Polystachya siederi Hermans, 2021

Hermans, Johan, Rajaovelona, Landy & Cribb, Phillip, 2021, New species in Orchidaceae from Madagascar, Kew Bulletin 76, pp. 39-56 : 49-54

Polystachya siederi Hermans sp. nov.

Type: Madagascar, Mahajanga prov., Bealanana area, 924 m, Jan. 2017, Sieder, Pertl, Prehsler & Andriantiana 7112 (leg. Hermans 8151), (holotype WU!) .

http://www.ipni.org/urn:lsid:ipni.org:names:77214549-1

A large erect creeping lithophytic herb up to 18 cm high, on a very short rhizome with the new growth emerging from the base or node of the old pseudobulbs, with a dense mass of thick glabrous greyish roots 2 – 3 mm in diam. Pseudobulbs conical to subcylindrical, 35 – 60 × 4 – 7 mm, longitudinally ridged, the older ones distinctly annulate at the old leaf joints, partly covered by several overlapping greyish-brown sheaths, 3 – 4 leaves towards the apex. Leaves linear-ligulate, attenuate at the tip, 8 – 15 × 1.2 – 1.5 cm, narrowed into a conduplicate petiole at the base, the basal one much smaller and almost bractlike, pale green. Inflorescence erect, surpassing the leaves, up to 18 cm, 1.2 – 2.1 mm in diam., densely racemose in the apical third with up to 9 flowers. Peduncle flattened – laterally compressed, almost glabrous towards the base but becoming echinatehirsute towards the rachis, generally with a prominent conduplicate leaf-like scale towards the middle (35 – 65 × 4 – 6 mm) and 2 – 4 thinner and much shorter amplectant sheaths. Rachis terete, not branched, 3 – 6 cm, flowering in succession, hirsute. Floral bracts lanceolate acuminate, persistent and thickened at the base, hirsute on the exterior especially towards the base, 3 – 3.2 × 1.5 – 2.1 mm. Flowers c. 10 × 9 mm, pure white except for the orange callus in the middle of the lip and farinose orange-yellow patch on the mid-lobe of the lip, the white ovary becoming green on fertilisation. Pedicel and ovary glabrous, fusiform, longitudinally ridged, 4.8 – 5.2 × 1.1 – 1.3 mm. Dorsal sepal ovate, acute, 5.1 – 5.5 × 3.4 – 3.7 mm, arching over the column. Lateral sepals spreading, forming an indistinct rounded mentum with the column foot, ovate, shortly acuminate, 5.4 – 5.6 × 5.4 – 5.9 mm. Petals oblanceolate, 4.2 – 4.3 × 1.9 – 2 mm, acute, incurved. Lip 5.1 – 5.3 × 4.9 – 4.8 mm, 3-lobed in the apical half, the lateral lobes porrect elliptic obtuse, the mid-lobe sub-triangular, ending in a short incurved apicule, margins rounded, the disc and mid-lobe minutely pubescent-farinose, disk with a distinct ovoid to almost globular callus in the centre, densely covered in farina. Column stout, 3-lobed, stigma rounded, transversally oblong, 2.5 – 3 × 1.5 – 1.6 mm; anther hemispherical with a rounded apical lobe, c. 1.1 × 1.2 mm; pollinia (2) ovoid, c. 0.8 × 0.6 mm. Seed capsule fusiform, c. 15 × 5 mm. Figs 9 , 10 .

RECOGNITION. Polystachya siederi is distinct in having ascending pseudobulbs on a very short rhizome, large leaf-like scales towards the middle of the inflorescence, an unbranched inflorescence densely racemose towards the apex, hirsute rachis and floral bracts, medium-sized flowers with ovate lateral sepals forming a short rounded mentum at the base, a lip three-lobed in the apical half, and a farinose disk with an ovoid to globular callus in the centre. Amongst the Polystachya Hook. ( Hooker 1824: t.103) species occurring in Madagascar, P. siederi has some vegetative and floral similarities to P. anceps Ridl. ( Ridley 1885: 473) , P. heckeliana Schltr. ( Schlechter 1913: 167) and P. rhodochila Schltr. ( Schlechter 1916: 323) but it differs in having an unbranched inflorescence, slightly smaller flowers, and more oval lateral sepals and lip shape; it also has much shorter floral bracts than P. rhodochila . In flower shape it is most similar to P. heckeliana but the lip is lobed in the apical half (vs the basal half), the callus is more distinct and the flower colour is also different. It shares a number of characteristics with several Polystachya from mainland Africa: the inflorescence, lip shape and callus are most similar to P. transvaalensis Schltr. ( Schlechter 1895: 28) and P. albescens Ridl. ( Ridley 1887: 199) and its variants but it is distinct from both by its shorter and broader pseudobulbs, unbranched inflorescence, and slightly smaller flowers with shorter and more rounded ovate lateral sepals.

DISTRIBUTION. Endemic to Mahajanga province in northern Madagascar, only known from the type collection.

HABITAT. In semi-shade in riverine forest on rock. Elevation 900 m.

CONSERVATION STATUS. Polystachya siederi is known from one location, outside a protected area in Sofia region, Mahajanga province. The habitat, the extent of occurrence and the area of occupancy face continuing decline due to fire and agricultural practices. Polystachya siederi is therefore assessed as Critically Endangered CR B1ab(i,ii,iii) + 2ab(i,ii,iii).

FLOWERING TIME. January.

ETYMOLOGY. Named for Anton Sieder of the Botanical Garden, University of Vienna, Austria; an intrepid explorer, grower and connoisseur of the orchids of Madagascar.

NOTES. Polystachya is a pantropical genus of about 250 epiphytic, lithophytic or terrestrial plants, with the greatest diversity in sub-Saharan tropical Africa ( Cribb in Pridgeon et al. 2014: 455). There are currently 23 Polystachya species recognised from the Madagascar, Mascarene and Comoro region, 16 of which are only found on Madagascar and three have a wider distribution outside the region.

Figures

Fig. 9. Polystachya siederi. A plant in habitat; B part of the rachis C detail of flower. PHOTOS: ANTON SIEDER.

Fig. 10. Polystachya siederi. A plant habit; B inflorescence; C floral bract, enlarged; D flower front view; E dorsal sepal; F lateral sepal; G petal; H lip; J column, front view; K column, side view; L anther cap, dorsal view; M anther cap, ventral view. Based on spirit material Sieder 7112. DRAWN BY JUDI STONE.