Didymaea is unusual morphologically in Rubiaceae in its habit as soft herbaceous scramblers with leaves that have subpalmate venation and juicy fruits. This genus has a rather localized range, compared to most Rubiaceae, in cool humid mountain areas from central Mexico to northern Panama. Didymaea is also characterized by opposite leaves, small, interpetiolar, bilobed stipules, axillary and terminal, shortly cymose inflorescences, the calyx limb reduced, the corollas small, white to purple, and four-merous, and black, drupaceous, didymous fruits. The inflorescences appear to be indeterminate, with leaves or perhaps foliaceous bracts borne at some of the branching points. The plants are characteristically variable morphologically, within a region and among plants of a population, possibly because these species are weedy and their growth is sensitive to microsite factors. 

The most widespread and commonly found species is Didymaea mexicana. Lorence (2009) noted that this species was long confused with Didymaea alsinoides, and he clarified the types and identities of both of these. The treatment of this genus here relies heavily on the Flora Mesoamericana treatment by Lorence (in Lorence et al., 2012), which presented a significantly different taxonomy from previous authors. In particular there Didymaea mexicana was cirumscribed more broadly, to include more variation. Lorence noted (2009: 74) that the taxonomic distinctions commonly used in Didymaea included "stem length, habit (e.g., climbing versus procumbent), degree and type of pubescence, leaf texture, number of secondary leaf veins, degree of inflorescence branching, flower number, and the ratio of corolla lobe to corolla tube length.... However, when a large number of collections are examined, these characters seem to vary independently or intergrade, making it difficult to delimit species". Lorence used some leaf features along with specific characters of stipule and fruit form and size to separate Didymaea species, and recognized (1999) seven species in the genus, but later (2012) synonymized four of them with Didymaea mexicana, leaving three recognized species in the genus. Because of the Standley's confusion over the identity of the name Didymaea alsinoides, his varieities of Didymaea alsinoides apply to the plants Lorence included in Didymaea mexicana. 

In addition to its local variation in vegetative form, Didymaea is an herb with annual stems and overall is relatively reduced in morphology so it thus presents relatively few taxonomic characters. Pacheco-Trejo et al. (2009) studied leaf anatomy and architecture of this genus, and distinguished eight species based on leaf form and size, arrangement of the secondary and higher-order venation, details of the bundle sheath, and trichome distirbution and cell number. Lorence (2012) noted that Pacheco-Trejo et al. found distinctions among these species, but found their anatomical distinctions incomplete without ecological, geographic, and overall morphological study and looked forward to the publication of such a work, which has not appeared yet. Subsequently Borhidi & Martínez (2012) described additonal Didymaea species, which have not been compared with previous treatments here because no material is available; the isotypes at MO that are cited in the protologues have not been located. 

Didymaea is similar to Galium, which has verticillate leaves that apparently lack stipules (these are foliaceous). Didymaea is also similar to species of Caryophyllaceae, which generally lack stipules and have a developed calyx, free petals, and a superior ovary. 

Author: C.M. Taylor.
The content of this web page was last revised on 7 May 2020.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

• Lorence, D. H. 2009. Lectotypification of Didymaea mexicana Hook. f. (Rubiaceae, Rubieae) and the identity of D. alsinoides (Schltdl. & Cham.) Standl. Acta Bot. Mex. 88: 73–79.
• Pacheco-Trejo, J., T. Terrazas & H. Ochoterena-Booth. 2009. Leaf architecture of the genus Didymaea Hook. f. (Rubiaceae). Pl. Syst. Evol. 281: 137–149.
• Borhidi, A. L. & E. M. Martínez Salas. 2012. Estudios sobre Rubiáceas méxicanas XXXIX. Dos especies nuevas de Didymaea Hook.f., y una especies nuevas de Mitracarpus Zucc. ex Schult. et Schult.f. in Roem. et Schult. (Spermacoceae). Acta Bot. Hung. 54(1–2): 73–79.