Diplacrum R.Br.

Wilson, Karen L. & Barrett, Russell L., 2023, Revision of the tropical genus Diplacrum (Cyperaceae: Bisboeckelereae) in Australia, Australian Systematic Botany 36 (2), pp. 143-156 : 144-146

Diplacrum R.Br. , Prodr. 240 (1810)

Scleria sect. Diplacrum (R.Br.) Kern , Blumea 11(1): 208 (1961).

Type: D. caricinum R.Br.

Pteroscleria Nees , in C.F.P. von Martius, Fl. Bras. 2(1): 196 (1842).

Type: P. guianensis Nees.

Sphaeropus Boeckeler , Flora 56: 89 (1873); Scleria sect. Sphaeropus (Boeckeler) J.Kern , Blumea 11(1): 208 (1961).

Type: S. pygmaeu s Boeckeler.

Small or dwarf annuals with reddish roots (rarely slender stoloniferous perennials). Culms tufted. Leaves basal and cauline, flat, without a ligule or contraligule. Inflorescence capitate, or branched with several small, dense clusters of spikelets, or spikelet clusters axillary at the remote nodes of a more elongated inflorescence axis. Primary involucral bracts leaf-like. Lateral spikelets in a cluster male, 0‒2; terminal spikelet female. Male spikelets with 1 persistent glume, each subtending 1 male flower; stamen 1(3). Female spikelets on a peduncle usually thickened at the apex at maturity, often strongly so and disc- or cup-like; glumes 2, distichous, keeled or not, persistent or deciduous with the fruit, encircling a (?)pseudo-terminal female flower; style 3-fid; style base deciduous, not thickened. Disc usually triangular, adhering to nutlet, usually paler. Nutlet subglobose to ovoid or narrow–obovoid, terete to trigonous in cross-section, inconspicuously 3-angled, surface glabrous or sparsely puberulous near the apex, otherwise smooth or longitudinally ribbed or reticulate, falling freely or enclosed by the persistent, thickened glumes.

This description covers the features of Diplacrum sens . strict. as well as the three species sometimes referred to the genus Pteroscleria (see discussion above). The features of the latter are given in parentheses where they differ from those of the former.

Key to Diplacrum species in Australia, Malesia and South-East Asia

1. Nutlet globose to ovoid....................................................................2

Nutlet narrow–ellipsoid to narrow–ovoid or narrow–obovoid...........5

2. Glumes tightly clasping nutlet and falling as a unit........................3

Glumes not tightly clasping nutlet, remaining on spikelet axis after nutlet falls.....................................................................................4

3. Glume of female spikelet with several prominent nerves, apex 3-lobed; nutlets globose to broad–ovoid, irregularly longitudinally ridged.......................................................................... D. caricinum

Glumes of female spikelet with only mid-nerve prominent, apex acute, not lobed; nutlets depressed–globose, irregularly reticulate............................................................................ D. reticulatum

4. Glumes with awn 0.8–1.2 mm long; nutlet with 3 basal swellings adjacent to disc......................................................... D. pygmaeum

Glumes without an awn; nutlet lacking basal swellings.................... ................................................................................ D. pygmaeopsis

5. Nutlet prominently ribbed longitudinally........................................6 Nutlet more or less 3-angled but without prominent ribs...... D. latzii

6. Nutlet 3-ribbed; leaves shorter than inflorescences; leaf blades 1.0– 1.5 mm wide ......................................................... D. exiguum

Nutlet 9–12-ribbed (some not complete); leaves exceeding inflorescences (up to 2.5 times as long); leaf blades 0.3–0.7 mm wide...... .......................................................................................... D. blakei

Diplacrum in Australia

Three species in Australia are apparently endemic ( Fig. 1 ), with only Diplacrum caricinum occurring more widely from Malesia to Japan, the Pacific, and India. Diplacrum exiguum (J.Kern) T.Koyama from Vietnam and D. pygmaeopsis from the Lesser Sunda Islands are not known from Australia, and we have seen no specimens of them. It is quite likely that the latter will be found in Australia, given the similarity of some habitats and the occurrence of many other sedge species in both of these regions.

Two or more of these tiny annual species may occur together, as shown by various mixed collections. They commonly grow in dense swards with other annual sedges on the damp margins of water-bodies.

Stamens are not obvious on most herbarium specimens because specimens tend to be collected when in fruit, at which stage the anthers have dropped and the male spikelets have withered, compressed by the expanding female spikelet above them, as noted by Bentham (1878) for D. pygmaeum .

Figures

Fig. 1. Distribution of Diplacrum species in Australia and Papua New Guinea based on data from the Australasian Virtual Herbarium (see https://avh.ala.org.au/, accessed June 2022). (a) D. caricinum. (b) D. blakei. (c) D. latzii. (d) D. pygmaeum.