Much-branched shrub, the branches stiff, densely sericeous, becoming spinescent at apex in age; stipules none; leaves decussate; petiole very short, eglandular; lamina small, narrowly elliptical or ovate, usually bearing 1 tiny marginal gland on each side near base, often hidden by hairs. Inflorescence terminal or lateral, often terminating a very short lateral shoot, an umbel of (1–) 2–4 flowers, each pedicel subtended by 1 bract and 2 smaller bracteoles, the peduncle none or very short, bracts and bracteoles eglandular. Flowers bilaterally symmetrical in all whorls; sepals abaxially densely tomentose, adaxially glabrous, pressed in against the androecium in anthesis, the anterior sepal eglandular, the 4 lateral sepals biglandular; petals yellow, abaxially ± densely sericeous, adaxially glabrous; stamens 10, glabrous, filaments 1/3–1/2 connate, the filament opposite posterior petal without an anther (always?), the other 9 filaments bearing subequal anthers, the connectives reddish, not exceeding the locules; pollen radially symmetrical, with 5 wide colpi, each colpus diorate, with a pore at each end; receptacle glabrous between androecium and gynoecium; gynoecium comprising 3 carpels completely connate in the ovary; ovary densely hirsute, all 3 locules developed and containing ovules; 2 posterior styles distinct, stout, glabrous, short-hooked at the apex and with large internal stigmas turned toward front of flower; anterior style present but rudimentary, short and slender without an obvious stigma. Fruit a dry, indehiscent, spherical, nutlike structure with a thick fibrous wall, each carpel bearing a crest at apex and a cluster of long vascularized setae with bulbous bases surrounded by white basifixed hairs; seeds probably 1 per fruit, maturing in a posterior locule (?). Chromosome number unknown.
One species, T. usillo, of dry vegetation in western Argentina (Catamarca, Córdoba, La Rioja, Mendoza, San Juan, San Luis, and Tucumán) at elevations of 600–2300 m on the eastern side of the Andes. [map]
Tricomaria is distinctive because of its habit as a small-leaved spinescent shrub, obviously adapted for growing in xeric conditions, and its geographic isolation in western Argentina. Niedenzu (1928) placed it with other genera bearing setae on the fruits, but except for that feature it has little in common with those genera, so it was no surprise to see it moved into a position sister to Dicella (Davis & Anderson, 2010 [pdf]). Those two genera have similar hairy yellow petals and indehiscent nutlike fruits, but the most striking similarity is in their styles, as can be seen by comparing our drawings of the gynoecia in the two genera. Some details of the description given above are open to question; this species would repay thorough study of its flowers and fruits by someone with access to good material from throughout its range of distribution.
Reference: O'Donell & Lourteig, 1943 [pdf].
Etymology: The name Tricomaria comes from the prefix tri-, meaning three- in Greek and Latin, and the word coma (Latin) or kome (Greek), meaning the hair of the head; coma was already used to denote a tuft of hairs on various plants in classical Latin. In this case the name refers to the fact that each of the three carpels of the ovary and fruit bears a tuft of long vascularized bristles. — Some authors have spelled the generic name "Trichomaria," but that is wrong; the original spelling was Tricomaria and that must be used. Moreover, the spelling "Trichomaria" reflects a mistaken belief that the name comes from the Greek word for a hair, trichoma.
Photos: T. usillo
Drawings: T. usillo