Phyllagathis nanakorniana ( Melastomataceae ) , a new species from Thailand

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INTRODUCTION
Phyllagathis Blume (Melastomataceae, Sonerileae) is an Old World genus endemic to Southeast Asia and comprising approximately 56 species (Cellinese 1997(Cellinese , 2002. Its distribution ranges from South China, Vietnam, Laos, Thailand, and West Malaysia to Sumatra and Borneo. It was originally described as tetramerous with two whorls of four stamens (Blume 1831a, b). However, merosity in the genus is a proven unstable character (Hansen 1992, Cellinese 1997, 2002, and a few species display a switch from tetramerous to pentamerous flowers (e.g., P. prostrata C.Hansen, P. rotundifolia (Jack) Blume, and P. scortechinii King) sometimes even within the same species (Cellinese, pers. observ.), and from diplostemony to haplostemony (e.g. P. tetrandra Diels). Cellinese (1997Cellinese ( , 2002 provided a comprehensive synopsis of Phyllagathis and its morphological characteristics, and proposed a wider concept of the genus to include several pentamerous and tetramerous monotypic and diatypic genera. Among these are Kerriothyrsus C.Hansen (Hansen 1988), endemic to Laos, and Tylanthera C.Hansen (Hansen 1990), endemic to Thailand.
The monotypic Kerriothyrsus displays flowers that can be either haplostemonous or diplostemonous with 4 antepetalous sterile staminodes (Hansen 1988). In general, the switch from diplostemonous to haplostemonous androecia is not uncommon in Phyllagathis and can be observed in other species (Hansen 1992, Cellinese & Renner 1997, and other genera in the family (Renner 1989(Renner , 1993. The only species of Kerriothyrsus, K. tetrandrus (M.P.Nayar) C.Hansen, is found in a few localities of Laos. Based on its morphological characteristics, K. tetran drus should be moved into Phyllagathis (Cellinese, in prep.).
Phyllagathis nanakorniana is morphologically similar to P. tu berosa and P. siamensis from Thailand, and the neighbouring K. tetrandrus.

MATERIALS AND METHODS
This work is based upon the study of specimens collected by Khanit Wangwasit and Monthon Norsaengsri in the Nongkai Province, Northeastern Thailand. Measurements of the vegetative and reproductive parts were taken from their collected material. Fruits and flowers were studied using a dissecting microscope. Data on the distribution and ecology were taken in the field. Habitat -On sandstone rock in dry evergreen forest. Etymology -We name this species in honour of Dr. Weerachai Nanakorn, former Director of the Queen Sirikit Botanic Garden in Chiang Mai, Thailand.

KEY TO THE SPECIES OF THAILAND
Note -Similar to P. siamensis but differs by having 8 dimorphic stamens, which include 4 fertile stamens and 4 staminodes. In addition, the leaf apex is subacute to round and obcordate, distinctly different from the narrowly acuminate leaves of P. siamensis.

DISCUSSION
Phyllagathis nanakorniana is an acaulescent, small herb, with a tuberous rhizome, very much resembling both P. siamensis and P. tuberosa in habit. Phyllagathis tuberosa has also similar scorpioid inflorescences, but it is much smaller and covered with glands and soft, patent hairs. Phyllagathis siamensis is a much larger herb with umbellate inflorescences, covered with a spare indument. Although similar in the overall habit, P. nanakorniana is clearly distinguished for having diplostemonous flowers, with four fertile stamens, and four staminodes.
Diplostemous flowers arranged in scorpioid cymes are also observed in Kerriothyrsus tetrandrus. This is a subacaulescent herb, overall similar in habit, sparsely covered with hairs and glands, and with opposite, isomorphic leaves. It lacks tuberous rhizomes and interestingly, some specimens exhibit haplostemonous flowers, with a loss of the staminodial whorl. Because all of the above species are known from a few localities and a limited number of specimens, their intraspecific variation cannot be assessed at this stage. However, it is important to point out their morphological affinities and potential phylogenetic relationship to one another.