Diplachne gigantea Launert, Bol. Soc. Broteriana, ser. 2a, 47: 394. 1974.

Snow, Neil, Peterson, Paul M., Romaschenko, Konstantin & Simon, Bryan K., 2018, Monograph of Diplachne (Poaceae, Chloridoideae, Cynodonteae), PhytoKeys 93, pp. 1-102 : 78-81

Diplachne gigantea Launert, Bol. Soc. Broteriana, ser. 2a, 47: 394. 1974. Figures 9 , 10

Leptochloa gigantea (Launert) Cope and N. Snow, Novon 8: 79. 1998.

Type.

ZAMBIA. Mbala (Abercorn), Saisi River (near Jerico), 27 Feb 1958, floodplain (in swamp) in ditch at side of embankment, LDF Vesey-Fitzgerald 1551 (holotype: K!; isotype: BM!, K!) .

Description.

Plants perennial. Culms to nearly 300 cm tall, 5-10 mm wide at base, round, erect, arising from thickly rhizomatous root crowns, branching; nodes glabrous; internodes mostly 5-18 cm long, firm but spongy (aerenchymatous) in centre. Leaf sheaths usually much longer than internodes, sericeous but becoming glabrous with age, the margins ciliate; ligules to 14 mm long, membranous, attenuate but becoming lacerated; blades 30-65 × 0.5-0.65 cm, linear, tightly involute on drying, glabrous to slightly scabrous above and below. Panicles 50-80 ×ca. 25 cm wide, exserted with 19-44 branches; the branches 12-18 cm long, alternate or whorled along the rachis, somewhat reflexed to ascending, flexuous, minutely scabrous, the axils scabrous to short pilose. Spikelets 8-13 mm long, distant near base of branches to imbricate at tips; florets (6-)10-15; callus glabrous; lower glumes 2.7-3.4 mm long, narrowly ovate to ovate, scabrous on midnerves, acute to obtuse; upper glumes 2.5-4.0 mm long, otherwise similar to lower glumes; lemmas 3.6-4.8 mm long, narrowly ovate to ovate, dark green, the lateral nerves faint, appearing glabrous but sparsely sericeous upon close inspection, apex acute to attenuate, awnless or with mucros to 1.3 mm long; paleas subequal to lemma, elliptic to narrowly ovate, sparsely sericeous on nerves; apex acute. Stamens 3; anthers 1.9-2.5 mm long, purple or yellow. Caryopses ca. 1.8 mm ×ca. 0.6 mm, elliptic in hilar side profile, depressed obovate in transverse section, hilar groove lacking, smooth but slightly uneven; pericarp apparently weakly adnate to the endosperm.

Leaf anatomy.

Not studied.

Stem anatomy.

The stellate aerenchyma of D. gigantea probably reflects adapations for its emergent growth habit in aquatic situations. A series of relatively large, elliptic air canals are subjacent to the epidermal layers (Fig. 10 ). Chlorenchymatous tissue is also present near the surface. Peripheral sclerenchymatous ring present and linked to outermost vascular bundles via girders. Inner sclerenchymatous ring canal tissue present. Sclerenchymatous rings surrounding vascular bundles located inside inner sclerenchymatous ring present. Kranz sheath cells present, with vascular bundles present in outer portions of sheaths; Kranz sheath cell canal tissue present. Vouchers: Simon & Williamson 2025 (BM); Smith 4126 (MO).

Chromosome number.

Unknown.

Phenology.

Flowering December through May.

Distribtution.

Native: A few widely scattered collections from Botswana, Tanzania and Zambia. Growing in edges of swamps and margins of slow moving rivers. Elevation 1250-1710 metres, although upper and lower ranges uncertain. (TDW: BOT, ZAM, TAN). Non-native: See below (cultivated) in Zimbabwe.

Conservation status.

Data Deficient ( IUCN 2012). However, detailed localities are provided below given the evident rarity of Diplachne gigantea , which apparently has not been collected since 1983. Focused attempts by the first author in January of 1996 to re-collect at the two localities of Pete Smith in the Okavango region of Botswana were unsuccessful, even after speaking with Smith regarding his knowledge of the species. To the best of the authors' knowledge, Smith is the last person to have collected the species. He knew of no vernacular name for the species in Botswana which, if it existed, might help in relocating efforts and indicated that each of the two populations from which he had collected were small. He confirmed, as some herbarium specimens labels and the stellate culm anatomy suggest, that the species is an emergent aquatic that can continue to grow on sandbars.

More specifically, on 28 and 30 January 1996 the first author hired a boat and driver at Drotskys Cabins in Botswana, located approximately 20 km south of the Namibian boarder (Caprivi Strip) and approximately 7 km southeast of the small town of Shakawe (Highway A 35). One entire day each was devoted to searching for D. gigantea moving southeast downstream on the Okavango River and returning by the same route on the opposite bank of the river and upstream and back from the same point of departure. No individuals were seen. Although only speculative, two hypotheses concerning the evidently limited distribution in relation to its habitat merit consideration. First, the stellate aerenchyma of the culm likely contributes to a relatively soft texture, which may render it particularly palatable for large grazers such as hippopotamus ( Hippopotamus amphibius ), which are common in the Okavango Delta and which consume large quantities of food. Second, given the extensive potential habitat for D. gigantea in the Okavango Delta, D. gigantea may occur more frequently than herbarium records suggest. However, given the paucity of collections to date, it likely is uncommon anywhere in its range.

Diplachne gigantea was cultivated in the early 1980s at the Lakeside National Botanic gardens in Zimbabwe (Browning 8; A, P). Gibbs Russell et al. (1991: 118) indicate that D. gigantea has been collected at the western edge of the Caprivi Strip in Namibia but the authors have not seen a voucher.

In summary, the data suggest that Diplachne gigantea is rare and additional knowledge of its distribution, ecology and relative abundance are critical for making a recommendation following IUCN standards.

Etymology.

The word Diplachne gigantea refers to the large stature of the species.

Vernacular name.

Giant diplachne.

Comments.

Diplachne gigantea is easily diagnosed from D. fusca by its much taller stature at maturity, its evidently obligate and usually emergent growth in hydric habitats, the stellate aerenchyma of the culms, the absence or near-absence of lemmatal macrohairs and the relatively lax panicle branches. The collection from cultivation at the Lakeside Botanic Gardens (see below) is indicated as growing from a "dense mat at the base", presumably meaning the culms arose from short rhizomes.

The fourth author personally recalled the species as being fairly commonly locally at the time of his collection in 1983 (Simon & Williamson 2025).

Specimens examined.

Botswana. Ngamiland: Okavango River, sandbank just recovering from being inundated by high waters during the previous six weeks, 18°34.25'S, 22°06.3'E, 8 May 1985, Smith 1387 (BRI, K, MO); In water 1 m deep in a backwater off the mainstream of the Okavango River among water lillies, reeds, 18°20.2'S, 21°49.9'E, Smith 4126 (K, PRE, SRGH [n.v.]) . Tanzania. Reported but not confirmed from the Iringa District ( Phillips 1974) . Zambia. Kabompo: Bog by river 100 yards from Customs point [near the Tanzanian border], McCallen Webster A 305 (BM, K). Mbala (Abercorn): 70 mi. S of Mwinilunga on Kabompo Rd., rooted in water, +/- 1250 metres elev. (ca. 4,100 ft), 25 Dec 1969, Simon & Williamson 2025 (K, BM, SGRH [n.v.]) . Zimbabwe. Cultivated at Lakside Botanic Gardens in 1983 from an unknown source, Browning 8 (A, P [P06795263]!) .

Figures

Figure 9. A-K Diplachne gigantea Launert A habit B inflorescence C ligule D spikelet E lower glume F upper glume G lemma H palea and lodicules I ovary and stigmas J stamen K caryopsis, dorsal view (left) and lateral view (right). Drawings from McCallum Webster A 305 (K), from Zambia.

Figure 10. Stem anatomy of Diplachne gigantea. Upper: Stellate aerenchyma (SA) that occupies much of the central portion of the culm (voucher: Smith 4126 [MO]). Lower: Outer portion of stem showing air canals (AC), chlorenchymatous tissue (CT), vascular bundles (VB), and sclerenchyma bands (SB) surrounding the air canals (voucher: Simon & Williamson 2025 [BM]).