Cyphostemmacalcarium, a new species of Vitaceae from the Ankarana Special Reserve, Madagascar

Abstract Cyphostemmacalcarium Rabarij & L.M.Lu, sp. nov., is herein described as a new species found on limestone outcrops in northern Madagascar. Its diagnostic morphological characteristics were compared to the species occurring in the Ankarana Special Reserve. We present detailed descriptions, illustrations, distribution map, and a preliminary conservation assessment of the species. An identification key to all known species of Cyphostemma from the Ankarana Special Reserve is also provided.


Introduction
The genus Cyphostemma (Planch.) Alston contains ca. 200 species, representing the second largest genus following Cissus within the grape family, Vitaceae Rabarijaona et al. 2020). Species of Cyphostemma are distributed mainly in Africa with a few species occurring in southern India, Thailand and southwest China (Dang et al. 2017;Wen et al. 2018). The genus is distinguished by several unique morphological characters: floral buds constricted at the middle, a floral disc of 4-large free glands, conspicuous stipules, and seeds with extra layers of endotestal sclereids covering the ventral infolds in cross-section (Wen 2007;Manchester 2007, 2011).
In Madagascar, Cyphostemma consists of ca. 25 species and displays substantial morphological diversity (Baker 1887;Descoings 1967; The Madagascar Catalogue Project 2019). Species are found in a diversity of habitats, including rainforests, savannas, dry thickets, dunes, and seasonal arid habitats such as the vegetation on limestones or an area within the reserve referred to as "Tsingy". Several of the species of Vitaceae from Madagascar exhibit features that are very unusual in the family, such as succulent shrubs or trees, rather than lianas, and the lack of leaf-opposed tendrils (Hearn et al. 2018). Northern Madagascar possesses the highest species diversity for Vitaceae across the island, with ca. 68% at the family level and ca. 56% for Cyphostemma (The Madagascar Catalogue Project 2019).
Since Cyphostemma species were reported to exhibit distinct morphotypes during their vegetative and flowering stages, we conducted an in-depth morphological investigation of all 25 described Cyphostemma species from Madagascar. Of the eight species of Cyphostemma in the Ankarana Special Reserve (Fig. 1), Cyphostemma ankaranense Desc., C. caerulans Desc., C. rutilans Desc., and the newly described species in this paper, all lack tendrils. However, the new species can be distinguished from the other three species based on several traits such as habit, stipule shape and size, leaf architecture, flower color, style length, and fruit shape (Table 1). We herein describe and illustrate this new species, assess its conservation status, and provide an identification key to all the species found in the Ankarana Special Reserve.

Phenology. Flowering and fruiting around May.
Etymology. The epithet of the species refers to the habitats on limestone outcrops. Distribution and habitat. It grows on limestone outcrops in northern Madagascar at an altitude of 90-300 m. (Fig. 1) Provisional conservation assessment. The new species is endemic to Madagascar with distribution restricted to its type locality. It is assessed here as Critically Endangered (CR) according to the IUCN Categories and Criteria (IUCN 2019). Even though the species occurs within a protected area, succulent plants are still highly sought after by collectors for their horticultural values. Seeds of Cyphostemma calcarium should therefore be collected, banked, and propagated to ensure its longterm conservation.
Taxonomic notes. This species is described from materials collected by Rakotovao C. et al. in 2013. It was initially identified as Cissus pileata Desc., but it clearly belongs to Cyphostemma in having constricted flower buds and floral disks with four free glands. These characters, together with its M-shaped endosperm as viewed in cross sections of the seeds and the presence of extra layers of endotestal sclereids covering the ventral infolds in cross-section, clearly distinguish the new species from Cissus L. A summary of some diagnostic characters that differentiate this new species from other shrubby species of Cyphostemma in Ankarana Special Reserve is provided in Table 1.