanical Garden herbarium, originally was
misdetermined as the superficially very similar R. cathartica. It was
redetermined during studies by Guy Nesom toward a forthcoming treatment of the
genus in the Flora of North America series. Elsewhere in the United
States, this taxon is still known thus far only from the Chicago metropolitan
area (Swink and Wilhelm, 1994), where it has become increasingly frequent
during the past few decades.