JARS v63n3 - A Reassessment of the Rhododendron verticillatum Low ex Lindl. Complex with a New Species, R. atrichum Argent and Two Subspecies, subsp. atrichum and subsp. dendrolepis (Argent) Argen

A Reassessment of the Rhododendron verticillatum Low ex Lindl. Complex with a New Species, R. atrichum Argent and Two Subspecies, subsp. atrichum and subsp. dendrolepis (Argent) Argen
George Argent
Edinburgh, Scotland

This article is reprinted with the kind permission of the editor from Rhododendrons, Camellias and Magnolias. 2009: 82-87, with the correction of the new species name, Rhododendron epilosum, to R. atrichum. The name R. epilosum is unfortunately a later homonym for R. xepilosum (J.J. Sm.) Argent.

Rhododendron verticillatum Low ex Lindl. was treated by Sleumer (1966) as having two forms: forma verticillatum and forma velutinum (Becc.) Sleumer. Forma velutinum was reduced to synonymy in Argent (2006) after an examination of the type specimen of forma verticillatum in the Cambridge herbarium which showed that this had hairs on the petioles and leaves, the important distinguishing characters for forma velutinum . There is an uncharacteristic vagueness about the distinctions between the two formae in Sleumer's Flora Malesiana account (Sleumer 1966). The flowering in cultivation of a Rhododendron collected by the Jermy, Mulu (Sarawak) expedition prompted a re-examination of herbarium specimens in the Kew and Edinburgh herbaria. This showed that there were both hairy-leaved and just scaly-leaved plants under Rhododendron verticillatum , further there were collections with rounded to lobed scales, stellate scales and distinctly dendroid scales which appear to form three distinct entities.

Rhododendron atrichum Argent ssp.
atrichum flowering at the RBG Edinburgh
Fig.1: Rhododendron atrichum Argent ssp. atrichum flowering at the RBG Edinburgh.
Photo by Lynsey Wilson

There is some similarity of leaf form between Rhododendron verticillatum from southern Sarawak and R. polyanthemum Sleumer from Sabah although these have been considered distinct both on corolla tube length (Fig. 2 A,B) and scale type (stellate in R. verticillatum (Fig. 5 above) and dendroid in R. polyanthemum (Fig.5 below). They also differ significantly in flower bud morphology; R. verticillatum (from near the type locality) having very distinctive buds with reflexed, broadly pointed and usually bright purple bract apices (Fig. 3B), while R. polyanthemum has smooth, green, spherical buds with the apices appressed except for the outermost bracts at the base of the bud where there is a transition from foliage leaves where a few bracts have spreading caudate apices. With few intervening collections however there was always the possibility that there was a cline of variation from south to north along the main mountain chain which essentially forms the border between Sarawak and Kalimantan.

Fig. 2: Difference in corolla
shape between (A) R. polyanthemum, and (B) = verticillatum/atrichum
Fig. 2: Difference in corolla shape between (A) R. polyanthemum , and (B) = verticillatum/atrichum .
Drawings by Louise Olley and Anna Dorward
Fig. 3: Bud morphology of (A) R.
atrichum ssp atrichum and (B) R. verticillatum.
Fig. 3: Bud morphology of (A) R. atrichum ssp atrichum and (B) R. verticillatum .
Drawings by Louise Olley and Anna Dorward

Collection Acc. 19913786 from the Mulu National Park in northern Sarawak conforms to Sleumer's interpretation of R. verticillatum forma verticillatum , being hairless on the leaves, petioles and only sparsely on the flowers but this name is no longer tenable as the type specimen with this name is hairy, clearly synonymous with the type of R. verticillatum forma velutinum . The new name Rhododendron atrichum Argent (Fig. 1) is proposed for this plant which is lacking simple hairs on the leaves and differs also in flower bud morphology (Fig. 3A). This new species does however have minute hairs on the very young stems; these are much shorter than in R. verticillatum (0.1 mm vs. 0.4 mm). Examination of all the materials under the name R. verticillatum in the herbaria at Edinburgh and Kew revealed however that there were two distinct forms of the new proposed species in northern Sarawak, which are proposed as distinct subspecies: those with rounded to stellate scales (subsp. atrichum ) and those with shortly dendroid scales (subsp. dendrolepis ). Subspecies dendrolepis is as yet poorly understood as it is at present only known from a few herbarium specimens and we are lacking information about the bud morphology; the shortly dendroid scales are reminiscent of the stellate scales of R. verticillatum and are intermediate in form between those of R. polyanthemum . However R. polyanthemum is distinct from all forms of R. verticillatum in having very short corollas (Fig. 2A). The corolla tube in R. polyanthemum is c. 20 mm whereas it is 30-40 mm long in R. verticillatum (Fig. 2B) and R. atrichum . There is a further complication in that this new species has flower bud morphology very similar to that of the wide ranging and variable R. longiflorum Lindl. although this species has much more slender flower buds c. 8mm wide compared with the c. 13mm of R. atrichum . These two species are also easily distinguished when living as R. longiflorum has its slender petiole distinctly grooved throughout its length while the petiole of R. atrichum is broader and rounded without a groove. Further collections especially from Sarawak are very desirable to give a more complete picture of the variation in this complex group of taxa.

Leaf surfaces showing scales and
hairs: (above) R. atrichum ssp. atrichum and (below) R. atrichum ssp. dendrolepis Fig 5: Leaf surfaces showing
scales and hairs: (above) R. verticillatum and (below) R. polyanthemum.
Fig. 4: Scanning electron micrographs (SEMs): Leaf surfaces showing
distribution and morphology of scales and hairs: (above) R. atrichum ssp. atrichum
(19913786) and (below) R. atrichum ssp. dendrolepis (S.48061) RBGE.
Fig 5: SEMs: Leaf surfaces showing distribution and morphology of
scales and hairs: (above) R. verticillatum (19871300) and
(below) R. polyanthemum (Collenette 73/79). RBGE.

Rhododendron atrichum Argent nom . nov . ≡ R. epilosum Argent, Rhododendrons, Camellias and Magnolias. 2009: 86, non Rhododendron x epilosum (J.J. Sm.) Argent -Rhododendrons of subgenus Vireya 44. 2006

Similar to R. verticillatum but differing in lacking the simple hairs on the leaves, having broader scales with less deeply divided lobes and having the bract apices appressed in bud.

Type: Warwick, Camus & Hapi MW 301 15 Nov. 1990. Ridge above camp 2 (SE of summit of G. Mulu). Gunong Mulu National Park, Sarawak. Epiphyte in a large tree c. 1000 m. Holotype E, isotypes BM; K; SAR.

Epiphytic shrub to c. 1m. Twigs densely brown-scaly and minutely hairy when very young (hairs to 0.1mm), 3-4 mm in diameter; internodes 2-6 cm. Leaves in 4-5 together in tight pseudowhorls usually with 1-3 much smaller leaves. Blade 8-13 x 30-55 mm, elliptic; apex acute to rounded; margin entire, flat; base broadly tapering to rounded, scaly initially on both sides, glabrescent above except for the mid-vein where the scales tend to persist, beneath, remaining laxly scaly for a long time. Scales very small, dark brown, rounded to lobed, the centre small, slightly impressed. Mid-vein broad impressed above; below, raised throughout most of its length and tapering gradually, lateral veins, slender, 4-6 per side, curved-ascending, slightly raised both above and below (dry). Petiole 20-25 x c.3 mm, smooth, rounded, ungrooved when fresh, grooved when dry, densely brown scaly.

Flower buds up to 15 x 13 mm, ellipsoid, green, or flushed with pink, the apices of the bracts appressed. Bracts broadly elliptic to rounded, the apices rounded, scaly all over outside and fringed with small dark brown scales. Bracteoles c.17 x 2 mm, filiform, slightly broader distally, glabrous except for some marginal scales distally. Inflorescence a full umbel with 6-8 flowers, which are erect to horizontal. Pedicels c.15 x 1 mm, laxly scaly and very laxly minutely hairy. Calyx oblique, c. 4 mm in diameter, sub-circular scaly outside. Corolla 50-75 x 40-60 mm, narrowly funnel-shaped, orange-red, the lobes darker than the tube; tube 30-40 x 5-7 x 15-20 mm, deeply grooved in the proximal part, densely to laxly scaly and with a few small hairs at the base outside, laxly hairy inside; lobes 30-35 x 16-20 mm, semi-erect to spreading horizontally, broadly elliptic. Stamens exserted to c.10 mm irregularly grouped around the mouth; filaments 40-50 x c. 1 mm, filiform, laxly hairy in the proximal third, glabrous distally; anthers c. 2 x 1 mm, oblongoid, slightly curved. Disc hairy at the upper margin, glabrous below. Ovary 5-6 x 3-4 mm, sub-cylindric-conical, abruptly contracted distally, densely covered with appressed hairs, and small brown scales; style held on the lower side of the tube, at first at the mouth, later exserted up to 15 mm, hairy and scaly for the lower 3/5 - 4/5, glabrous distally; stigma rounded. Fruit not seen.

Additional specimens: 306D (19913786) cultivated specimen from the type locality 23rd Nov. 1998 E!
Rhododendron atrichum subspecies dendrolepis (Argent) Argent comb. nov . R. epilosum subspecies dendrolepis Argent, Rhododendrons, camellias and magnolias 2009. RHS: 86-87.

Similar to R. atrichum subspecies atrichum but with shortly dendroid scales on the leaves.

Type: Abang Mohtar S 48061 29 Aug. 1984 Batu Laga, Kapit. Heath Kerangas forest. Holo K! duplicate collections KEP; L; SAN.

I am grateful to the curator of the Kew Herbarium for allowing the examination of their specimens and to Katherine Challis for pointing out the invalid status of R. epilosum . Frieda Christie has used her usual skill in obtaining the SEM's from the sometimes very imperfect material; Lynsey Wilson for the colour photographs and to Louise Olley and Anna Dorward for help with the diagrams. Robert Mill rendered the diagnoses into Latin and he and Professor John McNeill made very useful suggestions and corrections.

Argent G. 2006. Rhododendrons of subgenus Vireya . The Royal Horticultural Society: 388 pp.
Sleumer, H. 1966. Rhododendron. Flora Malesiana 6: 474-667.

George Argent
George Argent is a member of the Scottish Chapter. He is a leading authority on vireyas having published a number of papers and books on their taxonomy, including the major review mentioned above and a comprehensive book Rhododendrons of Sabah Malaysian Borneo.