Phytologia(November 1992) 73(5):350-352. A NEW SPECIES OF Tfl/D/lX(ASTERACEAE, HELIANTHEAE) FROM GYPSUM OUTCROPS IN NUEVO LEON, MEXICO Billie L. Turner Department of Botany, University ofTexas, Austin, Texas 78713 U.S.A. ABSTRACT A new species of Tridax, T. hintoniorum B. Turner, is described and illustrated from Nuevo Leon, Mexico where it occurs on gypsimi outcrops. It is most closely related to T. candidissima A. Gray, a gypseous endemic ofSan Luis Potosi, Mexico. KEY WORDS: Tndax^ Asteraceae, Heliantheae, Mexico Routine identification of Mexican Asteraceae has revealed the following novelty. Tridax hintoniorum B. Turner, sp. nov. TYPE: MEXICO. Nuevo Leon: Mpio. Galeana, between La Poza and Rio de San Jose, 1450m, gyp- sum hillsides, 31 May 1992, Hmton et ai 22031 (HOLOTYPE: TEX!; Isotype: MEXU!). Tridace candidissimaA. Gray similissedfoliislateoblanceolatis velobovati-eUipticis(vs. linearibus)et capitulisin pedunculis nudis 10-40 cm long (vs. eradiatis in pedunculis brevibus plerumque 2-8 cm longis) differt. Erect perennial herbs 30-85 cm high, stems loosely floccose with woolly hairs, among these are interspersed glandular trichomes, the latter uniseriate mm and up to 1.5 long. Leavesellipticoblanceolatetoovateelliptic,sometimes oblanceolate, mostly 5-12 cm long, 1-4 cm wide, gradually reduced upwards, floccose-tomentose, trinervate; petioles narrowly winged and grading into the blades. Heads hemispheric, single on rather stout naked peduncles, the latter 350 Turner: New Tndax from Mexico 351 Fig. 1. Tridax hintoniorum, from holotype. PHYTOLOGIA 352 volume 73(5):350-352 November 1992 10-40 cm long. Involucres 8-12 mm high, the bracts ca. 13, biseriate, sube- qual, lanceolate-elliptic, floccose or pilose, interspersed with glandular hairs. mm Receptacles 4-6 across, convex, the pales scarious, persistent, lacerate api- mm cally. Ray florets ca. 13, pistillate, fertile, the ligules yellow, 5-7 long, 3 mm lobed, the lobes ca. 3 long. Disk florets numerous, the coroUeis yellow, mm mm ca. 8 long, pubescent, the tubes ca. 3 long. Achene*broadly clavate, appressed pilose throughout, the pappus of ca. 20 linear-lanceolate fimbriate mm tawny bristles 6-7 long. ADDITIONALSPECIMENS EXAMINED: MEXICO. NuevoLeon: Mpio. Galeana, Rio de San Jose, 1490 m, gypsum hillside, 2 Oct 1991, Hinton et ai 21566{TEX)\ between La Poza and Rio de San Jose, 1840 m, gypsum hillside, 1 Jul 1992, Hinton et ai 22061 (TEX). Tridax hintaniorumis a very distinct species but clearly relates to T. can- didissima A. Gray (c./., Powell 1965, Taxonomy of Tridax . . .Brittonia 17:47- 96). Bothofthese arestrong perennials havingfloccoseor tomentose vestiture, and both are monocephalous with nongraduate involucralbracts and havesim- ilar achenes and corollas. Tridax hintaniorumis readily distinguished from T. candidissimain having radiate heads on long naked peduncles (10-40 cm long vs. 2-8 cm); in addition its leaves are very broad and mostly confined to the lower portion of the stem (vs. leaves linear and stems leafy throughout). In- terestingly, both species appear to be confined to gypsum outcrops. It is a pleasure to name this remarkable species for the remarkable Hinton family who have collected all of the specimens available to date. ACKNOWLEDGMENTS I am grateful to Guy Nesom for the Latin diagnosis and to him and A.M. Powell for reviewing the manuscript.