The pantropical genus Cissampelos L., together with African Antizoma Miers and the mostly Asian Cyclea Arn. ex Wight, were placed in subtribe Cissampelinae, one of the three subtribes in tribe Menispermeae, which together with seven other tribes were recognized by Diels (1910) in the family Menispermaceae. In studies based on ndhF, matK, trnL-F, ITS, rbcL, and atpB sequence data, subtribe Cissampelinae has been consistently recovered as monophyletic (Ortiz et al. 2007, Wang et al. 2007, Hoot et al. 2009, Jacques et al. 2011, Wang et al. 2012), although relationships within the subtribe remain unresolved (Jacques et al. 2011). The sampled species of Cissampelos form a clade that is sister to Cyclea (Ortiz et al. 2007, Wang et al. 2012, Ortiz et al. in prep.), but studies including the southern African Cissampelos capensis L. f. recovered a polyphyletic Cissampelos, with Cyclea (Hoot et al. 2009, Jacques et al. 2011), and Antizoma (Jacques et al. 2011) nested within. While Cissampelos capensis is recognized in Cissampelos (Botha 1980), at times it has been placed in Antizoma (Diels 1910), which is characterized by a shrubby habit, a spur borne along the stem [abaxial to the petiole], pistillate flowers with two sepals on opposite sides of the carpel, and two antesepalous petals (Miers 1851). The most common perianth condition in the pistillate flowers of Cissampelos is one sepal and one antesepalous petal, occasionally, more than one sepal and petal are found in some of the species (Diels 1910). Based on the fact that some species of Cissampelos display variation in sepal and petal number, the shrubby Cissampelos capensis was removed from Antizoma and included in the genus Cissampelos (Botha 1980). However, commonly overlooked is that while the pistillate flowers of Cissampelos may show variation in the number of sepals and petals, these are consistently located facing the adaxial slit of the carpel (Fig. 1A). On the other hand, the sepals and the antesepalous petals of Antizoma are usually located at both sides of the adaxial slit of the carpel (i.e., on the lateral sides of the carpel) (Fig. 1B), or around the carpel when more than two sepals/petals are present. The pistillate flowers of Cissampelos capensis have not only two sepals and two antesepalous petals but these are located in a similar manner as in Antizoma angustifolia Miers ex Harv. and in Antizoma miersiana Harv.

At present, the taxonomic status and phylogenetic affinities of Cissampelos capensis are unclear, similarly, the generic boundaries and relationships between Cissampelos, Antizoma, and Cyclea remain unresolved, and a re-assessment of all the genera in subtribe Cissampelinae is warranted, but is beyond the scope of this paper. Until further studies clarify the taxonomy and affinities of Cissampelos capensis, here it is still provisionally referred to as Cissampelos and we compare it to the new species Cissampelos arenicola as they both share superficial morphological resemblance in habit and foliage.

The last comprehensive revision of the genus Cissampelos recognized 20 species, nine of them distributed in the Americas, nine in Africa, one in Asia, and the pantropical Cissampelos pareira L. (Rhodes 1975). Species of Cissampelos are often found growing in open, disturbed habitats, and with a few exceptions, are morphologically variable, such that species limits are frequently difficult to discern. As new collections have accumulated since Rhodes’s (1975) revision, it has become evident that a number of collections from similar habitats in Bolivia and adjacent Paraguay, are conspecific but differ from all other described American species of Menispermaceae. Still, placing this material generically has been challenging. Because of the general similarity in leaf shape with the North American Cocculus carolinus (L.) DC., the specimens were initially identified as belonging to Cocculus DC. However, the Old World-centered Cocculus (Diels 1910, Troupin 1962), has only a few species in temperate and subtropical North America and Mexico, but none in South America. Examination of the minute staminate flowers of these Bolivian and Paraguayan plants allowed for the specimens to be confidently placed in Cissampelos, and the new species is described here as Cissampelos arenicola.

We studied all 13 herbarium collections of the new species housed at MO and at NY. Most of the collections have duplicates in several other herbaria but these were not available during this study. The specimens studied include male, female, and fruiting individuals, an infrequent situation in dioecious plants, such as Menispermaceae. Additionally, for comparison, we examined specimens for two other species that share some morphological similarities with the new species. These included: the Paleotropical Cissampelos mucronata A. Rich. (five specimens) and the southern African Cissampelos capensis (seven specimens). Measurements given in the species description refer to ranges of the mean values stemming from two to three replicate measurements of each structure and organ per individual voucher specimen. Before measuring, floral parts were first rehydrated, and endocarps were first boiled in water for a few minutes and the fleshy part of the fruit removed. Menispermaceae fruits develop asymmetrically so that the long axis do not necessary correspond with fruit length; endocarp measurement follow the convention of Ortiz (2012).

Although, the phylogenetic relationships of Cissampelos and the other genera in the Cissampelinae are still unresolved, Cissampelos is the oldest name in the clade and will therefore stand any future generic reassessment. While Cissampelos arenicola has not yet been included in any DNA based studies and as a result its phylogenetic affinities are not known, and despite the fact that the sepals and petals are variable in number, they are consistently located on the adaxial side of the carpel, a feature that is shared by all other studied species of Cissampelos with exception of Cissampelos capensis. Hence, we are confident that our new species belongs to Cissampelos s. str.

The genus Cissampelos is characterized by its dioecy, as are nearly all Menispermaceae. However, rare instances of monoecy have been reported by Miers (1871), and we observed monoecy in two collections (Nee 48487; 51401) both from Bolivia, in which a few pistillate flowers and immature fruits were observed in the same inflorescence that is predominantly staminate. Petals of pistillate flowers in this inflorescence resemble these of staminate flowers in being cupuliform. There are reports of sex switching in Menispermaceae such as Asian Tinospora cordifolia (Willd.) Miers, where complete switching in a plant from staminate to hermaphrodite and back to staminate flowers have been observed (Geetha et al. 2007). Only plants with staminate flowers exhibit this labile sex expression, while plants with pistillate flowers show consistency in their expression (Geetha et al. 2007, Malpotra et al. 2009). The report of monoecy in the American species Disciphania spadicea Barneby is based on observations of a single collection from Jalisco (Mexico) (Carrillo-Reyes et al. 2013) which had both staminate and pistillate inflorescences on the same plant. Our case is similar to that of Tinospora where inflorescences with mostly staminate flowers may have a few pistillate flowers.

Anther cells in Cissampelos arenicola vary from 6–10 (Table 1), with about 45% of the sampled 24 staminate flowers having an 8-locular synandrium. This condition is also rarely observed in Cissampelos grandifolia Triana & Planch., but is unknown in the remaining American species of Cissampelos recognized by Rhodes (1975), which all have a 4-, rarely 6-locular synandrium. Similar 8-locular synandria are most commonly found in the vining Cissampelos mucronata A. Rich. from tropical Africa. Cissampelos mucronata differs from Cissampelos arenicola, however, in several characters, most obviously, larger leaves that are densely light golden hispidulous-tomentose on both surfaces. Vegetatively, Cissampelos arenicola superficially resembles the southern African Cissampelos capensis, from which it can be distinguished by the staminate flowers with a 6–10-locular (vs. 4-locular) synandrium, and pistillate flowers having sepals and petals located at the adaxial side of the carpel only. The larger and scarcely ornamented endocarp of Cissampelos arenicola (Fig. 3A) is readily distinguished from both, Cissampelos capensis (4.2 × 5.5 mm), and Cissampelos mucronata (Fig. 3B–C), (4.3 × 4.7 mm), respectively (Table 1).