Didymocarpus puhoatensis (Gesneriaceae), a new species from Vietnam

Abstract Didymocarpus puhoatensis, a new species from Vietnam is described and illustrated with photographs. The new species is morphologically similar to D. brevicalyx and D. epithemoides, but can be easily distinguished by a combination of characters. So far, five species have been recorded in the genus Didymocarpus from Vietnam.


Introduction
The delimitation of the genus Didymocarpus Wallich (1819: 378) has varied considerably over time (Burtt 1998, Weber et al. 2000, Möller and Clark 2013, Li et al. 2015. Now approximately 70 species range from northwest India, eastwards through Nepal, Bhutan, northeast India, Burma (Myanmar), to southern China, Vietnam, Laos, Cambodia, Thailand, the Malay Peninsula and northwards to Sumatra (Weber and Burtt 1998, Weber et al. 2000, Möller et al. 2016. Only three species of this genus were found in Vietnam before 2012, then D. kerrii and D. purpureobracteatus were respectively reported as new record species for the flora of Vietnam (Phuong et al. 2012(Phuong et al. , 2014. Didymocarpus bonii [= Calcareoboea bonii], is now a synonym of Petrocodon bonii . So now there are four species of Didymocarpus recorded in Vietnam: D. kerrii, D. pulcher, D. poilanei and D. purpureobracteatus. During a floristic expedition to northern Vietnam in 2015, the authors observed a population of an interesting Gesneriaceae in Pu Hoat Nature Reserve, Nghe An province, Vietnam. It was confirmed that it is member of the genus Didymocarpus based on its disc-like stigma (Wang et al. 1998). Over the past two years, the living plants were monitored in the field and an ecological survey was carried out by the co-author in Vietnam and in the nursery of Gesneriads Conservation Centre of China (GCCC) in China.
After thorough comparisons of diagnostic morphological and anatomical features of similar taxa from China, Vietnam, Thailand and adjacent regions (Kiew 1990, Hilliard and Burtt 1995, Wang et al. 1998, Burtt 1998, 1999, Weber et al. 2000, Hilliard 2001, Nangngam and Maxwell 2013, Nangngam and Middleton 2014, Phuong et al. 2014, it is concluded that, as its morphological characters do not fit any known species, it is a new species to science and accordingly described herein. Its morphological characters are compared with the closely related species: D. brevicalyx Nangngam & D.J. Middleton (2014: 35) and D. epithemoides B.L. Burtt (2001: 92). Therefore, there are five species of the Didymocarpus recorded in Vietnam.

Material and methods
Measurements and morphological character assessments of the putative new species were performed and described using specimens' work by the current authors, living material observed in the field and also those cultivated at the Gesneriad Conservation Centre of China. All available specimens of Southeast Asian Didymocarpus kept in the following herbaria were examined: E, GH, HN, IBK, K, KUN, MO, PE, PH, US and VNMN. The images of type specimens were also obtained from Tropicos (http://www.tropicos.org), JSTOR Global Plants (http://plants.jstor.org) and the International Plant Names Index (http://www.ipni.org). All morphological characters were studied under dissecting microscopes and are described using the terminology presented by Wang et al. (1990Wang et al. ( , 1998.
Phenology. It flowers and fruits from June to September. Etymology. The specific epithet is derived from the type locality, Pu Hoat Nature Reserve, Nghệ An Province, Vietnam.
Distribution and habitat. This new taxon is an endemic species from Pu Hoat Nature Reserve of Vietnam. The species grows on limestone rocks in tropical monsoon forest with sufficient seasonal run-off water, at an elevation of 390 m a.s.l. It distributes much lower in altitude and the habitat is much hotter and more humid than other species with stems of the genus.
Preliminary conservation assessment. As population information of Didymocarpus puhoatensis is still unclear, it is not appropriate to make an assessment of the extinction risk faced by this new taxon. Thus, the category of Data Deficient (DD) should be appropriate, according to IUCN (2016) criteria. Fortunately, the known habitat of the species is protected as part of a nature reserve. Besides prolonged droughts and illegal logging in the area, there are other potential risks to the persistence of this new species.
Key to the species of Didymocarpus in the Vietnam

Discussion
It is morphologically similar to D. brevicalyx and D. epithemoides in having the calyx consisting of a tube, similar shape and colour of corolla, both morphological affinities being distributed in Thailand. However, D. puhoatensis can be clearly differentiated from both by several characters. The major differences between the species are outlined in Table 1.