A New Foliose-rooted Genus of Podostemaceae from Thailand with a Note on Root Evolution

2004·1.5 MB·English

by  Kato, Masahiro

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ISSN 1346-7565 Acta Phytotax .Geobot. 55 (2) 6;5-73 <2004)
A New Foliose-rootedGenus of Podostemaceaefrom Thailand
with a Note on Root Evolution
MASAHIRO KATO,SATOSHI KOI YOKO KITA
and
Department oj'Biolog Siccieanlce sG,raduate School qfScience, of n')kyo,
7-3-1llbngo,foio,oi13-O033,JLipan[1}iivenyio･

Bascd on comparative flor amolrphology, and supported by molecular phylogenetic data t,hc fo1iosc-root-
ed 71hawatcha ina'ilobata gen. et sp. nov. (subfam iPoldyostemoideae )is describe dfrom northern
Thailand S.imilarit aineds dissimilari tiin evcsgetative and reproductive characters between 1'kavv'atchaia
and related foliose-ro oAtsieadn genera arc discus¢sd and the consequence oftheir bearin gon gener-
ic recognition is noted. The marginal meristem, which give srise to the fblios rcoot, was probably estab-
lishcd at the base of a lineag leeadin tgo 77zawatcha iHab,inseniel alnda I'lydivbiyun ?A. key to the Asian
fbliose-roote dgenera is providcd.
Key words: Hdnsenietla ,Ilydlobrpum, Podostemaceae, Thailand, 77)awatcha itrailobat a(gen e,t sp. ney.)
The Podostemaceae are unusual aquatic angio- southern India .In a]1 the genera the folios reoot is
sperms that occur on rocks in waterfa11s and rapids. lobed and dorsiventr atlo the rock surface with
Vegetati vpelant sgrow submerged in swift-running adventitious sheots (tuf otfs leaves )and flowers
water durin gthe rainy season and become emerged s¢attered on the dorsa lsurfacc and adhesive root
and withered during the dry season when water hair son the ventral surface, The gcnus Htrnseniella
leve dlrops .Flowers generall yopen and seeds are set is characterized by the 4-ranked d,imorphi c(lobed
shortly after emergence. Adaptation sto those and simple) bracts c,a. 1O ovules per ovary (i, cea..
include 5 locule), 8-ribbed (Cuss1e9t92).
cxtremc environmental conditions various- per and capsule
ly flatten e(dnear clylyindrical, ribbon-shaped, or flydrobryum and Diplobryorm are distinguished
fbliose )cr,eeping and adhering (i nsorne species from Hdnseniella by the 2-ranked, entire, uniform
fioatin gro)ots. The most specialized is the fbliose bracts ,and 12-70 ovules per ovary (6-3 5ovules
root, as seen in Asian and African rnembers of per locule ).H.vdrobs:yum and Diplobr.vum differ
Podostemaceae. in that Ifydrobryu mhas 1O-14 ribs on the capsule,
There are fbur fbliose-roo tgeednera in Asia, and Diptobryunz is characterized by the 20-ribbed
Htinseniell al,lydrobilytt mp,ar tofDiplohtTum, and capsule (Cuss eIt972, 1992). Those diagnostic
part of Zaylanidium, all referred to subfamily characters have recently been revealed by fiel adnd
Podostemoideae. The firs tthree g¢nera are dis- herbarium research of the Podostemaceae for the
tribute din Thailan dand adj acent southeastern and Flora of Thailan dto be at leas tin par tuseless (M.
eastern Asia, and the las toccurs in Sri Lanka and Kato unpubl. data) ,An undcscribed spccies of
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Hanseniella has capsules with up to 16 ribs ,and an we came across a foliose-roo tspeedcies. It is dis-
undescribed species of Ilydrobryum has fewer tinct from all foliose-roote dgenera noted above,
ovules than the rest ofthe genus and capsules with particularl yin the bract morphology, and is de-
up to 20 ribs a,s in D. minutale, A new definiti iosn scribed here as a new genus and species. This pro-
therefore needed to reflect accurately the systemat- posa] is supported by a moiecular phylogenetic
ic relationships. The southern Asian Zaylanidium analysis (Kha & Kato in press )T.he fbllowing is a
differ fsrom all other genera in that the capsule key to the five genera with fblios eroots in Asia,
splits int o2 unequal valves. although the phylogenetic relationships ofDiplo-
During fiel dresearch in northern Thailand, bplyu mremain uncertain yet.
Key toFoliose-rooteGdeneraof Asia

1. Capsule splitting into2unequa lvalves .,,,,,.,,...,,,.,.,,,.,,,,.,,,,,.,,,...,,,.,,,,.,,,,,..,,.,,,.....,,.,,,
Zcytanidium (fbliose- srpecoieos:t eZLd olivaceum, Zl maheshvearii)
1 Capsule splitting into 2 equal valves,,...,.,,..,,..,,,..,,,,..,.,,.,,.,.,,,..,..,...,,..,,,,..,...........,,,,. 2
2. Floral shoot erect or suberect; bract s4-ranked, dimorphic, ventral ones (faci rnoogt surface) bilobed,
dorsa lones simp]e ,,...,,,.,..,,.,,,...,,,...,,,,.,...,,,..,,,,,..,,,..,,..,,,,..,,,,,,,,,..,,....,Iinnseniella
2, Flora lshoot appressed or strongly oblique to root surface; bract s2-ranked u,nifbrm.....,..,.,,..,.,,
"v-""",."".".-H",v"HH-,."",."v.H"-".HHH".."--,w".H.-".H"-""H-"-.3
3, Bract trilobed ,later allobes acute, small lower bract swith small round lobes at base ;capsule ribs 8-
1O ........................,...........,..,...................................,.......................T7iawatchaia
3. Bract simple, ovate; capsule rib s12-20 .....................................................................4
4.'Spathella longitudinally; 18-20,,,....,,...
splitting stalk ofovary straight or curved; capsule ribs
Diptobt:yum (foliose-r spoecoiets:ed D. mtnutale, D, vientianense)
4. Spathel lruapturing irregula rnelayr apex or rarely splitting longitudina l(liyfspat hspelilttlinag lon-
gitudinal tlhMen stalk of,ovary always straight; ribs ofcapsule 12-17) ;capsule ribs 12-20 (ifr i18b-s
2O,spathellarupturingirregularly)..,...,,..,.,............,.............,...,,.,,.,,.......,.H)'drob,:yum
Thawatchaia trilobat aM. Kato, Koi & Yl Kita, 2)Ipus :Northern THAILAND. M Kato, R.
gen. et sp. nov,, Figs ,1, 2, 3A-C imaichi ,S. Kbi & T VPbngprase r7tZ-1003 (holo
Radices crustaceae, irregularit lerobata ef;olia BKF; iso TI), Mae Wang stream, north of Doi
3 longa;bracteabeifariae,intharioNnatl,Park,ChiangMai,450 18038'
caespitosa, usque ad mm m alt,
usque ad 12, superne trilobata el,ob iacuti; fiores N, 98043'E, December 24, 2003; fi ,and fr
valde obliqui vel appressi; tepala 2; stamina 2, infra
medium connata; ovaria 2-locularia ,ellipsoidea; opecimen sexamined: Northern THAILAND. M.
Ktite,S,KZ)i,Z Kita& Z Pfongr)rasertZL-419,M 1<dto,

ovula in quoqu elocul o7-9; stigmata 2, aequalia, C. 71yutsumi& Z PPbngnrasert TL-502, M. Ikrato,C.

subintegra; capsula 8-10-costa taA. Hanseniella lsutsum i& T PPbngprase rZtIL-50 3M,ae Klang waterfa11,
et Ilydrobr:y obractei sbifaris t,rilobati sdiffert. Doi Inthanon, Chiang Mai, 450 m alt., 180 29' N, 980 40'
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. '1i'xxLxsN
it
F x
tr
pt
D x K't7.)
e
7
c
tw
FIG,1,Cf7tawatchai atrilobata. Illustrati oofnKsdto et aL ZL-809 and IL-1O03. A, Roet with tuft sofleaves. B. Root with fiowers,
Ybung flowe rD.. Flower. E. Ybung ftui tF,, Bract .Scale bar s= 3 mm in A, B; 1 mm in C-F,
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ge
J
'
FiG .2. Photographs of 11tawatchai atrilobala .A. Foliose root with tufts ofleayes on dorsa lsurime Plant on rock in nature. Scale
bar = 1 cm. B, C. Flower bud shoots, Asterisk sindicat elater ablract lobes .C, dried specimen (Kbu etto aL 71L-809 )S,cale bar =
1 cm in B; 1 mrn in C, D. Flowers with reclpuTple ovarics at anthesis. Scale bar = 1 ern. E. Dehisced capsules on withered root.
=
Dried specimen CKbt oet al. 71L-809 )Scale bar 3 mm.
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te
B
te
D

ge
s'
FiG. 3. TV!awatchai tarilobata .Kato et al, 7Z-809. A. Lengitudina slcction of fblios reoot with marginal meristem and protect itivse-
sue. B. Longitudin aslection ofendogenous shoot primordium .C. Cros ssection ofyoung flowe rN.ote tip ofcurved stigina between
stamcns on left D.. Longitudinal scction ofroot ofHansenietta heteiQph.vU aS,cale bars = 1OO m. mm, marginal meristem; ov,
,fi
ovary; pt ,protectiv teissue; sp, spathella; st, stamen; te, tepal.
E, fl .bud, st. Dec., Mar,; M. Kato, R. Jmaichi & Z and fr .Mar.; M 1<ket oR,. Iinaich &i T Pfongprase r7tZL-
Pfongpra,s '7e1rLt-S04 W,ang Kwai waterfa]1 near Forest 809, Mae Wang stream, north ofDoi lnthanon Natl. Park,
Protectio nStatio nabove Mae Klang waterfa11, Doi Chiang Mai, northern Thailand, 450 m alt., 18038'N,
Inthanen C,hiang Mai, 450 m alt. ,1 8" 30' N, 98"40' E, fi. 980 43 ' E, fi a.nd fr ,M,ar.
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Roots fblios e(crustace iorurseg)u,lar llyobed, to investig atthee Podostemacea oef Thailand fbr the
ca. O.3 mm thick ,with tuft sof leaye sscattered on Flora ofThailand, Mr. Thawatacha iWongprasert
dorsa lsurface, patent, surrounded by root rim; guided us to many differen tlocalit iien sThailand,
leaves 2-5 per tuft, 2-ranked, Iinear ,sheathed at includin gMae Klang waterfa11 and Mae Wang
base ,teret empicall 1y-,3 mm long .Flowering shoots stream, to collect Podostemaceae ,The Mae Wang
bearin gsolitary terminal flowers ,scattered on dor- (typ leocality p)opulatio wnas discovere bdy him,
sal surface ofroot, appressed or sometimes strong- In a molecular phylogeneti ctree based on
ly oblique to root surface, rarely ascending, 2-3 matK sequences, 77iawatcha iias roughly as remote
mm long, flowers strongly oblique; bract sthick, 2- from Hanseniella ( 1l 2 bp of 1530 bp sequenced) as
ranked, 4-6 per rank, upper ones trilobed or upper- from llydit)b,:y uimncludi nSg)?nst [y8t 8bips (mean)
most brac tsemetimes bilobe dwith ventral lateral from nine species], compared with the interspecifi
lobe reduced, T.5-3 mm long ,lobe sbluntl yacutc, ic differenc eamseng the species of HYdrobr:yum
ventral lobes smaller, lower bract ssmaller than the [50 bp (mean )(]Kit &a Kato in press) .The three
upper, ca, 1 mm long with small round lobes at genera and Cladopus form a monophyletic group,
base ;spathella thin ,mucronate with mucrones ca. which is in turn sister to the Zaytanidium-Pbly-
O.3 mm long ,ruptured irregula rnelayr apex; stalk of pleurum group ,This Asian group is elose to the
ovary (pedun celxterud)ed from spathella, 2-2.5 Africa annd American groir pofssubfatnily Podostemoi-
mm long ;tepals 2 on both sides of stamen, borne deae ,which are related to the other two subfarnilies,
above base ofovary stalk, 2-3.5 mm long, reaching Weddellinoideae and Tristichoid e(aKeit &a Kato
above middle of ovary, linea rs;tamens 2, connate 2001) .Morphologicall y7,'7tawatch aiis asimilar
near or above middle offilament, 3-5(-7 )mm long, to Hl nseniella in some characters, while more sim-
longe rthan ovary, anthers oblong-ellipsoid, ea. 1.2 ila rto IZydrobi:y uimn others (Tabl 1e; see also
long, Key), Zhawatchaia,however,isdistincftrom
mm withered shortly after rupturing of spathel-
la ;ovary 1, stalked (sta olfokvary ca. 2 mm long), Hanseniella in the flowe rshoot and flowe irnser-
ellipsoid, 2,5-3,5 mm long, locule s2, red-purple tions, number of ranks of bracts b,rac tuniformjty
when fresh s;tigmas 2, forked near base, equal, lin- and form, and number of ovules, and from
ear, entire, pointed ,O.6 -l mm long ,withered short- H.vdrobFlyum in the number ofbracts, brac tform,
ly after extruding from spathella; ovules 7-9(- l1) per and number of ribs on the capsule (Tabl 1e).
locule ,borne on marginal surface of septa or on Because ofevidence from both molecular and mor-
entire septa surface except in small lower central phologica llines w,e recognize 7Ziawatcha iaat the
area; capsule stalked (sta l2-k3 mm long), ellip- rank ofgenus, The genera are compared in more
flattene2d-,3 long,1 ribs8-1O. detaiblelow,
soid, mm mm wide,
Habitat: On rocks in waterfa11s and rapids, 7]liawatchai ias similar to Hlxnseniel lIal,ydto-
711tawatchai atrilobata occurs with other podo- br:yum ,par tofDiplobrlyum and part of ZIp)lanidium
stemads in Wang Kwai waterfa11 and in Mae Wang in the fblios reoots with tufts of leave sand fiowers
stream. scattered on the dorsal surface, and the fiower com-
Distribution T:hailand (Northern), posed of a spathe]la (cov ecrontaining a fiower
7Zixonomic note: The genus is dedicate tdo Dr. bud) ,two linea trepals ,two starnens (o rrarely one)
Thawatchai Santisuk and Mr, Thawatchai Wbng- on a common filamen (tandrope da)nd, a 2-locular
praser tD.z Thawatchai Santisu kguided us te Mae ovary with two stigmas (Fig s1,. 2, 3C). 711iawa-
Klang waterfall, but we faile tdo fin dthe plan ton the tchata is most similar to HansenieUa among the
fir svitsit in 1999, He encouraged one of us (M.K.)foliose-ro goetneedr ain the 1O or more lobed bracts,
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TABLE 1, Comparison betwee nTlha",atcha ianad Asian fbliose-rQot egdenera ofPodestemaceae. Data from Cusse t(1972,
Mathcw & Satheesh (1997 )K,ato & Fukuoka {2002 )K,ato & Kita (20e3 a)nd, M, Kato' sunpublished results.1992),
Characters Tliawatchaia HL!nseniella flydrob-,ttm Diplobrp'um' Zeytanidium*"
Ftower shoot apprcssed erect or suberect appressed appressed appressed
Flower ob]ique erect appressed, erect erect
rarely erect
No. ofbract ranks 24-6uniformtrilobedirr4'S7--89deqiumae]rgp-Iheic 21.4uniformsimplei2r2r-.3,uniformsirnple2I3o-n4g,u2n2i-f3o5neqmusailmlpgl-2colong.45-50unequalg
No. of bract sper rank
Bract unifonmity
Braetform bilobed
and simple
Spathellabreak*'" 1rr.4-6equal8, rarely long.
Ovulc no. per locule 4-29equalI2-20
Capsulevalyes
No. of capsule ribs 12r16
* fbliose-root espdecies: D, minutale andD, vientianense.
** foIiose-ro ospteecides/ Z. otivaceorm and Z maheshwarii
""* irr .i,rregu[ar lruyptured: long, ,longitudinal lspy]it.
less than 1O ovules per locul ea,nd les sthan 10 ribs the apex. It differs h,owever ,in the thick trilobed
on the capsule wall. Lobed bract salso occur in bract usp te 6 per rank, the 7-9 ovules per locul e(vs.
Claclopu swh,ich has subcylindrical or ribbon-like usually more than 1O in ftydtobF:yu mexcept in two
roots, and is sister to a clade ofthese genera and undescribed species with 4 or 6 ovules), and the 8-
flydrobo,um, 77iawatehaia and Hkenseniel lshaow 10 capsule ribs,
furthe arnatomical similarity in that the cortex ofthe Diplobryum comprises two foliose-rooted
root consists ofthree layer (sFig 3sA,, D). The dor- species and two other species with floatin cgy,lin-
sal and ventral cortex is composed of elongate drica laxes (roo tors shoots) presumabl ybranched
parenchyma cells, while the middle cortex is com- from adhering roots (Cuss e19t92, Kato & Fukuoka
posed ofshort cells. In comparison, the root cortex 2002), but perhaps the genus is ho tnatural. The
is uniform in Ilydtobi:} ,1u'amponicum (Ot aet al. foliose-roo tspeedcies, D. minutale from southern
2001) and Zqytanidiu molivaceum and Z mahesh- Vietnam and D, vientianense from Laos ,share the 4-
warii (Hiyam eat al. 2002). 6 simple bract son 2 ranks (2 or3 per rank), longi-
711iawatchai adiffe rfrsem Hdnseniell ain the tudinall ysplit spathella, and capsules with al)ou 2t0
flowe rshoots appressed or oblique to the root, the ribs .Each ofthe characters, howeveg is seen in dif-
uniform anq usually trilobe d2-ranke dbract st,he feren tspecies of LlydrobF:yum and a phylogenetic
thin spathella, and more than 5 ovules per locule. analysis is needed to make clear the systematics
The 4-ranked bract sof Hbnsenielia, tike those of ofllydrobryum and Diplobr:yt{ m.The phylogenet-
uallisi a(Uniy &al Mohan Ram 200i) ,are unusual ic relationship ofDiplobt:yum and 7)haveatchai ias
in Asian Podostemoideae, includin tghe clade of also uncertain, although the genera diffe rmorpho-
CladQpus ,HdnsenieUa ,ilydtobs;Fum and 7}hawa- logicall yto a considerable degree ([la b1l)e,
tchaia, Zeylanidium is a southern Indian and Sri
11hawatchaia is also similar to IlyclrobrlyumLankan genus with two foliose-roote sdpecies (Zl
in the flowe rshoots being oblique or appressed to olivaceum, Zl maheshwarii) and a few otheis with
the root surface, the bract s2-ranked and unifbrm, subcylindrical or ribbon-iike roots (Cuss e1t992,
and the spathella thin and ruptured irregular nleayr Mathew & Sathees h1997) .A molecular phylo-
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geneti cstudy indicate tshat the genus is mono- micranthera the secondary root fbrms exogenously
phyleti acnd remote from the group ofllanseniella, frem the later sailde ofthe hypocoty alnd no primary
flydrobryum and 71hawatchaia (Kit &a Kato 2001, shoet is formed.
in press) .Zeylanidium, lik eit4tijkrria, is character- In the root of 7-Ziawatcha itarilobata and
ized by the capsule with two unequal valves, com- Hlznseniell ahetempltyl tlhaere is a marginal meris-
pared with the equal-valved capsules of the group of tem covered by a protecti tviessue or root cap (Figs.
Hansenielia, Ilydrobryu mand 711iawatcha (ifaor 3A, D). Tufts of Ieave sor reduced shoots are
other characters, see Table 1). endogenous in the root. These characters are sbared
Tliawatchaia ,lik eHdnseniella, is endemic to not only by Hydrobrl Jy'uapmonicu m(Ot aet al,
northem Thailan di,n contrast to f]lydiobiy uwhmic,h 2001) (an dperhaps DiplohTlyum) but also by
is widely distribut efdrom nonhern India eastward to Zqylanidium olivaceum and Z, maheshwarii,
Japan and southward to Thailand (Cuss e1t992, although the protectiv etissu eis only slightly devel-
Kato & Kita 2003). [fakin ginto account these dis- oped in Z. maheshwarii (Hiyam aet al. 2002).
tributional data, the phylogeny with 71iiawatchaia Hiyama et al. (200 2hy)pothesize tdhat the protec-
and Hdnseniell baasa lin the fbliose-roo tgerdoup, tive tissue of the Zaylanidium species appeared sec-
and the occurrence ofmost species of Jlydiobtyum ondarily from the naked root meristem seen in Z
in nomhem Thailand, Thailan dis likel tyo be a cen- lichenoid eansd Z, subulatum.
ter of d{versificat oifo nthe Asian foliose-rooted The differen cbeetween the fblios eand rib-
Pedostemaceae .This suggested biogeograph yis bon-lik eroets is due to a differen cien the root
consistent with the high diversi otfyPodostemaceae meristem. In H)pdrobi:yum.ia pZeoyntainciduimu,m
- nearly 30 species - in Thailand w,hich is marked- olivaceum and Z maheshwarii, the marginal meris-
ly more than in neighboring regions (M. Kato tem along the rnargin ofthe root lobe is unifomi and
unpubl, data), deterrnin aitn ethe productio onfroot tissues (Ot aet
;Vbt eon root evoiution: The folio sreoot is still al. 2001, Hiyarna et al, 2002) ,It is likel ythat the
an enigmatic organ. Compared to the absorbing other species ofIlydtobFyum, 77iawatcha tirailobata
and anchoring ordinary root, it is multifunctional, i.e. and Hdnsenieila hetemplppl lhaave a similar root
adhesive to the rock surface by root hairs p;hoto- meristem. In comparison, the root of Cladopus
synthetic in place of reduced leaves w,hich may fakien iss inasrrow (ca 0.,6 mm wide) and that ofZ
dioxide lichenoides Z is1-2 The
absorb carbon and oxygen atid minerals; and and subulatum mm wide.
reproductive by preducing fiora lshoets. Phylo- apical meristem oftheir roots is indetermina tase ,in
genetic analyses show that the folios reoot ef the typical angiosperm roots, but compressed as is the
group comprising Ify(lrobf:y uHma,nsenieUa and extemal merphology (Hiyam eat aL 2002, Koi &
77iawatchaia ,and that ofZeylanidium, are probably Kato 2003) ,In a study ofthe developmenta alnato-
derived independently from a ribbon-like root sim- my of C. javanic woitrh s broadl yribbon-like (ca 4,
ila rto that in species of Cladoptt asnd Zaylanidium mni wide) roots, Koi & Kato (200 3sh)owed tha tthe
(e. gZ,, tichenoides )re,spectively (Kit &a Kato root meristem consists of an apical meristem and
2001, in press) ,A comparatiye study of seedlings marginal meristems on both sides ofit, Cladbpus
also supports this independen tevo]ution (Suzpkiaymanii is inteTmediat ebetween C. .fitkiens ainsd C,
et al. 2002) .In Z olivaceum the secondary root juvanic iuns the root morphology and meristem
form sendogenous]y in the hypocoty 1an,d a reduced stmcture, Koi & Kato (200 3i)nferr etdhat rednction
primary shoot lplumu lfber)ms betwJeen the cotyle- ofthe apical meristem or replacement by a marginal
dons but is ephemeral. In Uy Jg,rij77t hanidi I(p meristem was inyolv iend the evolution ofthe foliose
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root, The evolution may have occurred at the base Laos, Acta Phytotax ,Geobot, 53i l 15-120,
ofthe lineag leeadin gto T7iawatchai Had,nseniella --- & Y Kita ,2003, Taxonomic study ofPedostema-
and Lijdrobr)?u mI.t is perhap salso the case with ccae ofChina. Acta Phytotax. Geobot. 54: 87-97.
Kita,Y.& M. Kato.2001.Infrafamilial
Zt71anidium olivaceum and Zl maheshwarii phylegeny of
the aquatic angiospenn Podostemaceae inferre dfrom
Wlethank Tha-'atchaSiantisuTkh,awatchaiWbngprasert, thc nucleotide sequences ef the matK gene. Plant
Biol.3:156-163.

and R. [maichi for their help durin gthe cellecting trip sin & . Molecular phylogeny of Cladopus and
Thailand. We are also indebte dto reviewers for their use- Ib･'drobfl p(uPmodostema cPoedaoset,emoideae) with
fu1 suggestions, D. E. Bouffbrd for his reading the man- implicatio nfsor their biogeograph yin East Asia,
uscript with linguist ciercrections, and H. Okada for his Syst .Bot. (i npress)
advice on nomenclature. This study was partl ysupport- Koj, S,& M. Kato.2003.Comparativcdevelopmental

ed by a Grant-in-Ai dtbr Scientif iRcesearch from the anatomy of the root in three species ef Ciadopus
JapanSocietyforthePromotion
ofScience. (Podostemacea Aen)n.. Bot. 91: 927-937.
Mathewl C. J .& V/ K. Satheesh 1,997, Taxonomy and dis-
References
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Receivedfebruaf:), 12, 2004 ; accepted April 1 ,2004
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