Didymoplexis stella-silvae Hermans, 2021

Hermans, Johan, Rajaovelona, Landy & Cribb, Phillip, 2021, New species in Orchidaceae from Madagascar, Kew Bulletin 76, pp. 39-56 : 46-47

Didymoplexis stella-silvae Hermans sp. nov.

Type: Madagascar, Fianarantsoa prov., Mahakajy area, near Ifanadiana . c. 800 m, Dec. 1996, Hermans 8137 (holotype K!) .

http://www.ipni.org/urn:lsid:ipni.org:names:77214547-1

Erect holomycotrophic herb up to 9 cm tall, on a subterranean, stem-like rhizome, roots wiry, glabrous, c. 1.5 mm in diam. Rhizome terete, pale brown, elongate, a little verrucose, puberulent, ave. 12 × 0.3 cm. Leaves scale-like, at the base of the inflorescence, c. 5 × 3 mm. Inflorescence erect as an uninterrupted continuation of the rhizomatous stem, with 1 – 2 brown scarious scales towards the base. Peduncle slender, c. 1.5 mm in diam., with 2 – 3 short nodes, otherwise smooth, brown, bearing a short 3 mm scale below the rachis. Rachis apical, racemose with 1 – 4 flowers opening one at a time in succession. Floral bracts ovate, acute, 2 – 2.3 × 1.4 – 1.6 mm, verrucose abaxially. Flowers resupinate, spreading, reclining, overall 18 – 19 × 14 × 16 mm, crystalline white except for the yellow-orange lip callus and base, the lip with a greenish-yellow triangle abaxially. Pedicel and ovary fusiform, 8 – 11 × 0.8 – 1.1 mm, with longitudinal ridges during flowering, minutely verrucose, pale brown. Dorsal sepal connate to the petals for 2 – 3 mm at the base, erect, broadly lanceolate, 8.1 – 8.5 × 4 – 4.3 mm, subacute, verrucose-warty on the exterior. Lateral sepals connate to each other at the base for c. 3 mm, spreading, ovate, 9.8 – 10.2 × 5 – 5.5 mm, obtuse to subacute. Petals oblanceolate, 6.9 – 7.2 × 3.8 – 4 mm, acute, partly adnate to the dorsal sepal. Lip with a strap-like 3 – 5 mm foot at the base, blade then reniform, truncate, margins curved, undulate, becoming serrate towards the anterior part, overall 5.1 – 5.3 × 9.1 – 9.3 mm, base claw-like, extended into a bilobed callus irregularly laciniate at the anterior margin, the blade with two irregular longitudinal rows of long thorn-like hairs, the entire lip mobile and seems to retract back against the column, during or after fertilisation. Column slender at the base, with short, triangular stelidia and with obovate, rounded arms just below the stelidia, 3.8 – 4 × 3.4 – 3.5 mm; anther elliptic, disk-like, c. 1.2 × 1.9 mm. Figs 5 , 6.

RECOGNITION. Didymoplexis stella -silvae is a holomycotrophic plant with a thin erect inflorescence carrying few flowers opening successively and one at a time. The petals are adnate to the dorsal sepal and the lateral sepals adnate to one another. The mobile truncate reniform lip, undulate to laciniate at the margin, has two rows of thorn-like hairs, a bilobed callus becoming laciniate at the front, and a column with obovate, rounded arms just below the stelidia. The new species has some characteristics in common with a number of other Didymoplexis species but there are also several distinct differences. In the Africa – Madagascar region it has flowers about half the size of D. avaratraensis P.J.Cribb, Nusb. & L.Gaut. ( Cribb et al. 2013: 43) and D. africana Summerh. (Summerhayes 1952: 465) : the petals and sepals are also less fused, while the lip adornments and shape and callus are also very different. It has a similar habit and flower colour to D. verrucosa J.Stewart & Hennessy (1980 : 841), but D. stella-silvae has 1 – 4 flowers (vs 8 – 12), the sepals and petals are connate (vs fused to the middle), the lip reniform (vs transversely triangular) and two rows of long hairs on the lip (vs lines of warts from base to tip). It is closest to Didymoplexis recurvata P.J.Cribb, Nusb. & L.Gaut. ( Cribb et al. 2013: 44), recently described from northern Madagascar which is comparable in size and colour, the petals and sepals adnate toward their base, its lip shape and column wings being similar but D. stella-silvae has narrower petals, and the lip larger and broader (3.5 – 4 × 5 – 6 mm vs 5.1 – 5.3 × 9.1 – 9.3 mm). Didymoplexis recurvata also lacks the rows of long hairs on the disk and has a 3-lobed callus at the base (vs 2-lobed).

DISTRIBUTION. Endemic to eastern Madagascar, Fianarantsoa province, around Ifanadiana and Ranomafana National Park.

SPECIMENS EXAMINED. MADAGASCAR. Without specimen: observed by one of the authors (JH) and other researchers in Ranomafana National Park , at c. 1000 m .

HABITAT. Humid evergreen forest, in deep shade, in leaf litter and humus. Elevation 800 – 1070 m.

CONSERVATION STATUS. Recent observations of several colonies by one of the authors (JH) found an increasing threat by two different species of bamboo in the area; one tall Bambusa with its fallen leaves covering the forest floor and a smaller species forming an equally impenetrable mass. Some Asian Didymoplexis species are associated with bamboo but this may not be the case for this species as it was only found some distance away from invasive bamboos. Didymoplexis stella-silvae is distributed in SE Madagascar in the Vatovavy-Fitovinany region, Fianarantsoa. With two threat locations, found within the Ranomafana Protected Area, the extent of occurrence is estimated to be less than 5000 km 2 and the area of occupancy less than 500 km 2, in continuing decline due to habitat destruction caused by fire, the species is therefore assessed as Endangered EN under criteria B1ab(i,ii,iii) + 2ab(i,ii,iii).

FLOWERING TIME. December to January.

ETYMOLOGY. The name refers to the bright white flowers, resembling stars appearing on the forest floor.

KEW BULLETIN

NOTES. Didymoplexis Griff. ( Griffith 1844: 383) is a genus of 21 terrestrial, holomycotrophic, achlorophyllous species found in tropical and southern African, tropical Asia from India and the Ryukyu Islands eastwards through Malesia to Australia and the Pacific archipelagos to Fiji and Samoa ( Govaerts 2019). Until 2011 no true Didymoplexis had been recorded from Madagascar ( D. madagascariensis (Schltr. ex H.Perrier) Summerh. ( Summerhayes 1953: 131) is now considered Gastrodia madagascariensis Schltr. ex Martos & Bytebier ( Martos et al. 2015: 048). Cribb et al. recorded D. verrucosa from northern Madagascar in 2011 ( Cribb et al. 2011) and two years later ( Cribb et al. 2013) described D. avaratraensis and D. recurvata , also from northern Madagascar and wondered how many species of Didymoplexis there were on the island. Didymoplexis stella-silvae is another surprising discovery and brings the total to four, it is the first to be found in the central eastern forest of the island. The bright star-white flowers stand out in the deep shade of the forest; recently small colonies were observed and photographed in the Ranomafana National Park in a very well researched area near one of the main paths. It is likely that the species has rarely been seen because it has no leaves and the flowers only seem to open immediately after a significant rain shower and only remain open for less than a day, new flowers then open after the next rain shower. The pollination and lip mechanism deserves further research: the lip is hinged on a thin strap-like foot and was observed to retract against the column in a number of flowers, it is possible that the lip and its hairs act as a simple balance and flip back when visited by pollinators, it also possible that it just retracts after pollination has been effected or it may even be a trapping mechanism as has been observed in the South American orchid genus Porroglossum Schltr. ( Schlechter 1920: 82) .

Figures

Fig. 5. Didymoplexis stella-silvae. A plant in habitat; B detail of flower. PHOTOS: JOHAN HERMANS.