PROC. BIOL. SOC. WASH. 99(1), 1986, pp. 160-185 THE SYSTEMATICS OF THE GENUS PACHYSTACHYS (ACANTHACEAE) Dieter C. Wasshausen Abstract.— In 1847, Nees von Esenbeck described the genus Pachystachys with a single species, P. riedeliana, from Brazil. Later that same year, he pub-lished six additional species including P. coccinea (Aublet) Nees, a species that has long been confused with P. riedeliana. Although the two species are su-perficially similar, they can readily be separated by the longer, linear calyx segments of P. coccinea. Recent field activities in South America have shown that in number of species and in morphological diversity, the genus reaches its zenith in Amazonian Peru. Nine new taxa are currently proposed, whereas four of Nees' six species are reduced to synonymy. The genus is characterized by a prominent terminal spike of flowers with large, ringent, two-lipped corollas and two stamens that are about as long as the corolla, having equal, deeply sagittate anthers. Several species {P. coccinea, P. spicata) have been observed to be hummingbird pollinated, and the inflorescences of two additional species {P. ossolaea, P. puberula) are extensively covered with ants. However, floral morphology appears to preclude myrmecophily as a factor in the pollination system of the red-flowered rain-forested species of Pachystachys. All taxa have been studied in depth, including their pollen morphology, and analytical keys to the species have been prepared to facilitate the identification of the 1 2 species recognized in this treatment. The data from pollen morphology show only slight variation in the pollen of species of Pachystachys and support the reten-tion of the genus in Bremekamp's (1938) tribe Odontonemeae subtribe Grap-tophyllinae. The genus Pachystachys Nees is a Neo-these shrubs or subshrubs are highly prized tropical genus of 1 2 species. A member of in the garden. the subfamily Acanthoideae (=Imbricatae Recent field activities in South America Lindau, 1893), these plants occur along edges have shown that in number of species and of the lowland rainforest at relatively low in morphological diversity, the genus reach-elevations (below 1000 meters) from the es its zenith in Amazonian Peru. Since at-West Indies southward to Peru and Bolivia tempts to determine specifically the mate-and eastward to French Guiana and Ama-rial from these collections proved zonian Brazil. unsatisfactory, I have decided to present a Members of Pachystachys are perhaps best basic revision of the genus, known for their ornamental beauty, and many may be found in botanical gardens History throughout the warmer regions of the world. In 1847, Nees von Esenbeck described Their prominent terminal spikes with bright the genus Pachystachys with a single species, red flowers are an impressive sight when the P. riedeliana, from Amazonian Brazil. Lat-showy inflorescences reach full anthesis, and er that same year in de Candolle's Prodro-
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