New species in Orchidaceae from Madagascar

Five new species from Madagascar, Aerangis bovicornu (Vandeae: Angraecinae), Angraecopsis lemurelloides (Vandeae: Angraecinae), Didymoplexis stella-silvae (Gastrodieae), Habenaria crocodilium (Orchideae: Orchidinae) and Polystachya siederi (Vandeae: Polystachinae) are described and illustrated. A short summary of endemism and the history of description of orchids of Madagascar are given.


Introduction
The flora and fauna of Madagascar are well known for their exceptionally high levels of diversity and endemism (85% of its species), the island having also been identified as one of the most important biodiversity hotspots (Myers et al. 2000: 855). The Orchidaceae are the largest plant family on Madagascar (Callmander et al. 2011: 122) and novelties are still discovered frequently. A new tabulation by one of the authors (JH) of all orchids currently recognised (April 2019) from Madagascar and surrounding W Indian Ocean Islands shows that there are now a total of 1002 species, subspecies (17) and varieties (51) known from Madagascar. Eighty-five percent of these are endemic to Madagascar and this endemism increases to c. 95% when the surrounding archipelagos of the Mascarenes, Comoros and the Seychelles are included.
Many botanists and collectors have been involved in the discovery of this remarkable orchid flora but only a few are responsible for their formal description. Until the 1820's, only a handful of orchid species were recognised from Madagascar and most were then only known from the Mascarene Islands. In 1822, Louis Marie Aubert du Petit Thouars described and illustrated 31 species from the island (Thouars 1822). Between 1824 and 1850, John Lindley added another 15 species. Heinrich G. Reichenbach f. described a further 51 between 1850 and 1885, mostly based on collections by Johann Hildebrandt (Reichenbach 1881a & b) and Léon Humblot (Reichenbach 1885a & b). Between 1883 and 1886, Henry Ridley described 57 new species based on collections by various missionaries, mainly of the London Missionary Society, including William Deans Cowan (Ridley 1883), Richard Baron and Joseph Tregelles Fox (Ridley 1885(Ridley , 1886. Robert A. Rolfe added 23 species in the 1890's from a variety of sources but principally from commercial collectors. Rudolf Schlechter contributed no fewer than 282 new names to the list between 1911 and 1925 with most of them based on collections by the French explorer Henri Perrier de la Bâthie (Schlechter 1913(Schlechter , 1915(Schlechter , 1916(Schlechter , 1918a(Schlechter & b, 1922(Schlechter , 1924(Schlechter , 1925(Schlechter and 1932. Following Schlechter's death in 1925, Perrier de la Bâthie described many novelties (Perrier 1930(Perrier , 1934(Perrier , 1936a(Perrier , b, c, 1937(Perrier , 1938a(Perrier , b, c, d, 1939a(Perrier , 1951(Perrier , 1955 and revised a number of genera. Perrier's work culminated in the orchid volumes of the Flore de Madagascar (Perrier in Humbert 1939b& 1941. He described 195 novelties mainly based on his own collections but also those of Raymond Decary, Henri Humbert and others working on the island during the French colonial era between 1929 and 1955. From the early 1960s until 2013, Jean Bosser, in collaboration with others, described 102 new taxa (Bosser 1965(Bosser , 1969a(Bosser , b, c, d, 1970a(Bosser , b, 1971(Bosser , 1980(Bosser , 1989(Bosser , 2000 and published an illustrated orchid flora of the Island with Marcel Lecoufle in 2011 (Bosser & Lecoufle 2011). Recent work by Johan Hermans, Phillip Cribb and collaborators has added 57 new orchids to the flora (most notably Hermans et al. 2007Hermans et al. , 2017Hermans & Cribb 2014 & others in press) together with a new checklist, bibliography (Hermans et al. 2007) and Field Guide (Cribb & Hermans 2009). The most recent illustrated guide is by Hervouet (2018).
As part of ongoing research and fieldwork on the orchid flora of Madagascar at the Royal Botanic Gardens Kew, the University of Vienna and PBZT Antananarivo, specimens of putative new taxa were compared with descriptions, herbarium material and drawings of all the type specimens of the genus to ascertain their novelty. This has resulted in the recognition of a number of new species.

IUCN Red List assessments
The conservation status of the new species given in this paper are summaries of the full IUCN Red List assessments which will be completed and submitted for review and publication by IUCN once the species names are validly published and therefore available for assessment. All the assessments have been compiled based on current knowledge of these taxa, by one of the authors (Landy Rajaovelona), who is an IUCN Red List assessor, using the IUCN Red List Categories and Criteria (2012).
RECOGNITION. Aerangis bovicornu is a few-flowered compact plant with small flowers that do not open fully, ovate to elliptic leaves which are brownishorange on top and greyish-green underneath; the stellate flowers have a short horn-shaped spur and white perianth with an orange-brown ovary and spur and long rostellum and anther with a small rounded swelling at the apex. It is undoubtedly most closely related to Aerangis fastuosa (Rchb.f.) Schltr. (Schlechter 1914: 598) (Fig. 1C) but differs substantially in its flowers which are consistently a third of the size, the straight slender, up to 23 mm long, spur which is consistently much shorter (vs a minimum of 76 mm in A. fastuosa), the perianth tips and spur coloured brownish-orange (vs generally white), the narrower column apex and the obovoid anther with a small rounded swelling at the apex (vs conical with a long beak). The new species flowers from December to January whilst Aerangis fastuosa flowers earlier from June to November. Table 1  HABITAT. Epiphyte on small moss-and lichen-covered trees in remnants of forest on granite inselberg. Elevation c. 1800 m. CONSERVATION STATUS. Recent observations by one of the authors (JH) found only six plants remaining in the type locality, one bearing an immature seed capsule. The area has now been cleared for cultivation of Pelargonium, grown increasingly in the region for the production of essential oils. Aerangis bovicornu has an EOO less than 100 km 2 and an AOO less than 10 km 2 and only one threat location is identified. In addition, the number of mature individuals is estimated to be less than 50. The species is therefore assessed as Critically Endangered CR with Criterion D. FLOWERING TIME. December to January. ETYMOLOGY. The epithet refers to the typical shape of the spur, resembling a bull's horn, especially that of the local Madagascan zebu cattle. NOTES. The genus Aerangis Rchb.f. (Reichenbach 1865: 190) now comprises 59 species; they are largely epiphytes confined to Africa and the Madagascar region, apart from one species which occurs both in eastern Africa and Sri Lanka. There are 28 species in Madagascar, Réunion and the Comoros with 20 of them endemic to Madagascar. Reichenbach described it as having rugose leaves on the upper surface, a narrow lip and a long caudicula immersed in the spur. Two weeks later (Reichenbach 1881d: 844) added notes on a specimen from the same source and grown by Sir Trevor Lawrence, as equal to his type in all details, but having an obovate rounded lip, instead of a narrow acute one, and suggested either that M. Humblot's plant was a peloric form, or that there were two closely allied species. Joseph Hooker (1891: t.7202) later commented on Reichenbach's confusing descriptions. He added that the rugosity of the leaf occurs only after flowering and is hardly perceptible. He referred to the 1885 illustration of a Lawrence plant in Gardener's Chronicle (Masters 1885: 533) as a good likeness. Subsequently a great number of plants found their way into European collections and the variability of the species became evident as recognised by Perrier de la Bâthie in 1941 (pp. 93 -95) when he described six varieties of the species, none being validly published because they lacked Latin descriptions. He also mentioned that they may well be separate species, all coming from different localities, a long way from each other. All are only  represented by very few herbarium specimens and it is difficult to define constant characteristics. Five of the varieties, notably var. francoisii, var. grandidieriana, var. maculata, var. rotundifolia and var. vondrozensis (Perrier 1941: 93 -95) are generally accepted as local forms of one variable species (Stewart 1986a: 905;Hermans et al. 2007: 12;La Croix 2014: 149), the stated variation was mainly in the leaf shape and texture. The sixth, var. maculata (Perrier 1941: 95) was considered by Bosser (2006: 50) to be Aerangis punctata J. Stewart (1986bStewart ( : 1120. A seventh, Aerangis fastuosa var. angustifolia H. Perrier (1941: 95), was considered an immature form of Aerangis modesta (Hook.f.) Schltr., (Schlechter 1914: 600). At first sight Aerangis bovicornu could be considered another local variant of Aerangis fastuosa, especially as it comes from a relatively accessible region but closer examination proves otherwise. They share the leaf shape, short rachis, shape of the perianth and the long rostellum but size of all the floral parts, spur length, column and anther shape and colour are all very different. Table 2 outlines the main characteristics of the different variants, none corresponding with Aerangis bovicornu. The characteristics of the new species were found to be consistent within the local colony of about 20 plants which consisted of plants with flowers that were just opening and others that had almost wilted. Arching to pendent epiphytic herb 15 -21 cm long; roots basal, glabrous, greenish-white, 1 -2 mm in diam. Stem woody, up to 15 cm × 3 -4 mm, zigzag, flattened, the margins winged, carrying 12 -17 leaves, pale green with the lower part somewhat corrugate and brown. Leaves alter-nate, coriaceous, flat with an indistinct central vein, lanceolate-linear, 6.5 -8.1 × 0.5 -0.8 cm, narrowing towards the apex, attenuate, the base subcordate, surface somewhat glossy, pale green. Inflorescences emerging from the leaf-axils along the entire length of the stem, sometimes several produced from the same axil, 5 -8 cm long, with 3 -7 flowers. Peduncle wiry, 3 -5 cm, c. 0.5 mm in diam., with a short thin, brownish bract at the base peduncle, 1 -2 peduncle sheaths c. 3 × 1 mm. Rachis loosely racemose, wiry, 3 -5 cm, somewhat zigzag. Floral bracts amplexicaul, 3.1 -3.8 × 1.1 -1.4 mm, green becoming brown with age. Flowers small, c. 7 × 5 mm, yellowish-green. Pedicel and ovary turbinate, 9 -12 × 1.3 -1.6 mm. Dorsal sepal convex, ovate, acute, 2.1 -2.5 × 1.9 -2.2 mm. Lateral sepals partly fused to the petals at the base, broadly lanceolate, acuminate with the tip reflexed, 3.1 -3.4 × 1.2 -1.4 mm, margins recurved, the apex of the dorsal mid-vein mucronate before the apex of the leaf. Petals reflexed, ovate-triangular, acute, 1.4 -1.7 × 1.5 -1.7 mm, convex with a thickened mid-vein. Lip 3-lobed, 2.3 -2.9 × 2 -2.2 mm, the hypochile orbicular beneath with the obtuse lateral lobes partly enveloping the column, the epichile broadly ovate, concave, somewhat bullate with a few fine hairs at the base; spur incurved below the flower, 4.9 -5.4 × 0.6 -0.8 mm, narrow at the opening, slightly thickened in the apical half. Column, fleshy, short, c. 1.6 × 1.5 mm; rostellum 3-lobed, wings sub-orthogonal, with an acicular median tooth; pollinia on separate viscidia, oblong, c. 1 mm long; anther cap elliptic, c. 0.6 × 1.1 mm. Seed capsule turbinate, c. 20 × 5 mm, green. Figs 3, 4.

Angraecopsis lemurelloides
RECOGNITION. Angraecopsis lemurelloides is recognised by the long zigzag flattened stem with lanceolatelinear leaves alternate along its length, small flowers on the upper third of the serial wiry inflorescences, and flowers with petals and sepals that are fused at the base and with a mucronate tip to the mid-vein, the roundly 3-lobed lip with a concave epichile with wiry hairs at the base and the lateral lobes enveloping the column and a short inflated spur. Table 1. Comparison of differences and measurements of Aerangis bovicornu and A. fastuosa, based on 28 herbarium specimens and the types of the varieties of the latter.

Aerangis fastuosa Aerangis bovicornu
Dorsal sepal (mm) 17 -36 × 5.5 -9 7.8 -8.6 × 3.6 -3.9 Lateral sepals (mm) 19 -36 × 5.5 -8.5 8.7 -8.9 × 3.5 -3.7 Petals (mm) 19 -34 × 6 -10 8.3 -8.6 × 3.8 -4.1 Lip (mm) 20 -34 × 5 -13 7.6 -8.2 × 3.9 -4.1 Spur 70 -102 × 1 -1.8 mm, generally coiled 20 -23 × 0.9 -1.3 mm, straight Column 3 -4 × 2 -3 mm, apex broadened 3 -5 mm × 1 -3 mm, apex narrow Anther 2.2 -4 × 1.8 -2 mm, conical with long beak up to 1.4 × 1.6 mm, broadly obovoid with a small rounded swelling at the apex Flowering time June to November December to January It is very different in plant habit and flower morphology from Angraecopsis parviflora (Thouars) Schltr. (Schlechter 1914: 601), the only other Angraecopsis Kraenzl. (Kraenzlin 1900: 171) recorded from Madagascar. That species has a stem that is always much shorter than the leaf, a lip with three distinct attenuate lobes and a slightly longer spur. In habit and inflorescence it somewhat resembles A. trifurca (Rchb.f.) Schltr. (Schlechter 1915: 428) from the Comoros but it has a much longer stem, carrying leaves along its length, the flowers being about half the size, the spur much shorter and the lip very different (three distinct attenuate lobes in A. trifurca). It is closest to Angraecopsis pobeguinii (Finet) H.Perrier (1941: 81) from the Comoros in flower size and in having a lip with small rounded lateral lobes but the plant of A. pobeguinii is much smaller (3.5 cm long), the stem shorter (1 cm long), the leaves much smaller (c. 2 cm long), the spur is also much more clavate towards the apex. DISTRIBUTION Refers to the similarity of the flowers to those of the genus Lemurella Schltr. (Schlechter 1925: 366). NOTES. Morphologically the new species is somewhat ambiguous in its generic position. With a long stem, many wiry inflorescences and flowers with a threelobed lip with hairs at its base it resembles some species in the genus Lemurella and especially Lemurella culicifera (Rchb.f.) H.Perrier (1941: 334) but it lacks the non-resupinate flowers, has more flowers and the opening of the spur is narrow (vs funnel-shaped). It illustrates that the generic circumscription of both these genera is somewhat uncertain and needs further research. As determined by Bosser (2007), it fits more easily in the genus Angraecopsis. This genus, somewhat diverse and containing a confusing variety of species, was established by Kraenzlin (1900), based on Angraecopsis tenerrima Kraenzl. from Tanzania. Schlechter (1914Schlechter ( & 1918a extended the genus to seven species based on a spur with a narrow opening, entire side lobes of the lip and lip flat at the base, five being from mainland Africa and two from Madagascar, the Mascarenes and the Comoros. Perrier de la Bâthie (1941: 81) recognised five species in the Flore de Madagascar. Summerhayes (1951) in his revision of the genus recognised three sections: Angraecopsis, Cardiochilos and Coenadenium, mainly based on differences in the shape of the lip and column, with 14 species from the Afro-Madagascan region. Rasmussen (1978: 137) added further observations on the genus b u t d o u b t e d t h e g e n e r i c a n d s e c t i o n a l 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.

R E C O G N I T I O N
. Didymoplexis s tell a-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  (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 Erect terrestrial herb 20 -40 cm tall without the inflorescence, with two ellipsoid tubers, c. 2.5 -3 cm long, hirsute, roots wiry, clustered around the base of the stem c. 3 mm in diam. Stem not branched, 10 -30 × 0.5 -0.8 cm with 3 -4 leaf-like sheaths along its length dividing it into short segments, becoming progressively  larger toward the top, 3 -8 × 1 -3.5 cm. Leaves spreading, 3 -5, arranged spirally along the stem forming an apical crown, lanceolate, 7 -18 × 1. RECOGNITION. Habenaria crocodilium is distinct in being a tall plant with spirally arranged leaves forming an apical crown, having an inflorescence borne on a short peduncle, denticulate floral bracts and flowers that are large for the genus in Madagascar, with denticulate ovary ridges, bifid petals with equal lobes, a lip with three equal descending lobes, a slightly flattened clavate spur shorter than the pedicellate ovary, and long and strongly up-curved anther channels. Amongst the large and medium-sized-flowered species of Madagascar and mainland Africa it is somewhat similar in flower shape to H. beharensis Bosser (1969a: 293), H. cirrhata (Lindl.) Rchb.f. (Reichenbach 1865: 180) and H. clavata (Lindl.) Rchb.f. (Reichenbach 1865: 180) but it is very distinct in its habit with its leaves borne along the stem (vs apical), and much smaller flowers (half the size) with a much shorter and differently shaped spur. It has a similar spur and anther-channel shape to H. cornuta Lindl. (Lindley 1837: 208) but again differs by its distinct habit, smaller flowers and differently proportioned lip and petals. The short clavate spur is also found in H. cochleicalcar Bosser (1969a: 294), and its flowers are also similar in size but the habit of H. crocodilium is different and the lobes of the petals and lip are of different proportions (unequal vs equal). It is similar in plant habit to H. tianae P.J. Cribb & D.L.Roberts (2008: 199) but its flowers are smaller by at least 1 3 , the spur much shorter (12 mm vs 12 cm) and the anther channels a different shape. None of the other species listed above have the pronounced echinate-denticulate ovary, floral bract and exterior of the dorsal sepal. Habenaria crocodilium is distributed in E Madagascar, in the Alaotra-Mangoro and Analanjirofo regions, Toamasina province. Two threat locations were identified, although the two subpopulations are protected in Makira and Vakona lodge, the species habitat quality is continuing in decline due to shifting agriculture and forest logging. Recently, colonies in the type locality were destroyed by landslides. The species is therefore assessed as Endangered EN B1ab(iii) +2ab(iii). FLOWERING TIME. July to September. ETYMOLOGY. Referring to the type locality near a crocodile enclosure and the resemblance of the column and dorsal sepal to the head of a crocodile, plus the numerous teeth on the flowers and ovary. NOTES. Habenaria Willd. (Willdenow 1805: 44) is a genus of about 600 terrestrial orchids, widely distributed in tropical and subtropical regions of the Old and New World (Cribb in Pridgeon et al. 2001: 300 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 flattenedlaterally 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.