'J^ HARVARD UNIVERSITY Library of the Museum of Comparative Zoology iH THE VASCULAR FLORA OF ISLA SOCORRO, MEXICO GEOFFREY A. LEVIN AND REID MORAN Department of Botany San Diego Natural History Museum San Diego, California 92112, USA SAN DIEGO SOCIETY OF NATURAL HISTORY MEMOIR 16 1989 MCZ LiBRARY SEP 1 1 1989 HARVARD UNIVERSITY THE VASCULAR FLORA OF ISLA SOCORRO, MEXICO GEOFFREY A. LEVIN AND REID MORAN Department of Botany San Diego Natural History Museum San Diego, California 92112, USA SAN DIEGO SOCIETY OF NATURAL HISTORY MEMOIR 16 15 August 1989 The publication of this Memoir was made possible by a generous contribution from Norman C. Roberts Table of Contents ABSTRACT 4 RESUMEN 4 INTRODUCTION 5 CLIMATE 5 HUMAN IMPACTS 7 BOTANICAL COLLECTORS 7 Anthony and Stockton 8 Barkelew 8 Other Collectors 9 VEGETATION 10 BIOGEOGRAPHY 13 Sources of the Flora 14 Modes of Long-Distance Dispersal to Socorro 19 Evolution of the Flora 19 CATALOGUE OF SPECIES 20 Ferns and Fern Allies 20 Angiospermae: Dicotyledons 23 Angiospermae: Monocotyledons 59 EXCLUDED SPECIES 66 ACKNOWLEDGMENTS 66 LITERATURE CITED 67 APPENDIX 1. Vascular Flora of Isla Clarion 70 APPENDIX 2. Vascular Flora of Isla San Benedicto 71 Abstract. This is the first comprehensive report since 1931 on the vascular flora of Isla Socorro, a small isolated volcanic island in the Islas Revillagigedo off the west coast of Mexico. In the last 58 years the number of collections from the island has increased about seven- fold. The native flora comprises 117 species, 30 of them endemic to Socorro and 9 more to the Revillagigedos. Despite high endemism, there does not appear to have been an evolutionary radiation on the island. The flora is derived principally from western mainland Mex- ico, but particularly the endemic species show a strong affinity with plants of southern Baja California, and the closest relatives of a few species are found only in places even farther away. Dispersal by birds appears to account for the arrival of most species or their ancestors; drift and air flotation also are important among the non-endemics. Several species show increased woodiness on the island, and several others have denser pubescence or more compact inflorescences than do their mainland relatives. The vegetation is mostly low non-spiny scrub, but there is forest on the upper east slope and some grassland from shore to summit. Sheep and limited clearing for agriculture and for roads have disturbed the vegetation on the south side of the island and have allowed the naturalization of 47 introduced species. We describe six new taxa: Aegopogon solisii Levin, Bidens socorren- sis Moran and Levin, Botrychium socorrense W. Wagner, Caslilleja socorrensis Moran, Lepechinia hastata subsp. socorrensis Moran, and Salvia pseudomisella Moran and Levin. We make six new combina- tions: Chamaesyce anthonyi (T. S. Brandegee) Levin, based on Euphorbia anthonyi; Cynanclwm californicum (Benth.) Moran, based on Metastelma californicum: Cynanchum sonorense Moran, based on Metastelma minuliflorum Wiggins; Spermacoce nesiolica (Robin- son) Levin, based on Borreria nesiotica; Teucrium ton nsendii subsp. affme (T. S. Brandegee) Moran, based on T. affine; and Zapoteca formosa subsp. socorrensis (1. M. Johnston) Levin, H. Hernandez, & Moran, based on Calliandra socorrensis. We also refer to wider- ranging species the following taxa previously thought to be endemic to Isla Socorro: Bulbostylis sepiacea Krai, Bursera nesopola I. M. Johnston, Nicottana nesophila I. M. Johnston, f^peromia chrysolepida Trel., Phoradendron townsendii Trel., and I iguiera deltoidea var. townsendii Vasey & Rose. Appendices list the known floras of islas Clarion (41 native and 1 introduced species) and San Benedicto (formerly 10 but now 6 native species). Resumen. Este es el primer reporte comprehensivo desde 1931 sobre la flora de tejido vascular de la Isla Socorro, una isla volcanica aislada del grupo de las islas Revillagigedo en la costa occidental de Mexico. En los liltimos 58 anos, el niimero de colecciones hechas en la isla han incrementado per siete veces. La flora nativa comprende 117 especies, 30 de ellas endemicas a la Isla Socorro y 9 otras a las Revillagigedo. No parece existir una irradeacion evolutiva en la isla aunque existe gran endeminismo en ellas. La flora proviene prin- cipalmente del oeste del Mexico continental, pero en particular las especies endemicas demostran una gran afinidad con plantas de Baja California Sur, y parentela con unas cuanlas especies que se encuen- tran en lugares aiin mas lejanos. Dispersion por medio de aves parece ser la razon del arrivo de la mayoria de las especies o de sus ancestros; corrientes marinas y de aire tambien son importantes con las inendemicas. Varias especies en la isla demostran un incremento en su caracter maderoso, y algunas otras tienen una pubescencia mas densa o una inflorescencia mas compacta que la de sus parientes en el area continental. La vegetacion consiste, en su mayoria de mator- rales bajos y sin espinas, pero existe una selva en la cuesta superior oriental, asi como pastizales de la playa hasta la cima. Algo de gonado ovino, asi como claros para agricultura y creacion de caminos han molestado a la vegetacion en el lado sur de la isla, y han permitido la naturalizacion de 47 especies introducidas. Describimos seis taxa nuevas: .Aegopogon solisii Levin, Botrychium socorrense W. Wagner, Bidens socorrensis Moran y Levin, Castilleja socorrensis Moran, Lepechinia hastata subespecie socorrensis Moran, y Salvia pseudomisella Moran y Levin. Creamos seis combinaciones nuevas: Chamaesyce anthonyi (T. S. Brandegee) Levin, basandose en Euphor- bia anthonyi; Cynanchum californicum (Benth.) Moran, basandose en Metastelma californicum; Cynanchum sonorense Moran, basandose en Metastelma miimtiflorum Wiggins; Spermacoce nesiotica (Robin- son) Levin, basandose en Borreria nesiotica; Teucrium townsendii subespecie affine (T. S. Brandegee) Moran, basandose en T. a/fine; y Zapoteca formosa subespecie socorrensis (I. M. Johnston) Levin, H. Hernandez, y Moran, basandose en Calliandra socorrensis. Tambien referimos a especies plias las siguentes taxa, previamente creidas ser endemicas a Isla Socorro: Bulbostylis sepiacea Krai, Bursera nesopola I. M. Johnston, Nicotiana nesophila I. M. Johnston, Peperomia chrysolepida Trel., Phoradendron townsendii Trel., y I iguiera deltoidea var. townsendii Vasey y Rose. Los apendices describen las floras conocidas en las islas Clarion (41 especies nativas y una introducida) y San Benedicto (antes 10, pero ahora 6 especies nativas). The Vascular Flora of Isla Socorro, Mexico Geoffrey A. Levin and Reid Moran' INTRODUCTION The Islas Revillagigedo are four scattered volcanic islands, Clarion, Roca Partida, San Benedicto, and Socorro, in the Pacific Ocean off the west coast of Mexico (Figure 1). They are true oceanic islands, never having been connected to the mainland (Richards 1957, Jehl and Parkes 1983). Isla Socorro, the largest (210 km-) and highest (1040 m), lies 460 km south- southwest of Cabo San Lucas, Baja California Sur, and 580 km west of Cabo Corrientes, on the Mexican mainland. The earliest lists of vascular plants for the Revillagigedos are those of Vasey and Rose (1891) and Brandegee (1900b). The most important work on the flora has been the thorough study by Johnston (1931). Reporting mainly on the collections of H. L. Mason in 1925, Johnston also discussed all available earlier collections and analyzed the relationships of the flora. He listed 93 species known from Socorro and nine others he thought doubtful. He also listed 43 species from Clarion and 11 from San Benedicto; Roca Partida is a pinnacle devoid of vascular plants. Though a few later works have dealt specifically with Socorro plants (e.g., Svenson 1939, H. S. Gentry 1949, Howell 1949, Miranda 1960) and several scattered new records have appeared, no comprehensive flora has been published in the last half-century. This flora is mainly the fruit of Moran's trips to Socorro in 1957, 1978, and 1981 and Levin's trips in 1987 and 1988, but also draws on other sources. The number of collections from Socorro has grown from under 200 seen by Johnston (1931) to over 1200 seen by us. On the basis of these specimens and current tax- onomic treatments, we accept 117 species as native and 47 more as naturalized. We report 17 of the native species from Socorro for the first time, including four we propose as endemic new species, and we confirm for Socorro four species that Johnston thought doubt- ful. We also propose one endemic new species and one endemic new subspecies for plants already known on the island, but refer to wider-ranging species six taxa previously considered to be endemic to Socorro. In addition, we include much more information about species' abundance and distribution on the island than 'Department of Botany, San Diego Natural History Museum. could Johnston (1931), who had to rely on specimens with only scanty data. Most place names used in this paper can be found in Figure 1, which we adapted from Richards and Bratt- strom (1959). With three exceptions, these names appear on most recent maps of the island. Most Mexican maps use Cabo Regla for what we call Cabo Rule, and Cabo Chato for Cabo Middleton. We chose to use the older names in both cases. Caleta Castelan, the main landing at the south end of the island, is unnamed on most maps. Hanna (1926, fig. 5) showed it as the landing, but apparently considered it part of Bahia Braithwaite, and perhaps others did likewise. The cove received its current name shortly after the naval base was established in 1957 (Adem 1960). It has also been called Bahia Vargas Lozano, though we cannot find this name published before 1973 (Lewis 1973:178), and simply "The Cove" (Lewis 1973:174, U.S. Defense Mapping Agency Hydrographic/Topographic Center 1982). For two places without known proper names, we use descriptive names. The "summit plateau" is the relatively flat area east of the summit of Cerro Ever- mann at about 950 m, or about 90 m below the summit. The "caldera valley" is an undrained valley southeast of Cerro Evermann at an elevation of about 550 m. CLIMATE The Mexicans have been collecting meteorological data near the naval base on Isla Socorro since it was established in 1957. According to Medina (1978), mean monthly temperatures at the naval base vary between 19°C (January) and 28°C (July/August). The mean annual precipitation near sea level is 761 mm. Almost all the rain falls between July and October, half of it in August (mean of 140 mm) and September (mean of 240 mm). At lower elevations, the remainder of the year is quite dry, with no rain falling for months at a time. In contrast, the summit of the island frequently is engulfed by clouds and probably receives small amounts of moisture through much of the year. Thus, except at the summit, the climate is similar to that of the nearby Mexican mainland (Mosino Aleman and Garcia 1974:375-376). Levin and Moran Cabo Middleton Bahia Academy •18°50'N Playa Cabo Blanca Henslow. Punta Tosca •18M5' N Cabo Pearce -Lagunat Escondida Isia Roca Partida. •) ^ | g San BenedT Isia Socorro L Isia Clarion Bahia Braithwaite Kilometers 1 1 roo'w 1 10°55'W Fig. 1. Isia Socorro, Mexico, and the Islas Revillagigedos (inset). Adapted from Richards and Brattstrom (1959). Flora of Isla Socorro HUMAN IMPACTS Except for two short-lived attempts at colonization, Socorro remained uninhabited until 1957, when the Mexican government established a naval base on Cabo Rule (Richards and Brattstrom 1959). Since then, feral house cats apparently have caused the extinction of one endemic bird and threaten at least one more (Jehl and Parkes 1982, 1983). Goats have not been introduced yet, but sheep brought between 1869 and 1887- still survive on the southern part of the island. They now range mainly over the southeastern slope, probably being limited by lack of fresh water. A large, gently sloping area north of Huerta Grande, between 300 and 450 m, is called "Llano de los Borregos" by the Mexican sailors stationed on the island. Sheep have denuded most of this area, leaving a few dying trees and shrubs. This area is now covered with annuals, especially Mitracarpus hirtus, Aristida adscensionis, and other, introduced, grasses. (We provide authors only for species not in the Catalogue.) With the perennial cover essentially gone, erosion is rapid and several large arroyos have developed. Higher, in the forest to about 700 m, the sheep have left few herbs and little undergrowth. The trees are old, generally in poor con- dition and with a lot of dead wood; young trees apparently cannot get established. F. Miranda (pers. comm.), however, suggested that the vegetation may have been relatively sparse in these areas even before the sheep were introduced. He also suggested (Miranda 1960) that grasslands such as that on Cabo Rule may have been favored by browsing of the scrub by sheep. By 1903 only two weeds, Eragrostis ciliaris and Tribulus cistoides, had been found on Socorro. In 1955 Carlquist found Dactyloctenium aegyptium, and two years later Moran found only two more, Cenchrus echinatus and Eragrostis tenella. All three were scarce and apparently just getting started. Since the naval base was established in 1957, clearing for roads and for a -The date usually cited for the introduction of sheep is 1869 (Hanna 1925, Richards and Brattstrom 1959, Miranda 1960). This date is based on a report (quoted in Hanna 1925) prepared in 1923 by E. R. del Rip, a clerk in the American consulate in Manzanillo, Mexico. Though based on Mexican historical accounts, del Rip's report confuses information about Isla Socorro and other Mexican Islands, including Isla Guadalupe, and therefore may not be entirely reliable. An alternative date of between 1884 and 1887 derives from a letter (now in the University of Notre Dame library) dated I June 1887 from W. S. Lyon, an amateur botanist from California, to E. L. Greene. In it, Lyon stated that "some three years ago," i.e., in 1884, he was to have visited Socorro to evaluate the island's potential for sheep ranching. He did not make the trip for unspecified reasons, but reported that Californian sheep had since been introduced to the island. The possibility remains that Lyon and the men who had planned to send him to Socorro were unaware that sheep had been introduced in 1869. little planting has disturbed some parts of the south end, and traffic from the mainland has greatly increased. By 1988 the number of naturalized plants had grown to at least 47, mostly in these disturbed places and the overgrazed area north of Huerta Grande. More are sure to come. Although sheep might be expected to carry seeds up, the only weeds known to have reached the summit of the island are Eragrostis ciliaris, found there by 1957, and Mitracarpus hirtus, found there in 1988. The Mexican navy now maintains a crude trail to the peak, making direct introduction of foreign plants to the interior of the island at least conceivable. Visits to other parts of the island have also increased. For each plant newly found since 1957, even in a remote area, we must therefore ask whether it could have been introduced. Especially with plants that are rare or localized, it may be hard to tell whether they are new arrivals or had just been overlooked before. We have had this ques- tion with, for example, Conyza confusa, Glinus radiatus, Gnaphalium sphacilatum, Hibiscus pernam- bucensis, and Opiismenus hirtellus. In addition to plants brought in accidentally, several others have been planted intentionally. As part of a 1933 colonization study, scientists from the Mexican Escuela Nacional de Agricultura experimentally planted certain plants (Sanchez de la Peiia 1948), but apparently none have survived. Two coconut palms (Cocos nucifera L.), probably planted later, grew well at Caleta Grayson (Mason & Hanna 14606); others have since been planted at Playa Blanca, where the introduction of Hibiscus pernambucensis may also have been intentional. The Mexican navy planted a grove of Citrus aurantium L. in the forest on the east slope of Cerro Evermann above the caldera valley (Moran 29529), and the trees produce heavily. Many food and ornamental plants grow near the naval base and at Huerta Grande. Four cultivated plants. Acacia fame- siana, Cynodon dactylon, Lagenaria siceraria, and Solanum americanum, and one possibly cultivated, Chenopodium ambrosioides, are already naturalized locally. Others, such as Delonix regia (Bojer) Raf., Ipomoea fistulosa Martius ex Choisy, Psidium guajava L., and Terminalia catappa L., may become naturalized on the island, as they have elsewhere in tropical America. BOTANICAL COLLECTORS Table 1 summarizes the work of all the botanists we know to have collected on Socorro. Johnston (1931) discussed those whose specimens he had seen, namely, C. H. Townsend, A. W. Anthony and A. L. Stockton, Levin and Moran Table 1. Plant collections from Isla Socorro. Number of Collector Date Collecting Localities^ Specimens Herbarium'' Charles H. Townsend March 1889 BB 19 US A. W. Anthony (Alfred L. May 1897 north and south ends 41 seen uc Stockton) (of?) Frederick E. Barkelew May-July 1903 ?; reached upper slopes 54 seen (of 68) CASt; uc, GH Herbert L. Mason May 1925 BB to CE; CB; CG 83 CAS Octavio Solis May 1925 with Mason 16 seen (of?) MEXU Thomas Craig February-March 1928 BB few POM John Thomas Howell March 1932 BA; CC; BB to 600 m 76 CAS Jesiis Patino 1933 ?, reached CE 20 seen (of?) 11 MEXU Francis H. Elmore March 1939 BB RSA (formerly AHFH and LAM) H. L. Mason and November 1953 BA; CB; CG 39 UC G. Dallas Hanna James W. Warren November 1953 with Mason and Hanna 9 LA Sherwin Carlquist May 1955 BA; CB to slopes of CE; CG 40 RSA George Lindsay February 1956 CC 14 SD Reid Moran March 1957 CB to CE to BA; CC; CG 224 SD Faustino Miranda Januarv 1958 mostly S quarter of island, incl. CE 76 MEXU Richard S. Felger March 1967 CC to CE; PB 101 SD Roberto Cruz"' March 1967 CC to CE; PB 80? ENCB Luz Maria Villarrea! de Puga'-" March 1967 CC to CE; PB 88 IBUG Reid Moran April 1978 BA; CC to 850 m; CG to 350 m; PB 114 SD Reid Moran April 1981 CC to CE; PB 60 SD Geoffrey A. Levin April 1987 BB; CG to CE 72 SD Raymundo Dominguez September-October 1988 CC to 600 m 65 CIB Jose Luis Leon de la Luz December 1988 CC to CE; PB 82 CIB Geoffrey A. Levin December 1988 CC to CE 28 SD = BA, Bahia Academy; BB, Bahia Braithwaite; CB. Caleta Binner; CC, Caleta Castelan; CE, Cerro Evermann; CG, Caleta Grayson; PB, Playa Blanca. ''The location of the first (or best) set of each collection is shown by the herbarium abbreviation from Holmgren el ai. (1981). 'Specimens not seen for this study. F. E. Barkelew, and H. L. Mason. Because of ques- tions about the origin of some specimens, we review the itineraries of Anthony and Barkelew. Then we give a few notes about other collectors. Anthony and Stockton On the voyage of the Wahlberg in the spring of 1897, Alfred L. Stockton, a nephew of Katherine Brandegee, took charge of collecting plants (Brandegee 1899, 1900b). However, as with the voyage of 1896 when James M. Gaylord was botanist, the printed labels bear the name of A. W. Anthony, leader of the expedition. For simplicity, we follow the usual practice of citing the specimens as Anthony's. The Wahlberg sailed from San Benedicto to the southwest coast of Socorro on 3 May, later anchored around the southern part of the island, moved to the north end about 14 May, and left for San Benedicto and Clarion on 16 May (Anthony n.d.). Most of Anthony's specimens have only inclusive dates for the whole trip. Some are unnumbered, but others, evidently collected in large sets, have completely printed labels with numbers at least from 180 to 435. Specimens numbered 375-401 are labeled "Socorro," as are some unnumbered specimens. The numbers appear to be mainly chronological, since the sequence of label localities generally follows the itinerary of the trip. Some numbers are clearly out of chronological sequence, however. For example, no. 367 (Eschscholzia ramosa (E. Greene) E. Greene), from Isla Cedros, follows a series from San Jose del Cabo, although the Wahlberg did not return from San Jose to Cedros before sailing to the Revillagigedos. Thus it appears that the specimens were not numbered as collected and that the numbers therefore cannot be used to infer in what part of an island any specimen was collected, or even on which island. We see little reason to doubt the localities on any of Anthony's collections, however, except possibly for Nicotiana stocktonii (no. 382) and Scaevola plutnieri (no. 387); see Remarks under those species in the catalogue. Barkelew From 14 May to 9 July 1903, Frederick E. Barkelew collected in the Islas Revillagigedo for the California Academy of Sciences. The first set of his collections Flora of Isla Socorro and any notes he might have made were destroyed in the San Francisco fire of 1906, but fortunately several sets of duplicates already were distributed. According to Johnston (1931:13), the most complete sets are at UC and GH (herbarium acronyms follow Holmgren et al. 1981). Partial sets are at A, ARIZ, DS, F, MO, NDG, NY, POM, and US (Cronquist 1945, Lanjouw and Staneu 1954, Pfeifer 1970, Powell 1974). Johnston saw 66 numbered collections and we found three more, all numbered between 169 and 252. Some evidence suggests that Barkelew numbered his specimens consecutively as he collected, and nothing suggests otherwise. For example, he gave consecutive numbers to three strand plants known in the Revillagigedos only from the north end of Socorro and likewise to eight plants known there only from the higher elevations. Similarly, some other plants of related habitats fall close together in the sequence. Barkelew collected on San Benedicto, Socorro, and Clarion, in that order (Johnston 1931:13), but we have seen no labels for Clarion. The first eight numbers (169-176) are all labeled San Benedicto, and all but one are of species also collected there by others. All other specimens found are labeled Socorro, except no. 188, labeled San Benedicto. This is Dodonaea viscosa, not otherwise known from San Benedicto and not likely to have been overlooked on that barren island. Prob- ably the specimen is from Socorro or Clarion, where this plant does occur, but was mislabeled, as Johnston (1931:14) suggested. Number 218 is Psilotum nudum, not otherwise known from the Revillagigedos. With this single e.xcep- tion, numbers 177-244, so far as found, are all plants known to occur on Socorro, the second island Barkelew visited. Many, including 12 of the 13 specimens seen for numbers 227-244, are of species not known from San Benedicto or Clarion. Number 188 (Dodonaea viscosa) would therefore appear to be from Socorro. The seven specimens seen for the last eight numbers (245-249, 252; 245 is a mixed collection of two Ipomoea species), though labeled Socorro, are of species known to occur on Clarion, the last island visited. Three of these specimens (245 and 246) are of species not known from Socorro; two more (247 and 248) are of Melochia pyramidata and Macroptilium atropurpureum, which were not otherwise found on Socorro until 1978 and 1988, respectively, and appear to be new arrivals there. None of the last eight numbers is of a species known from San Benedicto. Thus it appears that Barkelew's specimens numbered 245-252 really came from Clarion but were mislabeled, as Johnston (1931:14) suspected. These specimens (245-252) include six of the seven species collected by Barkelew whose occurrence on Socorro Johnston (1931:14) doubted. Two of the species, Canavalia rosea and Cressa truxillensis, have since been collected on Socorro; however, because the plants are also known from Clarion, Barkelew's specimens (249 and 252) probably came from Clarion, as his numbers suggest. The seventh species whose occurrence on Socorro Johnston doubted is Caesalpinia bonduc. Because that plant is now known to occur on Socorro as well as Clarion and because the number (207) falls among those presumably from Socorro, there now seems to be no reason to doubt the label locality. The rest of Johnston's doubtful species have yet to be collected in undisturbed habitats on Socorro. In summary, we believe that Barkelew's numbers 245-252, though labeled Socorro, really came from Clarion; that his no. 188, labeled San Benedicto, really came from Socorro; and that his other collections are correctly labeled. We therefore exclude from the known native flora of Socorro the following species, whose only claim to inclusion rests on Barkelew's labels: Ipomoea indica (Burm. f.) Merr., /. halierca I. M. Johnston, Macroptilium atropurpureum, Melochia pyramidata, and Sophora tomentosa L. Other Collectors Octavio Soli's was with H. L. Mason on the Cali- fornia Academy of Sciences expedition in 1925 (Hanna 1926), but Johnston did not see his collections. We found 16 of his specimens from Socorro (MEXU, US). Eight bear numbers, all between 20 and 90, suggesting that he collected many more, though not necessarily from Socorro. Most of the 16 have dates, altitudes, and brief habitat notes, but not specific localities. Some specimens bear labels of Gustavo A. Rovirosa, and at least one still has the name of Rovirosa as collector. The dates are those of Solis's visit, however, and Rovirosa was not on the trip. Jesiis Patino's specimens have no specific localities or dates. One is labeled "500-900 meters," and two others (Castilleja socorrensis and Gnaphalium attenuatum) suggest that he reached the summit. Perhaps he was on the expedition of the Escuela Nacional de Agricultura (see Richards and Brattstrom 1959). Because he was traveling with others, Richard S. Felger had little time for notes and numbered his specimens later, not chronologically. Moran curated the specimens, combining some collections, renumbering some because of duplicated numbers, and adding approximate coordinates. Thus some localities may be in error. On the same trip, Roberto Cruz collected about 80 numbers (J. Rzedowski, pers. comm.) and Luz Maria Villarreal de Puga (pers. comm.) collected 88; we have not seen their specimens, though de Puga sent us a list of her collections. In addition, some 10 Levin and Moran faculty and students from the University of Guadala- jara have made limited collections in recent years (L. M. V. de Puga, pers. comm.)- In 1988, biologists from the Centro de Investi- gaciones Biologia (CIB), La Paz, Baja California Sur, began long-term studies on Isla Socorro. Two botanists, Raymundo Dominguez C. and Jose Luis Leon de la Luz, have collected on the island. Their studies and those by other CIB biologists may be expected to yield valuable collections in the future. Though the number of specimens from Socorro has increased considerably since Johnston wrote his flora in 1931, most collections still are from near the few landing sites or from between the south shore and the peak of Cerro Evermann via the upper east slope. Large areas remain unexplored, especially on the north and east slopes. As on the nearby mainland (Mosino Aleman and Garcia 1974:375-376), almost all the rain on Socorro falls between July and October (see Climate, above). Most collections are from March to May — late in the dry season. Until 1988, when Domin- guez, Leon, and Levin all collected between September and December, only Barkelew (May-July 1903) and Mason and Hanna (November 1953) collected during or near the wet season. Thus opportunities for inter- esting collecting remain for the hardy botanist. VEGETATION Miranda (1960) described the vegetation of Isla Socorro and illustrated it with several excellent photographs. From his observations, mostly of the southwest to southeast slopes of the island, he classified the vegetation into seven types, mainly found in suc- cessive zones from shore to summit: coastal halophytes, Croton scrub, guava scrub, fig forest (with or without guavas), Bumelia/guawa forest, Dodonaea scrub, and summit grassland. We propose no formal classification of the vegetation but discuss vegetation and species distribution under the following headings: shore, grassland, scrub, forest, and summit. Shore. — Ipomopsis pes-caprae brasiliensis and Jouvea pilosa are common on sandy beaches around the island (Figure 2). These are joined by Chamaesyce incerta and Heliotropium curassavicum near the north end and by Boerhavia coccinea, Physalis mimulus, and invading Fig. 2. Shore vegetation at Bahia Academy, with Ipomoea pescaprae subsp. brasiliensis and Jouvea pilosa (lower right). The hillside in the background is dominated by Conocarpus erecta (dark trees) and Crolon masonii (light shrubs). Flora of Isla Socorro Cenchrus echinatus at the south end. Canavalia rosea is locally common. At Playa Blanca, Scaevola plumieri is common on the outer beach (Caleta Trueno). Just back from the beach in the northern part of the island are thickets of Conocarpus erecta and groves of Hip- pomane mancinella. Hibiscus pernambucensis is invading and becoming common on and above the beach near Playa Blanca. Cyperus ligularis is locally common just behind the beach. On bluffs and cliffs near the sea the vegetation is rather open. Common plants include Nicotiana stocktonii and Perityle socorrosensis, both growing almost exclusively near the shore, and Chamaesyce anthonyi, Cheilanthes peninsularis, and Erigeron socor- rensis, all reaching the summit. Grassland. — In openings in the scrub near the coast on the southern part of the island, coarse grasses pre- dominate, notably Aristida vaginara, Cenchrus myo- suroides, Heteropogon contortus, and Schizachyrium sanguineum. Miranda (1960) suggested that some of these openings, particularly the large one at Cabo Rule, have resulted from the combined action of humans and sheep. Small grassy flats occur well beyond their influence, however, as in the scrub just south of Caleta Grayson. A large area on the southeast side of the island, north of Huerta Grande at 300-450 m, clearly denuded by sheep, supports not perennial grasses but annual grasses and forbs. Over much of the comparatively dry west slope of the island above Caleta Grayson, grasses (especially Schizachyrium sanguineum) predominate on slopes and ridges, whereas scrub and small trees of Ficus cotinifolia predominate in moister places such as arroyos and north-facing slopes. Scrub. — A fairly dense non-spiny scrub, generally about 1-2 m high, covers most of the island (Figures 3 and 4). Although it varies from place to place, several very widespread plants tend to give it a basic unifor- mity in most places. Chamaesyce anthonyi and Waltheria indica are common from shore to summit and often abundant; with them, usually in lesser numbers, are Acalypha umbrosa, Brickellia peninsularis amphithalassa, Dodonaea viscosa, Triumfetta socor- rensis, and Viguiera chenopodina. On the generally moister north slope of the island, Erigeron socorrensis and Spermacoce nesiotica also are abundant, and Preridium caudatum, Schizachyrium sanguineum, and Sorghastrum nutans are common. At low elevations on the south and east sides of the island, Croton masonii predominates, sometimes to the near exclusion of other plants, giving the scrub a grayish cast (Figure 3). On the lower north and west slopes it is also common, sometimes forming pure stands locally but generally sharing the dominance with Chamaesyce anthonyi, Dodonaea viscosa, Spermacoce nesiotica, and Waltheria indica. Fig. 3. Scrub northeast of Caleta Binner, composed almost exclusively of Croton masonii. 12 Levin and Moran ^^v'-^- '•'fv ^•^' Fig. 4. Lava flow on south side of Cerro Evermann at ca. 700 m. Dominant plants are Dodonaea viscosa (dark) and Lepechinia haslala subsp. socorrensis (light). Dodonaea viscosa, though common elsewhere as well, pre(iominates in dry rocky places such as talus slopes and lava flows (Figure 4), where some of its usual associates are lacking. Often with it in such places are Cordia curassavica and Lepechinia hastata socorrensis. Forest. — Fairly dense forest covers much of the wet middle east slope of the island (Figures 5 and 6) and extends to the canyons and shallow valleys to the north. Dominant trees are Bumelia socorrensis, Ficus cotinifolia (Figure 7), and Ile.x socorroensis. Also common are Guettarda insularis and Psidium sp.; less common are Meliosma nesites, Prunus serotina, and, at higher elevations, Oreopanax xalapense. On both trees and ground are the ferns Asplenium sessilifolium, Polypodium alfredii, and P polypodioides aciculare; other epiphytes include Peperomia socorronis, P. tetraphyUa, and various orchids. Lianas are uncommon but include Chiococca alba and Vernonia littoralis. Southward, at lower elevations, the forest becomes lower and more open, reduced to shrubby Ficus cotinifolia and Psidium sp. or, in places, to Psidium alone. In this area, roamed by the sheep, there is little herbaceous vegetation, and the shrubs look in poor condition. Bumelia socorrensis, Guettarda insularis, and Ile.x socorroensis extend as shrubs nearly to the summit of Cerro Evermann; shrubby Guettarda also extends to sea level. Tree-size Ficus cotinifolia reaches the north coast at Bahia Academy and Playa Blanca and even approaches the shore in the narrow arroyo just north of Caleta Binner. Summit. — The summit of Cerro Evermann is cooler and wetter because of the elevation and the cloud cap that often covers it. About the summit, above about 950 m and down the west slope, coarse grasses predom- inate, especially Aristida vaginata, Schizachyrium sanguineum, Setaria geniculata, and Sorghastrum nutans (Figure 8). Although the predominance of grasses might result from grazing sheep. Levin found exposed ridgetops north and west of the summit equally grassy but apparently not reached by the sheep. Patches of brush extend up the east slope, with a few shrubs and subshrubs reaching the summit, including Bidens socorrensis, Brickellia peninsularis amphithalassa, Bumelia socorrensis, Chamaesyce anthonyi, Dodonaea viscosa, Eupatorium pacificum, Guettarda insularis, Hypericum eastwoodianum. Ilex socorrensis, Lepechinia hastata socorrensis, Spermacoce Flora of Isla Socorro 13 Fig. 5. Forest on southeast side of Cerro Evermann at ca. 800 m. nesiotica, Thumfetta socorrensis, and Waltheria indica. These are also found in the forest or scrub below. Many herbs are found only on the small climatic island of the peak, mainly in grassy places on the east slope, though others extend lower, some even to sea level. Plants found only near the summit include Aegopogon solisii, Botrychium socorrense, Castilleja socorrensis, Centaurium capense, C. wigginsii, Conyza confusa, Daucus montanus, Gnaphalium attenuatum, G. sphacilatum. Hibiscus diversifolius, Linaria canadensis texana, Oplismenus hirtellus, Passiflora sp., Sporobolus purpurascens, and Triodanis perfoliata biflora. This com- bination of local and widespread species makes the summit vegetation particularly rich, containing about one-third of the species native to the island. BIOGEOGRAPHY The known native vascular flora of Isla Socorro now stands at 117 species (Tables 2, 3, and 4). Four of these are identified only to genus, mostly for lack of ade- quate material: Opuntia sp., Passiflora sp., Psidium sp., and Riibus sp. Of the 113 identified species, 30 (26.5%) are endemic to Socorro at the specific (27) or subspecific (3) level. Nine more (8%) are endemic to the Revillagigedos (7 specific, 2 subspecific). The rate of endemism for the Revillagigedos is 31.6% (43/136; see Appendices 1 and 2). Although 26.5% may seem a high rate of endemism, for oceanic islands other than atolls it is only moderate. Carlquist (1974:112) found a range of 12-94% for 11 diverse oceanic islands. Furthermore, the Socorro flora does not show adaptive radiation. As a measure of insular diversity, several authors (MacArthur and Wilson 1967, Simberloff 1970) have used the species/genus ratio. Balgooy (1969) preferred the genera/family ratio, and Carlquist (1974:110-116) used both and added the species/family ratio. Table 2 summarizes vascular plant diversity on Isla Socorro. For Socorro the species/genus ratio is 1.15 for all species and 1.23 for endemics. Only one genus (Cypenjs) has more than three species native to the island, and only in that genus, and perhaps in Psidium, are different species on the island closely related. In almost all genera with more than one species on the island, the species belong to different sections or subgenera. For Socorro the genera/family ratio is 2.17 and the species/family ratio is 2.49. Each of the three values for Socorro is near the lowest of those Carlquist (1974:112) calculated for 11 oceanic islands and is well below what he considered the threshold for radiation. 14 Levin and Moran Fig. 6. Forest on upper east slope of Cerro Evermann. Fern is Asplenium sessilifolium. Several factors explain the moderate level of endemism on Socorro and the Revillagigedos. First, Socorro is a relatively small island with little ecological diversity to promote radiation (MacArthur and Wilson 1967:173-174, Balgooy 1969, Carlquist 1974:92-100). Second, unlike island groups known for their high endemism, such as the Hawaiian Islands and the Galapagos, the Revillagigedos are not an extensive archipelago that might allow speciation on different islands followed by dispersal among the islands (MacArthur and Wilson 1967:175). Only three genera (Aristida, Aristolochia, and Erigeron) are represented by sister species on different islands. Third, Socorro is closer to the mainland than are most oceanic islands, so that dispersal to the island is hkely to be more fre- quent, reducing the probability of speciation (Mac- Arthur and Wilson 1967:175). Thus, as would be expected on a small, solitary oceanic island relatively near the mainland, a moderate number of new island species has evolved but no radiation has occurred. Sources of the Flora Table 3 summarizes the distribution off the island of each non-endemic species, and Table 4 summarizes the distribution of the putative closest relative of each endemic species. Table 5, which summarizes Tables 3 and 4, is an analysis of the geographic relationships of the flora. (For the purposes of this discussion we Table 2. Vascular plant diversity on Isla Socorro. Families Genera Species Native Introduced Native Introduced Native Introduced Ferns and fern allies Dicotyledons Monocotvledons 3 40 4 77 17 0 24 5 10 86 21 0 37 10 All vascular plants Total 47 54 102 131 29 117 164 47 Flora of Isla Socorro 15 Fig. 7. Ficus cotinifolia. J:^-' Fig. 8. Suminit of Cerro Evermann, with vegetation of coarse grasses and scattered low shrubs. 16 Levin and Moran Table 3. Socorro. Distributions and presumed modes of dispersal for non-endemic native plants of Isla Distribution bevond Socorro" Non-endemic Species < Abutilon californkum N Adianlopsis radiata S Arislida adscensionis W Asplenium fonnosum W Asplenium sessilifolium N Boerhavia coccinea W Bursera epinnata Caesalpinia bonduc W Canavatia rosea W Cardiospermuin haltcacabum W Calileya auraniiaca W Cenchnis myosuroides W Centaurium capense Cenlaurium nigginsii N Chamaesyce incena N Chtococca alba W Conocarpus erecta W Conyza confusa W Cordia curassavica W Cressa rruxillensis W Cynanchum californicwn Cynanchum sonorense N Cyperus iwwellii N Cyperus ligularis W Daucus montanus W Dodonaea viscosa W Elyiraria imbricata W Epidendrum tiitens S Epidendrum rigidum S F/citf colinifolia W Foresliera rhamnifolia Galium mexicanum W Glinus radiatus W Gnaphalium attenualum W Gnaphalium sphacilalum W Gossypium hirsutum Heliotropium curassavicum W Heieropogon contortus W Heterotoma cordifolia W Hibiscus diversifolius Hippomane mancinella S Ipomoea pescaprae brasiliensis W Ipomoea triloba W Jouvea pilosa W Lanlana velutina W Linaria canadensis lexana W Opiismenus hirtellus N Opuntia sp. Oreopanax xalapense W Fbssijlora sp. Pavonia hastata X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X X oeromia telraphxlla W X aseolus lunalus W oradendron cornmulatwn W lypodium alfredii s lypodium polypodioides aciculare w lystichum muricatum s X nulaca pilosa w X X E Mexico Pacific Islands C. & S. Am. S. Am., SE U.S., NE Mexico X X X X X X X X X X X X X X X X X X X X X X X X X D? A BB A A BV HI D D D A BB BM BM D? BI D BB BI D BB BB BM BM BB D BV? A A BI BI BB BM BB BB D D BM BM D? D D D D BI BM? BB BI BI BI D? BV D? BI A A A D Flora of Isla Socorro 17 Table 3 (continued) Non-endemic Species Distribution beyond Socorro^ U -a D. 1 X BI BI BI X A X A BI X D? BI X D X BB X BB X D? BI BB BI BI X BM BB X D? BI Fbtamogeton nodosus W X Primus serotina W Psidium sp. Psilotum nudum w X Pteridium caudalum w X Rhamnus sharpit s Rhvnchosia minima w X Rubus sp. Scaevola plumieri w X X Schizachyrium sanguineum w X X Seiaria geniculaia w X X Sida rhombifolia w X X Solanum madrense w Sorghasirum nutans w Sporobolus purpurascens s X Tourneforlia hartwegiana w X Triodanis perfoliata biftora w X \ 'iguiera chenopodina X Waltheria indica w X X Zanthowlum insulare N X ^We give distributions outside the primary source areas of western mainland Mexico, Baja Cali- fornia, and the West Indies only if the species does not occur in either western mainland Mexico or Baja California. We divide western mainland Mexico into two subregions: northwestern (N) for the coastal states from Colima north and southwestern (S) for the coastal states south of Colima; if a species occurs in both regions, we show "W" for western. Other geographic abbreviations: C. Am., Central America; E, eastern; NE, northeastern; S, southern; S. Am., South America; SE, southeastern. ''A, air flotation; BB. barbs, bristles, etc., caught on birds; BI, internally in birds; BM, mud on birds' feet; BV, viscid disseminules stuck to birds; D, drift. include under the term "endemic" both plants restricted to Socorro and other Socorro plants endemic to the Revillagigedos.) Among potential source areas we include not only Baja California and western mainland Mexico, the closest land masses, but also the West Indies because Johnston (1931) thought the Socorro flora bore some relationship to the West Indian. Table 5 clearly shows that the Socorro flora is mainly west Mexican. Fully 83% of the species occur also in western mainland Mexico or (endemics) have their closest relatives there, and ZS'Vo extend no farther. The Socorro flora is less related to the Baja Californian. Fewer than half the species (47%) occur also in Baja California or (endemics) have their closest relatives there, and very few (11%) extend no farther. Nearly half the Socorro species (46%) occur also in the West Indies or (endemics) have their closest relatives there, but all these plants extend beyond the West Indies, most to western or southwestern Mexico. (Johnston (1931) sug- gested that the closest relatives of two endemics, Ilex socorroensis and Pleurolhallis unguicallosa, are restricted to the West Indies, but we consider the rela- tionships of these species unclear.) Thus the relation- ship with the West Indian flora is not remarkable. Because Socorro is closer to Baja California than to mainland Mexico, the much stronger affinity of the flora with that of western Mexico may appear surpris- ing. Mainland Mexico, however, is a much larger and more diverse source for colonization than is tropical Baja California. Furthermore, the principal storm tracks run from the east or southeast (Mears 1944:104, Mosino Aleman and Garcia 1974:359-360), probably favoring dispersal of all but drift plants, which are favored by the current from the north. Finally, because the flora of southern Baja California shares many species with the mainland flora (Brandegee 1892, Wiggins 1960), the specific source for these species is not identifiable. Though both non-endemics and closest relatives of endemics are basically Mexican, the distributions of the two groups show some interesting differences (Table 5). Proportionally, many more of the non- 18 Levin and Moran Table 4. Distribution and presumed modes of dispersal for endemic plants of Isla Socorro and their closest relatives. (We also include here Socorro plants endemic to the Revillagigedos.) .5 < u D. 5 Acalypha umbrosa E X BM? Aegopogon solisii E W X BB Aristida vagina ta E N BB Aristolochia socorroensis E W X HI? Bidens socorrensis E W X BB Boirychium socorrense E w A BhckelUa pen insular is amphithalassa C X BB Bulbosiylis nesiolica R w X BM Bumelia socorrensis E w X X X BI Castilleja socorrensis E X BM? Ceslrum pacificum E w BI Chamaesyce anthonyi R w X X X BI? Cheilanthes peninsularis insularis C X A Coreocarpus insularis E X BB Croton masonii E N D? Cyperus duripes R w X X X BM Erigeron socorrensis E w BB Eupatorium pacificum E w BB Guettarda insularis E w X BI Hypericum eastwoodianum E X BM? Ilex socorroensis E 7 BI Lantana involucrata socorrensis E X SE Mexico, C. & S. Am. BI Lepechinia hasiata socorrensis E X BV? Meliosma nesites E w BI Nicotiana stockronii C NE Mexico BM? Paspalum longum E X C. & S. Am. BI f^peromia socorronis E w BV Perityle socorrosensis R X BB Physalis mimulus E w X X X BI Pleurolhallis unguicallosa E 7 A Psidium socorrense E 7 BI Salvia pseudomisella E w X X X BV? Sida nesogena E w X X X D? Spermacoce nesiolica C w X BM? Teucrium townsendii affine E N X BM Triumfetta socorrensis E w BB Verbena sphaerocarpa E w BI? lernonia liltoralis E w BB Zapoteca formosa socorrensis C w X D? ^C, also on Clarion; E, only on Socorro; R, on all the Revillagigedos. ''We give distributions outside the primary source areas of western mainland Mexico, Baja California and the West Indies only if the endemic species does not have closest relatives in either western mainland Mexico or Baja California. Abbreviations as in Table 3. ■^ Abbreviations as in Table 3. endemics are widely distributed in the American tropics. Indeed, plants that disperse so well would be expected to colonize the island repeatedly and so not to speciate there (MacArthur and Wilson 1967:175). Only 5% of the non-endemics are restricted to Socorro and Baja California, whereas fully 22% of the relatives of endemics are found only in Baja California. Re- striction to Baja California may result from limited dispersal ability, so that these plants would be unlikely to reach the island more than once and therefore more likely to speciate should they colonize it. Furthermore, Table 5 suggests that dispersal to Socorro generally is less likely from Baja California than from mainland Mexico — again with the result that plants from Baja California are more likely to speciate on the island. Similar causes may explain why just 5% of the non- endemics grow in neither Baja California nor western mainland Mexico but only in some more distant area Flora of Isla Socorro 19 Table 5. Geographic relationships of flora of Isla Socorro. We base percentages on the 74 identified non-endemics and on the 36 endemics for which we can identify closest relatives. Data for individual species can be found in the Catalogue of Species and are summarized in Tables 3 and 4. Western Mainland Mexico Baja Present California Restricted West Indies Not in Western Mexico or Baja California Widespread in American Tropics Present Restricted Present Restricted Non-endemics (74 spp.) 66 89% 17 23% 35 47% 4 5% 37 50% 0 0% 4 5% 38 51% Closest relatives of endemics (36 spp.) 25 69% 11 30% 17 47% 8 22% 14 39% 0 0% 3 8% 6 17% Total (110 spp) 91 83% 28 25% 52 47% 12 11% 51 46% 0 0% 7 6% 44 40% whereas 8% of the closest relatives of endemics grow only in a more distant area. Johnston (1931) consid- ered an even higher proportion of endemics to be derived from outside western Mexico and Baja Cali- fornia; our lower figure probably results from increased knowledge of plant distribution in Mexico and from refinements in taxonomy. Modes of Long-Distance Dispersal to Socorro Using the guidelines proposed by Carlquist (1974:54-82), we have decided what mode of long- distance dispersal seems most probable (or least improbable) for the ancestor of each plant taxon native to Socorro (Tables 3 and 4). Of the 117 species, 14 (12%) probably arrived by air flotation, 24 (21%) by barbs, bristles, etc., caught on birds, 31 (26%) inter- nally in birds, 17 (15%) in mud on birds' feet, 6 (5%) by viscid disseminules stuck to birds, and 25 (21%) by drift, either rare or frequent. These figures are mostly close to those Carlquist (1974:78) determined for the Revillagigedos using Johnston's (1931) flora. Our figure for air flotation is higher, however, probably because of the several newly reported ferns and orchids. Also, our figure for drift is lower and our figure for mud on birds' feet is higher. We guessed that many plants with small seeds and no obvious means of long- distance dispersal might have reached Socorro in mud on birds' feet, whereas Carlquist perhaps guessed drift instead. These two modes are the least distinctive, so we do not place much confidence in these figures. For the oceanic islands he studied, Carlquist (1974:78-82) interpreted high frequencies of air-borne and of barbed disseminules as reflecting (1) relative closeness of the island to the mainland and (2) a relatively dry climate, which favors plants with these types of disseminules over plants with fleshy fruits eaten by birds. On Socorro, most of the plants with fleshy fruits grow in the forest or wetter scrub. We also summarize means of arrival for the closest relatives of the 39 endemic species alone: 3 (8%) by air flotation, 10 (26%) by barbs, etc., caught on birds, 12 (31%) internally in birds, 8 (20%) in mud on birds' feet, 3 (8%) by viscid disseminules, and 3 (8%) by drift. The substantial decreases for air flotation and especially for drift may reflect the efficacy of these methods for dispersal to Socorro and the resultant reduced probability that plants arriving by these means will differentiate into new species. The increases for all types of dispersal by birds probably are simply the con- sequence of decreases for the other two types. The preponderance of dispersal by birds to Socorro is striking, both for the entire flora (67%) and for the endemics (85%). Jehl and Parkes (1982), documenting the avifauna of the Revillagigedos, listed six shorebirds, five gulls, and five warblers that are at least casual visitors to Socorro, and Levin has observed one more (Wandering Tattler, Heteroscelus incanus). In addition. Mourning Doves (Zenaida macroura) and Northern Mockingbirds (Mimus polyglottos) have recently col- onized the island (Jehl and Parkes 1982, 1983). Thus there appear to be enough birds visiting the island to account for their importance as dispersal agents. Evolution of the Flora Many of the non-endemic species on Isla Socorro differ from their mainland representatives in subtle ways, as might be expected from the isolation of the island populations. Though many of the differences, as well as the differences between the endemics and their closest relatives, appear to be random, certain pat- terns do emerge. Most notable is increased woodiness. The perennial endemics Bidens socorrensis, Chamaesyce anthonyi, Erigeron socorrensis, and Salvia pseudomisella all appear to be derived from herbaceous annual ancestors, and both C. anthonyi and E. socorrensis are well-developed shrubs. Hypericum eastwoodianum and Perityle socorrensis are also endemics that are shrub- bier than their mainland relatives, though those are perennial. Among non-endemics, only Conyza confusa appears to have become more woody on the island. 20 Levin and Moran Carlquist (1974:350-428) discussed insular woodiness at length. Among the possible causes he discussed, climatic moderation seems the most likely to apply on Socorro. Certain species among both endemics (Brickellia peninsularis amphithalassa, Lepechinia hastata socor- rensis, Sida nesogena) and non-endemics (Abutilon californicuin, Conyza confusa, Waltheha indica) have denser and/or longer pubescence than their mainland relatives. The reason for this is not obvious, but perhaps greater pubescence helps protect the leaves from intense sunlight or winds. Possibly winds also account for the inflorescences of /J. califomicum, S. nesogena. Verbena sphaerocarpa, and W. indica being more compact than those of their mainland relatives. CATALOGUE OF SPECIES This list includes all species of vascular plants known to be native or naturalized on Isla Socorro. An asterisk before the name means that the plant probably is not native. A list of species reported from Socorro but excluded from our flora follows the list of known species. Lists of synonyms are not necessarily complete but aim to include all names used for the taxa as they occur on Socorro. We summarize the known distribution on the island for each ta.\on, the distribution off the island for each non-endemic taxon, and the distribution of the putative closest relative for each endemic. We generally cite specimens only if the species is newly reported. We add brief descriptive and taxonomic notes as appropriate. Our descriptions of abundance are based on subjective impressions and are not quantifiable. FERNS AND FERN ALLIES Key to Families 1. Leaves scale-like; stems naked, dichotomously branched; sporangia trilocular PSILOTACEAE 1. Leaves foliaceous; stems clothed with scales and/or leaf bases, variously branched; sporangia unilocular 2. Stipes two-branched below the blade, one portion fertile with sporangia borne on an erect non-leafy panicle (only a stub remaining on old leaves), the other portion sterile and leaf-like OPHIOGLOSSACEAE 2. Stipes not branched below the blade; sporangia borne on the undersides of normal leaves POLYPODIACEAE sensu lata Ophioglossaceae Botrychium socorrense W. Wagner, sp. nov. (Figure 9) Type. — Isla Socorro, Mexico: uncommon in summit grassland at ca. 1000 m, slope ca. 1 km northwest of summit of Cerro Evermann, near 18°47'N, 110°58'W, 15 December 1988, Levin 2044 (\\o\o\.ypt: SD 124655; isotypes BM, MEXU, MICH, MO, UC, US). Planla 40-60 cm alta. Caulis erectus, usque ad 1 cm diametro. Primordia foliorum dense sericea pilis 5 mm longis. Radices usque ad 15, suberosae annulataeque. Lamina sterilis 4-pinnata, late deltoidea, 16 (13-20) cm longa, 25 (17-28) cm lata; segmenta arc- tissime imbricata; pinnulae ultimae pro parte maxima ovatae vel late lanceolatae, vadose vel profunde fissae, illae non ad medium fissae 10.5 (7-14) mm longae, 5.5 mm latae, margine crenulatae. Sporophorum 40-60 cm altum, fascicule sporangiorum ca. 10-15 cm longo. Typus: Le\in 2044 (SD 124655). Species B. schaffneri affinis sed statura majore, pinnis pinnulisque arctissime imbricatis, et segmentis ovatis vel late lanceloatis differt. Coarse fleshy perennial herb, 40-60 cm tall when mature, including the fertile structure (sporophore). Stem erect, up to 1 cm in diameter, covered by root and leaf bases. Roots up to 15 per plant, corky with closely spaced encircling ridges, and reaching to 5 mm in diameter (dried specimens). Leaf primordia densely covered with fine silky hairs up to 5 mm long, and enclosed by tubular leaf sheath. Leaf sterile by abor- tion of sporophore, or two-parted, with sterile photo- synthetic portion (trophophore) more or less horizontal when ahve, and an erect sporophore. Common stalk in mature leaves 4.5 (3-8) cm long. Trophophore stalk 15 (10-26) cm long, very robust, 5.5 (4-8) mm in diameter (dried). Trophophore blade broadly deltoid, 16 (13-20) cm long, 25 (17-28) cm wide, tetrapinnate. Pinnae alternate to opposite, the tips somewhat rounded. Pinnules that are cleft less than one-half the distance to main vein 10.5 (7-14) mm long, 5.5 (4-7) mm wide. Pinnule margins shallowly crenate, the crena- tions associated with vein endings. Sporophore stalk considerably larger than entire trophophore, 32 (25-42) cm long, topped by a sporangial cluster 10-15 cm long (as estimated from fragmentary axes). Paratypes. — Uncommon in grassland, ridge north- west of Cerro Evermann, 915 m. Levin 1814 (MICH, SD); north slope of Cerro Evermann near solfataras, 1000 m, Moran 5854 (SD); north ridge of Cerro Evermann, 950 m, Moran 5854A (SD). Distribution. — Endemic to Isla Socorro. Uncom- mon, in sparse grass and under brush, north and west slopes of Cerro Evermann above about 900 m. Relationships. — B. socorrense is most closely related to B. schaffneri L. Underw. of the mainland Mexican highlands. The Socorro plant differs in its extremely robust habit (40-60 cm tall rather than mostly 15-35 cm tall), its broadly ovate to lanceolate, relatively undivided, and roundish tipped leaf segments (rather than lanceolate to linear and more divided and pointed), and its more coarsely crenate leaf margins (rather than mostly entire to finely crenate). The Flora of Isla Socorro 21 Fig. 9. Leaves and pinnae of Botrvchium. A, B. schaffneri (Mexico, Guerrero, 6.5 miles from Tetipac on road to Ta.xco, Wagner and Wagner 80821: MICH); B, B. socorrense (Levin 2044; SD). Strongly overlapping blade divisions are especially distinctive. Figure 9 illustrates these differences, which are also maintained in immature plants, of which we have examined three examples of B. socorrense (Moran 5854). Extremely fine cutting like that frequently seen in B. schaffneri has not been documented for B. socor- rense. Perhaps environmental factors have played a role in the distinctive morphology of B. socorrense. Its habitat, among tall grasses, is similar to that of B. schaffneri, but at a considerably lower altitude (only 915-1000 m vs. 1650-3000 m, average 2500 m, for B. schaffneri). The climate of Isla Socorro is oceanic, however, and the frequent cloud cover near the summit may ameliorate the effects of the lower elevation. The Socorro grapefern should be further studied biosystematically, by means of common garden experiments as well as isozyme analysis. 22 Levin and Moran Polypodiaceae Key to Species 1. Blades digitate, with several pinnae arising from the same point Adiantopsis radiata 1. Blades pinnate or bipinnate, the pinnae arising from many points along the rachis 2. Blades once pinnate, the pinnae entire 3. Pinnae essentially glabrous fblypodium alfredii 3. Pinnae densely scaly on undersurfaces fblypodium polypodioides 2. Blades twice or more pinnate, or if once pinnate then the pinnae toothed to lobed 4. Fronds large, 1.2-3 m tall; blades coriaceous; sori continuous, marginal, covered by inrolled leaf margin Ptehdium caudatum 4. Fronds smaller, much less than 1 m tall; blades generally thinner; sori marginal or on undersurface of blade 5. Leaf segments essentially equilateral, equally divided along both sides; sori marginal to submarginal, covered by the inrolled leaf margin Cheilamhes peninsularis 5. Leaf segments clearly inequilateral, much more deeply divided on the upper side than on the lower side; sori on undersurface of blade, covered by indusium arising from blade surface 6. Pinnae with spinose teeth; sori round; indusia peltate Fblysrichum muricalum 6. Pinnae toothed, but the teeth not spinose; sori elongate; indusia attached along one side 7. Stipes dull; upper side of ultimate leaf segments with a basal lobe cut nearly to midrib; plantlets often produced near leaf apex Asplenium sessilifoUum 7. Stipes lustrous; upper side of ultimate leaf segments equally incised along entire length but not lobed at base; plantlets not produced Asplenium formosum Adiantopsis radiata (L.) Fee, Mem. Foug. 5:145. 1852. central and southern Baja California. In the protologue Distribution. — Occasional in rock crevices and on of var. insularis, Weatherby described specimens from damp forest floor, 550-800 m. Southern and eastern Comondii as intermediate, some individuals rather Mexico, West Indies, to South America. closely approaching var. insularis. Asplenium formosum Willd., Sp. PI. 5:329. 1810. Polypodium alfredii Rosenstock, Repert. Spec. Nov. Distribution. — Occasional on canyon walls and on Regni Veg. 22:15. 1925. Pecluma alfredii tree trunks and forest floor, from near shore to near (Rosenstock) Price, Amer. Fern J. 73:113. 1983. summit. Southern and western Mexico, West Indies, Distribution. — Common on soil, in rock crevices, to South America. and on trees in forest, 750-1000 m. Southern Mexico Remarks. — On the basis of Mason 1615 and 1631, to Costa Rica. Weatherby (in Johnston 1931:48) said the Socorro plant Remarks. — On the basis of Barkelew 236 and Mason differed from the typical form in its brown rachis and 1633, Weatherby (in Johnston 1931:50) reported this rather less divided pinnae, in these respects approaching fern as fblypodium pulchrum M. Martens & Galeotti. A. carolinum Maxon [A. formosum var. carolinum According to A. R. Smith, who annotated specimens (Ma.xon) C. Morton], of the Galapagos Islands. at SD (Felger 15817; Moran 5796, 25500, 25502, . , . ...r ,. T^ r, ,■ XM c m 29520), the Socorro plant is Polvpodium alfredii. Asplenium sessilifolium Desvaux, Berlin. Mag. 5:322. ^ 1811. Polypodium polypodioides (L.) Watt var. aciculare /I. pofos;>2WW Hieron. var. />;nsuw Hieron., Hedwigia Weath., Contr. Gray Herb. 124:33. 1939. 60:247. 1918. Distribution.— Common on trees in forest, 550-950 Distribution. — Common in forest shade, at least m. Southwestern Mexico and Central America. 700-825 m. Central and western Mexico to South Remarks. — Previously reported from Socorro by America. Weatherby, who cited Barkelew 236 in the protologue Remarks.— 'We follow Mickel and Beitel (1988) in to the variety. treating A. potosinum incisum, the name used by „ , . ,. . . ., ^ r-- »*■ r- c -.-lo ,,„,,.„, . . .,.,, ,. Polvstichum muncatum (L.) Fee, Mem. Foug. 5:278. Johnston (1931:49), as a synonym of yl. s?S5////o//ww. " 1852. Cheilanthes peninsularis Maxon var. insularis Weath., Distribution. — Uncommon on forest floor, at least Amer. Fern J. 21:25. 1931. 700-825 m. Southern Mexico to Jamaica, Haiti, and Distribution. — Fairly common on rocks and cliffs, Venezuela, especially near shore but also to summit. Endemic to Remarks. — Weatherby (in Johnston 1931:49) cited isias Clarion and Socorro; type from very dry north- Mason 1630 and 7677 and Barkelew 227. He wrote that facing wall of canyon above Caleta Grayson, 300 m, the undersurface of the pinnae seemed much more 4 May 1925, Mason 1616 (CAS 186601). fibrillose in the island plants than in most material Remarks. — The var. peninsularis is endemic to from the mainland. Flora of Isla Socorro 23 Pteridium caudatum (L.) Maxon, Proc. U.S. Natl. Mus. 23:631. 1901. P. aquilinum (L.) Kuhn var. caudatum (L.) Sadeb., Jahrb. Hamburg Wiss. Anst. 14 (Beih. 3):5. 1897. Distribution. — Fairly common on brushy slopes in upper half of island; down to ca. 500 m on south slope, to at least to 275 m on west slope above Caleta Grayson, and to 90 m on north slope above Bahia Academy. Florida, West Indies, most of Mexico, to northern South America. Psilotaceae Psilotum nudum (L.) P. Beauv., Prod. Aetheog. 106, 112. 1805. Distribution. — Reported for Socorro by Johnston (1931:48) on the basis of Barkelew 218; no new collec- tions. The distribution of plants with close collection numbers suggests that Barkelew's collection might have come from the forested upper part of the island, possibly on the north side. Widely distributed in tropics. ANGIOSPERMAE: DICOTYLEDONS Key to Families 1. Plants vines climbing by tendrils 2. Tendrils arising from inflorescence axis below flowers; leaves compound SAPINDACEAE 2. Tendrils arising from base of leaves; leaves simple 3. Tendrils branched; leaves cordate but not lobed CUCURBITACEAE 3. Tendrils unbranched; leaves irilobed PASSIFLORACEAE 1. Plants lacking tendrils 4. Plants cactus-like, with flattened, leafless, fleshy stems covered with spines CACTACEAE 4. Plants not cactus-like 5. Plants parasitic, attached to branches of shrubs or trees; leaves opposite VISCACEAE 5. Plants not parasitic, terrestrial, or if epiphytic and perhaps appearing parasitic, the leaves alternate or whorled 6. Inflorescence a dense head subtended by an involucre; sepals dry scales, bristles, or absent, white to brown but not green ASTERACEAE 6. Inflorescence various, but not an involucrate head; sepals generally green 7. Plants conspicuously woody: shrubs or trees 8. Leaves compound GROUP 1 8. Leaves simple 9. Leaves opposite GROUP 2 9. Leaves alternate 10. Leaves entire GROUP 3 10. Leaves crenate or toothed GROUP 4 7. Plants herbaceous or base slightly woody 11. Perianth absent or a single whorl (appearing to be sepals or petals, but not both) GROUP 5 11. Perianth of 2 or more whorls, differentiated into sepals and petals 12. Petals fused to each other at least at base to form a cup or tube, falling as a unit GROUP 6 12. Petals free from each other at least at base and usually throughout, falling singly or in pairs GROUP 7 GROUP 1: SHRUBS OR TREES WITH COMPOUND LEAVES 1 . Leaves palmate ARALIACEAE 1. Leaves pinnate 2. Leaflets entire; plants not aromatic; fruit a several-seeded legume FABACEAE 2. Leaflets crenate to serrate; plants aromatic; fruit of several one-seeded follicles RUTACEAE GROUP 2: SHRUBS OR TREES WITH OPPOSITE, SIMPLE LEAVES 1. Plants with milky juice; flowers unisexual, without perianth; inflorescence of a single carpellate flower and several staminate flowers subtended by an involucre EUPHORBIACEAE 1. Plants without milky juice; flowers perfect, or if unisexual then inflorescence not as above 2. Ovary inferior 3. Stipules absent; petals free; stamens more than 10 M YRTACEAE 3. Stipules present; petals fused; stamens 5 or 4 RUBIACEAE 2. Ovary superior 3. Flowers inconspicuous, petals none; fruit a one-seeded drupe; stone longitudinally ribbed OLEACEAE 3. Flowers conspicuous, petals present; fruit a capsule, 2-4 nutlets, or a two-seeded drupe 4. Petals free, bright yellow; stamens more than 10; fruit a many-seeded capsule HYPERICACEAE 4. Petals fused, white, pink, violet, or blue, never bright yellow; stamens 4 or 2; fruit 2-4 nutlets or a two-seeded drupe 5. Flowers strongly bilateral, solitary or in verticels LAMIACEAE 5. Flowers radial or nearly so, in dense terminal heads or spikes VERBENACEAE GROUP 3: SHRUBS OR TREES WITH ALTERNATE, SIMPLE, ENTIRE LEAVES 1. Branches producing aerial roots that may reach the ground to form secondary trunks; terminal buds enclosed in large stipules; flowers borne on the inside of a globose, fleshy receptacle, the fig MORACEAE 24 Levin and Moran 1. Branches not producing aerial roots; stipules, if present, small and not enclosing buds; flowers variously arranged, not inside a receptacle 2. Flowers and fruits in globose, conelike heads; leaf blade with a pair of glands at base COMBRETACEAE 2. Flowers and fruits not in conelike heads; leaf blade without glands at base 3. Fruit a winged capsule; leaves glutinous SAPINDACEAE 3. Fruit an unwinged capsule, drupe, or berry; leaves not glutinous 4. Plants succulent; ovary inferior; corolla strongly bilateral GOODENIACEAE 4. Plants not at all succulent; ovary superior; corolla radial or nearly so 5. Inflorescence one-sided and coiled; fruit drupaceous, containing four nutlets BORAGINACEAE 5. Inflorescence various, but not one-sided and coiled; fruit a berry or capsule 6. Fruit a one-seeded berry; plant with milky juice; stamens opposite corolla lobes, alternating with petaloid staminodes SAPOTACEAE 6. Fruit a many-seeded capsule or berry; plants without milky juice; stamens alternate with corolla lobes; staminodes none SOLANACEAE GROUP 4: SHRUBS OR TREES WITH ALTERNATE, SIMPLE, TOOTHED LEAVES 1. Plants aromatic; bark smooth, exfoliating in thin sheets; fruit a 1-seeded leathery capsule BURSERACEAE 1. Plants not aromatic; bark rough or if smooth, not exfoliating in thin sheets; fruit various, but if a capsule, containing more than 1 seed 2. Petals absent, at least in carpellate flowers; tlowers all unisexual; inflorescence a spike or raceme, with carpellate flowers below and staminate flowers above EUPHORBIACEAE 2. Petals present in all flowers; flowers perfect, or if some or all of them unisexual, then the inflorescence not as above 3. Fruit a spiny bur TILIACEAE 3. Fruit not spiny 4. Fruit a follicle, capsule, or schizocarp 5. Petals fused; stamens free from each other but fused to the corolla SOLANACEAE 5. Petals free; stamens fused to each other 6. Stamens many more than 10 MALVACEAE 6. Stamens 5 STERCULIACEAE 4. Fruit a berry or drupe 7. Inflorescence an open panicle; 3 outer petals considerably larger than inner 2; stamens 2, opposite 2 petals SABIACEAE 7, Inflorescence various, but not a panicle; all petals essentially the same size; stamens 5 or more 8. Flowers fascicled 9. Leaves broadly obovate, about as long as wide; petals 4; stamens alternating with petals; drupes one-seeded AQUIFOLIACEAE 9. Leaves elliptic or narrowly obovate, 2-3 times as long as wide; petals 5; stamens opposite petals; drupes three-seeded RHAMNACEAE 8. Flowers in spikes, racemes, or cymes 10. Petals free; stamens many more than 10; carpels free ROSACEAE 10. Petals fused; stamens 5; carpels fused at least in ovary II. Inflorescence a terminal spike; style 4-branched; ovary 4-celled BORAGINACEAE U. Inflorescence a lateral or terminal cyme; style unbranched; ovary 2-celled SOLANACEAE GROUP 5: HERBACEOUS PLANTS; PERIANTH NONE OR IN ONE WHORL I. Plants with milky juice; inflorescence of a single carpellate flower and several staminate flowers subtended by an involucre .... '. .EUPHORBIACEAE 1. Plants without milky juice; inflorescence various, but not as above 2. Plants epiphytic; perianth absent; flowers minute; inflorescence a dense elongate terminal spike with inconspicuous bracts PIPERACEAE 2. Plants terrestrial; perianth present; flowers larger; inflorescence various, if a terminal spike then the bracts conspicuous 3. Leaves appearing whorled 4. Perianth parts 4, fused; fruit a schizocarp RUBIACEAE 4. Perianth parts 5, free; fruit a capsule MOLLUGINACEAE 3. Leaves alternate or opposite 5. Leaves opposite NYCTAGINACEAE 5. Leaves alternate 6. Leaves compound; inflorescence a compound umbel APIACEAE 6. Leaves simple; inflorescence a spike or the flowers solitary 7, Perianth parts fused into a long tube; flowers strongly bilateral; fruit a 5-celled capsule. . . . ARISTOLOCHIACEAE 7, Perianth parts free or nearly so; flowers radial; fruit a utricle 8. Bracts and perianth dry, scarious; bracts spine-tipped AMARANTHACEAE 8. Bracts and perianth herbaceous; bracts not spine-tipped CHENOPODIACEAE GROUP 6: HERBACEOUS PLANTS; PETALS FREE 1. Leaves compound 2. Ovary inferior; inflorescence a compound umbel APIACEAE 2. Ovary superior; inflorescence a spike, raceme, or panicle, or the flowers solitary 3. Ovary 5-celled; fruit a schizocarp, separating into 5 spiny mericarps ZYGOPHYLLACEAE Flora of Isla Socorro 25 3. Ovary 1-celled; fruit a legume or capsule 4. Ovary with 2 carpels; placentalion parietal: fruit a capsule CAPPARIDACEAE 4, Ovary with 1 carpel; placentalion marginal; fruit a legume FABACEAE Leaves simple 5. Sepals 2; leaves succulent; ovary partly inferior PORTULACACEAE 5. Sepals 3 or 5; leaves not succulent; ovary completely superior 6. Sepals 3, caducous; petals 6; plants spiny PAPAVERACEAE 6. Sepals 5, persistent; petals 5; plants not spiny 7. Stamens 5 STERCULIACEAE 7. Stamens more than 10 8. Stamens free or nearly so; fruit an elongate capsule TILIACEAE 8. Stamens fused into a tube surrounding the ovary; fruit a schizocarp or + globose capsule MALVACEAE GROUP 7: HERBACEOUS PLANTS; PETALS FUSED Leaves opposite 2. Plant a twining vine with milky juice; stamens fused to stigma; fruit one or two follicles ASCLEPIADACEAE 2. Plant not twining; milky juice absent; stamens free from stigma; fruit a capsule or schizocarp 3. Ovary inferior; petals 4; leaves entire RUBIACEAE 3. Ovary superior; petals 5 or if appearing 4 then the leaves toothed 4. Stamens 5; anthers spirally twisted GENTIANACEAE 4. Stamens 4 or 2; anthers straight 5. Corolla radial or nearly so VERBENACEAE 5. Corolla strongly bilateral 6. Plants aromatic; fruit 4 nutlets LAMIACEAE 6. Plants not aromatic; fruit a capsule SCROPHULARIACEAE Leaves alternate or basal 7. Leaves basal; stems clothed with imbricated scale-like bracts ACANTHACEAE 7. Leaves scattered along stem, alternate; stems without scale-like bracts 8. Stamens more than 10, fused by the filaments but the anthers free MALVACEAE 8. Stamens 5 or 4, free or if fused by the filaments then the anthers also fused 9. Ovary inferior CAMPANULACEAE 9. Ovary superior 10. Corolla bilateral; stamens 4 SCROPHULARIACEAE 10. Corolla radial; stamens 5 11. Inflorescence one-sided and coiled; ovary 4-celled; fruit a schizocarp BORAGINACEAE 11. Inflorescence not one-sided and coiled; ovary 2-celled; fruit a capsule 12. Styles, style branches, or stigmas 2; sepals free nearly to the base CONVOLVULACEAE 12. Style and stigma 1; sepals fused through much of their length, only the lobes free SOLANACEAE Acanthaceae Elytraria imbricata (Vahl) Pers., Syn. PI. 1:23. 1805. E. tridentata Vahl, Enum. PL 1:107. 1804. E. squamosa (Jacq.) Lindau, Anales Inst. FisrGeogr. Nac. Costa Rica 8:299. 1895. Distribution. — Widespread on island, from shore to summit, but uncommon. Arizona and Texas to Brazil. Remarks. — Anthony's specimens are labeled E. tridentata, and Johnston (1931:96) called the plant E. squamosa. Both names are now considered synonyms of E. imbricata. Amaranthaceae *Ainaranlhus palmeri S. Watson, Proc. Amer. Acad. Arts 12:274. 1877. Distribution. — Weed in abandoned field, EI Ran- chito, north of Caleta Castelan {Felger 15863). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1967. Weed throughout southwestern United States, Baja California, to central Mexico; introduced in central and eastern United States. Apiaceae Daucus montanus Humb. & Bonpl. ex Sprengel in Roemer & Schultes, Syst. Veg. 6:482. 1820. Distribution . — Fairly common near summit of Cerro Evermann, 950-1040 m {Felger 15759; Levin 1815; Miranda 8739; Moran 5803, 29507). Not previously reported for Socorro. Northern Mexico to Chile. Aquifoliaceae Ilex socorroensis T. S. Brandegee, Univ. Calif. Publ. Bot. 4:90. 1910. Distribution. — Common in forest above ca. 550 m and occasional in scrub and grassland to summit; on north ridge down to 450 m. Endemic to Isla Socorro; type without e.xact locality, Barkelew 179 (\JC 136719). Remarks. — In the forest this is a tree to 10 m tall, with a branch spread up to 14 m; the largest trunk measured was 4.5 x 3.0 dm. In more exposed places, as on the north ridge and near the summit of Cerro Evermann, it is a shrub 2-3 m tall. It flowers from December to April. According to Johnston (1931:72), this tree has no 26 Levin and Moran obvious relations in Mexico or Central America but is related to /. repanda Griseb. of the West Indies. /. socorroensis looks much like the widespread Mexican /. tolucana Hemsley, however, differing mainly in leaf shape. Although leaf shape is similar in /. socorroensis and /. repanda, leaves of/, socorroensis have distinctly crenulate-serrate margins as in /. tolucana, whereas the leaf margins of /. repanda are wavy with at most a few weak crenulations and are often entire. Resolution of the relationships among these species awaits a modern revision of the genus. Araliaceae Oreopanax xalapense (Kunth) Decne. & Planchon, Rev. Hort. sen 4. 3:108. 1854. Distribution. — Uncommon in upper part of forest, 800-950 m. Jalisco and Veracruz to Panama. Aristolochiaceae Aristolochia socorroensis Pfeifer, Tax. Rev. Pentandr. Spp. Aristolochia 82, fig. 26. 1970. Distribution. — Occasional on low seabluffs and upper beach, Bahia Academy; south slope of Cerro Evermann, 800 m; abandoned field north of Caleta Castelan; Bahia Braithwaite. Endemic to Isla Socorro; type from Bahia Academy at 15 m, Moran 5910 (DS). Remarks.— Vasey and Rose (1890:149), Brandegee (1900b:27), and Johnston (1931:59) all reported this plant as A. brevipes Benth. Ahhough most Mexican herbaceous aristolochias have been called A. brevipes, Pfeifer (1970:87) considered that species restricted to lava fields in central Mexico. When he named /I. socor- roensis, Pfeifer (p. 72) also proposed A. islandica for the plants of Isla San Benedicto. Unfortunately, he did not discuss the relationships of either species. Whereas we accept these two endemic insular species, Wiggins (1964:407; 1980:97) included the Revillagigedos in the range of the wide-ranging species he called at first A. brevipes var. acuminata S. Watson and later A. por- phyrophylla Pfeifer. According to Pfeifer (1976), the correct name for this mainland species is A. watsonii Wooton & Standley. Asclepiadaceae Key to Species 1. Leaves ovate-lanceolate Cynanchum californicum 1. L-eaves linear Cynanchum sonorense Cynanchum californicum (Benth.) Moran, comb. nov. Metastelma californicum Benth., Bot. Voy. Sulphur 33, t. 18. 1844. Distribution .—Occasional around island, from shore at least to 450 m, climbing over shrubs (Barkelew 217; Dominguez 737; Felger 15872; Levin 1797; Moran 5865, 25466, 25486, 25519, 25533). Central and south- ern Baja California; a report from Jalisco is unlikely (Wiggins 1964:1122). Remarks. — On the basis of Barkelew's sterile and nearly leafless specimen, Johnston (1931:83) reported this plant as an unidentified species of Metastelma. While working on Metastelma, R. W. Holm identified our specimens of this and the next species and sug- gested that, following Woodson (1941), both species belong in Cynanchum. Cynanchum sonorense Moran, nom. nov. Metastelma minutijlorum Wiggins, Contr. Dudley Herb. 3:71. 1940; non Cynanchum minutijlorum Schumann, 1895. Distribution. — Occasional in low open Croton scrub, ridge south of Bahia Academy, 100 m (Moran 5873). Not previously reported from Socorro. Sonora and Sinaloa. Asteraceae Key to Species Corollas of all flowers strap-shaped and 5-toolhed at apex; plants with milky juice 2. Leaves serrate or with broad lobes Sonchus asper 2. Leaves pinnately parted into narrow lobes Sonchus lenerrimus Corollas of at least some flowers tubular and radial (disk flowers), marginal flowers often strap-shaped and 2-3-toothed at apex (ray flowers); plants without milky juice 3. Ray flowers conspicuous 4. Receptacle chaffy 5. Rays white Bidens socorrensis 5. Rays yellow 6. Plants less than 40 cm tall; leaves dissected into linear lobes Coreocarpus insutaris 6. Plants more than 1 m tall; leaves simple, ovate to deltoid Viguiera chenopodina 4. Receptacle naked 7. Leaves oblanceolate; heads paniculate, small, the disks less than 5 mm wide Erigeron socorrensis 1. Leaves ovate; heads solitary, larger, the disks 1 cm wide or more Birityle socorrosensis 3. Ray flowers minute or absent 8. Heads unisexual, the staminate above, the pistillate involucre modified into a spiny bur Ambrosia confertiftora 8. Heads all alike Flora of Isla Socorro 27 9. Leaves opposite 10. Phyllaries conspicuously striate, strongly graduated, the longest at least 1 cm long Brickellia peninsularis 10. Phyllaries not conspicuously striate, all about the same length, the longest less than 5 mm long Eupatorium paciftcum 9. Leaves alternate 11. Plants scandent shrubs climbing to over 2 m; leaves broadly ovate, over 4 cm wide Vernonia littoralis 11. Plants erect herbs up to about 1.5 m tall; leaves obovate, oblanceolate, or linear, rarely more than 2 cm wide 12. Plants silky but greenish, glandular-viscid; leaves coarsely serrate along their entire margin . . Pseudoconyza viscosa 12. Plants (except the upper leaf-surface) densely white-woolly, not glandular; leaves entire or with a few teeth near the apex 13. Leaves, at least the basal, with a few teeth near the apex; phyllaries subherbaceous, at least the outer with green or purplish tips Conyza confusa 13. Leaves all entire; phyllaries dry and scarious, entirely straw-colored or light brown 14. Pappus bristles united into a ring a base and falling as a unit; inflorescence spike-hke Gnaphalium sphacilatum 14. Pappus bristles free, not united at base, falling separately; inflorescence a panicle Gnaphalium attenuatum *Ambrosia conferliflora DC, Prodr. 5:526. 1836. Distribution. — Uncommon near the naval base, 15 m (Dominguez 747). Not previously reported for Socorro and evidently a recent arrival, first collected there in 1988. Southwestern United States to central Mexico; adventive in Hawaii and Puerto Rico. Bidens socorrensis Moran & Levin, sp. nov. (Figure 10) Type. — Isla Socorro, Mexico: fairly common in low vegetation at 950 m on the east slope near the summit of Cerro Evermann, near 18°46.5'N, 110°57.5'W, 4 April 1981, Moran 29509 (Holotype: SD 108329; isotypes: CAS. F, MEXU, US). Suffrutex hirsutulus 4-12 dm altus. Folia plerumque simplicia, laminis ovato-lanceolatis ad rhombico-ovatis acutis serratis 2-7 cm longis 1-3 cm latis, petiolis 0.5-1.5 cm longis. Capitula solitaria paucave radiata 2.5-3.5 cm lata. Involucra extra hispidula, bracteis exterioribus 8-9 lineari-ellipticis 5-8 mm longis triplinervatis apice processo conico indurato ca. 0.2 mm longo latoque munitis, bracteis interioribus oblongo-lanceolatis apice cihatis 7-9 mm longis multiner- vatis. Flores ligulati saepius 8, laminis albis subflavescentibus 12-15 mm longis 5-7 mm latis. Flores disci ca. 40, corolla subflava 5 mm longa. Achaenia lineari-elliptica atra apicem versus subflava 7-9 mm longa biaristata. Typus: Moran 29509 (SD 108329). Species B. albae radiatae et B. odorame affinis sed habitu perenni, foliis saepius simplicibus, floribusque ligulatis fere 8 distinguitur. Perennial herb, becoming somewhat woody at base, 4-12 dm tall, or even to 16 dm when supported by shrubs, simple or branching below, commonly with several ascending branches above. Herbage hispidulous, the trichomes simple, several-celled, crisped, tawny or whitish, 0.3-0.9 mm long, somewhat matted on young growth. Stems at first subquadrangular, the two faces below leaves convex, each with about 5-7 often reddish longitudinal ridges, the alternate faces narrower, concave, each with about 3 longitudinal ridges; at length stems terete and longitudinally ridged, to 1.5 cm thick at base; lower internodes ca. 0.5-5 cm long, the upper 3-13 cm long. Leaves petiolate, simple or lower sometimes 2-3-foIiolate, the blades rhombic-ovate to ovate-lanceolate, acute, cuneate below, 2-7 cm long. 1-3 cm wide, with ca. 7-15 veins departing on each side of midvein at ca. 45°, the margins rarely entire, mostly crenate-serrate in upper half to three-fourths, with ca. 6-30 teeth on each margin, or sometimes shallowly lobed, the petioles 0.5-1.5 cm long, often narrowly winged above, slightly connate at base. Heads soHtary or few and laxly corymbose, 2.5-3.5 cm wide, the peduncles 3-7 cm long, leafless or with few reduced leaves near base. Receptacle flat. Involucre hemispheric, ca. 7-10 mm wide, in fruit to 20 mm wide; outer phyllaries 8-9, linear-elliptic, acute, 5-8 mm long, 1-1.5 mm wide, conspicuously 3-nerved, densely hispidulous dorsally, with conic terminal process ca. 0.2 mm long and wide; inner phyllaries ca. 8, oblong-lanceolate, obtuse and ciliate at apex, 7-9 mm long, 2.5-3 mm wide, hispidulous dorsally where exposed, glabrous and somewhat petaloid ventrally, with ca. 20 close parallel red nerves, the wide margins nerveless. Paleae similar to inner phyllaries but 1.5-2 mm wide, glabrous except for ciliate apex. Ray florets sometimes 7 but mostly 8, neutral, the ovary 2 mm long, the tube 2-2.5 mm long, the blade white drying yellowish, oblong to broadly elliptic, rounded and shortly 3-toothed at apex, 12-15 mm long, 5-7 mm wide, with 7-12 reddish nerves. Disk florets ca. 40, 5-merous; corolla yellowish, ca. 5 mm long, with conspicuous reddish nerves alternating with lobes, the tube ca. 1.5 mm long, expanding abruptly into narrowly campanulate limb ca. 3 mm long and 1.5 mm wide, the lobes deltoid-ovate, acute, puberulent, ca. 0.75 mm long, somewhat reflexed; anthers ca. 2 mm long, sagittate at base; style bran- ches recurved, ca. 1.2 mm long including appendages, 0.3 mm wide, 0.15 mm thick, the appendage 0.3 mm long. Achenes black, yellowish at apex, minutely papillose, linear-elliptic, rhombic in cross section with each face bisulcate, 7-11 mm long, 0.8-1.0 mm wide, and 0.6-0.75 mm thick at middle, 0.5-0.7 mm wide at apex and just above base, bearing yellowish antrorse bristles ca. 0.1-0.4 mm long especially at apex and on 28 Levin and Moran iWi^W" SOCORRO rSl.AND. MEXICO Bidens socorrensis Moran Rays white. Fairly conmon in low vegetation, east slope rear siiirrnt (1040 m) of Cerro Evermann, Seen to siimii 1083^9 46', N. ' 29609 57',-» II,..,-... 950 - 4 April 1981 Fig. 10. Holotype of Bidens socorrensis. Flora of Isia Socorro 29 angles; aristae 2, often unequal, to 2.3 mm long when well developed bearing retrorse bristles, but one or both sometimes subobsolete. Paratypes. — Hillside above Playa Blanca, east- southeast of Cabo Henslow, 120 m, Moran 25488 (ENCB, SD, UC); brush-covered north ridge midway between Cerro Evermann and Bahfa Academy, 450 m, Moran 5862 (NY, SD); south slope of Cerro Evermann, 1030 m, Felger 15764 (ARIZ, BM, CAS, F, SD, TEX), Moran 5819 (F, SD); among rocks on ridge south of Cerro Evermann, 800 m, Miranda 8716 (MEXU, SD); same vicinity, Carlquist 354 (RSA); upper south slope of peak south of Cerro Evermann, 700 m, Moran 5781 (F, MO, SD, US); brushy ridge east-northeast of Caleta Grayson, 350 m, Levin 1799 (SD, TENN); slopes above Caleta Grayson, 325 m, Moran 25464 (BISH, DAV, ENCB, CIB, SD); without exact locality, Barkelew 241 (UC). Distribution.— Endemic to Isla Socorro. Widespread but not usually common in scrub on the northern two- thirds of the island, becoming more common near the summit. Relationships. — Bidens socorrensis belongs to the Bidens pilosa complex, which Sherff (1937, 1955) treated as a single species with many subordinate taxa. Melchert (1976) and Ballard (1986), however, divided the North and Central American plants into three species. From all three, B. socorrensis is easily distinguished by its perennial habit, mostly simple leaves, (7-)8-rayed heads, and longer (5-8 mm vs. 2-5 mm) outer phyllaries. In Ballard's treatment, which is more complete, B. socorrensis appears closest to B. alba var. radiata (Schuhz-Bip.) Ballard, which also has rays with blades about twice as long as wide, or to B. odorata Cav., which also has about eight linear outer phyllaries. B. alba radiata is a plant of moist low eleva- tions (generally below 600 m) in Florida, the West Indies, eastern Mexico, and northern Central America; B. odorata grows in the mountains and high plateaus of the southwestern U.S., central and western Mexico, and northern Central America, usually at elevations above 100 m (Melchert 1976, McVaugh 1984, Ballard 1986). It would be valuable to know the chromosome number of B. socorrensis, because B. odorata is a diploid (n = 12) whereas B. a. radiata, with n = 24, is a tetraploid (Ballard 1986). The two mainland species also differ in their leaf flavonoids (Ballard 1986). Dispersal. — Cax\c[\i\sx (1966, 1974:444-445) illustrated the achenes of B. socorrensis and discussed their dispersibility. Insular members of the B. pilosa complex have less efficient dispersal mechanisms than do mainland members. B. socorrensis, however, with its barbed aristae and hairy achenes, has retained more of the features that aid dispersal than have most insular Bidens. We note, though, that one or both aristae are sometimes subobsolete. Brickellia peninsularis T. S. Brandegee var. amphi- thalassa Robinson ex I. M. Johnston, Proc. Calif. Acad. Sci., ser. 4, 20:100. 1931. Distribution. — Rather common in brushy places from shore to summit. The variety is endemic to islas Socorro and Clarion; type from Isla Clarion {Mason 1555). Remarks.— This is a weak shrub, generally 1-2 m tall but shorter on exposed seaward slopes. The corollas are yellowish white, but the heads appear yellow because of the prominent yellow anthers. The var. peninsularis is endemic to southern Baja California. Conyza confusa Cronq., Bull. Torrey Dot. Club 70:632. 1943. Distribution. — Uncommon and local on northeast slope near summit of Cerro Evermann, 1020 m (Felger 15753, 15813; Levin 1820; Moran 29508). Not previously reported for Socorro. Central and western Mexico from Durango to Chiapas. Remarks.— L L. Strother, who identified the specimens, noted (pers. comm.) that the island mate- rial had more robust rosettes and more pubescent leaves than the mainland material he had seen. On this basis he felt that the plant was not a recent arrival on Socorro. Coreocarpus insularis (T. S. Brandegee) E. B. Smith, Brittonia 36:274. 1984. Leptosyne insularis T. S. Brandegee, Erythea 7:5. 1899. Coreopsis insularis (T. S. Brandegee) S. F. Blake, Proc. Amer. Acad. Arts 49:340. 1913. Distribution. — Locally common in scattered popula- tions from near Caleta Grayson up west and southwest slopes of Cerro Evermann at least to 700 m. Endemic to Isla Socorro; type without exact locality, Anthony 394 (UC 89282). Remarks. — From the root-crown, often 1 to 1.5 cm thick, grow several erect to arching or prostrate pere- nnial stems I to 8.5 dm long and 2 to 5 mm thick, which are conspicuously ringed with the somewhat persistent connate leaf bases. The peduncles are 4 to 23 cm long. Blake (1913) and Johnston (1931:103) thought this species most closely related to Coreopsis cylcocarpa S. F. Blake and its var. pinnatisecta (S. F. Blake) Crawford, plants of grassy plateaus above 1500 m in western Mexico, from Jalisco to Oaxaca (Crawford 1970, Smith 1984). E. B. Smith (pers. comm.) noted that these species are superficially similar but considered C insularis 30 Levin and Moran more closely related to Coreocarpus dissectus (Benth.) S. F. Blake, endemic to southern Baja California. Erigeron socorrensis T. S. Brandegee, Erythea 7:4. 1899. Distribution .—Brushy places from shore to summit; common in northern and central parts of island but uncommon in southern part, where perhaps reduced by sheep. Endemic to Isla Socorro; type without exact locality, Anthony 376 (UC 174214). Remarks.— This is a weak shrub 3 to 15 dm tall, with stems to 1.5 cm thick at the base. This species is closely related to E. crenatus Eastwood ex I. M. Johnston of Isla San Benedicto, which, in fact, Blake (1926:1499) earlier included in the same species. Johnston (1931:101-102) found the rela- tionships of these two species obscure but placed them in sect. Caenotus and suggested that their most pro- bable relatives were found in southern Mexico. No members of this section are known in Baja California. Eupatorium pacificum Robinson ex I. M. Johnston, Proc. Calif. Acad. Sci., sen 4, 20:99. 1931. Ageratina pacifica (Robinson ex I. M. Johnston) R. King & H. Robinson, Phytologia 19:215. 1970. Distribution.— Rather common in rocky places from 800 m to summit. Endemic to Isla Socorro; type from near the summit, Mason 1666 (CAS 186613). Remarks.— This is a shrub 6 to 10 dm tall, branching from the base. According to B. L. Robinson (in Johnston 1931:99), this species is similar to E. pazcuarense Kunth and E. conspicuum Kunth & Bouche, both of southern and western Mexico. H. Robinson (pers. comm.) thought E. pacificum probably more closely related to E. bustamenta DC, which extends from central Mexico to Panama and has other close relatives in western Mexico. Gnaphalium attenuatum DC, Prodr. 6:228. 1837. Distribution. — Uncommon, near summit of island, above 750 m. Central Mexico to Panama. Gnaphalium sphacilatum Kunth in Humb., Bonpl. & Kunth, Nov. Gen. Sp. 4:67. 1818. Gnaphalium pedunculosum 1. M. Johnston, Contr. Gray Herb. 11. 68:99. 1923. Distribution. — Rare in open spots in summit grassland, east slope near summit of Cerro Evermann (Felger 15760; Moran 29514). Not previously reported for Socorro. West-central Mexico and Baja California. The occurrence near the summit very likely is native, even though the first collection was not made until 1967. Remarks.— The Socorro material appears to belong to this species, but, as noted by McVaugh (1984: 450-451), the taxonomy of this species-complex is not well understood. Perityle socorrosensis Rose, Bot. Gaz. 15:118, pi. 13, fig. 9. 1890. Distribution. — Nearly confined to seacliffs, where it is common (Cabo Middleton, Bahia Academy, Playa Blanca, Caleta Grayson, southwest of Cabo Pearce, Caleta Binner, Cabo Rule). One old plant found in bare ground among scattered shrubs north of Huerta Grande at 330 m (Moran 29535). Endemic to the Revillagigedos, found on islas Socorro, San Benedicto, and Clarion; type from Isla Socorro without exact locality, but presumably from Bahia Braithwaite, Town- send s. n. (US). Remarks.— PoweW (1974) thought this species most closely related to R crassifolia T. S. Brandegee of south- ern Baja California, a plant that also is primarily coastal. Both species have 2n = 38 (Powell 1983). We have used Rose's original spelling rather than the more euphonious socorrensis used by some later authors (e.g., Powell 1974). *Pseudoconyza viscosa (Miller) D'Arcy, Phytologia 25:281. 1973. Conyza viscosa Miller, Gard. Diet. ed. 8. 1768. Blumea viscosa (Miller) Badillo, Revista Fac. Agron. (Maracay) 7(3):9. 1974. Distribution.— Corrwnon in disturbed areas north of Bahia Braithwaite (Levin 1767). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1987. Common weed from the West Indies and western and southern Mexico to northern South America. *Sonchus asper (L.) Hill, Herb. Brit. 1:47. 1769. Distribution.— Common on flat at east side of Laguna Escondida (Moran 29538). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1981. Cosmopolitan weed, native of Europe. ♦Sonchus tenerrimus L., Sp. PL 794. 1753. Distribution.— Occasional weed at roadside, naval base, Cabo Rule (Moran 25441). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1978. Widely naturalized from central California to southern Baja California; native of Europe. Vernonia littoralis T. S. Brandegee, Erythea 7:3. 1899. Eremosis littoralis (T. S. Brandegee) Gleason, N. Am. Fl. 33:100. 1922. Distribution .-Occasional, climbing over trees and shrubs, 300-900 m. Endemic to Isla Socorro; type Flora of Isla Socorro 31 without exact locality, Anthony s. n. (UC 86053). Remarks. — One plant (Moran 5760) was scandent, 7 m long, the trunk 9 cm thick, the bark thick, fur- rowed, almost black. Although it is not clear from the literature (Blake 1926, Gleason 1922, Jones 1973) to what species V. lit- toralis is most closely related, all authors agree that it belongs to a group ranging from southern Mexico to Honduras. Viguiera chenopodina E. Greene, Leaflets Bot. Observ. Crit. 2:154. 1911. Viguiera deltoidea A. Gray var. townsendii Vasey & Rose, Proc. U.S. Natl. Mus. 13:148. 1890. Type from Isla Socorro without exact locality but presumably from Bahia Braithwaite, Townsend s. n. (US). Distribution.— From shore to summit, common in Croton scrub and occasional above. Southern Baja California to Isla Tiburon, Sonora. Remarks.— On Socorro, this is a shrub 1-2.5 m high or clambering to 5 m, the trunk to 8 cm thick. Ahhough Blake (1918) maintained the Socorro plant as an endemic variety of V. deltoidea, he pointed out that it approached V. deltoidea var. chenopodina (E. Greene) S. E Blake. Johnston (1931:103) questioned the validity of separating these two taxa. On the basis of his studies of morphology, chemistry, and cytology, E. E. Schilling (pers. comm.) recognized V. cheno- podina as a distinct species. He noted (pers. comm.) that the Socorro material does not completely match any other sample of the species but does closely approach some material from islands in the Gulf of California. Furthermore, using flower buds Levin col- lected. Schilling (pers. comm.) found that the Socorro plant has 2n = ca. 54 and is therefore a hexaploid like V. chenopodina. He would therefore include the Socorro plant in V. chenopodina. Boraginaceae Key to Species 1. Plants herbs; fruit dry, separating into 2 or 4 bony nutlets Heliotropium curassavkum 1. Plants shrubs; fruit drupaceous 2. Leaves entire; inflorescence a panicle of coiled cymes Tournefonia harlwegiana 2. leaves toothed; inflorescence spike-like Cordia curassavica Cordia curassavica (Jacq.) Roemer & Schultes, Syst. Veg. 4:460. 1819. C. bre\'ispicata M. Martens & Galeotti, Bull. Acad. Roy. Sci. Bruxelles 11:311. 1844. C. socorrensisT. S. Brandegee, Erythea 7:5. 1898. Type from Isla Socorro without exact locality, Anthony 384 (UC). Distribution .-Occasional in arroyos in Croton scrub at south end and in Dodonaea-Lepechinia scrub on south slope up to 800 m; more common in mixed scrub on north and west sides of island, on alluvial flats sometimes in pure stands, as at Bahia Academy. Western Mexico from southern Baja California and central Sonora south. West Indies, Central America, and northern South America. Remarks. — Johnston (1949) only tentatively included C. brevispicata (and thus C. socorrensis) in C. curas- savica. Miller (1988), however, found that these and the numerous other segregates from C. curassavica inter- grade too much to allow their separation. Heliotropium curassavicum L., Sp. PI. 130. 1753. Distribution. — Occasional on upper beach in north- ern part of island (Playa Blanca, Bahia Academy). Widespread in tropical and subtropical America. Tournefortia hartwegiana Steudel, Nomencl. Bot. ed. 2, 2:693. 1841. Distribution.— Occasional in Croton scrub on hillsides near shore at south end (Caleta Binner, Caleta Castelan). Southern Baja California to Tamaulipas and Oaxaca. Burseraceae Bursera epinnata (Rose) Engl., Nat. Pflanzenfam. ed. 2, 19a:425. 1931. B. nesopola I. M. Johnston, Proc. Calif. Acad. Sci., ser. 4, 20:66. 1931. Type from Caleta Binner, Mason 1609 (CAS 186618). Distribution.— Vncommon, scattered in Croton scrub at south end (Caleta Binner; Cabo Rule; above Caleta Castelan, 150 m). Southern Baja California. Remarks.— J ohnsion compared B. nesopola with B. rhoifolia (Benth.) I. M. Johnston, now regarded as a synonym of B. hindsiana (Benth.) Engl. However, the Socorro plant seems essentially like B. epinnata— \f indeed that is distinct from B. hindsiana. Wiggins (1964:757) included B. nesopola in the synonymy of B. epinnata. Cactaceae Opuntia sp. D/5m'tor/o/7.— Uncommon in northern part of island and only near shore (Bahia Academy, Playa Blanca, Caleta Grayson); fairly common in southern part, in Croton scrub and in rather bare areas above, to 400 m. Description.— Shrnh 0.5-2 m high, in age with indefinite trunk. Joints subcircular to mostly obovate, 15-30 cm long, 13-23 cm wide, glabrous, green, with areoles ca. 60 per joint (both sides) and thus widely 32 Levin and Moran separated, adjacent ones ca. 2-5 (-7) cm apart. Areoles circular or slightly elongate, 4-11 mm long, 4-8 mm wide, with cushion of somewhat tawny wool project- ing ca. 2-3 mm from surface of joint, all spiny except sometimes the lowest. Glochids mostly at upper side of areole, yellow, mostly ca. 3-4 mm long and project- ing ca. 2 mm from surface of joint, a few longer. Spines mostly ca. 4-6 per areole, variously spreading, light yellow to whitish, becoming gray, acicular but com- monly ca. twice as wide as thick at base, the largest 1.5-4 cm long, to 1 mm wide at base, often somewhat twisted, scarcely barbed. Flowers 5-6 cm long, 4-5 cm wide. Ovary obpyramidal, ca. 2.5-3 cm long and 2-2.5 cm wide, papillose, with ca. 20-25 areoles mostly crowded near apex; the areoles circular, tawny, 2-3 mm wide, with yellow glochids and the upper each with 1-3 acicular yellow spines to 8 mm long. Petaloid perianth parts yellow, the inner cuneate-obovate, rounded to emarginate, 25-32 mm long, 20-25 mm wide. Inner filaments ca. 4 mm and outer ca. 9-13 mm long; anthers ca. 1.5 mm long. Style rosy, 16-24 mm long, ca. 5 mm thick above narrowed base, 3 mm thick above; stigma lobes greenish. Fruit red, obovoid to subglobular, truncate and scarcely umbilicate, 18-45 mm long, 20-35 mm thick. Seeds numerous (to ca. 70), irregular, 2.4 mm long, ivory (Figure llA-C). Plants at the north end are smaller — e.g., Moran 25495 (rather bare west slope, Playa Blanca, 25 m) 6 dm high, the joints 15-23 cm long, the fruit 18-30 x 20-32 mm (Figure IID). Remarks. — Having seen no specimens, Johnston (1931:80) reported this plant only as Opuntia species. Elzada Clover (H. S. Gentry 1949:71) identified sterile specimens collected by Elmore as O. occidentalis Engelm. & Bigelow, a plant described from southern California; that name is now considered a synonym of O. ficus-indica (L.) Miller, and the plants to which it has commonly been applied are considered a hybrid swarm derived from the introduced O. ficusindica and varieties of O. littoralis (Engelm.) Cockerell, native to Fig. 11. Opuntia sp. Undescribed species from Isia Socorro. A. habit of plant growing northwest of Cabo Pearce (Moran 25514); B, pad and fruit of same plant: C, flower of plant growing near km 6 on airport road (Moran 25516); D, pad and fruit of plant growing near Playa Blanca (Moran 25495). Flora of Isia Socorro 33 southern California (Benson and Walkington 1965). Brattstrom (1953), having collected the Opuntia of Socorro in flower and that of Clarion in fruit, consid- ered them to be of the same species. He described the plant briefly and identified it somewhat equivocally as 0. engelmannii Salm-Dyck ex Engelm. The Socorro Opuntia differs from O. engelmannii in being somewhat shorter and, more significantly, in having slender glochids clustered at the top of the areole rather than stout glochids distributed throughout the areole (Parfitt and Pinkava 1988). In Bravo-Hollis's (1978) treatment of Mexican Opuntia, it keys to series Dillenianae Britton & Rose, but it is not much like any of the species. The classification of the subgenus Opuntia seems somewhat artificial, and relationships among the species are poorly understood. Because we are so unsure of the proper placement of this species, we are unwilling to name it. Campanulaceae Key to Species 1. Flowers bilateral; stamens fused Helerotoma cordifoUa 1. Flowers radial; stamens free Triodanis perfoliala Heterotoma cordifolia (Hook. & Arn.) McVaugh, Bull. Torrey Bot. Club 67:143. 1940. Distribution. — Occasional in shade of upper forest and scrub on the north and east slopes above ca. 700 m, and among rocks on the northwest slope from ca. 900 m to near summit (Levin 1824; Moran 5825, 29502, 29513). Not previously reported for Socorro. Chihuahua and southern Sonora to Costa Rica. Triodanis perfoliata (L.) Nieuwl. var. biflora (Ruiz Lopez & Pavon) Bradley, Brittonia 27:114. 1975. Distribution. — Scarce in rock crevices near summit of Cerro Evermann (Levin 1817; Moran 5844). Not previously reported from Socorro. Oregon to the Cape region of Baja California; south-central and south- eastern United States south to central Mexico; South America. Capparidaceae *Cleome viscosa L., Sp. PL 672. 1753. Fblanisia viscosa (L.) DC, Prodr. 1:242. 1824. Distribution. — Uncommon south of the airstrip at 260 m (Dominguez 728). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1988. Pantropical weed, probably native to Asia; in the New World from the West Indies, western and southern Mexico to Nicaragua, and in Brazil. Chenopodiaceae *Chenopodium ambrosioides L., Sp. PI. 219. 1753. Distribution. — Locally common in bare areas, near km 6 on airport road, west of Cabo Pearce (Moran 25517), and north of Huerta Grande (Moran 29531). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1978. Weedy over most of temperate and tropical America, thought to be native from Mexico southward; introduced in the Old World. Called "epazote" and used as a condiment in Mexico and parts of Central America; possibly brought to Socorro intentionally. Combretaceae Conocarpus erecta L., Sp. PL 176. 1753. Distribution. — Locally common along shore and on hillsides up to 100 m, northern part of island (Playa Blanca, Bahi'a Academy). Widely distributed on shores in tropical America, from Florida, Tamaulipas, and central Baja California to Ecuador and Brazil; Galapagos Islands; tropical West Africa. Convolvulaceae Key to Species 1. Plants silky-canescent; leaves sessile; corolla white, about 5 mm long Cressa truxillensis 1. Plants glabrous or essentially so; leaves petioled; corolla pink to purple, more than 1 cm long 2. Leaves nearly orbicular, unlobed, the apex rounded or retuse; plants trailing and rooting at the nodes Ipoinoea pes-caprae 2. Leaves cordate, often 3-lobed; the apex acute; plants twining Ipomoea triloba Cressa truxillensis Kunth in Humb., Bonpl. & Kunth, Nov. Gen. Sp. 3:119. 1819. Distribution. — Abundant about brackish pool at Bahia Academy (Moran 5878). Alkaline or sahne soils from western United States south throughout Baja California and to central Mexico; South America. Ipomoea pes-caprae (L.) Sweet subsp. brasiliensis (L.) Ooststr., Blumea 3:533. 1940. Distribution. — Common on beaches around island (Bahia Academy, Playa Blanca, Caleta Grayson, Caleta Castelan). Circumtropical on beaches. Ipomoea triloba L., Sp. PL 161. 1753. Distribution. — Occasional in open and brushy places at least up to 250 m (Bahia Academy, along airport road at 250 m, Caleta Binner, Cabo Rule, north of Caleta Castelan). Southern Arizona, Florida, West Indies, southern Baja California through tropical America; naturalized throughout Old World tropics. Cucurbitaceae *Lagenaria siceraria (Molina) Standley, Publ. Field Mus. Nat. Hist., Bot. Ser. 3:435. 1930. 34 Levin and Moran Distribution. — Roadside ca. 1 km southeast of Laguna Escondida (Moran 29543). Not previously reported for Socorro; a recent introduction, first col- lected there in 1981 after having escaped from cultiva- tion at Huerta Grande. Native to the Old World but widely cultivated throughout the American tropics, often becoming naturalized. Euphorbiaceae Key to Species 1. Plants herbs or subshrubs; one carpellate flower and several staminate flowers enclosed in a cup-shaped gland-bearing involucre, the whole unit resembling a single flower 2. Leaves alternate, symmetrical at base; stipules glandular Euphorbia heierophylla 2. Leaves opposite, asymmetrical at base; stipules subulate, not glandular 3. Leaves entire 4. Plants herbaceous; stems glutinous, often covered with sand grains; involucres solitary in leaf axils Chamaesyce incerta 4. Plants subshrubs; stems not glutinous; involucres crowded in dense glomerules Chamaesyce anthonyi 3. Leaves toothed 5. Herbage and capsules glabrous Chamaesyce hyssopifolia 5. Herbage and capsules pubescent 6. Stems with spreading yellowish hairs; involucres crowded in dense glomerules Chamaesyce hirra 6. Stems with appressed white hairs; involucres few to several at nodes, but not in dense glomerules Chamaesyce thymifolia 1. Plants well-developed shrubs or trees; flowers in spikes or racemes, not enclosed in a cup-shaped involucre 7. Plants trees; leaves glabrous; fruit a 6-9-celled drupe Hippomane mancinella 7. Plants shrubs; leaves pubescent; fruit a 3-celled capsule 8. Pistillate flowers subtended by a foliaceous bract; pubescence simple Acalypha umbrosa 8. Pistillate flowers not subtended by a foliaceous bract; pubescence stellate Croton masonii Acalypha umbrosa T. S. Brandegee, Erythea 7:7. 1899. Distribution. — Widespread in brushy places from shore to summit, often abundant. Endemic to Isla Socorro; type without exact locality, Anthony 375 (UC 110724). Remarks.— Pax and Hoffmann (1924:124; 1931:130) assigned A. umbrosa to section Cuspidatae, which is mainly West Indian and South American. Standley (1923:630) and Johnston (1931:69) instead thought it closest to A. californica Benth., of southwestern Cali- fornia, Baja California, and northwestern Sonora, in section Betulinae. These sections are supposed to differ in their spikes: predominantly bisexual in Cuspidatae, unisexual in Betulinae. As Pax and Hoffmann pointed out, however, spikes may be unisexual or bisexual in both sections, the difference really being one of predominant condition. Johnston noted that in A. umbrosa many spikes are bisexual, with a few car- pellate flowers on the peduncle but otherwise staminate, and that some plants bear elongate, loosely flowered carpellate spikes. The many collections made since then show that most plants bear both carpellate spikes and bisexual spikes; most of these have only a few carpellate flowers at the base, but a few bear numerous carpellate flowers (e.g., Moran 5859); exclusively staminate spikes are rare. This condition appears to be more like that in section Betulinae, and A. umbrosa compares quite well with the American members of that section, all essentially restricted to Baja California. Final resolution of this problem awaits a modern revision of the genus. Chamaesyce anthonyi (T. S. Brandegee) Levin, comb. nov. Euphorbia anthonyi T. S. Brandegee, Erythea 7:7. 1899. Euphorbia clarionensis T. S. Brandegee, Erythea 7:7. 1899. Euphorbia anthonyi T. S. Brandegee var. clarionensis (T. S. Brandegee) 1. M. Johnston, Proc. Calif. Acad. Sci., ser. 4, 20:71. 1931. Type from Isla Clarion (Anthony 406). Distribution. — Widespread in open and brushy places from shore to summit, often abundant. Endemic to the Revillagigedos; the type of E. anthonyi from Isla San Benedicto (Anthony 369). Remarks. — At the shore this subshrub may be semiprostrate; inland it is arching to erect, to 4 dm tall, the stem to 1 cm thick at the base. With this species, Brandegee described E. clario- nensis, from Clarion. Johnston (1931:71) kept the plants of Clarion separate from those of San Benedicto and Socorro as E. anthonyi var. clarionensis, saying that they differed in having narrow white petaloid appen- dages to the involucral glands. However, some plants from Socorro also have such appendages (e.g., Moran 5917). Therefore, we follow Standley (1923:602) in treating these as synonyms. M. J. Huft (pers. comm.) thought C. anthonyi m.ost closely related to C. hypericifolia (L.) Millsp. and C. hyssopifolia (L.) Small, both of which range from the southern United States to South America. ♦Chamaesyce hirta (L.) Millsp., Field Mus. Nat. Hist., Bot. Ser. 2:303. 1909, var. hirta. Euphorbia hirta L., Sp. PI. 454. 1753. Distribution. — Locally common in disturbed places, along road to Cerro Evermann trailhead at 350 m (Levin 2035), near airstrip (Ledn 3515), at naval base (Levin 2029). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1988. Widespread weed of tropical America, ranging from the southeastern United States west to Arizona, south Flora of Isla Socorro 35 through Mexico to Argentina; adventive ,n the Old A-.m-^..r/o..-Dominant and often forming pure stands on hillsides and alluvial fans near shore at south ♦Chamaesyce hyssopifolia (L.) Small, Bull. New York ^"'^' ^''tending upward to about 250 m; abundant but Bot. Card. 3:429. 1905. Euphorbia hyssopifolia L., '^^^ dominant at north end to about 150 m; on west Syst. Nat. ed. 10, 1048. 1759. ' side above Caleta Grayson scattered to at least 350 m. Distribution. —Common locally in roads, airport Endemic to Isla Socorro; type from "the east slope," road between 190 and 250 m (Dominguez 7]0; Ledn ^"^"'^ ^^^'^ ^^^^ 186600). Mason evidently did not 3444; Levin 2032), northeast of Bahia Braithwaite ^'^'* ^^^ '°^^'" ^^^' ^'OP^- however, and the type prob- (Levin 1778), Caleta Castelan near main landing ^'''^ '^ ^^°^ "^^'' ^^^ia Braithwaite. (Moran 29495). Not previously reported for Socorro Remarks.—This is a shrub 0.5-3.5 m tall, branching and evidently a recent arrival; first collected there in *^™™ ^^^ ^^^^' ^^^""^ *he stem is 1-5 cm thick. The old 1981. Widespread weed of tropical America, ranging '^^^^^ °^^^" '"™ ^ beautiful orange-red before falling. from Florida west to Arizona, south through Mexico Johnston (1931:68) thought the relationships of C. to Argentina; adventive in the Old World. Wfl5o/?// obscure but perhaps closest to the widespread nu ■ ^ Mexican C. cortesianus Kunth. G. L. Webster who is Chamaesyce .ncerta (T. S^ Brandegee) Millsp., Field preparing a monograph on the Mexican croto^s (per /Wcm. T S B^ a^d P- r ',•■/?• f ''''''''' '^°"'"->' '^°"^^^ ^ — '' ^ ^^'^^'-^^ ^P-ie mo" 2 3i71 1 89^ '''' ^'''- '"■• '''■ ''°'''' '''''''' '° ^- '^'i'^^nensis Croizat, of Sinaloa, r>-' * u ,'■ ^ '^''^m which it differs conspicuously in its larger coarser D,.rnW;.-Common with Ipomoea pes-caprae on leaves with a different venation pattern upper beaches on north side of island (Bahia Academy, Playa Blanca). Sandy shores near La Paz, Baja Cali- *Euphorbia heterophylla L., Sp. PI. 453. 1753. fornia Sur, and from Bahia Kino, Sonora, to Mazatlan Distribution. —Few plants in disturbed soil, north of and Islas Tres Marias, Sinaloa (R. S. Felger, pers. ^^^'^ Braithwaite (Levin 1789). Not previously comm.). ' reported for Socorro and evidently a recent arrival; first *nu .^ - .. collected there in 1987. Widespread weed of tropical *Chamaesyce thyn. ol.a (U Millsp. Field Mus. Nat. America, ranging from Texas and Arizona through Hist., Bot. Sen 2:412. 1916. Euphorbia thymifolia Mexico to South America L., Sp. PI. 454. 1753. Distribution.— Common weed at naval base, Cabo Hippomane mancinella L., Sp. PI. 1191. 1753. Rule (Mo/HA? 25-^55); uncommon near airstrip (Doww- Distribution.— Forming groves in sandy soil just gue: 723, Ledn 3509). Not previously reported for behind beach on northern and western sides of island Socorro and evidently a recent arrival; first collected ^""^ growing as scattered shrubs on nearby hillsides up there in 1978. Widespread weed of tropical America, ^° '^° ^ ^^^^'^ Academy, Playa Blanca, Caleta ranging from Florida and the West Indies through Grayson). Sandy beaches, southern Florida, West Mexico to South America; adventive in the Old World. 'ndies, southern and western Mexico, to Colombia and ■ Venezuela; Galapagos Islands. Cro.on masomi I. M. Johnston, Proc. Calif. Acad. Sci., Remarks.-Thc larger trees are about 8 m tall, with ser. 4, 20:67. 1931. trunks sometimes 1.5 m thick. Fabaceae Key to Species 1. Leaves bipinnate 2. Leaflets 1-2 cm wide; fruit more than 3 cm wide, pricklv r-„„ , ■ ■ u _, 2. Uanets less than 7 mm wide; fruit less than 1.5 cm wide,' not prickly Caesalp.ma bonduc 3. Plants armed with paired white nodal spines 1-3 cm long a„ ■ f 3. Plants unarmed, nodal spines absent Acacm farnesmna 4. Plants well-developed shrubs, woody throughout; leaflets 2-6.5 mm wide; petiole and rachis without a nectary gland 5. Fruit linear, 4-]0 cm long and 3-4 mm wide, sessile . . . n^^^„„ri,. o <,■ 5 Fruit plUntir 7 <; t ^ ^^ i„„„ 0^-711 ■ j uesmanthus bicornutus D. rruii elliptic, Z.i-i.i cm long and 7-11 mm wide, stinitate \t , ■ 1 1. Leaves simply pinnate supnate Neptunia plena 6. Uaves even-pinnate, mostly with 3 pairs of leaflets; flowers somewhat bilateral but not pea-like; stamens free Senna obtmifniin 6. Leaves odd-pinnate or palmate with 3 leaflets; flowers strongly bilateral and clearly pea-hke; stamenraM ^9 of Xm fused 7. Leaves white-silky; lateral leaflets often lobed on outer side; fruit 6-10 cm long and 3-5 mm wide . ... Macroptilium atropurpureum 36 Levin and Moran 7. Leaves green, not silky; lateral leaflets sometimes asymmetrical at base but never lobed; fruit less than 5 cm long or, if longer, then 1 cm or more wide 8. Fruit transversely divided between the seeds, breaking into 1-seeded segments, which are clothed with hooked hairs . . . Desrnodium procumbens 8. Fruit not transversely divided between the seeds, longitudinally dehiscent along both margins, not clothed with hooked hairs 9. Leaflets and calyx with yellowish gland-dots; fruit less than 2 cm long Rhynchosia minima 9. Leaflets and calyx not gland-dotted; fruit more than 2.5 cm long 10. Plants erect; stamens alternately longer and basifixed, shorter and dorsifi.xed Crolataria incana 10. Plants prostrate or twining; stamens not dimorphic 11. Leaves orbicular, the apex rounded; fruit straight, more than 10 cm long and 2 cm wide Canavalia rosea 11. Leaves ovate to deltoid, the apex acute; fruit lunate, less than 6 cm long and 1.5 cm wide - . Phaseolus limatus *Acaciafarnesiana(L.) Willd., Sp. PI. ed. 4:1083. 1806. Canavalia apiculata Piper, Contr. U.S. Natl. Herb. Distribution. — Abundant in arroyo north of Bahia 20:566. 1925. Braithwaite (Levin 1771) and scattered along roadsides Distribution. — Common on shore at Cabo Middle- from there to the airstrip. Planted near Caleta Castelan. ton, the only place it has been collected, and probably Not previously reported for Socorro and apparently a elsewhere around the island. Pantropical on seacoasts. recent escape from cultivation. Southwestern United Johnston (1931:64) reported that Mason had seen but States and West Indies through Mexico to South not collected this plant at Caleta Grayson, and Miranda America; introduced to Old World tropics. (1960:132) said it is found on berms in bays and coves. Remarks. — This plant was first collected on Socorro Though Moran looked for it in several places, he found in 1978, when Moran found it only where it had been it only on the bare west slope of Cabo Middleton at planted near Caleta Castelan. In the subsequent 10 50 m (Moran 25534). Barkelew 249, though labeled years it spread along the road all the way to the airstrip. Socorro and so cited by Sauer (1964), is probably from Though it is spreading rapidly, it appears to be Clarion, where the plant is known from other coUec- restricted to disturbed sites. tions (see discussion under Botanical Collectors). Remarks.— We follow Verdcourt (1971:576-577) in Caesalpinia bonduc (L.) Ro.xb., Fl. Ind. ed. 1832, 2:362. accepting C. rosea as the correct name for this species. 1832. Guilandina bonduc L., Sp. PI. 381. 1753; op. cit., ed. 2, 545. 1762. pro parte. *Crotalaria incana L., Sp. PI. 716. 1753. Caesalpinia crista L., Sp. PL 380. 1753. pro parte. D/5m/>j/f/o/;.-Common in disturbed areas north of Caesalpinia bonducella (L.) Fleming, Asiat. Res. 11:159. Bahia Braithwaite (Levin 1761) and near naval base 1810. (Dominguez 750). Not previously reported for Socorro Guilandina socorroensis Britten & Rose, N. Amer. Fl. and evidently a recent arrival; first collected there in 23:338. 1930. Type from Socorro without exact 1987. Widespread weed of tropical America, ranging locality, Barkelew 207 (US). from the southeastern United States and West Indies Distribution. —SmaW colonies occasional in scrub in through Mexico to South America; introduced to Old northern half of island, from shore to at least 550 m. World tropics. Widespread in tropics and subtropics of both *Desmanthus cf. bicornulus S. Watson, Proc. Amer. hemispheres, generally near the shore. Acad. Arts 21:426. 1886. Although Britton and Rose attributed this plant to Distribution.— Uncommon along disturbed road- Socorro on the basis of Barkelew 207, Johnston sides northeast of Bahi'a Braithwaite (Zi'iw /77J). Not (1931:14,63) thought Barkelew's specimen probably previously reported for Socorro and evidently a recem came from Clarion instead. We believe it was indeed arrival; first collected there in 1987. Arizona and from Socorro; see discussion under Botanical Collec- western Mexico, from Baja California, Sonora, and tors. Whatever the source of the specimen, the plant Chihuahua to Guerrero, grows on both islands. Remarks.— When collected in April 1987, all the Remarks.— This plant has usually been called C. plants were in fruit and bore only a few battered leaves, crato, the name used by Johnston (1931:63), or C.^on- without more complete material, identification of ducella. The correct name, according to Dandy and species in this complex is difficult. Exell (1937), is C. bonduc (L.) Roxb. *Desmodium procumbens (Miller) A. Hitchc, Annual Canavalia rosea (Sw.) DC, Prodr. 2:404. 1825. Rep. Missouri Bot. Gard. 4:76. 1893. Canavalia maritima [Aublet] Thouars in Desv., J. Bot. Distribution. — Uncommon in disturbed soil at road- 1:80. 1813. sides. El Ranchito, north of Caleta Castelan (Moran Flora of Isla Socorro 37 25527), northeast of Bahi'a Braithwaite (Levin 1781), near the naval base (Dominguez 752); in 1988, seen by Levin along roadsides to at least 215 m. Not previously reported for Socorro and evidently a recent arrival; first collected there in 1978. Mexico from Sinaloa south, Central America, West Indies, northern South America, Galapagos Islands; introduced to tropical Africa and the Philippines. *Macroptilium atropurpureum (DC.) Urban, Symb. Antill. 9:457. 1928. Phaseolus atropurpureus DC, Prodr. 2:395. 1825. Distribution. — Uncommon near naval base at 15 m {Dominguez 708). Widespread from Texas through much of Mexico to Central and South America. This is the first definite record of this species for Socorro. A specimen collected by Barkelew (248) is labeled "Socorro" but, as Johnston (1931:14) pointed out, probably came from Clarion (see discussion under Botanical Collectors). Because no one found this plant on Socorro between 1903 and 1988 and because it has been found only in a disturbed area, it would appear to be a recent arrival. *Neptunia plena (L.) Benth., J. Bot. (Hooker) 4:355. 1841. Distribution. — Common along disturbed roadsides near naval base (Dominguez 740; Levin 2052) and northeast of Bahia Braithwaite (Levin 1776). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1987. Coasts from the West Indies, southern Texas, southern Baja California, and Sinaloa to South America; India. Phaseolus lunatus L., Sp. Pi. 724. 1753. Distribution. — Occasional on rocks and in brush on upper part of island, at least from 450 to 980 m. Known in its wild form from southern Mexico through Central America and in South America from Peru to Argentina. Rhynchosia minima (L.) DC, Prodr. 2:385. 1825. Distribution. — Local on hillside at Bahia Academy (Howell 8446; Moran 5908); uncommon in disturbed areas north of Huerta Grande (Leon 3435) and north of Bahia Braithwaite (Levin 1762). Reported by Johnston (1931:65) from Socorro without definite locality on the basis of Anthony s. n. Grear (1978) cited Moran's collection. Southern United States through Mexico to South America, possibly introduced from the Old World (Grear 1978). Though the plants in disturbed soil on the south side of the island might be introduced, those in undisturbed vegetation at the north end appear to be native. *Senna obtusifolia (L.) Irwin & Barneby, Mem. New York Bot. Gard. 35:252. 1982. Cassia obtusifolia L., Sp. PI. 377. 1753. Distribution. — Common in disturbed area near naval base, 15 m (Dominguez 746). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1988. Pantropical and subtropical weed, apparently native to the Americas, where it ranges from the southern United States and the West Indies through most of Mexico to Central and South America. Zapoteca formosa (Kunth) H. Hern, subsp. socorrensis (I. M. Johnston) Levin, H. Hernandez, & Moran, comb. nov. Calliandra socorrensis I. M. Johnston, Contr. Gray Herb. 70:71. 1924. Anneslia socorrensis (I. M. Johnston) Britton & Rose, N. Amer. Fl. 23:64. 1928. Calliandra sp. I. M. Johnston, Proc. Calif. Acad. Sci., ser. 4, 20:63. 1931. Distribution. — Occasional to locally common in scrub, Bahia Academy, 20-150 m; rare on upper east slope of Cerro Evermann. Endemic to islas Clarion and Socorro; type from Isla Socorro without exact locality, Anthony s. n. (UC 83534). Amplified description. — Shrub 1-1.6 m high, the crown to 2 m wide. Trunk single for up to half its height (when shrubs crowded), to 6 cm thick above slightly enlarged base; bark gray, somewhat mottled. Branches numerous, terete, glabrous, reddish or yellowish when young, the internodes 0.5-2.5 cm long. Stipules 2-5 mm long, 1.5-2.5 mm wide, oblong-lanceolate, acute, 5-7-nerved, persistent. Leaves (l-)2-4-jugate, the petiole 1.5-2.5 cm long, the rachis 5-8 mm long, the petiole, rachis, and rachilla reddish when young; pinnae 1.5-4.0 cm long; leaflets 4-12 pairs, 5-15 mm long, 2.0-6.5 mm wide, oblong, obtuse at apex, obtuse and strongly oblique at base, membranous, faintly veined, sparsely ciliate but otherwise glabrous. Peduncles axillary, (l-)2-4 per axil, 2-3 cm long, slender, glabrous. Heads with 12-20 sessile ephemeral flowers. Calyx 2.0-2.5 mm long, the lobes about as long as tube and glandular-puberulent on backs and margins. Corolla 3.5-4.5 mm long, glabrous, the lobes a little shorter than tube and recurved, acute. Stamens numerous, 1.5-2.0 cm long, white. Legumes somewhat veiny and light to rather dark brown without, pale yellow within, 2.5-6.5 cm long, 6-10 mm wide. Seeds 2-9 per pod, the shape variable, commonly irregularly subquadrangular-lenticular with circumference subacute but sometimes (when adjacent ovules develop) with one end or two opposite ends truncate, 3.0-5.6 mm long, 3.0-4.5 mm wide, ca. 2.5 mm thick. 38 Levin and Moran smooth and somewhat shining, mottled, each of the two main faces with a distinct brownish white pleurogram, the margins blackish (Figure 12). Collections examined. — Isla Clarion: without exact locality, Mason 1566 (CAS, GH); above Sulphur Bay, Howell 8371 (CAS, GH). Isla Socorro: without exact locality, Anthony s. n. (two sheets: UC 83534, the holotype, and UC 89257); ridge south of Bahia Academy, 20 m, Moran 5902 (ENCB, SD); locally common, flat top of ridge west of Bahia Academy, 90 m, Moran 25546 (ARIZ, BISH, BM, CAS, CIB, ENCB, F, GH, MEXU, MICH, MO, NY, RSA, SBBG, SD, TEX, US); same, grown from seed and flowering in San Diego (MEXU, SD); same, grown from seed and flowering at Huntington Botanical Garden (SD); near summit of island. Mason 1661 (CAS); few locally, east slope of Cerro Evermann, 980 m, Moran 5841 (SD, US); near summit of Cerro Evermann, Miranda 8732 (MEXU). Remarks. — Johnston (1931:62-63), working with a Fig. 12. Flowenng branch o( Zapolecaformosa subsp. socorrensis. Grown in San Diego from seed of Moran 25546. Scale bar equals 1 cm. few fruiting specimens only, recognized two species of Calliandra from the Revillagigedos, differing in number of pinnae and number and size of leaflets. He consid- ered both weak species, the first, C. socorrensis, close to C.formosa (Kunth) Benth. of western Mexico, and the second, an unnamed species, possibly close to C. portoricensis (Jacq.) Benth. of the West Indies and southeastern Mexico. Hernandez (1986) recognized Calliandra series Laetevirentes Benth., which includes C.formosa and C. portoricensis, as a new genus, Zapoteca, on the basis of differences in pollen, seedling morphology, chromosome numbers, and reproductive features. He (Hernandez in press) studied all the Socorro and Clarion specimens Johnston had seen and two more fruiting collections, and called them all Zapoteca formosa subsp. rosei (Wiggins) H. Hern., which ranges from southern Baja California and northern Sonora to Oaxaca. Moran's three collections, which Hernandez had not seen, fall within the range of variation of the other Revillagigedos collections. Three shrubs, grown from seeds of Moran 25546 by B. Knerr at the San Diego Zoo (one subsequently transplanted to Huntington Botanical Garden, San Marino, California), flowered in late September and October of 1980 and following years, allowing us to describe the flowers of Z. formosa subsp. socorrensis for the first time. In most respects the Socorro plant seems much like Z. formosa rosei, as Hernandez recognized. Z. formosa socorrensis has white filaments, however, whereas Z. formosa rosei has filaments that are white below and red to pink or purple above. Also, the leaves of Z. formosa socorrensis are oblong and often smaller than the obovate or oblong-obovate leaves of Z. formosa rosei. Thus it seems best to treat Z. formosa socorrensis as an endemic subspecies. In cultivation the flowers of Z. formosa socorrensis are diurnal, though short-lived. If they are diurnal in the wild, Z. formosa socorrensis is unique in the genus; all other species of Zapoteca have nocturnal flowers (Hernandez 1986). Gentianaceae Key to Species 1. Corollas deep pink within, 10-16 mm long; leaf apexes blunt Cenlaurium capense 1. Corollas light pink within. 8-10 mm long; leaf apexes mucronate Cenlaurium wigginsii Centaurium capense Broome, Madrono 24:241. 1977. Distribution. — Widely scattered in openings in low vegetation, east slope of Cerro Evermann at 900-1000 m (Moran 5849 (in part), 29519), not as common as the following. Not previously reported for Socorro. Flora of Isla Socorro 39 Southern Baja California, in the Sierra de la Giganta and the Cape region. Centaurium wigginsii Broome, Madrono 24:239. 1977. Distribution. — Common in low vegetation, east slope of Cerro Evermann at 950-1040 m (Felger 15765; Levin 1816; Moran 5849 (in part), 29506). Not previously reported for Socorro. Sierra Madre Occidental of Durango and Nayarit. Remarks.— The corolla opens in the morning and closes by midafternoon. Broome (1977) described this species as closest to C. strictum (Schiede) Druce of the Nueva Galicia region of western Mexico, differing in its divaricate branches, fewer and less conspicuous bracts, longer pedicels, and other respects. Most of the Socorro specimens, however, have much less divaricate branches, leafier stems, and shorter pedicels than shown in the original figure. Goodeniaceae Scaevola plumieri (L.) Vahl, Symb. Bot. 2:36. 1791. Distribution .—LocaWy common with Ipomoea and Jouvea on tombolo (spit) connecting Cabo Henslow with the main island and separating Caleta Trueno on the west from Playa Blanca {Felger 15778; Moran 25467); not on inner beaches of Playa Blanca. Beaches through- out the tropics of both hemispheres; on the Pacific coast of Mexico north to southern Baja California. Brandegee (1900b) reported this plant from Socorro on the basis of Anthony 387, but since that specimen is labeled Clarion, Johnston (1931:98) thought the report erroneous. Although the number alone is not decisive, it does fall with those from Socorro (375-401) rather than with those from Clarion (403-417); see discussion under Botanical Collectors. Furthermore, Brandegee obviously could have had information directly from the collector, though if he did, it is strange that he did not correct the label of the specimen in his own herbarium. Because the plant is now known to occur on Socorro and apparently has not since been collected on Clarion, we are now inclined to accept Brandegee's report rather than the label locality. Hypericaceae Hypericum eastwoodianum I. M. Johnston, Proc. Calif. Acad. Sci., ser. 4, 20:78. 1931. D/5m/7ur/o«.— Occasional in rocky places from shore to summit, more common in upper half of island. Endemic to Isla Socorro; type from cliff-faces at Caleta Grayson, Mason 1614 (CAS 186606). Remarks.— This is a shrub 2-7 dm tall, with fragrant foliage. Johnston (1931:79) said that the exact relationships of this species were obscure but probably were in south- ern Mexico or South America. Eastwood (1943) has since described the related H. peninsulare from the mountains of the Cape region of Baja California. These two species are very similar, supposedly differ- ing in sepal size and shape, capsule length, and stamen number. Collections of H. eastwoodianum made since Johnston's description show that the two species overlap in the sepal and capsule characters but that H. eastwoodianum is somewhat more shrubby than H. peninsulare. Lamiaceae Key to Species 1. Calyx strongly bilabiate, the upper lip with 1 tooth or blunt, the lower lip with 2 teeth; stamens 2; anther con- nectives elongate and articulating with the filament 2. Plants suffrutescent; corolla light blue; anther connec- tives straight, with a small tooth on the upper side near the middle; upper calyx lip rounded to retuse with mucro short to subobsolete . . .Salvia pseudomisella 2. Plants strictly herbaceous; corolla dark blue; anther con- nectives abruptly bent upward, not toothed; upper calyx lip clearly mucronate Salvia riparia 1. Calyx not bilabiate, with 5 nearly equal teeth; stamens 4; anther connectives not elongate 3. Plants annual herbs; flowers small, the calyx ca. 3 mm long and the corolla ca. 4 mm long Hyptis pectinata 3. Plants perennial herbs or weak shrubs; flowers larger, the calyx at least 5 mm long and the corolla at least 1 cm long 4. Leaves cinereous, hastate, serrate; inflorescence a cymose panicle Lepechinia hastata 4. Leaves glabrous, elliptic, attenuate at base, entire or nearly so; flowers solitary in axils of upper leaves Teucrium townsendii *Hyptis pectinata (L.) Poit., Ann. Mus. Natl. Hist. Nat. 7:474, t. 30. 1806. Distribution .—Common on disturbed roadsides, as along road to Cerro Evermann trailhead at 350 m (Levin 2033) and along arroyo, Caleta Castelan near main landing (Moran 25442, 29491); also one large plant on broken lava, northeast of Bahia Braithwaite (Levin 1786). Not previously reported for Socorro and evidently a recent arrival; first collected there in 1978. Common weed in tropical America. Lepechinia hastata (A. Gray) Epling subsp. socorrensis Moran, subsp. nov. (Figure 13) Type.— ls\a Socorro, Mexico: with low vegetation at 700 m on the rocky upper south slope of the peak south of Cerro Evermann, near 18°45.5'N, 110°57.75'W, 11 March 1957, Moran 5780 (Holotype: SD 49466; isotypes: CAS, MEXU, US). Frutex statu sterih fere 3-5 dm alius. Foha cinerea, laminis 3-13 cm longis 1-8 cm latis. Corolla alba, tubo 9-11 mm longo. Typus: Moran 5780 (SD 49466). A subspecie typica foliis minoribus coroUaque alba minore differt. 40 Levin and Moran i^^SV Jdli I -riiy ■ ;.i.! int >roIU while. ...:...«,. POISONED mm LPCP ■■'9 «NniFGOSOCItr( OF NATURAL HIS10R1 C«™ -^O 3 2044 093 361 152