Of the three generally recognized sections of Xyris, the smallest, sect. Pomatoxyris, confined to Australia and probably of no more than 20 species, is characterized by a basally 3-locular capsule.
The sect. Xyris, consisting perhaps of a hundred or more species, is characterized by a unilocular capsule and parietal placentation and is predominantly North American and pantropic. In South American Guayana we find only three species of the euxyrids: A', caroliniana, which is highly variable, plastic, and widespread; X. fallax, often confused with the above, well represented in Guayana, in coastal Guiana, and south to Mato Grosso in Brazil; X. erythema, endemic to Guayana and known at this time only from the Pakaraima Mountains, British Guiana.
It is the sect. Nematopus, with unilocular capsules and basal placentation, that provides the great majority of the neotropical taxa, possibly exceeding 160 species. A brief consideration of the distribution and geographic implications of these species is instructive. For convenience of reference we have placed in tabular form, under four categories (high-altitude Guayana endemic species, lowaltitude Guayana endemic species, species of Guayana-Brasilia distribution, and widespread species) the names of the 72 species of Xyris now known to occur in Guayana. In view of the world and particularly the widespread Western Hemisphere distribution of the genus, the degree of endemism achieved in Guayana is most remarkable. Of the 72 Guayana species, 60 or approximately 85 per cent are endemic.
As now recorded 34 species (one, X. erythema belonging to the sect. Xyris, 33 to the sect. Nematopus) are restricted to the higher (above 600 m ) areas of the Roraima sandstone sediments. A somewhat smaller grouping of 26 (all belonging to the sect. Nematopus), invariably occurring on sandy, usually moist, soil of savannas or sandbanks, is found at lower altitudes below 500 meters. Of these, X. involucrata often occurs in savannas up to 1200 meters altitude, and has, not surprisingly, been collected on similar habitats along the lower Rio Negro; X. uleana, while also predominantly of lower altitudes, has a similar geographic distribution.
Six species, all of low altitude except X. hymenachne (which in Guayana is scantily collected, and is confined to high altitude), have a common Guayana Brazilian Highland distribution. Of these, X. lomatophylla is rather broadly distributed in lowland Guayana, but is more sparingly found southward.
And finally six species (four of the sect. Nematopus, and two of the sect. Xyris) have a more general distribution in Guayana, coastal Guiana, Amazonia, and Brasilia. All these reach middle altitudes on the Roraima sediments.
In the Brazilian Highland and contiguous areas there is a second great center of distribution in which some 80 to 90 species are to be found. There is here also represented a high degree of restricted and broader endemism, and a residue of species of general and widespread distribution.
The two areas, then, that of Guayana to the north of the Amazon, and Brasilia to the south, are in m a n y respects analogous, namely as to history, physiography, and phyletic composition of the xyrid population. Very few identical species as indicated above occur in both areas, but many instances of close intraspecific relationships obtain.
It is suggested that Xyris in South America, with its present bicentric distribution, may have occupied at one time an essentially identical and continuous area, and that in subsequent history, by the intervention of the Amazon Basin, the original area became divided, and that since then a parallel evolution has proceeded without appreciable interchange.
Type species: Xyris indica Linnaeus.