Variability within Stegnosperma halimifolium Benth. (Phytolaccaceae)

Abstract

Species circumscription of the subtropical genus Stegnosperma (Phytolaccaceae) is made difficult by great variability within S. halimifolium, a common shrub in Baja California also present in Sonora, Mexico. The problem is compounded by the difficulty of discriminating between two of the three species of the genus, S. halimifolium and S. watsonii, by means of pre- viously suggested characters. Clustering and principal component analysis were applied to the problem using 60 morphological traits recorded from herbarium specimens of the three species and 75 characteristics from four population samples of S. halimifolium. Results demonstrate that S. halimifolium and S. watsonii are distinct and clearly distinguishable by a large suite of charac- teristics and new discriminators are indicated. Stegnosperma halimifolium is more variable than the narrowly distributed S. watsonii and the oldest of four sampled populations of the former spe- cies is from the Cape Region on the Pacific side of the peninsula. At various times the small, tropical genus Stegnosperma has been consi- dered to be a member of a monotypic tribe, or a member of a monotypic family. In current taxonomy the genus represents the monotypic tribe Steg- nospermoideae of the family Phytolaccaceae although Bedell (1980) raises the tribe to the family level. Mabry (1966) provided chemical data support- ing morphological evidence which dictates membership in the Caryophyl- lales. The genus is a shrub distinguished from other members of the Family Phytolaccaceae by the presence of petals, the 1-5 seeded capsules, and the bright red arils of the seeds. In Nowicke's (1968) discussion of proposed rela- tionships within the family, Stegnosperma stands out as one of several highly distinctive genera. The goals of this research have been to quantify the patterns of dissimilarity within S. halimifolium and to determine the probable relations among four populations of this species in the Baja Cali- fornia; and to determine if the separation of S. watsonii from the latter spe- cies is taxonomically sound. The genus includes only three species: S. cubense Rich., S. halimifolium Benth., and S. watsonii Rogers. A fourth species, S. scandens (Lunan ex Jackson) Standley, was noted by Rogers (1949) to be a nomenclatural syn- onym under S. cubense. We feel Stegnosperma cubense to be easily distin- guished from the other two species by its orbicular leaves, 3 to 4 carpellate flowers, and generally more compact racemes, although Gibson and Horak (pers. comm.) have seen plants of this species with racemes 10 cm long. It has the widest distribution of the three species, ranging from Sinoloa, Mex- ico, south to Nicaragua, Guatemala, Costa Rica, and the West Indies. Stegnosperma halimifolium is very similar in appearance to the most recently described species, S. watsonii. The latter is said to be distinguisha- ble from S. halimifolium by three distinguishing traits: few-flowered cymules, petals that are abruptly constricted at the base, and seeds with a lateral funicular scar. None of these traits provides a practical guide for dis- crimination. Inflorescences of the type specimen of S. watsonii and the other