A New Species of Bidens (Asteraceae) from San Luis Potosi, Mexico

Abstract

Bidens clavata, a new species from Mexico, is described and illustrated. This species is related to Bidens odorata, but can be distinguished morphologically by its clavate outer achenes and chemically by differences in the flavonoids accumulated in its leaves. During the course of a systematic study of Mexican Bidens, the following new species is recognized. Bidens clavata Ballard, sp. nov. (fig. 1).-TYPE: Mexico, San Luis Potosi, along Hwy 70, 7.7 mi W of Santa Catarina, km 205-206, 9 Oct 1971, Melchert, Ballard, & Hart 71-64B (holotype: IA!; isotype: CLEMS!). Capitula 2.4-3.1 cm in diametro ad anthesin. Phyllaria exteriores 7-10, 2-4 mm longae, 0.40.9 mm latae. Phyllaria interiores 8, 3-4 mm longae, 1.2-2 mm latae. Flosculi radii 5, 7-11 mm longi, 5-9 mm lati, ligulae albae aut roseae, 6-11-striatae. Flosculi disci 26-50, 2.5-4 mm longi. Achaenia glabra, dimorpha, exteriora clavata, 2.5-3 mm longa; interiora linearia, 5-9 mm longa. Chromosomatum numerus n = 12. Plants annual; stems erect, square, branched, 0.2-1.3 m tall, densely pilose; leaves petiolate, opposite, mostly 3-parted, 6-10 cm long, 5-8 cm wide, leaflets simple, margin serrate, ovate-lanceolate, moderately to densely pilose on both surfaces; heads 18-27, terminal, cymosely disposed, 2.4-3.1 cm broad at anthesis; involucre basally villous, outer phyllaries 7-10, linearspatulate, 2-4 mm long, 0.5-1 mm wide, green, ciliate, surface moderately pubescent, inner phyllaries 8, lanceolate, 3-4 mm long, 1.2-2 mm wide, dark brown, tomentose, margin hyaline; ray florets 5, ligules elliptic-obovate, apically subtruncate, 7-11 mm long, 5-9 mm wide, white to rosaceous, 6-11 striate; disc-florets 26-50,2.54 mm long, yellow; achenes 24-47, usually awnless, rarely biaristate, glabrous, dark brown to black, dimorphic, outer ones clavate, 2.5-3 mm long, innermost ones linear-clavate to linear, 59 mm long; chromosome number n = 12. Distribution. Bidens clavata is known only from the Sierra Madre Oriental of San Luis Potosi. It grows in moderately disturbed habitats and can be found on roadsides. Additional specimens examined. MEXICO. San Luis Potosi: Along Hwy 70, 17.3 mi E of San Luis Potosi, Melchert et al. 71-52 (IA); along Hwy 70, 21.5 mi E of San Luis Potosi, Melchert el al. 71-57 (IA); along Hwy 70, 27.8 mi E of San Luis Potosi, Melchert et al. 71-60 (IA); along Hwy 70, 41.2 mi E of San Luis Potosi, Melchert et al. 71-62 (IA); along Hwy 70, 41.8 mi E of San Luis Potosi, Melchert et al. 71-63B (IA); along Hwy 70, 7.7 mi E of Santa Catarina, Melchert et al. 71-64A (IA); Alvarez, Palmer 192 (F, GH, MO, MSC, NY, US). Bidens clavata most resembles Bidens odorata Cavanilles. Both are square-stemmed, 5-rayed, self-incompatible annuals with a chromosome number of n = 12. However, B. clavata differs from B. odorata by its glabrous, dimorphic achenes (fig. ic). The outer achenes are short clavate and the inner achenes are linear-clavate to linear. In contrast, the achenes of B. odorata are linear, obcompressed-quadrangulate, glabrous below and tuberculate-strigose above. Its outer and inner achenes differ only in size; the outer achenes are shorter. The leaf flavonoid chemistry of B. clavata is similar to B. odorata. Both synthesize the same chalcones, 3-0-methyl okanin 4'-glycosides and okanin-3'-glycosides, but B. clavata synthesizes glucosides of the flavones luteolin and apigenin instead of the flavonol quercetin, which is characteristic of B. odorata (Ballard 1986). Plants of B. clavata grown in the greenhouse rarely flowered and hybridization data were difficult to obtain. From the 12 intraspecific crosses attempted with B. odorata, the achenes formed often lacked embryos, and those with embryos usually died shortly after germination. Only one artificial hybrid grew to flowering. The pollen viability as expressed in pollen stainability was 11%, and the meiotic chromosome pairing consisted of 10 bivalents and 1 chain of 4.