Bromus japonicus Thunb., japanese brome. Annual, fibrous–rooted, 1—several–stemmed at base, mostly unbranched, ascending to erect, in range 20—50+ cm tall; shoots with a few basal leaves and to 6 cauline leaves, main axis sometimes bent at nodes (geniculate), pilose.
Stems (culms) cylindric, to 2 mm diameter, internodes to 55 mm long and mostly glabrous but short–hairy approaching node, nodes often glossy and purplish red when exposed; internodes hollow, nodes solid.
Leaves alternate distichous, simple with sheath; sheath closed to within 3—8 mm of ligule, ridged, narrowly membranous and often long–ciliate on margins of throat, pilose (glabrate on sheaths of some upper leaves), without lobes (auricles) at top; ligule membranous, obtuse, in range 1—2 mm long, jagged–fringed on margin, glabrous; collar narrow; blade linear, < 50—120+ × 2—4 mm, the widest near the base, flat, minutely toothed on margins, acuminate at tip, parallel–veined with midrib raised on lower surface, upper surface densely pilose, lower surface often glossy and short–pilose.
Inflorescence spikelets, in terminal panicles, panicle fully exserted from leaf sheath of flag leaf, ± open, to 130 mm long, mostly with 4—5 nodes, in flower with lax and spreading lateral branches (never drooping), becoming stiffer and more ascending in fruit, branchlet with 1 terminal spikelet, spikelet mostly with 6—8(—12) florets but at least terminal floret = reduced lemma with shorter awn, bracteate, awned; rachis 50—80 mm long, minutely scabrous, lateral branchlets 1—3 per node, < 65 mm long (to tips of awns), axis below spikelet wiry, 12—35 mm long, minutely scabrous.
Spikelet narrowly lanceoloid slightly compressed side–to–side, in range mostly 15—22 mm long + awns, not ribbed, essentially glabrous, breaking above glumes and between florets; glumes 2, unequal, ovate rounded on back (not keeled), in range lower glume 4.5—7 mm long and 3–veined or 5–veined, upper glume 5—8 mm long and 5–veined or 7–veined, veins reddish and short–scabrous above midpoint and lateral veins converging toward tip but terminating before merging with midvein, slightly ridged above midpoint, with membranous margins; lemma awned, lanceolate notched at top, 7—9 mm long (excluding awn), > glume, rounded on back, with membranous margins, 5–veined, not keeled but midvein raised approaching awn, the veins converging but not merging with midvein, tip 2–toothed and completely membranous, the teeth to 1 mm long, the awn arising at notch as extension of midvein, initially essentially straight later divergent or twisted, 5—13 mm long, aging purplish red; palea somewhat narrowly elliptic, mostly 4—5 mm long and < where awn arises on lemma (absent in sterile florets), 2–veined, 2–keeled, folded strongly inward along veins, flat between keels, membranous with keels green and short–ciliate, ± truncate at tip, surfaces glabrous, margins minutely toothed above midpoint.
Flower bisexual; perianth (lodicules) 2, acute–ovate, 0.6—0.7 mm long, translucent, at anthesis not bulbous at base thus flat, persistent; stamens 3, included; filaments slender, ca. 1 mm long, white; anthers basifixed, dithecal, 1.2—1.5 mm long, light yellow, longitudinally dehiscent; pollen light yellow; pistil 1; superior ovary, inversely obovoid, ca. 0.7 × 0.5 mm, translucent white, with a translucent, large terminal lobe, with short, ascending hairs, fleshy, 1–chambered with 1 ovule; styles 2, included, 1 mm long, colorless, threadlike with many wavy lateral branches.
Fruit achene (caryopsis), dispersed as floret tightly enclosed within adherent palea and awned lemma + a rachilla segment, awned dispersal unit commonly 16—18 mm long (straightened); achene elongate, relatively thin, and cupped–inrolled, ca. 5.4—5.8 × 0.7 mm, amber light brown, light brown, or pinkish light brown, acute at base, rounded at top with flap and tuft of short hairs on tip; lemma veins mostly not raised, surface minutely scabrous and scarious; palea conspicuously short–ciliate along keels with stiff hairs, scarious, adherent to achene, surfaces appearing glabrous; rachilla segment ± appressed, flattened on inner side and rounded on back, 1.6—1.9 mm long, scabrous on exposed side and glabrous on flat side; awn curved away from floret axis, 5—13 mm long.
A. C. Gibson