Lettowia, a new genus of Vernonieae from East Africa (Asteraceae)

Abstract A new genus, Lettowia H. Rob. & Skvarla is named for the single East African species originally described as Vernonia nyassae Oliv. Its pollen is lophate and triporate, with a perforated tectum restricted to the muri. The new genus is placed near Vernoniastrum in the subtribe Erlangeinae.


Introduction
Since Vernonia Schreb. is a strictly American and mostly eastern North American genus (Robinson 1999a, b), a continuing eff ort is under way to properly reassign the species of the Eastern Hemisphere that have been erroneously placed in that genus. In previous eff orts to resolve the species of Vernonieae from Africa, one species, Vernonia nyassae Oliv., was examined but left untreated. In a careful analysis of East African Vernonieae, Jeff rey (1988) keyed V. nyassae among species that are now mostly placed in the subtribe Erlangeinae, and the species was listed among those now mostly placed in the genus Vernoniastrum H. Rob. Robinson (1999a) studied but did not treat the spe-cies; however, its possible relation to Vernoniastrum had been considered. Th e failure to include the species in that genus was because of the lack of the distinctive bands of idioblasts in the achene walls, and the presence of an infl orescence of single elongate unbranched erect scapes each bearing a single capitulum. Vernoniastrum has branched infl orescenses with cauline leaves, and has bands of idioblasts in the achene wall that are visible with a hand-lens. For these reasons, the species is considered here to be related to Vernoniastrum but placed outside of that, hence in need of recognition as a new genus.
Th e present review includes a full description of the plant, including SEM studies of pollen, and a formal description of the new genus.

Preparation of pollen
Pollen from dried buds was obtained from herbarium sheets at the U.S. National Herbarium in Washington, D.C. Detailed light microscope analyses under a Wild light microscope using oil and immersion optics were made of pollen in the dry condition as well as after immersion in Hoyer's solution (Anderson 1954). Scanning electron microscope (SEM) observations were made after acetolysis treatment (Erdtman 1960). Lettowia nyassae pollen was examined with a Hitachi S-570 SEM (at the United States National Museum of Natural History) after coating with gold/palladium. Vernoniastrum nestor pollen was examined with a Zeiss Neon 40 EsB dual beam SEM/FIB after metal coating treatments with osmium thiocarbohydrazide and gold/palladium (Chissoe et al. 1995(Chissoe et al. , 1996. Images were digitally processed and the fi nal plates prepared using Adobe Photoshop 7. Description. Erect or decumbent perennial herbs from creeping rhizome or perennial root crown, all becoming erect, rosettiform, with erect pedunculate scapiform infl orescence; hirsute or pilose with long white hairs, hairs of stems, leaves and peduncles with cells uniseriate and with few short basal cells and long, acicular, rather stiff apical cell. Leaves alternate in loose basal rosette, mostly 2-4 cm long, petiole narrow, blade obovate, to 9 cm long and 1.5 cm wide, apically obtuse, cuneate into petiole at base, margins entire, slightly paler abaxially, pilose on margins and both surfaces, more densely pilose abaxially, few glandular dots adaxially, numerous dots abaxially. Scape mostly 5-11 cm long, densely and stiffl y hirsute, sometimes with small bract near middle, bearing 1 terminal head. Heads broadly campanulate, up to 2 cm high, 1.5-1.8 cm broad. Involucre with ca. 15-20 persistent, mostly subequal ovate-lanceolate bracts in ca. 2 series, up to ca. 1.5 cm long, apices acute, without acumination, densely pilose outside with long simple hairs, with 3 longitudinal veins, margins narrowly scarious, sometimes reddish. Receptacle epaleaceous; fl orets ca. 40 in a head; corollas lavender, ca. 12 mm long, narrowly funnelform distally from slender basal tube, tube ca. 7 mm long, with stalked narrowly capitate glands outside, throat ca. 0.7 mm long, lobes linear, ca. 4.3 mm long, with glandular dots outside and numerous stiff uniseriate hairs distally; anther thecae ca. 3 mm long, with tapering bases, with short clavate tails, apical appendage oblong, glabrous, with tenuous cell walls; style base with narrow annular node; with acicular sweeping hairs restricted almost completely to branches; achenes weakly 5-costate, densely sericeous on and between ribs with long setulae, setulae slightly split at tips, glandular dots present near base, without evident idioblasts, raphids linear; carpopodium narrowly annuliform, with small quadrate cells; pappus of ca. 40 persistent barbellate bristles ca. 8 mm long, mostly of even width, tapering at extreme tips, with outer series of short narrowly lanceolate squamae. Chromosome number not known. Chemistry not known. Pollen grains of Lettowia nyassae ( Fig. 2A, B) ca. 55 μm in diam in fl uid, ca. 45 μm dry, echinolophate, triporate, muri shortly echinate with 2-3 spicules on each mural segment, perforated tectum restricted to the muri. Th e lacunae are irregular in position and rather irregular in shape. Th e baculae are elongate, in a mostly single partially unaligned series under each murus, and they are fi rmly attached to the footlayer. In these features, the pollen is nearly like that of Vernoniastrum ( Fig. 2A, B), and the relationship to that genus in the subtribe Erlangeinae is assumed. In direct comparison with the single species of Vernoniastrum for which SEM study of the pollen is available (Fig. 2C-F), a number of subtle diff erences can be seen. In Lettowia , the lacunae are larger in general and less numerous (ca. 25-30 in Lettowia versus 35-40 in Vernoniastrum ), the spicules on the muri are shorter and less numerous, and the baculae under the muri are mostly in a single row. In both genera there are lacunae that are elongate as if two lacunae are joined, but these do not seem to be aligned in positions where colpi might be located.
Etymology. Th e genus is named for Colonel Paul von Lettow-Vorbeck, 1870-1964(Gunther 1956Hoyt 1981;Lettow-Vorbeck 1920), who led the defense of German East Africa in WWI until the surrender of Germany, and was remembered fondly by the people of the former German colony on a return visit after WWII.
Th e genus contains the following single species. Distribution. Th e species occurs primarily in the area of southwestern Tanzania north of lake Nyassa, and also in eastern Zambia, Ndola, Oct 1906, C.F.E. Allen 363