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40 Phytotaxa 27 © 2011 Magnolia Press RODDA & SIMONSSON
round, alternating with single glands, glabrous, a few long hairs at the junction between the sepals and the pedicel. Corolla revolute, white; lobes lanceolate, 911 mm long, acute at apex; free portion of lobes 78 × 4.55.5 mm, distance between each sinus 3.54.0 mm, abaxially glabrous, hirsute adaxially, hairs up to 0.3 mm long, lobe apex (ca. 1 mm long) glabrous, margins ciliate. Corona staminal (Fig. 1a,b,c) fleshy, dull white to light yellow, laterally spreading, ca. 3 mm high, 6.57.5 mm in diameter; corona lobes held at 3040 degrees to the filament tube, outer process rounded to obtuse, flattened, only partially folded beneath (Fig. 1c), inner process acute, held at about the same height as the anther appendages. Distance between center and outer corona process 3.33.6 mm; distance from center to corona sinus 1.51.8 mm; beneath corona, distance between filament tube and anther skirt (beneath guide rail) 0.60.8 mm. Pollinaria erect, ca. 830 × 430 μm; pollinia elongated, compressed, 660 × 220 μm, with a lateral pellucid margin; retinaculum 260 × 160 μm; translator 70100 μm long. Ovary lanceolate, about 1.7 mm long, light green. Fruits and seeds not seen. All measurements from fresh type material.
Phenology:Hoya rotundiflora is commonly seen flowering in cultivation during the summer months, which is consistent with the flowering periods of plants from a monsoonal area such as south Myanmar. A similar flowering season has been observed for Hoya pandurata Tsiang (1939: 125) and H. chinghungensis (Tsiang & P.T.Li) Gilbert et al. (1995: 9) both originating from this geographical area (personal observations).
Habitat and distribution:Little is known about the original habitat of this species. It has been observed to be difficult to grow and flower in constantly warm areas such as Bangkok (S. Somadee, personal communication) and therefore it is likely to inhabit higher elevated areas where winter temperatures are lower and where there is a greater disparity between day and night temperatures.
The type plant can be traced back to a market in Sangklaburi in Kanchanaburi province, Thailand, where it was first found in 2005. The plant was collected near the border in neighboring Myanmar but the exact locality has been kept secret by the seller.
IUCN Red List category:Population size and distribution range of Hoya rotundiflora cannot be estimated, as it is so far known from only a single collection. Due to the high horticultural interest in Hoya it is surprising that no further collections belonging to this taxon have been made since its first introduction into cultivation in 2007. This may suggest that the species may have a very restricted distribution range and small population size containing a limited number of mature individuals or its habitat may be inaccessible, for example being on steep karst formations. Further, Hoya populations are often under pressure because of frequent collection to supply the horticultural trade and therefore H. rotundiflora is hereby suggested as vulnerable according to IUCN Red List criteria (IUCN 2001).
Additional specimen examined:Ex Hort., 15 June 2010, Rodda Hort 2010/1 (L, SING, TO).
Discussion
Hoya rotundiflora is described from cultivated material growing in the personal Hoya collection of Torill Nyhuus (Stockholm, Sweden), where the species first flowered in Europe in September 2009. The ornamental potential of this species was apparent immediately after its discovery, and due to the efforts of Surisa Somadee, the plant became widely cultivated in Thailand. Nowadays it is commonly found in tropical areas as a garden ornamental and in colder regions as a house plant where it is known as Hoya sp. Square, due to the square or more precisely rectangular shape of the leaves. It is currently so common that it can be purchased in flower markets in Bangkok grown in hanging coconut husks (M. Rodda personal observation). Hoya rotundiflora was initially identified as H. lyi, a Chinese species, but after comparison with type material, it proved to be related but morphologically distinct. After comparison with herbarium material, H. rotundiflora was found to be similar to H. lyi and H. thomsonii Hooker (1883: 61), primarily due to the similar growth habit, as these two species are mainly pendulous plants, often growing tightly attached to limestone outcrops. Hoya rotundiflora can be easily differentiated from the related species by flower and leaf morphology and by flowering time. Hoya lyi and H. thomsonii present a flat to slightly concave corolla, whereas H. rotundiflora has revolute corolla lobes making the flowers look globose, hence the choice of the specific epithet (Fig. 1b). The corona lobes of H. rotundiflora are nearly erect, set at a 3040 degrees angle, while H. lyi has nearly flat (i.e., spreading) corona lobes, much resembling those of H. carnosa (L.f.) Brown (1910: 460). Hoya thomsonii instead presents a concave corona with a markedly depressed stigma. The leaves of H. rotundiflora present revolute margins and apex, giving the characteristic rectangular or pandurate appearance, while H. thomsonii and H. lyi have nearly flat, sometimes slightly concave leaves. The three species have a more or less conspicuous dark green margin on the leaf underside (Fig. 2), whereas the leaves of H. lyi and H. thomsonii have a more conspicuous indumentum on both leaf surfaces. These characters are often difficult to assess, since they are visible only in fresh material. A morphological comparison of the three Hoya species is presented in Tab. 1.
The flowering time of Hoya species has not been much investigated. Hoya rotundiflora, at least in cultivation in Europe, appears to flower in spring or early summer, up to early autumn. Hoya thomsonii and H. lyi in contrast, flower in autumn.
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Kew_Hoya   
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20 ส.ค. 54 00:40:21
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ได้ชื่อเสียที ...............Hoya rotundiflora...............แล้วมันเป็นแบบไหนหละ
Wow !!...........
