A Revision of Discocarpus (Euphorbiaceae)

As revised here, Discocarpus is interpreted to consist of three neotropical species: D. essequeboensis Klotzsch, D. gentryi S. M. Hayden, which is described and named herein as new to science, and D. spruceanus Mull. Arg. One previously accepted name, D. brasiliensis Klotzsch ex Mull. Arg., is reduced to synonymy of D. essequeboensis. Lectotypes are proposed for the two species previously described. One species is newly excluded from Discocarpus, as are three others, following previous literature. Foliar anatomy is described with a focus on epidermal sclereids, which are shown to occur on both epidermides. Evidence presented supports close relationships with Lachnostylis Turcz. and Amanoa Aubl.; little was found to support previous hypotheses concerning a relationship with Chonocentrum Pierre ex Pax & K. Hoffm. Discocarpus Klotzsch is a genus of trees found in seasonally flooded riparian habitats of northern South America, where they are components of the forest canopy. The plants are dioecious and bear small clusters of flowers in the axils of simple, alternate, entire leaves. Discocarpus was first described by Klotzsch (1841) who initially named, but did not describe, two species; he subsequently described one of these, D. essequeboensis Klotzsch (1843), based on Schomburgk collections from the Essequibo River region of Guayana. Omitting two nettles (Urticaceae) from Mexico and Nicaragua grossly misplaced in the genus, the next species of Discocarpus to be named was D. spruceanus Mull. Arg. (1863), based on collections of Richard Spruce from the Rio Negro of Brazil. Some 32 years after being first mentioned by Klotzsch, D. brasiliensis Mull. Arg. (1873) was formally named, based on a collection of von Martius from the early 19th century. Three taxa were added to the genus in the 20th century. The first addition was D. hirtus (L. f.) Pax & K. Hoffm. (Pax & Hoffmann, 1922), a consequence of synonymizing the South African genus Lachnostylis with Discocarpus. In current literature, however, Lachnostylis is treated as distinct from Discocarpus (e.g., Levin, 1986; Mennega, 1987; Webster, 1994b). The two most recently described species, 1 Financial support for the master's thesis research upon which this study is based was generously provided by the Graduate School of Arts and Sciences at the University of Richmond. We thank Rafael de Sd, Miles F. Johnson, Geoffrey A. Levin, Gary Radice, Dean Simpson, Donna M. E. Ware, and an anonymous reviewer for assistance, and the curators of BM, BR, C, E, F, G, GH, GOET, ILLS, K, L, LD, M, MANCH, MICH, MO, NY, OXF, P, R, RB, RSA, S, TCD, U, UC, URV, US, and W for the loan of herbarium specimens. 2 Department of Biology, University of Richmond, Richmond, Virginia 23173, U.S.A. ANN. MISSOURI BOT. GARD. 83: 153-167. 1996. 154 Annals of the Missouri Botanical Garden D. mazarunensis Croizat (1948) and D. duckeanus Jabl. (1967), were based on South American material. Jablonski (1967) accepted the five South American species noted above as distinct entities constituting Discocarpus, but his treatment indicated several gaps in the available data on these plants. For example, staminate or pistillate flowers were undescribed for several species. Furthermore, recent studies reveal that two of the species accepted by Jablonski were misplaced in Discocarpus, and presently available collections indicate the existence of a previously unrecognized species (Hayden, 1995). In addition, the issue of generic relationships is unresolved. Discocarpus was classified most recently in subfamily Phyllanthoideae Asch. tribe Wielartdieae Baill. ex Hurus. (Webster, 1994b), but earlier concepts of generic relationships have varied widely (see below). Moreover, reports of foliar sclereids in Discocarpus and Amanoa (Gaucher, 1902; Levin, 1986) suggest the new possibility of placement in tribe Amanoeae. This paper provides a revision of Discocarpus, including detailed descriptions of foliar anatomy and discussion of relationships. MATERIALS AND METHODS This revision is based on a total of 171 herbarium specimens of Discocarpus borrowed from 29 herbaria in the United States, Europe, and South America. Small samples of leaf tissue from the following collections were removed for anatomical study: Discocarpus essequeboensis Klotzsch, Jangoux & Bahia 294 (NY), Krukoff & Froes 11974 (NY), Maas et al. 7395 (U), Schomburgk 659 (U), Silva 4776 (NY), Smith 2692 (F); Discocarpus gentryi S. M. Hayden, Encarnacion 25065 (F), Vdzquez & Jaramillo 5487 (NY); Discocarpus spruceanus Mull. Arg., Davidse 27631 (NY), Wurdack & Adderly 43349 (NY). Half of each sample was mounted directly on stubs and sputter coated with a gold/ palladium mixture prior to observation with SEM. The second half of each sample was rehydrated by boiling in water with a few drops of Aerosol OT, dehydrated in tertiary butanol, embedded in paraffin, and sectioned at 10 jkm on a rotary microtome. Paraffin sections were stained in toluidine blue (Berlyn & Miksche, 1976) or a combination of saffranin and haematoxylin (Johansen, 1940). SYSTEMATIC TREATMENT Discocarpus Klotzsch, Archiv. Naturg. 7(1): 201. 1841. TYPE: Discocarpus essequeboensis Klotzsch. Figures 1-4. Dioecious trees (or shrubs), (3-)10-30 m tall, DBH 25-100 cm. Twigs glabrous to short-pilose, silvery gray to dark purplish red; lenticels raised, elongate, parallel with the axis; terminal buds acuminate, cylindric, glabrous to tomentose, 3-7 mm long, often with two basal knoblike protrusions, sometimes sexually dimorphic. Leaves alternate, simple, petiolate, glabrous, leathery; petioles 4-8 mm long, wrinkled; margins entire; base obtuse; apex acute to acuminate; venation pinnate; ultimate veins reticulate, orthogonal. Stipules fugaceous. Inflorescence axillary, 1-several flowers per node; flower clusters sessile, subtended by cupulate bracts; bracts ca. 1 mm long, 1 mm wide, glabrous to pubescent; staminate clusters several per node; pistillate clusters one per node. Staminate flowers sessile, congested, 10-30 per node; sepals (4) 5, 1.5-2 mm long, 1-1.5 mm wide, pilose; petals (0)5, delicate, hyaline, glabrous to pubescent, less than 1 mm long, linear, often fringed apically; disk extrastaminal, lobed; stamens (4) 5; filaments fused below the level of the disk, free portions 1.5-3 mm long; anthers elongate, 1 mm long, longitudinally dehiscent, exserted; pistillode segmented into two or three linear, pubescent, membranous filaments. Pistillate flowers 1-3(-5) per node; pedicels essentially lacking to 5 mm long; sepals 5, cupulate, 1.53 mm long, 1-2 mm wide, densely pubescent; petals (0-)5, hyaline, 0.5-3 mm long, up to 1 mm wide, pubescent; disk slightly 4obed; ovary 3-carpellate, subglobose, smooth or sculpted, densely pubescent; styles 3, parted to the base or nearly so, spreading horizontally, densely pubescent below; stigmas 3, dilated, lobed, horizontal or reflexed; ovules 2 per locule. Fruits symmetrically 3-lobed or asymmetrically subglobose, 6-12 mm tall, 6-15 mm diam., longitudinally dehiscent into 2, 3, or 6 mericarps, 1-3-seeded; pericarp ca. 1 mm thick, hard, brittle; surface smooth to deeply sculpted, densely pubescent; columella persistent. Seeds subglobose, ecarunculate; testa thin, shiny. Discocarpus can be distinguished from other woody genera of subfamily Phyllanthoideae by the combination of: deciduous stipules; dioecy; minute petal-bearing flowers produced in axillary clusters; lobed extrastaminal disks; finely reticulate xine on pollen grains; and styles that are not bifid, but terminate in three dilated, irregularly lobed stigmas. It is noteworthy that floral merosity and presence of petals are somewhat inconstant. Most flowers are 5-merous, but 4-merous flowers are not infrequently encountered. Most flowers examined showed petals to be present and isomerous with the sepals, but sometimes fewer petals, or even none, will be Volume 83. Number 2 Hayden & Hayden 155 1996 Revision of Discocarpus