8 



MEMOIRS OF THE NEW YORK BOTANICAL GARDEN 



[VOL. 10 



Abolboda ebracteata var. brevifolia Maguire, var. now 



Folio vulgo 3.0-4.5 cm longa, ca. 1.5 mm lata, basibus vaginatis, valde 7-9- 

 nervatis, marginibus conspicue scariosis; scapo ebracteato, bracteis basilaribus 

 exceptis; bracteis cephalaribus 6-8 mm longis et spica bracteis breviori; spicis 

 et floribus ut in var. ebracteata. 



Type. Flowers purple-blue, locally abundant in Sabana de Moyo, right hand 

 bank of Rio Orinoco, 10 km above mouth of Rio Ventuari, alt. 125 m, 31 Jul 

 1959, Amazonas, Venezuela, J. J. Wurdack & L. S. Adderley 43689 (holotype 

 NY). Paratype. Flowers purple-blue, apparently rare, Cacagual Savanna, alt. 

 135 m, Rio Atabapo, 13 Sep 1957, Vaupes, Colombia, Maguire, Wurdack & Keith 

 41439. 



The two populations, treated as varieties here, are quite distinct, and in their 

 respective localities quite uniform. Should they be found to retain this distinc- 

 tion while occurring in the same locality, it perhaps would then be better to 

 consider them to be distinct species. 



Xyris Linnaeus, Sp. PI. 42. 1753. 



Of the three generally recognized sections of Xyris, the smallest, sect. Poma- 

 toxyris, confined to Australia and probably of no more than 20 species, is char- 

 acterized by a basally 3-locular capsule. 



The sect. Xyris, consisting perhaps of a hundred or more species, is charac- 

 terized 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 : X. caroliniana, which is highly variable, plastic, and 

 widespread ; X. fallax, often confused with the above, well represented in Gua- 

 yana, in coastal Guiana, and south to Mato Grosso in Brazil ; X. erythema, en- 

 demic 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 tabu- 

 lar form, under four categories (high-altitude Guayana endemic species, low- 

 altitude 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 be- 

 longing 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- 



