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We present the generic description below, using the terminology of Jacobs & al. (1995). This is followed by a listing of species in the genus, with new combinations presented in bold face for three taxa currently included in Stipa L. that exhibit the characteristics of Nassella. Table 1 summarizes the distribution of each species.

Generic Description.─Plants perennial, cespitose, occasionally rhizomatous; basal branching intra- or extravaginal. Culms branching above the base in some species. Sheaths open; auricles absent; ligules membranous, sometimes pubescent or ciliate. Inflorescence an open or closed panicle, sometimes partially included at maturity. Spikelets with 1 floret and no rachilla extension. Glumes longer than the anthoecium, with (1)3-5 veins, usually long-tapering to an acuminate tip, frequently purplish at the base before drying out; callus well developed, short and blunt to long and sharp, glabrous or antrorsely strigose; lemma strongly convolute, coriaceous, with a crown at the apex, this varying from inconspicuous to conspicuous; pappus absent; awn centric or eccentric, scabrous, caducous or persistent, varying from short to long, from non-geniculate and only slightly twisted to twice-geniculate with a strongly twisted column, the column usually scabrous; palea up to 1/3(1/2) the length of the lemma, almost always glabrous and without veins; lodicules 2 or 3, if 3, the third somewhat shorter than the other two; style branches 2, not united at the base; anthers 1 or 3, when 3, often of two different lengths. Figs. 1, 2.

The morphological features that best distinguish Nassella from other species of the Stipeae are the combination of its strongly convolute lemma and short, glabrous palea. They are also the most useful field characters. The lemma margins overlap about 25-50%, the outer margin of the lemma usually lying over the lateral vein of the inner half of the lemma, sometimes extending to the midvein. This extended overlap, combined with the coriaceous texture of the lemma, makes it very difficult to open the floret to observe the palea or caryopsis. It is, however, possible to pinch off the callus with one’s fingernails or strong tweezers in such a way that the palea and flower remain attached to it and available for observation in the field. Other characters supporting the new generic boundary are those italicized in the above description, plus the lemma epidermal pattern (Thomasson 1976, 1978, 1979; Barkworth 1990; Torres 1997), and ITS sequence data (Jacobs & al. 2000).

The only other stipoid genus in which some species have strongly convolute, coriaceous lemmas is the Australian endemic Austrostipa S.W.l. Jacobs & J. Everett. They do not, however, have the reduced, glabrous, unveined palea that occurs in almost all species of Nassella, nor the distinctive lemma epidermal pattern (Thomasson 1976, 1978, 1979; Barkworth & Everett 1987), nor similar ITS sequences (Jacobs et al. 2000).

The geographic range of Nassella extends from southern South America to the southern portion of Yukon Territory, Canada, and the southwestern portion of the MacKenzie District, Northwest Territories, Canada. Its altitudinal range is from 0-5,000 m. It is particularly well represented in two regions: the Central Andean region that includes the great plateau or altiplano of Peru, Bolivia, north-central Chile, and northwestern Argentina and the plains of the Pampean Steppe that extend from east-central Argentina to southern Brazil and throughout Uruguay.

The ecological range of Nassella is considerable. Most species grow on plains, hill slopes, mountains, and tablelands or in valleys. They tend to occur in dry, open, sunny places in arid to semi-arid conditions. The genus is, however, also found in the humid grasslands of the southern part of Rio Grande do Sul, Brazil, Uruguay, and east-central Argentina. Several species of Nassella grow in the sub-humid and semi-arid grasslands of the province of Buenos Aires, Argentina. Others live in deserts or semi-deserts as components of grass-steppes or shrub-steppes, e.g., in the provinces of Patagonia and Puna, Argentina. Some taxa are present in xeromorphic and open woodlands such as the “Caldenia” woodlands of San Luis and La Pampa provinces of Argentina. A few species grow in the High Andean Zone, over 4,200 m above sea level, where they grow between rocks, protected from the wind, and only rarely in alpine meadows. No species of Nassella grow in tundra or wetland habitats.

Species list.─The list below includes all the named taxa of Nassella of which we are aware. If either of us has examined type material for a taxon, the nature and location of the type is given. In some cases, we have not been able to determine the nature of the type or even, in a few cases, positively confirm that the specimen examined is type material. We include the information that we have in order to assist other taxonomists. In general, types of South American species were examined by Torres and those of North American species by Barkworth. The species list, supplemented by bibliographic information for the various names and images of representative species, can be found at http://herbarium.usu.edu/stipeae/Nassella.htm. We have seen specimens that appear to represent new, unnamed taxa from western South America, but describing them is beyond our scope for the foreseeable future. We leave it to those with greater field knowledge of the genus in the countries concerned.