Solenandra in its current, expanded circumscription (Paudyal et al., 2018) includes 15 species of shrubs and small trees. These are characterized by a normally branched unarmed habit with chartaceous leaves; terminal inflorescences; generally 5-merous flowers; showy, salverform to narrowly funnelform, white, sometimes rather large corollas; rather slender exserted anthers; subcapitate stigmas exserted among the anther' septicidally capsular, rather small fruits; and numerous flattened, circular, marginally winged seeds. The flowers are at least sometimes fragrant, and variously diurnal or nocturnal. The plants contain bitter compounds, and are locally called "quina" and used as a remedy for fevers. Solenandra includes a relatively large number of species with morphologically varying forms in the Antilles, especially Cuba, similarly to some other Antillean genera that inhabit dry vegetation on limestone and serpentine (e.g., Scolosanthus, Schmidtottia, Phialanathus). Solenandra mexicanum is by ar the most widepread and commonly collected species, but it differs from the rest of the genus in its distribution in Mexico and Central America. In the Antilles, Exostema parviflorum (= Solenandra comb. ined.) is the most widespread and common, and Solenandra ixoroides the most morphologically variable.
The history of the circumscription and systematics of Solenandra is inextricable from the unusually complex history of Exostema. Exostema is a relatively old genus that is similar to Cinchona, so many of its species were originally described in that genus. Soon after its description Exostema became circumscribed rather broadly, to include what are now separated as several distinct genera (Paudyal et al., 2018) including Solenandra. Our current taxonomy separates most of the species of traditional Exostema into two genera, Exostema and Solenandra, with most of them in Solenandra.These two groups are supported by morphological and molecular characters, but their nomenclature has been somewhat confued. See the web page for Exostema for more details.
The taxonomy of Exostema and Solenandra species has has varied from a more traditional view of rather broadly circumscribed, geographically coherent species based on extenstive field work (e.g., McDowell, 1995), to a more detailed view of numerous infraspecific taxa diagnosed by vegetative details that sometimes are variable (Borhidi, 1989; Borhidi et al., 2017, 2018; the overlapping variation in diagnostic features was noted by these authors themselves). Additionally, older taxonomies of these plants often differed markedly in species circumscription from each other (e.g., Grisebach's taxonomic scheme's limitations were noted by Urban at various times) as well as from our modern view. Reconciliation of taxonomies among authors is further complicated for some modern taxonomic treatments by confused nomenclatural citations, with formally published names cited together with names that are "sensu" literature references. In particular the treatments by Borhidi (1989) and Borhidi et al. (2018), contain such confusions that have led to inaccurate citations, and to some intended new combinations that were inaccurately based on names that were not actually published (e.g., "Exostema valenzuelae A. Rich.", which was not accepted in its original publication and so is invalid; "Exostema parviflorum A. Rich.", which was not published in this work and an explicit reference there to Bonpland's name). The species taxonomy here follows McDowell (1995). Paudyal et al. (2018) appear to also have followed his taxonomy, and to have made nomenclatural transfers for species he recognized that Borhidi did not treat.
Solenandra is similar to Exostema, which has axillary inflorescences. Solenandra is similar vegetatively to Coutarea and these are often sympatric; Coutarea differs in its terminal inflorescences, funnelform pink corollas with the lobes much shorter than the tube, and larger capsules that are markedly flattened laterally.
McDowell (1995, 1996; McDowell & Bremer, 1998; McDowell et al., 2003) studied the systematics of Exostema in its traditional circumscription in excellent detail, including morphological and molecular analysis and extensive field work. Relatively early in the use of these techniques, McDowell & Bremer (1998) studied this group with cladistic methods and a combined morphological and molecular data. They found three well supported clades and the two odd South American species, Exostema maynense and Exostema corymbosum, basal to the rest of Exostema; they included one outgroup, Coutarea hexandra. McDowell & Bremer regarded their three Exostema clades taxonomically as sections, Sect. Exostema, Sect. Pitonia, and Sect. Brachyantha, and documented repeated ecological shifts within each groups, in contrast to a single characteristic pollination mode in each. McDowell et al. (2003) studied this group again with additional molecular sequence data and outgroups but more limited species sampling, and found a more complex result, with Exostema paraphyletic with respect to several other genera, similarly to the results Paudyal et al. (2018), and the three sectional clades no longer clearly supported. They considered their results preliminary in terms of making taxonomic changes due to limited their limited sampling and sequence data.
Based on the contemporaneous, unpublished molecular results of Johan Rova in his 1999 dissertation together with McDowell & Bremer's (1998) characterizations of their three Exostema clades, Borhidi (2002) separated Solenandra, which had long been synonymized with Exostema. Borhidi circumscribed Solenandra to comprise two of McDowell & Bremer's (1998) sections, and distinguished these two genera by various morphological features (2002: Table 1) including inflorescence position, flower size and biology (nocturnal and fragrant in Exostema vs. supposedly diurnal and lacking odor in Solenandra), ovules erect in Exostema vs. pendulous in Solenandra, stigma held above the stamens in Exostema vs. not above them in Solenandra, different pollen sizes, different capsule sizes, and some other features. Borhidi (2010) separated Solenandra again in his Rubiaceae treatment for Mexico, but later again synonymized it, without comment, with Exostema in his Rubiaceae treatment for Cuba (Borhidi, 2017). Shortly after that, Borhidi et al. (2018) separated Solenandra and Exostema again in Cuba based on Paudyal et al.'s (2018) results, and explained there that Solenandra had been separated by them eaerlier based on Borhidi (2002) and the molecular data of Manns & Bremer (2010), which also found Solenandra distinct from Exostema, but had then been synonymized following its synonymization by Lorence et al. (2012). Borhidi (2002) effectively transferred some but not all the species of McDowell's two Exostema sections to Solenandra, and Borhidi (1989) only addressed names for Cuban species' most of the remaining species were transferred to Solenandra by Paudyal et al. (2018), and Exostema parviflorum still awaits a valid name in Solenandra. [Note: The treatment of Lorence et al. was published in 2012, but preparation of the flora volume was a complex task that took significant time, and that Rubiaceae treatment was finalized in 2009-2010 before the publication of Manns & Bremer's analysis. - CMT]
Nomenclatural Notes. The circumscription and identity of Candolle's Sect.Pitonia were emended by McDowell (1996), and this has created some later confusion. Borhidi et al. (2018: 303) noted accurately that Candolle originally included the type of Exostema in this section, but McDowell (1996: 293) explicitly changed the circumscription of this section to exclude several of its species and was the first to lectotypify it. McDowell excluded from Sect. Pitonia the nomenclatural type of the genus, Exostema caribaeum, so it no longer corresponds to the typical species. and his lectotype, Exostema sanctae-luciae, is also the type of Borhidi's Sect. Floribundae so those are synonymous. Further confusion has arosen from Borhidi et al.'s (2018) statement that Candolle's Sect. Pitonia, described in 1830, included the type of Solenandra, but that is not accurate because Solenandra and its type species were not described until 1873. Another persistent nomenclatural confusion in this group has been the name Exostema valenzuelae, which is an illegitimate name that has been incorrectly accepted by various authors; this name corresponds to Solenandra ixoroides, for more details see the dicussion on the web pages for that species.
Author: C.M. Taylor
The content of this web page was last revised on 15 September 2021.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml