Erigeron jenkinsii (asteraceae: Astereae), a new species from the Río Mayo area of Sonora, México

3 Pages·1993·1.3 MB·

by  G L Nesom

Phytologia{Anguat 1993)75(2):118-120.
ERIGERONJENKINSII{ASTERACEAE: ASTEREAE), A NEWSPECIES
FROM THE RIO MAYO AREA OFSONORA, MEXICO
Guy L. Nesom
DepartmentofBotany,UniversityofTexas, Austin,Texas 78713 U.S.A.
ABSTRACT
Anewspecies, Erigeronjenkinaii,isdescribedfromtwocollections
fromtheRioMayoareaeastofAlamosinSonora,Mexico. Itisappar-
entlymostcloselyrelatedtoE.delphinifoliusbutisparticularlycharac-
terized(andcontrastedwiththelatter)byitsthin,elliptic-oblanceolate,
andrelativelysmallleaveswithmostlyentiremargins,opencapitules-
cence,smallheadswithfewflowers,andeglandularphyllaries.
KEYWORDS: Erigeron,Astereae,Asteraceae,RioMayo,Mexico
Exploration and intensivecollectingin the Rio Mayo region ofsouthern-
most Sonora has brought to light a new species of Erigeron L. The species
is named for its first collector, Philip D. Jenkins, Assistant Curator of the
Herbariumat theUniversityofArizonainTucson,
Erigeronjenkinsii Nesom, sp. nov. TYPE: MEXICO. Sonora: Rio Mayo
region,Vallecitos,nearSantaBarbara,ca.22kmbyairENEofAlamos;
ca. 27°07' N, 108°43'30" W; 30 Oct 1991, P. Jenkins 91-119 (HOLO-
TYPE: ARIZ!;Isotype: TEX!).
ErigerontidelphinifoHoWilld.similissedfoliistenuioribusellip-
tici-oblanceolatisplerumqueintegrisque,capitulismultominoribus
flosculispaucioribus,phyllariiseglandulosis,etacheniisnerviislat-
eralibustenuibusdiffert.
Annual herbs from aslender taproot. Stems mostly single from thebase,
15-60 cm tall, with 5-10 primary branches on the upper half, eglandular, the
upperportionsmoderatelytodenselystrigoseorstrigose-hirsutewithsharply
upturnedhairs0.1-0.5 mmlong,theselessdenseandspreadingbelow. Leaves
118
Nesom: NewErigeronfrom Mexico 119
eglandular, minutelyand sparsely strigose toshort-pilose,obovateorelliptic-
obovatetonarrowlyoblanceolate,not clasping,withadistinctpetiolarregion
1/10-1/8aslongastheleaf,withmarginsentireorsometimeswith1-2pairsof
coarseteeth,thelargestnearmidstem,25-40mmlong,5-13mmwide,reduced
insizeupwards but continuingrelativelylargeuntilimmediatelybeneaththe
heads where bracteate. Heads 4-6 mm wide, l-ca. 20 in a loose, paniculate
capitulescence barely recognizable as corymboid; phyllaries linear-lanceolate,
2-3 mm long, sparsely strigose to strigose-hirsute, eglandular or with a few
minuteglands. Rayflowers24-40 in 1-2 series,thecorollaswhite,(5-)7-9 mm
long, theligules 1 mm wide. Disc corollas 1.8-2.2 mmlong. Achenes 0.8-1.0
mmlong,sparselystrigose,the2lateralribsverythin;pappusof6-10 basally
caducous bristles nearly as long as the disc corollas, without an apparently
outerseries.
Additionalcollectionexamined: MEXICO. Sonora: EslopesoftheSierra
deAlamos,7kmSWofAlamos,ArroyoelHuirotal(=ArroyoelGuaje),Ran-
choUvalama,26°56'15"N,108°57'W;edgeofstreambedintropicaldeciduous
forest,650 m, 13 Oct 1992, T.R. &R.K. Van Devender 92-1310(ARIZ).
ThetwocollectionsofErigeronjenkinsiiareseparatedbyabout40kilome-
ters. Threeorfourplantswithslendertaproots,clearlyofannualduration,are
representedintheJenkinscollection. TheVan Devendersheet bearsasingle,
considerablymorerobustindividual,describedbyitscollectorsas"perfennial;"
therootismissing,however,andthereisnodoubt thatalloftheseplantsare
thesamespecies. Themarkedvariationinsizeamongthedifferentindividuals
ischaracteristicofannuds.
In its linear phyllaries, narrow white ray corollas sharply reflexing at the
ligule-tubejunction, and its few, basally caducous pappus bristles, Erigeron
jenkinsiiisclearlyamemberofErigeronsect.Polyactis(Less.)Nesom(Nesom
1989). In this sectionof20 species,thereareonly threepreviously described
annuals, twoofwhichoccurin northwestern Mexico: E. delphinifoHusWilld.,
whichappearstobetheclosestrelativeofthenewspecies,andE. inoptaiuaA.
Gray,whichhasdenselyglandularvegetativepartsandisrelatedtoadifferent
groupofspecies.
Thenewspeciesdiffersfrom ErigerondelphinifoHusWilld.inanumberof
features: (1) thinner, smaller, and mostly entire leaves (25-40 mm long and
5-13mmwidevs. 15-90mmlong,3-50mmwideand pinnatelydissected),(2)
opencapitulescence(vs. distinctlycorymboidthoughsometimesfew-headed),
(3)smallerheadswithfewerflowers(heads4-6mmwidewith24-40rayflowers
vs. 7-14 mmwidewith 110-240 ormorerayflowers),(4)eglandularphyllaries
(vs.minutelybutdistinctlygranular-glandular),and(5)acheneswiththinribs
(vs. thickribs).
Erigeron ddphinifoliusis a common species in central Durango and from
thereisscatteredsouthwardtotheTrans-volcanicRange,whereitisalocally
abundant colonizerofopen areas from Jaliscoeastward to Puebla and Tlax-
PHYTOLOGIA
120 volume 75(2):118-120 August 1993
cala. Atthenorthwesternextremityofitsrange,itisknownbyasingle,recent
collection from west-central Chihuahua (the only collection from that state,
notshownonthe1989 map)ofplantsrelativelytypicalinmorphologyforthe
species. Erigeronjenkinsiialsooccursat thenorthwestendoftherangeofE.
delphinifolius, but thetwoarecompletelyseparatedindistribution.
As noted earlier(Nesom 1989), Erigeron delphinifoliusis theonly species
ofsect.PolyactisthatrangesoutsideofthewesternSierraMadre. Further,the
centerofmorphologicalvariabilityforthisspeciesisinDurango,whereplants
commonly produce tripinnately dissected leaves. This is "near the center of
speciesdiversityfor thesection [primarilyChihuahua, Durango, and Sonora]
and it is almost certain that the widedistribution ofE. delphinifoliusacross
thetrans-volcanicmountainswas attainedafteritsevolutionaryoriginin the
western sierra" (p. 438). In this perspective, E. jenkinsii may have arisen
from the same ancestral stock that produced E. delphinifolius, a hypothesis
weightingannualduration as asynapomorphicfeatureofthetwospecies.
Occasional plants of Erigeron neomexicanus A. Gray produce a prepon-
deranceofentireleaves, but that species is a large-headed perennial usually
withstronglydissectedleaves,anditisalmostcertainlythesistertaxonofE.
oreophilusGreenman, another species with dissected leaves. Erigeron annu-
actia Nesom, the only other species in the section with entire leaves, occurs
inMichoacan andhas beenhypothesized tobecloselyrelatedto E. delphini-
foliusor derivedfromit (Nesom 1989). Relativeto E. jenkinsii, however, E.
annuactisislargerinstatureandproduceslarger,epetiolateleaves,glandular
phyllaries,andlargerheads withmorenumerousflowers,and thetwospecies
surelyhavehadindependentorigins.
ACKNOWLEDGMENTS
Ithank BillieTurner,RebeccaVan Devender,andTom Van Devenderfor
theircommentsandreviewofthemanuscriptandthestalTofARIZforaloan
ofspecimens.
LITERATURECITED
Nesom, G.L. 1989. Taxonomy of Erigeron sect. Polyactis (Compositae:
Astereae). Phytologia66:415-455.