Isomeris

Isomeris was first described by Forrest Nuttall in 1838 with only one species (monotypic). In 1888, Edward Greene (Pittonia 1: 200) transferred Isomeris to Cleome, a genus with 140 to 270 species; however, Isomeris continued to be accepted in the later California floras and manuals; e.g., Jepson 1936; McMinn 1939; Abrams 1944; Munz 1959; Vanderpool in The Jepson Manual 1993 & its Desert Manual 2002, and Stuart and Sawyer 2001. .

Phylogenetic analyses of chloroplast DNA proposed three more generic name changes during 2007 to 2015, Peritoma, Melidiscus, and Cleomella, because, as the authors had previously stated, their “well-resolved” species phylogeny is not supported at the genus level.” Y et, Iltis and Cochrane in 2007 (Novon 17(4) transferred Isomeris to Peritoma based on unresolved genus phylogeny in which the most closely related species were shown to be P. platycarpa (Cleome platycarpa Torrey 1874), followed by P. serrulata (Cleome serrulata Pursh 1814), the type species name for the genus Peritoma DeCandole (1824). Peritoma was adopted for TMJ2. In 2014, Patchell et al. showed Isomeris might be included in Melidiscus; however, in 2015, Isomeris was transferred to Cleomella by many of the same authors, and is currently recognized in the Jepson eflora (2025) and Kew (online). In the Sawyer et al. Manual of California Vegetation (MCV online accessed Feb 2023), ‘bladderpod’ has been recognized as a co-dominant for the “Ericameria linearifolia - Cleome isomeris Shrubland Alliance” within the “California Coastal Scrub” Group.

Nevertheless, Isomeris remains supported both by phylogeny and morphology in Jocelyn C. Hall, Kenneth J. Sytsma, and Hugh H. Iltis (2002).

“Within Cleomoideae there are two well-supported clades: (1) the North American endemic Cleomella, Oxystylis, Wislizenia, and Isomeris and (2) Cleome, Dactylaena, Dipterygium, Gynandropis, Podandrogyne, and Polanisia. The first clade has been studied, both with morphological (Iltis, 1957; Keller, 1979) and molecular (Vanderpool, Elisens, and Estes, 1991) approaches. These genera share a significant number of floral and vegetative features, including the possession of inconspicuous stipules, which are only found in a few other species of Cleomoideae (Iltis, 1957). In addition, there is specialization in fruit type found in these genera, with the replum of Oxystylis and Wislizenia so reduced that a bilocular fruit is produced” (Hall et al. 2002).

The transfer of Isomeris to Cleomella is not supported by the morphological differences of the fruit types for the Isomeris clade for that includes Cleomella, Wislizenia, and Oxystylis: Systematic Treatment of Fruit Types (Spjut 1994). New York Bot. Gard. Mem. 70,181 pp.): Isomeris (utricle), Cleomella (ceratium), Oxystylis (spiny achenarium), Wislizenia (camarium). Utricle: A small bladderlike or vaselike fruit composed of more than one carpel with a thin pericarp, often inflated, indehiscent or dehiscent. Ceratium: A capsular fruit with a persistent replum. Achenarium: a fruit of two carpels that separate at maturity as achenes. In Oxystylis the carpels develop into a burr-like fruit, which persists on the plant as interpreted by Fremont “ to present a singular appearance” (1845: “Report on the Exploring Expedition to the Rocky Mountains in the Year 1842”). If the fruiting carpels disperse collectively or mature ± simultaneously, the fruit then an achenosum. Camarium: A schizocarpic fruit of indehiscent fruitlets in which the seeds are not tightly adherent to the pericarp. Also, the type for Peritoma serrulata, and for the genus, does not have an inflated fruit.

Thus, the genus name for ‘bladderpod’ remains unsettled strictly from a molecular viewpoint; however, Isomeris should be recognized when there is strong support from morphological data. See Michael S.Y. Lee and Alessandro Palci (2015): Morphological Phylogenetics in the Genomic Age. Current Biology 25(19), Science Direct.

The most logical and simplest phylogeny is to recognize what the data support: Isomeris, clearly a distinct genus.

The fruit of Isomeris arborea was referred to as an utricle by Spjut (1994) because of its inflated character feature and tardily dehiscence. It varies from being nearly round (globose) to fusiform. Four varieties have been recognized based on differences in fruit shape (Jepson: A Flora of California 2: 10–11. 1936):

Var. arborea—Not clearly distinguished by Jepson (1936) from the other varieties. Type collected by Nuttall from San Diego (n.v.) distinguished by Vanderpool and Iltis in FNA (2010) by the “obovoid” shape of the inflated “capsule.” Illustration in Wiggins, Flora of Baja California (Stanford Univ. Press, 1980) for var. arborea shows long stipitate fruits that appear to initially inflate in the lower two-thirds and abruptly taper to a sharp point, and one mature utricle that is fully inflated and round at apex, much like var. globosa, but perhaps differing being less rounded to base. Wiggins (1980) indicated var. arborea occurs along the western edge of the Mojave and Sonoran Deserts to central Baja California peninsula, and on Isla Cedros.

Var. angustata Parish, type collected by Parish from Palm Springs, CA (n.v.). Described by Jepson (1936) to differ by the fruit being sharply attenuate at both ends and to occur from the Tehachapi Mountains south along the western border of the Sonoran Desert and into Baja California. Wiggins (1980) indicated var. angustata to occur from the Mojave Desert to central Baja California peninsula, and also on Isla Cedros. Vanderpool and Iltis (2010) reported its range extends further to Colima, Sonora and to islands south of the Baja California peninsula (Revillagigedo Islands).

Var. globosa Coville, type collected by Coville from Caliente (cf. CalPhotos image by Potts from Caliente). This variety appears to differ in having completely round to kidney shaped fruits, described by Jepson (1936) as varying from globose to abruptly short pointed. It is common in the foothills around the southern San Joaquin Valley with sporadic occurrences reported further south to coastal bluffs in San Diego Co. where it geographically overlaps more with var. arborea. It is interesting that it has not been reported to occur in Baja California.

Var. insularis Jepson, type from Santa Rosa Island, collected by T. Brandegee (n.v.). Distinguished by Jepson (1936) for the fruit tapering (“salverform”) to base and appearing “cuspidate” to apex, and by geographically restricted to the Channel Islands (Santa Catalina, Santa Rosa); however, Vanderpool and Iltis (2010) considered this a synonym of var. angustata, while also indicating that plants from Isla Cedros have been referred to as var. insularis. This is in contrast to Shreeve & Wiggins, in Vegetation and Flora of the Sonoran Desert (Stanford Univ. Press, 1964, I: 566) who indicated var. insularis was a synonym of var. arborea.

Additionally, several species of Isomeris may be distinguished by the length and curvature of the stipe relative to the length of the fruit Isomeris craggii Spjut ineditus, collected in San Bernardino County, has relatively short stipitate fruits; the stipe <1× the length of fruit. The fruit also differs in generally being tapered at both ends. It was initially recognized in the field by the bonsai-like habit and short flower scapes. Another tentatively proposed species, I. twisselmannii, has relatively long stipitate fruits, the stipe on average appearing 3× longer than fruit, and is erect or sigmoid. Isomeris arborea is generally recognized by the pendent fruits on recurved stipes or abruptly being reflexed near junction with stipe. Type specimens for the previously described species have not been studied (n.v.).

Isomeris arborea var. arborea Morongo Valley, CA Spjut 15758, Mar 2004	Isomeris arborea var. angustata Mojave, CA Mar 2005
Isomeris arborea var. arborea Smoke Tree Wash, Joshua Tree Natl. Park, CA Mar 2005	Isomeris arborea var. arborea Joshua Tree Natl. Park, CA Close-up of flower and immature utricle Mar 2005
Isomeris craggii ined. CA: Mojave Desert, San Bernardino Co.; Kelbaker Road, distinct for its bonsai appearance, shorter inflorescences, narrower fruits and leaves. Frequent along margins of wash, occurring with I. arborea. 19 and 31 Mar 2008, Spjut 16228	Isomeris arborea var. angustata West of Indian Avenue, north of Palm Springs, California. Feb 2008. Possibly near type locality.
Isomeris arborea var. angustata CA: Mojave Desert, San Bernardino Co.; Kelbaker Road, Mar 2008, Spjut 16228B. 31 Mar 2008	Isomeris arborea var. globosa Hillsides, Tejon Ranch near Bakersfield, CA, Mar 2010
Isomeris arborea var. globosa Kern River, riparian environment with sandy soil, Bakersfield, CA, 27 May 2006	Isomeris twisselmannii ineditus Distinct for the long stipitate erect fruit Southern Sierra Nevada, Kern River Canyon, steep rocky slopes, 12 May 2012
Trees and Shrubs of Kern County (Feb 2013) Isomeris. Low bonsai to tall tree-like shrubs; flowers yellow in terminal simple to branched scapes; fruits inflated, balloon or bladder-like. Generally regarded monotypic, the one species divisible into three or more varieties, three in Kern County; however, two additional species are apparent, one of which is in Kern County. the other in San Bernardino County. The genus Isomeris was placed in the genus Peritoma (Vanderpool and Iltis in FNA 7: 205–08, 2010) with 5 other species, all annuals with non-inflated fruits. Peritoma arborea is thus easily distinguished by its shrub habit and inflated fruit. However, these species also have been formerly classified in the genus Cleome from which Peritoma was subsequently distinguished by its “filaments inserted on a cylindric androgynophore” and “folded leaflets,” in contrast to the “discoid or conical androgynophore” and “flat leaflets” in Cleome. The decision here is to retain Isomeris for its shrubby habit and inflated fruits. At least three species are evident as delineated in the following key. Key to species and varieties of Isomeris 1. Fruit long stipitate, stipe ~3× longer than fruit, not recurved, not reflexed nor dangling, but straight to slightly sigmoid (S-shaped) or ascending; Southern Sierra Nevada, Kern River Canyon................................. Isomeris twisselmannii 1. Fruit short to moderately stipitate, stipe <2× length of fruit, recurved, or reflexed near junction with fruit, or dangling.................................................................... 2 2. Stipe <1× longer than length of fruit; flowers scarcely exserted from leaves; plants low, broader than high; known from the Mojave Desert, San Bernardino County, not in Kern County.........................................................................Isomeris craggii 2. Stipe 1–2× longer than length of fruit; flowers exserted from leaves; plants erect shrubs, taller than broad; widely distributed in California, including Kern County.... *Isomeris arborea* Three varieties distinguished as follows (trichotomy): ..... Fruit ±spherical. scarcely longer than wide (1–1.5×), like a balloon hardly tapered at ends; Valley foothill............. ........................... var. globosa Fruit inflated more towards apex, like a punching bag, 1.5–2.5× longer than wide, abruptly tapered at both ends; Southern Sierra Nevada, and adjacent desert slopes............................................................ var. arborea ...... Fruit inflated more to center and gradually tapered to base and/or apex, much like a hot green pepper; >2.5× longer than wide; Mohave Desert.......................................................................... var. angustata *Isomeris arborea* Nuttall 1838. Bladderpod [Peritoma arborea (Nuttall 1838) H.H. Iltis 2007. FNA, JM2; Cleome isomeris Greene 1888]. Shrub 1–3 high and broad with soft foul-smelling leaves and terminal linear arrays (racemes) of yellow flowers; the mains stems whitish gray, yellowish when slashed. Leaves digitately divided into three slightly elliptical leaflets, ~4–6× longer than wide, folded slightly along the midrib. acutely tapered to a pointed apex, on stalks (petioles) 1–2.5 cm long. Fruit balloon or bladder like (utricle). Common in washes and low sandy areas in the desert and foothills in Kern County, Channel Islands (Santa Rosa, Santa Catalina), from southern California to islands south of Cabo San Lucas in Mexico. Desert bladderpods generally flower anytime after rain. One shrub species with three varieties distinguished in JM2 by the shape of the pods (in the genus Peritoma that includes 5 other annual species). var. angustata Parish 1907, type from Palm Springs. Plants collected by Spjut from near Palm Springs for chemical studies have fruits gradually tapered to base and strongly inflated to apex; however, in JM2 the variety is defined as being fusiform, which means it is narrowed at both ends. Kern Co.: ~ 5 miles north of Randsburg near Saltdale; 10 miles north of Johannesburg along Hwy 395; Ridgecrest Highway at the east end of the El Paso Range; Iron Canyon, El Paso Range; east end of the El Paso Range on Highway 395 (near the Garlock Road); slopes above Mojave on Tehachapi Road, 670–1,066 m (CCH). *var. arborea,* type from San Diego. Fruits abruptly tapered to base. Kern Co.: Tehachapi Pass, 5 mi. from Mojave; mouth of Red Rock Canyon; Tehachapi Mountains - Bird Bath Spring in a fork of Pine Tree Canyon; Antelope Valley; Mojave Desert - mouth of Cache Canyon (lower Tehachapi Pass); Grapevine Canyon south of Inyo border, 731–1,158 m (CCH). var. globosa Coville 1892, type from Caliente (cf. CalPhotos image by Potts from Caliente). Fruit entirely balloon-like, not tapered at base. Foothills and valley floor Hills bordering the west and south parts of the Great Valley near Bakersfield,304–1,280 m. This variety is a dominant shrub in the lower foothills west of Hwy 58, but the species is not in MCV2. However, Isomeris arborea and Isomeris arborea-Ericameria nauseosa shrubland alliances proposed (Magney 2010) The original Jepson flora (1936) also recognized a fourth variety: Var. insularis Jepson, type from Santa Rosa Island. Distinguished by the fruit tapering (“salverform”) to base and appearing “cuspidate” to apex, and by its geographical restriction to the Channel Islands (Santa Catalina, Santa Rosa; Jepson 1936); however, Vanderpool and Iltis (2010) considered this a synonym of var. angustata. They also indicated that plants from Isla Cedros belong to var. insularis in contrast to Shreeve and Wiggins (in Vegetation and Flora of the Sonoran Desert (Stanford Univ. Press, 1964, I: 566) who indicated that var. insularis was a synonym of var. arborea. It is not clear whether all authorities since Jepson have studied the type specimens, and to what extent the varieties may occur together. A fifth variety or possibly a distinct species has been suggested by Spjut as being different in its bonsai habit with shorter flowering scape-like stems, which scarcely extend beyond the leafy stems, and by the fruit appearing narrowly ellipsoid and long tapered towards the apex into a sharp point. It was collected in San Bernardino Co and noted to be common along I-40 just east of the road to the Mojave Preserve. Isomeris twisselmanni ineditus. More study needed on mature fruits, appearing distinct for the erect long stipitate fruit, Kern River Canyon, ~750 m.

Phytochemical Referenes

Blua, M. J. and Z. Hanscom III. 1986. Isolation and characterization of glucocapparin in Isomeris arborea Nutt. J. Chem. Ecol. 12(6): 1449–1458 “Isomeris arborea (Capparaceae), is the only woody caper endemic to southern California and northern Baja. Methylglucosinolate, also known as glucocapparin, was the only glucosinolate found in I. arborea organs by paper chromatography of the thiourea derivatives and was quantitatively determined by gas chromatography by hydrolytic products. The concentration of glucocapparin ranged from an average of 4.6 mg/g wet weight in mature leaves to 5.2 mg/g wet weight in immature leaves. Buds averaged 6.2 mg/g wet weight and capsule walls 1.8 mg/g wet weight. Seeds contained an average of 14.3 mg/g wet weight of glucocapparin. Glucocapparin concentration was found to vary significantly among the mature leaves of individuals within a single population. This compound is known to be deleterious to nonadapted herbivores and may be implicated in the chemical defense mechanism of I. arborea.”