Schizostachyum mekongensis

Synonyms: Neohouzeaua mekongensis A. Camus, Bull. Mus. Natl. Hist. Nat. 28, 1922: 101; type: Thorel s.n. (P03652490, lectotype; cf. K. M. Wong in W. L. Goh & al., 2020: 117 [#1309]); S. Sungkaew & al., Thai Forest Bull., Bot., 49 (1), 2021: 142-150 [#1308]; Schizostachyum mekongensis T. Smitinand, Thai Plant Names, rev. ed. For. Herb., Roy. For. Dept., 2001: p. 470, invalid [#1003].

Thai names: ไผ่กะแสนดำ (phai ka saen dam) (Nong Khai); ไผ่หลอด (phai lot) (Trat), หลอด (lot) = tube. — T. Smitinand, 2001 [#1003]; BKF [#1368].

Distribution: THAILAND (North-East): Loei Province: Nong Khai; (North): Chiang Mai Province: Chiang Dao District: "Huay Tat", in the mixed deciduous forest by a stream, at 800 m altitude, common (Kew Herbarium [#1333]); (Central): Provinces of Phitsanulok and Nakhon Nayok; (West): Provinces of Kanchanaburi and Ratchaburi; (East): Trat Province; in open and dry evergreen forest, mixed evergreen forest, at 100–800 (1,200) m altitude. — LAOS.

Culm size: Height to 10 (15?) m, diameter to 4.5 cm.

Descriptions:

(1) "… Rhizomes short; pachymorph. Culms 1000–1500 cm long; woody. Culm-internodes terete. Lateral branches dendroid. Ligule an eciliate membrane. Leaf-blade base with a brief petiole-like connection to sheath. Leaf-blades lanceolate; 17–30 cm long; 25–40 mm wide. … [flowers and seeds described]." — Kew GrassBase [#1335].

(2) Habit erect; culms erect, not drooping; culm internodes thin-walled; culm-leaf auricles small, erect, with long bristles; culm-leaf blade lanceolate, deflexed; culm-leaf ligule 3 mm long, lacinate; foliage leaf blades medium-sized, glabrous; foliage leaf auricles small, with long bristles. — S. Dransf. & al., Thai Forest Bull. (Bot.) 31, 2003: 31, table 1 [#1061].

(3) "Arborescent, unarmed bamboo. Rhizomes pachymorph with short-necks, forming a clump usually with dense culms. Culms erect, 5–10 m tall, 1–4.5 cm in diameter, tips out-arching to drooping; internodes terete, 45–100 cm long, walls rather thin, dark to bluish green when young, covered with dense appressed white to pale hairs and white-waxy, yellowish green and glabrous when mature, with a narrow band of semi-persistent dense appressed pale hairs and white-waxy just below nodes; nodes not prominent. Branch buds at each node solitary, broadly ovate; prophyll abaxially glabrous, 2-keeled, keels ciliolate and slightly fimbriate towards apex. Branches usually developing from around mid-culm or higher upwards; mid-culm branch complement a cluster of (3–)5–7(–9) to many sub-equal branches at each node, 65–90 cm long, all developing from a solitary non-dominant primary branch axis. Culm sheaths deciduous, ca ⅓–½ as long as internodes, subcoriaceous, apex recessed, pale green to orange-green, back covered with appressed white hairs and a thin covering of white waxy substance, margins of the culm-sheath proper drying off earlier towards the apex; culm-sheath blades narrowly linear, green, spreading to reflexed, ca ⅔ or as long as the culm-sheath proper, covered with dense pale hairs adaxially and scattered pale hairs abaxially; auricles inconspicuous or replaced by a low dark thickened rim to ca 1 mm high, with dense, relatively erect pale bristles (oral setae) in a tidy row, to ca 10 mm long; ligule ca 1 mm tall, margin irregular toothed or laciniate and fringed with pale bristles to 5 mm long. Foliage leaves 5–11 per branchlet; pseudo-petioles 0.2–0.8 cm long; leaf sheaths 5.5–14 cm long, back covered with scattered, appressed pale hairs to glabrous, margins ciliate to glabrous; auricles absent, replaced by low rims along both sides of sheath apex, fringed with dense, relatively erect to apically curved pale bristles (oral setae), to ca 5 mm long; ligules short, ca 0.5 mm tall, margin fringed with pale bristles to 1.5 mm long; margin of outer ligule ciliolate; leaf blades lanceolate-oblong, 10–38(–40) by 2–6.5(–9) cm, adaxially and abaxially scaberulous to glabrous, base obtuse to acute or cuneate, apex acuminate (vegetative components described primarily from Sungkaew 1570). Synflorescence fully bracteate, the subtending lanceolate sheaths/bracts usually glabrous, paniculate, 40–50 cm long, much branched and dense, glabrous, usually on leafless branches, inflorescence units iterauctant (indeterminate), composed of pseudo-spikelets. Pseudospikelets slender, oblong or fusiform, (1.9–)2.2–2.4 cm long, apices acute, glabrous, basal bracts several-nerved; empty glumes 3–4, ovate-lanceolate, unequal, 5–9 mm long, apices mucronate or sub-aristate, glabrous; fertile floret 1, rachilla extension beyond floret absent or much reduced and slender. Florets with lemmas ovate-lanceolate, 1.4–1.7 cm long, margins involute, apices mucronate, glabrous, several-nerved; paleas 1.7–1.9 cm long, margins involute, apices attenuate, or bi-cuspidate with the tips 2 mm long, without keels, glabrous; lodicules absent; stamens 6, filaments connate into a narrowly cylindric tube, anthers ca 8 mm long, glabrous, apices obtuse; ovary oblong, glabrous, ending in a long rigid style ca 1.5 cm long; stigmas 3, plumose. Caryopses not seen. (Floral components described primarily from the lectotype)." — S. Sungkaew & al., 2021: 146, as "Neohouzeaua mekongensis" [#1308].

Comments:

(1) Prior to 2020, the available descriptions of vegetative parts of Neohouzeaua mekongensis are incomplete; identification cannot be based on it.  The type specimens comprise 3 sheets of flowering branches only (Herb. Mus. Paris [#1334]), without any leaves, which renders identification of non-flowering specimens difficult.

(2) The type specimens were collected by C. Thorel during the "Expédition du Mè-Khong" (1866-1868), possibly along the banks of the Mekong River or closely nearby, as the expedition went upstream by gunboat from Cambodia, a French protectorate at that time, to the former Kingdom of Siam (which included the vassal Kingdom of Laos). Localities of the specimens were given as "Ubon" and "Kemmarath", both located in nowadays Thailand (not in "Laos" as stated by the French): จังหวัดอุบลราชธานี, Ubon Ratchathani Province, with its northeastern part bordering the Mekong River, and อำเภอเขมราฐ, Khemmarat District, the northernmost district of Ubon, extending along the Mekong River.

(3) Plants received as "Neohouzeaua mekongensis" from Australia in June 2010 (→ BS-0500) [W3] turned out to be wrongly assigned to this species, and it possibly represents a species of Gigantochloa. Photos of "Neohouzeaua mekongensis", which are not this species but most likely the same species as BS-0500 are published in: BambooCraft.net (habit, node, culm sheath) [#1341]; BambooWeb.info "from Vietnam" (culm, shoot, culm sheath, leaves) [#1340]; TropicalBamboo.org (habit).

Characteristics: Habit unicaespitose, tight. Rhizome pachymorph, short. Culms straight, bending outwards and arching, ca. 5 m long [by Nov. 2020]. Young shoots conical, green, with culm-leaf blades spreading; emerge from July to October. Culm-internodes terete, 50–70 (85) cm long, dull green to bluish green, slightly rough, densely covered with whitish short fuzzy hairs, except for a smooth, glabrous, almost glossy lower section for ca. 4–5 cm above the nodal line; diameter to 1.7 cm [by Nov. 2020], easily splitting, walls thin, lacuna without(?) pith. Culm-nodes glabrous, smooth, almost glossy, not prominent; nodal line horizontal, with the sheath scar marginally (less than 1 mm) protruding; supranodal line obscure, without a ridge, ca. 10 mm above the nodal line; with a persistent whitish ring ca. 6 mm high below the nodal line; aerial roots none. Branch-buds solitary, subrotund, from the basal node up. Branches several, initially (3) 5–7 (9), subequal, slender, relatively short (70–90 cm long), initially developing from an acute angle; unbranched on the lower culm; branching intravaginal; rebranching (into a 3rd axis only). Culm-leaves early deciduous, on the unbranched lower nodes late deciduous. Culm-leaf sheaths parabolic, ca. 9–10 cm wide at the base, 15–18 cm long, length ca. one-fourth of the internode, thickly papery, green to yellowish brown when young, light straw-colored with an orange tint when dry, medium brown when old, with appressed, short, soft, whitish hairs when young, the hairs caducous or nearly so with age; apex concave-truncate, symmetrical; the base of the outer margin usually with a tiny, 1 mm high subcircular projection below the point of attachment; margins eciliate, involute when dry. Culm-leaf auricles inconspicuous, very low rims (less than 1 mm high), blackish when young, each adnate to the basal margin of the blade and extending along the entire sheath apex (shoulder), the margin of the auricles with 8–15 mm long erect white fringes. Culm-leaf ligule very low, less than 0.5 mm high, with 3–5 mm long fine white bristles. Culm-leaf blades papery, caducous, reflexed, linear-lanceolate; base constricted to ca. 3 mm at the junction with the sheath apex, ca. 8–13 cm long, often twisted when dry, initially adaxially somewhat short-hairy, green to dark green when young, light straw-colored when dry; apex long pointed; margins scabrous. Foliage-leaves (6) 7–9 (12) per branchlet. Foliage-leaf sheaths green and pale hirsute when young, straw-colored and glabrous when dry; apex truncate, glabrous, plain green or with a reddish tint when young; margins eciliate. Foliage-leaf auricles very low rims along both sides of the sheath apex, with fine, erect, long white fringes. Foliage-leaf ligule inconspicuous; outer ligule a low rim or ridge. Foliage-leaf blades lanceolate, 30–40 × (3) 4–8 (9) cm; glabrous above, puberulent beneath when young, medium green above and beneath; base rounded to wedge-shaped; apex attenuate; margins antrorsely scabrous; midvein proximally distinct and prominent, yellowish-green beneath; pseudopetiole 5–11 mm long. Flowers and seeds are unknown.