Argostemma cordatum Nuraliev, 2017

Nuraliev, Maxim S., Beer, Anton S., Kuznetsov, Andrey N. & Kuznetsova, Svetlana P., 2017, Argostemma cordatum (Rubiaceae), a new species from Vietnam, Phytotaxa 317 (1), pp. 42-52 : 43-48

Argostemma cordatum Nuraliev , sp. nov. ( Fig. 1–3 ).

Argostemma cordatum is characterized by a single large leaf per plant and differs from all other species of the genus with this feature by the small size of the whole plant (particularly the large leaf, which is 2.6–4.7 cm long) and invariably cordate base of large leaf. This species also differs in the following combination of morphological traits: plant completely glabrous, stipules minute and represented by papillate warts, inflorescences with all axes (peduncle and pedicels) elongated, anthers basifixed, coherent into anther cone and dehisce by longitudinal slits and a style slightly exserted.

Type:— VIETNAM. Dak Lak province: Lak district, Bong Krang municipality, Chu Yang Sin National Park, 11 km SSE of Krong Kmar village, in mixed forest, elevation ca. 1250 m a.s.l., N 12° 24’ 53’’, E 108° 22’ 56’’, 22 May 2014, M. S. Nuraliev, A. N. Kuznetsov, S. P. Kuznetsova 960 (holotype MW 0595623).

Plant epilithic, herbaceous, perennial, completely glabrous ( Fig. 1a , 2a ). Tuber ca. 6 mm in diam. in sicco, bearing several buds each with 5 pairs of cataphylls ( Fig. 1b ). Roots adventitious, attached at base of aerial stem (in cataphyllbearing nodes) as well as at tuber surface, dense, much-branched, with small nodules. Stem erect, unbranched, 2.2–5.6 [4.0] cm long ( Fig. 2b , 3a,b ), sometimes with 2 opposite foliaceous scales 1.3–2.5 [1.7] mm long and 0.7–1.0 [0.9] mm wide inserted at 1.1–4.0 [2.0] cm from stem base ( Fig. 1a ). Stipules minute, each of 2 represented by a wart covered by several papillae, whole structure less than 0.5 mm ( Fig. 1c ). Leaves opposite, in 1 pair, strongly anisophyllous, sessile. Large leaf always pointing down and inflorescence inclined towards it, with anthetic flowers facing the leaf. Large leaf ovate, 2.6–4.7 [3.8] cm long, 1.4–2.5 [1.9] cm wide, with 4–5(6) pairs of secondary veins, margin entire, apex acuminate to attenuate, base cordate ( Fig. 2c ); small leaf usually overlapping the large leaf, narrowly to broadly ovate, 2.5–11.8 [3.9] mm long, 1.8–7.7 [2.8] mm wide ( Fig. 1c ). Inflorescence single, terminal, dichasial with all axes elongated, unbranched to 2 times branched (sometimes unevenly), 1–5-flowered (or probably more) ( Fig. 2d , 3c ). Peduncle 5–20 [14.3] mm long; bracts on primary axis (= bracteoles of first order flower, single pair) green, 1.4–2.7 [2.0] mm long, 0.5–1.2 [0.9] mm wide; bracts on secondary axes much smaller. Pedicel 3.3–8.0 [5.2] mm long. Flower actinomorphic, 5-merous (except the gynoecium). Calyx shorter than corolla tube (not visible from above), green, with short tube and broadly triangular lobes ca. 0.5 mm long ( Fig. 2d , 3d ). Corolla broadly rotate (star-shaped), white with pale green area at base of the tube; corolla tube short, ca. 1 mm long; corolla lobes triangular-ovate, conspicuously 3- nerved at base, 3.5–5.2 [4.5] mm long, 2.0–2.5 [2.2] mm wide ( Fig. 2e,f , 3e ). Stamens inserted at base of corolla tube. Filaments free, tightly appressed to each other (much thinner and loose in sicco), straight, ca. 1 mm long.Anthers fused to each other postgenitally by lateral margins (forming an anther cone), basifixed, oblong with straight base, 2.9–3.5 [3.1] mm long, introrse, dehiscent by longitudinal slits, without apical appendages (sterile prolongations less than 0.3 mm long) ( Fig. 1d , 2f , 3e,f ). Filaments pale green, anthers light yellow at base gradually becoming white towards apex. Ovary inferior, 2-locular, 0.5–1 mm long, ca. 1 mm wide; style slightly exserted from stamens (for 0.5–0.8 [0.6] mm), filiform, 4.5–5.5 mm long; stigma globose. Fruit and seeds unknown.

Notes:—1. The whole genus Argostemma is often characterized by possessing perennial habit due to the presence of rhizomes or tubers ( Sridith 1999a, Puff et al. 2005, Tanaka et al. 2010, Choudhary et al. 2013). However, in the descriptions and drawings of some species, including those similar to A. cordatum (e.g. Sridith 1999b, 2009, 2012), no such long-lived (perennial) shoots are shown, and all the roots are attached to the annual stem. According to our observations of A. cordatum , the annual stem occupies lateral position on the tuber, and probably can be easily detached from it. Indeed, in our material most of the plants lack the tuber, and we suppose that it was the result of inaccurate collecting. In other species, similar reasons could lead to the apparent absence of perennial basal part of plants. The precise structure and branching pattern of the tubers in A. cordatum and its congeners remain unknown.

2. It is remarkable that in Argostemma cordatum the appearance of stamen filaments differs substantially in living and dried plants. It is possible that this effect takes place in some other species of the genus, and filaments described and illustrated as not touching each other are in fact tightly appressed, with the style not visible between them.

Taxonomic relationships:— Argostemma cordatum belongs to a morphologically distinct group of species which is characterized by strong anisophylly combined with presence of a single pair of stem leaves (or sometimes two pairs). As a result, these species possess a single large leaf in each plant, the other leaf (or three others) being minute and often stipule-like (see also Puff 2009). The other species from this group are A. humile , A. kurzii , A. monophyllum , A. phyllocharis , A. siamense , A. tenue , A. unifolioloides (often misspelt as “ unifolioides ”), A. unifolium ( Ridley 1915, 1923, 1927, Sridith 1999a, 1999b, 2009, 2012, Puff 2009) and a poorly known species A. begoniaceum Miquel (1857: 348) from Java ( Ridley 1927). The main morphological differences between species of this group are summarized in Table 1.

Argostemma cordatum differs from the above-mentioned related species by the smaller sizes of the whole plant and particularly the enlarged leaf. Furthermore, A. cordatum is distinctive in possessing an invariably cordate base of the large leaf, whereas the large leaf of all other species shows an acute, cuneate or rounded base in all or some of the individuals. Other important features of A. cordatum are: plant completely glabrous, stipules minute and reduced to papillate warts, all inflorescence axes elongated (not umbel-like), anthers basifixed, coherent into anther cone and dehiscent by longitudinal slits, style slightly exserted.

It should be noted that comparison of species from this group is complicated by the scarcity of illustrations of flower morphology: for most of them, including observed in recent years, photographs are not provided, and drawings are hardly informative, most probably prepared on the basis of dried material. At the same time, descriptions are too complicated and sometimes controversial. For instance, Sridith (2009) stated anthers of A. kurzii to be connivent into a cone-like structure, while later he described this species as having separated free stamens and opposed it to the species with anthers connivent or coherent into a cone-like structure ( Sridith 2012).

Etymology:—The specific epithet “ cordatum ” refers to the prominently cordate base of large leaf which distinguishes the new species from its relatives.

Distribution and habitat:— Argostemma cordatum is currently only known from Dak Lak province in Southern Vietnam ( Fig. 4 ), where it occurs in large populations in areas with appropriate habitats. It is found at elevation from 950 to 1250 m a.s.l., in forests (including shady stream banks) and inhabits vertical granite rock surfaces covered by mosses.

The type specimen was collected on slopes covered by polydominant mixed tropical mountain forest of middle elevation, characterized by a well-developed hydrological network. Granite rocks up to 2 m in diam. are common on the surface. The forest stand can be subdivided into three stories of which upper and lower ones are well developed. Trees of the first forest stratum are 20–22 m (up to 24–26 m) high, with trunks 40–70 cm DBH; canopies 2–4 m in diam., approaching each other but not overlapping. Trunks are covered by mosses from base to 4–7 m height.

Apart from the location of the holotype, A. cordatum was observed in several other places at a distance of up to 5 km from it. One of them (N 12° 23’ 50’’ E 108° 21’ 00’’, 1100 m a.s.l., see Fig. 3 ) is very close to the type locations of three other recently described species ( Averyanov et al. 2013, Nuraliev et al. 2015, Wood et al. 2017). In these publications, extended data on forest composition in this area are provided.

Phenology:—The plants observed in late May showed open flowers and numerous flower buds. In some individuals, inflorescences were considerably underdeveloped. Thus, the very beginning of flowering probably takes place in this season. As the inflorescence is dichasial, it can possibly develop subsequent branches of several higher orders after the first flower opens, and bear more flowers than indicated above. We therefore suppose that the flowering time is at least May–June.

Figures

FIGURE 1. Argostemma cordatum. A. General view of a plant with 1-flowered inflorescence. B. Tuber with roots, buds and annual shoot. C. Node with large leaf base, small leaf and stipules.D. Flower, longitudinal section.All drawn from type Nuraliev, Kuznetsov, Kuznetsova 960 by A.Beer.

FIGURE 2. Argostemma cordatum at type locality.A. General view of population. B. Flowering individual. C. Individual with 1-flowered inflorescence, view in plane of large leaf. D. Dichasium with flower buds. E, F. Flower, apical and oblique view. Nuraliev, Kuznetsov, Kuznetsova 960. All photos by M.Nuraliev.

FIGURE 3. Argostemma cordatum at second recorded locality. A, B. General view of a plant, showing its orientation relative to the ground. C. Twice branched inflorescence. D. Flower, view from below showing calyx and ovary. E. Flower, side view. F. Close-up of anther cone. All photos by M.Nuraliev.

FIGURE 4. Map of Southern Vietnam with type locality of Argostemma cordatum.