Macrocnemum includes perhaps 5 species of medium-sized to large trees (to 40 m tall) found in wet forests of the Neotropics. Macrocnemum has opposite petiolate leaves, well developed interpetiolar elliptic to obovate stipules, axillary inflorescences, homostylous protandrous 5-merous flowers, medium--sized tubular pink corollas with the lobes valvate-reduplicate in bud, bilocular inferior ovaries, and distinctive longitudinally loculicidal capsules with numerous winged seeds. The bark is smooth and peels in thin layers. The branching is characteristically sylleptic, with new stems originating from one axil of a lower node on developed stems, and starting their growth with a long curved first internode. The stipules are distinctive: these are flat and held erect and pressed together, and are rounded at their tips. The stipules are often if not usually fused at first in their lower portions. The leaves are characteristically wider above the middle, though their shape varies. The inflorescences were considered terminal and sometimes also axillary by Andersson (1994, 1995, and cited by subsequent authors), however more detailed review now shows that the inflorescences are consistently axillary with an arrangement similar to that of Chimarrhis. These inflorescences are borne in pairs in the uppermost axils of a stem, and the stem terminates in a vegetative bud that is sometimes small and easily overlooked but is present on all the stems seen. The flowers of Macrocnemum appparently mostly open at the same time, and are often described as sweetly fragrant and showy. The corolla lobes have a distinctive arrangement in bud, with the fleshy central portions of the lobes triangular in cross-section and meeting in a valvate arrangement while their marginal wings extend outward. These marginal wings are at least as wide as the central portion of the lobe, and are held flat and pressed together in bud so that the top of the corolla bud has a strongly winged or pleated form. The capsules of Macrocnemum are also distinctive in their dehiscence along the length of each side, with the valves remaining connected to each other and that entire dehisced fruit attached to the infructescences.

Macrocnemum has not been studied as a whole; an outline of the species that are currently recognized is presented here but is mainly based on a summary of the current literature, not a comprehensive review. Macrocnemum roseum is by far the most widespread and commonly collected species, and is morphologically a little variable; the other species of Macrocnemum are distinguished by usually one unusual feature, and have rather local ranges that are sympatric with Macrocnemum roseum. The broadest and most recent review of species of Macrocnemum was presented by Andersson (1994), who studied the Ecuadorian plants and first demonstrated the broad circumscription of Macrocnemum roseum. In a molecular systematic study, Kainulainen et al. (2010); as Condamineeae found Calycophyllum to belong to the Tribe Dialypetalantheae, and related generally to Rustia and Pentagonia.

Macrocnemum is widespread and rather commonly encountered, but as a genus not well known and it is often overlooked, especially when not in flower. It is similar in particular to species of Cinchona and some of its related genera of Tribe Cinchoneae, which also have capsular fruits and rounded interpetiolar stipules that are held erect and flattened together in bud, however those genera have distylous flowers, flatly valvate corolla lobes, and septicidal capsules that open from the top or the bottom with the valves then separating. Similar thin-barked smooth trunks are found in Capirona and Calycophyllum megistocaulum.

Author: C.M. Taylor
The content of this web page was last revised on 8 December 2022.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

• Andersson, L. 1995. Tribes and genera of the Cinchoneae complex (Rubiaceae). Ann. Missouri Bot. Gard. 82(3): 409–427.
• Kainulainen, K., C. H. Persson, T. Eriksson & B. Bremer. 2010. Molecular systematics and morphological character evolution of the Condamineeae (Rubiaceae). Amer. J. Bot. 97(12): 1961–1981.
• Andersson, L. 1994. Tribe 1. Cinchoneae. 50: 1–82. In G. W. Harling & L. Andersson (eds.) Fl. Ecuador. University of Göteborg, Göteborg.