Maguireothamnus is a genus with two species of low shrubs found in cloud forest scrub vegetation in the tepuis of the Guayana Highlands, in northeastern South America. These plants are characterized by their branched shrub habit; tissues without raphides; interpetiolar triangular stipules that are persistent; rather small, leathery, subsessile leaves; axillary inflorescences composed of a single pedunculate flower in each axil at a node below the stem apex; homostylous 5-merous flowers; deeply lobed calyx limbs; well developed salverform white corollas with a long slender tube and convolute lobes; and rather small woody ellipsoid capsules that are loculicidally dehiscent and contain numerous small angled seeds. The flowers are borne on axillary peduncles, or short-shoots, with a pair of well devleloped, often foliaceous bracts at the top, and then an articulated structure bears the flower and is here considered a pedicel in a reduced inflorescence. The bracts vary from similar to the vegetative leaves, to triangular and fused at the based in a structure similar to the calyculus of Coffea. The flowers have a form that is classically associated with hawkmoth pollination, and are reported to have a fragrance similar to that of gardenias (as in many white-flowered Rubiaceae). The calyx lobes are elliptic and relatively large, and the corolla lobes are acute to often somewhat prolonged and acuminate. 

Two species of Maguireothamnus have been recognized, and separated by details of pubescence. These are recognized here also, but distinguished somewhat differently here than previously (e.g., Delprete & Steyermark in Taylor et al., 2004). Delprete & Steyermark separated these species based only on presence vs. absence of pubescence on the ovary, hyanthium, and capsules. Plants with well developed pubescence have indument also on the stems and stipules, and have been included in Maguireothamnus tatei. A few plants from Ptari-tepui have sprase pubescence only on the stipule apices, and are here included in Maguireothamnus speciosus (e.g., Holst 3596, MO). Maguireothamnus has a generally reduced morphology, and varies in both species in leaf size and form and corolla size so additional characters to separate the species are subtle at best. However, these species differ to some extent in leaf shape and size, with leaves of Maguireothamnus tatei usually narrower and more acute apically. The specimens seen also differ between the species in the presence of a yellowed, filamentous fungus or lichen on the leaves, stems, and flowers: this is usually present and extenstively developed on Maguireothamnus speciosus but absent on plants of Maguireothamnus tatei. These species are also apparently allopatric. Several infraspeciric taxa of each of the species have been named; Delprete & Steyermark recognized two subspecies of Maguireothamnus speciosus, and none within Maguireothamnus tatei. 

The relationships of Maguireothamnus were considered for some time to be in the tribe Rondeletieae, but the tribe at that time was broadly circumcribed and deeply heterogenous. The genera of the Rondeletieae are now distributed into various smaller tribes, and Delprete & Cortés-Ballén (2004) found Maguireothamnus within thieir Sipaneeae. Here it was retrieved in several of their analyses as sister to Limnosipanea, which is very different morphologically. 

Maguireothamnus was included at one time within Chalepophyllum, which is found in the same region and has a similar habit and also has capsular fruits. However Chalepophyllum can be separated by its much smaller corollas with dense pubescence in the throat and its smaller capsules with septicidal dehiscence. Maguireothmanus is also similar to Hillia, which can be separated by its ligulate stipules that are erect and pressed together in bud and its septicidal capsules with flattened seeds with a tuft of long tichomes on one end. Maguireothamnus is similar also to Holstianthus, with funnelform red corollas.  

Author: C.M. Taylor.
The content of this web page was last revised on 14 April 2021.
Taylor web page: http://www.mobot.org/MOBOT/Research/curators/taylor.shtml

• Taylor, C. M., J. A. Steyermark, P. G. Delprete, A. Vincentini, R. Cortés-Ballén, D. C. Zappi, C. H. Persson, C. B. Costa & E. Anunciação. 2004. Rubiaceae. 8: 497–847. In J. A. Steyermark, P. E. Berry & B. K. Holst (eds.) Fl. Venez. Guayana. Missouri Botanical Garden Press, St. Louis.
• Steyermark, J. A. 1974. Rubiaceae. 9(1): 7–593. In T. Lasser (ed.) Fl. Venezuela. Fondo Editorial Acta Científica Venezolana, Caracas.
• Delprete, P. G. & R. Cortés-Ballén. 2004. A phylogenetic study of the tribe Sipaneeae (Rubiaceae, Ixoroideae), using trnL-F and ITS sequence data. Taxon 53(2): 347–356.