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The genus Isomeris Nuttall, generally considered monotypic, was recently included in the
genus Peritoma deCandolle by Vanderpool and Iltis in Flora North
America (FNA) 7: 205–08 (2010) that included five other species that
differ by their annual habit and non-inflated fruits, in contrast to the
shrub habit of P. arborea with its bladder-like fruits. These
species also had been formerly treated in the
genus Cleome; however, Peritoma is distinguished by
having
“filaments inserted on a cylindric androgynophore” and by the “folded leaflets,”
in contrast to the “discoid or conical androgynophore” and “flat leaflets”
of Cleome.
Another related genus Wislizenia,
which comprises both annual and shrub species, differs by its schizocarpic
fruit. Although Isomeris would seem to have a distinct fruit
and habit, it has lost its generic status in FNA.
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.).
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Isomeris arborea
var. arborea
Morongo Valley, CA
Spjut 15758, Mar 2004 |
Isomeris arborea
var. angustata
Mojave, CA
Mar 2005 |
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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 |
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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.
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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 |
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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 |
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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.
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